KBmec - Mining & Exploration Consulting

Chemical sediments of the palaeo and active Okavango alluvial fan, Botswana: evidence for variations in the processes of formation.

ZIETSCRIFT FÜR GEOMORPHOLOGY N.F., 2004, VOLUME 136, PAGE 89-112

Alexander I. Shaw & Andrew S. Goudie

School of Geography and the Environment, University of Oxford

Mansfield Road, Oxford, OX1 3TB, U.K.

ABSTRACT

Chemical sediments were investigated from a range of environments within the Okavango alluvial fan of northwest Botswana. These took the form of calcretes and sil-calcretes that displayed compound features which indicate some role for groundwater during formation. However, there was extensive evidence from stabilised abandoned areas of the floodplain that indicates pedogenic calcretization has occurred within the upper sections of the horizons. The alpha fabric groundwater calcretes are dominated by smectite and to a lesser extent in situ weathered alkali feldspars, mica and kaolinite. In addition, two sites showed extensive accumulations of sepiolite, which is believed to have formed through neoformation associated with local Mg enrichment of groundwater during more arid conditions. Transects through three modern islands, which are believed to be active sites of chemical deposition, showed that calcrete and authigenic silica, resulting in the formation of sil-calcretes and cal-silcretes, was accumulating in the form of nodules and grain coatings within the capillary fringe above the water table.