MORPHOLOGY, SEDIMENTOLOGY AND PALAEOHYDRAULIC SIGNIFICANCE OF LARGE GRAVEL DUNES, ALTAI MOUNTAINS, SIBERIA

Coarse-gravel bedforms which resulted from Pleistocene glacial outburs t floods are identified as subaqueous dunes. Comparison of the morphol ogy 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 morphol ogy within lesser-scale geophysical flows. The similarity of the steep est gravel dunes with equilibrium dunes in sand indicates that grain s ize is not a major factor in constraining primary duneform. Internal s tructure indicates that flow over 2-D dunes was relatively uniform but over 3-D bedforms flow was locally variable. Flow separation and comp lex streaming of flow occurred over the steepest 3-D dunes. Cross-beds are thin and few approach the angle of repose; consequently most dune s did not migrate primarily by avalanching but by stoss-entrained grav el transported over the crests rolling-down and depositing on the lee slopes. Lee-side sediments are often finer than the stoss-slope sedime nts, 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 slop e. However, silt-rich caps indicate that any height reduction was cont emporary with the final deposition of foresets. Post-flood modificatio n has been negligible although the modern topography is subdued by loe ss deposits within the dune troughs.