Formation of Inversely Graded, Downstream-Dipping, Low-Angle Cross-Stratification by Antidunes
Thomas R. Clifton
Antidunes migrating upstream during aggradation produce a distinctive internal structure that results from the upstream climb of the bed form. This structure differs markedly from the upstream-dipping foresets usually ascribed to antidunes. Most studies on antidune internal structures focus on equilibrium conditions where there is no net deposition and preservation potential of structures is limited.
Experiments where aggradation was induced in small coastal streams show that upstream-climbing antidunes produce low-angle, downstream-dipping, cross-stratification 1-10 mm thick. The stratification results from grain size and density segregation between the antidune trough and crest. Studies show that small, dense grains form a lag in the antidune trough while coarser, less-dense grains accumulate at the crest. The grain segregation causes inverse grading within the stratification as the antidune body and crest pass over the finer grained lag in the trough.
Upstream-dipping foresets are not commonly formed by this process as there is no avalanching of grains over the bed form. Temporal variations in flow velocity may produce upstream-dipping foresets bounded by the downstream-dipping cross-stratification, but these features are subtle.
The studies above suggest that antidune cross-stratification may be more common in fluvial deposits than previously thought. Deposits along the Toutle River in southern Washington show numerous examples of inversely graded, downstream-dipping, low-angle (up to 15°) cross-stratification 1-5 cm thick, quite similar to antidune cross-stratification produced experimentally. Local upstream-dipping cross-stratification bounded by the downstream-dipping strata strengthens the interpretation.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.