COLLABORATIVE RESEARCH: Neogene Sediments of the San Timoteo Badlands, Southern California: Implications along the San Andreas and San Jacinto Faults

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This proposal is to study the stratigraphy, biostratigraphy, and geochronology of the San Timoteo Badlands. It contains a sequence of nonmarine deposits about 12,000 feet thick that ranges in age from about 5 to 0.7 Ma, yields remains of fossil mammals throughout the sequence, and contains lithologies amenable to magnetostratigraphic and isotopic age analysis. The deposits, the Mt. Eden and San Timoteo formations, occur in well exposed outcrops of an anticline that plunges gently northwestward and is truncated on its southwestern edge by the currently active San Jacinto fault. The San Jacinto, the most active strand of the San Andreas fault system in southern California, underwent about 24 km of right slip during the past two million years. Paleocurrent data indicate that the sediments were transported to the San Timoteo basin from the north and northeast. Distinctive clast suites in the rocks are consistent with derivation from sources in those directions now northeast of the Badlands; some east of the present San Andreas fault (s.s). The San Timoteo beds thus are ideally situated in space and time to promote investigations of the slip history of elements of the San Andreas fault system during the past 5 million years. These beds are the only sequence that accumulated relatively continuously during this span of time in this part of southern California. A detailed analysis of age of these beds has never been attempted. The program of age analysis will include vertebrate paleontology and biostratigraphy, magnetostratigraphy and isotope chronology. If successful, the results will dramatically improve our ability to attempt a reconstruction of the sedimentary history of the San Timoteo beds, and palinspastic reconstructions of the tectonic block that contains them relative to the then adjacent terranes in this region. The data also will be valuable in addressing the timing of transfer of regional slip from the San Andreas fault strands (s.s) to the San Jacinto fault, will improve our ability to evaluate when and how the entire San Andreas system became effectively "locked" in this region; will sharpen the focus of local investigations of slip rates on the San Andreas, and on how regional shear between the Pacific and American plates is propagated through the Transverse Ranges province.