Wallrocks of the Central Sierra Nevada Batholith, California: A Collage 0f Accreted Tectono—Stratigraphic Terranes By WARREN j. NOKLEBERG GEOLOGICAL SURVEY PROFESSIONAL PAPER 1255 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Nokleberg, Warren J. Wallroeks of the central Sierra Nevada batholith, California. (Geological Survey Professional Paper 1255) Bibliography: 28 p. Supti of Docs. No.: l 19.162125 1. Batholiths—Sierra Nevada Mountains (Calif. and Nevi) It Title. ll. Series. Q5461 .N64 552’.3 81—607153 AACR2 For sale by the Superintendent of Documents, US Government Printing Office Washington, DC. 20402 CONTENTS Page Page Abstract .................................... 1 Cretaceous metavolcanic rocks ..................... 14 Introduction .................................. 1 Occurrence ............................... 14 Definitions ................................ 2 Stratigraphy and structure .................... 14 Acknowledgments ........................... 2 Kings terrane ................................ 14 Relation of multiple regional deformation to terrane Occurrence ............................... 14 accretion .................................. 2 Stratigraphy .............................. 15 Relation of terranes to intrusive epochs of granitic Structure ................................ 15 rocks ............................. 3 Evidence for Kings River fault ................. 16 Owens terrane ................................ 3 Critical differences from Goddard terrane ......... 16 Occurrence ............................... 3 Accretion of Kings terrane .................... 16 Stratigraphy .............................. 3 Merced River terrane .......................... 17 Structure ................................ 5 Occurrence ............................... 17 Evidence for Owens Valley fault bounding eastern Stratigraphy .............................. 17 margin ................................. 5 Structure ................................ 18 Critical differences between Owens terrane and meta— Evidence for Foothill suture ................... 18 sedimentary rocks of White-Inyo Mountains ...... 7 Critical differences from Kings terrane ........... 18 Accretion of Owens terrane ................... 8 Amalgamation and accretion of Merced River terrane . 19 High Sierra terrane ............................ 8 Foothills terrane .............................. 19 Occurrence ............................... 8 Occurrence ............................... 19 Stratigraphy .............................. 8 Stratigraphy .............................. 20 Structure ................................ 10 Structure ................................ 20 Evidence for Laurel-Convict fault ............... 11 Evidence for Melones fault .................... 21 Critical differences from Owens terrane ........... 11 Critical differences from Merced River terrane ..... 21 Amalgamation and accretion of High Sierra terrane . . 11 Accretion of Foothills terrane .................. 21 Goddard terrane .............................. 12 Mesozoic and Cenozoic accretionary tectonics along the Occurrence ............................... 12 western margin of North America ................ 22 Stratigraphy .............................. 12 Cenozoic accretionary tectonics of southern Alaska . . . 22 Structure ................................ 12 Comparison of the Mesozoic Sierra Nevada and southern Evidence for San Joaquin River fault ............ 13 Alaska ................................. 23 Critical differences from High Sierra terrane ....... 13 Conclusions .................................. 23 Amalgamation and accretion of Goddard terrane ..... 13 References cited .............................. 25 ILLUSTRATIONS Page PLATE 1. Geologic map showing Paleozoic and Mesozoic tectono-stratigraphic terranes, central and southern Sierra Nevada, California ........................................................... In pocket FIGURE 1. Generalized stratigraphic columns for Paleozoic and Mesozoic tectono-stratigraphic terranes, central Sierra Nevada, California ............................................................. 9 2. Simplified geologic map of southern Alaska showing major features of tectono-stratigraphic terranes ......... 24 TABLES Page TABLE 1. Principal occurrences, original lithologies, fossils and ages, and investigators of wallrocks and roof pendants constituting tectono-stratigraphic terranes in the central Sierra Nevada ................................ 4 2. Regional deformations recorded in tectono—stratigraphic terranes in the central Sierra Nevada ............. 6 III WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA: A COLLAGE OF ACCRETED TECTONO-STRATIGRAPHIC TERRANES By WARREN J. NOKLEBERG ABSTRACT Structural and stratigraphic analysis of the wallrocks of the central Sierra Nevada batholith, California, supports the hypothesis that the wallrocks constitute a collage of tectono-stratigraphic terranes that were accreted along an actively deformed continental margin. The terranes, represented by well-known pre-middle Cretaceous belts of wallrocks, with north-northwest trends, were successively accreted during Triassic and Jurassic time. Successive pulses of magmatism were interspersed with successive accretions of terranes and with successive deformations and movements along interarc faults. From east to west, the terranes consist of: (1) early Paleozoic metasedimen— tary rocks (Owens terrane); (2) late Paleozoic metasedimentary and unconformably overlying Permian and Triassic metavolcanic rocks (High Sierra terrane); (3) Jurassic metavolcanic rocks (Goddard ter- rane); (4) Late Triassic and Early Jurassic miogeosynclinal metasedimentary rocks and sparse metavolcanic rocks of the Kings sequence (Kings terrane); (5) late Paleozoic slate, marble, and metaquartzite of the Calaveras Formation and rocks similar to the Shoo Fly Formation—and underlying late Paleozoic ophiolite (Merced River terrane); and (6) Jurassic andesite and basalt flows, breccias, epiclastic mudstone, siltstone, volcanic graywacke, and conglomerate representing a volcanic arc suite and underlying Jurassic and older ophiolite west of the Melones fault (Foothills terrane). Each terrane generally has: (1) a distinct stratigraphic sequence with a narrow age range; (2) no nearby sediment source; (3) bounding thrust, re— verse, or strike-slip faults; and (4) a distinct structural history. After accretion, adjacent terranes were welded together and their bound- aries locally obliterated by episodic intrusion of granitic magmas. Major thrust, reverse, or strike-slip faults representing accretio- nary sutures separate the various terranes. The Owens Valley fault, a vertical, predominantly right lateral strike-slip fault, separates the Owens terrane from bedrock units to the east in the White—Inyo Mountains. The Laurel—Convict fault, an originally west dipping thrust, separates the Owens and High Sierra terranes. Triassic accre- tion and defamation along this suture defines the Sonoman orogeny in the eastern Sierra Nevada. The San Joaquin River fault, a major strike-slip fault, separates the High Sierra and Goddard terranes. The Kings River fault, a major right-lateral strike-slip fault, sepa- rates the Goddard and Kings terranes. Late Jurassic accretion and deformation along the Kings River and San Joaquin River faults be- tween the High Sierra, Goddard, and Kings terranes defines the Nevadan orogeny in the eastern part of the central Sierra Nevada. The Foothill suture, originally a major left-lateral strike-slip fault, separates the Kings and Merced River terranes. Middle Jurassic accretion along this suture marks the accretion of the Tethyan-de- rived Merced River terrane to the western margin of North America. The collision of the Merced River terrane with the western margin of North America may have caused the contemporaneous left-lateral offset of Precambrian crystalline terranes from southeastern Califor- nia to Sonora, Mexico. The Foothill suture may be the continuation of the left-lateral megashear in the central Sierra Nevada. The Melones fault separates the Merced River and Foothills terranes. Late Jurassic accretion and defamation along this suture, along with reactivation of the Foothill suture, defines the Nevadan orogeny in the western metamorphic belt. Comparison of the tectonics of the Mesozoic Sierra Nevada with that of Cenozoic southern Alaska reve- als many similarities. The Mesozoic Sierra Nevada may represent an Alaskan-type volcanic and plutonic are that formed along a tectoni— cally active continental margin. INTRODUCTION The geology of the wallrocks of the Sierra Nevada is a key to understanding the geology of the North American Cordillera and the relations between the Pa- leozoic and Mesozoic rocks of the Great Basin, Great Valley, and California Coast Range provinces. How- ever, the tectonic framework of the wallrocks of the Sierra Nevada is still very controversial. Currently, there are several major interpretations. A complex, faulted synclinorium is proposed by Bateman and others (1963), Bateman and Eaton (1967), and Bateman and Clark (1974). Inward-facing top directions and stratig- raphic sequences that are progressively younger toward the center of the batholith and symmetrical with its long axis are cited as evidence for the synclinorium. Accretion of Mesozoic country rocks onto the west side of the range against an Andean-type arc is proposed by Hamilton (1969, 1978), Schweickert and Cowan (1975), Davis and others (1978), and Schweickert (1978). East of the Melones fault, the Calaveras Formation and underlying ophiolite are cited by some of these authors as one example of an accreted terrane. Local occur- rences of Tethyan fossils in tectonic fragments of the Calaveras Formationi? indicate an exotic origin for at least parts of this unit. West of the Melones fault, the Jurassic wallrocks and underlying ophiolite, possibly representing an island-arc system, are also cited by some of these authors as another example of an ac- creted terrane. An anticlinorium that formed as a result 2 WALLROCKS OF THE CENTRAL'SIERRA NEVADA BATHOLITH, CALIFORNIA of multiple deformations is proposed by Kistler and others (1971). The multiply deformed and nearly verti- cal strata forming the wallrocks are cited as evidence for the anticlinorium. This paper analyzes the well-known belts of wall rocks in the central Sierra Nevada and proposes the hypothesis that the pre-middle Cretaceous belts consti- tute a collage of tectono-stratigraphic terranes that were accreted during volcanism and plutonism along a tectonically active continental margin. This analysis lists the depositional, structural, metamorphic, and magmatic histories for each terrane and shows the unique geologic history of each terrane and that large- scale tectonic movements are required for juxtaposition of terranes. Terranes are analyzed from east to west. Because most of the faults bounding terranes and the bulk of the study area is underlain by Mesozoic granitic rocks (pl. 1), emphasis is placed on the contact relations between wallrock sequences. Finally, this paper com- pares and shows that the stratigraphy, structure, and tectonics of the Mesozoic Sierra Nevada is extremely similar to that of southern Alaska in Cenozoic time. The Mesozoic Sierra Nevada may represent an Alaskan- type volcanic and plutonic arc that formed along a tec- tonically active continental margin. This paper represents the first attempt to analyze all of the wallrocks of the central Sierra Nevada batholith as a collage of accreted terranes. Because of considerable stratigraphic and structural complexity, future investigations may show that some terranes, particularly the Goddard, Kings, Merced River, and Foothills terranes, may each be a composite of several terranes. DEFINITIONS “Tectono—stratigraphic terrane” is defined as a fault- bounded geologic entity with distinct geologic history, stratigraphy, structure, and mineral deposits differing markedly from those of adjoining neighbors (Jones and Silberling, 1979; Beck and others, 1980). Each terrane is characterized by one or more distinctive, internally coherent stratigraphic sequences (Jones and Silberling, 1979). “Accretion” is defined as the juxtapositioning of a terrane into its present position. “Amalgamation” is defined as the juxtapositioning of two terranes in a loca- tion with subsequent drift and accretion at a site far removed from the site of amalgamation. “Arc” is de- fined as a linear belt of contemporaneous volcanic and plutonic rocks and adjacent wall rocks. “Suture” is de- fined as a major fault between tectono—stratigraphic ter- ranes. A key factor in terrane analysis is determining the timing of accretion and amalgamation by defining the oldest units that weld together two adjacent ter- ranes. In the central Sierra Nevada, such welding con- sists of either younger units that unconformably overlie adjacent terranes or plutonic rocks that intrude adja- cent terranes. ACKNOWLEDGMENTS I am grateful for the many discussions of this topic with D. L. Jones, R. W. Kistler, J. B. Saleeby, R. S. Schweickert, N. J. Silberling, R. C. Speed, and J. H. Stewart. I thank David L. Jones and Edwin D. McKee for their constructive reviews, and the numer- ous colleagues with whom I have had so many exciting discussions of Sierra Nevada and Alaskan geology over the last several years. RELATION OF MULTIPLE REGIONAL DEFORMATION TO TERRANE ACCRETION In a detailed study of regional deformations of the central Sierra Nevada, Nokleberg and Kistler (1980) show that (1) each generation of structures is charac- terized by folds of a particular style and orientation, with related cleavages, schistosities, lineations, and faults and (2) each generation of structures was formed during a regional deformation. The recognition of multi- ple generations of structures is based principally on superposition relations, especially the warping or re- folding of an earlier fold, schistosity, lineation, or cleav— age by a later fold. Other criteria used for distinguish- ing between generations include: (1) differences in the style of folding for a generation—generally more open for successively younger fold sets; (2) different orienta— tions of axial planes, schistosities, and cleavages; and (3) crossing of an older axial plane, cleavage, or schis- tosity by a younger structure. Associated with each re- gional deformation of this terrane and of other terranes to the west was a period of regional metamorphism (Nokleberg and Kistler, 1980). Usually the interiors of the larger roof pendants show the remnants of ' greenschist-facies regional metamorphism with develop- ment of penetrative fabric and extensive recrystalliza- tion along axial plane schistosities and fold axes formed during a particular deformation. Adjacent to granitic plutons, the margins of most roof pendants are contact metamorphosed to the albite-epidote or hornblende- hornfels facies of contact metamorphism, and the con- tact metamorphism has masked the earlier regional metamorphism and deformation. In the western metamorphic belt, greenschist-facies. regional meta- morphism generally prevails along with local areas of amphibolite—facies regional metamorphism (Behrman, 1978; Saleeby and others, 1978; Nokleberg and Kistler, 1980). In this study, each period of regional deformation OWENS TERRANE 3 and metamorphism is related to accretion, amalgama- tion, and (or) transport of terranes along sutures. RELATION OF TERRANES TO INTRUSIVE EPOCHS OF GRANITE ROCKS The oldest geologic units that weld together terranes in the Sierra Nevada are mainly individual granitic plu— tons, or age groups of plutons that define intrusive epochs (Evernden and Kistler, 1970). The intrusive epochs of granitic rocks occur along various linear age belts (Evernden and Kistler, 1970; Kistler and others, 1971). From oldest to youngest, the intrusive epochs (Evernden and Kistler, 1970) are: Lee Vining of Late Triassic and Early Jurassic age (210 to 195) my); Inyo Mountains of Early and Middle Jurassic age (180 to 160 my); Yosemite of Late Jurassic and Early Cretaceous age (148 to 132 my); Huntington Lake of Early Creta- ceous age (121 to 104 my); and Cathedral Range of Late Cretaceous age (90 to 79 my). Relative age as- signments of radiometric dates are from Sohl and Wright (1980). If each age belt of granitic rock is continous at depth, then the distribution of each belt could be analyzed to place limits on timing of terrane accretion. However, this analysis has inherent pitfalls. Sutures or major faults may occur within a volcanic and plutonic arc with immense strike slip displacement. Such displacement could result in alinement of coeval fragments of a com- posite batholith that originally formed in widely sepa- rated places. And in some areas of the central Sierra Nevada batholith major fragments are faulted against wallrocks; some of the faults may represent sutures. Examples of such sutures are the White Mountains fault zone (Crowder and Sheridan, 1972) and portions of the Melones fault in the western Sierra Nevada (Duf- field and Sharp, 1975). The welding of adjacent terranes by granitic rocks can only be demonstrated where indi- vidual plutons, or series of plutons, are observed in the field to intrude adjacent terranes. In the bulk of the batholith, the granitic rocks are considerably younger than the ages of movement on sutures. This relation allows that substantial movement on sutures was possi- ble before intrusion by granitic magmas. OWENS TERRANE OCCURRENCE The Owens terrane consists of early Paleozoic metasedimentary rocks that are exposed discontinu- ously in roof pendants in the eastern Sierra Nevada for about 140 km in a northwest-trending belt that ranges from about 20 to 25 km in width (pl. 1; table 1). This terrane, the oldest in the range, is best exposed in the Log Cabin mine and the eastern part of the Mount Morrison roof pendants. To the east, it is either intruded by Late Triassic and Early and Middle Juras— sic granitic plutons of the Lee Vining and Inyo Moun- tains intrusive epochs (Evernden and Kistler, 1970) or bounded by the Owens Valley fault discussed below. To the west, this terrane is either intruded by Late Triassic; Early Jurassic, and Late Cretaceous granitic plutons of the Lee Vining and Cathedral Range intru- sive epochs or faulted against the High Sierra terrane along the Laurel-Convict fault (Pl. 1). The Laurel-Con- vict fault in the Mount Morrison roof pendant, and the extension of this fault northward in the Gull Lake and Saddlebag Lake roof pendants (Brook and others, 1979; Kistler and Nokleberg, 1980), forms a suture between the High Sierra and Owens terranes (pl. 1). In the Gull Lake roof pendant, the fault occurs at the base of a thin Carboniferous marble unit, mapped by Kistler (1966b), about 0.8 km east of the east shore of Silver Lake (W. J. Nokleberg, unpub. data, 1975). In the Saddlebag Lake roof pendant, the western trace of the fault occurs at the base of a highly deformed, thin sec- tion of Carboniferous rocks on the slopes about 0.4 km west of the southwest shore of Saddlebag Lake (Brook, 1977; Brook and others, 1979). STRATIGRAPHY The dominant protoliths in the Owens terrane are calcareous sandstone, shale, and chert, with minor marl, limestone, and dolomite (table 1). These rocks are dated (pl. 1; table 1) by the occurrence of Ordovician graptolites in the Log Cabin mine roof pendant (J. H. Stewart, written commun., 1979), Ordovician to Siluri- an(?) graptolites in the Mount Morrison roof pendant (Rinehart and Ross, 1964), Cambrian Skolithos(?) and sziam(?) in the Big Pine Creek roof pendant (Moore and Foster, 1980). The nonfossilerous rocks of the east- ern parts of the Saddlebag Lake and Gull Lake roof pendants are correlated with the lower Paleozoic rocks of the Mount Morrison roof pendant on the basis of distinctive metamorphosed crossbedded quartzite and shale occuring in all three (Kistler, 1966a, b; Brook, 1977; Brook and others, 1979; W. J. Nokleberg, unpub. data, 1977). Rocks of this terrane are interpreted as slope-rise deposits in the Mount Morrison roof pendant (J. H. Stewart, oral commun., 1979), as near-shelf de- posits in the Bishop Creek roof pendant and as shelf deposits in the Big Pine Creek roof pendant (Moore and Foster, 1980). There appears to have been a facies change from shelf to slope deposition in the area now 4 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA TABLE 1.—Principal occurrences, original lithologies, fossils and ages, and investigators of wallrocks and roof pendants constituting tectono- stratigraphic terranes in the central Sierra Nevada Principal occurrences Log Cabin mine, eastern part of Saddlebag Lake, eastern part of Gull Lake, eastern part of Mount Morrison, Bishop Creek, Big Pine Creek, Casa Diablo, and Dinkey Creek roof pendants. mnflnmnofthMgmm, western part of Mount Dana, northern part of Ritter Range, western part of Gull Lake, eastern part of central Ritter Range, Oak Creek, western part of Mount Morrison, and Big Pine Creek roof pendants. Original lithologies Fossils and ages Owens terrane Calcareous sandstone, shale, and chert; minor marl, limestone, and dolomite. Ordovician to Silurian(?) graptolites in Mount Morrison and Bishop Creek roof pend- ants. Cambrian Skolithos(?) and Cruziana(?) in Big Pine Creek roof pendant. Ordo- vician graptolites in Log Cabin mine roof pendant High Sierra terrane Volcanic rocks: andesite to rhyo- dacite tuff, ash-flow tuff, and flows, conglomerate, volcanic breccia, andesite sills; minor marl, volcanic sandstone, and basalt flows. Sedimentary rocks: siliceous and calcareous mudstone, minor lime- stone, marl, conglomerate, quartz—sandstone, and chert. Volcanic rocks: 240 m.y. by Rb-Sr whole—rock isochron. Sedimentary rocks: Pennsylvanian conodonts, Mississippian(?), Pennsylvanian, and Permian(?) crinoids, corals, brachiopods, and pel ecypods. Goddard terrane Investigators Rinehart and Ross (1957, 1964), Bateman (1965), Kistler (1966a, b), Crowder and Sheridan (1972), Brook (1977), Brook and others (1979), Russell and Nokleberg (1977), w. J. Nokleberg (unpub. data, 1977), Moore and Foster (1980), J. H. Stewart (oral commun., 1980). Rinehart and Ross (1964), Bateman(1965L Huber and Rinehart (1965), Kister (1966a, b), Morgan and Rankin (1972), Brook and others (1974), Chesterman (1975), Russell (1976), Tobisch and Fiske (1976), Brook (1977), Russell and Nokleberg (1977), Tobisch and others (1977), Fiske and Tobisch (1978), C. H. Stevens, N. J. Nokleberg, and A. H. Harris (unpub. data, 1978), Brook and others (1979), Kistler and Nokleberg (1980). Central and western part of Ritter Range, eastern part of Boyden Cave, Mount Goddard, and Alabama Hills roof pendants. Andesite to dacite, ash-flow tuff, lava flows, tuff-breccia, tuff, lapilli tuff; minor basalt and rhyolite flows and tuff, lime- stone, and limy tuff. Early Jurassic pelecypod. 168 m.y. by Rb-Sr whole—rock isochron. 153 to 186 m.y. by U-Pb zircon techniques. Rinehart and Ross (1964), Bateman(1965L Bateman and Moore (1965), Moore and Marks (1972), Moore (1973), Lockwood and Lydon (1975), Tobisch and Fiske (1976), Girty (1977a, b), Russell and Nokleberg (1977), Schweickert and others (1977), Tobisch and others (1977), Moore (1978), Fiske and Tobisch (1978), Saleeby and others (1978), and Moore and others (1979L Kings terrane Metasedimentary rocks of Strawberry mine, western part of Boyden Cave, eastern part of lower Kings River, eastern part of Kaweah River, Tule River, Mineral King, eastern part of Yokohl Valley, and Kern Canyon roof pendants. Quartzite, arkose, limestone, marl mudstone, and calcareous sandstone; minor dacite to rhyodacite tuff, ash-flow tuff, breccia, and volcanic sandstone. Very minor andesite and basalt tuff and breccia. Early Jurassic(?) pelecypod, Late Triassic and Early Jurassic ammonites and pelecypods. 168 and 210 m.y. by U-Pb zircon techniques Durrell (1940), Macdonald (1941), Krauskopf (1953), Ross (1958), Moore and Dodge (1962), Christensen (1963), Nokleberg (1970, 1980), Moore and Marks (1972), Jones and Moore (1973), Girty (1977a, b), Schweickert and others (1977), Saleeby and others (1978), Moore and others (1979), Busby-Spera and others (1980), Saleeby and Sharp (1980). Merced River terrane Metasedimentary rocks and underlying ophiolite of the western meta— morphic belt east of the Melones fault. Similar rocks in north- eastern part of Oakhurst, western part of lower Kings River, western part of Kaweah River, and western part of Yokohl Valley roof pendants. Similar rocks in fault- bounded lenses within Bear Mountain fault. Sedimentary rocks: shale, quartzite, and limestone; minor marl, quartz- siltstone, chert, and conglomerate. Ophiolite: serpentinite, serpentin- ized peridotite and dunite, gabbro, diabase, basaltic pillow lava, pillow breccia, tuff—breccia, and tuff. Sedimentary rocks: Carboniferous and Permian fusulinids, part with lethyan origin; Carbon— iferous or Permian coral; Carboniferous brachiopod. Ophiolite: 270 to 305 m.y. by U-Pb zircon techniques on plagiogranite. Durrell (1940), Macdonald (1941), Taliaferro and Solari (1948), Clark (1954), Eric and others (1955), Clark (1960), Baird (1962), Clark (1964), Douglass (1967), Clark (1970), Nockleberg (1975), Duffield and Sharp (1975), Morgan (1976), Wetzel and Nokleberg (1976), Morgan and Stern (1977), Russell and Cebull (1977), Schweikert and others (1977), Behrman (1978), N. J. Nockleberg (unpub. data, 1978), Saleeby (1978, 1979), Saleeby and others (1978), Sharp and Saleeby (1979), Saleeby and Sharp (1980). Foothills terrane Metamorphosed volcanic arc rocks and underlying ophiolite of the western metamorphic belt west of the Melones fault. Similar rocks in southwestern part of Oakhurst roof pendant. Volcanic arc rocks: epiclastic mudstone, siltstone, and volcanic graywacke and conglomerate. Pre— dominately basaltic with lesser andesitic flows, pillow lava, and breccia. Ophiolite: serpentinite, serpen— tinized peridotite and dunite, gabbro, diabase, basaltic pillow lava and tuff—breccia, and chert. Volcanic arc rocks: Callovian and Kimmeridgian (Middle and Late Jurassic) ammonites and pelecypods. Ophiolite: 182 to 190 m.y. or older for possibly syngenetic diorites dated by U—Pb zircon techniques (Morgan, 1976); 200 to 300 m.y. by U-Pb zircon techniques on plagiogranite (Saleeby and others, 1979). Taliaferro and Solari (1948), Eric and others (1955), Clark (1960), Mannion (1960), lmlay (1961), Best (1963). Clark 1964, 1970), Duffield and Sharp 1975), Morgan (1976), Netzel and Nokleberg (1976), Morgan and Stern (1977), Russell and Cebull (1977), Behrman and Parkinson (1978), Schweikert (1978), Saleeby and Moores (1979), Saleeby and others (1979). represented by the metasedimentary rocks of the Bishop Creek roof pendant (Moore and Foster, 1980). The stratigraphic section of rocks forming the Owens terrane is fault bounded. Because of intense, multiple deformation, stratigraphic thicknesses are not well known. The minimum stratigraphic thicknesses of these OWENS TERRANE 5 rocks are estimated at about 2,500 to 5,150 m in the Mount Morrison roof pendant by Rinehart and Ross (1964). However, Russell and Nokleberg (1977) found that intense isoclinal folding caused considerable repeti- tion of strata and estimated the stratigraphic thickness as a few hundred meters. Minimum stratigraphic thicknesses are estimated at about 1,500 m in the Bishop Creek roof pendant and at about 250 m in the Big Pine Creek roof pendant (Moore and Foster, 1980). STRUCTURE Most of the Owens terrane is thrice deformed (table 2). The principal, and first, generation of structures in the Owens terrane was formed in a Devonian or Missis- sippian regional deformation, possibly the Antler orogeny (Russell and Nokleberg, 1977). Structures of this generation consist of appressed to isoclinal folds, parallel thrust and reverse faults, cleavage and schis- tosity parallel to faults and axial planes of folds, and lineations parallel to fold axes (Russell and Nokleberg, 1977; N okleberg and Kistler, 1980). The style of folding is predominantly flexural slip with a minor component of slip. Where bedding is developed in relatively incom- petent rocks, such as marble, it is commonly thinned on limbs and thickened in hinges, indicating flowage during folding. In areas relatively unaffected by later deformations, axial planes of major and minor folds and parallel schistosities have average strikes of north to N. 10° W. and nearly vertical dips (table 2). Most major and minor fold axes plunges gently north or south and indicate a first or near-first deformation of these rocks (Nokleberg and Kistler, 1980). The Devonian or Mississippian age of this regional deformation is best established in the Mount Morrison roof pendant where first-generation north-trending structures are restricted to the lower Paleozoic metasedimentary rocks (Russell and Nokleberg, 1977). Later (Mesozoic) structures occur in both the Owens terrane and other, younger terranes containing Missis- sippian(?) and Pennsylvanian rocks to the west, thereby limiting the first deformation of the Owens terrane to the Devonian or Mississippian (Russell and Nokleberg, 1977). The Laurel-Convict fault, which separates the Owens terrane from the High Sierra terrane to the west, forms a suture between a thrice-deformed terrane to the east and a twice-deformed terrane to the west. The Devonian or Mississippian age of deformation coin- cides with the Antler orogeny of the Great Basin (Rus- sell and Nokleberg, 1977). Asymmetric folds and thrust faults of the Log Cabin mine roof pendant and the steeply west dipping Laurel-Convict fault indicate tec- tonic transport from west to east, analogous to the An- tler orogeny (Burchfiel and Davis, 1972). The Laurel- Convict fault has been considerably steepened by Meso- zoic deformations (Russell and Nokleberg, 1977). Two younger deformations also occurred in the Owens terrane (table 2) (N okleberg and Kistler, 1980). The second, the Late Jurassic Nevadan orogeny, oc- . curred along N. 20° to 40° W. trends in all roof pendants of this terrane except the Log Cabin mine and Bishop Creek roof pendants, which were apparently shielded by Late Triassic granitic rocks of the Lee Vining intru- sive epoch. This deformation occurred during accretion of terranes to the west along the coeval Laurel-Convict fault, as discussed below. The third deformation oc- curred along N. 50° to 80° W. trends in the middle Cre- taceous in this terrane and other terranes to the west (table 2). This generation of structures refolded and offset older structures formed in the Late Jurassic Nevadan orogeny and in older deformations in the cen- tral Sierra Nevada (table 2) (Nokleberg and Kistler, 1980). EVIDENCE FOR OWENS VALLEY FAULT BOUNDING EASTERN MARGIN The eastern limit of the Owens terrane appears to be a Cretaceous or Tertiary suture, herein named the Owens Valley fault, that strikes north-south from Ben- ton Valley in the north through Owens Valley in the south. In the northern part of the. area, the Owens Val- ley fault separates the lower Paleozoic terrane in the eastern Sierra Nevada from the Permian to Jurassic metavolcanic rocks in the White Mountains to the east. Further south, the Owens Valley fault separates the Owens terrane of the eastern Sierra Nevada from the late Precambrian and Cambrian metasedimentary rocks in the White-Inyo Mountains. In the north, the evidence for the Owens Valley fault is that although highly deformed, the younger Permian to Jurassic metavolcanic rocks are nowhere observed, either east or west of the fault, to unconformably over— lie the late Precambrian and Cambrian metasedimen- tary rocks (Rinehart and Ross, 1957; Crowder and Sheridan, 1972). This relation indicates that the Permi- an to Jurassic metavolcanic rocks are bordered on their margin by major faults. In the south, the evidence for the fault is the lack of detailed correlation of Cambrian units between the Sierra Nevada and White-Inyo Mountains. General correlation of stratotypes has been made by Moore and Foster (1980) between the Cambri- an to Silurian(?) metasedimentary rocks in the Bishop Creek and Big Pine Creek roof pendants with late Pre- cambrian and Cambrian metasedimentary rocks in the White-Inyo Mountains. However, Moore and Foster (1980) were not able to correlate formations across Owens Valley, despite extremely well established 6 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA TABLE 2.—Regional deformations recorded in tectono-stratigraphic terranes in the central Sierra Nevada [Age of deformation includes average strike of fold axial planes, rock types, and age of rocks for each area. Ages of roof pendants or wallrocks. when known with some certainty, are given; for areas in which younger generations of structures are absent, age of adjacent granitic rocks is given] Devonian or Mississippian Area deformation Antler orogeny Triassic deformation Sonoman progeny Early and Middle Jurassic deformation Late Jurassic Nevadan orogeny Middie Cretaceous deformation Owens terrane Log Cabin mine roof pendant N.; (Kistler, 1966; N. J. metasedimentary rocks; Nockleberg, unpub. data, 0rdovician(?) and 1975). Si)urian(?). Eastern part of Saddlebag N. 8° N.; N. 25° N.; same rocks. Bordered by ......... N. 20° to 30° N.; Late Triassic granite- N. 50° to 60° N.; Lake roof pendant (Brook, metasedimentary rocks; same rocks. same rocks. 1972, 1974, 1977; Brook 0rdovician(?) and and others, 1979). Silurian(?). Eastern part of Mount N. 8° N.; __________________________________________ N. 23° w; N. 58° N.; Morrison roof pendant metasedimentary rocks; same rocks. same rocks. (Russell and Nockleberg, Ordovician and Silurian(?r). 1974, 1977). Bishop Creek roof pendant N.; ---------------------- Bordered by --------- Late Triassic granite- ----------------- (Bateman, 1965', Moore metasedimentary rocks; and Foster, 1980). 0rdovician(?) and Silurian(?). Dinkey Creek roof pendant N. 5° E.', ------------------------------------------ N. 25° N.; N. 60° N.; (Kistler and Bateman, metasedimentary rocks; same rocks. same rocks. 1966) Paleozoic(?). High Sierra terrane Western part of Saddlebag -------------------------- N. 20° to 30° N.; ------------------------------------------ N. 61° N.; Lake roof pendant (Brook, metasedimentary rocks, same rocks. 1972, 1974, 1977; Brook Mississippian(?); and others, 1979). metavolcanic rocks, Permian and Triassic. Western part of Mount Dana -------------------------- N. 25° N.; ------------------------------------------ N. 60° to 65° N.; roof pendant (Kistler, metavolcanic rocks; same rocks. 1966a, D; Russell, 1976). Permian and Triassic isochron. Northern part of Ritter -------------------------- N. 25° N.; ------------------------------------------ N. 50° N.; Range roof pendant metavolcanic rocks; same rocks. (Kistler, 1966a). Permian and Triassic isochron. Eastern part of central -------------------------- N. 30° N.; -------------------- N. 30° N.; N. 50° to 70° N.; Ritter Range roof pend- metavolcanic rocks; same rocks. same rocks. ant (Kistler, 1966a, b; Permian and Triassic Huber and Rinehart, isochron and Triassic 1965', Fiske and Tobisch, U-Pb zircon age. 1978). Central part of Mount -------------------------- N. 30° N.; -------------------- N. 30° N.; N. 62° N.; Morrison roof pendant metasedimentary rocks; metasedimentary rocks; same rocks. (Rinehart and Ross, 1964; Russell and Nokleberg, 1977). Pine Creek roof pendant (Bateman, 1965). Pennsylvanian and Permian(?). N. 22° N.; metasedimentary rocks; Pennsylvanian(?) and Permian(?). Bordered by ————————— Pennsylvanian and Permian; metavolcanic rocks; Permian to Jurassic. Late Triassic granite- Goddard terrane Central and western parts of Ritter Range roof pendant (Kistler, 1966a, b; Huber and Rinehart, 1965‘, Fiske and Tobisch, 1978). Mount Goddard roof pendant (Bateman and Moore, 1965; Chen and Moore, 1979). Eastern part of Boyden Cave -------------------------- roof pendant (Moore and Marks, 1972; Girty, 1977a, b; Saleeby and others, 1978). N. 30° N.; metavolcanic rocks; Permian, Triassic, and Early Jurassic isochrons and Pb-U zircon ages. N. 20° to 40° N.; metavoicanic rocks; pre-Late Jurassic. N. 15° to 20° N.; metavolcanic rocks; pre-Early Cretaceous. N. 50° to 70° N.; same rocks. N. 50° to 70° N.; same rocks. - Encircied by Early Cretaceous granite. OWENS TERRANE TABLE 2.—Regiorbal deformations recorded in tectorLs-stratigraphic terranes in the central Sierra Nevada—Continued Area Devonian or Mississippian Triassic deformation Early and Middle Sonoman orogeny Jurassic deformation Late Jurassic Nevadan Middle Cretaceous orogeny deformation Kings terrane Strawberry mine roof pendant (Nockleberg, 1970. 1980). Western part of Boyden Cave roof pendant (Moore and Marks, 1972; Girty, 1977a, b; Saleeby and others, 1978). Mineral King roof pendant (Christensen, 1963L ------------------- N.E.; metasedimentary rocks; Early Jurassic(?). ---------------------- N. 60° E.; metasedimentary rocks; Late Triassic and Early Jurassic. ---------------------- N. 20° to 30° N. and N. 55° to 60° w.; same rocks. N. 20° N.; Encircled by same rocks. Early Cretaceous granite. N. 30° N.; Encircled by metasedimentary rocks; Early Cretaceous Late Triassic. granite. Merced River terrane Stanislaus River near Camp 9 (Baird, 1962). Stanislaus River near Melones (Baird, 1962L Merced River (w. J. Nokleberg, unpub. data, 1972). Merced River (N. J. Nokleberg, unpub. data, 1977). ---------------------- N. 55° to 65° 5.; Calaveras Formation, quartzite; Late Paleozoic(?). ---------------------- N. 30° to 50° E.', Calaveras Formation, argillite; Late Paleozoic(?). ---------------------- N. 30° to 40° E.', Calaveras Formation, quartzite; Late Paleozoic(?). ______________________ N. 45° to 55° 5-; Calaveras Formation, argillite; Late Paleozoic(?). N. 20° to 30° N. ----------------- N. 20° to 30° w,; ................. same rocks. N. 20° w.; ----------------- same rocks. N. 20° to 30° w.; ................. same rocks. San Andreas (Duffield and Sharp, 1975). Angels Camp and Sonora quadrangles (Eric and others, 1955). Mariposa (Best, 1963) _______ San Andreas (Clark, 1970)--- Stanislaus River, east of Knights Ferry (Clark, 1964). N. 15° to 25° N.; ----------------- Mariposa Formation, Slate and Logtown Ridge Formation; Late Jurassic. N. 30° to 40° N.-, ----------------- Mariposa Formation, Slate and Brower Creek Volcanic Member; Late Jurassic. N. 15° to 30° N.; ----------------- Mariposa Formation, slate; Late Jurassic. N. 20° to 30° w.; ----------------- Salt Spring Slate and Copper Hill Volcanics, slate and volcanic rocks; Late Jurassic. N. 20° to 35° N.-, ----------------- Salt Spring Slate and Gopher Ridge Volcanics, slate and volcanic rocks; Late Jurassic. stratigraphic relations in shelf deposits in the White- Inyo Mountains and a distance of only 13 km between the metasedimentary rocks on opposite sides of the val- ley. Also, there is a transition from shelf deposits in the Big Pine Creek roof pendant to slope deposits in Substantial differences exist between the the Mount Morrison roof pendant which does not occur in the White-Inyo Mountains to the east and which ap- pears to be truncated by the Owens Valley fault. CRITICAL DIFFERENCES BETWEEN OWENS TERRANE AND METASEDIMENTARY ROCKS OF WHITE-INYO MOUNTAINS metasedimentary rocks of the Owens terrane and the late Precambrian and Cambrian metasedimentary rocks in the White-Inyo Mountains. Sparse fossils indicate 8 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA Cambrian, Ordovician, and Silurian(?) ages for the Owens terrane, whereas abundant fossils indicate late Precambrian and Cambrian ages for the sedimentary rocks in the White-Inyo Mountains. In addition, the metasedimentary rocks of the Owens terrane consist of both shelf and slope deposits, but the late Precambri- an and Cambrian metasedimentary rocks of the White- Inyo Mountains consist predominatly of shelf deposits, Also, the metasedimentary rocks of the Owens terrane are multiply deformed and metamorphosed, whereas the late Precambrian and Cambrian metasedimentary rocks of the White-Inyo Mountains are barely metamorphosed and less intensely deformed (Dunne and others, 1978). These differences and the evidence for the Owens Valley fault indicate the Owens terrane is displaced, may be allochtonous, and has been ac- creted to units to the east. ACCRETION OF OWENS TERRANE The timing of accretion of the Owens terrane can only be determined by the age of crosscutting units that weld the Owens terrane to terranes to the east. There is no known area where individual plutons of any Meso- zoic intrusive epoch can be observed in the field to in- trude and weld the Owens terrane to the Permian to Jurassic metavolcanic rocks or the late Precambrian and Cambrian metasedimentary rocks of the White-Inyo Mountains (Evernden and Kistler, 1970). The only other Mesozoic constraint on timing of accretion of the Owens terrane is the Late Jurassic Independence dike swarm, dated by U-Pb techniques as 148 my, which crosses Owens Valley near Independence (Moore and Hopson, 1961; Chen and Moore, 1979). Moore and Hopson (1961) suggest that this occurrence of the dike swarm across Owens Valley limits movement on any fault in the val- ley to less than a few tens of kilometers. However, recently discovered exposures of the dikes greatly en- large the width and length of the dike swarm (Chen and Moore, 1979). These new data shows the dike swarm is up to 60 km wide and strikes almost parallel to Owens Valley. Thus, major fault with several hun- dreds of kilometers of movement could readily occur in Owens Valley, strike subparallel to the dike swarm, and easily offset portions of the dike swarm to form the present-day pattern. The above analysis indicates accretion of the Owens terrane in the interval between the deposition of the Permian to Jurassic metavolcanic rocks in the White- Inyo Mountains and the inception of Tertiary volcanism in Owens Valley. To narrow this time interval, more data are needed on the ages of the metavolcanic rocks and on critical geologic relations in similar rocks further south in the Mojave Desert. These relations indicate the Owens Valley fault is a major tectonic boundary, possibly a suture, between two allochthonous terranes, the Owens terrane to the west and the late Precambri- an and Cambrian metasedimentary rocks of the White- Inyo Mountains to the east. The Owens Valley fault may be a major right-lateral strike-slip fault of Cenozoic age, possibly related to movement on the San Andreas fault system, the Death Valley fault of Cenozoic age, and the Las Vegas shear zone. Of considerable note is the inability of various workers to extend the Roberts Mountain thrust, formed during the Antler orogeny, and the vast differences in structures produced in the Antler orogeny in central Nevada and the Sierra Nevada (Burchfiel and Davis, 1972; Russell and Nok- leberg, 1977; Davis and others, 1978); these relations indicate considerable movement on the Owens Valley fault, thereby causing offset of the Roberts Mountain thrust as well as juxtaposition of different terranes. HIGH SIERRA TERRANE OCCURRENCE The High Sierra terrane consists of metamorphosed late Paleozoic metasedimentary rocks and unconforma- bly overlying Permian and Triassic metavolcanic rocks that are exposed discontinuously in roof pendants along the crest of the Sierra Nevada for about 140 km in a northwest-trending belt that averages about 5 km in width (pl. 1) (Kistler and Nokleberg, 1980). This ter- rane is best exposed in the Saddlebag Lake, Mount Morrison, and Pine Creek roof pendants. To the east, this terrane is either intruded by Late Triassic to Mid- dle Jurassic and Late Cretaceous granitic plutons of the Lee Vining, Inyo Mountains, and Cathedral Range in- trusive epochs (Evernden and Kistler, 1970) or faulted against the Owens terrane along the Laurel-Convict fault (pl. 1). To the west, this terrane is either faulted against the Goddard terrane along the San Joaquin River fault (pl. '1) or intruded by Late Cretaceous grani- tic plutons of the Cathedral Range intrusive epoch (Evernden and Kistler, 1970). The stratified Permian and Triassic metavolcanic rocks forming the western part of this terrane are faulted against the Jurassic metavolcanic rocks of the Goddard terrane in the cen- tral part of the Ritter Range roof pendant (Huber and Rinehart, 1965; Fiske and Tobisch, 1978; W. J. Nok- leberg, unpub. data, 1976). STRATIGRAPHY The dominant protoliths in the late Paleozoic metasedimentary rocks are siliceous and calcareous mudstone, with minor limestone, marl, conglomerate, quartz-sandstone, and chert (Kistler and Nokleberg, HIGH SIERRA TERRANE A 23.3 E swim wows ammo.“ 2:. 358526.... .«o moohsom 6.3 E 52» mummflofi: no .33. mo wmopgw .82 uni £3.82. 5 552% mansion Econ?“ 9.5.83 3...: amigwsgm 3620 .8523. .532... 9.3 .Efiwms E25 8 5.65. 9.8.. 3.52m .33.. 2: m...» = + .. .3 twin... .532... .....§._.. 8.6 2-5.8 .Ez. >58... 55.8 >525 iii: .96: 522...? .E: >38... iii: BED 6:: 9:55 Mo 8383 a... swim $3.... so “5896:. ER 5528.. .8 wm< .mocwtou mushyaom $.35 nnméxrzm .8 $22.9. 5min. 8.3.: €30.59. BE... -39 “53?... .8933 mos: «flaw .3552 2.8% 35:8 .mozwtma oEQEMEEum 5:803 2833). was 238...... .8. 2.538 oEmuawsnbm 63.....uocwwla BEER .525 5.3:: 9.852. a... 2...... .5625... 3.3.... .3E....... .23 4.32% .32... a... 5:258... .52... E. ....a .3...» .2232... 5.35585 iii. gnu-E. 3...... 8.3.8- .. “35 9.33.3. 2... 2.5.. SE8. .9 «=2... .52.... as 35.2. 5:83- .. 25.... «as: 2...... 5.3.8- ... 38.. 33...... £5 _.=_..__. gags... .a .3... f 13 x I < x r[ < k r < \ r! < K H m. ‘ mm. 2.2.... .s..§§. lm 255....6-3. .Efi W $2“... .235 MW . . . . m. .5... 95:3: E... 828...... mm :2. .3. Wm egg—18:85.. )w. 5.... mm .2... a... 2...... . m m .5... .33.... .23... mm as 2.52.... w. 53...... .x 2.8. 38... .E 2.. M 85...: ‘ W . “is: 5 . . 2.... 87:5 m. Em 3.5. .85.. .26: gun-n W . M. 3...... s. 3.3.... 2... ,u «a u. 3...... a...” “h... mm a... 9.2.... .3. E2523... \\ m. H m m. 2.23.... 1 852% Um, 3g :2 m w. 382.. £532 is .8: m. m Eat ass—5 Wm. M. mm. 2... 2.23:... 3....» 3...... M m E. .38.... 3.8? i... m m ..= .=... 9......“ .3...» in... mum. i... am .32.. 2532...... M W S... .225... is. 5.2. m. m. .5... g__.._.. .38.... is m 1 £2... 8.... 2 2325... .m m. .223... .8235. as... w w 2......» .5... m .33»... 6...... NW .2335: E3: 98.3.8 Wm .22.: 9.3.? 2 8:352: (m. H w u. .c..2u.§.=__.£_E-=a m / m m I. H u. . . I. . £552.85 [A m w 2.... «2-2.: a 2 E Eng. 2.... 878: “88229 3.5 can 258:5 2.: 9.5.5:. 382» «.5855 3...“ 9.32.... 21. fig... 2.5:... E. is. ”S... 8.... a 5.3.5.... :88 2.7.2.... £2... 2.2...5 35: .2353 9.3 5.35:5: 32.3.; “25.2.: “25.5.. 55.5.. mz> 3:35.20 10 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA 1980). The metasedimentary rocks of the High Sierra terrane contain (pl. 1; table 1) Pennsylvanian conodonts in an unnamed roof pendant northeast of Saddlebag Lake (W. J. Nokleberg and A. H. Harris, unpub. data, 1978); Mississippian(?) crinoid fragments and corals in the Saddlebag Lake roof pendant (Brook and others, 1979); Mississippian(?) brachiopods and pelecypods in the northern Ritter Range roof pendant (Brook and others, 1979); and abundant brachiopods, corals, and bryozoans of Pennsylvanian and Permian(?) age in the Mount Morrison roof pendant (Rinehart and Ross, 1964). The unfossiliferous metasedimentary rocks of other roof pendants in this terrane are correlated with the upper Paleozoic rocks of the Mount Morrison roof pendant on the basis of lithologic similarity. The stratig- raphic section is bounded by a fault at its base and by an angular unconformity at its top. Because of in- tense, multiple deformation, the stratigraphic thickness of the late Paleozoic metasedimentary rocks of this ter- rane is not well known but is estimated at about 1,600 m in the Mount Morrison roof pendant (Rinehart and Ross, 1964; Morgan and Rankin, 1972) and at about 1,600 m in the Pine Creek roof pendant (Bateman, 1965). In the western part of the Saddlebag Lake roof pendant, transposition of bedding into foliation by in— tense, penetrative deformation has made estimation of stratigraphic thicknesses impossible (Brook, 1977). The metasedimentary rocks of this terrane are interpreted as having originally been deposited in a shelf or slope environment (Kistler and Nokleberg, 1980). The dominant protoliths in the Permian and Triassic metavolcanic rocks are andesite to rhyodacite tuff, ash- flow tuff, and flows, conglomerate, volcanic breccia, and andesite sills, with minor marl, volcanic sandstone, and basalt flows (table 1) (Brook, 1977; Kistler, 1966a). In the Saddlebag Lake roof pendant, the sequence consists of a basal conglomerate, a thick rhyolite ash-flow tuff, andesite sills and tuff, and an upper unit of volcaniclas— tic rocks, tuff, and marl (Brook, 1977). The very distinc- tive basal conglomerate and overlying ash-flow tuff can be traced for 70 km from the Saddlebag Lake roof pen- dant in the north to the Mount Dana, Ritter Range, and Mount Morrison roof pendants in the south (Rus- sell, 1976; Kistler, 1966b; W. J. Nokleberg, unpub. data, 1976). Although identifiable fossils have not been found in the Permian and Triassic metavolcanic rocks, the rocks are dated Permian and Triassic (240 my.) by a Rb/Sr whole-rock isochron (Kistler, 1966a; Brook, 1977; Fiske and Tobisch, 1978; Nokleberg and Kistler, 1980). Because of intense, multiple deformation, the stratigraphic thickness of the Permian and Triassic metavolcanic rocks is unknown but is estimated at about 500 m in the Saddlebag Lake roof pendant (Brook, 1977) and at about 2,000 m in the Ritter roof pendant (Fiske and Tobisch, 1978). The metavolcanic and limy metasedimentary rocks of this terrane are in- terpreted as having formed in a shallow marine environ- ment. A major regional angular unconformity separates the late Paleozoic metasedimentary rocks from the overly- ing Permian and Triassic metavolcanic rocks. This un- conformity occurs along a 75-km stretch of the Sierra Nevada crest that includes the Saddlebag Lake, Mount Dana, Ritter Range, and Mount Morrison roof pendants (Kistler, 1966a, b; Russell, 1976; Brook, 1977; Russell and Nokleberg, 1977; Fiske and Tobisch, 1978). Inter- pretation of the contact between these two sequences as an angular unconformity rather than a fault is dis- cussed by Brook and others (1974) and Morgan and Rankin (1972). The angular unconformity separating the two rock sequences is manifested by (1) a marked change in lithologies, (2) the basal conglomerate in the Permian and Triassic metavolcanic rocks containing clasts similar to the underlying late Paleozoic metasedimentary rocks, and (3) truncation of layers in the late Paleozoic metasedimentary rocks with no indi- cation of shearing along the contact (Brook and others, 1974). The time hiatus represented by the unconformity may be quite short because of the Mississippian(?), Pennsylvanian, and Permian(?) age of the underlying metasedimentary rocks and the Permian and Triassic age of the overlying metavolcanic rocks. In the south- ern part of the Mount Morrison roof pendant, local faulting may occur along the unconformity (Morgan and Rankin, 1972). This unconformity represents a great change in the stratigraphy of the wallrocks and marks not only Permian and Triassic uplift and erosion, but also the cessation of marine sedimentation in the Paleo- zoic, and the inception of Andean-type arc volcanism in the latest Paleozoic and Mesozoic (Brook and others, 1974). STRUCTURE Most of the High Sierra terrane is either twice or thrice deformed (table 2). The principal, and first, de— formation of this terrane occurred in a Triassic regional defamation, possibly the Sonoman orogeny (Nok- leberg, 1979; Nokleberg and Kistler, 1980). Structures produced during this deformation consist of the north- northwest-trending Laurel-Convict fault (pl. 1), paral- lel-trending isoclinal folds and faults, and schistosity. Axial planes of major and minor folds and parallel schis- tosities have average attitudes of N. 20° to 30° W. with vertical dips (table 2). Major and minor fold axes plunge moderately to steeply northwest (Nokleberg and Kis- tler, 1980). The Triassic age of this regional deformation is best established in the Saddlebag Lake and northern Ritter HIGH SIERRA TERRANE 11 Range roof pendants where structures, including the northern extension of the Laurel-Convict fault, are in- truded by Late Triassic granitic rocks of the Lee Vining intrusive epoch (Evernden and Kistler, 1970; Nokleberg and Kistler, 1980). A similar age of defamation is also determined from the Pine Creek roof pendant where N. 20° to 30° W.-trending structures in Pennsylva- nian(?) and Permian(?) metasedimentary rocks are crosscut by relatively undeformed Late Triassic granitic rocks of the Lee Vining intrusive epoch, which restricts the deformation to Late Permian or Triassic. In both the Saddlebag Lake and northern Ritter Range roof pendants, the upright nature of folds and the nearly vertical dip of associated faults indicate mainly com— pression during this deformation. The moderately plunging fold axes in the Saddlebag Lake roof pendant (Brook, 1977; Nokleberg and Kistler, 1980) may indicate a component of strike-slip movement. Critical geologic data on structural history are lacking in the western part of the Mount Dana, and northern part of the Ritter Range roof pendants, and only a Tri- assic or Jurassic age of deformation can be determined for the N. 20° to 40° W.-trending structures (table 2) (Nokleberg and Kistler, 1980). Later, intense deforma- tion occurred during accretion of the Goddard terrane to the west during the Late Jurassic Nevadan orogeny, as discussed below (table 2). Coaxial, multiple deforma- tions along N. 20° to 40° W. trends may have occurred in both the Triassic and Late Jurassic in the High Sierra terrane (Nokleberg and Kistler, 1980) (table 2). A third deformation occurred along N. 50° to 80° W. trends in the middle Cretaceous (table 2). EVIDENCE FOR LAUREL-CONVICT FAULT The evidence for the Laurel-Convict fault is: (1) in the Mount Morrison roof pendant, juxtaposition of upper Paleozoic sedimentary rocks over lower Paleozoic sedimentary rocks along a steeply west dipping surface of shearing and intense deformation (Rinehart and Ross, 1964; Russell and Nokleberg, 1977); (2) in the Gull Lake roof pendant, juxtaposition of upper Paleozo- ic sedimentary rocks against lower Paleozoic sedimenta- ry rocks along a vertically dipping surface of tectonic imbrication and shearing located at the east side of a mapable unit about 1 km east of Silver Lake (Kistler, 1966b; W. J. Nokleberg, unpub. data, 1975); and (3), in the Saddlebag Lake roof pendant, juxtaposition of twice-deformed upper Paleozoic sedimentary rocks over thrice-deformed lower Paleozoic sedimentary rocks along a folded shear surface (Brook, 1977; Brook and others, 1979). In the Mount Morrison roof pendant, the Laurel-Con- vict fault dips steeply westward (Rinehart and Ross, 1964). The Laurel-Convict fault was originally interpre- ted by Rinehart and Ross (1964) as a faulted angular unconformity between lower Paleozoic rocks to the east (Owens terrane) and upper Paleozoic rocks to the west (High Sierra terrane). In this study, the concept of the Laurel-Convict fault representing a former unconfor- mity is rejected because nowhere in the eastern Sierra Nevada is the unconformity ever observed. Instead, a surface of intense deformation and shearing is generally observed. This fault has probably been rotated to steeper angles by the Nevadan orogeny, dicussed sub- sequently, and was originally a moderate to shallowly west dipping thrust fault. CRITICAL DIFFERENCES FROM OWENS TERRANE The main differences between the High Sierra and Owens terranes are (tables 1, 2): (1) a late Paleozoic and Early Triassic age for the High Sierra terrane com- pared to an early Paleozoic age for the Owens terrane; (2) the occurrence of predominantly siliceous and cal- careous mudstone and marble in the former sedimenta- ry rocks of the High Sierra terrane compared to the predominantly calcareous sandstone and shale in the former sedimentary rocks of the Owens terrane; (3) the occurrence of former volcanic rocks generated in an An- dean-type are setting in the High Sierra terrane com- pared to no volcanic rocks in the Owens terrane; and (4) the occurrence of three generations of structures in the Owens terrane compared to only the younger two generations of structures in the High Sierra ter- rane. These differences and the evidence for the Laurel- Convict fault between the High Sierra and Owens ter- ranes indicate that the High Sierra terrane has been either amalgamated or accreted to the OWens terrane. AMALGAMATION AND ACCRETION OF HIGH SIERRA TERRANE The Laurel-Convict fault is the locus of the Triassic overthrusting of the Owens terrane by the High Sierra terrane. The age of this overthrusting is generally simi- lar to the emplacement of the Golconda allochthon in western Nevada during the Sonoman orogeny as de- scribed by Speed (1979). However, units in the G01- conda allochthon or autochthon cannot be correlated with the High Sierra and Owens terranes, respectively (R. C. Speed, oral commun., 1979). Consequently, the Laurel-Convict fault is only a similar-age thrust fault or suture which, together with the other structures of the Triassic deformation, may represent the equivalent of the Sonoman orogeny in the eastern Sierra Nevada. This deformation and overthrusting defines the amal- gamation of the High Sierra terrane onto the Owens 12 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA terrane. To the north in Nevada, a late Paleozoic island arc was amalgamated onto the North American margin (Silberling and Roberts, 1962; Speed, 1979), whereas to the south, a Permian or Triassic Andean-type arc was amalgamated to the North American margin. The amalgamation also was the major tectonic event just prior to development of the Mesozoic volcanic and plutonic arc of the Sierra Nevada. Subsequently, the High Sierra and Owens terranes were welded together in the northern part of the study area by Late Triassic granitic plutons of the Lee Vining intrusive epoch (pl. 1; fig. 1) (Evernden and Kistler, 1970). Preliminary data on the offset of initial strontium isotope contours in eastern California suggest reactivation of the southern part of the Laurel-Convict fault with right-lateral dis- placement in the Jurassic (Kistler and others, 1980). Subsequently, the High Sierra and Owens terranes were welded together during intrusion of the Late Cre- taceous granitic plutons of the Cathedral Range intru- sive epoch (fig. 1). GODDARD TERRANE OCCURRENCE The Goddard terrane consists of metamorphosed vol- canic rocks of Jurassic age that are exposed discontinu- ously in roof pendants along and just west of the crest of the Sierra Nevada for about 190 km in a north-north- west-trending belt that ranges from about 6 to 33 km in width (pl. 1; table 1). This terrane is best exposed in the Ritter Range and Mount Goddard roof pendants. However, the metavolcanic rocks of this terrane in the Goddard, eastern part of the Boyden Cave, and Alabama Hills roof pendants have not been extensively studied. Much additional work, particularly on obtain- ing radiometric age data, is needed in these roof pen- dants. To the east, this terrane is either intruded by Late Cretaceous granitic plutons of the Cathedral Range intrusive epoch (Evernden and Kistler, 1970) or faulted against the High Sierra terrane along the herein named San Joaquin River fault (pl. 1). To the west, this terrane is (1) intruded by Early Cretaceous and Late Cretaceous granitic plutons of the Huntington Lake and Cathedral Range intrusive epochs (Evernden and Kistler, 1970), (2) faulted against Kings terrane along the Kings River fault (pl. 1), or (3) unconformably overlain by Middle Cretaceous metavolcanic rocks (pl. 1). In the central and western parts of the Ritter Range roof pendant, a major angular unconformity separates twice-deformed predominantly Jurassic metavolcanic rocks from overlying once-deformed middle Cretaceous metavolcanic rocks (Huber and Rinehart, 1965; Fiske and Tobisch, 1978; Nokleberg and Kistler, 1980). STRATIGRAPHY The dominant protoliths in the Goddard terrane are andesite to dacite lava flows, tuff-breccia, tuff and lapilli tuff, ash-flow tuff, and minor basalt and rhyolite flows and tuff, limestone, and limy tuff (table 1). Abundant specimens of the bivalve genus Weyla at one locality in the Ritter Range roof pendant (Huber and Rinehart, 1965) and radiometric determinations of 153 to 186 my. in the Ritter Range roof pendant (Fiske and Tobisch, 1978) indicate an Early and Late Jurassic age (pl. 1; table 1). At the eastern margin of the Goddard terrane in the Ritter Range roof pendant, U-Pb zircon ages range from 186 to 214 my. (Fiske and Tobisch, 1978); however, these older ages are from samples near the San Joaquin River fault between the Goddard and High Sierra terranes where there is considerable tectonic mixing of the two terranes (W. J. Nokleberg, unpub. data, 1976). The Goddard terrane in the northern part of the Mount Goddard roof pendant is dated as pre-Late Jurassic. These rocks are intruded by a sheared granodiorite which is dated by U-Pb zircon techniques as 158 my. (Chen and Moore, 1979). The Goddard ter- rane in the eastern part of the Boyden Cave roof pen- dant is dated as pre-Early Cretaceous (>140 m.y.). These rocks are intruded by Early Cretaceous granitic plutons of the Huntington Lake ’ intrusive epoch (Evernden and Kistler, 1970; Moore and others, 1979). The metavolcanic rocks of the Goddard terrane are either'fault bounded or unconformably overlain by the middle Cretaceous metavOlcanic rocks. A minimum thickness of a highly deformed section in the Ritter Range roof pendant is about 5,000 m (Fiske and To- bisch, 1978). The metavolcanic rocks of the Goddard terrane are interpreted as having formed in a shallow, but continually subsiding, marine basin in which volcan- ic arc flows, tuff, and breccia were being deposited dur- ing repeated marine transgressions and regressions (Fiske and Tobisch, 1978). STRUCTURE The Goddard terrane is twice deformed (table 2). The principal, and first, generation of structures was formed in the Late Jurassic Nevadan orogeny (Nokleberg and Kistler, 1980). Structures of this generation consist of north- northwest-trending faults, including the bound- ing San Joaquin River fault (pl. 1), moderately appres— sed to isoclinal folds, schistosity, and lineations (Nok- leberg and Kistler, 1980). Axial planes of major and minor folds and parallel schistosities show average strikes of N. 20° to 40° W. (table 2). Major and minor fold axes have moderate to steep plunges. In the Ritter Range roof pendant, several fault zones show extreme cataclasis occurring parallel to the strike of major units GODDARD TERRANE 13 and axial planes of major folds (W. J. Nokleberg, unpub. data, 1976). ' The Late Jurassic age of the Nevadan orogeny in the roof pendant near the crest of the Sierra Nevada is best established in the Ritter Range roof pendant. The rocks of the central part of the roof pendant contain N. 20° to 40° W.-trending structures with Early J uras- sic fossils and Jurassic radiometric ages and are uncon- formably overlain by Early Cretaceous metavolcanic rocks containing only younger structures (Fiske and To- bisch, 1978; Nokleberg and Kistler, 1980). These rela- tions bracket the deformation as Late Jurassic. In late Paleozoic and Mesozoic metavolcanic rocks (probably Permian through Jurassic) of the Mount Morrison roof pendant, intrusions by Late Cretaceous granitic rocks of the Cathedral Range intrusive epoch (Evernden and Kistler, 1970) indicate the deformation probably was be- tween Jurassic and Early Cretaceous time. In other roof pendants in this area, critical data are lacking and only a Mesozoic age of deformation can be inferred for the N. 20° to 40° W.-trending structures (Nokleberg and Kistler, 1980). The upright nature of folds and the nearly vertical dip of axial planes and fold limbs indicate mainly compression during this deformation, and the gentle to steep plunges of major and minor fold axes indicate compressional as well as strike-slip components of movement. A later deformation occurred along N. 50° to 80° W. trends in the middle Cretaceous (table 2) (Nokleberg and Kistler, 1980). Structures of this middle Cretaceous deformation are the only ones to occur in the middle Cretaceous metavolcanic rocks that unconformably ov- erlie the Jurassic metavolcanic rocks in the Ritter Range roof pendant (Tobisch and Fiske, 1976, Fiske and Tobisch, 1978). EVIDENCE FOR SAN JOAQUIN RIVER FAULT The San Joaquin River fault forms a suture between the Goddard and the High Sierra terranes. This fault is best exposed in the central part of the Ritter Range roof pendant (pl. 1). This fault occurs at the western limit of the Permian and Triassic metavolcanic rocks, along the piedmontite-bearing zone of Huber and Rinehart (1965), located about 0.5 km west of the head- waters of the San Joaquin River. The evidence for the San Joaquin River fault is: (1) intense shearing and cataclasis in the metavolcanic rocks in and adjacent to the piedmontite-bearing zone (W. J. Nokleberg, unpub. data, 1975); (2) truncation of units in both terranes along the contact (Fiske and Tobisch, 1978); and (3) jux- taposition of stratigraphic units of highly different ages, stratigraphy, and structure (tables 1, 2). Considerable tectonic mixing of the two terranes may occur along the San Joaquin River fault. CRITICAL DIFFERENCES FROM THE HIGH SIERRA TERRANE The stratigraphy, structure, and geologic history of the Goddard terrane differ greatly from those of the High Sierra terrane to the east. The main differences (tables 1, 2) are: (1) a Jurassic age for the Goddard terrane compared to a late Paleozoic and Early Triassic age for the High Sierra terrane; (2) greatly different stratigraphy for the former sedimentary rocks of the two terranes with abundant siliceous and calcareous mudstone, limestone, and marl in the High Sierra ter- rane compared to sparse limestone and limy tuff in the Goddard terrane; and (3) greatly different structural histories for the two terranes. The High Sierra terrane generally was thrice deformed: twice along N. 20° to 40° W. trends, in the Triassic Sonoman orogeny and in the Late Jurassic Nevadan orogeny, and once along N. 50° to 80° W. trends during the middle Cretaceous; whereas the Goddard terrane was twice deformed, dur- ing the later two periods (table 2). In addition, the High Sierra terrane is intruded by Late Triassic plutons of the Lee Vining intrusive epoch, whereas the Goddard terrane is intruded only by plutons of younger intrusive epochs. These differences and the evidence for the San Joaquin River fault between the Goddard and High Sierra terranes indicate that the Goddard terrane is al- lochthonous and has been amalgamated or accreted to the High Sierra terrane. AMALGAMATION AND ACCRETION OF GODDARD TERRANE The San Joaquin River fault, formed during the Late Jurassic Nevadan orogeny, is the locus of accretion of the Goddard and High Sierra terranes. Portions of the Early and Middle Jurassic granitic rocks of the Inyo Mountains intrusive epoch may have been also displaced along the San Joaquin River fault. These relations indi- cate the Late Jurassic Nevadan orogeny was an intense and widespread defamation that not only included accretion of terranes in the western metamorphic belt along the Melones fault (Davis and others, 1978) but also accretion of terranes to the east along major faults such as the San Joaquin River fault. Like the Melones fault, the San Joaquin River fault represents a suture along which there was major displacement and jux- taposition of diverse terranes within and during forma- tion of the volcanic and plutonic arc of the Mesozoic Sierra Nevada. The Goddard terrane is welded to the High Sierra terrane by Late Cretaceous granitic plu- tons of the Cathedral Range intrusive epoch (fig. 1) (Evernden and Kistler, 1970). This relation allows that additional relative movement between the two terranes was possible in the middle Cretaceous. 14 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA CRETACEOUS METAVOLCANIC ROCKS OCCURRENCE The Cretaceous metavolcanic rocks do not form a ter- rane, but instead unconformably overlie or intrude, and thereby weld together, the Kings and Goddard terranes (pl. 1; fig. 1). The Cretaceous metavolcanic rocks con- sisting mainly of metamorphosed andesite to rhyodacite lava, tuff, breccia, and sparse limy metasedimentary rocks are exposed in the Mount Dana, western part of the Ritter Range, Merced Peak, and Strawberry mine roof pendants (pl. 1). The Cretaceous metavolcanic rocks have been studied by Kistler (1966a), Russell (1976), Tobisch and Fiske (1976), Fiske and Tobisch (1978), and Nokleberg (1981). In the western part of the Ritter Range roof pendant, Early and middle Cretaceous metavolcanic rocks uncon- formably overlie the Early and Middle Jurassic rocks of the Goddard terrane. To the west, metamorphosed shallow intrusions of middle Cretaceous age intrude metasedimentary rocks of the Kings sequence in the Strawberry mine roof pendant (Nokleberg, 1980). These shallow intrusions are part of the Cretaceous metavolcanic rocks of the Merced Peak and adjacent roof pendants (pl. 1). The shallow intrusions and metavolcanic rocks form a magmatic sequence with the granodiorite of Jackass Lake and related plutonic rocks of middle Cretaceous age (Peck and others, 1977; Nok- leberg, 1980). The Cretaceous metavolcanic rocks of the Mount Dana roof pendant unconformably overlie the late Paleozoic and Triassic metasedimentary and metavolcanic rocks of the High Sierra terrane (Kistler, 1966a, b; Russell, 1976). STRATIGRAPHY AND STRUCTURE The dominant protoliths of the Cretaceous metavol- canic rocks are volcanic tuff, flows, breccia, ash-flow tuff, shallow intrusions, volcanic graywacke, and minor limestone, marl, shale, and conglomerate. Igneous rock composition ranges from basalt to rhyolite, with ande- site and dacite being most prevalent (Kistler, 1966a, b; Russell, 1976; Fiske and Tobisch, 1978; Nokleberg, 1981). The Cretaceous metavolcanic rocks are dated by Rb-Sr whole-rock and U—Pb zircon techniques as 97 to 100 my. in the Strawberry mine and Merced Peak roof pendants (Peck— and others, 1977; Nokleberg, 1981), as 98 to 127 my. in the Ritter Range roof pendant (Fiske and Tobisch, 1978), and as Early Cretaceous in the Mount Dana roof pendant (Nokleberg and Kistler, 1980). In the Ritter Range roof pendant, these rocks are interpreted as a portion of an Andean-type arc in which there was mainly subaerial and local lacustrine deposition of volcanic material (Fiske and Tobisch, 1978), or as a portion of an Andean-type arc deposited in shallow marine basins in the Mount Dana roof pen- dant (Kistler, 1966a; Russell, 1976). The Cretaceous metavolcanic rocks are generally once deformed. The defamation occurred in the middle to Late Cretaceous along mainly N. 50° to 70° W. trends (Nokleberg and Kistler, 1980). Structures consist of west-northwest—trending major and minor faults and folds, schistosity, and lineation. Fold axes and linea- tions generally plunge gently west-northwest. Conju- gate folding along northeast trends is also associated with the west—northwest-trending structures in parts of the Ritter Range roof pendant (Tobisch and Fiske, 1976) and in the Strawberry mine roof pendant (Nok- leberg, 1980). Regional metamorphism of the greensch- ist facies and penetrative deformation are slight to mod- erate. KINGS TERRANE OCCURRENCE The Kings terrane consists of abundant Triassic and Jurassic metamorphosed sedimentary rocks and sparse intermediate to silicic metavolcanic rocks. These rocks have been included in the Kings sequence by Bateman and Clark (1974), Schweickert and others (1977), Saleeby and others (1978), and Nokleberg (1980). In this paper, the Kings sequence is defined as the metasedimentary rocks and sparse metavolcanic rocks that are discontinuously exposed from the Strawberry mine roof pendant in the north to the Isabella roof pen- dant in the extreme southern Sierra Nevada. These ex- posures form a north-northwest-trending belt about 210 km long and from 25 to 50 km wide between the crest and western foothills of the Sierra Nevada (pl. 1; table 1). This terrane is best exposed in the Strawberry mine, western part of the Boyden Cave, and Mineral King roof pendants. The Dinkey Creek roof pendant is here excluded from the Kings sequence (pl. 1) because of stratigraphy and structure that differs markedly from the Kings se- quence. The Dinkey Creek roof pendant contains former mudstones, marls, limestone, and particularly orthoquartzite, as well as three generations of super- posed structures that are nearly identical to similar fea- tures in the early Paleozoic terrane to the east (Kistler and Bateman, 1966; Russell and Nokleberg; 1977; Bate- man and Clark, 1974; Nokleberg, 1981). The Dinkey Creek roof pendant is interpreted as a small slice of the Owens terrane that has been displaced and accreted against the Kings terrane (pl. 1). However, such tec- tonic juxtaposition of the Dinkey Creek roof pendant KINGS TERRANE 15 presents a major unsolved problem in Mesozoic Sierra Nevada tectonics. To the east, the Kings terrane is (1) intruded by Early Cretaceous and Late Cretaceous granitic plutons of the Huntington Lake and Cathedral Range intrusive epochs (Evernden and Kistler, 1970), (2) faulted against the Goddard terrane along the herein named Kings River fault (pl. 1), or (3) intruded by middle Cretaceous dikes and sills of andesite to rhyolite composition in the Strawberry mine roof pendant (pl. 1) (Nokleberg, 1970, 1981). To the west, this terrane is either intruded by Early Cretaceous granitic plutons of the Huntington Lake intrusive epoch (Evernden and Kistler, 1970) or faulted against the Merced River terrane along the Foothill suture (pl. 1). In the central part of the Boyden Cave roof pendant, the Kings River fault separates the less deformed metavolcanic rocks of the Goddard ter- rane to the east from the more higly deformed metasedimentary rocks of the Kings sequence and ter- rane to the west (Girty, 1977a, b; Moore and others, 1979). This fault occurs within and at the eastern mar- gin of the highly deformed chaotic unit (Saleeby and others, 1978; Moore and others, 1979). STRATIGRAPHY The dominant protoliths in the Kings terrane are quartzite, arkose, limestone, marl, mudstone, calcare- ous sandstone, and sparse dacite to rhyodacite tuff, ash- flow tuff, breccia, and volcanic sandstone (table 1). Pre- cambrian zircons in the thick quartzites (Moore and others, 1979) indicate a cratonal derivation. The volcan- ic rock and detritus composition is predominantly dacite with minor rhyodacite (table 1). These metasedimen— tary rocks of the Kings terrane are dated (pl. 1; table 1) by the occurrence of a late Mesozoic bivalve, most likely Inoceramus pseudomytiloides of Early Jurassic age, in the Strawberry mine roof pendant (Nokleberg, 1980) and pelecypods and ammonites of Late Triassic and Early Jurassic age in the Boyden Cave and Mineral King roof pendants (Christensen, 1963; Moore and Dodge, 1962; Jones and Moore, 1973; Saleeby and others, 1978). The metavolcanic rocks of the Kings ter- rane are dated (pl. 1; table 1) by U-Pb zircon ages of 210 and 168 my in the Mineral King and Yokohl Valley roof pendants, respectively (Early to Middle Jurassic) (Busby-Spera and others, 1980; Saleeby and Sharp, 1980). The stratigraphy of the Kings sequence is inter- preted by Saleeby and others (1978) and by J. S. Saleeby in Moore and others (1979) as a submarine fan system containing craton—derived sand, silicic volcanic- lastic units, silicic tuff, and ash-flow tuff, all interbed- ded with mudstone, carbonate, and marl. Submarine fan facies include massive channel deposits, midfan to basin plane deposits, shallow-water slide blocks, and olistostromes of limestone and sandstone. The sub- marine fan system was dispersed off the shelf of west- ern North America and westward onto accreted ocean floor represented by the Kings-Kaweah ophiolite belt (Saleeby, 1978). Because of intense multiple deforma- tion, the stratigraphic thickness of the strata forming the Kings terrane is not well known. The base and top of the stratigraphic sections are either faulted or in- truded by granitic plutons. The minimum stratigraphic thickness of these rocks is estimated at 887 m in the Strawberry mine roof pendant (Nokleberg, 1970, 1981) and at thousands of meters for the entire sequence by Saleeby and others (1978). STRUCTURE Most of the Kings terrane is twice or thrice deformed (table 2). The first generation of structures was formed in a Middle Jurassic deformation (Nokleberg and Kis- tler, 1980). Structures of this generation consist of northeast-trending open to isoclinal major and minor folds, parallel major and minor faults, axial plane schis- tosity, lineation, and areas of melange, broken forma- tion, and cataclasite. Axial planes of major and minor folds and parallel schistosities have average strikes of northeast to N. 60° E. (Girty, 1977a, b) (table 2). Major and minor fold axes have gentle to moderate plunges (N okleberg and Kistler, 1980). A maximum age for this Middle Jurassic deformation of the Kings terrane is de- termined by the structures occuring in Early Jurassic rocks in the Strawberry mine and western parts of the Boyden Cave roof pendants. A minimum age for this deformation is determined by the occurrence of super- posed structures formed in the Late Jurassic N evadan orogeny in (1) the western part of the Boyden Cave roof pendant (table 2) and (2) the Merced River terrane, which also contains the older, northeast-trending struc- tures (table 2) (Nokleberg and Kistler, 1980). These re- lations bracket the deformation occurring along north- east trends as Middle Jurassic. The generally upright nature of folds and the nearly vertical dip of axial planes and fold limbs indicate mainly compression during this deformation. The gentle to moderate plunges of major and minor fold axes indicate that this was the first de- formation of the Kings terrane. '1\vo younger deformations also occurred in portions of the Kings terrane (table 2). The second deformation occurred along N. 20° to 40° W. trends in the Late Jurassic Nevadan orogeny in the western part of the Boyden Cave and the Mineral King roof pendants (table 2). These Nevadan structures refolded and offset struc- tures formed in the Middle Jurassic deformation (Girty, 1977a, b); they are very similar in the Kings and God- dard terranes (table 2) (Nokleberg and Kistler, 1980). 16 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA The Nevadan structures consist of north-northwest- trending faults, including the Kings River fault (pl. 1), moderately appressed to isoclinal major and minor folds, schistosity, and lineations (Girty, 1977a, b; Moore and others, 1979; Nokleberg and Kistler, 1980). Axial planes of major and minor folds and parallel schis- tosities have average strikes of N. 20° to 30° W. (table 2), and major and minor fold axes generally have verti— cal to steep southeast plunges. The major contacts be- tween units of the western part of the Boyden Cave roof pendant strike north-northwest, parallel to the the deformational fabric of the rocks, and represent mostly tectonic contacts (Girty, 1977a, b; Moore and others, 1979). The timing of this deformation in the Kings ter- rane is bracketed by the earlier, Middle Jurassic defor- mation and by the crosscutting of all structures by the Early Cretaceous granitic rocks of the Huntington Lake intrusive epoch. The steep plunges of major and minor fold axes formed in the Late Jurassic Nevadan orogeny in the Kings terrane indicate mostly a strike-slip compo- nent of movement. The third deformation occurred along N. 55° to 60° W. trends in the Strawberry mine roof pendant in the middle Cretaceous (Nokleberg, 1970, 1980). The Boyden Cave and Mineral King roof pendants were ap- parently shielded from this deformation by the Early Cretaceous granitic rocks of the Huntington Lake intru- sive epoch (table 2). EVIDENCE FOR KINGS RIVER FAULT The Kings River fault forms a suture between the God- dard and Kings terranes (pl. 1) (Girty, 1977a, b; Saleeby and others, 1978; Moore and others, 1979). This predo- minantly strike slip fault occurs in the eastern part of the chaotic unit of the Boyden Cave roof pendant (pl. 1), just west of the quartz sandstone or flysch unit of Saleeby and others (1978). The critical evidence for the Kings River fault is: (1) intense penetrative deformation in the chaotic unit adjacent to the fault (Girty, 1977a, b; Moore and others, 1979); (2) juxtaposition of units with greatly different ages and stratigraphy (table 1); and (3) juxtaposition of strata with greatly different structural histories (table 2). CRITICAL DIFFERENCES FROM GODDARD TERRANE The stratigraphy, stucture, and geologic history of the Kings terrane differ greatly from those of the God- dard terrane to the east (tables 1, 2). The critical differ- ences are (tables 1, 2): (1) a Late Triassic and Early Jurassic age for the Kings terrane compared to a pre- Cretaceous, most likely Jurassic, age for the Goddard terrane; (2) the occurrence of predominantly former quartzite, arkose, limestone, and marl, and sparse in- termediate to silicic flows, tuff, and epiclastic rocks in the Kings terrane compared to predominantly former andesitic to rhyodacitic volcanic flows and tuff in the Goddard terrane; and (3) a greatly different structural history, including an older, northeast to N. 60° E.- trending set of folds, faults, and schistosity in the Kings terrane. In addition, there is no gradation between the massive metamorphosed flows of the Goddard terrane and the metasedimentary rocks and sparse metamorph- osed volcanic and tuffaceous rocks of the Kings se- quence (Girty, 1977a, b; Saleeby and others, 1978). The pre-Cambrian zircons in the quartzites of the Kings ter- rane (Moore and others, 1979) require a cratonal sources that does not exist adjacent to the Kings se- quence. The above differences and the evidence for the Kings River fault between the Goddard and Kings ter- ranes indicate the Kings terrane is allochthonous and has been accreted to the Goddard terrane. ACCRETION OF KINGS TERRANE The Kings River fault, formed in the Nevadan orogeny, is the locus of accretion of the Kings terrane to the Goddard terrane. Accretion consisted predomin- antly of right-lateral strike-slip movement and caused displacement of the Kings terrane from the Mojave re- gion to its present position (Saleeby and others, 1978). Prior to movement on the fault, the Kings terrane formed an important spatial and temporal tie between major sedimentation patterns of the southwest Cordill- era in the Mojave region and the continental margin of western North America (Saleeby and others, 1978). Movement of the submarine fan complex of the Kings sequence along strike-slip faults dissipated the strikes- slip component of oblique subduction (Saleeby and others, 1978). The Kings River fault represents a su- ture, along which there was major displacement and juxtaposition of diverse terranes within and during for- mation of the volcanic and plutonic arc of the Mesozoic Sierra Nevada. After accretion, the Kings terrane was welded to the Goddard terrane by Early Cretaceous granitic plutons of the Huntington Lake intrusive epoch (fig. 1) (Evernden and Kistler, 1970). In the Strawberry mine roof pendant, the Kings terrane is intruded by shallow dikes and sills of middle Cretaceous age (Nok- leberg, 1981) that are the subsurface equivalent of the middle Cretaceous metavolcanic rocks of the Goddard terrane. MERCED RIVER TERRANE 17 MERCED RIVER TERRANE OCCURRENCE The Merced River terrane consists of metamorphosed Carboniferous and Permian sedimentary rocks—the Calaveras Formation—and underlying ophiolite that are exposed either continously in the western metamorphic belt east of the Melones fault or discontinuously in roof pendants in the western Sierra Nevada in the area south of Mariposa (pl. 1). Within the study area, this terrane forms a north-northwest-trending belt about 310 km long and about 5 to 25 km wide (pl. 1). This terrane is best exposed along the Stanislaus and Merced Rivers in the western metamorphic belt, and in the lower Kings River, Kaweah Peaks, and Yokohl Valley roof pendants. To the east, this terrane is either intruded by various granitic plutons of Early Jurassic to Early Cretaceous age of the Inyo Mountains, Yose— mite, or Huntington Lake intrusive epochs (Evernden and Kistler, 1970; Sharp and Saleeby, 1979; Saleeby and Sharp, 1980) or faulted against the Kings terrane along the Foothill suture (pl. 1). To the west, this terrane is either intruded by various granitic plutons of the same intrusive epochs as to the east or faulted against the Foothills terrane along the Melones fault (pl. 1). In the northern part of the study area, the eastern part of the Calaveras Formation (map unit mq, pl. 1) in the western metamorphic belt may be the southward con- tinuation of the lower Paleozoic Shoo Fly Formation (Schweickert, 1978). Further north in the northern Sierra Nevada, the Calaveras Formation is thrust under the Shoo Fly Formation (Clark, 1976), and the fault between the two formations has been traced to the south end of the western metamorphic belt by Schweickert (1978). In this study, rocks similar to the Shoo Fly Formation are included in the Merced River terrane. In the central Sierra Nevada, rocks called Kings sequence, as defined by Bateman and Clark (1974), were assigned by Schweickeret and others (1977) to the upper part of their Calaveras Complex which includes rocks formerly assigned to both the Calaveras Formation and the Kings sequence. In this study, the Calaveras Formation and Kings sequence are interpreted as belonging to separate terranes. STRA\TIGRAPHY h r The dominant protliths in the metasedimentary rocks of the Merced River terrane are shale and quartzite with minor limestone, marl, quartz-siltstone, and chert (table 1). The dominant lithologies in the underlying ophiolite are peridotite and dunite, serpentinite, gab— bro, diabase, and basaltic pillow lava, breccia, tuff, and chert (table 2). Although commonly in fault contact with the metasedimentary rocks of the Merced River ter- rane, the ophiolite is observed to underlie and be the basement for the Merced River terrane (Saleeby, 1978, 1979; Saleeby and others, 1978; Sharp and Saleeby, 1979; Saleeby and Sharp, 1980). In the southern part of the study area, this ophiolite has been termed the Kings-Kaweah ophiolite belt (Saleeby, 1978; Saleeby and others, 1978). The metasedimentary rocks of the Merced River terrane east of the melones fault are dated (pl. 1; table 1) by sparse Carboniferous and Per- mian fusulinids, brachiopods, corals, and pelecypods in the northern part of the study area (Clark, 1964; Schweickert and others, 1977). The metasedimentary rocks of the Merced River terrane in a tectonically de- tached block west of the Melones fault and west of San Andreas (pl. 1) are dated (pl. 1; table 1) by Permian Tethyan fusulinids (Clark, 1964; Douglass, 1967); the metasedimentary rocks of the Merced River terrane in the Yokohl Valley roof pendant are also dated (pl. 1; table 1) by Permian Tethyan fusulinids (Schweickert and others, 1977; Saleeby and others, 1978). A late Pa- leozoic age for the ophiolite underlying the metasedimentary rocks of the Merced River terrane is indicated by U-Pb radiometric ages of 270 to 305 my. on zircon from plagiogranite dikes in the ophiolite (Saleeby, 1978; Saleeby and Sharp, 1980). Of considerable importance are the Permian Tethyan fusulinids found in a large tectonically detached block of the Calaveras Formation within the Bear Mountain fault and in highly deformed chert and argillite of the upper part of the Kings—Kaweah ophiolite in the Yokohl Valley roof pendant (Clark, 1964; Schweickert and others, 1977; Saleeby and others, 1978). On the basis of the sparse Permian Tethyan fusulinids and similar lithologies, the Merced River terrane is correlated with the Cache Creek terrane of British Columbia and simi- lar groups of rocks in northeastern Oregon and the Klamath Mountains containing Permian Tethyan fusulinids (Davis and others, 1978). There are conflicting interpretations of the stratig- raphy of the former sedimentary rocks of the Merced River terrane. Schweickert and others (1977) interpret the strata as a series of numerous shale and chert olis- toliths that accumulated on oceanic lithosphere in a marginal basin. The quartzites represent progradation of mature sands derived from the North American con- tinent to the east that were deposited across the basin margin. In this study, the strata are interpreted as a series of highly deformed fault-bounded tectonic blocks and lenses that are part of an accreted terrane. The strata may have been deposited in a marginal basin, 18 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA however, but in the wam-water, equatorial Tethys re- gion of the late Paleozoic. Because of intense, multiple deformation, and bounding of the stratigraphic section by faults, the stratigraphic thickness of the Merced River terrane is not known. A rough estimate of the stratigraphic thickness of the sedimentary rocks is thousands of meters (Schweickert and others, 1977). The base of the ophiolite and the top of the sedimentary units of the terrane are fault bounded (Schweickert and others, 1977; Saleeby and others, 1978). STRUCTURE Most of the Merced River terrane is thrice deformed (table 2). The second generation of structures was formed in a Middle Jurassic deformation (Nokleberg and Kistler, 1980). Structures of this generation consist of northeast- trending moderately appressed to isoclinal major and minor folds, parallel major and minor faults, axial plane schistosity, lineation, and zones of melange, broken formation, and cataclasite. Axial planes of major and minor folds and parallel schistosities have average strikes of N. 30° to 65° E. (table 2). Major and minor fold axes plunge moderately to steeply northeast and east (Nokleberg and Kistler, 1980). This defamation is related to formation of the Foothill suture (Nokleberg and Kistler, 1980). A minimum age for this defamation is indicated by the occurrence of superposed structures formed in the Late Jurassic Nevadan orogeny (table 2). A maximum age for this deformation is indicated by the occurrence of nearly identical and similar-trend- ing structures in the Late Triassic and Early Jurassic Kings terrane (table 2). These relations bracket the de- formation along northeast trends as Early and Middle Jurassic. A concordant U-Pb zircon age of 170 my. on a syntectonic pluton that intrudes the Calaveras Foma- tion along the Stanislaus River (Baird, 1962) and that contains the northeast-trending structures also indi- cates a Middle Jurassic age of defamation (Sharp and Saleeby, 1979). The generally upright nature of folds and the nearly vertical dip of axial planes and fold limbs indicate mainly compression during this defamation. The uniform regional moderate to steep northeast plunges of major and minor fold axes indicate that the Merced River terrane was first deformed into north- northwest—trending isoclinal folds that dipped moder- ately east (Baird, 1962). The third major defamation recognized in the Merced River terrane is attributed to the Late Jurassic Nevadan orogeny (table 2). This defamation produced structures with N. 20° to 30° W. trends and comprises north-northwest-trending isoclinal folds, cataclasite zones, and faults, including reactivation of the Foothill suture. Major and minor fold axes plunge steeply to vertically. The timing of the Nevadan defamation in the Merced River terrane is determined by superposi- tion of the Nevadan structures on the northeast-trend- ing structures formed in the Middle Jurassic and by the crosscutting of the Nevadan structures by the Late Jurassic granitic rocks of the Yosemite intrusive epoch (Nokleberg and Kistler, 1980) The Jurassic defama- tional histories of the Merced River and Kings terranes are very similar (table 2). The similar trend, style, and age of the Middle Jurassic and Late Jurassic structures in the two terranes indicate co-defomation during the Middle Jurassic deformation and the Late Jurassic Nevadan orogeny. EVIDENCE FOR FOOTHILL SUTURE A major fault, named the Foothill suture by Saleeby and others (1978), separates the Merced River and Kings terranes in the lower Kings River, Kaweah River, and Yokohl Valley roof pendants. In this study, the fault is just east of the chert unit which occurs just east of the Kings-Kaweah ophiolite belt (pl. 1). This location is at a major and highly deformed litholagic break between the sedimentary rocks of the Merced River and the Kings terranes (Nokleberg, 1975). The evidence for the Foothills suture is the juxtaposition of strata with different ages and stratigraphy (pl. 1; table 1) and the intense penetrative defamation and tectonic imbricatian of units of the Merced River and Kings terranes along the suture (Nokleberg, 1975). In the area from the lower Kings River roof pendant to the Yokohl Valley roof pendant, this fault occurs in an area of extensive submarine slide blocks and olistos- tromes which were derived from the Kings terrane to the east and which are intemixed with chert and argil- lite of the Merced River terrane (Saleeby and others, 1978). In the Yokohl Valley roof pendant, the fault is placed in this study immediately west of the site of the 168-m.y. age determined by Saleeby and Sharp (1980) for the Kings sequence and immediately east of the Pemian Tethyan fusulinid locality of Saleeby and others (1978) (pl. 1). Saleeby and others (1978) place the Foothill suture a few kilometers further east than shown in plate 1, in the area of extensive olistostromes. CRITICAL DIFFERENCES FROM KINGS TERRANE The stratigraphy, structure, and geologic history of the Merced River terrane differ markedly from those of the Kings terrane to the east (tables 1, 2). The main differences are: (1) a late Paleozoic age for the metasedimentary rocks (the Calaveras Fomation) and a late Paleozoic age for the underlying ophiolite in the Merced River terrane compared to a Late TriasSic and FOOTHILLS TERRANE 19 Early Jurassic age for the metasedimentary rocks and metavolcanic rocks of the Kings terrane; (2) the occur- rence of sparse former dacite and rhyodacite ash—flow tuffs and epiclastic rocks in the Kings sequence, whereas such rocks are not present in the Merced River terrane; (3) the occurrence of an ophiolite basement, representing oceanic lithosphere, under the Merced River terrane, whereas no ophiolitic basement occurs under the Kings terrane; (4) the occurrence of Tethyan fusulinids in the sedimentary rocks of the Merced River terrane; and (5) the occurrence of an older generation of north-northwest-trending structures in the Merced River terrane. The occurrence of Tethyan fusulinids in highly de- formed blocks in the Merced River terrane indicates that these exotic blocks and probably the enclosing strata were formed in the warm-water, equatorial Tethys region of the late Paleozoic. Subsequently, this terrane moved toward and was accreted against the western margin of North America (Davis and others, 1978; Saleeby and others, 1978; this study). This inter- pretation strongly contrasts with previous interpreta- tions of the Kings sequence as the upper part of the Calaveras Complex of Schweickert and others (1977). The preceding critical differences and the evidence for the Foothill suture between the Merced River and Kings terranes indicate the Merced River terrane is ailochthonous and has been amalgamated or accreted to the Kings terrane. AMALGAMATION AND ACCRETION OF THE MERCED RIVER TERRANE The Foothill suture is the locus of amalgamation of the Merced River and Kings terranes (Nokleberg, 1975; Saleeby and others, 1978). The common deformation of the Merced River and Kings terranes along northeast trends (table 2) is interpreted as having formed during the amalgamation of the two terranes in the Middle Jurassic. The northeast-trending folds, shear and catac— lasite zones, and schistosities that formed during this amalgamation probably represent northwest-southeast compression (Nokleberg and Kistler, 1980). Amalgamation of the Merced River and Kings ter- ranes appears to be contemporaneous with the left-lat- eral shear displacement along the western margin of North America (Silver and Anderson, 1974; Kistler and Peterman, 1978). This displacement is defined by offset along a left-lateral megashear of Precambrian crystal- line terranes and Paleozoic depositional trends from southeastern California to Sonora, Mexico (Silver and Anderson, 1974). The Early and Middle Jurassic age and the left-lateral sense of displacement of Precambri- an crystalline terranes and of Paleozoic depositional trends are very similar to those in the Middle Jurassic amalgamation and northwest-southeast compression of the Merced River and Kings terranes. The displace- ment of the crystalline terranes and depositional trends may have been caused by amalgamation of the Tethyan- derived Merced River terrane with the Kings River ter- rane on the western margin of North America. In the central Sierra Nevada, the Foothill suture may be the left-lateral megashear of Silver and Anderson (1974). The syntectonic Middle Jurassic granitic plutons within the Merced River terrane (Evernden and Kistler, 1970; Sharp and Saleeby, 1979) probably were intruded dur- ing this amalgamation and deformation. Prior to, or at the inception of, the Middle Jurassic amalgamation, the ophiolite of the Merced River ter- rane underwest 190— to 200-m.y.-old amphibolite facies regional metamorphism and was intruded by syntec- tonic 200-m.y.-old gabbro (Saleeby and others, 1978; Saleeby and Sharp, 1980). This regional metamorphic and plutonic event may have occurred with the first defamation of the Merced River terrane along north- northwest trends. According to Saleeby and others (1979), this metamorphic event may represent the in- itial stage of amalgamation. After the Middle Jurassic amalgamation, both the Merced River and Kings terranes and the Early and Middle Jurassic plutonic rocks intruding the Merced River terrane were displaced and accreted by right-lat- eral movement on the Kings River fault during the Late Jurassic Nevadan orogeny (Saleeby and others, 1978). During this movement and accretion, both terranes were penetratively deformed, resulting in the formation of the N. 20° to 30° W. structures (table 2) (Nokleberg and Kistler, 1980). Intense development of these Neva- dan structures along and adjacent to the Foothill suture indicates that the Foothills suture was reactivated dur- ing the Nevadan orogeny. After this accretion, the Merced River and Kings terranes were welded together by Early Cretaceous granitic plutons of the Huntington Lake intrusive epoch (fig. 1). F OOTHILLS TERRANE OCCURRENCE The Foothills terrane consists of metamorphosed Middle and Late Jurassic volcanic arc rocks, formed in either an Andean-type arc or island-arc environment, and underlying ophiolite that are exposed continuously in the western metamorphic belt west of the Melones fault and in the southwestern part of the Oakhurst roof pendant (pl. 1; table 1). Within the study area, this terrane forms a north-northwest-trending belt about 160 km long and about 25 km wide (pl. 1). It is best 20 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA exposed along the Stanislaus and Merced Rivers in the western metamorphic belt (pl. 1). To the east, this ter- rane is either intruded by Late Jurassic and Early Cre- taceous granitic plutons of the Yosemite intrusive epoch (Evernden and Kistler, 1970) or faulted against the Merced River terrane along the Melones fault (pl. 1). To the west, this terrane shows onlapping by the Meso- zoic Great Valley sequence and Cenozoic sedimentary and volcanic rocks along the east edge of the Great Valley. The Melones fault is a northeast-dipping high- angle reverse fault along which the Foothills terrane is thrust under the Merced River terrane (pl. 1). The fault is marked by intense shearing and penetrative de- formation, juxtaposition and tectonic imbrication of the diverse lithologies of the two terranes, and tectonically dismembered portions of the ophiolites underlying the Foothills and Merced River terranes (Clark, 1960; Schweickert and Cowan, 1975). West of the Melones fault, rocks of the Merced River terrane only occur as fault-bounded tectonic blocks (pl. 1). STRATIGRAPHY The dominant protoliths in the volcanic arc rocks of the Foothills terrane are epiclastic mudstone and silt- stone, volcanic graywacke and conglomerate, basaltic with lesser andesitic flows, pillow lava, and breccia (table 1). Local abundant detritus, derived from rocks similar to those in the Calaveras Formation of the Merced River terrane, including fragments of argillite, quartzite, conglomerate, and quartz veins, form sparse but important constituents of the epiclastic rocks, graywacke and conglomerate (Clark, 1964; Behrman and Parkison, 1978). The major formations constituting the volcanic arc rocks are the Logtown Ridge, and Mariposa Formations, upper part of the Penon Blanco Volcanics, and the Salt Spring and Merced Falls Slates (Clark, 1964). The Mariposa Formation and Salt Spring and Merced Falls Slates are correlative units that occur in various fault blocks west of the Melones fault (Clark, 1964). The dominant lithologies in the underlying ophiolite are peridotite and dunite, gabbro, diabase, basaltic pillow lava and tuff-breccia, and chert (table 1). The major formations constituting the ophiolite are the Gopher Ridge, Copper Hill, and Peaslee Creek Vol- canics, the lower part of the Penon Blanco Volcanics, and mostly unnamed bodies of peridotite, dunite, gab- bro, and diabase, (Clark, 1964). The various volcanic rocks are correlative units that occur in various fault blocks west of the Melones fault (Clark, 1964). The volcanic arc rocks of the Foothills terrane are dated (pl. 1; table 1) by the occurrence of Middle and Late Jurassic ammonites and pelecypods (Imlay, 1961; Clark, 1964) (table 1). The ophiolite has been dated as (1) 182 to 190 my (Early Jurassic) or older by U-Pb zircon techniques on possibly syngenetic diorites that intrude the Penon Blanco Volcanics in the upper part of the ophiolite (Morgan, 1976; Morgan and Stern, 1977) and (2) about 200 to 300 my. (late Paleozoic and Trias- sic) by U—Pb zircon techniques on plagioclase granite in the ophiolite (Saleeby and others, 1979). Additional isotopic investigations are needed to determine the original age of this ophiolite. If the 200- to 300-m.y. age for the ophiolite is correct then the ophiolites at the bases of the Merced River and Foothills terranes would have identical ages and possibly the same origin (Saleeby and others, 1979). Because of intense defama— tion, the stratigraphic thickness of the volcanic arc rocks is not well known. The base of the ophiolite and the stratigraphic top of the Foothills terrane are fault bounded (Clark, 1964; Schweickert and Cowan, 1975). The minimum stratigraphic thickness of the volcanic arc rocks is 1,830 m; the minimum stratigraphic thickness of the stratified rocks forming the upper part of the ophiolite is 915 m (Clark, 1964) The volcanic arc rocks forming the upper part of the Foothills terrane are interpreted as either Andean-type arc volcanic rocks shed from the Middle and Late Juras- sic volcanic arc of the Sierra Nevada (Wetzel and Nok- leberg, 1976; Saleeby and others, 1979) or island-arc rocks formed oceanward of North America (Schweic- kert and Cowan, 1975; Schweickert, 1978). The underly- ing ophiolite of the Foothills terrane with an Early Ju- rassic age is interpreted as oceanic lithosphere that formed during a Late Triassic to Middle Jurassic period of rifting that occurred either immediately adjacent to the volcanic arc of the Sierra Nevada (Wetzel and Nok- leberg, 1976) or adjacent to an island are forming ocean- ward of North American (Schweickert and Cowan, 1975; Schweickert, 1978). The underlying ophiolite of the Foothills terrane with a late Paleozoic and Triassic age can be most readily interpreted as older oceanic lithosphere upon which volcanic debris was shed from the Middle and Late Jurassic volcanic arc of the Sierra Nevada (Saleeby and others, 1979). STRUCTURE The only major deformation of most of the Foothills terrane was the Late Jurassic Nevadan orogeny (table 2). Structures of this deformation consist of the north- northwest-trending Melones and Bear Mountain faults, numerous other parallel faults, parallel-trending moder- ately appressed to isoclinal major and minor folds, axial plane schistosity, lineation, and zones of melange, bro- ken formation, cataclasite, and mylonite (Duffield and Sharp, 1975; Behrman, 1978; Nokleberg and Kistler, 1980). The Bear Mountain fault is one of several major faults that trend parallel to the Melones fault within the Foothills terrane. The zones of melange, broken for- FOOTHILLS TERRANE 21 mation, cataclasite, and mylonite trend north-north- west, parallel to the major faults and folds. Axial planes of major and minor folds and parallel schistosities have average strikes of N. 15° to 40°'W. (table 2); major and minor fold axes generally plunge moderately south- east (Nokleberg and Kistler, 1980), and the folds are commonly asymmetric and have a clockwise or right-lat- eral sense of movement (Wetzel and Nokleberg, 1976; Nokleberg and Kistler, 1980). The age of the Nevadan orogeny in the western metamorphic belt is bracketed by the early Late Jurassic (Kimmeridgian) age of the volcanic arc epiclastic rocks and by the Late Jurassic and Early Cretaceous (148 to 132 m. y.) age of the grani- tic plutons of the Yosemite intrusive epoch that cross- cut the major folds and faults, including the Melones fault, formed in this orogeny (Bateman and others, 1963; Bateman and Clark, 1974). A combination of right-lateral strike-slip and reverse dip-slip movement for this deformation is indicated by the southeast plunge of major and minor fold axes and by the clockwise asymmetry of the major and minor folds (Wetzel and Nokleberg, 1976; Nokleberg and Kistler, 1980). The structures produced in the Late Jurassic Neva- dan orogeny are almost identical to those in the Foot- hills, Merced River, and Kings terranes (table 2) Nok- leberg and Kistler, 1980). In all three, major and minor faults and folds strike N. 20° to 40° W., parallel to the sutures. Major and minor fold axes plunge moderately to steeply southeast in the Kings and Merced River terranes and moderately southeast in the Foothills ter- rane. These relations indicate a large component of strike-slip movement on faults and penetrative slip sur- faces in all three terranes, and a minor to moderate amount of reverse dip—slip movement on similar sur- faces in the Foothills terrane. EVIDENCE FOR MELONES FAULT The evidence for the Foothills fault system, of which the Melones fault is the easternmost branch, is excel- lently discussed by Clark (1960, 1964). The main evi- dence for the Melones fault is: (1) mappable belts of cataclastically deformed rock; (2) thin elongate pods of highly deformed serpentine, and ultramafic and mafic igneous rocks; (3) pervasive shearing; and (4) juxtaposi- tion of Upper Jurassic rocks of the Foothills terrane against the Calaveras Formation of the Merced River terrane. West of the Melones fault, the Calaveras For- mation does not occur stratigraphically under the Foothills terrane, but only as fault-bounded slivers in the melange of the Foothills fault system (Clark, 1960, 1964; Duffield and Sharp, 1975; Schweickert, 1978). One such large tectonic sliver of the Calaveras Formation occurs west of the Melones fault in the area west of San Andreas (pl. 1). CRITICAL DIFFERENCES FROM MERCED RIVER TERRANE The main differences between the Foothills and Merced River terranes are (tables 1, 2): (1) a Middle and Late Jurassic age for the former volcanic and sedi- mentary rocks of the Foothills terrane compared to a late Paleozoic age for the former sedimentary rocks of the Merced River terrane; (2) the occurrence of mainly former volcanic and epiclastic rocks formed in an An- dean-type arc environment in the Foothills terrane com- pared to mainly former shale, quartzite, limestone, and marl in the former sedimentary rocks of the Merced River terrane; (3) the occurrence of two older genera- tions of structures in the Merced River terrane; and (4) the occurrence of a Tethyan fauna in the sedimenta- ry rocks of the Merced River terrane. These differences and the evidence for the Melones fault between the Foothills and Merced River terranes indicate the Foot- hills terrane is allochthonous and has been accreted to the Merced River terrane. ACCRETION OF FOOTHILLS TERRANE The Melones fault, part of the structures formed in the Late Jurassic Nevadan orogeny, is the locus of accretion of the Foothills and Merced River terranes (pl. 1). Accretion and codeformation of both terranes occurred along north-northwest trends (Nokleberg, 1975; Schweickert and Cowan, 1975; Schweickert, 1978). Movement during accretion along the Melones fault consisted of predominantly right lateral strike slip with minor reverse dip slip (Nokleberg, 1975; Wetzel and Nokleberg, 1976; Nokleberg and Kistler, 1980). These data indicate the Foothills terrane, like the Merced River and Kings terranes, was migrating north- ward along the western margin of the volcanic arc of the Sierra Nevada before accretion. Intense deforma- tion occurred during accretion with development of cataclasite, mylonite, broken formation, and severe tec- tonic dismemberment of both terranes along the Melones fault. The Melones and Bear Mountain faults were intruded immediately after formation by various Late Jurassic granitic plutons of the Yosemite intrusive epoch. These relations suggest the Melones fault repre- sents a major accretionary suture between the Merced River and Foothills terranes and does not represent a former subduction zone as suggested by Schweickert and Cowan (1975), because the fault is intruded by nearly contemporaneous granitic magmas of the same deformational and magmatic cycle. The Foothills terrane is interpreted as having formed 22 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA during: (1) Middle and Late Jurassic island-arc vol- canism that occurred over slightly older ophiolite (Schweickert and Cowan, 1975; Schweickert, 1978); or (2) shedding of volcanic debris from the Middle and Late Jurassic volcanic arc of the Sierra Nevada and from the Calaveras Formation in the Merced River ter- rane onto ophiolite formed in Late Triassic to Jurassic ocean-floor rifting (Wetzel and Nokleberg, 1976) or onto ophiolite formed in the late Paleozoic to Triassic (Saleeby and others, 1979). The occurrence of abundant detritus, derived from rocks similar to those in the Calaveras Formation in the Merced River terrane, in the epiclastic rocks of the Foothills terrane (Clark, 1964; Behrman and Parkison, 1978) is strong evidence that the ophiolite forming the base of the Foothills ter- rane was adjacent to some portion of the volcanic and plutonic arc of the Sierra Nevada during deposition of the epiclastic rocks. After deposition of the epiclastic rocks, the Foothills terrane was telescoped and ac- creted to the Merced River terrane during the Late Jurassic Nevadan orogeny. Considerable right-lateral movement may have occurred during accretion in the Nevadan orogeny (Saleeby and others 1978; Nokleberg and Kistler, 1980). During the waning stages of the orogeny, the Foothills and Merced River terranes were welded together along the accretionary suture of the Melones fault by various Late Jurassic and Early Creta- ceous granitic plutons of the Yosemite intrusive epoch (fig. 1) (Evernden and Kistler, 1970; Bateman and Clark, 1974). MESOZOIC AND CENOZOIC ACCRETIONARY TECTONICS ALONG THE WESTERN MARGIN OF NORTH AMERICA One exciting area of geologic research in the last de- cade has been the development of the theory of micro- plate tectonics and the application of this theory to the tectonic evolution of the North American Cordillera during the Mesozoic and Cenozoic. Within the last few years several important articles have been published on the Mesozoic and Cenozoic accretionary tectonics of the North American Cordillera. These articles include studies of the accreted terrane of Wrangellia by Jones and others (1977), the collage of accreted terranes in the North American Cordillera from central British Col- umbia to central California by Davis and others (1978), the accretionary tectonics of the Western United States by Hamilton (1978), and the accreted terranes of the southern Sierra Nevada by Saleeby and others (1978). These papers stress that substantial portions of the western margin of the North American Cordillera con- sists of a collage of microplates or terranes that were accreted during the Mesozoic or Cenozoic. In a similar manner, a major purpose of this paper is to list and discuss data for the hypothesis that the wall rocks of the central Sierra Nevada batholith consti— tute a collage of terranes that were accreted onto the margin of an active volcanic and plutonic arc in the Tri- assic and Jurassic. However, a secondary purpose of this paper is to show there is a Cenozoic, that is a young analog, to the Mesozoic Sierra Nevada. After a review of recent studies of the North American Cor- dillera, the region of southern Alaska was selected as the best—known example of a young analog. CENOZOIC ACCRETIONARY TECTONICS OF SOUTHERN ALASKA The tectonic synthesis of the geology of southern Alaska has been the subject of various summary studies. Plafker (1972), Csejtey (1976), Jones and others (1977, 1981), Jones and Silberling (1979), Beikman (1980), show that southern Alaska, along with much of the entire North American Cordillera, consists of a series of accreted terranes into which have been in— truded granitic rocks of pre- and post-acretion ages. Periodic intrusion of granitic rocks in southern Alaska from late Paleozoic to late Tertiary is discussed by Smith and Lanphere (1971), Reed and Lanphere (1973), Richter and others (1975), and Hudson (1979). Hudson (1979) shows that many of the Mesozoic plutonic rocks of southern Alaska can be grouped into five calc-alkalic plutonic belts and further suggests that some of the plutonic belts may represent magmatic arcs that mig- rated or were rafted relatively northward to their pre- sent position in southern Alaska. The important features of southern Alaska, as deter- mined from the cited studies, are as follows (fig. 2). A collage of accreted terranes, such as the Wrangellia, Nixon fork, Chugach, Peninsular, and Prince William terranes, form the basement to the region. The best presently know terrane in southern Alaska is Wrangel- lia (fig. 2), which consists of late Paleozoic island-arc volcanic rocks and the overlying Triassic Nikolai Greenstone which initially formed near the Triassic equator and subsequently moved northward and was accreted onto North America in late Mesozoic or early Cenozoic time (Jones and others, 1977). The terranes are bounded by ancient and modern thrust and strike- slip faults or sutures such as the Denali, Castle Moun- tain, Border Ranges, and Contact faults. Volcanic and plutonic rocks of mainly Jurassic to Recent age have been intruded into or onto various terranes either be- fore or after accretion. Andean-type arc volcanism and contemporaneous plutonism appears to have occurred simultaneously with accretion of terranes. Belts of CONCLUSIONS , 23 nearly contemporaneous volcanic and plutonic rock ap- pear to have been displaced along major faults and su- tures within the volcanic and plutonic arc. Some of the older plutonic rocks may have been accreted along with enclosing terranes. Holocene volcanic rocks overlap ad- jacent terranes; locally, however, Quaternary volcanic rocks are offset by movement on sutures. Some su- tures, such as the Denali, Border Ranges, and Contact faults, are presently active. The modern sutures appear to be part of a complex system of faulting related to subduction and transform faulting south and east of southern Alaska. The Aleu- tian megathrust (fig. 2) is the surface projection of the subduction zone that dips gently northwestward under southern Alaska and separates the North American and Pacific plates (Pflaker, 1972). In the western part of southern Alaska, sutures trend northeast, parallel to the Aleutian megathrust, whereas in the eastern part, the sutures trend east-west or northwest, more parallel to the continental margin (fig. 2). Northwest-southeast convergence along the Aleutian megathrust appears to be the cause of right-lateral strike-slip movement along sutures, such as the Denali fault, in eastern Alaska. Convergence between two major plates along the Aleu- tian megathrust appears to be causing simultaneous for- mation of the late Cenozoic magmatic arc and displace- ment of terranes and volcanic and plutonic rocks. In summary, the above data and relations indicate that in Cenozoic time, a collage of tectono-stratigraphic ter- ranes were accreted during volcanism and plutonism along an actively deformed continental margin in south- ern Alaska. This tectonic setting is herein defined as an Alaskan-type arc. COMPARISON OF THE MESOZOIC SIERRA NEVADA AND SOUTHERN ALASKA There are several similarities between the Mesozoic Sierra Nevada and southern Alaska. First, both consist of a series of terranes separated by sutures. Second, plutonism and volcanism were interspersed with accre— tion of terranes in both areas. Third, sutures in both areas are predominantly right lateral strike-slip faults. Fourth, some terranes in both areas are allochthonous with respect to North America. Fifth, some terranes in both areas formed along the continental margin of North America, but subsequently migrated along su- tures into their present position. Sixth, some terranes are welded together by granitic and volcanic rocks. And seventh, in both areas, some granitic and volcanic rocks are displaced by movements on sutures. There are also several contrasts between the Mesozo- ic Sierra Nevada and southern Alaska, mostly resulting from comparing a modern-day arc with a Mesozoic vol- canic and plutonic are. First, granitic rocks are much more extensive and wall rocks are much less extensive in the Sierra Nevada than in southern Alaska, probably because of a greater depth of erosion in the Sierra Nevada. Second, displacement is still occurring along sutures in southern Alaska, whereas movement has ceased along sutures in the Sierra Nevada. Third, ter- ranes are commonly longer and wider in southern Alaska than in the Sierra'Nevada (fig. 2); however, some Alaskan terranes, such as the Maclaren terrane, are of comparable size to terranes in the Sierra Nevada (pl. 1; fig. 2). And fourth, the geologic history of ter- ranes in the two areas is very dissimilar. In summary, there is a very great similarity between the tectonic frameworks of southern Alaska and the Mesozoic Sierra Nevada. This great similarity strongly suggests that the Mesozoic Sierra Nevada formed in a manner analogous to that of southern Alaska and that the Mesozoic Sierra Nevada probably represents an Alaskan-type arc in which a series of terranes were accreted during volcanism and plutonism along an ac- tively deformed continental margin. CONCLUSIONS Stratigraphic and structural analysis of the age belts of wallrocks in the central Sierra Nevada indicates that the wall rocks constitute a collage of tectono-strati- graphic terranes that were accreted in an Alaskan-type are setting similar to that of southern Alaska in Cenozo- ic time. Timing of accretions of the various terranes range from Triassic to Late Jurassic (fig. 1). Movement and accretion along the Owens Valley fault, which bounds the easternmost Owens terrane, may have ceased as late as late Tertiary. Available data indicate that the four eastern terranes, the Owens, High Sierra, Goddard, and Kings terranes, originated along the western margin of North America before. detachment, movement, and accretion at their present loci. The two westernmost terranes, the Merced River and Foothills terranes, have origins far removed with respect to con- tinental North America. During the Triassic Sonoman orogeny, the High Sierra terrane was thrust over and accreted to the Owens terrane. The Goddard terrane was accreted to the High Sierra terrane during the Late Jurassic Neva- dan orogeny. The Merced River terrane and the cor- relative Cache Creek terrane to the north in British Columbia are exotic with respect to North America and were formed in the warm-water, equatorial Tethys re- gion of the late Paleozoic, before detachment, move- ment, and amalgamation along the western margin of North America. The Merced River terrane was amalga- mated to the Kings terrane along the Foothill suture 24 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA 156° 141° Yukon—Tanana 34° Delta Junction 0 K STRANDQ. ES CREE KI “\$pingst°”"<.‘“95§?n Broad Pass fiT' , o Medfra Nikon Fork 0 50 100 150 KILUMHERS L_____l—_|__l FIGURE 2,—Simplified geologic map of southern Alaska showing major features of tectono-stratigraphic terranes, faults, Mesozoic and Cenozoic igneous rocks, and Aleutian megathrust. Arrows indicate the present relative horizontal component of plate motion. Adapted from Plafker (1972), Jones and Silberling (1979), Beikman (1980), and Jones and others (1980). during the Middle Jurassic. This amalgamation coin- California to Sonora, Mexico. In the central Sierra cided with, and may have caused, the left-lateral offset Nevada, the Foothill suture may be the left-lateral 0f Precambrian crystalline terranes from southeastern megashear of Silver and Anderson (1974). After amal- REFERENCES CITED 25 EXPLANATION IGNEOUS ROCKS Quaternary volcanic rocks Quaternary and Tertiary volcanic rocks , Tertiary volcanic rocks Tertiary intrusive rocks Tertiary and Cretaceous volcanic rocks Tertiary and Cretaceous intrusive rocks _ Cretaceous intrusive rocks Cretaceous and Jurassic intrusive rocks Jurassic intrusive rocks SEDIMENTARY ROCKS Undifferentiated Cenozoic and Upper Cretaceous deposits Deformed Lower Cretaceous and Jurassic flysch Contact Fault bounding terrane—Dotted where inferred H—‘ Megathrust separating Pacific and North American plates tectono-stratigraphic FIGURE 2.—Continued gamation of the Merced River and Kings terranes, both terranes were offset by right-lateral strike-slip move- ment from the site of origin of the Kings terrane in the Mojave region to their present loci in the central Sierra Nevada. The westernmost, Foothills, terrane formed during a Middle to Late Jurassic period of An- dean-type arc volcanism in the central Sierra Nevada. Debris and flows from the volcanic arc and detritus from rocks similar to those in the Calaveras Formation in the Merced River terrane were shed onto oceanic lithosphere of either Late Triassic to Middle Jurassic or late Paleozoic and Triassic age. Telescoping and accretion of the Foothills terrane and northward migra- tion of the Foothills, Merced River, and Kings terranes occurred during the Late Jurassic Nevadan orogeny. REFERENCES CITED Baird, A. K., 1962, Superposed deformations in the central Sierra Nevada foothills east of the Mother Lode: California University Publications in Geological Sciences, v. 42, p. 1—70. Bateman, P. C., 1965, Geology and tungsten mineralization of the Bishop district, California: U.S. Geological Survey Professional Paper 470, 208 p. Bateman, P. C., and Clark, L. D. 1974, Stratigraphic and stuctural setting of the Sierra Nevada batholith, California: Pacific Geol- ogy, v. 8, p. 79—89. Bateman, P. 0., Clark, L. D., Huber, N. K., Moore, J. G., and Rinehart, C. D., 1963, The Sierra Nevada batholith—a synthesis of recent work across the central part: U.S. Geological Survey Professional Paper 414—D, p. D1—D46. Bateman, P. C., and Eaton, J. P., 1967, Sierra Nevada batholith: Science, v. 158, p. 1407—1417. Bateman, P. C., and Moore, J. G., 1965, Geologic map of the Mount Goddard quadrangle, Fresno and Inyo Counties, California: U.S. Geological Survey Geologic Quadrangle Map GQ—429, scale 1:62,500. Beck, Myrl, Cox, Allan, and Jones, D. L., 1980, Mesozoic and Cenozo— ic microplate tectonics of western North American: Geology, v. 8, p. 454—456. Behrman, P. G., 1978, Pre-Callovian rocks, west of the Melones fault zone, central Sierra Nevada foothills, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeog'raphy Symposium 2, p. 337—348. Behrman, P. G., and Parkison, G. A., 1978, Paleogeographic signifi- cance of the Callovian to Kimmeridgian strata, central Sierra Nevada foothills, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 349—360. Beikman, H. M., 1980 Geologic map of Alaska: U.S. Geological Sur- vey Map, scale 12,500,000. Best, M. G., 1963, Petrology and structural analysis of metamorphic rocks in the southwestern Sierra Nevada foothills, California: California University Publications in Geological Sciences, v. 42, p. 111—158. Brook, C. A., 1972, Stratigraphy and superposed deformations of parts of the Saddlebag Lake roof pendant, Sierra Nevada, California [M.A. thesis]: Fresno, California State University, 47 1974, Nature and significance of superposed folds in the Saddlebag Lake roof pendant, Sierra Nevada, California: Geologi- cal Society of America Abstracts with Programs, v. 6, pt. 3, p. 147—148. —1977, Stratigraphy and structure of the Saddlebag Lake roof pendant, Sierra Nevada, California: Geological Society of America Bulletin, v. 88, p. 321—334. Brook, C. A., Gordon, Mackenzie, Jr. Mackey, M. J ., and Chetelat, G. F., 1979, Fossiliferous upper Paleozoic rocks and their struc- tural setting in the Ritter Range and Saddlebag Lake roof pen- dants, central Sierra Nevada, California: Geological Society of America Abstracts with Programs, v. 11, p. 70. Brook, C. A., Nokleberg, W. J., and Kistler, R. W., 1974, Nature of the angular unconformity between the Paleozoic metasedimen- tary rocks and the Mesozoic metavolcanic rocks in the eastern Sierra Nevada, California: Geological Society of America Bulle- tin, v. 85, p. 571—576. Burchfiel, B. C., and Davis, G. A., 1972, Structural framework and evolution of the southern part of the Cordilleran orogen, western United States: American Journal of Sciences, v. 272, p. 97—118. Busby—Spera, C. J., Goodin, S. E., and Saleeby, J. B., 1980, Early Mesozoic quartz, sandstone—volcanic arc associations in the Kaweah River area, southern Sierra Nevada, California: Geologi- cal Society of America Abstracts with Programs, v. 12, p. 99—100. Chen, J. H., and Moore, J. G., 1979, Late Jurassic Independence dike swarm in eastern California: Geology, v. 7, p. 129—133 26 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA Chesterman, C. W., 1975, Geology of the Matterhorn Peak Quad- rangle, Mono and ’hiolumne Counties, California: California Divi- sion of Mines and Geology Map Sheet 22, scale 1:62,500. Christensen, M. N., 1963, Structure of metamorphic rocks at Mineral King, California: California University Publications in Geological Sciences, v. 42, p. 159—198. Clark, L. D., 1954, Geology and mineral deposits of Calaveritas quad- rangle, Calaveras County, California: California Division of Mines and Geology Special Report 40, 23 p. 1960, Foothills fault system, western Sierra Nevada, Califor- nia: Geological Society of America Bulletin, v. 71, p. 483—496. 1964, Stratigraphy and structure of part of the western Sierra Nevada metamorphic belt, California: U.S. Geological Survey Professional Paper 410, 70 p. ——-1970, Geology of the San Andreas l5~minute quadrangle, Calaveras County, California: California Division of Mines and Geology Bulletin 195, 23 p. 1976, Stratigraphy of the north half of the western Sierra Nevada metamorphic belt, California: U.S. Geological Survey Professional Paper 923, 26 p. Crowder, D. F., and Sheridan, M. F., 1972, Geologic map of the White Mountain Peak quadrangle, Mono County, California: U.S. Geological Survey Geologic Quadrangle Map GQ—1012, scale 1:62,500. Csejtey, Bela, J r., 1976, Tectonic implications of a late Paleozoic vol- canic arc in the Talkeetna Mountains, south-central Alaska: Geol— ogy, v. 4, p. 49—52. Davis, G. A., Monger, J. W. H., and Burchfiel, B. C., 1978, Mesozoic construction of the Cordilleran “collage,” central British Colum- bia to central California, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 1—32. Douglass, R. C., 1967, Permian Tethyan fusulinids from California: U.S. Geological Survey Professional Paper 593—A, p. A1—A13. Duffield, W. A., and Sharp, R. V., 1975, Geology of the Sierra foot- hills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p. Dunne, G. C., Gulliver, R. M., and Sylvester, A. G., 1978, Mesozoic evolution of rocks of the White, Inyo, Argus, and Slate Ranges, eastern California, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 189—208. Durrell, Cordell, 1940, Metamorphism in the southern Sierra Nevada northeast of Visalia, California: California University Publica- tions in Geological Sciences, v. 25, p. 1—118. Eric, J. H., Stromquist, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angels Camp and Sonora quadrang- les, Calaveras and Tuolumne Counties, California: California Di- vision of Mines and Geology Special Report 41, 55 p. Evernden, J. F., and Kistler, R. W., 1970, Chronology of emplace- ment of Mesozoic batholithic complexes in California and western Nevada: U.S. Geological Survey Professional Paper 623, 42 p. Fiske, R. S., and Tobisch, O. T., 1978, Paleogeographic significance of volcanic rocks of the Ritter Range pendant, central Sierra Nevada, California, in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the westerm United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 209—222. Girty, G. H., 1977a, Cataclastic rocks in the Boyden Cave roof pen- dant, central Sierra Nevada, California: Geological Society of America Abstracts with Programs, v. 9, p. 423. 1977b, Multiple regional defamation and metamorphism of the Boyden Cave roof pendant, central Sierra Nevada, California [M.A. thesis]: Fresno, California State University, 82 p. Hamilton, Warren, 1969, Mesozoic California and underflow of Pacific mantle: Geological Society of America Bulletin, v. 80, p. 2409— 2430. 1978, Mesozoic tectonics of the western United States, in How- ell, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleon- tologists and Mineralogists Pacific Coast Paleogeography Sym- posium 2, p. 33—70. Huber, N. K., and Rinehart, C. D., 1965, Geologic map of the Devils Postpile quadrangle, California: U.S. Geological Survey Quad- rangle Map GQ—437. Hudson, Travis, 1979, Mesozoic plutonic belts of southern Alaska: Geology v. 7., p. 230—234. Imlay, R. W., 1961, Late Jurassic ammonites from the western Sierra Nevada, California: U.S. Geological Survey Professional Paper 374—D, p. D1—D30 Jones, D. L., and Moore, J. G., 1973, Lower Jurassic ammonite from the south-central Sierra Nevada, California: U.S. Geological Sur- vey Journal of Research, v. 1, p. 453—458. Jones, D. L., and Silberling, N. J ., 1979, Mesozoic stratigraphy—The key to tectonic analysis of southern and central Alaska: U.S. Geological Survey Open-File Report 79—1200, 37 p. Jones, D. L., Silberling, N. J ., and Hillhouse, J ., 1977, Wrangellia—A displaced terrane in northwestern North America: Canadian Journal of Earth Sciences, v. 14, p. 2565—2577. Jones, D. L., Silberling, N. J., Plaflier, George, and Berg, H. C., 1981, Tectono—stratigraphic terranes of Alaska: U.S. Geological Survey Open-File Report 81—792, 20 p. Koenig, J. B., 1963, Geologic Map of California—Walker Lake Sheet (l:250,000): California Division of Mines and Geology. Kistler, R. W., 1966a, Structure and metamorphism in the Mono Craters quadrangle, Sierra Nevada, California: U.S. Geological Survey Bulletin 1221—E, p. E1—E53. 1966b, Geologic map of the Mono Craters quadrangle, Mono and 'I‘uolumne Counties, California: U.S. Geological Survey Geologic Quadrangle Map GQ—462, scale 1:62,500. Kistler, R. W., and Bateman, P. C., 1966, Stratigraphy and structure of the Dinkey Creek roof pendant in the central Sierra Nevada, California: U.S. Geological Survey Professional Paper 524—B, p. B1—Bl4. Kistler, R. W., Evernden, J. F., and Shaw, H. R., 1971, Sierra Nevada plutonic cycle; Part 1, Origin of composite granitic batholiths: Geological Society of America Bulletin, v. 82, p. 853— 868. Kistler, R. W., and Nokleberg, W. J ., 1980, Carboniferous rocks of the eastern Sierra Nevada: U.S. Geological Survey Professional Paper 1110—00, p. 0021 to 0026. Kistler, R. W., and Peterman, Z. E., 1978, Reconstruction of crustal blocks in California on the basis of initial strontium isotopic com- position of Mesozoic granitic rocks: U.S. Geological Survey Pro- fessional Paper 1071, 17 p. Kistler, R. W., Robinson, A. C., and Fleck, R. J., 1980, Mesozoic right-lateral fault in eastern California: Geological Society of America Abstracts with Programs, v. 12, p. 115. Krauskopf, K. B., 1953, Tungsten deposits of Madera, Fresno and ’I‘ulare Counties, California: California Division of Mines and Geology Special Report 35, 83 p. Lockwood, J. P., and Lydon, P. A., 1975, Geologic map of the Mount Abbot quadrangle, central Sierra Nevada, California: U.S. Geological Survey Geologic Quadrangle Map GQ—1155, scale 1:62,500. REFERENCES CITED 27 MacDonald, G. A., 1941, Geology of the western Sierra Nevada be- tween the Kings and San Joaquin Rivers, California: California University Publications in Geological Sciences, v. 26, p. 215—286. Mannion, L. E., 1960, Geology of the La Grange quadrangle, Califor- nia [Ph. D. thesis]: Stanford, Stanford, Univ., 173 p. Matthews, R. A., and Burnett, J. L., 1965, Geologic Map of Califor— nia—Fresno Sheet: California Division of Mines and Geology, scale 1:250,000. Moore, J. G., 1973, Geology of the Mount Pinchot quadrangle, south- ern Sierra Nevada, California: U.S. Geological Survey Bulletin 1130, 152 p. 1978, Geologic Map of the Marion Peak quadrangle, Fresno County, California: U.S. Geological Survey Quadrangle Map GQ— 1399, scale 1:62,500 Moore, J. G., and Dodge, F. C., 1962, Mesozoic age of metamorphic rocks in the Kings River area, southern Sierra Nevada, Califor- nia: U.S. Geological Survey Professional Paper 450—B, p. B19—21. Moore, J. G., and Hopson, C. A., 1961, The Independence dike swarm in eastern California: American Journal of Science, v. 259, p. 241—259. Moore, J. G., and Marks, L. Y., 1972, Mineral Resources of the High Sierra Primitive Area. Callifornia, with a section on Aeromagne- tic interpretation, by H. W. Oliver: U.S. Geological Survey Bulle- tin 1371—A, p. Al—A40. Moore, J. G., Nokleberg, W. J., Chen, J. H., Girty, G. H., and Saleeby, J. B., 1979, Geologic guide to the Kings Canyon High- way: Geological Society of America, Cordilleran Section Meeting, 33 p. Moore, J. N., and Foster, C. T., Jr., 1980, Lower Paleozoic metasedimentary rocks in the east—central Sierra, California: Cor— relation with Great Basin formations: Geological Society of America Bulletin, v. 91, p. 37—43. Morgan, B. A., 1976, Geologic map of the Chinese Camp and Moccasin quadrangles, Tuolumne County, California: U.S. Geological Sur- vey Geologic Quadrangle Map MF—840, scale 1:24,000. Morgan, B. A., and Rankin, D. W., 1972, Major structural break between Paleozoic and Mesozoic rocks in the eastern Sierra Nevada, California: Geological Society of America Bulletin, v. 83, p. 373945744. Morgan, B. A., and Stern, T. W., 1977, Chronology of tectonic and plutonic events in the western Sierra Nevada, between Sonora and Mariposa, California: Geological Society of America Abstracts with Programs, v. 9, p. 471—472. Nokleberg, W. J ., 1970, Geology of the Strawberry mine roof pen- dant, central Sierra Nevada. California [Ph. D. thesis]: Santa Barbara, University California, 156 p. 1975, Structural analysis of a collision between an oceanic plate and a continental plate preserved along the lower Kings River in the Sierra Nevada: Geological Society of America Abstracts with Programs, v. 7, pt. 3, p. 357—358. 1979, Accreted microplates in the central Sierra Nevada, California: Geological Society of America Abstracts with Pro— grams, v. 11, p. 120. ‘ —1981, Stratigraphy and structure of the Strawberry mine roof pendant, central Sierra Nevada, California: U.S. Geological Sur- vey Professional Paper 1154, 18 p. Nokleberg. W. J., and Kistler, R. W., 1980, Paleozoic and Mesozoic deformations in the central Sierra Nevada, California: U.S. Geological Survey Professional Paper 1145, 24 p. Peck, D. L., Stern, T., and Kistler, R. W., 1977, Penecontemporane- ous volcanism and intrusion in the Sierra Nevada batholith, California: IASPEI/IAVCEI Joint Assembly, Durham, Abstracts. Plafker, George, 1972, Alaskan earthquake of 1964 and Chilean earth- quake of 1960: Implications for are tectonics: Journal of Geophysi- cal Research, v. 77, p. 901—925. Reed, B. L., and Lanphere, M. A., 1973, Alaska—Aleutian Range batholith: Geochronology, chemistry, and relation to circum~ Pacific plutomism: Geological Society of America Bulletin, v. 84, p. 2583—2610. Richter, D. H., Lanphere, M. A., and Matson, N. A., Jr., 1975, Granitic plutonism and metamorphism, eastern Alaska Range, Alaska: Geological Society of America Bulletin, v. 86, p. 819—829. Rinehart, C. D., and Ross, D. C., 1957, Geologic map of the Casa Diablo Mountain quadrangle, California: U.S. Geological Survey Geologic Quadrangle Map GQ—99. 1964, Geology and mineral deposits of the Mount Morrison quadrangle, Sierra Nevada, California: U.S. Geological Survey Professional Paper 385, 106 p. Ross, D. C., 1958, Igneous and metamorphic rocks of parts of Sequoia and Kings Canyon National Parks, California: California Division of Mines and Geology Special Report 53, 24 p. Russell, L. R., and Cebull, S. E., 1977, Structural-metamorphic chronology in a roof pendant near Oakhurst, California: Implica- tions for the tectonics of the western Sierra Nevada: Geological Society of America Bulletin, v. 88, p. 1530—1534. Russell, S. J ., 1976, Geology of the Mount Dana roof pendant, central Sierra Nevada, California [M.A. thesis]: Fresno, California State University, 71 p. Russell, S. J ., and Nokleberg, W. J ., 1974, The relation of superposed deformations in the Mount Morrison roof pendant to the regional tectonics of the Sierra Nevada: Geological Society of America Abstracts with Programs, v. 6, p. 245. Russell, S. R., and Nokleberg, W. J ., 1977, Supefimposition and tim- ing of deformations in the Mount Morrison roof pendant in the central Sierra Nevada, California: Geological Society of America Bulletin, v. 88, p. 335—345. Saleeby, J. B., 1978, Kings River ophiolite, southwest Sierra Nevada foothills, California: Geological Society of American Bulletin, v. 89, p. 617—636. 1979, Kaweah serpentinite melange, southwest Sierra Nevada foothills, California: Geological Society of America Bulletin, v. 90, p. 29—46. Saleeby, J. B., Goodin, S. E., Sharp, W. D., and Busby, C. J., 1978, Early Mesozoic paleotectonic-paleogeographic reconstruction of the southern Sierra Nevada region, in H0well, D. G., and McDougall, K. A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 311—336. Saleeby, J. B., Mattinson, J. M., and Wright, J. E., 1979, Regional ophiolite terranes of California-Vestiges of two complex ocean floor assemblages: Geological Society of America Abstracts with Programs, v. 11, p. 509. Saleeby, J. B., and Moores, E. M., 1979, Zircon ages on northern Sierra Nevada ophiolite remnants and some possible regional cor- relations: Geological Society of America Abstracts with Pro- grams, v. 11, p. 125. Saleeby, Jason, and Sharp, Warren, 1980, Chronology of the structur- al and petrologic development of the southwest Sierra Nevada foothills: Geological Society of America Bulletin, Part II, v. 91, p. 1416—1535. Schweickert, R. A., 1978, Triassic and Jurassic paleogeography of the Sierra Nevada and adjacent regions, California and western Nevada, in Howell, D. G., and McDougall, K. A., eds., Mesozoic palegeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 361—384. 28 WALLROCKS OF THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA Schweickert, R. A., and Cowan, D. S., 1975, Early Mesozoic tectonic evolution of the western Sierra Nevada, California: Geological Society of America Bulletin, v. 86, p. 1329—1336. Schweickert, R. A., Saleeby, J. B., Tobisch, 0. T., and Wright, W. H., III, 1977, Paleotectonic and paleogeographic significance of the Calaveras Complex, western Sierra Nevada, California, in Stewart, J. H., Stevens, C. H., and Fritsche, A. E., eds., Paleo- zoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists Pacific Coast Paleogeography Symposium 1, p. 381—394. Sharp, W. D., and Saleeby, J. B., 1979, The Calaveras Formation and syntectonic Middle Jurassic plutons between the Stanislaus and Tuolumne Rivers, California: Geological Society of America Abstracts with Programs, v. 11, p. 127. Silberling, N. J ., and Roberts, R. J ., 1962, Pre-Tertiary stratigaphy and structure of northwestern Nevada: Geological Society of America Special Paper 72, 58 p. Silver, L. T., and Anderson, T. H., 1974, Possible left-lateral early to middle Mesozoic disruption of the southwestern North America craton margin: Geological Society of America Abstracts with Programs, v. 6, p. 955—956. Smith, A. R., 1964, Geologic Map of California—Bakersfield Sheet (1:250,000): California Division of Mines and Geology. Smith, T. E., and Lanphere, M. A., 1971, Age of sedimentation, plutonism, and regional metamorphism in the Clearwater Moun- tains region, central Alaska: Isochron West, no. 2, p. 17—20. RETURN gAgTH SCIENCES LIBRARY '3 I‘AVC I3 U.S. GOVERNMENT PRINTING OFFICE: 676-041l31—1983 Sohl, N. F., and Wright, W. B., 1980, Changes in stratigraphic nomenclature by the US. Geological Survey, 1979: U.S. Geologi- cal Survey Bulletin 1502—A, p. Al—A2. Speed, R. C., 1979, Collided Paleozoic microplate in the western United States: Journal of Geology, v. 87, p. 279—292. Strand, R. G., 1967, Geologic Map of California—Mariposa Sheet: California Division of Mines and Geology, scale 1:250,000. Strand, R. G., and Koenig, J. B., 1965, Geologic Map of California— Sacramento Sheet: California Division of Mines and Geology, scale l:250,000. Taliaferro, N. L., and Solari, A. J ., 1948, Geology of the Copperopolis quadrangle, California: California Division of Mines and Geology Bulletin, v. 145, 64 p. Tobisch, O. T., and Fiske, R. S., 1976, Significance of conjugate folds and crenulations in the central Sierra Nevada, California: Geolog- ical Society of America Bulletin, v. 87 , p. 1411-1420. Tobisch, O. T., Fiske, R. S., Sacks, S., and Taniguchi, D., 1977, Strain in metamorphosed volcaniclastic rocks and its bearing on the evolution of orogenic belts: Geologic Society of America Bulle- tin, v. 88, p. 23—40. Wetzel, N., and Nokleberg, W. J ., 1976, Plate tectonic and structural relations for the origin and deformation of the western metamor- phic belt along the margin of the central Sierra Nevada batholith: Geological Society of America Abstracts with Programs, v. 8, p. 420. or’rh Sciences Bldg. 642-2997 \\\ m‘ yéf‘""é‘i:iéz; ”an; Core KM—S, a Surface-to-Bedrock Record of Late Cenozoic ‘ Sedimentation in Searles Valley, California By GEORGE I. SMITH, VIRGIL J. BARCZAK, GAIL F. MOULTON, and JOSEPH C. LIDDICOAT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1256 The basin sedimentation history of Searles (dry) Lake from Miocene or early Pliocene time to the present UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES C. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Core KM—3, a surface-to—bedrock record of late Cenozoic sedimentation in Searles Valley, California. (Geological Survey professional paper 1256) Bibliography Supt. of Docs. no.: I 19.16:1256 1. Rocks, sedimentary. 2. Geology, stratigraphic—Cenozoic. 3. Geology—California—Searles Valley. 4. Borings—California—Searles Valley. 1. Smith, George Irving, 1927— II. Title. III. Series: United States. Geological Survey. Professional Paper 1256. QE471.C73 1982 551.7’8’0979495 82—600347 For sale by the Distribution Branch, US. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Page Abstract 1 History of sedimentation—Continued Introduction 1 From Miocene or early Pliocene time to 3.18 m.y. B.P.— History of coring in Searles Valley .............................................. 3 alluvial sand and gravel .................................................... 16 Methods of study 3 3.18-2.56 m.y. B.P.—Unit 1, Mixed Layer .......................... 16 Lithology 3 2.56-1.94 m.y. B.P.—Unit H ........................... Chemistry 7 1.94—1.27 m.y. B.P.—Unit G .................................................. 16 Mineralogy 9 1.27—1.00 m.y. B.P.—Unit F .................................................. 17 Dating 10 1.00—0.57 m.y. B.P.—Unit D+E ..17 Acknowledgments ' 11 0.57—0.31 m.y. B.P—Unit C ............ ..17 Stratigraphy 11 0.31—0.13 m.y. B.P.—Units A and B .. ...17 Summary 11 Rates of sedimentation 18 Description of units of the Mixed Layer ............................ 12 Trends in sedimentation and their significance ........................ 18 Unit A 12 Change from alluvial to lacustrine sediment .................... 18 Unit B ........ l3 Increases in the percentage of chemical sediment .......... 18 Unit C 13 Changes in composition of the lake water ............... .._18 Unit D+E .............. 14 Changes in carbonate minerals in mud layers .................. 19 Unit F 14 Paleoclimatic interpretation .......................................................... 20 Unit G ...... 14 History of Searles Lake 20 Unit H 14 History of Owens River flow, as indicated by the size of Unit I 15 Searles Lake 21 Alluvial sand and gravel .......................................................... 15 Relation, between Searles Lake sizes and Sierra Nevada Bedrock 15 glacial stages 22 History of sedimentation 15 Nature of the climate 3—1 m.y. B.P ...................................... 23 References cited ............ 23 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Graphic log of core KM—3, showing lithologies, stratigraphic-unit boundaries, magnetic stratigraphy, and hues of mud units 2. Stratigraphic variations in percentages of five acid-soluble components and the acid—insoluble residue in core KM-3 3. Stratigraphic variations in normative weight percentages of acid-soluble minerals commonly concentrated in saline layers of core KM—3 4. Stratigraphic variations in normative weight percentages of acid-soluble minerals and acid-insoluble residue commonly FIGURE 1. 2. 00 TABLE 1. 2. 3. concentrated in mud layers of core KM-3 Page Map showing positions of late Pleistocene and present-day lakes in chain that included Searles Lake .................................. 2 Map of Searles (dry) Lake, showing location of corehole KM-3, locations of cross section A—A' and coreholes plotted in figure 5, and approximate limits of dry lake 4 . Plots of chemical analyses of samples from core KM—3 for borate, lithium, potassium, and bromine .................................... 8 . Plots of arc-emission spectrographic analyses of samples from core KM—3 for manganese, molybdenum, titanium, and vanadium ........ 9 . Diagram of east-west section through 11 cores, showing stratigraphic relations between Bottom Mud and Units A and B of Mixed Layer, and lateral variations of subunits within Unit A ........ 13 . Graph showing reconstructed history of Searles Lake, based primarily on evidence from core KM—3 .................................. 21 TABLE S Page Summary of stratigraphic units in core KM—3 3 Generalized log of core KM—3 5 Sedimentation rates between paleomagnetically dated horizons in core KM—3 18 III CORE KM-3, A SURF ACE-TO-BEDROCK RECORD OF LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA By GEORGE I. SMITH, VIRGIL J. BARCZAK‘, GAIL F. MOULTON2, and JOSEPH C. LIDDICOAT3 ABSTRACT A 930—m core designated KM—3, recovered from an area near the center of Searles Lake in Searles Valley, Calif., records a history of nearly continuous sedimentation from Miocene or early Pliocene time to the present. The earliest sedimentary deposits, 220 m of reddish-brown alluvial gravel, rest on 15+ m of quartz monzonite bedrock. Lacustrine sedimentation in ancestral Searles Lake started 3.18 million years ago (m.y. BR) and left 693 m of various types of lake deposits that make up the rest of the fill in that part of the valley where the core was recovered. The lacustrine sediment is here divided into 14 informal strati- graphic units, three of which are new, on the basis of field logs, chemical and mineralogic data, and study of the preserved core and color photographs taken when the core was fresh. Quantitative reconstruction of the evaporite mineralogy, based on 254 analyses of the acid-soluble components and X-ray diffraction data, allows the details of chemical sedimentation to be documented. Ages ofcritical contacts are estimated from ”C data (younger sediment), paleomag- netically established horizons (older sediment), and interpolation and extrapolation from these levels. Apparent sedimentation rates in core KM-3, overall, average 22 cm/1,000 yr; most older intervals have rates between 10 and 30 cm/1,000 yr, whereas younger, less compacted deposits average 53 cm/ 1,000 yr. The upper five informal stratigraphic units beneath the surface of Searles Lake, described briefly above, extend to a depth of 69 m and represent approximately the past 0.13 m.y. The underlying lacustrine sediment, here referred to informally as the Mixed Layer, is subdivided into nine units (only seven of which can be separated in core KM—3), with the following lithologies, indicated depths, ages, and inferred depositional environments: Depth Lithology Age of Unit to base and color base Inferred lake character (in) of mud (m.y.) A+B ------- 114 0 Salinas and 0.31 Perennial, intermediate to mud, olive- shallow depths, fluctuating. brown. C--------- 166.6 Salines--------- .57 Dry (salt flat), briefly perennial. D+E ------- 227.7 Mud, olive— 1.00 Mostly perennial, intermediate brown, some depths, occasionally salines. desiccated. F --------- 291.1 Mud, light- to 1.28 Perennial, deep. dark-green. G- -------- 525.5 Salines and 2.04 Perennial, intermediate depths, mud, olive. periodically desiccated. H--------- 541.6 Mud, brown —————— 2.56 Dry (plays), briefly perennial. I--------- 693.4 Mud, olive ------ 3.18 Perennial, deep. lKerr-McGee Corp. Oklahoma City, OK 73125. 2Kerr-McGee Chemical Corp, Trona. CA 93562. 3Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964. The most extreme change in sedimentation in Searles Valley was the shift from alluvial to lacustrine deposition 3.18 m.y. B.P., possibly as a result of volcanic damming of a river channel across the Sierra Nevada that had previously allowed part of the Great Basin to drain externally. Subsequent changes in the mineralogy and amount of chemical sediment in the lacustrine deposits indicate marked fluctuations in lake level superimposed on a gradual increase in the salinity of the deeper lakes and a progressive change in the chemis— try of their waters. Upon concentration or desiccation, the composi- tion of the saline deposits changed accordingly. Paleoclimatic reconstructions indicate a decrease over time in the amount of runoff that fed the lake, partly owing to continuing uplift of the Sierra Nevada that created an enlarging rain shadow, although climatically induced variations in runoff caused fluctua- tions in inflow that exceeded this more gradual change. Correlation of the perennial lake deposits in Searles Valley with the Sherwin Till and the McGee Till of the Sierra Nevada—a reasonable relation because of the inferred hydrologic connection between these areas— _ suggests that the Sherwin Glaciation began 1.28 m.y. B.P., waned 1.00 m.y. B.P., but did not cease until 0.57 m.y. B.P., and that the McGee Glaciation may have extended from 3.18 to 2.56 m.y. B.P. INTRODUCTION Searles Valley, containing Searles (dry) Lake, lies near the southwest corner of the Basin and Range province and about midway between the south ends of the Sierra Nevada and Death Valley (fig. 1). Before the turn of the 20th century, the geomorphic record of abandoned shorelines and bars in Searles and nearby valleys allowed geologists to conclude that these val- leys once contained a chain of lakes that were nour- ished chiefly by waters flowing from the east side of the Sierra Nevada into the ancestral Owens River sys- tem (see summary by Smith, 1979, p. 6—7). Then, as now, the Owens River terminated in Owens Lake. When filled, Owens Lake overflowed southward into Indian Wells Valley to form China Lake; China Lake, in turn, overflowed eastward into Searles Valley to form Searles Lake. At their highest stages, Searles and China Lakes coalesced into one large lake that drained southeastward into a stream that led to Panamint Valley, to form Panamint Lake. That lake, filled only during the most intense pluvial episodes, 1 2 CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA overflowed eastward into Death Valley and thus con- tributed, along with the Mojave and Amargosa Rivers, to the formation of what has been designated Lake Manly. The existence of lakes older than those responsible for the geomorphic record in Searles and the other closed valleys has long been suspected. Cores from Owens, China, Searles, Panamint, and Death Valleys show that the sediment deposited in perennial lakes extends to depths of at least 200 to 300 m (Smith and Pratt, 1957; Hunt and Mabey, 1966, table 19). The age of this deeper sediment was commonly estimated at early or middle Pleistocene. Lakebeds of comparable 0 10 20 30 40 50 60 70 KILOMETERS age crop out along the flanks of these valleys, and projections of the fine-grained facies suggest that deep lakes once occupied large areas now covered by playa or alluvial sediment. Lakebeds, many slightly de- formed, also crop out east of Long Valley (Rinehart and Ross, 1957; Bailey and others, 1976), east of the central part of Owens Valley (Walcott, 1897; Hopper, 1947 p. 418), southeast of Owens Lake (Schultz, 1938, p. 78; Hopper, 1947, p. 415—416; Duffield and Bacon, 1977), north of Indian Wells Valley (Duffield and Bacon, 1977), south of Searles Valley (Smith, 1964, p. 40—42), and on the periphery of much of Death Valley (Hunt and Mabey, 1966, p. A69—A72). EXPLANATION Present— day playa or lake Pleistocene lake Present- day rivet Pleistocene rivet 40 MILES " Death Valley) FIGURE 1.—Positions of late Pleistocene and present-day lakes in the chain that included Searles Lake. METHODS OF STUDY 3 The core described in this report documents for the first time that lakes have existed in the depression now known as Searles Valley for more than 3 my, a length of time exceeding that assigned to the Quater- nary Period. The age of the sediment at 267 m in the deepest core previously available from Searles Lake was estimated to be 0.5 to 1.0 m.y. (Smith, 1979, p. 109). HISTORY OF CORING IN SEARLES VALLEY Several hundred cores representative of the upper 50 m of sediment beneath the present surface of Searles Valley have been described previously (Gale, 1914; Haines, 1959; Smith, 1979). These cores were mostly taken in the course of commercial exploration and development by the several industrial chemical companies that have been extracting concentrated brines from this saline deposit during most of the 20th century, although about 40 cores to this depth were obtained by the US. Geological Survey during the 1950’s to define better the geology of the deposit. The aggregate value of chemicals produced from the brines of Searles Lake is about $2 billion. Subsurface deposits to the depth of 50 m are alternating layers of mud“ (indicating large and relatively fresh lakes) and salts (indicating small saline or dry lakes), in about equal volumes. Two deeper cores, taken as part of special @mwnmmmdmwwdmmhmmnfimmwmdmwnumhmhxwtudeHML were moist and plastic when first extracted as core. The deeper sediment may have been more indurated and could be more correctly named using rock terms. but the field log implies an absence of induration by using the term “mud" throughout the lacustrine section. The field log describes the alluvial sand and gravel as “arkose,” implying greater induration. but we have used sediment terms for that part of the section as well because the lithology of these segments of the core can be more accurately described in this way. exploration programs, extend to depths of 191 and 267 m (Gale, 1914; Smith and Pratt, 1957). In 1967, the Kerr-McGee Chemical Corp. (KMCC) absorbed the holdings of the American Potash & Chemical Corp. in Searles Valley, including its fee land near the middle of Searles Lake. On July 5, 1968, KMCC started drilling an exploratory corehole in this area and designated it KM—3 (fig. 2). Coring reached bedrock in September 1968 at a depth of 915 m; an additional 15 m of drilling in bedrock brought the total depth on September 6 to 930 m. On June 3, 1976, permission was granted by D. A. McGee, Chairman of the Kerr-McGee Corp., (parent to KMCC), for the US. Geological Survey to study the company’s internal report, a highly detailed description of the core (Bar- czak and Petticrew, 1969), to make additional studies of the preserved core, and to publish a description of it using the company data. In this report, we (1) present a description of the core, with emphasis on its chemistry, mineralogy, and lithology; (2) interpret the various depositional environments indicated by its compo- nents and establish stratigraphic units on that basis (table 1); and (3) discuss the implied climates and their relation to Sierra Nevada glacial stages. METHODS OF STUDY LITHOLOGY Core KM—3, approximately 8 cm in diameter, was logged at the drill site by T. S. Melancon of KMCC, and several hundred lithologic units were identified and described. The saline minerals were identified at that time by megascopic methods, and the logged descrip- tions of the fine-grained mud zones included grain TABLE 1,—Summary of stratigraphic units in core KM—3 [Ages to 0.032 m.y. are based on 1“C dates; ages to 0.13 m.y. were estimated by extrapolation from depositional rates (Smith, 1979, p. 77-78). Older ages are based on extrapolation or interpolation between nearest dated paleomagnetic boundaries, located with accuracies ranging from 0.1 to 10.6 m (Liddicoat and others, 1980)] Depth Age Length of Stratigraphic to base Thickness Dominant of depositional unit (m) (m) lithology base period (m.y.) (103 yr) Overburden Mud---— 5.8 5.8 Mud 1-—------' ---------------- 0.006 6(?) Upper Salt-—------ 19.9 14.1 Salines --------------------- .010 4(7) Parting Mud---~--- 25.0 5.1 Mud ------------------------- .024 14 Lower Salt--—---'- 37.9 12.9 Salines and mud ------------- .032 8 Bottom Mud-------- 269.0 31.1 Mud ------------------------- .13 98 Mixed Layer Unit A+B—---- 114.0 45.0 Salines and mud ------------- .31 180 Unit C---—--- 166.4 52.4 Salines --------------------- .57 260 Unit D+E ------ 227.7 61.3 Mud, some salines— --------- 1.00 430 Unit F -------- 291.1 63.4 Mud------------—-------—---- 1.28 280 Unit G- ------- 425.5 134.4 Salines and mud------------- 2.04 760 Unit H—----—- 541.6 138.4 Mud— ----------------------- 2.56 520 Unit I -------- 693.4 151.8 -—do ------------------------ 3.18 620 Alluvial sand and 915.5 221.9 Coarse sand and gravel —————— ? ? gravel Bedrock—----—---- 929.6 14.1+ Quartz monzonite —————————— -— --- 1Not recovered as core by 104-3. 2Depth in core 289M, 150 in north of 104—3. 4 CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA 117°20' Edge ofdry-lake surfaceasshown : 1‘ on topographic w 3 map m in l> 35°45' Approximate edge of saline layers in Upper Salt and Lower > Salt(Smith, 1979, p. 20) I 0 ‘l 2 3 KlLOMETERS |___—_;l_l—l 0 1 MILE FIGURE 2.—Searles (dry) Lake, showing location of corehole KM—3, locations of cross section A—A’ and coreholes plotted in figure 5, and approximate limits of dry lake using two criteria. METHODS OF STUDY size, bedding character, and color. After logging, the core was split, and one face was photographed in color under standardized lighting conditions and with a gray density scale to assure uniformity of the prints. We later used these prints to modify and generalize the original field log and to assign standard rock colors (Goddard, 1948). The descriptive 10g included here (table 2) lists the most conspicuous lithologies, minerals, textures, and colors, on the basis of the field log, the photographic record, the detailed mineralogic and chemical data later obtained by Kerr-McGee personnel at their Technical Center in Oklahoma City, and some reexam- ination of the preserved cores. For brevity, this des- criptive log combines similar units so that it contains only about a fourth of the entries in the detailed log prepared for the chemical and mineralogic analysis and the company’s internal report; we used that detailed log, however, to compile the graphic lithologic log (pl. 1). TABLE 2.—Generalized log of core KM—3 Depth to base of Thickness unit of unit (m) (m) Description Not cored. Overburden Mud is included in this zone. Upper Salt—Salines, mostly trona and halite; abundant hanksite near top, borax at base; light- to medium-gray (N6—8) and yellowish- gray (5Y8/1), with some thin interbeds of olive-gray (5Y4/1) mud; mostly indistinctly bedded to massive, locally vuggy. Parting Mud.—Megascopic crystals of gaylus- site and pirssonite in soft mud composed of microscopic crystals of dolomite, halite, arag- onite, other evaporite minerals, and elastic sil- icates; light- to moderate-olive-gray (5Y3— 5/1—4); upper part finely laminated, lower part massive. Lower Salt—Seven saline layers interbedded with six mud layers; salines are mostly halite and trona in upper two layers, trona, halite, and burkeite in underlying two layers, and trona in lower three layers; interbedded mud layers contain megascopic crystals of gaylus- site and pirssonite; salines range in color from white through dark gray to yellowish orange (N5—8, 10YR6/6), mud from dark olive gray to brown (5Y4-6/1-4); salts poorly bedded to massive; some mud layers have thin laminar bedding. Bottom Mud—Mud containing megascopic gay- lussite crystals; mud is composed of micro- scopic crystals of dolomite, aragonite, calcite, and other carbonate minerals, and about 30 5.8 5.8 19.9 14.1 25.0 5.1 37.9 12.9 69.0 31.1 TABLE 2. —Gemmlized log of core KM —3—Continued Depth to base of unit (m) Thickness of unit (m) Description 90.8 95.4 99.8 114.0 124.0 130.4 135.6 151.2 166.4 178.6 21.8 4.6 4.4 14.2 10.0 6.4 5.2 15.6 15.2 12.2 percent acid-insoluble silicates and organic residues; thin-bedded to massive, with some laminar bedding; medium- to dark-brown, brownish-gray, and olive (5YR4/4 to 5Y3— 4/1—2). Discontinuous saline layers at 41.4 m (0.5 m thick), 48.5 m (0.4 m thick), and 54.4 m (0.8 m thick). Interval of poor core recovery; recovered core (3.4 m) is composed of mud containing megas- copic gaylussite crystals, massive, medium— to dark-brown and olive (5YR5/2 to 5Y3/2). Top of interval probably represents top of Mixed Layer. This and following three units probably represent Units A and B of Mixed Layer, of which most of the saline layers were lost dur- ing drilling (see text). Salines, mostly trona, containing mud; faintly bedded to massive; moderate-brown to olive (5YR4/4 to 5Y5/1). Mud, mostly acid-insoluble material, some dolomite; light-olive-gray (5Y5—6/1—2), thin- bedded. Salines, mostly trona, with small amounts of other minerals and extensive mud impurities; light- to dark-green and brown (5GY5/1, 5Y4—6/1, 5YR3/4); faint bedding in lighter colored salines, with interbeds of mud com- mon near base. Contact between Units B and C of Mixed Layer is at base of this interval. Salines, with some interbedded mud; saline minerals are mostly halite, with smaller amounts of trona and other evaporite miner- als; indistinct bedding, beds mostly I to 2 cm thick; salines light- to dark-olive-gray (5Y4— 7/1—2) and moderate-brown (10YR4—6/2-4). Salines and some mud; salines are about two- thirds halite and one-third trona, with some thenardite; yellowish-gray (5Y5—7/2); upper part of unit contains largest percentage of mud impurities. Salines, mostly halite, with minor trona and the— nardite; dark- to medium-gray (N3—5); upper part of unit contains mud impurities. Interbedded mud and salines; salines are mostly halite, with some trona; salines olive-gray (5Y4/1 to 5Y6/1), mud brownish-black (5YR2/1); saline layers, 0.3 to 0.6 m thick, con- stitute about one-third of zone. Salines, mostly halite, with smaller amounts of trona and thenardite, nearly pure in lower part; mostly gray to yellowish-gray (N4—7 to 5Y4—6/1); bedding 1 to 2 cm thick, some zones porous but most nonporous. Contact between Units C and D+E of Mixed Layer is at base of this interval. Mud containing megascopic crystals of gaylus- site and pirssonite, microscopic crystals of CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA TABLE 2.——Ge7wmlized log of core KM —3——Continued Depth to Depth to base of Thickness base of Thickness unit of unit ‘ unit of unit (m) (m) Description (m) (m) Description dolomite, halite, and probably other acid- 291.1 14.2 Mud,similar to that at 249.5 In. Contact between soluble minerals; brownish-black (5YR2/1) in Units F and G (new) of Mixed Layer is at base upper and lower part, moderate-brown of this interval. (5YR3/4) in middle. 294.4 3.3 Impure salines with mud impurities similar to 186.5 7.9 Mud, with interbedded salts at 179 and 184 m; interval above; salines, mostly halite and the- salts, in beds 0.1 and 0.6 m thick, are mostly nardite, are mottled aggregates surrounded halite and trona, with some thenardite and by mud. northupite; mud is composed largely ofmicro- 299.3 4.9 Mud, dusky-yellow-green (5GY5/2); upper part scopic crystals of dolomite and other carbo- mottled, lower part has buff laminae and thin nates; olive to brownish-black (5Y2/1 to beds. 5YR2/1). 306.3 7.0 Salines and some mud; salines are chiefly halite 192.0 5.5 Salines interbedded with mud containing scat- and thenardite, white to light-gray (N58); tered saline crystals; salines are mostly halite, mud pale-green (10G6/2); upper part faintly with subordinate trona, thenardite, and other bedded, lower part mottled. minerals; salines olive-gray and medium- to 324.3 18.0 Two saltbeds separated by mudbeds (see pl. 1); dark—gray (5Y6/1 to N4—6), mud dark-olive- salts, largely halite, massive, light-greenish- black (5Y1—2/1). gray (5GY6-8/1); muds massive, greenish-gray 196.1 4.1 Mud, dark-olive-black (5Y1—2/1). (5G6/1). 196.6 0.5 Salines, mostly halite; olive-gray (5Y4/1). 333.1 8.8 Three mud and two impure salt layers (pl. 1); 204.5 7.9 Mostly mud, with some disseminated saline mud pale-green (10G6/2), massive, with dis- crystals; brown (5YR3/4) in upper part, olive- persed salts; saline layers, consisting of mottled gray (5Y4/l) in lower. zones of lighter colored secondary crystals 207.4 2.9 Salines, with interbedded mud; salines are most- oriented randomly in mud matrix, are halite, 1y halite, distinctly bedded, averaging1cm in with some thenardite, glauberite, and thickness; salines yellowish-gray (5Y7/2), mud anhydrite. olive-gray (5Y4/1). 334.7 1.6 Impure salts, halite; dark-greenish-gray 210.9 3.5 Mud, moderate-brown (5YR4/4) in upper half, (5GY4/1). olive-black (5Y2/1) in lower part. 337.3 2.6 Mud containing some halite; brownish-gray 213.6 2.7 Salines, mostly halite, with mud impurities; (5YR4/1). olive-black(5Y2/1)tolight-olive-gray (5Y6/1); 341.1 3.8 Muddy salt grading downward into impure upper part faintly bedded, lower part massive mud; salts largely halite and anhydrite; mud ‘ to mottled. dark—greenish-gray (5GY4/ 1). 218.4 4.7 Mud, massive, olive-black (5Y3/1). 345.3 4.2 Mud with some dispersed salts; massive, faint 218.5 0.1 Salines, trona and halite. mottled coloring; olive-gray (5Y4/1). 227.4 8.9 Mud, massive, olive-black (5Y2/1). 405.7 60.4 Nine alternating salt and mud layers in nearly 227.7 0.3 Salines,trona and halite. Contact between Units equal volumes, with individual layers gener- D+E and F of Mixed Layer is at base of ally4t06m thick (see pl. 1); salts are halite and this interval. other saline minerals, mostly light- through 248.1 20.4 Mud, mostly grayish-olive (5GY4/1), with a medium-gray (N5—7) to light-olive—gray grayish-brown (5YR3/2) zone at 236—238 In (5Y6/1) and grayish-orange-pink (5YR7/2); and a greenish-gray (5GY6/2) zone at 244—245 mud greenish-gray (5G4-6/ 1) to dark-greenish- m. gray (5GY4/1); mud is massive except in 0.5- 2495 1.4 Mud, dark-greenish-gray (5GY4/1), mottled to m-thick zone below saltbeds, where it is thin faintly bedded; lower half extremely hard bedded; salts are faintly bedded to massive. (limestone). 413.3 7.6 Mud; inadvertently not photographed, but re- 271.4 21.9 Mud, with irregular concentrations of a few ported by field log as green to brown. mottled areas caused by light-colored dolomite 422.5 19.3 Core not recovered. or salts; mostly pale- to grayish-green (10G4— 425.5 22.3 Mud, soft and plastic, olive—black (5Y2/1). Con- 6/2), upper 3 m grayish-olive—green (5GY3/2); tact between Units G and H (new) of Mixed massive except near 260 and 268 m, where thin Layer is at base of this interval. to laminar bedding is defined by pale-orange 437.7 12.2 Mud, with small crystals of thenardite dispersed (10YR6-8/2—4) layers (dolomite?) randomly; more coherent than interval above; 276.9 5.5 Mud, similar to interval above but containing average moderate-brown (5YR3/4). searlesite. 444.6 6.9 Mud, soft and plastic, olive-black (5Y2/1). METHODS OF STUDY 7 TABLE 2.—Generalized log of core KM—3—Continued Depth to Depth to base of Thickness base of Thickness unit of unit unit of unit (m) (m) Description (m) (m) Description 449.6 5.0 Mud, more coherent than interval above; mod- 693.4 2.5 Mud and sand; faint to conspicuous thin beds, erate-brown (5YR3/4). light-olive-gray (5Y5/2). with streaks of pale- 451.4 1.8 Salts and mud; salts in thin beds and mottled brown (5YR5/2). Contact between Unit I of areas, yellowish-gray (5Y8/1), chiefly glauber- Mixed Layer and alluvial sand and grave] is at ite and anhydrite, with some halite; mud mas- base of this interval. sive, olive-gray (5Y4/1). 915.3 211.9 Pebbly arkosic sand and gravel, most commonly 482.5 31.1 Mud,moderate-brown (5YR3/4), with some zones moderate brown (5YR3-4/4), with zones that of light-olive—gray (5Y5-6/1—2); salts largely averagelightbrown(5YR6/4)in color between halite and anhydrite, both dispersed and con- 726—740 and 748—798 m; mostly coarse to very centrated in mottled zones. coarse sand, poorly sorted, containing quartz 483.4 0.9 Mud and some salts; mud olive-gray (5Y4/1), monzonite and volcanic-rock fragments, as thin bedded; salts are chiefly anhydrite and large as 15 cm in diameter; faintly bedded to halite. massive; not cored between 748.3—793.4, 804.7— 494.4 11.0 Mud, moderate-yellowish— to pale-brown 826.3, and 8391-9031 In. ‘ (10YR5/2-4), massive. 929.6 14.3 Quartz monzonite,light—tomedium—gray (N 5—7), 507.5 13.1 Mud, light-olive-gray (5Y5/2) to pale-yellowish- with pale-brown (5YR5/2) stains along frac- brown(10YR/2), 2-m-thick zone at 502 m is tures extending through cored interval; rock pale brown (5YR5/2). bit was used from 915.3 to 926.3 m, and so no 507.8 0.3 Mud and salt; mud light-olive—gray (5Y5/2); salts core was recovered. are mostly glauberite. 516.6 8.8 Mud and disseminated salts; light-olive-gray (5Y6/1) to pale-yellowish-brown (10YR6/2). 524.0 7.4 Mud; moderate-brown (5YR4/4) in upper part, yellowish-brown (10YR6/2) to pale-brown CHEMISTRY (5YR5/2) in lower part. 530-1 6'1 Mud? “pp," pa" greeniSh'E’ay (5GY6/1)' "m“ The half of the core not photographed was again tled, thin-bedded to masswe; lower part yel- . l . h . . cut lengthWISe, and one part was sent to the Kerr- ow1s -brown (10YR6/2), thin-bedded. _ , , , 541.6 11.5 Mud, mottled, pa]e_bmwn (5YR5/2) to pale_ McGee Technical Center in Oklahoma Cityfor minera- yellowish—brown (10YR6/2). Contact between logic and arc-emission spectrographic analyses; the linihts H and 11(neW) of Mixed Layer is at base other part was sent to the KMCC laboratories at Whit- 0 t is interval - tier, Calif. At Oklahoma City, the core was divided into 582.2 40.6 Mud, olive—gray (5Y4/l) down to 558 m, light- - - . - “New” (5Y6 /1)below that depth,with 2_m_ 254 Intervals on the bas1s of lithology, and chemical thick pale-brown (5YR5/2) zone at base. analyses of the same Intervals were performed at the 634.0 51.8 Mud, light-olive—gray (5Y56/1-2). Whittier laboratories. The percentage of the acid- 640.1 6-1 Mygigigwnish- black (5YR2/l) to olive-black insoluble residue, and the percentages of seven ele- _- , ments in the acid-soluble fraction (Na, K, C03, S04, C1, 6492 9'1 giig’ulglgglive'gmy GYM/1'2) and pale' B407, Br), were determined in all 254 samples, as well 658.7 9_5 Mud; grayish—olive (10Y4/2) in upper part, as the percentages of three more elements in the acid- yellowish-gray (5Y6/2) in lower part. soluble fraction (Ca, Mg, Li) in a slightly smaller 681.5 22.8 Mud, mostly pale-olive (10Y6/2) to yellowish- number of samples, gray (5Y7/2), “”th “"95 "ear'd've‘g’ay The results of the analyses on 254 samples have (5Y4/1) at 662’ 665’ and 669 m' been reduced here to 144 units by calculatin and 684.0 2.4 Tuff mixed with mud, grading down into pure , _ , , g tuff; impure tuff is olive-gray (5Y4/1). pure combining weighted averages for successmns of ana- tuffyellowish-gray(5Y7/2); wellindurated in lyses of similar materials. The percentages of acid- , basal 40 cm- _ . _ soluble Ca, Na, CO3, S04, and Cl, and of the acid- 690'4 6‘4 MW “1"?" sand'S‘ze‘ “med' ”“3"“ m 00,1“ insoluble'residue, are plotted in stratigraphic order on from olive gray (5Y4/1) to dark yellow13h l 2 S 1 t l d f C k b brown (10YR4/2) p ate . amp es no ana yze or . a are nown to e 6909 0_5 Tug, yellowish-gray (“GS/1), well-indurated, low and are plotted as If they contained none. Figure 3 crossbedded. plots the contents of K, Li, Br, and B407, which gener- BROMINE CONTENT. IN PERCENT POTASSIUM CONTENT, IN PERCENT IN PERCENT LITHIUM CONTENT, BORATE CONTENT, IN PERCENT .08“ .06 -* .04 -- .02 ‘- 0.5 0 5.2 ‘— -m ‘_' ,— Q Q CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA II 100 200 300 IIIIIIIIIIIIIIIIIIIIIIHI IIIIIIILII IIIIIIII III I I 600 700 800 900 I IIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIILLIIIHIIII I IIIIIII II ‘l I I II. 100 100 200 200 300 300 400 500 600 700 800 900 .05-I» | OLJ—IJIIIIIII I I‘IIIIIIII IIIIIIIIIIIII I II IIIIIIIIIIIIIIIIIIIIIIIIIIII 1 I IIIlIIIIIIIII I IIIIIIIIIIII I I 100 200 300 400 500 600 700 800 900 IIIIIII I IIIIIII‘IIILIALIIJJLIJIJWITIII III IIIIILIIIIIIII | IILEIIIII IIIIIIII I I I. 400 500 600 700 800 900 DEPTH, IN METERS FIGURE 3.—Chemical apalyses of samples from core KM~3 for borate (3407), lithium (Li), potassium (K), and bromine (Br). METHODS OF STUDY 9 ally exist in smaller amounts. On plate 2, the sums of acid-soluble cations and anions plus acid-insoluble material are less than 100 because the percentage of H20 was not determined, and the less abundant com- ponents (fig. 3) combine to account for several percent of the sample. Arc-emission spectrographic analyses KM—3. Most minerals reported to exist in amounts greater than about 5 percent were observed on X-ray diffraction charts, and some were checked microscop— ically. The names and chemical compositions of the nonclastic minerals found in core KM—3 are as follows: ' ' Mineral Composition for Mn, Mo, T1, and V 1n selected samples from core Analcime NaAlSiZOG-Hzo KM-3 (fig. 4) indicate the amounts of these elements Anhydrjte CaSO4 present in the total sample. A’agmte CaCOs Borax ........ N a2B407-10H20 Burkeite 2Nza.2304-Na2CO3 MINERALOGY Calcite CaC03 . _ . ‘ Celestite (Sr, Ba) SO X-ray diffract1on methods prov1de the prlmary Dolomite CaMg(C03): basis for identification of the mineral species in core Gaylussibe Cacoa-Nazcoa-snzo 300 -— (D E g E 5 200-- — z 3 2:; I z l g EE 100" § . § ‘ o z “- r‘n l m n o a I U H 1‘ J l l 0‘ I I ' I | I ' o 100 200 300 400 500 600 700 300 900 I9 4ooo« I a s :2) z 3 7 ‘ 2 F5 2000" 7 I‘ < z I I E E5 l “ 2 LL l l . 8 0_ IIIIIIIIIIIII II I , II o 100 200 300 400 soo 600 700 800 900 :2 1ooo~- >; a: Z 3 35 9 u.| Z .J 3 fig 500-- 5 E I: g § § 3 § § 2 E E v v fiEI—L—fi l I—gi r—‘fl I—‘n I‘H I—‘—I 8 0- ”“lll‘llllll llJlllllll‘l III“ [I] III IIIIIIII Illlflflll‘lllllllllll lIIIIIIl ll|llll|lllllll|JLi I II 1 1 II: o 100 200 300 400 500 600 700 800 900 3000-- (I) u, E 8 E 5 2000-— z z 3 < —.=' E ,2 g: 1000‘“ ’ 8 l 0_ HIHIIII Illlllllnl l o 100 200 300 400 500 600 700 800 900 DEPTH, IN METERS FIGURE 4.—Arc-emission spectrographic analyses of samples from core KM-3 for manganese (Mn), molybdenum (Mo). titanium (Ti), and vanadium (V). 10 CORE KM-3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA Mineral Composition Glauberibe NaZSO,CaSO, Gypsum CaSO‘-2H20» Halite NaCl Hanksite 9Na2804~2NaZC03-KC1 Heulandite ............................................................ CaO-A1203-6Si02-5H20 Magnesite MgCO3 Nahcolite _____________ NaHCO3 Northupite ............................................................. Na2003-Mg003-NaCl Pirssonite _____ .. CaCOs-NaZCO3-2H20 Searlesite _________________________________________________________________________ NaBSiZOG-HZO Thenardite ........................................................................................ NaZSO4 Tincalconite .......................................................................... NazB4O7-5H20 Trona ...................................................................... NaZCOaiNaHC03-2H20 Most of these minerals have been previously reported from Searles Lake (Smith and Haines, 1964; Smith, 1979), but six are new. Glauberite and anhy- drite, which occur in appreciable amounts in core KM—3, have not been reliably identified in other cores. Glauberite was confirmed by X-ray diffraction of sev- eral samples from intervals 341.1-345.3, 4255—4596, and 5136—5185 m; and anhydrite was confirmed in samples from intervals 334.4—340.5, 5185—5380, and 5462—6818 m. Gale (1914, p. 289, 297, 302—303) re- ported glauberite, anhydrite, and gypsum from Searles Lake, though probably without good basis; he presented no evidence of confirmation by petrographic examination and cited a specific occurrence only for glauberite—in a core from a shallow zone, since sampled by hundreds of cores without glauberite being reported. Small amounts of gypsum (at 385.6 m), heulandite (at 5855—6952 m), celestite (at 622.7 m), and magnesite (at 375.7 m), confirmed by X-ray dif- fraction of one or more samples from the depths indi— cated, also represent new minerals from Searles Lake. Quantitative estimates of the percentages of acid- soluble minerals in each analyzed unit were made by converting the chemical analyses into normative com- positions. A computer program was established that converted the percentages of major components into percentages of probable minerals, based on the suite of minerals normally observed in cores from Searles Lake plus the species detected by X-ray diffraction in that segment of core. Because many of the minerals found in the upper part of the core (for example, hank- site, burkeite, and northupite) were absent in the lower part, a slightly different program was used for sam— ples from below 304.8 m (1,000 ft). In the upper part, where trona was abundant, the program reconstructed its abundance, although it did not differentiate be- tween trona and nacholite; in the lower part, trona was eliminated as a phase, and glauberite anp anhydrite were added as more probable phases, according to the X-ray diffraction data. After calculation of the normative compositions, the mineral percentages were recalculated so that their sums, plus the acid-insoluble residue, equaled 100 percent. Of the 254 intervals selected for chemical analyses and normative reconstruction, 24 had a cation-anion imbalance greater than 0.1 mol percent/g, and 5 of these 24 intervals had a cation-anion imbal- ance greater than 0.2 mol percent/g. An imbalance of 0.1 mol percent Na, for example, means that 2.3 per- cent more or less than the reported amount would be required to form the normative suite. DATING The ages of the younger units in core KM—3 are based on 1“C dates and on estimates of the sedimenta- tion rates derived from them (Stuiver and Smith, 1979, p. 74—75); the ages of older units are based on paleo- magnetic stratigraphy (Liddicoat and others, 1980). Attempts are in progress to correlate the thick vol- canic ash layers in core KM-3 (near 685 m) with ash layers that crop out in other nearby areas, but the results thus far are inconclusive. The ages of all paleomagnetic horizons, and all cited dates from other sources that are based on K-Ar ages, have been cor- rected for the revised decay and abundance constants (Dalrymple, 1979) and conform to the polarity time scale of Mankinen and Dalrymple (1979). The precision of dating the contacts between units varies greatly, and the citing of their ages to the near- est 0.01 m.y. does not imply confidence at this level. Younger samples dated by 1“C techniques probably are accurate to within 5 to 10 percent when the laboratory uncertainty and sample contamination or bias errors are combined. Reasonable estimates of the age of the base of the Bottom Mud, discussed elsewhere (Smith, 1979, p. 77—78), vary by several tens of thousands of years. In the Mixed Layer, the ages of the bases of Units D+E, G, H, and I are best controlled because they lie near closely spaced paleomagnetically dated hori- zons; the ages of the bases of Units C and F have greater uncertainties. The base of Unit F is dated by interpolation between horizons that are separated from the dated contacts by many beds of salts that probably were first deposited more rapidly than the mud and then became horizons representing virtual nondeposition. The base of Unit C is dated by extrapo- lation from the nearby, but only approximately located, boundary between the Brunhes Normal and Matuyama Reversed Epochs at 185i10.0 m (Liddicoat and others, 1980, table 1), although evidence described in a later section suggests that the inferred age is nearly correct. The age of the base of Unit A+B is least well known because it is separated from the nearest paleomagneti- cally dated horizon by thick beds of salt and from the base of the Bottom Mud, itself dated only approxi- STRATIGRAPHY 1 1 mately, by more than 100 m of salts and mud. Ages of this contact, calculated using a number of reasonable assumptions, ranged from 0.2 to 0.4 my; the age of 0.31 my. (table 1) is based on extrapolation from the esti- mated age of the base of the Bottom Mud, using a net sedimentation rate of 25 cm/ 1,000 yr, midway between the 30-cm/1,000-yr value used in estimating the age of the Bottom Mud (Smith, 1979, p. 77) and a rounded value of 20 cm/ 1,000 yr based on the rates calculated for Units D through G (see table 3). ACKNOWLEDGMENTS The data in this report represent the contributions of many people in the Kerr-McGee Corp. and its sub- sidiaries. We express our appreciation first to D. A. McGee for granting us permission to make additional studies of this core and to publish our results. F. C. Hohne provided guidance and support throughout this project. We especially acknowledge theefforts of T. S. Melancon for his observations at the drill site and his care in cutting and shipping the core; of A. C. Gonzales for most of the core photography; of K. A. Smitheman, R. Pai-Ritchie, and coworkers in Whittier Calif., for analyzing the core; of C. H. Long and his group at the Kerr-McGee Technical Center, Oklahoma City, for the X-ray and arc-emission analyses; of R. M. Becker for writing the normative-mineral computer program; of R. W. Petticrew for assisting with core logging, materials-balance calculations, and compiling of data; of C. W. Cowie for compiling the data and cross sec- tions of Units A and B of the Mixed Layer; and of many others who contributed in numerous ways. G. Winston of the U.S. Geological Survey reduced the numerical data of the original Kerr—McGee Corp. report to the graphic form used in plates 2 through 4; and R. Aquino of the Survey helped compile the data presented in the tables. STRATIGRAPHY SUMMARY The sediment and rocks in core KM—3 are of three major types: Depth (m) 0—693 Description Mud and evaporites, deposited in standing water by perennial or seasonal lakes. 693—915 Alluvial sand and gravel, deposited by intermittent streams flowing on the surfaces of fans. 915—930 Quartz monzonite, crystallized at depth from magma. The lacustrine section, both mud and evaporites, is the main subject of this report, and that section is herein subdivided into 14 informal stratigraphic units, 3 of which are new. The alluvial sand and gravel and the quartz monzonite are also described briefly. Table 1 lists these units, their dominant lithologies, and their ages. The upper five units in the lacustrine section— informally designated the Overburden Mud, the Upper Salt, the Parting Mud, the Lower Salt, and the Bottom Mud—were defined and described previously (Flint and Gale, 1958, Smith, 1962, 1979; Smith and Haines, 1964); descriptions of them are repeated here only to the extent found in tables 1 and 2. All underlying lacustrine sediment was informally designated by Flint and Gale (1958) as the “Mixed Layer,” and six stratigraphic subdivisions of it, based on the 200 m of sediment from this zone known at that time, were proposed and defined by Smith (1962). The subdivi- sions of the Mixed Layer designated Units A (at the top) and B cannot be separated in core KM—3 but are described and redefined here on the basis of new data from other cores. Unit C is described as observed in core KM-3. The lithologies originally assigned to Units D and E can be identified in core KM-3 but the boundary between them cannot; because future work may determine a valid division, these two units are retained and described together here as Unit D+E. Unit F was defined by Smith (1962, p. C68) on the basis of about 25 m of sediment at the bottom of the deepest core available; it now appears that this section was the upper two-thirds of a unit composed of similar sedi- ment, and so the name is here retained, and the de- scription of the unit is amplified to include its lower part. Units G through I are identified, defined, and described here for the first time. The criteria used to establish Units A through F of the Mixed Layer (Smith, 1962; 1979, p. 108—109) were that they identify relatively thick sections of lacustrine sediment characterized by (1) the dominance of a cer- tain lithology (mud, salines, or a cyclic alternation of the two), or (2) a saline-mineral content that indicates a relatively consistent chemical or physical character of the lake in which the unit was deposited. The same criteria are used here, along with those described below that help separate lacustrine Units G through I of the Mixed Layer from each other and from the underlying alluvial sand and gravel. Coring since publication of the original definition of the Mixed Layer units has shown that the division between Units A and B of the Mixed Layer on the basis of saline mineralogy is impractical because there is a demonstrable lateral variation in the mineral compo- sition of the saltbeds. The mudbeds, however, record depositional events that affected large parts of the lake at the same time, and mud deposits are less likely than saltbeds to alter diagenetically. For these reasons, we are here modifying the earlier definitions of Units A 12 ‘ CORE KM-3~LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA and B and placing the boundary between them at the base of a distinctive mud layer. In distinguishing between all units, the color of the mud layers is taken as an important criterion in identi- fying modes of deposition that differ significantly. The hues of the mud zones are indicated on the graphic log (pl. 1) and for convenience are divided into groups that might be described as distinctly green, yellow, or orange (when light), or as olive, olive brown, or brown (when dark). Green and olive hues are considered to be evidence of deposition in a perennial lake that was characterized by reducing conditions in the accumu- lating layer of mud as a result of a great depth or stratification. Yellow and olive-brown hues are con- sidered indicative of deposition in a shallower or better oxygenated lake that produced only a partially reduc- ing environment in the mud. Orange or brown hues are inferred to indicate oxidizing environments. Thick zones having these orange and brown colors are thought to indicate an environment in which the accumulating lake sediment was exposed to the atmosphere during part of each year, as in a playa; thin zones having these colors may represent the top of a mud unit that was deposited in a reducing environ- ment but later exposed to the atmosphere long enough for oxidation and, possibly, soil-forming processes to be effective near the exposed surface. The criterion used to separate the Mixed Layer from the underlying alluvial sand and gravel is the great contrast in their depositional environments. In the Mixed Layer, restricted by definition to lacustrine deposits (including playa deposits), the sorting and bedding characteristics indicative of deposition in standing water are consistently observed, even though the water body may have been saline or ephemeral. In the alluvial sand and gravel, the dominance of red arkosic sand and the common presence of angular pebbles and cobbles clearly indicate subaerial deposi— tion as an alluvial fan. DESCRIPTION OF UNITS OF THE MIXED LAYER UNIT A Unit A of the Mixed Layer was originally defined as the uppermost zone composed of interbedded saline and mud layers in which the saline beds included trona and nahcolite but not halite (Smith, 1962, p. C68; 1979, p. 14). The upper contact was placed at the top of the uppermost saline layer. In many cores, this contact is difficult to identify with precision because the saline layers of this unit tend to be thin, lenticular, and impure. Therefore, practical considerations have led to the use of a combination of lithologic and electric-log criteria in placing the contact, although we are uncer- tain whether these indicators reconstruct a contact that has the same age in all places. Unit A is not well represented in core KM—3. Other cores from areas near corehole KM—3 show depths to the top of Unit A mostly between 60 and 70 m, and to the base mostly between 100 and 110 m. In core KM—3, only 16 percent of the interval 69-91 m is represented by recovered core, and none of it includes saline beds, although this segment probably repre- sents most of Unit A. In a more complete core (289M) drilled at a site 150 m north of corehole KM—3, layers of trona and nahcolite assigned to Unit A were recovered between 67.9 m and the base of that unit at 105.4 m; because we consider core 289M to be more representa- tive, it is plotted on the graphic log (pl. 1) beside core KM—3 to show this part of the section. A better understanding of this part of the section is provided by combining the data from several cores. Figure 5 presents an east-west cross section of Unit A, based on 11 cores along the line in figure 2. These cores show that Unit A of the Mixed Layer can be divided into two informally named subunits on the basis of their physical properties as revealed by electric logs and their lithology as recorded in core logs; these sub— units are here termed, in ascending order, the Main A Zone and the Upper A Zone. The Main A Zone is further subdivided into five saline layers (S-l through S—5) and five mud layers (fig. 5). All the saline layers except S—2 can be traced laterally over a large area. The basal saline layer in the Main A Zone, which resembles the overlying two saline beds, is composed predominantly of trona but also contains thin mudbeds and varies laterally in saline-mineral composition. The layer ranges in thick- ness from a few centimeters near the edges of the lake to 6 m in the center and has the largest areal extent of the Main A Zone salt layers (although it does not cover so large an area as the Unit B salts, described below). Near the edge of the lake, nahcolite has been observed in this layer; in the northern part of the body, massive halite has been found; and in the southern and eastern parts, thenardite and halite have been recovered, gen- erally from the lower part of the layer. The upper two saline beds of the Main A Zone, S—4 and 8-5, are com- posed of impure mixtures of trona and mud in which the trona was generally recrystallized into interlock- ing networks of acicular and tabular crystals. The basal mud zone in the Main A Zone is a con- spicuous bed, 3 to 5 m thick, composed of green, tan, or black mud, commonly laminated. This bed, which is characterized by megascopic crystals of gaylussite and trona in the upper part and by a significant per- centage of northupite crystals (1—20 mm diam) in the lower part, is easily identified on electric logs. The STRATIGRAPHY overlying four mud layers that separate the saline lay- ers in the Main A Zone are massive black marls con- taining megascopic crystals of gaylussite. The Upper A Zone includes a basal mudbed that is overlain by a saline zone characterized by numerous thin discontinuous beds of finely crystalline trona and nahcolite. This basal mudbed, commonly about 1 m thick, contains abundant crystals of gaylussite and is typically massive. UNIT B In core KM—3, Unit B of the Mixed Layer is not separable from Unit A, partly because much of the core in this interval was not recovered. The difficulty in separating them also stems from the original de- scription of Unit B (Smith, 1962, p. C68; 1979, p. 14), which relied on the smaller amounts of nahcolite and the presence of halite in Unit B as the major distinction between it and Unit A. We now find that some beds of trona and nahcolite of Unit A grade laterally into halite-rich zones, that Unit B contains several continu- ous halite-free beds of trona that can be traced over large areas, and that Unit B is also characterized by a 13 considerable amount of thenardite. Therefore, we here abandon the mineralogic distinction between these two units and, on the basis of either core or electric logs, separate Unit A from Unit B at the base of the conspicuous northupite-bearing mud layer described above at’the base of the Main A Zone of Unit A. Less information is available about the composition of Unit B than that of Unit A, although core data obtained since publication of the latest description of the unit indicate that the saline beds in Unit B, besides contain- ing thenardite, are areally more extensive than any saline bed in Unit A. UNIT C Halite dominates Unit C in core KM—3, as in the originally defined unit (Smith, 1962), and its abun- dance still provides a satisfactory method of separat- ing this unit from Unit B above and Unit D+E below. In the salt layers, bedding is mostly faint, thin, and defined by variations in silt or clay impurities; the colors range from medium to light shades of green or gray. The mud layers, most numerous in two zones near 140 and 150 m, are generally dark brown. A A' West Core designations East Dep‘h Stratigraphic 501 502 503 504 505 506 507 508 509 510 511 “eemme‘e's’ unit (E) (E) (C) (E) (E) (E) (C) (C) (C) (E) (0) 0 I 0 J/ i l E . l ) ) 1 1 1 ° 3.) , , W 2:: a: / 1 o E c 150 — m = ,3 — 50 < 31') E 200 —- 60 < 3 x 04 / e 2 \\/ _ 70 a D '8 >. 250 — 3 ‘E [Unit s-5 — 80 1: - ~— .§ § 5 ”:11, 300 —’ 9° '1'; " 'E D — 100 I 350 — ( ( — 110 1 “In? 0 500 1000 METERS __ 400 I 120 o 1000 2000 3000 FEET VERTICAL EXAGGERATION APPROXIMATELY X20 FIGURE 5.—East-west section through 11 cores, showing stratigraphic relations between Bottom Mud and Units A and B of Mixed Layer, and lateral variations of subunits within Unit A (Upper A Zone and Main A Zone with its saline subunits 8—1 to 8—5). Mud layers within Main A Zone are stippled. Letters in parentheses below well numbers indicate basis for stratigra- phic boundaries: (E), electric log; (C), core log. See figure 2 for locations of section line and cores. 14 Normative compositions of the saline layers in Unit C show that halite accounts for 65 to 95 percent of the total; thenardite makes up more than 15 percent of one zone and 5 to 10 percent of most others; and trona makes up 2 to 15 percent of most beds. The normative compositions of the mud layers are about half acid- insoluble material, with a varying amount of dolomite; the mud generally includes 25 percent or more saline minerals. UNIT D+E Unit D+E in core KM—3 is composed mostly of mud layers, although two thick saline zones occur in the lower half of the unit and several thinner ones in the upper half. Most of the mud layers are distinctly massive and generally dark olive brown to brown; the saline units are thin bedded and impure. Unit D was distinguished from Unit E in earlier reports (Smith, 1962, p. C68; 1979, p. 15) by the presence of nahcolite and trona in Unit D and the absence of these minerals in Unit E. In core KM-3, the presence of trona in two beds near the base-of the combined unit makes this criterion for separating Unit D from Unit E in- applicable. About equal amounts of pirssonite, dolomite, and acid-insoluble materials, with a little magnesite, char- acterize the normative compositions of the mud that constitutes most of Units D+E in core KM—3, although magnesite was not identified in any X-ray patterns from this unit. Halite, trona, and thenardite, in de- creasing order of abundance, make up the few saline layers. UNIT F Unit F, originally defined on the basis of the lowermost 25 m of nearly pure mud in core L—W—D (Smith, 1962, p. C62), is here redefined to include deeper sediment and a description of its basal contact. Almost the entire unit is mud, mostly green to olive; the upper part is very dark and nearly massive, whereas the lower part is lighter colored and mostly well bedded. The mud of this unit contains large amounts of acid-soluble pirssonite and dolomite, and smaller amounts of searlesite and (normative) magnesite; acid- insoluble materials generally account for 25 to 50 per- cent of the normative composition, and dolomite ac- counts for 10 to 20 percent of the total. Pirssonite ranges in abundance from less than 10 to more than 40 percent and averages about 20 percent; the absence of this mineral in deeper units provides a mineralogic criterion, in addition to the lithologic criterion, for CORE KM-3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA separating Unit F from Unit G.5 Normative saline- mineral percentages are uniformly low. UNIT G Unit G is a new unit defined here. Its most evident characteristic is the cyclic pattern of salt layers alter— nating with mud layers, each commonly 2 to 10 m thick and averaging 3.4 m in thickness; a total of 17 salt layers can be identified (pl. 1). Some of these mud layers are distinctly bedded, but many are mottled as if deformed by postdepositional bioturbation, soft- sediment deformation, or diagenetic saline-mineral growth. With three exceptions, the mud layers are light to medium green or greenish gray. Most of the salt layers are massive and coarse grained. About two- thirds are relatively mud free and have gray or yellow hues; the less pure beds are light green. The presence of numerous thick salt layers distinguishes this unit from Unit F above, and the green color of the mud layers distinguishes it from Unit H below. Except for a few beds in the lower third of the unit, the amount of normative dolomite in the mud is mostly between 10 and 20 percent, less than in Unit F above. The amount of calcite is about half that of dolomite but distinctly greater than in all but the basal zone of the overlying unit; normative and observed magnesite occur in one bed. Acid—insoluble minerals in the mud range from 50 to 75 percent. The salines are dominated by halite, but notable concentrations of thenardite occur near the top of the unit. A few percent of glauber- ite and anhydrite occur in beds composed of impure halite. UNIT H Unit H is another new unit defined here. The litho- logic features that distinguish it from the overlying and underlying units are its persistent yellow to brown colors, its massiveness and mottling, and its character- istic irregular concentrations (rather than beds) of anhydrite and glauberite. Much of the unit is com- posed of material estimated to be silt size. The normative composition of Unit H is domi- nated by acid-insoluble minerals, on the average ac- counting for 75 percent of the total. A few horizons have concentrations of thenardite, glauberite, and anhydrite with distribution and shapes similar to those of calcium carbonate in calcic soils; these sim- ilarities suggest crystallization of the sulfate minerals from vertically migrating capillary waters beneath 5The computer program used to calculate the normative compositions from chemical analyses was changed to exclude pirssonite and trona below 305 m, which is near the top of Unit G. The result is possibly an exaggeration of the abruptness with which these minerals die out downward, although neither the X»ray data nor the field log reports them below the depths shown on plates 3 and 4. HISTORY OF SEDIMENTATION 15 the floors of playa lakes. Some normative dolomite persists throughout the interval, but it is consistently sparse. UNIT I Unit I is the third new unit defined here. Its domi- nant lithology is mud. Except for the upper 3 m, which is darker and mottled, the unit is characterized by medium- to light-olive-gray color, thin faint bedding, and an absence of bedded saline minerals. Its contact with the overlying Unit H is placed at the top of the uppermost thick bed characterized by olive hues. The normative composition of Unit I is largely acid-insoluble minerals. Concentrations of normative anhydrite are found in the transitional zone near the top, and 5 to 10 percent normative dolomite character- izes most of the unit. Two layers of moderately indurated tuff lie near the base of Unit I. The upper bed consists of glass shards (refractive index, 1.495) in a fine-grained an- isotropic matrix; glass makes up about 50 percent of the rock. Also present are minor amounts of K- feldspar, plagioclase, biotite, an expansive clay, and an unidentified elongate mineral that has been entirely altered. The glass shards have acicular shapes and sharp points that indicate little, if any, stream trans- port. Analysis shows that 17 percent of this material is soluble in water and that the insoluble fraction consists of 66.4 weight percent SiOZ, 14.4 weight percent A1203, and 19.2 weight percent of other components. The vitreous material in the lower tuff bed is almost entirely altered to analcime; it also contains small amounts of sanidine, plagioclase, biotite, hornblende, an expansive clay, and opaque minerals. ALLUVIAL SAND AND GRAVEL More than 200 m of moderately well indurated coarse sand and gravel underlie the lacustrine units of the Mixed Layer. Although 60 percent of this interval was not cored, drilling characteristics gave no indica- tions of other lithologies. In the samples recovered, some zones have a very faint bedding defined by alinement of similar-size fragments, but most zones are massive. Colors typically are medium to dark brown, hues imparted chiefly by hematitic inclusions in the cement. Thin-section study of a sample from 836 m confirms that the sand grains are angular, poorly sorted, and composed of an arkosic-mineral assemb- lage; they are cemented and locally replaced by dolom- ite and analcime. ' Seven partial analyses of the acid-soluble fraction of this sand and gravel—presumably, mostly cement- ing and replacement minerals—differ little. The aver- age values and ranges of individual percentages are: A verage Range 0.76 - 1.3 .35 — 0.93 1.09 - 1.89 2.22 — 4.53 .46 — 0.76 1.25 - 2.15 75.57 — 87.32 Angular to slightly rounded fragments of igneous rocks ranging in size from 3 to 15 cm were recovered in the core at depths of 698, 699, 717, 718, 733, 742, 747, and 821 m. Thin-section study of a cobble from 698 m showed it to be a porphyritic granite composed of an estimated 50 percent K-feldspar, 15 percent plagio- clase (Angs). 5 percent quartz, 3 percent green horn- blende, 2 percent microcline, a trace of biotite, and about 20 percent calcite or dolomite that has replaced some minerals. Study of a cobble from 699 m showed it to be a dacite or andesite; its estimated composition was 20 percent normally zoned euhedral plagioclase (An45), 10 percent brown hornblende, and 2 percent pyroxene, in a matrix composed of crystalline to hemi- crystalline material. BEDROCK The bottom 15 m of core KM-3 is in quartz monzo- nite, the material that makes up almost all the bedrock west of Searles Valley but only part of it to the east. Because the upper 11 m of bedrock was penetrated with a rock bit, no core was recovered, and so the depth and character of the surface-weathering profile is not known, except that it did not extend below this depth. Below 11 m, the rock is slightly fractured, shows some iron oxide stains on the crack surfaces, but is not wea- thered. Thin-section study of a sample from 929 m shows the quartz monzonite to be medium grained (2—4 mm) and composed of an estimated 40 percent K- feldspar, 40 percent plagioclase (Anso), 8 percent quartz, 7 percent biotite, 4 percent hornblende, and trace amounts of apatite, sphene, zircon, and opaque minerals. HISTORY OF SEDIMENTATION The depositional environments responsible for the sediment present in the Mixed Layer and below it in core KM-3 can be reconstructed in large part by using the criteria established to separate the stratigraphic units, as described briefly in the previous section and in more detail elsewhere (Smith, 1979, p. 78-85, - 108—109). Each stratigraphic unit in the Mixed Layer (table 1) represents a long interval, most several hundred thousand years, when the depositional mode remained relatively constant even though the constant characteristic of that environment might have been 16 one of no change or of repeated cyclic change. Deposi- tional environments of the sediment younger than the Mixed Layer were described elsewhere (Smith, 1979, p. 79—96, 109—112); those environments responsible for the Mixed Layer units younger than Unit F are only summarized below, except where the environments inferred here differ from those suggested earlier. FROM MIOCENE OR EARLY PLIOCENE TIME TO 3.18 m.y. B.P.—ALLUVIAL SAND AND GRAVEL Coarse alluvial sand and gravel, probably derived from the ancestral Argus Range area to the west, built fans that sloped to a low point somewhere east of the site of corehole KM-3. Their arkosic composition indi- cates a plutonic-rock source area, as do most of the larger fragments. The southern Argus Range (fig. 1) is now composed largely of quartz monzonitic to granitic rocks. The absence of metamorphic-rock fragments in the core makes the ancestral Slate Range area, east of the drill site, an unlikely source for this sediment. A source for the dacitic cobbles is not known. The poorly sorted and bedded alluvial sediment indicates deposi- tion by intermittent streams, presumably in an arid or semiarid region. In contrast, the evidence provided by the pervasive red, orange, or brown color suggests that the terrane undergoing erosion was deeply weathered; possibly this coloration reflects an earlier climate that was humid, or a surrounding terrane that previously was tectonically stable long enough to become deeply weathered under semiarid conditions. It is not known when deposition began in the basin; the estimate here of Miocene to early Pliocene is based on an extension of observations made in the northern part of Searles Valley. There, late Tertiary volcanic flows, which rest on snail-bearing sediment no older than middle Miocene, were deposited on an east- sloping surface that predates the downwarping re- sponsible for the creation of the northern part of Sea- rles Valley (Smith and others, 1968, p. 13—14, 25; Smith and Church, 1980, p. 528-529, fig. 4). The age of the now—deformed volcanic rocks provides a lower limit for the age of the deformation that created the northern part of the valley. We assume that the same episode of deformation affected areas to the south, including the site of corehole KM-3, and that it was responsible for initiation of the basin of sedimentation that is now Searles Valley. 3.18-256 m.y. B.P.—UNIT I, MIXED LAYER Unit I of the Mixed Layer is composed almost entirely of brown to olive lacustrine sediment. The fine grain size, absence of saline layers, faint thin bedding, CORE KM—3—-—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA and low percentages of normative calcite and dolomite all indicate a persistent deep freshwater lake as the depositional environment. Thin zones at 582 and 640 m characterized by brown hues may record two periods of desiccation when oxidation occurred at the surface; if they do, the absence of accompanying salts would confirm the inferred freshness of the lake water. The light color of much of the sediment suggests either that the lake and its sediment originally had a low organic carbon content or that the carbon has been removed by diagenetic processes. 256-2 04 m.y. B.P.—UNIT H Unit H of the Mixed Layer is composed predomi- nantly of mottled faintly bedded yellow to brown clay and silt that is inferred to have been deposited in a playa or an intermittent shallow lake. Concentrations of glauberite and anhydrite, generally in the form of veins and pods, are inferred to have crystallized within the weathering zone or in the capillary zone; they may represent times when playa sedimentation slowed or ceased. Relatively brief periods of perennial lakes may be represented by the dark-olive mud near 440 m and the green mud near 526 m, but the three thin zones of yellow to olivebrown mud that lie between those layers contain disseminated salts that probably document them as products of ephemeral lakes. 2.04-1.28 m.y. B.P.—UNIT G Unit G of the Mixed Layer is characterized by cyclic deposition of mud and saline layers. A total of 17 saline layers (pl. 1), indicative of nearly complete or complete desiccation, are separated by mud with, in all but three layers, olive to green hues thought to be indicative of deep perennial lakes. The significance of the mottling of many of these mud layers is unclear, although some variation in this type of depositional depositional environment might be inferred. The mud layers have thicknesses that imply deposition over periods of several thousand to a few tens of thousands of years (Smith, 1979, p. 76—78). The mud-free saline layers have lithologies that are best explained by unin- terrupted deposition from a rapidly shrinking lake; beds of these thicknesses could be attained by crystal- lization over periods of less than half a century (Smith, 1979, p. 75—76). The less pure saline layers may repre- sent dry-season crystallization of salts that was inter- rupted by wet-season influxes of sediment and water, thus reflecting a slower overall process of deposition. The layers composed of bedded thenardite and halite are likelier to indicate periods of desiccation than are those characterized by only one saline mineral, because HISTORY OF SEDIMENTATION 17 cocrystallization of two phases generally indicates higher brine concentrations. Because of the inferred contrast in the depositional rates of salines and mud, most of the period represented by Unit G is believed to have been characterized by deposition of fine elastic material in a perennial lake that evaporated rapidly to dryness or near-dryness at least 17 times. 1.28-1.00 m.y. B.P.—UNIT F Unit F is mostly green to olive mud, much of it distinctly bedded; the green (almost blue) hues of the lower half of the unit are especially distinctive. Deposi- tion in a perennial stable lake seems to be the most likely explanation for the dominant lithologies and mineral components present. Two zones near the top, characterized by orange to brown hues, may represent periods of dryness during which the mud underwent oxidation at the surface. The absence of saltbeds at these times of possible desiccation implies that pre- vious high stands of the lake were deep enough to overflow, so that the accumulation of large amounts of saline components in the lake water was prevented. The small amounts of normative pirssonite, searlesite, trona, northupite, and magnesite mixed with elastic materials document the first accumulation of new components in the waters entering the basin, espe- cially near the end of this period, and, probably, their incorporation into the sediment interstitial brines. Other inferences about the style of sedimentation represented by the upper part of this unit, based on other cores, were made elsewhere (Smith, 1979, p. 108—109). 1.00-0. 57 m.y. B.P. —UNIT D+E The mud and subordinate amounts of bedded salts in Unit D+E appear to record a fluctuating and occa- sionally small or dry lake. The olive—brown to brown color of most of the mud layers suggests that even the more permanent lakes were well oxygenated. The con- tinued presence of several new saline minerals in the salt layers shows that the evolution of lake-water chemistry which began during the previous episode was continuing, and the several phases of minerals crystallized during periods of saline deposition proba- bly indicate desiccation rather than the existence of a shallow perennial saline lake that was crystallizing an incomplete suite of its dissolved salts. Evidence from outcrops suggests that the age assigned to the upper contact of Unit D+E is approxi- mately correct. In the Lava Mountains, which form the south edge of Searles Valley, the Christmas Canyon Formation (Smith, 1964, fig. 15) grades northward from its type section into about 30 m of lacustrine deposits that do not have their basal contact exposed and have a layer of volcanic ash 1 to 2 m below their top. This ash has been identified as Perlette type 0 (G. A. Izett, written commun., 1978), and its recalibrated age of 0.62 my. shows that a large lake existed in Searles Valley until shortly after that time. A zone of thin tuff beds, characterized by poorly defined platy shards similar to those of the Perlette ash, has also been identified in core KM-3 near the top of Unit D+E, between 168.6 and 169.8 m (R. L. Hay, written com— mun., 1981), but the tuff is too highly altered for posi- tive identification. We think it reasonable, therefore, to correlate the Christmas Canyon Formation with Units F and D+E in core KM—3 because their inferred depositional environments are similar and the interpo- lated age of 0.57 m.y. assigned to the upper contact of the subsurface Unit D+E is close to the probable age of the top of the Christmas Canyon Formation. 0.57-0.31 m.y. B.P.—UNIT C Saline layers dominate Unit C. Halite is the most abundant saline mineral, but the amounts of norma- tive thenardite and trona suggest that desiccation was complete most of the time and that Searles Lake was a dry salt flat during most of the 260,000 years assigned to this unit. The two zones near 140 and 150 m contain- ing numerous layers of brown mud represent sediment deposited in a succession of lakes that were perennial over periods of probably centuries to ten or more thou- sands of years, although this time represents only a minor part of the total. 0.31-0.13 m.y. B.P.-—UNITS A AND B Units A and B of the Mixed Layer are so incom- pletely represented in core KM—3 that a satisfactory reconstruction of the depositional environment is not possible from this core. Earlier studies (Smith, 1979, p. 84—85, 108—109) concluded that deposition took place in fluctuating shallow saline lakes whose anion compo— sition during deposition of Unit B was largely carbo- nate plus some chloride, whereas deposition of Unit A was from a more nearly pure sodium carbonate water. The presence of halite in some parts of the central facies of Unit A in the more recently obtained cores described here indicates that a significant amount of chloride was present in the lake waters responsible for both Units A and B—as it is in virtually all natural waters in this geologic province today. The halite- bearing layers may indicate desiccation. Those layers in Units A and B that appear to be truly monomineralic, composed of nahcolite or trona in 18 all parts of the depositional basin, we now infer to reflect depositional conditions similar to those sug- gested elsewhere to explain monomineralic beds of sodium carbonate and sulfate in the Bottom Mud (Smith, 1979, 85-86, 109—111). Those beds crystallized from lakes that had estimated salinities of between 5 and 15 percent, as well as large percentages of dis- solved salts whose solubilities vary greatly in response to temperature. (For example, winter cooling of Owens Lake surface waters produced monomineralic depos- its of natron that converted diagenetically within months to trona or nahcolite [Smith, 1979, p. 83].) The interbedded thin layers of mud in Units A and B are presumed to represent periods when the lake was slightly too deep and dilute for salts to crystallize out during winter, or periods when the winters were not so cold. RATES OF SEDIMENTATION The overall apparent sedimentation rate of the lacustrine sediment in core KM-3 is about 22 cm/1,000 yr. The rates of deposition of the sediment that lies between paleomagnetically dated horizons in core KM—3 (table 3) are mostly between about 10 and 30 cm/ 1,000 yr, although the net sedimentation rate for the paleomagnetically dated section between 185.0 and 683.9 m is 20.6 cm/1,000 yr. The anomalous rates calculated between closely spaced horizons that fall in Unit G probably reflect actual variations that charac- terize the deposition of sections that include salts, as opposed to those composed largely of mud deposited in perennial lakes. Units younger than the Mixed Layer TABLE 3.—Sedimentation rates between paleomagnetically dated horizons in core KM—S [Data from Liddicoat and others (1980, table 1)] Depth of Calculated Units in dated horizon ( Age ) depositional rate Mixed (m) m.y. (cm/103 yr) Layer 185.0:t10.0 0.73 11.7 D+E 204.9:k0.1 .90 - 23.3 D+E 221.2110.6 .97 22 5 [NE F G 378.Sil.1 1.67 10'2 ’G ’ 398.910.!» 1.87 6.7 G 408.3:t4.2 2.01 54'7 G 424.7tl.7 2.04 28.4 G H 447.4:t1.6 2.12 [‘3'0 ’11 456.014.6 2.14 ”'7 H 522.9tl.2 2.48 21'2 H I 616.2:t1.5 2.92 ”'9 ,I 643.112.6 3.01 21's I 651.7t2.2‘ 3.05 32'2 I 683.9-1:4.2 3.15 ' CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA have more rapid apparent rates of deposition. The rate from the base of the Bottom Mud to the surface is 53 cm/1,000 yr; rates in the Parting Mud range from 26 to 42 cm/1,000 yr, and in the Bottom Mud are 22 cm/ 1,000 yr; and rates for the units composed mostly of salines are somewhat higher (Smith, 1979, p. 1, 77). TRENDS IN SEDIMENTATION AND THEIR SIGNIFICANCE CHANGE FROM ALLUVIAL TO LACUSTRINE SEDIMENT The abrupt change from alluvial to lacustrine sed- iment at 693 m represents the most extreme change in depositional environments in core KM—3. In outcrops of Cenozoic sediment deposited in closed basins, alluv- ial sections that grade upward (or laterally) into lake deposits almost all have a transition zone composed of alternating lacustrine and alluvial material because closed-basin lake levels change from year to year, sometimes by several meters, and areas destined soon to be “permanently” covered by water commonly are inundated briefly one or more times before the final change takes place. The absence of such a transition near the alluvium- lakebed contact in core KM—3 suggests that the flood- ing of Searles Valley was a rapid event. Such flooding could have been due to one or more of the following causes. (1) The eruption of volcanic flows in the Dead- man Pass area (fig. 1) of the central Sierra Nevada 3.2 m.y. B.P. (Dalrymple, 1964) blocked the channel through which the ancestral San Joaquin River had previously cut across the Sierra Nevada and drained part of eastern California and Nevada (N. K. Huber, 1981); after that eruption, the integrated drainage just east of the Sierra Nevada appears to have been per- manently diverted into the ancestral Owens Valley and other valleys to the south. (2) The global climatic change that first generated ice sheets large enough to raise the 180 content of the oceans above that of their present interglacial level about 3.2 my RP. (Shackle- ton and Opdyke, 1977) may have caused a marked increase in precipitation in this continental area. (3) A major vertical displacement along the Garlock or another fault (fig. 1) created or raised the spillway threshold of Searles Valley, so that waters flowing into Searles Valley immediately formed a new or greatly enlarged lake that permanently covered the alluvial fan at the site of corehole KM—3. (4) Headward erosion by a stream that drained into Searles Valley captured the drainage from the east slopes of the ancestral Sierra Nevada area and diverted it from a former course that led to another area. The coincidence TRENDS IN SEDIMENTATION AND THEIR SIGNIFICANCE 19 between the documented ages of causes 1 and 2 and the age of the base of lacustrine sediment in core KM—3 (table 1) provides strong circumstantial evidence that either or both causes were responsible, although either cause 3 or 4 also is certainly possible. Of the two dated events, however, evidence that the flooding of Searles Valley was rapid favors strongly, in our view, the explanation that involves volcanic damming of the ancestral San Joaquin River at Deadman Pass over the more gradual global climatic change. INCREASES IN THE PERCENTAGE OF CHEMICAL SEDIMENT The low percentage of nonclastic minerals and the absence of bedded saline minerals in Units H and I suggest that the lakes in Searles Valley during those periods contained only small amounts of dissolved sol- ids. These lakes must have overflowed much of the time they were enlarged, and received little inflow during the time they were playas. When they dried— briefly, as suggested by the relatively thin zones of orange sediment in Unit I; or for long periods, as sug- gested by accumulation of the playa sediment that constitutes Unit H—the volume of dissolved solids was too small to form beds of salt minerals. The relatively insoluble salines that did accumulate as the lakes shrank to low levels are either disseminated through- out the clastic materials or concentrated in patchy zones that are inferred to be products of postdeposi- tional processes. Once lacustrine deposition of Units H and I had ceased, there followed a gradual, but inexorable, increase in the relative percentage of acid-soluble chemical sediment in the mud layers, accompanied by a relative decrease in clastic components (pl. 2). Dolomite and calcite increased most notably in Unit G, pirssonite and dolomite in Units F, E, and D, and gaylussite, dolomite, and aragonite in the younger units (pl. 4). This trend is believed to indicate a gradual decrease in the volume of water entering Searles Val- ley, which would have decreased the volume of clastic material and increased the salinity of the lakes in Sea- rles Valley. As explained elsewhere (Smith, 1979), dolomite, considered to be a primary mineral in the Searles Lake deposits, is probably an indicator of lake waters with a moderately high pH and total salinity (p. 81); pirssonite and gaylussite, though diagenetic min- erals, are considered indices of the amount of CaCO3 in the sediment before its reaction with N a-rich solutions (p. 101-103); and a large amount of inferred or observed CaCO3 in the sediment deposited in high-pH lake waters is believed to indicate a deposit formed in lakes that were chemically stratified (p. 79—80). CHANGES IN COMPOSITION 0]" THE LAKE WATER The chemical composition of the acid-soluble frac- tion of the cores (pl. 2) and the suites of nonclastic normative minerals (pls. 3, 4) allow the changing chemistry of the lake waters to be reconstructed. In Units H and I, the consistently small amounts of acid- soluble components, and their identification as rela- tively insoluble calcium-bearing minerals, show that the earliest lake waters in Searles Valley contained Ca”, Mg”, Na‘, S042", and 0032‘, and that these ions reached concentrations in the interstitial brines that were high enough to cause postdepositional crystalli- zation. Lake waters present when the saline layers in Unit G were deposited reached higher salinities than in any previous lake in the valley. These waters were dominated by Na+ and Cl‘, and the absence of norma- tive or observed sodium carbonate or sodium borate minerals indicates that the saline lakes were not alkaline. The carbonate in Unit F, nearly twice as abundant as in the underlying units, is mostly present in the mud layers as normative dolomite and pirssonite, and a little as calcite. The appearance of a large amount of normative pirssonite and small amounts of trona and northupite is not an artifact of the computer-program change; those minerals were added to the program for the normative-mineral assemblage and applied to data 17 m lower in the core than their first reported norma- tive occurrence. We interpret this change to mean that the lakes depositing the mud did contain a little more Na+ than previously, so that some normative trona, thenardite, and northupite formed, but that down- ward-moving interstitial Na*-rich brines later intro- duced all the sodium present in pirssonite (Hardt and others, 1972, fig. 20; Smith, 1979, p. 100-103; Fried~ man and others, 1982). The total amounts of carbonate in Unit D+E and Unit C are about the same as in Unit F, and the normative minerals show that slightly less of this component crystallized in the mud layers as dolomite and pirssonite, and more of it in the saline layers as trona and northupite. As in Unit G, high concentrations of 01’ continued to characterize the saline lakes, but an abrupt increase in the concentra- tion of S04? early during the deposition of Unit C resulted in the crystallization of more thenardite. The minerals in Units A and B indicate that beginning about 0.31 m.y. B.P., high—pH lakes contain- ing large amounts of Na” and 0032' in their waters occupied the basin. This compositional change we believe to be a consequence of the hydrothermal activ- ity that reached its peak about 0.3 m.y. B.P. in the Long Valley caldera area (Bailey and others, 1976, p. 738), which is considered to have been the chief source of 20 mineral components in the upper part of the Searles Lake deposits (Smith, 1976). Beds containing halite and thenardite, such as the lower three saline beds in the Main A Zone and parts of Unit B, document salini- ties exceeding about 20 percent. However, lake—water salinities appear to have remained in the range 5—15 percent for long periods, probably spanning hundreds to thousands of years, when winter cooling generated successions of monomineralic layers of crystals, such as those present in the upper part of the Upper A Zone, in the upper two saline horizons in the Main A Zone, and in parts of Unit B. Lake-level increases—possibly small—resulted in periods of comparable length when lake waters had lower salinities and salts did not form. Saline crystallization during winter requires salini— ties to be near their maximum during that season. Such a regime could have been provided by a climate in which most precipitation in the tributary area fell as snow in the mountains during winter; such a climate would have produced the maximum flow of fresh melt water during late spring and early summer and per— mitted winter crystallization of salines in a down— stream lake. The mud layers above the Mixed Layer—the Bot- tom Mud and the Parting Mud—also contain minerals indicating that Na+ and C032' continued to dominate the lake waters, although the CI" content also was high when the Parting Mud was deposited (Smith, 1979, p. 79-82). In the saline units of the overlying Lower Salt and Upper Salt, sodium carbonate and chloride min- erals crystallized in sequences dictated by their phase relations, and the appearance in these sequences of sodium and sodium-potassium sulfate minerals shows that these components reached greater concentrations in those lakes than at any earlier time. CHANGES IN CARBONATE MINERALS IN MUD LAYERS The major COg-bearing minerals in the mud units are gaylussite, pirssonite, calcite, aragonite, and dolomite (pl. 4). Their distribution is a result both of primary crystallization patterns that reflect lake chemistry and of diagenesis that reflects interstitial- brine compositions and migration patterns. Dolomite persists throughout the entire section, and, as noted above, its percentage increases gradually upward. Aragonite is present in most cores only at depths less than about 40 m (Smith, 1979, p. 104), although it is found in core KM—3 down to 65 m. Gaylussite, pirssonite, and calcite are almost mut- ually exclusive. Of these three, only calcite is detected below the base of Unit F; pirssonite is most abundant from the top of a transition zone at the base of Unit F to the middle of Unit C; and gaylussite dominates from CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA that horizon upward. The apparent replacement of primary calcite by diagenetic pirssonite and the abruptness of the base of the pirssonite-bearing zone suggest that this is the maximum depth to which inter- stitial Na+-rich brines moved downward before mov- ing outward from Searles Valley (Hardt and others, 1972, fig. 10; Smith, 1979, p. 100—103; Friedman and others, 1982). The change from pirssonite to gaylussite in Unit C is apparently controlled by temperature; the temperature at that depth is estimated to be near 35°C, which approximates the temperature that laboratory studies of these phases, corrected for higher salinity, would indicate for this mineral transformation (Smith, 1979, p. 103). PALEOCLIMATIC INTERPRETATION HISTORY or SEARLES LAKE A 3.2-my. history of the lakes in Searles Valley can be interpreted using the following assumptions: ( 1) Mud layers (except playa-lake sediment) represent perennial lakes, and salt layers saline or dry lakes; (2) green mud represents deep lakes, and yellow mud shallower lakes; (3) thin orange oxidized zones, soil- profile textures, and playa sediment represent per- sistent dry lakes; and (4) the ages of all beds are pro- portional to their positions between the nearest dated horizons. Figure 6 plots the generalized history of the lake inferred from these assumptions. Two aspects of this interpreted history are espe- cially notable. One is that during about three—quarters of the past 3.2 m.y., medium-size to large pluvial lakes have occupied Searles Valley; interpluvial regimes have been much less frequent, and their durations have shortened over time. The first part of this conclu- sion is similar to one based on deep-sea-core evidence indicative of polar-ice-sheet sizes and global climates; this conclusion infers that it is the interglacial climates during this period which are atypical (Shackleton and Opdyke, 1973, 1976; Ruddiman and McIntyre, 1976; van Donk, 1976). The other notable aspect of this his- tory is that some stratigraphic units, representing periods of hundreds of thousands of years, are charac- terized by little recorded climatic variation (Units C, F, H, 1), whereas other units, also representing periods of significant duration, are characterized by repeated variations (Lower Salt and Units A+B, D+E, G). Minor climatic variations during times when the lake was continuously large or overflowing (parts or all of Units F and I) might not be recorded by this type of evidence, although significantly more pluvial variations should be recorded by the deposits of arid periods (Units C, H). O a: l | LAKE LEVEL OR SALINITY U l PALEOCLIMATIC INTERPRETATION HISTORY OF OWENS RIVER FLOW, AS INDICATED BY THE SIZE OF SEARLES LAKE Variations in the volume of water that reached Searles Valley during its 3.2-my. lacustrine history are primarily interpreted to record variations in the regional climate as expressed by the volume of water flowing in the ancestral Owens River. During late Quaternary time, when its drainage area and the orc- graphic influence of the Sierra Nevada on precipita- tion in the Great Basin were about the same as at present, the volume of water carried by the Owens River and the lakes filled by it served as a relatively quantitative record of variations in the precipitation within its drainage area, especially the east slope of the Sierra Nevada. Estimates of this water volume, based on the amount calculated as necessary under present climatic conditions to offset evaporation from the sev- eral lakes in the chain, suggest that three times the present average flow of the Owens River would create a small saline lake in Searles Valley, five times the present flow would form a large, but slightly saline, lake, and six times or greater flow would cause over- flowing and thus create a freshwater lake (Smith, 1976, table 1; Smith and Street-Perrott, 1982). If pluvial-period air temperatures at that time were 5° to 10°C lower than at present and evaporation rates were reduced accordingly, inferred streamflow volumes would still have been 50 to 75 percent of these amounts. 21 Translation of changes in the flow of the Owens River into changes in regional precipitation is difficult; this relation is by no means linear, partly because of the multiplicity of hydrologic factors that control runoff. A more significant factor in Owens Valley is that most of the present runoff comes from less than 20 percent of the total drainage area, and when climatic change produced some runoff from additional areas, the total flow increased disproportionately (Lee, 1912, p. 9, 32—44). For some purposes, however, an accurate his- tory of the changes in streamflow is as useful as, or even more useful than, accurate data on the changes in actual precipitation. Expanding the lake history plotted in figure 6 to reconstruct the history of flow in the Owens River before late Quaternary time becomes more uncertain as the age of the deposits increases. The size of the drainage area 3.2 my. BB, the sizes and numbers of upstream lakes, and the evaporation rates could have differed substantially. Also, during the early part of that time, the Sierra Nevada had not yet been elevated to its present level, and so its effectiveness as a barrier to the eastward movement of airmasses bringing storms was less. The central part of the range is esti- mated to have been 950 m lower 3 my. B.P. than at present (Huber, 1981), and one climatologic model (see below) suggests that the precipitation in areas east of an ancestral Sierra Nevada crest that was lower in SEARLES LAKE Soils? % I! 34 \I l a: . g g Mixed Layer A2353. 3 5 UNIT UNIT UNIT UNIT UNIT UNIT UNIT and 9 A + B c D + E F G H I grave' l l I o l I I I I I I I l 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 AGE, IN MILLIONS OF YEARS FIGURE 6. —Reconstructed history of Searles Lake, based primarily on Core KM—3. Inferred lake level or salinity indicated by letters: A, Lake deep, . fresh, overflowing most of time. B, Lake deep, slightly saline, overflowing only part of time. C, Lake shallow, moderately saline, overflowing rarely if at all; thin saltbeds deposited intermittently as result of winter cooling. D, Lake shallow to dry, very saline; salts deposited during most of year. E, Lake a playa or salt flat most of year, flooded intermittently. Brief possible fluctuations during deposition of Units F, H, and I are not plotted. 22 elevation by that amount might have been 50 percent greater than now. This model would help explain an ancestral Owens River flow that was greater and thus able to support lakes that were larger and more per- sistent than those characterizing the past million years. However, the long dry period 2.56-2.04 my RP. and the sporadic dry periods between 2.04 and 1.28 my RP. must correspond to periods of markedly decreased precipitation and streamflow; geologically brief and reversible changes in drainage area, lake area, or mountain-range elevation during those times are unlikely. RELATION BETWEEN SEARLES LAKE HISTORY AND SIERRA NEVADA GLACIAL STAGES Correlations of the Tahoe and Mono Basin Glacia- tions with the Bottom Mud, of the Tioga Glaciation with the Parting Mud, and of the Tenaya Glaciation of Sharp and Birman (1963) with the upper part of the Lower Salt have been made (Smith, 1968, p. 307—308; 1979, p. 116, fig. 42). The record from core KM—3 now enables us to correlate the older Sierra Nevada glacial stages with Searles Lake history. We correlate the most extensive deposits of the Sherwin Glaciation with Unit F of the Mixed Layer and infer less extensive glacial deposits that are correlative with the less per- sistent pluvial period represented by Unit D+E. The age of the base of the Sherwin Till, therefore, would be about 1.28 m.y., an age allowed by Sharp’s (1968, p. 355) interpretation of “Sherwin outwash” that rests on basalt dated at 3.4 my. (recalc). The age of the top of Unit F in core KM-3 is estimated at 1.00 m.y. Sharp (1968, p. 361) estimated the weathered upper surface on the extensive deposits of the Sherwin Till that lie east of the present range to be a few tens of thousands of years older than the 0.73-m.y.-old Bishop Tuff, although he noted other evidence that could indicate an older age for the till. Although most of Unit D+E is older than 0.73 my, the inferred perennial lakes char- acterizing Searles Valley up to the end of deposition of Unit D+E and the nearby outcropping lacustrine sed- iment of the Christmas Canyon Formation indicate that pluvial conditions persisted in the area until approximately 0.57 my. B.P. Significant-size glaciers might have persisted in the deep valleys of the eastern Sierra Nevada 150,000 years longer than east of the range, where the Sherwin Till is overlain by the Bishop Tuff. In fact, evidence of such glaciation in the Sierra Nevada during that period was presented by Rinehart and Ross (1964, p. 68) and Bailey and others (1976, p. 732, 735), who reported ice-r'afted morainal material on an island in Pleistocene Long Valley Lake, which formed 0.75—0.65 m.y. B.P. (recalc) as a resur- gent dome grew in the Long Valley caldera. The 1- to CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA 2-m sizes of the blocks virtually require that debris— bearing glaciers extended to at least the edge of the lake (2,320—m elev), where blocks from them calved directly into the waters, floated eastward to the shore of the rising dome, and melted. Glaciers in this area extended as much as 125 m below this elevation during subsequent glaciations (Rinehart and Ross, 1964, pl. 1), whereas during the present interglacial period they terminate about 1,000 m above this elevation. This discussion revives a question posed earlier (Smith, 1968) on the nature of western Great Basin climate during the nearly half a million years between the pluvial episodes represented by Unit D+E and that represented by the Bottom Mud, which began approx- imately 0.13 m.y. B.P. Except for two zones of perennial-lake mud in Unit C (the thickest of which is 4 m, possibly representing 10,000-30,000 thousand years of pluvial climate), that unit appears to repres- ent 0.26 m.y. of dominantly interpluvial climate. The following 0.18 m.y., represented by Units A and B, is also dominated by salts deposited under conditions that apparently ranged from interpluvial to semipluv- ial. These two units contain numerous mud layers, although individual-bed thicknesses do not exceed a few meters. These mudbeds may each represent a few tens of thousands of years of lakes that were too deep to allow salt deposition, although most of the beds are much thinner and presumably represent periods only hundreds or thousands of years long. It appears from the lacustrine record in Searles Lake, therefore, that during 0.57-0.13 m.y. B.P., first interpluvial and then semipluvial climate characterized this part of western North America, and although sporadic periods of fully pluvial climate may have interrupted this regime, they rarely lasted more than a few tens of thousands of years. The absence of dated glacial deposits in the Sierra Nevada during this period allows an interpre- tation that major glaciations in that area were sim- ilarly brief or absent. The pluvial period represented by Unit I may be correlative with a glacial stage in the Sierra Nevada. The age which that unit represents (2.56—3.18 my.) includes the age assigned to the Deadman Pass Till (Curry, 1966), which is overlain and underlain by vol- canic rocks dated (using new constants) at 2.8 and 3.2 m.y., respectively; it also includes the possible age of the McGee Till, which rests on basalt dated at 2.7 m.y. (Dalrymple, 1963, p. 387). Recent studies by Huber (1981) suggest, however, that at 3 my. BF. (1) the crest of the Sierra Nevada near the Deadman Pass area was 950 m lower than now, (2) glaciers might not have’been able to form because of this lower elevation, and (3) the clast composition of the Deadman Pass Till indicates transport from the north rather than the west, a direc- REFERENCES CITED 23 tion that is difficult to reconcile with a glacial origin for the deposit. Huber, therefore, has suggested that the McGee Till is no more than 1.5 my. old and that the deposits at Deadman Pass may not be till. If Huber’s inference is correct about the unfavorability of summit elevations for glacier development 3.0 m.y. B.P., the pluvial deposits in core KM—3 that can be correlated with the McGee Till would be limited to those units also correlated with the Sherwin Till, namely, Unit F and parts of Unit D+E. But the great differences in the thickness and preserved extent of the Sherwin Till and the McGee Till (Rinehart and Ross, 1964, pl. 1, fig. 31) suggest that they differ in age, and the pluvial period identified here on the basis of Unit I in core KM—3 provides a climatic basis on which to infer an earlier glaciation in the ancestral Sierra Nevada—possibly the McGee Glaciation—that began during late Plio- cene time and ended about 2.6 m.y. BR, 1.3 m.y. before the onset of the Sherwin Glaciation. NATURE OF THE CLIMATE 3-1 m.y. BJ’. If the central Sierra Nevada was 950 m lower 3 my RP. (Huber, 1981) and if most of the mountain range was that much lower, precipitation in the area to the east at that time could have been significantly greater than at present. Assuming that large-scale atmospheric circulation and East Pacific sea-surface temperatures were similar to those of the present, and that we can apply low-level pseudoadiabatic charts as used elsewhere to model precipitation in this area (Smith and others, 1979, p. 173—174, fig. 2), we can estimate the decrease in aridity. For example, moun- tain barriers that are now about 2,500 m high (such as the Sierra Nevada south of lat 36° N.) would then have been about 1,550 m high and, therefore, would have adiabatically cooled the airmasses flowing over them about 65°C less than now and allowed them to retain almost 50 percent more moisture; barriers now 4,000 m high (such as the Sierra Nevada west of Owens Valley), then 3,050 m high, would have cooled passing airmasses almost 7°C less and allowed slightly more than 50 percent more moisture to reach areas to the east. Translating this increased amount of moisture into precipitation is difficult, but we can say that 3 my. RP, about 50 percent more moisture could have been available for condensation and precipitation within the ancestral Owens River drainage. If precipitation increased by 50 percent, the re- sulting increase in runoff would probably have been somewhat greater, possibly 75 to 100 percent. As stated above, however, the present topography of the drainage area, and reasonable projections of the ear- lier Pleistocene hydrology and climate, would not create permanent lakes in Searles Valley unless Owens River flow was several times greater than the present flow. If the drainage leading to Searles Lake was any- thing like it is now, then more than a lowering of mountain barriers would be required to explain lakes in the now dry closed basins of the western Great Basin, and warmer Pacific sea-surface temperatures and more humid climates seem to be indicated. East Pacific sea-surface temperatures 10°C above those of the present, for example, would result in nearly 90 percent more moisture in the eastward-moving air- masses at sea level and nearly 250 percent more at the level of the 950—m-lower range crest. It appears, then, that a persistent lake like the one that deposited Unit I in ancestral Searles Valley required a warmer and more humid regional climate than now, that the playa lake which followed and was responsible for Unit H required a significant drying of this climate, and that the cyclic deposition recorded by the sediment and salts of Unit G was due to repeated oscillation between two climatic balances, possibly centered around a cli- mate intermediate between these extremes. Unit F was deposited during a period that ended 1.0 my. BB, after 70 percent of this entire period of lacustrine deposition had elapsed; the then-existing drainage patterns and climatic regimes were presumably ap- proaching those that characterized late Quaternary and Holocene time, and this unit clearly documents a period of pluvial climate relative to that at present. REFERENCES CITED Bailey, R. A., Dalrymple, B. G., and Lanphere, M. A., 1976, Volcan- ism, structure, and geochronology of Long Valley caldera, Mono County, California: Journal of Geophysical Research, v. 81, no. 5, p. 725-744. Barczak, V. J., and Petticrew, R. W., 1969, The mineralogy and chemistry of exploratory well KM-3: Oklahoma City, Kerr- McGee Research Center Special Report 69—51—A, 3 v. Curry, R. R., 1966, Glaciation about 3,000,000years ago in the Sierra Nevada: Science, v. 154, no. 3750, p. 770—771. Dalrymple, G. B., 1963, Potassium-argon dates of some Cenozoic volcanic rocks of the Sierra Nevada: Geological Society of America Bulletin, v. 74, no. 4, p. 379—390. 1964, Cenozoic chronology of the Sierra Nevada, California: University of California Publications in Geological Sciences, v. 47, 41 p. 1979, Research note: Critical tables for conversion of K-Ar ages from old to new constants: Geology, v. 7, no. 11, p‘ 558—560. Dalrymple, G. 8., Cox, Allan, and Doell, R. R., 1965, Potassium- argon age and paleomagnetism of the Bishop Tuff, California: Geological Society of America Bulletin, v. 76, no. 6, p. 665—674. Duffield, W. A., and Bacon, C. R., 1977, Preliminary geologic map of the Coso volcanic field and adjacent areas, Inyo County, Califor- nia: US Geological Survey Open-File Map 77—311, scale 150,000, 2 sheets. Flint, R. F., and Gale, W. A., 1958, Stratigraphy and radiocarbon dates at Searles Lake, California: American Journal of Science, v. 256, no. 10, p. 689—714. 24 Friedman, Irving, Smith, G. I., and Matsuo, Sadao, 1982, Economic implications of the deuterium anomaly in the brine and salts in Searles Lake, California: Economic Geology, v. 77, no. 3, p. 694-702. Gale, H. S., 1914, Salines in the Owens, Searles, and Panamint Basins, southeastern California: U.S. Geological Survey Bul- letin 580—L, p. 251—323. Goddard, E. N., chairman, 1948, Rock-color chart: Washington, National Research Council. Haines, D. V., 1959, Core logs from Searles Lake, San Bernardino County, California: U.S. Geological Survey Bulletin 1045—E, p. 139—317. Hardt, W. F., Moyle, W. R., and Dutcher, L. C., 1972, Proposed water-resources study of Searles Valley, California: U.S. Geo- logical Survey open-file report, 70 p. Hopper, R. H., 1947. Geologic section from the Sierra Nevada to Death Valley, California: Geological Society of America Bul- letin, v. 58, no. 5. p. 393-432. Huber, N. K., 1981, Amount and timing of Late Cenozoic uplift and -tilt of the central Sierra Nevada, California—evidence from the upper San Joaquin River basin: U.S. Geological Survey Profes- sional Paper 1197, 28 p. Hunt, C. B., and Mabey, D. R., 1966, Stratigraphy and structure, Death Valley, California: U.S. Geological Survey Professional Paper 494—A, p. A1—A162. Liddicoat, J. C., Opdyke, N. D., and Smith, G. 1., 1980, Palaeomag- netic polarity in a 930-m core from Searles Valley, California: Nature, v. 286, no. 5768, p. 22-25. Lee, C. H., 1912, An intensive study of the water resources of a part of Owens Valley, California: U.S. Geological Survey Water-Supply Paper 294, 135 p. Mankinen, E. A., and Dalrymple, G. B., 1979, Revised geomagnetic polarity time scale for the interval 0—5 m.y. B.P.: Journal of Geophysical Research, v. 84, no. B2, p. 615-626. Rinehart, C. D., and Ross, D. C.. 1957, Geology of the Casa Diablo Mountain quadrangle, California: U.S. Geological Survey Geo- logic Quadrangle Map GQ—99, scale 1162500. ___1964, Geology and mineral deposits of the Mount Morrison quadrangle, Sierra Nevada, California: U.S. Geological Survey Professional Paper 385. 106 p. Ruddiman, W. F., and McIntyre, Andrew, 1976, Northeast Atlantic paleoclimatic changes over the past 600,000 years, in Cline, R. M., and Hays, J. D., eds., Investigation of late Quaternary paleo- ceanography and paleoclimatology: Geological Society of Amer- ica Memoir 145, p. 111—146. Schultz, J. R., 1938, A late Cenozoic vertebrate fauna from the Coso Mountains, Inyo County, California, paper 3 of Studies on Cenozoic vertebrates of Western North America: Carnegie Institution of Washington Publication 487, p. 75—109. Shackleton, N. J., and Opdyke, N. D., 1973, Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific Core V28-238: Oxygen isotope temperatures and ice volumes on a 105 year and 106 year scale: Quaternary Research, v. 3, no. 1, p. 39—55. _1976, Oxygen-isotope and paleomagnetic stratigraphy of Pacific core V28—239, late Pliocene to latest Pleistocene, in Cline, R. M., and Hays, J. D., eds., Investigation of late Quater- nary paleoceanography and paleoclimatology: Geological Soci- ety of America Memoir 145, p. 449—464. _1977, Oxygen isotope and palaeomagnetic evidence for early Northern Hemisphere glaciation: Nature, v. 270, no. 5634, p. 216—219. Sharp, R. P., 1968, Sherwin Till-Bishop Tuff geological relation- ships, Sierra Nevada, California: Geological Society of America Bulletin, v. 79, no. 3, p. 283—398. Sharp, R. P., and Birman, J. H., 1963, Additions to classical sequen- CORE KM—3—LATE CENOZOIC SEDIMENTATION IN SEARLES VALLEY, CALIFORNIA ces of Pleistocene Glaciations, Sierra Nevada, California: Geo- logical Society of America Bulletin, v. 74, no. 8, p. 1079—1086. Smith, G. I., 1962, Subsurface stratigraphy of late Quaternary de- posits, Searles Lake, California—A summary, art. 82 of Short papers in geology and hydrology: U.S. Geological Survey Pro- fessional Paper 450—C, p. C65—C69. 1964, Geology and volcanic petrology of the Lava Mountains, San Bernardino County, California: Geological Survey Profes- sional Paper 457, 96 p. 1966, Geology of Searles Lake—a guide to prospecting for buried continental salines, in Rau, J. L., ed., Second Symposium on Salt: Cleveland, Ohio, Northern Ohio Geological Society, Inc., v. 1, p. 167-180. 1968, Late Quaternary geologic and climatic history of Searles Lake, southeastern California, in Morrison, R. B., and Wright, H. E., Jr., eds., Means of correlation of Quaternary successions: Salt Lake City, University of Utah Press, v. 8, p. 293—310. 1976a, Origin of lithium and other components in the Searles Lake evaporites, California, in Vine, J. D., ed., Lithium resour- ces and requirements by the year 2000: U.S. Geological Survey Professional Paper 1005, p. 92—103. 1976b, Paleoclimatic record in the upper Quaternary sedi~ ments of Searles Lake, California, U.S.A., in Horie, Shoji, ed., Paleolimnology of Lake Biwa and the Japanese Pleistocene: Kyoto, Japan, Kyoto University, v. 4, p. 577—604. 1979, Subsurface stratigraphy and geochemistry of late Qua- ternary evaporites, Searles Lake, California, with a section on Radiocarbon ages of stratigraphic units, by Minze Stuiver and G. 1. Smith: U.S. Geological Survey Professional Paper 1043, 130 p. Smith, G. I., and Church, J. P., 1980, Twentieth-century crustal deformation in the Garlock fault-Slate Range area, southeast- ern California: Geological Society of America Bulletin, pt. 1, v. 91, no. 9, p. 524-534. Smith, G. L, Friedman, Irving. Klieforth, Harold, and Hardcastle, Kenneth, 1979, Areal distribution of deuterium in eastern Cali- fornia precipitation, 1968-1969: Journal of Applied Meteorol- ogy, v. 18, no. 2, p. 172—188. Smith, G. I., and Haines, D. V., 1964, Character and distribution of nonclastic minerals in the Searles Lake evaporite deposit, Cali- fornia: U.S. Geological Survey Bulletin 1181—P, p. P1-P58. Smith, G. I., and Pratt, W. P., 1957, Core logs from Owens, China, Searles, and Panamint basins, California: U.S. Geological Sur- vey Bulletin 1045—A, p. 1—62. Smith, G. I., and Street-Perrott, F. A., 1982, Pluvial lakes of the Western United States, chap. 10 ofWright, H. E., Jr., ed., Late- Quaternary environments of the United States: Minneapolis, University of Minnesota Press [in press]. Smith, G. I., Troxel, B. W., Gray, C. H., and von Huene, Roland, 1968, Geologic reconnaissance of the Slate Range, San Bernardino and Inyo Counties, California: California Division of Mines and Geology Special Report 96, 33 p. Stuiver, Minze, and Smith, G. I., 1979, Radiocarbon ages of strati- graphic units, in Smith, G. I., Subsurface stratigraphy and geochemistry of Late Quaternary evaporites, Searles Lake, California: U.S. Geological Survey Professional Paper 1043, p. 68-73. van Donk, Jan, 1976, 018 record of the Atlantic Ocean for the entire Pleistocene epoch, in Cline, R. M., and Hays, J. D., eds., Investi- gations of Late Quaternary paleoceanography and paleoclima- tology: Geological Society of America Memoir 145, p. 147-163. Walcott, C. D., 1897, The post-Pleistocene elevation of the Inyo Range and the lake beds of Waucobi Embayment, Inyo County, Cali- fornia: Journal of Geology, v. 5, no. 4, p. 340—348. AENICES LIBRARY -xrth Srienqes Bldg. 642-2927 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY the W’ (13° , _ . *7”; x i\\ Saddleoag‘ Lake \ ”\V y'7 240 Knights Ferry 0 I Ritter ~ Flange hms hmv<%oms \ 186 to 250 Mount Morrison "EJ Stravaérry Mine y.~/ ’ 3 \.\\./ I/x o Sanger .. a \ , \ ’:Bi‘sh0p\ ‘\ Creek IX 7 Bishop 0 ‘ ~ I Big Pine i ‘Créek PROFESSIONAL PAPER 1255 PLATE 1 (I80 I 1.10 mu CONTACT—Dashed where intruded by younger granitic rocks FAULT—Dashed where intruded by younger granitic rocks in THRUST FAULT— Dashed where intruded by younger granitic rocks é STRIKE SLIP FAULT—With right-lateral movement. Dashed where intruded by younger granitic rocks —)—> ANTICLINE—Showing trace of axial surface and direction of plunge. Approximately located .J FOSSIL LOCALITY WITH FOSSIL AGE RANGE J-K, Jurassic or Cretaceous J, Jurassic EJ, Early Jurassic LE—EJ, Late Triassic and Early Jurassic P, Permian lP—P, Pennsylvanian and Permian IP, Pennsylvanian C—P, Carboniferous and Permian 360 m M—K, Mississippian through Cretaceous M, Mississippian O~S(?), Ordovician and Silurian(?) O, Ordovician C, Cambrian 1153; o RADIOMETRIC AGE LOCALITY—Showing values of Rb-Sr iso- chron or U—Pb zircon age of meta—igneous rocks Kern; Canyon I i . or. m0 130° ((8 Base from H, W. Uliver (written commun; 1978) with additions and modifications from 15V?“ 0 5 ill 15 20 25 30 MILES ‘ Geology adapted from Bakerfield, Fresno, Marioosa, Sacramento, San Jose, and Walker Lake sheets the sheet of the Geologic Map of California listed in the geologic credit note I I I , I l I J I I I I of the Geologic Map of California (Koenig, 1953; Smith, 1954; Mathews and Burnett, 1955; Strand E g; 0 10 20 30 40 KILOMEIERS - A andKoeni-g, l965; Strand, 1967], with modifications from investigators listed in tables 1 and 2. 2 9 - Radiometric ages from Kistler (1986a), Morgan (1978), Brook (1977), Morgan and Stern (1977), Peck g 25 CALIF and others [1977), Fiske and Tobiseh (197B), Saleeby and others (1978), Chen and Moore (1979), r E Moore and others (1979], BushyrSpera and others (1981), Nokleherg llQBll, Nokleberg and Kistler APPROXIMATE MEAN \-/ [1980), and Saleehy and Sharp (1980). Fossil locations and ages from lmlay (1961), Clark (1964), DEcLiNArioN,l9aa T Hinehart and Ross [1984), Huber and Rinehart (1965], Douglass ((967), Jones and Moore ((973), AREA 0‘ MAP Schweickert and others [1977), Saleehy and others (1978), Brook and others [1979), J. H. Stewart (oral common, (979), BusbyrSpera and others (1980), Kistler and Nokleherg (1980), Moore and Foster (1980), and Nokleberg (1981) EXPIANATION - Metavolcanic rocks (Cretaceous). 4 t Granitic rocks (Mesozoic) Metamorphosed andesite to rhyo- dacite flows, tuff, and breccia FOOTHILLS TERRANE MERCED RIVER TERRANE KINGS TERRANE GODDARD TERRANE HIGH SIERRA TERRANE OWENS TERRANE I o" n Slate, metagraywacke, metatuff, and - Quartzite, marble, calc—silicate grano~ , ‘ , Argillite and chert with minor quartzite - Quartzite, slate, marble, metasand- - Metavolcanic rocks (Jurassic). Pre» - Metavolcanic rocks (Permian and Tri— W Metasedimentary rocks (Cambrian to metabreccia (Middle and Late fels, and minor argillite (Car- and marble (Carboniferous and stone and metadacite tuft, ash—flow dominantly metamorphosed assic) Metamorphosed andesite to 7 A . a Silurian (9)). Metasandstone, pelitic Jurassic) boniferous and Permian). Includes Permian). Includes part of the Cala- tuff and volcanic breccia (Late Tri- andesite to dacite flows, tuff, ash— rhyolite tuff, ash-flow tuff, flows, hornfels, and minor marble and part of the Calaveras Formation veras Formation assic and Early Jurassic) flow tuff and breccia breccia, and conglomerate calc-silicate homfels Metavolcanic rocks (Early and Middle Jurassic). Predominantly meta» basalt flows, pillow lava, and brec- Metavolcanic rocks (Pennsylvanian cia I ' I and Permian). Predominantly metabasalt flows, pillow lava, and _ Ultramafic and mafic igneous rocks breccia (pre-Middle Jurassic) ‘ Ultramafic and mafic igneous rocks (Pennsylvanian and Permian) CENTRAL AND SOUTHERN SIERRA NEVADA, CALIFORNIA Unconformity W Metasedimentary rocks, (Carboniferous and Permian). Pelitic hornfels, mar- ble, calcasilicate hornfels, and minor quartzite PALEOZOIC AND MESOZOIC TECTONO-STRATIGRAPHIC TERRANES, UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY ANALYSES OF ACID—SOLUBLE COMPONENTS IN CORE KM—3 (Values are weight percentages of samples dried at room temperature) SULFATE CONTENT, IN CHLORINE CONTENT, IN WEIGHT PERCENT WEIGHT PERCENT CARBONATE CONTENT, IN WEIGHT PERCENT SODIUM CONTENT, IN WEIGHT PERCENT CALCI U M CONTENT, IN DEPTH, DEPTH, WEIGHT PERCENT IN IN METERS FEET 0 5 10 15 20 0 10 20 30 40 50 60 0 10 20 30 40 50 60 0 10 20 30 40 50 60 0 1O 20 30 40 50 60 0 _ 0 I I I l I I I I I I I I I I I I I I I I -._;I_I_, 50- 100 — 150 - 50- 200 - I 5 5 J; _i :51: I; I: I g i ——: 250 — H H WIT 500 - 550 — — — = 600 — 650 - 200 - 700 — E g % ~%- 750 ~ — — — — 800 — ~ — — d 250 — - 850 — — — — — 900 — — — — I 950 — — — — — 300 — 1000 I — — — I I I WHIIIULUU 1050 — 1100 — 350 — 1150 — 1200 -1 1250 — 1300 — 400 - 1350 — 1400 — 1450 — 450 - 1500 — 1550 - 1600 — 500 ' 1650 — 1700 — 1750 — WW WIWWIIIW 550 _ 1800 1850 — 1900 — 1950 — — — _ _ 600 — I 2000 — — _ _ 2050 I — ~ _ J 2100 — — - _ _ I 650 I 2150 — — — _ _ 2200 — — _ I _ 2250 — — _ _ _ 70° ‘ 2300 — — _ _ _ 2350 — — _ _ _ 2400 — _ _ _ _ 2450 — _ _ _ _ 750 — I- 2500 d — — _ TI 2550 — — _ _ _ 2600 I I I I I 2650 l I I I I :II I _ 2800 850 - | I I I | 2850 — _ _ _ _ 2900 I I | I l l I I I 900 — 2950 3000 I I———I | I I~——I I‘——I I'—I 3050 I 1 I I I I I I I I I I I TI I I I I I *1 I I I I I ‘I PROFESSIONAL PAPER 1256 PLATE 2 ACID—INSOLUBLE RESIDUE, Stratigraphic IN WEIGHT PERCENT unit 0 10 20 30 4O 50 60 70 80 90 I I J I I Overburéen Mud I I l I Upper Salt Parting Mud ’ Lower Salt / pt " .,-<'\“~_ Bottom Mud /“ i t 4T: §5\‘ UnitA+B UnitC UnitD+E .1 UnitF Alluvial sand and gravel l | I I I I | I l i‘leTERIOR—GEOLOGICAL SURVEY, RESTON, VA—1982—GS1767 STRATIGRAPHIC VARIATIONS IN PERCENTAGES OF FIVE ACID —SOLUBLE COMPONENTS AND THE ACID -INSOLUBLE RESIDUE IN CORE KM—3 @275 ODE LO meI OZ< .>Im .zDSmm _>m>m:w J_mkm ncm ucmm _E>::< mama paSJQABH quwew IUSAB DQSJBAGH QUE-)9] qoodg |BLUJON ssneg fl Q E H _l luaAg |euuoN uogunau f 1uaA3|euuoN [BAnp|O 8mm: 92:0 ucm $0062.. E0: >Lamhmzmtm 0:952). 3:0 mEEmcs SF: 8.60 «oz qoodg DBSJBAGH eweAnww BEoNcoE Ntmso \ / / 622m 95 Ucmm o_wovt< NEIEHD A'ILONILSIG 03H HO BONVHO AWLONILSICJ MOTIBA AILONLLSIG UDE mcEmsoS com m+Q~ED 1ueA3 |euuoN O||[UJBJ€[‘ ZO:._ 62: So. _mc_mto vim/Eu w EBB 90E :0 mmmm -POO._OIE._U v Irma Manama H mil; >m>m5m A<2OOJOMO omNH mmmai ‘ZZOHmmmmOma EOEMHZ. MIR “—0 Emzhgmma mmhflrw DthD UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 1256 GEOLOGICAL SURVEY PLATE 3 THENARDITE NORTHUPITE DEPTH, DEPTH, HALITE CONTENT, IN TRONA CONTENT, IN HANKSITE CONTENT, IN BURKEITE CONTENT, IN CONTENT, IN BORAX CONTENT, IN CONTENT, IN Stratigraphic IN IN WEIGHT PERCENT WEIGHT PERCENT WEIGHT PERCENT WEIGHT PERCENT WEIGHT PERCENT WEIGHT PERCENT WEIGHT PERCENT unit METERS FEET 0 2'5 50 715 100 0 25 50 75 100 0 25 50 75 0 25 50 0 10 2'0 0 10 20 0 1'0 20 0 _ 0 I I I I I I I I I I I I I I I OvfiflMud 50_ _ 1 _ [J _ _ _J _ UpperSalt —_—_—I __ Parting Mud E, :I 100 “:1 ‘i ‘ ‘j 5 I L°WE r53” =:=== NAHCOLITE 150 T "' I I I I ‘I ‘ 50 - Bottom Mud 200 — ~ _ 250 d — _ I I I :l: I 300 — — 1 — UnitA+ B 100 — 350 — _ _ 400 — — — — — 450 — — —:;i — — :F’ I:; 3 l H bum LI 3 Unit C 150 ‘5 500 _ == _ I] _ 550 — _ _ __ I] j 600 - — _ _ _ UnitD+E I 700 — 41A 1 TI 1 8 0 — _ 5 GLAUBERITE CONTENT, ANHYDRITE CONTENT, IN WEIGHT PERCENT IN WEIGHT PERCENT 0 10 20 30 0 5 10 900 - - 950 -I ‘ 950 - 950 ‘ - ‘ _.____ 300 — 1000 I , , I — - — , 2 fi—j__, 1050 — _ E 2 _ 1100— y; ”I 7 I T 350'" 1150-— r—f'l _ __ _ _ UnltG 1200 - — — _ _ I250 — — _ _ _ :1 1300— - — — _ 400 " ———fi :I 1350- I—\_ LJLI 1400 — — _ 2 _ 1450 - — _ _ _ 450 — :I 1500 — — — — _] 1550— ‘ ‘ ' ‘—_'— _ UM” 1600 — _ _ 500 — 1650 — — _ “I " ‘I 1750— — _ _ 550 ._ 1800 — — — _ 1850 - — _ _ 1900 — — _ _ 1950 — — — _ 600 — 2000 - — _ - UnitI 2050 — — I j I *1 2100— _ 650j 2150 — _] 2200 I . _] 2250 — _ *Ifi—‘I 70° ‘ 2300 — 2350 — 2400 ~ 2450 — 750 — 2500 — 2550 — 2600 — 800 - I AIIuviaI sand 2650 — and gravel 2700 — 2750 I] 2800 — 850 — 2850 - 2900 - 900 — 2950 T 3000 —:I , J— 3050 I I I I fiINTERIOR—GEOLOGICAL SURVEY, RESTON, VA—1982—GB1767 STRATIGRAPHIC VARIATIONS IN NORMATIVE WEIGHT PERCENTAGES OF ACID—SOLUBLE MINERALS COMMONLY CONCENTRATED IN SALINE LAYERS OF CORE KM —3 PROFESSIONAL PAPER 1256 UNITED STATES DEPARTMENT OF THE INTERIOR PLATE 4 GEOLOGICAL SURVEY MAGNESITE CONTENT, DEPTH, DEPTH, GAYLUSSITE CONTENT, PIRSSONITE CONTENT, CALCITE CONTENT, ARAGONITE CONTENT, DOLOMITE CONTENT, IN WEIGHT ACID—INSOLUBLE RESIDUE, Stratigraphic IN WEIGHT PERCENT IN WEIGHT PERCENT IN WEIGHT PERCENT IN WEIGHT PERCENT IN WEIGHT PERCENT PERCENT IN WEIGHT PERCENT unit IN IN METERS FEET 0 10 20 30 40 0 10 20 30 40 0 10 20 30 0 10 20 0 25 50 0 25 0 25 50 75 100 I l 4 I l I I Overburmden Mud 0 _ 0 I I I I l I I I I I I | I TI Upper Salt 50 _ _ T Parting Mud = F. 7:.- TIIT Lower Salt I ' I ‘WWHHQIIIIU 100T : :2 150 __:: _ _ _ 50 T Bottom Mud 200 _ I 250 — UnitA+B 300 — I _ _ 100 — 350 7 — — TTT—I—I 400 T — _ 450 T T T 150— :I 500 T _ T UnitC 550 I — _ 600 — -1 _ _ 650 T _ 700 _ —_———I_AL 75° ‘ ‘ SEARLESITE CONTENT, _ IN WEIGHT PERCENT 0 10 20 WWW II“ II UnitD+E I I I 200 T J0 I W VIII 800 — — — 250 — 850 — — — 850 — it F 900 _ — _ 950 T T T 300 _ 1000 T | I I fl ‘ 1050 T — TI :I IIOOI 350 T 1150— _J:_, _ :I J T W 1200 T T 1250 _ _ 1300 T _ 400 T 1W1 1350 _ _ 1400 T T 1450 T _ II 450 _ 1500 T _ 1550 T T UnItH % E J 1600 T _ 500 T 1650 T _ LIL] 1700 T _ I750 _ _ 1800 T — 550 T 1850 T _ 1900 T _ 1950 T _ 600 T 2000 T _ 2050 T __ 2100 T _ 650 — 215M _ 2200 T _ 2250 T _ 700 J 2300 — _ 2350 T _ 2400 T _ 2450 — _ _ 750 — _ 2500 T — _ 2550 T — - _ 2600 T — 4 800 T I I AIIuvIaI 2650 _ — _ sand and gravel Z: I I i I 850 ” 2800 — — _ 2850 T - _ 2900 T _ _ 2950 T - _ - I -I - I 3050 I I I I I I ' 900 T I I I *I 5:} INTERIOR—GEOLOGICAL SURVEV, RESTON, VA—1982—GB1767 STRATIGRAPHIC VARIATIONS IN NORMATIVE WEIGHT PERCENTAGES OF ACID -SOLUBLE MINERALS AND ACID-INSOLUBLE RESIDUE COMMONLY CONCENTRATED IN MUD LAYERS OF CORE KM—3 :6? 0‘ m men} ' Coloradanffi/atuml Res was gjf. ”t 7; Weqfflw StateEngmeer ‘ , ;, ‘7 * ‘Dmvé? Bowdquater Commmams} and '5 l , ' g; ‘E‘AJmns Arapahot, Dougzas Elm maxim 6'an i; a" giBedrock Aquifers m the Bedrock Aquifers in the Denver Basin, Colorado— A Quantitative Water-Resources Appraisal By s. G. ROBSON U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1257 Prepared in cooperation with the Colorado Department of Natural Resources, Ofllee of the State Engineer; Denver Board (yr Water Commisxz‘oners; and Adams, Arapahoe, Douglas, Elbert, and El Paso Counties UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Robson, Stanley G. Bedrock aquifers in the Denver basin, Colorado. (Geological Survey professional paper ; 1257) “Prepared in cooperation with the Colorado Department of Natural Resources, Office of the State Engineer; Denver Board of Water Commissioners; and Adams, Arapahoe, Douglas, Elbert, and El Paso counties.” Bibliography: p. Supt. of Docs. no.: I 19.16:1257 1. Aquifers—Colorado—Denver Region. 2. Water, Underground—Colorado—Denver Re- gion—Mathematical models. 3. Water, Underground—Colorado—Denver Region—Data process— ing. I. Colorado. Office of the State Engineer. 11. Title. III. Series. GB1199.3.CGR63 1987 553.7'9’0978883 ‘ 84—600228 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 CONTENTS Page Abstract 1 Introduction 1 Purpose and scope 2 Location and features of the study area 2 Related studies 2 Acknowledgments 6 The natural hydrologic system 6 Stratigraphy 6 Aquifer characteristics 9 Water levels 18 Recharge and discharge 23 Water quality 27 The simulated hydrologic system 31 Model description 31 The steady-state model 32 The transient-state model 36 Model simulations 38 Base conditions 40 Ground-water development plans 42 Satellite well field 42 Metropolitan well field 52 Pumpage for park and golf course irrigation 59 Bedrock storage of municipal water 61 Conclusions 66 References cited 68 Supplemental information 69 Historical pumpage estimates 69 Future pumpage estimates 70 Modeling errors and limitations 70 ILLUSTRATIONS [Plates are in pocket] Map showing measured 1978 potentiometric surfaces of bedrock aquifers in the Denver basin, Colorado. Map showing model-calculated 1978 potentiometric surfaces of bedrock aquifers in the Denver basin, Colorado. Maps showing model-calculated 2050 potentiometric surfaces of bedrock aquifers in the Denver basin, Colorado: 3. Model simulation STDY-BASE. 4. Model simulation HALF-BASE. 5. Model simulation FULL-BASE. PLATE 5".”2" 1 2. Map showing mean annual precipitation in study area 3. Generalized geologic sections through the Denver basin 9. Maps showing: 4. Location and extent of bedrock aquifers 8 5. Depth to the base of the bedrock aquifers 10 Total thickness of water-yielding material in the bedrock aquifers 12 14 16 use . Map showing location of study area and trace of generalized geologic sections 3 4 5 . Location of faults in the Laramie-Fox Hills aquifer north of Denver . Transmissivity of the bedrock aquifers Confined storage coefficient of the bedrock aquifers and location of water-table and confined conditions ——— 20 wmsw -III IV FIGURE TABLE 10. 11. 12. 13. 14. 15. 16. 17—24. 25. 26—31. 32. 33—34. 35—36. 37. dam-Amph- . Porosity and specific-yield statistics for water—yielding materials . Typical bedrock water quality and drinking water standards . Regional-scale steady-state water budget for the bedrock aquifers . Transient-state 20‘year water budget for the bedrock aquifers . Distribution of FULL pumpage estimate for periods 1979—85 and 2046—50 . Transient-state water budgets for 1978 (calibration run) and 2050 (STDY-BASE run, HALF-BASE run . Metropolitan well-field pumpage . Park and golf course irrigation pumpage cuppa CONTENTS Water-level hydrograph for the Arapahoe aquifer near the Colorado State Capitol —————————————————————————— Map showing measured water-level changes in the aquifers between 1958 and 1978 ————————————————————————— Graph showing estimated pumpage from bedrock wells, 1958—78 Map showing dissolved-solids and dissolved-sulfate concentrations in the bedrock aquifers ——————————————————— Map showing size and distribution of grid blocks used in the transient-state model ————————————————————————— Diagram showing mean-square—error configuration during steady-state calibration ————————————————————————— Graph showing historical and projected pumpage for the bedrock aquifers Maps showing incremental water-level declines for a satellite well field in the: 17. Dawson aquifer, using STDY pumpage estimate 18. Denver aquifer, using STDY pumpage estimate 19. Arapahoe aquifer, using STDY pumpage estimate 20. Laramie—Fox Hills aquifer, using STDY pumpage estimate 21. Dawson aquifer, using FULL pumpage estimate 22. Denver aquifer, using FULL pumpage estimate 23. Arapahoe aquifer, using FULL pumpage estimate 24. Laramie— Fox Hills aquifer, using FULL pumpage estimate Water-level hydrographs for the Laramie—Fox Hills aquifer at Parker Maps showing incremental water-level declines for a metropolitan well field in the: 26. Denver aquifer, using STDY pumpage estimate 27. Arapahoe aquifer, using STDY pumpage estimate 28. Laramie—Fox Hills aquifer,~using STDY pumpage estimate 29. Denver aquifer, using FULL pumpage estimate 30. Arapahoe aquifer. using FULL pumpage estimate 31. Laramie— Fox Hills aquifer, using FULL pumpage estimate Map showing incremental water— level decline in the Arapahoe aquifer due to park and golf course pumpage, using HALF pumpage estimate Maps showing incremental water-level change in the Arapahoe aquifer at site 1: 33. After 4 years of injection, using STDY pumpage estimate 34. After 1 year of pumping, u'sing STDY pumpage estimate Maps showing incremental water-level change in the Arapahoe aquifer at site 2: 35. After 4 years of injection, using STDY pumpage estimate 36. After 1 year of pumping, using STDY pumpage estimate Graphs showing hypothetical effects of modeling conditions on simulation accuracy ———————————————————————— TABLES Estimated total and recoverable ground water in storage in the Denver basin and FULL-BASE run) Satellite well-field pumpage Page 22 24 26 28 33 34 39 44 45 46 47 48 49 50 5 1 52 53 54 55 56 57 58 60 62 63 64 65 72 Page 15 18 27 35 38 . 40 41 43 52 59 CONVERSION FACTORS Inch-pound units used in this report may be converted to International System of units (SI) by using the following coversion factors: Multiply By To obtain acre 4.047x10—1 hectare acrefoot (acre-ft) 1.233 X 10 '3 cubic hectometer acre-foot per year (acre-ftJyr) 1.233><10—3 cubic hectometer per year cubic foot (ft’) 2.832 X 10—2 cubic meter cubic foot per second {ft’ls} 2.832><10_2 cubic meter per second foot (ft) 3.048X10’1 meter foot per day (ft/d) 3.048x10‘1 meter per day foot per mile (ft/mi) 1.894X10‘1 meter per kilometer foot per year (ftJyr) 3.048x10‘l meter per year foot squared (ft’) 9.290X10‘2 meter squared foot squared per day (ft’ld) 9.290><10_2 meter squared per day gallon per minute (gal/min) 6.309X10'2 liter per second gallon per day (gal/d) 3.785 liter per day inch (in) 2.540 centimeter inch per year (in/yr) 2.540 centimeter per year inch squared per pound (inf/lb) 1.450><10_1 kilopascal‘l mile (mi) 1.609 kilometer square mile (mi’) 2.590 square kilometer temperature, degrees Fahrenheit (°F) °C=5/9(°F—32) degrees Celsius National Geodetic Vertical Datum of 1929 (N0 VD of1929): A geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called mean sea level. BEDROCK AQUIFERS IN THE DENVER BASIN, COLORADO-— A QUANTITATIVE WATER-RESOURCES APPRAISAL By S. G. ROBSON ABSTRACT The Denver metropolitan area is experiencing a rapid population growth which is requiring increasing supplies of potable water to be pumped from bedrock aquifers in order to meet demand. In an effort to determine the ability of the aquifers to continue to meet this de- mand, the Colorado Department of Natural Resources; the Denver Board of Water Commissioners; and Adams, Arapahoe, Douglas, Elbert, and El Paso Counties joined with the US. Geological Survey in undertaking a hydrologic evaluation of the ground-water resources of the basin. This evaluation involved mapping aquifer extent, thickness, structure, hydraulic characteristics, and water-level and water-quality conditions. Ground-water modeling techniques were then used to simulate aquifer response to various pumpage estimates and ground-water development plans. The Laramie—Fox Hills aquifer (the deepest aquifer) underlies the 6,700-mi2 study area and is overlain by the more permeable Arapahoe aquifer, the Denver aquifer, and the Dawson aquifer, which crops out in the southern part of the study area It is estimated that 270x106 acre-ft of recoverable ground water is in storage in these four bedrock aquifers. However, less than 0.1 percent of this volume of water is stored under confined conditions. The larger volume of water stored under unconfined conditions will be available for use only when the water levels in the confined aquifers decline below the top of the in- dividual aquifer, allowing water-table conditions to develop. Annual precipitation on the Denver basin supplies an average of 6,900 ft’Is of water to the area; about 55 fta/s of this recharges the bedrock aquifers, principally through the Dawson Arkose. The direc- tion of ground-water movement is generally from ground-water divides in the southern part of the area northward toward the margins of the aquifers. Pumpage has ranged from about 5 ft’Is in 1884 to about 41 ft‘/s in 1978. Pumpage exceeds recharge in the metropolitan area and has caused water-level declines (1958-7 8) to ex- ceed 200 ft in a 135-mi2 area of the Arapahoe aquifer southeast of Denver. A quasi—three-dimensional finite-difference model of the aquifer system was constructed and calibrated under steady-state and transient-state conditions. Steady-state calibration indicated that lateral hydraulic conductivity within the aquifers is about 100,000 times larger than vertical hydraulic conductivity between the aquifers. Transient-state calibration indicated that between 1958 and 1978. 374,000 acreft of water was pumped from the aquifers, produc- ing a 90,000-acre-ft net decrease in the volume of water in storage in the aquifers. During this time, pumpage also changed the rates of in- teraquifer flow, induced additional recharge, and caused capture of natural discharge. Three 1979-2050 pumpage estimates were made for use in simulating the effects of various ground-water development plans. Simulations, using each of these pumpage estimates, indicated that by the year 2050, large water-level declines could occur, particularly in the deeper aquifers. Maximum water-level declines of 410, 1,700, and 1,830 ft were produced, using the small, medium, and large pumping rates. Four plans for supplementing the Denver water supply include pumping a satellite well field, pumping a municipal well field, pump- ing to irrigate parks, and injecting water during periods of low demand for later use during periods of peak demand. Model simula- tion of these plans indicated that the satellite well field will yield twice as much water as the municipal well field but will produce larger and more widespread water-level declines in the four aquifers. The municipal well field would not significantly affect water levels in the Dawson aquifer. Pumping the Arapahoe aquifer to supply irrigation water to selected parks was shown to produce only small water-level declines in the aquifer. Results of simulating injection—pumpage well fields at two locations indicated that injection rates could range from 1.7 to 10 ft‘Is, depending on the choice of site. The volume of water that could be stored in the bedrock aquifer thus is sensitive to the hydrologic characteristics of the chosen site. More study is needed to evaluate water-chemistry compatibility of native and injected water. INTRODUCTION Between 1970 and 1980, 238,000 additional housing units were constructed in the Denver metropolitan area in conjunction with a population increase of 353,000 people. This growth produced a 220—percent increase in the number of housing units in Douglas County and a 250- to 400-percent increase in the number of housing units in suburban cities such as Broomfield, Federal Heights, Glendale, ThOrton, and Westminster (US. Bureau of the Census, 1981). The growth in housing and population has produced a corresponding increase in demand for potable water. In an effort to provide additional water to meet part of these demands, the Denver Water Department has con- structed the Foothills Water Treatment Complex at a cost of about $170,000,000, the first phase of which will provide an additional 125 million gal/d of treated water to the metropolitan area. However, this facility will pro- vide additional water to only about 60 percent of the 1,590,000 people in the metropolitan area. In areas not supplied by the Denver Water Department, additional water must be obtained from surfacewater and ground- water sources through numerous smaller water agencies and from private wells. 2 BEDROCK AQUIFERS IN DENVER BASIN, COLO. In rural or suburban areas without access to surface- water supplies, bedrock aquifers commonly are the sole source of water, and water requirements in these areas are met by more than 12,000 municipal, industrial, domestic, and stock wells completed in the bedrock aquifers. Increasing demands for ground water have produced more than 500 ft of water-level decline in local areas and in excess of 200 ft of decline in a 240«mi2 area to the north, east, and south of Denver. The magnitude of present demands, coupled with ex- pected in future demands, has led to concern over the in- creased ability of the bedrock aquifers to meet future water requirements. In an effort to better understand and manage this vital water supply, the Colorado Department of Natural Resources, Office of the State Engineer; the Denver Board of Water Commissioners; and Adams, Arapahoe, Douglas, Elbert, and El Paso Counties joined with the US. Geological Survey in funding a cooperative hydrologic study of the principal bedrock aquifers in the Denver basin. Work on this study began in 1978. PURPOSE AND SCOPE The purpose of the study was to provide a broad- scale, comprehensive evaluation of the ground-water resources of the entire Denver ground-water basin. This evaluation involved documenting the past and present hydrologic and geologic conditions in the aquifers. The ultimate objective was to use modeling techniques to evaluate the probable effects of future pumpage on ground-water supplies. Specific elements of this work include: 1. Identification and mapping of the extent, struc- ture, thickness, and depth of the four principal bedrock aquifers in the basin. 2. Determination of the hydraulic characteristics of the water-yielding strata in the four aquifers. 3. Determination of the past and current water-level conditions and rate of water-level change in the aquifers. 4. Estimation of rate and distribution of natural recharge, discharge, and pumpage. 5. Determination of the general water quality in the aquifers and the suitability of the water for potable use. 6. Construction and calibration of a multilayer, mathematical model of the aquifer system for use in estimating the probable effects of future pumpage on the water levels in the aquifers. LOCATION AND FEATURES OF THE STUDY AREA The Denver ground-water basin underlies a 6,700-mi2 area in Colorado extending from the Front Range of the Rocky Mountains east to near Limon, and from Greeley south near Colorado Springs (fig. 1). The Denver metropolitan area is located on the west-central margin of the basin in a broad valley formed by the South Platte River and its tributaries. Land-surface altitudes in the basin range from 4,500 ft above sea level near Masters on the northeast to as much as 7,500 ft in the mountainous areas north of Colorado Springs. Sur- face drainage is generally to the north in the area north of El Paso County and to the south or southeast in El Paso County. Alluvial aquifers ranging in thickness from 10 to 150 ft are present in the valleys of the larger streams. These aquifers supply water for irrigation of commercial crops in the northern onethird of the basin and at a few other scattered locations. The Denver basin has a semiarid continental climate with 50-70 in. of mean annual potential evaporation and only 11—18 in. of mean annual precipitation (fig. 2). About 70 percent of the precipitation falls during the 6-month period from April through September, with areas of higher altitude receiving more precipitation. For example, Greeley, at an altitude of 4,663 ft, receives 11 in. of mean annual precipitation (32 in. of snowfall); Denver, at an altitude of 5,280 ft, receives 14 in. (56 in. of snowfall); and Monument, at an altitude of 6,961 ft, receives 18 in. (83 in. of snowfall). Record high temper- atures in the basin range from 100 to 105 ° F, and record low temperatures range from -30 to -40 °F. Mean monthly temperatures in Denver range from 70°F in July and August to 32°F in December and January (Hansen and others, 1978). A mean annual precipitation rate of 14 in./yr occurs in the Denver basin and produces an average of 5.0 million acre-ft of water per year, equivalent to a continuous flow of 6,900 ft3/s. Most of this volume of water is lost through evaporation, transpiration, and runoff, and less than 1 percent sup- plies recharge to the bedrock aquifers. Four bedrock aquifers occur in water-yielding strata of the Fox Hills Sandstone, Laramie Formation, Arapa- hoe Formation, Denver Formation, and Dawson Arkose (fig. 3). The aquifers are termed the Laramie—Fox Hills aquifer, the Arapahoe aquifer, the Denver aquifer, and the Dawson aquifer. RELATED STUDIES In 1883, it was discovered that flowing wells could be constructed in the bedrock formations near downtown Denver. This led to a rapid increase in the number of INTRODUCTION 106° 135° 410 f \5 N!\ ' £3 “a " \ as -JJ (k K 5“ nuts, ‘. s” O _ , 2( . ' ~ (5 BOU -. 40° figs 1 -oulder ., x . -_ _ mngNGTor ( L- Denver basm (CeMIJTS y ._,- a study area :j/(Georgé - ~ ~QLEA' CREEKI my —-—~_% ' 3d “a A I r l airplay , - A R K I1— I if: ' 5 / v |I 4 fl // ,. . ‘ 7' fjfl'; " .1 Two—Wis J x ~ -M I ””le \.~; "i <.; .131"; 33;» j I- y “l x; \: «sf . . ’1. ‘\ /"] 40 EU MILES | c——c= I 40 BI] KILUMETEHS FIGURE 1.—Location of study area and trace of generalized geologic sections. BEDROCK AQUIFERS IN DENVER BASIN, COLO. 105° 1040 ”\V' l \_ i T LARIMER | Greek}, \ i Big I} Loveland’ ° \ I Jackson 0 ”MAYO” 3 Riverside I DR“ Res , p6 MORGAN a South . ”9 «6 Empire 1' i, / WELD Res ‘~ V . FonG "Morgan iJ‘ 2 / /’ x (L m .f I 9 r‘ x g \g b] > a ‘19? i . k5 -$/\ ' «9/3 x I ~ e .3 k ‘b -,— ---------- “fl..-_1._./_- --- - "T“"'" “ c3 \ q 1 ) 1 / i' f \ ; ? I '§‘ , .n ADAMS} / j g , w 5/ ’/ f // \ : / ‘ f l j R 6/ ( / / i I ’ -_7’§}:a:‘72'§__L-__ -_- - E 'f i ' ' j) . 63 A ’ - IWASHINGTON ‘ . 2 I f i ) | ! EXPLANATION —74— LINE OF EQUAL MEAN ANNUAL PRECIPITATION— Interval 2 inches ( / / / Color-do ° Springs L‘ K | x \ Modified from Hansen [1 5 ill 15 MILES and others [1973} i—efiw fl 5 10 15K|LDMETERS FIGURE 2.—Mean annual precipitation in study area. INTRODUCTION 5 FEET 7000 FEET 7000 6000 DENVER Watkins 6000 South Platte River 5000 — 5000 4000 4000 Pierre Shale 3000 3000 B I FEET FEET 9000 ‘L 9000 800° — 3000 Precambrian rocks _ — 7 7000 Dawson Arkose 000 6°00 ’ 6000 5000 — 5000 4000 — Pierre Shale 4000 3000 3000 c I FEET FEET 8000 — 8000 " l 7000 _ Colorado Castle Rock Conglomerate _ 7000 Springs E Dawson Arkose ‘ EE - o 6000 _ Watkins E 6000 Denver Formation : 5000 — §, 5000 Arapahoe Formation 4000 _ Laramie Formation _ 4000 Pierre Shale 3000 3000 0 ll] 20 30 MILES [l 10 2|] 3U KlLUMETEHS VERTICAL EXAGGERATIUN X 32 NATlUNAL GEUDETIC VERTICAL DATUM 0F1929 FIGURE 3.—Generalized geologic sections through the Denver Basin. (Line of sections located in fig. 1.) such wells (from 80 in 1884 to 400 in 1890) and was the vided a more comprehensive evaluation of the geology subject of the first study of the hydrology of the and hydrology of the formations in the area. Since this bedrock aquifers near Denver (Cross and others, 1884). early work, numerous authors have studied the geology Subsequent work by Emmons and- others (1896) pro- and hydrology of parts of various geologic units in the 6 BEDROCK AQUIFERS IN DENVER BASIN, COLO. basin. Chronic and Chronic (1974), Hampton (1975), Hampton and others (1974), Anna (1975), and Norris and others (1984) provided indexes to more than 1,800 reports dealing with the geology and hydrology of the Denver basin. Therefore, only those reports that have particular relevance to the hydrology of the bedrock aquifers are discussed further in this section. McConaghy and others (1964) and Major and others (1983), for example, presented data for water wells com- pleted in the bedrock aquifers. These data were the bases for several interpretative hydrologic studies. Romero and Hampton (1972) and Romero (1976) were the first of the more recent studies to show structural mapping of bedrock aquifers. Romero (1976) undertook a relatively comprehensive hydrologic investigation of the entire basin which served as the initial framework for this study. Hillier and others (1978) also provided useful hydrologic mapping of part of the Arapahoe aquifer, and Schneider (1980) provided a hydrologic evaluation for part of the Laramie—Fox Hills aquifer in Boulder County. Results of these studies were reviewed and incor- porated with new data to provide a more complete and extensive evaluation of the hydrology of the entire basin. Maps showing geologic structure of the top and base of each aquifer; depth to the base of the aquifer; sandstone thickness in the aquifer; 1958 and 1978 water-level altitudes; water-level change from 1958 to 1978; and concentrations of dissolved solids, dissolved sulfate, and hardness were prepared as part of this study. This information has been presented for the Dawson aquifer (Robson and Romero, 1981a), the Denver aquifer (Robson and Romero, 1981b), the Arapahoe aquifer (Robson and others, 1981a), and the Laramie—Fox Hills aquifer (Robson and others, 1981b). Robson (1983) presented estimates of hydraulic charac- teristics of the bedrock aquifers, such as hydraulic con- ductivity, transmissivity, porosity, specific yield, and storage coefficient. Norris and others (1984) provided an overview of the hydrology of the alluvial and bedrock aquifers in the Denver basin, including a more extensive discussion of surface-water relations and the hydrology of the alluvial aquifers than is contained in this report. ACKNOWLEDGMENTS This study was conducted as a joint effort of the US. Geological Survey and the Colorado Department of Natural Resources, Office of the State Engineer. John Romero, Andrew Wacinski, and Stanley Zawistowski of the State Engineer’s Office were an integral part of the team of scientists working on this project; their efforts materially contributed to many of the maps and other results of the study. The basic data collected during this study were provided mainly through the generous coop- eration of private well owners, managers of commercial facilities, and private consulting geologists and hydrol- ogists. This cooperation significantly increased the data base for the study and aided in the timely completion of the work. Collection of field data and review of reports and technical procedures were performed by numerous employees of the US. Geological Survey and the cooper- ating agencies. The contributions of all of the above- mentioned individuals is gratequy acknowledged. THE NATURAL HYDROLOGIC SYSTEM STRATIGRAPHY The Denver ground-water basin is part of the larger Denver structural basin that extends from Colorado in- to western Nebraska, Kansas, and eastern Wyoming. Tectonic movement has produced more than 20,000 ft of structural relief between the Precambrian basement rocks that occur in the Front Range, in the structural depression, and on the Las Animas and Chadron arches in eastern Colorado and western Nebraska (fig. 1). The structural basin is asymmetrical. Low-angle dips occur on the eastern flank of the basin from the Las Animas arch to the synclinal axis in the Denver-Cheyenne area. Steeply dipping to overturned beds occur on the western flank of the basin near the mountain front (fig. 3). Faulting and deformation associated with mountain-building geologic movement beginning in Cretaceous time have created the markedly asym- metrical features of the basin (McCoy, 1953). Faulting along the southwestern edge of the basin has truncated Cretaceous and Tertiary sedimentary rocks and left them in contact with Precambrian rocks on the up- thrown fault block (fig. 3). Along the northwestern margin of the basin, faulting has offset and tilted the Cretaceous and older sedimentary rocks and, in Boulder and Jefferson Counties, has produced scenic outcrops with steeply dipping beds (Tweto, 1979). The Flatirons (a local sandstone outcrop) is an example of the results of this structural deformation. Strata yielding usable quantities of potable water oc- cur in the Fox Hills Sandstone, Laramie Formation, and Arapahoe Formation of Late Cretaceous age; in the Denver Formation of Late Cretaceous and early Tertiary age; and in the Dawson Arkose of Tertiary age. These formations attain a maximum combined thickness of 3,200 ft in an area about 20 mi south of Castle Rock. The Pierre Shale of Late Cretaceous age underlies the Fox Hills Sandstone and is considered to be the base of the water-yielding units because of its thickness, which exceeds 5,000 ft, and minimal permeability. These formations occur in a sequence of THE NATURAL HYDROLOGIC SYSTEM 7 layers that form an ellipsoidal, bowl-shaped ground- water basin having structural features similar to those in the underlying structural depression (fig. 3). The Laramie—Fox Hills aquifer is the deepest and most extensive aquifer in the basin, underlying a 6,700-mi2 area between Greeley and Colorado Springs. The limit of this aquifer in the area south of the South Platte River also is the limit of the study area (fig. 4). The Laramie—Fox Hills aquifer occurs primarily in the lower sandstone units of the Laramie Formation and the upper sandstone and siltstone units of the underly- ing Fox Hills Sandstone. However, sandstones in the upper 100—200 ft of the Pierre Shale may form aquifers in the area northwest of Denver and at a few other scat- tered locations; the uppermost of these sandstones like- ly are hydraulically connected to those in the Fox Hills Sandstone and are considered in this study to be part of the Laramie—Fox Hills aquifer. The part of the Laramie—Fox Hills aquifer within the Fox Hills Sandstone is generally 150—200 ft thick and is composed of an overlying bed of very fine grained silty sandstone 40—50 ft thick underlain by 100—150 ft of shaly siltstone and interbedded shale. The part of the Laramie-Fox Hills aquifer within the Laramie Forma- tion is generally 50-100 ft thick and is composed of very fine to medium-grained sandstone with interstitial silt and clay. Locally, the sandstone is separated into upper and lower members by interbedded shale 10—20 ft thick. A shale bed 5-20 ft thick generally separates the Laramie part‘of the aquifer from the Fox Hills part. The 400—500 ft of Laramie Formation overlying the Laramie—Fox Hills aquifer form an upper confining layer for the aquifer. Laramie strata above the aquifer are composed of gray to black shale, coal seams, and minor amounts of gray siltstone and sandstone. The lowermost coal seams are useful in identifying the upper limit of the aquifer. The subbituminous to lignitic coal seams range in thickness from a few inches to about 10 ft and are present in several stratigraphic horizons. These seams have been extensively mined along the northwest and southwest margin of the Laramie Formation. The locations of about 300 abandoned or in- active coal mines in these two areas have been shown by Colton and Lowrie (1973) and Kirkham (1978). The thickness of the Laramie—Fox Hills aquifer ranges from zero at the aquifer boundary to between 200 and 300 ft in the central part of the basin. Altitudes of the base of the aquifer are shown by Robson and others (1981b) to range from 5,700 ft in the area be- tween Lirnon and Truckton to less than 3,500 ft in the lowest part of the structure near Cherry Creek Reservoir. Depths to the base of the aquifer range from 1,500 to 2,000 ft in most of the Denver metropolitan area, as shown in figure 5A. Thickness of water-yielding materials within the aquifer averages about 150 ft and ranges from zero to almost 250 ft (fig. 6A). In the northwest part of the basin, near Boulder County, numerous northeast-trending faults have offset strata in the Fox Hills Sandstone and Laramie Formation. Marked differences in water-level altitudes in wells occur near some of these faults, possibly due to the fault zone funtioning as a barrier to the movement of ground water (Schneider, 1980). The locations of some of these faults have been mapped by Colton and Lowrie (1973) and Tweto (1979) (fig. 7), but the locations of others can only be inferred from features such as the predominantly northeast trending alignment of lower Boulder Creek, lower Saint Vrain Creek, Big Dry Creek, and Coal Creek. Prominent topographic features, such as the mesas 4 mi southeast of Boulder and the west bank of the South Platte River between Brighton and Commerce City, also have a northeast trend. The Arapahoe aquifer (fig. 4) consists of a 400 to 700-ft-thick series of interbedded conglomerate, sand- stone, siltstone, and shale. Shale is more prevalent in the northern onethird of the area where the aquifer can sometimes be subdivided into an upper, middle, and lower part. The upper and lower parts generally consist of 150-200 ft of sandstone and siltstone interbedded with less prevalent zones of shale, whereas the middle part consists of about 100 ft of relatively homogeneous shale. The conglomerates, sandstone, and siltstones are normally light to medium gray with local very light gray and grayish-green beds. These colors are generally darker in the upper 100—200 ft of the formation near its boundary with the overlying Denver Formation. Shales are normally medium gray and silty. The larger propor- tion of conglomerate and sandstone with respect to shale, the absence of significant carbonaceous beds, and a generally lighter color distinguish the Arapahoe For- mation from the underlying Laramie Formation and the overlying Denver Formation. Individual conglomerate and sandstone beds in the Arapahoe Formation are generally lens shaped, moderately consolidated, and range in thickness from a few inches to 30 or 40 ft. The beds may be so closely spaced that they form a single hydrologic unit that is 200—300 ft thick in some areas. , Altitudes of the base of the Arapahoe aquifer have been shown by Robson and others (1981a) to range from more than 6,000 ft near the southern margin of the basin to less than 4,000 ft in the lowest part of the struc- ture near Parker. Depths to the base of the aquifer ex- ceed 2,600 ft near Black Forest, but range from 500 to 1,500 ft in most of the Denver metropolitan area (fig. 53). Although the total thickness of the Arapahoe aquifer commonly ranges from 500 to 700 ft, only 200 to 300 ft of water-yielding material is generally present, as shown in figure GB. BEDROCK AQUIFERS IN DENVER BASIN. COLO. 105° 104° I ! .. .“—"—r—"--—-——-—-—"1 \ l LARIMER | \ i L 1 . Big 730,, 0ove and \ l 4030!: Riverside ‘ Res I . South | -_ in 40°1 «- r . 4' . r‘ l a i I I / “’1 ) 'iWASHINGTON .3 J i JEFFERS '1!— ______ I I . leon 1 K \5 K LINCOLN \\ .— EXPLANATION ¥ \ 1 LARAMIE- \ FOX HILLS _______ \ AOUIFER Comma ARAPAHOE 59"“35 \N AQUIFER a (, \ 'm L, 1 DENVER \ *— ,‘i , g \ } AOUIFER Peuntain («6‘ | ' \_ \ \ ’3‘ c k ( DAWSON \ . I \1 \ AQUIFER L , I , \ Modified from Hansen 0 5 ‘0 ‘5 ““3 and others [1978] U 5 10 15 KILUMHERS FIGURE 4.—Location and extent of bedrock aquifers. THE NATURAL HYDROLOGIC SYSTEM 9 The Denver aquifer (fig. 4) consists of a 600- to 1,000-ft-thick series of interbedded shale, claystone, siltstone, and sandstone in which coal and fossilized plant remains are common. Distinguishing characteris- tics of the unit are its olive, green-gray, brown, and tan colors; the presence of coal; and the preponderance of shale and claystone compared to other rock types. The predominant olive and green-gray colors in the forma- tion are due to the presence of iron-rich sediments de- rived from erosion of basaltic and andesitic lavas. Denver rocks are thus distinguished from the generally lighter colored rocks found in the overlying Dawson Arkose and the underlying Arapahoe Formation. Water-bearing layers of sandstone and siltstone occur in poorly defined, irregular beds that are dispersed within relatively thick sequences of claystone and shale. In- dividual sandstone and siltstone layers commonly are lens shaped and range from a few inches to as much as 50 ft thick. Structural mapping by Robson and Romero (1981b) indicates that the altitudes of the base of the Denver aquifer range from more than 6,400 ft in the southern part of the basin to less than 4,600 ft in the lowest part of the structure, extending from Aurora to Parker. Depth to the base of the aquifer exceeds 2,100 ft near Black Forest but is between 100 and 1,000 ft in most of the Denver metropolitan area (fig. 50). Because of the preponderance of shale and claystone in the formation, the thickness of water-yielding materials in the aquifer is proportionally less, generally ranging from 100 to 300 ft, as shown in figure 6C. The sediments that occur in the Dawson aquifer (fig. 4) consist primarily of conglomerate, sandstone, and shale, varying from light gray to yellowish brown, with beds of pale-green shale in some areas. In general, the conglomerates and sandstones are coarse grained and poorly to moderately well consolidated. In most of the aquifer in Arapahoe, Douglas, and Elbert Counties, a layer of shale 100—250 ft thick separates an upper and lower sequence of conglomerate, sandstone, and minor amounts of shale. In the southern part of the aquifer, the intervening shale is absent, and conglomerate, sand- stone, and minor amounts of shale occur in a continuous sequence 600 ft or more thick. Individual conglomerate and sandstone beds are usually lens shaped and range from a few inches to as much as 200 ft thick. Con- glomerate and sandstone beds that are penetrated by one well may be of a different thickness or may be ab- sent in an adjacent well because of this lens-shaped layering. Structural altitudes of the base of the Dawson aquifer range from more than 6,600 ft in the southern part of the aquifer to less than 5,300 ft in the center of the structural low near Parker (Robson and Romero, 1981a). Depths to the base of the aquifer exceed 1,000 ft in several areas but range from 300 to 1,000 ft over most of the aquifer, as shown in figure 5D. The Dawson aquifer generally ranges from 200 to 900 ft thick and contains 100—400 ft of water-yielding material, as shown in figure 6D. AQUIF ER CHARACTERISTICS The ability of an aquifer to transmit water depends on the thickness and permeability of the water-yielding material. A greater thickness of water-yielding material (or material of greater permeability) allows larger volumes of water to move through the aquifer. Hydraulic conductivity is a common measure of the ability of a unit volume of material to transmit water under a given set of temperature and pressure condi- tions. Transmissivity is the product of hydraulic con- ductivity and thickness, and thus incorporates the effects of both thickness and permeability in measuring the ability of an aquifer to transmit water. The hydraulic conductivity of the water-yielding materials in the bedrock aquifers was estimated through use of aquifer tests, specific-capacity tests, and laboratory analyses of undisturbed rock samples. Robson (1983) showed that hydraulic-conductivity values in the Laramie-Fox Hills aquifer range from more than 6 ft/d near Littleton to less than 0.05 ft/d along the northwest margin of the aquifer. The largest. hydraulic conductivity (7 ft/d) in the study area occurs in the Arapahoe aquifer south of Littleton; however, values less than 0.5 ft/d occur in the central part of the Arapahoe aquifer. Hydraulic conductivity in the Denver aquifer ranges from 0.5 to 1.5 ft/d; in the Dawson aquifer hydraulic conductivity ranges from 0.2 to 3.0 ft/d. The transmissivity of the aquifers was also mapped by Robson (1983). Transmissivity ranges from zero at the edge of each of the aquifers to over 1,000 ftz/d in the Laramie—Fox Hills aquifer, 2,100 ftzld in the Arapahoe aquifer, 400 ft2/d in the Denver aquifer, and 1,200 ftz/d in the Dawson aquifer (fig. 8). By comparison, the transmissivity of the alluvial aquifer along the South Platte River between Brighton and Platteville averages about 13,000 ft’ld (Hurr and Schneider, 1972). The 10 BEDROCK AQUIFERS IN DENVER BASIN . COLO. A — LARAMIE — FOX HILLS AQUIFER D — DAWSON AQUIFER A “9—5 \ A _ ’ arkcr lo 0 ' o D 5 ID 15 MILES U 5 10 15KILUMETEHS FIGURE 5.—Depth to the base of the bedrock aquifers. A, Laramie—Fox Hills aquifer, B, Arapahoe aquifer; C, Denver aquifer; D, Dawson aquifer. (See fig. 4 for geographic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 1 1 B — ARAPAHOE AOUIFER C— DENVER AQUIFER EX PLANATIO N —7000——-L|NE OF EQUAL DEPTH FROM LAND SURFACE TO THE BASE OF THE AOUIFER— |nterva|500 feet FIGURE 5.—Cont.inued. 12 BEDROCK AQUIFERS IN DENVER BASIN, COLO. A - LARAMIE — FOX HILLS AQUIFER D — DAWSON AQUIFER [I 5 10 15 MILES fl 5 10 15KILOMETEHS FIGURE 6.—Total thickness of water~yielding material in the bedrock aquifers. A, Laramie-Fox Hills aquifer, B, Arapahoe aquifer; C, Denver aquifer; D. Dawson aquifer. (See fig. 4 for geographic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 13 B — ARAPAHOE AOUIFER C— DENVER AOUIFER Cherry Creek Re v {3 OX ‘Park I AS 0 EX PLANATIO N —100— LINE OF EQUAL THICKNESS OF WATER-YIELDING MATERIAL— |nterva|100 feet FIGURE 6.—Continued. 14 BEDROCK AQUIFERS IN DENVER BASIN , COLO. 105° _ Modified from Bolton 0 5 10 15 MILES and anrie, 1973 E l J l I l I and Tweto, 1979 U 5 11] 15 KILDMHEHS EXPLANATION LIMIT OF LARAMIE FOX HILLS AQUIFER fi—FAULT—Bar and Ball on downthrown side —————— LIMIT OF AREA WHERE GROUND~WATER MOVEMENT MAY BE AFFECTED BY FAULTING FIGURE 7.—Location of faults in the Laramie-Fox Hills aquifer north of Denver. THE NATURAL HYDROLOGIC SYSTEM 1 5 TABLE 1.—Pomsity and specific-yield statistics for water-yielding materials (Data, with exception of number of samples, in percentage] Porosity data based on Specific yield data Laboratory analysis Interpretation of based on laboratory of samples geophysical logs analysis of samples Number Standard Number Standard Number Standard Aquifer Mean Range of samples deviation Mean Range of samples deviation Mean Range of samples deviation Dawson ——————————— 32 18—46 20 6.7 (1) 25-30 5 (1) 18 3.6—34 18 8.4 Denver ———————————— 31 15-44 24 6.0 29 24-34 19 2.9 14 .2—29 11 9.7 Arapahoe ---------- 30 12-46 33 7.3 30 22—35 52 3.8 18 3.3—33 25 8.4 Laramie-Fox Hills -- 32 21—44 42 6.1 32 24-36 21 2.5 20 4.8—38 29 9.1 1Insufficient. data bedrock aquifers are thus only about one-tenth as transmissive as this alluvial aquifer. This fact accounts for the smaller well yields normally encountered in bedrock wells and affects the water budget and poten- tial yield of the entire basin. The ability of an aquifer to store water depends on the thickness and porosity of the water-yielding material and, to a lesser extent, on the compressibility of the water and rock. If the void space in a porous rock is physically drained of water, as occurs during a decline in the water table, the volume of water released from storage may be expressed as a percent of the rock volume. This ratio is called specific yield. The volume of water that will not drain from the rock adheres to the rock structure. The ratio of this volume of water to the volume of rock is called specific retention. Porosity, which is a measure of the pore volume of the rock, is mathematically equal to specific yield plus specific retention. The mean porosity of the water-yielding materials in the bedrock aquifers was estimated by Robson (1983) to range from 29 to 32 percent, and the mean specific yield was estimated to range from 14 to 20 percent, as shown in table 1. Estimates of specific retention ranged from about 12 to 17 percent. Thus, a cubic foot of water- yielding material typically would contain about 0.3 ft3 of water in the void space of the rock (porosity = 30 per- cent), and, by drainage, would yield about 0.15 ft3 of water to a well (specific yield = 15 percent), and would retain about 0.15 ft3 of water in permanent storage in the rock structure (specific retention = 15 percent). The porosity data shown in table 1 are based on either laboratory analyses of undisturbed rock samples or on interpretation of neutron-density geophysical logs (Rob- son, 1983). Laboratory porosity determinations were made on about 120 undisturbed bedrock samples, taken mainly from excavations or outcrops. The samples were chosen to represent the general character of the water- yielding materials in each aquifer. Neutron-density geophysical logs were interpreted to provide a second determination of porosity of these water-yielding materials. Porosity determinations from 97 siltstone, sandstone, and conglomerate intervals shown on 50 logs indicate that the mean in situ porosity of these intervals is in good agreement with the mean porosity based on laboratory analyses from other locations (table 1). Laboratory specific-yield determinations were made on about 60 samples of the water-yielding materials. Results of this work and data from McConaghy and others (1964) indicate that the specific yield of in- dividual samples may range from about 1 to 38 percent. This large range is due to the variable composition of the aquifer materials, clay content being a principal fac- tor. A moderately consolidated sandstone with minimal clay content generally will have a relatively large por- osity and corresponding large specific yield. A clayey sandstone, by contrast, can have a relatively large porosity but a small specific yield due to the presence of the clay (Todd, 1967). Because of the variable composi- tion of the permeable materials, specific yield can be ex- pected to vary considerably from one water-yielding bed to another and from one area to another in the basin. 16 BEDROCK AQUIFERS IN DENVER BASIN, COLO. A — LARAMIE - FOX HILLS AQUIFER , 'NVERJ."“ \. ,/ K t. _____ —9*-———-—¥—.— r — D — DAWSON AOUIFER U 5 10 15 MILES ll 5 10 lfiKlLUMETERS FIGURE 8.-Transmissivity of the bedrock aquifers. A, Laramie—Fox Hills aquifer; B. Arapahoe aquifer; C, Denver aquifer, D, Dawson aquifer. (See fig. 4 for geographic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 1 7 B — ARAPAHOE AQUIFER éLBBRT C— DENVER AQUIFER “99/ ‘ b) O O ¢ .. V ,. 9‘3)? X9 , . \ ‘c \J/ $1 ‘ ' I V/ \__ \ / I EL PASO 39/ I EXPLANATION ———100— LINE OF EOUALTRANSMISSIVITY— Interval, in feet squared per day, is variable FIGURE 8.—Conti.nued. 18 BEDROCK AQUIFERS IN DENVER BASIN. COLO. ‘ Water also may be released from storage in an aquifer without physically draining water from the void space in the rock. This occurs as a result of the elastic prop- erties of water and rock, and is measured by an aquifer characteristic called the confined storage coefficient. Fatt (1958) and Clark (1966) presented data indicating that rocks similar to those in the Denver basin have a compressibility ranging from 7X10‘7 to 7X10‘6 inf/lb. These and other data were used by Robson (1983) to estimate confined storage coefficients ranging from less than 2X10—4 to more than 8X10“ (fig. 9). Although a cubic foot of water-yielding material typically would contain about 0.3 ft3 of water in storage, it would yield only about 2X10'6 ft8 of water (about 1 drop) as a result of the compressibility of the water and rock. The volume of water released from storage (or taken into storage) in the aquifers is thus drastically different, depending on whether gravity drainage of rock void space occurs under water-table conditions, or whether compressive release of water from storage occurs under confined con- ditions. The specific yield and confined storage coeffi- cient have an important effect on water-level changes in the aquifers and will be discussed further in the section “The Transient-State Model.” The location of water- table and confined conditions in the bedrock aquifers is shown in figure 9. The lateral extent of these areas may be altered by changes in water levels in the aquifer; thus, figure 9 represents approximate conditions as of 1978. As indicated in figure 9, water-table conditions generally are present in the near-surface parts of an aquifer, and confined conditions occur at greater depth in the aquifer and in underlying aquifers. About 470x 10‘ acre-ft of water is in storage in the bedrock aquifers in the Denver basin. Of this amount, about 270x106 acre-ft is estimated to be theoretically recoverable by gravity drainage of the aquifers. Aquifers generally cannot be completely drained by wells, so this volume of recoverable water represents a theoretical upper limit on yield, not a pratical limit. The distribution of total and recoverable water by aquifers is shown in table 2. TABLE 2.—Estimated total and recoverable ground water in storage in the Denver basin [Storage in millions of mfeet] Total water Recoverable water Aquifer in storage in storage Dawson ———————————————————————————— 48 27 Denver ———————————————————————————— 89 42 Arapahoe —————————————————————————— 150 90 Laramie—Fox Hills ——————————————————— 180 110 Total 467 269 By comparison, the Denver Water Department has a water-storage capacity of about 0.25X106 acre-ft in a principal mountain reservoir (Dillon Reservoir), and the Denver metropolitan area annually uses about 0.31 X106 acre-ft of water (A. Udin, Engineering Science, oral commun, 1983). The recoverable water in storage in the Denver basin is thus about 1,000 times the volume of Dillon Reservoir and would constitute slight- ly less than a 1,000-yr supply for the metropolitan area if such massive use of the ground water were feasible. As shown in figure 9, confined conditions occur in most of the area of the bedrock aquifers. In spite of the large area of confined conditions, the volume of water in confined storage in these areas is only about 0.3X106 acre-ft. This is the volume of water that would be re- leased from storage in the aquifers as water levels in confined areas decline to the point where water-table conditions first develop in each aquifer. This volume constitutes less than 0.1 percent of the total volume of ground water in storage in the basin. These figures in- dicate that if ground-water development is to use the large volume of water in storage, water levels ultimately must be lowered such that water-table conditions develop in larger areas of the basin. Only under water- table conditions does gravity drainage of the aquifer allow large volumes of water to be removed from ground-water storage. WATER LEVELS The water-table conditions shown in figure 9 are pres- ent in the near-surface parts of the aquifers in the out- crop areas. A water table is present when the water level in a well lies below the top of the water-yielding zone. . However, water in the bedrock aquifers generally is con- fined by overlying and underlying shale and clay strata of low hydraulic conductivity. As a result, the water level in a well may be considerably above the top of the water-yielding strata (a confined aquifer). Near Aurora, for example, the top of the Laramie—Fox Hills aquifer occurs at a depth of 1,500 ft, but the depth to water in wells completed in the aquifer is only about 250 ft. Depths to water in wells completed in the confined aquifers thus reflect the varying fluid pressures en- countered in strata of different depths at different times. These pressures are expressed in terms of altitude of water level in a well (head) in feet above mean sea level. A map showing lines of equal head is referred to as a potentiometric surface map. Prior to about 1885, heads in the Denver, Arapahoe, and Laramie—Fox Hills aquifers were large enough to cause bedrock wells in the valley of the South Platte River near Denver to flow with considerable pressure at THE NATURAL HYDROLOGIC SYSTEM 19 the land surface (Emmons and others, 1896). The sub- sequent rapid decline in head in one of these wells, com- pleted in the Arapahoe aquifer, is shown in figure 10. The head in this well has ranged from about 60 ft above land surface in 1883 to as much as 340 ft below land sur- face in 1960. Since 1960, heads have risen, probably as a result of a decrease in pumpage in the downtown area. In other parts of the basin, depths to water in 1978 have ranged from near zero in some low-lying undeveloped areas to more than 1,000 ft in other areas. In 1978, the measured altitude of the potentiometric surface in the Dawson aquifer ranged from a high of 7,500 ft in the area near Black Forest to a low of 5,500 ft near Englewood, as shown on plate 1. A ridge in the potentiometric surface is located near a line from Palmer Lake to Rattlesnake Butte and forms a ground- water divide. Ground water north of the divide moves in a northerly direction; ground water south of the divide moves in a southerly direction. The altitude of the potentiometric surface and the direction of water move- ment are controlled primarily by the altitude of the stream channels in the area, the aquifer characteristics, and the magnitude and distribution of precipitation. This occurs because the rate of precipitation recharge to the Dawson aquifer generally exceeds the ability of the aquifer to transmit water over long distances. The ex- cess ground water is discharged to nearby streams, allowing the altitude of the stream channels to affect the altitude of the potentiometric surface and the direc- tion of the ground-water movement. As a result of this condition, the areas with the highest land-surface altitude (near the Black Forest, for example) also have the highest potentiometric surface altitude. In addition, more water is available for recharge in these high areas because of greater precipitation at the higher altitudes (fig. 2). Vertical as well as lateral movement of water occurs in the Dawson aquifer. Lower potentiometric surface altitudes in the underlying Denver aquifer allow water to move down through the Dawson aquifer into the Denver aquifer. This vertical component of flow is small in comparison to the lateral component of flow but con- stitutes an important source of recharge for the under- lying aquifers. In most of the northwestern part of the Dawson aquifer, water-level changes have ranged from less than 50 ft of rise to less than 50 ft of decline during the 20-year period from 1958 to 1978. However, water-level declines near Castle Rock have exceeded 100 ft, and declines near Parker have exceeded 50 ft, as shown in figure 11D. The 1958—78 water-level change data from wells at a few scattered‘locations in other parts of the Dawson aquifer generally show water-level rises or declines of less than 30 ft, with no consistent pattern in these areas. In 1978, the measured part of the potentiometric sur- face in the Denver aquifer ranged from a high of 6,800 ft in the southern part of the aquifer to a low of 5,000 ft near Commerce City (pl. 1). In the central part of the aquifer, water-level data are unavailable, and the poten- tiometric surface cannot be defined. In the northern, eastern, and southern parts of the aquifer, water generally is moving from the south-central part of the area toward the margins of the aquifer. Relatively sharp bends in the potentiometric contours occur near some small streams as a result of water moving from the aquifer into the stream valleys. Near the western edge of the aquifer, water is moving either approximately parallel to the aquifer limit or east from the aquifer limit toward the South Platte River. Ground water flows into a major trough in the potentiometric surface extending along Plum Creek and the South Platte River to the area northeast of Commerce City. The trough originally was shallower, being formed by the natural discharge of ground water into the South Platte River and its tributaries, but it has been deepened during the past hundred years by flowing wells and pumpage in the Denver metropolitan area. Between 1958 and 1978, water-level declines in the Denver aquifer have exceeded 200 ft in an area east of Denver, and declines exceeding 50 ft have occured in large areas along the eastern and southern edges of the metropolitan area (fig. 110). Near Cherry Creek Reser- voir, the water level in the uppermost part of the Denver aquifer rose more than 50 ft between 1958 and 1978, probably as a result of recharge from the reservoir. The water-level change data from wells at a few scattered locations in other parts of the Denver aquifer usually show water-level rises or declines of less than 50 ft, with no consistent pattern in these areas. In the Arapahoe aquifer, the measured part of the 1978 potentiometric surface ranged in altitude from a high of 6,500 ft in the southern part of the aquifer to a low of 4,900 ft near Brighton (pl. 1). In the central part of the aquifer, reliable water-level data for the Arapahoe aquifer are unavailable, and the potentiometric surface cannot be accurately defined. In the northern, eastern, and southern parts of the aquifer, water generally is moving from the south-central part of the area toward the margins of the aquifer. Near the western edge of the aquifer, water is moving either approximately parallel to the aquifer limit or is moving east from the aquifer limit toward the South Platte River. Ground water flows from the west, northwest, and southeast into a major trough in the potentiometric surface extending along the South Platte River to an area northeast of 20 BEDROCK AQUIFERS IN DENVER BASIN. COLO. A — LARAMIE — FOX HILLS AQUIFER Cherry Creek Res "'\ :1 \ : \ |_I i leton D — DAWSON AOUIFER : K 9/ BLBERT S i W , 0 5 10 15 MILES fl 5 10 15K|LOMETEHS FIGURE 9.—-Confined storage coefficient of the bedrock aquifers and location of water-table and confined conditions. A, Laramie—Fox Hills aquifer; B, Arapahoe aquifer; C, Denver aquifer; D, Dawson aquifer. (See fig. 4 for geographic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 21 B — ARAPAHOE AQUIFER C- DENVER AQUIFER EXPLANATION AREA WHERE WATER-TABLE CONDITIONS COMMONLY OCCUR IN THE UPPER PART OF THE AQUIFER AREA WHERE CONFINED CONDITIONS COMMONLY OCCUR —2—— LINE OF EQUAL STORAGE COEFFICIENT FOR THE CONFINED WATER-YIELDING MATERIALS, 1978— Dimension- less units X 10", Interval is 2 X 10‘“ FIGURE 9.—Continued. 22 BEDROCK AQUIFERS IN DENVER BASIN, COLO. .1 5300 1 1 I y I I l I LU > B 5 LAND SURFACE ALTITUDE U) L” 5200 — > O m < E w 5100 — LL E 1.0 S t 5000 — 5 < .J Lu 5 4900 , _l a: a < E 4800 1 l 1 | I I 1 l 1 I 1 1880 1900 1920 1940 1960 1980 2000 FIGURE 10.—Water—level hydrograph for the Arapahoe aquifer near the Colorado State Capitol. Brighton. The trough originally was shallower prior to well drilling in the area and has been deepened and ex- panded during the past hundred years by flowing wells and pumpage. Water-level declines in the Arapahoe aquifer have ex- ceeded 200 ft in a 135-mi2 area southeast of Denver, and have exceeded 50 ft in a much larger, but less well de- fined, area (fig. 113). Water-level rises have exceeded 100 ft in a 60-mi2 area under the north-central part of the Denver metropolitan area; in other parts of this area, only moderate water-level changes have occured. This pattern of water-level change is the likely result of recharge and a decrease in the rate of pumping in the metropolitan area, coupled with an increase in pumping in the surrounding suburban areas. The 1958—78 water- level change data for wells at a few scattered locations and other parts of the Arapahoe aquifer generally show water-level rises or declines of less than 50 ft. The 1978 altitude of the measured potentiometric sur- face in the Laramie—Fox Hills aquifer is highest (6,300 ft) in the southern part of the aquifer and lowest (4,600 ft) near the northeast margin of the aquifer where ground water discharges to the alluvium of the South Platte River and its tributaries (pl. 1). In the central part of the aquifer, reliable water-level data for the Laramie—Fox Hills aquifer are unavailable, and the potentiometric surface cannot be accurately defined. North of El Paso County, water typically is moving to the north or northeast; south of the El Paso-Douglas County line, water generally is moving to the south or southeast. In and near Boulder County, local faulting appears to have segmented the aquifer and allowed markedly different water levels to occur in adjacent segments. The resulting potentiometric surface is distorted near these segments, and the direction of water movement in this area is complex. Schneider (1980) provided more detailed maps of the poten- tiometric surface and additional discussion of the hydrology in this area. In the northwestern part of the Denver basin, ground water in the Laramie-Fox Hills aquifer moves toward a major trough in the poten- tiometric surface. This trough extends from Littleton through Northglenn and Brighton and northeast toward Masters. A similar, but less extensive, trough has been shown to have been present in this area in 1970 (Romero, 1976). The trough has been formed by pump- _ age from wells in the area and has expanded and deep- ened as withdrawals have increased. In an 80-mi2 area near Brighton, water levels in the Laramie—Fox Hills aquifer declined more than 200 ft between 1958 and 1978. Declines in excess of 100 ft have occurred in an area extending from north of Denver to northeast of Brighton, and in the area north- east of Littleton, as shown in figure 11A. Water-level changes measured in wells at a few scattered locations in other parts of the aquifer usually indicate water-level rises or declines of less than 50 ft. In spite of their outward appearance, bedrock forma- tions have elastic properties and are compressible when subjected to stress such as that created by large water- level declines (Mayuga and Allen, 1970). For example, land-surface subsidence due to elastic compression of the bedrock aquifers in the Denver metropolitan area is estimated to range from 0.07 to 0.7 in. per 100 ft of water-level decline in the Laramie—Fox Hills aquifer and to range from 0.2 to 2.0 in. per 100 ft of water-level decline in the Arapahoe aquifer. The 400 ft of water- level decline in the Arapahoe aquifer between 1883 and THE NATURAL HYDROLOGIC SYSTEM 23 1960 (fig. 10) thus may have produced between 0.8 and 8.0 in. of land-surface subsidence in the metropolitan area. Resurvey of bench-mark altitudes on first-order level lines would provide a means of checking the ac- curacy of these subsidence estimates; however, this needed resurvey is not scheduled for completion before late 1984 (R. Cohen, National Geodetic Survey, oral commun., 1984). Lacking corroboration, these sub- sidence estimates based on rock-compressibility data from other areas (Fatt, 1958; Clark, 1966) must be con- sidered as approximations. Land-surface subsidence also may occur in suburban areas where more recent water-level declines have oc- cured. Subsidence in these areas is likely smaller than in central Denver due to smaller water-level declines. Sub- sidence due to the elastic compression of the formations is partly reversible, and large water-level rises may pro- duce some recovery in the land-surface altitude. Sub- sidence due to inelastic compaction of formation materials generally is not reversible. However, the consolidated nature of the formations indicates that in- elastic compaction probably is not a major contribution to subsidence in the Denver basin. If future development of the ground-water resources produces large water-level declines in the aquifer, several feet of land-surface subsidence might occur. Results of subsidence due to water-level declines in Arizona, California, Idaho, Nevada, and Texas Were shown by Bull (1975) and Holzer (1977) to include damage to well casings, disruption to surface drainages and ditches, development of surface fissures, and disruption to gravity-flow sewage lines. The severity of these conditions is affected by the magnitude and distribution of subsidence. The rate of ground- -water movement is controlled by the hydraulic conductivity, porosity, and gradient of the potentiometric surface in the aquifer. These factors vary from one area to another in the basin but, in general, produce rates of movement that are much slower than those commonly found in surface streams. The average rate of movement in the Dawson or Arapahoe aquifers, for example, may range from 5 to 200 ft/yr. The Denver or Laramie—Fox Hills aquifers have rates of movement ranging from about 1 to 100 ft/yr. These figures represent average rates of move- ment in the aquifers and do not consider the larger rates of movement that occur near pumping wells. If porosity and hydraulic conductivity are similar in two areas, the area with the more closely spaced potentiometric con- tours (steeper gradient) will have a greater rate of ground-water movement; thus, the potentiometric- surface maps shown on plate 1 provide an indication of the relative rates of ground-water movement in the basin. RECHARGE AND DISCHARGE The mean annual precipitation in the basin supplies an average of 6,900 fts/s of water to the area. However, almost all of this water is lost through surface runoff, evaporation, and transpiration by vegetation, allowing only a small part of the precipitation to replenish the bedrock aquifers. In the outcrop area of the formations, recharge occurs as deep infiltration of precipitation in the highland areas between stream channels or as in- filtration of water from alluvial aquifers located above the water level in the bedrock aquifers. In the central part of the basin, downward movement of water from overlying bedrock aquifers is an important source of recharge to most of the underlying aquifers. Although the rate of vertical movement is small in comparison to the rate of lateral movement, the large areas involved allow significant volumes of water to move between aquifers. In the case of the Laramie—Fox Hills aquifer, however, vertical movement of water through the thick shales of the Laramie Formation is probably insignificant. Most water moves laterally through the permeable sandstone strata from areas of recharge toward areas of discharge. This can occur on both a local and regional scale. On a local scale, water moves from the highland recharge areas in the outcrop through the upper part of the aquifer, or through perched aquifers, to the discharge areas in nearby stream valleys. On a regional scale, water moves from outcrop recharge areas, or the central part of the study area where recharge occurs from overlying aquifers, into deeper parts of the aquifer and ultimately discharges in more remote stream valleys. In these stream valleys, water from the bedrock discharges into the streams or into the alluvial aquifers along the stream channels, or it is consumed by vegeta- tion growmg in the valleys. In addition to these long-standing processes of natural recharge and discharge, relatively recent discharge is occurring from pumping wells. In areas where pumping has caused significant water-level declines, natural discharge may no longer occur, and pumpage may be the only form of ground-water discharge. In these areas, the interaquifer movement of water may be enhanced, halted, or reversed, depending on the relation of the heads in adjacent aquifers. The Denver basin has a long history of water use from the bedrock aquifers. In 1883, a well drilled for coal ex- ploration in sec. 29, T. 3 S., R. 68 W., was abandoned because of the large, uncontrollable flow of water en- countered. By 1884, about 80 wells had been drilled into the Denver and Arapahoe aquifers in an approximately 4-Ini2 area near secs. 22, 27, 28, 32, 33, and 34 of T. 3 S., R. 68 W. Cross and others (1884) estimated that in 1884, 24 I BEDROCK AQUIFERS IN DENVER BASIN, COLO. A — LARAMIE - FOX HILLS AQUIFER D — DAWSON AQUIFER ...2 ELBERT X \W/ "5‘ ./ U 5 1D 15 MILES 0 5 1O 15K|LOMETEHS FIGURE ll.—Measured water-level changes in the aquifers between 1958 and 1978. A, Laramie-Fox Hills aquifer; B, Arapahoe aquifer, C, Denver aquifer, D, Dawson aquifer. (See fig. 4 for goegraphic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 25 C— DENVER AQUIFER B — ARAPAHOE AQUIFER Cherry leek Res /9 Emma 93““) EXPLANATION AREA OF WATER LEVEL RISE —— 700—- LINE OF EQUAL WATER- LEVEL CHANGE—IntervaI 50 and 100 feet. Dashed where inferred FIGURE 11.-Continued. 26 BEDROCK AQUIFERS IN DENVER BASIN, COLO. uncapped flowing wells produced about 4.5 ftsls of discharge from the aquifers. Emmons and others (1896) estimated that 400 bedrock wells had been drilled by 1890, but that decreasing heads in the aquifers had caused the discharge to decreased to about 2.3 ft3/s. Barton (1905) reported that heads in the Denver and Arapahoe aquifers were 50 to 200 ft below land surface in the Denver area by 1905, requiring that water be pumped to the surface in most areas. In 1963, the US. Geological Survey compiled a preliminary tabulation of bedrock pumpage in the basin. This work included municipal, commercial, and industrial pumpage from most wells in the basin and indicated a pumping rate of 12.5 ft3/s. Work by Romero (1976) contain the only other known pumpage estimate for the bedrock aquifers, but it is not usable for this work because it is an estimate of the maximum pumpage allowed under Colorado statutes rather than actual pumpage. 50 Because of limited data, it was necessary to estimate the total bedrock pumpage in the Denver basin for the model period 1958—78. Data were adequate to make annual estimates of municipal pumpage during this period, total pumpage for the years 1960, 1974, and 1977 also could be estimated. These estimates and results of ground-water modeling indicated that total pumpage was 3.0 to 4.5 times the municipal pumpage. This relation and the annual municipal pumpage were used to estimate the annual total pumpage for the entire 1958—78 period. Further discussion of the procedure used to estimate pumpage is contained in the “Supple mental Information” section at the end of this paper. Estimates of total pumpage for wells completed in the bedrock aquifers ranged from 15 fth in 1958 to as much as 41 ft3/s in 1978. As shown in figure 12, a gradual in- crease in pumpage occurred between 1958 and 1962, with little change between 1962 and 1975, followed by a Municipal pumpage Total pumpage 40— 20— PUMPAGE, IN CUBIC FEET PER SECOND 0 1958 1960 1965 1970 1975 1978 FIGURE 12.—Estimated pumpage from bedrock wells, 1958—78. THE NATURAL HYDROLOGIC SYSTEM greater increase in pumpage between 1975 and 1978. The latter pumpage increase is probably due to (1) an in- crease in population and development in suburban areas that rely on bedrock wells and (2) the effects of outside water-use restrictions imposed on metropolitan customers by the Denver Water Department during the period 1977—82. The 1978 pumping rate is slightly less than the mean annual natural recharge to the entire bedrock-aquifer system. Most of this pumpage occurs within a 30-mi radius of downtown Denver and has caused major water-level declines in the bedrock formations. Between 1975 and 197 8, about onehalf of the bedrock pumpage from the basin was obtained from the Arapahoe aquifer. This is to be expected, considering the large well yields, good quality water, and relatively shallow depths of the aquifer under the metropolitan area. The other three aquifers contributed 15 to 20 per- cent of the total pumpage during 1958—78. Measured discharge from bedrock wells is highly variable but typically ranges from 2 to 700 gal/min. Small discharges of several gallons per minute are com- monly found in domestic and stock wells or in larger municipal or industrial wells in areas of low trans- missivity. Large-capacity wells in areas of high trans- missivity usually yield several hundred gallons per' minute, with reported discharges of as much as 800 gal/min. Wells in the Arapahoe aquifer generally have the largest yield, folldwed by those in the Dawson, Laramie—Fox Hills, and Denver aquifers. 27 WATER QUALITY Water in the Dawson aquifer generally is of excellent chemical quality, meeting drinking-water standards (US. Environmental Protection Agency, 1976, 1977; Colorado Department of Health, 1978) for public water supplies in most of the area (table 3). The water gener- ally contains a preponderance of dissolved calcium and bicarbonate ions, and thus is classified as a calcium bicarbonate—type water. Sodium bicarbonate— or sodium sulfate—type water occurs in a few isolated areas, usually near the margin of the aquifer. Dissolved- solids concentrations range from less than 100 mg/L in the south-central part of the Dawson aquifer to more than 1,000 mg/L near the northern edge of the aquifer, as shown in figure 13D. Dissolved-solids concentrations and the pH of the water are smallest near the ground- water divide in the area to the east of Palmer Lake. Dissolved-iron concentrations generally range from 20 to 100 ug/L, which is less than the limit of 300 ug/L recommended for public water supplies (U.S. En- vironmental Protection Agency, 1977). However, dissolved-iron concentrations between 8,000 and 85,000 ug/L have been measured in water samples from a few wells at scattered locations. Dissolved-sulfate con- centrations commonly range from 4 to 10 mg/L in the central part of the aquifer to more than 700 mg/L in an isolated area at the northern margin of the aquifer (fig. 130). In this area, sulfate concentrations exceed TABLE 3.-Typical bedrock water quality and drinking water standards [Values in milligrams per liter unless micrograms per liter (ug/L) indicated] Typical analysis for water from bedrock aquifers near Denver Drinking water standard for public water supplies Laramie- (us. Environmental Protection Dissolved Dawson Denver Arapahoe Fox Hills Agency. 1976; 1977; Colorado constituent aquifer aquifer aquifer aquifer Department of Health, 1978) Calcium 30 11 31 4.2 Iron 80 ag/L 30 ug/L 170 ag/L 100 ug/L 300 ug/L Magnesium 2.7 .4 3 .9 Manganese 20 ng/L 10 pg/L 30 ag/L 10 [lg/L 50 ug/L Potassium 3.5 1.0 4.1 2.8 Sodium 12 57 140 270 Bicarbonate 120 150 250 640 Carbonate 0 0 0 0 Chloride 3.7 3.8 57 43 250 Fluoride .5 1.6 1.1 2.7 1.8 (for Denver) Nitrate as N .11 .05 .04 .03 10 Phosphate .03 0 4.1 2.8 Silica 40 13 9.6 14 Sulfate 1 2 13 1 10 7.6 250 Hardness as calcium carbonate —————————————————————— 86 29 90 14 Dissolved solids 164 175 479 662 1,000 28 BEDROCK AQUIFERS IN DENVER BASIN, 001.0. A —- LARAMIE - FOX HILLS AQUIFER Cherry Creek L: D - DAWSON AQUIFER l] 5 1!] 15 MILES U 5 10 15KILUMETERS FIGURE l3.—Dissolved-solids and dissolved-sulfate concentrations in the bedrock aquifers. A, Laramie—Fox Hills aquifer; E, Arapahoe aquifer; C, Denver aquifer; D, Dawson aquifer. (See fig. 4 for geographic location of aquifers.) THE NATURAL HYDROLOGIC SYSTEM 29 B — ARAPAHOE AQUIFER ' 1.9-] ,r—x- C — DENVER AOUIFER EXPLANATION DISSOLVED SULFATE CONCENTRATIONS IN MILLIGRAMS PER LITER Less than 25 25 to 50 Greater than 250 —600-— LINE OF EQUAL DISSOLVED SOLIDS CONCENTRATION— lnterval, in milligrams per liter, is variable FIGURE 13.—Continued. 30 BEDROCK AQUIFERS IN DENVER BASIN, COLO. the recommended drinking-water standard of 250 mg/L for public water supplies. A typical analysis of water from the north-central part of the Dawson aquifer is shown in table 3. For com- parison, selected drinking-water standards for public water supplies also are shown. Concentrations of dis- solved constituents in the bedrock water that exceeds these standards generally are not harmful but they may affect the color, odor, or taste of the water. For ex- ample, sulfate concentrations in excess of the standard may produce a laxative effect in people unaccustomed to drinking the water; large concentrations of iron or manganese may affect the taste of the water and stain plumbing fixtures and laundered clothing; large dissolved-solids concentrations may impart a mineral- ized taste to the water. Large concentrations of other constituents also may be objectionable, although no drinking-water standard has been set. Hardness, for ex- ample, is objectionable because hard water may leave a scaly deposit on the inside of pipes, steam boilers, and hot-water heaters; it requires more soap to make a good lather than does soft water; and it roughens washed skin and clothing. Hardness is classified, in terms of calcium carbonate, as soft water (0—60 mg/L), mod- erately hard water (61—120mg/L), hard water (121- 180 mg/L), and very hard water (more than 180 mg/L). Water in the Denver aquifer is also of good chemical quality, meeting drinking-water standards for public water supplies in most of the area. A typical analysis of water from the northwestern part of the aquifer is shown in table 3. Water in the central part of the aquifer is classified as a calcium bicarbonate type. Near the margins of the aquifer, a sodium bicarbonate— or sodi- um sulfate—type water is more common. The calcium bicarbonate water occurs as a result of the calcium bicarbonate water in the overlying Dawson aquifer moving down into the Denver aquifer. As the calcium bicarbonate water moves laterally through the Denver aquifer, the water is naturally softened by cation exchange on the clay minerals that abound in this predominantly silty and clayey formation. This process increases the dissolved-sodium concentration in the water and decreases the dissolved-calcium concentra- tion. As the water in the Denver aquifer moves beyond the limit of the overlying Dawson aquifer, other factors affect the chemical composition of the water. In the outcrop areas, the availability of oxygen in the soil and in the bedrock can lead to the formation of soluble minerals in these sediments and in the coal and other organic material that are common in the formation. Part of the precipitation that falls on the outcrop area percolates downward, carrying some of these soluble minerals from the soil, rock, and coal into the Denver aquifer. This process, coupled with the cation exchange, produces the sodium bicarbonate- or sodium sulfate— type water found near the margins of the aquifer. Dissolved-solids concentrations are less in the central part of the aquifer, near the source of recharge from the overlying Dawson aquifer, as shown in figure 13C. The concentrations of dissolved solids increase to as much as 1,000 mg/L as the water moves toward the north, east, and south margins of the aquifer. Dissolved-iron concentrations generally range from 10 to 150 ug/L; however, concentrations as much as 6,600 ug/L are found in water from a few widely scattered wells. Measured concentrations of dissolved sulfate range from 2 mg/L in the central part of the Denver aquifer to as much as 2,700 mg/L in the northern part of the aquifer (fig. 13C). Concentrations in excess of 250 mg/L occur in a 300-mi2 area along the northern margin of the aquifer and in a few isolated areas in the southern part of the aquifer. Water in the Arapahoe aquifer generally is of good chemical quality and also meets drinking-water stan- dards for public water supplies in most of the area. A typical analysis of water from the Arapahoe aquifer southeast of Denver is shown is table 3. The water in this aquifer is classified as a sodium bicarbonate type. Calcium bicarbonate—type water also occurs in the aquifer at scattered locations and in the area between Sedalia and Colorado Springs. Water in the Arapahoe aquifer is similar in type to that found in the overlying Denver aquifer, due in part to the downward movement of water from the Denver aquifer to the Arapahoe aquifer. Dissolved-solids concentrations seem to be less in the central part of the aquifer, near the source of recharge from the overlying Denver aquifer. As shown in figure 133, the concentrations increase to more than 2,000 mg/L in some areas as the water moves toward the margins of the aquifer. This occurs as the result of soluble minerals being carried into the aquifer from near-surface sources. Dissolved-iron concentrations generally range from 20 to 200 ug/L; however, concen- trations as much as 6,500 ug/L occur in a few widely scattered wells. In areas of strong reducing conditions in the Arapahoe aquifer, sulfate minerals and organic material may be reduced to hydrogen sulfide and methane gases. When these gases are present in high concentrations, water pumped from the aquifer may effervesce, have a putrid odor, and be of marginal value for many uses. Although this condition is uncommon in the Arapahoe THE SIMULATED HYDROLOGIC SYSTEM 31 aquifer, a few occurrences have been reported. Measured concentrations of dissolved sulfate range from 5 mg/L in the southeast part of the aquifer to as much as 1,500 mg/L near the northern margin of the aquifer, and are in excess of 250 mg/L in large areas along the east and northwest margins of the aquifer (fig. 133). Water in the Laramie—Fox Hills aquifer generally is classified as a sodium bicarbonate type. Sodium sulfate-type water is found along the northern and eastern margins of the aquifer. A typical analysis of water from the southeastern part of the Denver metropopitan area is shown in table 3. Data are not available to define the apparently small concentrations of dissolved solids in the central part of the aquifer. The concentrations increase to more than 1,200 mg/L in three areas near the northern and eastern margins of the aquifer (fig. 13A). In each of these areas, the Laramie—Fox Hills aquifer is overlain by the shaly up- per part of the Laramie Formation and is beyond the edge of the overlying Arapahoe aquifer. When the direc- tion of ground-water movement is considered, poor quality water in these areas seems to occur as a result of soluble minerals being carried into the aquifer from sur- face sources or from sources within the upper part of the Laramie Formation. Dissolved-iron concentrations generally range from 20 to 200 ng/L. Concentrations of 42,000 and 79,000 [Lg/L have been found in two wells near Colorado Springs, and concentrations of about 1,000 pg/L have been found at a few other widely scat- tered wells. In areas of strong reducing conditions in the aquifer, sulfate minerals and organic material may be reduced to hydrogen sulfide and methane gases. When these gases are present in high concentrations, water pumped from the aquifer may have a putrid odor, ef- fervesce, and be of marginal value for most uses. Dissolved-sulfate concentrations range from less than 2 mg/L south of Denver to more than 1,200 mg/L east of Boulder. Concentrations in excess of 250 mg/L occur in four areas near the northern, eastern, and southern margins of the aquifer (fig. 13A). THE SIMULATED HYDROLOGIC SYSTEM Ground water flow models are mathematical tools used to further our understanding of a ground-water system and to aid in evaluating the hydrologic changes that can occur as a result of changes in water use. Steady-state models are used to evaluate the hydrologic conditions in a basin prior to man’s development of the water resources. These models provide estimates of the long-term average recharge and discharge to the aquifers and relate the effects of transmissivity, aquifer configuration, and recharge and discharge to the pristine water levels in the aquifers. Transient-state models are used to evaluate the hydrologic conditions that result from man’s alteration of the natural system by such means as pumping wells, building reservoirs, and diverting water from streams. A transient-state model may estimate the timedependent response of an aquifer system over a historical period (1958—78 in this study), or it may estimate the aquifer response over a future time period. The quality of any model, whether it be a hydrologic model or a plastic model airplane, can be judged by how well the model represents the prototype. Because models are simplifications of the prototype in either size (the airplane model) or complexity (the hydrologic model), an exact correspondence between the model and prototype never can be achieved. What must be achieved, however, is a correct representation of the essential aspects of the prototype. In hydrologic models, this means that the model must function in a manner similar to that of the prototype without con- sideration of all the complexities of the prototype. The correspondence between model and prototype function is judged during calibration. Calibration is accomplished by simulating water-level altitudes over a historical period and comparing these results to measured water-level altitudes for the same period. Differences between the model and prototype response are noted, and the geohydrologic data are checked and reinterpreted in an effort to resolve the dif- ferences. When the geoghydrologic data used in the model and the model computations agree satisfactorily with the corresponding measurments of the prototype, the model is considered to be calibrated and ready for further study and use. The process of judging the ade- quacy of the calibration is not quantitative; rather, it is a qualitative procedure in which the hydrologist must judge the degree of calibration against the worth of the data and the intended use of the model. MODEL DESCRIPTION Modeling in this study is based on the equation gov- eming the flow of ground water through a porous medium in three dimensions. The equation may be expressed as: 32 BEDROCK AQUIFERS IN DENVER BASIN, COLO. va gt)+a(T athEg‘K ah EAT»: 8x 37 w a; 225’: 6h Sat— +bW (1) man) ’ where h = hydraulic; head, WT” = the x and y components of transmis- sivity, b = saturated thickness, K2Z = the vertical component of hydraulic conductivity, S = storage coefficient, and mem = the rate of recharge or discharge ex- pressed as a function of location and time. In order to solve this equation for a heterogeneous, anisotropic aquifer with irregular boundaries, the area of the aquifer is subdivided into blocks in which the aquifer properties are assumed to be uniform (fig. 14). The continuous derivatives in equation (1) are replaced with finite-difference approximations for the derivatives at points located at the center of each block. Because 40 rows, 24 columns, and 4 layers of blocks were used in the transient-state model, a total of 40X24X4=3,840 equations in 3,840 unknowns is generated. The set of 3,840 simultaneous finite-difference equations is solved on a digital computer, using the iterative, strongly im- plicit procedure described be Trescott (1975) and Trescott and Larson (1976). The four layers in the model represent the four aquifers in the basin. Vertical hydraulic conductivity is simulated between the layers, using the quasi—three- dimensional procedure decribed by Trescott (1975). Storage coefficients may be either confined or uncon- fined, depending on the altitude of the computed heads in relation to the altitude of the top of the aquifer. Where water-table conditions are present, the transmis- sivity is adjusted in response to changes in the satu- rated thickness of the aquifer. The linearity of the basic equation is maintained by modifying transmissivity and storage coefficient at the timestep level, and equal- interval time steps are used in most simulations. The data arrays used in the program are indexed to allow computer storage of only non-zero values. This signifi- cantly reduces the computer core requirements by eliminating storage space formerly needed for zero values, which occur outside the aquifer limits in each layer. Input to the model program consists primarily of operational parameters, which control the internal operation of the program, and geohydrologic character- istics of the aquifers. Operational parameters include such items as the number of rows, columns, and layers in the model; configuration of the aquifers; number of time steps; duration of pumping periods; and format of output. Geohydrologic characteristics describe the geologic and hydrologic conditions in each grid block for each of the four aquifers, including altitude of the poten- tiometric surface at the start of the simulation, storage coefficient and transmissivity of the aquifers at the start of the simulation, vertical leakance between aquifers, structural altitudes of the top and bottom of each aquifer, and location and pumping rate of wells in each aquifer. Model output consists of row, column, and layer tabulations of the model-calculated heads and water- level declines from the starting conditions. Heads calculated by the model represent the average altitude of the potentiometric surface in the area of the grid block. If a pumping well is simulated in the grid block, the model-calculated head also is the average head in a hypothetical 100 percent efficient well of radius re located in the center of the block, where re=ql4.81, (2) in which ri=Ax=Ay is the dimension of the grid block. Trescott and others (1976) provided additional discus- sion of the‘procedure used to calculate the head in a pumping well from the model-calculated head in a grid block. It is important to note that the model-calculated head in a grid block is not equivalent to the head in a pumping well. This must be calculated separately, using the model-output data and equation (2). A water budget also is calculated as part of the model output. This budget summarizes all the recharge, dis- charge, and storage terms developed in the computa- tions and presents an error term used to judge the computational accuracy of the finite-difference approxi- mation to equation (1). THE STEADY-STATE MODEL A steady-state model was used to investigate the un- disturbed hydrology of the bedrock aquifers. This model simulates long-term, constant hydrologic conditions prior to impacts of man and was used to produce estimates of the water budget and the rate of vertical leakance between adjacent aquifers. In contructing this model, an equal-interval grid of blocks 3 mi on a side, consisting of 41 rows, 27 columns, and 4 layers, was used. The aquifer limits and transmissivity distributions shown in figure 8 were ROW NUMBER THE SIMULATED HYDROLOGIC SYSTEM COLUMN NUMBER 1 2 3 45 10 15 20 21 22 23 24 1 _ 12:3 10 20 25 30 35 36 37 38 39 4O [l 5 1t! 15 20 MILES 0 5 10 15 20 KILDMETERS FIGURE 14.—Size and distribution of grid blocks used in the transient-state model. 33 34 BEDROCK AQUIFERS IN DENVER BASIN, COLO. used, and both starting head and storage coefficient were set to zero, as is common practice in this type of simulation. Constant-head nodes were specified in the outcrop areas of the four aquifers at points of recharge and discharge as indicated by the potentiometric- surface maps. Constant—head nodes located at these points provide recharge or discharge to the model. Water-level measurements in 31 selected wells were available to define the pristine water-level conditions in the basin. Near Denver, measurements made prior to 1885 were used for this purpose; in other areas, more re- cent'measurements were suitable because of the more measurements were of adequate depth and areal distribution to define the pristine heads at a number of points in each aquifer. Calibration consisted of varying the vertical hydraulic conductivity between the aquifers until the model-calculated heads were in close agreement with the pristine heads measured in each aquifer. The level of agreement was judged by use of the mathematical ex- pression call mean square error (MSE), calculated as: . . . . . . — 2 2 limited hlstorrcal decline 1n water level. The MES—AH +3 . (3) F 30,000 — F'— 35,000 - 30,000 D LLI I < 8 — 25,000 0‘) }_ LU LU LL — 20,000 E cc 0 t D: n: r- 15.000 3 - c: RUN1 K21,2,3=O.0 g C — 10.000 U: z < LU \ E h 5000 - 15.000 RUNZ K21,2,3=3.0X10—5 ‘ O 9 10.000 7 0(6 79 7; 2 ¢ ‘74. 47 m l— / o <6 7 <6 ¢ 6’ % \ 6‘ 5000 RUN 3 K11 :30 x10-6 K2 2,3 =30 x10-5 0 4, ‘5 10,000 Q + d‘ EXPLANATION ’ K11 - Vertical hydraulic conductivity 5000 between Laramie-Fox Hills and Arapahoe aquifers, in RUN 4 K21 = 8 5 X 10 7 feet per day K22 = 6.0 X 10‘6 0 K22 - Vertical hydraulic conductivity K13 = 3 0 X 10 5 between Arapahoe and Denver 7 .7 .7 7 aquifers, in feet per day 06 06 06 06/ / K23 - Vertical hydraulic conductivity {:6 {:0 fix ?6 between Denver and Dawson ’9 ’5’ ’9 ’9 5000 aqutfers, in feet per day RUN 5 1 =00 2 =2.6 x 10-5 3 =3.5 x 10-5 0 FIGURE 15.—Mean-square-error configuration during steady-state calibration. THE SIMULATED HYDROLOGIC SYSTEM 35 where n A712 — E (Hci—HmiVn 2, i=1 n —- S2 = E (AH,—AH)2/(n—1), i=1 AH = Hc_Hm ! H c = the computed head at a grid block where a well is located, Hm = the measured head at the corresponding well, and n = number of wells. MSE thus provides a method of measuring how well the computed heads agree with the measured heads. Large values of MSE indicate poor agreement, whereas small values indicate good agreement. Several values of ver- tical hydraulic conductivity (K) were tried in the model, and the resulting MSE was calculated, as shown in figure 15. For example, when it was assumed that there was no vertical connection between any of the aquifers, a large MSE was produced in the Denver and Arapahoe aquifers (run 1, fig. 15), indicating that this was not a good assumption for these aquifers. However, this may be a reasonable assumption for the Laramie—Fox Hills aquifer, as indicated by the small MSE produced in this layer. Subsequent simulation runs were used to investi- gate the effects of various combinations of vertical hydraulic conductivity until a uniformly small value of MSE was produced in all of the aquifers (run 5, fig. 15). The calibration results indicate that the vertical hydraulic conductivity through the Dawson, Denver, and Arapahoe aquifers is about 3X10‘5 ft/d. Lateral hydraulic conductivity of these aquifers ranges from about 0.05 to 7.0 ft/d, with 3.0 ft/d a common value. The ratio of lateral to vertical hydraulic conductivity is thus about 1X10“, indicating that these aquifers are about 100,000 times more permeable in the lateral direction than in the vertical direction. In terms of water move- ment, this means that water-level changes due to pump- ing primarily will spread laterally through the aquifer and will have much less effect on water levels in over- lying or underlying aquifers. The vertical hydraulic conductivity is accurate only to the degree that the lateral hydraulic conductivity was correctly estimated. In the central part to the deeper aquifers, no data are available to define the lateral hydraulic conductivity. In this area, the vertical hydraulic conductivity is also less accurately defined. The regional-scale steady-state recharge and discharge for the four principal bedrock aquifers as calculated by the steady-state model are shown in table 4. Local-scale recharge and discharge are not con- sidered in this table because their inclusion would not be representative of the effective recharge and discharge for the aquifers. This water budget represents long-term average flow conditions and does not consider temporal changes in hydraulic conditions, such as pumpage or annual variations in precipitation. It can be seen in table4 that the Dawson aquifer receives most of the recharge (40.6 fta/s) and also supplies most of the discharge (33.4 ftsls), principally to the drainage areas of Plum, Cherry, Kiowa, and Monument-Fountain Creeks. The difference between the recharge and discharge for the Dawson aquifer (7.2 fts/s) is the rate of water move- ment from the Dawson to the underlying Denver TABLE 4.—RegionaJ-scale steady-state water budget for the bedrock aquifers [Values in cubic feet. per second] Aquifer Laramie— Recharge or Discharge Terms Dawson Denver Arapahoe Fox Hills Total Precipitation Recharge 40.6 5.5 2.8 5.8 54.7 Discharge to principal drainage area of: Plum Creek 6.1 1.1 0.3 —— 7.5 Cherry Creek 10.3 .2 -— -— 10.5 South Platte River .3 2.2 2.4 0.5 5.4 Box Elder Creek 2.6 .2 .9 .1 3.3 Lost Creek -— 1 .6 . .9 Kiowa Creek 5.9 .2 .2 .7 7.0 Bijou Creek .6 2.1 2.1 2.1 6.9 San Arroyo—Badger Creek —— —— -— 1.1 1.1 Big Sandy Creek .2 .3 .5 .2 1.2 Rush—Steel Fork Creek —- —— .1 .5 .6 Black Squirrel Creek 0.4 0.7 .5 .2 1.8 Monument—Fountain Creek 7.0 .3 .5 .2 8.0 Total discharge 33.4 7.4 8.1 5.8 54.7 36 BEDROCK AQUIFERS IN DENVER BASIN, COLO. aquifer. Recharge to the Denver aquifer occurs from the Dawson aquifer and from infiltration of precipitation in the outcrop areas (5.5 ft3/s), with discharge principally occurring in the drainage areas of Bijou Creek and the South Platte River. Recharge to the Arapahoe aquifer occurs from the Denver aquifer (5.3 ft’ls) and from in- filtration of precipitation in the outcrop areas (2.8 fts/s). Discharge from the Arapahoe aquifer is primarily into the drainage areas of Bijou Creek and the South Platte River. The Laramie-Fox Hills aquifer does not receive significant recharge from the overlying Arapahoe aquifer; as a result, recharge and discharge occur in the outcrop areas, and each totals 5.8 fta/s. Principal discharge areas for the Laramie-Fox Hills aquifer are the drainages of Bijou and San Arroyo-Badger Creeks. The largest discharge from the four bedrock aquifers is into the drainage areas of (1) Cherry Creek, (2) the Monument-Fountain Creek combined area, (3) Plum Creek, (4) Kiowa Creek, (5) Bijou Creek, and (6) the South Platte River. The accuracy of the water budget is directly related to the accuracy of the hydraulic conductivity data used in the steady-state model. Independent determination of the water budget accuracy is difficult due to unfavor- able geologic and hydrologic conditions along most stream valleys. These conditions prevent direct measurement of bedrock aquifer discharge. However, discharge from the Dawson, Denver, and Arapahoe aquifers to Monument Creek can be measured; Livingston and others (1976) indicated that 7.5 ft3/s of discharge occurs in the reach underlain by these aquifers. This compares favorably with the 7.8 ftsls of discharge calculated by the model (table 4). Results of the steady-state simulations provide both hydrologic insight to the functioning of the aquifer system and information required in transient-state modeling. Successful calibration of the steady-state model indicates that the relationships between recharge, discharge, and water movement described in the previous sections of this report are essentially cor- rect. In addition, the water budget and vertical hydraulic-conductivity results provide a new and quan- titative definition of the hydrologic characteristics. These results must be available if the more complex phase of transient-state modeling is to be successful. THE TRANSIENT-STATE MODEL The transient-state model expands the simulation ability of the steady-state model by considering time- varying hydrologic conditions. Transmissivity, storage coefficient, and specific yield are no longer held in- variant but are allowed to change in response to changing head conditions in the model. Variable pum- page, induced recharge, and capture of discharge also are considered in the transient-state model. Pumpage variations are simulated as a stepped se- quence of pumping periods, three of which were used in the calibration of the transient-state model. The pump- ing periods are of 3 years’ duration (1959—61), 13 years’ duration (1962—74), and 4 years’ duration (1975—78). The model pumpage is held constant at the average pumping rate for each period. Induced recharge occurs when the head in an aquifer declines and allows additional water to enter the aquifer, as from an overlying alluvial aquifer, for exam- ple. The rate of induced recharge increases as larger head declines occur, ultimately reaching a constant rate due to the effects of anisotropic sediments in and under the alluvial aquifer. Capture of discharge occurs when the head in an aquifer declines and reduces the rate of discharge from the aquifer. A decreasing rate of flow from a bedrock spring is an example of captured discharge. The model program was modified to allow in- duced recharge or captured discharge to increase as a linear function of head decline until a maximum recharge rate of 0.133 ftS/s per square mile of alluvial- bedrock aquifer interface was reached. A constant rate of induced recharge was then simulated. Data are not available to document the maximum recharge rate; however, testing of various rates in the model indicated that reasonable head response could be achieved with a value of 0.133 ft3/s. A variable-interval grid was used in the transient- state model. Grid blocks ranged in size from 1.5X1.5 mi in the west-central part of the basin to as much as 12X 14 mi in the outlying eastern parts of the basin. The greater grid-block density in the Denver metropolitan area allowed greater resolution of head conditions in this critical area. Calibration of the transient-state model was per- formed by calculating the potentiometric heads in the aquifer during the period from 1958 to 1978. The model- calculated 1978 heads, shown on plate 2, were then com- pared to the measured 1978 heads, shown on plate 1, to judge the simulation ability of the model. The agree- ment between the two groups of maps generally is very good, with minor disagreement limited to local areas. This indicates that the transient-state model is a reasonable simulator of the response of the prototype. The principal reasons for disagreement between the two groups of maps are (1) inadequate information on the location and rate of pumping from bedrock wells, (2) the inability of the model to simulate small-scale anisotropy due to faulting near Boulder County, and (3) partial vertical stratification of heads in the Dawson and Denver aquifers. As discussed in the “Supplemental Information” sec- THE SIMULATED HYDROLOGIC SYSTEM 37 tion of this paper, adequate data are not available to make accurate estimates of historical pumpage. The model-calculated 1978 heads are thus affected by errors in the location or rate of pumping, and these errors may be responsible for local differences in head between the two map groups. If pumpage data of better accuracy had been available, closer agreement between com- puted and measured heads probably would have been obtained. Faulting in the Laramie—Fox Hills aquifer near Boulder County has markedly affected the local ground- water altitude and direction of movement. The extent of the area affected by faulting has not been determined, but model results indicate that it may extend through much of the area north of Westminster and west of Brighton (fig. 7). Anisotropic transmissivity was mod- eled in the Laramie—Fox Hills aquifer in this area in an attempt to simulate the large-scale hydrologic effects of the faults. Although an anisotropy ratio of Tx/Ty=25 was found to substantially improve the simulation re- sults, the anisotropic effects of the faults could not be fully simulated. This was primarily due to the coarse- ness of the model grid in relation to the spacing of the faults and the noncoincident alignment of the northeast- trending faults with the east-west—oriented model grid. Because of this limitation, the model will not provide valid simulations of head conditions in the part of the Laramie—Fox Hills aquifer strongly affected by faulting. When model runs indicated insufficient water-level decline in the water-table parts of the Dawson and Denver aquifers, the accuracy of the pumpage data was initially suspected. However, the modeled pumpage in a 60-mi2 area near Parker was found to be in reasonable agreement with a more accurate and detailed pumpage estimate made in this area. Further testing of the model indicated that proper water-level declines could be pro- duced when a confined storage coefficient was used in a larger area of these aquifers. The model thus indicated that although water-table conditions occur in the upper parts of these aquifers, at the depths of most wells the aquifers are confined and are not in direct hydraulic con- nection with the water table. Such partial vertical stratification of heads is normally simulated by use of two or more model layers for each aquifer. Time and computer-core limitations precluded this major revision of the model and required use of a simplifying assump- tion in order to allow the existing model to be used. It was assumed that confined conditions would occur in the central part of the Dawson aquifer until the water level declined to the upper perforations in most well cas- ings. This distance was estimated to be about 100 ft. Therefore, the computer program was modified such that as soon as water-level declines exceeded this value, unconfined conditions would occur, and specific yield replaced storage coefficient in the model node. A similar modification to the model code allowed the Denver aquifer to remain temporarily confined in part of the outcrop immediately beyond the Dawson aquifer. Head- dependent conversion to unconfined conditions still occurred in this area. Because this assumption only ap- proximates the more rigorous multilayer simulation, it may be responsible for some of the differences between computed and measured heads in the calibration of the Dawson and Denver aquifers. This revision allows the model to more correctly simulate the head conditions at the depth of most wells in the Dawson and Denver aquifers. It also requires that the 1978 potentiometric surface shown in Robson and Romero (1981a, 1981b) be revised to show the head in only the deeper parts of the aquifers. The revised 1978 potentiometric surface is shown on plate 1. It is important to note that although the model pro- duces a reasonable simulation of the response of the pro- totype, it may not be unique in this ability. If the temporal and spatial variations in such hydrologic characteristics as natural recharge and discharge, pumpage, lateral and vertical hydraulic conductivity, specific yield, and storage coefficient are well defined by field data, little question remains as to the correct values for the associated model parameters. If these characteristics are poorly defined by field data, the cor- rect values of the associated model parameters are less certain. In this case, the range of acceptable values for the model parameters might be such that more than one combination of values could produce an acceptable model calibration. In the case of the Denver basin model, lateral hydraulic conductivity and specific yield are the parameters best defined by field data. Vertical hydraulic conductivity and natural recharge and discharge were determined through steady-state model- ing and are compatible with, but probably less accurate than, the lateral hydraulic conductivity. Indirect tech- niques were used to arrive at estimates of storage coeffi- cient and pumpage, and a larger range of values is possible. Because of the uncertainties associated with the definition of all of these hydrologic characteristics, a combination of model parameters different from those used might also produce an acceptable model calibra- tion. The differences between the results of the two models probably would not be large due to constraints on the choice of parameters values. However, the ques- tion of uniqueness takes on new importance when model simulation time periods and pumping rates greatly exceed those used in calibration. Further discussion of modeling errors and limitations may be found in the “Supplemental Information” section of this paper. This information should be taken into account before using results of model simulations. 38 BEDROCK AQUIFERS IN DENVER BASIN , COLO. TABLE 5.—Transient—state 20year water budget for the bedrock aquifers [Values in acre-feet] 20-year steady— 20-year transient-state state period calibration period 1958-78 Net Net decrease in Net Precipitation Ground-water interaquifer ground-water Net . interaquifer Aquifer recharge discharge flow Pumpage storage recharge . flow Dawson 588,000 —484,000 —104,000 —30,000 7,000 120,000 —97,000 Denver 80,000 - 1 07,000 27,000 —66,000 38,000 96,000 —68,000 Arapahoe 41,000 —1 18,000 77,000 —219,000 31,000 23,000 165,000 Laramie—Fox Hills 84,000 —84,000 0 -59,000 14,000 45,000 0 Totals 793,000 —793,000 0 —374,000 90,000 284,000 0 The calibrated model is a valuable hydraulic tool, for it provides quantitative information about the hydrologic system. A water budget for the 20 year transient-state calibration period is of particular in- terest in this respect. A transient-state water budget is more complex than is a steady-state water budget and must be used with caution to avoid misinterpretation. The transient-state water budget for 1958—78 for each aquifer is shown in table 5. A corresponding 20-year steady-state water budget also is shown for purposes of comparison. ~ During a 20-year steady-state period, the aquifer system, as a whole, would have received 793,000 acre-ft of recharge and would have discharged an equal volume of water. During this 20-year period, 104,000 acre-ft of water would have moved from the Dawson aquifer into the Denver aquifer, and 77,000 acre-ft of water would have moved from the Denver aquifer into the Arapahoe aquifer. This produced a net inflow to the Denver aquifer of 27,000 acre-ft (104,000—77,000). During the 20-year transient-state calibration period, a total of 374,000 acre-ft of water was pumped from the bedrock aquifers. This pumpage, coupled with the ef- fects of pumping prior to 1958, produced a net im- balance between recharge and discharge (net recharge). (Note that recharge and discharge were equal under steady-state conditions.) The net recharge of 284,000 acreft is due to the cumulative effects of pumpage from 1883 to 1978 and occurs as the result of capture of natural discharge, induced recharge of additional water from surface sources, and additional recharge supplied by man’s activities (infiltration from Cherry Creek Reservoir, for example). The cumulative effect of pumpage prior to 1978 altered the interaquifer flow from the steady-state values. Flow from the Dawson aquifer to the Denver aquifer was reduced from 104,000 acre-ft to 97,000 acre- ft, and flow from the Denver aquifer to the Arapahoe aquifer was increased from 77,000 acre-ft to 165,000 acre-ft. This produced a net outflow from the Denver aquifer of 68,000 acre-ft (165,000—97,000) under transient conditions, as compared to a net inflow of 27,000 acre-ft under steady-state conditions. The net change in the volume of water stored in the aquifers is the difference between the pumpage, net recharge, and net interaquifer flow. The net decrease in the volume of ground water in storage totaled 90,000 acre-ft during the 20-year transient period. Because of model limitations, it is not known how much of the 374,000 acre-ft of pumpage came from a decrease in ground-water storage and how much came from net recharge and net interaquifer flow. MODEL SIMULATIONS These model simulations consider only geologic and hydrologic conditions and do not consider economic, legal, or social factors. In addition, the model simula- tions are not predictions of future conditions that will occur in the aquifers. Rather, they are predictions of conditions that could result from a specified rate, distribution, and duration of pumpage. The simulations will predict future conditions in the aquifer only if the modeled pumpage accurately represents future pump- age and the model accurately simulates the prototype. The accuracy of the following model simulations varies, depending on the particular conditions being simulated. Factors affecting this accuracy include (1) the quality of the model calibration, (2) the uniqueness of the model, (3) the duration of the simulation period compared to the duration of the calibration period, and (4) the rate of simulated pumping compared to the rate of calibration pumping. These factors, discussed in some detail in the “Supplemental Information” section of this paper, effects the accuracy of the calculated head distributions, drawdowns, and mass balances. Simula- tions involving short time periods and small rates of pumping are probably the most accurate. Long simula- tion periods and large rates of pumping, when combined MODEL SIMULATIONS 39 1 50 1 i i _i D Z 8 bu) 100 - E Historical Projected 0' pumpage pumpage *— Lu Lu LL 9 ca _ D U E ”I 0 g 50 — STDY pumpage estimate _ E D - LL 0 l i i | | | | | 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 FIGURE 16.—I-Iistorical and projected pumpage for the bedrock aquifers. with the uncertain uniqueness of the model, can produce large errors. The magnitude of these simulation errors is difficult to determine, for they are the result of a complex interaction of conditions that may produce par- tially offsetting errors. As a result, special caution must be taken in using the results of the long-term, heavy pumped simulations. The calibrated transient-state model is converted to simulation use by means of two principal revisions. The first involves changing from a 1958 to a 1978 starting head distribution, and the second involves replacing 1958—7 8 pumpage with 1979-2050 pumpage. These and other minor revisions allow the model to calculate heads in the aquifers over all or part of a 72-year simulation period (1979-2050). Three 1979—2050 pumpage estimates have been made for simulation use (fig. 16). The first is termed the “FULL” pumpage estimate and is calculated as a con- stant fraction of the projected water-supply require- ments of the Denver metropolitan area (see “Supple mental Information”). This estimate is intended to be a reasonable estimate of the maximum rate of future pumpage that might be expected to occur to the year 2050. A second, more conservative, estimate was calculated as one-half of the FULL pumpage estimate and is termed the “HALF” pumpage estimate. The third, still more conservative, estimate is based on the assumption that no increase in pumpage will occur after 1983. This steady rate of pumping is called the “STDY” pumpage estimate and is intended to be a reasonable estimate of the minimum rate of future pumping that might be expected to occur to the year 2050. Because it is not possible to foretell what the actual rate of future pumping will be, it is not known which of these three estimates will most closely represent future pumpage. As a result, the three pumpage estimates are used to show how the aquifer can be expected to respond to large, medium, and small rates of future pumping. Colorado State law limits the annual pumpage of a well to 1 percent of the volume of recoverable water in storage in the aquifer under the well owner's land. Because of this statute,ithe three pumpage estimates were adjusted to prevent the pumping rate from exceeding that allowed by the land area near the wells. This was done on the basis of the area of the grid block in which pumpage was estimated to occur. If pumpage for a particular grid block exceeded 1 percent of the volume of water in storage in the aquifer under that grid block, the pumpage for that aquifer was set to the l-percent rate and was not allowed to increase further. If little or no pumpage was shown in adjacent grid blocks, the pumping rate was sometimes allowed to exceed the l-percent limit by considering a larger land area underlying parts of adjacent grid blocks. Limiting 40 BEDROCK AQUIFERS IN DENVER BASIN, COLO. TABLE 6.—Distn'bution of FULL pumpage estimate for periods 1979—85 and 2046—50 [Values in cubic feet per second] Counties Aquifer Adams Arapahoe Boulder Denver Douglas Elbert Jefferson Weld Totals Distribution during 1979—85 Dawson ------------------ 0 1.01 0 0 6.59 0.60 0 0 8.20 Denver —————————————————— 1.89 2.54 O 0.31 1.53 .06 1.44 0 7.77 Arapahoe ---------------- 8.63 4.57 0.06 .91 4.83 0 3.54 0.73 23.27 Laramie-Fox Hills ————————— 1.52 .98 1.00 .40 1.47 0 .31 2.24 7.92 Totals ---------------- 12.04 9.10 1.06 1.62 14.42 .66 5.29 2.97 47.16 Distribution during W50 Dawson —————————————————— 0 5.66 0 0 23.62 0.60 0 0 29.88 Denver —————————————————— 1.89 8.60 0 .30 4.52 .06 .80 0 16.17 Arapahoe ———————————————— 14.82 13.34 .06 1.48 33.05 0 5.46 .73 68.94 Laramie-Fox Hills ————————— 4.83 6.66 1.39 1.05 15.91 0 .54 3.56 33.94 Totals ................ 21.54 34.26 1.45 2.83 77.10 .66 6.80 4.29 148.93 the maximum pumping rate from grid blocks had the most pronounced effect on the FULL pumpage estimate. This caused the decrease in slope of the FULL pumpage curve shown in figure 16 and only slightly af- fected the slope of the upper end of the HALF pumpage curve. The varying rates of pumpage shown in figure 16 were modeled by use of eight pumping periods. The first period was 7 years (1979—85); followed by six periods, each of 10 years (1986—2045); and ended with a period of 5 years (2046—50). The distribution of pumpage during the first and last pumping period is shown in table 6. This distribution is based primarily on the expected location of future ground-water use as estimated in 1974 by the Denver Water Department (1975). These estimates were updated to consider the more recent pat- terns of use as indicated by permits issued for construc- tion of new bedrock wells during the period 1978-82. As shown in table 6, the largest increase in pumpage is ex- pected in Douglas, Adams, and Arapahoe Counties. The Arapahoe aquifer is expected to be the source of most of the increased pumpage; the Laramie—Fox Hills and Dawson aquifers supply lesser amounts of water. The pumpage estimates do not consider future pumpage in the outlying and primarily rural parts of Elbert, El Paso, and Morgan Counties. BASE CONDITIONS The first series of model simulations was made to estimate the aquifer response to the three pumpage estimates. These simulations also serve as the bases for comparison to subsequent model simulations, which will include specific changes in pumpage or recharge. Potentiometric-surface maps, areas of water-level decline, and water-level hydrographs are shown for each simulation to document the hydrologic changes. Model-run “STDY-BASE” simulates the hydrologic response of the aquifers to the STDY pumpage estimate. The resulting water-level conditions are shown on plate 3. Results indicate that under the most conservative pumping conditions (STDY), 1979—2050 water-level declines in excess of 100 ft could occur in the Dawson aquifer north of Castle Rock and in a 120-rni2 area of the Denver aquifer between Aurora and Franktown. In the Arapahoe aquifer, declines exceed 250 ft in a 300-mi2 area of southern Arapahoe County and northcentral Douglas County. The most extensive declines occur in the Laramie—Fox Hills aquifer, where declines exceed 300 ft in a 440-mi2 area extending from Commerce City to Castle Rock. The larger water-level declines in the deeper aquifers are due to the hydrologic characteristics of the aquifers and to an increased rate of pumpage from the deeper aquifers. In the Laramie—Fox Hills aquifer near Parker, for example, the aquifer presently is confined and under about 1,700 ft of head. The relatively small transmissiv- ity and small storage coefficient will allow 1,700 ft of rapid water-level decline to occur in this area before un- confined conditions develop. By contrast, in the Dawson aquifer, unconfined conditions already occur, and much smaller rates of water-level decline are pro- duced by pumpage. MODEL SIMULATIONS 41 A water budget for the final year of the model- calibration period (1978) and the final year of run STDY-BASE (2050) is shown in table 7. The small increase in pumpage between 1978 and 2050 produces correspondingly small adjustments in the other compo- nents of the water budget. The net recharge has in- creased from 23.18 ft3/s in 1978 to 27.44 ft3/s in 2050, and the net decrease in ground-water storage has gone from 13.83 ft3/s to 17.80 ftsls. These changes in the water budget are affected by pumpage prior to 197 8 but are primarily due to post-1978 pumpage. Thus, a 1978—2050 pumpage increase of 8.23 ft3/s primarily causes a 4.26-ft3/s increase in net recharge and a 3.97-ft3/s decrease in the rate of ground water pumped from storage between 1978 and 2050. About 52 percent of the increased pumpage is derived from an increase in net recharge, and 48 percent is derived from ground- water storage. Model-rlm HALF-BASE simulates the hydrologic response of the aquifers to the HALF pumpage estimate. The results of this simulation are shown on plate 4. In the Dawson aquifer, 1979 to 2050 water-level declines of more than 200 ft occur to the north of Castle Rock and near Cherry Creek Reservoir. Water-level declines of 200—300 ft are shown to occur in the Denver aquifer in a 110-rni2 area between Cherry Creek Reser- voir and Franktown. Water-level declines of as much as 800 ft are present in the Arapahoe aquifer; declines in excess of 600 ft occur in a 160-miz area around Parker. In the Laramie—Fox Hills aquifer, water-level declines of 1,500—1,700 ft are present in a 130-mi2 area west of Parker. A comparison of the STDY-BASE and HALF-BASE simulations indicates that the larger pumpage in the HALF-BASE run creates much larger water-level declines in the aquifers. The largest declines occur in the deepest aquifers and cause a significant change in the direction of ground-water movement in the Laramie—Fox Hill aquifer. In the STDY-BASE run, a single cone of depression is present in the Laramie—Fox Hills potentiometric surface near Fort Lupton. In the HALF-BASE run, this depression is deepened, and a second cone of depression is formed in northern Douglas County. As a result of these two cones of depression, most of the water present in the northern half of the Laramie—Fox Hills aquifer moves toward the cones of depression and no longer discharges near the northern limit of the aquifer. The 2050 water budget for run HALF-BASE is shown in table 7. The increase in 2050 pumpage from 45 ftsls in run STDY-BASE to 100 fta/s in run HALF- BASE causes proportionally larger volumes of water to be removed from storage. This is caused by physical limitations on the volume of induced recharge and cap- tured discharge that may occur. When net recharge is TABLE 7.—Tmnsient-state water budgets for 1978 (calibration run) and 2050 (STDY-BASE run, HALF-BASE run, and FULL-BASE run) [Values in cubic feet per second] Net decrease in Net Aquifer Pumpage ground-water Net interaquifer storage recharge flow 1978 Water budget from calibration run Dawson -5.94 0.95 11.95 —6.96 Denver -6.83 3.69 6.21 —3.07 Arapahoe —18.89 7.10 1.74 10.03 Laramie— Fox Hills —5.37 2.09 3.28 0 Totals -37.01 13.83 23.18 0 2050 water budget from run STDY-BASE Dawson —7.65 1.82 14.00 --8.17 Denver —6.55 3.65 6.84 —3.94 Arapahoe -23.22 9.15 1.96 12.1 1 Laramie— Fox Hills —7.82 3.18 4.64 0 Totals —45.24 17.80 27.44 0 2050 water budget from run HALF-BASE Dawson -19.14 10.66 17.66 -9.18 Denver —11.21 10.38 7.43 —6.60 Arapahoe —46.92 28.99 2.15 15.78 Laramie— Fox Hills —23.32 17.26 6.06 0 Totals -100.59 67.29 33.30 0 2050 water budget from run FULL-BASE Dawson —29.19 20.25 18.64 -9.70 Denver —15.59 15.44 7.90 —7.75 Arapahoe -68.88 49.14 2.29 17.45 Laramie— Fox Hills -33.81 27.23 6.58 Totals —147.47 112.06 35.41 0 limited in this manner, an increase in pumpage must be balanced by increased removal of ground water from storage. Interaquifer flow also can be seen to increase more slowly than pumpage. The changes in the water budget from 1978 to 2050 are affected by pumping prior to 1978, but primarily are due to post-1978 pumpage. The increase in pumpage from 1978 to 2050 is 63.58 ftals. This causes a corre- sponding increase in net recharge of 10.12 ft3/s and a 53.46-ft3/s increase in the rate of ground water pumped from storage. Thus, 16 percent of the increased pump- age is derived from an increase in net recharge, and 84 percent is derived from ground-water storage. 42 BEDROCK AQUIFERS IN DENVER BASIN, COLO. Model-run FULL-BASE simulates the hydrologic response of the aquifers to the FULL pumpage estimate. The resulting water-level conditions are shown on plate 5. Results indicate that under the max- imum pumpage condition (FULL), 1979—2050 water- level declines of 200—3 10 ft occur in the Dawson aquifer near Parker, southeast of Cherry Creek Reservoir, and north of Castle Rock. Water-level declines of as much as 420 ft occur in the Denver aquifer, and declines exceed 200 ft in a 280-mi2 area extending from near Aurora to near Larkspur. In the Arapahoe aquifer, a maximum 1979—2050 water-level decline of 1,000 ft occurs near Parker. Declines in excess of 600 ft are present in a 370-mi2 area extending from near Cherry Creek Reser- voir through most of northern Douglas County. Under FULL pumpage, the largest water-level declines appear in the Laramie—Fox Hills aquifer. Declines ranging from 1,500 to 1,830 ft are present in a 220-mi2 area ex- tending from Cherry Creek Reservoir to near Dawson Butte southwest of Castle Rock. In the later years, the rate of decline diminishes dramatically in some areas (note the hydrograph for Parker, for example), due to the development of unconfined conditions in the aquifer. The 2050 water budget for run FULL-BASE is shown in table 7. Pumpage is shown to increase from 37.01 ft3/s in 1978 to 147.47 fta/s in 2050 (run FULL-BASE), a net increase of 110.46 fta/s. This increase in pumpage is primarily responsible for a 1978—2050 increase in net recharge of 12.23 ft3/s and a decrease in ground water in storage of 98.23 ft3/s. Under FULL pumpage condi- tions, 11 percent of the increased pumpage is derived from an increase in net recharge, and 89 percent is derived from ground-water storage. The simulations indicate that future rates of water- level decline could vary considerably, depending on the rate of future pumping. The hydrographs on plates 3, 4, and 5 show that rates of water-level decline range from near zero in many areas to over 40 .ft/yr in the Laramie—Fox Hill aquifer near Parker, for example. This large rate of decline has not yet been experienced in the basin aquifers on a broad-scale, continuing basis. By comparison, rates of water-level decline, shown in figure 10, averaged about 11 ft/yr between 1885 and 1910, and averaged about 3 ft/yr between 1910 and 1960. If future rates of water-level decline exceed several tens of feet per year on a continuing basis, economic, social, and political constraints will limit further development of some ground-water resources. Factors affecting the economic constraint include (1) in- creasing costs for energy needed to lift water greater distances to the surface, (2) increasing well construction costs as additional wells are needed to offset the decreasing yield of existing wells, and (3) increasing costs for well modifications needed to install more powerful pumps at greater depths in the wells. GROUND-WATER DEVELOPMENT PLANS In 1981, a committee called the Metropolitan Water Roundtable was formed, with Colorado Governor Richard D. Lamm as chairman, to evaluate the future water-supply needs of the Denver metropolitan area. A ground-water task group of the Roundtable submitted its final report to the committee in May 1982. In this report, four ground-water use scenarios were outlined as possible methods of supplementing the water require- ments of the metropolitan area. These scenarios in- volved (1) pumping a satellite well field located in an undeveloped area of the basin and piping the water to the metropolitan area; (2) pumping a well field located within the metropolitan area; (3) using bedrock wells to provide irrigation water for city parks and other public land presently irrigated with treated municipal water; and (4) using the bedrock aquifers for temporary storage of treated municipal water by recharging and, later, pumping the aquifers. These scenarios are used as the bases for the following sequence of model simulations. In some instances, it was necessary to estimate specific details of the plans because information provided by the task group was somewhat generalized. SATELLITE WELL FIELD The model was used to investigate the hydrologic response of the aquifers to pumpage from a hypothetical satellite well field located on 36 mi” of land in T. 6 S., R. 65 W. in eastern Douglas and western Elbert Counties. The location of the well field and pumping rates were provided by Bishop Associates, Inc.,1 a subsidiary of Engineering Science, Inc. They are the prime contractors to the US. Army Corps of Engineers and are involved in developing a system-wide environmental impact statement for the Denver Water Department. An economic analysis of the satellite well field was made as part of the system-wide environmen- tal impact statement. This phase of the work was done by Engineering Science, Inc., in cooperation with the Denver Water Department, the US. Army Corps of Engineers, and the US. Geological Survey. 1Use of firm names in this report is for identification purposes only and does not con- stitute endorsement by the US. Geological Survey. MODEL SIMULATIONS 43 TABLE 8.—Satellite well-field pumpage Pumpage (cubic feet Aquifer per second) Dawson 1 1.1 Denver 10.2 Arapahoe 12.5 Laramie-Fox Hills —————————————————————————— 7.2 Total 41.0 It was assumed that the satellite well field would be pumped at the maximum rate allowed under Colorado law and that the pumping would commence at the start of the 72-year simulation period (table 8). The model simulated the effects of this pumpage in addition to that indicated by the STDY and FULL pumpage estimates. The head distributions calculated in these simulations were then subtracted from those calculated in the STDY-BASE and FULL-BASE simulations to produce maps showing water-level declines due only to satellite well field pumpage. These incremental water- level declines are shown in figures 17—24 for the STDY and FULL pumpage conditions. Total 1979—2050 water-level declines due to the satellite well field may be obtained by adding the incremental declines to the cor- responding declines shown on plates 3 and 5 for the appropriate base simulation. The incremental declines are markedly differentyfor the upper and lower aquifers and also are markedly dif- ferent for the STDY and FULL pumpage conditions. The larger water-level declines in the deeper aquifers are mainly due to the confined conditions present in these aquifers, as opposed to the unconfined conditions pres- ent in the upper aquifer. In the unconfined upper aquifer, gradual rates of water-level decline occur because of the large specific yield of this aquifer. In a confined aquifer, much more rapid rates of water-level decline may occur because of its smaller storage coeffi- cient. The rapid rate of decline can lead to large water- level declines over a period of time if the starting head in the aquifer is well above the top of the aquifer. This situation is most pronounced in the deeper aquifers, leading to their larger incremental water-level decline (figs. 17 and 20). The difference in incremental water-level decline be- tween the STDY and FULL simulations (figs. 20 and 24) is illustrated by the two hydrographs for the aquifer near Parker shown in figure 25. The lines labeled STDY-BASE and FULL-BASE are the water levels calculated from the base simulations for the STDY and FULL pumpage conditions. The lines labeled STDY- SWF and FULL-SWF are the water levels calculated from the satellite well-field simulations for the STDY and FULL pumpage conditions. Quantities A and B are the incremental water-level declines shown for Parker (figs. 20 and 24). The difference between the incremental declines in figures 20 and 24 is the result of the amount of confined head that is pumped off the aquifer under the base conditions. Under FULL-BASE conditions, rapid water-level declines occur until unconfined condi- tions begin to develop near Parker, at which point the hydrograph (fig. 25) flattens markedly. More rapid declines initially occur under FULL-SWF conditions; however, this hydrograph also flattens in response to unconfined conditions, and a small incremental decline (B) results. This does not happen under STDY condi- tions. Neither the STDY-BASE or the STDY-SWF hydrographs are affected by unconfined conditions, and the two hydrographs become parallel with about 900 ft of incremental water-level decline (A) between them. In the Dawson aquifer, incremental water-level declines due to the satellite well field of as much as 500 and 400 ft occur under STDY and FULL pumpage con- ditions (figs. 17 and 21, respectively). In the Denver aquifer, incremental declines exceed 600 and 500 ft under STDY and FULL pumpage conditions (figs. 18 and 22, respectively). In the Arapahoe aquifer, in- cremental declines in excess of 100 ft take place in a 1,040-mi2 area under STDY pumpage conditions (fig. 19). This is about 11 times'the area of similar declines shown to occur in the Arapahoe aquifer under FULL pumpage conditions (fig. 23). In the Laramie—Fox Hills aquifer, incremental water-level declines exceed 1,300 ft under STDY conditions (fig. 20) and exceed 600 ft under FULL conditions (fig. 24). In less analytical terms, the satellite well-field simula- tions show that the amount of water-level decline produced by the well field will vary, depending on the - structural depth of the pumped aquifer and the amount of other pumpage that is occurring in the basin. An equivalent amount of pumpage from a deep aquifer and a shallow aquifer will produce deeper and more widespread water-level declines in the deep aquifer than in the shallow aquifer. The incremental water-level declines caused by the satellite well field will be rela- tively large if little other pumpage is occurring in the aquifer and will be smaller and less widespread if a large rate of other pumping is taking place in the aquifer. The large, widespread, incremental declines shown to occur in some cases would cause existing shallow wells in some aquifers to go dry and would reduce the yield of other, deeper wells in affected areas. In either case, it would be necessary to redrill or re-equip wells in order to maintain the previous pumping rate. BEDROCK AQUIFERS IN DENVER BASIN , COLO. 105°00' 104°30' EXPLANAT'ON u 5 10 15 MILES AREA OF SATELLITE WELL FIELD I] 5 II] 15 KILOMETERS '— 100— LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 500 FEET WATER- LEVEL DECLINE, 1979- NATIONAL GEUDETIC VERTICAL DATUM OF 1929 2050—Interval. in feet. is variable AQUIFER LIMIT FIGURE l7.—Increment.al water-level declines for a satellite well field in the Dawson aqlfifer, using STDY pumpage estimate. MODEL SIMULATIONS Il]5°0(]' 104°30' 40° 00' 39° V 30' EXPLANAT'O" u 5 II] 15 MILES . AREA OF SATELLITE WELL FIELD I] 5 III 15 KILUMETEHS — 100— LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 500 FEET WATER-LEVEL DECLINE, 1979- NATIONAL GEUDETIC VERTICAL DATUM 0F1929 2050—Interval. in feet. is variable AQUIFER LIMIT FIGURE 18.—Incremental water-level declines for a satellite well field in the Denver aquifer, using STDY pumpage estimate. 45 BEDRQCK AQUIFERS IN DENVER BASIN, OOLO. II]5°UI]' 104°30' EXPLANATION [l 5 II] 15 MILES - AREA OF SATELLITE 1 WELL FIELD U 5 IO 15 KILUMETERS -— 700— LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 50!] FEET WATER-LEVEL DECLINE, 1979- NATIONAL GEUDETIC VERTICAL DATUM UF I929 2050—Interval, in feel, is variable AQUIFER LIMIT FIGURE l9.—-Incrementa1 water-level declines for a satellite well field in the Arapahoe aquifer, using STDY pumpage estimate. MODEL SIMULATIONS 105° 00' 104°3II' 40° I my 39° 30' EXPLANATION I] 5 10 15 MILES - AREA OF SATELLITE WELL FIELD I] 5 II] 15 KILUMETEHS —— 700 — LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL SUI] FEET WATER- LEVEL DECLINE, 1979- NATIONAL GEODETIC VERTICAL DATUM DF 1929 2050—Interval, in feet Is variable AQUIFER LIMIT FIGURE 20,—Incremental water-level declines for a satellite well field in the Laramie—Fox Hills aquifer. using STDY pumpage estimate. BEDROCK AQUIFERS IN DENVER BASIN, COLO. 105°00' 104°3ll' 2? ~ Mi ; W J NW” 4* 4”? ..”j 68 «£7- " .21-‘ -__.__.. .63" ¢__g_a I .~ . , g : 55. 0|] yette I n hton 65 fworse Creek? a I V/ \‘v esservm'r 3 W l l I r-f' / 96 fix ,) V / I‘ E quet‘,‘ ;' I , ‘ I ”alw‘ism'asbur I‘, /, I V520: La 6 PR5, ,, ‘ ! EXPLANATION 0 5 II] 15 MILES AREA OF SATELLITE WELL FIELD V I] 5 II] 15 KILOMETERS — 100— LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 500 FEET WATER- LEVEL DECLINE, 1979- NATIONAL GEODETIC VERTICAL DATUM OF 1929 2050—lnterval, in feet. is variable 'AQUIFER LIMIT FIGURE 21.——Incremental water-level declines for a satellite well field in the Dawson aquifer, using FULL pumpage estimate. MODEL SIMULATIONS 105°00' 104°30' 40° 00' 39° 30' 0116-5 EXPLANAT'ON «u 5 ID 15 MILES AREA OF SATELLITE I WELL FIELD , I] 5 10 I5 KILOMETERS — 700—- LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 50!] FEET WATER-LEVEL DECLINE, 1979- NATIONAL GEDDETIC VERTICAL DATUM OF 1929 2050—Interval, in feet, is variable AQUIFER LIMIT FIGURE 22.—Incremental waterlevel declines for a satellite well field in the Denver aquifer, using FULL pumpage estimate. 49 50 BEDROCK AQUIFERS IN DENVER BASIN . COLO. 105°IJU' 104° 30' J A, 8;me . I j . NI fl _ .2- * _,.¢._62 _..__L63' 7 a at rl hton —65 9/ \< L[arse Creek W x « I ?aservoir I -' g 1 (5;; D 59 fieaaha EXPLANATION 0 5 10 15 MILES AREA OF SATELLITE WELL FIELD _ ll 5 II] 15 KILOMETEHS CONTOUR INTERVAL 500 FEET NATIONAL GEUDETIC VERTICAL DATUM OF 1929 — 700— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE, 1979- 2050—lnterval, in feet, is variable AQUIFER LIMIT FIGURE 23.—Incremental water-level declines for a satellite well field in the Arapahoe aquifer, using FULL pumpage estimate. MODEL SIMULATIONS 105°llll' 104° 30' EXPLANATION 0 5 10 15 MII£S - AREA OF SATELLITE l — J WELL FIELD l] 5 10 15 KILDMETERS — 100— LINE OF EQUAL INCREMENTAL CONTOUR INTERVAL 500 FEET WATER- LEVEL DECLINE, 1979- NATIONAL GEDDETIC VERTICAL DATUM 0F1929 2050—lnterval, in feet. is variable AQUIFER LIMIT FIGURE 24.—Incrementa] water-level declines for a satellite well field in the Laramie-Fox Hills aquifer. using FULL pumpage estimate. 51 52 BEDROCK AQUIFERS IN DENVER BASIN. COLO. 5600 v . . , . i I I r . _ _ STDY- BASE _ _ — ‘~~..__- Simulation 5200 — ___________ _ - ‘ """"""" r ‘ ' "_/ FULL- BASE ' d _ Simulation > — \ LL] -‘ > _ _ _ 5 3 4800 — a W B- Incremental T > water-level ‘ g ‘ ' decline shown < r44 on figure 24 fl |_ _ _ _ LU LU u. _ i - _ E u; 4400 — — ~ — o D , - _ E STDY-SWF —1 _ Simulation , _ _ < J ' ’ FULL-SWF _ . | . _ 4000 _ A - Incremental _ _ Simu ation _ water-level _ , ~ decline shown on figure 20 _ _ _ i , """""" >3 < I l l l I 1 I I l l 3600 o o E 8 8 too .9 8 8 g E 8 8 8 .°_‘ 8 N N YEARS FIGURE 25.—Water-level hydrographs for the Laramie—Fox Hills aquifer at Parker. METROPOLITAN WELL FIELD This series of simulations shows the effects of pump- ing a hypothetical well field located on 36 mi2 of land in the western half of T. 4 S., R. 67 W., and the eastern half of T. 4 S., R. 68 W. The well field would be primarily located in the southeastern part of the City and County of Denver. It is assumed that the field would be pumped at the maximum rate allowed under Colorado statutes without regard for the fact that the land is owned by numerous individuals and organizations. This assump- tion presently applies to the metropolitan communities of Federal Heights and Northglenn, which have been permitted to pump water under private property after obtaining the consent of property owners who are sup- plied with water by the city water departments. The assumption could also apply to the metropolitan well field if some similar action were taken in this area. Pumping is assumed to commence at the start of the 7 2-year simulation period (table 9). Only the three deeper aquifers are involved because of the absence of the Dawson Arkose in this area. Although both the satellite well field and the metropolitan well field are of equal area, the difference in aquifer characteristics allows only about one-half as much water to be pumped from the metropolitan well field under Colorado law. The model was run to simulate the effects of the metropolitan well-field pumping in conjunction with the STDY and FULL pumpage estimates. The head distributions calculated in these two simulations were subtracted from those calculated in the STDY-BASE and FULL-BASE simulations to produce maps of water-level decline resulting from only metropolitan well-field pumping. These incremental water- level—decline maps are shown in figures 26—31. Total 197 9—2050 water-level declines for the metropolitan well field may be calculated by adding the incremental TABLE 9.—-Metropolitan well-field pumpage Pumpage {cubic feet Aquifer per second) Dawson 0.0 Denver 5.78 Arapahoe 7.63 Laramie-Fox Hills —————————————————————————— 8.58 Total 21.99 MODEL SIMULATIONS 1050 my 104°30' ‘3 0 5 10 15 MILES 0 5 10 15 KILUMEI’ERS CONTOUR INTERVAL SUI] FEET NATIONAL GEODETIC VERTICAL DATUM DF 1929 EXPLANATION AREA OF METROPOLITAN WELL FIELD ——700—— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE, 1979- 2050—lnterval, in feet. is variable AQUIFER LIMIT FIGURE 26.—Incremental waterlevel declines for a metropolitan well field in the Denver aquifer, using STDY pumpage estimate. 54 BEDROCK AQUIF‘ERS IN DENVER BASIN . COLO. 105° 00' "WW 4; I] 5 10 15 MILES I] 5 IO 15 KILOMETERS CONTOUR INTERVAL 500 FEET NATIONAL GEOOETIC VERTICAL DATUM OF 1929 EXPLANATION AREA OF METROPOLITAN WELL FIELD —100— LINE OF EQUAL INCREMENTAL WATER- LEVEL DECLINE, 1979- 2050—Interval, in feet. is variable AOUIFER LIMIT FIGURE 27 .—Inc1emental water-level declines for a metropolitan well field in the Arapahoe aquifer, using STDY pumpage estimate. MODEL SIMULATIONS 105°on' 104°” 0 o 5 1o 15 MILES 0 5 ‘ 10 15 KILIJMEI’ERS CONTOUR INTERVAL 500 FEET NATIONAL GEUDETIC VERTICAL DATUM OF 1929 EXPLANATION AREA OF METROPOLITAN WELL FIELD —700—— LINE OF EQUAL INCREMENTAL WATER- LEVEL DECLINE, 1979- 2050—lnterval, in feet, is variable AQUIFER LIMIT FIGURE 28.—Incremental water-level declines for a metropolitan well field in the Laramie—Fox Hills aquifer. using STDY pumpage estimate. 55 56 BEDROCK AQUIFERS IN DENVER BASIN. COLO. 105° 00' 104°3U' ‘27 c/M fig v- /» . 0 5 11] 15 MILES fl 5 10 15 KILUMETEHS CONTOUR INTERVAL 500 FEET NATIONAL GEDDETIC VERTICAL DATUM OF 1929 EXPLANATION AREA OF METROPOLITAN WELL FIELD —700-—- LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE, 1979- 2050—Interval, in feet. is variable AOUIFER LIMIT FIGURE 29.——Incremental water-level declines for a metropolitan well field in the Denver aquifer, using FULL pumpage estimate. MODEL SIMULATIONS 105° my 104°30' Q I] 5 10 15 MILES 0 5 1|] 15 KILOMETEHS CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 EXPLANATION AREA OF METROPOLITAN WELL FIELD —-—100--— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE, 1979- 2050—lnterval. in feet, is variable AOUIFER LIMIT FIGURE 30.—Incremental water-level declines for a metropolitan well field in the Arapahoe aquifer, using FULL pumpage estimate. 57 58 BEDROCK AQUIFERS IN DENVER BASIN. COLO. 105° 00' I [14°30' e 0 5 10 15 MILES l] 5 II) 15 KILUMETERS CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUMDF I929 EXPLANATION AREA OF, METROPOLITAN WELL FIELD —700—-— LINE OF EQUAL INCREMENTAL WATER~LEVEL DECLINE, 1979- 2050—lnterval, in feet. is variable AOUIFER LIMIT FIGURE 31.—Incremental water-level declines for a metropolitan well field in the Laramie-Fox Hills aquifer. using FULL pumpage estimate. MODEL SIMULATIONS 59 declines to the corresponding declines shown on plates 3 and 5 for the appropriate base simulation. As in the case of the satellite well field, the magnitude and areal distribution of incremental declines vary markedly with aquifer depth and pumpage estimate used. When the STDY pumpage estimate is used, incremen- tal water-level declines in the three aquifers range from as much as 500 ft in the Denver aquifer to as much as 1,350 ft in the Laramie—Fox Hills aquifer. Under FULL pumpage conditions, these declines range from as much as 430 ft in the Denver aquifer to as much as 390 ft in the Laramie—Fox Hills aquifer. In the Denver aquifer, incremental water-level declines in excess of 100 ft occur in a 90-mi2 area under STDY pumping conditions (fig. 26) and in a 60-mi2 area under FULL pumping conditions (fig. 29). In the Arapahoe aquifer, these 100-ft declines occur in a 260-mi2 area under STDY conditions (fig. 27) and in a 50-mi2 area under FULL conditions (fig. 30). The most extensive declines, which occur in the Laramie—Fox Hills aquifer under STDY pumping conditions, are in excess of 200 ft in a 1,120-mi2 area (fig. 28). By contrast, these declines extend over only a 50-mi2 area under the FULL pumpage estimated (fig. 31). Results of this simulation indicate that large changes in water level can take place as a result of pumping a metropolitan well field. Larger and more areally exten- sive water-level declines are produced in the deeper aquifers. However, the magnitude of water-level decline produced by the well field can vary considerably, de- pending on how much other pumpage is occurring in the bedrock aquifers. The largest incremental declines are produced when small (STDY) rates of other pumping are present; conversely, there are small incremental declines when large (FULL) rates of other pumping are considered. A comparison of the results from the satellite well- field simulations (figs. 17—24) with those from the metropolitan well-field simulations (figs. 26—31) in- dicates that the aquifer characteristics and the smaller rate of pumping from the metropolitan well field com- TABLE 10.—Park and golf course in'igation pumpage Pumpage from Arapahoe aquifer Public land (cubic feet. per second) Bible Park 0.21 Cheesman Park, City Park and Golf Course ——————— .66 Ruby Hill Park .27 Willis Case Golf Course —————————————————————— .36 Total 1.50 bine to produce less incremental water-level decline in the metropolitan well field. Pumping from the metropol- itan well field also has less extensive effects in each of the pumped aquifers. In addition, the metropolitan well field would not tap the Dawson aquifer and, thus, would have mimimal effect on water levels in this aquifer. The comparatively small quantity of water that could be pumped from the metropolitan well field is a hydrologic constraint on this plan. PUMPAGE FOR PARK AND GOLF COURSE IRRIGATION The Denver Water Department supplies treated municipal water for irrigation of all or part of several parks and golf courses in Denver. If part of this public land could be irrigated with ground water, additional treated water would be available for higher priority use. This simulation is intended to show the effects of pump- ing the Arapahoe aquifer to supply water for irrigation of Bible Park, Cheesman Park, City Park and Golf Course, Ruby Hill Park, and Willis Case Golf Course. The 1.50-ft3/s total pumpage (table 10) was modeled to take place at a constant annual rate, and the effects of seasonal variation in rate are not considered. As a result, the model shows the average annual water-level altitudes in the aquifer, but it does not show maximum or minimum seasonal altitudes, which could be signif- icantly different from the average altitudes at locations near the pumping wells. At greater distances from the pumping wells, the water levels produced by the con- stant pumping will more closely approximate those pro- duced by a seasonal variation in pumpage. A 27-year (1979—2005) simulation period was used with the HALF pumpage estimate. As in previous simulations, incremental water-level declines were calculated to show only the effects of the pumping for park and golf course irrigation. The 1979—2005 incremental water-level declines in the Arapahoe aquifer produced by irrigation pumpage under HALF pumpage conditions are shown in figure 32. The hydrograph shows the average water- level altitude near City Park between 1978 and 2005 with park pumpage (HALF-PARK) and without park pumpage (HALF-BASE). Results indicate that 30—40 ft of average water-level decline could occur in the 2.25-mi2-area grid blocks near the parks and golf courses. Declines in excess of 5 ft primarily extend to the southeast from the pumping sites, over a 380-mi2 area. Maximum water-level declines in the Denver aquifer usually do not exceed a few feet because of the poor vertical connection between the Denver and Arapahoe aquifers. 60 BEDROCK AQUIFERS IN DENVER BASIN, COLO. 105°00' 104°30' HALF- BASE Simulation 00' HALF- PARK Simulation _I Lu > Lu .1 < Lu U) u.I > O m < '— Lu I.I.l u. E u? D D l: .— _I < EXPLANATION ll 5 1t] 15 MILES —5— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE, 1979- 0 5 ‘0 '5 K'LOMETERS 2005—lnterval 5 and 10 feet CONTOUR INTERVAL 500 FEET LIMIT OF ARAPAHOE AQUIFER NATIONAL GEODETIC VERTICAL DATUM OF 1929 FIGURE 32.—Incremental water-level decline in the Arapahoe aquifer due to park and golf course pump- age, using HALF pumpage estimate. \ Results of this simulation indicate that relatively Arapahoe aquifer to supply irrigation water to selected small incremental water-level declines will be produced public land. This simulation considers pumping from in the metropolitan area as a result of pumping the only the Arapahoe aquifer. If two or three of the MODEL SIMULATIONS 61 aquifers were used to jointly supply the water, the resulting incremental declines in each aquifer could be less than that shown in figure 32. Further work is needed to evaluate engineering and economic consid- erations before the feasibility of this water supply would be known. BEDROCK STORAGE OF MUNICIPAL WATER The municipal water-distribution system for Denver, like all well-designed systems, is built with adequate capacity to meet the peak water demands of the customers. Because of seasonal variations in demand, the system capacity is strained during summer months when lawn irrigation is required and demands are large; conversely, it has excess capacity during winter months when most vegetation is dormant and water demands are small. Increased water demands due to an extension of the service area or increased per capita consumption can require large capital outlays for expansion of water- collection, treatment, and distribution facilities. If the bedrock aquifers in the metropolitan area could be used as storage reservoirs for treated municipal water, the need for costly system expansion might be delayed or, in some cases, eliminated. Such reservoirs might be operated by injecting treated municipal water into the aquifers through wells during winter months when excess treatment and distribution capacity is available in the municipal system. The injected water then would be available for later withdrawl to help meet summer peak demands on the municipal system. It is likely that little or no additional treatment of the pumped water would be required prior to municipal use. The bedrock reservoir could be operated on either an annual or multiyear cycle. If the volume of water in- jected during one winter were balanced by an equal amount of pumping the following summer, an annual cycle would result with no net change in the volume of ground water in storage. This plan of operation prob- ably would not require the availability of additional municipal water supplies; it simply allows for more effi- cient use of existing water supplies, treatment, and, distribution facilities by storing water in the aquifers rather than in surface reservoirs. The impacts of this plan on the aquifers primarily consist of large seasonal variations in water level near the injection-pumping wells. If the inj ection-pumping well field were located in or near Denver (an area of relatively few wells), the seasonal fluctuation in water level would not signif- icantly affect large numbers of wells. Additional study is needed to evaluate economic considerations and water-chemistry compatibility problems between the native and injected water before feasibility of injection is known. If the reservoir were operated on a multiyear cycle, treated municipal water would be injected during periods of low system demand over a span of several years. The water would be held in the aquifer for use during an exceptionally dry year when demands might exceed system capacity or available supplies of raw water. This plan of operation would require the availability of additional water during years when inj ec- tion exceeded pumpage. The impacts of this plan on the ground-water resources were investigated by use of model-simulation techniques. The multiyear inj ection-pumpage cycle was simulated in the Arapahoe aquifer at two sites in the southeastern suburban area. Site 1 is located on 2.25 mi2 of land to the west of Cherry Creek Dam in T. 5 S., R. 67 W. Site 2 is of similar area and is located in northern Douglas County near secs. 16 and 17 of T. 6 S., R. 67 W. Total in- jection rates for the well fields at each site were estimated by model simulation such that the water level at the sites would not rise above land surface by the end of a 4-year injection period. Thus, the model injection rates are controlled by the hydrology of the site. The in- jection rate at site 1 was 1.7 fta/s and at site 2 was 10.0 fts/s. The difference between the two injection rates is due to larger aquifer transmissivity and greater depth to water at site 2. Four years of injection followed by 1 year of pumping were simulated. Pumping rates, calculated to remove one-half of the injected water, are 3.4 fts/s at site 1 and 20.0 fta/s at site 2. Incremental water-level changes are calculated as the difference be- tween the STDY-BASE and STDY-INJ estimate (4-year injection simulation) and the STDY-BASE and STDY-PMP estimate (1-year pumping simulation). The STDY pumpage estimate was used for the simulations because it causes minimum water-level decline in the aquifers and will produce a conservative estimate of the injection rates. Larger rates of injection would be possi- ble under HALF or FULL pumpage estimates. Results indicate that there could be as much as 198 ft of incremental water-level rise in the Arapahoe aquifer under site 1, and rises in excess of 50 ft could be present in a 47-rni2 area surrounding the site (fig. 33). After 4 years of injection, the well field is pumped for 1 year, forming a cone of depression at the center of the former recharge mound produced by the injection. As shown in figure 34, remnants of the recharge mound are present, mainly to the east and south of site 1. Water-level declines in excess of 50 ft occur in a 21-mi2 area near the pumping site. At site 2, there is as much as 424 ft of incremental water-level rise under the site as a result ‘of the larger rates of injection. Water-level rises in excess of 50 ft are present in a 448-mi2 area surrounding the site (fig. 35). When 1 year of pumpage is simulated to take place after 62 BEDROCK AQUIFERS IN DENVER BASIN , COLO. 105° [10' 104°3ll' EXPLANATION — —10— -“ LINE OF EQUAL INCREMENTAL WATER-LEVEL RISE—Interval 40 and 50 feet LIMIT OF ARAPAHOE AQUIFER [I 5 11] 15 MILES l I I I If r | I 0 15 KILUMETEFIS CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 FIGURE 33.——Incnemental water-level change in the Arapahoe aquifer at site 1, after 4 years of injection, using STDY pump- age estimate. 4 years of injection, a cone of depression forms with an excess of 400 ft of drawdown at the site, as shown in figure 36. Again, remnants of the recharge mound formed by the injection are still present in surrounding areas. In the same time span, injection at site I placed 4,900 acre-ft of water into storage and injection at site 2 placed 29,000 acre-ft of water into storage. The water- level rises shown in figures 33 and 35 represent pressure changes in the confined Arapahoe aquifer and do not in- dicate the lateral extent of movement of the injected water. The water injected at site 1 could, theoretically, be contained Within an area of aquifer 3,100 ft in di- ameter. At site 2, the injected water could, theoretically, OI MODEL SIMULATIONS 63 105°Ull' 39° 30' EXPLANATION —50— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE—Interval 50 and 100 feet — —10 — — LINE OF EQUAL INCREMENTAL WATER- LEVEL RISE—Interval 10 feet LIMIT OF ARAPAHOE AOUIFER 104°30' 35’ c? 15 MILES | I J 15 KILOMETERS CONTOUR INTERVAL 500 FEET NATIONAL GEDDETID VERTICAL DATUM OF 1929 FIGURE 34.—Incremental water-level change in the Arapahoe aquifer at site 1, after 1 year of pumping, using STDY pump- age estimate. be stored in an area of aquifer 5,800 ft in diameter. Thus, although pressure changes due to injection ex- tend over large areas, the injected water would be con- tained in a local area near the well field and would be available for removal by subsequent pumping of the in- jection wells. These simulations indicate that the choice of a site for bedrock storage of municipal water is critical, for it can drastically affect the volume of water that can be stored in, and pumped from, the aquifer. Both the Denver and Laramie—Fox Hills aquifers underlie sites 1 and 2. If these aquifers were utilized in addition to the Arapahoe aquifer, injection and pumping rates much larger than those simulated for the Arapahoe aquifer alone could be 64 BEDROCK AQUIFERS IN DENVER BASIN, COLO. “15°00 EXPLANATION —70— LINE OF EQUAL INCREMENTAL WATER-LEVEL RISE—Interval 50 and 100 feet LIMIT OF ARAPAHOE AQUIFER 104°30' ._ I J i ii 1) firm?“ -‘ \ _ ”an 5 ID 15 MILES I I J | 5 10 15 KILUMETEHS 137—1: _. CONTOUR INTERVAL 500 FEET NATIONAL GECIDETIC VERTICAL DATUM OF 1929 FIGURE 35.—Incremental water-level change in the Arapahoe aquifer at site 2, after 4 years of injection, using STDY pump- age estimate. possible. Also, injection rates for the Arapahoe aquifer may be conservative (based on STDY pumpage. estimate). Therefore, it may be possible for large volumes of water to be injected, stored, and pumped from the bedrock aquifers near Denver. If more water were injected than pumped, a net increase in the volume of ground water in storage would result, with beneficial effects for surrounding well users. The effects of one MODEL SIMULATIONS 65 105° 00' 39° 30' EXPLANATION —100— LINE OF EQUAL INCREMENTAL WATER-LEVEL DECLINE — Interval 100 and 200 feet ——50—— LINE OF EQUAL INCREMENTAL WATER-LEVEL RISE—Interval 50 feet LIMIT OF ARAPAHOE AOUIFER 104°30' f 5 III 15 MILES J l 5 10' 15 KILUMETEFIS ca——c CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 FIGURE 36.—Incremental water-level change in the Arapahoe aquifer at site 2, after 1 year of pumping, using STDY pump- age estimate. 5-year cycle of injection and pumpage are shown in figures 33 through 36. The cones of depression shown to occur at the end of the 1-year pumping phase would be short-lived, for subsequent injection in the next cycle would quickly raise the water levels in this area. Further work is needed to evaluate engineering and economic considerations and to investigate the chemical com- patibility of the native and injected water before the feasibility of such a system can be known. 66 BEDROCK AQUIFERS IN DENVER BASIN, COLO. CONCLUSIONS Transmissivity is a measure of the ability of an aquifer to transmit water. It is shown to range from zero near the margins of the bedrock aquifers to as much as 1,000 ftz/d in the Laramie—Fox Hills aquifer, . 2,100 ftz/d in the Arapahoe aquifer, 400 ftz/d in the Denver aquifer, and 1,200 ft2/d in the Dawson aquifer. The area of highest transmissivity is located south of Littleton in the Laramie—Fox Hills and Arapahoe aquifers. In the Denver and Dawson aquifers, the area of highest transmissivity is farther south, near Castle Rock and Palmer Lake. In general, larger well yields may be obtained in areas of larger transmissivity; thus, the bedrock aquifers south of Littleton offer the greatest potential for ground-water resource develop- ment. This area also is well suited for artificial recharge by deep-well injection due to the large transmissivity and the deep water levels. It is estimated that 470x10“ acre-ft of water is in storage in the bedrock aquifers of the Denver basin. Of this amount, about 260><10'3 acre-ft is theoretically recoverable through gravity drainage of the aquifers. However, less than 0.1 percent of the total volume of water in storage in the basin is stored under confined conditions. Thus, if ground-water development is to make use of the large volume of water in storage, water levels in the aquifers must decline to allow release of water from unconfined storage at the water table. Development of the ground-water resources in this manner will become increasingly difficult as water-level declines require not only the redrilling of existing shallower wells but also drilling of larger numbers of deeper wells to offset decreasing well yields. In spite of their outward appearances, bedrock forma- tions have elastic properties and are compressible when subjected to pressure, such as that caused by large water-level declines. It is estimated that the 1883-1960 water-level decline in the Arapahoe aquifer in Denver may have produced from 0.8 to 8.0 in. of subsidence in the land surface. Subsidence also may be occurring in surrounding areas but is likely of lesser magnitude. Water-level rises since 1960 may have produced some rebound in the surface elevation in Denver if elastic compaction was primarily responsible for the initial subsidence. If future pumpage causes large water-level declines in the aquifers, as the model results indicate, several feet of land-surface subsidence might occur. Severe subsidence in other areas has caused damage to well casings, development of surface fissures, and disruption in water flow through sewage lines, ditches, and other surface conveyances. Resurveys of bench- mark altitudes in the metropolitan area are needed to provide direct measurement of changes in land-surface elevation. An alternative would be the installation of compaction-monitoring equipment on a few unused wells in areas of large water-level decline. Either proce- dure would allow monitoring of subsidence and would provide advanced warning of subsidence-related problems. The mean annual precipitation on the Denver basin supplies 6,900 ft3/s of water to the area, of which only about 55 fta/s recharges the bedrock aquifers. Under long-term steady conditions, the Dawson aquifer is esti- mated to receive the majority of the recharge (40.6 ftals) and also to supply the majority of the discharge (33.4 fts/s), primarily to streams. Under these condi- tions, interaquifer flow is 7.2 fti’ls from the Dawson aquifer to the Denver aquifer and 5.3 fta/s from the Denver aquifer to the Arapahoe aquifer. No significant interaquifer flow occurs between the Arapahoe aquifer and the Laramie-Fox Hills aquifer. The largest natural discharge from the bedrock aquifers is into the drainage area of Cherry Creek, followed by the Monument- Fountain Creek combined area, Plum Creek, Kiowa Creek, Bijou Creek, and the South Platte River. Estimates of discharge from bedrock wells range from about 5 ftS/s of flow from uncapped wells in 1884 to be- tween 15 and 41 fta/s of pumpage between 1958 and 1978. Most of the 1958—78 pumpage occurred within a 30-mi radius of Denver and represents an overdraft on the aquifers in this area, which has caused major water- level declines in the bedrock formations. Estimates of pumpage from bedrock wells are difficult to obtain because of lack of data. The importance of this limita- tion is illustrated by modeling results, which indicate that the rate of pumping will significantly affect future water levels in the aquifer. If the major users of bedrock aquifers submitted annual pumpage data to a central agency, such as the State Engineers Office of the Col- orado Department of Natural Resources, the ground- water resources of the basin could be more effectively studied, and better model simulations of future water- level conditions would be possible. Three 1979—2050 pumpage estimates were made for use in simulating effects of various ground-water development plans. These estimates represent large, medium, and small rates of increase that might be ex- pected to occur in future pumpage from the bedrock aquifers. Results of model simulation of the small pumpage estimates indicate that maximum 1979—2050 water-level declines of 410 ft will take place in the Laramie—Fox Hills aquifer, 280 ft in the Arapahoe aquifer, 120 ft in the Denver aquifer, and 280 ft in the Dawson aquifer. Simulation of the medium rate of pumpage indicates maximum 1979—2050 water-level declines of 1,700 ft in the Laramie—Fox Hills aquifer, 800 ft in the Arapahoe aquifer, 300 ft in the Denver CONCLUSION 67 aquifer, and 280 ft in the Dawson aquifer. When the large pumpage estimate is used, maximum 1979—2050 water-level declines of 1,830 ft occur in the Laramie—Fox Hills aquifer, 1,000 ft in the Arapahoe aquifer, 420 ft in the Denver aquifer, and 310 ft in the Dawson aquifer. These results indicate that large water- level declines will primarily occur in the deeper aquifers when medium or large rates of pumping are considered. Rapid water-level declines take place in these aquifers until water-table conditions develop nearby, at which time the rate of water-level decline decreases. Large water-level declines can be delayed by limiting the rate of increase in pumpage. This has begun on a local basis through use of stringent water-conservation practices or through land-use zoning that limits popula- tion density and thereby limits pumpage. Minimal rates of increase in pumpage are shown to produce moderate water-level declines in the aquifers and will assure pro- longed availability of bedrock water supplies. An alternative to limiting pumpage would be to en- courage pumping from the shallow aquifers where water-table conditions allow slower rates of water-level decline. However, this would accelerate the water-level declines in these aquifers and would cause the shallower wells commonly used as domestic water supplies to go dry more quickly. If large water-level declines are allowed to occur, economic factors likely will cause some users of the deep aquifers to seek sources of water from the shallow aquifers or from surface sources. If these other sources are not available, continuing water-levels declines in the deep aquifers are likely. It is important to note, however, that none of the pumpage estimates produced significant dewatering of the aquifers. Thus, future pumping in the basin to the year 2050 probably will not cause the aquifers to go dry; rather, water will still be available in the aquifer, but the cost of maintaining in- itial rates of pumping will increase markedly as water levels decline and individual well yields decrease. Although water still may be available in the aquifer, wells that do not fully penetrate the aquifer may go dry as a result of the declining water levels. Four ground-water development plans that consider possible methods of supplementing the municipal sup- ply for Denver were simulated. These plans involve (1) pumping a satellite well field and piping water to the metropolitan area, (2) pumping a well field located in the metropolitan area, (3) using bedrock wells to provide ir- rigation water for city parks, and (4) using the bedrock aquifers for temporary storage of municipal water for later use. Model simulations of the effects of pumping the satellite and municipal well fields indicate that larger incremental water-level declines will be produced by the well-field pumpage if small rates of other pump- age are occuring in the basin. Conversely, smaller in- cremental water-level declines are produced by the well-field pumpage if large rates of other pumping are occurring in the basin. Comparison of the effects of pumping the satellite and metropolitan well fields in- dicates that the metropolitan well field will produce only about one-half as much water as the satellite well field. However, pumping from the metropolitan well field will have a less adverse effect on water levels in surrounding aquifers. The Dawson aquifer, for example, is not affected by the metropolitan well field due to the location of the well field. Both well fields offer the ad- vantage of providing supplemental water from a source separate from the current surface-water sources. As such, the well fields might provide an emergency or backup source for part of the metropolitan area. Further analysis of both well fields are needed to determine the feasibility of this method of supplementing Denver’s water supply. Simulations of the effects of pumping the Arapahoe aquifer to supply irrigation water for several city parks and golf courses indicate that 30 to 40 ft of incremental water-level decline are produced near the parks. These model results do not consider the seasonal variation in water level that would occur as a result of pumping for summer irrigation use. Seasonal water levels would differ from the average annual decline shown by the model. If pumpage were to occur from the Denver or Laramie—Fox Hills aquifers, in addition to the Arapahoe aquifer, lesser rates of incremental water- level decline would be produced in the Arapahoe aquifer. Again, evaluation of other constraints will affect the feasibility of this water source. If the bedrock aquifers could be used to store treated municipal water during periods of low demand on the municipal system, additional water could be pumped back into the system at a later date to help meet peak water requirements. Model simulation of two injection- pumping well fields in the Arapahoe aquifer indicates that a field located in northern Douglas County would allow injection rates of 10 fta/s, whereas a field located near Cherry Creek Reservoir would allow an injection rate of only 1.7 ft3/s. If, after 4 years of injection at these rates, the well fields are pumped at twice these rates for 1 year, a cone of depression would form near the former center of the injection-recharge mound. Remnants of the recharge mound are shown to be present to the north, east, and south of the cone of depression after 1 year of pumping. A total of 4,900 acre-ft of water was injected in 4 years near Cherry Creek Reservoir, and a total of 29,000 acre-ft of water was injected in northern Douglas County. Results indicate that large volumes of water may be injected into the bedrock aquifer and subsequently removed by pumping. The choice of loca- 68 BEDROCK AQUIFERS IN DENVER BASIN, COLO. ’ tion of the injection-pumping site is critical to achieving large flow rates. Additional work is required to evaluate other constraints and to investigate chemical compati- bility of the native and injected water. REFERENCES CITED Anna, L. 0., 1975, Map showing availability of hydrologic data pub- lished as of 1974 by the US. Environmental Data Service and by the U.S. Geological Survey and cooperating agencies, Colorado Springs—Castle Rock area, Front Range Urban Corridor, Colorado: US. Geological Survey Miscellaneous Investigations Series Map I—857—D, scale 12100.000, 1 sheet. Bull, W. B., 1975. Land subsidence due to ground-water withdrawl in the Los Banos—Kettleman City area, California, Part 2, Subsid- ence and compaction of deposits: US. Geological Survey Profes- sional Paper 437—F, p. F1—F90. Chronic, Felicie, and Chronic. John, 1974, Bibliography and index of geology and hydrology, Front Range Urban Corridor, Colorado: US. Geological Survey Bulletin 1306, 102 p. Clark, S. P., Jr., ed., 1966, Handbook of physical constants: Geo logical Society of America Memoir 97, 587 p. Colorado Department of Health, 1978, Primary drinking water reg- ulations for the State of Colorado: Denver, Water Quality Control Division, 60 p. Colton, R. B., and Lowrie. R. L., 1973, Map showing mined areas of the Boulder-Weld coal field, Colorado: US. Geological Survey Mis- cellaneous Field Studies Map MF—513, scale 1224,000. 1 sheet Cross, C.W., Chisholm, F. F., Chauvenet. Regis, and Van Deist, P. H., 1884, The artesian wells of Denver: Colorado Scientific Soci- ety, Proceedings, v. 1, p. 76—108. Darton, N. H., 1905, Geology and underground water resources of the central Great Plains: US. Geological Survey Professional Paper 32, 433 p. Denver Regional Council of Governments, 1978, Regional water sup ply plan; preliminary report: Denver Regional Council of Govem- merits, 59 p. Denver Water Department, 1975, Metropolitan water requirements and resources 1975—2010: Prepared under the direction of the Denver Regional Council of Governments for the Colorado State Legislature, Metropolitan Denver Water Study Committee, v. I, 146 p.;v. 11,383 p.; v. III, 153 p. Emmons, S. F., Cross, C. W., and Eldridge, G. H., 1896, Geology of the Denver basin in Colorado: US. Geological Survey Monograph No. 27. Fatt, I., 1958, Compressibility of sandstones at low to moderate pressures: American Association of Petroleum Geologists Bulletin, v. 42, no. 8, p. 1924—1957. Hampton, E. R., 1975, Map showing availability of hydrologic data published by the US. Environmental Data Service and by the US. Geological Survey and cooperating agencies, Greater Denver area, Front Range Urban Corridor, Colorado: US. Geological Survey Miscellaneous Investigations Series Map I—856—C, scale 1:100,000, 1 sheet. Hampton, E. R., Clark, G. A., McNutt, M. H., 1974, Map showing availability of hydrologic data, Boulder—Fort Collins—Greeley area, Front Range Urban Corridor, Colorado: US. Geological Survey Miscellaneous Investigations Series Map I—855—C, scale 1100.000, 1 sheet. . Hansen, W. R., Chronic, John, and Matelock, John, 1978, Climatography of the Front Range Urban Corridor and vicinity, Colorado: US. Geological Survey Professional Paper 1019, 59 p. Hillier, D. E., Brogden, R. E., and Schneider. P. A., Jr., 1978, Hydrology of the Arapahoe aquifer in the Englewood—Castle Rock area, south of Denver, Denver basin, Colorado: US. Geological Survey Miscellaneous Investigations Series Map I—1043, scale 1:100,000, 2 sheets. Holzer, T. L., 1977, Ground failure in areas of subsidence due to ground-water decline in the United States, in International Sympo sium on Land Subsidence, 2d, Anaheim, 1976, Proceedings: Inter- national Association of Hydrological Science—Association Interna- tionale des Sciences Hydrologiques Publication No. 121, p. 423—433. Hurr, R. T., and Schneider, P. A., 1972, Hydrologic characteristics of the valley-fill aquifer in the Brighton reach of the South Platte River valley, Colorado: US. Geological Survey Open-File Report 72—332, 2 p. Kirkham, R. M., 1978. Coal mines and coal analyses of the Denver and Cheyenne basins, Colorado: Colorado Geological Survey Open- File Report 78—9. 104 p. Livingston, R. K., Klein, J. M., and Bingham, D. L., 1976, Water resources of El Paso County. Colorado: Colorado Water Conserva- tion Board, Colorado Water Resources Circular 32, 85 p. Major, T. J ., Robson. S. G.. Romero, J. C., and Zawistowski, Stanley, 1983, Hydrogeologic data from parts of the Denver basin, Col- orado: US. Geological Survey Open-File Report 83-274, 425 p. Mayuga, M. N., and Allen, D. R., 1970, Subsidence in the Wilmington oil field, Long Beach, California, U.S.A., in Symposium on Land Subsidence, International Hydrologic Decade, Tokyo, 1969, Pro- ceedings: International Association of Hydrological Sciences— Association Internationale des Sciences Hydrologiques Publication No. 88, p. 66—79. McConaghy, J. A., Chase, G. H., Boettcher, A. J., and Major, T. J., 1964, Hydrogeologic data for the Denver basin, Colorado: Colorado Water Conservation Board Basic-Data Report 15, 224 p. McCoy, A. W., 111, 1953, Tectonic history of the Denver basin, Colo rado: American Association of Petroleum Geologists Bulletin, v. 37, no. 8, p. 1873—1875. Norris, J. M., Robson, S. G., and Parker, R. S., 1983, Summary of hydrologic information for the Denver coal region, Colorado: US. Geological Survey Open-File Report 84—4337, 68 p. Robson, S. G., 1983, Hydraulic characteristics of the principal bedrock aquifers in the Denver basin, Colorado: US. Geological Survey Hydrologic Investigations Atlas HA-659, scale 1:500,000, 3 sheets. Robson, S. G., and Romero, J. C., 1981a, Geologic structure, hydrology, and water quality of the Dawson aquifer in the Denver basin, Colorado: US. Geological Survey Hydrologic Investigations Atlas HA—643, scale 1:250,000, 3 sheets. ___1981b, Geologic structure, hydrology, and water quality of the Denver aquifer in the Denver basin, Colorado: US. Geological Survey Hydrologic Investigations Atlas HA—646, scale 1:500,000, 1 sheet. Robson, S. G., Romero, J. C., and Zawistowski, Stanley, 1981a, Geologic structure, hydrology, and water quality of the Arapahoe aquifer in the Denver basin, Colorado: US. Geological Survey Hydrologic Investigations Atlas HA—647, scales 1:500,000 and 1:250,000, 3 sheets. Robson, S. G., Wacinski, Andrew, Zawistowski, Stanley, and Romero, J. C., 1981b, Geologic structure, hydrology, and water quality of the Laramie-Fox Hills aquifer in the Denver basin, Colorado: US. Geological Survey Hydrologic Investigations Atlas HA—650, scale 1:500,000, 3 sheets. Romero, J. C., 1976, Ground-water resources of the bedrock aquifers of the Denver basin, Colorado: Colorado Division of Water Resources, 109 p. SUPPLEMENTAL INFORMATION / 69 Romero, J. C., and Hampton, E. R., 1972, Maps showing the approx- imate configuration and depth to the top of the Laramie—Fox Hills aquifer, Denver basin, Colorado: US. Geological Survey Miscella- neous Investigations Map I-791, scale 1:500,000, 1 sheet. Schneider, P. A., Jr., 1980, Water-supply assessment of the Laramie—Fox Hills aquifer in parts of Adams, Boulder, Jefferson, and Weld Counties, Colorado: US. Geological Survey Open-File Report 80-327, 21 p., scale 150,000, 6 sheets. Todd, D. K., 1967, Ground water hydrology: New York, John Wiley, 336 p. ’I‘rescott, P. C., 1975, Documentation of finite-difference model for simulation of three-dimensional ground-water flow: US. Geological Survey Open-File Report 75-438, 107 p. ’I‘rescott, P. C., and Larson, S. P., 1976. Documentation of finite- difference model for simulation of three-dimensional ground-water flow, Supplement to Open-File Report 75—438: US. Geological Survey Open-File Report 76—591, 20 p. 'I‘rescott, P. C., Pinder, G. F., and Larson, S. P., 1976, Finite difference model for aquifer simulation in two dimensions with results of numerical experiments: US. Geological Survey Tech- niques of Water-Resources Investigations, Book 7, Chapter Cl, 116 . 'I‘weto,p Odgen, 1979, Geologic map of Colorado: US. Geological Survey Special Geologic Map, scale 1:500,000, 1 sheet. US. Bureau of the Census, 1981, Final population and housing unit counts, Colorado (1980): Report PHC 80—V—7. US. Environmental Protection Agency, 1976, National interim primary drinking water regulations: EPA 570/9—76-003, 159 p. __1977, National secondary drinking water regulations: Federal Register, v. 42, no. 62, Thursday, March 31, 1977, Part 1, p. 17143—17147. SUPPLEMENTAL INFORMATION HISTORICAL PUMPAGE ESTIMATES Pumpage from bedrock wells for the period 1958—78 was estimated by first assessing the annual municipal pumpage for this period. This value was converted to total pumpage by use of a conversion factor calculated on the basis of data for the years 1960, 1974, and 1977. Municipal pumpage values were based, to the extent possible, on pumpage records provided by municipal water agencies for the period 1958-78. The quality of these records varied considerably, requiring that estimates be provided for missing and erroneous data. Estimates for missing data were based on per capita consumption and population in the service area. In some cases, erroneous data could be corrected or ad- justed by, for example, separating pumpage from bedrock and alluvial wells. In other cases, correction was not possible, and data of questionable accuracy were included in the municipal pumpage estimate. The resulting municipal pumpage data are probably ac- curate within about: 20 percent. The municipal pumpage from bedrock wells is shown to range from 5 ftals in 1958 to 9 fta/s in 1978 (fig. 12). Between 1960 and 1977, municipal pumpage probably constituted 20 to 30 percent of the total bedrock pumpage. More detailed bedrock-pumpage estimates were tabulated for 1960, 1974, and 1977 in order to allow calculation of a conversion factor: F=P,/Pm, (4) where P, represents total pumpage and Pm represents municipal pumpage. The pumpage estimates were tabulated using four categories of water use: (1) municipal, (2) domestic and stock, (3) commercial and industrial, and (4) irrigation. Municipal pumpage for 1960, 1974, and 1977 was obtained from the previous estimates. Domestic pumpage was estimated on the basis of a per capita use of 175 gal/d and a mean occupancy rate of three people per house (US. Bureau of the Census, 1981). This is equivalent to a pumping rate of 0.6 acre- ft/yr per domestic well, assuming one domestic well per household. Yields from stock wells may vary con- siderably, but a value of 0.6 acre-ft/yr also seems reasonable for these wells. The number of active bedrock wells in each of the four categories can be ap- proximated from well data available for the years 1960, 1974, and 1977. The number of active domestic and stock wells multiplied by the yield per well (0.6 acre- ft/yr) produced the initial estimates of domestic and stock pumpage. Because of uncertainty in the number of domestic and stock wells in use and the average pumping rate for these wells, the accuracy of this estimate is probably only about 1'30 percent. Bedrock pumpage for domestic and stock use increased from about 10 percent of the total pumpage in 1960 to about 30 percent of the total pumpage in 197 7. Commercial and industrial pumpage estimates were based on the average yield of wells in this category. Average yield was calculated from measured discharge of 20 typical bedrock wells (5 percent of the total number of commercial and industrial wells) and was adjusted for a pumping cycle of 4 hours pumping per day, 5 days per week, 52 weeks per year. An average yield of 9 acre-ft/yr per well was obtained and multiplied by the number of active wells in this category to initially estimate the commercial and industrial pumpage. Because little data were available on which to base these estimates, the resulting pumpage for commercial and industrial use is of lower accuracy than is the domestic and stock pump- age estimate. Pumpage for this water-use category rang- ed from about 30 percent of the total pumpage in 1960 to about 20 percent in 1974 and 1977. Irrigation pumpage was estimated in a manner similar to that used for cormnercial and industrial pumpage. Average yield for these wells was calculated from measured discharge of 10 typical bedrock wells 70 BEDROCK AQUIFERS IN DENVER BASIN, COLO. (7 percent of the total number of irrigation wells) and was adjusted for a pumping cycle of 12 hours’ pumping per day for a 100-day irrigation season. An average yield of 41 acre-ft/yr per well was obtained and multiplied by the number of wells in this category to estimate the initial irrigation pumpage. Again, little data are available to document these estimates, and the resulting pumpage for irrigation wells is probably of lower accuracy than are the other pumpage estimates. On the basis of these estimates, irrigation pumpage like- ly constituted about 25 percent of the total pumpage in 1960, 1974, and 1977. The sums of the pumpage estimates for the four categories of water use were calculated for 1960, 197 4, and 197 7. The factor defined in equation (4) ranged from 3.0 to 3.7 for the above period, indicating that total bedrock pumpage is about three to four times the magnitude of municipal bedrock pumpage. This pump- age factor was used to make initial estimates of total pumpage from municipal pumpage for each year from 1958 to 1978. Subsequent modeling of the ground-water system indicated that these pumpage estimates were too small to be consistent with the water-level changes and hydrologic characteristics of the aquifers. A pump- age factor ranging from 3.5 to 4.5 was found to provide more satisfactory modeling results and was used to estimate total pumpage from municipal pumpage. The revision of the initial pumpage estimate is reasonable because (1) the 0- to 20-percent increase in the pumpage factor is within the error of estimate for the initial pumpage, and (2) the initial pumpage estimate is one of the more poorly documented parameters used in the model and is thus subject to revision on the basis of other, better-defined model parameters. FUTURE PUMPAGE ESTIMATES Municipal pumpage estimates for 1979—2050 were based on the estimated water requirements for the Denver metropolitan area as presented in a study of the metropolitan water requirements for 1975—2010 (Denver Water Department, 1975). In making the municipal pumpage projections, it was assumed that the historical ratio of bedrock supply rates to surface- water supply rates would be maintained in the future. The resulting pumpage was a constant fraction of the increasing total water demand for the metropolitan area. This pumpage probably is an overestimate of future municipal pumpage because, historically, pump- age has not increased as rapidly as has total water demand. In addition, the population and water-demand estimates contained in the Denver Water Department (1975) report were considered to be too large by the Denver Regional Council of Governments and were revised downward by the Denver'Regional Council of Governments (1978) for their purposes. Pumpage estimates for private users or new (post-1975) municipal suppliers were based on the assumption that this pumpage will increase at the same rate as the projected supply requirements of municipal suppliers located to the southeast of the metropolitan area. Municipal suppliers in this area were chosen because most of the new (1978-82) well construction has been in this area. Water-supply requirement data for Aurora, Willows Water District, Parker Water District, Denver Southeast Suburban Water and Sanitation District, Silver Heights Water District, and the town of Castle Rock were used to determine the future rate of increase in water requirements. This rate was applied to the permitted pumping rate for new wells in the area to distribute the pumpage over 1979—2010. This pumpage was added to a base pumpage and the municipal pump- age to obtain the total pumpage estimate for 1979—2010. The base pumpage was included to repre- sent nonmunicipal pumping in established areas where pumpage is not expected to increase. Pumpage estimates for the period 2010 to 2050 were based on a linear extrapolation of the 1975—2010 total pumpage estimate. This pumpage estimate is termed the “FULL” pumpage estimate in this paper. A second pumpage estimate was made in order to provide a more conservative alternative to the initial (FULL) estimate. This estimate was arbitrarily taken to be one-half of the FULL estimate and is termed the “HALF” pumpage estimate. A third, still more conservative, estimate was made by assuming that no increase in pumpage could occur after 1983. This estimate is termed the “STDY” pumpage estimate. MODELING ERRORS AND LIMITATIONS In both the steady-state and transient-state calibra- tions, differences between measured and calculated water-level altitudes are due to errors associated with (1) the simplifying assumptions made in describing the aquifer system, (2) the computational scheme used to approximate the solution to the basic equation, (3) the aquifer characteristics, (4) the initial conditions specified for a model simulation, (5) the determination of water-level altitudes in the prototype, and (6) the rate and distribution of recharge and discharge. Errors associated with the description of the aquifer system usually result from the simplifying assumptions that are made in gaining a conceptual understanding of the operation of the prototype, Simplifying assump- tions also are required in order to reduce the complexity of the system to a level that can be simulated by the model. Examples of simplifying assumptions include [K A— SUPPLEMENTAL INFORMATION 71 the assumption that a uniform vertical head distribu- tion occurs within each aquifer, or the assumption that storage coefficient is either confined or unconfined through the entire thickness of each aquifer. Errors associated with simplifying assumptions are probably of small to moderate importance in both models and may be partly responsible for differences between measured and calculated heads. Errors associated with the computational scheme result from the numerical approximation of the solution to the governing equation and are also of small impor- tance in both models. In the transient-state model, these errors become more pronounced as the size of the grid blocks become large in the outlying parts of the study area. Use of transient-state simulations should be limited to areas having small to moderate grid spacing, in order to reduce error and provide greater resolution. Aquifer characteristics such as transmissivity and vertical hydraulic conductivity are a small source of error in the steady-state model and, with storage coef- ficient, are a moderate to large source of error in the transient-state model. The value of storage coefficient can significantly alter the computed heads in the transient-state model. Because the effective value of this characteristic is not well documented, differences ‘ between calculated and measured water levels may be due, in part, to use of an inappropriate storage coeffi- cient in the transient model. The water-level altitudes at the start of a transient- state simulation period must be specified. The model uses these altitudes as the starting point for further head computations. Errors in these and other initial conditions may be carried forward and may affect the heads calculated at later times. These effects tend to diminish with the duration of the simulation period, so that heads calculated by the transient-state model are normally not seriously affected by initial condition errors. Water-budget computations also can be affected by initial condition errors if the duration of the simula- tion period is relatively short. For this reason, use of the water budget to determine percent of pumped water derived from storage might lead to erroneous results if the change in storage is strongly affected by incorrect initial conditions. Errors in prototype water-level altitudes are of moderate importance in the calibration of both the steady-state and transient-state models. These errors are due to sampling errors, such as measuring a recently pumped well or measuring a well completed in the wrong aquifer, or are due to interpretation errors, such as incorrectly extrapolating equal-potential lines into areas without data or incorrectly computing a water- level altitude. An important interpretation error can be produced by assuming that the water level in a well represents the average water level in the aquifer. This assumption is particularly troublesome in aquifers with numerous partially penetrating wells, such as the Dawson and Denver aquifers. Some differences between measured and calculated water levels, therefore, may be due not to model deficiencies, but to errors in the pro- totype water levels, which normally serve as the stan- dard in judging the calibration of the model. In this situation, the model results may be a more accurate representation of the water-level conditions than the measured water levels. Errors associated with recharge and discharge include errors in estimating the rate, distribution, or duration of natural recharge, natural discharge, and historical pumpage. Natural recharge and discharge are a moderate source of error in the calibration of the transient-state model; however, historical pumpage is probably the largest single source of error in the calibra- tion of this model. Because historical pumpage is not well defined by data, this parameter was most readily adjusted during calibration to produce better agree- ment between measured and calculated water-level altitudes. The lack of data to define both historical and future pumpage estimates limits the type of model simulations that should be made. As an illustration, consider the following three questions that could be asked in a modeling study: 1. What will be the future pumpage, and how will the prototype respond to this pumpage? 2. What will be the prototype response to a specified rate of pumpage? 3. What will be the difference in response of the pro- totype to two specified rates of pumpage? The model-generated answer to the firSt question will contain errors resulting from all of the abovementioned sources, but will be most profoundly affected by errors in estimating future pumpage. Because it is impossible to foretell what the rate, distribution, and duration of future pumpage will be, the model results also will be an incorrect prediction of future conditions. The answer to the second question will not contain errors in model results due to errors in pumpage estimates because the rate of pumping has been specified. The answer to the third question involves comparison of model results for two specified rates of pumping. This eliminates the effects of initial conditions, so this model result is not affected by either pumpage errors or initial condition er- rors. Simulations on the Denver basin model are made using the latter technique whenever possible to improve the accuracy of the model results. The accuracy of a model simulation is affected by ( 1) the quality of the calibration, (2) the uniqueness of the 72 BEDROCK AQUIFERS IN DENVER BASIN, COLO. CALIBRATION SIMULATION 2 — A 2 - COMPUTED HEAD CASE1 UNIQUE MODEL SIMULATION PERIOD= CALIBRATION PERIOD SIMULATION PUMPAGE= CALIBRATION PUMPAGE CASE 2 NON-UNIQUE MODEL SIMULATION PERIOD = CALIBRATION PERIOD SIMULATION PUMPAGE = CALIBRATION PUMPAGE 0 IO 20 0 10 20 CASE3 UNIQUE MODEL SIMULATION PERIOD= 2 X CALIBRATION PERIOD SIMULATION PUMPAGE= a CALIBRATION PUMPAGE 0 10 20 0 IO 20 3O 40 HYPOTHETICAL DIFFERENCE BETWEEN COMPUTED AND MEASURED HEAD, IN FEET CASE 4 UNIQUE MODEL SIMULATION PERIOD= CALIBRATION PERIOD SIMULATION PUMPAGE =2 X CALIBRATION PUMPAGE _2 I _2 I o 10 20 o 10 PERIOD, IN YEARS FIGURE 37 .—Graphs showing hypothetical effects of modeling conditions on simulation accuracy. model, (3) the duration of the simulation period com- pared to the duration of the calibration period, and (4) the rate of pumping during the simulation period com- pared to that during the calibration period. These four effects are illustrated in elementary form in figure 37. The most accurate simulations are possible when the model simulation period and pumping rate are less than, or comparable to, the calibration period and pumping rate. In the case of a unique model (fig. 37, case 1), the accuracy of the calibration is indicative of the accuracy of the simulation. If at the end of a 20-year calibration period the calculated head in the model is 1 ft above the observed head, then a similar head error could be ex- pected at the end of a 20-year simulation period. If the model is not unique (see the section “Transient- State Model”), a second model exists that will calibrate as well as, or better than, the original model and will be based on an alternate intrepretation of the field data. If such a model exists, constraints on the interpretation of the field data normally require that the second model be similar to the original model. In an extreme case, the second model might produce a calibration head distribu- tion that is the mirror image of the head distribution of the original model. In the example of case 2 (fig. 37), the second model might produce a calculated head 1 ft below the observed head, although the original model produces a calculated head 1 ft above the observed head. Both models are calibrated to within 1 ft of the m—r w SUPPLEMENTAL INFORMATION 73 observed head but differ from each other by 2 ft. A similar accuracy could be expected in the simulation results. This indicates that simulations from a non- unique model could be different from those produced by a second, equally acceptable model, and this difference could be of greater magnitude than the accuracy of the calibration. If the duration of the simulation period is much L greater than the duration of the calibration period, errors that accumulate through time can lessen the ac- curacy of the simulation results. As shown in case 3 (fig. 37 ), the calibrated head is 1 ft above the observed head at the end of a 20-year calibration period. By the end of a 40-year simulation period, the calculated head is 2 ft above the observed head. Thus, if all of the errors in a model calibration are of a cumulative nature, a simulation of twice the duration of the calibration will exhibit about twice the error of the calibration. The rate of recharge or discharge (primarily pumpage in this case) during the simulation period also can affect the accuracy of the simulation results. If all of the errors in a model calibration are due to incorrect model response to pumpage, then a doubling of the pumping rate in a simulation could lead to an approximate doubling of the simulation error over that shown by the calibration (fig. 37, case 4). The degree to which the above conditions affect the simulation results of the Denver basin model is difficult to determine. Errors in a model calibration or simula- tion are normally not due to one cause alone, but are a complex interaction of many causes. The net result is often a series of partially offsetting errors that defy quantitative description. In general terms, the shorter duration model simulations involving small pumping rates are probably the most accurate; the longer dura- tion simulations involving large pumping rates are probably the least accurate. It can be seen from the simulation results shown in figure 37 that the cumulative effect of errors from a nonunique model used for a long simulation period with a large rate of pump- ing could be large. Although is is unlikely that those errors would be fully additive, the results of such long- term, heavily stressed simulations must be used with special caution. US, GOVERNMENT PRINTING OFFICE 1987~773A047 46,029 REGION NO. 8 75 I *"f » . PIP PREPARED IN COOPERATION WITH THE { I 3 EI ‘2. Iv I ‘ if \1 . I 257 COLORADO DEPARTMENT OF NATURAL RESOURCES, OFFICE OF THE STATE ENGINEER; \\~ DEPARTMENT OF THE INTERIOR THE DENVER BOARD OF WATER COMMISSIONERS; AND ADAMS, ARAPAHOE, DOUGLAS, \ . ‘ . L US. GEOLOGICAL SURVEY ELBERT, AND EL PASO COUNTIES “ ~\.,_ PROFESSIONAL PAPER 1257 PLATE 1 R'mw‘ 389W: MSW-WWW , NEW ‘ ', .- ‘ mm. 3 " , “1W . ,MDW' 134‘700’ “W ......R'58W' 357‘” R'IUW' IT 89W Raswmm' RWWA “WW NEW. 8 613W, 304“”30' mm R 62W. “If"? ., ITEM” I04 “0 359W; .I 358W -IIWW‘..- 2 v , , ‘ ’ " "" "‘ ‘ ' ’ "V H ' ""\_ V T V ‘ ‘ 7 ,, - "MW 77 . I \ , ‘7 _ , x... . 7, ,.. I ' «_ h . Q} a O ‘ A Iv; A O ‘ I 5N I 5N TEN. Gucdr , ’KK asters QVIIIEM? . T 3‘ MN TAN: \rr‘ {.4 I Q’Pec GKICYC‘SIT [ I RESERVOIR . 76 , ,7, I ‘ « u {VII/ton{C‘Kv _/ : I I. MN 00 9?? mm, ‘ . * . .7 R68 j _ 3 . : I) T-3N. I? MN, IIZN. Tm MN. . T, I N. 1W “N TIN. a , ‘ 40°00' AQQQO' “9 90 WW i . ms, INS HS us. I 11.28. T 23 T23 W3 T.3s,_ T38. G _ I as ms 7148. T 53 7, 53 ms. Ll7 T. SS, \1 I I'BS' T63 w_, . " V , I ; " ‘ ;, TISS- 39Q30': {13930 39°30 \ . i H; , , ‘ w“ 9 ,k ", [13930 TBS 119$. ‘ . Teas. \Lémon I r108 V103. I INS. \ IIIS. , \1 \l mzs. T123. 39000, 39 00 39-00 mzs. I. ~ Faicm E v ‘ a .' - ,_ . u"- ’ \ 0 I » I / ‘ . . . ._ \ \ " I 7.133. I ‘ ' \660 " " " , , ’ . " " ‘: INS. I148. LIBS. I\\ _ F,» Tm yyyyy I ms was. [SUMMER I I MSW, I I MIN. 7 LIB mm. ’ww I me RESW. 101;“00’ MQW MSW 357‘” / ’Rzas'xwx": I Ram, REz/Iw.FITTIAOUVSMW, ne2w I Rimw IRSGW. 104000“ ”59‘” “SW ”SW- LARAMIE-FOX HILLS AQUIFER ARAPAHOE AQUIFER WOW mew, RISSWMWGG’ Fifi/”W ,. "II-Rwy IWEW. Rssaw 304038‘ R 23w 7 R 52W- . ,R'EIIW: .. ,E'SGW' Toma" Risw‘ R'SSW' HEW “735W; 8-6 ITEM: IQ‘IOOI_ H 59W; .I'.57W;-,, O I“; V‘ C’Peckh Giicres‘i KL I ( x J1 x r] (5”? RESERVOIR\ \ / Pl]? ML’_£OI>{\ \ > as” 1 T. 3 N ”as I I WW" " I I (5°; I Vailey ’3 ea? VaIIey ,9 ' ,yalmoné a) 3 W 7 k5 48°00 ’ 40°00' 40°00 IX; ” 40900 ’ TIS, TIS TIS, 7.18M: I TSS. ,. T. 68. 39030’ TBS. . _ ». ms. ,, ,. I , , 1 « z 7 "A . \ V \ -, I E~-n;‘>‘\ Q , _, <4 I I _ ’ I ,\ .r - I" I L‘ \> ‘ I . “TR" M‘ ._ 1‘" ‘ I [I wrap“ ‘ a , mysl \ I4 ' ,_ . \ 5.: ' I.“ x ‘ TIQS. I. \ \ , ’ x I a , x > ; -' , . , ' _ :QEUOQ’ a , , _ ‘ , _, 'IGECOD’ i , MSW. , r' , . . " _ ‘ ~ ‘ < ‘ .\ {WSW I EXPLANATION AREA WHERE WATER-LEVEL DATA ARE INADEQUATE TO DEFINE POTENTIOMETRIC SURFACE —6000 — LINE OF EQUAL WATER-LEVEL ALTITUDE—Interval 100 fed 0 DATA POINT LIMITOF AQUIFER “W I MS. TISS, :33 U) I I “7/: .,\ , TIES. U I ":364039 WW. DENVER AQUIFER DAWSON AQUIFER R, gmgmam' R, 59 w. Raw. Base from US Geological Survey State base map, 1969 SCALE 11500 000 10 O 10 20 MILES 10 O 10 20 KILOMETERS I-——-I I—I I—I J CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP SHOWING MEASURED 1978 POTENTIOMETRIC SURFACES OF BEDROCK AQUIFERS IN THE DENVER BASIN, COLORADO GE 75 PG _ 3232, PREPARED IN COOPERATION WITH THE ' ~ A , 3- * \Io V257 COLORADO DEPARTMENT OF NATURAL RESOURCES, OFFICE OF THE STATE ENGINEER; DEPARTMENT OF THE INTERIOR THE DENVER BOARD OF WATER COMMISSIONERS; AND ADAMS, ARAPAHOE, DOUGLAS, . PROFESSIONAL PAPER 1257 U. S. GEOLOGICAL SURVEY ELBERT, AND EL PASO COUNTIES PLATE 2 RI32W‘ 7 VRR‘W- RRRW; R757"?! 7 , R 79W» :33sz RSSWIGF‘GO’ RHIW 7 WWW? 3.653;. mm. 3:33.323 53 23:5in. ,, _ 3.57w RMW 7 MW! IRR RR! R 59W; RSRW’ ._R,57W:.. '1) O ' I ’ I I I I; I 5N I {IN T. 5N. : GOOdrIc rdm ’ 3x I}: R R asters rgptafi’d T494 I fIN MI ' ’ TAN TEN TBN. I'.3N. ’ ’ n 3'3 I, ‘ , . / . , 3: f v 4 5‘ fl ‘ , ' ‘ C} 1‘ 3 TWIN TXN T,2N. \4‘3" hangar/v " ~ » , , I _ Kg .2 . C? _ ,v. I _ _‘ . 1 ,7 T g; 2_L// ' ’ ‘ I Sauih Raggew'\ I JWLS } «H1 TAN LIN, \LEJIYfithL' 3“? , 40000 40 00 ITS, 3 TIMES Caiy I : .3. ¥ I0 ,‘ j ‘ R R 43$”? ‘7‘“ . “ \{éamei’ Wm! I, SS. 3 , 39°33 Ramah i 3 _ .11 :7 3, Hg was. ~183I€I€3n3k3 Ema“! I ./ * , I ISL/ENC» ,_ ‘ ..... I ' . ' I _ F T3128, 33333 ' _ I R a ' """" ‘ , ' _ 33°03 335003" IDSRUD’ ' 8.68W II saw “5T”? H38 TMS 3.343 I us. KISS“, MISS I 1.63 Tues. I V : “2 F , I: \1 _ ,rx, :7 ‘ 0 RI (2 ,3 ‘ r‘ , > \\ , 3/, { MSW W R RSSW '7 III-ISM 334030“ S33v3 M Rééw' ' ' Rem; RIIIIW, 10mg 9- 39w RISESW‘ R 57W ITEM 3 65W RBIIWV ”3332' 3.333, Razw RBBW. III/1003' 359w. “SW “57‘” LARA IE-FOX HILLS AQUIFER ARAPAHOE AQUIFER . R RM , R 5“” R'RRW'IRR’W, R'RR'W R RR 33 saw 3 Saw ”('3 R 5333 R 53W 2 R 55;W: , .RRR‘W AERRRRR R’QW RRRR MRW- MBWIOSWO’ MW- 3 323w, 334w 234033 RSSW, RRRW' 104000, RRQW' “7W “L - I 1’ MI I ”m" L “S x ,, ‘ " \ H "TH" ”Rd/VI- 2 ----- ,___\\ 1 i I .’ , II! If” ‘ P Law} I53: I m ISN. GOOd """" (i betiard T 4 N. g ('30,; M N. > I 2N T. 3N. ’Qggen 3'er g I ’ > MN IZN Ream R In - SOII‘III 90;:ng Vaéiey :TN. \ TIN MN. 40°33 RRRRR" 43333 R “ODOR, :3 MS, HS 1: 2' TIS. J J Barr: Lake \ I R ‘R7\\ ‘I ' ' \R - W , ”IV , / I »- , """ R , . "H TERR-ST 2 \ L._\ >\ 3 ' " V ' ' I- T353 I“ - MS GIGS-K T5333 ISS T T683 3339 33333 I953 I238, Ramah I US I, H.228. 303 33333 I, 32:; I :38 WW K5333 ., TIA? ; ‘ ,, T I" 9 ‘ MR" HIS EXPLANATION I I53. TASS 7' 753' —6‘000— LINE OF EQUAL WATER-LEVEL ALTITUDE—Interval 100 feet .3 LIMIT OF AQUIFER /_ : 1" ”I .' \_ 7" . 1 -_ 2 FIGS 1 IRS’ T1582 " 35W" “533‘ " ff ’; 1' 332332. 33333 W 33333 333333! {3“ 59W 5358131 MM 815% HEW R'ybmy”miéggl 353333 4 ”375233 RSIW fi'ggwimfiéb" H.58W Rssvv, RS7W. Base from US Geological Survey State base map, 1989 SCALE 1:500 000 10 o 10 20 MILES 10 o 10 20 KILOMETERS l--—II-—I|—l CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP SHOWING MODEL-CALCULATED 1978 POTENTIOMETRIC SURFACES OF BEDROCK AQUIFERS IN THE DENVER BASIN, COLORADO GE ’75 P0 W7, PREPARED IN COOPERATION WITH THE I’L ’5 7 COLORADO DEPARTMENT OF NATURAL RESOURCES, OFFICE OF THE STATE ENGINEER; ONAL PAPER 125“; DEPARTMENT OF THE INTERIOR THE DENVER BOARD OF WATER COMMISSIONERS; AND ADAMS, ARAPAHOE, DOUGLAS, PROFESSI PLATE 3 U. S. GEOLOGICAL SURVEY ELBERT, AND EL PASO COUNTIES , 4: “ 4°00 RVBEIW, I‘VDW- FISBW. II,aaw.I05°00' 354w 104030 4800 9-701”: _ _ WWW IIIsIIII/IesI‘UIII mm, RVII‘IBW‘I ,. IIIIIIW. BMW. IOIIDSII’ mm , 05sz 3, k 7 3117““ ., 19 .1 7 , ' =\‘ / \fiM/g ‘* - s I. i ' » ‘ 1 “071% _- ‘ " ' ‘“ f ’ " I I Jig; ‘ ‘. , , ’31 2U " <‘ ( ,A £11 {0 ’91} L’ ‘ , , __ ,_ 1 \r 30 O by , _ , ,‘ I y,\ ‘‘‘‘‘ \3 i TSN » I, * "I 3 MIN, 3 I \LBI' I I 5300 I I | l "102 [L2 ‘/ ‘ _ » ,3 ‘ 0 ardm' PIVENé/IIL ,I I 'LIK o ,3 HE‘szI‘pI/om It; ‘ ‘ z T / n , '— T, 4N. T 4N, ~ _ ' GIIcrest / 1‘ 48000 , fig, 1 I + I I i I S N ‘ ,I x 1.5”, (3‘ em 6 Res L J r3! T 3N TEN, 1% “Tmmas‘: 4800 I a I 5500 . . I . TZN. i 1 I '33 I120. - g 400 L? g‘ ‘ . ‘ C I r—I‘ : 3 . . “' KI asbur @ 1 4m» 80th Rog‘ger‘x” Wflflfi .e \ “M M, ' 1..“me 2*; '7 I“? ' I ‘ W ’ .7 5000 I l I I I I , .. 52 v , I X 1 3 E u . I f , \ ~31 pectVaIIey ‘°_’ \ 3 3 ,; I‘II I ’f‘f? (RI/Eh pn§7\ w (/1) ‘ ”I53: 1*" .4. 4o " 7 ~ “A“ I54 1.." . .. 40000 a; y m Greek ’ , " G 00 I (/1 [:1 3 I’ fifty/e ‘ ,‘LICII‘SO Greek 3 warm} _ ' LOUISVI‘ e ,__ _3 I? ' . w’eservcir ‘ 118110160 1 I” ,1 , 0’ 2‘ 1'15 Ij/ “(1 C,» ' I (f 1300 a 3‘ I Spy", \ t1 1 ,/1<9I>' 3 VI" I T328, 379,; \ /.f / ‘ {gas 17 La e\) 3 -. DeeITIaII } , ,I , 39°30 : 0 LO 0 N 6000 6000 , P 0 L0 ‘ O N 1 55000 55000 '5: l\ .— CD I/‘;'" ,1 M r! / Rama‘n :3 I Ramah EXPLANATION ‘ , I EXPLANATION T 118 1979-2050 WATER-LEVEL T‘ H S 3 1979.2050 WATER-LEVEL “ \ DECLINE, IN FEET 1 "’ DECLINE IN FEET I "" 0- 50 - 0.100 I ITIM: T128. 100-200 39°00 I I I I 50150 39 00 I, I23. I , Xi) - 150-250 j: 3 7,; 200-300 105300; _ 105000" 300-400 R' 58 M 250-280 II 58 W: M33. . 400.410 I I I I —5000— LINE OF EQUAL WATER-LEVEL I 1 8 ALTITUDE—InteNal 100 0 and 200 feet —--6000— LINE OF EQUAL WATER-LEVEL N ALTITUDE—Interval 100 P LIMIT OF AQUIFER andzooIeeI H43 mas, III 600° LIMIT OF AQUIFER ; I- 6000 7 I E a g E A I I I .3 N “I“ * '3 o < 5600 I T153. 3 O O O '_ _ i\ 0 LO ; a: o o _l 1 .— N N < 56000 O 0 YEARS LO 5 R 8 i WATER-LEVEL HYDROGRAPH YEARIS ; 3 WATER—LEVEL I-HYDROGRAPH } (,7: 55000 . (I: I I , I o T 185. ‘2 1 K' I\ 0 ID I\_ I A ; II 0) O O i x; \ \ 701500,? R 55w. RESAW 304930: 053w 0020/, MW mow, 304000 MSW- MBW- 057w. RBEW. R 54w. IIIIIchII msw, IIIWW RI70WI mew, mam/405000 RI67W, MSW MSW 8.82m. 104330, M3“, MM. 37 8'51 w. 3 figmw’ 104000 31539031, 3 R 58W, MAIL Rimw. 0540/. 104030, RSBW, RBZW. IIEIW , , wIIIIIIW, 310000 R;§9W',,m ‘1 ’" I M ' 5 7 I 7 ' “ ' ' ‘I ' " M I 1 II 1 .2“; AGKSON I; I g ‘. ‘EI’I’VOI 1.5 N. T. 5N. 3,1”1I/ T. s N. ’ 4:0 \ mm RIVERSI I/AI’N 1/ 95351 TAN , w MN TAN, / 4798mm 1 1 I GIICwSI‘: 1 1002 , M : 1 REESE}? vom If 3/ RESERV 305 T. 3 N / 5700 MR m Canal I é ‘ f‘l‘ // \ r I ‘3 iugLak 9 , i nesbur : 1, 30001 RoggeI \ Egg“? Luw NI ”gap, ~ :3. I, ' ‘1' 1 1 T V I S (fa \‘L P603; e0tVaI$ey 1‘ ”<\: I" I a W \ V 1‘ \X 1 : almont Q); (Ly; ‘3’“ Re I I A - I I. I» Y : 1 40°00 40000, :I V ,. ,_ I ‘ 1. 59 ,. 40°00 "‘1 ,1 L/ ,1 g 65 /' \ 5 {arse Creek ,3] ,. (3 \/ _ : 3 LOUISVI/II IE "5 \. 3 ?eserv01r 1, : \ :50) I\ I 0‘ x, ,I " I [0' ' ‘ 3 Qy/l C3, . I, , I _, I TISMIH » S e $1 WI 1-13- /' X 1 i 3 Q I '11 gI emefSK‘ , ,7 3 f9 3 I g! ,0; _ , I‘ , ’ M I ma. Ix: fI‘IgI-irston La. 6500 I 6300 9, I 8 _ W 5800 60000 I ,9 '5 a, .— IJabIeTszock 1 EXPLANATION EXPLANATION T113 _ , , ‘ I. II S: I. II S, I“ 3 , ’. . " . ’ ‘ ’ , , _ \ 1979-2050 WATER-LEVEL 1979-2050 WATER-LEVEL I - 4 ‘ , __ g r :I’ I," ' ‘1 I """ 1 DECLINE IN FEET DECLINE, IN FEET W: , ” " " -. I: I , ' I 7 I 0- 50 ,. 0' 50 >8Iack£ 1.128: : I ‘03; c‘ m, ”23' 39000: 3900' 53100 3000: 5 I Agape j 39°00: 50-100 mm, I. \ , I128 ‘ If ' 100-200 100-120 105000, R 68 8, 68W __ E 200-280 6000—— LINE OF EQUAL WATER-LEVEL K. 138', 138: I I I3 ‘3‘ ALTITUDE—Interval 100 ._\ 6000 LINE OF EQUAL WATER LEVEL ' ” ‘ and 200 feet \ — __ _ . LIMIT OF AQUIFER ASJITLéng—S Ifntetrval an ee 1- 6000 I I I I I I E I LIMIT OF AQUIFER g ' I443 I43. E 6000 I I ' ' ' "HAS. u; - “II.J D ' Z 3 W —, t ' 3 S D < 5600 I I | I I I I I: I9 8 8 II “ 5 L a) O O I .35: < 5600 IICMOII 1.153: .— N N O o 0 YEARS 5 8 8 WATER~LEVEL HYDROGRAPH '- N YEARS N l 60 WATER-LEVEL HYDROGRAPH I. 168. 3 L I grew a 65W R 6m 110403017 REM/1 ‘.'Iéé'w, III4I30' 063w VII WW, II 68W. IOADIIOV’WRFSQWI Base from US Geological . Survey State base map, 1969 SCALE 1:500 000 10 O 10 20 MILES 10 0 10 20 KILOMETERS I—I I—I I—I CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP SHOWING MODEL-CALCULATED 2050 POTENTIOMETRIC SURFACES OF BEDROCK AQUIFERS IN THE DENVER BASIN, COLORADO, MODEL SIMULATION STDY—BASE Q E 75 ‘Plp S212 \I I I2 67 PROFESSIONAL PAPER 1257 PREPARED IN COOPERATION WITH THE COLORADO DEPARTMENT OF NATURAL RESOURCES, OFFICE OF THE STATE ENGINEER; DEPARTMENT OF THE INTERIOR THE DENVER BOARD OF WATER COMMISSIONERS; AND ADAMS, ARAPAHOE, DOUGLAS, U. s. GEOLOGICAL SURVEY ELBERT, AND EL PASO COUNTIES PLATE 4 5400 I . I I I I 405ch RBJIW. 1040m- REBW. 4800 NW mew, RESSW 105000 R 85W RGSW. 0.5m. 04°30 0,5301 0,62w, NOW 104 00 ”W; (I 0 . ‘ » I50. I 5” 5000 7 \y Goodnc _ 50m * - ,‘ '39 MN. _ > 4500 L , I3N 5500 - 4100 _ E L211. 5000 WE ' I I SGIIIh ,_ N aIIey I IN, I” “N 4000' 40 00 1.13. T. I s. 7.03. . I109 39°30 I 09°30 i 6000 _ 6000 L I o E Ln 0 _ ' N 5500 i 5500 — 5100 _ O r\ O) 5000 — _ — [U / I. IIS T. IIS. 4500 — F I A mm. 7 39000 ‘ 39°00 4300 ’ I 320 o o - ‘ B 8 “I, » EXPLANATION .— N ' 4‘ -o It 4m 7 ~ 1979-2050 WATER-LEVEL “59,3058“, “ ‘ E L ,, ' DECLINE IN FEET ' ' E ‘5‘ 8 I033 1133 8 0.200 EXPLANATION 200-400 1979-2050 WATER-LEVEL DECLINE IN FEET 400-600 I: :49 T. M 3. 0-200 600-800 1 200-500 ' ' —— 6000 -— LINE OF EQUAL WATER- LEVEL ‘ ALTITUDE—Interval 100 _ 50‘“ 000 and 200 feet “cm 1000 1500 LIMIT OF AQUIFER ' ' E 6000 6000 . ' . . I . Lu 0 0 LL 0 In 1500-1700 J , Z 8 8 , ‘ "£793 ‘3 7“ - "v I I \ I)\\1 —6000— LINE OF EQUAL WATER-LEVEL 3 5 > 3 > g , I , 1 K. ALTITUDE—Interval 100 ; , . , .. I _ , .- I E I , , , , ,‘ . , ’ " L 0 I “ and 200 feet ‘- ~/ , ‘ . . ' _I I . - . > . 7, . 7 0... ”W . . 96W 9650/. 06416, 104130 RES-NV <1 5600 I,“ ~ I p; . I 40' , 06.00. 0220/, Imw. RBOW. 304000 R-S9W- MSW H57W- LIMIT OF AQUIFER > E g E RHhW KISW. RMW. 0 30 3 J 56000 , , I . LARAMIE-FOX HILLS AQUIFER T N mas “ ARAPAHOE AQUIFER : WATER-LEVEL HYDROGRAPH g r 3 I; ,— Zz‘ 5600 ’ .‘3 8 8 m 0 O ,_ N N YEARS WATER-LEVEL HYDROGRAPH RISSW. 8.68W.305”00’ IIIIIW, g 55w. 359V“ ”4003/ ”SW, , R “7W ’ R 55W RBEW, II 80W. I04030’ R 63 , " ' .\' - 3‘“; ‘I' ‘ I 50. ,5 utter Lake -- LLLLLL asters flies” ‘ ‘ MN . - Weak OPEC" 06 !I (500766! L K,\ I, 7' L 7 “WW 7' EMPIRE "\ /’ . . . 3 — EESERVO/E \‘ W/ ' 2 / 2;, , I: I Wimp-9x- . . \_ ~orf Mnm M ‘Défke I} .3 ‘ " REFS]: : .I C. ”.01 x, I”: “$30, 0 .1 0 1110.773 9/ 5 k“: . ‘a Em“ .. ,,,,, 2; \_ / (I ~ 3 3),] /E% T.3N. I (7’ 1,7. , ’ A? 1/ 5500 _ I I , ,, l I , 50000 I <3¥§I“‘9I‘j;,.,r\ I\ 1 ‘m g_> 40000 ‘" ' I, I 5. I25, 135. AS. I650 ;_ \;- herry Creek 6300 / 5800 """ O [\ O) EXPLANATION 1979-2050 WATER-LEVEL EXPLANATION T. I? S. T H S DECLINE IN FEET 1979-2050 WATER-LEVEL 0-50 DECLINE IN FEET 50-100 .1 , .1 .- 0 00 BIacha-r ,. LIES. 3000’ ' 39000 , 100-200 7 m- I , 100450 3909‘ . I125. L. 200-300 . - ~ - - 150‘200 . “Egfigw 105000 ,I . ' 5800 FEICOII 7 , R'IISW‘ — 6000— LINE OF EQUAL WATER-LEVEL _ m 200'280 3 ALTITUDE—IntewallOO " ' ’ ( — I I I I I I I I I“ I35 and 200 feet I _ —6‘000—- LINE OF EQUAL WATER-LEVEL LIMIT OF AQUIFER I ALTITUDE—Interval I | I I I E 6000 63000 o O 100 and 200 feet E '8 8 8 —— LIMIT OF AQUIFER 2 I105, " N N I- 6000 , , L. I E I“, I 148, I LL ‘3 if V, z I: i m- I— o 3’ 5600 I N I a O O O , 3 — 6 8 5 , . ~ ~‘ , 5 m N N lI‘I’LICIKtO“ U- \ LIE-S, < 5600 7 I58 ‘— YEARS ._ ,- [ I x ‘ _ WATER-LEVEL HYDROGRAPH ' ‘ YEARS ! 1’ WATER-LEVEL HYDROGRAPH 77777 RSBW M 67 W, ‘3; ,. ‘ .1 . .x‘ L. 11.61,: I I "7 0600/ 0.6.5071 R 84W, I0 30 063%!- Base from US. Geological Survey SIaIe base map, 1969 SCALE 1:500 000 10 o 10 20 MILES 10 o 10 20 KILOMETERS I=I I——-I I——I ; I CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP SHOWING MODEL-CALCULATED 2050 POTENTIOMETRIC SURFACES OF BEDROCK AQUIFERS IN THE DENVER BASIN, I ‘ COLORADO, MODEL SIMULATION HALF-BASE QE 75 P0 SUIZ “S. , PREPARED IN COOPERATION WITH THE \) I 25 7 COLORADO DEPARTMENT OF NATURAL RESOURCES, OFFICE OF THE STATE ENGINEER; ' DEPARTMENT OF THE INTERIOR THE DENVER BOARD OF WATER COMMISSIONERS; AND ADAMS, ARAPAHOE, DOUGLAS, PROFESSIONAL PAPER 1257 U. S. GEOLOGICAL SURVEY ELBERT, AND EL PASO COUNTIES PLATE 5 5500 I I I I I 5000 I I I I - 6 533W 0.520;. In 70W 0,5210, RSIW. mow, 104°00’ 869w, ’ 50, I - ' ‘ 1:50. - 1‘ £13 I """""""""" III-Is 5000 — 4500 ’ , MI: MN , (LI? — f/sz ‘ ,',, 4200 4500 A \ WW I 3N I 30 , E\ L 5500 \ _ \ - _ \. WWW-- , 21;” . 7 a I 20.? - I70. 40000 " K0 313300 E H * fl ‘4’ EMA“. I1... I. ; / , ,I '3, - 5" I52 0 I a 5000o ‘— I U x \ I T3903 , r I B a I " 2 IN. '— »,« / 2 K I e /" v" I2 Iioyt, A I r: on ‘ 0‘55”” @0650 I I I I 65317161.? I 00 ‘ ' I i 4300 44 5 \ I, 2. I I6!) SarrELake ; I If I I Q: ’ 5” I '\ 1: i I I 1‘ g IIIIIIIIIIIIII I ‘ I ‘ I I I M I .II. , I : I I ‘ I [It 2.1013. L; 52 .. I I _ 4000 i :3 _ _ < I: f I , - i f I3 I E. - - ,. LEONE). LIMJA’SIII 3600 l | | I I I 8 ‘ : 3W : 8 8 8 v 8 I8 6000‘ T I. 1 “96%) 7 _ v R 69W 55 ‘ I” was 6000 I I I I I I 2 5500 A - 202200 , 7 E I ’ I I I I I I O O E ' 0 Ln 0 O N N _ . 5500 A I 5000 — - / , - m , .. r .I. ,1 faTabie R003 1 I 5 _Table Reck INT: 7 MI I; 73 I) I 10'} I; .2 'I“ « . I /’»- - w M I \\~ _ I 5800* / i _ I\V/ I k] V I /, x , 7" , \J '6\‘ INS f ~ ’/ 1 I / - .RavéesIIakg/‘BIIIXE /I' I \\ ~ 1‘ / East/00m »I » ‘ ,- : I ,5; ; _\ 7 , , , ,, , , I \V ' 5000 I I I0 I I I ‘ 7' ‘ A, I? o 8 I HZ“ 4500 7 EIackIForast» x " I 9 g ‘9‘ 3‘“ IVUI‘IIIIIIWI‘ 30 00 . i- em ’ ' 0 ' T ‘2"; \ ‘ I03 I\ ' \I \v — 4 I . - EXPLANATION I I I I I I l L, 1., I\ I 0* v I 42002 8 g gigw * 1979-2050 WATER-LEVEL ”5&8“, °’ 8 8 ' 3 ' I -7 DECLINE IN FEET ' ' I I I I I I ‘— {138 6» I . I T 333‘ I I I I I I I I 1' j I , - ‘1, " ' I [7 l I , I 2 _y_ I I . ' 0 0-200 7 . I , I I» _ In ,'5 , ,Ii3 , . r g EXPLANATION a I -. 200.400 5 1979-2050 WATER-LEVEL zl j Noam *3 I Q »I I _ DECLINE IN FEET I» 340 : 2.; , = I I, 540. 400-600 1.40 . __ I I ,; Hf} I 2 8 0200 I ‘ _. . 600-800 a 200-500 , /‘ TI 3 '- 800-1000 w 1 . I)", I J :I_ v _ I 1153 500-1000 ‘ 2 I _ 5 i 7 — 6000— LINE OF EQUAL WATER-LEVEL 1,, ' 1 3- ~ I I I I I ALTITUDE—Interval 100 1000-1500 A and 200 feet 2 __ I LIMIT OF AQUIFER 1500-1830 2 _ f E 6000 I y I K I ‘ 2 , I, was. I i i ”J ,0 It —-—6‘000-- LINE OF EQUAL WATER-LEVEL I, _, I _ I I ‘ K 5500<3 ' ‘ g ' ' ' ' 0 “2L » § I I, / V, ,, I , . " V F» I " , , ~‘* " I, I "' ’ ’ U I\ o In '1 .IcIIIL IL .. .I I , , ~ ~ , ~ - -- :nLdT 1221(1)ng Intewalloo 5 62W 3 55W» “W” ”4530’ ”53W R 52‘” M I M 3’ 8 8 “a ’ 0,5510. 0.640: IGA’”38’ 083w, RISJW R 5010 I04°II0 RIEQW MBW- 057W 2) I: _ F LIMITOFAQUIFER LARAMIE-FOX HILLS AQUIFER g I ARAPAHOE AQUIFER E I 2 8 P. 2 9 8 8 3: YEARS g WATER-LEVEL HYDROGRAPH I. E < 5600 ‘ I? 8 8 2 a 8 YEARS WATER—LEVEL HYDROGRAPH If W #52310 RSSWIOSQCI‘J' , 353W? , “W . 0 WW II Iggw 35821405000, Raw 05601 00510. MAW» IDA-.3” “3W, 5.5207. II 61 w, _ IIIIow. 10400 5590/. _ II 53w. 0.5m, Fifi 0 Q 9I 3 O O 1/ II W I I50, ”N, TEN, T \ Vleédona". I III I4 N T 4 N, a GIIcresI EMPIRE REBEHVO/f} Mfigon'f“; 1m. “"5"! ”I TEN. 5500 lug Lake _ T,?N TIZN. 5000 I IN. UN- 0 B F 40°00 40°00 I Is. Mjarshy »‘ 1.13, I HS 125 I23. \ >31". 5200 ##me ‘ ‘ ' {S If» ' \I- I area CIty ‘\ I: ' I 1.38, I33. ., I; W ItiQMémIIv 6000 I 48. T2 43 I;- c‘: arsro L0 63‘ , ‘3 I55. I53, I65. 3 I as. 3300’ I “ I 39°30 90> I / 5300 I _ 6500 5800 I I E 6000 0') O E '5 EXPLANATION 7 EXPLANATION I T H S I \ 9 3 1979-2050 WATER-LEVEL ‘ 1979-2050 WATER-LEVEL , f DECLINE IN FEET DECLINE IN FEET ' é, »- I 6" of “I 0- 50 0-100 ~ ,5 was, I 353000 - 39'0“ 50-100 3000' ' 100-200 “I 223 I m _' T428, l“ L 100-200 200-300 10500" 200310 R,58W, 300-420 T I33 LINE OF EQUAL WATER LEVEL —— 6000 — LINE OF EQUAL WATER— LEVEL —6000— - _ t I ALTITUDE—Interval 100 :‘ggflggo 2;“ ““1200 fem LIMIT OF AQUIFER LIMIT OF AQUIFER P 6000 I— 6000 a ' ‘ I ' ' ' IIAs. \ Lu 0 LL Lu I 5 LL ' __ m E E r,‘ I F LII u; I o I; I I \ I I 5 I: l I IICHIIII L; \\ I < 56000 (I) o <( , 56000 O I?) I ‘ , B 8 8 a 8 o 930 1 .— N N .— N v N YEARS , YEARS WATER-LEVEL HYDROGRAPH I WATER-LEVEL HYDROGRAPH w I__/ 17“ I, I, ,, £_I z , - -- 1 - ' -- ii» -- , 7 _~ » ~ ' r - ~~ ’ R Z _ _ *5 . 7 R 55w. HIISIIW 10430 863W 0.6M 001W II 00.0; 0100’ 0.52va RIBSW RIb7W. MSW; ‘ MW 8 MW, “3039' mm, I1 620/. 9151 w, 06001204000 059w, H.58W 0.57W, Base from US. Gemagical Survey State base map, 1969 SCALE 1:500 000 10 0 10 20 MILES 10 O 10 20 KILOMETERS L—I I—I I——I \— L1 CONTOUR INTERVAL 500 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAP SHOWING MODEL-CALCULATED 2050 POTENTIOMETRIC SURFACES OF BEDROCK AQUIFERS IN THE DENVER BASIN, COLORADO, MODEL SIMULATION FULL-BASE