Subsurface Structure in the Montezuma Hills, Southwestern Sacramento Basin

Abstract

Abstract The Montezuma Hills, in the southwestern corner of the Sacramento Basin, are underlain by a deep, asymmetric, south-plunging graben that is bounded on the east by the Midland Fault System and on the west by the Kirby Hills Fault System. The thickest and most complete Paleogene stratigraphic section in northern California is preserved within this graben, and the base of the Tertiary lies below 11,000 ft along the graben axis. Commonly refered to as the “delta depocenter”, the graben is herein termed the Rio Vista Basin. Both west-dipping and east-dipping normal faults of the Midland Fault System are present along and east of the eastern boundary of the Montezuma Hills. These faults form a complex network of north-to-northwest striking faults. Paleocene strata are offset by as much as 500 ft, but these faults die out upward and do not offset Neogene strata. West of the Midland system, the Sherman Island Fault System forms a wide, arcuate zone of east-dipping normal faults that dissects the gently-dipping eastern flank of the basin. Mapped vertical offsets on faults of this system range from less than 100 to over 400 ft on Paleocene horizons. Activity in this system progressed from east to west as older faults died out and new faults developed farther west. In the northwestern corner of the Montezuma Hills, the Sherman Island system converges toward several slightly older faults that dip in the opposite direction, forming a horst block that is partially responsible for the Denverton Creek gas field. These faults trend nearly north-south and offset Paleocene strata by as much as 300 ft, but they die out upward near the base of the Eocene section. West of the Rio Vista Basin axis, Paleogene strata dip to the east and are offset by several east-dipping normal faults along the western edge of the Montezuma Hills. These faults strike north-south and offset Paleocene strata by as much as 1000 ft. At least one west-dipping reverse fault is also present in this area, and offsets Eocene strata by approximately 100 ft. These faults are herein termed the Montezuma Fault System.