Gray shale rolls in western Kentucky
Although shale can fill paleochannels, causing rolls and cutouts, there are also special types of gray shale rolls that can cause discontinuities in coals of the Illinois Basin. In some cases, these types of rolls may also be caused by channel processes shortly after peat burial, but in other cases, they appear to be caused by tensional forces in the roof after burial.
Nelson (1983) described gray shale rolls of the Illinois Basin as lens-shaped pods of gray shale or siltstone similar in composition to the roof rock immediately above a coal seam. In some cases, they may incorporate black shale or limestone into the roll, if those rocks overlie thin gray shales in the immediate roof. Gray shale rolls in the Illinois Basin are a few feet to tens of feet wide and are commonly several feet thick. Shale rolls tend to be elongate and curved or straight in plan view. Most rolls are hundreds of feet long, but may be as much as 1,000 feet in length (Krausse and others, 1979). Large shale rolls typically cause splitting or splaying of the upper part of the coal seam around the shale lens. They also may cause the coal and floor beneath the coal to bow downward. Internal lamination in shale rolls commonly shows folding or rolling (similar to broad rolls or ball-and-pillow structures). Small offset faults (sometimes called “slips” by miners) are common.
Gray shale rolls are similar to clay veins or dikes, other types of discontinuities, in that they may cause the coal to (1) bow downward or change elevation, (2) interdigitate with the upper part of the coal, and (3) be associated with small compactional faults that extend into the coal and overlying roof. They differ from clay dikes in that they tend to be filled with shale rather than clay, and are more lens-shaped than vein-shaped. Like clay veins or other types of roof rolls, they may also be associated cut by small compactional fractures and faults, and slickensides.
These types of rolls have been encountered in several western Kentucky coal seams, including the W. Ky. No. 4 (Greb and others, 2001). In the No. 4 coal, the rolls occurred in a parallel set with relatively straight orientations. Similar straight-oriented swarms of shale rolls have been encountered above the Herrin (Illinois No. 6, W. Ky. No. 11) coal seam in Illinois, where they may trend parallel to the local trend of the contact between the gray Energy Shale Member (Illinois term), and the harder black Anna Shale Member (Illinois term) Shelburn Formation, in the mine roof (Nelson, 1983; Krausse and others, 1979). In at least one mine in southern Illinois, gray shale rolls follow a narrow sinuous trend of dark Anna Shale in the immediate roof (Bauer and DeMaris, 1982; Nelson, 1983). Straight-oriented rolls (especially those occurring in parallel sets) may have formed from tectonic stresses and tensional cracks, while sinuous rolls more likely formed from channel processes during deposition of the roof strata.