ABSTRACT

The central Appalachian Shelf, approximately bounded on the northeast by the Precambrian crystallines of the Adirondacks, the Cincinnati-Algonquin arch to the northwest, and including the western portion of a geosynclinal terrane lying to the southeast, received sediments discontinuously throughout the Paleozoic. Major carbonate sequences were deposited during late Cambrian-early Ordovician (Conococheague-Beekmantown), medial Ordovician (Black River, Trenton), medial Silurian (Lockport), late Silurian-early Devonian (Tonoloway, Keyser, Helderberg), and medial Devonian (Onondaga). Minor, but equally interesting, carbonate units were deposited in the medial Devonian (various thin limestones within the Hamilton), and early late Devonian (Tully).

Recent environmental stratigraphic studies of these carbonate units show a great variety of lithofacies and biofacies (Conococheague, Wilson, 1952; Zenger, 1969; Beekmantown, Sando, 1957; Donaldson, 1969; Black River, Textoris, 1968; Walker, 1969; Trenton, Cameron, 1968; Lockport, Zenger, 1965; Tonoloway, Gwinn and Clack, 1964; Rondout-Keyser, Harper, 1969a, 1969b; Keyser-Helderberg, Head, 1969; Laporte, 1967, 1969; Onondaga, Lindholm, 1967; Portland Point, McCave, 1968; and Tully, Heckel, 1966). Despite this diversity, four major facies suites can be recognized.

Tidal flat deposits consisting of laminated, dolomitic, mudcracked, intraclastic rocks with low faunal diversity and containing algal structures are well developed among the older carbonate units (Conococheague, Beekmantown, Black River, upper Lockport, Tonoloway, Rondout-Keyser, and lower Helderberg). These rocks represent supratidal and intertidal environments.

Shallow subtidal deposits consisting of biomicrites (usually well burrowed), biosparites (frequently current stratified), and occasional oosparites, with relatively diverse and abundant biotas, are particularly common in the middle portion of the Cambrian to Devonian sequence (upper Black River, Trenton, middle Lockport, Rondout-Keyser, Helderberg, and Onondaga). These rocks record restricted to open marine environments lying above, or slightly below, effective wave base.

Deep subtidal deposits consisting of well burrowed, argillaceous and quartz silty biomicrites with abundant and diverse biotas are more typical of the younger part of the sequence (Onondaga, Portland Point, and Tully). These rocks represent open marine environments below effective wave base.

Carbonate build-ups are found throughout, from Cambrian to Devonian, and occur either as small algal mounds (Conococheague, Beekmantown, and Lockport), as tabulate or stromatoporoid biostromes (Black River, Lockport, Rondout-Keyser, Helderberg, and Tully), or as bioherms dominated by rugose and tabulate corals (middle Lockport, Helderberg, and lower Onondaga). Fossil diversity and abundance are usually high within the biostromes and bioherms.

As might be expected, the temporal distribution of these broad facies suites parallels the 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 bordering geosynclinal areas to the southeast the nearshore parts of these environments were flooded by land-derived clastics; greater subsidence of the shelf areas also seems to have occurred so that deeper-water carbonates became more common.

Carbonate build-ups had somewhat more complex reasons for their development. While related to the general tectonic regime in that they are found near the more slowly subsiding, shallow parts of the basin, they were also dependent on good marine circulation and the local paleogeography.