The sedimentology of diamondiferous deflation deposits within the Sperrgebiet, Namibia

Abstract

Abstract provided by author:

Under these conditions loose grains on the bed, including garnets and diamonds between 0 and -2 phi (1. 0 to 4. 0 mm) in diameter, are rapidly sorted by size, density and shape. As stone pavement surfaces are reworked, creep erosion ripples, with an upwind-facing concave crest comprised of very coarse sand and granules. The segregation of garnet grains along the crests of these bedforms indicates that like-sized particles move predominantly as kinematic waves

This study has established that sandstorm events contribute 20 to 50 percent of the annual sandflow transported towards the Namib Sand Sea. For the period February to May, 1987, about 7 to 8 tonnes of sand per metre width was transported along the aeolian transport corridor generated at Bakers Bay. Some 23 percent of this material moved along the corridor in two to three days of very high sandflow. This measurement contradicts previous estimates, based upon wind data analysis that failed to take cognizance of interaction between the aeolian system and the marine environment which ultimately determines sediment input to the deflation basin

Stone pavement bed configurations are determined by the spatial distribution of sandflow through the deflation basin. In areas of moderate sandflow, surfaces stabilised against deflation are characterised by regularly spaced, east-west oriented rows of pebbles with their long axes preferentially oriented transverse to the southerly surface-wind flow. Beneath aeolian transport corridors, the dense packing arrangements of granules and small pebbles comprising the creep bedload protect the substrate from deflation. The most stable configuration is attained when particles develop an imbricate shape-fabric, with their upper surfaces inclined into the saltation load derived from the south. The long axes of these particles are commonly aligned parallel with the prevailing wind. The presence of cumulate soil textures has been noted beneath surfaces stabilised in this way

Raindrop impact during high velocity northerly winds is able to destroy the stable bed configurations created by creep processes. The exposure of the underlying substrate to rainsplash erosion, and deflation as the surfaces dry out, probably results in surface lowering prior to rearrangement of the bed into a stable configuration by creep

Dendritic tributary networks generated by surface run-off during heavy rainfall generally occur at the northern ends of endoreic basins, and feed streams which flow south into ponded water bodies at the base-level of the ephemeral systems, which are sometimes below sea-level. Although stream transport is highly ephemeral, sediment dispersal is rapid during these events, and coarse-grained aeolian deposits on the floor of endoreic basins are commonly reworked. A repetitive cycle of transport is therefore observed, whereby aeolian processes essentially transport material to the north, and ephemeral streams redistribute material to the south

Cyclical reworking of sediments by aeolian and ephemeral stream processes is reflected by computerised analysis of diamond dispersal in endoreic basins. The patterns produced also illustrated the extent to which diamonds have been transported into successive basins to the north. This transport has primarily occurred by creep, but experiments using garnet grains with a similar size distribution show that the entrainment of some diamonds into saltation probably also occurred. East-west oriented obstacles transverse to the creep-migration direction provided sites for aeolian heavy mineral segregation, which can be examined using kinematic wave theory

Analysis of the Cenozoic stratigraphic record reveals a similar pattern of sedimentation to that which has previously been defined in the Kuiseb River area further north. Suggested new correlations include the assignment of the Blaubok and Gemsboktal gravels to the Lower to Middle Miocene, together with the Kalkrucken sandstone and the Strauchpfutz carbonate. The aeolian Fiskus Sandstone Beds, which were formerly believed to be Lower Miocene, were probably deposited between the Latest Miocene and the Early Pleistocene. Their deposition in a tidally-flooded pan environment, at about 25 m above sea-level, suggests that tentative correlation with the 30 m Pleistocene transgression is possible. This implies that minimal crustal warping of the Plio-Pleistocene beach elevations along the west coast has occurred, contrary to previous concepts

Dateable stratigraphic units broadly constrain the duration of the phases of regional development that are identified. Middle Eocene (Upper Buntfeldschuh Formation) to Lower Miocene (Rooilepel sandstone) aeolian deposits, which are correlated with the Tsondab Sandstone Formation of the Central Namib, provide evidence of an early arid phase, which may represent a proto-Namib. This phase was terminated during the Lower Miocene, when widespread alluvial activity led to the deposition of the Grillental Beds, Kalkrucken sandstone, Gemsboktal and Blaubok gravels, and the Arries Drift Gravel Formation. A regionally extensive pedogenic hardpan calcrete formed during the early Late Miocene, prior to the Namib Desert Phase, which is represented by the aeolian Fiskus Sandstone Beds and the Annental sandstone

The silcrete-capped topographic residuals occurring within the study area, which probably correlate with the African palaeo-surface, might have been associated with the initiation of aridification on the west coast during the Late Cretaceous to Early Tertiary. The preservation of the Grillental Beds and the Rooilepel sandstone in depressions below the elevation of the silcrete show that the silcrete had been incised prior to the Lower Miocene. This reduces the time span represented by the African erosion cycle. The extent and location of sand seas within the region has varied with time. Periods of sand sea expansion occurred during marine regressions, when it is thought that aeolian accretion occurred along the former western coastal margins of ergs, as sea-level was lowered, and the dynamic aeolian tract was displaced to the west. However, it is likely that sand seas contracted during transgression, when areas which were formerly within the depositional basin came to lie within the new limit of the deflation basin. As shown by the Fiskus Sandstone Beds, this resulted in the development of regional bounding: surfaces caused by aeolian erosion

The previous model of endoreic basin placer formation invoked progressive deflation and surface lowering of a pre-existing diamondiferous sediment pile. Had such a body existed, it is likely that erosion during the Lower Miocene alluvial phase would have redeposited the material to the west on the continental shelf, which was exposed by regression at the time. In the new model proposed here, the Bogenfels Basin is identified as the site of a major south-facing re-entrant embayment, in which marine placer bodies were deposited during transgressions. Non-active (palaeo) yardangs along the embayment's western edge prove that the area has been traversed by an aeolian transport corridor during regressions. Former marine placer bodies would then have been subjected to reworking by the aeolian system, and the occurrence of optimal conditions for creep transport would have resulted in diamond input to the endoreic basin system represented by the Daheimtal-Idatal conduit. Subsequent cyclical reworking of the diamondiferous sediments by the aeolian and ephemeral stream systems, coupled with continued transport of the diamonds to the north into successive basins, would have led to the formation of the placer bodies