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Predictive models that characterize coal bed discontinuities enable mine operators to be better judges of the size, geometry, and influence of these features in unmined portions of the coal bed. An analysis of depositional environments in initial geologic studies of prospective mine properties will indicate the specific types of coal bed discontinuities that can be anticipated.
The type of coal bed discontinuity and the frequency of occurrence are functions of the depositional environment. Peat, the precursor of coal, accumulated in swamps that may form on fluvial plains, on delta plains, and in littoral areas. Using generalized depositional models for these environments, the types of coal bed discontinuities that can be expected and an estimate of the likelihood of their occurrence can be determined. Coal beds deposited on fluvial plains generally are thin, erratic, and discontinuous because of the highly oxidizing character of this environment. Discontinuities due to irregular topography and fluvial channel activity are common, as displayed by Upper Pennsylvanian and Permian coal beds of the Dunkard basin. Coal beds formed in deltaic settings are generally thick and laterally extensive, as they commonly infill broad interdistributary areas. These coal beds are typically plagued by discontinuities associated with distributary channels (e.g., avulsion, splays). The Freeport and Kittanning coal beds in west-central Pennsylvania demonstrate features characteristic of delta-plain coals and the discontinuities that beset them. Coal beds deposited landward of barrier bar sequences generally are irregular and are interrupted by tidal channels and washover deposits. The Pocahontas 3 coal bed of southern West Virginia exhibits many of the characteristics of this paralic setting. Although some clastic dikes may be depositionally related, such discontinuities, as well as faulting, may be overprinted by tectonic activity on coal beds from any depositio al environment.
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