LATE PLEISTOCENE STRATIGRAPHY OF THE ANCESTRAL CHINA LAKE SEDIMENTS, INDIAN WELLS VALLEY, SOUTHEASTERN CALIFORNIA

During a recent Navy study to characterize the hydrogeology of the Indian Wells Valley (IWV) in southeastern California, cores from 15 soil borings and surface soil samples were dated using accelerator mass spectrometry determinations of carbon-14. The purpose of the radiocarbon analyses was to provide age control and stratigraphic correlation of lacustrine olive clay and silts. Ten samples yielded dates ranging from 14,690 to 32,220 years before present (ybp), a time frame that bounds the period of the last major Pleistocene lake in IWV, known as the ancestral China Lake. This time frame correlates well with the last major Sierra Nevada glacial advance, or stade, as marked by a dramatic increase in rock flour abundances seen in USGS cores from the upstream Owens Lake. Previous studies date this last stade at 30,500 to 15,000 ybp. Owens Lake outflow into IWV increased during this time, creating a sizeable ancestral China Lake. The first-encountered clays seen in the IWV cores are not coeval, however, because the clays represent sedimentary facies intercalated with fan-delta and deltaic sequences located at various distances from the lake’s primary depocenter. The distribution of the lacustrine clays is generally consistent with a conceptual shoreline map developed during this study. Shallow sediments under the current China Lake playa appear to be Late Pleistocene in age, likely reflecting removal of younger sediments by Holocene wind scour and eolation. Six deeper and older core samples ranged in age from 31,350 to 46,010 ybp, confirming that a stable lake persisted in the basin during this time. Two gastropod shells apparently from a near-shore death assemblage yielded dates of 14,060 and 12,825 ybp. The fossil horizons appear to reflect the salinization of the lake and declining freshwater input from upstream Owens River prior to the final infilling during the Younger Dryas age. The river surged again one last time, recorded in overbank/distributary deposits of the delta plain that yielded a date of 11,215 ybp. In the southeastern IWV, a gypsum-rich silty clay hummock dated at 10,070 ybp puts the desiccation of the last large ancestral China Lake at the close of the Pleistocene.

Approved for public release; distribution is unlimited.