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ABSTRACT: Antidune Behavior and Stratification

Thomas R. Clifton

The nature of internal stratification produced by migrating antidunes depends strongly on the behavior of the bed forms and their overlying surface waves. Flow over antidunes segregates fine, dense grains in the trough and coarser, less dense grains at the crest. Given uniform rates of migration and aggradation, this configuration should produce inversely graded strata that dip downstream at a uniform angle. Upstream-dipping backset laminae are not necessarily preserved. Experiments that induced aggradation in small channels, however, produce a more complex structure. Box cores from these channels typically show laminae with variable upstream and downstream dips and localized downcutting.

The antidunes observed in the experiments showed the common cyclic behavior of building in amplitude until the overlying surface wave becomes unstable and breaks. This behavior affects both the migration and aggradation rates of the bed form. As antidunes build in amplitude, the shear stress increases in the trough and decreases at the crest. These changes cause increased deposition on the stoss side of the antidune, increasing the migration rate of the bed form. Additionally, the rise in shear stress in the trough mobilizes more sediment, leading to a reduction in the rate of net deposition.

With this behavior, laminae resulting from the translation of the antidune trough will climb upstream, with the climb angle decreasing upsection. As the antidune amplitude increases, sediment transport in the trough can reach a point where downcutting will occur. The strata observed in the box cores are inferred to form in this manner. Changes in antidune behavior should produce structures with a range of internal geometries.

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