II 7 DAYS Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona EOLOGICAL SURVEY PROFESSIONAL PAPER 1259 |,k-....- ntrPARTMENT i-EPOS"' Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona By ROBERT G. BOHANNON GEOLOGICAL SURVEY PROFESSIONAL PAPER 12 5 9 A study of the age, nomenclature, lithology, and tectonic history of Miocene continental deposits in the transition zone between the Great Basin and Sonoran Desert UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984UNITED STATES DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Bohannon, Robert G. Nonmarine sedimentary rocks of Tertiary age in the Lake Mead region, southeastern Nevada and northwestern Arizona. (Geological Survey Professional Paper; 1259) Bibliography; 72 p. Supt. of Docs. No.: 119.16: 1. Rocks, Sedimentary. 2. Geology, Stratigraphic—Miocene. 3. Petrology—Mead, Lake, region (Ariz. and Nev.) I. Title. II. Series. QE471.B682 552'.5'0979312 81-607893 AACR2 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402CONTENTS Page Abstract ...................................................... 1 Introduction .................................................. 2 Acknowledgments ........................................... 3 Stratigraphic nomenclature .................................... 3 Historical development..................................... 3 Proposed stratigraphic nomenclature........................ 6 Age of the Tertiary rocks...................................... 9 Description of rock units..................................... 14 Horse Spring Formation.................................... 15 Rainbow Gardens Member................................ 15 Distribution and thickness........................ 15 Lithology......................................... 15 Clastic and siliceous lithologies.............. 15 Carbonate and sulfate lithologies.............. 18 Sedimentary structures ........................ 19 Facies and their distribution.................. 20 Environments of deposition........................ 22 Thumb Member.......................................... 23 Distribution and thickness........................ 23 Lithology......................................... 23 Clastic rock types............................. 23 Carbonate and sulfate rocks.................... 28 Tuffaceous rocks and volcanic flows............ 28 Sedimentary structures ........................ 29 Page Thumb Member-Continued Depositional environments.......................... 29 Facies and provenance.............................. 31 Bitter Ridge Limestone Member.......................... 33 Distribution and thickness......................... 33 Lithology.......................................... 34 Calcareous lithologies.......................... 34 Noncalcareous lithologies....................... 36 Sedimentary structures ......................... 36 Facies and depositional environments............... 37 Lovell Wash Member..................................... 41 Distribution and thickness......................... 41 Lithology.......................................... 42 Carbonate rocks, claystone, and chert........... 42 Tuffaceous and clastic lithologies.............. 44 Sedimentary structures ......................... 46 Depositional environments.......................... 48 Informally named rock units................................ 49 Red sandstone unit..................................... 49 Rocks of the Grand Wash trough......................... 54 Muddy Creek Formation...................................... 56 Paleogeologic and paleotectonic evolution...................... 58 References cited............................................... 67 ILLUSTRATIONS Plate 1. Geologic and paleogeologic maps of the Lake Mead region.......................................................In pocket Page Figure 1. Location map of the Lake Mead region..................................................................................... 3 2. Diagram showing general lithology, stratigraphy, previous nomenclature, and proposed nomenclature for Tertiary rocks of the Lake Mead region.................................................................................. 5 3. Maps showing Tertiary geology and sample sites in the Muddy Mountains area. Virgin Mountains-Grand Wash Cliffs area, and Frenchman Mountain area....................................................................... 13 4. Measured stratigraphic sections of the Rainbow Gardens Member of the Horse Spring Formation...................... 16 5. Photograph of the resistant limestone unit in the upper part of the Rainbow Gardens Member.......................... 19 6. Photograph of the magnesite-bearing strata within the Rainbow Gardens Member....................................... 20 7. Map showing distributions of the five lithofacies of the Rainbow Gardens Member................................. 21 8. Generalized stratigraphic sections of the Thumb Member of the Horse Spring Formation............................ 24 9. Photographs of parallel, continuous bedding within sandstone of the Thumb Member................................ 26 10. Map showing distributions of conglomerate and fine-grained facies of the Thumb Member and relative abundances of rock types..................................................................................................... 27 11. Photograph of a continuous, well-defined conglomerate bed in the Thumb Member.................................... 28 12. Photograph of one of the green tuff units in sandstone of Thumb Member.............................................. 28 13. Diagram of lithology and bedforms of a microsection in the Echo Wash area........................................... 29 14. Photograph of rocks measured for the Echo Wash microsection....................................................... 30 15. Map showing northern basin margin of the Thumb Member............................................................ 31 ill 16027IV NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Page Figure 16. Photographs of the northern margin of the Thumb basin................................................................. 32 17. Map showing distribution of outcrops of the Bitter Ridge Limestone Member of the Horse Spring Formation........ 33 18. Photographs of bedding characteristics in the Bitter Ridge Limestone Member . . . . :.......................... 35 19. Photomicrograph of sample from type section of Bitter Ridge Limestone Member................................... 36 20-23. Photographs of sedimentary structures in the Bitter Ridge Limestone Member: 20. Conglomerate interfingering with limestone............................................................... 37 21. “Teepee” structures...................................................................................... 38 22. Large circular depressions on a dip slope................................................................ 39 23. Stromatolitic structures................................................................................. 40 24. Map showing distribution of outcrops of the Lovell Wash Member of the Horse Spring Formation................... 41 25. Photographs of three lithofacies in carbonate and claystone beds of the Lovell Wash Member..................... 43 26. Photomicrographs of three characteristic textures of limestone in the Lovell Wash Member....................... 44 27. Photograph of a small chert dome from Lovell Wash Member, showing original bedding................................. 45 28. Photograph of thick, well-exposed tuff unit in Lovell Wash Member.................................................. 45 29-31. Photomicrographs of samples from the Lovell Wash Member: 29. Shard-rich, matrix-supported tuffaceous rock............................................................. 46 30. Reworked, partially altered tuffaceous rock.............................................................. 47 31. Pumice lapilli in sparry calcite matrix.................................................................. 47 32-36. Photographs of the Lovell Wash Member: 32. “Teepee” structure in thick limestone bedset............................................................. 48 33. Contorted thin bedding................................................................................... 48 34. Eggshell-like hemispheroids.............................................................................. 49 35. A large carbonate mound.................................................................................. 50 36. Rip-up structure......................................................................................... 51 37. Map showing distribution of informal rock units................................................................ 51 38. Generalized geologic map of red sandstone unit in White Basin.................................................. 52 39. Photograph of red sandstone, siltstone, and claystone in the red sandstone unit................................ 53 40. Photomicrograph of sandstone from the red sandstone unit....................................................... 54 41. Photograph of Paleozoic carbonate rocks abutting conglomerate of the red sandstone unit along the Muddy Peak fault . . 54 42. Generalized geologic map of the Grand Wash trough area......................................................... 55 43. Map showing present distribution and probable original extent of Muddy Creek Formation in the Lake Mead region . . 57 44. Cross section from a point north of Muddy Peak to area southwest of Gale Hills..................................... 65 TABLES Page Table 1. Selected previously published K-Ar ages for Tertiary rock samples from the Lake Mead region...................................... 10 2. Zircon fission-track ages of samples from Tertiary rocks of the Lake Mead region............................................. 11 3. Analytical data for zircon fission-track age determinations.................................................................. 12NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN THE LAKE MEAD REGION, SOUTHEASTERN NEVADA AND NORTHWESTERN ARIZONA By Robert G. Bohannon ABSTRACT Tertiary sedimentary rocks form a broad belt north of Lake Mead in eastern Clark County, Nev., and western Mohave County, Ariz., between Las Vegas and the Grand Wash Cliffs. This belt steadies the diffuse boundary between the Great Basin and Sonoran Desert sections of the Basin and Range structural province and is directly adjacent to the Colorado Plateaus province. Detailed study of surface exposures of these rocks has provided information pertinent to their lithology, chronology, and stratigraphy and has resulted in changes in stratigraphic nomenclature and in concepts of the Tertiary tectonic evolution of the Lake Mead region. Three stratigraphic terms of formational status are endorsed in this report: the Horse Spring, Muddy Creek, and Hualapai. Four mappable stratigraphic units within the Horse Spring Formation are given formal member status: from oldest to youngest these are the Rainbow Gardens, Thumb, Bitter Ridge Limestone, and Lovell Wash Members. Two stratigraphic units unconformably above the Horse Spring at most localities and apparently older than most of the Muddy Creek are left informally designated as the red sandstone unit and the rocks of the Grand Wash trough. The Rainbow Gardens Member encompasses the lower part of the Thumb Formation of Longwell (1963), the lowest Tertiary beds of the Gale Hills Formation of Longwell and others (1965), most of the Horse Spring Formation of Longwell (1928), the Cottonwood Wash Formation of Moore (1972), and the formation at Tassai Ranch of Lucchitta (1966). The Thumb Member comprises the upper part of the Thumb Formation of Longwell (1963), much of the lower Gale Hills Formation of Longwell and others (1965), and the upper part of the Horse Spring Formation of Longwell (1928). The Bitter Ridge Limestone and Lovell Wash Members represent parts of the Gale Hills Formation of Longwell and others (1965), and the lower and middle parts, respectively, of the Horse Spring Formation of Longwell (1963). The red sandstone unit consists of the uppermost beds in both the Gale Hills Formation of Longwell and others (1965) and the Horse Spring Formation of Longwell (1963). Rocks commonly assigned to the Muddy Creek Formation near Grand Wash are called the rocks of the Grand Wash trough herein. The Muddy Creek Formation is also reevaluated to the extent that the term is restricted to rocks demonstrably continuous with those of the type locality. A new principal reference section is defined for the Horse Spring Formation in White Basin and Bitter Spring Valley. The type sections of the Rainbow Gardens, Bitter Ridge Limestone, and Lovell Wash Members are at the localities for which they were named, and the type locality for the Thumb is in Rainbow Gardens. Twenty-two fission-track age determinations on zircon extracted from basal parts of airfall tuff beds in the Horse Spring Formation and the two informally named rock units are concordant with previously published K-Ar age determinations. These determinations indicate that the Thumb Member ranges from 17.2 to possibly 13.5 m.y. old, the Bitter Ridge Limestone Member from 13.5 to about 13.0 m.y. old, and the Lovell Wash Member from 13.0 to 11.9 m.y. old. The Rainbow Gardens Member has not been directly dated, but is suspected to be no older than about 20 m.y., making the Horse Spring Formation Miocene in age with a possible total age range from about 20 to 11.9 m.y. The red sandstone unit is 11.9 to 10.6 m.y. old, and the rocks of the Grand Wash trough are as old as 11.6 m.y. by fission track ages and as young as 10.9 to 8.44 m.y. by K-Ar determinations. Only two age determinations are available for the Muddy Creek Formation: an 8-m.y. age from interbedded basalt and an age of 5.88 m.y. for the Fortification Basalt Member near the top of the Muddy Creek. The Rainbow Gardens Member occurs between Frenchman Mountain and Grand Wash, ranges from 50 to 400 m in thickness, and includes clastic rocks ranging from conglomerate to claystone, several types of carbonate rocks, evaporites, and chert. A basal conglomerate occurs, and carbonate lithologies dominate the upper portions of the member, but the middle parts are lithologically diverse. Five lithofacies delineated above the basal conglomerate include the clastic-carbonate, tuff-limestone, gypsiferous, gypsum-limestone, and magnesite facies. All are thought to have been formed in lacustrine and marginal-lacustrine environments developed above a widespread gravel on a pediment surface. The Thumb Member occurs from Frenchman Mountain to the eastern Virgin Mountains, appears to be as thick as 1300 m or more, and consists of clastic lithologies, ranging from siltstone to breccia, and laminated gypsum. Carbonate lithologies are rare. Fine-grained clastic constituents and gypsum are widespread, forming a lacustrine depositional facies, and coarse-grained clastic lithologies form alluvial lithofacies adjacent to faulted basin margins. The lake is thought to have periodically dried, and the alluvial fans are thought to have formed along high, steep margins. The Bitter Ridge Limestone Member is distributed between Frenchman Mountain and White Basin, is about 300 to 400 m thick, and at most locations consists of parallel-bedded limestone. Gypsiferous sandstone and conglomerate in Lovell Wash and sandstone, intra-formational breccia, and limestone near Lava Butte compose a clastic lithofacies. The stromatolitic limestone is thought to have originated in a lake with very shallow water, while the other facies are thought to be alluvial and to have resulted from syntectonic sedimentation at subaerial sites near lake margins. 12 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION The Lovell Wash Member occurs between Frenchman Mountain and White Basin, is about 450 m thick at maximum, and consists of limestone, dolomite, claystone, sedimentary tuff, tuffaceous sandstone, and arenaceous tuff. An isolated lithofacies of sandstone and siltstone occurs near Black Mesa north of Callville Bay, and a conglomerate lithofacies is present in Lovell Wash. A lacustrine environment is favored for most of the member. Tuffaceous rocks either were deposited directly into the lake by airfall or were transported in by fluvial processes. Clastic lithofacies are thought to have originated near active basin-margin faults as alluvial deposits. The red sandstone unit occurs in White Basin and east of Frenchman Mountain, is as thick as 500 m, and consists of sandstone, gypsiferous sandstone, tuff, and conglomerate. The rocks of the Grand Wash trough occur in the Grand Wash region, are thicker than 500 m, and consist of sandstone, gypsum, conglomerate, tuff, and limestone. The red sandstone unit is thought to be a playa-lake deposit in which sand accumulated by eolian activity, gypsum was deposited in the vadose zone of existing sediment, and conglomerate was shed off active basin-margin fault scarps. The rocks of the Grand Wash trough probably accumulated in a closed basin also. The Muddy Creek Formation is distributed throughout areas of low elevation in the local part of the Basin and Range province. Its thickness is unknown, and it consists of bedded siltstone, sandstone, gypsum, gypsiferous siltstone, and conglomerate near basin margins. It is thought to have formed during basin- range development in alluvial, fluvial, and lacustrine environments associated with valleys having internal drainage. The Tertiary sedimentary rocks have been deformed and fragmented by 65 km of left slip along the Lake Mead fault system, 40-60 km of right slip along the Las Vegas Valley shear zone, crustal extension of possibly 100 percent or more in a S. 70° W. direction, and stratal tilting associated with normal faulting. These structural elements are thought to have become active after early stages of sedimentation and to have interacted with one another during sedimentation. A palinspastically restored pre-Rainbow Gardens Member paleogeologic map indicates north-northeast trending, west-dipping thrusts of the Sevier belt west of a topographically featureless autochthon slightly deformed by a large north-trending arch cored by Precambrian crystalline rocks. Between 20 and 17 m.y. ago, the Rainbow Gardens Member was deposited in a broad, shallow sag with low-relief margins developed on the nose and northeastern flank of the arch between the thrusts and the present position of the Colorado Plateau. The Thumb Member was deposited at the same site in a deep fault-bounded trough between 17 and 13.5 m.y. ago, possibly as a result of synchronous initial activity on the Lake Mead fault system and the Las Vegas Valley shear zone. The Bitter Ridge Limestone and Lovell Wash Members occur only northwest of the Lake Mead fault system, and they record major activity on that system and the Las Vegas Valley shear zone about 13 m.y. ago. Crustal extension south of Lake Mead accompanied this activity. The red sandstone unit and the rocks of the Grand Wash trough formed in grabens and on the downthrown sides of tilted mountain blocks, which overprinted earlier structures and may represent early Basin and Range deformation between 12 and 10 m.y. ago. The Muddy Creek Formation filled Basin and Range valleys and overlapped most of the basin-forming structures. INTRODUCTION In the Lake Mead region, between Las Vegas, Nev., and the Grand Wash Cliffs in Arizona (fig. 1), are abundant, widespread exposures of nonmarine and probable nonmarine sedimentary rocks of Tertiary age. The oldest of these rocks were deposited upon Mesozoic and Paleozoic rocks which at that time were nearly undeformed except where directly affected by Cretaceous thrust faults. These oldest Tertiary rocks are now highly deformed. They are exposed in mountain ranges and valleys of the Basin and Range structural province, and apparently they predate the development of that province in the Lake Mead region. The youngest of the Tertiary sedimentary rocks, on the other hand, are confined to the valleys of the Basin and Range province, are relatively undeformed, and rest with angular unconformity on all of the older Tertiary and pre-Tertiary rocks. Tertiary deposition was synchronous with the formation of Basin and Range structure, and its style records the historical development of the province. Sedimentation evolved from an early pattern of regional accumulation in a broad downwarp, through a period of deposition in response to strike-slip and associated normal faulting, into final stages of sedimentation in localized Basin and Range valleys. The final stage of basin filling is still active in many parts of the province, but in most of the Lake Mead area, deposition has halted subsequent to downcutting of the Colorado River system, which has initiated a period of dissection and erosion. The complex Tertiary stratigraphy of the Lake Mead region had not been previously mapped or studied in detail, and its description and interpretation are the chief purposes of this report. Recent detailed geologic mapping in the Valley of Fire (Bohannon, 1977a), in Bitter Spring Valley and White Basin (Bohannon, 1977b), and in the Gale Hills (Bohannon, 1983) provides the basis for the description and the framework for the interpretation. Previously published stratigraphic nomenclature provides no adequate basis for regional mapping and correlation, and it is reevaluated and revised accordingly herein. The early nomenclature evolved over a period of decades, during which time many different geographically restricted stratigraphic names were introduced. There apparently was no understanding of the stratigraphic framework of the region. Ages of the Tertiary rocks were not well understood even in the light of limited K-Ar dating reported by Anderson and others (1972). In the absence of good chronologic and stratigraphic information, it was not possible for previous workers to relate Tertiary rocks regionally, and thus, the described Tertiary geologic history of this part of the Great Basin and the Colorado Plateau was equivocal. This report focuses on revisions and refinements of stratigraphic nomenclature, stratigraphic correlation, rock description, rock age, sedimentary history, and sedimentary and tectonic deposi-tional environments of all of the Tertiary rocks of the Lake Mead region. The evolution of this sedimentarySTRATIGRAPHIC NOMENCLATURE 3 leadow galley./ NEVADA UTAH ARIZONA Buffington Pockets 3 intain MEAD ■5000 25 KILOMETERS l°45' CONTOUR INTERVAL 1000 FEET Figure 1.—Location map of the Lake Mead region showing the general topography, major physiographic features referred to in the text, and the generalized distribution of Tertiary sedimentary rocks (shaded areas). sequence is examined in the light of its relation to the development of the Basin and Range province and its differentiation from the Colorado Plateaus province. ACKNOWLEDGMENTS The U.S. Geological Survey began studying this area in 1974 as part of a national appraisal of lithium resources, because concentrations of lithium within some of these rocks is abnormally high. Other geologists that contributed to this study in its early phases include J. D. Vine, R. K. Glanzman, and Elizabeth Brenner-Tourtelot. Special thanks are extended to R. E. Anderson, who continuously gave time and attention to the project. Fission-track dates were determined with the aid, instruction, and assistance of Chuck Naeser. National Park Service personnel, especially the rangers at Echo Bay, were extremely helpful and cooperative. Comments resulting from manuscript reviews by R. E. Anderson and T. D. Fouch greatly influenced the report. STRATIGRAPHIC NOMENCLATURE HISTORICAL DEVELOPMENT Two stratigraphic names of formational status, the Muddy Creek and Horse Spring, have been widely accepted in the region, although little attention has been paid to the age and stratigraphic position of the rocks for which the names are commonly used. Stock (1921) named the Muddy Creek Formation for outcrops of nearly undeformed strata in the bluffs of Meadow Valley north of Overton, Nev. Longwell (1921, 1922) named apparently older rocks exposed in a hogback near Horse Spring in the southern part of the Virgin4 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Mountains the Horse Spring Formation. The type locality of the Horse Spring was not precisely located, and much of the lithologic description was from Overton Ridge 40 km to the northwest (fig. 1). In the Virgin Mountain-Grand Wash Cliffs area, stratigraphic nomenclature further evolved with the introduction of three new names and the widespread application of the term Muddy Creek Formation. Longwell (1928, 1936) proposed the name Tassai Wash Group for rocks that unconformably overlie the Permian Kaibab Limestone at the north end of Wheeler Ridge. On lithologic grounds he considered these rocks to be of probable Tertiary age and a possible correlative to the Horse Spring Formation. Lucchitta (1966) tentatively supported that lithologic correlation. Moore (1972) named the Cottonwood Wash Formation for Tertiary rocks unconformably above Cretaceous(?) and Jurassic rocks in Cottonwood Wash. Both the Tassai Wash and Cottonwood Wash are exposed within 25 km of Horse Spring and are lithologically and stratigraph-ically similar to the Horse Spring Formation in its type locality. These new names appear to have been proposed as a result of a lack of understanding of regional stratigraphy. Slightly deformed to undeformed rocks that unconformably overlie the Horse Spring, the Tassai Wash, and the Cottonwood Wash in the Grand Wash region have been referred to as the Muddy Creek Formation (Longwell, 1928, 1936; Lucchitta, 1966, 1972; Blair, 1978). However, no similarity of detailed lithology, no age correlation, and no direct connection with the Muddy Creek type locality of Stock (1921) was demonstrated for these rocks by the authors. A thick limestone unit overlying the rocks called Muddy Creek in the area of Wheeler Ridge and the Grand Wash Cliffs was named the Hualapai Limestone (Longwell, 1936). Lucchitta (1972) and Blair (1978) consider the Hualapai to be a member of the Muddy Creek Formation, thereby redefining the upper contact of the Muddy Creek and reducing the stratigraphic rank of the Hualapai. These relations are summarized in column 4A of figure 2, except that the Tassai Wash Group and Cottonwood Wash Formation are not shown in the column owing to lack of space. These two units are equivalent to the lower half of the Horse Spring Formation. At Overton Ridge near the type locality of the Muddy Creek Formation, Longwell (1921, 1928, 1949) described two formations of Tertiary and possible Tertiary age older than the Muddy Creek—the Horse Spring Formation and the Overton Fanglomerate. Carbonate rocks (largely dolomite and magnesite) of the Horse Spring Formation were described as conformably overlying conglomerate of the Overton Fanglomerate. The Over-ton Fanglomerate was, in turn, described as unconformably overlying the Cretaceous Baseline Sandstone. However, as a result of detailed geologic mapping, Bohannon (1976, 1977) reduced the rank of the Overton Fanglomerate to the Overton Conglomerate Member of the Baseline Sandstone. Bohannon (1977a) mapped an unconformity, not previously recognized, beneath a thin conglomerate that is conformable with the overlying carbonate rocks that Longwell (1921, 1928, 1949) referred to as Horse Spring Formation. Bohannon (1976, 1977a) includes the thin conglomerate, which was formerly part of the Overton Fanglomerate, with the Horse Spring Formation. The relations described by Longwell (1949) are shown in column 3A, and those described by Bohannon (1976, 1977a) are shown in the lower left part of column SB, both on figure 2. South of the Muddy Mountains, in the Gale Hills, Bitter Spring Valley, and White Basin, widespread exposures of Tertiary rocks were originally described by Longwell (1921, 1928) as the Horse Spring Formation. Longwell and others (1965) later included rocks thought by them to be the Cretaceous Willow Tank Formation, the Cretaceous Baseline Sandstone, the Cretaceous and Tertiary(?) Overton Fanglomerate, and the Tertiary Horse Spring Formation together as the Cretaceous(?) or Tertiary(?) Gale Hills Formation. By means of detailed geologic mapping, I (Bohannon, 1977b, and unpublished mapping) differentiated the Willow Tank Formation, Baseline Sandstone, and Horse Spring Formation. I did not use the term Gale Hills Formation, and I subdivided the Horse Spring Formation into three informal members (Bohannon, 1977b). The terminology of Longwell and others (1965) as it applies to the rocks of White Basin and Bitter Spring Valley is shown in column 2A of figure 2. The scheme used by Bohannon (1977b) is not depicted there but is discussed further below. As part of a reconnaissance study of the geology along part of the Colorado River, Longwell (1963) named the Thumb Formation for exposures of predominantly clastic beds in the vicinity of Rainbow Gardens near Frenchman Mountain. He considered the Thumb Formation to be possibly temporally correlative with the Willow Tank Formation or Baseline Sandstone and on this basis assigned a possible age of Cretaceous or Tertiary to it. Light-colored carbonate beds stratigraph-ically above the Thumb Formation were correlated with the Horse Spring Formation, and widespread exposures of slightly deformed rocks that unconformably overlie both of these units were mapped as Muddy Creek Formation. The above scheme is diagrammed in column 1A of figure 2. For a regional analysis of strike-slip faulting, Bohannon (1979) subdivided many of the above rock units into informal members. These members were based on detailed stratigraphic and lithologic studies as well as ageSTRATIGRAPHIC NOMENCLATURE 5 2A 1A £ o si ■? a c c “ o (/> ra oj E £ o o Linz Aztec Sandstone through Moenkopi Formation East of Frenchman Mountain 2B White Basin and Bitter Spring Valley METERS I- 1000 L- 0 3B Muddy Creek Formation ~. r - r. --i-r.'-rr — c. r*. — r o o m~ rr.' o~ — —— Red sandstone unit o o £>a o7 *• « oof Horse Spring Formation Thumb Member ss\ i .V*? W.f i.*W :-.il Rainbow Gardens Member Overton Conglomerate Member, Baseline Sandstone Overton Ridge HA 4B Virgin Mountains to the Grand Wash Cliffs Sandstone Conglomerate Siltstone and shale Unaltered and slightly altered tuff EXPLANATION Altered tuff Gypsum Limestone Faulted section ■ a Dolomite; with magnesite Sandy limestone and intrafor-mational limestone breccia Unconformity 14.9 Isotope or fission-track age determination, in millions of years before present Figure 2—General lithology and stratigraphy, previous stratigraphic nomenclature (A column), and proposed stratigraphic nomenclature {B column) of the Tertiary rocks of the Lake Mead region. Ages of the rocks are generalized from tables 1 and 2. data, but do not provide a completely adequate regional nomenclature scheme. The problems inherent with the nomenclature pre- sented above are numerous. Historically, where terms such as Muddy Creek and Horse Spring have been used regionally, it has been without proper attention to6 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION stratigraphic details. This problem has been compounded by the large-scale displacement and distortion along the left-slip Lake Mead fault system and the right-slip Las Vegas Valley shear zone and in large areas of crustal extension, all of which were active during sedimentation. These large structures have subsequently fragmented once-continuous rocks, leaving them widely separated geographically (Anderson, 1971, 1973; Bohannon, 1979). Published stratigraphic nomenclature was proposed before this complex depositional and structural history was known, and in the absence of detailed stratigraphic information. Inconsistencies of this nomenclature are discussed below in the light of lithologic data presented in brief descriptions of four composite stratigraphic sections, and age interpretations derived primarily from tuffaceous beds within those sections (fig. 2). PROPOSED STRATIGRAPHIC NOMENCLATURE The system of stratigraphic nomenclature proposed in this report retains some of the most widely used geologic names, but reorganizes them slightly. Some names are abandoned, and three new formal names are proposed as a result of detailed geologic mapping. Formal nomenclature is used only where regional geologic, stratigraphic, and structural interpretations substantiate its use. Informal nomenclature is employed where regional relations are uncertain and stratigraphic continuity cannot be demonstrated. Because a basic understanding of stratigraphic and lithologic character of the units is necessary to make the new nomenclature meaningful, brief descriptions, which are keyed to figure 2, are provided below for the area east of Frenchman Mountain, the White Basin-Bitter Spring Valley area, Overton Ridge, and the Virgin Mountains-Grand Wash Cliffs area. More complete presentations of lithologic and age data follow in other sections of this report. East of Frenchman Mountain, in Rainbow Gardens, the type section of the Thumb Formation of Longwell (1963) contains two distinct mappable units totaling 1,200 m in thickness, and it unconformably overlies Mesozoic formations ranging from the Jurassic and Triassicf?) Aztec Sandstone through the Middle(?) and Lower Triassic Moenkopi Formation. The lower mappable unit includes 30 m of conglomerate overlain by interbedded red to pink sandstone and gray limestone under a resistant, poorly bedded, light-colored limestone. The lower unit is wedge shaped and varies from more than 300 m thick near Las Vegas Wash to less than 30 m thick northwest of Lava Butte. The upper mappable unit, which is almost 1,000 m thick, is primarily red and brown sandstone, but also contains gypsum, conglomerate, breccia, altered green sedimentary tuff beds, partially altered gray tuffaceous beds, and some lava flows. Radiometric ages suggest that the upper unit ranges from possibly older than 17.2 m.y. to possibly as young as 13 m.y. Overlying Longwell’s (1963) Thumb Formation, 700 m of carbonate rock, tuff, and red sandstone that he called Horse Spring Formation can be subdivided into three distinct mappable units. A resistant lower unit, nearly 200 m thick, is pale brown and buff, arenaceous, thick-bedded limestone and intraformational limestone breccia and conglomerate. Above that is a lithologically complex unit of white limestone, white and gray tuff, tuffaceous sandstone, gray and white clay beds, and brown chert. An age from interbedded volcanic rocks in the vicinity of the contact between the two units suggests that they are about 13 m.y. old. A thin unit of red sandstone with gray and white air-fall tuff beds is the highest mappable unit, although the nature of its basal contact is uncertain. Age interpretations from the red sandstone section indicate that it ranges from 11.2 to 10.6 m.y. old. Brown conglomerate, pink siltstone and sandstone, and white gypsum are conformably higher than all of the rocks described above. No dates have been determined for these rocks in the Frenchman Mountain area They have been mapped as the Muddy Creek Formation (Longwell, 1963; Longwell and others, 1965). In White Basin and Bitter Spring Valley, the Tertiary part of the Gale Hills Formation (Horse Spring Formation of Bohannon, 1977b) is similar in lithology and age to the combined Thumb and Horse Spring Formations (Longwell, 1963) east of Frenchman Mountain. At most localities in White Basin and Bitter Spring Valley, these Tertiary rocks unconformably overlie the Baseline Sandstone. In Echo Wash, though, they overlie the Aztec Sandstone. In Bitter Spring Valley, the Gale Hills, and Echo Wash, the lower part of the Tertiary section contains two mappable units like those of the Thumb Formation of Longwell (1963) at Frenchman Mountain. The lowest unit consists of a thin (10- to 50-m-thick), continuous basal conglomerate overlain by poorly bedded white gypsum. In Echo Wash this gypsum is, in turn, succeeded by a thick lenticular body of wavy-bedded yellow limestone, which is also included in the lowest unit. The upper mappable unit, which is about 1,100 m thick, consists of brown parallel-bedded sandstone, conglomerate and lenticular bodies, and beds of thinly laminated white gypsum. Continuous altered green tuff beds are common in both mappable units. Fission-track and K-Ar dates suggest that these units, whose total thickness is 1,300 to 1,400 m, range in age from 15.6 to 14.9 m.y. Bohannon (1977b) mapped these rocks as theSTRATIGRAPHIC NOMENCLATURE 7 lower member of the Horse Spring Formation and correlated them on a lithologic basis with the Thumb Formation east of Frenchman Mountain. In White Basin, the upper part of the Tertiary section consists of three different units, which are like those of the Horse Spring Formation of Longwell (1963) east of Frenchman Mountain. About 300 to 400 m of light-buff, pale-yellow, and tan, wavy- and parallel-bedded limestone exposed at Bitter Ridge makes up the lowest of the three. This lowest unit is overlain by a lithologically complex middle unit of white limestone, white and gray tuff and tuffaceous sandstone, gray and white clay beds, and brown chert. The youngest unit, 400 m thick, consists of red sandstone with abundant gray and white sedimentary tuff beds and sparse, discontinuous gypsum lenses. The three units total 1,200 m in thickness. One date from the middle unit suggests that it is about 13 m.y. old. Further age data suggest that the youngest unit may range from as old as 11.9 m.y. to as young as 11.0 m.y. Bohannon (1977b) abandoned the use of the term Gale Hills Formation, mapped the lowest unit of this sequence as the middle member of the Horse Spring Formation, and combined the two uppermost units as the upper member. At Overton Ridge, the Horse Spring Formation, as mapped by Bohannon (1976, 1977a), is in a stratigraphic position similar to that of the lowest mappable unit in the Thumb Formation of Longwell (1963) east of Frenchman Mountain and the lowest Tertiary unit in Bitter Spring Valley. Here a continuous basal conglomerate 50 m thick is overlain by pink dolomite and magnesite. These rocks, which unconformably overlie the Baseline Sandstone, are in turn overlain by 200 m of brown bedded siltstone and poorly bedded conglomerate. At one location, a thin unit of red sandstone with gray and white tuff beds overlies the latter beds and has an apparent age of 12.5 to 15.6 m.y. An unknown thickness of pink siltstone, pink sandstone, and brown conglomerate unconformably overlies all of the above rocks and constitutes the type locality of the Muddy Creek Formation (Stock, 1921). In the Virgin Mountains at Wechech Basin and at Horse Spring, the type Horse Spring Formation contains two mappable units, the upper of which is the same age as the upper unit in the Thumb Formation of Longwell (1963) in Rainbow Gardens. Bohannon (1979) described the lithologic similarities between basal units in the two areas. The similarities include the overall wedge shape of the two units and their stratigraphy, which consists, from base to top, of a basal conglomerate above Mesozoic rocks, red sandstone with calcite filled “tubelets,” and resistant, poorly bedded, “fragmental” limestone. The upper unit in the Virgin Mountains, which consists of 1,200 m of brown, parallel- bedded sandstone, white laminated gypsum, conglomerate, and altered, green tuff beds, is lithologically similar to the upper unit of the Thumb Formation at Rainbow Gardens and much of the lower member of the Horse Spring Formation of Bohannon (1977b) in Bitter Spring Valley. Age data from Wechech Basin suggest that the upper unit ranges from 16.3 to 15.9 m.y. old and that the two units correlate with those of the Thumb in Rainbow Gardens and the lower member of the Horse Spring in Bitter Spring Valley. However, no rocks known to occur in the type section of the Horse Spring Formation at Horse Spring or in Wechech Basin have the correct lithology, stratigraphic position, or age to correlate with the rocks mapped as Horse Spring Formation near Rainbow Gardens by Longwell (1963). At Cottonwood Wash, at Grand Wash, and between Wheeler Ridge and the Grand Wash Cliffs, a sequence of red sandstone, gray and white tuff beds, conglomerate, gypsum, and limestone, which is commonly called the Muddy Creek Formation (Longwell, 1928, 1936; Lucchitta, 1972, 1979; Blair, 1978), unconformably overlies beds thought to be equivalent to the lower unit described above in Wechech Basin. At the above-named places the red sandstone is lithologically similiar to the stratigraphically highest unit in White Basin and to rocks that occur unconformably beneath the Muddy Creek Formation east of Frenchman Mountain. The interpreted age range of 11.6 to 8.4 m.y. for this red sandstone unit suggests that it correlates temporally both with the latter units and possibly with the lower part of the Muddy Creek at its type locality. The unit commonly called the Hualapai Limestone is apparently part of the Muddy Creek Formation in Detrital Valley and is also gradational above the red sandstone described above in Grand Wash, but lies on Precambrian rocks between the two valleys. The nomenclature of the rocks of the Lake Mead region is redefined below. The Horse Spring Formation as herein redefined has its principal reference area, designated in this report, in Bitter Spring Valley and White Basin. This area contains the thickest, best exposed, and most lithologically complete section of these rocks known. As redefined herein, the Horse Spring Formation includes all the Tertiary rocks below the Muddy Creek Formation of Stock (1921), with the exception of the informal unit of red sandstone with gray and white tuff beds which occurs east of Frenchman Mountain and in White Basin. Although the principal reference area is remote from Horse Spring, the name is retained because of its time-honored acceptance and use. A principal reference section 2,100 m thick is defined between longitudes 114°36' and 114°37' W. and latitudes 36°15'45" and 36°19'30" N. (Muddy Peak 15-minute quadrangle).8 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION This part of Bitter Spring Valley, Bitter Ridge, and White Basin contains a complete north- to northwestdipping section, which includes the base and top of the Horse Spring Formation and representative strata of four mappable units herein designated as members. The base of the Horse Spring Formation is defined at the regionwide unconformity above Mesozoic formations, upper Paleozoic formations, and the Cretaceous and Tertiary!?) Baseline Sandstone. The top of the Horse Spring Formation is defined at the well-developed contact between the white limestone, white and gray tuf-faceous rocks and light-gray claystone of the uppermost Horse Spring, and the overlying unit of red sandstone (fig. 2). Type sections for the four formal members of the Horse Spring Formation have been chosen in areas where the best exposures and most representative lithologies of each member are present. These locations do not necessarily correspond to the location of the Horse Spring principal reference section defined above. The oldest member of the Horse Spring Formation is herein named the Rainbow Gardens Member for exposures in Rainbow Gardens. The type section, which is about 300 m thick, is designated at latitude 36°08' 30" N. and longitude 114°57'30" W. in the Henderson, Nev., and Frenchman Mountain, Nev., 7 Vi-minute quadrangles. This member includes the oldest mappable unit within Longwell’s (1963) Thumb Formation east of Frenchman Mountain, the oldest Tertiary mappable unit in the Bitter Spring Valley area, most of the Horse Spring at Overton Ridge, and the lowest unit in the Horse Spring Formation of Longwell (1921, 1922) in the Virgin Mountains area At its type section, the base of the Rainbow Gardens Member corresponds to the base of the Horse Spring Formation where it unconformably rests on the Moenkopi Formation. The type section dips to the southeast, and its top is defined at the top of the resistant limestone that forms a well-developed hogback throughout Rainbow Gardens. Complete location-by-location lithologic descriptions, detailed stratigraphic sections of the Rainbow Gardens Member, and more exact definitions of member boundaries can be found below in the section “Description of Rock Units.” The Thumb Member of the Horse Spring Formation is redefined here as the rocks stratigraphically above the Rainbow Gardens Member and below the carbonate rocks of the Bitter Ridge Limestone Member. The principal reference section of the Thumb Member, which is as much as 1,000 m thick, is defined in the Rainbow Gardens area at about 36° 07'30" N. latitude and 114°56'45" W. longitude in the Henderson, Nev., and Frenchman Mountain, Nev., 7Vi- minute quadrangles. Here, the Thumb rests conformably on the upper limestone of the Rainbow Gardens Member. This member includes the upper mappable unit of Longwell’s (1963) Thumb Formation east of Frenchman Mountain. Although the name Thumb Valley no longer appears on local topographic maps, the name “Thumb” is retained for the Thumb Member because of its time-honored acceptance and use. Complete location-by-location descriptions accompanied by measured stratigraphic sections can be found below in the section “Description of Rock Units.” The limestone that composes the hogback of Bitter Ridge between Bitter Spring Valley and White Basin is herein defined as the Bitter Ridge Limestone Member of the Horse Spring Formation.1 The base of the Bitter Ridge Limestone Member is defined at the top of the stratigraphically highest sandstone bed of the Thumb Member, and the top of the Bitter Ridge is defined at the top of the uppermost resistant limestone bed, which is overlain by less resistant gray and white tuffaceous beds. The type section, which is about 300-400 m thick, is at Bitter Ridge at latitude 36°18'30" N. and longitude 114°35'00" W. in the Muddy Peak, Nev., 15-minute quadrangle. In this area, the member dips to the north. Complete lithologic descriptions are included in a subsequent section of this report. The youngest member of the Horse Spring Formation is herein called the Lovell Wash Member and is named for rocks stratigraphically above the Bitter Ridge Limestone Member in Lovell Wash. The type section in Lovell Wash is defined at latitude 36°12'45" N. and longitude 114°42'30" W. in the Hoover Dam, Nev., 15-minute quadrangle, where about 300 m of the member is exposed in a syncline. The unit of red sandstone and siltstone that contains gray and white tuff beds and discontinuous gypsum lenses in White Basin and the area east of Frenchman Mountain is not included with the Horse Spring Formation, because it possibly is unconformable with the Horse Spring. There is also a lithologically and chronologically similar unit in Grand Wash and between Wheeler Ridge and the Grand Wash Cliffs. These two units apparently have distributions and sedimentary-tectonic histories different from those of the Horse Spring Formation. In the Grand Wash area, the red sandstone is commonly referred to as Muddy Creek Formation (Longwell, 1936; Lucchitta 1966, 1979), but is confined within local basins (Lucchitta, 1966) that are not continuous with the basin in which the principal reference section of the Muddy Creek was deposited. Furthermore the red sandstone in the Grand Wash area is older than the Muddy Creek Formation at its principal reference section. For the above reasons, 1 There are two ridges locally called Bitter Ridge. The other is in the Virgin Mountains and is not composed of Bitter Ridge Limestone Member.AGE OF THE TERTIARY ROCKS 9 these units are left informally designated as the red sandstone unit in White Basin and east of Frenchman Mountain, and as the rocks of the Grand Wash trough in the Grand Wash area, throughout this report. The Hualapai Limestone presents a special nomenclature problem because it is commonly called a formal member of the Muddy Creek Formation in recent literature (Blair, 1978; Blair and Armstrong, 1979; and Luc-chitta, 1972, 1979). Its member status is chiefly based upon the fact that limestone is interbedded with red sandstone at the base of the main body of Hualapai Limestone at Grapevine Mesa, and the red sandstone has traditionally been termed Muddy Creek. In this report, however, the red sandstone that is interbedded with the limestone of the Hualapai is considered to be part of the rocks of the Grand Wash trough rather than part of the Muddy Creek. On this basis, it is better to refer to the Hualapai as part of the rocks of the Grand Wash trough rather than as a member of the Muddy Creek. However, the Hualapai extends to the west of Grapevine Mesa beyond the western limits of the underlying red sandstone, where it rests directly on Precam-brian crystalline rocks. It ultimately extends into Detrital Valley, where it rests on rocks called Muddy Creek Formation. Because the stratigraphic relations of the Hualapai with the Muddy Creek in Detrital Valley are incompletely understood owing to a paucity of published maps and accurate descriptions there, and because the Hualapai interfingers with informally named rocks in the Grand Wash trough, it is left unassociated with either the rocks of the Grand Wash trough or the Muddy Creek; it is restored here to its original forma-tional rank (Longwell, 1936) and referred to simply as the Hualapai Limestone. The Muddy Creek Formation of Stock (1921) is slightly reevaluated in this report in that the use of that term is restricted somewhat. Only rocks that can be demonstrated to have been stratigraphically continuous with those of the Muddy Creek type locality are called Muddy Creek Formation. AGE OF THE TERTIARY ROCKS The precise age of the Tertiary nonmarine sedimentary rocks in the Lake Mead region has been poorly doc-mented largely because fossils are rare to nonexistent, and because some important unconformities were not detected. Although Anderson and others (1972) report 24 K-Ar determinations from igneous rocks associated with the nonmarine sedimentary rocks, the authors did not agree among themselves (Anderson and others, 1972, p. 283) that the age determinations were representative of the age of the sedimentary rocks. The disagreement focused upon whether the ages were determined from lava flows interbedded with the sedimentary rocks or from younger shallow intrusive rocks, and on whether or not the dates from demonstrated extrusive rocks were reset by thermal effects from nearby intrusive rocks. Several additional factors must be considered concerning the dates presented by Anderson and others (1972). The lower part of the section was never dated by them, even in Rainbow Gardens, and they still considered it to be possibly Cretaceous in age. Also, saline minerals present throughout the Horse Spring Formation suggest that the associated ground water was capable of altering volcanic rocks2 and of subsequently chemically adjusting the apparent K-Ar ages. In addition, the ages reported by Anderson and others (1972) were grouped and referred to in terms of the confusing system of stratigraphic nomenclature in existence at the time and they, therefore, must be reevaluated in the light of newly documented stratigraphic relations. Twenty-five fission-track age determinations were made on zircon extracted from basal tuff beds interpreted to be of air-fall origin interbedded with the other Tertiary sedimentary rocks. The fission-track dating method was chosen for several reasons: zircon is present, is not susceptible to alteration, and has a high resistance to track annealing upon heating. Also, fission-track dating of zircon, which is rich in uranium, works especially well in Tertiary rocks. The age-determination procedure follows that of Naeser (1976), and lab work was done in Naeser’s lab under his guidance. Two important assumptions must be made when evaluating fission-track age determinations as primary ages. It must be assumed that the dated rock has not endured a heating event later than its original formation, or that it has not undergone a prolonged cooling history. Also, it must be assumed that the dated grains are not detrital and have had the same thermal history as the encompassing rock. The inferred air-fall origin and the limited stratigraphic thickness of the dated tuffs suggest that they have not undergone a slow cooling history, and their mineralogy indicates that they have not been postdepositionally heated beyond the annealing temperature of zircon. Naeser (written communication, 1978) has demonstrated that the approximate annealing temperature of zircon through geologic time is about 175°-200°C. The presence of both heu-landite3 and analcime in the dated tuffs suggests that 2Many of the dated rocks are highly altered. 3Both clinoptilolite and heulandite are present in these tuffs. Ordinarily, when X-ray diffraction techniques are used, heulandite is masked by clinoptilolite. By means of heating experiments outlined by Mumpton (1960), Harry Starkey (written communication, 1978) demonstrated the presence of heulandite in the dated tuffs.10 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION they have not been subjected to temperatures above about 150°-175°C for long time periods (Coombs, 1971; p. 324, fig. 3). In most cases, the presence of glass or zeolite adhering to the dated grains after grinding and mineral separation attests to the nondetrital nature of the grains. An individual example (sample 40 from Overton Ridge) in which detrital contamination may present a serious problem is discussed separately below. The relevant K-Ar dates presented by Anderson and others (1972) are shown in table 1, the 19 new fission-track dates are given in table 2, and the supporting data are summarized in table 3. The most important dated samples are located on three regional diagrams of the Muddy Mountains area, the Virgin Mountains-Grand Wash Chffs area, and the Frenchman Mountain area (fig. 3). East of Frenchman Mountain, six fission-track and K-Ar samples date the Thumb Member of the Horse Spring Formation. The Rainbow Gardens Member has not been dated owing to a paucity of datable rocks. Fission-track samples 20, 21, 22, and 34 date the Thumb Member and suggest a range from 16.2 to 13.2 m.y. Sample 34 is the youngest of the samples from the Thumb Member at 13.2 m.y.; it is probably anomalously young because its age overlaps with those of stratigraphically higher samples. Two samples from the Thumb Member have K-Ar ages reported by Anderson and others (1972) that appear to fall within the above fission-track range (samples 12 and 13; 17.2±3 and 15.6±3 m.y. respectively). These samples are from unaltered core rock of a pyroxene-ohvine andesite that can be demonstrated to be a lava flow interstratified with the sedimentary rocks (R. E. Anderson, written commun., 1978). Algally laminated carbonate incrustations and domes deposited on the upper surface of this andesite testify to its origin as a lava flow. Several of the K-Ar samples reported by Anderson and others (1972) from the Thumb are much younger than the above age range (samples 7, 8, and 9). Although these samples are possibly from extrusive igneous rocks interstratified with the Thumb Member, later chemical analyses indicate that the sampled rocks are altered and contain as much as 8 percent K20. This excess potassium explains their anomalously young K-Ar ages; these samples should be disregarded (R. E. Anderson, written communication, 1978). Table 1 .—Selected previously published K-Ar ages for Tertiary rock samples from the Lake Mead region, Nevada and Arizona [Ages are taken from Anderson and others (1972, table 1) except as noted. Data have not been recalculated to new constants] Sample No. This Orig.1 2 3 Rock type North West rept. latitude longitude 1 1 Ash-flow tuff 36°38'20" 114°31'30‘ 2 2 do 36°38'20" 114°31'40' 3 3 do 236°30' 114°36' 4 4 Tuff? 36°13' 114°48' 5 5 do 36°14' 114°45'30' 6 6 Basaltic lahar 36°09' 114°46' 7 7 Mafic alkalic igneous rock. 36°06'20" 114°57'22' 8 8 Biotite-homblende rhyodacite. 36°07'50" 114°56'25' 9 9 do 36°07'50" 114°56'25‘ 10 10 Mafic alkalic igneous rock. 36°07'15" 114°58'35' 11 11 do 36°07T5" 114°58'35' 12 12 Pyroxeneolivene andesite lava. 36°07'27" 114°56'38' 13 13 do 36°07'27" 114°56'38' 14 15 Basalt lava 35°56'39" 114°39'25' 15 21 do 35°49'40" 114°38'20' 16 22 do 36°10T0" 114°41'20' 317 23 do 36°02'45" 114°39'35' 18 24 do 36°09'55" 114°46' 19 (4) do 36°02'48' 114°39'36' General locality Rock unit Northern Muddy Mts., near Glendale. SE. of Muddy Mts.------- Possibly Hiko, Racer Canyon or Harmony Hills Tuff. Thumb Member---------------- SW. of Muddy Mts.— do TT .L • Rainbow Gardens, of Red Needle. NE. Igneous rocks of Lava Butte Rainbow Gardens Thumb Member Rainbow Gardens, of Red Needle. SE. do SE. of Hoover Dam— Malpais Flattop Mesa rnllvillo Wn«h Fortification Basalt Member, Muddy Creek Formation. Fortification Hill ‘Sample numbers shown by Anderson and others (1972, table 1). 2Probably an incorrect coordinate, as Anderson and others (1972, table 1), describe this location as "near Bitter Spring" and show it at about 36°15' N. on their map. 3Now superseded by sample no. 19. 4Unnumbered sample horn Damon and others (1978). K-Ar age (m.y.) 21.3 ±0.4 19.6 ±0.8 15.3 ±0.7 14.9 ±0.5 15.1 ±0.5 13.2 ±0.5 10.9 ±1.1 13.4 ±0.7 11.7 ±2.0 11.8 ±1.0 11.8 ±0.7 17.2 ±3 15.6 ±3 4.9 ±0.4 5.8 ±1.0 11.3 ±0.3 10.6 ±1.1 11.1 ±0.5 5.88±0.18AGE OF THE TERTIARY ROCKS 11 Table 2.—Zircon fission-track ages of samples from Tertiary rocks of the Lake Mead region, Nevada and Arizona [Analytical data are given in table 3) Sample Rock type North No. latitude 20 Green air-fall tuff 36°11'10' 21 Silver-gray air-fall tuff 36°11'10 22 do 36°10'47' 23 Green air-fall tuff 36°17'27' 24 do 36°18T5' 25 White air-fall tuff 36°19'34' 26 Silver-gray-air-fall tuff 36°19'34 27 do 36°19'55' 28 Gray air-fall tuff 36°12'37' 29 Silver-gray air-fall tuff 36°20'45' 30 do 36°30'55 31 Ash-flow tuff 36°39'10 32 Silver-white air-fall tuff. 36°08'25‘ 33 White air-fall tuff 36°09'20 34 Green air-fall tuff 36°08'20 35 do 36°29'13' 36 do 36°29'25' 37 White air-fall tuff 36°20'20 38 Green air-fall tuff 36°17'45 39 Silver-gray air-fall tuff 36°30'55' 40 do 36°30'55‘ 41 Gray air-fall tuff 36°07'37‘ 42 Gray air-fall tuff 36 ° 07'37 ‘ 43 do 36°07'37' West Locality description longitude 114°54'45" 5 km NE. of Lava Butte — 114°55'00" do 114°54'15" do 114°34'05" Bitter Spring Valley 114°36'12" do 114°38T0" White Basin 114°38T0" do 114°39'05" do 114°42'15" Lovell Wash 114°40'28" White Basin 114°28'55" Overton Ridge 114°31'52" Northern Muddy Mtns. near Glendale. 114°55'10" SE. of Lava Butte 114°54'15" East of Lava Butte 114°56'30" 1.5 km SW. of Lava Butte. 114°09'40" Wechech Basin 114°09'14" do 114°07'58" Horse Spring area 114°29'45" Echo Wash 114°28'55" Overton Ridge 114°28'55" do 114°01'35" Pierce Ferry area 114°01'35" do 114°01'35" do Fission- Stratigraphic unit track age (m.y.) Thumb Member--------------------------- 14.8±1.4 ----------------do-------------- 16.1 ±1.5 ----------------do-------------------- 16.2±0.8 ----------------do-------------------- 15.4±0.8 ----------------do-------------------- 15.6±1.0 Red sandstone unit-------------- 11.9±0.9 ----------------do-------------- 11.7 ± 1.3 ----------------do-------------- 11.2 ± 1.1 Lovell Wash Member--------------------- 13.0±0.8 Red sandstone unit-------------- 11.0±0.9 ----------------do-------------------- 15.6±0.9 Unknown (possibly equivalent to 20.7 ±1.2 Hiko, Racer Canyon, or Harmony Hills Tuff). Red sandstone unit-------------------- 10.6±0.9 ----------------do-------------- 11.2± 1.2 Thumb Member-------------------------- 13.2±0.9 ----------------do-------------- 15.9 ±1.0 ----------------do-------------- 16.3 ±1.9 Rainbow Gardens Member.--------- 15.1 ±0.8 Thumb Member-------------------------- 15.3±2.0 ----------------do-------------------- 15.0±0.8 ----------------do-------------------- 12.5±0.9 Rocks of Grand Wash trough 10.8±0.8 (below level of Hualapai Ls.). Rocks of Grand Wash (several 11.6±1.2 meters below sample 41). Rocks of Grand Wash trough 11.1 ± 1.3 (same level as sample 41). Five fission-track ages and three K-Ar age determinations from tuffs of an interpreted air-fall origin from the Thumb Member south of the Muddy Mountains and in the Virgin Mountains fall in the range of ages derived from that member near Frenchman Mountain. One fission-track sample4 from the Rainbow Gardens Member near Horse Spring gave an age of 15.1 ±0.8 m.y., but this seems young because nearby samples from the overlying Thumb are slightly older. These older samples (samples 35 and 36) indicate that the Thumb Member in Wechech Basin is about 15.9 to 16.3 m.y old. A similar age range of 15.6-13.3 m.y. is indicated for the Thumb Member by fission-track dates of samples from Bitter Spring Valley and Echo Wash south of the Muddy Mountains (samples 23, 24, and 38). Anderson and others (1972) report three K-Ar ages for samples from tuffs within the Thumb Member in the Gale Hills and in Bitter Spring Valley. These samples (samples 3, 4, and 5) are from rocks grouped by Ander- 4Sample 37, obtained from and located by Richard Glanzman (written commun., 1978); dated by the author. son and others (1972) as Horse Spring Formation, range from 15.3 to 14.9 m.y., and have ages concordant with other ages from the Thumb Member of that formation. The Thumb Member exposed at Overton Ridge is dated by samples 30, 39, and 40 and ranges in age between 15.6 and 12.5 m.y. Samples 30 and 39 are concordant, whereas sample 40 is discordant. The dated tuffs are separated by no more than a few meters strati-graphically. There is no apparent reason for the age discrepancy, but it is possible that detrital zircons are present in the older samples. This explanation, however, is not considered likely as the two older determinations are the concordant ones. The ages of the Bitter Ridge Limestone and Lovell Wash Members of the Horse Spring Formation are interpreted from two K-Ar dates, from one fission-track date, and from abundant bracketing dates from strati-graphically higher and lower units. The two K-Ar determinations, 13.4±0.7 and 11.7±2.0 m.y. (samples 8 and 9; Anderson and others, 1972), are both for the rhyodacite of Lava Butte, which occurs stratigraph-ically between the Thumb Member and the base of the12 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Table 3.—Analytical data for zircon fission-track age determinations shown in table 2 [Samples analyzed using the external detector method described by Naeser (1976). Determinations made in the laboratory of C. W. Naeser. Decay constant for spontaneous fission, Xj=7.03X10 ^yr total decay constant for 2®®U, X(j=1.55IX10 ^yr atomic ratio 2®®U/2®®U, 1=7.252X10 thermal neutron fission cross section for 2®®U, a=580X 10^ cm2] Correlation Sample No. Field No. Lab No. Fossil-track density1 (Xg) (10® tracks/cm2) Induced-track density1 (Xj) (10® tracks/cm2) Neutron flux (0) |1015 n/cm2) Number of grains coefficient of total counts (r) u content (ppm) 20 1-121-1 2,044 1.15 (211) 4.69 (862) 1.01 8 0.811 134 21 1-121-2 2,045 1.03 (224) 3.87 (842) 1.01 8 .792 110 22 1-121-3 2,046 2.87 (450) 10.7 (1,682) 1.01 6 .981 305 23 1-67-270 2,047 2.73 (714) 10.7 (2,798) 1.01 10 .900 305 24 1-67-271 2,048 1.80 (253) 6.89 (968) 1.00 6 .958 198 25 1-67-272A 2,049 3.38 (370) 17.0 (1,856) 1.00 6 .802 490 26 1-67-272B 2,050 .938 (181) 4.79 (924) 1.00 8 .740 138 27 1-67-273 2,051 1.04 (205) 5.55 (1,094) .998 8 .808 198 28 1-96-174 2,052 2.20 (381) 10.3 (1,748) .996 8 .938 298 29 2-131-73 2,053 1.80 (321) 9.70 (1,728) .994 8 .775 281 30 1-28-63 2,054 1.84 (375) 6.90 (1,278) .992 6 .979 203 31 174-94-1 2,055 2.69 (239) 7.68 (894) .990 6 .918 223 32 1-13-2 2,056 .701 (140) 3.90 (780) .988 7 .980 114 33 1-13-1 2,057 .821 (133) 4.33 (702) .986 6 .844 126 34 1-13-3 2,058 2.41 (391) 10.8 (1,746) .984 8 .834 316 35 1-23-1 2,059 1.98 (321) 7.31 (1,184) .982 6 .922 214 36 1-23-2 2,060 2.83 (595) 10.2 (2,138) .980 8 .118 300 37 GP-5 2,061 2.50 (536) 9.64 (2,070) .978 8 .947 284 38 1-69-165 2,062 .896 (121) 3.41 (460) .976 6 .718 101 39 1-28-X1 3,171 3.28 (633) 13.75 (2,656) 1.05 8 .961 377 40 1-28-X2 3,172 1.51 (331) 7.62 (1,666) 1.05 8 .894 209 41 2-80-la 3,173 1.55 (248) 9.03 (1,658) 1.05 6 .943 248 42 2-80-lb 3,174 .91 (232) 4.96 (1,260) 1.05 8 .698 136 43 2-80-2 3,175 1.03 (215) 5.85 (1,216) 1.05 8 .577 160 ‘Numbers in parentheses show total number of tracks counted in each sample. Bitter Ridge Limestone Member. The rhyodacite has some features that indicate it is intrusive and others that suggest it is extrusive, and many of its contacts are fault-bounded. Nonetheless, a conglomerate composed of clasts of rhyodacite similar to that of Lava Butte lies stratigraphically within the Bitter Ridge Limestone Member near Lava Butte. This conglomerate probably indicates either the rhyodacite of Lava Butte was subaerially exposed, eroded, and transported into the Bitter Ridge Limestone or that it domed through and erupted into that member. It is inferred then that the age of the rhyodacite of Lava Butte dates either the lower beds of the Bitter Ridge Limestone Member or the contact between that member and the Thumb Member. A fission-track age of 13.0±0.8 m.y. was obtained from the Lovell Wash Member in Lovell Wash. Unwelded ash-flow tuffs and overlying carbonate rocks are exposed near Glendale, Nev. Two K-Ar ages from the volcanic rocks have been reported by Anderson and others (1972), one K-Ar age has been reported by Shafiqullah and others (1980), and one fission-track age was derived in the present study. The K-Ar ages of 21.3±0.4 and 19.6±0.8 m.y. reported by Anderson and others (1972, samples 1 and 2), the date of 20.0±0.8 m.y. reported by Shafiqullah and others (1980, sample 91, p. 252), and the fission-track age of 20.7 ± 1.2 m.y. (sample 31) are concordant. These rocks have traditionally been referred to as Horse Spring Formation (Anderson and others, 1972; Longwell, 1921, 1949; and Longwell and others, 1965), but they are older than the rest of the Horse Spring Formation, and no other ash-flow tuffs are known from that formation. Thus, their age range should not be used to define the age of deposition of the Horse Spring. Ekren and others (1977) describe several synchronous or nearly synchronous ash-flow deposits from Lincoln County, Nev., including the Hiko, Racer Canyon, and Harmony Hills Tuffs, that may temporally correlate with the ash flows near Glendale. Additional geologic mapping and stratigraphic studies may clarify the relation between these ash flows and the Horse Spring Formation. Available data indicate that the Horse Spring Formation is Miocene in age and ranges from older than 17.2 m.y. to possibly 11.9 m.y. old. Although undated at Figure 3 (facing page).—Tertiary geology and important sample sites in the Muddy Mountains area (A), Virgin Mountains-Grand Wash Cliffs area (J3), and Frenchman Mountain area (C).AGE OF THE TERTIARY ROCKS 13 114°30' 00' 06' 40 •39 EXPLANATION Tertiary rocks: Volcanic rocks of Lava Butte Hualpai Limestone Muddy Creek Formation Undifferentiated Fortification Basalt Member Red sandstone unit Horse Spring Formation Lovell Wash Member Bitter Ridge Limestone Member Thumb Member Rainbow Gardens Member Mesozoic rocks Mesozoic and Paleozoic rocks, undifferentiated Paleozoic rocks - Contact—Dashed where approximately located - Fault—Dashed where approximately located; dotted where concealed Strike and dip of bedding plane Locality of date sample (tables 1 and 2)14 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION the present time, the Rainbow Gardens Member is not considered to be much older than about 20 m.y. The overlying Thumb Member appears to range from about 17.2 m.y. to possibly 13.5 m.y. old. The Bitter Ridge Limestone Member is apparently as old as 13.4 m.y. or slightly older, and the Lovell Wash Member is probably no younger than 11.9 m.y., which is the oldest age determined for the overlying red sandstone unit. The age of the red sandstone unit that overlies the Horse Spring Formation is indicated by fission-track ages of two samples from near Frenchman Mountain and four samples from White Basin. Four fission-track age determinations provide the basis for the age interpretation of the rocks of the Grand Wash trough, which underlie the Hualapai Limestone between Wheeler Ridge and the Grand Wash Cliffs. Two K-Ar determinations reported by Blair (1978) date the Hualapai Limestone in Detrital Valley. Near Frenchman Mountain, samples 32 and 33 from the red sandstone unit are 10.6 and 11.2 m.y. old, respectively, whereas samples 25, 26, 27, and 29 from White Basin indicate that this unit ranges from 11.9 to 11.0 m.y. old. West of the Grand Wash Cliffs, the stratigraphically and structurally lowest dated beds of the rocks of the Grand Wash trough range from 10.8 to 11.6 m.y. old (samples 41, 42, 43, and 44). Blair (1978) reports a K-Ar age of 10.9± 1.1 m.y. from a basalt “intercalated with the * * * lower part of the Muddy Creek Formation.” The basalt of the Muddy Creek Formation sampled by Blair (1978) is in Detrital Valley. Blair (1978) also reports an 8.44 ±2.2 m.y. age from a tuff within the Hualapai Limestone. A basalt flow that overlies the rocks of the Grand Wash trough has been dated by Damon and others (1978) at 3.80±0.11 m.y. and 3.79±0.46 m.y. old. It appears that in the Muddy Mountain-Frenchman Mountain area the red sandstone unit is Miocene in age with a possible range from about 11.9 to 10.6 m.y or younger. In the Grand Wash Cliffs area, the rocks of the Grand Wash trough and the Hualapai Limestone are, respectively, older than 11.6 m.y. and possibly as young as about 4 m.y. Although the age range of the Muddy Creek Formation has not been determined in the Lake Mead region, Anderson and others (1972) and Damon and others (1978) report several K-Ar ages from the Fortification Basalt, which they refer to as a member of the Muddy Creek Formation. The Fortification Member apparently was extruded into the basin of the Muddy Creek Formation during the late stages of Muddy Creek deposition. The ages derived for the Fortification Basalt Member are summarized in table 1. The age I consider the most reliable for the Fortification is that reported by Damon and others (1978) of 5.88±0.18 m.y., because it was derived using new techniques limiting the effects of abundant CaC03 in the dated sample. Also, the latter date is concordant with the earlier ages for Fortification samples from south of Lake Mead, where not as much CaC03 is present in the basalt. Eberly and Stanley (1978, sample 124, p. 928) report an age of 8 m.y. derived from a basalt flow interstratified with the Muddy Creek Formation near the Overton Arm of Lake Mead. No age range is given for this sample, and examination of the outcrop from which it was taken indicates zeolitization and alteration of plagioclase. Although the Muddy Creek Formation is the most poorly dated of the Tertiary rocks, it appears to be no older than about 10.6 m.y. near Frenchman Mountain, where it overlies the dated red sandstone unit, and at Fortification Hill it may be no younger than about 6 m.y. DESCRIPTION OF ROCK UNITS Throughout the descriptive part of this report, terms will be used that might have ambiguous meanings. Thus, definitions of potentially ambiguous terms are included in this section. Bedding terminology used is that of Reineck and Singh (1975, figs. 135 and 136, p. 82 and 83), which is modified from Campbell (1967), and from McKee and Weir (1953). Using this terminology, bedding relations are described as parallel or nonparallel and as continuous or discontinuous. The terms even, wavy, and curved are used to describe surfaces of beds. Beds of equal thickness are referred to as uniform beds. Terms such as tabular, lenticular, and wedge shaped are self-explanatory; they describe the shapes of individual beds, bedsets (see below), or mappable units. Bed thickness is commonly given in metric units. However, in some instances bedding is simply described as thin or thick following the definitions of those terms in Reineck and Singh (1975, fig. 136, p. 83). The term bedset is used in this report to describe groups of two or more beds of the same character. Bedsets can be simple (one lithology) or composite (more than one lithology). On a larger scale, formations and members are subdivided using the terms unit and facies. A unit is a group of beds and (or) bedsets lithologically distinct from other units. The term facies is simply used to describe the aspect or character of the sediment within beds of one and the same age (Pettijohn, 1975). Two types of facies are discussed: lithofacies (such as carbonate-sandstone) and depositional facies (such as lacustrine).DESCRIPTION OF ROCK UNITS 15 HORSE SPRING FORMATION RAINBOW GARDENS MEMBER DISTRIBUTION AND THICKNESS The thickness of the Rainbow Gardens Member varies considerably, as is illustrated by figure 4, which shows the regional distribution of 11 surface stratigraphic sections. In Rainbow Gardens (sec. A, fig. 4), the Rainbow Gardens Member is wedge shaped. Southwest of section A, along strike, the member is thicker than 300 m, but northeast of section A the upper parts of the member undergo an abrupt lithofacies change into lithologies of the Thumb Member. Consequently, the Rainbow Gardens Member thins to less than 50 m in this direction. A similar northward thinning occurs in the Virgin Mountains, where the member is also wedge shaped. Here sections K and F (fig. 4) indicate that the member is thicker than 300 m in their vicinity, but it thins to less than 100 m at section D (fig. 4). The Rainbow Gardens Member is thinnest in the Gale Hills and in Bitter Spring Valley, where it is only about 60 m thick (sec. G, fig. 4). In its easternmost exposures near the Grand Wash Cliffs (secs. E and J), the member may be thicker than 275 m, but because it is unconformably overlain by younger deposits, its original thickness cannot be measured. In general, the thickest section of the Rainbow Gardens Member is about 400 m thick and the thinnest about 50 m thick. The regional distribution and palinspastic restoration of these thickness trends are discussed below. LITHOLOGY The lithologically varied Rainbow Gardens Member includes clastic sedimentary rocks ranging in grain size from conglomerate to claystone, several types of carbonate rocks, evaporite beds, and chert. Conglomerate occurs at the base of the member at every known exposure, and, in general, carbonate lithologies dominate its upper parts. The greatest diversification is in the middle and lower middle parts of the member, where sandstone, siltstone, claystone, limestone, dolomite, gypsum, chert, and conglomerate are all interbedded. Clastic and Siliceous Lithologies The resistant, blocky-weathering gray, gray-brown, and red-brown basal conglomerate varies considerably in thickness from about 50 m to as little as 1 or 2 m. It has a well-defined, scoured basal contact where it uncon- formably overlies Mesozoic and upper Paleozoic formations. The upper contact of this basal unit is also well defined and is planar at most locations. However, channel-form beds are exposed in stratigraphic sections B (Echo Wash) and C (Overton Ridge). In these channels, which are at least 3 m deep, there is limited interfingering of conglomerate with the overlying rocks. Most clasts in the basal conglomerate are of gray cherty limestone similar in lithology to rocks of the Kaibab and Toroweap Formations of Permian age, and red and red-brown sandstone that resembles sandstone of Mesozoic age. Other clasts include gray limestone and brown quartzite similar in lithology to other local Paleozoic rocks. In southernmost Rainbow Gardens, granitic and gneissic clasts occur. Limestone clasts, which range in relative abundance from 60 to 90 percent, are the dominant clast type, whereas the percentage of sandstone clasts ranges from 10 to 40. The limestone clasts are commonly subangular, and the sandstone clasts are subrounded to round. Clast sorting is moderate. Clasts range in diameter from as little as a few centimeters to as much as 1 m. Clast-supported texture is present. The matrix makes up about 20 percent of the rock and is chiefly gray crystalline limestone or arenaceous limestone, though a red-brown calcareous sandstone occurs locally. The matrix imparts a hard, well-indurated aspect to the rock and produces blocky, resistant outcrops. Bedding character varies within the basal conglomerate. Locally the conglomerate is unbedded; but more commonly the bedding is poorly defined, is both even and curved, and is discontinuous. In some outcrops, the bedding appears parallel from a distance but is lenticular in detail. In many cases it is defined only by clast trains and clast size changes. Internal low-angle trough crossbeds occur at many locations and range in amplitude from a few centimeters to 1 m. Conglomeratic beds occur in the middle and lower middle parts of the exposed sections at Rainbow Gardens, the Echo Hills, Wechech Basin, and Cottonwood Wash (secs. A, /, D, and E, respectively, fig. 4). In general, these beds consist of thin, discontinuous zones of conglomerate in sandstone or of short, stubby lenses at the base of sandstone beds. The clasts are small, rarely exceeding a few centimeters in diameter, and they are composed chiefly of subangular to subround resistant limestone and cherty limestone that resembles limestone of the Kaibab and Toroweap Formations. In Wechech Basin, some of the clasts are well-rounded, hard, small (less than 10 cm in diameter), pieces of quartzite that closely resemble clasts of the locally exposed Upper Triassic Shinarump Member of the Chinle Formation.16 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION wmmA mnmmuL iTil I n ii i i VC C M F VF U N M R VR Figure 4.—Measured stratigraphic sections of the Rainbow Gardens member of the Horse Spring Formation with inset showing geographic distribution of outcrops. The Member overlies Mesozoic and upper Paleozoic beds unconformably at all sections, and it is overlain conformably by the Thumb Member at all sections except E, H, and J. The left side of each column depicts bulk grain size: VC, very coarse; C,DESCRIPTION OF ROCK UNITS 17 u****0" r~* r L.^i i, i ±---r I— • •=C ~J=Z T/.T/.T •. . ■ 1 7— • — • -; — ■1 --T7 ~:.r r.:: VC C M F VF N M RVR coarse; M, medium; F, fine; V, very fine. The right side of each column depicts relative resistance to weathering: N, nonresistant; M, moderately resistant; R, resistant; VR, very resistant.18 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Red, red-brown, and brown sandstone with grain-supported texture is a major component of the middle and lower middle parts of the stratigraphic sections at Rainbow Gardens, Wechech Basin, Cottonwood Wash, the northern Black Mountains, Echo Hills, and Pigeon Wash (secs. A, D, E, H, I, and J respectively, fig. 4). The sand grains are composed chiefly of medium-grained, rounded quartz, but in rare cases coarse-grained sandstone, grit,5 and siltstone are interbedded with the medium-grained sandstone. The coarser grained types are composed of subangular grains of carbonate rocks. Silt- and clay-size matrix is rare to absent. Sand grains are cemented by calcareous cement. Bedding is parallel, wavy, and continuous overall, but some beds have small-scale internal cross-laminations. It is apparent that Mesozoic clastic rocks, especially the Aztec Sandstone, were the source for the sandstone beds because of the similarity in sand grains, but the direction of transport is uncertain. Red-brown and brown siltstone is present within the sandstone, and within gypsiferous sandstone (mentioned below), but it is minor in volume. Its color and its association with the sandstone suggests that it too may have been locally derived from Mesozoic clastic rocks. White and light-gray, nonresistant, calcareous clay-stone beds are abundant in the Lime Ridge and Cottonwood Wash sections and are present in the section at Horse Spring (secs. K, E, and F repectively, fig. 4). Similar beds also occur at Rainbow Gardens (sec. A, fig. 4). These claystone beds are in the middle and lower middle parts of the Rainbow Gardens Member and are associated with clayey carbonate rocks and tuffaceous rocks. The claystone beds are as thick as 1 m and are continuous over lateral distances as great as 1 km, but internal bedding, laminations, or structures were not observed owing to insufficient exposure. X-ray data suggest that the clay includes species with 0.7-, 1.0-, and 1.5-nm peaks (R. K. Glanzman, written communication, 1979). These claystone beds are inferred to have formed as the byproduct of alteration of tuffaceous minerals. This inference is based on relict texture in which clay occurs mixed with micrite in the shape of pumice lapilli and fine-grained pumice fragments. Dark-green, black, and white chert is interbedded with limestone, claystone, and tuff. This chert occurs in thin, continuous beds that have well-defined bases and tops; in small, irregular lenses; and in lenticular beds a few centimeters thick. The continuous chert beds are generally black or green and are interbedded with claystone and carbonate beds, whereas the lighter-colored chert is more lenticular and occurs in carbonate beds. 5Super coarse grained sandstone or very fine grained conglomerate. Chert is most common in the middle of the member in the southern Virgin Mountains, but it also occurs in sparse amounts in the upper carbonate unit of the northern Virgin Mountains. Carbonate and Sulfate Lithologies White, light-buff, light-gray, and brown limestone and dolomite, and arenaceous limestone and dolomite are the dominant lithologies above the basal conglomerate in the Rainbow Gardens Member. Eight of the eleven stratigraphic sections have a thick unit of resistant carbonate rocks, commonly limestone, in their upper part. Most of the middle parts of the stratigraphic sections contain many limestone and dolomite beds that are interbedded with other rock types. The thick, resistant upper limestone units (fig. 5) form hogbacks and consist of 25-cm- to 1-m-thick uneven, wavy beds that are parallel to one another and relatively continuous. Bedding is defined primarily by erosional differences and is obvious from a distance but is commonly difficult to detect on close inspection. Finegrained limestone with wavy internal laminations (evident in hand specimen) is common. However, many other bedsets and beds have a tufa-like appearance: they are porous, do not appear to be internally bedded or laminated, and have irregular and indistinct bedding contacts. In addition to these tufa-like beds, intraforma-tional limestone breccia occurs in the lower parts of some resistant limestone units. Breccia clasts are about 5 cm in diameter and are similar in lithology both to the matrix that surrounds them and to other nearby limestone beds. All the limestone probably contains siliceous detritus, and some beds contain enough sand to be called arenaceous limestone, but sand content is gradational. The arenaceous detritus is medium-grained quartz sand that is moderately well rounded. In the middle parts of the studied stratigraphic sections, many individual limestone, dolomite, and arenaceous limestone beds range in thickness from a few centimeters to a meter. These beds are continuous, have well-defined bedding surfaces, are a little more resistant than the surrounding rocks, and are composed of medium- to fine-grained crystalline carbonate, which may represent some degree of recrystallization. In hand specimen, many exhibit wavy internal laminations. Dolomite is more prevalent in the fine grained beds (R. K. Glanzman, written communication, 1979). There is a gradation from relatively pure limestone beds into calcareous sandstone beds, and all the beds probably have some siliceous clastic material in them. At Overton Ridge, the Rainbow Gardens Member contains bright white, extremely fine grained sedimen-description of rock units 19 Figure 5 — Resistant limestone unit in the upper part of the Rainbow Gardens Member near Rainbow Gardens. View to the north-northeast. tary magnesite and dolomite (fig. 6). Bedding is parallel, even, continuous, and fairly uniform, ranging in thickness from a few centimeters to 40 cm. This magnesitebearing unit is underlain by pink limestone and dolomite that is calcite rich at its base and dolomite rich at its top. Above the limestone and dolomite, at the base of the magnesite-bearing unit, dolomite is the dominant mineral and calcite is absent. Dolomite and magnesite vary in proportion to each other throughout the magnesite-bearing unit. Trioctohedral smectite, rich in lithium (probably hectorite), is a common associate of the magnesite and dolomite and makes up as much as 30 percent of the rock. A less abundant associate of the magnesite is celestite, which occurs disseminated throughout the rock and in a 1-cm-thick bed or bedding-parallel vein. White, light-gray, and pink gypsum is the dominant lithology directly above the basal conglomerate in studied stratigraphic sections B, G, and I (fig. 4) and throughout Bitter Spring Valley and the Gale Hills. The gypsum forms thick units, some relatively pure, that are interrupted only by thin, brown, red-brown, and pink, fine-grained sandstone and siltstone beds and rare gray limestone beds. Silt and possibly clay are disseminated in some of the gypsum and apparently cause its pink color. Bedding is obvious from a distance because of color changes, and it appears to be irregular, uneven, and discontinuous, but close inspection reveals that it is indistinct and that veins are common. The gypsum may be recrystallized, and it may have been bedded or laminated when it was deposited. Sedimentary Structures One sedimentary structure, not described above, which occurs in the middle parts of several of the studied stratigraphic sections of the Rainbow Gardens20 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 6.—Magnesite-bearing Rainbow Gardens Member at Overton Ridge. The uniform, parallel, even bedding is apparent in the white magnesite. The lower limestone- and dolomite-bearing unit beneath the magnesite is less obvious. The high ridge is composed of the basal conglomerate. View to the north. Member, is a system of branching, mostly vertically oriented calcite-filled tubes (fig. 4). These tubes are found in beds of limestone and dolomite, arenaceous limestone and dolomite, and calcareous sandstone. They are about a centimeter or less in diameter, and some are as long as 1 m. Several different origins are possible for these tubes. They could be root or burrow casts, but it is more likely that they represent filled escape pipes formed by fluid expulsion related to sediment dewatering during compaction. This inorganic origin is favored because no signs of organic activity are recognized in the rocks. The mineralogy of associated strata (such as gypsum) suggests salinities that were not compatible with indigenous biologic activity, except perhaps that of algae. Facies and Their Distribution The Rainbow Gardens Member is readily subdivisible on a lithologic basis. One obvious lithologic subdivision consists of the basal conglomerate, but this unit does not qualify as a lithofacies because it cannot be demonstrated to be laterally equivalent to any other lithology in the member. However, lateral lithologic (facies) variation is recognized above the basal conglomerate. Five lithofacies are recognized: (1) A lithofacies of interbedded sandstone, siltstone, conglomerate, claystone, limestone, and dolomite, which is overlain by a resistant unit of limestone, is called the clastic-carbonate facies. (2) A similar lithofacies of tuffa-ceous carbonate rocks, sandstone, and claystone, which is also overlain by limestone is called the tuff-limestone facies. (3) A gypsiferous lithofacies occurs and is referred to herein by that name. (4) A lithofacies of gypsum overlain by limestone and a lithofacies of interbedded gypsum and limestone are grouped, forming the gypsum-limestone facies. (5) A lithofacies of magnesite and dolomite is called the magnesite facies. The present distribution of the five facies is controlled, to a large extent, by Tertiary faults, especially the Lake Mead fault system of Anderson (1973) and Bohannon (1979). The paleogeographic significance of the facies cannot be evaluated without palinspastic restoration of the lateral slip on these faults. Such a restoration is attempted in the section entitled “Paleogeologic and paleotectonic evolution.” For now, the facies are described in their present configuration relative to major geographic features and important fault traces (fig. 7). The clastic-carbonate facies is exposed north of the Lake Mead fault system in Rainbow Gardens, between branches of the fault system 30-40 km northeast of Rainbow Gardens in the northern Black Mountains, and southeast of the main branches of the fault system in Wechech Basin 65 km northeast of Rainbow Gardens. An isolated exposure of this facies is at the north end of Wheeler Ridge in Pigeon Wash, about 25 km southeast of Wechech Basin. In Rainbow Gardens, the clastic-carbonate facies thins to the north and becomes sandier in a zone that is transitional to the gypsiferous facies. Indescription of rock units 21 Meadow V//alley ^ EXPLANATION Glendale ■4., : Of q \Logandale | Tuff-limestone fades Gypsiferous fades Gypsum-limestone fades Magnesite fades 14000' Wechecir, Basin/ Buffington i Pockets 5J Jumbo'' Peak MEAD Figure 7.—Distributions of the five lithofacies of the Rainbow Gardens Member, and locations of related prominent faults of the Lake Mead fault system of Anderson (1973) and Bohannon (1979). the Black Mountains, the clastic-carbonate facies is fault bounded and cannot be demonstrated to grade into other facies. However, in Wechech Basin, excellent exposures indicate a gradual transition of this facies into the tuff-limestone facies to the south toward Horse Spring. This transition is also exposed in Cottonwood Wash. The isolated exposure of the clastic-carbonate facies in Pigeon Wash is southeast of the transition described above, beyond the area where the tuff-limestone facies is exposed. The tuff-limestone facies, the thickest of the five facies, is confined to the Virgin Mountains region and is widespread. It is best exposed between the Lime Ridge and Gold Butte faults, two left-separation faults that are associated with the Lake Mead fault system (Bohannon, 1979), but the northern transition of this facies with the clastic-carbonate facies is exposed on both sides of the Lime Ridge fault. The southern transition of this facies with the Pigeon Wash exposure is covered. The gypsiferous facies occurs north of the Lake Mead fault system in the Gale Hills and Bitter Spring Valley. Although it is the thinnest of the five lithofacies, it is exposed over a wide area North of Rainbow Gardens, it is gradational into the clastic-carbonate facies. To the east, the gypsiferous facies grades into the gypsum-limestone facies, which is distinguished from it by (1) a thick unit of wavy-bedded limestone stratigraphically above the gypsum in the area north of Echo Wash and (2) limestone interbedded with the gypsum on the north flank of the Echo Hills. The latter exposure is within 1 km of the clastic-carbonate facies in the northern Black Mountains. Although the gypsum-limestone facies of the Echo Hills and the clastic-carbonate facies of the Black Mountains are different in composition and22 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION no gradation can be observed between them, their close proximity can be explained by the intervening Bitter Spring Valley fault, a major branch of the Lake Mead fault system. The Bitter Sping Valley fault bounds the gypsiferous and gypsum-limestone facies to the south, but to the north, they may once have intergraded with the magnesite facies. The magnesite facies occurs only at Overton Ridge and is isolated from the other facies across faults and large covered areas. Based upon present distribution, it is assumed to be most closely related to the gypsum-limestone facies. However, the closest exposures of these two facies are 25 km apart. ENVIRONMENTS OF DEPOSITION The basal conglomerate probably was deposited as a gravel veneer on a widespread pediment surface developed on Mesozoic and upper Paleozoic rocks. The tabular, sheetlike deposit is no thicker than 50 m and thins in places to as little as 1 m, but it is present in all exposures of the member. Despite its thinness, it apparently once covered a large geographic area6 The conglomerate has a well-defined upper contact, which shows evidence of erosion in the form of channels and scours, and there is little or no evidence of any interfingering of the upper part of the conglomerate with the overlying rocks, except in those channels. In most places it has indistinct, discontinuous bedding typical of alluvial or stream deposits. However, some parts of it are unbedded, and these might be the result of mass flow. The cement and matrix are chiefly crystalline carbonate minerals, a composition which suggests vadose-zone crystallization. A major unanswered question about this unit is the degree to which it is diachronous. Most alluvial conglomerates are diachronous to some extent, and they commonly grade laterally into time-equivalent finer grained rocks. However, no such gradation has been observed for this conglomerate, so it is interpreted to have been deposited prior to the other facies of the member. Therefore, if the conglomerate is at all diachronous, its older parts must have been subaerialiy exposed prior to deposition of the overlying beds. Such a history is compatible with a pediment gravel model. The rocks overlying the basal conglomerate probably were deposited in a complex system of lake, lakeshore, marginal lake, playa, spring, and alluvial environments that developed upon the pediment gravel deposit. Al- though detailed bed-by-bed discrimination of these environments has not been attempted, a general evaluation of the most likely environments for these strata is undertaken here. A thick, resistant limestone unit composes the upper third of both the clastic-carbonate and tuff-carbonate facies, and its lithology is essentially the same in both, indicating little environmental difference during its deposition. Beneath that limestone unit, however, contrasting lithologies indicate significant environmental differences between the two facies. Both facies may have been deposited near the margins of a large lake or playa system. The facies include clastic rocks that appear to have been locally derived and that were probably transported by fluvial processes. The bedding, however, is parallel, indicating a flat depositional surface. The tuff-carbonate facies could be a stream deposit, but its source must have been nearby coeval volcanic deposits rather than local bedrock. Many features of the upper limestone unit, such as its wedge shape, its “tufa-like” appearance, and its irregular bedding, suggest that it may have been deposited near springs that fed into the basin. The gypsum-limestone and gypsiferous lithofacies probably represent lacustrine deposits. Because the gypsum is recrystallized, it is not possible to tell whether standing water existed during its deposition, or whether it was deposited in the vadose zone of existing sediments, but the coexistence of gypsum and limestone suggests evaporative concentration in a permanent lake with evaporite crystal growth both in the lake and in the bounding sediments. Because the available exposures are few and the study of them was limited, it is not known whether deposition occurred in one or more lakes. In the magnesite lithofacies, magnesite occurs with dolomite and smectite clay in extremely fine grained, parallel, even, continuous, uniform beds that contain ripple marks and plant fragments. These features collectively suggest a lacustrine sedimentary origin for the magnesite facies, and the uniform increase in the Mg-Ca ratio through time suggests an orderly chemical evolution of lake water during deposition. In the lower beds of this deposit, calcite is dominant, dolomite is present, and magnesite is absent; but higher in the section, dolomite increases in abundance relative to calcite. In the upper part of the section, magnesite is present where calcite is absent. The carbonate minerals may have evolved from high-calcium to high-magnesium mineral species simply by using up the available calcium in the lake waters through time. Deposition in a nearly closed drainage system seems likely. 6After palinspastic reconstruction it appears that this deposit covered nearly 3,000 km2.DESCRIPTION OF ROCK UNITS 23 THUMB MEMBER DISTRIBUTION AND THICKNESS The Thumb Member has a distribution similar to that of the Rainbow Gardens Member (fig. 8), but it does not occur at Pigeon or Cottonwood Wash. It is thicker than the Rainbow Gardens Member, although its thickness is variable and is difficult to determine because of faulting. In unfaulted, complete stratigraphic sections, such as those in the Gale Hills and in Bitter Spring Valley, the Thumb Member is as thick as 1300 m. At Rainbow Gardens (studied stratigraphic section A, fig. 8) the principal reference section of the Thumb Member is at least 850 m thick, but its top is faulted. Near Lava Butte, north of stratigraphic section A, the Bitter Ridge Limestone Member conformably overlies the Thumb Member, but a large fault in the Thumb makes section measurements there uncertain. The Thumb Member dips to the east at Rainbow Gardens and probably is present in the subsurface between there and the Gale Hills. Surface exposures of the Thumb also occur north, northwest, and south of Frenchman Mountain, and it is likely that the Thumb Member occurs in the subsurface west of Frenchman Mountain. The Thumb Member is widespread in the Gale Hills, where a complete and relatively unfaulted stratigraphic section (B on fig. 8) indicates that it is about 1,200 m thick. Between the Muddy Mountains and locality B, the member appears to be thicker than 1,200 m, but it is repeated by faults and neither its top nor its base is exposed. Surface exposures of the Thumb are bounded to the south by a zone of faults. However, it is thought that the member occurs in the subsurface south of that fault zone. There are no outcrops of the Thumb north of the Gale Hills in the Muddy Mountains or in California Wash, and facies and provenance information described below suggest that it was never deposited there. Relatively unfaulted stratigraphic sections in Bitter Spring Valley and Echo Wash (sections C and D respectively, fig. 8) indicate the member’s maximum thickness of 1,300 m. Surface exposures in Bitter Spring Valley are bounded to the south by the Bitter Spring Valley fault. Only thin, discontinuous remnants of the member occur in the northern Black Mountains between the Bitter Spring Valley and Hamblin Bay faults. At the north end of Bitter Spring Valley, the Thumb dips to the north under the Bitter Ridge Limestone Member. It underlies at least the southern part of White Basin but probably does not underlie the northern part. Instead, the Thumb apparently abuts against a steep subsurface buttress unconformity under the southern part of the basin. In Echo Wash, the Thumb is unconformably overlain by younger deposits. Isolated outcrops near the west shore of the Overton Arm of Lake Mead suggest that the Thumb is present in the subsurface throughout the Overton Arm area north of the eastward-projected trace of the Bitter Spring Valley fault. In the Virgin Mountains, surface exposures of the Thumb Member occur from Wechech Basin, where studied stratigraphic section E (fig. 8) attains a minimum thickness of 1,150 m, to the Gold Butte fault. The exposures near Horse Spring (fig. 8) are isolated from all others and are atypical, being composed entirely of unbedded breccia East of Wechech Basin only isolated, small exposures occur and the eastern depositional extent of the Thumb is not known. Although limited in thickness and extent, the exposures of probable Thumb Member at Overton Ridge (not measured and not depicted on fig. 8) are the most northerly known for that member. These exposures, which dip to the east, indicate that the Thumb is present in the subsurface in the Muddy and Virgin River valleys, but its extent is uncertain. LITHOLOGY The Thumb Member is chiefly clastic, consisting of sandstone, siltstone, conglomerate, and breccia, but gypsum occurs at many localities in thick, pure units. Carbonate, predominately limestone, is present locally. Sandstone is the most widespread rock type, but conglomerate also occurs at all locations except Echo Wash. Clastic Rock Types The conglomerate-sandstone ratio varies abruptly with geographic locality. In the southern part of Rainbow Gardens, conglomerate makes up about 50 percent of the member, sandstone constitutes about 30 percent, and other rock types the remainder. North of Rainbow Gardens, the sandstone content of the member increases to more than 90 percent. In the Gale Hills, sandstone makes up about 75 percent and conglomerate 20 percent of measured section B (fig. 8). North of the measured section near the Muddy Mountains, conglomerate is dominant. In southern Bitter Spring Valley, sandstone is the most abundant rock type except in the conglomeratic southwest part of the valley. In northern Bitter Spring Valley, the lower and upper parts of studied stratigraphic section C (fig. 8) are dominated by sandstone, but the middle part, especially in the north-central part of the valley, is conglomeratic. The clastic24 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION BITTER RIDGE LIMESTONE MEMBER TOP NOT EXPOSED EXPLANATION TOP NOT EXPOSED TOP NOT EXPOSED wmm \'/>7 7/ */ /y /* rrni J/////77///////K //'./ Z/jiy/y_ o° rm - rm CGSF C NMRV Sandstone Sandstone and siltstone Conglomerate Breccia Gypsum and gypsiferous lithologies Limestone beds Andesite flows d £> a & & Air-fall tuff beds ~ 200 - 100 L 0 Vertical scale of measured sections Frenchman Mountain Rainbow Gardens/^ \#DESCRIPTION OF ROCK UNITS 25 rocks in the stratigraphic section in Echo Wash (section D, fig. 8) are nearly all sandstone, but a thin lens of conglomerate is exposed at one locality about 1,000 m from the base of the section. The stratigrahic section in Wechech Basin (section E, fig. 8) is chiefly sandstone and gypsiferous sandstone, but about 20 m of conglomerate is exposed stratigraphically high in the member. South of Horse Spring, breccia and conglomerate are the only rock types present. Most of the sandstone is fine grained, brown, and parallel bedded, but some is lenticularly bedded, fine to coarse grained, and red brown. The brown, parallel-bedded type occurs in Rainbow Gardens near Lava Butte and in nearly all of the Gale Hills, Bitter Spring Valley, Echo Wash, and Wechech Basin. The red-brown, lenticularly bedded type occurs only in parts of Rainbow Gardens and interbedded with conglomerate in other areas. At Rainbow Gardens, the coarse-grained sandstone composes much of the member north of Lava Butte and in the vicinity of stratigraphic section A (fig. 8). Brown, fine-grained sandstone occurs in parallel, continuous beds of uniform thickness. Some bedding surfaces are even, but others are undulatory (fig. 9). Although regionally these beds vary from as thin as 1 cm to as thick as 50 cm, in any one exposure bedding thickness commonly does not vary by much more than 15 cm. The most common bed thickness is between 8 and 15 cm. Many individual beds and most definable bedsets are continuous, though accurate measurements of lateral continuity are not possible because of colluvial cover and offsets on abundant small faults. Some thin beds are laterally discontinuous, but these are generally part of continuous bedsets. Only rare thick beds are lenticular, and these commonly consist of the coarser grained rocks. Most individual bedding surfaces are defined by abrupt changes in grain size. Most of the sandstone ranges from medium grained to very fine grained, but coarse-grained sandstone and siltstone also occur. Most grains are composed of quartz and carbonate minerals with minor amounts of feldspar, lithic fragments (chiefly of carbonate rocks), and micas. Point counts of 100 grains indicate carbonate grain abundances from 50 to as much as 70 percent, and quartz abundances of 25 to 45 percent. Other constituents occur in amounts ranging from a trace to as great as 5 percent. In most samples, matrix and cement minerals compose 20 to as much as 40 percent of the rock. Both carbonate-clay and gypsum cements occur. The carbonate-clay mineral cement is composed chiefly of fine-grained crystalline to amorphous carbonate minerals that fill pores between grains. The brown color of the cement indicates contamination of the carbonate with clay minerals. Large gypsum crystals surround many of the sand grains, giving some samples of the sandstone a texture that resembles poikilitic texture in igneous rocks. Because neither type of cement leaves many pores, the porosity and permeability are low. It is not uncommon to find both types of cement in one sample. Conglomerate of the Thumb Member has bedding, clast types, clast percentages, and clast-matrix percentages that vary geographically. The following discussion of clast types and percentages is keyed to figure 10, which depicts the results of pebble counts and the geographic distribution of sandstone versus conglomerate. In the southern part of Rainbow Gardens (locality 7, fig. 10), most of the conglomerate is interbedded with sandstone in nonparallel, discontinuous, uneven, non-uniform beds that range in thickness from a few centimeters to 2 m. Bedding is defined by grain-size changes between sandstone and conglomerate, and trough crossbedding, defined by clast trains, is common. The rock has a clast-supported texture, and the clasts are moderately sorted and moderately rounded. The largest clast is 70 cm in diameter, but most average about 25 cm. Some continuous conglomerate beds have well-defined, parallel bases and tops. Clasts in these beds are supported by mud, silt, and sand; are only 2 to 5 cm in diameter; occupy about 25 percent of the rock; and exhibit inverse grading near the base of the bed. All the conglomerate at Rainbow Gardens grades into sandstone to the north. This lithofacies change is especially well displayed by one thick, well-defined hogback forming a unit that thins considerably to the north and that grades entirely into sandstone in the vicinity of Lava Butte. In the northern part of the Gale Hills and Bitter Spring Valley (localities 1, 2, and 4 on fig. 10) the conglomerate is coarse-grained, thick-bedded, and abundant. Continuous, well-defined, but poorly sorted and unbedded conglomerate is common near the resistant Figure 8 (facing page).—Generalized stratigraphic sections of the Thumb Member of the Horse Spring Formation with map showing geographic distribution of outcrops. All five stratigraphic sections are conformably underlain by Rainbow Gardens Member. Sections A, B, D, and E on this figure overlie, respectively, sections A, G, B, and L) on figure 4. Section C on this figure has no counterpart on figure 4, but Rainbow Gardens Member does occur beneath it. The left side of each section depicts bulk grain size: C, conglomeratic; G, fine-grained conglomerate or grit; S, sandstone; F, finer grained than sandstone. The right side of each section depicts relative resistance to weathering: N, nonresistant; M, moderately resistant; R, resistant; V, very resistant.26 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 9—Examples of parallel, continuous bedding within sandstone of the Thumb Member. A, View south towards the Black Mountains across Echo Wash (not visible) shows extreme lateral continuity of bedding in a stratigraphic section of sandstone with even bedding surfaces. B, An outcrop in Callville Wash, north of Callville Bay, shows undulatory bedding. Photo case on the left is 50 cm square. Paleozoic carbonate units of the Muddy Mountains. This bedding contrasts with the lenticular, discontinuous, unevenly bedded conglomerate with moderate clast sorting in more southerly exposures (localities 9, 10, 11, 12, and 5 on fig. 10). Most of the conglomerate in the Gale Hills and Bitter Spring Valley has clast-supported texture, but some of the well-defined, unbedded units have matrix-supported clasts. Clast diameters of one meter and greater occur adjacent to outcrops of Paleozoic strata, but clast size decreases toDESCRIPTION OF ROCK UNITS 27 114°00' 113°45' CONTOUR INTERVAL 1000 FEET Figure 10.—Distributions of the conglomerate facies and the fine-grained facies of the Thumb Member, relative abundances of clast types, and locations of major faults of the Lake Mead fault system. the south to about a 25-cm average. The unbedded units commonly have well-defined, parallel, inversely graded bases and parallel tops (fig. 11.). Carbonate clasts predominate over sandstone clasts in the conglomerate (fig. 10, localities 1, 2, 4, 5, 6, 9, 10, 11, and 12), and all clasts are moderately rounded. A small percentage of volcanic clasts occur in a conglomerate lens north of Echo Wash (fig. 10, locality 6). Conglomerate occurs in Wechech Basin, but exposures are limited. Its bedding is discontinuous, nonparallel, and uneven. The clasts, the largest of which is 10 cm, are in grain contact, and the matrix is of well-indurated sandstone. Clasts are a mix of carbonate rock types, granitic types with rapakivi granite, and gneiss (fig. 10, locality 13). Conglomerate is apparently more abundant in the southern part of Wechech Basin. Large lenses, small discontinuous beds, and widespread exposures of breccia occur within the Thumb Member in Lovell Wash, in Rainbow Gardens, and north of the Gold Butte fault (localities 3, 8, and 14, respectively, fig. 10). In Rainbow Gardens, nearly mono-lithologic breccia of the Thumb Member occurs in unstratified, unsorted lenses with angular clasts more than 1 m in diameter. Little or no matrix matter is present, and the clasts of rapakivi granite and other granitic rocks are tightly fitted. Brenner-Tourtelot (1979) shows the location of most of the breccia lenses in the Thumb between Rainbow Gardens and Lake Mead Boulevard (see fig. 3C for the location of Lake Mead Blvd.), but she also shows some at other localities that occur in younger deposits. In Lovell Wash, clasts of rapakivi granite, other granitic rocks, gneiss, and carbonate rocks occur in thin, discontinuous breccia beds that are interbedded with sandstone. North of the Gold Butte fault, bedding appears to be nonexistent and the clasts, which consist of gneiss, rapakivi granite, other granitic28 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 11.—A continuous, well-defined conglomerate bed within the Thumb Member of the Horse Spring Formation. This particular bed occurs in Bitter Spring Valley and is interbedded with parallel-bedded sandstone. It is not close to outcrops of Paleozoic strata, although conglomerate beds of this type are more prevalent near such outcrops. The well-defined base of the bed is in shadow, and there is a thin zone of inverse clast grading near its base. The texture is generally matrix-supported. rocks, carbonate rocks, and mafic igneous rocks, are as large as several meters in diameter. Carbonate and Sulfate Rocks The best exposure of gypsum, a primary constituent of the Thumb Member, occurs in the steep sides of Echo Wash, where thick, pure units as much as 20 m thick occur in the upper part of stratigraphic section D (fig. 8). Strata average about 15 cm thick in the blocky-weathering gypsum and are parallel, wavy, continuous, and uniform. Internal laminations, defined by changes in shade from white to dark gray, are wavy, continuous, parallel, and a few millimeters thick. At other locations in some recrystallized units of gypsum, the parallel laminations are not visible. Some of this recrystallization is a near-surface weathering phenomenon; the fine internal structure is only visible in outcrops in deeply incised drainages where rapid erosion is taking place. Possibly all the gypsum had finely laminated primary texture. In places, the gypsum contains sandstone, silt-stone, and clay. Limestone of the Thumb Member occurs in the southern part of the Gale Hills, in Bitter Spring Valley and north of Echo Wash. In nearly all places, it is dense, very hard, fine-grained, yellow and gray lithographic limestone and is resistant to weathering. It occurs in thin (2- to 10-cm-thick), even, well-defined, parallel beds that are interbedded with sandstone. In Bitter Spring Valley the lithographic limestone is beneath the Bitter Ridge Limestone Member. North of Echo Wash, limestone near the Rogers Spring fault grades south into sandstone. Tuffaceous Rocks and Volcanic Flows Pale green homblende-biotite tuff beds, bedsets, and units (fig. 12) occur interbedded with all of the of the Thumb Member except the conglomerate. These tuff units also occur in the Rainbow Gardens Member, the Bitter Ridge Limestone Member, and the lower part of the Lovell Wash Member, but they are most common in the Thumb. The tuffs range in thickness from 15 cm to 10 m and have uniform, even, discontinuous internal Figure 12.—One of the green tuff units (light-colored bed in center of photo) interbedded with sandstone of the Thumb Member in the Gale Hills. This particular unit is about 2 m thick. Units such as this provide zircons for fission-track age determinations.DESCRIPTION OF ROCK UNITS 29 crossbedding and parallel bedding. The thickest units commonly contain unbedded basal layers as thick as 20 cm, which are possibly original, unreworked ash falls. Throughout the tuffs, euhedral andesine, sanidine, biotite, and hornblende phenocrysts are set in a matrix of green clinoptilolite and heulandite altered from the original glass and glass shards. Quartz is present in minor amounts but is rarely euhedral. The lava flows of the Thumb Member in Rainbow Gardens were not studied in detail and are not reported on herein. Sedimentary Structures Environmentally significant textural features of many conglomerate beds include their matrix support, their basal inverse clast grading, and their well-defined bases and tops. The lack of internal structures in some of the beds is also environmentally significant. These nearly structureless beds differ from other conglomerate units, in which internal trough crossbeds are well defined, clasts are supported by a sandy matrix, and the boundaries between conglomerate and sandstone units are poorly defined. Flat bedding contacts are present at the base of many conglomerate beds. These flat contacts may indicate deposition on a low-relief surface without large initial dips. Another simple, but significant, sedimentary structure occurs within many gypsum units in the form of thin, parallel laminations in thicker parallel beds. Perhaps the most interesting sedimentary structures within the Thumb occur in its widespread parallel-bedded sandstones. To evaluate the structures in these beds, a detailed microsection was measured in Echo Wash, where they are well exposed. Figure 13 is a diagram of the microsection and figure 14 is a photograph of the rocks. The parallel-bedded sandstone shown in figure 14 is typical of much of the Thumb, and the structures associated with those beds are probably representative of structures throughout a wide area One coarse-grained sandstone bed that was measured contains abundant small-scale cross-laminations, but its upper and lower 2 cm are not laminated. Several upward-fining sandstone and siltstone beds are exposed, and many of these have small-scale crosslaminations in their lower parts, whereas others are not laminated. One inversely graded bed grades upward from nonlaminated and parallel-laminated siltstone into cross-laminated sandstone. Shale chips occur in one of the beds, and another contains large-scale trough crosslaminations. Many irregular and wavy bedding surfaces occur, suggesting possible scour. In general, parallel laminations and structureless bedding appear to be associated with siltstone beds, whereas most sandstone ROCK TYPES EXPLANATION BEDFORMS Small-scale crosslaminae Large-scale trough cross-laminae Shale chips Parallel laminae Wavy and irregular laminae Structureless 0 J Figure 13.—Lithology (A) and bedforms (B) of a microsection in the Echo Wash area (See photo, fig. 14.) The left side of the lithologic column (A) depicts grain size: C, coarse-grained sandstone; M, medium-grained sandstone: F, fine-grained sandstone: S, siltstone. The right side depicts relative resistance to weathering: N, nonresistant; M, moderately resistant; R, resistant. beds appear to contain small-scale cross-laminations or wavy irregular laminations. Where bedding surfaces are exposed, oscillation ripple marks can be observed. No mud cracks were observed. DEPOSITIONAL ENVIRONMENTS The parallel and continuous bedding in most of the Thumb Member may indicate deposition on a flat, even surface such as a lake bottom. Such a setting seems30 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 14.—Rocks measured for the Echo Wash microsection (fig. 13). Distance showing along measuring rod at edge of photo is 1.4 m. reasonable for the origin of gypsum and limestone, and it is also a likely environment of deposition for the parallel-bedded sandstone. Some of the conglomerate beds that have continuous, even bases parallel to underlying bedding were probably also deposited in a lacustrine environment. Standing water was probably always present during the deposition of the laminated gypsum and the dense, hard limestone beds, but it may only have been sporadically present during the deposition of the parallel-bedded sandstone and siltstone. It seems prudent to interpret the parallel, thin laminations in gypsum as structures that formed by precipitation in standing water. The water depth is uncertain, but the fact that the thin laminations were preserved suggests that it was moderately deep and that bioturbation was rare. The presence of limestone and gypsum suggests saline, alkaline water. Much of the conglomerate and associated sandstone has discontinuous, lenticular, uneven, nonuniform, poorly defined bedding suggestive of fluvial deposition. These thick deposits, which probably represent alluvial fan accumulations, interfinger with the parallel-bedded sandstone thought to be lacustrine in origin. Also, debris-flow beds of nearly unstructured conglomerate with well-defined, flat bases, matrix-supported clasts, and inverse basal clast grading are interbedded with both the alluvial lenticularly bedded fanglomerate and the lacustrine parallel-bedded sandstone. Analysis of the microsection depicted on figure 13 indicates standing water was present during deposition of some of these beds. Oscillation ripple marks, the even, unscoured bedding surfaces, and abundant small-scale cross-laminations are consistent with deposition in a permanent lake with slow-moving currents. Some bedding surfaces are uneven and irregular, and may be scoured or may have been deposited during times of high flow. Small- and large-scale cross-stratification indicates rapid deposition in a fluid medium and suggests possible storm activity accompanied by deposition of a large sediment load. The lowest bed in the microsection is coarse-grained sandstone, which occurs above an irregular scoured base and has small-scale crossstratification. This bed is similar to subaqueous flows, such as distal turbidites. The sequence suggests that sediment-charged water entered the lake during a storm and resulted in deposition of the lowest coarse-grained bed. Rapid subaqueous deposition probably continued to form the overlying siltstone and sandstone beds with small-scale cross-laminations and irregular bedding. The thin gypsum layer was probably deposited in the aftermath of rapid storm deposition, after the water had cleared. Two graded beds of fine-grained sandstone and siltstone containing small-scale cross-laminae deposited above the gypsum indicate further clastic influx. The bedding surface at the top of the upper graded bed is wavy and uneven, suggesting lacustrine wave action or subaerial eolian scour. Above the wavy surface, a structureless bed of sandstone with shale chips at its base indicates rapid deposition accompanied by erosion and incorporation of the underlying bed. The bed overlying the structureless sandstone coarsens upward from unstructured to parallel-laminated siltstone into cross-laminated sandstone, as if formed in a small delta prograding into the lake. At the top of the coarsening-upward bed is an irregular bedding surface that could have been caused by scour or wave reworking. A sandstone bed with trough cross-laminations above the wavy surface suggests rapid deposition. The cross-laminated bed is succeeded by laminated siltstone and several interbedded cross-laminated sandstone and unlaminated siltstone beds without evidence of exposure or scour. The above scenario is speculative, and it illustrates only a likely sequence of events occuring during deposi-DESCRIPTION OF ROCK UNITS 31 tion of the lacustrine sandstone. Certainly, there are other areas where the relative abundances of the described structures are different and where such things as uneven bedding surfaces are more common than they are at the location of the microsection. Also, ratios of coarse-grained to fine-grained detritus obviously must have varied considerably at different geographic localities during deposition, but only numerous closely spaced sections could document the nature of such changes. FACIES AND PROVENANCE The Thumb Member, as described above, consists of lacustrine (fine-grained) and alluvial (coarse-grained) facies, whose geographic distributions are shown on figure 10. These two depositional facies can be considered to be temporally equivalent. Analysis of the distribution and character of the alluvial facies shows that the Thumb had a northern source chiefly of upper-plate rocks above the Cretaceous Muddy Mountain thrust and a southerly source of lower-plate carbonate rocks and granitic and gneissic basement rocks in the Gold Hill area of the southern Virgin Mountains. Figure 15 is a diagram of the northern basin margin of the Thumb against the Muddy Mountains. In the northern part of the Gale Hills, the alluvial facies is very coarse grained at its contact with the resistant Paleozoic rocks of the Muddy Peak area (figure 16A). Although this contact is chiefly faulted and has been termed the Gale Hills fault (Bohannon, 1979), at some localities it appears to be a steep buttress unconformity. At other localities, thrust relations exist along the Gale Hills fault where intact mountain-size blocks of Paleozoic rocks have been emplaced above parts of the Thumb. The predominant clast type in the alluvial facies is Paleozoic carbonate rock from the upper plate of the Muddy Mountain thrust, but some Aztec-like clasts of red sandstone, probably from the lower plate, also occur. The Gale Hills fault locally marked the margin of the Thumb basin, and the apparent steepness of the marginal contact as well as the coarse grain size of the alluvial facies suggest that it was a high-relief margin. The trend and location of this margin are shown on figure 15 by the solid dots southwest of Muddy Peak. North of Echo Wash two resistant ridges of Paleozoic rocks, East and West Longwell Ridges, extend southwest from the eastern part of the Muddy Mountains, forming hogbacks that dip to the southeast. The rocks of the alluvial facies of the Thumb rest depositionally on the older rocks of West Longwell Ridge on a steep buttress unconformity (fig. 16B). The alluvial facies rocks 114°45' 114°30' EXPLANATION Post-Thumb basin fill Thumb Member of the Horse Spring Formation Paleozoic rocks of the Muddy Mountains thrust: Upper plate Lower plate Location of buttress unconformity or fault between Thumb Member and upper plate • Exposed o Inferred --------- Contact ^ ^ A Muddy Mountains thrust fault—Sawteeth on upper plate Figure 15.—Location of the northern basin margin of the Thumb Member of the Horse Spring Formation. The margin is marked by buttress unconformities or faults where the alluvial facies of the Thumb abuts the Paleozoic rocks in the upper plate of the Muddy Mountain thrust. The location of the margin is inferred where it is covered or eroded. Rocks in the upper plate of the Muddy Mountain thrust are distinct stratigraphically from those in the lower plate. The allochthonous stratigraphy is defined by geologic mapping. The autochthonous stratigraphy is defined from lithologic logs of core from the two drill holes shown here. abruptly interfinger with the rocks of the lacustrine facies in the valley between the two ridges. Clasts in the alluvial facies are composed of local Paleozoic rocks, indicating that the facies was derived from the ridge or from the White Basin area beyond the ridge to the west. Presently White Basin is a graben in which younger members of the Horse Spring Formation are exposed. However, the buttressing of the coarse-grained alluvial Thumb on the narrow, basin-bounding hogback suggests that the area now occupied by the basin may have been an uplifted source area for at least the oldest part of the Thumb. The solid dots north of Echo Wash on figure 15 show the location of the buttress unconformity. In northern Bitter Spring Valley, between the Gale Hills fault and the buttress unconformity described above, a small outcrop of Paleozoic rocks is overlain by32 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 16.—Rock strata along the northern margin of the the Thumb basin. A, In the area of Muddy Peak, the conglomerate facies of the Thumb Member (low hills in foreground) abuts Paleozoic strata in the upper plate of the Muddy Mountain thrust (mountains in backgound). The contact is the Gale Hills fault. B, Along West Longwell Ridge, a prominent, southwest-trending ridge north of Echo Wash, rocks of the alluvial facies of the Thumb (at top and right) have been deposited on these same upper-plate rocks (at bottom and left). The contact is a buttress unconformity. conglomerate of the alluvial facies deposited on a buttress unconformity. Both the unconformity and the conglomerate have been faulted. The youngest part of the Thumb, which is lacustrine, overlaps the outcrop of Paleozoic rock, the older Thumb conglomerate, and the faulting. The overlapping lacustrine facies dips north under White Basin, but the outcrop of Paleozoic rocks and its associated conglomerate may indicate that the basement of the Thumb is close to the surface and that a steep, high-relief basin margin probably existed in northern Bitter Spring Valley during the deposition of the oldest part of the Thumb Member. The younger lacustrine facies apparently lapped across this margin into the area of southern White Basin, although evidence of equivalent rocks on the northern side of the basin is limited. Thin, discontinuous conglomerate beds occur beneath the Bitter Ridge Limestone Member in northern White Basin and possibly are Thumb equivalents. Two dots on figure 15, southwest of the outcrop of Paleozoic rocks in northern Bitter Spring Valley, indicate the location of the faulted buttress unconformity. The pattern of conglomerate indicates that a high-relief basin margin existed south of the Muddy Mountains during much of the deposition of the Thumb and that the youngest lacustrine facies overlapped that margin at some locations. The margin was apparently irregular, extending from the site of the Gale Hills fault in the Gale Hills, through northern Bitter Spring Valley, northwest to Echo Wash. North of this margin the highland was composed of the Paleozoic rocks of the upper plate of the Muddy Mountain thrust (fig. 15). South of the margin the Thumb and Rainbow Gardens Members were deposited upon Mesozoic rocks that occur in the lower plate of that thrust. This contrast and the significance of the Gale Hills fault are illustrated by examination of the area between Muddy Peak and the Bowl of Fire. North of the basin margin and Muddy Peak, the Muddy Mountain thrust dips south, towards the Bowl of Fire. Cambrian, Ordovician, Devonian, and Missis-sippian rocks above the thrust also dip south. Upper-plate stratigraphy is distinct from that exposed in the lower plate at Frenchman Mountain. Although rocks of the upper plate dip south and the thrust apparently roots in that direction, the Shell #1 Bowl of Fire drill hole in the Bowl of Fire (only about 6 km south of Muddy Peak) penetrated a lower-plate sequence similar to that at Frenchman Mountain (Bohannon and Bach-huber, 1979, p. 592). This finding suggests that the Gale Hills fault formed a high-relief basin margin and was an important structural boundary. This structural boundary extends from the Gale Hills to Echo Wash and forms the southern margin of the upper plate of the Muddy Mountain thrust. Its trace, shown on figure 15, has been made irregular by later fault displacements. Exposures of Precambrian granite and gneiss and Paleozoic carbonate rocks at Gold Butte apparently were the source of conglomerate and breccia both locally in the Virgin Mountain region and 65 km to the southwest in Rainbow Gardens. In Rainbow Gardens, the alluvial facies contains granitic and gneissic clasts, and breccia lenses are commonly composed entirely of rapa-kivi granite. Anderson (1973, p. 12-13) and LongwellDESCRIPTION OF ROCK UNITS 33 and others (1965, p. 43) concluded that the conglomerate and breccia of the Thumb in Rainbow Gardens were derived from the south. This conclusion seems compatible with the transition of the alluvial facies into sandstone of the lacustrine facies from south to north. Anderson (1973) also concluded that the conglomerate, with its abundant clasts of Paleozoic and Mesozoic rocks, could not have been derived from the nearby River Mountains or northern Black Mountains, because volcanic rocks equivalent to and older than that conglomerate rest on the gneissic and granitic basement rocks there. Recent radiometric ages of the Thumb confirm this conclusion. Anderson (1973) further reasoned that the rapakivi granite-bearing breccia could not have come from due south of Rainbow Gardens because such granite does not occur there in the basement terrane. The Gold Butte area south of the Virgin Mountains is the closest locality where volcanic cover is absent and rapakivi granite is exposed. Because the coarseness of the conglomerate and breccia suggests a nearby source, Anderson (1973) proposed that the Frenchman Mountain area has been transposed 65 km southwest from the Virgin Mountains by strike-slip displacement on northeast-trending faults. This thesis was further refined by Bohannon (1979) and is elaborated upon in a later section of this paper. Clasts of rapakivi granite occur in small isolated lenses in the Thumb in Lovell Wash. These occurrences of granite are not a part of the alluvial facies in the Gale Hills, but are part of the lacustrine facies. Anderson (1973) suggested these granite-clast lenses had a source in the Gold Butte area south of the Lake Mead fault system. I concur. Conglomerate and breccia of the Thumb Member in the Virgin Mountains appear to have been locally derived from the Gold Butte area. Coarse-grained breccia thought to be part of the Thumb occurs directly north of the Gold Butte fault and probably records activity on that fault. This breccia apparently fines to the north into alluvial rocks interbedded with lacustrine rock in Wechech Basin. The clasts in both the alluvial rocks and the breccia match lithologies exposed in the Gold Butte area BITTER RIDGE LIMESTONE MEMBER DISTRIBUTION AND THICKNESS The Bitter Ridge Limestone Member is exposed only north and northwest of the Lake Mead fault system (fig. 17), unlike the Rainbow Gardens and Thumb Members, which are widespread in the Virgin Mountains. At some locations, such as in the western Muddy Mountains, the Figure 17.—Geographic distribution of outcrops (shaded) of the Bitter Ridge Limestone Member of the Horse Spring Formation. Two major faults of the Lake Mead fault system are shown for reference. The solid dot in the southern part of Lovell Wash indicates the position of conglomerate that interfingers with limestone. Bitter Ridge Limestone Member rests depositionally on Paleozoic rocks beyond the northern depositional limits of both older members. The easternmost exposures of the member occur at Bitter Ridge, its type section, between White Basin and Bitfer Spring Valley. West of there it occurs near Buffington Pockets in narrow ridges throughout the southeastern Gale Hills, and in numerous exposures south of California Wash and north of Frenchman Mountain. Although isolated from and lithologically dissimilar to other exposures of the member, the rocks exposed in the same stratigraphic interval near Lava Butte are included with the Bitter Ridge Limestone Member herein. The Bitter Ridge Limestone Member rests conformably on the Thumb Member except where it unconformably overlies Paleozoic rocks, and the Lovell Wash Member is conformable above the Bitter Ridge Limestone Member at most locations. At its type section the Bitter Ridge Limestone Member is about 375 m thick and dips north under White Basin. In the eastern part of White Basin it abuts against the White Basin fault. At the western side of the basin, near Buffington Pockets, the Bitter Ridge dips to the southeast, indicating that the basin is synclinal and that the member is present in the subsurface in at least the western part of the basin. The member probably occurs in the subsurface of much of the northern part of the basin as well, but it is not exposed along the northern basin margin. At the northern margin the overlying Lovell Wash Member and the informal unit of red sandstone appear to buttress against the Paleozoic rocks. This relation indicates that the34 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Bitter Ridge Limestone Member either was eroded or was not deposited there. In the northern Gale Hills, south and southwest of California Wash, the Bitter Ridge Limestone Member dips slightly to the north and onlaps Paleozoic rocks west of Muddy Peak. It projects into the subsurface under southern California Wash, and although its maximum northern extent is not known, it could be continuous throughout California Wash. The Bitter Ridge Limestone Member overlaps the Gale Hills fault and is depositional on Paleozoic rocks to the north and on the Thumb Member to the south. The unit is exposed throughout the northwestern Gale Hills to the region north of Frenchman Mountain. Near Lava Butte, rocks in the same stratigraphic position as the Bitter Ridge Limestone Member are 180 m thick and dip east, but they are not continuous at the surface with other exposures of the member. These lithologically distinct rocks are probably present in the subsurface east of Lava Butte, but it is not known whether they continue into typical Bitter Ridge strata there. Northeast of Lava Butte, the underlying Thumb Member and the overlying Lovell Wash Member are in contact, and no rocks equivalent to the Bitter Ridge Limestone Member are exposed. LITHOLOGY The sole rock type of the Bitter Ridge Limestone Member at most localities is uniform light-brown, yellow, and pale-pink crystalline limestone. A thin unit of red and yellow sandstone occurs near the top of the member at Bitter Ridge. This sandstone unit thickens in the Lovell Wash area, where it is gypsiferous. In Lovell Wash there is an abrupt lithofacies change from limestone to conglomerate (fig. 17). Near Lava Butte the member consists of brown and pale-red-brown arenaceous limestone, intraformational arenaceous limestone breccia, and calcareous sandstone. These beds also include a minor amount of dark brown conglomerate. Calcareous Lithologies Light-tan, yellow, and pale-pink limestone of the Bitter Ridge Limestone Member has parallel, wavy, continuous, uniform bedding that ranges from about 1 to 20 cm in thickness. Some bedsets, which are commonly 15 to 80 cm thick, weather differentially and masquerade as individual thick beds, but close inspection shows these bedsets commonly consist of as many as 20 individual beds (fig. 18). Individual beds can be distinguished from one another by differences in weathering which are probably due to grain size and (or) textural differences. Texturally the rock is highly porous and has both laminated and unlaminated aspects (figs. 185 and O. The laminations are very thin, wavy, relatively continuous in some beds but discontinuous in others, and nearly parallel to bedding surfaces. Unlaminated beds have large vugs and resemble tufa In thin section the Bitter Ridge is composed of very fine- to coarse-grained crystalline limestone. This limestone consists of a rhythmic alteration of thin laminations of very fine grained crystalline calcite (possibly micrite) and thicker laminations of mediumgrained crystalline calcite (spar) with an interlocking grain texture. In most rocks, the primary bedding and grain types have not been well preserved, owing to post-depositional disruption of the laminations and recrystallization. However, some fine-grained thin laminations preserve laterally linked micro-hemispheroids, which impart a stromatolitic texture. Thinner laminations are generally dark because of impurities, and coarser grained laminations consist of relatively clear calcite crystals. Laminations are wavy, discontinuous, nearly parallel, and of uniform thickness. Figure 19 is a photomicrograph of a sample in which some original texture is observable. Where original depositional features have not been well preserved, the laminations are less distinct and are discontinuous and wavy. Lumps of fine-grained mud or crystalline carbonate with diffuse edges are alined to form the irregular laminations. These lumps consist of rounded masses of the disrupted original laminae, long stemlike features that may be plant fragments, and rounded individual sand-size crystals of calcite. Impurities such as mediumgrained, angular, well-sorted quartz, feldspar, and biotite constitute roughly 5 percent of the limestone. These grains are supported by carbonate cement and are evenly disseminated. Medium- to coarse-grained crystalline carbonate minerals fill some of the pores and have cemented many of the grains. Other carbonate rocks occur in the Bitter Ridge Limestone Member in Lovell Wash and near Lava Butte. Intraformational limestone breccia is inter-bedded with limestone in the upper part of the member in Lovell Wash. This breccia occurs in several 1- to 2-m-thick beds that have well-defined, even-surfaced, parallel bases and tops. Its clasts are well sorted and locally matrix-supported; they range from a few centimeters to 20 cm in diameter and are composed of limestone similar to that common elsewhere in the member. Southeast of Lava Butte, arenaceous limestone, limestone intraformational breccia, and calcareous sandstone exhibit discontinuous, wavy, nonparallel, nonuniform bedding. Unbedded limestone intraformational breccia is also common and in many places gradesDESCRIPTION OF ROCK UNITS 35 Figure 18.—Bedding characteristics in the Bitter Ridge Limestone Member of the Horse Spring Formation. A, Typical bedding in the member at its type section at Bitter Ridge. Notebook in center of photograph is 22 X 13 cm. B, Detailed view of typical bedding and laminations within the member in the area south of California Wash. Pencil near top is about 15 cm long. C, Detailed view of tufa-like bed in the member south of California Wash. Pencil at right provides scale. laterally into bedded and laminated limestone. Normal bedding has apparently been disrupted to form the clasts in the intraformational breccia; such disruption also commonly occurs in lenses associated with clayey fine-grained limestone. Discontinuous, large-scale trough crossbeds are common in the calcareous sandstone and arenaceous limestone. However, some of the thicker, ledge-forming sandstone units have parallel, discontinuous bedding with only low-angle crossbedding. Most of the sandstone is composed of detrital36 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 19.—Photomicrograph of sample from the type section of the Bitter Ridge Limestone Member at Bitter Ridge. The laterally linked micro-hemispheroids are visible in the photograph, as are light-hued pore fillings of crystalline carbonate. Plane-polarized light. grains of limestone and dolomite that were probably derived from nearby Paleozoic rocks. Noncalcareous Lithologies A unit of red and yellow sandstone and gypsum within the upper part of the Bitter Ridge Limestone Member in Lovell Wash is 110 m thick, but thins to less than 30 m on the north side of Bitter Ridge. The sandstone occurs in parallel, even, continuous, uniform beds that average 20 cm thick; it is red to red brown where the cement is calcareous, but yellow where gypsum is the cement. Gypsum occurs in the middle part of the unit and has disrupted bedding, but the disrupted beds are internally stratified. Gypsum and sandstone are gradational vertically. In southern Lovell Wash the entire upper part of the Bitter Ridge Limestone Member and the lower part of the overlying Lovell Wash Member grade into conglomerate near the trace of the Bitter Spring Valley fault and an unnamed fault that trends towards Frenchman Mountain (fig. 17). The lithofacies change between limestone and conglomerate is abrupt, taking place within about 30 m, as congomerate south of Lovell Wash interfingers with limestone in the wash bottom. Conglomerate is only a minor constituent north of the wash. Bedding in the conglomerate is roughly parallel on a large scale, but is discontinuous, nonparallel, and curved on a small scale. Scour features, channels, and large-scale trough crossbeds are present. The texture of the conglomerate is clast-supported and the clasts are of dominantly Paleozoic carbonate rocks, which range from 50 cm to less than 5 cm in diameter, are moderately sorted, and are rounded. One of the most interesting features of the clasts in the area of the facies change is a concentrically laminated rindlike coating of carbonate minerals (fig. 20). A thin unit of dark-brown conglomerate containing clasts of the dacite of Lava Butte occurs in the Bitter Ridge Limestone Member near Lava Butte. These clasts are set in a hard siliceous matrix. The presence of this conglomerate confirms the subaerial exposure of the dacite plug at Lava Butte during deposition of the Bitter Ridge. The subaerial exposure of the dacite indicates that it did not intrude the Bitter Ridge and that it predates that unit. Sedimentary Structures Rare laterally linked micro-hemispheroids, apparent in some thin sections (fig. 20), and the wavy, stromato-litic bedding characteristic of the Bitter Ridge suggest that stromatolitic features are widespread. Another common sedimentary structure is a “teepee” structure, which, in cross-sectional view, is an upward convergence of beds forming an inverted V with coarse crystalline carbonate minerals in the central part of the V (fig. 2L4). The “teepees” range from a few centimeters to as large as 50 cm in height, and they may involve one bed or several. They are not closely spaced either laterally or vertically. They are exposed on at least one bedding-plane surface, where they form a polygonal pattern of pressure ridges about 10 to 50 cm in height and about 25 m across (fig. 2 IB). Several other rare, but important, sedimentary structures occur in the upper part of the Bitter Ridge Limestone Member in Lovell Wash. Large circular depressions (fig. 22) are exposed on some bedding-plane surfaces. The largest is nearly 25 m in diameter, and its center is 1.5 m lower than the surrounding bedding-plane surface. Several also have raised rims Vt m above the surrounding surface. All are oriented along a curved line, and several large cracks in the limestone correspond to the trend of the depressions. All have large cracks in their centers. The depressions appear to be a local phenomenon, but it is not possible to tell how widespread they may be without further detailed study. Also, several types of large stromatolitic features occur in the upper limestone unit above the red and yellow sandstone. The most notable of these are large clusters of both laterally and vertically linked hemispheroids that form stromatolites as high as 50 cm shaped like icecream cones (figs. 23 A, B, and Q. These clusters areDESCRIPTION OF ROCK UNITS 37 Figure 20.—Conglomerate interfingering with the Bitter Ridge Limestone Member in the southern part of Lovell Wash. Note the calcareous coating on many of the clasts. Largest clasts are about 15 cm in diameter. laterally separated from one another by as little as a few centimeters to as much as 20 m, and the normal bedding between them bows upward at their margins. Many of the beds associated with the clusters contain smaller laterally linked hemispheroids that weather into eggshell-like chips (algal rinds). These smaller stromato-litic features are much more abundant than the larger clusters. Below the red and yellow sandstone unit, other occurrences of stromatolites include bedding-plane exposures of small (1-2 cm) domes that are associated with a minor amount of chert (fig. 23D) and low-relief hemispheroids that form a polygonal pattern on the bedding plane (fig. 23JE7). FACIES AND DEPOSITIONAL ENVIRONMENTS The Bitter Ridge Limestone Member is chiefly stromatolitic limestone, which can be subdivided into two subfacies: the wavy-bedded subfacies, which is the most widespread, and the stromatolitic mound subfacies, which apparently occurs only in the upper one-fourth of the member in Lovell Wash. A conglomeratic lithofacies, also exposed in Lovell Wash, is the southern stratigraphic equivalent of the stromatolitic mound subfacies, and it is distributed near large faults. Near Lava Butte the Bitter Ridge consists of sandstone and intraformational breccia, which are grouped with the conglomerate as the clastic lithofacies. Both subfacies of the stromatolitic limestone appear to have originated as sediment in a lacustrine environment. The tabular nature of the member; the parallel, continuous bedding; and the fine, uniform laminations of the wavy-bedded subfacies support the lacustrine origin. A lacustrine environment provides the even, widespread surface necessary to develop such bedding, whereas few other nonmarine environments could. Apparently the lake was relatively free of detrital influx, because the limestone contains only widely dispersed grains of siliceous clastic material and no sand-sized38 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 21—“Teepee” structures (pressure ridges) in the Bitter Ridge Limestone Member. A, Cross sectional view of a “teepee” in the Bitter Ridge area The measuring stick is marked in decimeters. Feature is formed by the displacing action of crystallizing carbonate matrix minerals. B, View of a bedding-plane exposure of the polygonal pattern made by the teepee structures in Lovell Wash. The polygons are about 25 m across.DESCRIPTION OF ROCK UNITS 39 _ , „ -x - • 3*? ' ^ * ' K+~ \ X --t' v-.; %. -% # » * ’ ■ < k * *fcr* *n#V *r O X/ t- \ < k^r" -* t‘" Figure 22.—Large circular depressions on a dip slope in the Bitter Ridge Limestone Member exposed in the north bank of Lovell Wash. A, Distant view of most of the features. Note their arcuate alinement along cracks, the size range of the features, and the man standing in the largest of them for scale. B, Closeup view of the largest depression. The measuring rod in the lower part is 6.5 m long.40 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 23.—Stromatolitic structures in the Bitter Ridge Limestone Member. A, Three adjacent hemispheroids. The one on the left is a single vertically linked hemispheroid. B and C, Examples demonstrating the ice-cream-cone shape of most of these features. D, Small mounds exposed in relief on a bedding-plane surface. E, Polygonal pattern made by small stromatolitic mounds on a bedding-plane surface. grains of detrital carbonate. Siliceous grains and the fragments of plant material are thought to have been transported into the lake by eolian processes. The wavy bedding, laterally linked micro-hemispheroids, large ice-cream-cone-shaped stromatolitic clusters, large vertically linked hemispheroids, and the beds containing eggshell-like chips are interpreted to be algal in origin. Because these stromatolitic features are widespread, the algal lacustrine environment is considered to characterize the member, and the lake was probably shallow enough that most of its bottom was within the photic zone. Coarse-crystalline, vuggy limestone is interpreted to have formed as tufa deposits whose origin is closely tied with local spring activity. Other features possibly indicative of spring activity are the large circular depressions that occur near the conglomeratic facies, which is interpreted below to be a basin-margin deposit. These depressions may have originated by dissolution of limestone at the site of springs, and their raised rims may be the result of carbonate mineral precipitation during overflow of spring water. Similar features are known in modem spring environments, but the water surrounding these types of springs is commonly no deeper than the marginal rim of the spring depression. The depth of water present during Bitter Ridge deposition is not known, but if the stromatolitic features are indeed algal, standing water, no deeper than the photicDESCRIPTION OF ROCK UNITS 41 zone, was probably present throughout deposition. Thin laminations in the limestone may indicate seasonal variations in carbonate mineral precipitation in an alkaline lake with little or no indigenous biota Wave action might have caused the disrupted and redeposited laminations. The domes and clusters of small domes probably indicate a water depth at least as great as the relief on any individual dome, which is as much as 5 cm, or they could have formed near the alternately wet-dry lake margin. The “teepee” structures are interpreted to have formed as a postdepositional feature. Cementation and the growth of crystalline carbonate minerals in the intergranular spaces is thought to have caused an increase in volume of the newly deposited bed. This volume increase resulted in its lateral expansion and the formation of a polygonal network of “pressure ridges” on the bedding surface. Shinn (1969) attributed these features in the marine environment to a similar process of submarine cementation. The clastic facies is interpreted to have originated in a localized alluvial fan that spread northward into the lake. Probably this conglomerate indicates activity and uplift on the Bitter Spring Valley fault and (or) the unnamed fault between Lovell Wash and the north side of Frenchman Mountain. At Lava Butte the clastic rocks, which show evidence both of clastic influx and reworking of previously deposited limestone, may be the result of a similar type of activity probably on the unnamed north-trending fault that passes east of Lava Butte (fig. 3Q. LOVELL WASH MEMBER DISTRIBUTION AND THICKNESS The Lovell Wash Member, like the Bitter Ridge Limestone Member, occurs only northwest of the Lake Mead fault system (fig. 24). However, unlike the Bitter Ridge it is not widely distributed or continuous between exposures. It is exposed in White Basin, in Lovell Wash, around Black Mesa north of Callville Bay, and between Lava Butte and Gypsum Wash. In the southeast limb of the syncline in White Basin it is 450 m thick and dips to the northwest and west. The Lovell Wash is also exposed in the west limb in western White Basin, where it rests depositionally on Paleozoic rocks at the northern margin of the basin. It probably occurs throughout the subsurface of the basin, but no outcrops of it are known north of White Basin. At its type section in Lovell Wash at least 250 m of the member is exposed in a syncline, but its top is not exposed there. Between Lava Butte and Gypsum Wash the member measures 170 m, but numerous small faults and covered intervals make this value speculative. At Lava Butte the Lovell Wash dips east and southeast, and it is probably present in the subsurface east of there. Several isolated 115°00' 114°45' 114°30’ Figure 24.—Geographic distribution of outcrops (shaded) of the Lovell Wash Member of the Horse Spring Formation. Two major faults of the Lake Mead fault system are shown for reference.42 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION exposures occur between Lava Butte and the widespread outcrops at Black Mesa, where the member undergoes a facies change to clastic rocks near the Bitter Spring Valley fault. Although widely separated by areas of erosion, the isolated outcrops of the Lovell Wash Member are probably remnants of a once continuous deposit, as is manifest from their lithologic similarity. The Lovell Wash Member is conformable above the Bitter Ridge Limestone Member in Gypsum Wash, where it and the Thumb are in contact. The Lovell Wash Member is unconformably overlain by the red sandstone unit in White Basin and at Lava Butte and unconformably overlain by the Muddy Creek Formation at Black Mesa LITHOLOGY The Lovell Wash Member is chiefly white limestone and dolomite; gray and white claystone; and gray and brown tuff, tuffaceous sandstone, and arenaceous tuff. Brown chert occurs with the carbonate rocks at many locations. At Black Mesa, near the Bitter Spring Valley fault, there is a facies change from the above rock types into pink, brown and red-brown sandstone and siltstone. In southern Lovell Wash, in a similar change, conglomerate and sandstone interfinger with the above rock types. Northwest of Black Mesa, several small intrusive igneous bodies have slightly altered the carbonate and clay minerals of the Lovell Wash. Carbonate Rocks, Claystone, and Chert Carbonate rocks and claystone are combined for purposes of discussion because they are interbedded in the two most common of four definable carbonate-rich litho-facies. These lithofacies are all gradational into one another, but they can be defined by their bedding thickness, the presence or absence of claystone, and the nature of the carbonate rock. In the most widespread lithofacies (medium-bedded limestone-claystone lithofacies), limestone, dolomite, and claystone are present in medium, nonresistant, rhythmically interstratified beds in which the ratio and relative bedding thickness of the carbonate and claystone varies (fig. 25A). Dolomite is the less common of the two carbonate mineral species present, but appears to occur over a wide area. Most of the carbonate beds are resistant, and these define the wavy, parallel, uniform bedding; some beds are discontinuous. Bedding commonly varies from 5 to 20 cm in thickness, but thins considerably in places where it is gradational into a lithofacies composed of thin-bedded limestone and claystone interbedded in V2- to 2-cm beds (fig. 25B). In the thin-bedded lithofacies the relative resistance between limestone and claystone is about equal, claystone is commonly more abundant than limestone, and bedding is parallel, discontinuous, and wavy. Both the medium- and thin-bedded claystone-bearing lithofacies occur with two carbonate-rich lithofacies in which claystone is absent. One of the claystone-free lithofacies is medium-bedded limestone similar to that in the medium-bedded carbonate-claystone facies, and the other claystone-free lithofacies is poorly bedded to unbedded limestone. The two claystone-free lithofacies are not known to occur together. In the medium-bedded claystone-free facies, which is common in the northern part of White Basin, parallel, wavy, continuous beds range from 5 to 15 cm in thickness, but bedsets as much as 30 cm thick weather differentially, giving the rock a thick-bedded appearance. This facies resembles the Bitter Ridge Limestone Member, but it is distinguished from the latter by its stratigraphic position and slightly lighter color. The unbedded to poorly bedded claystone-free lithofacies occurs in isolated lenses and amorphous masses that range from a few meters in width and 1 m in thickness to tens of meters in thickness and width (fig. 25Q. The gray and brown coarse crystalline limestone of the unbedded lithofacies is abruptly gradational into both of the clay-rich facies and is common throughout the Lovell Wash except in the northern part of White Basin. Bedding in the unbedded to poorly bedded lithofacies is discontinuous, extremely wavy (undulatory), nonparallel, and not of uniform thickness. Many vertically stacked hemispheroids as large as 1.5 m across occur, and chert is also present in the limestone of this facies. Petrographic analysis reveals that unordered to poorly laminated crystalline carbonate minerals (mostly calcite) are dominant in the imbedded to poorly bedded limestone facies and common in the other facies. In this vuggy dismicrite,7 sparry calcite and chert form irregular masses between larger zones of fine-grained crystalline carbonate material (micrite) that is irregularly structured and contains undulatory laminations (fig. 26A). The vuggy dismicrite is commonly fragmental, and the micrite forms large clumps cemented by sparry calcite. Other common textural types in the medium-bedded carbonate-claystone and medium-bedded claystone-free facies include oolitic grainstone as well as packstone and grainstone composed of a variety of carbonate fragments. The oolitic grainstone (fig. 26B) consists of loosely packed, grain-supported ooids that vary in their degree of preservation: in some, ’Texture is like that described by Folk (1968), but the use of this term here does not imply disruption of previously deposited sediment.DESCRIPTION OF ROCK UNITS 43 Figure 25.—Three of the four lithofacies in the carbonate and claystone beds of the Lovell Wash Member. A, Medium-bedded lithofacies of carbonate rocks and claystone rocks showing parallel, even, wavy, uniform, and mostly continuous bedding. Rhythmically interbedded carbonate rocks and claystone are visible. B, Thin interbeds of limestone (light bands) and claystone (dark bands). C, Large lens of poorly ordered limestone that contains chert. Measuring tape is 1.5 m long. both radial and concentric banding, a central core or grain, and well-defined outlines are preserved, but at the other extreme are those in which internal structures and sharp outlines have been erased by recrystallization or were not initially formed. The ooids range from 0.5 to 0.1 mm in diameter, are very well sorted, and are44 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 26.—Photomicrographs of three different characteristic textures of limestone in the Lovell Wash Member. A, Unordered, poorly laminated dismicrite with sparry calcite (light-colored material) filling a large, irregularly shaped area at right and several smaller, micritic, poorly laminated areas. B, Oolitic limestone with loosely packed cemented ooids (darker areas) and pore spaces (lighter areas) between them. C, Grainstone consisting of grains of calcite crystals (dark subrounded grains), ooids, and lumps. Some quartz grains (rounded white grains) are also visible. cemented by fine-grained crystalline calcite and chert. The grainstone and packstone consist of grain-supported calcite crystals, whole and fragmental ooids, microcrystalline lumps, and large, unoriented fragments of rock showing laminations that contain some laterally linked micro-hemispheroids (fig. 26C). Many grains have eroded, irregular boundaries that indicate dissolution, and most are cemented by fine-grained calcite. The limestone and dolomite contain 1 to 5 percent medium- to fine-grained, subangular detrital quartz, andesine, sanidine, volcanic lithic fragments, biotite, and hornblende. In most samples these grains are dispersed throughout the rock. However, in some dismicrite samples the grains are floating in the crystalline carbonate mineral matrix that apparently fills pores. Macroscopic chert, associated with carbonate rocks, occurs in small lenses, thin beds, mounds, domes, amorphous masses, and flat, irregularly shaped pods. The small lenses and thin beds, commonly of green chert, occur chiefly in Lovell Wash, where clastic rocks and tuf-faceous beds are interbedded with carbonate rocks. Mounds, domes, and amorphous masses are brown to purple and occur throughout the member in association with the unordered to poorly laminated limestone facies and the thinner bedded claystone-bearing facies. Flat, irregularly shaped pods occur along bedding planes in the thin-bedded carbonate-claystone facies. Much of the chert occurs as thin rinds on the largest domes and mounds, but smaller domes (5 to 25 cm) are commonly all or nearly all chert. Original bedding is preserved in the chert at places where a small percentage of the original calcite is also preserved. Figure 27 is a photograph of a vertically linked hemispheroid in a small chert dome whose interior is bedded calcite. Tuffaceous and Clastic Lithologies Tuffaceous rocks are gradational from unaltered tuffs, some of which might represent unreworked ash falls, to slightly tuffaceous sandstone. Most of the tuffaceous rocks are reworked to some extent, and many include grains derived from Paleozoic and Mesozoic rocks in addition to pumice shards and phenocrysts. Nontuffaceous clastic rocks are present around Black Mesa (north of Callville Bay) and in southern LovellDESCRIPTION OF ROCK UNITS 45 sr- Figure 27.—Small chert dome from the Lovell Wash Member in which original bedding is preserved. The light bedding in the interior of the dome is composed of bands of calcite that are only partially replaced by chert. At the margins of the dome the bedding and the calcite are totally replaced. Wash. They include sandstone, siltstone, and a minor amount of conglomerate. The latter occurs near the Bitter Spring Valley fault. Units of gray tuff and tuffaceous sandstone vary from a meter to tens of meters thick and have parallel, continuous, even bedsets of low- and high-angle crosslaminae that vary from 25 cm to 1 m thick (fig. 28). The most common type of cross-stratum is low angle and has flat, parallel, even, continuous surfaces. Unstruc- tured, thin, laterally discontinuous tuff beds at the base of thicker tuffaceous sandstone units may represent primary air-fall tuff. Petrographic analysis reveals contrasts in mineralogy an^ texture that may represent degrees of reworking of the tuffaceous material. Although some samples are composed almost entirely of coarse-grained shards and (or) pumice lapilli as much as 1 mm in diameter, most have medium- to coarse-grained phenocrysts or reworked grains of quartz, andesine, sanidine, biotite, and hornblende. The percentage of phenocrysts and reworked grains ranges from less than 1 to nearly 100 percent. In the higher percentage samples, shards and lapilli are absent. Other rocks contain variable percentages of coarse-grained carbonate lithic fragments composed of fossiliferous micrite, apparently derived from Paleozoic carbonate rocks, and rounded quartz, apparently derived from Mesozoic sandstone, in addition to tuffaceous fragments. Shard- and lapilli-rich samples generally have a sparry matrix in which the shards and phenocrysts are supported (figs. 29A and B); but in samples with more alteration or apparent reworking, grain support is common, and both clay and finegrained crystalline calcite matrices occur (figs. 30A and B). Where present, shards are generally delicately preserved, phenocrysts and detrital grains are angular to subangular, and carbonate lithic fragments are rounded. The coarse-grained shards contain fresh glass and have unaltered margins, but nearly all of the pumice lapilli are wholly or partially altered to clay (fig. 31). Because the lapilli have fine texture, it is apparent Figure 28.—One of the thickest, best-exposed units of tuffaceous rocks in the Lovell Wash Member, photographed at the type section in Lovell Wash. The parallel thick bedding, defined by erosional differences in individual beds, is evident, but the beds are also parallel laminated and cross laminated. Many beds have current lineations on their surfaces.46 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION that the amount of readily alterable surface area controlled alteration patterns. At Black Mesa north of Callville Bay, pink, brown, and red-brown sandstone and siltstone are abundant next to the Bitter Spring Valley fault, and they grade northward into white and gray carbonate rocks and tuff away from the fault. Widespread younger volcanic cover, complex pre-volcanic structure that affects the Lovell Wash Member, and local alteration of the member near small intrusive igneous bodies make detailed investigation of this facies change difficult, and none was attempted. Like the sandstone and siltstone of the Thumb Member, the Lovell Wash Member next to the Bitter Spring Valley fault consists chiefly of medium- to very fine-grained subangular to subrounded carbonate lithic fragments and quartz grains. Lenses of granite-bearing breccia are present at Callville Bay, but appear to be contained within the Bitter Spring Valley fault zone and may not relate to the lithofacies change described here. Conglomerate with a rindlike coating of carbonate matter on its clasts (fig. 20), which probably is related to this facies change, interfingers with both the Lovell Wash and Bitter Ridge Limestone Members in Lovell Wash. This rock type and the facies change associated with it were described in the section on the Bitter Ridge Limestone Member. Sedimentary Structures In the medium- and fine-bedded carbonate-claystone and the medium-bedded limestone facies, “teepee” structures, small-scale disrupted bedding, “eggshell” stromatolitic beds, and vertically stacked cherty hemispheroids and domes are the most conspicuous sedimentary structures. ‘Teepee” structures are commonly 25 to 50 cm high, involve one or several individual beds, and appear to form large polygonal patterns on bedding-plane surfaces (fig. 32). Many of the thin limestone beds, especially those interbedded with claystone or with claystone partings, are contorted into small-scale folds and boudins (fig. 33), but overlying and underlying beds are not deformed. Some limestone beds contain very thinly laminated stromatolitic structure that consists of many laterally linked and vertically Figure 29.—Photomicrographs of shard-rich, matrix-supported tuf-faceous rock from the Lovell Wash Member. Both photographs are in cross-polarized light. A, Delicately preserved shards (dark areas) set in a sparry calcite matrix (lighter areas). Shards do not appear to be abraded; sample may represent an ash fall. B, Slightly abraded shards set in a sparry calcite matrix. Some reworking is evident in the roundness of many shards and in the fact that the sample was taken from a cross-stratified rock.DESCRIPTION OF ROCK UNITS 47 Figure 31.—Photomicrograph of pumice lapilli from the Lovell Wash Member, set in a sparry calcite matrix. The lapilli are slightly altered to clay. Calcite occurs in some vesicles and holes. Cross-polarized light. stacked hemispheroids no larger than a few centimeters across (fig. 34). These limestone laminae that resemble stacked eggshells commonly are separated by clay-stone, but claystone is not always present. Small, high-relief stromatolitic mounds occur in the beds with eggshell laminae, and these attain a maximum height of about 10 cm and a relief above the surrounding bedding of as much as 2 cm. Other stromatolitic features are the chert-rich vertically stacked hemispheroids like those described above and shown in figure 27. However, chert is also commonly present in amorphous masses and irregular shapes that have no stromatolitic structure. Many outcrops of the unbedded to poorly bedded limestone facies could be termed carbonate mounds because they vertically and laterally interrupt the more Figure 30.—Photomicrographs of reworked, partially altered tuf-faceous rocks from the Lovell Wash Member. A, Fine-grained abraded shards and phenocrysts or reworked grains set in a fine grained, crystalline, calcite matrix. Cross-polarized light. B, Delicately preserved, grain-supported shards with clay matrix. The clay is thought to have been altered from very fine grained shards and pumice lapilli. Plane-polarized light.48 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 32.—“Teepee” structure in a thick bedset of limestone in the Lovell Wash Member. The “teepee” is about 60 cm high and has a relief above the surrounding bedding of possibly 20 cm. Part of the bedding plane is visible and the “teepee” extends onto that plane in two directions, to form part of a polygonal pattern. widespread, more consistent bedding and lithology of other facies, but are isolated from one another geographically (fig. 35^4). Local upturning of the lowest flanking beds marginal to the mounds suggests that they stood in syndepositional relief. Common internal sedimentary structures include large domes, large vertically stacked hemispheroids, and large dish-shaped structures. The domes, which are as much as 70 cm tall and 1 m across, occur in lines and groups and are differentially resistant (fig. 35B). Chert rings the tops of many domes and covers others in a thin rind. Large vertically stacked hemispheroidal laminations make up the internal structure of most domes. The large dish-shaped structures are commonly 3 m across and may be as deep as V2 m; some apparently had as much as Vi m of relief above surrounding bedding (fig. 35C). Chert commonly occurs around the margins of these circular features, and most of them have teepee-like internal structure. They vary in size, but rim height is exaggerated around many of the smaller ones, and the resultant structures are large grotesque mounds with small central depressions. Clusters of these irregularly oriented mounds commonly form large chert-rich masses. Sedimentary structures in tuff and tuffaceous sandstone include parallel, planar, large-scale, low-angle cross-stratification, rare high-angle cross stratification, current lineations on bedding-plane surfaces, and rip-up structures at the base of some beds. Figure 36 illustrates an example of rip-up structure. Figure 33.—Contorted thin bedding of the Lovell Wash Member. Claystone partings are evident as dark, less-resistant rocks. Numbered intervals on measuring rod are decimeters. DEPOSITIONAL ENVIRONMENTS Despite the complexity of the lithofacies of the Lovell Wash Member and the different local conditions implied by it, the majority of the rocks appear to have originated as sediment deposited in a lacustrine environment. The parallel, uniform bedding of the medium-bedded limestone-claystone facies, the stromatolitic beds, the “teepee” structures, and the abundance of in situ carbonate units collectively indicate lacustrine sedimentation. Lithologic complexities, such as different textural types and sedimentary structures, suggest that several subenvironments probably coexisted in the lake. Unordered to poorly laminated dismicrite is interpreted to be lithified tufa deposited in the lake by direct precipitation of carbonate minerals near springs. Oolitic limestone is thought to have lithified from oolitic sand that formed in clear saline and alkaline water with oscillatory wave action. Other types of grainstone andDESCRIPTION OF ROCK UNITS 49 Figure 34.—Thin limestone laminae with eggshell texure in the Lovell Wash Member in Lovell Wash. Thin claystone partings are evident, as are the individual domes and the relief on those domes. The knife at left is about 10 cm long. packstone appear to have resulted from in situ reworking of fragments from previously formed beds. All the stromatolitic features are interpreted to be of algal origin, and the chert that is commonly associated with them is almost certainly diagenetic. “Teepees” probably resulted from subaqueous cementation, which increased the volume of the bed and led to its lateral expansion and compression. Dismicrite, in the carbonate mounds of the unbedded to poorly bedded limestone facies, is thought to have developed directly at spring orifices, as are the large bowls, which are interpreted to be spring pots. Tuffaceous rocks appear to be composed chiefly of reworked material that is thought to have been derived from regional ash falls that were locally concentrated in the lake. Rip-ups and parallel laminations suggest high flow regimes and rapid deposition. Contaminants from local sources, chiefly Paleozoic carbonate rocks, appear to have mixed with the tuffaceous material during transportation, because deposits thought to be original ash falls do not contain them. Much of the claystone that is associated with the carbonate rocks is interpreted to be an alteration product of fine-grained shards and pumice lapilli. Sandstone, siltstone, and conglomerate that occur near the Bitter Spring Valley fault are interpreted to have been derived from south of that fault and shed north into the lake. These deposits may record activity on the Bitter Spring Valley fault. INFORMALLY NAMED ROCK UNITS Informally named rock units overlie the Horse Spring Formation near Lava Butte, in White Basin, and in the vicinity of the Grand Wash Cliffs. Although existing maps show them to have been included locally with the Horse Spring Formation or Muddy Creek Formation, they are herein separated from those stratigraphic units and are subdivided into two geographically separated informal units (fig. 37) based on reasoning explained in the section on stratigraphic nomenclature. Near Lava Butte one of the units, the red sandstone unit, overlies the Lovell Wash Member on a possible angular unconformity and is in turn overlain by the Muddy Creek Formation. Although an accurate stratigraphic measurement has not been obtained, the unit appears to be thinner than 100 m. In the western part of White Basin, the red sandstone conformably overlies the Lovell Wash Member. However, it is apparently unconformable above that member in the central part of the basin, where it is 500 m thick and is unconformably overlain by Quaternary!?) rocks. Based on lithologic similarities and equivalent ages, the separated outcrops of the red sandstone unit are thought to have once been continuous between White Basin and Lava Butte. Those near Lava Butte dip east and are probably continuous in the subsurface for several kilometers. Although in White Basin the red sandstone is confined by present basin geometry in most directions, to the south it was once probably more extensive. The other geographically separated unit, the rocks of the Grand Wash trough, occurs throughout the Grand Wash-Grapevine Mesa region, where it rests on an angular unconformity above the Rainbow Gardens Member of the Horse Spring Formation. An accurate measurement of the total thickness of the rocks of the Grand Wash trough is not possible because subsurface information is not available, but the exposed part of the unit is thicker than 500 m in the Grapevine Mesa area RED SANDSTONE UNIT Red sandstone rhythmically interbedded with siltstone and claystone composes most of the red sandstone unit, but gray and white air fall and reworked air-fall tuff beds and bedsets are common in its lower part. At the western margin of White Basin, adjacent to the50 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 35.—A large carbonate mound in the Lovell Wash Member 1 km southeast of Lava Butte, and some of its internal structure. A, The facies change between the coarse-grained crystalline limestone on the south (dark beds at left) and the medium-bedded carbonate and clay-stone on the north (light-colored beds in center and at right). The darker, unbedded coarse-grained limestone interfingers with the lighter colored medium-bedded carbonate and claystone in this exposure, but at some places the contact is abrupt and nearly vertical. B, An example of one of the large domes in the mound. Both a chert ring and a rind are evident. Numbered intervals on measuring rod are in decimeters. C, Large, circular, dish-shaped structures with chert rims. Muddy Peak fault, a basin-margin conglomerate litho-facies interfingers with a unit at least 100 m thick that consists of the red sandstone, tuff, and gypsiferous sandstone. Gypsum and gypsiferous sandstone are present in central White Basin near the contact with the underlying Horse Spring Formation (fig. 38). DetailedDESCRIPTION OF ROCK UNITS 51 Figure 36.—Rip-up structure in the tuff of the Lovell Wash Member in Lovell Wash. Clasts of the underlying limestone bed are incorporated into the tuff. Hammer at right is about 35 cm long. geologic mapping of the area near Lava Butte is incomplete, but both red sandstone and tuff are known to be exposed there. The red sandstone, siltstone, and claystone are in-terbedded in parallel, nonuniform beds that vary in thickness from 2 cm to 1 1/2 m. Resistant, even-surfaced sandstone beds are the thickest, averaging 20 cm, whereas the erosionally recessive claystone and finegrained siltstone beds average 5 to 7 cm. Nearly all beds appear to be massive internally and most are continuous, but some of the thinnest ones are discontinuous (fig. 39). Bedsets as much as 1 m thick of uniform, even, 10- to 20-cm-thick sandstone beds are also common. Petrographic analysis of the sandstone indicates subrounded to angular, grain-supported, very well sorted grains of quartz, plagioclase, and carbonate lithic fragments with minor amounts of diopside, biotite, chert, and hornblende. These grains are chiefly cemented by clean sparry calcite, but in some cases large crystals of gypsum surround grains, giving those 115°00' 45' 30' 15' 114°00' Figure 37.—Geographic distribution of informal Tertiary rock units discussed in the text. Geology of the Cottonwood Wash-Grapevine Mesa region after Lucchitta (1966) and Blair (1978).52 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION 114°40' 114°35' Figure 38.—Generalized geology of the red sandstone unit in White Basin. Three major lithologic facies are shown, as are some of the faults that cut the unit or were active during its deposition. This is the largest single exposure of this unit and represents 85 percent of known outcrop area samples a texture similar to poikilitic texture in igneous rocks. Grain size ranges from 0.05 to 1.0 mm in diameter, and clay or silt matrix is not present (fig. 40). White and gray reworked tuff beds and bedsets total less than 5 percent of the unit, but are a distinct part of its lower half. These beds range from 25 cm to 2 m in thickness and most have thin, parallel, even, discontinuous internal bedding. Unbedded tuffs a few centimeters thick occur at the base of many beds and bedsets and are interpreted to be unreworked ash falls. Low-angle, large-scale cross-strata are also common in some tuffs, as are contorted beds and wavy laminations. Unaltered, delicately preserved glass shards and partially altered pumice lapilli, the two most abundant constituents of the tuffs, commonly occur in a matrix of clay, but they are chiefly grain-supported. Phenocrysts and detrital grains are angular and include feldspar, biotite, sanidine, diopside, and quartz. The clay-mineral matrix, which is chiefly smectite, is interpreted to be an alteration product of very fine grained glass fragments and pumice lapilli because such alteration is observable in the coarser-grained lapilli present in most thin sections. Adjacent to the Muddy Peak fault in western White Basin, conglomerate interfingers to the east with the other lithologies of the red sandstone unit (figs. 38 and 41). Bedding, although not well defined, appears to be discontinuous, nonparallel, and nonuniform, ranging from a few centimeters to several meters thick. Texture of the conglomerate is clast-supported, and the sub-angular clasts of Paleozoic carbonate rocks range from 5 to 40 cm in diameter. The conglomerate occurs onlyDESCRIPTION OF ROCK UNITS 53 Figure 39—Red sandstone, sUtstone and claystone in the red sandstone unit about 2 km northeast of Lava Butte. Parallel, nonuniform, continuous beds are evident. A tuff bed (lighter gray bed at top of small cliff) 1 m thick is visible in the central part of the view. where resistant Paleozoic carbonate rocks, which were its source, are present directly across the fault to the west (figs. 38 and 41). North of Muddy Peak, these Paleozoic rocks have been tectonically removed from the area north of the intersection of the Muddy Mountain thrust with the high-angle Muddy Peak fault. The Triassic(?) and Jurassic Aztec Sandstone is juxtaposed to the red sandstone unit and upper Horse Spring Formation in this area At the point of fault intersection, the conglomerate of the red sandstone unit undergoes an abrupt facies change into sandstone and tuff. This relation suggests that the Muddy Peak fault incurred only vertical displacement (Bohannon, 1979). Gypsum and gypsiferous sandstone in the lower part of the unit in central White Basin are poorly bedded and coarsely crystalline. Veins and irregular deposits of gypsum are common. The red sandstone, siltstone, and claystone and the white and red-brown gypsum and gypsiferous sandstone are interpreted to have been deposited in a playa-lake environment. (In this paper, a playa is considered to be dry virtually all of the time.) The fact that bedding is parallel and continuous suggests a flat surface of deposition, but the lack of water-related sedimentary structures suggests the predominantly dry environment. The paucity of internal stratification within sand- stone beds possibly indicates sediment disruption by burrowing creatines or by expansion of the sediment during cement crystallization. The latter interpretation is favored because the gypsum, gypsiferous sandstone, gypsum cement, and carbonate cement are all thought to have crystallized between grains of previously deposited sediment in the vadose zone. Most of the gypsum and gypsiferous sandstone lacks well-defined bedding and is coarsely crystalline, and much of it is sandy and silty. Gypsum cement with “poikilitic” texture and clean sparry calcite cement in the sandstone both suggest postdepositional introduction and crystallization. Inasmuch as the gypsum and carbonate minerals form nearly pure deposits in some places and are merely cement or pore-space filler in others, the vadose crystallization apparently was variously effective, and the occurrence of gypsum veins indicates postdepositional recrystallization and remobilization. The chief precement sediment was clean, well-sorted sand inter-bedded with silt and clay. The good sorting of this sand suggests eolian transport, and its grain mineralogy indicates a combination of volcanic and carbonate sources. The presence of clay and silt is also compatible with these sources. The tuff beds and bedsets are interpreted to be ash-fall deposits largely reworked by streams from the surrounding terrain. The conglomerate on the west54 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Figure 40.—Photomicrograph of sandstone from the red sandstone unit. Well-sorted, grain-supported grains of quartz, plagioclase, carbonate lithic fragments, biotite, hornblende, rare pumice, and rare diopside are present in the rock. The cement is clean sparry calcite. Crossed nichols. side of White Basin is thought to have developed as a short, steep alluvial fan shed to the east into the playa lake during uplift of the Muddy Peak block by activity on the Muddy Peak fault. The total geographic extent of the playa environment is not well known, but it may have continued as far south as the Lake Mead fault system and as far west as Lava Butte. No evidence of it exists south of the Lake Mead fault system except possibly in the Grand Wash area, where some of the rocks of the Grand Wash trough are lithologically and temporally similar to the red sandstone unit. ROCKS OF THE GRAND WASH TROUGH Although not part of this study, the rocks of the Grand Wash trough have been mapped, described, and interpreted with those of the Muddy Creek Formation by Lucchitta (1966, 1972, 1979), Blair (1978), Blair and Figure 41.—Paleozoic carbonate rocks in the steep, rugged east flank of Muddy Peak (background) abut the smooth-surfaced conglomerate of the red sandstone unit (foreground) along the Muddy Peak fault, a high-angle vertical-slip fault. These Paleozoic rocks are the probable source of the clasts in the conglomerate, which inter-fingers with red sandstone and other fine-grained lithologies about Vt km east of the area shown here. View is to the northwest. others (1979), Blair and Armstrong (1979), and Bradbury and Blair (1979). Brief descriptions provided herein are based chiefly on Lucchitta’s work, but the other authors have contributed much information on the Hualapai Limestone. The basic lithologic subdivisions shown on figure 42 are slightly modified from Lucchitta (1966, p. 87, fig. 18). The red sandstone and siltstone facies shown on figure 42 bears a strong resemblance to the red sandstone unit of White Basin. The two units have a similar rock type, bedding, and color and both contain nearly identical, coeval tuff beds in the their lower parts. Lucchitta (1966, p. 85-86) describes these rocks as “gypseous, bright-red to brick-red sandstone-siltstone * * * composed chiefly of quartz grains coated with iron oxide * * * interbedded with layers of impure limestone” east of the Wheeler fault, a large fault bounding the west side of Wheeler Ridge. “These rocks are persistently fine-grained even near areas of considerable relief.” Lucchitta (1966, p. 92 and 93) also describes the tuff beds as “ranging in thickness from less than an inch to more than 5 feet * * * commonly very pure, persistent, and of uniform thickness * * * colored variously tan, pale blue, pale green, light gray, and white * * * commonly composed almost exclusively of very fresh delicate glass shards. UnbrokenDESCRIPTION OF ROCK UNITS 55 EXPLANATION Hualapai Limestone Lithologic subdivisions of the rocks of the Grand Wash trough: Conglomerate With clasts of crystalline rock With clasts of Gold Butte Granite of Longwell (1936) With clasts of sedimentary rocks Red sandstone and siltstone with tuff beds Gypsum Contact—Queried where uncertain Figure 42.—Generalized geology of the Grand Wash trough area, modified from Lucchitta (1966, p. 87, fig. 18). Vertical order of lithologies in the legend does not imply relative age; no chronologic sequence has been established, and many of the lithologic units on the map are laterally equivalent facies. glass bubbles are locally prominent. Green to brown biotite, commonly euhedral, is the other abundant constituent, but rarely exceeds 5 percent of the rock. In places, nodules and laminae of hard, vitreous, dark gray material, probably chert or chalcedony, are abundant. Euhedral to subhedral, little-abraded crystals of quartz, feldspar, biotite, and amphibole were noted * * Three distinct conglomerate lithofacies, described by Lucchitta (1966), are differentiated on the basis of clast lithology and include one with Gold Butte Granite of Longwell (1936) (rapakivi granite) and other crystalline clasts, another dominated by several types of crystalline clasts but lacking Gold Butte Granite, and one dominated by sedimentary clasts (fig. 42). Lucchitta (1966, p. 88, table 6) describes the overall nature of the conglomerate as red brown, pinkish brown, and light gray with very poorly sorted, subrounded to angular clasts that range from clay-size particles to boulders 30 feet (9.2 m) in diameter. Stratification ranges from chaotic to distinct and generally becomes better defined upward. Lucchitta (1966, p. 88, table 6) describes the Hualapai Limestone as impure, vuggy to dense, and finely crystalline with strata ranging from thin laminae to beds 10 feet (3.1 m) thick. He refines his description (Lucchitta, 1979) by adding that the Hualapai is the uppermost member of the Muddy Creek and in most places forms the thin upper cap of that formation, but it is also present as thick masses and as interbeds lower in the section. It is a maximum 300 m thick and consists of silty and sandy limestone that grades downward and laterally into limey sandstone, siltstone, and mudstone. Blair (1978), Blair and others (1979), Blair and Armstrong (1979), and Bradbury and Blair (1979) offer no further basic lithologic description of the Hualapai, but they have examined the chert, present in western exposures, and some sedimentary structures in the limestone at Grapevine Mesa, in detail. They found several different types of diatoms and some colloform algal stromatolites. Analysis of these organic constituents indicated to them that the Hualapai Limestone formed in brackish to saline water and possibly even in marine conditions. In addition, their interpretations of carbon and oxygen isotope data indicate saline, if not marine, conditions. They further infer that the Hualapai Limestone was deposited in a marine estuary formed as part of the Bouse embayment (Bouse Formation of southwest Arizona). All of the interpretations presented by Blair (1978), Blair and others (1979), Blair and Armstrong (1979), and Bradbury and Blair (1979) are equivocal, and a marine origin for the Hualapai Limestone is still uncertain. Longwell (1936), Lucchitta (1966, 1979), and Hunt56 NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION (1969) disagree with marine interpretations for the origin of the Hualapai. These authors have proposed that the Hualapai Limestone was deposited in one or several inland lakes that developed above the playa deposits of the red sandstone in the Grand Wash trough and above Precambrian crystalline rocks and clastic beds of the Muddy Creek Formation to the west of the trough. Hunt (1969) further suggested that the Hualapai lake was fed directly by the ancestral Colorado River. Longwell (1936) and Lucchitta (1966, 1979), on the other hand, suggest that the lack of fluvial or deltaic deposits in the Hualapai indicates that the Colorado was not in existence during its deposition. Also, Lucchitta (1979) has proposed several different subbasins that were controlled by local tectonics and that contain facies indicative of local sources and interior drainage. Although published interpretations of the origin of the Hualapai Limestone are conflicting, none place constraints on the relation between the red sandstone unit of White Basin and the red sandstone that underlies the Hualapai Limestone in the rocks of the Grand Wash trough. Conversely, information presented herein on the Tertiary rocks of the Lake Mead region does not severly constrain interpretations of the origin of the Hualapai. The constraining factor in the direct comparison of the two red sandstone lithologies is the lack of regionally consistent descriptive information bearing on the internal characteristics of the rocks of the Grand Wash trough. These two isolated units appear to be similar both lithologically and temporally, and both apparently formed during the same graben-forming tectonic regime. However, any postulated depositional ties between these two units are conjectural because of the large distance separating them and the intervening Lake Mead fault system. Formally assigning the rocks of the Grand Wash trough to the Muddy Creek Formation is also not warranted. The Muddy Creek and the rocks of the Grand Wash trough may partially overlap in age and both were deposited in interior basins, but no direct connection of the two has been demonstrated, and they are lithologically dissimilar in many ways. MUDDY CREEK FORMATION The distribution and probable maximum extent of the Muddy Creek Formation is depicted on figure 43. Although widespread, it is chiefly confined to lower elevations such as Mormon Mesa, where its exposed stratigraphic top is no higher than about 540 m. The highest known occurence is the Fortification Basalt Member at Fortification Hill at an elevation of 1128 m. From its type locality north of Glendale, it is continuous into the valleys of the Muddy and Virgin Rivers and California Wash, and it is interpreted to be continuous into Detrital Valley, Las Vegas Valley, and the valley of the Colorado River south of Fortification Hill. The probable original extent shown on figure 43 was drawn at the major break in slope along margins of the above valleys. In places, such as the Virgin and Boulder Basins, the Muddy Creek may have accumulated to higher elevations and thus had a greater extent. At Mormon Mesa its top is exposed and the maximum extent shown is probably realistic. The base of the Muddy Creek Formation is exposed only at the margins of the valleys, and its true thickness is not known; however, dissection in the valleys of the Muddy and Virgin Rivers reveals exposures that indicate it is at least 215 m thick. Drilling in Detrital Valley indicates a minimum thickness of 425 m. The most widespread rocks of the Muddy Creek are interbedded pink sandstone, siltstone, and claystone. Gypsum, gypsiferous sandstone and siltstone, and arenaceous gypsum are common, and conglomerate is also present at most of the basin margins. These rock types are gradational into one another, but large areas of homogeneity occur. In the Muddy and Virgin River valleys and the north half of California Wash the dominant rocks are sandstone, siltstone, and claystone, but conglomerate is present at the east flank of Overton Ridge, on the northwest flank of Black Ridge, and in southern California Wash. The gypsiferous facies occurs sporadically around the Overton Arm and is dominant in northern Detrital Valley and in the vicinity of Virgin Basin. Around Boulder Basin, Frenchman Mountain, and Las Vegas Valley all three facies occur, but their distribution is poorly understood. Bedding in the pink sandstone, siltstone, and claystone is chiefly parallel, even, continuous, and moderately uniform in thickness. Discontinuous wavy bedding, lenses, and channels are prominent near basin margins. Bedding thickness ranges from 1 cm to Vi m, and internal parallel laminations occur at a low angle to bedding in most of the beds. Small-scale trough crosslaminae occur in many of the lenticular beds and channel deposits. Bedding is defined by abrupt grain-size changes and resistant sandstone beds, but bedsets consisting of several beds of uniform grain size also occur. Petrographic analysis reveals that most of the sandstone is clay-rich arkose and subarkose with 0.1- to 0.02-mm grain-supported, moderately sorted grains of quartz, feldspar, calcite, muscovite, biotite, chert, and clay lithic fragments. Carbonate-clay matrix constitutes as much as 90 percent of some samples, but most are 30 percent matrix. Halite and glauberite are known from surface ex-DESCRIPTION OF ROCK UNITS 57 MOUNTAINS MOUNTAINS Muddy Peak A Frenchman Mountain Lava Butte MEAD LAKE RIVER MOUNTAINS Fortification Hill 115°00' 45' 30’ 114°15' 1 1 20 KILOMETERS 1 _J1 1 \0 \ % \ U / | c EXPLANATION ( 1 w Muddy Creek Formation: o 1 j I Present outcrops \ J NORTH & V* 1 1 \ £ Probable original ^ n/S - / , c extent ^ l Millerton Lake Quadrangle, West-Central Sierra Nevada, California—Analytic Data GEOLOGICAL SURVEY PROFESSIONAL PAPER 1261 Millerton Lake Quadrangle, West-Central Sierra Nevada, California—Analytic Data By PAUL C. BATEMAN and ALAN J. BUSACCA GEOLOGICAL SURVEY PROFESSIONAL PAPER 1261 Modal and chemical data on and isotopic ages of the plutonic rocks of the Millerton Lake quadrangle UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1983UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Bateman, Paul Charles, 1910- Millerton Lake quadrangle, west-central Sierra Nevada, California—analytic data. (Geological Survey professional paper 1261) Bibliography Supt. of Docs, no.: I 19.16:1261 1. Rocks, igneous. 2. Geological time. 3. Geology—California—Madera County. I. Busacca, Alan J. II. Title. III. Series: United States. Geological Survey. Professional Paper 1261. QE461.B42 1983 552.3’0979481 82-600354 For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304CONTENTS Page Abstract................................................................................. 1 Introduction ............................................................................ 1 General geology.......................................................................... 1 Sampling and analytical methods ......................................................... 1 Tbnalite south of Black Mountain......................................................... 2 Tbnalite of Blue Canyon.................................................................. 2 Tonalite south of the Experimental Range................................................. 3 Leucotonalite of Ward Mountain..............................1.......................... 3 Biotite granodiorite .................................................................... 3 Biotite granite ......................................................................... 3 Aplitic leucogranite..................................................................... 3 U-Pb age data............................................................................ 3 References cited......................................................................... 4 ILLUSTRATIONS Page Figures 1-9. Simplified geologic maps of Millerton Lake quadrangle showing: 1. Locations of chemically analyzed and isotopically dated samples ..................................... 6 2. Volume-percent quartz................................................................................. 8 3. Volume-percent potassium feldspar .................................................................... 9 4. Volume-percent plagioclase........................................................................... 10 5. Volume-percent mafic minerals........................................................................ 11 6. Volume-percent biotite .............................................................................. 12 7. Volume-percent hornblende............................................................................ 13 100 hornblende g .................... .................................................................................. 14 biotite + hornblende................................................... 9. Bulk specific gravity ............................................................................... 15 10. Plots of modes of granitic rocks ............................................................................ 16 11. Plot of norms of granitic rocks ............................................................................. 17 TABLES Page Table 1. Chemical analyses, norms, and modes of representative granitoids............................................................. 18 2. U-Pb age determinations on zircon from granitoids ......................................................................... 20 IIIMILLERTON LAKE QUADRANGLE, WEST-CENTRAL SIERRA NEVADA, CALIFORNIA—ANALYTIC DATA By Paul C. Bateman and Alan J. Busacca ABSTRACT More than 300 samplesof plutonic rocks were collected from the Millerton Lake quadrangle; 249 of these were analyzed modally, 13 were analyzed chemically, and zircon from 6 samples was dated isotopically by the U-Pb method. The results are given in tables and diagrams, and the brief text provides a background for understanding and interpreting the data. The U-Pb zircon ages of the most widespread granitoids are about 114 m.y. Except for scattered small bodies of granite and aplitic leuco-granite, all the granitoids are poor in potassium and include tonalite, leucotonalite, and biotite granodiorite. In these low-potassium rocks, potassium feldspar (K-feldspar) generally is in thin stringers interstitial to plagioclase, quartz, and the mafic minerals. This texture is interpreted to indicate that, when the magmas were emplaced and began to solidify, they contained crystals of plagioclase, quartz, and mafic minerals, but not K-feldspar. K-feldspar crystallized from interstitial melt. The presence locally of subequant crystals of K-feldspar indicates that, with continued crystallization and falling temperature, the melt phase of the magma became saturated with K-feldspar and precipitated crystals. INTRODUCTION The Millerton Lake quadrangle is in the western foothills of the central Sierra Nevada, and the southwest corner extends into the Central Valley of California. California State Highway 41 from Fresno to Yosemite National Park passes through the east side of the quadrangle, and branching roads provide access to all parts of the quadrangle. This report supplements the geologic map of the Millerton Lake quadrangle (Bateman and Busaca, 1982) by providing modal and chemical data and isotopic uranium-lead ages for the plutonic rocks. These data are contained in the maps, diagrams, and tables; the brief text provides a background for understanding and interpreting these data. A nontechnical summary of some of the more interesting aspects of the geology of the quadrangle accompanies the geologic map. GENERAL GEOLOGY The quadrangle is underlain chiefly by granitoids and by small bodies of diorite and gabbro and remnants of metamorphosed sedimentary and volcanic rocks. The tonalite of Blue Canyon is the most extensive granitoid, and it encloses the metamorphic remnants and is intruded by most of the other, less extensive, granitoids. Cenozoic volcanic and sedimentary rocks locally rest on the plutonic and metamorphic rocks, especially in the south-central and southwestern parts of the quadrangle, but only the sedimentary deposits of the Central Valley in the southwest corner of the quadrangle are shown on the simplified geologic maps accompanying this report (figs. 1-9). The analytic data pertain only to the granitoids. These rocks occur in separate plutons, which generally are either in sharp contact with one another or are separated by thin, discontinuous septa (screens) of metamorphic rock. However, in several places, hybrid zones occur between different granitoids and between granitoids and metamorphic rocks. These hybrid zones consist of intricately branching dikes of the younger rocks into the older and of fragments of varying size of the older rocks in the younger. The granitoids of the Millerton Lake quadrangle differ from granitoids farther east in the Sierra Nevada in that they generally contain little potassium feldspar (K-feldspar), having a low potassium content, and almost no magnetite. Comparison of the compositions of the Millerton Lake granitoids with granitoids farther east in the Sierra Nevada can be made by referring to reports of analytic data for the Kaiser Peak (Bateman and Lockwood, 1970), Huntington Lake (Bateman and Wones, 1972), Mount Abbot (Lock-wood, 1975), and Shaver Lake quadrangles (Bateman and Lockwood, 1976). SAMPLING AND ANALYTICAL METHODS Samples were collected in the course of geologic mapping, and although we made no effort to follow a rigid sampling pattern, we did try to collect samples about 1.6 km apart of the freshest and most representative rocks. More than 300 samples of plutonic rocks were collected; 249 of these were analyzed modally to l2 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA determine their mineral content (figs. 2-7), 13 were analyzed chemically for their major elements (table 1), and zircon from 6 samples was analyzed to determine isotopic uranium-lead ages (table 2). Samples collected for modal and chemical analyses averaged about one kilogram, and samples collected for age dating averaged about 50 kg. The chemical analyses were made in the rapid rock analysis laboratory of the Geological Survey, and the isotopic U-Pb ages were determined in the laboratory of T. W. Stern, both in Reston, Virginia. Modal analyses were made by point counting on sawed slabs of at least 70 cm2 on which plagioclase was stained red and K-feldspar yellow (Norman, 1974). The number of counts on most slabs of the tonalite of Blue Canyon ranged from 1,120 to 1,330 and averaged about 1,200, whereas the counts on the other granitoids were fewer, ranging from 1,040 to 1,140 and averaging about 1,100. Fewer counts were made on the less extensive granitoids because they are more deeply weathered than the tonalite of Blue Canyon, making it difficult to collect large fresh samples. The percentages of hornblende and biotite on the tonalite of Blue Canyon were determined by point counting on thin sections and apportioning the counts to the total mafic content determined on stained slabs. The number of counts on the slabs would be sufficient to permit assigning limits of error of less than ±3 percent at the 95-percent confidence level, the limits diminishing with the decreasing abundance of minerals, except for the fact that the points were counted at spacings of 1.75 to 1.81 mm, whereas the grain size of the rocks ranges up to 5 mm or more. Tb place limits of error on the counts, the points counted should be at least as far apart as the distance across the largest grains (Van der Plas and Tbbi, 1965). Nevertheless, counts on samples collected within a few meters of one another at different times and for different purposes show good agreement and indicate that both the counting errors on individual samples and the variations among closely spaced samples are small. Modes of two groups of closely spaced samples from the tonalite of Blue Canyon are illustrative: Group 1 Group 2 MLc-19 MLc-23 MLc-151, MLb-5 5 MLb-69 Quartz...................... 24 23 25 23 24 K-feldspar............... 2 3 1 4 tr. Plagioclase ................ 55 51 56 52 51 Biotite..................... 13 15 13 18 20 Hornblende................... 6 8 6 4 5 Tbtal ................ 100 100 101 101 100+ TONALITE SOUTH OF BLACK MOUNTAIN The tonalite south of Black Mountain underlies a small area in the southeast corner of the quadrangle. The rock is medium-grained hornblende-biotite tonalite and closely resembles the tonalite of Blue Canyon both compositionally and texturally. Because primary foliation in the tonalite of Blue Canyon truncates strong, probably cataclastic, foliation in the tonalite south of Black Mountain, the tonalite south of Black Mountain is considered to be the older rock. Sample MLd-17, the only sample collected from the tonalite south of Black Mountain, is strongly foliated and typical. TONALITE OF BLUE CANYON The tonalite of Blue Canyon occurs in two facies, a widespread facies that is characterized by conspicuous blocky hornblende prisms and a much less extensive facies that lacks conspicuous hornblende prisms. Despite the apparent abundance of hornblende in the blocky hornblende facies, biotite is more abundant than hornblende in most samples of both facies, and the boundary value of is about 25 (fi£- 8)- The color index of the blocky hornblende facies ranges from 10 to 30, wheras the color index of few samples from the biotite facies exceeds 20. The normative composition of plagioclase ranges from An44 to An49 in samples of the blocky hornblende facies and from An37 to An4i in the biotite-rich facies (table 1). Another difference in the two facies is that many samples from the blocky hornblende facies contain more plagioclase than samples from the biotite-rich facies. In both facies, K-feldspar is interstitial in all but a few samples where it occurs in subequant grains. Extensive cataclasis reduced the grain size of many samples and doubtless destroyed some subequant K-feldspar grains, but the fact that K-feldspar is mostly interstitial in undeformed samples indicates that subsequent K-feldspar grains were never abundant. We interpret these relations to indicate that the biotite-rich facies crystallized at a somewhat lower temperature and (or) in the presence of a higher volatile content than the blocky hornblende facies but that the two facies constitute a continuum. The spread of modes along the plagioclase-quartz side of the modal plot (figs. 10, 11) indicates that plagioclase was the first felsic mineral to crystallize and was followed by quartz. At the time the magma began to solidify, it was saturated in both these minerals as well as in hornblende and biotite. Interstitial K-feldspar crystallizedMILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 3 from interstitial melt and was not present in the magma as crystals. As the temperature fell and the magma solidified, subequant K-feldspar crystals began to precipitate locally from the melt phase of the magma. TONALITE SOUTH OF THE EXPERIMENTAL RANGE The tonalite south of the Experimental Range is a distinctive uniformly fine-grained tonalite that contains biotite but no hornblende. Sparse K-feldspar is interstitial. The pluton intrudes the tonalite of Blue Canyon and is intruded by the leucotonalite of Ward Mountain. LEUCOTONALITE OF WARD MOUNTAIN The leucotonalite (trondhjemite) of Ward Mountain forms two plutons in the west half of the quadrangle, the larger and more northerly Ward Mountain pluton and the more southerly Experimental Range pluton (fig. 1). Undeformed rock typically is medium grained, equigranular, and light colored, and deformed rock is fine grained and gneissic. Biotite is the sole mafic mineral and generally constitutes less than 10 percent of the rock, but several samples in a belt that extends across the south-central part of the Ward Mountain pluton contain 12 to 14 percent, and one sample contains 24 percent (fig. 6). Most samples of the leucotonalite contain less than 5 percent of interstitial K-feldspar, and many samples contain none. However, samples in the northwest and south-central parts of the Ward Mountain pluton contain 6 to 29 percent, and two samples in the west side of the Experimental Range pluton contain 15 and 16 percent. Most of the K-feldspar in samples that contain more than 5 percent is in subequant grains. These relations indicate, as do similar relations in the tonalite of Blue Canyon, that the magma contained no K-feldspar crystals when it was emplaced and began to solidify but that, with continued crystallization and falling temperature, the melt phase became saturated with K-feldspar and subequant crystals precipitated in increasing abundance in the last parts of the leucotonalite to solidify. BIOTITE GRANODIORITE Scattered masses of biotite granodiorite are all composed of similar-looking rock except that some have been cataclastically deformed and some have not. TVpical undeformed rock is medium-grained biotite granodiorite. The color index ranges from 4 to 8 except in the Great Bend pluton and in the smaller pluton east of the Great Bend pluton in which the biotite content ranges between 5 and 15 percent. The grain size is reduced in strongly foliated cataclastically deformed rocks, and grain boundaries appear diffuse, probably because of granulation. BIOTITE GRANITE Biotite granite forms small plutons at and just east of Rock Mountain and a somewhat larger pluton at Corlew Mountain along the eastern part of the south boundary of the quadrangle. The granite in the plutons at and east of Rock Mountain is strongly deformed, whereas that in Corlew Mountain is not. The undeformed granite in Corlew Mountain is fine grained and has a weak primary foliation that is shown by the preferred orientation of biotite. The deformed granite at and east of Rock Mountain appears to have been medium grained before it was deformed, but now consists of clasts as much as 4 mm across in a very fine grained matrix. Although the granites in the two areas are of about the same composition, they may not be consanguineous. APLITIC LEUCOGRANITE Scattered small masses of aplitic leucogranite are composed of fine-grained felsic rock that generally contains no more than 2 percent biotite. In a few places, coarse-grained pegmatitic rock of the same composition is present. Their compositions indicate that these rocks crystallized at minimum magmatic temperatures. U-Pb AGE DATA Zircon from five samples of the tonalite of Blue Canyon and one sample of the leucotonalite of Ward Mountain were dated by the isotopic U-Pb method as part of a program of dating rocks of the central Sierra Nevada (Stern and others, 1981). Only the 206Pb/238U ages are considered reliable. The five ages on samples of the tonalite of Blue Canyon range from 110 to 124 m.y., and the age on sample MLa-12 of the leucotonalite of Ward Mountain is 115 m.y. The leucotonalite of Ward Mountain intrudes the tonalite of Blue Canyon and is therefore younger. If the age of 124 m.y. on sample MLc-10 is omitted, all the other ages on the4 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA tonalite of Blue Canyon fall in the narrow range of 110 to 115 m.y. The age on sample MLc-10 can be questioned because earlier determinations on this same sample yielded much older and obviously incorrect ages, which suggests that this determination also is incorrect. An evaluation of all the U-Pb ages on the tonalite of Blue Canyon, including ages from samples collected in adjoining quadrangles, suggests an optimum age of about 114 m.y., approximately the same as the U-Pb age on sample MLa-12 of the leucotonalite of Ward Mountain. For additional information, see the paper by Stern and others (1981). REFERENCES CITED Bateman, P. C., and Busacca, A. J., 1982, Geologic map of the Miller-ton Lake quadrangle, west-central Sierra Nevada, California: U.S. Geological Survey Geologic Quadrangle Map GQ-1548, scale 1:62,500. Bateman, P. C., and Lockwood, J. P„ 1970, Kaiser Peak quadrangle, central Sierra Nevada, California—analytic data: U.S. Geological Survey Professional Paper 644-C, p. C1-C15. ______ 1976, Shaver Lake quadrangle, central Sierra Nevada, California—analytic data: U.S. Geological Survey Professional Paper 774-D, p. D1-D20. Bateman, P. C., and Wones, D. R., 1972, Huntington Lake quadrangle, central Sierra Nevada, California—analytic data: U.S. Geological Survey Professional Paper 724-A, p. A1-A18. Lockwood, J. P., 1975, Mount Abbot quadrangle, central Sierra Nevada, California—analytic data: U.S. Geological Survey Professional Paper 774-C, C1-C18. Norman, M. B., II, 1974, Improved techniques for selective staining of feldspar and other minerals using amaranth: U.S. Geological Survey Journal of Research, v. 2, no. 1, p. 73-79. Stern, T. W., Bateman, P. C., Morgan, B. A., Newell, M. F., and Peck, D. L„ 1981, Isotopic U-Pb ages of zircon from the granitoids of the central Sierra Nevada, California: U.S. Geological Survey Professional Paper 1185, 17 p. Streckeisen, A. L., chairman, 1973, Plutonic rocks: Classification and nomenclature recommended by the IUGS Subcommission on the Systematics of Igneous Rocks: Geotimes, v. 18, no. 10, p. 26-30. Van der Plas, L., and Tbbi, A. C., 1965, A chart for judging the reliability of point counting results: American Journal of Science, v. 263, no. 1, p. 87-90.FIGURES 1-11; TABLES 1, 26 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA THORNBERRY MOUNTAIN PLUTON 35 O'NEALS LOBE" MOUNTAIN VIEW PEAK <-ROOF PENDANT ■''Mixed Kbtband " metamorphic rocks EXPERIMENTAL RANGE PLUTON/ ROCK MOUNTAIN PLUTON •, -GREAT ■ -BEND ' PLUTON. MILLERTON RIDGE' •;V PLUTON U 'v, PINCUSHION MOUNTAIN ROOF PENDANT :K-TACK-TOEROOF PENDANt Mlllerton Lake CORLEW MOUNTAIN - ../'..PLUTON Cenozoic deposits MLa-MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 7 EXPLANATION Aplitic leucogranite // // * /, V. ' ' '' v' Kbg v\ -,Kgd - ii / J , » , / Biotite granite Biotite granodiorite \ " /„\ Kw ^ \ ^ \ Leucotonalite of Ward Mountain ■ ■ Kex ■ • Tonalite south of the Experimental Range Tonalite of Blue Canyon Kbl, blocky hornblende facies Kblb, biotite-rich facies Tonalite south of Black Mountain mx Intrusive breccia Diorite Gabbro Metasedimentary and metavolcanic rocks T T— + + + + + + + + XXX XXX XXX XX X > co CD o uJ o < cc o Contact Dashed where inferred Modally analyzed sample o Chemically analyzed sample X Sample dated isotopically by the U-Pb method Figure 1.—Millerton Lake quadrangle showing the principal geologic units and the locations of modally analyzed, chemically analyzed, and isotopically dated samples. The letters in the upper left of each quadrant (MLa- etc.) prefix the sample numbers within the quadrants. Numbers refer to sample numbers in tables 1-2.8 MILLERTON LAKE QUADRANGLE, CALIFORNIA—ANALYTIC DATA 3 MILES 3 KILOMETERS 119°30 119°45‘ 37°15' \ ' *34 -x' ' ' ' •'51 - '«31 / 1 *,2\1/'28''i S ' \ l N \ I / s" ' | Z°* ' . . / ^ 1 I ' \ \ ^ ' Ax' " M 0, ' o- >,v I ' „ \ / V I .1 Figure 2.—Millerton Lake quadrangle showing volume-percent quartz. Explanation in figure 1.MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 9 119°45' 37°15' 119°30 37°00' 0 12 3 MILES 1 --i—H-------H---------1 0 12 3 KILOMETERS Figure 3.—Millerton Lake quadrangle showing volume-percent potassium feldspar. Explanation in figure 1.10 MILLERTON LAKE QUADRANGLE, CALIFORNIA—ANALYTIC DATA 119°45 119°30 37°15' 37°00' 3 MILES 3 KILOMETERS Figure 4. Millerton Lake quadrangle showing volume-percent plagioclase. Explanation in figure 1.MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 11 0 12 3 MILES 0 12 3 KILOMETERS 119°30 119°45' 37°15' 37°00' Figure 5.—Millerton Lake quadrangle showing voltime-percent mafic minerals. Explanation in figure 1.12 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 3 MILES 3 KILOMETERS Figure 6.— Millerton Lake quadrangle showing volume-percent biotite. Explanation in figure 1.MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 13 119°45 119“30 0 12 3 KILOMETERS Figure 7.—Millerton Lake quadrange showing volume-percent hornblende. Explanation in figure 1.14 MILLERTON LAKE QUADRANGLE, CALIFORNIA—ANALYTIC DATA 119°45 119°30 37°15' 37°00' 3 MILES 3 KILOMETERS Figure 8.—Millerton Lake quadrangle showing 100 hornblende biotite +hornblende' ExPIanati°" in fi^«-e 1.MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 15 0 12 3 KILOMETERS Figure 9.— Millerton Lake quadrangle showing bulk specific gravity. Explanation in figure 1.16 Other constituents MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA Other constituents EXPLANATION Tonalite of Blue Canyon-biotite-rich facies Figure 10.—Plots of modes of granitic rocks. Classification plan by Streckeisen (1973).MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 17 Quartz Figure 11.—Plot of norms of granitic rocks.18 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA Table 1.—Chemical analyses, norms, [ Rapid rock chemical analyses in weight percent; analyst. H. Smith, under the supervision of Floyd Brown. CIPW norms in weight percent. Modes in volume percent: felsic minerals and total apportioning counts on thin sections to total mafic Tonalite of Blue Canyon Blocky hornblende facies Biotite-rich facies Tonalite of the Experimental Range Sample - MLa-19 MLb-6 MLb-l 2 MLb-69 MLc-30 MLc-154 MLd-9 MLd-52 MLc-10 MLd-27 MLd-59 MLc-45 Chemical analyses S102 64.3 65.0 46.8 63.6 65.0 65.7 61.6 61.6 67.2 66.4 63.3 68.8 Al2°3 16.5 16.7 21.3 16.9 16.6 16.5 17.0 16.9 16.7 16.7 16.3 16.8 Fe2°3 .82 1.2 1.0 1.1 .76 .97 1.8 2.0 .79 1.0 1.1 .77 FeO 4.1 3.6 7.2 4.0 2.8 3.0 3.9 3.8 2.6 3.0 3.9 2.2 MgO 2.6 2.1 5.7 2.3 2.5 1.8 2.6 2.6 1.5 1.4 2.0 .93 CaO ■ 5.0 5.0 11 .4 5.4 5.2 4.8 5.9 6.0 4.2 4.4 4.5 3.8 Na20 3.4 3.5 2.1 3.3 3.1 3.6 3.5 3.2 3.9 4.0 3.7 4.3 k2o 1.9 1.6 .38 2.1 1.3 1.9 1.8 1.9 1.7 1.5 2.1 1.3 h2o+ • .84 .89 1.6 .81 1.0 .83 .80 .93 .70 .66 .73 .23 H20 ■ .04 .00 .15 .08 .14 .05 .08 .04 .08 .18 .13 .49 T102 • .64 .68 1.3 .62 .41 .53 .88 .84 .47 .41 .55 .38 P205 17 .17 .15 .15 .12 .15 .21 .21 .16 .18 .20 .15 MnO ■ .07 .06 .09 .06 .05 .05 .07 .08 .05 .06 .08 .03 C02 .01 .02 .08 .06 .02 .02 .02 .01 .02 .04 .00 .01 Sum 100 100 100 100 99 100 100 100 100 100 99 100 CIPW norms Q 20.48 23.11 21.56 19.52 25.73 23.21 17.03 18.22 25.72 24.74 19.62 28. .45 c .19 .57 — .00 .97 .20 .00 .00 1.24 .94 .24 1. .79 or 11.23 9.46 7.76 12.41 7.68 11.23 10.64 11.23 10.05 8.95 12.70 7. .68 ab 28.77 29.61 34.18 27.92 26.23 30.46 29.61 27.08 33.00 34.17 32.04 36, .38 an 23.63 23.57 23.73 25.10 24.89 22.71 25.36 26.14 19.66 20.85 21.51 17. .81 di .00 .00 .22 .27 .00 .00 1.97 1.79 .00 .00 1 1 1 1 1 ■ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ■ 1 1 1 1 1 1 1 1 1 1 1 1 • 1 1 12.40 9.84 9.61 11.12 10.16 8.41 9.86 9.69 7.17 7.68 10.72 5. .15 1.19 1.74 1.61 1.60 1.10 1.41 2.61 2.90 1.15 1.46 1.63 1. .12 il 1.22 1.29 .96 1.18 .78 1.01 1.67 1.60 .89 .79 1.07 .72 ap .40 .40 .38 .36 .28 .36 .50 .50 .38 .43 .49 .36 cc .02 .05 — .14 .05 .05 .05 .02 .05 — — .05 Total 99.53 99.64 100.01 99.62 97.87 99.05 99.30 99.17 99.31 100.01 100.02 99. .48 Modes Quartz 18 26 21 24 25 23 21 16 27 31 15 31 Potassium feldspar— 1 .5 0 .5 0 1 1 1 3 0 7 1 Plagioclase 55 56 58 51 55 57 58 53 55 54 57 57 Biotite 17 14 13 20 14 13 13 16 14 14 -1 11 Hornblende 7 3 8 5 6 6 6 12 1 1 J 0 Total 98 99 100 100 100 100 99 98 100 100 101 100 Bulk specific gravity 2.77 2.75 2.73 2.76 2.73 2.75 2.78 2.78 2.71 2.70 2.74 2. .69 ^Sum of mafic mineralsMILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA 19 and modes of representative granitoids mafic minerals determined by counting 1,000 to 2,000 points on selectively stained slabs of at least 70-cm2 area—analyst, Oleg Polovtzoff; hornblende and biotite determined by minerals—analyst, Alan Busacca. nd, not determined] Leucotonalite of Ward Mountain Aplitfc leuco- granite Hornblende gabbro Tonalite of the Experimental Range Leucotonalite of Ward Mountain Aplitic leuco- granite Hornblende gabbro MLa-12 MLc-43 SJ-1 MLc-84 Mlh-10 MLc-77 MLc-45 MLa-12 MLc-43 SJ-1 MLc-84 MLb-10 MLc-77 Chemical analyses—Continued 72.1 70.4 70.9 74.8 46.8 53.9 68.8 72.1 70.4 70.9 74.8 46.8 53.9 15.9 16.1 16.2 14.4 21.3 17.5 16.8 15.9 16.1 16.2 14.4 21.3 17.5 .48 .46 .27 .91 1.0 .8 .77 .48 .46 .27 .91 1.0 .8 1.1 1.4 1.3 .68 7.2 6.6 2.2 1.1 1.4 1.3 .68 7.2 6.6 .58 .61 .73 .32 5.7 4.5 .93 .58 .61 .73 .32 5.7 4.5 2.5 3.0 2.3 1.2 11.4 7.5 3.8 2.5 3.0 2.3 1.2 11.4 7.5 4.4 4.5 4.3 3.6 2.1 3.2 4.3 4.4 4.5 4.3 3.6 2.1 3.2 2.2 1.7 2.2 3.7 .38 1.3 1.3 2.2 1.7 2.2 3.7 .38 1.3 .61 .75 .16 .64 1.6 1.9 .23 .61 .75 .16 .64 1.6 1.9 .05 .05 .75 .05 .15 .19 .49 .05 .05 .75 .05 .15 .19 .24 .27 .22 .15 1.3 1.2 .38 .24 .27 .22 .15 1.3 1.2 .16 .14 .04 .07 .15 .35 .15 .16 .14 .04 .07 .15 .35 .03 .03 .03 .04 .09 .10 .03 .03 .03 .03 .04 .09 .10 .02 .02 .05 .03 .08 .10 .01 .02 .02 .05 .03 .08 .10 100 99 99 101 99 99 100 100 99 99 101 99 99 CIPW norms—Continued 31.79 30.02 31.19 36.79 5.44 28.45 31.79 30.02 31.19 36.79 __ 5.44 2.17 1.78 2.70 2.53 — — 1.79 2.17 1.78 2.70 2.53 — — 13.00 10.05 13.20 21.86 2.31 7.93 7.68 13.00 10.05 13.20 21.86 2.31 7.93 37.23 38.08 36.94 30.46 18.24 27.93 36.38 37.23 38.08 36.94 30.46 18.24 27.93 11.23 13.84 11.32 5.31 48.83 30.48 17.81 11.23 13.84 11.32 5.31 48.83 30.48 .00 .00 .00 .00 6.73 4.57 .00 .00 .00 .00 .00 6.73 4.57 2.73 3.32 3.73 1.12 12.85 19.28 5.15 2.73 3.32 3.73 1.12 12.85 19.28 __ 6.66 __ — 6.66 — .70 .67 .40 1.32 1.49 1.20 1.12 .70 .67 .40 1.32 1.49 1.20 .46 .51 .42 .29 2.53 2.35 .72 .46 .51 .42 .29 2.53 2.35 .38 .33 .96 .17 .36 .86 .36 .38 .33 .96 .17 .36 .86 .05 .05 .00 .07 — — .05 .05 .05 .00 .07 99.74 98.65 100.86 99.92 100.00 100.04 99.48 99.74 98.65 100.86 99.92 100.00 100.04 Modes—Continued 34 31 26 35 nd nd 31 34 31 26 35 nd nd 6 4 10 24 nd nd 1 6 4 10 24 nd nd 53 58 60 36 nd nd 57 53 58 60 36 nd nd 7 7 5 5 nd nd 11 7 7 5 5 nd nd 0 0 0 0 nd nd 0 0 0 0 0 nd nd 100 100 101 100 nd nd 100 100 100 101 100 nd nd 2.67 2.67 2.64 2.64 nd nd 2.69 2.67 2.67 2.64 2.64 nd nd20 MILLERTON LAKE QUADRANGLE, CALIFORNIA-ANALYTIC DATA Table 2.—U-Pb age determinations on zircon from granitoids [From Stern and others, 1981, table 1] Sample Rock Unit Ages (m.y. ) Parts per million Atomic ratios 2°fPb 238y 207Pb 235y 208pb 232Th Pb U Th 208pb 2 0 6pb 207pb 206pb 2°“Pb 206pb MLa-12 Leucotonalite of Ward Mountain— 114.8 112.6 ... 86.97 1556.2 -114.2 0.81137 0.35868 0.02112 MLb-69 Tonalite of Blue Canyon 114.3 113.9 76.3 7.12 372.3 138.9 .13709 .070198 .00150 MLc-51 (Jo 110.3 105.2 99.8 4.13 218.2 46.5 .14391 .07768 .00216 MLc-154 115.1 120.8 112.0 4.24 222.1 57.2 .13467 .07152 .00141 MLd-52 do 112.3 112.4 117.3 3.80 194.5 47.3 .16131 .07930 .00211 MLc-10 Tonalite of Blue Canyon, biotite- rich facies 123.6 119.4 120.1 5.06 234.8 57.71 .15999 .07919 .00220PROFESSIONAL PAPER 1260 PLATE 1 UNITED STATES DEPARTMENT OF THE INTERIOR MzRze MzRze RzpCm Aeromagnetic ORODIN Del ME MzRze Mzfte KUSKANAX BATHOLITH MzRze StAtflALLj l rsGNEISS' RzpCm ftpCm MzRze ftpCm W'/ Ya TMza 'YYZ. BRITISH )CO! Mzfte CANADi Mzfte unitf.d STAT1 Northport[j ftpCm i' /^Y^/jLMgtaline Falls/ p Mzfte Loomis ftpCm KETTLE GNEISS xDOME Mzfte TMzb ,-TMza Omak Mzfte Pend Oreille Lake Mzfte ftpCm MibNire f jrand (Coulee 'igure ioeur d’Alene Postfalls l||p w ieur d’Alene \ Lake Kellogg Aeromagnetic gjCheney Ephrata Oakesdale C5 Ritzville ftpCm Mzfte ftpCm TMzb Mzfte Mzfte Mzfte Okanagan Lake Tsv Penticton ftpCm iremeas, MzRze ftpCm Northport Figure 10.—Baydjarakhs, pyramid-shaped remnants resulting from thawing of ice wedges, left bank Aldan River, 224 km above junction with Lena River. From Washburn (1973, fig. 10.2, p. 234. ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIADISTRIBUTION AND THICKNESS OF UPLAND SILT 17 stand for many years. Along the lower Aldan River the cliffs are as much as 25 m high. The stability of these cliffs may be due to the angularity of the grains and the strengthening effects of concretionary rods and tubes. DISTRIBUTION AND THICKNESS OF UPLAND SILT The silt is thickest near the major rivers draining glaciated areas, but absent on the flood plains and low flood plain terraces. It is especially thick along the south side of the Aldan River where it blankets all the high terraces and the tops of ridges and hills. The blanketing distribution is also recorded in the central and eastern part of the Vilyuy River basin (Alekseev, 1970) (fig. 2). Rusanov (1968, fig. 5) and Sudakova (1969, fig. 11) describe the thick cap of silt on the terraces on the left limit of the lower Aldan (fig. 11). Along the Lena River, silt is 10-25 m thick on the edge of the west side of the valley but thins rapidly to a featheredge west of Yakutsk (fig. 3). On the Tyungyulyu Terrace it reaches a maximum thickness of 60 m near Syrdakh in the general region between the junction of the Aldan and Lena Rivers (table 2). On the edge of the Tyungyulyu Terrace adjacent to the the Bestyakh Terrace the silt is very thin, but it rapidly thickens eastward (Solov’ev, 1973, p. 34) and northward toward Syrdakh (Are, 1973). All thicknesses of loess reported from the river bluffs and inland toward Tyungyulyu and Maiya are at elevations no higher than 200 m above the level of the rivers. The silt is thin and locally absent on the Magan Terrace. The silt is only a few meters thick on glacial and glaciofluvial deposits (fig. 3) on the north (right) side of the Aldan River (Vangengeim, 1961; Katasonov and Solov’ev, 1969). Just southwest of the study area (fig. 3), on the steep limestone cliffs of the "Pillars of the Lena” (fig. 12), the loess is thin to absent 200 m or more above the Lena River, but on the north side (left limit) of the river it is 7-25 m thick on the terraces (fig. 13), which are only 30-40 m above the river. In general, silt is thickest in the bottoms of small valleys and on lower slopes and thinner on the highlands, especially on the Magan Terrace. Are (1973, p. 16-20) describes 60 m of silt at Syrdakh (fig. 3), but he does not state if this thickness is in a small valley or on the general terrace level. Hilltops, where the original silt thickness has not been supplemented by slopewash, are probably the only place where total original thickness can be approximated, modified only by the amount removed by erosion. Some scarps of the terraces along the Lena River and almost all scarps along the south side of the Aldan River are capped with 10-35 m of silt. This does not represent the original thickness of the silt as the upper part has been washed away (Sudakova, 1969). In general, it appears from exposures west of Pok-rovsk (figs. 3,14) that the silt is thicker on the Abalakh Terrace than on the Tyungyulyu Terrace. Solov’ev (1959, p. 43) states that the Abalakh Terrace is covered With "limnitic-alluvial clay,” 60 m thick, the top part pierced with ice veins. Alases, which form best in thick silt, are well developed on both the Tyungyulyu and Abalakh Terraces. Detailed measurements of loess thickness over a large area, similar to those made in Illinois by Smith (1942), in Iowa by Davidson and Handy (1952) and Ruhe (1969), and in parts of Alaska by Pewe (1955; Pewe and Holmes, 1964) have not yet been made in central Yakutia. As seen from the air, however, the loess forms a widespread cover, evident in the well-developed dendritic drainage patterns on the terraces (fig. 7). The thickness reported here (table 2) include measurements from exposures and bore holes. Although silt has plainly washed into depressions in some localities measured, no coarse sediment was observed, so the measurements may give a reliable estimate of the total amount present. The thick deposits on the south side (left limit) of the Aldan River face the enormous apron of glaciofluvial 80-meter-high terrace Figure 11.—Loess-capped sand terraces near Mamontova Gora along the Aldan River, Yakutia. Slightly modified from Rusanov (1968, fig. 5).00 Figure 12.—Lena River valley from atop "Pillars of the Lena,” Cambrian dolomitized limestone, 255 km upstream from Yakutsk. Loess-covered terrace on left limit in distance. Photograph 3465 by T. L. Pewe, August 1, 1973. (Reproduced by the courtesy of Zeitschrift fur Gletscherkunde und Glazial-geologie.) ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIADISTRIBUTION AND THICKNESS OF UPLAND SILT 19 Table 2.—Thickness of silt in central Yakutia, Siberia, U.S.S.R. Thickness site no. (fig. 3) Thickness (m) Location Source Sediment sample (fig-3) i — 30-35 Chuysakaya Mountain, right limit of the Aldan River, 30 km upstream from mouth of Tumara River. Katsanov and Solov’ev (1969, p. 21) Katasonov and Ivanov (1973, p. 18). A,B 2 - 25-32 Tettigi, left limit of the Aldan River, 130 km from mouth. Katasonov and Solov’ev (1969, p. 22). 3 - 25-30 40-65 Rossypnoy Perekat, left limit of the Aldan River, 244 km from mouth. Katasonov and Solov’ev (1969, p. 22) Katasonov and Ivanov (1973, p. 24). C 4 — 5-8 285 km from the mouth on the right limit of the Aldan River, overlies glaciofluvial material. Katasonov and Solov’ev (1969, p. 23). G,H,I 5 .. 25 310 km from the mouth on the left limit of the Aldan River at Mamontova Gora. Pewe and others (1977). 6 - <1 1 00 cn Krest Khal’dzhay, right limit of the Aldan River, 360 km from mouth. Katasonov and Solov’ev (1969, p. 27) Vangengiem (1961, fig. 20). 7 __ 60 Syrdakh. Are (1973, p. 16, 20). J 8 __ 50 Tyungyulyu. Are (1973, p. 12). M 9 __ .0 53 km west of Yakutsk. Pewe, 1973 field data. N 10 __ 2+ Bluff west of Yakutsk, 0.5 km from the edge. Pewe, 1973 field data. O 11 — 5-101 Tabaga Cape, left limit of Lena River, 30 km south of Yakutsk. Katasonov and Solov’ev (1969, p. 12). 12 — .0 On Bestyakh terrace 2 km south of N. Bestyakh Village. Pewe, 1969 field data. P 13 — 8+ At alas, Bestyakh-Maiya Road, edge of Tyungyulu Terrace. Pewe, 1973 field data. D 14 — .0 On Bestyakh Terrace 25 km south of N. Bestyakh Village. Pewe, 1969 field data. 15 __ 10+ 7 km southeast of Maiya Village. Katasonov and Solov’ev (1969, p. 41). 16 - 31 Abalakh Terrace near Abalakh. Katasonov and Solov’ev (1969, p. 43). 17 __ 20 Churapacha. Solov’ev (1973, p. 30). 18 25 On the Abalakh Terrace, 6 km west of Pokrovsk Village. E. M. Katasonov (oral commun, 1973). Z 19 __ 7 12 km upstream from Pokrovsk on the left limit of the Lena River. Pewe, 1973 field data. BB, CC 20 - 10-15 On the Tyungyulyu Terrace, 1-5 km west of Pokrovsk. E. M. Katasonov (oral commun, 1973). X, Y 21 - 7 Ilankjo Village, left limit of the Lena River, 70 km upstream from Pokrovsk. Pewe, 1973 field data. FF Figure 13.—Loess-covered terrace of Ilanskjo Village on Cambrian dolomitic limestone, 70 km upstream from Pokrovsk on left limit of Lena River. See figure 3. Loess is 7 m thick at gulley.20 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA terrain and glacial moraines emanating from the Verkhoyansk Range to the north (fig. 3). Such a distribution is very similar to distribution in central Alaska where thin loess is present on the apron of glaciofluvial debris emanating from the towering Alaskan Range and terminating at the Yukon-Tanana Upland to the north mantled by loess of great thickness (Pewe, 1955,1968). Although loess thicknesses of 10-35 m are reported from northern Yakutia (Tomirdiaro and others, 1974), the thickest deposits in the Republic appear to be along the Lena and Aldan Rivers in central Yakutia. This relation, too, is similar to the distribution of loess in central Alaska where loess as much as 60-100 m thick occurs on the north side of the Tanana River near Fairbanks (Pewe, 1955, 1968; 1982), the thickest known loess deposits in the State. COLOR AND TEXTURE The unfrozen upland silt is commonly tan, although grayish-tan silt is not rare; when wet or frozen, it is brown to black. In many localities, the frozen silt is gray to black and has thin dark carbonaceous layers and iron-stained bands and mottling. As an aid to understanding the mechanical composition of the upland silt, analyses were completed on 27 samples; analyses were also done on several samples of river silt and eolian and fluvial sand (table 1). The mechanical analyses (granulometric analyses) were made at the Centre de Geomorphologie in Caen, France, by sedimentation in water. Grains smaller than 20 /im were separated by sieving. For comparison, silt samples from other countries were analyzed in the same labora- tory by the same method: windblown dust from Tempe, Arizona, and loess collected by Pewe from Alaska, New Zealand, and Czechoslovakia, and from China (near Peking) collected by R. J. E. Brown (table 3). The silt examined is well sorted, especially on highlands away from river flood plains and sand terraces and from low areas where silt has been retransported short distances. On the high edges of major alases, as, for example, near Maiya or Syrdakh (fig. 3) (samples R, J and K; table 1), mechanical analyses (fig. 15) reveal that the sediments are 70-80 percent silt, 1-15 percent sand, and about 15 percent clay2. Because the clay content is low, the material has little plasticity. In general, the mechanical analyses of upland silt throughout the area show that the mechanical composition and texture of the tan silt is remarkably uniform (figs. 16,17,18) (table 3). The range of texture of the 27 samples is shown by the shaded envelope in figure 16, the average texture is sand 17 percent, silt 70 percent, and clay 13 percent. Solov’ev (1959) states that on the Abalakh Terrace near Churapacha, the upper clayey loam is 70-90 percent silt and is unbedded. Tomirdiaro (1975b) states that this uniform texture exists from the Aldan River to the north coast and even to the Novosibirsk Archipelago. Some samples near the sandy Magan or Bestyakh Terraces may be as much as 40 percent very fine sand (fig. 19, samples Q and O) and some of the silt adjacent to the Aldan and Lena River flood plains may be 20-25 percent fine sand (fig. 19, BB, 2In this report the U.S. Department of Agriculture classification is used: very fine sand, 0.10-0.05 mm; silt, 0.05-0.005 mm; clay, 0.005-0.002 mm. Figure 14.—Upland silt mantling terraces west of Pokrovsk, showing location of silt samples. Vertical exaggeration 150x. Stratigraphic relations and silt thicknesses from E. M. Katasonov (oral commun., July 30, 1973).COLOR AND TEXTURE 21 Table 3. —Mechanical properties of sand and loesslike silt from central Yakutia and loess and ash from other parts of the world [Sedimentological terms from Folk (1974). Compiled by J. Bales. See Table 1 and cumulative-frequency curves (figs. 15-21, 23, 26-29)] Sediment sample (fig. 3) Material Graphic mean (Mz) Median (Md) Inclusive graphic standard deviation (sorting) (o/) 4> Inclusive graphic skewness (Sk.) Graphic kurtosis (Kg) A _Loess 6.20 5.44 2.33 0.58 1.43 B Sandy loess. 4.78 4.57 1.60 .50 2.49 C -Loess 6.58 5.57 2.58 .65 1.42 D Loess - - 6.14 5.89 1.97 .36 1.47 E Sand-silty river alluvium 4.39 3.78 1.75 .48 1.05 F -Loess 6.71 5.80 2.44 .61 1.12 G -Loess _ - 7.47 6.70 2.67 .42 1.08 H Loess 7.43 6.75 2.77 .39 1.07 I -Loess 7.14 6.88 2.14 .27 1.06 J -Loess 5.80 5.27 2.15 .49 1.41 K Loess 5.58 5.27 1.87 .43 1.86 L Clayey loess 6.90 5.80 3.02 .53 .97 M -Loess 6.27 5.72 2.54 .40 1.19 N Sand 1.93 1.84 .69 .29 1.05 O -Sandy loess 4.94 4.64 3.24 .23 1.02 P -Eolian sand 2.98 2.84 .55 .43 .89 Q -Sandy loess 5.03 4.18 2.99 .51 1.90 R Loess 6.27 5.11 2.45 .84 1.96 S -Loess 6.90 5.80 2.97 .48 1.05 T Loess 6.64 5.72 2.47 .58 1.15 U -Loess 7.53 6.27 3.20 .59 1.08 V Loess 7.33 6.38 2.74 .52 1.01 W Loess 6.27 5.44 2.69 .52 1.44 X -Loess 7.21 5.64 3.28 .70 1.11 Y Loess 7.05 5.8 2.99 .54 .93 Z Loess 7.19 6.06 6.33 .63 3.24 AA Alluvial sand 2.16 2.15 .59 -.11 1.51 BB Sandy loess 7.59 5.51 4.68 .58 1.15 CC -Loess 8.43 6.27 4.25 .63 .84 DD .Sandy loess 5.94 5.01 3.10 .43 1.34 EE Eolian sand .83 .71 .57 .36 1.17 FF Loess 7.41 5.89 3.36 .64 1.01 GG -Silt from limestone 5.63 5.21 1.40 .45 1.33 Tomi -Loess 5.45 5.44 1.70 .23 3.83 Dust, German 5.42 5.50 .95 .17 1.02 Volcanic Ash, Fairbanks 6.24 6.15 1.28 .026 1.23 Dust, Kansas 6.84 5.90 2.41 .44 1.26 Dust, Arizona 7.80 6.65 2.87 .46 1.47 Loess, New Zealand 8.10 7.80 2.83 .10 .92 Loess, Czechoslovakia. 7.37 5.95 2.96 .56 .98 Loess, Alaska 5.84 5.50 1.36 .26 1.12 Loess, China 7.94 7.65 2.84 -.10 .83 Loess, Illinois 6.07 5.95 1.23 .075 1.64 CC, and DD) similar to the sandy loess adjacent to braided glacial streams in Alaska (Pewe and Holmes, 1964; Trainer, 1961). The clay content is 10-20 percent where the silt is little transported. Where the silt is reworked, as in valley bottoms, alases, or thermokarst lakes, it contains( more clay (20-40 percent) and minute organic fragments (figs. 20, 21, samples G, H, FF). Samples taken close to the surface (probably from the B soil horizons) are also slightly higher in clay (fig. 20, samples L, 25 cm below surface and S, 16 cm below the surface). Detailed sampling of silt for clay content in relation to soil age and depth, location, and rate of silt accumulation (see Ruhe, 1969; Smith, 1942) remains to be done on these deposits. Samples taken at various depths from several sections (for locations see fig. 3) demonstrate the vertical uniformity of the silt. Three samples (G, H, and I, taken at depths of 1, 7, and 23 m, respectively) from a 25-m-high cliff at Mamontova Gora (Pewe and others, 1977), two samples (U and V, from 2 and 3 m deep, respectively) from Churapacha, and two samples (BB and CC, from the bedrock surface and 1 m above, respectively; fig 22) from the silt overlying dolomitic limestone at a quarry 12 km south of Pokrovsk (fig. 3) and very similar in texture (table 3 and figs. 19 and 21) and other sedimentologic parameters (table 3). Similar vertical uniformity is apparent in a 29.5-m-thick section of loess on the north coast of Yakutia (Tomirdiaro and others, 1974, table 1). The samples from the limestone quarry emphasize the independence of the silt from the underlying bedrock. In an attempt to gage lateral uniformity samples (W, X, Y, Z) of silt were taken at various distances (fig. 14)22 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA DIAMETER, IN MILLIMETERS Figure 15.—Cumulative-frequency grain-size curves for loess from edge of alases on Tyungyulyu Terrace, 40 km west of Yakutsk. See figure 3 for locations.COLOR AND TEXTURE 23 tttt_i i i i nr EXPLANATION -• • •— Average of 27 upland silt samples $ 1 xx M X s /v\ s AV\\ Xx Xs s\' X I | i XX XX xVvxN X sV & sx: X $ I n XXX |f|; nV s\" '\' $ x XX 111 —- a; vX X yv 1 X s\> ' \ \ \ 1 i $ x ^x W\X\ \X\XX \VxV\ 'vxVnN Xx / | vj A \y xxx XXX xxx y / 1 / X \ § $ V lx \ \ \ sNN \v A' /\v A\Vs s\' I 1. 11: 1 5 s\ \ 1 XX vvllx 1 1 °1.0 0.5 0.1 0.05 0.01 0.005 0.001 0.0005 0.0001 DIAMETER, IN MILLIMETERS Figure 16.—Average cumulative-frequency grain-size curve for 27 samples of upland silt on high terraces in central Yakutia. Range of texture of samples shown by cross-hatched area.24 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA Figure 17.—Cumulative-frequency grain-size curves for loess blanketing terraces along lower Aldan River. Distance of sample locations up Aldan River from mouth: A and B, 30 km; C and D, 244 km; F, 284 km. See figure 3 for locations.COLOR AND TEXTURE 25 Figure 18.—Cumulative-frequency grain-size curves for loess mantling Tyungyulyu Terrace west of Lena River. See figure 3 for locations.26 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA 1.0 0.5 0.1 0.05 0.01 0.005 0.001 0.0005 0.0001 DIAMETER, IN MILLIMETERS Figure 19.—Cumulative-frequency grain-size curves for upland loess near sandy terraces (Q and O) and near sandy flood plains (BB, CC, and DD) along Lena River. Samples BB and CC are in same section, respectively 1 cm and 70cm above bedrock. See figures 3 and 22.CUMULATIVE FREQUENCY, IN PERCENT COLOR AND TEXTURE 27 0 il hi I Tl --------------------------1111 —----------------------1111 ------------------- 1.0 0.5 0.1 0.05 0.01 0.005 0.001 0.0005 0.0001 DIAMETER, IN MILLIMETERS Figure 20.—Cumulative-frequency grain-size curves for reworked upland loess and B soil horizon illustrating relatively high clay content. See figures 3, 12.CUMULATIVE FREQUENCY, IN PERCENT 28 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA 1 0 0,5 0.1 0.05 0.01 0.005 0.001 0.0005 0.0001 DIAMETER, IN MILLIMETERS Figure 21.—Cumulative-frequency grain-size curves for retransported upland loess at various depths in section, illustrating vertical uniformity of texture. Samples G, H, I, Mamontova Gora; U, V, Churapacha. See figures 3, 30, and table 1.MINERAL AND CHEMICAL COMPOSITION 29 away from the Lena River in the Pokvorsk area to determine whether the percentages of sand, silt, and clay changed (see Ruhe, 1969, p. 71). Within distance covered, only 6 km, no significant changes were seen in the the size distribution (fig. 23). MINERAL AND CHEMICAL COMPOSITION A number of chemical and mineral analyses of representative samples were made at the Centre de Geomor-phologie, Caen, France. Heavy mineral analyses were made of 27 samples of the upland silt and of 3 samples of local fluviatile and eolian sand for comparison. The minerals were separated by bromoform (2.87 sp gr) after washing with HC1 (20 percent) and sieving in three granulometric fractions. Fourteen samples were analyzed for 11 oxides. Thirty samples were analyzed for calcium carbonate content alone, and 16 by X-ray for types of clay minerals present. Petrographic examination reveals that the silt grains, although slightly iron-stained, are angular and fresh. The mineralogy of the loesslike silt is nearly uniform (table 4); typical samples contain abundant quartz, mica, and feldspar, with a lesser amount of heavy minerals. These include 30 to more than 70 percent hornblende; 12-30 percent epidote, and lesser amounts of hypersthene and garnet, and very small amounts of zircon, anatase, tourmaline, rutile, disthene, monazite, METERS x Excavated Figure 22.—Location of samples of upland silt mantling dolomitic limestone on right limit of Lena River, 12 km upstream from Pokrovsk. and other heavy minerals. The analyses show that, in addition to mineral uniformity in the loesslike silt geographically, there is vertical uniformity. Samples from various levels in the cliff at Mamontova Gora (table 1, samples G, H, I) and two samples (BB-CC) from a section over limestone 12 km upstream from Potrovsk (fig. 22) are practically the same mineralogically and chemically with respect to each other. The absence of heavy minerals in the limestone that underlies the silt upstream from Yakutsk along the Lena River (sample GG, table 4) confirms the independence of the silt and bedrock. On a local scale, the mineralogy of floodplain sandy-silty alluvium from near the mouth of the Tatta River (sample E) is very similar to nearby upland silt (sample S, table 4). Sands of the Magan and Bestyakh terraces (samples N and P, table 4) are also similar to adjacent silts. On a larger scale, differences in heavy-mineral content between samples from the Lena River and the lower Aldan River may indicate general regional differences despite the small number of samples. In both, hornblende, epidote, hypersthene, and garnet constitute 80-90 percent of the heavy minerals present. The samples from the Lena River area, however, are considerably higher in hornblende and epidote and lower in hypersthene and garnet than those from the lower Aldan River. Percentages are: hornblende, 61.0 for the Lena River samples, 46.8 for the Aldan River samples; epidote, 24.1 and 20.0; hypersthene, 1.4 and 8.5; garnet, 5.6 and 12.7. A “mineralogy index” defined as ,, (epidote + hornblende) , ,, M = —-------------------------hypersthene gives garnet M = 14.9 for the Lena area and M = 4.6 for the Aldan (fig. 24). To determine the statistical confidence level for the mineralogical differences described here (fig. 24), we calculated a probability value (P), using the "t” test. This value was less than 0.005 and indicates a high degree of confidence. Further, to determine whether the mineralogic differences merely reflected differences in grain-size distribution, we compared textural parameters (defined by Folk, 1974) for the samples from the two areas. Despite the mineralogic differences, the average values of these parameters are essentially the same for the two groups (table 5). Thus, the mineralogic differences seem to reflect a real division between the upland silt of the Lena River area and that of the lower Aldan River. Chemical analyses (table 6) shows that the upland silt is nearly constant in chemical composition over large areas. The chemical analyses of the upland silt on the30 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA Table 4.—Percentage distribution of heavy minerals in sediments from Yakutia, Siberia, U.S.S.R. [Sample No. 30, loess from Peking, China, collected by R. J. E. Brown, National Research Council of Canada, Ottawa, 1973. Analyses made at Centre de Geomorphologie, Caen, France] Sediment sample (fig. 3) A c D E F G H I J Mineral Loess Loess Loess Allu- vium Loess Loess Loess Loess Loess Hornblende 61.6 42.4 56.5 44.7 31.4 58.8 45.4 46.3 63.0 Epidote 20.2 28.0 15.6 17.4 17.2 19.8 12.1 27.1 27.3 Hypersthene 5.8 1.7 14.4 16.3 11.2 7.7 12.1 3.9 .6 Garnet 8.2 5.9 10.6 13.9 19.4 11.4 16.8 11.9 3.6 Sphene .7 .8 .7 1.5 .4 1.5 .6 1.8 Zircon .7 .8 .3 1.4 8.9 .4 1.1 5.6 Staurolite .3 5.9 6.7 .6 Pyroxenes __ (monoclinic) .5 4.1 5.5 .6 .6 Anatase .3 .8 .5 1.1 1.1 .7 .6 1.8 Tourmaline .3 1.7 .3 .4 1.1 Rutile .7 .3 .5 .4 1.1 .6 Disthene .3 .8 .4 .6 Sillimanite . . Andalusite . .3 8.4 3.7 Chloritoid . .2 .3 .7 Topaz _ _ .. .6 Monazite - .3 .6 .6 Total . 99.7 97.2 99.9 100.0 100.0 100.0 97.8 100.1 99.9 10 0.5 o.l 0.05 0.01 0.005 0.001 0.0005 0.0001 DIAMETER, IN MILLIMETERS Figure 23.—Cumulative-frequency grain-size curves for upland loess. Distances from Lena River: W, 10 m; X, 1 km; Y, 5 km; Z, 6 km. See figures 3, 13.FIELD RELATIONS 31 Table 4.—Percentage distribution of heavy minerals in sediments from Yakutia, Siberia, U.S.S.R. -Continued Sediment sample (fig. 3) M N 0 P R s T w X Mineral Loess Sand Loess Sand Loess Loess Loess Loess Loess Hornblende 68.6 55.1 51.7 60.8 54.5 69.9 63.1 60.3 73.6 Epidote 25.5 33.9 29.7 19.3 13.0 21.3 25.4 26.2 19.9 Hypersthene .7 .5 1.7 .3 .4 Garnet 2.6 5.1 9.9 4.1 7.1 3.9 3.1 4.4 2.1 Sphene 1.8 1.9 3.3 2.3 1.7 .9 3.5 1.9 1.1 Zircon .4 3.5 1.6 9.3 15.3 1.4 1.7 2.4 1.6 Staurolite .6 5.6 1.2 — Pyroxenes — — — — — — — (monoclinic) 1.0 — — — 1.0 .8 .5 Anatase . .4 .7 1.9 .3 .8 .5 Tourmaline .3 .3 Rutile .5 1.0 1.6 .5 Disthene 1.2 .5 .7 Sillimanite .2 Andalusite .6 1.1 .5 .3 — Chloritoid Topaz Monazite. — — — — — — — — .5 Total 100.0 100.0 99.9 99.9 98.8 99.8 99.3 100.0 99.8 Sediment sample (fig. 3) Y z AA BB cc DD EE FF GG 30 Silt Sandy from Minerals Loess Loess Sand loess Loess Loess Sand Loess limestone Loess Hornblende 67.8 61.8 37.3 60.6 58.8 43.7 46.5 56.4 57.8 Epidote 22.3 24.1 45.6 24.1 23.6 30.9 30.0 28.7 34.5 Hypersthene .9 1.2 1.1 2.9 2.9 CO .2 Garnet 4.3 7.3 10.4 5.6 8.4 9.1 10.3 4.9 3.6 Sphene .9 2.1 4.4 2.4 4.6 4.1 3.4 u c 2.2 Zircon ._ . .9 2.1 4.1 2.8 2.4 5.8 2.5 1.5 .2 Staurolite .5 .1 .8 >» Pyroxenes — — > a — (monocl inic) .9 — .6 -C — Anatase .9 .5 1.0 1.6 1.4 .8 .5 .2 Tourmaline is u .4 Rutile .5 .5 .4 .8 .8 1.6 3 CO .8 Disthene .5 1.0 .4 .4 .3 .5 Sillimanite .1 c Andalusite .1 .9 z Chloritoid Topaz Monazite — — — — — — — — .5 .5 .5 1.6 .4 1.1 .4 Total 99.9 99.9 99.9 99.9 100.0 99.7 99.9 100.2 99.9 50-m terrace along the Aldan River presented by Rusanov (1968, table 8) are almost the same as those listed for the same area in this paper. The chemical analyses are also similar to those analyses of loess along the Main River in northeast Siberia (Tomirdiaro, 1972, table 2). No chemical or mineralogical analyses are readily available for the bedrock in the Verkhoyansk Range or upstream on the Aldan or Lena Rivers. The calcium carbonate composition of the silt ranges from less than 2 percent to about 7 percent and is fairly consistent over the area under consideration (table 7). All loesslike silt samples analyzed by X-ray contain montmorillonite, chlorite, and illite, and most samples contain kaolinite. FIELD RELATIONS The massive silt overlies various substrates with a sharp contact: limestone along the Lena River, alluvial32 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA Table 5.—Average grain-size values of samples of upland silt along the Lena and Aldan Rivers in south-central Yakutia, U.S.S.R., analyzed for heavy-mineral content [Parameters and word descriptions from Folk, 1974, and Folk and Ward, 1957. See figs. 2,24] Aldan River Lena River Mechanical Group Group property (6 samples) Remarks (15 samples) Remarks Graphic mean M, 6.91 6.60 Median M(1 6.27 5.33 Inclusive graphic standard deviation Very poorly Very poorly (sorting), alif Inclusive graph- 2.43 sorted 3.26 sorted ic skewness, .44 Strongly Strongly Sk Graphic kur- coarse-skewed .66 coarse-skewed tosis, K(; 1.20 Leptokurtic 1.35 Leptokurtic sand on the left limit of the Aldan River, and glacioflu-vial deposits at places on the right limit of the Aldan River. The upland silt is massive, showing little or no stratification or jointing except in the retransported facies; however, near valley bottoms where some silt has been retransported, crude stratification is apparent. The faint stratification consists of iron oxide-stained horizons, organic films, vague colored bandings of short lateral extent and thin seams of ice, "Taber ice,” which have grown parallel to the very indistinct bedding of the retransported silt. Differences of moisture content at certain horizons outline faint stratification. In no instance does the faintly developed stratification resemble fluvial or lacustrine bedding. It is only in the frozen Figure 24.—Location of loess samples along Lena River and lower Aldan River analyzed for heavy-mineral content. Heavy lines encircle two different mineral suites based on average percentage of composition and differences of "mineralogy index,” M. See text for explanation of equation. Only 10 samples of Lena River silt analyzed for hypersthene.FOSSILS 33 Table 6.—Chemical analysis of Quaternary loess from central Yakutia, Siberia, U.S.S.R. [All calculations in weight percent. Analyses made at Centre de Geomorphologie, Caen, France. Samples collected by Troy L. Pewe] Sediment sample (fig-3) Sediment Si02 A1203 Fe203 Ti02 MaO CaO MgO k2o Na20 P,o5 h2o Loss on ignition A Loess 59.01 11.84 4.60 0.715 0.163 7.19 1.43 2.56 2.18 0.81 2.25 7.79 G Loess 59.94 12.32 5.36 .724 .145 4.83 .974 2.82 2.04 .53 3.14 7.15 H Loess 58.49 1.10 5.21 .736 .152 4.82 .958 2.74 2.08 .497 3.01 8.48 I Loess 59.12 12.81 6.70 .715 .217 4.21 .855 2.73 1.73 .55 3.60 6.59 -J Loess 64.31 11.92 4.42 .669 .149 4.91 1.055 3.12 2.61 .421 1.32 4.26 0 -Loess 69.19 11.29 2.92 .72 .151 4.48 .826 3.14 2.58 .370 1.45 3.87 s Loess 61.77 12.32 4.76 .723 .173 5.09 1.01 2.89 2.20 .629 3.05 5.42 X -Loess 61.20 11.29 3.48 .622 .146 7.25 1.02 2.90 2.50 .331 2.51 6.24 BB Sandy loess 64.91 12.38 3.97 .725 .173 3.82 .813 2.91 2.27 .551 3.26 3.97 CC .Loess 63.04 12.66 3.98 .786 .138 4.02 .820 2.89 2.22 .510 3.82 4.81 DD Loess . 65.37 10.79 3.60 .628 .174 4.73 .975 2.97 2.45 .435 1.62 4.84 FF Loess 57.91 11.40 4.59 .715 .165 7.04 1.16 2.71 2.26 .566 1.00 8.88 GG Silt from limestone 2.49 .76 .86 .07 .142 32.44 17.05 .32 0 .350 .25 45.39 Table 7.—CaCO:l in Quaternary sediments from central Yakutia, Siberia, U.S.S.R. [Analyses made at Centre de Geomorphologie, ^aen, France. Samples collected by Troy L. Where stratification is visible, however, minor folds and faults are common and, near the boundaries of ice wedges, the strata are commonly bent upward. Sediment Weight sample percent (fig. 3) Sediment CaC03 A B D E G H I J K L M N O T_____ W____ X .... Y ____ Z_____ AA .. BB __ CC __ DD __ EE __ FF __ GG 301 __ Loess_____________ Loess_____________ .Loess____________ .Alluvium_________ Loess_____________ Loess_____________ Loess_____________ .Loess____________ .Loess____________ .Loess____________ .Loess____________ Sand______________ Loess_____________ .Loess____________ Loess_____________ Loess_____________ Loess_____________ .Loess____________ Loess_____________ Loess_____________ Sand______________ Sandy loess_______ Loess_____________ Loess_____________ Sand______________ Loess_____________ Silt from limestone Loess_____________ 6.8 ~2 =3 <2 <2 =3 4.5 <2 <2 <2 =3 <2 <2 <2 =2 4.5 7.6 4.7 <2 <2 <2 <2 <2 <2 7.6 Dolomite 6.3 ‘Collected in China by R.J.E. Brown, 1973. or freshly thawed sediments that the bedding can be seen; after the silt has been thawed for years, it is tan instead of gray to black, and most of the indistinct bedding can no longer be seen. This retransported silt has the same texture and composition as the tan loesslike silt away from the depressions (figs. 15, 20, and 21; tables 3, 4, and 6) but is distinguished by a higher ice content and a fetid odor, as well as by faint stratification. Because of the uniform texture, composition, and color of the silt, structural disturbance is rarely evident. FOSSILS The upland silt represents the greatest repository of late Quaternary flora and fauna in central Yakutia. Fossils in glacial and flood-plain deposits of equivalent age are less abundant and less well preserved, especially the Pleistocene mammal fossils. Fossil remains are uncommon in the silt on ridges or low hills but common in lowlands and slight depressions where retransported silt has accumulated. This report does not consider in detail the flora and fauna of the silt, already reported by Alekseev (1961), Vangengeim (1961), Rusanov (1969), Motuzko and others (1969), Markov (1973), Pewe and others (1977), and other investigators. In summary, the silt deposits are characterized by numerous remains of Mammuthus primigenius of a late type, Equus caballus, subsp. B (small form), Bison priscus aff. deminutus W. Grom, muskox, moose, caribou, and other large mammals and rodents such as Lemmus and Dicrostonyx. These fossils are especially common on the terraces of the Aldan River valley. Some, such as the large horse and the long-horn bison, are older fossils. These older remains may be from the lower levels of the silt or reworked from the underlying sand terraces (Vangengeim, 1961, p. 34). Although a considerable number of specimens in central Yakutia have been identified, it remains to relate individual fossils to specific stratigraphic units within the silt as suggested by Pewe and others (1977). It is not uncommon for many of the skeletons to be found with several of the bones joined together, and Vangengeim (1961, p. 38-39) reports recovery of an entire skeleton. Some of the widely reported carcasses, or partial carcasses, of ice-age mammals, including the Berezovka mammoth, have come from this silt (Shilo,34 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA 1978). Partial carcasses of Pleistocene mammals have also been recovered from the perennially frozen retransported silt in Alaska (Pewe, 1975a). Aquatic fossils in the silt include two mollusk species collected by Pewe on the south side of the Aldan Valley: Valvata piscinalis Mull, and Valvata cristata (O. F. Muller) (identified by J. Vasatko, Czechoslovakia Academy of Science, Brno, CSSR, written commun., 1974). Solov’ev (1959) reports aquatic mollusks at various depths in the upper of two major clayey loam layers near Churapacha. The retransported frozen silt contains a varied megaflora in the form of peat, sticks, twigs, pods of plant remains, and isolated tree limbs and stumps. Study of such specimens and pollen analysis indicate the predominance of herbaceous tundra and forest-tundra vegetation at the time of silt accumulation (Vangengeim, 1961; Giterman and Golubeva, 1967). ORIGIN Although the origin of loess has been a controversial subject worldwide for about 100 years, most of the discussions have centered on the loess in China, central United States, and later, Alaska. Less is known about the probable origin of the widespread loesslike silt in the lowlands of unglaciated eastern Siberia. Marine, estuarine, lacustrine, fluvial, weathering, and eolian processes have been invoked to explain the loesslike silt. The marine and estuarine hypotheses have had little support; but the lacustrine, fluvial, and residual hypotheses, either separately or in combination, have had widespread support and are currently strongly held by Soviet workers, especially by researchers working with frozen ground. In recent years, however, the eolian hypothesis has been advocated by some workers (To-mirdiaro and others, 1974; Tomirdiaro, 1975b; Pewe and others, 1977). MARINE AND ESTUARINE HYPOTHESES Many of the river valleys in central Yakutia, especially in the north, are less than 100 m above sea level. It has been suggested that much of the loesslike silt could result from inundation by an advancing sea (Wright, 1902). To deposit such uniform silt at both high and low elevations, however, flooding the valleys many hundreds of meters deep would have been required (fig. 2). Moreover, deltas, shorelines, and beaches would be present if the water level had been constant for a long period of time. The silt would stratify and would contain large amounts of clay, mud cracks or ripple marks, and marine fossils, but none of these features are present. WEATHERING HYPOTHESIS Berg (1960) proposed that loess forms in place as the underlying rock breaks down. His hypothesis exerted a profound influence over many Soviet scientists, but now it seems to have little support in the western part of the Soviet Union. In Yakutia, however, many workers have argued more recently that the loesslike silt there is a product of breakdown by seasonal freezing and thawing (Sudokova, 1959; Popov, 1965, 1967, 1972, 1976; Agadzhanyan and others, 1973; Danilova, 1973), a combination of frost action and chemical weathering (Chigir, 1972; Konishchev, 1972; Konishchev and Rogov, 1972), or chemical weathering alone, a "loessification” process in place (Morozova, 1971). According to this theory, the upland silt of central Yakutia is in the initial stages of loess formation and not yet as well formed as the loess of the Russian plains. If the upland silts were formed in place by disintegration of local country rock, the following features would be expected: 1. The mineralogy of the silt and the underlying rock would be similar, allowing for differential resistance to weathering. In central Yakutia, the silt overlies many substrates, including dolomitic limestone, alluvium, poorly consolidated sands, glacial outwash, and till of diverse provenance. In all areas, the mineralogy and chemistry of the silt is similar despite the great diversity of underlying material. In the southern part of the area, the upland silt lies directly on dolomitic limestone, but samples (W, X, Y, Z, BB, CC, DD, and FF; fig. 3) overlie the limestone and could not have been derived from the weathering, either chemical or mechanical, of the underlying rock. These samples contain a suite of heavy minerals similar to other samples, minerals not present in the underlying bedrock (tables 1, 2, and 4). The percentage of calcium carbonate in these samples is actually about the same as in the rest of the mapped area, or slightly less; yet, one would expect a tremendously high concentration of calcium carbonate in loess if it had originated from the underlying limestone. On top of dolomitic limestone cliffs 147 m above the Lena River at the Pillars of the Lena, a sample (tables 1 and 4) of weathered limestone was taken from a crevice deep in the rock in a dry environment. Although the sample is a well-sorted silt, it contains no heavy minerals and is pure calcium carbonate (fig. 27). The silt occurs high on the bluffs and could not have originated from some other local bedrock at higher altitudes and been transported to the area (figs. 13, 14, and 22). It is evident, therefore, that the silt in these localities did not come from the underlying or nearby bedrock.ORIGIN 2. Sediments produced by rock disintegration would undoubtedly contain some large particles, especially of the more resistant minerals, but such resistant particles are scarce, At some localities, angular pieces of resistant limestone as much as 5 cm in diameter occur at the base of the loesslike silt, probably broken from the underlying limestone by frost action. The massive loesslike silt overlies these frost derived fragments on a sharp contact that marks (fig. 22) a great change in textural, chemical, and mineralogical composition. 3. If silt accumulated on top of a hill or a flat-lying area through the disintegration of underlying bedrock, the material should be finer and more weathered at the surface than that closer to bedrock. The vertical uniformity of size distribution evidenced at Mamontova Gora (fig. 21), at consecutive depths of 2 m (U) and 3 m (V) (fig. 3) at Churapacha on the Abalakh Terrace, and by samples BB and CC in loesslike silt overlying dolomitic limestone (figs. 19 and 22) contradicts this expectation. 4. Mechanical disintegration, probably the domi- nant process of bedrock weathering in subarctic climates, depends largely on the action of alternate freezing and thawing (cryolithic processes). Where a thick layer of breakdown residuum develops, it might be expected to shield the underlying rock from further disintegration. On hillsides, the silt might be removed as fast as it forms, but it would accumulate on flat hilltops and ridges. The depth of seasonal freeze and thaw of the ground in central Yakutia ranges from 1 to 3 m with deepest penetration in gravel and least penetration in silt. Under present climatic conditions seasonal freeze and thaw of bedrock probably does not occur under a protective mantle of silt thicker than 1-2 m. In past colder periods, seasonal penetration of frost may have been slightly deeper in the nonperennially frozen ground. It is true that, in the past, well-drained silt may have been perennially frozen, but if no frequent alternation of freeze and thaw occurred, little disintegration would take place. Because much more than 3-4 m of upland silt is present on the relatively flat terraces and uplands, including the highest terrace, it is clear that the material could not have originated in place from seasonal frost action. 5. Residual silt on hilltops, ridges, plateaus, or ter- races would contain no carbonaceous layers or soil horizons if the material were formed by 35 a progressive downward disintegration of the underlying bedrock. Sections of the silt do, however, reveal carbonaceous layers and soil horizons. In summary, a number of characteristics that would be expected in the loesslike silt if it were formed by disintegration in place are not observed. Together the lack of these characteristics argue convincingly against the disintegration hypothesis. LACUSTRINE AND FLUVIATILE HYPOTHESES The most popular explanation of the origin of the upland silt in central Yakutia is that the loesslike silt is a combination of lacustrine and alluvial deposits formed on great flood plains and marshy plains (Popov, 1953; Solov’ev, 1959; Boyarskaya and Malayeva, 1967; Katasonov and Solov’ev, 1969; Motuzko and others, 1969; Agadzhanyan and others, 1973; Ivanov and Katasonov, 1973; Katasonov and Ivanov, 1973; Konishchev, 1973; Popov, 1973). Periglacial researchers concerned with various forms of ground ice favor this hypothesis because of the faint stratification where the silt has flowed into shallow depressions when large amounts of segregated ice melts. The quantity of ground ice present indicated, they thought, that the silt had been deposited on vast partly flooded plains. It has long been known (Bunge, 1884; Toll, 1895) that ground ice of various types and quantity (fig. 6) is widespread in the loesslike silt cover of the high and low terraces of central Yakutia, as well as on the coastal plain of the north. The lacustrine-alluvial hypothesis evolved over the years in an attempt to explain the different types and distribution of ground ice. According to Tomirdiaro and others (1974), until the early 1950’s of the 20th century, it was believed that most of the ice in the ground was remnant ice and snow from Pleistocene glaciers buried by clayey alluvium; this idea is the so-called glacial firn theory. It is now known that the glaciers were not that extensive (fig. 4) and that the foliated ground-ice wedges (figs. 6, 25) formed in thermal contraction cracks in the perennially frozen ground. Many scientists thought that the ice wedges must have grown syngenetically in the saturated sediments of a gigantic flood plain and invoked great floods and colossal lacustrine-alluvial plains to account for the great vertical extent of the ice, from modern flood plains to high terraces (Popov, 1953, 1965). A number of factors combine to make the fluvial and lacustrine hypotheses unlikely. Fluviatile silt is well stratified and crossbedded, individual beds are lens shaped, and, though individual beds are fairly well sorted, samples collected over a broad area or from a thick section are poorly sorted. The upland silt, however, is unstratified and uniform in texture. Moreover, though fluviatile silt is abundant on the flood plains of the Lena and Aldan Rivers, it is typically of larger grain36 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA size than the upland silt (fig. 26 and table 3, sample E). It is also difficult to explain, in the context of the fluvial hypothesis, the presence of as much as 25 m of silt on bedrock bluffs as much as 50-100 m above the river level and the character of the silt as a nearly continuous blanket over a surface with 100-200 m of relief. The lacustrine hypothesis is also improbable for several reasons: 1. No known barriers capable of damming water 200 m deep, 150 km up the Lena River from Yakutsk and 300 km up the Aldan River from its mouth existed in the area where the silt was deposited. 2. No features such as preserved shorelines, wave-cut benches, and deltas, which would be expected to form in a lacustrine environment, are evident in the silt. 3. Lacustrine deposits have a limited vertical ex- tent, but the loesslike silt occurs at various elevations and even on the moraines at the foot of the Verkhoyansk Range, hundreds of meters above the Aldan River. 4. Lacustrine silt would be stratified and might even contain varves. The only stratification in the upland silt is produced by isolated carbonaceous layers and iron-stained horizons, and, in most places, stratification is absent. Crude stratification is outlined by concentrations of ground ice and by slight differences in grain size. In the retransported materials that were carried by slopewash to shallow depressions, however, this crude stratification disappears upon thawing and complete drying of the silt in cliffs. 5. An appreciable amount of clay could be expected in the deposits of any large lake. The average cumulative-frequency curve of 27 samples from the upland silt has a very low content of clay, only about 5-15 percent. 6. As would be expected of lacustrine deposits, the upland silt bears little lithologic relation to the underlying rock. This characteristic, however, is typical of any transported sediment. 7. Some forms of life would have existed in these lakes. So far as we are aware, no lacustrine fossils have been found in the high upland silt, although gastropods and pelecypods have been found in the reworked material of the valley bottoms. The fossils in the valley bottoms probably were derived from thermokarst lakes (alases) that existed at the time of silt accumulation. 8. Land fossils would be absent or scarce in lacus- trine deposits, as they are in the nonretrans-ported upland silt. But land fossils are preserved in the upland silt although they are fragmentary and show evidence of transport. These features, however, are to be expected in Figure 25.—Panorama of late Quaternary silt containing ice wedges exposed at Mamontova Gora, left limit of Aldan River, 310 km upstream from junction with Lena River. View downstream. Photographs 3427, 3428, 3429, and 3430 by T. L. P£w6, July 22, 1974. (Reproduced by the courtesy of Quaternary Research.)ORIGIN 37 transported sediments regardless of the transport mechanism. 9. Mud cracks and ripple marks would be expected in lacustrine silt, but none have been observed. All but two of these features of lacustrine deposits are contrary to the observed nature of the upland silt. The two features that are consistent are typical of deposits transported by any mechanism. The observed characteristics of the silt in central Yakutia cannot be explained by the lacustrine and fluviatile hypotheses alone. Moreover, radiometric and paleontologic dating show that the silt accumulated in middle and late Pleistocene glacial times (Vangengeim, 1961; Sher, 1971; Pewe and others, 1977), not during a warm period when melting would inundate the region. The remains of steppe animals such as Siaga (Sher, 1967, 1969, 1971) and steppe tundra (Yurtsev, 1972; Hopkins, 1976; Mathews, 1976) confirm the general dryness of the climate. EOLIAN HYPOTHESIS Although the idea that the silt in central Yakutia is eolian in origin is not new, it has not been described and evaluated in detail. Obruchev (1945), a longtime Soviet leader in loess studies, mentioned that loess existed in central Yakutia. Volkov and others (1969) have demonstrated that the widespread silt in southwest Siberia is eolian in origin. Recently, Tomirdiaro (1972) and To-mirdiaro and others (1974) recognized that the silt in northern Yakutia is loess. Vangengeim (1961, p. 37) referred to the silt as dusty loam, "the formation of Figure 26.—Cumulative-frequency grain-size curves for sand from Lena-Aldan Rivers area. E, flood-plain alluvium, UstTatta Village; N, sand, top of Magan Terrace, 53 km west of Yakutsk; P, sand dune on Bestyakh Terrace; AA, alluvial sand, 7 km west of Pokrovsk; EE, eolian sand from cliff-head dune, 40 km upstream from Pokrovsk on Lena River. See table 1, figure 3.38 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA which the majority of geologists have correlated with unusual physical-geographical conditions in glaciated regions.” Rusanov (1968) mapped all the silt on the high terraces of the lower Aldan River valley as loess. Giter-man and Golubeva (1967) referred to the sediments on the high terraces as loess but did not provide supporting data. Fedorovich (1972) recently mentioned that the study of the silt in Yakutia should be undertaken to provide new contributions to loess science. Our studies of the silt in southcentral Yakutia, and comparisons with similar deposits in other parts of the world, support the identification of the deposits as loess. Features and characteristics of eolian-deposited silt fall into three groups: field relations, mechanical composition, and petrographic character (Smith and Frazer, 1935). FIELD RELATIONS Topographic distribution.—Loess characteristically mantles preexisting topography, "filling up the gulleys, covering minor depressions, lying deepest in the depressions and thinning-out up the flanking slopes of the higher ridges” (Barbour, 1930, p. 468). The silt in central Yakutia conforms closely to this pattern. It is draped over river terraces of varous levels and thinly over the glacial outwash and moraines emanating from the Verkhoyansk Range. It is absent on the flood plain, low floodplain terraces, and the sandy Bes-tyakh Terrace. It is not known why the silt is absent from the Bestyakh Terrace, possibly because the terrace surface, part of a late Pleistocene sandy flood plain of the Lena River, was a source of windblown silt, not an area of accumulation. In general, the high Magan Terrace has little or no loess because: (1) the terrace, in most areas, is distant from the major rivers (fig. 3); (2) the Lena River is narrow and has a small flood plain where it is near this terrace and; (3) the terrace was upwind from a major glacial flood plain, the source of silt. Independent lithologic character of the deposits.— Wind-deposited material may differ mineralogically and texturally from the underlying material. In south-central Yakutia, uniform silt covers glacial till, dolomi-tic limestone, poorly consolidated sandstone, and alluvium. At these several localities, the fine texture of the silt contrasts strongly with the texture of the underlying material, but is very similar to that of the typical tan upland silt in south-central Yakutia (fig. 4) and northern Yakutia (fig. 27). Decrease in thickness away from source of supply. — The greatest thickness of the silt blanket is near its source, the flood plains and low terraces of the Lena and Aldan Rivers. The silt is 2-5 m thick on the bluff of the Magan Terrace west of Yakutsk and thins westward to a featheredge within a few kilometers (fig. 3). Such marked thinning is consistent with present-day westerly prevailing winds and may indicate similar wind patterns during the Pleistocene from the west. Absence of distinct stratification.—Absence of distinct stratification, a characteristic of loess, is a conspicuous feature of the upland untransported silt. The only suggestion of stratification is imparted by thin carbonaceous horizons and iron-stained horizons. Color and texture.—Wind-deposited silt, like that described here, is generally light colored except where stained by solutions or carbonaceous inclusions; the texture is generally uniform and the clay component makes up no more than 10-15 percent of the sediment. Association with other evidences of wind action.— Smith and Fraser (1935, p. 19) state that sand dunes and ventifacts may be expected near the source of material. Well-developed, vegetated sand dunes cover the entire narrow Bestyakh Terrace adjacent to the wide Lena River from Pokrovsk to the mouth of the Aldan River (fig. 3). The eolian forms exist as ridges 2-10 m high and as small merged parabolic dunes. An active cliff-head dune 20 m high occupies the edge of the Bestyakh Terrace on the right limit of the Lena River about 40 km upstream from Pokrovsk. This dune and other dunes are evidence of eolian action still going on where sediment source areas are nearby. Small dust clouds raised today from the almost completely vegetated braided-stream deposits of the major rivers in central Yakutia suggest that enormous clouds of dust may have come from the unvegetated braided streams present during the Pleistocene. It has been demonstrated by many workers that no more wind is necessary to blow this dust than is present today, as winds up to 20 m/s are more than strong enough to bear quantities of dust in both perigla-cial areas and hot desert areas (Bryan, 1927, p. 39-40; Pewe, 1951; Warn, 1953, p. 70-71; Pewe and others, 1976; Pewe and others, 1981). Chigir (1972), however, believes that much higher wind velocities are necessary and that such winds were produced by the nearness of glaciers. Instead, only larger source areas than now available are necessary, in our opinion, and they were provided by the wide unvegetated, braided Lena and Aldan Rivers in Pleistocene time. Fossils of air-breathing animals.—Fossils of air-breathing animals should logically be expected to be present in the silt if the material is eolian. Middle and late Pleistocene vertebrate fossils are distributed widely in the silt blanketing the terraces on the Lena, Aldan, and Vilyuy Rivers (for example, Alekseev, 1961; Vangengeim, 1961; Rusanov, 1968; and others). Fossils present include remains of mammoth, horse, bison, and other mammals. The preservation of many of the specimens shows evidence of very little transportation. No land gastropods have been reported in the uplandORIGIN 39 silt in Yakutia. Russell (1944, p. 34) regarded land gastropods as being characteristically present in loess. Some other loess deposits, however, in the upper Mississippi Valley and most of the loess in Alaska, do not contain land gastropods (Pewe, 1955, p. 720-721). Also, Dr. T. C. Yen (oral commun., June 27, 1950), former paleontologist at the United States National Museum, Washington, D.C., reported that much of the loess of China and the Rhine Valley contained no terrestrial gastropods so this evidence is not conclusive. MECHANICAL COMPOSITION If the upland silt is loess, the grain size and degree of sorting would be similar to that of windblown dust or volcanic ash that has been transported a considerable distance (Udden, 1898, p. 31-60; Swineford and Frye, 1945, p. 252; Warn and Cox, 1951, p. 559; Pewe, 1955, fig. 11; and Pewe and others, 1976), or similar to silt known to be loess. Cumulative-frequency curves of mechanical analyses of windblown dust from Kansas (Swineford and Frye, 1945, p. 252) and Germany (Zeuner, 1949, p. 27) and Arizona (Pewe and others, 1981) are similar to the average cumulative-frequency curve of the upland silt from central Yakutia (fig. 28). It was thought that a great similarity of grain size and degree of sorting between the upland silt of Yakutia and deposits of silt of known eolian origin elsewhere in the world, would constitute a strong argument for the eolian hypothesis. For comparison, samples of silt long known to be loess were collected from Czechoslovakia, Alaska, Illinois, China, New Zealand, France, and Uzbekistan. All but one of the samples were analyzed by the same method in the same laboratory as the Yaku-tian samples. The cumulative-frequency curve of a typical sample from Yakutia is strikingly similar to curves of loess from other parts of the world (fig. 29). PETROGRAPHIC CHARACTER Angular grains have long been considered typical of loess (Twenhofel, 1932; Charlesworth, 1957), and scanning-electron-microscope analysis supports that assumption (Cegla and others, 1971). The silt grains from Yakutia are clearly angular, especially the light minerals such as quartz. It has been suggested that loess grains should also be largely unweathered and clay minerals almost absent (Smith and Fraser, 1935; Figure 27.—Cumulative-frequency grain-size curves of silt from Yakutia. Sample GG is silt from weathered dolomitic limestone in crevice 147 m above Lena River, 250 km south of Yakutsk. Sample Tomi is loess from northern Yakutia (Tomirdiaro and others, 1974, table 2).40 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA Charlesworth, 1957), though the degree of weathering would depend on the age and rate of deposition of the deposit and the effects of diagenetic processes. The silt from Yakutia, like middle and late Pleistocene loess from other parts of the world, is relatively fresh but shows some weathering under the scanning electron microscope. The heavy-mineral content of the silt in the Lena and Aldan Rivers area (table 4) confirms the independence of the silt from the underlying deposits, in contrast to Sudakova’s (1969, p. 55) claim that the microscopic component is inherited from the underlying rocks. It also indicates the general uniformity of the silt, though it can be used to distinguish the Lena River and Aldan River flood plains (fig. 24). Heavy-mineral content in loess has been used to distinguish source areas in many parts of the world (Kay and Graham, 1943; Pewe, 1955; Frye and others, 1962; Goldthwait, 1968; Journaux and others, 1969; Codarcea and others, 1972). Summary.—Many points of similarity between the characteristics of loess and the loesslike silt in central Yakutia support the eolian hypothesis. In light of this evidence we conclude that the silt on the terraces and plateaus of the major river valleys such as the Lena, Aldan, and Vilyuy Rivers in south-central Yakutia is loess. AGE AND CORRELATION Glaciers pushed southward and westward from the Verkhoyansk Range in pre-Wisconsinan and Wisconsin-an time, advancing almost as far as the Aldan River (fig. 4). During advances of glaciers from this range and ranges farther southeast and of continental glaciers to the west (fig. 4), braided silt-laden melt-water streams such as the Lena, Aldan, and Vilyuy Rivers flowed over wide vegetation-free flood plains and spread glacial flour over the outwash plains. Winds picked up some of the silt on the flood plains and outwash plains and redeposited it as loess on the lowlands, mainly the high terraces and low plateaus. At least two major periods of loess deposition are recorded in the upland silt in central Yakutia, according Figure 28.—Cumulative-frequency grain-size curves for upland silt (sample K) from near Yakutsk, volcanic ash from Fairbanks, Alaska, and modem wind-deposited dust from Germany (Zeuner, 1949, p. 27), Kansas (Swineford and Frye, 1945, p. 252), and Tempe, Ariz. Samples from Alaska, Arizona, and Siberia collected by T. L. Pewe. Siberian and Arizona samples analyzed at Centre de Geomor-phologie du Centre National de la Recherche Scientifique, Caen, France; Alaskan sample analyzed by U.S. Army Corps of Engineers, Rock Island, 111.AGE AND CORRELATION 41 to Pewe and others (1977). On the basis of biostrati-graphic studies and radiometric dating, we believe that the loess here is middle and late Pleistocene in age, correlative to Maximum Glaciation and later glaciations equivalent to pre-Wisconsinan and Wisconsinan glaciations in North America. On the basis of biostratigraphic studies, Vangengeim (1961) believes that the loess first began to accumulate during the time of the Maximum Glaciation (Samarov) in the Verkhoyansk Range. Further, the silt deposits on the terraces, at least the upper part, are characterized by a late Pleistocene fauna consisting largely of numerous remains of Mammuthus primigenius of late type, Equus caballus, subsp. B. (small form), Bison priscus aff. deminutus W. Grom, and other species. The fossils are found not only at Mamontova Gora, but also elsewhere on the terraces of the Aldan River valley (Vangengeim, 1961, p. 38-39). This fauna has been described at Mamontova Gora by Motuzko and others (1969, p. 63). Giterman and Golubeva (1967, p. 237) believe that the loess on the high terraces is correlative with the Samarov (Maximum) and Zyryan Glaciations. Much of the stratigraphic information is from the well-known section at Mamontova Gora, where perennially frozen ice-rich silt and associated flora and vertebrate fauna of late Quaternary age are well exposed. It is true that on some of the higher terraces along the Aldan, mammalian remains of older types are also present, as reported by Motuzko and others (1969) and Vangengeim (1961). This is to be expected, because the older silt could be present on the older, higher terraces. In fact, Motuzko and others (1969, p. 65) report remains of a long-horned bison and large horse in the lower silt of the 50-m-high terrace. At Mamontova Gora, Vangengeim (1961, p. 33) reports remains of an early-type mammoth and large horses in an exposure in the northern part of the area but late-type mammoths and small horses elsewhere in the area. She (1961, p. 34) believes that the older fauna was derived from the base of the silt section or from the top of the underlying sand. Alekseev (1961, table 3) reports some early radiocarbon dates of wood from central Yakutia and one from an exposure of "clay loam/sandy loam” at a depth of 17 m on the right bank of the Aldan River, 50 km from its Figure 29.—Cumulative-frequency grain-size curves for upland silt from central Yakutia (sample K) and loess from Czechoslovakia, Alaska, Illinois, China, New Zealand, France, and Uzbekistan. Samples from all areas except Illinois analyzed at Centre de Geomorphologie du Centre National de la Recherche Scientifique, Caen, France; Illinois sample analyzed by U.S. Army Corps of Engineers, Rock Island, 111. All samples except those from France collected by T. L. Pewe; sample from France collected by Andre Joumaux.42 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA mouth. The wood fragment was dated by the solid carbon method as being more than 20,000 years old. Radiocarbon dates for what appears to be the upper loess unit where exposed in the 50-m-high terrace at Mamontova Gora have been presented in Markov (1973, p. 4—5, 163). These dates are on samples from depths of 1-8 m but not from one continuous section or from one locality; therefore, the depth below the surface does not define stratigraphy. The dates and sample depth reported are: 26,800±600 years B.P., 5.5 m; 40,600±500 years, 3 m; and 44,000± 1,900 years, 8 m. We believe that the nature of erosion and deposition of these sediments is such that at a depth of 3 m in one area, the sediments may be 20,000 years old, whereas at this depth at another locality, the age may be 40,000 years. At Mamontova Gora, truncation of the top of the upper loess unit is indicated by an unconformity. In an attempt to better understand the stratigraphy of the loesslike silt, Pewe in 1973 collected carefully located organic specimens in a vertical section at Mamontova Gora (Pewe and others, 1977). This section comprises two thick silt units and an overlying thin (1-2 m thick) silt layer of loess and retransported loess of Holocene age (fig. 30). The radiocarbon dates on the 1973 samples ranged from about 42,000 to 46,000 years B.P. for the thick upper loess and greater than 46,000 years B.P. for the thick lower loess. Samples collected by M. S. Ivanov and V. V. Kastukevich in 1974 indicate a range from about 34,000 to greater than 56,000 years B.P. for the upper loess and greater than 56,000 years B.P. for the lower loess (fig. 30). Location and dates of the radiocarbon samples, dating organizations, and location of silt samples for mineralogical and chemical examinations area shown in figure 30. On the basis of the biostratigraphy, radiocarbon dates, and relation to the glacial record of nearby areas, we believe that the age of the upper thick loess in south-central Yakutia and the middle silt unit of Mamontova Gora, dated at 26,000 to greater than 56,000 years B.P., is Wisconsinan. This age correlates to the Sartan and Zyryan Glaciations and included inter-stadials and interglaciations of Siberia. Although the youngest date now available from the upper loess on the 50-m-high terrace at Mamontova Gora (26,000 years) does not provide evidence that the age of the loess corresponds to the time of the Sartan Glaciation, supposed to have begun about 25,000 years ago (Kind, 1972, p. 56-7; 1975, fig. 1), the unconformity at the top of this unit indicates truncation of the section. Kind (1972, p. 57) cites a date of 20,900±300 years B.P. from loesslike material on a terrace on the Yenisei River. Until younger dates are determined from the upper part of the upper thick loess and dates are obtained from the lower part of the Holocene silt, the relation of the upper loess Figure 30.—Diagrammatic stratigraphic section of perennially frozen late Quaternary silt in the exposure of the 50-m-high (Abalakh) terrace at Mamontova Gora, left limit of the Aldan River 310 km from its mouth, central Yakutia. Radiocarbon material dated in U. S. and loesslike silt samples collected by T. L. P6w6, 1973. Radiocarbon material dated in U.S.S.R. collected by M. S. Ivanov and V. V. Kastukevick, 1974 (P. I.Melnikov, written commun., June 6, 1975). From P6we and others, 1977 (fig. 3). (Reproduced by the courtesy of Quaternary Research.) (1) Radiocarbon date on a root 1 m below the surface, SI 1968 (SI numbers indicate laboratory number of Smithsonian Institution, United States). (2) Depth to permafrost is about 1-2 m under the forest. Flat-topped ice wedges indicate a lower permafrost table with thawing down to the top of ice wedges sometime in the past. (3) Loess sample G. See figure 21 for mechanical analyses; tables 3, 4, and 6 for chemical and mineralogical analyses of samples G-I. (4) Radiocarbon date from Permafrost Institute, Yakutsk. (5) Radiocarbon date on tree fragments, SI 1965. (6) Loess sample H. (7) Radiocarbon date from two small fragments of unidentified late Quaternary mammal ribs, SI 1972. (8) Radiocarbon date on wood fragments, SI 1967. (9) Radiocarbon date on wood fragments, SI 1966. (10) Loess sample I. (11) Crossbedded brown sand of early and middle Pleistocene age.SUMMARY 43 to the Sartan Glaciation is speculative. The base of the upper loess is more than 56,000 years old, and possibly as old as 100,000 years. The top may be as young as 10,000 years. No definitive dates are yet available to mark the boundaries, but we believe the boundaries will prove to be of the order of those ages. The lower loess is definitely beyond the range of conventional radiocarbon dating. It may be Wisconsinan in age, but a Samarov age is a strong possibility, correlative with the Maximum Glaciation of Siberia and older than the last interglacial period. Mammalian remains support the hypothesis that the loess first began to accumulate in Maximum time (Vangengein, 1961; Giter-man and Golubeva, 1967); if it did, interglacial beds may be found separating it from the Wisconsinan beds. Alekseev (1970) believes that the silt beds along the lower Vilyuy River with large ice wedges and mammalian remains were deposited in the second half of the Pleistocene. He suggests that they represent a large interval of time, from the Riss Glaciation to the end of the Wiirm Glaciation. It is hoped that additional dating of loess samples from the units exposed on the high terraces along the Aldan River and elsewhere will soon be undertaken. SUMMARY Loesslike silt mantles the upland terraces and low plateaus throughout south-central Yakutia, reaching maximum thickness along the south side of the Aldan River valley and the east side of the Lena River valley. It is well exposed in the river cuts in these areas, the greatest thickness recorded being more than 60 m on the Tyungyulyu Terrace. The texture and mineral composition are uniform throughout central Yakutia, whether the silt overlies limestone, poorly consolidated sandstone, alluvium, or glacial outwash. All loesslike silt samples examined contain a high percentage of quartz and feldspar and lesser amounts of hornblende, epidote, hypersthene, and garnet. Most samples contain a small amount of sphene, zircon, and rutile. The loesslike silt is massive except where retransported and has developed crude stratification. Vertebrate fossils, not common in the untransported silt, are very common in the retransported silt in shallow depressions. No land snails, typical in loess deposits in some areas, have been found in these deposits. Hypotheses advanced to explain the origin of the upland silt proposes marine, estuarine, lacustrine, fluvial, residual, or eolian sources, or a combination of these. A marine or estuarine source is unlikely because of the absence of evidence of deltas, shorelines, beaches, clay, mud cracks, ripple marks, and marine or brackish-water fossils. Moreover, such an origin would require a rise of sea level or a subsidence of the land of many meters in late Pleistocene time, an inundation not borne out by geologic evidence. The upland silt is not of fluviatile origin because (1) it is unstratified and contains no individual beds or lenses of other sediments, (2) it occurs as a nearly continuous blanket over a surface of irregular topography with relief of 200 or 300 m and is more than 200 m above the major rivers; and (3) no fresh-water fossils are present. The lacustrine hypothesis is strongly supported by workers studying the ice wedges in the perennially frozen ground. They believe that the loesslike silt is a combination of lacustrine and alluvial deposits formed on great flood plains and marshy plains. But there are several reasons for believing this origin unlikely. No evidence of shorelines, wave-cut beaches, or deltas are present; nor are mud cracks, ripple marks, or freshwater fossils found. Neither stratification nor an appreciable amount of clay exists in the silt. No definite upper limit, to be expected under a lacustrine hypothesis, is present. The hypothesis that loesslike silt is a product of the breakdown in place of the underlying rocks, mainly by frost action, has also received strong support from many workers. Evidence against this hypothesis includes: (1) the minerals in the silt are not everywhere similar to the underlying bedrock, (2) no large particles of more resistant minerals are present, (3) the silt does not become progressively coarser toward bedrock, (4) mechanical disintegration by alternate freezing and thawing could not produce untransported silt 60 m thick on flat terraces, and (5) the silt contains undisturbed carbonaceous layers. Evidence for the eolian origin of the upland silt is abundant: (1) the silt mantles older topography, (2) it is lithologically independent of the underlying material, (3) stratification is indistinct or absent, (4) it is associated with sand dunes, (5) it contains fossils of air-breathing land animals, (6) its sorting and texture are similar to those of loess and windblown dust from many places elsewhere in the world, and (7) the grains are angular and relatively fresh. Central Yakutia has not been glaciated. However, glaciers from the Verkhoyansk Range on the north and west as well as glaciers in the ranges south and east and the continental glacier on the west almost surrounded the interior of Yakutia during times of glacial maxima (fig. 4). We believe the upland silt to be loess deposited mainly during periods of glacial expansion by winds blowing across the unvegetated outwash plains and flood plains of glacial streams. REFERENCES CITED Agadzhanyan, A. K., Boyarskaya, T. D., Glusenkova, N. I., and Sudakova, N. G., 1973, Paleogeography and stratigraphy of44 ORIGIN AND CHARACTER OF LOESSLIKE SILT, YAKUTIA, SIBERIA MamontoVa Gora, central Y akutia: Biuletyn Peryglacjalny, v. 29, p. 19-36. Alekseev, M. N., 1961, Stratigraphy of continental Neogene and Quaternary deposits in the Vilyuy Depression and the lower Lena River Valley; Akademiya Nauk SSSR, Geologicheskiy Institut, Trudy, v. 51, 199 p. 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Note: Translations by CRREL (Cold Regions Research and Engineering Laboratories) available from Federal Scientific and Technical Information, Springfield, Va. 22151.GPO 687-040/17t *