DEPARTMENT OF THE INTERIOR 3 1 Ijyýc TO ACCOMPANY MAP 1-1728 U. S. GEOLOGICAL SURVEY SM

LATE TERTIARY AND QUATERNARY GEOLOGY OF THE TECOPA BASIN, SOUTHEASTERN CALIFORNIA By John W. Hillhouse

ABSTRACT INTRODUCTION SCOPE OF THE INVESTIGATION ,Stratigraphic units in the Tecopa basin, located in The objectives of this study were to establish the southeastem California, provide a framework for interpreting Quaternary climatic change and tectonism distribution, age, and structure of Quaternary deposits in along the present Amargosa River. During the late Pliocene the Tecopa basin. This information provides a basis for and early Pleistocene. a climate that was appreciably interpreting past episodes of faulting and climatic change in wetter than today's sustained a moderately deep lake in the Amargosa River drainage system. The Tecopa basin is the Tecopa basin. Deposits associated with Lake Tecopa ideal for studies of Quaternary history because erosion has consist of lacustrine mudstone, conglomerate, volcanic ash, clearly exposed the stratigraphy, and the deposits of and shoreline accumulations of tufa. Age control within the Pleistocene Lake Tecopa have proven to be datable. lake deposits is provided by air-fall tephra that are Volcanic ash beds within the lake deposits have been correlated with two ash falls from the Yellowstone caldera, chemically correlated with isotopically dated volcanic the Lava Creek (0.62 Ma) and Huckleberry Ridge (2.02 sources in the Yellowstone (Wyoming) and Long Valley Ma) Tuffs, and one from the Long Valley caldera, the (California) calderas (Izett, 1981; Sama-Wojc~cki and Bishop Tuff (0.73 Ma). Paleomagnetic determinations from others, 1984). In addition, the continuous sedimentary deposits in the Tecopa basin are consistent with the ages record of the lake beds provides an opportunity for of the ashes. Extrapolations of ages using average paleomagnetic correlations. With age control offered by the sedimentation rates suggest that the beds of Lake Tecopa, volcanic ashes, magnetozones in the lake deposits can be which accumulated to a minimum thickness of 72 m, are correlated with the geomagnetic polarity time scale 0.5 to 3 m.y. old. In the central part of the basin, volcanic (Mankinen and Dalrymple, 1979). glass has been replaced by authigenic silicate minerals that Proterozoic and Cambrian sedimentary rocks and formed in porewater of extremely high salinity and Tertiary volcanic rocks of the region were given only a alkalinity. Therefore, Lake Tecopa occupied a closed basin cursory examination as far as establishing source terranes during the latter part, if not all, of its 2.5-million-year for the Quaternary alluvium. No efforts were made to history. describe stratigraphy, faults, or other structures in the Sometime after 0.5 m.y. ago, the lake developed an bedrock of the ranges. Information on the bedrock geology outlet across Tertiary fanglomerates of the China Ranch of the Tecopa area is found in reports by Hazzard (1937). Beds leading to the development of a deep canyon at the. Mason (1948), Noble and Wright (1954), and Troxel and south end of the basin and establishing a hydrologic link Wright (1976). between the northern Amargosa basins and Death Valley. In constructing the geologic map, contacts between After a period of rapid erosion, the remaining lake beds Quaternary units that have developed distinctive were covered by alluvial fans that coalesced to form a geomorphologies were marked on aerial photographs pediment in the central part of the basin. Remnants of (scale 1:47,200) and then transferred to the topographic these fans are capped by desert soil horizons composed of: base (scale 1:48,000) with a stereographic plotter. These (1) a clay-rich B-Horizon, (2) a thin bed of vesicular tan silt units were checked and described during fieldwork in the (A-horizon), and (3) densely packed pavement with well winter of 1981. The lake deposits and outcrops of the developed desert varnish. The B-horizon of one such fan thicker volcanic ashes were mapped in the field on aerial yielded an age of 160,000 ± 18,000 years by the uranium photographs and contacts were then plotted on the trend method. The Tecopa basin bears no evidence of topographic base. Several stratigraphic sections were Lahontan-age lake deposits. Holocene deposits consist of measured in the lake beds. unconsolidated sand and gravel in the Amargosa River bed Paleomagnetic investigations of the lake deposits, and its deeply incised tributaries, a small playa near made primarily in 1971-74, and supplemented in 1978. Tecopa, alluvial fans without pavements, and small sand involved the collection of oriented sediment cubes. dunes. The pavement-capped fan remnants and the Sampling was limited to steep-walled outcrops to obtain Holocene deposits are not faulted or tilted significantly, relatively unweathered material. The samples were given a although basins to the west, such as Death Valley, were standard treatment in alternating magnetic fields to remove _ tectonically active during the Quaternary. Subsidence of unstable magnetic components and to reveal the original the western basins strongly influenced late Quaternary depositional remanent magnetization (McElhinny, 1973). rates of deposition and erosion in the Tecopa basin. Magnetozones were established from several equivalent 90110384 930305 1 PD IWASTE ,1l-11 PDR stratigraphic sections that were closely correlated by STRATIGRAPHY PROTEROZOIC, AND CAMBRIAN ROCKS volcanic ash beds. ARCHEAN. PHYSIOGRAPHY The oldest rocks within the study area are granitic in the •, The Tecopa basin is in the southeastern part of lnyo gneiss at the southwest end of the Nopah Range and which County, California. about 30 km east of the south end of Ibex Hills. The gneiss contains metadiorite dikes to Death Valley. In this report "Tecopa basin" refers to the Chesterman (1973) assigned an Archean age. Exposed in Hills and the part of the Amargosa River drainage that lies chiefly the rugged banded cliffs of the eastern Dublin succession of between the towns of Shoshone and Tecopa and includes Resting Spring Range is a nearly complete the southeast end of Greenwater Valley and the southern uppermost Proterozoic to Middle Cambrian marine half of Chicago Valley (fig. 1). sedimentary rocks. The sequence consists of the, The Tecopa basin can be considered a southern ascending, Noonday Dolomite (Proterozoic), Johnnie Quartzite (Proterozoic). extension of the Amargosa Desert, which generally lies Formation (Proterozoic), Stirling (Proterozoic and Lower between Eagle Mountain on the south (25 km north of Wood Canyon Formation Shoshone) and the Bullfrog Hills on the north (120 km Cambrian), Zabriskie Quartzite (Lower Cambrian), Carrara and the northwest of Shoshone). The areal extent of the Tecopa Formation (Lower and Middle Cambrian), basin is approximately 500 km2. The region is Bonanza King Formation (Middle and Upper Cambrian) characterized by arid sparsely vegetated valleys and bare TERTIARY VOLCANIC ROCKS rugged mountain ranges. A dominant feature of the Volcanic flows and tuffs, predominantly of rhyolitic Tecopa area is the Amargosa River, which is dry except composition, are exposed in the western Dublin Hills and applied the during infreouent thunderstorms or near permanent the Resting Spring Range. Haefner (1976) springs at Shoshone and Tecopa. North of Shoshone as far name "Shoshone Volcanics" to the pyroclastic rocks of the' as Eagle Mountain, the river bed follows a nearly linear Dublin Hills. The Shoshone Volcanics. which consist of are part of a large channel, which has cut approximately 25 m into the pumice lapilli tuff and welded tuff, the surrounding alluvial piedmont South of Shoshone, the Tertiary volcanic terrane exposed throughout river bed broadens into a braided plain surrounded by Greenwater Range and the Black Mountains. The welded badlands of low rounded hills, mesas, and deeply dissected units are crystal poor in the Dublin Hills. but elsewhere plagioclase. alluvial fans. A small playa forms the floor of the they contain abundant phenocrysts of depression near Tecopa. South of Tecopa, the river hornblende, and biotite. The unwelded units consist of descends through a narrow canyon, of which steep wails blocks of andesite and rhyolite, rare clasts of dolomite and -ire locally more than 100 m high. The basin is flanked by quartzite, and a devitrified pumice lapilli matrix. Haefner equivalent to )anded brightly colored rocks of the Resting Spring Range (1976) considered the Shoshone Volcanics \---'on the east and the Dublin Hills on the west (fig. 1). The part of the "older volcanics" that underlie the Greenwater Sperry Hills, which are bisected by the canyon of the Volcanics of the Funeral Peak quadrangle (Drewes, 1963). Amargosa River, make up the southern margin of the Potassium-argon ages, principally from biotite, range from Tecopa basin. Mean annual precipitation ranges from 7 cm 6.32 to 8.02 Ma for the "older volcanics" and range from in the valleys (-460 m elevation) to 15 cm in the higher 5.32 to 5.47 Ma for the Greenwater Volcanics (Fleck, parts of the Resting Spring Range (-1,000 m elevation) 1970). Volcanic flows at the northwest end of the Dublin (Winograd and Thordarson, 1975). Hills have yielded K-Ar ages of 7.9 Ma (plagioclase) and PREVIOUS WORK 8.7 Ma (glass) according to E. W. Hildreth and R. E. Drake Sheppard and Gude (1968) reviewed the early (oral commun., November, 1982). geologic studies of the Tecopa area, most of which Pyroclastic rocks of Tertiary age are exposed along the concerned exploration for nitrates, borates, and pumicite. east flank of the Resting Spring Range and make up the General geology of the Tecopa basin is summarized by prominent butte that is 4 km northeast of Shoshone. An Mason (1948) and Noble and Wright (1954). Detailed excellent example of these pyroclastic rocks is exposed in geologic maps, at 1:24,000 scale, were published for the the road cut where California Highway 178 crosses the southeast quarter of the Tecopa quadrangle (Wright, 1974) Resting Spring Range (Troxel and Heydari, 1982). The cut and the northeast quarter of the Shoshone quadrangle exposes devitrified pumice tuff, welded tuff, and vesicular (Chesterman, 1973). Tertiary volcanic rocks in the Dublin vitrophyre, dated by K-Ar methods at 9.5 Ma (R. E. Drake, Hills were studied by Haefner (1976). In general, emphasis oral commun., November, 1982). Although the distribution of previous work was on the bedrock geology, so the of volcanic rocks of the Resting Spring Range is shown on Quaternary deposits received only cursory examinations. geologic maps by Mason (1948) and Noble and Wright Pleistocene Lake Tecopa, which was named and (1954), detailed descriptions, isotopic ages, or the relation described in a brief article by Blackwelder (1936), has of these volcanic rocks to similar units farther west have been the subject of several geochemical and mineralogical not been published. studies. Sheppard and Gude (1968) described the TERTIARY BASALT of stratigraphy and approximate distribution of the lake beds Isolated remnants of basalt are exposed 1 km north in a study that focused on the formation of authigenic Shoshone and in the vicinity of the southern Dublin Hills. ,ilicate minerals, particularly zeolites, within volcanic ashes The basalt forms black rounded hills strewn with blocks as ,f the basin. Starkey and Blackmon (1979) investigated much as several meters in diameter. The olivine-rich basalt and highly \s._.the distribution and origin of clay minerals in the lacustrine is fine grained, moderately porphyritic, deposits. vesicular. Just north of Shoshone the basalt overlies buff-

2 116'45' 1160 36030' -

350450 boundaryreTeopmPsn of HIILLS

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0 10 15MILES

FIGURE 1.-Major geographic features of the Amargosa region. Dotted line indicates boundary of Tecopa basin. 3 colored tuff that consists of rounded pumice cobbles, ancestral Spenry Hills, which consisted primarily of volcanic rock fragments, and pumiceous matrix. Tertiary(?) granite and Archean gneiss. The ancient fan has Chesterman (1973) assigned a Quaternary age to the been modified by extensive faulting. \.basalt and stated that the unit overlies deformed lacustrine Lenses of white pumice fill ancient channels within the deposits of Pleistocene Lake Tecopa. However. tuffaceous fanglomerate. The lenses, as much as 5 m thick, consist of lake beds were found on top of the eroded basaltic mound pumice clasts in a white lapilli matrix. The clasts are well on the east side of the Amargosa River, 1 km north of rounded and are from 1 to 5 cm in diameter. Pumice Shoshone. Inasmuch as the oldest deposits of Lake lenses are generally no more than a few tens of meters Tecopa are late Pliocene (see section on "Age of Lake wide, although one extensive pumice bed is exposed in the Tecopa"), the basalt is presumably of Tertiary age. In the northern Amargosa canyon, 3 km south of Tecopa. This 5 Dublin Hills, the basalt and related tuff directly overlie m-thick bed is a complex of pumice pebbles and cobbles Paleozoic carbonate rocks, so the maximum age of the interbedded with pumice lapilli and coarse arkosic volcanic unit is poorly constrained. Because the sandstone. The clasts are well rounded and present in stratigraphic relation of the basalt to the Shoshone laminar graded (upward fining) beds as much as 20 cm Volcanics is not demonstrated and because basalt thick. Pumice cobbles are rich in sanidine phenocrysts. eruptions occurred sporadically in the Amargosa region The pumice bed is traceable southward from the throughout the late Tertiary and Quaternary, only tentative fanglomerate into gypsiferous mudstones. The crudely conclusions can be made concerning the timing of basaltic bedded fan deposit is finer grained to the south and volcanism at Shoshone. eventually interfingers with buff and pale-pink mudstones. The basalt may be related to volcanism that produced which are well bedded and contain abundant gypsum. part of the Funeral Formation (Drewes, 1963: Denny and Total exposed thickness of the mudstone is approximately Drewes, 1965), which includes a large area of basalt flows 400 m. The gypsum beds suggest a shallow lacustrine in the eastern Greenwater Range. In fact, Noble and environment or playa, where evaporation was dominant. Wright (1954) described the Shoshone basalt as part of The mudstones are extensively faulted and deformed into their Funeral Fanglomerate of Pliocene and Pleistocene(?) tight plunging folds. age (renamed the Funeral Formation by Drewes, 1963) in The China Ranch Beds overlie and incorporate their generalized geologic map of the Death Valley region. material from the Cambrian and older bedrock. Tertiary(?) If this tentative correlation is correct, then the basalt is granite from the Sperry Hills, and the Tertiary volcanic younger than 5.4 Ma, which is the isotopically determined rocks. The beds underlie the deposits of Lake Tecopa and age of the Greenwater Volcanics that underlie the Funeral form the southern margin of the lake basin. In a few Formation in the Funeral Peak quadrangle. A basalt flow places, a slight angular unconformity is indicated where in the type section of the. Funeral Formation near Ryan lake-age gravels filled depressions in the eroded and tilted yielded a whole-rock K-Ar age of 4.03-±0.12 Ma China Ranch Beds. Noble and Wright (1954) and Wright (McAllister, 1973). (1974) assigned a Pliocene and Pleistocene(?) age to the TERTIARY CHINA RANCH BEDS China Ranch Beds on the basis of stratigraphic relations. Debris flows, fanglomerate, pumice beds, and Because the beds unconformably underlie lake beds of gypsiferous mudstone in the Sperry Hills compose the earliest Pleistocene age, the China Ranch Beds are China Ranch Beds, which are well exposed in the considered to be wholly Tertiary. Amargosa canyon south of Tecopa. Mason (1948) In many ways, the China Ranch Beds are similar to originally applied the name to the light-colored mudstones the Funeral Formation of the Death Valley region. For and gypsiferous beds that crop out in the vicinity of China example, the Funeral Formation consists of coarse fan Ranch. However, Wright (1974) redefined the unit to deposits with large slump blocks, playas, and pumiceous include the coarse fanglomerate that interfingers with the tuff (Drewes, 1963), as are found in the vicinity of China gypsiferous mudstones. The fanglomerate lithology of the Ranch. The presence of huge blocks and megabreccias China Ranch Beds is prevalent in the Alexander Hills (fig. within the fanglomerates suggests that uplift and 1) and in the western part of the Sperry Hills. accelerated erosion were affecting the highlands Exposures of the fanglomerate south of Tecopa Pass, throughout the region. This tectonic activity, which consist of bodies of monolithologic conglomerate probably occurred during the Pliocene, was accompanied interbedded with heterogeneous material (Wright 1974). by local volcanic activity. The monolithologic conglomerates, which are present as QUATERNARY LAKE BEDS, CONGLOMERATE. AND TUFA lenses as long 300 m, are slump blocks of bedrock derived The deposits of Lake Tecopa (fig. 2a) are divided into from the nearby ranges. West of the Amargosa River, the three informal units that are mapped separately: (1) fanglomerate is a heterogeneous unit of gneiss, granite, lacustrine mudstone, (2) conglomerate, and (3) tufa. and dolomite boulders. Quartzite, metasedimentary rocks, Deposits of Lake Tecopa occupy the low region between and volcanic rocks are common but less abundant The Shoshone and Tecopa and the southern end of Chicago .subrounded boulders, which are 1 to 5 m in diameter, are Valley. The maximum areal extent of the lake was set in a sand and gravel matrix of similar lithology. In the approximately 250 km2. The southern margin of the lake cliffs of the Amargosa canyon, where at least 100 m of the was originally buttressed by fanglornerate of the China fanglomerate is exposed, the poor sorting and . crude Ranch Beds, now breached by the Amargosa River. . bedding of the unit is readily apparent The unit originated as debris flows and stream deposits derived from the

4 B Oal

FIGURE 2a.-Schematic geologic cross section of the deposits of Lake Tecopa: lacustrine mudstone unit (Qtlm) including tufts A, B, and C of Sheppard and Gude (1968), conglomerate unit (Qtc), and tufa. Also shown are basement rocks (TAu) composed of Archean gneiss, Proterozoic and Paleozoic sedimentary rocks, Tertiary volcanic rocks, •a well as older Quatemary alluvium (Qa2 ), and Holocene alluvium (Qaj). mudstone Lacustrine be massive, especially in the central part of the basin. A cumulative thickness of 72 m of mudstone, For the most part, the lake sediments are moderately claystone, and volcanic ash is exposed (fig. 2b), although indurated, but tend to disaggregate easily during wetting. the level of erosion has not reached the base of the lake However, localized concentrations of calcite produce deposits in the central part of the basin. Stratigraphic resistant ledges and beds especially near Shoshone, where section 1 was measured 1.5 km northeast of Shoshone, spring water and the Amargosa River entered the lake, and section 2, 2 km east of Shoshone, and section 3 was also near the Greenwater fan. Sediments along the lake measured near the southern end of the Dublin Hills. The perimeter tend to be richer in CaCO than the sediments sediments have pastel colors ranging from buff and pink 3 of the central basin. Sandstone beds, commonly cemented near the lake margins to green and brown in the central by carbonate, form nodules and flagstones as much as 50 basin. Badland topography developed on the lacustrine cm wide and 5 to 10 cm thick. The eroded surface of the mudstone unit generally consists of low rounded mounds lake beds is littered with a lag concentration of carbonate with knobby surfaces resembling popcorn, caused by the flagstones. wetting and drying of expandable clay minerals. Near the Several volcanic ash beds, ranging in thickness from 1 lake margins, the deeply dissected lake beds form vertical cm to 4 m, are present in the lake deposits. Of the 12 walls. Several thick volcanic ash beds form orange or white ashes that are recognized, three form thick beds that are ledges in the walls of the arroyos. resistant thinner The dominant lithology is mudstone that consists of mappable throughout the basin; whereas, the discontinuous. Sheppard and Gude (1968) clay, silt, and minor amounts of sand. The mudstone ashes are of the three thick ashes, and contains calcite, clay minerals, mineral and rock fragments, provided detailed descriptions their nomenclature has been followed in refemng to the and gypsum fills cracks. Authigenic zeolites and potassium as tufts A, B, and C (from youngest to oldest). feldspar are also present in the mudstone. Claystone, ashes the fresh ashes are white or pale gray and which breaks with a brittle conchoidal fracture, is In general, friable. They are composed of rhyolitic vitreous interbedded with mudstone and is most common in the extremely of crystal and rock fragments. middle part of the lake beds. The predominant detrital clay shards and minor amounts are sharp, suggesting direct minerals are illite, lithlum-rich saponite, and The basal contacts of the ashes the upper contacts are montmorillonite, (Starkey and Blackmon, 1979). Grain air fall into the basin, and with the overlying mudstones. In many places, sizes of the sedimentary rocks become coarser toward the gradational have multiple fine beds, main inlet north of Shoshone and toward the tributaries of the upper parts of the thick ashes grading, and small-scale crossbeds indicative Greenwater and Chicago Valleys. Lenses of sandstone and ripple marks, fresh ash is subrounded pebbles mark the channels of ancient streams of reworking by water currents. Locally, form resistant ledges. at the inlets. The subangular sand grains consist of quartz, cemented by CaCO, to fresh ash feldspar, biotite, and silicic volcanic-rock fragments. The Except near the northern lake margin, where the glass shards are generally altered to dominant lithologies of the pebbles are basalt and silicic predominates, by volcanic rocks. zeolites, potassium feldspar, and searlesite. As described of the tufts The mudstones are generally well bedded and nearly Sheppard and Gude (1968), the authigenesis with fresh glass at the outer flat lying, although the beds dip approximately 1' toward follows an areal zonation way to zeolite (phlllipsite, clinoptilolite, the center of the basin. Minor warps, slumps, and perimeter, giving and finally depressions were noted in a few limited areas (see fig. 5, and erlonite) in the Intermediate zone, of the basin. Sheppard and Gude, 1968). The claystone beds tend to potassium feldspar and searlesite at the center 5 SECTION METERS ~35

30~

20

SECTION 10 SECTION METERS 3 METERS 2 Tuff 45 23 -

20 40

020

EXPLANATION Tuff C

Claystone

Mudstone Nodules 10 Sandstone Root casts

Conglomerate Paleosol

Tuff

0-E FIGURE 2b.-Ceasured stratigraphic sections in the lacustrine mudstone unit Locations: Section 1, SE, sec. 19 and SW¼A sec. 20, T. 22 N., R. 7 E.; Section 2, SEIA sec. 29 and NE¼A sec. 32, T. 22 N., R. 7 E.; Section 3, NW¼ sec. 19, T. 21 N., R. 7 E. 6 Altered tuff is punky, porous, has a blocky fracture, and make up 1 to 10 percent of the ash. Resistant ledges of tuff forms yellowish or greenish ledges. B at the southweit side of the basin contain authigenic Tuff C-The lowermost thick ash, tuff C, is zeolites and potassium feldspar as replacement products of stratigraphically 17 m above the deepest exposure of the the glass shards. Orange ledges of the altered ash have lake beds. Tuff C is 10 to 75 cm thick and is best exposed abundant spherulitic phillipsite within the zeolite facies. In along the west side of California Highway 127, where the the potassium feldspar facies, the tuff is green or brown. altered tuff forms a resistant ledge containing green Tuff A-Tuff A, which is 0.5 to 4 m thick, is exposed nodules of silicified clay. Distinctive features of the tuff are around the perimeter of most of the basin, but was not the highly contorted and well-laminated upper part and the recognized in Chicago Valley. The 4-8 m interval that deformed base, which is generally warped into a wavelike separates tuff B from the overlying tuff A generally consists surface with amplitudes of 50 cm and wavelengths of of green brittle claystone. Tuff A is overlain by about 10 several meters. Although the cause of the deformation 30 m of lacustrine sandy mudstone and conglomerate; no remains a matter of debate, the wavy base is probably a ashes were found above this tuff. The best exposures of load feature, but the contorted upper part may be due to tuff A are in an abandoned pumicite quarry, 1 km south turbidity flow. Seismic shaking has been proposed as an of Shoshone, where the 4-m-thick bed consists of silver alternative cause of the deformation. gray vitric tuff with many laminar interbeds several Although the glass shards of tuff C are predominantly centimeters thick. The tuff contains only minor amounts of altered to potassium feldspar minerals, a small pocket of locally derived detritus, although there is ample evidence fresh ash is preserved near the eastern margin of the basin that the upper part of the bed has been reworked by water (SW¼/4 sec. 24, T. 21 N., R. 7 E.). Fresh ash is mainly of currents. Well-preserved ripple laminations indicate a platy bubble wall shards and 1-5 percent rock and crystal south-southeast current direction. In Greenwater Valley, fragments. Zircon phenocrysts with glass selvages are tuff A is 2 m thick and overlies siltstone containing present. The bed is difficult to trace in the rounded hills of abundant root casts. On a nearby isolated outcrop of the central basin, because the outcrops have thick mantles Cambrian dolomite, the ash was deposited on talus, of weathered material. suggesting deposition by direct airfall. In the Sperry Hills. Tuff B-Tuff B is approximately 18 m stratigraphically the 2.5-m-thick bed was deposited on gravels which above the base of tuff C. The stratigraphic interval between interfinger with the lake beds. In general, the substantial the two tufts, which is predominantly claystone, is poorly thickness of the tuff provides a continuous exposure, exposed except at the southwest part of the lake basin. except near the ancestral Amargosa River channel where Tuff B is thick (0.3 to 3.5 m) and can be traced almost the ash is mixed with sand and gravel and forms continuously from the Sperry Hills, along California discontinuous lenses. The lenses exhibit chaotic rip up "•'- Highway 127 to Shoshone, then along the west flank of structures, crossbeds, and ripple marks. the Resting Spring Range to Chicago Valley. The southern Where fresh, tuff A is predominantly composed of exposures of tuff B are mostly associated with deep-water platy glass shards approximately 0.1 mm in diameter. Rock mudstones, whereas elsewhere the tuff is exposed in and crystal fragments typically comprise less than 1 percent shallow fluviatile deposits near the ancient shore. The of the ash. Where the tuff forms resistant ledges, it displays deep-water exposures contain very minor amounts of alteration features similar to those of altered tuff B. locally derived rock fragments. In contrast, on the lower part of the Greenwater fan, the tuff has a clean vitreous Conglomerate base beneath layers containing an abundance of local This unit consists of fanglomerate and pebbly fluviatile carbonate and volcanic detritus. The thicker accumulations deposits that interfinger with the lacustrine mudstone unit. of the ash tend to be massive except near the base where The unit generally forms cobble-strewn low rounded hills fine laminations are common. Gypsum-filled root casts are that develop dendritlc outlines in plan view. Typical abundant throughout the ash bed in the shallow-water exposures of the conglomerate unit are seen in the gravelly depositional environments. Near the southern tip of the piedmont that flanks California Highway 127 north of Resting Spring Range, tuff B was deposited by direct air Shoshone, where the unit consists of coalescing alluvial fall onto alluvium, as suggested by the presence of fans at the foot of the surrounding ranges. The fan abundant fossilized plant stems in the base of the ash. In deposits, which are at least 50 rn thick, consist of sand, the channels of the ancestral Amargosa and Chicago gravel, and subangular cobbles composed of dolomite, Valley tributaries, tuff B was deposited as a complex of quartzite, basalt, and silicic volcanic rocks. Pebble and channel fillings and tuffaceous sandy beds. Just northeast cobble sizes increase from 1 to 8 cm near the center of the of Shoshone on the east side of the Amargosa River, the valley to 20 cm at the foot of the bedrock ranges. Near the ash is interbedded with mudstone that was deposited on present channel of the Amargosa River, the conglomerate Tertiary basalt consists of rounded pebbles, 1 to 4 cm in diameter, and The freshest examples of tuff B are near Shoshone sandy channel fillings that were probably deposited by an and along the Resting Spring Range, where the ash stands ancestral river. These fluviatile sediments are interbedded out as a white band between underlying pink mudstone with tufts A and B and a 1-m-thick mudstone bed. and overlying green claystone. Fresh ash is mainly Small remnants of the conglomerate unit are exposed composed of distinctive pumice shards (as much as 0.5 at the foot of the Dublin Hills, the Sperry Hills, and the "mm) having spindles of elongate tubular bubbles. Volcanic Resting Spring Range. Where a direct association with lake rock fragments and crystals, mainly biotite and plagioclase, beds cannot be demonstrated, the remnants are

7 reedlike plant stems. Caprock composition varies from more CaCO with distinguished from the younger alluvium by a cemented gravel to nearly pure fine-grained 3 due to mounds have rounded dendritic morphology, greater induration stacked flattened pores or fenestrae. A few depositional layers, although carbonate cementation, and generally steeper knobby surfaces of concentric travertine tends to be solution-pitted slopes near the range fronts. A shallower slope most of the mounds are capped by jagged because the range present in the younger alluvial deposits blocks of tufa rubble. since the discontinuous benches fronts have been incised more deeply The tufa forms two parallel relation is well meters apart conglomerate unit was deposited. This that are about 100 m apart laterally, several conglomerate fan follow the eastern demonstrated in the Dublin Hills where vertically, and which apparently gravels that have Features of the remnants stand above younger pediment shoreline of the ancient Lake Tecopa. the conglomerate sands are locally shallower slopes. Although clasts within shoreline such as beach gravels and of pedogenic typical mounds carry partial rinds of CaC03, thick zones preserved in tufa, although the more mudstone beds caliche have not developed. appear to have grown as wedges within is a fan and B. The tufa In the Sperry Hills, the conglomerate unit near the stratigraphic levels of tuffs A of the China Tecopa existed, deposit that overlies the eroded surface mounds apparently formed while Lake This beds and tufa Ranch Beds with a slight angular unconformity. because all mounds are associated with lake the west postdate the contact is best exposed in the uppermost 10 m of rubble forms clasts within alluvial deposits that the units. wall of the Amargosa canyon. Compositionally, lacustrine mudstone and conglomerate from tht precipitation Quaternary conglomerate unit is difficult to distinguish Accumulations of tufa probably formed by it was derived from runoff or spring water the China Ranch fanglomerate because of CaCO 3 when carbonate-rich However, Gude, 1968). No the granite and gneiss clasts of the older unit. entered the alkaline lake (Sheppard and of the cracks was the morphology and structural features evidence of spring vents or carbonate-filled deposits Therefore. the conglomerate unit are distinctive: (1) Younger fan found in the lake beds near the mounds. whereas the to the ancient consistently slope toward the lake basin; alinement of tufa mounds is probably related more of springs along China Ranch fanglomerate has generally steeper, shoreline rather than more recent activity clast sizes variable dips; (2) the younger unit has smaller a fault zone. is much and large boulders (>1 m) are rare; (3) faulting of Lake Tecopa in the China Ranch Beds; and (4) although Age more extensive of the Tecopa ash conglomerate unit is interbedded with tuff Tephmchronolog--CCoffelations the Quaternary sources are listed in contain the pumice cobble beds with Isotopically dated volcanic A south of Tecopa, it does not from the Ranch Beds. table 1, using K-Ar ages that were calculated lenses that characterize the China (Steiger current decay constants and '-ArP'Ar abundance correlated the Tufa and Jager, 1977). Izett and others (1970) foot Bishop Tuff in the The white flat-topped mounds along the western middle Tecopa ash bed, Tuff B, with the of Califomia. Tuff B, also of the Resting Spring Range are thick accumulations Long Valley caldera of eastern of the tephra that was tufa. The tufa commonly consists of dense CaCO3 called the Bishop ash bed, is part powdery white the Bishop Tuff was caprock, 2 to 4 m thick, over a layer of ejected from Long Valley when grades and others, 1965; CaCO . The powdery base, which commonly erupted, 0.73 m.y. ago (Dalrymple 3 unit, generally The correlation is downward into the lacustrine mudstone Mankinen and Dalrynmple, 1979). and contains fossil ostracodes, thin-walled bone fragments,

TABLE 1.--Correlation and dating of Lake Tecopa ash beds air-fall ashes (Preferried Source of air-fall Ash beds of Lake Tecopa; nomenclature Other correlative in parentheses) ash of Sheppard and Gude (1968) and name this report and Lava Creek B ash of the Lava Creek Peaulette-like ash, Type 0 (Idtt Group), Tuff A 1970, 1972; bItt, 1981; Sarna- Tuff (of the Yellowstone others, 0.62 Ma by K-Ar dating and others, 1984) Wyoming; age Wojdcld method (Christliansen and Blank, 1972; lbtt and Wilcox, 1982) (Ash of the Lava Creek Tuff) Bishop ash (biztt and others, 1970; Bishop Tuff, ageLong 0.73 Valley Ma caldera,by K-Ar dating Tuff B SmethodCalifornia; (Dalrymple and others, 1965; Sarna-Woicick and others, 1964) Mankinen and Dalrymple, 1979)

(Ash of the Bishop Tuff) Hucldeberry Ridge Tuff (of the Pearlette-like ash, Type B (bitt, 1981. Yellowstone Group). Wyoming Tuff C Sarna-Woicicid and others, 1984); age (Chrisansen and Blank, 1972); age Fission-track age, 1.6 Ma (C. E Meyer, 1.9=±0.1 Ma by fission-track dating 2.02±0.08 Ma by K-Ar dating method M. J. Woodward, and A. M. Sarna (Naeser and others, 1973) (Reynolds, 1977; Love and Wojcicki, oral commun., Sept, 1981) Christiansen, 1980)

(Ash of the Huckleberry Ridge Tuff) 8 horizon at stratigraphic intervals of 15 cm (fig. 3). The mineralogic similarities, shard morphology, founded on polarity of each horizon was interpreted according to the analyses of major elements, spectrographic microprobe method of Hillhouse and others (1977), and a generalized of 11 trace elements in the glass shards, and analysis magnetostratigraphy, which combines data from all Further substantiation of the correlation paleomagnetism. columns, was constructed (fig. 4). by Sama-Wojcicki and others (1984) in their was provided The interval from tuff A down to tuff B (ash of the in the Western States. The method study of tephra Bishop Tuff) is consistently of normal polarity. As shown in analysis to determine employs neutron-activation figure 3, a polarity transition is found approximately 50 cm of 20 minor and trace elements in volcanic abundances below tuff B (Hillhouse and Cox. 1976). This transition. abundances are combined with results of glass. These which was confirmed by spot sampling at five localities analysis for major elements in making regional microprobe around the basin, is undoubtedly the transition from the of air-fall tephra. An excellent match of correlation Matuyama Reversed Chron to the Brunhes Normal Chron: was determined for tuff B and basal elemental abundances the transition has an estimated age of 0.73 Ma (Mankinen pumice lapilli of the Bishop Tuff. and Dalrymple, 1979). The position of this transition just The chemistries of tufts A and C match the elemental below the ash of the Bishop Tuff is consistent with the of the family of tephra that erupted from the abundances paleomagnetism of dated volcanic units in Long Valley and 1,050 km northeast of the 'Tecopa Yellowstone caldera, the Western States. widespread air-fall deposits, which were basin. These As shown in figure 4, two normally polarized originally named the Pearlette ashes for exposures found in magnetozones below tuff B are tentatively correlated with the Midwest, are now known to be the air-fall equivalents the Jaramillo (0.97-0.90 Ma) and Olduvai Normal of welded tufts in the Yellowstone caldera (Izett and others, Subchrons of the Matuyama Reversed Chron (1.87-1.67 1972). Tuff A, the youngest Tecopa ash, correlates 1970, Ma). Finally, at the bottom of the stratigraphic section. tuff with the Lava Creek B ash of the Lava Creek Tuff, dated C is within a reversed magnetozone that is consistent with K-Ar methods at 0.62 Ma (Izett, 1981; Christiansen and by the older part of the Matuyama Reversed Chron (2.48 1972; lzett and Wilcox, 1982). The original Blank, 1.87 Ma). This correlation is in agreement with K-Ar dating founded on petrographic similarities, correlation, (2.02 Ma) of the Huckleberry Ridge Tuff. which is the stratigraphy, and major element chemistry, is further presumed source of tuff C. The fission track age (1.6 Ma) substantiated by neutron-activation analysis of trace and determined from Tuff C is not consistent with this minor elements (Sama-Wojcicki and others, 1984). The interpretation of the magnetic stratigraphy. Correlations are name "ash of the Lava Creek Tuff" which relates tuff A to made with considerable uncertainty in the lower part of the the source area, is now preferred over the original name, Tecopa lake beds for two reasons: (1) despite cleaning "Pearlette-like ash, type 0", that was assigned by Izett and treatments, postdepositional remagnetization of normal others (1970). polarity persists throughout the lower horizons at Lake Sarna-Wojcicki and others (1984) and lzett (1981) Tecopa, and (2) in the polarity time scale, the placement tentatively correlate the lower Tecopa ash, tuff C, with the and duration of normal polarity subchrons have not been Pearlette ash bed of Meade County, Kansas. The ash older fully resolved for the lower part of the Matuyama. bed in Kansas, referred to as "Pearlette-like ash, Type B", Depositional rates--On the basis of time lines was correlated with the Huckleberry Ridge Tuff of the provided by the ash beds, rates of deposition were Yellowstone Group (Naeser and others, 1973). The age of approximately 0.06 m/1,000 yr between tuffs A and B and the Huckleberry Ridge Tuff is 2.02 t 0.08 Ma, as measured 0.015 m/l,000 yr between tufts B and C. By extrapolating .the K-Ar method (Reynolds, 1977; Love and by these rates to cover the beds of the lacustrine mudstone Christlansen, 1980). Fission-track dating of the type-B unit that are above tuff A and below tuff C, a cumulative Pearlette ash of Meade County, Kansas, yielded an age of age of 3.0-0.5 Ma was obtained for the lake beds. These 1.9±0.1 Ma (Naeser and others, 1973). However, limits should be considered rough estimates because the analysis of fresh tuff C from the Lake Tecopa preliminary rates of deposition vary by a factor of four within the beds gives a fission-track age of 1.6 Ma (C. E. Meyer, M. section. In addition, the maximum age of the lake is very J. Woodward, and A. M. Sarna-Wojcicld, oral commun., uncertain because the base of the deposits is not exposed 1981). September, in the central basin. Paleomagnedlc Stratignaphy-Paleomagnetic work in According to the geologic time table of van Eysinga the Lake Tecopa beds is consistent with the chronology (1978), the lake beds are late Pliocene and early that is established from the volcanic ashes. The Pleistocene in age. The Plocene-Pleistocene boundary correlation method involves establishing the paleomagnetic (beginning of the Olduvai Normal Subchron) is a few pattern of magnetozones, defined by the magnetic polarity meters above tuff C. The younger age limit of the lake of oriented specimens, for comparison with the beds is slightly younger than the early-late Pleistocene time scale (Manldnen and Dalrymple, geomagnetic polarity boundary (Brunhes-Matuyama transition). 1979). Oriented specimens were collected from four Paleontology-Sheppard and Gude (1968) described sections, which collectively represent the lake stratigraphic localities of fossils found prior to 1966 in the deposits of from the base up to tuff A. Three oriented specimens, beds Lake Tecopa. The upper part of the beds contains rare within a lateral distance of 50 cm, were collected collected horse, camel, mammoth, and muskrat fossils, too horizons about 1 m apart. This sampling at stratigraphic fragmentary for age-diagnostic identification. Sheppard and procedure was used at three of the sections. The remaining Gude also reported several localities of ostracodes and section was sampled more densely: four specimens per 9 THICKNESS LITHO- PALEO- INCLINATION IN DEGREES DECLINATION IN DEGREES IN METERS LOGY MAGNETIC-90 -60 -30 0 30 60 90 270 300 330 0 30 60 90 120 150 180 210 240 270 INTERVALS i , I , ,1 , , I , , 1 , ,I I , I I , _ l _ _. • I

Bishop Ash bed (,.) I I

U3 z x 0

10 14 em

1.0 Ileo " 1 ""I"

o~s o0&e 01. 2.0 - emI EI

3.0 - ami 30 I A i I

4.00 - I I 'me 40 - " ICI I

FIGURE 3.-Paleomagnetic data used in determining transition from the Matuyama Reversed Chron to the Brunhes Normal Chron (0.73 Ma). Transition is present 50-75 cm below the ash of the Bishop Tuft. Sample locality is 2 km east of Shoshone. LITHOLOGY THICKNESS GENERAL- LAKE PALEOMAGNETIC GEOMAGNETIC POLARITY IN METERS IZED TECOPA INTERVALS TIME SCALE MAG- (Mankinen and Dalrymple, 1979) STRATI- YEARS GRAPHIC NETO- AGE IN MILLION SECTION ZONES LAKE TECOPA

Lava Creek -0.5 A (0.6 Ma)

Brunhes Normal ,Chron 0.73

0.90 Bishop -B-- (0.73 Ma) 097 -1.0

30 Jaramillo Normal Subchron (of Matuyama Reversed Chron)

-1.5

Olduvai ' Normal Subchron-' 1.67 (of Matuyama Chron) 20 , _..R eversed

Huckleberry Ridge 1.87 12.02 = 0.08 Me) -2.0 2.01 2.04

= 12.2.12 14

10

ý !2.48 -2.5

0- 0 FIGURE 4.-Correlatlon of Lake Tecopa magnetozones with geomagnetic polarity time scale of Manldnen and Dalrymple (1979). Stippled areas denote normal polarity, white areas denote reversed polarity. 11 Thickness of the older alluvium ranges from a thin and varieties) from the upper lake diatoms (42 species gravelly veneer (30 cm) in the central basin to thick species indicated a middle to beds. Three of the diatom wedges (100 m) of bouldery conglomerate at the range and the ostracodes did not contradict late Pleistocene age. fronts. Clasts consist of subangular dolomite. quartzite, and a Pleistocene age. silicic volcanic rocks from the surrounding ranges, and tufa collections of vertebrate fossils Since 1971, several and zeolitic tuff from the lake beds. The gravelly deposits the vicinity of Tecopa (table 2). have been taken from are crudely interbedded with sand, are poorly indurated. are held by the Los Angeles County These collections and lack substantial deposits of pedogenic caliche. In the History (D. P. Whistler, written Museum of Natural central basin, the older alluvium was deposited as a true Because the Tecopa fauna has not been commun.. 1983). pediment on the eroded surface of the lake beds, the are preliminary. Slightly studied in detail, the identifications eroded surface having several meters of relief. C (localities 4-6, table 2) the larger below the level of tuff The morphology of the older alluvium is planar in the is dominated by camels, including one vertebrate fauna central basin, and gives way to gently concave slopes that generic level. The same area has taxa that is new at the rise about 100 m from the basin floor to the range fronts. fossils, which are housed yielded abundant micromammal The unit forms coalescing, conical fans with steep slopes at Department collections at the in the Earth Sciences the foot of the bedrock ranges. The older alluvium is Riverside. Although the collections University of California, typically deeply dissected by the modem washes, so that they include a number of are not formally described, the unit forms fingerlike mesas near the margin of the lake microtine rodents related to pack rats that are recognized beds. indicators (Repenning. 1980; as valuable biostratigraphic A distinguishing feature of the older alluvium is its 1982). Evidence of Mammuthus May and Repenning, surface of desert pavement and advanced development of the upper part of the section is (locality 1, table 2) from soil. The desert pavement consists of planar surfaces of age. consistent with a late Pleistocene densely packed pebbles and cobbles, predominantly 2-5 QUATERNARY OLDER ALLUVIUM cm in diameter. Scattered larger cobbles and boulders as Older alluvium (unit Qa2 ) consists of remnants of much as 1 m in diameter commonly protrude above the alluvial fans which now form prominent mesas and raised close-packed surface. Clasts were derived from the capped by desert pavement. These remnants areas underlying fan deposits; although atop the tufa benches, generally stand 1 to 10 m above the active washes and are the pavement consists of flat.CatCO 3 plates. Color of the receiving sediment A good example is the no longer pavement surface ranges from dark red brown to black. fan in Greenwater Valley. abandoned depending on the degree of varnish development.

TABLE 2.-Vertebrate fossil collections of Natural History [Locations are shown on map. List compiled by D. P. Whistler, Los Angeles County Museum (LACM)] 1. Locality LACM 1209 (sec. 35, T. 21 N., R. 6 E.) Mammuthus (mammoth) Equidae, small species (horse) cf. Hemiauchenia (llama) 2. Locality LACM 1210 (sec. 7, T. 20 N., R. 7 E.) Equidae, large species cf. Camelops (camel) cf. Titanotyiopus (large camel) cf. Hemlauchenia 3. Locality LACM 3772, 6805 (Approximate location: sec. 33 and 34, T. 21 N., R. 7 E.) Mastoionidae (mastodon) Equldae, small species dc Taunotylopus cf. Hemiauchenia Anuilocapridae, large species (antelope) 4. Locality LACM 7104 and 5. Localty LACM 7106 (sec.27, T. 21 N., FR7 E) cf. Camelope cf. T'anotylopus cf. Hemiauchenia 6. Locality LACM 7108, 7109, 7111, 7112, 7113, 7132 (sec. 23, T. 21 N., R. 7 E.) Phoenicopteridae (flamingo) Mastodontidae Equidae, small species Equidae, large species cf. Camelops cf. ritanotylopus cf. Hemiauchenia Miolabinae, new genus and species (camel) 12 ranges beyond the older alluvial fans. Downcutting has Quartzite and dolomite clasts acquire little or no varnish, been rapid in Amargosa canyon as indicated by Holocene while volcanic clasts have the darkest accumulations. The terraces now standing 3-4 m above the active stream bed. is broken by occasional risers as much as 10 planar surface Major areas of recent deposition are the Amargosa River cm high. bed, the playa, and areas of sheetwash on the older fan Directly beneath the armor of pebbles is a layer almost deposits. These places of deposition appear to be entirely composed of brown silt (A-horizon), 10-30 cm temporary storage areas for material that will eventually be thick, which contains spherical cavities 1-3 mm in swept through Amargosa canyon by floodwaters. diameter. Origin of this silt bed has been explained as the processes, or alternatively, as product of desert soil-forming LATE TERTIARY AND QUATERNARY Denny (1965) a widespread loess deposit. For example. STRUCTURAL FEATURES weathering process attributed the silt to a mechanical A major episode of faulting and folding occurred after and drying, which causes larger rock driven by wetting deposition of the China Ranch Beds and prior to lifted to the surface. Hoover and others fragments to be deposition of the beds of Lake Tecopa. High-angle faults origin for the silt, mainly (1981) favored an aeolian with vertical offsets as much as 100 m are common in the the silt has a uniform composition despite the because China Ranch fanglomerates in Amargosa canyon. These material. lithology of underlying faults do not cut the Quaternary conglomerate unit or the is generally underlain by a buff or The vesicular silt Lake Tecopa beds. The China Ranch Beds are extensively B-horizon. The layer, which rarely reddish-brown pebbly folded as well as being faulted, although folding is more is enriched in clay, iron oxide, exceeds 50 cm in thickness, prominent in the lacustrine mudstone unit than in the Pebbles near the base of this zone and calcium carbonate. fanglomerate. The presence of megabreccia and huge slide mm) undercoating of CaCO commonly have a thin (1-3 3 blocks within the China Ranch fanglomerates suggests that others, 1966). but the soil lacks dense (stage I of Gile and deformation was contemporaneous or followed soon after Although the pavement and soil horizons CaCO 3 horizons. deposition of the beds. This time interval, probably on the older alluvium, they are commonly developed occurring during the Pliocene, was also marked by volcanic of the Quaternary conglomerate unit locally cap remnants activity which supplied pumice to the nearby drainages. fronts. Therefore, of lakebed age, especially near the range Only minor faulting occurred in the Tecopa basin after Quaternary conglomerate can be similar the surface of the the major Pliocene episode. High-angle faults having of the older alluvium. Such areas have been to the surface vertical displacements of as much as 3 m were recognized conglomerate, because the soil mapped as the Quaternary in a few areas of the Lake Tecopa beds, most commonly veneer. forms only a thin in the low hills northeast of Tecopa Hot Springs. to date the Quaternary older allluvium, "Inan effort Deformation, probably due to differential compaction of uranium-trend method (Rosholt, 1978) samples for the the lake beds, is localized and minor. For example, a small from a gravel pit 2.5 km east of Shoshone. were collected dome with dips as much as 8. is exposed beside California the B-horizon of the soil yielded an Samples taken from Highway 127, 3 km south of Shoshone. Disturbed beds 18,000 yr B. P (J. N. Rosholt, oral age of 160.000± that contain tuff B are exposed at the southwest end of October 1981). commun., Valley, but these features were probably HOLOCENE ALLUVIUM Greenwater caused by slumping of channel walls during early erosion The younger alluvial deposits (unit Qal) consist of of the Tecopa beds. Careful examination of aerial unconsolidated sand, silt, and conglomerate in active and photographs and field checks failed to detect any faults recently abandoned stream channels. The main channel is within the Holocene or older Quaternary alluvial deposits. the Amargosa River bed which is a braided complex of Although the linear course of the Amargosa River suggests washes and gravel bars with microrelief of 0.3-2. m. that it follows a fault, no evidence of such a fault was Tributary washes that originate on the range fronts are found in the flanking terraces. Apparently, the faults which commonly deeply incised (as much as 15 m) into the older caused uplift of the Dublin Hills and Resting Spring Range fan deposits. Near the ranges, the washes contain a ceased activity prior to deposition of the Lake Tecopa subangular cobbles, and boulders of mixture of sand, beds. quartzite, and basalt, whereas, the Amargosa dolomite. The base of tuff B provides a datum for detecting River bed predominantly consists of sand and subrounded large-scale tilt due .to regional deformation or faulting cobbles. Pavements and varnish are poorly developed and within the basin. Although elevation of the tuff ranges from soils are absent in the recently abandoned washes, which 430 to 540 m above sea level, almost all of the variation are presumed to be of Holocene age. In the central part of is attributable to a gentle dip (10) of the beds toward the the Lake Tecopa beds, the recent deposits consist of loose center of the basin. Compaction and the original lake silt and clay which form a small playa. When dry, the playa bottom topography can account for the gentle basinward surface develops a white efflorescence of halite and dip. Near the lake margin, the elevation of tuff B ranges gypsum (Starkey and Blackmon, 1979). The only from 495 m at Shoshone to 468 m at the northern tip of Holocene aeolian deposits of the basin are a few dunes the Sperry Hills. The difference in elevation allows a and southwest-trending streaks of sand near Tecopa Hot maximum south and downward tilt of 0. 1. The maximum Springs. allowable tilt in the east-west direction is 0.2' downward to The recent features suggest a regime in which erosion the west. Therefore, the region has not been tilted over deposition within the Tecopa basin. has dominated appreciably since about 500,000 years ago. Deeply incised washes are carrying coarse debris from the

13 CONCLUSIONS ago (Hoover and others, 1981). As the supply of material to the alluvial fans waned, three million years ago, the Tecopa basin was About and as stream channels deepened, parts of the fans were an alluviated depression, closed at the south end by thick abandoned as sites of deposition, and soils and pavements '• fanglomerates of the China Ranch Beds. These beds were began to form on surfaces of the fans. The Tecopa basin perhaps uplifted by faulting to form a dam for Lake bears no evidence of a pluvial lake of Lahontan age, At this time, climatic conditions favored the Tecopa. probably because the basin has not been closed since the formation of a moderately deep lake rather than a playa, Amargosa River canyon formed. In the Holocene, the rate as indicated by geochemical studies and depositional of erosion apparently increased, because the deepening of of the lake beds (Sheppard and Gude, 1968; features stream channels dominated the building of new fans. Blackmon, 1979). The presence of saline Starkey and ACKNOWLEDGMENTS minerals, magadilte, and certain zeolite minerals indicate I have benefitted from geologic discussions with that Lake Tecopa, at least in its later stages, was highly Wilfred J. Carr, David L. Hoover, and W. C. Swadley of alkaline and saline due to evaporation. Therefore. the lake the U.S. Geological Survey, and Richard L. Hay of the did not have a permanent outlet during its long history. of Illinois. Appreciation is expressed to Andrei The existence of Lake Tecopa for over 2 million years University M. Sarna-Wojcicki (U.S. Geological Survey) for assistance raises the question of whether the Amargosa region was with the geochronology of Lake Tecopa, to Brigitte Smith considerably less arid during the early Pleistocene as of the University of Paris for help during the paleomagnetic compared to today. One way to examine this question is study, and to D. P. Whistler (Los Angeles County Museum to determine whether current amounts of spring discharge of Natural History) for information concerning the Tecopa runoff would support a lake in the Tecopa basin if an and vertebrate fossils. outlet did not exist. The ratio of drainage basin area to the area of Lake Tecopa is approximately 42:1 (Snyder and REFERENCES CITED that others, 1964). Observations from Nevada indicate Blackwelder, Eliot, 1936, Pleistocene Lake Tecopa labs.]: a potential annual evaporation exceeds precipitation by Geological Society of America Proceedings for 1935. factor of roughly 5 to 25 times (Winograd and Thordarson, p. 333. 1975). The factor may exceed this range in the Tecopa Chesterman, C. W., 1973, Geology of the northeast basin, because the average elevation (460 m) is relatively quarter of Shoshone quadrangle, lnyo County, low. If we assume that for the entire drainage area less California: California Division of Mines and Geology than 10 percent of the total precipitation would reach the Map Sheet 18, scale 1:24,000. factor of lake as runoff, then an evaporation/precipitation Christiansen, R. L, and Blank, H. R., Jr., 1972, Volcanic maintenance of a lake in the \... 4.2 or greater would preclude stratigraphy of the Quaternary rhyolite plateau in Tecopa basin. Additional water from springs, such as the Yellowstone National Park: U.S. Geological Survey group at Ash Meadows which annually supply about Professional Paper 729-B, p. B1-B18. 3 17,000 acre-ft (22-million m ) (Winograd and Thordarson, Dalrymple, G. B., Cox, A., and Doell, R. R., 1965, 1975), could bring roughly 3 in. (8 cm) of water to the lake Potassium-argon age and paleomagnetism of the yearly, and offset only 5 percent of the potential annual Bishop Tuff: Geological Society of America Bulletin, v. evaporation. Therefore, the current water supply is 76, p. 665-675. sustain a inadequate to balance evaporation and thereby Denny, C. S., 1965, Alluvial fans in the Death Valley perennial lake in the Tecopa basin. region, California and Nevada: U.S. Geological Considering the inadequacy of current water sources Survey Professional Paper 466, 62 p. or spring to sustain a lake in the basin, precipitation Denny, C. S., and Drewes, Harald, 1965, Geology of the discharge must have been substantially greater, and Ash Meadows quadrangle, Nevada-California: U.S. evaporation lower, during the existence of Lake Tecopa Geological Survey Bulletin 1181-L4 p. L1-L56. a 3.0-0.5 m.y. ago. Inflow and evaporation maintained Drewes, Harald, 1963, Geology of the Funeral Peak sediment balance while the basin steadily filled with quadrangle, California, on the east flank of Death Creek Sometime after deposition of the ash of the Lava Valley: U.S. Geological Survey Professional Paper Tuff, 0.6 m.y. ago, the barrier at the south end of the lake 413, 78 p. lack of was breached, probably by overflow. The general Fleck, R. J., 1970, Age and tectonic significance of'volcanic due tectonic activity during this period rules out breaching rocks, Death Valley area, California: Geological as the to tilting or faulting. The lake drained rapidly Society of America Bulletin, v. 81, p. 2807-2816. between the Amargosa canyon developed, creating a link Gile, L. H., Peterson, F. F., and Grossman, R. B., 1966, the upper Amargosa basins and Death Valley. Following Morphological and genetic sequences of carbonate creation of this link, subsidence of Death Valley due to accumulation in desert soils: Soil Science, v. 101, p. faulting could affect rates of erosion in the Tecopa basin. 347-360. the eroded More than about 160,000 years ago, Haefner, Richard, 1976, Geology of the Shoshone surface of the Lake Tecopa beds was partly buried by Volcanics, Death Valley region, eastern California: alluvial fans which extended from the nearby ranges. California Division of Mines and Geology, Special Although the change from an erosional regime to one of Report 106, p. 67-72. change is deposition remains unexplained, a climatic Hazzard, J. C., 1937, Paleozoic sections in the Nopah and favored, because the alluviation appears to have occurred Resting Spring Mountains, Inyo County, California: throughout the Amargosa region 80,000-430,000 years 14 California Journal Mines and Geology, v. 33, no. 4, p. tectonics: Cambridge University Press, New York, 358 273-339. p Hillhouse, Jack, and Cox, Allan, 1976, Brunhes-Matuyama Naeser, C. W., lzett. G. A., and Wilcox, R. E., 1973, polarity transition: Earth and Planetary Science Zircon fission-track ages of Pearlette family ash beds in Letters. v. 29. p. 51-64. Meade County, Kansas: Geology, v. 4, p. 187-189. Hillhouse. J. W.. Ndombi. J. W. M., Cox, A.. and Brock, Noble, L. F., and Wright, L. A., 1954, Geology of the Death Valley region, California, A., 1977. Additional results on paleomagnetic central and southern R. H., editor. Geology of southern stratigraphy of the Koobi Fora Formation east of Lake in Jahns, Turkana (Lake Rudolf), Kenya: Nature, v. 265, p. California: California Division of Mines, Bulletin 170. 411-415. Chapter 2, p. 143-160. between Hoover. D. L., Swadley. W. C.. and Gordon, A. J., 1981, Repenning, C. A., 1980, Faunal exchanges Correlation characteristics of surficial deposits with a Siberia and North America: Canadian Journal of description of surficial stratigraphy in the Nevada Test Anthropology, v. 1, p. 37-44. the Site region: U.S. Geological Survey Open-File Report Reynolds, R. L., 1977, Comparison of the TRM of Pearlette 81-512. 30 p. Yellowstone Group and the DRM of some Research, v. 84, p. lzett. G. A.. 1981, Stratigraphic succession, isotopic ages, ash beds: Journal of Geophysical partial chemical analyses. and sources of certain silicic 4525-4536. volcanic ash beds (4.0 to 0.1 m.y.) of the Western Rosholt, J. N., 1978, Uranium-trend dating of alluvial United States: U.S. Geological Survey Open-File deposits; in R. E. Zartman, ed., Fourth International Report 81-763, 2 sheets. Conference, Geochronology, Cosmochronology, and lzett, G. A., and Wilcox, R. E., 1982, Map showing Isotope Geology: U.S. Geological Survey Open-File localities and inferred distributions of the Huckleberry Report 78-701, p. 360-362. Ridge, Mesa Falls, and Lava Creek ash beds (Pearlette Sarna-Wojcicki, A. M., Bowman, H. R., Meyer, C. E., family ash beds) of Pliocene and Pleistocene age in Russell, P. C., Woodward, M. J., McCoy, Gail, Rowe, the western United States and southern Canada: U.S. J. J., Jr., Baedecker, P. A., Asaro, Frank, and Michael, Geological Survey Miscellaneous Geologic Helen, 1984, Chemical analyses, correlations, and Investigations Series map 1-1325, 1:4,000,000. ages of upper Pliocene and Pleistocene ash layers of Izett. G. A., Wilcox, R. E., and Borchardt, G. A., 1972, east-central and southern California: U.S. Geological Correlation of a volcanic ash bed in Pleistocene Survey Professional Paper 1293, 40 p. deposits near Mount Blanco, Texas, with the Guaje Sheppard, R. A., and Gude, A. J., 3d. 1968, Distribution pumice bed of the Jemez Mountains, New Mexico: and genesis of authigenic silicate minerals in tuffs of Quaternary Research, v. 2, p. 554-578. Pleistocene Lake Tecopa, Inyo County, California: Izett, G. A., Wilcox, R. E., Powers, H. A., and U.S. Geological Survey Professional Paper 597, 38 p. Desborough, G. A., 1970, The Bishop ash bed, a Snyder, C. T., Hardman, George, and Zdenek, F. F., Pleistocene marker bed in the western U.S.: 1964, Pleistocene lakes in the Great Basin: U.S. Quaternary Research, v. 1, p. 121-132. Geological Survey Miscellaneous Geologic Love. J. D., and Christiansen, A. C., 1980, Preliminary Investigations Map 1-416, scale 1:1,000,000. correlation of stratigraphic units used on 10 x 20 Starkey, H. C., and Blackmon, P. D., 1979, Clay geologic quadrangle maps of Wyoming: Wyoming mineralogy of Pleistocene Lake Tecopa, Inyo County, Geological Association Guidebook, Annual Field California: U.S. Geological Survey Professional Paper Conference, v. 31, p. 279-282. 1061, 34 p. Mankinen, E. A., and Dalrymple, G. B., 1979, Revised Steiger, R. H., and Jager, E., 1977, Subcommission on geomagnetic polarity time scale for the interval 0-5 Geochronology: Convention on the use of decay m.y. B.P.: Journal of Geophysical Research, v. 84, p. constants in geo- and cosmochronology: Earth and 615-626. Planetary Science Letters, v. 36, p. 359-362. Mason, J. F., 1948, Geology of the Tecopa area, Troxel, B. W., and Heydari, Ezat, 1982, Basin and range southeastern California: Geological Society of America geology in a roadcut, in Geology of Selected Areas in Bulletin, v. 59, no. 4, p. 333-352. the San Bernardino Mountains, western Mojave May, S. R., and Repenning, C. A., 1982, New evidence *Desert, and Southern Great Basin, California, Cooper, for the age of the Old Woman Sandstone, Mojave J. D., Troxel, B. W., and Wright, L. A., eds., Desert, California: in Cooper, J. D., compiler, Geological Society of America, Cordilleran Section, Guidebook for the Geological Society of America guidebook, p. 91-96. Cordilleran Section Meeting, Anaheim, California, p. Troxel, B. W., and Wright, L A., eds., 1976, Geologic 93-96. features, Death Valley, California: California Division McAllister, J. F., 1973, Geologic map and sections of the of Mines and Geology Special Report 106, 72 p. Amargosa Valley borate area--southeast continuation van Eysinga, F. W. B., 1978, Geological time table, 3rd of the Furnace Creek area-Inyo County, California: Edition: Elsevier Scientific Publishing Company, U.S. Geological Survey Miscellaneous Investigations Amsterdam, The Netherlands, 1 Sheet Series Map 1-782, scale 1:24,000. McElhinny, M. W., 1973, Paleornagnetism and plate

15 Winograd, 1. J., and Thordarson, William, 1975, Wright L. A., 1974, Geology of the southeast quarter of Hydrogeologic and hydrochemical framework, south the Tecopa quadrangle, San Bernardino and Inyo central Great Basin, Nevada-California: with special Counties, California: California Division of Mines and reference to the Nevada Test Site: U.S. Geological Geology Map Sheet 20,, scale 1:24,000. Survey Professional Paper 712-C, p. C1-C126.

16 In q tH "'.- DEPARTMENT OF THE INTERIOR TO ACCOMPAN"Y MAP !. '28 U S GEOLOGICAL SURVEY

LATE TERTIARY AND QUATERNARY GEOLOGY OF THE TECOPA BASIN, SOUTHEASTERN CALIFORNIA By John W. Hillhouse

ABSTRACT INTRODUCTION Stratigraphic units in the Tecopa basin, located in SCOPE OF THE INVESTIGATION southeastern California. provide a framework for The objectives of this study were to establish the interpreting Quaternary clirnanc change and tectonrsm distribution, age, and structure of Quatemary deposits n along the present Amargosa Rivet During the late Pliocene the Tecopa basin. This information provides a basis for and early Pleistocene. a climate that was appreciably interpreting past episodes of faulting and climatic change tn wetter than today's sustained a moderately deep lake in the Amargosa Rwver drainage system. The Tecopa basin is the Tecopa basin. Deposits associated with Lake Tecopa ideal for studies of Quaternary history because erosion has consist of lacustrine mudstone. conglomerate, volcanic ash. clearly exposed the stratigraphy, and the deposits of and shoreline accumulations of tufa. Age control within the Pleistocene Lake Tecopa have proven to be datable lake deposits is provided by air-fall tephra that are Volcanic ash beds within the lake deposits have been correlated with two ash falls from the Yellowstone caldera, chemically correlated with isotopically dated volcarn the Lava Creek (0.62 Ma) and Huckleberry Ridge (2.02 sources in the Yellowstone (Wyoming) and Long Valley Mal Tuffs. and one from the Long Valley caldera. the (California) calderas (Izet. 1981: Sarna-Wogc-cki and Bishop Tuff (0.73 Ma). Paleomagnetic determinations from others. 1984). In addition, the continuous sedimentary deposits in the Tecopa basin are consistent with the ages record of the lake beds provides an opportunity for of the ashes. Extrapolations of ages using average paleomagnetic correlations. With age control offered by the sedimentation rates suggest that the beds of Lake Tecopa, volcanic ashes, magnetoones in the lake deposits can be which accumulated to a minimum thickness of 72 m, are correlated with the geomagneti polaty time scale 0.5 to 3 m.y old. In the central part of the basin, volcanic (Mankinen and Dalrymple, 1979). glass has been replaced by authigenic silicate minerals that Proterozoic and Cambrian sedirentary rocks and formed in porewater of extremely high salinity and Tertiary volcanic rocks of the region wee grien only a alkalinity. Therefore. Lake Tecopa occupied a closed basin cursory examination as far as establishing source terames during the latter part, if not all, of its 2.5-million-year for the Quaternary alluvium. No efforts were made to history. describe srigraphy, faults. or other structures in the Sometime after 0.5 m.y. ago. the lake developed an bedrock of the ranges. Informaion on the bedrock geology outlet across Tertiary fanglomerates of the China Ranch of the Tecopa am is found in re by Hazard (1937). Beds leading to the development of a deep canyon at the Mason (1948). Noble and Wright (19541. and Troxel and south end of the basin and establishing a hydrologic link Wright 41976). between the northern Amargosa basins and Death Valley. In constructing the geologic map, contacts betwen After a period of rapid erosion, the remaining lake beds Quaternary units that have developed ditncve were covered by alluvial fans that coalesced to form a geomorphologle were marked on aerial photographs pediment in the central part of the basin. Remnants of (scale 1:47.200) and then transerred to the topographic these fans are capped by desert soil horizons composed of base (•sale 1:48,0001 with a stweog ic plover. These (1) a clay-rich B-Horizon, (2) a thin bed of vesicular tan silt units were checked and described during fieldwrk in the (A-horizon). and (3) densely packed pavement with well winter of 1981. The ak deposits and outcrops of the developed desert varnish. The 8-horizon of one such fan thcker volcanic ashes were mapped in the f on aerial yielded an age of 160.000±="18,000 years by the uranium photographs and con€cts were then plotted on the trend method. The Tecopa basin bears no evidence of topographic base. Several smratgraphlc sections were Lahontan-age lake deposits Holocene deposits consist of measured in the lake beds unconsolidated sand and gravel in the Amargosa Riv bed Paleomagnefc Investigations of the lake deposits. and its deeply incised tributaries, a small pha near made primarily in 1971-74, and supplemented in 1978. Tecopa. alluvial fans without pavements, and small sand involved the collection of oriented sediment cubes dunes. The pavement-capped fan remnants and the Sampling was limited to steep-walled outcrops to obtain Holocene deposits are not faulted or tilted significantly. relatively unweathered material. The samples wer given a although basins to the west. such as Death Valley, were standard treatment in alternating maneft fields to remove tectonically active during the Quatemary. Subsidence of unstable magnetic components and to reveal the original the western basins strongly influenced late Quatemary depositional remanent magnetization (McElhinny, 1973) rates of deposition and erosion in the Tecopa basin Magnetozones were established from several equivalent 9303110364 930305 PM1 uAoarr I stratgraphic sectons thai were closely correlated bi, STRATIGRAPHY volcanic ash beds ARCHEAN PROTER(.ZOIC A•) CAMBRIAN ROCKS PH'YSIOGRAP.4\" The oldest rocks within the study area are granitic The Tecopa basin is :n the southeastern part of [nvo gneiss at the southwest end of tne Nopah Range and :n tme County, Cahfornta about 30 km east oi me south enc of Ibex Hills The gneiss contains metadcionte dikes to which Death Vailey In this report Tecopa basin refers to the Chesterman (1973) assigned an Archean age Exposed :n part of the Amargosa River drainage that lies chiefly the rugged banded cliffs of the eastern Dublin Hills and the between the towns ot Shoshone and Tecopa and mncludes Resting Spring Range is a nearly complete succession of the southeast end of Greenwater Vaiiev and the southern uppermost Proterozoic to Middle Camibnan manne half of Chicago Valley (fig 1I sedimentary rocks The sequence consists of the The Tecopa basin can be considered a southern ascending. Noonday Dolomite (Proterozoic,. Johnnie extension of the Amargosa Desert. which generally lies Formation (Proterozoic). Stirling Quartzite iProterozo,c, between Eagle Mountain on the south J25 km north of Wood Canyon Formation (Proterozoic and Lower Shoshonei and the Bullfrog Hills on the north (120 km Cambrian), Zabnskie Quartzite (Lower Cambrani. Carrara northwest of Shoshone) The areal extent of the Tecopa Formation 1Lower and Middle Cambnani. and the basin is approximately 500 km2 The region is Bonanza King Formation (Middle and Upper Cambnan) characterized by arid sparsely vegetated valleys and bare TERTIARY VOSCANIC ROCKS rugged mountain ranges. A dominant feature of the Volcanic flows and tufts. predominantly of rhyohnc Tecopa area is the Amargosa River, which is dry except compositiocn, are exposed tn the western Dublin Hills and during intr. :ent thunderstorms or near permanent the Resting Spring Range. Haefner 1976; applied the springs at Sr none and Tecopa. North of Shoshone as far name -Shoshone Voicanics" to the pyroclastic rocks of -he as Eagle Mountain. the nver bed follows a nearly linear Dublin Hills. The Shoshone Volcanics. uhicn consist of channel, which has cut approximately 25 m into the pumice lapilli tuff and welded tuff. are part of a large surrounding alluvial piedmont South of Shoshone. the Tertiary volcanic terrane exposed throughout the river bed broadens into a braided plain surrounded by Greenwater Range and the Black Mountains The welded badlands of low rounded hills, mesas, and deeply dissected units are crysal poor in the Dublin Hills but elsewhere alluvial fans. A small playa forms the floor of the they contain abundant phenocrysts of plagioclase. depcession near Tecopa. South of Tecopa, the river homblende. and biodtt The unwelded units consist of descends through a narrow canyon. of which steep walls blocks of andesite and rhyolite, rare clasts of dolomite and are locally more than 100 m high. The basin is Ranked by quartzite. and a devitrified pumice lapilli matrix Haefner banded brightly colored rocks of the Resting Spring Range (1976) considered the Shoshone Volcanics equivalent to on the east and the Dublin Hills on the west 11ig. 1). The part of the "older volcanics" that underlie the Greenwater Sperry Hills, which are bisected by the canyon of the Volcanics of the Funeral Peak quadrangle (Drewes. 19631 Amargosa River, make up the southern margin of the Potassium-argon ages, principally from biotite. range from Tecopa basin Mean annual precipitation ranges from 7 cm 6.32 to 8.02 Ma for the "older volcanics" and range from in the valleys (-460 m elevation) to 15 cm in the higher 5.32 to 5.47 Ma for the Greenwater Volcanics (Fleck, parts of the Resting Spring Range (-1.000 m elevation) 1970). Volcanic flows at the northwest end of the Dublin I Vinograd and Thordarson. 1975). Hills have yielded K-Ar ages of 7.9 Ma (plagioclase) and PREVIOUS WORK 8.7 Ma (glass) according to E. W. Hildreth and R E. Drake Sheppard and Gude 11968) reviewed the early (oral commun.. November. 19821. geologic studies of the Tecopa area. most of which Pyroclastic rocks of Tertary age are exposed along the concerned exploration for nitrates. borates. and purnicite. east flank of the Resting Spring Range and make up the General geology of the Tecopa basin is summarized by prominent butte that is 4 km northeast of Shoshone. An Mason (1948) and Noble and Wright (1954). Detailed excellent example of these pyrocastic rocks is exposed in geologic maps. at 1.24.000 scale. were published for the the road cut where California Highway 178 crosses the southeast quarter of the Tecopa quadrangle (Wright 1974) Resting Spring Range (Troxel and Heydari. 1982). The cut and the northeast quarte of the Shoshone quadrangle exposes devitrfted pumice tuff. welded tuff, and vesicular (Chesterman. 1973). Teomy volcanic rocks in the Dublin vitrophyre. dated by K-Ar methods at 9.5 Ma (R. E Drake. Hills were studied by lae'fner (1976). In general, emphasis oral commut.. November, 1982). Although the distribution of previous work was on the bedrock geology, so the of volcanic rocks of the PRsting Spring Range is shown on Quaternary deposits received only cursowy examiatlont. geologic maps by Mason (1948) and Noble and Wnght Pleistocene Lake Tecopa. which was named and (1954), detailed descriptions. isotopic ages, or the relation described in a bnef article by Blackwelder (1936). has of these volcanic rocks to similar units farther west have been the subject of several geochemical and mineralogical not been published. studies. Sheppard and Gude (1968) described the TERTIARY BASALT stratigraphy and approximate distribution of the lake beds Isolated remnants of basalt are exposed 1 km north of in a study that focused on the formation of authigenic Shoshone and in the vicinity of the southern Dublin Hills silicate minerals, particularly zeolites, within volcanic ashes The basalt forms black rounded hills strewn with blocks as of the basin Starkey and Blackmon (1979) investigated much as several meters in diameter The olivine-rich basalt the distnbunon and oigin of clay minerals in the iacustrine is fine graimed. moderately porphyntiic. and highly deposits vesicular Just north of Shoshone the basalt overlies buff-

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FIGURE I -MajoT gsogrphic featume of d-.e Amrgoaý rego Dotted line indicates boundaxV of Tecopa bamn. 3 colored tuff thai consists of rounded pumice cobbies ancestral Sperry Hills which consisted pnrmani,. ,t volcanic rock fragments and pumiceous matrx Chesterman ,1973; assigned a Quatemar, age to rhe beer. modified by extensive faulting basalt and stated that tne unit overites detorr.ed iacustnrine Lenses of white pumice fill anc:ent channels within (lie .angiomerate deposits of Pleistocene Lake Tecopa However tuffaceous The lenses as much as 5 m tnic,. consist of ,axe beds were 'ound un top of the eroded basaltic mound pumice clasts in a white ;apilli man'.x The clasts are well ,)n the east side of the Amargosa River 1 km norm of rounded and are from I to 5 cm :n diameter Pumice Shosnone lnasrruc as !he ,idest .deposits of LaKe tenses are generally no more than a few tens of meters Tecopa are late Pliocene see wsec'-, on "Age of LaKe alde although one extensive pumice bed :s exposed in the 'recopa ,. the basalt is presurrarl, .)! Tertian. age In Ohe northern Amargosa canyon. 3 km south ot Tecopa This 5 Dublin Hills the basalt and reiated rutf directly overlie .n thick bed is a complex of pumice pebbles and cobbles Paleozoic carbonate rocKs. so the maximum age of the !nterbedded with pumice iaplih and coarse arkosic volcanic unit is poorly constrained Because the sandstone The clasts are well rounded and present -n strat3graphic relation of the basait to the Shoshone laminar graded 'upward fining beds as muci as 20 cm Volcaiics is not demonstrated and because basalt thick Pumice cobbles are nch in sarudine pienocrysts eruptions occurred sporadicalkt n the Amargosa region The pumice bed is traceable southward from the throughout the !ate TertarV and Quaternary oniv tentative fanglomerate .nto gypsaferous mudstones The crudel' conclusiomn can be made concerning me timing of basaltic bedded tan deposit is finer grained to the south and volcanism at Shoshone eventually interfingers wnth buff and pale-pink mudstcnes The basalt may be related to vokcanism that produced which are well bedded and contain abundant gypsum part of the Funeral Formation Drewqs. 1963. Denny and Total exposed thickness of the mudstone is approximatel', Drewes. 19t6? which includes a large area of basalt flows 400 m The gypsum beds suggest a shallow lacustrmne in the eastern Greenwater Range In fact. Noble and environment or playa. where evaporation was dom:nant Wright (1954) described the Shoshone basalt as part of The mudstones are extensively faulted and deformed into their Funeral Fanglomerate of Pliocene and Pleistocenet• 1 tight plunging folds age trenamed the Funeral Formation by Drewes. 19631 in The China Ranch Beds overie and ,corporate their generalized geolooc map of the Death Valley region mat"eal from the Cambrian and older bedrock Teniaryt B It this tentative cormlatkon is correct, then the basat is granite from the Sperry Hill. and the Ternary volcanic younger then $ 4 Ma. which i• the isoioptcally determined rocks The beds underlie the deposits of Lake Tecopa and age of the Greenwater VoIcanics thai underle the Funeral form the southern mnargn of the lake basin In a few Formation in the Funeral Peak quadrangle. A basalt Now places, a slight angular unconformity is indicated where in the type section of the Funeral Formaton ruar Ryan la-keage gravels filed depresson in the eroded and alted vielded a whole-rock K-Ar age of 403*:0 12 Ma China Ranch Beds. Noble and Wright 11954) and Wright tMcAlliter. 1973) (1974) assgied a Pliocene and Pleistocene()) age to the TERTIARY CHINA RAN'CH MM China Ranch Beds on the bas of stratigraphic relations Debris flows. fanglomerate, pumice beds, and Because the beds unconformiably underle lake beds of psifrvous rmudstone in the Speny Hll compo the earlis Pesocene age. the China Ranch Beds are C"':;i4 Ranch Beds, which are well expoed in the considered to be wholly Tertiary Amarii.ja canyon south of Tecopa. Mason 11948) In many ways. the China Ranch Beds are similar to onginaly applied the name to the light-colomd mudstones the Funera Foirnation of the Death Valley region For anrt .Mpfemus beds that crop out inthe vinitty of China ewmple. the Funeral Formation consists of coarse fan Ranch However. Wright 119741 redefined the unit to deposits with lar•ge slwnp blocks. playas. and pumiceous include the cowk fIanglomemte that tnwwfqmns wth the tuff (Drewes. 1963). as fefound in the vicinity of China psaferous mudstone The ktanlomere uilow of the Ranch. The presence of huge blocks and megabrecctas China Ranch Beds is peeslknt in the Meander His ifig. wAthm t fanglonerates snggests that uplift and I and in the westn pen of the Spena H accelerated erosion were affecting the highlands Exposures of the fmnglomerwe south of Tecopa Pas, throughout the region. This tectonic activity, which consis of bodie of monohtholo~c conglomnerate probably occurred during the Plocene. was accompanied vntw*wIdded with NheWm nos iaenal IWns. 1974) by local volcanic actvity. T'h monoathokoc conglomnies. which awe present as QUATIEflY LAKE BEDS CON•G•O•M1TE. A-N4DTUFA lenSe a long 300 m. ae slump blocks of bedrock derived The deposits of Lake Tecopa (fi. 2a) are divited into f•rn the nearby ranges West of the Anargu FbRe the three informal unts that are mapped separately dII tarinklrrierte is a het•e•oneous unit of gnesw gramite, iacusaine mudszone. (2) conglomerate. and (3) tufa anrd dolonute boulders Quimtt, metasednentury rocks. Deposits of Lake Tecopa occupy the low region between end vokanIc Mrok6 aM commnoI but less abUndant The Shoshone an-d Tecopa and the southern end of Chicago submunded boulders. wrich are I to S rn in dianetir are Valey The rnaxtmum aeal extent of the lake was wt m a sand and gravel matrix ot wsimar tholmof In the approxmiarely 250 kl 2 The southern margin of the lake ch•s of thei Arnargosa canyon. where at Wast 100 m of the was onginatly buttressed by fangiomerate of the China anglOmerate ,s exposed. the poor sorting and crude Ranch Beds now breached by the Amnargosa River bedding o the unit 's readdy apparent The unit orignated as dJetb flco.s and stream deposats derived from the

'4 East-T- -

-_ Tuff A . Tufa, Tuff B _Qt'm Ga. West

FIGURE 2a -- Schematic geologic cross section of the deposits of Lake Tecopa. lacusinne mudstone uni (Qdmi including tuffs A. B. and C of Sheppard and Gude (19681. conglomerate unit (Qtc), and tufa Also shown are basement rocks ITAu) composed of Archean gneiss. Proterozoic and Paleozotc sedimentary rocks. Tertiary volcanic rocks, as well as older Quam,,mary alluvium (Q&21. and Holocene alluvium (Qa1I

Lacustnnv mudstone be massive, especially in the central part of the basin thickness of 72 m of mudstone. A cumulative Foe the most pat. t lake sediments are moderately (Mg. 2b). although claystone. and volcanic ash is exposed indurated, but tend to dagregate easily during wetting. the level of erosion has not reached the base of the lake However. ocalized concenuations of calcite produce in the cental part of the basin. Suatigraphic deposits resisiant ledges and beds espciallly near Shoshone. where I was mneured 1 5 km northeast of Shoshone, secton spring water and the Ainargoua Rive entered the lake, arid ection 2. 2 km east of Shoshone. and section 3 was also near the Greenwater fan Sediments along the lake measured near the southern end of the Dublin Hilk The permne tend to be richer in CaCO than the sediments sediments have pastel colors ranging fuom buff and pink 3 of du central basin. Sandstone beds. comnmonly cemented near the lake margins to green and brown in to central form nodules and flagstones as much as 50 Bedland topography doveloped on the lacustine by carbonate, besn cm wide and 5 to 10 an thik The eroded surface of the mudatone unit generally consms of low rounded mounds lak beds is littered with a lag concenfradon of carbonate with knobby surfaces resembling popcorn, caused by the expqndabh clay minerals. Ner the wetting and dring of Several volcanki ash beds, ranging in thickness from 1 the deel dissected lake beds form vertical lake mains. an to 4 m. are present in the lake deposits. Of the 12 walls, rsveral thick volcanic ash beds form orange or white ashs that am recognized& three form thick beds that are reistant ledges in the walls 01 the arroyos. the basin; whereas, the thinner lthology is mudstone that consists of mappable throughout The dominant ashes are dcontinuouw. Sheppwd and Gude (1968) amounts of sund The mudstone clay, silt and minor of the three thick ashes, and contains calcts day minerah, minmea and rock fragments. provlided detailed descriptions the irommdanre hs been followed in rfrring to the and gypsum fis cracks. Authigenbc amoim and poausum ashes as W A B. and C (fom youngest to oklest). feldsiar a also present in thi nAdste. Claystone. In gneral, the fresh ashs are white or pale gray and which breaM wit a brtt conchoidl fracture, is interbedded with mudsbna and is mos common in the extremely frible. They ar composed of rhyolilic vitreous crystal and rock fragments. middk parn of te War beds. The predominant demrtal day shads and minor amounts of of the ashes m sharp. suggesting direct minrals we S• ithur-n-Tich saponite. and TM baal con%= air i into the basn, end the upper contam are montironllornt (Steasy and Backmon, 19791. Grain sue of the sedirneaby rocks become coaser toward the grada nl with the overyn mudstone. In many places. tick ashes have multiple flne beds. main inlet north of Shoshmm and toward the ribuades of the upper pants of th. smal-scale croesbeds indicative Chicago Valleys. Lnas of sandstone and tipple marks. gradin and Greenwater and Locally. fresh ash is pebb4ls mak the channels of ancient streams of rewordug by water current subrounded by CaCO0 to form *eimntledges. at the mnlets The subangular sand grins consist of quar cemented feldspar biodt. and sillc vokarnt-cock fragent, The Except ner t•h northern ake mangn. where fresh ash basalt and scic preomnatsthi ejm shardsa- genera*l altered to dominant Wtoogies of tM pebble am As described by volcanic rocks. moon, potassium feldspa. and searnift. the tuffs The mudston• ar generaly we bedded and nearty Sheppard and Gude (1968). the autdgenesls of flat Nt although "e beds dip appox•mately V owad follows an amal onadion with fresh glas at the outer the center of the basn. Minor warps, slumps, and perimeter, giving way to zeole (phllipsite, clnoptilolte. and erlnt) in the Intermediate zone, and finally depowo weme noted in a few limited areas (see fi. 5. Sheppard an Gude. 1968). The clayone beds tend to potassium feldsa and searnleuite at the center of the basin -5 SECTION

wo

- '~-~-SECTION SErTION E`--R 3 ME'~~2 Tuff A

20 -

______Tuff B\.

-z - 4.

0-- 20

EXPLANATION Tuff C

Mudstone Nodules

10 Sandstone Root ca~st

Cm~ornlonate P4a806aI

Tuft

FIGURE 2b.-4leasured slyatigt iphic jectic~tis. in the lacustrine mudstone unit. Locations. Section 1, SE1'. sec. 19 and SW*- sec 20. T 22 N R. 7 E.. Section 2. SE-. sec. 29 and NE1/4 sec. 32, T 22 N., . 7E.,Sect~on3. NW 1,4sec. 19,.T 21N ,R. 7E 6 Altered tuff is punky. porous, has a blocky fracture, and make up 1 to 10 percent of the ash Resistant ledges of rur. forms yellowish or greenish ledges B at the southwest side of the oastn contain authigenwc Tuft C-The lowermost thick ash. ruff C is zeolites and potassium feldspar as replacement products of stratigrapnicaily 17 m above the deepest exposure of the the glass shards Orange ledges of the altered ash have lake beds Tuff C is 10 to 75 cm thick and is best exposed abundant sphen•litic phillipsite within the zeohte facis In along the west side of California Highway 127. where the the potassium feldspar facies, the tuff is green or brown altered tuff forms a resistant ledge containing green Tuff A-Tuff A. which is 0 5 to 4 m thick. is exposed nodules of silicified clay Distinctive features of the tuff are around the penmeter of most of the basin, but was not the highly contorted and well-iarnnazed upper part and the recognized in Chicago Valley The 4-8 m interval that deformed base. which is generally warped into a wavelike separates tuff B from the overlying tuff A generally consists surface with amplitudes of 50 cm and wavelengths of of green brittle claystone Tuff A is overlain by about I0 several meters. Although the cause of the deformation 30 m of lacustrnne sandy mudstone and conglomerate, no remains a matter of debate, the wavy base is probably a ashes were found above this tuff The beA exposures of load feature, but the contorted upper part may be due to ruff A are in an abandoned pumicite quarry. I km south turbidity flow. Seismic shaking has been proposed as an of Shoshone. where the 4-m-thick bed consists of silver alternative cause of the deformation. gray vitr"c tuff with many lamtnar interbeds several Although the glass shards of tuff C are predominandy centimeters thick The ruff contains only minor amounts of altered to potassium feldspar minerals, a small pocket of locally derived detritus, although there is ample evkdence fresh ash is preserved near the eastern margin of the basin that the upper part of the bed has been reworked by wmter (SW¼ sec. 24. T. 21 N.. R. 7 E.). Fresh ash is mainly of currents. Well-preserved ripple laminations indicate a plary bubble wall shards and 1-5 percent rock and crystal south-southeast current direction. In Greenwater Valley fragments. Zircon phenocrysts with glass selvages are tuff A is 2 m thick and overlies sitsrone containing present The bed is difficult to trace in the rounded hills of abundant root casts. On a nearby isolated outcrop of the central basin, because the outcrops have thick mantles Cambrian dolomite, the ash was deposited on talus of weathered material. suggesting deposition by direct airfall In the Sperry I-Ills. Tuff B-Tuff B is approximately 18 m stranigraphically the 2.5-m-thick bed was deposited on gravels which above the base of ruff C. The stratgraphic interval between interfinger with the lake beds. In general. the substantial the two tuffs. which is predominantly claystone, is poorly thickness of the tuff provides a continuous exposure. exposed except at the southwest part of the lake basin. except near the ancestral Arnargosa iver channel where Tuff B is thick (0.3 to 3.5 m) and can be traced almost the ash is mixed with sand and gravel and forms continuously from the Sperry Hills, along California discontinuous lenses. The lenses exhibit chaotic rip up Highway 127 to Shoshone, then along the west flank of structures, crossbeds. and tipple marks. the Resting Spring Range to Chicago Valley. The southern Where fresh. tuff A is predominantly composed of exposures of tuff B are mostly associated with deep-water piety glas shards approamately 0.1 mm in diameter Rock mudstones, whereas elsewhere the tuff is exposed in and crystal fragments tyally comprtise less than percent shallow fluviatlle deposits near the ancient shore. The of the ash. Where the tuff forms resstant ledges, it diplays deep-water exposures contain very minor amounts of alteration features similar to those of altered cuff B locally derived rock fragments. In contrast, on the lower part of the Greenwater fan, the tuff has a clean vitreous Congionmeate base beneath layers containing an abundance of local This unit conto of fanglomerat and pebbly .uviatl carbonate and volcanic det1ttus. The thicker accumulations deposits that interbiger with dth lacusrine mudszone unit of the ash tend to be massive except near the base where The unit generally forms cobble-strewn low rounded hills fine laminations are common. Gypsum-filled root casts are that develop dendritic outlines in plan view Tpscal abundant throughout fi ash bed in the shallow-water exposures of the conglomerate unit ame sen m the gramevy depositionai environments. Near the southern tip of the pwidmont that flanks Califoa Highway 127 north of Resting Spring Range, tuf B was deposited by direct air Sho•hone, where the unit como a of coalesci alluvial fall onto alluvium, a suggested by the presence of fans at the foot of the surroundin range. The fan abundant fossilized plant smen in the base of the ash. In deposits, which ane at least 50 m uhick. consist of saint the channels of the ancestal Amatgosa and Chicago gravel, and subangular cobbles compomed of dolomite. Valley tributaries, tuff B was deposited as a complex of quartzte basalt, and sick volcanic rocks. Pebble and channel Rlngs and tuffaceous sandy beds. Just northeast cobble siz increase from 1 to 8 cm near the center of the of Shoshone on the east side of the Arnargos River, the alley to 20 cm at the foot of the bedrock ranges, Near the ash is interbedded with mudstone that was deposited on present channel of the Amargosa River. the conglomerate Tertiary basalt consists of rounded pebbls 1 to 4 cm in diameter, and The freshest examples of tuff B are nea Shoshone sandy channel filngs that were probably deposited by an and along the Raiting Spring Range. where the ash stands ancestral rver. These fluviatile sediments are interbedded out as a white band between underlying pink mudstone with tuffs A and B and a I -m-thick mudstone bed. and overlying green claystone. Fresh ash is mainly Small remnants of the conglomerate unit are exposed composed of distinctive pumice shards (as much as 0 5 at the foot of the Dublin His, the Sperry Hils. and the mm) havng spindles of elongate tubular bubbles. Volcanic Resting Spring Range. Where a direct association with lake rock fraqments and crystals. mainly biotite and plagioclase, beds cannot be mmonstrated. the remnants are reedlike plant stems Caprock composition vanes from the younger alluvium by a more distinguisýhed from cemented gravei to nearly pure fine-grained CaCO, wth morphology, greater induration due to rounded dendntic stacked flattened pores or fenestrae. A few mounds have and generally steeper depositional carbonate cementation, knobby surfaces of concentrc travenmne layers. although A shallower slope tends to be slopes near mhe range fronts. most of the mounds are capped by )agged solution-pitted deposits because the range present in the younger alluvial blocks of tufa rubble more deeply since the fronts have been incised The tufa forms two parallel discontinuous benches deposited. This relation is well conglomerate unit was that are about 100 m apart laterally. several meters apart the Dublin Hills where conglomerate fan demonstrated -n vertically, and which apparendy follow the eastern younger pediment gravels that have remnants stand above shoreline of the ancient Lake Tecopa Features of the Athough clasts within the conglomerate shallower slopes. shoreline such as beach gravels an,: sands are locally of CaCO , thick zones of pedogenic carry part•al rinds 3 preserved in rufa. although the more typical mounds developed caliche have not appear to have grown as wedges within mudstone beds the conglomerate unit is a fan in the Sperty Hills. near the stratigraphic levels of ruffs A and B The tufa the eroded surface of the Ch.na deposit that overlies mounds apparently formed while Lake Tecopa existed. a slight angular unconformity. This Ranch Beds with because all mounds are associated with lake beds and tufa in the uppermost 10 m of the west contact is best exposed rubble forms clasts within alluvial deposits that postdate the canyon Compositionally. the wall of the Amargosa lacustrine mudstone and conglomerate units unit is difficult to distinguish from Quaternary conglomerate Accumulations of tufa probably formed by the precipitation fanglomerate because it was denved from the China Ranch of CaCO when carbonate-rich runoff or spring water clasts of the older unit. However. 3 the granite and gneiss entered the alkaline lake (Sheppard and Gude. 1968) No and structural features of the the morphology evidence of spring vents or carbonate-filled cracks was distinctive. 11) Younger fan deposits conglomerate unit are found in the lake beds near the mounds Therefore. the the lake basin, whereas the consistently slope toward alinement of tufa mounds is probably related to the ancient generally steeper, more China Ranch fanglomerate has rather than more recent activity of springs along clast sizes shoreline variable dips, 02) the younger unit has smaller a fault zone. and large boulders (>I m) are rare, (3) faulting is much more extensive in the China Ranch Beds; and (4) although Age of Lake Tecopa the Quaternary conglomerate unit is interbedded with tuff Tephrwhmnoiogw-Correlatl-is of the Tecopa ash A south of Tecopa. it does not contain the pumice cobble beds with isotopically dated volcanic sources are listed in lenses that characterize the China Ranch Beds. table 1. using K-Ar ages that were calculated from the current decay constants and *ArP'Ar abundance ISteiger Tufa and Jager, 1977) bIet and others (1970) correlated the mounds along the western foot The white flat-topped middle Tecopa ash bed, Tuff B. with the Bishop Tuff in the are thick accumulaltons of of the Resting Spring Range Long Valley caldema of eastern Calfornia Tuff B. also of dense CaCO tufa, The tufa commonly consists 3 called the Bishop ash bed, is part of the tephra that was a layer of powdery white caprock, 2 to 4 m thick. over eected from Long Valley when the Bishop Tuff was base, which commonly grades CaCO 3. The powdery 0.73 my. ago JDalrymple and others, 1965. mudstone unit. generally eruped. downward into the lacusuine Manldnen and Dalrymple. 1979). The correlation is contains fossd ostracodes. thin-walled bone fragments. and

TABLE I .--ConIoton and dating of Lake Tecopa ash beds Other correlatve air.fall ashes (Prefesed Sou"e o1 a--al Ash beds of Lake Tecopa. nomenclature ash of Sheppard and Gude (1968) and name in pmnendsme) this report Laa . .e...ahf h Lav •ee Tuff A P~arlst.-llhe ash. Type 0 UnlitU and LavaTuff Creek(of the B Yelawstoenash of the LavaGroup). Creek others, 1970. 1972. litet, 1981. Sam&a Wiyomlni age 0.62 Ma by K-Ar dating Wqcicki and others. 1984) method (Chroamn and Blank. 1972. [= and W#Wc 198Z) (Ash of the Lava Creek Tuff) Bishop Tuff, Long Valley a•ldei Rbshop ash (laem and othems 1970 Calfotma age 0.73 Ma by K-Ar dating Tuff B 1964) Sarna-Wolicicki and others. method (Day•mple and others. 1965: Mankinen and Dalrymple. 19791 (Ash of the Blishop Tuff) Hucidebenry Ridg Tuft tof the Tuft C Pearktett-Ike ash. Type B (lieU, 1981; Yelloswio Groupl. Wyo• n other. 1984); age E Meyer. Samna-Wolacki and (Chrmawmn and Blank 1972). age Ftsson-treck age. 1 6 Ma (C Ma by hwsson-uw~k dating Sama 19:=0 1 2.02-0 08 Ma by K-Ar dating method M J Woodward. and A. M (Naesef and others. 1973) WoK•mic. oral commun. Sept. 1981) (Reynold, 1977, Love and Chrulstansen. 19801 (Ash of the Huckleberriv Ridge TWAf 8 founded on mnneralogic similarities, shard morphology. horizon at stoatigraphic intervais of 15 cm (fig. 3) The microprobe analyses of major elements spectrographic polantv of each horizon was interpreted according to the analysis of II trace elements in the glass shards, and method of Hillhouse and others 1977). and a generalized paleomagnetsm Further substantiaton of the correla-on magnetostraugraphy. which combines data from all was provided by Sama-Wojcicki and others (1984M in their columns. was constructed (fig. 4). the study of tephra in the Western Stares. The method The interval fromn tuff A down to ruff B (ash of employs neutron-acnvatoon analysis to determine Bishop Tuff) is conssently of normal polarity As shown in abundances of 20 minor and trace elements in volcanic figure 3. a polarity transition is found approximately 50 cm glass These abundances are combined with results of below tuff B rHilihouse and Cox. 1976) This transwon. microprobe analysis for major elements in making regional which was confirmed by spot sampling at five localities correlation of air-fail tephra An excellent match of around the basin, is undoubtedly the transition from 'he elemental abundances was determined for tuff B and basal Matuyama Reversed Chron to the Brunhes Normal Chron. pumice lapilli of the Bishop Tuff the trarmtion has an estimated age of 0 73 Ma (Mankinen The chemistries of tufts A and C match the elementail and Dalrymple, 19791 The position of this transinon just abundances of the family of tephra that erupted from the below the ash of the Bishop Tuff is consistent with the Yellowstone caldera. 1,050 km northeast of the -Tecopa paleomagneusm of dated volcanic units in Long Valley and basin. These widespread air-fall deposits, which were the Western States. unginally named the Pearlette ashes for exposures found in As shown in figure 4. two normally polarized the Midwest. are now known to be the air-fall equivalents magnetozones below tuff 8 are tentatively correlated with of welded tuffs in the Yellowstone caldera (Izeur and others. the Jaramilfo (0.97-090 Mai and Olduvaa Normal 1970. 1972). Tuff A. the youngest Tecopa ash, correlates Subchrons of the Matuyama Reversed Chron Q1 87-i 67 with the Lava Creek B ash of the Lava Creek Tuft, dated Ma). Finally, at the bottom of the stratigrphic section, tuff by K-Ar methods at 0 62 Ma (zen. 1981. Christiansen and C is within a reversed magnetozone that is consistent with Blank. 1972; lent and Wilcox. 1982). The original the older part of the Matuyama Reversed Chron [2.48 correlation, founded on petrographic similarities, 1 87 Ma). This correlation is in agreement with K-Ar dating strafigruphy, and major element chemistry. is further (2.02 Ma) of the Huckleberry Ridge Tuff. which is the substantiated by neutron-activation analysis of trUce and presumed source of tuff C. The fission track age (16 Mal minor elements (Sarna-Wolckli and others, 1984). The determined from Tuff C is not consistent with this name "ash of the Lava Creek Tuff" which relates tuff A to interpretation of the magnetic stratigraphy. Correlations are the source area. is now preferred over the original name. made with considerable uncertainty in the lower part of the "Pearlette.i ash, type 0". that was assigned by liet and Tecopa lake beds for two reasons: (1) despite cleaning others (1970). tmatnent. __stdpositional remagnetizagton of normal Sama-Wojlcki and others (1984) and Iteu (1981) polaft persists throughout the lower horizons at Lake tentatively correlate the lower Tecopa ash, tuff C, with the Tecopa. and 12) In the polarity time scale, the placement older Pearlette ash bed of Meade County, Kansas. The ash and duration of normal polarity subchrons have not been bed In Kansas, referred to as "Peareta-like ash, Type B", fuly resolved for the lower part of the Matuyama. was correlated with the HucilebeTy Rkige Tuft of the Depodlon alsa-On the basis of time lines Yellowstone Group (Namer and others. 19731. The age of provided by the ash beds. rates of deposition were the Huckleberry Ridge Tuft is 2,02 r 0.08 Ma. as measued approximately 0.06 mi1.000 yr betwen tufts A and B and by the K-Ar method (Reynol 1977; Love and 0.015 nV1.000 yr between u B and C. By vxtapolating Christiansmn. 1980). FIssion-trck daing of the wpe-B these rotes to cove the beds of the lacustine mudstone Peadette ash of Meade County. Kansas yiedded an age of unit that - above tuff A and below tuft C, a cumulative 1.9:0.1 Ma (Naeser and ote.m 1973). 1-lowever. age of 3.0-0.5W was ob•tined for the lake beds. These preliminary Analysis of fresh tuf C from the Lake T[opa uits should be considered rough estmates because the beds Ovem a fion-tmack age of 1.6 Ma (C. E Meyer. M. rates of depofton. vary by a factor of four within the J. Woodward. and A. 4 Sama-Wqdkcld oral communr. scton. In additio the maximum age of the lake is very September, 1981). uncrtain becae the base of the deposits is not exposed Paernr&S SmWtgphy-Pakeomagnetic work in in the c.tal bast the Lake Teo=ps b bIcotitnt wih the chronolow Accoviding to the geologic ne table of van Eysinga that is esabihd fro the volcanic ashes. The (1978). the lake beds are late Pliocene and early pialeomanelc correlation method Invokes eabishing the Pestocene in age. The Phocene-Pleistocene boundary pattern of ngyneoones. defined by the magnec polait (begnin=g of the Olduvu Normal Subchron) is a few of oriented specimens. for comparbon with the metews above tuff C. The younger age limit of the lake geomagneti polarty time scal (Mankinen and Dmrymnple, beds is sightly younger than the early-late Pleimcene 1979), Orine specimens wer collected from four -ondrlBwnhe*4Matumra transtion)I. - -graphsections, which coGweve irepreset the lake Paleonwokw-Sheppard and Gude (1968) described bedsi from the base up to tiuft & Three ouented speomens, localItie of fotils found prior to 1966 in the deposits of collemd within a laterual disance of 50cm. were collected Lake Tecopa. The upper part of the beds contains rare at smaigraphic horizots about I m apa This samwig horse, cam& mammoth. and muskrat fossils. too procedure was used at three of the sections. The remainin fragmentary for age-diagnostic identification. Sheppard and secton was sampled mor dersey four specimens per Gi•d also reported several localities of ostracodes and 9 THICKNESS LITHO- PALEO I INCUtdATION IN 2ECAIEES DECLINATION IN DEGREES IN METERS LOGY MAGNETI I o~ ) b ~3 * 4~ , " I 0AO1tý 1 e 9 Vi1ID L k -INTERVAS 9 0JD 060 9

Bishop I I bed 9

- II

8 I. I. %0III I im4

an.

I I lil

FIGURE 3 -Pdl~omagnetic data used in determining transiflon Iror the MatuVanmt Rieversed Chrun to~ 11-WBrurihes Normal Chron (0i 73 Mjl Transifion is present 50-75 cm below the dbh of the Rishop Tull Sample kwocdlity ik 2 km east of Stuhuhne LITHOLOGY THICKNESS GENERAL-: LAKE ;PALEOMAGNETCiC GEOMAGNETIC POLARITY IN METERS IZED TECOPA' INTERVALS TIME SCALE STRATI- MAG- WMankinen and Dalrymple. 1979) NETO- Gi P NOAGE IN MILLION YEARS SGRAPHIC SECTION ZONES: LAKE TECOPA

Lava Creek__ ___ -0-5 (0.6 Mal

- Brunhes Normal SChton, 0"3 401

Bishop 090 i0.73 M W 097 , - I I 097 - .

- I -1.0 - I

30 - "Jaramillo Normal Subchron -of Matuynma -- Reversed Chron)

Olduvoi Normal Subchron "- 7 lot Mat•ama -

Huckerry"- Ridgis W8 12.02 0,0.01 Mal

- z -2.0

214

10

2 4 -2.5

0 FIGURE 4 -Correlaton of Lake Tecopa magnetozones with geomagnetc pclarty le scale of Manldnen and Dahympl: (1979) Stppled areas denote normal porla"tI, white areas denote reversed pol-nty diatoms ;42 species and vanetes) from the upper lake Thickness of the older alluvium ranges from a thin beds Three of the diatom species mndicated a miadie to jTavelly .qeneer .30 -." :n the central basin to ,nicn late Pleistocene age. and the ostracodes did not contradict wedges ADO0 ml of bouldery conglomerate at the range a Pleistocene age fronts ClasL consist of subanguiar dolomite quartile, and Since 1971 several coilections of vertebrate fossils siLcic volcanic rocks from the surrounding ranges. and ,vua have been taAen from the vicinity ol Tecopa ;table 21 and zeoitiic tuff from the lake beds The gravelb. deposits These collecoons are held by the Los Angeles County are crudely interbedded with sand. are poordy indurated Museum of Natural History D P Whtsder wntten and lack substantial deposits of pedogenic caitche In the commun. 1983) Because me Tecopa :auna has not been central basin, the older alluwum was deposited as a true i,tudied in detail, the identifications are preiminary Slightly pediment on the eroded surface of tme lake beds. the below the levei of tuff C ilocalities 4-6. table 2) the larger erod- surface having several meters of relief vertebrate fauna is dominated by camels, including one !he morphology of the older alluvium ,s planar in the taxa that is new at the genenc level The same area has central basin. and gives way to gently concave slopes that yielded abundant micromammal fossils, which are housed nse about 100 mnfrom the basin floor to the range tronts in the Earth Sciences Department collections at the The unit forms coalescing. conical fans with steep siopes at University of California. Riverside Although the collections the foot of the bedrock ranges The older alluvium is are not formally descnbed, they include a number of typically deeply dissected by the modem washes, so that microtine rodents related to pack rats that are recognzed the unit forms fingerhke mesas near the margin of the laKe as valuable biostratigraphic indicators lRepenning. 1980. beds. May and Repenning, 19821 Evidence of Mammuthus A distinguishing feature ol the older alluvium is its locality 1 table 2) from the upper part c; the section !s surface of deser pavement and advanced development of consistent with a late Pleistocene age sod. The desert pavement consists of planar surfaces of QUATERNARY OLDER ALLVIUM densely packed pebbles and cobbles, predominantly 2-5 Older alluvium (unit Qa2 ) consists of remnants of cm in diameter. Scattered larger cobbles and boulders as alluvial fans which now form prominent mesas and raised much as 1 m in diameter commonly protrude above the areas capped by desert pavement. These remnants close-packed surface. Clasts were denved from the generally stand I to 10 m above the active washes and are underlying fan deposits: although atop the tufa benches. no longer receiving sediment A good example is the the pavement consists of flat CaCO3 plates Color of the abandoned fan in Greenwater Valley pavement surface ranges from dark red brown to black. depending on the degree of vanish development

TABLE 2.--Venebvte fossil collections (Locations am shown an map List compiled by D P Whiaer. Los Angole County Museum of Natural History ILACM)] I LocalltyLACM1209(sec. 35. T 21N,R6E) Mammuthua (mammoth] Equidae, snall spedes (homn) ct. Hernauchwto (4m11) 2 LocaiftLACM1,210(w 7, T 20N.R 7E) Eqwdlu areIap pcs cf. Ca'eiips (camel) cf Trewlcyepus (lar" camel) d. HernmbaWio 3 Locality LACM 3772.680 (Approimate lo•aton' sec 33 and 34. T 21 N. R 7 E Masmdonadw (muadoni EW&. smnall Vwc" di. Thwaonoptya di. Hemiachri Antocapnda, lag e specms (antelope) 4 Locally LACM 7104 arn 5 LocabyLACM?7106 sec.27, T 21NR 7El cf Camefop di Thanctyfpsa Cf Hemuichenm b LocaiyLACM 7108. 7109. 7111. 7112, 7113. 7132 fsec23. T 21N.R 7EE Phosý opd!1armnvgol l Mastdonudw Equxde. sinai spies Eqwidae. lampe species cf Camielo

cf Hemratchrm MtolabsNae. new gerus and speces Icamel) 12 Quartite and dolomite clasts acquire little or no varnish. ranges beyond the older alluvial fans Downcutnng has while volcanic ciasts have the darxest accumuiaflons Te been rapid in Amargosa canyon as indicated by Hoiocene planar surtace is broken by occasional nsers as much as 10 terraces now standing 3-4 m above the active stream bed cm high Major areas of recent deposition are the Ama"gosa River Directi, beneath the armor of pebbles is a layer almost bed. the playa, and areas of sheerwash on the older fan entirely composed of brown silt iA-horizon). 10-30 cm deposits. These places of deposition appear to be thick, which contains spherical cavities 1-3 mm in temporary storage areas for matenai that will eventually be diameter Orign of this silt bed has been explained as the swept through Amargosa canyon by floodwaters product ot desert soil-torming processes. or alternatively. as a widespread loess deposit. For example. Denny 09651 LATE TERTIARY AND QUATERNARY STRUCTURAL FEATURES attributed the silt to a mechanical weathering process A major episode of faulting and folding occunred after driven by wetting and divng, which causes larger rock Beds and prior to fragments to be lifted to the surface Hoover and others deposition of the China Ranch of Lake Tecopa. High-angle faults (19811 favored an aeolian origin for the silt. mainly deposition of the beds in the because the silt has a uniform composition despite the Aith vertical offsets as much as 100 m are common lithology of underlying matenal. China Ranch fanglomerates in Amargosa canyon. These conglomerate unit or the The vesicular silt is generally underlain by a buff or faults do not cut the Quatemary Beds are extensively reddish-brown pebbly B-honzon. The layer, which rarely Lake Tecopa beds The China Ranch folding is more exceeds 50 cm in thickness. is enriched in clay, iron oxide. folded as well as being faulted, although in the lacustrine mudstone unit than in the and calcium carbonate. Pebbles near the base of this zone prominent and huge slide commonly have a thin (1-3 mmn)undercoating of CaCO fanglomerate. The presence of megabreccia 3 suggests that (stage I of Gile and others. 19661 but the soil lacks dense blocks within the China Ranch fangiomerates CaCO, horizons. Although the pavement and soil horizons deformation was contemporaneous or followed soon after are commonly developed on the older alluvium, they deposition of the beds. This time interval, probably locally cap remnants of the Quaternary conglomerate unit occumng during the Pliocene. was also marked by volcanic of lakebed age. especially near the range fronts Therefore, activity which supplied pumice to the nearby drainages the surface of the Quaternary conglomerate can be similar Only minor faulting occurred in the Tecopa basin after to the surface of the older alluvium. Such areas have been the major Pliocene episode. High-angle faults having mapped as the Quaternary conglomerate, because the soil vertical displacements of as much w, 3 m were recognized forms only a thin veneer in a few areas of the Lake Tecopa beds, most commonly in an effort to date the Quaternary okler alluvium. in the low hills northeast of Tecopa Hot Springs of samples for the uranium-trend method tRosholt 1978) Deformation, probably due to differential compaction were collected from a gravel pit 2.5 km east of Shoshone. the lake beds. is localzed and minor. For example, a small Samples taken from the B-horLzon of the soil yielded an dome with dips as much as S is exposed beside California age of 160.000± 18.000 yr B P JI N. Roshoilt oral Highway 127. 3 km south of Shoshone. Disturbed beds commun., October 1981). that contain tuff B are exposed at the southwest end of HOLOCENE ALLUVIUM Greenwater Valley, but these features were probably by slumping of channel walls during early erosion The younger alluvial deposits (unit Qa1) consist of caused unconsolidated sand, st and conglomerate in active and of the Tecopa beds, Careful examination of aerial recently abandoned stream channelst The main channel is photographs and field checks fadied to detect any faults the Amargosa River bed which is a braided complex of within the Holocene or older Quaternarv alluvia deposits washes and gravel ban with mincrorelef of 0.3-2 m. Although the linear course of the Amargosa River suggests Tributary washes that oiginate on the range fronts are that it follows a fault no evidence of such a fault was commonly deeply inded (as much as 15 m) into the okler found in the flaninog terraces. Apparently, the faults which fan deposits. Near t ranges, the washes contain a caused uplift of the Dublin Hils and Resting Spring Range mixture of sand. subangular cobbles, and boulders of ceased activity prior to deposition of the Lake Tecopa dolomite, quarftl. and basalt whereas, the Amargosa beds. River bed predominantly corstsis of sand and subrounded The base of tuff B provides a datum for detecting cobbles. Pavement and varnish are poody developrd and large-scale tilt due to regional defornation or faulting sods are absent in the recently abandoned washes, which within the basin. Although elevation of the tuff ranges from are presumed to be of Holocene age. in th. central part of 430 to 540 in above sea level, almost all of the variation the Lake Tecopa beds. the recent deposfts consist of loose is attributable to a gentle dip W) of the beds toward the sit and clay which forn a small playa. When dry. the playa center of the basin. Compaction and the original lake surface develops a white efflorescence of halite and bottom topography can account for the gentle basinward gypsum IStarkey and Blackmon. 1979). The only dip. Near the lake margin, the elevation of tuff B ranges Holocene aeclian deposits of the basin are a few dunes from 495 m at Shoshone to 468 m at the northern tip of and southwest-tending streaks of sand near Tecopa Hot the Sperry Hills. The difference in elevation allows a Spings maximum south and downward tilt of 0. IV The maximnu The recent features suggest a regime in which erosion ailowable tilt in the east-west direction is 0 2ý downv-ard !o has dominated over deposition within the Tecopa basin. the west Therefore. the region has not been r'ieA Deeply incised washes are carnring coarse debns from the appreciably since about _5-,00.000 years ago

1.3 CONCLUSIONS ago (Hoover and others. 1981) About three million years ago. the Tecopa basin was As the supply of material to the adluvial fans waned. an alluviated depresson. closed at the south end by thick and as stream channels deepened. parts of the tans were fanglomerates of the China Ranch Beds These beds were abandoned as sites of deposition, and soils and pavements perhaps uplifted by faulting to form a dam for Lake began to form on surfaces of the fans The Tecopa basin Tecopa At this time, climatic conditions favored the bears no evidence of a pluvial lake of Lahontan age. formanon of a moderately deep lake rather than a playa. probably because the basin has not been closed since the as indicated by geochemical studies and depositional Amargosa River canyon formed In the Holocene. the rate features of the lake beds ýSheppard and Gude. 1968. of erosion apparently increased, because the deepening of Starkey and Blackmon. 1979) The presence of saline stream channels dominated the building of new fans minerals, magadilte. and certain zeolite minerals indicate ACKNOWLEDGMENTS that Lake Tecopa. at least in its later stages, was highly I have benefitted from geologic discussons with alkaline and saline due to evaporation. Therefore. the lake Wilfred J Can, David L. Hoover. and W C Swadley of did not have a permanent outlet during its long history. the U S. Geological Survey. and Richard L Hay of the The existence of Lake Tecopa for over 2 million years University of Illinois. Appreciation is expressed to Andrei raises the question of whether the Amargosa region was M. Sama-Woicicki (U S Geological Survey) for assistance considerably less and during the early Pleistocene as with the geochronology of Lake Tecopa. to Brigitte Smith compared to today One way to examine this question is of the University of Pars for help dunng the paleornagneuc to determine whether current amounts of spring discharge study. and to D P Whistler fLos Angeles County Museum and runoff would support a lake in the Tecopa basin if an of Natural History) for information concerning the Tecopa outlet did not exist. The ratio of drainage basin area to the vertebrate fossils. area of Lake Tecopa is approximately 42.1 (Snyder and others, 1964). Observations from Nevada indicate that REFERENCES CITED potential annual evaporation exceeds precipitation by a Blackwelder. Eliot. 1936, Pleistocene Lake Tecopa tabs I factor of roughly 5 to 25 times (Wlnograd and Thordarson. Geological Society of America Proceedings for 1935. 1975) The factor may exceed this range in the Tecopa p. 333. basin, because the average elevation (460 m) is relatively Chesterman. C. W.. 1973, Geology of the northeast low. If we assume that for the entire drainage area less quarter of Shoshone quadrangle, Inyo County. than 10 percent of the total precipitation would reach the California: California Division of Mines and Geology lake as runoff, then an evaporataonrprecipitation factor of Map Sheet 18, scale 1 24,000 4 2 or greater would preclude maintenance of a lake in the Chrisansen. R. L., and Blank. H. R.. Jr. 1972, Volcanic Tecopa basin. Additional water from springs, such as the stratiraphy of the Quaternary rhyolite plateau in group at Ash Meadows which annualy supply about Yellowstone Natiorud Park- U.S. Geokocal Survey 17.000 acre-ft (22-million mi) (Winograd and Thordarson, Professional Paper 729-B, p. 81-818, 1975). could bnng roughly 3 in. (8 cm) of water to the lake Dalrymple. G. B.. Cox, A,. and Doell, R. R., 1965. yearly, and offset only 5 percent of the potential annual Potassium-argon age and paleomagnetism of the evaporation. Therefore, the current water supply is Bishop Tuff: Geological Soc" of America Bulletin. v inadequate to balance evaporation and thereby sustain a 76, p. 665-675. perennial lake in the Tecopa basin Denny. C. S., 1965, Alluvial fans in the Death Valley Considering the inadequacy of current water sources region. California and Neva& U.S. Geokxlical to sustain a lake in the basin, precipitation or spring Survey Professional Paper 466 62 pý discharge must have been substantially greater, and Denny. C. S., and Drewes, Haald, 1966, Geology of the evaporation lower, during the existence of Lake Tecopa Ash Meadows quadrangle, Nevada-California, U S. 3.0-0.5 m y ago. Inflow and evaporation maintained a Geological Survey Buktin 1181-L p. L1-%56 balance while the basin steadily filled with sediment Drewes. Harald, 1963. Geolog of the Funeral Peak Sometime after deposition of the ash of the Lava Creek quadrangle. California. on the east flank of Death Tuff. 0 6 m.y ago. the batrier at the south end of the lake Vale. U.S. Geologic Survey Professional Paper was breached, probably by overflow. The general lack of 413, 78 p. tectonic activity during this period rules out breaching due Fleck. R. J., 1970. Age and tectonic significance ofvoicanic to tlting or faulting. The lake drained rapidly as the rocks, Death Valley are Cabtxnonw Geological Amargosa canyon developed, creating a link between the Society of America Bulletin, v. 81, p. 2807-2816. upper Amargosa basins and Death Valley. Following the Gile, L H.. Petefwn. F. F., and Growman, Rf B., 1966. creation of this link. subsidence of Death Valley due to Morphological and geneic sequences of carbonate faulting could affect rates of erosion in the Tecopa basin accumulation in desert sodls: Soil Science, v 101. p. More than about 160.000 years ago, the eroded 347-360. surface of the Lake Tecopa beds was partly buried by Haefner. Richard. 1976, Geology of the Shoshone alluval fans which extended from the nearby ranges Volcanics. Death Valley region, eastern Califomia Although the change from an erosional regime to one of California Division of Mines and Geology, Special deposition remains unexplained, a climatic change is Report 106. p 67-72 favored, because the alluvianon appears to have occurred Hanard. J. C. 1937. Paleozoic sections in the Nopah ar throughout the Amargosa region 80.000--43,0.000 years Resting Spring Mountains, Inyo County, Califomra

14 California Journal Mines and Geology. v 33. no 4. p tectonics- Cambridge University Press. New York. 358 273-339 P Hillhouse. Jack. and Cox. Allan. 1976. Brunhes-Matuyama Naeser. C W, lzern, G. A.. and Wilcox. R. E.. 1973. polarxy transition Earth and Planetary Science Zircon fission-rrack ages of Pearlerte family ash beds in Lerters v 29. p 51-64 Meade County. Kansas Geology. v. 4, p. 187-189 Hillhouse. J W Ndombi. J W M. Cox. A and Brock. Noble. L F. and Wright. L. A. 1954, Geology of the A. 1977. Additional results on paleomagnetic central and southern Death Valley region, California. srrangraphv of the Koob, Fora Formation east of Lake in Jahns. R H. editor. Geology of southern Turkana ,Lake Rudolf Kenya Nature. v 265. p California California Division of Mines. Bulletin 170 411-415 Chapter 2. p 143-160. Hoover D. L.. Swadley W C and Gordon. A. J. 1981. Repenning. C A. 1980. Faunal exchanges between Correlation charactenstics of surficial deposits with a Siberia and North America Canadian Journal of description of surficial stratigrapl'y in the Nevada Test Anthropology. v 1. p. 37-44 Site region U S. Geological Survey Open-File Report Reynolds. R L. 1977. Comparison of the TRM of the .81-512. 30 p Yellowstone Group and the DRM of some Pearlette lzent. G A. 1981. Strangraphic succession, isotopic ages. ash beds Journal of Geophysical Research. v 84. p partial chemical analyses, and sources of certain silicic 4525-4536 volcanic ash beds (4.0 to 0 1 m.y) of the Western Rosholt, J. N. 1978, Uranium-trend dating of alluvial United States U S Geological Survey Open-File deposits; in R. E. Zartman, ed.. Fourth International Report 81-763. 2 sheets. Conference, Geochronoklogy. Cosmochronology. and lIze. G A. and Wilcox, R E. 1982, Map showing Isotope Geology: U-.S. Geologia Survey Open-File localities and Lnferred distributions of the Hucklebeny Report 78-701. p. 360,-362. Ridge. Mesa Falls. and Lava Creek ash beds (Pearlette Sama-Wolcicki. A. M.. Bowman. F. R., Meyer. C. E. family ash beds) of Pliocene and Pleistocene age in Russel. P. C.. Woodward. M. J.. McCoy, Gai. Rowe, the western United States and southern Canada U S. J. J.. Jr.. Baedecker. P A.. Asaro. Frank, and Michael. Geological Survey Miscellaneous Geologic Helen, 1984, Chemical analyses. correlations, and Investigations Senes map 1-1325. 1.4,000,000 ages of upper Pliocene and Pleistocene ash layers of Izert. G. A.. Wilcox. R. E., and Borchardt G. A.. 1972, east-central and soutermnCahlfomni U.S. Geologca Correlation of a volcanic ash bed in Pleistocene Survey Professional Paper 1293. 40 p. deposits near Mount Blanco, Texas, with the Guale Sheppard. R. A.. and Gude. A J., 3d, 1968, Distribution pumice bed of the Jemez Mountains. New Mexico: and genesis of authigenic silcate minerals in ruffs of Quaternary Research, v 2. p 554-578 Pleistocene Lake Tecopa. Inyo County. California lzett. G A, Wilcox. R E. Powers, H Ak. and U.S. Geological Survey Professional Paper 597. 38 p Desborough, G. A., 1970. The Bishop ash bed. a Snyder. C. T.. Hardman, George. and Zdenek- F. F.. Pleistocene marker bed in the western U.S.. 1964, Pleistocene lakes in the Great Basin: U S Quaternary Research. v 1, p. 121-132. Geological Survey Miscelneous Geologic Love. J. D.. and Christiansen, A. C., 1980. Preliminary Investigatuons Map 1-416, scale 1:1.000,000 correlation of stratigraphic units used on I' x r Starkey, H. C.. and Blackman, P D., 1979. geologic Clay quadrangle maps of Wyoming. Wyoming mineralogy of Plitocene Lake Tecopa, Inyo County. Geological Association Guidebook Annual Field California: U.S. Geological Survey Professional Paper Conference, v. 31. p. 279-282. 1061, 3 4 p. Mankinen. E A., and Dalryrmple. G. B., 1979. Revised Steiger, R. H.. and Jager, E.. 1977, Subcommlssion on geomagnetic polarity time scale for the interv 0-5 Geochronolog. Convention on the use of decay m.y. B.P: Journal of Geophysics Research. v. 84. p. constants in geo- and coenochronology: Earth and 615-626. Planetary Science Lens, v. 36. p. 359-362, Mason. J. F. 1948. Geology of the Tecopa area. Troxel, B. W. and Heydari. Ezat 1982, Basin and range southeastern Caifornia: Geological Society of America geology in a roadcut, in Geolog of Selected Areas in Bulletin. v. 59. no. 4, p. 333-352 the San Bernardino Mountains. western Mojave May. $ R_. and Repenning, C. A., 1982. New evidence Desert and Southern Great Basin. Cahfornii Cooper, for the age of the Old Woman Sandstone, Mojave J. D.. Troxel, B. W., and Wright L. A.. eds. Desert, California: in Cooper, J. D.. compiler. Geological Guidebook Society of America, Cordifleran Section. for the Geological Society of America guidebook p. 91-96. Cordilleran Section Meeting, Anaheim, California, p. Troxel. B. W., and Wright L A, eds.. 1976. Geologic 93-96. features. Death Valley, Caifoma: Caifomia Division McAJlister. J. F. 1973. Geologic map and sections of the of'Mines and Geology Special Report 106. 72 p. Amargosa Valley borate area--southeast continuation van Eysinga, F. W. Ba, 1978. Geological time table. 3rd of the Furnace Creek area-lnyo County. California Edition: Elsevier Scientific Publishing Company. U S Geological Survey Miscellaneous Investigations Amsterdam. The Netherlands, I Sheet. Series Map 1-782, scale 1 24,000. McElhinny. M W 1973. Paleomagnetism and plate

15 Winograd. I. J.. and Thordarson. William. 1975, Wright. L. A.. 1974. Geology of the southeast quarter of Hydrogeolocgc and hydrochemical framework, south the Tecopa quadirangie, San Bernardino and Inyo central Great Basin. Nevada-Califomia: with special Counties. California California Divison of Mines and reference to the Nevada Test Site U S Geological Geology Map Sheet 20. scale 1 24.000 Survey Professional Paper 712-C. p C1-C126.

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