Paleoslumps in coal-bearing strata of the Breathitt Group (Pennsylvanian), Eastern Kentucky Coal Field, U.S.A

Abstract

The benefits of geologic analysis for roof-control studies and hazard prediction in coal mines are well documented. Numerous case studies have illustrated the importance of recognizing geologic features such as paleochannels, coal riders, and kettlebottoms in mine roofs. Relatively understudied features, in terms of mining, are paleoslumps .Paleoslumps represent ancient movement and rotation of semi-consolidated sediment. Because bedding in paleoslumps is deformed or inclined, these features cause instability in mine roofs, haul roads, surface highwalls, and other excavations. Various types of paleoslumps above coals in the Eastern Kentucky Coal Field were studied in order to aid in their recognition and prediction in mines. The paleoslumps studied all showed characteristic slump-deformation features, although some differences in magnitude of deformation and overall slump size were noted. Coals beneath slumps often exhibited folding, reverse displacements, truncation, clastic dikes, and locally increased thickness. Slumps are inferred to have been triggered by a wide range of mechanisms, such as loading of water-saturated sediment on rigid substrates, synsedimentary faulting, and over-pressurization of channel margin and bar slopes. Analysis of paleoslumps in underground mines, where paleoslumps are viewed from beneath rather than in profile is difficult, since characteristic bed rotation may not be conspicuous. Sudden increases in bed-dip angle inferred from changes in rock type or bedding contacts in the roof; occurrence of bounding, polished rotation surfaces; or roof irregularity and occurrence of loading features may indicate the presence of paleoslumps. Another key to recognition may be the sudden appearance of over-thickened coal, which can occur because of slump-created paleotopography, synsedimentary faults, and slump-generated overthrusting. In addition, steeply inclined, folded, or transported coal marginal to paleoslumps can create apparent increases in coal thickness in cores. Although thick coals are obviously a target of exploration, anomalously thick coals may actually indicate adjacent paleoslumps accompanied by hazardous roof conditions and loss of seam thickness.