ABSTRACT The Searles Lake Formation in Searles Valley, southeastern California, represents deposition of the paleo–Owens River into a Pleistocene and Holocene pluvial terminal lake. A prior 32–10 ka estimated age for the upper part of the Searles Lake Formation relied on uncalibrated, conventional radiocarbon dates. We present accelerator mass spectrometer radiocarbon dates that indicate the base of the Searles Lake Formation at the Poison Canyon type section is 46 ka. That age is consistent with paleomagnetic data at Poison Canyon and the Tire Farm locality, which record high-latitude Southern Hemisphere virtual geomagnetic poles that we assign to the 41 ka Laschamp excursion. The presence of Searles Lake at 46–43 ka also is consistent with a Pacific storm track that extended south of 37.5°N at that time. At the head of Salt Wells Valley–Poison Canyon, sediments that we interpret as a Searles Lake highstand were radiocarbon dated at 14.1 ka.

Abstract Driven by requests to provide carbonate analogs for subsurface hydrocarbon exploration in rift settings, we have identified and described select examples, summarized them from a carbonate perspective, and assembled them into a GIS database. The analogs show a spectrum of sizes, shapes and styles of deposition for lacustrine and marginal marine settings, wherein the types of carbonates inferred from seismic and cores (emphasis on microbialites, tufas, and travertines) can be illustrated.

From the late Miocene to the middle Pliocene, the current drainage basin of the Owens River probably consisted of a broad, moderate-elevation, low-relief plateau with radiating drainage toward the Pacific Ocean, the northwestern Great Basin (now Lahontan drainages), and the Mojave and Colorado drainages. This plateau probably contained shallow basins, created by an extensional pulse at 12–11 Ma, at the present locations of major valleys. Between 4 and 3 Ma, this plateau was disrupted by a rapid westward step of extensional Basin and Range Province tectonism, which reactivated the Miocene faults and resulted in a linear north-south valley (the Owens Valley) with high mountain ranges on each side. This tectonic event resulted in geographic isolation and fragmentation of aquatic habitats and may have been a critical driver for speciation of aquatic organisms. Subsequent to this remarkable transformation of the landscape, the predominant influence on aquatic habitats has been very large, climate-driven fluctuations in the regional water balance that have resulted in the repeated interconnection and disconnection of the various basins that make up the paleo–Owens system. The magnitude of these fluctuations appears to have increased markedly since the early Pleistocene. Searles Lake has generally been the terminus of the Owens River, but at least once, probably at ca. 150 and/or ca. 70 ka, the system overflowed into Death Valley. During the last interglacial (marine isotope stage 5) and the Holocene, Owens Lake has been the terminus, but apparently not frequently before. These very large fluctuations in the water balance undoubtedly produced large shifts in the nature and distribution of aquatic habitats over geologically short periods of time, as well as repeatedly creating and severing connections between various parts of the larger drainage basin. This dynamic hydrological system provided the setting, and no doubt much of the impetus, for speciation, extinction, and distribution of aquatic species within the paleo–Owens system, but any paleohydrological causes will have to be extracted from a complex history.

Owens Lake has existed for most of the past 800,000 yr, but the sequence of interconnected lakes and streams of which it was often part, the Owens River cascade, last flourished during late Pleistocene time. A fluctuating, increasingly saline, terminal lake survived into the late Holocene until upstream water diversions to the Los Angeles Aqueduct began in 1913. Shoreline fragments and beach stratigraphy indicate that the lake reached its highest late Pleistocene level around 23.5 ka, during the Last Glacial Maximum, when it was fed by meltwaters from Sierra Nevada glaciers and spilled southward to Searles Lake and beyond. The lake then fell to relatively low levels after 16.5 ka before experiencing terminal Pleistocene oscillations related to hydroclimatic forcing, which involved changing regional precipitation regimes rather than major inputs from Sierra Nevada glaciers. Two major transgressions occurred. The first culminated around 14.3 ka and was probably related to a cooler, wetter regional climate. The second culminated around 12.8 ka and was linked to the earlier wetter phase of the Younger Dryas cold event. However, the high late Pleistocene shoreline is deformed, and the highest beach ranges in elevation from 1140 m to 1167 m above sea level. If the terminal Pleistocene lake overflowed, as suggested here, then its outlet has also been raised since 12.8 ka. This deformation appears to have involved uplift of the Coso Range magmatic complex relative to subsidence and faulting within the Owens Lake graben between the Sierra Nevada and Inyo Mountains frontal faults. Such deformation confounds simple hydroclimatic explanations of lake behavior and must be incorporated into models that seek to interpret the changing form and geochemistry of Owens Lake and the frequency of its spillage southward to Searles Lake.

Little is known for certain about early Wisconsin (isotope stage 4) lakes and glaciers of the Great Basin. A moderate lake-level rise in the Bonneville basin is not well dated, but on the basis of amino-acid and radiocarbon ages, is thought to be early Wisconsin in age. A moderate rise of lakes in the basins of Lake Lahontan is dated as ca. 50 ka by U-series ages on tufa, but may have occurred earlier. In the southern Great Basin, Searles Lake fluctuated at levels below the threshold connecting it with Panamint Valley, and Panamint Valley apparently did not contain a large lake during the early Wisconsin. The glacial record is even less-well dated than the lacustrine record. The extent of glaciers in and around the Great Basin during the early Wisconsin is not known; ice extent was certainly greater than at present, but probably was less than the late Wisconsin maximum in most glaciated valleys. Further work is necessary to refine lacustrine and glacial chronologies, and to investigate the causes of lake vs. glacier expansion. Important clues to these questions will come from detailed studies of lacustrine and glacial sequences in different parts of the Great Basin.