ABSTRACT

Non-pedogenic calcrete is forming as a thin sheet (10-50 cm thick) just above the water table in the subsurface of a barrier dune terrain in South Western Australia. The calcrete is forming in response to the regional evapo-transpirative regime. The annual rainfall is high but of short duration, and it infiltrates rapidly into the sandy substrates. Relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain. The vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone, thereby precipitating CaCO₃. Where plants are not drawing phreatic water, calcrete is developing by evaporation of water at the top of the zone of capillary rise.

Various structure types of calcrete are developed in the study area, namely rhizoconcretionary, mottled, massive, and laminar forms; these occur in a definite stratigraphic sequence related to maturity of profile, vegetation cover, and thickness of the vadose zone. Rhizoconcretionary calcrete forms in the vadose zone; mottled and massive calcrete, frequently capped by laminar calcrete, develop in the zone of capillary rise. These latter calcrete types form a sheet that is ubiquitous and parallel to the water table.

Since it is a feature of the water table, the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities. At the local level the various structural types of calcrete may he used as paleowater table indicators.