LATE QUATERNARY VALLEY CALCRETES IN THE KALAHARI

Calcretes that form in non-pedogenic settings have been widely reported in the geomorphological and geological literature, yet their petrological characteristics are poorly understood in comparison with pedogenic varieties. This may be because there are assorted types of non-pedogenic calcrete (often 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. As a step towards expanding our knowledge of such calcretes, this study describes the petrology, micromorphology, and mode of development of one non-pedogenic calcrete type, 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. On the basis of these results, the Late Quaternary environmental context of calcrete formation is discussed