GPR as a tool to elucidate the depositional processes of giant gravel dunes produced by late pleistocene superflooding, altai, siberia

摘要

Understanding the development of bedforms in coarse gravel deposits is important for hydrogeological, engineering purposes and for the interpretation of ancient sedimentary sequences. Coarse-gravel bedforms int he Altai area (Siberia), which result from Pleistocene glacial outburst floods have been identified as subaqueous dunes. GPR surveys of several dunes have illucidated the internal structures. Sediment analysis and stratigraphic mapping of sections have 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.