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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG

Abstract


Aspects of the Geologic History of the California Continental Borderland, 1976
Pages 266-293

Middle Miocene Paleogeography, Santa Cruz and Santa Rosa Islands

David G. Howell, Hugh McLean

Abstract

A thick sequence of middle Miocene volcaniclastic rocks is exposed on the southwest part of Santa Cruz Island and the east part of Santa Rosa Island. The sequence is thickest and the rocks are generally coarser grained in the south-central part of Santa Cruz Island. Grain size and bedding thickness markedly decrease across the trace of the Christi anticline toward the southwest shore of Santa Cruz Island. On Santa Rosa Island coarse-grained, thick-bedded units are complexly interstratified with fine-grained, thin-bedded units. Westward across Santa Rosa Island the finer grained facies increase in relative abundance. Paleocurrent directions inferred from sedimentary structures show south to southwest flow on Santa Cruz Island and a westward flow on Santa Rosa Island.

The volcaniclastic rocks have been divided into seven lithofacies: (1) massive unsorted conglomerate facies, (2) well-bedded cobble conglomerate facies, (3) well-bedded reverse to normally graded cobble-conglomerate facies, (4) thin- to thick-bedded sandstone facies, (5) very thin-bedded sandstone facies, (6) thin-bedded to massive siltstone facies, and (7) primary volcanic rock facies.

Sediment flow processes are inferred to be mostly high-concentration debris flows and viscous and inertial grain flows in the eastern areas that progressed southwest and westward and gradually transformed into high- and low-concentration turbidity flows. Hemipelagic deposits are best developed in the central part of Santa Rosa Island.


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