Aeolian architecture, bedform climbing and preservation space in the Cretaceous Etjo Formation, NW Namibia

Sets of aeolian cross‐strata within the Cretaceous Etjo Formation of NW Namibia are bounded by a hierarchy of surfaces, the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour. The base of the main aeolian succession is characterized by a basin‐wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building. Interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5° in an upwind direction (SW). Perpendicular to the palaeowind, interdune surfaces form 500‐m‐wide troughs, signifying crestline sinuosity within the original bedforms. Superimposition surfaces are inclined at 5–10° in a downwind direction and indicate the migration of crescentic oblique dunes over larger, slipfaceless transverse draa. Reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross‐strata lie parallel to them, rather than downlap onto them. Analysis of the geometry of these bounding surfaces, together with the orientation of the cross‐strata within the sets that they bound, has enabled the detailed morphology of the original bedforms to be reconstructed. The maximum preserved thickness of individual aeolian sets varies systematically across the basin, from 52 m in the basin depocentre to only 8 m at the basin margin. The set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin, rather than the presence of smaller bedforms. Similarly, a temporal reduction in the angle‐of‐climb, rather than a reduction in bedform size, is considered to be responsible for an upward decrease in preserved set thickness. Reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin.