The Miocene Monterey Formation records the deep basinal phase of a major late Tertiary cycle of basin formation and filling associated with wrench fault tectonism. Over much of coastal California, Neogene facies show a progression from Oligocene nonmarine and neritic rocks to Miocene basinal shales and siliceous rocks and then to upper Miocene to Pliocene turbiditic and neritic clastic rocks. In many places the Monterey consists of a basal calcareous facies, a middle transitional phosphatic facies, and an upper thick siliceous facies composed of diatomaceous rocks and their diagnetic equivalents (cherts, porcelanites, etc). Hydrocarbon-producing turbidite sandstones occur in these basinal rocks in the southern San Joaquin Valley and in redeposited pelletal phosphorites in the central Coast Ranges.

By analogy with modern environments of phosphate formation off South America and Africa, Monterey phosphatic shales probably represent phosphatization of shelf-slope-basin muds near the boundaries of the oxygen minimum zone. Nodular and pelletal phosphorites may also have formed near the boundaries of the oxygen minimum zone, but on sediment-starved bank tops and shelves. The widespread siliceous facies represents rapidly deposited diatom ooze that records high plankton productivity spawned by late Miocene climatic deterioration and intensified upwelling. Abundant organic matter, ecologic inferences from faunal data, and sedimentary structures such as alternations of massive and laminated cycles indicate that these siliceous rocks also formed as basin, slope, and shelf deposits within or near the fluctuating boundaries of a well-developed oxygen minimum zone--a depositional environment similar to the present Gulf of California and western margin of South America.

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