LATE NEOGENE DEPOSITION IN THE LAST CHANCE RANGE, EASTERN CALIFORNIA

Hypotheses regarding the paleo-Owens River include possible flow from Owens Valley east into Eureka Valley. If true, then the Last Chance Range (LCR) is the only barrier to direct flow of the Owens River into Death Valley to the east. Previous geologic studies identified Cenozoic conglomerate with intercalated basalt flows both atop and along the western flank of the LCR. We present detailed geologic mapping, ⁴⁰Ar/³⁹Ar dating, geochemical, and provenance data for these late Neogene deposits to determine the age and paleocurrent directions. Atop the LCR, tilted conglomerate, sandstone and a 3.5 Ma rhyolite ash bed are overlain by flat-lying basalt flows. Clast provenance is consistent with depositional source areas on top of the LCR; geomorphic evidence indicates that the basalt flowed east to west. On the western flank, the late Neogene deposits are mapped as one unit with conglomerate surrounding rhyolite ash beds and a basalt flow. Mapping indicates that there are two separate conglomerate deposits. We dated an ash bed in the east-tilted conglomerate by ⁴⁰Ar/³⁹Ar at 3.5 Ma. Clast provenance indicates an east-to-west paleocurrent that records exposure of Mississippian bedrock. The second, west-flank conglomerate also has east-to-west paleocurrent indicators and an ash bed. Glass shard composition correlates this ash bed with the 0.77 Ma Bishop ash bed. X-ray fluorescence analysis of the basalt flows atop and on the west flank of the LCR indicates that the two flows have the same source. Our conclusions are that the LCR was a topographic high at least since 3.5 Ma depositing sediments into Eureka Valley; the Eureka Valley fault zone is stepping basinward progressively uplifting basinal deposits.