ABSTRACT: The Midland Fault Zone, Southern Sacramento Basin, California
PEPPER, MIKE W., Consulting Geologist, Loomis, CA, and DANE S. JOHNSON, Consulting Geologist, Clovis, CA
Paleogene tectonic activity in the delta depocenter of the southern Sacramento basin has resulted in the formation of a system of north-south-trending faults along the eastern flank of the depocenter. The Midland fault zone, a syndepositional growth fault, is the dominant fault of the system. Numerous synthetic and antithetic normal faults emanate obliquely from, or parallel to, the Midland fault throughout its 55-mi length. En echelon faults are characteristic of the northern part of the fault zone.
Initial tectonic activity along the Midland fault zone occurred in the middle Paleocene, with renewed uplift during late Paleocene. The syndepositional character of tectonic activity along the Midland fault zone is documented by the differential offset of strata across the main branch of the fault zone. Average offset of the Cretaceous Starkey sandstone is 1200 ft in the south (T1N), 2500 ft in the Rio Vista area (T4N), and 300 ft in the north (T7N). Typical offset at the top of the Eocene Domengine Sandstone is 300 ft in the south, 400 ft in the Rio Vista area, and 50 ft in the north, demonstrating progressive growth-fault movement through time. Movement along the Midland fault zone probably ceased during the Oligocene, as the faults do not appear to offset sediments within the Oligo ene Markley submarine canyon.
The resultant complex of stratigraphy and faulting of the Cretaceous, Paleocene, and Eocene deposits along the Midland fault zone has resulted in the establishment of several prolific gas fields along most of its length. These gas fields include the Dutch Slough, Rio Vista, Lindsey Slough, Bunker, Millar, and Winters.