San Gregorio-San Simeon-Hosgri Fault, Point Sur, California

Clarence A. Hall, Jr.

The San Gregorio-Hosgri fault zone (SGHFZ) is present in the Point Sur region, coastal central California. The principal trace of the fault is near vertical, locally marked by right-deviated stream channels, suggesting recency and right slip. A beheaded stream channel (the local northeastern dip of fault traces in the fault zone) and late Cenozoic uplift (creating a remarkable coastline) also suggest compression normal to the present San Andreas system.

A Miocene rock sequence west of the SGHFZ consists of Vaqueros Sandstone, Rincon Shale, and overlapping upper Miocene facies of the Pismo Formation. These rocks are correlated with rocks of similar age in the Morro Bay region, 105-135 km south and east of the SGHFZ. The absence of late middle Miocene Monterey Shale in this and the Morro Bay-Cambria area is noteworthy. Absence may be coincident with cessation of wrench and pull-apart tectonics following the middle Miocene and initiation of compression tectonics during late Miocene in this region. Miocene rocks depicted by others as being in thrust-fault (Sur thrust) contact with older rocks are within late Cenozoic landslides and are not bounded by a thrust fault. The Sur fault is present, but faulting is Paleogene in age, younger than the Upper Cretaceous (Maastrichtian) rocks it faults.

Coherent slabs of Upper Cretaceous rocks (Pfeiffer slab), which crop out north of Cooper Point and south of the Little Sur River west of the SGHFZ, are correlated with the Cambria slab 95 to 135 km to the south.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.