Abstract: Pleistocene Sedimentation in the San Andreas Fault Zone North of San Francisco: Evidence for Tectonic Deformation and Sea-Level Changes

Karen Grove

Two Pleistocene formations are exposed around Tomales Bay, in the valley along the San Andreas fault (SAF) zone near Point Reyes. Based on our thermoluminescence dates from fine-grained sediments, the Millerton Formation (MF) is 134 ± 12 ky old and the Olema Creek Formation (OCF) is 132 ± 28 ky old (analyses by G. Berger, 1994). Both formations contain estuarine sediments (confirmed by analyses of sampled diatom assemblages) deposited during oxygen isotopic stage 5 high stands of sea level, and alluvial sediments deposited during subsequent low stands. The formations have been deformed into broad folds by fault movements and show interactions between the tectonic and climatic influences. The MF (>70 m thick) consists of 4 or more fining-upward transgressive s quences (alluvial to estuarine) separated by angular unconformities. At the northern end of the outcrop belt, two strands of the SAF (last active in 1906) cut through and deform MF sediments. The OCF (>170 m thick) also consists of interbedded estuarine and alluvial sediments; these sediments were probably deposited within a transtensional zone that was later compressed and folded. Sediments in the OCF were derived from granitic basement exposures that have been dextrally offset from the site of deposition. The OCF age and amount of offset from the source area provide a minimum right-lateral slip rate of 18 mm/yr. Estuarine sediments now exposed at 70 m elevation provide evidence of uplift within the central part of the valley. Basement rocks of the Franciscan Complex appear at the su face in the southern part of the fault valley; they dip northward toward Tomales Bay, where they were encountered at 300 m depth in a bore hole (8.5 km north of surface outcrops).

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California