Backbeach Deflation Aprons: Morphology and Sedimentology

Abstract

Backbeach deflation aprons are heterogeneous, coarse clastic deposits associated with active transverse dune fields. The apron comprises variously angular, rounded, fractured, cracked, polished, and ventifacted clasts of bedrock (local and exotic), beachrock, and bioherm fragments (coral and worm reef). Terrestrial and anthropogenic clasts (from archeological sites, deflated dunes, flotsam, and shipwrecks) are also present. The unit forms as a lag deposit on top of a gently sloping (ca. 0.008), planar sandy deflation surface and is typically only one clast thick. It is up to 100 m wide and extends for tens of kilometers along the coast. It is replaced landward by a steep (ca. 0.1) deflated soil surface armored with a layer of iron concretions. Backbeach deflation aprons are compound deposits that result from marine storm, eolian, and human activity since the mid- Holocene. The depositional model proposed involves periodic storm deposition of coarse clasts (of beachrock, bedrock, and biogenic origin) and flotsam (shipwreck and other floating debris) on the backbeach or dunes. Some clasts are deposited directly into interdune depressions and enter the deflation plain immediately. Others are deposited initially on dune surfaces. Dune migration results in winnowing of the storm-deposited coarse clasts (together with rhizoliths and any other coarse material in the dune) until they come to rest on the deflation surface. The lag deposit is subsequently buried and uncovered multiple times by dune migration, during which time it is exposed to polishing and subaerial weathering. Consequently, the lag comprises a variety of materials that reflect both storm and eolian processes. The clasts exhibit variable degrees of rounding, polishing, and ventification according to how long they have been in the deposit.Backbeach deflation aprons are characteristic of sandy beaches along approximately 475 km (. 10% of the total length) of the South African east coast. They develop in association with large-scale transverse dunefields on the backbeach, which in turn depend on reversing shore-parallel winds and high sediment abundance. The variable degree of polishing and weathering, planar surface, low elevation, and back-beach location along extensive shoreline distances distinguish backbeach deflation aprons from tsunami or marine storm deposits.