EVIDENCE FOR FLUVIAL INCISION NEAR THE FRASNIAN-FAMENNIAN CONTACT (LATE DEVONIAN) IN THE CENTRAL APPLACHIAN BASIN OF MARYLAND

Within the central Appalachian basin the upper Foreknobs Formation consists of thinly interbedded marine siltstone and shale, with two prominent blanket sandstone intervals, named the Brierly Gap and Pound sandstones. Along the western edge of the Foreknobs outcrop belt, along the Allegheny structural front, and in the subsurface of the Appalachian Plateaus Province farther to the west, the Brierly Gap and Pound sandstones exhibit only nominal variations in thickness and lithologic character. However, study of the upper Foreknobs Formation in more eastern outcrop areas, within the Ridge and Valley Province of Maryland, reveals that these two sandstone intervals are enclosed within a succession of upper Foreknobs strata that are partially nonmarine. In these more proximal depositional areas the upper Foreknobs contains intervals of interbedded lenticular fluvial sandstones and red paleosols. Furthermore, the Brierly Gap and Pound sandstones exhibit dramatic localized thickening to nearly 50 m and a multistoried character. Each sandstone unit is comprised of up to three individual fining upward sandstone successions exhibiting a sharp, convex downward, basal contact, and lag conglomerates composed of shale clasts and fossilized tree trunks.

The apparent shoreward change of the upper Foreknobs from marine to nonmarine suggests a gradual drop in sea level during deposition of the facies enclosing these two Brierly Gap and Pound sandstones. Furthermore, the localized dramatic thickening and multistoried attributes of these two sandstone layers appear to reflect a sharp drop in base level that caused fluvial incision of the regional trunk streams.

The Brierly Gap Sandstone has been correlated with a late Frasnian sea level lowering that preceded the European lower Kellwasser event and the Pound Sandstone has been interpreted to be contemporaneous with the upper Kellwasser event. The character of the proximal upper Foreknobs Formation in Maryland suggests that late Frasnian global events may were not instantaneous within the Appalachian basin. Additionally, progradation of terrestrial facies and channel incision of the main trunk streams reflect both local and global environmental changes, likely climatically driven.