Mining geology of the principal resource coals of the Illinois Basin

The Middle Pennsylvanian Herrin and Springfield coals contain the majority of the Illinois basin’s resources. Many mining issues of these two seams are directly related to the geology of the region, including regional stress, faulting, and facies changes. Some of these conditions are widespread, while others are locally restricted. The long history of mining in these seams has been associated with many difficulties and hard-learned lessons related to mining geology especially concerning aspects of coal quality and roof geology along paleochannels, moisture-sensitivity in shales, fractures and “slips” in shales, floor heave, lateral stresses, clay dikes (veins), and coal balls. Both seams occur within classic “cyclothems,” resulting from sequences of deposition between lowstand and highstand deposition. Transgressive, marine black shales and limestones blanket the coals across much of the basin, except near syndepositional channels where wedges of gray silty shale and interbedded shale and sandstone intervene between the coal and transgressive shale. Differences in the thickness, chemistry and strength of various shales in each sequence influence coal quality and roof conditions in underground mines. Associations of mining geology issues not specifically related to tectonics and horizontal stress, are (1) syndepositional channel and channel margin conditions, (2) post-depositional channels, (3) gray-shale wedge margins, and (4) marine shale roofs; especially where limestones thicken. Many of these features are similar to both seams, while others are more common in the Herrin, than Springfield Coal. Similar conditions occur above other coal beds in the Illinois Basin, as well as coal-bearing strata in other basins. Herein we review horizontal stress, fractures, coal quality, partings, coal balls, clay dikes (veins), dips and swales, floor heave, moisture sensitivity in shales, shale rolls, limestone bosses and cutouts, paleochannels and cutouts, stackrock conditions, paleoslumps, coal riders, kettlebottoms, and concretions in the roof. Hopefully, this summary will be useful for understanding seams with similar geology, but with less research on mining geology or shorter mining histories.