Pseudo‐anticlines and other structures in some calcretes of Botswana and South Africa

Abstract

AbstractDeformation structures, within some Quaternary calcretes of Botswana and South Africa, have been classified into five types. Type 1 folds are small‐scale (< 2 m wavelength) anticlines in shale or sheet calcrete, separated by wedges of calcrete, polygonal in plan. The anticlines have resulted from horizontal expansion and buckling caused by the disruptive growth of the calcrete wedges. Type 2(a) folds are medium‐scale, strongly convoluted features in sheet calcrete and bedrock and are attributed to the introduction of calcrete along major vertical joint planes. Type 2(b) folds are large wavelength (> 20 m), but low amplitude (< 1 m), anticlines in hardpan calcrete; structural evidence suggests dominantly horizontal compression within the hardpan due either to an overall increase in volume of the calcrete, or to the disruptive formation of calcrete in joints in the synclinal areas. The mineralogy of the calcretes is dominated by low‐Mg calcite which may have inverted from a high‐Mg form; the deformation in type 1 and 2 folds was probably caused by the disruptive and displacive growth of calcite during calcretization. Type 3 folds are saucer‐shaped depressions in hardpan calcrete and may be due to the removal of soluble salts below the folded layer during or after calcretization. Finally, type 4 folds are small, diapiric anticlines resulting from the upward injection of swelling clays into calcrete or calcretized shale.