THE SEQUENCE STRATIGRAPHIC CONTEXT OF THE FLUVIAL ARCHITECTURE ASSOCIATED WITH THE PITTSBURGH COAL IN SOUTHEASTERN OHIO

The Pittsburgh Coal, the most extensive coal in the Appalachian basin, marks the base of the Monongahela Group in southeastern Ohio and occurs approximately 70 m above the last documented marine limestone in the region; however, a diverse trace fossil assemblage recovered within the uppermost Conemaugh Group shows that the interval directly below the coal was at least marine-influenced. Closely spaced, detailed measured sections along an extensive roadcut in Athens County, Ohio, reveal that the sediments that bracket the Pittsburgh Coal are dominated by lacustrine and palustrine mudstones and limestones with lesser amounts of red to gray blocky mudstones (paleosols), and medium- to fine-grained sandstones and siltstones (fluvial channels and crevasse splays). The sediments are grouped into six facies associations that show an upward change in fluvial style from anastomosing to braided. The facies also form five 4th-order sequences in which interfluve paleosols represent both the Falling Stage and Lowstand Systems Tracts, and braided fluvial deposits both the Lowstand and early Transgressive Systems Tracts (TST). Palustrine limestone deposition was mainly restricted to the late TST and was followed by either anastomosed or meandering fluvial deposits of the Highstand Systems Tract. Variations in grain size, bioturbation, and paleosol development within the sequences are interpreted as products of changes in the moisture balance associated with a regional rise and fall in base level. The fluvial style varied in response to a rapid increase in subsidence-generated accommodation that was sufficient to offset the overall 3rd-order fall in eustatic sea level documented from the equivalent interval in the Mid-Continent. The widespread extent of the coal suggests that the accommodation was the result of major tectonic basin reorganization.