Abstract: Preliminary Comparison of two Upper Cretaceous Conglomerates: The Strata of Anchor Bay and an Unnamed Unit East of the Pilarcitos Fault, San Mateo County, California; and Implications for Motion of the San Andreas, San Gregorio, and Pilarcitos Faults

BURNHAM, KATHLEEN, Stanford University, Stanford, CA 94305- 2115 ([email protected])

Two Upper Cretaceous conglomerates, 180 km apart on opposite sides of the Pilarcitos-San Gregorio-northern San Andreas faults, bear such a striking resemblance that they pose the possibility of 175 to 185 km offset of these faults, and may resolve the apparently disparate offsets of the northern and central segments of the San Andreas Fault. These conglomerates are the Strata of Anchor Bay, at Fish Rock Beach near the town of Anchor Bay, on the Gualala Block terrane; and an unnamed unit just east of the Pilarcitos Fault, east of the town of Half Moon Bay, on the Permanente Terrane. This unit is here informally designated the Pilarcitos conglomerate.

These two conglomerates incorporate mafic pebbles and cobbles and carbonate clasts - notably Coralliochama orcutti. The matrix and the suite of particular varieties of conglomerate clasts, are uncommon in pebble or cobble conglomerates, and mafic clasts have been reported from only a handful of distant sites in the Coast Ranges. None of the twenty other Upper Cretaceous and Paleogene conglomerates in this study share the Anchor Bay/Pilarcitos suite of clasts.

These results suggest a possible 175 to 185 km right-lateral offset of the northern San Andreas fault, with either the San Gregorio or the Pilarcitos Fault assuming the motion further south. A 175 to 185 km San Gregorio Fault offset correlates with allowable offsets of conglomerates of Point Reyes and Point Lobos, as well as of other features.

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