ABSTRACT: Stratal Geometries Constrain Paleodepth to Top of Low-Oxygen Bottom Water, Lower Campanian (Cretaceous) Pierre Shale, Eastern Wyoming

Frederick B. Zelt

The Sharon Springs Member of the Pierre Shale is an organic-rich clayshale that was deposited in the early Campanian in the central and eastern parts of the Western Interior seaway. The Sharon Springs Member contains as much as 8% total organic carbon, abundant fecal pellets, phosphate nodules, high concentrations of marine vertebrate remains, and numerous altered ash beds. Previous workers have cited these as evidence for enhanced paleoproductivity due to upwelling. However, existing K-Ar age dates and a regional age model indicate that the rate of organic carbon accumulation in the Sharon Springs was only moderate (0.49g C/cm²/1000 yr). The physical stratigraphy of the basal Pierre Shale in the eastern Powder River Basin indicates that the Sharon Springs Memb r is a condensed section that was deposited under a stratified water column. Seismic data and well logs show that the Sharon Springs occurs at the base of large clinoforms. Stratal geometry was used to reconstruct paleobathymetry at Sharon Springs time. Well logs were also used to map the western limit of organic-rich facies, which is interpreted to represent the western limit of low-oxygen bottom water. The paleobathymetric map was combined with the facies map to document the paleodepth of the transition from oxygenated shallow water to low-oxygen bottom water. The depth to the top of low-oxygen bottom water in Sharon Springs time was approximately 600 ft (180 m) in eastern Wyoming.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990