Record of glacial–eustatic sea-level fluctuations in complex middle to late Pennsylvanian facies in the Northern Appalachian Basin and relation to similar events in the Midcontinent basin

Abstract

Pennsylvanian cycles in the Northern Appalachian Basin (NAB) were historically considered to result from delta-lobe switching, and more recently from sea-level fluctuation with sandy deltas prograding during highstand. These interpretations are revised using new data from cores and outcrop exposures. Thick (> 5 m) channel deposits with a marked erosion surface at their base cutting down across previous cycles are re-interpreted as incised valley fill (IVF) deposits in paleovalleys, because the basal erosion surfaces are widespread, and thus reflect a record of lowstand. Most common are simple paleovalleys that contain mainly sandy fluvial deposits. Compound paleovalleys with sequence boundaries above the basal erosion surface, contain terrestrial, estuarine, and marine deposits. Early to late highstand deposits in interfluvial parts of the cycles are dominated by shale and mudstone, with paleosols, coals, and local non-marine limestone, which reflect floodbasin to lacustrine conditions. These reinterpretations are applied to previously and newly recognized cycles in ascending order: Upper Kittanning, Lower Freeport, Upper Freeport Leader (new), Upper Freeport, Piedmont (new), Mahoning, Mason interval (locally includes Upper New Galilee in the north), and Brush Creek, across a 300-km arc in the Northern Appalachian Basin. These deposits accumulated in a ‘high shelf’ setting that experienced fewer marine transgressions, and were interrupted by more frequent exposure and downcutting, in contrast to the thicker and more complete succession with more numerous marine units in the Midcontinent. Magnitudes of highstand transgressions into this basin, deduced from the up-dip extent of marine and brackish fossil assemblages, were greatest for the Brush Creek, less so for the Upper Kittanning and Mahoning, and least for the Lower Freeport, Upper Freeport Leader, Piedmont, and Mason. The anomalous basin-wide fresh-water roofshales and equivalents of the Upper Freeport coal may reflect ponding of fresh water to the north by an elevated area that prevented marine encroachment at the southern opening of the basin along the Kentucky–Ohio border. Maximum thicknesses of IVF deposits in each cycle are used as a proxy for estimating the minimum depth of lowstand incision preceding transgression. While the thickness of IVF deposits in most cycles ranges from 6 to 20 m, the two thickest are 30+ m thick, one associated with the pre-Mahoning paleovalley and the other with the immediately post-Mahoning paleovalley. The extent of transgression of each cycle in the NAB is roughly equivalent to those of their age-correlative major marine cyclothems (Pawnee through Swope) in the Midcontinent. Moreover, the amount of incision between them is also roughly equivalent to the amount of regression between each of the roughly coeval cyclothems in the Midcontinent, with the deepest incisions in the NAB (pre-Mahoning and post-Mahoning) corresponding to the greatest southward withdrawals of the sea in the Midcontinent (Memorial and Seminole/Hepler) during this same interval.