Chemostratigraphic and lithostratigraphic variability of the Eocene Kreyenhagen Formation in the Kettleman Area, San Joaquin Basin, California

Abstract

The Eocene Kreyenhagen Formation is a widespread siliceous, organic-rich mudstone of the San Joaquin Basin (SJB). However, in comparison to the highly studied siliceous Monterey Formation, studies of the Kreyenhagen are limited. This study completes a stratigraphic characterization of the Kreyenhagen with emphasis on chemostratigraphy to understand its compositional variability and depositional history. The Kreyenhagen was subdivided into 8 zones (A-H) and correlated across 128 wells to define its stratigraphic framework in the Kettleman area. Cross-sections and isopach maps show an eastward thinning from 1,100 to less than 600 feet. Petrophysical estimates of TOC and clay volume were coupled with bulk and trace elemental geochemistry as proxies of detrital input, paleo-productivity, and benthic redox conditions. The lower Kreyenhagen (H-E) does not display significant enrichment in any major compositional components. The middle Kreyenhagen (E-C) is the most distinctive, encompassing the zone of greatest TOC and enrichment in elemental proxies for anoxia (D). Also distinct in the middle Kreyenhagen is an upward trend of decreasing detritus and increasing biogenic quartz content. Lastly, the upper Kreyenhagen (B-A) cycles between low and moderately high values of detritus, biogenic quartz, and redox proxy character. Regional gridding of compositional data reveals a trend of high TOC sub-parallel to the present-day basin axis and an eastward increase in detritus. The Kreyenhagen succession records at least one major transgressive-regressive cycle. The middle interval likely represents the period of highest sea-level, indicated by high TOC and enrichment of trace elemental proxies for anoxia. Additionally, the vertical increase in biogenic components may reflect an increase in primary productivity associated with known middle-late Eocene climatic cooling events. Lateral compositional trends may indicate that 1) some terrigenous sediment was delivered from the east from the Sierran Magmatic Arc, and 2) a northwest trending oxygen deficient environment, potentially caused by restricted ocean circulation associated with the uplifted Franciscan subduction complex. Findings reveal spatial and temporal changes in benthic environments within the northern SJB during the Eocene. Further, relationships between geochemical proxies and lithofacies demonstrate the depositional controls on composition in siliceous, organic-rich mudstones.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019