PALEOENVIRONMENTAL HISTORY OF FIVE LATE MIDDLE PENNSYLVANIAN CORES (CARBONDALE GROUP) FROM THE ILLINOIS BASIN, SOUTHWEST INDIANA: A COMBINED GEOCHEMICAL AND GEOPHYSICAL APPROACH

Geochemical analyses were performed on five cores of the Carbondale Group from Vanderburgh County, Indiana. This new geochemical data is compared to existing lithological descriptions and geophysical data to interpret the depositional setting in Southern Indiana during the Late Middle Pennsylvanian.

In reverse stratigraphic order the cores are: Core 1 - West Franklin Limestone; Core 2 – Grey shale containing marine fossils over the Springfield Coal; Core 3 – Grey mudrock with marine fossils above the organic-rich Excello Shale which overlies the Houchin Creek Coal; Core 4 – Grey blocky mudrock above the Survant Coal; Core 5 – Grey blocky mudrock above the Seelyville Coal. Black shales overlying coals are a common lithologic feature and reflect marine incursion onto the continental environment. Tracking the evolution of intracratonic basins containing these shales is complicated when diagnostic marine fossils are absent and requires a paired geochemical and geophysical approach to interpret water column chemistry and modes of organic matter deposition. Metal concentrations (Mo, V, Cr, Fe, and U) were correlated with gamma logs and lithology. Total organic carbon (TOC) was calculated from natural gamma ray and neutron density, and measured directly by combustion. Extensive pyritization observed above the coals in Cores 2 and 3 indicates marine flooding, as sulfate-rich marine waters covered terrestrial anoxic peat. Redox-sensitive metal concentrations (Mo, V, Cr, Fe, and U) increased above each coal seam. Metal enrichments may reflect immobilization and sequestration within an anoxic water column. TOC for all samples calculated from natural gamma ray and neutron density borehole geophysical tool measurements ranged 0 to > 20 wt%.

Our results are consistent with a dynamic equatorial lowland environment where sediment influx and environmental conditions are controlled by a complex mixture of glacio-eustatic forces with extensive lateral flooding episodes responding to perturbations in Gondwanan glaciation, deltaic sediment transport, and a shallow basin with little accommodation space. In times of glacial advance and regression, shallow-water circulation within the Illinois Basin may have become restricted. Proxies for basin isolation will be explored in future geochemical analyses.