ABSTRACT

Thick to very thick beds of sand to gravel-matrix, limestone-clast conglomerate constitute an important lithology of the Richmond slice of the allochthonous Hamburg klippe of southeastern Pennsylvania. Sedimentary structures in the conglomerates can be explained by both debris flow (planar clast fabric, nondeforming "plugs") and grain-flow (reverse grading) mechanisms. However, the lack of a cohesive mud matrix and the great thickness of the beds rule out either one of these sediment-gravity-flow types. Previous studies have illustrated that thick flows of poorly sorted, gravel-size debris can move over relatively low slopes by density-modified grain flow, a cross between true unimodal grain flow and debris flow. The coarse-grained matrix conglomerates of the Richmond slice can be exp ained as ancient density-modified grain-flow deposits. In addition, the presence of layers of normally graded level and parallel- to cross-laminated sand-size material on top of some of the conglomerates is suggestive of increased turbulence toward the top of the flow. This is probably related to either backward shearing of sediment and water in front of the moving sediment mass, creating turbulence on top of the flow, or expulsion of pore fluid during movement of the main sediment mass.