Morphology, sedimentology and palaeohydraulic significance of large gravel dunes, Altai Mountains, Siberia

Coarse‐gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two‐dimensional (2‐D) transverse dunes and 3‐D cuspate and lunate dunes developed in coarse gravels is comparable with sand‐dune morphology within lesser‐scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2‐D dunes was relatively uniform but over 3‐D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3‐D dunes. Cross‐beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss‐entrained gravel transported over the crests rolling‐down and depositing on the lee slopes. Lee‐side sediments are often finer than the stoss‐slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly planed‐down during falling stage because lee‐side cross‐beds tend to be steeper than the angle of the preserved lee slope. However, silt‐rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post‐flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs.