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ABSTRACT: Anatomy of an Organic-Rich Condensed Section: The Campanian Sharon Springs Member of the Pierre Shale, Powder River Basin

T. S. Loutit, A. E. Bence, J. Smale, J. D. Shane

The Sharon Springs Member of the Pierre Shale is an early Campanian organic-rich rock deposited over a wide region of the Cretaceous Interior Seaway. Widespread exposure and detailed subsurface cross sections provide an ideal setting to reconstruct details of its depositional system. The Sharon Springs, whose organic matter component is dominated by marine algae, has concentrations of total organic carbon that average about 5 wt. % in the eastern Powder River basin to about 13 wt. % in eastern South Dakota. Thicknesses range from approximately 140 ft in the west, near Redbird, Wyoming, to about 10 ft near Chamberlain, South Dakota. The Sharon Springs section near Redbird can be divided into two parts, a lower shale having smectite-dominated clays, total organic carbon (TO ) = 2-5 wt. %, TOC/S = 0.1-0.2, and Ni(Ni+V) = 0.2, and an upper shale with more illite-rich clay, TOC = 5-8 wt. %, TOC/S = 0.2-0.6, and Ni/(Ni+V) = 0.05-0.1. Progressive changes in La/Sm from the lower unit (La/Sm = 2-6) to the upper unit (La/Sm = 7-10) and in K2O, AI2O3, and TiO2 reflect changes in the provenance of the clastic component. The two units can be correlated over a large area of the Interior Seaway using inorganic and organic chemical markers and physical stratigraphy.

The bulk inorganic and organic properties of the Sharon Springs, as well as the molecular characteristics of the soluble organic fraction, suggest that it was deposited at relatively low sedimentation rates under anoxic bottom-water conditions in a stratified water column.

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