Abstract: General Geology of Santa Rosa Island, California

DIBBLEE, THOMAS W., JR., and HELMUT E. EHRENSPECK Dibblee Geological Foundation, Santa Barbara, California

Santa Rosa Island is roughly 200 km² (80 mi²) of hilly terrain, severely dissected by narrow canyons, eroded from mostly marine sedimentary formations of Eocene to middle Miocene age. Much of the northern and western part of the island is beveled and covered by marine terraces and a veneer of Pleistocene wind-deposited sand.

The sedimentary sequence in ascending order is: Eocene marine South Point Sandstone and Cozy Dell Shale; disconformably overlain by Oligocene-lower Miocene terrestrial Sespe Formation; conformably overlain by lower Miocene marine Vaqueros Sandstone and Rincon Claystone; conformably overlain by middle Miocene marine Monterey Shale with volcaniclastic rocks; grading upward into middle Miocene marine Bechers Bay Formation of tuffaceous sandstone, locally with basal San Onofre Breccia, and minor volcanic conglomerate upsection. The exposed sequence aggregates roughly 2000 m (6500 ft), with an additional 3400 m (1 1~It) of marine clastic rocks of late Cretaceous?to Eocene age known from deep test drilling. The section up through the lower Monterey Shale is intruded by pods and sills of basalt-diabase.

Santa Rosa Island is transected by the east-striking Santa Rosa Island fault of left-slip movement, as indicated by deflected stream courses. Left slip increases westward to as much as 11 km (8 mi) as suggested by displaced rock units. A minor south-side-up component is evident from the generally higher and more rugged terrain to the south.

Directly north of the Santa Rosa Island fault, Miocene formations are gently uparched along it and weakly downfolded farther north, with minor subparallel faults. South of the fault, Miocene formations are gently compressed into northwest- to westtrending folds with adjacent subparallel faults. More complexly faulted and folded Eocene formations are exposed along the southwest and south coast of the island.

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