Abstract: Facies relations in the San Onofre Breccia: Evidence for Uplift and Volcanism in the Early Miocene, Santa Cruz Island

BOLES, JAMES R., Department of Geological Sciences, University of California Santa Barbara, CA 93106

The San Onofre Breccia exhibits marked lithofacies change from northeast to southwest on Santa Cruz Island. In upper Posa Canyon, the San Onofre is 380 m of coarse conglomerate/breccia. Rare thin sandstone preserve oysters in growth position and indicate very shallow marine conditions. Two miles to the southwest, coastal outcrops of San Onofre Breccia are 90 m of thin-bedded conglomerate, interbedded sandstone, and minor siltstone. Redeposited oyster debris is common. These coastal exposures are considered to be the paleo offshore equivalent of the upper Posa Canyon outcrops.

In upper Posa Canyon, the lower 2/3 of the formation is dominated by schistose metasedimentary clasts including distinctive blueschist, and diorite. A garnet-amphibolite clast is contemporaneous with and petrologically similiar to outcrops on Catalina Island. The dominant class type in the upper 1/3 of the sequence is a light-colored felsic volcanic which indicates nearby contemporaneous volcanism. The coastal outcrops are dominantly felsic volcanics with subordinate blueschist clasts. This sequence is petrologically equivalent to the upper third of the Posa Canyon section.

The San Onofre Breccia records the uplift of high pressure metamorphic rocks followed by active volcanism during the transition from normal plate convergence to oblique convergence, rotation and extension. Pre-rotation plate reconstructions, sedimentology, and transport directions indicate very locally derived westerly sources; a surprising conclusion considering that rifting occurred to the east and south of the Santa Cruz Island block.

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AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California