Abstract: Refinements in Age and Depositional Environment of the Eocene Mabury Formation, San Joaquin Valley, California

Michael P. Gring, Richard L. Squires

The Mabury Formation, crops out along the west side of San Joaquin Valley in Kern and Kings counties. Since 1943, when the formation was named at its type section in the Mabury Hills south of Devils Den, there has been disagreement as to the geologic age (early vs. middle Eocene) and environment of deposition (shallow vs. deep). Our analysis of the calcareous nannofossils and facies distribution of the formation in the typesection area (63-m thick) revealed that the geologic age is latest early Eocene, CPl1 (Discaaster ladoensis) zone, to earliest middle Eocene, CP12a (Discoasteroides kuepperi) subzone, and the depositional environment is submarine channels filled with discontinuous turbidite sandstones and a localized limestone conglomerate. Other than tubes of the olychaete worm Rotularia (Rotularia) tinajasensis and discocyclinids, macrofossils within the sandstones have been pulverized into fossil-hash concentrations. An unusual transitional nannofossil species that is morphologically between Discoaster lodoensis and Discoaster sublodoensis was found in the Devils Den area.

We also recognized the Mabury Formation to crop out in two other areas: Media Agua Creek, 40 km south of Devils Den where the section is 10 m thick, and Avenal Ridge, 10 km northwest of Devils Den where the section is 230 m thick. At Avenal Ridge, the Mabury Formation is equivalent to the Acebedo Formation. The age of the Mabury Formation in these two areas is approximately the same age as that at Devils Den, but there is deepening from Avenal Ridge (shallowest), to Devils Den (moderate depth), to Media Agua Creek (deepest). At Media Agua Creek, there is a basal-cobble conglomerate containing fragments of hermatypic colonial corals and large gastropods.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994