Development of paleogeographic concepts related to deposition of major units of the Silurian-Devonian carbonate sequence (late Middle Silurian through early Middle Devonian) in the Appalachian basin has been hampered by lack of stratigraphic information concerning the basin’s vast subsurface area. In order to gain a better understanding of subsurface stratigraphic relations, units such as the Silurian Lockport Formation and the Devonian Onondaga Limestone have been correlated from their type areas throughout the Appalachian subsurface by means of a network of gamma-ray and sample logs. The resulting subsurface stratigraphic framework provides data which fill the large gaps between Appalachian outcrops.
Outcrop and subsurface studies indicate that the “type” Lockport in western New York is probably equivalent to Middle Silurian Niagara and Upper Silurian “A-1” strata in Michigan. Facies studies of the Appalachian Niagara and “A-1” indicate the presence of a Middle Silurian topographic basin in Ohio which is similar to the Middle Silurian (Niagaran) basin in Michigan, although considerably smaller. Thick, dolomitized Niagara and “A-1” carbonate banks rim the basin and grade into thinner, argillaceous dolomites and limestones in the basin interior. Numerous pinnacle reefs are scattered through the basinal area, and a thin, widespread sheet of “A-1” anhydrite is present across the basin floor. Contrary to previous reports and speculations, no evidence has been found for major Niagaran topographic basins in the New York-Pennsylvania-West Virginia area. Instead, a broad, shallowwater, open-marine shelf apparently existed east of the topographic basin in Ohio. The shelf was primarily a site of carbonate deposition dominated by numerous patch reefs, interreef muds, and banks of calcarenite. Most of the shelf sediments have been dolomitized. Farther east, the reefs and sand banks are replaced by thin-bedded ostracod-rich shales, limestones, and dolomites which probably represent the deposits of extensive tidal flats that bordered an eastern land area.
Informal unit designations “A” through “G,” established for the Michigan basin Upper Silurian Salina Group, are equally applicable to the Salina Group of the Appalachian basin. In particular, shaly units “C,” “E,” and “G” can be correlated with gamma-ray logs over hundreds of square miles of both basins. Comparison of the Middle Silurian topographic basin position in Ohio to the location of the Early Devonian Helderberg topographic basin indicates a 200-mi (320 km) southeastward shift of the Appalachian basin topographic axis. The known distributions of Salina halites “B,” “D,” and “F” also show a pronounced southeastward shift through time. Positions of individual halite units are intermediate to the deep water areas of the Lockport and Helderberg and may represent deposition in the interior of topographic basins during Late Silurian episodes of low sea level.
Middle Devonian Onondaga paleogeographic patterns indicate a recurrence of widespread, reefal Niagaran-type carbonate deposition in the Appalachians. Shallow-water carbonate shoals and patch reefs developed along the margins of an extensive starved basin. Hydrocarbon-bearing Onondaga pinnacle reefs have been discovered in the transition zone between shelf and basin, and may represent only a small portion of an extensive Devonian pinnacle-reef belt in the Appalachian basin.