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Recognition Of Milankovitch Orbital Forcing Patterns In Shelf Facies Of The Lower Devonian New Creek And Corriganville Formation Of Central Pennsylvania
Orzechowski, Christopher
Orzechowski, Christopher
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1995
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Earth and Environmental Science
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http://dx.doi.org/10.34944/dspace/8649
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-Âfall 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-Âorder 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.
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