Geology of the Minturn 15-Minute Quadrangle, ,J Eagle and Summit Counties, Colorado By OGDEN TWETO and THOMAS S. LOVERING GEOLOGICAL SURVEY PROFESSIONAL PAPER 956 Prepared in cooperation with the Colorado Mining Industrial Development Board Geology of the region surrounding a major zinc mining district and the Vail recreational area, with emphasis on the Minturn Formation of Pennsylvanian age UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 21977 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 77-600036 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001-02995—2 CONTENTS Abstract ................................................ Introduction Geography .......................................... History of investigation ............................... English and metric units .............................. Rock formations ......................................... Precambrian rocks Biotite gneiss .................................... Cross Creek Granite and related migmatite and diorite ........................................ Cross Creek Granite .......................... Migmatite .................................. Diorite ...................................... Inferred history .............................. Age and correlation .......................... Cambrian System .................................... Sawatch Quartzite ............................... Peerless Formation .............................. Ordovician System ................................... Harding Sandstone .............................. Devonian and Mississippian Systems ................... Chaffee Group ................................... Parting Formation ........................... Dyer Dolomite ............................... Gilman Sandstone ........................... Leadville Limestone (or Dolomite) ................. Pre~Belden unconformity and Molas Formation ......... Pennsylvanian and Permian Systems .................. Belden Formation ................................ Mintum Formation .............................. Subdivision Lithology ................................... Sedimentary features ........................ Type section ................................. Clastic unit A ............................... Clastic unit B and the dolomite bed of Dowds . . . Clastic unit C and the reef dolomite of Lionshead Clastic unit D Clastic unit E and Wearyman and Hornsilver Dolomite Members ......................... Robinson Limestone Member ................. Clastic unit F ................................ Elk Ridge Limestone Member ................. Clastic unit G ............................... White Quail Limestone Member ............... Clastic unit H ............................... ’6 In N a MQOIOICDODODH 12 13 14 14 15 15 19 21 22 23 24 24 27 28 30 32 33 34 38 39 41 43 43 43 43 44 44 44 45 45 45 45 46 Rock formations — Continued Pennsylvanian and Permian Systems — Continued Minturn Formation — Continued ‘Jacque Mountain Limestone Member ......... Changes in thickness and facies .............. Fossils, age, and correlations ................. Origin ...................................... Maroon Formation ............................... Stratigraphic relations ....................... Thickness ................................... Character ................................... Fossils and age .............................. Origin ...................................... Triassic System ...................................... Chinle Formation ................................ Jurassic System Entrada Sandstone Morrison Formation .............................. Cretaceous System ................................... Dakota Sandstone ............................... Upper Cretaceous and Tertiary igneous rocks ........... Pando Porphyry sill .............................. Dike rocks ...................................... Volcanic rocks ................................... Physiography and upper Tertiary and Quaternary unconsolidated deposits ............................ Tertiary and Pleistocene (7) colluvium Pre-Bull Lake glaciations ............................. Bull Lake Glaciation ................................. Eagle River and tributaries from Sawatch Range” Gore Creek drainage ............................. Piney River Pinedale Glaciation .................................. Landslide ........................................... Alluvium Structure ............................................... Gore fault ........................................... GoreRange ............... Sawatch Range ...................................... Central sedimentary belt ............................. Bedding faults in Gilman area ......................... Economic geology ........................................ Type section ............................................. Index ................................................... Page 46 47 48 52 53 53 54 54 55 55 56 56 57 57 57 58 58 58 58 61 62 63 65 65 66 66 67 67 68 68 69 69 71 74 75 75 77 77 79 89 93 P'I'MQI CONTENTS ILLUSTRATIONS Page PLATE 1 Geologic map and sections of the Minturn quadrangle ...................................................... In pocket FIGURE 1. Index map of west-central Colorado showing geographic setting of the Minturn quadrangle ......................... 4 2. English-metric conversion scales ............................................................................. 5 3 —7. Photographs: 3. Canyon of the Eagle River at Belden and Gilman ........................................................ 7 4. Cross Creek Granite .................................................................................. 10 5. Migmatite from the Gore Range ....................................................................... 12 6. Gneissic diorite ...................................................................................... 13 7. Sawatch Quartzite in contact with Precambrian rocks ................................................... 15 8. Index map of Gilman area and canyon of the Eagle River ..................................................... 18 9 —1 2. Photographs: - 9. Thin-bedded dolomite of Dyer Dolomite ............................................................... 27 10. Belden Formation in roadcut on US. Highway 24 ...................................................... 35 11. Channeled contact between Belden and Minturn Formations ............................................ 35 12. Cliff exposures of lower part of the Minturn formation ................................................. 39 13. Diagram showing subdivisions and general character of the Minturn Formation ................................... 40 14. Photographs of crossbedded gritty dolomite and conglomeratic dolomite in Minturn Formation in the Pando area ..... 42 15. Photograph of dolomite reef in Minturn Formation at Lionshead ................................................. 44 16. Photograph of J acque Mountain Limestone Member of the Minturn Formation .................................... 46 17. Photomicrograph of biotitic oolitic limestone in Jacque Mountain Member of the Minturn Formation ................ 47 18. Photograph of columnar structure in sill of Pando Porphyry ..................................................... 59 19. Photomicrograph of Pando Porphyry showing sericitic alteration ................................................ 60 20. Photomicrograph of Pando Porphyry showing chloritic alteration ................................................ 60 21. Photograph of glacial topography in Gore Range ............................................................... 63 22. Photograph of dip slopes on flank of Sawatch Range ............................................................ 63 23. Photograph showing typical topography in area of Minturn Formation ........................................... 64 24. Map showing relation of Minturn quadrangle to major structural features and Colorado mineral belt ................ 70 25. Geologic sketch map and section of Gore fault on south slope of Bald Mountain .................................... 72 26. Photograph of deformed strata in Gore fault zone at head of Spraddle Creek ...................................... 73 TABLES Page TABLE 1. General stratigraphic section in the Minturn quadrangle . . . . . . .- ............................................ 6 2. Chemical analyses, norms, and modes of Cross Creek Granite as compared with Boulder Creek Granite .............. 1 1 3. Approximate modes of Cross Creek Granite as measured on stained polished slabs 6 ~10 square inches in size ........ 12 4. Fossils in the Belden Formation .............................................................................. 38 5. Fossil collection localities, Minturn Formation ................................................................. 49 6. Megafauna of the Robinson Limestone Member of the Minturn Formation ........................................ 51 7. Fusulinds in the Robinson Limestone Member of the Minturn Formation ......................................... 52 8. Fauna of the White Quail Member of the Minturn Formation .................................................... 53 MEASURED SECTIONS Page Section of the Sawatch Quartzite ......................................................................................... 16 Section of the Peerless Formation ......................................................................................... 20 Section of the Harding Sandstone ......................................................................................... 23 Section of the Parting Formation ......................................................................................... 25 Section of the Dyer Dolomite ............................................................................................. 27 Section of the Gilman Sandstone ...................................... _ ................................................... 29 Section of the Leadville Dolomite ......................................................................................... 31 Emended type section of the Belden Formation and section of the Molas Formation .......................................... 35 Type section of the Minturn Formation .................................................................................... 79 GEOLOGY OF THE MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO By OGDEN TWETO and THOMAS S. LOVERING ABSTRACT The Minturn quadrangle is an area of about 230 square miles (600 square kilometres) in the mountains of central Colorado, 75 miles (120 kilometres) west of Denver. In its northeastern part it includes a segment of the high and extremely rugged Gore Range, and in its southwestern part it includes the northeastern flank of the Sawatch Range. These two ranges consist mainly of Precambrian rocks. A lower but mountainous area between them, which occupies a large part of the quadrangle, consists of sedimentary rocks, mainly of Pennsylvanian and Permian age. Mining operations at Gilman, in the southern part of the quad- rangle, were long the chief industry in the quadrangle. The Gilman district is credited with a production through 1972 of about $328 million in zinc, silver, copper, lead, and gold. The ore deposits of this district are discussed in a separate report.1 Since the early 1960’s a skiing and recreational industry has burgeoned at Vail, in the middle part of the quadrangle, and has surpassed mining as an economic ac- tivity in the area. ' The rocks exposed in the Minturn quadrangle are of six major categories: (1) Precambrian crystalline rocks, (2) a thin but economically significant sequence of pre—Pennsylvanian Paleozoic sedimentary rocks, (3) a thick sequence of Pennsylvanian and Per- mian sedimentary rocks, (4) a thin sequence of Mesozoic sedimen- tary rocks preserved in an area of only about a square mile (2.6 square kilometres) in the northwest corner of the quadrangle, (5) scattered Upper Cretaceous and Tertiary intrusive igneous and volcanic rocks, and (6) unconsolidated Quaternary surficial deposits, mainly of glacial origin. The Precambrian X rocks exposed in the Gore and Sawatch Ranges consist mainly of migmatite and granitic rocks, but they also include biotite gneiss, diorite, and dike rocks, such as pegmatite and aplite. Biotite gneiss, the oldest rock, was widely converted to migrnatite during early phases of granitic intrusion. Biotite-quartz diorite and minor hornblende diorite form many small bodies that were emplaced during and following migmatization and in advance of the main bodies of granitic rocks. The granitic rocks are formally defined here as the Cross Creek Granite. Extensive bodies of this granite in the Sawatch and Gore Ranges are inferred to be parts of a single large batholith. The Cross Creek Granite varies in composition from granodiorite to granite, but most of it is quartz monzonite that is near granodiorite in composition. The granite bodies are typically concordant with the enclosing gneisses and have gradational con» tacts with the gneisses. The Cross Creek is dated isotopically as about 1,700 million years in age. The sequence of pre-Pennsylvanian Paleozoic rocks consists, from the base upward, of the Sawatch Quartzite, Peerless Formation, ‘The report on the ore deposits nl’the Gilman district. Eagle County. has been dosxgnaled as Geological Survey Professional Paper 1017. Harding Sandstone, Chaffee Group, and Leadville Limestone (or Dolomite). These rocks are exposed only in small areas, principally along the canyon of the Eagle River and on the lower slopes of the Sawatch Range; some are exposed also in small fault slices along the Gore fault, a major fault along the southwestern side of the Gore Range. The Gore fault marks the western border of an ancient high- land that was elevated repeatedly during Paleozoic time, especially late Paleozoic. Consequently, all the Paleozoic formations thin or pinch out toward the Gore Range. The Sawatch Quartzite, of Late Cambrian age, rests with profound unconformity upon a planar surface cut over Precambrian rocks. The Sawatch consists almost entirely of medium-bedded medium-grained white or lightly tinted quartzite, but it contains local lenses of fine conglomerate at the base and scattered small lenses of brown- weathering dolomitic sandstones in its middle part. It is about 200 ft (feet) (60 m (metres) ) thick near the Eagle River and about 100 ft (30 m) thick where preserved along the Gore fault. The Peerless Formation, of Late Cambrian age, conformably over- lies the Sawatch Quartzite and locally is in gradational contact with the Sawatch. The Peerless consists of dolomitic sandstone, sandy dolomite, dolomite, shaly dolomite, and dolomitic edgewise con— glomerates. The rocks are locally glauconitic and ferruginous, and they are variously colored brown, maroon, green, and buff. The Peer- less is 65—70 ft (20 m) thick near the Eagle River, and a maximum of 20 ft (6 m) of it is preserved near the Gore fault. The Harding Sandstone, of Middle Ordovician age, lies with ero- sional unconformity upon the Peerless Formation. The unit consists of white quartzite, green sandstone and conglomerate, and gray and green shale. As exposed along the Eagle River, it ranges in thickness from 14 to 50 ft (4‘15 m) and it may be as much as 80 ft (24 m) thick in some of the mine workings at Gilman. It is absent near the Gore fault. As defined in this report, the Chaffee Group, of Late Devonian and Early Mississippian(?) age, consists from the base upward of the Parting Formation, Dyer Dolomite, and Gilman Sandstone. Near Gil- man, the Parting Formation lies with erosional and slight angular unconformity upon the Harding Sandstone; in other areas it overlaps older sedimentary formations, and in places along the Gore fault it lies upon Precambrian rocks. The Parting consists mainly of coarse- grained tan to white quartzite and conglomerate; locally, green shale is also a prominent component. The Parting is typically about 45 ft (14 m) thick near the Eagle River, though it locally reaches 65 ft (20 m). A maximum of about 20 ft (6 m) is preserved near the Gore fault. The Dyer Dolomite lies conformably and with locally gradational contact upon the Parting Formation. The Dyer consists of thinbed- ded gray and buff dolomite. It is 75~80 ft (23-24 m) thick near the Eagle River and is absent near the Gore fault. The Gilman 1 2 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Sandstone, formerly classed as a member of the Leadville Limestone, lies with erosional unconformity upon the Dyer Dolomite and is over- lain with erosional unconformity by the Leadville as here redefined. The Gilman consists of sandstone, dolomite, chert, and breccia in various proportions. It is typically about 20 ft (6 m) thick but ranges from 10 to 50 ft (3—15 m) in thickness. In the mineralized area near Gilman, the Gilman Sandstone has been considerably modified in composition and structure by solution and collapse. The Leadville Limestone (or Dolomite) as here redefined consists of carbonate rocks of Mississippian age overlying the Gilman Sandstone and underlying either the regolithic Lower Pennsylvanian Molas Formation or the Middle Pennsylvanian Belden Formation. In most of Colorado, the Leadville is a limestone, but across the width of the Colorado mineral belt — a distance of as much as 40 mi (65 km) — it is entirely a dolomite. In the Minturn quadrangle, the boundary between the limestone and the dolomite facies is in the valley of Cross Creek, midway between Gilman and Minturn. Northwest of Cross Creek, the Leadville consists of light-gray-weathering foraminiferal limestone, and it is referred to as the Leadville Limestone. Southwest of Cross Creek, the Leadville consists of fine- grained dark-gray dolomite and various recrystallized dolomite facies, referred to as Leadville Dolomite. The fine-grained dark dolomite is concluded to have replaced original limestone during an early stage of Laramide orogeny in Late Cretaceous time, and various recrystallizations of this material occurred later. The top of the Lead- ville is a karst erosion surface marked by local pockets of regolithic silt referred to the Molas Formation. Because of the uneven karst surface, the thickness of the Leadville varies widely over the region. Along the Eagle River in the Minturn quadrangle, the Leadville is 110—140 ft (33—42 m) thick; it is absent at the Gore fault. The Lead- ville is the principal host rock of ore deposits in the Gilman district. Regolithic silt of the Molas Formation is present only locally at the top of the Leadville and is generally only a few inches to a few feet thick. Where present, it was mapped with the Belden Formation. As seen in mine workings, material of the Molas fills solution channels and caves in the Leadville beneath the karst surface, though no layer of Molas may be evident at this surface. The Leadville, or locally the Molas, is overlain by as much as 10,500 ft (3,220 m) of predominantly elastic rocks of Pennsylvanian and Permian age. These rocks are divided into three formations, the Belden Formation, about 200 ft (60 m) in maximum thickness, the overlying Minturn Formation, as much as 6,300 ft (1,920 m) thick, and the Maroon Formation, as much as 4,200 ft (1,280 m) thick. The Belden Formation consists of interbedded black shale, limestone, and fine-grained sandstone. Fossils from the type section, near Gilman, indicate an early Middle Pennsylvanian (Atokan) age. An emended type section is presented. The Belden is about 200 ft (60 m) thick near the Eagle River, and it pinches out northeastward toward the Gore fault, probably by nondeposition. The Minturn Formation, a type section of which is presented in this report, consists predominantly of grit, conglomerate, and sandstone in lenticular beds. These rocks are highly arkosic, micaceous, coarse grained, and poorly sorted. They are mainly gray or of various light pastel colors, but a zone 400—700 ft (120—210 in) above the base is dull red, and a zone several hundred feet thick at the top is bright red. The elastic rocks are inferred to be marine- margin piedmont deposits derived from a highland east of the Gore fault. Several beds of marine limestone are intercalated in the coarse clastic rocks, particularly in the upper half of the formation. The lower half contains a few thinner beds of dolomite and, at several horizons, scattered dolomite reefs. Seven of the most distinctive and persistent limestones or dolomites were designated as members of the Minturn in the Pando area, immediately south of the quadrangle (Tweto, 1949): the Wearyman, Hornsilver, Resolution, Robinson, Elk Ridge, White Quail, and Jacque Mountain Members. All are present in the Minturn quadrangle, but only the last four are widespread. Fossils from the limestones indicate that the Minturn is Middle Pennsylvanian (Atokan and Des Moinesian) in age. The Minturn overlaps the eroded edges of all older formations and extends onto Precambrian rocks near the Gore fault. It is about 6,300 ft (1,920 m) thick near the Eagle River, but it thins abruptly by onlap against the old highland near the Gore fault; there, the Robinson Limestone Member, 4,200 ft (1,280 m) above the Belden in the area farther west, is almost in contact with the granite. Westward, the Minturn rocks become finer grained, and they intertongue with gypsum of the Eagle Valley Evaporite near the western boundary of the quadrangle. The Jacque Mountain Limestone Member marks the top of the Min- turn Formation. The Maroon Formation resembles the Minturn in lithology, but it is entirely red, and, in general, it is less coarse grained. It is about 4,200 ft (1,280 m) thick in the northwestern part of the quadrangle and thins eastward toward the Gore fault. The Maroon is un- fossiliferous inthe Minturn quadrangle, but, from stratigraphic rela- tions and scant fossils found elsewhere, it is concluded to be Middle and Late Pennsylvanian and Early Permian in age. In a small area on Red and White Mountain, in the northwestern part of the quadrangle, the Maroon is overlain by Mesozoic rocks comprising the Chinle Formation, Entrada Sandstone, Morrison For- mation, and Dakota Sandstone. These units have a total thickness of about 535 ft (160 m). The Upper Cretaceous and Tertiary igneous rocks of the quad- rangle are (1) Pando Porphyry, of Late Cretaceous age, in a sill that intrudes the Belden Formation in the Gilman-Red Cliff area; (2) patches of basalt and tuff of Miocene age in the Piney River area; and (3) scattered dacitic dikes of probable middle Tertiary age in the ’ Gore Range. The unconsolidated deposits consist of (1) a thick col- luvium of Pliocene and Pleistocene(?) age in the Red and White Mountain—Piney River area, (2) glacial tills of pre-Bull Lake, Bull Lake, and Pinedale ages, (3) landslide deposits, and (4) strvam alluvium and gravels of Pleistocene and Holocene age. The Minturn quadrangle is divided structurally into three main units, corresponding to the parts in the Gore Range, the Sawatch Range, and the intervening area. The Gore Range is a large fault block of Precambrian rocks, only a part of which is included in the quadrangle. The Gore fault, the bounding fault on the southwestern side of this block, is a complex fault that has several strands. It origi- nated in Precambrian time and underwent movements at many times from then to the late Tertiary. It is, in general, a vertical or steep normal fault. The Sawatch Range, only a small part of which lies within the quadrangle, is a huge anticline. The range consists largely of Precambrian rocks in the core of the anticline; a thin cover of Paleozoic sedimentary rocks, broken by a few small faults, forms dip slopes on the northeastern flank of the anticline, southwest of the Eagle River. The area between the Gore and Sawatch Ranges is broadly syn- clinal and only moderately deformed. The principal folds are three north- to northwest'trending synclines arranged echelon in a north- west-trending line. The southeastern — or Black Gore — syncline has a broad, gently dipping southwestern limb that is a part of the flank of the Sawatch anticline and a narrow northeastern limb that turns up steeply against the Gore fault. The middle — or Vail — syn- cline is a bowed, doubly plunging syncline that is prominently ex- posed on the sides of the valley of Gore Creek at Vail. The north- western ———- or Red and White — syncline occupies most of the area between the mouth of Gore Creek and the Piney River. The southeastern nose ofthe syncline is blunt and forms an abrupt north- west-dipping monocline along the north side of Gore Creek from the Eagle River to Red Sandstone Creek. This monocline separates a southern area that is structurally a part of the flank of the Sawatch Range anticline from a northern area that is part of a large struc- INTRODUCTION 3 tural basin that lies northwest of the quadrangle. All three of the synclines are accentuated in the subsurface because of the thinning of the Paleozoic formations toward the Gore Range. The sedimentary rocks are broken by steep faults in places, but most of the faults have displacements of less than 100 ft (30 m). In the mine workings and canyon walls at Gilman, bedding faults are numerous, and steep faults are rare. The bedding faults, though in— conspicuous, played an important role in ground preparation prior to mineralization at Gilman. Faults inthe Precambrian rocks in this area are reactivated fractures associated with the Homestake shear zone, a broad northeast-trending Precambrian shear zone that lies mainly to the south of the quadrangle. Most of the faults terminate upward, at the base of the Sawatch Quartzite. INTRODUCTION GEOGRAPHY The Minturn quadrangle is an area of about 230 mi2 (600 km?) in the mountains of central Colorado, 75 mi (120 km) west of Denver (fig. 1). In its northeastern part it includes a segment of the high and extremely rugged Gore Range, and in its southwestern part it in- cludes the northeastern flank of the Sawatch Range. The Eagle River, which drains all but the northernmost part of the quadrangle, flows along the base of the Sawatch Range. A broad northwest-trending belt be- tween the Eagle River and the high crestal ridge of the Gore Range is mountainous but generally lower than the high crests to the northeast and southwest. Easy access to the area is provided by US. Highway 6 and I—70 which follow Gore Creek westward across the middle of the quadrangle. Access from the south is pro- vided by US. Highway 24 and the Denver and Rio Grande Western Railroad, which follow the Eagle River northwestward. Until recent years, population of the quadrangle was centered in the three small towns of Red Cliff, Gilman, and Minturn, along the Eagle River. In the early 1960’s, a fourth settlement, Vail, was established as a ski resort on Gore Creek near the mouth of Mill Creek (pl. 1). Mining conducted in the Red Cliff -Gilman area was long the principal industry within the quadrangle. Red Cliff, the oldest settlement in the quadrangle, was established as a mining town in 1879. The center of mining operations later shifted to Gilman, which since 1918 has been a “company town” of the New Jersey Zinc Co. The mines at Gilman are a principal source of employment for the residents of Minturn also, but Min- turn is, in addition, a railroad town that was established as a base for the extra locomotive equipment necessary to move trains over the Continental Divide at Ten- nessee Pass, 12 mi (19 km) south of Red Cliff. About 85 percent of the quadrangle is in the White River and Arapahoe National Forests. The forest land supports a summertime sheep-grazing industry and a small lumbering industry. It also supports a skiing and recreational industry which, though only recently established, has surpassed , mining as a principal economic activity in the quadrangle. Superb mountain scenery, the virtual absence of habitations outside the valleys of the Eagle River and Gore Creek, and snow and slope conditions ideal for skiing make the area at- tractive for such activities. The Gore Range, the predominating topographic feature, rises 5,000 ft (1,525 m) above the valley of Gore Creek at Vail to a crest-line more than 13,000 ft (4,000 m) in elevation. Below the steep and craggy rock slopes of this intensely glaciated range is a broad area of forested, rounded ridges that extends southwestward to the foot of the Sawatch Range. Many grassy slopes and ridges dot the forested area, which is characterized by dense growths of spruce and alpine fir on the north-fac- ing slopes and by lodgepole pine, aspen, and scattered Douglas-fir on the south-facing slopes. HISTORY OF INVESTIGATION The earliest geologic investigations in the Minturn quadrangle were made by Peale (1874, 1876), who described the general stratigraphic sequence and struc- ture along the Eagle River and mapped the area in rapid reconnaissance for the Colorado Atlas (Hayden, 1877). Peale’s work provided the geologic framework for various reports on the mines of the Red Cliff—Gil- man area (for example, Olcott, 1887; Guiterman, 1890; Means, 1915), although it was gradually supplemented by extrapolation of geologic information from the Lead- ville district, 20 mi (32 km) to the south, with which the Red Cliff —Gilman area has many parallels (Emmons, 1886; Emmons and others, 1927). The first detailed geologic map of any part of the quadrangle was prepared by Crawford and Gibson (1925), who mapped the area along the canyon of the Eagle River from 1 mi north of Gilman to about 5 mi (8 km) south of Red Cliff and described the ore deposits.’ Results of private studies of the ore deposits and vicinity, begun as early as 1912 by the New Jersey Zinc 00., are summarized in reports by Borcherdt (1931) and by Radabaugh, Merchant, and Brown (1968). Geologic mapping of the Minturn quadrangle and study of the Gilman ore deposits in light of the regional geology was begun by us in 1940 for the US. Geological Survey in cooperation with the Colorado Metal Mining Fund, a predecessor of the present (1975) Colorado Mining Industrial Development Board. Study of the mineralized area occupied most of the first short field season, and mapping of the quadrangle on a close reconnaissance basis at a scale of 1:48,000 was largely accomplished in the field season of 1941, except for the 107° GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIE, COLORADO 40° I | __._I w H I T E a I v E R | P L A T E A U o o O G U -l H r a E < < kl {D r /. 6 4t°°/ o“°/°° | r—v \\ /00° ‘N Glanwood C" Springs I ’e,‘ Roan” Fryingp.” o Fulford \\ *DENVER Area of flour- 1 PITKIN CO GUNMs% o l Trouuro Mount-In I INDEX MAP OF COLORADO Mount of mo® §\ @ r3 :4! oBteckenndge/ Holv Crou X Pando S 3 / u" '1 1 a? 2 o 9" . “ H l 4‘ \ Kokomo / z s” V o _ EAGLE CO q“ Q \ ‘ , Climax DEF/l ILKIN C0 ‘F Tlnnossee u: E "9* )‘fie l Pu: / o f‘:«‘ f 5 ‘00 ' mAlmao k 8( o :- Leadville' t o s o , Z a q ~ "‘1 ’- [5 ¥ 9; 0‘ O - I ~ ,5 o 0:, '5’ ! a go < <01 "\ mo 3 S O a 7/7 0 10 |||:[||ll‘lllll ll Illll I O 10 20 30 40 MILES | 40 Kl LOMETRES FIGURE 1.-—Index map of west-central Colorado showing geographic setting of the Minturn quadrangle (patterned). Adjoining quadrangles are (1) Mount Powell, (2) Dillon, (3) Mount Lincoln, and (4) Holy Cross. area of Precambrian rocks in the Gore Range. Most of the area in the Gore Range area was mapped during 3 weeks in 1942, and the project was then recessed, owing to wartime pressures for other work. A summary of results to that date (Lovering and Tweto, 1944) was ac- companied by the geologic map of the quadrangle, which at that time contained many imperfections and was still blank in the extreme northeast corner. Follow- ing work in a few critical areas and in several mines by Tweto in 1946, a summary of the ore deposits was published in 1947 (Tweto and Lovering, 1947). Owing to other demands, the project remained dormant for several years after that date. The last essential stratigraphic and structural fieldwork was completed by Lovering during brief field seasons in 1960—63, and the glacial geology was studied intermittently through the early 1960’s by Tweto. Last improvements in the map of the area in the Gore Range were made in 1969 (Tweto and others, 1970). With apologies for this long history of delay, we here report on the general geology of the Minturn quad- I rangle. A separate report (Lovering and others, 1977), deals with the ore deposits of the Gilman district and with the alteration history of the Leadville Limestone. In the many years since the work on the Minturn quad- rangle started, studies of various aspects of the geology and of the ore deposits were made by others, and the science of geology made marked advances in concepts, capabilities, and techniques. In the sections that follow we present the results of our interrupted studies as in- tegrated with the later studies of others, without at- tempting either to achieve balance in the scope of treat- ROCK FORMATIONS 5 ment of the various topics or to pursue many topics to the extent that the state of the science nowadays per- mits. We acknowledge with thanks petrographic data supplied by Tom G. Lovering on our scattered samples of sedimentary and dike rocks from outside the mineralized area. The geologic map accompanying this report will be found to be geometrically inaccurate in many places, owing to inaccuracies in the topographic base used in the original geologic mapping. Some of the more glaring inaccuracies in the base were latter corrected photo- grammetrically, but much of the map remains as plot- ted on the original base, which was a preliminary ver- sion, at a scale of 1 : 48,000, of the 1934 edition of the Minturn 15—minute quadrangle topographic map. ENGLISH AND METRIC UNITS Thickness listed in the stratigraphic sections in this report are in feet, as are the contours on the topographic base and the elevations, derived from the contours, in the geologic cross sections. Scales for con- verting thickness measurements and elevations to metric units are shown in figure 2. Both English and metric values are indicated for other measurements ex- cept petrographic dimensions which, in accord with convention, are entirely metric. Conversion units are as follows: English In murrlrs Inches (in.) multiplied by 2.54 = centimetres (cm); Feet (ft) multiplied by 0.3048: metres (m); Miles (mi) multiplied by 1.609: kilometres (km); Square miles (miz) multipled by 2.6: square kilometres (km2). Mam m English- Millimetres (mm) multipled by 0.039: inches; Centimetres (cm) multipled by 0.394: inches; Metres (m) multipled by 3.281: feet; Kilometres (km) multipled by 0.621: miles. ROCK FORMATIONS The rocks of the Minturn quadrangle are divided into many formations or map units (table 1), but on the basis of their occurrence and geologic connotations they fall into six main groups: (1) Precambrian crystalline rocks, (2) a thin sequence of pre-Pennsylvanian Paleozoic sedimentary rocks, (3) a thick sequence of Pennsylvanian and Permian sedimentary rocks, (4) a thin sequence of Mesozoic sedimentary rocks in a small area on Red and White Mountain, (5) scattered Upper Cretaceous and Tertiary intrusive and volcanic rocks, and (6) unconsolidated surficial deposits. FEET METRES l,- 100 . 300m— FEET METRES -‘ 13,000.4—4000 :— 75 ‘- P3500 200 —: 10.000 —_ 3,000 :- 50 _—2.500 .. ‘—2,ooo FEET~ METRES -’ 10 3 100 —~ 5.000—— 1,500 :e 25 _ 2 :- _—1.000 5 -- 1 :- ‘— 500 o o 0—— o o—— 0 FIGURE 2.——English-metric conversion scales. The Precambrian rocks, which are principally gra- nite and migmatite, form the high part of the Gore Range in the northeastern part of the quadrangle, and except for a thin cover of sedimentary rocks, they also form the bulk of the flank of the Sawatch Range in the southwestern part of the quadrangle. They are com- pletely covered by sedimentary rocks in the area be- tween the two ranges. The pre-Pennsylvanian Paleozoic rocks, mainly quartzites and dolomites, are exposed principally in the canyon of the Eagle River (fig. 3) and adjoining lower slopes of the Sawatch Range, although some of them are exposed also in thin fault slices along the Gore fault in the Gore Range. On the flank of the Sawatch Range these rocks form extensive smooth gentle slopes (fig. 22). These slopes are approximate dip slopes, but because they are slightly gentler than the dip, the various formations are in shingled arrangement, with younger units appearing succeSsively downslope and northward. Although thin, aggregating only about 550 ft (168 m), the pre-Pennsylvanian rocks are of special interest as the principal host rocks of the ore deposits of the quadrangle. The Pennsylvanian and Permian rocks, in contrast, are more than 10,000 ft (3,050 m) thick and are essen- tially devoid of known mineral deposits in this quad- rangle. They occupy the entire area between the Eagle River and the Precambrian rocks of the Gore Range. In this area they form a northwest-trending en echelon GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO TABLE 1. — General stratigraphic section in the Minturn quadrangle Age Name Thickness, feet Character (meters) Holocene and late Pleistocene Alluvium and landslide 0—200? Alluvium, terrace gravels, pond sediments, and dePOSits (0—90?) landslide debris. Pleistocene Glacial deposits 0—300? Tills of Pinedale, Bull Lake, and pre-Bull Lake ages. (0 —90?) Pleistocene(?) and Pliocene Colluvium 0-75 Sandstone blocks and supergene chert in dirt matrix. (0 —23) Miocene Volcanic rocks 0—200 Tuff and basalt. (0—61) Miocene, Oligocene(?), and Intrusive igneous rocks Quartz latite, dacite, and quartz basalt porphyries, in Late Cretaceous sills and dikes. Early Cretaceous Dakota Sandstone 150— 160 Medium-bedded to massive light-gray sandstone; is (46—49) dark gray, thin bedded, and shaly at top; locally conglomeratic at base. Late Jurassic Morrison Formation 250 lnterbedded light-gray sandstone, green, gray, and (76) purple shale, and gray limestone. Entrada Sandstone 60 Massive, cross-bedded buff to orange sandstone. (1 8) Late Triassic Chinle Formation 70 Red and purple siltstone, mudstone, and fine-grained (21) sandstone; Gartra Member, at base, is 10—25 ft (3—7.5 m) of coarse white sandstone and conglomerate. Early Permian, Late Maroon Formation 1,700—4,200 Red sandstone, siltstone, grit, and conglomerate. and Middle (518-1281) Pennsylvanian Middle Pennsylvanian Minturn Formation 2,100‘6,300 Grit, conglomerate, sandstone, and shale in lenticular (640- 1,921) bodies, and some intercalated limestone and dolomite in persistent beds; predominantly gray but red in upper part and in an irregular zone near base. Some of the limestones are named members; see figure 13 for subdivisions. Belden Formation 0—200 Dark-gray to black shale, limestone, and minor (0-61) sandstone, in thin beds. Early Pennsylvanian Molas Formation 0—10 Gray, yellow, and brown regolithic silt and clay (0-3) containing abundant chert fragments. Early Mississippian Leadville Limestone 0—— 140 Dark-gray limestone or dolomite; massive in upper part; (or Dolomite) (0—43) medium bedded and cherty in lower part. Dolomite is extensively recrystallized. Early Mississippian(?) ca Gilman 0—50 lnterbedded yellow-gray sandstone, sandy and cherty and Late Devonian g Sandstone (0- 1 5) dolomite, and breccia. Late Devonian 2 Dyer Dolomite 0—80 Thin-bedded gray dolomite. gt: (0—24) .2 Parting 0-65 Coarse-grained white to tan quartzite and 0 Formation (0—20) conglomerate; subordinate interbedded green shale. Middle Ordovician Harding Sandstone 0—80? White, gray, and green sandstone and quartzite, and ' (0 —24?) green shale. Late Cambrian Peerless Formation 0—70 Brown, red, green, and buff sandy dolomite, dolomitic (O —2 1) sandstone, dolomite, and dolomitic shale; irregularly glauconitic and ferrug‘inous. Sawatch Quartzite 0—220 Medium- to thick-bedded, medium-grained white (0—67) quartzite. Precambrian X Cross Creek Granite Coarse-grained, generally porphyritic gneissic to massive quartz monzonite or granodiorite. - Diorite Mainly biotite-quartz diorite; gneissic to massive. Gneisses Mainly migmatite; some biotite gneiss. PRECAMBRIAN ROCKS ' 7 FIGURE 3.—-Canyon of the Eagle River at Belden (in canyon bottom) and Gilman (at top of cliffs). Cliffs of stratified rock in middle part of canyon wall are Sawatch Quartzite, which lies on Precambrian rocks. Discontinuous cliffs higher on canyon wall are Chaffee Group and Leadville Dolomite. Vertical distance between Belden and Gilman is about 600 ft (200 in). line of synclines. The sedimentary rocks terminate abruptly to the northeast at the Gore fault. This fault flanks the Gore Range and separates the sedimentary and crystalline rocks in the northeastern part of the quadrangle. The Mesozoic sedimentary rocks are preserved only as a cap about 535 ft (163 In) thick on Red and White Mountain, near the northwest corner of the quad- rangle. The Upper Cretaceous and Tertiary igneous rocks oc- cur only in scattered small bodies, the largest of which is a quartz latite porphyry sill intercalated in the basal Pennsylvanian rocks along the canyon of the Eagle River. The sill is of Late Cretaceous age. Tuff and basalt in small areas in the northwestern corner of the quad- rangle are of Miocene age, and small dikes in the Gore Range are of probable middle Tertiary age. The unconsolidated materials are principally glacial drift, which is widespread throughout most of the quad- rangle. The high part of the Gore Range, however, was swept clean by the glaciers, and glacial drift is rare there. PRECAMBRIAN ROCKS The Precambrian rocks were mapped only in recon- naissance and were not studied in great detail. Hence, they are divided into only four units on the map (pl. 1), although many other units might be distinguished in more detailed studies. The oldest and least abundant of these units is biotite-quartz-plagioclase gneiss, referred to as biotite gneiss. This gneiss is similar to, and correl- ated with, the old gneiss of the Front Range and other areas of Precambrian rocks in Colorado, where it has been assigned names such as Idaho Springs Formation (Ball, 1906) or Black Canyon Schist (Hunter, 1925). Presumably, this old gneiss was once far more abun— dant in the Gore and Sawatch Ranges than it is now, but most of it was converted to migmatite or destroyed during the plutonic-metamorphic episode in which a granite here called the Cross Creek Granite was emplaced. Hence, far more migmatite than biotite gneiss is depicted on the map (pl. 1). The migmatite and granite are accompanied in many places by a gneissic biotite diorite that seems to grade 8 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO into both rocks. The diorite is abundant in most areas where both granite and migmatite are present, but much of it is in small bodies that were not distinguished in mapping. Only the larger bodies of dioritic rocks are shown on the map, and in most of them the diorite is mixed with granite, migmatite, and pegmatite. BIOTITE GNEISS The predominating rock of the unit here called biotite gneiss is a dark- to medium-gray strongly foliated gneiss consisting of alternating layers that are rich, ' respectively, in biotite or in quartz and plagioclase. The grain size is variable, and in the facies in which the biotite is coarse and abundant, the rock is a schist. Some of the gneiss contains sillimanite either as scat- tered needles or as waxy white clots, and some contains small dull-red almandite garnets. In places the gneiss contains lenses or layers of calcvsilicate rock or of quartzite a few inches to a few feet thick. and locally it contains a little hornblende gneiss or amphibolite. In general these other rock varieties are much less abun- dant in the biotite gneiss of the Minturn quadrangle than they are elsewhere in the Precambrian of Col- orado. The biotite gneiss consists of 20—40 percent quartz, 30v60 percent plagioclase (oligoclase/andesine, An;,,,_;,3), and 15—30 percent biotite. Sillimanite, garnet, cordierite, microcline, or hornblende may con- stitute as much as 10 percent of some varieties, and magnetite, apatite, and zircon are ubiquitous minor components. Microcline generally constitutes no more than a few percent of the rock, if present at all, except in varieties grading into migmatite, in which it may be a major component. In composition and general character the biotite gneiss is closely similar to that of the Front Range, described in detail by Sims and Gable (1964, p. C14—C19) and by Moench (1964, p. A11—A16). Although only scattered bodies of biotite gneiss are shown on the geologic map (pl. 1), the gneiss is more abundant than indicated, as it occurs also in many small bodies within the migmatite and the granite. It is, however, much less abundant than in many other areas of comparable size in the Precambrian terranes of Col« orado. Viewed regionally, this sparsity is a fairly local feature associated with the Cross Creek Granite and accompanying migmatite. The gneiss is much more abundant a few miles south of the quadrangle in the Sawatch Range, a few miles north of the quadrangle in the Gore Range, and a few miles east of the quadrangle in the Tenmile Range. The biotite gneiss is the oldest rock recognized in the Minturn quadrangle, just as similar gneiss (together with associated gneisses not present in the quadrangle) are the oldest rocks recognized in the Front Range and elsewhere in Colorado. Isotopic dating by the whole- rock rubidium-strontium method indicates that similar and presumably correlative gneisses of the Front Range and the Black Canyon of the Gunnison were formed by metamorphism 1.7 to 1.8 by. (billion years) ago (Hedge and others, 1967: Peterman and others, 1968; Hansen and Peterman, 1968). Age of the sedimentary rocks that were parent to the gneiss remains undetermined but, as suggested by isotopic data, probably does not ex- ceed 2 by. (Z. E. Peterman, oral commun, 1970). CROSS CREEK GRANITE AND RELATED M IGM AT IT E AND I) IORIT E The Precambrian rocks of the Gore and Sawatch Ranges in the Minturn ‘quadrangle are predominantly Cross Creek Granite, which is accompanied in most places by closely related diorite and migmatite. These three kinds of rocks are intimately mixed, grade into one another, and seem to be joint products of a major episode of granitic intrusion and attendant plutonic metamorphism. Because of the intermixing and in- tergrading, mapping of these rocks is a highly subjec- tive process, and maps made by different workers, or even by the same worker at different times, are likely to differ appreciably. (IROSS CREEK GRANI'l‘li The rock unit here designated the Cross Creek Gran- ite2 is an inhomogeneous batholithic unit ranging in composition from granodiorite to granite. The unit forms the northern end of the Sawatch Range, where it occupies an area of about 50 mi? (130 kmz), and it takes its name from Cross Creek in its type area, where it is well exposed in clean and fresh glaciated outcrops. The unit also forms the bulk of the Gore Range, not only within the Minturn quadrangle but also in adjoining parts of the range in the Dillon quadrangle to the east and in the Mount Powell quadrangle t0 the north (Tweto and others, 1970). The granite bodies in the two ranges are so similar that they are inferred to be parts of a single large batholith. This inference is supported by magnetic data (Tweto and others, 1970, p. C33, pl. 1) which suggest that the granite is continuous in the sub- surface between the Gore and Sawatch Ranges near the latitude of Gore Creek. So far as is known, the Cross Creek Granite is restricted to the northwest side of a major Precambrian fracture zone called the Homestake shear zone (Tweto and Sims, 1963). This broad northeast-trending zone is 2The term Cross Creek Granite was used in the 1944 preliminary report (hovering and Tweto, 1944) but has not been formally introduced and published. In the interim “granite of Cross Creek" has been used (Pearson and others, 1966; Bergendahl, 1969; Tweto, 1974). PRECAMBRIAN ROCKS 9 centered only a few miles south of the Minturn quad- rangle in the Sawatch Range (Tweto, 1974) and some of its border fractures extend into the southwestern part of the Minturn quadrangle (fig 24). In the Gore Range the shear zone is exposed only to the east of the Min- turn quadrangle, south of the latitude of Gore and Black Gore Creeks. As the Homestake shear zone had a left-lateral horizontal displacement of at least several miles (Tweto and Sims, 1963, p. 1003—1004), the Cross Creek batholith is possibly displaced with respect to some unknown continuation to the south or southeast. More likely, however. the shear zone coincides with the edge of the batholith because border facies characterize the batholith along the shear zone. Thus, there may be no offset extension of the batholith southeast of .the shear zone. FIELD RELATIONS The Cross Creek batholith is characterized by very abundant inclusions of partly granitized gneiss, or mig- matite, and by complex relations with bordering wallrocks. The inclusions of gneiss range in size from small fragments only inches across to many square miles. In some areas, as on the west slope of the Gore Range just south of the northern boundary of the Min« turn quadrangle, the rock of the batholith is essentially a breccia of gneiss fragments a few inches to several feet in diameter in a matrix of granite. More typically, however, the inclusions are remnants of layers of biotite gneiss or migmatite that have been distended or engulfed and have been partly assimilated by the gran- ite. The gneiss bodies are generally but not everywhere elongate parallel to the foliation in the gneiss, and the primary foliation in the granite, though younger, is parallel to this same direction. Reaction between gran- ite and gneiss was extensive, and hence the contacts between the two rocks are commonly vague or com- pletely gradational. In border areas of the batholith, long tongues of gneiss showing these features project into the granite, and, conversely, irregular but basically concordant bodies of the granite project into the gneiss. Along or near many contacts between granite and gneiss are lenticular bodies of diorite ranging from a few feet to a few thousand feet in length. The diorite grades both into granite and into biotite gneiss or mig- matite, but some of it is at least slightly older than the granite, as it is cut by the granite. Moreover, gneissic diorite occurs as inclusions in the granite, and in some of these the foliation is discordant with that in the granite, although dimensionally the inclusions parallel the foliation in the granite. In many places, granite, diorite, and migmatite are so closely intermixed and are so gradational that only the predominating type could be mapped at the scale used. Most areas of contact between Cross Creek Granite and migmatite or diorite are characterized by abundant pegmatite and aplite. The pegmatite is of several different ages, as some predates the diorite, some post- dates the diorite but predates the granite, some is a facies of the granite, some seems to have replaced gran- ite, and some is in sharply defined dikes cutting the granite. In contrast, most aplite is in dikes that cut the granite, although some is in irregular bodies that grade into the granite and seems to be a textural facies of the same age as the granite. CHARACTER In its most typical facies, the Cross Creek Granite is a medium- to coarse-grained irregularly porphyritic gray to pinkish-gray slightly to markedly foliated quartz monzonite. Departures from this norm are common, however. Appearance of the rock, and also the composi- tion, varies with amount, color, and size of potassium feldspar (microcline) crystals, which are erratically dis- tributed. In some of the rock the potassium feldspar is bright rose or salmon pink, and, if abundant, it gives the rock a pink cast, but in other parts of the rock the potassium feldspar is light gray to white and in- conspicuous. Some parts of the rock contain only very little potassium feldpsar, in inconspicuous small grains (fig. 4A), whereas other parts are characterized by abundant coarse grains or crystals of the feldspar (fig. 4B). In some places these crystals attain lengths of 2 in. (5 cm) and constitute as much as 50 percent of the rock. In places, the large potassium feldspar crystals are oriented so as to give a pronounced fluxion structure or grain to the rock, but in other places they have random orientation. If oriented, the feldspar crystals may or may not correspond in orientation to foliation and lineation defined by other minerals in the rock. Degree of foliation and lineation likewise ranges widely. Most of the rock is gneissic in some degree, but some is essentially structureless. A complete sequence exists from structureless to strongly foliated, banded varieties that resemble, or grade into, coarse-grained biotite gneiss. Coarse banding or streakiness generally characterizes the foliated varieties. The banding may be compositional or textural or both. In some places it reflects only slight differences in biotite or feldspar con- tent between layers or streaks, but in others it is due to streaks of diorite, or to lenses or layers of partly assimilated, or granitized, migmatite or biotite gneiss. In general, the granite is most strongly foliated in the vicinity of gneiss bodies and is only weakly foliated where uncontaminated by gneiss. In lithology and mode of occurrence, the Cross Creek Granite closely resembles the Boulder Creek Granite of the Front Range (Lovering and Tweto. 1953, p. 8—16; Sims and Gable, 1967, p. E29-E35). 10 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO FIGURE 4.—Cross Creek Granite. A, Nonporphyritic granodiorite facies; B, porphyritic granodiorite facies. PETROORAPHY AND COMPOSITION The predominant quartz monzonite facies of the Cross Creek Granite is composed of 35—50 percent oligoclase, 15—30 percent potassium feldspar, 20—30 percent quartz, 8—15 percent biotite, and minor amounts of accessory minerals. The oligoclase, quartz, and biotite are in fairly constant ratio to each other in the various facies of the rock, but the proportion of potassium feldspar to these other minerals ranges widely. With decrease in the potassium feldspar, the . rock grades toward granodiorite, and with increase it grades toward true granite. Detailed study of the many different facies of the Cross Creek Granite has not been attempted. In table 2, results of chemical and modal analyses of a sample judged to be representative of the most common variety of the quartz monzonite are presented along with simi- lar data for quartz monzonite typical of the Boulder Creek Granite (or Granodiorite). Approximate modes as determined from polished slabs stained to identify the potassium feldspar are given in table 3, illustrating the variability of the rock even in single samples. The Cross Creek Granite typically has a hyp- automorphic-granular and seriate porphyritic texture. In general, the potassium feldspar crystals are the largest in the rock, although locally aggregates of quartz are larger. Exclusive of the potassium feldspar crystals, the grains have a maximum diameter of about 8 mm, and an average of about 2 mm. Most thin sec- tions exhibit a rude gneissic structure due to weak preferred orientation of biotite and to some segregation of biotite in bands. In the more gneissic varieties, quartz and plagioclase grains are also elongated and crudely oriented. ' The potassium feldspar grains generally are in the form of prisms partly or wholly bounded by crystal faces. The crystals vary widely in size, and as brought out by staining, they may range in a given slab of rock from 2 mm to 5 cm in maximum dimension. Almost all the potassium feldspar shows the grid twinning of microcline, and many of the larger crystals can be seen in hand specimen to be Carlsbad twins. The microcline is slightly to markedly perthitic, having a content of albite lamellae that ranges from only a percent or two to as much as 25 percent. » In many localities the potassium feldspar crystals show clear evidence of being younger than ’the other constituents of the rock. In outcrop, this is shown by the gradation of granite with abundant pink feldspar crystals into small pegmatite dikes and, in places by the orientation of the feldspar crystals athwart the gneissic structure in the rock. In thin section, the microcline shows evidence of having replaced other minerals, par- ticularly plagioclase and biotite, and it commonly con- tains inclusions of these and other minerals, including ‘quartz, apatite, sphene, magnetite-ilmenite, and minor untwinned potassium feldspar. In some samples the plagioclase and biotite inclusions are altered, although the microcline is fresh. In some samples also, the inclu- sions show cataclastic effects, such as bent and frac- tured twinning lamellae in plagioclase or crumpled biotite leaves, even though the enclosing microcline is not deformed. Thus, the microcline crystals are more in PRECAMBRIAN ROCKS 11 TABLE 2. — Chemical analyses, norms, and modes of Cross Greek Granite as compared with Boulder Creek Granite I [Chemical analyses of Cross Creek Granite by M. Seerveld, U.S. Geological Survey. Data for Boulder Creek Granite from Sims and Gable (1967, p. E34) leaders (. . .) indicate not detected] Cross Creek Boulder Creek Granitel Grenite‘ Chemical analyses (in weight percent) SiO2 ........... 69.46 64.37 A1203 .......... 15.14 15.86 Fe203 .......... 1.22 1.78 FeO ........... 1.62 3.04 M30 ........... .88 1.69 0210 ........... 2.02 2.37 N320 .......... 3.24 3.09 K20 ........... 4.60 5.00 MnO ........... .04 .05 H20+ .......... .55 .52 H20" .......... .04 .08 TiOz ........... .39 .72 P205 ........... .25 .32 CO2 ............ .03 .23 Cl ............. .02 .03 F .............. .07 .12 S .............. .00 .14 B30 ........... .15 .23 SrO ............ 08 . . . Subtotal . 99.80 99.64 Less 0 .......... 03 .13 Total . . . . 99.77 99.51 Bulk density . . . . 2.65 2.66 Powder density . 2.69 2.73 Norma Quartz ......... 29.10 19.44 Orthoclase ..... 24.46 29.47 Albite .......... 27.25 26.20 Anorthite ...... 9.73 10.29 Hypersthene . . . . 2.92 6.34 Magnetite ...... 1.86 2.55 Ilmenite ....... .76 1.37 Corundum ...... 1.12 1.63 Apatite ........ .34 .66 Pyrite .......... . . . .48 Fluorite ........ .07 .16 Calcite ......... . . . .50 Model (in volume percent) Quartz ......... 27.1 18.3 Potassium feldspar ...... 25.6 34.4 Plagioclase ..... 36.6 32.9 Biotite ......... 9.2 1 1.9 Muscovite ...... Trace 1.0 Magnetite- ilmenite ...... .4 .6 Apatite ........ .2 .4 Zircon ......... 1 .2 Calcite ......... .3 Composition of plagioclase . . . An26 An27 ' Porphyritic quartz monzonite, top of cliffs at knob outlined by 10,800-fl; contour on north shoulder of Mount of the Holy Cross, Holy Cross 15-minute quadrangle 1 mi (1.6 km) south of Minturn quad- rangle line (See 'l‘weto, 1974). 1 Quartz monzonite, Mount Pisgah. Central City quadrangle in Front Range, from Sims and Gable (1967, p. E34). the nature of porphyroblasts than of phenocrysts, as they seem to have grown in the rock after the other constituents had crystallized, and after the stress en- vironment that produced foliation among the other con- stituents had changed or had disappeared. It is not known at this time, however, whether all the microcline in all the Cross Creek Granite is paragenetically late like this or whether this is a feature only of the batholithic border areas, which happen to be the ones most studied. The normal plagioclase of the Cross Creek Granite is oligoclase, which ranges in composition from An22 to An28 in different specimens. In some samples that con- tain late microcline, the oligoclase grains have narrow, sharply defined outer rims that are more sodic and generally less altered than the main grains. The plagioclase of the rims is mostly oligoclase with a com- position of An” to Anls, but locally it is albite with a composition of An6 to An“). Locally, grain contacts be- tween microcline and oligoclase are marked by patches of antiperthitic intergrowths of microcline in oligoclase. Most quartz in the granite exhibits strong strain shadows and is somewhat fractured. Some quartz was evidently introduced with the microcline, however, as some grains of the strained and fractured quartz are surrounded by aggregates of finer grained, less deformed quartz. The fine-grained quartz also occurs in microscopic veinlets cutting early quartz and other minerals, including the late microcline. Biotite of the granite is green brown where fresh, but in most localities it is altered and is green, or faded. It is accompanied by exsolved iron and titanium oxides. Accessory minerals of the granite are magnetite, il- menite, pyrite, apatite, sphene, and zircon. The pyrite is present only locally, and the apatite and sphene are ir- regularly distributed, being relatively abundant in some thin sections and absent in others. Muscovite is present in small amounts in most thin sections, occur- ring both as an alteration product of biotite and in association with late quartz veinlets. All samples of the Cross Creek Granite studied show cataclastic effects in some degree, and some samples show at least two generations of cataclasis, one that preceded introduction of the large microcline crystals and one or more that followed. Evidence of the early cataclasis .is shown by deformed inclusions of plagioclase, biotite, and apatite in undeformed microcline and by replacement relations of microcline against granulated aggregates of other minerals. Younger cataclasis is shown by mortar structure along the edges of the microcline crystals and, where defor- mation was severe, by jagged fracture zones zigzagging back and forth along cleavages in the microcline. Most of the alteration observed in inclusions in microcline 12 GEOLOGY, MIN'I‘URN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO seems to be associated with such fractures; hence it is interpreted to be younger than the microcline rather than older. MIGMATITE The migmatite map unit includes several varieties of rocks intermediate between granite or diorite and biotite gneiss. One common variety is of the classical type, consisting of alternating thin layers of dark biotite-rich gneiss and light-colored granitic or peg- matitic material (fig. 5). The layers in this rock are generally less than 1 in. (2.5 cm.) thick, but the granite or pegmatitic layers swell in places into knots or lenses several inches to a few feet thick. The gneiss layers are similar compositionally to biotite gneiss, consisting es- sentially of biotite, quartz, and plagioclase but locally containing microcline, sillimanite, or garnet. In some places, as in the outcrops near the mouth of Homestake Creek at the south edge of the quadrangle, milky blue cordierite is a prominent constituent of the gneissic layers. The granitic layers consist essentially of quartz, microcline, and plagioclase but generally contain prom- inent muscovite, also. They are typically richer in quartz and iron oxides than most granite. Banded mig- matite occurs principally in the area west of the Eagle River between the mouth of Homestake Creek and Fall Creek and in the higher parts of the Gore Range. Another variety of migmatite consists of biotite gneiss which has been partly granitized by introduction of feldspars in grains or crystals 0.2—0.8 in. (0.5—2 cm) long, forming a prominently speckled rock that super- ficially resembles a coarse granite but which consists in large part of biotite gneiss. The introduced, or newly crystallized, feldspars in such rock are generally com- plex perthitic and myrmekitic intergrowths of different feldspars and quartz. Migmatite of this type occurs in many small bodies in both the Sawatch and Gore Ranges, either bordering granitic rock or isolated from it. FIGURE 5.—Migmatite in the Gore Range, showing typical mixture of biotite gneiss (dark) and granitic materials (light), and streaking and crenulation resulting from rock flowage. Another variety of migmatite is intimately associated with, and grades into diorite. It consists of layers, laminae, and wisps of biotite gneiss separated by thin layers of diorite or embedded in larger bodies of diorite. Relations observed at the outcrop in many places sug- gest that the diorite in such occurrences formed in part by reaction with biotite gneiss (fig. 6) and that the mig- matite of this occurrence is a mixture of modified gneiss with the diorite. Migmatite of this kind and associated diorite characterize the border of the Cross Creek batholith along Cross Creek at the quadrangle bound- ary and immediately southward, and it is also widespread along the border zones of banded migmatite bodies in the Gore Range. TABLE 3. — Approximate modes of Cross Creek Granite as measured on stained polished slabs 6- 10 square inches in size [Determined by R. C. Pearson, US. Geological Survey] Sample .................... 1 2 3 s13b ....................... A B c B c D A B C Quartz ............. 30.2 26.3 29.4 29.7 25.6 25.8 30.0 29.6 23.9 . 26.1 Potassium feldspar . . 19.4 26.8 24.4 14.3 24.6 15.5 15.9 27.9 25.9 27.8 Plagioclase ......... 39.1 37.7 36.4 41.0 37.3 42.6 40.6 34.0 38.2 36.6 Biotite .............. 9.8 8.8 8.8 14.3 11.8 15.1 12.5 7.4 11.3 9.2 Accessory minerals . . 1.5 .4 1.0 .7 .7 1.0 1.0 1.1 .7 .3 Totals ........ 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 SAMPLE DESCRIPTlONS AND IJOCALITIES 1. Porphyritic quartz monzonite, cliffs on east side of canyon of the Eagle River at Gilman. 2. Porphyritic quartz monzonite, between 8,450- and 8,5004%. contours in bottom of valley of Cross Creek, on southeast side of creek. 3. Porphyritic quartz monzonite, railroad cut on east side of canyon of the Eagle River, 2,100 ft (700 In) north of mouth of Rock Creek. PRECAMBRIAN ROCKS 13 Still another variety of material mapped as mig- matite is coarsely interlayered igneous rock, such as granite, quartz monzonite, granodiorite, or diorite, and biotite gneiss or banded migmatite. In such material, comparatively pure and presumably intrusive igneous rock in generally concordant layers a few inches to many feet thick alternates with biotite gneiss or banded migmatite in layers of similar thickness. Material of this kind is widespread in the Gore Range. Where the gneiss fraction predominates, it was mapped as migmatite, and where the igneous fraction predomi- nates, it was mapped as granite or diorite. Thus, much of the rock mapped as Cross Creek Granite in this range differs only in degree from the migmatite unit. Finally, breccias of gneiss fragments in a granite matrix in the Gore Range were mapped as migmatite where the fragments predomiminate, and as granite where the matrix predominates. DIORITE Several varieties of diorite occur in close association with the Cross Creek Granite, but they have not been studied in detail. Most of the varieties are older than the Cross Creek, as they are cut by the granite or by pegmatites related to it, or occur as inclusions in the granite, or have been partly granitized by the granite. Some varieties seem to have crystallized at the same time as the granite, however, and at least one variety is younger than the granite. As a group, the diorites are thought to be closely related to the Cross Creek batholith in age and origin. The diorites occur in count- less small bodies as well as in the scattered larger ones indicated on the map (pl. 1). Most of these bodies are concordant, paralleling the foliation in bordering gneiss or granite, but crosscutting bodies of diorite are also found. Many diorite bodies are laced by dikes of diorite pegmatite consisting of andesine, quartz, and biotite. The most abundant variety of diorite is dark gray, medium-grained, slightly gneissic, biotite—quartz diorite. Locally, this rock contains minor hornblende. Biotite-quartz diorite is closely associated with mig- matite, especially near granite contacts, and as noted above, part of it formed by recrystallization of biotite gneiss (fig. 6). The bulk of the diorite, however, is believed to be magmatic and intrusive in origin. The diorite typically consists of 30—-45 percent plagioclase, which is andesine, An32_44, 20—33 percent biotite, 15—30 percent quartz, and 2—5 percent magnetite-il- menite and other accessory minerals. Where partly gra- nitized, the diorite contains a few percent of potassium feldspar, and more quartz, less biotite, and a slightly more sodic plagioclase than the normal rock. Where the adjoining rocks include amphibolite, the diorite generally contains hornblende, and where they include FIGURE 6.—Gneissic diorite showing structure inherited from original migmatite. It occurs in upper Piney River area, Gore Range. calc—silicate rock, it contains as much as several per- cent of epidote. Where cut by granite pegmatites, the diorite is commonly recrystallized to a form rich in coarse biotite. Various gneissic diorites and diorite gneisses are similar in composition and occurrence to the diorite just described. They are believed to be fundamentally the same rock but were subjected to a magmatic or tectonic movement environment that induced a gneissic struc- ture. Hornblende and hornblende-biotite diorites occur principally in dikes or small pluglike bodies, although some are of the same occurrence as the biotite-quartz diorites just described. Most of the hornblendic diorites thus appear to be younger than the biotite-quartz diorites, but only rarely are they seen to cut Cross Creek Granite. Most of the hornblendic diorites lack foliation, though a few are markedly gneissic. The 14 GEOLOGY, M'INTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMlT COUNTIES, COLORADO plagioclase of the hornblendic diorites is as calcic as labradorite (An55), and quartz is only a minor consti- tuent or is absent. On the whole, the suite of diorites accompanying the Cross Creek Granite is similar to———although more abundant than — the suite accompanying the Boulder Creek Granite of the Front Range (Lovering and Tweto, 1953; Harrison and Wells, 1959; Sims and Gable, 1967). INFERRED HISTORY From field relations and petrography, development of the Cross Creek Granite batholith is inferred to have begun with the intrusion of small bodies of diorite and the migmatization of preexisting biotite gneiss in ad- vance of the rising batholith of granitic rocks. The early diorite reacted extensively with the biotite gneiss, transforming some of the gneiss into diorite and diorite migmatite. Concurrently, other biotite gneiss was mig- matized and was riddled by early pegmatites in advance of the main batholith. The batholith was emplaced as magma of a composition ranging from granodiorite to calcic quartz monzonite. Emplacement occurred under conditions that promoted extensive reaction between magma and wallrocks, causing extensive assimilation and granitization of gneiss as well as the formation of granite-gneiss mixtures, here classed as migmatite, along with the earlier or banded migmatite. The biotite gneiss and migmatite wallrocks were plastic in this en- vironment, and they were deformed by and along with the granite of the invading and moving batholith, resulting in parallel foliations and generally concordant contacts. Minor diorite and abundant pegmatite developed within the batholith and its border zones, both during and following batholith emplacement. Movement of the batholith, or parts of it, continued as crystallization proceeded, resulting in local deforma- tion of early minerals. Finally, potassium essential to the formation of microcline, and accompanying sodium and silicon were introduced into the essentially consoli- dated rocks from deeper levels in the batholith. Microcline crystals of porphyroblastic habit, sodic rims of oligoclase, and late quartz veinlets and rims formed as a result, transforming much of the primary grano- diorite and calcic quartz monzonite to a more alkalic quartz monzonite or, locally, to true granite. Petrographic relations would allow an interpretation that the microcline is a product of some geologic event later than the emplacement and crystallization of the Cross Creek batholith and that the rocks of the batholith were deformed and even altered in the in- terim. Field relations and isotopic dating indicate, however, that the microcline is related to the crystallization history of the batholith and that the present microcline-bearing rock is a product of a single continuous process rather than of two processes separ- ated by a geologically appreciable time interval. The microcline is restricted to the granitic rocks of the batholith or to metasedimentary rocks immediately bordering granite, and hence it is most likely related to . the batholith in origin. If it were related to some other geologic event, such as the intrusion of a younger gran- ite at depth, then it would not be likely to be so exactly coincident in occurence with the batholith but would extend either less or more widely. Similarly, if the microcline were significantly younger than the rest of the rock, then microcline-bearing facies should yield younger isotopic ages than other facies, but as dis- cussed in the following section, the greatest ages ob- tained thus far are on the porphyritic, microcline-bear- ing facies. AGE AND CORRELATION As indicated previously, the Cross Creek Granite resembles the Boulder Creek Granite (or Granodiorite) of the Front Range both in composition and occurrence. Both granite units range in composition from grano- diorite to true granite, are characteristically gneissic, are generally concordant and syntectonic, have grada- tional contacts resulting from reaction with their gneissic wallrocks, and are accompanied by diorites that range in age from pregranite to postgranite. These characteristics are those of the oldest (Precambrain X) of three general age groups of granites recognized in Colorado (Tweto, 1964; Hutchinson and Hedge, 1967). The Cross Creek Granite is accordingly classed on geologic grounds as a member of the old group and as an approximate—if not exact—correlative of the Boulder Creek Granite. Isotopic dating corroborates the geologic correlation. Age of the Boulder Creek Granite is established as 1.70 by (Peterman and others, 1968) or 1.71 by (Hutchin- son and Hedge, 1967). Age of the Cross Creek Granite, on the basis of a recently determined six-point rubidium-strontium isochron, is firmly established as 1.71 by. (C. E. Hedge, written commun., 1974). The samples used to establish this isochron came from the Cross Creek and Grouse Creek drainages just west of the Minturn quadrangle. Some samples from Cross Creek valley within the Minturn quadrangle and from the Gore Range in the Dillon quadrangle, analyzed earlier by Hedge (written commun., 1968) also yielded ages of about 1.7 by Other samples, however, gave younger ages, as did samples dated by the potassium- argon method (Pearson and others, 1966, p. 1115). The younger ages probably reflect heating and consequent migration of elements in local areas during the second episode of granitic intrusion in Colorado, 1.35—1.45 b.y. ago. Granites of this age, which include the Silver CAMBRIAN SYSTEM 15 Plume Granite of the Front Range, the St. Kevin Gran- ite of the Sawatch Range, and many others, have not been observed in the Minturn quadrangle but occur as podiform dikes in Cross Creek Granite just east of the quadrangle, south of Gore Creek. CAMBRIAN SYSTEM Rocks of Cambrian age in the Minturn quadrangle and surrounding region are assigned to two formations, the Sawatch Quartzite and the Peerless Formation, both of Late Cambrian age. The Sawatch Quartzite rests with profound unconformity upon a virtually planar surface cut over Precambrian rocks of various kinds. The contact between the quartzite and the pre- dominantly dolomitic rocks of the Peerless Formation is gradational in most places but locally is sharply defined. Both units were included in the Sawatch Quartzite as originally defined by Eldridge (1894) in the Crested Butte area, on the western side of the Sawatch Range. In the Leadville area, where these strata were earlier known as “Lower Quartzite” or “Cambrian quartzite,” the part nowadays comprising the Peerless was commonly distinguished as “transition shale” or “Red-cast beds” (Emmons, 1886, p. 58—60). These in- formal terms were supplanted in 1932 when Behre (1932) assigned the reddish dolomitic strata to the Peerless Shale Member of the Sawatch Quartzite. Because the term “shale” is a misnomer as applied to the Peerless in most places and because the unit is readily mappable over a wide area, it has been classed as the Peerless Formation since 1947 (Singewald, 1947). SAWATCH QUARTZITE The Sawatch Quartzite consists of about 200 ft (60 m) of nearly uniform medium- to thick-bedded light-col- ored quartzite. The quartzite is perhaps the most resis- tant rock in the quadrangle, and it generally forms cliffs or ledges wherever exposed. On the dip slopes, however, it breaks down to sharply angular fragments or blocks and is poorly exposed. The quartzite is well displayed in the walls of Eagle Canyon where it forms nearly vertical cliffs rising abruptly above somewhat gentler cliffy slopes of Precambrian rocks (fig. 3). It is exposed also in the walls of canyons southwest of the Eagle River, though it is partly buried by talus along most of these canyons. It is not seen northeast of the canyon of the Eagle River, except in a few narrow fault slices along the Gore fault, the largest of which is on the southern spur of Bald Mountain, west of Booth Creek (fig. 25). The surface of Precambrian rocks on which the quartzite rests seems to be planar wherever exposed in the Minturn quadrangle (fig. 7), but only a mile or two south of the quadrangle, in the area of the Homestake shear zone, the surface has a relief of as much as 50 ft (15 m) (Tweto, 1949, p. 160; Tweto and Sims, 1963, p. 1008). In most places the Precambrian rocks beneath the quartzite are weathered and aresoft and crumbly to depths of a few inches to a few feet, but in some places they are fresh. Where bedding—plane movement has oc- curred at the base of the quartzite, the soft, altered Pre- cambrian rock is deformed into ridges or mounds as much as 5 ft (1.5 m) high, giving a deceptive ap- pearance of relief in the depositional surface. Basal beds of the Sawatch Quartzite are generally somewhat coarser grained than the remainder of the quartzite, and locally they contain lenses of quartz granule conglomerate a few inches thick. The granules in such conglomerate are typically 1/8—1/4 in. (3—6 mm) in diameter and are well rounded; they consist of quartz of various colors, white predominating. In most places the basal 10—-30 ft (3—9 m) of the quartzite is gray, pink, or light tan, but in some places this part is white like the remainder of the formation. The main body of the Sawatch is predominantly vitreous white quartzite made up of well-sorted rounded and subrounded medium-sand quartz grains. Along the canyon of the Eagle River, the zone between 75 and 160 feet (23 and 48 m) above the base of the quartzite con- tains scattered lenses of brown-weathering dolomitic sandstone or sandy dolomite. This feature was not noted elsewhere in the quadrangle or adjoining areas, FIGURE 7.—Sawatch Quartzite (bedded rock) resting upon nearly planar surface cut over Precambrian rocks (at base of photo- graph). Roadcut. on US. Highway 24, 600 ft (180 m) northwest of high bridge near Red Cliff. 16 GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO though it exists also in Glenwood Canyon, 40 mi (65 km) to the west (Bass and Northrop, 1963, p. J4—J7). A few green shaly partings occur on bedding planes in the quartzite sequence, and, near Gilman, a shaly sandstone bed 4—6 ft (1.2—2.0 m) thick about 70 ft (21 m) above the base of the quartzite serves as a useful stratigraphic marker. The upper 30—40 ft (9—12 m) of the quartzite is very vitreous and brittle and has a crackled appearance in outcrop. This quartzite is white in the Eagle Canyon area, but more commonly in the region it is pink, particularly in the uppermost part. In the Gilman area, the upper vitreous quartzite contains brecciated and mineralized beds that are known locally as the Rocky Point zone. One or more of these beds, each 1— 4 ft (0.3—1.2 m) thick may be present in any part of the upper vitreous quartzite, but in most places they are in the upper middle part, 10—20 ft (3—6 m) below the top of the Sawatch Quartzite. The quartzite of the Rocky Point zone is stained yellow by the oxidation products of pyrite and forms a conspicuous color band near the top of the quartzite cliffs. As seen in thin sections, quartzite of the Sawatch is somewhat variable; the sand grains in most beds are well sorted and rounded, but in some beds they are poorly sorted and are subrounded to subangular. The grains range from 0.2 to 1.3 mm in diameter, though in most of the rock they are 0.25—0.5 mm. In some sam- ples, the sand grains are in a matrix of very small (0.05 mm) angular quartz fragments that resemble a microbreccia. Many of the larger quartz grains contain inclusions of tourmaline, sillimanite, or rutile, and they commonly show strain shadows and lines of fluid inclusions. Accessory detrital minerals are not abun- dant but include muscovite, biotite, chert, potassium feldspar, sphene, green tourmaline, and zircon. The tourmaline is especially consistent and is seen in all sections of the quartzites. Feldspar is present only in the uppermost quartzite beds, and it is abundant in sandstone of the overlying Peerless Formation. The feldspar grains generally are fresh and most of them are microcline, though some untwinned potassium feldspar is present also. The Sawatch Quartzite is 190—220 ft (58—67 m) thick in the canyon area of the Eagle River, and it has a similar thickness of about 200 ft (60 m) at the northern end of the Sawatch Range to the west, at the 12,000-ft contour west of the head of West Grouse Creek. The quartzite thins to the northeast and to the south of the canyon area. In the fault slice on Bald Mountain, it con- sists of about 100 ft (30 m) of white quartzite and relatively coarse quartzite conglomerate lying on Pre- cambrian rocks and overlain by red hematitic mudstone of the Peerless Formation. Near Pando, 5 mi (8 km) south of Red Cliff, the quartzite thins to about 120 ft (37 m) upon crossing the Homestake shear zone and then thins gradually southward to about 100 ft (30 m) in the Leadville area (Tweto and Sims, 1963, p. 1008). The character of the Sawatch Quartzite is shown in the following stratigraphic section which was measured as a standard of reference for studies in the mine work- ings. Section of the Sawatch Quartzite [Measured along railroad at north end of canyon of the Eagle River, beginning 2,550 ft (778 m) south of highway overpass. See fig. 8] Thickness Distance (feet) above base (feet) Sawatch Quartzite: Top of formation ...................... 33. Quartzite, white, vitreous, medium- bedded ........................... 32. Sandy quartzite, white to gray, medium- to thin-bedded, fine- grained, locally slightly glauconitic; weathers conspicuous ocherous brown. Rocky Point zone of Gilman mines ............................ 3.0 31. Quartzite, white, fine-grained, vitreous, medium-bedded .................. 30. Dolomite, tan, medium-grained to coarsely crystalline, rough weathering ....................... 1.0 29. Quartzite, white with some pink bands, medium- to fine-grained, vitreous, locally slightly dolomitic; becomes dolomitic and red toward base. Wavy contact with unit below ............ 28. Dolomitic shale, orange-red with purple blotches, finely sandy and micaceous, thin- to medium -bedded. Wavy contact with unit below ...... 1.0 27. Quartzite, light-gray, medium grained, massive; is dolomitic and pink near base and grades on strike into dolomite ......................... 4.7 26. Quartzite, medium-bedded ........... 2.0 25. Dolomite, tan-pink .................. .9 24. Quartzite, white, vitreous, medium- to thick-bedded ..................... 4.0 23. Dolomitic sandstone and quartzite in thin beds; crossbedding weathers in relief. Dolomitic sandstone is buff to pink; quartzite is white ............ 4.0 22. Shale and dolomite in alternating thin beds, purplish-pink; forms shelf along cliff ........................ 2.5 21. Quartzite and dolomitic sandstone, gray and pink, thin-bedded ........ 2.0 20. Quartzitic sandstone, white, medium- grained, sugary, massive ........... 7.8 19. Quartzitic sandstone, white, medium~ grained, sugary, and interbedded brown, medium-grained, soft sandy dolomite ......................... 18. Quartzite sandstone, white, medium- grained, sugary, massive ........... 6.0 220.3 11.7 208.6 205.6 10.5 195.1 194.1 12.0 182.1 181.1 176.4 174.4 173.5 169.5 165.5 163.0 161.0 153.2 12.0 141.2 135.2 Section of the Sawatch Quartzite— Continued Thickness Distance Sawatch Quartzite — Continued 17. Quartzite, white, vitreous, fme- to medium-grained, massive; contains interbedded brown, coarse, soft, sandy dolomite in short lenses 2 in. to 3 ft thick which become less abundant downward. Quartzite beds are 1 —5 ft thick; quartzite is argillaceous near base of unit, and at 18 ft above base, contains 7-in. shaly bed ........................ 16. Limy and shaly sandstone, thin- and irregular -bedded; weathers dark chocolate brown; contains thin micaceous laminae with numerous dark-brown fossil fragments in some laminae. Micaceous at base. White alum efflorescence on outcrop. Unit is bounded above and below by bed- ding-plane slips ................... 15. Sandstone, light-gray, medium- grained; mottled with dark shell fragments of inarticulate brachiopods; is thin bedded and irregular bedded .................. 14. Quartzite, light-tan, vitreous, medium- bedded. Weathered surface is marked by thin horizontal pits 1/3-1/4 in. long .......................... 13. Quartzitic sandstone, light-tan-gray, knobby-weathered. Contains abundant small brachiopods (Dicellomus sp.) and shell fragments on weathered surface .............. 12. Quartzite, light-gray to light-tan, vitreous, medium- to fine-grained, thin- to medium-bedded. Beds separated by thin micaceous partings with knobby or lenticular structure. At base of unit is 6-in. bed of coarse-grained, light-gray, quartzitic sandstone with abundant fossil fragments, chiefly brachiopods 11. Sandy quartzite, light-gray, fine- grained, massive .................. 10. Shaly sandstone, light-bluish-gray, very thin bedded, finely micaceous; slightly pink on weathered surfaces . 9. Quartzite, white, medium- to fine- grained, gritty .................... 8. Covered ............................ 7. Quartzite, white, medium- to coarse- g‘rained, massive- to medium-bedded; clay speckled (arkosic?); weathers mottled tan. Thin lens of fine conglomerate with thin shaly layer above it at 7 in. from top ........... 6. Sandstone, gray-white, finely conglomeratic, quartzitic .......... 5. Quartzite, white, massive; lower 3 in. strongly sheared; bedding-plane slips above and below unit .............. (feet) 62.1 6.0 1.6 1.5 1.8 6.0 3.3 1.0 1.0 15.5 2.3 2.0 CAMBRIAN SYSTEM 17 above base (feet) 73.1 67.1 65.5 64.0 62.2 56.2 52.9 52.3 51.3 50.3 34.8 32.5 30.5 Section of the Sawatch Quartzive — Continued Thickness Distance (feet) above base ' . ' (feet) Sawatch Quartz1te — Continued 4. Quartzite, white, vitreous, massive, medium-grained. 4-in. shaly zone at base of interval; strong shear slip 5 ft above base ....................... 3. Quartzite, white, gritty, coarse- g'rained; has thin sandy partings . . . . 2. Sandy quartzite, white to pink, medium- to coarse-grained, massive; weathers with vertical tubular holes from 1/2 to 21/2 in. long ............. 1.0 1. Conglomeratic and quartzitic sandstone, light-gray, coarse- grained, medium‘ to thin-bedded. Wavy basal contact ............... 6.0 24.5 7.5 17.0 16.0 16.0 0 Total measured thickness of Sawatch Quartzite ............ 220.3 Precambrian granite: Top 6 in. to 3 ft is soft, altered, and sheared. As indicated in the preceding stratigraphic section, fossil fragments are abundant in certain beds in the lower half of the Sawatch Quartzite, but fossils that are complete enough to identify are rare. The only fossils identified from the Sawatch Quartzite in the Minturn quadrangle are tiny inarticulate brachiopods assigned to the upper Cambrian genus Dicellomus by Dr. W. C. Bell of the University of Texas (oral commun., 1941). These fossils are from unit 13 of the preceding stratigraphic section. Better preserved and more abun- dant brachiopods from about the same level in the Sawatch in the Pando area, a few miles south of Red Cliff, also were identified by Bell as Dicellomus sp. (Tweto, 1949, p. 159). Later, as quoted by Berg and Ross (1959, p. 107), Bell reported brachiopods in the Sawatch (presumably in the collections discussed here) to be “Dicellomus of the pectenoides and nanus types, both ‘lower’ Dresbachian.” These determinations by Bell represent almost the only reliable information on fossils from the Sawatch. Trilobites are mentioned in the early literature, but they seem to have come mainly from the strata now assigned to the Peerless Formation (Johnson, 1934, p. 20). Trilobite fragments can be dis- tinguished among the fossil fragments in the quartzite of the Gilman and Pando areas, but most of the frag- ments are of brachiopods. The quartz sand that later became the Sawatch Quartzite has been widely recognized to have been formed as the Late Cambrian sea gradually transgressed over a surface of very low relief cut in the Precambrian rocks. The rocks at this surface were weathered, and the quartz residue from them was an abundant source of quartz sand. This sand probably was in part derived by direct wave action on the 18 3° 30’ GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO 106°22’30" I , i / // /‘\ : (x x ‘ \) ) \\ / / R \® 5°! E \ :1 2‘, \\ §\ \ 5\ ’2 \ -\ o \ <\ \ 0‘ 8 1| 4 \ '6 \9 'a I 0 \ .(\ ‘1; \3- \\ 6) Rock 2 m. x 3‘ \°9@1 © g ) I E *- 9, l I 9‘ I; a I: / ,c .‘° a. 5' ig ( , ’=‘ a c: 2 \ ‘/ g o \\ Wilkesborre mm 0“ \ / (Eu/e mine) 9. ‘3‘ 2 / Gilmsn / z / s I, T; “ /® " x" O m It." 2 Newhouee / 0 ° tunnel @ a— 9 Belden 3 i - mine 9 Belden 7/ F 18 E 17 Ben Butler 2 g \g . v” "m" g T.BS. O 1 .9 3. Vi Sent. Cruz ‘\ mine /— 3 {‘fi '70 Ground Hog ( mine 6 ® '9 "i- ’L¢ . MEASURED SECTIONS 9 . Red Cliff »—. Line of action ‘ 20 ® Sewetch Ouenxite end Peerieu , Formation ® Herding Sendstone (in Newhouee tunnel) © Perting Form-tion © Dyer Dolomite, Glimen Sendetone, end Leedviiie Dolomite © Belden Formetion Iron (emended type section) Mount-[n © Mlnturn Formation (lower eegment) 29 0 7: 1L 1 ' I O .5 i KILOMETRE FIGURE 8.—Gi1man area and canyon of the Eagle River showing locations of mines referred to and measured sections CAMBRIAN SYSTEM 19 weathered rocks and was in part brought to the sea by streams from land that lay to the east, as regional rela- tions indicate that the transgression was generally eastward (Lochman-Balk, 1956, p. 565—574), and paleocurrent data indicate westward transport of the sand on the sea bottom (Seeland, 1968). The occurrence of fresh detrital microcline and biotite in the uppermost beds of the Sawatch indicates that by that time streams somewhere had removed the weathered mantle and were eroding fresh Precambrian rock. Whether this fresh rock was later covered by Sawatch sediments or remained an island is not known, because the Sawatch has been removed by erosion from so much area since its deposition. However, the rapid thinning of the Sawatch between the Eagle River and Bald Mountain and the relatively coarse conglomerate in the Sawatch at Bald Mountain suggest that land, or an island, may have lain somewhere in the vicinity of the present Gore Range. PEERLESS FORMATION The Peerless Formation consists of 65—70 ft (20—21 m) of highly varied but predominantly dolomitic rocks. In most places the lower 20—30 ft (6—9 m) of the Peerless is a sandy dolomite or dolomitic sandstone which commonly is glauconitic and locally is ferruginous or chloritic. This dolomite or sandstone has a slabby ap- pearance in outcrop even though individual beds are as much as 3 ft (1 m) thick, and it weathers dark brown to dark maroon; thus, it contrasts markedly with the un- derlying white quartzite of the Sawatch. Thin beds of white or pink quartzite in the lower 5—10 ft (1.5—~3 m) of the dolomite or sandstone make the contact with the Sawatch a gradational one in many places. Abundant iron-bearing material characterizes the lower part of the Peerless. The sandstones locally con- tain red earthy hematite, either in thin beds or as a matrix of the elastic grains. Some of the hematite ap- pears oolitic, but generally the “oolites” are quartz grains heavily coated with hematite. In places the sandstones contain lenses that are as much as 50 per- cent glauconite, and in other places they contain abun— dant bright-green iron-rich chlorite, which petrographically appears to be authigenic rather than detrital. Dolomite beds of all the Peerless, but especially of the lower part, commonly contain thin limonitic laminae generally accompanied by sand grains or argillaceous matter, that weather in relief, giving the rock a wavy laminated appearance. Locally, minute limonitic veinlets form a reticulate network in the dolomite. Bright-red hematitic mudstone is also com- mon in the Peerless; near the Gore fault on Bald Moun- tain, the Peerless is 20 ft (6 m) thick and consists en- tirely of the red mudstone. . The middle part of the Peerless consists of tan, buff, _ maroon, and pale-green thin-bedded sandy dolomite, dolomitic shale, dolomite, mudstone, and minor micaceous shale. This unit is soft and nonresistant, and it generally weathers to covered slopes. The upper part of the formation is somewhat thicker bedded than the middle part, and it is also more purely dolomitic, though it, like all the Peerless, differs in lithology from place to place. On the dip slopes west of Gilman and along Cross Creek, the upper 40 ft (12 m) of the Peerless is pure dolomite which is thin bedded, buff, and coarsely crystalline. The dolomitic rocks of- the Peerless are characterized by color mottling and by great variety in sedimentary structures. Bedding planes are wavy, and many are coated with mud or mica or are spotted with mud lumps. Fucoidal markings and worm trails are common on the bedding planes. Ripple marks are conspicuous, and many thin beds have mud cracks filled with material that contrasts in color or composition with the rest of the rock. Many thin beds of the dolomite are flat-pebble or edgewise conglomerate that consists of slightly abraded pebbles of maroon or purple dolomite in a buff crystalline dolomite matrix. Chips of maroon clay, mudstone, or micaceous shale are also scattered through some of the conglomerates. Other beds are mottled buff and purple, but pebbles cannot be dis- tinguished in them. Some such beds also have various unidentified small curly color markings, as well as con- centric structures that may be either concretionary or algal, or both. Many of the markings of various kinds are maroon, as are some beds of the dolomite, and hence the term “Red-cast beds” was once applied to these rocks. As a unit, however, the part of the Peerless above the dark lower beds is mainly buff, though it is splotched or is streaked maroon and pale green in places. As seen in thin section, the sandy dolomites and dolomitic sandstones of the Peerless consist largely of dolomite, quartz, and feldspars in various proportions, though glauconite and chlorite are also abundant con- stituents of some samples. The feldspar grains include both microcline and sodic plagioclase, which generally are fresh or only slightly altered. Chlorite, in small ir- regular flakes and sparse larger, rounded flakes, is most abundant in the arkosic varieties of the rocks. Glauconite is in rounded grains that are among the coarsest in the rocks. Common accessory minerals are muscovite, zircon, and tourmaline; hematite or limonite coats many of the elastic grains. The quartz and feldspar grains are subrounded to subangular and well- 20 GEOLOGY, MIN TURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO sorted. They range in maximum size from 0.2 mm in some beds to 0.5 mm in others. In some of the dolomitic sandstones the dolomite is concentrated in discrete microlenses or pockets in an otherwise quartzitic rock, whereas in other sandstones the dolomite is a matrix or cement for the clastic grains. In the sandy dolomite, the elastic grains are less well sorted than in the sandstones, and they are scattered unevenly through the dolomite. Character of the Peerless in the vicinity of Gilman is shown by the following stratigraphic section. As may be seen by comparison with sections measured 1.6 and 2.0 mi (2.6 and 3.2 km) south of Red Cliff (Tweto, 1949, p. 164-465), the Peerless differs markedly from place to place. Section of the Peerless Formation [Measured southward along railroad at north end of canyon of the Eagle River, beginning 1,800 ft (550 m) south of highway overpass. See fig. 8] Thickness Distance (feet) above base Harding Sandstone: (feet) Sandy quartzite and arkosic sandstone, white, medium- to fine-grained, massive, slightly crossbedded. Peerless Formation: Top of formation ........................ 15. Flat-pebble dolomite conglomerate, purplish-pink to buff, medium- and thin-bedded; weathers light chocolate brown; contains partings of purple conglomeratic shale ........ 2.0 14. Flat-pebble dolomite conglomerate, mottled purplish-pink and light- green, thin-bedded; contains partings of fissile greenish-gray to greenish- buff micaceous dolomitic shale ..... 6.0 13. Shaly dolomite, mottled green and gray-buff, thin-bedded; weathers with fine siliceous ridges on weathered surface ................ 1.5 12. Dolomitic shale, greenish-gray to buff- gray, thin-bedded ................. 1.0 1 1. Flat-pebble dolomite conglomerate, light-buff, thin-bedded; grades downward into green and purple- pink fissile, sandy shale ........... 1.2 10. Covered ............................ 5.0 66.5 64.5 58.5 57.0 56.0 54.8 49.8 9. Flat-pebble dolomite conglomerate, mottled purple and tan, medium- 1.5 bedded, buff-weathering ........... 8. Shale, fissile, greenish-gray, fine- 1.5 grained .......................... 7. Dolomite, mottled light-tan and green, medium- to thin-bedded, green-buff 1.5 weathering ....................... 6. Dolomitic shale, light-tan, thin-bedded 1.0 5. Flat-pebble dolomite conglomerate, light-tan with local purple mottling; contains shale fragments and some laminae of green shale ............ 1.0 48.3 46.8 45.3 44.3 43.3 Section of the Peerless Formation — Continued Peerless Formation — n i CO tnued Thickness Distance (feet) above base 4. Dolomite, greenish- to light-tan, fine- (feet) grained, massive, somewhat argillaceous; weathered surfaces are brown and threaded with thin limonitic argillaceous ridges; 4-in. glauconitic shale bed in middle and 2-in. bed at base .................. 3.3 3. Sandy dolomite, light-gray, coarsely crystalline, medium-bedded; has coarse green glauconitic grains and coarse quartz sand. Weathers tan, with quartz grains and argillaceous ridges in relief, giving rough surface. Contains purple and green shaly fragments in a few thin seams, and near top, a seam of purple, micaceous shale ............................ 2. Sandy dolomite, pink, massive, brown- weathering, medium-crystalline; has thin coarsely crystalline bands. Weathers rough, with coarse crystals and sand in relief. Grades downward into pink dolomitic crossbedded sandstone ........................ 5.0 4.0 l. Sandstone, white, fine-grained, chalky- looking; has thin beds of pink sandy dolomite at top; medium bedded and crossbedded; weathers brown. Conformable contact with underlying quartzite .............. 4.0 0 Total measured thickness of the Peerless Formation ......... Sawatch Quartzite: Vitreous white quartzite. 40.0 31.0 9.0 66.5 Fossils found sparingly in the Peerless Formation at various localities in central Colorado establish the for- mation as middle Late Cambrian (Franconian Stage) in age. We found no fossils Within the quadrangle, but Crawford and Gibson (1925, p. 35) reported the Upper Cambrian trilobite Saukia pepinensis and Resser (1942, p. 66) reported the trilobites Ellipsocephaloides butleri and Briscoia in material from “near Gilman” acquired by C. D. Walcott prior to the mid-1920’s. The trilobites reported by Resser are indicative of the Franconian Stage, as are others (Ptychaspis, Idahoia) reported by Berg and Ross (1959) from the Manitou Park area near Colorado Springs. A trilobite from a bed 44 ft (13 m) above the base of the Peerless in the Holy Cross quad- rangle, 1.6 mi (2.6 km) south of Red Cliff was identified by Josiah Bridge of the US. Geological Survey (written commun., 1950) as Pterocephalia cf. P. sanctisabae Roemer? and as “characteristic of the Elvinia-Cam- araspis zone of the Franconian Stage.” Brachiopods from the same locality were identified by W. C. Bell (reported in the Bridge communication) as Obolus 0RDOVICIAN SYSTEM 21 maeschae Lochman, “characteristic of the Cedaria and Crepicephalus zones of the early Late Cambrian (Dresbachian stage).” Later, however, brachiopods from this same collection were reported by Berg and Ross (1959, p. 107) to have been tentatively identified by Bell as “Dicellomus? cf. mosaica of the Conaspis zone or an unnamed species from the Cedaria zone.” (The Conaspis zone is in the Franconian Stage, above the Elvinia-Camaraspis zone and below the Briscoia zone). These data on the age of the Peerless are significant in the evaluation of the unconformity at the top of the Peerless. In the Gilman area, the Peerless is overlain unconformably by the Middle Ordovician Harding Sandstone, and on Bald Mountain and locally in the Pando area (Tweto, 1949, p. 163) it is overlain by the Upper Devonian Parting Formation. In most of the Sawatch Range region, however, it is overlain by the Lower Ordovician Manitou Dolomite (Johnson, 1944). At Glenwood Canyon, 40 mi (64 km) west of Minturn, additional Cambrian strata intervene between the Peerless and the Manitou. Bass and Northrop (1953; 1963) divided the strata of Cambrian age at Glenwood Canyon into two formations, the Sawatch Quartzite, 517 ft (158 m) thick, and the predominantly dolomitic Dotsero Formation, about 100 ft (30 m) thick. Although a well-defined unit with lithology characteristic of the Peerless is present in the area (observation by Tweto), Bass and Northrop did not distinguish it as Peerless because it is overlain by about 150 ft (45 m) of quartzites typical of the Sawatch. They, therefore, placed these quartzites and the 69 ft (21 m) of under- lying Peerless equivalent (unit 10 of Sawatch Quartzite stratigraphic section; Bass and Northrop, 1963, p. 6) in the Sawatch Quartzite, which accounts for the relatively great thickness of 517 ft (158 m) reported for this unit, as contrasted with the 220 ft (67 m) at Gil- man. The carbonate rocks containing Upper Cambrian fossils above the quartzite were assigned by Bass and Northrop (1953) to the Dotsero Formation as redefined from the original usage of Bassett (1939). Since the redefinition of the Dotsero, it has been a common practice to correlate the Dotsero and Peerless (for example, Berg, 1960; Stevens, 1961), but this is in error as indicated not only by the presence of a unit with typical Peerless lithology and thickness within the Sawatch as applied by Bass and Northrop, but also paleontologically. The Dotsero is characterized by fossils of late Late Cambrian age (Trempealeauan Stage) as determined by A. R. Palmer (Bass and North- rop, 1953, p. 896), whereas the Peerless is older, of the Franconian—and possibly even Dresbachian—Stage, as discussed above.lThis difference in age and the pres- ence of 250 ft (75 m) of Cambrian strata between the Peerless equivalent and the Manitou in Glenwood Can- yon strongly suggest that in areas where the Manitou lies on the Peerless some Cambrian strata were removed by erosion before deposition of the Manitou. Such an erosional break between the Upper Cambrian and Lower Ordovician was noted by Berg and Ross (1959) in the Colorado Springs area and as a general feature in Colorado by Tweto (1968a, p. 561). It is thus likely that in the Minturn quadrangle some part of the Peerless as well as formerly overlying Cambrian strata were removed by erosion prior to Early Ordovician time and that an additional part of the Peerless may have been removed by erosion in pre- Middle Ordovician time, when the Manitou Dolomite was eroded from the area, as discussed below. Some of the eroded Cambrian rocks presumably were carbonate rocks similar to those of the Dotsero Formation. The central Colorado Upper Cambrian sequence of the Sawatch Quartzite, Peerless Formation, and Dot— sero Formation or equivalent is remarkably similar to the Middle Cambrian sequence of the Grand Canyon, described in detail by McKee (1945). Like the Bright Angel Shale of the Grand Canyon, the Peerless displays a very extensive combination of lithic types that indi- cate shallow’water deposition, rapidly changing en- vironments of deposition, disturbed and noncontinuous deposition, and repeated regressive-transgressive shifts within the broadly transgressive sequence. The abun- dant flat-pebble conglomerates of the Peerless indicate repeatedly agitated waters and breaks in sedimenta- tion, especially as they include fragile chips of clay or coarse mica shale that can only have been torn by wind or waves from other Peerless sediments nearby. These chips, and also mudcracks, suggest repeated exposure to the air, as on mudflats. In combination, features, such as the shale chips, glauconite, ripple marks, fucoid markings, the conglomerates, and irregular bedding planes, suggest deposition in shallow water. The abun- dance of iron, as expressed by hematite, limonite, glauconite, and authigenic chlorite, suggests slow deposition under conditions of restricted circulation and high salinity, and the variety of iron minerals sug- gests changing conditions within this framework (James, 1966). Color mottling in the nonconglomeratic dolomites also suggests these conditions (McKee, 1945, p. 75—77). ORDOVICIAN SYSTEM In central Colorado the Ordovician System comprises three formations—the Manitou Dolomite, Harding Sandstone, and Fremont Limestone. These formations, of Early, Middle, and Late Ordovician ages, respec- tively, are separated one from the other and from post- Ordovician rocks by unconformities that represent 22 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO widespread erosion after deposition of each unit (Lover- ing and Johnson, 1933; Johnson 1944; Sweet, 1954). As a result, the three formations are preserved only in remnants of their original extent. Of these remnants, the Manitou is by far the most extensive and the Hard- ing and Fremont are areally much more restricted. The distribution of these remnants isof economic concern because the Manitou and Fremont are mineralized in many mining districts in central Colorado. The Manitou was referred to as the “White limestone” or “Yule limestone” in older literature in many of the mining districts (Emmons, 1886; Emmons and others, 1927). In much of the region surrounding the Minturn quad- rangle, such as the Leadville area and adjoining Mos- quito Range, the western side of the northern Sawatch Range north of Aspen, Glenwood Canyon, and the White River Plateau, the Manitou is the only Ordovi- cian formation present. In the Minturn quadrangle, however, the Manitou is absent and the Ordovician is represented only by the Harding Sandstone. This occur- rence of the Harding constitutes an outlier 25 mi (40 km) north of the general area of Harding occurrence, as shown on a map by Sweet (1954, fig. 1). The closest known approach of the Manitou to the Minturn quad- rangle is in the Pando area about 8 mi (13 km) south- southeast of Red Cliff, where the Manitou tapers to an eroded edge unconformably beneath the Harding (Tweto, 1956). The Manitou is also present about 9 mi (14 km) southeast of the Minturn quadrangle at Mayflower Gulch in the Kokomo district (Tweto, 1949, p. 156), where it is 20 ft (6 m) thick, lies on Pre- cambrian rocks, and is overlain by the Parting Forma- tion. The Fremont Limestone is even less widespread than the Harding; its nearest exposures are more than 30 mi(48 km) south of the Minturn quadrangle. HARDING SANDSTON E Strata assigned to the Harding Sandstone of Middle Ordovician age consist of white and green quartzite, sandstone, and shale a few feet to perhaps 80 ft (24 m) thick that lie unconformably on the Peerless Formation and are unconformably overlain by the Parting Forma- tion. No fossils have been found in these rocks, but as the rocks are lithologically similar to the Harding exposed in many places along the sides of the Sawatch Range farther south, and as they clearly overlie the Manitou Dolomite south of the Minturn quadrangle in the area between Pando and Tennessee Pass (Tweto, 1956), they are assigned with confidence to the Harding. The Harding is continuous, although of variable character and thickness, from the north end of the can- yon of the Eagle River southward to the quadrangle boundary. Farther south, it is in discontinuous lenses (Tweto, 1949), and it finally pinches out completely about a mile north of Tennessee Pass (Tweto, 1956). It is continuous westward from the canyon area at least to Beaver Creek, about 2 mi (3 km) west of the quadrangle boundary, but it must pinch out farther west. At Fulford, 7>mi (11 km) west of Beaver Creek, only the Manitou is reported between the Peerless and Parting (Gabelman, 1950). The Harding must also pinch out eastward from the canyon area as it is absent at Bald Mountain and at Mayflower Gulch in the Kokomo dis- trict. The Harding is varied in composition but commonly consists of massive white quartzite in discontinuous lenses at the base; thin-bedded green and maroon sandstone, quartzite, and conglomerate in the middle part; and green shale and sandstone in the upper part. At the north end of Eagle Canyon, the Harding consists of 6 ft (2 m) of massive white quartzite overlain by about 10 ft (3 m) of purple-mottled green clay shale. One—half mile farther south it is represented by a single massive 25-ft (7.5-m) bed of white quartzite. In the Newhouse tunnel of the Eagle mine it is 39 ft (12 m) thick and consists of 22 ft (7 m) of basal white quartzite overlain by 17 ft (5 m) of green-gray quartzites, arkosic quartzites, and sandy green shales. In roadcuts 1.5 mi (2.4 km) south of Gilman, the Harding is only 14 ft (4 m) thick and consists of 2—3 ft (0.6—1 m) of basal white quartzite overlain by 11—12 ft (3.4—3.7 m) of interbed— ded pink, gray, and green quartzite, conglomerate, san- dy shale, and clay shale. On the slopes of the Sawatch Range the Harding consists of at least 35 ft (10.7 m), and possibly as much as 50 ft (15 m), of massive white quartzite. In the lower levels of the Eagle mine the Harding has an apparent thickness of as much as 80 ft (24 m) and consists of 30 ft (9 m) of white and green quartzites overlain by about 50 ft (15 m) of soft varieg- ated clay shale. The shale has been deformed by low- angle faults wherever seen, however, and it may have been thickened by repetition of beds. Although the Harding normally appears to be merely disconformable with the formations below and above it, small angular discordances exist. In the Pando area, a discordance of as much as 6° between the Peerless and the Harding was measured in cliff exposures, and some of the basal white quartzite occupies steep-sided chan- nels cut in the Peerless (Tweto, 1949, p. 166 ~169). In the Newhouse tunnel at Gilman (fig. 8), a discordance of 2° was measured between the Harding and the overlying Parting Formation. Throughout the canyon area and southward into the Pando area, the Harding locally grades at the top into a bed of tough massive green clay from 1 to 2.5 ft (0.3 to 0.75 m) thick that is characteristically marked by dark purple spots and streaks. This clay layer shows no bed- DEVONIAN AND MISSISSIPPIAN SYSTEMS 23 ding but in places displays a slightly kneaded structure. In places where the Harding is discontinuous, such as south of Red Cliff, this massive clay bevels the Harding and lies directly on the Peerless Formation. The clay bed is interpreted as a residual soil developed on the older rocks prior to the deposition of the overlying Up- per Devonian beds. It is a product of unconformity and technically is not a part of either the Harding or the Parting, but its thinness dictates that it be mapped with one or the other. In the Minturn quadrangle it was generally included in the Harding because of its grada- tional relation with the Harding, but in the Pando area it was included by Tweto (1949, p. 170 —173) with the Parting because it lies above the surface of angular dis- cordance between the Harding and Parting. The Harding Sandstone commonly weathers to a partly or completely covered slope, and the best ex- posures are found in mine openings. The following sec- tion was measured in the Newhouse tunnel, which crosscuts the stratigraphic section from Precambrian rocks at the portalto the Chaffee Group. Section of the Harding Sandstone [Measured in Newhouse crosscut tunnel, Gilman. See fig. 8] Thickness Distance . (feet) above base Parting Formation: (feet) White quartzite, uneven-grained, finely conglomeratic. Unconformity, 2° discordance ................. . 39.2 Harding Sandstone: 16. Quartzite, green, uneven-grained, sandy and shaly; contains subangular quartz pebbles as much as 1/4 in ........................... 2.3 36.9 15. Shale, green, nodular, sandy. Nodules are pink quartzite, irregular in shape and 1/4—1 in. long ................. 1.2 35.7 14. Shale, green, sandy ................. .2 35.5 13. Quartzite, white, massive, glassy, fine- grained .......................... 2.6 32.9 12. Quartzite, medium~grained; lower part is arkosic and feldspars are kaolinized ........................ ' .5 32.4 11. Sandstone, green, shaly .............. .2 32.2 10. Quartzite, greenish-gray, medium- grained; stained pinkish brown in scattered l/i-in. angular spots ...... 3.0 29.2 9. Shale, green, sandy ................. .1 29.1 8. Quartzite, light-green-gray, medium- grained, massive; bottom 3-in. layer is arkosic and contains many white kaolinized grains in slightly chloritic matrix ........................... 1.8 27 .3 7. Shale, green, sandy ................. .1 27.2 6. Quartzite, light -gray to greenish-gray, medium-grained, with a few 1/2- to 1- in. pinkish-brown layers ........... 4.8 22.4 5. Quartzite, greenish gray, fine-grained .2 22.2 4. Shaly sand parting .................. .3 21.9 3. Quartzite, white, medium-grained . . . . 3.5 18.4 Section of the Harding Sandstone — Continued Thickness Distance (feet) above base Harding Sandstone — Continued (feet) 2. Quartzite, white and gray banded; alternating coarse- and fine-grained layers ........................... 1. Quartzite, white, fine-grained; contains a few very small black grains ....... 1.7 0 Total measured thickness of the Harding Sandstone .......... 16.7 1.7 39.2 Peerless Formation: Dolomite, brown, micaceous, medium-grained sandy. Petrographic examination of two samples of sandstone from the Harding, one fine-grained and the other coarse-grained, shows that the sandstone consists of quartz, abundant interstitial sericite, minor calcite in scattered small lenses less than 1 mm in length, and ac- cessory tourmaline, green biotite, chlorite, il- menite/leucoxene, sphene, and zircon. Unlike the sandstones of the Peerless, feldspars are absent except as inclusions in some of the quartz grains. The feldspar of this occurrence is an untwinned variety of low refrac- tive index. In the fine-grained sandstone the quartz is in well-sorted grains 0.1—0.2 mm in diameter which have recrystallized to produce irregular interlocking grain boundaries; sericite is in elongated shreds and flakes that have a strong preferred orientation parallel to the bedding but are unevenly distributed. In the coarse‘grained sandstone the quartz grains are poorly sorted and range from 0.1 to 2 mm in diameter. Some of the larger grains are well rounded but most are irregu- lar, with interlocking boundaries. Hydromica or sericite fills in around quartz grains and also fills fractures cut- ting them. A sample of green siltstone from the Hard- ing contains flakes of muscovite as much as 0.2 mm long in a matrix of sericite, clay minerals, and amorphous limonite. DEVONIAN AND MISSISSIPPIAN SYSTEMS The Devonian and Mississippian rocks of the Min- turn quadrangle and surrounding region consist of a basal quartzite, typically about 40 ft (12 m) thick, and overlying carbonate rocks, typically about 250 ft (75 m) thick. Originally, these strata were divided into two units, the Parting Quartzite and the Leadville Limestone or “Blue Limestone” (Emmons, 1882, 1886). Kirk (1931) later restricted the Leadville to “limestones of Mississippian age,” and assigned the carbonate rocks in the lower part of the Leadville of previous usage, along with the Parting strata, to the Upper Devonian Chaffee Formation. Kirk designated the basal quartzite the Parting Quartzite Member of the Chaffee Forma- 24 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO tion, and Behre (1932, p. 60) later designated the carbo- nate strata the Dyer Dolomite Member. In restricting the Leadville and defining the Chaffee, Kirk selected as the lower boundary of the Leadville an unconformity at the base of a thin sandstone and brec- cia unit then known to occur in the Leadville area (Em- mons and others, 1927, p. 34), in the Mosquito Range (Behre, 1929, p. 38), and in the Gilman area (Crawford and Gibson, 1925, p. 36). This sandstone and breccia unit was later found to be far more widespread and was designated the Gilman Sandstone Member of the Lead- ville Limestone (Tweto and Lovering, 1947; Tweto, 1949). Thus, in the usage of the US. Geological Survey since 1931, the Devonian and Mississippian rocks of central Colorado have been divided into two formational units: (1) the Chaffee Formation, consisting of the Parting Quartzite Member and the Dyer Dolomite Member, and classified as Upper Devonian; and (2) the Leadville Limestone (0r Dolomite), consisting of the Gilman Sandstone Member and an unnamed carbonate rock member, and classified as Lower Mississippian. Geologic mapping and stratigraphic studies by many workers since 1931 have established that the Parting, Dyer, and Gilman are each a widespread mappable unit in central and northwestern Colorado. Further, the pre- sence of an unconformity between the Gilman and the overlying carbonate rocks of the Leadville was established (Tweto, 1949, p. 179; Banks, 1967, p. 41), and a close relationship in lithology and origin between the Gilman and the Dyer was recognized. In the White River Plateau area, where the Dyer consists of a lower limestone unit and an upper dolomite unit (Bass and Northrop, 1963, p. 21), Campbell (1970) distinguished these units as members and raised the Dyer and Part- ing in rank to formation and the Chaffee in rank to group. The classification of Campbell is here adopted with modification. The Parting is designated “Formation” rather than “Quartzite” because it has a mixed lithology or is largely shale in many places. The Dyer is designated the Dyer Dolomite. The Gilman is removed from the Leadville and is designated the Gilman Sandstone, thereby restricting the Leadville to carbon- ate rocks above the Gilman and below the Pennsylva- nian strata. The Chaffee Formation of former usage, plus the Gilman, is designated the Chaffee Group. The Gilman is placed in the Chaffee Group because of its close relation to the Dyer in character, origin, and prob- ably in age. Thus, the Chaffee Group consists, from the base upward, of the Parting Formation, Dyer Dolomite, and Gilman Sandstone. As discussed in the following sections, the Parting Formation and the lower half or more of the Dyer Dolomite are established to be Upper Devonian. The up- per part of the Dyer Dolomite and the Gilman Sandstone might be either Late Devonian or Early Mississippian in age. Accordingly, the Dyer Dolomite and Chaffee Group are referred to the Upper Devonian and Lower Mississippian(?). The Gilman Sandstone is referred to the Upper Devonian or Lower Mississippian. CHAFFEE GROUP The Chaffee Group is exposed in the Minturn quad- rangle only in the area near the Eagle River and—in part—in small fault slices along the Gore fault in the Gore Range. Near the Eagle River the group is 140—165 ft (43—50 m) thick—a normal thickness for the region. However, the Chaffee Group thins northeastward toward the Gore Range. In this direction, it overlaps the eroded edges of older formations, and its eroded edge is in turn overlapped by the Pennsylvanian Minturn For- mation. In fault slices near the Gore fault, the Parting Formation is the only part of the group preserved beneath the Pennsylvanian rocks. These relations, among others, led Lovering and Johnson (1933) to the concept of a persistent highland in the area of the Gore and Front Ranges in early as well as late Paleozoic time. Rocks of the Chaffee Group are resistant, and they commonly crop out in cliffs. In the canyon of the Eagle River, they form the lower part of the upper cliffs (fig. 3), which rise above a slope that represents the Harding Sandstone and Peerless Formation. Though thin, the rocks of the Chaffee Group are of special economic interest because they—along with the overlying Leadville Dolomite—are the host rocks of the principal ore deposits at Gilman, Leadville, and several other mining districts. In such areas, and at Leadville especially, the thin sandy units comprising the Parting and the Gilman provide the principal stratigraphic con- trol in a sequence of mineralized and altered dolomites. In areas where the Dyer and Leadville Dolomites have been replaced by jasperoid, the sandstone in the Gil- man—though extensively replaced also—is particularly significant as a stratigraphic marker. Areas of jasperoid northwest of Gilman and west of Minturn are indicated on plate 1 and are discussed in the report on the ore deposits by T. S. Lovering, Tweto, and T. G. Lovering (1977). PAR’I‘ING FORMATION In the southwestern part of the Minturn quadrangle, the Parting Formation consists of 40—65 ft (12~20 In) of predominantly quartzitic rocks. It lies unconform- ably on the Harding Sandstone in this area, though a few miles to the south it locally lies on the Peerless For- mation (Tweto, 1949). As exposed in fault blocks along the Gore fault in the Gore Range, the Parting is 10—30 DEVONIAN AND MISSISSIPPIAN SYSTEMS 25 ft (3—9 m) thick and rests, in different places, on Pre- cambrian rocks, on Sawatch Quartzite, or on a thin remnant of the Peerless Formation (fig. 25). In the Minturn quadrangle and nearby areas, the Parting Formation consists chiefly of quartzite and quartzite conglomerate, but locally it contains abun- dant turquoise-green shale which in places is streaked and is mottled maroon. The quartzite is typically light tan to white, poorly sorted, coarse grained, thick bedded to massive, prominently crossbedded, and vitreous. Quartzite conglomerate, characterized by well-rounded to angular white and pink quartz pebbles 1/4—2 in. (0.6-5 cm) in diameter, generally is present at the base and occurs also in scattered lenses throughout the unit. ‘Quartzite of the Parting can generally be distinguished from that of the Sawatch—or from the white quartzite present locally in the Harding—in isolated exposures by the coarse and uneven grain, the presence of clear quartz as contrasted to cloudy white quartz of the Sawatch and Harding, and, commonly, by tan color of the rock. The quartzite of the Parting is composed almost en- tirely of quartz grains but contains sparse interstitial sericite and a few detrital grains of zircon and leucox- ene. Some of the quartz grains contain small inclusions of muscovite, green tourmaline, and slender needles of sillimanite, together with lines of minute fluid inclu- sions. Many of the grains show marked strain shadows. The quartz grains in the nonconglomeratic beds range in size from 0.1 to more than 1 mm; most of them were originally rounded but recrystallization and secondary quartz overgrowths have produced an irregular in- terlocking texture. The green shale in the Parting occurs in local thin beds between the heavy quartzite beds and also in thick lenses that locally constitute as much as half the thick- ness of the formation, as at the north end of the canyon of the Eagle River, south of Cross Creek. Much of the shale is sandy, and some of it contains thin beds of vitreous white quartzite. Thin sections show that the green shale is composed of alternate layers of fine arkosic sand and green chloritic material. The arkosic layers are composed chiefly of quartz, microcline, and a pale-green micaceous mineral tentatively identified as a chlorite. Small flakes of detrital muscovite are com- mon in the arkosic material, and the accessory detrital minerals are tourmaline, sphene, and zircon. Character of the Parting Formation is illustrated by the following section, measured in the canyon between Red Cliff and Gilman. As measured in sections a few miles farther south (Tweto, 1949, p. 171—173), the up- permost sandstone and quartzite beds of the Parting are dolomitic, and the contact with the overlying Dyer Dolomite is gradational. Section of the Parting Formation [Measured in cut along US. Highway 24, 0.6 mi (1 km) northwest of high bridge over the Eagle River one-halfmile (0.8 km) west of Red Cliff. See fig. 8| Distance above base (feet) Thickness Dyer Dolomite: (rm) Covered, 10 ft. Parting Formation: Approximate top ........................ 15. Quartzite, white, vitreous, coarse- grained, uneven-grained ........... 1.0 14. Sandy quartzite, light~yellowish-gray, thick-bedded; contains white vitreous quartzite fragments and bands of intraformational conglomerate ofthese fragments in sandy quartzite ................... 4.0 13. Quartzite, white, massive, vitreous, coarse, uneven-grained, slightly conglomeratic; contains buff-gray specks of altered feldspar .......... 4.0 12. Quartzite, light-gray, vitreous, even- grained, medium— to fine-grained; abundant buff flecks. Weathers with hackly surface .................... 5.0 11. Quartzite, white, coarse-grained, slightly sandy, many clear quartz grains as much as 1/4 in. in diameter 7.0 10. Sandy quartzite, banded white and greenish~gray, coarse-grained; contains clear quartz granules ..... 2.5 9. Quartzite, white, vitreous, fine-grained; contains very few buff specks ...... 7.0 8. Quartzite, light tan, very coarse and poorly sorted; locally conglomeratic; glassy quartz pebbles .............. 1.5 7. Sandstone, shaly, greenish—gray, brown-weathering, fine-grained. Grades down into green-gray shaly and arkosic friable sandstone ...... 1.5 6. Sandstone, quartzitic, conglomeratic, contains some hematite; grades down into shaly sandstone ......... .5 5. Quartzite, conglomeratic, white, micaceous, vitreous. Contains pebbles of clear and pink quartz and l/2-in. angular fragments of gray shaly sandstone that may be from Harding Sandstone ............... 1.5 4. Sandy shale and shaly sandstone, light- greenish gray, fine-grained, nonmicaceous .................... 4.0 7.5 3. Quartzite, light-pink, medium~grained, vitreous, finely banded. Grades downward into white, vitreous, poorly sorted quartzite; is finely conglomeratic at base. Fragments of clear quartz ...................... 5.0 2.5 2. Quartzite, banded pink and gray, coarse-grained, conglomeratic; has abundant fragments of pink quartz as much as 3/8 in. in diameter. Many grains of kaolinized feldspars as much as 1/3 in. in diameter ......... 1.5 1.0 47.0 46.0 42.0 38.0 33.0 26.0 23.5 16.5 15.0 13.-5 13.0 26 Section of the Parting Formation — Continued Parting Formation — Continued Thickness Distance 1. Clay-shale, tough, green with purple (feet) ”3:23” bands and spots; is micaceous; has kneaded appearance. Probably a regolith .......................... 1.0 0 Total measured thickness of the Parting Formation ....... 47'0 Harding Sandstone: Quartzite, white, vitreous, fine- and even-grained. In many places the quartzite beds of the Parting con- tain rusty cavities that can be seen to be molds of brachiopods, pelecypods, or crinoid stems, but generally these are too vaguely imprinted in the coarse quartzite matrix to be more closely identified. On the dip slope traversed by the Tigiwon Road 1.2 mi(1.9 km) north- west of Gilman, somewhat better preserved casts and molds were found in abundance in the Parting. These fossils were identified by Edwin Kirk of the US. Geological Survey (written commun., Feb. 5, 1953) as: Schizophoria striatulata var. qustralis Kindle Spirifer (Cyrtospirifer) whitneyi (Hall) Productella sp. Paurorhynca endlichi (Meek) Aviculopecten .9 sp. (fragmentary) Kirk stated “Although poorly preserved, the above listed fossils can be recognized. It is a typical Ouray (Upper Devonian) fauna.” A generally similar group of fossils has been reported from the Parting in the southern Sawatch Range (Dings and Robinson, 1957, p. 15). More recently, C. A. Sandberg of the US. Geologi- cal Survey (written commun., 1971) has reported a con- odont fauna containing ?Clydgnathus ormistoni from the Parting in Glenwood Canyon. Except for these three invertebrate collections, the chief fossils found in the Parting in central Colorado are fish remains (Bryant and Johnson, 1936; Denison, 1951; Bass and Northrop, 1963, p. J20—J21). On the basis of the fish fossils, particularly the genus Bothriolepis, the Parting has been classed as early Late Devonian in age (Denison, 1951; Poole and others, 1967). On the other hand, the brachiopod Paurorhynca endlichi and the conodont .9Clydgnathus ormistoni reported above indicate a late Late Devonian age. This difference in age probably is an expression of different levels of fossil occurrence in the Parting, though it does establish that some of the Parting is younger than was previously known. The Parting is a transgressive unit that progressively thins and becomes younger north- eastward. Most of the fish localities are on the south- west side of the Sawatch Range, 60 mi (97 km) south- west of the Minturn quadrangle, in thin-bedded limy, shaly, and sandy strata nearly 100 ft (30 m) below the GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Dyer. The brachiopod locality in the Minturn quad- rangle, in contrast, is in quartzite no more than 20 ft (6 m) below the Dyer, and the conodont locality is only 21 ft (6.4 m) below the Dyer. One fish locality is in the Mosquito Range north of Salida, in a red shale unit that underlies the typical quartzite of the Parting. This shale unit extends discontinuously northward to the Leadville area, but it is absent in the Minturn quad- rangle. Beds thought by Behre and Johnson (1933) to represent this shale at Gilman are of different character and are herein assigned to the Harding For- mation. From regional studies of the Upper Devonian rocks, and frdm the fact that most of fish remains in the Part- ing and correlative units are of fresh or brackish water forms, Denison (1951) concluded that the Parting represents nearshore, shallow-water marine deposits, possibly including freshwater stream-channel and . flood-plain deposits. Character of the Parting in the Minturn quadrangle supports such a conclusion. The widely occurring molds of brachiopods and crinoids in- dicate marine deposition, but the lenticular conglomer- ates suggest channel deposition, particularly as they show crossbedding of the stream gravel-bar type. General increase in coarseness of the Parting north- eastward suggests a source area and depositional margin not far northeast of the Minturn quadrangle, if not at the site of the Gore Range. From studies of crossbedding and grain size in the Parting from Min- turn westward to Rifle, Campbell (1967) concluded that the lower part of the Parting was derived from sources to the east and the upper part was derived from sources to the north. Local areas of nondeposition of the Part- ing were recognized by Singewald (1931) in the Alma distrct, 16 mi (25 km) southeast of the Minturn quad- rangle. Throughout the region, the Parting Formation is separated from underlying rocks by a major uncon- formity. The youngest formation known beneath this unconformity is the Fremont Limestone of Late Ordovi- cian age, and regionally the Parting bevels formations of all ages from Fremont down to Precambrian. Though the unconformity was long thought to represent erosion in Silurian time, the discovery of Silurian and Upper Ordovician limestones in diatremes in the Front Range (Chronic and others, 1969) along with other, indirect evidence suggests that both the Fremont and a Silurian limestone may once have been widespread over the state. If so, these rocks were eroded in Early and Middle Devonian time, when most of the Rocky Mountain region was a land area (Poole and others, 1967; Sandberg and Mapel, 1967). The paleontological data just discussed indicate that in the area of the Minturn quadrangle erosion probably continued through the early part of the Late Devonian also. DEVONIAN AND MISSISSIPPIAN SYSTEMS DYER DOLOMITE The Dyer Dolomite lies conformably, locally with gra- dational contact, upon the Parting Formation and is un- conformably overlain by the Gilman Sandstone. The Dyer is uniformly 75— 80 ft (23- 24 m) thick in most of the area near Gilman and southward to Leadville. In a few places, however, the overlying Gilman Sandstone fills broad channels cut to depths of as much as 25 ft (7.5 m) into the Dyer, reducing the thickness of the Dyer to as little as 50 ft (15 m). The Dyer consists almost entirely of dolomite, which characteristically is fine grained and thin bedded and breaks into small, sharp, hackly fragments (fig. 9). The dolomite is gray to black; much of it is finely laminated in shades of gray. The laminations are wavy, and Campbell (1970) has interpreted them as stromatolitic in origin. In outcrop, the lower half to two-thirds of the member weathers light buff or yellowish gray, and the upper part weathers dark brown to bluish gray. Argillaceous matter coats many of the bedding planes and occurs also in scattered beds of shaly breccia in the lower, yellow-weathering unit. Such beds are bounded by wavy bedding surfaces that are evident minor dis- conformities or diastems; they commonly have a bleached-looking, light gray or yellow color and lie upon dolomite that is similarly bleached to depths of a few in- ches. Such argillaceous breccia zones are interpreted as weathered zones formed during periods of interrupted deposition and temporary exposure. Other beds in both the lower and upper parts of the member have a fine FIGURE 9.—Thin-bedded dolomite characteristic of Dyer Dolomite in roadcut of abandoned highway 0.3 mi (0.5 km) northwest of Gil- man. Hammer (arrow) shows scale. 27 breccia structure but are without argillaceous matter or bleaching. These are interpreted-as wave breccias. Within the canyon of the Eagle River and in adjoin- ing mines, the Dyer contains severaldistinctive and persistent beds that are useful as stratigraphic markers. A 7-ft (2.1-m) bed that contains abundant black chert in small nodules and lenticular stringers is present 15 ft (4.5 m) above the base. Thin but persistent shale partings occur at 22 and 32 ft (6.7 and 9.8 m) above the base. At 45 ft (14 m) above the base is a bed of dense black dolomite 3—8 ft (1—2.4 m) thick that shows in cliff exposures as a black band at or near the boundary between the yellow- and the blue-gray- weathering parts of the member. At the base of this black bed is a thin sandy stratum known locally as the “sand grain marker.” The “sand grain marker” is typically 1— 2 in. (25* 5 cm) thick, but locally it is as thin as 1/4 in. (6 mm) or as thick as 5 in. (13 cm). It consists of dark dolomite sprinkled with rounded, frosted quartz sand grains generally 0.5- 1 mm in diameter. In some places the sand grains are so abundant as to constitute a sandstone. In others, they are so sparse as to require very close scrutiny for identification; nevertheless, this thin stratum has proved to be remarkably persistent and widespread (Lovering and Tweto, 1944, p. 23; Tweto, 1949, p. 175; Banks, 1967). Other sandy zones of similar character occur locally in the upper part of the Dyer, and Campbell (1970) has noted the presence of disseminated quartz sand grains and local thin stringers of sandy dolomite throughout the uppermost part of the Dyer in the White River Plateau area. The frosted, well-rounded grains of the “sand grain marker” and other, local, sandy layers are interpreted to be of eolian origin, blown from coastal dunes into the shallow waters and tidal flats in which the carbonate muds of the Dyer accumulated. Character of the Dyer is illustrated by the following stratigraphic section, measured in the canyon of the Eagle River. Section of the Dyer Dolomite [Measured along first gully north of Rock Creek, beginning at abandoned highway grade below US. Highway 24. See fig. 8] Thickness Distance (feet) above base Gilman Sandstone: ”“0 Quartzite and dolomite breccia. Unconformity. Dyer Dolomite: 16. Dolomite, gray, brown- to gray- weathering, finely crystalline, brittle, thin-bedded ............... 15. Dolomite, dark-bluish-gray, finely crystalline, brittle, hackly, thin- bedded, and finely banded ......... 3.5 14. Dolomite, tan-gray, finely crystalline, brittle, thick-bedded .............. 77.8 9.0 68.8 65.3 2.0 63.3 28 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Section of the Dyer Dolomite—— Continued Thickness Distance (feet) above base (feet) Dyer Dolomite — Continued 13. Dolomite, grayish -black, finely crystalline, thin-bedded; contains 1/5—1/4 in. mud lumps at top ........ 4.0 12. Dolomite, black, dark-brownish-gray- weathering, medium-crystalline to finely crystalline, medium -thin- bedded ........................... 5.0 11. Dolomite, shaly, gray, thin-bedded . . . . .8 10. Dolomite, blue-black, finely crystalline, massive. Weathers dark blue gray to black. “Black marker bed.” 2- to 5-in. bed of sandy dolomite at base is “sand grain marker” .............. 8.0 9. Dolomite, dark-gray; weathers light buff to greenish gray; is thinly banded; forms massive bed ......... 8. Dolomitic shale, dark-gray, thin- bedded; grades upward into overlying bed. “Shale marker” .............. 3.0 7. Dolomite, argillaceous, dark-bluish- gray grading to gray downward, green-buff-weathering, fine-grained, irregularly bedded ................ 9.0 6. Shale, dolomitic, green, thin-bedded . . 1.0 5. Dolomite, cherty, light gray, buff - weathering, fine-grained, thin. bedded. Chert is black, in scattered lenses as much as 18 in. long and 3 in. thick. “Cherty marker” ......... 7.5 4. Shale, calcareous, gray‘green, fissile . . 0.5 3. Dolomite, light-gray, gray-weathering, very fine grained, thin-bedded ...... 4.0 2. Shaly dolomite, light-gray, thin bedded 0.5 1. Dolomite, light-gray, finely crystalline, medium-bedded; weathers buff and to smoothly rounded ledge. Contains a few shale partings. Basal 3 ft is very thinly banded, irregularly bedded, and knobby weathering. Conformable contact with quartzite below ............................ 59.3 54.3 53.5 45.5 10.0 35.5 32.5 23.5 22.5 15.0 14.5 10.5 10.0 10.0 0 Total measured thickness of Dyer Dolomite ..................... 77.8 Parting Formation: Sandy quartzite, white and pinkish-white, rust—spotted, coarse- and uneven-grained; is slightly dolomitic in irregular areas. Under the microscope, typical Dyer Dolomite is seen to consist of dolomite grains averaging about 0.05 mm in diameter, most of which contain minor amounts of opaque dark carbonaceous matter as “dust.” This dusty organic matter probably accounts for the dark color of the rock. A few tiny irregular masses, of quartz and of a clay interpreted as halloysite, are also present; no other minerals were observed. Fossils are rare in the Dyer Dolomite in the northern Sawatch and Mosquito Ranges, but they have been found in several localities farther west and south. We found none in the Minturn quadrangle, though the Up- per Devonian brachiopod Spirifer whitneyi var. animasensis Girty was reported by Crawford and Gib- son (1925, p. 37—38). At Glenwood Canyon, 40 mi (64 km) west of Minturn, the Dyer contains a basal unit of limestone that has yielded an abundant fauna of Upper Devonian brachiopods (Bass and Northrop, 1963, p. J 21—26). Similar fossil assemblages have been reported from several localities in the lower part of the Dyer on the western side of the Sawatch Range (Johnson, 1944, p. 329—330) and from the southern end of the range (Dings and Robinson, 1957, p. 15). The fossils from the lower part of the Dyer are the basis for the assignment of the Dyer to the Upper Devonian, though close affinities with Lower Mississippian faunas have been recognized (for example, P. E. Cloud, in Bass and North- rop, 1963, p. J26). Though the upper part of the Dyer has been classed as Devonian by many authors, it is poorly fossiliferous and its age is not well established. Helen Duncan of the US. Geological Survey (in Morris and Lovering, 1961, p. 87) reported that a sparse coral fauna “suggests * * * Early Mississippian age.” Both Hallgarth (1959) and Rothrock (1960) concluded from subsurface studies in western Colorado that the upper part of the Dyer passes westward into limestones classed as Lower Mississip- pian on the basis of lithology and sparse fossils. In southwestern Colorado and adjoining Utah, Baars (1966, p. 2089) reported Early Mississippian endothyrid foraminifera in the upper part of the Ouray Limestone, with which the Dyer is generally correlative. Therefore, we class the Dyer as Late Devonian and Early Mississippian (?) in age. GILMAN SANDSTONE The Gilman Sandstone is a thin but widely persistent unit of sandstone, breccia, and dolomite that lies with erosional unconformity upon the Dyer Dolomite and is overlain with erosional unconformity by the Leadville Limestone (or Dolomite). The unit is typically about 20 ft (6 m) thick in the Minturn quadrangle and surround- ing region, but it thins locally to about 10 ft (3 m), and in a few places it thickens to as much as 50 ft (15 In). Where thick, the sandstone fills broad channels cut into the underlying Dyer Dolomite. One such channel was observed in the Eagle mine, and others occur south of the quadrangle. The Gilman is varied in lithology, but in most places a major part is sandstone or sandy dolomite. A basal bed of sandstone 1—2 ft (0.3—0.6 m) thick is commonly pre- sent. This bed is generally overlain by a few feet of in- terbedded sandstone and dolomite in beds that pinch and swell, and this is overlain by lenticular bodies of breccia, dolomite, and sandstone or sandy dolomite. Chert is abundant in some of the breccia and dolomite. DEVONIAN AND MISSISSIPPIAN SYSTEMS 29 The top of the Gilman is marked in many places by a bed of structureless brown-gray lithographic dolomite called the “waxy bed” by the mine geologists at Gilman and also referred to by that name by Engel, Clayton, and Epstein (1958) and by Banks (1967, p. 41). This bed is irregular in thickness, ranging from a few inches to several feet, and is generally separated from underly- ing strata by a wavy contact. A more pronounced wavy contact separates it from overlying bedded carbonate rock of the Leadville. Though cut out by unconformity in places, the “waxy bed” has been traced throughout the region from the White River Plateau to the Sangre de Cristo Range (Engel and others, 1958, p. 376; Banks, 1967, p. 36—42). In cliff exposures of the Chaffee Group and Leadville Limestone (or Dolomite), the Gilman generally shows up as a yellowish-gray massive unit separating thin- bedded gray dolomite of the Dyer below from thick-bed- ded gray carbonate rocks of the Leadville above. In such exposures, the “waxy bed” commonly forms a shallow indentation in the cliff. In mineralized areas, the Gilman displays composi- tional and structural features that are interpreted as products of solution processes. These features are dis- cussed in the report on the ore deposits (Lovering and others, 1977). In most unmineralized areas solution features are absent and the sandstone and sedimentary breccia of the Gilman are essentially unmodified. Pre- vious descriptions of the Gilman by us (Lovering and Tweto, 1944; Tweto, 1949) were based on studies in the Gilman and Leadville mineralized areas, and they con- sequently emphasized solution features that are now known to be of restricted occurrence in the Gilman. Where not modified by solvent action, the unconformity at the base of the Gilman is a smooth but wavy surface that commonly shows relief of several in- ches in only a few feet along strike, and a relief of several feet over longer distances. Where modified by solvent action, this surface is irregular and is marked by abrupt pits and pinnacles that have relief of as much as several feet. Strata above the modified surface show local sag structures and internal faults and are cut by dikelets of black clay, breccia, quartz sand, or dolomite sand. Sandstone of the Gilman is yellow to light gray and consists of well-sorted, rounded quartz grains about 0.5 mm in diameter cemented by dolomite, calcite, or silica. Some of the sandstone is speckled with minute particles of white clay. As judged from several thin sections, the sandstone is devoid of detrital heavy minerals. The sandstone closely resembles that of the “sand grain marker” and other thin sand lenses in the Dyer Dolomite, and, like them, it is probably of eolian origin, though obviously reworked in water. Except in the “waxy bed”, the dolomite in the Gilman is in part identical to the gray, fine-grained, finely lami- nated dolomite in the underlying Dyer and in part a breccia or edgewise conglomerate of dolomite frag- ments cemented by dolomite. Both varieties locally con- tain sparse to abundant quartz grains similar to those of the sandstone, and both varieties occur in cherty or chert-free forms. The “waxy bed,” in contrast, is devoid of quartz grains and chert and shows no lamination or bedding. The chert in the Gilman is predominantly black, but a light-gray to white variety is also present. The chert occurs principally in sharp to somewhat abraded frag- ments. The fragments are most abundant in the dolomite breccias, but they are scattered through the laminated dolomite also. The chert has been studied in detail by Banks (1970) who distinguished an early variety that was deposited from hypersaline waters penecontemporaneously with the enclosing dolomite, though the chert was broken and redistributed by wave action as dolomite deposition proceeded, and a late chert that transects primary sedimentary structures in the dolomite. Banks attributed the late chert to ground- water action during karst erosion after deposition of the Leadville Limestone. A section of ‘the Gilman Sandstone in the Gilman area follows. In this locality, the Gilman shows effects of solution and collapse, and also of hydrothermal alteration in some of the dolomite. Sections in other areas show less breccia and little or no hydrothermal alteration (Tweto, 1949, p. 179—180). Section of the Gilman Sandstone [Measured in gully in cliffs 0.4 mile northwest ofGilman, at abandoned highway grade below US. Highway 24. See fig. 8) Thickness Distance Leadville Dolomite: ’feeU “$59530“ Light-gray finely crystalline dolomite. Unconformity, wavy surface. Gilman Sandstone: 8. Dolomite “waxy bed"; is dark gray, brown-gray weathering, dense, uniform; hackly fracture; 3-in. layer of black chert at base .............. 9.0 7. Dolomite, hydrothermally altered; is brownishgray, porous, vuggy; has zebra< >< Pelmatozoans: Crinoid columnals ......... X X . . . . . . . . . Cidarid spines (Echinoid) . .. R . .. i . . .. . X Brachiopods: Lingula carbonaria Shumard? .............. 7 Lingula tighti Herrick ...... ... ... ... ... ... ... A A Orbiculoidea sp ............ ... R ... ... X .i. , Derbyia crassa (Meek and Hayden) ................ C Derbyia? sp. indet ......... .1. ... X ... R ... Eolissochonetes sp ......... C C C C C X Orthotichia schuchertensis Girty .................. C ? . . . , . . Echinoconchus sp .......... . . i . . . . i . R Antiquatoniu coloradoensis (Girty) ................ C ? C Linaproductus prattenianus (Norwood and Pratten)? . X X ... X ... ... ... ... Linoproductussp.indet R Anthracospirifer rockymontanus (Marcou) , Anthracospirifer sp. indet . i . Condrathyris perplexa (McChesney) .......... _ .. A A X 5% Pelecypods: Nuclopsis cf. N, girtyi Schenck ................ R Nucloidea sp .............. R . . . . i . . . Parallelodon tenuistriatus (Meek and Worthen) ..... R . i. C Edmondiasp ......... . Pasidonia sp ............. Aviculopecten cf, A, eaglensis Price ................... X X X . . . . . . C R Pernopecten cf. R ohioensis Newell ................ R Pectenoid, genus and species indet . . Myalina sp 1 . . . Septimyalina? sp. indet . Bakeoellia sp , . . . Schizodussp , i . . Schizodus sp. indet . . . Permopharus sp .......... Astartella concentrica (Conrad) ............... C Pelecypod indet .......... . xo 5 >< xxx} x} 3 x>< ><§ :17} E Gastropods: Knightites (Cymatospira) sp . R Knightites (Retispira) sp . . . . C Glabrocingulum sp. indet . . . ? Pleurotomaracean, gen. and sp.indet ................ X Murchisonia? sp. indet ..... R R . . i . . . . . . R Straparollus (Euomphalus) sp ...................... X Cephalopods: Pseudorthoceras knoxense (McChesney) ? .......... R Trilobites: Pygidium undetermined . . .. ... ... R As shown on an isopach map by Brill (1944, fig. 2) the Minturn quadrangle is high on the side of the trough in which the Belden was deposited, and, as previously noted, the depositional margin of the Belden must he no more than a few miles east of the type section. The age relations previously discussed are in accord with this distributional pattern. Near the center of the trough, in the Glenwood Springs area where the Belden is thick, the lower part is Morrowan in age and the upper part is TABLE 4 COLLECTION LOCALITIES AND DESCRIPTIONS 98697PC In type section of Belden Formation, in roadcut of U.S. Highway 24 on north side of Rock Creek, opposite Gilman, in limestone and shale about 100 ft (305 m) stratigraphieally above base of Belden. (Approximately unit 13 of emended type section.) Collected by J. S. Williams, T, S, Lovering, and Ogden Tweto, July 20, 1941. Same as 9869—PC, but 1007150 ft (30.5*45.7 m) farther east along highway and about 60 ft (18.3 in) higher stratigraphically, or about 160 ft (48.7 m) above base of Belden. (Approximately unit 21 of emended type section.) Same as 9869~PC and 9870—PC, but 100 ft (30.5 m) farther east along high- way, near top of Belden. (Approximately unit 33 of emended type section.) 9870—PC 9871*PC 98927PC From shale above porphyry sill in Belden Formation, on mountainside north of high bridge carrying U.S. Highway 24 over the Eagle River one-half mile (0.8 km) southwest of Red Cliff. (Probably 20‘30 ft (6—9 m) above base of Belden.) Collected by L. G. Henbest. Sept. 28, 1945. From undetermined stratigraphic position in Belden Formation, on west slope of southern prong of Battle Mountain, between 9.200» and 9,300-ft contours, 0.4 mi (0.6 km) N. 40° W. of high bridge carrying US. Highway 24 over the Eagle River 05 mi (0.8 km) southwest of Red Cliff. Collected by L. G. Hen- best, Sept. 1945. 13048430 In type section of Belden Formation, in uppermost part. Locality about same as 9871APC, Collected by Ogden Tweto, E. L. Yochelson, G. W, Weir, and R. E. Davis. Aug. ll, 1952. 13053iF’C From Belden Formation in roadcut on US. Highway 24, 1.2 mi (airline) north of Gilman. In cut on west side of highway and about 75—80 ft (22.8v24.3 m) above Leadville Dolomite. Collected by E, L. Yochelson, G. W. Weir, and R. E. Davis, Sept. 12, 1952. 14580—PC From 8-ft (2.4-m) unit of limestone with shaly partings (unit 4 in Tweto, 1949, p. 228), 49 ft (14.9 m) above base of Belden Formation, on south bank of Silver Creek, NE’liSWVisec. 33, T. 6 S., R. 80 W., Holy Cross quadrangle, 2.5 mi (4 km) southeast of Red Cliff. Collected by Mackenzie Gorden, Jr., and E. L. Yochelson, July 25, 1953. l2089—PC Atokan. In the Minturn quadrangle, where the Belden is thin, sedimentation began later and only strata of Atokan age were deposited. MINTURN FORMATION The Minturn Formation comprises as much as 6,300 ft (1,920 In) of elastic rocks and subordinate interbed- ded carbonate rocks of Pennsylvanian age lying above the Belden Formation and below the Maroon Forma- tion. Except as covered in places by the Maroon Forma- tion, the Minturn occupies almost all the area between the Eagle River and the Gore fault, and it is thus the most widespread geologic unit in the quadrangle. Seg- ments of the formation are well exposed in the walls of many of the canyons (fig. 12), but exposures are generally poor in the intercanyon areas. Thus, the for- mation is not exposed in its entirety in any one place in the quadrangle. The main body of the Minturn Formation in the Min- turn quadrangle is a coarse clastic facies lying between a shoreline of deposition along the flank of the present Gore Range and a fine-grained and evaporitic facies just west of the quadrangle. The formation thins in both these directions. Toward the Gore Range, which is at the western edge of the late Paleozoic Front Range highland, the formation thins by onlap, or the shin- gling-out of lower beds against the old highland. Near the Gore fault, the entire lower half, or more, of the for- mation is missing, as discussed in a following section. In the opposite direction, toward the evaporite basin, the thinning is far less pronounced and is internal, as shown by diminishing thicknesses between carbonate marker beds. PENNSYLVANIAN AND PERMIAN SYSTEMS 39 FIGURE 12.——L0wer part of the Minturn Formation, in cliffs on the east side of the Eagle River near Two Elk Creek. Vertical distance from up- per switchback in highway to skyline is 1,200—1,400 ft (365-427 In). The clastic rocks that make up the bulk of the forma- tion are varied but in general are highly arkosic, coarse grained, and poorly sorted. They are extremely lenticu- lar and many of the beds or lenses change rapidly in lithology in short distances. The rocks are also poorly exposed over wide areas. Consequently, the elastic rocks present great difficulties in the tracing of stratigraphic levels and the determination of detailed geologic structure. In contrast, the limestone and dolomite beds interbedded with the elastic rocks are relatively persistent and are moderately well exposed. They constitute the only reliable stratigraphic markers in this thick formation. Though only a few of the carbo- nate beds have proved to be widely persistent, many others serve as local markers useful in stratigraphic bridging from area to area. In mapping, nearly all the principal carbonate beds were “walked out” as a means of establishing stratigraphic control and determining the presence or absence of faults. SUBDIVISION In defining the Minturn Formation, Tweto (1949) designated seven of the principal carbonate beds or zones as members in the Pando area (fig. 13). The lower three of these—the Wearyman, Hornsilver, and Resolu- tion Dolomite Members—at about 2,600, 2,900, and 3,700 ft (793, 884, and 1,128 m), respectively, above the base of the formation, were newly named at that time. The upper four—the Robinson, Elk Ridge, White Quail, and Jacque Mountain Limestone Members—were named earlier in the adjoining Kokomo district, where they are the host rocks of ore deposits (Emmons, 1898; Koschmann and Wells, 1946), The Jacque Mountain, the highest persistent limestone in the Pennsylvanian- Permian sequence, is the uppermost unit of the Min— turn Formation and defines the top of the formation (Tweto, 1949). Not all of the seven carbonate members are persis- tent throughout the Minturn quadrangle. Further, the members were defined in the Pando area after most of the mapping in the Minturn quadrangle was completed, and no attempt was subsequently made to trace each one through the Minturn quadrangle. Consequently, only the Robinson, Jacque Mountain, and—to a lesser degree—the White Quail are distinguished widely on the geologic map (pl. 1). The other members are dis- tinguished locally. In the Robinson, which consists of several limestone beds separated by clastic rocks, only 40 GEOLOGY, MIN TURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO MINTURN TYPE SECTION \ Jacque Mountain Limestone Member: Mottled light-bluish-gray and pinkish- gray limestone. Contains oolitic, algal- pellet, sandy, and miceceous limestone beds F Clastic unit H: Grit, conglomerate, sand» stone, and shale White Quail Limestone Member Clastic unit G Elk Ridge Limestone Member \> Clastic unit F: Grit, sandstone, shale, and siltstone Robinson Limestone Member: Several beds of light-bluish-gray fossiliferous lime- stone, 5—65 ft (1.5—19 m) thick, and thin beds of dolomite, separated by FEET IMETRES) 6,308 (1,923) ' '1' U Cl C > E! a 6,140 1 (1,567) 2 4,979 L r" (1,518) B E *7 4,426 (1,349) 1 l l T 3,680 . (1,122) X E '5 U C U 3 9. 3 > 21355-7? (e70) . E E 0 2,524 F: (769) g .E a 'U S >‘ E o k i 1,490_ (454) 1,024 , (—7 312 ( ) Red and WHY 2 a: 376 _ * (1 15) Greenlsh HWY 0 l elastic rocks > Clastic unit E: Grit, sandstone, shale, and siltstone; mostly in thin beds ‘—-'Probable equivalent of Hornsilver Dolomite Member: Thin dolomite beds in black shale J Probable equivalent of Wearyman Dolomite Member: Thin dolomite beds in black shale Clastic unit D: Grit and conglomerate in thick massive beds Reef dolomite of Lionshead Clastic unit C: Grit, sandstone, shale, and minor amounts of dolomite Dolomite bed of Dowds Clastic unit 3: Grit, sandstone, shale, and conglomerate; corresponds in general with lower red zone Clastic unit A: Sandstone, shale, and dolo- mite, thin- and even-bedded FEET {ME TRESI 5,900 (1,793) 4,930 (1,503) 4,750 (1 ,448) 4,160 (1,268) 3,700 (1,128) 2,900 (884) 2,600 (793) 1 ,800 (549) 1 ,000 (305) 500 (152) PAN DO SECTION Jacque Mountain Limestone Member White Quail Limestone Member Elk Ridge Limestone Member Red and (my Robinson Limestone Member } Dolomite reef zone Resolution Dolomite Member Gray and pale tints of yellow, green, and pink Hornsi lver Dolomite Member Weary man Dolomite Member } Dolomite reef zone } Zone of black dolomite in thin but per- sistent beds Lower red zone: Massive grit and conglom- erate Grey and "n Even-bedded zone FIGURE 13,—Subdivisions and general character of the Minturn Formation in Minturn quadrangle, and comparison with stratigraphic section in Pando area, Holy Cross quadrangle. PENNSYLVANIAN AND PERMIAN SYSTEMS 41 the thickest and most prominent beds are dis- tinguished. These beds are not necessarily the bound- ing carbonate beds of the member. Clastic rocks between the various carbonate beds or members are all generally similar except that some of . them differ in color, and some units have either more massive or thinner bedding than other parts of the for- mation. For purposes of reference, the elastic rocks are divided into lettered units A—H, bounded by the carbo- nate members or by certain carbonate beds of more local occurrence, as indicated in figure 13. Attempts have been made in the past to subdivide the thick sequence of Pennsylvanian and Permian rocks in the region on the basis of color, as some of the rocks are gray and some are red, but the color boundaries migrate stratigraphically by as much as hundreds of feet in short distances and can be only indefinitely located in thick zones of interbedded red and gray rocks. Thus, ex- cept in a very general way, color is unreliable as an in— dicator of stratigraphic position. The Minturn Forma- tion is predominantly gray, or pale tints of green, yellow, and pink, but it contains a dull red zone in its lower part and a zone of brighter red at its top. In the middle part of the Minturn quadrangle, the lower red zone extends from about 375 ft (114 m) above the base of the Minturn Formation to about 1,075 ft (328 m) above the base; the lower half of this zone is almost en— tirely red, and the upper half is alternating red and gray. In the Pando area a few miles to the south, in con- trast, the lower red zone occupies the interval between 500 and 1,000 ft (152 and 305 In) above the base of the formation (Tweto, 1949, p. 194, 220—222). Similarly, in the Minturn quadrangle, the upper red zone lies about 4,750 ft (1,450 m) above the base of the formation and consists of 700 ft (215 m) of alternating red and gray rocks overlain by 800 ft (245 m) of almost entirely red rocks. In the Pando area (Tweto, 1949), the upper red zone lies 4,300 ft (1,310 m) above the base and consists of about 600 ft (185 m) of alternating red and gray rocks overlain by about 950 ft (290 m) of entirely red rocks. l.l'l‘ll()l.()(;\' Exclusive of the volumetrically minor carbonate beds, the Minturn Formation consists of interbedded— or interlensed—grit, sandstone, conglomerate, shale, and siltstone, with grit the predominating and charac- terizing rock type. As applied here, grit is coarse grained, poorly sorted in size and shape of grains, markedly feldspathic, generally micaceous. and friable to firmly cemented. In most of it, a conspicuous fraction of the grains is very coarse sand (1—2 mm in diameter), and much of it contains abundant particles of granule size (2_4 mm). as well as scattered pebbles, or even cob« bles several inches in diameter. Quartz is the most abundant constituent, but a large fraction of the grains is pink feldspar (microcline) derived from Precambrian pegmatites; plagioclase also is an abundant but less conspicuous component. Many of the feldspar grains are sharp-edged cleavage fragments. Coarse, ragged flakes of detrital muscovite are common in the grit; flakes of dark mica are somewhat less common. In a zone roughly 500—2500 ft (150fl760 m) above the base of the Minturn, the grit also contains rather abundant particles of dark—green chlorite phyllite. Pebbles in the grit are mainly quartz of the bull—quartz type, derived from Precambrian pegmatites and quartz veins, but some are feldspar fragments or rocks of various types, such as pegmatite, granite, and gneisses. The pebbles are well rounded to sharply angular. Pebbles of sedi- mentary rocks are rare and are found principally in the lower part of the formation. - Most of the sandstone in the Minturn Formation is also feldspathic, micaceous, and poorly sorted; it differs from the grit only in being finer grained and, com- monly, in having a smaller proportion of angular grains. Though some relatively pure quartz sandstone is present in the Minturn, particularly in the lowermost part, most of the sandstone and grit is arkose by the definition of Pettijohn (1957, p. 291). Some beds of sandstone and grit contain abundant dark rock and mineral grains and might be called graywackes in the old sense of the term, but they lack the fine-grained matrix that is nowadays implicit in the term (Pettijohn, 1957, p. 301). Petrographic study by Boggs (1966, p. 1414) of 68 samples of sandstone showed very wide ranges in mineral compositions and a mean composi- tion among the detrital grains of 50.5 percent quartz, 34.3 percent feldspar, 6.4 percent micas, 6.3 percent rock fragments, minor amounts of mud coatings and heavy minerals, and a trace of clay minerals. Boggs found that the cement is dominantly calcite but is silica in some samples. His samples apparently came from along Gore Creek and westward along the Eagle River, where rocks of the Minturn are generally finer grained and less feldspathic than those to the south. Most shales and siltstones of the Minturn Formation are micaceous and sandy, though some gray to black clay shale is also present. Some of the micaceous shale contains as much as 50 percent of detrital mica in flakes 1—3 mm in diameter. Sand and silt in the shale and siltstone are generally arkosic. Boggs (1966, p. 1416) made X-ray diffraction studies of 26 samples of these rocks and found illite present in all samples, chlorite in about half the samples, and mixed-layer Clays in several. No discrete kaolinite or montmorillonite were found. In a broader study of the clay mineralogy of the Pennsylvanian rocks in central Colorado, Raup (1966) found that the clay minerals in the finer grained rocks of the Minturn Formation are 42 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO A FIGURE 14,—Dolomite in the Minturn Formation in the Pando area. A, Crossbedded gritty dolomite 200 ft (60 m) below base of Robinson Limestone Member, 0.5 mi (0.8 km) south of Minturn quadrangle boundary. B, Conglomeratic dolomite, a local facies of the Hornsilver Dolomite Member, 1.5 mi (2.4 km) south of Minturn quadrangle boundary. dominantly illite, mixed-layer illite-montmorillonite, mixed-layer chlorite-vermiculite, and, in the lower part of the formation, some kaolinite. The clay and mica fractions in the samples tested ranged from a trace to 100 percent and averaged less than 50 percent. A suite of 12 samples from near Gilman came mainly from the lower red zone in the Minturn. Carbonate rocks of the Minturn Formation exhibit many unusual lithologic features resulting from the coarse-elastic environment of deposition. Almost all the carbonate beds are locally sandy or gritty, or even con- tain pebbles. Many grade laterally into grit, congrlomer~ ate, or sandstbne, passing through such odd facies as conglomerate made up of quartz and pegmatite pebbles in a matrix of dolomite, or dolomite containing 50 per- cent by volume of coarse arkosic grit (fig. 14). The car- bonate rocks are also prominently micaceous in places. Extreme examples are biotitic limestone that contains as much as 10 percent detrital biotite in a matrix of otherwise pure limestone and “schistose” muscovitic limestone that contains so much muscovite in coarse flakes oriented parallel to the bedding that the rock resembles a coarse mica schist. The limestones of the Minturn are in part very fine grained or sublithographic and in part calcarenitic, con- sisting of limestone grains and fossil fragments. Most limestone beds or units contain both of these varieties. Boggs (1966), using the classification of Folk (1959), classified most of the limestone as micrite (lithified car« bonate mud), biomicrite (carbonate mud with small fossils or fossil fragments), and oomicrite (oolitic carbo- nate mud). In a detailed study of the Robinson Limestone Member, Tillman (1971) identified four main facies of limestone: (1) oolite, (2) tubular Foraminifera micrite, (3) phylloid algae facies of biomicrite, and (4) stromatolite facies of laminated micrite. Bedding planes in the limestones are typically rough and scaly, and many near the tops of the limestone units are coated with films of yellowish- or greenish-gray argillaceous matter. In places, such bed— ding surfaces are studded with small fossils, such as fusulinids. Some limestone units locally contain chert. Most of the limestones are bluish or brownish gray on fresh fracture, but they weather a distinctive light bluish gray. In this respect particularly, the limestones closely resemble those in other Pennsylvanian forma- tions of the region, as in the Hermosa Formation of southwestern Colorado and in the Madera Formation of northern New Mexico. Dolomite in the Minturn is of three main varieties: (1) Evenly and generally thin-bedded, finely crystalline, dark-gray to black dolomite, most of which weathers brownish gray; (2) massive, vuggy, light-gray to black, brown-weathering, crystalline reef (biohermal) dolomite; and (3) coarsely crystalline, light—gray, buff- to tan-weathering, vuggy hydrothermal dolomite formed either by replacement of limestone or by recrystallization of earlier dolomite. The thin-bedded dark dolomite is generally in layers only 1—5 ft (0.3~1.5 m) thick. This dolomite commonly is interbedded with black shale, and many of the dolomite beds grade on strike into black shale. Some of the dolomite contains abundant black chert, either as long lenticles or as very irregular, scraggly bodies. Also, some of the dolomite is appreciably phosphatic, for it reacts strongly to the qualitative ammonium molybdate test for phosphorus. PENNSYLVAN IAN AND PERMIAN SYSTEMS 43 The dolomite reefs occur either as isolated bodies sur- rounded by clastic rocks or as abrupt bulges on thin layers of bedded dolomite or limestone. Most of the reefs have steep and ragged sides; a few have smooth and nearly vertical sides abutted by coarse grits. The reefs are typically a few hundred feet in diameter and 25—100 ft (7.5—30 m) high. However, some are as small as 25 ft (7.5 m) across and 5—10 ft (1.5‘3 m) high; others are as large as 1 mi (1.6 km) in diameter. The largest reef in the quadrangle, partly exposed in the bottom of the valley of Two Elk Creek 3 mi(4.8 km) above its mouth, has a visible thickness of almost 500 ft (152.4 m) and a maximum horizontal dimension, in- cluding a projecting layer, of about a mile. This and many other reefs contain abundant fossil fragments, particularly in their outer part; some contain areas with concentrically laminated algal structures. The reefs evidently were deposited as algal and bioclastic limestone, and were subsequently dolomitized, perhaps diagenetically. SEDIMENTARY FEATURES Many of the lenses of clastic rocks in the Minturn Formation are crossbedded, some of them spectacularly so. Most of the crossbedding is high-angle, medium- scale planar in the classification of McKee and Weir (1953), but simple and trough crossbedding also occur. Scour structures, in addition to those recorded by planar crossbedding, are common also. The scours are of two general magnitudes: (1) small filled channels cut as much as several feet into underlying strata, and (2) channels hundreds of feet wide—seen in cliff ex- posures—filled by entire beds or lenticular bodies of grit or conglomerate. Other sedimentary features present in some of the rocks are ripple marks of various kinds and sizes, mud cracks, mud-chip conglomerates, small clastic dikes, raindrop impressions, and, rarely, salt casts. TYPE SECTION In defining the Minturn Formation, Tweto (1949) designated the cliffs and area east of Minturn as the type locality but did not present a type section measured at that locality. Instead, he presented a representative section measured in the Pando area, about 10 mi (16 km) to the south. In 1963, T. S. Lover- ing measured a detailed section of the Minturn at the type locality, presented at the end of this report. That section is here designated the type section of the Min- turn Formation, and the section in the Pando area (Tweto, 1949, p. 207-227) is designated a reference sec- tion. The main features of the two sections are com- pared in figure 13. Because the type section was necessarily measured over a horizontal distance of several miles in rocks that are shingled against an old highland, the thicknesses measured in the type section are not necessarily a measure of the thickness of the Minturn at any given locality. As shown by cross sections (pl. 1), the distance between the Robinson Limestone Member and the base of the Minturn Formation decreases northeastward toward the Gore Range. This is due to progressive pinchout eastward of elastic beds in the lower part of the formation. The western boundary of the area affected by this pinching is not known but is almost cer— tainly west of the outcrops of the major limestones. Therefore, the vertical distance from any given horizon to the base of the formation probably is less in most localities than that indicated by measurement of stratigraphic thickness along a horizontal course. CLASTIC UNIT A The basal unit of the Minturn Formation, here refer- red to as Clastic unit A, is characterized by relatively fine-grained and evenly bedded Clastic rocks. The rocks of the unit constitute a transition zone between the un- derlying shaly and limy Belden Formation and the overlying coarse-grained, lenticular, arkosic grits of the main body of the Minturn Formation. Unit A is 375 ft (115 m) thick in the Minturn type section and about 500 ft (150 m) thick in the Pando section. It consists largely of green-gray and tan sandstone and shale but includes many thin beds of dolomite and a little con- glomerate. Grit is essentially absent, thereby dis- tinguishing this unit from the remainder of the forma- tion, and the sandstones and conglomerates are less feldspathic than those in overlying rocks. Raup (1966) . found appreciable kaolinite in the shales of this unit, just as in those of the Belden Formation below. (ILASTIC L'NIT B AM) T111; 1)()L()Ml'l‘li 15111) 01: Downs Clastic unit B, about 650 ft (200 m) thick, consists of grit, sandstone, shale, and conglomerate lying between unit A and the top of a distinctive dolomite bed here referred to as the dolomite bed of Dowds. (Named for Dowds siding near the junction of Gore Creek and the Eagle River.) Unit B corresponds in general to the lower red zone in the area near Minturn and is characterized by alternating thin and thick strata. The dolomite bed of Dowds is about 6 ft (2 m) thick; it is mottled greenish gray and black and is cherty. This bed of dolomite is the lowest carbonate bed in the Minturn that is persistent and distinctive enough to serve as a stratigraphic marker. (ILASTIC L'Nl'1‘(: AND THE REEF 1)()1.()M1'1‘11 01“ LIONS} IliAl) Clastic unit C, about 450 ft (135 m) thick, consists of strata between the dolomite bed of Dowds and the top of 44 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO FIGURE 15.——Dolomite reef in Minturn Formation at Lionshead, on steep slope above Minturn. Note abrupt termination to right (ar- row). a reefy dolomite zone referred to as the reef dolomite of Lionshead. Unit C is similar in lithology to Clastic unit B except that it is gray rather than red and contains a few thin beds of dolomite. The reef dolomite of Lionshead consists of discrete reefs strung at intervals along a thin and otherwise inconspicuous bed of dark dolomite. A typical reef in this unit forms the landmark of Lionshead on the slope above Minturn (fig. 15). This reef is 48 ft (14 m) thick but abruptly pinches laterally to two thin dolomite beds separated by several feet of grit. (ELASTIC UNI'I' l) Clastic unit D. about 1,000 ft (305 m) thick, consists of massive grits and conglomerates between the reef dolomite of Lionshead and the top of a massive grit bed about 200 ft (60 m) thick that is persistent through the cliffs on the east side of the Eagle Valley from Two Elk Creek to Gore Creek (grit marker bed of pl. 1). Clastic unit D constitutes the most massive part of the Minturn Formation. Many of the thickest beds are markedly len- ticular; one bed that forms a cliff 200 ft (60 m) high on the shoulder north of Two Elk Creek pinches in a quarter ofa mile (0.4 km) along strike to 5 ft (1.5 m). In the Pando area. thin beds of dolomite and also many dolomite reefs occur in this stratigraphic interval, but they are absent at Minturn. (ELASTIC LINl'l‘ li AND \VliARYMAN AND HORNSILVICR l)()l.()MI'l‘l-Z MliMBl-lRS Clastic unit E, about 1.100 ft (335 m) thick. consists of varied Clastic rocks in beds thinner than those in unit D. The base of the unit is marked by the Wearyman Dolomite Member or probable equivalent. The Hornsilver Dolomite Member—or equivalent——is about 330 ft (100 m) above the base. The Wearyman and Hornsilver both change in character northward in the Minturn quadrangle. The Wearyman is a light-colored reef (stromatolitic) dolomite 15—75 ft (4.5~ 22 m) thick in the Pando area (Tweto, 1949, p. 198) and in the Wearyman Creek and Turkey Creek areas in the southern part of the Minturn quadrangle. On Battle Mountain the Wearyman thins. grades into bedded dolomite, and becomes discontinuous. Farther north, at the Minturn type section at Game Creek, its probable equivalent is a 27-ft (8-m)unit of dark-gray shale that contains thin beds of dolomite (unit 86. Minturn type section). Similarly, the Hornsilver, an 18~28 ft (5.5-8.5 m) of distinctive light-weathering dolomite in the Pan- do area and in southern part of the Minturn quad- rangle. also thins and changes in character on Battle Mountain. In the part of the Minturn type section at Game Creek. the Hornsilver equivalent probably is a dolomitic sandstone and thin beds of dolomite in a pre- dominantly clastic unit 54 ft (16.5 m) thick (unit 94, Minturn type section). ROBINSON LIMESTONE M liM BER The name Robinson was first applied by Emmons (1882, p. 220; 1898, p. 2) to a group of three limestone beds separated one from the other by 80—100 ft (24 ~30 m) of elastic rocks at the Robinson mine in the Kokomo district. Tweto (1949. p. 201) defined the Robinson as a member of the Minturn Formation in the Pando area and adjacent Kokomo district. There. the Robinson con- sists of 3v5 beds oflimestone separated by elastic rocks in a zone 300~~400 ft (90—120 m) thick and about 4.200 ft (1,280 m) above the base of the Minturn Formation. In the early mapping ofthe Minturn quadrangle. Lover- ing and Tweto (1944) called the limestones of this zone the “Lime Cliffs," but subsequent mapping in the area between the Minturn quadrangle and the Kokomo dis- trict (Tweto. 1953) established that these limestone beds are the same as the Robinson, and the term “Lime Cliffs" was dropped (Tweto. 1949, p. 201). In the area of the Minturn type section in the Min turn quadrangle, the Robinson member is 746 ft (228 m) thick; it consists of six carbonate beds. each 5—65 ft (1.5-20 m) thick. separated by much thicker intervals of elastic rocks. The increased number of limestone beds and thickness of the member in this area as con- trasted with the Pando area result from facies Changes in the Resolution Dolomite Member of the Pando area. Northward from the Pando area, thin-bedded dolomite of the Resolution Member grades into thick-bedded to massive light—bluish-gray fossiliferous limestone characteristic ofthe Robinson. as does also an overlying zone of dolomite reefs and dolomitic grit (fig. 13). These lower limestone beds are accordingly included in the PENNSYLVANIAN AND PERMIAN SYSTEMS 45 Robinson, and the Resolution Dolomite Member is not recognized except in the southernmost part of the quad- rangle. The greatest amount of limestone known in the Robinson is along Gore Creek east of the mouths of Mill and Spraddle Creeks. In this area the member includes as many as four major limestone beds 25—75 It (7.6—22.9 m) thick, as well as several minor beds. From this vicinity, the limestone beds decrease in number and thickness westward toward the evaporite basin, and the limestone also changes in character to a non- resistant and inconspicuous shaly, flaggy non- fossiliferous brownish-gray limestone. On the spur north of Dowds, the Robinson contains no more than two well-defined limestone beds and is a vaguely defined unit of interbedded shaly limestone, shale, and sandstone. Northwestward from this spur, gypsum ap- pears in this limy-shaly zone and gradually supplants the limestone. Cross sections diagramming the changes in the Robinson from Booth Creek westward have been presented by Boggs (1966, fig. 5), and a facies classifica- tion of the Robinson over a wider region has been made by Tillman (1971). Individual limestone beds within the Robinson ex- tend for distances of a few thousand feet to several miles. Generally, as one bed pinches out, it is overlap- ped by another at a somewhat different stratigraphic level. Thus, the limestone beds have the form of broad lenses, and the member as a whole consists of overlap- ping lenses of limestone in a matrix of elastic rocks. In places, thin beds of dolomite are also present in the Robinson, and a few limestone beds have dolomite caps. Such dolomite is apparently of early, perhaps diagenetic, origin and is distinct from late hydrother- mal dolomite, which irregularly replaces the gray limestone in many places in the southern part of the quadrangle. - In places, limestone beds of the Robinson swell abruptly into massive, vuggy bioherms or reefs consist- ing of algal bodies, shell fragments, and calcarenite. Most of these bioherms are dolomite, but some are limestone. Good examples, among many, are on the south wall of Gore Creek valley just east of Mill Creek, and on the west wall of the canyon of the Piney River near Moniger Creek. The Robinson contains a large assemblage of fossils, as discussed under a following heading, but it is charac- terized particularly by brachiopods, pelecypods, and fusulinids. CLASTIC UNIT F Unit F (fig. 13) consists of about 450 ft (138 m) of grit, sandstone, shale, and siltstone, much as in elastic unit E. In the Minturn quadrangle, the lowermost light-red beds of the upper red zone in the Minturn Formation oc- cur in the upper part of this unit, though most of the unit is gray. In the Pando area, in contrast, the lowest red beds of the upper red zone are in the middle part of the Robinson Limestone Member (fig. 13). ELK RIDGE LIMESTONE MEMBER The name Elk Ridge Limestone was first applied by Koschmann and Wells (1946, p. 67) to two beds of limestone separated by 175—225 ft (52—68 m) of red clastic rocks in the Kokomo district. The Elk Ridge was designated a member of the Minturn Formation by Tweto (1949, p. 202) who noted that in the Pando area it is a single limestone bed 7.5~21 ft (2.3—6.4 m) thick. In the Minturn quadrangle the member is also a single bed and, as elsewhere, is discontinuous, passing abruptly into black shale in places. In the area of the Minturn type section it is 30 ft (9 m) thick, but in most places it is no more than half this thickness. It has not been identified with certainty north of Gore Creek. The limestone in the Elk Ridge is in part dark, fine grained, and thin bedded, and in part bluish gray like the Robin- son. This lighter variety is mottled pale pink in many places and is generally sandy and locally oolitic. CLASTIC UN IT G About 130 ft (40 m) of grit, sandstone, and shale bet- ween the Elk Ridge and White Quail Limestone Mem- bers constitutes clastic unit G. In the Minturn type sec- tion these rocks are gray or pink, but to the southeast in the Pando area and the Kokomo district they are pre- dominantly red. WHITE QUAIL LIMESTONE MEMBER The name White Quail was first applied by Emmons (1898) to limestone beds that are the major host rocks of ore deposits in the Kokomo district. Koschmann and Wells (1946, p. 67) further described this unit in the Kokomo district as two, and locally three, beds of dark fossiliferous limestone 5—30 ft (1.5—9 m) thick, separ- ated by clastic rocks, in a zone about 200 ft (60 m) thick. Limestones of this zone were designated the White Quail Limestone Member of the Minturn Forma- tion by Tweto (1949, p. 203). In the Pando area, the zone is reduced to a single 10-ft (3-m) bed of dark-col- ored oolitic limestone or locally to about 10 ft (3 m) of dolomite and black shale. It thickens northward in the Minturn quadrangle, however, reaching 35—50 ft (10*15 m) in the general area of Mill, Booth, and Mid- dle Creeks. From this area it thins westward- toward the evaporite basin and also changes to silty limestone or calcareous siltstone, just as does the Robinson. It has not been identified farther west than the mountain spur north of Dowds. Throughout the area, the White Quail contains lenses 46 GEOLOGY, MINTU'RN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIE, COLORADO of black shale, and parts of it grade abruptly along the bedding into clastic rocks, causing marked differences in thickness from place to place. In general, the upper beds grade into grit, and the lower beds into shale or siltstone. In the southern part of the quadrangle and in the Pando area, the White Quail is a dark—bluish- or greenish-gray, dark-weathering, irregularly oolitic limestone that in many places contains scattered gastropods, cephalopods, and pelecypods. It maintains these general characteristics northward to Gore Creek, . except that the weathered surfaces gradually change to light grayish yellow. At Mill Creek the White Quail con- sists of a lower limestone unit about 15 ft (4.5 m) thick and an upper one about 20 ft (6 m) thick, separated by 10—12 ft (3-3.6 m) of dark shale. The limestones are in alternate thick and thin beds, and the thin beds are calcarenites consisting almost entirely of shell frag- ments. North of Gore Creek the limestone changes from dark gray to light gray and assumes many of the characteristics of the Jacque Mountain Limestone Member higher in the section. For this reason, it was misidentified as J acque Mountain in the original map- ping (Lovering and Tweto, 1944) in the Spraddle Creek area, near the Gore fault. In this area much of the limestone is foraminiferal, accentuating the oolitic ap- perance, and on weathered surfaces it is characterized by very abundant pinhole cavities. It contains thin beds that consist largely of fossil fragments, and contains scattered coiled cephalopods in the thicker beds. It was classified in this general area by Boggs (1966, p. 1408*1410) as biomicrite, oomicrite, and algal oomicrite. CLASTIC UNIT H Grit, conglomerate, sandstone, and shale 1,000—1,200 ft (305—365 m) thick lying between the White Quail Limestone Member below and the Jacque Mountain Limestone Member above are referred to as clastic unit H. This unit is almost entirely red, though locally it contains some gray beds in its lower part. Through most of the area the red color is bright, rang- ing from orange red to maroon, but as the evaporite basin is approached, the color becomes dull red, or grayish red. The clastic rocks of the unit include much conglomerate and are in general coarser than in most of the other clastic units except in unit D. JACQUE MOUNTAIN LIMESTONE MEMBER The Jacque Mountain Limestone Member, which, defines the top of the Minturn Formation (Tweto, 1949), is the most persistent and consistent limestone bed in the Pennsylvanian and Permian sequence. The limestone was named by Emmons (1898) for Jacque Mountain (later called Jacque Peak) in the Kokomo district. It has been traced from the Kokomo district northwestward' beyond the northern boundary of the Minturn quadrangle, and it has been identified with reasonable certainty in a separate area of Pennsylva- nian and Permian rocks 15 mi (24 km) farther north- west along the Colorado River near McCoy (Murray, 1958, p. 54). It also has been identified along the Conti- nental Divide east of the Mosquito-Tenmile Range (ob- servation by Tweto; also Singewald, 1951, p. 11; Brill, 1952, p. 819). Like all the limestones, the Jacque Moun- tain fades out in the evaporite basin just west of the Minturn quadrangle. Lovering and Mallory (1962) traced it from the Gore Creek area westward to a ter- minus on the north‘side of the Eagle River at a point almost exactly on the. quadrangle boundary. Boggs (1966) reported that it extends discontinuously about 5 mi (8.1 km) farther west along the north side of the Eagle River, but W. W. Mallory (written commun., 1968) has stated that this extension cannot be con- firmed. The Jacque Mountain (fig. 16) is typically 20—25 ft (6—7.6 m) thick and consists of gray to light-bluish- gray fine-grained limestone with a distinctive combina— tion of features. It is generally oolitic in some part, par- ticularly in the upper beds. Some part or all of the limestone commonly contain clastic materials, such as grit, sand, or micas (fig. 17), and in many places the FIGURE 16.—Jacque Mountain Limestone Member of the Minturn Formation, on ridge between the heads of Mill and Two Elk Creeks. PENNSYLVANIAN AND PERMIAN SYSTEMS 47 FIGURE 17.—Photomicrograph of biotitic oolitic limestone in Jacque Mountain Limestone Member of Minturn Formation, showing fresh and altered biotite flakes (dark), and oolites recrystallized to large single crystals. Crossed polars. limestone contains lenses of grit, ' conglomerate, or siltstone. Locally, the oolites and clastic grains define a crossbedded structure in an otherwise sublithographic limestone (micrite). Beds of intraformational limestone conglomerate are common. Some beds are welded ag- gregates of irregular pellets a few millimeters in diameter, at least some of which seem to be algal. In places the limestone contains concentric algal struc- tures a few inches to 2 ft (7 cm to 0.6 m) in diameter. In many exposures, some bedding planes are marked by cusps and hollows an inch or two across that look like disorganized cross-ripples. Pink mottling of the gray or bluish-gray limestone is common, particularly in the intraformational conglomerates, and locally some beds are pink throughout. Stylolites in the limestone are typically micaceous and are stained red by hematite. Throughout, but perhaps most commonly in the upper oolitic beds, the limestone contains widely scattered coiled cephalopods 2-4 in. (5—10 cm) in diameter, and locally it a15u contains straight cephalopods, high- spired gastropods, and fragments of pelecypods. These fossils are generally recrystallized to sparry calcite. In at least one occurrence, at the head of Mill Creek, oolites in the top bed of the Jacque Mountain are very soft but coarsely crystalline and were tentatively iden- tified as gypsum. Boggs (1966, p. 1410) classified the limestone in the Jacque Mountain as biomicrite, oomicrite, and algal oomicrite in the east-central part of the quadrangle, and as micrite and intraclast and stromatolitic limestone near the evaporite basin. A series of sections of the limestone along Gore Creek and the Eagle River has been diagrammed by Lovering and Mallory (1962). CHANGES IN THICKNESS AND FACIES The Minturn Formation thins rapidly toward the Gore Range by onlap, or the shingling out of the lower units (pl. 1, secs. A—A’, B—B', E—E’),On the mountain southeast of the junction of Gore and Black Gore Creeks, typical blue-gray fossiliferous limestone of the Robinson Member is separated from Precambrian rocks by only 50—100 ft (15—30 m) of grit. Similar rela- tions are known on Copper Mountain, in the northern part of the Kokomo district (Tweto, 1949,‘ p. 195; Bergendahl and Koschmann, 1971, pl. 1) where limestone of the Robinson member also lies within 50 ft - (15 m) of Precambrian rocks. The same relations also are known or reasonably inferred at several places along the Gore fault north of Gore Creek, as at Bighorn and Pitkin Creeks and near the head of Middle Creek. As the base of the Robinson Limestone Member lies 3,700—4,200 ft (1,130-1,280 m) above the Belden as measured from the valley of the Eagle River (fig. 13), the relations just described indicate that a minimum of about 4,000 ft (1,220 m) of elastic rocks beneath the Robinson pinches out between the Eagle River and the flank of the Gore Range. Much of the thinning evi- dently is localized near the Gore fault, for more than a thousand feet (305 m) of grit is exposed beneath the Robinson at Booth Creek, only a mile or two (1.6 or 3.2 km) from the fault. As discussed in the section on struc- ture, the Gore fault is an ancient fault that in Penn- sylvanian time formed an abrupt border between the highland area to the east and the basin of sedimenta- tion to the west. In the area along the fault clastic rocks of the Minturn Formation are very coarse. Conglomer- ates exposed in fault slices on the south slope of Bald Mountain contain abundant 2-ft (0.6-m) boulders and some as large as 4 ft (1.2 m) in diameter. Westward from the vicinity of the Gore fault, the elastic rocks become finer grained toward the evaporite basin. A partial section measured on the western side of the mountain spur north of Dowds showed predominant sandstone and siltstone and only 5 percent of grit in clastic unit E. Clastic units F and G contain no grit, 15 percent sandstone, 78 percent shale and siltstone, and 7 percent carbonate rocks. These proportions contrast with predominant grit and abundant conglomerate farther east, as described in the Minturn type section. The section near Dowds also illustrates the thinning be- tween carbonate members of the Minturn. The White Quail and Robinson are separated by about 500 ft (150 m) of elastic rocks, in units F and G, in contrast to 700 ft (213 m) in the Minturn type section. Clastic unit E is reduced even more severely, from 1,100 ft (335 m) in the type section to only 350 ft (105 m). Clastic units C and D have the same thickness at Dowds as in the type section, but it should be noted that this part of the type 48 GEOLOGY, MIN TURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO section is the same distance from the depositional shoreline as in the locality near Dowds. Westward from Dowds, the Minturn intertongues with, and grades into, gypsum of the Eagle Valley Evaporite (Mallory, 1971, pl. 2). On the north side of the Eagle River, thick units of gypsiferous shale and siltstone alternate with thin units of gypsum, most of which is silty. South of the river, near Stone Creek, gyp- sum is much more abundant, but its stratigraphic rela- tions are obscured by slumps and landslides. This gyp- sum probably is the same as that in two beds that are exposed near Avon, 2 mi (3.2 km) to the west. Burchard (1911, p. 362) described these as a lower bed 130 ft (40 m) thick and an upper bed 50—75 ft (15—23 In) thick, separated by 40—50 ft (12—15 In) of clay, calcareous shale, and shaly limestone. Because of the small area of exposure and the obscurity due to slumping, Eagle Valley Evaporite is not distinguished on the quadrangle map (pl. 1). FOSSILS, AGE, AND CORRELATIONS The Minturn Formation and equivalents have been classed as mostly or entirely Middle Pennsylvanian in age by many workers (Roth and Skinner, 1930; Brill, 1944, 1952; Henbest, 1946; Tweto, 1949; Stevens, 1962; Bass and Northrop, 1963; Murray and Chronic, 1965). Extensive fossil collections from the Minturn quad- rangle, Pando area, and Kokomo district establish that the lower five-sixths of the formation, up through the White Quail Limestone Member, is Atokan and Des Moinesian in age. The age of the uppermost part, com— prising clastic unit H and the Jacque Mountain Limestone Member, is not so definitely established but is also classed as Pennsylvanian and very likely Des Moinesian, as indicated below. Diagnostic fossils—both megafossils and fusulinids—— are most abundant in the limestones of the Robinson Limestone Member. Megafossils are also found at various levels from the base of the formation up to the Robinson, but fusulinids have not been found lower than the Hornsilver Dolomite Member. Above the Robinson, the Elk Ridge has yielded a few megafossils and fusulinids. The White Quail has yielded diagnostic megafossils but the few fusulinids found in it are too poorly preserved for identification. Though searched extensively for identifiable fossils, the Jacque Moun- tain has yielded only a sparse molluscan fauna. Data on fossils are summarized below from reports by Mackenzie Gordon, Jr., and E. L. Yochelson of the US. Geological Survey (written commun., Feb. 16, 1966) on 31 collections of megafossils, and by L. G. Henbest (written commun., May 5, 1958 and Oct. 2, 1958) on 44 collections of fusulinids. The localities of the fossil col- lections to which we refer in the present report are listed in table 5. Gordon and Yochelson noted that colonies of the cor- al Chaetetes are the principal fossils in the lower part of the Minturn Formation, and that this form “ranges through about 3,000 feet of beds in this area, the lowest recorded occurrence being approximately 1,420 feet above the base of the Minturn.” From a shale bed about 40 ft (12 m) above the base of the Minturn (loc. 14582—PC) they reported: Linoproductus cf. L. prattenianus (Norwood and Pratten) Juresania nebrascensis (Owen) Aviculopecten sp. From a dolomite reef at a stratigraphic level below the Wearyman Dolomite Member and above the reef dolomite of Lionshead‘ (locs. 9875—PC and 9879—PC) Gordon and Yochelson reported: Chaetetes sp. Desmoinesia nana (Meek and Worthen)? Linoproductus sp. indet. Anthracospirifer rockymontanus (Marcou) opimus (Hall) Condrathyris perplexa (McChesney) Streblochondria cf. S. tenuilineata (Meek and Worthen) Lima? sp. indet. Edmondia sp. Weideyoceras sp. From the Hornsilver Dolomite Member (loc. 14579—PC) Gordon and Yochelson reported: Caninoid coral undet. Chaetetes sp. Multithecopora ? Sp. Derbyia crassa (Meek and Hayden)? Kozlowskia ? sp. A Anthracospirifer rockymontanus (Marcou) Condrathyris perplexa (McChesney) Crurithyris planiconvexa (Shumard) Cleiothyridina orbicularis (McChesney) Composita ovata Mather Beecheria bouidens (Morton) Glabrocingulum sp. indet. From limestone about at the stratigraphic level of the Resolution Dolomite Member, or possibly of the Hornsilver Member (loc. 9874—PC) they reported: Chaetetes sp. Cidarid spines Meekella striatacostata (Meek) Antiquatonia coloradoensis (Girty) Linoproductus prattenianus (Norwood and Pratten)? Anthracospirifer rockymontanus (Marcou) Condrathyris perplexa (McChesney) ? Composita subtilita (Hall)? and stated: Occurrence of these fossils with a small tumid species of Fusulinella indicates a late Atokan age for the collection, according to Henbest (oral commun., 1966). The megafossils are all rather long- ranging Pennsylvanian species not restricted to beds of Atokan age. PENNSYLVANIAN AND PERMIAN SYSTEMS 49 TABLE 5. — Fossil collection localities, Minturn Formation, Minturn quadrangle and vicinity, Colorado [Abbreviations for 15-minute quadrangle are: M, Minturn; HC, Holy Cross; ML, Mount Unmlnl Cogzction Quadrangle Locality Description and remarks 9864-PC M Turkey Creek, about 5.5 mi (9 km) northeast of Red Lower of two prominent beds of limestone of Robinson Cliff, near mouth of tributary gulch from north that Member exposed at this locality. enters Turkey Creek at 10,050-ft contour. 9865—PC M Roadcut on US. Highway 6 southeast of junction of Limestone of Robinson Member. Gore and Black Gore Creeks, 0.25 mi (400 m) south of highway bridge over Gore Creek. 9867 -PC M Meadow at head of Spraddle Creek between 10,600- Limestone of White Quail Member. and 10,800-ft contours, and outcrops on slope on northwest side of creek; about 0.75 mi £12 km) southwest of top of Bald Mountain. 9868-PC M Knob marked by closed 10,450-ft contour on ridge on Limestone of White Quail Member. southeast side of Spraddle Creek, about 1.7 mi (2.7 km) south-southwest of top of Bald Mountain. 9874-PC M Chest of ridge between Spraddle and Booth Creeks, at Limestone bed probably at about stratigraphic level of 1 1,300-ft contour. Resolution or Homsilver Member. 9875-PC M Two Elk Creek, about 1.7 mi (2.7 km) from its mouth, Dolomite reef stratigraphically below Wearyman near 8,7 50-ft contour at trail and creek. Dolomite Member. 9876—PC M Ledges in hay meadow on north side of Gore Creek Limestone of Robinson Member. and US. Highway 6, 0.5 mi (0.8 km) west of Red Sandstone Creek. (Locality was later covered by Highway 1—70.) 9879—PC M Same as 9865—PC. 12062—PC HC Summit of Ptarmigan Hill (12,154—ft survey point) Dolomite and black shale of White Quail Member. 0.35 mi (0.56‘km) south of boundary of Minturn Unit 101 of Pando stratigraphic section (Tweto, quadrangle. See Pando map (Tweto, 1953). 1949, p. 208). 12063-PC HC Southwest shoulder of Ptarmigan Hill. See Pando Calcareous shale equivalent to fourth highest of five map (Tweto, 1953). limestone beds in Robinson Member. Unit 55 of Pando stratigraphic section (Tweto, 1949, p. 212). 12093—PC HC Same as 12063-PC. 12094—PC HC Same as 12062—PC. 13049—PC M Two Elk Creek, at mouth of tributary gulch from Limestone boulders from ledges in Robinson Member north, at 9,100-ft contour. higher in gulch. 13057-PC HC Same as 12063—PC. 13058—PC ML Ridge crest outlined by 11,750-ft contours, 3,400 ft Limestone of Jacque Mountain Member. (1,020 in) east of common corner of Mintum, Holy Cross, Mount Lincoln, and Dillon quadrangles, and 500 ft (150 m) south of Dillon quadrangle boundary. See Pando map (Tweto, 1953). 13059—130 M Same as 9864—PC. 13060—PC M Same as 9867—PC. 13061 —PC M Same as 9867 -PC. 13062—PC M Same as 9865—PC. 13064-PC M Ridge between Turkey and Wearyman Creeks, 200 ft Limestone of J acque Mountain Member. (60 m) below crest of mountain with triangulation station Shrine on it at elevation 11,876 ft. 50 GEOLOGY, MINTURN l5-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO TABLE 5. — Fossil collection localities, Minturn Formation, Minturn quadrangle and vicinity, Colorado — Continued (blgzction Quadrangle locality Description and remarks 14578—PC n ML 100 ft (30 m) east of crest of ridge followed by Eagle Limestone of White Quail Member. County—Summit County boundary,rat 12,250-ft contour; 1.4 mi (2.25 km) south of Dillon quadrangle boundary. See Pando map (Tweto, 1 953). 14579—PC' HC Tributary gulch of Resolution Creek, NW1/4 sec. 36, Dolomite and limestone of Hornsilver Member. T. 6 S., R. 80 W., at 10,700-ft contour. See Pando map (Tweto, 1953). 14582—PC M Roadcut on US. Highway 24 on north side of Rock Green-gray shale with limestone nodules, 40 ft (12 m) Creek, 0.25 mi (400 m) north of Gilman. above base of Mintum Formation. 14583—PC M Same as 9867~PC. 14584—PC M Same as 9867 —PC. (5757—5760 HC Southwestern shoulder of Ptarmigan Hill. See Pando Collections from limestone beds of Robinson Member map (Tweto, 1953). in Pando measured section (Tweto, 1949, p. 211 —212): f5757—unit 42; f5758—unit 39; f5759— unit 55; f5760—unit 60. {5761 —5766 ML Type locality of Robinson Member, in Kokomo From the three limestone beds present in the district, quarry at site of former town of Robinson. Robinson Member at this locality: f5761b—lower (Locality was later buried beneath tailings ponds). bed; f5761, f5762, f5762b— middle bed at base, middle, and top, respectively; f5765, f5766—upper bed at base and 5 ft (1.5 m) above base, respectively. f5790 ML Nearly same as 14578—PC, but on crest of ridge, Limestone of White Quail Member. between 12,300- and 12,350-ft contours. {7036-7039 M Lime Creek, about 3 mi (5 km) above mouth, NW1/¢ From three lowest limestone beds of Robinson sec. 3, T. 6 S., R. 80 W., in clearing on slope above Member: f7036—base of upper bed, 90 ft (27 m) Lime Q‘eek trail. above second bed; f7037—upper part of second bed; f7038—nodular limestone within second bed; f7 039—lowest bed. Megafossils from the Robinson Member are listed in table 6, which includes one collection, 13049—PC, from float judged to be from the Robinson but not included in the table as prepared by Gordon and Yochelson, who also stated: Brachiopods characteristic of the Robinson Member and ap- parently restricted to it in this region are: Mesolobus mesolobus (Nor- wood and Pratten), Chonetinella jeffordsi Stevens, Desmoinesia muricatina (Dunbar and Condra), Antiquatonia aff. A. hermosana (Girty), and Neospirifer coloradoensis Stevens. These fossils suggest a. correlation between the Robinson Member and units 7 to 9 of Stevens (1962) in the McCoy area, about 30 miles northwest of Minturn. Fusulinids from systematic collections from the Robinson Member in three localities in or near the Min- turn quadrangle are listed in table 7, as collected and identified by L. G. Henbest. Of a collection made earlier by Tweto at the same locality as megafossil collections 9864—PC and 13059~PC (table 5), Henbest said (writ- ten commun., April 29, 1941): Two early species of Fusulina are present. 'One is new, and the other is either Fusulina taosensis Needham 1937, or a very closely related form. Though the stratigraphic record of these two species is but poorly known, the evolutionary status of these forms indicates early Des Moines age or perhaps a position in the upper part of the Lampasas Series of Cheney (1940). Fusulina taosensis was originally found in the lower part of the Magdalena Group east of Taos, N. Mex. I think that the evidence for an age at least as early as the early Des Moines is good. . Other Foraminifera noted by Henbest from the collec- tions reported in table 7 are: Bradyina sp. Calcitornella sp. Calcivertella Sp. Climacammina Sp. Cribrostomum sp. Earlandia perparva Plummer Earlandia Sp. Endothyra sp. Globovalvulina sp. Monotaxis sp. Orthovertella sp. Polytaxis sp. Qzawainella sp. .Serpulopsis sp. Spiroplectammina sp. Tetrataxis millsapensis Cushman and Waters Tetrataxis sp. Trepeilopsis gnandis Cushman and Waters Trepeilopsis sp. PENNSYLVANIAN AND PERMIAN SYSTEMS 51 TABLE 6. — Megafauna of the Robinson Limestone Member of the Minturn Formation [X. present; ?, questionable occurrence] Collection No. —PC —PC Name 9864—PC 13059-PC 9865—PC 9879—PC 3062—PC 9876-PC 2063-PC 12093 13057 13049-PC 1 i 3 >< >< ><><><{ 1 Corals: Caninia sp ............................. . . . Lophophyllidiid corals ................... X Chaetetes of. C. milleparaceous ' Milne-Edwards and Haime ............ X Bryozoans : Fenestellid ............................. Echinoderms: Crinoid stems .......................... X Echinoid spines ......................... . . . . . . X Brachiopods: Derbyia crassa (Meek and Hayden) ...... . . . X . . . X Derbyia? sp. indet ...................... X . . . . . Meekella striatocostate (Cox) ............. . . . X X Mesolobus mesolobus (Norwood and Pratten) ............... . . . . . . . . . Chonetinella jefi‘ordsi Stevens ............ . . . . . . . . . X . Krotovia maccoyensis Stevens? ........... . . . . . . i . . . . . X Krotovia? sp ........................... Desmoinesia muricatina (Dunbar and Condra) of. D. ingrata (Girty) ................ . .. Antiquantonia aff. A. hermosana (Girty) , . X sp ................................. . . Linoproductus prattenianus (Norwood and Pratten) ............... ? sp. indet ........................... Punctospirifer kentuckensis (Shumard) . . . . . . Anthracospirifer rockymontanus (Marcou) . X Neospirifer coloradoenis Stevens ......... Crurithyris planicanuexa (Shumard)? ..... Condrathyris perplexa (McChesney) . . Composite subtilita (Hall) ......... >4 §>< ><>< fxxxxxxxfx xfxx x §><><><§ ><><§ >< sp ........................ Beecheria cf. B. bovidens (Morton) . . . . . . . . . Koalowskia‘? sp.A ...................... X Pelecypods: Pernopecten cf. P. ohioensis Newell ....... . . . . . . . . . . . . X sp .......................... .. Concardium sp ..... Schizodus sp ....... Sphenotus? sp . . . . Astartella sp ....... x} xxx Gastropods: Bellerophontid indet ..................... X Knightites (Cymatospira) mantfortianus (Norwood and Pratten) ............... Warthenia tabulata (Conrad) ............ Platyceras (Orthanychia) parva (Swallow) . High-spired gastropod ................... Gastropods indet ....................... >4 {*1 x} xxx: Cephalopods: Pseudarthoceras knoxense (McChesney) . . . . . . X . . . X NOTE: In reports prepared later than this table, E. L. Yochelson (written commun., Nov. 14, 1966) and Mackenzie Gordon, Jr., and W. J. Sando (written commuxi., Nov. 21, 1966) reported the brachiopods Antiquatonia coloradoensis (Girty) and Crurithypis sp., the gastropod (" "" sp., and ‘ ‘ ' ‘5 corals in " " from red-mottled cherty limestone of the Robinson Member 1 mi (1.6 km) west of Bald Mountain. In summarizing the results of studies of the Foraminifera, Henbest said: All of the fusulinid bearing units, except those with the problematic fauna of £7036, have foraminiferal faunas that indicate a Middle Pennsylvanian age. Of these, none younger than the middle part of the Middle Pennsylvanian was recognized. The fauna repre- sented by 97036 suggests an early Late Pennsylvanian age, but the fora‘miniferal evidence is opposed by field evidence. Tweto observed that the zone of 17036 is overlain at another locality by beds contain- ing Mesolobus sp. A definite solution of this problem remains to be determined. In addition to the fusulinid species from the Robinson reported by Henbest, other species were reported by Tillman (1971, p. 599) as identified by G. J. Verville: F usulina curta, F. distenta, F. truncatulina, E plattensis, and Wedekindellina coloradoensis. Fauna of the White Quail Member is shown in table 8 as reported by Gordon and Yochelson (written com- mun., 1966), who noted that: “Maximites cherokeensis was described originally from the Mulky coal of the Cherokee Shale in Henry County, Missouri, and except for the White Quail speci- mens this genus and species is not known elsewhere.” Regarding Mesolobus euampygus, they also noted that: this is the only bed in the Minturn section in which this species has been found. This occurrence suggests a correlation between the White Quail Member and unit 15 of the section recorded by Stevens (1962, p. 618—624) in the McCoy area, the only bed in which he en— countered M euampygus. Most collections of microfossils from the White Quail proved to be devoid of identifiable fusulinids, but in one collection (f5790) Henbest identified “Profusulinella sp. or immature specimens of a higher fusulinid.” Fossils in the Jacque Mountain Limestone Member as reported by Gordon and Yochelson (written com- mun., 1966) include (locs. 13058—PC, 1/3064—PC): Schizodus sp. Permophorous sp. Dolorothoceras sp. Domatoceras sp. indet. Gordon and Yochelson stated: The Domatoceras * * * is distinct from the species in the White Quail Member. It is too poorly preserved to name formally but proba— bly is the same species that was collected by Koschmann and Williams near the head of Searles Gulch in the Kokomo district [Koschmann and Wells, 1946, p. 69]. This earlier specimen was described and figured as Domatoceras sp. (of Colorado) by Miller and Youngquist (1949, p. 46, 47, pl. 15, figs. 1-7) in their volume on American Permian Nautiloids. Despite this implication of Permian age, Gordon regards the Jacque Mountain species as rather closely related to Cherokee age forms such as Domatoceras umbilicatum Hyatt and D. williamsi Miller and Owen and rather unlike most known Permian‘species. The genera so far recorded from the Jacque Mountain Member range widely through Pennsylvanian and Permian rocks. There is no demanding reason, especially when one considers the rapidity of sedi- mentary accumulation in the Minturn section, to regard the Jacque Mountain Member as any age but merely slightly younger than the White Quail Member. In summarizing the age of the Minturn Formation, Gordon and Yochelson stated: the Minturn Formation is regarded as Middle Pennsylvanian in age, and includes beds of Atokan and Des Moinesian age. The Robinson and White Quail Members are dated as Des Moinesian in age by the 52 GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIFS, COLORADO TABLE 7. — Fusulinids in the Robinson Limestone Member of the Minturn Formation Robinson Member type locality (Mount Lincoln Ptarmigan Hill (Holy Cross quadrangle) Lime Creek (Minturn quadrangle) quadrangle) ”use“ “a?” N... “a?” Name 4 f5760 Fusulina pristina Thompson 3 15759 Fusulina sp. T7036 ?Iowanella sp. aff. I. winternensis f5766 Wedekindellina euthysepta (Thompson, Verville, (Henbest) and Lokke) Fusulina rockymontana Roth and Skinner f5765 Fusulina rockymontana Roth and Skinner 2 f5757 Fusulina sp. (early form) P7038 Fusulina illinoisensis f5762b Wedekindellina euthysepta? insultnella or Fusulina sp. Dunbar and Henbest (Henbest) Millerella sp. {7037 Wedekindellina euthysepta Fusulina sp. (Henbest) or W. excentrica {5761 Fitsulina rockymontana Roth (Roth and Skinner) and and Skinner Fusulina sp. {5762 Wedekindellina sp. 1 f5758 Fusulina sp. cf. F. rockymontana f7039 Fusulina novamexicana f5761—b Wedekindellina excentrica Roth and Skinner Needham Roth and Skinner ? Fusulinella devexa Thompson Wedekindellina euthysepta Wedekindellina euthysepta (Henbest) (Henbest) Millerella-like foraminifer megafossil content. The Hornsilver and Robinson Members are dated as early Des Moinesian in age by fusulinids. The presence of Fusulinella in a collection of unknown horizon dates at least some of the lower part of the Minturn Formation as late Atokan in age. As indicated by the preceding statements of Gordon and Yochelson, and by Stevens (1962), some of the limestones of the Minturn correlate with limestones in the McCoy area, 12 mi (19 km) northwest of the Min- turn quadrangle, where the term “McCoy formation” was formerly applied to the Pennsylvanian rocks (Don- ner, 1949). On the basis of fossils and, in some cases, physical stratigraphy, the Minturn also is approx- imately or in some part is equivalent to the Paradox Formation as used by Bass and Northrop (1963) in the Glenwood Springs area, to the Gothic Formation of Langenheim (1952) on the western side of the Sawatch Range, to the Morgan Formation of northwestern Col- orado and northeastern Utah (Thomas and others, 1945) to the Hermosa Formation of southwestern Col- orado (Bass, 1944; Henbest, 1946), to the lower part of the Fountain Formation of the eastern side of the Front Range (Mallory, 1958), and to the upper part of the Sandia and the lower part of the Madera Formations of the Magdalena Group of northern New Mexico (Hen- best, 1946; Brill, 1952; Myers, 1968). ORIGIN The Minturn Formation records two alternating en- vironments of deposition: (1) a marine facies, compris- ing carbonate rocks, black shales, intertongueing evaporites, and probably some even-bedded siltstones and sandstones, and (2) a nonmarine, largely fluviatile, facies comprising the bulk of the grits and conglomer- ates and at least some of the associated sandy and 'micaceous shales. A nonmarine origin of many of the coarse clastic rocks is not directly proved but is inferred from their extreme lenticularity, very poor sorting, general coarseness of grain, extensive crossbedding, channel structures, and the presence in some of them of land plants. The plant remains are not abundant but in- clude fossil tree roots (stigmaria), Lepidodendron, Sigillaria, Walchia, fern impressions, and rare petrified wood. In addition, rush, reed, and twiglike impressions are fairly common. Although some of these are marine as indicated by their occurrence in fossiliferous dolomites, most are probably terrestrial, as also are thin coaly or carbonaceous seams in well-defined cyclothemic sequences (Tweto, 1949, p. 211~214; Brill, 1952, p. 820). Distribution of the two general classes of rocks sug— gests that normal shallow-water marine sedimentation was interrupted repeatedly by floods of coarse clastic debris derived from the highland area immediately to the east. This debris probably was deposited in the form of shallow-water deltas, bar and back-bar deposits, river flood-plain deposits, and alluvial fans in a recurrent piedmont. When marine conditions pre- vailed, they extended literally onto the highland, as shown by the limestones on Precambrian rocks near the Gore fault, and when fluviatile conditions prevailed, they extended to the margins of the evaporite basin. PENNSYLVANIAN AND PERMIAN SYSTEMS 53 TABLE 8. — Fauna of the White Quail Member of the Minturn Formation IX, resent; ?, questionable occurrence] P Collection locality and N05. Spraddle Ptarmigan Radio Creek Hill Ridge 9867—PC 12062-PC 14578—PC 9868—PC 12094-PC 13060—PC 13061—PC 14583—PC ‘ 14584-PC Name Plants: Reedlike plant ................... Lepidodendron sp ................. ><>< Echinoderms: Echinoid spines .................. . . . X Brachiopods: Derbyia crassa (Meek and Hayden)? _ , , . , , X Mesolobus euampygus (Girty) ...... . . . X . . . Linoproductus prattenianus (Norwood and Pratten) ? ......... X Anthracospirifer aff. A. rockymontanus (Maroon) ......... . . . . . . ’ X Condrathyris perplexa (McChesney) . . . X Composita sp ..................... X . . . Pelecypods: - Nuculoidea? sp. indet ............. . . . . . . Polidevcia sp ..................... . . . X Myalina sp ....................... X . . . Auiculopecten sp ...... . . . Schizodus sp. A 3x xxxxE Gaetropods: Bellerophon sp. indet .............. X Knightities (Retispira?) sp ......... . . . . . . Euphemites sp .................... . . . X Bellerophontid sp. indet ........... X . . . Glabrocingulum? sp .............. . . . X 2 we Cephalopods: Brachycycloceras sp ............... . . . Kianoceras? sp ................... X Mooroceras normale Miller, Dunbar, and Condra ..................... . . . Metacoceras sp. A ................. X X Domatoceras sp. A ................ X . . . Liroceras sp ...................... X Ephippioceras ferratum (Cox) ...... X Maximites cf. M. cherokeensis (Miller and Owen) .............. X 3x NNNNN Boggs (1966) and Mallory (1971) have interpreted the sediments of the evaporite basin as products of shallow hypersaline waters in the center of the trough that extended through west—central Colorado, and with this we concur. The open sea with normal marine condi- tions lay to the northwest, in the area of the Morgan Formation, which closely resembles the marine facies of the Minturn. Evidently, the waters of the Morgan sea repeatedly swept southeastward along the border of the Front Range highland, depositing the marine beds of the Minturn Formation, and just as repeatedly, they are forced back by deposition of the coarse clastic sedi- ments. To what extent subsidence in the basin or trough, pulses of uplift in the highland, or climatic fac- tors each controlled the interplay between spreading of the sea and the building of elastic dams in it is unap- praised, but most likely, all three were involved. From the absence of kaolinite in all but the basal part of the combined Belden and Minturn, and from the abundance of fresh feldspars, both Raup (1966) and Boggs (1966) concluded that the clastic sediments of the Minturn were derived from a source area—the highland to the east—that was undergoing rapid ero- sion under conditions of a semiarid to arid climate. Parallels in composition and sedimentary structure bet- ween the Minturn rocks and modern sediments in parts of the semiarid West suggest that this was so. Hubert (1960), however, concluded that because of the generally red color of the Fountain Formation—an analog of the Minturn—lateritic weathering under humid conditions prevailed. This interpretation of red sediments in the Minturn and Fountain has been refuted by Walker (1967). MAROON FORMATION STRATIGRAPHIC RELATIONS The Maroon Formation as redefined by Tweto (1949) consists of the Pennsylvanian and Permian red beds lying above the Minturn Fromation. As applied in the Minturn quadrangle, the Maroon includes all the strata—as much as 4,200 ft (1,280 m) in thickness— between the J acque Mountain Member of the Minturn and the Upper Triassic Chinle Formation. The base of the Chinle, which in most places is marked by a distinc- tive sandstone—the Gartra Member—was also recog- nized as the top of the Maroon Formation by Bass and Northrop (1963) in the Glenwood Springs quadrangle and, farther west, by Donnell (1954; 1958). Throughout the area west and northwest of the Min- turn quadrangle, however, other units have been recog- nized by various authors in the few hundred feet of strata immediately beneath the Chinle. Stratigraphically lowest of these, lying 100—750 ft (30—230 m) below the Chinle, is a distinctive white to orange sandstone called a tongue of the Weber Sandstone in the Maroon Formation by Donnell (1954) and Bass and Northrop (1963), called Weber Sandstone by Brill (1944) and Murray (1958), and called Schoolhouse Sand by Thompson (1949) or Schoolhouse Tongue by Brill (1952). About 50—150 ft (15—45 m) above the tongue of the Weber and 50—100 ft (15—30 m) below the Chinle, in the area west of Glenwood Springs is a cherty dolomite bed a few feet thick desig- nated the South Canyon Creek Dolomite Member of the Maroon Formation by Bass and Northrop (1950), who recognized it as a probable tongue of the Phosphoria Formation. All the strata between the tongue of the Weber and the Chinle, including the South Canyon, were assigned 54 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO to the State Bridge Formation by Brill (1944; 1952), although in one measured section (1944, p. 643) he assigned these strata to the Dinwoody or Moenkopi For- mations. At its type locality at State Bridge, 10 mi (16 km) northwest of the Minturn quadrangle, the State Bridge is distinguished from the underlying Maroon Formation by a change from the predominating coarse red sandstones of the Maroon to red siltstone and shale (Donner, 1949). A few miles northwest of State Bridge, Murray (1958) found that the tongue of the Weber ends abruptly; he inferred an erosional unconformity bet- ween the Maroon and State Bridge, and such an uncon- formity has subsequently been confirmed elsewhere in the region (Freeman, 1971). The part of the State Bridge above the South Canyon Member (Stewart and others, 1972), or above an arbitr- ary datum 100 ft (30.4 In) higher (MacLachlan, 1959; Oriel and Craig, 1960), has been correlated on lithologic grounds with the Lower Triassic Moenkopi Formation. It is not yet established whether equivalents of the Weber, South Canyon, and State Bridge or Moenkopi are included within the Maroon Formation as applied in the Minturn quadrangle. More likely, these units are younger than any of the Maroon in the quadrangle. The tongue of the Weber pinches out about 12 mi (20 km) west of the quadrangle, in the area between Walcott and Eagle. The South Canyon or a probable equivalent—the Yarmony Limestone Member of Sheridan (1950)—has not been identified closer to the quadrangle than Walcott or the State Bridge area. The Maroon within the quadrangle contains much siltstone in its upper part, and thus it might include strata equivalent to the lower part, at least, of the State Bridge. However, siltstone is abundant throughout the Maroon, even in the coarse-grained facies of the Pando and Kokomo areas. Therefore, the mere presence of siltstone in the red-bed sequence does not establish pre- sence of the State Bridge. THICKNESS In the Minturn quadrangle the Maroon is largely restricted to the area north of Gore Creek, although a few small patches are present on high peaks south of the creek. The formation has a maximum thickness of about 4,200 ft (1,280 m) near Red and White Mountain, as determined from structure sections (pl. 1, secs. A-A’, C—C'), but it thins rapidly eastward toward the Gore Range. In the valley of the North Fork of the Piney River 2—3 mi (3—5 km) north of the quadrangle, the Maroon is only 1,700 ft (518 m) thick in the area just west of the Gore fault. East of the fault, where it lies on Precambrian rocks, only 100—300 ft (30—90 m) of it is present beneath the Chinle Formation (Tweto and others, 1970). CHARACTER The Maroon Formation resembles the Minturn For- mation in lithology—except that it is almost uniformly red, contains only very minor carbonate beds, and grades irregularly upward into a predominating fine- grained facies. A measured section of the part of the formation preserved on Jacque Peak in the Kokomo district (Tweto 1949; 1958) shows that the lower 2,000 ft (610 m) of the formation consists of: sandstone, 35 percent; siltstone and shale, 25 percent; grit, 22 per- cent; and conglomerate, 18 percent. All these rocks are feldspathic and micaceous, just as are those of the un- derlying Minturn Formation. Some of the conglomerate beds contain abundant cobbles greater than 4 in. (10 cm) in diameter, and some contain 18-in. (45-cm) boulders. Many of the siltstone beds are calcareous, and some contain thin beds or lenses of fine-grained gray limestone. In the area north of Gore Creek, the lower 3,000—3,500 ft (900—1,050 m) of the Maroon is moderately well exposed along Red Sandstone Creek and along the jeep road on the ridge to the west of this creek. The exposed rocks are principally brick-red sandstone, siltstone, grit, and conglomerate, just as on Jacque Peak, though collectively a little less coarse in grain. A few nonpersistent beds of gray dense un- fossiliferous limestone 2—5 ft (0.6—1.5 m) thick are present among the clastic rocks. The upper LOGO-1,500 ft (300-450 In) of the Maroon is poorly ex- posed but seems to consist mainly of brick-red sandstone, siltstone, and grit. Much of the siltstone is limy, and some on the slopes above the Eagle River is gypsiferous. One thin bed of mottled gray and red algal limestone about 3,700 ft (1,130 m) above the Jacque Mountain can be traced with difficulty for several miles across the area (pl. 1). We consider it unlikely that this bed correlates with the South Canyon Member or the Yarmony Member of Sheridan (1950) because the red beds above it include much sandstone and grit. Detrital material in siltstone and silty limestone in the upper part of the Maroon is of the same arkosic composition as that in the grits. In the siltstones, quartz, microcline, plagioclase, muscovite, and chlorite are principal constituents; magnetite is a rather abun- dant accessory mineral, and tourmaline and zircon are less abundant. The coloring matter is chiefly earthy red hematite or brown limonite concentrated at the grain boundaries. Some of the plagioclase grains are slightly altered, but many are fresh. In the silty limestones, the calcite matrix encloses abundant angular fragments of quartz and subordinate chert, microcline, plagioclase, muscovite, chlorite, and magnetite. The microcline and plagioclase are fresh. The magnetite is in rounded grains. Earthy red hematite coats the elastic grains PENNSYLVANIAN AND PERMIAN SYSTEMS and, in some of the limestones, this coating is accom- panied by small but well defined crystals of red hematite. FOSSILS AND AGE No diagnostic fossils have been found in the Maroon in the Minturn quadrangle, and very few have been found in it or in closely associated units elsewhere in the region. A piece of fossil wood from a level about 200 ft (60 m) below the Chinle Formation on Red and White Mountain was tentatively referred to the genus Dadox- ylon by R. A. Scott of the US. Geological Survey (writ- ten commun., 1961), who noted that this species occurs in all the post-Silurian periods of the Paleozoic. Limestone samples from two localites were examined by P. E. Cloud, Jr., of the US. Geological Survey (writ- ten commun., Feb. 7, 1961). Cloud found no microfossils in the insoluble residues. From thin-section study, he reported abundant fecal pellets 0.4-1.2 mm in diameter in a mottled pink and gray limestone from the saddle between Buffer and Indian Creeks. In a light- gray subaphanitic limestone from sec. 8, T. 5 S., R. 81 W., he found—in different specimens—tiny ostracods and “subspherical to bladder shaped objects about 0.3 mm in diameter and up to 1.2 mm long that suggest a siphonaceous alga similar to Gymnocodium.” Cloud noted that if the objects are gymnocodian algae, they imply warm shallow water and a Permian age. The ostracods, which were examined by specialist G. I. Sohn of the US. Geological Survey, were unidentifiable. Fossils found in the South Canyon Dolomite Member near Glenwood Springs indicate an age similar to that of the Permian Phosphoria and Kaibab Formations (Bass and Northrop, 1950; 1963). Poorly preserved fossils from the Yarmony Limestone Member of Sheridan (1950) were classed as Middle or Late Pen- nsylvanian or Permian in age by N. D. Newell (Brill, 1942). Far to the south, in the Salida area, reptilian re- mains occur in the Sangre de Cristo Formation at a ' level 1,800 ft (550 m) above a limestone unit that Brill (1952) correlated with the Jacque Mountain. The rep- tilian fossils were classed as Early Permian (Wolfcam- pian) in age by Brill (1952), but later collections from the same locality were classed as Late Pennsylvanian (Missourian) by Vaughn (1969). Thus, the Maroon Formation of the Minturn quad- rangle and neighboring areas probably is of Pen— nsylvania and Permian age. It is underlain without evi- dent stratigraphic break by rocks of Middle Pennsylva- nian (Des Moinesian) age and overlain by rocks of Early Permian (Leonardian) age, and reptilian fossils in the lower middle part of the generally equivalent Sangre de Cristo Formation have been assigned either to the Late Pennsylvanian or the Early Permian. 55 ORIGIN The Maroon Formation records a continuation of the sedimentation that produced the Minturn Formation, but under somewhat changed environmental condi- tions. The average finer grain of the Maroon as con— trasted to that of the Minturn and the general decrease in grain size upward within the Maroon suggest that the source land area to the ,east became progressively less mountainous. Presumably, the abrupt mountain front that existed along the Gore fault in Minturn time gradually was transformed by erosion to a gentler front which migrated eastward through Maroon time, thus continually increasing the distance between the main ‘source of sediments and the site of deposition. The ab— sence of fossiliferous marine limestones, such as those of the Minturn, and the absence of any strata indicat- ing a marine origin—except near the evaporite basin— suggest that the pattern of alternating marine and ter- restial conditions that characterized theMinturn gave way to dominantly terrestial conditions of sedimenta— tion. The sediments of the Maroon in the Minturn quad- rangle are interpreted as stream channel and flood- plain deposits at the east and as grading westward into coastal plain or tidal flat and local lagoonal deposits. In the lower part of the formation, these deposits inter- tongue westward with marine evaporites. The thin and nonpersistent limestones in the Maroon probably formed in desiccation ponds or lagoons rather than in marine waters. Farther west, carbonate rocks, such as those of the South Canyon, represent marine condi- tions. The consistent red color of the Maroon also indicates a change in environment from that of the bulk of the Minturn, though the uppermost part of the Minturn was also affected by the change. Raup (1966) concluded from the clay mineral assemblage and particularly from the absence of detrital kaolinite in the fine- grained red beds of the Minturn, Maroon, and State Bridge Formations that the source material could not have been laterite, as has been often assumed, but that the source material was a product of weathering in a semiarid to arid environment. Walker (1967) concluded from an extensive study of red beds in the Maroon and Minturn and from close mineralogic parallels with red beds that are forming at present in Baja California that the red color formed after the deposition of the sedi- ments, by alteration of iron-bearing minerals, in an arid environment. We agree in general with these con- clusions but would qualify them somewhat. The widespread occurrence of evaporites and eolian deposits in the Upper Pennsylvanian and Permian rocks of the Rocky Mountain region (McKee, Oriel, and others, 1967) suggests a regionally arid climate, as does the absence of plant and animal remains in the Maroon 56 GEOLOGY, MIN TURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIITS, COLORADO Formation. The abundance of fresh feldspars, even in the siltstones of the Maroon, and the presence of unaltered detrital magnetite and biotite suggest mechanical weathering in an arid environment rather than chemical weathering in a humid, laterite-produc- ing environment. However, hematite does occur as dis- crete, apparently detrital flakes in company with unaltered detrital biotite and magnetite in some of the siltstones and impure limestones. Such hematite must have been transported to the site of deposition rather than forming there by alteration or by subsequent redistribution of ferric iron by ground water. Assuming, as the clay mineralogy seems to dictate, that lateritic soils did not exist as a source of the detrital hematite, a source might nevertheless be produced by normal weathering in an upland somewhat less arid than the environment of deposition. One of the factors contributing to the redness of the red beds is the presence of films of hematite in minute fractures within quartz clasts, giving them a pink or red cast. Such quartz occurs as pebbles even in the gray, buff, and green conglomerates of the Minturn Forma- tion. Reconnaissance of the entire Gore Range (Tweto and others, 1970) has revealed that quartz of this character occurs in many hematite-stained fracture zones in the Precambrian rocks of the range. Rocks of the Gore fault zone, for example, are stained red by hematite and contain the hematite-impregnated quartz through widths of hundreds of feet in places. Presum- ably, this hematite is a product of geologically recent weathering. However, the Gore fault and many of the numerous other faults in the range originated in Pre- cambrian time (Tweto and others, 1970); thus, they were in existence and were exposed to weathering in Pennsylvanian and Permian time. Such faults or frac- ture zones might have been a source of some hematitic material in the red beds. Most of the hematite, however, probably resulted from alteration of iron-bearing minerals in the depositional environment, as demon- strated by Walker (1967), and from redistribution by ground water of iron liberated by alteration. Alteration of iron-bearing minerals and redistribution of iron need not have been, in their entirety, penecontemporaneous with sedimentation. Indeed, some evidently occurred much later. The lower red zone in the Minturn, for ex- ample, transects bedding in a wavy manner that sug- gests ground-water control; in part at least, this red color may be related to the present topography. On the other hand, the occurrences of typical red-bed rocks, such as pebbles or fragments in nonred conglomerates of the upper Minturn, and of red siltstone chips in dense gray limestone in the Maroon, though rare, indicate that red beds were in existence at the time these con- glomerates and limestones were deposited. TRIASSIC SYSTEM CHINLE FORMATION The Upper Triassic Chinle Formation is the only Triassic unit recognized in the Minturn quadrangle. The Chinle is preserved in the quadrangle only in an area of less than a square mile on the northeastern slope of Red and White Mountain. In this area, and also in the Mount Powell quadrangle to the north, it consists of a basal white sandstone or conglomerate 10—25 ft (3—7.5 m) thick overlain by red siltstone. The basal sandstone or conglomeate, earlier referred to the Shinarump Member (Lovering and Tweto, 1944), was later classed as the Gartra Member of the Chinle (Poole and Stewart, 1964). On Red and White Mountain the Gartra seems to be conformable with the underlying Maroon Formation, but a few miles to the northeast, in the Mount Powell quadrangle, it fills shallow channels in the top of the Maroon and in places shows an angular discordance of 1°—2° with the Maroon. The Chinle is overlain unconformably by the Jurassic Entrada Sandstone. 0n the northeast shoulder of Red and White Mountain, the red siltstone of the Chinle is about 70 ft (21 In) thick. About a mile to the west, on the west and northwestern ridge of Red and White Mountain, Poole and Stewart (1964) measured 225 ft (68 m) of the siltstone between the Gartra and the Entrada. Thus, a significant unconformity between the Chinle and Entrada is indicated; this pre-Entrada un- conformity has been widely recognized in Colorado. As exposed within the small area on the side of Red and White Mountain, the Garta Member is about 10 ft (3 m) thick and consists of coarse-grained gray sandstone. On the northwestern side of Red and White Mountain, Poole and Stewart (1964; also F. G. Poole, written commun., 1956) found the Gartra to consist of 25 ft (7.6 m) of crossbedded conglomeratic sandstone. The pebbles consist of quartz, chert, and quartzite and are as much as 3 in. (7.6 cm) in maximum dimension. The sandstone also contains silicified wood in frag- ments or in segments of logs. A few miles north of the boundary between the Minturn and Mount Powell quadrangles, the Gartra is in some places a massive crossbedded coarse-grained sandstone and in others a conglomerate characterized by abundant silicified wood (Tweto and others, 1970). The red siltstone of the Red and White Mountain area is divided by Poole and Stewart (1964) into a lower mottled member and an upper red siltstone member. The mottled member, about 25 ft (7.6 m) thick, consists of red and purple mudstone, siltstone, and sandstone. The red siltstone member consists of brick red siltstone, much of which is calcareous, and subordinate fine- grained red sandstone. As seen in thin section, the J URASSIC SYSTEM 57 siltstone from Red and White Mountain consists of silt- sized angular fragments of quartz and equant grains of calcite, fairly abundant accessory leucoxene in rounded grains, and interstitial red hematite. Very sparse, small flakes of muscovite are also present but no detrital feldspar fragments were observed. The calcite appears to be detrital rather than a matrix cement as it is in the Maroon siltstones. The scarcity of mica and absence of feldspar suggest that the rock is more mature than the siltstones of the underlying Maroon and Minturn For- mations; possibly the Chinle rocks were derived in part from the reworking of red beds in these older forma- tions. JURASSIC SYSTEM Two Upper Jurassic formations, the Entrada Sandstone and the Morrison Formation, were mapped in the Red and White Mountain area, the only locality where rocks of Jurassic age are preserved in the Min- turn quadrangle. The Morrison of this locality and also of the area immediately north of the quadrangle (Tweto and others, 1970) contains much sandstone in its lower part. Some of this sandstone may be equivalent to units of the Sundance Formation as recognized in the State Bridge area by Pipiringos, Hail, and Izett (1969) or to the Curtis Formation of Baker, Dane and Reeside (1936). In the area just north of the Minturn quadrangle, the Entrada is absent and the Morrison rests, successively eastward, on Chinle, Maroon, and—a few miles east of the Gore fault—on Precambrian rocks (Tweto and others, 1970). Some part of the erosion that destroyed the Chinle or reduced it to thin remnants may be at- tributed to the pre-Entrada period of erosion discussed in the preceding section. A part, however, was caused by pre-Morrison erosion that removed the Entrada in a belt near the Gore fault and beveled an even surface across rocks of the Maroon Formation and remnants of the Chinle. East of the Gore fault, this surface beveled Precambrian rocks also. ENTRADA SANDSTONE As exposed on Red and White Mountain, the Entrada consists of about 60 ft (18 m) of cliff-forming, massive, bluff-to orange-weathering crossbedded sandstone. A generally similar character and thicknesses ranging from 62 to 109 ft (19 to 33 m) are reported in localities from 8 to 20 mi (13 to 32 km) west of the quadrangle (Sheridan, 1950). In outcrop, the Entrada appears to be conformable with the underlying Chinle Formation and the overlying Morrison, but regional relations indicate that both contacts are unconformities. The sandstone of the Entrada on Red and White Mountain is compact, homogeneous, and fine grained. It consists of well-sorted subangular to subrounded equant sand grains about 0.1 mm in diameter and of small amounts of interstitial clay and_orange goethite. The scattered coarse sand grains that characterize the Entrada in many places were not noted at this locality. The grains of the sandstone are mostly quartz, but chert is fairly abundant and a few grains of fresh microcline and slightly argillized plagioclase are also present. The interstitial clay is a mixture of hydromica and kaolinite, which locally contains small specks and irregular masses of goethite. No detrital heavy minerals were observed in thin section. MORRISON FORMATION In the small area of exposure on Red and White Mountain, the Morrison Formation consists of about 250 ft (76 m) of sandstone and shale unconformably overlying the Entrada Sandstone and unconformably underlying the Cretaceous Dakota Sandstone. This thickness is small as compared with about 500 ft (150 m) of Morrison in the southern part of the Mount Powell quadrangle (Tweto and others, 1970) and the 350—400 ft (105—120 m) shown on a regional isopach map (Craig and others, 1955). The abundance of sandstone in the Morrison at Red and White Mountain suggests that only the lower part of the formation is present and that the upper part was eroded prior to deposition of the Dakota. The Morrison of Red and White Mountain, and the lower half of the formation as exposed just north of the Minturn quadrangle, consists of predominant sandstone, subordinate interbedded green and gray clay shale, and a few beds of fine-grained gray limestone. The upper part of the formation—north of the quadrangle—is mainly variegated shales. The sandstone is characterized by uneven, lenticular bedding and by abundant particles of white clay that give the rock a chalky appearance. The clay occurs in- terstitially and also as discrete grains or granules. Some beds of the sandstone contain scattered granules and pebbles of chert and quartz. Thin sections show that the grains in the sandstone are poorly sorted, subangular to subrounded, and from 0.2 to 2 mm in diameter. In most parts of the rock these grains have the interlocking sutured grain boundaries of a quartzite, but in small irregular areas a few millimeters in diameter, the individual sand grains are separated by a matrix of fine-grained calcite. In the basal sandstone, the detrital sand grains consist largely of quartz, with subordinate chert, calcite, and fresh microcline, in order of decreasing relative abundance. A few rounded detrital grains of zircon and green tour- maline are present also. Sandstone above the basal bed 58 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIE, COLORADO is similar in character except that it contains chalky white grains of argillized feldspar and a few rounded masses as much as 2 mm in diameter of white clay. Specks of light-brown limonite averaging about 1 mm in diameter are abundantly and evenly disseminated through some of the rock. Limestone of the Morrison is typically dense, thin bedded, lithographic, and medium gray to light bluish gray. Some of it contains irregular olive-gray chert nodules as much as one-half inch (1 cm) long. Some beds, and particularly a 5- to 10-ft (1.5- to 3-m) thick bed that is 10—15 ft (3)—4.5 m) above the base of the for- mation, contain abundant spherical algal structures 1 —3 mm in diameter that were identified as charophyte remains by Richard Rezak, formerly of the US. Geological Survey (oral commun., 1951). The presence of charophytes is conclusive evidence of the nonmarine origin of this limestone (Peck, 1957, p. 1). CRETAC EOUS SYSTEM DAKOTA SAN DSTONE The Dakota Sandstone is the youngest formation in the sequence of consolidated sedimentary rocks preserved in the Minturn quadrangle. Only two small remnants of the Dakota—on the northeastern slope of Red and White Mountain—remain in the quadrangle, but the sandstone is widespread t0 the northwest and north. Just north of the quadrangle (Tweto and others, 1970), the Dakota is 150—160 ft (45—48 m) thick. Ex- cept for 25—35 ft (75—105 m) of dark~gray thin-bedded shaly sandstone at the top, it consists entirely of light- gray sandstone. In some places the sandstone is medium to thick bedded and contains thin argillaceous seams and nodules; in others it is very massive, crossbedded, and is composed of a clean quartz sand. Lenses of conglomerate a few inches thick occur locally at the base of the sandstone or scattered through the lower 5—10 ft (1.5—3 m) of it. At localities farther north, as much as 40 ft (12 m) of conglomerate is pres- ent at the base of the Dakota. Pebbles in the conglomer- ate are typically about one-half inch (1 cm) in diameter, are well rounded, and consist of chert, quartz, and white silicified volcanic rock. The sandstone of the Dakota is brittle but is hard and resistant, and it has a tendency to fracture into blocks that slide on the shales of the underlying Morrison For- mation and slopes below. An area of several square miles east of Red and White Mountain is littered with blocks of Dakota, and it is evident that the two small patches of Dakota bedrock near the top of the mountain are the last remnants of an extensive sheet that has been destroyed by sliding of detached blocks. UPPER CRETACEOUS AND TERTIARY IGNEOUS ROCKS Igneous rocks younger than Precambrian occur only in scattered areas in the Minturn quadrangle. They in- clude (1) a persistent sill of quartz latite porphyry in basal strata of the Belden Formation along the canyon of the Eagle River; (2) scattered small dark dikes in the Gore Range; and (3) patches of basalt and tuff on the sides of the Piney Rivery valley in the northwestern corner of the quadrangle. The sill is a northern exten- sion of the group of porphyry bodies that characterize ‘ the Colorado mineral belt at this general longitude. The northwestern edge of the main belt of abundant and varied porphyry bodies is in the Pando area, about 5 mi (8 km) south of the Minturn quadrangle. PAN DO PORPHYRY SILL The quartz latite porphyry of the sill exposed along the canyon of the Eagle River was named the Pando Porphyry by Tweto (1951), who traced it to the Lead- ville area, where distinctive altered facies ofit had been known as “White porphyry" and “Mount Zion porphy- ry” (Tweto, 1956). Throughout the area between Gil- man and Leadville, the Pando Porphyry occurs prin- cipally in one or more sills near the base of the Belden Formation, but it also forms sills in the Sawatch Quartzite and the Minturn Formation. The sills become thinner and less numerous northward; from near Pan- do northward into the Minturn quadrangle, only one sill is present. This sill—in the Belden—is more than 100 ft (30 m) thick near the quadrangle boundary and about 80 ft (24 m) thick at Gilman. North of Gilman it tapers more rapidly and apparently comes to a wedge end in a covered area south of the mouth of Two Elk Creek. Geologic relations show that the Pando is the earliest and most widespread of all the porphyries in the mineral belt in the southern Gore and Mosquito Ranges. Isotopic dating by the K-Ar method established a Late Cretaceous age of about 70 my (million years) for it (Pearson and others, 1962). The Pando Porphyry is altered wherever exposed in the Minturn quadrangle and through most of the region to the south. The only known unaltered occurrences of it are in the center of a source plug or stock north of Leadville. The widespread pervasive alteration is deuteric. In mineralized areas, a later hydrothermal alteration is superposed on the deuteric alteration, but as the chemistry’of the two stages was generally simi- lar, the second alteration merely accentuated the first one, and the two are difficult to distinguish. The deuterically altered porphyry consists mainly of UPPER CRETACEOUS AND TERTIARY IGNEOUS ROCKS 59 an aphanitic groundmass; phenocrysts constitute only 1—10 percent of the rock. The phenocrysts are prin- cipally altered plagioclase, typically in prisms 2-4 mm long, and smaller shreds of altered biotite. Quartz phenocrysts are. generally present in smaller amounts, and potassium feldspar and smoky muscovite phenocrysts locally are scattered sparsely through the rock. The altered porphyry is light gray to orange gray or pinkish gray and weathers buff to yellowish gray. As seen in thin section the deuterically altered rock consists of a slightly trachytoid groundmass and sparse phenocrysts. The phenocrysts are mainly plagioclase (oligoclase) and biotite, invariably strongly altered, but also they include quartz in rounded to subhedral grains and occasional grains of moderately fresh potassium feldspar. The groundmass consists of these same minerals together with anorthoclase. Sphene and apatite are minor accessory minerals. In outcrop, sills of Pando Porphyry generally are characterized by a thin platy structure near and parallel to the contacts and by a crude columnar struc— ture in the interior (fig. 18). Primary flow structures, described in detail by Tweto (1951), include textural layering, mineral orientations in the outer parts of a sill at right angles to the orientation in the body of the sill, intrusive-stage folds and faults, and the platy parting, which was produced by differential laminar flow. These features were interpreted by Tweto to indicate a relatively viscous magma as compared with other porphyries that do not show these features. The struc— tural features indicate intrusion from the south- southeast, the general direction of the presumed source pluton near Leadville. Chill zones in the Pando Porphyry sills generally are only a few inches thick, but locally they were thickened to as much as 3 ft (0.9 m) by the intrusive-stage drag folds and thrust faults. The contacts between the chilled and unchilled porphyry are sharp discon- tinuities that reflect differential flow while the sill mag- " ma cooled and solidified. The chilled porphyry is glassy in appearance, though microscopic study shows that most of it is finely crystalline. Sedimentary rocks in contact with Pando Porphyry generally show only slight metamorphism. Black shale, FIGURE 18.—Columnar structure in sill of Pando Porphyry exposed in cut on US. Highway 24, one-half mile (0.8 km) north of Gilman. Thin- bedded strata of the Belden Formation are above the sill, and a karst pinnacle of Leadville Dolomite casts shadow on embankment below highway. 60 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO the common wallrock of the porphyry, is only slightly hardened and slightly bleached for a few inches adjoin- ing the contact. Black limestone is slightly bleached but otherwise unaltered. Where the Pando intrudes Lead- ville Dolomite 2-3 mi (3—5 km) south of Red Cliff, the gray dolomite is slightly reddened for about 2 in. (5 cm) from the contact. As seen in thin section, the dolomite is finely brecciated and is cut by minute veinlets of quartz and hematite. The dolomite also contains a few specks of hematite and a few blades of sericite that may have been introduced from the sill. In contrast to the negligible effect of the sill on its wallrocks, the wallrocks seem to have affected the deuteric alteration within the sill, for the type of altera~ tion seems to correlate with the type of wallrock. Where the Pando Porphyry sill intrudes shale of the Belden Formation, the sill is characterized by a sericite— anorthoclase type of alteration. In contrast, small sills of Pando in Sawatch Quartzite just south of the quad- rangle in the Pando area (Tweto, 1953) are charac- terized by chlorite-allophane alteration. The difference in alteration with difference in wallrocks suggests that the wallrocks either influenced or caused deuteric alteration, probably by supplying water to the cooling sills which, asjudged by metamorphic effects and struc- tural features, were intruded as “dry” and viscous mag- mas. Where the enclosing rock is shale, the dominant mineral in the Pando Porphyry is moderately coarse- grained sericite which makes up a large part of the groundmass of the porphyry. Biotite is altered to coarse sericite or muscovite accompanied by calcite, leucoxene, and small crystals of included apatite (fig. 19). Chlorite is absent. The feldspar phenocrysts are irregularly FIGURE 19.—Pando Porphyry showing phenocryst of biotite altered to sericite, leucoxene, and calcite, and sericitic groundmass. Crossed polars. replaced by an isotropic clay identified microscopically as allophane. Both the feldspar and the allophane are veined by anorthoclase or by a more sodic plagioclase, which remains fresh. In the remaining allophanized portions of the feldspar grains, fine-grained sericite later formed abundantly. Except in the altered biotite grains, calcite or other carbonates are lacking in the deuterically altered porphyry, though they are generally present where the rock was further altered by late hydrothermal solutions. Pyrite is locally present in small amounts in the deuterically altered porphyry. Quartz and apatite are unaltered. Where the Pando Porphyry has quartzite walls, biotite in the porphyry is changed to chlorite, and feldspars are extensively allophanized. The chlorite is accompanied by minor amounts of sericite, muscovite, montmorillonite, and leucoxene (fig. 20). Some chlorite is partly replaced by montmorillonite which, in turn, is replaced by sericite. Both the plagioclase and the potassium feldspar are strongly allophanized, and the allophanized plagioclase is partly altered to hydromica (about 10 percent) with some sericite. Potassium feldspar phenocrysts are strongly allophanized and contain many small irregular masses of chlorite which probably represent former inclusions of biotite. Moderately coarse sericite is abundant in the ground- mass where it apparently represents groundmass biotite. Pyrite and carbonate are absent in the chloritized porphyry in quartzite and no evidence of silicification was observed. The early allophane in both rocks and the accom- panying chlorite in the one suggest only the addition of water and some leaching of iron. The abundant sericite and accompanying anorthoclase and late sodic FIGURE 20.—Pando Porphyry showing biotite phenocryst altered to chlorite and opaque oxides. Potassium feldspar phenocryst at left is allophanized. Plain transmitted light. UPPER CRETACEOUS AND TERTIARY IGNEOUS ROCKS 61 plagioclase of the other rock suggest addition of alkalies at a later stage. At the time of intrusion, the wallrocks were almost certainly water saturated, as indicated by mudstone dikes near some porphyry contacts in the Pando-Leadville area; these dikes must have been emplaced as mud slurries. A sill of relatively dry mag- ma intruded into saturated rocks might first absorb water nearly free of solutes, perhaps in the vapor phase, and then, with passing time and the transfer of heat farther into the wallrocks, water containing alkalies leached from shales might be supplied. Where the wallrocks consisted almost entirely of clean quartz sandstone or quartzite, alkalies were not available in quantity, and alteration stopped at the allophane- chlorite stage, except that alkalies liberated by allophanization of the feldspars and chloritization of biotite became available to form the relatively minor hydromica and sericite of these rocks. In the shale en— vironment, the alkali-bearing solutions from the wallrocks probably contained a relatively high propor- tion of potassium, inasmuch as the potassium feldspar crystals are little sericitized, whereas biotite is com— pletely altered to sericite and leucoxene. The formation of potassium and sodium feldspars would decrease the ratio of alkalies to hydrogen ion, and, as shown in the diagrams of Hemley and Jones (1964, figs. 1 and 2), this would shift the field of equilibrium from feldspar towards mica and ultimately to kaolinite-pyrophyllite. The shift from feldspar to mica is evident in the mineralogic relations in the altered porphyry; but the kaolinite-pyrophyllite stage was not reached, probably because equilibrium in saturation between wallrocks and sill was reached in a relatively short time, ending the transfer of solution to the sill. During the process of deuteric alteration, iron and magnesium were largely expelled, though some iron was fixed locally as pyrite or siderite. DIKE ROCKS Scattered small dikes of dark fine-grained igneous rocks occur along faults in the Precambrian rocks of the Gore Range, especially near the northern boundary of the quadrangle. The dikes are somewhat more abun- dant northward in the Mount Powell quadrangle but are absent southward through the remainder of the Gore Range (Tweto and others, 1970). Only one small dike has been found in the sedimentary rocks; this dike cuts grit of the Minturn Formation on Pitkin Creek about one-half mile (0.8 km) from Gore Creek. The dikes are latite, dacite, and quartz basalt porphy- ries, some of which have lamprophyric characteristics. In the Slate Creek area and northward into the Mount Powell quadrangle, most of the dikes more than 4 or 5 ft (1.2 or 1.5 m) wide contain sanidine phenocrysts in their inner zones, but the small dikes are aphanitic. These dikes are inferred to be related to the trachytic intrusive and volcanic center at Green Mountain in the Mount Powell quadrangle (Tweto and others, 1970). The dikes are considerably altered deuterically and therefore cannot be closely characterized petrograph- ically. They seem to have consisted originally of a groundmass of small andesine laths and magnetite grains in a matrix of low-index feldspar, possibly anorthoclase, and phenocrysts of labradorite, potassium feldpsar, hornblende, augite, and minor biotite and quartz. Some contain as much as 3 percent apatite. Calcite and chlorite are abundant alteration products, and one dike, 1.2 mi (1.9 km) southwest of Upper Slate Lake, contains the zeolite scolecite in abun- dance. The rock is classed as a lamprophyric latite. Dacite porphyry dikes on the western side of the Gore Range, on the slopes above the upperPiney River, are characterized by abundant phenocrysts of andesine, some of which are as much as an inch (2.5 cm) in length and by small anhedral grains of hornblende. The groundmass is a very fine grained aggregate of oligoclase, quartz, potassium feldspar, biotite, and mag- netite. Sphene is a relatively abundant accessory mineral, and apatite, epidote, and allanite are present in small amounts. A short dike on the ridge between Bighorn and Pitkin Creeks and the dike in the Minturn Formation near the mouth of Pitkin Creek are quartz basalt. In the dike on the ridge, some of the quartz is in rounded grains that may be xenocrystic, although quartz occurs also in the groundmass. In the dike on Pitkin Creek, quartz occurs both in the groundmass and as 2—3 mm phenocrysts with square cross-section, suggesting original cristobalite. The plagioclase phenocrysts of the basalt porphyries are labradorite, An58_60, and the plagioclase of the groundmass is andesine, An40. The dike on the ridge contains about 25 percent of pyroxene in the form of aegerine-augite and pigeonite. The pigeonite rims and replaces the aegerine-augite. The dike on Pitkin Creek contains 11 percent augite and 5 percent biotite. The magnetite content of the two rocks is 5 and 8 per- cent, respectively. The dike rocks are considered to be of middle to late Tertiary age. The basalts presumably are related to the basalts of the Piney River which, as shown in the following discussion, are Miocene in age. The latites and dacites seem to be related spatially and composi- tionally to the Green Mountain intrusive-volcanic center in the Mount Powell quadrangle. Geologic evi- dence suggests that this center is of late Tertiary age (Tweto, 1957), although a single fission—track age of about 30 my. (Naeser and others, 1973) suggests a late Oligocene age. 62 GEOLOGY, MIN TURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO VOLCANIC ROCKS Basalt and tuff or volcanic ash occur in patches high on the sides of the Piney River valley in the north- western corner of the quadrangle. Basalt caps a ridge and knob northeast of the junction of Meadow Creek and the Piney River between the 9,500- and 9,800-ft contours, and it occurs also in a small patch high on the east slope of Red and White Mountain at the 10,300-ft contour. Tuff or ash crops out in a narrow belt at the top of the cliffs on the west side of the canyon of the Piney north of Dickson Creek, between the 9,400- and 9,7 50-ft contours. The tuff probably underlies much of the area covered by surficial deposits on the east and north slopes of Red and White Mountain (pl. 1). These occurrences are erosional outliers of a moderately extensive basaltic volcanic field in the Piney River area and along the Colorado River north of the quadrangle. As mapped by Brennan (1969) and Donner (1949), this volcanic field extends from about 3 mi(5 km) north of the Minturn quadrangle northwest- ward about 14 mi (22 km) to Yarmony Mountain, north of the Colorado River. The volcanic sequence consists of superposed basalt flows which locally are separated by beds of volcanic ash or by lenticular fluviatile deposits. Donner (1949) distinguished at least nine flows, each 25—150 ft (76—457 m) thick, in the State Bridge area; he also mapped a persistent bed of tuff and breccia be- tween the fifth and sixth flows. Taggart (1962) recog- nized more than 25 flows, each 5—40 ft (15—122 m) thick, on Piney Ridge, east of the Piney River, and he noted thin beds of calcareous tuff between some of them. In the lower valley of the Piney River, tuffaceous sedimentary rocks several hundred feet thick overlie the basaltic rocks (Brennan, 1969). On the basis of lithology, vertebrate fossils, and intercalated ash beds, the sedimentary rocks might be assigned to the Troublesome Formation of Middle Park, the North Park Formation, or the Browns Park Formation, all of Miocene age (Izett, 1968; also oral commun., 1970). The tuff in the Minturn quadrangle crops out for about a mile (1.6 km) northwestward from the Dickson Ranch, at a level about 800—900 ft (240—270 In) above the Piney River. It lies on a gullied surface with relief of as much as 75 ft (22.8 m) cut over strata of the Maroon Formation. It is overlain by 50—75 ft (15—228 m) of surficial materials, which are discussed in the following section. Maximum exposed thickness of the tuff is about 200 ft (61 m). The tuff is light brown to yellowish white, coherent and tough but porous and friable, and distinctly “light” in weight, or specific gravity. Much of the tuff contains charcoal specks, and the upper 50 ft (15 m) contains abundant chunks of White opal as well as fragments of opalized twigs or rootlets. Bedding is faint or absent. As seen microscopically, the tuff con- sists principally of pyroclastic materials but contains a minor fraction of foreign materials, such as microcline, rounded quartz grains, epidote, muscovite, and proba- bly some biotite. The pyroclastic fraction consists in part of faintly anisotropic, turbid, partly devitrified glass and in part of isotropic glass shards, crystal frag- ments of plagioclase, potassium feldspar, biotite, and hornblende. Refractive index of the glass is 1.49 as determined by G. A. Izett of the US. Geological Survey. The basalt in the Minturn quadrangle is near an- desite in composition and is notable for the presence of rather abundant potassium feldspar in the ground- mass. The rock is dark gray, vesicular, and aphanitic, except for a few plagioclase phenocrysts as much as 4 mm long. These phenocrysts are conspicuously zoned and have cores of labradorite with rims of oligoclase. Grains of augite about 1 mm in diameter constitute about 25 percent of the rock. Olivine is present in scat- tered grains about the same size as the augite. Both the augite and the olivine are extensively altered to id- dingsite and to a fine-grained low-index chlorite. The groundmass is a very fine grained mixture of twinned acicular andesine, tabular orthoclase, and sparse ac- cessory magnetite. Flows near State Bridge were described by Donner (1949) as olivine-bearing hy- persthene andesites and olivine basalts. The stratigraphic relationship between the tuff and the basalt in the Minturn quadrangle is not evident, . owing to the isolated exposures. Absence of basalt beneath the tuff suggests that the tuff may be the older, but the two rocks could intertongue, or if the basalt were discontinuous, the tuff could be the younger. Further, the precise relation of the tuff t0 the sequence of tuffs and siltstones farther north is unknown, but on the basis of general similarity in character, they are assumed to be of about the same age. Fossil dog remains from the tuffaceous sedimentary rocks on the lower Piney River (E 1/2, sec. 1, T. 3 S., R. 83 W.) were described as Cynodesmus casei and assigned to the early Miocene by Wilson (1939). Reappraisal of this fossil and study of other vertebrate fossils found later in these strata led G. Edward Lewis of the US. Geological Survey (written commun., May 1, 1970) to assign a late MiOCene age. Lewis referred Wilson’s specimen to Tomarctus thomsoni (Matthew) rather than to Cynodesmus casei and, from other fossil collections in the area, identified: 1. The dogs Amphicyon sp. and Tomarctus sp. ‘ 2. The oreodont Brachycrus sp. close to B. vaughani Schultz and Falkenbach and B. wilsoni Schultz and Falkenbach 3. The horse Merychippus sp. close to M isoncsus (Cope) Basalt from Yarmony Mountain in the Piney River PHYSIOGRAPHY AND UPPER TERITARY AND QUATERNARY UNCONSOLIDATED DEPOSITS 63 FIGURE 21,—Rugged topography formed by glaciation in Gore Range. View southwestward from Upper Slate Lake. basalt field has been dated isotopically as 21—24 my (early Miocene) in age (Mutschler and Larson, 1969). As the basalts underlie the fossiliferous tuffs and siltstones, this age is consistent with the late Miocene age of the vertebrate fossils. PHYSIOGRAPHY AND UPPER TERTIARY AND QUATERNARY UNCONSOLIDATED DEPOSITS Physiographically, the Minturn quadrangle consists of three main units, corresponding to the threefold divi- sion in its bedrock geology. The Gore Range, on the northeast, is characterized by deep canyons and knife— edge ridges created by intense glaciation (fig. 21). The flank of the Sawatch Range, on the southwest, is an area of shallower glacial canyons, separated by broad, evenly inclined dip slopes (fig. 22) that rise southwest- ward toward high and rugged peaks that lie outside the quadrangle. The broad area between the two ranges, corresponding in general to the area underlain by the Minturn and Maroon Formations, is an area of smooth ridges and slopes and deep stream valleys (fig. 23). The fluvial and glacial erosional processes that pro- duced these landscapes also produced deposits, such as stream gravels, moraines, and colluvial blankets. Although the preserved deposits are small in com- parison to the volume eroded, they provide a record of the character and timing of the erosional processes. This record begins with local and generally scanty deposits of late Tertiary age and extends with increas- ing clarity through the Pleistocene Epoch to the pres- ent. The earliest deposits related to the existing topogra- phy and physiography are the tuff and basalt of the Piney River area at the north edge of the quadrangle. These volcanic materials were deposited in a broad valley that was centered approximately over the pres- FIGURE 22.—Dip slopes on flank of Sawatch Range. View northwest- ward across mouth of Bishop Gulch; canyon of Cross Creek in mid- dle distance. 64 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO FIGURE 23.—Typical topography in area of Minturn Formation. View northwestward across valley of Gore Creek toward Bald Mountain (on skyline). Gore fault (arrow) is in front of crest of Bald Moun- tain; Precambrian rocks in back of the fault. ent valley. Within the quadrangle, where the valley of the Piney River was greatly deepened by glacial erosion and related stream cutting, remnants of the old valley bottom as defined by the volcanic rocks are several hundred feet above the present stream (pl. 1, sec. D—D’). Farther north, however, the volcanic rocks ex- tend to the bottom of the present valley, indicating that the valley was both wide and deep in Miocene time. Other streams in the sedimentary terrane, such as Gore, Red Sandstone, and Turkey Creeks and the Eagle River probably occupied similar valleys in a rolling up— land of low relief in the Miocene, but no direct evidence of this has been found. Aside from the volcanic rocks in the old valley of the Piney River, the oldest deposit related to the present physiography is a thick and old colluvium that mantles the high slopes in the northwestern part of the quad- rangle. The colluvium covers some of the tuff along the Piney River; hence, it is younger than late Miocene. Shallow cirques of the earliest recognized glaciation are incised below some of the colluvium-mantled slopes, and in places the colluvium seems to extend beneath ancient glacial drift. The colluvium is, therefore, preglacial. It is judged to be Pliocene and possibly early Pleistocene in age. The glacial history of the Pleistocene Epoch is repre- sented by an exceptionally rich record, as almost every drainage of consequence in the quadrangle was glaci- ated at least once. This record takes the form both of erosional features, such as canyons and cirques, and of depositional features, such as moraines and valley trains of outwash gravels. Studies made subsequent to our main field work in the Minturn quadrangle indicate that as many as nine distinct episodes of glacial ad‘ vance and retreat occurred in the mountains of central Colorado (Tweto, 1961). Of these, two are tentatively correlated with the pre-Bull Lake glaciations defined by Richmond (1960, 1964); two are correlated with the Bull Lake Glaciation; three are correlated with the Pinedale Glaciation (Blackwelder, 1915; Richmond 1960); and two are Neoglacial or Holocene. All nine are represented by deposits in the Minturn quadrangle, but they are depicted only in two main groups on the map: (1) pre-Bull Lake and (2) Bull Lake and Pinedale un- divided, although deposits of these two glaciations are separately distinguished in a few critical areas. Deposits of the two Neoglacial episodes exist in many of the cirques of the Gore Range but were not mapped. Relicts of the later of these, if not of both, are repre- sented by many ice-cored rock glaciers in the high cirques. The glacier shown on the map (pl. 1) at the head of Black Creek was active in 1942 when dis- covered by Lovering, but it had degenerated to a boulder- and snow-covered body of stagnant ice by 1969 when it was revisited by Bruce Bryant in connection with the study of the Gore Range Primitive Area (Tweto and others, 1970). The glacial epoch was a time of pronounced erosion of valleys and canyons, both by glacial ice and by streams. At the time of the pre-Bull Lake glaciations, deep can- yons, such as those of the Eagle River and upper Gore Creek, did not yet exist. The early Eagle Glacier, for ex- ample, occupied a broad valley whose bottom was at the level of Gilman and the present canyon rims, as shown by the relation of ancient glacial drift to the topogra- phy. By the time of the succeeding Bull Lake Glacia- tion, the canyon had been cut to a depth of about 400 ft (120 m) in hard rocks, as shown by the location of 3 till of early Bull Lake age on the canyon sides. By the time of the Pinedale Glaciation, the canyon had reached its present depth of 500~600 ft (150—7180 in), or was even a trifle deeper. No deepening of the valleys has occurred since the Pinedale Glaciation and, in fact, many of the valleys have been aggraded by a combination of stream action, colluvial processes, growth of alluvial fans, and landsliding. Some part of the canyon cutting was certainly ac- complished by the glaciers, but the bulk of it seems to have been produced by stream erosion, as the cutting occurred after the valley or canyon was occupied by one glacier and before it was occupied by the next. Melting of ice many cubic miles in volume upstream from a can— yon may have been a factor in the canyon cutting, but it probably was not the only factor. Canyon cutting occur- red after early glaciation in many places in the Cor~ dilleran region (Richmond, 1965), suggesting that climatic or orogenic factors might also have been in- volved. PHYSIOGRAPHY AND UPPER TERTIARY AND QUATERNARY UNCONSOLIDATED DEPOSITS 65 TERTIARY AND PLEISTOCENEC") COLLUVIUM The upper slopes and tops of smooth ridges northeast and southeast of Red and White Mountain are mantled in places by thick colluvium that contains abundant blocks oflight-colored sandstone from the Dakota, Mor— rison, and Entrada Formations. In other places, bedrock surfaces on the Maroon Formation are littered with the sandstone blocks, or with irregular masses or nodules of varicolored chert, or both. Gradation of sandstone-bearing colluvium into areas of isolated sandstone blocks resting on bedrock suggests that the isolated blocks are residual from the erosion of the col- luvium. Some of the sandstone blocks are partly replaced by varicolored chert. The same chert has also locally replaced grit and thin limestone beds of the Maroon Formation at the erosion surface on which the blocks rest. This suggests that the chert-forming proc- ess was related to weathering at and following the time in which the sandstone-bearing colluvium accumul- ated. The colluvium consists of angular and subangular sandstone fragments of various sizes up to several feet across, scattered limestone fragments, and pieces of chert in a brown sandy or clayey matrix. Gullies through the colluvium indicate that it is as much as 50-75 ft (15—228 m) thick. Because the colluvium is on slopes below the small area of Mesozoic rocks cap- ping Red and White Mountain and contains no materials other than Mesozoic rocks and minor debris from the Maroon Formation, it is believed to represent an apron of debris that formed as cliffs of the Mesozoic sandstones retreated toward the crest of Red and White Mountain. The debris accumulated on the side of the broad old valley of the Piney River, and it extended over the volcanic rocks that also coat the sides of that old valley. Because these volcanic rocks are Miocene and possibly late Miocene in age, and because the colluvium predates the earliest recognized glaciation, the col- luvium is regarded as Pliocene and possibly early Pleistocene in age. The chert associated with the colluvium is distinctive in its varied color and in the extreme irregularity of the pieces or lumps in which it occurs. It differs markedly from the olive-gray chert that is present in minor amount in the Morrison Formation on Red and White Mountain and could not have been derived from that source. The chert is scattered over the broad, mature erosion surface that forms the divide between the Piney and the Eagle Rivers southeast of Red and White Moun- tain. It is especially abundant in the channel of an in- termittent stream that joins Buck Creek from the north at about the 10,000-ft contour. Most of the loose chert is free of matrix materials, but some pieces show remnants of sandstone. limestone, or grit. Chert in sandstone blocks is in globular or veinlike forms. Chert in limestone and grit of the Maroon For- mation at the stripped erosion surface is in small, sharply angular, blocky bodies. The chert is white to dark gray and various shades of yellow, red, and brown; much of it is variegated. Most of it is dense, but the larger lumps commonly contain vugs lined by quartz or—rarely—by calcite. In all types of oc- currence, the chert generally shows several genera- tions of deposition. As seen in thin section, chert that has replaced limestone at the outcrop of the Maroon Formation consists in large part of fine-grained quartz, the earliest silica mineral. Thin layers of fine-grained hematite coat this quartz. The quartz is cut by veinlets of coarser chalcedony in fibers that have an ordered ar- rangement and that maintain optical continuity across the hematite bands. The chalcedony is in turn coated by quartz in microvugs, and some of these openings con- tain still later growths of chalcedony, quartz, or calcite. A good example of chert that has replaced arkosic red sandstone of the Maroon Formation was seen in a boulder near the head of Buck Creek. The chert, in an irregular body about 3 ft (1 m) long, has a dark-red hematite-rich core. This core is veined and is rimmed by salmon—pink chert that contains far less hematite. The sandstone adjoining the chert is pitted by solution cavities and is decolorized in a band about an inch wide. The occurrence, character, and paragenesis of the chert together indicate that the chert formed under supergene conditions, at and near the base of the col- luvium covering an old erosion surface. The successive generations of deposition recorded in the chert indicate fluctuating conditions that could well reflect fluctua- tions in amount and in composition of descending ground waters. The leaching of hematite in red beds by the chert-depositing solutions, and the wide ranges in iron content suggested by color contrasts in the chert, indicate that solutions that were able to dissolve hematite existed at times. Because hematite is stable in most natural solutions of inorganic composition, these features strongly suggest that the solutions contained organic compounds. Vegetation at the surface of the colluvium would have been a ready source for such com- pounds. PRE-BULL LAKE GLACIATIONS During the two pre-Bull Lake glaciations more of the quadrangle was covered by ice than in any subsequent time. Glaciers existed during one or both of the early glaciations in several areas that were never glaciated again: in tributary gulches on both sides of Wearyman, Two Elk, and Mill Creeks, and at the heads of Turkey, 66 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Timber, Lime, Willow, Game, Spraddle, Freeman, and Dickson Creeks. The largest of the early glaciers oc- cupied the valley of the Eagle River from the southern edge of the quadrangle northwestward to Minturn. This glacier originated more than 15 mi (24 km) to the south of the quadrangle, and glaciers in the valleys of Fall and Cross Creeks were tributary to it within the quad- rangle. Deeply weathered till that is the chief evidence of this glacier occurs in scattered patches, either as blanketlike deposits without morainal form or as damlike bodies in the valleys of sidestreams and gulches. One of the larger till bodies lies on the dip slope west of Gilman, where it extends from a little above the canyon rim upslope to about the 9,500-ft contour. Loca- ' tion of this and other remnants of the till indicate that the valley occupied by the glacier descended rapidly from the level of Gilman, 550 ft (170 In) above the pre- sent Eagle River, to the level of a rock bench on the north side of Martin Creek, 250 ft (75 In) above the valley floor at Minturn. Morainal remnants at this locality are the most distal that have been found; if the glacier extended farther, the evidence has been destroyed. Trunk glaciers also existed in the Gore Creek and the Piney River drainages during the pre-Bull Lake glacia- tions. Along Gore Creek, evidence of these glaciations is in the form of scattered blanketlike deposits of till high on the valley walls, above the lateral moraines of Bull Lake age. The old glaciers in the Piney River drainage spread southward over the drainage divide into the drainages of Red Sandstone and Indian Creeks. The ridge between Indian and Freeman Creeks, for exam- ple, is capped by old till that contains boulders of Dakota Sandstone. The sandstone is thought to have been derived from an underlying colluvial blanket and to have originated on Red and White Mountain, though, conceivably, it could have come from remnants of a sedimentary cover on the Gore Range. The older of the two pre-Bull Lake tills-is typically red brown to brown and contains soft, weathered boulders in a tough, clayey matrix. Where extensively eroded, however, it is buff to light brown and sandy. The younger till has the same general characteristics but is a lighter shade of brown and not quite so clayey and tough. In many of the smaller old cirques and glacial valleys, these tills arehefavily mantled with colluvium derived from the cirque walls, and hence they are shown on the map as “landslide and colluvium.” Weathering and colluvial creep have destroyed or have buried any cliffs that existed in the old cirque walls, forming basins with characteristic steep smooth slopes at the heads of minor stream valleys. The Vail ski area owes the excellence of its ski runs to this modified glacial topography. BULL LAKE GLACIATION The Bull Lake Glaciation was characterized by the longest and thickest glaciers ever to occupy the Min- turn quadrangle. The glaciation was in two episodes or stades, separated by a period of time long enough to allow appreciable modification of the moraines of the first stade and some weathering of the till before the second glacial advance occurred. Large lateral moraines along the valley sides hundreds of feet above the valley bottoms are the hallmark of these two glacial advances. Terminal moraines are inconspicuous because they were extensively eroded by the meltwater streams from younger glaciers with fronts farther up the valleys. In general, the glaciers of the earlier stade extended farther down the valleys than did those of the later stade, but in many places ice of the later stade reached higher levels on the valley walls than in the early stade. Because of this and also because ice of the late stade ex- tensively eroded the lateral moraines of the early stade in places, morainal evidence of the early stade of the Bull Lake is much less abundant than for the late stade. Although the two sets of Bull Lake moraines differ preceptibly in degree of modification and weathering, together they are intermediate between the generally formless and deeply weathered pre-Bull Lake morainal deposits and the hummocky and bouldery moraines of fresh till characteristic of the Pinedale Glaciation. The Bull Lake moraines typically form benches on the valley walls, but where unimpeded by such walls, they form ridges. These ridges generally have smooth slopes with few or no boulders lying on them, in contrast to rough and bouldery morainal ridges characteristic of the Pinedale. Stream adjustment to the Bull Lake moraines is complete, and extensive segments of the moraines have been eroded in the process. EAGLE RIVER AND TRIBUTARIES FROM SAWATCH RANGE In the early stade of the Bull Lake, a glacier deep enough to have covered Iron Mountain at Red Cliff en- tered the quadrangle from the south and extended at least to Minturn. Morainal evidence of this glacier is scanty. Patches of lateral moraine lie just above the canyon rim west of the river and, near the lower end of the canyon, till of this age lies on bedrock 150 ft (45 m) above the river. Although the glacier was very large south of the quadrangle, it seems to have tapered rapidly between Red Cliff and Minturn. It may have reached Minturn only because of nourishment from the Fall Creek and Cross Creek tributary glaciers. During the late stade of the Bull Lake, the Eagle PHYSIOGRAPHY AND UPPER TERTIARY AND QUATERNARY UNCONSOLIDATED DEPOSITS 67 Glacier terminated about at Red Cliff. No terminal moraine remains, and probably none was formed, for all the drainage from a very extensive glacial system was here funneled into the narrow canyon of the Eagle River, presumably as a torrent that might have carried away almost all of the debris of the terminal area. Bull Lake glaciers in the valley of Fall Creek formed prominent high compound lateral moraines that extend for 3—4 mi (5—6.5 km) along the valley. Ice of the early stade joined the Eagle Glacier at the level of a bedrock sill 250 ft (76 m) above the present canyon, and that of the second stade formed a small terminal moraine one- half mile (0.8 km) short of the mouth of this hanging valley. Ice of both stades also spilled prongs southward into the valley of Peterson Creek. The big lateral moraines lie on pre-Bull Lake tills, as along Notch Mountain Creek, and extensive blankets of the early tills extend upslope from them. The Cross Creek drainage contained large Bull Lake glaciers that originated in cirques several miles south- west of the quadrangle. Unlike Fall Creek and many other valleys, Cross Creek valley contains only short segments of lateral moraines of any age, presumably because glacial movement and erosion were too vigorous to allow them to form or survive. In the ter- minal moraine area at the mouth of Cross Creek, only the northern portions of the Bull Lake terminal moraines are preserved. These are in the form of high morainal ridges. The older ridge curves northward, as if joining a glacier in the valley of the Eagle River, but the younger one extends straight eastward to a truncated front above the Eagle River where it rests on bedrock 60—75 ft (18—23 m) above the valley floor. Clearly, the late Bull Lake Cross Creek Glacier, and also the next succeeding Pinedale Glacier, forced the Eagle River eastward against the valley wall, causing erosion that accounts for the spectacular cliffs of the Minturn Formation between Minturn and the mouth of Eagle Canyon (fig. 12). The valleys of Grouse and West Grouse Creeks were occupied by Bull Lake glaciers that were very narrow but had lengths of 5—6 mi (8—10 km). Moraine of the late stade of the West Grouse Glacier is at the level of the Eagle River below Minturn. An even narrower glacier of probable Bull Lake age descended the canyon of Stone Creek, at the western edge of the quadrangle, to a small terminal moraine between the 9,000- and 9,500-ft contours. GORE CREEK DRAINAGE The Gore Creek glaciers of Bull Lake time were the largest in the quadrangle. In both stades, ice reached levels 1,100—1,300 ft (335—400 In) above the present valley bottom in the vicinity of Black Gore, Bighorn, and Pitkin Creeks. The glacier of the early stade de- scended Gore Creek to a terminal moraine area just southwest of the Gore Creek School, or to a point about 1.5 mi (2.5 km) from the Eagle River. The terminal moraine is much dissected, and as shown in road cuts,it rests on a bedrock surface that is 40—50 ft (12—15 m) above the present stream. The glacier of the second stade was about 2 mi (3 km) shorter; it extended to a terminal moraine area in the vicinity of Red Sandstone Creek. This moraine is also extensively dissected. The largest remnant is a terraced deposit outlined by the 8,250-ft contour just east of Red Sandstone Creek. Other remnants to the south of this are knobs and ridges of till separated by channels cut into the till by Gore Creek, probably in Pinedale time. In both stades, the glacier coming from the head of Gore Creek, 3-4 mi (5—6.5 km) east of the quadrangle boundary, was heavily augmented by tributary glaciers within the quadrangle. Massive glaciers also descended Black Gore Creek from sources in the West Tenmile drainage east of the quadrangle. The ice from this source spilled through Vail Pass, bearing a load of telltale porphyries that are foreign to the Gore Creek drainage. A prominent lateral moraine of this origin caps the ridge east of Timber Creek at the 10,500-ft contour. Other major tributaries of the Gore Creek glaciers came from the Bighorn, Pitkin, and Booth Creek drainages. Smaller tributaries from Spraddle Creek, Middle Creek, and possibly from Mill Creek existed dur— ing the early stade of the Bull Lake. Middle Creek also contained a glacier in the late stade, but this failed by about a mile to join with the Gore Glacier. The two forks of Red Sandstone Creek contained glaciers in both stades that formed large moraines southeast of Lost Lake. These glaciers extended only to the forks of the stream; thus, they failed by 4—5 mi (6.5-8 km) to reach the Gore glaciers. PINEY RIVER The Piney River drainage contained very large glaciers in both stades of the Bull Lake. The bulk of the ice came from what is essentially a single elongated cirque extending 3.5 mi (5.5 km) from the Booth Creek drainage divide to the sharp bend in the Piney River. Smaller amounts came from a hanging valley or cirque on the south side of Mount Powell, just north of the quadrangle, drained by the stream that joins the Piney River at the big bend and from a cirque drained by East Meadow Creek. Massive lateral moraines that were formed in the two stades of the Bull Lake and in the earliest stade of the Pinedale border the Piney River valley from near the big bend southwestward and west- ward for 6 mi (10 km). At their upper ends, these 68 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO moraines are 1,200+1,500 ft (360—450 In) above the present stream. Ice of the early stade of the Bull Lake spilled into the East Meadow Creek drainage on a wide front, forming extensive moraines in that valley. Ice of the second stade spilled into the East Meadow Creek drainage on a much smaller scale, through a saddle northwest of Piney Lake. The lower ends of the lateral moraines of the early stade, as well as any former ter- minal moraine, have been removed by erosion. The end of the eroded north lateral moraine is on the spur southeast of the mouth of Meadow Creek, 450 ft (135 m) above the Piney River. The end of the eroded south lateral moraine plugs the valley of Dickson Creek at Dickson Ranch, 550 ft (165 m) above the river. In con- trast to the early moraines, the north lateral moraine of the late stade descends to a remnant of a terminal moraine in the canyon bottom. This indicates that the bedrock floor of this part of the canyon has remained at about the same level since Bull Lake time. PINEDALE GLACIATION With a few exceptions glaciers of the Pinedale Glacia- tion were far less extensive than the Bull Lake glaciers. The glaciation occurred in three episodes, or stades, of ice advance and retreat. Of these, the first was by far the most extensive, and in a few drainages in the region it equaled and even exceeded the late Bull Lake glaciers. Glaciers of the second and third stades of the Pinedale were everywhere much smaller than those of the first stade, and in many of the smaller drainages, they were absent. Moraines of the Pinedale glaciers are hummocky, bouldery, and little modified. The till in them is sandy rather than clayey and, except as it might be colored by rocks, such as red beds, it is generally light gray in con- trast to the yellow, buff, or brown colors characteristic of the older tills in surface or near-surface exposures. Soils are only weakly developed on the Pinedale Till and, practically speaking, are absent in many places. The largest glaciers in the Minturn quadrangle dur- ing the Pinedale were in the valleys of Cross Creek and the Piney River. No glacier existed in the part of the Eagle Valley that is within the quadrangle, and Gore Creek Valley did not have a trunk glacier. An early Pinedale glacier did occupy the canyon of upper Gore Creek east of the quadrangle, but it terminated in the area between the mouths of Black Gore and Bighorn Creeks. Early Pinedale glaciers of Bighorn, Pitkin, and Booth Creeks reached the Gore Valley and deposited small moraines on its floor. Middle Pinedale moraines are a short distance up the canyons, and late Pinedale moraines are in the midportions of the canyons. On Cross Creek, unlike all other drainages in the quadrangle, glaciers of all three stades of the Pinedale descended to the terminal moraine area near the mouth of the creek. On the north side of this morainal area, the early Pinedale glacier formed a morainal ridge almost as high as the Bull Lake ridges. The ridge descends more rapidly than the Bull Lake ridges, however, and turns into a broad, low, complexly ridged terminal moraine near US. Highway 24 and the Eagle River. The middle Pinedale glacier was split near its terminus by the hill of rock in the center of the morainal area (pl. 1) and formed two morainal lobes within the terminal moraine horseshoe of the early Pinedale. The southern lobe rests on a low valley flat excavated out of part of the early Pinedale moraine. The late Pinedale glacier formed small morainal loops immediately south and west of the bedrock hill. In the Piney River valley, the early Pinedale glacier was as high on the valley wall as the Bull Lake glaciers near Piney and Lost Lakes. From about this place, however, it descended to a prominent southwestward- sloping morainal bench 1.5 mi (2.5 km) west of Piney Lake and formed a small terminal moraine on the floor of the Piney River valley just above the mouth of Dickson Creek. The small terminal moraines of the middle and late stades of the Pinedale are in the vicinity of Piney Lake. LAN DSLIDE The Minturn and Maroon Formations contain many incompetent shaly beds that make ideal surfaces for landsliding where the beds dip toward valleys. Addi- tionally, the weak and platy-weathering rocks of these formations readily form a heavy “slopewash” or col- luvium that creeps down the slopes and accumulates in slidelike piles in the basins or valleys. All the major valleys that cut the Minturn and Maroon Formations and many of the smaller ones have their slides splat- tered with landslides and colluvial accumulations. The larger slides are mainly of the dip-slope slide type, but some follow ground broken by faults, and many are on slopes oversteepened by glacial erosion. Most of the con- spicuous slides are postglacial in age, and some are very recent or modern. Incipient slides are evident in many places where open tension fissures as much as several feet wide and 20 ft (6 In) deep occur on hillsides that slope in the general direction of the dip. Such fissures (pl. 1) are especially common in a belt that extends from the head of Game Creek to the slopes south of Mill Creek and were probably caused by glacial oversteepening of Gore Valley. Clear evidence of recent movement was seen at the east end of a fissure that starts at the 10,600-ft con- tour on the nose extending north from a knob on the STRUCTURE 69 Game Creek—Gore Creek Divide 2.1 mi (3.38 km) N 78° E of the mouth of Game Creek. Turf extends across the eastern end of the slowly widening fissure, but a few feet to the west a bare root 2 in. (5 cm) thick from a pine tree growing on the north wall extends through the air horizontally into a crack in the south wall about 20 in. (50 cm) away. The pine was cut down in 1963 and the tree rings indicated an age of 68 years. The two walls of the fissure must have been next to each other when the root first crossed into the south wall, as otherwise the root would have grown vertically downward, if it grew at all. Either the walls have moved apart so gradually that the growth of the root has kept it from breaking as the separation proceeded or else the roots were strong enough to withstand the pull of faster movement that took place after the tree was partly grown. The data show that the walls have separated at a minimum rate of about one-third inch (8 mm) per year, and it is proba- ble that the separation of the walls began many years after the pine seed first sprouted. Elsewhere, as on the slopes south of Vail, many open fissures show evidence of very recent movement. The relation of the fissured area shown on the geologic map (pl. 1) to Vail indicates that the possibility of sudden mass movement of the ground on the northward-dip- ping bedding planes in this area should be carefully ap- praised. The large landslide at Whiskey Creek and another opposite Dowds constrict the valley of the Eagle River for about a mile downstream from Dowds. These slides forced the Eagle River against the northeast wall of its earlier valley, causing steep and cliffy slopes to be formed there. The construction of I—70 across these slides in 1969—70 induced much heaving and slumping of the slide material, indicating that the slides are still unstable. The Whiskey Creek slide overrides glacial outwash gravels that probably are as young as Pinedale; hence, it is probably postglacial in age. Farther west, near Stone Creek, gypsum has slid or flowed over these same gravels, reducing the width of the valley bottom from its former extent. ALLUVIUM Morainal areas in the valleys have many pockets and channels filled with alluvium or reworked glacial drift, and most of the stream courses are bordered by alluvial deposits. The landslide just above the mouth of Gore Creek and the terminal moraine at the mouth of Black Gore Creek both have formed natural dams in the past that impounded lakes above them. Delta deposits of crossbedded sand have formed in these lakes, and rem- nants of such deposits remain along the sides of the valleys. These deposits are included with “alluvium” on the map (pl. 1) and so also are terrace gravels and fanglomerates. STRUCTURE The Minturn quadrangle contains elements of three major structural units—the Gore Range uplift on the northeast, the Sawatch Range uplift on the southwest, and a broad, northwest-trending, generally synclinal area of sedimentary rocks between the two major uplifts. The Gore Range is a fault-block range bordered on its southwestern side by the Gore fault—or fault zone—the largest and most complex fault in the quad- rangle. The Sawatch Range, in contrast, is an anticlinal uplift of great size—90 mi (145 km) long and 40 mi (65 km) wide (fig. 24). The Minturn quadrangle includes only the eastern flank of the northward-plunging north end of the anticline. In this area, the flank of the anti- cline is disrupted only by minor faults, but several miles south of the quadrangle the eastern flank is disrupted by major graben faults of the upper Arkansas River valley (Tweto and Case, 1972). Structural development of both the Gore Range and the sedimentary basin to the west was closely in- fluenced by a long history of movement on the Gore fault. This fault was active in Precambrian, Paleozoic, Laramide (Late Cretaceous and early Tertiary), and late Tertiary times, and there is suggestive evidence of movement in Quaternary time. During the latter part of Paleozoic time, and perhaps intermittently earlier, the fault formed the western edge of a highland that ex- tended eastward beyond the crest of the present Front Range—the Front Range highland of the Ancestral Rockies. (See Lovering, 1929.) As a structural and topographic unit created out of a part of the old high- land, the Gore Range came into existence in Laramide time, but it was much modified and elevated as a fault block in late Tertiary time, accounting for its present relief (Tweto and others, 1970). The Gore fault was also the border of the basin in which the Belden, Minturn, and Maroon Formations ac- cumulated. Thus, it is not only a fault but also a zone of abrupt unconformity and wedgeout of sedimentary rock units. Folding or upturning of the sedimentary rocks along the fault may have begun in Pennsylvanian time. Folding that produced the present broadly synclinal structure of the sedimentary basin resulted from uplift of the Gore and Sawatch Ranges in Laramide time, but a minor part of it may have occurred in the late Terti- ary, inasmuch as the volcanic rocks north of the quad- rangle are synclinally folded. In the Sawatch Range area, no evidence has been found anywhere of uplift prior to formation of the Sawatch anticline in Laramide time. This anticline was 70 GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO 107° 106° 1 EXPLANATION ' __T__ Fm" M l D D L E It! and boll on downthrown aid. 93"” o Kremmlln; An‘lcllno Svnclino P A R K Fold: _ 40° Showing men of axial piano and dlmtion of plum of ui: o Falrplay SOUTH «,0 PARK 9 FIGURE 24. —Relation of Mintum quadrangle (outlined) to major structural features and Colorado mineral belt (patterned). STRUCTURE 71 wedgeout of thousands of feet of sedimentary rocks elevated early in the Laramide orogeny, before the Gore Range (Tweto, 1975). Rise of the anticline tilted the sedimentary rocks northeastward through much of the Minturn quadrangle, forming the flank of the central syncline. This uniform tilt, or homoclinal dip, still characterizes a large area east of the Eagle River and south of Gore Creek. North of Gore Creek and, especially, from near the mouth of Gore Creek west- ward, a more complicated structural pattern now exists. Near the western border of the quadrangle, at least, this pattern is related to diapiric movement of gypsum as a result of excavation of the valley of the Eagle River. Folds and faults in this area probably have been developing almost continuously from the Laramide to the present. ’ GORE FAULT The Gore fault delimits the western edge of the Pre— cambrian terrane of the Gore Range for a distance of at least 45 mi (72 km), from the Mosquito fault in the Ten- mile Range, several miles southeast of the Minturn quadrangle, to the Colorado River, 15 mi (24 km) north of the quadrangle (fig. 24). Through most of its course in the Minturn quadrangle the fault brings rocks of the Minturn Formation against Precambrian rocks, but in the north part of the quadrangle the Maroon Formation lies against the fault, and farther north various Mesozoic formations are against the fault. In most places the Gore fault is not a single fracture but a wide and complex fault zone. Most of the fractures in this zone are in the Precambrian rocks, on the north- east or upthrown side of the fault surface that sepa- rates the Precambrian and the sedimentary rocks, though strands of the fault are present also in the sedi- mentary rocks in places. Many of the fractures in the fault zone are of Precambrian age, as will be discussed further; others are younger, but it is difficult to establish the time of origin of most fractures. Main periods of later movement along the fault zone, whether on reactivated or newly formed faults, oc- curred in the late Paleozoic, the Laramide, and the late Tertiary. Precambrian origin of many of the fractures in the Gore fault zone is indicated by several lines of evidence (Tweto and Sims, 1963; Tweto and others, 1970): (1) the occurrence of Precambrian intrusive rocks such as peg- matite, aplite, and mafic diorite as dikes along the faults in places, or intruded into mylonitic rocks; (2) the presence of mylonitic rocks of Precambrian aspect beneath undeformed Pennsylvanian rocks and the oc- currence of cobbles of the mylonite in the Pennsylva- nian conglomerates; (3) the presence of intensely sheared rocks beneath much less deformed Devo- nian(?) quartzite; and (4) the occurrence of little deformed dike rock dated isotopically at 1 by. on a fault of the Gore system at the Colorado River (Barclay, 1968). Evidence of pre-Pennsylvanian Paleozoic movement along the Gore fault is shown by stratigraphic and fault relations. Of the pre-Pennsylvanian formations (table 1), the Harding Sandstone, the Dyer Dolomite, the Gil- man Sandstone, and the Leadville Limestone are ab- sent from upthrown fault blocks along the Gore fault. Along with the Manitou Dolomite of the area southeast of the quadrangle, they are concluded to have been ero- sionally truncated both in pre-Late Devonian and in pre-Pennsylvanian times and to wedge out beneath the Minturn Formation in a zone near and parallel to the Gore fault (Levering and Johnson, 1933; Tweto and others, 1970). The Sawatch Quartzite, Peerless Forma- tion, and Parting Formation reach the Gore fault (fig. 25), but the Sawatch is thinned to only 100 ft (30.5 m) beneath the Peerless, suggesting a possible “high" in the area of the Gore Range even in Cambrian time. The Peerless is only 20 ft (6 m) thick and tapers to a vanish- ing edge beneath the Parting Formation, indicating, along with absence of Ordovician rocks, extensive ero- sion before Late Devonian time. The Parting is excep- tionally coarse grained and conglomeratic wherever ex- posed along the Gore fault, suggesting a land area in the vicinity of the Gore Range in Late Devonian time. Fault relations suggest not only a land area but also active movement along the Gore fault zone in Paleozoic time. In the western of two fault blocks on the south slope of Bald Mountain (pl. 1; fig. 25), the Parting For- mation lies on Precambrian rocks and is overlain by coarse Pennsylvanian conglomerate with an angular discordance of 17°. In the eastern fault block, one-half mile (0.8 km) away and 500 ft (150 m) lower, the Part- ing is underlain by the Peerless and Sawatch and over- lain without angular discordance by strata of the Min- turn Formation. The relations suggest that a north- trending fault—part of the Gore fault system—between the two blocks was active prior to deposition of the Part- ing and, again, prior to deposition of the Minturn. Similarly, in the valley of Black Gore Creek one-half mile (0.8 km) from Gore Creek, white quartzite and quartz granule conglomerate thought to be Parting but possibly Sawatch is turned up almost vertically, whereas rocks of the Minturn Formation 100 ft (30 m) away dip gently, suggesting a marked angular uncon- formity. Major movement occurred on the Gore fault in Penn- sylvanian and Permian time, as indicated by the 72 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Z EXPLANATION Mlmurn Formation II, Ilmutona bod Pa rung Formation Purina Formntion 9 Sowuch Quartzito Prombrinn rocks Contact /5 _|_ Strike and dip of bod: Fault hr and ball on downthrown aid: 106° 20' J 0 1000 2000 3000 F E ET ; | | I I | | T O 300 600 900 MET H ES FIGURE 25.—-Geolog'ic sketch map and section of Gore fault zone on south slope of Bald Mountain. STRUCTURE 73 against Precambrian rocks in and along the Gore fault zone (pl 1, secs. A—A’, B-B’, E-E'). The occurrence of the Robinson Limestone Member of the Minturn For- mation only 50—100 ft (15*30 m) above Precambrian rocks—0r above thin patches of the Parting Formation lying on Precambrian rocks—on the mountain southeast of the junction of Gore and Black Gore Creeks has been noted in the discussion of the Minturn Formation. The best example of the abrupt wedgeout of the sedimentary rocks against the fault-scarp front of the old highland is in the Mount Powell quadrangle 3—4 mi (5—6.5 km) north of the Minturn quadrangle, where sedimentary rocks are' preserved east of the fault. There, the entire Minturn and all but the uppermost 100—300 ft (30~90 m) of the Maroon wedge out in a zone no more than 3—4 mi (5—6.5 km) wide along the Gore fault (Tweto and others, 1970). In this area, the fault may even have been active in Jurassic time, as suggested by a marked difference in thickness of the Morrison Formation on the two sides, but no evidence of a scarp in Late Triassic time is seen in the Chinle For- mation. Most of the movement that placed sedimentary rocks in fault contact with Precambrian rocks and that caused folding and overturning of the type shown in figures 25 and 26 is inferred to be of Laramide origin. However, it is difficult to separate with certainty the effects of Laramide fault movement from those of later Tertiary movements. Laramide deformation is proved more by regional geologic relations than by local ones, inasmuch as no sedimentary rocks younger than the Dakota are preserved within the quadrangle. On'the northeastern side of the Gore Range, conglomerates of late Miocene(?) age show by their abundant content of Precambrian rocks that the range had been uplifted and had been stripped of its cover of Morrison and younger sedimentary rocks by that time, which was prior to the marked uplift in late Tertiary time (Tweto and others, 1970). The uplift that led to stripping of the sedimentary cover was almost certainly the Laramide in this range just as in most other major ranges in C01- orado. Extensive late Tertiary uplift and fault movement in the Gore Range has been documented by Tweto, Bryant, and Williams (1970). Some of this movement may have occurred along the strand of the Gore fault that separates the sedimentary and the Precambrian rocks, but most of it occurred along faults of the Gore FIGURE 26.——Deformed strata of Minturn Formation in Gore fault zone at head of Spraddle Creek. View northwestward. One strand of fault lies in covered area between vertical strata of White Quail Limestone Member in center and inclined strata of Robinson Member and un- derlying Minturn rocks at right; main fault, which brings Precambrian rocks against the Minturn is just out of view to right. 74 GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO fault zone within the Precambrian rocks. In the upper Piney River area, the late movement occurred on the series of long parallel faults of north-northwest trend (pl. 1). These are reactivated Precambrian faults. In the Gore Creek area, the late movement occurred on the fault strand that lies a short distance east of the sedi- mentary rocks. This strand is thought to be of Laramide origin. From the north boundary of the quadrangle to Bald Mountain, the main strand of the Gore fault—separa- ting the sedimentary and crystalline rocks—trends south—southeastward in a fairly straight line. At Bald Mountain, this fault intersects a west-northwest-trend- ing fault zone in a complexly faulted area. The west- northwest-trending zone is a reactivated Precambrian fault zone that extends all the way across the range (pl. 1; Tweto and others, 1970) and is one of a family of per- sistent faults of this trend in the Gore Range. The line of displacement and upturning of sedimentary rocks turns eastward along the west-northwest-trending zone for about 2 mi (3 km) to beyond Booth Creek, and then it turns south-southeastward again. In the area of this bend, the fault is intersected by the faults with late dis- placement extending from the upper Piney River area. Southeast of the bend, an older unit of the Gore fault extends to a broad east-trending fault zone along Gore Creek, and the line of displacement jogs eastward along this zone. A younger strand of the fault—probably Laramide and Tertiary—cuts diagonally across the salient outlined by the two older faults in almost a straight line from Pitkin Creek to Gore Creek and beyond (pl. 1). In the area of the salient, near the mouths of Black Gore and Big Horn Creeks, other strands of the fault cut the sedimentary rocks. These are poorly delineated because of widespread cover of glacial deposits and uncertainties in distinguishing the effects of faulting as opposed to unconformity in an area with only scattered small outcrops. A branch of the Gore fault extends south-southeast- ward up Black Gore Creek and then southward along Timber Creek, ultimately connecting with faults of the Pando area (Tweto, 1953). Also. old strands of the fault in Precambrian rocks near the mouth of Black Gore Creek project beneath strata of the Minturn Formation on the slopes east of the creek. From Gore Creek, the main young strand of the Gore fault extends into the adjoining Dillon quadrangle, where it intersects sedi- mentary rocks once again and forms a boundary bet- ween these and the Precambrian rocks (Tweto and others, 1970). Most of the faults in the Gore fault zone are vertical or are steep normal faults (pl. 1, secs. A—A’, E—E’), and the zone as a whole is interpreted to be essentially ver- tical. However, reverse and even low—angle thrust faults are present locally. They probably formed in response to local stress conditions or in response to expansion of the Precambrian massif as it rose. Near Booth Creek, the fault strand between the Precambrian and the sedi- mentary rocks is a steep reverse fault that dips 80°—85° N. As shown in figure 25, section A—A’, this fault is in- ferred to steepen to vertical at depth; the northward dip near the surface is interpreted to reflect expansion of the upthrown block of Precambrian rocks. Near Middle Creek, low-angle reverse and thrust faults are present in a small area at the edge of the Gore fault zone (pl. 1, map and sec. A—A’). The reverse and thrust faults are in the tip of a wedge between southeast- and south- trending faults in the area where the main Gore fault begins to turn eastward. The south-trending fault has a displacement of several hundred feet in the Gore fault zone (pl. 1, sec. A—A') but only a minor displacement farther south. Exposures are too poor in the small area of thrust faulting to reveal details of the complex struc- ture there, but it is likely that the thrust fault is in a thin fault block underlain by an upward-steepening reverse fault as shown on plate 1 (map and sec. A—A '). GORE RANGE The Gore Range has two major categories of struc- tural features—the internal structure of early origin in the Precambrian rocks and later faults. The Pre- cambrian gneisses were highly deformed plastically and then were sundered by granite intrusion, forming a gigantic breccia of gneiss blocks in a matrix of granite (pl. 1). Thus, the gneisses are structurally disorganized, and no attempt was made to study them in detail. In general, a broken irregular tongue of gneisses extends south-southeast along the crest of the Gore Range to the head of Bighorn Creek. The Cross Creek Granite which surrounds and intrudes this mass of gneisses has a structure that is nearly concordant with the broad outline of the area of gneissic rocks. To the west and east the foliation or planar structure of the granite strikes north-northwest; south of the area of metamorphic rocks, the structure of the granite strikes northeast or easterly. The north-northwest foliation trend along the western side of the range is paralleled by fractures of the Gore fault zone and probably in- fluenced the trend of parts of the Gore fault. The many faults in the Precambrian rocks of the range are of two main orientations, north-northwest and nearly east—west. Faults of the set trending north- northwest are long and straight, and most of them dip almost vertically. The set that trends nearly east—west includes a few widely spaced persistent faults that ex~ tend across the entire range (Tweto and others, 1970) and many shorter faults that subdivide the long blocks STRUCTURE 75 outlined by the north-northwest-trending faults. Faults of'both sets are typically fracture zones several feet to a few hundred feet wide, though a few narrow locally to a single fault plane. Many of the fracture zones are altered, and in general, the zones are marked by gullies and saddles on the slopes and ridges of otherwise hard rock. The density of the fault pattern in the Pre- cambrian rocks contrasts markedly with the pattern of sparse faults in the bordering sedimentary terrane (pl. 1). This suggests that most of the faults in the range are Precambrian in age, as do other features noted in the discussion of the Gore fault. Many of the faults, however, were reactivated in late Paleozoic, Laramide, and late Tertiary time. One of the north- northwest—trending faults of the upper Piney River area, nearly 2 mi (3.2 km) east of the main Gore fault (pl. 1), contains in one area narrow slices of down- dragged red beds of the Minturn or Maroon Formation, indicating probable Laramide displacement of hundreds of feet. SAWATCH RANGE The part of the Sawatch Range included in the Min- turn quadrangle consists of a core of Precambrian rocks covered by a thin mantle of sedimentary rocks that dip northeastward off the range, nearly in dip slopes (pl. 1, sec. B—B'). As in the Gore Range, the Precambrian rocks have an early fold structure and a later fracture structure, and the main elements of the fracture structure predate the Paleozoic rocks. A major Precambrian structural feature, the Home- stake shear zone, lies just south of the quadrangle (fig. 24). This master zone, which trends northeast and con- sists of several individual shear zones in a belt 7—8 mi (11-13 km) wide, separates a metamorphic terrane to the southeast from the granitic terrane of Cross Creek Granite to the northwest (Tweto and Sims, 1963; Tweto, 1974). Fringe shear zones or faults of the Home- stake zone project into the Minturn quadrangle in the vicinity of Notch Mountain, Fall, and Peterson Creeks. They are exposed only locally, however, because of a widespread cover of Paleozoic rocks and glacial and col- luvial deposits. The strongest of these fracture zones extends northeastward for about a mile along the slope northwest of Notch Mountain Creek and then disap- pears beneath the Sawatch Quartzite (pl. 1). It is prob- ably represented in the canyon of the Eagle River by some of the northeast-trending faults and veins in the Precambrian rocks near Gilman. The Ben Butler mine (fig. 8), for example, is on small veins in or near a wide shear zone that resembles the Precambrian shear zones, and both the veins and the shear zone end abruptly upward against the smooth and unbroken basal bed of the Sawatch Quartzite. Similarly, the San- ta Cruz vein (fig. 8) is a wide and strong fracture zone in the Precambrian rocks but barely affects the overly- ing Sawatch Quartzite. These fracture zones and mines are described in the report on the Gilman district (Lovering and others, 1977). Among faults of Laramide or younger age in this part of the Sawatch Range, the largest are in the area north of the latitude of Minturn, at the north end of the Pre- cambrian core of the range. A prominent fault of east— northeast trend, downthrown to the north, extends from the Eagle River at the mouth of West Grouse Creek, to a fault along Stone Creek; west of Stone Creek, an en echelon fault of the same trend and dis- placement extends west-southwestward out of the quadrangle at least 2 mi (3 km). The two echelon faults are north-dipping normal faults and have displace- ments of 200—300 ft (60—90 m). No sign of the eastern ' fault could be found east of the Eagle River at Game Creek; the fault is inferred to end against a fault along the river. Evidence of a northwest-trending fault, upthrown to the northeast, along the river is seen in the repetition of the Gilman Sandstone on the two sides of the river at Minturn, and in the position of the dolomite bed of Dowds near Dowds (pl. 1, sec. B-B‘). The fault along Stone Creek, very near the western border of the quadrangle, trends north-northeast and is upthrown about 250 ft (76 m) on its southeastern side. Its course along lower Stone Creek is uncertain because of slumping of shale and gypsiferous strata in the Belden and Minturn Formations. In the area of the lower Stone Creek and Whiskey Creek, strikes and dips of the strata change erratically in short distances. A thrust fault that brings grits of the lower part of the Minturn Formation over gypsiferous strata cuts through this area (pl. 1) . This fault probably is not a fundamental tectonic element of the Sawatch Range but is primarily a product of deformation and mass slumping at the edge of the gypsum basin. A fault along Whiskey Creek may be of the same origin. This fault is exposed only in a zone of vertical strata on the bank of the Eagle River'lt projects southward beneath the landslide along Whiskey Creek but is not evident in the bedrock at the head of the slide. Neither is it seen to the north, across the Eagle River. There, it is inferred to end against east-west faults in a small area of land- slide. CENTRAL SEDIMENTARY BELT The broad belt of sedimentary rocks between the Gore and Sawatch Ranges is predominantly synclinal in structure. Three northwest- to north-trending syn- clines, which are arranged echelon in a northwest- trending line, dominate the area structurally, though 76 GEOLOGY, MIN TURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO other folds are present. The southeastern syncline, called the Black Gore syncline (pl. 1), closely parallels the Gore fault in the area from upper Mill Creek to Turkey Creek. The middle syncline, called the Vail syn- cline, extends from the Spraddle Creek area southward to the Two Elk Creek drainage. The northwestern and largest syncline, called the Red and White syncline, ex- tends from lower Buffer Creek northwest to and beyond Red and White Mountain. As shown in the cross sec- tions (pl. 1), these synclines are more pronounced at depth than at the surface because of the thinning of all the Paleozoic formations—and particularly the Min- turn Formation—toward the Gore Range. The Black Gore syncline (pl. 1, sec. E—E') is markedly asymmetric, with a wide, gently dipping southwestern limb and a narrow and steeper northeastern limb. Though interrupted by minor flexures, the south- western limb is essentially homoclinal and is a part of the flank of the Sawatch anticline. The northeastern limb is a Gore Range structure and in part is due to drag along the Gore fault. As expressed in the rocks ex- posed at the surface—high in the Minturn Formation— the axis of the syncline is sinuous parallel to the Gore fault and lies less than a mile from the fault. As seen in cross section, however, the main synclinal axis is a mile farther southwest, owing to the thickening of the Min- turn Formation in that direction. The Black Gore syn- cline dies out on the ridge between Mill and Gore Creeks, and it is overlapped on the west by the Vail syn- cline. The Vail syncline (pl. 1) is a bowed, north-trending doubly plunging syncline that is prominently exposed on the sides of the valley of Gore Creek at Vail. The syn- cline is longitudinally faulted, and north of Gore Creek the east limb is turned up steeply against the fault (pl. 1). The northern part of the syncline, north of Gore Creek, is bounded on the west by a small anticline cen- tered over Middle Creek. South of Gore Creek, the west limb is the homoclinal flank of the Sawatch anticline. The fault zone that extends the length of the Vail syncline and beyond is called the Spraddle Creek fault zone. This fault zone extends south-southwestward from the Gore fault zone in the Bald Mountain—Sprad- dle Creek area nearly to Gilman. In the Bald Mountain area it is part of a complex of fault blocks where the Gore fault turns eastward. In this area it shows wide differences in amount of displacement, from as little as 100 ft (30 m) to more than 900 ft (275 m), reflecting both the differential movement of fault blocks and probable pre-Pennsylvanian and Pennsylvanian move- ments. From the head of Spraddle Creek, the fault ex- tends southwestward into the eastern flank of the Vail syncline, separating steeply dipping beds of the syn- clinal flank from gently dipping beds to the east. At Gore Creek the fault bends southward, slicing across the eastern flank of the Vail syncline to Two Elk Creek. Farther south, a series of short en echelon faults in the Minturn Formation suggests that the zone of deforma- tion in the basement rocks persists to the vicinity of Gilman and might even project to fractures of the Homestake shear zone. Through most of its length, the Spraddle Creek fault is downthrown to the east, and from Gore Creek southward the displacements are less than 100 ft (30 m). The Red and White syncline is a large and nearly symmetric syncline that occupies most of the area be- tween the mouth of Gore Creek and the Piney River (pl. 1, sec. B—B’). In much of this area, rocks of the Maroon Formation—and also of the Chinle and younger formations in a small area on Red and White Mountain—are disturbed by many gentle flexures; thus, the syncline is scarcely evident from the attitudes of the strata as observed on the surface. The southeastern nose of the syncline is blunt and is an abrupt northwest-dipping monocline (pl. 1, sec. C— C') that extends along the north side of Gore Creek from the Eagle River to Red Sandstone Creek where it flattens somewhat and turns northward and then northwestward. On the spur north of Dowds, east—west faults with as much as 1,000 ft (305 m) of displacement increase the structural displacement along the monocline, inasmuch as they are downthrown to the north. In effect, the curving monocline or synclinal nose separates a southern area that is structurally a part of the flank of the Sawatch Range from a northern area that is part of a large structural basin in the area [bordered by the Sawatch Range, White River Plateau, and northern Gore Range (figs. 1, 24). In the Vicinity of Dickson Creek, a small but sharp north-trending anticline—the Dickson anticline—is superposed on the lower northeastern flank of the Red and White syncline (pl. 1, sec. A—A’). The Dickson anticline enlarges northward and is a major structural feature along the Piney River near the quadrangle boundary (pl. 1, sec. D—D’). The anticline is cut acutely by two northwest-trending faults that have opposite displacements. The faults define a long narrow horst that is upthrown about 200 ft (60 m) on the northeastern side and nearly 1,000 ft (305 m) on the southwestern side (pl. 1, sec. D—D’). The area between the Dickson anticline and the Gore fault is occupied by the East Meadow anticline, which trends east, almost at right angles to the Dickson anticline (pl. 1, map and sec. B—B’). At the intersection of the two anticlines, on the slope southwest of the mouth of Meadow Creek, the tight nose of the East TYPE SECTION 79 1970, p. 91) the mineralized part is less than 100 ft2 (9 m2) in area and is not of itself of commercial signifi- cance. The geochemical anomalies and most of the vein material found along the faults in the Gore Range were concluded by Tweto, Bryant, and Williams (1970) to be products of metal—bearing solutions that passed through the fracture system in the Precambrian rocks enroute either to hot springs at the surface or to mineral deposits in sedimentary rocks now eroded away. In part, at least, these solutions were introduced into the fracture system after the late Tertiary uplift of the Gore Range; thus, they reflect a mineralization epoch younger than those of major mining districts nearby. However, some part of the mineralization and most of the alteration probably are products of earlier Tertiary or Laramide hydrothermal activity. In the part of the Sawatch Range included within the quadrangle, little evidence of mineralization is seen. Jasperoid that has replaced carbonate rocks of the Dyer and Leadville Dolomites is present near Minturn, as indicated on plate 1, but it is nearly barren of metals other than iron (T. G. Lovering, 1972, p. 79—81). In the area southwest of Gilman, where the carbonate rocks are no longer preserved, short veins of jaspery quartzite breccia or of hematitic breccia occur in the Sawatch Quartzite in several places. As judged from the small size of prospect diggings on many of these veins, the jaspery and hematitic materials are barren of metal values in the commercial range, but, to our knowledge, they have not been tested geochemically for trace metals. Except in the Gilman district, no evidence of mineralization was observed in the Precambrian rocks of the Sawatch Range. The rapid urbanization of the valley of Gore Creek has created a large demand for sand and gravel. The quadrangle is not well endowed with these materials. Because of the urbanization, deposits in the valley of Gore Creek are eliminated from availability. Other stream valleys in the sedimentary terrane contain little gravel, and what exists is of poor quality because it is derived from weak and inhomogeneous sedimentary rocks. The only large and readily accessible potential source of sand and gravel of good quality is in the moraines near the mouth of Cross Creek. These moraines consist almost entirely of materials derived from Precambrian rocks; though containing boulders, they could become a source of sand and aggregate of good quality. Moraines in the valley of the Piney River area are also potential sources of sand and gravel. However, these moraines contain a fraction of red sedimentary rocks from the Minturn and Maroon Formations; hence, they might not be as suitable for aggregate as the moraines of Cross Creek. They are also far less accessible than those of Cross Creek. TYPE SECTION Type section of the Minturn Formation [Section begins at small knob at elevation 11,500 ft on ridge between Mill Creek and Two Elk Creek, midway between the mountains with elevations 1 1,820 and 1 1,223 ft . approximately sec. 22, unsurveyed T. 5 S., R. 80 W., Minturn 15-minute quadrangle, 1934 edition (pl. 1). Section measured southward down spur toward Two Elk Creek. Section is successively offset, as indicated, and ends at intersection of US. Highway 24 and Rock Creek, 0.25 mi (400 in) north of Gilman, approximately SE‘I. sec. 13, unsurveyed T. 6 S., R. 81 W. Measured by T. S. Lovering, 1963] Thickness Distance (feet) above base (feet) Maroon Formation: Sandstone and siltstone, highly micaceous, thin-bedded; weather medium reddish gray. Conformable contact ......................... Minturn Formation: J acQue Mountain Limestone Member: 135. Limestone. Upper part contains oolitic beds that grade laterally into mottled light- and dark-gray pseudoconglomerate consisting of algal nodules in lighter limestone matrix; cephalopods and fragmentary or poorly preserved brachiopods present but uncommon. Middle part is light gray, medium grained, and medium bedded. Lower 10 ft is thin bedded and irregular bedded, fine grained, and mottled pale pinkish gray and pale green. Pink color cuts across bedding ...... 31 Clastic unit H: 134. Sandstone, siltstone, and shale. Sandstone and siltstone are red, weather pale grayish red; many sandstone beds are crossbedded; siltstone is micaceous and platy. Shale is light green, weathers green and medium grayish red; breaks in fissile chips ...................... 330 133. Shale, red and green; and minor interbedded red sandstone and sfltstone ......................... 35 132. Conglomeratic grit in massive resistant bed ....... ' ....................... 1 2 131. Grit and interbedded sandstone and minor shale, medium- to dark- grayish-red; some green mottling on fresh fracture; is thin to medium bedded. Grit is calcareous .......... 108 130.Sandmone,darkqed;hassonw interbedded calcareous siltstone and l/-2.-in. beds of silty limestone ....... 25 129. Conglomeratic grit and interbedded sandstone and siltstone. Grit is light- greenish-gray; weathers light pinkish gray; is thin bedded; contains abundant 1. to 2-in. pebbles of quartz and fel'sitic Precambrian rocks and some pebbles of fresh mafic rock; grit is highly arkosic and some is calcareous. Siltstone is micaceous . and greenish gray ................. 100 128. Conglomerate; consists of pebbles, 4- to 6-in. cobbles and ofa few 12—in. 6.308 6,277 5,947 5.912 5,900 5,792 5,767 5,667 80 Minturn Formation — Continued Clastic unit H —- Continued GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Type section of the Minturn Formation — Continued (feet) 1 28. Conglomerate — Continued boulders of Precambrian rocks in green-gray grit matrix ............. 127. Grit and conglomeratic grit, and minor interbedded sandstone and siltstone, as in unit 128 ..................... 126. Limestone, moderate- to pale-red; has - light-green spots; weathers light pinkish gray; is thin bedded, flaggy, and medium grained; contains abundant muscovite throughout, and upper layers also contain abundant medium- to fine-grained biotite and pink feldspar ..................... 3 125. Siltstone, sandstone, and shale, interbedded. Siltstone and shale are light green and weather reddish gray; sandstone is medium grayish red; sandstone and siltstone are very thin bedded to thin bedded; weather in flaggy to platy ledges; most beds are limy; some sandstones are crossbedded. Thin—bedded conglomerate with pebbles 1—2 in. in diameter at‘ 40 ft above base ....... 124. Grit and sandstone, interbedded, light- pinkish—gray; weather medium to dark grayish red. Grits are about half feldspar and half quartz; contain scattered quartz and pegmatite pebbles 1‘/2 in. in maximum size; are calcareous. Conglomeratic grit bed near top of unit contains limestone fragments and grades laterally into limestone and calcareous grit. In lower middle part, unit is interbedded grayish—red and grayish- green siltstone and moderate-red sandstone. Near base, unit is moderate-red to grayish-red, fine- to medium-grained thin-bedded sandstone; is calcareous; contains fresh mafic minerals grains ........ 123. Shaly siltstone, sandstone, and grit, interbedded. Siltstone weathers medium grayish green. The sandstone weathers light brownish gray. Sandstones are medium to coarse grained, some grades into grit; consists of quartz and mica with some feldspar and mafic minerals. Both grit and siltstone contain abundant feldspar in a green, probably chloritic, matrix. Grit predominates in the interval from 10 to 50 ft above the base of unit. White Quail Limestone Member: 122. Dolomite, light-pinkish-gray, dense, thin-bedded; top 6-in. layer is reefoid, porous; weathers yellowbrown ..... 4 35 115 73 180 70 Thickness Distance above base (few) 5,632 5,517 5,514 5,441 5,261 5,191 5,187 Type section of the Minturn Formation — Continued Minturn Formation — Continued White Quail Limestone ——- Continued 121. Limestone, medium~bluish-gray; weathers medium light gray, except light bluish gray at top; is fine grained, medium crystalline, and medium to thin bedded; Locally oolitic; where oolitic, is light brownish gray. Weathers in slightly rounded slabby blocks; forms low ledge ............................ 8 120. Gritty limestone and interbedded calcareous grit in about equal proportions. Limestone is medium blue gray; weathers medium light blue gray; contains feldspar, quartz, and mica; grades into grit by increase in elastic material. Both grit and limestone are crossbedded ...... 119. Limestone, mottled dark-gray and brownish-gray; weathers medium light bluish gray; is medium to fine crystalline and thin bedded; forms massive low cliff .................. 118. Covered. Probably interbedded shale, shaly limestone, and thin-bedded grit .............................. 10 117. Limestone, dark-gray; weathers medium bluish gray; is thin bedded 5 Thickness Distance (feet) above base (feet) 5,179 10 5,169 14 5,155 5,145 5,140 [Section is offset 1 mile west on White Quail Limestone Member to spur extending south- southwest from 11,223-ft mountain on divide between Two Elk and Mill Creeks. Section continues south from this point down nose to base of basal limestone bed of Robinson Member, about 120 ft (37 m) above Two Elk Creek] Clastic unit G: 116. Sandstone, arkosic grit, and shale, interbedded. Sandstone is light greenish gray; weathers grayish green; is very thin bedded to thin bedded, fine to medium grained, micaceous, and arkosic; is dolomitic in lower part. Grit is pale greenish gray; weathers pinkish gray; has abundant fresh pink feldspar in fragments as much as 1/2 in. in diameter ......................... Elk Ridge Limestone Member: 115. Limestone, light-gray to dark-blue- gray; weathers light to medium blue gray; is fine grained to dense, thin to medium bedded, except thick bedded at base; some bedding in lower part distorted by domed algal structures. Uppermost bed is black, dark gray weathering. At 20 ft above base, is 8- in. dolomite bed; weathers brownish gray; has bunched parallel grooves on bedding planes. Clastic unit F: 114. Quartz grit, light-gray, well-sorted; consists almost entirely of gritty quartz grains in calcareous cement 8 131 5,009 ................ 30 4,979 4,971 Type section of the Minturn Formation —— Continued Minturn Formation — Continued Clastic unit F — Continued 113. Sandstone, grit, and shale, interbedded. Grit predominates in upper third of unit; is light pinkish gray; contains abundant pink feldspar. Sandstone is light yellowish gray to light greenish gray, thin bedded to very thin bedded, fine to medium grained, except that it contains coarse mica; lowest bed contains abundant plant material ......................... 112. Shaly siltstone and shale. Dark-gray shale in upper part grades downward into shaly siltstone that weathers dark gray to brownish red and to platy to fissile chips. Grades upward into dark shale ................... 111. Conglomeratic grit and gritty sandstone, pale-greenish-gray; weathers pinkish gray to light red; is very thin bedded and micaceous . . . . 110. Dolomite, pale-bluish-gray; weathers pale brownish gray; is medium bedded, medium grained, and fossiliferous ...................... 5 109. Shaly siltstone and arkosic sandstone, light-yellowish-g'ray to light-pinkish: gray, thin-bedded; at 30 ft above base is 15-ft ledge of coarse-grained crossbedded sandstone containing abundant muscovite flakes and angular quartz fragments as much as 1/2 in. in diameter. Unit forms sandy slope covered with thin platy fragments of the sandstone; coarse pebbles and cobbles about 120 ft above the base indicate a concealed conglomerate ..................... Robinson Limestone Member: 108. Limestone, medium- and irregular- bedded; weathers light blue gray blotched with irregular yellow-brown areas of argillaceous material. Unit is poorly exposed; forms low terracelike change of slope ......... 5 107. Mostly covered but sparse outcrops show interbedded grits, conglomeratic grits, and yellowish- gray shaly siltstone ............... 106. Limestone, light-blue gray, fine- grained, thin—bedded. Bedding is very irregular; contains abundant fossils and fossil fragments; fossils recrystallized to pinkish-white coarse calcite. Unit crops out in sporadic low ledges ................ 15 105. Covered, probably sandstone and siltstone ......................... 104. Limestone, medium- to light-gray; weathers light bluish gray and is medium to thick bedded; contains 115 145 90 190 125 65 TYPE SECTION Thickness Distance (feet) above base (feet) 4.856 4.711 4,621 4,616 4,426 4,421 4,296 4,281 4,216 81 Type section of the Minturn Formation — Continued Minturn Formation — Continued Thickness Distance Robinson Limestone Member — Continued (rm) ”3;?“ 104. Limestone — Continued abundant fossils. In middle upper part, fossils dolomitized and yellowish and pinkish gray, giving rock a mottled appearance. Productids are abundant in lower layers. Upper part of unit forms smoothly rounded cliff 10—15 ft high 103. Shale and micaceous siltstone, interbedded, yellowish- to pinkish- gray. Upper part of unit is concealed in covered slope. Fault repeats 50 ft of section, and thickness of unit is corrected accordingly ............. 102. Limestone and subordinate dolomite, Upper 8—10 ft is dolomite with reef structure, medium yellowish gray; weathers brownish gray; is medium coarse grained, thin bedded, and slabby; forms ledge 8 ft high. Remainder is light-bluishgray, medium< to thick-bedded limestone; has nodular structure ............. 101. Sandstone and siltstone, shaly, yellowish- to pinkishgray .......... 100. Dolomite, light-gray; weathers medium brownish gray; is thick bedded, medium crystalline; weathers in low rounded ledge .................... 8 99. Micaceous sandstone and siltstone, interbedded, light-greenish—gray; weathers light yellowish gray to light pinkish gray. Sandstone is medium grained, moderately even grained, and thin to medium bedded; contains abundant mica and argillized plagioclase grains. Unit is poorly exposed .......................... 98. Limestone and dolomite, reefoid. Limestone is light brownish gray; weathers medium blue gray; is thin to thick bedded. Dolomite is granular, mostly medium grained but locally coarse grained; vuggy at base. Top of reef forms cliff 10~30 ft high ............................. 40 4,176 150 4,026 43 3,983 80 3,903 3,895 150 3,745 65 3,680 [Section is offset 4 miles northwest on limestones of Robinson Member to point at elevation 9,650 ft on ridge between Game and Gore Creeks. At this locality. limestone of unit 98 is 120 ft thick. Section continues southwest into valley of Game Creek and alongjeep road in valleyl Clastic unit E: 97. Sandstone, siltstone, and minor dolomite. Sandstone and intergrading siltstone are greenish gray; weather medium yellowish brown; are thin bedded, arkosic, and micaceous; many beds are dolomitic; some contain abundant mafic grains. Dolomite is brownish gray; weathers medium yellow; is fine grained; Minturn Formation — Continued GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTHB, COLORADO Type section of the Minturn Formation — Continued Clastic unit E — Continued we” 97. Sandstone, siltstone, and minor dolomite — Continued ‘ . contains fine quartz grains; occurs in scattered thin beds. Unit forms smooth slope; exposed only in jeep trail ............................. 96. Dolomite, medium-blue-gray; weathers light brownish gray; is very finely crystalline and thin to medium bedded ........................... 6 95. Sandstone, conglomeratic grit, shaly siltstone, and shale. Unit is mostly greenish-gray, thin- to medium- bedded arkosic sandstone, with minor interbedded siltstone, shale, and conglomeratic grit. Mest of unit weathers gray to light greenish gray. Many of sandstone beds grade into arkosic grit; some appear to be gypsiferous; none are calcareous; all weather readily. Conglomeratic grit contains pebbles 1/2 in. or less in diameter and local lenses of conglomerate. Shale is dark gray to greenish gray and finely micaceous. Siltstone is greenish gray to medium green, shaly, highly micaceous, and chloritic. Beds range in thickness from a few inches to a few feet and alternate in a random way. Unit weathers to a smooth slope and is not well exposed ...................... 94. Probable equivalent of Hornsilver Dolomite Member of Pando area. Dolomite, grit, sandstone, and shale. Upper part is sandy dolomite grading into dolomitic sandstone; is very micaceous; weathers orange brown; breaks into angular blocks and slabs. Middle part is interbedded conglomeratic grit and dolomitic sandstone; weathers brown speckled with orange-brown spots; is strongly crossbedded. Gritty dolomite at base is overlain by black shale with interbedded black dolomite in thin beds, up to 10 ft above base ........ 93. Sandstone and grit (80 percent), shaly siltstone (10 percent), shale and dolomite (10 percent). Unit weathers pale yellowish brown to light orange brown and is thin to medium bedded. Unit has a few lenses of quartz- pebble conglomerate; pebbles are mostly less than 1 in. in diameter, but a few are as much as 2 in. Unit forms slope with a few ledges of sandstone cropping out ............ 92. Shale, shaly siltstone, sandstone, and grit. Shale is dark gray; weathers medium gray. Siltstone is gray; weathers medium gray to brown and 234 530 54 70 Thickness Distance above base (feet) 3,446 3,440 2,910 2,856 2,786 Type section of the Minturn Formation —— Continued Minturn Formation — Continued Thickness Distance Clastic unit E — Continued (few ”$55)” 92. Shale, shaly siltstone, sandstone, and grit -— Continued light brown. Both shales and siltstones contain abundant plant remains. Sandstone and gritty sandstone weather light bluish gray to light yellowish gray; are thin to medium bedded, and contain scattered plant remains. Thin sandstone beds have current ripple marks ........................... 91. Grit, conglomeratic, gray to brownish- gray, medium- to thick-bedded, unevenly calcareous. Pebbles are chiefly quartz .................... 90. Siltstone, shale, and gritty sandstone, interbedded. Siltstone is micaceous, very fine grained, and grades into shale. Sandstone is arkosic and light green; weathers medium grayish green; is thin to medium bedded and medium grained to gritty. Unit has 1~ ft bed of sandy calcareous dolomite about 30 ft above base. Unit forms smooth slope broken by ledgy layers of sandstone ...................... 89. Sandstone, dolomitic and gritty, yellowish-gray mottled with light yellowish-brown, medium-bedded . . . 5 88. Grit, conglomeratic; weathers pale brownish gray to pale gray; is poorly cemented and poorly sorted; is medium to thick bedded; contains scattered pebbles 1/2 —3 in. in diameter that include wide variety of Precambrian rocks. Unit forms covered slope with sporadic outcrops in rounded forms .................. 87. Siltstone and gritty sandstone, arkosic; weather brownish gray to light yellowish gray; are thin bedded to very thin bedded; contain abundant plant remains .................... 6 86. Probable equivalent of Wearyman Dolomite Member of Pando area. Shale (80 percent), silty dolomite (10 percent), and micaceous sandstone (10 percent). Shale is dark gray; weathers light gray; is micaceous. Dolomite is gray; weathers brown; occurs as beds 1-3 in. thick. Sandstone is greenish gray; weathers brown; micaceous ................. 60 2,726 15 2,711 55 2,656 2,651 94 2,557 2,551 27 2,524 [Section is offset about 1.4 miles south on contact between probable Wearyman equivalent and top of underlying grit marker bed (pl. 1) to point N. 6° E. of Minturn Ranger Station, at top of highest bold cliffs at approximate elevation of 9,975 ft. Section continues southward down cliffs] Clastic unit D: 85. Grit marker bed. Conglomerate and grit are light gray to pale green; TYPE SECTION 83 Type section of the Minturn Formation — Continued Type section of the Minturn Formation —- Continued Mintum Formation — Continued Thickness Distance Minturn Formation — Continued Thu-linen Distance Clastic unit D — Continued ““0 “50;: fife Clastic unit 1) _ Continued (feet) “7:25” 85. Grit marker bed —— Continued 2 81. Grit and sandstone -—- Continued weather pale pinkish gray to pale thick bedded with a few greenish gray; are medium bedded nonpersistent layers of very thin and crossbedded; alternate layers of bedded micaceous sandstone in fine- and coarse—grained upper half. Grit is conglomeratic, conglomerate and interbedded grit poorly cemented, and slightly give banded appearance. Most dolomitic, Pebbles and cobbles in the pebbles are less than 3 in. across, but grit are as much as 8 in. across, some are as much as 1 ft; are mostly erratically distributed, subangular, granitic and metamorphic rocks, and consist of Precambrian rocks subangular to subrounded, tending and minor green micaceous arkosic toward flat ellipsoidal shapes. Unit sandstone and brown-weathering forms highest sheer cliff on upper dolomite. In profile unit forms high slope of Eagle Valley east of Minturn 193 2,331 cliff with rounded forms ........... 101 2,124 84. Sandstone, grit, and minor interbedded 80. Grit, sandstone, and shale. Grit is pale siltstone and shale. Sandstone and greenish gray; weathers light grit are arkosic, micaceous, reddish greenish gray ; CODSiStS 0f feldspar, green; weather light brownish gray quartz, and abundant mica; is thin to light greenish gray; are mostly bedded and flaggy. Sandstone is thin bedded and slabby or flaggy. coarse to medium grained. Shale is Shale is dark olive gray and fissile. medium greenish gray; weathers There are ripple marks about 3 in. light grayish green; is micaceous and from crest to crest at 27 ft above base laminated; weathers to fissile chips 55 2,069 of unit ........................... 33 2,298 79. Dolomite, micaceous, conglomeratic, 83, Sandstone, grit, siltstone, and medium-pinkish-gray; weathers light conglomerate, interbedded; are light yellowish brown to medium orange green; weather pale greenish gray; brown; is medium bedded and slabby; are thin to thick bedded. Many beds contains subrounded to subangular contain coarse and poorly sorted pebbles of Precambrian rocks 1/2 ~63 fresh feldspar grains in a slightly in. across ......................... 2 2.067 dolomitic matrix 0f pyritic and 78. Grit, pale-bluish-gray; weathers pale micaceous sandstone. Some yellowish gray to pale orangish gray; sandstone and grit bEdS show is arkosic, poorly cemented, and very penecontemporaneous slumping and thick bedded; has grains 2—8 mm in minor faulting. Conglomerates diameter; contains shales in small contain cobbles as much as 8 in. lenses both parallel to bedding and across; most conglomerate layers are crosscutting at steep angle; steep nonpersistent and fill 01131111915. lenses are 1 in. thick and 6—10 in. Persistent conglomerate bed 2ft long ............................. 23 2,044 above base Of ‘jmlt is overlain by very 77. Grit (80 percent), shale (15 percent), thin. beddedmIcaceous sandstone or and dolomite (5 percent), lammatedsdtstone . . I ............ 55 2,242 interbedded. Grit is light gray; 82' SllFStone’ gr It‘ and dolomite, weathers pale greenish gray to light interbedded, thm: to medmm- yellowish gray; is conglomeratic and bedded. Siltstone ls pale green; dolomitic, medium bedded; contains weathers pale greenish gray; 1s shaly, quartz pebbles as much as 3 in. in coarsely micaceous;.contains diameter. Shale is dark gray; abundant grit. Gm 1slam1nated; weathers light gray; is laminated and consrsts of coarse quartz grains in fissile. Dolomite is dark-gray; Chlont,” and micaceous matrix. weathers medium brown; is fine to P°1°mltels medium gray; weathers medium grained; is thin to medium light orange brown; ranges from bedded and flaggy; occurs as dolomitic sandstone through sanfly lenticular beds ................... 90 1,954 dolomite to conglomeratic dolomite 76 Grit shale and sandstone with lenticular reefy masses of ' . ’ ' ’. dolomite . . . . 17 2.225 interbedded, pale-yellowish-gray, """"""""""" thin. to thick-bedded. Grit lS conglomeratic and calcareous; [Section is offset 1,000 ft north on unit 82 and continues down cum] contains pebbles as much as 8 in. across; is in well-defined beds with 81. Grit and sandstone, pale-pinkish-gray irregular but nearly parallel tops and to pale-yellowish-gray. Unit is mostly bottoms; grades into very thin 84 GEOLOGY, MIN TURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Type section of the Minturn Formation — Continued Type section of the Minturn Formation ~ Continued Minturn Formation — Continued Thickness Distance Minturn Formation — Continued Thickness Distance Clastic unit D —- Continued (feet) above base Clastic unit D f Continued (feet) above base 76. Grit, shale and sandstone — Continued (few 69. Grit and sandstone, light-gray to nearly (feet) bedded micaceous sandstone at top white; weather pale brownish gray to of unit. Shale layers are thickest in medium light grain; is thin to very middle of unit .................... 27 1,927 thick bedded, coarsely crossbedded, 75. Shale, very dark gray; weathers gray; poorly sorted, and poorly cemented. contains minor interbedded Some beds are dolomitic; are light brownish-gray thin-bedded brown and cavernous where micaceous grit and siltstone. Unit is weathered. Grit contains pebbles of mostly covered ................... 23 1,904 quartz, Precambrian rock, and 74. Grit and conglomerate, with siltstone dolomite """""""""""""""""" 28 1333 and shale partings. Some layers are 68. Shale (70 percent) and dolomite (30 dolomitic, changing on strike to percent), interbedded. Shale is dark gritty dolomite that pinches and gray; weathers light to dark gray; ls swells in thickness from a few inches laminated to very thin bedded, and to 2 ft, Unit weathers light greenish dolomitic near base. Dolomite m gray to light grayish brown; is platy upper part is dark gray; weathers to slabby; forms cliff. Upper part light yellow brown to light orange contains quartz pebbles 1/2 _3 in. brown; is thin to medium bedded, across in matrix of poorly sorted sand dense, and almost lithographic; and grit .......................... 33 1,871 middle Part is medium brownish 73. Conglomerate, light-brownish-gray to gray: and $1“ny conglomeratic; ls light-greenish-gray, very thick plnk,15h gay’ micaceous, and . bedded, poorly cemented, poorly medium fine grained near base. Unit sorted. Pebbles are subrounded to forms ledgy slope, mostly covered . . . 95 1,538 subangular, mostly of Precambrian Clastic unit C: rocks but a few of dolomite; matrix is 67. Reef dolomite of Lionshead, dark-gray; coarse calcareous grit. Unit forms weathers medium brown; is fine to cliff pocked by caverns several feet medium grained, medium to thick high along vertical joints .......... 46 1,825 bedded, and slabby or flaggy; 72. Grit (60 percent), shale (25 percent), - contains poorly preserved fossils; is and dolomite (15 percent), irregularly vuggy, With calcite interbedded. Unit is thin to medium crystals in the was At 20-35 ft ‘ bedded. Grit is greenish gray to above base Conglomeratic grit butts brownish gray; weathers pale brown; against side 0f reefin steep bUt contains brown-weathering sedimentary contact; dolomite above carbonate grains. Shale is light gray. the grit contains abundant quartz Dolomite is dark gray to brownish pebbles. Upper surface 0f reefis gray; weathers medium brown; is irregular ------------------------- 48 L490 fine grained and gritty ............ 36 1,789 66. Sandstone, siltstone, and grit, 71. Grit and conglomerate with a few shale interbedded. Sandstone and grit are partings. Unit is pale greenish gray hght greenish gray to pale red; to light brownish gray and thin to weather grayish brown F0 orange thick bedded. Conglomerate is poorly brown and, locally, to bright red; are sorted; has pebbles 2—6 in. across in calcareous, dolomitic, thm t9 grit matrix. Unit forms prominent medium bedded, and flaggy; ‘5 cliff 50 ft high .................... 54 1,735 EFOSSbedded in middle Ohm“- 70. Shale, siltstone, grit, conglomerate, and :iltitonedis dir: ggeenish grayhand dolomite, interbedded. Shale is dark us y re [,0 1g t rown, weat ers . . dark greenish gray, medium reddish gray; weathers medium gray; is , . laminated and fissile. Siltstone and gray, and medium orange brown; ls grit are greenish gray to brownish very micaceous and very thin bedded 38 52 gray; are thin to medium bedded and to fiss1le """"""""""""" 1'4 very micaceous; siltstone is 65. Sandstone and minor shale, medium‘ laminated. Conglomerate is pale ~ light-gray; weather light yellowish pinkish gray; weathers pale greenish ' gray; are micaceous, medium gray; is poorly cemented; most of grained, medium well sorted, and pebbles are quartz. Some fine_ medium to thick bedded ........... 13 1.439 grained grit and siltstone contain 64. Shale (60 percent), siltstone (20 abundant plant fragments, percent), and sandstone (20 especially rushlike leaves .......... 74 1,661 percent), interbedded. Shale is dark Type section of the Minturn Formation — Continued Minturn Formation — Continued Clastic unit C - Continued 64. Shale, Siltstone, and sandstone — Continued greenish gray; weathers medium greenish gray; is interlayered with laminated micaceous Siltstone that weathers to fissile chips. Sandstone is medium light gray, weathers dark gray to orange-brown; is thin to medium bedded ................... 63. Shale and dolomite. Shale is dark greenish gray; weathers dark grayish green; is silty, laminated, and fissile. Dolomite is medium grayish green; weathers medium grayish brown; is medium grained, medium bedded, and flaggy; contains abundant white- and-black mica and fine-grained sand ............................. 62. Grit, light-grayish-green; weathers light greenish gray; is arkosic, locally conglomeratic, medium bedded, and flaggy; contains a few Siltstone partings ......................... Shale, dark-gray to darkgreenish-gray; weathers dark greenish gray; is very thin bedded to fissile; contains plant remains in lower part. In middle of unit is 18-in. bed of medium-bluish- gray, medium-orange-brownA weathering dolomite .............. Grit. conglomeratic, pale-greenish—gray to pale-pinkish-gray, calcareous: weathers light pinkish gray; is very thin bedded and platy at top, massive in middle. and very thin bedded at base ............................. Shale and minor shaly Siltstone, black: weathers dark greenish gray to dark gray, in part is finely micaceous . . . . Sandstone and grit, greenishA and pinkish-gray; coarse grit at top of unit and medium-fine-grained sandstone at base. Unit forms prominent continuous ledge with irregular profile .................. Grit 150 percent). shale (30 percent). and sandstone {20 percent), interbedded, lightgrayish-green. Sandstone and grit are thin to medium bedded. Sandstone is medium fine grained and micaceous. Shale is micaceous and fissile ...... 61. 60. 59. 58. 57. 56. Shale. shaly Siltstone, and grit. grayish- green: weather greenish gray. Grit is sandy to conglomeratic: forms ledges 2-4 ft thick on shaly slope ......... 55. Shale and dolomite. interbedded. Shale is light greenish gray, micaceous. and fissile. Dolomite is in beds 2-4-1” thick. except 4-ft bed near top of unit TYPE SECTION Thickness Distance (feet) 57 20 45 14 18 10 17 33 70 19 above base (feet) 1,382 1,362 1,317 1.303 1.285 1.275 1.258 1.225 1,155 1.136 85 Type section of the Minturn Formation —— Continued Thickness Distance Minturn Formation — Continued Clastic unit C —— Continued 54. Sandstone and conglomeratic grit, interbedded, pinkish-gray to light- gray except maroon to pink at base. medium thin-bedded to massive, strongly crossbedded. Grit contains scattered pebbles, and a few lenses of conglomerate ..................... 53. Sandstone, shaly Siltstone, and shale; top is approximately top oflower red zone in section in this area. Sandstone is hematitic and chloritic; weathers dark red; is very thin to medium bedded; forms ledges 1 »2 ft thick. Shaly Siltstone and shale are red and laminated or fissile ........ 52. Sandstone and grit, mottled light- greenish-gray and light—green; weather light pinkish gray. Grit is arkosic, locally conglomeratic, very micaceous, slightly calcareous, and crossbedded ...................... 51. Sandstone (40 percent), Siltstone (30 percent), and shale (30 percent), interbedded; weather medium red, except some ledges weather light grayish green. Shale is red to dark green and hematitic to chloritic. Sandstone is red to medium greenish gray. Coarse conglomerate layer 10 ft below top of unit contains pebbles of Precambrian rocks and of greenish micaceous shale and Siltstone ...... Clastic unit B: 50. Dolomite bed of Dowds: Cherty dolomite, medium-gray: weathers light grayish brown; is thin to very thin bedded and flaggy, with greenish-gray shale films on bedding planes. Chert is light to medium gray; some chert lenticles cut bedding in dolomite at low angle . . . . 49. Grit, sandstone. and Siltstone, pinkish- gray to lightvgreenish-gray; weather medium red with light-gray streaks. Unit is medium to thick bedded; has local lenses of micaceous conglomerate. Upper part of unit is very thin bedded hematitic shaly Siltstone ......................... 48. Sandstone (70 percent), shale (20 percent), and Siltstone (10 percent) interbedded. Sandstone is partly hematitic and dark red, and partly arkosic. chloritic. and gray; is thin to medium bedded, poorly sorted, and fine grained to gritty; includes some graywacke. Shale is medium green to dark greenish gray; weathers light green: is laminated and fissile. Siltstone is shaly, finely micaceous, hematitic, and laminated; some (feet) above base (feet) 20 1,116 21 1,095 14 1,081 51 1,030 6 1,024 26 998 86 Minturn Formation — Continued Clastic unit B — Continued GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Type section of the Minturn Formation — Continued 48. Sandstone, shale, and siltstone — Continued hematitic layers show ripple marks, rill marks, and possible raindrop impressions ...................... 52 47. Grit, sandstone, and siltstone. Poorly sorted conglomeratic grit alternates with well-sorted mediumgrained arkosic sandstone containing abundant fresh pink feldspar grains. Grit is light greenish gray; weathers light gray to brownish gray; is thin bedded, but weathers in massive rounded forms. A little shaly Siltstone occurs in middle of unit . . . 37 46. Shale, mudstone, and siltstone, interbedded; are thin to medium bedded. Shale is green; weathers light green. Mudstone is light brownish green; weathers yellowish brown to medium red; contains abundant medium- to coarse-grained muscovite, black mica, and other mafic minerals-in a very fine grained slightly chloritized matrix ......... 8 45. Grit, conglomeratic, pale-gray; weathers light gray to yellowish gray; is thin to medium bedded and flaggy; contains fresh feldspar, chloritized mafic minerals, and scattered 1 —3 in. quartz pebbles. ................ 12 44. Sandstone (80 percent) and shale (20 percent), interbedded. Sandstone is medium gray; weathers light brownish gray to medium red; is poorly sorted and gritty; contains fresh pink feldspar, chloritized coarse hornblende and biotite, and abundant white mica. Shale is light greenish gray ..................... 23 43. Sandstone and grit,, light-greenish- gray; weather light yellowish brown; are thin to medium bedded, crossbedded, and flaggy. Layers of well-sorted, medium~grained arkosic sandstone alternate with grit containing abundant quartz and Precambrian pebbles in highly micaceous matrix. Some layers contain abundant limonite spots and carbonized plant remains; also contain minor thin interbeds of fissile greenish-gray shale . .' ....... 25 42. Shale (70 percent), and sandstone (30 percent), interbedded. Shale is micaceous, silty, and light green, except lower 6 ft is red. Sandstone is pinkish gray; weathers light brownish gray; is arkosic, medium bedded, and locally crossbedded . . . . 10 Thickness Demure (feet) above base (feet) 946 909 901 889 866 841 831 Minturn Formation —— Continued Clastic unit B — Continued Type section of the Minturn Formation — Continued 41. Sandstone and minor interbedded siltstone. Uppermost bed is a medium-red fine-grained hematitic sandstone, Sandstone below is light greenish gray to light brownish gray; weathers pale greenish gray to pale pink; is medium to thick bedded, medium grained, well sorted, and arkosic; contains abundant brown limonite spots; has chlorite matrix. Siltstone is shaly, light greenish gray, very thin bedded to laminated, and contains abundant mica and chlorite .......................... 10 40. Covered ............................ 28 39. Sandstone, arkosic, conglomeratic, medium -gray; weathers dirty brownish yellow; is medium to thick bedded, faintly crossbedded; is interbedded with light-grayish- yellow fine~grained micaceous thin- bedded sandstone ................. 14 38. Sandstone and grit, light-gray to light- yellowish-gray; weather light yellowish brown to medium grayish brown. Sandstone is medium grained, well cemented, and thin bedded; weathers to platy slabs. Grit is thin to medium bedded, flaggy, and locally crossbedded; contains small quartz pebbles. A 5-ft bed of conglomeratic grit 35 ft above base of unit shows both trough and planar crossbedding ..................... 44 37. Covered slope ....................... 35 36. Sandstone, grit, and shale, interbedded. Sandstone is light greenish yellow, and thin to very thin bedded; consists of fine grained quartz and abundant medium-grained mica. Grit is light grayish white, very arkosic, locally conglomeratic, thin to medium bedded, flaggy, and crossbedded. Shale is pale grayish yellow; weathers light olive yellow; is micaceous and very thinly laminated ........................ 55 35. Grit and interbedded shale. Grit is conglomeratic; contains green chloritic particles; is medium grayish red to medium grayish brown; weathers pale brownish yellow to dark grayish red; is thick bedded; contains 3/4-in. quartz pebbles. Shale is dark grayish red; weathers medium grayish red; is very thin bedded to fissile .................. 94 34. Sandstone, gray, medium-grained, crossbedded ...................... 10 Mic-Imus nuance (feet) above base (feet) 821 793 779 735 700 645 551 541 Type section of the Mintum Formation - Continued Minturn Formation — Continued Clastic unit B -— Continued 33. 32. 31. Grit and conglomerate; has shale partings. Unit is light pinkish gray; weathers light grayish white to pale pinkish red with black seams; is very thick bedded; has planar crossbedding; contains many pebbles of green chloritic phyllite as well as pegmatite, granite, and metamorphic rocks; pebbles are as much as 3 in. across. Local channeling occurs between units . . . Grit and conglomerate, light-yellowish- brown to light-pin'kish-gray; weather medium orange brown to medium grayish red; are medium to very thin bedded, flaggy, and crossbedded. Pebbles are similar to those in unit 33 ............................... Grit and sandstone, arkosic, light- reddish-gray to light -yellowish-gray; weather dark orange brown; are thin to medium bedded, and flaggy ...... 30. Grit, grayish-red; weathers dark grayish red; is thin to medium bedded, flaggy, and arkosic; contains minor muscovite and chloritic phyllite .......................... TYPE SECTION Thickness Distance (feet) above base 20 92 35 18 (feet) 521 429 394 376 [Interval between unit 30 and top of Deadville Limestone is largely covered. Section is offset 3.7 mi southward to slope on north side of Rock Creek, north of Gilmsn, to corresponding stratigraphic position above Inadville. Section ends at top of amended type section of Bolder: Formation. (See fig. 8)] Clastic unit A: 29. 28. 27. 26. 25. 24. 23. Grit; is in part conglomeratic; grades into quartz-pebble conglomerate, locally crossbedded ............... Shale, hematitic, dark-red; marks approximate base of lower red zone Shale and sandstone, interbedded. Shale is green and fissile. Base of unit is 3-ft bed of brown-speckled gray sandstone ................... Sandstone and shale, interbedded. Shale is light green; forms 15-ft bed at top of unit. Sandstone is pinkish gray ............................. Dolomite; contains abundant fossil fragments ........................ Sandstone, greenish-gray; weathers dark orange brown; is medium to thick bedded, and, in part, crossbedded; is arkosic and sideritic. Greenish micaceous shale in beds 1 ft or less thick occur in lower 15 ft of unit Conglomerate and sandstone. Upper part is a light-pinkish-gray medium- bedded crossbedded medium- to fine- grained sandstone containing thin beds of conglomerate and lenses of 18 31 71 35 358 353 322 251 249 214 87 Type section of the Minturn Formation — Continued Minturn Formation — Continued Clastic 23 22 21 20 19. 18. 17. 16. 15. 14. 13. unit A — Continued . Conglomerate and sandstone — Continued green and red shale. This grades downward into pinkish-gray medium- to thick-bedded conglomeratic sandstone, and this grades into dark-red conglomerate at base. Conglomerate contains 1-in. pebbles of white and pink quartz, dark chert, and finely micaceous green shale fragments; matrix is micaceous ........................ . Shale, micaceous, green with dark- maroon-red streaks .‘ .............. . Sandstone (70 percent), and shale (30 percent), interbedded. Sandstone is light greenish gray and thin to medium bedded; contains pebbles as much as V2 in. in diameter in a matrix of fine to coarse quartz grains. Shale is medium green; weathers light green; is very thin bedded ........................... . Grit, conglomeratic, light-green, weathers medium brown; is micaceous and poorly sorted; contains quartz pebbles 1/2 in. in diameter and minor feldspar ....... Shale, micaceous, green; mostly covered .......................... Quartz-pebble conglomerate, light- gray; weathers light orange brown; is thick bedded; matrix is medium to coarse grained; pebbles are l/4-—l/2 in. in diameter Dolomite, thin- to medium-bedded; is in persistent layers; has thin shale parting at top .................... Shale and sandstone, interbedded. Shale is fissile and thin bedded. Sandstone is medium bedded, fine grained to gritty, and micaceous; has clay cement ...................... Conglomerate and sandstone, light— gray; weather light brownish gray; are medium to thick bedded; have pebbles up to 3/4 in. that are mostly quartz but some that are chert, sandstone, and quartzite. Sandstone at base has irregular upper surface Shale, dolomite, and grit, interbedded. Unit is mostly dolomite and dolomitic grit in upper half and mostly shale in bottom half. Shale is platy and dolomitic in middle and fissile at bottom. Grit is very dolomitic; contains dolomite nodules Dolomite and shale. 1.5-ft dolomite bed at top is medium bedded, medium fine grained, and nonmicaceous; has Thickness Distance (feet) above base (feet) 22 192 1 191 9 182 1 181 17 164 5 159 2 157 8 149 8 141 7 134 88 GEOLOGY, MINTURN 15-MIN UTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Type section of the Minturn Formation —— Continued Minturn Formation — Continued Clastic unit A — Continued 13. Dolomite and shale — Continued few shale partings. Dolomite is underlain by 3-ft bed of fissile black noncalcareous shale with lenticular gritty dolomites 1 -3 in. thick. 1 ft of gritty dolomite is at base ........... 12. Shale and grit, interbedded. Shale is micaceous and pink; weathers light gray to yellow brown. Grit is chloritic, with some shale partings . . 11. Sandstone, light-greenish-gray; weathers medium orange brown to brownish gray; is poorly sorted, fine to coarse grained, very micaceous and crossbedded; has layers 1 —6 ft thick; shale partings 1 in. to several inches thick occur between beds . . . . 10. Shale (80 percent) and dolomite (20 percent), interbedded; beds are l —3 in. thick. Shale is fissile in upper part and thin platy in lower part ........ 9. Conglomerate. Pebbles are subangular to subrounded, l/4—1.5 in. across but mostly about 1/2 in.; are mostly quartz, but some are shale and black or gray chert (from Leadville(?) Limestone); some are coarse white quartzite (from Parting(?) Formation); Precambrian pebbles are minor. Rock contains spots of coarse-grained recrystallized carbonate and sparse grains of chalcopyrite; weathers with many vugs, which probably represent leached carbonate ................ 8. Shale and grit, micaceous; is very thin bedded in upper part and medium bedded at base. Grit is mostly quartz and mica ......................... 7. Shale and dolomite, interbedded; is in equal proportions. Shale is medium gray and thin bedded. Dolomite is gritty, greenish gray to brownish gray ............................. 6. Shale, dolomite, and grit, interbedded. Shale is medium gray; weathers light blue gray; is irregular and very thin bedded; contains lenses of grit as much as 1 ft thick and 10 ft long. Dolomite is greenish gray; weathers orange brown; is argillaceous, gritty, and micaceous; contains some chlorite; has angular quartz fragments as much as ]/2 in. in diameter ......................... 5. Sandstone and minor interbedded micaceous shale. Sandstone is medium grained, poorly sorted, micaceous, chloritic, and thin to thick bedded; occurs in lenticular beds; is crossbedded at top of unit, Thickness Distance (feet) above base 22 12 12 (feet) 128 120 98 90 82 70 58 49 Type section of the Minturn Formation — Continued Minturn Formation -— Continued Thickness Distance Clastic unit A — Continued “bi/laid“ 5. Sandstone and minor interbedded micaceous shale — Continued with foreset beds as much as 10 ft long ............................. 33 4. Shale and micaceous dolomite, interbedded. Shale is medium gray to dark gray; weathers medium light gray. Dolomite is micaceous and medium greenish gray; weathers brownish gray; occurs in lenticular nodular beds 1—8 in. thick ......... 25 3. Grit, arkosic, medium-gray to light- greenish-gray, medium-grained; contains many thin partings of micaceous shale; grit bed locally cuts down into underlying shale as much as 30 in. and thickens correspondingly .................. 23 2. Shale (80 percent), grit (15 percent), and dolomite (5 percent), interbedded. Shale is dark gray; weathers medium gray; contains arkosic grit in lenses 3~12 in. thick and 10—50 ft long. Dolomite is medium light gray; weathers light brownish gray .................... 14 1. Sandstone, gritty, medium-light-gray; weathers light gray, except orange- brown stain onjoints from weathering ofiron-bearing carbonate; is massive to thick bedded; lowest bed shows planar and trough crossbedding; contains sparse ‘/2-in. quartz pebbles mixed with poorly sorted medium-fine to coarse quartz and altered feldspar grains; also contains abundant small grains and blebs of brown-weathering carbonate, presumably siderite ..... 0 Channeled contact. Belden Formation: Black shale and limestone. NOTE —— Summary of measured thicknesses of major units. Metres J acque Mountain Limestone Member .......... 9.45 Clastic Unit H ............................... 331.1 White Quail Limestone Member ............... 15.55 Clastic Unit G ............................... 39.95 Elk Ridge Limestone Member ................. 9.15 Clastic Unit F ............................... 168.6 Robinson Limestone Member ................. 227.45 Clastic Unit E ............................... 352.45 Clastic Unit D ............................... 300.6 Clastic Unit C ............................... 154.9 Clastic Unit B ............................... 199.4 Clastic Unit A ............................... 1 14.6 Total measured thickness of the Minturn Formation .......................... 1,923.2 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO 89 REFERENCES CITED Baars, D. L., 1966, Pre-Pennsylvanian paleotectonics—Key to basin evolution and petroleum occurrences in Paradox basin, Utah and Colorado; Am. Assoc. 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A Page Abstract .................................... 1 Age, Belden Formation ..... . 37 Cambrian ............................. 15 Cretaceous .......................... 58 Cross Creek Granite .......................... 14 Devonian .................................... 23 Dyer Dolomite ...................... . . 28 Gilman Sandstone ................... . 30 Jurassic System ............... . . 57 Leadville Limestone ................. 32 Maroon Formation . . . . ...... 55 Minturn Formation . . . ........... 48 Miasissippian ................... 23 Molas Formation . ..................... 33 Ordovician ............................. 21 Parting Formation ........................... 26 Peerless Formation ........................... 20 Pennsylvanian ....................... . . 33 Permian ........................... 33 Sawatch Quartzite .......... , 1 7 Silurian ............................ 26 Tertiary ..................... 58 Triassic System ..................... 56 Algal structures ..................... 58 Allophone ................................ 60 Alluvium , . . . ...................... 69 Amphicyon sp ................................ . 64 Ancestral Rockies ........................... . 33 Anthracaspirifer optimus ............. . 48 rockymontanus .................... 48 Antiquatoriia coloradoensis .. .. .48 hermasana .......................... 50 Arapahoe National Forest . ............ 3 Aviculopecten eaglensis . . ................. 37 sp. .............................. 26, 48 Avon, gypsum . ............................ 48 Azurite .......................................... 78 B Bald Mountain, fault blocks ....................... 71 Minturn Formation, boulders .................. 47 Sawatch Quartzite ............................ 15 thinning, Sawatch Quartzite .................. 19 Banks, quoted . . ............................. 31 Barite ........................................... 78 Battle Mountain, concealed ores ......... . . . 78 Battle Mountain formation .............. . . . 34 Bedding faults .............. . . 77 Belden, pre-, unconformity ............... 32 Belden Formation ................... 34 age ................................... 37 bedding faults . .................... 77 channels . ........................... 35 fossils ....................................... 38 Gordon, Mackenzie, Jr., and Yochelson, E. L., quoted ........... 37 Henbest, L. G., quoted ...... . 37 section, emended type ............... 35 sill ........................... 58 Belden mine .......................... 34 Belden Shale Member ............. 34 Ben Butler mine ............................... 75 Bighorn Creek, copper-bearing quartz veins ........ 78 dike .................................. . . 61 Minturn Formation .......................... 47 INDEX [Italic page numbers indicate major references} Page Biotite gneiss ..................................... 8 Black Canyon Schist ..§ ................. , . 7 Black Gore Creek, glaciers .............. , .67 Minturn Formation ......... . . 47 upturned conglomerate .............. 71 uranium ........................ 78 Black Gore syncline ..................... 76 Blue Limestone ........................ 23 Bolts Lake, outcrop knob . ........................ 32 Booth Creek, Robinson Limestone Member ......... 45 Bornite ..... . . 78 Bothriolepis ..................................... 26 Boulder Creek Granite ................ . 9, 10 Brachycrus uaughani ................... . . 64 wilsoni .................. . . 64 sp. .................................. 64 Bradyina sp. ...................... 50 Breccias, gneiss ........................... 13 granite ........................... 13 Bright Angel Shale ............................... 21 Briscoia .......................................... 20 Breecheria bauidens ....................... 48 Browns Park Formation .................. . 64 Buck Creek, chert ............... . 65 Bull Lake Glaciation ........................... 66 C Calcite veinlets ................................... 77 Calcivertella sp. .................................. 50 Caleitornella sp. ................................. 50 Cambrian System ...................... . . . 15 Caninoid coral undet ................... . . . 48 Carlsbad twinning ........... . . , 10 Central sedimentary belt ............... 75 Chaetetes sp. ...................... 48 Chaffee Formation . . . . .................. 23 Chaffee Group ........................ 24 Chalcopyrite .................................. 78 Channels, Belden Formation ...................... 35 Dyer Dolomite .......................... . , . 28 Channel cuts, Dyer Dolomite ................. . . . 27 Charophytes ....................... . 58 Cherokee Shale, fossils ................... 51 Chen ...................... . . . 65 Banks, quoted ..................... 31 Gilman district . . . . ............. 27 Gilman Sandstone . . ...................... 29 Chert breccia marker . ...................... 31 Chill zones ....................................... 59 Chinle Formation ............................. 53, 56 Garta Member .................... . . 56 Red and White syncline ............ . 76 uranium ................. . . 78 Chanetinella jeffordsi ....... Cidarid spines ........ Cirques, old, Vail .......... Clastic dikes, Minturn Formation .................. 43 Clastic unit A, Minturn Formation ................. 43 described .................................... 87 Clastic unit B, Minturn Formation . . . . .43 described ..................... . . 85 ripple marks ................ . . 86 Clastic unit C, Minturn Formation . . ............ 43 described .................................... 84 Page Clastic unit D, Minturn Formation ................. 44 described .............................. 82 ripple marks . . , ................. 83 rushlike leaves ............................... 84 (Hastic unit E, Minturn Formation ................. 44 described ........................... . , 81 ripple marks ...................... . . 82 Clastic unit F, Minturn Formation . . . .45 described ............................. 80 Clastic unit G, Minturn Formation . . .......... 45 described .................................... 80 Clastic unit H, Minturn Formation ................. 46 described .................................... 79 Cleiathyridina orbicularis ......................... 48 Climacammina sp. .......................... . . . 50 Clydgnathus ormistoni ...................... . . . 26 Coal, Belden Formation ............... . . 34 Minturn Formation ............... . . 52 Coffee Pot Member ............. 3O Colluvium .............. ' .............. 65 chert ..................... 65 Colorado Atlas ........................... 3 Compasita ovata . .................... 48 subtilita .................................. 48 Condrathyris perplexa ............................. 48 Condruthvris perplexa ........................ . l 48 Copper Mountain, Minturn Formation ......... . 47 Cordierite ........................... , 12 Cretaceous System .................... 58 Cribrostomum sp ............ Cross Creek, glaciers ,,,,,,, moraines ....... sand ........ Cross Creek batholith . diorite . . . . .............................. 12 history ...................................... 14 Cross Creek Glacier .............................. 67 Cross Creek Granite ...................... 7, 8, 74, 75 age, C. E. Hedge, analyst ............... 14 cataclasis ........................... 11 diorite ................... 13 feldspar ........................... 10 M. Seerveld, analyst . ............... 11 Crurithyris planiconvexa . . . ................. 48 Curtis Formation ........................ 57 D Dacite porphyry dikes ..... Dadoxylon ............ Dakota Sandstone . . . . Deluge Creek, quartz vein, bornite ................. 78 Derbyia crassa .......................... . . . 48 Desmoinesia muricatina .................. .37, 50 nana ...................... . . . 48 Devonian System ........................ 23 Dicellomus ........................ 17 mosaica ................................ 21 Minus .......................... 17 pectenoides .......................... 17 Dickson anticline ................................. 76 Dickson Creek, anticline ................. . .76 tuff ............................... . . 64 Dickson Ranch, tuff .............................. 64 93 94 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Page Page Page Dikes ....................................... 58, 61 Fossils — Continued Fossils — Continued diorite .................................. 13 Briscoia .............................. , . 20 Qzawainella sp ........................... 50 Dillon quadrangle, Cross Creek Granite ............ 8 Calcivertella sp. ................... , . , . 50 Resolution Dolomite Member .............. 48 Dinwoody Formation . . , l ...................... 54 Caleitornella spr .................. '50 Saukio pepinensis ...................... 20 Diorite ....................................... 12, 13 Caninoid coral undet .................. 48 Sawatch Quartzite ............. Dolomite bed of Dowds, described .................. 85 Chaetetes Sp. ----------------------- 48 Schizadus sp ............... . Dowds ..................................... ,_ , 75 Cherokee Shale ....................... 51 Schizophoria striatulata australis Dolomite bed, discontinuous banded ................ 31 Chonetinella jeffordsi ................ 50 Serpulopsis 5p, .............. Dolomite caps, Robinson limestone Member _ . . , . , 45 Cidarid spines ........................ 48 Sigillaria ................ Dolomite reefs, Minturn Formation ............ , , 43 Cleiothyridina orbicularis , . l , ......... 48 Spiri/er centronatus . . , Dalorothoceras sp. ......................... , , 51 Climacammina sp. ......................... 50 (Cyrtospirifer) Whitney; . Domatoceras umbilicatum ................... . , 51 Clydgmzthus ormistoru' . .............. 26 whitneyi animasensis . , williamsi ............................. , i 51 Composite 011030 ----------------------- 48 Spiroplectammina spl , l . spl indet. ............................. , . . . 51 subtilita ........................ 48 stromatolites ........ Dotsero Formation .................... , . 21 Condrathyris perplexa , ................. 48 Tetrataxis millsapensis . Dowds, dolomite beds, described ....... Cribrostomum sp ........................ 50 sp, ............................... 50 landslides ...................... _ . Crurithyris planiconvexa iiiiiiiiiiiiiiiiiii 48 ’I‘igiwon Road ............................ 26 Dowds dolomite beds, Minturn Formation . Cynadesmus casei . . 1 ................... 64 Tomarctus thomsoni , l . ..................... 64 Dowds siding, Minturn Formation ............ 45 Dadoxylon ............................ 55 Trepeilopsis gnandis , ..................... 50 Dyer Dolomite Member .................. 24, 27 Derbyia crassa .............................. 48 sp, ................................ 50 age ................................... 28 Desmoinesia muricatina .................... 37, 50 Walchia ..................................... 52 bedding faults ............................ 77 mm; ................................. 48 Wearyman Dolomite Member ................. 48 feasils ............................... 28 Dicellomus mosaica ........................... 21 Wedekindellina coloradoensis .................. 51 jasperoid ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 79 nanus l . . , ........................... 17 Werdeyoceras sp. ............................. 48 section ................................... 27 peCtenoides . , ......................... 17 wood ........................................ 56 Dolorothoceras sp ............................. 51 Fossil markings, fucoid ........................... 19 Domatacems umblicatum .................... 51 worm trails .................................. 19 E williamsi . 1 ........................... 51 Fossil plants, Minturn Formation llllllllllllllllll 52 sp, indet, ................................ 51 Fossil wood, Maroon Formation .................. , l 55 Eagle Canyon, Sawatch Quartzite ' """"""" 1'5 Earlandia perparva ........................... 50 Fremont Limestone, exposures .................... 22 Eagle Glad" ------------------------------- 66 Spl ............................. ..50 Front Range highland ...................... 33, 3s, 69 Eagle mine """""""""""""" 28’ 30 Edmondia sp. ............................ 48 Fountain Formation .............................. 52 badding {“1“ ------------------- 77 Ellipsocephaloides butleri ......... i 20 Fusulina curta ............................. . . , 51 Harding Sandstone ‘ ‘ """"""""" 22 Endothyra scitula ...................... 32 disienta . , ........................... l . , 51 Eagle River, canyon ' ‘ ‘ ‘ """""""""" 5 sp, ........................ 37, 50 plattensis ............................ . . . 51 en echelon faults ‘ ' """"""""" 75 fecal pellets .............................. 55 taosensis ............................. . . 50 glaciation """"""""""""""" 66 Fusulina curta ........................ 51 truncatulina ......................... . , . . 51 glacier """"" ' """""""""" 66 distenta ........................... 51 Fusulinella ................................... 37, 48 Eagle Valley Evaporite .................... 34, 48 plattensis ___________________ 51 Earlandia perparva , l ....................... 50 taasensis ______________________ 50 sp. ...................................... 50 truncatulina lllllllllllllllllll 51 G East Meadow anticline , ....................... 76 Fusulinella IIIIIIIIIIIIIIIII 37‘ 48 Economic geology -------------------------------- 77 Girvanelln sp. ...................... 37 Galena ...................................... 78 Edmondza sp, .................................... 48 Glabrocingulum sp, indet. _ ‘‘‘‘‘‘‘‘‘‘‘ 43 Game Creek, landslides IIIIIIIIIIIIIIIIII 68 Elk Ridge Limestone Member, Globoualvulina sp ...................... 50 Minturn Formation ,,,,,,,,,,,,,,,,,,,, 44 Minturn Formation """""""""" 45 Gymnocodium .............................. 55 Garnet, almandite ........................... 8 described .................................... 80 Hornsilver Dolomite Member ''''''''''''''''' 48 Garta Sandstone Member lllllllllllllll 53’ 56 {058113 ‘ ' ' ‘ Idahoia ...................................... 20 fossil wood ................................ 56 grooves ...................................... 80 Juresania nebrascensis .1 """"""""""" 48 uranium """""""""""""""""""""""" 78 Ellipsocephaloid“ bum" ----------------------- 2° Kozlowskia sp. A ...................... 48 Gilman ...................................... 3 Elvinia-Camaraspis zone .......................... 20 Lepidodendron ‘ ' H _______________ 52 Homestake shear zone IIIIIIIIIIIIIIIIIIIII 75 Endathyra SCI-"dd """""""""""""""""" 32 Lima sp ...................................... 48 Gilman mining district, host rocks ................. 24 SP' """"""""""""""""""""" 37' 50 Linoproductus prattenianus .................... 48 are deposits .............................. 77 Entrada Sandstone """""""""""" 33' 56‘ 57 sp. indetl ................................ 48 Gilman Sandstone Member . . ............... 24 Maroon Formation l . Gilman Sandstone ......................... 28 F Maximizes cherokeensis ........................ 51 age ..................................... 30 Meekella striatacostata ........................ 48 chert ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 29 Fall Creek ................................. . . 12 Merychippus isonesus ......................... 64 Minturn ................................. 75 glaciers .............................. . . 67 Mesolobus euampygus ......................... 51 paleogeography , ...................... 30 Homestake shear zone ............. l 75 mesolobus ............................ 37, 50 section ................................... 29 Faulting .................................... 69 sp. ............................... 51 Solution features . , ,,,,,,,,,,,,,,,,,,,,,,,, 29 Faults, Gore Range ...................... 74 Millerella sp .................................. 37 waxy bed ................................ 29 Fecal pellets, Maroon Formation .............. 55 Minturn Formation ........................... 48 Giruanella sp1 . , . . .............................. 37 Flow structures ......................... 59 Monotaxis sp, ................................ 50 Glabrocingulum sp, indet. ......................... 48 Fluid inclusions, Parting Formation ........... 25 Morrison Formation .......................... 58 Glaciation, Bull Lake 4 , ,,,,,,,,,,,,,,,,,,,,,,,,, 66 Fluorite .................................... 78 Multithecopora sp, ............................ 48 Eagle River .................................. 66 Fluxion structure ......................... 9 Neospirifer coloradoensis ...................... 50 Glaciations, pre-Bull Lake ,,,,,,,,,,,,,,,,,,,,,,,, 65 Fossils, Amphicyon sp, .................. 64 Obolus maeschae ............................. 21 Glenwood Canyon, Sawatch Quartzite .............. 16 Antiquatonia coloradoensis l .............. 48 opalized wood ................................ 64 Globovaluulina sp. ................................ 50 Anthracospirifer optimus ...................... 48 Orthouertella sp .......................... . . 50 Gordon, Mackenzie, Jr., and Yochelson, Anthracospirifer rockymontalms ................ 48 Osagia ................................... , . 37 E. L., quoted ......................... 37 Antiquatonia hermosana , l ................ 50 Paurorhynca endlichi ................. . 26 Minturn Formation, fossils ............. 48, 50, 51 Aviculopecten eaglensis . . . ................. 37 Peerless Formation ................... , . 20 Gore Creek, Elk Ridge Limestone Member .......... 45 sp ............................. 26, 48 algae .......................... , , 19 glaciers ............................... .67 Belden Formation . , . . ................. 38 Permophorous sp .................... . , 51 gravel ................................ . . . 79 Bothriolepis ......................... 26 Polytaxis sp ...................... , . 50 Minturn Formation .................. . 45, 47 Brachycms uaughani ................... 64 Produ'ctella sp, ........................ 26 sand ..................................... 79 wilsoni ........................... 64 Profusulinella sp ..................... 51 White Quail Limestone Member ............. 46 Bmdyina sp. ........................... 50 Pterocephalia sanctisabae ................ 20 Gore fault ..................... 5, 7, 34, 38, 69, 71, 76 Breecheria bovidens .......................... 48 Ptychaspis ................................. 20 described .................................... 74 Page Gore fault — Continued Maroon Formation . ..................... 54‘ red beds ............................. 56 slices ............................... 24 veins .................................. 78 Gore Range ..................... 3, 5, 63, 74 diorite .................................. 12 migmatite ............................ 12 uplift .................................. 69 west, dikes .......................... 61 Gore River, glacier .......................... 66 Gore Valley, landslides .................. 68 Gothic Formation .......................... 52 Gravel ................................ l . 79 Green Mountain, volcanic center ....... . . 61 Grit marker bed ....................... . 82 Gymnacodium .................................... 55 H Harding Sandstone ........................... 22 bedding faults ...................... . 77 section .............................. . i . , 23 Hedge, C‘ H., analyst, Cross (keek Granite . r 14 Hematite .............................. , . 60 Henbest, Li G., quoted .................... r . 37 Minturn Formation, fossils ............ . . , 50 Hermosa Formation ........................... 42, 52 Homestake Creek, migmatite ...................... 12 Homestake shear zone ................... 8, 16, 75, 76 Sawatch Quartzite ....................... , . r 15 Hornsilver Dolomite Member ...................... 82 fossils .................................... . 48 Minturn Formation ........................... 44 H Idaho Springs Formation ................. Idahoia ................................. Igneous rocks ................................... Jacque Mountain Limestone Member ..... . l , . 34 age ................................... l , 48 described ............................ . r 79 fossils .............................. r . 51 Minturn Formation .................. . . . l 46 stylolites ............................. l . . . 47 Jacque Peak, Maroon Formation .......... . . 54 Jasperoid .................................. , . . 79 Jasperoid replacement ...................... . . . 24 Jurassic system .................................. 57 Juresania nebrascensis ............................ 48 K, L Kaibab Formation ............................ 55 Kanawha Formation ......................... 37 Kokomo district, Minturn Formation . . r ..... 44 Kozlowskia sp. A ................................. 48 Landslides ....................................... 68 Stone Creek ........................... 75 Leadville Dolomite ........................... 30 bedding faults ....................... . . 77 jasperoid ............................. r . 79 origin ................................... . . 30 section . r 31 Leadville Limestone ....................... . 23, 30 age ................................... . . 32 correlation ........................... . . 32 facies boundary ........................ , . 30 fossils ................................. r . 32 Lepidodendron ................................... 52 Lima sp ..................................... . l 48 Lime Cliffs ....................................... 44 Linaproductus prattenianus ........................ 48> sp. indet. .................................... 48 Lionshead. reef dolomite. described ................ 84 Lionshead reef dolomite. Minturn Formation ....... 43 ————i INDEX M Page Madera Formation ............................ 42, 52 Malachite ................................... . 78 Manitou Dolomite, exposures ...................... 22 Maroon formation ......................... 33, 34, 53 age ................................... 55 chert .......... correlations . . . . fossils ,,,,,,,,,,, fossil wood ....... landslides ........... paleogeography ....... Red and White syncline ‘ 1 thickness ............. Martin Creek, rock bench ..... Maximites cherokeensis .......................... 51 McCoy, Jacque Mountain Limestone Member ....... 46 McCoy formation ............................. 52 Meadow Creek, anticlines ................... 76 basalt .............................. 64 Mechelle striatacostata ....................... 48 Merychippus isonesus ................ . 64 Mesolobus euampygus ................... . l . 51 mesalobus .......................... 37, 50 ..... 51 1 1 Middle Creek, anticline ..................... . l 76 Minturn Formation .................... . . 47 Minturn Formation ........................... 45 thrust faults .............................. . 74 Migmatite .H. .12 Millerella sp ...................................... 37 Mill Creek, head, Jacque Mountain Limestone Member ................... 47 landslides .................................. 68 Minturn Formation ........................... 45 White Quail Limestone Member ............... 46 Minerals, allphone ................................ 60 anhydrite ............................. 34 azurite ........... barite ............ bornite ............................... 78 calcite veinlets ........................... 77 feldspar, Carlsbad twins .................. 10 chalcopyrite ............................ 78 coal .................................. 34 cordierite ........................... 8, 12 epidote ................................ . 13 fluorite .................................. 78 galena ....................................... 78 garnet ................................. 8 Gilman ores ............................ 77 gypsum ........................... 34, 38 hematite .............................. 60 malachite ........................... 78 opal .................................... 64 phosphate ........................... 42 pyrite .................................. 78 scolecite ........................... . 61 sericite .............................. , 60 siderite .............................. 78 sillimanite .............................. . . 8 sphalerite ............................... . 78 uranium .............................. . 78 Minturn .................................. . . . 3 Gilman Sandstone ........................... 75 Minturn Formation ........................... 34. 38 age .......................................... 48 Black Gore syncline ...................... . . 76 boulders, Bald Mountain ...................... 47 chert ........................................ 42 elastic unit A . . ......................... 43, 87 clastic unit B r , ......................... 43, 85 elastic unit C . . ....................... 43, 84 elastic unit D , ........................ 44, 82 clastic unit E . . ......................... 44, 81 clastic unit F r . ........................ 45, 80 clastic unit G ., .................... 45, 80 clastic unit H ......................... 46, 79 coal ....................................... 52 Page Minturn Formation — Continued correlations ........................ 48, 52 dolomite beds of Dowds .................. 43 dolomite reefs ......................... 43 Elk Ridge Limestone Member ............. 45 facies changes ........................ 47 fossils ................................ 48 fossil plants ...................... . 52 Gore fault ........................... . 38 Hornsilver Dolomite Member ........... l 44 Jacque Mountain limestone Member . . . .46 Kokomo district ....................... l 44 landslides ............................... l 68 lithology . . . . ....................... . . 41 origin ................................ . . 52 paleogeography ......................... . , 52 phosphate .................................. 42 reef dolomite of Lionshead .................... 43 Resolution Dolomite Member .................. 44 Robinson Limestone Member .................. 44 sedimentary features ......................... 43 subdivisions ............................... 39 thickness changes .................. 47 thinning, Gore fault l , . .................. 47 type section ........................... 43, 79 Wearyman Dolomite Member ................. 44 White Quail Limestone Member ............... 45 Mississippian System ............................. 23 Moenkopi Formation ...................... 54 Molas Formation ........................... 32 age ................................. 33 ores .................................... 33 regolith ............................... 32 section ...................................... 35 Mongier Creek, Robinson Limestone Member ....... 45 Monolaxis sp. .................................... 50 Moraines, Bull Lake time ............... . .66 Cross Creek lllllllllllllllllllllllllll . . 32 Fall Creek ............................ , 67 Moraine material, West Tenmile area .............. 67 Morgan Formation ............................ 52, 53 Morrison Formation .............................. 57 chert ....................................... 65 fossils ................................. 58 malachite ,, . l ............................ 78 Mosquito fault ................................. 71 Mosquito -Tenmile Range, Jacque Mountain Limestone Member ......... 46 Mount Powell quadrangle. Cross Creek Granite ...... 8 Mount Zion porphyry l . ........................ 58 Mulky coal ................................ 51 Multithecopora sp. ............................... 48 Mud-chip conglomerates, Minturn Formation ....... 43 Mudcracks, Minturn Formation ................... 43 Mudstone dikes .................................. 61 N,O Neospirifer coloradoensis New Jersey 7jnc Col ........ Newhouse tunnel, Harding Sandstone .............. 22 Nodules, chert ................................... 65 North Fork Piney River, Maroon Formation ................... 54 North Park Formation ............................. 64 Notch Mountain Creek, Homestake shear zone ........................... 75 Obolus maeschae ................................. 21 Opal ................................... 64 Opalized wood ............................. 64 Ordovician System ...................... 21 Ore deposits ............................... 77 concealed ........................... 78 emplacement ....................... 79 host rocks .......................... 24 Leadville Dolomite ................ 30 Rocky Point zone ............................ 77 i 96 GEOLOGY, MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO Pa e Ore deposits — Continued g Page Page White ngilrlaimestone Member ............... 45 Red Sandstone Creek, Maroon Formation .......... 54 Stromatolites, fossils ........................... 27 Orthouertella sp ............................ 50 moraine ..................................... 67 Structure ........................................ 69 Osagia ......................................... 37 Reef dolomite of Lionshead , . . ., .............. 43 Stylolites, Jacque Mountain Limestone Member ..... 47 described ......................... 84 Sundance Formation .............................. 57 P References cited .................................. 89 Synclines, sedimentary belt ...................... 75 Resolution Dolomite Member, . “ Mi t F t‘ .................. 44 Panda Porphyry sill .............................. 58 {mils " “m ”'1'” 1°" 48 T Sm“ The: """"""""""""""" :9 Ripple marks, Minturn Formation ................. 43 Tennessee Pass """"""""""""""""" 3 P dew]: ruc 2,195 """"""""" 9 Robinson Dolomite Member, fossils ................ 50 l-Iarding Sandstone """"""""" 22 are ox ormalon ......................... 52 Robinson Limestone Member, Tertiary ........... _. ...................... 58 Parting Formation ------------------- j: Minturn Formation _ . ........... 44 Tetrataxis millsapensrs . . . .............. :3 age ................................... , 5 .................................. de 0 bed .............. h ............ 81 p. besides faults -------------------- -77 mile: .................................... 42 sswon Ross. fossils ------------- 26 defined --------------------------- - - 24 fossils """""""""""""""""""" 48 51 Till, pre-Bull Lake time .................. 66 flu“? inclusions ‘ """""""""" ‘ ‘ 25 Robinson mine .......................... :44 Tomarctus thomsoni """""""" ' 64 fossils """""""""""""""""" 26 Rock Creek, faults, concealed ores ........ . . , . 76 Tongue faults """""""""""""""" 77 Gore fault land area . ..................... 71 Rocky Point zone ......................... 16, 77 Trepeilopsis gnandis ........................ . 50 paleogeosraphy .- .. ............. 26 5p, ................................... 50 section ---------------------------------- 25 S Triassic System ., ....................... 56 Parting Quartzite Member Troublesome Formation ........................... 64 gaugwhygvffg’fihl ---------------- 15 19 Si new-n Gianna ................................ 15 Turkey Creek, nun, ,chslcosyn‘te ------------------ 78 eer ess 0 a 10 """""""" Salt casts, Minturn Formation . ................ 43 Minturn Formation """""""""""" 44 age ----------- Two Elk Creek, Minturn Formation , , . ........... 44 1 Sand ................................... 79 . a gale ‘- - - if. t‘ Sand grain marker .................... 27 sill ......................................... 58 W? ying on}?! ions ' ' Sandia Formation ............... . . 52 Pa eogeograp y ' ' ' Sangre de Cristo Formation ............. l . 55 U, V ripple marks , . . . . ti Santa Cruz vein ........................ . . 75 —S L , h‘ hl d P secl on' ' ' Sy t """"" Saukia pepinensis , . . ................... . l . 20 Uncompahgre an u.ls lg an """"""""" 33 ennsy vanian s em """ Sawatch anticline . . ....................... 76, 77 Uplifts """""" _ """"""""""""""" -' ' '69 Permian System ............................ 33 Upper Slate Lake, dike ..................... 61 Sawatch Range .................... 5, 63, 75 , Permopharous sp ............................. 51 Sawatch Qua rtzi te 16 Uranium ............................ 78 Peterson Creek, Homestske shear zone H ..... 75 u m . “ """""""""" 69 Urbanization .................................... 79 Phosphate, Minturn Formation ............... 42 p """"""""""""""" Ph h _ F t‘ 53 55 Sawatch quartz .............. . , . ............. 15 V _ 3 cap ona orma ion """"""""""" ’ Sawatch Quartzite, bedding faults , . ...... 77 all """ , """"""""""""""""" ' ‘ ' ' Physiography ...... . . 63 fossils 17 landslides . . ............. l , .69 Pinedale Glaciation . ..................... 68 """ . """"""""""" ‘ ' Vail syncline . .. .......... 7e , . Gore fault high .................. . . 71 . , Pinedale 'I‘ill ................................... 68 jasperoi d 79 Vail ski area .............................. 66 Piney Ridge, volcanic flows . ............... 64 """""""""""""""" ' , Volcanic rocks ................................... 64 , , paleogeography l . ................... . , . l7 Piney River, basalt ......................... 64 . , Rocky Point zone l ................. 16, 77 fault slices ........................ 75 l gla i; s 66 68 section ................................... 16 W c r .............................. , . l l moraines, sand .................. . 79 Schigclizurg’ Bald Mountain ‘ ' """""" :1) Walchia ..................................... 52 North Fork, Maroon Formation ....... . 54 . .p' ‘ ' i """"""" l """""" W ave breccias ................................ 27 Schlzaplwrta strmtulata australts . ..... 26 . tuff and basalt ...................... . a 63 Waxy bed, Gilman Sandstone i ................. 29 . . Schoolhouse Sand ......................... 53 . Pink brecc1a .............................. 31 Schoolhouse Ton e 53 Weber 8T“ ------------------------------ 33 Pitkin Creek, dike . . . . ...................... 61 , g“ """"""""" ' ' Weber Sandstone ........................... 53 , , ScoleCite .................................. . . . 61 Minturn Formation . ,,,,,,,,,,,,,,, 47 Seerv ld M anal st Cross Creek Granite 11 Weber shale ..................................... 33 Pitkin Lake, prospects ,,,,,,,,,,,,,,,,,,,,, 73 Se , ,f ’ " y ’ ' ' ' ' ' '60 Wearyman Creek, dolomite beds, concealed ores . . . . 78 Polytaxis sp ............................ 50 serlu [e I ', """"""""""""""""""""" 50 Minturn Formation ........................... 44 Porphyry sill ............................. 35 86 "I" 7pm SP' """"""""""" Wearymnn Dolomite Member. Minturn Formation . . 44 , , ‘ Shaly limestone ..................... 33 , Pottsvxlle Formation ................... 37 Shinarum Member 56 described ------------------------------- 82 Pre-Bull Lake Glaciations .................. 65 Sid “e p """""""""""" 78 fossils .................................. 48 Precambrian rocks ..................... _ , 7 . er , """""""""""""""" Wedekindellina coloradoensis . . .................. 51 , . Sigillaria ................................. 52 deformation, granite ..................... ‘74 Sill Belde F r ati n 58 Werdeyoceras sp. ................................. 48 faulting ,,,,,,,,,,,,,,,,,,,,,,,,,,, ’ hn o m o """"""" '35 West Grouse Creek, Sawatch Quartzite ............ 16 Productella 5p, ,,,,,,,,,,,,,,,,,,,,,,, 26 S‘ll' W'Fty‘y """"""""""""""" ‘ West Grouse Glacier ......................... 67 Profusulinella sp .............................. 51 3:1 irriianifer. iati Whiskey Creek, landslides ----------------- 69 n m n , . , Pterocephalia sanctisabae. , l .............. 20 , ur a o 0 .5 ”IN-St fault ---------------------- . . . 75 . Silver Plume Granite . . Ptychaspis ................................ 20 , White limestone l . ................... . . , 22 . Slate Creek, dikes ....................... . Pyrite ........................................... 78 . White porphyry ................................. 58 Slumping, Stone Creek ........................... 75 . , , , , White Quail Limestone Member, Solution features, Gilman Sandstone ............... 29 Minturn Formation 45 Q, R South Canyon Creek Dolomite Member ......... 53, 54 age """""""""""" 48 fossils ............................... 55 , : .................................... _ , described .............................. 80 Quartz latlte porphyry ....................... 58 Sphalerite .......... ~ ...................... 78 fossils 51 Qzawamella sp ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 50 SPIN/gr centror-iatusWA: . , - i ------------ ‘ :2 White River National Forest ...................... 3 _ , _ _ , ( y nos”, mf ‘7) ","eyl """"""""" ' 6 Willow Creek, dolomite reefs, concealed ores l , . , . 78 Raindrop impression, Minturn Formation .......... 43 whltneyl animasensls ..................... . 28 Wyoming formation 34 Red and White Mountain ................. , , 7 Spiroplectammina sp. ............................. 50 """"""""""""""""""""" Chinle Formation ..................... . , 56 Spraddle Creek, Minturn Formation ............... 45 Y Z colluvium ............................ 65 White Quail Limestone Member ............... 46 ’ Entrada Sandstone . . . .................... 57 Spraddle Creek fault zone .................... 76 Yarmony Limestone Member . i , . ,,,,,,,,,,,,,,, 54 fossil wood ....................... 55 concealed ores .................. 7B fossils .............................. 55 Maroon Formation ....................... 54 fluorite vein ........................... 78 Yarmony Mountain, volcanic field . , . , ........... 64 Morrison Formation ................. 57 State Bridge Formation .............. 34, 54 Yochelson, E, L., and Gordon, tuft“ .................................... 64 State Bridge, volcanics .................... . . 64 Mackenzie, Jr, quoted ................ 37 uranium ....................... , , 78 Stone Creek, fault .......................... l , 75 Minturn Formation, fossils ............. 48, 50, 51 Red and White syncline ...................... 76 gypsum ............................. 48 Yule limestone ................................... 22 Red»cast beds ........................ 15, 19 thrust fault l . , . . , . ..................... 75 Red Cliff ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 3 Streblochondria tenulineata ........................ 48 Zebra-rock structure .............................. 78 aU.S. Government Printing Office: 1977-777-034/2 Region 8 — 9? UNITED STATES DEPARTMENT OF THE INTERIOR Prepared in cooperation with the «9? PROFESSIONAL PAPER 956 GEOLOGICAL SURVEY \ COLORADO MINING INDUSTRIAL DEVELOPMENT BOARD Q) ‘ PLATE 1 a Q o i o 196 3O, 372°0°mAE. 373 374 l 375 376 377 378 25’ 379 , . aq w. (MOUNT POWELL) 383 385 20' 385 388 389 1 780 00° FEET391 392 106 {33°45 39 45 I , w . » - , , 4 ~ ,. K ' , « - x ~ - , - , : z , , ,- 2 I ~ . ,_ CORRELATION or MAP UNITS 4400 Holocene and Pleistocene I! ' \ t ‘\ , 44000"""1-N. i \ QUATERNARY a I Pleistocene m ,u" . o -- , 4399 ' ' " P16 I (9 d PI, QUATERNARY(?) A is ocene .) an iocene AND TERTIARY I I' Miocene ‘ ' ‘ Td‘j TERTIARY , I , , :‘ , £90 000 I Oligocene(.) ' ‘ y ,1»: FEET _ , ‘I ~ n .E Upper Cretaceous . , 1* ,. ,, ' CRETACEOUS _' _ ' ,1, Lower Cretaceous I 4‘ . *I , 4397 Unconformity 8 ‘e a ‘ I , \‘t ‘ Unconformity Upper Jurassic JURASSIC U) ‘ 4396 Unconformity Je Q: ~ I / ,1 / ‘ ,1 ,; it: } Upper Triassic } TRIASSIC "RC ” / ii I ’ " II E Re It 4 Unconformity E * Morin ‘ ' . ~ » Ple Lower Permian and Upper PERMIAN AND A V i i ‘ 4395 '5 and Middle Pennsylvanian PENNSYLVANIAN 2 0969 7,, it _ I I T . d (1 hit ' PM 15%? 39 IA; SI: -» -r/»,;{ [meq is ‘I _ , » lee l __ 9 _ - » “ ‘ \K [Pmrc { 4394 IPmr , A , [th 4394 . [me . . gm <:::::: L Middle Pennsylvanian > PENNSYLVANIAN rdl dd 4393 c 4393 rpm [Pb 1 J Unconformity 4392 , . . . Ml Lower Mississ1pplan MISSISSIPPIAN ,1 Unconformity 40’ 40’ Lower Mississippian(?) MISSISSIPPIAN(?) I _i_ and Upper Devonian AND DEVONIAN 4391 Unconformity 4391 Oh } Middle Ordovician > ORDOVICIAN Unconformity 4390 Upper Cambrian CAMBRIAN 4390 PRECAMBRIAN X E P: SYMBOLS ON SECTIONS ONLY 8 23 - MISSISSIPPIAN Z T0 CAMBRIAN [2 iv; [L . WEST - } PRECAMBRIAN x 8 E 3 (V 2 g '2 E > O o c: Li 9 4386 DD 4385 [0' DESCRIPTION OF MAP UNITS T 5 Alluvium (Holocene and Pleistocene) . S 4385 Landslide and thick colluvium (Holocene and Pleistocene) 4385 7 , i 5 Glacial drift (Pleistocene) of Pinedale and Bull Lake ages undivided 314/ I f" r “' g L / (fizz/{V ( s \. Of Pinedale age / (~\\\\\\. 0° 2 $33)) « j g9 Of Bull Lake age I? /{ I33 4384 g; Glacial drift (Pleistocene) of pre—Bull Lake age o as \ Preglacial colluvium (Pleistocene? and Pliocene) Basalt (Miocene) Tuff (Miocene) 4383 " ’ . . . . 1,1 Dike rocks (Miocene and Oligocene?) — DaCIte and quartz basalt MDc 35/ Pando Porphyry (Upper Cretaceous) — Quartz latite porphyry; in sill , ,, Dakota Sandstone (Lower Cretaceous) — Light—gray sandstone 43 82 ‘ Morrison Formation (Upper Jurassic) —- Interbedded light-gray sandstone, vari- colored shales, and gray limestone Entrada Sandstone (Upper Jurassic) — Buff to orange sandstone '30 Chinle Formation (Upper Triassic) — Red and purple siltstone 43 81- PIPm Maroon Formation (Lower Permian and Upper and Middle Pennsylvanian) - Red sandstone, siltstone, and grit IS Limestone bed ‘ - , 4 ,f . . , 5 . I v I I 7 I i. . ‘ I hm Ii . ,- A. _ , I I . le Minturn Formation (Middle Pennsylvanian) — Grit, conglomerate, sandstone, and 4380 ’ _ - r ' ' , ' ,7 ' ‘ ' , i f ‘ , ' 12 ‘_ ‘ , . ' (H , , " : " .. m \j , " , c . I " ~. . T i ..... shale, and intercalated beds of carbonate rocks, predominantly gray, but red in . .: i 7 1 _: ' ‘ . ‘ " ‘ ‘ i ' ' , - ,_ " 'i.‘ _ ' - , . ' ' ' I t I upper part and in irregular zone near base \ lej Jacque Mountain Limestone Member ‘ ‘0 , vi Imeq White Quail Limestone Member ‘ 3 8 ‘~ 8 IPme Elk Ridge Limestone Member 8 E) 11’er Carbonate beds in Robinson Limestone Member \‘1‘ ‘ . . (N 4378 ler Resolution Dolomite Member Blue lines representing car- \ bonate units are dashed 2 Ith Hornsilver Dolomite Member } where approximately 10- TL . I 4 , . cated or inferred; dotted E ’l‘ 2‘, , leW Wearyman Dolomite Member where concealed hi 3 i LLLLLLL (I) i 1 gm Grit marker bed 3 /i 4377 * ------- / 'i; I, . . 8 Jiiii “I? rdl Reef dolomite of Lionshead 9 I III! I Ii ;_ _ dd Dolomite bed of Dowds t i. . ii i"! i ' ' a I\II I i , c Carbonate beds )1 \ \I‘ b x 4375 . . . . l-\ \ L/ / IPb Belden Formation (Middle Pennsylvanian) — Gray to black shale and thin beds of \ \\ \ \/ . . . . . . 4376 \ / limestone. Local thin patches of regolithic mudstone of Molas Formation in- p \\ . cluded at base (Lower Pennsylvanian) iI . \/ Leadville Limestone (or Dolomite) (Lower Mississippian) — Light-gray limestone ’/! , '/ / or dark-gray dolomite , 43 . . , , 3% , 75 Chaffee Group (Lower Mississippian? and Upper Devonian) — Cons1sts, from top 4375 .. A’ - " ' , E x ‘ / . , ‘ ' / If I down, of Gilman Sandstone, Dyer Dolomite, and Parting Formation I ”5 , E I . ‘ Parting Formation (Upper Devonian) — Along flank of Gore Range Harding Sandstone (Middle Ordovician) — White and green sandstone and shale 43 000 . ' . % . '7 74 m N" Peerless Formation (Upper Cambrian) — Red, buff, and green dolomite and brown 4374 I "j .I Iéi/ 1 I-IfI i sandstone I . I 610 000 i/ - ' l ' » ‘ r/ ’ Sawatch Quartzite (Upper Cambrian) — White quartZite / V‘ \ i 1 I i I I// // r’ .4 ,2. FEET t’ j I . =» _ i g“ . : I , I r%’ 05/7 ya\ -._. Cross Creek Granite (Precambrian X) — Granodiorite and quartz monzonite, por- ti . - A; . i L IE , . . its , e i o“ ” / 1‘44: //”25» a .. Phyritic in part 39030, 1 _ , , iv r. r t . , . i , 3. i \ . // / \ 39030, . I . I . I 106° I i1 720 000 FEET 373 374 3756 xc 379 €p 380 I (HOLY C 88) cp £25 383 384 385 20’ R. 80 W385 O, 03er o , Diorite (Precambrian X) — Biotite—quartz diorite and hornblende diorite 3O Xm F‘ANDO 5 MI. 391000m.E 106 15 I LEADV/LLE 25 MI. /,;7 Migmatite (Precambrian X) Base from US. Geological Survey 1162,500, 1934—50 SCALE 1248 000 Geology by T. S. Levering and 00 Interstate Highway 70 as of 1970 1 17% O 1 2 3 MILES Ogden Tweto, 1940—41, 1946, 1961 1,) Biotite gneiss (Precambrian X) — Biotite—quartz-plagioclase gneiss . . l—l l—-l l——l l—l I-———l F—-—-—-—i l————% 10,000—foot grid based on Colorado coordinate (/ SYMBOLS ON SECTIONS ONLY system, central zone 1 .5 o 1 2 3 KILOMETRES o 1000—metre Universal Transverse Mercator grid H H H H H "— ‘ *fl O< - Leadville Limestone, Chaffee Group, Harding Sandstone, Peerless Formation and tiCkS, lone 13, shown in blue I CONTOUR INTERVAL 50 FEET 1” Sawatch Quartzite, undivided APPROXIMA E MEAN ”‘CL'N‘T'ON'B” DATUM IS MEAN SEA LEVEL - Precambrian rocks, undivided Contact — Dashed where approximately located; dotted where concealed 3: u.l +4 75 8 A E ZI A’ O _i_L... Fault — Showing dip. Dashed where approximately located; dotted where con- E Q— E -‘ 9 ~ .2 8 cealed. Bar and ball on downthrown side 0 ' .3 U § IQ I V) . o O is I 0: m ‘5‘- _ 3 E '3 8 O o o o o b k b ' ‘d 8* g“ I Q o l: 5' a: 3 g 8 o o o I 0 Car onate roc 5 replaced y JasperOi co \— ,_ m - I I Folds — Showing approximately located trace of axial plane ’ §— I ”Pm l 8 _ —— Anticline — o °° | I i °° —-——- Syncline 8 ' i iPmI I _ 0 RE 8 I I L o _ g Strike and dip of bedding 0 ‘ 0 <0 I I 8 ’3 Inclined — _ i I “ T —l— Vertical 8 I I GORE FAULT ZONE 0 70 0 9r — I I — 8 —a— Overturned O — st 0 * o - I I II — 8 60 . . . . r O I I / I _ + Strike and dip of foliatlon O - §E i :8: _ 37*— Plunge of lineation I _ —°— Vertical SEA LEVEL— E _ SEA LEVEL -—--———- Line of measurement, type section of Minturn Formation Surficial deposits not shown El g .‘E 5i: 3 is 3: E.) H u.l O * o B m I U" 0 Lu D It 0 . SAWATCH RANGE g g] a I <2 g Bo 3 2 d1.) 0 “z" e i I73 “‘ - n —8 | in N x 2 .1 3 ~ F r MW, mCs 9" 0| 0 Z - E 5 | k 2 o l» <3 E EEEWE‘W—‘fiimm :; a: w <( O. D i U E i— ; E o g k] I o z i?» § E1 A 2 2 E E 8 8 , E Lu > I“: 5 I "5 g in < kl o 8— O. i n: cc m a 5 l 5. a: Ple r8 — in l m — E3: Is I lej L 8 ‘11 i i o — E Ple i ‘ °° l o — i 0 §‘ 8 l Pm —8 o“ 1 N I _ i L Projection north of _ 8 i quadrangle boundary 0 e ‘ ' — 8 8 L i 8 O .— - | _ “I3 8 o g o — _ o N a _ SEA LEVEL J _ E Surficial deposits not shown ‘2 S A LEVEL 2 Lu u] a: o A: 44 .5: :5 2I m m z w 4‘ o L o C 9r< a 3 5 9'n 5 S C’S 8* 5H: “ -’< l— *4 E ' " D E’ 0i u.l ’- I o 0 _I - Oi u.l 5 3, § Q i— J | 0' m E < "1 ”’I '~ § 3 N — eTic ml 3| D a: 3 g "’I '4 i. IPmr A .. E o r 44 § 3 E I‘ g :5, E o o_ 8 u 3 8 g L g | I 5 Q: Q 8 m T _ I '5 g 9 F8 0 I PEPm U _ o_ I O O _ 8 ii I ' "' g o 1 I lej 8 E o I I 8 N 8e 1 8 ‘8 (O I 8 1 _ i O I E 8 _ i 8 8 7 V I Pm I 8 O O _ i I _ o E I I 9 o 8_ I I 8 _ N I I 8 _ ' 1 Pt: I M SEA LEVEL — I Thin surficial deposits not shown _SEA LEVEL IJJ LU ': E w I d g u.l E Lu 2 _ O z D o l- _ Z - z x .l “ <21 3 5 < ,3: SAW CH 2 (z) 2' E E 2 g E w— o LIJ AT ‘ _ O~ " e «’1 8D oz :5 De RANGE ,4 gt pl‘.’ 5 5; 3; 96 0 R E a“ A N G EE’O q- W > _ 0) -~ -°~ E in u: 8 0'” *4 e: P E " 8 g I f” o x °< 9 E m w 3| r: is 8 ‘1’ N _ Red and White 2 0 g» t: I o a Q ‘— 5 Mountain Je+Jm "J a .S E In 3 g f E o 8 - ‘m m ‘14 Profile of prevolcanic valley 8 O ,4 {N [w E o E 8 q— t Q o 0 93 O 8 O m 9 9 (9) rrrrrr El GORE FAULT ZONE /, g. 8 _ _ » [Pm 1 /- ‘_ 0') ° mms/ o o I. 0552‘ “E g 8 r E 8 Strand of O o o O O Homestake O N o _ L o N o o O ‘0 shear zone 0 N to ‘9 8 Postvolcanic surficial deposits not shown Surficial deposits not shown nus. Government Printing Office: 1977-77 7-034/2 GEOLOGIC MAP AND SECTIONS OF THE MINTURN 15-MINUTE QUADRANGLE, EAGLE AND SUMMIT COUNTIES, COLORADO @5‘75 L»! F‘w ,w; ‘c. , Wm 1" 0 U: 453 Lead in the Environment GEOLOGICAL SURVEY PROFESSIONAL PAPER 957 , (c I \\~4ff Q , «‘ \ENCE “6%“ i l 1 DEC 15 1976 --.‘n Lead in the Environment T. G. Lovering, Editor GEOLOGICAL SURVEY PROFESSIONAL PAPER 957 A compilation of papers on the abundance and distribution of lead in rocks, soils, plants, and the atmosphere, and on methods of analysis for lead used by the US. Geological Survey UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPPE, Secretary GEOLOGICAL SURVEY V. E McKelvey, Director Library of Congress Cataloging in Publication Data Main entry under title: Lead in the environment. (Geological Survey Professional Paper 957) Bibliography: p. 1. Lead 2. Geochemistry. 3. Lead—Analysis. I. Levering, Tom Gray, 1921- II. Series: United States Geological Survey Professional Paper 957. QE516.P3L4 553'.44 76—7962 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001—0291 l-l CONTENTS Page Summary, by T. G. Lovering ........................................................................................................ 1 Inorganic chemistry of lead in water, by J. D. Hem ......................................... 5 Organic chemistry of lead in natural water systems. by R. L. Wershaw ............................ 13 Distribution of principal lead deposits in the continental United States, by A. V. Heyl .. .. l7 Migration of lead during oxidation and weathering of lead deposits, by Lyman C. Huff ......... 21 Lead in igneous and metamorphic rocks and in their rock-forming minerals, by Michael Fleischer ............................................................................................................... 25 Abundance of lead in sedimentary rocks, sediments, and fossil fuels, by T. G. Lovering.. 31 Lead content of water, by M. J. Fishman and J. D. Hem ................................................ 35 Lead in soils, by R. R. Tidball ................................................... 43 Lead in vegetation, by H. L. Cannon ........................................................................................... 53 Lead in the atmosphere, natural and artificially occurring lead, and the effects of lead on health, by H. L. Cannon .................................................................................................. 73 Analytical methods for the determination of lead, by F. N. Ward and M. J. Fishman .............. 31 ILLUSTRATHDNS 938‘! FIGURE 1. Diagram showing fields of stability for solid species and dominant solute species in system Pb+H20 as functions of pH and redox potential ................................................................................................................................................................... 6 2. Diagram showing fields of stability for solids and solubility of lead in system Pb+COT+S+H.,O at 25°C and 1 atm pressure... 7 3. Map showing principal lead deposits of the United States ............................................................................................................. 18 4. Graph and sketch showing relationship between unnamed vein and geochemical anomaly in residual soil, Porters Grove Range, Wis ...................................................................................................................................................................... 22 5. Histograms of lead distribution in granite, granodiorite, basalt, and gneiss .............................................................. 26 6. Histograms showing frequency of occurrence of lead in K-feldspar from granitic pegmatite and granitic rocks and in plagioclase from pegmatite ............................................. 28 7. Graph showing lead content of common sedimentary rocks 32 8. Map and histogram showing distribution of lead in soils and other surficial materials of the United States .. 48 9. Map and histogram showing concentration and distribution of lead in soils of Missouri . .............................. 50 10. Graph showing comparative lead content of soils and plants near Ameka lode, Nigeria ..... 60 11. Graph showing lead in grass collected for 1,000 feet from highway in 1961 and in 1969 ........................ 67 TABLES Page TABLE 1. Standard Gibbs free energies of formation of lead species and related solutes ................................................................................ 5 2. Chemical equilibria and solubility equations for lead species in hydroxide and carbonate domains ........................................... 7 3. Observed and calculated saturation concentrations of lead in US. surface waters collected during October and November 1970 ........................................................................................................................................................................... 9 4. Range of copper, lead, and zinc content of soils collected near ore veins .................................................... 22 5. Ore-metal content of stream-sediment samples (minus 80-mesh) collected in and downstream from the Tombstone district, Arizona .......................................................................................................................................................................... 23 III IV TABLE 6. KOCDV 10. ll. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. . Published estimates of lead contents of igneous and metamorphic rocks .................... . Summary of published analyses of lead content of igneous and metamorphic roeks.. . Range of variation of Ba/Pb ratio in igneous and metamorphic rocks ........................................... CONTENTS Mean lead contents of plants compared with mean lead contents of associated soils in both nonmineralized and mineralized areas ........................................................................................................................................................................ Lead content of feldspars ........................................................................................... Lead content of other rock-forming minerals ............................................................................................................................ Maximum and minimum concentrations of lead found in surface waters of the United States and Puerto Rico from 1960 to 1971 ................................................................................................................................................................................ Lead concentrations in selected surface waters of the United States, ........................................................................... Maximum and minimum concentrations of lead found in ground waters of several States and Puerto Rico from 1960 to 1971 .................................................... - ............................................................................................................................ Lead concentrations in residual soils from Wisconsin, grouped according to pH ......................................................................... Lead concentrations in selected soils developed under a variety of soil-forming factors" ................... Lead concentrations in selected Scottish soils developed from different parent materials.. Mean concentrations of lead in selected parent material types and derivative soils ......... Lead concentrations in tropical soils developed on different parent materials ............................................................................... Distribution of lead concentration and organic matter in soil profiles from the tropical climatic zone ....................................... Distribution of lead concentration in soil profiles from the temperate climatic zone .............................................. Cycling of lead in different environments ........................................................................................ Normal lead concentrations in various classes of vegetation ......................................... Lead in garden produce ..................................................................................................................................... Seasonal variations of lead in trees and soils... . ' Concentrations of lead in unpeeled and peeled garden produce from Maryland. ........................ Effects of lead and other metals on growth from four plant families .............................................. Lead in conifer twigs, needles, and subjacent mull, Coeur d’Alene district, Idaho. Lead content of vegetation and soils from several mining districts ......................................................... Maximum lead and zinc contents of plants and drained peat, Orleans County, N.Y. .............................. Lead indicator and accumulator plants rooted in mineralized ground ........................................................................................... Lead contents of plants and soils as related to smelter contamination in Oklahoma and Colorado ............................................. Comparison of lead content between on-road and off—road samples of some soils and plants in Missouri ..................... Lead concentrations in surface air in 1967 from selected sites along the 80th Meridian ............................... Lead emissions in the United States, 1968 ............................................................................ Normal lead content of some uncontaminated natural substances ..................................................................................... List of minerals and alloys in which lead is a major constitutuent ................................................................................................ LEAD IN THE ENVIRONMENT—SUMMARY By T. G. LOVERING Lead is a soft, heavy, malleable, relatively inert metal with a low melting point (327.4°C), which has been known and used by man since ancient times. It has the chemical symbol Pb (from its Latin name plumbum), atomic number 82, and atomic weight of 207 .2, making it the heaviest of the common metals. It is the stable end product formed by the radioactive decay of uranium. Metallic lead was used in antiquity for jewelry, plumbing, and cooking utensils. In modern times metallic lead and lead alloys are extensively used for storage battery plates, sheathing for electrical cables, small-caliber ammunition, shielding for X-ray apparatus and atomic reactors, type metal, bearing metal, and solder. Lead compounds are also important as components in the manufacture of paint, ceramics, and glass and as an anti’ knock additive in gasoline. In 1970 the United States produced about 570,000 tons of lead and consumed about 1,380,000 tons. Much of the excess consumption was derived from recycled scrap metal, and the rest was imported chiefly from Canada and Mexico. Ores of lead and zinc are often closely associated in deposits formed by replacement of limestone or dolomite. Lead ore is commonly present together with ores of copper, zinc, silver, arsenic, and antimony in complex vein deposits that are genetically related to silicious igneous intrusive rocks, but lead ore may occur in a variety of igneous, metamorphic, and sedimentary host rocks. Although both metallic lead and the common lead minerals are nearly insoluble in pure water, they are appreciably soluble in' certain organic acids; likewise, some of the lead compounds produced industrially are also considerably more water soluble than the element. If lead enters the human body in soluble form it accumulates there and can cause lead poisoning. Airborne lead in smoke from smelters, or from the coking of coal with a high lead content, may produce toxic effects in plants, grazing animals, and humans exposed to the smoke for a long time. Lead poisoning may also result from the ingestion of lead-bearing paint by small children or of lead leached from pottery glazes by the citric acid in fruit juices. Lead compounds released into the atmosphere in the exhaust fumes of automobiles have produced abnormally high concentrations of lead in the blood of individuals continuously exposed to these fumes for long periods of time, but as yet there is no established instance of lead poisoning resulting directly from this source. MIGRATION OF LEAD IN THE NATURAL ENVIRONMENT Natural concentrations of lead in lead ore deposits do not normally move appreciably in normal ground or surface water, because any lead dissolved from primary sulfide ore tends to combine with carbonate or sulfate ions to form insoluble lead carbonate or lead sulfate or else to be adsorbed by ferric hydroxide. Mechanical disintegration and transportation of these insoluble lead compounds can remove lead from the surface of lead ore bodies and disperse it to some extent. Lead can also be leached by acid waters, particularly those that are rich in organic material, and travel in solution as soluble lead organic complexes. In this form it can be taken up by plants and enter the food chain, but examples are rare. In regions characterized by alkaline, neutral, or saline waters and soils, naturally occurring forms of lead do not enter either water or plants except in very minute traces. Lead minerals in a non- reactive host rock such as sandstone or quartzite have been known to dissolve in acid waters in amounts toxic to vegetation in a small area, but there are no known instances of lead poisoning in humans related to the natural occurrence of lead. ABUNDANCE OF LEAD IN ROCKS, SEDIMENTS, FOSSIL FUELS, AND MINERALS The average abundance of lead in the Earth’s crust is approximately 15 ppm (parts per million), which is equivalent to half an ounce of lead per ton of rock. The lead contents of the common rock types that make up the crust of the earth range from about 30 ppm for granitic rocks, rhyolite, and black shale to about 1 ppm for evaporite sediments, basalt, and the ultramafic igneous rocks such as dunite, which are rich in iron and magnesium and poor in silica (table 36). Unconsolidated continental sediments have a mean lead content very close to the mean crustal abundance of 15 ppm, but unconsolidated deep sea muds are appreciably l 2 LEAD IN THE ENVIRONMENT richer in lead, containing an average of about 60 ppm lead, and concentrations as high as 0.2 percent lead have been reported in sediments precipitated from hot brines in the Red Sea (table 36). Coal contains an average lead concentration of about 10 ppm, and petroleum an average of less than 1 ppm of lead (table 36). Most of the lead in these fossil fuels concentrates in their ash when they are burned, although it has been estimated that during industrial burning coal may release as much as 6 percent of its original lead content into the atmosphere. Petroleum produces far less ash than coal, and inorganic constituents are much more highly concentrated in its ash. Although lead is a major constituent of more than 200 known minerals (table 37), most of these are very rare, and only three are commonly found in sufficient abundance to form minable lead deposits. These three are galena, the simple sulfide of lead (PbS); anglesite, the lead sulfate (PbSO4); and cerrusite, the lead carbonate (PbC03). Galena is a common primary constituent of sulfide ore deposits; anglesite and cerrusite normally form by the oxidation of galena close to the surface. Lead is also present in trace amounts in many of the common rock-forming minerals (table 36). The amount of lead in any one of these minerals varies widely, and the greater the normal lead content of such a mineral, the greater the observed variation is likely to be. For instance, potash feldspar (orthoclase or microcline) generally con- tains the most lead of any of the common silicate minerals; yet, whereas samples from one group of pegmatite dikes in Norway yielded 280 ppm lead, similar samples from another group of pegmatite dikes in the same region con- tained less than 10 ppm. The maximum amount of lead this mineral can contain is unknown, but 2,800 ppm lead has been reported in a sample of a green variety of micro- cline called amazonite. The common silicate minerals found in igneous rocks, in order of decreasing lead content, are (1) potash feldspar; (2) plagioclase feldspar and muscovite mica; (3) pyroxenes, amphiboles, and biotite mica; and (4) quartz. The common minerals of chemically precipitated sedi- mentary rocks (calcite, dolomite, gypsum, and halite) all normally contain less than 10 ppm lead. The lead content of the sedimentary clay minerals is extremely variable but is commonly on the order of 10—20 ppm. NATURAL ABUNDANCE OF LEAD IN WATER The concentration of lead in river water is low under natural conditions (table 36). Although small amounts of lead are widely distributed as a minor constituent in rock and soil minerals, lead is only slowly released by weathering processes. Even where the element is concentrated in ore deposits, the low solubility of lead in water that contains dissolved carbon dioxide species and has a pH near neutrality generally will maintain concen- trations of lead in solution below a few tenths of a milli- gram per litre. The waterborne element tends to be complexed by relatively insoluble organic matter and may also be extracted from water by organisms. The median concentration of lead in river and lake water of the United States is about 2 )ig/l (micrograms per litre). Concentrations of lead in seawater range from a few hundredths of a microgram per litre (a value of 0.03 pg/l is widely quoted) in the deeper parts of the ocean basins to 0.4 p.g/l observed at several places both nearshore and far offshore in surface waters of the Pacific Ocean. The higher nearshore and near-surface concentrations, however, are ascribable to atmospheric fallout of lead particles or washing out of such particles by rainfall. Concentrations of lead in rainfall over the United States have been reported to average 34 )ug/l over a 7-month period in 1966 and 1967, with a median value of about 10 ,ug/l. When the distribution of sampling points is correlated with reported concentrations, results show strongly the effects of indus- trial air pollution near cities. ABUNDANCE OF LEAD IN SOILS Soil samples collected from nearly a thousand localities throughout the conterminous United States ranged in lead content from < 10 to 700 ppm and had a mean lead concen- tration of 16 ppm (Shacklette, 1971); only 6 percent of these samples contained more than 30 ppm of lead (table 36). Many of the soil samples with lead content in excess of 100 ppm were obtained from localities in western Colorado, but others came from widely scattered isolated sites. Lead content of young residual soils is strongly influenced by that of the parent rock from which they are derived; however, this relationship is modified, and may be obscured, by other factors in mature soils developed on deeply weathered parent material. These factors include oxidation and reduction reactions, linking of organic compounds by lead ions (chelation), base exchange reactions by clay, adsorption of lead by hydroxides of iron and manganese, local solution and transportation by organic acids, and cycling by vegetation. In general, lead is more mobile in acid soils than in alkaline soils, tending to be leached out of the former and to form residual concen- trations in the latter. Relatively high total-lead concen- trations in alkaline soil may reflect residual concentra- tion of lead in an insoluble form, which is not available to plants. LEAD IN VEGETATION Lead occurs naturally in small amounts in all plants (table 36). The concentration of lead in vegetation varies not only with the individual species, but also as a complex function of climatic variations, parts of the plant, composition of the soil in which the plant grows and of the rock from which this soil is derived, and finally the SUMMARY 3 effects of artificial contamination of both the water that nourishes the plant and the air that surrounds it. Anomalously high concentrations of lead in plants may reflect natural contamination from lead dsposits or arti- ficial contamination of the plant’s environment by man. Extensive analyses of plants from primitive areas, unaffected by either of these sources of contamination, are required to establish normal background values for lead in natural vegetation. Median concentrations of lead in the ash of uncon- taminated natural vegetation are highest for lichens (1,000 ppm), an order of magnitude lower in mosses (100 ppm), about 30-50 ppm in evergreen trees, and about 15-30 ppm in deciduous trees, shrubs and grasses; however, all of these types of vegetation exhibit a wide range in lead content (commonly an order of magnitude) on both sides of the median. The ash of uncontaminated domestic fruit and vegetable samples from the United States has a median lead content close to 10 ppm, which is the lower limit of the analytical method. The observed range for lead in the ash of these foods is from <10 to 100 ppm. The lead content of these plants expressed in dry weight varies from about one-fourth to less than one-tenth the lead content of the ash. Thus the highest observed lead-in-ash content of 100 ppm, for a tomato sample, corresponds to a dry-weight lead content of about 7 ppm and to a lead content for a fresh tomato of less than 1 ppm. Studies of seasonal variation in the lead content of trees suggest that lead concentration is highest in early spring at the beginning of the growing season, declines during the summer, and rises again in the fall. Lead also tends to concentrate‘in certain parts of a growing plant. In trees, the highest lead concentrations are usually found in the older twigs; somewhat less lead occurs in the young twigs, seeds, and trunk wood, still less in the leaves or needles, and least of all in the roots. On the other hand, lead content of the leaves of certain vegetables appears to be higher than that of their stems, and the lead in fruits and root vegetables is largely concentrated in the skin or peel. There appears to be a general tendency for lead to be more abundant in plant ash than in the soil, and more abundant in the soil than in the bedrock, but there are many exceptions. The knowledge that certain plant species have the ability to absorb anomalous amounts of lead from lead- rich soils and their underlying, parent materials has been used as a biogeochemical tool in prospecting for lead deposits for a quarter of a century. These accumulator plants include certain species of both evergreen and deciduous trees, as well as many shrubs and smaller plants. Contamination of food and forage crops by artificial lead compounds contained in insecticide sprays, automobile exhaust fumes, and industrial smoke is a matter of concern to public health workers. Anomalously high lead concentrations have been found in leafy vegetables and grasses grown in proximity to major highways, in crops grown on soil with a long history of treatment with lead-bearing insecticides, and in crops exposed to fallout from smelter smoke. LEAD IN THE ATMOSPHERE Lead enters the atmosphere largely in the exhaust fumes from internal combustion engines and, to a lesser extent, from the smoke produced by / large-scale industrial burning of coal. Consequently the lead content of the air is highest in urban industrial areas and lowest in rural areas (table 36). The average lead concentration in the air of large metropolitan areas is about 2.5 rig/m3. In rural areas it is less than 0.5 ,ug/m3. The amount of lead present in the air at any particular place varies with traffic density, air temperature, and atmospheric conditions. The lead- bearing particles in the air are heavy and tend to collect in low areas with poor air circulation; lead concentrations greater than 40 pg/ma have been measured in the air of vehicular tunnels. ‘In spite of this tendency of lead to accumulate close to the ground, traces of it enter the upper atmosphere and are carried widely around the earth to return to the surface in rain or snow. The lead content of a snow sample collected in 1965 from the Greenland ice cap was approximately 0.4 lug/l of melted snow as compared to about 0.05 ,ug/l in melted ice that had formed in 1864. METHODS FOR DETECTION 0]" LEAD Lead may be determined in water, soil, and rock, and in the ash of plants and organic fuels by a wide variety of methods. The analytical methods employed for both water and solids in the laboratories of the US. Geological Survey include optical emission spectrography and atomic absorption spectrophotometry. In addition, X-ray, spectrographic, and colorimetric methods are employed for the analysis of rock, mineral, and soil samples; electron microprobe analysis of polished ore samples is used for the nondestructive determination of lead in high concen- trations in small mineral grains. Water samples are filtered to remove solid particles and are then acidified. In the atomic absorption method a lead-scavenging organic compound is first added to the water and then extracted with an insoluble organic fluid (methyl isobutyl ketone); this fluid is then introduced into the flame of the atomic absorption spectrophotometer. In the emission spectrographic method, lead and other metals are first precipitated by adding a suitable reagent to the water sample. The precipitate is filtered, dried, and introduced into the arc of the emission spectrograph for p the simultaneous determination of the metals present. The optical emission spectrograph is the most widely used laboratory tool for chemical analysis of solid samples that do not contain large amounts of organic carbon, because this method provides a simultaneous analysis for a large number of elements on a very small quantity of 4 LEAD IN THE ENVIRONMENT sample material. In routine operation it will normally determine lead in concentrations as low as 5 ppm. Specific analyses for small traces of lead in such materials are made by the atomic absorption method, which, in routine operation, will give good results for lead in amounts as small as 1 ppm, and, with special sample preparation, this method can be used to detect even smaller amounts. Colorimetric methods with a sensitivity to about 20 ppm of lead are used for rapid field tests in reconnaissance geo- chemical exploration. UNITS AND NOTATION The amount of lead in natural materials is expressed in different ways for different substances by the authors of this report. Lead concentrations are generally reported in the following units: For water— }; g/l=micrograms per litre mg/l=milligrams per litre For solids— ppb=parts per billion ppm=parts per million lug/g=micrograms per gram oz/ton=ounces per ton For air— ng/m’=nanograms per cubic metre p. g/m3=micrograms per cubic metre g/ha=grams per hectare Conversion factors— ] ng=1 nanogram=0.000000001 gram 1 ,ng=1 microgram=0.000001 gram 1 mg=l milligram=0.001 gram 1 ppb=l part per billion=0.000001 percent 1 ppm=l part per million=0.0001 percent 1 part per thousand=0.l percent 1 oz/ton=3l parts per million=0.003l percent 1 g/metric ton (t)=l part per million=0.0001 percent 1 ha=l hectare=l,000 square metres Symbols >=greater than <=less than ~ =approximately INORGANIC CHEMISTRY OF LEAD IN WATER By J. D. HEM The concentration of dissolved lead in most natural water systems, which usually contain dissolved carbon dioxide and have a pH near 7, is very low—commonly less than 10 ,ug/l—partly because lead combines readily with carbonates, sulfates, and hydroxides normally present in such waters to form compounds of low solubility. Highly saline waters or strongly acid waters may contain appreciably higher concentrations of lead in solution, but such waters tend to be very restricted in distribution. STABILITY FIELDS OF INORGANIC LEAD COMPOUNDS IN NATURAL WATER SYSTEMS An outstanding characteristic of lead is its tendency to form compounds of low solubility with the major anions of natural water. The hydroxide, carbonate, sulfide, and more rarely, the sulfate of lead may act as solubility controls. Over most of the Eh-pH region, in which water is stable at 25°C and 1 atm (atmosphere) pressure, the divalent form, Pb+2, is the stable species of lead. The more oxidized solid PbOz, in which lead has a +4 charge, is stable only in a highly oxidizing environment; thus Pb+4 probably has very little significance in controlling the behavior of lead in aqueous geochemical systems. If sulfur activity is very low, metallic lead can be a stable phase in alkaline or near-neutral reducing conditions in the presence of water. Fields of stability for solid species of lead in the system Pb+H20 at 25°C are shown in figure 1. A dissolved activity of solute lead species of 10""32 mol/l (equivalent to l g/ l) was used to locate solid-solute boundaries. The stability fields of solids in the Eh-pH diagram would be enlarged at the expense of the areas of Pb+2 and Pb(OH)3' if a higher dissolved lead activity were assumed. The boundaries in figure 1 were calculated from ‘ thermodynamic data for lead assembled in table 1. The data were selected from published literature, mostly from the compilation of Wagman and others (1968). The range of published free energy values is rather large for some of the species of lead important in this study, and the choice of values can influence stability and solubility calculations substantially. Standard Gibbs free energy values compiled in the National Bureau of Standards Technical Notes (Wagman and others, 1968) should be mutually compatible, but for a few of the lead species either no values are given or values in other publications appeared to be in better agreement with the consensus of published solubility information. Free energy values for PbS and PbCOs in table 1 were taken from Robie and Waldbaum (1968) because they seemed to give solubilities nearer those reported by most investigators, as quoted by Sillén and Martell (1964). No value for the hydroxy-carbonate solid or PbSO, aq is given in either the compilation by Robie and Waldbaum or that of Wagman and others. The value given by Wagman and others for PbOH+ is not compatible with stability data for this complex given by Sillén and Martell (1964). The value chosen here is calculated from a stability constant published by Faucherre (1954), which agrees better with the considerable number of values for this ion determined by other investigators, even though it was calculated at 20°C rather than 25°C. The discrepancy introduced by this deviation from standard temperature is negligible. Faucherre’s data were obtained for a solution of 0.06 ionic strength, and have been adjusted to zero ionic strength using the Debye-Hiickel equation. Faucherre’s work also provided data for the polynuclear ion Pb4(OH)4 +4. Latimer’s (1952) compilation of free energy data gave a value for Pb(OH)2c nearly 8 kcal less negative than the one given in table 1. The rather wide range in thermodynamic data adds a considerable factor of uncertainty to the solubility calculations. The mixed oxide Pb304 c apparently is less stable than Pb(OH)2 c in the system specified for figure 1 and does not appear in the Eh-pH diagram. The Pb(OH)2 c value given in table 1 results in a large domain for this species in the Eh-pH diagram and almost eliminates the mixed hydroxy carbonate from consideration. The natural occurrence of this mineral, however, does suggest that the difference between the stabilities of mixtures of PbCOg and Pb(OH)2 and of the basic carbonate likely favors the basic carbonate. If the free energy value of Latimer is used for the basic carbonate, the species has a narrow stability field near pH 8, and this field would be considerably enlarged if the carbon dioxide species concentration were assumed to be higher than the 10'3 mol/l used in drawing the boundaries in figure 2. 6 LEAD IN THE ENVIRONMENT The system specified for figure 1 is highly simplified. To be more applicable to systems involving ordinary surface or underground water, the effects of dissolved carbon dioxide and sulfur species must be included. If a fixed total activity of dissolved carbon dioxide species and a fixed total activity of dissolved sulfur species are specified, stability fields of lead carbonate, sulfate, and sulfide can be shown. Figure 2 is an Eh-pH diagram for the system Pb-COz-HrzO-S at 25°C and 1 atm pressure. Total activities of dissolved C02 and sulfur species are both 10-3 molar. Areas of stability of solids and the solubility of lead are shown in figure 2. Table 2 gives equations used for the solubility calculations in that part of the diagram where Pb(OH)2 c and carbonate or hydroxy carbonate are dominant. As specified here, the activities of these dissolved carbon dioxide species do not necessarily involve a gas phase. Specifying a fixed partial pressure of carbon dioxide, as is sometimes done in solubility calculations of this type, tends to lead to a very high, probably unrealistic, concen- tration of carbonate in solution at the higher pH’s. FrOm figure 2 it is evident that the solubility of lead is very low (less than 1 pg/l) between pH 8.5 and 11, and is 1.2 y | , l ‘ l ' | ' I r Water oxidized Pboz 0.4 — PM2 0.2 m _ Eh, in volts Pmom; —o.2 — ~ _0.4 a — 0.6 — Water reduced _0_8.I.I.| 0 FIGURE l.—Fie1ds of stability for solids species (c; patterned) and dominant solute species in system Pb+H20 as functions of pH and redox potential. Dissolved lead activity 10'“? mol/l at 25°C and 1 atm pressure. TABLE 1.—Standard Gibbs free energies of formation of lead species and related solutes. [c, solid state; aq, dissolved] Free energies Formula AG°f(kcal/mol) Source of data Pb+2 at] —5.83 Wagman and others (1968). PbOH+i aq —51.41 Calculated from Faucherre (1954). Pb(OH)2 aq -95.8 Wagman and others (1968). Pb(OH)5- aq —137.6 Do. Pb4(OH),+‘ aq —225.6l Calculated from Faucherre (1954). PbCl+ aq —39.39 Wagman and others (1968). PbCl 2 aq —71.05 Do. . Pb504 aq —187.38 Calculated from Kolthoff, Perllch, and Weiblen (1942). PbO c 2 —44.91 Wagman and others (1968). PbO c 3 —45.16 Do. Pb(OH)2 C —108.1 Do. PbS C —22.96 Robie and Waldbaum (1968). PbC03 c —l50.3 Pb3(OH)2(C03)2c -409.1 Do. Latimer ( 1952). PbSO4 c —194.36 Wagman and others (1968). PbOz c —51.95 Do. Pb304 c —143.7 D0. H20 ‘ —56.69 Do. OH” aq —37.59 Do. H28 aq —6.66 Do. HS' aq 2.88 Do. 804—2 aq -l77.97 Do. HSO,‘ aq —180.69 Do. H2C03 aq —l48.94 Do. HCOs' aq —l40.26 Do. C03 ‘2 aq —126.17 Do. ‘At 20°C. 2Yellow. ’Red. ‘Liquid. also low in all strongly reducing systems down to pH 2. H0wever, between pH 6 and 8 the solubility of lead is a rather complex function of pH and total dissolved C02 and sulfur species. Lead can be relatively soluble in a dilute natural water below pH 6.5. In such a solution if the activity of dissolved CO2 species were lower than 10'3 molar, the lead solubility would be greater than shown in the diagram. Some dilute river waters thus could be a favorable chemical environment for the solution of lead. Within the field of stability for PbSO4, the solubility of lead is a function of the total sulfate concentration. This control is most effective at rather low pH and when sulfate concentrations are high. The dissolved ion-pair PbSO4 aq is not a dominant form of dissolved lead unless sulfate activity exceed 1072'?2 mgl/l. In a solution of this type ionic strength would be rather high, requiring a stoichio- metric sulfate concentration of at least 500 mg/l to produce the specified activity ofSO‘f—2 ions. The PbSO“ aq complex does influence the solubility of lead shown in fugure 2, however, and was taken into account in locating the solubility lines. The solubility of lead is near 5 mg/l in most of the PbSO4 c field; within this area solubility is constant above pH 2 because of the specified constant sulfur activity. Below pH 2 the lead solubility increases because some of the sulfate is converted to HSO; at very fi—— INORGANIC CHEMISTRY OF LEAD IN WATER 7 l ' l l l l ‘ ‘ Water oxidized Pb dissolved (pg/l) Pb dissolved ( [Lg/l) O ‘_' 0.6 0.4 :2 '0 > .S .5 LIJ —O.2 Pb dissolved -0.4 # (pg/l) —O.6 — Water reduced _0_ l l I l | l | 80 2 4 6 8 10 12 14 pH FIGURE 2.—Fields of stability for solids (c, patterned) and solubility of lead in system Pb+COz+S+H20 at 25°C and 1 atm pressure. Ionic strength 0.005. low pH. Under reduced conditions the stable solid is galena, PbS, which has a very low solubility above pH 2. The lead solubilities and regions of stability of solids shown in figures 1 and 2 are derived from theoretical chemistry, and certain variables are artificially fixed so that the results can be expressed on a two-dimensional graph. In effect, the defined systems retains only two degrees of freedom. Although some natural systems—for example, ground water in a mineralogically homo- geneous aquifer—may have constraints enough to conform to a simple equilibrium model, the usual river or lake water is subject to changing conditions that increase the number of degrees of freedom, and a close con- formance to equilibrium is unusual. However, the strong influence on lead solubility of the pH and alkalinity of the water may be reflected in analyses of river water, because the effect may be strong enough to be discerned even if equilibrium is not attained. Although available data are neither entirely reliable nor adequate in quantity, the best current information suggests that many surface waters in the United States carry concentrations of lead in solution whose values are close to those predicted from solubility calculations such as those represented in figure 2. TABLE 2.——Chemical equilibria and solubility equations for lead species in hydroxide and carbonate domains Chemical equilibria Equation Source of constant [1’b*""][1"1'l']’2 = 10 “5 Calculated from data in table 1. [Pb0H+][H+][Pb+21-I = 10 4-12 Do. [Pb(OH)2][H+]2[Pb+2]-1 = 10—17.16 Do. [Pb.-][H+13[Pb+21-l = 10—28-08 D0. = 10 17.95 D0. = 10 +09 Do. [Pba.(0H)4+‘]lH+l‘le+2] " [Plfizllflco{llH’rl’l [Pbso.][so.—2]—1[Pb+2]-l = 10 2-62 Do. [HC03-][H+][H2C03]-1 10 +35 Carrels and Christ (1965). [COs-Z][H][HC03‘]'1 = 10 40-83 Do. [H2C03]pC02" = 10 4-47 Do. Solubility equations ‘ H, O [HCOs'] [c0341 2C02,dlss.=[ 2C Shyncoa—Wcog-z [Pb+2] [Pb0H+]4[Pb4(0H>4+‘] 2Pb - =-—+‘——+_——_’ diss. 7Pb+2 ’YPbOH+ 'YPb.(OH)4+“ [PbSOA] [Pb(OH)3-] +[Pb(OH)2l+—-————7Pb(OH)3_ + Obtaining the solubility of Pb(OH)2 c by the use of data in table 1 leads to elimination of stability regions for several lead oxides shown in similar diagrams published by others. The field for PbOz is so small that it has very little significance, and it is not shown in figure 2. There are no areas of stability for the mixed valence oxide Pb304 or for the anhydrous PbO species. The low solubility of lead shown on the diagram, however, is a function of both hydroxide and carbonate effects and is likely to be real. The basic lead carbonate Pb3(OH)2 (C092, because of the rather low C02 concentration specified, has a small field of stability in the system defined in figure 2. However, there is only a small difference in free energy of formation between the actual double compound and a mixture of lead hydroxide and lead carbonate. The determination of which species might be formed in the vicinity of pH 8 by precipitation from a supersaturated solution could be mainly a matter of relative rates of reaction. Whether the stable solid is basic carbonate or a mixture of hydroxide and carbonate, the calculated equilibrium solubility of lead is essentially the same. Other solids mentioned in the literature include hydroxy sulfates, nitrates, and chlorides of lead, none of which are stable in the system as defined. The solubility of lead phosphate is low enough to be a factor in some natural waters; generally, phosphate ionic species are not present in sufficient amounts in river and lake water to control the solubility of lead. Other possibly significant solute species of lead have been described in the literature. The chloride complex PbCl+ will not be significant in solutions having less than about 1,000 mg/l of chloride. It could be a significant * 8 LEAD IN THE ENVIRONMENT factor in saline water. A considerable amount of data has been obtained on polynuclear hydroxide complexes of lead such as Pb4(OH)4+4 and Pb3(OH4 *2 For the most part, such species have been studied in more concentrated solutions of lead than the upper limit represented in figure 2. From the thermodynamic data it can be shown that polynuclear hydroxide species are not dominant factors in the aqueous chemistry of lead unless the concentration of lead is over 10 ‘2 molar (Faucherre, 1954) and other complexing anions, such as sulfate and chloride are absent or in low concentrations. It seems reasonably certain that such conditions are rarely, if ever, attained in natural geo- chemical systems. Carbonate complexes such as PbCO‘3 aq may be important in some waters but are not considered here. EFFECTS OF TEMPERATURE AND IONIC STRENGTH Except as noted, all the thermodynamic data in tables 1 and 2 are for a temperature of 25°C. This standard temperature is somewhat higher than that of most stream waters in the United States. At lower temperatures the ‘ solubilities of most minerals are somewhat decreased. However, the solubility of some carbonates increases as temperature declines. In a system having a C02 gas phase, the solubility of C02 also increases as temperature goes down and the dissociation reactions are affected. Strictly speaking, solubility calculations made for 25°C are applicable only to systems at that temperature. If one is interested in testing a solution for adherence to equilibrium solubility at some other temperature, for example, at 5°, all the measurements on the system that would be affected by temperature should be made at 5°, and the results compared with solubility calculations adjusted to apply at 5°. Approximate temperature corrections for equilibrium constants can be made using standard chemical thermodynamic relationships. The alkalinity and pH measurements are seldom all made at the field temperature, however. Converting such values to ones that would represent some other temperature is not a simple matter. ‘ Alkaline-earth metal carbonates decrease in solubility as temperature increases. The effect of temperature on solubility of lead carbonate does not appear to have been closely studied. A few stability constants quoted by Sillén and Martell (1964, p. 140, 141) suggest the solubility of lead carbonate may decrease with decreasing temperature, but temperature effects do not appear to be large. It does not seem worthwhile to calculate the solubility of lead at temperatures below 25° until more study has been given. However, the effect is probably not very large between 25° and 0°C, when compared to other factors that influence field application of solubility calculations. Solutions of high ionic strength may retain sub- stantially more lead in solution than do more dilute waters. Ion pairing effects also become increasingly important at high ionic strength. The calculations given here do not apply to solutions whose total dissolved-solids concentration is much above 5,000 mg/l. INORGANIC CONTROLS OF LEAD SOLUBILITY As noted earlier in this discussion, lead forms carbonate, sulfate, sulfide, hydroxide compounds of low solubility. It is possible to show by equilibrium solubility calculations (fig. 2) that the concentration of lead species in solution should be less than 10 pg/l at ordinary Earth-surface temperatures when HCOs concentration is 61 mg/l or more and pH is between 7.6 and 12.6. A great many natural surface and ground waters are in this concentration range. If the pH is constant and bicarbonate alkalinity is increased, the solubility is proportionately decreased throughout the pH range characteristic of most fresh- water. Calculations described by Hem and Durum (1973) showed that many river waters in the United States have lead concentrations near the solubility limits imposed by their pH levels and contents of dissolved C02 species. The comparison of observed lead concentrations with theoretically calculated ones can never be expected to give more than a very approximate indication of what is happening in river water. The influences of changing temperature and pH on solubility may be substantial. It is not unlikely that apparent supersaturation could occur, perhaps as the result of kinetic barriers to crystallization of small particles. Close adherence to a solubility model can be expected to be rare or nonexistent; however, some waters have a so much greater capacity for lead solution than others that even a crude model can be useful. Although many waters do not have sufficient contact with lead compounds to be able to attain saturation, the oppor- tunity for such contact is particularly favorable in urbanized areas or wherever there is heavy automobile traffic and extensive utilization of leaded gasoline. In such areas, the rainfall itself, along with dry fallout of particulate material from the atmosphere, can supply enough lead so that surface runoff reaching streams could contain substantial amounts—enough to reach satura~ tion at times. Lazrus, Lorange, and Lodge (1970) calculated that as much as 138 g of lead is deposited by rainfall per month on each hectare (104 m2) in some parts of the northeastern United States. If the runoff in such an area is at a rate of about 50 cm of water annually (a commonly observed rate), the runoff would have to have an average lead concentration of 330 ug/l to remove lead at a rate of 138 g/ ha/ mo. Higher or lower concentrations could be calculated for other areas and conditions. This figure for average concentration does indicate, however, that a large potential supply of lead is available for transport in runoff. Concentrations as high as 330 ,ug/l could be stable in water having a pH near 6.5 and an alkalinity of about 25 INORGANIC CHEMISTRY OF LEAD IN WATER 9 mg/l HC03. Water having these properties is common in runoff from areas in New York and New England. Thus, the potential for high lead concentrations in river and lake waters does exist in some places, and careful monitoring of river and ground water quality is desirable. In some other areas the composition of runoff water will be less favorable for lead solution. In many places the average pH and alkalinity are so high that less than 1 pg of lead could be retained in solution at equilibrium. The most comprehensive data on lead concentrations in . river water of the United States are included in a report .by Durum, Hem, and Heidel (1971). However, their compila- tion does not include either the bicarbonate or pH deter- minations needed to apply solubility calculations in testing for adherence to equilibrium conditions. Some of the sampling sites were points at which pH and alkalinity measurements are regularly made, and some applicable measurements were found in the computer-stored data for water quality stations. Of the 720 samples obtained for the Durum, Hem, and Heidel study, 70 could be correlated with applicable pH and alkalinity determinations that had been made either on a sample obtained the same day as the sample for lead, or on one obtained within a few days, if the composition of the source was relatively constant (as in large lakes, for example). Lead was detectable in 39 of these samples, as shown in table 3. This table gives a calculated range of lead concentra- tion for each sample, based on pH and alkalinity. The lower number is the solubility in a very dilute solution; the TABLE 3.—Observed and calculated saturation concentrations of lead in U.S. surface waters collected during October and November 1970 [Modified from Durum, Hem, and Heidel, 1971, and unpublished USGS records] Pb Date sam 1e 1 Sample source (055 19:31, ected 2 HCO: 2 HCO: pH (“g/l) except as noted) (Mg/l) (log mol/l) Calculated range Observed Raritan River near Manville, N.] 12 75 -2.85 7.5 5.0— 15.0 3 Millstone River near Manville, NJ 12 48 —3.09 7.4 12.0— 35.0 5 Manasquan River at Squankum, NJ 13 60 ~2.96 7.1 18.0— 60.0 4 Delaware River at Trenton, N._] ............ 13 45 —3.12 7.7 4.0— 9.0 5 St. Johns River near Melbourne, Fla ...................................... 14 113 —2.66 7.6 3.0— 9.0 1 St. Johns River at Jacksonville, Fla ........................................ 2 103 -2.75 7.9 2.0— 5.0 2 Plantation Road Canal near Fort Lauderdale, Fla... 29 167 —2.49 7.6 2.0- 6.0 4 Caloosahatchee River near Olga, Fla ........................ 29 199 —2.43 8.2 .6— 1.5 1 Peace River at Arcadia, Fla ..................................................... Nov. 6, 1970 51 -3.05 7.9 3.0— 10.0 3 Phillipi Creek at Sarasota, Fla.... ....... 6 158 —2.56 8.0 1.2— 3.0 1 Alifia River at Lithia, Fla ........ 7 42 -3.09 7.2 15.0— 50.0 1 Swift Creek at Facil, Fla ........................ 6 15 —3.42 6.7 150.0— 400.0 1 Ochlockonee River near Havana, Fla... 15 32 —3.21 7.1 25.0— 80.0 4 Coosa River at Childersburg, Ala ........................................... 14 77 —2.83 7.4 5.0- 20.0 4 Alabama River at Claiborne, Ala ............................................ 9 50 —3.06 7.5 9.0— 25.0 9 Sipsey Fork near Grayson, Ala ........................... 22 38 —3.05 6.8 40.0— 150.0 6 Mahoning River, Ohio-Pennsylvania boundary... . 8 88 —2.85 7.3 8.0— 25.0 1 Cataloochee Creek near Cataloochee, N.C .............................. 13 9 —3.53 7.2 40.0— 100.0 20 Tennessee River at Kentucky Dam, Ky. (near Paducah)... . 16 66 —2.90 7.5 6.0— 18.0 4 Washington Creek at Windigo, Mich .................................. 15 68 —2.87 7.4 6.5- 20.0 2 St. Marys River, Sault Ste. Marie, Mich... ..... 12 50 -3.04 7.7 5.0— 12.0 6 St. Clair River at Port Huron, Mich ....................................... 19 95 '—2.79 8.3 1.0— 4.0 5 Detroit River at Detroit, Mich ................................................. 16 98 -2.78 8.2 1.5- 4.5 4 Missouri River at St. Joseph, Mo .............. 14 186 —2.45 8.0 .9— 2.5 5 Mississippi River at East St. Louis, Ill .. 13 229 —2.39 8.0 .8— 2.5 7 North Sylamore Creek near Fifty Six, Ark.. .. 30 146 —2.54 7.3 4.0— 12.0 10 Arkansas River below Little Rock, Ark .................................. 7 100 —2.72 7.6 3.5— 10.0 11 Kiamichi River near Big Cedar, Okla .............. 10 —3.74 7.9 20.0— 60.0 84 Las Vegas Wash near Boulder City, Nev ..... Nov 19, 1970 258 —2.33 8.0 .8— 2.0 2 Santa Ana River below Prado Dam, Calif ....................... 318 —2.40 8.2 .5— 1.2 34 Merced River, Happy Isles Branch, Yosemite, Calif .............. 20 9 —3.76 7.1 100.0— 300.0 2 Elder Creek near Branscomb, Calif ......................................... 7 75 —2.80 7.5 4.5— 12.0 2 North Fork Quinault River, near Amanda Park, Wash ......... 15 44 -3.08 7.4 10.0— 30.0 2 South Fork Coeur d’Alene River near Smelterville, Idaho ..... 8 7 -3.24 5.8 10000—50000 5 Hayden Creek near Hayden Lake, Idaho ................................ 21 47 —3.07 7.7 5.0— 15.0 3 Snake River near Shelley, Idaho ....... .. 6 150 —2.57 8.2 .8— 2.0 3 Rock Creek near Twin Falls, Idaho ..... 19 335 '—2.24 7.8 1.0— 3.0 2 Campbell Creek near Spenard, Alaska. .. 16 48 —3.06 7.9 5.0— 10.0 2 Ship Creek near Anchorage, Alaska ........................................ 16 59 —2.90 7.9 5.0— 15.0 1 10 LEAD IN THE ENVIRONMENT higher represents the effect of increased ionic strength. Temperature effects are not directly recorded, but will generally have the effect of slightly decreasing the solubility of carbonates as temperature decreases. Most samples probably were 10°—15° cooler than the standard 25°C used in the calculations. The influence of tempera- ture on lead solubility in these systems needs more investigation. Of the 39 concentrations of lead determined, 12 are within the predicted solubility range and an additional 10 are reasonably close (within a factor of 2) to upper or lower limits. The possible experimental and sampling errors are probably enough to explain these discrepancies. The remaining 17 samples include 15 that are well below saturation, a condition to be expected in a high per- centage of the samples, owing to lack of sufficient oppor- tunity to dissolve lead. There were only two samples in which a substantial degree of supersaturation occurred—one from the Mississippi River at East St. Louis, which was sampled during a high stage and may have been carrying colloidal particulate lead that could pass through the filter used in clarifying the sample, and the other from the Santa Ana River below Prado Dam, in the Los Angeles area of California, where lead fallout rates are probably very high, causing a situation also con- ducive to accumulation of colloidal particulate lead. The principal value of these solubility calculations is in the aid they give to understanding the freshwater part of the geochemical cycle of lead and in the suggestion that hazards of increased lead in water supplies may be most severe in areas where runoff has both a relatively low dissolved-solids concentration and a low pH. It is not intended to imply that the equilibrium aspects of lead chemistry are the only important ones. Factors not con- sidered here may be operating to prevent high lead concen- trations from being reached in solutions that are below saturation. However, until more evidence is obtained that such factors are preventing lead from appearing in runoff, it seems obvious that lead concentrations in river and lake water require close attention. NONSOLUTE LEAD IN WATER A significant fraction of the total content of lead carried by river water may be in an undissolved state. This nonsolute lead can consist of colloidal particles in a hydrosolic suspension, a characteristic form of many metals having sparingly soluble hydroxides. It may also be present as larger undissolved particles of lead carbonate, oxide, hydroxide, or other lead compound, and can be incorporated in other components of the particulate lead of the runoff, either as sorbed ions or as surface coatings on sediment mineral particles. It can also be carried as a part of organic suspended matter in both living and nonliving forms. The concentrations of lead usually reported represent an arbitrarily defined solute fraction, separated from the nonsolute fraction by filtration. Most filtration techniques cannot be relied upon to remove all colloidal- size particles, and on acidification of the filtered sample, as usually is done for preservation before analysis, the colloidal material that passed through is dissolved and is reported in that form. Usually the solids removed from a surface water sample by filtration are not analyzed for lead. Exceptions are the analyses reported for rivers in the USSR by Konovalov, Ivanova, and Kolesnikov (1968), for which the only determinations of lead were on material removed by filtration. Their assumption appears to have been that solute lead was insignificant. Samples of ground water are not generally filtered before analysis. Content of colloidal material in such waters is probably negligible, however, owing to natural filtration effects during recharge and movement through the aquifer. Even the lead in rainfall can be mainly particulate. Samples of rain were collected at the US. Geological Survey in Menlo Park, Calif, during the period January—April 1971. Filtration through membranes with 0.10- [rm-diameter pores removed as much as 90 percent of the lead. Several samples of runoff from a small stream draining part of the city of Palo Alto, Calif., were collected during this period. One of these contained 90 ,ug/l of lead, about 90 percent of which could be removed by filtration. The sediment contained 0.11 percent lead. Kopp and Kroner (1970, p. 14) reported lead in only 5 of 228 samples of suspended material obtained from composited river and lake samples. In one of these, however, the concentration was equivalent to 120 ,ug/l in the original composite. Dry atmospheric fallout of lead was measured during ' May and June of 1971 at Menlo Park, Calif. These measurements were made by exposing, for 1—2 weeks on the building roof, a large shallow polyethylene container in which distilled water was maintained at a depth of a few centimetres. The water and sediment accumulated were then analyzed for dissolved and particulate lead. This particulate fallout consistently contained 0.09 to about 0.10 percent of lead, and the fallout rate of lead ranged from 30 to 66 g/ ha/ mo. N o measurable amount of rain fell during this period. Rates and composition of fallout obtained in this set of observations are similar to those reported earlier by Chow and Earl (1970) for the San Diego, Calif., area. Before a meaningful estimate can be made of the effectiveness of runoff in transporting lead away from areas where it has been deposited by atmospheric fallout and rain, it will be necessary to obtain more information on the amounts of lead transported in nonsolute form, especially during periods when runoff rates are high, as well as corresponding information on the amount carried in' solution. At present such data are almost totally INORGANIC CHEMISTRY OF LEAD IN WATER 11 nonexistent. A laboratory study of sorption of lead by cation exchange (Hem, 1976) indicated that a major part of the lead in stream water may be adsorbed on suspended sediment. REFERENCES CITED Chow, T. J., and Earl, J. L., 1970, Lead aerosols in the atmosphere— Increasing concentrations: Science, v. 169, p. 577—580. Durum, W. H., Hem, J. D., and Heidel, S. G., 1971, Reconnaissance of selected minor elements in surface waters of the United States, October 1970: US. Geol. Survey Circ. 643, 49 p. Faucherre, Jacques, 1954, Sur la constitution des ions basiques métalliques—2, Le plomb: Soc. Chim. France Bull., v. 21, p. 128—142. Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals and equi- libria: New York, Harper 8c Row, 450 p. Hem, J. D., 1976, Geochemical controls on lead concentration in stream water and sediments: Geochim. et Cosmochim. Acta, v. 40 (in press). Hem, J. D., and Durum, W. H., 1973, Solubility and occurrence of lead in surface water: Am. Water Works Assoc. Jour., v. 65, no. 8, p. 562—568. Kolthoff, I. M., Perlich, R. W., and Weiblen, D., 1942, Solubility of lead sulfate and lead oxalate in various media: Jour. Phys. Chemis- try, v. 46, p. 561. Konovalov, G. S., Ivanova, A. A., and Kolesnikov, T. Kh., 1968, Dis- persed and rare elements dissolved in the water and contained in the suspended matter of the main rivers of the U.S.S.R., in Geochemistry of sedimentary rocks and ores [in Russian]: Moscow Izdatel’stvo Nauka, 435 p. Kopp, J. F., and Kroner, R. C., 1970, Trace metals in waters of the United States—A five year summary of trace metals in rivers and lakes of the United States (Oct. 1, 1962-Sept. 30, 1967): US. Dept. Interior, Federal Water Pollution Control Admin., 29 p. and app. Latimer, W. M., 1952, The oxidation states of the elements and their potentials in aqueous solutions [2d ed.]: New York, Prentice-Hall, 392 p. Lazrus, A. L., Lorange, Elizabeth, and Lodge, J. P., Jr., 1970, Lead and other metal ions in United States precipitation: Environ- mental Sci. and Technology, v. 44, p. 55—58. Robie, R. A., and Waldbaum, D. R., 1968, Thermodynamic properties of minerals and related substances at 298.15°K (250°C) and one atmosphere (1.013 bars) pressure and at high temperatures: U.S. Geol. Survey Bull. 1259, 256 p. Sillén, L. G., and Martell, A. E., 1964, Stability constants of metal- ion complexes [2d ed.]: London Chem. Soc. Spec. Pub. 17, 754 p. Wagman, D. D., Evans, W. H., Parker, V. B., Halow, I., Bailey, 5. M., and Schumm, R. H., 1968, Selected values of chemical thermo- dynamic properties: U.S. Natl. Bur. Standards Tech. Note 270-3, p. 187—195. ORGANIC CHEMISTRY OF LEAD IN NATURAL WATER SYSTEMS By R. L. WERSHAW rNatural soil-water and sediment-water systems are extremely complex, consisting of a myriad of interacting organic and inorganic components. In this section we shall be concerned with the interaction of one of the inorganic components, lead, with the organic components of these systems. The organic components of a soil-water system are an extremely diverse group of compounds (Saxby, 1969), including carbohydrates, amino acids, phenolic and quinonic compounds, organic acids, nucleic acids, enzymes, porphyrins and other heterocyclic compounds, lipids, terpinoids, and humic materials. (See Saxby, 1969; Spakhov and Spakhova, 1970; Greaves and Wilson, 1969; Kowalenko and McKercher, 1971; Ivarson and Sowden, 1969; and Kononova, 1966, for more complete discussions of the organic compounds in soils.) In addition to the natural organic compounds present in soil, streams and lakes contain organic sediments and suspended solids that have been derived from municipal, agricultural, and industrial wastes. These wastes are made up of carbohydrates, proteins, nucleic acids, enzymes, lipids, and many of the other organic compounds of living systems. Oils, plasticizers, polymers, and an enormous number of other organic compounds are discharged to natural waterways by manufacturing and chemical industries. The interaction of lead with the organic components of soil-water and sediment-water systems is still not well understood, but we shall review the work that has been done and attempt to draw what conclusions we can from the data available. A number of studies have shown that the organic matter of soils and stream sediments apparently binds metals and reduces their mobility. However, as Mortensen (1963) has pointed out, although many workers have suggested that soil organic matter forms complexes with metals, and that these complexes are important in soil formation and plant nutrition, the evidence for existence of metal-soil organic matter complexes in nature is largely circumstantial. Although a relatively large number of publications are available on the interaction of metals with organic materials of soils and waters (see Mortensen, 1963; Saxby, 1969), very few of the papers have dealt with interactions of lead and organic material, and of these, most have been concerned with lead in soil-water systems; there are very few on the interaction of lead with organic materials in streams and lakes. SURFACE WATER—SEDIMENT SYSTEMS Herrig (1969) and Hellman (1970a, b, 1971 ) have studied the heavy metals in the waters and sediments of the Rhine River and its tributaries. Herrig found that both inorganic and organic lead compounds were being introduced into rivers as industrial and municipal wastes; the most common of these were lead oxide, metallic lead, lead stearate, lead palmitate and tetraethyl lead. Much of this lead was concentrated in the suspended and bottom sedi- ments of high organic content. The sediments were more effective in removing lead than in removing copper or zinc (Hellman, 1970a). The sediments of the lower Rhine contained about 10 times the amount of heavy metals found in those of the upper Rhine and its tributaries. Lead concentrations as, high as 0.2 percent were found in some of the sediments; high heavy-metal concentrations in the sediments were associated with high concentrations in the river water. The high concentrations are orders of magnitude higher than the natural background levels and can only be due to municipal and industrial pollution. The high concentration of lead in sewage sludge indicates that municipal waste can be a major contributor of lead pollution in some streams (Gross, 1970). Even though the sediments remove large quantities of heavy metals from the water of rivers, they are still not efficient enough to totally decontaminate the water, and therefore the river water often contains high heavy-metal concentrations. Hellman (1970a) measured lead concentrations in the water as high as 85 ,u.g/1, and the average for the length of the Rhine river was 51 ,ug/ 1. The lead in the organic components of a sediment enters the sediment from two sources: (1) The plant and animal remains that provide the organic compounds to' the sediment, and (2) the water that comes into contact with the sediment. The chelation of lead by the chemical components of living organisms tends to cause lead to accumulate in living tissue (MacLean and others, 1969; 13 14 LEAD IN THE ENVIRONMENT Danielson, 1970; Bryce-Smith, 1971). This accumulation of lead is no doubt a major source of lead in organic sediments. However, both the organic and inorganic components of sediments will also remove lead from water. Therefore, as has been pointed out by Hellman (1970a), organic sediments will tend to decontaminate surface waters contaminated with lead and other heavy metals. However, one must also remember that sediments will tend to supply lead to waters with low lead concentrations in order to maintain chemical equilibrium. Therefore, even if the lead pollution of a surface water were to cease immediately, the sediments would continue to supply lead to the water and to its fauna and flora. CHEMISTRY OF THE INTERACTIONS Saxby (1969) has reviewed the literature of metal- organic chemistry in the geochemical cycle. He found that lead has been detected in the carbonaceous fractions of soils, coals, petroleum, bituminous rocks and asphalts, shales, sedimentary sulfides, and phosphate rocks. In general, however, very little is known about the molecular associations of metals in geological materials. Therefore, in our discussion we shall have to draw heavily upon studies that have been made on synthetic organic systems and medically important biological systems; however, the general chemical principles elucidated in these studies should also hold true for natural soil-water systems. Two different types of organic lead compounds may be found in nature: (1) the so-called organo—lead compounds, in which the lead is covalently bonded to carbon atoms and the metal complexes, and (2) chelates, in which the lead is ionically bonded to organic ligands. Lead normally exhibits both divalent and tetravalent states in organic compounds. When lead is covalently bonded to carbon it is generally more stable in the tetravalent state; relatively stable divalent lead derivatives are known, however. (See Shapiro and Frey, 1968, for a more complete discussion of lead bonding in organic compounds.) Covalently bonded organo-lead compounds in natural systems undergo slow oxidation by air and photolysis when exposed to light. Most of these compounds except those in which the lead-carbon bonds are highly polar, such as the perfluoroalkyl lead compounds and RstX and RszX compounds in which X is an anion, are not hydrolyzed by water. In coordination compounds, such as metal complexes and chelates, ionic lead is normally divalent; however, organo-lead cations in which the lead is tetravalent may also be complexed by organic ligands. Most of the detailed studies on the complexing of lead and other metals by organic ligands have been conducted by biochemists on the components of living systems; the same ligands however are present in the organic material of soils, natural waters, and sewage effluents. Lead is generally most strongly bound to sulfur containing ligands and to phosphoryl groups (Passow, 1970), but it also forms coordination bonds with other ligands. Vallee and Wacker ( 1970) have reviewed the chemistry of metalloproteins and have pointed out that lead interacts with proteins and enzymes by binding to carboxyl groups or sulfhydryl groups. The binding of lead and other heavy metals to a wide variety of enzymes inhibits the activity of the enzymes (Morrow and others, 1969; Hernberg and Nikkanen, 1970; Vallee and Wacker, 1970). Li and Manning (1955) have demonstrated that, in general, lead forms more stable complexes with proteins than do cadmium, zinc, or copper. The binding of lead to enzymes and the consequent inhibition of enzymatic activity will undoubtably affect the biological reactions that take place in streams, soils, and sewage treatment facilities, but unfortunately there are very few data on the interactions of lead with soil and water biochemical systems. However, the poisoning of enzyme systems in natural waters by heavy metals can be expected to drastically alter the rates of decomposition of organic wastes in these systems. Heavy-metal cations may not always act as enzyme poisons in soil systems. Ladd and Butler (1970) have shown that divalent cations are effective in reducing the inhibition of the enzyme protease by humic acid. It appears that the humic acid binds to the enzyme, thereby preventing it from combining with substrate. If metal cations are present in the system, active sites on the humic acid will apparently preferentially bind to the cations and be unavailable for binding the enzyme. Lead also interacts with polynucleotides and viruses, Lead ions will cause the depolymerization of some polynucleotides such as ribonucleic acid (RNA) (Farkas, 1968) and deoxyribonucleic acid (Eichhorn, 1962; Izatt and others, 1971). In the depolymerization of RNA it appears that the lead binds the phosphate groups. This neutralizes the charge on the phosphate groups and renders the phos- phodiester bonds more susceptible to hydrolysis by hydroxyl groups (F arkas, 1968). In the binding of lead to tobacco mosaic virus molecules the lead may displace protons of the hydroxyl groups (F raenkel-Conrat, 1965). In at amino acids, in more complex molecules con- taining a amino acids, and in molecules similar to 8- hydroxyquinoline and quinoline acid, lead and many other metals form five-membered chelate rings with reactive ligands. Charles and F reiser (1952), in their work on five-membered chelate rings, have measured the stability constants of lead chelates of o-aminophenol and o-aminobenzenethiol. Their data indicate that the order of decreasing stability for metal o-aminophenol is as follows: Cu, Ni, Zn, Co, Pb; for o-aminobenzenethiol, however, the order is Cu, Ni, Pb, Zn, Co. Lead forms more stable complexes with o-aminobenzenethiol than with o- ORGANIC CHEMISTRY OF LEAD IN WATER l5 aminophenol, apparently because the lead-sulfur bond is stronger than the lead-oxygen bond. Other groups of compounds such as carbohydrates, carboxylic acids, lipids, phenols, and oxygenated isoprenoids that have electron donor ligands similar to those of the proteins mentioned above would also be expected to complex lead. In this regard Hoogeveen (1970) has demonstrated that lead is bound by phospholipids. Martin, Ervin, and Shepherd (1966) and Martin and Richards (1969) have shown that copper, zinc, iron, and aluminum form complexes with soil polysaccharides. These metal-polysaccharide complexes are generally more resistant to decomposition in soils than are uncomplexed polysaccharides. Although there are no experimental data on lead-polysaccharide complexes in soil, one would expect them to have properties similar to those of other heavy-metal polysaccharide complexes. It is well established that soil humic materials such as humic acid and fulvic acid form complexes with metals (Kononova, 1966). Aleksandrova (1967) has classified the interactions of metals with humic materials into three categories: (1) Formation of heteropolar salts (an ion exchange phenomenon between strong bases and humic materials), (2) formation of coordination complexes, (3) formation of adsorption complexes with nonsilicate sesquioxides (adsorption of humus on sesquioxide gels). Most of the interactions of lead with humic materials that have been studied fall under the second category. Humic acids and fulvic acids form stable complexes with lead (11) ions and other metal ions; these complexes may be separated from a mixture of humic acid or fulvic acid metal complexes by gel permeation chromatography (Miicke and Kleist, 1965; Klocking and Miicke, 1969). Therefore, it should be possible to isolate intact, humic- metal complexes from soils and sediments. Bondarenko (1968) found that the presence of fulvic acid in water increased the rate of solution of lead sulfide 10—60 times over a water solution at the same pH that did not contain fulvic acid. At pH values near 7, soluble lead- fulvic acid complexes were present in solution; at initial pH values between 7.4 and about 9,‘ the lead-fulvic complexes partially decomposed, and lead hydroxide and carbonate were precipitated. At initial pH values of about 10, the amount of lead—fulvic acid complexes again increased. Bondarenko (1968) attributed this increase to dissociation of phenolic groups at high pH values, a phenomenon which increases the complexing capacity of the fulvic acid. In addition to interactions in which a lead ion is bound by a single organic molecule, a single metal ion can be bound by two different organic molecules. It has been demonstrated, for example, that polyvalent cations act as bridges between clay minerals and humic and fulvic acids (Greenland, 1971). SUMMARY AND CONCULSIONS The complexing of lead by most of the common sulfur-, phosphorus-, oxygen- and nitrogen-containing ligands means that lead will accumulate in both the living and nonliving organic components of soil-water and sediment-water systems. The living and non-living organic components are not independent of each other, but are constantly interacting, as the living components metabolize the nonliving components of the system and then die, contributing their remains to the pool of non- living compounds in the system. Some of the lower forms of life are consumed in the process by the higher forms of life and in this way chemical elements from the sediments are introduced into the food chain of the system; this food chain often ends with man. The fate of heavy metals in this process has not been elucidated; however, the sediment in a contaminated surface-water body will serve as a large reservoir which can provide lead and other metals to the biota of the system even after heavy metal pollutants have ceased to be introduced into the system. Unfortunately, in the case of heavy metals, much more attention has been paid to heavy metals dissolved in the water phase of surface water than to the complexed metals in the sediment phase which, in general, probably contain a considerably higher amount of metals than the water. The sediments will also act as a reservoir for providing dissolved heavy metals to the water when contamination levels are low. High concentrations of lead in soils and sediments will have a marked effect upon enzyme systems, acting in some instances to reduce inhibition by other components of the systems. The alteration of the activity of some of the most basic components of living systems is certain to have a profound effect upon the biological and chemical pro- cesses taking place in natural systems. ACKNOWLEDGMENT This work could not have been completed without the diligent efforts of Miss Sarah Booker who did most of the literature search. REFERENCES CITED Aleksandrova, L. N., 1967, Organomineral humic acid derivatives and methods of studying them: Soviet Soil Sci. 1967, no. 7, p. 903—913. Bondarenko, G. P., 1968, An experimental study of the solubility of galena in the presence of fulvic acids: Geokhimiya 1968, p. 631—636; translated in Geochemistry Internat., v. 5, p. 525—531. Bryce-Smith, D., 1971, Lead pollution—A growing hazard to public health: Chemistry in Britain, v. 7, p. 54—56. Charles, R. G., and Freiser, Henry, 1952, Structure and behavior of organic analytical reagents—2, Stability of chelates of o-amino- phenol and of o-aminobenzenethiol: Jour. Am. Chem. Soc, v. 74, no. 6, p. 1385—1387. Danielson, Lennart, 1970, Gasoline contains lead: Swedish Nat. Sci. Research Council, Ecological Research Comm. Bull. 6, 45 p. Eichhorn, G. L., 1962, Metal ions as stabilizers or destabilizers of the deoxyribonucleic acid structure: Nature, v. 194, p. 474—475. 16 LEAD IN THE ENVIRONMENT Farkas, W. R., 1968, Depolymerization of ribonucleic acid by plum— bous ion: Biochimica et Biophysica Acta, v. 155, p. 401—409. Fraenkel-Conrat, Heinz, 1965, Structure and function of virus proteins and of viral nucleic acid, in v. 3, of Neurath, Hans, ed., The proteins—Composition, structure, and function: New York, Aca- demic Press, p. 99—151. Greaves, M. P., and Wilson, M. J., 1969, Adsorption of nucleic acids by montmorillonite: Soil Biology and Biochemistry [Oxford], v. 1, p. 317-323. Greenland, D. J., 1971, Interactions between humic and fulvic acids and clays: Soil Sci., v. 111, no. 1, p. 34—41. Gross, M. G., 1970, Preliminary analyses of urban wastes, New York metropolitan region: Stony Brook, N.Y., Marine Sciences Re- search Center, New York State Univ., 35 p. Hellmann, Hubert, 1970a, Die Absorption von Schwermetallen an den Schwebstoffen des Rheinseine Untersuchung zur Entgiftung des Rheinwassers (ein Nachtrag) [Absorption of heavy metals by sus- pended solids in the Rhine River]: Deutsche Gewasserkundliche Mitteilungen, v. 14, no. 2, p. 42—47; English abs. available in Chem. Abs., v. 73, no. 48360b, 1970. 1970b, Die Charakterisierung von Sedimenten auf Grund ihres Gehaltes an Spurenmetallen [Characterization of sediments on the basis of their trace metal content]: Deutsche Gewasserkundliche Mitteilungen, v. 14, no. 6, p. 160—164; English abs. available in Chem. Abs., v. 73, no. ll5'626d, 1971. 1971, Bestimmung von Metallen in Flusschlammen mit Hilfe der Rontgenfluorescenz—Bedeutung fur die Praxis [Deter- mination of metals in river sludge by eray fluorescence— Application in actual practice]: Zeitschr. Anal. Chemie, v. 254, no. 3, p. 192-195; English abs. available in Chem. Abs., v. 74, no. l46178p, 1971. Hemberg, Sven, and Nikkanen, J., 1970, Enzyme inhibition by lead under normal urban conditions: The Lancet, v. I, no. 7637, p. 63—64. Herrig, Hans, 1969, Untersuchen an flusswasser inhaltsstofen [River water substances]: Gas- und Wasserfach, Wasser-Abwasser, v. 110, no. 50, p. 1385—1391; English abs. available in Chem. Abs., v. 72, no. 355562, 1970. Hoogeveen, J. Th., 1970, Thermoconductometric investigation of phosphatidylcholine in aqueous tertiary butanol in the absence and presence of metal ions, in Maniloff, Jack, Coleman, J. R., and Miller, M. W., eds., Effects of metals on cells, subcellular elements, and macromolecules [Rochester Conf. on Toxicity, 2d, 1969, Proc.]: Springfield 111., Charles C. Thomas Pub., p. 207-229. Ivarson, K. C., and Sowden, F. J., 1969, Free amino acid composition of the plant root environment under field conditions: Canadian Jour. Soil Sci., v. 49, p. 121—127. Izatt, R. M., Christensen, J. J., and Rytting, J. H., 1971, Sites and thermodynamic quantities associated with proton and metal ion interaction with ribonucleic acid, and their constituent bases, nucleosides, and nucleotides: Chem. Rev., v. 71, no. 5, p. 439—481. Kliicking, Renate, and Miicke, Dietrich, 1969, Isolierung wasser- loslicher Huminsauren (Fulvosauren) aus ihren Blei, (II—— Chelatverbindungen [Isolation of water soluble humic acids (fulvic acids) from their lead, (2)-—Chelates]: Zeitschr. Chemie, v. 12, p. 453-454; English abs. available in Chem. Abs., v. 72, no. 660202, 1970. Kononova, M. M., 1966, Soil organic matter—Its nature, its role in soil formation and in soil fertility [2d English ed.: translated from Russian by T. Z. Nowakowski and A. C. D. Newman]: London, Pergamon Press, 544 p. Kowalenko, C. G., and McKercher, R. B., 1971, Phospholipid com- ponents extracted from Saskatchewan soils: Canadian Jour. Soil Sci., v. 51, p. 19—22. Ladd, J. N ., and Butler, J. H. A., 1970, The effect of inorganic cations on the inhibition and stimulation of protease activity by soil humic acids: Soil Biology and Biochemistry [Oxford], v. 2, p. 33—40. Li, N. C., and Manning, R. A., 1955, Some metal complexes of sulfur-containing amino acids: Jour. Am. Chem. Soc., v. 77, no. 20, p. 5225—5228. MacLean, A. J., Halstead, R. L., and Firm, B. J., 1969, Extract- ability of added lead in soils and its concentration in plants: Canadian Jour. Soil Sci., v. 49, p. 327—334. Martin, J. P., Ervin, J. 0., and Shepherd, R. A., 1966, Decomposition of the iron, aluminum, zinc, and copper salts or complexes of some microbial and plant polysaccharides in soil. Soil Sci. Soc. America Proc., v. 30, p. 196—200. Martin, J. P., and Richards, S. J., 1969, Influence of the copper, zinc, iron, and aluminum salts of some microbial and plant poly— saccharides on aggregation and hydraulic conductivity of Ramona sandy loam: Soil Sci. Soc. America Proc., v. 33, p. 421-423. Morrow, J. J., Urata, G., and Goldberg, A., 1969, The effect of lead and ferrous and ferric iron on o-aminolevulic acid synthetase: Clin. Sci. [London], v. 37, p. 533—538. Mortensen, J. L., 1963, Complexing of metals by soil organic matter: Soil Sci. Soc. America Proc., v. 27, p. 179—186. Miicke, Dietrich, and Kleist, Horst, 1965, Papierelektrophoretsche untersuchungen an Huminsaure-metallverbindungen [Paper electrophoresis of metal-humic acid compounds]: Albrecht-Thaer- Archiv, v. 9, no. 4, p. 327—336; English abs. available in Chem. Abs., v. 63, no. 9097a, 1965. Passow, Hermann, 1970, The red blood cell—Penetration, distribution, and toxic actions of heavy metals, in Maniloff, Jack, Coleman, J. R., and Miller, M. W., eds., Effects of metals on cells, sub- cellular elements, and macromolecules [Rochester Conf. on Tox- icity, 2d, 1969, Proc.]: Springfield, 111., Charles C. Thomas, p. 291—340. Saxby, J. D., 1969, Metal-organic chemistry of the geochemical cycle: Rev. Pure and App]. Chemistry, v. 19, June, p. 131—149. Shapiro, Hymin, and Frey, F. W., 1968, The organic compounds of lead: New York, Interscience, 486 p. Spakhov, Yu. M., and Spakhova, A. S., 1970, Composition of free water-soluble organic compounds in the rhizosphere of some tree species: Soviet Soil Sci., v. 2, no. 6, p. 703—710. Vallee, B. L., and Wacker, W. E. C., 1970, Metalloproteins, v. 5 of Neurath, Hans, ed., The proteins—Composition, structure and function: New York, Academic Press, 192 p. DISTRIBUTION OF PRINCIPAL LEAD DEPOSITS IN THE CONTINENTAL UNITED STATES By A. V. HEYL The principal lead deposits in the continental United States are shown in figure 3. Most of the deposits and depositional districts are located within a few major tectonic and geographic regions of the United States. They are, from east to west, (1) the Appalachian foldbelt (Blue Ridge, Ridge and Valley, and Piedmont provinces) extending from Maine to Alabama; (2) several low domed uplifts within the little-disturbed craton of the greater Mississippi Valley (Central Lowland); (3) the Ouachita Mountains foldbelt (Ozark Plateaus) of Arkansas and Oklahoma; (4) the Rocky Mountains belt from Mexico and western Texas northward to western Wyoming and eastern Idaho (Middle and Southern Rocky Mountains); (5) the cordillera, which includes the Basin and Range province of New Mexico, Arizona, Nevada, western Utah, southeastern California, and southern Idaho, as well as the Sierra Nevada, Oregon Plateaus (Columbia Plateaus), Northern Rocky Mountains, and Cascade Mountains, and the main mountain ranges of Alaska (Pacific Mountain System); and (6) the Pacific foldbelt (Pacific Border province), which forms a narrow band in western California. The deposits in the Mississippi Valley region (Central Lowland and Ozark Plateaus), Rocky Mountain. belt (Northern and Southern Rocky Mountains provinces), and the cordillera (Basin and Range province) are the largest in the United States and have produced most of the commercial lead during the history of the United States. It is worthy of note, however, that the actual areas of lead deposits in these three regions are so small that even if all are taken into account they probably would not markedly increase the total crustal abundance of lead within these three regions. The deposits outside of these three regions are locally numerous, but they have not been major commercial sources of lead. Lead is commonly associated with zinc and silver, and less commonly with gold, copper, fluorine, barium, cadmium, antimony, bismuth, and arsenic. In most districts lead is subordinate to zinc in total tonnage. The most notable exception is the southeastern Missouri district where lead is far more abundant than zinc (fig. 3, Ice. 1). This district has been our major source of lead since the 1870’s. Most of the major districts in Utah also have more abundant lead than zinc and silver has been a major byproduct. Likewise, in the Coeur d’Alene district of northern Idaho (fig. 3, loc. 3), which dominates lead production in the West, total lead production also exceeds that of zinc; this district is also the major source of silver in the United States. Silver is associated with lead in most deposits in the western States, the quantity ranging from a minor byproduct to a major value of the ore. Many of the lead deposits mined in the foldbelts of the southern Rocky Mountains and in the Basin and Range region of the cordillera during the 19th and early 20th centuries were oxidized silver-enriched deposits worked mainly for their silver content. The deposits in the Appalachian foldbelt (Blue Ridge, Piedmont, and Ridge and Valley provinces) are mainly zinc or copper-zinc deposits with relatively minor lead. A few of the largest deposits, such as the most productive zinc deposits in New Jersey, Pennsylvania, Maine, and east Tennessee, are almost free of lead. A few deposits, such as those at Edwards and Balmat, N .Y. (fig. 3, loc. 5), Blue Hill, Maine, and Austinville, Va. (fig. 3, loc. 6), contain enough lead to provide a commercial byproduct. Hundreds of other small deposits that contain lead are known throughout the Appalachian foldbelt. The deposits in the Ouachita Mountain belt (Ozark Plateaus province) have also produced mainly zinc in the past, and lead is locally only abundant enough to provide a minor byproduct. The cordillera and Rocky Mountain belt (Basin and Range and Southern Rocky Mountains provinces) contain many silver-bearing lead and lead-zinc deposits, and local concentrations of deposits are widely scattered throughout these regions. In the southern Rocky Mountains they are most abundant in the Colorado mineral belt (fig. 3, loc. 9), the San Juan Mountains, Colo. (fig. 3, 10c. 10), and the Wasatch Range, northern Utah. In the cordillera, deposits are scattered throughout the Basin and Range and the 17 l8 LEAD IN THE ENVIRONMENT mxm£< .aofiw— “35:: . 5%? 595$: 35H mix Sam humus—O .mom v::o~U . «SUN-2 595%: 3:3: “Em Mam ma . 93: 55.5% $5— 89$ 6:: £020 EE< 62933 .mw:.:Qm 5.53 . as: .2396 EEO 6:5 Su=m> away— .Euswim £13552 .33 . :53 £032 303:0:00 :5 35 fish . :53 591.3. 3:8: :5 :5; 2E. . JED .fluEmmv :Em HE: 939:5..— :mm . . M w m 32‘.me *0 9.55 :8QO __ w.>_amo_o .5 5:55 9:55. ,, «Em 35. we 32.sz *0 95.5 .5 Sign 9:55. _, _ gm: :8: E 30:55 Jo 5: 9 5+9. EmnEsz _ | £2.sz EB .3655 w _ «85>th oEaEmewffi to Emu—50m. SE52 .53:qu SEEN US“ 0:?— 253 . 3562 «33:3: 3:8: uzm BEG: . .55 5:38 or: E 235:: 3:8: “Em 53:5 . SEQ .zau 66.23:. hum—EM :ofiwiv— U5: mwiam 9.58% 3:3: “En .3:ng 6:5 30:3” . a:o~i< 6.813% 3:8: HEN EOE—Em: . a:o~t< .3:me 3:3: 38 REESE ‘ m:o~i< drawn. van—us . 8:32 252 £8.53: 3:8: uEm .uu>c:3.~ 4:550 8:82 262 5.:qu 5:23:93: mEaEuE 5233 £51.31 3.5200 "in .Eaim .Busm 4.3502 £35m Saw—.333 33:20: 5:23.: 35302 9.3 0—8.0 anon £8552 0:20—00 EOEE 35:32 :2; :aw :3 75:1: 23830 £0:sz 5:5 :5 .:Km< «EU BM 65233..— .E: if .> .< E fiasco 33:8 Umw— Mfiwwmwbofi wO «SUMO a: fiuum: ohm mmvhm H0 335me .fimoH mo mEOu OOOdOH GNSH Whoa :MSSOU OH ESQ—Sv— ohm ho UMUZUOHQ exam: awe: mwfiwgm flaw—H: 2.: Gm mammOn—QU Wauwullfi. HMOO—nm wmmhmzonzx com o mmfiz com o :2, L; «w, ‘ / ,AJ<.—.m<00u~ ,/ W x y L “ m 4 mmquOmm >m.._._<>w_f .3 .2 .N. .2 S .m wait} z_<._.z:oS_L_mkz_ “5:3— cEw fins—EEO #5322 diam-5 :0me =55:on 05 ‘0 2:80: #51: uimmmi 2:. .>.z.§§fi.;§£ . .m> .oo::~>~.u=..>:m~m=< . osfigfiifi 580 . A530— .mmoEE .EEOSSC >~=m> 5333:: 3an .2:ng «£3638 Emfirw>w Z_w 2.42.2305. >¥UOm a 22:23:: w ozPbSO4 Galena Anglesite PbS+C02+H20+202—-—-'>PbCQS+H2804 Galena Cerussite Cerussite usually is formed at a pH above 6 and anglesite at a pH below 6 (Garrels, 1960, p. 170-171). Where pyrite is present, Fe *3 in solution may facilitate oxidation of the galena. The C02 required in the formation of cerussite is provided by the carbon dioxide content of soil gas or, in many instances, by the carbonate content of nearby limestones. Part of the lead may also be incorporated in clay minerals or in iron oxide coatings of complex composition. If the primary ore is rich in pyrite, a large part of the lead may become incorporated in the iron- oxide-rich gossans which characterize the weathered outcrops of such deposits. The common supergene lead minerals are quite insoluble in natural waters. In weathered ore, dark- colored fragments of galena commonly are coated with a light-colored layer of anglesite or cerussite, showing the process of oxidation in progress. These oxidation products of lead do not leach away in soil moisture, but remain in the weathered rock. Many lead deposits have a high content of zinc, which oxidizes to soluble products that do leach away from the weathered ore. The behavior of lead during weathering thus contrasts sharply with that of zinc, sulfur, and other soluble oxidation components of the original ore. The lead content of weathered lead ore commonly is just as high as that of the original unweathered ore. In the lead mining district of southwestern Wisconsin, residual concentrations of weathered galena fragments were found over some of the lead veins (Huff, 1952). These residual concentrations were left by the decomposition and mechanical washing away of the lighter minerals. As these residual concentrations had a high lead content and were easy to mine, they formed an important source of ore during the early development of this district. Such residual concentrations of lead are not common, but they do show the characteristic insolubili ty and limited leaching of lead minerals during weathering. LEAD IN SOILS NEAR ORE DEPOSITS As the weathered outcrops of lead deposits contain a large amount of lead, it might be suspected that soils on or near such deposits also would have a high lead content. Study in many places has shown this to be true. Some representative data (table 4) show that lead contents of 1,000 to 10,000 ppm are common in soils near outcrops of lead deposits. 21 22 LEAD IN THE ENVIRONMENT TABLE 4.—Range of copper, lead, and zinc content of soils collected near one veins [Veins are classified in order of decreasing grade as C, commercial; SC, subcommercial; and M, mineralized (Huff, 1952, p. 536)] Classification Copper Lead Zinc VE‘“ High Low High/low High Low High/low High Low High/low (Ppm) (ppm) ratio (ppm) (ppm) ratio (ppm) (ppm) mi" Unnamed vein, Porters Grove Range, Iowa County, Wis ............. SC .................. 4,400 110 44 2,600 400 6 Iron King vein, Yavapai County, Ariz ........................................... SC 720 180 4 7,000 200 35 5,500 520 11 Collins East vein, Mammoth St. Anthony mine, Pinal County, Ariz ................................................................................... C .................. 4,500 430 10 .................. Unnamed copper vein, Pima County, Ariz... .. M 930 170 ..................................... Apache vein, Gila County, Ariz ........................................................ M .................. 220 15 14 .................. Pittsburgh vein, Coeur d’Alene district, Shoshone County, Idaho: Upper traverse ............................................................................. C .................. 12,000 70 170 580 200 3 Lower traverse .................................................. C .................. 13,000 130 100 1,800 230 8 Chicago vein, Blackbird district, Lemhi County, Idah C 1,600 220 7 .................................... Malachite vein, Jefferson County, Colo: West traverse .................... SC 5,300 50 106 .................. 500 80 5 East traverse ........................................................................ . M 400 50 8 .................................... Union Copper veins, Gold Hill district, Cabarrus County, N.C..... SC 720 150 5 2,500 150 17 680 220 3 Most supergene lead minerals are soft and disintegrate mechanically during weathering, so that much of the lead probably is concentrated in the fine-size fraction of the soil. Most geochemical exploration studies of soil are based upon analysis of a fine-size fraction, such as the minus-80-mesh fraction. These studies yield high lead values, indicating concentrations in the fines. Few analyses of all size fractions of soil have been made, however, and so little data are available on the relative concentration of lead in the various size fractions. The dispersion of lead in residual soil is demonstrated effectively by study of soil samples collected near lead-rich veins (Huff, 1952). Soil samples collected across a lead-zinc vein in Wisconsin (fig. 4) show maximum lead content immediately over the vein. Soil on the hillside above the vein contains background amounts of lead. On the hillside below the vein, the lead content of the soil gradually decreases away from the vein through dilution as the soil creeps downhill. The distribution of zinc in the soil is somewhat different than that of lead. Probably this difference is caused by partial dispersion of zinc by solution in soil moisture, whereas the dispersion of lead is almost completely mechanical. Some of the ore metals show a marked tendency to leach from the surface or A horizon of residual soils and to concentrate in the lower or B horizon, so that the B horizon attains a higher metal content than the A. Such leaching and redeposition has been recognized for zinc and copper but not for lead. Wherever such comparisons have been made the A horizon contains as much or more lead than does the B (see Keith, 1969, table 1). In the southwestern Wisconsin lead-zinc district residual soil derived from lead deposits is overlain by a silty layer of eolian loess. This loess is believed to be largely of Pleistocene age. Although the loess has been in contact with lead-rich residual soil for thousands of years, study shows that there has been little, if any, diffusion of lead in solution from the lead-rich soil into the loess DISTANCE ALONG TRAVERSE, IN FEET 0 100 200 300 400 | ' I ' I I I ' I ' E if 3 4000 i E I IJJ D. m 3000 |_ § u “- 5“} I\ z —' I \ . I I- Z 2000 I 0 Lu I \ E I \ a. O I O\"70 O I \ -‘ \O—-O E 1000 m E ll I I ll FIGURE 4.—Relati0nship between unnamed vein and geo- chemical anomaly in residual soil, Porters Grove Range, Iowa County, Wis. From Huff (1952, p. 524). (Kennedy, 1956, p. 187—223). Similar studies made where glacial till .overlies lead ore show little or no diffusion of lead in solution upward into the glacial till. Surface soils may contain highly anomalous amounts of lead for 5 miles (8 km) or more from a major source of contamination such as a lead smelter. (Canney, 1959). In Yugoslavia, near a lead smelter which has operated for centuries, as much as 24,000 ppm lead has been measured in contaminated soil (Dj uric and others, 1972). These high MIGRATION OF LEAD DURING OXIDATION AND WEATHERING 23 concentrations result from airborne dust particles rich in lead which have been incorporated in the soil. The minimal leaching rate of lead from soils indicates that most of the lead added to surface soils from contaminating sources will remain in these soils indefinitely. LEAD IN STREAM SEDIMENTS In most mining districts the lead which creeps downhill in soils eventually is transported to creeks, or washes, where it is incorporated by running water into stream sediments. Geochemical exploration studies show that stream sediments containing anomalous amounts of lead from mineralized areas can be traced downstream several miles from the source of the lead. It is assumed that the lead in such sediments, like that in soils, is mostly concentrated in the fine particles. Here again few analyses have been made of different size fractions to determine the extent of concen- tration in the fine particles. Several studies of panning concentrates indicate that lead may also be concentrated in part in heavy minerals. In one area in New Mexico the heavy minerals containing lead have been identified (Griffitts and Alminas, 1968, p. 8) as fragments of limonite and wulfenite. Anglesite and cerussite are too soft to survive stream transportation as sand grains and are rarely found in heavy-mineral concentrates. Reconnaissance mineral exploration of many large areas has been accomplished by sampling stream sediments and by analyzing these samples for lead as well as for other ore metals. A good example is a study in New Brunswick (Boyle and others, 1966). Stream-sediment samples commonly contain less than 100 ppm of lead (in the minuSo80-mesh-size fraction); near known lead deposits they may contain more than 5,000 ppm of lead. Representative data for stream sediments collected by the author near Tombstone, Ariz., are given in table 5. TABLE 5.—0re—metal content of stream-sediment samples (minus 80- mesh) collected in and downstream from the Tombstone district, Arizona [N.a., not analyzed] Sample Sample locality Metal content (ppm) No Lead Zinc Silver Molybdenum Copper Tombstone Gulch: Near town and rich mines ...... 4,500 4.300 26.0 120 175 38 0.5 mi (0.8 km) downstream from Sample 1 ..................... 5,000 7,000 33.0 50 300 3 0.7 mi (1.1 km) downstream from Sample 1 ....... . 5,500 6,000 46.0 16 500 2 1.9 mi (3.0 km) do from Sample 1 ....... 850 720 9.1 4 80 32 2.6 mi (4.2 km) downstream from Sample 1 and 0.4 mi (0.6 km) above Walnut Creek .................................... 550 460 4.] <4 30 Walnut Creek: 18 0.8 mi (1.3 km) below mouth of Tombstone Gulch ........... 118 130 N.a. N.a. 30 7 1.4 mi (2.2 km) below mouth of Tombstone Gulch........... 90 83 1.3 <4 N.a. LEAD IN MINE DRAINAGE Measurable amounts of lead are rarely detected in either mine waters or drainage from mines. Representative data obtained near lead and zinc deposits in New Brunswick indicate a lead content of as much as 3,300 ppm in stream sediments but less than 0.001 ppm in stream waters. The water sample with the highest lead content, 2 ppm, was collected within a mine (Boyle and others, 1966, p. 24). In the same waters, the zinc content is as much as 1,000 ppm. The contrast shows the relative insolubility of lead. These results and other available data indicate that during weathering and erosion, little if any lead is removed in solution from lead deposits. PLANTS Plants growing in lead-rich soil absorb some lead from the soil. Later, when the plants die and decay, this lead is returned to the soil. The renowned geochemist V. M. Goldschmidt, after his chemical studies of coal ashes (1937), theorized that lead, as well as many other trace elements present in soil, tends to be absorbed by plants and plant humus. Thus, wherever soils are rich in lead, the plants also are likely to be rich in lead. The extent to which plants accumulate lead from the soil (Webb and Millman, 1951; Worthington, 1955; and Malyuga, 1964) can be judged by the lead content of plants growing near areas of lead mineralization. Some studies have indicated an anomalous lead content in some plants but not in others (Cannon, 1960, table 3). Relative amounts of lead accumulated by various species can be evaluated by comparing the lead content of plant ash (which contains nearly all the inorganic constituents) with the lead content of the soil in which the plants are growing. Table 6 gives data of this type for three areas. In most of these areas the lead content of plants is higher than that of the soil in the nonmineralized areas and lower than that of the soil in the mineralized area. In other words, the plants show less accumulation of lead from lead-rich soils. The reasons for this relationship between soils and plants with respect to lead is not completely understood. However, it does indicate that the ability of plants to accumulate lead from soils is limited. Data given in another section of this report indicate considerable differences among plant species in ability to accumulate lead. Considering the large number of plant species, it is quite possible that some common plants which have never been investigated accumulate large amounts of lead. The mineralized areas, with high lead content in their soils, provide excellent environments to study the lead content of common, non-agricultural plant species. Because of the current interest in lead dispersion, additional studies of common plants in mineralized areas seem desirable to identify all species which accumulate lead. 24 LEAD IN THE ENVIRONMENT TABLE 6.—Mean lead contents of plants compared with mean lead contents of associated soils in both nonmineralized and mineralized areas [Data from the Mississippi Valley district from Keith (1969, p. 358—356); data from Alaska from Shacklette (1960, p. 13102-13103); data from the Tombstone district collected by the author] Lead (ppm) Nonmineralized area Mineralized area Upper Mississippi Valley Soils: Residual, A horizon ............................ 18 124 Plant ash: Elm, stems ........................................... 102 78 Maple, stems... 119 135 Oak, stems ........................................... 150 99 Mahoney Creek lead-zinc deposit, Alaska Soils ......................................................... 20 1,300 Ash of all plant species sampled ............ 90 160 Tombstone district, Arizona Soils: Alluvial ................................................ 35 3,905 Plant ash: Mesquite, stems ................................... 53 239 CONCLUSIONS The supergene dispersal of lead, unlike that of some other ore metals, takes place mostly by mechanical processes. Chemical changes are involved in weathering but the weathered products are relatively insoluble and are not leached from the rock by soil moisture. These soft insoluble lead minerals are incorporated in the fine-size fractions of the soil and stream sediments. They are dispersed by gravity and can be traced in ever-diminishing concentrations a long distance from their source. The low solubility of the supergene lead minerals limits the amount of lead which dissolves in mine waters or is accumulated by plants. Available data on the lead content of mine waters indicate that it is normally very low. Some plants accumulate lead from lead-rich soil near ore deposits; this accumulation varies according to species and may amount to several hundred parts per million in the ash of some species. However, as shown elsewhere in this report, high concentrations in plants have also been observed near lead smelters and near highways where plants absorb lead from industrial and automobile exhaust fumes. REFERENCES CITED Boyle, R. W., Tupper, W. M., Lynch, J., Friedrich, G., Ziauddin, M., Shafiqullah, M., Carter, M., Bygrave, K., 1966, Geochemistry of Pb, Zn, Cu, As, Sb, Mo, Sn, W, Ag, Ni, Co, Cr, Ba, and Mn in the waters and stream sediments of the Bathhurst—Jacquet River district, New Brunswick: Canada Geol. Survey Paper 65-42, 50 p. Canney, F. C., 1959, Geochemical study of soil contamination in the Coeur d'Alene district, Shoshone County, Idaho: Mining Eng., v. 11, no. 2, p. 205~210. Cannon, H. L., 1960, Botanical prospecting for ore deposits: Science, v. 132, no. 3427, p. 591—598. Djuric, Dusan, Kerin, Zarka, Graovac-Leposavic, Ljubica, Novak, Ljiljana, and Kop, Marija, 1972, Environmental contamination by lead from a mine and smelter: Archives Environmental Health, v. 23, no. 4, p. 275—279. Carrels, R. M., 1960, Mineral equilibria—At low temperature and pressure: New York, Harper and Bros., 254 p. Goldschmidt, V. M., 1937, The principles of distribution of chemical elements in minerals and rocks [7th Hugo Miller Lecture]: Chem. Soc. Proc. [London], Jam—June [1937], p. 655—673. Griffitts, W. R., and Alminas, H. V., 1968, Geochemical evidence for possible concealed mineral deposits near the Monticello Box, northern Sierra Cuchillo, Socorro County, New Mexico: U.S. Geol Survey Circ. 600, 13 p. Hawkes, H. E., and Webb, J. S., 1962, Geochemistry in mineral ex- ploration: New York, Harper and Row, 415 p. Huff, L. C., 1952, Abnormal copper, lead, and zinc content of soil near metalliferous veins: Econ. Geology, v. 47, no. 5, p. 517—542. Keith, J. R., 1969, Relationships of lead and zinc contents of trees and soils, Upper Mississippi Valley district: Soc. Mining Eng. Trans, v. 244, no. 3, p. 353—356. Kennedy, V. C., 1956, Geochemical studies in the southwestern Wis- consin zinc-lead area: U.S. Geol. Survey Bull. 1000—E, p. 187-223. Malyuga, D. P., 1964, Biogeochemical methods of prospecting: New York, Consultants Bur., 205 p. Shacklette, H. T., 1960, Soil and plant relationships at the Mahoney Creek lead-zinc deposit, Revillagigedo Island, Southeastern Alaska, in Short papers in the geological sciences: U.S. Geol. Sur- vey Prof. Paper 400—B, p. B102—B104. Webb, J. S., and Millman, A. P., 1951, Heavy metals in vegetation as a guide to ore—A biogeochemical reconnaissance in west Africa: Inst. Mining and Metallurgy Trans, v. 60, pt. 2, [no.] 537, p. 473—504. Worthington, J. E., 1955, Biogeochemical prospecting at the Shaw- angunk Mine [N.Y.]—A case study: Econ. Geology, v. 50, no. 4, p. 420—429. LEAD IN IGNEOUS AND METAMORPHIC ROCKS AND IN THEIR ROCK-FORMING MINERALS By MICHAEL FLEISCHER LEAD IN IGNEOUS AND METAMORPHIC ROCKS Most of the recent estimates of the lead content of igneous and metamorphic rocks (table 7) are based principally on the spectrographic analyses by Wedepohl (1956). Since the publication of his work, several hundred determinations by a variety of methods have been published. These are summarized in table 8 and are shown graphically for selected rock types in figure 5, in which the rocks analyzed have been classified under the names given by the respective authors. The data are in good general agreement with the averages previously proposed by Wedepohl and other researchers. It will be noted that the median values are generally slightly lower than the arith- metic averages, a consequence of the inclusion of a few very high determinations; thus, it is believed that the median value is generally more meaningful than the arithmetic average. The data indicate that, within the probable error of the averages, the lead contents of plutonic rocks are generally about the same as those of their volcanic equivalents (table TABLE 7.—Published estimates of lead contents of igneous and meta- morphic rocks Rock Type Lead (ppm) Silicic rocks (granite, rhyolite) ....................... ‘20.0 Granite ....................... 220.0 Granite, high Ca 315.0 Granite, low Ca . ........ 319.0 Granodiorite ............................................................................ 215.0 Intermediate rocks (diorite, andesite) ..................................... 115.0 Quartz diorite ........................................ 2 8.0 Diorite ..................................................................................... 210.0 Alkalic rocks ........................................................................... 2 312.0 Ultramafic rocks .................................................................... 2 3.0 1 .1 3 1.0 Rhyolite and obsidian .. 221.0 Dacite ....... : ............. 211.0 Basalt and gabbro ..... .0 .0 .0 ‘Vinogradov (1956, 1962). ’Wedepohl (1956). lTurekian and Wedepohl (1961). . 8), granites correspond to rhyolites, diorites to andesites, and alkalic rocks to trachytes and phonolites. The lead contents increase with increasing silica content and, in general, with content of potassium, but even within a single pluton or igneous complex correla- tions such as Pb/K, Pb/ Rb, and Pb/ Ba show wide varia- tions. It is very doubtful that these ratios are useful as a tool for correlation. They are even less useful as a basis for prediction of lead contents. A few typical examples from recent papers for the variation of the Ba/ Pb ratio are listed in table 9. The average lead content for gneiss in table 8 is slightly lower than that for granitic rocks, but there are not sufficient data to permit drawing any conclusions as to whether lead is gained or lost during metamorphism, except to say that metasomatic reactions leading to the development of K-feldspar result in increased lead content. Milovskiy and Matveyeva (1970) studied three instances of granitization and found increases of average lead content in two, where lead increased from 13 to 30 ppm and from 8 to 18 ppm; in a third example the lead content remained constant at 40 ppm Pb. The distribution of analyses for granite, granodiorite, basalt, and gneiss is shown in figure 5. LEAD IN ROCK-FORMING MINERALS OF IGNEOUS AND METAMORPHIC ROCKS From consideration of the ionic radii of the rock- forming elements of igneous rocks (values are from those of Whittaker and Muntus, 1970, for coordination number 6), one would expect lead (Pb+2, 0.126 nanometres) to be concentrated in minerals of potassium (K , 0.146 nm), such as K-feldspars, biotite, and muscovite, and to occur to a lesser degree in minerals of calcium (Catz, 0.108 nm) and sodium (Nat, 0.108 nm). In igneous rocks one would also expect very close geochemical coherence between lead and barium (Batz, 0.144 nm) and between lead and strontium (Srt 2, 0.121 nm), and fair coherence of lead with rubidium (Rb*, 0.157 nm). In reality, however, although there is a general parallelism between the contents of lead and potassium, 25 26 PERCENT OF SAMPLES LEAD IN THE ENVIRONMENT 25 20 25 Granite (414 samples) Granodiorite (286 samples) 40 30 Basalt (324 samples) 35 30— 25— Gneiss (118 samples) 10 20 30 40 50 60 70 Pb, IN PARTS PER MILLION FIGURE 5.—Histograms of lead distribution in granite, granodiorite, ba 80 \. salt, and gneiss. 90 100 LEAD IN IGNEOUS AND METAMORPHIC ROCKS 27 TABLE 8,—Summary of published analyses of lead content of igneous and metamorphic rocks. [Leaders (...) indicate not determined] Lead content (ppm) Rock type N;::{:::f Range Agrggezm Median Granitic rocks ........................... 536 0—200 25.0 18 Granodiorite, adamellite 317 0— 80 22.0 16 Diorite, quartz diorite.... 122 0— 76 14.0 11 Alkalic rocks ............... 153 0—500 22.0 16 Ultramafic rocks ....................... 34 0— 37 2.0 Rhyolite, obsidian.... 273 0—200 21.0 18 Latite, quartz latite.. 49 0- 50 25.0 21 Dacite, rhyodacite 121 0—300 12.0 11 Andesite ....................... 203 0—150 12.0 8 Basalt, gabbro, diabase .. 372 0—100 7.5 4 Trachyte, phonolite .................. 33 0— 60 18.0 16 Gneiss ........................................ 274 0— 80 20.0 12 Schist ........... 81 0—100 15.0 15 Amphibolite... 51 0— 50 11.0 9 both in igneous rocks and in the individual minerals of those rocks, examination in detail shows extremely wide and unsystematic variations in the K/Pb ratio. The same may be said of the relation between lead and rubidium contents; likewise, the ratios of lead to barium and lead to strontium vary over an extremely wide range and show no systematic relation to crystallization trends. In discussing this problem, Heier (1962 p. 426) stated, Lead does not show any simple relation to any of the other elements substituting for potassium in K-feldspars. Because Pb+2 is both divalent and smaller than K+ , it should be strongly captured according to classical distribution rules. However, the data "" ‘1 * show that lead tends to be enriched in the most fractionated (pegmatite) K-feldspars. This contradiction to Goldschmidt’s rule is related to the large electro- negativity value of Pb”, and the consequent increased covalent nature of the Pb—O bonds as compared to the K-0 bonds. DISTRIBUTION OF LEAD AMONG ROCK-FORMING MINERALS Less than a dozen analyses have been published in which lead content has been determined for all the consti- tuents of a given rock, and the material balances for these are not very good, presumably because of the low spectro- graphic sensitivity for lead. The few available analyses (Nockolds and Mitchell, 1948; Tauson and Kravchenko, 1956; Zlobin and Gorshkova, 1961) all show that 70—95 percent of the total. lead in the rocks is present in K- feldspar plus plagioclase. It should be noted that some of the accessory minerals of granitic rocks, especially those containing radioactive elements (for example, monazite, xenotime, uraninite, thorite, zircon, allanite, titanite), commonly contain far greater concentrations of lead than major rock-forming minerals; the contribution of accessory minerals to the total lead content of the rock, however, is generally small. Because galena occurs in small amounts in many rocks, attempts have been made (Arnaudov and others, 1967; TABLE 9.—Range of variation of Ba/Pb ratio in igneous and meta— morphic rocks Rock type Range of Ba/ Pb ratio Reference Rhyolite: Pitchstone ................... 60—300 Carmichael and McDonald (1961). Pantellerite ................. 4— 56 Gibson (1972). Andesite ......... 20— 44 Peltz and others (1971). Do .............. 30-153 Taylor and others (1969). Olivine basalt 36—129 Cummings (1972). Ankaramite.... 75—360 Gunn and others (1970). Dolerite ................. 8— 35 Walker (1969). Granodiorite ................... 17—2 10 Savu and others (1971). Granite .......... 23— 76 Do. Do 2— 90 Saba and others (1968). Gneiss ............................. 7—132 Khaffagy (1971). Arnaudov and Pavlova, 1971) to measure ”sulfide lead” (by leaching with a solution containing 25 percent NaCl+0.5N HCl) and “sulfate lead” (by leaching with a solution containing 25 percent NaCl); these experiments show that in nearly all samples of feldspars and muscovites, 70—95 percent of the lead present is in the silicate molecule. LEAD CONTENT OF ROCK-FORMING MINERALS The feldspars, as stated previously, are the principal carriers of lead in igneous and metamorphic rocks. Their contents of lead vary widely from locality to locality, and even in different samples from a single locality, but general trends are evident from table 10 and figure 6, where data for orthoclase and microline are grouped together. For rocks of granodiorite composition the content of lead is only slightly greater in K-feldspar than in coexisting plagioclase (indeed, a few analyses show more lead in the plagioclase), but the lead content of the plagioclase decreases with decreasing content of calcium. Consequently, the ratio of lead in K-feldspar to lead in plagioclase increases towards the granites proper and TABLE lO.—Lead content of feldspars [Leaders (...) indicate not determined] Number PNPPm) Rock type of analyses Range Average Median K-feldspars (miaocline and orthoclase) Rhyolite, latite, quartz latite1 ...... 162 2— 115 47 Granitic rocks2 ...................... 344 0— 150 36 20 Granitic pegmatite3.. 428 0— 560 72 30 Amazonite, granitic pegmatite 174 <5—13,500 477 200 Plagiodue Rhyolite, latite, quartz latite ....... 3 13—14 14 14 Granitic rocks2 ...... .. 128 1—52 13 10 Granitic pegmatite 90 < 5—148 46 35 lThese are mostly sanidine, 2Gramte, granodiorite, adamellite, gnelss. IExcluding amazomte. LEAD IN THE ENVIRONMENT 35 * — 30 K-feldspar from granitic pegmatite that _ excludes amazonite (428 samples) 2° I I | | 15 K-feldspar from granitic pegmatite that —- includes amazonite (174 samples) 35 I I I I I 30 — PERCENT OF SAMPLES 25 ' K-feldspar from granitic rocks (344 samples) 20 — 15 '— 30 I I I I I 25 — 20 Plagioclase from pegmatite (90 samples) 10 ~ | I | | | | | I l 00 100 200 300 400 500 600 700 800 900 >950 Pb, IN PARTS PER MILLION FIGURE 6.—Frequency of occurrence (percent of total samples) If lead in K-feldspar from granitic pegmatite that excludes amazonite, from granitic pegmatite that includes amazonite, and from granitic rocks, and in plagioclase from pegmatite. LEAD IN IGNEOUS AND METAMORPHIC ROCKS 29 further increases in the feldspars of granitic pegmatite. The K-feldspars of very late stages of granitic pegmatite (including the amazonite) show very high contents of lead, reaching maximum levels of 1.35 percent for amazonite (Alker, 1959) and 1.10 for green orthoclase (Cech and others, 1971). Plots correlating lead content with Rb/ Pb and Ba/ Pb ratios show that these ratios tend to decrease as lead content increases, but the data scatter is so great that these ratios probably are not useful for correlation or prediction. Data on other rock-forming minerals are assembled in table 1 1. Most of the data on muscovite are from Bradshaw (1967), who found significantly higher contents of lead in samples from granites associated with mineralization. The data of Parry and Nackowski (1963) and of Lovering (1969, 1972) indicate that the lead content of biotites from mineralized granitic rocks is slightly higher than in those from unmineralized granitic rocks; it is possible, however, that some of the variation is due to regional differences not connected with processes of mineralization. The average for chlorites seems to be higher than would have been anticipated; it may reflect introduction of lead during hydrothermal alteration, but more work is clearly needed. The low contents in quartz, pyroxene, amphibole, garnet, and olivine are as expected. Very little work has been done on the fate of lead in the weathering of igneous rocks. The data of Butler (1953, 1954), however, indicate that most of the lead is taken up by minerals of the clay-size fraction. SUMMARY Averages for lead in the major types of igneous and metamorphic rocks as shown by recent analyses are in good agreement with the averages suggested by Wedepohl (1956). In the normal igneous or metamorphic rock series, the lead content increases with silica content and there is a rough correlation between contents of lead and potassium, but the variations are so great that ratios of lead to potassium, rubidium, or barium are not of much value for correlation or prediction. No distinct differences were noted between extrusive rocks and their intrusive equivalents. The data do not clearly show whether lead is gained or lost during metamorphism, but indicate gain of lead during feldspathization or other types of potassium— metasomatism. Most of the lead in igneous and metamorphic rocks is contained in feldspar; although some minor accessory minerals commonly contain more lead, their contri- bution to the total lead content of the rock is small. Late- stage K-feldspars of granitic pegmatite usually have the highest lead content of the common rock-forming minerals. The lead content of the late micaceous minerals (muscovite, biotite, and chlorite) is generally lower than TABLE ll.—Lead content of other rock-forming minerals Number of Pb (ppm) Mineral and rock type analyses Range Average Muscovite: Granitic rocks: Mineralizedl .......... 55 4—100 21 Unmineralizedl . 99 1-450 7 Granite pegmatite.... 21 9— 34 18 Schist .............. 8 <10— 77 42 Eclogite ............................. 2 (2) 10 Biotite: Granitic rocks .......................... 585 < 1—450 29 Silicic volcanic rocks 17 < 10— 50 10 Schist ........................... 73 5—120 20 Pyroxenite ......................... . 1 None 9 Biotite and phlogopite: Granite pegmatite .................... 11 no data 13 Chlorite3 .................... 39 7—220 64 31 0— 50 3 4196 O— 59 55 547 0— 34 ‘10 21 1— 46 8 Olivine ............................................ 9 0— 6 2 ‘Granite, granodiorite, gneiss. 2Berth analyses yielded 10 ppm. 5Nearly all from granitic rocks. ‘Excluding two analyses of 216 and 843 ppm Pb. 5Excluding one analysis of 150 ppm Pb. that of the feldspars, but higher than that of the other common rock-forming minerals. Micas from mineralized granitic rocks appear to contain slightly more lead than those from unmineralized rocks of similar composition. REFERENCES CITED Alker, Adolf, 1959, Ein Amazonit pegmatit bei Pack, Steiermark: Abh. Mineralogie Landesmuseums Joanneum, Graz., 1959, p. 1-6. Arnaudov, V., and Pavlova, M., 1971, The presence of lead in some mica-bearing pegmatites from southern Bulgaria [in Bulgarian, with Russian and English summ.]: Bulgar. Akad. Nauk Geol. Inst. Izv. Ser. Geokhim., Mineral, Petrog., v. 20, p. 21—29. Arnaudov, V., Pavlova, M., and Petrusenko, Sv., 1967, Lead content in certain amazonites [in Bulgarian, with Russian and English summ.]: Bulgar. Akad. Nauk., Geol. Inst. Izv. Ser. Geokhim., Mineral, Petrog., v. 16, p. 41-44. Bradshaw, P. M. D., 1967, Distribution of selected elements in feldspar, biotite, and muscovite from British granites in relation to mineralization: Inst. Mining Metallurgy Trans., sec. B, v. 76, [no.] 727, p. B137-B148. Butler, J. R., 1953, The geochemistry and mineralogy of rock weathering—1, The Lizard area, Cornwall: Geochim. et Cosmo- chim Acta, v. 4, no. 4, p. 157—178. 1954, The geochemistry and mineralogy of rock weathering— 2, The Nordmarka area, Oslo: Geochim. et Cosmochim. Acta, v. 6, no. 5, p. 268—281. Carmichael, Ian, and McDonald, Alison, 1961, The geochemistry of some natural acid glasses from the north Atlantic Tertiary volcanic province: Geochim. et Cosmochim. Acta, v. 25, no. 3, p. 189—222. Cech, F., Misar, Z., and Povondra, P., 1971, A green lead—containing orthoclase [with German summary]: Tschermaks Mineralog. U. Petrog. Mitt, v. 15, p. 213—231. Cummings, David, 1972, Mafic and ultramafic inclusions, Crater 160, San Francisco volcanic field, Arizona, in Geological Survey re— search 1972: US. Geol. Survey Prof. Paper BOO-B, p. B95—B104. Gibson, I. L., 1972, The chemistry and petrogenesis of a suite of pantellerites from the Ethiopian rift: Jour. Petrology, v. 13, no. 1, p. 31—44. 30 LEAD IN THE ENVIRONMENT Gunn, B. M., Coy-Yll, Ramon, Watkins, N. D., Abranson, C. E., and Nougier, Jacques, 1970, Geochemistry of an oceanite-ankaramite- basalt suite from East Island, Crozet archipelago: Contr. Minera- logy and Petrology, v. 28, no. 4, p. 319—339. Heier, K. S., 1962, Trace elements in feldspars—A review: Norsk Geol. Tidsskr., v. 42, no. 2, p. 415—454. Khaffagy, Mahmoud, 1971, Zur Geochemie der Spitzer Gneisse und der Paragesteinsserie des Kamptales, Niederosterreich: Verh. Geol. Bundesanstalt, 1971, no. 2, p. 171-192. Lovering, T. G., 1969, Distribution of minor elements in samples of biotite from igneous rocks, in Geological Survey research 1969: U.S. Geol. Survey Prof. Paper 650—B, p. B101—BlO6. 1972, Distribution of minor elements in biotite samples from felsic intrusive rocks as a tool for correlation: U.S. Geol. Survey Bull. 13l4—D, 29 p. [1971]. Milovskiy, A. V., and Matveyeva, S. S., 1970, Behavior of elements during the granitization of rocks [in Russian]: Geologiya Rudnykh Mestorozhd, 1970, no. 3, p. 3—22; translated in Internat. Geology Rev., v. 14, p. 623—638, 1972. Nockolds, S. R., and Mitchell, R. L., 1948, The geochemistry of some Caledonian plutonic rocks—A study in the relationship between the major and trace elements of igneous rocks and their minerals: Royal Soc. Edinburgh Trans. [1946], v. 61, pt. 2, p. 533—575. Parry, W. T., and Nackowski, M. P., 1963, Copper, lead, and zinc in biotites from Basin and Range quartz monzonites: Econ. Geol- ogy, v. 58, p. 1126—1144. Peltz, Sergiu, Vasiliu, Cecilia, and Udrescu, Constanta, 1971, Petro- logy of magmatites from the Neogene subvolcanic zone, East Carpathians [in Rumanian with French and English summaries]: Romania Com. Stat Geologiei Anuarul, v. 39, p. 177—256. Saha, A. K., Sankaran, A. V., and Bhattacharyya, T. K., 1968, Trace- element distribution in the magmatic and metasomatic granites of Singhbhum region, eastern India: Neues Jahrb. Mineralogie Abh., v. 108, no. 3, p. 247—270. Savu, Haralambie, Vasiliu, Cecilia, and Udrescu, Constanta, 1971, Petrologic and geochemical study of the synorogenic and late orogenic granitoids from the Susita pluton, south Carpathians [in Rumanian, with French and English summaries]: Romania Corn. Stat Geologiei Anuarul, v. 39, p. 257—296. Tauson, L. V., and Kravchenko, L. A., 1956, Characteristics of lead and zinc distribution in minerals of the Caledonian granitoid of the Susamyr batholith in central Tian-Shan [in Russian]: Geok- himiya, 1956, no. 1, p. 81—89; translated in Geochemistry, 1956, no. 1, p. 78—88. Taylor, S. R., Capp, A. C., Graham, A. L., and Blake, D. H., 1969, Trace element abundances in andesites—2, Saipan, Bougainville, and Fiji: Contr. Mineralogy and Petrology, v. 23, no. 1, p. 1—26. Turekian, K. K., and Wedepohl, K. H., 1961, Distribution of the ele- ments in some major units of the Earth’s crust: Geol. Soc. America Bull., v. 72, p. 175—192. Vinogradov, A. P., 1956, Regularity of distribution of chemical ele- ments in the Earth’s crust [in Russian]: Geokhimiya, 1956, no. 1, p. 6—52; translated in Geochemistry, 1956, no. 1, p. 1—43. 1962, Average contents of the chemical elements in the principal types of igneous rocks of the Earth’s crust [in Russian with English summary]: Geokhimiya, 1962, no. 7, p. 555—571; translated in Geo— chemistry, 1962, no. 7, p. 641—664. Walker, K. R., 1969, The Palisades sill, New Jersey—A reinvestigation: Geol. Soc. America Spec. Paper 111, 178 p. Wedepohl, K. H., 1956, Untersuchungen zur Geochemie des Bleis [with English summ.]: Geochim, et Cosmochim. Acta., v. 10, no. 1—2, p. 69—148. Whittaker, E. J. W., and Muntus, R., 1970, Ionic radii for use in geo- chemistry: Geochim. et Cosmochim. Acta, v. 34, no. 9, p. 945—956. Zlobin, B. 1., and Gorshkova, M. S., 1961, Lead and zinc in alkalic rocks and their bearing on some petrologic problems [in Russian with English summary]: Geokhimiya, 1961, no. 4, p. 283—292; translated in Geochemistry, 1961, no. 4, p. 317—328. ABUNDANCE OF LEAD IN SEDIMENTARY ROCKS, SEDIMENTS, AND FOSSIL FUELS By T. G. LOVERING SEDIMENTARY ROCKS AND SEDIMENTS Lead is present in very small amounts in the common ' sedimentary rocks. Analytical data from the US. Geological Survey’s rock analysis storage system for more than 2,500 sedimentary rock samples from various parts of the United States show average lead concentrations of about 32 ppm for carbonaceous shale, 23 ppm for siltstone, muds tone, 'claystone, and noncarbonaceous shale, 17 ppm for sandstone, and l 1 ppm for limestone and dolomite. For all these rock types, median values are lower than the averages (fig. 7), and even the carbonaceous shale, which has the highest average lead content, contains only about 1 ounce of lead per ton (31 g/ t) of average rock. Comparable data on the lead content of these and other sedimentary rocks are found in the literature. Selected samples of carbonaceous shale from various formations in the Western United States contained from <10 to 70 ppm lead, with a median of 15 ppm (Davidson and Lakin, 1961, 1962). Vine (1966) collected four sets of black shale samples from various parts of the country and found median values in them ranging from 10 to 30 ppm of lead, and Wedepohl (1971) obtained an average lead value of 23.8 ppm from 200 bituminous shale samples taken from both the United States and Europe. Likewise, in an additional 73 samples of shale of Mesozoic to Holocene age, from Europe, Korea, Japan, the United States, and Trinidad, Wedepohl found an average of 23.8 ppm lead with a standard deviation of 15 ppm. Tourtelot (1962, pl. 4) found that the lead content of samples of Pierre Shale taken from widely separated localities in Montana, Wyoming, and South Dakota showed a narrow range of from 7 to 30 ppm with a median at 15 ppm. Krauskopf (1955, p. 416) gave an average value for lead in shale of 20 ppm; Newman (1962, p. 425) estimated the average lead content of Triassic mudstones on the Colorado Plateau as 13 ppm, and Cadigan (1971, p. 41—42) calculated a geometric mean of about 12 ppm lead for these rocks. Published data on the lead content of normal sandstone are somewhat less abundant than those for shale and silts tone. Krauskopf ( 1955, p. 416) gave an average range of 10—40 ppm lead for sandstone. Wedepohl (1971, p. 241) found a mean value of 19.2 ppm lead in 45 Paleozoic and Mesozoic quartz sandstones from Germany. Lapchinskii and Lapchinskaya (1966, p. 123—127) reported an observed range of 16—52 ppm for lead in Carboniferous sandstone from Shebelinsk, Russia, and Razdorozhnyi (1966, p. 203—206) obtained a modal value of 10 ppm lead from sandstone samples of similar age from the Donets Basin of Russia. Our data on normal lead in carbonate rocks (limestone and dolomite) are comparable with the average abundance of 814 ppm estimated by Graf (1960, p. 71) and with the average of 9 ppm given by Wedepohl (1971, p. 241) for 124 samples of European carbonate rocks. Semiquantitative analyses of several hundred samples of phosphate rock from the Phosphoria Formation in Idaho (Sheldon and others, 1953; McKelvey and others, 1953) suggest that the normal lead content of this rock ranges from 10 to 100 ppm and that many of the samples contain >100 ppm of lead, indicating a slightly greater concentration of lead in phosphate rock than in carbonaceous shale. Bauxite, on the other hand, appears to be more comparable to the carbonate rocks in its lead content; samples of Arkansas bauxite had an average content of 7 ppm and a maximum content of 70 ppm lead (Gordon and Murata, 1952). However, some published analyses of European phosphorites and bauxites suggest the reverse relationship in the relative concentrations of lead in these two rocks. Orekhov (1968) reported a maximum value of 30 ppm lead in samples of lower Tertiary phosphorites of the Rostov region in Russia. Maksimovic (1968) found a range of 16—155 ppm, with an average of 69 ppm lead, in 120 samples of bauxite from Herzegovina, Yugoslavia. Gyérgy (1957), however, gave lead concentrations in bauxite samples from western Hungary that are comparable to those reported for Arkansas bauxite. The normal lead content of marine evaporite rocks appears to be even lower than that of the sedimentary carbonate rocks. Stewart (1963, p. Y33-Y39) presented 31 32 several analyses of these marine rocks and of the chloride and sulfate minerals of which they are composed. The highest lead value he gave is 5.4 ppm for a core sample of halite (NaCl); most of the halite samples, and also those of potash evaporites, contained less than 1 ppm of lead. The low lead content of potash evaporite sediments is somewhat surprising in view of the tendency for lead to concentrate in the potash feldspars of igneous rocks. The presence of available lead in the marine environment is indicated by relatively high lead concen- trations in samples of unconsolidated marine sediments. Turekian and Wedepohl (1961, p. 186) reported an average of 45 ppm lead in deep sea clay from the Atlantic and 110 ppm in corresponding samples from the Pacific. Ericson, Ewing, Wollin, and Heezen (1961, p. 229—231) analyzed more than 100 argillaceous bottom sediments from the Atlantic Ocean and the Caribbean Sea. These samples gave a median lead value of 60 ppm and a maximum of 240 ppm. Riley and Skirrow (1965, p. 49) reported an average 45MIIIII IIIII _ .. Sandstone Noncarbonaceous siltstone (733 samples) and shale (677 samples) M “mm 20 30 50 70 100 200 0 use 10 20 30 50 70 100 200 3 Pb, IN PARTS PER MILLION : M g 70 I I I I I I w 65 Limestone and Carbonaceous _ Lol- dolomite shale ,_ 60 (877 samples) (2865amples) _ z B 55 _ a: E 50 _ 0 10 20 30 50 7O 0 10 2O 30 50 70 100 200 Pb, IN PARTS PER MILLION FIGURE 7.——-Lead content of common sedimentary rocks (from US. Geological Survey’s rock analysis storage system). M, median; E, average. LEAD IN THE ENVIRONMENT of 162 ppm of lead in deep sea clay samples; they believed more than half of this lead to have been derived from seawater (p. 57). Similarly, in deep sea nodules of iron and manganese oxides, they obtained nearly 1,000 ppm lead, which they thought was adsorbed from seawater by hydrous colloidal oxides of iron and manganese. Marine sediments deposited in shallow water appear to contain appreciably less lead than those deposited in deep water. Nearshore samples from the Pacific Ocean average only 20 ppm lead (Riley and Skirrow, 1965, p. 49), and similar lead contents are reported from Mediterranean Sea sediments (Bilyavskii, 1969), and Baltic Sea sediments (Lubchenko, 1970). However, in tectonically active areas, such as the Red Sea, where volcanic emanations and hydrothermal solutions mingle with marine water, lead content of the sediments can be greatly increased. Sediment samples from the bottom of hot brine pools in the Red Sea contain as much as 0.2 percent lead (Hendricks and others, 1969, p. 434). This contrast between lead contents of deep and shallow marine sediments suggests that lead entering the marine environment has a tendency to move outward and down- ward toward the deeps. Such a theory would help to explain the low lead in evaporite sediments that formed in shallow basins; most of the lead may be presumed to have migrated seaward from these basins before the brines became sufficiently concentrated for the evaporite minerals to precipitate. Higher lead content of deep marine sediments may also be caused, in some places, by lead-bearing juvenile emanations entering the deep sea basins. The normal lead content of unconsolidated terrestrial stream sediment appears to be comparable to that of shale. Holman (1963) found an average of 18 ppm lead in stream sediment samples from Nova Scotia. Analyses of more than 5,500 stream sediment samples from a large area in Maine yielded a mean of 25 ppm lead (F. C. Canney and Maurice Chaffee, written commun., 1971). FOSSIL FUELS The normal lead content of coal appears to be inter- mediate between that of shale and of the carbonate rocks, averaging about 10 ppm. Rao (1968) estimated that the lead content of Alaskan coal samples ranged from 1 to 50 ppm and averaged 10 ppm. Published lead analyses of organic fuels samples are normally given as percent lead in ash, inasmuch as lead cannot be determined by ordinary analytical methods in the presence of large amounts of organic carbon. The values given by Rao for the lead content of the ash of Alaskan coal samples are approxi- mately one order of magnitude higher than his estimated values for the raw coal, ranging from 20 to 400 ppm and averaging 100 ppm. Abernathy, Peterson, and Gibson ( 1969) presented data on the lead content of the ash of large numbers of coal samples from the western, interior, and LEAD IN SEDIMENTARY ROCKS, SEDIMENTS, AND FOSSIL FUELS 33 eastern coal provinces of the United States. The average lead in ash was given as 29 ppm for the western province, 131 ppm for the interior province, and 55 ppm for the eastern province. The published data suggest that the lead content of coal tends to decrease as its rank increases. Duel and Annell (1956) found lead contents in ash of low-rank-coal samples ranging from 100 to 1,000 ppm. Nunn, Lovell, and Wright (1953, p. 56) estimated a range of 10-100 ppm in the lead content of anthracite ash; and Chow and Earl ( 1970, p. 46) found a range of 10.0—33.8 ppm of lead in the ash of anthracite samples from Pennsylvania and Rhode Island. Headlee and Hunter (1955, p. 151—155) ran some experiments on the partitioning of lead in coal among the products of combustion. They concluded that the ashing techniques used in the laboratory retain essentially all of the lead in the ash, but that ordinary burning of coal in industrial processes volatilizes about 6 percent of the lead originally present in the coal. If we use this figure, and assume a lead content of 10 ppm (0.001 percent) in the coal, it would require the combustion of approximately 10,000 tons of coal to introduce 1 pound (0.45 kg) of volatilized lead into the atmosphere. Industrial burning of coal samples with a lead content of 0.037 percent in the ash produced soot with a lead-in-ash content of 0.4 percent, and the ash of coal tar derived from this coal contained 2.84 percent lead (Headlee and Hunter, 1955, p. 155). The lead content of petroleum, although generally lower than that of coal, is extremely variable. Hyden (1961 , p. 44—59) published extensive tables showing the lead content of crude oil samples from different parts of the country. Lead ranges from < 0.001 to 11.4 ppm and averages 0.025 ppm in the oil, and it ranges from < 0.0015 to 300 ppm and averages 20 ppm in the ash. Donnell, Tailleur, and Tourtelot (1967) reported a maximum lead content of 200 ppm in the ash of samples of Alaskan oil shale. REFERENCES CITED Abernathy, R. F., Peterson, M. J., and Gibson, F. H., 1969, Spectro- chemical analyses of coal ash for trace elements: U.S. Bur. Mines Rept. Inv. 7281, 30 p. Bilyavskii, G. A., Mitropolskii, A. Yu., and Romanov, V. I., 1969, New data on the distribution of trace elements in the columns of bottom sediments in the Mediterranean Sea [in Ukranian]: Akad. Nauk Ukrayin RSR, Dopov'i'd'i', ser. B, v. 31, no. 12, p. 1059—1061; abs. in Chem. Abs., v. 72, no. 69311f, 1972. Cadigan, R. A., 1971, Geochemical distribution of some metals in the Moenkopi Formation and related strata, Colorado Plateau region: U.S. Geol. Survey Bull. 1344, 56 p. Chow, T. J., and Earl, J. L., 1970, Lead and uranium in Pennsylvanian anthracite: Chem. Geology, v. 6, no. 1, p. 43—49. Davidson, D. F., and Lakin, H. W., 1961, Metal content of some black shales of the western United States, in Short papers in the geologic and hydrologic sciences: U.S. Geol. Survey Prof. Paper 424—C, p. C329—C331. 1962, Metal content of some black shales of the western comer- minous United States, pt. 2, in Short papers in geology and hydro- logy: U.S. Geol. Survey Prof. Paper 450—C, p. C74. Deul, Maurice, and Annell, C. S., 1956, The occurrence of minor ele— ments in the ash of low-rank coal from Texas, Colorado, North Dakota, and South Dakota: US. Geol. Survey Bull. 1036-1-1, p. 155—172. Donnell, J. R., Tailleur, I. L., and Tourtelot, H. A., 1967, Alaskan oil shale: Colorado School Mines Quart, v. 62, no. 3, p. 39—43. Ericson, D. B., Ewing, Maurice, Wollin, Goesta, and Heezen, B. G., 1961, Atlantic deep-sea sediment cores: Geol. Soc. America Bu11., v. 72, p. 193—285. Gordon, Mackenzie, Jr., and Murata, K. J., 1952, Minor elements in Arkansas bauxite: Econ. Geology, v. 47, no. 2, p. 169—179. Graf, D. L., 1960, Geochemistry of carbonate sediments and sedi- mentary carbonate rocks—Pt. 3, Minor element distribution: Illinois State Geol. Survey Div. Circ. 301, 71 p. Gyfirgy, Bardossy, 1957, Bauxite in the region of Szoc and Nyirad [in Hungarian]: Magyar Allami F'dldt. Intéz. Evkohyve 46, p. 433—454. Headlee, A. J. W., and Hunter, R. G., 1955, Changes in the concentra- tion of the inorganic elements during coal utilization, in Char- acteristics of minable coals of West Virginia: West Virginia Geol. Survey [Rept.], v. 13a, p. 150—159. Holman, R. H. C., 1963, A regional geochemical reconnaissance of stream sediments in the northern mainland of Nova Scotia, Canada: Canada Geol. Survey Paper 63—23, p. 1—19. Hendricks, R. L., Reisbeck, F. B., Mahaffey, E. J., Roberts, D. B., and Peterson, M'. M. A., 1969, Chemical composition of sediments and interstitial brines from the Atlantiss II, discovery and chain deeps, in Degens, E. T., and Ross, D. A., eds., Hot brines and recent heavy metal deposits in the Red Sea: New York, Springer- Verlag, p. 407—440. Hyden, H. J., 1961, Distribution of uranium and other metals in crude oils: U.S. Geol. Survey Bull. 1100—B, p. 17-99. Krauskopf, K. B., 1955, Sedimentary deposits of rare metals: Econ. Geology, 50th Anniversary Vol [1905—1955], pt. 1, p. 411—463. Lapchinskii, Yu. G., and Lapchinskaya, L. V., 1966, Trace elements in carboniferous formations of the Shebelinsk natural gas deposits [in Russian]: Ukrain. Nauchno—Issled. Inst. Prirodnykh Gazov Trudy 1966, no. 2, p. 123—127; abs. in Chem. Abs., v. 67, no. 8393a, 1967. Lubchenko, I. Yu., 1970, Lead in recent Black Sea sediments [in Russian]: Akad. Nauk SSSR Doklady, v. 193, no. 2, p. 445—448. McKelvey, V. E., Armstrong, F. C., Gulbrandson, R. A., and Cambell, R. M., 1953, Stratigraphic sections of the Phosphoria formation in Idaho, 1947—48, pt. 2: US. Geol. Survey Circ. 301, 58 p. Maksimovic, Z., 1968, Distribution of trace elements in bauxite deposits of Herzegovina, Yugoslavia [in English]: Acad. Yugoslave Sci. Arts, Trav. Com. Int. Etude Bauxites, Oxydes, Hydroxydes Alum. 1968, no. 5, p. 63—70; abs. in Chem. Abs., v. 69, no. 88747g, 1968. Newman, W. L., 1962, Distribution of elements in sedimentary rocks of the Colorado Plateau—A preliminary report: U.S. Geol. Survey Bull. 1107—F, p. 337—445. Nunn, R. C., Lovell, H. L., and Wright, C. C., 1953, Spectrographic analysis of trace elements in anthracite: Anthracite Conf., 11th, Lehigh Univ. 1953, Trans., p. 51—65. Orekhov, S. Ya., 1968, Mineralogy and structural types of phosphorites of the Rostov region [in Russian]: Rostov na Donu Gosudar- stvennogo Universitet, Uchenye Zapiski, v. 53, no. 9, p. 273—287. Rao, P. D., 1968, Distribution of certain minor elements in Alaskan coals: Alaska Univ. Mineral Industry Research Lab. Rept. 15, 47 p. Razdoroshnyi, V. F., 1966, Copper, zinc and lead in rocks of small folding zone of Donets Basin (Lugansk geological district) [in Russian]: Polez. Iskop. Ukr. 1966, p. 203-206; abs. in Chem. Abs., v. 67, no. 83966b, 1967. Riley, J. P., and Skirrow, G., eds., 1965, Chemical oceanography, V. 2: London, New York, Academic Press, 508 p. 34 LEAD IN THE ENVIRONMENT Sheldon, R. P., Werner, M. A., Thompson, M. E., and Pierce, H. W., 1953, Stratigraphic sections of the Phosphoria formation in Idaho, 1949, pt. 1: US. Geol. Survey Circ. 304, 30 p. Stewart, F. H., 1963, Marine evaporites: U.S. Geol. Survey Prof. Paper 440—Y, p. Yl—Y52. Tourtelot, H. A., 1962, Preliminary investigation of the geologic setting and chemical composition of the Pierre Shale, Great Plains region: U.S. Geol. Survey Prof. Paper 390, 74 p. Turekian, K. K., and Wedepohl, K. H., 1961, Distribution of the ele- ments in some major units of the Earth’s crust: Geol. Soc. America Bull., v. 72, no. 2, p. 175—192. Vine, J. D., 1966, Element distribution in some shelf and eugeosyn- clinal black shales: U.S. Geol. Survey Bull. 1214—E, p. E1—E3l. Wedepohl, K. H., 1971, Zinc and lead in common sedimentary rocks, app. to Lavery, N. G., and Barnes, H. L., Zinc dispersion in the Wisconsin zinc-lead district: Econ. Geology, v. 66, p. 240—242. LEAD CONTENT OF WATER By M. J. FISHMAN and J. D. HEM The lead content of water is usually described in terms of concentrations of the dissolved element. For research purposes, the U.S. Geological Survey, in testing water samples for trace elements, considers dissolved material to be particles small enough to pass through a 045- ,um— pore-diameter membrane. Although this is an arbitrary operational definition and some colloidal material may pass through the filter, the 045- ,um size limit is widely used. Where data are given here on dissolved lead, they may be assumed to represent material that passed through a 0.45- [Tm-porosity filter. Until recently very little attention has been given to the minor-element content of material caught on the filter, and it is not possible to obtain from published literature any clear indication of the importance of the suspended fraction in the total quantity of lead transported by streams. There is some evidence, however, that, at times, considerable amounts may be present in suspension in streams carrying direct runoff from urbanized areas. LEAD IN SURFACE WATERS A considerable amount of information on lead concen— trations in surface waters has been obtained since the late 1950’s. However, the observations tend to be sporadic and scattered, and the details of the chemical behavior of the element and its occurrence in any single stream have not been fully explored. Kleinkopf (1960) reported a mean lead concentration of 2.3 [Lg/l in waters from 440 lakes in the State of Maine, with a range of 003—115 ,ug/l. Durum, Heidel, and Tison (1960) reported concentrations of a suite of minor elements, including lead, for samples from major streams throughout the world. Data from their report and supplementary unpublished analyses in U.S. Geological Survey files show a range of lead concentrations from 0 to 200 #g/l, with an average of 8.4 ,u.g/l. There were 93 samples in this group, representing 27 streams, mostly in North America. Lead occurred in concentrations above the detection limit in more than 90 percent of the samples. The samples in the study by Durum, Heidel, and Tison were filtered before analysis, but the filter only removed particles over 1 pm in diameter. Consequently these lead values may include some from material usually found in suspension as well as that in solution. Since 1960, the U.S. Geological Survey has performed more than 1,600 lead determinations on surface~water samples collected throughout the United States. These samples were analyzed for lead by spectrographic and atomic-absorption techniques, which are discussed by M. J. Fishman (this report). Most of the samples were from one of three sources: (1) public water supplies, (2) water courses downstream from major municipal or industrial complexes, and (3) U.S. Geological Survey hydrologic bench—mark stations. The dissolved-lead concentrations in these samples, representing surface waters in all 50 states as well as Washington, DC, and Puerto Rico, range from “not detected” to 890 ug/l. However, lead only rarely occurs in amounts exceeding the U.S. Public Health Service (1963) drinking water standard of 50 [Lg/l lead. Only 13 of the more than 1,600 samples analyzed, or less than 1 percent, contained concentrations in excess of 50 ,ug/l. Of the 13 samples, only 3 contained more than 100 ,u.g/l lead, or more than twice the U.S. Public Health Service limit. Eighty—six percent of the samples contained less than 10 )1. g/l. Most surface waters, except for water courses down- stream from major municipal and industrial complexes, evidently contain less than 10 ,ug/l of dissolved lead. Table 12 summarizes, by States, the maximum and minimum concentrations of lead found in surface waters of the United States. Also included in the summary are the percentage of samples containing less than 10 [Lg/l and the number of samples that exceeded the U.S. Public Health Service drinking water limit. The data in this table, which cover the period from 1960 through 1971, were obtained from Durfor and Becker (1964), and Durum, Hem, and Heidel (1971), and from miscellaneous spectro- graphic data from analyses performed in the U.S. Geological Survey laboratory in Denver, Colo., under the direction of P. R. Barnett. Table 13 shows lead concentrations in selected surface waters of the United States, as determined from miscellaneous spectrographic analyses. Several in-depth studies of surface water in individual States and regions have also been carried out. Silvey (1967) reported an average concentration of 5.7 lug/1 of lead in streams of California in which the element could be 35 36 LEAD IN THE ENVIRONMENT TABLE l2.—Maximum and minimum concentrations of lead found in surface waters of the United States and Puerto Rico from 1960 to 1971 (ND, sought but not found] Number of Percent Lead samples equaling of samples concentration Number of or exceeding having ( ug/l) samples mandatory maximum less than —_____ for drinking water' lOug/l Maximum Minimum Alabama 76 1 96 50.0 ND Alaska.... 9 0 100 5.0 < 1.0 Arizona.. 19 0 89 12.0 ND Arkansas ................. 32 0 81 20.0 < .8 California ............... 40 0 68 34.0 < 1.0 Colorado ....... 70 0 89 30.0 ND Connecticut.. 50 l 72 50.0 < 1.0 Delaware ................. 4 0 25 23.0 2.0 District of Columbia ............ 4 0 100 5.8 < 1.0 Florida... 37 0 100 6.0 < .7 Georgia. 22 0 41 32.0 < 1.0 Hawaii... 8 0 100 < 1.0 <1.0 Idaho ...................... 26 0 100 6.0 < 1.0 Illinois .................... 23 0 48 25.0 2.5 Indiana. 46 0 89 20.0 ND Iowa ....... l4 0 100 5.7 < 1.0 ‘Kansas l4 0 100 7.5 ND Kentucky .. 10 0 60 20.0 < 1.0 Louisiana ............... 19 0 95 11.0 < 1.0 Maine ....... 7 l 57 890.0 5.0 Maryland ......... 29 0 93 43.0 < 1.0 Massachusetts. 24 1 75 87.0 1.3 Michigan ................ 25 0 92 15.0 2.0 Minnesota ............... 30 0 97 17.0 < 1.0 Mississippi 11 0 82 10.0 < 1.0 Missouri 103 0 84 38.0 < .5 Montana 25 0 96 23.0 < 1.0 Nebraska... 21 0 81 40.0 < 1.0 Nevada .................... 8 0 100 5.0 < 1.0 New Hampshire ..... 5 l 80 70.0 4.0 New Jersey ............. 150 2 88 240.0 < 1.0 New Mexico. 34 0 91 10.0 ND New York ............... 211 0 92 22.0 < 1.0 North Carolina ...... 27 0 70 32.0 < 1.0 22 0 86 37.0 < 1.0 31 0 100 7.9 ND 16 2 75 94.0 < 1.0 14 0 100 < . 1.0 < 1.0 Pennsylvania .......... 65 l 71 55.0 2.0 Rhode Island .......... 5 0 100 8.0 3.4 South Carolina ....... 17 0 94 11.0 < 1.0 South Dakota ......... 22 0 68 35.0 < 1.0 Tennessee ............... 21 2 67 390.0 < 1.0 Texas ..... 46 0 98 11.0 ND Utah ...... 28 0 89 12.0 < 1.0 Vermont 3 1 0 50.0 13.0 Virginia ........ 16 0 81 42.0 < 1.0 Washington ............ l7 0 100 7.0 < .5 West Virginia ......... 22 0 86 44.0 < 1.0 Wisconsin ........ 18 0 83 26.0 2.0 Wyoming ...... 37 0 289 3.0 < 1.0 Puerto Rico ............ 40 0 85 19.0 < 1.0 Total ............... 1,673 13 ........... Percent of total samples ....... 86 ........... '50 3/1, as established by the US. Public Health Service (1963). 2Inc udes four values above 10 ug/l. detected. However, lead concentrations were below the detection limit (0.6 ,ug/l) in 78 percent of the stream samples analyzed. A comprehensive investigation of the distribution of dissolved lead in Florida surface waters was carried out by the US. Geological Survey in cooperation with the Bureau of Geology, Florida Department of Natural Resources, and other State and local agencies (C. S. Conover, written commun., 1971). More than 180 samples were analyzed; concentrations of lead ranged from 0 to 40 )1. g/l. The samples were collected during low (May 1970) and high (September 1970) streamflow conditions. The data for May 1970 indicate the following: (1) lead was . found in about 60 percent of the samples in concen- trations ranging from 1 to 30 leg/1; (2) 78 percent of all samples had lead concentrations equal to or less than 10 pig/l; and (3) 4 percent of all samples had concentrations of 30 [Lg/l. For September 1970, the following was observed: (1) lead was found in about 73 percent of the samples in concentrations ranging from 1 to 40 ug/ l; (2) 75 percent of all samples had concentrations equal to or less than 10 [L g/l; and (3) 7 percent of all samples had lead concen- trations equal to or greater than 30 )tg/l. Several other investigators have reported the lead content of various lake and river waters. Boswell, Brooks, and Wilson (1967) reported that the lead concentration in the bottom waters of Lakes Joyce and Hoare in Antarctica were 340 pg/l and 83—91 [Lg/l, respectively. The lead content of the water of the Orange River at Vioolsdrif, Cape Province, Union of South Africa, was reported by DeVilliers (1962) to range from less than 0.001 to 136 ppm (1,360 [Lg/l), based on 24 samples collected over a period of 1 year. Zhukhovitskaya, Zamyatkina, and Lukashev (1966) reported that lead concentrations in streams of the upper Dnieper basin ranged from 0.01 to 0.55 ppb. Lukashev, Zhukhovitskaya, and Zamyatkina (1965) found that lead concentrations in the surface waters of the Poles’e territory near Pripyat in the Belorussian S. S. R. ranged from 2 to 13.3 ppb. Krainov and Korol’kova (1964) analyzed mineral waters of the Lesser Caucasus and found a maximum lead concentration of 40 ,ug/l. Turekian and Kleinkopf (1956) determined lead in 439 stream and lake waters of Maine by a semiquantitative spectrographic procedure. The average concentration of lead was 0.26 ppb. Heidel and Frenier (1965) reported on the lead concentration of 156 surface waters, including some samples from the estuary of the Patuxent River basin, Maryland. The lead content ranged from 0.9 to 11 ,u.g/l, with an average of 5 )Lg/l. A survey of the concentration of lead and other trace elements in the Colorado, Columbia, Ohio, Mississippi, and Missouri Rivers and in the Great Lakes was presented by Kroner and Kopp (1965). In only a few samples did the lead concentration exceed the US. Public Health Service drinking water standard. For most samples the concentration of lead was below the detection LEAD CONTENT or WATER 37 TABLE 13.—Lead concentrations in selected surface waters of the United States Sample locality Dale f’f Lead Concentration Reference collect1on ( [J g /1) Alabama. Sipsey Fork, near Grayson ............................................ Sept. 10, 1970 l J. R. Avrett (oral commun., 1971). Coosa River at Gadsden... ..... . Nov. 30, 1970 2 Do. Cahaba River, near Centreville ..................................... Feb. 4, 1971 1 Do. Arizona: Wet Bottom Creek, near Childs .................................... Sept. 16, 1968 < 1 N. B. Carmony (written commun., 1971). Canal (Gila River), near Arlington .............................. Sept. 18, 1968 < 1 130-, Arkansas: Nov. 6, 1967 5 U.S. Geological Survey (1968a). Bayou Meto, near Lonoke ............................................ Bayou Bartholomew, near McGehee" Nov. 7, 1967 < 5 Do. White River, at Clarendon” Nov. 6, 1967 4 Do. Saline River, near Rye ........... Nov. 7, 1967 < 2 Do. Ouachita River, at Arkadelphla... Nov. 8, 1967 3 Do. Little River, near Ashdown ...... .. Nov. 9, 1967 1 Do. Black River, at Black Rock ........................................... Nov. 16, 1967 < 4 Do. Colorado: Arkansas River, at Granite ............................................ Jan. 19, 1967 30 U.S. Geological Survey (1967a). South Platte River, near Henderson ............................. Jan. 15, 1971 4 R. Brennan (written commun., 1971). Connecticut: Naugatuck River, at Beacon Falls... . ...... Oct. 5, 1967 30 U.S. Geological Survey (1968b). Hockanum River, near Rockville ................................. Aug. 14, 1968 26 Do. Florida: Sopchoppy River, near Sopchoppy ............................... Mar. 10, 1970 6 R. L. Malcolm (written commun., 1971). Iowa: Big Sioux River, at Akron ............................................. Apr. 4, 1967 3 US. Geological Survey (1967c). Massachusetts: Connecticut River, at Northfield .................................. July-NOV. 1970 30 C. E. Knox (written communu 197111- (composite) Hoosic River, below Williamstown .............................. July-Dec. 1970 3 D0. (composite) Missouri: West Fork Black River, at Westfork .............................. Oct. 24, 1967 5 U.S. Geological Survey (1968c). Turkey Creek, near Joplin .......... .. Mar. 19, 1968 25 Do. Spring River, near Waco... June 26, 1968 < 4 Do. Sedalia Lake, near Sedalia. Oct. 29, 1970 1 A. Homyk (written commun., 1971). James River, near Springfield ..... .. Feb. 16, 1971 < 4 Do. Missouri River, near St. Louis ...................................... Mar. 12, 1971 4 Do. Montana: Blackfoot River, near Lincoln ....................................... Aug. 22, 1969 < 3 U.S. Geological Survey (1969). Nebraska: Dismal River, near Thedford... ............................. June 13, 1967 13 L. R. Petri (written commun., 1971). Missouri River, at Omaha ......................................... Mar. 19, 1971 2 A. Homyk (written commun., 1971). New Hampshire: Pemigewasset River, at Woodstock ............................... July-Oct. 1970 70 C. E. Knox (written commun., 1971)1. (composite) New Jersey: Passaic River, at Chatham ............................................ Aug. 28, 1970 9 P. W. Anderson (written commun., 197l)‘. Whippany River, at Morristown. .. do 5 Do. Rockaway River, at Boonton ...... Sept. 1, 1970 3 Do. Hackensack River, at River Vale. Sept. 2, 1970 12 Do. Millstone River, near Manville... Sept. 3, 1970 < 2 Do. Raritan River, near Manville...... do < 2 Do. Assunpink Creek, at Trenton... .. do < 3 Do. Delaware River, at Trenton ........................................... Sept. 29, 1970 2 Do. New Mexico: Pecos River, at Artesia .......... Apr. 7, 1970 < 2 K. Ong (written commun., 1971)1. Rio Mora, at Terrero ......... .. Oct. 5, 1970 .7 Do. Mogollon Creek, near Cliff ........................................... Oct. 14, 1970 .8 Do. New York: Allegheny River, at Salamanca ..................................... Sept. 21, 1967 < 3 US Geological Survey (1967b). Oneida Lake, at Brewerton ............ .. Oct. 1, 1969 < 2 R..J Archer( written commun., 1971)1. Susquehanna River, at Johnson City Apr. 6, 1970 1 Do. Lake Champlain, at Crown Point ....... June 2, 1970 7 Do. Niagara River, at City of Niagara Falls... Nov. 16, 1970 < 5 Do. Lake Ontario, at Oswego ..................... Dec. 9, 1970 4 Do. Hudson River, at Poughkeepsie ........... Dec. 16,1970 2 Do. St. Lawrence River, at Alexandria Bay Dec. 14, 1970 < 3 Do. Black River, at Watertown ............................................ Feb. 22, 1971 1 Do. North Carolina: Neuse River, at Raleigh ................................................ July 9, 1970 .7 R. Heath (written commun., 1971). South Dakota: Belle Fourche River, at Wyoming—South Dakota boundary .................................................................... Oct. 13, 1970 <30 F. C. Boner (written commun., 1971). Wyoming: Laramie River, at Laramie ........... . ................................ Apr. 23, 1970 < 7 Do. Cache Creek, near Jackson ......... Sept. 1, 1970 < 2 Do. North Platte River, below Casper.. Jan. 13, 1971 < 8 Do. Bighorn River, at Kane Jan. 26, 1971 < 5 Do. lSee Pt. 2, Water Quality Records, of the U.S. Geological Survey‘s Water Resources Data series, for the respective State and water year. 38 LEAD IN THE ENVIRONMENT limit of the spectrographic method used to analyze the samples. The data of Durum, Hem, and Heidel (1971) show a dis- tinct regional pattern of dissolved-lead distribution in river water in the United States. Concentrations above the detection limit occurred in nearly all sources sampled in the States along the Atlantic Coast and in most sources in thickly populated regions around the Great Lakes. In the more thinly populated regions west of the Mississippi, a high proportion of the sources had less than 1 ,ug/l of lead, but relatively high concentrations appeared again along the Pacific Coast in streams of the Los Angeles, San Fran- cisco,‘ and Seattle areas. There were areas of higher con- centrations in lead mining regions in Wisconsin; near the Ozark Mountains, and in northern Idaho and adjacent areas. The distribution pattern may be explained by con- sidering the sources of environmental lead and by the solu- bility of lead carbonate and hydroxide from industrial wastes in river water. Livingstone (1963) stated that from data then available it seemed likely that the global mean lead content for lakes and rivers ranged from 1 to 10 ppb. Data cited here suggest that this is a reasonable estimate for the dissolved fraction. In general, the presence of a few tens of micrograms per litre of lead in solution in river water appears to be most common in areas of heavy automobile traffic and extensive industrial development. In the vicinity of lead ore de- posits, the content of lead in river water may also be in this range or higher. Lead can be carried as a colloidal suspension of hydrox- ide in river water, and it may also be present as a coating on other mineral particles, or as ions sorbed on mineral surfaces. The available data do not clearly show the importance of suspended lead. Konovalov, Ivanova, and Kolesnikov ( 1968) determined lead concentrations in particulate material carried by 33 rivers in the U.S.S.R.; no data are given in their report for dissolved forms of lead. Concentrations of lead associated with sediment were as high as 152 ,ug/l of the original water sample, but most were below 40 lug/l. The analyses for lead in solution apparently all showed concentrations below detection, but it is not certain that the methods used give results that are comparable with those of other investigations cited here. LEAD IN PRECIPITATION AND RUNOFF WATERS A large amount of lead is used each year in the United States as a gasoline additive. This lead is mostly dispersed in the atmosphere, and has a readily measurable influence on the composition of rainfall—and, hence, of runoff waters—especially in the more thickly populated parts of the country. Lazrus, Lorange, and Lodge (1970) reported the average concentration of lead in rainfall to be 34 ,ug/l for 32 precipitation measuring stations throughout the United States during a 7-month period ending in March 1967. The concentrations of lead in rainfall in the north- eastern part of the country, however, are known to exceed this average substantially. In addition, many of the dilute, relatively low pH solutions that constitute the usual river and lake waters in this region have a high capacity for retaining lead in solution at equilibrium. Many such waters could attain lead concentrations amounting to several hundred micrograms per litre before reaching chemical saturation. The potential for higher concen- trations in the surface waters of industrialized and thickly settled regions does exist, although concentrations in this range have not yet become common enough to be brought to light by the rather thin distribution of sampling and analysis thus far accomplished. The amounts of lead used as gasoline additives each year in the United States are enough to give an average lead concentration near 150 )u g/l for all the runoff leaving the conterminous 48 States in the average year. This figure, of course, indicates only that lead is available in amounts that are of considerable potential hydrochemical significance. Just how much lead might be expected to accumulate in a particular stream at a given time depends on many other factors. The greatest contributions to runoff in most large streams come from areas where population is relatively sparse and lead is less abundant. A considerable fraction of the lead brought down in rain or snow can be expected to be inter- cepted and retained by soil and vegetation; however, the direct runoff from urbanized areas may contain sub- stantial lead concentrations at times, and further study will probably show that such occurrences are not rare. It is important that studies of lead in river water be continued, with special efforts to measure solubility parameters (pH and alkalinity) and to determine the particulate fraction of lead in runoff, especially in early stages of runoff events. LEAD IN GROUND WATERS The lead concentrations found in ground waters of many States and Puerto Rico are summarized in table 14. Again, spectrographic techniques were used for deter- mining the lead in most of these samples. Atomic- absorption techniques were used for a few samples. Of the 353 samples anlayzed, only two contained lead concen- trations exceeding the US. Public Health Service drinking water standards. Eighty percent of the samples contained less than 10 p.g/l of lead. The amounts of lead in ground waters used for public supplies in some of the larger cities of the United States were reported by Durfor and Becker (1964). Most of the samples represented treated water, and treatment may have influenced some of the concentrations. Results of 57 analyses, many representing mixed water from several wells, were given. Detectable concentrations of lead were present in 30 of these; the remainder had concentrations below detection. The lower limit of detection in these solutions was variable and rather high, generally above 2 LEAD CONTENT OF WATER 39 TABLE l4.—Maximum and minimum concentrations of lead found in ground waters of some States and Puerto Rico 1960 to 1971. [ND, sought but not found] Lead concentration Number of Percent samples equaling of samples Number of or exceeding containing ( pg/l) samples mandatory maximum less than __.—__ for drinking water‘ 10 ug/l Maximum Minimum Alabama ............ 2 0 100 ND ND Arizona... 9 0 56 40 ND California... 7 0 14 46 5.7 Colorado ............ 103 0 96 30 < .5 Connecticut ....... l 0 100 < 8 < 8 Florida ............... l4 0 78 40 ND Georgia 1 0 100 < 2.6 < 2.6 Hawaii 3 0 100 5.8 .6 Illinois. 2 O 100 7.5 ND Indiana... 3 0 67 21 ND Kansas ............... 3 O 267 4 ND Kentucky 5 0 100 4 < 2 Louisiana 4 0 100 4.5 ND Missouri ..... 43 0 95 10 < 2 Montana ............ 8 0 50 < 35 < 4 Nebraska ............ l 0 100 ND ND New Jersey ........ 8 0 75 < 22 1 New Mexico ...... l3 0 31 30 ND New York .......... 19 0 89 10 ND North Carolina. 1 0 100 < 3 < 3 Ohio .................. 26 O 54 30 < 3 Pennsylvania ..... 27 0 74 29 < 2 Tennessee .......... 5 0 100 3.2 < 1.7 Texas ...... l4 0 86 38 ND Utah .................. l l 0 62 ..... Virginia ...... l 0 100 < 9 < 9 Washington... 2 0 50 11 < 2.6 Wisconsin... 1 O 100 7.4 ..... Wyoming... 20 l 90 240 < 1 Puerto Rico 6 0 33 (3) (3) Total .......... 353 2 ........... Percentof total samples ....... 80 ........... ‘50 pg/l, as determined by U.S. Public Health Service (1963). 2Includes one value above 10 ug/l. 3All values are reported as "less than,” because of high dissolved solids. ,1. g/l. The highest value observed was 62 pg/l, but this was a definite anomaly; most concentrations observed were between 2 and 10 [Lg/l. Lead has a very low solubility in water that contains moderate concentrations of bicarbonate ions and has a pH near 8. Many ground waters display these properties, and thus are expected to be low in dissolved lead concentration. Silvey (1967) found detectable amounts of lead in 17 percent of the samples from springs and in 15 percent of the samples from wells and oil-field brines collected at various sites in California. The average concentration of lead for the water in these samples was 17 [Lg/l for the springs and 2.8 [Lg/l for the wells and oil-field brines. However, one of the spring samples contained 143 [Lg/l, which strongly influenced the reported average. The compilation of analyses of ground waters by White, Hem, and Waring (1963) contains a few values for lead concentrations, although the element was not generally determined. The highest reported concentration was about 1,400 [Lg/l in water from Wilbur Spring, Colusa County, Calif. Several other saline or thermal waters contained a few tens or hundreds of micrograms per litre, but most analyses for lead indicated concentrations below detection limits. In general, the ground waters that contained significant concentrations of lead were either high in chloride or low in pH, and had relatively high temperatures. More than 100 spring-water samples were collected east of the Continental Divide in Colorado (E. C. Mallory, Jr., written commun., 1971). The dissolved-solids content of these samples ranged from less than 50 mg/l to more than 28,000 mg/l. Lead concentrations were generally low; 68 percent of the samples contained 1 ug/l or less. Only four samples contained more than 10 ,ug/l, and the maximum concentration was 30 ,ug/l. Lead concentrations in Missouri ground waters were also found to be generally low (E. C. Mallory, Jr., written commun., 1971). Of the 43 samples analyzed, 42 contained less than 10 ,ug/l of lead; the remaining sample contained 10 lug/l. Kosolapova (1963) reported that the lead content of subsurface waters in the Olenek River basin of Russia ranged from 5 to 90 )1. g/l. Goleva, Polyakov, and Nechayeva (1970) reported 15 analyses of ground waters associated with ore deposits and 21 analyses of mineral and saline ground waters from various localities in the USSR. In the waters from ore deposits, the highest lead concentration found was 1,680 p. g/l, but all the rest were below 100 ,ug/l; the water containing the highest concentration was strongly acid, with a pH below 1.0. The mineral and saline waters generally contained from 1.9 to 11.4 ,ug/l of lead. Acid mine drainage samples in some areas contribute large quantities of iron, manganese, aluminum, and other elements to surface waters. A number of acid mine waters from Pennsylvania and Maryland and one from West Virginia have been analyzed by a U.S. Geological Survey laboratory in Denver, Colo. The concentrations of iron, aluminum, and manganese in these samples are high, showing maximum values of 190,000, 130,000, and 13,000 ,u g/l, respectively. On the other hand, the lead concen- trations were low; 86 percent of the samples contained less than 10 ug/l lead, and 80 percent contained less than 5 [1. g/l. LEAD IN THERMAL WATERS Several investigators have made chemical studies of the lead content of hot-springs waters. The lead content of the waters at Stubic, Yugoslavia, was reported by Miholié (1945) to be 0.003 mg/ kg (approximately 3 ug/l ). In seven hot springs of Shikabe, Hokkaido, Japan, Uzumasa and Akaiwa (1960) determined lead concentrations ranging from 0.09 to 0.36 mg/l. Noguchi and Nishiido (1969) deter- 40 mined lead in Tateyama-jigokudani hot springs in Toyama Prefecture, Japan. Eighteen samples were analyzed colorimetrically with dithizone, and the concen- trations of lead found ranged from 0.00 to 1.25 mg/l. Minami, Sato, and Watanuki (1957, 1958) analyzed hot- spring waters of Tamagawa, Akita Prefecture, Japan. In 10 springs along the main Tamagawa stream, the lead concentrations ranged from 0.98 to 1.8 mg/l. In 10 springs in the Yukawa River, the average lead content was 1.29 mg/l. Goleva, Polyakov, and Nechayeva (1970) gave analyses of four thermal waters from the USSR. One, from a fumarole at Ebeko volcano, had 34.7 ,ug/l lead. The other three (carbonate and nitrogen-carbonate brines), obtained from the thermal area of Cheleken, had from 1,210 to 4,000 ng/l. These waters were very saline, containing more than 200 g/l of dissolved solids, and some of the other waters of this area were reported to contain up to 6 mg/l of lead. LEAD IN SEAWATERS Tatsumoto and Patterson (1963) determined lead concentrations in seawaters off southern California using isotope dilution techniques. The lead content ranged from 0.08 to 0.4 ,ug/l and averaged 0.2 pg/ 1. In deep waters, the concentration did not vary much and averaged 0.03 [Lg/l. Skurnik-Sarig, Zidon, Zak, and Cohen (1970) were unable to detect lead in Atlantic Ocean waters. However, in the Mediterranean Sea, the lead content at Ashdod and Palmachim, Israel, was 340 and 170 ,ug/l, respectively. In Black Sea samples, Belyaev (1966) reported that the lead content was 3.6 ppb. This value represents an average of determinations made in the course of 5 years at a number of sites. Loveridge and others (1960) stated that lead in seawater is associated with the suspended solids that are removed by filters that retain particles 1 pm in diameter or larger. The values found for dissolved lead in their study ranged from 0.6 to 1.5 ug/l. REFERENCES CITED Belyaev, L. 1., 1966, Distribution and content of trace amounts of heavy metal elements in Black Sea waters [in Russian]: Akad. Nauk Ukrain. SSR, Inst. Morskogo Gidrofiz Trudy, v. 37, p. 199—213; abs. in Chem. Abs., v. 66, no. 118688w, 1967. Boswell, C. R., Brooks, R. R., and Wilson, A. F., 1967, Trace element content of the Antarctic Lakes: Nature, v. 213, p. 167—168; abs. in Chem. Abs., v. 66, no. 58736y, 1967. DeVilliers, P. R., 1962, The chemical composition of the water of the Orange River at Vioolsdrif, Cape Province: Rep. Suid-Afrika, Dept. Mynwese, Ann. Geol. Opname, v. 1, p. 197—208 [1963]; abs. in Chem. Abs., v. 62, no. 10220h, 1965. Durfor, C. N., and Becker, Edith, 1964, Public water supplies of the 100 largest cities in the United States, 1962: U.S. Geol. Survey Water-Supply Paper 1812, 364 p. Durum, W. H., Heidel, S. C., and Tison, L. G., 1960, World-wide runoff of dissolved solids: Comm. Surface Waters, Internat. Assoc. Sci. Hydrology General Assembly, Helsinki 1960, Internat. Assoc. Sci. Hydrology Pub. 51, p. 618-628. LEAD IN THE ENVIRONMENT Durum, W. H., Hem, J. D., and Heidel, S. G., 1971, Reconnaissance of selected minor elements in surface waters of the United States, October 1970: U.S. Geol. Survey Circ. 643, 49 p. Goleva, G. A., Polyakov, V. A., and Nechayeva, T. P., 1970, Distribu— tion and migration of lead in ground waters [in Russian]: Geok- himiya 1970, p. 344—357; translated in Geochemistry Internat., 1970, p. 256—268. Heidel, S. G., and Frenier, W. W., 1965, Chemical quality of water and trace elements in the Patuxent River basin: Maryland Geol. Survey Rept. luv. 1, 40 p. Kleinkopf, M. D., 1960, Spectrographic determination of trace elements in lake waters of northern Maine: Geol. Soc. America Bull., v. - 71, p. 1231—1242. Konovalov, G. S., Ivanova, A. A., Kolesnikov, T. Kh., 1968, Trace and rare elements dissolved in water and carried by suspended sediments of principal rivers of the U.S.S.R., in Geochemistry of sedimentary rocks and ores: Moscow, Izdate l’suo “Nauk", 435 p. Kosolapova, M. N., 1963, Microcomponents in natural waters of the Olenek River basin: Akad. Nauk SSSR Yakutskogo Filiala Sibirsk. Otd. Trudy, Ser. Geol. 1963, no. 16, p. 56—74; abs. in Chem. Abs., v. 59, no. 9673f, 1963. Krainov, S. R., and Korol’kova, M. Kh., 1964, Distribution of some trace elements in the mineral waters of the Lesser Caucasus: Vsesoyuz. Nauchno-Issled. Inst. Gidrogeol. i Inzh. Geol. Trudy [N.S.], no. 9, p. 72—93; abs. in Chem. Abs., v. 61, no. 10439e, 1964. Kroner, R. C., and Kopp, J. F., 1965, Trace elements in six water systems of the United States: Am. Water Works Assoc. Jour., v. 57, no. 2, p. 150—156. Lazrus, A. L., Lorange, Elizabeth, and Lodge, J. P., Jr., 1970, Lead and other metal ions in United States precipitation: Environ- mental Sci. and Technology, v. 44, p. 55—58. Livingstone, D. A., 1963, Chemical composition of rivers and lakes: U.S. Geol. Survey Prof. Paper 440—6, 64 p. Loveridge, B. A., Milner, G. W. C., Barnett, G. A., Thomas, A. M., and Henry, W. M., 1960, Determination of Cu, Cr, Pb, and Mn in sea water: Atomic Energy Research Estab. [GL Britain] [Rept.] R—3323, 36 p.; abs. in Chem. Abs., v. 54, no. 24118h, 1960. Lukashev, K. 1., Zhukhovtskaya, A. L., and Zamyatkina, A. A., 1965, Heavy metals in surface waters of the Poles’e territory near Pripyat in the Belorussian S.S.R. [in Russian]: Akad. Nauk Beloruss. SSR Doklady, v. 9, no. 3, p. 183—186; abs. in Chem. Abs., v. 63, no. l584e, 1965. Miholié, Stanko, 1945, The chemical analysis of the hot-spring water at Stubic: Rad. Hrvatske Akad. Znanosti i Umjetnosti. Razreda Mat-Prirodslov, v. 278, no. 86, p. 195—211; abs. in Chem. Abs., v. 40, no. 4158-6, 1946. Minami, Eiichi, Sato, Gen, and Watanuki, Kunihiko, 1957, Arsenic and lead contents of the hot springs of Tamagawa, Akita Prefecture—[PL] 1: Nippon Kagaku Zasshi, v. 78, p. 1096—1100; abs. in Chem. Abs., v. 52, no. 11323e, 1958. 1958, Arsenic and lead contents of the hot springs of Tamagawa, Akita Prefecture—[PL] 2: Nippon Kagaku Zasshi, v. 79, p. 860—865; abs. in Chem. Abs., v. 52, no. 18968i, 1958. Noguchi, Kimio, and Nishiido, Toshio, 1969, Behavior of copper, zinc, and lead in Tateyama-jigokudani hot springs in Toyama Prefec- ture [in Japanese]: Nippon Kagaku Zasshi, v. 90, no. 8, p. 781-786; abs. in Chem. Abs., v. 71, no. 105046p, 1969. Silvey, W. D., 1967, Occurrence of selected minor elements in the waters of California: U.S. (Geol. Survey Water-Supply Paper 1535—L, 25 p. Skurnik-Sarig, Sarah, Zidon, Moshe, Zak, 1., and Cohen, Yves, 1970, Lead determination in natural saline waters by UV spectrophoto- metry: Israel Jour. Chemistry, v. 8, no. 3, p. 545—549; abs. in Chem. Abs., v. 73, no. 101881a, 1970.. Tatsumoto, M., and Patterson, C. C., 1963, The concentration of common lead in sea water, in Geiss, J., and Goldberg, E. D., LEAD CONTENT OF WATER 41 compilers, Earth science and meteoritics: New York, Interscience, p. 74—89. Turekian, K. K., and Kleinkopf, M. D., 1956, Estimates of the average abundance of Cu, Mn, Pb, Ti, Ni, and Cr in surface waters of Maine: Geol. Soc. America Bull. 67, p. 1129—1131. U.S. Geological Survey, 1967a, Water resources data for Colorado, 1967—Pt. 2, Water quality records: Denver, Colo., U.S. Geol. Sur- vey, Water Resources Div., 101 p. 1967b, Water resources data for New York, 1967—Pt. 2, Water quality records: Albany, N.Y., U.S. Geol. Survey, Water Resources Div., 160 p. 1967c, Water resources data for South Dakota, 1967—Pt. 2, Water quality records: Huron, S. Dak., U.S. Geol. Survey, Water Resources Div., 165 p. 1968a, Water resources data for Arkansas, 1968—Pt. 2, Water quality records: Little Rock, Ark, U.S. Geol. Survey, Water Re- sources Div., 133 p. 1968b, Water resources data for Connecticut, 1968—Pt. 2,Water quality records: Hartford, Conn., U.S. Geol. Survey, Water Re- sources Div., p. 129—247. 1968c, Water resources data for Missouri, 1968—Pt. 2, Water quality records: Rolla, Mo., U.S. Geol. Survey, Water Resources Div., p. 189—299. 1969, Water resources data for Montana, 1969—Pt. 2, Water quality records: Helena, Mont., U.S. Geol. Survey, Water Re- sources Div., 234 p. U.S. Public Health Service, 1963, Public Health Service drinking water standards [rev. 1962]: U.S. Public Health Service Pub. 956, 61 p. Uzumasa, Yasumitsu, and Akaiwa, Hideo, 1960, Minor constituents of the springs of Shikabe, [PL] 58, of Chemical investigations of hot springs in Japan: Nippon Kagaku Zasshi, v. 81, p. 912—915; abs. in Chem. Abs., v. 55, no. 3883b, 1961. White, D. E., Hem, J. D., and Waring, G. A., 1963, Chemical composi- tion of subsurface waters: U.S. Geol. Survey Prof. Paper 440—F, 67 p. Zhukhovitskaya, A. L., Zamyatkina, A. A., and Lukashev, K. I., 1966, Trace elements in the Upper Dnieper waters [in Russian]: Akad. Nauk Beloruss. SSR Doklady, v. 10, no. 11, p. 891—893; abs. in Chem. Abs., v. 66, no. 49131v, 1967. LEAD IN SOILS By R. R. TIDBALL INTRODUCTION Investigation of the lead content of agricultural soils was first stimulated by the known toxic effects of lead on plants, animals, and man; more recently, the develop- ment of geochemical prospecting techniques has resulted in analysis of nonagricultural soils for anomalous lead concentrations related to mineralization. Lead is among the trace elements listed as harmful to various plants and animals (McMurtrey and Robinson, 1938, p. 808; Swaine, 1955, p. vi), and the toxicity of lead is of continuing concern in human health (US. Public Health Service, 1966; Kehoe, 1971). Early interest in the concentration of lead in soil came as a result of pollution by effluent from smelters (Holmes and others, 1915) and pesticide sprays in orchards (Jones and Hatch, 1937, 1945). Geochemical prospecting activities, which greatly expanded in the 1930’s (Hawkes, 1957, p. 314), stimulated interest in back- ground concentrations of metals in soils as an aid to identifying mineralized zones. More recent interest stems from the contamination of soils along highways by auto- motive exhaust (Cannon and Bowles, 1962; Singer and Hanson, 1969; Ault and others, 1970; Dedolph and others, 1970; Motto and others, 1970; Schuck and Locke, 1970; Connor and others, 1971). The natural concentration of any trace element in soil can be viewed in at least two ways. First, the concen- trations of such elements may be expressed as total amounts, Which include all modes of occurrence ranging from water-soluble salts to relatively immobile forms locked within the crystal lattice of primary minerals. Second, the concentrations may be expressed as extractable amounts that are soluble in a specified solvent. Such extractable quantities will vary depending on the solvent used and the strength of that solvent. Extractable quantities are of principal interest to the agricultural worker (Brewer, 1966, p. 216) because of efforts to equate extractable with “available” quantities for given plant species. The objective in this report is to examine the soil as a natural reservoir of lead. Therefore, total concentrations are given here because they indicate the potential ability of the soil to supply lead (Mitchell, 1964, p. 331). In contrast, the actual availability, as estimated by extractable amounts, depends on the interaction of many locally variable factors. Average values are commonly expressed by either the arithmetic mean or the geometric mean. The familiar arithmetic mean is the sum of the values divided by the number of values. However, frequency distributions of trace-element concentrations in soils tend to be more nearly symmetrical on a logarithmic scale; therefore, a logarithmic transformation is appropriate. The best estimate of the most typical value in a log-normal distri- bution is given by the geometric mean, which is the anti- logarithm of the arithmetic mean of the log values (Miesch, 1967, p. B1—B2). The arithmetic mean will always be greater than the geometric mean. Many authors fail to identify which kind of mean they have used; therefore, the arithmetic mean is assumed to have been used in the literature cited in this report. CHEMISTRY OF LEAD IN SOILS The migration and ultimate distribution of lead within the soil result from combinations of factors that include chemical processes such as oxidation and reduction reactions, adsorption of cations on the exchange complex, chelation by organic matter and by other metal oxides, and cycling by vegetation. Most of these processes are in turn influenced by the regional factors involved in soil formation—climate, biota, topography, and, especially, parent material, all operating through time (Jenny, 1941). Entrapment is one form in which lead occurs in the soil. A study of the soils in Bulgaria, for instance, showed that most of the lead occurred as inclusions in iron and aluminum hydroxides and in calcium carbonate (Iordanov and Pavlova, 1963). A’small amount of lead was also present as pyromorphite (Pb5Cl(PO4)3). Lead tends to associate with the soil minerals in other ways as well. Lead is presumed to be adsorbed readily onto the exchange complex of clay minerals and is replaced only with difficulty (Mitchell, 1964, p. 337). Divalent lead should be adsorbed more strongly than monovalent potassium which has a similar ionic size (Goldschmidt, 1954, p. 402). The enrichment of lead in the B horizons of 43 44 LEAD IN THE ENVIRONMENT some soils has been attributed in part to the presence of excess clay (Presant, 1971, p. 57). Lead also associates with amorphous iron sesquioxide and to a lesser extent with aluminum sesquioxide (Mitchell, 1964, p. 337). Although he did not specifically study lead, Jenne (1968) found that hydrous manganese and iron oxides play a predominant role in controlling the fixation of several heavy metals in the transition series. The sesquioxides and hydrous oxides may occur as surface coatings on particles of all sizes and, therefore, can exert an influence that is out of proportion to their concentration. Lead was found concentrated in that portion of the soil that was soluble in acidified hydrogen peroxide (Taylor and McKenzie, 1966). The soluble portion includes manganese minerals, organic matter, and other soluble minerals and salts. Presant (1971) found lead associated with free iron oxides in the B horizons of New Brunswick soils, but the correlation coefficient was not significant. In general, most metals tend to be more available under acid conditions than under alkaline conditions (Hodgson, 1963, p. 141—144). In one study, plants grown in soils to which lead had been added took up more lead from acid soils than alkaline soils, but the effect was confounded somewhat by differing amounts of organic matter (MacLean and others, 1969). A summary of the composi- tion of soils from Wisconsin, shown in table 15, suggests that greater total amounts of lead are found in the more alkaline soils; the pH-dependent stability of metal organic chelates may aid in explaining this distribution. Presant (1971) found negative correlations between total lead and pH in all soil horizons except the A2 horizon. The effect of pH may be the indirect consequence of microbiological activity which in turn controls the oxidation and reduc- tion of iron and manganese (Hodgson, 1963, p. 146). Studies of the distribution of the heavy metals within the soil profile based on pH alone may fail to give consistent results. Instead, the sorption-desorption exchange process should be viewed in terms of both the Eh and pH of the soil-water system, (Jenne, 1968, p. 342). The movement of metals during the weathering process by means of organic chelating agents is another important TABLE 15.—Lead concentrations in residual soils from Wisconsin, grouped according to [21-] [Data from computer storage of the U.S. Geological Survey; samples collected by H. T. Shackletle; spectrographic analysis by J. C. Hamilton; lower limit of detection 10 ppm. Means and deviations calculated according to methods of Miesch (1967). Detection ratio, the number of samples containing detectable concentrations versus the number of samples examined] Lead concentration Soil H Detection Geometric mean Geometric 1) ratio (PPm) deviation Less than 6.5 105:133 17 1.84 6.5-7.5 1551168 21 2.21 Greater than 7.5 52:57 31 2.73 process in the chemistry of lead in soil. The following dis- cussion is based on a review of these processes by Pauli (1966). The creation of organic chelating agents by biologic activity serves as one of the most effective processes of metal mobilization. These agents are either plant products, microbial metabolites, or humic compounds. The humic compounds, also known as humic acids (Drozdova, 1968), are three-dimensional, interlinked, aromatic polymers that result from the reaction of heteropolycondensation. This reaction splits simpler compounds and reconstitutes them into a more complex heteropolycondensate (polymer), some units of which are possibly linked together (chelated) by heavy metal cations. As the molecular weight of the polymer is build up through additional polycondensation, a complex lattice structure with diverse interconnections is developed. Thus cations, molecules, and even mineral particles may be enmeshed within a protective “cage.” Clay minerals, especially those encrusted with iron oxides and hydroxides, adsorb the polymers, and even accommodate the smaller molecules in interlayer positions. Metals initially become chelated according to their sus- ceptibility, competition from other metals present, and the nature of the complexing agent. The continued fixation of the metal ion depends on the stability of the polymer and its ability to resist the forces of degradation. The metal binding force generally diminishes with increasing acidity, as we have seen. Experiments with calcium- humate and kaolin showed that lead was held by both sub- stances at pH 5 but was released completely at pH 1.5 (Iordanov and Pavlova, 1963). Further, the complex polymers are more resistant to microbial attack than the simpler organic compounds. Cate (1959) suggests that the metal ions might be released from the polymer by an auto- catalytic decomposition process. He speculates that in the podzolization process, lead is cycled to the ground surface by vegetation, is complexed by the humic polymers, is translocated down in the profile, and finally, is released for subsequent precipitation at the lower depth. Cate’s model may approximate reality but it apparently fails to account for the lack of abundant movement of lead in weathering profiles. _ Finally, the amounts and types of organic matter present appear to provide an important control on the movement of heavy-metal ions in soil systems. Evidence that suggests the importance of organic matter is summarized (Jenne, 1968, p. 340) into four categories: (1) known chelating ability of both synthetic and natural organics, (2) ability of plant material extracts to leach metals from soil material, (3) positive correlation found between concentrations of metal and organic matter of the soil, and (4) release of metals during oxidation treatment of organic matter by hydrogen peroxide. The association of lead and organic matter is not always empirically LEAD IN SOILS consistent however. For example, the results in table 20 for tropical soils show no apparent lead-organic matter relationship. INFLUENCE OF PARENT MATERIAL Parent material has been recognized in some cases as a fundamental control on the concentration of lead in the soil (Swaine and Mitchell, 1960; Hodgson, 1963; Mitchell, 1964, p. 321; Fleming and others, 1968) and, among the various regional factors in soil formation, it thus deserves special attention. The lead concentration of immature soils tends to be correlated directly with its concentration in the parent material. Thus most of the averages reported for lead in soils are similar to the average concentration of 16 ppm (Goldschmidt, 1954, p. 398) of lead in the Earth’s upper lithosphere. As weathering progresses to a more advanced stage, other pedogenic factors may modify the distribution of lead within the soil profile (Wells, 1960; Tiller, 1963). DISTRIBUTION OF LEAD IN SOILS An extensive review of the literature on the lead concen- tration in soils was given by Swaine (1955, p. 83—87). In most soils from nonmineralized areas, concentrations range from 2 to 200 ppm. Average concentrations in these soils are generally between 5 and 25 ppm. Vinogradov (1959, p. 155—157) reported an average of 10 ppm lead in Russian zonal soils (see table 16); these results compared well with data from the literature, which were generally in the range of 10—50 ppm. Other selected summary statistics i, 45 from the literature and unpublished data of the U.S. Geological Survey are shown in table 16. Typical concentrations of lead in topsoils range from 10 to 30 ppm. The concentration of lead in selected Scottish soils developed in a cool humid climate is shown in table 17 (Swaine and Mitchell, 1960). The higher concentrations of lead appear to be assOciated with granitic parent material. Unpublished data from files of the U.S. Geological Survey, which are summarized in'table 18, indicate no important differences between the concentration of lead in the parent material and that in the soil. Other data given in tables 19, 20, and 21 illustrate a range of lead concen- trations in soils developed from different parent materials. The majority of values are less than 50 ppm and often less than 30 ppm. In general, the results are inconclusive in establishing soil/ parent material relationships, because the natural variations in parent-material composition probably include the range of lead values observed among the soils. Further investigation in particular localities of interest to determine the magnitude of variation in both the soils and the parent material is necessary before meaningful relationships can be recognized. The trace-element relationship between soil and parent material, if it exists, should permit prediction of the composition of the one from knowledge of the other. Oertel (1961 ) concluded on the basis of soils from Australia and Tasmania that such a prediction could not be satis- factorily reached; that is, a linear equation failed to describe the relationships accurately. However, a similar study of selected Belgian soils by Prabhakaran Nair and TABLE l6.—Lead concentrations in selected soils developed under a variety of soil-forming factors [Leaders (...) indicate no data] Same descri tion Number of Am— Mean (ppm)‘ Deviation Source of data p samples Minimum Maximum Finland, soils from moraine, sand, silt, clay, and peat 16 ...... Vuorinen (1958). Earta an}? Mariisk regions, U.S.S.R., soils 4 30 .. Borisova (1959). astern uropean p ain, soils, A horizon l3 <10 43 210 211:18 V' d l . Kagakhstan region, U.S.S.R., desert soils, mogra 0v < 959) umus horizon 20 ll ...... D b l’ k' Kazakhstan region, U.S.S.R., salinized soils, 0 rovo 5 1y (1960). humus horizon 6 10 ....... Do. Westfalen-Lippe,_ Federal Republic of Germany, . agricultural $0115 15 68 30 ....... Balks (1961). Westfalen-Lippe, Federal Republic of Germany, grassland soils 12 79 34 ....... Do. Dahomey, (Africa), tropical topsoils 5 3 23 ....... Pinta and Ollat (1961). People s'Republic of China, sells 11] 26 ....... Fang, Sung, and Bing (1963). Nov051birsk region, U.S.S.R., $0115 43 10 30 ....... Viller and Khrapov (1963). U.S.S.R., gray forest 5011s ' 25 10 45 ....... Akhtyrtsev (1965). New Illruns‘Wick, Canada, podzol sorls, A horizon 53 - l3 ....... Presant and Tupper (1965). Kox’vinskii (Kamen region, U.S.S.R., alpine .soils, humus horizons 100 ....... Mikhailova and Mikhailov (1967). Silesia, Poland, topsorls 94 328 ....... Roszyk (1968). Missouri, U.S.A., subsoils under cedar trees, wgsfcfoll’glli U S A 5 i1 ($21.0. 1000 :3 3 ....... Connor and others (1971). , . . ., o s < > 22.60 H.T.Sh kl tt . , . Kentucky, U.S.A., red-yellow podzolic soils, ' ac e e (unpub data 1972) A horizons 96 < 7 96 514 3£1.51 Connor and others (1976). ‘Arithmetic mean assumed unless otherwise noted. {Arithmetic mean and standard deviation estimated by methods described by Miesch (1967). 5Geometric mean and geometric deviation estimated by methods described by Miesch (l967). ——— 45 LEAD IN THE ENVIRONMENT Cottenie (1971) reached just the opposite conclusion. The TABLE 19 —Lead concentrations in tropical soils developed on different accuracy of prediction improved by grouping the soils [Went materials . . . D l N l ' P' . ' ' Wlthln taxonomic Classes. [ ata mm a ovtc and inta (1969) Leaders ( ) indicate no data] Parent material Soil type Pb (ppm) TABLE 17.—Lead concentrations (in ppm) in selected Scottish soils Igneous: developed from different parent materials Basic rocks F erruginous 88.0 [Data from Swaine and Mitchell (1960). Soil profile averages calculated by weighting 2:22:56 Ferrallltic :33 each horizon according to thickness] Acid rocks 14:0 Volcanic ash 9.0 Parent material Soil Pb Metamorphic: Igneous: Marble 19.0 Serpentine till 12 0116.155 . 400 Olivine gabbro 13 SChlSl Ferrugmous 22.0 Andesitic moraine 18 , Granitic till 25 Sedlmemary: . . Limestone Ferrallltic 41.0 Metamorphic: Ferruginous 21.0 Granitic gneiss 42 Sandstone do ...................... 20.0 Quartz-mica schist till 54 Slate 18 Sedimentary: TABLE 20.—Distribution of lead concentration and organic matter in Sandstone till 12 sotl profiles from the tropical climatic zone Parent material Soil depth Organic matter Soil Pb and soil type (cm) (percent) (ppm) TABLE l8.—Mean concentrations of lead in selected parent material types and derivative soils Dahomey. Africa (Pinla and Ollat, 1961) [Unpublished data of the U.S. Geological Survey. Statistics calculated according to Miesch Clay 0_10 3.0 23 (1967). Leaders (...) indicate insufficient data for calculation. Detection ratio, the number (ferrallitic soil) 40—60 .8 20 of samples containing detectable concentrations versus the number of samples examined] 180—190 .3 27 Range (ppm) . , Unknown 0—20 1.0 9 Sample material Detection Geomemc Geometric (ferruginous soil) 40-60 .6 1 1 ratio Minimum Maximum mean (ppm) deviation 80—100 .6 14 Cambrian and Lower Ordovician rock, weslem mton, western United States' Tertiary §efdim§nts 0‘15 2-3 l5 (ferralllttc 5011) 20—30 .6 20 Sandstone: 80—90 ‘4 22 Soil 33:80 < 20 130 16 12.23 375—385 2 20 Parentmaterial 3:80 < 20 36 <20 ...... Madagascar (Nalovic and Pinu, 1969) Shale: _ 5011 _ 28:72 <20 870 12 753.45 Marble (red-brown 0—18 8.2 21 Parent materlal 28:72 < 20 1,300 12 ' $3.33 (ferrallitic soil) 18—40 4.3 17 40—85 .2 23 Carbonate: 85—130 .2 18 Soil 25:76 <20 350 11 $4.07 130—280 .1 14 Parent material 12: 76 < 20 120 < 20 ...... Calcareous material 0—15 1.1 24 Nonmineraliud areas of Kentucky, Missouri, and Wisconsin2 “Bungmous 5011) 15‘200 “9’ 20 200—240 .3 24 Sandstone, siltstone, > 240 ~1 18 and quartzite: _ Soil 38:52 <10 30 11 $1.62 Alluvwm 48-11% 1g 13 P re t ter‘al 6:11 10 50 9 12.45 _ ‘ a n ma 1 < ~ 110—320 .2 22 Dolomite: 320—350 .2 1? Soil 37:33 <10 70 15 z 1.90 >350 2 25 Parent material 11:16 <10 70 13 $2.33 Limestone: x GEOGRAPHIC DISTRIBUTION OF LEAD IN SOILS Soil 27:27 15 50 23 +1.43 Parent material 5:8 < 10 20 9 $1.56 OF THE UNITED STATES - - The distribution of lead concentration in soils and other Gramte, rhyollte: . . . . . Soil . 12:12 10 70 18 32.25 surf1c1al materlals at 964 Sites throughout the United Parentmateml 6:6 15 50 25 “‘75 States is shown in figure 8. The symbols on the map 1Collected by A. T. Miesch and J. J. Connor; analyzed on direct reader spectrograph ICPYCSCIH concentrations Within selected frequency Classes by R. G. Havens; lower limit of detection, 20 ppm. Soil samples from surface horizon. - - - - - 2Collected by H. T. Shacklette; semiquantitative spectrographic analysis by J. C. Hamilton; as Shown 1n the accompanYIng hlstogram. Th6 dlstrl- lower limit of detection, 10 ppm. Soil values based on averages of samples from all horizons. bution map is modified from Shaklette, Hamilton, LEAD IN SOILS 47 TABLE 2l.—Distribution of lead concentration in soil profiles from the temperate climatic zone Parent material Soil depth or horizon Soil Pb (ppm) Wales (Archer, 1963) Depth (cm) Dolerite 0—5 150 5—1 8 100 18—46 100 Pumice tuff 0—5 80 5—15 20 15—23 8 23-30 25 Rhyolite 0—10 180 10—25 60 Mixed glacial drift 0—5 200 5—41 100 Ireland (Fleming and others, 1968) Horizon Granite Ap 25 (Brown podzolic soil) 31 20 32 20 C 25 Shale A1 25 (Brown podzolic soil) B2 10 C 15 Limestone A1 45 (Gray-brown podzolic soil) A2 60 132: 50 C 50 Depth (cm) Peat over granite 0—8 12 8-20 30 20—41 3 41—46 11 Bedrock 20 New Brunswick, Canada (Presant and Tuppcr, 1965)‘ Horizon Mixed parent materials L-H 73 (nonmineralized terrain) Ae 13 B, 36 82 28 C 65 Wisconsin (unpub. data, U.S. Geological Survey2 Horizon Mixed parent materials A 24 (nonmineralized terrain) B 14 C 22 ‘Average (arithmetic?) lead in each horizon computed from 53 podzol profiles. 2Samples collected by H. T. Shacklette; analyses by J. C. Hamilton. Geometric means are estimated from a number of soil samples, as follows: A, 146 samples (no lead detected in 11 samples); B, 70 samples (no lead detected in 17 samples); C, 97 samples (no lead detected in 20 samples). Boerngen, and Bowles (1971) with 101 additional sample values added. The sampling sites were located at approxi- mately 50-mile intervals, and samples were collected either along highways or within various geologic study areas. There is some risk that the distribution shown has been modified by lead fallout along the highways. However, collectors were asked to select quiet rural roads if possible, to avoid roadcuts or fills, to move away from the roadside, and to sample at a depth of about 8 inches (20 cm). Studies in the vicinity of highways show that the amount of lead decreases very sharply within the first 6 inches (15 cm) depth of soil (Lagerwerff and Specht, 1970; Motto and others, 1970). Thus, the risk of modification by fallout is believed to be minimal. The reader should avoid inferences about any single value, but rather examine only patterns of larger regions. The largest regional patterns examined by Shacklette and others (1971) were “eastern” soils versus “western” soils. This division between east and west was established along the 97th meridian because that was the approxi- mate boundary between Marbut’s (1935) pedocals of the west and pedalfers of the east.1 It also corresponds approxi- mately to the division between moist soils of the east and dry soils of the west (U.S. Geological Survey, 1970, p. 86). The mean lead concentration in eastern soil samples is 14 , ppm as compared with 18 ppm for western soil samples. Although the difference is small, it appears to reflect a real difference in the background lead content of soils of the two regions (statistically significant at the 95 percent confidence level). The histogram in figure 8 shows that about 94 percent of the samples have lead concentrations that are equal to or less than 30 ppm. About 58 percent of the samples have concentrations equal to or less than the mean of 16 ppm. Thus, most of the sample sites across the country are typified by concentrations of 15—30 ppm, with few distinc- tive local geographic variations. Regions having many sites with lead concentrations below average include the Atlantic Coastal Plain and Gulf Coastal Plain from North Carolina to Texas, the High Plains of west Texas and southeastern New Mexico, the Lake States of Michigan, Wisconsin, and Minnesota, and parts of the northern Great Plains, particularly the Sand Hills of Nebraska. There were even fewer regional instances of above-average concentrations, although most of the sites in Colorado exhibit above-average lead concentrations: four samples with concentrations of 100 ppm were collected in the western half of Colorado. Otherwise, high concentrations in the range of 100—700 ppm are found only atwidely scat- tered and isolated sites in the United States. The distribution of lead in soil has been studied in detail in Missouri, which has large lead deposits and a well- established lead mining industry. Samples of agricultural soils were collected extensively throughout the State from the surface horizon (0—6 in. (0—15 cm) depth), through the lPedocal and pedalfer are Marbut’s terms for mature soils with and without a lime horizon, respectively. 48 128° 126° 124° LEAD IN THE ENVIRONMENT 0 122° 120° 118° 116° 114° 112° 110° 108° 106° 104° 102° 100° 98° 96° 48 e I / I I I I I I I I I I I I i l 46°\ [\77\‘~\\ \“\~‘ ‘— _-§_______I}__-Jl' 000 I000 0 o o o \ 44°\ % g 0 0‘0 O 0 0 8 9 oh 6 0 Go 'I g g 0 ° OI‘e O O G “———__ 0. 42°\ mr£77\~~‘a-_ 3r ‘0; J (3 “ l OI) 099000IOO figl o I 0 $ 0 Ole 4O \ 7‘ Q I O 0 0 0 ’_ G Y—‘ '3 _§————_ I \ O 0 0 0 G. va( 9 $7 9 O O 0 o o o o ‘7\_g_ O 0* G O G R 38 \ .! \“?*a~ o “to I 0 \ .. __§———————— O Cl C a o, .. .0..o. lo 0....fi 36 K e O 0’ . o O . o 0 o o I o 0 o O . . I O C 2‘ l O o O O 0? 73'+~\Q_0 . O o . O . 34° 9° 9 “ —— 0I. o 0 O 0 e o . O 0 0° 06 0 .0 _—h—_I O O O 0 0 O 1 g $ 0 O O O I e e o '. o o , ° 0 O 0 o o o o o '0 o i“ 9 o o. o J 00 0 j ‘3”- l A uh OI Q 2 O O 6‘3 “/70 30°\ 7. Q C . O O O C O 0 00 O 8 .I _‘ O O O . o 0 no 2%?wa e \o 0 o 6,0 O o 0 o \ 0 O O 0 28°\ \ O C l (30 O G O \ “- o ‘\ 9r \\3 0 0 ° 0 < '- \ o O (3 l 26 2 \ 0 \‘ 0 0 United States Number Number of Range Estimated Geometric Geometric . samples of samples Minimum Maximum arithmetic mean deviation 1 24° \ samples <10 ppm mean \\3 East of 97° long ---- 428 92 <10 300 17 14 1.9 '9‘ West of 97° long - - - -536 54 <10 700 22 18 1.9 Total ---------- 964 144 <10 700 20 16 1.9 22°\ I l I l I I I I I l l I 118° 116° 114° 112° 110° 108° 106° 104° 102° 100° 98° 96" FIGURE 8.—Distribution of lead in soils and 0th Map adapted from Shacklette, Hamilton, B Neiman. er surficial materials at 964 sites throughout the United States. Symbols correspond oemgen, and Bowles (1971), supplemented with an additional 101 sites, data courtesy LEAD IN SOILS 49 48° /42° ,22° {1 __ _— 1" o . 00 0 ° 0 o 7-"; GT' 01— .0 o .\ O O O O O SYMBOLANDPERCENTAGE /~ P 0 I ‘0 o 90 \‘0 OF TOTALSAMPLES I I x \ O “f“ o\ 0 sl‘q 0 1—3 0 o 9 +0 0 O 0 X0 3 0 ~ I, 0 "' L I 5 . / O -t > O l— ‘ b 9 O b g o O cg o l O O J 9 O O O O E 0 o o o 0.0 \\ I o o o 8 c ______ I “"7 ”1% ‘ ‘ I e I O O 0 " Y ’ O \ LL ’ O O 0’ O . ~ I ‘0' (9 O O o ,; - 4/ e k l.0 O \\ 100 - o - o o , l. O / I / O 1 OO ‘ O O o 100 200 300 400 500 MILES ‘ I oomoooogsg 88 l I I I I 4 °. FF'NmmV‘FFflgmh F I I I I I 55,4 v o 100 200 300 400 500 KlLOMETRES -: -'° LEADI'N PARTS PER M'LL'ON I l l l l l \ \ \ \ \ \ 96° 94° 92° 90° 88“ 86° 84" 82° 80° 78° 76° 74“ to classes of lead concentration as shown on histogram. Location of the symbol is approximately at the point of sample collection. of H. T. Shacklette. Semiquantitative spectrographic analysis in US. Geological Survey laboratories by J. C. Hamilton and H.G. 50 cooperation of Missouri farmers and the Extension Division of the University of Missouri. Ten samples were collected in each of the 114 counties of the State. The samples were analyzed semiquantitatively for 45.elements by emission spectrograph (Myers and others, 1961). The lower limit of determination for lead by this method is 10 ppm. The distribution of lead revealed by the analyses is generalized in figure 9, and is adapted from a more detailed distribution map by Tidball (1972, p. 39). The inclusion of a few high lead values from the lead mining districts results in an estimate of the mean of Missouri soils of 20 EXPLANATION LEAD, IN PARTS PER MILLION 70—7,000 Lead-belt district 0 Southern limit of glactatlon Northern limit of Mississippi Alluvial Plain O 50 100 150 MILES 100 150 KILOMETRES 500 1,140 samples Arithmetic mean (estimated) = 22 Geometric mean = 20 250 Geometric deviation = 1.6 FR EOUENCY omooo o oo o Frwmmgomoooog 8 Pt—N Nom 0 Pt— I\ LEAD, IN PARTS PER MILLION FIGURE 9.—Histogram showing lead concentrations in Missouri soil samples and map showing general distribution throughout the State. Physical divisions from Fenneman (1946); mining district from Kiilsgaard, Hayes, and Hey] (1967); statistics calculated according to Miesch (1967). LEAD IN THE ENVIRONMENT ppm as compared to the mean for eastern United States soils of 14 ppm (fig. 8). The highest lead concentrations were found in soils in the southeast Missouri lead belt. These values may represent either natural occurrences of lead in the soil or aritifical occurrences resulting from mining activity. Some highly anomalous samples, ranging in concen- tration from 700 to 7,000 ppm and collected from agri- cultural fields on the flood plain of Big River, are believed to reflect the mineralized zone. Upstream from the sample sites Big River flows through the center of the mining district that has produced more than 90 percent of the lead ores mined in southeast Missouri since the deposits were discovered in the area in 1701 (Kiilsgaard and others, 1967, p. 50). Other samples with lead concentrations ranging from 50 to 150 ppm were obtained from uplands within the lead-belt areas and may reflect conditions less influenced by mining activity. The Big River flood-plain samples also contain anomalous concentrations of barium which are believed to originate from important near-surface barite deposits that are located just northwest of the lead belt. Numerous small barite deposits also occur along the northern flank of the Ozark Plateaus.These deposits have accessory galena (lead sulfide) which may explain the band of above- average lead values occurring in the soils of this area. Other mining districts in southwest Missouri have not been clearly delineated by the soil samples. Below-average concentrations of lead were found in two large areas of the State, as shown in figure 9. One of the areas in northwest Missouri is associated with prominent surfical deposits of glacial material that at one time probably covered the entire landscape, but subsequent erosion has exposed the underlying rock in many places. From 67 to 100 percent of the land is covered with loess in deposits ranging in depth from 4 feet ( 1 m) to more than 32 feet (10 m) (National Research Council, 1952). The average (geometric mean) concentration of lead in loess in the bluffs along the Missouri River in this area in north- west Missouri was estimated to be 15 ppm (Connor and Ebens, 1972, p. 12; R. J. Ebens, oral commun., 1971). The second area of low-lead soils is in southeast Missouri and comprises the Mississippi Alluvial Plain and the central part of the Ozark Plateaus. Soils on the alluvial plain area are developed on alluvial materials of both Tertiary and Quaternary ages. Soils of the Ozark Plateaus area are on a very old landscape being developed from highly weathered residuum of carbonate rocks of Cambrian and Ordovician ages. These soils tend to have below-average concentrations of numerous trace elements in additon to lead. The remainder of the soils sampled throughout a wide part of the State are characterized by near-average concen- trations of lead that range from about 15 to 30 ppm. Most LEAD IN SOILS 51 of this area exhibits considerable local variation within this midrange of lead concentration. REFERENCES CITED Akhtyrtsev, B. P., 1965, Content of trace elements in gray forest soils of the central chernozem belt [in Russian]: Agrokhimiya, 1965, no. 9, p. 72—80; abs. in Chem. Abs., v. 64, no. 3242c, 1966. Archer, F. C., 1963, Trace elements in some Welsh upland soils: Jour. Soil Sci., v. 14, no. 1, p. 144—148. Ault, W. U., Senechal, R. G., and Erlebach, W. E., 1970, Isotopic composition as a natural tracer of lead in the environment: Environmental Sci. and Technology, v. 4, no. 4, p. 305—313. Balks, R., 1961, Lead content of soil [in German]: Kali-Briefe, Fachgeb 1, 1961, no. 11, p. 1—7; abs. in Chem. Abs., v. 57, no. l295e, 1962. Borisova, E. N., 1959, Natural lead concentration in the soil and in food products [in Russian]: Kazanskii meditsinskii Zhurnal, 1959, no. 4, p. 88-90; abs. in Chem. Abs., v. 55, no. 9752a, 1961. Brewer, R. F., 1966, Lead, in Chapman, H. D., ed., Diagnostic criteria for plants and soils: Riverside, California Univ., Div. Agr. Sci., p. 213—216. Cannon, H. L, and Bowles, J. M., 1962, Contamination of vegetation by tetraethyl lead: Science, v. 137, p. 765—766. Cate, R. B., Jr., 1959, Organic translocation of metals: Southeastern Geology, v. 1, no. 3, p. 83—93. Connor, J. J., and Ebens, R. J., 1972, Geochemical survey of geologic units, in US. Geological Survey, Geochemical Survey of Missouri—Plans and progress for third six-month period (July- December, 1970): US. Geol. Survey open-file report, p. 5—15. Connor, J. J., Shacklette, H. T., Ebens, R. J., Erdman, J. A., Miesch, A. T., Tidball, R. R., and Tourtelot, H. A., 1976, Background geochemistry of some rocks, soils, plants, and vegetables in the conterminous United States: US. Geol. Survey Prof. Paper 574—G. Connor, J. J., Shacklette, H. T., and Erdman, J. A., 1971, Extra- ordinary trace-element accumulations in roadside cedars near Centerville, Missouri, in Geological Survey research 1971: US. Geol. Survey Prof. Paper 750—13, p. Bl51—B156. Dedolph, Richard, Ter Haar, Gary, Holtzman, Richard, and Lucas, Henry, Jr., 1970, Sources of lead in perennial ryegrass and radishes: Environmental Sci. and Technology, v. 4, no. 3, p. 217-223. Dobrovol’skiy, V. V., 1960, Microelements in several soils and soil- forming parent materials of Kazakhstan: Soviet Soil Sci. 1960, no. 2, p. 120-127. Drozdova, T. V., 1968, Role of humic acids in concentrating rare ele- ments in soils: Soviet Soil Sci., 1968, no. 10, p. 1393—1396. Fang, C. L., Sung, T. C., and Bing, Yeh, 1963, Trace elements in the soils of northeastern China and eastern Inner Mongolia [in Chinese]: Acta Pedologica Sinica, 1963, v. 11, p. 130—142; abs. in Chem. Abs., v. 60, no. 9048h, 1964. Fenneman, N. M., 1946, Physical divisions of the United States [map], with Characteristics [of the sections], by Fenneman, N. M., and Johnson, D. W.: U.S. Geol. Survey, scale 1:7,000,000. Fleming, G. A., Walsh, T., and Ryan, P., 1968, Some factors in- fluencing the content and profile distribution of trace elements in Irish soils: Internat Cong. Soil Sci., 9th, Adelaide, Australia, Trans., v. 2, p. 341—350. Goldschmidt, V. M., 1954, Geochemistry: Oxford, Clarendon Press, 730 p. Hawkes, H. E., 1957, Principles of geochemical prospecting: U.S. Geol. Survey Bull. 1000—F, p. 225—355. Hodgson, J. F., 1963, Chemistry of the micronutn'ent elements in soils, in Norman, A. G., ed., Advances in agronomy, v. 15: New York, Academic Press, p. 119—159. Holmes, J. A., Franklin, E. C., and Gould, R. A., 1915, Report of the Selby Smelter Commission: U.S. Bur. Mines Bull. 98, 528 p. Iordanov, N., and Pavlova, M., 1963, Geochemistry of lead in soils [in Bulgarian]: Izvestiya na Instituta Obshch. Neorganichna Khimiya Bulgarska Akademiya na Naukite, 1963, no. 1, p. 5—14; abs. in Chem. Abs., v. 60, no. 6657g, 1964. Jenne, E. A., 1968, Controls on Mn, Fe, Co, Ni, Cu, and Zn concen- trations in soils and water—The significant role of hydrous Mn and Fe oxides, in Baker, R. A., ed., Trace inorganics in water: Washington, D. G, Am. Chem. Soc. (Advances in Chemistry Ser. 73), p. 337—387. Jenny, Hans, 1941, Factors of soil formation—A system of quantitative pedology [lst ed.]: New York, McGraw-Hill, 281 p. Jones, J. S., and Hatch, M. B., 1937, The significance of inorganic spray residue accumulations in orchard soils: Soil Sci., v. 44, p. 37—63. 1945, Spray residues and crop assimilation of arsenic and lead: Soil Sci., v. 60, p. 277—288. Kehoe, R. A., ed., 1971, Papers read before the conference on inorganic lead, Amsterdam 1968: Archives Environmental Health, v. 23, no. 4, p. 245—311. Kiilsgaard, T. H., Hayes, W. C., and Heyl, A. V., 1967, Metallic mineral resources, in US. Geological Survey and the Missouri Division Geological Survey and Water Resources, Mineral and water re- sources of Missouri: US. 90th Cong., lst sess., Senate Doc. 19, p. 41—63. Lagerwerff, J. V., and Specht, A. W., 1970, Contamination of roadside soil and vegetation with cadmium, nickel, lead, and zinc: Environ- mental Sci. and Technology, v. 4, no. 7, p. 583—586. MacLean, A. J., Halstead, R. L., and Firm, B. J., 1969, Extractability of added lead in soils and its concentration in plants: Canadian Jour. Soil Sci., v. 49, p. 327—334. McMurtrey, J. E., Jr., and Robinson, W. 0., 1938, Neglected soil constituents that affect plant and animal development, in Soils and men—Yearbook of agriculture 1938: Washington, US. Govt. Printing Office, p. 807—829. Marbut, C. F., 1935, Soils of the United States, Pt. 3 of Atlas of American agriculture: Washington, US. Govt. Printing Office, 98 p. Miesch, A. T., 1967, Methods of computation for estimating geo- chemical abundance: U.S. Geol. Survey Prof. Paper 574—B, 15 p. Mikhailova, R. P., and Mikhailov, I. S., 1967, Geochemical character- istics of soils above the timberline in the Kos'vinskii Kamen area [in Russian]: Rastitel’nost’ krainego Severa SSSR 1967, no. 7, p. 146-150; abs. in Chem. Abs., v. 69, no. 4389f, 1968. Mitchell, R. L., 1964, Trace elements in soils, in Bear, F. E., ed., Chemistry of the soil [2d ed.]: New York, Reinhold Publishing Corp., p. 320—366. Motto, H. L., Daines, R. H., Chilko, D. M., and Motto, C. K., 1970, Lead in soils and plants-Its relationship to traffic volume and proximity to highways: Environmental Sci. and Technology, v. 4, no. 3, p. 231—237. Myers, A. T., Havens, R. G., and Dunton, P. J., 1961, A spectro- chemical method for the semiquantitative analysis of rocks, min- erals, and ores: U.S. Geol. Survey Bull. 1084—1, p. 207-229. Nalovic, L., and Pinta, M., 1969, Recherches sur les elements-traces dans les sols tropicaux—Etude de quelques sols de Madagascar [with English summary]: Geoderma, v. 3, no. 2, p. 117—132. National Research Council, Committee for the Study of Eolian De- posits, 1952, Pleistocene eolian deposits of the United States, Alaska, and parts of Canada [map]: Geol. Soc. America, scale 122,500,000, 2 sheets. Oertel, A. C., 1961, Relation between trace-element concentrations in soil and parent material: Jour. Soil Sci., v. 12, no. 1, p. 119—128. Pauli, V. W., 1966, Some recent developments in biogeochemical re- search: Alsace-Lorraine Service de la Carte Geologique, Bulletin [Strasbourg], v. 19, no. 3—4, p. 221-240. 52 LEAD IN THE ENVIRONMENT Pinta, M., and Ollat, C., 1961, Récherches physico-chimiques des Elements—traces dans les sols tropicaux—l, Etude de quelques sols du Dahomey [with English summary]: Geochim. et Chosmo- chim. Acta., v. 25, no. 1, p. 14—23. Prabhakaran Nair, K. P., and Cottenie, A., 1971, Parent material— soil relationship in trace elements—A quantitative estimation: Geoderma, v. 5, no. 2, p. 81—97. Presant, E. W., 1971, Geochemistry of iron, manganese, lead, copper, zinc, arsenic, antimony, silver, tin, and cadmium in the soils of the Bathurst area, New Brunswick: Canada Geol. Survey Bull. 174, 93 p. ‘ Presant, E. W., and Tupper, W. M., 1965, Trace elements in some New Brunswick soils: Canadian Jour. Soil Sci., v. 45, p. 305—310. Roszyk, E., 1968, Lead in some very fine sandy soils of the lower Silesia: Roczniki gleboznawcze [Warsaw] 19, p. 123—132; abs. in Chem. Abs., v. 70, no. 5882u, 1969. Schuck, E. A., and Locke, J. K., 1970, Relationship of automotive lead particulates to certain consumer crops: Environmental Sci. and Technology, v. 4, no. 4, p. 324—330. Shacklette, H. T., Hamilton, J. C., Boerngen, J. G., and Bowles, J. M., 1971, Elemental composition of surficial materials in the conterminous United States: U.S. Geol. Survey Prof. Paper 574—D, 71 p. Singer, M. 1., and Hanson, L., 1969, Lead accumulation in soils near highways in the Twin Cities metropolitan area: Soil Sci. Soc. America Prod., v. 33, p. 152—153. Swaine, D. J., 1955, The trace-element content of soils: Common- wealth Bur. Soil Sci. Tech. Commun. 48, 157 p. Swaine, D. J., and Mitchell, R. L., 1960, Trace—element distribution in soil profiles: Jour. Soil Sci., v. 11, no. 2, p. 347-368. Taylor, R. M., and McKenzie, R. M., 1966, The association of trace elements with manganese minerals in Australian soils: Australian Jour. Soil Research, v. 4, no. 1, p. 29—39. Tidball, R. R., 1972, Geochemical survey of soils, in U.S. Geological Survey, Geochemical Survey of Missouri—Plans and progress for sixth six-month period (January—June, 1972): U.S. Geol. Survey open-file report, p. 19—57. Tiller, K. G., 1963, Weathering and soil formation on dolen'te in Tasmania with particular reference to several trace elements: Australian Jour. Soil Research, v. 1, no. 1, p. 74—90. U.S. Geological Survey, 1970, National atlas of the United States of America: Washington, D. C., 417 p. U.S. Public Health Service, 1966, Symposium on environmental lead contamination: U.S. Public Health Service Pub. 1440, 176 p. Viller, G. E., and Khrapov, V. 5., 1963, Content of trace elements in soils of the Baraba area in Novosibirsk Region [in Russian]: Mikroelementy v Sibiri, Inform. Byul. no. 2, p. 3—5; abs. in Chem. Abs., v. 62, no. 10249g, 1965. Vinogradov, A. P., 1959, The geochemistry of rare and dispersed chemical elements in soils [2d ed.]: New York, Consultants Bur., 209 p. Vuorinen, J., 1958, On the amounts of minor elements in Finnish soils: Maataloustieteelinen Aikakauskirja [Helsinki], 1958, v. 30, p. 30—35; abs. in Chem. Abs., v. 52, no. 14049e, 1958. Wells, N., 1960, Total elements in topsoils from igneous rocks—An extension of geochemistry: Jour. Soil Sci., v. 11, no. 2, p. 409—424. LEAD IN VEGETATION . By H. L. CANNON Lead occurs naturally in small amounts in all plants, but the average concentration of lead in vegetation in highly populated countries has risen in the last several decades owing to man’s activities. Because of this contamination from artificial lead, it is important that information on lead concentrations in plants be docu- mented as to both date of collection and location with respect to sources of manmade lead. Information from primitive areas on lead in vegetation is therefore of prime value and should be collated and preserved. Only by comparison with such information can the effects of present-day contamination be properly assessed. Finally, averages for lead contents in vegetation are affected by natural variation in and among plants owing to differences in seasonal uptake, species, parts of the plant, and lead concentrations in the substrate. THE BIOGEOCHEMICAL CYCLING OF LEAD Bormann and Likens (1967) have pointed out that in the biological community the vertical extensions of the terrestrial ecosystem will be delimited by the top of the vegetation and by the depth to which roots penetrate into the regolith, and that the terrestrial ecosystem participates in the various larger biogeochemical cycles of the earth through a system of inputs and outputs. Thus, if we consider the geologic input, the weathering of the rocks will provide to the soil both elements incorporated in primary and secondary minerals and soluble ions that may be dissolved in the soil solution or adsorbed on the clay- humus complex. The degree of accumulation or loss in the ecosystem will vary with the response of a particular element to erosion and weathering. MacLean, Halstead, ( and Firm (1969) studied the uptake of lead by oats and alfalfa in soils with different levels of lead chloride, (<- “ organic matter, and pH. They found that low humus content and high acidity of the soil increase lead uptake. ' Lead, although not readily soluble, is absorbed by plants and stored in woody tissue to a considerable degree. An unpublished study of the uptake of lead by native plants and vegetables growing on various rock types in Hagerstown valley, Maryland, made by the author in 1958—59, suggests that there is a buildup of lead in the terrestrial ecosystem, inasmuch as median values increase from 11 ppm in rocks to 33 ppm in residual soils to 93 ppm in the ash of native vegetation in this particular area. Unfortunately at that time no consideration was given to the possibility of atmospheric lead, and the higher values in soils and vegetation probably reflect contamination from car exhaust. A similar progressive increase from rocks to vegetation was found by Lounamaa (1956), who studied native vegetation on various rock types in Finland and found lead to be concentrated in twigs and roots. Elsewhere, native vegetation and soils, largely on sandstone from the Navajo Reservation in New Mexico, were collected by the author in 1961 and 1962 using the same collection methods as those used in Maryland, but at sites considerably more remote from roads. Analyses showed a mean concentration of 20 ppm lead and a range of 5—700 ppm in the ash of 101 native plants that were rooted in soils having a mean of 18 ppm lead and a range of only 10—20 ppm. In both Maryland and New Mexico, the leaves of deciduous trees and the two most recent years of twig growth and needles of conifers were collected. The vastly different means of lead in plant ash from New Mexico (20 ppm) and from Maryland (93 ppm) are further interpreted to suggest the possibility of atmospheric contamination in the relatively more populated area of Maryland. A comparison of the cycling of lead from soil to tree leaves to humus for uncontaminated and con- taminated areas is given in table 22. .. The data suggest that lead is not concentrated in the , humus under ordinary conditions, but that in areas of mining and smelting lead contamination of the humus and soil is largely unavailable to the vegetation or is held in the root and is not translocated to the upper portions of the plant to any appreciable degree. Bolter and others (1972) made a more recent study of the cycling of lead in the so-called Viburnum Trend, or New Lead Belt, in southeastern Missouri. Several hundred soil and vegetation samples were collected and analyzed to delineate areas of anomalous high concentrations caused by the mining and smelting activity. Elevated heavy metal 53 54 LEAD IN THE ENVIRONMENT TABLE 22.—Cycling of lead in different environments [Leaders (...) indicate no data] Pb (ppm) Leaves Roots 5011 horizons Source . . Humus (In ash) (in ash) A B Uncomaminated ground jelferson County, Colo, (on schist; 14 mi (1 km) from road): Populus Iremuluides (aspen). 65 35 50 70 20 Arer glabrum (maple)... 50 100 30 30 30 Pinus ponderosa (pine) , 200 1:10 50 30 20 Pseudotsuga taxi/Olin (D g 200 60 50 30 20 Washington County, Md. (on Limestone; 200 11(60 m) from road): Carya rordiformis (hickory) ....... 100 ...... 20 30 Caryn ovala (hickory) ......... 200 iiiiii 30 20 Fruxinus americana (ash) ....... 150 ...... 15 15 Cedar County, Mo. (on limestone); Juniperus uirginiana (cedar) .. 140 ...... 80 30 Quertus stellala (oak) ............. 40 iiiiii 83 iiiiii Contaminated ground Joplin, Mo, (Lonlaminationbydraina efrom n' Betula nigra (birch) 340 1,600 1,700 2,000 Quertus muhlenbergia 150 iiiiii 3,200 1,290 Bartlesville, Okla. (contamination from smelter): Populus deltoide: (poplar). 120 ...... 2,500 2,500 Allium porrum (onions) ...... 200 1,000 125 Asparagus officinalis (aspar 25 ...... 1,000 125 concentrations in soils were found to be mainly restricted to the humus and to the A horizons. A large part of the lead found in samples of leaves of vegetation was thought to be lead present on the surface of the leaves. BACKGROUND LEAD CONCENTRATIONS IN VEGETATION Before manmade contamination can be evaluated or the effect of mineralized ground appraised, the normal concentrations of lead that might be expected in various types of vegetation must be determined in samples from primitive areas with no manmade contamination and from results of sampling many years ago before manmade contamination had occurred. Warren and Delavault (1962) proposed, on the basis of analyses of uncontaminated Canadian plants, that the normal lead content of the twigs of trees be considered as 2.5 ppm in the dry weight and 50 ppm in the ash. They also suggested that the normal lead content of vegetables and cereal grains, which absorb less lead than trees, be taken as ranging from 0.1 to 1.0 ppm in the dry weight and from 2 to 20 ppm in the ash. Mitchell (1963) reported that in spring and summer pasture herbage normally contains less than 1 ppm lead in the dry weight, but that in autumn and winter it may contain several times this level. However, unless the samples are collected from ground known to be unmineralized and are carefully screened to exclude any possibility of manmade contamination, a few very high values will unduly raise the arithmetic mean. For this reason, a better measure of the expected “normal” value is the median concentration, as has been calculated for various classes of natural vegetation and given in table 23. The lead medians for 193 deciduous tree tips (all from paloverde trees) and 131 leaves and stems (mostly from creosotebush) are considerably higher than values for other plants and may represent greater absorption of lead by these species. Large concentrations of lead are found in lichens and mosses, which are very slow growing. The normal levels of lead in garden vegetables are more diffi- cult to establish, inasmuch as they are generally grown only in populated areas that are subject to airborne pollu- tion and contamination from sprays. The results of analyses for lead in vegetables have been studied most intensively by H. V. Warren of British Columbia (Warren, 1972; Warren and Delavault, 1971). Median values from his data and those from Kehoe (1961) are given in table 24. A concentrated effort has recently been made by Warren (1972) to determine the “normal” (presumably median) concentrations of lead and other metals in vegetables. He concluded that variations in trace-element content of vegetables of both urban and rural areas are greater than have been generally realized and suggested a normal range for lead of 16—40 ppm in the ash or 1.6—2.0 ppm in the dry weight. These values are twice those he suggested earlier (Warren, 1961). Median values for lead in vegetables that I collected in 1961—63 in New York, Maryland, and New Mexico are also shown in table 24. In many of the samples the amounts of lead were below the limit of detection by the spectro- graphic method used. All the samples of beet tops and onions and most of the cabbage samples had 10 ppm or more lead in the ash. The median lead in the dry weight of 111 vegetables was 5,000 3,288 2,775 Needles.. 425 100 >5,000 927 712 Mull ......... 437 70 >5,000 1,564 1,105 Western white pine Twigs .............. 37 200 >5,000 3,354 2,394 Needles.. 37 100 >5,000 2,777 1,538 Mull ................ 37 100 >5,000 3,030 2,094 Grand fir Twigs .............. 42 1,000 >5,000 2,938 2,739 Needles .. 42 200 >5,000 779 634 Mull ................ 42 300 >5,000 1,940 1,591 Western hemlock Twigs .............. 4 2,000 >5,000 4,250 3,942 Needles... 4 500 >5,000 925 851 Mull ................ 4 1,000 >5,000 2,250 2,060 Sub alpine fir Twigs .............. 40 200 >5,000 2,280 1,710 Needles... 52 100 >5,000 734 642 Mull ................ 52 100 >5,000 1,219 953 Lodgepole pine Twigs .............. 39 200 >5,000 1,851 1,342 Needles .. 39 150 >5,000 1,419 876 Mull ................ 37 150 >5,000 1,048 651 Ponderosa pine Twigs .............. 5 150 1,500 710 412 Needles. .. 5 150 500 270 246 Mull ................ 5 100 1,000 400 306 All species Twigs .............. 549 150 >5,000 3,080 2,478 Needles. 605 100 > 5,000 1,039 735 Mull ................ 616 70 >5,000 1,611 1,117 Lead frequency (number of occurrences in :11 species studied) Lead 122m) Twi s Needles Mull 0 0 0 0 70 0 0 1 100 0 4 9 150 1 9 1 200 11 41 27 300 9 69 55 500 7 89 83 700 24 151 15 1,000 32 118 120 1,500 46 46 67 2,000 104 36 119 3,000 109 13 68 5,000 115 14 29 >5,000 91 15 22 LEAD IN VEGETATION 61 development of the root system, and mobility of lead in the particular geochemical environment. Brown and Meyer (1956) have also tested plants as a prospecting tool in the Edwards-Balmat district in New York and obtained positive results over known orebodies, although there was a wide variation in absorption by species. A very thorough pilot study by Hornbrook (1969) of the Silvermine lead deposit, Cape Breton Island, Nova Scotia, indicates the scope and limits of geobotanical and biogeo- chemical prospecting methods. The data included 4,050 spectrographic determinations for lead in organic soil and plant material. The lead deposits are overlain by till that showed anomalous concentrations of lead, although the extent of the anomaly had been modified by glacial action. The pattern of lead anomalies in plant organs of different ages was compared, and anomalous concentrations in bark and twigs were sufficient to outline the extent of the ore. Although twigs contained more lead than bark, bark provided a more reliable basis for interpretation. The differences in lead concentration for different ages of twigs were not significant for prospecting purposes. Several studies of lead uptake by plants in mineralized areas have been made by US. Geological Survey scientists. Shacklette (1960) made a study of vegetation and soils at the Mahoney Creek lead—zinc deposit in southeastern Alaska. The results given in table 29 show that mean lead concentrations increase from 90 ppm in the ash of plants from unmineralized ground to 160 ppm in the ash of plants rooted in soil over the vein. Shacklette suggested that the significance of differences between background, halo, and vein values of a species may be judged by the standard error. If a difference between two values is as great as or greater than twice the standard error, it can be considered a significant difference, not a sampling error. Keith (1969) reported on similar studies of three deciduous tree species in the Upper Mississippi Valley lead-zinc district, where he collected 152 soils and 256 plants in mineralized areas and 306 soil and 368 plant samples in nonmineralized areas (table 29). It is apparent that, as expected, stems contain at least twice as much lead as leaves, but, contrary to expectations, lead is not notice- ably concentrated in the trees growing on mineralized ground and is actually higher in nonmineralized areas. On the other hand, zinc (not shown here) was significantly concentrated in trees on mineralized ground. Keith believed the anomalously low concentrations of lead to be due to a higher pH in mineralized limestones in contrast to the surrounding areas of sandstone and glacial drift. A study of lead in vegetation of the Ruby Hill lead-silver mining district in Arizona by Maurice Chaffee (written commun., 1970) shows the increase in lead uptake in an acid sulfide environment (table 29). The data show a remarkable concentration of lead in the older wood of sage and, in other vegetation, about a tenfold increase in lead over the levels in vegetation of the Upper Mississippi Valley limestone district. Studies by Wallace (1971) show no effect of calcium on lead uptake by bushbeans, but a large increase in root absorption of lead when iron is added and a decrease with increments of NaHC03. Lead accompanied by zinc and cadmium may also acCumulate in spring-fed peat bogs that drain dolomites of relatively high, but noneconomic, lead concentrations. When these peats are drained for “muck” farming of commercial vegetable crops, the metals become oxidized and are thus available to the vegetation to such an extent that vegetables cannot be grown in certain areas of highly mineralized peat. Such a situation developed on the “Manning muck” in Orleans County, N.Y., in 1938, but, during the last 30 years, the oxidized metals have been leached from the muck by ground and surface waters and neither wild plants nor vegetables now contain anomalous amounts of lead (Cannon, 1955, 1970). Some of the actual lead and zinc contents for the years 1946 and 1967—68 are given in table 30. It is unfortunate that vegetables grown on the “Manning muck” were not analyzed for lead by Staker (1942) in 1938, as the median zinc content he recorded at that time was 140 ppm, and by 1946 the zinc content of native vegetation had dropped to one-third that level. Presumably, lead levels would have shown the same sharp decrease. In the 1946 collections, native vegetation (37 samples) had a median of 10 ppm and a range of 2—58 ppm lead in the dry weight; in the 1967—68 collections, native vegetation (7 samples) had a median of 4.7 ppm and a range of < 05—102, and the vegetables (15 samples) had a median of <0.5 ppm and a range of < 0.5—6. LEAD INDICATOR AND ACCUMULATOR PLANTS Certain species of plants that are tolerant of high-lead soils have been used as indicators in prospecting. These species may have adapted to living exclusively on rocks or soils that supply unusual amounts of a particular element, or they may have acquired an immunity to large amounts of an element by being able to reject the metal at the root site. Some indicator plants that have been found universally associated with a particular mineral assemblage absorb and concentrate large amounts of one or more metals in their storage tissues; others that do not concentrate large amounts of the metals in question may be species of wide distribution that favor mineralized ground under certain local conditions, because of a reduction in competition or a change in acidity, water conditions, or availability of major plant nutrients (Cannon, 1971). Inasmuch as lead and zinc commonly occur together in ore deposits, it is difficult to establish positively that a particular plant is an indicator of lead. Plants that have 62 LEAD IN THE ENVIRONMENT TABLE 29.—Lead content of vegetation and soils from several mining districts [Number of samples, where specified, shown in parentheses] Material analyzed Plant par! Mean lead contents (ppm) Mahoney Creek Lead-zinc deposit, Alaskal Background (3 standard error) Halo Vein Menziesia ferrugz'nosa 8-in. older stems .......................... 140110 160i20 350160 (rusty menziesia). (20) (10) (5) Do ........... Leaves and young stems ............. 40 $10 60120 1101:30 (20) (10) (5) Vaccim'um ovalifolium 8-in. stems and leaves ................. 40110 60120 100130 (whortleberry). (20) (10) (5) Tsuga heterophylla 10-in. stems and needles ............. 160110 160120 140i40 (hemlock). (18) (9) (5) Gaultheria shallon lO-in. stems and leaves ................ 50i10 170i120 110140 (Wintergreen). (15) (8) (4) Vaccinium pamifolium 8-in. stems and leaves ................. 50110 40110 180i100 (whortleberry). (4) (4) (2) Picea sitchensis B-in. stems and needles ............... 5O 50 150 (Spruce); (1) (1) (2) Mean of all plants. 90110 110i20 160120 Soil samples ...................................................................................... 20i10 110i40 1,3001690 Upper Mississippi Valley district2 Nonmineralized areas Mineralized areas In ash of In dry wt In dry wt I'n ash of In dry wt In dry wt plants of plants of soils plants of plants oi soils (368) (368) (306) (256) (256) (152) Ulmus sp. (elm) ........................ 102 4.9 29 3 Acer sp. (maple) ........................ 119 4.7 44 4.5 Quercus sp. (oak) ...................... . .. 150 6.8 Leaves ........................................ 61 2.8 Soils: A Horizon ............................................................................................................................ B Horizon .. C Horizon ............................................................................................. Ruby Hill mining district, Arizona5 In ash In dfl wt Pinus edulis (pinyon) ............... Needles ........................................................... 500 12.0 1- to 2-year twigs... .. 500 13 Needles .................. 500 ‘ 11 1- to 2—year twigs... 500 13 Needles .................. 700 14 1- to 2-year twigs ............................................ 700 16.8 Artemisia m'dentata (sage) ........ 1—year growth ................................................. 700 ' 42 Older wood and leaves. .. 5,000 270 1-year growth ................................................. 1,500 90 Older wood and leaves ................................... >5,000 >220 Juniper sp. (juniper) .................. 1-year growth ................................................. 200 7.2 ’ d .. 100 3.8 200 8.4 Cercocarpos ledifolius 700 36 (mountain mahagony). 700 36 2,000 88 Median .............................................................................................. 700 15.4 Range .......................................................................................................... lOO—>5,000 3.8—>220 ‘Collected by H. T. Shacklette; analyzed by D. R. Marx. {Analyzed by Maurice DeValliere and J. C. Hamilton. 5Analyzed by E. L. Mosier. LEAD IN VEGETATION 63 TABLE 30.—Maxirnum lead and zinc contents, in parts per million dry weight, of plants and drained peat, Orleans County, _N.Y [N, not detected. Analysts: H. W. Lakin, F. N. Ward, Laura Reichen, H. Almond, F. Grimaldi, H. Bloom, T. H. Harms, C. S. E. Papp. From Cannon (1970)] Year of Lead ,Zinc collection Plant Soil Plant Soil Portulaca oleracea (purslane). 1946 42 67 10,000 26,000 Ambrosia elatior (ragweed 1946 50 250 4,800 88,000 Amaranthus retro/locus (pigweed). 1946 6 250 2,000 88,000 Urtlm dioeia (nettle) ...... 1946 2 250 1,000 88,000 Sedge .............................. 1946 55 250 600 88,000 Arctium minus (burdock).. 1946 58 110 390 4,200 Salix 5p. (willow) ............... 1946 4 28 860 5,100 Populus tremuloides (aspe 1946 4 110 860 4,200 Solanum nigrum (nightshade)... 1946 10 67 10,000 26,000 Solanum tuberosum (potato) ..... 1967—68 2.4 105 104 9,900 Daueus carrota (carrots). 1967—68 2.0 38 156 5,200 Allium cepa (onions) ,,,,, 1967—68 N 16.5 44 600 Brassica oleracea (cabbag 1967-68 N 16.5 150 2,800 Cucurbita moschata (squas 1967-68 N 16.5 225 2,800 Urtim diocia (nettle) .............. 1967-68 4 7 23 487 6,200 Pilea pumlla (clearweed) ........................ 1967—68 5 0 38 450 31,000 Rubus occidentalis (wild raspberry)... 1967—68 7.5 38 487 6,200 Salix sp. (willow) ................................... 1967—68 10.2 23 490 1,100 been reported as indicators or accumulators of lead are given in table 31. Plants that have been mentioned in the literature as being indicators of lead deposits in the Mississippi Valley lead-zinc district include white birch, cottonwood, and wild indigo. The association of Amorpha canescens (leadplant) with lead that has been mentioned in the early literature has not been verified. In northern Australia, zone of copper-lead—zinc minerals permits a more careful‘study of the tolerance of the various species that grow there (Cole, Provan, and Tooms, 1968). Gomphrena canescens R. B. (globe amaranth) and Polycarpea synandra F. Muell. var. gracilis (a pink), two of the indicator species, were found to tolerate 50,000 ppm zinc and 5,000 ppm lead in the soil. At another deposit, Polycarpea glabra (a pink), Bulbostylt's barbata (bulletwood), and Eriachne mucronata indicated lead-zinc ores, the last concentrating as much as 50 ppm lead in the dry weight. Although no trees were considered to be indicators of lead ores in studies that have been made by H. V. Warren in Canada and H. T. Shacklette in Alaska, very high contents of lead have been found in Douglas-fir, mountain hemlock, alder, birch, and heather. Shacklette (1965) described the distribution of a liverwort (Cephalozz'a bicuspidata) as entirely covering the surface of the ore, to the exclusion of all other species, at the Mahoney Creek lead-zinc mine in Alaska. The exposed ore contained 12.9 percent lead. The concentrations of lead in Canadian hemlock, arborvitae, and several common adventive weeds from mineralized, drained peat in New York were also high, the lead content in the dry weight of coniferous species exceeding that of the peat. Ledum palustre in Finland was found by Salmi (1959) to accumulate high lead contents compared to other species. Thyssen (1942) has reported a grass, Molina altissima, growing at Innerstetal, Germany, to contain 35,600 ppm lead in the ash. The grass species, Agrostis tenuz's, appears to have adapted specifically to high-lead soils, and seed from selected heavy-metal-tolerant races of this species is being used as cover on mine dumps and tailings in Wales (Jowett, 1964; Wild and Wiltshire, 1971). Some naturally lead poisoned areas have been studied and described in detail by Lag and Bqllviken (1974). Occurrences of naturally lead poisoned soil and vegeta- tion have been found in 5 different areas in Norway where deposits of galena occur in the bedrock. In the initial stages of poisoning, the forest trees and Vaccinz'um spp. (blueberry) are replaced by Deschampsia flexuosa (hairgrass), which grows profusely but without seed. Where lead poisoning is more advanced, the vegetation is abnormal or dying, and in highly mineralized soils, the ground is barren. D. flexuosa responds to increasing contents of lead in the soil with a corresponding decrease in the plant-Pb/soiI-Pb ratio. After a low ratio of 0.6 percent is reached, at level of 1 percent lead in the soil, the ratio remains constant with increasing soil lead until the plant is killed. This defense mechanism of D. flexuosa, combined with a restricted availability of lead in the soil, effectively counteracts poisoning of the plant. Because plant growth is retarded, podzolization of the soil in these poisoned areas is lacking and erosion of morainic soils is increased, resulting in a stony soil surface. Lag and B¢lviken believe the search for areas showing signs of natural heavy-metal poisoning to be a valuable method of geochemical prospecting. ANOMALOUS LEAD CONCENTRATIONS IN PLANTS RESULTING FROM MAN’S ACTIVITY LEAD ACCUMULATION RELATED TO MINING AND SMELTING ACTIVITY The activities of man in the mining, milling, and smelting of ores have released considerable lead into the environment. These accumulations, because they occur as the result of activities in populated areas and because they may be released in a more available form than the original sulfide ores, may have a greater effect on health and disease than the lead in vegetation rooted in mineralized ground. Extensive studies are being conducted in Great Britain and Japan, where health problems have arisen in populations living in contaminated areas. In Japan, lead analyses have been made of agricultural products grown near a large zinc refinery in Annaka City, where a chronic disease, itai itai, has been shown to be caused by contamination of water and rice crops by cadmium from the refinery (Kobayashi, 1972). Mulberry 64 LEAD IN THE ENVIRONMENT TABLE 31.—Lead indicator and accumulator plants rooted in mineralized ground Indicator (1) Maximum lead content (in ppm ) refined Plant species or Locality In plants In soil Reference accumulator (A) (ash) (dry weight) (dry weight) Conifers Pseudotsuga menzz'esia A British Columbia ......................... 2,200 ................ Warren and Delavault (1960). (Douglas-fir twigs). Pseudotsuga taxifolia A Coeur d’Alene, Idaho .......................... 130 20,000 Kennedy ( 1960). (Douglas-fir tips). Larix occidentalis A do ................................................. 100 1,000 Do. (larch tips). Pinus monticola A do ................................................. 100 20,000 Do. (white pine tips). Tsuga mertensiana A Alaska ........................................... 4,000 .............. H. T. Shacklette (written commun., (mountain hemlock twigs). 1960). Tsuga canadensz's A New York (peats) ................................ 40 10 Cannon (1955). (Canada hemlock tips). . Ts'uga heterophylla A British Columbia ......................... 1,100 .............. Warren and Delavault ( 1960). (Pacific hemlock stems). . Thuja occidentalis A do ................................................. 19 13 Do. (eastern arborvitae tips). Thuja plimta A British Columbia ......................... 3,100 .............. Do. (giant arborvime stems). Picea sp. A Wisconsin ..................................... 1,300 .............. Thyssen (1942). (spruce wood). Equisetum amense A Warren, N.H. (ore) ....................... 420 140 ....... Cannon, Shacklette, and Bastron (horsetail stalks). (1968). D0 .......................................... A Warren, N.H. (tailings) ............... 344 86 21,000 Do. Deciduous trees Alnus crispus A Alaska ........................................... 5,000 ....... H. T. Shacklette (written commun., (American green alder). 1960). Alnus sinuala A do .......................................... 4,000 ....... Do. (Sitka alder) Populus sp. I Wisconsin .......................................................... A. V. Heyl (oral commun., 1948). (cottonwood twigs). Do .......................................... A Missouri ....... . .. 400 45 5,000 Cannon and Anderson (1971). Betula populifolz'a A Alaska ........................................... 5,000 .............. H. T. Shacklette (written commun., (gray birch). 1960). Do .......................................... I Wisconsin .......................................................... A. V. Heyl (oral commun., 1948). Betula glandulosa A British Columbia ......................... 25,000 ....... Warren and Delavault (1960). (birch stems). Salix sp. A Harz Mountains, Germany .......... 17,300 .............. Thyssen (1942). (willow root). Betula lutea A Joplin, Mo ................................... 4,600 175 2,000 Cannon ( 1971). (yellow birch twigs). Shrubs Calluna sp. A Alaska ........................................... 3,000 .............. H. T. Shacklette (written commun., (Alaskan heather). 1960). Vaccim'um canadense A Warren, N.H. ............................... 238 81 21,000 Cannon, Shacklette, and Bastron (dewberry fruit). (1968). Holodiscus discolor A Coeur d’Alene, Idaho .......................... 77 20,000 Kennedy (1960). (rockspirea leaves). Ledum palustre A Finland ......................................... 600 ....... l600 Salmi (1959). (Labrador tea, twigs). Herbs A renaria (M inuartia) verne A North Wales ................................. 47,000 .............. H. V. Warren (written commun., (leadwort). 1972). Portulaca oleracea A New York (peat) .................................. 42 ‘67 Cannon, Shacklette, and Baston (common purslane). (1968). Ambrosia elatior A do ................................................. 50 1250 Do. (common ragweed). Arctium minus A do ................................................. 58 1110 D0. (small burdock). Polycarpea glabra IA Australia ............................................................ Nicolls. Provan, Cole and (pink). Tooms (1964—65). Tusilago far/am I Siegerland, Germany ........................................ Linstow (1929); Dom (1937). (coltsfoot). Baptism bracteata I Wisconsin .......................................................... H. L. Cannon (unpub. data, 1948). (wild indigo). LEAD IN VEGETATION 65 TABLE 31.——Lead indicator and accumulator plants rooted in mineralized ground—Continued Indicator (1) WWW—"2* Plant species or Locality In plants In soil Reference accumulator (A) (ash) (dry weight) (dry weight) Herbs—Continued \ Polycarpea synandra v. gracilis IA Australia ..................................................... 5,000 Cole (1965). (pink). Gomphrena canescens (globe amaranth leaves) ........... IA do ................................................. 49 5,000 H. E. King (written commun., undated). (globe amaranth flowers) ......... IA ....... 114 5,000 Do. (globe amaranth stems) ............ IA ....... 51 5,000 Do. Bulbostylt's barbata IA ....... ....... Nicolls, Provan, Cole, and (bulletwood). Tooms (1964—65). Eriachne mucronata IA do ................................................. 50 ....... Do. Campanula alpina IA Caucasus ....................................... 1,000 .............. Starikov, Konovalov, and (alpine bellflower). Brushtein (1964). Grand Molim'a altissima IA Innerstetal, Germany ................... 35,600 .............. Thyssen (1942). (Indian grass). Deschampsia flexuosa I Norway ......................................... 2,000 99 17,200 Lag and B¢lviken (1974). A grostt's tenuis (subsp.) I North Wales.. .......................... Jowett (1964). Calamagrostis epigeios A Poland .................................. >100 .............. Sarosiek (1959). (reed grass). Ferns Dryopten's cristata A Wisconsin ............................................ 30 71 Cannon, Shacklette, and Bastron (woodfern). (1968). Dryopterz's spp. A Norway ......................................... 4,100 410 23,300 Lag and B¢lviken (1974). Pryopten's spzmlosa ..................... A New Brunswick ................................................. Schmidt (1955). Liveron Cephalozia bicuspidata ................ I Alaska ......................................................... 129,000 Shacklette (1965). Algae Spirogyra type .............................. A Warren, N.H. ............................... 9,420 6,600 (2) Cannon, Shacklette, and Bastron (1968). lPeat. 2Water in which algae were growing contained 16 ppm total heavy metals. growing on the hills above the refinery contained 41—160 ppm lead in the dry weight of the leaves, depending on the distance from the refinery. Leafy vegetables contained 4.4—260 ppm lead in the dry weight, root vegetables < 0.02—63 ppm, and fruits of vegetables < 0.4-1 1.0 ppm. A particular moss that covers the garden soils contained 370 ppm lead in the dry weight, 7,010 zinc, and 61 ppm cadmium. The lead uptake thus varies with species and plant part as well as diStance. It was observed that the principal source of metals in the vegetation was by absorption through the roots from polluted soil rather than from direct deposition of metals from the air. A comprehensive study by the US. Environmental Protection Agency was conducted in the Helena Valley area of Montana to determine the effects on vegetation of pollutants from the American Smelting and Refining Co. smelter (Hindawi and Neely, 1972). The lead content of vegetation was found to decrease with distance from the stack. Gordon (1972) reported samples from east Helena of plants and the soils in which they were grown to contain lead in the dry weight, in parts per million, as follows: Vegetation Soil lead lead Washed lettuce ................................ <10— 26 49- 550 Unwashed lettuce . . 26-460 370-1,400 Conifer needles ................................ 40— 125 370—1,100 Hindawi and Neely (1972) grew alfalfa, pinto beans, carrots, beets, petunias, and tobacco in greenhouses in vermiculite, exposing the growing plants to ambient air, and in garden soils outdoors, at the same distances from the stack, in order to test how much lead might be absorbed from the soil and how much from the air in the vicinity of a smelter. Their results, reported for unwashed produce, in parts per million, are as follows: Distance from Local garden Absorbed from smelter Vermiculitc soil soil (mi) (km) (A) (B) (B‘A) 0.4 0.6 7.4 48.3 140.9 .8 1.3 3.0 5.4 2.0 2.5 4.0 1.5 3.0 1.5 4.5 7.2 .6 1.0 .4 ‘Believed to reflect contamination by duststorm that arose during sampling. 66 LEAD IN THE ENVIRONMENT If one assumes that the plants grown in vermiculite in the greenhouse but receiving ambient air are receiving the same atmospheric lead insult as the outdoor plants, then by subtraction it would appear that about half of the lead measured in unwashed produce may be absorbed from the soil and half from the air. During the 19th century, several parts of Wales were intensively mined for lead. Fields adjacent to and down- stream from the mines became contaminated by airborne and waterborne heavy-metal compounds; these fields still contain high concentrations of these metals. Work by Alloway and Davies ( 1971) showed that herbage, consisting largely of the grasses F estuca and Lolz'um, and soils in this area have the following lead contents in parts per million dry weight: Standard Range Mean deviation Herbage ............................. 30— 100 63.2 24.5 Soil (total) ........ 90—2,976 1,652.0 878.3 Soil (availablel) 16—1,020 323.8 292.7 1Acetic acid extractable Radishes were then grown in 15 gardens of north Cardiganshire, some close to and some remote from mines. The radishes had lead contents ranging from 4.6 to 33.1 ppm dry weight, with a mean content of 13.1 ppm and standard deviation of 7.4. Lagerwerff, Brower, and Biersdorf (1972) collected soil and grass samples along 4 transects extending 6,684 feet (2,400 m) from a smelter in Galena, Kan., near the Missouri border, after the smelter had discontinued opera- tions involving lead production. In the direction of prevailing winds, lead content of the unwashed grass ranged from 98.4 ppm (dry weight) at 1,082 feet (330 m) from the smelter to 16.2 ppm at 6,684 feet (2,400 m). They also observed that the pastures near the smelter had been invaded by switchgrass (Pam'cum virgatum) and broomsedge (Andropogon virginicus), which are both tolerant of the combination of low pH and high heavy- metal concentration in the soil. In 1968 the U.S. Geological Survey sampled birch and oak trees, their humus, and the alluvium in which they were growing, along a stream in a forested mining area near Joplin, Mo.; old mine diggings and pits were visible under the undergrowth. Although this locality had no air contamination from lead at the time of sampling, concen- tration of lead in the bark was comparable to that in the first-year twigs. The lead content was higher in the birch, which is an accumulator, than in the oak. Results of the study are as follows (number of samples collected are given in parentheses): Lead (ppm) In ash In dry weight Betula lulea (birch) (2): Leaves ....................................... 345 25.0 Twigs (lst-year) .. 4,600 175.0 Wood ................ 900 14.0 Bark ..... 2,900 73.0 Roots ........................................ 1.600 50.0 Lead (ppm) In ash In dry weight Quereus muhlenbergia (oak) (2): Leaves ....................................... 140 6.4 Twigs (lst-year) 1,200 48.0 Wood ................. 375 9.4 Bark.. 1,500 178.0 Humus ................... 1,700.0 Alluvium ....................................................... 2,000.0 Smelters and refineries, commonly located in well- populated areas, may contribute considerable amounts of lead to the atmosphere; this airborne lead, in turn, contaminates the soil and vegetation. Warren, Delavault, and Cross (1966) found as much as 1,600 ppm lead in the ash of birch leaves growing 3 miles (5 km) from the Trail smelter in British Columbia. Studies have been made by the U.S. Geological Survey near several zinc-lead smelters. Analyses of plants and soils collected near the Bartlesville, Okla., and Leadville, Colo., smelters are given in table 32. The tree sampled at Leadville was the nearest pine to the smelter able to grow in the contaminated soil and was considerably dwarfed. The smelter had not been operative for 15 years at the time of sampling. TABLE 32.—Lead contents of plants and soils as related to smelter eon- taminalion in Colorado and Oklahoma [Leaders (...) indicate no data. From Cannon and Anderson (1971); analysts, T. F. Harms and C. S. E. Papp] Lead (ppm) Sample No. Material analyzed In ash Converted D413- (0 dry wt Bartlesville, Okla. 1,500 ft (460 In) from smelter 387 Populus deltoides (cottonwood), leaves ................................................. 120 l 1.2 410 Populus delloides (cottonwood), wood ................................................. 190 2.5 411 Populus deltoides (cottonwood), bark ................................................... 500 34 406 Top 0.5 in. (1.25 cm) soil .. ..... 2,500 ...... 407 1—5 in. (25—127 cm) soil ..................... 2,500 ...... 7,000 ft (2,100 m) from smelter 390 Ulmus ameriama (elm), leaves ............ 120 7.2 388 Asparagus offieinalz's (asparagus), tops ................................................... 40 2.4 389 Allium porrum (onion), bulb... . 200 11 408 Humus ............................. . 1,000 ...... 409 Soil ............................................. 125 ...... Leadville, Colo.l 1,000 ft (300 m) from smelter 452 Can'x sp. (sedge), tops .......................... 500 49 3,200 ft (975 m) from smelter 453 Pinus edulis (pine), needles ................. 2,500 40 454 Pinus edulis (pine), branch wood (including bark) ............................... 18,000 342 ‘Smelter inoperative since 1956. LEAD IN VEGETATION LEAD ACCUMULATION RELATED TO AGRICULTURAL ACTIVITIES Contamination of the soil due to agricultural activities stems generally from efforts to reclaim or increase the fertility of the land, or from sprays used on crops to combat insect or fungal infestation. Tailings from lead-zinc mines are occasionally used for fertilizer, but a study by Hawkes and Lakin (1949) in east Tennessee where such practice is common showed that the amounts added were small and did not affect the level of lead in the soil. Residual lead can be detected in soils that have been sprayed in the past and this lead may be translocated to the edible parts of produce. Warren (1961) compared trees in orchards in Okanogan County, Wash., that had been sprayed 10 years earlier with trees that had never been sprayed. The unsprayed trees contained less than 1 ppm lead whereas sprayed trees contained 40, 50, and 100 ppm. A garden in Canadaigua, N.Y., which had been sprayed repeatedly, but which was well protected from street contamination, was sampled by the author and found to contain 300 ppm lead in the soil, 700 ppm in the ash (44 ppm in dry weight) of raspberry canes, and 300 ppm lead in the ash (12 ppm in dry weight) of the berries. LEAD ACCUMULATION RELATED TO URBAN AIR POLLUTION The effects of lead in the atmosphere on the lead content of vegetation were pointed out by Warren and Delavault (1960), who found higher values in Canadian plants collected near roadsides than in those remote from roads. Warren and Delavault (1962) further investigated this anomaly by sampling similar vegetation in two localities in England—in an area of Sussex remote from a road and in London. Their results for twigs of the previous year’s growth showed these lead contents, in parts per million: Species Sussex locality London locality Dry weight Ash Dry weight Arh Lime .............. 0.4 8 5 50 Yeco (needles) .4 7 6 100 Willow ........... 1.0 35 2 30 Birch ...... 1.0 36 8 160 Oak.... .8 30 20 280 Ash ................. 2.0 30 14 160 Hazel ...................... 2.0 50 52 680 In Maryland the lead content of the ash of vegetables collected near roads was also shown to be higher than that of vegetables farther from roads (Cannon and Bowles, 1962), as shown in the following table (analyst, E. F. Cooley): Distance from road Number of samples Lead cantenl (ppm) (ft) (m) analyzed Mean Range 1— 25 4— 8 29 80 10—500 25— 50 8—1 6 29 66 10—700 50—500 16—160 45 45 500 >160 28 20 <10—200 67 To test the significance of these findings, Cannon and Bowles (1962) sampled grass in four directions for 1,000 feet (304 m) from major highways in the Denver area in early June of 1961. The samples were washed in tapwater and analyzed using an emission spectrograph. The lead contents along all four traverses decreased logarith- mically with distance from the highway, from 30 ppm to a background of 2 ppm at 900 feet (274 m). In 1969, the samples were re-collected in early June along the only traverse remaining in pasture. The lead content had risen about 1,000 percent and had spread considerably farther from the highway. The highest lead content, 222 ppm dry weight, was found at a distance of 5 feet (1.5 m) from the highway, and grass at 1,000 feet (304 m) contained 28 ppm (fig. 11) (Cannon and Anderson, 1971). Washed grass contained less than unwashed, although the samples were collected during a period of heavy rains. Salmi (1969) reported a decrease in lead content of Sphagnum tuscum (moss) on the surface of a bog at distances of 0—984 feet (0—300 m) from a road in Finland. At about 33 feet (10 m) on one traverse, 27 ppm lead was measured, in contrast to a norm of 2 ppm. Purves and Mackenzie (1969, 1970) reported on significant differences in lead content of herbage in parklands of Scotland. The mean for rural herbage was 30.3i2.3 ppm and that for urban (Edinburgh) herbage was 44.6:42 ppm in dry weight. On the other hand, the inner leaves of cabbage collected from rural and urban areas showed significant METRES 1 3.05 61.0 1000 I ‘ 305 |_ I 9 LL! 3 E o 100— Z 9 _| :‘ E u: Lu 4:. (ll-J 75,0 Car/ \ 961 s 24 CC --., ( hr 5 1O - ""WAS/‘(Eo s .v \ n E d < Lu .. 'J ..... 1 I 1 l | l 1 | ‘— u') o o o o o oo _ m e a 8 E8 FEET ‘- DISTANCE FROM HIGHWAY (WEST 6TH AVENUE, DENVER, COLO.) FIGURE 11.—Lead in grass collected for 1,000 feet (304 m) from highway in 1961 and 1969. 68 LEAD IN THE ENVIRONMENT differences in boron, molybdenum, and zinc, but none in lead. Unwashed privet leaves were collected by Everett, Day, and Reynolds ( 1967) in areas close to and remote from highways in England. Leaves near highways had an average of 86 ppm dry weight, and leaves remote from highways had an average of 45 ppm dry weight. AU.S. Geological Survey study in Missouri (Connor, Erdman, and others, 1971) found that roadside contamina- tion has a much greater effect on the lead content of vegeta- tion than it does on soils or rock outcrops. Thirty-two samples of cedar and 60 samples of grass were collected from locations both on (< 50 ft (15 m) away) and off ( > 500 ft ( 152 m) away) secondary roads. The collections from these locations consisted of the terminal branches of cedar (Juniperus virginiana) and above-ground portions of grasses at a specified stage of development. The mixed grasses were mostly Triodia flavens, Festuca elatior, and Setaria viridis. The samples, which were not washed, were analyzed by emission spectrography. The results are shown in table 33. The differences in _both cedar and grass are significant at the 0.05-ppm probability level; those in subsoils and rocks are not. These data show that lead reported in roadside soils is confined to surface soils, and that soils at a depth of several inches are unaffected. Cedar, which is an evergreen and thus exposed to contami- ination throughout the year, contained no more lead than did the grass. That cedar is an accumulator of lead was shown by one sample collected near Centerville, which contained 1,200 ppm lead dry weight (Connor, Shacklette, and Erdman, 1971). Further investigation has shown a geometric mean of 348 ppm dry weight in 15 cedar samples from the area. It is believed that the soil here has been contaminated by ore dust from trucks transporting ores from mine to smelter. These data can be compared with those of Lagerwerff and Specht ( 1970), who sampled grass at 8, 16, and 32 feet (about 2, 4, and 9 m) from major freeways near Washington, D. C., and soil at these sites at depths of 0—5, 5—10, and 10—15 cm. Lead contents in grass and soil decreased with distance; the levels of lead in grass ranged from a maximum of 68.2 ppm dry weight at 8 feet (2 TABLE 33.—Comparison of lead content between on—road and off-road samples of some soils and plants in Missouri [Data from Connor, Erdman, and others, 1971; analyst, Harriet Nieman] Sample Number of ' Lead content (ppm)l Locality type samples On-road2 Off-road3 Various ................... Cedar branch tips. 32 16 7 D0 .................... Soil ....................... 32 l7 14 Kansas City, Mo ..... Grass ..................... 30 29.0 9.7 Do .................... Soil ....................... 30 10 14 Pacific, Mo ............. Grass...'.. 30 17.0 4.5 Do .................... Soil ....................... 30 10.0 3.5 1Dry weight, geometric mean. 2Samples taken <50 ft (15 m) from road. 55amples taken >500 ft (152 m) from road. m) to a minimum of 7.5 ppm at 32 feet (9 m) and in surface soil ranged from 540 ppm to 55 ppm. The data also show a reduction in lead content with depth. Dedolph, Ter Haar, Holtzman, and Lucas (1970) were able to correlate the amounts of lead in grass with concentrations in air. At 40 feet (12 m) from the road, the air contained 2.3 u g/m3 and the grass averaged 15 ppm dry weight, and at 120 feet (36 m), the air contained 1.7 ug/m3 and the grass 8.4 ppm lead. An increase in the lead content of plant foliage in the winter months has been reported by several workers and generally attributed to increased exposure time to air- pollution lead. Mitchell and Reith (1966) found an increase in pasture herbage from 1 ppm lead (dry weight) in early autumn to 30—40 ppm in the winter. However, they attributed the increase to either a translocation of lead from roots to tops during the winter months or a loss of organic matter through respiration rather than to surface contamination from the air. The effects of washing on the lead values varies with season and rainfall conditions. In the study reported by Cannon and Anderson (1971), on grass collected in the spring shortly after a heavy rain, no great difference in lead content between washed and unwashed samples was noted. Ganje and Page ( 1972) compared the lead content of washed with unwashed samples of lima-bean leaves and pods, corn leaves, sugar-beet leaves, and tomato fruits collected in California near the Santa Ana and Riverside Freeways; they found a 30—70 percent decrease, depending on such factors as the extent of plant surface exposed, roughness of surface, and duration of exposure. Motto, Daines, Chilko, and Motto ( 1970) also found a 140 percent decrease in washed grass collected within 225 feet (68 m) of highways. Rains ( 1971 ), on the other hand, could not wash lead from Avena fatua (wild oats) that had grown in a lead- fallout region, nor could he extract it with water. He was, however, able to extract 50 percent with 0.01 M HCl and 95 percent with 0.01 M NH4 EDTA. Whether the lead that cannot be washed from roadside plants is absorbed directly from the air through the leaves or is taken up from the contaminated soil has been debated. Certainly there can be no doubt concerning absorption from the air in regard to Tillandsia usneoides (Spanish moss), which has no root system and obtains all essential elements from the atmosphere. Analyses of 123 samples of Spanish moss collected by H. T. Shacklette (written commun., 1971) from the Atlantic and Gulf Coastal Plain areas ranged from 3.2 to 230 ppm lead in dry weight and had a geometric mean of 192 ppm; the contrast between samples from urban industrial areas and rural areas indicated the influence of atmospheric pollution in raising lead levels. Wherry and Buchanan (1926), who analyzed Spanish moss for some of the essential plant nutrients, found no difference between washed and unwashed samples. Martinez, N athany, and Dharmaraj an LEAD IN VEGETATION ‘ 69 (1971) in most instances found no significant difference between unwashed samples of Spanish moss and those washed with ammonium acetate, but were able to extract more than one-third of the lead from a sample collected near a four-lane highway. These plants, however, have a highly developed system for extracting nutrients from the air and cannot be considered as representative of the entire plant kingdom. Riihling and Tyler (1968) grew plants from roadside seed in contaminated soils obtained near highways. The plants contained only 5-10 ppm lead in the dry weight of the plant, whereas the same species of weeds that grew in the same soils along the roadside contained 68—950 ppm lead in the dry weight of washed samples. These data indicate absorption of lead from the atmosphere. Crops were grown by Ter Haar (1970) both in greenhouses supplied with filtered and ambient air and in plots planted in rows perpendicular to a busy highway. Of the 10 crop plants studied, only leaf lettuce, bean leaves, and the husk of sweet corn showed significant increases in lead content in unfiltered air compared to filtered air. In crops grown 30-520 feet (9—158 m) from a highway, soybeans and snap beans also showed higher lead contents nearer the highway. He concluded from these studies that natural lead in the soil is the main source of lead in edible crops and that airborne lead contributes only 0.5 to 1.5 percent of the lead content of the US. diet. More sophisticated experiments (Dedolph and others, 1970) at the Argonne Laboratory consisted of growing six harvests of ryegrass and three harvests of radishes in chambers ventilated with filtered and unfiltered air and in a field adjoining a highway traversed by about 29,000 vehicles per day. The data obtained from analyses of coded samples were subjected to statistical evaluation. Clearly defined differences in plan t-lead content attributable to differences in the lead concentration of the atmospheric environment could be discerned in leaves of both species. Lead-enriched water applied to the plants had no effect on their lead content. Atmospheric lead had no effect on the lead content of the root or edible portion of the radish. They concluded that the radish and rye leaves contain on the order of 2.5 ppm lead dry weight, which is soil derived. Lead concentrations above this base level (maximum 15 ppm) reflect the average lead concentration in the atmosphere in which the plants were grown. Schuck and Locke (1970) investigated crops routinely grown in fields near highways in the vicinity of Riverside, Calif, and concluded that lead particulate particles are not absorbed but exist as a topical dust coating, of which at least 50 percent can be removed by washing. In a cauli- flower head, interior florets contained less lead than did the outer florets; maximum amounts were contained in the outer leaves of cabbage heads growing 25 feet (7 m) from the highway. No evidence of absorption through the roots from water or from the high-lead soils that had been treated in the past with lead arsenate was observed. Zimdahl and Arvik (1972) believed that the leaf cuticle is a primary barrier to en trance of atmospheric lead into the leaf and plays a more important role than stomatal penetration. Experiments with penetration of lead through cuticle membrane from leaves and fruits showed that less than 1 percent of the lead is able to penetrate the cuticular barrier. Whether atmospheric lead can be absorbed through the stomata of the leaf and translocated to other parts of the plant has not been determined, although lead isotopic studies should be able to resolve this question. Chow (1970) has shown that the isotopic composition characteristic of gasoline lead is of greater abundance in grass near highways and in surface soils than in soils at depth. Ault, Senechal, and Erleback (1970) studied tree rings and found no significant difference in isotopic ratio (Pbm/Pbm) with age. The ratios in all rings of wood were lower than that of roadside grass or of gasoline lead. The lead abundance in the outermost tree rings (0.34 ppm) was twice that of the 15- and 30-year-old rings (0.15 ppm). Either the uptake of lead was less in the past or lead is removed from the xylem layers after formation—perhaps along the radial ray cells of the woody part of the tree. Algae, mosses, lichens, and others of the lower plant groups, because of their slow growth and unusually high concentrations of metals, have been proposed by several scientists as indicators of lead contamination. Ruhling and Tyler (1968) suggested that mosses growing on trees in urban areas be used as an index for surveying deposition of ' airborne heavy metals. They found that the contents of lead in mosses in southern Sweden had risen from about 25 to 100 ppm from 1860 to 1968. Martinez, Nathany, and Dharmarajan (1971) suggested that Spanish moss, which is not a true moss but a highly specialized epiphytic angio- sperm, be used as a sensor for lead in the Gulf States; they were able through this medium to establish gradients from line-and-point sources to pollutants. Rains (1971) found a particular species of Avena fatua (wild oats) to be highly tolerant of lead and suggested its use as an indicator. Two sources of pollution-caused lead in the atmos- phere that have not been mentioned in previous sections have been reported in British Columbia and England. Warren, Delavault, Fletcher, and Wilks (1971) described an episode in Richland, B.C., in 1970, when dust falling in a particular area of the town was found to contain 4.2 percent lead, soil within a radius of a few hundred feet contained as much as 4,000 ppm lead, and forage grown in this locality contained from 1.5 to 8.5 percent lead in the ash. Lead in unwashed vegetables contained as much as 6.5 ppm in wet weight but a maximum of 0.6 ppm after washing. Although, for the most part, the lead is not absorbed into the plant tissue, the lead on the plants was being consumed by cattle. The industrial plant credited 70 with being the source of the lead pollution has been closed voluntarily until control devices can be installed. Dunn and Bloxam (1932) reported on livestock deaths in England, which, although widely separated, were all on pastures near coke ovens. Soil downwind from the ovens contained as much as 7 ppm lead and grass contained as much as 46 ppm in the dry weight. Soils and grass upwind from the ovens had considerably lower lead contents. Further analysis showed the chimney soot to contain 342 ppm lead and the flue soot, 68 ppm. SUMMARY Plants absorb available lead from soils. Natural lead in plants growing in uncontaminated and unmineralized areas averages 2 ppm in the dry weight. As much as 350 ppm lead has been measured in the dry weight of vegeta- tion rooted in soils developed from rocks containing lead ores; concentrations of as much as 664 ppm have been reported in vegetation subjected to the industrial and agri- cultural activities of man. A large percentage of lead in soil is not available to plants, and only in mining areas where high lead levels in soils occur is the growth of plants seriously affected. Of greater environmental significance is atmospheric lead from lead smelters, other types of industrial plants, and automobile emissions. Concen- trations of lead in or on plants exposed to contaminated air decrease logarithmically with distance from the source. There is no clear evidence that lead from the atmosphere enters into the plant, except in the case of Spanish moss, which is highly specialized for absorbing atmospheric elements into the plant tissue. Washing and preparation of vegetables reduces the lead content to levels acceptable for human consumption, with the possible exception of lettuce and other leafy vegetables that are consumed raw; however, atmospheric lead in or on forages remains a hazard to livestock. Lead levels are increasing markedly and should be monitored on a continuous basis in urban and industrial areas. REFERENCES CITED Alloway, B. J., and Davies, B. E., 1971, Heavy metal content of plants growing on soils contaminated by lead mining: Jour. Agr. Sci. [Cambridge], v. 76, p. 321-323. Ault, W. U., Senechal, R. G., and Erlebach, W. 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L., and Arvik, J. H., 1972, Lead in soils and plants: Sym- posium on Environmental Chemicals—Human and Animal Health, Fort Collins [Colorado] 1972 (sponsored by U.S. Environ- mental Protection Agency and Inst. Rural Environmental Health, Colorado State Univ.), Proc., p. 33—40. LEAD IN THE ATMOSPHERE, NATURAL AND ARTIFICIALLY OCCURRING LEAD, AND THE EFFECTS OF LEAD ON HEALTH By H. L. CANNON The amount of lead introduced into the environment from natural sources appears to be small compared to that made available in one way or another by man. This lead, in turn, is being absorbed by man at rates estimated by Patterson (1965) to be 30 times higher than inferred natural rates. Any discussion of lead in the environment must consider, therefore, the sources and effects of contamination lead in the world today. We are all exposed to lead in the air we breathe and in the food and water we ingest; furthermore, workers in particular industries and residents of particular areas are also exposed to occupa- tional and industrial lead. NATURAL ATMOSPHERIC LEAD The amount of lead from natural sources that is released into the atmosphere is small, as shown by the following estimates by Patterson (1965): Source Lead (pg/m3) Silicate dust from soils ...................................... 0.0005 Volcanic halogen aerosols .............. 00003 Volcanic silicate smoke ................... 000006 Forest fire smoke ......................... .000006 Aerosolic sea salts ............................... .000001 Meteorites and meteoritic smoke ....................... .000000002 The calculation of these values involved many assump- tions, and the total of about 0.0005+ u g/m3 in the atmos- pheric reservoir is probably low. One source of lead in the atmosphere that may be important in mineralized areas is the exudate from vegetation. Curtin, King, and Mosier (1974) showed that heavy metals, including lead, are given off in vapor by vegetation; lead in condensed tree exudate measured from 1 to 12 ppb. Although the exudate in non- mineralized areas has a much lower lead content, the overall contribution is considerable. A small amount of lead is also released from the decay of radioisotopes. Inasmuch as emanations from the volcano, Mauna Loa, Hawaii, affect the general circulation of the trade winds, Kruger (1969) measured Pb210 levels as a means of studying atmosphere chemistry and transport processes. The comparatively large values of Pb210 in Hawaii indicate that little is removed from the atmosphere during the trade wind transport over the open ocean. The release of Pb210 from radon decay has been estimated at 2260le12 curies per m2 per year for annual rainfall in Great Britain, with a mean air residency time of about 4 weeks (Hill, 1960). Because of man’s widespread contamination of the air with lead throughout the world, data on natural levels of lead in the atmosphere are difficult to obtain. Certainly, in noncontaminated areas of the world, lead in the atmosphere averages less than 0.01 ttg/m3 and perhaps less than 0.001 ug/mS. A median lead value of 0.019 ug- m3 was reported by the US. National Air Pollution Control Administration (1968) from 29 nonurban collec- tions made in 1965; likewise, no significant lead (< 0.001 p g/m3) was detected in the fourth quarter of that year at six stations in remote areas of the country, using especially sensitive methods of analysis. LEAD CONTAMINATION OF THE AIR Although the quantity of lead ingested from food and water is much higher than that taken in from urban air, inhaled lead is much more readily absorbed; probably some 20—40 percent of lead absorbed is derived from the air (Carroll, 1970). In general, the higher the atmospheric level the higher the blood level of lead and, presumably, the total body burden. The differences in the concen- trations of lead in surface air collected by the Atomic Energy Commission along their 80th Meridian network in 1967 are shown in table 34 (Harley, 1970). In data compiled by McCaldin (1966), mean atmospheric lead concentrations for particular sets of samples ranged from 0.3 ng/m5 of lead in 149 samples from suburban stations of the National Air Sampling Network to 44.5 ng/m3 in 7 samples collected from Boston’s Sumner Tunnel. Lazrus, Lorange, and Lodge (1970) reported on atmospheric precipitation samples collected from a nationwide network of 32 stations in the United States. The lead values averaged for each station ranged from 0 to 138 g of lead deposited on 1 hectare by 1 cm of precipitation. An increase with time in lead in the atmosphere has been shown by Murozami, Chow, and Patterson (1969) who studied the annual ice layers in dated sections in northern Greenland. The lead content of the ice was less than 0.0005 ug/kg in 800 B.C.; 0.01 p g/kg in 1750, at the beginning of the Industrial Revolution; 0.07 ug/kg in 73 74 LEAD IN THE ENVIRONMENT TABLE 34.—Lead concentrations in surface air in 1967 from selected sites along the 80th Meridian [Data from U.S. Atomic Energy Commission (cited in Harley, 1970)] Site sampled Latitude Lead (Hg/m”) Thule, Greenland .............................................. 70° N. <0.01 Moonsonee, Canada.. ..... 56° N. 0.06 New York, N.Y ........... 41° N. 2.5 Sterling, Va .......... 39° N. .74 Miami, Fla ......................................................... 26° N. 1.7 Bimini, Bahama Islands.. ..... 26° N. 0.10 San Juan, P.R..... . 18° N. 0.80 Balboa, Panama .......... 9° N. 0.23 Guayaquil, Ecuador.. ..... 2° S. 0.35 Lima, Peru ......................................................... 12° 5. 0.50 Chacahaya, Peru ................................................ 16° S. 0.09 Antofagasta, Chile. ..... 24° 5. 0.06 Santiago, Chile .............. 33° S. 0.87 Punta Arenas, Chile ........................................... 53° S. 0.06 1940; and 2.5 u g/kg in 1965. Lead contents of Antarctic ice are much lower, probably because much less lead is introduced by man into the air of the southern hemisphere than into that of the northern hemisphere. Estimates of the residence time of lead in air range from 1 to 4 weeks depending on the size of the particulate matter with which the lead is associated (Lagerwerff, 1972). Measurements of aerial concentrations of Pb210 show an upward transport of the finer particulates into the tropo- sphere and beyond. The aerosols then travel toward the poles, where they become concentrated and sink again into the atmosphere. The cycle is closed by surface air currents which move toward the equator. The most significant contribution of lead to the atmosphere is from the combustion of leaded gasoline, as table 35 shows. According to TerHaar and Bayard (1971) the lead originates in the antiknock fluid, which contains PbBrz, PbBrCl, Pb(OH)Br, (PbO)2PbBr2, and (PbO)2PbBrCl. After burning, the compounds recombine, so that after 18 hours 75 percent of the bromides and 30—40 percent of the chlorides disappear, and the proportion of lead carbonates and lead oxides are increased. A study of the aerosols in Fairbanks, Alaska, by Winchester, Zoller, TABLE 35.—Lead emissions in the United States, 1968 [Data from U.S. Environmental Protection Agency (cited in National Research Council, 1972)] Lead emitted Emission source (tonnes/yr) (tons/yr) Gasoline combustion ......................................... 181,000 199,000 Coal combustion ..... 920 1,000 Fuel oil combustion ............. 24 26.5 Lead alkyl manufacturing... 810 890 Primary lead smelting ....................................... 174 192 Secondary lead smelting .................................... 811 892 Brass manufacturing ............ 521 573 Lead oxide manufacturing .. .. 20 22 Gasoline transfer ................................................ 36 39.5 Total ........................................................ 184,316 202,635 Duce, and Benson (1967) showed that although the weight ratio Clsz averages close to that of ethyl fluid (0.34) the Br:Pb ratio is 4 times less than that of ethyl fluid (0.39). This information supports the theory that lead halide particles, formed initially by combustion of ethyl fluid, lose bromine by oxidation and volatilization. The extent of the lead contribution to the atmosphere from ethyl gasoline was proved by Chow and Johnstone (1965), who determined that the isotopic composition of the lead from ethyl gasoline and of aerosols from the Los Angeles basin were the same. The lead from car exhaust was identified also in mountain snowsland in the surface waters of the ocean. This identification followed a joint study by the U.S. Public Health Service and the petroleum industry of the lead in the atmosphere of three cities—Cincinnati, Philadelphia, and Los Angeles—which was conducted from June 1961 to May 1962 (Ludwig and others, 1965), during which time 3,400 samples were collected from 20 stations. They found an average concentration of 1.4 ng- m3 in Cincinnati, 1.6 ng/m3 in Philadelphia, and 2.5 ng- m3 in Los Angeles. Concentrations as high as 44 ng/m3 were measured in a vehicular tunnel. The highest concen- trations were found during the autumn and winter months and in the early morning hOurs. Other studies in the United States and elsewhere have substantiated these findings. Temperature inversion phenomena in San Diego, which prevent the vertical movement of air above the base of the inversion layer, were studied by Chow and Earl ( 1970). Although pollution in San Diego is relatively low compared to that of other cities, average weekly lead concentrations of as much as 8 ng/m3 were measured. A study by Daines, Motto, and Chilko (1970) has shown that a relationship exists between the amount of lead in the air and traffic volume, proximity to the highway, engine acceleration, and wind direction. They also found that traffic markedly affected lead content of air only in a relatively narrow zone along the highway, with the lead content decreasing more than 50 percent in the interval from 10 to 150 feet (3 to 46 m). Ault, Senechal, and Erlebach (1970) demonstrated that there are significant differences in lead isotopic ratios found in rock, soils, grasses, tree leaves, normal air particulate, and those found in coal, fly ash, gasoline, and fuel oil. The Pb205/ Pb204 ratio of 18.2 in air within 500 feet (152 m) of a turnpike approached that of gasoline (18.3), whereas at greater distances the ratio averaged 18.7. A less serious source of atmospheric lead is in fly ash which originates mainly from the burning of coal by power plants, steel mills, cement plants, and other industries. In a recent U.S. Geological Survey study of the coal and fly ash produced by power plants in the South- west Energy Program, it was found that coal being produced for 10 power plants had a median content of 50 ppm lead in ash (73 samples), with a range of 30—200 ppm. Bottom ash had a median content of 30 ppm, with a range LEAD IN THE ATMOSPHERE AND THE EFFECTS ON HEALTH 75 of 20—30 ppm; fly ash had a median content of 50 ppm lead with a range of 20—70 ppm. At the Four Corners plant, in San Juan County, N. Mex., fly ash collected in the stack was compared with the effluent falling on vegetation and soil of the surrounding area. The fly ash in the stack averaged 70 ppm lead and the effluent only 20 ppm. Analyses of the top one-half inch of soil along two 6.5-mile (10.4-km) traverses from the power plant indicated, for 1.5 miles (2.4 km), a lead content above the 20 ppm reported by Shacklette, Hamilton, Boerngen, and Bowles (1971) as average for US. soils, but the lead value at no point was greater than 30 ppm (Cannon and Anderson, 1972). A major but relatively local source of atmospheric lead is the effluent from mining and smelting operations. Djurié, Kerin, Graovac-Leposavic, Novak, and Kop ( 1971) reported on a detailed study of lead levels in air, water, plants, and urine in the environs of a lead smelter in Yugoslavia. The smelter lies in a basin, has been operative since 1746, and currently produces 22,000 tons (19,800 t) of lead annually. The stack had no filtering devices until 1969. Samples of air collected in 1967 from three villages in the valley had a lead content ranging from 1.3 to 84.0 ng/ m3, with considerable variation on different sampling dates. Lead contents in soils ranged from 91 to 24,880 mg / kg, compared with 0.8—374 mg/kg in uncontaminated soil. The river‘that received effluent from the smelter ranged in lead content from 685.12 mg/l at the smelter to 2.89 mg/l 22 km below the smelter. Above the smelter the river contained only 0.001—0.02 mg/l. Analyses of food and forage show relatively low lead levels in peeled produce but high levels of lead in lettuce and hay; the lead levels in hay are toxic to cattle if the hay is used exclusively and continuously as forage. Milk, meat, and honey were also contaminated, the lead content in milk being 40 times the national average in the United States. Lead in the environ- ment is being absorbed by the inhabitants at levels sufficient to induce a physiologic response. A study has been made of the distribution of lead around two smelters in Toronto, Canada, and of blood lead levels in the local families. The lead fallout was shown to originate from episodic large-particulate emissions from near-ground sources rather than from stack fumes. Between 13 and 30 percent of the children living in the contaminated areas had absorbed excessive amounts of lead, but probably the lead was ingested from contaminated dusts rather than from drinking water, home-grown vegetables, air, or paint pica. Of the children who had absorbed excessive lead, most of those selected for special study showed metabolic changes; 10—15 percent of this selected group showed subtle neurological dysfunctions and minor psychomotor abnormalities (Roberts and others, 1974). Cases of lead poisoning have been reported among workers and residents near a lead smelter in India where the daily flue emissions contained almost 2,000 lb (900 kg) of lead; in Colombia, families living in lead foundries exhibit chronic symptoms, and many children die from lead poisoning (Haley, 1969). Lead poisoning in 75 children in Smeltertown, an area of 120 families near El Paso, Tex., where lead has been smelted since 1887 (Wall Street Journal, 1972a, b), has recently shown the urgency of the problem in this country. An intensive monitoring program has been initiated to study the effects on the environment of the so-called New Lead Belt (Wixson and others, 1972) in Missouri, which accounted for more than 80 percent of the total United States lead production in 1973. Lead levels in soils, air, plants, and water are being carefully checked and the effects of lead pollution on grazing animals are being studied. Long—term studies should contribute much to our knowledge of the long-range effects of low-level lead contamination on the health of animals, including man. Cigarettes represent a minor but common source of lead. Although most of the lead remains in the ash, an intake of 20 ng per pack from the smoke has been estimated by Horton (1966). Studies by the Air Pollution Control Administration have demonstrated that in urban areas smokers have 1.24 times more blood lead than nonsmokers and in rural areas, 1.42 times more blood lead (Hofreuter, 1960). LEAD CONTAMINATION OF WATER AND FOOD Pollution lead may be ingested from several sources. A main source of lead poisoning in underprivileged children is pica, or an abnormal craving to ingest odd things including fragments of lead-based paint from cribs and walls (Haley, 1969). A decrease in plumbism from this source can be expected as current laws ordering the substitution of titanium-based paints on nursery furniture become effective. Cooking in earthenware pottery with a lead glaze or the use of such pottery as containers for acid foods can be a cause of lead poisoning (Klein and others, 1970). Vegetables and fruits are easily contaminated by lead-arsenate sprays and, although less lead arsenate is used than formerly, a buildup of the metals occurs in the soils of old orchards where a reservoir of lead is available to crops long after the use of lead arsenate has ceased. Experi- mental work by Jones and Hatch (1945) showed that crops grown in soil taken from an orchard with a spray record of 22 years assimilated from 1 to 3 times more lead in the above-ground portions and 2 to 8 times in the below- ground portions than the same crops grown in unsprayed soil; a maximum lead value of 35 ppm in dry weight of peeled eggplant was recorded. Lead may also be introduced into cultivated land through the use of tailings from lead-zinc mines or of high-lead limestones for fertilizer. Warren, Delavault, and Cross (1966) have reported on the use of a limestone containing 45 ppm lead for fertilizer on land which then produced oats containing 3—4 ppm lead in the dry weight. 76 LEAD IN THE ENVIRONMENT The soil and vegetation in the vicinity of mining operations (even though long defunct) may be contaminated by waste and spoil piles. The lower part of the Tamar Valley district of west Devon and Cornwall in Great Britain is a richly mineralized area which was mined for base metals in the 19th century. Soils from pastures and gardens contain unusually high contents of lead; garden soils had a maximum of 522 ppm total lead and 36 ppm available lead (extractable with acetic acid) (Davies, 1971 ). Radishes were used as a convenient test crop to measure actual uptake. They contained as much as 74 ppm lead in the ash. Davies (1972) concluded that the inhabitants of this valley are exposed to a greater lead insult than similar populations elsewhere in Great Britain. Drinking water standards have been set by the US. Public Health Service at 0.5 mg/l maximum for lead. Although ground-water lead levels have remained relatively unaffected, man has increased lead in surface waters through agricultural practices, fallout of lead alkyl aerosols, contact with scrap and waste heaps, and mining activities. However, according to Patterson (1965), in rural areas the atmospheric washout of lead alkyl products should immediately be fixed in the clay fractions of the soil and thus make little contribution to the surface waters except through storm sewers. Sewer treatment plants contribute lead to surface waters, although about 65 percent of the lead in drinking water is removed in treat- ment processes, according to Horton (1966). He stated that lead is frequently present in unfiltered surface waters at levels well above the maximum permissible concen- tration but is generally much lower in filtered water. Analyses of 42 sewage sludges from rural and industrial towns in England and Wales show very large concen- trations of certain trace elements. The sludges contained from 120 to 3,000 ppm lead and had a median content of 700 ppm in dry weight (Berrow and Webber, 1972). Contributions of lead to ground and surface waters from rainfall can range as high as 0.49 mg/l in areas of industrial contamination or high traffic volume. However, Lazrus, Lorange, and Lodge (1970) have shown that there is, on the average, twice as much lead in atmospheric precipitation as in municipal water supplies, suggesting that the lead precipitates or is adsorbed on suspended matter and is thus filtered out in processing. Ettinger ( 1966) showed maximum lead in surface water to be associated with metal-working and chemical industries, but concluded that only a fraction of the lead in surface water remains dissolved. Lead piping and lead solder used with copper tubing may also be a significant source of lead in water distribu- tion systems. According to Patterson (1965) many of our largest cities use lead pipe exclusively for water service connections. She has estimated a per capita contribution of 5 pg/l of lead from lead used in water distribution systems. EFFECTS OF INGESTED LEAD ON LIVESTOCK In animals, the toxic effects of lead ingested in contaminated vegetation have often been noted. Horses, especially young ones, are particularly susceptible to lead poisoning. Chronic lead poisoning in six foals on three different properties within 5 miles (8 km) of the Trail smelter in British Columbia and subsequent poisoning of introduced horses has been described by Schmitt and others (1971 ). The symptoms were loss of weight, muscular weakness, stiffness of joints, and harsh, dry coats. A thorough investigation showed that young horses were more susceptible than old horses, and that lead rather than zinc, arsenic, or cadmium was the cause of the illness. Soil, forage, and blood levels of the horses on the three properties were as follows: Properly Distance from Lead in top Lead in forage Mean lead in smelter 1 inch soil (ppm dry weight) blood (miles) (ppm) my 100 g) A 2.0 750 38 22.6 B 1.5 620 90 31.6 C .5 2,900 264 37.6 In all but a few fruits and vegetables grown within 5 miles (8 km) of the smelter, lead concentrations were considered to be acceptable for human consumption, and human- urine lead levels were not elevated. The death of four horses that had been corralled on the tailings pond of an old gold mine near Evergreen, C010,, was investigated by D. M. Sheridan and H. L. Cannon of the US Geological Survey (unpub. data, 1970). The drinking water for the horses contained high concen- trations of fluorine but the autopsy report showed lead poisoning. A noxious weed, Grindelia squarrosa, evident in the hay fed to the horses, which had been cut at a city country club, was subsequently found to contain 1,540 ppm lead in the dry weight. Inasmuch as the grass in the hay contained only 5.2 ppm, Grindelia appears to be an accumulator of toxic levels of lead. Horses subsequently died in a pasture near Centerville, Mo., where 15 samples of cedar were reported by Connor, Shacklette, and Erdman (1971) to contain as much as 1,200 ppm lead in the dry weight, with a geometric mean of 348 ppm dry weight. The area was believed to have been contaminated by lead from ore trucks. An outbreak of acute lead poisoning among cattle pastured near a lead smelter was studied by veterinarians from the University of Minnesota (Hammond and Aronson, 1964). The forage contained 148—285 ppm lead in the dry weight. Acute bovine lead poisoning resulted from long-term continuous intake of contaminated forage, whereas horses in the same environment responded by long-delayed onset of the classical syndrome of chronic equine plumbism. These and other case histories suggest that airborne lead absorbed by and adhering to plants may be more hazardous to grazing animals than to man. LEAD IN THE ATMOSPHERE AND THE EFFECTS ON HEALTH 77 OCCUPATIONAL LEAD Plumbism from occupational lead used in automobile body manufacture and repair, foundries, electrotype printing, storage battery manufacture, secondary smelters, and munition plants was a hazard in this country before 1945. Improved occupational practices since that time and the introduction in 1945 of safe working standards for lead levels in air and urine have been responsible for decreasing exposure to airborne lead in dust and fumes by several orders of magnitude (Stokinger, 1966). In a study of workers who are regularly exposed to gasoline additives, it was found that garage mechanics show elevated lead levels in urine, but the levels are below the present acceptable limits. Although inhalation intoxication and skin absorp- tion have caused problems in other countries (Haley, 1969), controls instituted in the United States have resulted in safe handling of lead alkyls, tetraethyl lead, and tetra- methyl lead during manufacture. Nevertheless, after the manufactured products leave the plant, they may still pose a hazard through careless handling by the consumer; for instance, acute plumbism among children has been caused by burning old battery cases for fuel in fireplaces and cookstoves (National Research Council, 1972). The contributions of lead to the environment that result from man’s activity are much greater, therefore, than the lead that is available in the environment from natural sources. Considerably improved controls and monitoring programs are indicated. ABSORPTION AND EFFECTS OF LEAD ON HEALTH The human body tends to discriminate against the heavier metals in favor of the lighter, using the lighter in metabolic processes and being poisoned by the heavier (Patterson, 1965). The chemistry of lead is similar to that of barium in its behavior in the body, particularly in regard to deposition in and mobilization from the skeleton. Lead in animals (including man) is concentrated largely in bone, although small amounts of lead occur in other types of tissue. Amounts that have been reported are as follows (Haley, 1969): Tissue Lead (fig/100 g) Long bone .......................................................... 670—3590 Flat bone .............. 210—l,110 Liver ......... 40— 280 Kidney .. 15— 160 Muscle .. 10— 170 Brain .................... 10— 90 Bone represents the greatest storage site of lead, which is bound to bone by absorption or exchange at the surfaces of bone salt crystals, or is combined with the sulfate in the organic matrix. However, according to work by Kehoe (1961), the average individual appears to be in lead balance; the body burden of lead is established early in life and does not change appreciably throughout the life span. Haley (1969) reported lead concentrations in blood from 10 to 26 ug/ ml; however, the lead levels of blood can be raised experimentally to as much as 450 u g/ 100 g by heavy dosages in food and air, with accompanying increases in urinary and fecal lead (Kehoe, 1964). Lead interferes with normal maturation of erythroid elements in the bone marrow and inhibits hemoglobin synthesis in precursor cells (Kench and others, 1952). Lead also affects prophyrin metabolism and interferes with the activity of several enzymes. Although less than 10 percent of lead ingested by man is 'absorbed, 20—25 percent of respiratory intake may be retained and absorbed. Epidemiologic studies of blood lead levels show a logarithmic regression on estimated atmospheric exposure, according to Goldsmith and Hexter (1967), who postulated that further increases in atmospheric lead will result in predictable, progressively higher lead levels in the population. In a group of workers who were exposed to industrial lead and who had blood lead levels of 0045—0. 14 mg/ 100 g, the serum transaminase activity was increased, suggesting that subclinical liver impairment may occur at levels below the “safe” level of 0.07 mg/ 100 g (Hofreuter, 1960). Experiments by Schroeder, Vinton, and Balassa (1963) with lead-fed rats showed increased mortality rates in both sexes and renal and hepatic accumulations of lead similar to those in adult humans. However, there is evidence that the average intake of lead from food has not changed from its level during the three-decade period prior to 1965, because the increase in environmental lead in food has been balanced by the decrease in use of lead in agricultural chemicals and in food processing and packaging (Lewis, 1965). In children, overt manifestations of acute lead poisoning, plumbism, differ from those of adults, and the mortality rate is relatively high. Symptoms may include anemia, gastrointestinal distress, and encephalopathy. The last may result in early death, permanent symptoms of brain damage, or complete recovery (Haley, 1969). Adult or chronic lead poisoning requires years to develop to the critical level, which is recognizable by symptoms such as headache, muscle pains, constipation, abdominal tenderness, colic, weight loss, and fatigue (Johnstone, 1964). Kidney damage from lead has not been identified in the United States although it is commonly reported in European countries; this difference may be due to poorer industrial hygiene in European countries. Renal lesions, which may be caused by lead ingestion, have been shown to lead to hypertension and may also predispose to gout as a result of defective urate secretion (Emmerson, 1965). An important symptom of chronic plumbism is the neuro- muscular involvement, usually of the extensor muscles, as in wrist drop. Chronic exposures to lead in the water supply have been shown to result in increased mis- carriages and stillbirths; the pregnant woman and her 78 LEAD IN THE ENVIRONMENT fetus are highly susceptible to lead poisoning (Wilson, 1966). The poisoning of industrial workers with organic lead alkyls—tetraethyl or tetramethyl lead—causes a different set of symptoms called the acute brain syndrome. These symptoms include irritability, insomnia, emotional instability, tendon reflexes, and tremor. Delusions and hallucinations may occur (Sanders, 1965). However, lead alkyls decompose rapidly and are largely handled physio- logically as inorganic water-soluble lead (Tepper, 1966). Although, according to Haley, there are no documented cases of lead poisoning in children or adults attributable to airborne lead from the combustion of leaded gasoline, continuous monitoring has been advised and realistic lead tolerance levels advocated after further study. REFERENCES CITED Ault, W. U., Senechal, R. G., and Erlebach, W. E., 1970, Isotopic composition as a natural tracer of lead in the environment; Environmental Sci. and Technology, v. 4, no. 4, p. 305—314. Berrow, M. L., and Webber, J., 1972, Trace elements in sewage sludges: Jour. Sci. Food and Agriculture [London], v. 23, p. 93—100; abs. in Chem. Abs., v. 76, no. ll7245u, 1972. Cannon, H. L., and Anderson, B. M., 1972, Trace-element content of the soils and vegetation in the vicinity of the Four Corners Power Plant, Pt. 3 of Coal resources work group report: U.S. Dept. Interior, Southwest Energy Study, open-file report, 44p. Carroll, R. E., 1970, Trace element pollution in air, in Hemphill, D. D., ed., Trace substances in environmental health—3, [3d Ann. Conf. Proc.]: Columbia, Missouri Univ., p. 227-231. Chow, T. J., and Earl, J. L., 1970, Lead aerosols in the atmosphere— Increasing concentrations: Science, v. 169, p. 577—580. Chow, T. J., and Johnstone, M. S., 1965, Lead isotopes in gasoline and aerosols of Los Angeles Basin, California: Science, v. 147, p. 502—503. Connor, J. J., Shacklette, H. T., and Erdman, J. A., 1971, Extra- ordinary trace-element accumulations in roadside cedars near Centerville, Missouri, in U.S. Geological Survey research 1971: U.S. Geol. Survey Prof. Paper 750—B, p. 8151—3156. Curtin, G. C., King, H. D., and Mosier, E. L., 1974, Movement of elements into the atmosphere from coniferous trees in subalpine forest of Colorado and Idaho: Jour. Geochem. Explor., v. 3, no. 3, p. 245—263. Daines, R. H., Motto, Harry, and Chilko, D. M., 1970, Atmospheric lead—Its relationship to traffic volume and proximity to highways: Environmental Sci. and Technology, v. 4, no. 4, p. 318—322. Davies, B. E., 1971, Trace metal content of soils affected by base metal mining in the West of England: Oikos [Copenhagen], v. 22, no. 3, p. 1—7. 1972, Occurrence and distribution of lead and other metals in two areas of unusual disease incidence in Britain: Internat. Sym— posium on Environmental Health Aspects of Lead, Amsterdam 1972, Proc., p. 125—134. Djuric', Dusan, Kerin, Zarka, Graovac-Leposavic, Ljubica, Novak, Ljiljana, and Kop, Marija, 1971, Environmental contamination by lead from a mine and smelter: Archives Environmental Health, v. 23, no. 4, p. 275—279. Emmerson, B. T., 1965, The renal excretion of urate in chronic nephro- pathy: Australian Annals of Medicine, v. 14, p. 295—303. Ettinger, M. B., 1966, Lead in drinking water: Symposium on Environ- mental Lead Contamination, 1965, U.S. Public Health Service Pub. 1440, p. 21—27. Goldsmith, J. R., and Hexter, A. C., 1967, Respiratory exposure to lead—Epidemiological and experimental dose-response relation- ships: Science, v. 158, no. 3797, p. 132—134. Haley, T. J., 1969, A review of the toxicology of lead: Am. Petroleum Inst. Air Quality Mon. 69—7, 53 p. Hammond, P. B., and Aronson, A. L., 1964, Lead poisoning in cattle and horses in the vicinity of a smelter: Conf. Veterinary Medicine, 1963, New York Acad. Sci. Annals, v. 111, no. 2, p. 595—611. Harley, J. H., 1970, Discussion of sources of lead in perennial ryegrass and radishes: Environmental Sci. and Technology, v. 4, p. 225. Hill, C. R., 1960, Lead-210 and polonium-210 in grass: Nature, v. 187, p. 211—212. Hofreuter, D. H., 1960, Evaluating the health hazards of exposure to lead and carbon monoxide: Air Pollution Medical Research Conf., San Francisco 1960 [Proc.]; available from U.S. Dept. Health, Education, and Welfare, Public Health Service, Air Pollu- tion Div., Cincinnati, Ohio 45226. Horton, R. J. M., 1966, Major sources of lead pollution: Symposium on Environmental Lead Contamination, 1965, U.S. Public Health Service Pub. 1440, p. 137—142. Johnstone, R. T., 1964, Clinical inorganic lead intoxication: Archives Environmental Health, v. 8, no. 2, p. 250—255. Jones, J. S., and Hatch, M. B., 1945, Residues and crop assimilation of arsenic and lead: Soil Sci., v. 60, p. 277—288. Kehoe, R. A., 1961, The metabolism of lead in man in health and disease: Harben Lecture, no. 1, 1960, 18 p.; reprinted in Jour. Royal Inst. Public Health and Hygiene, v. 24, p. 81—97. 1964, Metabolism of lead under abnormal conditions: Archives Environmental Health, v. 8, no. 2, p. 235—243. Kench, J. E., Lane, R. E., and Varley, H., 1952, Urinary coproporphy- rins in lead poisoining: British Jour. Industrial Medicine, v. 9, p. 1334137; abs. in Chem. Abs., v. 46, no. 11450h, 1952. Klein, Michael, Namer, Rosalie, Harpur, Eleanor, and Corbin, Richard, 1970, Earthenware containers as a source of fatal lead poisoning: New England Jour. Medicine, v. 283, no. 13, p. 669—672. Kruger, Paul, 1969, Lead-210 in surface air along the slopes of Mauna Loa volcano, Hawaii: U.S. Atomic Energy Comm. Rept. SU-326- PA-16-3, 24 p.; available only from U.S. Dept. Commerce Natl. Tech. Inf. Service, Springfield, Va. 22161. Lagerwerff, J. V., 1972, Lead, mercury and cadmium as environmental contaminants, Chap. 23 in Micronutrients in agriculture: Madison, Wis., Soil Sci. Soc. America, p. 593—636. Lazrus, A. L., Lorange, Elizabeth, and Lodge, J. P., Jr., 1970, Lead and other metal ions in United States precipitation: Environ- mental Sci. and Technology, v. 4, no. 1, p. 55—58. Lewis, K. H., 1965, The diet as a source of lead pollution: Symposium on Environmental Lead Contamination, 1965, U.S. Public Health Service Pub. 1440, p. 17—20. Ludwig, J. H., Diggs, D. R., Hesselberg, H. E., and Maga, J. A., 1965, Survey of lead in the atmosphere of three urban communities, a summary: Am. Indus. Hygiene Assoc. Jour., v. 26, p. 270-284. McCaldin, R. 0., 1966, Estimation of sources of atmospheric lead and measured atmospheric lead levels: Symposium on Environmental Lead Contamination, 1965, U.S. Public Health Service Pub. 1440, p. 7~l5. Murozami, M., Chow, T. J., and Patterson, C. C., 1969, Chemical concentrations of pollutant lead aerosols, terrestrial dusts, and sea salts in Greenland and Antarctic snow strata: Geochim. et Cosmochim. Acta, v. 33, p. 1247—1294. National Research Council, Division of Medical Sciences, Committee on Biological Effects, 1972, Lead—Airborne lead in perspective: Natl. Acad. Sci., 330 p. Patterson, C. C., 1965, Contaminated and natural lead environments of man: Archives Environmental Health, v. 11, p. 344—360. Roberts, T. M., Hutchinson, T. C., Paciga, J., Chattopadhyay, A., Jervis, R. E., and VanLoon, J.. 1974, Lead contamination around LEAD IN THE ATMOSPHERE AND THE EFFECTS ON HEALTH 79 secondary smelters—Estimation of dispersal and accumulation by humans: Science, v. 186, p. 1120—1122. Sanders, L. W., 1965, Lead excretion and health of antiknock blenders: Archives Environmental Health, v. 10, no, 6, p. 886—892. Schmitt, Nicholas, Brown, Gordon, Devlin, E. L., Larsen, A. A., McCausland, E. D., and Savile, J. M., 1971, Lead poisoning in horses, an environmental health hazard: Archives Environmental Health, v. 23, no. 3, p. 1855-195. Schroeder, H. A., Vinton, W. H., Jr., and Balassa, J. J., 1963, Effects of chromium, cadmium, and lead on the growth and survival of rats: Jour. Nutrition, v. 80, no. 1, p. 48—54. Shacklette, H. T., Hamilton, J. C., Boerngen, J. C., and Bowles, J. M., 1971, Elemental composition of surficial materials in the con- terminous United States: U.S. Geol. Survey Prof. Paper 574—D, 71 p. Stokinger, H. E., 1966, Recent history of lead exposure in U.S. industry, 1935—1965: Symposium on Environmental Lead Contamination, 1965, U.S. Public Health Service Pub. 1440, p. 29—35. Tepper, L. B., 1966, Under what circumstances is direct contact with lead dangerouSP: Symposium on environmental lead contamina- tion, 1965, U.S. Public Health Service Pub. 1440, p. 59—62. TerHaar, G. L., and Bayard, M. A., 1971, Composition of airborne lead particles: Nature, v. 232, p. 553—554. U.S. National Air Pollution Control Administration, 1968, Air quality data from national air surveillance networks and contributing State and local networks [1966 ed.]: U.S. Natl. Air Pollution Control Admin. Pub. APTD 68—9, 157 p. Wall Street Journal, 1972a, El Paso mayor seeks Federal aid in area hit by lead poisoning: New York, Wall Street Jour., March 24, 1972. 1972b, Lead-poisoning issue eclipses pollution suit fought against Asarco: New York, Wall Street Jour., March 31, 1972. Warren, H. V., Delavault, R. E., and Cross, C. H., 1966, Mineral con- taminations in soil and vegetation and its possible relations to public health, in Pollution and our environment: Canadian Council of Resources Ministers Natl. Conf. [Montreal], Back- ground Paper A3—3, 11 p. Wilson, A. T., 1966, Effects of abnormal lead content of water supplies on maternity patients: Scottish Medical Jour., v. 11, p. 73—82. Winchester, J. W., Zoller, W. H., Duce, R. A., and Benson, C. S., 1967, Lead and halogens in pollution aerosols and snow from Fairbanks, Alaska: Atmospheric Environment, v. 1, no. 2, p. 105—119. ‘ Wixson, B. G., Bolter, Ernst, Gale, N. L., Jennett, J. C., and Purushothaman, K., 1972, The lead industry as a source of trace metals in the environment: Environmental Resources Conf. on Cycling and Control of Metals, Columbus, Ohio, Battelle Memorial Inst., Proc., 11 p. ANALYTICAL METHODS FOR THE DETERMINATION OF LEAD By F. N. WARD and M. J. FISHMAN DETERMINATION OF LEAD IN SOILS AND ROCKS Methods of chemical analysis with enough sensitivity to measure the low concentrations of lead in soil an rock samples are numerous. Such methods are based on familiar operations of optical emission spectrography, X- ray spectrography, molecular absorption (colorimetry), and atomic absorption. In a review of available methods, a certain amount of judgment has to be exercised in choosing those to be discussed; accordingly, the following discussion will be devoted to the methods most commonly used by the US. Geological Survey. OPTICAL EMISSION SPECTROGRAPHY Optical emission spectrographic methods are used to determine the lead content of soils, rocks, and minerals at the 5- to lO-ppm level, with a relative standard deviation of 10—20 percent. Such precision is attained with a semi- quantitative procedure variously designated as the three- step or six-step semiquantitative spectrographic method. Better precision is attained with strictly quantitative procedures, but the semiquantitative methods are so widely used in the Geological Survey that some discus- sion of them is in order. Quantitative data are achieved by comparing the spectra from unknown soil and rock samples with spectra of standard mixtures obtained under identical conditions of instrumental parameters, sample handling, and so on. The standard spectra are produced from mixtures of different amounts of pure compounds incorporated in matrices whose chemical compositions are similar to those of the host materials of the unknowns. The mixtures of pure compound plus matrix material are accordingly referred to as standard mixtures or simply as standards. Within an order of magnitude, the concentration of lead in standards differs by a common factor, the cube root of 10 (Myers and others, 1961; Barnett, 1961 ). An explanation of the theoretical basis of this common factor is beyond the scope of this paper. It suffices to say that within an order of magnitude a series of lead standards would contain the following amounts of lead: the first standard, 1 percent; the second standard, 0.464 percent; the third, 0.215 percent, and so on. Ward and others (1963) stated that these values could be rounded off respectively to l, 0.5, 0.2, and 0.1 percent. Within an order of magnitude change in lead concen- tration, three standards were used and the method was called a three-step semiquantitative spectrographic method. The differences between the standards were called concentration ranges, and these ranges were rather broad. Many spectrographers felt that by inserting additional standards at the logarithmic midpoints of these ranges they could achieve better precision and accuracy. The method henceforth became known as a six-step semi- quantitative spectrographic method. After using both three-step and six-step procedures for a few years many Survey spectrographers concluded that the value of the additional midpoint standards for obtaining more accurate results was debatable and that the time required to prepare the midpoint standards as well as the extra space required on the photographic plate might offset any advantage. Currently both methods are used in the Geological Survey, although most appraisal studies and exploration programs need only the three-step, or three—standards, procedure. In applying the method, the spectrographer prepares a standards plate having spectral lines produced by lead concentrations of 1.0, 0.5, 0.2, and 0.1 ppm, and he designates the midpoints of the ranges delineated by these standards as 0.7 0.3, and 0.15 ppm, respectively. He compares the spectral line of lead from an unknown with the line produced by one of these standards or, if the unknown lies at a midpoint between two standards, with the range delineated by two standards lines. This range is often called a bracket and the midpoint further specified as the geometric midpoint. Shacklette and others (1971, p. 225) described this method for reporting semiquantitative results as follows. The values obtained by spectrography were reported in geometric brackets having boundaries 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, 0.12 and so forth, percent; the brackets are identified by their respective geometric midpoints, 1.0, 0.7, 0.5, 0.3, 0.2, 0.15, and so forth. Thus, a reported value of 0.3 percent for example, identifies the bracket from 0.26 to 0.38 percent as the analyst’s best estimate of the concentration present. The precision of a reported value is approximately plus or minus one bracket at the 68- 81 82 LEAD IN THE ENVIRONMENT percent level of confidence, and plus or minus two brackets at the 95- percent level. The lead content of the unknown is thus reported with one of the following numbers: 1.0, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, and the reported value for lead will always be one of these numbers expressed as a power of 10 that depends on the relative concentration. Ideally, the composition of the matrix or material to which pure lead compounds are added to form this series of standards should approximate the composition of the samples as closely as possible, especially with regard to major components. These major components cause changes in the sample excitation, which, along with the spectral lines and physical nature of the samples, affect spectral lines and intensities of the lead analyte. These effects are magnified because of the predominance of the matrix in the standard mixture. For highest accuracy, therefore, standards would have to be incorporated in several different kinds of matrices—for instance, gossan, silicate, or carbonate—and ultimately the number of standards would become unwieldy. In practice, a compromise is made, and most lead deter- minations are made on the basis of standards prepared in one or two matrices modified to approximate the vast number of geologic materials analyzed. Further modi- fications—such as the addition of graphite to the unknown sample in a 2:1 ratio—are made to promote uniform excitation and inhibit the effect of major components in the matrix. In these semiquantitatiVe methods, spectra of unknowns are compared visually with spectra of standards, and occasionally faulty comparisons along with possible computational errors give rise to shocking results which usually prove embarrassing. Aware of these possibilities, spectrographers of the US. Geological Survey worked for several years to develop systems in which human participation was minimized and as many operations as possible were performed instrumentally. The problem was twofold. First, they had to make an instrument to read the line intensities recorded on the plates; however, each plate is unique and has to contain built-in flagging so that the instrument will measure the intensity of the most appropriate line or lines of the elements of interest including lead. Second, the Survey spectrographers had to devise computer programs to perform as many computations as possible, including corrections for matrix and background, and present the final readout in a format useful to geologists. The computer program was devised first and is described by Helz, Walthall, and Berman (1969); the instrument development came later and is given by Helz (1973). Sub- sequent applications and improvements are described by Dorrzapf (1973). X-RAY SPECTROGRAPHY In the US. Geological Survey, lead determinations are made routinely by X-ray spectrographic methods. These methods are not generally considered as trace methods, but after chemical pretreatment sensitivities as low as 50 ppm with a relative standard deviation of about 20 percent are achieved. MOLECULAR ABSORPTION (COLORIMETRY) For more than 25 years, trace amounts of lead in soils and rocks have been determined by molecular absorption methods. The common name for this process—colorimetry—is descriptive, but the process is one of absorption. Under specific conditions lead reacts with certain organic reagents—for instance, dithizone—to form colored species which can be extracted into immiscible solvents such as carbon tetrachloride or xylene. The absorption of the colored species by these solvents occurs maximally at discrete frequencies, and the amount of absorption can be related to the concentration of lead in the soil or rock sample. Molecular absorption methods of lead analysis are so numerous that a thorough review is beyond the scope of this paper, but procedures included by Sandell (1959) illustrate some of the available methods. For real samples of igneous rocks the sensitivity of the method is as low as 1 ppm (Thompson and Nakagawa, 1960). Rapid field methods based o®the reaction of lead with dithizone have a sensitivity limit of 20 ppm, which is adequate for reconnaissance studies. ATOMIC ABSORPTION SPECTROMETRY Atomic absorption methods for lead in soils and rocks as used in our laboratory were described by Ward and others (1969). Sensitivities as low as 1 ppm are routinely achieved, and under special conditions, such as by using a Delves spoon, the sensitivity can be lowered by one or two orders of magnitude. Some typical lead values determined routinely by an atomic absorption method are as follows: five repeat deter- minations on a sandstone yielded 12—16 11 g/ g lead, with a relative standard deviation of 12 percent; corresponding values on a granite were 22—24 11 g/ g, with a relative standard deviation of 5 percent. ELECTRON PROBE The electron probe is useful for determining lead concentrations greater than about 0.1 percent. In ordinary soils and vegetation and in many rocks, the lead concen- tration is too low for measurement by the probe. In minerals, however, the local concentration of lead may easily exceed the threshold amount, and the probe is most useful for determining such enrichments. DETERMINATION OF LEAD IN VEGETATION One routine spectrographic method for determining lead in plant ash calls for mixing equal parts of ash with quartz to simulate the silicic matrix in which the standard powders are incorporated. The obvious advantage is that ANALYTICAL METHODS FOR THE DETERMINATION OF LEAD . 83 the same standard plates can be used on plant ash and silicic rock samples. The obvious disadvantage is that the sensitivity given above for lead in soils and rocks by optical emission spectrography is only 10—20 ppm. This sensitivity is adequate for many species of vegetation, but is inadequate for some specific applications. Recently Mosier (1971, 1972) incorporated standard powders in a plant-ash base and, with a split-slit technique using a Hartmann diaphragm and a step filter assemblage, achieved a simultaneous recording of 35 elements including such volatile elements as silver, arsenic, bismuth, antimony, and others. Using this procedure Mosier determined amounts as small as 1 ppm lead in plant ash, with a relative standard deviation of 20 percent. Trace amounts of lead in organic matter can also be measured by analytical methods based on molecular absorption (colorimetry) and atomic absorption. Methods based on X-ray spectrography may also be used after special sample treatment, but many different species contain less than threshold amounts and such X-ray methods are less useful than other procedures discussed here. ' Molecular absorption methods for determining lead in vegetation are similar to those for soils and rocks except in sample size and treatment before analysis. Usually a 2-g sample of air-dried and ground vegetation is used; the minimum lead content that can be determined is near 0.25 ppm. Atomic absorption methods for lead in vegetation are again similar to those for lead in soils and rocks, except in sample size and treatment. Ashing is necessary and can be accomplished either with oxidizing acids or by ignition in a muffle furnace at temperatures of 450°—500°C under controlled conditions to avoid losses. Detection limits for lead of 0.02 p g/ ml are quoted in the literature (Perkin-Elmer Corporation, 1971). Such limits are achieved with conventional atomic absorption tech- niques, but with a flameless procedure, Fernandez and Manning (1971) detected as little as 0.4 [Jg/l. Ward and others (1969) defined sensitivity in more practical terms, and, using a new procedure, they achieved a sensitivity of l u g/ g in plant ash. Five samples of fir ash contain 30—60 u g/ g lead. On the basis of 4 percent ash, these values are equivalent to 1.2—2.4 u g/ g lead in air-dry sample. DETERMINATION OF LEAD IN WATER The literature on analytical methods for the deter- mination of lead in water is voluminous. U.S. Geological Survey personnel have periodically reviewed the literature of analytical chemistry applied to water analysis; the last such review was published in 1973 (Fishman and Erdmann, 1973). These reviews are a good source of references to published descriptions of analytical procedures for lead, and we will not attempt to duplicate these references here. The principal methods used by the U.S. Geological Survey for determining lead in aqueous solutions are atomic absorption spectrophotometry and emission spectroscopy. For example, the analyses for this report, which were performed by U.S. Geological Survey personnel using these techniques, and which are described by M. J. Fishman (this volume), were determined on water samples filtered at the time of collection through 0.45- ;; m-membrane filters and acidified with nitric acid to a pH less than 3. Atomic absorption spectrophotometric methods are being used routinely for the direct determination of many trace elements; however, lead normally occurs in fresh- water at concentrations less than can be directly detected by atomic absorption. Therefore, a preconcentration procedure is essential if lead is to be determined by atomic absorption. Brown, Skougstad, and Fishman (1970) described a rapid simple accurate and sensitive precon- centration procedure in which lead is chelated with ammonium pyrolidine dithiocarbamate (APDC) at a pH of 2.8, and the chelate is then extracted with methyl isobutyl ketone (MIBK). The extract is aspirated into the flame of an atomic absorption spectrophotometer. Water containing from 1.0 to 20.0 )1 g/l lead may be analyzed by this procedure; higher concentrations must be reduced by dilution. None of the substances commonly occurring in natural water interferes with this method. F ishman and Midgett (1968) reported that results obtained by this procedure are in good agreement with those obtained spectrographically. With the emission spectrograph, lead in water is determined simultaneously with many other minor elements. Three methods are described by Barnett and Mallory (1971). The first method consists of evaporating the water and analyzing the residue, and is used for the analysis of samples whose dissolved-solids concen- trations do not exceed about 1,000 mg/l. This method is sensitive, precise, and reasonably accurate. The lower limits of detection vary with the quantity of dissolved solids; however, in order to achieve lower limits of detection for water samples containing more than 1,000 mg/l dissolved solids, it is necessary to separate lead and other minor elements from the major constituents prior to analysis. In the second method, lead and 20 other metallic elements are precipitated with thioacetamide, and the resultant sulfides are converted to oxides and analyzed. With the third procedure, 18 elements, including lead, are precipitated by complexing with tannic acid, thionalide, and 8-hydroxyquinoline. The precipitates are ashed and the resulting oxides analyzed. REFERENCES CITED Barnett, P. R., 1961, An evaluation of whole-order, lé-order, and 1/5- order reporting in semiquantitative spectrochemical analysis: U.S. Geol. Survey Bull. 1084—H, p. 183—206. 84 LEAD IN THE ENVIRONMENT Barnett, P. R., and Mallory, E. C., Jr., 1971, Determination of minor elements in water by emission spectroscopy: U.S. Geol. Survey Techniques Water-Resources Inv. TWI 5—A2, 31 p. Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, Methods for collection and analysis of water samples for dissolved minerals and gases: U.S. Geol. Survey Techniques Water-Resources Inv. TWI 5—Al, 160 p. Dorrzapf, A. F ., Jr., 1973, Spectrochemical computer analysis—argon- oxygen d-c arc method for silicate rocks: U.S. Geol. Survey Jour. Research, v. 1, no. 5, p. 559—562. Fernandez, F. J., and Manning, D. C., 1971, Atomic absorption analysis of metal pollutants in water using a heated graphite atomizer: Atomic Absorption Newsletter, v. 10, no. 3, p. 65—69. Fishman, M. J., and Erdmann, D. E., 1973, Water analysis: Anal. Chemistry, v. 45, no. 5, p. 36lR—403R. Fishman, M. J., and Midgett, M. R., 1968, Extraction techniques for the determination of cobalt, mickel, and lead in fresh water by atomic absorption, in Baker, R. A., ed., Trace inorganics in water: Washington, Am. Chem. Soc. (Advances in Chemistry Ser. 73), p. 230-235. Helz, A. W., 1973, Spectrochemical computer analysis—Instrumenta— tion: U.S. Geol. Survey Jour. Research, v. 1, no. 4, p. 475—482. Helz, A. W., Walthall, F. G., and Berman, Sol, 1959, Computer analysis of photographed optical emission spectra: Appl. Spectroscopy, v. 23, no. 5, p. 508—518. Mosier, E. L., 1971, A method for semiquantitative spectrographic analysis of plant ash for use in bio-geochemical exploration [abs.]: Soc. Appl. Spectroscopy Natl. Mtg., 10th, St. Louis 1971, Program, p. 65. 1972, A method for semiquantitative spectrographic analysis of plant ash for use in biogeochemical and environmental studies: Appl. Spectroscopy, v. 26, no. 6, p. 636—641. Myers, A. T., Havens, R. G., and Dunton, P. J., 1961, A spectro- chemical method for the semiquantitative analysis of rocks, minerals, and ores: U.S. Geol. Survey Bull. 1084—1, p. 207—229. Perkin-Elmer Corporation, 1971, Analytical methods for atomic absorption spectrophotometry [rev. ed.]: Norwalk, Conn., Perkin- Elmer Corp., 584 p. Sandell, E. B., 1959, Colorimetric determination of traces of metals [3d ed.]: New York, Interscience, 1032 p. Shacklette, H. T., Hamilton, J. C., Boerngen, J. G., and Bowles, J. M., 1971, Elemental composition of surficial materials in the con- terminous United States: U.S. Geol. Survey Prof. Paper 574—D, 71 p. Thompson, C. E., and Nakagawa, H. M., 1960, Spectrophotometric determination of traces of lead in igneous rocks: U.S. Geol. Survey Bull. 1084—F, p. 151—164. Ward, F. N ., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used in geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull. 1152, 100 p. Ward, F. N., Nakagawa, H. M., Harms, T. F., and VanSickle, G. H., 1969, Atomic-absorption methods of analysis useful in geo- chemical exploration: U.S. Geol. Survey Bull. 1289, 45 p. TABLES 36 8c 37 86 LEAD IN THE ENVIRONMENT TABLE 36—Normal lead content of some uncontaminated natural TABLE 37.—List of minerals and alloys in which lead (Pb) is a major substances constituent—Continued {Estimated from data given in this report] Name Formula Material Median Normal range Knuth—Continued ppm C I h h 15 U k Carbomtes—Continued “1le I eeart ............................................... n nown - - Pb (so ) (C03)2(OH)2 Common rocks: Leadhilluge ..... 4 4 ‘ Granite. 18 10 ~ 100 mm“ gxgalfggal4(w4)os SHZO '2 (“1’ : '32 Pbm(C03)e(0H)eO('-’) 4 <1 — 25 PbsCaaCu2(COa)s(0H)e - 61120 15 <1 — 50 Pb1(804) (003)2(0H)2 Pb4Cu(C03) (504)2(C110H)20 Gneiss ........... 12 <1 — 40 Am hibolite. 10 <1 — 25 : 511113510“ ..... 15 10 30 g . szABCla (ROI-[)2 Sillstone and shalt 15 5 - 50 . Pb2C1 (010102 Carbonaceous shale 20 5 — 70 Relate. PbDCflsA83C121(0H)16 ‘ H20 Limestone 1.1. 5 3 - 15 Chloroxrphite PbacuC 12(0H) 202 Sediments: Cotunnite ...... PbC12 ‘ 15 5 — 40 60 30 — 150 Cumengne“ Pb4Cu4'Clg(OH)3 - H20 15 5 50 abolerte.. sz CuClz (0H)4 Fwdlente. PbaCh (OHM 10 3 — 30 Laurionite .. PbCl (0H .02 .005- 1 Lomttoite .. Pb706C12(?) Potassium feldspar ..... 50 10 — 150 Matlockite . PbFCl Plagioclase feldspar l5 5 — 50 Mendipite Pb3C1202 Muscovite mica 20 5 — 70 Naderiten PbSb02C1 Biotite mica, 30 5 — 100 Pmlaurionite. PbC1(0H) Hornblendem 10 3 — 30 Penfieldltc ..... Pb2C13(0H) Quartz... 3 1 — 10 Calcite... S l — 15 Pei-glue PbCuC121(0H)2 gill-he PbgB(CO-) 0s nite. 1‘5” Pseu oboleite‘ Pb5Cn4C11Co(0H)s 21-120 Pseudocotunmte. KszCl 4(?) Schwartzember 'te ........... Pb 10 )2C10 0H) 8 0.1 — 100 Unnamed (v. 5 , p. 1814). szé16%103)‘2()22( 2 10(2) 10' :1 01513 “dams: . 2 _ ‘ 10 Schwartzember te ............ Pba(103)2C1402(0H)2 ' ‘ Unnamed (v. 5 , p. 1814). Pba(103)2C1902' I‘ 5“” Molybdates, tungstates: Damn. wa04 0.02 < 0.001— 0 1 PbWO4 PbMoO4 ppm in ash szSb200(0 ,OH) Vegetation: H2 Evergreen trees. 30 10 - 100 (Na Ca,Pb<>)2U2(0 OHM Deciduous tree 25 10 - 50 ”(Mn+2 Mn;)sO1e Shrubsw 25 <10 - 50 Pb2U501-1 41-120 Grasses 20 10 — 100 Mosses 100 10 —l,000 Fourmarierite ..... PbU4013- 41-120 Fruits a < 10 <10 — 30 Hematophanite. Pb4Fe409(OH,C1)2 hitharge 131......6 13:? Nicaragua] agneto um tte.. n 2 e60 Massicot ............... P130, orthorhomlliic TABLE 37.-—List of minerals and alloys in which lead (Pb) is Masuyite ........ . .......... Pb,U oxide(?) 1 . Metavandendnesscheite.. PbU-1022 - nHzO, n less than 12 a major constituent Minium... p.3304 IMlurdoehite.l’bCu11014 [Compiled by Michael Fleischer. This list includes minerals considered to be valid or attneme P1302 doubtful species. Minerals containing two anions are listed under both, with the exception Plumboferrite ........... PbFe40- of compounds containing halides. For example. hidalgoite, PbAl,(SOt)(ASOt)(OH)5, is gwgggfizrochlore 9:13“ng ,Ca)2.be203(0H) listed both under sulfates and under phosphates, arsenates vanadates. and antimonites; Rankamaite (Na, ln( P130 ,Li)s(Ta Nb HA1)11(0 0H)30 however, mimetite, Pb5(ASO‘)3CI is listed under phosphates, arsenates, vanadates, and Richetite ...... Pb,U oxide(7) antimonites, but not under halides. References for unnamed minerals, given in paren- . . theses, 'show volume and page number of American Mineralogist where the mineral [SJergzlitlfhé-Z . EEET¥$E?2):B? 1s descrlbcd] Vandendnesschene PbU-1022 . 121-120 \lesendorfite. (Pb,Ca)U207 . 21.120 Unnamed (v. 5 pbsom Name Formula Phos hates, arsenntes vanadates, antimonites: yldonite" PbCu3(A504 )2(0H )2 Alloy: Beudantite.1 PbFez1( A504) ( SO4 ) (OH )4 Rmrl L L Pb4MnFe(V04)4. 2H20 Carminite... PbFe2(AsO4 )2( OH ) 2 laggibopauadjnjtem N3Pb2 Caryinite... (Ca, Na Pb)5(Mn, Mg)4(ASO4)5 ................... Pd Pb.Bi . Unnamed Ev. 43,13. 464) NEPb ) gheivtetite... 5:2FV2?1;0 )(SO )(OH) Unnamed v 5 1063 .. P ' or. 1 9. e3 4 4 4; P' ) “”3!” Curientt. wa 02)2(vo4)2- 51120 Descloizite .. PbZn ( V04) (0H) Minerals Dewmdtite .1 PM U02)2( P04) 2 ~ 31-120 A . ‘ guftiten..." ggcfigsso 41):)OH) H rsenttes: . umontite.. ( ). l ) (0 3H 0 Finnentanite. Pb5(AsOa)3Cl Ecdemite.. Pl'>t1AS2(2)13C144 2 )“ 2 Trigonite ....... PbsMnH(A803)a Ferrazite .. (Pb 3030002 ' 311200) Fomacite. (Pb,Cu):1 [(CI',AS)0412(0H) Carbgétates: (C ,Pb)B (C0 )2 02 erite. ..... a 12 s Francevillite (Ba Pb)(UO ) (V0 ) -5H20 Caedon1te.. Pb5Cu2(SO4)s(C03)2 (011).; Gabrielsonite Pbifemsooion 2 H) ’ 2 Ceruss1te . PbCO Georgiadesi qu ( A804 ) C13 Dundastte.. PbA12(C03)2(0H)4 2H20 Gra ite ..... (Th, Pb,Ca)PO4 H20 Hydrocerussne. Pba (C03 )2( 0H )2 Hal imond Pb2( U02) ( As04 ) 2 TABLE 37.——List of minerals and alloys in which lead (Pb) is a major constituent—Continued TABLES 87 TABLE 37.—List of minerals and alloys in which lead (Pb) is a major constituent—Continued Name Formula Name Formula Minnow-Continued Minerals—Continued PL ‘ L 111 ‘ r‘ominued Sulfides, selenides, tellurides—Cominued Hedy 11111111.. .. (Ca,Pb)5(ASO4)3C1 N430“, “”252 Hid goite PbAls(SO4)(A804)(0H)s NLaPP Hinsdalite (PbSr)A13( P04) (80.) (OI-1)., Pbe—j‘e H'figelite... Pb2(U02)3(ASO4)2(0H)4 41-120 - P (”Elfin Lusungite (Sr,Pb)Fes(Po4)2(0H)5 . 1.120 Unnamed v.55, p. 1067 (Pd,Pb)aAs Mimetitc.. Pb5(AsO4)3C1 Monimolite: (Pb1Ca)aSb208(7) Momamitg Pb(Cu,Zn ) (V04) (OH) Sulfosalts: Mounarimte PbFe2( V01 )2( 0H )2 Aikinite ..... PbCuBng Parsonsne... Pb2(U02) (P002 - 2H20 Andglll'litz PbAnggSa 3311 au Pb A S Plumbo mmiteu PbA13(PO4)2(Ol-I)5 - 1-120 Benjaminite Pb;((siil.28)23i4so Przheva skith Pb(U02)2(P04)2 1 41-120 Berryite ..... Pb2(Cu A8)3Bi5311 Pyrobelonitp" Pan(vo1C)l(o 11) ' Pyromorphite. Pba( P )3C Bonchevite PbBi45-1(?) Renafdlle ------- Pb( U02) 4 ( P104 )2(OH ) 4- -7H20 Boulangeri PbaSb4S11 , . Boumomte PbCquSa Sahlmne. ..... P131 4( Aso4 )209014 Bursaite. P13513148] 1 Schulte111te.. PbHAsO4 Carmina Pb,Bi,S Tsumebipe PbgCl—l(oP04) (504) (CH) ' xanaduiit Pb5(V 0”) C1 golsal‘iitqt... ..... ggzglzssri, S auquelm ym ne. 3 n4 2 4 U d 51 258) Pb2Cu(Cr04) (P04 ) (0H) gadslclmite :géfblszgzg nname v. , p. . pb A 121 out; 8.1 3 s Unnamed v. 52, p. 1585) pbfiréAss&))2((Sgfi))§9%)zo(7) Du renoysne P11215255 Unnamed v. 47, p. 418;. pb F, arsenate Unnamed v. 47, p. 418 . “£2“ amnm PbaA82Sbssls silicates: Pbasnasbgs“ Alamosite... PbSiOas, PbsAgfisb5512 Barysilite Pb4MnS Ga] ht; “““ Eggs-gs“) Ekanite (Eh Pgficéo Fe Pb)2Sin020 eno 1smu11te. 1_ ‘ sperite... ( a n 10 4 Geocronite Pb SbA A855 . 1’) Ganomaliteu PbaCuSmOn gizgghfl SEECuBme 1 5530(1)) Hancockite ..... (Pb, Ca ”51-12011 is=eo)a(51o.);1(OI-1) “ ‘5 9 Hemihedrite Pbloln(C104)a( SiO 4) )2F2 83:31:36“; 11:29.?be S yyalpzekite l()1>’111:c§,_11a)1135111Ho11(011,11) 9‘ 5)” ‘3 3301!: a e 1110111 (OH ) Joesmithite. (Pb Ca ,Mn) (Mg,Fe)4Fe ”Show ‘ gmmtfitw 53258181331153” (0H )4(010H)8 gatchite" h (Pb {112) 2A3A5285 Kn ...... Pb UO- 3.0 . H 0 eterqmor-p 1tc. P11151185“, Keii‘iigleite PbéMn: 2'3lSi1-4011 2 Hutchinsomte (Pb' TI)2(Cu,Ag)AS5Sm Larsenite" szn5i04 - Pb F b Margarosaiute P :(Ca1M")ZSi3°9 £3313?"- (Pl‘) Te'ls) 1311134 S23 ' 1 7 Melanotekite ..... P 21:6251209 Kobelliie gg2(3g,sg)2ss M01 bdoph 11ite. Pb Mg. Si 0 (011) 225 2“ 0‘ gfsgnilte ____ y Pg:Ca:Si320271Ci2 2 Pbu(Ag,Cu)2AS4S13 uma s1t_e.. P 4A12(SiOa)7 . weep . ......s...,...o..,...so... .. 121111;...» 1c en urgl e.. PbgCaA12S110024(01-1)a 3‘12“th $512-$125? .82 a cone. 1 S 16 5 Sulfaéenséfeiggmates, selenates,selenites,tcllur:;e(sc F M) (so glen) Marrite Pb Ag A553 eaverite u e 4)2 11 Beudantite" PbFe3(AsO.1)?SO 004)( fienegmmte Pgls:1Cqu-1824 Ealrflloniteu PbaCu2(SO.1)s(C03)(0H)a N:;;;;;: {11, 31%; ’5‘” 55 ....... 5 , ‘ - 0' “9 PbF°3(P°4)(S°4)(°H)6 Ney1te ..... 1’11(Cu,1«11;_)213115118 820mm k .. ... .. PbC1-04 Nuffieldite" Pb’ 0Cu431mS21 mesmae eritc Pb2Cu (U0 ) (SeO ) (0H) 1 2H 0 Fleischgritc. PbaGe?SO4)22(2OH)as-63H20 a 2 gwyileeite ......... Ag2Pb58b5S15 a”??? ... (Pb,Cu ) 3[ ( Cr,As ) 04]2( 0H) Pfiggmfgonite. ggflggssboSu .. ' 1') B 17 em e ntc Pmen(C1'04)a(Sio4)2F2 Platy nite PbBiz(Se,S)s gidal ‘fiite" Pg A111 (S34) 128504 g (OI-[)3 P1ayfairite.. PbleSbmS43 ms a te.. b,S O OH ignite. i’bCral . iizo 4H 4“ )6 fizglflghme ?ggpnb33txs 153 tone... Pb G so 011 ’ 3 5 ‘0 1...... 2E "2°“ ’2 121.112.11.11. 1113111105” 73 b12 2:1 {gadiiilliteu P134 ($04) (C0102 ( OH)2 Sakharovaite. (Pb Fe)(Bi, Sb)2S4 iname ...... PbCu SO )(OH - Moctczumite. Pb(Ui)2)iTeos))22 g‘c‘flmm'g" ““5254. Molybdorilenite.. PbSc03 Selimgnfw :g‘efifils‘s” Nasledovne ...... Pan3A14(C03)4(SO4)05 ' 51120 gefiifexite phasbgsm3 glsacherite ....... 5:268:94ggg4)011) or y1 e """ Pb"(Sb’As)22SW sarizawaite u ( 2( Palmierite..(K1Na):Pb(§04)2 a Starryne Pb' 2(SSb’ As) ”527 §f°°‘gi§9°h}9"e" $320205 iigiiiigite 1;:511s (Sb, B ) s t F . . 5 ' “m 11110111 e °“(50‘)4(°H)° Twmnng. Pb(Sl_), 119:5." germ“ te ..... ngcéiszsse1AOHhMSOO2 Ustarasne... Pb(B1,Sb)aSw c mei erite ( , u) 0 (0H) (7) - Susannite.. Pb4(S04) (C611 )2(012‘i)2 $fifi‘¥e~~~ “’2‘ 5" A,.~1)2S5 1.1.1.1111. gg2gu<595) <50» (03> 11211311111. 53115335211111. au ue1n1tc P . . - 43 q 2 “( 4" mu m Mime..- 911131165011 Wherry ite Pb4Cu(C03)(SO4)2(C1,01-1)20 zmken'“ 9111151111521 Unnamed (v 51, p 253)" szCu(AsO4)(SO4)(0H) Unnamed v.5, p.136) ..... PbaAs489 Sulfideshselenides, tellurides: Unnamed V. 55,p .1067 . Pb,Ag,Sb,S ta1te..... ..... PbTe Unnamed v. 55,]: 31445;. Pb,Ag,Cu,Bi,S Betekhtmite" Cu10(Fe,Pb)Se Unnamed v.54, p990 (Pb,Ag Bi)Cu4Bi,r,Sn Clausthahte.. PbSe Unnamed v 38, p 525 Pb,Ag,éu,Fe Sb,S Galena......... PbS Unnamed (v 55.1» 533)... PbsBiuSn ’ An ' -- plants 61, 76 Agricultural waste Air pollution, Alaska, air pollution Alkalinity, solubility.. Allznite ’ A' ‘ eesqlrinvide 44 Amphibole ........... Analytical methods. Ande ite A . Angle ite Anomalous concentrations .. Antarctica ................ surface water. Antimony ......... 1 Arsenir 1 Associations, ore deposits .................. Atlantic Coastal Plain, concentration.. Atmosphere ................................... 1, 2, 3, 38, 54, 66, 67, 73 3, 35, 32 .. l, 3, 38, 67 Atomic absorption spectrometry Automobile exhaust... B Barley, toxicity sensitivity ............................................. 58 Basalt l, 25 Bauxite 5| Biogeochemistry .......................................... . 53 Biotin: 2, 25 Black Sea... 40 Blood -’ 77 Bluegrass, toxicity sensitivity 58 Bone ‘ 77 Brain “ British Columbia, livestock poisoning Bulgaria, soil ........................................ C Calcite 2 Carbonate ..................................................... l, 2, 5, 7, 8, 21 Cattle, poisoning Oerrusite .............. ('helatinn Chemistry ............................................................... 5, 13, 43 Cu :mnlre 75 Cincinnati, air pollution .............................................. 74 Clay 2, 21 Clay tone 5] Goal 2, 32 Coeur d’Alene district, Idaho . 17, 57, 60 Colloidal particles ........ Colorado, concentration .. livestock poisoning ................................................ 76 ‘ 66 soil 2 vegetation 56 Colorado mineral belt. 17 Colorado River....... Colorimetric analysis Columbia River Consumption, U.S. .. 1 Coordination compounds .. 14 Copper, association ......... 1 plant toxicity resistance. Corn, toxicity resistance ..... INDEX [Page numbers of major references are in italic] D Page ml" inn 78 Deoxyribonucleic acid ............................. l4 Deposits, distribution ...... I 7 mineral associations . I 7 Detection ...... 3 Diorite 25 Distribution, ore deposits 17, 46 Dolomite .................................................................. l, 2, 81 Dunite 1 E Electron probe ........ Emotional instability 78 Encephalopathy ............................................................ 77 Evaporite l, 32 F Feldspar ........ 2, 25, 27, 32 Flavoproteins .. 58 Florida, surface water ................................................... 36 Food 75 Fossil fuels ................................................................. 2, 32 G Galena.. ........... 2, 7, 2l, 27 Garnet 29 Gasoline additive .............................................. l, 38, 74, 77 Gastrointestinal system .. 77 Geochemical halos....... . 21 (‘neiu 25 Gout 77 Grain 75 toxicity sensitivity. 58 Granite . 1, 25 Cmnndr‘nnte 25, 27 (‘nuee 3 i Great Britain, health problems 63 soils .................................... 76 Great Lakes 36 Cmnlsmd 3 Ground water ............................................................. 6, 38 Gulf Plain, concentration ............................................. 47 Gypurm 2 H Halite Hallucinations .. Halos Health hazards ........................................................... 7}, 77 High Plains, concenu'ation .. 47 Horses, poisoning... 76 Hot-springs water 39 Hydrosolic suspensron 10 Hydroxides. 5, 7, 8 Hyperten inn 77 I Idaho, surface water .......... vegetation Igneous rocks Indimtor plants Industrial safety Industrial waste .................................................. 2, 3, 13, 38 Page Inorganic chemistry, water ........................................... 5 Insertiridp 3 Insomnia ....................................................................... 78 > Ionic radii ................ 25 Ionic strength, solubility 8 IowaI vegetation 54 Iron seeqrriovide 44 J, K, L Japan, health problems ........... 63 thermal water.................... 39 Kidney -’ 77 Lakes 2, 36 Lane County, Oreg. 19 Leaching ................. 1 Lichen 3 I ime lone L 21, 31 Limonire . 23 Liver 77 Livestock, poisoning ..................................................... 76 Los Angeles, air polluuon 74 M . . 2 Maryland, surface water ................................................ 36 vegetarion 5% 54, 57 Mediterranean Sea... 40 Metabolic effect .. Metamorphic rocks. Mira Michigan, concentration 47 Migration ....................... ..I, 21, 32, 53 Minerals . . 2, 25 Mining 23, 39, 63, 76 Minnesota. concentration. . 47 vegetation 54 Misrzntiaoe 77 Mississippi River ....... 36 Missouri, ground water 39 livestock poisoning ................................................ 76 mil 50, 53 vegetation 53 66, 68 Missouri district.. . 17 Missouri River 36 Molecular absorption .................................................... 82 Mona7ite 27 Montana, vegetation ..................................................... 65 Mosses 3 Mud tnne Municipal waste.. Muscovite ......... Nebraska, concentration.. Neuromuscular damage New Lead Belt, Mo.... New Mexico, air pollution.. veoetatton New York, vegetation O Oats 75 toxicity sensitivity .. 90 Occupational hazards . Ocean water Ohio Rive Oil-field brine Oil shale .. Oklahoma, smelter Olivine Optical emission spectrography. Ore deposits effect on sods. Organic chemistry, wate Organic compounds Oxidation Ozark Mountains, surface water. P Paint Perfluoroalkyl compounds Pesticide PeLrnleum pH, solubility ............9, 38, 44 Philadelphia, air pollution 74 Phnnnlim 25 Phosphate- 7 Phosphoria Formation, Idaho 3] Phosphorus, plant toxicity resistance .......................... 58 First 75 Plagioclase 2 Plants ....... .2, 23, 53 analysis. 82 Poisoning... . 1, 73, 77 Polysaccharide complexe l5 Pollution, air 1, 38, 54, 66, 67, 73 coil 66 water ............................................................... 13, 32, 35 Polynucleotides .............. l4 Poppy, toxicity resistanc i. 58 Potash fold nar 2 Pottery glaze ............................................................... 1, 75 Precipitation 38 Production, U.S. . l Prospecting. .21, 43, 56, 59, 61 plants 3 Pyromorphite ................................................................ 43 Pyrnxene 2, 29 Q, R Quarn 2, 29 Quarm'te l Radioactive deca 73 Rainfall Red Sea Rhynlite INDEX Page Ribonucleic actd l4 Rivers 2 Runoff water .............. 38 Rural v. urban pollutio 3 S San Diego, air pollution ............................................... 74 San Juan Mountains Sandstone ................ Santa Catalina Island, Calif 19 Saturation, surface water ..... Seasonal variation, concentration in plants Seawater ..................................................... Sedimentary rocks Sediments ............. Shale l, 3] Shasta copper district, Calif ......................................... 19 Shrubs . 3 Siltstnne 3] Silver 1 Smelting ................................................................. 3, 63, 75 Soil ............. Missour Solubility .................... Spectrographic analys Springs Stability fields, inorganic compounds 5 Stillbirth 77 Stream sediments .......................................................... Z3 Subsurface water Sulfate Sulfide ..................... Supergene processes Supersaturation, surface water Surface water ........................... Surface water—sediment systems 13 Suspended particles ................. .10, 35, 88 Sweetpeas, toxicity sensitivity ....................................... 58 T Thermal water ............... 39 Thermodynamics, water .. 8 Thorite 27 Titanite ......................................................................... 27 Tomatoes, toxicity sensitivity. 58 Tombstone, Ariz ........ 23 Toronto, air pollution 75 Toxicity, animals. 76 humans 1, 73, 77 plants 57 Trachyte .. 25 Trees 5! 56, 57, 60 Troposphere... 74 Page Ultramafirs 1 Union of South Africa, surface water .. Uraninite Urban v. rural pollution U.S.S.R., ground wate surface water ........ thermal water ................. V Vegetation 2, 23, 53, 73, 75 analysis. 82 Viburnum Trend, Moi... 53 Violets, toxicity sensitivity. .. 58 Viruses 14 W Wales, mining ............................................................... 66 vegetatinn 54 Wasatch Range 17 Washington County, M .. 57 Waste 13 Water ...................................................................... 2, 35, 76 analysis .................. inorganic chemistry. ocean ................. organic chemistry . pollution.. . 23 solubility“ 1, 5, 7, 8, 23, 35, 38, 39 subsurface .......... 6, 38 surface ...... , 6, 35, 36, 40 thermodynamics Weathering......l..... Wheat, toxicity sensitivity. Wisconsin, concentration .. 47 ore deposits.... 21 surface water .. 38 ngPtalinn 54 Wulfenite 23 X, Y, Z X-ray spectrography ................................................... 3, 82 Xenotime 27 Yugoslavia, air pollution 75 thermal water ........... 39 Zinc, migration ............................................................. 22 ore deposits ................. 1 plant toxicity resistance 58 Zircon 27 fi U.Sr GOVERNMENT PRlNTING OFFICE l976—777-034/IE i RETURN EARTH SCIENCESVLIEBP' T“??? MEMO?“ E 7 6‘ 7 .. ,, _ O . J 6: DAY u . ‘76“? ‘ f ' , Calderas of tho San Juan Volcanic Field Sonthwestern Colorado MCI, M. i ~“IGEOLQ‘GICAL SURVEY PROFESSIONAL PAPER 958 MAR 2 \976 , w . ‘29:. \é‘l y; mg": LL :4 _; Calderas of the San Juan Vole anic Field, Southwestern Colorado By THOMAS A. STEVEN and PETER W. LIPMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 958 Eighteen major ash-flow tuff sheets were deposited and perhaps as many related calderas developed during emplacement of an underlying shallow batholith in late Oligocene time UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPPE, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Steven, Thomas August, 1917- Calderas of the San Juan volcanic field, southwestern Colorado. (Geological Survey Professional Paper 958) Bibliography: p. Supt. of Docs. no.: 119.161958 1. Calderas—Colorado—San Juan Mountains. 2. Volcanic ash, tuff, etc.—Colorado—San Juan Mountains. 3. Batholiths— Colorado—San Juan Mountains. I. Lipman, Peter W., joint author. II. Title. III. Series: United States Geological Survey Professional Paper 958. QE524.sss 551.2’1’09788’38 75—619377 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001-02783-6 CONTENTS Abstract ...................... . .................................................. Page] Central San Juan caldera complex— Continued Page introduction ............................................ 1 Bachelor caldera ..................................................................... 19 General geology .................................................... 3 Mammoth Mountain(?) caldera ............................................. 20 Early eastern calderas ................................................... 4 Source 9f the Wason Park Tuff ------------------------------------------- 22 Platoro and Summitville calderas.. 4 San Luis and Cochetopa Park calderas ................................ 22 Bonanza caldera ........................... 6 Compound subsidence of the San Luis caldera ............ 25 Western San Juan caldera complex 7 Subsidence of the Cochetopa Park caldera ........... 26 Ute Creek caldera ................................................................... 7 Resurgence of the San Luis caldera ............. 27 Lost Lake caldera (buried) .................................................... 8 Postcaldera lavas .............................. 27 San Juan, Uncompahgre, and Silverton calderas... ..... 10 Creede caldera ........................................... 27 Uncompahgre and San Juan collapses .......... 10 Late general magmatic uplift. ............... 29 Postcollapse lavas and sediments ................ ll Block-faulted area... ...... 30 Intracaldera ash-flow tuffs .................... 12 Discussion ................................... 30 Silverton collapse ............................... 12 Development of the batholith ................................... 30 Resurgent doming.. ........................................ 12 Relation of ash-flow eruption and caldera subsidence... 31 Lake City caldera ......................................................... 13 Differentiation in local cupolas ................................... .. 31 Central San Juan caldera complex ...................................... 16 Resurgence ................................................ 32 Mount Hope caldera ......................................................... .. 15 Mineralization.. 33 La Garita caldera ................................................................... 18 References cited .................................................................... 34 ILLUSTRATIONS FIGURES 1-25. Maps: Page 1. Calderas in the San Juan volcanic field in relation to Bouguer gravity field... 2 2. Generalized geology of the Platoro and Summitville calderas ................................................................................................... 5 3. Distribution of La Jara Canyon Member of Treasure Mountain Tuff in relation to Platoro caldera ............................ 6 4. Distribution of Ojito Creek and Ra Jadero Members of Treasure Mountain Tuff in relation to Summitville caldera... 6 5. Geologic map of Ute Creek and Lost Lake calderas ...................................................................................................... 8 6. Distribution of Ute Ridge Tuff in relation to Ute Creek caldera .............................................................. 9 7. Distribution of Blue Mesa Tuff in relation to Lost Lake caldera ..................................... 9 8. Distribution of Dillon Mesa Tuff in relation to Uncompahgre caldera ......................................................................... 10 9. Western San Juan caldera complex after subsidence related to eruption of the Sapinero Mesa Tuff ...................................... ll 10, Distribution of Sapinero Mesa Tuff in relation to San Juan and Uncompahgre calderas ....................... 12 11. Distribution of Crystal Lake Tuff in relation to Silverton caldera ............................................................... 13 12- Western San Juan caldera complex after subsidence of the Silverton caldera and general resurgence .................................... 14 13- Generalized geology of the western San Juan caldera complex showing distribution of rocks related to the Lake City caldera ................................................................................................................................................................................... 15 14. Generalized geology of the Mount Hope caldera and adjacent areas ........................................................ 17 15. Distribution of the upper member of Masonic Park Tufi in relation to Mount Hope caldera... .......... 18 16. Restored Bachelor and La Garita calderas ................................................................................................................. 20 17. Distribution 'of the Fish Canyon Tuff in relation to the La Garita caldera. 21 18. Distribution of Carpenter Ridge Tuff in relation to Bachelor caldera ............ 21 19. Distribution of Mammoth Mountain Tuff in relation to probable source area ....................................................... 22 20. Distribution of Wason Park Tuff in relation to possible source area ....................................................................................... 22 21. Generalized geology of the San Luis and Cochetopa Park calderas in relation to remnants of the Bachelor and La Garita calderas ...................................................................................................................................................................... 23 22. Generalized geology of the Creede and San Luis calderas in relation to remnants of the Bachelor and La Garita calderas.. 24 23- Distribution of Nelson Mountain Tuff in relation to San Luis caldera ................................................ 26 24. Distribution of the Cochetopa Park Tuff in relation to Cochetopa Park caldera... 26 25- Areas where erosional remnants of Snowshoe Mountain Tuff are preserved in relation to the Creede caldera ...................... 28 TABLE Page TABLE 1. Ash-flow stratigraphy in the San Juan volcanic field ....................................................................................................................... 4 III CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN ‘ COLORADO By THOMAS A. STEVEN and PETER W. LIPMAN ABSTRACT Calderas in the San Juan volcanic field in southwestern Colorado formed largely in late Oligocene time (30-26 m.y. ago) in response to recurrent large-volume ash-flow eruptions. The ash-flow deposits over- lie a coalescing assemblage of early Oligocene (35-30 m.y.) andesitic stratovolcanoes that formed the southwest part of a widespread composite volcanic field in the southern Rocky Mountains. A nearly one- to-one relationship exists between large-scale pyroclastic eruptions and calderas: 18 major ash-flow sheets have been identified; 15 calderas are known, 2 are postulated on indirect evidence, and another one may possibly be identified in the northeast part of the volcanic field. In general, the different caldera cycles confirm the successive stages of development described by Smith and Bailey in 1968, except that few calderas demonstrate all stages of activity. On the other hand, almost every stage is exceptionally well developed in one or more of the calderas. The development of the calderas is believed to chronicle the emplace- ment of successive segments of an underlying shallow batholith that is indicated by a major gravity low having sharp marginal gradients. Early calderas in the eastern part of the field formed in areas of clustered andesitic volcanoes and are not clearly associated with the main gravity low. These early calderas are believed to have formed above local high- level magma chambers that developed in the roots of the volcano clusters before the main body of the batholith rose to shallow depths. Post- collapse volcanics are largely of andesitic composition, indicating that only limited volumes of silicic differentiates formed at the tops of these chambers and that these differentiates were depleted by the ash-flow eruptions. The western San Juan caldera complex also formed in an area of clustered earlier andesitic volcanoes, but are above the western part of the batholith indicated by gravity data. Large volumes of silicic differentiates formed within the batholith and were spread widely by ash-flow eruptions. Five calderas formed within a period of about 2 m.y. (in the interval from 29 to 27 m.y. ago), and contrasting lithologies of the ash- flow sheets related to the calderas require sequential development of cupolas and magmatic differentiation within them. Postsubsidence lavas that were erupted after emplacement of the most voluminous of these ash-flow sheets were largely of mafic quartz latite to andesite composi- tions, indicating temporary depletion of silicic differentiates in the source magma chamber. In the same area, a sixth caldera formed 4-5 m.y. later in response to eruption of a petrologically distinct ash-flow tuff believed to have had an origin different from the earlier ash-flow tuffs. Development of the central San Juan caldera complex began about 28 m.y. ago, during the period of ash-flow eruptions and caldera collapses in the western San Juan Mountains, and was largely complete by the end of the Oligocene, 26 m.y. ago. During this 2-m.y. span, recurrent pyro- clastic eruptions caused deposition of eight major ash-flow sheets and formation of at least seven calderas. The calderas are above the main eastern segment of the gravity low and are believed to mark the culminating upward movement of magma in this part of the batholith. Contrasting lithologies of sequential ash-flow sheets, which were derived from clustered and, in some places, nested caldera sources, require rapid development of successive cupolas above the batholith of and local differentiation within them. Most of the postsubsidence lavas that were erupted late in the different caldera cycles are coarsely porphyritic quartz latites composition'ally related to the associated ash-flow tuffs; apparently even the most voluminous ash—flow eruptions did not deplete the silicic differentiates at the top of this part of the batholith. Con— current eruption of andesitic rocks from scattered volcanoes not closely associated with the calderas is evidence of the presence of more mafic un- differentiated magma at depth, however. The life span of the batholithic magma chamber, as indicated by ash- flow eruptions and caldera subsidence, appears to have been brief. Voluminous andesitic material was erupted from widely scattered centers throughout early Oligocene time (35-30 m.y. ago). Toward the end of this period, the first local magma chambers 10-30 km across had risen to shallow depths beneath some of the major volcano clusters, and had differentiated sufficiently to supply large volumes of silicic ash. Within the next 4 m.y. (30-26 m.y.), the main batholith rose to shallow depths in segments indicated by the main caldera complexes; vast quantities of ash were erupted and numerous calderas collapsed into the partly evacuated magma chambers. However, within another 4 m.y. (by 22 m.y. ago), the batholith had congealed sufficiently to allow a younger, petrologically distinctive magma to penetrate to comparably shallow depths and retain its compositional identity. INTRODUCTION The San Juan Mountains, southwestern Colorado (fig. 1), consist mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in middle Tertiary time (Steven and Epis, 1968; Steven, 1975). This remnant is an eroded volcanic plateau (Steven, 1968), in which coalescing early andesitic volcanoes were widely overlain by the silicic tuffs of 18 major and several minor ash-flow sheets and by related lavas and breccias (Lipman and others, 1970). The sources of all the larger ash-flow sheets were near-surface magma chambers that were rapidly evacuated during voluminous pyroclastic eruptions, thereby causing collapse of overlying calderas. This paper reviews the history of the ash-flow field and its related calderas and summarizes the general relations that seem common to most individual cycles of pyroclastic eruption CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 108° 107° 106° N ‘ U 0 \250 _ Outline of San Juan volcanic field 38° _ Mountains >. E E > \ .2 9 3 ‘55 _l C l'B a: San Juan Basin -250 37° 1 I m 0 10 20 30 4O 50 KILOMETRES l_ | | l I | EXPLANATION —1— Fault—Bar and ball on downthrown side —-300— Bouguer gravity oontours—Hachures point CALDERAS (In order m Toward closecl gravity Iowa. Contour of increasing age) ® Caldera Interval 10 mI|l_Igals. Gravnry data from LC Lake City Plouff and Paklser (1972, fig. 3) C Creede CP Cochetopa Park ’_~ . \’ ,‘ Buried or inferred caldera 8" San Luis “ B Bachelor LG La Garita MH Mount Hope S Silverton DENVERo SJ San Juan UN Uncompahgre COLORADO L Lost Lake U Ute Creek 33" Ju?" _ SM Summitville volcanlc field P Platoro Bz Bonanza LOCATION OF SAN JUAN VOLCANIC FIELD FIGURE 1,—Calderas in the San Juan volcanic field (patterned) in relation to Bouguer gravity field. GENERAL GEOLOGY 3 and caldera subsidence in the San Juan field. The sequence of events we have determined for each of the calderas conforms well to the succession of stages in the development of a typical resurgent caldera described by Smith and Bailey (1968), although few of the San Juan calderas demonstrate all stages of activity. On the other hand, almost every stage is exceptionally well developed in one or more of the San Juan calderas. GENERAL GEOLOGY The general evolution of the San Juan volcanic field has been described by Lipman, Steven, and Mehnert (1970) and will be outlined only briefly here. Volcanic activity began in latest Eocene or earliest Oligocene time, probably between 40 and 35 my ago.1 The early rocks are largely intermediate in composition (andesite, rhyodacite, and mafic quartz latite) and were erupted from many scattered stratovolcanoes. These volcanoes were especially active in the interval from 35 to 30 my. ago, and the products derived from them coalesced into a composite volcanic field covering more than 25,000 kmz. About 30 my ago the character of volcanic activity changed markedly to predominantly pyroclastic eruptions, and large—volume quartz latitic (and rhyolitic ash flows spread widely from many centers. Most of the larger sheets show evidence of compound cooling, and evidently were formed by many individual ash flows that followed one another in rapid succession. The earliest ash flows came largely from the northeastern and southern parts of the San Juan field, and were erupted from clusters of the early stratovolcanoes; caldera collapse resulting from the ash-flow eruptions largely destroyed the upper parts of these volcanoes. Postsubsidence eruptions around these early calderas were largely of intermediate- composition lavas and breccias that commonly are virtually indistinguishable from the early intermediate rocks of the composite volcanic field. Beginning about 29 my ago, ash-flow eruptions broke out in the western part of the San Juan volcanic field where five calderas formed in less than 2 my The first two of these calderas are largely covered and are imperfectly understood, but the last three evolved in a manner generally similar to the early calderas farther east. They developed in an area of clustered andesitic central volcanoes whose vent areas were largely destroyed by caldera subsidence. Postsubsidence eruptions here were also mainly of intermediate-composition lavas and breccias that closely resemble those of the early volcanoes. About 28 my ago, while ash flows were still erupting and calderas forming in the western San Juan Mountains, ‘Except where otherwise noted, all specific age designations are from Lipman, Steven, and Mehnert(1970) or Steven.Mehnert, and Obradovich (1967). major pyroclastic eruptions began in the central part of the San Juan volcanic field. A sequence of eight major ash- flow sheets formed, and caldera subsidences have been identified or inferred at all the ash-flow source areas. Post- subsidence eruptions around most of the central San Juan calderas were of viscous quartz-latitic and rhyolitic lavas closely related in composition to the ash-flow tuffs. Although some more mafic volcanoes were active during this same interval, they were not closely associated in space with the developing calderas. Ash-flow activity terminated in the central San Juan Mountains about 26.5 m.y. ago. In early Miocene time, about 25 my. ago, the character of the erupted material changed from the andesitic and derivative rocks that formed the early San Juan volcanoes and succeeding ash-flow tuffs to fundamentally basaltic materials with some associated high-silica alkali-rich rhyolites. This change approximately coincided with inception of basin-and-range faulting in the adjacent San Luis Valley segment of the Rio Grande trough (Lipman and Mehnert, 1975). Fundamentally basaltic eruptions in the San Juan field continued intermittently until about 5 my ago. The only large—volume rhyolitic ash-flow tuff deposited during the period of fundamentally basaltic activity is the Sunshine Peak Tuff, about 22.5 my. old (Mehnert and others, 1973a), which formed from ash flows that accumulated in and around the concurrently developing Lake City caldera in the western part of the San Juan volcanic field. In all, 15 calderas are now known in the San Juan volcanic field, and indirect evidence suggests that at least two and perhaps three more exist. A large negative Bouguer gravity anomaly underlies the area containing most of the calderas (fig. 1), and is believed to reflect a major underlying batholith (Plouff and Pakiser, 1972). Sharp gradients at the margins of the anomaly indicate that the top of the batholith is relatively shallow. The change from eruption of intermediate- composition rocks by widely scattered early strato- volcanoes to eruption of the more silicic ash flows probably took place as this batholith rose and differentiated beneath the central part of the field. When the roofs of the more differentiated and gas-charged cupolas of the batholith failed, great volumes of ash were erupted rapidly, and unsupported segments collapsed to form the calderas. The sequential development of the calderas is believed to reflect the progressive emplacement of the different high-level plutons of a composite batholith. The calderas in the San Juan volcanic field became comprehensible to us only after the complex stratigraphy of the related ash-flow units and associated rocks was determined by regional mapping of the Durango 1°x2° quadrangle (Steven, Lipman, Hail, and others, 1974) and 4 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO adjacent parts of the Montrose l°x2° quadrangle. This regional work has led to major revisions in stratigraphic nomenclature established by earlier studies of local areas (Steven, Lipman, and Olson, 1974; Lipman and others, 1973). The volcanic stratigraphy of the ash-flow units as understood in 1975 is given in table 1. For a more complete summary of the total volcanic stratigraphy, the reader should consult the above references and particularly the explanation of the Durango quadrangle map. EARLY EASTERN CALDERAS The oldest calderas in the eastern part of the San Juan field—the Bonanza caldera and the nested Platoro and Summitville calderas—are widely separated and the related ash-flow sheets do not overlap. Thus, the relative ages of eruption and caldera development at the two centers are uncertain (table 1). All these cycles are younger than the dated rocks in the early intermediate-composition volcanoes (34.7-31.1 m.y.) and are older than the Fish Canyon Tuff (27.8 m.y.). The only age relations among these rocks that can be told directly by superposition are those between the several members of the Treasure Mountain Tuff that are derived from the Platoro and Summitville calderas. K-Ar ages of the Treasure Moun- tain Tuff related to the Platoro and Summitville calderas appear older than those obtained from tuffs related to any of the calderas in the western San Juan Mountains, but present data do not permit interpretation of age relations between the Bonanza caldera and any of the other older calderas. Both the Bonanza caldera and the Platoro and Summit- ville calderas formed within clusters of earlier andesitic stratovolcanoes, and they are on or just outside of the margin of the shallow batholith that gravity data suggest underlies the San Juan volcanic field (fig. 1) (Plouff and Pakiser,’ 1972). The Bonanza caldera is located near the northeast end of a narrow gravity low that extends east- northeast from the main anomaly; this caldera probably is localized above a satellitie pluton. The Platoro and Summitville calderas are outside the sharp gradient along the southeast side of the main gravity low, and thus are probably not above the near-surface part of the main batholith. Quite possibly these early calderas developed above local high-level magma chambers that formed in the roots of earlier volcanoes before the main batholith had risen to its present near-surface position. PLATORO AND SUMMITVILLE CALDERAS The Platoro and Summitville calderas in the south- eastern part of the volcanic field (fig. I) constitute a composite collapse structure about 20 km in diameter that formed as a result of recurring eruptions of ash flows of the Treasure Mountain Tuff (Lipman and Steven, 1970; Lipman, 1975a, b). The Platoro caldera and the nested younger Summitville caldera (fig. 2) formed within a cluster of six or seven intermediate-composition strato- volcanoes of the Conejos Formation. These central volcanoes had been extensively eroded and the inter- vening basins filled with the resultant detritus, producing a widespread low-relief surface. Ash-flow activity began 30-29 m.y. ago when at least 500 km3 of phenocryst-rich quartz-latitic ash that now constitutes the La Jara Canyon Member of the Treasure Mountain Tuff was erupted from sources in the Summitville-Platoro region and spread 30- 40 km in all directions (fig. 3). Caldera collapse began before these eruptions were complete, and the late ash flows forming the La Jara Canyon Member ponded within the collapsing caldera to a thickness of more than 800 m. Similar concurrent eruption and collapse characterized other large calderas‘in the San Juan Mountains, when the TABLE l.—Ash-flow stratigraphy in the San Juan volcanic field Estimated Ash-flow unit v:ll:me Dominant composition Age (m.y.) Related caldera ( "1) Sunshine Peak Tuff ....................................... 100—500 Silicic rhyolite ............................................ 22.5 Lake City. Snowshoe Mountain Tuff . .. > 500 Quartz latite ............................ 26.5(P) Creede. Cochetopa Park Tuff ......... < 100 Zoned rhyolite to quartz latite. . > 26.4 < 26.7 Cochetopa Park. Nelson Mountain Tuff.. >500 ....do ........................................ >26.4 < 26.7 San Luis. Rat Creek Tuff ............ < 100 Rhyolite. >26.4 < 26.7 Early stage San Luis. Wason Park Tuf ............... 100—500 ....do ........................................ >26.4 < 26.7 Unknown. Mammoth Mountain Tuff. ..... 100—500 Zoned rhyolite to quartz latite. 26.7 Do. Carpenter Ridge Tuff ............. >500 Rhyolite, locally zoned to quartz latite. >26.7 <27.8 Bachelor. Crystal Lake Tuff ............ 25—100 Rhyolite ...................................... >26.7 <27.8 Silverton. Fish Canyon Tuff ................................ > 3,000 Quartz latite.. 27.8 La Garita. Masonic Park Tuff (upper member). ..... >500 ....do ........... 28.2 Mount Hope. Sapinero Mesa Tuff ....................................... >l,000 Rhyolite.. .......................... > 27.8 <28.4 Uncompahgre and San Juan. Dillon Mesa Tuff ........................................... 25—100 > 27.8 <28.4 Uncompahgre(?). Blue Mesa Tuff ..... . 100—500 . > 27.8 28.4 Lost Lake(?). Ute Ridge Tuff ............................. >500 Ute Creek. Treasure Mountain Tuff: . _ Ra Jadero Member .................................. 100—500 < 29.8 Summitvtlle Ojito Creek Member .......... . 40—70 < 29.8 Do. La Jara Canyon Member... ..... >500 Platoro. Bonanza Tuff ........................................................ Bonanza. EARLY EASTERN CALDERAS 5 106° 30' Precaldera andesitic rocks (Conejos Formation) 0 5 if Fault—Hachures on downthrown side Precaldera andesitic rocks (Conejos Formation) Precaidera andesitic rocks (Conejos Formation) 10 15 KILOMETFiES | | . FIGURE 2.—-Generalized geology of the Platoro and Summitvrlle calderas. Control moderate to good where boundarles of calderas are shown by solid symbols; conjectural where shown by open symbols. roofs of the magma chambers lost support before eruptions were completed. The thick La Jara Canyon tuffs within the Platoro caldera are topographically and structurally high as a result of resurgent uplift shortly after collapse. Early resurgence is demonstrated by the presence in the core of the caldera of monolithologic talus breccias that were derived from the La Jara Canyon Member and that inter- tongue with lavas that filled the structural moat adjacent to the resurgent block. The resurgent core forms a nearly unbroken block that dips homoclinally to the southwest, in contrast with the fractured domical uplifts that characterize many other known resurgent calderas. After resurgence was virtually complete, the marginal moat of the Platoro caldera was filled by as much as 1 km of dark andesitic lavas and interbedded volcaniclastic sedimentary rocks of the lower member of the Summit- ville Andesite. These andesitic lavas, in essence, represent 6 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO O 100 KILOMETRES FIGURE 3.—Distribution of La Jara Canyon Member (diagonal lines) of Treasure Mountain Tuff in relation to Platoro caldera (P) and San Juan volcanic field (shaded). Base for figures 3, 4, 6—8, 10, ll, 15, 17—20, and 23—25 from US. Geological Survey, Colorado State map, 1:500,00, 1968. a continuation of the same type of volcanic activity that characterized the development of the Conejos Formation, with which they are readily confused in the field. A younger collapse structure, the Summitville caldera, occupies the northern part of the Platoro caldera (fig. 2). This caldera apparently formed when ash-flows constituting the upper sheets of the Treasure Mountain Tuff, including the Ojito Creek and Ra Jadero Members, were erupted after the Platoro caldera moat was nearly filled by lavas of the lower member of the Summitville Andesite. The Ojito Creek and Ra Jadero tuffs (fig. 4) are nearly coextensive with those in the La Jara Canyon Member and their constituent ash must have been erupted from the same general area. Although the volumes of these upper two members are much less than that of the La Jara Canyon Member (table 1), they are large enough to suggest associated caldera collapse (Smith, 1960, fig. 3). In .addition, the Summitville caldera is indicated by (1) a large-displacement (800+ m) arcuate fault marking the main ring-fracture fault on the southeast side of the caldera (fig. 2), (2) fragmentary exposures of the topo- graphic wall, especially on the northeast side, and (3) the concentration of the products of late igneous activity and mineralization around the margins of the caldera. The caldera probably was not resurgent, but was filled by thick lavas of the upper member of the Summitville Andesite, and many key geologic relations are largely buried. In 108C 107° “’T'l Q‘ka n l Montros 3r 38a P :53 3 rings. ‘2 100 KILOMETRES w FIGURE 4.—Distributi0n of Ojito Creek and Ra Jadero Members (diagonal lines) of Treasure Mountain Tuff in relation to Summit- ville caldera (S) and San Juan volcanic field (shaded). § 37° -- 4-. _ 0 addition, extensive late intrusion and hydrothermal alteration further obscured relations. Porphyritic rhyodacitic to rhyolitic lavas and genetically related dikes and granitic stocks were emplaced repeatedly around the margins of the Platoro and Summit- ville calderas during the interval between 29 and 20 my ago, with dated events at 29.1, >26.7< 27.8, 25.8, 22.8, and 20.2 m.y.; these shallow intrusions and associated rocks locally were hydrothermally altered and mineralized. BONANZA CALDERA A caldera in the vicinity of the old mining camp of Bonanza in the northeast part of the San Juan volcanic field has been postulated by Karig (1965), Mayhew (1969), Bruns (1971), Knepper and Marrs (1971, p. 261), and others. The oldest rocks in the Bonanza area are andesitic to latitic flows and breccias of the Rawley Andesite and Hayden Peak Latite (Burbank, 1932; Mayhew, 1969) that form an interfingering assemblage. We interpret these rocks to be the near-source facies of one or more local volcanoes equivalent in age to the Conejos Formation near the Platoro area. These rocks are overlain by the Bonanza Tuff, a densely welded quartz-latitic ash-flow tuff that once formed a widespread sheet over much of the northeastern part of the San Juan volcanic field. Near Bonanza, remnants of this sheet must have been deeply faulted into the older andesitic pile, probably as a result of caldera subsidence. Younger andesitic flows and breccias it WESTERN SAN JUAN CALDERA COMPLEX 7 cover at least some of the Bonanza Tuff that occurs within the subsided block (Knepper and Marrs, 1971, pl. 1). Few generalizations can be made about the Bonanza caldera from available published data. However, we consider it significant that, as in the Platoro caldera complex, subsidence took place within an area of older intermediate—composition volcanoes and that post- subsidence lavas are predominantly andesitic in composition. WESTERN SAN JUAN CALDERA COMPLEX Six calderas in the western San Juan Mountains are located above the main western lobe of the gravity low that has been interpreted to represent a shallow batholith. The first five of these calderas, the Ute Creek, Lost Lake, San Juan, Uncompahgre, and Silverton, formed within the brief span of about 2 m.y., and the five related ash-flow deposits superficially resemble each other enough to have once been included within a single formation (the now- abandoned Gilpin Peak Tuff of Luedke and Burbank, 1963). Of these five calderas, only the Ute Creek caldera is not clearly within the gravity low, but instead is located at the sharp gradient along the south side of the gravity low (fig. 1). This structure may have formed above a local magma chamber that either was too deep or, after ash-flow eruptions, did not retain enough relatively light silicic differentiate at its top to be detected by gravity measure- ments. The other four early calderas are well within the area of the gravity low, and, of these, three are closely clustered and have overlapping cycles of development. These cycles are believed to represent high-level magmatism and volcanic eruption related to the main upward movement of magma in this western part of the batholith. The last subsidence structure, the Lake City caldera, formed about 5 my after the earlier calderas subsided, and the associated ash-flow tuff contrasts markedly in composition and appearance with the earlier ash-flow tuffs. During the 5-m.y. interval, the main batholith is believed to have congealed sufficiently for a petro- logically distinct batch of magma to penetrate to shallow depths and still retain its compositional identity. UTE CREEK CALDERA The southern margin of the caldera that formed during eruption of the Ute Ridge Tuff is exposed for about 3 km along the canyon of Ute Creek (figs. 5 and 6). Elsewhere, the eastern and northern topographic wall of the caldera can be located closely at only two places, one near the mouth of Ute Creek and the other along the south side of Pole Creek Mountain, near the eastern end of the mountain. The downdropped block within these limits may have subsided as a trapdoor that has no ring-fracture zone along its west side. Along Ute Creek, intracaldera Ute Ridge Tuff, more than 300 m thick, is juxtaposed against Precambrian melasyenite and quartzite and an older unnamed crystal- poor ash-flow tuff of Tertiary age, whereas an outflow layer of the Ute Ridge Tuff is only about 130 m thick and extends out over an irregular surface cut on the older rocks. The probable, caldera-boundary fault between the thick section of Ute Ridge Tuff and the adjacent rocks is occupied by a quartz monzonite intrusive (fig. 5); another similar intrusive cuts the thick section of Ute Ridge Tuff north of lower Ute Creek. In the vicinity of Ute Creek, a younger ash-flow sheet, the Blue Mesa Tuff, unconformably overlies the Ute Ridge Tuff, quartz monzonite intrusives, and Precambrian rocks. A chilled vitrophyre at the base of the Blue Mesa is in contact with all these older rocks, and north of Ute Creek a thin lens of andesite occurs locally along the unconformity. Development of the structural discordance between the thick section of Ute Ridge Tuff and the juxtaposed older rocks thus closely accords in time with eruption of the Ute Ridge Tuff, and seems most easily accounted for by caldera subsidence related to that eruption. Subsidence concurrent with eruption is indicated by the thick section of Ute Ridge Tuff within the downdropped block, in contrast with the relatively thin outflow sheet. Although we saw no evidence for post- subsidence resurgence, two quartz monzonite intrusive bodies along and near the southern margin apparently were emplaced late in the Ute Creek caldera cycle. The topographic caldera wall along the east side of the downdropped block is closely controlled near the mouth of Ute Creek, where the top of a buried hill of older andesitic volcanic breccias (San Juan Formation) is exposed a few hundred metres east of the thick section of Ute Ridge Tuff that lies within the caldera. Several kilo- metres northwest, across the Rio Grande, another buried hill of andesitic breccias of the San Juan Formation is exposed along the south side of Pole Creek Mountain. The Ute Ridge and Blue Mesa Tuffs wedge out against this hill, which may represent a remnant of the northeast caldera wall. Thick Ute Ridge Tuff is excellently exposed along the Rio Grande inside the west-facing semicircular are described by the intrusive-filled fault along Ute Creek and the two buried hills of andesite breccia. The rude layers representing the many successive ash flows in the Ute Ridge Tuff dip about 5° east as part of a much later regional tilting that affected the whole western San Juan area, but otherwise are undeformed westward from the arcuate wall for 12-15 km. The tops of at least two buried hills of Precambrian crystalline rocks extending well up into the Ute Ridge Tuff are exposed along the Rio Grande 3-9.5 km west of the arcuate wall. These incompletely exposed relations suggest that the 107°30' Rio Grande Needle Mountains CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 107°15’ f «(a may . d I\_J\1lo Kl LOMETRES FIGURE 5.—Geologic map of Ute Creek (U) and Lost Lake (L)calderas. DCr. Precambrian rocks; 0, precaldera volcanic rocks; u, Ute Ridge Tuff; b, Blue Mesa Tuff; y, postcaldera volcanic rocks; black, intrusive rocks. Open rectangles indicate approximate boundaries of postulated buried calderas. source of the Ute Ridge Tuff was in a hilly area cut on older Precambrian rocks and Tertiary andesitic rocks, and that the pyroclastic eruptions caused concurrent subsidence of a trapdoor block that was faulted along the south, east, and north but probably only downwarped on the west. The fragmentary structural and topographic wall exposed in places along Ute Creek and Pole Creek Mountain reflects the abrupt boundaries of subsidence in these directions, whereas the downwarped(?) western margin is obscured by the relatively flat lying upper part of the Ute Ridge Tuff exposed along the Rio Grande. LOST LAKE CALDERA (BURIED) A distinctive arcuate drainage pattern along the upper Rio Grande (fig. 5) reflects a buried caldera related to eruption. of the ash composing the Blue Mesa Tuff. All rocks in this area dip regionally about 5° east and northeast; eastWard across the arcuate drainage pattern, however, the dips increase to 10°- 1 5° and then flatten again to form an arcuate monocline. Fracturing related to this monocline provided a zone of weakness later etched out by stream erosion. The monocline involves the Dillon Mesa and Sapinero Mesa Tuffs and younger units in this area, and reflects minor late subsidence around the western periphery of an older buried caldera. The western margin of the largely buried Lost Lake caldera closely follows the arcuate monocline. The wall of the caldera is closely controlled near the mouth of Ute Creek, where a hill of older andesitic mudflOw breccia protrudes up into the ash-flow section and represents a WESTERN SAN JUAN 108° 107° 106° — car—‘1' V *9 ””233 ‘ G U N o N/ \( lida / o . ontros’a it) T un u \\ I U MESA ——- — - -—“" \ R s \ __ __ _ I ELL \ Vii-:1) o E 38 -¢ j. N I IS 7’ LLE TO —- Alam ‘ I ' ES _ :L<-\ P L I p gosa . ‘ 8 rings . ,- ur n o ,/ g g l A c H L E i§ ' 37. <: J s 0 100 KILOMETRES |___|____|P_I__—J———J FIGURE 6.—Distribution of Ute Ridge Tuff (diagonal lines) in relation to Ute Creek caldera (U) and San Juan volcanic field (shaded). wedge-shaped septum between the Ute Creek and Lost Lake calderas (fig. 5). Farther north, the northwes tern wall of the Lost Lake caldera follows Lost Trail Creek for about 5 km; this segment of the caldera is marked, on the west side of the creek, by early, intermediate-composition andesitic rocks in the wall of the caldera and, on the east side, by caldera-filling rocks. No constraints are known on the east side of the caldera, however, and the projected margin (fig. 5) is drawn simply to close out the eastern side of a circular area that conforms to the arcuate monocline and drainage pattern to the west. The Lost Lake caldera, like several others in the San Juan volcanic field, possibly may have subsided as a trapdoor block with an incomplete ring-fracture zone and hinged eastern margin. The oldest rocks exposed within the Lost Lakes caldera are the nearly identical Dillon Mesa and Sapinero Mesa Tuffs. These rocks form the lower parts of prominent cliffs along the Rio Grande, and are best exposed along the north side of the river opposite the mouth of Ute Creek. At this locality, the lower cliffs consist of Dillon Mesa Tuff; the upper 150 m of this unit is well exposed but, as the base is everywhere below the level of the river, its total thickness is not known—although it probably greatly exceeds the 0- 100 m of Dillon Mesa Tuff exposed on Pole Creek Mountain west of the caldera. The overlying Sapinero Mesa Tuff, which forms the upper cliffs, is about 130 m thick, about the same as it is on Pole Creek Mountain. Evidently the Dillon Mesa ponded within and passively filled the caldera, and the succeeding Sapinero Mesa Tuff was deposited across it without appreciable thickening. CALDERA COMPLEX 9 The Lost Lake caldera cuts off the Ute Ridge Tuff (fig. 5), and in turn was largely filled by the Dillon Mesa Tuff. These stratigraphic relations limit the time of caldera subsidence to the period during which the ash of the Blue Mesa Tuff was erupted. Of all the major ash-flow sheets in the upper Rio Grande area, only the Blue Mesa Tuff lacks an exposed source. The Blue Mesa is a widespread sheet (fig. 7) that pinches out against Precambrian rocks in the Needle Mountains on the south, extends Westward to the eroded edge of the volcanic rocks near Telluride, and wedges out to the north against older volcanic rocks north of the Gunnison River, more than 80 km north of the upper Rio Grande area. To the east, the Blue Mesa Tuff is covered by younger rocks and its distribution is unknown (Steven, Lipman, Hail, and others, 1974). The Blue Mesa Tuff and older rocks are sufficiently well exposed north and west of the upper Rio Grande area that a caldera source can be eliminated in most of these areas. The source therefore most likely lies within the covered part of the sheet—and thus in the Lost Lake caldera. 108a 107° .. ..---| ( 63%,, l U N. ON/ \ (Mon un I v'. MESA ——— -" "' R S i... __ .__‘_| N ., ____.qu E 38° rx/ . s l 17 Lung; |--— _ _Alam H LL"? P L ' P gosa . l 8 rings . . ur ngo I _/ g l A c H L E S g u 370 ‘1 | s 0 100 KILOMETRES l____.____|___|___L——J FIGURE 7.—Distribution of Blue Mesa Tuff (diagonal lines) in relation to Lost Lake caldera ('L) and San Juan volcanic field (shaded). The Blue Mesa Tuff is a uniform—textured, moderately phenocryst poor, densely welded ash-flow tuff over most of its exposed extent. In the upper Rio Grande area, however, the phenocrysts are larger and more abundant than else- where, and the unit is weakly compositionally zoned, becoming more mafic upward. A rhyodacitic lava flow also occurs locally between the Blue Mesa and the over- 10 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO lying Dillon Mesa Tuff on the north side of Pole Creek 108° "370 106.. Mountain and just west of the arcuate monocline; this is 37 —- ~-~--j _ . . . @102 n G U N the only known locality of lava-flow act1v1ty at thls 72% i horizon. These areally restricted variations suggest a I “0“"05 R nearby source for the Blue Mesa Tuff. N o evidence was seen for resurgent doming of the Lost Lake caldera after subsidence. However, only the upper parts of the caldera fill can be seen, and the oldest rocks exposed are along the margins of the caldera, where the effects of resurgence would have been minimal. Thus, a low dome could exist a depth and not be reflected by the younger rocks that cover the caldera area. SAN JUAN, UNCOMPAHGRE, AND SILVERTON CALDERAS The geologic history of the caldera complex in the western San Juan Mountains has recently been reinter- preted by Lipman, Steven, Luedke, and Burbank (1973), and the discussion here is taken largely from them. The western San Juan Mountains were the site of a cluster of intermediate-composition central-vent volcanoes in early Oligocene time, 35-30 m.y. ago. The near-source facies of these volcanoes consists of complex accumulations of lavas, breccias, and pyroclastic debris, which pass laterally into coalescing volcaniclastic aprons consisting pre- dominantly of mudflow breccias and, at the margins, of conglomeratic and other stream-worked debris. The clustered volcanoes formed part of the great field of early intermediate-composition volcanic rocks that covered much of the southern Rocky Mountains in early Oligocene time (Lipman and others, 1970; Steven and Epis, 1968; Steven, 1975). Ash-flow sheets from nearby sources at the Ute Creek and Lost Lake calderas covered the lower flanks and coalescing outflow aprons of these volcanoes, and wedged out against highlands built up around the central vents. UNCOMPAHGRE AND SAN JUAN COLLAPSES The volcanic cycles that led to the‘development of the San Juan, Uncompahgre, and Silverton calderas probably began when the relatively small volume of material constituting the Dillon Mesa Tuff (table 1; fig. 8) was erupted shortly before 28 my ago. No direct evidence can be marshalled to tie this eruption with any specific increment of subsidence, but the unit seems to be radially distributed around the area of the Uncompahgre caldera, and is thickest and most densely welded near the margins of this possible source area. Renewed eruptions of similar rhyolitic ash-flow tuff material about 28 my ago led to widespread emplacement of the Sapinero Mesa Tuff, and to simultaneous collapse of the Uncompahgre and San Juan calderas (fig. 9). The Sapinero Mesa Tuff spread widely (fig. 10) from its source area at these calderas, and had an estimated volume in excess of 1,000 km3. It has been traced northeastward for more than 90 km, northward 65-70 km, and south- 38° Ala %23 5 £322. pF/ L E O 100 KILOMETRES w FIGURE 8.—-Distribution of Dillon Mesa Tuff (diagonal lines) in rela- tion to Uncompahgre caldera (U) and San Juan volcanic field (shaded). eastward 40-45 km. Apparently it wedged out southward against the older highland of Precambrian rocks in the Needle Mountains. The western flank of the volcanic pile has been removed by erosion, and the original extent of the Sapinero Mesa Tuff in this direction is not known. Outflow Sapinero Mesa Tuff is generally less than 100 m thick, whereas the intracaldera Eureka Member is at least 700 m thick in places and the base is only locally exposed. This disparity in thickness between outflow and intra- caldera facies is common among the San Juan calderas, and is interpreted to indicate subsidence concurrent with- eruption; the intracaldera accumulation is generally somewhat younger than most of the outflow sheet. Approximate contemporaneity of subsidence of the Uncompahgre caldera and eruption of the Sapinero Mesa Tuff can be demonstrated convincingly on the basis of geologic evidence (Lipman and others, 1973), and similar general relations of intracaldera and outflow accumulations of Sapinero Mesa Tuff for the San Juan caldera imply a similar timing. Subsidence of the two calderas was accompanied by widespread landsliding and avalanching of the steep caldera walls, and abundant debris from these sources is interleaved marginally with the intracaldera ash-flow accumulations (Eureka Member) in both calderas. By this process the topographic walls of the two calderas actually merged locally to form a single depression with a distorted dumbbell shape: the two main subsided blocks were joined across a low divide when the septum between them largely caved away (fig. 9). The simultaneous development of adjacent but separate WESTERN SAN JUAN CALDERA COMPLEX ll ”Ti / , \ / / \ x -« x4***Q/i / y //=\ X // X / xx xx xx \ Topographic rim Xx i 380— x UNCOMPAHGRE 4‘ ' xx CALDERA 99 \ l / >0 i5 9“ ll lu {l / (a)? 3 H , is, l SILVERTON / e l CALDERA / st \ 3 y a \\ / 5 / / \ // // \ / / \H: y/ <\ Structural margin \\ y%& \%%% EXPLANATION -——|'1—I"'l—I" Fault—Hachures on downthrown side, where known 0 5 10 15 KILOMETRES l 4| FIGURE 13.—Generalized geology of the western San Juan caldera complex showing distribution of rocks related to the Lake City caldera. Resurgence of the Lake City caldera produced a simple dome characterized by outward dips of 20°-25° on its flanks and a northeast-trending apical graben over its distended crest. The trend of this graben reflects reactivation of the trends of the earlier Eureka graben system, which was related to resurgence of the Uncompahgre and San Juan calderas. Most of the mapped faults of the Lake City resurgent structure have relatively small dis- placements—10-50 m—and some seem to be little more than cracks that localized weak hydrothermal alteration. Chaotic caldera-collapse breccias are widely exposed below the intracaldera tuffs on the southwest side of the Lake City caldera, and the resurgence appears to have been somewhat asymmetrical, with maximum uplift in this area. Resurgence resulted from upward movement of magma which crystallized as a shallow stock of granite porphyry. Gently dipping upper contacts of the granite porphyry are exposed at the bottoms of several deep erosional valleys within the core of the Lake City caldera, and the most intensely altered rock within the resurgent dome occurs around margins of the granite porphyry. Parts of the northern ring fault are occupied by a short, thick, discontinuous ring dike, which broadens and becomes coarser grained downward. The top of the granite porphyry is within 1 km of the top of the Sunshine Peak Tuff indicating crystallization at very shallow depths beneath the resurgent dome. Most of the original topographic wall of the Lake City caldera has been eroded, and only a few small remnants are 16 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO preserved southwest of Lake City. The present valleys of Henson Creek and the upper Lake Fork of the Gunnison River, which define a striking elliptical drainage pattern just outside the structural boundary of the Lake City caldera, are not directly controlled by any major fault structures. They more likely developed in debris that accumulated in a low topographic moat along the margin of the resurgent dome of the Lake City caldera outside the limits of the subsided block, and were subsequently super- imposed onto older rocks of the caldera wall. CENTRAL SAN JUAN CALDERA COMPLEX The central San Juan caldera complex developed above the eastern part of the gravity anomaly (fig. 1) between 28.2 and 26.5 m.y. ago. Within this brief span of less than 2 m.y., eight major ash-flow sheets were erupted, at least seven calderas formed, and numerous local volcanic rock units accumulated. These caldera cycles thus had an average duration of only about a quarter of a million years, and in at least one case, the cycles of two nearby calderas overlapped in time. We believe that these intense pyroclastic eruptions and resulting caldera subsidences marked the culmination of upward movement of magma to form the eastern part of the underlying batholith. As the roof above this segment of the batholith became progressively thinner, it was breached by numerous ash~flow eruptions of great magnitude, and related calderas subsided at the ash-flow source areas. The development of the numerous clustered and partly overlapping calderas that followed one another in rapid succession, and the common marked contrasts between lithologies—ranging from phenocryst-rich quartz latites to phenocryst-poor rhyolites—of successive ash-flow deposits, required rapid congealing of the upper parts of the local cupolas and equally rapid reestablish- ment of new cupolas and magmatic differentiation within them. The pyroclastic eruptions and caldera subsidences apparently ceased when the upper part of the batholith congealed to a thickness sufficient to contain the magmatic pressures. MOUNT HOPE CALDERA The Mount Hope caldera (fig. 14) is almost completely buried by younger rocks, primarily the Fish Canyon Tuff, but fragmentary evidence suggests that it was the source of two phenocryst-rich quartz latite ash-flow sheets of the Masonic Park Tuff. The lower sheet has been recognized only within 10-15 km of the caldera, but the upper sheet (fig. 15) extends 30 km west of the caldera, to the margin of the San Luis Valley 50 km east, and into New Mexico 75 km southeast (Steven, Lipman, Hail, and others, 1974). The topographic wall along the north margin of the Mount Hope caldera is exposed for about 6 km. This wall cuts sharply across the lower ash-flow sheet of the Masonic Park Tuff, andesitic flows and breccias of the Sheep Mountain Andesite, thick quartz latite flows of the volcanics of Leopard Creek, and the upper ash-flow sheet of the Masonic Park Tuff. Total relief along the exposed section of this wall is about 500 m. The caldera is filled with Fish Canyon Tuff that is at least 1.4 km thick and extends over adjacent rocks on the south, east, and north- east sides of the caldera. Adjacent to the caldera, the Fish Canyon Tuff generally rests directly On the upper sheet of the Masonic Park Tuff. The exposed segment of the topographic wall indicates that the caldera subsided after the ash flows forming the widespread upper sheet of Masonic Park Tuff were erupted (28.2 m.y. ago), and the basin formed by sub- sidence was in turn filled passively by the next succeeding major ash-flow sheet, the Fish Canyon Tuff (27.8 my old). In addition to these stratigraphic constraints on the age of the caldera, other evidence, as follows, indicates that the Masonic Park Tuff was derived from a source within the Mount Hope caldera: (1) the position of the caldera well within the Masonic Park ash-flow sheet (fig. 15) and adjacent to the thickest sections of this formation; (2) inter- tonguing of the Masonic Park Tuff with andesite lavas of the Sheep Mountain Andesite and with other locally derived lavas near the rim of the Mount Hope caldera; and (3) the presence near the south and southwest edges of the caldera of bedded welded tuffs at the base of the upper ash- flow sheet of the Masonic Park that are believed to represent agglutinated ash fall, a depositional process only likely close to the eruptive source (Lipman, 1975a, fig. 26). An earlier complex history of eruptions from the same general source is indicated by scattered exposures north- west and west of the Mount Hope caldera and in the highly faulted area southwest of the caldera (fig. 14). In these areas, the several individual ash flows within the composite older ash-flow sheet of Masonic Park Tuff intertongue with dark andesite flows and breccias of the Sheep Mountain Andesite that apparently formed a number of local volcanoes near the western margin of the later Mount Hope caldera. Just northwest of the Mount Hope caldera, the intertongued assemblage of Masonic Park Tuff and Sheep Mountain Andesite is overlain by thick coarsely porphyritic quartz latite lavas (volcanics of Leopard Creek), and these in turn are overlain by the thick upper sheet of Masonic Park Tuff (Steven and Lipman, 1973). We interpret these relations to indicate early concurrent eruptions of quartz latitic ash flows of Masonic Park Tuff and andesitic lavas and breccias from local Sheep Mountain volcanoes. The ash-flow eruptions were apparently of small volume, and it is not known whether they were accompanied by caldera subsidence. Viscous quartz—latitic lavas forming a thick local assemblage (volcanics of Leopard Creek) were erupted along the CENTRAL SAN JUAN CALDERA COMPLEX 17 107° | 37° _ 30' Younger volcanics Younger volcanics l / Younger volcanics Y Older volcanics 0 5 l I 10 15 KILOMETRES | 4| FIGURE l4.—Generalized geology of the Mount Hope caldera and adjacent areas. 0, precaldera (older) volcanic rocks; Tmp, Masonic Park Tuff; Tsm, Sheep Mountain Andesite; Tf, Fish Canyon Tuff; Th,, Huerto Formation; TI, volcanics of Leopard Creek; TC, Carpenter Ridge Tuff; y, volcanic rocks younger than Carpenter Ridge Tuff. Open rectangles outline the buried caldera. Heavy lines indicate faults; buried. northwest side of the postulated source area following accumulation of the older Masonic Park ash-flow tuffs and Sheep Mountain andesitic flows and breccias. Renewed eruptions of quartz-latitic ash flows identical in composition and appearance to the older sheet of Masonic Park Tuff spread great volumes of tuff in all directions from the source area. thereby forming the widespread upper sheet of Masonic Park Tuff and resulting in sub- sidence of the Mount Hope caldera. The Mount Hope caldera was not resurgently domed immediately after subsidence to any discernible extent; the Fish Canyon Tuff filled a hole more than 1.4 km deep and bar and ball or hachures on downthrown sides; dotted where the floor of the fill is not exposed. Later broad uplift of the caldera area is indicated by the progressive overlap of younger ash-flow units onto the eastern side of the caldera fill (fig. 14). This uplift apparently was more a trapdoor uplift than a dome, with the western side preferentially uplifted. This later uplift probably was roughly con— current with major faulting west and southwest of the Mount Hope caldera that will be discussed later. The Mount Hope caldera forms a type generally transitional between the earlier and later calderas of the San Juan field. Mount Hope is like the earlier calderas in its associated andesitic volcanism and the quartz-latitic composition of 18 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 107° “”1 P 305 3 rings . ACHIL l O 100 KILOMETRES FIGURE 15.—Distribution of the upper member of Masonic Park Tuff (diagonal lines) in relation to Mount Hope caldera (M) and San Juan volcanic field (shaded). the erupted tuffs; yet this caldera is well within the main gravity anomaly (fig. 1) and is associated with more silicic lava flows, as represented by the volcanics of Leopard Creek. LA GARITA CALDERA The La Garita caldera (fig. 16), the largest in the San Juan volcanic field, ranks among the great calderas in the world. The original topographic basin was at least 40 km across from north to south, and the subsided block within this basin appears to have been nearly 30 km across. The original east-west diameter of the caldera was somewhat less, but the precise dimensions cannot be established because the western part of the La Garita caldera has since been destroyed by later caldera subsidences. The La Garita caldera is located near the center of the San Juan volcanic field, above the east-central part of the triangular-shaped batholith indicated by gravity data (fig. 1). Subsidence took place 27.8 m.y. ago (Lipman and others, 1970, p. 2340), in response to eruption of more than 3,000 km?’ of the phenocryst-rich quartz-latitic ash of the Fish Canyon Tuff (fig. 17). This unit spread over an area of more than 15,000 kmz; it is 30-200 m thick over wide areas and accumulated to a thickness of more than 1.4 km within the concurrently subsiding caldera. Whereas many of the older San Juan calderas formed near clusters of earlier andesitic volcanoes (the Bonanza, Platoro-Summitville, San Juan, Uncompahgre, and Mount Hope calderas), any such relationship is more difficult to determine for the La Garita caldera because of its size. The intrusive core of an older volcano is exposed along the eastern wall of the La Garita caldera, and other volcanic centers are near the east, west, and north sides of the caldera, but exposures are inadequate to indicate whether these volcanoes were concentrated near the caldera. A more important localizing factor may have been the position of the La Garita caldera with respect to the under- lying batholith. The caldera occupies much of the roof above the broadest part of the batholith indicated by the gravity anomaly (fig. 1); this part of the roof was re- peatedly disrupted by ash-flow eruptions and caldera sub- sidences throughout the less than 2 my that remained of the period of intermediate to silicic volcanic activity. The recurrent volcanic activity and related subsidences here must have reflected equally active high-level magmatism throughout the eastern part of the batholith. Little can be reconstructed of the early development of the La Garita caldera. The floor of the caldera is nowhere exposed, and the eastern and northern margins that survived later caldera subsidences are marked by a structural moat deeply filled with younger volcanic deposits. Little air-fall ash occurs below the outflow sheet, so premonitory eruptions seem to have been minor. In addition, no compositional zoning was noted anywhere in the outflow sheet of Fish Canyon Tuff, and the basal rocks seem indistinguishable from those elsewhere in the unit. Apparently, large volumes of ash were erupted quite suddenly from a major chamber containing relatively homogeneous phenocryst-rich quartz-latitic magma. The disparity in thickness between the intracaldera (more than 1.4 km thick) and outflow parts (generally less than 200 m thick) of the Fish Canyon Tuff requires sub- sidence concurrent with eruption. Compaction foliation and rude layering, marked in places by less-welded partings, are virtually parallel throughout the caldera fill, and indicate that block subsidence was concurrent with filling. Little igneous activity or sedimentation seems to have occurred immediately after subsidence, and the first demonstrable event was broad resurgent doming of the core. The crest of the dome was relatively flat over an area at least 5 km across, and the preserved eastern and north- eastern flanks dip 5°-15° radially outward from the crest. Minor tensional faults separate flat from inclined segments of the core, and also separate inclined segments that dip in different directions. Maximum relief from the northeast moat to the crest of the dome was more than 1.4 km. The western and southwestern flanks of the resurgent dome have since caved into younger calderas, and have been covered by younger volcanic units. Most of the moat is deeply filled by younger volcanic rocks, or has been destroyed by younger caldera sub- sidences. The lower fill, along the northeast side of the CENTRAL SAN JUAN CALDERA COMPLEX 19 caldera, consists of Carpenter Ridge Tuff, in places showing evidence of having accumulated in shallow water. Minor tuffaceous sedimentary rocks exposed locally in this area probably represent deposition by small streams or in local ponds. No late ring-fracture igneous activity is evident along the eastern or northern sides of the La Garita caldera, but the major. rhyolite dome along Miners Creek, partly exposed in a deep canyon 6 km west of Creede (Steven and Ratté, 1965), is older than the Carpenter Ridge Tuff, and may have erupted through the western ring-fracture zone of the La Garita caldera. The eastern wall of the La Garita caldera apparently stood especially high, and most subsequent units from the central San Juan caldera complex were trapped against it. Elsewhere, the younger ash flows overtopped the La Garita caldera rim and spread widely. This high part of the caldera wall marks the truncated end of a line of major older andesitic volcanoes that extended west-northwest from the Summer Coon center 10 km north of Del Norte (Lipman, 1968), through the Baughman Creek and upper La Garita Creek centers, to the Sky City center exposed in the eastern topographic wall of the caldera (fig. 16). These volcanoes were deeply eroded prior to eruption of the Fish Canyon Tuff, but protruded through all but the youngest subsequent ash-flow units. BACHELOR CALDERA The Bachelor caldera, the source of rhyolite ash flows that formed the widespread Carpenter Ridge Tuff (fig. 18), collapsed along the west margin of the La Garita caldera (fig. 16). The age of the Carpenter Ridge Tuff and of the Bachelor caldera is bracketed by K-Ar ages of 27.8 m.y. for the older Fish Canyon Tuff and 26.7 m.y. for the younger Mammoth Mountain Tuff. The outflow Fish Canyon and Carpenter Ridge Tuffs are separated by andesite flows and breccias of the Huerto Formation that formed a series of volcanoes near the present edge of volcanic rocks south and southwest of the Bachelor caldera. After eruption of the andesites, the area southwest of the Mount Hope caldera (fig. 14) was intricately downfaulted toward the caldera complex, as described later. During the Huerto eruptions and later faulting, magma accumulated in a newly developed high- level chamber beneath the west side of the La Garita caldera, and differentiated to a silicic phenocryst-poor rhyolite which erupted to form the Carpenter Ridge Tuff. Southeast of the caldera, the Carpenter Ridge is locally compositionally zoned from rhyolite upward into quartz latite. Eruption of the Carpenter Ridge Tuff was accompanied by subsidence of an oval-shaped caldera about 15 by 25 km across. Tuff more than 1.5 km thick accumulated within the subsiding caldera, whereas the outflow sheet generally is less than 400 m thick. As in other calderas in the San Juan Mountains, these relations are believed to indicate subsidence concurrent with eruption. The floor and eastern wall of the Bachelor caldera are exposed locally northeast of Creede. In this area, a rough fault—block topography marked by tectonically shattered rocks, fault scarps, and talus breccia developed on the older rocks (Steven and Ratte’, 1965, p. 17). These fault blocks underlie the Bachelor caldera fill and form the eastern caldera wall, where they mark the progressive breaking down of the resurgent core of the La Garita caldera toward the younger subsidence feature to the west. In part these fault blocks may represent inward sliding of the Bachelor caldera wall toward the main ring-fracture zone during Carpenter Ridge eruptions. Similar relations on the west side of the caldera may be represented by the intertonguing of Shallow Creek Quartz Latite with the intracaldera Bachelor Mountain Member of the Carpenter Ridge Tuff, as described by Steven and Ratte’ (1965, p. 23). This intertonguing was mapped in the early 1950’s before any of the calderas in the central San Juan Mountains had been recognized. In retrospect it seems plausible that the layers of Shallow Creek Quartz Latite within the Bachelor caldera fill may instead be avalanche and landslide breccia derived from a major quartz latite dome (fig. 8) that formed the nearby over- steepened caldera wall, a relationship which has been found in many other intracaldera fills in the San Juan Mountains. This suggestion needs to be checked in the field. Three small ash flows resembling Fish Canyon Tuff appear to intertongue with the intracaldera Bachelor Mountain Member of the Carpenter Ridge Tuff on the east side of the Bachelor caldera (Steven and Ratte', 1965, p. 18). These relations, if valid, indicate that neighboring cupolas on the underlying batholith were sufficently locally restricted to permit contrasting magmas to coexist locally and earlier magma types to persist while the Carpenter Ridge rhyolite was differentiating. The core of the Bachelor caldera was resurgently domed shortly after subsidence, and a fragmented cross section of the dome can be seen in the north wall of the younger Creede caldera. The top of the Carpenter Ridge Tuff is more than 700 In higher at the crest of the dome than it is near the eastern and western ring-fracture zones. Tensional fractures that formed during doming dropped local blocks nearly 500 m near the crest of the uplift, and longitudinal normal faults, including the ancestral Amethyst fault in the Creede mining district (Steven and Ratte', 1965, p. 55-56), extended north-northwest down the axis of the uplift. These faults marked the initial fracturing of an area that was recurrently broken during later volcanic episodes, and was widely mineralized at the time when the ores of the Creede mining district were deposited. 20 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 107° l qa$$$u¥ %$‘ 6 'F-p. f4 ‘hx % ‘ *5. X X XX 5.“ / ¢=1==§\ \ %/ ///¢ \\\ \X / // \\ X % / \\ \ % / \ \ 380_ /§ L/ 4§$§ Resurgent core \ \ f % \ l i: //% / / ‘I l lt% LA GARITA / / l f fl / CALDERA l ,I i k Intrusive core, Sky City \ / < Fault (hachures toward downthrown side) volcanic center ‘ w 1 “ I '/ ‘ / f e” 4‘» 1.3 Structural margin\, 7 SHALLQW CREEK g VOLCANlIC DOME z / i K r \ I 5 it W :1 v” OCreede \ f {:45 BACHELOR CALDEM\\ ‘1 \ //l E 3 \\ \\ / { /// Ag / \ \ / f=¢ // X \\ // f l’ \ \\§==§/ I’ f X 2’ Topographic rim? x / 1’ \\ ’7 \**%+4—*’ 0 5 ‘IO 15 KILOMETRES | FIGURE l6.—Restored Bachelor and La Garita calderas. Control moderate to good where boundaries are shown by solid symbols; conjectural where shown by open symbols. Small plugs of rhyolite cut the intracaldera tuffs near the center of the Bachelor caldera. These plugs are most abundant in the rocks that are cut by tensional faults and that were pervasively brecciated during resurgent doming. Evidence is ambiguous, but the rhyolite plugs may have been emplaced before brecciation and, thus, prior to resurgence. No postresurgence igneous activity related to the Bachelor caldera cycle has been recognized. The shattered intracaldera Bachelor Mountain Member of the Carpenter Ridge Tuff on the resurgent dome was altered during the waning stages of the Bachelor caldera cycle. The shattered tuff was intensely silicified and largely healed to a massive rock. The K20 content of the silicified rock increased markedly from about 5 percent to as much as 11 percent (Ratte’and Steven, 1967, p. H13); this increase is reflected by abundant very fine grained orthoclase in the matrix. In the northern part of the Bachelor caldera, the intracaldera tuffs were variably bleached and altered, and disseminated pyrite was erratically introduced. No significant economic mineral deposits related to the Bachelor caldera cycle are known, however. MAMMOTH MOUNTAING) CALDERA Indirect evidence suggests that subsidence took place during eruption of the Mammoth Mountain Tuff, but the CENTRAL SAN JUAN CALDERA COMPLEX 21 108° 107° 106° A 108° 107° 106“ _ Quwi <\ R 5 1— @7— (‘ “if 72 ,n N ON 72 _. G N ’14? I / \l lida / 7226b ‘ \‘ lida / ontros \‘ I .Montros n \x _ ._L—— \\ _ ___‘._ l \ ‘11“"“~' . ‘i"“"- \ «a sea , -——-1iou A '_'EbU A '"l E '1 38° 38" 5 Tell Tell ,\/\\ ,\/\\ / .._ / __ é N J A 2/ é N J A — — _ _ l I _ _ _ l l . e I . e | Al . ,7 “5'52. r_ _ am ,7 “Egg. [—7— _Ala 0 V HLH P L I P ~L‘- P L I p 1) g ( s <3 . 3 5:2- ' IN n 0 ' LII' n O / r ' A c H L E / r g ‘ A c H L E . g | ‘ § l e ‘ § § I 370 V 370 ‘3 | e 0 100 KILOMETRES o ‘ 100 KILOMETRES |___|___A__#|————‘——J L___|____J——l———‘——l FIGURE l7.—Distribution of the Fish Canyon Tuff (diagonal lines) in relation to La Garita caldera (G) and San Juan volcanic field (shaded). postulated caldera has been largely destroyed by the younger Creede caldera and covered by younger rocks. The Mammoth Mountain eruptive cycle began with local episodic pyroclastic eruptions that formed a sequence originally called Farmers Creek Rhyolite by Steven and Ratté (1964, 1965) and Ratte’ and Steven (1967); subsequent regional work has indicated that these rocks were primarily local products of premonitory eruptions in the Mammoth Mountain cycle (Steven, Lipman, Hail, and others, 1974; Steven and Ratte, 1973) that were deposited just northeast of the postulated source beneath the younger Creede caldera. The early episodic pyroclastic eruptions progressed into a pulsating, nearly continuous, ash-flow eruption that flooded (fig. 19) the lower parts of the rough topography left by subsidence and resurgence of the La Garita and Bachelor calderas (fig. 1). Most of these Mammoth Mountain ash flows welded into coherent, dense tuff that shows obscure compound cooling (Ratté and Steven, 1967, p. H18-H33). The Mammoth Mountain Tuff accumulated to a thick- ness of more than 500 m in the moat of the resurgent Bachelor caldera, where it is strongly zoned from phenocryst-poor rhyolite at the base to phenocryst-rich quartz latite at the top (Ratté and Steven, 1964, 1967). The Mammoth Mountain Tuff wedges out against and is absent over the top of the resurgent core of the Bachelor caldera, and was contained on the northeast and south- west by the outer topographic wall of the caldera. A 300— to 400-m:thick sheet of densely welded quartz latite tuff of the FIGURE 18,—Distribution of Carpenter Ridge Tuff (diagonal lines) in relation to Bachelor caldera (B) and San Juan volcanic field (shaded). Mammoth Mountain Tuff extends southeast from the postulated source within the subsequently formed Creede caldera to the southeast topographic wall of the La Garita caldera where it wedges out abruptly. Remnants of crystal- rich Mammoth Mountain Tuff 5-20 m thick are preserved near the Continental Divide 35-40 km south of Creede, indicating that the unit spread widely in this direction over the smooth top of Carpenter Ridge Tuff. Near the postulated source, the products of the Mammoth Mountain eruptive cycle show structural relations that may indicate nearby caldera subsidence related to the ash-flow eruptions. The rudely layered rocks at the base of the section—the Farmers Creek Tuff (Farmers Creek Rhyolite of Steven and Ratté, 1965)—and most of the overlying phenocryst—poor tuff are inclined 10°—15o NE. The dip of this compaction foliation flattens in the younger quartz-latitic rocks; the change in dip is about coincident with the change in lithology and takes place within a vertical span of about 50 m. This change in dip is interpreted to reflect structural disturbance during Mammoth Mountain eruptions, a structural disturbance perhaps related to caldera collapse at the source vents. Most of any Mammoth Mountain caldera that may have existed was destroyed by the younger Creede caldera. The size or shape of the postulated caldera is unknown, except that it must be within the topographic wall of the Creede caldera, which exposes pre—Mammoth Mountain rocks on its northern, eastern, and southwestern segments. Numerous viscous quartz-latitic lava flows that probably 22 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 108° 3 _ ~____: I Q‘k;22&u l l M 0% l ‘ ontros w 7‘ _ __ _ l '11- _. "\. N -— {W A '1 38° Tell 7 ‘\ gs N J A l . e I /17 LLEEZ "~‘— _ _ 75 “L1 PL I P _. s 5:22. 3/ ./ éwngo iA CH LE 2 «4 e ' 1 37° #‘l i l J § 0 100 KILOMETRES ;|—_I~l_i__i__l FIGURE 19,—Distribution of Mammoth Mountain Tuff (diagonal lines) in relation to probable source area (S) and San Juan volcanic field (shaded). accumulated near their own vents overlie the Mammoth Mountain Tuff east and northeast of the Creede caldera and may represent the outer parts of flows and domes erupted marginally around the postulated Mammoth Mountain caldera. SOURCE OF THE WASON PARK TUFF The Wason Park Tuff is a simple cooling unit of rhyo- litic ash-flow tuff that spread widely over the central San Juan volcanic field (fig. 20) and seems centered on the younger Creede caldera (fig. 1). North of the Creede caldera, the Wason Park fills the upper parts of the same rough topography that confined the Mammoth Moun- tain Tuff. It is largely confined to the moats around the re- surgent La Garita and Bachelor calderas where it rests on thick tongues of Mammoth Mountain Tuff, and it thins or wedges out laterally against the resurgent cores 0r topo- graphic walls of these calderas. To the west, south, and southeast, however, the Wason Park Tuff spread widely as a thin sheet on top of older ash~flow sheets. The Wason Park Tuff has a volume greater than 100 km3 (Ratté and Steven, 1967, p. H34) and is sufficiently voluminous that subsidence should have resulted at the source (Smith, 1960, fig. 3), but no caldera structure is exposed within the area covered by the sheet. The only area large enough for subsidence related to Wason Park Tuff is under the younger Creede caldera, which is near the center of the area of distribution of the thickest and most densely welded Wason Park Tuff. White pumice blocks charac- teristic of the Wason Park Tuff are larger near the Creede 108° 107° 106° “ «st-“i l <. s l ’02:- “ l G U N o N/ \ /\ ’ ‘9”? ' (New; ,Montros Il' un . \ x M} ’I‘ .\ /‘ UR JgESA —— _ _. __.l-- k ' \ \ E h r l l '___ A‘Iah \ ' P I 3) £32: /i7‘ 3 A c H L E y I . | i s 0 100 KILOMETRES W FIGURE 20.——Distribution of Wason Park Tuff (diagonal lines) in rela- tion to probable source area (s) and San Juan volcanic field (shaded). caldera than in the distal parts of the sheet (Ratté and Steven, 1967, fig. 16), and may indicate proximity to source. A local lava flow closely similar in lithology to the densely welded Wason Park Tuff underlies the Wason Park along the northeast rim of the Creede caldera (Ratté and Steven, 1967, fig. 158), again suggesting a nearby source. SAN LUIS AND COCHETOPA PARK CALDERAS Although completely separate subsidence structures, the San Luis and Cochetopa Park calderas (figs. 21 and 22) are so intertwined in evolution that they are considered together. Furthermore, the San Luis caldera appears to be a compound structure consisting of two overlapping subsided blocks that formed in sequence during separate periods of ash-flow eruptions. Stratigraphic uncertainties preclude confident inter— pretation of the evolution of these calderas. The history of past usages of the Rat Creek, Nelson Mountain, and Cochetopa Park as rock-stratigraphic units chronicles the confusion that has stemmed from attempts to correlate virtually identical rock types from area to area; nearly every report dealing with these units that has been pub- lished since 1964 treats them differently. This report continues this confusing practice because none of the previous usages permits reconstruction of a coherent history of evolution of the calderas. In its type locality in the Creede mining district (Steven and Ratté, 1964, 1965), the Rat Creek Tuff consists largely of soft white nonwelded to slightly welded zeolitized tuff, CENTRAL SAN JUAN CALDERA COMPLEX 23 107° COCHETOPA PAR K CALDERA Fault (hachures on downthrown side) \ \ "0 XX \ "9» x o 9’0 38 — \ go i‘ \ g l LA GARITA l ’ l CALDERA l l FStructural margin 1' l / I / / EARLY STAGE 7 Nelson I / SAN LUIS Mountain l / CALDERA B Tuff l f I f if 0 5 10 15 KILOMETRES If I LL #l FIGURE 2l.—Genera1ized geology of the San Luis and Cochetopa Park calderas in relation to remnants of the Bachelor (B) and La Garita calderas. Control moderate to good where boundaries are shown by solid symbols; conjectural where shown by open symbols. E, general area of the Equity mine; SLP, San Luis Peak. with a ledge of densely welded tuff 10-30 m thick just above the middle. The Rat Creek is overlain by Nelson Mountain Tuff, a densely welded quartz latite containing 15-30 percent phenocrysts. Steven and Ratté (1965, p. 37) included in the Nelson Mountain the large mass of propylitized densely welded tuff (the Equity Quartz Latite as used by Emmons and Larsen, 1923) that fills the lower part of the San Luis caldera, although they could not prove physical continuity. After extensive regional mapping in the central and northern San Juan Mountains, Steven, Lipman, and Olson (1974, p. A80) concluded that the densely welded rocks in the San Luis caldera were probably the intra- caldera equivalent of the Rat Creek Tuff, and called them the Equity Member of that formation. Overlying densely welded tuffs on the north and northeast sides of the caldera were correlated with the lithologically identical Nelson Mountain Tuff at its type locality. These overlying welded 24 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 107° 1 lfl} _,_ / K * 4 l x X r \ **‘v~ / // \ Topographic rim \\ \ / a9: '2' =§%\\ \ " \\ 7b \ l’ \\ 099,. X ‘t [I \ 9%,. \ J ( —> l ,,,l \ 06’ \ 33°~ w F. , SAN LUIS \ ”a \ V (mill: ' ,, : CALDERA \ — Postpalgena) r g ~ a , l‘Structural margin 4 :lfvtaix: > y l i l '1: fl;- \gst; LA GARITA ' i A ~» é « % - A (intrusivei - CALDERA l I EARLY STAGE I i [I I’)’ SAN LUIS Fault (hachures on downthrown side) / CALDER? I, / 2’ l I / l [I / l 1 t I l l( / l I l / l / i / / / / )’ CREEDE { CALDERA { / RIO GRANDE GRABEN )’ d—-& r O 5 1O 15 KILOMETRES l FIGURE 22.—Generalized geology of the Creede and San Luis calderas in relation to remnants of the Bachelor (B) and La Garita calderas. Control moderate to good where boundaries are shown by solid symbols; conjectural where shown by open symbols. PR, Point of Rocks volcano; S, Spar City. ~ tuffs extend into their apparent source area in the Continued work in the northwestern San Juan Cochetopa Park caldera to the northeast. Younger post- Mountains has shown that something is wrong with this subsidence tuffs filling the Cochetopa Park caldera were interpretation of the stratigraphic assemblage. Major called the CochetOpa Park Member of the Nelson densely welded ash-flow tuffs in the Lake City area (40 km Mountain Tuff, northwest of Creede) almost certainly are equivalent to the CENTRAL SAN JUAN CALDERA COMPLEX Nelson Mountain Tuff at its type locality in the Creede mining district, but details of distribution and volume make it highly unlikely that they were derived from the Cochetopa Park caldera. This forces the following tenta- tive conclusions: 1. The correlation of Nelson Mountain Tuff in its type locality in the Creede district with lithologically identical tuffs northeast of the San Luis caldera probably is wrong. 2. The original correlation of Nelson Mountain Tuff with the densely welded (Equity) tuffs in the San Luis caldera by Steven and Ratté (1964, 1965) probably is correct. The Equity Member is therefore removed from the Rat Creek Tuff and assigned to the Nelson Mountain Tuff. 3. The densely welded tuffs north and northeast of the San Luis caldera, apparently derived from the Cochetopa Park caldera, constitute a separate unit younger than the Rat Creek and Nelson Mountain Tuffs. We there- fore remove the Cochetopa Park Member from the Nelson Mountain Tuff and raise it to formational rank as the Cochetopa Park Tuff. The interpretations that follow assume that these tenta- tive conclusions are generally correct, but repeated examinations of critical field relations and petrographic studies have failed to ~develop the firm criteria for dis- tinguishing these units that will be required for confident evaluation of our evolutionary model. COMPOUND SUBSIDENCE OF THE SAN LUIS CALDERA As currently (1975) understood, the Rat Creek Tuff appears to be a relatively low volume assemblage of poorly welded rhyolitic ash-flow tuffs confined largely to the vicinity of the Creede mining district (Steven and Ratté, 1965). Several factors imply a nearby source: (1) at least one local volcano of Rat Creek age is well exposed in the heart of the Creede district (Steven and Ratté, 1965, p. 35); (2) a local ledge of densely welded tuff occurs just above the middle of the Rat Creek Tuff within the Creede district; and (3) a small subsidence structure (caldera) of Rat Creek age is located along the northwest side of the Creede district. This last structure, described below, is here considered an early stage in the development of the compound San Luis caldera (fig. 21). The regional sheet of Rat Creek Tuff ranges widely in thickness because of rough underlying topography. It is 150-200 m thick through the central part of the Creede district, but to the northeast it wedges out completely against an eroded fault scarp cutting the resurgent core of the La Garita caldera. North of the Creede district, the Rat Creek is cut off by the topographic wall of the main San Luis caldera. 25 In the early stage of the San Luis caldera along Miners Creek northwest of the Creede district (fig. 21), soft zeolitized tuffs that are physically continuous with type Rat Creek Tuff fill a partly exposed subsidence structure that has a steep arcuate fault along the south and south- west sides. To the north and northeast, this caldera is largely covered by younger rocks and relations are almost totally obscured; the map pattern (figs. 21 and 22) implies that the northern part of the caldera caved into the main San Luis caldera which developed shortly afterward. Soft tuffs of the Rat Creek accumulated to a thickness of more than 350 m within the early caldera and are at least twice as thick as nearby correlative outflow tuffs. The intracaldera tuffs are flat lying and show no evidence of resurgence. The faulted margin is largely obscured by massive landslides, but it can be estimated to within a few metres along one gully where it appears to be very steep. Elsewhere the fault is indicated by juxtaposition of thick intracaldera Rat Creek Tuff and flat-lying older rocks in the wall. Two intrusives were emplaced along or near the faulted margin. One is a major neck, more than 1 km across, that clearly occupies the fault; the other forms a low knob, 500 m across, completely surrounded by landslide debris. Several features indicate a Rat Creek age for the early caldera. The wall outside the faulted margin includes rocks as young as Wason Park Tuff, which is the next oldest ash-flow unit in the San Juan Mountains, and locally may include the overlying andesite of Bristol Head which is directly beneath the Rat Creek Tuff just east of the early caldera. The local ledge of densely welded tuff in the upper part of outflow Rat Creek Tuff extends westward to the vicinity of the early caldera, but is nowhere found within the caldera, although the area of its projected position is well exposed. However, the caldera margin, whose development probably accounts for this discordance, is covered by surficial debris and younger lavas. The caldera fill is overlain by typical Nelson Mountain Tuff that extends from its type locality westward across the area of the early San Luis caldera. The Nelson Mountain forms an unbroken rim across the trend of the older faulted caldera margin without any change in thickness or evidence of deformation. The main San Luis caldera (figs. 21, 22, and 23) is now thought to have subsided concurrently with eruption of the Nelson Mountain Tuff, and at least 1.5 km of phenocryst-rich quartz latite accumulated within the sub- siding basin, whereas the outflow sheet nearby is only about 300 m thick. This thick mass of intracaldera ash welded into a dense, nearly homogeneous rock with only a few local less-welded partings. Later propyllitic alteration further homogenized the rock and obscured partings. The base of the intracaldera Nelson Mountain Tuff is exposed , locally within the north-central part of the caldera core 26 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 107° 106° GL3 1N iO‘N <\\ iii 1 ' // ‘WW /) / r/a’x / \\ IV/ (LMESA 7A,:- — - -J— Rs ,/// \\ fi‘ \ E 3 /{ .\ \ J ‘ rm f" : \/\\;\¥"K L. '/ A‘lak; . :r i 35:22.1 / N CH (LE 0/; a H 4 0 100 KILOMETRES FIGURE 23.—Distribution of Nelson Mountain Tuff (diagonal lines) in relation to San Luis caldera (S) and San Juan volcanic field (shaded). where densely welded quartz latite directly overlies a local hill, probably a volcanic dome, of older rhyolite. Some probable Carpenter Ridge Tuff is exposed on the north flank of this hill, but its relations with the adjacent rhyolite are obscure. N0 soft tuff representative of Rat Creek ash flows is exposed on this buried hill, but the lower parts of the caldera floor where such tuff deposits might more reasonably be expected are nowhere exposed. Subsidence of the main San Luis caldera produced a broad basin nearly 15 km across. Intermediate to silicic lavas and breccias of the volcanics of Stewart Peak accumulated within the northern and eastern parts of this basin, and were locally accompanied by deposits of stream and lake sediments (figs. 21 and 22). Most intracaldera lavas around the eastern side of the caldera are rhyodacite and quartz latite, but perlitic rhyolite flows are abundant along the north margin. All known dike and neck feeders for the intracaldera lavas are within the caldera core. The eastern margin of the caldera block again subsided locally during eruption of the intracaldera lavas, as indicated by a fault with at least 500 m of throw that places the lower part of the volcanics of Stewart Peak within the caldera against older Fish Canyon Tuff in the wall. The fault passes under unbroken Cochetopa Park Tuff to the north, and ledges of this younger unit representing successive ash flows both to the north and south intertongue with younger lavas on the Stewart Peak sequence, providing an upper age limit for the renewed subsidence. SUBSIDENCE OF THE COCHETOPA PARK CALDERA. Ash flows of the Cochetopa Park Tuff that were deposited in the San Luis caldera area (fig. 1) during accumulation of the intracaldera volcanics of Stewart Peak had their source in the Cochetopa Park area, some 30 km northeast. Minor phenocryst-poor rhyolite ash accu- mulated near the source, but the eruptions soon changed to crystal—rich quartz latite that is seemingly identical with the Nelson Mountain Tuff. The Cochetopa Park ash flows followed the nearly filled moat around the northern side of the La Garita caldera (fig. 1) and inter-tongued with the volcanics of Stewart Peak within the San Luis caldera (fig. 24). 0 100 KlLOMETRES W FIGURE 24.—Distributi0n of the Cochetopa Park Tuff (diagonal lines) in relation to Cochetopa Park caldera (C) and San Juan volcanic field (shaded). In contrast with the larger San Juan calderas, where subsidence occurred concurrently with ash-flow eruptions, the Cochetopa Park caldera collapsed after major eruptions had ceased,» as indicated by lack of thickening of the intracaldera tuff. In further contrast with the larger calderas, the Cochetopa Park caldera did not form a complete circular structure, but subsided as a trap- door bounded by a horseshoe-shaped fault. The south- western margin did not fracture, but merely bent down- ward and tilted eastward. A trough, probably representing a graben, formed along the northwest side of the caldera core, and a tilted, northeast-trending ridge of Cochetopa Park Tuff dipping 5°-l 0° E. extended beyond the middleof the caldera (fig. 21). An inner ring-fracture zone can be postulated to account for major subsidence (fig. 21). CENTRAL SAN JUAN CALDERA COMPLEX 27 Maximum displacement on the northeastern side of the trapdoor is 700-800 m, as judged from the height of the topographic wall in this direction. After the inner trapdoor block had subsided, or perhaps concurrently with subsidence, the walls of the caldera slumped inward along arcuate faults that nearly surround the downfaulted parts of the caldera. The breakaway zone near the hingeline of the trapdoor on the west side of the caldera is a splintered area of many faults and jumbled relations between blocks. Elsewhere, the slumped blocks are bounded by curved, linking faults that are concave toward the subsided trapdoor. Locally along the northeast side, no slumping took place. Minor pyroclastic eruptions after subsidence of the Cochetopa Park caldera deposited local moderately to densely welded ash-flow tuffs near the hingeline and thick nonwelded pumiceous ash-flow tuff within the caldera east of the medial ridge (fig. 21). The trough (graben?) northwest of the medial ridge was filled with sandy tuffaceous stream deposits and a few layers of air-fall tuff. The two facies merge in the northeastern part of the caldera about on trend with the medial ridge. Layering in the tuffaceous caldera fill is flat and shows no resurgence of the caldera core. In places the caldera fill covers arcuate faults of the outer slumped zone and provides an upper limit on the age of that faulting. A thick rhyolite flow with a prominent black vitrophyre at its base was erupted through the caldera till about on trend with the medial ridge, and eroded remnants still persist as the feature called Cochetopa Dome. RESURGENCE OF THE SAN LUIS CALDERA Whereas minor resurgence of the San Luis caldera may have preceded or accompanied eruption of the intra- caldera volcanics of Stewart Peak, most resurgence took place later. Densely welded layers of the Cochetopa Park Tuff, representing many successive ash flows from the Cochetopa Park caldera, intertongue with the upper volcanics of Stewart Peak and are involved in this resurgent uplift. North of the caldera, the Cochetopa Park Tuff is inclined less than 10° in various directions, but near the caldera margin the layers are bent up along a curving hingeline and dip 20°-25° radially outward from the resurgent core. In part, this hingeline is marked by a fault with little displacement and the change in dip is sharp; elsewhere no fault is apparent and the change in dip is less abrupt. South of the caldera, resurgence is locally marked by an abrupt change in dip from flat layers of Nelson Mountain Tuff outside the caldera to layers dipping 15° or so southward off the dome within the caldera. Near the Equity mine in the northern part of the Creede district, however, local resurgence uplifted a triangular block, 1.5-3 km on a side, nearly a kilometre (Steven and Ratté, 1965). This resurgence was somewhat asymmetrical to the San Luis caldera. The small remnant of the early subsided block along the southwest side of the caldera is not domed, and evidence of resurgence is apparently limited to the main caldera and to an area extending about a kilometre beyond the eastern and northeastern structural margin of the caldera. The edge of resurgent uplift on the north side of the San Luis caldera is just inside the outer topo- graphic wall, and possibly is outside the buried structural margin. The minimum structural relief caused by resurgence is a little less than a kilometre, as indicated by the elevation difference between the top of flat-lying Nelson Mountain Tuff outside the caldera and the top of San Luis Peak within the caldera. On the north side of the caldera, this difference is about 1.4 km, whereas on the south side it is about 0.7 km; as discussed later, this contrast reflects tilting caused by late general uplift of the roof of the batholith that underlies the central part of the San Juan volcanic field. POSTCALDERA LAVAS Resurgence of the San'Luis caldera was followed by development of a line of volcanoes that extends westward from the caldera about 14 km. The eastern part of these postcaldera volcanic rocks, as shown in figures 21 and 22, consists of coarsely porphyritic lavas and breccias (the quartz latite of Baldy Cinco) that were erupted from many local vents, some within the western part of the San Luis caldera. Over most of their extent, these lavas and breccias rest on flat ledges of densely welded Nelson Mountain Tuff or Cochetopa Park Tuff, but on the east they abut and wedge out against tilted volcanics of Stewart Peak and Nelson Mountain Tuff in the resurgent core of the San Luis caldera. Apparently most of the western third of the caldera was once covered by these rocks. Lavas of similar age and composition (quartz latite of Rambouillet Park) (Steven, 1967) extend farther southwest, toward the south- east margin of the Uncompahgre caldera. CREEDE CALDERA The Creede caldera (fig. 22) formed in response to eruption of the Snowshoe Mountain Tuff about 26.5 m.y. ago and is the youngest subsidence structure in the central San Juan caldera complex. Its form is clearly reflected in the modern landscape. (See frontispiece, Ratte' and Steven, 1967, for a color panorama of the Creede caldera.) The excellently preserved topographic form of this structure has resulted primarily from burial of all but the higher parts of the resurgent core by stream and lake sediments of the upper Oligocene Creede Formation that were not removed by erosion until late Cenozoic time (Steven, 1968, p. 1 14). Development of the Creede caldera was considered in detail by Steven and Ratte’ (1965, p. 58-62), and Smith and Bailey (1968, p. 625-626) used it as one of their seven examples of typical resurgent cauldrons. 28 The Creede caldera subsidence was localized along the southwest margin of the La Garita caldera (fig. 22); it obliterated the south part of the Bachelor caldera and the larger part of any calderas related to eruptions of the Mammoth Mountain and Wason Park Tuffs. Initial eruptions of the phenocryst-rich quartz latite forming the Snowshoe Mountain Tuff spread a thin, poorly welded sheet over the flat surfaces of earlier ash- flow tuffs in adjacent areas (fig. 25). Subsidence began shortly thereafter and proceeded concurrently with eruption. More than 1.4 km, and perhaps more than 2 km, of nearly uniform crystal—rich ash accumulated within the subsiding basin; most of this is densely welded, but a few partings are less welded. 107° 106 7 ~ —-~—‘ < K- @2ng GU N ON/ ‘CJ—Z /‘ ‘ ‘Nw; \\ 0%; , ontros iv 1‘ un ( \‘ x _.J,_ IV \ 38° I I/ LLE. TO -—- Alan\s\ /L/ P L ES i— _ '— v " ' P gosa )7 ;. j S rings / / as ur ngo I § I A C H L E O § .§ I 37° V l l. ‘ O 100 KILOMETRES FIGURE 25.—Areas where erosional remnants of Snowshoe Mountain . Tuff are preserved (diagonal lines) in relation to Creede caldera (C) and San Juan volcanic field (shaded). Tongues of talus and rockfall breccia (Steven and Ratte', 1965, p. 42) extend into at least the upper 700 m of the intracaldera Snowshoe Mountain Tuff along the western side of the caldera, and probably are present but unexposed elsewhere within the intracaldera tuffs. These breccias clearly indicate subsidence concurrent with accumulation of the Snowshoe Mountain Tuff, and demonstrate that the developing caldera wall exposed rocks as old as Wason Park Tuff at least episodically during subsidence. The successive rude layering and compaction foliation in the Snowshoe Mountain Tuff are virtually parallel, and indicate that the core of the caldera sank as a coherent mass, rather than in piecemeal blocks. Final subsidence left a basin 12-15 km across, with steep walls rising 1—1 .4 km above the flat floor. These walls were CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO unstable, and great masses fell off to form tongues of rock- fall breccia that extended over the caldera floor. The north- east wall of the caldera, where densely welded Mammoth Mountain and Wason Park Tuffs overlie soft tuffs, was particularly susceptible to avalanching, and a broad crescent-shaped scallop, 8 km wide and 3 km deep, developed in the outer wall. Breccias derived from the northeast wall of the caldera (Steven and Ratte', 1965, p. 42- 43) have been identified more than halfway across the caldera, 8-10 km from their source. The core of the Creede caldera was strongly domed after subsidence, and the center of the dome was uplifted more than 1.5 km above the structural moat left around the periphery. The eastern part of the uplift is a simple half dome with the layers of Snowshoe Mountain Tuff dipping radially outward 25°-45°. A deep north-trending graben extends across the center of the uplift; displacement is minor near the north and south ends of the graben, but exceeds 700 m near the center of the' dome. Internal structure of the graben is complex (Steven and Ratte', 1965, pl. 1), but its general form is a fractured keystone block across the distended top of the resurgent dome. The west part of the resurgent core was less regularly uplifted; the northern flank was tilted northwest and broken by faulting and the western flank was tilted generally west- ward and was progressively more uplifted toward the south. A small dome of autobrecciated rhyolite (Point of Rocks volcano) formed along the west side of the Creede caldera at some time just preceding, during, or immediately after resurgent doming. Pebbles of similar rhyolite have been found in some avalanche breccias caught in jumbled fault blocks along the keystone graben, suggesting that this volcano may have formed before resurgence. On the other hand, the eruption may have followed resurgent uplift, inasmuch as a local exposure shows the feeding neck to be nearly vertical where it cuts across strongly inclined layers of Snowshoe Mountain Tuff. Resurgence of the Creede caldera was followed by eruption of flows and domes of Fisher Quartz Latite at places around the periphery and by deposition elsewhere of the stream and lake sediments and travertine of the Creede Formation. Closely accordant K-Ar age deter— minations of Fisher lavas indicate that this stage in the Creede caldera cycle took place about 26.4 m.y. ago. Most of the postcaldera lavas were erupted from centers north- east and south of the caldera. Ash-flow deposits in the con- currently deposited Creede Formation are most abundant toward the south, indicating that Fisher centers in this direction supplied much of the ash that elsewhere was reworked into the predominant stream-and lake-sediment facies. Travertine in the Creede Formation was deposited widely around the periphery of the caldera; consideration of the timing of deposition and of carbon isotope ratios led CENTRAL SAN JUAN CALDERA COMPLEX Steven and Friedman (1968, p. B32-B33) to conclude that the carbonate was derived from underlying sedimentary carbonate units by resurgent magma rising into the roots of the Creede caldera. The Creede Formation is presently exposed over a vertical range of more than 700 m; the bottom of the basin of deposition is not exposed and the top of the formation is eroded. The original thickness of the formation was more than a kilometre and was possibly as much as 1.4- km. The final major stage in structural disruption that has been recognized in the vicinity of the Creede caldera was strong local faulting accompanied by mineralization in the Creede mining district adjacent to the north margin of the caldera (Steven and Ratté, 1965) and in the Spar City mining district along the south margin of the caldera (Steven, 1964). Steven (1972) and Steven and Eaton (1975) have interpreted the faulting and mineralization in the Creede district to have resulted from local intrusion of a stock into the roots of a preexisting broken zone. This intrusion may have been the terminal stage of the Creede caldera cycle or, more probably, may have been a later unrelated event, inasmuch as K—Ar age determinations on adularia from the OH vein in the Creede district indicate that mineralization took place there about 24.6 m.y. ago (M. A. Lanphere, P. M. Bethke, P. B. Barton, written commun., 1973), nearly 2 m.y. after caldera subsidence. LATE GENERAL MAGMATIC UPLIFT A broad zone extending from the Platoro caldera complex northwest through the central San Juan caldera complex was broken by normal faults late in the period of caldera development. Faults that developed at this time extend along the crest of the eastern part of the gravity low (fig. 1); these faults are thought to reflect uplift and distention related to a general buoyant rise of the whole eastern part of the batholith, where evidence for con- current magmatism is strong. Faulting took place in two general segments—the Rio Grande graben (figs. 1 and 22) extending southeast from the central caldera complex, and the Clear Creek graben (fig. 1) extending tangentially northwest from the caldera complex. Some faults in the Rio Grande graben were recurrently active, particularly near the northwest end, where we recognized several increments of displacement related to the progressive breakdown of the southwestern half of the La Garita caldera. Recurrent movement is also evident at the southeast end of the graben system, where Miocene basalts of the Hinsdale Formation are locally involved. The main graben faulting, which integrated some of the earlier faults into a regional pattern, took place during the Creede caldera cycle. Many of the faults cut Nelson Mountain Tuff and are in turn overlapped by unbroken Fisher Quartz Latite that was erupted late in the Creede caldera cycle. The northwest-trending faults are generally parallel to the trend of the underlying batholith. 29 This same part of the batholith was the source of repeated ash—flow eruptions and caldera subsidences during and just before graben development, indicating high-level activity in the underlying magma chamber. Some Rio Grande graben faults terminate to the southeast against a north—trending fault that closely parallels the eastern margin of the gravity low—again implying a close relationship between the faulting and distention of the roof above an active segment of the batholith. As shown on figure 1, the Clear Creek graben (Steven, 1967) extends tangentially northwest from the southwest side of the central San Juan caldera complex. Its trend is parallel to the southwest side of the gravity low in the south-central part of the volcanic field. Where the graben extends across the gravity low toward the north, it separates an area of then-recent ash-flow eruptions and caldera subsidences to the east from an area of older ash- flow eruptions and caldera subsidences to the west. The graben reflects distention of the roof of the batholith, and it also marks relative uplift of the area to the east (Steven, 1967), toward the area of concurrently active magmatism. To the north, faulting changes from a graben to a west- facing normal fault and then to a northwest-facing monocline that bends around the northwest side of the San Luis caldera and dies out as it approaches the margin of the gravity low. As discussed earlier, the Nelson Mountain Tuff is about 0.7 km higher to the south of the San Luis caldera than it is to the north—a discordance believed to measure some of the late general uplift of the area above the batholith. The Clear Creek graben began to develop during the closing stages of the San Luis caldera cycle, and may have been about concurrent with the Creede caldera cycle. The west-facing normal fault at the north end of the graben already existed when the quartz latite of Baldy Cinco was erupted, inasmuch as lavas of this unit poured over and covered a scarp related to the fault. Relations are ambiguous near the Creede caldera at the southeast end of the graben, where widespread glacial till obscures much of the bedrock. We saw no evidence that the graben existed when the Creede Formation was being deposited, yet none of the faults can be demonstrated to 'cut the Creede Formation. At least one fault on trend with the Clear Creek graben cuts Fisher Quartz Latite flows in the Spar City mining district on the south side of the Creede caldera, but this might reflect late reactivation of an earlier fault. The Rio Grande and Clear Creek grabens are thus believed to reflect general distention and minor buoyant uplift of the eastern part of the batholith, where intense ash-flow activity and caldera subsidence demonstrated concurrent high-level magmatism in the underlying batholith. The faulting dies out southeastward and north- westward as it approaches the margins of the related gravity low, and no related graben faults of this age are 30 CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO present in the western part of the gravity low where ash- flow eruptions and caldera subsidences are older. However, the intricate pattern of veins and small faults that radiate outward from the Silverton caldera may reflect analogous broad uplift during waning stages of volcanism in that area. BLOCK-FAULTED AREA An area west and southwest of the Mount Hope caldera is also broken by faults that were recurrently active during the period of caldera formation. This structurally some— what anomalous area seems localized where the south margin of the gravity low, and presumably of the sub- volcanic batholith (fig. 1), extends southwest across a thick wedge of Paleozoic-Mesozoic sedimentary rocks marking the northern extension of the San Juan Basin (fig. 1). The faults have the general pattern of a short ladder (fig. 14), with two northeast-trending faults bounding an area cut into narrow blocks by irregularly northwest trending steep faults. Several periods of movement can be discerned. The oldest faulting followed deposition of the inter- tongued Masonic Park welded tuffs and Sheep Mountain andesitic lavas, and preceded deposition of the Fish Canyon Tuff. A later period of faulting is indicated in the eastern part of the highly faulted area, where andesitic breccias and minor flows in the Huerto Formation thin abruptly from more than 1,500 m to about 70 m thick eastward across a buried scarp on the Fish Canyon Tuff. This scarp probably marks a local fault that was active between deposition of the Fish Canyon Tuff and the Huerto Formation. The main faulting followed eruption of the andesitic lavas and breccias of the Huerto Formation. The ladder- shaped pattern of faults formed at this time and the general result was to depress the area toward the caldera area to the north. These faults are superimposed on the steep gravity gradient along the south side of the underlying batholith (fig. 1), and the general effect of the faulting was to warp a local segment of the roof downward toward the batholith across its southwestern margin. This faulting did not coincide with any particular ash-flow eruption, but rather followed accumulation of episodically erupted andesitic lavas and breccias at a number of local volcanic centers. The movement thus probably resulted from magmatic movements within the batholith and distention along its south margin. This distention may have been in part localized by the relatively incompetent prevolcanic sedi- mentary sequence in this area. Many of the faults that formed during the main post- Huerto faulting were reactivated later, after the Carpenter Ridge Tuff accumulated. In places, this reactivated move- ment was in the same direction as the earlier movement, and in other places, the direction of displacement was reversed. Most of the post-Carpenter Ridge faulting appears to have reflected minor readjustments in the already broken roof of the batholith. The youngest faults within this area reflect shallow gravity sliding of the south flank of the volcanic pile out over underlying Cretaceous shales. Fracturing took place along crescentic faults facing southward toward the edge of the volcanic plateau (fig. 14). The rocks enclosed within these faults were dropped downward toward the south, and tilted northward 30° or more by concurrent rotation. These faults are related to the present erosional scarp along the south side of the volcanic plateau, and are not directly related to the older faults that resulted from magmatic movement during late Oligocene volcanic activity. DISCUSSION DEVELOPMENT OF THE BATHOLITH Calderas and related subsidence structures in the San Juan volcanic field are situated within a marked negative gravity anomaly that is believed to reflect an underlying shallow batholith. Successive ash-flow eruptions and caldera collapses in the volcanic pile above this batholith probably mark the local culminations of upward move- ment of magma: when the roof of an individual chamber became so thin that it failed, voluminous ash was erupted rapidly and a caldera collapsed into the partly evacuated magma chamber. Resurgent upward movement of magma domed many of the calderas and caused extrusion of lavas along the earlier formed structures. High-level magmatism, as manifested by volcanic eruptions and related structural adjustments, diminished at each center as the underlying cupola crystallized. Using this model, we can trace the development of the high-level batholith beneath the San Juan volcanic field from the histories of the successive calderas. The early calderas are widely scattered and are either on the margins of the gravity low or on outward projections from it. Most of these early calderas developed within clusters of older andesitic stratovolcanoes, and the postsubsidence eruptions were commonly of intermediate lavas similar to the older andesites. We interpret these calderas to have developed above isolated high-level cupolas of magma that developed in the roots of older volcanoes before the main body of the batholith rose to its present position. The upper parts of these cupolas differentiated to quartz latite and low-silica rhyolite, which formed the ash-flow tuffs associated with the early calderas, but the quantity of silicic material was limited, and the postsubsidence lavas were from the underlying relatively undifferentiated andesitic magma. The later calderas and associated structures are above the main body of the batholith as indicated by gravity data. At least 12 separate calderas formed within about 3 my This intense activity is believed to reflect the rise of the DISCUSSION batholith in two main high-level segments corresponding to the two main caldera complexes. The segment beneath the western San Juan Mountains rose to shallow depths about a million years before the central San Juan segment, but high—level magmatic activity in each segment over- lapped in time. The top of the batholithic mass of magma . was extensively differentiated—to phenocryst-poor rhyolite in the upper parts of local cupolas, and to large Volumes of phenocryst-rich quartz latite beneath. Ash—flow eruptions in the western San Juan Mountains depleted the more silicic differentiates in the upper parts of the magma chambers, and the postsubsidence lavas were more mafic intermediate-composition rocks from progressively greater depths in the chambers. In the central San Juans, _ however, even the largest ash-flow eruptions did not exhaust the differentiated material in the underlying chambers, and the postsubsidence lavas that were erupted around the calderas are typically porphyritic quartz latites which are related in composition to the associated ash flow tuffs. Local andesitic volcanoes not closely associated with the calderas tapped deeper, little-differentiated parts of the batholith throughout the period of ash-flow eruptions and caldera subsidences. The Lake City caldera in the western part of the San Juan volcanic field, which is related to eruptions of the petrologically distinct alkali rhyolite of the Sunshine Peak Tuff, collapsed about 4 my later than the youngest major eruptions of andesitic and derivative rocks from the central and western parts of the San Juan volcanic field. This late caldera is believed to have formed above a later high-level magma chamber unrelated to the earlier segments of the batholith. These interpretations suggest some time limitations on the magmatic life span of a shallow, composite batholith 100 km across. After about 5 my (during the period from 35 to 30 my ago) of intensive eruption of andesitic lavas from many scattered centers, local magma chambers 10-30 km across had risen to shallow depths beneath some of the larger volcano clusters, and had differentiated sufficiently to supply large-volume eruptions of silicic ash. Within the next 4 my (30-26 my) the batholith evolved from this collection of scattered chambers to a broad shallow mass of extensively differentiated magma, and then to virtual dormancy. Within another 4 my (26-22 my), the lower part of the batholith had congealed sufficiently to permit a younger, petrologically distinctive body of magma to work its way up to similar shallow depths, while still retaining its compositional identity. RELATION OF ASH-FLOW ERUPTIONS AND CALDERA SUBSIDENCE An essentially one-to-one relationship exists between major eruptions of ash flows and subsidence of calderas in 31 the San Juan volcanic field. Unresolved is the volume of ash required to make this maxim operative. Most of the ash-flow sheets we have mapped have either very small (<10 km3) or moderate (100-500 km3) to large (>500 km3) volumes; caldera subsidence is known or suspected to have been associated with all the moderate-volume sheets and invariably accompanied the large-volume sheets. Several of the calderas associated with moderate—volume ash-flow sheets did not form complete circular collapse structures, but subsided as trapdoors hinged on one side. Such calderas generally were not resurgently domed after subsidence. Calderas associated with large-volume sheets, on the other hand, generally are complete subcircular structures, and commonly were resurgently domed after collapse. All large-volume ash-flow units are much thicker within the calderas, typically by an order of magnitude, than in the surrounding outflow areas. This relationship is most easily explained by subsidence concurrent with eruption. Many of the large—volume units are compo- sitionally zoned from early rhyolite to later quartz latite. In most, the rhyolite is confined to the base of the outflow sheet, commonly near the source, and the exposed intra- caldera tuff is entirely quartz latite. This suggests that by the time ash-flow eruptions had removed sufficent magma to cause collapse, the rhyolitic material at the top of the chamber was usually exhausted. Alternatively, initiation of collapse may have disrupted the vent systems and caused tapping of deeper, less differentiated parts of the magma chamber. No generalization can be made concerning the beginning of subsidence relative to composition of the ash being erupted, however, as the relative volumes of rhyolitic versus quartz-latitic ash vary widely from one sheet to another. At the extremes, no rhyolite at all has been found at the base of the Fish Canyon Tuff, which forms the largest sheet in the San Juan field, whereas rhyolite dominates in both the outflow and intracaldera facies of the large-volume Sapinero Mesa and Carpenter Ridge Tuffs. DIFFERENTIATION IN LOCAL CUPOLAS Evidence for independent differentiation at separate volcanic source areas supports the idea that the successive calderas developed above local cupolas that projected above the general top of the batholith. This postulate is inherent in our interpretation of the development of the widely scattered early calderas, but appears valid for the clustered calderas in the western and central San Juan complexes as well. In the western San Juan caldera complex, contrasting phenocryst-rich quartz-latitic Ute Ridge Tuff and phenocryst-poor rhyolitic Blue Mesa Tuff were erupted sequentially from calderas only a few kilometres apart, and individual magma chambers with separate 32 differentiation seem required. The later series of eruptions of rhyolitic Dillon Mesa and Sapinero Mesa Tuffs, progressively more mafic lavas and breccias of the Burns and Henson Formations, and rhyolitic Crystal Lake Tuff, all from within the confines of the San Juan- Uncompahgre-Silverton caldera cluster, chronicle the sequence of depletion of the differentiated magma at the top of one major cupola, establishment of another cupola, further differentiation, and, finally, renewed eruptions of regenerated rhyolite. Inasmuch as only about 2 my intervened between eruptions of the Ute Ridge and Crystal Lake Tuffs, these sequential high-level magmatic processes must have progressed rapidly. The major La Garita caldera in the central San Juan Mountains occupied much of the roof of the eastern segment of the batholith. The associated ash flows show virtually no evidence of compositional zoning; evidently, little highly silicic material had accumulated at the top of the broad magma chamber that supplied the enormous quantities of ash for the Fish Canyon Tuff. The magma chambers that developed successively along the west side of the La Garita caldera, however, were strongly differentiated. Rhyolite is a major constituent in the Carpenter Ridge, Mammoth Mountain, and Wason Park Tuffs, which followed in succession from closely associated source areas. Each of these units has distinctive phenocryst characteristics requiring separate develop- ment, and the first two are strongly compositionally zoned upward from phenocryst-poor rhyolite to phenocryst-rich quartz latite. All three of these units evidently developed under conditions that permitted local differentiation within sequentially formed restricted chambers. The later Rat Creek, lflelson Mountain, Cochetopa Park, and Snowshoe Mountain Tuffs are from more widely scattered centers, for which local cupolas can readily be postulated. Except for the rhyolitic Rat Creek Tuff, these units are mostly composed of closely similar phenocryst-rich quartz latite, although the Cochetopa Park Tuff has some rhyolitic ash at its base. The overlap in time of the areally separate San Luis and Cochetopa Park caldera cycles also indicates concurrently developing, separated cupolas containing individually differentiated batches of magma. Considering the number of calderas that developed sequentially within the approximately 4 my of ash-flow activity above the San Juan batholith, differentiation must have been relatively rapid to generate silicic magma at the tops of the individual cupolas. This is particularly true for the succession of Fish Canyon, Carpenter Ridge, Mammoth Mountain, and Wason Park Tuffs, where the related calderas and source areas apparently overlap and the individual high-level magma chambers had to develop and differentiate in sequence. CALDERAS OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO RESURGEN CE Two broad types of magmatic uplift and general uplift related to calderas in the San Juan volcanic field have been recognized—in one, the uplift was closely confined to the caldera and the immediately adjacent area and occurred soon after collapse; in the other, however, broader uplift involved major segments of the roof of the batholith and occurred over an extended period. These two types of magmatic uplift merge and are, in places, difficult to distinguish. Virtually none of the smaller calderas in the San Juan volcanic field show evidence of resurgent doming after subsidence. Fairly clear-cut relations indicating no resurgent doming are found at the Summitville, Ute Creek, Silverton, and Cochetopa Park calderas, as well as at the buried or postulated Lost Lake, Mammoth Mountain, and Wason Park calderas. Of these, the Silverton, Cochetopa Park, and probably the Ute Creek calderas subsided as trapdoor blocks with horseshoe- shaped incomplete ring-fracture zones interrupted by monoclinal hinges on one side; the buried Lost Lake caldera possibly may have subsided in a similar manner. The Summitville caldera is deeply buried by a fill of andesite flows, and the postulated Mammoth Mountain and Wasson Park calderas were largely or completely destroyed by younger subsidences. Most major San Juan calderas were resurgently domed shortly after subsidence. The Platoro, La Garita, Bachelor, San Luis, Creede, and Lake City calderas display such doming particularly well, and the Bonanza caldera may belong to this group. These are the typical resurgent cauldrons so well described by Smith and Bailey (1968), in which uplift was a definite stage in the caldera cycle (stage V) and was closely confined to the collapsed block and a narrow belt around the margin. Uplift clearly seems to have resulted from the rise of a central pluton beneath the subsided block, and often was accompanied by escape of magma along the marginal ring-fracture zones. The uplifted blocks within these calderas range from nearly symmetrical domes cut by tensional fractures (the Creede, Lake City, San Luis, and probably La Garita and Bachelor calderas) to tilted trapdoor blocks (the Platoro caldera). Less well defined is the long-continued uplift of the area of the western San Juan caldera complex. Although some typical resurgence—in the sense described by Smith and Bailey (1968)—of both the San Juan and Uncompahgre calderas may have taken place, the main uplift was of long duration and involved an oval-shaped area that included both calderas. It began shortly after collapse of the San Juan and Uncompahgre calderas, continued through filling by locally derived lavas and sediments of the Burns and Henson Formations, eruption of the Crystal Lake tuff and resultant subsidence of the Silverton caldera, and further filling by ash flows from intracaldera sources in DISCUSSION the central San Juan Mountains. Quite possibly the total period of recurrent uplift spanned a million or more years. This long-continued activity may have resulted in part from general buoyant uplift by the major magma chamber that underlay the whole western caldera complex. Another example of late general uplift is the Mount Hope caldera. We saw no evidence for postcollapse resurgence, and the succeeding Fish Canyon Tuff appears to have passively filled a deep depression. After filling, however, the whole area of the caldera was broadly upwarped, and so several of the overlying ash-flow sheets wedged out laterally against a low dome in the caldera area. Again, slight buoyant uplift above either a persisting cupola of magma, or a later renewed incursion of magma, would account for the relations seen. This type of uplift, and to a lesser extent that displayed in the Uncompahgre and San Juan calderas in the western part of the volcanic field, seems transitional to the broad buoyant uplift that ‘ affected the whole eastern part of the shallow batholith, as described in the section on “Late magmatic uplift.” MINERALIZATION Only about a third of the calderas in the San Juan volcanic field are significantly mineralized (Steven, Luedke, and others, 1974). These calderas all had complex postsubsidence histories involving recurrent intrusion and extrusion of magma along the ring-fracture zones and related grabens. Some mineralization may have taken place during terminal stages of the associated caldera cycle, but the association of mineralization with a given caldera cycle generally seems tenuous, and the caldera seems principally to have provided fractures that guided later igneous intrusion and hydrothermal activity. The Creede mining district occupies a radial graben on the north side of the Creede caldera. Faulting began during resurgence of the Bachelor caldera, and was reactivated several times later during the volcanic history of the area. The last faulting took place either late in the Creede caldera development cycle or shortly thereafter, when a generally equidimensional area 5-6 km across just outside the Creede caldera wall was broken, probably by intrusion of a stock at depth (Steven and Eaton, 1975). Important silver, lead, zinc, copper, and gold ores were deposited widely in fractures in the heart of the broken area (Steven and Ratte', 1965). The Creede caldera subsided about 26.5 m.y. ago, whereas the ores were deposited about 24.6 m.y. ago (M. A. Lanphere, P. M. Bethke, and P. B. Barton, written commun., 1973). These dates are perhaps too widely separated for the mineralization to be considered a terminal phase of the Creede caldera cycle of development and should, instead, be considered a later, unrelated event. Mineralization around the nested Platoro and Summit— ville calderas is even less closely tied to the caldera cycles. 33 The calderas formed 29-30 m.y. ago in response to the repeated ash-flow eruptions of the Treasure Mountain Tuff (Lipman and Steven, 1970; Lipman, 1975a). The ring-fracture zones of these calderas were the sites of repeated igneous intrusion that began shortly after caldera subsidence and continued to about 20 m.y. ago. Significant mineral deposits seem to be concentrated where ring fractures of the Summitville and Platoro caldera complex intersect a regional northwest-trending fault zone (fig. 1) in the Summitville, Stunner, and Platoro mining districts. Other mineralized areas in or near this complex, at Crater Creek, Jasper, and Cat Creek, are also localized by caldera-margin faulting and igneous activity. The ores at Summitville have been dated by K-Ar methods as 22.4 m.y. old (Mehnert, Lipman, and Steven, 1973b), _, and thus are much too young to be tied to the main caldera cycles. The western San Juan Mountains have a complex history of mineralization with several distinct periods of ore deposition extending over an interval of about 15 m.y. in late Tertiary time (Lipman and others, 1976). In the Lake City area, scattered mineral deposits, currently of limited economic significance, occur in the intrusive cores of intermediate-composition stratovolcanoes that were active between 35 and 30 m.y. ago, before any of the ash- flow eruptions that resulted in caldera collapses. Significant vein and disseminated mineralization occurred within northern parts of the Uncompahgre caldera after it collapsed about 28 m.y. ago, but before collapse of the Lake City caldera during eruption of the Sunshine Peak Tuff 22.5 m.y. ago. This timing is indicated by mineralized areas cut off by the younger subsidence and fragments of mineralized rock in land- slide debris within adjacent parts of the Lake City caldera fill. Additional vein and disseminated mineralization occurs within and adjacent to the Lake City caldera, and is therefore younger than 22.5 m.y. old. Major veins that follow faults of the Eureka graben (fig. 13) between the Lake City and Silverton calderas seem also to have formed after collapse of the Lake City caldera, although the graben faults existed as pre—Lake City structures. Later mineralization seems indicated by the following field relations: (1) We saw no altered rocks or vein fragments in landslide breccias within the Lake City caldera adjacent to the Eureka graben, although veins and mineralized rocks along graben faults extend to the structural margin of the caldera. (2) At Engineer Pass on the northwest side of the Eureka graben, silicic quartz porphyry intrusions that cut the Sunshine Peak Tuff, which was erupted from Lake City caldera, occur at the junction of a hydrothermally altered breccia pipe with associated mineralized veins and have yielded K-Ar and fission-track ages of 15-22 m.y. (H. H. Mehnert and C. N. Naeser, written commun., 1974). These silicic quartz porphyry intrusives, which form a well -defined northeas t— trending belt extending about 40 km from northwest of 34 Silverton to north of Lake City, range from quartz latite to silicic rhyolite and granite. They are petrologically distinct from dated intrusions of the main cycle of ash flows and caldera collapse, which occurred at about 28 m.y., but they have close petrographic affinities to magmas of the 22.5-m.y. Lake City cycle. In the productive Red Mountain district, on the west side of the Silverton caldera, intense alteration and breccia—pipe mineralization are similarly associated with silicic quartz-porphyry intrusions that have been dated at 22-23 m.y. (H. H. Méhnert and C. N. Naeser, written commun., 1974), suggesting that mineralization in this area is young and genetically unrelated to the Silverton caldera cycle. The ages of the major vein systems south- east and northwest of the Silverton caldera are less certain, but at least some of the major veins southeast of Silverton cut quartz-porphyry intrusions similar to those in the Engineer Pass and Red Mountain areas, and replacement ores associated with the rich veins on the northwest side of the Silverton caldera have yielded adularia K-Ar ages of 11- 17 my (F. S. Fisher and H. H. Mehnert, written commun., 1974). Thus, much—perhaps all—of the productive mineralization structurally associated with the Silverton caldera appears to have occurred at least 6—10 m.y. later than formation of the caldera 28 my ago. The caldera appears to have acted mainly as a structural control, with some mineralization genetically associated with quartz- porphyry intrusions that are petrologically distinct from volcanic rocks erupted during the caldera—forming process. The primary function of calderas in mineralization thus appears to be the preparation of zones of weakness in the roofs of major magma-chambers. If conditions at depth are favorable, some of these zones are the sites of recurrent igneous intrusion and extrusion, locally accompanied by hydrothermal activity and mineralization. Whether this activity is the terminal stage of a specific caldera cycle, or is later and generally independent, seems accidental at our present incomplete state of knowledge. REFERENCES CITED Bruns, D. L., 1971, Geology of the Lake Mountain northeast quad- rangle, Saguache County, Colorado: Colorado School Mines M.S. thesis, 79 p. Bruns, D. L. Epis, R. C., Weimer, R. J., and Steven, T. A., 1971, Stratigraphic relations between Bonanza center and adjacent parts of the San Juan volcanic field, south-central Colorado, in New Mexico Geol. Soc. Guidebook 22d Ann. Field Conf., San Luis Basin Colorado, 1971: p. 183—190. Burbank, W. 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C., 1964, Revised Tertiary volcanic sequence in the central San Juan Mountains, Colorado, in Short papers in geology and hydrology: U.S. Geol. Survey Prof. Paper 475-D, p. D54-D63. 1965, Geology and structural control of ore deposition in the Creede district, San Juan Mountains, Colorado: U.S. Geol. Survey Prof. Paper 487, 90 p. 1973, Geology of the Creede quadrangle, Mineral and Saguache Counties, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-1053. Varnes, D. J., 1963, Geology and ore deposits of the South Silverton mining area, San Juan County, Colorado: U.S. Geol. Survey Prof. Paper 378-A, 56 p. fiUj. GOVERNMENT PRINTING OFFlCE: 1976—677-340/52 1% ‘59