LATE NEOGENE GEOLOGY OF THE LAST CHANCE RANGE: IMPLICATIONS FOR PALEOHYDROLOGY OF THE DEATH VALLEY AREA, EASTERN CALIFORNIA

Paleohydrologic reconstruction of the eastern California Basin and Range province provides greater understanding of the effects of past hydrology and its tectonic evolution. Key localities are Eureka Valley and the Last Chance Range (LCR) northeast of Death Valley. Earlier hypotheses suggest that the paleo-Owens River, which later terminated in Death Valley, flowed into or through Eureka Valley. Using tephrochronology and 40Ar/39Ar dating we identify ~3.5 Ma tuffs similar to the Long Canyon rhyolite dome in the Sierra Nevada, the 3.5-3.3 Ma tuff of Curry Canyon, the 3.3 Ma tuff of Mesquite Spring, and the 3.2 Ma tuff of Kit Fox Hills intercalated with LCR sediments. Using stratigraphy and trace element chemistry, basalt to basaltic andesite flows, originating from the LCR, are divisible into pre-5 Ma, pre-3.5 Ma and post-3 Ma units. Clast counts and basalt flows indicate that during the Pliocene, a channel developed from north to south and descended through the center of the LCR. Along the normal-fault bounded mountain front, a depositional lobe transgressed to the north, meeting another locally-derived fan at Hanging Rock Canyon, and formed a bajada in Eureka Valley. Poorly sorted sandstone, landslide breccia and quartzite-boulder debris-flow deposits indicate the north-south channel system was a high-energy depositional system. Additionally, in the bajada, green mudstone with interbeds of gypsum found underlying the tuff of Mesquite Spring point to the presence of a Pliocene saline pluvial lake. By the early Pleistocene the mountain-front fault stepped basinward, uplifting the bajada, raising local base level, and terminating the north-south drainage system. Local alluvial-fan deposition at Hanging Rock Canyon continued. The range-bounding fault stepped basinward again during the middle Pleistocene. In summary, the paleo-drainage reconstruction indicates that the LCR was a topographic high during the Pliocene with a mafic volcanic center. Gypsiferous lake deposits indicate that at 3.3 Ma a saline lake existed in Eureka Valley when freshwater lakes existed in Searles Valley and Death Valley. The Eureka Valley lake conditions (high calcium and sulfate) versus that of the contemporaneous Searles Valley lake make flow of the paleo-Owens River (high sodium) into Eureka Valley unlikely.