CROUCH, JAMES, J. K. Crouch & Associates, Inc., Santa Barbara, CA
ABSTRACT: Geology, Evolution, and Petroleum Potential of the Oceanside Basin
"Oceanside basin" is an informal name for the offshore inner borderland region between San Clemente and La Jolla, California. The central trough of this Neogene basin is bounded by the San Joaquin high on the north, the Coronado bank on the south, the Newport-Inglewood fault zone (NIFZ) on the east, and the Palos Verdes-Coronado bank fault system on the west. A relatively deep arm of the basin (the Capistrano embayment) extended into the present-day onshore area between Dana Point and San Clemente during the late Miocene-early Pliocene. The present-day physiography of the offshore part of the Oceanside basin actually consists of a shelf, slope, and slope apron rather than a basin. As delineated by isopachs, however, this region is underlain by a substantial thickness of Neogene strata that define a former trough that was about 60 km long and 15-25 km wide. The physiography of the former basin has been greatly modified by post-Miocene contraction and strike slip that, together, caused structural inversion of the basin fill, the development of a fold and thrust belt, and a narrow band of en echelon folds along the offshore NIFZ.
The Oceanside basin began as a graben or half graben in the early Miocene and persisted as a structural depression throughout much of the Neogene. The east flank of the central trough (the offshore NIFZ) marks the pre-Neogene location of the southern margin of the Western Transverse Ranges (WTR) block and the locus of initial Neogene rifting (breakaway) within the inner borderland. The basin formed when this former forearc (WTR) was rifted and then rotated clockwise, away from the westernmost flank of the Peninsular Ranges. Rifting or extension was similar to Basin and Range style tectonics. Accordingly, basement flooring the central trough is interpreted to be a metamorphic core complex that consists chiefly of Catalina Schist. A nearly flat-lying, abandoned detachment fault marks th boundary between this schist basement and the overlying Neogene fill.
During its depositional phase, the Oceanside basin was probably similar to the present-day San Diego trough. Subsurface well and seismic data indicate that it was the site of organic-rich hemipelagic (Monterey-type) sediments and that these deposits were interbedded with large volumes of sand-rich turbidites fed from numerous submarine channels. This, together with the fact that the Oceanside basin is directly on trend with the Torrance-Wilmington and Newport-Inglewood producing trends, makes it
an attractive exploration target. The basin, however, appears to lack a sufficient "kitchen area;" if one does exist, it is significantly smaller than those (i.e., the Wilmington graben and the central trough) believed to have sourced many of the prolific Los Angeles basin fields. Moreover, local, state, and federal opposition to offshore drilling makes it unlikely that the petroleum potential of the Oceanside basin will ever be tested.
AAPG Search and
Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach,
California, May 5-7, 1993.