PLIOCENE LACUSTRINE TRANSGRESSION IN THE WESTERN SALTON TROUGH, SOUTHERN CALIFORNIA: IMPLICATIONS FOR REGIONAL TECTONICS AND EVOLUTION OF THE COLORADO RIVER DELTA

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 middle or late Pliocene time. The transition from Pliocene Diablo to Plio-Pleistocene Borrego Fm records transgression of the Borrego lake over a large lacustrine-delta system during initiation and early growth of the SFBB. This transition is thick, interbedded, and very gradational. Lithofacies span a spectrum from sand-rich (Diablo) to clay-rich (Borrego) end members and include: thick cross-bedded sandstone (Colorado River channel); tabular sandstone and mudstone (delta plain and lake-margin bays); and claystone, marlstone and siltstone (distal Borrego lake). Upper Diablo paleocurrent data in the NE San Felipe Hills show overall transport to the SW, with ~180 degrees of scatter. Paleotransport in the Borrego Mt to Borrego Sink area is toward the SE. The lower ~450 m of Borrego Fm fines laterally west across the SFBB, with thick beds of Colorado River sand in the east pinching out into claystone in the west. Farther west in the SW Borrego Badlands, lacustrine fines are interbedded with thick conglomeratic sandstone units that indicate proximity to the western margin of the lake. A syn-Borrego oblique-sinistral fault zone in the SW Borrego Badlands, when restored for bedding dip, appears to be a zone of moderately NW-dipping intrabasinal normal faults.

During Borrego time 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. The older Diablo-Borrego transition, however, is very complex. We tentatively infer that initiation of a large dextral-normal fault on the NE flank of the Fish Creek and Vallecito Mts disrupted the formerly through-going Colorado River drainage, diverted river water into the SFBB, and created a large marshy (palustrine) lowland with highly disorganized drainage and multiple delta lobes. This large fault may represent syn-kinematic breakup of the upper plate of the west Salton detachment system, or it may be a younger cross-cutting fault. Continued subsidence and transgression drowned the delta complex in the SFBB and created the perennial Borrego lake, which was fed by the Colorado River and filled the Salton Trough NE of the Fish Creek and Vallecito Mts.