Tectono‐sedimentary Evolution of Early Pennsylvanian Alluvial Systems at the Onset of the Alleghanian Orogeny, Pocahontas Basin, Virginia

Abstract

AbstractForeland basin strata provide an opportunity to review the depositional response of alluvial systems to unsteady tectonic load variations at convergent plate margins. The lower Breathitt Group of the Pocahontas Basin, a sub‐basin of the Central Appalachian Basin, in Virginia preserves an Early Pennsylvanian record of sedimentation during initial foreland basin subsidence of the Alleghanian orogeny. Utilizing fluvial facies distributions and long‐term stacking patterns within the context of an ancient, marginal‐marine foreland basin provides stratigraphic evidence to disentangle a recurring, low‐frequency residual tectonic signature from high‐frequency glacioeustatic events. Results from basin‐wide facies analysis, corroborated with petrography and detrital zircon geochronology, support a two end‐member depositional system of coexisting transverse and longitudinal alluvial systems infilling the foredeep during eustatic lowstands. Provenance data suggest that sediment was derived from low‐grade metamorphic Grenvillian‐Avalonian terranes and recycling of older Palaeozoic sedimentary rocks uplifted as part of the Alleghanian orogen and Archean‐Superior‐Province. Immature sediments, including lithic sandstone bodies, were deposited within a SE‐NW oriented transverse drainage system. Quartzarenites were deposited within a strike‐parallel NE‐SW oriented axial drainage, forming elongate belts along the western basin margin. These mature quartzarenites were deposited within a braided fluvial system that originated from a northerly cratonic source area. Integrating subsurface and sandstone provenance data indicates significant, repeated palaeogeographical shifts in alluvial facies distribution. Distinct wedges comprising composite sequences are bounded by successive shifts in alluvial facies and define three low‐frequency tectonic accommodation cycles. The proposed tectonic accommodation cycles provide an explanation for the recognized low‐frequency composite sequences, defining short‐term episodes of unsteady westward migration of the flexural Appalachian Basin and constrain the relative timing of deformation events during cratonward progression of the Alleghanian orogenic wedge.