PLIOCENE DEVELOPMENT AND MIS-4 FRESHWATER LAKE DEPOSITS OF DEEP SPRINGS VALLEY, WESTERN GREAT BASIN, INYO COUNTY, CALIFORNIA: IMPLICATIONS FOR REGIONAL TECTONICS AND PALEOCLIMATE

Deep Springs Valley (DSV) is a hydrologically isolated valley between the White (north and west) and Inyo (south) Mountains that is commonly excluded in regional paleohydrologic and paleoclimate studies. The ~200-800 m high Deep Springs ridge (DSr) (informal name), formed by slip on the Deep Springs fault, separates DSV from Eureka Valley. Here we present tephrochronology, clast counts, paleontology, and optically stimulated luminescence (OSL) data that confirms when DSV formed and a freshwater lake in DSV during Marine Isotope State 4 (MIS-4). We correlate the 3.13 Ma ash interbedded with conglomerate on DSr to the tuff of Mesquite Flat. Clasts of Jurassic granitoid, Paleozoic metasedimentary rocks, and basalt found in the 3.13 Ma fluvial conglomerate and a similar 3.335 Ma conglomerate in Eureka Valley confirm the conglomerate provenance is the White Mountains and that the Pliocene conglomerate flowed across DSV before the valley formed. Previous studies showed that uplift of DSr defeated DSV-crossing streams sometime after eruption of the Bishop Tuff, hydrologically isolating DSV after 0.772 Ma. Finely bedded siltstone, sandstone and gravel lacustrine deposits are mapped on the western piedmont of DSV. The upper 4 m of lacustrine deposits yielded OSL dates of 64.5-61.5 ka. A sandstone bed near the base was dated at 83.3 ka. Fossil gastropod, ostracodes, and plants indicate a nearshore, freshwater lake environment. Presently, the lacustrine deposits (1658 masl) are 24 m below the lowest wind gap into Eureka Valley at Soldier Pass (1682 masl). We conclude that a freshwater lake occupied DSV during MIS-4; however, overflow into Eureka Valley is unclear. Evidence for MIS-4 glaciation in western North America is sparse; however, a wetter climate in the western Great Basin is implied by a transition from closed and saline to overflowing lake conditions at Owens Lake ca. 73 ka, and extensive alluvial fan deposition occurred in Death Valley during MIS-4.