LAKE ANDREI: A PLIOCENE PLUVIAL LAKE IN EUREKA VALLEY, EASTERN CALIFORNIA

Eureka Valley is an endorheic basin between Owens Valley and Death Valley in the eastern Great Basin, California, that was presumably occupied by a pluvial lake. Eureka Valley is notably fishless and without springs, which may explain why it is omitted from the hypothesized interconnected pluvial lakes and rivers of eastern California. Early maps speculated that a lake once filled the valley. Geomorphic and geologic evidence suggest that prior to 0.77 Ma, streams crossed Deep Springs Valley, the adjacent valley to the northwest, and flowed into Eureka Valley via the Soldier Pass, Wyman Creek, and Cottonwood Creek wind gaps. The evidence also suggests that Deep Springs Valley pluvial lake most likely spilled over into Eureka Valley through Soldier Pass during the early to middle Pleistocene. It was speculated that during the late Pliocene or early Pleistocene, the paleo-Owens River may have flowed east out of Owens Valley and either terminated in Eureka Valley or flowed through Eureka Valley and into Saline Valley to the south. Regional geologic maps of Eureka Valley identify Neogene sedimentary deposits. However, descriptions are often brief and numerical dates elsewhere in Eureka Valley are limited to whole-rock K-Ar ages of volcanic rocks that range from 10.0 Ma to 1.1 Ma. In this study, we completed geologic mapping, tephrochronology, and ⁴⁰Ar/³⁹Ar dating of rhyolite tuffs interbedded with four Neogene sedimentary sequences in Eureka Valley including previously mapped lake deposits. We identified six different tuffs in these sequences, but only found gypsum-bearing, green, fine-grained, and bedded sediments that we interpret as lake deposits on the Last Chance Range piedmont. We interpret these deposits as evidence that a perennial-saline lake existed in eastern Eureka Valley during the Pliocene (ca. 3.5 Ma). The gypsum beds are consistent with an Owens River source, but do not exclude a local water source. The 3.509 ± 0.009 Ma tuff of Last Chance and the 3.506 ± 0.010 Ma tuff of Hanging Rock Canyon were identified in Eureka Valley. The tuffs allow correlation of the Eureka Valley deposits with other Pliocene sedimentary units in the region, and indicate that the pluvial lake, which we informally named Lake Andrei, coincides with the glacial climate that was present during the late Pliocene Marine Isotope Stages MG5 to M2.