Lithologic patterns in the Energy Shale Member and the origin of "rolls" in the Herrin (No. 6) Coal Member, Pennsylvanian, in the Orient No. 6 Mine, Jefferson County, Illinois

Abstract

The Energy Shale Member, an argillaceous wedge associated with the Walshville Channel and Herrin (No. 6) Coal Member, consists of a lower dark-gray shale unit and an upper light-gray shale unit in the Orient No. 6 Mine of the Freeman United Coal Mining Company. The dark-gray shale has laminae about one-eighth of an tach thick, is abundantly carbonaceous, and is characterized by the presence of an expandable clay mineral. The light-gray shale has layers as much as two inches thick, contains less carbonaceous matter, lacks expandable clay minerals, and is more coarsely grained than the dark-gray shale. Thus the dark-gray shale unit is inferred to have been deposited in small, shallow water bodies within the swamp. In contrast, the light-gray shale appears to have resulted from widespread inundation of the swamp, perhaps by crevasse splays and other overbank deposits from the contemporaneous Walshville Channel. The light-gray shale is characterized by the presence of rolls--elongate shale bodies of lenticular cross-section that penetrate downward into the coal. Rolls and their associated slips, small slicken-sided normal faults, are most abundant along the margins of dark-gray shale bodies and trend more or less parallel to the boundary separating the two rock types (Krausse et al, 1979). Elsewhere in southern Illinois coal names where the roof rock is uniformly composed of light-gray shale, there are few rolls. These relationships suggest that local accumulation of dark-gray shale in depressions on the peat terminated peat deposition and caused compaction of the underlying peat. Later, when the light-gray shale deposit smothered the entire swamp, the topography of the swamp was inverted. This resulted because peat in previously uncompacted and uncovered areas condensed more than the already compacted peat beneath the dark gray shale lenses. Thereafter, rolls arose through slumping of light-gray shale, which covered the newly created mound-like masses of dark gray shale and previously compacted peat.