LATE DEVONIAN MARINE-NONMARINE TRANSITION IN THE CATSKILL CLASTIC WEDGE: CURIOSER AND CURIOSER

Recent highway construction in east-central West Virginia has produced a series of extensive outcrops of Upper Devonian strata. The Foreknobs and Hampshire formations exposed near Elkins, WV were deposited during the last stages of regression associated with the Catskill Clastic Wedge. Physical and biogenic sedimentary structures and a diverse assemblage of marine invertebrate fossils indicate that the Foreknobs was deposited under normal marine conditions. In contrast, strata of the Hampshire Formation were the product of a spectrum of nonmarine depositional processes, as indicated by pedogenic sedimentary structures, fluvial channel deposits and lags, fossil plant debris, and a lack of invertebrate fossils.

Features observed in these exposures suggest that the marine-nonmarine transition may not have been simply the result of a gradual fall in relative sea level. Within the uppermost Foreknobs, beds of mudstone with polygonal mudcracks overlie intensely bioturbated sandstones containing marine trace fossils. These strata are interbedded with a meter-thick debris flow with inverse grading and oblate quartz pebbles up to 5 cm in long dimension. Overlying this sequence is approximately 60 meters of shale with marine trace fossils and a variety of tool marks. Tool marks include drag marks up a meter in length and 10 cm in width and a series of smaller structures produced by flattened pebbles that appear to have slid or been pushed a short distance along the muddy bottom and then left in place. The marine-nonmarine transition recorded in these strata appears to have been accompanied by relatively rapid, localized oscillations in sea level. Desiccation features suggest arid conditions; the debris flow and unusual tool marks may be ice-contact or glacial in nature as has been suggested for the Late Devonian-Early Mississippian Spechty Kopf Formation in Pennsylvania.