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Journal of the Geological Society

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Calcrete and other soils in Quaternary limestones: structures, processes and applications

Quaternary limestones commonly include descriptions of calcrete, caliche and soils which embrace both surficial deposits in which dissolution and cementing phenomena interact on a calcareous substrate, and soils or other sediments where carbonates are precipitated as cements. Three genetic groups are recognized, in which (1) a soil profile is developed by surface alteration of existing marine sediments; (2) deposits accumulate superficially on a lithified surface; and (3) soil-like materials have been added to unconsolidated marine sediments by surface processes including infiltration.

Sediments range from unaltered marine grainstones through poorly sorted wackestones to mudstones. Diagnostic structures include pisoliths and other glaebules. laminae, grain aggregates, birdseyes and other vugs, and rootlets as well as fractures and brecciation not found in other environments. The most common precipitated carbonates are micrites, but needle fibres, whiskers and prismatic crystals occur. Crystal morphology is controlled in part by the level of supersaturation. rapid evaporation leading to micrite precipitation and whiskers, but also by the pressure of organic contaminants. Distribution of cements and structures is irregular, relating to percolating groundwater streams. Cements may be isopachous but meniscus structures, cement bridges and microstalactites occur. These vadose textures may not be developed where evaporation rates are high. Irregular areas of replacive (neomorphic) spar and displacive calcite growths occur in some examples but criteria for recognition are not firmly established.

Dated recent examples lie between 400 and 18,300 years B.P. but deposits on Aldabra and in Kenya are intercalated with marine sediments which extend to 200,000+ years B.P. The recognition of these terrestrial intervals within otherwise marine sequences enables sea level changes to be identified and correlated with other climatic data to separate local patterns of uplift or subsidence.

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