Recognition Of Milankovitch Orbital Forcing Patterns In Shelf Facies Of The Lower Devonian New Creek And Corriganville Formation Of Central Pennsylvania

Abstract

Application of the Milankovitch model of allocyclicity to the New Creek and Corriganville Formations in central Pennsylvania reveals vertically consistent and laterally correlative stacking patterns of fifth-order and sixth-order cycles in below-wave-base shelf facies. Overlying an unconformable third-order sequence boundary (Keyser-New Creek boundary), the formational succession consists of progressively deeper fifth-order sequences traceable throughout Pennsylvania. The New Creek Formation is one fifth-order sequence, consisting of shallow-shelf, bioturbated calcarenite packaged into three meter-scale allocycle or (PACs). This fifth-order sequence, incomplete because of hiatus at the third-order boundary, is asymmetric, shallowing to peritidal facies in the uppermost Pac at Tyrone. In general, the Corriganville Formation is a complete fifth-order sequence consisting of five sixth-order cycles, but is incomplete at Tyrone where the basal PAC is missing. PAC 1 was not deposited at Tyrone because this area was not flooded by the first precessional rise in the Corriganville fifth-order sequence. Unlike New Creek PACs, which are internally gradational, Corriganville PACs contain distinct highstand and lowstand portions separated by a sea-level-­fall surface. Precession-driven eustacy is responsible for the primary cyclic fabric of this stratigraphic interval. Eccentricity functioned as a modulator by enhancing the precessional affect at the fifth-order boundaries and by dampening the precessional affect within the fifth-­order sequence and producing a general shallowing-upward trend. Recognition of these cyclic patterns, at the sixth and fifth-order scale, lends support to the concept of a genetic hierarchy of allocycles.