Seven study areas located over 320 km of coastal plain in southwestern Australia show the distribution of groundwater and vadose calcrete within Holocene coastal sands (age <7,100 yr BP,) over a regional climate gradient. The calcrete exhibits a range of structures that include rhizoconcretionary, mottled, massive, and laminar types. Groundwater calcrete, composed of mottled, massive, and laminar types, occurs as a thin sheet in the zone of capillary rise just above the water table. Vadose calcrete is predominantly rhizoconcretionary. At all sites where it is well developed the groundwater and vadose calcretes are closely associated with vegetation. Calcrete is well developed in southern areas that are humid. Further north, as the rainfall decreases and evaporation increases, calcrete is less common until it is absent or rare at the semiarid northern limit of the study region. Its limit of occurrence is correlated with a region whose rainfall is ca 800 mm/annum, evaporation is ca 1,900 mm/annum, and mean summer temperature is 23 degrees C. This study shows, therefore, that a trend toward a drier climate may not be conducive to the development of calcrete, although such a view prevails in the literature.

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