STRATIGRAPHIC AND BIOFACIES PATTERNS IN THE MIDDLE PENNSYLVANIAN MAGOFFIN MARINE UNIT IN THE APPALACHIAN BASIN, USA

The Magoffin marine unit is a Middle Pennsylvanian age interval of mar ine strata that directly overlies the Taylor, Copeland, and correlativ e coal zones in the Appalachian Basin. For this study the Magoffin was measured, described, and sampled at 17 localities along a northeast t o southwest transect in the center of the Middle Pennsylvanian outcrop belt in eastern Kentucky and West Virginia. Throughout the study area the base of the Magoffin is characterized by a thin, dark, highly fos siliferous limestone with a brachiopod-dominated fossil assemblage. Th e limestone base is usually overlain by a fining-upward sequence consi sting of fossiliferous dark shales or mudstones with mollusk-dominated assemblages. These dark mudstones include a fissile black shale with a distinctive Posidonia fauna deposited over part of the study area. T he lower, fining sequence is overlain by a thicker, coarsening sequenc e bearing brachiopod-dominated fossil assemblages. The lower beds of t he Magoffin, particularly the basal limestone, are persistent and rela tively uniform throughout the study area. In contrast, strata in the u pper part of the Magoffin sequence show a high degree of geographic va riability, with localities in the southwestern half of the study area showing two successive, thick, coarsening-upward sequences of strata, while those to the northeast record a single thinner coarsening-upward sequence. The widespread, uniform nature of the basal Magoffin limest one appears to indicate rapid transgressive flooding of the coal-swamp and associated environments accompanied by a hiatus in elastic influx into the Magoffin seaway. Nearshore brachiopod faunas were replaced b y deeper-water, possibly dysaerobic-adapted mollusk faunas as transgre ssion progressed, culminating in the fissile black shales and monotaxi c Posidonia fauna deposited beneath a localized pycnocline during maxi mum transgression. The onset of regression is indicated by the reverse of the stratigraphic sequence of faunas observed during transgression , and by the return of rapid elastic influx into the basin.