Pedogenic gypcrete formation in arid central Australia

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In arid Australia, various gypsum sediments are widely distributed around salt lakes and along present and ancient drainage lines. They are primarily precipitated from saturated surface and subsurface brines and may be later transported by water or wind. When these primary gypsum sediments are exposed at the land surface and above the ground-water table, they tend to be altered by pedogenic processes, resulting in the development of a pedogenic gypcrete. Pedogenic gypcretes are white, powdery and massive and are commonly cemented to some degree, with a polygonal hardened ‘skin’ on the exposed surface. It consists of very fine (mostly < 0.1 mm) gypsum crystals, distinctively different from the underlying parent gypsum sediments which usually consist of larger (> 0.5 mm) crystals and have lacustrine, aeolian or other primary sedimentary features. A pedogenic gypcrete usually contains > 80% gypsum with minor quartz sands, clays, carbonate and heavy minerals. Various pedologic features have been developed such as channels, cutans, pedotubules and secondary carbonates. The major processes of gypcrete formation include dissolution, leaching and recrystallization. Dehydration possibly occurs, helping to break down larger crystals. However, dehydrated forms of sulphates tend to be rehydrated to gypsum when any water is available from rainfall or atmospheric moisture. The TL dating of co-occurring quartz sands in gypsum dune sequences indicates that a 0.5–1.0 m pedogenic gypcrete horizon may form in a period of up to several thousands years. In central Australia, at least three episodes of pedogenic gypcrete formation have been dated as 80–100 ka, 35–50 ka and in the Holocene. These pedogenic gypcrete formations followed major gypsum deposition episodes in salt lakes of central Australia and indicate intervals of stable land surface during which both gypsum deposition and reworking were negligible.