ABSTRACT: The Kirby Hill Fault
MACKEVETT, NAT H., Consultant, Bakersfield, CA
The Kirby Hill Fault (KHF), which crops out on the east side of the Kirby Hill gas field in Sec. 30, T4N, R1E (MDB&M), was first recognized by surface mapping in the 1930s. Kirby Hill, which is some 40 mi northeast of San Francisco, geologically and topographically reflects a complex faulted structural high nearly surrounded by lowlands of the alluviated tideland and marshland of Sacramento River delta and associated bays and sloughs. The rolling low-lying Montezuma Hills (up to 285 ft) impinge Kirby Hill on its easternmost margin. Natural waterways, Nurse Slough and Montezuma Slough, provide northwestern and southwestern boundaries, respectively. Kirby Hill rises in elevation from sea level to 361 ft and covers about 700 ac of uplifted area in Sec. 25, T4N, R1N, and Sec. 30, T4N, R1E (MDB&M). Outcrops ranging from the upper Eocene Markley and Nortonville formations to Pliocene-Miocene nonmarine Tehama formation, are exposed at Kirby Hill, and show generally easterly dips ranging from 30 to 80 degrees, with the norm in the 30-40 degrees easterly range.
Early exploration and gas discoveries, evolving structural interpretations, the projection of the Kirby Hill fault northwest and southeast of the Kirby Hill gas field based on surface and subsurface data, and the imprint of Neogene compressional and transpressional tectonics on the older arc-trench extensional tectonics that persisted through the Paleogene are examined. Fault displacement of 7500 ft has been documented, with recurrent movement from Upper Cretaceous through upper Eocene. Recent lateral movement is suspect but not proven.