Stratigraphic units in the Tecopa basin, located in southeastern California, provide a framework for interpreting Quaternary climatic change and tectonism along the present Amargosa River. During the late Pliocene and early Pleistocene, a climate that was appreciably wetter than today's sustained a moderately deep lake in the Tecopa basin. Deposits associated with Lake Tecopa consist of lacustrine mudstone, conglomerate, volcanic ash, and shoreline accumulations of tufa. Age control within the lake deposits is provided by air-fall tephra that are correlated with two ash falls from the Yellowstone caldera, the Lava Creek (0.62 Ma) and Huckleberry Ridge (2.02 Ma) Tuffs, and one from the Long Valley caldera, the Bishop Tuff (0.73 Ma). Paleomagnetic determinations from deposits in the Tecopa basin are consistent with the ages of the ashes. Extrapolations of ages using average sedimentation rates suggest that the beds of Lake Tecopa, which accumulated to a minimum thickness of 72 m, are 0.5 to 3 m.y. old. In the central part of the basin, volcanic glass has been replaced by authigenic silicate minerals that formed in porewater of extremely high salinity and alkalinity. Therefore, Lake Tecopa occupied a closed basin during the latter part, if not all, of its 2.5-million-year history.
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