SEQUENCE STRATIGRAPHIC EXPRESSION OF THE WAXING AND WANING OF THE GONDWANAN ICE SHEET: EXAMPLES FROM THE CARBONIFEROUS OF THE APPALACHIANS

The Mid-Carboniferous sedimentary record in southern West Virginia and southwestern Virginia is dominated by siliciclastic units derived from the fold-and-thrust belt to the southeast and the Archean Superior Province to the north. Fourth-order stratigraphic sequences between 30-120 m thick and of ca. 400 k.y. duration are recognized in both Upper Mississippian and Lower Pennsylvanian successions. The sequences (cyclothems) are unconformity-bounded, and include incised-valley-fills and stacked coastal plain deposits. Major incised-valley fills that range in scale from 15 to >100 km wide and average 30 m thick are oriented longitudinally along the foreland basin axis. In contrast, minor incised-valley-fills are orthogonal to the basin axis and range in scale from 1-30 km wide and 5-30 m thick. Paleovalley fills at both scales consist of braided-alluvial facies grading upwards into estuarine deposits that locally contain well-preserved tidal rhythmites. Maximum flood deposits of carbonaceous mudstones are overlain by highstand deposits. Upper Mississippian highstand deposits consist of coastal plain red mudstones with intercalated channel sandstones, crevasse deposits, and lacustrine limestones; or progradational deltaic deposits. Lower Pennsylvanian highstand facies are dominated by stacked, progradational deltaic parasequences. The fourth-order sequences are arranged in retrogradational-progradational composite third-order sequences that consist of (1) major incised valley fills that constitute lowstand-transgressive systems tracts, (2) fossiliferous limestones and/or mudstones related to major drowning events, and (3) a progradational set of minor incised-valley-fill dominated sequences that make up highstand systems tracts. Fourth-order sequence development is attributed to eccentricity-driven glacioeustacy during the early stages of the Gondwanan glaciation. Composite sequences reflect long-term (2- 4 Ma) changes in accommodation probably related to tectonically driven eustacy, but possibly a product of long-term glacioeustatic fluctuation. The stratigraphic hierarchy recognized in these Carboniferous successions is consistent with icehouse stacking patterns.