Kalahari valley calcretes: their nature, origins, and environmental significance

Calcretes that form in non-pedogenic settings have been widely reported in the geomorphological and geological literature, yet they are still poorly understood in comparison with pedogenic varieties. This may be because there are assorted types of nonpedogenic calcretes (often loosely referred to as groundwater calcretes, but encompassing groundwater, phreatic, open valley or confined channel calcrete types) forming within vadose and phreatic environments in different geomorphological settings. Relatively few studies have described the detailed petrological characteristics of such calcretes, leading Wright and Tucker (Calcretes; International Association of Sedimentologists Reprint Series, (1991) Vol. 2, Blackwell Scientific Publications, Oxford, p. 10) to suggest that ‘‘much more work is needed to define the ‘groundwater calcrete facies’ and to devise criteria for its recognition’’ so that the various non-pedogenic calcretes can be identified within the Quaternary as well as further back in the geological record. Most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls. Thus, the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types. As a step towards expanding our knowledge of the variety of non-pedogenic calcretes, this paper describes the petrology, micromorphology, and mode of development of one of these types of calcrete, namely that which forms within valley locations (but not in definite fluvial channels). Late Quaternary to Holocene valley calcretes are described from various dry valleys (mekgacha) across the Kalahari region of central Botswana, with the majority of samples collected from trunk and tributary valleys of the Okwa. Samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology. In general, most samples consist of grains of quartzose Kalahari sand cemented by fine crystalline, often glaebular, grain-coating and pore-filling micrite. Cement types are fairly consistent and not as micromorphologically variable as has been noted for other Kalahari calcretes. Biological inputs are prevalent in many samples and include networks of calcified rootlets. These characteristics reflect the fact that the calcretes formed in a relatively near-surface environment with relatively high rates of evaporation.