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Biofacies Zonation of Middle Miocene Benthic Foraminifera, Southeastern San Joaquin Basin, California

Hilary Clement Olson

The quantitative distribution of benthic foraminifera across the middle Miocene margin of the southeastern San Joaquin basin constitutes a useful tool in applying benthic biofacies zonation to the interpretation of marine paleoenvironments. A middle Miocene transect (near the Luisian/Relizian boundary) was completed across the margin of the southeastern San Joaquin basin near Bakersfield, California. Surface and subsurface fauna encompass strandline through bathyal environments. Quantitative analyses of these fauna result in a useful biofacies zonation for the middle Miocene which can be applied to the interpretation of middle Miocene paleobathymetric and paleogeographic reconstructions, basin analysis, and subsidence histories of the San Joaquin basin. In addition, these data suggest that vertical faunal migration of continental slope fauna has occurred between the middle Miocene and Recent.

During the early and middle Miocene, marine temperatures were warmer than today and lower latitudinal gradients prevailed. Stepwise climatic cooling since the middle Miocene has been accompanied by the latitudinal adjustment of surface iostherms, strengthening of the permanent thermocline, and the associated migration of temperature-sensitive planktonic and benthic biofacies. Sedimentologic and seismic evidence in the southeastern San Joaquin basin suggests that present-day lower bathyal biofacies may have been at shallower depths during the middle Miocene. Such migrations would have a significant impact on paleoenvironmental interpretations. Middle Miocene faunal transects from the southeastern San Joaquin basin are compared with equivalent Holocene transects from the eastern Pacific and differences are discussed in light of this proposal.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.