Abstract: Middle Devonian (Eifelian) Carbonates, Northern and Central Appalachian Basin: Sequence Stratigraphic Framework

C. E. Brett, C. A. Ver Straeten

The coeval Eifelian Onondaga (NY), Buttermilk Falls (eastern PA), and Selinsgrove limestones (Central PA) were deposited daring a time of relative tectonic quiescence between two early tectophases (I & II) of the Devonian Acadian Orogeny. Four member-level subdivisions of the carbonate succession (=Edgecliff, Nedrow, Moorehouse, and Seneca Mbrs. of NY terminology) as well as key marker beds and K-bentonites are recognizable all across the northern and central Appalachian Basin. The Onondaga-Buttermilk Falls-Selinsgrove limestones comprise the bulk of two apparent third-order cycles.

The base of the Edgecliff Member (= base of Cycle 1) is a complex unconformity across central to western NY and northwestern PA (subsurface) where it onlaps a subsiding peripheral bulge associated with the end of Acadian Tectophase I. This sequence boundary becomes conformable between eastern NY and most of south-central PA.

The Cycle 1 transgressive systems tract (TST) comprises the Edgecliff and Nedrow Mbrs.; in central NY at least nine smaller-scale cycles are recognized within the Edgecliff (4) and Nedrow (5) Mbrs. Widespread black shales (top Nedrow Mbr., central NY and PA) may represent maximum flooding. General shallowing-upward trends through the lower to middle Moorehouse Mbr. (ca. 8-10 small scale cycles) represent the highstand systems tract (HST) of Cycle 1.

Relatively coarse, crinoidal grainstone to packstone facies, locally resembling the Edgecliff Mbr., mark the upper Moorehouse Mbr. A generally fining-upward trend above a slightly irregular disconformity marks the lower part of the TST of Cycle 2 (uppermost Moorehouse and Seneca Mbrs.); thin condensed bone beds mark a major marine flooding surface at the top of the carbonate succession or within basal Hamilton black shales (Bakoven Mbr., NY; Shamokin Mbr., PA). The HST of Cycle 2 is represented by upper Bakoven and Stony Hollow Mbrs. (NY) and lower to middle parts of the Purcell and Turkey Ridge Mbrs. (PA), all capped by a thin limestone unit at the top of the Union Springs Fm. (Hurley Mbr., NY). The widespread and apparently synchronous nature of large and small scale cycles in the A palachian Basin suggests eustatic control of the carbonate succession; however, specific facies patterns and variable erosion on the unconformities reflect Acadian tectonics.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York