Paleoenvironmental constraints on Paleozoic shale deposition in the midcontinent United States

Trace element paleoenvironmental proxies were used to constrain depositional environments for several black shales of the midcontinent United States to better understand the formation of metalliferous shales. These shales range in age from Cambrian to Pennsylvanian. The proxies evaluated were for paleoredox (U/Th, U-[Th/3], Ni/Co, V/Cr, V/[V+Ni], Mo concentration, Mo/total organic carbon), basin restriction (Cd/Mo, Co × Mn) and paleosalinity (Sr/Ba). The results of the paleoredox proxies indicate a range of depositional conditions from oxic to dysoxic to anoxic. The findings suggest that the Cambrian Mt. Simon, Eau Claire, and Tunnel City samples in the northern part of the study area were deposited under oxic marine conditions influenced by upwelling. The Ordovician black shales from the Ouachita Mountains and the Pennsylvanian shales from the Cherokee and Forest City Basins were likely formed under anoxic, open marine conditions. The basin restriction and paleoredox proxies suggest decreasing oxygen levels during the deposition of the Ordovician shales, whereas the paleosalinity proxy, the Sr/Ba ratios, during this time suggests decreasing salinity. The Devonian Chattanooga Shale from the Ozark Dome and the New Albany Shale from the Illinois Basin were likely deposited under similar anoxic to dysoxic conditions. Paleoredox proxies suggest that the Mississippian Fayetteville Shale in the Ozark Dome formed under a range of oxic to anoxic conditions. Similarly, the Pennsylvanian Atoka and Jackfork Formations in the Ouachitas were deposited under oxic marine conditions. The results of this study also highlight the importance of using multiple proxies to interpret paleoenvironments.