CONTRASTING FLUVIAL STYLES WITHIN LATE PENNSYLVANIAN STRATA OF THE DISTAL APPALACHIAN FORELAND BASIN

Variations in fluvial style within a basin can provide important clues to the changes in external forcing functions during basin evolution. Two well-exposed sandstones near the base of the Monongahela Group (Late Pennsylvanian) in southeastern Ohio, which are in close proximity both laterally and vertically, show significantly different fluvial styles. The lower example is up to eight-meters thick, fines upward from medium- to very fine-grained sandstone, and shows a pronounced offlap up-section into the adjacent sediments. The laterally adjacent facies consist of thin, fine-grained sandstones interbedded with laminated mudstones that show a pronounced dip away from the main sandstone body. Within ten meters of the lower sandstone the entire eight-meter thick interval is composed of gray laminated mudstones with scattered thin lenticular sandstones. The upper sandstone body is >11 m thick and fines upward from coarse- to fine-grained. The facies adjacent to the upper sandstone consist of a complex assemblage of thin nonmarine, limestones interbedded with red to gray blocky mudstones. Neither sandstone contains lateral accretion surfaces.

Both thick sandstones represent fluvial channel deposits. The offlap of sandstone and lateral facies change to laminated mudstone within the lower interval are interpreted as channel aggradation and the progradation of a levee into adjacent lacustrine sediments. These facies, combined with an absence of lateral accretion, are consistent with an anastomosed fluvial deposit. The upper sandstone was formed by a more energetic bedload system that is bounded by paleosols. The lateral facies, coarser grain size, and lack of lateral accretion surfaces suggest deposition by an incised braided fluvial system. The close vertical proximity of the two fluvial systems in a distal foreland basin setting implies an abrupt change in tectonic subsidence and/or climate during the Late Pennsylvanian.