The Anza-Borrego Desert State Park®, southern California, U.S.A., located within the Salton Trough, provides a geological record of complex, tectonically driven environmental change spanning Miocene through Pleistocene time. The earliest sedimentary records reflect pervasive marine conditions throughout most of the region. Late Miocene through Pliocene aged strata have both marine and non-marine elements. Late Pliocene and early Pleistocene sediments are primarily non-marine deposits accumulated within and around the geographic limits of “Lake Cahuilla”, a late-Tertiary precursor to the modern Salton Sea. The Borrego Formation, consisting mostly of lacustrine silts and sands with intermittent rheocrene silts, contains Lake Cahuilla sediments deposited across the Pliocene-Pleistocene transition.

Samples collected from the Borrego Formation, western Borrego Badlands, Anza-Borrego State Park®, California contain a variety of fossilized, non-marine ostracode species. Many of the fossiliferous deposits contain gastropod, crustacean, and charaphyte remains as well. Overall abundance data identify at least three major peaks in general ostracode abundance across the section. Species richness also peaks several times, however, the peaks in abundance and richness do not coincide stratigraphically. Candona sp. cf. patzcuaro is by far the most abundant species. Specific water chemistry fluctuations are indicated by Limnocythere staplini (bicarbonate depleted, saline water) and other Limnocythere species. Preliminary results indicate that water supply and water sources varied during the late Pliocene at Anza-Borrego, with times of abundant groundwater discharge, indicated by spring ostracode species, gastropods, and charaphytes, and times of limited water supply, indicated by Candona sp. cf. patzcuaro and L. staplini . Tertiary climate variations, as evidenced by the interpretation of regional lacustrine microfauna, help establish climatic trends to which the proliferation and later demise of the Plio-Pleistocene megafauna may be tied.