Slickensides are highly polished and/or striated surfaces that form as a result of tectonic movement or differential compaction of sediment. Common to coal-bearing strata, slickensides can occur on practically any plane of weakness subjected to elevated stress. They can be found along joints, cleats, faults, and other structural and sedimentary features and occur in practically all rock types. However, slickensides occur most frequently in finer-grained rock types such as mudstone and shale. Slickensided fractures are often analogous to slips.
Laird et al. (1985) distinguished two types of slickensides in coal deposits of the Western U.S. The first type, referred to as compaction slickensides, form contemporaneously in fine-grained strata near sandstone units. The second variety, referred to as structural slickensides, exhibit higher deformation caused by more intense pressure. Structural slickensides form adjacent to and beneath paleochannels, at depths exceeding 450 m and under the influence of high horizontal stress. The same classification terms are adopted here. However, structural slickensides incorporate a much broader spectrum of genesis.