LATERAL WATERMASS GRADIENTS RECORDED IN ORGANIC-RICH FACIES OF THE LATE DEVONIAN APPALACHIAN AND ILLINOIS BASINS, USA

The Late Devonian is known for extensive deposition of black shales across vast epicontinental seas that flooded Laurentia. Black shales have traditionally been associated with anoxia, which—in the case of the Late Devonian—may be related to enhanced nutrient loading as the spread of vascular plants led to increased weathering and pedogenesis. The watermass structure of modern silled basins is controlled by a complex set of processes, however, including density and solute gradients related to freshwater-seawater mixing, organic carbon export rates, and the degree of basin restriction, with many of these parameters being difficult to constrain in the geologic record. Here, we present paleo-redox and paleosalinity (Sr/Ba) data from six drill cores that form a transect from northeast to southwest through the Cleveland Shale of the Appalachian Basin and from one core through the Chattanooga Shale of the southern Illinois Basin. Iron speciation reveals the persistence of water column euxinia across the Appalachian and Illinois basins, with the possible exception of ferruginous conditions in the most basinal part of the Appalachian Basin. δ³⁴S_py reveals a distinct trend from 0 to –40‰ up-section through the Cleveland Shale, with a similar trend recorded over a longer time interval in the Chattanooga Shale. This combination implies progressive marine transgression, enhanced connectivity to the marine sulfate reservoir, and intensification of syngenetic pyrite formation in a euxinic water column. Trace metal enrichment patterns differ between the two basins, which can be related to the degree of basin restriction constrained by our paleosalinity data. Brackish conditions are recorded throughout the Cleveland Shale, indicating that the Appalachian Basin, like the modern Black Sea, contained a strongly restricted watermass. Under these restricted conditions, Mo and U became progressively drawn down from the water column as euxinia intensified. A shift from brackish (higher salinity than the Cleveland Shale) to fully marine salinity is recorded through the Chattanooga Shale, however, along with a progressive increase in Mo and U. This may reflect Mo and U replenishment from the open ocean in a less restricted setting, possibly enhanced by intensification of local euxinia. Ultimately, these data highlight the utility of a combined paleo-redox/paleosalinity approach in ancient watermass reconstruction.