Chemostratigraphic and lithostratigraphic framework of the Eocene Kreyenhagen Formation: Kettleman area, northern San Joaquin Basin, CA

Abstract

The Eocene Kreyenhagen Formation is a widespread siliceous, organic-rich mudstone of the San Joaquin Basin. However, it is significantly understudied in comparison to the prolific Miocene Monterey Formation. This study completes a subsurface characterization of the Kreyenhagen at the Kettleman area to understand its vertical and lateral compositional variability. We integrate well log analysis with commonly used indicators of redox (Mo, U, Cr), productivity (biogenic SiO2, P, Ba), and detritus (Al2O3, TiO2) to determine the various oceanographic and depositional conditions that persisted during Kreyenhagen time. The Kreyenhagen is subdivided into 8 zones (A-H) and correlated across 72 wells. Cross-sections and isopach maps show eastward thinning from 1,100 ft to less than 600 ft across a 35 mile transect. Its structure mirrors the present-day geometry of known oil fields, including Coalinga and Kettleman North Dome. On the basis of well log and geochemistry data, we identify three Kreyenhagen members: a lower calcareous silty mudstone (H-F), a middle organic-rich clayey mudstone enriched in redox-sensitive trace elements (E-D), and an upper siliceous silty mudstone with high paleoproductivity signals (A-C). This vertical facies trend is generally consistent with previous outcrop studies of the Kreyenhagen type section on Reef Ridge. Laterally, the greatest enrichment of TOC, redox-sensitive trace elements, and biogenic silica occur sub-parallel to the present-day basin axis. These properties decrease eastward while clay volume, titanium, and aluminum increase. We interpret the Kreyenhagen to record at least one transgressive-regressive cycle with contemporaneous climatic cooling. Specifically, a transgression with the initiation of suboxia, a sea-level highstand with anoxic-sulfidic benthic conditions and clastic starvation, and a regression with high input of detritus and biogenic silica. The gradual upward switch from a calcareous to a siliceous composition may reflect known climatic cooling and upwelling intensification along the middle Eocene California Margin. Molybdenum-U and Th-U covariation patterns suggest frequent redox fluctuations and the possible operation of an Mn-Fe-oxide particulate shuttle. Lateral compositional trends imply that eastern areas were proximal to a Sierran clastic sediment source while western areas were distal and exhibited stronger redox conditions.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)