A Geochemical Analysis of Five Late Middle Pennsylvanian Cores (Carbondale Group) From the Illinois Basin, Southern Indiana

Broach, Clinton M.; Gilhooly, William P.; Elliott, William S.; Smith, Christopher

The sequential deposition of coal, black shale, and mudrock in five Late Middle Pennsylvanian cores from southern Indiana were studied to better understand the mode of deposition in near-shore intracratonic basins flooded by shallow epeiric seas. We use a combined approach of lithologic descriptions, geophysics, and geochemistry to interpret marine/nonmarine environmental changes. Proxies for basin isolation (degree of pyritization [DOP] and sulfur isotopes) and organic matter contributions (total organic carbon [TOC], total nitrogen, and stable carbon isotopes) were used to track water column redox conditions and sediment source parameters.

Cores are summarized as follows: C1 - West Franklin Limestone; C2 - Grey shale containing marine fossils over the Springfield Coal; C3 - Grey mudrock with marine fossils above the Excello Shale overlying the Houchin Creek Coal; C4 - Grey mudrock with no marine fossils above the Survant Coal; C5 - Grey mudrock with no marine fossils above the Seelyville Coal. Coal-mudrock transitions were analyzed to examine the effect of transgressive sulfate-rich marine water on an extensive peat environment prevalent in southern Indiana during the Pennsylvanian. Comparing TOC, total nitrogen, and δ13C values provides proxies for both the abundance and nature of organic matter present while total sulfur, δ34S, and DOP values reflect the water chemistry conditions. Geochemical conditions at time of deposition are vital in determining whether waters were conducive for marine fauna and directly impact the type of organic matter deposited. Results show mud with high metal enrichments (Mo, V, Cr, Fe, U, Cu, Zn, Ni, and Pb) above coals and may reflect periods of dysoxia and possibly euxinia (anoxic and sulfidic). The majority of the mudrocks have lower concentrations of these metals, which may reflect changes in source parameters (riverine or marine) or redox conditions.

Geochemical results reveal a dynamic environmental system where water column geochemistry varies with cyclothemic transgression/regression cycles and reveal the effects of shallow epeiric seawater flooding on the extensive peat lands of equatorial Late Middle Pennsylvanian southern Indiana. Explaining the lack of marine fossil evidence in Cores 4 and 5 and their prevalence in Cores 2 and 3 through the use of C, N, and S isotopes will be the focus of future research and will hopefully shed light on the geochemistry of mudrock deposition within intracratonic basins.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013