--> Organic-Rich Sediments From the Mid-Cretaceous Western Interior Seaway: A High-Resolution, Multi-Proxy Investigation

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Organic-Rich Sediments From the Mid-Cretaceous Western Interior Seaway: A High-Resolution, Multi-Proxy Investigation

Abstract

The greenhouse world of the Cretaceous marked a time in the Earth's history during which there were numerous short-lived but extreme climatic perturbations, known as oceanic anoxic events (OAEs). These are synonymous with deposition of organic-rich sediment, carbon isotope excursions, and extreme biotic turnover due to prevailing anoxic conditions within oceanic realms. In particular, OAE2, one of the most globally extensive and best-studied OAEs marks the Cenomanian-Turonian stage boundary. The Middle Cenomanian Event (MCE) occurred ∼2 Myr prior to OAE2 and is thought to be a “precursor event” and a major turning point within the mid-Cenomanian in terms of climatic and oceanographic systems. Thus, determining climatic drivers and environmental responses to these events is key to understanding the effects of OAEs on the global carbon cycle. The Western Interior Seaway (WIS) of North America has many mid-Cretaceous successions in which the preservation of organic matter is excellent, such as the AMOCO Rebecca K. Bounds-1 core from Kansas. Spanning from the Albian-Cenomanian boundary to the Cenomanian-Turonian boundary, a high-resolution multi-proxy study of this core section (5 cm to 90 cm intervals) provides a detailed insight into the ecological, environmental, and climatic variability along the eastern margin of the central WIS. This multi-proxy investigation consists of total organic carbon (TOC), organic carbon isotope (δ13Corg) and biomarker analyses, Rock-Eval pyrolysis, trace element geochemistry via ICP-MS and XRF, backscatter electron microscopy and organic petrology, along with palynofacies analyses. Terrestrial and marine palynological biostratigraphy through the whole core section provides key observations that can be used to fill gaps prior to the well-defined ammonoid and inoceramid biostratigraphic framework established for the WIS from the mid-Cenomanian onwards. This framework also permits biostratigraphic correlation of RKB-1 with Pueblo, CO, the location of the type sections for the Graneros Shale, Thatcher Limestone Member, and the GSSP for the Cenomanian-Turonian boundary and to key climatic events identified in other WIS localities (e.g. Eagle Ford Gr., Texas).