Origin of Eocene Depositional Sequences in the Sacramento Basin, California: The Interplay of Tectonics and Eustasy

Sullivan, Raymond¹, Morgan D. Sullivan² (1) San Francisco State University, San Francisco, CA (2) Chevron Energy Technology Company, Houston, TX

The Eocene strata in the Sacramento Basin can be subdivided into six depositional sequences, and each of these can be correlated with a third order regressive-transgressive cycle observed on the global coastal onlap curve. Two types of unconformity bounded sequences are recognized. The first type is associated with submarine canyon formation. These canyons were filled with bathyal shales and turbiditic sandstones, and are represented by the Meganos "C', Capay and Sidney Flat/Kellogg shales. The second type of sequence is associated with predominantly estuarine/fluvial sands. Examples include the Hamilton, Domengine and Ione sands. The close correlation between the chronologic occurrence of these sequences and the global sea level chart suggests a strong eustatic control on the timing of the depositional cycles in the basin. Tectonism, however, is interpreted to have controlled the location of the submarine canyons and the estuarine/fluvial incisements since they stack vertically and follow the tilt of the basin toward the depocenter in the southwest margin of the basin. The interplay of tectonism and relative sea level change also was a determining factor as to which type of depositional sequence formed in the basin. If subsidence rates exceeded the rate of relative sea level fall, less of the shelf was exposed, and submarine canyons were formed. Conversely, when the rate of relative sea level fall exceeded subsidence rates, then the shelf was exposed, and widespread fluvial and estuarine systems developed on the basin margin.