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Geologic studies in the Appalachian region have shown that many parameters of coal beds (thickness, continuity, roof and floor rock, sulfur and trace-element content, and ash) can be attributed to the depositional environment in which the peat beds formed and to the tectonic setting at the time of deposition. With an understanding of the depositional setting of the coal seam and contemporaneous tectonic influences, the characteristics and variability of many of these parameters can be predicted.
Coals formed in "back-barrier" environments tend to be thin, laterally discontinuous, high in sulfur, and to exhibit severe roof problems. Therefore, they are not generally important as minable coals. Coal beds deposited in the "lower delta-plain" environment are relatively widespread with fewer roof problems but generally are thin and show a highly irregular pattern of sulfur and trace-element distribution. Conversely, "upper delta plain-fluvial" coals are low in sulfur, are thick locally, but are commonly discontinuous laterally. Despite these problems, some "lower delta-plain" and "upper delta plain-fluvial" coals are successfully mined. However, most important seams in the Appalachian area are in the transitional zone between these two environmental facies. In this transition zone thick coals attain a relatively high degree of lateral continuity and are usually low in sulfur.
Contemporaneous tectonic influences are superposed on changes in seam character attributed to variations in environments of deposition. Rapid subsidence during sedimentation generally results in abrupt variations in coal seams but favors lower sulfur and trace-element content, whereas slower subsidence favors greater lateral continuity but higher content of chemically precipitated material.
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