The San Felipe-Borrego basin (SFBB) is a large sub-basin in the western Salton Trough that has evolved independently of the Fish Creek-Vallecito sub-basin since Pliocene time. The Pliocene Diablo Formation (Fm..) accumulated in the Lower Colorado River and delta system, which covered the full width of the Salton Trough before the SFBB and Fish Creek-Vallecito sub-basin became separate basins. The Diablo Fm.. in the SFBB is overlain by the Plio-Pleistocene Borrego Fm.., which records the transgression of a large regional lake over the Lower Colorado River and delta during initiation and early growth of the SFBB. The age of the Diablo-Borrego contact is poorly known and will be dated with magnetostratigraphy (B. Housen and Dorsey, in progress). The Diablo-Borrego transition is interbedded and gradational, and shows large variations in thickness and complexity around the SFBB. Lithofacies range from sandrich (Diablo) to clay-rich (Borrego) end members and include: very thick cross-bedded sandstone (Colorado River channel); tabular bedded sandstone and mudstone (delta plain and lakemargin bays); and claystone, marlstone and siltstone (Borrego Lake). These facies record a transgressive lacustrine-delta setting during the Diablo-Borrego transition. Mudstone and claystone units in the Diablo Fm. yield benthic foraminifers (Ammonia beccarii), green algae (Chara), and smooth-shelled ostracodes, suggesting lagoon or estuary environments with a possible connection to the Gulf of California. Microfossils in the Borrego Fm. include Chara, ostracodes, micromollusks, diatoms, and rare benthic foraminifers, suggesting possible migration of marine faunas into the Borrego Lake through channels of the lower Colorado River. Lithofacies of the Borrego Fm. are more consistent with a perennial lake (not lagoonal) depositional environment. Paleocurrent data in the Diablo Fm. show overall transport to the SW in the NE San Felipe Hills, and toward the SE in the Borrego Sink area. Restoring paleocurrents for younger faults, we infer that the Colorado River entered the Salton Trough northeast of what is now Salton City during Diablo deposition. This implies approximately 125 km of right-lateral slip on the San Andreas fault since Diablo time, as suggested by Winker and Kidwell (1986). The significance of this slip amount will not be known until the age of the top of the Diablo Formation is determined. During younger Borrego deposition, the Colorado Delta probably formed a low topographic barrier that separated the Borrego Lake in the NW from the Gulf of California in the SE, similar to the modern setting. The older Diablo-Borrego transition, however, is poorly understood. We believe climate was not the main control on lake formation because regional climate records indicate cooling and drying in the SW U.S. at ~3.0-2.5 Ma (e.g., Forester, 1991; Smith, et al., 1993). If the Colorado River entered the Salton Trough NE of presentday Salton City, it could not have also created a topographic barrier to the SE. This suggests that multiple delta lobes and/or a structural barrier may have been present SE of the lake to isolate it from the gulf. Also, a tectonic barrier would likely be required to isolate the Fish Creek-Vallecito sub-basin from Colorado River input starting ~2.8 Ma (Winker, 1987) and initiate lake transgression in the SFBB. Two possible structural controls on basin segmentation are: (1) initiation of the Fish Creek Mountains fault on the NE flank of the Fish Creek and Vallecito Mountains; or (2) syn-depositional growth of a large NW trending anticline. Ongoing research will allow us to test these hypotheses. References Forester, R.M., 1991. Pliocene climate history of the western United States derived from lacustrine ostracodes: Quaternary Science Reviews, 10, p. 133-146. Smith, G.A., Y. Wang, T.E. Cerling, and J.W. Geissman, 1993. Comparison of a paleosol-carbonate isotope record to other records of Pliocene-early Pleistocene climate in the western United States: Geology, 21, 691-194. Winker, C.D., 1987. Neogene stratigraphy of the Fish Creek– Vallecito section, southern California: implications for early history of the northern Gulf of California and Colorado delta: Ph.D. dissertation, University of Arizona, Tucson, 494 p. Winker, C.D., and S.M. Kidwell, 1986. Paleocurrent evidence for lateral displacement of the Colorado River delta by the San Andreas fault system, southeastern California. Geology, 14, 788-791.