CONTROLS ON A ~50 M LATE PLEISTOCENE LAKE IN THE SALTON TROUGH

Lake Cahuilla formed in the Salton Trough (ST) episodically over at least the last ~2 ka, forming a shoreline at ~13 masl that coincides with the lowest part of a divide on the Colorado River fan near Cerro Prieto, Mexicali Valley. When the river avulsed north, fan-dammed lakes in the ST reached ~13 m and drained southward across the divide (sill) to the Gulf of California. Thomas and Stanley described higher shoreline deposits, at ~50 masl, on both sides of the ST that locally bear freshwater fossils. They attributed the deeper lake to an ancient sill in the Colorado River fan that was later downwarped. Our studies of the older lake deposits identified several new exposures, linking shoreline deposits discontinuously for >100 km along both sides of the ST at ~50 masl. The deposits represent highstand beaches and nearshore deposits of sand, silt, and carbonate rock; they lie on old alluvial fan deposits, deformed Pleistocene to Pliocene mudstone, and granite. Previous studies dated 3 shells and 15 tufa samples at 26 to >50 cal ka BP using conventional radiocarbon dating. We used AMS 14C, U-series, and luminescence methods to improve uncertainty. Preliminary dates are varied but point to either a ~35 ka or ~65 ka highstand, or both. Beach stratigraphy is consistent with a single highstand. The consistent beach elevations indicate negligible deformation along the margins of the ST in the US, requiring that a high barrier in Mexico existed. Colorado River fan deposits that aggraded to ~50 m along the eastern margin of Sierras El Mayor and Cucapah represent this barrier. They were dated by Armstrong et al. (2010) by luminescence at 30-40 ka. This aggradation coincides with late MIS 3 and may have been driven by increasing discharge and sediment delivery to the Colorado River fan as climate became wetter. Because the fan crosses many of the most active strands of the transtensional plate margin in the Mexicali Valley, large-magnitude earthquakes may have been associated with subsequent fan degradation. We conclude that, despite the presence of the major plate-boundary San Andreas and Imperial faults in the ST center, its margins in the US have been relatively stable for >30 ka. The Colorado River fan is one of the most dynamic geomorphic features in the ST, aggrading and degrading on a time scale of thousands of years with minor tectonic controls in the Mexicali Valley, thereby controlling lake levels in the ST.