Abstract: Tectonic and Depositional Environment of the Middle and Upper Cenozoic Sequences of the Coastal Southern California Region

Thomas W. Dibblee, Jr.

Within the coastal region of southern California southwest of the San Andreas fault, the middle Cenozoic sequence of sedimentary and volcanic rocks ranges from the latest Eocene-Oligocene to late Miocene. It is composed largely of terrestrial deposits in inward areas, marine pelagic sediments in outward areas.

During the Oligocene, much of the region of pre-Oligocene fore-arc basin sediments and underlying basement terranes was emerged and elevated by crustal movements of the Ynezan orogeny. During and following this orogeny, clastic sediments of the lowest part of the middle Cenozoic sequence were deposited in valleys and undersea as this region party subsided.

In the early Miocene, recurrent crustal movements of the Lompocan orogeny affected much of this region, when local volcanism preceded widespread subsidence under a submerged shelf. Biogenic and argillaceous sediments of the middle Cenozoic sequence accumulated on its outward part, mostly terrestrial sediments on its inward parts.

Profound crustal movements during the Coast Range orogeny in the late Cenozoic reelevated the basement terrane of the Peninsular and central Transverse Ranges and emerged and elevated the western Transverse and southern Coast Range areas. Emergence of these areas divided the submerged shelf into local basins that subsided deeply during the late Cenozoic and filled with a great thickness of clastic sediments derived from nearby mountains that were elevated contemporaneously.

Regional evidence suggests that western California has been subsiding continuously since the late Mesozoic, but the mobile belt, including coastal southern California, has been increasingly elevated by compressive movements.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994