Correlation and Paleogeography of Upper Part of Helderberg Group (Lower Devonian) of Central Appalachians

Abstract

Physical stratigraphy and environmental relations in the upper part of the Helderberg Group have clarified the interpretation of biostratigraphic data in a previously enigmatic area of Appalachian Lower Devonian correlation. The Licking Creek Limestone, a marine carbonate shelf facies, is subdivided and its members (lower, Cherry Run Member; upper, Little Cove Member) are traced along the eastern outcrop belt from south-central Pennsylvania southward to central Virginia. Toward the northwest the Cherry Run increases in thickness and intertongues with the lower two thirds of the Shriver Chert, an offshore calcareous cherty silt and mud which occupies the depositional basin axis. The Little Cove rises stratigraphically toward the northwest and is traced into the upper part f the Shriver Chert. Lithologic and faunal differences of the equivalent units are caused by the differing, but time-equivalent, adjacent environmental settings (shallow shelf-basin axis). Lithostratigraphic evidence indicates that the upper contact of the Licking Creek-Shriver strata rises in the section in a northwesterly (basinward) direction and that the Licking Creek of the eastern outcrop belt intertongues with the lower two thirds of the Shriver in its type area. Biostratigraphic evidence indicates that faunal elements are added to the upper part of this interval as the contact rises. This evidence also suggests that the Licking Creek of the eastern outcrop belt and the lower two thirds of the Shriver are Helderbergian and the upper third of the Shriver is Deerparkian. Paleogeographic reconstruction of these correlations shows that (1) shoreline and facies boundaries are slightly oblique to the outcrop belt; (2) shallow carbonate facies tended to migrate westward (basinward) during this time, indicating a regressive trend just prior to Oriskany Sandstone deposition; and (3) the basin margin pre-Oriskany unconformity does not extend over large areas of the depositional basin itself. These conclusions are of potential significance for gas exploration in the upper Helderberg and overlying Oriskany Sandstone in relating subsurface stratigraphic facies and structure to the observed surface trends and correlations.