Recent carbonate sediments on Ambergris Cay, British Honduras, occur as a thin veneer of supratidal and intra-island lagoonal deposits, incompletely mantling an irregular Pleistocene limestone surface. Both sediments and rock exhibit different degrees of diagenetic modification. The supratidal mud flats usually adjoin very shallow hypersaline ponds, where sediments are subjected to extremes of chemical and physical environmental conditions; the raised rims of the mud flats prevent rapid drainage after periods of heavy rainfall or sea-water flooding.

Etching by rainwater and boring by algae tend to destroy or comminute sediment particles on the mud flats. Furthermore, extensive blue-green algal mats commonly are associated with a near-surface crust of dolomitized sediment. Degree of induration of this crust is related to the degree of dolomitization. Low pinnacles of Pleistocene limestone, where exposed near the dolomite crust, also have been partly dolomitized. Recent sediment particles commonly are recrystallized to cryptocrystalline carbonate, without mineralogic change, prior to burial.

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Storm-tossed sand and cobbles on windward beach ridges show other diagenetic effects, including disintegration caused by decay of organic matrices and by solution of particles below the fresh-water table. Conversely, cementation and pore filling in some beach-ridge sands represent incipient lithification.

The extensive outcrops of Pleistocene limestone afford a study of post-lithification diagenesis affecting lithofacies which are analogous with nearby Recent sedimentary facies. Replacement of most of the component grains of this rock by low-magnesium calcite, a change not seen in Recent sediments, tends to obliterate boundaries of recognizable grains. Boring by various organisms, leaching by percolating water, and filling of pores further modify the rock's texture; however, its primary fabric remains readily recognizable.

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