Abstract

The Kirby Hill Fault Zone (KHFZ), which crops out on the Kirby Hill gas field in Section 30, T4N - R1E and in Section 25, T4N - R1W (MDB&M), was first recognized by surface mapping in the 1930’s. Kirby Hill, which is 40 miles northeast of San Francisco, geologically and topographically reflects a faulted structural high that is nearly surrounded by lowlands of the alluviated tideland and marshland of the Sacramento River delta and associated bays and sloughs. Strata ranging in age from late Eocene (Nortonville and Markley formations) to Miocene and Pliocene (nonmarine deposits) are exposed at Kirby Hill. Dips vary widely, but mostly east dips ranging from 30° to 80° are predominant.

Early exploration and gas discoveries, evolving structural interpretations, the projection of the Kirby Hill Fault northward and southward of the Kirby Hill gas field, and the imprint of Neogene compressional tectonics on the older arc-trench extensions tectonics that persisted throughout the Paleogene are examined. Fault displacement of 7500 ft. has been documented, with recurrent movement from Late Cretaceous through late Eocene. Neogene lateral movement across the KHFZ appears negligible, based on the mapped distribution of the “Black Diamond Coal Seam” in the Eocene Domengine Formation.