Environmental Aspects of Middle Ordovician Limestones in Central Appalachians: ABSTRACT

Abstract

Black River and Trenton limestones of the outcrop belts in West Virginia and Maryland were deposited on a gentle carbonate ramp that sloped eastward into a deep-water shale basin. The overwhelming sediment type on the ramp was lime mud, deposited below wave base. Water turbidity and circulation fluctuated, which precluded many epifauna. Burrowing infauna, however, were common. The consistency of the mud varied from soft to firm, and hardgrounds developed locally. The more coherent muds were probably stabilized by early dewatering and cementation. Another common sediment type, fossiliferous lime mud, represents patches of organisms that inhabited the muddy substrate. These communities, dominated by echinoderms, trilobites, and brachiopods, had both low densities and divers ties. Patches were initially established by large, flat brachiopod pioneers but did not greatly expand because of the high physiologic stress and the soft consistency of adjacent substrate. Occasionally, bioclastic sands were produced by storms reworking skeletal grains of the patches. These storm deposits cut into underlying sediments, and the bioclastic debris was clearly locally derived. Other skeletal sands, containing abundant calcareous algae and Tetradium corals as well as peloids and intraclasts, were deposited above wave base on shallower portions of the ramp. Rare cross-laminated peloid sands were confined to small lenses and channels at various depths, and intermittent bottom currents were probably responsible for their deposition. Into progressively deeper water on the ramp, keletal sediments decreased in abundance; storm- and current-laid sediments also decreased; and shales increased. Carbonate sedimentation eventually ended when the ramp facies were overstepped by basinal shales. End_of_Article - Last_Page 1917------------