MUDROCK VARIABILITY ABOVE COALS OF THE CARBONDALE GROUP (DESMOINESIAN) IN CORES FROM VANDERBURGH COUNTY, INDIANA

The USI 1-32 well is located at 37.951°N and 87.670°W on the campus of the University of Southern Indiana in Vanderburgh County, Indiana. The total depth of the well is 780 feet, with core intervals from 395 to 415, 495 to 515, 540 to 560, and 655 to 675 feet depth that sampled parts of the Petersburg Formation (Springfield and Houchin Creek Coals) and Linton Formation (Survant and Seelyville Coals) of the Carbondale Group respectively. Our lithologic descriptions and interpretations provide further insight into sea level change and resultant paleoenvironmental conditions that occurred in Southern Indiana during the Late Middle Pennsylvanian.

The youngest coal, the Springfield, is overlain by black to dark gray, faintly laminated mudrock with abundant disseminated pyrite and brachiopods (Barroisella campbelli and Dunbarella sp.), indicating shallow marine environments. The Houchin Creek Coal is capped by a 4 cm thick layer of pyrite that represents a significant reaction boundary, most likely resulting from sulfate reduction of marine waters that came into contact with the coal. This pyrite layer is overlain by dark gray to black, faintly laminated, calcareous mudrock with several vertical calcite-filled fractures that transition upward into a dark gray, laminated mudrock. Approximately 1.5 m above the coal, there are rugose corals, crinoidal debris, and ostracods, consistent with a marine environment. In contrast, the Survant Coal is overlain by gray, faintly laminated mudrock that contain sparse plant fragments. In addition, the mudrocks above the Survant Coal lack significant pyrite and are interpreted to represent continental deposition rather than marine. The oldest coal, the Seelyville, is overlain by moderate gray, faintly laminated mudrock with sparse 1 to 3 cm diameter siderite concretions. The base of this mudrock interval contains rugose corals, attesting to marine conditions.

The variability of mudrocks above coals in the Carbondale Group suggests that diverse depositional processes were responsible for the burial and preservation of coal. Furthermore, pyrite content of coals may be directly related to the depositional setting of overlying mudrocks. Mudrock layers with abundant pyrite are interpreted to represent hypoxic/anoxic conditions consistent with restricted marine environments.