Formation of Diagenetic Dolomite in Coastal Sabkha Along Arabian (Persian) Gulf

Abstract

Aragonitic intertidal sediments are being dolomitized in the subsurface supratidal environment of a sabkha along the south shore of the Arabian (Persian) Gulf as a result of the percolation of wind-driven marine-derived brines. The development of abundant diagenetic dolomite results from an optimum combination of high mMg++/mCa++ ratio fluids, a rapid flow rate which is related to the flooding frequency and sediment permeability, and a shoreline configuration that maintains the proper conditions for a sufficient length of time. Strongly dolomitized areas are localized along the flanks of remnant channels and in the lee of islands that have been engulfed by the prograding sabkha. The dolomitizing process takes about 1,000 to 1,500 years. Most diagenetic dolomite forms at the expense of primary aragonite according to the reaction: Mg++ + 2CaCO3(arag) ^rarr CaMg(CO3)2 + Ca++. Dolomite forms under the following conditions: (a) chloride concentration between 3.25 and 3.75 mCl-/Kg; (b) mMg++/mCa++ greater than about 6; (c) pH between 6.3 and 6.9; (d) minimum PCO2 of 10-2 to 10-3 atm; (e) temperature between 25 and 40°C; (f) saturation with respect to gypsum; and (g) reducing environment in association with hydrogen sulfide. Other recent dolomite occurrences in the Bahamas, Florida, and Bonaire are significantly different in terms of stratigraphy, degree of lithification, and evaporite mineral association from those in the Arabian Gulf.