Early Tertiary Sequences in the Southwestern Sacramento Basin

Raymond Sullivan and William E. Steinkraus

The early Tertiary strata of the southwestern Sacramento Basin are composed of alternating successions of marine sandstones and shales that total more than 2000 meters in thickness. The cyclic pattern of sedimentation is interpreted as representing third-order depositional sequences that formed along a tectonically active margin in early Tertiary times. The relative fall of sea level at the onset of each sequence resulted in the cutting of fluvial valleys on the shelf and submarine canyons on the slope. These incisements were filled during the late lowstand by estuarine sandstones on the shelf and turbidite sandstones and mudstones on the slope. The overlying transgressive and highstand systems tracts are made up predominantly of neritic and bathyal mudstones and siltston s. Regional tectonism also influenced sedimentation and north-south structures, such as the Midland and Kirker Pass faults which were active throughout Tertiary times. Another major structure that was important at this time was the Stockton Arch, an east-west uplift normal to the structural trend. Movements along this structure resulted in marked changes in the Tertiary succession on the north side of the Stockton Arch. Tertiary rocks in the Byron area rest unconformably on Cretaceous shales of early late Campanian age. Uplift of the Stockton Arch occurred during Paleocene, early Eocene, and mid Eocene times. As a result, the early Tertiary sequences on the northside of the Stockton Arch includes several major unconformities within the succession which demonstrates the importance of this transverse structure in regional stratigraphic interpretations.

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