Peat accumulates in areas of low relief and gradient, but through geological time, the peat is buried in sedimentary basins and forms coal seams that can be folded by tectonic events. Many coal fields are affected by folds that impact or influence mining methods or CBM development.
Fold styles are numerous and vary from broad, open folds with gentle dips to narrow, closed structures characterized by steeply dipping or overturned beds (Picture 1). Fold wavelengths can vary from many miles to the microscopic scale.
Folding is a primary result of the effects of differential stress and many fold types originate from the effect of horizontal stress (Williamson, 1967). Folds may be classified either genetically or descriptively. The standard types of folds include anticlines, synclines, domes, and monoclines (Pictures 2 and 3). Drag folds may develop in the vicinity of faults. Folds may form also by soft sediment deformation and slumping. Folds are usually derived tectonically or through differential compaction.