Holocene and Pleistocene Calcareous Crust (Caliche) Profiles: Criteria for Subaerial Exposure

Abstract

ABSTRACT Surficial, calcareous crust profiles, hard, irregular, subhorizontal, calcareous laminae, surrounded by crumbly, chalky carbonate, form at the surface of many limestone successions in semi-arid areas. Detailed study of Holocene profiles, developed in late Pleistocene reef limestones on northern Barbados, reveals that diagenesis of limestone in such profiles creates a diagnostic, repeatable set of textures and fabrics. The original limestone is altered by brecciation, recrystallization (to microspar), micritization and boring. The CaCO3 thus put into solution and augmented by CaCO3 from fallout and salt spray is reprecipitated as calcite (less than 4 mole percent MgCO3), in the form of crystals often, quite different from those in the vadose zone below. Calcite crystals are precipitated as flower spar, micrite, random needle fibres, and tangential needle fibres. Several of these crystal morphologies are similar to those precipitated from highly supersaturated solutions or solutions that contain appreciable amounts of other dissolved ions. These 4 basic crystal types are not only precipitated as void-filling cement, but, more important, combine to form characteristic structures such as oolith-like coated particles, pelletoids, and the laminar crusts themselves. The diagenetic textures and fabrics described above are also the basic units of many Florida and Middle East calcareous crust (caliche) profiles, suggesting that the above features may be characteristic of calcareous crust profiles in general. Fossil calcareous crusts and associated features, buried in the Pleistocene limestone succession on northern Barbados without alteration, indicate that these features may well be preserved as indicators of subaerial exposure in the fossil record.