Geologic features that influenced coal bed thicknesses in the eastern part of the Illinois Basin

In the east-central part of the Illinois Basin, mappable thicknesses of coalbeds vary by several hundred percent (from less than 3 feet to more than 6 feet) across distances ranging from a few hundred feet to several hundred miles. The formation of Middle Pennsylvanian coalbeds in that area was affected by the La Salle Anticlinal Belt, by contemporaneous deposition of linear sandstones, and by differential volumetric changes around underlying features. These underlying features include earlier Middle Pennsylvanian linear sandstones (75 to 150 feet deeper), Lower Pennsylvanian sandstones filling paleovalleys of the Mississippian-Pennsylvanian unconformity (300 to 700 feet deeper), embayments and passes of a Silurian carbonate bank (1,900 feet deeper), and individual Silurian reefs. These influences interacted to produce abnormally thick (greater than 4.5 feet) deposits of the Seelyville, Survant, Springfield, and Hymera Coal Members (Pennsylvanian) within three geographically restricted areas (mining districts) in west-central Indiana. The intermittent influence of preexisting features on Middle Pennsylvanian deposition indicates that significant depositional hiatuses occurred during Middle Pennsylvanian time.