VARIABILITY IN MUDROCKS OVERLYING COAL SEAMS OF THE CARBONDALE GROUP (PENNSYLVANIAN) IN SOUTHWESTERN INDIANA

The USI 1-32 well is located at 37.951°N and 87.670°W in Vanderburgh County, Indiana. The total depth of the well is 780 feet, with three cored intervals: Springfield Coal of the Petersburg Formation (395 to 415 feet), Survant Coal of the Linton Formation (540 to 560 feet), and the Seelyville of the Staunton Formation (655 to 675 feet). Our lithologic descriptions and interpretations provide further insight into sea level change and resultant paleoenvironmental conditions that occurred in southwest Indiana during the Late Middle Pennsylvanian.

For this study, mudrocks overlying the coals were cut, polished, and described to identify sedimentological variation. The mudrock overlying the Springfield Coal is gray to black with discrete wavy laminations and abundant disseminated pyrite. There is a gradual increase in bioturbation from an ichnofabric of 1 directly above the coal to 4 two feet above the coal. Trace fossils include Chondrites, Palaeophycus, Phycosiphon, and Planolites. Discrete laminations within this interval also contain broken skeletal debris, including brachiopod and molluscan fauna. The mudrock overlying the Survant coal is greenish gray to light gray, finely laminated with discrete bioturbated intervals consisting primarily of Palaeophycus with an ichnofabric of 2. Additionally, disk-shaped, 1 to 4 cm diameter siderite concretions occur sparsely in the mudrock interval above the Survant Coal. Above the Seelyville Coal, the mudrock is gray to dark gray, finely laminated with disseminated pyrite and sparse phosphate nodules consisting of aggregates of peloids. There is minor bioturbation in the mudrock overlying the Seelyville Coal.

The variation in bioturbation and mineral content implies diverse depositional settings and environmental conditions of mudrocks overlying coals in the Middle Pennsylvanian of southwest Indiana. For example, the mudrocks overlying the Springfield and Seelyville coals are interpreted to be the result of deposition associated with a transgressive marine setting. In contrast, the mudrocks overlying the Survant Coal are interpreted to be deposited in a continental setting with minimal marine incursion. The variation in depositional settings of these mudrocks ultimately dictates the sulfur content of their respective underlying coal.