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Salt-Related Structures in Northern Appalachian Basin

Philip Towey

The Plateau province of the northern Appalachian basin is characterized by a series of sharp, detached, thrust-faulted anticlines roughly parallel with the Allegheny Front. In southwestern Pennsylvania and northern West Virginia, major thrusting of Alleghenian age is dominantly from the east, with numerous smaller thrusts from the west. Although the principal decollement is in the Silurian Salina Group, evidence of deeper detachment zones close to the front is abundant. In central and northern Pennsylvania, however, major thrusting is from the west. In those areas, thrusts from the east are rare to absent. Folds there are thin skinned above a decollement in the Salina Group, with no evidence of deeper detachment.

Some recent workers have dismissed thrusting from the west as merely back thrusts of Alleghenian age, but seismic and well information indicate that the structural history of the Plateau province was dominated by a tectonic regime characterized by faults from the west and later overprinted near the structural front by the structures of the Alleghenian orogeny with its compression from the east. The basic idea is not new, but evidence in support of it has not been widely published.

A sequence of seismic examples from southern New York to southwestern Pennsylvania illustrates the evolution of this process, involving Silurian salt removal and flowage under gravity, from its simplest beginnings to its most complicated interaction with Alleghenian structures. Evidence is shown to suggest that the process began at least as early as during Marcellus deposition (Middle Devonian) and probably earlier. Characteristics of this structural style, such as collapse, normal faulting, and compensation, are also exhibited. Perhaps of some significance is the implication that these structures may not have originated in response to any of the major orogenic episodes to which the basin has been subjected. The evidence suggests that the salt began to move in response to less than 1, 00 ft of overburden. In some parts of the province that amount of overburden occurred during the Acadian orogeny, but that may be more coincidental than causal, at least in a tectonic sense.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.