WEBER, GERALD E., Earth Sciences Dept., UCSC, Santa Cruz, CA; JEFFREY M. NOLAN, Nolan Associates, Santa Cruz, CA; and ERIK N. ZINN, Consulting Geologist, Santa Cruz, CA

Abstract: Map of Quaternary Deposits and Faulting Along the San Gregorio Fault Zone, San Mateo and Santa Cruz Counties, California

We have prepared a revised geologic strip map along the San Gregorio fault zone, from Tunitas Creek in San Mateo County to Davenport Landing in Santa Cruz County. The map, based on a combination of aerial photo interpretation and field work is a revision of the 1980 Open-File Report of Weber and LaJoie. Our revision of the marine terrace stratigraphy allows correlation of marine terraces across the San Gregorio fault zone, and a more reliable determination of regional uplift rates and both horizontal and vertical slip rates on the fault zone.

Re-mapping of critical areas and the inclusion of information from exploratory trenching studies has refined the pattern of faulting, particularly near Point Ano Nuevo. Here, between Whitehouse Creek and the south shore of the point a small constrictive bend in the fault zone subjects that area to tectonic compression. Small reverse-oblique faults act as crossover faults between a series of left stepping en echelon Riedel shears along the Frijoles fault. To the south of this area a small Holocene graben has formed between the two principal faults within the fault zone.

The contact between the Cretaceous Pigeon Point Formation and the Pliocene Purisima Formation in Arroyo de los Frijoles appears to be a "buttress" unconformity that has been offset by oblique (reverse and rightlateral) slip along the Frijoles fault. Re-mapping also indicates that the large structural block between the two principal fault traces of the fault zone (Pomponio structural block) is being slivered by numerous small oblique (reverse and right-lateral) slip faults.

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