ABSTRACT

Laminar and massive calcretes are forming in the vadose zone of the Quaternary calcarenite coastal dunes of South Australia. Calcrete formation is intimately associated with a near surface soil zone of intense plant root growth and an abundant soil biota. The calcretes contain many fabrics indicative of caliche formation including meniscus cements, micrite envelopes, rhizoconcretions. dissolved grains, nodules, zones of fractured sediment and intraclast breccias. Within Pleistocene dunes and especially within calcrete zones. the individual sand grains (including quartz grains) are often coated with a rind or envelope of micrite. This envelope is perhaps formed by fluctuations in the volume and chemistry of the film of attached water which surrounds each grain. On the mesoscale many So th Australian coastal calcretes contain intraclast horizons which show interesting "clast-within-clast" fabrics, and some of the clasts show an unusual stellate form. The formation of these intraclasts is controlled by the episodic nature of calcrete development and dune-sand emplacement within a coastal dune setting. Recognition of these various calcrete fabrics in pre-Quaternary carbonate sequences can be used to refine further the distinction between subaerial exposure surfaces and marine hardgrounds.