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Along the southern shores of the Persian Gulf, Holocene carbonate sediment infilling of coastal lagoons has generated extensive salt-encrusted, supratidal surfaces or coastal sabkhas, which are the site of penecontemporaneous dolomitization. Recent diagenetic dolomite is present mainly in the unlithified, upper-intertidal facies sediments, although minor quantities are at greater depths in other sediment facies. The aragonite fraction of the sediment is replaced, and the process may be complete (100% dolomitization) within 1,500 years after primary sediment deposition. Dolomitization is local in effect, depending on a combination of fluids high with ^mMg⁺⁺/^mCa⁺⁺ ratios, a rapid interstitial flow rate (2-3 cm/yr), and a high surface-flooding f equency; the flanks of flood channels and peninsulas best achieve these requirements, and their primary sediments may be completely dolomitized. The local distribution of intensively dolomitized sediments is related to rapid sedimentation and associated shoreline progradation (approximately 2 m/yr); at slower progradation rates, more extensive dolomitization would ensue. The diagenetic dolomite occurs as 1-4 micron rhombs, is nonstoichiometric (Ca_51-54), is partly ordered, has ^dgrO¹⁸ = +2.5 to +3.7 ^pmil, and ^dgrC¹³ = +4.0 to +4.4 ^pmil. Radiocarbon ages increase progressively inland from the present shoreline (1,500-3,500 years ago); these dates, however, reflect more the age of the original sediment than the age of the dolomitization event, most of th carbon in the reaction being derived from the original aragonite. The dolomitizing fluids are of marine origin and have the following characteristics: 3.25-3.75 ^mCl^-/Kg: ^mMg⁺⁺/^mCa⁺⁺ ratios between 7 and 22, pH between 6.3 and 6.9, a minimum ^pCO₂ of 10^-3 to 10^-2 atmospheres, temperatures between 25° and 40°C and at saturation with respect to gypsum and celestite. Associated early diagenetic minerals within the coastal sabkha sediments include huntite, magnesite, gypsum, bassanite, anhydrite (nodular), celestite, and halite. Many ancient dolomites of probably analogous, penecontemporaneous, sabkha origin can be shown to have undergone later diagenetic recrystallization w th a concomitant coarsening of grain size, approach to a stoichiometric composition, an increase in ordering, and a loss of trace cations.

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