Paleo-redox Conditions during Deposition of the Devonian New Albany Shale (Illinois Basin) and Correlative Black Shales in the Eastern US: Multiple Perspectives from Geochemistry, Paleoecology, and Petrography

Lazar, Ovidiu Remus¹, Juergen Schieber¹ (1) Indiana University, Bloomington, IN

Deposition of organic matter-rich mud into a persistently anoxic epicontinental sea has been a widely held view regarding the formation of Late Devonian hydrocarbon source rocks of the eastern U.S. Recent sedimentologic and geochemical investigations suggest that conditions may have been anoxic for some units, but more commonly must have been in the dysoxic realm. This study, conducted on the New Albany Shale of the Illinois Basin for better discrimination of different oxygenation states, utilized high resolution core descriptions, geochemical analyses, and petrographic investigations. Geochemical paleo-redox proxies, such as DOP, V/Cr, V/(V+Ni), Ni/Co, U/Th, and Mo/Al, give inconsistent results and clash with core observations (intensity of bioturbation and indicators of current activity). Pyrite framboid size distributions for all analyzed samples suggest that bottom waters were depleted in oxygen at the time of mud deposition but not persistently anoxic. Agglutinated benthic foraminifera, organisms that require at least some oxygen in order to persist at the seafloor, are widespread in the black shales of the New Albany Shale. They are also common in other portions of the Late Devonian black shale complex, covering the entire geographic range from Tennessee to New York. Converging paleoecological (bioturbation) and petrographic (framboids, foraminifera) views of Late Devonian black shale environments suggest an oxygen stressed environment like that of the modern Santa Barbara Basin, rather than the persistent anoxic/euxinic setting envisioned by many previous investigators.