Stratigraphic analysis of the Mabury Formation, Kern and Kings Counties, California

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 versus middle Eocene) and environment of deposition (shallow versus deep). 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, CPll (Discoaster lodoensis) 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 polychaete worm Rotularia (Rotularia) tinajasensis and discocyclinids, macrofossils within the sandstones have been pulverized into fossil-hash concentrations. An unusual transitional nannofossil species that resembles both Discoaster lodoensis and Discoaster sublodoensis was found in the Devils Den area. The Mabury Formation crops out in two other areas: Media Agua Creek, 40 km south ofDevils Den where the section is 10-m thick, and Avenal Ridge, 10 km northwest ofDevils Den where the section is 480-m thick. At Avenal Ridge, the Mabury Formation is equivalent to the Acebedo and Avenal Formations. 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.