Geochemistry of the Cretaceous Mowry Shale in the Wind River Basin, Wyoming

Abstract

The siliceous nature of the Mowry Shale distinguishes it from many of the well-studied organic-rich mudstones of the Cretaceous Western Interior Seaway. Available models of organic enrichment in mudstones rarely incorporate detailed biomarker, bulk organic, inorganic, and mineralogy data. Here, we used these data to evaluate how variations in organic matter source, productivity, dilution, and preservation modulated organic matter accumulation during the deposition of the Mowry Shale, while also demonstrating the benefits of this integrated approach. An organic stable carbon isotope vertical profile for the Mowry Shale is presented to test whether the Mowry Shale was deposited during oceanic anoxic event 1d (OAE 1d), thereby contributing to organic enrichment in the Mowry Shale. Samples were analyzed from three thermally immature drill cores collected from the Wind River Basin, Wyoming to advance our understanding of the conditions that led to the formation of the Mowry Shale. Results from bulk organic and inorganic geochemistry and biomarkers show that redox-driven preservation is closely coupled with stratigraphic changes in organic matter content in intermediate to distal settings of the Mowry Shale. Biogenic silica dilution decouples productivity and organic enrichment, such that the interval that was deposited during the highest productivity is offset from the interval containing maximum organic content. These findings contrast with previous studies that identified productivity as the primary driver of organic enrichment in distal settings of the Mowry Shale. The middle and lower Mowry Shale contain primarily marine organic matter, whereas the relative contribution of terrestrial organic matter increases in the upper Mowry Shale and overlying Frontier Formation as relative sea level declined and the western shoreline prograded eastward. Finally, the organic stable carbon isotope profile of the Mowry Shale together with recent radioisotopic dating suggest that the Mowry Shale postdates OAE 1d by ∼1−2 million years (m.y.), providing another example of temporal offset between organic matter accumulation in the Cretaceous Western Interior Seaway compared to other regions that record global OAEs.