Abstract
Continental borderlands are commonly the site of complicated depositional and paleoceanographic relationships. The late Miocene San Joaquin Basin was part of such a complex borderland along the tectonically active margin of central California. Basins within this borderland exhibit complex lithofacies and biofacies patterns. A detailed analysis of bathyal lithofacies and foraminiferal biofacies within the late Miocene Stevens Sandstone indicates that climatic and eustatic fluctuations played a large role in controlling these patterns. A combined sedimentologic and micropaleontologic approach provides information concerning both the depositional history of a portion of this basin and the paleoceanographic history of the marine water that filled the basin. Lithofacies analysis of core samples from the Coles Levee area emphasizes lithologic variations, small-scale sedimentary structures, and bedding characteristics of sandstones. The rocks examined fit a modified Mutti and Ricci Lucchi (1972) lithofacies classification and form the basis for two lithofacies cross sections, one parallel and the other normal to the depositional trend of the Stevens Sandstone. The cross sections illustrate great depositional variability across the inferred Stevens submarine fan system and episodes of rapidfan progradation and retrogradation in response to lobe switching and eustatic fluctuations. A quantitative analysis offoraminiferal assemblages within the Stevens Sandstone defines four major biofacies: 1) an agglutinated spp. biofacies; 2) a Bolivina vaughani biofacies; 3) an Uvigerina subperegrina biofacies; and 4) a mixed calcareous biofacies. By comparison with foraminiferal distributions in the modern southern California borderland, the stratigraphic distribution of Stevens biofacies is interpreted to represent systematic variations in oxygen concentration of basin waters. Such variations can reflect either a eustatic or climatic control. Episodes of carbonate dissolution in the Stevens may also be climatically controlled and may correlate to global carbonate-dissolution events.
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