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Calcretes are generally considered to be tropical arid carbonated soils, the formation of the laminar crust being the last stage. Often situated beneath soft soils, this hard crust forms a relatively impermeable layer covering the lower calcrete horizons. The origin of this crust (usually attributed to an accumulation of calcium carbonate within the soil by leaching processes) and its implications in calcrete genesis will be discussed.

Petrophysical measurements (porosimetry, bulk density), micromorphological observations and laboratory experiments indicate that the laminar crust is a surficial deposit, biogenic in origin, and genetically unrelated to the overlying and underlying calcrete horizons. The crust is formed by sequential layers of trapped material of aeolian origin which form the skeleton, and remains of organic matter within crystals mixed with micrite, which is often associated with fibro-radial calcitic spherulites. The significance of these spherulites has been demonstrated in the laboratory where light-dependent cyanobacteria precipitated calcite crystals identical to laminar crust spherulites in structure and chemistry. All of these features confirm the stromatolitic nature of the laminar crust.

If laminar limecrusts are considered to be fossil formations of biogenic origin rather than the ultimate facies of calcium carbonate accumulation within calcareous soils, genetic models of calcretes must be re-interpreted. In identifying the laminar crust as a palaeosurface, the presence of the soft overlying sediments (soils) must be reinterpreted as the result of some later geodynamic depositional process.