Abstract

Upper Cretaceous and Paleogene strata in the forearc Sacramento Basin have been affected by fault activity associated with a north-northwest trending syndepositional growth fault system. This system, the Midland Fault System, is the dominant structural feature in the southern Sacramento Basin.

Syndepositional movement along the Midland Fault System has resulted in a thickening of Upper Cretaceous and Paleogene strata along the western (downthrown) side of the growth faults, and the formation of domal structures on the downthrown side of growth faults at the Bunker, Lindsey Slough, and Rio Vista gas fields. Late Paleocene-early Eocene growth fault activity resulted in the formation of the domal structure at the Rio Vista Gas Field, as evidenced by the thinning of the Anderson Sand and Meganos Shale near the crest of this structure. The relatively constant thickness of the McCormick Sand and Martinez Shale west of the Midland Fault, and the increased rate of post-Martinez Paleogene deposition stepping westward across the fault system, indicates a Late Paleocene through Eocene age for most Midland Fault System activity in the Rio Vista area. Early Eocene movement along the Midland Fault System has resulted in the removal of the Hamilton Sand, Meganos Shale, Anderson Sand, Martinez Shale, and the upper portion of the McCormick Sand east of the fault system, with only the lower portion of the McCormick Sand remaining east of the fault system.