PALEOENVIRONMENTAL AND PALEOCLIMATE RECORD OF THE PLEISTOCENE LAKE TECOPA BEDS, SOUTHEASTERN CALIFORNIA

Sedimentary deposits of the Pleistocene Lake Tecopa beds are used to interpret the paleoenvironmental settings and paleoclimate in the Tecopa basin from the early to latest middle Pleistocene (2.4 to 0.3 Ma). The early Pleistocene (ca. 2.4 to 1.1 Ma) deposits of the Lake Tecopa beds record deposition in small saline, alkaline lakes and playas with surrounding mud and sand flats and adjacent alluvial fans. Ancestral Amargosa River gravels are first observed in fluvial deposits in the northern part of the basin at ca. 1.1 Ma and correspond with formation of intermediate to small lakes. Between ca. 1.0 and 0.3 Ma, six lake phases to form intermediate to basin-filling lakes alternated with periods of substantial dryness in the basin. The basin-filling LC lake, which reached its acme during deposition of the 0.63 Ma Lava Creek B ash bed, may have briefly overflowed the sill along the Tecopa Hump at the south end of the basin. The post-Lava Creek B strata reflect primarily alluvial, fluvial, eolian and groundwater discharge depositional processes, punctuated by intermediate to basin-filling lakes (Sh lake, high lakes 1 and 2). The LCB ash bed and older lacustrine strata exhibit extensive zeolitization and clay authigenesis, characteristic of saline, alkaline-lake deposits, but the post-LCB ash bed lacustrine strata have only minor zeolite and clay alteration suggesting fresher water conditions and a change in the hydrologic state of the basin. Sedimentological observations along with shoreline elevation data provide evidence for intermittent overflow of basin-filling lakes after ca. 0.63 Ma. Dry periods between the lake phases indicate substantial decrease of lake size, erosion along basin margins and prominent eolian deposition. Based on limited tephrochronology and estimated sedimentation rates the alternating wet and dry cycles in the Tecopa basin follow a roughly 40-ka cyclicity between ca. 2.4 and 1.1 Ma and 100-ka cyclicity from 1.0 to 0.3 Ma, indicating prominent Milankovich-scale frequency of climate variations typical of the Pleistocene. The amplitude of climate variations inferred from lake extent in the basin from 1.0 to 0.3 Ma correlates to composite MIS and regional paleoclimate records with prominent basin-filling lakes during MIS 16, 12 and 10, and protracted dry periods during MIS 21, 13, and 11.