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Abstract

The rugged karst terrain developed on the dolostones of the Miocene Cayman Formation (Fm) on Grand Cayman includes numerous large cavities that formed through the activity of tree roots. The surfaces of those cavities are coated with laminated calcrete crusts up to 8 cm thick that are formed of an alteration zone, an accretionary zone, and final infill of the cavities. These crusts are formed of various laminae, including dolostone with root traces, alveolar septal structures, peloids, micritic and microsparitic laminae, micrite with bioclasts, and pisoliths. Features such as microborings, spores, needle-fiber calcite and micro-rods are common in all parts of the calcrete crust. Calcrete formation was initiated as the roots and associated microorganisms generated the cavities. Later on trapping and binding processes and organically induced precipitation of carbonate allowed the formation of the accretionary (mostly laminar) part of the calcrete. The last phases of crust formation took place when ponded waters filled the cavities. The calcrete crusts developed on the Cayman Formation dolostones record a very specific setting for calcrete formation and constitute a good example of non-horizontal calcrete crusts.