Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis

Abstract

The calcretes of the Kalahari are amongst the thickest in the world representing pedogenic episodes in a semi‐arid climate during Pliocene to Recent times. The descriptive morphological terminology of Netterberg is used to describe the calcrete types and a differentiation into simple and composite profiles is made. A pedogenic/diagenetic scheme has been constructed using all available data. Early calcite cementation is induced by two mechanisms. Slow evaporation and/or CO2 loss causes the precipitation of low‐Mg calcite, whereas rapid evaporation and/or CO2 loss precipitates predominantly high‐Mg calcite, in thermodynamic disequilibrium with the low Mg/Ca ratio vadose water, and minor aragonite (which transforms rapidly to low‐Mg calcite). High‐Mg calcite is also precipitated from high Mg/Ca ratio vadose waters in calcretes developed on Mg‐rich host lithologies and by capillary rise from shallow groundwaters in saline depressions. Calcite precipitation may be passive (cement), displacive or replacive, in the latter released silica migrating down‐profile to precipitate length‐slow chalcedony, clinoptilolite (saline conditions), length‐fast chalcedony and megaquartz (non‐saline conditions). Displacive introduction of calcite takes place from highly supersaturated solutions due to rapid evaporation (with CO2 loss) of vadose waters. During low‐Mg calcite precipitation (in a ‘closed’ system) the Mg concentration of the resulting solution increases. This, combined with Mg released during high‐Mg to low‐Mg calcite transformation, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. Vadose dolomite is often present whilst some dolomite may be precipitated at the mixing‐zone of vadose and phreatic waters. The proposed model applies to the Kalahari calcretes, although it may perhaps be extrapolated to other areas. Further detailed studies, involving analyses of pore water chemistry, soil microclimate, and trace element and isotopic analyses of individual cements are necessary.

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

“…Moreover, calcretes contain important information that help interpret ancient ecosystems, their palaeogeography, and the tectonic, climatic and sedimentary regimes in which they formed. A good defmition of a calerete is that proposed by Watts (1980) after modifYing that of Goudie (1973): "pedogenic caleretes are terrestrial materials composed dominantly, but not exclusively, of CaC0 3 , which occur in states ranging from nodular and powdery to highly indurated and result mainly from the displacive and/or replacive introduction of vadose carbonate into greater or lesser quantities of soil, rock or sediment within a soil profi le". This defi nition only refers to pedogenic ca1cretes, however Wright and Tucker (1991) later proposed a wider use of the term ca1crete to include, according to the initial ideas of Netterberg (1980), the effects of shallow groWl dwaters.…”

Fluoride dynamics in the granitic aquifer of the Wailapally watershed, Nalgonda District, India

“…Concentrations of MgO in the analysed calcretes are 454 usually <3 weight % but reach up to 15.6 weight % (Table 5). Watts (1980) …”

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“…Una vez constituidos los nódulos opalinos por reemplazamiento o precipitación directa de sílice, debido a que son recientes (Pleistoceno-Holoceno) y a que se encuentran en un clima muy árido con alta evaporación y falta de agua, no sufren prácticamente procesos de disolución, ni transformación a cuarzo (envejecimiento). La aparición sólo de calcedonita como cemento de escasas y pequeñas grietas, indica que cuando se produce esta cementación el ambiente no es ni sulfatado, ni rico en Mg (Kastner, 1980), características bastante frecuentes en silcretas de clima árido (Watts, 1980;.…”