Extensive dolomitization of Holocene sediments is occurring on humid supratidal flats adjoining lagoons in the interior of Ambergris Cay. One such flat (Tomas Savannah) caps 3 discrete emergent beach-ridge and washover-swale sand and mud systems that prograded over an irregular, karstic surface of Pleistocene limestone. The Scytonema-covered sediments of the flats are dominantly Mg-calcitic peneroplid (foram) sands and micrites, with subordinate amounts of aragonite from cerithids and bivalves. The relict beach ridges stand as much as 30 cm above mean sea level, whereas adjoining swales are nearly perennially bathed in waters of varying salinity. The most-landward swale occupies a bowl on the Pleistocene surface and is the locus of dolomitization on the flats. The sedimen s of the swale are a 0.7 m thick section of graded sandy muds in which 3 superposed dolomitic crusts (each 10-15 cm thick) are present; the middle and upper crust are dated at 1,700 and 905 yr B.P. (±130), respectively. The volume of dolomite on this one flat alone is 17,000 m³, which formed at a startling rate of 14 cm/1,000 yr. Each crust grades downward to an equal thickness of unconsolidated deposits. The upper crust contains 64-68% protodolomite, the remainder is Mg-calcite micrite and altered peneroplids; most of the skeletal aragonite has been removed by dissolution. The dolomite is microcrystalline and has selectively replaced the host micrite of the sediments. Average porosities and permeabilities of the crusts are 45-52% and 5.9-7.2 darcys, respectively.

The hydrology of the flats is complex, and involves first-order, seasonal cycles of (minor) hypersaline to normal-marine through fresh interstitial and standing waters on the flats. Superimposed on these cycles, meteoric input results not only in lagoonward flood recharge through the flats, but also upward-charging from the subjacent karst aquifer. In addition, the semidiurnal tidal flux appears effective in introducing variable-salinity lagoon waters to the ambient pore fluids on the flat. The significant amount of dolomite found here compares with or exceeds that in the Coorong, both areas containing more dolomite than other Holocene supratidal occurrences. Such complex hydrologic regimes in humid areas may be requisite for the formation of regionally extensive, penecontemporaneous eritidal dolomites.

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