Stratigraphic Architecture of Early Pennsylvanian, Coal-Bearing Strata: A Case Study from Buchanan, Dickenson and Wise Counties, Virginia

Lower Pennsylvanian, coal-bearing, siliciclastic strata of the central Appalachian Basin define a southeasterly thickening clastic wedge and were deposited in continental to marginal marine environments influenced by high-amplitude relative sea level fluctuations. Detrital zircon geochronology indicates that sediment was derived from low-grade metamorphic and Grenvillian-Avalonian terranes of the Alleghanian orogen towards the southeast and, in part, from the Archean Superior Province to the north. Immature sediments, including sublithic sandstone bodies, were deposited within a southeast-northwest oriented transverse drainage system. Texturally and mineralogically mature quartzarenites were deposited in strike-parallel elongate belts along the western periphery of the basin. These mature quartzarenites are braided fluvial in origin and were deposited within northeast-southwest oriented axial drainage head-watered in a northerly cratonic source area. The contemporaneity of transverse and axial fluvial systems defines a trunk–tributary drainage system operating in the central Appalachian foreland basin during the early Pennsylvanian.

Sequence stratigraphic analysis of Lower Pennsylvanian strata in Buchanan, Dickinson and Wise Counties, Virginia is based on gamma ray and density well log interpretation coupled with two continuous core logs and outcrop observations. Multiple cross sections reveal a stratigraphic architecture consisting of unconformity bounded, 4th-order sequences consisting of upward-fining, transgressive incised valley fill (alluvial to estuarine) deposits that are capped by thin intervals of fine-grained strata (condensed sections) and overlain by upward-coarsening, progradational deltaic deposits. The 4th-order sequences of ~400 k.y. duration are stacked into 3rd-order composite sequences of ~2.0 Ma duration in which regional marine horizons define maximum flooding surfaces. Regionally extensive coal beds are developed in close association with condensed sections and also occur within progradational deltaic deposits. Formation of coal beds in the central Appalachian basin of southwest Virginia is attributed to extrabasinal, glacio-eustatic controls and intrabasinal deltaic processes related to channel avulsion and delta lobe switching.