OBSERVED CHANGES IN PROCESS SEDIMENTOLOGY OF NAMIB LINEAR DUNES USING GROUND PENETRATING RADAR

Linear sand dunes are found in large, low latitude deserts in Namibia, the Sahara and the Arabian Peninsula. Winds blow oblique to the dune long axis, often with at least two different seasonal orientations that combine to transport sand parallel to the long axis. In the Namib Sand Sea, linear dunes are 0.5-2.0 kilometers wide can reach hundreds of kilometers long and often exceed 100 meters high. 200-MHz Ground Penetrating RADAR (GPR) profiles acquired along the flanks of large linear dunes in the Namib Sand Sea captured well-expressed bedforms. In the GPR profiles two Reflectivity intervals are recognized: (1) the upper interval consists of gently dipping, planar reflectors that are well-layered and display a relatively uniform character. This upper interval represents deposits of large, 2D, superimposed, flanking dunes that are oriented perpendicular to the linear dune, but migrate parallel to the linear dune. This interval drapes a thicker, more complex interval. (2) The lower interval displays two patterns: (a) Bowl-shaped patterns that are indicative of trough cross-stratification which is produced by dunes with a 3D wave front moving approximately parallel to the axis of the linear dune. (b) Tabular to sigmoidal patterns that indicate 2D dune fronts migrating approximately perpendicular to the axis of the linear dune. Patterns (a) and (b) often cross cut each other indicating active migration of superimposed dunes on the flank of the dune during this phase of deposition. The most prominent single reflector on the profile separates interval (1) and (2)—this surface displays a variety of reflection termination including toplap, downlap and onlap. These terminations suggest that the reflector was a prominent erosional surface. This surface delineates a major change in process sedimentology from an active dune migration/building phase below to a sediment-draping phase above. This change in phase is evidently indicative of changing factors (wind regime/strength for example).