Chemostratigraphic and lithostratigraphic framework of the Eocene Kreyenhagen Formation, Kettleman area, central San Joaquin Basin, California

Abstract

ABSTRACT The Eocene Kreyenhagen Formation is a widespread siliceous, organic-rich mudstone within the San Joaquin Basin, but it is less studied than the Monterey Formation. This study characterizes the Kreyenhagen Formation in the Kettleman area to define its vertical and lateral variability on the basis redox conditions (Mo, U, Cr), paleoproductivity (biogenic SiO2, P, Ba), and detrital input (Al2O3, TiO2) to determine the dominant environmental conditions during deposition. The Kreyenhagen Formation was correlated across 72 wells over a 4600 km2 (1776 mi2) area, which revealed an eastward thinning from 335 m (1100 ft) to less than 183 m (600 ft). We identified three informal members on the basis of log response and bulk/trace geochemistry: a lower calcareous silty mudstone, a middle organic-rich clayey mudstone, and an upper siliceous silty mudstone. Spatially, the greatest enrichment of total organic carbon, redox proxies, and biogenic silica occurs along Kettleman North Dome. These properties decrease eastward as clay volume, titanium, and aluminum increase. We interpret the Kreyenhagen Formation to record one transgressive-regressive cycle with contemporaneous climatic cooling: a transgression with initial suboxia and calcareous plankton productivity, a highstand with anoxic-euxinic benthic conditions and clastic starvation, and regression with elevated biogenic silica input. The upward transition from a calcareous to siliceous composition may reflect known cooling and upwelling intensification on the middle Eocene California margin. Mo/U and Th/U patterns suggest variable redox conditions across space and time. Lateral compositional trends indicate that eastern areas were proximal to a Sierran clastic sediment source, while western areas were distal and more anoxic.