DETAILED SEDIMENTOLOGICAL STUDY OF THE WEST FRANKLIN LIMESTONE MEMBER (DESMOINESIAN TO MISSOURIAN) OF THE SHELBURN FORMATION (UPPER PENNSYLVANIAN) OF SOUTHWESTERN INDIANA

Southwest Indiana is underlain by gently, westward dipping (2^o to 3^o) Middle to Upper Pennsylvanian strata on the southern margin of the Illinois Basin consisting of recurring deposits of limestone, sandstone, shale, and coal. USI 1-32, an exploratory coalbed methane well drilled in 2009, is located at 37.951°N and 87.670°W south of the campus of the University of Southern Indiana in Vanderburgh County, Indiana. The total depth of the well is 237.7 m with a core recovered from the West Franklin Limestone from 23.2 to 30.5 m.

From the cored interval, the basal unit of the West Franklin Limestone consists of skeletal packstone with broken and abraded brachiopods, crinoids, and fusulinids. This basal unit is overlain by 0.41 m of dark gray, laminated to bioturbated claystone. The middle limestone (4.96 m) is made up of 2.77 m of mottled, brecciated skeletal wackestone to packstone overlain by 2.19 m of interbedded skeletal packstone, grainstone, and rudstone containing nodular chert with broken fragments of brachiopods, bryozoans, rugose and tabulate corals, and crinoids. The middle limestone is overlain by 1.31 m of light greenish gray, bioturbated silty claystone with sparse organic stringers. The uppermost limestone is composed of skeletal packstone with broken and abraded algae, brachiopods, bryozoans, crinoids, forams, and gastropods. The geophysical well log from USI 1-32 suggests a thickness of 0.51 m for the basal limestone; and another core from the USI Groundwater Lab (less than 3 km from USI 1-32) provides a thickness of 2.41 m for the upper limestone of the West Franklin.

The alternation of siliciclastic and carbonate sediments in the West Franklin Limestone Member may be due to: (1) the complexity of shallow transitional marine settings; (2) interruption of carbonate deposition by avulsion and subsequent delta migration with increasing influx of detrital sediment; and/or (3) localized variation in depositional environments due to penecontemporaneous grabens that formed from reactivated faults of the Rough Creek-Shawneetown Fault Zone. The latter is further supported by thickness variation and limited lateral extent of the overlying Inglefield Sandstone. Finally, there is no evidence of deep water deposition in this part of the Illinois Basin during the late Desmoinesian to early Missourian.