Origin of cross-stratal layering in fluvial conglomerates, Devonian Malbaie Formation, Gaspé, Quebec
Planar cross-bedded conglomerates in proximal braid-plain deposits of the Devonian Malbaie Formation of eastem Gaspe, Quebec, display a distinctive cyclic layering of three facies: conglomerate with sand matrix (A); matrix-free conglomerate, in which the voids are filled by spany white calcite cement (C); and coarse sandstone (B). The sequence was previously interpreted as ABC, but is here reinterpreted as ACB. B forms cross-strataparallel layers. Facies A and C together make up the gravel layers, with C passing down-foreset into A across a matrix-cement interface which dips at low angles in the same direction as the foreset. The sequence results from migration of gravel-sand bed forms across bar tops ,with deposition of gravel at bar margins as foresets and infiltration of sand into the gravel fabric from blackflow currents induced by flow separation at the foreset crests. Infiltration proceeds until the framework voids are plugged by granules and coarser sand grains, after which sand and granules may continue to accumulate on the plugged surface to form a discrete layer. Occasional peaks on the matrix - open framework interface record sites of more effective infiltration. Elongate peaks, strike-parallel or downdip, possibly reflect organisation of the sand load into npples across the foreset at the time of its infiltration, or they may reflect regular variation in grain size or packing, and hence ingress potential, of the framework. Partial sequences in which one or another of the layers is missing apparently result from variation in the efficiency of sand transport up the foreset coupled with variation in the efficacy and timing of framework plugging, or variable sorting in bar-top bed forms.