Near-surface structure of a large linear dune and an associated crossing dune of the northern Namib Sand Sea from Ground Penetrating Radar: Implications for the history of large linear dunes on Earth and Titan
Résumé
We imaged the near-surface sedimentary structures of a large linear dune, flanking dune forms and an associated crossing linear dune never before studied in the northern Namib Sand Sea using 200-MHz Ground-Penetrating Radar (GPR). The dry, uniform sandy conditions and wavelength used allowed for highly detailed observations of sedimentary structures to depths of ∼ 12 m across a >1km lateral scan. Sedimentary features observed in the main linear dune include scouring and abrupt changes in strata such as trough cross stratification (TCS), onlap, downlap, truncation and avalanche-related bedding, all a result of complex sand transport conditions. Different phases of deposition have produced an opposed succession of strata on each side of the dune. These successions alternate 2-dimensional (2D), or bedform instability mode features with 3-dimensional (3D), or fingering mode features, separated by a clear process boundary. These alternating successions reflect a change in the dominant wind environment in the recent past. The changing winds may feed into the building and overall stability of this dune field and may be a model for conditions in other large linear/longitudinal dune fields. The subsurface structure of an oblique crossing linear dune demonstrates sand transport generally down the dune long axis in the direction predicted from modern, ERA-Interim model as well as paleoclimate model winds. This suggests relatively long-term stability of this intermediate-sized landform and the potential long-term coexistence of large dunes and secondary forms. These studies have implications for the extensive sand seas of Titan, where lack of large secondary forms may indicate a simple wind regime over long time periods.