Paleo-Oceanographic Significance of Cretaceous and Cenozoic Diatomites Along Eastern Pacific Margin: ABSTRACT Free

Diatomaceous mudstones and laminated diatomites punctuate the upper Mesozoic and Cenozoic marine sequence of California. These largely bathyal deposits provide a clear record of upwelling, primary productivity, and development of oxygen minima along the eastern margin of the North Pacific Ocean during the past 80 million years. Marine diatomites exposed in uplifted continental margin sequences in California include the uppermost Cretaceous Marca Shale Member of the Moreno Formation, the middle and upper Eocene Kreyenhagen Formation, and the middle to upper Miocene Monterey Shale. All three of these deposits contain pelletal or nodular phosphorite and represent fossil analogs of various Recent basin plain, slope, and outer shelf settings in which organic-rich diatomaceous sediments are currently accumulating beneath the well-developed oxygen minimum layer of the marginal eastern Pacific. Moreover, each of these units forms a known or potential source rocks for hydrocarbons in this region. The deposition and preservation of Cretaceous, Eocene, and Miocene diatomites along the California margin each demand: (1) a period of intensified upwelling, primary productivity, and associated development of oxygen minima via climatically inducted accelerations of atmospheric and oceanic circulation; (2) coincident reductions in the flux of terrigenous clastic material to the continental margin through eustatic and/or tectonic adjustment of adjacent strandlines and pathways of sediment distribution; and (3) tectonic production of appropriate continental margin depocenters. Each diatomaceous unit can be correlated with a major climatic event or threshold associated with increased polar refrigeration, resultant increases in the pole-to-equator thermal gradient, compression of middle and low latitude surface circulation, and associated intensification of upwelling and siliceous productivity.