ABSTRACT

Subaqueous sand-flow cross strata are formed by the flowage of sand down the slipfaces of submerged bedforms and isolated steps. Such cross strata make up the great bulk of the foreset deposits of shallow-stream megaripples and bars. Characteristics of sand-flow cross strata that form in fine to medium sand on subaqueous slipfaces less than 2 m high include steep dip angles (generally 27-34°), little if any curvature in dip cross sections, distinct segregation of grain types, downward coarsening along the cross stratum and generally upward coarsening (inverse grading) across the cross stratum, loose gram packing, absence of interbedded grainfall or other deposits, small range of cross-stratum thickness within a set, increasing cross-stratum thickness with increasing slipface heigh , and great width (lateral extent) relative to cross-stratum thickness and slipface height. In coarse, subaqueous sand, the sand-flow cross strata tend to be narrower (more lenticular in horizontal sections). Flume experiments suggest that each cross stratum is produced by one in a series of intermittent sand flows and that the avalanching is generally not closely controlled by bedforms that spill over the brink of the slipface.

Subaqueous cross strata that formed in fine to medium sand on relatively low slipfaces typically differ from eolian cross strata that formed under similar conditions. Grainfall deposits are less common on subaqueous slipfaces, probably because avalanching tends to be more nearly continuous there than on eolian slipfaces. Another difference is that subaqueous sand-flow cross strata tend to be wider (more extensive laterally) than eolian ones, in part because avalanching tends to be more nearly continuous on subaqueous slipfaces and in part because submerged sand is even less cohesive than dry eolian sand.