Gravel Dune Stratigraphy in the Altai Mountains, Siberia
Understanding the development of bedforms in coarse gravel deposits is important for hydro-geological, engineering purposes and for the interpretation of ancient sedimentary sequences. Coarse-gravel bedforms in the Altai area (Siberia), which result from Pleistocene glacial outburst floods have been identified as sub-aqueous dunes. GPR surveys of several dunes have elucidated the internal structures. Sediment analysis and stratigraphic mapping of sections has been used to calibrate the characteristic radar facies pattern. A striking feature is the continuous, subhorizontal reflections in the GPR profiles, in section parallel to the palaeoflow. These reflections separate sets of inclined reflections, which are identified as cross-sets consisting of alternate layers of bimodal and well-sorted open-framework gravel. The steeper reflection cross-sets are interpreted as indicative of avalanching processes on steep dune lee-sides, whereas low angle reflectors represent the sorting process associated with deposition of bedload sheets on low-angle dune lee-sides or on the back of the stoss-side. The geometry of the continuous reflections distinguishes different distinct sets of superimposed gravel cross-beds. This structure is interpreted as representing the unsteady downstream migration of 2-D dunes, about 2 m high in an aggrading setting.
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