THORNBURG, JENNIFER, and GERALD E. WEBER
Earth Sciences Department, University of California, Santa Cruz, California

Abstract: Historic Sedimentation in San Gregorio Creek: Implications for Absence of Geomorphic Expression of Recent Surface Rupture along the San Gregorio Fault Zone, San Mateo County, California

In coastal San Mateo County, California there is no geomorphic expression of faulting where the Coastways trace of the San Gregorio fault zone crosses the floodplains and low terraces of small stream valleys. This lack of evidence for Holocene faulting on stream valley floors supports the argument that the San Gregorio fault zone is inactive or has a low slip rate.

Recent lateral erosion along San Gregorio Creek formed a cut bank exposing fluvial sediments that lie across the mapped trace of the Coastways fault. These sediments were expected to record evidence of late Holocene surface rupture along this trace of the San Gregorio fault zone.

After cleaning and logging 115 m of this cut bank, it became evident that the 4-5 meter thick blanket of sediments exposed in the cut bank were less than 150 years old. Artifacts such as boards nailed to a post and a shotgun shell cap attest to the youth of the sediments. These artifacts constrain the maximurn age of the fluvial deposits to post-1850 A.D. We believe the rapid and massive sediment deposition is the result of large storms in the 1860s combined with the effects of logging in these coastal watersheds.

A gravel dike found near the western end of the exposure represents a historic liquefaction event, that must have formed in response to the 1906 earthquake on the nearby San Andreas fault. Since the dike intrudes through most of the thickness of the fluvial deposits, it constrains the timing of deposition as prior to 1906 A.D.

Rapid historic sedimentation is apparently not unique to San Gregorio Creek, but appears to be present in other nearby coastal stream valleys. The extreme youth of the sedimentary fill explains why there is no geornorphic expression of the active San Gregorio fault zone across these coastal valleys.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California