Early Eocene Uplift of Southernmost San Joaquin Basin, California

Stephen A. Reid, Brett F. Cox

Stratigraphic studies in the southern San Joaquin basin and in the El Paso Mountains of the southwestern Great Basin corroborate a hypothesized early Eocene regional uplift event. Eocene uplift and erosion of the southernmost San Joaquin basin south of Bakersfield were recently proposed because an early Paleogene fluviodeltaic sequence in the El Paso Mountains (Goler Formation) apparently had no seaward counterpart to the southwest. New microfossil data (coccoliths) indicate that marine deposits near the top of the Goler Formation are uppermost Paleocene (nannofossil zone CP8) rather than lower Eocene, as reported previously. These data (1) confirm that the oldest known Tertiary strata south of Bakersfield (Eocene Tejon Formation) are younger than the uppermost Goler Form tion and (2) seem to restrict uplift to the earliest Eocene.

Other evidence of early Eocene uplift was obtained from wells in the southern San Joaquin basin. West of Bakersfield, middle Eocene strata rest directly on crystalline basement; uppermost Cretaceous (Maastrichtian) and Paleocene strata are absent and apparently were eroded.

Well data also indicate a trough-shaped sandy and conglomeratic submarine canyon and deep-sea fan deposit (Mushrush sandstone member of the Lodo Formation) which is up to 800 ft thick, 11 mi wide, and at least 23 mi long, northwest of Bakersfield. Grain size decreases north-westward along the axis of the trough, implying a source to the south-east. Correlations suggest an early Eocene (Penutian) age for the trough fill. We propose that the uppermost Cretaceous and Paleocene deposits were eroded and the Mushrush trough was cut and filled mainly in response to earliest Eocene uplift. The uplift was transverse to the northwest-trending forearc basin. Thus, it was distinct from late early Eocene (pre-Comengine Formation) regional tilting and uplift, which produced northwest-trending struc ures. Early Eocene uplift probably played only a minor role in the southward termination of pre-Maastrichtian parts of the forearc basin, which we instead attribute to massive uplift of the southernmost Sierra Nevada during the early(?) Late Cretaceous.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.