STRATIGRAPHY OF THE PLEISTOCENE OCOTILLO FORMATION, BORREGO BADLANDS, SOUTHERN CALIFORNIA: BASINAL RESPONSE TO EVOLUTION OF THE SAN JACINTO FAULT ZONE

Pleistocene sedimentary rocks in the Borrego Badlands (BB) of southern California were deposited between the Clark (CF) and Coyote Creek (CCF) strands of the San Jacinto fault zone (SJFZ) during fault-zone evolution (Bartholomew, 1970; Pettinga, 1991). The Ocotillo Formation overlies the Borrego Formation along an abrupt but conformable contact estimated to be ~1.1 to 1.2 Ma (Remeika and Beske-Diehl, 1996; this study), and has a composite thickness of ~630 m. The Ocotillo Fm includes the 759-ka Bishop ash ~200 m above its base, and is overlain along a low-angle unconformity by the ~350- to 450-ka Fonts Point Sandstone (Ryter, 2002).

Two conformable members are recognized in the Ocotillo Fm. The lower member contains two sub-members that fine upward from sandy conglomerate to siltstone, and show lateral increase in thickness and grain size to the SW (sub-mbr 1) and NE (sub-mbr 2). Coarse-grained facies represent distal alluvial fans and desert washes that prograded from the NE and SW margins of the basin, passing laterally into lacustrine siltstone and claystone in the central BB. The upper member coarsens eastward from pebbly sandstone to cobble-boulder conglomerate (debris-flow facies) derived from the Santa Rosa Mountains NE of the CF. Clast counts record appearance of Pliocene Diablo Formation clasts in the western BB (lower sub-mbr 1), and an up-section increase in mylonite, marble, and Diablo clasts in the eastern BB (lower sub-mbr 2 and upper mbr). Clast imbrications indicate transport toward the central BB from the NE and SW basin margins, with some evidence for axial transport to the S and SE.

The data are consistent with a model for growth of a pull-apart basin due to transfer of slip from the CF to the SE CCF. Elevated topography NE and SW of the basin was produced by oblique slip on the CF and CCF, accompanied by slight tilting toward the faulted basin margins. Progressive clockwise rotation of the basin away from Coyote Mountain during deposition of the Ocotillo Fm (Scheuing, 1991) is compatible with a pull-apart basin geometry and will be tested with new paleomagnetic data. Abrupt progradation of upper-member coarse detritus into a lacustrine depocenter (represented by the lower mbr) records reorganization of the SJFZ at ~600 to 700 ka and appears to coincide with initiation of the NW CCF (Dorsey, 2002).