Additional Details

Description

11/12/2009 Abstract Searles Valley is an arid, closed basin lying 70 km east of the south end of the Sierra Nevada, California. It is bounded on the east and northeast by the Slate Range, on the west by the Argus Range and Spangler Hills, and on the south by the Lava Mountains; Searles (dry) Lake occupies the north-central part of the valley. During those parts of late Pliocene and Pleistocene time when precipitation and runoff from the east side of the Sierra Nevada into the Owens River were much greater than at present, a chain of as many as five large lakes was created, of which Searles Lake was third. The stratigraphic record left in Searles Valley when that lake expanded, contracted, or desiccated, is fully revealed by cores from beneath the surface of Searles (dry) Lake and partly recorded by sediments cropping out around the edge of the valley. The subsurface record is described elsewhere. This volume includes six geologic maps (scales: 1:50,000 and 1:10,000) and a text that describes the outcrop record, most of which represents sedimentation since 150 ka. Although this outcrop record is discontinuous, it provides evidence indicating the lake's water depths during each expansion, which the subsurface record does not. Maximum- depth lakes rose to the 2,280-ft (695 m) contour, the level of the spillway that led overflowing waters to Panamint Valley; that spillway is about 660 ft (200 m) above the present dry-lake surface. Several rock units of Tertiary and early Quaternary ages crop out in Searles Valley. Siltstone and sandstone of Tertiary age, mostly lacustrine in nature and locally deformed to near-vertical dips, are exposed in the southern part of the valley, as is the younger(?) upper Miocene Bedrock Spring Formation. Unnamed, mostly mafic volcanic rocks of probable Miocene or Pliocene age are exposed along the north and south edges of the basin. Slightly deformed lacustrine sandstones are mapped in the central- southwestern and southern parts of the study area. The Christmas Canyon Formation and deposits mapped as older gravel and older tufa are extensively exposed over much of the basin floor. The older gravel unit and the gravel facies of the Christmas Canyon Formation are boulder alluvial gravels; parts of these units are probably correlative. The lacustrine facies of the Christmas Canyon Formation includes the Lava Creek ash, which is dated at 0.64 Ma; the older tufa deposits may be equivalent in age to those sediments. Most of this study concerns sediments of the newly described Searles Lake Formation, whose deposition spanned the period between about 150 ka and 2 ka. Most of this formation is lacustrine in origin, but it includes interbedded alluvium. To extract as much geologic detail as possible, criteria were developed that permitted (1) intrabasin correlation of some thin outcrop units representative of only a few thousand years (or less), (2) identification of unconformities produced by subaerial erosion, (3) identification of unconformities produced by sublacustrine erosion, and (4) correlation of outcrop units with subsurface units. The Searles Lake Formation is divided into seven main units, many of which are subdivided on the five larger scale geologic maps. Units A (oldest), B, C, and D are dominantly lacustrine in origin. The Pleistocene-Holocene boundary is placed at the top of unit C. In areas that were a kilometer or more from shore at the time of deposition, deposits of units A,B, and C consist of fine, highly calcareous sand, silt, or clay; nearer to shore they consist of well-sorted coarse sand and gravel. Unit A has been locally subdivided into as many as four subunits, unit B into six subunits, and unit C into six subunits. The finer facies of units A, B, and C contain such high percentages of Caco3 that they are best described as marl. Sediments of unit C, and to a lesser extent those of unit B, are laminated with light- to white-colored layers of aragonite, calcite, or dolomite(?) that may represent seasonal deposits. Unit D, which is divided into two facies, is a pinkish silt with halite efflorescences that is locally overlain by a thin lacustrine gravel. Units AB, BC, and CD, which are interbedded between the two single- letter units that make up their names, include both alluvial and lacustrine deposits. Unit AB is divided into as many as seven subunits, unit BC into two subunits, and unit CD is undivided. Where these units are alluvial, their lithologies range from pebble sand to boulder gravel that is poorly sorted and bedded. Soils developed on each unit are distinctive, and in most instances they serve as a basis for tentatively identifying each unit throughout the basin (as well as constraining the possible ages of overlying and underlying lacustrine sediments). Where these units are lacustrine, they are most commonly shallow water, well sorted sand. The depositional intervals of the units that make up the Searles Lake Formation are estimated to be as follows (all estimated ages are in 'uncorrected 14C years'): unit A, 150 ka to 32 ka; unit AB, 32 ka to 24 ka; unit B, 24 ka to 15 ka; unit BC, 15 ka to 13.5 ka; unit C, 13.5 ka to 10.5 ka; unit CD, 10.5 ka to 6.0 ka; unit D, 6.0 ka to 2.0 ka. These ages are determined primarily by correlation with subsurface deposits that are dated by 14C years dates on organic carbon and U- series dates on salts. Late Holocene deposits are mapped as (1) older alluvium, (2) playa silt, (3) windblown sand and colluvium, (4) windblown dune sand, and (5) active alluvium. Tufa, deposited by a combination of organic and inorganic processes, is widespread. Major periods of growth of the relatively structureless 'lithoid' variety of tufa occurred early during deposition of unit A, and deposition of the nodular, tubular, or lobate 'nodose' variety of tufa occurred near the end of unit B time and the beginning of unit C time. Volumetrically, most of the tufa grew near the 1,800-ft (550 m) and just below the 2,280-ft (695 m) levels; the largest masses occur along the west side of the basin (see cover image) and in an area named The Pinnacles, where towers as high as 30 m are found. Fossil snails, clams, and ostracods are found in many parts of the Searles Lake Formation, but they are especially abundant near the paleoinlet area. The snails and clams indicate fresh water conditions, which occurred most commonly in the inlet area. Ostracods tolerate a wider range of salinities and occur more widely. Vertebrate fossils were not found. Shorelines, the most obvious geologic expressions of former lakes, are abundant around Searles Valley. Erosional shorelines have cut as much as 100 m into brecciated bedrock; depositional shorelines (beaches or tufa benches) are common, but their deposits tend to be thin. Gracefully curving offshore bars and spits, 2 to 10 m high, are prominent in the north and south ends of Searles Valley and near the middle of its west side. They were deposited subparallel to the shoreline, but their crests vary in elevation by as much as 10 m and it is clear that they were not deposited a uniform distance from shore. Most bars are asymmetrical, having slopes as steep as 25 deg on the uphill side but no more than 5 deg on the downhill side. Several faults are mapped as having displacements of late Cenozoic age. The left-lateral Garlock Fault, 250 km long, bounds Searles Valley on the south and is the most geologically significant fault in the area. It displaces several of the mapped units of late Quaternary age but does not displace active or older alluvium deposits of late Holocene age. A prominent graben along the east side of the valley, and normal-fault extensions of it to the north and south, also represent Holocene displacements; alluvial deposits of the early Holocene unit CD are displaced, but late Holocene older alluvium deposits are not. Certain paleolimnological aspects of the lakes that deposited the Searles Lake Formation can be reconstructed. Lake-water chemistry and pH are approximated from subsurface data on the mineralogies of salt layers, from outcrop data such as the mineralogy of salt efflorescences, and from the reconstructed salinities indicated by ostracod or mollusk suites. Laminated sediments are believed to indicate salinity stratification in the depositing lake; this requires the lower saline layer to be sufficiently dense to keep the stratification stable. Even when the lake was stratified, however, large amounts of Caco3 were deposited near the paleoinlet, where Ca-bearing inflow waters soon mixed with the high-pH, CO3-rich lake waters, creating a 'chemical delta'. Nearshore high-energy environments of deposition in Searles Valley lakes produced beaches and bars composed of coarse sand and gravel. Many of these deposits are now well cemented by calcite. During deposition of the Searles Lake Formation, the deposits of unit A were composed of larger fragments than were the later deposits of unit B, which, in turn, were coarser than those of unit C, apparently indicating a long-term decrease in storm-wind intensities. Storm-current directions, reconstructed from the fragment composition of offshore bars, also indicate differences in storm-wind directions at different times and lake levels. Combining the subsurface evidence of lake history with the evidence described herein allows the history of lake fluctuations to be reconstructed for the period between about 150 ka and the present. Between 150 ka and 140 ka, Searles Lake rose from a depth below the present dry-lake surface to its spillway level, now approximately at elevation 2,280 ft (695 m), 780 ft (235 m) above its floor at that time. From then until about 118 ka, it overflowed into Panamint Valley. Following this period of overflow, lake level lowered and fluctuated at intermediate levels until 35 ka. There was possibly a brief period of overflow at about 50 ka. These intermediate- to high-level stands were separated by brief lower level stands that are estimated to have occurred at periods centered at 107 ka, 79 ka, 69 ka, and 45 ka. Between about 35 ka and 23 ka, Searles Lake fluctuated every 1 to 2 thousand years between intermediate depths (when silt and clay were deposited) and shallow depths (when it deposited salts); it desiccated briefly at about 28 ka. This low- to intermediate-depth stage was followed by one of increased inflow, culminating in a period of overflow at about 16 ka. After falling to a low level between 15 ka and 13.5 ka, the lake again rose twice to overflow levels, then fell to desiccation levels at about 10.5 ka. Searles Lake has remained mostly dry since its desiccation at 10.5 ka, except for a period centered at about 3.5 ka when the lake surface rose to a level of about 1,800 ft (550 m). Translating this record of lake fluctuations into paleohydrologic and paleoclimatic histories is complicated by uncertainties as to which of the several components of climate affected runoff volumes and lake-surface evaporation. A simplified model, however, suggests that the flow of the Owens River stayed between 2.5 and 4.5 times its present flow volume for most of the past 150 k.y. Its flow exceeded this range only about 14 percent of the time, and it fell below this range only 4 percent of the time - which includes the present. In fact, the past 10 k.y. is clearly the driest part of the past 150 k.y. Over the full 3.2-m.y. period known from subsurface data, the climatic record is influenced by a geologic factor: the ever-increasing 'rain shadow' produced by continuing uplift of the Sierra Nevada. Meteorological principles and subsurface evidence from Searles Valley and elsewhere support the existence of this climatic trend toward aridity, yet the longest sustained period of desiccation of Searles Lake was in the first third of that 3.2-m.y. period, as was the longest period of continuous perennial-lake deposition. This and many other lines of evidence show that the lake oscillations in Searles Valley have been products of climatic changes that exceed the impact of geologic and topographic changes. The durations of the most persistent and extreme climatic regimes in the Searles Valley area resemble phenomena found in the deep-sea record that have a 413-k.y. cycle length, which can be attributed to the Earth's orbital perturbations. During 'intermediate' climatic regimes, however, the shorter duration, less extreme climatic changes in the Searles Lake record appear to reflect the orbital perturbations of shorter cycles that may have controlled the timing of high-latitude glacial events. It remains to be seen whether the glacial history of the east slope of the Sierra Nevada followed a timetable dictated by cooler temperatures related to high- latitude glaciation, or one that followed the 413-k.y. precipitation cycles recorded downstream by Searles Lake.

Survey Date

2009

Print Date

2009

Height In Inches

11.000

Width In Inches

0.750

Length In Inches

8.500

Two Sided

Yes

Pieces

5

Languages

English