Stream‐driven, high‐density gravelly traction carpets: possible deposits in the Trabeg Conglomerate Formation, SW Ireland and some theoretical considerations of their origin

Abstract

ABSTRACTWithin high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition.