Evidence for Paleo-Dune Fields in the Upper Formation of Aeolis Mons, Gale Crater, Mars

Part of the upper formation of Aeolis Mons (informally called "Mt. Sharp"), exhibits a prominent cliff-bench morphology. HiRISE (High Resolution Imaging Science Experiment) images of the surfaces of these benches reveal complex bedding patterns that are traceable at the scale of hundreds of meters. These include concentric patterns and sets of lineations bounded by truncation surfaces. Using a HiRISE digital terrain model (DTM), we have determined that these patterns cannot be explained by planar bedding (e.g. local topographic highs and lows do not correspond to concentric patterns as would be expected for planar bedding alone). Instead we interpret the patterns to be the plan-view exposure of cross-bedding, recording the migration of ancient aeolian dunes. Using the HiRISE DTM, we measured the local orientations of the beds that form the observed patterns. The measured dips are almost all <30°, consistent with dune slopes limited by the angle of repose. The lowest-elevation bench exhibits a concentration of dip directions toward the southwest, indicating dune migration in that direction. The bench above that, which has the most clearly exposed bedding patterns, has a broader distribution of dips, ranging in azimuth from 210° to 330° (NW to SW). This indicates net westward dune migration. In contrast, the dips on one of the higher elevation bench surfaces have a bimodal distribution toward azimuth 45° (NE) and 150° (SE), consistent with transverse bedforms with curved, out of phase crests or reversing linear dunes with crests oriented east-west. Our observations indicate that the observed portion of the Upper Formation of Aeolis Mons is composed of cross-bedded sandstone formed by the migration of hundred-meter-scale three-dimensional aeolian bedforms. The presence of such dunes indicates that the area was likely a sediment sink at the time of deposition, and the varying bedding dip directions between the major benches suggests that they represent distinct periods of deposition under varying wind regimes.