Paleozoic Carbonate Facies of Central Appalachian Shelf: ABSTRACT

The central Appalachian shelf, which received sediments discontinuously throughout the Paleozoic, is bounded on the north by the Precambrian crystalline rocks of the Adirondacks, on the west by the Cincinnati arch and on the east by geosynclinal basins. Major carbonate sequences were deposited during the Late Cambrian-Early Ordovician (Conococheague, Beekmantown), Middle Ordovician (Black River, Trenton), Middle Silurian (Lockport), Late Silurian-Early Devonian (Tonoloway, Keyser, Helderberg), and Middle Devonian (Onondaga). Minor, but equally interesting, carbonate units were deposited during the Middle Devonian (several thin limestones within the Hamilton) and Late Devonian (Tully). Recent environmental stratigraphic studies of these carbonate rocks show a great variety of lithofacies and biofacies. Despite this great diversity, four major facies complexes can be characterized. Tidal flat deposits, consisting of laminated, dolomitic, mud-cracked, intraclastic rocks with low faunal diversity and algal structures, are well developed in the older carbonate units (Conococheague, Beekmantown, Black River, upper Lockport, Tonoloway, Keyser, and lower Helderberg). These rocks were formed in supratidal and intertidal environments. Shallow subtidal deposits, consisting of biomicrite (generally well burrowed), biosparite (in many places current stratified), and some oosparite with relatively diverse and abundant biotas, are particularly common in the middle part of the Cambrian-Devonian carbonate sequence (upper Black River, Trenton, middle Lockport, Keyser, Helderberg, and Onondaga). These rocks record restricted- to open-marine environments above, or slightly below, effective wave base. Deeper subtidal deposits, consisting of well-burrowed impure biomicrite with less abundant and less diverse biotas, are more typical of the younger part of this interval (Onondaga, Portland Point, and Tully). These strata formed in open-marine environments below effective wave base. Carbonate buildups are found throughout the Cambrian-Devonian either as small algal mounds (Conococheague, Beekmantown, Lockport), tabulate or stromatoporoid biostromes (Black River, Lockport, Keyser, Helderberg, Tully), or as bioherms dominated by rugose and tabulate corals (middle Lockport, Helderberg, and lower Onondaga). Fossil diversity and abundance are greatest within the biostromes and bioherms. As might be expected, the temporal distribution of these broad facies complexes parallels the Paleozoic tectonic history of the central Appalachians. Thus, during times of tectonic stability carbonate tidal-flat and shallow subtidal deposits were abundant. As tectonism increased in the eastern geosynclinal terranes, the nearshore areas of these environments were flooded by land-derived terrigenous clastics. Greater subsidence of the shelf areas also seems to have been general, with the result that deeper water carbonate became more common. Carbonate buildups seem to occur in a wider variety of environmental situations. Although related to the general tectonic regime, they also were dependent on good marine circulation and local paleogeography. End_of_Article - Last_Page 728------------