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Sugarloaf Key is an area of active tidal pumping where Holocene dolomitization is occurring. Calcium-rich dolomite is found in a 0.25-10-cm thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted Pleistocene Miami oolite. Radiocarbon ages of the crust range from 160 to 1,420 y.B.P. with a corresponding increase in dolomite content from 0% to 80%.
The relatively high permeability of the underlying Pleistocene oolite and low permeability of the Holocene carbonate mud results in a tidal lag between surface waters and the partly confined aquifer. Consequently, seawater is pumped upward and downward through the Holocene sediments during spring tides. The highest concentrations of dolomite are found where the sediment layer is thinnest and tidal pumping is most effective. Limited analyses of surface and subsurface water samples taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially Florida Bay water, very slightly modified by sulfate reduction.
The earliest diagenesis of the sediment is by precipitation of dolomite cement which occurs as 0.1 to 0.3 µ subrounded crystallites that show no distinct crystal form. During further cementation, and somewhat later replacement, dolomite forms as 1 to 5 µ euhedral rhombs. The dolomite rhombs, which are poorly ordered, result from the recrystallization of many smaller preexisting crystallites. X-ray diffraction data indicate that the recrystallized dolomite is better ordered and less calcium-rich than the dolomite composed of crystallites.
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