70 Million Years of Coastal Upwelling in California; Source Rocks and Paleoceanography

C. Fonseca

The Late Mesozoic-Cenozoic marine sequence of California displays a unique record of coastal upwelling and productivity in the form of distinctive diatomaceous and organic-rich deposits including the upper Cretaceous-lower Paleocene Moreno Formation, the Eocene Kreyenhagen Formation and the Miocene Monterey Formation. Unique sedimentological characteristics of these ancient upwelling systems include a) Finely laminated biosiliceous-rich shales (>30% biogenic silica content), b) Distinctive laminae composed by monospecific diatom resting spores, c) Good source rock quality (>300 mg HC/mg org C), and d) High accumulation rates in mid water anoxic conditions. Detailed study of individual laminae in sediments of these formations revealed concentration of monospecific di tom resting spores and an abundance of Stephanopyxis sp. and Coscinodiscus sp. Like Recent upwelling systems, preserved laminations of monospecific resting spores reflect strong seasonal upwelling that lead to high organic matter production and enhancement of anoxia. The presence of spores in the ancient and modern upwelling systems is evidence that diatoms have adapted and successfully competed in the highly productive California Margin since the Late Cretaceous.

The Moreno, the Kreyenhagen and the Monterey Formation account for a significant portion of major petroleum source beds in California and contain an important record of coastal upwelling and paleoceanographic change in the northeastern Pacific Ocean over the past 70 million years. It is suggested that potential Late Maestrichtian source rocks could have been deposited along other favorable upwelling areas of the Eastern Pacific Rim.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California