Diatomaceous shales of the Kreyenhagen Formation underlie much of the western San Joaquin valley of California and contain a remarkable record of middle and late Eocene paleo-oceanographic events along the evolving continental margin of this region. The Kreyenhagen Formation is similar in many respects to the much studied Miocene Monterey Formation of California with both units constituting known or potential sources of mature hydrocarbons due largely to their unusual biogenic character and similar but not identical depositional histories. Lithofacies and biofacies patterns within the Kreyenhagen indicate that it was deposited at upper and middle bathyal depths with the occurrence of laminated diatomites marking the intersection of a well-developed oxygen minimum layer ag inst continental slopes and basin sills analogous to Holocene continental margin settings off Peru, California, and in the Gulf of California. Quantitative analysis of benthic Forminifera within a 565 ft (172 m) thick sequence of Kreyenhagen shale in the Tumey Hills area of Fresno County illustrates that three low diversity - high abundance biofacies, dominated respectively by Bulimina corrugata, B. microcostata, and Bolivina spiralis, are indicative of various slope and base-of-slope environments characterized by differing levels of dissolved oxygen as dictated by the thickness and intensity of Eocene oxygen minima. Moreover, Bolivina spiralis appears to be homeomorphic with Suggrunda eckisi, a Neogene species with an apparent affinity for oxygen minimum facies. The sequential appearanc of massive and laminated diatomites and associated low oxygen megafaunal and microfaunal associations within the Kreyenhagen Formation allows estimates to be made of dissolved oxygen values in the Eocene water masses which, in turn, imply variations in climatically tuned rates of upwelling, siliceous productivity, and mid-water circulation as a function of the deteriorating global climate during middle and late Eocene time. Finally, the biogenic character of the Kreyenhagen diatomites within a Paleogene stratigraphic sequence otherwise dominated by terrigenous clastics suggests that deposition of this unit was coincident with a period of reduced flux of diluting terrigenous sediment to the continental margin likely induced by eustatic and/or tectonic adjustment of the Eocene strandline. /P>

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