Description
A wide variety of studies have been published on the geology of Northern San Diego County, especially for areas near San Onofre Nuclear Generating Station. These studies are of structure and/or stratigraphy, onshore, offshore, and on different epochs. This thesis presents a more comprehensive geologic report to fill gaps in our knowledge of the area. Field mapping and interpretation of offshore seismic profiles helped in the reevaluation of previous studies. Description and extent of onland formations were used to correlate onland formations to offshore seismic stratigraphic units. High resolution transducer and lower resolution sparker profiles provided excellent detail of offshore structures and stratigraphy near San Onofre Nuclear Plant and moderate to poor detail in the southern part of the study area. Eocene Santiago Formation and middle Miocene San Onofre Breccia form extensive onland deposits. They dip southwest, extending into the southern offshore area where they outcrop beneath a thin cover of Pleistocene sediments within an anticlinorium. Miocene Monterey and Pliocene Capistrano Formations are restricted to isolated coastal outcrops projecting offshore to extensive, folded and faulted outcropping seismic units. The Monterey and Capistrano Formations are formed into a major structural trough in the northern offshore area allowing preservation of thick Pleistocene deposits. The trough is bounded on the west by a presently active major, faulted anticlinorium called the South Coast Fault Zone. Slightly tilted and faulted offshore Pleistocene sediments restricted to the synclinal trough to the north thin to the south, where they cover the entire nearshore continental shelf. Onshore Pleistocene terrace deposits blanket the coastal area except where highly elevated and eroded as in the San Onofre Mountains. Very thin Holocene deposits cover the nearshore continental shelf, forming a lense-shaped deposit disrupted slightly along the South Coast Fault Zone. During Eocene, marine and nonmarine sediments accumulated along a coastal zone, on a low relief erosional surface within the study area followed by Oligocene and early Miocene nondeposition. Middle Miocene was a time of great tectonic unrest which created local highlands and basins. An unusual breccia prograded into the area eroded from a rising western highland. Later, rapid subsidence of the study area and highland occurred and deep basinal shales and sandstones were deposited. Renewed Pleistocene deformation along the modern nearshore continental break elevated an eastern portion of the previous basin deposits into the wave zone. From Pleistocene to modern day, sediments have accumulated in thin wedges on the newly formed wave cut shelf, filling in structural lows and terrace cuts along the rising coast.
