GEOCHEMICAL ANALYSIS OF THE WEST FRANKLIN LIMESTONE MEMBER OF THE SHELBURN FORMATION (UPPER PENNSYLVANIAN) OF SOUTHWESTERN INDIANA

During the Middle to Late Pennsylvanian (~300 Ma) southern Indiana was a tropical inland, (epeiric) sea characterized by repeated fluctuations in sea-level, due to glaciation in Gondwana. These shallow tropical seas were potentially prone to anoxia and euxinia. We investigated paleo-redox of the upper part of the West Franklin limestone (referred to as the lower bench), a unit consisting of one to three limestone beds (1 to 3 m each) separated by dark grey to greenish grey, blocky to laminated mudrock (1 to 2 m). The formation is continuous from the southwestern portion of Indiana into southeastern Illinois and western Kentucky. The West Franklin limestone is rich with pyrite up to 0.7 wt%. Although pyrite accumulation in shale is controlled by sulfate availability, reactive iron delivery, and quantity of organic matter, it remains to be demonstrated whether the abundant pyrites formed in the West Franklin Limestone coevally or during diagenesis after burial and due to the same controls.

To address these questions we are using reactive iron, pyrite concentrations, and sulfur isotopes as paleoenvironmental proxies to interpret change in redox and other environmental factors. Using data from pyrite extraction, sulfur concentration can be used in conjunction with reactive iron totals as a proxy for determining freshwater, brackish-normal marine, or hypersaline dominated environments. XRD data gives an insight into the mineral composition and shows that there was a change is environmental factors that shifted the system between producing carbonates and shales. Sequential iron extractions can determine mineral speciation of Fe, revealing how much iron is retained in the sediment as sulfides, oxides, or carbonates. In addition, the ratio of highly reactive iron to total iron (Fe_HR/Fe_T) is a proxy of oxic, anoxic, or euxinic (anoxic and sulfidic) conditions.