ABSTRACT

Late Middle-Late Pennsylvanian (middle Allegheny and lower Conemaugh) marine units in the Appalachian basin represent the distal edges of glacial-eustatic marine incursions of the Midcontinent sea onto the detrital apron of the Appalachian highlands. Because these marine units typically overlie coal beds, the main environment of formation of this type of coal bed appears to have been that of a coastal swamp migrating ahead of transgression. As base level rose, the water table continually rose, and the swamp was further nourished and surrounding vegetation kept thick enough to inhibit detrital influx by rainfall from the expanding nearby source of moisture. Even in parts of the Appalachian cyclic succession with no marine units (upper Allegheny, Monongahela, Dunkard), the coal beds rep esent the wettest part of the climate cycle of Cecil (1990), sandwiched between detrital units representing seasonal wet-dry climates when sediment was most readily mobilized, and antipodal to nonmarine limestones representing the driest climates. Throughout this time, the Appalachian basin was in the center of the tropical zone of a nearly accreted megacontinent, quite distant ( 4000 km) from permanent oceanic sources of moisture. Climate modeling without considering inland seas or orographic effects on winds shows that the tropical zone of such a large land mass would have suffered from a severe deficit in rainfall and soil moisture. Although further modeling shows that the Appalachian orogenic highlands would have drawn in moisture to ncrease rainfall in their vicinity, they also may have impeded the normal westward flow of moisture,laden easterly winds. The western source for enough moisture to form coal beds was most likely the North American Midcontinent sea. During major highstand, this sea would have covered about 2 million km² and provided both high base level and abundant nearby moisture for enough continual rainfall to keep the coastal peat swamps widespread, fresh, and relatively detritus-free in the Appalachian basin. In contrast, during major lowstand it would have covered only about 200,000 km² with its shoreline about 1500 km away, and it apparently provided less consistent moisture to the Appalachian basin then, because most nonmarine limestones and paleosols formed there during the e times contain evidence of drier climates. Therefore, upper Allegheny, Monongahela, and Dunkard coal beds most probably formed when the Midcontinent sea was approaching highstand and stood just west of the presently preserved outcrop. Thus, they likely represent the coastal lowland equivalent of a marine unit at highstand farther west. This constrained time of peat formation and preservation, in a variety of coastal environments prior to and during marine highstand, is responsible for the discrete and widespread nature of the coal-bed interval in a typical depositional cycle (or cyclothem), a pattern that stands in contrast to the complicated relations of overlying and underlying detrital units, which reflect the segregation of sand and mud into a complex mosaic of local deltaic and ter estrial subenvironments that developed when sea level rainfall and the water table were lower.