SEQUENCE STRATIGRAPHY OF FORELAND BASIN DEPOSITS: A CASE STUDY FROM THE EARLY PENNSYLVANIAN POCAHONTAS BASIN, SOUTHWESTERN VIRGINIA

High resolution sequence stratigraphic architectural analysis of the coal bearing Breathitt Group within the Pocahontas Basin of Virginia is based on regional flooding surfaces and bounding discontinuities. These surfaces allow for a unified stratigraphic model at both regional, basin and individual coal bed methane field scales. Integration of a new Department of Energy and four industry donated cores with outcrop measurements and geophysical well log data aids sequence model construction and verification. This high resolution record of accommodation change throughout deposition of the Breathitt Group improves the understanding of the sequence stratigraphic expression of its >30 economically important coal seams and associated siliciclastic components.

The Pocahontas Formation is interpreted as dominantly non-marine, with fluvio-deltaic avulsion dominating 4th and 5th order sequences and moderated by an eustatic overprint. The Bottom Creek Formation is interpreted as non-marine with increasing estuarine and deltaic facies upwards. The sudden appearance of diagnostic marine ichnofabrics at the Hensley Shale Member at the base of the Alvy Creek Formation in several cores and at comparable outcrops signifies an important change in depositional style of the Pocahontas Basin. Increasing facies association rhythmicity, estuarine facies thicknesses, and brackish-marine fauna and ichnofabrics in the uppermost Bottom Creek and Alvy Creek Formations suggest strongly eustatic influences common to Early Pennsylvanian cycles elsewhere in the Appalachian Basin and globally.

These stratigraphic trends also suggest the prolonged westward expansion of Pocahontas foreland basin accommodation space and regional acceleration of tectonic subsidence, changing from being generally overfilled with dominantly fluvial sediments to increasingly estuarine deposits as basin subsidence exceeded rates of sand sediment supply and deposition. Cyclic episodes of maximum eustatic drop combined with periodic decelerating foreland basin subsidence allowed for a quartz pebble bearing southwesterly flowing fluvial system to incise into adjacent lithic molasse deposits.