ABSTRACT

Recent surface and buried dolomite crusts and massive dolomite beds on Ambergris Cay, Belize have formed on topographically low-relief, peritidal flats whereon the hydrologic system is characterized by a dominant phreatic diagenetic overprint. As a consequence of their low subaerial exposure index, such dolomites and associated undolomitized surface sediments lack many of the textural and microfabric features that are considered diagnostic of the peritidal environment. The progressive dolomitization of precursor sandy, fossiliferous sediments (wackestones to packstones) is accompanied by a textural change to dolowackestones with allochem ghosts, and a porosity system that resembles, but is distinct from, syndepositional fenestral fabrics formed in unconsolidated or partly consolidated peritidal sediments. The microcrystalline dolomite cements in these rocks are predominantly of phreatic aspect, and only rare examples of possible vadose diagenetic fabrics are observed. These texturally anomalous peritidal dolomites, characterized by a markedly subtidal appearance, may have fossil counterparts in many ancient carbonate tidal-flat sequences.