A High-Resolution, Multi-Proxy Study of Organic-Rich Mudrocks From the Mid Cenomanian Event in North America

Abstract

The Mid Cenomanian Event (MCE) is one of several extreme Cretaceous climatic events, which are generally characterized by extended anoxic conditions through the water column and/or at the sediment-water interface, and thus referred to as oceanic anoxic events (OAEs). Whilst in-depth studies of the MCE are in their relative infancy, the event is considered a major turning point within the mid-Cenomanian in terms of climatic and oceanographic systems. However, the nature of the MCE is not well understood, with ambiguity over whether it represents a truly global perturbation to the global carbon cycle, or is a prelude to OAE2 (∼93.9 Ma), the most pronounced global climatic event of the mid-Cretaceous greenhouse world. The MCE has been studied within the Rebecca K. Bounds-1 core from Kansas using a multi-proxy approach. Total organic carbon (TOC), organic carbon isotope (δ13Corg), trace element (TE) geochemistry, biomarker, and palynofacies data, along with SEM and microscopy provide a high-resolution, integrated insight into the nature of the MCE within the Western Interior Seaway (WIS). Key palynological bio-events and isotope stratigraphy allow correlation with other successions in the WIS (e.g. Eagle Ford Gr., Texas), whilst compound-specific isotope analyses allow for discrimination between local and global carbon isotope signals through comparisons between Kansas and the Demerera Rise (tropical North Atlantic). The MCE initiated during the early part of the 2nd order Greenhorn transgression connecting surface waters from the Tethys with those of the Boreal influenced epicontinental WIS. High TOC levels (≤9.2%) associated with a positive δ13Corg excursion identify these study sites as one of few localities where the MCE is characterized by organic-rich mudrock deposition. Consistent with global records, the event can be split into two phases: MCE1a and MCE1b, both with distinctive δ13Corg peaks (+1.0‰ and +1.2‰ respectively). The relationship between TOC and δ13Corg differ dramatically between the two stages of the event, with an inverse trend present during MCE1a and, conversely, a concordant trend during MCE1b. Redox-sensitive TE enrichment is mostly absent throughout the MCE, indicating long-term anoxic conditions did not develop in the study area. However, through integrated geochemical and visual analyses of organic matter, and using statistical techniques, this study clearly delimits four environmental facies through the MCE in Kansas, defining a terrestrial-marine shift with periods of intensified water column stratification indicated by high Mo, U and pyrite during times of maximum TOC preservation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016