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Lake Tecopa In The Eastern Mojave Desert History Essay

Lake Tecopa was a shallow, hypersaline lake that occupied a single basin in the Eastern Mojave Desert at the ancient terminus of the Amargosa River. It had a drainage area of about 6,000 km2, most of its water came from its headwaters in Yucca, Timber, Shoshone Mountains and Paiute Mesa, and at its highest (and final) stage covered an area of about 235 km2. Bedrock in the ranges surrounding the basin (Nopah and Resting Springs Ranges to the east, Sperry Hills to the south, Greenwater Range, Black Mountains, and Ibex Hills to the west) consists of Neo-proterozoic through lower Paleozoic carbonates, quartzite, and Tertiary volcanic rocks, except for the Sperry Hills, which consists of the Tertiary China Ranch beds, a sequence of debris-flows, fanglomerate, pumice beds and gypsum bearing mudstone. Sediments coeval with Lake Tecopa are designated the Lake Tecopa Allogroup, which is divided into four alloformations by three tephra layers; each alloformation represents several lake cycles or parts of the lake cycles. Shallow lake conditions existed until about 1 m.y. ago, then began a trend of gradually higher lake maxima culminated ~186,000 years ago. Afterward, Tecopa Valley became breached, triggering the dissection of the basin fill exposing one of the longest climatic-stratigraphic records in the southwestern United States. The following four major erosion cycles (stream incision, pedimentation, alluviation and soil development) gave formation to four widespread pediment-terrane surfaces. The Quaternary strata are faulted, locally tilted and many displacements are clear; these sediments are the proof of late Quaternary tectonism, erosion, and groundwater recharge during pluvial episodes due to the presence of “injection springs pipes”.

Introduction: Location and Past events

The basin of former Pleistocene Lake Tecopa is in the southeastern part of Inyo County, California, about 30 km east of the south end of Death Valley. The nearest large city is Las Vegas, Nevada, which is located about 97 km to the east. The Tecopa basin can be considered a southern extension of the Amargosa Desert, which generally lies between Eagle Mountain on the south (25 km north of Shoshone) and the Bullfrog Hills on the north (120 km northwest of Shoshone). The town of Shoshone is located in the northern part of the basin on State Route 127, while the town of Tecopa is in the southern part. The lake basin is bounded on the west by Dublin Hills, Ibex Hills, and the southern part of the Greenwater Range, on the east by the southern parts of the Nopah Range and Resting Spring Range, and to the south by are the Sperry Hills, through which the Amargosa River flows. The areal extent of the Tecopa Basin is approximately 500 km2 and a dominant feature of the Tecopa area is the Amargosa River, which is dry except during infrequent thunderstorms or near permanent springs at Shoshone and Tecopa (Fig.1 - 2)

Fig.1. Map of the entire drainage basin of the Amargosa River. Important highlands are shaded. Drainage boundary above the threshold that enclosed Lake Tecopa is shown by heavy dashed line, maximum area that was inundated by Lake Tecopa by vertical lining with shaded border. Circles X indicates site of the proposed nuclear-waste repository at Yucca Mountain. B = Beatty; P = Parhump; S = Shoshone; T = Tecopa. (Morrison, R.B., and Mifflin, M. D., 2000)

Fig.2. Principal features of Tecopa Valley (Morrison, R.B., 1999)

Tecopa Valley is a hot, near-hyperarid desert; the mean-annual temperature at Shoshone is 9,5°C, and summer temperatures frequently exceed 45°C (114°F), which is only slightly cooler than in the Death Valley. The mean-annual precipitation is about 7 cm in Tecopa Valley and only a few centimeters more in the adjacent highlands, even if precipitation is highly variable and results chiefly from occasional summer rainstorms. Lake Tecopa, within this valley, was central in a long-term climatic transition zone between the subtropical-arid and the north –temperate climatic zones, whose northern boundary fluctuated over at least 6 degrees of latitude during Quaternary climatic cycles, with big effects on stream flows, lake levels, water tables and ground-water potentiometric surfaces. Over time Lake Tecopa’s depth and area fluctuated repeatedly with changing climatic and tectonic conditions and this is testified by many evidences in the sediments. However Lake Tecopa, even at its higher stages, was a comparatively small high-pH lake with weak wave action and small sediment supply. It rose to its all-time highest lake stage about 186,000 yrs ago, inundating 235 km2, ~ 2,8% of the drainage basin. Afterward Tecopa valley was breached, commencing through drainage via a deep canyon cut by the Amargosa River into the Death Valley; the subsequent erosion created extensive badlands and exposed more than 100 m of shallow to deep lacustrine sediments intercalated with subaereal, playa and paludal sediments that led the geologists to establish correlations within the basin, as well as to determine changes in lake level through time.

Previous works and investigations

The Lake Tecopa area has been the subject of many studies in the past. During their investigation of the nitrate beds, Noble and Mansfield described the mudstones and saline deposits in 1922. Noble (1926) gave a brief description of part of the Amargosa valley, while Melhase, during the same year, described the geology and the mining operations of “armagosite”, which he claimed to have named in 1920. This clay, which is actually Montmorillonite, at the time was known as “soap rock” or “natural soap”. Dietrich (1928) described the clay beds as trending north and south, as being 1.8-2.4 m thick, and as being overlain by 1.2-1.8 m of volcanic ash. Noble (1934) gave a more complete description of the rock formations in the area which led to the detailed work of Hazzard (1937) on the Paleozoic stratigraphy of the Nopah region. Foshag and Woodford (1936) were the first ones to point out the presence of Sepiolite in Lake Tecopa stating: “In the bentonitic clay of Ash Meadows [40 Km north of Shoshone] there are numerous lumps of mixed lime, magnesium carbonates and a magnesium silicate close to sepiolite”, but they did not give an exact sample site location. Wright and Toxel (1954) described the lake beds as mostly siltstone with subordinate layers of volcanic ash and bentonite clay, whereas Sheppard and Gude (1968) reviewed the early geologic studies of the Tecopa area describing the stratigraphy of the lake beds in a study that focused on the formation of authigenic silicate minerals in the tuffs. These deposits were described in detail and successfully used as stratigraphic markers in many correlations. Tertiary volcanic rocks in the Dublin hills were studied by Haefner (1976) whereas Starkey and Blackmon (1979) investigated the distribution and origin of clay minerals in the lacustrine deposits. In general the majority of the works related to Lake Tecopa have been mainly associated to the bedrock geology instead of the Quaternary deposits.

Historical geology

The earliest events recorded in the Tecopa area are represented by the Archean granite gneiss that could have intruded a body of rock eventually eroded. The deposition of the thick Pahrump series over the erosion surface characterized the late pre-Cambrian period, whereas conformable sedimentary sequences represented the great Cambrian system in the Nopah Range. Several unconformities in the post-Cambrian Paleozoic sequence in the northern Nopah Range reflect only weak diastrophism (Hazzard, 1937a). The deposition of calcareous sediment such as Bonanza King and Cornfield Springs formations may have continued at intervals until the Permian, whereas, between the Carboniferous and the Tertiary, the older rocks were uplifted and partly eroded in a probably cumulative movement. It is even suggested that the east-going thrust on the east flank of the Nopah Range, which put Cambrian on Carboniferous, is a Laramide fault and is the oldest one in the area. The Tertiary history of the region began with extrusion and probably intrusion, closely followed by deposition of ash and debris in Miocene – Pliocene lake beds. After an intense period of tectonic activity that interposed fingers of Cambrian in the Tertiary volcanic west of Shoshone, erosion developed within the ranges and lake beds and basalt were deposited. The final elevation of the range blocks and the depression of the valley were accomplished by broad northwesterly warping of the very complex basement of these deposits; the flanks of the resulting anticlines were elongated by normal faults, called “Basin Range faults”. After the ranges were thus blocked out, a Pleistocene lake occupied much of the intermont valley area. Lake deposition was followed and accompanied by transport and deposition of great volumes of gravel establishing an outlet across the gravel ridge southward and many settling movements: those movements caused the lake deposits to warp forming minor faults. The present alluvial cycle followed.

Stratigraphy

The stratigraphic column in the Death Valley area includes Pre-Cambrian, Paleozoic, Tertiary and Quaternary deposits. In the Tecopa area, no post-Cambrian Paleozoic rocks, except a few Carboniferous, are present. According to Hillhouse, J. W. (1987) we can distinguish:

Archean, Proterozoic, and Cambrian rocks

The oldest rocks within the study area are granitic gneiss at the southwest end of the Nopah Range and in the Ibex Hills. Chesterman (1973), analyzing metadiorite dikes in gneiss, assigned them an Archean age. In the rugged cliffs of the eastern Dublin Hills is exposed a nearly complete succession of uppermost Proterozoic to Middle Cambrian marine sedimentary rocks. The sequence, ascending, consists of Noonday Dolomite (Proterozoic), Johnnie Formation (Proterozoic), Stirling Quartzite (Proterozoic), Wood Canyon Formation (Proterozoic and Lower Cambrian), Zabriskie Quartzite (Lower Cambrian), Carrara Formation (Lower and Middle Cambrian), and the Bonanza King Formation (Middle and Upper Cambrian).

Tertiary Volcanic Rocks

Volcanic flows and tuffs, predominantly of rhyolitic composition, are exposed in the western Dublin Hills and the Resting Spring Range. Haefner (1976) applied the name “Shoshone Volcanics” to the pyroclastic rocks of the Dublin Hills. The Shoshone Volcanics, which consist of pumice lapilli tuff and welded tuff, are part of a large Tertiary volcanic terrane exposed throughout the Greenwater Range and the Black Mountains. An excellent example of the pyroclastic rocks is exposed in the road cut where California Highway 178 crosses the Resting Spring Range (Troxel and Heydari, 1982). The cut exposes devitrified pumice tuff, welded tuff, and vesicular vitrophyre, dated by K-Ar methods at 9.5 Ma (R.E. Drake, oral commun., November, 1982).

Tertiary Basalt

Isolated remnants of basalt are exposed 1 km north of Shoshone and in the vicinity of the southern Dublin Hill the basalt and related tuff directly overlie Paleozoic carbonate rocks. The basalt forms black rounded hills strewn with blocks as much as several meters in diameter. Just north of Shoshone the basalt overlies buff-colored tuff that consists of rounded pumice cobbles, volcanic rock fragments, and pumiceous matrix. Chesterman (1973) assigned a Quaternary age to the basalt and stated that the unit overlies deformed lacustrine deposits of Pleistocene Lake Tecopa. Because the stratigraphic relation of the basalt to the Shoshone Volcanics is not demonstrated and because basalt eruptions occurred sporadically in the Amargosa region throughout the late Tertiary and Quaternary, only tentative conclusions can be made concerning the timing of basaltic volcanism at Shoshone.

Tertiary China Ranch Beds

Debris flows, fanglomerate, pumice beds, and gypsiferous mudstone in the Sperry Hills compose the China Ranch Beds, which are well exposed in the Amargosa canyon south of Tecopa. Mason (1948) originally applied the name to the light-colored mudstones and gypsiferous beds that crop out in the vicinity of China Ranch. However, Wright (1974) redefined the unit to include the coarse fanglomerate that interfingers with the gypsiferous mudstones. The fanglomerate lithology of the

China Ranch Beds is prevalent in the Alexander Hills and in the western part of the Sperry Hills. Lenses of white pumice in a white lapilli matrix, 5 m thick, fill ancient channels within the fanglomerate; the pumice bed is besides traceable southward from the fanglomerate into gypsiferous mudstones extensively faulted and deformed. The presence of huge blocks and megabreccias within the fanglomerates suggests that uplift and accelerated erosion were affecting the highlands throughout the region. This tectonic activity, which probably occurred during the Pliocene, was accompanied by local volcanic activity. Moreover, the gypsum beds suggest a shallow lacustrine environment or playa, where evaporation was dominant.

Quaternary Lake Beds, Conglomerate, and Tufa

During the middle to late Pleistocene, the Amargosa River was dammed by alluvial fan deposits south of the present-day site of Tecopa (Sheppard and Gude, 1968). Waters began to accumulate behind the dam with simultaneous deposition of fine-grained detritus. Later, the ash fall, which subsequently produced tuff C, was laid down, followed by the deposition of more detrital mudstone. The deposition of these mudstones was also interrupted periodically by light ash falls, one of which was large enough that it has been termed the “intermediate tuff”. The ash fall which produced tuff B was next deposited. By this time the concentration of the magnesium and the pH of the lake water had increased to the point that some sepiolite was being formed by the reaction of the magnesium-rich waters and the silica from the ash falls which had been taken into solution by the lake waters. More detrital material was then deposited as mudstone, followed by another ash fall which produced tuff A. Again, magnesium-bearing waters of the lake combined with the plentiful silica to produce even more sepiolite in association with tuff B. This was the last major ash fall although the presence of montmorillonite and sepiolite above tuff A indicates that later there was at least one minor ash fall. Detrital material was deposited as long as the lake existed. The deposits of Lake Tecopa (fig. 2a), that occupy the low region between Shoshone and Tecopa and the southern end of Chicago Valley, are divided into three informal units that are mapped separately: (1) lacustrine mudstone, (2) conglomerate, and (3) tufa.

Lacustrine mudstone

A cumulative thickness of 72 m of mudstone, claystone, and volcanic ash is exposed, although the level of erosion has not reached the base of the lake deposits in the central part of the basin. Badland topography developed on the lacustrine mudstone unit generally consists of low rounded mounds with knobby surfaces caused by the wetting and dying of expandable clay minerals. Several volcanic ash beds, ranging in thickness from 1cm to 4 m, are present in the lake deposits. Of the 12 ashes that are recognized, three form thick beds that are mappable throughout the basin. Sheppard and Gude (1968) provided detailed descriptions of the three thick ashes: Tuffs A, Tuff B, and Tuff C (from youngest to oldest).

Tuff C – Tuff C is the lowermost thick ash; stratigraphically 17 m above the deepest exposure of the lake beds, came 2.1 Ma ago from the Yellowstone park area as a member of the “Pearlette family” of tephra. Tuff C is 10 to 75 cm thick and is best exposed along the west side of California Highway 127, where the altered tuff forms a resistant ledge containing green nodules of silicified clay. Distinctive features of the tuff are the highly contorted and well-laminated upper part and the deformed base, which is generally warped into a wavelike surface with amplitudes of 50 cm and wavelengths of several meters;

Tuff B – This layer records a gigantic phreatic eruption ~ 758 ka from the Long Valley caldera, near Bishop, California. Tuff B is approximately 18 m stratigraphically above the base of Tuff C. The stratigraphic interval between the two tuffs, which is predominantly claystone, is poorly exposed except at the southwest part of the lake basin. Tuff B is thick (0.3 to 3.5 m) and can be traced almost continuously from the Sperry Hills, along California Highway 127 to Shoshone, then along the west flank of the Resting Spring Range to Chicago Valley. The southern exposures of tuff B are mostly associated with deep-water mudstones, whereas elsewhere the tuff is exposed in shallow fluvial deposits near the ancient shore. The freshest examples of tuff B are near Shoshone and along the Resting Spring Range, where the ash stands out as a white band between underlying pink mudstone and overlying green claystone;

Tuff A - Tuff A is the youngest member of the “Pearlette family” of tephra from the Yellowstone Park area; is 0.5 to 4 m thick, and is exposed around the perimeter of most of the basin, even if it was not recognized in Chicago Valley. The 4-8 m interval that separates Tuff B from the overlying Tuff A generally consists of green brittle claystone. Tuff A, whose best exposures are 1 km south of Shoshone, where the 4-m-thick bed consists of silver gray vitric tuff with many laminar interbeds several centimeters thick, is overlain by about 10- 30 m of lacustrine sandy mudstone and conglomerate. The tuff contains only minor amounts of locally derived detritus, although there is ample evidence that the upper part of the bed has been reworked by water currents: well-preserved ripple laminations indicate a south-southeast current direction.

Conglomerate

This unit consists of fanglomerate and pebbly fluviatile deposits that interfinger with the lacustrine mudstone unit. Typical exposures of the conglomerate unit are present in the gravelly piedmont that flanks California Highway 127 north of Shoshone, where the unit consists of coalescing alluvial fans at the foot of the surrounding ranges. The fan deposits, which are at least 50 m thick, consist of sand, gravel, and subangular cobbles composed of dolomite, quartzite, basalt, and silicic volcanic rocks. Small remnants of the conglomerate unit are exposed at the foot of the Dublin Hills, the Sperry Hills, and the Resting Spring Range.

Tufa

The white flat-topped mounds along the western side of the Resting Spring Range are thick accumulations of tufa. The tufa commonly consists of dense CaCO3 caprock, 2 to 4 m thick, over a layer of powdery white CaCO3 that generally contains fossil ostracodes, thin-walled bone fragments, and plant stems. Caprock composition varies from cemented gravel to nearly pure fine-grained CaCO3 with stacked flattened pores or fenestrae. The tufa forms two parallel discontinuous benches that are about 100 m apart laterally, several meters apart vertically, and which apparently follow the eastern shoreline of the ancient Lake Tecopa. Features of the shoreline such as beach gravels and sands are locally preserved in tufa, although the more typical mounds appear to have grown as wedges within mudstone beds near the stratigraphic levels of tuffs A and B (Sheppard and Gude, 1968).

Quaternary Older Alluvium

Older alluvium consists of remnants of alluvial fans capped by desert pavement. These remnants stand 1 to 10 m above the active washes and are no longer receiving sediment. A good example is the abandoned fan in Greenwater Valley. Clasts consist of subangular dolomite, quartzite, and silicic volcanic rocks from the surrounding ranges, and tuff and zeolitic tuff from the lake beds. The morphology of the older alluvium is planar in the central basin, and gives way to gently concave slopes that rise about 100 m from the basin floor to the range fronts. A distinguishing feature of the older alluvium is its surface of desert pavement and advanced development of soil. The desert pavement consists of planar surfaces of densely packed pebbles and cobbles, predominantly 2-5 cm in diameter. Clasts were derived from the underlying fan deposits, although two different theories developed over time. Denny (1965) attributed the formation of the silt to a mechanical weathering process driven by wetting and dying, which causes larger rock fragments to be lifted to the surface. Hoover and others (1981) favored an aeolian origin for the silt, mainly because the silt has a uniform composition despite the lithology of underlying material.

Holocene Alluvium

The younger alluvial deposits consist of unconsolidated sand, silt, and conglomerate in active and recently abandoned stream channels. In the central part of the Lake Tecopa beds, the recent deposits consist of loose silt and clay which form a small playa. When dry, the playa surface develops a white efflorescence of halite and gypsum (Starkey and Blackmon, 1979). The only Holocene aeolian deposits of the basin are a few dunes and southwest-trending streaks of sand near Tecopa Hot Springs. By the way the Holocene terraces now standing 3-4 m above the Amargosa Canyon suggest a rapid downcutting. (Fig. 3)

Fig.3. Schematic geologic cross section of the deposits of Lake Tecopa: lacustrine mudstone unit (Qtlm) including tuffs A, B, and C of Sheppard and Gude (1968), conglomerate unit (tc), and tufa. Also shown are basement rocks (Tau) composed of Archean gneiss, Proterozoic and Paleozoic sedimentary rocks, Tertiary volcanic rocks, as well as older Quaternary alluvium and Holocene alluvium. (Hillhouse, J. W., 1987)

Lake Tecopa Allogroup

The Lake Tecopa Allogroup is a sequence of four alloformations: the Spanish Trail Alloformation (AF) (that is the lowest), Greenwater fan AF, Shoshone Spring AF, and Amargosa AF. The three prominent tephra layers that bound and separate these AFs are the already described “Tuff A” (Lava Creek B tephra layer, 665 ka), “Tuff B” (Bishop tephra layer, 758 ka), and “Tuff C” (Huckelberry Ridge tephra layer, 2,1 Ma)

Spanish Trail AF (2 to 5 Ma)

This AF includes all sediments below the 2,1 Ma Tuff C. These are shallow-lake, playa, and fine basin-interior alluvium and are exposed only in the south-central interior of Tecopa Valley. Lithology of exposed Spanish Trail sediments indicates that sedimentation rates were at least as low as those of late Greenwater Fan time.

Greenwater Fan AF (2 to 0,74 Ma)

The Greenwater Fan AF consists of sediments between Tuff C and B, 2,1 Ma – 758 ka. It has three members:

The lower member is a shallow-lake, playa and fine basin interior alluvium;

The middle member, ~1,2 Ma – 0,9 Ma, is entirely playa and playa margin alluvium, recording a long lasting arid interval (Reheis and Morrison, 1997);

The upper member is shallow-lake, playa, and basin interior fine alluvium, that record the beginning of a long series of lake cycles with up-trending maxima.

Shoshone Spring AF (738 to 620 ka)

Deposition of the Shoshone Spring AF started with that of Tuff B (758 ka), at about midpoint of a lake transgression, shortly before the maximum of the first moderately high lake cycle of Lake Tecopa. Then ensued a long, deep lake recession marked by eolian and reworked sand. Finally came the early part of a moderate lake cycle whose rise continued until somewhat after the deposition of Tuff A. Exposures of the Shoshone springs AF are in the intermediate piedmont of the Tecopa basin; they consist of shallow-lake sand, silt, and clay, with a tongue of loessial and colluvial sandy silt and paleosols at least as low as 430 m altitude. Sedimentation rates were considerably greater than in the Greenwater Fan or earlier time.

Amargosa AF (620 to about 160 ka)

The Amargosa AF comprises the Lava Creek B tephra layer and overlying sediments as young as the youngest Lake Tecopa deposits. After deposition of the Lava Creek B tephra layer, lake level rose to about 500 m, then fell below 485 m, and finally rose to Lake Tecopa’s all-time maximum at 550 m about 160 ka. At this maximum Lake Tecopa was between 45 and 90 m deep. Lacustrine sediments of the Amargosa AF range from pebble gravel to sand, silt and clay. Although the Amargosa AF has the deepest lake and coarsest sediments of the entire Lake Tecopa Allogroup, its shore-gravel beds are local. The pebbly beds usually represent ancient lake bars, spits, and shore terraces, and they are best developed in the upper part of the basin, east and north of Shoshone, because the chief sediment supply was the Amargosa River.

Fig.4. Distribution of the Spanish Trail, Greenwater Fan, Shoshone Spring, and Amargosa Alloformations in Tecopa Valley as indicated by outcrops of Tuffs A, B, and C (Huckleberry Ridge, Bishop, and Lava Creek B tephra layers, respectively). T – Tecopa, S – Shoshone, circled ! – lowest point in the divide between Tecopa and China Ranch basins east of the Amargosa Canyon, 503 + 3 m altitude. (Morrison, R.B., and Mifflin, M. D., 2000)

Minerals: where and why

Quartz

Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. Stratigraphically the greatest amounts of quartz in this area are found in the youngest sediments above tuff A. In the center of the basin the sediment analyzed by Harry C. Starkey and Paul D. Blackmon in the work “Clay Mineralogy of Pleistocene Lake Tecopa, Inyo County, California”, contains no quartz. Changes in source and volume of the incoming sediments could easily account for stratigraphic variation in the quantity of quartz and changes in lake water chemistry: during periods of fresh water incursion the pH of the lake waters would have been lowered producing conditions in which the quartz would have been more stable. Conversely, during dry periods, evaporation of the lake waters would have caused an increase in salinity and pH.

Carbonates

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is found throughout the lake beds, and outside the lake former boundaries. However, some sediments near the entrance of present intermittent streams or close to the mountains, where runoff from storms would most likely have been heavy, showed little or no calcite, as in the area where Greenwater valley enters the Lake Tecopa basin. The largest amount of calcite is present at the northern end of the valley where the Amargosa river flowed into the lake: in that area the fresh waters of the river mixed with the carbonate rich lake brines of high pH and calcite precipitated.

Lithian Saponite

Saponite is a soft, massive, and plastic mineral that belongs to the Montmorillonite group. It usually exists in veins and cavities in Serpentinite and basalt and its name is derived from the Greek sapo, soap. The most widespread clay mineral in the area is Lithian Saponite, which is found not only within the lake Tecopa basin, but also in Chicago Valley and along the Amargosa river upstream. Sand and Ames (1958) said, in their investigation of altered silica volcanics, that saponite is often formed as a result of hot-springs activity where pyroclastic are deposited in alkaline layers.

Montmorillonite

Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. It is the main constituent of the volcanic ash weathering product, bentonite. Montmorillonite, in the lake Tecopa area, appears as an authigenic mineral that has two types of origin. “Amargosite”, found just west of Shoshone, is a soft, sticky and moist white montmorillonite, despite the desiccating effect of the present climate. This clay was formed by the hydrothermal alteration of a volcanic ash by warm springs full of salt (Melhase, 1926). The other type of montmorillonite is found very close to the tuffs deposited in the lake itself and was formed by the action of the saline lake waters on the siliceous glass of the tuff.

Sepiolite

Sepiolite is a clay mineral, a complex magnesium silicate. The name comes from a perceived resemblance of the material to the porous bones of the cuttlefish or sepia. On account of its low specific gravity and its porosity, it may float upon water, hence its German name “Meerschaum” (sea foam). Sepiolite is one of the most abundant of the possibly authigenic clay minerals found in the mud and siltstone beds adjoining the tuffs in the Tecopa basin. This sepiolite is most prevalent in a narrow band near the upper limit of the stratigraphic level of tuff A: most of the sepiolite is found within a meter or two of the tuff, usually just below its base. A few other, scattered, sepiolite-bearing sediments were detected near tuff B, and occasionally throughout the stratigraphic column above tuff B, especially near the lake margins. Little or no sepiolite has formed directly in the tuff formations, except occasionally in clay partings, which were deposited in the tuffs.

Several possible sources were considered for the sepiolite in the Tecopa basin sediment:

Clastic deposition was ruled out because most of the sepiolite was concentrated in the vicinity of the ash beds; a more uniform distribution throughout the strata would be expected if the sepiolite were of clastic origin;

Direct diagenesis from the volcanic ashes is unlikely. Little or no sepiolite was found within the tuff formations. It occurred most frequently within a few feet above or below the ash beds;

A transformation of a parent smectite clay to a sepiolite, as suggested by Parry and Reeves (1968) for sediments in pluvial Mound Lake, Tex., was also considered. There is even the possibility that slight dissolution of the mineral in a high-pH environment may have supplied some of the silica and magnesium necessary to authigenically form the sepiolite;

The sepiolite may have been precipitated directly from solution. The principal mode of sepiolite precipitation involved probably even post-depositional factors.

Hydrogeology

Past conditions

During the last pluvial episode hydrogeologic conditions were very different in the Tecopa Valley. The potentiometric surface rose 50-100 m higher than now, causing groundwater discharge at many sites now inactive, such as the fault zone along the western piedmont of the Resting Spring Range. During the two main pluvial episodes (~250-125 ka and ~100-50 ka), two types of groundwater discharge structures, called tufa mounds and “injection spring pipes”, were created in different parts of the Tecopa Valley.

Tufa

Tufa deposits in the Tecopa Basin reflect the response of arid groundwater regimes to wet climate episodes. Two different types of tufa are present, informally defined as “an easily disaggregated, fine grained mixture of calcite and quartz” (friable tufa - southwest Tecopa Valley), and “hard, vuggy micrite, laminated carbonate, and carbonate-cemented sands and gravels” (indurated tufa - eastern margin of Lake Tecopa). On the basis of field relations, petrography and isotope systematics, friable tufa represent discharge of regional groundwater directly into evaporated water of pluvial Lake Tecopa during wet climate periods. On the other hand, indurated tufa represents discharge of groundwater along faults conduits. However, these deposits include ledges stratabound within lake sediments, from which could be inferred that discharge occurred near the lake margin, although some deposits clearly postdate the lake.

Injection spring pipes

On Greenwater Fan scores of extinct artesian-springs structures are exposed “injection sprigs pipes” (name given by R.B Morrison). These are unusual features roughly cilindrical and 2-30 m in diameter, probably resulted from tectonic activity that produced local pull-aparts with small faults and random fissures providing conduits for these features. These pipes tend to converge downward, resembling classic dikes, and are entirely in the lower ~10m of the Amargosa AF, overlying highly contorted Tuff A, that is above a silty clay bed that became liquified by an earthquake while still water-saturated, ~ 479 ka (E. Larson, University of Colorado, written commun., 1990)

Fig.5. A: Diagrammatic cross section through a typical tufa mound complex on the western piedmont of the Resting Spring Range. It formed by intermittent carbonate deposition from artesian water rising in a fault zone, induced by temporary rises in the potentiometric surface during more than a single middle Pleistocene pluvial event. B: Diagrammatic cross section through a typical injection spring pipe in Greenwater Fan. It formed by sudden forceful artesian-water injection up a fissure conduit during a severe earthquake in early Amargosa AF time. The pipe originated from a single tectonic event during a pluvial episode, and its surface expression was a “mud spring”. Lasting at least decades, perhaps centuries and occasionally trapping animals. (Morrison, R.B., 1999)

Present conditions

The lowest parts of southern Tecopa Valley are groundwater discharge areas, marked by perennially moist ground, coated with salt and other saline minerals; these zones also manifest slow artesian seepage. The oases of Shoshone, Tecopa, Tecopa Hot Springs, and Resting Springs are the artesian springs that well up under pressure from depth along conductive faults and fissures. Such discharge indicates an extensive potentiometric groundwater surface positioned above the lower parts of the Tecopa Valley and fed from the Deep Acquifer System (DAS).

War Eagle Mine

The War Eagle Mine is located 12 kilometers east of Tecopa, at the south end of the Nopah Range, and operated from 1912 until 1957. Despite the extensive mine workings and large tonnage of ore removed from this mine and the adjacent Columbia and Noonday Mines, just a little has been published about it. Mineralization appears to lie along a nearly north-trending vein which dips moderately to the east. A brief examination of the adjacent mines suggests the main ore vein is offset by a post mineral fault system which strikes N 65 W. A series of northeast trending fractures with steep northwest dips are thought to control localization of the ore bodies (Newman and Stewart, 1951). It is even known that the existing crosscut was extended for more than 175 meters and that the access was via a series of raises and an inclined shaft. Ore mineralization was restricted to argentiferous galena, but traces of chalcopyrite and pyrite were noted in stopes. Native gold has been reported (Newman and Stewart, 1951).

Fig.6. Partial Geologic Map of the War Eagle Mine


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