Abstract

Santa Rosa Island is roughly 220 km² (85 mi²) of hilly terrain, severely dissected by narrow canyons, eroded into mostly marine sedimentary formations of Eocene to middle Miocene age. Over much of the northern and western part of the island, this series is beveled by marine terraces and covered by a veneer of alluvium, Pleistocene dunes and drift sand.

The sedimentary sequence in ascending order is as follows: 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 marine volcaniclastic rocks; grading upward into middle Miocene marine Beechers Bay Formation of tuffaceous sandstone, locally with basal San Onofre Breccia, and minor volcanic conglomerate up section. The exposed sequence aggregates roughly 2000 m (6500 ft), with an additional 3400 m (11,000 ft) 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, left-slip Santa Rosa Island fault, as indicated by deflected stream courses. Left-slip increases westward to as much as 11 km (8 mi) as indicated by displaced rock units. A minor south-side up component is evident from the generally higher and more rugged terrane south of the fault.

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