MARINE MARGIN AND BRAIDED STREAM DEPOSITIONAL ENVIRONMENTS RECORDED IN LOWER PENNSYLVANIAN ROCKS, NORTHERN CUMBERLAND PLATEAU, TENNESSEE

Much of the northern Cumberland Plateau in Tennessee is held up by resistant sandstone of the Rockcastle Conglomerate, which is underlain by siltstone, shale, coal, and fine-grained sandstone of the Fentress Formation. Common sandstone facies in the Fentress Formation include: (1) fine-grained sandstone characterized by bioturbation that has obliterated fine-scale bedding and by marine trace fossils (e.g., Zoophycos, Conostichus, Asterosoma) and (2) thickly bedded fine-grained sandstone in laterally continuous upward-fining beds < 5 cm thick characterized by diverse and abundant bedding plane trace fossils, including the starfish burrow, Asteriacites. In spite of the absence of body fossils, the presence of marine trace fossils and intensity of penetrative bioturbation reflects marine-dominated or influenced salinity conditions, as typical of a tidal flat or a distal distributary mouth bar, both of which are consistent with the observed sedimentary structures and lithology.

The lower part of the Rockcastle Conglomerate was interpreted in the early 1980's as recording deposition in a barrier system, based on the gradational lower contact with the clearly marine-margin Fentress Formation and bolstered by its quartz-rich composition (> 90% quartz). However, a subsurface study based on well logs integrated with facies description and analysis and paleocurrent data from outcrops indicates that the Rockcastle Conglomerate was deposited in braided streams. The facies, including several trough and planar crossbedded sandstone facies, are typical of braided stream sequences, as is the temporal and spatial arrangement of facies. Paleocurrent directions consistently are to the SSW, and sandstone and coal thickness trends parallel the paleocurrent direction. None of the facies are deeply or intensely bioturbated, and marine body and trace fossils are not found. The interpretation of the units individually is clear, but the processes responsible for delivery of massive amounts of quartz-rich sand to the coast in braided stream systems remain elusive.