MINERALOGICAL AND GEOCHEMICAL INDICATORS OF THE PALEOENVIRONMENT OF PLUVIAL LAKE THOMPSON, WESTERN MOJAVE DESERT, CALIFORNIA

Lake Thompson was a large lake that existed in the western Mojave Desert during late Quaternary time, extending beyond the margins of modern Rogers and Rosamond playas. Relict shoreline features indicate that the lake covered an area of 950 km² to a minimum depth of 20 m, but, apart from a possible link to nearby Koehn Lake, there were no connections to other pluvial lakes in the region. Cores recently obtained from depths of up to 35 m document the paleoenvironmental evolution of former Lake Thompson. The upper 3 to 5 m of the cores contain sandy sediment typical of latest Pleistocene beaches and Holocene dunes and playas. These unconformably overlie silty clay and clay, laminated in some core segments, that represent deposition in a deeper perennial lake. Preliminary AMS ages show that this lake existed from >30,000 to ~17,000 ¹⁴C years b.p. Smectite and illite are the dominant clay minerals, with the relative proportions varying both within and between cores. In one core (RW2), smectite averages 66±10% of the clay fraction and shows a general increase with depth. %CaCO₃ decreases over the same interval. Based on comparisons with the modern playa, these trends are consistent with deposition in a large lake under humid conditions, but with a drying trend from >30,000 b.p. to the end of the lake episode in this core. However, a second core (TL1) has an average smectite content of 36±15% that shows an apparent decrease with depth. Analcime is a common mineral in these sediments, so this may result from post-depositional alteration of smectite by alkaline fluids. Variability in composition between successive samples in cores may provide clues to rapid climate swings superimposed on the general trends.