STRATIGRAPHIC FRAMEWORK OF THE LOWER PENNSYLVANIAN LEE AND POCAHONTAS FORMATIONS IN SOUTHWESTERN VIRGINIA: EVIDENCE FOR CONTEMPORANEOUS TRUNK AND TRIBUTARY RIVER SYSTEMS

Despite numerous coal exploratory studies, basic stratigraphic relationships of Lower Pennsylvanian strata in the central Appalachian basin remain poorly understood. In particular, the possible contemporaneity of the Lee and Pocahontas formations has been much debated. In southwestern Virginia, the Pocahontas Formation consists of immature lithic sandstone, siltstone, mudstone whereas the Lee Formation is dominated by clean quartzose sandstones. Using a dense dataset of wireline logs, it is possible to construct two-dimensional subsurface cross-sections of the Lee and Pocahontas formations. Subsurface correlations reveal that economically important coal beds, despite localized scour and removal, are continuous from the Pocahontas into the Lee Formation. Thus, these two formations are coeval rather than the Lee Formation unconformably overlying the Pocahontas Formation as suggested by some previous workers. Previous studies have demonstrated that, Lee and New River quartzose sandstone bodies of Kentucky-southwestern Virginia and West Virginia, respectively, progressively onlap the northwestern margin of the basin. Subsurface correlation indicates that the uppermost intervals of the Sewanee and Warren Point sandstone members of the Lee Formation are more widespread and continuous to the southeast than previously documented. Based on the subsurface correlation, composition and geometry of sandstone bodies and paleocurrent data, the Pocahontas and Lee formations are considered to represent tributary and trunk systems, respectively. Pocahontas sediments were derived from incipient Alleghanian tectonic highlands to the southeast whereas the Lee sandstones were deposited in a braided fluvial system draining down the axis of the foreland basin from a northerly source. The resolution of basic stratigraphic relationships and sandbody geometry on a small scale can provide insight for regional stratigraphic correlations.