Kalahari valley calcretes: their nature, origins, and environmental significance
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文摘
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 non-pedogenic 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.