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Climatically Forced Progradation During Transgression? Supercritical-Flow Signature of an Extreme Fluvio-Deltaic Flood in the Late Carboniferous Pennine Basin (UK)

Abstract

Modern sequence-stratigraphic models increasingly need to expand consideration of allogenic forcing to various factors interfering with the well-established role of base level at basin scale. Climate remains a particularly difficult variable to isolate in very ancient successions, for which accurate chronological correlation and proxy-based quantification of paleoenvironmental conditions are seriously limited. However, sedimentary facies analysis still represents a fundamental conceptual tool to recognize process-patterns typical of specific climatic contexts. Recent developments show that fluvial systems in monsoonal settings are subject to prolonged hydrological inactivity alternating with brief (inter)annual phases of possibly extreme discharges, recognizable by sedimentological attributes such as great volumes of supercritical-flow deposits, vegetation-induced structures within channel fills and coarse overbank facies. Distally linked, flood-prone deltas, with higher preservation potential, should also present distinctive traits. Late Carboniferous (Bashkirian) sandstones of the Lower Kinderscout Grit (Millstone Grit Group, UK) were deposited at subequatorial latitudes in fluvio-deltaic and shallow-marine settings of the Pennine Basin, north of the Variscan Orogen, during early assemblage of the Pangean megacontinent. Outcrops frequently feature large, wavy geometries and unusual architectures traditionally difficult to interpret. The unique stratal configuration of deltaic deposits at Derby Delph Quarry (West Yorkshire) comprises thick sets of giant, rhythmically undulating sedimentary structures in massive, poorly sorted sandstones, with fully aggradational architecture. Recent insights on sediment beds under supercritical currents allow to interpret these deposits as geologically instantaneous progradation of a proximal delta front from a long-lived hyperpycnal current which accreted cyclic steps, bedforms identified only recently under high-energy flow conditions. The paleogeographic, paleoclimatic and sequence-stratigraphic context indicate that the region was subject to a tropical seasonal climate that enhanced megamonsoonal circulation on the eastern margin of the early Pangean landmass. Exceptional fluvial floods were able to transfer large volumes of sediment basinward, especially during interglacial phases of marked seasonality and base-level rise, forcing a high progradational efficiency for clastic systems locally also during transgressions.