Alluvial Architecture of the Early Pennsylvania Sharon Formation in Northeastern Ohio

Abstract

In northeastern Ohio, excellent exposures of the Sharon Formation allow study of the architecture (3-D geometry) of these gravel- and sand-bedload stream deposits. Specific architectural elements include gravel bar-platform deposits (including bar head, bar core, and bar tail sub-elements), suprabarplatform deposits (laminated sand sheets and chute channel-fills), bar-margin foreset deposits, and sandy 2-D and 3-D dune deposits. Paleochannels had a depth-to-width ratio of 1:10 (r2 = 0.69) for gravelbedload streams and 1:40 (r2 = 0.89) for sand-bedload streams. Channel paleoslopes were between 0.3 to 1.1 m/km and transported clasts with D95 = 5.6 cm. These data are consistent with modern, braided streams. In this region, Late Mississippian to Early Pennsylvanian glacio-eustatic baselevel fall resulted in subaerial erosion of the underlying marine shales and formation of paleovalleys. Subsequent baselevel rise created accommodation space that was filled by deposition of the Sharon Formation in two separate phases: (1) backfilling of paleovalleys and (2) unconfined fluvial depositional systems after the paleovalleys were filled and overtopped. The transition of fluvial systems from confined to unconfined probably resulted in braidplain widening and changes in bank materials, explaining observed changes in paleohydraulics and fluvial sedimentology of the unit.