TEMPORAL AND SPATIAL WATERMASS EVOLUTION DURING DEPOSITION OF MIDDLE TO UPPER DEVONIAN ORGANIC-RICH SHALES OF THE ILLINOIS BASIN, NORTH AMERICA

Recent attention to the dynamic depositional nature of black shales (BS) clearly demonstrates the need for comprehensive studies that include basic sedimentology and detailed petrography (conventional and SEM), as well as geochemical analysis of major and trace elements. Recently, the acquisition of large elemental datasets through the use of handheld X-ray fluorescence (HXRF) has enabled paleoenvironmental reconstruction at exceptionally high resolution. Furthermore, the development of paleosalinity proxies alongside existing redox-sensitive proxies provides a powerful toolkit for unraveling depositional conditions of BS and basinwide variations in paleoredox and paleosalinity through time. In the present contribution, we analyze the depositional conditions and hydrographic evolution of the semi-enclosed epicontinental Illinois Basin during deposition of the Middle-Upper Devonian New Albany Shale. Two cores have been selected representing the deepest southwestern basin depocenter in SW Illinois and another from shallower water in the same sub-basin of central-east Indiana. Microfacies analysis in conjunction with HXRF at ~30 cm resolution defines detrital input differences, nutrient availability, and origins of the main biogenic versus detrital components. Paleosalinity (B/Ga, Sr/Ba, S/TOC) and paleoredox proxies (Mo, U, V, C_org/P) are measured by a C-S analyzer and by ICP-MS. We are testing the hypothesis that previously established eustatic sea-level rises defined by high enrichment in trace metals and the development of laminated organic-rich mudstones, as well as reduction in the content of detrital input elements (Zr, Ti, Al, Si) and coarser silt-size particles. These eustatic rises, moreover, should be accompanied by increases in salinity due to marine incursions indicated by higher values of B/Ga, S/TOC, and Sr/Ba. Background salinity conditions are expected to be brackish, however, due to the influence of freshwater input during lower sea levels in the relatively restricted dysoxic to anoxic epicontinental basin.