GDP—’76" / a , arias 2" (I 2' m: 1/ ‘ , ~ uaRARY V~ q 07. — n N _ _ Precambrian Geology of the United States; An Explanatory Text to Accompany the Geologic Map of the United States GEOLOGICAL SURVEY PROFESSIONAL PAPER 902 l U." .90. JAN 21 ‘977 Precambrian Geology of the United States; An Explanatory Text to Accompany the Geologic Map of the United States By PHILIP B. KING GEOLOGICAL SURVEY PROFESSIONAL PAPER 902 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976 0998’? UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPPE, Swrrrtm)‘ GEOLOGICAL SURVEY V. E. McKelvey, Dirertm‘ Library of (‘ongress Cataloging in Publication Data King, Philip Burke, 1903- I’reeambrian geology of the United States. (Geological Survey Professional Paper 902) Includes bibliographical references. Supt. of Docs. No.2 1 l9.16:902 1. Geology. Stratigraphic~Pre-(‘ambrian. 2. GeologyiUnited States. 1. United States. Geological Survey. Geologic map of the United States. ll. Title. 111. Series: United States. Geological Survey. Professional Paper 902. 011,653.1(55 551.7'1'097 75-619035 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001-02839—5 CONTENTS Page Page Abstract __________________________________________________ 1 Exposed Precambrian rocks of the United States—Continued Distribution ______________________________________________ 1 Cordilleran Region _____________________________________ 44 Data for correlation ________________________________________ 2 Central Rocky Mountains ______________________________ 45 Paleontological data ____________________________________ 2 Precambrian W ____________________________________ 45 Radiometric data ______________________________________ 3 Precambrian complex of southwestern Montana ______ 46 Geologic applications of radiometric dating ______________ 6 Precambrian X ____________________________________ 48 Classification of Precambrian rocks ________________________ 6 Precambrian Y ____________________________________ 50 Classification in Minnesota ____________________________ 6 Northern Rocky Mountains ____________________________ 51 Classification in Canada ________________________________ 7 Precambrian Y ____________________________________ 51 Discussion of Canadian classification ____________________ 9 Precambrian of central Idaho ______________________ 52 Classification on Geologic Map of United States of 1932 -1 10 Precambrian Z ____________________________________ 53 Later usage of US. Geological Survey __________________ 10 Southern Rocky Mountains ____________________________ 54 Interim classification of 1972 __________________________ 11 Precambrian X gneiss complex ______________________ 55 Representation of Precambrian on Geologic Map of the United Precambrian X and Y granitic rocks ________________ 56 States ______________________________________________ 12 Precambrian of Needle Mountains __________________ 56 Exposed Precambrian rocks of the United States ____________ 13 Eastern Great Basin __________________________________ 56 Lake Superior Region __________________________________ 13 Crystalline basement (Precambrian X) ______________ 58 Precambrian W -_____; _____________________________ 22 Big Cottonwood Formation (Precambrian Y) ________ 59 Precambrian X ____________________________________ 24 Mineral Fork Tillite and Mutual Formation (Precam- Precambrian of northern Wisconsin ________________ 26 brian Z) _________________________________________ 59 Keweenawan Supergroup of Precambrian Y ________ 27 Uinta Mountain Group (Precambrian Y) _____________ 59 Precambrian Y rocks older than Keweenawan ______ 28 Supracrustal rocks of the allochthon (Precambrian Z) __ 61 Precambrian Z ____________________________________ 28 Supracrustal rocks of Utah-Nevada border __________ 62 Adirondack area ______________________________________ 29 Southern Basin and Range province ____________________ 62 Northern Appalachian region __________________________ 29 Crystalline basement of Arizona (mainly Precambrian Precambrian Y of western part ____________________ 29 X) ______________________________________________ 63 Precambrian Z of eastern part ______________________ 31 Crystalline basement of southern California (mainly The Avalonian belt ________________________________ 33 Precambrian X) __________________________________ 64 Central and Southern Appalachian region ______________ 33 Supracrustal rocks in Arizona (mainly Precambrian Y) 66 Blue Ridge belt ____________________________________ 33 Pahrump Group of eastern California (Precambrian Y Precambrian Y ____________________________________ 34 and Z) __________________________________________ 69 Precambrian Z ____________________________________ 34 Precambrian of western Texas (mainly Precambrian Y) Precambrian of Piedmont province __________________ 39 Precambrian Z supracrustal rocks of western Basin and 69 South-central United States ____________________________ 41 Range province __________________________________ 72 Ozark area ________________________________________ 41 Discussion and synthesis ___________________________________ 74 Arbuckle and Wichita Mountains __________________ 41 Acknowledgments __________________________________________ 79 Llano uplift ______________________________________ 42 References cited __________________________________________ 79 Regional problems ________________________________ 42 ILLUSTRATIONS Page FIGURES 1—5. Maps of the United States, showing surface distribution of Precambrian rocks as represented on the Geologic Map of the United States: 1. Map units W, X, Y, and Z, and metamorphic complexes of probable Precambrian age ____________________ 4 ’ 2. Rocks of Precambrian W _______________________________________________________________________________ 14 3. Rocks of Precambrian X _______________________________________________________________________________ 16 4. Rocks of Precambrian Y ______________________________________________________________________________ 18 5. Rocks of Precambrian Z ______________________________________________________________________________ 20 6. Geologic map of part of the Lake Superior Region ____________________________________________________________ 23 7. Stratigraphic chart showing Precambrian units northwest and southwest of Lake Superior in Minnesota, Michigan, and Wisconsin ___________________________________ ____________________________________________________________ 25 In IV FIGURE TABLE 11. 12—17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. egewwe CONTENTS Page Map of north-central United States, showing arcuate pattern of surface and subsurface upper Precambrian rocks (Y and Z) ______________________________________________________________________________________________________ 30 Map of south-central Vermont, showing part of the Green Mountain uplift and the Athens and Chester domes A--- 32 Map of northern part of Blue Ridge uplift in Virginia, Maryland, and Pennsylvania, showing Precambrian Z rocks, Precambrian Y basement, and adjacent Phanerozoic rocks ________________________________________________ 35 Stratigraphic diagram across Blue Ridge uplift in northern Virginia, showing Precambrian Z and Cambrian ______ 37 Maps of: 12. Part of Blue Ridge uplift in the border region of Virginia, North Carolina, and Tennessee, showing Precambrian and Paleozoic units ______________________________________________________________________________ 38 13. Southwestern end of Blue Ridge belt in southern North Carolina and Tennessee, and northern Georgia, showing Ocoee Supergroup, related units of Precambrian Z, and Precambrian Y basement ____________________ 40 14. Llano uplift, central Texas, showing Precambrian and surrounding Phanerozoic rocks ____________________ 43 15. Part of south-central United States, showing subsurface extent of late Precambrian and Early Cambrian supracrustal felsic volcanic rocks __________________________________________________________________ 44 16. Central Rocky Mountains in Wyoming, South Dakota, and Montana, showing outcrops of Precambrian rocks 46 17. Part of western United States and southern Canada, showing Precambrian W rocks from the Lake Superior Region to the Central Rocky Mountains ____________________________________________________________ 47 Map showing Precambrian rocks in the Black Hills, western South Dakota ____________________________________ 49 Map of northern Medicine Bow Mountains, Wyoming, showing rocks of Precambrian W and X and surrounding Phanerozoic rocks ______________________________________________________________________________________ 50 Map of Southern Rocky Mountains in Colorado and New Mexico, showing Precambrian rocks, the late Paleozoic geanticlines, and the Colorado Mineral Belt ______________________________________________________________ 55 Map showing Precambrian X and Y units in Needle Mountains, southwestern Colorado __________________________ 57 Synoptic diagram showing relations of units of Precambrian X and Y in Needle Mountains and their implications in the Precambrian history of the area __________________________________________________________________________ 58 Geologic map of northeastern Utah, showing Precambrian X, Y, and Z rocks in eastern Great Basin, and the adjoining mountains and plateaus to the east ______________________________________________________________________ 60 Map of central Arizona showing relations of Yavapai Series and other Precambrian X rocks ____________________ 65 Section showing Vishnu Schist and Unkar Group (Precambrian X and Y) in the Shinumo area, Grand Canyon, northern Arizona, and the truncation of their block—faulted structure by Cambrian deposits; section of Butte fault in eastern Grand Canyon; idealized section showing disruption and distention of Apache Group and Troy Quartzite by sills and dikes of intrusive diabase ________________________________________________________________________________ 67 Map of Van Horn area, west Texas, showing Precambrian rocks and their relations to surrounding Phanerozoic rocks 70 Synoptic section across Precambrian rocks of Van Horn area, west Texas, showing structural relations of the different units and their implications in the Precambrian history of the area ________________________________________ 72 Stratigraphic diagram showing relations between late Precambrian (Z) and lower Cambrian units exposed in different areas of the western Basin and Range province __________________________________________________________ 74 Maps of the United States and parts of Canada and Mexico, showing evolution of the North American continent during Precambrian time ______________________________________________________________________________________ 76 Map of western United States, showing western known extent of Precambrian rocks ____________________________ 78 TABLES Sequence and classification of Precambrian rocks of Minnesota, 1951—70 ________________________________________ 7 . Ages of orogenic events in Canadian Shield, as determined by different radiometric methods ____________________ 9 Comparison of recent classifications proposed for the Precambrian of North America ____________________________ 11 Precambrian supracrustal rocks of Arizona __________________________________________________________________ 68 Precambrian rocks of western Texas __________________________________________________________________________ 71 Precambrian Z—Lower Cambrian formations in western Basin and Range province ______________________________ 73 PRECAMBRIAN GEOLOGY OF THE UNITED STATES; AN EXPLANATORY TEXT T0 ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES By PHILIP B. KING ABSTRACT Precambrian rocks are at the surface in about 10 percent of the area of the United States, but are more extensive beneath the Phanerozoic rocks, especially in the Central Interior Region. Exposures occur in southward-projecting parts of the Canadian Shield in the Lake Superior Region and Adirondack Mountains, and in smaller inliers farther south in the Central Interior. Precambrian rocks emerge in the higher uplifts produced by Phanerozoic deformations in the Ap- palachian and Cordilleran mountain belts to the east and west, but are very scantily represented close to the Pacific Coast. Radiometric dating indicates that the Precambrian rocks vary widely in age, from as much as 3,550 my to about 600 my, rocks with the latter ages being conformable or nearly so with the succeeding Cambrian. The radiometric data, assisted to a minor extent by scanty primitive fossils, make possible correlation of the rocks of different exposures, and they also permit a subdivision of Precambrian rocks and time into named subdivisions. In advance of a worldwide agree- ment on nomenclature the US. Geological Survey uses an interim subdivision into Precambrian W, X, Y, and Z, which correspond broadly with the Archean, Aphebian, Helikian, and Hadrynian of the official Canadian classification. The radiometric data indicate peaks of abundance of ages at differ- ent levels, which express significant historical events—times of orogeny, of orogenic cycles, or of magmatism with or without orogeny. The principal events occurred 2,500—2,750, 1,600—1,850, 1,300—1,400, and 900—1,100 million years ago, and are named (following Canadian usage) the Kenoran, Hudsonian, Elsonian, and Grenvillian events, respectively. The events have been recorded at many places through- out the United States, Canada, and Mexico, and occur between or in the latter parts of the named subdivisions. Different events characterize certain areas, thereby delimiting provinces in the Precambrian terrane. The oldest provinces are in northern Minnesota (an extension of the Superior province of Canada), and in Wyoming and southern Montana; they contain Pre- cambrian W rocks that yield Kenoran and earlier dates. Younger provinces are to the south. Precambrian X rocks with Hudsonian dates are extensive in the Southern province of the Lake Superior Region, and also through much of the southern part of the Cordilleran region. A poorly defined province with Elsonian dates is indicated by subsurface data in the southern part of the Central Interior Region, and plutons with Elsonian ages are widely distributed in the Pre- cambrian X rocks of the southern Cordillera. Crystalline rocks of Precambrian Y with Grenvillian dates form a wide belt in the south- eastern United States, especially in the Appalachian region. By the time of Precambrian Y, however, a large part of the remain- der of the North American continent, in the United States and elsewhere, had been stabilized into a craton, and received supracrust- al sediments and volcanics that were only moderately deformed, or remained undeformed during Precambrian time, producing units such as the continental Keweenawan Supergroup of the Lake Superior Region, the marine Belt Supergroup of the northern Cordil- lera, and the Grand Canyon Supergroup and others farther south. During latest Precambrian time, or Precambrian Z, accumulation of supracrustal sediments and volcanics occurred mainly along the eastern and western sides of the continent, in the Appalachian and Cordilleran regions—in the east on a crystalline basement produced by the Grenvillian event, in the west lying with moderate discordance on Precambrian Y supracrustal rocks. However, in the coastward part of the Appalachian region is the Avalonian belt of Precambrian Z rocks, an exotic element which seems to have been joined to the North American continent by plate movements during Paleozoic time. It includes supracrustal rocks in the Carolina Slate Belt of the southern Appalachians, as well as farther northeast in Canada, but in south- eastern New England it is represented by extensive granitic plutons that are unconformable beneath the Lower Cambrian, with radiomet- ric dates of 570 my In most of the United States the Precambrian is separated from the Cambrian by a marked unconformity and hiatus; Middle or Upper Cambrian rocks overlie Precambrian Y or older rocks. However, in the mountain belts to the east and west, supracrustal rocks of both Precambrian Z and Lower Cambrian were deposited, and the bound- ary between the Precambrian and the Phanerozoic is less obvious. The problem is most acute in the southwestern part of the Basin and Range province where Precambrian and Cambrian are parts of a thick conformable sequence of fine-grained sediments, so that there is no clear physical or fauna] boundary between them. In this account, following a statement of general principles, the Precambrian rocks of the different areas of exposure are reviewed, described, and correlated in turn. The units selected for description are in terms of modern morphology, which correspond only broadly with the provinces of Precambrian time—the Lake Superior Region, the Adirondack Mountains, the Northern and Southern Appalachian regions, the south-central United States in the Interior Lowlands, the Central Rocky Mountains, the Northern Rocky Mountains, the Southern Rocky Mountains, the eastern Great Basin, and the south- ern Basin and Range province. In general it is assumed that the descriptions can be understood by reference to the Geologic Map ofthe United States, but to clarify certain subjects, maps on larger scales or maps which illustrate special features are included. A final discussion and synthesis deals with the larger Precambrian problems, some still obscure, including the origin and evolution of the continent during Precambrian time, and the possible participation of the continent in plate tectonics. DISTRIBUTION Precambrian rocks underlie all the Central Interior Region of the United States and large parts of the moun- tain belts east and west of it. However, they are covered 1 2 PRECAMBRIAN GEOLOGY OF THE UNITED STATES extensively by Phanerozoic rocks and form the surface of only about 10 percent of the country. By contrast, in Canada to the north Precambrian rocks form the sur- face of nearly half the country, mainly in the Canadian Shield. The largest exposures of the Precambrian in the United States are in southern extensions of the Cana- dian Shield—in the Lake Superior Region of Minnesota, Michigan, and Wisconsin, and in the Adirondack Moun- tains of northern New York State (fig. 1). Older maps (such as the Geologic Map of the United States of 1932) imply that an even larger area of Precambrian occurs in the Appalachian Region to the east and southeast; large parts of this supposed Precambrian are now known to be of Paleozoic age, although authentic Precambrian does emerge in the higher uplifts through much of the length of the chain. In the Central Interior, Precambrian is exposed only in small, widely spaced areas on the crests of a few uplifts; additional knowledge of the Precam- brian of this region is afforded by subsurface data. In the Cordilleran Region, a large area of Precambrian (mostly the supracrustal Belt Supergroup) extends across the Northern Rocky Mountains of western Mon- tana and northern Idaho. Farther south in the Rocky Mountains the outcrops of Precambrian are smaller, but many of them (as in Colorado) are closely spaced. Similar small but closely spaced areas of Precambrian occur in the southern part of the Basin and Range prov- ince in Arizona and adjacent States. No Precambrian is known within 200 miles (320 km) or more of the Pacific Coast, except in the Transverse Ranges of southern California. DATA FOR CORRELATION Prime requisites for representation of any group of rocks on a regional or national geologic map are adequate classification and correlation, but these are difficult to achieve in the Precambrian. Many parts of the Precambrian have been strongly deformed, metamorphosed, and injected with plutonic rocks; moreover, even where their primary sedimentary structures are well preserved, their fossil remains are sparse and enigmatic. While it is true that their struc- tural complexity is perhaps no greater than that of many Phanerozoic terranes whose sequences and ages have been deciphered, the few fossils in Precambrian rocks are not of the diagnostic value of those used for stratigraphic purposes in younger rocks. In the absence of normal criteria for classification and correlation, various indirect methods were formerly used in deciphering the Precambrian record. The earth was assumed to evolve during the Precambrian, from a molten, disordered condition (“Azoic” or “Archean” time) to a better ordered condition when more familiar sedimentary and volcanic processes prevailed ("Prot- erozoic” or "Algonkian” time). Assumptions were made as to the nature of Precambrian orogenic processes— supposedly universal cycles of deformation, plutonic in- jection, and peneplanation, applicable throughout a shield, or to even larger regions. Where sequences of Precambrian rocks could be worked out by conventional laws of superposition, they were compared and corre- lated with other sequences, even far distant, using as starting points supposed type areas, such as the Lake Superior Region.1 These early efforts failed to take into account various geological factors that are better un- ;: derstood now, such as the actual great length of Pre- cambrian time—at least five times longer than Phanerozoic time. They are merely of historical interest today. Great progress in understanding the Precambrian has been made in recent decades. Radiometric dating has made it possible to bring together many hitherto unrelated items of the larger history, and even to make a beginning in stratigraphic correlation. With this as- sistance, more can now be deduced as to the geochemical evolution of the earth, leading to inferences on worldwide events, such as a time of iron formation, the times of beginning of carbonate and of evaporite sedimentation, and times of glaciation. However, with the possible exception of the latter, these have only very general application to stratigraphic work. More to the point, radiometric dating has assisted in understanding the fossil record, such as it is, and to suggest at least rudimentary zonation. Moreover, much wider areas of Precambrian rocks have been geologically mapped which, coupled with radiometric dating, has assisted in understanding regional Precambrian history that is no longer restricted to a few classical and supposedly typi- cal areas. PALEONTOLOGICAL DATA The fossil record is influenced by the evolution of life on the earth, but during Precambrian time evolution was probably very slow at first, and did not accelerate until much later. Classification of Precambrian fossils is difficult because even major groups of organisms must have become extinct during the long timespans involved; even in the succeeding Early Cambrian there are shelly invertebrate groups that are not assignable to any existing phyla (Glaessner, 1968, p. 586). 1The strongest statements of these propositions were in the textbooks of the time. whose authors were eager to generalize the results of the field geologists; statements by the field geologists themselves (With the exception of Lawson, 1914) were more qualified. Ajudicious appraisal of the status of Precambrian problems is contained in C. K. Leith’s presidential address to the Geological Society of America in 1933 (Leith, 1934), and his strictures have been well justified by later developments. DATA FOR CORRELATION 3 During the first three-quarters of Precambrian time the only remains or traces of life are those of primitive bacteria and plants. The most prominent of these re- mains are the stromatolites, which are biogenic sedimentary structures probably produced by algae; they include stratiform, nodular, and columnar carbo- nate structures. All are notoriously variable in form and no doubt were much influenced by local environ- mental conditions. Nevertheless, when specimens of the more distinctive columnar forms have been studied through sequences long enough, and over areas wide enough, they seem to have changed sufficiently with time to permit division into zones dated radiometrically between 1,600 and 1,350 my, 1,350 and 1,000 my, and 700 and 500 my These express very slow evolutionary changes—two orders of magnitude slower than in Phanerozoic biostratigraphic zones (Glaessner, 1968, p. 587). Stromatolite zonation has been most success- fully applied in the Soviet Union (Raaben, 1969; Cloud and Semikhatov, 1969) where Precambrian stroma- tolite-bearing rocks can be studied across the whole expanse of northern Eurasia, but similar studies are in progress in Precambrian areas elsewhere. The earliest authentic metazoan fossils occur in strata not far beneath the Cambrian with ages of 600 to 700 m.y.—especially in the Ediacaran of South Austra- lia, the Vendian of northern Eurasia, and a scattering of other formations and localities in the Eastern Hemis- phere (Glaessner, 1971). The only reported occurrences in North America are in southeastern Newfoundland (Conception Group) (Misra, 1971, p. 979—980), in North Carolina, and in eastern California (Deep Spring Formation) (Cloud and Nelson, 1966). Some or most of the forms occur at all localities, indicating a well- characterized fauna—various primitive coelenterates, and forms with less certain affinities that probably be- long to extinct phyla (Glaessner, 1961, p. 73—77; ; Sokolov, 1973, p. 209—215). They were soft-bodied ani- 3 mals, whose imprints are preserved on bedding surfaces at unusually favorable situations. Although the strata in which they occur are clearly older than the Cam- brian, there is some philosophical justification for con— sidering them a basal unit of the Paleozoic, younger than the Precambrian as formally defined (Cloud, 1968, p. 36—37). Hard-shelled fossils, such as archeocyathids and trilobites, only appear in the Cambrian itself, for reasons that are still debated (Cloud, 1968, p. 42—49). RADIOMETRIC DATA Dating by radiometric methods has advanced far beyond the first few determinations on uranium and thorium ore minerals nearly three-quarters of a century ago. Aside from suggesting the possibilities of the method and the great length of geologic time, these first determinations were of little geologic use because of the rarity of the minerals, and because most kinds of rocks do not contain them, hence were as yet undatable. Sub— sequently, and especially during the last few decades, many other methods have been devised, some of them applicable to ordinary rocks. At the same time, how- ever, the hazards and pitfalls of the radiometric methods of dating have become more apparent. The lead-alpha method of dating zircon gives generalized results and is useful as a reconnaissance tool, but has little value for detailed work. The potassi um-argon method uses potassium-bearing minerals such as biotite, muscovite, and hornblende, hence has wide application to common igneous and metamorphic rocks. It is therefore useful for sampling and appraisal of wide areas of Precambrian rocks (as in the Canadian Shield). The results are mostly consistent among themselves, and thus indicate the relative ages of different units and provinces. However, the ages ob- tained in the Precambrian are rather consistently less than those by the other methods mentioned below, owing to gradual loss of argon from the mineral lattices. Because of differences in their molecular structure, this loss is greatest in biotite, less in muscovite, and least in hornblende. Also, argon is lost during the cooling that succeeds time of igneous injection or of metamorphism, and it does not become fixed in the mineral until the tempera- ture descends to a lower level. Thus many dates are “cooling dates” that are younger than the actual times of injection and metamorphism. These differences are least in low-grade metamorphic rocks and greatest in high-grade rocks of granulite facies that underwent the deepest burial and the greatest subsequent uplift and erosion. The rubidium-strontium method is not subject to the loss of a gas daughter product as in the potassium-argon method, and hence yields more reliable dates, but it has several of its own problems. Both elements are subject to gain or loss during metamorphism, and there is dis- agreement as to the Rb87 half—life decay constant. De— pending on the constant adopted, the dates obtained on Precambrian rocks by the rubidium-strontium method may differ by 6 percent, or 150 my. at 2,500 my ago. The most nearly absolute figures for primary crystal- lization are those obtained from uranium-lead and lead-lead methods, but the elements to be analysed are rare. The methods were originally applied to uranium and thorium ore minerals which did not have wide geological application; but uranium and lead also occur in minute amounts in the common accessory minerals zircon, monazite, apatite, and sphene, for which analyt- ical procedures are very exacting. Although fewer dates PRECAMBRIAN GEOLOGY OF THE UNITED STATES FIGURE 1.—Map of the United States showing surface distribution of Precambrian rocks as represented on the Geologic Map of the United States (map units W, X, Y, and Z). Also shown are metamorphic complexes (map units ms and ml—m4), which probably include rocks of Precambrian age. DATA FOR CORRELATION EXPLANATION Precambrian rocks % Metamorphic complexes FIGURE 1.——Continued. 6 PRECAMBRIAN GEOLOGY OF THE UNITED STATES can be obtained by uranium-lead and lead-lead methods, they are useful as controls for the less exact results obtained by the other methods. GEOLOGICAL APPLICATIONS OF RADIOMETRIC DATING These various methods indicate the times of crystalli- zation during igneous intrusion, and of metamor- phism—aside from the cooling factor. Dating of Pre- cambrian supracrustal rocks of sedimentary and vol- canic origin is more difficult. Direct dating from primary minerals in the Precam- brian supracrustal rocks in the Canadian Shield is largely unsuccessful, and the dates obtained commonly express merely the age of the metamorphism (Stockwell, 1968, p. 692). Elsewhere, it has been possi- ble in a few places. Glauconite and argillite in the little deformed or metamorphosed Belt Supergroup of the Northern Rocky Mountains are susceptible of dating by potassium-argon and rubidium—strontium methods; also, potassium-argon determinations have been made on hornblende from the Purcell Lava and associated sills interbedded in the Belt sediments (Obradovich and Peterman, 1968, p. 739—740). Zircons from the late Pre- cambrian felsic lavas of low metamorphic grade in the Blue Ridge of the Central Appalachians have been suc- cessfully dated by the uranium-lead method (Rankin and others, 1969). For dating the Precambrian supracrustal rocks of the Canadian Shield and other complex areas, recourse must generally be had to indirect methods, which bracket the times of accumulation between maximum and minimum limits. The maximum age of a sequence is indicated by the age of the plutonic and metamorphic rocks of its basement; the minimum age is indicated by the age of its metamorphism, or by the age of the igne- ous rocks that intrude it. CLASSIFICATION OF THE PRECAMBRIAN ROCKS With the new data available, proposals are being made in many parts of the world for reclassification of the Precambrian rocks. The subject is under considera- tion by the Subcommission on Precambrian of the In- ternational Commission on Stratigraphy, Kalervo Rankama, chairman. The subcommission is working toward an agreement on Precambrian classification and nomenclature that will meet worldwide acceptance, but such an agreement is still a matter for the future. The worldwide implications do not concern us here; our in- terest is in the interim problems of classification of the Precambrian in North America, and specifically in the United States. CLASSIFICATION IN MINNESOTA One of the first efforts to make effective use of radiometric data to classify the Precambrian was in Minnesota (Goldich and others, 1961) that refined and revised an earlier classification based largely on con- ventional geologic criteria (Grout and others, 1951). The State of Minnesota includes nearly half of the area of Precambrian rocks of the Lake Superior Region in the United States. Moreover, its sequence of Precambrian rocks is ,much like that across Lake Superior in north- ern Michigan and Wisconsin, so that any classification arrived at in Minnesota has applications over a wider area. Table 1 summarizes the various classifications pro- posed for Minnesota, including that of 1961. From the table, it is apparent that basic concepts of the Minnesota sequence have changed little through the years, but that significant changes have been made in classifica- tion and terminology. A notable change in 1961 was the transfer of the Animikie Group from Late Precambrian to Middle Precambrian and the Knife Lake Group from Middle Precambrian to Early Precambrian, as a result of dating the Penokean orogeny at 1,700 my and the Algoman orogeny at 2,500 my (the column for 1968 indicates that these dates are actually greater). These extreme ages were incompatible with the relative youth previously assumed for the two groups. The so-called “Laurentian orogeny,” previously considered to divide the Early and Middle Precambrian, was now downgraded to a minor role in the Early Precambrian. Emendations after 1961 include renaming the so- called “Grenville orogeny” of Minnesota the “Keweenawan igneous activity”; even though the event is broadly correlative with the true Grenvillian orogeny farther east, it was essentially anorogenic in Min- nesota. Also, in the classification of 1970 and 1972, the absolute distinctions between the Knife Lake and Keewatin Groups are discarded, as the sediments of the one and the volcanics of the other have variable mutual relations. With this, the so-called “Laurentian orogeny” and its accompanying epoch of granite intrusion disap- pears; granites within the Lower Precambrian are now interpreted as local phenomena. Nevertheless, as shown in 1968, extremely ancient rocks occur in south- western Minnesota, dated at 3,550 my. Radiometric dating indicates major events in Min- nesota at 2,700—2,750 m.y., 1,850 my, and 1,100 m.y., designated the Algoman orogeny, Penokean orogeny, and Keweenawan igneous activity (= Grenville orogeny). We will find these events again in the Cana- dian Shield in Canada, and elsewhere, and will inter- pret them as important markers for classifying the Pre- cambrian of North America. CLASSIFICATION OF PRECAMBRIAN ROCKS TABLE 1.—Sequence and classification of Precambrian rocks of Minnesota, 1951—70. Grout and others 119511 Goldich and others 119611 Goldich 11968! Sims :hIdSMh333l:19721 Cambrian Cambrian Cambrian Cambrian Unconformity 600 m.y. 600 m.y. Late Precambrian Keewanawan System Medial volcanics intruded by Duluth Complex (Grenville orogeny, 1,100 m.y.) Later Precambrian Keweenawan Group Medial volcanics intruded by Duluth Gabbro Late Precambrian Sediments and medial North Shore Volcanic Group, in- truded by Duluth Complex (Keeweenawan igneous activ- ity, LOGO—1,200 m.y.) Upper Precambrian Keweenawan Series Medial volcanics intruded by Duluth Complex Unconformity Penokean orogeny, 1,700 m.y. Middle Precambrian Granitic intrusives Huronian System Animikie Group Animikie Group Penokean orogeny, 1,600—1,900 m.y.) Middle Precambrian Granitic intrusives Animikie Group Penokean orogeny Middle Precambrian Granitic intrusives Animikie Group Algoman orogeny, ‘ 2,500 m.y. ‘ Early Precambrian Granitic intrusives Temiskamian System Knife Lake Group Unconformity Medial Precambrian Algoman intrusives Knife Lake Group Laurentian orogeny, age? Unconformity Earlier Precambrian Pre-Knife Lake intrusives Granitic intrusives Ontarian System Keewatin Group Coutchiching? Older rocks Keewatin Volcanics Soudan Iron-Formation Algoman orogeny, 2,400—2,750 m.y. Early Precambrian Granitic intrusives Knife Lake Group , Algoman orogeny Lower Precambrian Granitic intrusives Metasedimentary and metavol- canic rocks, with various mu- tual relations Laurentian oro en a e? . . . . g y, g Granitic 1ntrus1ves, older than Granitic intrusives part of metasedimentary rocks Keewatin Group Coutchiching? Older rocks, 3,300—3,550 m.y. Gneiss and schist, southwestern Minnesota CLASSIFICATION IN CANADA A more far-reaching reclassification of the Precam- brian rocks on the basis of radiometric dating has been made by the Geological Survey of Canada. This de- serves lengthy consideration, as it involves our neighbor to the north and its geological survey, as well as the largest exposure of Precambrian rocks in North America. The reclassification was carried out under the leadership of Clifford H. Stockwell for use on the new Geologic and Tectonic Maps of Canada then in prepara- tion (1969), and was based on an accelerated program of mapping the Precambrian rocks of the country and of radiometric dating, chiefly by the potassium—argon method. Outcrops of Precambrian rocks are nearly uninter- rupted in the Canadian Shield in the central and east- ern part of the country, except for submerged parts such as Hudson Bay, and for the area of Phanerozoic cover in the Hudson Bay Lowland. This vast Precambrian area was once thought to be a homogeneous body, as implied on the Geologic Map of North America of 1912, hence subject from time to time to universal cycles of orogeny and peneplanation. Field studies during the last half- century have demonstrated, on the contrary, that it is inhomogeneous, and divisible into provinces with dif- ferent rocks and histories, that developed indepen- dently during Precambrian time. Increasing knowledge has heightened the distinctions between the provinces and has sharpened their boundaries. Many of the boundaries are structural lineaments, emphasized further by geophysical anomalies; some are strati- graphic, where supracrustal rocks of a younger province overlap the basement of an adjoining older province. Of the provinces of the Canadian Shield, only a few bear directly on Precambrian problems in the United States: the Superior province of ancient rocks which includes the Lower Precambrian of Minnesota (see above); the Southern province of somewhat younger rocks, which includes the remainder of the Lake Superior Region in the United States and Ontario; and the Grenville province farther east, which extends into the Adirondack Mountains of New York State. The Pre- cambrian of the United States no doubt includes other extensions of the shield provinces, and additional prov- inces, but they are less apparent at the surface because of the interrupted outcrops. Radiometric dating has underscored the discreteness of the provinces. Each has its own characteristic peak of 8 PRECAMBRIAN GEOLOGY OF THE UNITED STATES abundance of dates, well expressed in histograms (for example, Stockwell, 1964, fig. 2). A scattering of older and younger dates also occurs, the older expressing earlier orogenic events nearly overwhelmed by the dominant events, the younger being from dike rocks and other anorogenic intrusives. The dominant sets of dates in the different provinces are interpreted as having been produced by orogenies (Stockwell, 1961, p. 111—113). Orogeny is defined as a period of mountain building, accompanied by folding, metamorphism, and granite intrusion, each orogeny being followed by a long period of uplift, erosion, and cooling before the next set of supracrustal rocks was laid down. The scatter of dates in the rocks of each province may extend over a span of as much as 300 my, but this is interpreted as partly the result of analytical error; the actual duration of an orogeny is believed to be 100 my. or less. In order to refine further the orogenic times, the available dates have been analyzed statistically (Stockwell, 1964, p. 4—7), using those from a single prov- ince, by a single method (for example, potassium- argon), and of orogenic origin (rather than relicts of earlier events, or of postorogenic events). The statistical analysis for each province yields a mean on the Gaus- sian or probability curve, and a standard deviation. The mean figure is interpreted as representing the probable climax of an orogeny, and the mean minus the standard deviation the probable end of this orogeny. Three principal orogenies are recognized in the Canadian Shield, the Kenoran (= Algoman of Min— nesota), the Hudsonian (= Penokean of Minnesota), and the Grenvillian (= Keweenawan igneous activity of Minnesota). Each orogeny has its “type region” in one of the provinces; "it is hoped that, eventually, it may be possible to select much smaller areas for type regions, while still retaining the present geological definitions and still containing rocks and minerals that are suita- ble for dating by a variety of methods on a variety of minerals” (Stockwell, 1972, p. 3). The Kenoran has its type region in the Superior province, where it has a mean age of2,490 my; the Hudsonian its type region in the Churchill province, where it has a mean age of 1,935 my; and the Grenvillian its type region in the Gren- ville province, where it has a mean age of 945 my These orogenies may be poorly expressed or absent in other provinces. The Grenvillian is unique in the Gren— ville province, and has no orogenic counterparts elsewhere in the shield; the Hudsonian is missing in the Superior province, but it recurs in the Southern prov- ince, and in some of the far northern provinces. Besides these, an additional Elsonian orogeny was proposed, based on a scatter of radiometric dates in the Nain province of Labrador, with a mean age of 1,370 m.y. (Stockwell, 1964, p. 2). Later work demonstrates that the events represented by these dates are not orogenic; instead, they were produced by adamellite (quartz monzonite) and anorthosite intrusions into rocks already consolidated by the Hudsonian orogeny (Taylor, 1971, p. 580—582). The Elsonian is more prop- erly termed an "event” (King, 1969, p. 35; Stockwell, 1972, p. 3). As indicated earlier, the potassium-argon method on which these radiometric ages are based has many ad- vantages, but the dates obtained are consistently younger than those obtained by other methods. Sub- sequent to the work summarized here, the orogenic periods have been checked by a smaller number of uranium-lead and rubidium-strontium determinations, all of which indicate older, and probably truer ages (Stockwell, 1972), as shown in table 2. Besides the major Precambrian orogenic events rec- ognized by Stockwell in the Canadian Shield, lesser events late in the Precambrian have been described in other parts of Canada, mostly insecurely dated radiometrically and not necessarily of the same age— the East Kootenay and Racklan orogenies in the Cordil- leran province (Douglas and others, 1970, p. 373) and the Avalonian orogeny in the Appalachian province (Poole and others, 1970, p. 232—233). Ofthese, the latter is of the greatest interest here because of its probable extension into the Eastern United States; the evidence will be treated at greater length later (p. 33, 39). The radiometric and orogenic data just summarized have been used to redefine the sequence of Precambrian rocks in Canada. The Precambrian of Canada has tradi- tionally been divided into Archean and Proterozoic Eons, and these and their subdivisions are now more precisely defined with the aid of the new data: Archean prior to the end of the Kenoran orogeny, Lower Prot— erozoic between the ends of the Kenoran and Hudson- ian orogenies, Middle Proterozoic between the ends of the Hudsonian and Grenvillian orogenies, and Upper Proterozoic between the end of the Grenvillian orogeny and the beginning of the Phanerozoic. Each orogenic event is thus placed within the preceding time division, and the end of the orogeny is considered to mark the upper boundary of the subdivision. New names are proposed for the subdivisions of the Proterozoic (Stockwell, 1964, p. 7—9): Aphebian for Lower Proterozoic, Helikian for Middle Proterozoic, and Hadrynian for Upper Proterozoic. The names are de— rived from Greek roots: Aphebian from “aphebos,” or old maturity; Helikian from "helikia," or maturity; and Hadrynian from “hadrynes,” or young maturity. Further subdivisions can then be created; for example, the Helikian is divided into Paleohelikian and Neohili- kian, bounded by the Elsonian event. The new names CLASSIFICATION OF PRECAMBRIAN ROCKS 9 TABLE 2.—Ages of orogenic events in Canadian Shield, as determined by different radiometric methods [Based on Stockwell, 1964, 1972] End of event in millions of years (zmeun minus Event standard deviation) K/Ar U/Pb Rb/Sr Rb/Sr Constant 1.47 constant 1.39 Grenvillian orogeny 880 ca. 1,000 ca. 1.010 ca. 1.070 Elsonian event 1,280 7’ 1,400 Hudsonian orogeny 1.640 ca. 1,800 ? 1.750 ? 1,850 Kenoran orogeny, 2,390 ca. 2.560 ? 2.540 ? 2,690 make it possible for there to be many subdivisions within the Precambrian (or specifically within the Prot- erozoic), instead of the three descriptive categories of “lower,” “middle,” and ”upper” that are available in the English language, and they avoid such unfortunate ex- pressions as “lower upper” and “middle lower” which have sometimes been used for smaller subdivisions. DISCUSSION OF CANADIAN CLASSIFICATION The classification of the Precambrian set forth above has been accepted by the Geological Survey of Canada for use in its published maps and reports, but it has been criticized by other geologists (for example, Goldich, 1968, p. 722; James, 1972a, p. 1132; 1972b, p. 2085) in' the following terms: (1) The statistical method of defining orogenies and subdivisions is questionable, as it depends on the valid- ity of the areal unit selected for analysis, the effective- ness of the sampling, and whether the dates selected rather than discarded represent a single population. (2) Reliance on the potassium-argon method of dat— ing produces unreliable results for determining the ages of the units. (3) The wide scatter of dates within each province is difficult to reconcile with the assumption that they were produced by a single orogeny, rather than by an orogenic cycle comprising many successive orogenies (King, 1969, p. 33; compare James, 1960, p. 107). (4) Orogenies have been discredited as the funda- mental basis for stratigraphic classification in the Phanerozoic, and their value for this purpose in the Precambrian should be no greater. (5) Archean has been differently defined as to age limits from one country to another, and from one geologist to another. (6) The new names proposed for subdivisions of the Proterozoic are unfamiliar and cumbersome, and do not clearly indicate their sequential relations. (7) New names for major units of the Precambrian should not be proposed unilaterally, but by interna- tional agreement. The reader can judge for himself between these ad- verse criticisms and the Canadian Viewpoint just sum- marized. Here, discussion of only one item, the Archean, is desirable. The term “Archean” has been widely used for more than a century for the oldest visible rocks of the earth, which are supposed to have special characters. “By later Precambrian time, the patterns of sedimentation, mountain building, and crustal evolution seem to have been much the same as they are now. The Archean is commonly thought to have been different—a time when the atmosphere and oceans were unlike the present, a time prior to crustal organization into cratons and geosynclines, a time unique in earth history” (Pet— tijohn, 1972, p. 133). Moreover, significant geochemical differences have been discerned between rocks formed during the “Archean” and the “Proterozoic,” or before and after about 2,500 my ago (Engel and others, 1974, p. 852). One of the original areas in which the Archean was recognized is the Canadian Shield, and especially the Superior province, a terrane consisting of linear belts or islandlike areas of supracrustal rocks, interspersed with or surrounded by a more extensive sea of intrusive granite. The supracrustal rocks include metavolcanics that are mainly andesitic and basaltic greenstones; and metasediments which, where best preserved, are graywackes and slates with interbedded conglomerate and iron formation, and elsewhere are migmatized quartz-mica schists and paragneisses. Their extreme age is demonstrated in places by unconformable rela- tions of both the supracrustal rocks and granites be- neath the middle Precambrian rocks, and by radiomet- ric dating. Similar terranes are recognized in the shield areas of other continents (for example, Australia and South Africa), and have likewise been called Archean. The term Archean has also, of course, been mis- applied to any thoroughly metamorphosed basement, especially before the period of radiometric dating. Thus, the metamorphic basement of the Appalachian region was commonly called "Archean,” until radiometric dat- ing demonstrated that it was not consolidated until about 1,000 my ago, at the time of the Grenvillian orogeny of the Canadian Shield. These misapplications aside, a worldwide survey of usage indicates much diversity of judgment as to the date of termination of the Archean (Rankama, 1970, p. 214, 216), with proposed dates from less than 2,000 my to nearly 3,000 my Proposals for a termination at less than 2,000 my seem to have little merit; the main problem is regarding diverse proposals for dates be- tween 2,000 and 3,000 my Some of the latter dis- 10 PRECAMBRIAN GEOLOGY OF THE UNITED STATES crepancies represent differences in field observations and analytical methods and can be adjudicated. Other discrepancies are probably genuine; perhaps "Archean” conditions ended at different (but everywhere ancient) times from one shield area to another. The ancient features of the earth, expressed by the rocks and the conditions that these imply, seem to be unique and well characterized, whether they be called “Archean” or by some other name. The problem is how to translate these concepts into a definition of strati— graphic value. Valid definitions can be proposed in specific areas, such as the Canadian Shield, but difficul- ties arise when they are expanded into a definition of worldwide application. It therefore remains to be seen whether such a worldwide definition can be worked out, or whether the term Archean must be discarded. CLASSIFICATION ON GEOLOGIC MAP OF UNITED STATES OF 1932 The Geologic Map of the United States of 1932 was ‘ compiled before the development of meaningful radiometric dating and was the last major publication of the US; Geological Survey which used the subdivisions “Archean” and “Algonkian” that had been standard in Survey publications for the preceding half-century. The classification used on this map is illustrated by the following abstract of its legend: Lake Superior Region Algonkian Keweenawan: sedimentary, Akl; volcanic, Akv Huronian: lower, middle, and upper, Ahl, Ahm, Ahu Archean Keewatin Series, ARk Precambrian undivided Precambrian intrusives, in New England and the Adirondacks Adirondacks Algonkian? Adirondack batholith, Ab Archean? Older igneous rocks, ARi Grenville Series, mg New England Algonkian? Younger sedimentary schists, As Archean? Older sedimentary and igneous gneisses, flgn Appalachian Region Algonkian? (Glenarm Series) Wissahickon Schist: oligoclase-biotite schist, Awh albite-chlorite schist and garnetiferous phyllonite, Awl schist with igneous injections, Awl’ , Cockeysville Marble and Setters Formation, Acs Granite, gabbro, and hornblende gneiss, Agn Mylonitized granite gneiss and hornblende gneiss, Agg Volcanic rocks, Av Archean? Older gneiss, 4:; gm Midcontinent Region Algonkian? Gneiss, schist, and quartzite, Agn Granite, porphyry, and gabbro, Agr Great Plains Algonkian? Sedimentary schist and quartzite, As Intrusive rocks, Ai Rocky Mountains Algonkian Belt Series: undivided, Ab; lower part, Abl; upper part, Abu Archean Archean rocks, A? Granite, A? g Pacific Coast Region, Great Basin, and Columbia River Plateau Precambrian Granite, diabase, and other intrusive rocks, p€g Schist, gneiss, and granite, p-C LATER USAGE OF U.S. GEOLOGICAL SURVEY When first proposed by the U.S. Geological Survey, the Archean and Algonkian were conceived to be periods or systems in a Proterozoic Era, which were time-stratigraphic units comparable in scope and prob— ably in length to the Phanerozoic periods or systems. In actual practice in Survey publications, however, they were used empirically, Archean for dominant plutonic and metamorphic rocks and Algonkian for dominant supracrustal rocks. By 1933 the results had become so incongruous that these subdivisions were abandoned, and the pre- Phanerozoic rocks were designated by the title Pre- cambrian alone. Any subdivisions made were applied informally as lower and upper (early and late) or as lower, middle, and upper (early, middle, and late), and were used in a relative sense in local areas, without respect to any overall classification and correlation; the informal terms might thus vary in absolute age from one area to another. This procedure was useful in studies of particular areas, but was without value for regional work. This classification was nevertheless followed on the U.S. Geological Survey’s Geologic Map of North Amer- ica of 1965, where the Precambrian was divided in many areas into lower Precambrian (p631) and upper Precambrian (pCu), with unrealistic and sometimes misleading results. On the U.S. Geological Survey’s Tectonic Map of North America of 1969 a more detailed interim classifi- cation of the Precambrian was used, for purposes of this map only. The Precambrian was divided into Archean, Lower Proterozoic, Middle Proterozoic, and Upper Prot- erozoic, following Canadian usage that had prevailed up to 1963, to enable effective use to be made of Canadi- an tectonic data that were being contributed to the map. CLASSIFICATION OF PRECAMBRIAN ROCKS 11 The classification was also extended to Greenland on the northeast, and to the United States and Mexico to the south on the basis of radiometric data then avail— able. INTERIM CLASSIFICATION OF 1972 By the time compilation of the present Geologic Map of the United States began in 1967, it was clear that major improvements could be made in the representa- tion of the Precambrian on the Geologic Map of 1932, partly resulting from increased knowledge of the local Precambrian sequences, partly from correlation of the different sequences by radiometric dating. The experi— ence of the Canadian geologists in the Canadian Shield indicated the general lines that a revised classification of the Precambrian of North America would assume, and the experience of compiling the Tectonic Map of North America demonstrated that such a classification could be extended to the Precambrian of the United States. Compilation of the Precambrian for the Geologic Map therefore proceeded on this basis. In 1970, to verify the results of the compilation, and to produce an interim classification of the Precambrian for use on the map and in other Survey publications, the US. Geological Survey appointed a Special Panel con— sisting of M. D. Crittenden, Jr., Chairman, J. E. Harri- son, and J. C. Reed, Jr., to advise the Geologic Names Committee and the Chief Geologist. After Survey ap— proval, their recommendations were published as Note 40 of the North American Stratigraphic Commission (James, 1972a). During its deliberations, the panel reviewed the vari- ous units and their age assignments that were shown on the Geologic Map, enlisting the advice of Z. E. Peterman and C. E. Hedge, geochronologists ofthe US. Geological Survey. Various minor corrections and improvements were made in the age assignments of various units, but the four gross subdivisions shown on the Geologic Map were verified. The panel therefore recommended an interim adop— tion of these subdivisions. However, rather than apply formal names to them, as in Canada, it was recom- mended that they be designated informally by the let- ters W, X, Y, and Z. These letters would be especially useful for map symbols, as there was no likelihood of their being confused with any other symbol (other pos- sible letter sequences, such as A, B, C, and D, were already preempted by map symbols for other systems). The letter W was used for the oldest recognized subdivi— sion, thus providing for the possibility that still older Precambrian subdivisions might be separated later, which could be symbolized by preceding letters of the alphabet. The boundaries between the subdivisions ”were selected so as to split as few of the known episodes of sedimentation, orogeny, or plutonism as possible” (James, 1972a, p. 1129), hence were initially based on geologic features. Nevertheless, they were not intended to correspond to natural events such as orogeny or plutonism; once established, they were defined by geo- chronology alone. The basis for the proposed classification thus differs from the basis for the Canadian classification, in which the boundaries are defined by natural features or events whose ages were established by radiometric means. The opposing rationales reflect the different geologic condi- tions in the two countries. In Canada Precambrian rocks are exposed nearly continuously over vast ex- panses of the Canadian Shield, so that regional geologi- cal features are an evident and obvious means of classi- fication. In the United States outcrops are relatively small and some are so widely spaced that identification of regional geological features are necessarily much more subjective. Here, the only assured means of classi- fying the rocks of an outcrop is by age alone. Despite these differences, the major subdivisions of the Pre- cambrian in Canada and the United States are much the same and are broadly correlative from one country to the other. The two classifications, and the earlier one in Minnesota, are compared in table 3. Like all stratigraphic schemes, the interim classifica- tion of the US. Geological Survey creates problems when applied in detail. New radiometric data sometimes improve the dating of rocks or events (see Stockwell, 1972). "The most sig- nificant practical difference between subdivision based on geochronology and that based on stratotypes is that revision in age of the given body of rock would result in TABLE 3.—Comparison of recent classifications proposed for the Pre- cambrian ofNorth America [Numbers are ages in millions of years. In the first column, numbers combine the resultscf various analytical methods; in the second column first number is by K-Ar method, second by U/Pb; in the third column numbers are arbitrary} M In nosota. 1961 1968. 1970 Canada. 1964. 1972 US. Geological Survey. 1972 Upper cheenawan igneous Hadi‘ynian Precambrian 7. 800 ‘dCtiVlty 3 Grenvillian urogeny 100071.200 Yo 880 11,000 Precambrian : Helikian Precambrian Y Penokcan orogeny : 1.700 a 1.600 O 9.. Hudsonian orogeny 1.640 11,800» Middle Precambrian Aphebian Precambrian X 2.500 Algoman orogeny 2.500 Lower Precambrian Kenoran orogeny 2,390 l2.560| Archean Precambrian W 12 PRECAMBRIAN GEOLOGY OF THE UNITED STATES reassignment of the rock unit in the time scale, rather than readjustment of the time scale itself” (James, 1972a, p. 1131). Under the Canadian scheme the boundaries of the subdivisions are revised in age; under the United States scheme the boundaries remain fixed and the specific rock unit is moved from one subdivision to another, even though this might result in geologi- cally unnatural groupings. In general, the boundaries between the subdivisions were carefully enough chosen by the Special Panel so that most such problems will be avoided, but some will certainly arise. The defined age boundary between Precambrian Y and Z remains problematical. An 800-m.y. boundary was chosen by the panel on the assumption that it was the age of termination of deposition of the Belt Super- group in the Northern Rocky Mountains. This date is no more than an approximation, as the termination is merely bracketed between determined dates of 930 and 760 my; it is suspect because the lower part of the Precambrian Z Windermere Group that lies uncon- formably on the Belt to the west has been dated between 820 and 900 my (For details, see p. 53.) Further, the upper part of the Precambrian Z supracrustal rocks in the Central Appalachians has been dated at 820 my; these rocks lie on Precambrian Y infracrustal rocks with the greatest discordance in the sequence below the Triassic. It would be intolerable to place this discord- ance, along with the infracrustal rocks below and the supracrustal rocks above, all in Precambrian Y. The proposed boundary at 800 my is therefore ignored on both the Geologic Map and in the ensuing text, where the most workable boundary is found to be about 100 my earlier. REPRESENTATION OF PRECAMBRIAN ON GEOLOGIC MAP OF UNITED STATES The interim subdivisions of Precambrian W, X, Y, and Z are used on the Geologic Map to classify the units in the different sequences, and to correlate these se- quences with those in other parts of the country. Differ- ent categories of rocks are indicated in the same manner as in the Phanerozoic. Each Precambrian subdivision thus contains representatives of stratified sedimentary rocks, volcanic rocks, plutonic or intrusive rocks, and metamorphic rocks, shown in separate columns in the legend. However, Precambrian continental and eugeo- synclinal deposits are either not separated or not rec- ognized. The arrangement of the stratified rocks in the legend indicates that in at least some areas the methods used in the Phanerozoic can be applied; the Belt Supergroup of Precambrian Y can even be subdivided on the Geologic Map in parts of northwestern Montana and northern Idaho. Because the assignment of strata to one of the new subdivisions or another will not be familiar to most users, representative units in different areas are listed more completely in the legend than for the subdivisions of the Phanerozoic rocks. The volcanic rocks, although placed in a separate column in the legend, are impor- tant components of the stratified sequences in some areas, as in Precambrian W and Y of the Lake Superior Region, and Precambrian Z of the Appalachian Region. Among the Precambrian plutonic rocks the most ex- tensive are granitic, but mafic categories are separately shown in Precambrian W and Y. Assignment of the plutonic rocks to one subdivision or another is based partly on their geologic relations to the surrounding country rocks, but more upon their radiometric dating. The ages determined for the granitic rocks indicate that many of them formed during the later stages of a sub- division, but in Precambrian Y an earlier suite is exten- sive; the granites of Precambrian W include both the terminal plutonics, and undifferentiated earlier ones. The metamorphic rock units, in general, are com— plexes so greatly altered as to preclude the application of normal stratigraphic analysis. The orthogneisses originated from plutonic rocks and the paragneisses from sedimentary or volcanic rocks. In places, the latter include some bodies of rock capable of more detailed analysis, but in such small areas that it would be fruit- less to separate them on the scale of the present geologic map. On the map, the ages assigned to the metamorphic rocks are based primarily on their time of metamor— phism, assuming that the original rocks were mostly formed during the time of the same subdivision, but in places they may include relict rocks formed during ear- lier subdivisions that have been overwhelmed by the later and dominant metamorphic event. These results are summarized on the accompanying maps (fig. 2—5), which show the surface distribution of rocks of the different major subdivisions, as represented on the Geologic Map. To give added meaning to the figures, the rocks of each subdivision are divided into three classes: (1) Sedimentary and volcanic supracrust- al rocks (including their metamorphic equivalents in the earlier subdivisions),2 (2) intrusive and plutonic rocks (including those of both felsic and mafic compo— sition), and (3) metamorphic rocks (paragneisses and orthogneisses). 2The word "supracrustal" has been defined briefly as referring to "rocks that overlie the basement." In this account, the term supracrustal is used for Precambrian sedimentary and volcanic rocks that were laid down on the surface of the earth, on a basement of rocks that have had a more complex metamorphic and plutonic history. Ideally, they are exemplified by such units as the little deformed or metamorphosed Keweenawan and Belt Supergroups. However, differences between "supracrustal" and "basement" rocks are relative, and distinc» tions between them become subjective and blurred in places. Thus, this account describes many units as "supracrustal" even though they have been deformed and metamorphosed, because they are Clearly of sedimentary and volcanic origin, and contrast with more enig- matic paragneisses and orthogneisses. LAKE SUPERIOR REGION 13 EXPOSED PRECAMBRIAN ROCKS OF THE UNITED STATES3 The following is a survey of the Precambrian rocks exposed at the surface in the United States, to explain the representation adopted on the Geologic Map. It ex- pands the explanation of these rocks in the legend. In the legend, the rocks are categorized by age and charac- ter (sedimentary, volcanic, plutonic, etc.); here, it is better to treat all the rocks of each province collectively, in order to demonstrate their mutual relations, and the reasons for assigning particular rock units to one or another of the broad age divisions. The exposed Precambrian rocks are only a small part of the Precambrian of the United States; much larger areas are concealed beneath Phanerozoic rocks, espe- cially in the Central Interior Region, between the Ap- palachian and Cordilleran mountain belts, where they are known from drill data. The concealed Precambrian rocks have been extensively investigated, especially during a project of Goldich, Muehlberger, Lidiak, and Hedge (1966). Here, these concealed rocks will be men- tioned only to suggest connections between the rocks of the various areas of exposure. In this account the results of many fundamental pieces of research will be summarized, but these are not always credited with a citation. Literature references are made primarily: (1) to recent publications that up- date the earlier records, (2) to summary reviews that contain references to earlier publications, and (3) to publications which contain information on radiometric dating. The account is illustrated in part by maps and diagrams, the maps being mostly on scales larger than those of the Geologic Map, which show rock units, struc- tures, and the names of localities which could not be represented on the Geologic Map itself. Features not illustrated by the maps and diagrams in the text are believed to be adequately represented on the main Geologic Map, to which the reader should refer. Extensive use will be made of radiometric data to justify the classifications and correlations that are made, and specific ages are cited where appropriate. In general discussions, however, I believe it is clearer to use names rather than numbers for the broad groupings of ages within a few hundred million years of each other that express orogenic, plutonic, metamorphic, or other significant events in the Precambrian history of North America. For this purpose the names used in the Cana- dian Shield are adapted in this text: The Kenoran with ages around 2,500 my, the Hudsonian with ages 3Previous official reviews of the Precambrian ofthe United States by Van Hise (1892) and Van Hise and Leith (1909) appeared more than halfa century ago. They provide interesting comparisons with the present review, both in the amounts ofdata available, and in geologic concepts. around 1,700 my, the Elsonian with ages around 1,300 my, and the Grenvillian with ages around 1,000 my. It is true that in the United States various local names have been used for comparable events, some proposed earlier, some later; for example, Algoman and Peno- kean orogenies in the Lake Superior Region, St. Fran- cois igneous activity and Llano orogeny in the South Central States, and Black Hills and Mazatzal orogenies in the Cordilleran Region. These names add precision to local discussions because they can be tied to specific dates within the particular area, but in a regional re- view such as this they obscure the broader relations. LAKE SUPERIOR REGION4 The most extensive outcrops of Precambrian rocks in the United States are in the region west and south of Lake Superior. Precambrian forms the northern half of Minnesota, the western half of the northern peninsula of Michigan, and a large part of northern Wisconsin. Also properly part of the region are outlying areas to the south, such as that of ancient gneisses in the Minnesota River valley, of Sioux Quartzite that extends into South Dakota, and of Baraboo Quartzite in central Wisconsin. The region is a southern extension of the Canadian Shield, the northwestern part belonging to its Superior province, and the southeastern part to its Southern province. The Lake Superior Region in the United States and adjacent Canada has been one of the longest known and most intensively studied Precambrian terranes in North America, particularly because of its wealth of mineral resources such as the great deposits of iron ore north and south of the lake and the copper deposits of the Keweenaw Peninsula. Moreover, it contains a long record of Precambrian rocks and events, all the major divisions (W, X, Y, and Z) being represented in some form or another. Their various supracrustal sequences total more than 150,000 ft (46,000 m) of strata, and the record is further diversified by several times of major or minor orogeny, and of plutonic and volcanic activity. The Precambrian rocks and structures have remained virtually untouched by Phanerozoic disturbances, in contrast to the Precambrian of most of the other regions of the United States which we will consider later. For these reasons, there has long been a temptation to regard the Precambrian sequence of the Lake Sueprior Region as the North American standard, to which the Precambrian of other regions is to be compared and correlated. This View, however, would fail to take into ‘For a recent compendium of the geology of the part of the Lake Superior Region in Minnesota, see Sims and Morey (1972, especially p. 274155). This includes recent data not available when the present summary was prepared; the more important revisions are in- cluded here. 14 PRECAMBRIAN GEOLOGY OF THE UNITED STATES FIGURE 2.——Map of the United States, showing surface distribution of rocks of Precambrian W as represented on the Geologic Map of the United States. LAKE SUPERIOR REGION FIGURE 2.—Continued. EXPLANATION Metamorphosed supracrustal rocks Plutonic and intrusive rocks (felsic to maflc) Metamorphic rocks (paragneiss and orth ogneiss) 15 16 PRECAMBRIAN GEOLOGY OF THE UNITED STATES FIGURE 3.—Map of the United States, showing surface distribution of rocks of Precambrian X as represented on the Geologic Map of the United States. LAKE SUPERIOR REGION \ \ “ \ \‘ ‘\ tr > j 1 r 'u ........ ’\____‘_.-—--.j , ’ \ ‘2 _/ ‘\ i?» \ \\ “- FIGURE 3.—Continued. EXPLANATION Metamorphosed supracrustal rocks Plutonic and intrusive rocks (felsz‘c to mafic) Metamorphic rocks (paragneiss and orthogneiss) 17 18 PRECAMBRIAN GEOLOGY OF THE UNITED STATES FIGURE 4.—Map of the United States, showing surface distribution of rocks of Precambrian Y as represented on the Geologic Map of the United States LAKE SUPERIOR REGION FIGURE 4.—Continued. EXPLANATION Supracrustal rocks Plutonic and intrusive rocks (felsic to mafic) M Metamorphic rocks (paragnez‘ss and orthogneiss) 19 20 PRECAMBRIAN GEOLOGY OF THE UNITED STATES FIGURE 5.——Map of the United States, showing surface distribution of rocks of Precambrian Z as represented on the Geologic Map of the United States. LAKE SUPERIOR REGION FIGURE 5.—Continued. EXPLANATION Supracrustal rocks Felsic plutonic rocks 21 22 account the great length of Precambrian time and large gaps in the record in even so complete a sequence, as well as the quite different tectonic and sedimentary regimes in other parts of North America. PRECAMBRIAN W. The northwestern and western part of the Precam- brian area in Minnesota is an extension of the Superior 1 PRECAMBRIAN GEOLOGY OF THE UNITED STATES probably intermingled in various combinations from place to place (Pettijohn, 1943, p. 980—981). Unconform- ities above the volcanics are of local extent, and grada- . tional or interbedded relationships occur in other ‘ places. province of the Canadian Shield, a domain of the an- i cient rocks of Precambrian W age (= Archean of Canada). Its rocks are well exposed toward the north- ‘ east, as well as westward along the Canadian border as far as the Lake of the Woods. Farther southwest out- ; crops are sparse; there is an extensive cover of thick glacial drift and of the thin intervening Cretaceous Coleraine Formation (King and Beikman, 1974, fig. 13), so that representation of the Precambrian here must be largely by subcrop methods, especially by deductions from geophysical surveys. The Superior province in Minnesota (as in adjoining i Ontario) is a great body of supracrustal rocks, probably 1 more than 50,000 ft (15,000 m) thick in all, partly metavolcanics (WV), partly metasediments (W), and equally extensive bodies of granitic plutonic rocks (Wg). The volcanics, traditionally called Keewatin Group, include the Ely Greenstone of northeastern Minnesota with the Soudan Iron-formation in its upper part (com— mercially productive in the Vermillion district). Much of the greenstone is basaltic, but intermediate and felsic varieties are present also. Pillow structure is ubiqui- tous, except where obscured by deformation and metamorphism, and indicates subaqueous eruptions. The superincumbent sediments—the Knife Lake Group of northeastern Minnesota and comparable units in On- tario (Temiskaming, etc.)—are dominantly graywacke, with local thick lenses of conglomerate and minor slate; quartzite and limestone are Virtually lacking. Graded bedding and related features in the graywackes indi- cate they they are turbidites, formed subaqueously in a tectonic environment (Pettijohn, 1943, p. 966—968). The relation of the sediments to the volcanics has been variously interpreted ever since A. C. Lawson began fieldwork in the Lake of the Woods area in 1883, and has given rise to some ofthe classic controversies of North American geology. It is now clear that most of the sediments overlie the volcanics, but a prevolcanic ter- rane (Coutchiching) has been claimed, especially in ad- jacent Ontario. In places, at least, the superincumbent sediments lie unconformably on the volcanics, and some granites intrude the volcanics but not the sediments, giving rise to the concept of a far-reaching “Laurentian orogeny” between the two. Actually, these problems are not fundamental, as volcanic and sedimentary units are A case in point is stratigraphic relations in the Ver- million district of north—eastern Minnesota, where many of the classic concepts of the Precambrian W rocks originated. Modern mapping (Sims, in Sims and Morey, 1972, p. 49—62) has indicated greater stratigraphic complexity than originally supposed; in essence, the Ely Greenstone (or local representative of the Keewatin Group) is followed by a unit of Knife Lake sediments, and this by a second volcanic body of Keewatin type, the last two merging into the main mass of Knife Lake sediments in the eastern part of the district. The whole sequence is conformable, and there is no evidence for any major orogenic interruption, as was formerly be— lieved. Of the older granites (traditionally but inappropri— ately called "Laurentian”) the only example that has been cited in Minnesota is the Saganaga Granite on the International Boundary in the northeastern corner of the State (fig. 6). It clearly intrudes the Ely Greenstone, and the adjacent Knife Lake sediments lie on its eroded surface. However, it intrudes other parts of the Knife Lake, and its radiometric age does not differ greatly from that of the surrounding rocks. Probably its pluton was emplaced at shallow depths, and quickly unroofed during the early part of Knife Lake sedimentation (Sims, in Sims and Morey, 1972, p. 53). The remaining granites (termed Algoman) intrude all the supracrustal rocks of the province: The Vermil- lion Granite forms a body 80 mi (130 km) long east-west and 30—40 mi (50—65 km) wide north—south along the International Boundary, and the Giants Range Granite farther south extends for more than 100 mi (160 km) along the northern edge of the Mesabi Range, where it is overlain unconformably by the Animikie Group (Pre- cambrian X). The Algoman (= Kenoran) orogeny deformed and metamorphosed the supracrustal rocks and emplaced the Algoman granites. The orogeny has been dated be— tween 2,400 and 2,750 my, on the basis of a variety of radiometric methods (Goldich and others, 1961, p. 69— 74). However, there are unexplained discrepancies be- tween uranium-lead, rubidium-strontium whole-rock, potassium-argon and rubidium-strontium mineral ages. Available radiometric data seem to suggest that all of the features in the Precambrian W complex of northern Minnesota—accumulation of the volcanics and sediments, and their deformation, metamorphism, and plutonism—were created during a remarkably LAKE SUPERIOR REGION 23 92° 90° //\/‘,\/\~,\’IT£I /\ — /’ \I 1.3) /\,l/ \/_\ \—-/\/’ J'x'c ” 1\fl/IC\E\/\\l r \1 4 Keweenawan ‘ LYSl A Supergroup granite Animlkie Group and Marquette r x x * Range Supergroup (with iron Granite formations in black) 7/ .. \ \ a .Pw‘a. Keewatin and Knife Lake Groups, ‘ + + i and related rocks (with minor iron formations in black) Granite and granite gneiss FIGURE 6.—Geologic map ofpart ofthe Lake Superior Region, showing localities and map units mentioned in the text. Generalized from geologic maps of United States (1974) and Canada (1969). 24 short interval between 2,700 and 2,750 m.y. ago (Gold— ich, in Sims and Morey, 1972, p. 32—34). South of the area just discussed, in southwestern Minnesota, Precambrian granites and gneisses (Wg, Wgn) appear along the Minnesota River valley (Goldich i and others, 1961, p. 123—146). Here, radiometric deter- minations have yielded a scatter of dates, with some as ‘ low as 1,850 m.y. (an overprint of the Penokean ; (= Hudsonian) event), and others, by lead-lead methods 1 on zircons from 2,870 to 3,280 m.y. A concordia plot ‘ suggests an original age of 3,550 m.y. (Goldich, 1968, p. 718—720), so that these rocks are among the oldest recorded in North America.5 South of Lake Superior in Michigan and Wisconsin, old rocks are exposed beneath the Marquette Range Supergroup (Precambrian X) in the higher folds, and have been identified as <‘Archean” (that is, Precambrian W) since the earliest surveys. Most of the rock is granite gneiss, probably mainly Algoman, but Keewatin-type greenstone occurs to the north in the Marquette district, and farther south is the Dickinson Group of arkose, schist, and amphibolite (fig. 2); it is in contact not only with the Algoman granite, but with an older granite gneiss (James, 1958, p. 31—33). This region has been more heavily involved in younger Precambrian events (such as the Penokean orogeny) than the region north- west of Lake Superior, so that radiometric dating has produced varied results. Nevertheless, feldspar rubidium-strontium ages and the diffusion age of zir- cons establish the age of the basement gneisses at near 2,700 m.y. (Aldrich and others, 1965, p. 462), or about as old as the Precambrian W rocks of northwestern Min- nesota. Gneisses east of the Minnesota River valley (the McGrath Gneiss of central Minnesota and the basement gneisses of Michigan and northern Wisconsin) are simi- lar petrographically and in metamorphic history to those along the Minnesota River but have so far failed to yield dates as ancient. Nevertheless, Morey and Sims (1976) suggest that they may all be part of the same terrane—a sialic protocontinent against which the greenstones and graywackes of northern Minnesota and elsewhere in the Superior province were built in later Precambrian W time. PRECAMBRIAN X Southeast of the Superior province is the Southern province, which forms the remainder of the Lake Su- perior Region. The boundary between them is in north- 5The oldest radiometrically dated rocks in North America, and among the oldest in the world, are those of the Godthaab area, western Greenland, where quartszeldspathic gneiss as with some shreds ofiron formation have been dated at more than 3,750 my. (Moorbath and others, 1972). Very ancient Precambrian rocks are suspected from geological evidence in parts of the Canadian Shield in Canada, but so far lack radiometric verification. PRECAMBRIAN GEOLOGY OF THE UNITED STATES ern Minnesota and adjacent Ontario, where rocks of Precambrian X lie with right-angle unconformity on rocks of Precambrian W and dip away from them south- eastward. Within the Southern province, rocks of Pre— cambrian X are extensive northwest and south of Lake Superior, in Minnesota, Wisconsin, and Michigan, flanking on each side the Keweenawan rocks (Precam- brian Y) that occupy the trough of the Lake Superior syncline. They contain all the commercially exploited iron deposits of the Lake Superior Region (shown in red on the Geologic Map), except those in Precambrian W of the Vermillion district: the Gunflint district of Ontario, the Mesabi and Cuyuna districts of Minnesota, and the Gogebic, Menominee, Marquette, and other districts of Wisconsin and Michigan (fig. 1). (The outcrops of iron formations in the various districts are commonly re- ferred to as “ranges,” hence such terms as “Mesabi Range”) The supracrustal rocks of Precambrian X northwest of Lake Superior are the Animikie Group, named long ago for the Thunder Bay district in Ontario, whence the group can be traced westward with little interruption into Minnesota. South of Lake Superior, the obvious stratigraphic and lithologic equivalents of the Animikie are in the middle of a more comprehensive sequence, the Marquette Range Supergroup (Cannon and Gair, 1970) (fig. 7). The Animikie of the northwestern area begins with a discontinuous basal quartzite lying unconformably on Precambrian W, followed by a persistent iron formation several hundred feet thick (Biwabic of Mesabi district), and topped by the Virginia Slate many thousands of feet thick. This iron formation (and those of Precambrian X elsewhere) is an alternation of ferruginous chert (= taconite), slate, and stromatolitic beds, whose weathered products were the readily exploited iron de- posits of past decades. The Virginia Slate is interbedded argillite and graywacke, a turbidite deposit not unlike the much older Knife Lake. In the Cuyuna district southwest of the Mesabi dis- trict the sequence is much the same, but the iron forma- tion is separated from the Precambrian W rocks on the west by a poorly exposed, wider stratigraphic interval. It may include pre-Animikie Precambrian X rocks com- parable to the Chocolay Group south of Lake Superior (Marsden, in Sims and Morey, 1972, p. 227—230). These lithologic components reappear in the Mar- quette Range Supergroup south of Lake Superior. Iron formations like the Biwabik occur in each of the princi— pal districts (Gogebic, Marquette, Menominee), again with basal quartzites and great overlying bodies of “slate” (argillite and graywacke). Here, however, the sequence is thicker, more diverse, and interrupted by unconformities, so that it has been divided into four LAKE SUPERIOR REGION GOGEBIC—KEWEENAWAN DISTRICTS,M|CH.—WIS. MENOMINEE AND ' ADJACENT DISTRICTS MlCH.—WIS. MESABl—VERMILION DISTRICTS, MINN YO _ _ q C o. E 3 MARQUETTE m 2 DISTRICT Q >._ 55>— MICH. 3 0 3 9 5 3 ‘fi 9 gm : |\ a: _ o _ 8 Baraga g- :g ‘6 m ><~ .EE‘ 5 Group _g :0 “L 5 _ _ n: g = w 9 g Menomi g 0 0 Group :1 5.” a" 3- <0 :a 3 Chocolay E u Grou ...... E p . . . .. 3- _ : — a g s 3 o ‘z 3 c ._ 5.- o 0 a»: .s— :5 -s 5 .—I .E >1 4 D LITHOLOGIC SYMBOLS Rocks of volcanic Rocks of sedimentary Crystalline rocks, mostly origin origin of igneous origin + + + + + + + Basalt Sandstone and Gabbro and granite quartzite Age 1,100 m.y. 0 O O —/ >:l. 27 The thickness of Keweenawan supracrustal rocks and their associated intrusives is well over 50,000 ft (15,000 m) in the trough of the syncline but thins out- ward and may not have extended far beyond its present limits (White, 1966, p. 28—32); accumulation of the Ke- weenawan and downwarping of the syncline were con- temporaneous. Although the rocks have been gently to steeply tilted they have not been folded or metamor- phosed. The Keweenawan sequence begins with thin basal sandstones preserved discontinuously on both the north and south flanks of the syncline. They are overlain by a great sequence of amygdaloidal basaltic to andesitic lavas in persistent thin to thick flows, with minor rhyo- lites (Portage Lake Group to southeast, North Shore Group to northwest). Observed sequences of the lavas are 15,000—25,000 ft (4,500—7,600 m) thick, but are in- complete and the total is clearly much greater. Flow structures in the lavas demonstrate that they spread out in both directions from the axis of the trough, against the present slope of its flanks. Evidently the rate of buildup of the lavas exceeded the rate of downwarping of the trough, and produced an outward slope (White, 1960, p. 368—371). Paleomagnetic studies indicate that the lower lava flows have reversed polar— ity and the uper lava flows normal polarity, which suggests a possible criterion for stratigraphic subdivi- sion (Craddock, in Sims and Morey, 1972, p. 285—286). Succeeding the lavas on the southeast shore are the elastic, continental sediments of the Oronto Group, as much as 15,000 ft (5,000 m) thick. The first deposits are coarse conglomerates made up largely of volcanic clasts (Copper Harbor), but the main body (Freda) is red ar- kosic sandstone and interbeded micaceous siltstone, de- rived from erosion of surrounding highlands of earlier Precambrian crystalline rocks. The thin Nonesuch Shale, which separates the lower conglomerates from the Freda, contains organic compounds, microfossils, and crude oil. Sedimentary structures in the sandstones indicate transport from the highlands toward the axis of the trough (Hamblin, 1961, p. 2—6), indicating that, unlike the volcanic buildup, the sedimentary buildup did not keep pace with the subsidence of the trough. The lower part of the Keweenawan is invaded by mafic intrusives, the largest being the Duluth Complex northwest of Lake Superior, a lopolith 150 mi (240 km) long and as much as 50,000 ft (15,000 m) thick near its center, injected near the base of the Keweenawan. It is a multiply-layered intrusive, mainly gabbro but with an- orthositic phases, and a granophyre phase at the top. Smaller mafic bodies south of Lake Superior are at about the same stratigraphic level, the largest being the Mellen Gabbro of the Gogebic district (fig. 6). The intru- sives are deep-seated manifestations of the same 28 “Keweenawan igneous activity” that produced the lavas. Radiometric dates of the Keweenawan rocks have been obtained from the felsic differentiates of the mafic intrusives and lavas. Felsites from the North Shore and Portage Lake Volcanics, the granophyric facies of the Duluth Complex and Mellen Gabbro, as well as other igneous rocks, have all yielded ages between 1,120 and 1,140 m.y. by uranium-lead determinations on cogenet- ic zircons, suggesting a narrow pulse of magmatic ac- tivity (Silver and Green, 1963, 1972). Dates by potassium-argon and rubidium-strontium methods have a greater span, between 1,100 and 1,300 m.y. (Goldich and others, 1961, p. 95; Goldich, in Sims and Morey, 1972, p. 35—36), but are less reliable. An age of 1,075 m.y. has been proposed for the Nonesuch Shale of the succeeding Oronto Group (Craddock, in Sims and Morey, 1972, p. 185). The Lake Superior syncline and its Keweenawan rocks are merely an exposed segment of a much larger tectonic feature (fig. 8). Prominent gravity and magnet- ic anomalies demonstrate that the trough and its as- sociated mafic igneous rocks extend another 600 mi (960 km) southwestward beneath the Paleozoic cover into northeastern Kansas (E. R. King and Zietz, 1971), and somewhat vaguer geophysical data suggest that the trough turns southeastward near the end of Lake Superior, to extend for an unknown distance beneath the southern peninsula of Michigan (Gray and others, 1973). The whole structure thus has a curiously arcuate form, concave toward the south—a product of crustal rifting of subcontinental dimensions late in Precam- brian time. PRECAMBRIAN Y ROCKS OLDER THAN Kl‘ZWEENAWAN Southwest of the Keweenawan area is the Sioux Quartzite of southwestern Minnesota and southeastern South Dakota which forms a plateaulike terrain 200 mi (320 km) long, partly concealed by glacial drift; the similar Barron Quartzite forms a smaller area on the west flank of the Wisconsin arch. The Sioux is warped gently into an axial trough and has a thickness of about 3,000 ft (900 m); the Barron is much thinner. Inter- bedded in the Sioux are layers of "pipestone” (argillite) which have yielded potassium-argon date of 1,200 m.y. (Goldich and others, 1961, p. 49), probably related to the mild deformation. A well in northwestern Iowa pene- trated Sioux Quartzite interbedded with rhyolite layers, the latter yielding an apparent rubidium- strontium age of 1,470 m.y. (Austin, in Sims and Morey, 1972, p. 450); regardless of whether the rhyolite is in— trusive or extrusive, this date suggests a minimum age for the formation. PRECAMBRIAN GEOLOGY OF THE UNITED STATES The Sioux and Barron Quartzites are approximately correlative with the Sibley Group north of the Lake Superior syncline in Ontario, which lies stratigraph- ically between the Animikie and the Keweenawan. All three units formed after the Penokean orogeny, but before the accumulation of the Keweenawan Super- group, during early Precambrian Y. Farther south, in the Baraboo area of central Wiscon- sin, Precambrian rocks project through the surrounding lower Paleozoic strata in a partly exhumed monadnock, and have long been classic for student work because of their proximity to many Middlewestern universities. (My own first field experience was at Baraboo during a summer field course of the State University of Iowa.) The rocks of the sequence at Baraboo much resemble those of the lower part of the Marquette Range Super- group (Precambrian X) in the Lake Superior Region to the north—a thick lower quartzite, followed by slate, iron formation, and dolomite—and were called "Huron- ian” in the older reports. Nevertheless, these rocks over- lie felsic volcanics with rubidium-strontium age of about 1,600 m.y. (Dalziel and Dott, 1970, p. 8-10), so that they are younger than those with which they have been compared. Evidently they formed during the early part of Precambrian Y, like the Sioux and Barron Quartzites. PRECAMBRIAN 7. Near the axis of the Lake Superior syncline, between the main body of the Keweenawan and the overlapping Upper Cambrian, is another body of sandstones, known as the J acobsville in Michigan, the Bayfield in Wiscon— sin, and the Fon du Lac and Hinckley in Minnesota. At one time or another, geologists have assigned these sandstones to the Keweenawan or to the Cambrian, but they are unconformable with both and are probably part of neither. Observed sequences of the sandstones are as much as 5,000 ft (1,500 m) thick, and geophysical surveys suggest that they may be 7,000 ft (2,100 m) thick on the southeast side of the Keweenaw Peninsula. They are red sediments like the underlying Keweenawan, but they are more cleanly washed, being quartzites rather than arkoses, with a less varied heavy mineral as- semblage. Their sediment transport was again toward the axis of the Lake Superior syncline (whereas that of the Cambrian is mainly southward) (Hamblin, 1961, p. 6—13), so that subsidence of the trough continued, but dips of the sandstones are much lower than those of the Keweenawan. These sandstones are probably the representatives of Precambrian Z in the Lake Superior Region, and they are so indicated on the Geologic Map, although definite radiometric proof is not available. NORTHERN APPALACHIAN REGION ADIRONDACK AREA The Adirondack area of Precambrian rocks of north- ern New York State is a domical uplift 120 mi (195 km) across, nearly encircled by Paleozoic rocks, but con— nected northwestward along the Frontenac axis with the Grenville province of the Canadian Shield, of which it is an extension. The Precambrian area includes two contrasting parts: a northwestern lowland 40 mi (65 km) broad, dominantly of medium-grade metasedimentary rocks, and the Adirondack Moun- tains to the southeast of high-grade gneisses and exten- sive plutonic rocks; the two parts are juxtaposed along the Highland Boundary fault, downthrown toward the lowlands. The Grenville Group of the lowlands (Y) is a meta- sedimentary sequence about 15,000 ft (4,500 m) thick (Engel and Engel, 1954, p. 1018), more than half of which is calcite or dolomite marble, and the remainder quartz-feldspar gneiss and minor quartzite. The rocks have been plastically folded and refolded, and meta- morphosed to amphibolite grade (with sillimanite). They contain many concordant lenses and pods of hornblende granite (Yg2), now with phacolithic struc- ture but probably intruded before or during the folding (Buddington, 1939, p. 152—158). The rocks of the mountains are a complex of para- gneiss (Ym), orthogneiss (an), syenite (Y5), and anor- thosite (Ya), metamorphosed to granulite facies, prob- ably at a deep level in the crust. The most prominent component is the anorthosite, covering 14 percent of the area and forming mountainous massifs, the largest of which is the Mount Marcy body 50 mi (80 km) across. The syenite (mangerite), in smaller areas, may be genetically related. The orthogneisses include both granitic and charnockitic varieties. The paragneisses were the host rocks of the others and have been corre- lated with the Grenville Group to the northwest, al- though they contain less marble. The origin and sequence of the plutonic rocks has long been debated, and many views have been expressed. Buddington (1939, p. 197—235) believed that they were introduced as magmas, the anorthosite, syenite, and charnockite successively before the deformation, the granite during the major orogeny and metamorphism. At the opposite end of the spectrum is a proposal that all the plutonic components were remobilized from a deep- er level, or basement, the mobilities ranging from slight in the anorthosite to a maximum in the granite; the more mobile the component the more transgressive the rock, hence the younger its apparent age (Walton and de Waard, 1963). The Adirondack Precambrian rocks, like those of the rest of the Grenville province, yield characteristic 29 Grenvillian radiometric dates of 1,000—1,200 my, and large parts of them in the province have been called Grenville Series in a broad sense.7 The Geologic Map follows present Canadian usage (Emslie, 1970, p. 124— 125) in restricting the Grenville Group to the recogniz- able metasedimentary rocks of the original Grenville area in southern Quebec and Ontario and the adjacent lowland of New York State. Igneous and metamorphic events in the lowlands have ages of 1,160—1,200 m.y. (Silver, 1963); structural evidence in Canada suggests that the group itself is Paleohelikian (=early Precamb- rian Y) (Emslie, 1970, p. 125). The other Precambrian rocks of the Adirondack area are likewise classed on the map as Precambrian Y, but not as Grenville. The Adirondack anorthosite contains zircons which have been dated between 1,020 and 1,100 my by uranium-lead methods; similar ages, but none older, have been found in the associated orthogneisses and pegmatites (Silver, 1968, p. 250). These dates record the time of granulite metamorphism, but it is claimed from the characteristics of the zircons that this is the age of the magmatic crystallization as well. Nevertheless, the Adirondack anorthosites are part ofa chain of massifs that extends 1,000 mi (1,600 km) north-northeastward, diagonally across the Grenville province, into the Nain province of eastern Labrador. Those of the latter province, outside the region of Gren— villian influence, yield Elsonian ages of about 1,400 my, and it has been suggested that the other massifs, many with apparently younger dates, have been re- worked during the Grenvillian event (Stockwell, 1964, p. 3). Be that as it may, emplacement of the anorthosite massifs appears to have been a unique event in earth history; all known massifs, both in eastern North America and elsewhere, are datable within a span of a few hundred million years of middle Precambrian time, as we shall see when considering the anorthosite bodies farther west in the United States (p.50, 66). All are shown on the Geologic Map as Ya. NORTHERN APPALACHIAN REGION PRECAMBRIAN Y OF WESTERN PART To the southeast and south of the Adirondack area, in the western part of the Northern Appalachians, Precambrian rocks with Grenvillian ages emerge in the higher uplifts, where they have been reworked during the various Paleozoic orogenies. They form 7The name "Grenville" has been extended from the original rocks ofthe Grenville Group to include a more comprehensive Grenville Series, a province and its northwestern tectonic front, and an orogeny. These extensions have been condemned by G111u1y<1966, p. 104—108), but in the absence ofany acceptable substitute, common usage must prevail—provided care is taken by the geological author to specify clearly which of the several "Grenvilles" he is referring to. 30 PRECAMBRIAN GEOLOGY the basement of the Green Mountains of Vermont, the Berkshire Hills of Massachusetts, the Hudson Highlands of New York State, and the Reading Prong of New Jersey and Pennsylvania. The uplifts are links in a chain that extends from the Long Range in Newfoundland to the south end of the Blue Ridge in Georgia. The basement is overlain by Lower 95 OF THE UNITED STATES Cambrian and younger Paleozoic geosynclinal rocks—basal miogeosynclinal quartzites on the west (€q, Cheshire and Poughquag), and more varied eugeosynclinal clastics and volcanics on the east (6e). The uplifts are vergent westward or northwest- ward, and become increasingly allochthonous south- 85° 50° N OR TH ,\)‘ MINNESOTA DAKOTA OLDER PRECAMBRIAN OLDER PRECAMBRIAN ILLINOIS 5 / KENTUCKY l | f 400 MILES [ r I I 400 KILOMETRES FIGURE 8.—Map of north-central United States, showing the arcuate pattern of surface and subsurface upper Precambrian rocks (Y and Z). (Based mainly on Craddock, in Sims and Morey, 1972, p. 283.) NORTHERN APPALACHIAN REGION ward. The Green Mountains and Berkshire Hills are anticlinoria, the first with a steep west flank, the second overthrust westward. The Hudson Highlands and Reading Prong have commonly been interpreted as fault-bounded horsts, but modern work indicates that the Precambrian of the Reading Prong, at least, is part of a floored nappe with roots farther southeast (Drake, 1970, p. 286—289). The smaller Precambrian bodies east and southeast of the main chain of uplifts are even more complexly involved in the Appalachian deformations. Those in the cores of the Chester and Athens domes in the Connecticut Valley of southeastern Vermont have risen diapirically into a thick pile of eugeosynclinal strata. Those south of the Hudson Highlands (Fordham and Yonkers Gneisses) have been plasticly folded and refolded with the lower Paleozoic rocks ofthe New York City Group (Hall, 1968, p. 124). The Precambrian rocks are dominantly paragneisses, EXPLANATION Probable Precambrian Y and Z rocks in southern Michigan subsurface PRECAMBRIAN Z Bayfield Group and related sandstone units PRECAM BRIAN Y Oronto Group Middle Keweenawan mafic intrusives NV Middle Keweenawan basaltic lavas Sioux Quartzlte and related units PRECAMBRIAN X and W E Older Precambrian metamorphic rocks Edge of Phanerozoic rocks FIGURE 8.—Continued. 31 with interbedded quartzite and marble units and minor intrusive orthogneisses. Many details of the subdivision and pattern of the gneisses are shown on the modern State Maps on a scale of 1:250,000, but this is impractical on the much smaller scale of the Geologic Map of the United States, where they are indicated merely as paragneiss (Ym). The pattern of the units in the Green Mountains uplift, as shown on the Vermont Map (fig. 9), is discordant to its elonga- tion and crosses it nearly at right angles, although somewhat curved as a result of the Paleozoic uplift. The rocks underwent a Precambrian metamorphism to high amphibolite grade in the Green Mountains and granulite grade in the Reading Prong, but they were metamorphosed again and retrograded during the Appalachian orogenies. As would be expected, the radiometric data reflect this complex metamorphic history. Relict Grenvillian dates of 900—1,100 m.y. have been obtained from the Green Mountains and Hudson Highlands by uranium-lead and related methods (Tilton and others, 1960, p. 4175; Faul and others, 1963, p. 3, 7). Determinations by rubidium-strontium and potassium-argon methods on rocks in the uplifts and southeastward yield mainly ages of about 360 m.y. that express the time of Paleozoic metamorphism, but there is a scatter of intermediate dates that ex- press either genuine events, or a resetting of original Grenvillian ages by the later metamorphism (Long and Kulp, 1962, p. 984—987). PRECAMBRIAN Z OF EASTERN PART None of the Precambrian Y basement is found east of the Connecticut Valley, but younger Precambrian is mapped in widely separated areas in eastern New England. In western Maine the oldest rocks of the Boundary Mountains anticlinorium form the Chain Lakes massif, and are largely highly metamorphosed paragneiss, quartzite, and amphibolite. They are cer- tainly pre-Ordovician and might be Cambrian, but a Precambrian? age has been suggested for them (Boone and others, 1970, p. 11); on the Geologic Map they are indicated as Z with a metamorphic over- print. Farther southeast in Maine, near Islesboro on an island in Penobscot Bay, metamorphic rocks in a small horst have yielded a 900 m.y. date by rubidium-strontium methods and are cut by 600 m.y.-old pegmatites (Stewart, 1974, p. 89—90); they are likewise mapped as Precambrian Z. In Rhode Island and southeastern Massachusetts, adjoining the Pennsylvanian Narragansett basin, is a much larger area of late Precambrian rocks. It in— cludes on the east the Dedham Granodiorite and on 32 PRECAMBRIAN GEOLOGY OF THE UNITED STATES 73° 43°3o' E... Z O E. a: W, >, 43 7 2 °3o' EXPLANATION Undifferentiated Paleozoic Undifferentiated Cambrian Cheshire Quartzite Cavendish Formation Quartzite and marble 20 MILES 4| | 20 KILOMETRES FIGURE 9.—Map showing part of the Green Mountain uplift in south-central Vermont, and the Athens and Chester domes east of it, showing superposition of north—south Paleozoic (= Appalachian) trends on east-west Precambrian trends (mainly Grenvillian). Generalized from Geologic Map of Vermont (1961). the west various granitic orthogneisses (Milford, Northbridge, Scituate, etc.). A key locality for stratigraphic relations is Hoppin Hill, Mass, near the northeastern corner of Rhode Island, where fos- siliferous Lower Cambrian strata lie on the eroded surface of granodiorite (Dowse, 1950); however, the old rocks of the hill are separated from the rest by Pennsylvanian cover. Radiometric determinations by the rubidium-strontium method on the Dedham‘ Granodiorite and Northbridge Gneiss yield ages of 591 and 569 my respectively; the granodiorite at Hop- pin Hill yields an age of 514 my but this may have been downgraded during the pre-Paleozoic weathering. The true age of all the granitic rocks in the area may be near CENTRAL AND SOUTHERN APPALACHIAN REGION 570 m.y. (Fairbairn and others, 1967, p. 324); they are represented on the Geologic Map as Zg. Large enclaves in the orthogneisses west of the Nar- ragansett basin are an earlier supracrustal sequence, the Blackstone Series which is 15,000 ft (4,500 m) or more of schist, quartzite, and greenstone (Quinn, 1971, p. 8—14); like the plutonic rocks, the supracrustal rocks are included in Precambrian Z. THE AVALONIAN BELT The Precambrian rocks of southeastern New Eng- land are an extension of those of the Avalonian belt (= Avalon platform) of the Appalachian province in Canada—a domain of late Precambrian (Z) supra- crustal and magmatic rocks and events different from those farther northwest—typified in the Avalon Peninsula of southeastern Newfoundland, but rep- resented also in Cape Breton Island and southeastern New Brunswick (Poole and others, 1970, p. 231—235; Rodgers, 1972, p. 512—514). In Newfoundland the belt includes basal volcanics intruded by the Holyrood Granite, followed by a thick sequence of clastic de- posits, the whole overlain unconformably by the Lower Cambrian; the granite has been dated at 575 m.y. (later recalculated at 610 m.y.). An “Avalonian orogeny” has been postulated between the granite and volcanics and the succeeding elastic deposits (Poole and others, 1970, p. 232—233), but relations have been plausibly reinterpreted as a product of volcanic and depositional events, punctuated by local disturbances, that do not express an “orogeny” in the usual sense (Hughes, 1970; Hughes and Bruckner, 1971). Nevertheless, the term “Avalonian” is appealing and is widely used, in the same manner as the term “Grenvillian” discussed earlier (footnote 7). It can appropriately be applied to a terrane of well- characterized rocks and structures of late Precam- brian and early Paleozoic age in eastern Canada and the United States, whether or not this involves a narrowly defined “Avalonian orogeny.” In the north- western part of the Appalachian region, Lower Cambrian strata with an Olenellus fauna lie on a 1,100-m.y.-old Grenvillian basement. By contrast, in the Avalonian belt to the southeast, Lower Cambrian strata with a Paradoxides fauna lie on a 600-m.y.-old Avalonian basement (Wilson, 1969, p. 282). The Cambrian of the Avalonian belt is much more akin to the Cambrian of the southern British Isles and western Europe than to the Cambrian of the remain- der of North America (Palmer, 1967, p. 143—144), suggesting that the belt may be an extension of its trans-Atlantic counterparts which was joined to North America by plate collision during Paleozoic time. South of New England the Avalonian belt seem- 33 ingly extends into the metamorphic rocks of the Piedmont province and their buried extensions be- neath the Atlantic Coastal Plain (p. 39). CENTRAL AND SOUTHERN APPALACHIAN REGION8 In the Central and Southern Appalachians, the principal occurrence of identifiable Precambrian rocks is in the Blue Ridge province, a mountainous belt that lies between the Valley and Ridge province and the Piedmont province from southern Pennsyl- vania to northern Georgia. No Precambrian rocks are exposed in the Valley and Ridge province, but dated Precambrian emerges in some of the higher uplifts of the Piedmont province, and Precambrian is probably also included in the undeciphered metamorphic com- plex (m) of the inner Piedmont and the less-metamor- phosed strata of the Carolina Slate Belt. Compared to the Canadian Shield, all the Pre- cambrian of the Central and Southern Appalachians is rather young. Even its crystalline basement yields dates no earlier than Grenvillian, and is accordingly classed as Precambrian Y. The great body of supra- crustal rocks above it is therefore Precambrian Z, and is, in fact, the greatest development of this division in the United States, even exceeding that in the western Cordillera (fig. 5). In the Central and Southern Appalachians, as in the Northern, the Precambrian is heavily involved in the Paleozoic orogenies. The basement, which underwent deformation during the Grenvillian event, was reworked and its metamorphic fabric retro- graded. By contrast, the Precambrian supracrustal rocks were not significantly deformed during Pre- cambrian time, and owe all their present structural and metamorphic complexities to deformations dur- ing the Paleozoic. BLUE RIDGE BELT The northern segment of the Blue Ridge is an anticlinorium, vergent westward, about 15 mi (25 km) broad near the Potomac River, but widening southward. In central Virginia low-angle thrusts ap- pear along its northwestern border, and the belt be- comes increasingly allochthonous. The extent of transport on the thrusts in the Tennessee-North Carolina segment is suggested by windows southeast of their leading edges, notably the Grandfather Mountain window on the southeastern side of the belt. From North Carolina to the edge of the Coastal Plain in Alabama the southeastern tectonic boundary of the Blue Ridge belt is the Brevard zone of high- angle faults. The Precambrian of the Blue Ridge is overlain by Paleozoic geosynclinal deposits, the belt marking the 5For details available through 1966, see King (1970, p. 17—54); the present account in- cludes later observations and rex‘lsions 34 PRECAMBRIAN GEOLOGY OF THE UNITED STATES approximate boundary between their miogeosynclinal l Hatcher, 1973.) Much of the Ym unit on the map is and eugeosynclinal parts. Along the northwestern flank the basal miogeosynclinal deposits are the Lower Cambrian quartzites and clastics of the Chilhowee Group (€q), whose mature sediments con- trast with the immature sediments of the Precam- brian supracrustal sequence (Z). In places the Chilhowee transgresses across them onto the base— ment (Y), perhaps because this flank of the Blue Ridge was near the original northwestern limit of the Precambrian supracrustal rocks. The contrast fades on the southeastern flank of the Blue Ridge, where the Precambrian supracrustal rocks and the Paleozoic eugeosynclinal rocks ('Ce) are more alike and more accordant (fig. 10). PRECAMBRIAN Y In the Maryland-Virginia segment of the Blue Ridge the basement of the anticlinorium is a plutonic complex (an) of granodioritic and granitic ortho- gneisses and migmatites, with one small body of anor- thosite (Ya). Traces of earlier host rocks of undeter- mined age occur in places, nearly destroyed by granitization. All the complex is hypersthene-bearing and charnockitic, and underwent metamorphism to granulite grade during the Grenvillian event. The plutonic basement extends southwestward into the Tennessee-North Carolina segment to form the Cranberry, Max Patch, and similar gneisses. Radiometric determinations on rocks of the com- plex in northern Virginia yield dates by uranium- lead and related methods of 1,070—1,150 m.y., and by rubidium—strontium and potassium—argon methods of 880 and 800 m.y., respectively; similar results have been obtained in the Tennessee-North Carolina seg- ment (Tilton and others, 1960, p. 4175—4176). In the latter segment rubidium-strontium whole-rock de- terminations on many of the basement units yield ages between 1,025 and 1,250 my (Fullagar and Odom, 1973, p. 3076—3077); it is suggested that the basement is a 1,200—1,300-m.y.-old crust that was remobilized during the Grenvillian event 1,050 my ago, Without the addition of new material. Distinctions between the basement and its cover become blurred farther southwest, in the border re- gion of North Carolina and Georgia (fig. 7). The rocks have passed into the high amphibolite (sil— limanite) phase of Paleozoic metamorphism (Hadley and Nelson, 1971), and are thrown into large-scale recumbent folds or nappes (Hatcher, 1971, p. 41—42; 1973, p. 683). On the Geologic Map, considerable areas are shown as Precambrian Y (paragneiss and schist, Ym), based on the best information available in 1971, but this will require revision on the basis of later work, in part still in progress (for example, biotite gneiss and interbedded amphibolite, which is probably a lower unit of Precambrian Z. True base- ment is probably represented by the Whiteside Gran- ite and related rocks, but even these are in the cores of nappes, and rootless in part. On the northwest edge of the Blue Ridge belt near Cartersville, Georgia, the Corbin Granite (gneiss) has sometimes been interpreted as a Paleozoic intru- sive, but uranium-lead determinations on zircons show that it has an age of 1,100 my (Odom and others, 1973); it and probably the adjacent Salem Church Granite are therefore basement to the sur- rounding Precambrian Z Ocoee Supergroup. Farther southwest, near the edge of the Coastal Plain in Alabama is the Kowaliga Gneiss (= “biotite augen gneiss” of the Alabama State Map of 1926), which was proposed as basement rock (Bentley and Neath- ery, 1970, p. 19—20) and is so represented on the Geologic Map; however, radiometric determinations yield ages no greater than 550 m.y., so this assign- ment is suspect. PRECAMBRIAN Z The supracrustal rocks of the Blue Ridge belt lie unconformably on the deeply eroded surface of the basement—the greatest structural break in all the Appalachian stratified sequence below the base of the Triassic. Although this relation is fundamental to the Precambrian geology of the region, it was curi- ously misapprehended for a long period; in the northern Blue Ridge the plutonic rocks were thought to intrude the supracrustal rocks, and were so rep- resented on the Geologic Map of the United States of 1932. It was not until much later that Jonas and Stose (1939) deduced the true relation, a deduction abundantly confirmed by subsequent investigations. The supracrustal rocks are an extensive and varied suite, broadly of the same age, although not all their mutual relations have been determined with cer- tainty. Volcanic rocks are common in the northwest and north, but most of the remainder are immature clastic sedimentary rocks. In Maryland and northern Virginia, the dominant supracrustal unit on the northwestern flank of the Blue Ridge is the Catoctin Greenstone (Zv), a body of mafic lava as much as 5,000 ft (1,500 m) thick, origi- nally basaltic, into which felsic lavas interfinger northward. The mafic lavas were spread out in flows several hundred feet thick, under terrestrial condi- tions; many of them are amygdaloidal and some of them contain well—preserved columnar jointing (Reed, 1969, p. 21—32). Between the lavas and the eroded surface of the basement is commonly a thin sedimen- tary layer (Swift Run Formation) (fig. 10). CENTRAL AND SOUTHERN APPALACHIAN REGION 0 o o 79 78 77 o 40— :‘Char‘n‘belsburrg -___—___—__—__n. Washivsmmpzc EXPLANATION MESOZOIC AND CENO ZOIC Cretaceous and Tertiary Triassxc Coastal Plain deposits Newark Group and mafic in trusives a PALEOZOIC 38 ~— Sedimentary rocks Basal Cambrian Metamorphic and In Valley and Ridge elastic deposits plutonic rocks province Chilhowee group In Piedmont province PRECAM BRIAN Z VA av Catoctin Greenstone Swift Run Lynchburg Old Rag Metarhyolite included Fbrmation Formation Granite to north PRECAM BRIAN Y ‘ :\\' ‘ ‘Q \\\ ‘ m Y Granitic Roseland orthogneiss Anorthosite 0 1 0 0 M | L ES HI I I I I | I I I I | I 0 100 KILOMETRES FIGURE 10.—Map of northern part of Blue Ridge uplift in Virginia, Maryland, and Pennsylvania, showing relations of Precambrian Z rocks to underlying Precambrian Y basement, and to adjacent Phanerozoic rocks. Compiled from state geologic maps, and other sources. 36 Across the anticlinorium to the southeast, the lower sedimentary unit expands into the Lynchburg For- mation (Z), a mass of medium- to coarse-grained tur- bidites at least 10,000 ft (3,000 In) thick, with lenses of bouldery conglomerate in the lower part derived from the plutonic basement (Rockfish Member). The Catoctin lavas thin out above the Lynchburg, but are the principal marker for separating it from the simi- lar and apparently conformable Cambrian eugeosyn- clinal deposits (6e, Evington Group) (fig. 11). Rela- tions on the southeastern flank of the anticlinorium are obscured by the higher grade of Paleozoic metamorphism (amphibolite grade) and many of the rocks are schistose or gneissic. Farther southwest in the Blue Ridge, near the Virginia-North Carolina boundary, volcanic rocks are again prominent on the northwestern flank (fig. 12). Thick bodies of rhyolite form the middle part of the Mount Rogers Formation, and both rhyolite and basalt occur in the Grandfather Mountain Formation (in the Grandfather Mountain window, hence also of northwestern facies). Congeneric with the volcanics is the Crossnore Plutonic Group, which includes many moderate-sized granitic plutons (Zg) that are embedded in the adjacent basement rocks. These southwestern volcanics are associated with greater volumes of elastic sediments than are those of the Catoctin. The upper sedimentary unit of the Mount Rogers Formation includes red, rhythmically bedded siltstone, and coarse diamictite formed of boulders of the basement plutonics, very much like the late Pre- cambrian diamictites in several parts of the Cordil- lera to which have been ascribed a glacial origin (Rankin, 1970, p. 232). On the southeastern flank of the Blue Ridge, the broad band of metasedimentary supracrustal rocks continues from the Lynchburg area through southern Virginia into North Carolina. In the latter segment it is the Ashe Formation (Rankin, 1970, p. 232-233), a thick mass of fine- to medium-grained biotite- muscovite gneiss; with interbedded amphibolite (Zv), especially in the lower part, that originated from mafic volcanic rocks. In the Spruce Pine pegmatite district, North Carolina, west of the Grandfather Mountain window, the Ashe lies in a southwest- plunging synclinorium, in the keel of which, on the heights of Mount Mitchell, it is overlain by rocks lithically like the Great Smoky Group of the Ocoee Supergroup (Hadley, 1970, p. 249). The Ocoee Supergroup (Z) dominates the south— western segment of the Blue Ridge belt, extending along its strike for more than 175 mi (280 km), from Asheville, North Carolina, to Cartersville, Georgia, and across it for 40 mi (65 km) or more; it projects in PRECAMBRIAN GEOLOGY OF THE UNITED STATES high ranges, such as the Great Smoky Mountains (King and others, 1968, p. 3—9) (fig. 13). The Ocoee is a great mass of nonvolcanic clastic sedimentary rocks; partial sequences as much as 25,000 ft (7,600 m) thick have been observed but the total is unde- termined. Along its southeastern side the Ocoee lies unconformably on the basement orthogneisses and paragneisses (an, Ym); on its northwestern side it is succeeded disconformably by the Chilhowee Group (€q); also, a belt of synclinally infolded younger rocks near its center (19) includes the Murphy Mar- ble that contains sparse lower Paleozoic fossils (McLaughlin and Hathaway, 1973). The Ocoee has been divided into the contrasting Walden Creek, Snowbird, and Great Smoky Groups (not differentiated on the Geologic Map but shown in fig. 13), which evidently formed in different parts of the original sedimentary basin, but they have been so telescoped by Paleozoic thrusting that most of their original relations to each other are now lost. In places one of the groups can be found in sequence with another, but it is likely that all of them were extensively intergradational. The varied clastics of the Walden Creek probably formed on an unstable shelf along the northwestern margin of the basin; the Snowbird is an intermediate facies; and the Great Smoky is a deepwater continental rise deposit. Much of the Great Smoky is a medium- to coarse-grained quartz-feldspar turbidite with prominent graded bed- ding, with which thin to thick units of dark sulfldic argillaceous rocks are interbedded. South of the main Ocoee area, along the southeast- ern edge of the Blue Ridge belt, strips of paraschist and paragneiss altered from rocks like the Great Smoky extend to the Coastal Plain border in Alabama, where they are the Heard Group of Bentley and Neathery (1970, p. 14—18). Associated with them, and mainly underlying them, are biotite gneisses and interbedded amphibolites, sometimes called Precambrian Y basement (p. 31), but more likely comparable to the volcanic rocks in the lower part of the Ashe Formation farther northeast. An exceptional feature in northeastern Georgia is the Tallulah Falls dome, exposing a quartzite formed of nearly pure siliceous sand; following a suggestion of Burchfiel and Livingston (1967, p. 252) we have speculatively correlated it on the Geologic Map with the Lower Cambrian Chilhowee Group (€q). How- ever, Hatcher (1973, p. 683) interprets the dome as a culmination in a much broader recumbent nappe, and considers the quartzite to be a unit near the middle of the Precambrian Z sequence. Although the general position of the Precambrian Z supracrustal rocks of the Central and Southern 37 CENTRAL AND SOUTHERN APPALACHIAN REGION $20335on bad: 53» ans .Awwm d .mmmC .8585 15" .8885 no ummmm .2 25mm E 323 mm 92% 2: 9E 28:53 .8»an .533 .EmO was N :ESESPE «a win: no now—£8 mfiBoSm ,Eiwhtw 5858: E fizn: mem 3am of $33.. Emmagmcmfi mung—03$ Sawfly uEmmesgumlAH 559m .mnEwS. \wumLmancoU ...mzxoom ..oEo Ucm .>.E coo... fiflmcmozto 2:523 :_n__>_.OU U_InEO_>_<._.m:>_ wOQE wDJm :wN/x ,. mtcmho any. 20 Fww ///i /, cotmELO... . .... r5250; :. . B . . . 3&8. . £33 .. . _.m,mdw..:.m_. .A ...wm>>I._.IOZ 38 PRECAMBRIAN GEOLOGY OF THE UNITED STATES l'r — .w paw 7/— ILOMETRES EXPLANATION MESOZOIC TriaSSic Newark Group PALEOZOIC Lower and Middle Paleozoic granite Includes mafic intrusives near Martinsville. Va. Metamorphic and plutonic rocks In Piedmont province Sedimentary rocks In Valley and Ridge province IIHII Basal Cambrian clastic deposits Chilhowee Group Evington Group (in north) and Alligator Back Formation (in south) PRECAMB RIAN Z Crossnore granitic Catoctin Greenstone rocks In north — Lynchburg Formation (in north) and Ashe Formation (in south) PRECAM BRIAN Y ‘_\ / LYE‘RI Granitic orthogneiss § .\ Mount Rogers and Grandfather Moun- tain Formations _L_A_L._ .l—J—L _‘_—— Faults Thrusts. normal, and strike-slip FIGURE 12.——Map of part of Blue Ridge uplift in the border region of Virginia, North Carolina, and Tennessee, showing Precambrian Y and Z, and Paleozoic units. Compiled from Rankin, Espenshade, and Shaw (1973, p. 6, 8), Conley and Henika (1973, p. 41, pl. 4), and other sources. CENTRAL AND SOUTHERN APPALACHIAN REGION Appalachians between their Precambrian Y (Grenvil- lian) basement and the Lower Cambrian is plain, the precise stratigraphic positions and ages of the differ- ent parts are as yet uncertain. The sediments of the Ocoee Supergroup, for example, contain detrital zir- cons with lead-alpha dates of 820—1,000 m.y. (Carroll and others, 1957, p. 186—188), which express merely the age of the basement from which they were de- rived; other dates from the Ocoee are around 350 m.y. and record the age of their Paleozoic metamorphism (Kulp and Eckelmann, 1961, p. 410— 413). Even fewer data are available for the other sedimentary components of Precambrian Z, such as the Lynchburg and Ashe Formations. The most specific information on the age of the su- pracrustal sequence has been obtained from uranium-lead determinations on zircons from the fel- sic volcanic rocks, which indicate an original age of 820 m.y. and an episodic lead loss at 240 m.y. (the latter probably at the time of the Appalachian orogeny) (Rankin and others, 1969). Dated specimens were obtained from felsic volcanics associated With the Catoctin Greenstone in southern Pennsylvania, from the Mount Rogers Formation in Virginia, and from the Grandfather Mountain Formation in North Carolina. No determinations are possible by this method on the mafic volcanics, but by extrapolation the 820 m.y. age probably applies to the Catoctin Greenstone as well; as indicated earlier, the Catoctin overlies the sedimentary part of the supracrustal sequence (Swift Run and Lynchburg) in the northern Blue Ridge, and thus sets a terminal date for Pre- cambrian Z in this segment. The similar ages from the Mount Rogers and Grandfather Mountain For- mations farther southwest are less decisive, as their fel- sic volcanics lie farther down in the local sequences. PRECAMBRIAN OF PIEDMONT PROVINCE The extensive Piedmont province southeast of the Blue Ridge belt is a domain of crystalline rocks that were mobilized and subsequently consoldiated during the orogenies of Paleozoic time. In this respect it resem- bles the crystalline area of New England in the North- ern Appalachians, but whereas the stratigraphic se- quence in New England is now fairly well known, much of that in the Piedmont is still poorly understood. Rep- resentation of the Piedmont province on the Geologic Map was assembled from the best data available in 1971, but investigations are actively in progress which will modify this representation in many places. Precambrian basement rocks, the Baltimore Gneiss (an), form the cores of half a dozen mantled gneiss domes in eastern Maryland and adjacent Pennsylvania 39 that have risen steeply into the supracrustal eugeosyn- clinal rocks of the Glenarm Series ('Ce). Radiometric determinations on zircons from the gneiss by uranium- lead and related methods yield ages of 1,000—1,100 m.y., whereas biotite from the gneiss yields rubidium- strontium and potassium-argon ages of 300—400 m.y., expressing the time of Paleozoic metamorphism (Tilton and others, 1958). At the northern edge of North Carolina, north of Winston-Salem (fig. 12), is the Sauratown anti- clinorium, more than 50 mi (80 km) long and 15 mi (25 km) broad, whose core exposes biotite gneiss and schist, and minor granitic gneiss, which are flanked by Pre- cambrian Z Ashe Formation. The granitic rocks of the core have yielded an age of 1,192 m.y. by lead-lead determinations on zircons (Rankin and others, 1973, p. 19). The only proved basement rocks farther southwest in the Piedmont province are the Woodland Gneiss and Jeff Davis Granite near Warm Springs, western Geor- gia, which have yielded uranium-lead ages of 1,000 m.y. (Odom and others, 1973; Sandrock and Penley, 1974). They lie beneath, but may intrude a metasedimentary sequence shown as Z and IE on the Geologic Map. All these are components of the Wacoochee belt which is bordered on both north and south by major faults, so that their relations to the adjacent Piedmont rocks on each side is undetermined. Most of the country rock of the Piedmont province (aside from the abundant plutons) is shown on the Geologic Map as unclassified metamorphic complex (In) and as Cambrian eugeosynclinal deposits (€e,€v). In North and South Carolina and adjacent States the eugeosynclinal deposits are the low-grade metamorphic sedimentary and volcanic rocks of the Carolina Slate Belt. The metamorphic complex, distinguished by its higher metamorphic grade, is partly equivalent, but may probably be partly older. The rocks of the Slate Belt are shown as Cambrian on the map mainly on the basis of the occurrence of Middle Cambrian Paradoxides in southern North Carolina, but the sequence evidently contains older components. Farther north, Lynn Glover III and his associates have found Ediacaran (= Ven- dian) type fossils at a locality on the Little River 12 miles north of Durham, NC. They are the imprints of soft-bodied wormlike animals, preserved on bedding surfaces of the volcaniclastic strata. Comparable fossils occur in the Precambrian Z Conception Slate of the Avalon Peninsula, southeastern Newfoundland. At the north end of the Slate Belt in southern Virginia the Slate Belt rocks, the adjacent gneisses, and the as- sociated intrusives have yielded an array of dates by uranium-lead methods ranging from 575 to 620 m.y., 40 suggesting an event of supracrustal accumulation, magmatic activity, and mild deformation near or a little l before the beginning of the Cambrian (= Virgilina de- I formation of Glover and Sinha, 1973, p. 247). These features have little resemblance to the late Precambrian—Early Cambrian features to the north- west in the Blue Ridge province, but the precise limits of the two terranes are still uncertain. In part of North Carolina they are juxtaposed along the Brevard zone, but in other places, as noted above, Grenvillian base- o 84 PRECAMBRIAN GEOLOGY OF THE UNITED STATES ment and its supracrustal cover extends into the north- western part of the Piedmont. Be that as it may, the late Precambrian—Early Cambrian rocks and events in the Piedmont most closely resemble those of the Avalonian belt farther northeast in the Appalachians (p. 33; Rod- gers, 1972, p. 514—516). Like them, the Piedmont rocks may have formed in a realm far away from the north- western belts of the Appalachians and were brought against them by plate collision during Paleozoic time (Odom and Fullagar, 1973, p. 140—146). 0 82 \ :l ‘ .' _Carters +ZC+ , I / - Q mél /\/\\ \ \‘l’ \\ \/\I/ :3; .\ I’Qw Za , , Pine Spr ce \ \ / // ’ 4/ . ’55 //’ , t// 7 45 a, ”7% / / / /// / ////// .-'. ”55/ $\§\ \ A. \\\ ~\\ \\ . \\\\\\\\\\\\ \ \\‘\§§‘\\\‘\§§§§\\\‘\ \\ \\\\ \\\ \\ ' \\\§ ‘ \ . ‘ _ \\ \ '100MILES‘I J I 100 KipoMETRES :3 \ FIGURE 13.—Map of southwestern end of Blue Ridge belt in southern North Carolina and Tennessee, and northern Georgia, showing Ocoee Supergroup, its subdivisions, and related rocks of Precambrian Z, as well as their Precambrian Y basement. Compiled from State geologic maps, Hadley and Nelson (1971), Hurst (1973), Hatcher (1973), and other sources. SOUTH-CENTRAL UNITED STATES SOUTH-CENTRAL UNITED STATE59 The wide Interior Region of the United States, be- tween the Appalachian and Cordilleran orogenic belts, is a domain of little deformed Phanerozoic rocks a few hundred to many thousands of feet thick, through which their basement emerges only in small, widely separated areas. In the southern part of the region the principal basement exposures are in the Ozark uplift of Missouri, the Arbuckle and Wichita Mountain uplifts of southern Oklahoma, and the Llano uplift of central Texas. 5For a summary ofinformation available through 1966, see Flawn and Muehlberger (1970, p. 73—143); additional data on geochronology and subsurface relations are given by Muehlberger, Hedge, Denison, and Marvin (1966). EXPLANATION PALEOZOIC Metamorphic and plutonic rocks In Piedmont province Sedimentary rocks In Valley and Ridge province Basal Cambrian Alaskite elastic deposits In Spruce Pine Chilhowee Group district 7 '7 Cataclastic rocks Mafic and In Brevard fault ultramafic intrusives zone PRECAMBRIAN Z OCOEE SUPERG ROUP Walden Creek Group Crossnore granitic rocks Great Smoky Muscovite gneiss Group and schist — V 7 a \ , Snowbird Ashe Formation Biotite gneiss Group and amphibolite PRECAM BRIAN Y \\ .. \ / \ §\ ,\§ \lY gn// \§\\ [QQ‘ Granitic orthogneiss Max Patch and Cranberry Gneiss Layered gneiss and migmatite S Sillimanite isograd FIGURE 13.—Continued. 41 Elsewhere, especially in Kansas, Oklahoma, and Texas, drilling has penetrated the basement in many places, and affords information on the extent of the various units beyond their outcrops. Stratigraphic relations and radiometric dating indicate a Precambrian age for all the basement rocks, except those in the Wichita Moun- tain area, which are Early Cambrian (6g). OZARK AREA On the crest of the Ozark dome, in the St. Francois Mountains of southeastern Missouri, Precambrian rocks are exposed in an area of about 600 mi2 (1,550 kmz). At the beginning of the Paleozoic transgression they projected in a rough terrain with as much as 2,000 ft (600 m) of relief, which was buried and variously overlapped by Upper Cambrian and Lower Ordovician deposits; present outcrops result from partial exhuma- tion of this terrain.10 The southwestern and larger part of the St. Francois Mountains is formed of stratified rhyolite and other felsic volcanic rocks (Yv), mainly flows but with inter- bedded tuff and breccia, dipping at low angles in various directions. The northeastern part of the mountains con- sists of several varieties of granite (Ygi). The granites intrude the volcanics, probably in thick sills at shallow depths in the crust, but both granites and volcanics are compositionally much alike, and both yield ages of about 1,500 my by uranium-lead methods (Bickford, 1972). They thus express a closely related set of events, the “St. Francois igneous activity” (Muehlberger and others, 1966, p. 5313). Rubidium-strontium ages are consistently lower, and may record a minor later event at about 1,300 my. On the western and southwestern edges of the Ozark uplift are some smaller outcrops of granitic rocks, also of nearly the same age. Those at Spavinaw, Okla., are on the exhumed tops of hills of the Precambrian erosion surface, but those at Decaturville, Mo., and Rose, Kans. (the latter included in Ti on Geologic Map) are rootless bodies brought to the surface by Phanerozoic disturb- ances. ARBUCKLE AND WICHITA MOUNTAINS The Arbuckle and Wichita Mountains of southern Oklahoma (fig. 15) are exposed parts of an intracratonic orogenic belt of Paleozoic age, composed of horstlike faulted uplifts, separated by deep troughs containing strongly deformed Paleozoic strata. The Arbuckle Mountains are underlain by Precam- brian granitic basement, which emerges in a horst at l°An alternative proposal is worth mentioning—that most ofthe Precambrian outcrops in the St. Francois Mountains are klippen of a former overthrust sheet that had been trans- ported 230 mi (370 km) northward from the Ouachita orogenic belt in Arkansas (Wheeler, 1965). 42 the eastern end. Westward, the basement of the horst is overlapped by Upper Cambrian; eastward, it passes be- neath Cretaceous Coastal Plain deposits but continues in subsurface 45 mi (72 km) farther, to the front of the Ouachita orogenic belt. The principal unit is the coarse, porphyritic Tishomingo Granite, but there is also a finer grained Troy Granite, as well as minor younger diorites and dike rocks. Both the Tishomingo and Troy have yielded ages in the range of 1,320—1,400 m.y. by rubidium-strontium and other methods (Ham and others, 1964, p. 126—140). The basement of the Wichita Mountains (and their largely buried extension to the east-southeast) is differ- ent and younger. It is a varied assemblage of floored felsic and mafic plutons embedded in supracrustal vol- canics and sediments, all with ages of about 525 m.y., hence early Cambrian (Ham and others, 1964, p. 35— 37); because of their small surface extent they are grouped on the Geologic Map as €g (Cambrian granitic rocks). Basement of Wichita type extends into the Tim- bered Hills uplift at the west end of the Arbuckle Moun- tains. ‘ LLANO L‘PLIFi In central Texas, south of the Arbuckle-Wichita orogenic belt, Precambrian rocks are exposed in an area of 2,000 mi2 (5,200 kmz) on the crest of the Llano uplift (fig. 14). The uplift is a structural high at the edge of the North American craton, little disturbed by Phanerozoic movements except for high-angle block faulting. Cam- brian and younger Paleozoic rocks slope northward and westward away from the Precambrian into the craton, and all of them are overlapped by Cretaceous deposits that dip southeastward beneath the Gulf Coastal Plain. A short distance southeast of the edge of the Cretaceous overlap the rocks of the Llano uplift adjoin in subcrop the much more deformed Paleozoic rocks of the Ouachita orogenic belt. The country rocks of the Precambrian basement are the felsic Valley Spring Gneiss and the mafic Packsad- dle Schist (Ym), folded along northwest-trending axes and derived from an original supracrustal sequence no less than 20,000 ft (6,000 m) thick. In these are embed- ded granitic rocks (Yg2), which form more than a third of the exposed area, as well as minor granite porphyry and pegmatite dikes, and a single ultramafic body (um). The granites were emplaced in three plutonic series, of which the youngest (Town Mountain) is the most exten- sive; it includes more than half a dozen nearly circular plutons 10 mi (16 km) or more across. Efforts to obtain radiometric ages from the Llano Precambrian extend back three-quarters of a century to calculations from the rare-earth minerals in the Bar— ringer Hill pegmatite (Becker, 1908, p. 134). These re- sults are only of historical interest, and reliable dates PRECAMBRIAN GEOLOGY OF THE UNITED STATES were not obtained until much later. The granites of the three plutonic series yield ages of 1,030 m.y. by rubidium-strontium and potassium-argon methods (Zartman, 1964), and 990—1,070 m.y. by uranium-lead methods on zircon; ages from the Valley Spring Gneiss by rubidium—strontium methods are 1,120 m.y. (Zartman, 1965). The cycle of metamorphism and intru- sion had a span of about 100 m.y. and is a Grenvillian event, termed for local purposes the "Llano orogeny” (Muehlberger and others, 1966, p. 4522); the rocks in- volved in it are classed as Precambrian Y. REGIONAL PROBLEMS It will be observed from the outcrop data just pre- sented that there are three general ages of basement rocks in the south-central United States—approxi- mately 1,000 m.y. in the Llano area, 500 m.y. in the Wichita area, and 1,200—1,400 m.y. in the Arbuckle and Ozark areas, each of which has also been recorded in buried basement rocks near the outcrops. The 1,000- m.y. ages mark a Grenvillian province that probably connects with the surface and subsurface Grenville province east of the Mississippi River, although there is a wide intervening gap where basement rocks have not been reached by drilling. The 500 m.y. ages represent a Cambrian basement province unique in the North American interior. The 1,200—1,400 m.y. ages north ofit recall the Elsonian event in the eastern Canadian Shield, and have been recorded in buried rocks over a wide expanse of the Interior Province, northward to the Wisconsin arch in the Lake Superior region, and east- ward to the buried front of the Grenville belt in Ohio and Kentucky. The regional meaning of the 1,200—1,400 m.y. set of dates is not clear. Do they express an age province like those in the Canadian Shield, with its own complex of metamorphic and plutonic rocks and with well-defined strucural boundaries against other provinces? Or does it result from extensive overprinting of later events on an earlier province? Available evidence is not decisive, because so much of it has been obtained from drill data, and so little from outcrops, but the second possibility seems more likely: (1) The boundaries of the region are poorly defined, the Elsonian dates being mingled on the north with Hudsonian dates, and on the south with Grenvillian dates. (2) Many of the dates recorded in subsurface are from plutonic bodies that might be younger than the complex in which they are embedded. (3) This situation is true in the few outcrop areas. In the Nain province of the Canadian Shield, the Wiscon- sin arch of the Lake Superior Region, and the Precam- brian of the Southern Rocky Mountains, plutons with 31 SOUTH-CENTRAL UNITED STATES Elsonian ages (Ygi) are thickly spaced in metamorphic and plutonic complexes with Hudsonian ages (Xm, Xg). To some extent, this later plutonism has updated the ages in the surrounding complexes. (4) Many of the Elsonian ages in the south-central States are from volcanic and other supracrustal rocks. An impressive feature of the concealed basement of this region is the wide extent of little-deformed felsic volcanics with Elsonian and younger ages, that pre- sumably overlie earlier complexes (fig. 15). In Missouri 43 they have yielded ages of 1,200—1,350 m.y. (as in the St. Francois Mountains), in northeastern Oklahoma ages of 1,150—1,300 m.y., in the Texas Panhandle ages of 1,100—1,200 m.y., and in the Wichita belt ages of 525 my. (Muehlberger and others, 1966, p. 5422 and fig. 3). Associated with the Panhandle volcanics is a very ex- tensive stratiform body of intrusive gabbro of somewhat younger age. On the Geologic Map, we have provided for the plutonic rocks with 1,200—1,400 m.y. ages in unit Ygl \\ KW\ /‘ / ’4 g, . f3; 50 MILES l; l l | l I | I I I I 0 50 KILOMETRES EXPLANATION V m Cambrian Ordovician to Cretaceous Pennsylvanian V V / v m! / / / A a 2 Packsaddle Schist (mafic paraschist) Valley Spring Gneiss (felsic paragneiss) Granite Ultramafic rocks (dashed lines indicate subsurface extent) FIGURE 14.—Map of Llano uplift, central Texas, showing Precambrian Y metamorphic and plutonic rocks, and their relation to surround- ing Phanerozoic rocks. Compiled from Geologic Map of Texas (1937), Flawn and Muehlberger (1970, p. 78), and other sources. 44 (“older Precambrian Y granitic rocks”); metamorphic and supracrustal rocks with these ages are not distin- guished separately from the remainder of Precambrian Y. CORDILLERAN REGION The outcrops of Precambrian rocks in the western United States are in the Cordilleran Region, a domain of later Phanerozoic orogenies which have raised the Precambrian to the surface in the higher uplifts. These outcrops are separated by 500 mi (800 km) or more of PRECAMBRIAN GEOLOGY OF THE UNITED STATES Phanerozoic cover from those in the Lake Superior Re— gion and elsewhere in the central United States, but much has been learned about the connections between them from drilling in the intervening plains. The Precambrian provinces and structures are mostly transverse to the Phanerozoic structures and landforms of the Cordillera, and are only grossly related to them. Nevertheless, it will be useful to describe the Precambrian in terms of the modern morphology. Under the first heading we will therefore deal with the ancient Precambrian crystalline rocks (mainly Precam- \ Front I Range ' I KANSAS / COLORADO I | I I MISSOURI 3‘ Fr, Moqains . | . - \ Sangre ' i I. \ - \ _ _ ‘- \fl ' de Cristo - _ ______ l t~ 1200-1350 Mountains “ — - - ‘2 f — —( / in 1100—1200 NEW MEXICO I 1100—1200 TEXAS ~——*-_J ~ Van Horn area Llano uplift OKLAHOMA 3. L- l \ ( LOUISIANA ) l l \ / M l 2(1)0 MILES I 200 KILOMETRES EXPLANATION Subsurface extent of supracrustal felsic vol- canic rocks (approx- imate ages in millions of years) Outcrops of Early Cambrian basement rocks Outcrops of Precambrian rocks FIGURE 15.—Map of part of south-central United States, showing subsurface extent of late Precambrian and Early Cambrian supra- crustal felsic volcanic rocks. Based on Muehlberger, Hedge, Denison, and Marvin (1966, p. 5422), and Bayley and Muehlberger (1968). CENTRAL ROCKY MOUNTAINS brian W) of the Central Rocky Mountains in Wyoming and southern Montana; and following this the later Precambrian supracrustal rocks (Precambrian Y and Z) of the Northern Rocky Mountains in western Montana and adjacent Idaho. In a like manner, we will deal with the somewhat younger Precambrian crystalline rocks (mainly Precambrian X) of the Southern Rocky Moun- tains in Colorado and New Mexico; followed by the supracrustal rocks (Precambrian Y and Z) of the eastern Great Basin in Utah and adjacent States. In a final section, we will describe the varied Precambrian rocks of the southern Basin and Range province in Arizona and adjacent States. CENTRAL ROCKY MOUNTAINS For purposes of this account, the Central Rocky Mountains are the ranges of Wyoming and southern Montana, and the Black Hills of South Dakota. They are irregularly disposed, widely spaced, broad-backed mountain uplifts, many of which expose large areas of Precambrian rocks in their cores; they are separated by even broader areas of downwarped Phanerozoic rocks, whose plains and plateaus are more or less confluent with the Great Plains to the east. On the west and northwest they are bordered by the more closely crowded ranges of the main Cordilleran thrust belt. The Precambrian cores of many of the ranges project a mile or more above their surroundings, and some peaks at- tain altitudes of as much as 13,000 ft (4,000 m). The larger areas of Precambrian in the Central Rocky Mountains are in the Black Hills of western South Dakota, well to the east of the others; in the Laramie and Medicine Bow Ranges of southern Wyoming; in the Wind River Mountains farther west in Wyoming and the Bighorn Mountains farther north; and in the Bear- tooth Mountains which straddle the boundary between northwestern Wyoming and southern Montana (fig. 16). Smaller Precambrian areas in some of the intervening ranges and to the northwest provide partial connections between the larger areas. The dominant structures of the Central Rocky Moun- tains are a product of late Cretaceous-early Tertiary (Laramide) orogeny, in which the Precambrian base- ment participated. Although the Phanerozoic rocks are steeply tilted or faulted at the edges of the uplifts, their Precambrian cores were raised mainly as rigid blocks. As a result, the Precambrian rocks and their structures have been so little modified by Laramide and other Phanerozoic deformations that the effects can be disre- garded. The Precambrian of the Central Rocky Mountains is an extension of that of the Superior province of the Canadian Shield (fig. 17), and most of it is ancient crys- 45 talline rocks with Kenoran or even earlier dates (Pre- cambrian W); however, younger dates are reported in places in the subsurface of the intervening area (Gold- ich, Lidiak, Hedge, and Walthall, 1966, p. 5400, fig. 1), and there are important areas of outcrop of younger supracrustal rocks (Precambrian X) to the southeast and east. The southeastern boundary with the younger crystalline rocks of the Southern Rocky Mountains is a major structural discontinuity (Mullen Creek—Nash Fork shear zone) that crosses the Medicine Bow and Laramie Ranges, and can be traced in subsurface more than 200 mi (320 km) farther northeastward beneath the Great Plains. The northwestern boundary is the stratigraphic overlap of the Belt Supergroup (Precam- brian Y) in central Montana. PRECAMBRIAN W The rocks in nearly all the ranges of the Central Rocky Mountains in Wyoming are gneiss (Wgn) and granite (Wg), and share a complex history that has only partly been deciphered. They have been studied in greatest detail in the Beartooth Mountains during a project under the direction of the late Prof. Arie Polder- vaart (Eckelmann and Poldevaart, 1957; and later re- ports). Here and elsewhere, the oldest rocks are para- gneisses, originally a thick supracrustal sequence of dominantly pelitic sediments and minor volcanics, that have been plastically folded and refolded, regionally metamorphosed to amphibolite grade, and partly con- verted to migmatite and granite; in addition, there are some postkinematic granite plutons, and the whole complex is crisscrossed by diabase dikes, formed during a late tensional phase. Radiometric determinations on the rocks of all the ranges by potassium-argon, rubidium-strontium, and uranium-lead methods have yielded rather consistent Kenoran ages of about 2,750 m.y., but this seems to express merely the later orogenic events. Zircons of detrital origin from the gneisses of the Beartooth Moun- tains have yielded ages in excess of 3,100 my and express an earlier event upon which the Kenoran event was superposed (Catanzaro, 1966, p. 9—11; Butler, 1966, p. 61). In the other ranges an earlier event of this kind can be inferred from the structural relations, but this has not been confirmed by radiometric dating. A unique feature of the northwestern Beartooth Mountains is the Stillwater Complex (Wmi), a body of layered chromite-bearing mafic and ultramafic rocks with an exposed length of 30 mi (48 km) and a preserved thickness of 18,000 ft (5,500 m) (Jones and others, 1960, p. 283—286). It intrudes and overlies the prevailing gneisses and dips steeply away from them, under the unconformably overlying Cambrian on the flank of the 46 range. The complex is younger than the 3,100-m.y.-old gneisses which it invades, and older than a 2,700-m.y.- old quartz monzonite which truncates its eastern end. Potassium-argon and rubidium-strontium dating of the complex itself yields conflicting results (Kistler and others, 1968; Fenton and Faure, 1969), but it was prob- ably emplaced nearer the later limiting date than the earlier. Some of the ranges farther south in Wyoming expose downfolded belts of supracrustal rocks (W) much like those in the Superior province of northern Minnesota. In the South Pass (Atlantic City) district at the south end of the Wind River Mountains one of these belts contains 15,000 ft (5,000 m) or more of strata, beginning with basal iron formation and quartzite, followed by a thick body of turbidites, and greenstones with pillow PRECAMBRIAN GEOLOGY OF THE UNITED STATES structure (Bayley, 1968, p. 502—598). These are older than the Louis Lake Granodiorite to the north with an age of 2,690 my PRECAMBRIAN COMPLEX OF SOUTHWESTERN MONTANA Precambrian crystalline rocks are exposed in south- western Montana between the Beartooth Mountains and the Cordilleran thrust front on the west, in the Madison, Jefferson, Tobacco Root, Ruby, and other ranges. On the Geologic Map they are represented as Precambrian W like those in Wyoming, but they are somewhat more varied, their ages are less certain, and they are more involved with Phanerozoic features, such as Laramide plutons (Kg3), Cenozoic volcanism, and block faulting. Three general rock types recur in the different "MT U} o C‘. E V M0NTA11A_ _ _ __‘ ' ' " ' " "vG‘YOMING g Owl Creek Mountains Q3 )1— -—-——————‘”—v‘16 l °——O 200 MILES J I 200 KILOMETHES FIGURE 16.—Map of Central Rocky Mountains in Wyoming, South Dakota, and Montana, showing outcrops of Precambrian rocks and localities mentioned in text. CENTRAL ROCKY MOUNTAINS ranges, called the Dillon (a granitic orthogneiss), the Pony (a mafic paragneiss), and the Cherry Creek (a sequence of metasediments and metavolcanics) (Scholten and others, 1955, p. 350—352; Reid, 1963); the first two are mapped as Wgn and the third as W. Their structure is complex and their mutual relations are still 47 debated, but the components of the Cherry Creek are sufficiently distinctive to suggest that it may be a valid stratigraphic unit (H. L. James, oral commun., 1973). In the type Cherry Creek area in the Jefferson Range it includes mica schist, pillow lava, iron formation, quartzite, and dolomite marble (Hadley, 1969a, b); al- 0 SASKATCHEWAN ~_ \ ‘_\_ o MONTANA O—y-O i— 200 MILES | l 200 KILOMETRES EXPLANATION Outcrops of rocks of Precambrian W Outcrops of rocks of Precambrianw Outcrops of younger Precambrian with known age greater than 3,200 m.y. m Surface and subsurface extent of supracrustal rocks of Precambri— an Y and Z (with ages of 2,500 m.y. or greater) rocks (Precambrian X, Y, and Z) 9 Wells which have yielded dated specimens of Precambrian rocks (solid circles with ages of 2,400— 2,700 m.y., open circles with ages of l,600-l,800 m.y.) FIGURE 17.-—Map of part of western United States and southern Canada, showing surface and subsurface extent of Precambrian W rocks in the Superior province of the Lake Superior Region, and westward to the Central Rocky Mountains. Compiled from geologic maps of United States (1974) and Canada (1969), and Goldich, Lidiak, Hedge, and Walthall (1966, p. 5390). 48 though here classed as Precambrian W, the prominent bodies of quartzite and marble are more characteristic of younger parts of the Precambrian in other regions. Radiometric dating by potassium-argon and rubidium-strontium methods provides‘ equivocal re- sults. The rocks of the ranges toward the southeast yield dates in excess of 2,600 m.y., but identical rocks farther northwest have dates in the range of 1,600—1,800 m.y.; in addition, in the northernmost exposures are 175 my dates produced by proximity to Laramide plutons (Giletti, 1966, p. 4031—4035). Apparently a Precam— brian W terrane with original Kenoran dates has been downgraded toward the northwest by Hudsonian events. The westernmost granitic rocks of Dillon type yield rather consistent 1,600 my. ages and are therefore mapped as Xg; they may represent a younger pluton that is at least partly responsible for the mixing of Kenoran and Hudsonian dates. Farther north, Precambrian crystalline rocks reap— pear in the core of the Little Belt Mountains uplift, where they form the basement of the Belt Supergroup. The varied rocks include paragneiss, migmatite, gran— ite gneiss, and diorite. Radiometric determinations by a variety of methods yield dominant ages of about 1,900 m.y., but zircons from the paragneiss and migmatite have ages as great as 2,450 my (Catanzaro, 1966, p. 13—15). Here, as in southwestern Montana, Hudson- ian dates are mingled with Kenoran dates, and the Little Belt crystalline rocks are accordingly mapped as Wgn. The mingling of Kenoran and Hudsonian dates in southwestern Montana and the Little Belt Mountains seems to represent a gradational boundary between two major Precambrian provinces, analogous to the Superior and Churchill provinces of the Canadian Shield. Here, however, in contrast to conditions in the Shield, there seems to be no sharply marked structural or stratigraphic boundary between the two provinces. A final comment should be made on the crystalline rocks in the core of the mantled gneiss domes of the Albion Range in southern Idaho and northwestern Utah, for which rubidium—strontium whole-rock iso— chron yields an age of 2,460 my (Armstrong and Hills, 1967, p. 118—120). This occurrence is 200 mi (320 km) west of the Precambrian W rocks in the Central Rocky Mountains, and represents the farthest known exten- sion of the Superior province in the United States. PRECAMBRIAN X In the eastern and southeastern part of the Central Rocky Mountains, as here delimited, younger Precam— brian supracrustal rocks (X) are emplanted in the pre— vailing ancient crystalline terrane (Wgn, Wg). They form most of the exposed Precambrian in the Black PRECAMBRIAN GEOLOGY OF THE UNITED STATES Hills, and smaller areas in the Hartville uplift and Medicine Bow Mountains to the southwest. All of them have yielded Hudsonian dates and are classed as Pre- cambrian X, but considerable differences in lithology and sequence among the several areas preclude more exact correlations. In the Black Hills of western South Dakota, Precam- brian rocks are exposed in a northward-elongated oval area of about 900 mi2 (2,300 km?) on the crest of the dome (fig. 18). All the Precambrian was surveyed in reconnaissance during the early part of the century by Sidney Paige (in Darton and Paige, 1925). Economic work was done later near the Homestake gold mine at the north end of the area (Noble and Harder, 1948; Noble and others, 1949) and in the pegmatite district in the southern part (Page and others, 1953), but com— prehensive regional mapping is of rather recent date (Redden, 1963, 1968; Ratté and Wayland, 1969; Bayley, 1970, 1972a, b, c). The rocks are a sequence of metamorphosed sedi- ments and minor volcanics more than 40,000 ft (12,000 m) thick, steeply or isoclinally folded along northerly axes, and in places curiously refolded. In the northern half they form a gross synclinorium plunging toward the south. Here, the lower part of the sequence is in the Nemo district on the eastern side (Runner, 1934), which adjoins basement granite on Little Elk Creek (Wgn) with a Kenoran age of 2,500 my (Zartman and Stern, 1967). A thick basal quartzite is succeeded un— conformably by an equally thick conglomerate with as- sociated beds of iron formation, schist, and limestone. The upper part of the sequence, which forms the rest of the exposure to the west, is a eugeosynclinal deposit originally laid down as graywacke, slate, graphitic slate, chert, and pillow lava. It contains several thin but very persistent formations of ferruginous cherty rock, one of which (the Homestake Formation) contains the gold ore at the Homestake Mine. Less is known of the overall stratigraphic sequence in the southern hills. Basement rocks with a Kenoran date (Wgn) project in a mantled gneiss dome at Bear Moun- tain on the western side. Farther east, at Harney Peak and elsewhere, large granitic plutons (Xg) have domed the already folded and faulted supracrustal rocks (Run- ner, 1943) and are surrounded by swarms of pegmatites. The granites and pegmatites have been dated by potassium-argon and uranium-lead methods at 1,620— 1,680 m.y., and were intruded during late kinematic or postkinematic phases of the “Black Hills” (= Hudson- ian) orogeny (Goldich, Lidiak, Hedge, and Walthall, 1966, p. 5401) At the crest of the northern Medicine Bow Mountains of southern Wyoming is another great sequence of su— pracrustal rocks, that was deciphered years ago by Blackwelder (1926). It lies on basement gneisses (Wgn) on the north, and dips steeply and homoclinally south- ward to the Mullen Creek—Nash Fork shear zone, which 104° CENTRAL ROCKY MOUNTAINS 103°3o' \: .\ , \. \ :/,Harney§ />/\\ \/V zl/\Pea'k /:\/\\:,-\. \\ :\'\ \ l\ / z I separates it from the crystalline complex of the Central Rocky Mountains (Xm, etc.) (fig. 19). At the base of the sequence, which totals 35,000 ft (11,000 m) in all, is a 49 EXPLANATION E Tertiary intrusive rocks Paleozoic (with Upper Cambrian at base) PRECAMBRIAN X Harney Peak Granite l,620—1,680 m.y. EUGEOSYNCLINAL SEQUENCE Schist and slate, with thin to thick bodies of graywacke and quartzite (stippled) ' / l a ,XC,“ ~ ‘ § Pillow basalt, iron formation, and slate or schist 3'32-beig' Slate; and quartzite (stipple) with Homestake Iron— formation at base in north MIOGEOSYNCLINAI. SEQUENCE Quartzite, conglomerate, iron— formation, and limestone PRECAMBRIAN W Little Elk Granite (in northeast) and granite gneiss at Bear Mountains (in southwest) 2,500 m.y. o——o 20 MILES I I l 20 KILOMETRES Contacts within map units Precambrian faults Phanerozoic faults FIGURE 18.—Map showing Precambrian rocks in the Black Hills, western South Dakota. Compiled from many sources, including Paige (in Darton and Paige, 1925), Noble and Harder (1948), Noble and others (1949), Redden (1963, 1968), Ratté and Wayland (1969), and Bayley 1970,1972a,b,c) 50 PRECAMBRIAN GEOLOGY OF THE UNITED STATES thick body of quartzite (Deep Lake Formation),followed ' Rubidium-strontium whole-rock isochrons indicate unconformably by a more varied set of formations thatthe Medicine Bow sequence is olderthan 1,550 m.y. (Libby Group), including probable tillite in the lower and younger than the basement gneiss at 2,350—2,400 part, several prominent quartzite units, and higher up, m.y. (Hills and others, 1968, p. 1776). slates, greenstone volcanics, and carbonates with abundant, well-preserved stromatolites (Houston and others, 1968, p. 15—38). The general aspect of the se- In the Laramie Range east of the Medicine Bow quence is miogeosynclinal, in contrast to the eugeosyn- Mountains, the position of the Mullen Creek-Nash clinal aspect of most of the Black Hills sequence. Fork shear zone is occupied by an anorthosite body (Ya) PRECAMBRIAN Y 106° | EXPLANATION Quaternary (glacial deposits and alluvium) Tertiary (Paleocene to Miocene) 41°3o' Mesozoic and Paleozoic Sherman Granite (1 ,335 m.y .) *anfl ++a Gneissic granite BAGGOT (L410 m.y.) ROCKS _..“\“._-...“_-_; X "'1 T < ' > Paragneiss (1,715 m.y.) V 8 A : >1 @454» Libby Group 3 8 3 "3. o" g, T %‘ .. o SIERRA 24y k \ @g 0- u- // Deep Lake a) v Undiffe entiated Precambrian Formation / MADRE 41° / Vl‘LOMING__ COLORADO 0 2'0 MILES Orthogneiss and l | r l | paragneiss (2,400 m.y.) 0 20 KILOMETRES Mafic intrusives (various Precambrian ages) m Cataclastic rocks in shear zone FIGURE 19.—Map of northern Medicine Bow Mountains, Wyo., showing rocks of Precambrian W and X, and their relation to surrounding Phanerozoic rocks. The Mullen Creek—Nash Fork shear zone separates the Precambrian provinces of the Central and Southern Rocky Mountains. Generalized from Houston and others (1968) and Hills and others (1968). NORTHERN ROCKY MOUNTAINS with an area of about 300 mi2 (780 km?) (Newhouse and Hagner, 1957); it intrudes the ancient gneisses (Wgn) which form the northern part of the range, and it is intruded in turn by the Sherman Granite (Ygi) which forms the southern part. The anorthosite has a minimum age of 1,510 m.y. by potassium-argon methods on hornblende (Hills and Armstrong, 1971), and the granite has been dated at 1,410 m.y. NORTHERN ROCKY MOUNTAINS The Northern Rocky Mountains in this account are the mountainous terrain of western Montana and northern Idaho. Most of the southwestern half of the mountains is occupied by the great plutonic mass of the Mesozoic Idaho batholith (Kg, Kgn), but a large part of the remainder, northward across the International Boundary into Canada, is formed of supracrustal rocks of later Precambrian age—mainly the Belt Supergroup (Y), but including the less extensive younger Winder- mere Group (Z) on the northwest. PRICCAMBRIAN Y” The Belt Supergroup, or "Beltian” (= Purcell Super- group in Canada), is exposed nearly continuously across an area of about 30,000 mi2 (78,000 km2) in the United States and an additional 10,000 mi2 (26,000 km?) in Canada—the greatest expanse of well-preserved Pre- cambrian supracrustal rocks in the country. Through- out much of this expanse the Belt is merely tilted or warped, broken into coarse—textured fault blocks, and subjected only to the lower grades of metamorphism—a truly remarkable preservation of rocks so ancient through the 900 m.y. of succeeding Precambrian and Phanerozoic time. The Belt remained little disturbed until the Mesozoic and early Tertiary Cordilleran orogenies (Laramide and earlier). As a result of these orogenies, much of the Belt is allochthonous, having been transported scores of miles eastward along the Lewis and other low-angle thrusts, across the Paleozoic and Mesozoic rocks of the Cordil— leran miogeosyncline and foreland. Southwestward, also, it is invaded and disrupted by the Idaho batholith and other Mesozoic plutons, near which it is regionally metamorphosed to amphibolite grade. In this western area there is, besides, a gneissic terrane (Ym) which exceeds the adjacent Belt in its metamorphic complex- ity, but which may have been converted from Beltian rocks by the plutonic activity (Clark, 1973). Through a large part of its extent, the Belt is the “For a summary ofdata on the Belt Supergroup available up to 1963, see Rossi1963,1970)2 later information and developments for the part in Canada are given by Price (1964! and Gabrielse (1972, p. 52%528), and for the part in the United States by Harrison (1972! and Harrison and others (1974!. 5.1 youngest rock preserved, but outliers of Paleozoic occur on it in places, and it passes beneath Paleozoic and younger rocks at the edges. The next overlying unit is generally the Middle Cambrian Flathead Quartzite, which is separated from the strata beneath by little or no structural discordance. This relation has created a persistent misconception that the Belt must be very late Precambrian, if not Early Cambrian; Daly (1912, p. 174—190) even proposed an elaborate correlation of most of the Belt with the Lower and Middle Cambrian formations farther north in the Rocky Mountains. Much later, Deiss (1935) demonstrated the extensive regional truncation of the Belt beneath the Middle Cambrian deposits (although his results are somewhat vitiated by an assumption of constant thickness of the different Belt formations). Moreover, a decade earlier Walker (1926, p. 13—20) had discovered the later Precambrian Windermere Group unconformable over the Belt on the northwest, thus proving that the Belt itself was much earlier Precambrian than anyone had hitherto sus- pected. (See p. 53.) Along its southeastern side, in the Little Belt Moun— tains and the Three Forks region of southwestern Mon- tana, the Belt lies on older Precambrian crystalline basement (Wgn) with Kenoran and Hudsonian dates (p. 46—48). Near Three Forks the Belt abuts southward against a rough and partly upfaulted highland of the crystalline rocks, near which it assumes a coarse boul- dery facies (LaHood Formation), quite unlike the nor- mal fine—grained Belt sediments (McMannis, 1963); this is a local feature. An eastward wedging out of the Belt deposits on their basement must also exist north of the Little Belt Mountains, where the line of overlap is now concealed beneath the Lewis thrust plate. Farther west in the expanse of Belt rocks their basement nowhere reaches the surface; moreover, all the rocks west of the Belt area are younger, so that there is no indication of any western borderland. Along the eastern edge of its exposure, as in the Belt Mountains and Glacier National Park, the Belt is about 20,000 ft (6,100 m) thick, and is readily divisible into half a dozen or a dozen distinctive formations, including two prominent carbonate units (Newland = Altyn be- low, Helena : Siyeh above), several units of bright red argillite, and (near the International Boundary) the Purcell Lava, the only volcanic rock in this sequence, or elsewhere in the Belt. This facies is marginal to the main area of Belt de- posits to the west. Near the probable center of its origi- nal depositional basin, observed partial sequences of the Belt are 50,000 ft (15,000 m) thick, and the whole thickness probably exceeds 65,000 ft (20,000 In). Here, contrasts in the deposits have faded and formation boundaries are blurred. Most of the deposits are 52 siltstones, grading on the one hand into argillites and on the other into fine-grained quartzites. Red colors have turned to drab; the carbonates become calcareous siltstones with a few thin limestone interbeds. A re- markable feature of the sequence is the indication nearly throughout of deposition in shallow water, as shown by mud cracks, cut-and-fill, casts of salt crystals, and other sedimentary structures. All indications are that this great mass of fine sediment was derived from cratonic areas to the east and southeast. As will be seen presently, radiometric data indicate that accumulation of the Belt sediments occupied a span of nearly 500 m.y.—a length equal to most of Phanerozoic time since the beginning of the Ordovician. Even the great known total thickness of the Belt seems inadequate the account for this time span on the as- sumption of continuous sedimentation. This has led to a proposal that the Belt sequence must contain several hidden unconformities, expressing lengthy time gaps (Obradovich and Peterman, 1968, p. 746). It is true that some unconformities have been observed from place to place, but they seem to be minor and local. The best guess may be that there was “long semicontinuous dep- osition of sediments, interrupted by many hiatuses” (Harrison, 1972, p. 1237). Many different local names have been given to sub- divisions of the Belt Supergroup from one district to another, both in the United States and Canada, and correlations between them have been much debated. Increasing knowledge in recent decades has clarified most of the relations and has led to recognition of gross regional subdivisions. Within the main area of Belt outcrops their pattern is sufficiently coarse textured that they can be distinguished on the Geologic Map. On the map, it is therefore feasible to represent a unit Y1, of siltstones, argillites, and quartzites, that includes the Prichard Formation and Ravalli Group of the south- western area; a unit Y2, of carbonates and calcareous siltstones, that includes the Wallace Formation on the west and the Helena and Siyeh Limestones farther east; and a unit Y3, again siltstones, argillites, and quartzites, that comprises the Missoula Group. Similar gross subdivisions are also recognizable in some of the smaller outlying areas, especially to the southeast, but cannot be shown on the scale of the map; these areas are indicated merely as undifferentiated Y. The fresh appearance of the Belt rocks and their well-preserved sedimentary structures have impelled geologists since the days of Walcott (1899, p. 235-239) to search for the remains of fossils. The search has re- vealed abundant and well preserved stromatolites in the carbonate rocks at many levels, which seem to be capable of at least local zonation (Rezak, 1957); and other probable organisms such as bacteria. Traces of PRECAMBRIAN GEOLOGY OF THE UNITED STATES more advanced, metazoan forms of life have also been claimed, such as burrows, trails, and shell fragments, but these are questionable and some of them are clearly inorganic sedimentary structures. A supposed brachiopod, Obolella montana Fenton and Fenton, is evidently an algal stromatolite (Cloud, 1968, p. 27—29). Radiometric data on the age of the Belt are rather extensive for a Precambrian stratified sequence, but some of the evidence that they afford is indirect, and some of it is equivocal and conflicting (Obradovich and Peterman, 1968; Harrison, 1972, p. 1234—1238). Abso- lute age limits of the Belt sequence are set by the 1,700 my dates from its crystalline basement and the 760 my age of a vein cutting its upper part (Garnet Range Formation of Missoula Group). From the Belt sediments themselves, dates have been obtained at nearly a dozen levels from base to top on glauconite, biotite, and argil- lite by rubidium-strontium and potassium—argon methods, which range from more than 1,300 my. to less than 900 m.y.; also, the Purcell Lava and associated intrusives near the base of the Missoula Group (Y3) have yielded a potassium-argon date of about 1,100 my In addition, dates of about 1,500 my have been ob- tained from gneisses (Ym) probably derived from lower Belt rocks, but their significance is questionable. Avail- able evidence thus suggests that Belt sedimentation took place over at least 400 my, and perhaps as much as 500 my Further evidence regarding the time of termination of Belt sedimentation is afforded by dates from the succeeding Windermere Group. PRECAMBRIAN OF CENTRAL IDAHO In east-central Idaho, southeast of the Idaho batholith and north of the Snake River Plain, the Paleozoic in the various ranges is underlain by a thick sequence of elastic sedimentary rocks which are com— monly considered to be equivalents of the Belt Super- group and are accordingly shown on the Geologic Map as Precambrian Y. Their character and sequence have recently been summarized by Ruppel (1975). Their total thickness probably exceeds 30,000 ft (9,150,m), but their structure is complex, largely owing to Mesozoic deformation, and the whole sequence is not exposed in any one area. They have been subjected to low-grade regional metamorphism in the chlorite and biotite zones, and additional metamorphism has been superposed to the northwest near the batholith. The sequence is divided into the Yellowjacket Formation below, followed by the Lemhi Group of five formations, and the Swager Formation. The sedimentary rocks of the area resemble those of the typical Belt in being a very thick sequence of elastic sediments, lying stratigraphically between the older NORTHERN ROCKY MOUNTAINS Precambrian crystalline rocks to the east and the over- lying Paleozoic. In detail, however, the rocks of the two areas have surprisingly little in common. In contrast to the dominantly silty rocks of the Belt, those of central Idaho are mainly fine- to medium-grained feldspathic sandstones. Limestone and dolomite are virtually lack- ing, and stromatolites are rare. The sandstones show none of the shallow-water sedimentary structures of the Belt rocks. Observers have been unable to find any points of resemblance between details of the two se- quences which would suggest correlations, and a pro- posed correlation based on general sequence (Ruppel, 1975, p. 18) is tenuous indeed. One reason for the differ- ences seems to be that the central Idaho sequence of Precambrian sediments and the overlying Paleozoic have been transported a long distance eastward from their original site of deposition by Mesozoic thrusting; accumulating evidence indicates that the distance of transport was more than 100 mi (160 km). Two Pre- cambrian sedimentary basins with different environ- ments of deposition have evidently been brought into juxtaposition. Additional data on the’Precambrian of central Idaho have been presented by Armstrong (1975). He finds that granitic gneisses near Salmon, Idaho, shown on the Geologic Map as an eastern lobe of the Idaho batholith, have yielded ages of about 1,500 my by rubidium- strontium methods, hence are early Precambrian Y. This has many implications. It suggests that the rocks that extend across and nearly bisect the middle of the batholith are of early Precambrian age. On the Geologic Map, indeed, an extremely metamorphosed part of them is represented as Xm, but Armstrong believes that the remainder, shown on the map as metamorphosed Belt (Y) or as “border phase of the Idaho batholith” (Kgn) deserve the same classification also. Furthermore, the 1,500 m.y.-old gneisses apparently intrude the central Idaho Precambrian sedimentary sequence just dis- cussed, which would imply that it is of pre-Belt age— either early Precambrian Y or Precambrian X. The observations so far made on these interesting problems are as yet insufficient to provide positive answers, and they deserve much further investigation. PRECAMBRIAN Z Overlying the Belt Supergroup on its northwestern border is the Windermere Group, a younger Precam- brian supracrustal sequence. Its typical development is in the Purcell and Selkirk Mountains of southeastern British Columbia, whence it extends northward through most of the length of the Canadian part of the Cordillera (Gabrielse, 1972, p. 529—531). It also projects southward into the northeastern corner of Washington 53 State, as in the Metaline district (Park and Cannon, 1943, p. 7—13), but is preserved here only in small dis— connected remnants (Z). Parts of the Windermere have been known for many years; the part along the Interna- tional Boundary was the “Summit Series” of Daly (1912, p. 141—159), who thought it was equivalent to the Belt farther east. Recognition of these different parts as a new and hitherto unknown entity first came with Walker (1926, p. 13—20), whose discovery was one of the major contributions to North American Precambrian geology of this century. Nevertheless, the Windermere has remained strangely unknown, ignored, or misin- terpreted by most geologists in the United States, even until today. The Windermere lies unconformably on the Belt and marks the beginning of a new cycle of sedimentation. Local discordances between the two sequences are slight, but regionally the Windermere lies on different units of the upper Belt; its basal beds contain abundant clasts of the Beltian rocks, a few of which have been metamorphosed. The unconformity expresses an event called the “East Kootenay orogeny” which involved epeirogenic uplift in the Purcell Mountains, and local plutonic intrusions (Gabrielse, 1972, p. 528—529). Within the main geosynclinal area on the west, the Windermere sedimentary cycle continued nearly un- broken into the Paleozoic. (In fact, the higher units of Walker’s original Windermere, the Hamill, Lardeau, etc., have since yielded Lower Cambrian fossils, and are now excluded.) Eastward and marginally, a disconfor- mity develops at the top; in the Banff-Jasper segment of the Rocky Mountains the Windermere (Miette Group) is unconformable below the Lower Cambrian quartzites. No fossils have been reported in the restricted Winder- mere, except for Chuaria in the Miette Group (Gussow, 1973), like that in the Chuar Group of the Grand Can- yon (p. 68). The Windermere in southeastern British Columbia and adjacent, Washington is as much as 15,000—20,000 ft (4,600—6,100 m) thick—modest compared with the preceding Belt deposits, but impressive nevertheless. Near the International Boundary the sequence com- prises (from below upward) the Toby ( = Shedroof) Con- glomerate, the Irene (= Leola) Volcanics, and the Horsethief Creek (2 Monk) Formation, the latter fol— lowed by the Hamill (= Gypsy) Quartzite of Lower Cambrian age. (Farther south in Washington the first two units are combined as the Huckleberry Formation). The Irene is a mafic pillow lava and interbedded tuff that wedges out a short distance north of the boundary. The Toby is a regional deposit of variable thickness; much of it is coarse diamictite, formed of clasts of all sizes, largely of Beltian rocks, but including a few of granite and gneiss from the craton farther east. Diamic- 54 tites recur in the lower part of the Monk (probably connecting with the main body of the Toby farther north), but most of it is a heterogeneous mixture of phyllite, carbonate rocks, and quartzite. The Toby (and the basal Monk) was probably a glacial marine deposit, fed by ice on adjoining lands to the east, seemingly the local expression of a worldwide epoch of refrigeration that occurred late in Precambrian time (Aalto, 1971, p. 778—7 84). The time of beginning of Windermere sedimentation (and by implication the end of Belt sedimentation) is an important level in the Precambrian evolution of the Cordilleran region, but is indicated by only sparse radiometric data. Determinations on the granitic stocks in the Purcell area that are thought to have originated during the “East Kootenay orogeny” yield equivocal results—potassium-argon dates of 705—770 my and a rubidium-strontium isochron of 1,260 my (Gabrielse, 1972, p. 528); if the latter is near the true age, the intrusives must have been emplaced during deposition of the later Belt sediments. The volcanics in the lower part of the Windermere in Washington State (Irene equivalent) have recently yielded potassium-argon dates on whole rocks and mineral separates of 829—918 m.y. (Miller and others, 1973), which suggests that Windermere sedimentation probably began about 300 my before the beginning of the Cambrian. SOUTHERN ROCKY MOUNTAINSl2 The Southern Rocky Mountains are the ranges that extend southward from Wyoming, through the center of Colorado, into northern New Mexico. These ranges, like those of the Central Rocky Mountains, are broad- backed uplifts that expose large areas of Precambrian rocks in their cores, but they differ from those farther north in being closely crowded together rather than dispersed, so that their intervening lowlands are much narrower. Eastward, the Southern Rocky Mountains front abruptly on the Great Plains, whereas westward they merge with the Colorado Plateau through inter- mediate ridges and plateaus (fig. 20). Facing the Great Plains is the Front Range, a massive upland 250 mi (400 km) long and 30—60 mi (50—95 km) broad; it branches northward in Wyoming into the Laramie and Medicine Bow Ranges, and terminates in southern Colorado in the appendage of the Wet Moun- tains. South of this termination the Sangre de Cristo Mountains rise from behind and form the frontal ridge southward into New Mexico, to their own termination near Santa Fe. West of the Front Range is the equally I2For a useful summary of Precambrian rocks and events in the Southern Rocky Moun- tains, and their relation to Phanerozoic rocks and events, see Tweto, 1968, p. 555—571. PRECAMBRIAN GEOLOGY OF THE UNITED STATES lengthy but narrower Park Range, and beyond that the short and massive Sawatch Range, 90 mi (145 km) long and 40 mi (65 km) broad, which culminates in the high- est summit of the Rocky Mountains (Mount Elbert, 14,431 ft, 4,399 m). Beyond the Sawatch Range are lower uplifts, still roofed over by Phanerozoic strata, in which Precambrian is revealed in the deeper cuts and canyons: the White River Plateau to the northwest, the Uncompahgre Plateau to the west, and nearer at hand, the uplift along the Gunnison River, whose Black Can- yon exposes Precambrian rocks in sheer walls 2,300 ft (700 In) high. Southwest of the Sawatch Range is the broad Cenozoic volcanic field of the San Juan Moun- tains, at the southwestern edge of which the Precam- brian projects again in the domical Needle Mountains. The gross surface features of the Southern Rocky Mountains, as farther north, are a product of late Cretaceous—early Tertiary (Laramide) orogeny, but here plutonic and volcanic activity was greater. One result of the activity is the Colorado mineral belt that extends diagonally northeastward across all the ranges (Tweto and Sims, 1963, p. 993—996), containing most of the prolific mineral deposits of Colorado and marked by numerous faults, veins, and intrusive stocks, as well as the large Mount Princeton pluton in the southern Sawatch Range (Kg3, Ti). Relations in the Southern Rocky Mountains are com- plicated further by Phanerozoic orogenic events earlier than the Laramide, especially during the later Paleo- zoic, when geanticlines and troughs were created that had a somewhat different pattern from the Laramide structures—a Front Range geanticline on the sites of the Front Range and northern Park Range, and an Uncompahgre-San Luis geanticline on the sites of the Uncompahgre Plateau and San Juan Mountains (fig. 20) (Mallory, 1972). In the Southern Rocky Mountains the gross patterns of the Precambrian rocks and structures are plainer than farther north because of the close proximity of the ranges, but they are confused in detail because of the more complex Phanerozoic events. Confusion is great- est in the Colorado mineral belt, where the Laramide plutonism, faulting, and mineralization are superposed on ancestral shear zones that originated during Pre- cambrian time. Erosion and sedimentation resulting from the Paleozoic orogenies produced contrasts be- tween the strata that lie on the Precambrian from place to place: lower Paleozoic shelf deposits in the troughs (as in the Sawatch Range), upper Paleozoic elastic deposits on the flanks of the geanticlines (as in the Front Range and Sangre de Cristo Mountains), and Mesozoic strata on the crests of the geanticlines (as in the Uncompahgre Plateau). SOUTHERN ROCKY MOUNTAINS PRECAMBRIAN X GNEISS (IOMPLEX The Precambrian of the Southern Rockv Mountains is 55 a complex of paragneisses, in which are embedded numerous small to large granitic plutons. South of the Mullen Creek—Nash Fork discontinuity in southern law — UTAH —__ ~__ COLORADO .__—._._ ___‘__ ' §__ ‘__ ___ EXPLANATION 7 Z Colorado mineral belt Geanticlines of late Paleozoic time \\ n \\ l/ I! Q: Outcrops of Precambrian rocks 9 _—__~_ _‘_._ ‘_._ I WYOMING COLORADO Laramie NEW MEXICO O-r-O 100 200 MILES i | I I 100 200 KILOMETRES FIGURE 20.—Map of Southern Rocky Mountains in Colorado and New Mexico, showing outcrops of Precambrian rocks, outlines of the late Paleozoic geanticlines and the Colorado Mineral Belt, and localities mentioned in the text. Outlines of Colorado Mineral Belt and Paleozoic geanticlines after Tweto and Sims (1963, p. 997, 1007) and Mallory (1972, p. 132). 56 Wyoming all the Precambrian rocks yield Hudsonian and later dates and no Kenoran dates are known. All the rocks of the Southern Rocky Mountains are there- fore Precambrian X or younger and no rocks of Pre- cambrian W are identifiable, if indeed they ever existed. The dominant paragneiss (Idaho Springs Formation of Front Range) is a biotitic quartzo-feldspathic gneiss derived from an original thick geosynclinal sequence of shale and graywacke, in which are numerous lenses and interbeds of amphibolite (Swandyke Gneiss of Front Range), derived from original volcanic rocks. The only prominent variant is a thick synclinal mass of quartzite at the mountain front northwest of Denver (Wells and others, 1964). The gneisses have been plastically de- formed into steep folds along northwest to west- northwest axes, and metamorphosed to almandine- amphibolite grade. Relicts of an earlier, more open fold- ing of about the same trend can be detected in places, and superimposed on both sets of structures is a later cataclastic deformation that produced northeast- trending shear zones, especially in the Colorado min- eral belt about midway along the Front Range and in the northern Sawatch Range (Tweto and Sims, 1963, p. 996-1005). The main deformation of the paragneisses has been dated by rubidium-strontium methods on whole-rock and feldspar samples at 1,750 m.y., the earlier deforma- tion and the original accumulation of the sediments could have been no more than 100 my earlier (Hedge and others, 1967, p. 555); the gneisses are accordingly classed as Xm on the Geologic Map. They may be the eugeosynclinal equivalent of the miogeosynclinal Pre- cambrian X rocks of the northern Medicine Bow Moun- tains (Hills and others, 1968, p. 1777). PRECAMBRIAN X AND Y GRANITIC ROCKS Embedded in the paragneisses are granitic rocks which form nearly half the area of Precambrian expo- sure. In the Front Range and elsewhere they are divisi- ble into three groups of different ages, each younger group emplaced at progressively shallower levels in the crust (Peterman and Hedge, 1968, p. 753—754). The oldest group (Xg), exemplified by the Boulder Creek Granite of the Front Range, forms concordant plutons in the paragneisses and is synorogenic to their principal deformation, with Hudsonian ages of about 1,700 my The much more extensive middle group (Ygl), exemplified by the Sherman Granite of the Laramie Range and the Silver Plume Granite of the Front Range farther south, is broadly contemporaneous with the final cataclastic deformation of the gneisses, and yields Elsonian ages of 1,390—1,450 m.y. The youngest group (Ygz), or Pikes Peak Granite, occurs PRECAMBRIAN GEOLOGY OF THE UNITED STATES only in the southern part of the Front Range, where it forms a great pluton with an exposed area of 1,200 mi2 (3,100 km2), and several satellite bodies; it is post- orogenic and has a Grenvillian age of about 1,040 my (Hedge, 1970). I’RECAMBRIAN OF NEEDLE MOUNTAINS The Precambrian of the Needle Mountains, at the edge of the San Juan volcanic field in southwestern Colorado, is more varied than elsewhere in the South- ern Rocky Mountains, hence has long intrigued geolo- gists, but relations have only been clarified recently by detailed mapping and by radiometric dating (Barker, 1969; Bickford and others, 1967, p. 1660—1661) (fig. 21). An older metamorphic complex (Xm) consists of the Vallecito Conglomerate with clasts derived from still older terranes, followed by the Irving Formation of am- phibolite, paragneiss, and metagraywacke. The com— plex was steeply folded along northerly to northeasterly axes, metamorphosed to amphibolite grade, and in- vaded by the synkinematic Twilight Granite and the postkinematic Tenmile Granite, with rubidium-stron- tium ages of 1,780 my and 1,700—1,720 m.y., respec— tively. Lying with right-angled unconformity on the deeply eroded edges of the metamorphic and plutonic rocks is theUncompahgre Formation (Y), a supracrustal se- quence of quartzite and interbedded slate more than 8,000 ft (2,400 m) thick; it was steeply folded along west-northwest axes before intrusion of the Eolus Granite with an age of 1,460 my (Ygl). Still younger minor granites with ages of 1,350 my intrude the rocks of the older complex in places. The time of accumulation and deformation of the Un- compahgre Formation is bracketed between the age of the youngest preceding granite (1,700 m.y.- and the age of the oldest succeeding granite (1,460 my) (fig. 22). The formation is therefore early Precambrian Y (Paleohelikian of the Canadian classification), and thus probably largely older than the Precambrian Y Belt Supergroup of the Northern Rocky Mountains. Its def- ormation is an Elsonian event (the “Uncompahgre orogeny” of local terminology)—a deformation of which there is little indication elsewhere in the western United States. EASTERN GREAT BASIN The eastern Great Basin in the western half of Utah and the eastern edge of Nevada is a region of interior drainage leading mainly into Great Salt Lake; it is a terrain of isolated or nearly isolated ranges that project from broad expanses of lowland floored by late Cenozoic deposits. Along its eastern border more cohesive plateaus and ranges face it in prominent escarpments, EASTERN GREAT BASIN of which the most notable are the Wasatch Mountains in the north, whose summits stand 7,000 ft (2,100 m) above the basin floor. Also included in this account are 000 9 ° 00000 o oSilvertgn o co 0 o°°g°°°oo° O .90 000 a ' oo°o°° .0 1/ 0 20 Ml LES I; l I l I I l I I 0 20 KILOMETRES EXPLANATION , o \ I \\ \ my ‘ Tertiary volcanic rocks Mesozoic and Paleozoic Trimble Granite Eolus Granite Diorite and gabbro 1,350 m.y. 1,460 m.y. < ” W -_ IIIIHIIII fixing ‘ A E a Uncompahgre Formation Tenmile Granite 1,700—1,720m.y. Twilight Granite Gneiss Irving Formation Vallecito Conglomerate (quartzite with argillite (and Bakers Bridge Granite) 1,780 m.y. layers) FIGURE 21.—Map showing Precambrian X and Y units in Needle Mountains, southwestern Colorado. After Barker (1969), with Phanerozoic rocks added from other sources. 57 the Uinta Mountains, actually an outpost of the South— ern Rocky Mountains, which extend 150 mi (250 km) eastward from the Wasatch Mountains at midlength. 58 FIGURE 22.—Synoptic diagram showing relations of units of Precam- brian X and Y in Needle Mountains, southwestern Colorado, and their implications in the Precambrian history of the area. Letter symbols the same as in fig. 21. After Barker (1969, p. A8). The gross forms of the region are of younger origin than those in the Rocky Mountains to the east and north—products of a late Cenozoic disruption that out- lined the ranges and lowlands of the Great Basin by block-faulting, and separated it from the Wasatch Mountains and other uplands on the east. The late Cenozoic disruption is superposed on an earlier Cordil— leran fabric, largely a product of Mesozoic orogenies, and especially of a mid-Cretaceous (Sevier) orogeny; this, in turn, is superposed on a preceding Phanerozoic miogeosynclinal regime. The late Cenozoic boundary between the disrupted region on the west and the more stable region on the east follows a persistent zone of weakness (Wasatch line), which had previously been the front of the Sevier orogenic belt, and the edge of the preceding miogeosyncline. The traces of the frontal thrusts of the orogenic belt are close to the block-faulted late Cenozoic boundary, lying west of it in the central Wasatch Moun- tains and south of the Wasatch Mountains, and east of it in the northern and southern segments of the Wasatch Mountains. About 100 mi (160 km) west of the frontal thrusts, near the Utah-Nevada border, a zone of décol- lement in the lower part of the miogeosynclinal se- quence emerges in the cores of the ranges which is probably geneticallyrelated to the frontal thrusts to the east. Beneath it is an infrastructure that was highly disturbed and metamorphosed during the Mesozoic orogenies. Within the region here considered, most of the ex- posed bedrock is Phanerozoic, but the underlying Pre- cambrian emerges in small areas on the lower slopes of some of the ranges—mainly Precambrian Y and Z su— PRECAMBRIAN GEOLOGY OF THE UNITED STATES pracrustal rocks, but including some Precambrian X crystalline basement toward the east (fig. 23). As the frontal thrusts of the Mesozoic orogenic belt involve more than 30 mi (50 km) of eastward transport, great contrasts exist between the Phanerozoic and Precam- brian rocks in the autochthon beneath the thrusts, and in the allochthon of the upper plates, contrasts which are especially evident in the Wasatch Mountains. Autochthonous Precambrian rocks occur in the 60—mi (95 km) middle segment of the Wasatch Mountains and in the Uinta Mountains to the east. Allochthonous Pre- cambrian rocks lie above them in the upper plate of the frontal thrust (Willard thrust) in the northern segment of the Wasatch Mountains, but do not come to the sur- face at the front of the upper plate in the southern segment. In the discussion which follows, the au- tochthonous Precambrian will be treated first, after which the allochthonous Precambrian of the northern Wasatch Mountains and farther north and west will be considered. (IRYSTALLINE BASEMENT (l’RECAMBRIAN X) Crystalline basement (Xm) of the autochthon is ex- posed in the central segment of the Wasatch Mountains, in one of the islands in Great Salt Lake immediately to the west (fig. 23), and on the northern edge of the Uinta Mountains, 150 mi (250 km) to the east. The Farmington Canyon Complex (Eardley and Hatch, 1940a) forms the frontal ridge of the Wasatch Mountains for 25 mi (40 km) between Ogden and Salt Lake City. It is a body of felsic paraschist of amphibolite grade, derived from an original sedimentary sequence more than 10,000 ft (3,000 m) thick, that has been thoroughly permeated by granitic and pegmatitic material. About 15 mi (25 km) farther south the Little Willow Formation forms a small outcrop between the mouths of Big and Little Cottonwood Canyons (not shown on Geologic Map, but see fig. 23); it is likewise a paraschist but lacks injected material, in this respect resembling the Red Creek Quartzite of the Uinta Moun- tains (see below). The Little Willow is succeeded by Precambrian Y supracrustal rocks, but these wedge out northward, and the Farmington Canyon is overlain directly by the basal Cambrian Tintic Quartzite. The Farmington Canyon has yielded Hudsonian dates of 1,640—1,700 m.y. by potassium-argon methods on hornblende, and somewhat younger dates by rubidium-strontium methods on muscovite (Whelan, 1970, p. 15—17). No Precambrian ages have been ob- tained from the Little Willow, and only dates between 27—29 m.y. that were produced by the nearby Tertiary plutons. Near the east end of the Uinta Mountains on their north side the Red Creek Quartzite forms a small inlier EASTERN GREAT BASIN at the base of the Precambrian Y Uinta Mountain Group (Hansen, 1965, p. 22—32). Determinations on muscovite from the Red Creek by rubidium—strontium and potassium-argon methods have yielded ages of 2,320 m.y. and 1,520 m.y., respectively. The larger figure is probably near the minimum age of the forma- tion, which would place its accumulation in the early part of Precambrian X, or even in Precambrian W. Nevertheless, its general aspect is much like the Pre- cambrian X rocks to the east and west, and it is so represented on the Geologic Map. BIG (IOTTONVH )Ol) FORMATION (I’RFCAMBRIAN Y) In the Cottonwood area southeast of Salt Lake City, the Little Willow crystalline basement is followed un- conformably by the Big Cottonwood Formation (Y), a 16,000-ft (5,000 m) sequence of quartzites and interbed- ded variegated shales (Eardley and Hatch, 1940b, p. 819—820; Crittenden and others, 1952, p. 3—4). Its rocks are not metamorphosed except near the Tertiary plutons, and they dip gently eastward beneath the Pre- cambrian Z and Cambrian supracrustal rocks farther back in the mountains. Ripple marks, crossbedding, and mud-flake conglomerates are well preserved, indicating deposition in shallow water. Like the other Precam— brian supracrustal rocks of the eastern Great Basin, no radiometric data are available on the age of the Big Cottonwood, but on the basis of relations to the rocks above and below, it is commonly believed to be equiva- lent to some part of the Precambrian Y Belt Supergroup of the Northern Rocky Mountains. MINERAL FORK 'l'II.I.I'I‘E AND MI'TL'AI. FORMATION (I’RECAMBRIAN 7.) In the upper reaches of Big Cottonwood and adjacent canyons, two higher Precambrian units intervene be— tween the Big Cottonwood Formation and the Cam- brian Tintic Quartzite—the Mineral Fork Tillite and Mutual Formation (Z) (fig. 23). The Mineral Fork Tillite, or diamictite, is a massive graywacke in which are embedded numerous clasts of all sizes up to large boulders, with interbedded layers of quartzite and laminated argillite. Some of the clasts are striated, and all are of Precambrian crystalline base- ment, such as granite gneiss, quartzite, and dolomite. The deposit thickens and thins over the eroded surface of the Big Cottonwood Formation, reaching more than 1,000 ft (300 m) in broad, smooth-bottomed basins, and nearly disappearing in the intervening areas. A glacial origin for the deposit was proposed by various early geologists, such as Blackwelder (1932, p. 301—303), and has been reaffirmed by some modern observers (Crit- tenden and others, 1952, p. 4—6), but questioned by 59 others (Condie, 1967, p. 1,341—1,342), who compare it with subaqueous mudflows and turbidites of other re- gions. Such features could, of course, be one of the man- ifestations of a general glacial episode, and the reality of such an episode is strongly suggested by the regional occurrence of the Mineral Fork and correlative diamic- tites throughout the eastern Great Basin (Crittenden and others, 1972). The Mutual Formation is a body of red-purple quartzites and red to green shales as much as 1,200 ft (360 m) thick, which lies on the eroded surface of the Mineral Fork, and is truncated in turn by the Tintic Quartzite. The position of the Mineral Fork and Mutual Forma- tions between the Big Cottonwood Formation and the Tintic Quartzite implies a late Precambrian, and prob— ably a Precambrian Z age, comparable to that of the Windermere Group in the Northern Rocky Mountains. L'IN’I'A MOIfNTAIN GROUP (I’RECAIVIBRIAN Y) The Uinta Mountains, like the other ranges of the Central and Southern Rocky Mountains, are a broad- backed anticlinal uplift, in part faulted on the flanks, with a large area of Precambrian exposed in the core. The core rocks are, however, not a crystalline basement, but a thick supracrustal clastic sequence, the Uinta Mountain Group (Y). The Uinta Mountain Group lies on the crystalline basement of the Red Creek Quartzite, exposed near the eastern end of the range, and is moderately unconform- able beneath the Paleozoic strata on the flanks, which include discontinuous thin Cambrian units at the base—Middle Cambrian to the west, Upper Cambrian to the east. The group is broadly arched, in conformity with the general uplift of the range, and is virtually unmetamorphosed. In the eastern exposures a complete sequence between the Red Creek Quartzite and the Paleozoic is 25,000 ft (7,600 m) thick (Hansen, 1965, p. 33); farther west, where the basement does not crop out, no more than about 10,000 (3,000 m) is exposed (Wallace and Crittenden, 1969, p. 129). The Uinta Mountain Group contains various mappa- ble subdivisions, but the only formally named unit is the Red Pine Shale, at the top in the western half of the range, a body of dark shale, siltstone, and minor quartzite as much as 5,000 ft (1,500 m) thick. The un- derlying main body of the group is dominantly quartzite and arkose, with shale only as thin interbeds. Three different facies are recognizable, representing contrast- ing sedimentary environments: deltaic-fluvial, fluvial-flood plain, and paralic-neritic (Wallace and Crittenden, 1969, p. 134—137). Sediments were derived from bordering lands of crystalline basement to the 60 north and northeast, and were transported westward along the axis of the depositional trough. Quartzites and arkoses are especially coarse and massive in the eastern exposures, where they contain thick wedges of pebble and cobble conglomerate formed of rounded quartz and quartzite clasts (Hansen, 1965, p. 36—37). Both the present extent of the Uinta Mountain Group beyond its outcrops, and its original extent, are difficult 113° PRECAMBRIAN GEOLOGY OF THE UNITED STATES to determine because the Uinta Mountains are flanked north and south by the Green River and Uinta basins, filled by Phanerozoic sediments so thick that their base has not been reached by drilling. Nevertheless, the group seems to be an unusual eastward extension of Precambrian supracrustal rocks into a domain other- wise formed of crystalline basement. Such crystalline rocks (with Kenoran dates) are known some distance to 40 Q _, ”a... 50M|LES l J I SOKILOMETRES O o 0 0000000 0 a o n n 0 FIGURE 23.—Geologic map of northeastern Utah, showing Precambrian X, Y, and Z rocks in eastern Great Basin, and the adjoining mountains and plateaus to the east, which are parts of the allochthon and autochthon of the Cretaceous Sevier orogenic belt. Com- - piled from Geologic Map of Utah (1963), with additions from published and manuscript data of M. D. Crittenden, Jr. EASTERN GREAT BASIN the north, near the Wind River Mountains, and others (with Hudsonian dates) are known from drilling along the south edge of the Uinta basin (Muehlberger and others, 1966, p. 5425). These occurrences define vaguely an east-trending belt of Precambrian supracrustal rocks that is probably a primary feature, as attested by the sedimentary facies in the group itself. The belt is comparable to the aulacogens of the Soviet geologists, or early sedimentary troughs which extend transversely into the craton. The westward prolongation of the Uinta Mountain trough is in the Cottonwood area of the autochthonous central segment of the Wasatch Mountains, with its sequence of Precambrian Y and Z supracrustal rocks, already described. Its northern flank is in the northern part of the autochthonous segment, where the supra— crustal rocks are missing, and the basal Cambrian lies directly on the earlier crystalline basement of the Farmington Canyon Complex (fig. 23). The age of the Uinta Mountain Group has aroused speculation since the first geological explorations of the Uinta Mountains a century ago. Former proposals that it is of early or even late Paleozoic age are obsolete, as its unconformable position beneath Cambrian strata is now established. Modern speculation centers around its precise correlation with the Precambrian supracrustal units in the autochthonous segment of the Wasatch Mountains to the west. Is it equivalent, wholly or in part, to Precambrian Y Big Cottonwood Formation, or EXPLANATION m D , O Quaternary intermontane deposits T) '. Tertiary postorogenic (includes some Upper Cretaceous) 61 to the younger Precambrian Z Mineral Fork and Mutual Formations? No tillites (diamictites) like those in the Mineral Fork occur in the Uinta Mountains, yet a considerable part of the Uinta Mountain Group beneath the Red Pine Shale is lithically much like the Mutual Formation, and has been so correlated. Nevertheless, the marked variations in sedimentary facies within the Uinta Mountain Group itself warn of the dangers of correlations on lithology alone, and it might be an on- shore phase of the Big Cottonwood Formation. The best present judgment on paleogeographic grounds seems to be that the Uinta Mountain Group is Precambrian Y (Crittenden and others, 1972, p. 337), a decision adopted on the Geologic Map. Recently a whole rock isochron by rubidium-strontium methods of 950 my. has been ob- tained from the Red Pine Shale at the top of the se- quence (Zell Peterman, written commun. 1974), which confirms this inference. SUPRACRUSTAL ROCKS OF THE ALLOCHTHON (PRECAMBRIAN Z) Along the eastern edge of the Great Basin, Precam- brian rocks are exposed in widely separated ranges of 300 mi (480 km), from southeastern Idaho to south- central Utah. In Idaho, they occur near Pocatello, in ranges immediately south of the Snake River Plain. In Utah, they are exposed in the northern segment of the Wasatch Mountains, in the Promontory Range west of Tertiary intrusive rocks deposits ROCKS OF THE ALLOCHTHON -_‘Z.- Precambrian Z supracrustal rocks Cambrian Paleozoic ROCKS OF THE AUTOCHTHON Precambrian Z supracrustal rocks Cambrian Paleozoic and Mesozoic “ Xw/ k Farmington Canyon Little Willow Precambrian Y Complex Ibrmation supracrustal rocks FIGURE 23.—Continued. 62 it, and in the Sheeprock, Dugway, Canyon, and Beaver Ranges further south (fig. 23). Except for rocks on one island in Great Salt Lake west of the autochthonous segment, those of all these exposures are allochthonous, and in the upper plates of thrusts of the Sevier orogenic belt. All the rocks are supracrustal and part of Pre- cambrian Z. No Precambrian Y supracrustal rocks are visible, and with one minor exception, no crystalline basement; the extent of the older Precambrian rocks in the area, if they exist, is indeterminate. Equivalents of the two relatively thin, unconform— ity-bounded Precambrian Z units of the autochthon (Mineral Fork and Mutual) occur in the allochthonous rocks, but here they are widely separated in a much thicker conformable sequence. In the well-known out- crops on the upper plate of the Willard thrust in the northern Wasatch Mountains, the sequence beneath the Cambrian is 13,000 ft (4,000 m) thick, but it reaches 20,000 ft (6,100 m) in the structurally more complex sequence near Pocatello, as deciphered by D. E. Trimble (Crittenden, Schaefer, Trimble, and Woodward, 1971, p. 582—594). Partial sequences preserved in the ranges farther south are thinner. In the upper plate of the Willard thrust in the Wasatch Mountains the supracrustal sequence bottoms on a thin wedge of crystalline basement (not on Geologic Map, but see fig. 23), which has been dated by rubidium-strontium methods on muscovites as between 1,600 and 1,800 my (Crittenden, McKee, and Peter- man, 1971), or approximately correlative with the au- tochthonous Farmington Canyon Complex exposed to the south. Tillite (diamictite) like that in the Mineral Fork oc- curs 1ow in nearly all the sequences, and in the Pocatello sequence contains an interbedded member of greenstone flows and tuffs, reminiscent of the Irene Volcanics intercalated in the diamictites of the Win- dermere Group farther north. A unit lithically identical with the Mutual Formation of the autochthon occurs much higher, the intervening strata being shale and siltstone with one or more thick quartzite units and in places a thin limestone layer. The rocks above the Mutual equivalent are mainly quartzites, traditionally called Brigham, Tintic, or Prospect Mountain depend- ing on locality and considered to be basal Cambrian. However, only their upper parts can be proved paleon- tologically to be of Cambrian age, and the lower parts may be Precambrian Z; these lower parts are now mostly given other formational names. A volcanic brec- cia in the Browns Hole Formation, between the Mutual and the upper quartzites, has yielded an argon-argon date on hornblende of 570 my (Crittenden and Wall- ace, 1973, p. 128), which indicates that it lies close to the Precambrian-Cambrian boundary. PRECAMBRIAN GEOLOGY OF THE UNITED STATES SUPRACRL‘S'I‘AI. ROCKS OF UTAH—NEVADA BORDER To the west, near the border between Utah and Nevada, Precambrian Z supracrustal rocks reappear in various mountains, from the Pilot Range 150 mi (250 km) southward to the southern Snake Range, and westward for 50 mi (80 km) into Nevada. They much resemble the supracrustal rocks just described, but are separated from them by a broad gap, mostly occupied by the Great Salt Lake Desert, and their structural setting differs. All of them lie in an infrastructure beneath the regional décollement mentioned earlier, and have been subjected to low- to medium-grade metamorphism dur- ing the Mesozoic orogenies. The rocks are termed the McCoy Creek Group from a locality in the Schell Creek Mountains, Nevada, where there is a 9,000-ft (2,700 In) sequence beneath the Cambrian Prospect Mountain Quartzite (Misch and Hazzard, 1962, p. 307—320). The group is nearly as thick in the Deep Creek Range, and 3,600 ft (1,100 m) of the upper part is preserved in the southern Snake Range. The rocks include several persistent quartzite units separated by units of argillite and siltstone, with a few minor layers of marble. The sequence in the Deep Creek Range, farther east than the rest, includes several hori- zons of “tillitic schist,” originally a diamictite of sand and silt with widely dispersed dropstones up to boulder size, of granite, gneiss, and quartzite; it is the water—laid distal end of the tillites (diamictites) farther east in the Great Basin. The lowest rocks exposed in the sequences are com- monly the most strongly metamorphosed, but are con— formable with those above, and no earlier basement emerges south of the Albion and Raft River Ranges (p. 48). The upper beds have a sharp boundary with the overlying Prospect Mountain Quartzite, but the succes- sion is seemingly conformable from one to the other. SOUTHERN BASIN AND RANGE PROVINCE In the final section of this review the exposed Pre- cambrian of the southwestern United States will be considered, from southern California 800 mi (1,300 km) eastward to western Texas, and 700 mi (1,100 km) or more north from the Mexican border. Although this large region is diverse in terms of modern morphology and Phanerozoic structure, its Precambrian rocks have a certain homogeneity that facilitates description. Moreover, the largest area of Precambrian outcrop and the most significant sequences are in Arizona, and these furnish standards with which the remainder can be compared. The southern Basin and Range province is a terrain of block-faulted mountains and intervening lowlands and deserts, much like that of the eastern Great Basin SOUTHERN BASIN AND RANGE PROVINCE treated in the previous section. Here, however, most of the drainage is exterior, leading into the Colorado River and its tributaries. In the lower “desert” region of southwestern Arizona and southeastern California, many of the block ranges have been so eroded that their original structural forms have been lost, and they stand as islands in a much broader sea of lowlands. On the other hand, in the “mountain” region of Arizona farther north, along the edge of the Colorado Plateau, the block ranges coalesce so that outcrops of Precambrian rocks are nearly continuous, except for outliers and downfaulted strips of Phanerozoic rocks. Included in this account are also the classic, long-known Precam- brian rocks that form the lower walls of the Grand Canyon within the Colorado Plateau, and the Precam- brian rocks of the Transverse Ranges of southern California, both lying in Phanerozoic morphological and tectonic settings different from the rest. The most extensively exposed Precambrian rocks in much of the region, and especially in Arizona, are the crystalline basement of Precambrian X—paraschists, paragneisses, orthogneisses, and granites—in which are embedded a few younger plutons of Precambrian Y. Lying on their deeply eroded surface, and preserved in smaller areas, are little-deformed supracrustal rocks of Precambrian Y, including such units as the Grand Can- yon Supergroup and the Apache Group of Arizona. Younger supracrustal rocks of Precambrian Z form still smaller areas, mainly in southwestern Nevada and eastern California. As in the Southern Rocky Moun- tains and eastern Great Basin, no rocks of Precambrian W have been identified in the region, if indeed they ever existed. CRYSTALLINE BASEMENT OF ARIZONA (MAINLY PRECAMBRIAN X) In Arizona (and elsewhere in the southwestern United States) the crystalline basement is divided on the Geologic Map into metasedimentary rocks (X), or- thogneiss and paragneiss (Km), and granitic rocks (Xg, Ygi, and Yg2). These units are modified from those of the Geologic Map of Arizona of 1969 as follows: Our unit X includes schist, greenstone, rhyolite, and Mazatzal Quartzite (units p-Csc, p6 gs, p€ry, and p€m of the Arizona Map); because of the small scale of the United States Map the metavolcanics are grouped with the metasediments. Our unit Xm includes Precambrian gneisses (p€gn), as well as so-called “Mesozoic” and “Cretaceous-Tertiary” gneisses ( Mzgn, TKgn), which are largely Precambrian in origin, but were reworked during Phanerozoic orogenies. Our units Xg, Ygi, and Yg2 are the granite, quartz monzonite, and quartz diorite of the Arizona 63 Map (pegr), which we have subdivided according to their radiometric ages. The Precambrian diorite and pyroxenite (p€di, p-pr) shown on the State Map are too small to be shown on the United States Map. The metamorphic rocks of Arizona traditionally have been called the Vishnu Schist to the north in the Grand Canyon, the Yavapai Schist in the central region, and the Final Schist in the southeastern region, all sup- posedly more or less correlative. More information is available now on all these units, although the schists and gneisses of the desert region to the southwest re- main poorly understood. Modern radiometric work in- dicates that all these metamorphic rocks and most of the granites which intrude them have Hudsonian ages be- tween 1,650 and 1,850 my, thus placing them in the later part of Precambrian X, but they are not necessar- ily correlative, and some of them are clearly younger than others. The Vishnu in the Grand Canyon includes the quartzose, micaceous Vishnu Schist (restricted) and the mafic Brahma Schist, the first derived from sediments, the second from volcanics; the whole forming an origi- nal sequence tens of thousands of feet thick. This was steeply folded along northeast axes, metamorphosed, and pervasively injected by the Zoroaster Granite (Maxson, 1961) (not separated on the map). The Zoroas- ter has yielded an age of 1,725 my by uranium—lead methods on zircons, which indicates the minimum age of the whole assemblage (Pasteels and Silver, 1966), although dates as low as 1,390 my. have been obtained from it and the adjoining schists by rubidium-strontium methods (Giletti and Damon, 1961, p. 640). The Pinal Schist of southeastern Arizona, as rep- resented in the Dragoon quadrangle, is a similarly thick body, derived from original graywacke and slate with interbedded felsic and mafic volcanics, steeply folded along northeast axes, metamorphosed to greenschist or amphibolite grade, and intruded by granodiorite and granite (Cooper and Silver, 1964, p. 11—34). The rhyo- lites have yielded an age of 1,715 my and the intrusive granodiorite an age of 1,615—1,630 m.y. by uranium- lead methods on zircons, suggesting that these rocks are somewhat younger than those in the Grand Canyon. The Yavapai Schist and associated metamorphic rocks of central Arizona, exposed in broader, more con- tinuous areas than the Vishnu and Pinal, are more varied. They have been deformed along northeast- trending axes and subjected to greenschist or low am- phibolite grades of metamorphism, but original sedimentary and volcanic structures are commonly well preserved. Wilson (1939, p. 1117~1129) was one of the first to demonstrate that the Yavapai rocks are divisible into distinctive, mappable formations of which he named nearly a dozen that he correlated between dis- 64 tricts. His original classification has been amplified and emended by further mapping, and by radiometric dat- ing that has shown more clearly the relative ages of the units. The type Yavapai area is in the center of the State near Prescott and Jerome, where the sequence includes the Ash Creek and Big Bug Groups, each about 20,000 ft (6,100 m) thick, both composed of felsic and mafic vol- canic and volcaniclastic rocks, and interbedded sedi- ments (Anderson, 1968, p. 14—17) (fig. 24). They are separated from each other by granitic plutons and by a major north-south strike-slip fault, but radiometric dat- ing by uranium-lead methods on zircons indicates that the Ash Creek is the older, the collective age of the two groups being between 1,775 and 1,820 my. (Anderson and others, 1971). On this basis the two are considered to represent a time-stratigraphic unit, and are called Yavapai Series. In separate fault blocks in the same area the Texas Gulch Formation lies with basal con— glomerate on the Brady Butte Granodiorite. Formerly the Texas Gulch was supposed to represent the base of the Yavapai sequence, but the granodiorite has an age of 1,770 my, so that the Texas Gulch is the youngest rock in the district, and is excluded from the Yavapai Series as now defined. In the Mazatzal Mountains farther east, which were studied in most detail by Wilson, the Yavapai rocks include several units of greenstone, rhyolite, and vol- caniclastic sediments which are so complexly faulted that their original sequence is conjectural. Only the Red Rock Rhyolite is in stratigraphic continuity with the uppermost rocks, the prominent, well-bedded Mazatzal Quartzite and the minor underlying Maverick Shale and Deadman Quartzite (Wilson, 1939, p. 1134—1137). The Red Rock has been dated by uranium-lead methods at 1,715 my. (Silver, 1965), suggesting that a consider- able part of the rocks in the Mazatzal Mountains is younger than the Yavapai Series as now restricted. In the Diamond Butte area a little farther east, on the north flank of the Sierra Ancha, Gastil (1958) has mapped in detail rocks like those in the Mazatzal Moun- tains, forming a 20,000—ft (6,100 in) sequence without top or base, divided into eight named formations, mainly volcanic or volcaniclastic. The apparent equiva- lent of the Mazatzal Quartzite (Houden Formation) is near the middle, and is followed by younger volcanics apparently unrepresented farther west. Other areas of Yavapai-type rocks in central Arizona could be mentioned (see, for example, Livingston and Damon, 1968, p. 765—769), but the above are sufficient to indicate their features, their complexities, and their problems in correlation; further studies are needed be— fore the rocks of the whole area can be integrated into a single picture. PRECAMBRIAN GEOLOGY OF THE UNITED STATES The metamorphic rocks of central Arizona are exten- sively invaded by synkinematic and postkinematic granitic plutons (Xg). In the Prescott-Jerome area and farther west these have uranium-lead ages between 1,760 and 1,775 m.y., suggesting that their intrusion was partly contemporaneous with the later volcanism of the Yavapai Series. Farther east and southeast, as far as the Final Schist area mentioned earlier, similar granites are slightly younger, with ages of about 1,660 my. A later suite of granitic rocks (Ygi), typified by the Ruin and Oracle Granites of the districts between the Sierra Ancha and Tucson, has been dated as between 1,420 and 1,460 m.y. by various methods, and a single body in Weaver Mountain south of Prescott (Ygz) has yielded a uranium-lead age of 1,000 my The deformation, metamorphism, and plutonism of the crystalline basement of central Arizona (and elsewhere in the southern Basin and Range province) preceded the accumulation of the Precambrian Y sup— racrustal rocks described below, which lie on its trun- cated, deeply eroded surface. This represents the “Mazatzal Revolution” (orogeny) of Wilson (1939, p. 1 161). For a time this orogeny was thought to have an Elsonian age of 1,350—1,550 m.y. (Giletti and Damon, 1961, p. 642), but this interpretation was based on insuf- ficient sampling, mainly of postorogenic plutons; its true Hudsonian date is shown by more complete studies to lie between 1,660 and 1,715 my. (Wasserburg and Lanphere, 1965, p. 736; Silver, 1965). CRYS'I'ALIJNI‘I BASEMENT OF SOUTHERN CALIFORNIA (MAINLY PRI‘ZCAMBRIAN X) Crystalline basement like that in Arizona, and again dominated by Hudsonian dates, is exposed in many of the ranges in southern California, from Death Valley on the north, southward through the Mojave Desert, into the Transverse Ranges. In this region, the basement rocks have been more involved in and overprinted by the effects of Phanerozoic orogenies than those farther east. In southern Death Valley Precambrian crystalline basement forms the prominent, rugged part of the Black Mountains on the eastern side, and inliers in the later Precambrian supracrustal rocks (Y and Z) of the Panamint Range on the western side. In the Panamint area, earlier paragneisses and orthogneisses are cut by intrusives which have yielded Hudsonian dates of 1,720—1,780 m.y. by uranium—lead methods on zircons, indicating the minimum age of the whole complex (Silver and others, 1962). Rubidium-strontium and potassium—argon ratios in the rocks have been so dis- turbed by Mesozoic metamorphism that they give unre- liable results (Wasserburg and others, 1964, p. 4400). Smaller, more dispersed outcrops of basement occur SOUTHERN BASIN AND RANGE PROVINCE 112°3o' 34°3U o 1 "1°0 o<° o oo ..o.°...o . ,I_ , °°o 00 _,l\/ :00 °o° ‘ ' \—/\‘-’ 000000000 ’\/l\‘|/\:)\// 00200 |\ _|-/ l o o o o /\’\\:).(g —‘\ o°oo° \7 \f ’I\ \/’ \\\ I l °o "0' 0 / '\ \— / — ‘ \ \ / ,2 o 0 ° ://—\/ PIN/I fl", ‘/'\/i\,|Tl—/l\l\ o00000500 \/\ \, I /\/\ \ \ \ \ 0° °°° 0 D~ ,\l\/ I—/\’\< 1“,\’\‘, (I, 000000 20M|LES 20 KILOMETRES EXPLANATION . o oc'TV°. '0 -. Quaternary Tertiary sediments Tertiary volcanic rocks Paleozoic lntermontane deposits <-‘X\g T YAVAPAI SERIES _ \ 2 1,770—I,820 m.y. Texas Gulch Formation Granodiorite and quartz 7 i 7 Unconformable on ranodiorite diorite l,760—1,770 m.y. - g / '4 E Big Bug Group Ash Creek Group Phanerozoic faults Precambrian faults FIGURE 24.—Map of Prescott-Jerome area, central Arizona, showing relations of Yavapai Series and other Precambrian X rocks. Afizer Anderson and others (1971, p. C4—C6), with Phanerozoic rocks added from other sources. 65 66 farther south in the Mojave Desert. Dating of these rocks by potassium—argon and rubidium—strontium methods indicates Hudsonian metamorphism of the country rock about 1,650 my ago, but a few of the plutons (as in the Marble Mountains) have an Elsonian age of about 1,400 my (Lanphere, 1964, p. 396—397); these have not been separated from the Precambrian X rocks on the Geologic Map. Of greater interest than these are the Precambrian rocks of the Transverse Ranges to the southwest, which are nearer the Pacific Coast than any others in the United States. They are part of the crystalline complex of the rugged San Gabriel and San Bernardino Moun- tains, whose peaks project to altitudes of 10,000 ft (3,000 m) or more. The mountains are upthrust blocks within the network of strike-slip faults of coastal California, and lie on opposite sides of the master San Andreas fault (Dibblee, 1968). The crystalline complexes of the two ranges exhibit a remarkable array of metamorphic and plutonic rocks of diverse ages, including upper Mesozoic eugeosynclinal rocks (Pelona Schist, u Nhe), Paleozoic miogeosynclinal quartzites and marbles (uE’), Mesozoic granitic plutons with ages of 75—90 my. and 160—170 m.y. (Kg), and the upper Paleozoic Mount Lowe Granodiorite with an age of 220 my (E’g3). These lie in a Precambrian matrix, now preserved only in shreds and patches, or ortho- gneiss and paragneiss of granulite facies (Xm), into which a large body of anorthosite (Ya) has been in- truded in the western part of the San Gabriel Moun- tains (Crowell and Walker, 1962, p. 242—261). South of the Tranverse Ranges, across the Los Angeles Basin and San Gorgonio Pass are other crystal- line massifs of the Peninsular Ranges, but their rocks are curiously different, and are all Paleozoic and Mesozoic in origin. In the Transverse Ranges, geologic studies and radiometric dating of the Precambrian rocks (especially by uranium-lead methods on zircons) indicate a com- plex structural and metamorphic history, during both Precambrian time and later (Silver and others, 1963; Silver, 1971). Supracrustal sedimentary and volcanic rocks accumulated between 1,680—1,75O m.y. ago, and were deformed and metamorphosed to amphibolite grade. They were invaded by granodiorite and quartz monzonite 1,650—1,680 m.y. ago, and the whole was subjected to a major orogeny 1,425—1,450 m.y. ago that refolded the rocks and raised them to granulite metamorphic grade. At about 1,220 my ago the body of anorthosite and associated gabbro and syenite was in- truded into the complex; there were no further Pre- cambrian events, but the anorthosite was greatly dis- turbed and sheared during the Phanerozoic (Carter and Silver, 1971). The diverse Precambrian events in the crystalline PRECAMBRIAN GEOLOGY OF THE UNITED STATES rocks of the Transverse Ranges contrast markedly with the simpler events in the regions to the east and north, where there was merely a Hudsonian metamorphism and plutonism, and a rare Elsonian plutonism. The contrast lends credence to the interpretation derived from geological evidence that the rocks of these ranges are far away from their original positions, whence they have been transported by strike-slip movements along the San Andreas and related faults, into the foreign environment of the Pacific border region. Traces of simi- lar rocks occur in the Orocopia Mountains and nearby ranges northeast of the Salton Sea (Crowell and Walker, 1962, p. 222—242), but even these are merely in wedges in the broader fault network. The original sites of all these rocks are still farther southeast, in some region as yet unidentified. SUPRACRUSTAL ROCKS IN ARIZONA (MAINLY PRECAMBRIAN Y) Lying on the truncated and deeply eroded edges of the crystalline basement just described, especially north- eastward toward the cratonic Colorado Plateau, are unmetamorphosed and only lightly deformed supra- crustal sedimentary rocks, with minor interbedded lavas and intrusive diabase. These are shown on the Geologic Map of Arizona of 1969 as Grand Canyon Series, peg; Apache Group, p€a; Troy Quartzite, p€t; and diabase, p€db. On the Geologic Map of the United States all these rocks, including the diabase but exclud- ing the Chuar Group at the eastern end of the Grand Canyon, are grouped together as unit Y. The Chuar, for reasons indicated later, is labeled Z. The supracrustal rocks are exemplified especially by the well-known Grand Canyon Supergroup exposed in the depths of the Grand Canyon, but the Apache Group and Troy Quartzite farther south in Arizona are very much like it and probably correlative in part. As with the Belt Supergroup of the Northern Rocky Mountains, their fresh appearance belies their ancient age, leading to a first impression that they are early Paleozoic—an impression dispelled early in the Grand Canyon by Walcott (1895, p. 313—314), but which persisted much later in central Arizona, until disproved by Darton (1925, p. 34—36). In the frequently visited, prominent exposures in the main segment of the Grand Canyon, the lower part of the Grand Canyon supergroup (Unkar Group) is tilted, block-faulted, and truncated by the flat-lying Middle Cambrian Tapeats Sandstone (fig. 25). The upper part (Chuar Group) in the seldom visited eastern alcoves of the canyon is less faulted and synclinally downwarped. Some of the faults were displaced only during the Pre- cambrian, but others were reactivated later and offset the Paleozoic rocks by varying amounts. The most spec- tacular example is the Butte fault in the eastern part of SOUTHERN BASIN AND RANGE PROVINCE 67 Vishnu Schist / / mi /U/://O///[//CO/ 2 MILES NORTH l l 3 KILOMETRES A Chuar Butte Kaibab Limestone (P) Tapeats Sandstone (C) BUTTE " ‘ FALIJLT | \\ .\, ~ l -\_\\\Sua ai Formation ——d , ' '\+4%1:: //,/ R we I‘Limestone ’ //, / // // / '- \ Muav Limestone // '.-\\\\\ (C) 3000 FEET EAST 1000 METRES U) I; B ,— Lu 2 O 8 i- .—. Basal Paleozoic sandstones (Cambrian in south, Devonian in north) LU . . ~ [L .T‘roy Quartzite_- ca O N o O In D g // “ n ’I H 1 Dripping Spring Quartzite __ ;b— _ _— _ _—_ _ _ Pioneer Shale_ ___. _ _—_ —’—_—___— o a o 4 MILES l I 1 II I I l I C 0 5 KILOMETRES FIGURE 25.—A, Section showing Vishnu Schist and Unkar Group (Precambrian X and Y) in the Shinumo area, Grand Canyon, northern Arizona, and the truncation of their block-faulted structure by Cambrian deposits. Note, however, that the major fault on the right underwent recurrent reversed displacement after Paleozoic time. After Noble (1914, section B—B’). B, Section of Butte fault in eastern Grand Canyon, Ariz., showing Precambrian downthrow to the left and post-Paleozoic downthrow to the right, each accompanied by steep dragging of the beds. After Walcott (1889, p. 53). C, Idealized section, showing disruption and distention of Apache Group and Troy Quartzite by sills and dikes of intrusive diabase. Based on outcrops in the Sierra Ancha and nearby localities, central Arizona. After Shride (1967, p. 67). 68 the canyon, which was downthrown 5,000 ft (1,500 In) to the west in Precambrian time and 2,700 ft (820 m) to the ‘ east after Paleozoic time, each displacement being ac- companied by steep dragging of the beds (Walcott, 1889) l (fig. 25). The Apache Group and Troy Quartzite were little disturbed during Precambrian time, except forl profuse injection of diabase sills and connecting dikes that have much disrupted and greatly distended the 1 sequence (fig. 25). Along the Colorado Plateau margin, as in the Sierra Ancha, they are almost as little dis- ‘ turbed by Phanerozoic movements as in the Grand Canyon, but farther southwest they share the complex block faulting of the succeeding Paleozoic strata. The supracrustal rocks in the Grand Canyon and central Arizona are divisible into persistent, distinctive formations, which are listed in table 4. The Apache Group and Troy Quartzite are obvious equivalents of the Unkar Group and contain identical rocks, but the order of the lithic units is strangely different in the two areas. The Bass Limestone is nearly at the base of the Unkar sequence and the Mescal Limestone is a thousand feet (300 m) or more above the base of the Apache. The red Hakatai Shale is above the Bass and the ,red Pioneer Shale is below the Mescal. The Troy Quartzite is at the top of the central Arizona sequence and the Shinumo Quartzite is beneath thick higher formations of the Unkar Group. Each set of formations persists within its own area, and the reasons for the reversals from one area to the other are not apparent. The differences in thickness of the sequences in the two areas are also of interest; the Apache and Troy are less than half as thick as the Unkar Group. The first two units may be a shelf or platform facies, farther away from the center of the depositional basin than the Unkar. The Troy Quartzite has been poorly understood until recently (Shride, 1967, p. 44—45); its full thickness and subdivisions could only be deciphered from detailed work, which involved untangling the structure pro- duced by the many diabase sills (fig. 21). Even after the Precambrian age of the underlying Apache Group was established, the Troy was long considered to be partly or wholly of Cambrian age, and equivalent to the Middle Cambrian Bolsa Quartzite. Actually, the Troy is over- lain unconformably by the Bolsa, or by sandy phases of the succeeding Cambrian Abrigo Limestone and Devo- nian Martin Limestone (Krieger, 1968). Even though the Precambrian quartzites are everywhere overlain by Paleozoic sandstones and quartzites, the Troy is in— truded by diabase and the higher strata are not; their basal beds frequently contain diabase debris, including cobbles and boulders in a few places. Both the Mescal Limestone and Bass Limestone con- tain stromatolites at several levels; those in the Mescal PRECAMBRIAN GEOLOGiY OF THE UNITED STATES TABLE 4.—Precambrian supracrustal rocks of Arizona Central Arizona (Barton, 1925: Shride, 1967) Grand Canyon (Walcott, 1895; Noble, 1914; Ford and Breed, 1973) Cambrian Cambrian or Devonian Major unconformity Major uncont'ormity Grand Canyon Supergroup Chuar Group, 6.000 ft 12,000 mi Sixty Mile Formation Kwagunt Formation Galeros Formation Disconformity 1'200“ Troy Quartzite, (360 m) maximum Disconl'o r m i t y Unkar Group, 5,500 ft (1,700 m) Nankoweap Formation Apache Group, 1,250—1,600 ft (38(L490 mi Mescal Limestone (basalt flow in upper part) Dripping Spring Quartzite (with Barnes Conglomerate member! Pioneer Shale (with Scanlan Conglomerate Member! Major unconl'ormity Rama erardenasu Basalt Dox Sandstone Shinumo Quartzite Hakatai Shale Bass Limestone Hotauta Conglomerate M aj o r unconformity Crystalline basement Crystalline basement are comparable to a lower Middle Riphean form and to a Middle Riphean to Vendian form of the sequences in the Soviet Union (Cloud and Semikhatov, 1969, p. 1031). Other fossils have been reported in the Arizona supra- crustal rocks, but nearly all of them are inorganic sedimentary structures. Diabase sills in the Apache Group and Troy Quartzite of the Sierra Ancha have been dated by uranium-lead and potassium-argon methods at 1,150—1,200 m.y. (Silver, 1960; Livingston and Damon, 1968, p. 769). The Apache and Troy are older than the diabase and younger than the 1,420—1,460-m.y.-old granitic rocks (Ygi) in the underlying basement (p. 64). Both the diabase sills in the Unkar Group and the Rama (= Cardenas) lavas near the top of the group yield rubidium—strontium ages of about 1,100 m.y.; potassium-argon ages from the same rocks of 800—900 m.y. suggest a later heating event (McKee and Noble, 1974). The Chuar Group, or upper unit of the Grand Canyon Supergroup, is rather different from the supracrustal rocks so far considered. It is a thick body of varicolored argillites, with thin stromatolite-bearing limestones at a dozen or so levels, and occasional beds of chert, oolite, and sandstone (Ford and Breed, 1973). Shales in the upper part of the group contain the small circular car- bonaceous structures Chuaria, once thought to be primitive brachiopods, but now interpreted as crushed spheres of microplanktonic algae (Ford and Breed, 1972). The dating of the preceding Rama (= Cardenas) lava shows that the Chuar is younger than 1,100 my, so that it is either very late Precambrian Y, or even a part of Precambrian Z. On the Geologic Map it is SOUTHERN BASIN AND RANGE PROVINCE speculatively indicated as Z, although this is by no means proved. PAHRUMP GROUP OF EASTERN CALIFORNIA (PRECAMBRIAN Y AND Z) Supracrustal rocks, in part like those in Arizona, reappear in the southern part of the Death Valley area of eastern California, where they form the Pahrump Group (labeled Y on the Geologic Map, although the upper part probably includes rocks of Precambrian Z, as indicated below). The group is preserved in a belt ex- tending 80 mi (130 km) northwestward from the Kings- ton Range east of Death Valley to the Panamint Range west of it, northeast and southwest of which younger strata lie directly on the crystalline basement (Xm) (Wright and Troxel, 1967, p. 938—939). The group is a package of supracrustal rocks par- titioned by unconformities from the older and younger Precambrian below and above, but inhomogeneous in- ternally, and with considerable lateral variation. It is divisible into the Crystal Spring Formation, Beck Spring Dolomite, and Kingston Peak Formation, which total 5,000 ft (1,500 m) thick in the Kingston Range, but reach up to 7,000—8,000 ft (2,100—2,440 m) farther west (Wright, 1968, p. 9—10). The Crystal Spring Formation lies on the basement, is 3,000—4,000 ft (900—1,200 m) thick, and is formed of lithic units much like those in the Unkar and Apache Groups of Arizona, including quartzites and shales below and above, and medial limestones or dolomites with associated chert. It is extensively invaded by diabase sills, one of which has widely altered the medial carbonates to commercial grades of talc. The Beck Spring Dolomite is a massive body that attains 1,000 ft (300 m) in the east, but which wedges out westward and southwestward. The upper unit of the group, or Kingston Peak Forma- tion, differs from any of the supracrustal rocks to the east in Arizona. It is a body 1,000—2,500 ft (300—7 60 m) thick of conglomerate or diamictite and associated shaly or sandy layers, some of which contain widely dispersed dropstones. The diamictites contain small to large clasts of crystalline basement, Crystal Spring and Beck Spring sediments, and diabase like that intruding the Crystal Spring Formation. Within the area of expo- sure the Kingston Peak is slightly unconformable on the underlying parts of the group, but they must have been sharply eroded elsewhere to provide the clasts in the diamictites. The formation is angularly truncated northeastward by the Noonday Dolomite at the base of the main Precambrian Z sequence, but elsewhere the discordance is slight or nonevident. Stromatolites occur in both the Crystal Spring and Beck Spring carbonates; those in the former are com- 69 parable to forms in the Middle Riphean to lower Upper Riphean of the Soviet Union. Stromatolites in the Beck Spring are associated with eucaryotic nannofossils, in- dicating the very early existence here of precursors of the metazoans (Cloud and others, 1969). No reliable radiometric dates have been obtained on the rocks of the Pahrump Group or the diabase intrusives in the Crystal Spring, but the two lower formations are quite compar- able to the Unkar Group, the Apache Group, and the Troy Quartzite in Arizona, and like them may have an age of about 1,100—1,42O m.y. The diamictites of the Kingston Peak Formation have much the same character as the diamictites farther north in the Cordilleran province (Mineral Fork, Toby, etc.), and likewise may be of direct or indirect glacial derivation (Johnson, 1957, p. 368—369; Crittenden and others, 1972, p. 339). Like the comparable deposits farther north, they are probably to be assigned to the early part of Precambrian Z; here, however, they are unconformable with the main body of Precambrian Z above. PRECAMBRIAN OF WESTERN TEXAS (MAINLY PRECAMBRIAN Y) In the Basin and Range province east of southern Arizona, in southwestern New Mexico, and western Texas, small outcrops of Precambrian rocks occur in the structurally higher parts of the ranges, in a terrain otherwise dominated by Phanerozoic rocks. Those in New Mexico are mainly Precambrian X metamorphic and plutonic rocks, but those in Texas are more varied and of younger ages, including supracrustal rocks of Precambrian Y that are 250 mi (400 km) or more east of those in Arizona. In Texas, Precambrian is exposed in the Franklin Mountains north of El Paso at the extreme western end of the State, in the Van Horn area 100 mi (160 km) farther southeast, and in two small patches in the inter- vening area. Near Van Horn, Precambrian rocks (shown on the Geologic Map as X, Y, and Z) emerge in several fault blocks in an area of about 225 mi2 (580 kmz) sometimes rather inappropriately called the "Van Horn dome” (fig. 26). In the Franklin Mountains they are almost as varied as at Van Horn (although marked only as Ygz on the Geologic Map), but are exposed only in a narrow 14-mi (23 km) strip along the east face of the range. The rock sequences in the two areas are shown in table 5. In western Texas and southeastern New Mexico the next youngest unit above the Precambrian is the Bliss Sandstone of latest Cambrian or earliest Ordovician age, but this is not preserved everywhere, and elsewhere the Precambrian is followed directly by upper Paleozoic or even Cretaceous strata. In the PRECAMBRIAN GEOLOGY OF THE UNITED STATES 70 Franklin Mountains the Precambrian supracrustal rocks are disrupted by the Precambrian granitic intru— sive, yet the inclination of their strata conforms closely to that of the overlying Paleozoic. In the Van Horn area 31 °——O EXPLANATION Quaternary intermontane Cretaceous sedimentary rocks, in places overlain by Tertiary 20 MILES J I 20 KILOMETRES Permian and Ordovician carbonate rocks. Basal Ordovician Bliss Sandstone shown where deposits volcanic rocks PRECAMBRIAN ”659”“ I | | \x s s Fifi/4’3 an»: $52” Hazel Formation Northern limit of strong deformation in Precam- brian Y rocks Van Horn Sandstone Allamoore Formation Low-angle fault of Precam- Rhyolite intrusive into Carrizo Mountain Formation v-v—v High-angle faults, mainly of brian age Cenozoic age FIGURE 26.—Map of Van Horn area, west Texas, showing Precambrian rocks, and their relations to surrounding Phanerozoic rocks. Compiled from King and Flawn (1953), and other sources. Carrizo Mountain Formation SOUTHERN BASIN AND RANGE PROVINCE TABLE 5.—Precambrian rocks of western Texas [Symbols on left are those used on Geologic Map of United States] Franklin Mountains (Harbour. 1960: p. 11—12: Harbour, 1972! \'an Horn area (King and Flawn, 1953; Flawn and Muehlberger, 1970, p. 8&1071 Bliss Sandstone (Lower Ordovician and Upper Cambrian! le Bliss Sandstone (Lower Ordovician» ”’7. Structural unconformity Structural unconl'ormity Z Van Horn Sandstone, 800 ft (260 ml Structural unconformity 7 Hazel Formation. 5,000? ft (1,500? m) (red sandstone, with conglomerate below) Ygl Granite Unconformity Intrusive contact _ Rhyolite extrusives, 1.800 ft (600 m) Lanoria Quartzite, 2,600 ft (790 ml Mundy Breccia, 07190 ft (0.63 m! Allamoore Formation, 3,000? It (900? (basalt agglomeratel m) (limestone, volcaniclastic sedi- ments. lavas, and diabase intrusivesr Castner Limestone, 1,100 ft (350 ml (with diabase sills) Sequence broken Carrizo Mountain Formation. 19,000 (1 I5.800 ml minimum (clastic metased- X iments. intruded by Sllls of metarhyolite and metagabbrol Base not exposed Base not exposed Not shown on Geologic Map all the Precambrian supracrustal rocks except the Van Horn Sandstone have been orogenically deformed in what has been termed the “Van Horn mobile belt” (Flawn, 1956, p. 32)—in contrast to those in the Franklin Mountains and those farther west that have been discussed earlier. Within the mobile belt the Carrizo Mountain Forma- tion is to the south and is followed successively north- ward by the Allamoore and Hazel Formations; the Van Horn Sandstone is a postorogenic deposit that lies indis- criminately on the rest. However, the Carrizo Mountain metasediments are not in contact with the Allamoore, but are separated from it by large intrusive bodies of metarhyolite which adjoin the Allamoore along a major low-angle fault, the Streeruwitz thrust. For about 3 mi (5 km) north of the thrust trace the Allamoore and Hazel are strongly folded and thrust, but the deforma- tion decreases rapidly beyond, and the Hazel in its northern exposures is nearly horizontal (fig. 27). Metamorphism also decreases northward. The Car- rizo Mountain Formation is of amphibolite grade in its southern exposures and contains much pegmatite; in its northern exposures it is of greenschist grade but it has been retrograded near the Streeruwitz thrust, and the rhyolite along the thrust has been converted to mylo- nite with conspicuous south-plunging lineation. The Allamoore Formation has been hydrothermally altered 71 to jasperoid close to the thrust, and some of the lime- stone layers farther north have been selectively con- verted to talc by the same process; blue alkali am- phibole and white asbestiform amphibole (richterite) occur in places (Rohrbacher, 1973, p. 6—13). Elsewhere in the disturbed belt neither the Allamoore nor the Hazel Formation are much metamorphosed, although some of their weaker layers show marked slaty cleav- age. Traditionally, the Carrizo Mountain Formation has been considered the oldest unit in the sequence, and on the Geologic Map this presumed age has been expressed speculatively by classifying it as Precambrian X. How- ever, there is little confirmation of this in the known geologic and radiometric data; alternatively, the Car- rizo Mountain may originally have been a conformable downward sequence beneath the Allamoore, or it might have been a more internal, eugeosynclinal facies of the Allamoore (Flawn and Muehlberger, 1970, p. 105—106). The Streeruwitz thrust might even have been a major suture in the Precambrian terrane that juxtaposed con- trasting sequences which were originally far apart, but exposures are too limited for proof of this possibility. The limestone of the Allamoore is identical with that of the Castner in the Franklin Mountains, and both contain stromatolitic layers. Both, in turn, strikingly resemble the limestones of the Mescal and Bass in Arizona and the Crystal Spring in California. The talc deposits in the Allamoore, like those in the Crystal Spring, are commercially productive, and are being mined on a large scale (Rohrbacher, 1973, p. 1). The Hazel and Allamoore Formations are intricately folded together in the deformed belt north of the Streeruwitz thrust, but the two are mostly separated by zones of shearing and thrusting, so that their original contact is seldom preserved. It must have been uncon- formable, because the lower part of the Hazel is a con- glomerate composed largely of clasts derived from the Allamoore: limestones (including a few marmorized pieces), and the lavas and mafic intrusives. Besides these, the conglomerate contains a few clasts of red granite and rhyolite porphyry like those in the Franklin Mountains and elsewhere north of the Van Horn area, implying that the Hazel is not only younger than these, but younger than all the supracrustal formations in the Franklin Mountains. The Hazel Formation is a very thick deposit of two contrasting facies: coarse, poorly sorted, poorly rounded conglomerates below, and fine-grained, almost silty, thinly laminated red sandstones above. Passage from one facies to the other is by interbedding, yet they are seldom intergradational—few of the conglomerates have a red sandy matrix, and few of the sandstones are pebbly. It is tempting to compare these conglomerates with the diamictites of the Kingston Peak Formation of 72 / , / Streeruwntz /thrust CARRIZO MTS /;/ PRECAMBRIAN GEOLOGY OF THE UNITED STATES BEACH MTN 4M|LES NORTH | l J 5 KILOMETRES FIGURE 27.—Synoptic section across Precambrian rocks of Van Horn area, west Texas, showing structural relations of the different units and their implications in the Precambrian history of the area. Letter symbols are the same as those on fig. 26; black lenses in unit Yr are mafic intrusives. After King (in King and Flawn, 1953, p. 104). California, but verification requires further field review. Whatever the relations between the Allamoore and Hazel may have been, the climactic orogeny in the Van Horn mobile belt came later, after the deposition of the Hazel Formation. This orogeny resulted in the north- ward emplacement of the Carrizo Mountain Formation and its intrusive rhyolites along the Streeruwitz thrust, their retrograde metamorphism, and the deformation of the Allamoore and Hazel immediately to the north. By this deformation the Allamoore was thrown into north- facing recumbent folds and thrust over the Hazel. Be— sides these fold and thrust structures there are some curious patches farther north of highly crumpled Al- lamoore resting on nearly flat-lying Hazel that may have been emplaced during the orogeny as detached gravity slides. Radiometric data on the west Texas Precambrian are incomplete, but partly clarify some of its geologic and orogenic problems (Wasserburg and others, 1962, p. 4023—4031). Radiometric determinations by potassium-argon, rubidium-strontium, and a few by uranium-lead methods have been made on the granites of the Franklin Mountains and the nearby Hueco Mountains, on rhyolites from the Pump Station Hills north of the Van Horn area, and on metarhyolites and pegmatites in the Carrizo Mountain Formation; all yield dates of about 1,100 my The dates define a widespread igneous event that is younger than any of the supracrustal rocks of the Franklin Mountains, and by implication younger than the Allamoore Formation of the Van Horn area. On the other hand, the event must have been older than the Hazel Formation, which contains a few clasts of the felsic igneous rocks, and it is therefore also earlier than the climactic orogeny of the Van Horn mobile belt. The 1,100 my dates are comparable to the dates determined on mafic intrusives in the Precambrian Y supracrustal rocks of Arizona, and they are also com— parable to the dates obtained on the infracrustal metamorphic and plutonic rocks of the Llano uplift, 300 mi (480 km) to the east in central Texas (p. 42); similar dates have been obtained even nearer at hand from basement rocks of the “Texas craton” that have been drilled into west of the Llano uplift (Wasserburg and others, 1962, p. 4035—4036). The west Texas region thus marks the closest approach in the western United States of Precambrian Y supracrustal rocks to infra- crustal rocks of the Grenvillian orogenic belt. The Van Horn mobile belt exposed in the Van Horn area is a tantalizingly small segment of what must be a major tectonic feature of the Precambrian in this part of North America, but one whose further extent and trend are unknown. The succeeding Van Horn Sandstone is postorogenic, and lies with right-angled unconformity on all the ear- lier Precambrian formations; it is a red, arkosic, coarse, conglomeratic, continental deposit, probably laid down on compound alluvial fans that were largely fed from highlands to the north (McGowen and Groat, 1971). Its conglomerates contain clasts of the Allamoore and Hazel and of the mylonitized rhyolites from the upper plate of the Streeruwitz thrust to the south. However, the most prominent components are rounded cobbles and boulders of red granite and rhyolite porphyry like those exposed in the Precambrian areas to the north- west. The Van Horn is tilted at low angles in various directions rather than folded, and it was block-faulted and beveled before the basal Ordovician Bliss Sandstone was deposited on it. In older reports the for- mation was classed as Cambrian, but it is quite unlike any Cambrian elsewhere in the Southwestern States, and is almost certainly late Precambrian; on the Geologic Map it is marked as Precambrian Z. PRECAMBRIAN Z SUPRACRUSTAL ROCKS OF WESTERN BASIN AND RANGE PROVINCE Besides the Precambrian supracrustal rocks so far considered, another great sedimentary body in south- SOUTHERN BASIN AND RANGE PROVINCE ern Nevada and eastern California extends conform- ably through Precambrian Z and the Lower Cambrian. It is exposed in many of the ranges from the Spring Mountains near Las Vegas westward beyond Death Valley, where it is 13,000 ft (4,000 m) thick between the Pahrump Group and crystalline basement below, and the Middle Cambrian above (Wright and Troxel, 1966). Farther northwest it is exposed in the Inyo and White Mountains of California and adjacent Esmerelda County, Nevada, where it is as much as 21,000 ft (6,400 m) thick without visible base (Nelson, 1962); this includes Walcott’s type Waucoban Series (=Lower Cambrian Series). Approximately the upper third of the sequence contains diagnostic Lower Cambrian fossils; traces of fossils occur in beds lower down, but most.of them are barren; the proper level of the Precambrian- Cambrian boundary in the sequence is problematical (see below). A comprehensive review has been made by Stewart ' (1970) of the stratigraphy of the units in this rock body, with results summarized in table 6. As indicated by the table, the recognizable formations in the sequence fall naturally into three belts from east to west (or southeast to northwest), in each of which is a set of widely recognizable rock units, that cannot be traced directly into the units of the other belts because of disconnected exposures. Hence there are some uncer- tainties as to correlation, although fairly satisfactory results can be obtained by matching successive meas- ured stratigraphic sections. On the Geologic Map of the United States the lower part of the sequence is indicated as Z and the upper part is included in unit 6. Compilation of the map was com~ pleted before the results of Stewart’s survey became available, and was based on different assumptions. The N oonday Dolomite and Stirling Quartzite of the central belt were thought to be correlative with the lithically similar Reed Dolomite and Campito Formation (= Sandstone) of the much thicker western sequence, whereas Stewart places both of the last two at a higher stratigraphic level. Moreover, it was assumed that the bases of the Stirling and Campito were a “natural” base of the Cambrian; whereas Stewart places the base of the Cambrian higher up, showing not only that the bound- aries in the two areas are not correlative, but that no “natural” boundary exists in a conformable sequence of this kind. These discrepancies, while seemingly funda- mental, actually do not greatly distort the representa- tion on the small scale of the Geologic Map. The Precambrian Z-Lower Cambrian supracrustal body of the western Basin and Range province is a great sedimentary wedge that was built along the western edge of the North American continent in much the same manner as the Precambrian Y supracrustal Belt de- posits were built farther north in the Cordillera several 73 TABLE 6,—Precambrian Z—Lower Cambrian formations in western Basin Range Province. [Based on Stewart 11970, p. 6!. Double line is base ofCambrian on US. Map; dashed line from Stewart] Western belt Central belt Eastern belt Middle Cambrian Middle Cambrian Middle Cambrian Mule Spring Limestone Bright Angel Shale Carrara Formation Saline Valley Formation Tapeats Sandstone Zahriskie Quartmte Harkless Formation Unconformity Poleta Formation Wood Canyon Campito Formation m: Formation Hiatus Deep Spring Formation Reed Dolomite Stirling Quartzite Wyman Formation Johnnie Formation Base not exposed Noonday Dolomite Unconformity Pahrump Group and crystalline basement Crystalline basement hundred million years earlier. Like the Belt deposits, it was derived from sedimentary waste derived from the craton, which accumulated to great thickness in a tec- tonically quiet regime (Stewart, 1970, p. 64—66). The wedge thickens from a few hundred feet in the Grand Canyon and elsewhere along the edge of the Colorado Plateau to more than 21,000 ft (6,400 m) in the western belt 175 mi (280 km) distant. In the central belt are thick units of quartzite and fine conglomerate that per- sist for long distances north-south along the strati- graphic strike, but which fade in the western belt, in the thickest part of the wedge, into fine-grained sandstone, intertongued with siltstone, shaly siltstone, and carbo- nate rocks (fig. 28). The problem of the Precambrian-Cambrian boundary in this deposit is more acute than in any other part of the United States. Fossil control disappears downward in a conformable sequence, in which no “natural” sedimen- tary separation exists. In most of the country there is no problem, as Precambrian and Phanerozoic rocks are separated by prominent unconformities and large hiatuses. Even on the opposite side of the continent, in the Southern Appalachians, where both Precambrian Z and Lower Cambrian are again represented, there is in most places a rather obvious “natural” boundary at the base of the Chilhowee Group. In the deposits in the western Basin and Range prov- ince, olenellid trilobites, archeocyathids, and other 74 WESTERN BELT CENTRAL BELT PRECAMBRIAN GEOLOGY OF THE UNITED STATES EASTERN BELT MIDDLE Inyo Mountains Death Valley Nopah Range Las Vegas Colorado Plateau CAMBRIAN Monola Emigrant Mule Spring 2 Fm Fm /Limestone Bonanza King Dolomite ES 1‘- o P . “’0: Wall? Muav Limestone ECO— Bright Angel Shale 43 Tapeats Sandstone o . METRES FEET 5000 """ 15,000 N Z S g Noonday E Dolomite < 0 NJ I u. Base 0 0 not exposed LITHOLOGIES (Greatly generalized) VIA Dolomite and limestone Shale and minor carbonate beds Sandstone and siltstone El Lowest occurrence of diagnostic Cambrian fossils Quartzite FIGURE 28.—Stratigraphic diagram showing relations between late Precambrian (Z) and Lower Cambrian units exposed in different areas northwestward across the western Basin and Range province, from the edge of the Colorado Plateau east of Las Vegas, Nevada, to the Inyo Mountains, California. Compiled from Stewart (1970, pl. 2—3). Length of area about 240 mi (400 km). diagnostic fossils of the Lower Cambrian are fairly abundant in the upper part, down to the middle of the Wood Canyon Formation in the central belt and the middle of the Campito Formation in the western belt, possibly at nearly the same stratigraphic level. This level is used by Stewart (197 0, p. 7) to define the base of the Cambrian, and this may be the best practical solu— tion in a situation of this kind. Nevertheless, indications of metazoan life extend some distance lower. The lower half of the Wood Canyon in the central belt contains fossil tracks and worm bor- ings. The middle part of the Deep Spring Formation in the western belt contains Rusophycus and Cruziana, which are sitz—marks and crawl—tracks formed by trilo- bites and other arthropods, that resemble markings in proved Cambrian strata (Cloud and Nelson, 1966, p. 766—768). About 350 ft (105 m) lower in the formation is a ribbed shell like the problematical genus Pteridin- ium (=Plagi0gomus) which occurs in the Ediacaran, Vendian, and related latest Precambrian units of the Eastern Hemisphere. Near the boundary between the Deep Spring and the Reed Dolomite, 600 ft (180 m) beneath, is the mollusklike shell Wyattia, resembling globorilids found in Cambrian rocks. Below the strata in which these remains occur, valid fossil control vanishes; tubular structures of probable algal origin occur in the Noonday Dolomite (Stewart, 1970, p. 15), and the presence of eucaryotic nannofossils in the Beck Spring Dolomite of the Pahrump Group has already been noted; but both of these can be confidently relegated to the Precambrian. In summary, part of the sequence under discussion is clearly Precambrian Z and part is clearly Lower Cam- brian, but there is no obvious boundary between them. Whatever boundary or boundaries might be selected depend less on the data afforded by the rocks themselves than on the predilections of individual stratigraphers. DISCUSSION AND SYNTHESIS The purpose of the preceding review has been to out- line the regional features of the Precambrian rocks of the United States, insofar as they relate to representa- tion of their outcrops on the Geologic Map of the United States. By its very nature the review is thus not a philosophical or speculative treatise on the Precam- brian rocks or the history that they imply. Neverthe- less, some generalizations emerge that can be sum- marized here. It is apparent from the review that the Precambrian of North America (and specifically the Precambrian of Canada and the United States) is not an indecipherable complex of rocks older than the earliest stratified and fossiliferous Phanerozoic rocks. Nor is it an “Archean” complex of crystalline rocks and a "Proterozoic” or “Al- gonkian” body of less deformed and metamorphosed DISCUSSION AND SYNTHESIS stratified rocks—or, in other terms, an "early” and a “late” Precambrian. Radiometric dating, whatever its defects and pitfalls in detail, has greatly amplified and refined the picture, which will continue to be improved in the future. Using this and other criteria, the Pre- cambrian can now be subdivided and correlated from one region to another, and the results can be rep- resented on regional geologic maps, such as those of Canada (1969) and the United States. Radiometric dating underscores the great length of Precambrian time—from more than 4,000 m.y. ago to about 600 m.y. ago, or about seven times the length of Phanerozoic time. During this vast interval the earth evolved from its primitive state to one more like that of modern times, with changes in the crust, the hydro- sphere, and the atmosphere that influenced the nature of geologic processes (Cloud, 1968, p. 48—51). Never- theless, the basic laws of matter and energy existed throughout, so that uniformitarian principles apply, at least in modified form. Thus, as during the Phanerozoic, processes of defor- mation and plutonism operated in orogenic belts at the same times as cratonic conditions existed elsewhere, and there were no universal Precambrian orogenies, as was formerly believed. Also, if processes of plate tec- tonics operated during Phanerozoic time, they must have existed during Precambrian time as well, al- though the obscurity of the record in these ancient rocks precludes the nature of these processes from being more than speculative. Rates of volcanic and sedimentary accumulation could not have been drastically different from those of Phanerozoic time. It follows that sequences of Precam— brian supracrustal rocks, although voluminous in many areas, can only record small samples of the inordinately long span of Precambrian time. The Precambrian se- quences in supposedly typical areas, such as the Lake Superior Region, must contain many gaps that are probably represented by volcanism and sedimentation in other areas. Radiometric dating of Precambrian rocks indicates that there are peaks of abundance of dates during spans of several hundred million years, between which there are spans as long or longer with few or no dates. The times of abundance express the Kenoran, Hudsonian, Elsonian, Grenvillian, and Avalonian events of Canada and the United States. These events have been inter- preted as orogenies, but most of them more likely repre- sent orogenic eras or cycles, like the Appalachian and Cordilleran orogenic cycles during Phanerozoic time. As during the Phanerozoic, the effects of the cycles are concentrated in provinces or belts, where the dates are mainly the products of infracrustal metamorphism and plutonism. Comparable dates, if present outside these 75 belts, express merely anorogenic or cratonic processes, such as volcanism, sedimentation, and stray intrusive activity. Various maps showing radiometric age provinces of parts or all of North America have been compiled (for example, Gastil, 1960, p. 10; Engel, 1963, p. 146; Gold- ich and others, 1966, p. 5386; King, 1969, p. 38—39). Outside the shield, where exposures are less continuous and more reliance must be placed on subsurface data, these maps are sometimes misleading in detail, because they fail to discriminate between dates of orogenic and anorogenic origin. More expressive, although much more subjective, are sequential maps showing inferred conditions during different parts of Precambrian time (fig. 29). The maps contribute some evidence, but only partial answers to the question of the evolution of the North American continent. How it was originally created and how it grew has been debated. Some of its continental crust must be very ancient ("Precambrian V” or “Katarchean”); rocks older than 3,200 m.y. have been dated radiometrically in southwestern Minnesota, along the Montana-Wyoming border, and in southwest- ern Greenland (marked by black triangles in fig. 29A). _ Other areas of very ancient rocks are suspected elsewhere from geologic evidence but as yet lack radiometric proof. Elsewhere, the main body of Pre- cambrian rocks is younger, Precambrian W (“Archean”) or later. One proposed model of the “Katarchean” and “Archean” rocks is that they were "components of emerging proto-cratons and interspersed, subparallel, relatively simatic orogenic belts, presumably involving oceanic spreading centers, arcs, and interarc basins, and subduction zones. By 2,500 m.y. B.P., however, the more ‘granitic’ proto-cratons converged, telescoping many oceanic, arc-interarc, and borderland environ- ments into subparallel series of synclinoidal ‘greenstone’ belts” (Engel and others, 1974, p. 843; see also Engel, 1963, p. 146—149; Goodwin, 1974). By the end of Precambrian W time, cratons had been stabilized by the processes referred to in the Superior province in the center of the continent, and in the Slave and Wyoming provinces to the northwest and southeast (fig. 29A). Precambrian W rocks have also been recog- nized in the Churchill and Grenville provinces of the Canadian Shield, but they were reworked by sub- sequent orogenies and not stabilized until later. Follow- ing the Kenoran event at the end of Precambrian W time, progressively larger areas of the continent were converted into craton. Stabilization of a province is in- dicated not only by its internal plutonic and metamor- phic history, but also by unconformable overlaps of younger deposits along its edges—for example, the overlap of Precambrian X rocks around the edges of the 76 PRECAMBRIAN GEOLOGY OF THE UNITED STATES Superior province (fig. 298). The last part of the Pre- phic and plutonic activity occurred at a time when the cambrian continent to attain stability was the Grenvil- remainder of the continent was craton. The contrast is lian belt on the southeastern margin, whose metamor- dramatic between the Grenville orogenic belt and the o v . 1,600 m.y. A i B a son 1000 MILES o 500 iooo MILES FIJ'l—[Ll—‘l—Q—T—l HWH—gr—ég o 500 1000 KILOMETRES 0 son 1000 KILOMETRES 150° 140° 130° 120°1io°1oo° 90° 30° 70° 60" 50° 40" 150° 140° 130° izo°no°ioo° 90" so“ 70" 60° 50° 40" ' , r3. ,. ~ l . esw ’¥~ ‘. ‘Ly‘i ‘. V r 2,» _ as ‘oo V unamp‘m 10° 30° 900 m.y. i D 500 1000 MILES 0 500 1000 MILES weasel—#4 0 500 1000 KILOMETRES 0 500 1000 KILOMETRES FIGURE 29.—Maps of the United States and parts of Canada and Mexico, showing evolution of the North American continent during Precambrian time: A, At close of Precambrian W (following Kenoran event, about 2,500 m.y. ago). B, At close of Precambrian X (following Hudsonian event, about 1,600 m.y. ago). C, Near middle of Precambrian Y (following Elsonian event, about 1,350 m.y. ago). D, Near close of Precambrian Y (following Grenvillian event, about 900 m.y. ago).E, At end of Precambrian (about 600 m.y. ago). No provisions have been made for possible later tectonic distortions. The maps are similar to those of Muehlberger and others (1967, p. 2374—2377), but have been greatly modified from later data, and from predilections of the present author. DISCUSSION AND SYNTHESIS 77 little-disturbed great sedimentary embankment of the Belt Supergroup along the opposite western margin of the continent (fig. 29D). No Kenoran dates are known in the southern part of the continent, south of Wisconsin and Wyoming (fig. 29A), where all the dates on the crystalline rocks are ° 140° 130° lzo°uo°loo° 9o°so° 70° so“ 50° 40° 150 ~ Avalonian belt, age 550—700 m.y., at- tached to North America during ‘ Paleozoic tim- 0 500 1000 MILES 0 500 1000 KILOMETRES EXPLANATION \q 6V0 - Outlines of Precambrian outcrops Unconformable overlap of supra- crustal deposits on older cra- \ / tons (dip symbols show homo- \,/ clinal structure Outer known limits of Precam- brian rocks (and approximate, edge at" continent at end of Precambrian time) a- Orogenic areas (that were mobile — during the event shown on map; .. trends of folding indicated in part) Faults (that were probably active during the event shown on map ’— a A Rocks older than 3,200m.y.(where proved by radiometric dating) Areas affected by 1,350 my (Elsonian) event Other areas probably formed of Precambrian rocks that are older than the event shown on the map A ‘90: Granite and anorthosite plutons Metamorphic geosynclinal rocks _ in Grenville orogenic belt Cratonic areas (stabilized during preceding events) Supracrustal sedimentary and vol- canic deposits (continental deposits stippled) o 0 “Fl: Occurences of diamictites (tillites at least in part) FIGURE 29.—Continued. younger (fig. 293). There is a strong possibility that no rocks of Precambrian W (“Archean”) ever existed in much of the southern area, suggesting that this part was added to the continent after the Kenoran event. Similarly, there is a notable absence, in surface or subsurface, of any Precambrian rocks in a large area in the western United States, west of the line shown in figure 30, and here there is much evidence that the crust was oceanic during Precambrian time, and was not made into continent until Paleozoic time or later. Even the westward projection of known Precambrian rocks nearly to the Pacific Coast in southern California prob- ably reached its present position by shifts of crustal blocks late in Phanerozoic time. In the maps of figure 29 this line is shown as the approximate western edge of the North American con- tinent at the end of Precambrian time. Similar lines are shown on the maps along the southern and southeast- ern sides of the continent. The boundary on the south indicates the margin of the Paleozoic Ouachita orogenic belt, where no Precambrian basement has been proved; possibly an original continental crust in this area has been removed by drift during Phanerozoic time to a position south of the Gulf of Mexico. The boundary on the southeast indicates the outer known limit of rocks of the Grenvillian belt; it is true that Precambrian rocks of younger ages in the Avalonian belt lie beyond this through much of the length of the Appalachian chain, but these were probably added to the continent by plate collision during the Phanerozoic. Available evidence indicates that after the Kenoran event the North American continent was a cohesive body, gradually enlarging by accretion—whatever its movements or its relations in space may have been to other continental plates. The only clear indication of an addition to the continent by plate collision is that of the Avalonian belt just referred to. ‘ Final Precambrian time (Precambrian Z or “Hadryn- ian”) has been poorly appreciated because of its scanty representation in the Central Interior—at most perhaps by continental deposits like the Bayfield Group of the Lake Superior Region, and by part of the volcanic and elastic rocks in the Wichita trough farther south (fig. 29E). Major depositional events had now shifted to the eastern and western margins of the continent, in the Appalachian and Cordilleran belts, where marine sed- iments and minor volcanics accumulated, forming sequences quite as impressive as those of earlier Pre- cambrian times, that lead upward with only slight in- terruption into the Paleozoic geosynclinal deposits. Precambrian Z deposits on the east are less mature than those of the succeeding Paleozoic, indicating accumula- tion under conditions of some tectonic disturbance. Those on the west, especially those now preserved in the 78 PRECAMBRIAN GEOLOGY OF THE UNITE]? STATES OREGON PRECAMBRIAN UNKNOWN \ IN OUTCROP \ ‘~ I I AND SUBSU£F\ACE ‘\\j \‘\ l I / I I / NEVADA / \\ /e [”9 / UTAH ' CALIFORNIA l W 9 I 4,] / II PRECAMBRIAN BENEATH' \\ / 7 I , ” \J / PHANEROZOIC \‘3 I STRATA 200 MILES | J I 4 | 200 Kl LOMETRES °——° EXPLANATION W/A Outcrops of supracrustal rocks (Precambrian Y and Z) Outcrops of metamorphic and plutonic rocks (Precambrian W and X) Western known limit of Precambrian rocks FIGURE 30.——Map of western United States, showing western known extent of Precambrian rocks. REFERENCES CITED western Basin and Range province, formed under condi- tions of crustal stability quite the equal of those accom- panying the earlier Belt sedimentation in the same region. Diamictites occur near the base of the Precambrian Z deposits throughout much of their extent in the Cordil- leran belt on the west, and at two localities in the Ap- palachian belt on the east. It is tempting to correlate these with the extensive glacial deposits of late Pre- cambrian time that have been proved in many of the other continents, and to consider all of them as a possi- ble time marker. Absolute proof of glacial origin is not available for all the diamictites in the United States and Canada, and the known pole positions of the time do not accord well with the supposed refrigeration. Nevertheless, the regional extent of the deposits and the variety of their component clasts point to control- ling conditions quite different from mere mudslides or other locally triggered deposits. The much debated question of the boundary between the Precambrian and the Cambrian need not concern us greatly here. Throughout the Central Interior and the eastern part of the Cordilleran belt rocks younger than earliest Cambrian lie unconformably on Precambrian rocks, which are partly supracrustal, but in more places a crystalline infracrustal basement. The question of the boundary only arises in the interiors of the Appalachian and Cordilleran belts, where the latest Precambrian (Z) and the Cambrian are present in the same sequences. Here, the rocks were orogenically deformed during Phanerozoic time and the outcrop bands of the debated rocks are very narrow, so that for purposes of the Geologic Map of the United States the question can be disregarded. A significant point is that Precambrian and Cambrian are not necessarily unconformable (as they are in the craton), and that no "Lipalian” or lost interval separates them. ACKNOWLEDGMENTS This review could not have been made without the information, aid, and counsel of those of my colleagues on the staff of the Geological Survey who are also in- terested in the Precambrian rocks of the United States. Among the many who have so contributed are Charles A. Anderson, Helen M. Beikman, Max D. Crittenden, Jr., Harold L. James, Zell E. Peterman, Gershon D. Robinson, and John H. Stewart, some of whom have reviewed parts or all of the manuscript during its vari- ous stages of evolution. Needless to say, however, the report is neither a collective statement of their opinions, nor a definitive official statement of the US. Geological Survey; I assume sole responsibility for the views ex- pressed. 79 I am also deeply grateful for the advice and inspira- tion I have received, during or before the preparation of the report, from Preston E. Cloud, Jr., of the University of California, Santa Barbara, and Clifford H. Stockwell of the Geological Survey of Canada, whose larger in- sights into Precambrian problems have done much to sharpen my own perceptions. REFERENCES CITED Aalto, K. R., 1971, Glacial marine sedimentation and stratigraphy of the Toby Conglomerate (Upper Proterozoic), southeastern British Columbia, northwestern Idaho, and northeastern Washington: Canadian Jour. Earth Sci., v. 8, no. 7, p. 753—787. Adams, F D., Bell, Robert, Lane, A. C., Leith, C. 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W., 1967, Isotopic age and geologic relationships of the Little Elk Granite, northern Black Hills, South Dakota, in Geological Survey research 1967: US. Geol. Survey Prof. Paper 575—D, p. D157—D163. ‘ r.y[ lr/ 5/ 7 DAYS @g76 F0 «1‘ 903 QARTH'The Paleozoic and Mesozoic Rocks; :lENCES .IBRAE‘V MsA Discussion to Accompany the Geologic Map of the United States GEOLOGICAL SURVEY PROFESSIONAL PAPER 903 The Paleozoic and Mesozoic Rocks; A Discussion to Accompany the Geologic Map of the United States By PHILIP B. KING and HELEN M.'BEIKMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 903 i 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 King, Philip Burke, 1903— The Paleozoic and Mesozoic rocks. (Geological Survey Professional Paper 903) Bibliography: p. 72—76. 1. Geology, Stratigraphic—Paleozoic. 2. Geology, Stratigraphic—Mesozoic. 3. Geology—United States. I. King, Philip Burke, 1903— Geologic map of the United States. II. Beikman, Helen M., joint author. III. Title. IV. Series: United States Geological Survey Professional Paper 903. QE654.K54 551.7'2'0974 76—21806 For sale by the Superintendent of Documents, US. Government Printing Oflice Washington, DC. 20402 Stock Number 024—001—02843-3 CONTENTS Page Page Abstract __________________________________________________ 1 Paleozoic plutonic rocks—Continued Introduction ______________________________________________ 1 Lower Paleozoic granitic rocks (Pgl) __________________ 38 Cambrian ________________________________________________ 1 Middle Paleozoic granitic rocks ( Pg2) __________________ 39 Marine stratified rocks (6) ____________________________ 2 Upper Paleozoic granitic rocks (Pga) __________________ 39 Eugeosynclinal deposits (6e) __________________________ 4 Paleozoic mafic intrusives (E’mi) ______________________ 42 Ordovician and Cambrian __________________________________ 7 Metamorphic complexes ____________________________________ 42 Marine stratified rocks (0-6) ___________________________ 7 Triassic and Permian ______________________________________ 42 Ordovician ________________________________________________ 7 Eugeosynclinal deposits (r74) __________________________ 42 Marine stratified rocks (0) ____________________________ 7 Triassic __________________________________________________ 43 Eugeosynclinal deposits (0e) __________________________ 9 Colorado Plateau ______________________________________ 43 Silurian and Ordovician __________________________________ 10 Great Plains __________________________________________ 46 Eugeosynclinal deposits (SOe) __________________________ 10 Appalachian Region __________________________________ 46 Silurian __________________________________________________ 10 Jurassic and Triassic (J Ta) ________________________________ 47 Marine stratified rocks (S) ____________________________ 10 Jurassic __________________________________________________ 47 Eugeosynclinal deposits (Se) __________________________ 12 Rocky Mountain Region ______________________________ 47 Devonian and Silurian ____________________________________ 13 Pacific coastal area ____________________________________ 48 Marine stratified rocks (DS) ____________________________ 13 Continental deposits (Jc) ______________________________ 48 Eugeosynclinal deposits (DSe) __________________________ 13 Lower Mesozoic __________________________________________ 48 Devonian ________________________________________________ 14 Marine stratified rocks (1 Nb) __________________________ 48 Marine stratified rocks (D) ____________________________ 14 Eugeosynclinal deposits (1 Nke) ________________________ 49 Continental deposits (D2c, D3c) ________________________ 16 Volcanic rocks (1 MW) __________________________________ 51 Eugeosynclinal deposits (De) __________________________ 16 Cretaceous ________________________________________________ 51 Lower Paleozoic __________________________________________ 17 Lower Cretaceous ____________________________________ 51 Marine stratified rocks (I?) ____________________________ 17 Texas ____________________________________________ 51 Cratonic deposits ______________________________________ 17 Atlantic Coastal Plain ____________________________ 54 Miogeosynclinal deposits ______________________________ 20 Rocky Mountains __________________________________ 55 Rocks of outlying areas ________________________________ 20 California and Oregon ____________________________ 55 Eugeosynclinal deposits (lPe) __________________________ 21 Washington ______________________________________ 55 Mississippian ____________________________________________ 22 Upper Cretaceous ____________________________________ 56 Pennsylvanian ____________________________________________ 24 Western Gulf Coastal Plain ________________________ 56 Permian __________________________________________________ 27 Eastern Gulf Coastal Plain ________________________ 56 Appalachian Region ___________________________________ 28 Atlantic Coastal Plain ____________________________ 57 Southwestern United States ____________________________ 28 Great Plains and Rocky Mountains _________________ 57 Midcontinent Region __________________________________ 29 Pacific coastal area ________________________________ 59 New Mexico __________________________________________ 32 Continental deposits (Kc) ______________________________ 59 Northern Arizona ____________________________________ 32 Eugeosynclinal deposits (Ke) __________________________ 60 Cordilleran Region ____________________________________ 33 Volcanic rocks (Kv) ____________________________________ 60 Eugeosynclinal deposits (Pe) __________________________ 33 Upper Mesozoic ___________________________________________ 61 Upper Paleozoic __________________________________________ 33 Upper Mesozoic eugeosynclinal deposits (uNke) __________ 61 Marine stratified rocks (11!?) __________________________ 33 Cretaceous eugeosynclinal deposits (Ke) ________________ 62 Mississippian ________________________________________ 33 Mesozoic plutonic and intrusive rocks ______________________ 62 Pennsylvanian ________________________________________ 36 Triassic granitic rocks (Tag) ____________________________ 62 Permian ______________________________________________ 36 Triassic mafic intrusives (Tqi) __________________________ 63 Outlying miogeosynclinal rocks ________________________ 37 Jurassic granitic rocks (J g) ____________________________ 63 Eugeosynclinal deposits (uPe) ________________________ 37 Jurassic mafic intrusives (Jmi) ________________________ 68 Paleozoic plutonic rocks ____________________________________ 38 Cretaceous granitic rocks (Kg) ________________________ 68 Cambrian granitic rocks (6g) __________________________ 38 Cretaceous intrusive rocks (Ki) ________________________ 71 References cited __________________________________________ 72 ILLUSTRATIONS FIGURES 1—19. Maps of: Page 1. Eastern United States, showing areas mapped as Cambrian on Geologic Map of United States ____________ 3 2. New England, showing positions of tectonic features ____________________________________________________ 5 III IV CONTENTS FIGURES 1—19. Maps of—Continued NOECHAOJ 13. 14. 15. 16. 17. 18. 19. . Eastern United States, showing areas mapped as Ordovician on Geologic Map of United States ____________ . Eastern United States, showing areas mapped as Silurian on Geologic Map of United States ______________ . Eastern United States, showing areas mapped as Devonian on Geologic Map of United States ____________ . Western United States, showing areas mapped as lower Paleozoic on Geologic Map of United States ______ . Eastern Cordilleran Region, showing surface and subsurface extent of lower Paleozoic on Geologic Map of United States __________________________________________________________________________________ . Eastern United States, showing areas mapped as Mississippian on Geologic Map of United States ________ . Eastern United States, showing areas mapped as Pennsylvanian on Geologic Map of United States ________ 10. 11. 12. United States, showing areas mapped as Permian on Geologic Map of United States ______________________ Western United States, showing areas mapped as upper Paleozoic on Geologic Map of United States ______ Eastern part of Cordilleran Region, showing surface and subsurface extent of upper Paleozoic on Geologic Map of United States ____________________________________________________________________________ United States, showing areas mapped as Paleozoic plutonic rocks on Geologic Map of United States ______ United States, showing areas mapped as lower Mesozoic stratified rocks on Geologic Map of United States __________________________________________________________________________________________ United States, showing areas mapped as Cretaceous stratified rocks on Geologic Map of United States __-- United States, showing areas mapped as Mesozoic and Cenozoic plutonic and intrusive rocks on Geologic Map of United States ________________________________________________________________________________ Western United States, showing areas of Triassic granitic rocks as mapped on Geologic Map of United States __________________________________________________________________________________________ Western United States, showing areas mapped as Jurassic granitic rocks and mafic intrusives on Geologic Map of United States ________________________________________________________________________________ Western United States, showing areas mapped as Cretaceous grantitic rocks and Cretaceous intrusive rocks on Geologic Map of United States ________________________________________________________________ Page 8 11 15 18 19 23 26 30 34 35 40 44 52 64 66 67 69 THE PALEOZOIC AND MESOZOIC ROCKS; A DISCUSSION TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES By PHILIP B. KING and HELEN M. BEIKMAN ABSTRACT This report deals with the Paleozoic and Mesozoic rocks that are exposed within the area covered by the Geologic Map of the United States and treats them in terms of the map legend. They are there- fore discussed in chronological order, from oldest to youngest. Under each age, the stratified rocks in the most complete sequences are ldescribed first, followed by combinations such as Devonian and Silu- rian, Jurassic and Triassic, and finally by lower Paleozoic, upper Paleozoic, and lower Mesozoic. Next, special lithologic types of each age are taken up, the continental deposits, eugeosynclinal deposits, and volcanic rocks. At the end of the discussions of both Paleozoic and Mesozoic Eras are summaries of the plutonic rocks formed dur- ing those eras. The better known and most extensively exposed rocks—the ‘Paleozoic sequence of the Central Interior and the Cretaceous of the Coastal Plains and Rocky Mountains—are summarized briefly, as 1these are well covered in an extensive literature. More details are given for rocks of each age in the Appalachian and Cordilleran mountain belts, especially the eugeosynclinal deposits, because they have been poorly understood until recently. Although the text is designed primarily to justify the representa- tion of the units shown on the map, it also amplifies the necessarily rief descriptions of the units in the legend, and sufficient data are Eiven to indicate the general lithologies of the units as exposed in the ifferent areas, which on the map are shown primarily as time- stratigraphic rather than rock-stratigraphic entities. INTRODUCTION The following text is a discussion and exposition of the different units of Paleozoic and Mesozoic rocks that Ere represented on the Geologic Map of the United tates. It is thus partly an expansion and justification of the legend of the Geologic Map, but something is also said regarding the nature and origin of the rocks involved and the stratigraphy of the stratified rocks. On the other hand, it is not a complete treatise on these rocks, for such a treatise would far exceed the objec- tives of the report. ‘ In the text, the better known and most extensively exposed sequences are summarized briefly and with little documentation: the Paleozoic of the Central Interior and the Cretaceous of the Coastal Plains and Rocky Mountains. Many accounts of these rocks have been published, and they are well described in the more detailed textbooks of historical geology and stratigraphy—for example, those of Dunbar (1969), Kay and Colbert (1965), and Kummel (1970). Correla- tion of the formations of the Paleozoic and Mesozoic systems, with useful annotations, may be found in the charts prepared by committees of the National Re- search Council that were published between 1940 and 1960 by the Geological Society of America. In addition, some regional surveys are available, the most notable of which is the Geologic Atlas of the Rocky Mountain region (Mallory, 1972). More details and more documentation are given of the complex Paleozoic and Mesozoic rocks in the Ap- palachian and Cordilleran mountain belts in the east and west, understanding of which is now increasing as a result of geologic work during the last few decades. These rocks are partly eugeosynclinal, partly crystal- line, are generally poorly fossiliferous, and contain embedded plutonic rocks. Besides new fossil dis- coveries, much light has been thrown on the ages and relations of these rocks by radiometric dating, as indi- cated by citations in the text. The Geologic Map represents only the surface geologic features of the country, and the text accord- ingly deals only with the exposed rocks; little or no mention is made of the concealed rocks that have be- come known from subsurface studies. The text is illustrated primarily by small-scale maps of the United States, which show the extent of the dif- ferent Paleozoic and Mesozoic systems or other gross units represented on the Geologic Map. CAMBRIAN The Cambrian System, the lowest unit of the Phanerozoic Eon, is represented on the Geologic Map of the United States by marine stratified rocks (-€), with basal Lower Cambrian clastic rocks (€q) differentiated in places; by eugeosynclinal deposits (6e) and as- sociated volcanic rocks (CV); and by a few Cambrian granitic rocks (-€g) described with the other Paleozoic plutonic rocks in a later section. 2 PALEOZOIC AND MESOZOIC ROCKS MARINE STRATIFIED ROCKS ('6) Cambrian strata are most extensively exposed in the cratonic area of the Central Interior Region (fig. 1). Here they form a wide band along the southern margin of the Precambrian rocks of the Lake Superior Region, from Minnesota across Wisconsin into the Upper Peninsula of Michigan; they also form an area sur- rounding the Precambrian rocks in the Ozark dome in Missouri. Smaller bands of outcrop adjoin the Precam- brian of the Adirondack dome in New York State, the Arbuckle and Wichita Mountains in southern Ok- lahoma, and the Llano uplift in central Texas. Cam- brian rocks also form long narrow bands in the folded and faulted miogeosynclinal belt of the Appalachian Region east of the Central Interior Region. The Cambrian System is extensive as well in the Cordilleran Region west of the Central Interior, but for the most part its outcrops and those of succeeding lower Paleozoic systems are so narrow and discontinu- ous that they are all merged on the Geologic Map into a single unit (15’). In a few areas, however, Cambrian outcrops are sufficiently extensive for representation, notably in the Great Basin and in the thrust belt ex- tending northward from southeastern Idaho into northwestern Montana. The Cambrian is also shown separately in Arizona, from the Grand Canyon to the southeastern part of the State; the thin overlying De- vonian System, elsewhere classed as lower Paleozoic, is here merged with the upper Paleozoic (uP). The Cambrian is the oldest system containing shelly fossils suitable for stratigraphic analysis, and it has been elaborately zoned and correlated. At least 10 fos- sil zones are recognized. The Cambrian has been di- vided into a Lower, Middle, and Upper Series— sometimes termed the Waucoban, Albertan, and Croixan (or St. Croixan) (Lochman-Balk, 1972, p. 61). These are not represented on the Geologic Map, for the large outcrops in the Central Interior are all Upper Cambrian, and the Middle and Lower Cambrian Series appear only in the mountain belts to the east and west, where the outcrop bands are too narrow to be sub- divided. In most places the Cambrian System overlies the Precambrian with a large hiatus and profound uncon- formity. Throughout the Central Interior, Upper Cam- brian lies on deformed rocks 500 to 2,000 my (million years) older (Precambrian Y, X, and W). In contrast, in some of the geosynclinal sequences in the mountain belts to the east and west, where both Lower Cambrian and Precambrian Z stratified rocks are present, the stratigraphic break between them is slight or absent, creating problems in classification. These problems are V most acute in the southwestern part of the Great Ba- sin, between Las Vegas, Nev., and the Inyo Mountains, Calif, where fine-grained Precambrian Z and Lower Cambrian strata form a conformable sequence as much as 21,000 ft (6,400 m) thick, with diagnostic Cambrian fossils Only in the upper third. Here and elsewhere, however, the problematical rocks form outcrops so small on the scale of the Geologic Map that for our purposes they can be disregarded. The top of the Cambrian System is generally con- formable with the Ordovician but is definable by paleontological means. The “Ozarkian System” which was proposed by E. O. Ulrich in the early part of the century for a unit between the Cambrian and the Or— dovician has now been discredited, and its proposed components have been assigned to one system or the other. The only difficulties in mapping the boundary are in parts of the Appalachian miogeosyncline where Upper Cambrian and Lower Ordovician rocks are parts of a thick mass of carbonates, as in such units as the Knox Group (Knox Dolomite of older reports) in the Southern Appalachians. By detailed stratigraphic work the Cambrian part of the Knox (Copper Ridge Dolomite or Conococheague Limestone) can be sepa- rated from the Ordovician part, but components of the two ages are not everywhere shown on the source maps; in such places they have been divided arbitrarily on the Geologic Map. From New Jersey northward, however, the outcrop belts are narrower and more complex, and so it has been necessary to merge the Cambrian and Lower Ordovician carbonates into a single unit (06), even though data for their separation are available in places (see section “Ordovician and Cambrian”). The miogeosynclinal sequence in the Appalachian Region begins with Lower Cambrian clastic deposits (€q), which are separately shown on the Geologic Map where their outcrop belts are sufficiently wide. They are typified by the Chilhowee Group of Tennessee, Vir- ginia, and Maryland, 3,000 ft (900m) or more thick, with conglomerates and arkoses in the lower part and prominent quartzite layers separated by shale and siltstone above. Similar rocks flank the western edges of the Green Mountains and Berkshire Hills uplifts in western New England, with the thick Cheshire Quartzite at the top. Shelly fossils of the Olenellus zone occur only in the upper formations of the elastic de- posits, although trace fossils such as Scolithus are found much lower. The lowest parts are unfossilifer- ous, hence are classed as Cambrian (?). Throughout the length of the Appalachian miogeosyncline, the elastic deposits are succeeded abruptly by a great carbonate sequence more than 10,000 ft (3,000 m) thick that in- cludes the remainder of the Lower Cambrian, the Mid— dle and Upper Cambrian, and the Lower Ordovician Series. CAMBRIAN GULF OF MEXICO FIGURE 1,—Eastern United States, showing areas mapped as Cambrian on Geologic Map of United States. Includes units of marine stratified rocks (£3), basal Lower Cambrian elastic rocks (-eq), Lower Ordovician and Cambrian carbonate rocks (O€), eugeosynclinal deposits (-6e), and volcanic rocks (€v). 4 PALEOZOIC AND MESOZOIC ROCKS The miogeosynclinal sequence of the Cordilleran Region, in the Great Basin west of the "Wasatch line,” also begins with Lower Cambrian clastic deposits known from place to place as the Brigham, Tintic, and Prospect Mountain Quartzites. For consistency, it would have been interesting to have separated these on the Geologic Map, but although they are as thick and prominent as those in the Appalachian miogeosyncline, their outcrops are more discontinuous and patchy and too small for representation. In this region, as in the Appalachian miogeosyncline, the clas- tic deposits are succeeded by carbonate rocks, in this case of Middle and Late Cambrian age, with thicknes- ses in the classic sections in the House Range, western Utah, and the Eureka district, east-central Nevada, of 8,200 and 5,400 ft (2,500 and 1,600 m), respectively. Throughout the Central Interior Region, the basal Cambrian deposits are also sandstone, but they are the basal deposits of the cratonic sequence and are all of Late Cambrian age except in the extreme west (parts of the Rocky Mountains and Colorado Plateau), where some of them are as old as Middle Cambrian. In north- eastern New York State the Potsdam Sandstone on the flanks of the Adirondack dome is closely adjacent to the Cheshire Quartzite of the basal Lower Cambrian clas- tic deposits across Lake Champlain to the east, but it is of Late Cambrian age like the other basal cratonic de- posits farther west in the Central Interior. The type Croixan Series in the border region of Minnesota and Wisconsin is nearly all different varieties of sandstone and about 1,000 ft (300 m) thick; fossils throughout it permit its division into three stages and eight zones (Bell and others, 1956). Cambrian (-6) is shown on the Geologic Map in the two core areas of the Ouachita Mountains foldbelt— the Broken Bow uplift of southeastern Oklahoma and the Benton uplift of southwestern Arkansas. The unit so shown is the Collier Shale, traditionally classed as Cambrian but from which early Ordovician (Tremado- cian) conodonts have recently been collected (Repetski and Ethington, 1973). The age designation on the map is therefore erroneous, but it at least illustrates the structurally highest parts of the foldbelt. EUGEOSYNCLINAL DEPOSITS (€e) Cambrian eugeosynclinal deposits are shown on the Geologic Map throughout much of the length of the Appalachian Region, east of the miogeosynclinal belt. In the Cordilleran Region, where present, they are merged with the other lower Paleozoic eugeosynclinal deposits (1 E’e). Characteristic components of the eugeosynclinal deposits are volcanic rocks (6v), which are differentiated on the Geologic Map where they underlie sufficiently large areas. Cambrian rocks of eugeosynclinal facies form an outcrop belt in western New England that extends along the east flank of the Green Mountains and other Precambrian uplifts from the Canadian border through Vermont and Massachusetts to Connecticut (fig. 2). In Vermont, north of the plunging end of the Precambrian, Cambrian rocks extend across the Green Mountains anticlinorium to adjoin the Cambrian miogeosynclinal rocks, and so a transition between them can be worked out (Cady, 1960, p. 539—543). The dominant carbonate rocks of the latter change east- ward into argillaceous and coarser clastic rocks (now schistose), with lenticular volcanic units, and the se- quence thickens dramatically to more than 20,000 ft (6,000 m). Fossils are virtually absent, but correlations can be reasonably established with the fossiliferous miogeosynclinal sequence to the west and with dated units along the strike in Canada to the north, suggest— ing that Lower, Middle, and Upper Cambrian Series are all represented. Cambrian eugeosynclinal deposits are also mapped in the Taconic area west of the Green Mountains in western New England and eastern New York State, where they are allochthonous on the Cambrian and Ordovician miogeosynclinal rocks. Here again, the se- quence is dominantly argillaceous or silty, with minor coarser layers; however, it is no more than a few thousand feet (300—600 m) thick, and interbedded vol- canic rocks are rare (Zen, 1967, p. 14—22). Parts of the sequence are fossiliferous and indicate that much of it is of Lower Cambrian age, although Middle and Upper Cambrian fossils have been found in places. According to present beliefs, the rocks -of the Taconic sequence formed in the transition zone between the miogeosyn- clinal carbonates and the thick eugeosynclinal rocks at a site a little east of the present Green Mountains axis and were transported as one or more slices, largely by gravity sliding, onto a sea floor where Middle Ordovi— cian deposits were still accumulating. Small areas of Cambrian rocks are mapped much farther east in New England, in eastern Maine and Massachusetts; however, most of these rocks are poorly defined paleontologically, and their age is suggested mainly by their relations to overlying beds or by radiometric dating. The Grand Pitch Formation at the base of the eugeosynclinal sequence in northeastern Maine is a red slate containing the trace fossil Old- hamia, which occurs elsewhere in Cambrian rocks. South of Boston in Massachusetts are the much better dated Hoppin Slate and Weymouth Formation with Lower Cambrian fossils and the Braintree Slate with Middle Cambrian Paradoxides, the faunas being of At- lantic facies, unlike the North American faunas in the Appalachian miogeosyncline to the west. Unfortu- CAMBRIAN 5 Narragansett . Basin 0 so 100 MILES l . 1 I I ' I 1 l o 50 100 150 KILOMETRES FIGURE 2.—N6W England, showing positions 0f tectonic features referred to in the discussions of the Cambrian, Ordovician, Silurian, and Devonian eugeosynclinal rocks. 6 PALEOZOIC AND MESOZOIC ROCKS nately, all these occurrences are very small—some little more than specimen localities—and so it is im- practical to mark them on the Geologic Map. The map indicates as Cambrian the somewhat larger outcrops of the Westboro Quartzite northwest of Boston, but its age is uncertain; it may be of late Precambrian age, as was deduced by Emerson (1917, p. 24). In the Central Appalachians, the principal eugeosynclinal deposit of Cambrian age is the Glenarm Series of the Piedmont province in Maryland and adjacent States (Hopson, 1964, p. 54—128; Higgins, 1972). At the base, next to the domes of Precambrian Baltimore Gneiss, are the Setters Formation (quartz schist and quartzite) and the Cockeysville Marble, fol- lowed by the great clastic mass of the Wissahickon Formation, more than 20,000 ft (6,000 m) thick. Al- though the Wissahickon consists of high-grade metamorphic rocks, it contains abundant relict sedimentary structures which indicate that it is a flysch1 or turbidite deposit. Locally interbedded in the Wissahickon is the thick lens of the Sykesville Forma- tion, once interpreted as a granitic intrusive but actu- ally a coarse submarine slide derived from an eastern source, containing heterogeneous clasts and blocks of sedimentary and metamorphic rocks. Toward the southeast, next to the Coastal Plain, the Wissahickon interfingers with and is overlain by the James Run Formation of volcanic and volcaniclastic rocks. In the western Piedmont of Maryland, the Wissahickon is re- placed by and partly overlain by shallow-water phyl- 1ites, marbles, and basalt flows (Ijamsville Phyllite, etc.) which are represented on the Geologic Map as Cambrian stratified rocks (€). The age of the Glenarm Series has long been dis- puted, being placed in late Precambrian or early Paleozoic time. Hopson (1964, p. 203—207) proposed a late Precambrian age because enclosed granitic plu- tons have ages of 470 to 550 my and of about 425 my The events separating the oldest of these plutons from the formation of the Glenarm Series were inferred to have been sufficiently prolonged that the series must have formed before Cambrian time. However, Higgins (1972, p. 1008—1009) demonstrated that the older group of supposed grantitic intrusives (470—550 m.y.) is actually an assemblage of metamorphosed sedi- ments; hence the dates define the maximum age of the Glenarm, or perhaps its true age—that is, Cambrian rather than Precambrian. 1The term "flysch" is derived from the original Flysch of the European Alps and is used consistently in this report for a peculiar assemblage of thinly interbedded sandy and shaly rocks; the sandy rocks commonly show grading and other structures which suggest that they were derived from turbid flows, and the shaly rocks represent interludes of pelagic sedimentation. Flysch is a deep-water deposit that commonly accumulated in linear troughs in mobile belts. Not included are other synorogenic deposits (to which the term has some- times been misapplied), many of which formed in quite different environments. Southwestward in central Virginia, in apparent con- tinuity with the Glenarm, is the Evington Group of the Lynchburg area (Espenshade, 1954, p. 14—21; Brown, 1958, p. 28—38). On the flank of the Blue Ridge, resting on the Precambrian Z Lynchburg Formation (or the Catoctin Greenstone, where present), is the Candler Phyllite, about 5,000 ft (1,500 m) thick, which is fol- lowed by several thousand feet of more varied strata—schist, marble, and quartzite—with about 1,000 ft (300 m) of greenstone metavolcanic rocks at the top. The Evington Group contains no fossils, and no radiometric dates are available; from its relations to the Lynchburg and by comparison with the Glenarm, it is presumably early Paleozoic (Cambrian?) and is so represented on the Geologic Map (6e). Still farther southwestward, near the Virginia— North Carolina border, and more or less in continuity with the Evington Group, is the Alligator Back Forma- tion (Rankin and others, 1973, p. 17—19). It overlies the Precambrian Z Ashe Formation (= Lynchburg Forma- tion) and consists of laminated graywacke and pelite with “pinstripe” structure; volcanic rocks interfinger northeastward. Although there is no clear evidence of age, it is likewise indicated as Cambrian (6e) on the Geologic Map. A much larger area of Cambrian eugeosynclinal de- posits (-€e) forms the Carolina Slate Belt of the South- ern Appalachians, extending as a wide band across the Piedmont province from southern Virginia through North and South Carolina to eastern Georgia (Conley and Bain, 1965; Sundelius, 1970). In central North Carolina the band is 130 mi (210 km) wide, but it nar- rows to the northeast and southwest. The Slate Belt contains a sequence of gently deformed low-grade metamorphic clastic and volcanic rocks more than 30,000 ft (9,100 m) thick. These are adjoined to the northwest and southeast by higher grade metamorphic rocks, in part the metamorphosed equivalents of the Slate Belt rocks, but probably mainly older. They are intruded here and there by granitic and mafic plutons with ages of 520 to 595 my (Fullagar, 1971, p. 2852— 2854). Areas of low-grade elastic and volcanic rocks like those in the Slate Belt also occur farther southeast and are encountered in many drill holes beneath the Atlantic Coastal Plain. In North Carolina, a rather consistent stratigraphy can be worked out and mapped in the rocks of the Slate Belt. Below (with the base not exposed) is the thick Uwharrie Formation of rhyolitic and rhyodacitic flows and pyroclastics. Above are thinner units of laminated shales, mudstones, and siltstones, alternating with volcanic-rich units. The subdivisions are not shown on the Geologic Map, but the volcanic rocks (6v) are sepa- rated where they underlie sufficiently large areas. The ORDOVICIAN 7 age of the rocks in the Slate Belt can be indicated only in a general way. At one locality in southern North Carolina, are a few Paradoxides of probably Middle Cambrian age (St. Jean, 1973). From rocks in the same lgeneral area, an Ordovician age of 440 to 470 my was obtained by the lead-alpha (Pb/alpha) method, but this age seems to be unreliable and too young. A considera- ble part of the sequence is probably late Precambrian (Z). In northern North Carolina, on the Little River 12 miles (20 km) north of Durham, Lynn Glover III and his associates have found Ediacaran (= Vendian) type fossils, which are the imprints of primitive wormlike animals on the bedding surfaces of volcaniclastic strata. At the north end of the belt in Virginia, radiometric ages in excess of 600 my have been ob- tained from the slate belt rocks (Glover and Sinha, 1973). In the Cordilleran Region, the only authentic Cam- brian eugeosynclinal deposits known to us are the Scott Canyon Formation which forms a small area in the south part of Battle Mountain, north-central Nevada. Limestones interbedded in its cherts, shales, and greenstones contain archeocyathids of Lower or Middle Cambrian age. It is mapped as part of the lower Paleozoic eugeosynclinal deposits (lPe). The Cam- brian(-€) shown in Battle Mountain and nearby ranges is the Harmony Formation, an arkosic turbidite of Late ambrian age, which is classed as a “transitional” facies rather than eugeosynclinal (Roberts and others, 1958, p. 2829—2830), hence is included in the normal ‘stratified sequence on the Geologic Map. ORDOVICIAN AND CAMBRIAN MARINE STRATIFIED ROCKS (0-6) As indicated above, the Appalachian miogeosyncli- al rocks include a thick sequence of carbonates that extends from the Lower Cambrian into the Lower Or- ovician System. In the Central and Southern Ap- Ealachians, it has been possible to separate the Cam— rian and Ordovician components on the Geologic illap, but from New Jersey northward the outcrop belts J re narrower and more complex and have been merged into a single unit (0-6). 1 In New Jersey and southern New York, the subdivi- ions are apparent in places but have not been worked out regionally, and they are shown as undivided units 11 the source maps—the Kittatinny and Wappinger imestones of the northwestern belts and the Inwood and Stockbridge Marbles of the metamorphic belt east nd southeast of the Hudson Highlands. Farther north, i the Champlain Lowlands of Vermont and adjacent New York, the subdivisions of the Cambrian and I‘lower Ordovician carbonates have been worked out and mapped in detail (see Geologic Map of Vermont, 1961) but cannot be represented on the scale of the Geologic Map. In the eugeosynclinal area of New England, the source maps likewise indicate some of the units as “Ordovician-Cambrian” (meaning Ordovician or Cam- brian), but these have been arbitrarily assigned to one system or the other on the Geologic Map. ORDOVICIAN The Ordovician System is represented on the Geologic Map of the United- States by marine stratified rocks (0), divided in part into Lower, Middle, and Upper Series (01, O2, 03), and by eugeosynclinal de- posits (Oe) with associated volcanic rocks (Ov). The * Ordovician is shown separately in the eastern two- thirds of the country; from the Rocky Mountains west- ward it is merged with the other lower Paleozoic sys- tems (lP, lPe). MARINE STRATIFIED ROCKS The most extensive exposures of Ordovician strata are in the cratonic area of the Central Interior Region, where they are nearly flat lying or are gently tilted on the flanks of the broad domical uplifts (fig. 3). In the north, they form a broad band of outcrop between the Cambrian and Silurian sequences around the west, south, and east flanks of the Wisconsin dome, extend- ing from Minnesota to the Upper Peninsula of Michi- gan and as far south as Illinois. Farther south, they form much of the surface of the Ozark dome in Mis- souri and Arkansas—largely Lower Ordovician with the Middle and Upper Ordovician Series in narrow bands around the edges. Middle and Upper Ordovician rocks form the crestal areas of the Cincinnati and Nashville domes farther east, where the Lower Or- dovician rocks is not exposed. The three series also encircle the Adirondack dome in New York State, and smaller Ordovician outcrops occur in the Arbuckle and Wichita Mountains and the Llano uplift in Oklahoma and Texas. Intervening areas in the Central Interior are covered by younger strata, but the presence of Or- dovician rocks beneath them is known from drilling. Ordovician rocks also crop out in long bands throughout the length of the miogeosynclinal belt of the Appalachian Region east of the Central Interior Region and emerge in the core areas of the Ouachita Mountains foldbelt to the south. The three series of the Ordovician System are shown separately throughout the Central Interior, as well as in parts of the Appalachian miogeosynclinal belt in New York and Pennsylvania. In the remainder of the miogeosynclinal belt, the Ordovician outcrop bands are PALEOZOIC AND MESOZOIC ROCKS GULF OF MEXICO "‘8: FIGURE 3.—Eastern United States, showing areas mapped as Ordovician on Geologic Map of United States. Includes units of marine stratified rocks (0) and their subdivisions (01, 02, 03), eugeosynclinal deposits (0e), volcanic rocks (0v), and a few of lower Paleozoic (19) in the Southern Appalachians, ORDOVICIAN 9 ‘ too narrow for subdivision and are shown as a single unit (0). On the other hand, the Lower Ordovician 1 Series in the Ozark dome forms such an extensive area that further separation is needed to illustrate the geol- ‘ ogy. Here, the series is divided at the base of the J effer- son City Dolomite into units 01a and 01b, although ‘there are no fundamental paleontological or sedimen- tological differences between the two parts. (Somewhat . similar subdivisions were made on the Geologic Map of 1932). ‘ The Lower Ordovician or Canadian Series (01) is llargely carbonate, much of it dolomite, which succeeds similar carbonates of the Upper Cambrian Series (as noted above). In many places the two components were not separated in early reports, resulting in units such las the Knox 0f the Southern Appalachians, the Ar- buckle of the Arbuckle and Wichita Mountains, Ok- llahoma, and the Ellenburger of the Llano area, Texas; relations are now clarified, and these and similar units lare classed as groups. Throughout the Appalachian miogeosyncline, the series is about 2,000 ft (600 m) lthick, but it has a maximum thickness of about 5,000 ft (1,500 m) in the deep trough adjoining the Arbuckle land Wichita Mountains. It thins in the cratonic area, being about 1,000 ft (300 m) thick in the Ozark dome land no more than a few hundred feet thick on the flanks of the Wisconsin dome (Prairie du Chien Group). 1 The Middle Ordovician or Mohawkian Series (02)2 is more varied, largely limestone and shale but with no- table units of sandstone in the lower part. Its abun- dantly fossiliferous strata have been the field of labor pf many paleontologists and stratigraphers, and it has been minutely subdivided and correlated from place to place. The details do not concern us here, but a few general items are worth noting. ‘ In the northern Midwestern States, the basal Middle Ordovician unit is the Saint Peter Sandstone, a clastic sheet of vast extent probably derived from the Cana- dian Shield (Dake, 1921, p. 221—224). To the north it ies on a rough surface eroded on Lower Ordovician rocks, but southward it fingers out into carbonate ormations. Eastward in the Appalachian miogeosyncline, the Middle Ordovician limestones and shales with shelly fossils give way to shales and coarser elastic rocks in which the principal fossils are graptolites. These shaly ocks include some important belts of flysch, such as the Normanskill Formation of New York State and the artinsburg Formation of the Central Appalachians 2Terminology ol' the Middle Ordovician Series is confused. According to some usage (for example. Twenhoi‘cl and others, 1954 l. the Middle Ordovician is the Champlainian, which is silbdinded into the Chazyan below and the Mohawkian (Black River and Trenton Lime- stones and equivilentst above. All these names are derived from the classic sequence in New York. (which continues upward into the Upper Ordovician, although it is all marked as 02 on the Geologic Map) (McBride, 1962, p. 39—43). These shaly rocks (02) form the autochthonous substratum of the allochthonous Taconic rocks in eastern New York and western New England. Southeast of the Hudson Highlands, they pass into a higher grade metamorphic facies (Manhat- tan and Berkshire Schists) which extends eastward in Connecticut to an obscure boundary (“Cameron’s line”), possibly a thrust, which separates them from the eugeosynclinal Hartland Schist (0e). Graptolite shales with some interbedded sandstone also compose the Lower and Middle Ordovician Series in the Ouachita Mountains foldbelt (Mazarn Shale, Blakeley Sandstone, and Womble Shale). The Upper Ordovician or Cincinnatian Series (03) is typified by exposures in the Cincinnati dome of Ohio and adjacent States, where its shales and limestones (Eden, Maysville, and Richmond Groups) are richly fossiliferous. Other shales, the Maquoketa, form the Upper Ordovician in the northern Midwestern States. Farther southwest, the Upper Ordovician sequence (and part of the Middle Ordovician) is mainly cherty limestone, the Viola of the Arbuckle and Wichita Mountains, which passes into the Bigfork Chert in the nearby Ouachita Mountains. Eastward toward the Ap- palachian belt, the uppermost Upper Ordovician pas- ses into redbeds, in part continental—the Queenston Shale of northwestern New York and the J uniata and Sequatchie Formations farther south—which are post- orogenic to the Taconian deformation in the foldbelt itself. EUGEOSYNCLINAL DEPOSITS Ordovician eugeosynclinal deposits and associated volcanic rocks (Ov) are mapped in numerous areas in all the New England States, but especially on the west and east flanks of the Connecticut Valley synclinorium, with smaller less continuous areas to the east and southeast. Their stratigraphy and ages are best deciphered in the north, where the rocks are least metamorphosed and fossils obtainable in places. The stratigraphy of the higher grade metamorphic rocks farther south, as in Massachusetts and Connecticut, is indicated by comparisons with or by actual tracing from these better known rocks. The eugeosynclinal rocks of the Connecticut Valley are in a homoclinal belt that extends south from the Canadian border through Vermont and Massachusetts into Connecticut, between the Cambrian rocks of the Green Mountains and other uplifts on the west and the Silurian and Devonian rocks of the synclinorium; they are mainly phyllites and schists but include lenticular bodies of sandstone and volcanic rocks; in northern 10 PALEOZOIC AND MESOZOIC ROCKS Vermont and adjacent Canada some of the units con- tain graptolites. The eugeosynclinal rocks east of the Connecticut Valley from New Hampshire southward are in the Bronson Hill anticlinorium—actually a highly irregu- lar chain of domes much entangled with plutons and with pronounced nappe structures. Volcanic compo- nents are much greater here than to the west; most of the sediments are tuffaceous, and the thick Am- monoosuc Volcanics occur near the middle (Billings, 1956, p. 12—21). The anticlinorium may have origi- nated as a volcanic arc in the eugeosyncline. Above the Ammonoosuc is the Partridge Formation, a black sulfidic argillaceous rock that continues southward into the Brimfield Schist of Massachusetts. The Or— dovician sequence in the Bronson Hill anticlinorium is more than 20,000 ft (6,000 m) thick, but its base is not exposed; it is overlain unconformably by the fossilifer- ous Silurian Clough Quartzite which also truncates the Highlandcroft Plutonic Series (E’ g1), intrusive into the Ordovician. Along the continuation of the belt across northern Maine, Middle Ordovician graptolites occur at various places in shales above the volcanic rocks. An interest- ing variant in northeastern Maine is the Shin Brook Formation of tuffaceous sediments and volcanic brec- cias that contains a large assemblage of Middle Or- dovician shelly fossils (brachiopods, trilobites, and so forth), probably formed in a shoal area in the eugeosyncline (Neuman, 1964). Small areas of Ordovician eugeosynclinal rocks are shown on the Geologic Map as overlying the Cambrian sequence in the allochthonous Taconic area of eastern New York. They are graptolite—bearing shales and graywackes like the Normanskill Formation of the surrounding autochthon but include strata of Early as well as Middle Ordovician age. The only authentic Ordovician formation known to us in the eugeosynclinal area south of New England is the Arvonia Slate of central Virginia that contains abundant (though much deformed) shelly fossils of Middle or Late Ordovician age (Brown, 1969, p. 25—26); it lies unconformably on metamorphic and plutonic rocks of earlier Paleozoic age. Also shown as Ordovi- cian (Oe) on the Geologic Map are the Quantico Slate south of Washington, DC, and the Peach Bottom Slate of southern Pennsylvania, but although traditionally they have been correlated with the Arvonia, their age is less certain and they may be older (Higgins, 1972, p. 972). The existence of Ordovician rocks elsewhere in the Piedmont of the Central and Southern Appala- chians is a possibility, but no evidence for them is available; present indications are that most of the Piedmont supracrustal rocks are older. SILURIAN AND 0RDOVICIAN EUGEOSYNCLINAL DEPOSITS (5013) In northeastern Maine (Aroostook County) the hy- brid category “Silurian and Ordovician” is used on the Geologic Map to designate the Carys Mills Formation and some related deposits. These form a sequence about 12,000 ft (4,000 m) thick of calcareous silty and sandy rocks that extends from the Middle Ordovician into the Lower Silurian Series (Caradoc to Llandov- ery), as indicated by graptolites and other fossils (Pav- lides, 1968, p. 8—13). Use of the hybrid term in this area demonstrates the lack of any Taconian orogenic activ- ity and consequent structural discordance, such as oc— curs between the Ordovician and Silurian Systems in much of the remainder of New England. Although the Carys Mills is calcareous and lacks any volcanic mate- rial, it is not miogeosynclinal. It is, instead, a calcare- ous flysch laid down in the depths of a part of the eugeosyncline that was far from any tectonic or vol- canic areas. SILURIAN The Silurian System is represented on the Geologic Map of the United States by marine stratified rocks (S), divided in part into the Lower, Middle, and Upper Silurian Series (S1, 82, S3), and by eugeosynclinal de- posits (Se) with associated volcanic rocks (SV). The Silurian is shown separately in the northeastern third of the country; in the Southern Appalachians and the Ouachita Mountains it is combined with the Devonian (DS), and from the Rocky Mountains westward it is combined, where present, with the other lower Paleozoic systems (19, lPe). MARINE STRATIFIED ROCKS (S) As with the systems below and above, the Silurian is most extensively exposed in the cratonic area of the Central Interior Region, where the strata are gently tilted off the flanks of the broad domical uplifts (fig. 4). Silurian outcrops are less extensive, however, than those of the adjoining systems, and in places they are truncated, or nearly so, by the systems above. The most prominent band of outcrops encircles the Michigan ba- sin, from northern Ohio, Indiana, and Illinois, through eastern Wisconsin, the Upper Peninsula of Michigan, and southern Ontario (see Geologic Map of Canada), and across the Niagara Gorge in western New York. A large detached area lies southwest of the Wisconsin dome in eastern Iowa and northwestern Illinois. Small remnants are preserved on the flanks of the Nashville and Ozark domes farther south. The New York out- crops bend around the northeastern end of the Al- legheny synclinorium into the miogeosynclinal belt of SILURIAN GULF OF MEXICO ‘3' FIGURE 4.—Eastern United States, showing areas mapped as Silurian on Geologic Map of United States. Includes units of marine stratified rocks (S) and their subdivisions (S1, Sz, 83), eugeosynclinal deposits (Se), volcanic rocks (Sv), and Silurian and Ordovician eugeosynclinal deposits (SOe). 11 12 PALEOZOIC AND MESOZOIC ROCKS the Appalachians, where Silurian rocks form numer- ous bands encircling the folds of the Valley and Ridge province from Pennsylvania into Virginia, but from southern Virginia southwestward the bands become so narrow that they are merged with the Devonian (DS), as in the Ouachita Mountains to the west. According to North American usage, the Silurian is divided into the Lower Silurian or Alexandrian Series (81) (Oswegan or Albion of earlier usage), the Middle Silurian or Niagaran Series (Sz), and the Upper Silu- rian or Cayugan Series (S3). This practice conflicts with usage in Great Britain and elsewhere in western Europe, where only a Lower Silurian (Llandovery) and an Upper Silurian (Wenlock and Ludlow) Series are recognized, the boundary between them being in the middle of the North American Middle Silurian (Berry and Boucot, 1970, p. 13—16). Nevertheless, the subdivi- sions of the Silurian System, where shown on the Geologic Map, follow the conventional North American usage. The three series of the Silurian are separated on the Geologic Map throughout the broad outcrop areas in the north, but the Middle Silurian (S2) is the most ex- tensive and accounts for a large part of the outcrop area, partly because of its resistant carbonate forma- tions. The Lower Silurian Series (Si) is inconsequential except in New York and the Iowa-Illinois area, and the Upper Silurian is only prominent at the surface in northern Indiana and Ohio. It is best developed downdip, in subsurface beneath the Michigan basin and Allegheny synclinorium. The Silurian is not sub- divided on the map in the smaller areas in the south- ern part of the Central Interior, or in the Appalachian miogeosynclinal belt. Throughout the Central Interior the Silurian se- quence is accordant with the Ordovician below and the Devonian above, although separated from them at many places by a hiatus of greater or lesser magnitude. At the Falls of the Ohio near Louisville, for example, Middle Silurian carbonates (Niagaran) are overlain without discordance by Middle Devonian carbonates (Onondaga equivalent), the contact actually being within a single layer. Nevertheless, the Geologic Map indicates a low-angle truncation of the Silurian by the Devonian in Indiana, Illinois, and Iowa. Eastward along the edge of the folded rocks of the northern Appalachians, a structural unconformity de- velops at the base of the Silurian sequence, reflecting the Taconian orogeny in this part of the foldbelt. The discordance is prominent west of the Hudson River in southern New York, northern New Jersey, and eastern Pennsylvania but fades out to the south. Northwest— tilted Lower Silurian sandstones and conglomerates (Shawangunk and Tuscarora) overlie highly disturbed Middle and Upper Ordovician flysch (Normanskill and Martinsburg). A significant outlier of the unconformity occurs at Becraft Mountain east of the Hudson, where Lower Devonian rocks overstep the Silurian and lie directly on deformed and faulted Cambrian and Or- dovician rocks of the Taconic allochthon. Carbonate rocks dominate the cratonic area of the Central Interior. The Niagaran Series in particular, in the middle of the sequence, is a sheet of dolomite that extends westward from New York State to Iowa. One of its stronger layers, the Lockport Dolomite, forms the rimrock of Niagara Falls. The Niagaran carbonates are studded wtih mound reefs, some of them 400 ft (120 m) thick, which grew on a sea floor of marked relief. In New York, however, the lower part of the Niagaran (Clinton Group) is shaly and contains beds of red iron ore. Limestones and shales, rather than dolomite, are more prominent in the Lower and Upper Silurian Se- ries, and the latter, or Cayugan, contains large volumes of evaporites, especially rock salt. These have mostly been leached back from the outcrops and are known mainly in subsurface. Cayugan time marked a climax in the sinking of the Michigan basin, and the series is 4,000 ft (1,200 m) or more thick in its center (beneath the cover of younger Paleozoic strata); nearly half of it is salt (Cohee, 1965, p. 217—218). The Cayugan salt deposits have long been exploited commercially, both in Michigan and the Appalachian Region; drilling for salt long preceded drilling for oil in the same areas. In the Appalachian miogeosyncline the Silurian car- bonates give place to clastic deposits, in part nonma- rine, related to the Taconian orogeny and its after- maths. Especially prominent are the ridge-making sandstones of the Lower Silurian, known from place to place as the Shawangunk, Tuscarora, Clinch, and other local terms. In Pennsylvania the Upper Silurian rocks include the Bloomsburg Redbeds, as much as 5,000 ft (1,500 m) thick, which thin westward and in- tertongue with gray shales and limestones. EUGEOSYNCLINAL DEPOSITS (Se) Silurian eugeosynclinal deposits and associated VOl- canic rocks (SV) are mapped in the New England States north of Connecticut and Rhode Island. None are known in the Piedmont province of the Central and Southern Appalachians, and probably they do not exist there. As in the adjacent systems, the stratigraphy is plainest to the north and northeast, where the meta- morphic grade is low and fossils are relatively abundant—especially in northern and eastern Maine, but also in a peculiar narrow belt of low-grade meta— morphism along the Connecticut River in western New Hampshire. DEVONIAN AND SILURIAN 13 In the northwestern belts flanking the Connecticut K Valley synclinorium, the Silurian sequence lies with structural discordance on the Ordovician, and at a few places in northern Maine it is also moderately uncon- formable below the Devonian. It is thin—generally lit- tle more than 1,000 ft (300 m) thick—and includes im- portant bodies of quartz sandstone and carbonate (Shaw Mountain Formation and Northfield Slate on the west flank in Vermont, Clough Quartzite and Fitch Formation on the east flank in New Hampshire, all Middle Silurian). The units in New Hampshire form such narrow outcrop belts and the structure is so com- plex that they are indicated only in places on the Geologic Map, although they are actually fairly con: tinuous. The Silurian rocks of these belts are more of miogeosynclinal than of eugeosynclinal facies, and their inclusion with “Se” on the Geologic Map- is mis- leading. The deposits represent an interlude in north- western New England, following the Taconian orogeny, between the eugeosynclinal regimes that dominated Ordovician and Devonian time. Southeastward into the Merrimack synclinorium in New Hampshire and Maine, the Silurian sequence it- self becomes eugeosynclinal and thickens dramatically to more than 15,000 ft (4,600 m), as shown by its wide outcrop bands on the Geologic Map. The Merrimack Group in the south is mainly slates and schists with a few more sandy units and minor volcanic rocks. A band of Silurian slates more than 30 mi (50 km) broad ex- tends northeastward through central Maine, nearly across the State. .Nearer the coast, volcanic rocks (Sv) dominate. They are well displayed in the Eastport area at the eastern tip of Maine, where they are Virtually unmetamor- phosed and contain fossils at numerous levels that in- dicate Middle and Late Silurian ages. Farther south- west, metamorphism is greater and fossil control is sparse. Brachiopods and ostracodes in the Ames Knob Formation of the Penobscot Bay area are of Late Silu- rian age. A few Late Silurian ostracodes have been found in the Newberry Volcanics north of Boston in eastern Massachusetts, and the barren Lynn and Mat- tapan Volcanic Complexes south of it may be of roughly the same age. DEVONIAN AND SILURIAN MARINE STRATIFIED ROCKS (DS) On the Geologic Map, the Devonian and Silurian Systems are combined into a single unit in the miogeosynclinal belt of the Southern Appalachians and in the Ouachita Mountains. The outcrop belts of both systems become narrow in southwestern Virginia, partly from steepening of the structure, more from thinning of the sequences. The great wedge of Devonian clastic rocks that is promi- nent in New York and Pennsylvania thins and be— comes inconsequential in Virginia and beyond. In Tennessee the combined thickness of the two systems is about 1,000 ft (300 m), and in Alabama no more than a few hundred feet. In Alabama the units are the Silu- rian Red Mountain Formation, with red iron ores like those of the Clinton in New York that are exploited commercially at Birmingham, and the Devonian Frog Mountain Sandstone. Mapped also as undivided Devonian and Silurian in Alabama are the upper rocks of the sequence in the Talladega belt, southeast of the area of miogeosyncli- nal rocks. These include the persistent Cheaha (= But- ting Ram) Sandstone, followed in the south by the Jemison Chert with abundant shelly fossils of Early Devonian age, and farther north by the Erin Slate from which fossil plants of supposed Carboniferous age have been collected. The slates and phyllites in the Tal- ladega sequence beneath the Cheaha Sandstone are indicated on the map as lower Paleozoic (1?) ,(see be- low). In the Ouachita Mountains, the Devonian and Silu- rian (DS) of the Geologic Map comprise the Blaylock Sandstone with Lower Silurian monograptids; the Missouri Mountain Slate, unfossiliferous but probably also Silurian; and the Arkansas Novaculite. The first two are prominent in the southern outcrop belts but insignificant farther north, whereas the Arkansas is persistent and forms mountain ridges that encircle the older Paleozoic rocks of the core areas. It is a few hundred to a thousand feet (80—300 m) thick and is a condensed sequence with conodonts indicating that it embraces all the Devonian Period and the lower part of the Mississippian (Kinderhookian) as well (Hass, 1951). EUGEOSYNCLINAL DEPOSITS (DSe) Many of the source geologic maps in New England designate units by the hybrid term “Devonian and Silurian” or "Devonian or Silurian,” indicating either that they contain rocks of both ages (as indicated by fossils or other less direct evidence) or that there is uncertainty as to which system they should be as- signed. The Geologic Map of Maine (1967) lists more than 20 such hybrid map units. For the most part, we have arbitrarily assigned such units to one of the two systems on the Geologic Map, on the basis of the rpre- ponderance of evidence for one or the other, but this has not been possible in the Merrimack synclinorium that extends from southeastern Maine and New Hampshire to Massachusetts. The broad band of Silurian eugeosynclinal rocks that 14 PALEOZOIC AND MESOZOIC ROCKS extends across southeastern Maine is separated from the Devonian eugeosynclinal rocks by bands nearly as broad labeled on all the source maps as “Devonian or Silurian,” which it would be presumptuous on our part to attempt to classify. Those on the northwestern flank are calcareous silty and sandy rocks (Madrid and Fall Brook Formations) which overlie strata with Upper Silurian (Ludlow) fossils and underlie equivalents of the Devonian Seboomook Formation; those on the southeastern flank (Vassalboro and Berwick Forma- tions) similarly overlie fossiliferous Upper Silurian but lack Devonian rocks at the top (Osberg and others, 1968; Ludlum and Griffin, 1974). The Geologic Map also extends the southeastern belt of “Devonian or Silurian” to the better dated rocks near Penobscot Bay to include rocks that were poorly understood at the time of compilation. Surveys now available indicate that this area is more heterogeneous than realized and includes not only Devonian and Silurian but also older Paleozoic and possibly even Precambrian rocks (Os- berg, 1974). On the west flank of the Merrimack synclinorium next to the Bronson Hill anticlinorium, the Geologic Map indicates a broad band of “Devonian or Silurian” in southern New Hampshire, Massachusetts, and Con- necticut. On the Geologic Map of New Hampshire (1955), the part in that State was mapped as Devonian Littleton Formation, but there and in Massachusetts later work has indicated the existence of complex nappe structures that involve not only the Devonian but rocks as old as the Ordovician Partridge Formation (Thompson and others, 1968). Not all these com- plexities are yet resolved, and the noncommittal de- signation of DSe was recommended by geologists of the Survey who are working in New England. The eastern boundary of the area, as shown on the Geologic Map, is unsatisfactory and probably does not express the true relations. DEVONIAN The Devonian System is represented on the Geologic Map of the United States by marine stratified rofcks (D), divided in part into Lower, Middle, and Upper De- vonian Series (D1, D2, D3); by Middle and Upper Devo- nian continental deposits (D2c, D3c); and by eugeosyn- clinal deposits (De), with associated volcanic rocks (Dv). The Devonian, like the Silurian, is shown sepa— rately only in the northeastern third of the country; in the Southern Appalachians and the Ouachita Moun- tains it is combined with the Silurian (DS), and from the Rocky Mountains westward it is combined with the other lower Paleozoic systems (19, lE’e). MARINE STRATIFIED ROCKS (D) As with the systems below and above, the Devonian is most extensively exposed in the cratonic area of the Central Interior Region (fig. 5), where the strata are gently tilted on the flanks of the broad domes and ba- sins. The most prominent area of outcrop is that in southwestern New York and adjacent Pennsylvania and Ohio, where the strata dip gently southwestward into the Allegheny synclinorium. It displays the classic Devonian sequence, known since the early days of the New York Survey a century and a half ago, to which much of the system in the rest of the country has fre- quently been compared. Somewhat narrower bands of outcrops encircle the Cincinnati dome and Michigan basin in Ohio, Indiana, and Michigan, and a large out- lying area of southwest-tilted Devonian strata occurs in Iowa and adjacent Minnesota and Illinois. The New York outcrops bend around the northeastern end of the Allegheny synclinorium into the miogeosynclinal belt of the Appalachians, where the Devonian rocks form numerous bands among the folds of the Valley and Ridge province through Pennsylvania into Virginia. Beyond Virginia they thin to such an extent that they are combined with the Silurian (DS). According to North American usage, the Devonian is divided into the Lower Devonian Series (Helderberg and Oriskany) (D1), the Middle Devonian Series (Onondaga, Hamilton, and Tully) (D2), and the Upper Devonian Series (consisting of the remainder of the system) (D3). The resulting subdivisions are quite un- equal, the Lower Devonian being thin and inconstant and the Upper Devonian very thick—as is well illus- trated on the Geologic Map by the relative widths of the three series in New York. As a substitute, five named series have been proposed: Ulsterian, Erian, Senecan, Chautauquan, and Bradfordian, the last three in the Upper Devonian (Cooper and others, 1942, p. 1732). In addition, the European stage names (Gedinnian, Coblenzian, Eifelian, Gevetian, Frasnian, and Famennian) have come into increasing use in North America. The three series of the Devonian System are differ- entiated on the Geologic map throughout the broad areas in the north, which for the most part consist about equally of Middle and Upper Devonian rocks. The Lower Devonian Series appears only as a narrow band in New York State; where present elsewhere, it is combined with the Middle Devonian. The Devonian is not subdivided on the map in the smaller outcrop areas in the Central Interior, nor in the Appalachian miogeosynclinal belt. The Devonian sequence of the eastern outcrops in New York and Pennsylvania is 12,000 to 15,000 ft (3,600—4,500 m) thick, forming the apex ofa great clas- tic wedge of Middle and Upper Devonian deposits, partly continental (see below), that is frequently re- ferred to as the “Catskill delta.” The wedge is a product DEVONIAN GULF OF MEXICO "‘ FIGURE 5.—Eastern United States, showing areas mapped as Devonian on Geologic Map of United States. Includes units of of marine stratified rocks (D) and their subdivisions (D1, D2, D3), Devonian and Silurian marine stratified rocks (DS), continental deposits (D2c, D3c), eugeosynclinal deposits (De), volcanic rocks (Dv), Devonian and Silurian eugeosyn- clinal deposits (DSe), and volcanic rocks (DSV). 15 16 of the Acadian orogeny that was in progress in the Appalachian foldbelt to the east, and it thins westward into the Central Interior Region, as well as southwest- ward along the strike in the Appalachian miogeosyn- clinal belt. The Lower Devonian strata and the Onondaga Limestone precede the development of the clastic wedge and consist of thin persistent shale and lime- stone units, as‘ well as one prominent sandstone layer, the Oriskany. The Onondaga and its equivalents extend far westward into the Central Interior, for ex- ample, to the Falls of the Ohio at Louisville, Ky., men- tioned earlier. In New York, the Middle and Upper Devonian coarse clastic rocks intertongue westward with finer grained sandstones, gray shales, and thin limestones; these in- tertongue in turn with black shales (Cooper, 1933; Chadwick, 1935), the transitions in each successive part being displaced a little farther west. West of New York the Middle and Upper Devonian sequences are no more than a few thousand feet (300—600 m) thick. There are important limestone units in the Middle De- vonian, with coral and stromatoporoid reefs and banks, but the Upper Devonian is shaly as far west as the Mississippi River. Upper Devonian black shales with various spans of age are extensive in the Midwestern States, including such units as the Ohio, New Albany, and Antrim Shales. They are followed in places by similar shales of Early Mississippian (Kinderhookian) age, creating problems in placing the Devonian-Mississippian bound- ary on the Geologic Map. This is the case, for exam- ple, in the heavily drift covered border region between Indiana and Michigan, underlain by the Antrim, New Albany, Ellsworth, Sunbury, and Coldwater Shales, the middle three being indicated on the State Maps as "Devonian and Mississippian”; on the Geologic Map the systemic boundary is arbitrarily located between the New Albany and the Ellsworth. South of Virginia the sole representative of the black shales is the thin very persistent Chattanooga Shale (included in D8 on the Geologic Map), mostly Late De- vonian but including Early Mississippian beds in places, that is notable not only for its conodont fauna (Hass, 1956) but also for radiometric dating of its uraniferous shales at 350 my. It lies with a hiatus on earlier Devonian rocks, which it oversteps to rest on strata as old as Ordovician, producing a low-angle re- gional unconformity traceable as far west as Oklahoma. ' CONTINENTAL DEPOSITS (D2C, Dsc) As indicated above, the proximal part of the Middle and Upper Devonian clastic wedge in New York and PALEOZOIC AND MESOZOIC ROCKS Pennsylvania is formed of continental deposits— conglomerates, coarse sandstones, and redbeds that in- clude a fossil forest at Gilboa, N.Y., of late Middle De- vonian (Tully) age. The continental deposits project in the heights of the plateaulike Catskill Mountains that overlook the Hudson Valley. The extent of the conti- nental deposits indicated on the Geologic Map is based on the Geologic Maps of New York and Pennsylvania. Small patches of little-deformed red continental de- posits, in part plant bearing, also occur within the Ap- palachian foldbelt in eastern Maine, which are younger than the Acadian orogeny and lie on upended Devonian and earlier Paleozoic eugeosynclinal rocks. The Mapleton Sandstone of Aroostook County is late Middle Devonian, and the Perry Formation of the Eastport district at the eastern tip of Maine is Late Devonian; the ages of other occurrences are less cer- tain. For convenience, all are grouped as Middle Devo- nian (D2c) on the Geologic Map. EUGEOSYNCLINCAL DEPOSITS (De) Devonian eugeosynclinal deposits and associated volcanic rocks (Dv) are prominent in New England, but none are known in the Piedmont province of the Cen- tral and Southern Appalachians. All the eugeosyncli- nal DeVonian rocks in New England are Lower Devo- nian (or no younger than low Middle Devonian)—in contrast to the scanty Lower Devonian Series in the miogeosyncline and craton to the west—and mark the final climax of eugeosynclinal subsidence and sedimentation prior to the Acadian orogeny. Devonian deposits, well dated by fossils, form broad outcrop belts in Vermont, New Hampshire, and Maine; narrower more metamorphosed extensions have been traced into Massachusetts and Connecticut. Devonian phyllites and schists form a belt 25 mi (40 km) wide in the trough of the Connecticut Valley synclinorium in eastern Vermont and are as much as 15,000 to 20,000 ft (4,500—6,000 m) thick. The western part (Waits River Formation) is calcareous and the eastern part (Gile Mountain Formation) is quartzose; they have complex mutual relations, partly from in- tergradation, partly from major nappe structures. Metavolcanic rocks are rare, except for an amphibolite layer in the eastern part and fossils are rather sparse. Equivalents extend southward in a narrower belt into Massachusetts, where they include the long-known fossil locality at Bernardston. A small patch (Wepawaug Schist=upper part of Orange Phyllite of earlier usage) emerges from beneath the Triassic cover west of New Haven, Conn. (Fritts, 1962). The northeastern part of the Connecticut Valley synclinorium in northern Maine is dominated by the Seboomook Formation, a mass of deep-water shaly and LOWER PALEOZOIC sandy turbidites as much as 20,000 ft (6,000 m) thick (Boucot, 1961, p. 169—171). However, a shoal-water belt extending from Moosehead Lake northeastward for 90 mi (140 km) to north of Mount Katahdin re- ceived sandy deposits (Tarratine and Matagamon Formations). Early Devonian (Oriskany and early Onondaga) fossils are abundant in the shoal-water de- posits and sparse in the deep-water deposits. Lying on the shoal-water deposits at Mount Kineo, Traveler Mountain, and elsewhere are thick masses of rhyolitic volcanic rocks (Dv), probably erupted from calderas in an island arc (Rankin, 1968). They are succeeded by several thousand feet (300—600 In) of additional sediments—the shallow-water Tomhegan Formation at Moosehead Lake and the brackish-water or terres- trial Trout Valley Formation at Traveler Mountain. , The former contains Oriskany shelly fossils, the latter fossil plants of early Middle Devonian age. The Trout Valley Formation is unmetamorphosed and little de- formed and is either a late orogenic or a postorogenic deposit. In the Bronson Hill anticlinorium of New Hamp- shire, east of the Connecticut Valley synclinorium, the Silurian is overlain by the Littleton Formation of shaly and sandy rocks with a volcanic member near the mid- dle (mainly tuffs and breccias); it has been variably metamorphosed to phyllite or to garnet and sillimanite schists and gneisses (Billings, 1956, p. 27—35). Lower Devonian (Oriskany) fossils are well preserved in the low-grade belt along the Connecticut River, and a few have been recovered even in the high-grade rocks farther east. The Littleton is about 4,500 ft (1,400 m) thick in the Bronson Hill anticlinorium, but like the preceding Silurian it thickens eastward into the Mer- rimack synclinorium to 16,000 ft (4,900 m). The Devonian and Silurian rocks of the Merrimack synclinorium in New Hampshire continue southward into east—central Massachusetts, but relations here have been clouded by the occurrence of Carboniferous (Pennsylvanian?) plants at the “coal mine” near Wor- cester, and so for many years the whole complex of deposits was assigned to the Carboniferous system—as it was on the Geologic Map of the United States of 1932, following Emerson (1917). The main body of the ; rocks is, however, lithically very different from the au— 3 thentic Pennsylvanian of the Narragansett and Boston basins to the southeast, and it was involved in the mid-Paleozoic (Acadian) deformation. Present judg- ment is that the rocks at the “coal mine” are merely a remnant of a younger formation enclosed tectonically in much older rocks. On the Geologic Map, Pennsylva- nian (IP) is marked in a small patch at the fossil local- ity, and the surrounding rocks are classed as Silurian and Devonian (Se, De). 17 LOWER PALEOZOIC From the Rocky Mountains westward, the Cam- brian, Ordovician, Silurian, and Devonian Systems form such small outcrops, either singly or together, that they are combined on the Geologic Map of the United States into a unit of lower Paleozoic, with a distinction between marine stratified rocks (19) and eugeosynclinal deposits (lPe). In a few areas the Cam- brian sequence (£3) forms outcrops sufficiently exten- sive for representation; elsewhere it is merged with the other systems. The same designation is also used for some small outcrops of lower Paleozoic rocks farther east in the United States, as explained below. MARINE STRATIFIED ROCKS (IP) The lower Paleozoic marine stratified rocks are of several different kinds—cratonic deposits, miogeosyn- clinal deposits, and miscellaneous rocks of outlying areas—which it is appropriate to describe separately. CRATONIC DEPOSITS Lower Paleozoic cratonic deposits similar to those in the Central Interior Region extend across the Central and Southern Rocky Mountains, the Colorado Plateau, and the Basin and Range Province of New Mexico and Arizona; there they are exposed in narrow bands of tilted strata along the edges of the uplifts of Precam- brian rocks (fig. 6). The Cambrian is shown separately in Arizona, but in some parts of the Basin and Range Province all the lower Paleozoic is missing. The lower Paleozoic cratonic deposits are no more than a few hundred or few thousand feet (60—600 m) thick in any of the outcrops, and each system has its own pattern of distribution and thickness independent of the others, reflecting in part the shifting through time of the epicontinental seas (fig. 7). Sequences at any locality are thus incomplete, lacking one or more systems or parts of systems. Details of distribution and thickness of the systems in outcrop and subsurface are illustrated in the “Geologic Atlas of the Rocky Moun- tain Region” (Mallory, 1972), to which the reader is referred. Especially notable is the complete absence of Silurian rocks from any outcrop area except in south- ern New Mexico, although its former presence in places is suggested by remnants preserved in dia- tremes in northern Colorado. Equally interesting are the areas where the lower Paleozoic rocks are missing entirely and upper Paleozoic or lower Mesozoic rocks lie directly on Pre- cambrian. Some of these areas, such as the Front Range in Colorado and Wyoming and the Uncom- pahgre Plateau in western Colorado, were the sites of geanticlines that were raised in later Paleozoic time, 18 PALEOZOIC AND MESOZOIC ROCKS FIGURE 6.—Western United States, showing areas mapped as lower Paleozoic on Geologic Map of United States. Includes units of marine stratified rocks (15’), eugeosynclinal deposits (lPe), Cambrian (£3), and Lower Ordovician (01). LOWER PALEOZOIC A CAMBRIAN B ORDOVICIAN C’ SILU RIAN D DEVONIAN FIGURE 7.—Eastern part of Cordilleran Region, showing surface and subsurface extent of the different systems grouped as lower Paleozoic on Geologic Map of United States: A—Cambrian (lines L, M, and U indicate maximum extent of Lower, Middle, and Upper Cambrian Series), B—Ordovician,’ C——Silurian, D—Devonian. Compiled from Geologic Atlas of Rocky Mountain Region (Mallory, 1972) and other sources. 19 20 when older deposits, if they had ever existed, were eroded. Other areas, such as the block ranges of north- ern New Mexico, are parts of the “Transcontinental arch,” a paleotectonic feature that extended south- westward from the Lake Superior Region, upon which many of the Paleozoic systems either never deposited or were laid down so thinly that they were removed later. Relations are well illustrated in the New Mexico ranges, where representatives of all the lower Paleozoic systems occur in the south but with each one thinning and wedging out northward toward the site of the arch until none remain. Westward from southern New Mexico, only Cam- brian rocks and a thin Devonian formation (Martin Limestone) persist into Arizona. Here, it is appropriate on the Geologic Map to represent the Cambrian (€)but to merge the Devonian with the Mississippian and Pennsylvanian into a unit of upper Paleozoic (uE’). MIOGEOSYNCLINAL DEPOSITS / The miogeosynclinal lower Paleozoic rocks form all or large parts of many of the ranges in the Northern Rocky Mountains and the eastern Great Basin. Here, the Cambrian System can be separated at many places on the Geologic Map. The miogeosynclinal deposits do not differ in either lithology or origin from the cratonic deposits, but they are greatly thicker and have a more complete se- quence. In Utah, the change from craton to miogeosyncline takes place near the present western edge ‘of the Colorado Plateau along the “Wasatch line”—a tectonic boundary with ancient antecedents. West of it in the Great Basin, Lower Cambrian strata wedge in at the base of the sequence, and all the suc- ceeding lower Paleozoic deposits thicken; Silurian rocks, so notably missing from the craton to the east, make their appearance. Contrasts between the two lower Paleozoic sequences have [been further em- phasized during the Cretaceous Sevier orogeny, when the two were telescoped along great thrusts along the “line.” In the Great Basin of western Utah and eastern Nevada, Middle Cambrian through Devonian se- quences are characteristically 10,000 to 15,000 ft (3,000—4,500 m) thick and overlie 3,000 ft (900 m) or more of Lower Cambrian clastic deposits. The long- known sequence at Eureka, east-central Nevada, is 14,500 ft (4,300 m) thick and is composed of 60 percent limestone, 30 percent dolomite, 8 percent shale, and 2 percent quartzite (Nolan and others, 1956). A notable sandy unit above the basal clastics is the Middle Or- dovician Eureka Quartzite (= Swan Peak Quartzite) about 300 ft (100 m) thick, which spreads across most of the eastern half of the Great Basin, like the nearly PALEOZOIC AND MESOZOIC ROCKS contemporaneous Saint Peter Sandstone of the Central Interior, and like the Saint Peter is derived from areas of crystalline rocks in the craton. Not only are the Cambrian and Lower Ordovician rocks of carbonate facies, as in the Appalachian miogeosyncline, but also so are the younger lower Paleozoic systems; the strata abOve the Eureka Quartzite in the Eureka district (Upper Ordovician, Silurian, and Devonian) are about 6,000 ft (1,000 m) of limestone and dolomite. Some of the lower Paleozoic miogeosynclinal strata are involved in a highly metamorphosed plastically de- formed infrastructure (-6 and 1?, with metamorphic overprint) which emerges in windows from beneath the less altered Paleozoic rocks in the Ruby Range, north- eastern Nevada, and in tectonically similar situations to the east and north. Mapped with the lower Paleozoic miogeosynclinal rocks is the so-called “transitional assemblage” of for- mations (-C and I?) which are tectonically entangled with rocks of the eugeosynclinal or “western as- semblage” east and northeast of Winnemucca, north- central Nevada (Roberts and others, 1958, p. 2817), but which have features not entirely like either the miogeosynclinal or the eugeosynclinal deposits. They include a thick quartzite like the Lower Cambrian clastic deposits farther east, various overlying grapto- lite shales, and the remarkable Upper Cambrian Har- mony Formation, an arkosic turbidite of unknown prov- enance which is interleaved tectonically with quite different Paleozoic and Mesozoic rocks in many ranges. ROCKS OF OUTLYING AREAS Besides the cratonic and miogeosynclinal rocks of the Cordilleran Region, some rocks in small areas farther east, in Texas, Oklahoma, and the Southern Appalachians are mapped as undivided lower Paleozoic. The Marathon region of western Texas is a small— scale replica of the Ouachita Mountains foldbelt, and its Cambrian, Ordovician, and Devonian rocks are broadly similar to the lower Paleozoic of the Ouachitas. They are exposed in several anticlinoria which are too small on the scale of the map to permit subdivision. In the Arbuckle and Wichita Mountains of southern Oklahoma, the Upper Cambrian Reagan Sandstone and overlying carbonates are separately mapped (‘6), but the succeeding lower Paleozoic sequence is not subdivided (15’). Much of the latter is Ordovician, which includes the very thick carbonates of the Lower Ordovician Arbuckle Group, but thin Silurian and De- vonian units (Hunton Group and Woodford Chert) occur at the top. The designation lower Paleozoic (l?) is used for rocks in a few outcrop belts in the Piedmont province of LOWER PALEOZOIC the Southern Appalachians; their precise ages are un- certain, but they appear to be younger than the rocks adjacent to them. Their occurrence is as follows: (1) Slates and phyllites in the Talladega belt, Alabama, below the Cheaha Sandstone and associated fossiliferous rocks (DS); they may be Ordovician, but both older and younger ages have been claimed. (2) The Wedowee Formation and Ashland Schist in the next belt to the southeast, composed of rocks like those in the lower part of the Talladega sequence but more metamorphosed and more involved with plutonic rocks. \ (3) Rocks of the Wacoochee (= Pine Mountain) belt of the southern Piedmont in Georgia and Alabama (Hollis Quartzite, Chewacla Marble, and Manchester Schist), which lie with apparent unconformity on or— thogneisses (an) that have yielded 1,.000-m.y. radiometric dates. (4) Rocks of the Murphy Marble Belt in northeast- ern Georgia and southwestern North Carolina. They overlie and are synclinally downfolded into the Ocoee Supergroup (Z) and include the Murphy Marble near the top. The marble has been correlated with the Lower Cambrian Shady Dolomite on physical re- semblance, and a few poorly preserved Paleozoic fossils have been recovered from it. (5) Low-grade schists of the Chauga belt on the southeastern side of the Brevard fault zone in north- western North Carolina. (6) Rocks of the Kings Mountain belt on the South Carolina-North Carolina border east of the Brevard fault zone, which include distinctive units of schist and quartzite, and the Gaffney Marble. EUGEOSYNCLINAL DEPOSITS (l Pe) Lower Paleozoic eugeosynclinal deposits occur mostly in Nevada and California, the only exceptions shown on the Geologic Map being a few small areas in south-central Idaho. In contrast to the carbonate- quartzite facies of the miogeosynclinal belt to the east, they are a facies of clastics, cherts, and volcanics. All of them are either continental margin or "off the conti- nent” deposits, but they probably formed in diverse environments, which are difficult to reconstruct be— cause of the wide separation of the different groups of exposures. The eugeosynclinal rocks in Nevada crop out in the ranges of the Great Basin in a belt 80 mi (130 km) wide that extends south-southwest across the center of the State from the Idaho border to the California border. For more than half this breadth, the eugeosynclinal rocks are allochthonous on the contemporaneous miogeosynclinal rocks along a major surface of move- ment, the Roberts thrust, which formed during the Ant- 21 ler orogeny of late Devonian and early Mississippian time. The miogeosynclinal rocks appear as windows in the different ranges, partly or wholly surrounded by the overlying eugeosynclinal rocks. The lower Paleozoic sequence is overlain by postorogenic Missis- sippian and Pennsylvanian deposits (Diamond Peak Formation, Battle Formation, and others). The lower Paleozoic eugeosynclinal sequence in Nevada is probably as much as 50,000 ft (15,000 m) thick, although not all of it is preserved in any one area (Roberts and others, 1958, p. 2816—2817). On the aver- age, shale constitutes 20 to 40 percent; sandstone, graywacke, and quartzite 10 to 30 percent; and vol- canic rocks from a few percent to 30 percent. The vol- canic rocks are andesitic and basaltic pillow lavas and associated pyroclastics. The most extensive component is of Ordovician age (the shaly Vinini Formation and the equivalent sandy Valmy Formation farther north- west). Rocks of other ages occur only in smaller areas. The Cambrian (Scott Canyon Formation) is rep- resented only in a small outcrop in the \south part of Battle Mountain. The Silurian and Devonian rocks (Elder Sandstone and Slaven Chert) are typically de- veloped in the northern Shoshone Range, although they occur sporadically elsewhere. Graptolites are common in the Ordovician and Silurian and are the chief means of zonation and correlation; other fossils are more sparse, both here and in the lower and high- er beds. In the Sierra Nevada of eastern California, lower Paleozoic eugeosynclinal rocks occur both east and west of the Jurassic and Cretaceous granitic rocks of the Sierra Nevada batholith in the core of the range. Those on the eastern side, west and northwest of Owens Valley, are preserved only in roof pendants up to 19,000 ft (5,800 m) thick and are mostly Ordovician (Rinehart and others, 1959, p. 941—944). Those on the west side extend southward along the foothills for more than 100 mi (160 km) from Taylorsville past Placer- ville and are collectively termed the Shoo Fly Forma- tion (although other names have been given to differ- ent parts in the past) (Clark and others, 1962; McMath, 1966, p. 178—179). Fossils of Silurian age have been obtained in one area near Taylorsville; other parts are barren but might include Ordovician as well as Silu- rian rocks. The Shoo Fly attains a thickness as great as 50,000 ft (16,000 m), with no base Visible, and consists of weakly metamorphosed phyllite and minor chert, siltstone, and quartzose sandstone, and some tuff and greenstone. It is overlain unconformably toward the northeast by the dacitic volcanics of the Sierra Buttes Formation which contain Devonian ammonoids in quartzite lenses (Anderson and others, 1974). On the west it is adjoined by the Calaveras Formation which 22 PALEOZOIC AND MESOZOIC ROCKS is generally considered to be upper Paleozoic, but the contact is mostly faulted. In northern California, lower Paleozoic eugeosyncli- nal rocks occur in the eastern subprovince of the Klamath Mountains, Where they are so entangled with ultramafic rocks and Jurassic granitic plutons that only fragments are preserved in any area and no com- plete sequence is known. In the north, west of Mount Shasta, are the Duzel and Gazelle Formations (Hotz, 1971, p. 7—8) of shale, volcanic graywacke, chert, and lenticular limestone, the one in thrust contact with the other. Shelly fossils (corals, brachiopods, and trilo- bites) in the limestone lenses indicate that the Duzel is Late Ordovician(?) and the Gazelle is Silurian. No younger Paleozoic rocks are known in the area. Farther south, in the Redding area, is a mass of Devo- nian volcanics—the Copley Greenstone that includes andesitic pillow lavas, and the local overlying body of Balaklala Rhyolite. They are capped by cherty shales and local limestone banks of the Kennett Formation with Middle Devonian fossils, shoal water deposits that probably accumulated on the crest of a volanic island arc. The Devonian rocks are overlain by the Mississippian Bragdon Formation, but the contact is seemingly tectonic. The central metamorphic belt of the Klamath Moun- tains is a thrust slice west of and tectonically beneath the rocks of the eastern subprovince. It is formed of Salmon Hornblende Schist and Abrams Mica Schist, which have yielded K/Ar radiometric ages of 270—329 my and Rb/Sr ages of 380 my, indicating that the original rocks are Devonian and older and had a pro— tracted mid-Paleozoic metamorphic history (Hotz, 1971, p. 10—141). They might be metamorphic equiva- lents of some of the lower Paleozoic eugeosynclinal rocks farther east, and they are indicated on the Geologic Map as 199, with metamorphic overprint. MISSISSIPPIAN The Mississippian System (M) is portrayed on the Geologic Map of the United States in the eastern two- thirds of the country; from the Rocky Mountains west- ward it is merged with the other upper Paleozoic sys- tems into a single unit (uP, uEe). Within the region Where it is mapped, the Mississippian is classed as marine stratified rocks; some continental deposits occur in the northeastern part of the Central Appala- chians but are not separated. No eugeosynclinal de- posits are distinguished; they are missing in the Ap- palachian Region, where eugeosynclinal conditions were terminated by the Devonian Acadian orogeny. Eugeosynclinal deposits are known only in the ex- treme western part of the United States, where those of Mississippian age are merged with the rest of the upper Paleozoic (uE’e). The Mississippian System is essentially the same as the Lower Carboniferous of Europe, and the differences are not fundamental; the top of the Lower Carbonifer- ous is placed between the Visean and Namurian Stages, at a level within the American Chesterian Series (Weller and others, 1948, p. 107—109). The US. Geological Survey defers to European usage by desig- nating the Mississippian and Pennsylvanian as “the Carboniferous Systems.” The American Mississippian is divided into the Kinderhookian, Osageian, Merame- cian, and Chesterian Series, the first two being com- bined on the Geologic Map as M1, the other two being labeled M2 and M3. The first two have also been called Lower Mississippian and the second two Upper Missis- sippian, but these are not useful for purposes of the Geologic Map. As with the other Paleozoic systems, the Mississip- pian is most extensively exposed in the cratonic area of the Central Interior Region, where it forms broad bands of gently tilted strata between the crests of the domes and the depths of the basins (fig. 8). From the type region along the upper Mississippi River in Iowa and Illinois, outcrops are nearly continuous south- westward around the Ozark dome, southeastward around the Illinois basin around the Nashville and Cincinnati domes, and on the flank of the Allegheny synclinorium. A large detached area in the lower peninsula of Michigan surrounds the Michigan basin. Where the outcrop belts are sufficiently wide, the sys- tem is divided into the three parts (M1, M2, M3). Mississippian rocks also form narrow outcrop bands throughout the miogeosynclinal belt of the Central and Southern Appalachians and occur farther southwest in the deformed areas of the Ouachita and Arbuckle Mountains of Arkansas and Oklahoma and the Marathon region of western Texas. The Mississippian strata are generally conformable with the Devonian; the black shales in the upper part of the latter are followed by black shales in the Kin- derhookian series. Throughout much of the Interior Region, Mississippian rocks are unconformable be- neath the Pennsylvanian, the discordance being most prominent toward the north, where the Chesterian series is missing at the top of the Mississippian and the Morrowan and Atokan series are missing at the base of the Pennsylvanian. In Iowa and northern Illinois, the Pennsylvanian sequence bevels the Mississippian at a low angle, to the west lying on different subdivisions of the Mississippian sequence and farther east on rocks of earlier ages down to the Ordovician. The Mississippian sequence is 6,000 ft (1,800 m) or MISSISSIPPIAN Q “a ‘ \W \ GULF OF MEXICO ‘ ., FIGURE 8.—Eastern United States, showing areas mapped as Mississippian on Geologic Map of United States. Includes units of marine stratified rocks (M) and their subdivisions (M1, M2, M3). 23 24 more thick in the Appalachian miogeosyncline. To the northeast, in eastern Pennsylvania, it is largely conti- nental (Pocono and Mauch Chunk Formations), with a few thin coal beds in the loWer part. Southwestward along the strike it is marine, with much limestone in the middle (in the Meramecian series) and with shales and sandstones below and above. In Alabama, the lower part of the Chesterian series is the Floyd Shale, which oversteps eastward in the Valley and Ridge province onto rocks as old as Early Ordovician. The upper part is the Parkwood Formation, a sandy unit which may extend conformably upward into the basal Pennsylvanian. Westward into the Central Interior, the Mississip- pian rocks thin to little more than a few thousand feet (300—600 In) and are mainly limestone. Oolitic lime- stones in the Meramecian series near Bedford, Ind., are extensively quarried for building stone. The Kinder- hookian and Osageian limestones of Iowa and Illi- nois are famous for their crinoids. Farther south, the Osageian carbonates are siliceous in such units as the Fort Payne Chert near the Nashville dome and the Boone Chert on the south flank of the Ozark dome. The Chesterian Series at the top is more heterogeneous than the rest, with much sandstone and many little units of shale and limestone. Near the common corners of Missouri, Oklahoma, and Arkansas, there are discrepancies between the State Maps in representation of the subdivisions of the Mississippian System. In southwestern Missouri the map shows Osageian (cherty limestones) (M1), Meramecian (limestones) (M2), but no Chesterian (M3). In northeastern Oklahoma the map shows Boone Chert (presumably Osageian), followed by Chesterian rocks; much the same units continue eastward into Arkansas. The Chesterian series wedges out northeastward near the northeastern corner of Oklahoma; the Meramecian of Missouri must continue southwestward into the upper part of the Boone Chert, but its extent is uncer- tain. On the Geologic Map, the Meramecian series is shown as wedging out southwestward in Oklahoma, beyond which Osageian rocks (M1) are shown in con- tact with Chesterian (M3); rocks of Meramecian age may be present also but are not represented on the Geologic Map. A very different facies of the Mississippian sequence from those considered so far occurs in the foldbelt of the Ouachita Mountains in Arkansas and Oklahoma and in its outlier in the Marathon region of western Texas. Here, novaculite is succeeded abruptly by a great flysch sequence. The novaculite is a condensed se- quence; conodonts in the Arkansas Novaculite indicate that it includes all the Devonian Period and the early Mississippian (Kinderhookian) as well. The flysch, by PALEOZOIC AND MESOZOIC ROCKS contrast, is a very thick body of elastic turbidites, laid down in a rapidly subsiding trough. In the Ouachita Mountains the lower two formations, the Stanley Shale and J ackfork Sandstone, are as thick as 22,000 ft (6,700 m), thinning to the north. In the northern part of the mountains, the succeeding Johns Valley Shale and Atoka Formation are nearly as thick. Equivalent formations in the Marathon region are thinner, al- though still of impressive proportions. The age of the lower part of the flysch sequence has long been disputed. At one time or another, the whole has been called Pennsylvanian by some, and all the sequence into the lower part of the Johns Valley Shale has been called Mississippian by others. Fossils are scarce and have been variously interpreted. Plant re- mains, broken and transported, have yielded ambigu- ous testimony (Miser and Hendricks, 1960, p. 1831). However, conodonts in the lower part of the Stanley are clearly Meramecian (Hass, 1950), and other fossils higher up are Chesterian; goniatites in the Jackfork are Morrowan (Gordon and Stone, 1969). The Missis- sippian fossils in the Johns Valley Shale are in trans- ported blocks, derived from the Caney Shale in the foreland to the northwest. On the Geologic Map, the Stanley Shale is labeled Mississippian (M) on the assumption that it is of Meramecian and Chesterian age, and the Jackfork Sandstone is labeled earliest Pennsylvanian (I? 1a), or of Morrowan age. The Tesnus Formation of the Marathon region includes equivalents of both the Stanley and Jackfork; it contains Mississipian cono- donts in the lower part and Pennsylvanian plants in the upper part. For convenience, all the Tesnus is labeled Mississippian (M) on the Geologic Map. PENNSYLVANIAN The Pennsylvanian System (I?) is portrayed on the Geologic Map of the United States in the eastern two- thirds of the country; from the Rocky Mountains west- ward it is merged with the other upper Paleozoic sys- tems into a single unit (uP, uPe). Within the region where it is mapped, the Pennsylvanian is classed as marine stratified rocks, even though it includes coal measures and other land-laid deposits, especially to- ward the east. No eugeosynclinal deposits are distin- guished; they are known only in the extreme western part of the country, where those of Pennsylvanian age are merged with the rest of the upper Paleozoic (u E’e). The Pennsylvanian Period is essentially the same as the Upper Carboniferous of Europe; the Namurian Stage extends a little lower than the Pennsylvanian, into the American Chesterian Series. The Namurian, Westphalian, and Stephanian Stages, and others of European usage, are not widely referred to in North PENNSYLVANIAN America. Instead, classification is based on two sets of subdivisions—the coal measures sequence in the east consisting of the Pottsville, Allegheny, Conemaugh, and Monongahela Groups and the marine sequence in the west comprising the Morrowan, Atokan, Des- moinesian, Missourian, and Virgilian Series. Correla- tions between the two sets of units are known. The Morrowan and Atokan are approximately Pottsville; and the Desmoinesian, Missourian, and Virgilian are approximately Allegheny, Conemaugh, and Monon- gahela, respectively, except that the Conemaugh em- braces somewhat more strata above and below than the Missourian. On the Geologic Map the two sets of’ names are used interchangeably; the Morrowan, Ato- kan, and Pottsville are indicated as H’i, the succeeding units as P2, “’3, and W4. ThepPennsylvanian has also been subdivided into Lower, Middle, and Upper Pennsylvanian, the Lower Pennsylvanian being the , Morrowan, the Middle Pennsylvanian the Atokan and ‘ Desmoinesian, and the Upper Pennsylvanian the Mis- sourian and Virgilian; this is not useful for purposes of 1 the Geologic Map. Exposures of the Pennsylvanian sequence are mainly in the cratonic area, where it forms the centers of the basins and is the youngest Paleozoic rock pre- served (fig. 9). Along the front of the Appalachian foldbelt, it forms the trough of the Allegheny synclinorium in Pennsylvania, West Virginia, Ken- tucky, and Alabama. A little of the Permian Dunkard Group overlies it in the north, and some outliers of , Pennsylvanian rocks are preserved to the east in the folds of the Valley and Ridge province. Farther west, the Pennsylvanian forms a large area in the Illinois basin of Illinois, Indiana, and Kentucky, and a smaller area in the center of the Michigan basin. West of the Mississippi River it underlies an even larger area that extends from Iowa to Texas, with an extension east- ward in the Arkoma basin of Oklahoma and Arkansas between the Ozark dome and the Ouachita Mountains 2 foldbelt; the strata are mostly tilted westward beneath Permian rocks in the Prairie Plains homocline. All these areas were originally more nearly continuous; some of them are still almost connected by intervening outliers, and the sequences in the different areas can be closely matched with each other. The Pennsylvanian System has been intensively studied, partly for its great deposits of coal toward the east and for its importance in petroleum exploration farther west. Its continental and marine deposits have an unprecedented multiplicity of lateral and vertical variations, accompanied by extreme persistence of many thin sedimentary units and a remarkable cyclic pattern of deposition in many regions, all suggestive of extreme tectonic stability. The record of land plants 25 and marine animals of the time is voluminous (Moore and others, 1944, p. 659). The Pennsylvanian sequence is generally unconfor- mable on the underlying Mississippian, especially in the northern Midwestern States, where it truncates the various subdivisions of the Mississippian at a low angle and extends downward across strata as old as Ordovician. Conformity with the Mississippian rocks occurs only along the edges of the Appalachian and Ouachita foldbelts. The Pennsylvanian is generally conformable with the Permian above in the Allegheny synclinorium and Prairie Plains homocline, and there have been considerable differences through the years in the placement of the boundary between the two sys- tems. The Pennsylvanian , sequence is about 3,000 ft (900 m) thick in the Allegheny synclinorium, thinnest in the north and thicker southwestward mainly be- cause of increase in the Pottsville Group at the base. In Alabama, Pennsylvanian rocks are 10,000 ft (3,000 m) thick, with the Pottsville alone represented. In most of the Illinois basin to the west the sequence is less than 2,000 ft (600 m) thick, but it thickens to the south to about 3,000 ft (900 In). West of the Mississippi River, the Pennsylvanian rocks are less than 2,000 ft (600 m) thick in the northern part'of the Prairie Plains homo- cline, but they thicken southward in Oklahoma and Arkansas, mainly by wedging in of the Morrowan and Atokan Series at the base. Along the front of the Ouachita Mountains in Arkansas, the Atoka Forma- tion alone is 18,000 ft (5,500 m) thick, but it thins rapidly northward across the Arkoma basin to a feath- eredge along the front of the Ozark dome 80 mi (130 km) to the north. The Pennsylvanian rocks are dominantly nonmarine and coal bearing to the east and dominantly marine to the west, but the two types of deposits are complexly interfingered in very thin units. Through much of the Interior Region, the sequence consists of cyclical units, or cyclothems. In the Illinois basin, a typical cyclothem begins with sandstone, resting on a channeled surface, followed by sandy shale, freshWater limestone, under- clay, and coal, which is overlain by gray and black shale and marine limestone (Wanless, 1962, p. 49—50). In the Allegheny synclinorium, nonmarine elements dominate and the coals are thicker, but fossiliferous marine limestones persist into Ohio and western Pennsylvania (Branson, 1962, p. 199). In Kansas, each cyclothem is dominantly marine shale and various kinds of limestone, but a coal bed sometimes occurs in the lower part (Merriam, 1963, p. 103—108). Each cy- clothem and its subdivisions are traceable for scores or hundreds of miles. The ultimate cause of the cyclical sedimentation is uncertain, but the rock record indi- 26 PALEOZOIC AND MESOZOIC ROCKS GULF OF MEXICO ., FIGURE 9.—-Eastern United States, showing areas mapped as Pennsylvanian on Geologic Map of United States. Includes units of marine and nonmarine stratified rocks (IP) and their subdivisions (P1, P1a,|P2,|P3,IP4). PERMIAN cates fluctuating deposition of level surfaces near sea level under conditions of great stability. In the northern Midwestern States, sedimentological studies indicate that the sandstone beds were derived from the north, from crystalline areas in the Canadian Shield, and some of the channels in which they were transported have been traced. Farther south and southeast, a greater proportion of the clastics was de- rived from the Appalachian and Ouachita foldbelts. In Alabama, the thick Pottsville sequence is all shallow-water, with coal beds at intervals throughout. It joins in the subsurface beneath the Mississippi Em- bayment with the equally thick or thicker Atoka For- mation of the Arkoma basin, which is a deep-water flysch or turbidite that filled a rapidly subsiding trough along the northern edge of the Ouachita foldbelt. A depth change between the two is indicated and is duplicated in the Atoka itself, which changes northward toward the Ozark uplift into a shallow- water deposit more like the Pottsville. The underlying Jackfork Sandstone ( lP 1a) and Johns Valley Shale, of Morrowan age, are also flysch facies. A remarkable feature of the Johns Valley in the frontal belts of the Ouachita Mountains is the occurrence of wildflysch, or beds containing blocks and slabs a hundred feet or more (30—60 m) across of older forma- tions of the cratonic sequence to the north and north- west. Similar block beds occur in the Marathon region of western Texas, but at a somewhat higher level, in the Haymond Formation of Atokan age. In the Arkoma and Ardmore basins north and west of the Ouachita Mountains, an important discontinuity is indicated at the top of the Atokan Series by a moder- ate unconformity at the base of the Desmoinesian and by the occurrence in the Desmoinesian of conglomer- ates derived from the Ouachita Mountains. These suggest that much of the deformation in the Ouachita foldbelt had been completed by the end of Atokan time. This is confirmed by drilling in the Coastal Plain south of the Ouachita Mountains, where fossiliferous Des- moinesian carbonates lie undeformed on steeply folded Ouachita rocks. The Pennsylvanian sequence in north-central Texas is separated from that in the remainder in the Prairie Plains homocline to the north by overlapping Creta- ceous deposits but is broadly similar. It has long been divided into the Bend, Strawn, Canyon, and Cisco Groups. The Bend Group (including the prominent Marble Falls Limestone) is of Morrowan and Atokan age and is unconformable beneath the Strawn Group which overlaps it from the east. The Strawn and Can- yon Groups are broadly equivalent to the Desmoines- ian and Missourian Series, but the original Cisco 27 Group has now been partitioned between the Virgilian Series and the basal Permian Wolfcampian Series. Most of the outcrops of Pennsylvanian age have been divided on the Geologic Map into the four divisions (Pl, “’2, P3, and W4), but for various reasons the Pennsylvanian is not divided in the smaller outlying areas. In the Marathon region of western Texas, the undivided Pennsylvanian (IP) includes the Dimple Limestone and Haymond Formation, which are flysch deposits of Morrowan and Atokan age, and the thinner shallow-water Gaptank Formation that embraces the rest of the Pennsylvanian System into the Virgilian Series. In Pennsylvania and Maryland the Pennsylva- nian is undivided in the outlying downfolds in the Val- ley and Ridge province—the Anthracite basins, the Broadtop basin, and the Georges Creek basin. The subdivisions of the main Pennsylvanian area are rec- ognizable in these outliers, but their outcrops are too narrow for representation on the scale of the Geologic Map. Also shown as undivided Pennsylvanian (H’) are the rocks of the Narragansett and Boston basins in south- eastern New England, whose precise correlation with the standard sequences is less certain. They lie uncon- formably on earlier Paleozoic and Precambrian metamorphic and plutonic rocks and are postorogenic to their deformation (Quinn and Oliver, 1962). How- ever, they themselves are steeply folded and along the south coast are intruded by the late Paleozoic Nar- ragansett Pier Granite (Pga). The rocks of the Nar- ragansett basin are at least 10,000 ft (3,000 m) thick and consist largely of sandstone and shale of gray to black color, with some redbeds in the lower part in the north. Conglomerates composed of pebbles and boul- ders of the surrounding rocks occur at the base and at intervals higher up in the sequence, and there are a few beds of coal, largely altered to graphite. Fossil plants are fairly abundant and indicate a middle Pennsylvanian age. The rocks of the Boston basin to the northeast are 5,000 ft (1,500 m) or more thick and differ somewhat from those of the Narragansett basin. A few late Paleozoic plants have been collected but do not furnish data for firm correlation. The rocks of the Boston basin may be older than those of the Narragan- sett basin. A younger, or Permian, age has been claimed for them on the basis of an alleged tillite (Squantum Tillite Member of Roxbury Conglomerate), but this deposit is probably neither Permian nor a tillite. PERMIAN The Permian System (P) is shown on the Geologic Map of the United States in a small area in the Ap- 28 palachian Region, in extensive areas in the Midconti- nent Region, and in parts of the Cordilleran Region. Elsewhere in the Cordilleran Region, from the Rocky Mountains westward, it is merged with the remainder of the upper Paleozoic (uE’, uE’e). Most of it is classed as marine stratified rocks, although some of it is actu- ally of continental origin. In a few areas in California, Permian eugeosynclinal deposits (Pe) are separated from the remainder of the upper Paleozoic. In the Mid- continent Region and Colorado Plateau, the Permian is divided into its four series (P1, P2, P3, P4), and some of these are divided into smaller subdivisions (P2a, P2b, Psa, Pab). the meaning of which is explained below. The Permian is the youngest system of the Paleozoic Era. Its type area is in European Russia, where it was proposed by Murchison in 1841 to supplant older terms such as Dyas that had been in use in western Europe for Paleozoic strata above the Carboniferous (Dunbar and others, 1960, p. 1764—1767). In the United States, the type area is in the richly fossiliferous marine strata of western Texas, where exposures in various moun- tain ranges (especially the Glass and Guadalupe Mountains) are divided into the Wolfcampian, Leonar- dian, Guadalupian, and Ochoan Series (P1, P2, P3, P4) (Adams and others, 1939). The Permian has also been divided into Lower and Upper Permian (with a some- what uncertain boundary between them in the United States), but these are not useful for the Geologic Map. The position of the base of the Permian has fluc- tuated through the years, the common tendency being to move it downward. As originally defined by Murchi- son, the lower division was the Kungurian Series, and the underlying Artinskian Series was included in the Carboniferous System. Later it was realized that the Artinskian was, in fact, younger than any Carbonifer- ous rocks in western Europe, and so it was accordingly added to the Permian System. At present, the base of the Permian is placed below the Sakmarian Series (originally a lower division of the Artinskian), which is . equivalent to the Wolfcampian in the United States. This basal unit is especially characterized by the zone of the fusulinid Pseudoschwagerina. At most places the Permian sequence is conformable on the underlying Pennsylvanian. The contact is un- conformable in parts of west Texas, as a result of orogenies in the adjacent Ouachita foldbelt. Through- out the United States the Permian is everywhere un- conformable with the overlying Mesozoic rocks and especially with the Triassic System where it is present. In the west Texas standard area, the overlying strata are Upper Triassic. In the Cordilleran Region farther west, Lower Triassic rocks are common, but the high- est Permian stage is Leonardian or Guadalupian; so, a considerable gap exists, representing latest Permian K PALEOZOIC AND MESOZOIC ROCKS time. Commonly, the unconformity at the top is not structural, and in parts of the Cordilleran Region the Permian and Triassic Systems are rigidly parallel for long distances, even though a hiatus exists between them. APPALACHIAN REGION The Dunkard Group (P1) occupies the deeper part of the Allegheny synclinorium in Pennsylvania, Ohio, and West Virginia as erosional remnants on top of the Monongahela Group ( IP 4) (fig. 10). Its preserved thick- ness is about 1,200 ft (400 m), but it was probably originally much thicker. The Dunkard is dominantly nonmarine. A few limestone beds in the northwestern exposures suggest vague marine connections, but the remainder is nonmarine shale, partly red, and sandstone. A few coal beds occur but not as abundantly as in the Pennsylvanian System, prompting the early term “Upper Barren Measures” for the group. As in the Pennsylvanian strata beneath, the different rock types are arranged in cyclical order. Fossils in the Dunkard Group are sparse (Berryhill, 1967; Barlow, 1972). They include vertebrates, insects, freshwater invertebrates, and plants. Only the plants offer much indication of precise age; most of them are characteristic Pennsylvanian types, but they also in- clude Callipteris conferta, which is commonly consid- ered to be a Permian index fossil and is the chief basis for the traditional classification of the group as early Permian. Nevertheless, this assignment has been questioned by some who would prefer to place the group in the late Pennsylvanian. On the Geologic Map, the Dunkard Group is classed as early Permian (P1), or approximately equivalent to the Wolfcampian Series farther west. SOUTHWESTERN UNITED STATES Permian rocks extend continuously in a wide band in the Midcontinent Region from Nebraska to central Texas, where they are part of the west-tilted Prairie Plains homocline (fig. 10). They are also exposed in west Texas and eastern New Mexico on the opposite flank of the intervening West Texas Permian basin. The central part of the basin is covered by Mesozoic and Cenozoic rocks, but the connection between the Permian rocks on the two sides has been traced in de- tail by closely spaced drilling. An outlying area to the south, in the Glass Mountains, duplicates the forma- tions of northern west Texas. Farther west in New Mexico, Permian rocks are exposed on the flanks of - many of the block mountains, and still farther west in northern Arizona they spread across the southern part of the Colorado Plateau to the western end of the Grand Canyon. Inall these areas, dips are sufficiently PERMIAN low and outcrops correspondingly so broad that the Permian rocks are subdivided in detail on the Geologic Map. The thickness of the Permian sequence is about 1,000 to 3,000 ft (300—900 In) in Kansas, thinning northward to disappearance in Nebraska and thicken- ing southward to 10,000 ft (3,000 m) in surface sections in west Texas and to as much as 18,000 ft (5,500 m) in the subsurface. In the Grand Canyon area to the west, it is about 2,000 ft (600 m) thick. ‘Classification of the Permian sequence in the region is based on standard sections in west Texas, where the Wolfcampian, Leonardian, Guadalupian, and Ochoan Series (P1, P2, P3, P4) are named and defined. All the series except the Ochoan at the top are open-sea marine deposits and are richly fossilferous. The Ocho- an is an evaporite deposit, the thick Castile Anhy- drite and Salado Halite, with the thin Rustler Dolo- mite at the top. The Rustler contains the only fossils, a scanty fauna of brachiOpods and mollusks that are unmistakably Paleozoic. Difficulties are encountered in extending the west Texas subdivisions outside the type area; assignment of beds near the middle to either the Leonardian or the Guadalupian Series is especially controversial, as explained below. The west Texas strata formed in a circumscribed area, the Delaware basin, that was nearly surrounded by carbonate barriers, culminating in the great Capitan Limestone reef in the upper part of the Guadalupian. Outside the basin, deposits were laid down in shallower water, in evaporite pans, or on low surfaces that received redbed deposits. Large numbers of the fossils characteristic of the basin fail to extend into these outside areas. Many problems thus exist regarding the proper classification of the much larger area of Permian rocks to the north, resulting in some uneasy compromises on the Geologic Map. These will probably satisfy no dedicated stratigrapher but are explained in the correlation diagram included at the bottom of the legend. In each general area, a feasi- ble subdivision is possible, but correlations between the areas that are implied on the map are not necessar- ily realistic. MIDCONTINENT REGION In the Midcontinent Region, from central Texas to Nebraska, Permian outcrops are continuous, and equi- valent units are traceable from one end to the other. The only complication is near the Arbuckle and Wichita Mountains in southern Oklahoma and north- ern Texas, where the Wolfcampian and lower part of the Leonardian Series (upper Cisco Group and Wichita Group) pass into a red arkosic continental facies (PIC, Pzac). On the map, the southern edge of these deposits in Texas is drawn to include localities where verte- 29 brate fossils are abundant. Subdivisions are difficult to trace through the red continental deposits. The Cole- man Junction Limestone, which is the dividing marker between the Wolfcampian and Leonardian Series farther south, disappears, but a uraniferous sandstone near its level persists and is identifiable by radiometry (Chase, 1954). South of the continental deposits the upper part of the Cisco Group and the Wichita Group (here originally called the “Albany Formation”) are marine fossiliferous shales and limestones. North of the continental deposits, the Wolfcampian Series (Ad- mire, Council Grove, and Chase Groups of Kansas) is likewise marine shales and limestones, but the lower Leonardian (Sumner Group) consists of red deposits and includes in subsurface the thick salt beds of the Wellington Formation. The higher Leonardian (sz) is red throughout and includes the Clear Fork Group in Texas and the NippewallaGroup in Kansas. These red deposits, and those higher up in the Permian, are classed with the marine deposits on the map. They con- sist of persistent layers of red shale, gypsum, and thin carbonates that are widely traceable; probably they were laid down on broad level surfaces which main- tained at least tenuous connections with the sea. Above these strata in the Midcontinent Region are two well-defined bodies of rocks, the El Reno (= Pease River) Group and the Whitehorse Group (Psa, Psb), each marking a distinct cycle of sedimentation and each at least locally having a disconformity at the base. At the base of the El Reno is sandstone (San Angelo or Duncan), followed by red shales (Flowerpot and Dog Creek) which contain variable thicknesses of gypsum and dolomite (Blaine). In Texas, the Blaine contains a sparse invertebrate fauna including the ammonoid Perrinites hilli.*Correlation with the west Texas standard section is controversial. The Perrinites occurs no higher than the Leonardian Series, but downdip to the west, in the subsurface, beds equivalent to the Blaine contain lower Guadalupian parafusulinids (Parafusulina rothi). On the Geologic Map, the group is classed as lower Guadalupian (Paa), but with misgivings. The succeeding Whitehorse Group (Psb) is a more uniform deposit than the beds below, with a lighter red color, and is divided into the Marlow Formation (shaly) and the Rush Springs Sandstone. Above the Rush Springs is the Cloud Chief Gypsum, generally excluded from the group but part of the same depositional cycle. Channel sandstones in the Marlow Formation contain a small fauna of marine invertebrates (Newell, 1940) identical with those in the backreef upper Guadalupian rocks (Artesia Group) in southeastern New Mexico, and correlation with these beds is further assured by subsurface tracing across the West Texas basin. 30 PALEOZOIC AND MESOZOIC ROCKS FIGURE 104—United States, showing areas mapped as Permian on Geologic Map of United States. Includes units of marine and nonmarine stratified rocks (P) and their subdivisions (P1, P2, P22}, P2b, P3, P3a, P3b, P4),continental deposits(P1c, Pzac), and eugeo- synclinal deposits (Pe). Does not include many small outcrops in Cordilleran Region which are merged with the remainder of the upper Paleozoic (uP , uE‘e). 31 PERMIAN OF MEXICO GULF FIGURE 10.—Continued. 32 The uppermost Permian deposits of the Midconti- nent Region are the redbeds of the Quartermaster Formation, which are discontinuously exposed in Texas and Oklahoma‘beneath Mesozoic and Cenozoic deposits. The basal layer in Texas, the Claytonville Dolomite, is traceable westward in the subsurface into the Salado Halite and Rustler Dolomite of the Ochoan Series, and the Quartermaster must be the shoreward edge of the great evaporite sequence of the West Texas Permian basin; it is shown as P4 on the Geologic Map. NEW MEXICO In eastern New Mexico, rocks equivalent to those in the Midcontinent Region reappear on the western margin of the West Texas basin, where they are rep- resented by the Abo Sandstone, Yeso Formation, Glorieta Sandstone, San Andres Limestone, and Ar- tesia Group. The Abo Sandstone is a red continental deposit con- taining plant and vertebrate fossils that interfingers southward near the Texas border with the marine Wolfcampian Hueco Limestone and hence is classed as PIC. Some marine Wolfcampian rocks persist locally at the base (Bursum Formation) but are not separated on the Geologic Map from the Pennsylvanian (uE’). The Yeso Formation is a redbed sequence, with beds of carbonate and evaporite, of lower Leonardian age (P2a). The Glorieta Sandstone is a thin deposit that interfingers laterally with both the Yeso and San Andres units. The San Andres Limestone has by far the largest surface area of the Permian rocks of eastern New Mexico and is the caprock of many mountain ranges and plateaus. Like the El Reno Group of the Midcontinent Region, to which it appears to be broadly equivalent, its age with respect to the west Texas standard sequence is controversial. Its brachiopods and other invertebrate fossils, including Perrinites hilli, are Leonardian forms, but Guadalupian parafusulinids occur, especially in the eastern exposures. The lime- stone is not divided on the source map (Geologic Map of New Mexico of 1965), and it is all indicated on the Geologic Map as Leonardian (P2b). Very likely, how- ever, it is a composite unit. During recent mapping in the southeastern part of the State, Kelley (1971, p. 7— 14) divided it into the Rio Bonito, Bonney Canyon, and Fourmile Draw Members. The Rio Bonito, or thick- bedded lower part, is the only member represented in the type San Andres Mountains to the west and is the most likely candidate for a Leonardian age. The two higher members, and especially the evaporitic Four- mile Draw Member at the top, are more likely Guadalupian, and the Fourmile Draw seems to be traceable into the middle Guadalupian beds in the Guadalupe Mountians. 'PALEOZOIC AND MESOZOIC ROCKS The succeeding Artesia Group (Psb) is clearly equiv- alent to the upper Guadalupian of the Guadalupe Mountains and has been traced in the subsurface into the Whitehorse Group of the Midcontinent Region. It is partitioned horizontally into five formations (Gray- burg, Queen, Seven Rivers, Yates, and Tansill), but each of these changes facies northward from backreef carbonates in the Guadalupe Mountains into evapo- rites and redbeds in the Pecos Valley. The overlying Ochoan evaporitic formations (P4) in the southeastern corner of the State are part of the type Ochoan Series. NORTHERN ARIZONA The Permian of northern Arizona is divided into the Supai Formation, Hermit Shale, Coconino Sandstone, and the Toroweap and Kaibab Limestones. The Kaibab and Toroweap form by far the greatest area of exposure, as they are the limestone caprock of this part of the Colorado Plateau. The Toroweap is a minor local unit, formed in a separate sedimentary cy- cle, but it is not greatly different in age from the Kaibab. Rather abundant brachiopods and other inver- tebrate fossils indicate that both are Leonardian (P2b); they are seemingly equivalent to the San Andres Limestone in New Mexico to the east, although the surface continuity is broken by cover of Mesozoic deposits. The three Permian formations beneath are more heterogeneous and have a wider range in age, but they occupy much smaller outcrop areas. The white Coconino Sandstone is a continental dune deposit without fossils except for vertebrate tracks. The red Hermit Shale below contains an abundant flora, prob- ably of early Leonardian age. The thicker red Supai Formation is largely Wolfcampian. It interfingers with marine Wolfcampian rocks (Pakoon Formation) at the western edge of the plateau, but at a few places elsewhere the Supai includes some strata of Pennsyl- vanian age in the lower part. Because of the small outcrop area of these formations, they are all marked on the Geologic Map as lower Leonardian (P2a). Northward in the Colorado Plateau in Utah, the Permian System emerges in small to large areas in the higher uplifts but is not subdivided (P). The largest area is in the Monument unwarp in the south part of Utah, where the rocks consist of units of sandstone like the Coconino and units of redbeds; these units are classed as members of the Cutler Formation, which has an age range nearly equal to the whole Grand Canyon sequence. A little limestone, classed as Kaibab but probably younger, appears at the top of the Permian in the San Rafael Swell but in areas too small to be differ- entiated. UPPER PALEOZOIC CORDILLERAN REGION Marine Permian rocks, with age ranges from Wolfcampian to Guadalupian (= Phosphoria), are about 5,000 ft (1,500 m) thick in the eastern Great Basin of Nevada and Utah and in many places form sufficiently large areas to be differentiated on the Geologic Map (P); elsewhere, the Permian is merged with the remainder of the upper Paleozoic (uP). EUGEOSYNCLINAL DEPOSITS Permian eugeosynclinal deposits in the western part of the Cordilleran Region are mostly merged with the other upper Paleozoic deposits (u Be) or are included in the Triassic and Permian eugeosynclinal deposits ' (‘5 Fe), but they are separately shown in a few areas in northern California. In the Taylorsville area of the northern Sierra Nevada are the volcanic and volcaniclastic Arlington, Reeve, and Robinson Formations, which in a few places contain large parafusulinids of Permian age and in others the Permian fish Helicoprion (McMath, 1966, p. 179—180). In the eastern part of the Klamath Mountains far- ther north are the McCloud Limestone, Nosoni Forma- tion (mudstone and tuff), and the Dekkas andesite, about 6,000 ft (1,800 m) thick. All are fossiliferous; fusulinids and corals of Wolfcampian and Leonardian age occur in the McCloud, and fossils in the Dekkas are of Guadalupian (Capitan) age (Albers and Robertson, 1961, p. 21—30). / UPPER PALEOZOIC From the Rocky Mountains westward, the Missis- sippian, Pennsylvanian, and Permian Systems form such small outcrops, either singly or together, that they are combined on the Geologic Map of the United States into a unit of upper Paleozoic, with a distinction between marine stratified rocks (u E’) and eugeosyn- clinal deposits (u Pe). In some areas, as explained above, the Permian (P) forms outcrops sufficiently ex- tensive for representation and partial subdivision; elsewhere, it is merged with the other systems. MARINE STRATIFIED ROCKS (in?) As in the lower Paleozoic systems, the marine stratified rocks of the upper Paleozoic systems of the Cordilleran Region formed partly‘in a cratonic area , and partly in a miogeosynclinal area, the geographic ‘ positions in each being essentially the same (fig. 11). However, the deposits in the two areas are much more varied than before, reflecting, in large part, the greater crustal unrest of later Paleozoic time. 33 The shifting patterns of distribution of the different systems in the cratonic area are illustrated in the “Geologic Atlas of the Rocky Mountain Region” (Mal- lory, 1972; see also fig. 12). The three systems were originally nearly continuous in the miogeosynclinal area to the west, although their continuity is now bro- ken by subsequent erosion or by burial beneath younger deposits. MISSISSIPPIAN Mississippian rocks are widely distributed in the cratonic area, the only exception being in parts of Col- orado and northern New Mexico. In Colorado, they may originally have been more extensive but were re- moved by erosion during the disturbances of Pennsyl- vanian and later times; however, in much of northern New Mexico they were probably never deposited. In the exposures in the mountain ranges of the cratonic area, the Mississippian sequence is from 500 to 2,000 ft (150—600 In) thick. A striking feature is the great sheet of carbonate rocks of middle Mississippian age (mainly Osageian, partly Meramecian, but with variable upper and lower limits from place ‘.to place), typified by the Madison Limestone of the Northern Rocky Mountains but including the Rundle of the Canadian Rocky Mountains, the Pahasapa of the Black Hills, the Redwall of the Grand Canyon, the Escabrosa of southern Arizona, and other named units in local areas. Higher Mississippian rocks, including Chester- ian, are of more local extent, and lowest Mississippian rocks, or Kinderhookian, are scantily developed. In the miogeosynclinal area to the west, the Missis- sippian sequence is generally thicker, but in variable amounts, indicating mild tectonic activity. It is 7,000 ft (2,100 m) thick in an irregular area in northwestern Utah, where it represents the initial deposits of the Oquirrh basin, a paleotectonic feature characterized by thick shallow-water deposits laid down in an area of unusual subsidence that culminated during Pennsyl- vanian time. The Mississippian deposits are mainly limestone but include some shale units; they are nota- ble for including an unusually complete sequence from Devonian to Pennsylvanian, or from Kinderhookian through Chesterian time. Another maximum of 7,000 ft (2,100 m) of Mississippian strata occupies a linear trough farther west in central Nevada, along the edge of the Antler orogenic belt, whose deposits are of quite different character. The lower Mississippian unit (Joana Limestone) is inconsequential, and most of the sequence is middle or upper Mississippian clastic deposits—the Chainman Shale and the overlying or interfingering quartzites and conglomerates of the Diamond Peak Formation. They are part of a clastic wedge derived from the orogenic belt to the westrand 34 PALEOZOIC AND MESOZOIC ROCKS {7 f’ If... . 1‘:.".{,");‘ FIGURE 11,—Western United States, showing areas mapped as upper Paleozoic on Geologic Map of United States. Includes units of marine stratified rocks (u E’), Mississippian rocks (M), Pennsylvanian rocks (IP, P1, P2, P3, P4), Permian rocks (P, P 1, Pic, P2, P2a, P2ac, P2b, P3, P3a, P3b, P4), clastic wedge, ge deposits (uPc), and eugeosynclinal deposits (uE‘e, Pe). UPPER PALEOZOIC A MISSISSIPPIAN B PENNSYLVANIAN M l » \ly— Front Range , ,\/\ anticiine Paradox basin J C PERMIAN D PALEOTECTONICS FIGURE 12.—Eastern part of Cordilleran Region, showing surface and subsurface extent of the systems grouped as upper Paleozoic on Geologic Map and some of the paleotectonic features of the time: A, Mississippian. B, Pennsylvanian. C, Permian. D, Paleozoic tectonic features. Compiled from Geologic Atlas of Rocky Moun- tain Region (Mallory, 1972) and other sources. l 35 36 are separately shown on the Geologic Map as a elastic wedge facies (u E’c). Also so mapped are the Mississip- pian Milligen Formation and the Pennsylvanian Wood River Formation of south-central Idaho, which are clastic wedge deposits derived from orogenic belts farther west. PENNSYLVAN IAN Pennsylvanian deposits in the cratonic area are much more varied than in any of the preceding Paleozoic systems, owing mainly to intracratonic orogenic activity on the site of the Southern Rocky Mountains, which produced the Colorado system of structures, or "Ancestral Rocky Mountains” (Mallory, 1972). Two geanticlines were raised: the Front Range geanticline on the site of the present Front Range and some ranges to the west, and the Uncompahgre gean- ticline that extended southeastward from the Uncom- pahgre Plateau of western Colorado across the site of the San Juan Mountains into northern New Mexico. The northeastern geanticline was raised somewhat earlier than the southwestern, although clastic de- posits were shed off both of them through Pennsylva- nian into Permian time. Between the two geanticlines was a deeply subsiding linear basin, the Central Col- orado trough, which received as much as 12,000 ft (3,700 m) of clastic sediments during Pennsylvanian time. Few structures attributable to the Pennsylvanian orogeny can be identified, and their record is mainly derived from the red clastic deposits along the flanks, including such units as the Fountain Formation on the eastern edge of the Front Range and the Sangre de Cristo Formation of the Sangre de Cristo Mountains (derived from the southeastern end of the Uncom- pahgre geanticline). Southwest of the Uncompahgre geanticline in southwestern Colorado and eastern Utah is the Paradox basin, another area where Pennsylvanian deposits are over 7,000 ft (2,100 m) thick; here, however, the greater part of the sequence is evaporite, mainly halite, but includes layers of potash salt. North and south of the Colorado system in the cratonic area, the Pennsylvanian is no more than 1,000 ft (300 m) or so thick and largely carbonate; it includes the Amsden and Tensleep Formations of Wyoming, the main part of the Naco Group in southern Arizona, and the Magdalena Group in New Mexico. In central New Mexico, the Magdalena rests directly on Precambrian rocks in most places; Mississippian rocks are sporadic and the lower Paleozoic sequence wedges out north- ward in the southern part of the State. The Pennsylvanian sequence is not more than 2,000 or 3,000 ft (600—900 m) thick in most of the PALEOZOIC AND MESOZOIC ROCKS miogeosynclinal area, but it thickens to as much as 13,000 ft (4,000 m) in the Oquirrh basin of northwest- ern Utah. Most of this sequence is the Oquirrh Forma- tion, whose base is of Morrowan age and whose upper part extends into Wolfcampian time (not divided on the Geologic Map). The formation consists of interbedded limestones and sandstones, the latter made up of fine- grained clean quartz sands. The sands could not have been derived from any orogenic area; apparently they came from the craton (where similar sandstones of lesser thickness occur) and were trapped in a unique restricted area of exceptional subsidence in the miogeosyncline. Farther west in the miogeosyncline, in the Ely and Eureka areas, Nevada, the Pennsylvanian is rep- resented by the Ely Limestone, which succeeds the clastic wedge deposit of the Diamond Peak Formation. Pennsylvanian rocks also occur in the Winnemucca area stillifarther west, where they are unconformable on ”transitional” and eugeosynclinal lower Paleozoic rocks of the Antler orogenic belt. Some is limestone, but the conglomeratic middle Pennsylvanian Battle Formation is included. Also included in the upper Paleozoic in this area on the Geologic Map are Permian units, the upper part of the Antler Peak Limestone and the Edna Mountain Formation, the latter with high Permian (Phosphoria) fossils (Roberts and others, 1958, p. 2841—2844). PERMIAN As indicated above, the more extensive areas of Permian rocks in the Cordilleran Region are sepa- rately mapped and in part subdivided; however, nar- rower belts of outcrop elsewhere are merged with the remainder of the upper Paleozoic. Through most of the cratonic area the Permian se- quence is no more than 1,000 or 2,000 ft (300—600 m) thick. Near the previously formed uplifts in Colorado, it is mostly red elastic deposits, an upward continua- tion of those of the Pennsylvanian, but finer grained. Southwestward and southward, carbonate rocks ap- pear in the Leonardian Series (separately mapped as the Kaibab, Toroweap, and San Andres Limestones, P2b). Northward, an important component is the com- plex of marine deposits that includes the Park City and Phosphoria Formations, the first shallow-water lime- stone, the second deep—water phosphatic shale 'and chert. Although the older part of the complex is Leonardian, the main body is Guadalupian and con- tains many of the same marine fossils as are found in the standard Guadalupian section in west Texas. Eastward across Wyoming, the complex of marine de- posits intertongues with redbeds of the Chugwater and Goose Egg Formations. UPPER PALEOZOIC 37 In the miogeosynclinal belt to the west, thicknesses of Permian rocks as great as 10,000 ft (3,000 m) are recorded in parts of northwestern Utah, mostly an up- ward continuation of the Oquirrh Formation, and are included in the upper Paleozoic (u E’) on the Geologic Map. Somewhat thinner sequences elsewhere in west— ern Utah and in eastern Nevada form prominent areas of outcrop and are shown as undivided Permian (P). The westernmost of these, the Carbon Ridge and Gar- den Valley Formations of the Eureka district, are dom- inantly clastic deposits that lie unconformably on the rocks beneath—the Carbon Ridge on miogeosynclinal rocks, the Garden Valley on lower Paleozoic eugeosyn- clinal rocks (Vinini Formation) (Nolan and others, 1956, p. 65—68). OUTLYING MIOGEOSYNCLINAL ROCKS Fragments of upper Paleozoic miogeosynclinal rocks, mingled with other rocks of varied kinds and ages, crop out in various parts of southern California. In the Mojave Desert are the Orogrande Series and Fairview Valley Formation, mainly carbonates, which contain a few upper Paleozoic fossils. To the south in the San Bernardino Mountains are the Saragossa Quartzite and Furnace Limestone, forming roof pendants in the Mesozoic plutons. Mississippian fossils are found in the Furnace, but a wider age range is suggested by recent studies (Steward and Poole, 1974), which indicate the presence of strata as old as Lower Cambrian. Smaller remnants of similar rocks occur in the San Gabriel Mountains to the west. More enigmatic is the Sur Series of the Coast Ranges, which forms part of the basement of the Santa Lucia Mountains south of Mon- terey. It is a body of high-grade schists and gneisses, with some large masses of marble. Its age is undeter- mined, but the series is indicated on the Geologic Map as upper Paleozoic miogeosynclinal rocks (uP), with metamorphic overprint. EUGEOSYNCLINAL DEPOSITS (uPe) In north-central Nevada, upper Paleozoic eugeosyn- clinal deposits closely adjoin the upper Paleozoic miogeosynclinal deposits (Battle Formation, and so forth) already referred to, but they lie in the upper plate of the Golconda thrust that was emplaced during the Sonoma orogeny of late Permian and early Triassic time. The principal rocks are of Permian and possible Pennsylvanian age, but units of Mississippian age occur in outlying areas (Silberling and Roberts, 1962, p. 16—18). In the Sonoma Range near Winnemucca and some nearby ranges are the Pumpernickel and Havallah Formations, with a total thickness of more than 15,000 ft (4,500 m). The Pumpernickel is mostly dark bedded chert with minor volcanic rocks, and the Havallah is mainly fine-grained sandstone, with some interbedded chert and argillite. The Havallah contains Wolfcamp- ian or early Leonardian fusulinids; it also contains re- worked Atokan fusulinids in the lower part. A few Pennsylvanian conodonts have been recovered from the otherwise barren Pumpernickel Formation, suggesting that it is Pennsylvanian. Recent observa— tions (John M. Stewart, oral commun., 1973) suggest that thick units of Havallah and Pumpernickel lithol- ogy are interbedded, and so the two may be not far apart in age. In the East Range farther west and the Independ- ence Range to the north are the Inskip and Schoonover Formations of chert, argillite, limestone, and volcanic rocks, from both of which a few Mississippian fossils have been collected (Fagan, 1962). In California, upper Paleozoic eugeosynclinal de- posits occur both east and west of the Jurassic and Cretaceous batholith that forms the core of the Sierra Nevada. On the eastern flank near Owens Valley, the Ordovician rocks referred to earlier are succeeded by 7,500 ft (2,300 m) of siliceous hornfels with upper Paleozoic fossils (Rinehart and others, 1959, p. 941— 945). On the western flank is the thick mass of the Calaveras Formation, cherts, slates, and volcanics, largely unfossiliferous but generally classed as upper Paleozoic (Clark, 1964). In the Taylorsville area at the north end of the Sierra Nevada is an array of forma- tions, dominantly pyroclastic, composed of dacite, andesite, and basalt (McMath, 1966, p. 179—180). The lower beds contain Mississippian fossils, and the higher are Permian (separately mapped as Pe). In the southeastern part of the Klamath Mountains, the Devonian sequence is succeeded by the Bragdon Formation, a mass of shale and sandstone, with minor volcanics and some conspicuous layers of conglomer— ate. It is overlain by the dominantly volcanic Baird Formation. The Bragdon and Baird contain Mississip- pian and Pennsylvanian fossils. The succeeding rocks, beginning with the McCloud Limestone, are separately mapped as Permian eugeosynclinal deposits (Pe). In the Blue Mountains uplift of north-central Ore- gon, upper Paleozoic eugeosynclinal rocks are exposed in small areas beneath the more extensive lower Mesozoic eugeosynclinal rocks. Relations are best shown in the Suplee area at the western end of the uplift, where the rocks are least altered and are abun- dantly fossiliferous (Merriam and Berthiaume, 1943; Dickinson and Vigrass, 1965, p. 14—16). The lowest unit contains Mississippian and Devonian inverte- brates, the middle Pennsylvanian plants, and the 38 upper early Permian fusulinids and other inverte— brates. The lower unit is limestone and sandy lime- stone; the middle is mudstone, conglomerate, and chert; and the upper is largely volcanic, with irregular fossiliferous limestone lenses. The Paleozoic rocks are structurally unconformable below the Triassic. Much farther northeast, near Baker and Sparta, the Elkhorn Ridge Argillite and Clover Creek Greenstone contain both later Permian fusulinids and Triassic fossils (Bostwick and Koch, 1962) and are shown on the Geologic Map as part of the Triassic and Permian eugeosynclinal deposits (EPe). In many of the mountain ranges in northern Washington, immediately south of the Canadian bor- der, upper Paleozoic eugeosynclinal deposits, in part metamorphosed, are associated with lower Mesozoic eugeosynclinal deposits but are much disrupted by Mesozoic plutons. They include the Chilliwack Group on the west flank of the northern Cascade Range, the Hozameen Group on the east flank, and still farther east the Anarchist Series. All these units are graywackes, cherts, and volcanics, with occasional lenses of fossiliferous limestone. For the most part, they are of Permian, or Permian and Triassic age, but the Chilliwack Group has yielded Devonian and lower Pennsylvanian as well as Permian fossils (Misch, 1966, p. 115—117). PALEOZOIC PLUTONIC ROCKS Plutonic rocks of Paleozoic age are extensive in the crystalline part of the Appalachian Region (New Eng- land and Piedmont provinces), and smaller bodies occur in Oklahoma and parts of the Cordilleran Region (fig. 13). They include both felsic (granitic) rocks and mafic rocks. The granitic rocks are divided into four categories according to age—Cambrian, lower Paleozoic, middle Paleozoic, and upper Paleozoic; the mafic rocks are not subdivided, as they form much , smaller areas which are not feasible to separate and not all their relative ages are known. CAMBRIAN GRANITIC ROCKS (6g) Plutonic rocks of Cambrian age occur only in a unique tectonic and igneous province that extends northwestward from the Wichita Mountains area of southern Oklahoma into the Rocky Mountains of southern Colorado. Although the rocks are of varied compositions, all are shown as granitic on the Geologic Map. The most extensive plutonic rock is the Wichita Granite, which crops out along the axis of the Wichita Mountains for 65 mi (97 km). Its Cambrian age is at- tested by dates of 535 to 550 my obtained by U/Pb, PALEOZOIC AND MESOZOIC ROCKS .Rb/Sr, and K/Ar methods (Ham and others, 1964, p. 60—79). It is a floored body which underlies and in- trudes the congeneric effusives of the Carlton Rhyolite and which overlies the somewhat older Raggedy Mountain Gabbro, itself a floored intrusive. Extensions of these rocks into surrounding areas are known from drilling, and the Carlton Rhyolite reappears to the east in two small outcrops in the Timbered Hills of the western Arbuckle Mountains. Drilling also indicates that the units mentioned overlie supracrustal rocks not exposed at the surface—the Navajoe Mountain Basalt and the Tillman Metasedimentary Group, the latter a sequence of graywackes at least 15,000 ft (4,500 m) thick that fills a deep trough bordered to the north and south by older Precambrian crystalline rocks. The Wichita Mountains rocks contrast with the Tishomingo Granite and other basement rocks of the eastern Arbuckle Mountains (Ygi), which have yielded radiometric ages of 1,320—1,400 m.y. Although of Cambrian age, the plutonic and extru- sive rocks of the Wichita Mountains are part of the basement of the Midcontinent Region. Like the older Tishomingo Granite to the east, they are overlain with rough erosion surface by the Upper Cambrian Reagan Sandstone at the base of the cratonic sequence. In the Wichita Mountains, both the granitic rocks and the adjacent lower Paleozoic sedimentary rocks project as peaks and knobs that have been partly exhumed from the surrounding Permian redbeds that formerly buried them. In Colorado, three small alkalic complexes lie in ter- ranes of Precambrian crystalline rocks, two in the Wet Mountains at the south end of the Front Range and another at Iron Hill north of the San Juan Mountains. These have yielded radiometric ages of 520—580 my and are likewise Cambrian. Probably they lie on a northwestern prolongation of the Wichita Mountains plutonic province (Parker and Sharp, 1970, p. 3; Olson and Marvin, 1971). LOWER PALEOZOIC GRANITIC ROCKS (Pgl) Lower Paleozoic granitic rocks, with ages of about 400 to 500 my (Ordovician and Cambrian), occur throughout the length of the crystalline part of the Appalachians. In New England, they include several different plutonic series. In the Connecticut Valley of western New Hamp- shire, five small bodies of the Highlandcroft Plutonic Series intrude the Ordovician rocks, and one of them is truncated and overlain unconformably by the Clough Quartzite, proving its pre-Silurian age. The plutonic rocks were originally quartz monzonite, but they are much sheared and chloritized. In the Bronson Hill anticlinorium to the east is the PALEOZOIC PLUTONIC ROCKS more extensive Oliverian Plutonic Series, which eX- tends southward from New Hampshire into Mas- sachusetts and Connecticut (Naylor, 1968). The Oli- verian characteristically is a series of elongate domes, whose foliation is accordant with that of the surround- ing and overlying Ammonoosuc Volcanics. The outer part of each body is weakly foliated gneiss, but com- monly there is a core of massive granitic rock, which partly crosscuts the mantling gneisses. Both gneiss and granite have yielded Pb/Pb and Rb/Sr ages of 440 to 450 my. Present belief is that the gneissic rock was metasomatized from felsic volcanics underlying the Ammonoosuc, into which the more massive core rocks were intruded as magmas; The Oliverian plutonic rocks formed at a deeper crustal level than the High- landcroft plutonic rocks, in an environment of plastic deformation. Another group of lower Paleozoic granitic rocks is in southeastern New England, represented by the alkalic Cape Ann, Peabody, and Quincy Granites, north and south of Boston. Formerly it was believed that they were late Paleozoic, an account of their fresh appear- ance and their apparent affinities with other young alkalic granitic rocks farther northwest, but Pb/Pb de- terminations on zircons yielded ages of 435 to 452 my. Evidently this region was outside of and southeast of the region of Acadian orogeny, where only earlier in- fluences prevailed (Zartman and Marvin, 1971). In the Piedmont province of the Central and South- ern Appalachians, the existence of many bodies of lower Paleozoic granitic rocks is indicated by field rela- tions, supported in many places by radiometric dating. All of them precede the regional metamorphism, which occurred about 380 to 420 my. ago (Butler, 1972). Many dates have been obtained by Fullagar (1971) by Rb/Sr whole—rock methods on plutonic rocks in the Piedmont province, and especially in North and South Carolina; these dates were extrapolated on the Geologic Map to related plutons in surrounding areas. The oldest group of plutons has ages of 520 to 595 m.y., hence are Cambrian and Ordovician. One of the plu— tons, the Farrington igneous complex near Chapel Hill, NC, intrudes the early Paleozoic rocks of the Carolina Slate Belt and resembles other plutons in the Slate Belt rocks; they are probably congeneric with the volcanic effusives of the belt. Another, the Hatcher complex in central Virginia, is unconformable beneath the Ordovician Arvonia Slate. In Maryland, various small granitic plutons (not shown on the Geologic Map) have ages of about 440 my (Hopson, 1964,'p. 199— 201). MIDDLE PALEOZOIC GRANITIC ROCKS (Pg2) Middle Paleozoic granitic rocks with ages of 350 to 400 my. (Devonian) are more plentiful than the lower 39 Paleozoic granitic rocks, especially in New England. Their principal representative in New England is the New Hampshire Plutonic Series (Billings, 1956, p. 125—129), which forms many plutons in and east of the Bronson Hill anticlinorium, and its extensions south- ward into Massachusetts, northeastward into Maine as far as Mount Katahdin, and into northeastern Ver- mont. The earlier members of the sequence are concord- ant gneissic bodies, probably synorogenic to the Aca- dian orogeny, such as the Bethlehem Gneiss and Kinsman Quartz Monzonite, which form thick sheets in the lower part of the Littleton Formation. Later members are cross-cutting plutons of binary granite. The later granites of New Hampshire have been dated at 380 m.y., and the Mount Katahdin pluton in Maine at 358 my The coastal plutons of eastern Maine may belong to a distinctly younger, late Devonian plutonic series (Chapman, 1968, p. 386—388); they are nearly circular cross-cutting granitic bodies, embedded in somewhat earlier mafic intrusives. ' In the Piedmont province of North and South Carolina granitic plutons with ages of 385 to 415 my are common in the plutonic complex of the Charlotte belt, which adjoins the Carolina Slate Belt on the northwest (Fullagar, 1971, p. 2854—2856). In the western United States, a single granite body of middle Paleozoic age has been identified in the Beaverhead Range along the eastern border of Idaho. It has been dated by K/Ar methods at 441 my (Schol- ten and Ramspott, 1968, p. 18—21), but there is some reason to suspect that the granite might actually be Precambrian (Armstrong, 1975, p. 447—448). UPPER PALEOZOIC GRANITIC ROCKS (E’ga) Upper Paleozoic granitic rocks, with ages of 250 to 300 my (Pennsylvanian and Permian) occur chiefly in the Piedmont province of the Southern Appalachians. The only known occurrence in New England is the Narragansett Pier Granite (and the minor associated Westerly Granite) on the south coast of Rhode Island, which intrudes the Pennsylvanian rocks of the Nar- ragansett basin. It has yielded ages of 240 my. by K/Ar methods, 259 my. by Rb/Sr methods on biotite, and 299 my by Rb/Sr whole-rock methods (Quinn, 1971, p. 51). The White Mountain Plutonic Series of northern New England was formerly assumed to be of late Paleozoic age, but it is now known from radiometric dating to be Jurassic (J g). The upper Paleozoic granitic rocks in the Piedmont province occur as discrete plutons in the Carolina Slate Belt and eastern edge of the Charlotte belt in North and South Carolina and Georgia. They are cross— cutting postmetamorphic bodies with ages of about 300 my. (Fullagar, 1971, p. 2856—2857). The oldest of the PALEOZOIC AND MESOZOIC ROCKS 40 -\. —\_ -‘., \——‘ .i i ‘- i' ‘- L i. , —--—--—--—--—--—--—-7' I K 7" 1 .- i .1 i l l:_ ___________________ s I \-"-\3, \\ '\ i ll‘ ____________________ _ i i i ................. i_._____.__ 1— ________ _ ___________ i .I I .i } L ‘zik i Ml‘un . i, txqka“- /"\“\ FIGURE 13.—Unifcd States, showing areas mapped as Paleozoic plutonic rocks on Geologic Map of United States. Includes units of Cambrian granitic rocks(€g), lower, middle, and upper Paleozoic granitic rocks (Pgl, E’g2, Pga), and mafic intrusives ( Pmi). GULF PALEOZOIC PLUTONIC ROCKS OF MEXICO FIGURE 13.—-—Continued. 41 42 group is the Petersburg Granite next to the Coastal Plain overlap in eastern Virginia, which is post- metamorphic like the rest and which has been dated at 330 m.y. by U/Pb determinations on zircon (Wright and others, 1975). (It is shown on the Geologic Map with the middle group of granites, sz). The youngest of the group is the Siloam Granite in the southeastern Piedmont of Georgia, which has yielded a Rb/Sr whole-rock age of 269 m.y. (Jones and Walker, 1973). Northwest of the other plutons, near Atlanta, is the Stone Mountain Granite, which has yielded ages of 280 m.y. by Rb/Sr methods and 294 m.y. by K/Ar methods (Smith and others, 1968; Whitney and Jones, 1974). The only recorded upper Paleozoic granitic pluton in the western United States is the Mount Lowe Granodiorite of southern California, part of the plutonic complex in the San Gabriel Mountains north of Los Angeles which includes rocks ranging in age from Precambrian to Cretaceous. The Mount Lowe body has a radiometric age of 220 m.y., near the Permian-Triassic boundary (Silver, 1971, p. 194). PALEOZOIC MAFIC INTRUSIVES (szi) Mafic rocks, mainly gabbro and diorite, intrude the crystalline rocks throughout the length of the Ap— palachian Region, but in smaller masses than the granitic rocks. They have various Paleozoic ages and various relations to the adjacent granitic rocks and to the regional metamorphism, but they are not sub- divided on the Geologic Map because of their small dimensions and because the relations of many of them are not known with certainty. Many of the mafic intru- sives are phases of the same plutonic series as the granitic rocks with which they are associated, but some are certainly older. METAMORPHIC COMPLEXES The rocks in a few areas in the United States are so strongly metamorphosed and complexly deformed, and have yielded so little indications of their original ages, that they are mapped as metamorphic complexes (m), without any age designation. They form extensive tracts in the Piedmont province of the Southern Ap- palachians and smaller areas in northern Washington, in the Cascade Range and ranges east of it. Here, in- stead of classifying the rocks as to age or sequence, they are divided into schist and phyllite (ms), felsic paragneiss and schist (m1), mafic paragneiss (horn- blendite and amphibolite) (m2), migmatite (m3), and felsic orthogneiss (granite gneiss) (m4). The metamorphic complexes of the Piedmont prov- ince are probably of Precambrian Z (late Precambri- an) and early Cambrian age. In many places they seem PALEOZOIC AND MESOZOIC ROCKS to lie stratigraphically beneath the Cambrian eugeo- synclinal deposits ('CV) of the Carolina Slate Belt and other areas, but other parts seem to be merely the more highly metamorphosed equivalents of the Cambrian rocks. Much more field investigation will be needed to untangle the true relations of the metamorphic com- plexes and the Cambrian sequence from one area to another. In the Virgilina area along the Virginia— North Carolina border, radiometric dating indicates rocks with ranges in age from 575 m.y. to 740 m.y. that were deformed between 575 and 620 m.y. ago (Glover and Sinha, 1973). The older rocks are more metamorphosed than the younger ones in the Virgilina synclinorium; however, there is no clear break be- tween them, and they are much alike, suggesting that there was no significant depositional break between the Cambrian eugeosynclinal deposits (6e) and the ad- jacent metamorphic complex (In). The metamorphic complexes in the northern Cas- cade Range are of undertermined age but appear most likely to be middle and upper Paleozoic. They are older than Paleocene and Cretaceous sediments on the flanks of the range and are probably older than earlier Mesozoic rocks of the area; they are involved in orogenic deformation that preceded the formation of these rocks. A few radiometric ages have been obtained of about 250 m.y., suggesting that the rocks them- selves are Permian and older (Misch, 1966, p. 107— 115). The rocks in the western part of the range make up the Shuksan metamorphic suite, with dominant blueschist metamorphism, which overlies less metamorphosed Paleozoic and Mesozoic metamorphic rocks along a major low-angle thrust. The Shuksan is separated from the Skagit metamorphic suite farther east, with greenschist metamorphism, by a major high-angle fault. The Skagit suite includes the Cas- cade River Schist (ms) and the higher grade ,mig- matized Skagit Gneiss (m1). Both sets of metamorphic rocks were derived from eugeosynclinal sediments and volcanics but otherwise have little in common; so their original relations to each other are unknown. East of the Cascade Range is the so-called "Colville batholith,” actually a metamorphic complex much like the more famous Shuswap Complex in British Colum- bia to the north. It consists of a core of paragneiss (m1), overlain by an outward-dipping body of granitic or— thogneiss (m4) (Fox and Rinehard, 1973). TRIASSIC AND PERMIAN EUGEOSYNCLINAL DEPOSITS 13 Fe) A rather diverse assemblage of eugeosynclinal de- posits is present in Oregon, California, and Nevada, in TRIASSIC which volcanic rocks are an important component and which contain both Permian and Triassic fossils. In the Blue and Wallowa Mountains of northeastern Oregon are various units of argillite and greenstone (Elkhorn Ridge Argillite and Clover Creek Greenstone; Gilluly, 1937, p. 14—26), which contain oc- casional limestone lenses. Some of these lenses contain Leonardian and younger Permian fusulinids, others Triassic fossils (Bostwick and Koch, 1962). A little farther east, along the Snake and Salmon Rivers in western Idaho, are the Seven Devils Volcanics, which likewise contain both Permian and Triassic fossils. All these units are very thick eugeosynclinal accumula- tions, structurally complex and partly metamorphosed. Whether the Permian and Triassic parts represent continuous sequences, or whether several units are represented, is undetermined. Farther west in the Blue Mountains area, the Triassic sequence is unconforma- ble on the Permian and is grouped with the Jurassic as lower Mesozoic (1 Me) on the Geologic Map. Extending the length of the Klamath Mountains in southwestern Oregon and northern California is the wide band of the Western Paleozoic and Triassic belt (Irwin, 1966, p. 21—24; Hotz, 1971, p. 11—13). It forms a structural block tectonically beneath that of the Cen- tral Metamorphic belt and tectonically above that of the Western Jurassic belt, and it is entangled with J uras- sic granitic plutons and masses of ultramafic rocks. The rocks themselves are fine-grained clastic sedi- ments, bedded cherts, mafic volcanic rocks, and lenses of crystalline limestone. These have been given vari— ous local names, such as Applegate Group and Chan- chelulla Formation, but the limits of such units are uncertain. Permian ammonoids and fusulinids have been collected in some places, and Triassic fossils in others; fossils of older Paleozoic ages have been re- ported but lack modern verification. Most of the rocks have been subjected to low greenschist-grade meta- morphism, but there are some areas of higher grade metamorphic rocks along the eastern side, such as the Stuart Fork Formation (shown by overprint on the Geologic Map). An exceptional area of low-grade schists at Condrey Mountain on the Oregon—California border forms a subcircular window, surrounded by higher grade schists; on the Geologic Map, these rocks are doubtfully correlated with those of the Western Jurassic belt (1 Mze) but may be older. In northwestern Nevada the Koipato Group of mainly nonmarine rhyolitic and andesitic lavas and associated volcaniclastic rocks unconformably under- lies Middle Triassic sediments. It is more than 10,000 ft (3,000 m) thick in its type area in the Humboldt Range, with no base Visible, but it thins to disappear- ance about 40 mi (65 km) to the east, where it overlies 43 upper Paleozoic eugeosynclinal rocks (Havallah and Pumpernickel Formations, uPe) that were deformed by the Sonoma orogeny. Its upper part contains Lower Triassic ammonoids. The Permian fish Helicoprion has also been reported, but its occurrence is suspect; Pb/ alpha dates of 230 to 290 my (Permian) have been obtained but require verification (Silberling, 1973, p. 349—351). Farther south is the volcanic Pablo Formation and the associated sedimentary Diablo and Excelsior F or— mations, which contain Permian and Triassic fossils and are apparently broadly correlative with the Koipato. The Diablo lies unconformably on deformed Cambrian and Ordovician eugeosynclinal rocks (1 Be) (Silberling and Roberts, 1962, p. 26—28). TRIASSIC The Triassic, the lowest system of the Mesozoic Era, is scantily represented on the Geologic Map of the United States. It is separately shown in the Colorado Plateau, in a few of the ranges of the eastern Rocky Mountains, on the western and eastern borders of the Great Plains in New Mexico and Texas, and in a series of fault troughs in the Appalachian Region from New England to South Carolina. In most of the Cordilleran Region, it is merged, where present, with the Jurassic, as a unit of lower Mesozoic (le , lthe). Elsewhere in the United States, and especially in the Central Inte- rior Region, Triassic rocks are missing and were proba- bly never deposited (fig. 14). The Triassic System is divided into the Lower Mid- dle, and Upper Triassic Series, but these terms are not used on the Geologic Map. The stage terms in the Alps of Europe (Scythian, Anisian, Ladinian, Karnian, No- rian, and Rhaetian) are frequently used in discussions of the stratigraphy of the marine Triassic rocks of the west (Reeside and others, 1957, p. 1455—1456). Most of the rocks shown as Triassic on the Geologic Map are nonmarine and dominantly red colored, but these rocks are not specifically designated as continen- tal deposits. Except in the Colorado Plateau, all of them are Upper Triassic. The Triassic marine deposits of the Cordilleran Region are grouped with Jurassic in the lower Mesozoic unit. COLORADO PLATEAU In the Colorado Plateau, the Triassic system is rep- resented by the Moenkopi and Chinle Formations—the first Lower and low Middle Triassic, the second Upper Triassic—With a combined thickness of about 2,000 ft (600 m). At the top is the Glen Canyon Group, also partly of Triassic age, which is shown on the Geologic Map as a unit of Jurassic and Triassic ('Fe). 44 PALEOZOIC AND MESOZOIC ROCKS J‘1._...__.._l FIGURE 14.——United States, showing areas mapped as lower Mesozoic stratified rocks on Geologic Map of the United States. Includes units of Triassic and Permain eugeosynclinal deposits (Ti Pe), Triassic ( Ti ), Jurassic and Triassic (J T: ), Jurassic (J, (Jo), and undivided lower Mesozoic (1M, lMe, 1Mv). GULF ' TRIASSIC OF MEXICO FIGURE 14.—Continued. 45 46 The Moenkopi Formation is a red fine-grained evenly bedded deposit, with thin beds of gypsum, and a few of limestone toward the west. Most of it is non- marine, but it grades westward into marine deposits, where it contains the Early Triassic ammonoid Meekoceras in the lower part. Elsewhere, the only fos- sils are vertebrate tracks and bones. The overlying Chinle Formation is more sandy and brilliantly colored. The persistent Shinarump Con— glomerate at the base was laid on the eroded surface of the underlying Moenkopi Formation. The Chinle is en- tirely nonmarine and contains vertebrates, fossil wood, and a few freshwater invertebrate shells. The Chinle extends eastward beyond the edge of the Moenkopi, into northwestern New Mexico. GREAT PLAINS The Dockum Group, another red Upper Triassic unit, forms extensive outcrops in eastern New Mexico and northwestern Texas, on each side of the Tertiary caprock of the Great Plains, and is continuous in the subsurface within the intervening area, where it is the final deposit of the West Texas Permian basin. In the center of the basin, it is about 2,000 ft (600 m) thick, but it is thinner in the outcrops. Various local names have been given to subdivisions, such as Santa Rosa Sandstone in New Mexico and Tecovas and Trujillo Formations in Texas, but they are inconstant and have no general significance (ED. McKee, in McKee and others, 1959, p. 13—14). The Dockum is entirely non- marine; vertebrates have been found at many places and indicate an approximate correlation with the Chinle Formation farther west. APPALACHIAN REGION Rocks of the Upper Triassic Newark Group form a series of faulted troughs in the southeastern part of the Appalachian Region from southern New England to South Carolina. Another fault trough of Newark rocks is in Nova Scotia to the northeast, and rocks like the Newark Group, probably also in fault troughs, have been penetrated by drilling in the Atlantic and Gulf Coastal Plains in Georgia, Alabama, and Florida, and as far west as 'southern Arkansas (Eagle Mills Forma- tion). 9 In the Appalachian Region, the Newark rocks lie on the deeply eroded edges of the Paleozoic rocks and the crystalline rocks of the Piedmont. The fault troughs broadly parallel the trends of the earlier structures, however, and in North Carolina, at least, are symmet- rically placed on each side of the metamorphic climax in the Piedmont rocks. Commonly, the troughs have PALEOZOIC AND MESOZOIC ROCKS the form of half-grabens, with the beds tilted toward a master fault on one side or the other, which was active during sedimentation. The half-grabens are symmetri— cally placed; thus, the Connecticut Valley Triassic has its master fault on the east, and the New Jersey— Pennsylvania Triassic, en echelon to the west, has its master fault on the northwest. The Deep River— Wadesboro basin and the Dan River basin in North Carolina have a similar arrangement. In places, there is some evidence that the Triassic deposits were origi- nally continuous between the opposing troughs; in western Connecticut a small downfaulted outlier in the Pomperaug Valley duplicates the sequence in the larger Triassic areas to the east and west. In addition to the master faults, the Triassic rocks are displaced by many other faults that mostly formed after the close of sedimentation. The faults are tensional (taphrogenic) features and were probably produced by rifting as- sociated with the opening of the Atlantic Ocean during Mesozoic time. Along part of the northwestern border in Pennsyl- vania, the highest Triassic beds overlap the Paleozoic rocks without surface rupture, but the master fault probably lies beneath and had ceased its activity before the end of Triassic deposition. On the Geologic Map, the fault is represented as continuous. The rocks of the Newark Group are all nonmarine conglomerates, sandstones, siltstones, and shales, complexly interfingered; many of them are conspicu- ously red, but some are gray or black. In the Connect- icut Valley, the Newark Group is about 12,000 ft (3,700 m) thick, and in the New Jersey—Pennsylvania area at least 20,000 ft (6,000 m); lesser thicknesses occur in the narrower basins farther south. Most of the coarser beds are arkosic. In New Jersey and Pensylvania, the first sediments are arkoses de- rived from crystalline highlands to the southeast, and coarse debris from across the border fault to the north- west appears only above these arkoses. In both the Connecticut Valley and the New J ersey—Pennsylvania area, the deposits next to the border faults are coarse fanglomerates; some fanglomerates in Pennsylvania are formed of carbonate clasts (“Potomac marble”), de- rived from Paleozoic formations to the northwest. Near the middle, in both the Connecticut Valley and New Jersey—Pennsylvania areas, are dark-gray to black lacustrine shales (for example, the Lockatong Forma- tion), which finger out into the red coarse sediments (D. B. McLaughlin, in Reeside and others, 1957, p. 1491—1494). In the southern basins there are more gray strata, and important beds of coal; the latter have been mined in the Richmond basin, Virginia, and the Deep River basin, North Carolina. J URASSIC 47 In the Connecticut Valley and in New Jersey, three basalt flows are interbedded in the upper partflav). Both here and farther south, the Newark rocks contain thick sills of diabase (Hi), the most prominent of which is the Palisades sill of eastern New Jersey that over- looks the Hudson River and which has been dated radiometrically at 190—200 m.y. (Erikson and Kulp, 1961). In addition, numerous vertical diabase dikes (shown on the Geologic Map) in the Triassic rocks extend far out into the older rocks of the Piedmont province (King, 1971). Various nonmarine (freshwater or continental) fos- sils are abundant in parts of the Newark Group, in- cluding fishes and plants in the dark shaly beds and vertebrate bones and tracks in the coarser red sedi- ments. They indicate roughly a Late Triassic age, al- though opinions differ as to precise correlations with standard sections elsewhere. JURASSIC AND TRIASSIC (J13) The designation Jurassic and Triassic (J13) is used in the western part of the Colorado Plateau for the rocks of the Glen Canyon Group—the Wingate Sandstone, the Moenave and Kayenta Formations, and the Navajo Sandstone (Baker and others, 1936, p. 4—6). Their age is uncertain between Triassic and Jurassic, as fossils are very sparse and relations to better dated rocks in surrounding areas are ambiguous; present judgment is that the Wingate is Upper Triassic and the Navajo Lower Jurassic. However, the group is a cohesive unit, characterized by great cliff-forming sandstones that are a very prominent geomorphic feature in the west- ern part of the plateau, and it has not been subdivided on some of the source maps (Geologic Map of Arizona, 1969); so, it seems best to portray the group on the Geologic Map as a separate unit (J'E). Eastward in C01- orado and New Mexico, the group thins out and loses its distinctive character, and thus the separate desig- nation is dropped. The Glen Canyon Group is commonly 1,000 to 2,000 ft (300—600 m) thick, but the different components vary from place to place; the Navajo Sandstone is thickest toward the west where the Wingate Sandstone is thin or absent, and the Wingate is thickest toward the southeast and extends beyond the featheredge of the Navajo. Both the Wingate and Navajo are massive cliff—forming sandstones, with prominent festoon crossbedding, and are probably largely of eolian origin; the Wingate is characteristically red, and the Navajo white or buff. The intervening Moenave and Kayenta Formations are thinner bedded sandstones that form a topographic bench between the Wingate and Navajo Sandstones, and the Kayenta contains lenses of mudstone and impure limestone. JURASSIC The Jurassic System is represented on the Geologic Map of the United States by marine stratified rocks (J) in the Southern Rocky Mountains, the Colorado Plateau, and the Pacific coastal area of California and Oregon and by continental deposits (J c) in a few areas in the northern part of the Interior Region. Elsewhere in the Cordilleran Region, it is merged with the Trias- sic System into a unit of lower Mesozoic (1M2, lee). In the remainder of the country, and especially in the Interior and Appalachian Regions, Jurassic rocks are absent and were probably never deposited (fig. 14). Jurassic rocks are well developed beneath the Gulf Coastal Plain, where they have been extensively explored by drilling, but none of them are exposed. The Jurassic is divided into the Lower, Middle, and Upper Jurassic Series. In addition, the western Euro- pean stage terms (Lias, Bajocian, Callovian, Oxford- ian, Kimmeridgian, Portlandian or Tithonian, and others) are commonly used in discussions of Jurassic stratigraphy in North America. None of these subdivi- sions are shown on the Geologic Map of the United States, because of the narrow outcrop belts. ROCKY MOUNTAIN REGION Along the western edge of the Rocky Mountain Re- gion, the Jurassic rocks are miogeosyclinal and marine. The Arapien Shale and evaporites of central Utah are 5,000 to as much as 10,000 ft (1,500—3,000 m) thick, and farther north, from the northern Wasatch Mountains into the thrust belt of southeastern Idaho, a more varied assemblage of formations is about 5,000 ft (1,500 m) thick (mostly shown as MA: on the Geologic Map), including the Nugget Sandstone, Twin Creek Limestone, Preuss Sandstone, and Stump Sandstone, which range from Lower Jurassic into Upper Jurassic. In the Colorado Plateau, the equivalent to the Nugget is probably the Navajo Sandstone, and the higher strata have equivalents in the San Rafael Group, 1,000 to 2,000 ft (300—600 m) thick (Baker and others, 1936, p. 6—9): the Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville For- mations. Most of the group is nonmarine, but the Car- mel and Curtis contain fossiliferous marine tongues, by which they can be linked with the miogeosynclinal strata to the northwest. To the north, in the Central and Northern Rocky Mountains, equivalent beds are all marine and form the Sundance Formation in the south and the more comprehensive Ellis Group farther north (mostly shown as 1M: on the Geologic Map). 48 Above these deposits throughout the Rocky Moun- tain Region is the latest Jurassic Morrison Formation, about 250 to 750 ft (75—230 m) thick, nonmarine throughout and probably a flood-plain and lacustrine deposit, and composed largely of variegated mudstones, but with sandstone units locally, especially in the southwest part. It spreads over an area of 655,000 mi2 (1,680,000 kmz) in the Rocky Mountains and Great Plains to the east and was produced during a remarkable period of quiescence in a region that was tectonically restless both before and after. The Morri- son is notable for its abundant remains of dinosaurs and other reptiles, but it also contains fossil freshwater pelecypods and plants. The Colorado Plateau formations extend eastward across the Southern Rocky Mountains and form nar- row bands of outcrop where they are turned up along the eastern flanks of the Front Range and Sangre de Cristo Mountains; they thin to a featheredge in the Great Plains beyond. Mention should also be made of the Upper Jurassic Malone Formation, exposed in a single area close to the Rio Grande in western Texas (Albritton and Smith, 1965, p. 25—38), which is the sole representative in the United States of the Jurassic miogeosynclinal rocks that are extensive in the Sierra Madre Occidental to the south in Mexico. PACIFIC COASTAL AREA l Along the western side of the Sacramento Valley in northern California, the Upper Jurassic (Portlandian or Tithonian) Knoxville Formation is exposed for 120 mi (190 km) and forms the base of the “Great Valley sequence” of upper Mesozoic strata. The rocks of this sequence dip homoclinally eastward beneath the val- ley and are up to 50,000 ft (15,000 m) thick; the Knox- ville part alone is 16,000 ft (5,000 m) thick (Bailey and others, 1964, p. 124—130), but it ends abruptly near the 40th parallel at a transverse fault, north of which the Lower Cretaceous rocks (Paskenta) form the base of the sequence. The Knoxville is thin-bedded shale and sandstone (flysch), with some layers of pebbly mudstone, that contains a sparse fauna of B uchia piochii, Inoceramus, belemnites, and various am- monoids. This fauna is Tithonian, but B uchia rugosa, a Kimmeridgian fossil, has recently been discovered in the basal layers (Jones, 1975). Paleocurrents indicate that the Knoxville sediments were derived from the northeast, probably from the Sierra Nevada and Klamath Mountains, whose Jurassic and older rocks had been deformed by the Nevadan orogeny of mid— Jurassic time. The Knoxville was deposited on an ophiolitic or oceanic crust, now preserved along its western edge as PALEOZOIC AND MESOZOIC ROCKS a great sheet of serpentinite, followed by diabase, pil- low lava, and radiolarian chert, which pass upward into the normal Knoxville sequence (Bailey and others, 1970). The serpentinite and overlying Knoxville are thrust westward over the partly coeval Franciscan Formation. Other Knoxville-type Upper Jurassic rocks crop out in places at the base of the Great Valley sequence farther south along the edge of the valley, but they are too small to separate from the Cretaceous on the Geologic Map. Jurassic rocks are also shown on the Geologic Map in the structurally complex coastal area of southwestern Oregon, where they are juxtaposed against other Mesozoic rocks—Upper and Lower Cretaceous (uK, 1K), the Dothan Formation (Franciscan equivalent, uMze), the Galice Formation (lee), and masses of ser- pentinite. The largest Jurassic unit is the Otter Point Formation (Koch, 1966, p. 36-43; Coleman, 1972, p. 12—14), a thick graywacke-shale unit, including much volcanic material (pillow lavas and pyroclastic debris), that contains Buchia piochii and other Tithonian fos- sils. CONTINENTAL DEPOSITS 0c) In the northern part of the Interior Region, the Paleozoic rocks are capped in places by red continental deposits, which palynological studies demonstrate are of Early Jurassic age (Cross, 1967), although they have erroneously been assigned to the Pennsylvanian or the Permian System in the past. The largest area is in the center of the Michigan basin, where 300 to 400 ft (90— 120) m) of red shale and sandstone, with minor gyp- sum, overlie the Pennsylvanian sequence (Cohee, 1965, p. 220). The Fort Dodge Gypsum and associated redbeds similarly overlie Pennsylvanian rocks in a smaller area in north-central Iowa. Another area of Jurassic continental deposits is mapped in northwest- ern Minnesota next to the Canadian border but is ap- parently known mainly from drilling beneath the gla- cial cover; it is the south end of an extensive belt in Manitoba, lying between the Cretaceous sequence on the west and the Paleozoic on the east. LOWER MESOZOIC In extensive areas of the Cordilleran Region, the Triassic and Jurassic rocks are combined on the Geologic Map into a unit of lower Mesozoic, which in- cludes marine stratified rocks (1N2), eugeosynclinal deposits (lNhe), and volcanic rocks (llev). MARINE STRATIFIED ROCKS (le) The lower Mesozoic marine stratified rocks are rela- tively thin cratonic deposits in Wyoming and Montana LOWER MESOZOIC but are thicker miogeosynclinal deposits in southeast- ern Idaho and westward in the Great Basin of Utah and Nevada. They also change westward from domi- nantly continental deposits to dominantly marine deposits. The cratonic deposits, as exposed on the flanks of the mountain uplifts, are 500 to 1,500 ft (150—500 m) thick and comprise the redbeds of the Chugwater Formation (Upper Triassic) below, followed by the marine Sun- dance or Ellis Formations, and topped by the continen- tal Morrison Formation (Jurassic). The Triassic red- beds wedge out northward in southern Montana, and only the Jurassic units persist beyond. . In the miogeosynclinal belt of southeastern Idaho, the Triassic sequence thickens to nearly 7,000 ft (2,100 m) and the Jurassic to 5,000 ft (1,500 m), and there is a more complex array of formations—the Dinwoody, Thaynes, and Ankareh Formations in the Triassic, and the Nugget Sandstone, Twin Creek Limestone, and Preuss and Stump Sandstones in the Jurassic. The Idaho Triassic section is notable for containing the thickest and most complete sequence of Lower Triassic in the world, and it contains ammonoids at many levels, including Meekoceras (Kummel, 1954, p. 165). The basal Dinwoody Formation lies with nearly paral- lel bedding and no indication of erosion on the Permian Phosphoria Formation, although separated from it by a considerable hiatus. The thicker, overlying Thaynes Formation is dominantly limestone, although with silty and sandy members, and intertongues with red- beds toward the craton. Jurassic rocks do not extend beyond western Utah, but the Triassic System is well represented in northeastern Nevada (Elko County), where the sequence is mostly limestone and as much as 3,000 ft (900 m) thick (Clark, 1957, p. 2200—2209). A quite different set of lower Mesozoic miogeosyn- clinal deposits occurs in west-central and southwestern Nevada. It is separated from those just described by a 100-mile (160 km) gap, which was probably a land bar- rier. In the Sonoma Range and elsewhere near Win- nemucca in north-central Nevada are two sequences, the Augusta to the east and the Winnemucca to the west, that were juxtaposed by moderate westward thrusting during later Mesozoic time (Silberling and Roberts, 1962, p. 19—25). Both lie in part on the Koipato Formation ('EPe), but they have no formations in common. The Augusta sequence was deposited nearer the shore to the east, and its lower part passes eastward into conglomerates and coarse clastics (China Mountain and Panther Canyon Formations); much of the rest is limestone. Where fully developed, the sequence is about 8,000 ft (2,400 m) thick. The Winnemucca sequence formed farther from shore and passes westward into deep-water turbidites and even- 49 tually into eugeosynclinal deposits. In its miogeosyn- clinal phase to the east, its lower part (Prida and Natchez Pass Formations) is largely limestone, but the higher parts are shaly and sandy; the Winnemucca se- quence reaches a thickness of 10,000 ft (3,000 m). The Augusta sequence is of Middle and—Upper Triassic age, but the Winnemucca sequence includes Lower Jurassic rocks at the top. About 60 mi (100 km) south of the exposures of these sequences is the Luning sequence of latest Triassic and Early to Middle Jurassic age (Silberling and Roberts, 1962, p. 28—33), which also was deposited west of a shoreline. The greater part of the sequence is the Lun- ing Formation, as much as 8,000 ft (2,500 m) thick, largely of carbonate rOcks, including coral reefs. Later on, local folding and thrusting occurred, and so the' coarse clastics and fanglomerates of the Dunlap For- mation at the top rest on different older parts of the sequence from place to place, and even on pre-Triassic rocks. EUGEOSYNCLINAL DEPOSITS (lee) Small areas of lower Mesozoic rocks, intruded by Mesozoic granitic rocks, occur in western Nevada and to the west in the Sierra Nevada of California. They change westward from miogeosynclinal to eugeosyn- clinal deposits; on the Geologic Map the line of separa- tion is made between unit J '13 (shale, mudstone, siltstone, and sandstone) and unit J'Fi (shale, sandstone, volcanogenic clastic rocks, andesite, and rhyolite) as represented on the compilation for the new Geologic Map of Nevada. Lower Mesozoic eugeosynclinal deposits, again as- sociated with Mesozoic granitic rocks, are more exten- sive in the Sierra Nevada. In the eastern Sierra the thickest sequence is in the Ritter Range roof pendant west of the head of Owens Valley, where there are 30,000 ft (9,000 m) of intermediate to felsic pyroclastic rocks (Rinehart and others, 1959, p. 945). Early J uras- sic fossils occur about a third of the way up in the sequence, and so there is a possibility that unrecog- nized Triassic and later Jurassic rocks may be present. The largest area of lower Mesozoic eugeosynclinal deposits in the Sierra Nevada, however, is in the west- ern foothills from the Mother Lode belt westward, where there is about 15,000 ft (4,500 m) of volcanic rocks and volcaniclastic sediments (Clark, 1964, p. 6—8). These include the “Mariposa Slate” of the Gold Belt folios, as well as “porphyrite,” "diabase”, and "amphibolite” that were once considered to be intru- sive but are now known to be supracrustal volcanic deposits. (They were erroneously grouped with the Sierra Nevada granitic rocks on the Geologic Map of 50 the United States of 1932.) The rocks have been sub- jected to low-grade greenschist metamorphism, but fossils occur at various places which indicate ages ranging from Middle to Late Jurassic (Bajocian to Kimmeridgian). The base of the sequence is generally juxtaposed against serpentinite which was probably original oceanic crust, and no Triassic is present. The sequence is also largely older than the Knoxville For- mation at the base of the Great Valley sequence to the west, which is largely Portlandian (=Tithonian), al- though it includes some Kimmeridgian at the base (Jones, 1975). Modern work indicates that these Jurassic rocks ac- tually consist of several sequences of unlike forma- tions. The eastern sequence includes the original Mariposa Formation at the top, of slate, tuff, and graywacke, underlain by the Logtown Ridge Forma- tion of andesitic volcanic breccia, pillow lava, tuff, and sandstone. The western sequence begins with the Gopher Ridge Volcanics of basaltic, andesitic, and rhyolitic pyroclastic rocks and lavas, followed by the Salt Spring Slate (possibly equivalent to I the Mariposa), the Merced Falls Slate, and the Copper Hill Volcanics which resemble those of the Gopher Ridge. Both sequences are island-arc deposits which origi- nally formed far apart, but which are now closely ad- joined as a result of faulting and subduction (Schweic- kert and Cowan, 1975, p. 1329—1331). Toward the north the two sequences are separated by the 20-mile (32-km) Smartville terrane, which is ophiolitic ocean floor facies, with pillow basalts underlain by sheeted dike complexes and intrusive gabbro. This terrane wedges out southward between the sequences, leaving a narrow belt of melange containing a great variety of tectonic blocks, including Permian fusulinid-bearing limestone of unknown original provenance (Duffield and Sharp, 1975). In the southern part of the Sierra Nevada, the pre- vailing granitic rocks contain many small roof pen- dants of supracrustal rocks, most of which are undated but all of which are shown on the Geologic Map as lower Mesozoic eugeosynclinal deposits (1Mze). In the Taylorsville area at the north end of the Sierra Nevada, the Triassic System is represented by the Hosselkus Limestone and Swearinger Slate, two relatively thin nonvolcanic abundantly fossiliferous units of Late Triassic age. They are followed on Mount Jura by a 13,000-ft (4,000-m) sequence of elastic and volcaniclastic rocks, subdivided into many units Whose fossils indicate a nearly complete sequence from base to top of the Jurassic System (McMath, 1966, p. 181— 182). In a somewhat similar sequence along the strike to the north in the easternmost belt of the Klamath Mountains, 9,000 ft (2,700 m) of Triassic rocks overlie PALEOZOIC AND MESOZOIC ROCKS the Permian sequence and are succeeded by 7,000 ft (2,100 m) of Jurassic rocks; A very different sequence of lower Mesozoic eugeosynclinal deposits forms the Western Jurassic belt of the Klamath Mountains for its entire length, from southwestern Oregon into northern California (Irwin, 1966, p. 24—25; Hotz, 1971, p. 13—14). It is faulted against the Western Paleozoic and Triassic belt ('FiPe) on the east and the Franciscan Formation (=Dothan Formation) on the west. Its rocks constitute the Galice Formation, volcanic below and clastic above, that is as much as 15,000 ft (4,600 m) thick and that is dated by fossils as of Oxfordian and Kimmerid- gian age; it has commonly been compared with the Mariposa Formation of the western Sierra Nevada. Northeast of the Klamath Mountains, in the Blue Mountains uplift of north-central Oregon, lower Mesozoic and Paleozoic rocks are again exposed. Those in the eastern part are mainly mapped as Triassic and Permian eugeosynclinal deposits ('EPe), but the stratigraphy in the western part is clearer and the rocks less metamorphosed; a typical sequence occurs in the Suplee-Izee area (lee) (Dickinson and Vigrass, 1965, p. 17—67). Lying unconformably on the Paleozoic is a sequence about 25,000 ft (7,600 m) thick of Late Triassic (Karnian) to early Late Jurassic (Callovian) age, the different parts themselves separated by angu- lar unconformities. Its sediments are largely argillites and siltstones, with interbedded sandstones and minor limestones, but lavas and volcaniclastic rocks occur in nearly all the units and dominate the upper third of the sequence. Far to the south, in the Peninsular Range of south- ern California, small to large bodies of supracrustal rocks are invaded by the Cretaceous Peninsular batholith. Most of these rocks are lower Mesozoic, al- though Paleozoic (uE’e) rocks may occur in the San J acinto Range to the east. The rocks are least metamorphosed and the sequence is plainest at the northwestern end, in the Santa Ana Mountains (Yerkes and others, 1965, p. 23). Below is the Bedford Canyon Formation, at least 20,000 ft (6,000 m) thick of argillite and slate, with some sandstone and limestone; it is overlain with angular unconformity by the San- tiago Peak Volcanics, several thousand feet thick. Fos- sils in the Bedford Canyon indicate an early Late Jurassic (mainly Callovian) age (Imlay, 1962, p. 98— 100). (Similar fossils occur in the Santa Monica Slate, exposed in an inlier northwest of the Los Angeles ba- sin.) The Santiago Peak Volcanics have not been dated; they are overlain unconformably by Upper Cretaceous rocks and may be of latest Jurassic or even Early Cre- taceous age, like similar rocks farther south in Baja California. The Julian Schist in the core of the Penin- CRETACEOUS sular Range has not been dated, but it may be a metamorphic phase of these formations. In the Mojave Desert region, between the Peninsular Range and the Sierra Nevada, many small areas are represented on the Geologic Map as lower Mesozoic eugeosynclinal deposits (lee). They include the Side- winder Volcanics and 0rd Mountain Group, of andesite and rhyolite flows and volcaniclastic rocks, generally believed to be Triassic. In the Clark Mountains farther east, however, volcanics overlie the Jurassic Aztec Sandstone. Similar rocks of less certain ages occur southeastward as far as the Colorado River and are classed as lower Mesozoic on the Geologic Map. In southwestern Arizona the State Map shows units of “Mesozoic gneiss” and “Mesozoic schist”; for purposes of the Geologic Map, the first is assumed to be Precam- brian reworked by Mesozoic orogenies and the second to be lower Mesozoic eugeosynclinal deposits. VOLCANIC ROCKS (lev) Terrestrial volcanic rocks (as contrasted with the eugeosynclinal volcanics already discussed) occur in several small areas, unrelated to each other, in widely scattered parts of the United States. The Moat Volcanics of New Hampshire (Billings, 1956, p. 35—37) are associated with and probably con- generic with the Jurassic White Mountain Plutonic Series and are preserved in downdropped blocks sur- rounded by ring dikes of the plutonic rocks. They con- sist of rhyolite flows and breccias up to 11,000 ft (3,300 In) thick and lie unconformably on Paleozoic rocks that were deformed and metamorphosed during the Aca- dian orogeny. They contrast surprisingly with the Upper Triassic sedimentary and volcanic rocks of the Newark Group not far to the south in New England. In the northeastern part of the Cortez Range in north-central Nevada is the Pony Trail Group of vol- canic wacke, tuff, and rhyolite flows about 10,000 ft (3,000 m) thick (Muffler, 1964, p. 20—39). It is not in contact with the adjacent Paleozoic rocks, but it is overlain in part by the Cretaceous Newark Canyon Formation (Kc) and is intruded by Jurassic granitic plutons. These volcanic rocks contrast with the lower Mesozoic miogeosynclinal rocks not far to the east and west but probably accumulated on the land barrier which separated the two groups of miogeosynclinal de- posits. In some of the ranges close to the Mexican border in southern Arizona is another assemblage of lower Mesozoic volcanic rocks (Hayes and Drewes, 1968, p. 51—54), which lie unconformably on Paleozoic rocks and are overlain unconformably by the Lower Creta- ceous Bisbee Group. They consist of a lower volcanic unit as much as 10,000 ft (3,000 m) thick of rhyodacite 51 and andesite flows and tuffs, a middle redbed unit about 2,000 ft (600 m) thick, and an upper volcanic unit about 7,000 ft (2,100 m) thick of silicic flows and tuffs. Fossils are rare and not diagnostic, but Pb/alpha and K/Ar determinations indicate that the sequence ranges in age from Early Triassic to Early Jurassic. The volcanics are, further, intruded by Jurassic gran- itic rocks that have yielded a Pb/alpha date of 184 my CRETACEOUS In terms of the Geologic Map of the United States, the Cretaceous System, or uppermost division of the Mesozoic, by far overshadows the Jurassic and Triassic Systems, as well as many of the Paleozoic systems, not only in the wide extent of its exposures but in the vari- ety and complexity of its formations. The Cretaceous sequence forms a nearly continuous band of outcrop in the Atlantic and Gulf Coastal Plains and expands in Texas into the hill and plateau country to the northwest (fig. 15). It is even more extensive in the central and northern Great Plains and westward in the Rocky Mountains. Other outcrops occur in the Pacific coastal area through California into southwest- ern Oregon. Most of the Cretaceous rocks so rep- resented are marine, but the marine deposits in the Rocky Mountains change westward in the Great Basin into continental deposits (Kc). The Cretaceous is divided into the Lower Cretaceous and Upper Cretaceous Series (lK, uK), whose local rep- resentatives in the Gulf Coastal Plain are called the Comanche and Gulf Series. The Lower Cretaceous comprises the Neocomian, Aptian, and Albian Stages, and the Upper Cretaceous the Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maestrichtian Stages of the European classification. LOWER CRETACEOUS TEXAS The Comanche Series of the Lower Cretaceous is mainly of Albian age but may include some Aptian rocks at the base. The remainder of the Aptian stage, and the Neocomian, are unrepresented in Texas, al- though they are well displayed in Mexico to the south, where they have been called the Coahuila Series (Im- lay, 1944, p. 1005—1007). The Comanche Series departs from the European classification by including some Cenomanian units at the top (Del Rio Clay and Buda Limestone); the discrepancy is not fundamental, as these are very thin units. The Comanche Series is divided into the Trinity, Fredericksburg, and Washita Groups (1K1, 1K2, 1K3), which are separately represented on the Geologic Map in the broad outcrop area that extends across central 52 PALEOZOIC AND CMESOZOIC ROCKS FIGURE 15.—The United States, showing areas mapped as Cretaceous stratified rocks on the Geologic Map of the United States; Lower and Upper Cretaceous are separately shaded. Includes units of Lower Cretaceous (1K, 1K1, 1K2, u M e) and of Upper Cretaceous (uK, uKl, uK2, uK3, uK4, Kc, Ke, Kv) age. :_ ........ —é: ! 3. l~ ’7’ x z’ I)| —— ----- a , J O / Q g G U L F 0 F CRETACEOUS MEXICO FIGURE 15.—Continued. EXPLANATION Upper Cretaceous Lower Cretaceous 53 54 and northern Texas. (For a summary of the Comanche Series, see the useful but now somewhat outdated pres- entation by Adkins (1932, p. 272—400).) In west Texas, where the outcrop bands are narrow and the structure more complex, the separation is not made, nor is it made in the small outlying areas of the Comanche Series in southern Arizona, Oklahoma, and southern Kansas. The Trinity Group (1K1) is an irregular basal deposit that lies on the eroded surface of Paleozoic and Triassic rocks. Where best developed near Austin, it is 800 ft (240 m) or more thick and consists-of the Travis Peak Formation below of sands, clays, and thin limestones and the overlying Glen Rose Formation of ledge- making limestones alternating with softer marls. To- ward the north and west, the group thins and finally disappears; sandstone tongues increase in prominence and finally dominate altogether. The Fredericksburg Group (1K2), 500 ft (150 m) or more thick, contains the thick widespread rudistid-bearing Edwards Lime- stone, with the more marly Walnut Clay and Coman- che Peak Limestone below and the Kiamichi Forma— tion above. The Washita Group (1K3) in north Texas consists of marls and clays with thin interbedded lime- stones, divided into an array of formations. Southward toward Austin it thins into a more condensed sequence, the Georgetown Limestone, with the thin well-marked Del Rio Clay and Buda Limestone at the top. The units in western Texas are similar but have some local vari- ations. In a broad area from Austin to the Pecos River and beyond, the Comanche Series lies nearly flat and forms the Edwards Plateau. Much of the surface of the plateau is not formed of the Edwards Limestone, how- ever, but of similar limestones of the Washita Group, and lower formations are penetrated only in canyons along the edges. The extent of the Washita is much greater than has been shown on previous maps (Geologic Map of United States of 1932; Geologic Map of Texas of 1937), where the base of the Washita had been placed erroneously 100 to 200 ft (30—60 m) too high, resulting in a notable difference in map pattern in this flat-lying terrane. In the southern part of the plateau, the simple stratigraphy farther north breaks down, and the Fred- ericksburg and Washita Groups merge into a massive reef deposit, the Devils River Limestone (Smith, 1970, p. 43—44). (On the Geologic Map, this facies change is ignored and the Fredericksburg-Washita boundary is sketched arbitrarily through the deposit.) The Devils River Limestone reef is the only surface exposure in the United States of a regional feature—a south-facing shelf break and barrier reef that extends far eastward in the subsurface beneath the Gulf Coastal Plain and PALEOZOIC AND MESOZOIC ROCKS at the surface southwestward across the Rio Grande into Mexico. From northern Texas, the outcrop belt of the Coman— che Series extends eastward across southern Ok- lahoma into southwestern Arkansas, but its top is progressively truncated by the unconformity at the base of the Upper Cretaceous Woodbine Sand (uKi) until none remains, although it continues in full de- velopment in the subsurface to the south. Westward from western Texas, small outcrops of Comanche Series in southern New Mexico and in southeastern Arizona, where it is represented by the Bisbee Group, are as much as 10,000 ft (3,000 m) thick (Hayes and Drewes, 1968, p. 55—56); they are typically developed in the Mule Mountains near Bisbee. Here, the middle part is the Mural Limestone, with Trinity and possibly Fredericksburg marine fossils, and the Morita Formation below and Cintura Formation above are pinkish sandstones and siltstones, largely terres- trial. The Glance Conglomerate at the base lies on a rough surface eroded on the Paleozoic rocks and the lower Mesozoic volcanics and likewise contains some interbedded lava. .Northwestward, the Mural Lime- stone wedges out, and the terrestrial deposits alone remain. North of Texas, small outliers of Comanche rocks are scattered over the Permian rocks in western Ok- lahoma, and a larger remnant occurs at the edge of the Great Plains in southern Kansas, where the series is represented by the Cheyenne Sandstone and Kiowa Shale (Merriam, 1963, p. 60—61). The latter contains thin limestones and shell beds whose fossils are of Washita age, the older units having disappeared by overlap. Farther north, the Comanche rocks largely wedge out, although their thinned equivalents may be represented in the Dakota Sandstone (uKi). ATLANTIC COASTAL PLAIN East of Arkansas, Lower Cretaceous rocks are miss- ing for a long distance at the edge of the Coastal Plain, and their next appearance is in northern Virginia, Maryland, and New Jersey, where they form the Potomac Group, typically developed near Washington, DC. (Spangler and Peterson, 1950, p. 62—69). The Potomac Group consists of terrestrial sandstones, sandy shales, and clays, with local beds of gravel and lignite; it is as much as 800 ft (240 m) thick and has been divided into the Patuxent, Arundel, and Patapsco Formations. Equivalents in New Jersey are in the Raritan Formation which is partly Upper Cretaceous. Some of the'nonmarine Tuscaloosa Formation of North Carolina, generally classed as Upper Cretaceous, may also be of Potomac age. The Potomac group contains CRETACEOUS fossil plants at many levels, and the Arundel Forma— tion contains reptiles like those of the Morrison For- mation of the Rocky Mountains. According to com- monly accepted correlations, the group is of Neocomian age at the base but extends into the Aptian Stage. ROCKY MOUNTAINS Lower Cretaceous rocks, mostly a few hundred feet (60—100 m) thick, are extensive in the Rocky Moun- tains and in the subsurface in the Great Plains to the east, but on the Geologic Map they are for the most part merged with the Dakota Group (uK1) of the Upper Cretaceous on the Geologic Map. The Dakota itself is a problematical unit: In its original area in eastern Ne- braska it is all Upper Cretaceous; in other areas the so-called Dakota is Lower Cretaceous; in still others the Dakota Group includes formations of both ages. In the Black Hills of South Dakota and Wyoming, for example, rocks mapped as uK1 include the Inyan Kara Group (Lakota Sandstone, Fuson Shale, and Fall River Sandstone, the latter the Dakota Sandstone of original reports), the Skull Creek Shale, Newcastle Sandstone, and Mowry Shale, the last of late Albian age—in other words, all Lower Cretaceous. In northeastern New Mexico and southwestern Colorado, the thin marine Purgatoire Formation, a tongue of the Comanche Series, is included in uKi. Similar‘combinations, or others with varied terminologies, occur in other areas. On the GeOlogic Map, the only Lower Cretaceous rocks represented as such are shown in the west and northwest, where the Lower Cretaceous rocks are thicker and not involved in the “Dakota problem.” In the thrust belt of southeastern Idaho and western Wyoming, the Lower Cretaceous (1K) includes the Gannett Group, 3,500 to 5,000 ft (1,000—1,500 m) thick (Eyer, 1969); the Bear River Formation, 500 ft (150 m) thick; and the Wayan Formation, 3,000—4,000 ft (900— 1,200 m) thick, which range in age from Neocomian through Albian. Only the thin Bear River Formation contains marine elements; the Gannett and Wayan are continental tectonic deposits related to the growth of the thrust belt, with much conglomerate and coarse- grained sandstone and interbedded red and purple mudstone. Pauses in tectonic activity during deposi- tion of the Gannett Group are indicated by two units of lacustrine limestone. ’ In west-central Montana the Lower Cretaceous series is represented by the Kootenai Formation, about 1,000 ft (300 m) thick of nonmarine conglomerate and purplish or greenish shale and mudstone, probably of Aptian age. It lies unconformably on other nonmarine deposits, probably equivalent to the Jurassic Morrison Formation and is succeeded by marine shales that are themselves of high Lower Cretaceous age at their base. 55 CALIFORNIA AND OREGON The Lower Cretaceous rocks on the western side of the Sacramento Valley in northern California are part of the "Great Valley sequence” and form the Shasta Series, which has traditionally been divided into the Paskenta and Horsetown Formations, although other stratigraphic names are now used. It overlies the Upper Jurassic Knoxville Formation and is more than 17,000 ft (5,100 m) thick, ranging in age from Neoco— mian to Albian (Bailey and others, 1964, p. 130-133). At the north end of the valley, it oversteps the Knox- ville and lies directly on the eroded surface of the de- formed Jurassic and older rocks of the Klamath Moun- tains and their embedded plutons. The lowest beds are nearly identical with those of the Knoxville beneath and are distinguished mainly by a different species of Buchia. Higher up, they are somewhat more varied, with layers of graywacke, conglomerate, and mudstone, and thin limestone interbeds, the whole still a turbidite (flysch) deposit like the preceding Knox- ville. To the west within the Coast Ranges are long narrow outliers of Lower Cretaceous rocks, which are in fault contact with the adjacent Franciscan as- semblage and differ notably from it in their lack of metamorphic rocks, greenstone, and serpentinite even though sparse fossils in the Franciscan prove that part of it is younger. These outlying areas of Lower Cre— taceous are evidently klippen of the Coast Range thrust sheet which forms the base of the main Great Valley sequence to the east. Lower Cretaceous rocks are also exposed in the southern Coast Ranges and along the west side of the San Joaquin Valley, but their occurrence here is more sporadic than farther north. In the structurally complex area of southwestern Oregon, small areas of Lower Cretaceous rocks are part of the Myrtle Group. The Myrtle lies unconforma— bly on the Jurassic rocks of the Galice Formation and is faulted against the Dothan Formation (= Francis- can). Above its basal conglomerates are rhythmically bedded sandstones and mudstones with interbedded shelly layers, probably mainly shallow-water deposits with only a few deep-water turbidites. WASHINGTON In the northern part of Washington State, in the Methow River valley east of the Cascade Range, Cre- taceous rocks occupy a downfaulted trough 20 mi (32 km) wide that extends'southward from the Canadian border nearly to the Columbia River and for an even greater distance northward into British Columbia (Barksdale, 1975, p. 24—50). The rocks are thick and steeply folded but not metamorphosed like those that 56 flank them on the east and west. The rocks are all clastic and include lithic sandstone, arkose, black shale, and chert- and granite—pebble conglomerate, of marine origin (in the Buck Mountain, Goat Creek, Panther Creek, Harts Pass, and Virginian Ridge For- mations), followed by continental arkose (Winthrop Sandstone), and topped by andesite tuff, breccia, and flows (Midnight Peak Formation). Total thickness is as much as 50,000 ft (15,000 m). The sequence is fos- siliferous, and ranges in age from Neocomian to Cenomanian, with possible; Jurassic at the base; the volcanics at the top (KV) may be Turonian. All the rocks are represented as Lower Cretaceous (1K) on the Geologic Map. UPPER CRETACEOUS WESTERN GL'LF (IOASTAL PLAIN The well-developed Upper Cretaceous sequence in Texas, the Gulf Series, provides a standard of reference for sequences elsewhere in the Coastal Plain. (For a useful summary, now somewhat outdated, see Adkins (1932, p. 400—516). Correlations are presented by Stephenson and others (1942).) On the Geologic Map, it is divided into the Woodbine Sand (uKi) of Cenoma- nian age; the Eagle Ford Shale and Austin Chalk (uK2) of Turonian, Coniacian, and Santonian age; the Taylor Marl (uKa) of Campanian age; and the Navarro Group (uK4) of Maestrichtian age. Most of the exposed Upper Cretaceous is normal neritic fossiliferous shales, marls, and chalks; however, marginal basal clastic rocks occur in the Woodbine, and terrestrial coal- bearing beds occur in the Navarro near the Rio Grande. _ The Gulf Series is everywhere unconformable on the Comanche Series in surface outcrops. The Woodbine Sand truncates all the groups of the Comanche east- ward into Arkansas. Southwestward, the Woodbine it- self wedges out on the unconformity and disappears near Waco. Beyond, in west Texas, the Eagle Ford Shale (uK2) lies with a hiatus on the Washita Group (1K3). The Gulf Series is followed at all places by a disconformity, which separates it from the overlying Midway Group of Paleocene age. The Woodbine Sand (uKi) is mainly poorly consoli- dated sand, in part leaf bearing and probably non- marine, which intertongues with clays, some of them lignitic but some of them oyster-bearing nearshore brackish water deposits. In places it contains much vol- canic material and in Arkansas includes interbeds of gravel. , The Eagle Ford Shale (uKz) in its typical area is marine black shale but changes laterally into marls, and in much of west Texas it is calcareous flagstone PALEOZOIC AND MESOZOIC ROCKS (Boquillas Flags). The Austin Chalk is a solid body of white chalk in its type area, but with a tongue of mar] and clay (Bonham) in the middle toward the northeast. The typical chalk weathers to a rich black soil, exten- sively planted in cotton. The Taylor Marl or Group (uKs) is more varied than the units that precede it. In its typical area it is largely marl, but northeastward in Texas and Arkansas it con- tains many traceable units of sand and chalk that are separately named, and for some distance west of San Antonio the marls are partly or wholly replaced by the reef deposit of the Anacacho Limestone. The sand units are marginal deposits that indicate the ephemeral exis- tence of nearby shorelines. The Navarro Group (qui) is equally varied and has been divided into many named formations. North of Austin, for example, it consists of the Neylandville Marl, Nacatosh Sand, Corsicana Marl, and Kemp Clay. Near the Rio Grande, the lower half of the group passes into terrestrial coal-bearing deposits (Olmos Forma- tion), which are more important in the Sabinas basin in Mexico to the south. EASTERN GULF COASTAL PLAIN Upper Cretaceous rocks reappear in the eastern part of the Gulf Coastal Plain, beyond the wide gap of the Mississippi Embayment where they are covered by Cenozoic deposits, and exhibit the same lithologies as in Texas, but arranged in a different order. (For a sum- mary, now somewhat outdated, see Stephenson (1926, p. 231—245). Some of the later developments are pre- sented by Eargle (1953).) Lower Cretaceous does not emerge in the region; the basal deposit is Upper Cre- taceous throughout. The sequence is best developed in Alabama, where it comprises the Tuscaloosa Formation (uKi), approxi- mately equivalent to the Woodbine Sand; the Eutaw Formation (uKz), approximately equivalent to the Eagle Ford and Austin Formations; the Selma Chalk (uKa), approximately equivalent to the Taylor Marl; and the Ripley Formation (uK4), approximately equiva- lent to the Navarro Group. The Tuscaloosa Formation consists of irregularly bedded nonmarine sands, clays, and gravels, in places lignitic and with fossil plants at many levels. The Eutaw Formation is a marine deposit, mainly glauconitic and micaceous sand with some interbedded clay. The Selma is a thin—bedded to massive chalk, much like the Austin although one stage younger (uK3 rather than uKz), but it fingers out into sands and clays northwestward in Mississippi and Tennessee and eastward in Georgia. The Ripley, like the Eutaw, is marine sands and clays, in part glauconitic. Toward the head of the Mississippi Embayment, in Kentucky CRETACEOUS and Illinois, all the lower part of the Upper Cretaceous sequence wedges out by overlap, and so the Ripley lies directly on Paleozoic rocks. ' Eastward in Georgia, South Carolina, and North Carolina, the Eutaw Formation wedges out, and so the higher units (uK3 and uK4) lie with a hiatus on the Tuscaloosa Formation (uKi). In eastern Georgia and in South Carolina, the outcrop belt of the Upper Creta- ceous rocks is much interrupted by overlapping Ter- tiary deposits, and the next large outcrop area is in southern North Carolina, where the Upper Cretaceous extends nearly to the coast along the broad upwarp of the Cape Fear arch. Here, the Tuscaloosa is a non- marine sandy and gravelly deposit as it is in Alabama, but the higher Black Creek and Pee Dee Formations (uK3, uK4) differ from the Alabama units, being marine clays, sands, and marls. ATLANTIC COASTAL PLAIN The Upper Cretaceous sequence is concealed by over- lapping Tertiary deposits in northern North Carolina and throughout Virginia, but it reappears in Maryland and extends through New Jersey into the New York City area, resting on the Lower Cretaceous Potomac Group, which it bevels to the northeast (Spangler and Peterson, 1950, p. 15—52). The Upper Cretaceous comprises the Raritan, Magothy, Matawan, and Monmouth Formations (or Groups), which span the whole period from Cenoma- nian to Maestrichtian, but they are all thin units and their outcrop bands are so narrow that the whole is represented on the Geologic map as an undivided unit (uK). The Upper Cretaceous is marine, in contrast to the Potomac Group, and is composed of sands, clays, and marls, including many beds of highly glauconitic green- sand. GREAT PLAINS AND ROCKY MOUNTAINS By far the largest area of outcrOp of Upper Cretaceous rocks in the United States is in the central and north- ern Great Plains and the contiguous Rocky Mountains (fig. 15). These rocks are the product of a single great seaway that connected the Gulf of Mexico on the south and the Arctic Ocean on the north. The eastern feath- eredge of the deposit is in eastern Kansas and western Iowa and‘ Minnesota, whence it extends 800 mi (1,300 km) westward through the Rocky Mountains to the front of the Cordilleran thrust belt in the Northern Rocky Mountains and the eastern Great Basin. Throughout this area the Upper Cretaceous was origi- nally a continuous deposit; it is now interrupted in the Rocky Mountains where it has been eroded from the uplifts, and it is covered in many areas in the Great Plains by Tertiary deposits. 57 There is a vast literature on the Upper Cretaceous in the region which it would be fruitless to attempt to document—on its stratigraphy, paleontology, sedimen- tology, and economic potential—but most of the publi- cations deal with special areas or problems. General syntheses are few; Cobban and Reeside (1962) have presented the correlations, and a general survey of the rocks and their problems appears in the “Geologic Atlas of the Rocky Mountain Region” (McGookey and others, in Mallory, 1972, p. 190—228). The gross units of the Upper Cretaceous in the region are the Dakota Group (uKi), the Colorado Group (uK2), the Montana Group (uKa), and the Laramie and as- sociated formations (uK4). These are approximately equivalent to the Woodbine Sand, the Eagle Ford Shale and Austin Chalk, the Taylor Marl, and Navarro Group, respectively, of the western Gulf Coastal Plain, and the symbols are usedinterchangeably between the two regions, although they are not precisely correlative in detail. Thus, the Dakota Group (uKi) toward the east is basal Upper Cretaceous, but as mapped farther west it includes Lower Cretaceous and in places is entirely Lower Cretaceous; the upper part of the Montana Group (uKa) includes equivalents of the lower part of the Navarro Group. The Upper Cretaceous sequence is 2,000 ft (600 m) thick or less in the eastern Great Plains, but it thickens to 20,000 ft (6,000 m) at the front of the Cordilleran thrust belt on the west. As these thicknesses imply, the dominant sediment source was in the Cordilleran re- gion to the west, where orogenic deformation was in progress during much of the period and was accom- panied by thrusting, volcanism, and batholithic intru— sion, whose erosion products were shed eastward into the Cretaceous-seaway. The eastern side of the seaway provided only minor sediment sources, only the basal Dakota Sandstone along the eastern margin appears to have been derived from the craton. The Upper Cretaceous sequence in the Great Plains is relatively simple. The Dakota Sandstone (uKi) at the base is a terrestrial marginal deposit. The succeed- ing Colorado Group (uK2) includes dark shale and widespread carbonate deposits—the thin Greenhorn Limestone in the Benton Group below, and the thicker Niobrara Chalk above, which is much like the Austin Chalk of the western Gulf Coastal Plain and of about the same age. The dark marine Pierre Shale dominates the Montana Group (uKa) above, but it is topped by thinner marginal deposits of the Fox Hills Sandstone, This is succeeded by terrestrial coal-bearing deposits known from place to place as the Laramie, Lance, or Hell Creek Formations (uK4), which are overlain, mostly conformably, by the similar Paleocene terres- trial deposits of the Fort Union Formation. 58 Along the eastern margin of the Upper Cretaceous outcrop its subdivisions are thin and heavily drift covered; so, they are not divided on the Geologic Maps of Iowa or Minnesota, nor on the Geologic Map of the United States. In considerable areas in northern Min- nesota, the thin unconsolidated Coleraine Formation of Upper Cretaceous age lies between the glacial drift and the Precambrian basement (Sloan, 1964) but is not shown on the Geologic Map (King and Beikman, 1974, fig. 13). Complications develop westward in the Rocky Moun- tain Region. Carbonate deposits, such as the Niobrara, fade out in the marine shales, and in the shales appear westward-thickening wedges of coarser clastics— shallow-water sandstones at their proximal ends changing distally into coal-bearing terrestrial de- posits. A minor wedge, the Frontier Formation, occurs low in the Colorado Group in western Wyoming, but the main wedges, which are higher up, near the middle of the Montana Group, are known in the Southern Rocky Mountains and Colorado Plateau as the Mesaverde Formation and in Montana as the Judith River Formation. The Mesaverde wedge partitions the marine shales into the Mancos Shale below and Lewis Shale above. The Mesaverde wedges have irregular distal ends, and so a unit referred to as Mesaverde at one place may be higher or lower stratigraphically than the Mesaverde at another and the ages of the enclosing marine shales may also differ accordingly. These intertonguing relations pose problems for rep- resentation on the Geologic Map. It would be instructive to be able to differentiate on the map between the marine deposits and the clastic wedges with their ter- restrial deposits, but the various wedges produce so complex a pattern that it is not feasible on the scale of the map. On the Geologic Map the divisions shown—the Dakota, Colorado, Montana, and Laramie—are there- fore solely time-stratigraphic, regardless of facies at any particular place. The Montana Group covers an enormous area in the northern Great Plains, occupying nearly half of North and South Dakota, and its vast Pierre Shale has no regionally distinguishable subdivi- sions. In the plains of Montana, however, clastic wedges such as the Judith River Formation make subdivision possible. Here, in order to clarify the geology and bring— out the structure, the Montana Group is divided into two parts, uKsa and uK3b, using the base of the Judith River Formation as the line of separation. In northeastern Utah, along the south edge of the Uinta basin, the Castlegate Sandstone in the clastic wedge of the Mesaverde Group is traceable westward into the coarser Price River Formation. The Price River becomes a red bould/ery piedmont deposit on the edge of the Wasatch Mountains, where it lies uncon- PALEOZOIC AND MESOZOIC ROCKS formably on a rough erosion surface of strongly de- formed older Mesozoic and Paleozoic rocks (Spieker, 1946, p. 130—132). Moreover, beneath the unconform— ity is an older coarse piedmont deposit, the Indianola Formation, of Colorado age. There is no unconformity at the base of the Indianola, but it is obviously related to a newly deformed terrane not far to the west. The Mesaverde clastic wedge in this segment can be di- rectly related to orogenic activity in the Cordilleran belt to the west, and relations are probably similar for most of the Upper Cretaceous elastic wedges of the Rocky Mountain Region, although the actual connec- tions are not so clearly preserved as in Utah. The dif- ferent pulses of orogenic activity produced a succession of transgressions and regressions in the Upper Cre- taceous deposits, of which there are four principal ones; transgressions are produced when the marine shales spread westward, and regressions when the clastic wedges advanced eastward (McGookey and others, in Mallory, 1972, p. 206). The piedmont deposits in Utah of Colorado and Mon- tana age are manifestations of the Sevier orogeny (Armstrong, 1968, p. 444—449), in the Cordilleran miogeosyncline in Utah, southeastern Idaho, and southwestern Wyoming, that resulted in eastward transport of thick miogeosynclinal Paleozoic rocks over thinner cratonic sequences on a series of gently dip- ping thrust faults. These episodes of thrusting began during the Jurassic and culminated during Late Cre- taceous time, although there were minor episodes as late as the Eocene Epoch (Armstrong and Oriel, 1965, p. 1857—1861). Different in style and time from the Sevier orogeny is the type Laramide orogeny in the eastern ranges of the Central and Southern Rocky Mountains, generally assumed to have terminated the Cretaceous Period in those areas. The Laramide orogeny resulted mainly in upthrusting of the ranges and depression of the inter- vening basins. These movements began in Late Cre- taceous time, as indicated by thickness variations in the higher Cretaceous basin deposits in the Central and Southern Rocky Mountains, but the principal un- conformities resulting from the orogeny are between the Paleocene and Eocene deposits, rather than at the top of the Cretaceous sequence. In the Northern Rocky Mountains, however, from Montana northward into Alberta, thrusting like that produced during the Sevier orogeny continued later and reached its climax during Laramide time (latest Cretaceous and Paleocene). The true top boundary of the Cretaceous System in the Rocky Mountains was long controversial—the “Laramie question” which was debated for many years following the work of the Hayden Survey a century ago CRETACEOUS (Merrill, 1904, p. 647—658). It was observed that Triceratops, the last of the dinosaurs, was found in the Laramie beds, whereas Tertiary mammals were abun— dant in the succeeding Fort Union Formation; nevertheless, it was claimed that the fossil plants in ‘ both the Laramie and Fort Union were of Tertiary as- pect. During Laramie and Fort Union time, the Rocky ‘ Mountains were rising, as shown by erosional debris in these formations, and it was assumed that an immense ‘ unconformity lay concealed in these deposits in the plains, which would presumably mark the top of the Cretaceous. Much futile effort was expended in a search for this unconformity. These questions are now largely resolved, and the vertebrate and paleobotani- ‘ cal evidence reconciled. Concepts have been further clarified by recognition of the Paleocene as a separate series of the Tertiary Period and by classification of the Fort Union as Paleocene rather than Eocene. PACIFIC COASTAL AREA Upper Cretaceous rocks of the Pacific coastal area occur mainly in California, and in a few minor exten- sions in southwestern Oregon. In northern California they are the upper part of the “Great Valley sequence,” and have sometimes been called the Chico Series. This term is inappropriate, as the type Chico is a thin near— shore deposit of Campanian age that overlaps the basement rocks of the Sierra Nevada, whereas the main body on the west side of the valley is a more complete sequence of deeper water deposits more than 15,000 ft (4,500 In) thick, divisible into a number of formations (Bailey and others, 1964, p. 133—135). In one segment it consists of the Venado, Yolo, Sikes, Funks, Guinda, and Forbes Formations, which are of Cenomanian to Campanian age. All of them are inter- bedded sandstone, siltstone, and shale—a typical tur- bidite or flysch sequence—the different formations being distinguished mainly by varying proportions of the coarse and fine clastic components. In places the lower part contains lenses of slumped material, includ- ing large boulders of quartz diorite, evidently derived from Sierra Nevada basement, against which the de- posit probably overlaps in subsurface not far to the east. Upper Cretaceous rocks extend southward along the west side of the San Joaquin Valley and are well dis- \ played along the eastern flank of the Diablo Range, where they are 25,000 to 30,000 ft (7,600—9,000 m) thick, with the thick Panoche Formation below and the thin Moreno Shale above. The Panoche is again an alternation of sandy and shaly beds, a turbidite or flysch sequence, but it locally contains thick lenses of conglomerate that includes clasts of porphyry and granitic rocks. The Panoche lies in places on thin rem- 59 nants of Lower Cretaceous and Jurassic rocks, but its base is mainly faulted against the Franciscan rocks of the Diablo Range along a segment of the Coast Range thrust. The Upper Cretaceous sediments were derived mainly from the Sierra Nevada to the east, and there is no detritus that can be attributed to the crystalline basement of the Salinian block, across the San An- dreas fault immediately to the west. Within the Salin- ian block itself, late Upper Cretaceous rocks (Campa- nian and Maestrichtian) of the Asuncion Group, of a different facies, lie on the crystalline rocks. Other thick sequences of Upper Cretaceous rocks are preserved in places farther south in the Coast Ranges and in the western part of the Transverse Ranges, and Upper Cretaceous deposits lie unconformably on the Jurassic sequence (Bedford Canyon and Santiago Peak Formations) in the Santa Ana Mountains, at the northwestern end of the Peninsular Range of southern California. These last range in age from Cenomanian to Campanian and are divided into the Trabuco, Ladd, and Williams Formations. At the eastern edge of the Klamath Mountains near the California-Oregon border, deformed and metamor- phosed Mesozoic and Paleozoic eugeosynclinal rocks are overlain by the Upper Cretaceous Hornbrook For- mation, which dips gently eastward beneath the Eocene volcanic rocks of the Cascade Range. It is a body of sandstone, siltstone, and mudstone about 2,500 ft (760 m) thick, with marine fossils of Cenomanian and Campanian age at several levels (Peck and others, 1956). CONTINENTAL DEPOSITS (Kc) Continental deposits of Cretaceous age (Kc) are separately mapped in parts of the Cordilleran Region. The designation is used especially for the coarser poorly stratified deposits. Excluded are the finer grained stratified deposits more intimately associated with the normal marine sequence, such as the Lower Cretaceous Gannett Group and Wayan and Kootenai Formations, and the Upper Cretaceous coal-bearing terrestrial wedges of the Mesaverde and other forma- tions. It is used especially in central Utah for the coarse piedmont deposits next to the Sevier orogenic belt, such as the Indianola and Price River Formations. Farther west, in the Great Basin, are small isolated areas of Cretaceous continental deposits that probably formed in local basins. They are typified by the Newark Canyon Formation of the Eureka district, central Nevada (Nolan and others, 1956, p. 66—70), which is a heterogeneous deposit about 2,000 ft (600 m) thick of siltstone and conglomerate, with many layers of freshwater limestone, that lies on the truncated and eroded surface of all the Paleozoic formations of the 60 district. The limestone contains gastropods of Early Cretaceous age, as well as plant and fish fossils. About 150 mi (250 km) farther northwest, in the Jackson Mountains of northwestern Nevada, are other small areas of Cretaceous continental deposits (Willden, 1958), which lie on Triassic and Permian eugeosynclinal rocks ('FzPe). The King Lear Formation consists of conglomerate and siltstone with some beds of freshwater limestone that contain Lower Cretaceous gastropods like those in the Newark Canyon Forma- tion. It is overlain in places by another conglomeratic deposit, the unfossiliferous Pansy Lee Formation, which likewise predates the Tertiary volcanic rocks of the area. Both units are indicated as Cretaceous conti- nental deposits (Kc) on the Geologic Map. In southern Nevada, east of Las Vegas, is another set of Cretaceous continental deposits, which lie uncon- formably on deformed older rocks and are themselves much deformed (Longwell and others, 1965, p. 41—45). On the eastern flank of the Muddy Mountains are the Willow Tank Formation and Baseline Sandstone, about 4,000 ft (1,200 m) thick, which contain fossil plants of middle Cretaceous age. The Willow Tank and Baseline are succeeded by the mass of the much coarser Overton Fanglomerate, which contains large blocks and slabs of the Paleozoic formations. Farther west, southeast of Frenchman Mountain, is the Thumb Formation, much like the Willow Tank and Baseline Formations, but containing lenses of coarse breccia composed of Precambrian metamorphic rocks. All these units are synorogenic deposits, laid down while deformation was in progress in the region. In the ranges of the southwestern desert region of Arizona, many areas of Mesozoic sedimentary rocks are indicated on the State Map (1969). They are a de- formed sequence of shale, sandstone, limestone, and conglomerate, less metamorphosed than the rocks on which they lie and overlain unconformably by Tertiary volcanic rocks. Although unfossiliferous, they are classed as Cretaceous continental deposits (Kc) on the Geologic Map. Far to the north, in the Purcell Trench north of Pend Oreille Lake in northern Idaho, are small areas of the Sandpoint Conglomerate, formed of clasts of the Belt rocks, lying in a terrane of the Belt Supergroup and Cretaceous plutons (Harrison and others, 1972, p. 6). No data for its age are available; it was once assigned to the Paleozoic, but the preponderance of evidence now suggests that it is Cretaceous (Kc) and is so indi- cated on the Geologic Map. EUGEOSYNCLINAL DEPOSITS (Ke) Cretaceous eugeosynclinal deposits are dealt with at the close of a later section entitled “Upper Mesozoic.” PALEOZOIC AND MESOZOIC ROCKS VOLCANIC ROCKS (Kv) Small areas of volcanic rocks of Cretaceous age (Kv) are shown in various parts of the Cordilleran Region on the Geologic Map. Of these, the most significant are those surrounding the Boulder batholith in‘west-central Montana (Robin- son and others, 1968, p. 563—569). The Elkhorn Moun- tain Volcanics which adjoin the batholith and form part of its roof are remnants of a volcanic accumulation that was probably originally 10,000 ft (3,000 m) thick, the lower part mainly andesite and rhyodacite breccia and lava, the middle part rhyolite welded tuff, and the upper part erosional debris derived from the lower members. It lies unconformably on Upper Cretaceous sedimentary rocks of Santonian age and is overlain unconformably by middle Eocene volcanic rocks. Radiometric dating has yielded ages of about 73 to 78 my. and suggests that the eruptive episode lasted for about 4 my. in Campanian time. The period of erup- tion overlapped that of emplacement of the Boulder batholith itself, which has been dated between 71 and 82 my Northeast of the Elkhorn Mountain Volcanics and the Boulder batholith, in the outer thrust zone of the Northern Rocky Mountains, are the Adel Mountain Volcanics, which are somewhat younger, more alkalic, and petrographically different. Farther southeast in Montana, between the Crazy Mountains and Bearpaw Mountains, the volcanic rocks of the Grey Cliff field lie on various Upper Cretaceous units as young as the Hell Creek Formation (uK4) but are indicated on the source maps as Cretaceous. The Late Cretaceous and Paleocene rocks of the Crazy Mountains basin themselves contain large quantities of andesitic debris but, are included in the stratified sequence on the Geologic Map. Other areas of Cretaceous volcanic rocks (Kv) are shown on the Geologic Map in southwestern New Mexico and southeastern Arizona. Some of them are interbedded with fossiliferous sedimentary rocks as old as Lower Cretaceous, and others follow conformably on the highest Cretaceous sediments. The Geologic Map of Arizona (1969) likewise indicates as Cretaceous the older volcanic rocks of many of the ranges in the southwestern part of the State. Evidence for their age, however, is inconclusive, and the Cretaceous designa- tion is based mainly on structural evidence; they are cut by intrusives of supposed "Laramide” age and are unconformably overlain by undoubted Tertiary vol- canic rocks. Some of these rocks may indeed be Cre- taceous, but an early Tertiary age for most of them seems to accord better with the volcanic sequence in adjoining States; they are therefore marked as lower Tertiary volcanics (lTv) on the Geologic Map. UPPER MESOZOIC UPPER MESOZOIC UPPER MESOZOIC EUGEOSYNCLINAL DEPOSITS (uMe) The upper Mesozoic eugeosynclinal deposits are primarily represented by the Franciscan Formation, or assemblage, which dominates the coastal region of California, with extensions northward into southwest- ern Oregon and southward into Baja California (Bailey and others, 1964). The Franciscan forms much of the surface of the Coast Ranges north of San Francisco Bay and extends into Oregon as the Dothan Formation. Similar rocks extend westward to the coast, but part of them in this segment are treated separately as “Cre- taceous eugeosynclinal deposits” (Ke). South of San Francisco Bay the Franciscan forms the basement northeast of the San Andreas fault as far as the south end of the San Joaquin Valley. Southwest of the San Andreas fault in this latitude is the different basement terrane of the structural block of Salinia, composed of the metamorphic Sur Series (uE) and intrusive Cre- taceous granite (Kg), but the Franciscan reappears along the coast in the southern Coast Ranges, beyond the Nacimiento fault. South of the Transverse Ranges in southern California is a large area of Franciscan rocks, mainly offshore, whose presence is indicated by small outcrops on Santa Catalina Island and in the Palos Verdes Hills and by abundant blueschist and other Franciscan debris in the San Onofre Breccia of Miocene age along the coast. The Franciscan is a chaotic partly metamorphosed assemblage of graywacke and shale, With interbedded pillow basalt, radiolarian chert, and minor limestone, in which masses of serpentinite and other ultramafic rocks are embedded; its thickness is indeterminate but is on the order of 30,000 ft (9,000 In) or more. Fossils are rare, but enough have now been discovered to indi- cate that it includes rocks of Late Jurassic (Tithonian) to Late Cretaceous (Turonian and even Campanian) age, approximately coeval to the “Great Valley se- quence” to the east and younger than the mid-Jurassic Nevadan orogeny. No base of the Franciscan is known, but it probably lies on an oceanic crust. The as- semblage is a submarine trench and ocean-floor deposit that has been crowded and subducted against the con- tinent to the east and added to it during late Mesozoic and early Tertiary time. The next strata in deposi- tional contact above the Franciscan are Tertiary and commonly Miocene; older Tertiary and Cretaceous rocks which adjoin it are commonly faulted against it. Parts of the Franciscan are a melange of tectonically disordered blocks and slabs of all sizes, fermed of rocks of heterogeneous lithologies, origins, and ages (Hsu, 1968). Other parts are straight-forward sequences, or “broken formations” at most. In those parts of the 61 northern Coast Ranges where the structure has been studied in most detail, thick units of melange alternate with thick units of more straightforward sequences. In these areas, the Franciscan is found to be divided into a succession of east-dipping tectonic slices, the higher slices to the east containing the oldest rocks of Jurassic age and the lower slices to the west containing rocks of successively younger Cretaceous ages. Further compli- cations and disorder are produced by north-northwest- trending strike-slip faults of the San Andreas fault family, which further sheared and displaced the rocks during Tertiary time. Strike-slip faulting of large displacement is probably responsible for the introduction of the crystalline basement mass of Salinia between the two Franciscan areas in the southern Coast Ranges. It is a reasonable assumption that all the now-separated bodies of Fran- ciscan rocks originally formed a continuous body in a deep-water offshore regime and that Salinia is a sliver of continental rocks, detached from some area farther south and moved into a position foreign to it. The gran- ites of Salinia are shown by radiometric dating to be of Cretaceous age, as young as or younger than the Fran- ciscan rocks that adjoin them; yet, their only contacts are faults, and there is no indication of any contact metamorphism in the Franciscan or any debris in the Franciscan derived from Salinia. The potassium feldspar content of the graywackes in the Franciscan and the Great Valley sequence is of interest (Bailey and others, 1964, p. 139—147). The graywackes in the Great Valley sequence show a pro— gressive increase in potassium feldspar from Jurassic to Upper Cretaceous time, suggesting that during this interval the granitic plutons of the Sierra Nevada and Klamath Mountains were becoming more and more unroofed and subject to erosion. Remarkably, graywackes in the coeval Franciscan immediately to the west contain little or no potassium feldspar, except in the coastal belt (Cretaceous eugeosynclinal deposits, Ke). Possibly the source of the Franciscan graywackes was different from that of the Great Valley sequence. The Franciscan rocks have been rather pervasively metamorphosed to the blueschist (high-pressure low- temperature) facies (Bailey and others, 1964, p. 89— 112). For the most part, this is barely perceptible; a zeolite facies with laumontite is near the coast, and farther inland the graywackes have been jadeitized, Without altering their primary sedimentary struc- tures. Higher grade blueschist with pumpellyite, glaucophane, and lawsonite occurs in a band along the eastern side of the Franciscan area, close to the Coast Range thrust, and its extreme phase has been named the South Fork Mountain Schist (Blake and others, 1967); it is marked by a metamorphic overprint on the 62 Geologic Map. A large outlying area, the Colebrooke Schist, occurs in southwestern Oregon (Coleman, 1972, p. 27—58). The schists decrease in grade and intensity downward from the sole of the fault, and so rocks on the ridgetops are more metamorphosed than those in the intervening valleys. Radiometric dating by the K/Ar method indicates that the metamorphism of the Franciscan rocks has a range in age from 70 to 150 m.y., or about the same time span as the age of the assemblage itself as indicated by fossils, showing that metamorphism went on hand in hand with the sedimentation (Suppe and Armstrong, 1972). The old- est dates are in the South Fork Mountain Schist on the east, and younger dates are farther west. Also shown on the Geologic Map as upper Mesozoic eugeosynclinal deposits (with a metamorphic over- print) are an assortment of metamorphic rocks of greenschist facies east of the Franciscan area in south- ern California. They include the Pelona Schist, close to the San Andreas fault in the San Gabriel and San Ber- nardino Mountains, the Orocopia Schist in the desert ranges east of the Salton Trough farther southeast, and the Rand Schist in the Mojave Desert to the north- east (Ehlig, 1968). Although these have been called Precambrian, they are everywhere in fault contact with the true Precambrian as well as with the late Paleozoic Mount Lowe Granodiorite, and they lack the pervasive plutonism of the Precambrian rocks. Various lines of indirect evidence suggest that the schists are 70 to 100 my old. The original rocks were interbedded graywackes, siltstones, and shales; they may be more or less equivalent to the Franciscan rocks to the west, but in a greenschist rather than a blueschist facies. In the northern part of Washington State, on the western flank of the Cascade Range, is the Nooksack Group, which contains fossils of Late Jurassic and Early Cretaceous age (Kimmeridgian to Valanginian) (Misch, 1966, p. 118) and is therefore shown as upper Mesozoic eugeosynclinal deposits (u Mze) on the Geologic Map. The Nooksack is a flyschlike sequence of graywackes and siltstones containing volcanic detritus and a few lenses of conglomerate, the whole a deep- water deposit laid down under conditions of tectonic unrest. As described, the Nooksack somewhat re- sembles the Franciscan but does not have its chaotic structure. CRETACEOUS EUGEOSYNCLINAL DEPOSITS (Ke) In the northern Coast Ranges, a western belt of Franciscan-type rocks differs significantly from the main body of the Franciscan farther east; the belt ex— tends for 150 mi (250 km) along the coast south of Cape Mendocino and for as much as 30 mi (50 km) inland. Its rocks have less structural disorder than those farther PALEOZOIC AND MESOZOIC ROCKS east and are mainly graywackes; interbedded lava and chert are rare. The graywackes contain appreciable quantities of potassium feldspar, in contrast to the Franciscan farther east (Bailey and others, 1964, p. 140); graywackes containing potassium feldspar in the San Francisco Peninsula might be correlative. The rocks of the coastal belt are the youngest part of the Franciscan assemblage; at least part of the belt is of Late Cretaceous age, as indicated by occasional fossils, and it is accordingly mapped as Ke. However, dino- flagellates and angiosperm pollen from many localities in the belt are definitely Eocene (Evitt and Pierce, 1975), thus greatly extending the time during which rocks of the Franciscan assemblage accumulated. MESOZOIC PLUTONIC AND INTRUSIVE ROCKS Plutonic and intrusive rocks of Mesozoic age are a significant component of the western part of the Cordil- leran Region, where they form about 10 percent of the surface (fig. 16). They are less abundant in the eastern part of the Cordillera (where the intrusives are mainly Tertiary) and are minor in the eastern United States, except for the granitic White Mountain Series in northern New England. All the Mesozoic periods are represented, although plutonic rocks of Cretaceous age are by far the most abundant. The ages of the plutonic and intrusive rocks are indicated by their relations to enclosing rocks and more often by radiometric dating. Radiometric dates are now available for most of the important plutonic bodies in the United States, and such ages can be extrapolated further to undated adja- cent bodies of similar character. TRIASSIC GRANITIC ROCKS (F9) Granitic rocks of Triassic age occur in a few places in the western part of the Cordilleran Region (fig. 17). One group of quartz monzonite and granodiorite plu- tons runs along the eastern edge of the Sierra Nevada in eastern California, from Bishop northwestward past Mono Lake, and into some of the ranges of western Nevada. The intrusive episode represented, designated as the Lee Vining epoch (Evernden and Kistler, 1970, p. 19), has been dated by K/Ar methods at between 195 and 210 m.y., or Middle to Late Triassic. These rocks are intruded by the more prevalent Jurassic and Cre- taceous plutonic rocks of the same area. In the eastern part of the Blue Mountains of north- eastern Oregon, near Sparta, a group of much-sheared albitized granites are distinctly older than the quartz diorite and granodiorite plutons of Cretaceous age (Gilluly, 1933, p. 66—67). Relations to adjacent supra- crustal rocks and intrusives suggest that they are of Triassic age, which has been confirmed by a few MESOZOIC PLUTONIC AND INTRUSIVE ROCKS radiometric dates in the range of210 to 250 m.y. (G. W. Walker, oral commun., 1971). Several bodies of Triassic granitic rocks occur in the crystalline complex of northern Washington. In the core of the Cascade Range is the Marblemount Quartz Diorite, which predates the Cretaceous metamorphism of the range and is overlain unconformably by the Cas- cade River Schist (ms) (Misch, 1966, p. 105). Dating of zircons by U/Pb methods yields ages of215 to 220 m.y., or Early Triassic (Mattinson, 1970), and later meta- morphism has not reset the zircon ages. In the Okano- gan Range to the east are the granitic rocks of the Toats Coulee Magma Series (Hibbard, 1971, p. 3029— 3031), which is composed of granodiorite, tonalite, and tonalite porphyry and which is premetamorphic or synmetamorphic to a Late Triassic orogeny and older than the adjacent plutons of the Cretaceous Horseshoe Basin Magma Series. The rocks have yielded a K/Ar age of 194 m.y., or Late Triassic. TRIASSIC MAFIC INTRUSIVES (hi) Mention has already been made of the diabase intru- sives in the Newark Group of the Appalachian Region from New England to Virginia. The thick Palisades sill of eastern New Jersey has been dated radiometrically at 190 to 200 m.y. (Erikson and Kulp, 1961). jURASSIC GRANITIC ROCKS (lg) Jurassic granitic rocks are more extensive than those of the Triassic in the western part of the Cordil— leran Region (fig. 18). They also occur in northern New England (fig. 16). They are, however, by no means as extensive as implied on the Geologic Map of the United States of 1932, where all the Mesozoic plutonic rocks of the Cordilleran Region were assigned to the Jurassic; a large part of these is now known to be of Cretaceous age. Jurassic granitic rocks are absent, for example, in all the plutons of the northern part of the Cordillera in the United States, in Montana, Idaho, Washington, and northeastern Oregon. In California, Jurassic granitic rocks occur in the Sierra Nevada and the Klamath Mountains. In the Sierra Nevada, the Jurassic granitic rocks occur east and west of the axis of the range, which is formed of Cretaceous rocks of the main Sierra Nevada batholith (Bateman and Wahrhaftig, 1966, p. 115—122). Field re- lations indicate more than one Jurassic intrusive epoch; some plutons in the western foothills are trun- cated by the Melones fault zone, whereas others are not. This is confirmed by K/Ar dating, which indicates an early episode of intrusion with ages of 160 to 180 m.y., called the Inyo Mountains intrusive epoch and represented chiefly east of the Sierra Nevada, and a 63 later episode with ages of 132 to 148 m.y., called the Yosemite intrusive epoch and represented on the west- ern slope of the Sierra Nevada (Evernden and Kistler, 1970, p. 17—19). (These are not separated on the Geo- logic Map.) The Jurassic granitic rocks of the western flank are dominantly quartz diorites and granodior- ites, less silicic than the Cretaceous granitic rocks of the main batholith. In the foothills the Jurassic grani- tic rocks form large equidimensional plutons embed- ded in the Jurassic eugeosynclinal rocks, but they merge to the east into a more continuous body which forms the western part of the main batholith, as at the lower end of Yosemite Valley (El Capitan Granite, and so forth). Except for a few minor Cretaceous plutons, all the granitic rocks of the Inyo and White Mountains are Jurassic of the first epoch. The granitic rocks of the Klamath Mountains of northern California and southwestern Oregon are all Jurassic and form large diorite and granodiorite plu- tons elongated parallel with the trends of the country rocks; they are most abundant in the Central Metamorphic belt and the Western Paleozoic and Triassic belt (Hotz, 1971, p. 15—17). Three epochs of intrusion are recognized by radiometric dating: from 165 to 167 m.y., from 145 to 155 m.y., and from 127 to 140 m.y., the first and last broadly equivalent to the two intrusive epochs in the Sierra Nevada. (Again, these are not differentiated on the Geologic Map.) The Shasta Bally pluton at the south end of the mountains, of the last intrusive epoch, is overlain unconformably by the earliest Lower Cretaceous rocks of the Great Valley sequence. Eastward in the Great Basin of Nevada, Jurassic granitic rocks (as dated radiometrically) seemingly have a random distribution with respect to the Cre- taceous granitic rocks. All the plutons, as exposed at the surface, are smaller than those in the Sierra Nevada, but the true dimensions of some are obscured by Tertiary cover. The easternmost Jurassic pluton is the Panther Spring Granite, intrusive into the Cam- brian strata of the House Range in western Utah and dated at 143 m.y. In northern New England, far from the areas just discussed, is the White Mountain Plutonic Series, named for its prominent development in central New Hampshire but with outlying plutons in Vermont on the west and Maine on the east (Billings, 1956, p. 129—135, 145—146). It is a set of fresh crosscutting in- trusions younger than the orogenies in the Paleozoic rocks and principally forms ring dikes and cauldron subsidences, but with one large batholith (actually an aggregate of coalesced ring dikes). It consists of alkalic rocks, with some mafic end members, but mainly of quartz syenite and alkali granite, of which the most 64 PALEOZOIC AND MESOZOIC ROCKS FIGURE 16.—The United States, showing in separate patterns areas mapped as Mesozoic and Cenozoic plutonic and intrusive rocks on Geologic Map of the United States. Mesozoic rocks include units of Triassic plutonic and intrusive rocks ( ‘fig, ‘5 i). Juras- sic plutonic rocks (Jg, Jmi), and Cretaceous plutonic and intrusive rocks (Kg, Kgl, Kg2, Kg3, Kgn, Ki). Cenozoic rocks include units of Tertiary intrusive rocks (Ti). NW] ‘ =_ ———————— —z ! L ‘\\ j i .1 J G U L F MESOZOIC PLUTONIC AND INTRUSIVE ROCKS "\ \ Q EXPLANATION \ (K, Cenozom plutomc rocks i i ‘ . ! \ , v : r V - ’ x ----- —. \. c:-——-""\--—»-—--—~s ; v - \. _.. K \ > Mesozoic plutonic rocks ’53-“ .I' \\ \ a :7 \»§ K . . 7') Q g i \ .\ 0 F M E X 1 C 0 “v FIGURE 16.—Continued. 65 66 PALEOZOIC AND MESOZOIC ROCKS FIGURE 17.——Western United States, showing areas of Triassic granitic rocks (13g) as mapped on Geologic Map of the United States. 67 MESOZOIC PLUTONIC AND INTRUSIVE ROCKS ./ \,~\-\ \.. ,/ 1 “““““““““““““““““ 1/ ‘5‘ '1 "—"m‘—”—"_" fififififi /' \. I E : 'L. I \ “(\‘ (I I, i / {A 'l k .1 .L--—————._.._..—.———_-; 1". Ar ---- \ ..... i | m. __________ , 1 .-’ ’ i .’ ’ i I . s I ....... I“ ,, -\~.\_ I I “““““““ V“ ‘I .' \i " ~ \3‘ V, L I \‘ '0 I “\"7, _________________ J: ‘\ / : ‘‘‘‘‘ i all, n .‘l :l ----- / / ! _____________ _ I ’ i i ' ,- —\..\..\__\__\,_\_ ! ,‘xl / ---------------- J --------------------- i 1‘ """"" ‘ i ' ' i ' ’ i I L“ : I \ R‘” t ,1, i 1%" \~‘\ 0 I: I \‘~J__\-‘__\ r """ ‘k --------- J \. \\‘ \X \| .\ ”\N \\ ./ \\ FIGURE 18.—Western United States, showing areas mapped as Jurassic granitic rocks (Jg) and mafic intrusives (Jmi) on Geo- logic Map of the United States. 68 prominent is the Conway Granite. The age of the White Mountain Series is commonly quoted as 180 m.y., or Early Jurassic, on the basis of concordant U/Pb, K/Ar, and Rb/Sr determinations on the Conway Granite (Lyons and Faul, 1968, p. 312). A wider range of sampling of the different White Mountain plutons reveals a much greater spread of ages—from 110 to 185 m.y., or from Early Jurassic into Early Cretaceous time (Poland and others, 1970). The White Mountain epoch thus overlaps that of the Monteregian intrusives in Canada to the north, which extend in a chain for 150 mi (250 km) northwestward from near the border to Montreal and which have Cretaceous ages of 84 to 123 my. The White Mountain and Monteregian intrusives are evidently closely related, both sequentially and magmatically. JURASSIC MAFIC INTRUSIVES Gmi) In the Stillwater and West Humboldt Ranges of west-central Nevada are some areas of diorite and gabbro, which have been emplaced as tabular masses at shallow depths and are associated with basaltic lavas (Page, 1965). They have been dated by K/Ar methods at 150 m.y., or Late Jurassic. CRETACEOUS GRANITIC ROCKS (Kg) The dominant granitic rocks of the Cordilleran Re- gion are of Cretaceous age. They occur throughout the length of California and into Oregon, with outlying bodies in Nevada and Arizona, and in Montana, Idaho, and Washington (fig. 19). Some of them form small to moderate—sized plutons, but in places the plutons are aggregated into large batholiths, such as the Peninsu- lar Range batholith of southern California, the Sierra Nevada batholith farther north, and the Idaho batholith in the mountain area of central Idaho. In places, the rocks are divided on the Geologic Map ac- cording to age into Lower Cretaceous granitic rocks (Kgi), Upper Cretaceous granitic rocks (ng), and latest Cretaceous granitic rocks (Kgs); the gneissic border rocks of the Idaho batholith are also differ- entiated (Kgn). Elsewhere, the Cretaceous granitic rocks are not divided (Kg). The best known Cretaceous granitic rocks are those of the Sierra Nevada, which form a continuous body 25 mi (40 km) or more wide along the crest of the range for its entire length (Bateman and Wahrhaftig, 1966, p. 116—125). Two general times of emplacement are represented—the Huntington Lake intrusive epoch with ages of 104 to 121 m.y., or Lower Cretaceous (Kgi), and the Cathedral Range intrusive epoch with ages of 79 to 90 m.y., or Upper Cretaceous (Kg2), which PALEOZOIC AND MESOZOIC ROCKS forms the main body (Evernden and Kistler, 1970, p. 17). The first consists of quartz diorite, quartz monzo- nite, and granodiorite. The second, represented by the Tuolomne Intrusive Series, includes the Sentinel Granodiorite, Half Dome Quartz Monzonite, Cathedral Peak Granite, and Johnson Granite Porphyry, and is more siliceous than the older Cretaceous and Jurassic intrusives of the Sierra Nevada. The Late Cretaceous age of the youngest granitic rocks of the Sierra Nevada raises interesting questions as to their relation to sedimentation of the Great Valley sequence, which was in progress during this time to the west. During emplacement of the batholith, the site of the Sierra Nevada was probably being raised and eroded, and the batholith surface unroofed, to provide the vast accumu- lation of Cretaceous sediments in the Great Valley. Farther south is the equally large mass of the Penin- sular Range batholith (“batholith of southern Califor- nia”), which extends past San Diego into Mexico, where it forms the backbone of Baja California as far south as the 29th parallel (Larsen, 1954). Unlike the Sierra Nevada batholith, it was intruded during a single epoch. In the United States it cuts Upper J uras- sic rocks and in Baja California cuts Lower Cretaceous rocks as young as Albian; its deeply eroded surface is overlain by undeformed Upper Cretaceous rocks of Campanian and Maestrichtian age. In southern California it has been dated by U/Pb methods as be— tween 109 and 120 m.y., and in Baja California as be— tween 100 and 115 my (Armstrong and Suppe, 1973, p. 1385). In both areas, K/Ar dates decrease in the easternmost exposures to as little as 80 m.y., but these probably reflect cooling events related to greater depth of erosion of this part of the batholith. Like the Sierra Nevada batholith, the Peninsular Range batholith is composed of many plutons, which vary in composition from gabbro, through granodiorite and tonalite, to granite. In the Salinian block of the Coast Ranges west of the Sierra Nevada, various granitic plutons invade the Sur Series (uPz) and form parts of the Santa Lucia, Gabilan, and Santa Cruz Ranges, as well as the Farallon Is— lands, Point Reyes, and Bodega Head farther north. They include quartz diorite, adamellite, granodiorite, and granite. Dating by K/Ar methods yields ages as young as 77 m.y., but the time of intrusion is clearly older, as the eroded surfaces of the plutons are overlain by the Asuncion Group of Late Cretaceous (Campa- nian) age (Compton, 1966, p. 288—287). Probably this is a “cooling date,” representing the time when argon could be retained in the rock, after uplift from the deep crustal level indicated by the high amphibolite and granulite metamorphic facies of the enclosing Sur MESOZOIC PLUTONIC AND INTRUSIVE ROCKS FIGURE 19.—Western United States, showing areas mapped as Cretaceous granitic rocks (Kg, Kgi, ng, Kg3, Kgn) and Creta- ceous intrusive rocks (Ki) on Geologic Map of the United States. 69 70 Series. An Rb/Sr whole-rock date from the Santa Lucia Range of 117 m.y., or Early Cretaceous, is probably closer to the actual time of intrusion. Similar young Cretaceous ages have been obtained from the granitic rocks of the San Gabriel and San Bernardino Mountains and probably are also "cooling dates” (Evernden and Kistler, 1970, p. 22; Armstrong and Suppe, 1973, p. 1383). Dating by U/Pb methods suggests plutonic events between 160 and 170 m.y. and 75 to 90 m.y. (Silver, 1971). East of the Sierra Nevada in western and northern Nevada, granitic rocks form many small plutons, which are shown by radiometric dating to be partly Jurassic, partly Cretaceous, and Tertiary. The Cre- taceous granites have ages of 87 to 105 m.y. and 68 to 71 m.y., approximately the same as those of the two intrusive epochs in the Sierra Nevada, (Silberman and McKee, 1971). In the Mojave Desert region of southern California and the desert ranges of southwestern Arizona are many small to moderate-sized bodies of Mesozoic granitic rocks, which mostly yield K/Ar and U/Pb ages of 64 to 95 m.y., or Late Cretaceous, but also yield from 150 to 165 m.y. and from 190 to 200 m.y., or Jurassic and Triassic (Armstrong and Suppe, 1973, p. 1383— 1384). However, the extent of the rocks of different ages is imcompletely known, and except for one pluton in the Clark Mountains of the eastern Mojave Desert, all are classed as Cretaceous (Kg) on the Geologic Map. The Idaho batholith sprawls across the mountains of central Idaho, from the Snake River Plain to north- western Montana, with an area of about 16,000 mi2 (42,000 km2) (Ross, 1936). It plunges southward be- neath the Cenozoic volcanic rocks of the Snake River Plain, but it may be nearly connected in this direction with the Sierra Nevada batholith, as numerous in- liers of granitic rocks emerge from beneath the Ter- tiary cover in southwestern Idaho and northwestern Nevada. The Idaho batholith intrudes rocks of the Belt Supergroup (Y) on the northeast, lower and upper Paleozoic miogeosynclinal rocks (19, u?) on the southeast, and lower Mesozoic eugeosynclinal rocks (lee) on the southwest. It is bordered, especially on the north, by a wide zone of regional metamorphism, where parts of the Belt formations reach sillimanite grade. Most of this regional metamorphism preceded the actual emplacement of the batholith, suggesting that its site had been subjected to a considerable period of prior crustal heating (Hietanen, 1962, p. 97—99). Large inclusions in the batholith have been metasomatized and converted into gneisses that fade out into the surrounding intrusive. The oldest supra- PALEOZOIC AND MESOZOIC ROCKS crustal rocks which overlie it are the Casto Volcanics (lTV) of Eocene age, and it is intruded by many small plutons of early Tertiary age (Ti). The batholith underlies a rough wilderness area, and while parts of it have been mapped in fair detail, large parts are still poorly known or even unexplored. The surface outline of the batholith is highly irregular, with projections of granitic rocks into the surrounding country rock and many small to large inclusions or pendants of country rock within the batholith. It is nearly bifurcated near the middle by a belt of inclu- sions, shown on the map as metamorphosed Belt supergroup (Y), older Precambrain (Km), and border phase of the batholith (Kgn). In this area, an eastward projection of granitic gneisses, mapped as part of the batholith, has proved from radiometric determinations to be 1,500 m.y. old, or early Precambrian Y (Armstrong, 1975, p. 440—441). The parts north and south of this belt of inclusions may be respectively termed the Bitterroot lobe and Atlanta lobe of the batholith. The batholithic rocks are mainly granodio- rite and quartz monzonite. So far as is known, they do not form many individual plutons like those that characterize the Sierra Nevada and Peninsular Range batholiths; instead, large areas are of nearly uniform composition, and compositional changes from one part to another appear to be gradational. Along parts of the periphery, however, is a more mafic gneissic border phase (Kgn). Emplacement of the batholith may have extended over a considerable period, and some rocks included with it may be much older, such as the 1,500-m.y.- old granitic gneisses mentioned above. The main period of emplacement appears, however, to have been during the Cretaceous. Radiometric determinations have yielded equivocal results, probably due in part to con- tamination with Precambrian materials and to updat— ing during Tertiary plutonism. Present evidence indi- cates that the Atlanta, or southern lobe, has an age of about 70 to 100 m.y., or comparable to the last plutonic event in the Sierra Nevada. The Bitterroot lobe is ap- parently somewhat younger, with an age of about 80 m.y., or close to that of the Boulder batholith to the east (Armstrong, 1975, p. 445). Many granitic plutons occur northwest of the Idaho batholith along the Canadian border from the Purcell Trench of northern Idaho to the northern Cascade Range of Washington State. They are the southward extensions of plutons in the western Cordillera of British Columbia. Near the Columbia River they plunge southward beneath the cover of the Miocene Columbia River lavas; they may not continue much farther, as much of the lava was probably erupted onto an oceanic crust. For the most part, the granitic rocks MESOZOIC PLUTONIC AND INTRUSIVE ROCKS are of Cretaceous age and have yielded radiometric ages close to 100 m.y., or middle Cretaceous time, al- though several plutons of Triassic age have already been noted and others of early Tertiary age (Ti) occur, especially toward the west. The eastern plutons invade the Belt Supergroup; others farther west lie in Paleozoic miogeosynclinal rocks of the Kootenai arc, and those beyond in Paleozoic and Mesozoic eugeosyn- clinal rocks (Yates and others, 1966, p. 55). They in- trude eugeosynclinal rocks as young as Middle Juras- sic and are overlain unconformably by plant—bearing Upper Cretaceous rocks, but abundant granitic debris first appears in Eocene conglomerates. East of the Idaho batholith in western Montana is an array of younger Cretaceous granitic plutons, of which the largest and best known is the Boulder batholith. They are classed on the Geologic Map as "latest Cre- taceous granitic rocks” (Kgs) and are commonly re- ferred to as “Laramide” intrusives. Emplacement of these granitic rocks overlaps in time that of the Upper Cretaceous granitic rocks (Kg2) of the Sierra Nevada; however, this emplacement continued later, and the associations of the two sets of rocks are quite different. The Boulder batholith extends north-northeast transverse to the regional trends of the enclosing rocks, with an area of about 2,300 mi2 (5,700 km2) and a length of about 60 mi (100 km) (Robinson and others, 1968). It invades the Belt Supergroup and the Paleozoic and Mesozoic rocks as young as the Elkhorn Mountain Volcanics (Kv) of Campanian age; it is over— lain unconformably by the middle Eocene Lowland Creek Volcanics (lTv). The batholith is a composite mass of a dozen or more plutons of calc-alkalic rocks which range in composition from syenogabbro to alas- kite but are dominantly quartz monzonite and granodiorite. Nearly three-fourths of the batholith was emplaced between 71 and 82 m.y., and thus it partly overlaps the eruption of the Elkhorn Mountain Vol- canics (73 to 78 m.y.), as well as the thrust faulting of the country rocks which occurred during Campanian and Maestrichtian time (approximately between 66 and 80 my). The mode of emplacement and the structure of the batholith remain controversial. Was it a steep-walled intrusive, descending into the depths from its roof of Elkhorn Mountain Volcanics (Klepper and others, 1971, p. 1580)? Or was it a shallow floored body that was open to the sky, on whose surface the Elkhorn Mountain Volcanics congealed as a sort of slag (Hamil- ton and Myers, 1967, p. C6-C9)? Probably the truth lies somewhere between these two extremes. Another group of “Laramide” plutons (Kga) of smal- ler individual dimensions is in the Mineral Belt of Col- orado, mostly in the Front Range (Tweto, 1968, p. 71 564—565). They have yielded radiometric ages of about 60 to 7 O m.y., and clasts derived from them have been identified in the Paleocene deposits of the Denver basin to the east. These plutons are part of a chain of intru- sive rocks that extends southwestward along the Min- eral Belt past the San Juan Mountains, but most of those west of the Front Range are younger, of early Tertiary age (Ti). Other "Laramide” granitic rocks (Kg3) occur in southern Arizona, partly intermingled with but mostly to the east of the earlier Mesozoic granitic rocks (J g, Kg) (Armstrong and Suppe, 1973, p. 1385). CRETACEOUS INTRUSIVE ROCKS (Ki) Small bodies of intrusive rock of Cretaceous age occur at the inner edge of the Gulf Coastal Plain in southwestern Arkansas and central Texas, associated with tuffaceous rocks interbedded in the Upper Cre- taceous marine sequence. Southeast of Little Rock in Arkansas, two sizeable knobs of nepheline syenite project through the early Tertiary deposits of the Midway and Wilcox Groups, which contain their weathered products, including commercial deposits of bauxite (Gordon and others, 1958, p. 60—71). A little farther west, in the folded Paleozoic rocks of the Ouachita Mountains, are numerous plugs of similar rock, of which the largest and best known is that at Magnet Cove, and many satellitic dikes. About 80 mi (130 km) to the southwest, at the edge of the Coastal Plain near Murphreesboro, four small pipes of diamond-bearing peridotite cut the Lower Cretaceous Trinity Group (Miser and Purdue, 1929, p. 99—117), containing the only abundant diamonds in the United States. Genetically related to the intrusive rocks are beds of volcanic tuff and tuf- faceous sandstone in the lower part of the Upper Cre- taceous in the same area—the Woodbine Sand (uKi) and the Tokio Formation of Austin age (uK2)—s0me of which are as much as 125 ft (40 m) thick (Ross and others, 1929). In central Texas, near the Balcones fault zone at the edge of the Coastal Plain, another group of Cretaceous intrusive rocks extends from Austin 150 mi (250 km) westward past Uvalde (Lonsdale, 1927, p. 9—46). They are small plugs and laccoliths of nepheline basalt and phonolite, which intrude Cretaceous rocks as young as the Austin and Taylor Formations, or somewhat higher than the intrusive and volcanic rocks in Arkan- sas. As in Arkansas, the associated marine deposits of the Austin and Taylor contain much volcanic debris, as well as several layers of bentonite. The Pilot Knob in- trusive near Austin has sometimes been referred to as a fossil volcano, for which there is some support in the 72 abundant volcanic debris in the surrounding Austin and Taylor Formations. Nevertheless, the intrusive rocks everywhere out these formations, indicating a younger age. 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