Abstract: Sedimentary Response to Tectonic and Climatic Controls: Quaternary Coastal Deposits north of San Francisco, California

GROVE, K., San Francisco State University, San Francisco, CA ([email protected])

Quaternary deposits in the San Andreas fault valley, adjacent to the Point Reyes Peninsula, record the influences of tectonic deformation and sea-level fluctuations in varying ways. The sedimentary record includes two Pleistocene formations -- Olema Creek (OCF) and Millerton (MF) -- and overlying alluvial and estuarine deposits. Although separated at the surface by overlying deposits, water- and oil-well data show subsurface continuity of the OCF and MF, which numerical age estimates indicate were deposited during the oxygen-isotope stage 5 highstand of sea level, beginning about 125 ka.

The OCF was deposited at the landward end of a paleoestuary, where fault-related subsidence and uplift are the most evident controls on depositional facies. The sedimentary sequence alternates complexly among estuarine, fluvial channel and overbank, pond, and alluvial-fan environments. Marine (estuarine) deposits occur toward the base, and the sequence generally coarsens upward, reflecting lowered sea levels following the substage 5e maximum.

The MF was deposited at the seaward end of a paleoestuary, where sea-level fluctuations are more clearly expressed. The formation consists of fining-upward sequences, from alluvial gravel to estuarine mud, separated by angular unconformities. The fining-upward sequences probably reflect transgressions associated with substage sea-level fluctuations (~20,000 yr recurrence interval) and the unconformities reflect deformation associated with earthquakes during interim lowstands (~200 yr recurrence interval).

Sea-level and climate changes during subsequent isotopic stages 4-1 caused alternating periods of deposition and incision that are recorded in sediments overlying the OCF and MF and in terracing of the entire Quaternary suite.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California