PALEOCEANOGRAPHIC CONDITIONS THAT RESULTED IN THE ACCUMULATION OF ORGANIC MATTER IN THE MIDDLE PENNSYLVANIAN HERMOSA GROUP, SOUTHWESTERN SHELF, PARADOX BASIN, UTAH

The Middle Pennsylvanian Hermosa Group exposed along Honaker Trail, Utah contains a series of black mudrocks that are cyclically interbedded with carbonates and coarser clastics. This study reconstructs the paleoceanographic conditions that persisted during Hermosa deposition with emphasis on times of black mudrock deposition. The character (isotopic and stoichiometric) and abundance of organic matter and bulk concentration of metals are used to constrain the conditions responsible for the accumulation of organic matter. Most mudrock horizons in the section were deposited under oxic to suboxic conditions resulting in low total organic carbon (TOC) accumulations (<0.5 wt.%). Two intervals, the A Shale and the upper portion of the Chimney Rock Shale, were deposited under anoxic-nonsulfidic conditions resulting in modest TOC accumulations (0.89 to 1.13 wt.%). One horizon, the lower portion of the Chimney Rock Shale, was deposited under anoxic-sulfidic conditions resulting in the highest TOC accumulations in the section (up to 6.18 wt.%). The organic matter throughout the Chimney Rock Shale displays characteristics indicative of deposition under anoxia, such as a negative δ¹³C_org excursion, a positive δ¹⁵N excursion, and elevated C_org/N ratios. The geochemical results integrate well with published sequence stratigraphy. The time of highest relative sea level coincides with the stratigraphic horizon characterized by the most reducing conditions.