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Structures of Pennsylvanian coal seams in Sullivan County, Indiana, reflect deeper structural components, of which regional dip is dominant. Other components of structure result from 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 valleys 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 multi-story sandstones, and others with claystone. Thickness of fill has intermediately scaled effects on overlying coal structures. The combined effects of "lower" Pennsylvanian thickness and sandstone content res lt in updip and downdip undulations in elevation of the coal seams along the regional strike, with an amplitude of up to 25 ft (8 m) and a magnitude of ±2 to 3 mi (3 to 5 km). The resultant oriented, linear, structure highs parallel the trend of the underlying paleoridges. (3) Silurian pinnacle reefs form small, circular features with a diameter of 1 to 2 mi (1.5 to 3 km) and closures of 25 to 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 program that penetrates only 150 ft (45 m) of Pennsylvanian strata. The cost of testing nonreef structures can be halved by termination at a Mississippian horizon if 50 ft (15 m) of closure cannot be substantiated. Below the Seelyville Coal, units examined demonstrate basin-margin convergence.
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