ASSESSING THE CONTROLS ON STRATIGRAPHIC PATTERNS IN THE SYNOROGENIC CLASTIC WEDGE OF THE CENTRAL ALLEGHANY OROGEN (Invited Presentation)

The Appalachian basin preserves a Pennsylvanian synorogenic clastic wedge that has served as the focus for study and debate regarding the influence of allogenic and autogenic controls on gross stratigraphic architecture. In particular, the term “cyclothem” was coined in efforts to use a Waltherian conceptual framework to explain the cyclic patterns observed in Pennsylvanian coal-bearing strata; ultimately these facies shifts came to be viewed as a result of extra-basinal forcing mechanisms, primarily climate and eustatic cyclicity. However, attempts by Wilkinson et al. (2003) to quantify cyclicity in cyclothems revealed that coal-bound packages lacked truly cyclic sedimentary facies patterns, suggesting a re-assessment of the fundamental controls on Alleghanian foreland stratigraphic architecture is warranted. As a case study, the Middle Pennsylvanian Allegheny Formation (MPAF) exposed in central WV represents a coal-bearing interval deposited in the equatorial lowlands of the overfilled Alleghany foreland and is commonly regarded as the non-marine, proximal expression of typical cyclothems. Paleobotany and palynology of coals indicate increasing aridity over 1 m.y. of MPAF deposition (307 to 306 Ma) and suggests alternating development of peat wetlands and alluvial systems within the MPAF are 5^th order stratigraphic cycles. We use sedimentological, compositional and paleohydrogical analysis of the MPAF multi-storey fluvial channel bodies, to assess the influence of varying water/sediment flux on stratal architecture. Our observations indicate the evolution of fluvial style recorded by MPAF sandstones and influx of sandy clastic detritus into the Alleghany foreland signals changes in drainage hydrology driven by paleoclimate. During climate cycles with greater seasonality, water and sediment flux increases to produce alluvial clastic deposition that largely blankets the central Alleghany foreland region. In contrast, during cycles with reduced seasonality, extensive wetland environments spread updip on the depositional profile above the basin base level and across the distal foreland. Increasing quartz content of MPAF sandstones suggests this the variable climate regime possibly influenced orogen unroofing patterns.