TERTIARY CALCIC SEDIMENT COVERS AND CALCR ETES IN NAMIBIA - ORIGIN AND GEOMORPHIC SIGNIFICANCE

Widespread Tertiary sediment covers and calcretes in Namibia can be co rrelated with the development of the main planation surface. It formed during the Lower Tertiary. This surface extending from the Angola-Nam ibian border to the Karas Mountains in the south meets the Namib uncon formity surface by passing the Midnamibian escarpment gap. The formati on of the Tertiary sediment covers is a consequence of the aridificati on of South West Africa caused by Antarctic bottom water circulation. It has to be emphasized that the uplift of the Windhoek Highland is a very important factor for the Midtertiary sediment and carbonate suppl y. The extension, thickness and facies of these deposits reflect the f urther surface development: In the N and NE the sediments fill deep su bsiding basins. South of the highland they cover the rocks of the Nama -and Karoo-sequence only in small amounts. Miocene calcrete formation were taking place meanwhile surface development was continuing in the escarpment gap. There the planation surfaces and valleys were also fil led up and overcrusted with increasing aridity during the Upper Miocen e. Since the Pliocene until today erosion processes reexposed the main plantation surface. Partially valley incision was taking place. Calcr ete carbonates deriving from fluvial erosion in the southern and weste rn part and deflation in the eastern part of Namibia keep the process of calcrete multiplication going on. The well known silty deposits fro m the Kuiseb, Hoarusib or Hoanib partly originates from remobilization of Tertiary deposits and calcretes.