Aspects of the Cenozoic geology in the Kuiseb valley, central Namib Desert

Abstract

Abstract by author:

The flushing action of the Kuiseb River appears to be the principal factor in checking the general northerly extension of the linear dunes. In contrast, the north- to northeastward migration of crescentic dunes across the Kuiseb Delta is facilitated by the infrequency of floods in the distal course and the high-energy, south - southwesterly coastal wind regime. As the monitoring period was comparatively short, it was realised that a concomitant investigation into the Cenozoic history of the Kuiseb Valley was necessary in order to understand more fully the boundary relationship between the main Namib Sand Sea and the Kuiseb River

The Cenozoic succession in the Kuiseb Valley west of the Escarpment comprises 10 principal sedimentary units which have been placed in a formal, updated lithostratigraphic framework and in a provisional geochronologic sequence. The accumulation of the Cenozoic succession, which took place mostly under arid to semi-arid conditions, reflects 5 major, regional events. (i) Post-Gondwana evasion phase: The Late Precambrian basement had been planed by the End Cretaceous - Earliest Tertiary to form the surface on which the Cenozoic sediments were initially deposited. (ii) Proto-Namib palaeo-desert phase: During this arid period, which spanned most of the Palaeogene, a thicker and a more extensive cover of desert sediments was laid down compared with that in the modern main Namib Sand Sea. (iii) Karpfenkliff fluvial phase: A change to a wetter, but nevertheless semi-arid, climate in the Miocene was responsible for the formation of a well-developed proto-Kuiseb/Gaub drainage system west of the Escarpment. (iv) Pedoqenic phase: In the End Miocene, a semi-arid climate and landform stability brought about the widespread development of pedogenic calcretes in the surficial sediments. (v) Namib Desert phase: This phase has been associated with the full establishment of the cold-water Benguela Current System in the Late Miocene. Aridification of the Namib tract has been a progressive trend through the Late Cenozoic to the present-day extreme-arid conditions

The incision of the Kuiseb drainage system west of the Escarpment and the development of the main Namib Sand Sea date from the comparatively recent, Late Cenozoic Namib Desert phase. The earliest association between the Kuiseb River and the main Namib Sand Sea is dated provisionally to the Early - Middle Pleistocene. Although the palaeo-hydrological regime of the Late Cenozoic Kuiseb drainage system has been ephemeral, the Kuiseb River has, nevertheless, been maintained as the northern boundary of the main Namib Sand Sea during the Quaternary. In the lower valley sector, however, the distal course of the Kuiseb River has been displaced some 30 km northwards from the vicinity of Sandwich Harbour to its present position over a period from the Middle Pleistocene to the Recent. This shift was probably caused by northward encroachment of the main Namib Sand Sea, the development of which dates from the Pliocene and has occurred under a southerly quadrant wind regime comparable with that of the present-day

Dunes from the main Namib Sand Sea will block the Lower Kuiseb River only in an extended period of no substantial surface flow (floods). This condition is likely to be fulfilled if prolonged dry periods affect the Kuiseb catchment or if the Kuiseb floodwaters are impounded, for example in man-made dams, in the catchment and upper canyon sector