ABSTRACT
The Pleistocene Carpinteria basin is an east-trending northward-verging, faulted syncline containing up to 4,000 ft (1,220 m) of partially intertonguing Santa Barbara and Casitas Formations deposited on previously folded pre-Pleistocene strata with up to 80° discordance. Structures subcropping against the unconformity indicate that most of the deformation in the Santa Ynez Mountains prior to deposition of the Pleistocene Santa Barbara Formation was by folding. During that time, the Carpinteria area was the northern margin of the offshore Ventura basin; subsequent uplift during middle to late Pleistocene time isolated the Carpinteria basin north of the Red Mountain fault.
Quaternary faults in the area are either south-dipping reverse faults related to bedding slip in pre-Pleistocene strata or north-dipping reverse faults that truncate bedding and are seismogenic. The Rincon Creek fault dips 35 to 60° south, steepening to nearly vertical at depth, where it apparently passes into bedding within the Sespe Formation. The Rincon Creek fault offsets late Pleistocene marine terraces and is itself deformed by the Red Mountain fault. The Red Mountain fault dips 55 to 63° north at the surface and steepens to 75° north with depth; it also steepens westward south of the Summerland Offshore oil field to 85° north. Vertical separation decreases westward from 14,750 ft (4,500 m) north of the Rincon field to 1,150 ft (350 m) at Rincon Point and 330 ft (100 m) south of Summerland. The main branch of the Red Mountain fault offsets a 45,000 year old marine terrace, but not a 4,500 year old terrace. The Summerland Offshore oil field is situated within a disharmonically folded anticline in which severely deformed, structurally incompetent Miocene mudstone overlies broadly folded, competent Oligocene sandstone. Because the anticline formed after deposition of the Santa Barbara Formation, oil could not have migrated into this field until middle to late Pleistocene time.
