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ABSTRACT: Separation of Bed Form and Bar Form Cross-Stratification in Sandy Fluvial Deposits

D. C. Mohrig, J. Dungan Smith

The beds of fluvial channels in sandy depositional systems are mantled with numerous periodic or quasiperiodic topographic elements that can be assigned to one of two hydrodynamic groups: bed form or bar form. Bed forms (ripples and dunes) arise from convergences in the sediment transport field that are associated with boundary shear stress variations produced by vertical adjustment of the flow over periodic, small-scale morphological features; bar forms are the result of adjustments in this field caused by deflection of the flow around larger scale topographic elements. Owing to these differences, the former has heights that are only a fraction of the water depth (less than half), whereas the latter closely approximates the formative flow depth.

Sandy channel fills are often characterized by stacked sets of foresetted beds. Each of these cross-stratified layers documents bar form or bed form migration and deposition. To develop a precise hydraulic reconstruction of channel flow the bed form/bar form distinction must be made for each layer and the relationships of successive layers must be carefully accounted for. Our work on the fills of modern and ancient low-sinuosity channels indicates that bar form cross-stratification is readily distinguishable from dune stratification by both the frequent preservation of topset-related strata and a systematic decrease in grain size along the lower portions of foreset laminae. Only when the salient features of the channel-bed morphologies are recognized and characterized in a proper hydr dynamic model can a reliable paleohydrologic reconstruction be made.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990