--> --> Abstract: Sequence Stratigraphy of Miocene Syntectonic Strata in the California Borderland, by C. J. Stuart; #90992 (1993).

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STUART, CHARLES J., Unocal Science and Technology, Brea, CA

ABSTRACT: Sequence Stratigraphy of Miocene Syntectonic Strata in the California Borderland

The southern California borderland has undergone a complex and poorly understood tectonic evolution since the East Pacific Rise impacted it about 29 Ma. One result of Miocene tectonism was deposition of conglomerate wedges composed of diverse clast types. Borderland conglomeratic formations (Vaqueros, Topanga, San Onofre Breccia, and Blanca Formations) are partly characterized by detritus derived from Catalina Schist (blueschist rich) and diorite (similar to the Willows Diorite on Santa Cruz Island) basement and Miocene volcanic terranes. They are grouped into a succession of three principal lithofacies types, in ascending order: (1) diorite rich, (2) blueschist rich, and (3) volcanic rich (dacitic, andesitic, and basaltic clasts). Deep-marine sediments of the Monterey Formation trans ressively overlie volcaniclastic sediments and locally unconformably overlie blueschist-rich sediments or basement rocks. The three conglomerate types comprise mostly lowstand systems tracts in four depositional sequences identified in the region. Each sequence is bounded by unconformities and was deposited during a unique tectonic event: (1) diorite-rich and blueschist-rich conglomerate, respectively, during two stages of compression(?), uplift, and erosion of borderland basement rocks; (2) followed by extension or relaxation of faults with resultant volcanism and generation of epiclastic volcanic sediments; and (3) this, in turn, was followed by rapid subsidence, transgression, and deposition of the Monterey Formation. The succession of lowstand conglomerates occurs in older sequences n the Northern Channel Islands than south of the islands. This suggests that whereas the Miocene tectonic evolution of both areas was similar, stages in that process occurred earlier in the north and then shifted to a more southerly part of the borderland.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.