Origin of coal seam structures, Sullivan County, Indiana

Structures of Pennsylvanian coal seams in Sullivan County, Indiana, reflect deeper structural components, of which regional dip is dominant. Other components of structure result form differential compaction. The effects of these components are characterized by their closure, size, shape, and orientation. (1) The Mississippian unconformity surface is characterized by parallel valley with up to 300 ft (91 m) of local relief. (2) The composite lower Pennsylvanian section below the Seelyville Coal has variable sandstone content. Some paleovalleys are filled with multistory sandstones, and others with claystone. (3) Silurian pinnacle reefs from small, circular features with a diameter of 1 to 2 mi (1.5 to 3 km) and closures of 25 tio 50 ft (8 to 15 m) on Pennsylvanian coal seams, 50 ft (15 m) on the Aux Vases Shale, and 150 ft (45 m) on the New Albany Shale. (4) The distributions and standard deviations of thicknesses, dips, and grain size of the sedimentary rocks between the coal seams demonstrate that seams above the Seelyville Coal were deposited in parallel and have concordant modern structures. Specific facies between seams have limited influence on the overall structure. Coal structures in the Illinois basin can be defined by a drilling programmore » that penetrates only 150 ft (45 m) of Pennsylvanian strata. Below the Seelyville Coal, units examined demonstrate basin-margin convergence.« less