Depositional Environments and Sequence Stratigraphy of Upper Mississippian Strata in the Central Appalachian Basin
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Transgressive and highstand deposits within several sequences contain tidal rhythmites. The prodeltaic Pride Shale member (Bluestone Formation) preserves a hierarchy of submillimeter-to meter-scale cycles that reflect a spectrum of tidal periodicities. The abbreviated character of these microlaminated rhythmites is suggestive of a distal, subtidal setting wherein neap tides were of insufficient strength to transport sand/ silt. Decimeter-scale bundling of semimonthly cycles is ascribed to seasonal fluvial discharge. Meter-scale, multi-year cycles may reflect the 18.6-year lunar nodal cycle.
Sequence development likely reflects fourth-order (~400 k.y.), Gondwanan glacioeustatic cycles. The character and relative stratigraphic position of paleoclimatic indicators within the sequences suggests a link between eustasy and patterns of global-scale atmospheric circulation. Calcic vertisols and lacustrine carbonates in coastal plain successions are suggestive of seasonal, semiarid climatic conditions during highstand progradation. Leached paleosols and coals that underlie sequence boundaries and occur within estuarine fills are suggestive of humid conditions during late highstand through early transgression. This pattern may reflect fluctuations in monsoonal circulation whereby the latitudinal shift of seasonal moisture was restricted to the equatorial zone during glaciations.
The fourth-order sequences stack into two (2-4 Ma) composite sequences that consist of: 1) a basal retrogradational interval comprised of a major paleovalley-fill sequence overlain by a thick aggradational sequence set made up of fluvial/ coastal plain sequences (TST); 2) a marine interval that demarcates maximum flooding; and 3)(where preserved) a progradational sequence set consisting of minor incised valley-fill sequences (HST). These composite sequences document accommodation change that may reflect global tectono-eustasy.
- Doctoral Dissertations