Abstract
A difference in the depositional environments of the Upper Permian and the Upper Jurassic carbonate rocks exposed in central Saudi Arabia led to a difference in the initial dolomite geochemistry; i.e. Ca: Mg stoichiometry and Sr-content. The Khuff dolomites are characterized by: (i) well-ordering (high R-value) (ii) stoichiometric nature (with an average CaCO3 of about 1 mole %) (iii) and low Sr-content (from 55 ppm to 183 ppm) relative to Holocene marine dolomites. The Khuff carbonates were deposited in a sabkha-like, hypersaline environment, where the increased salinity favored the precipitation of a better-ordered and more stoichiometric dolomite than would be expected to form had the dolomites formed in a normal marine environment. Later recrystallization of the Khuff dolomite also may have increased the degree of ordering in the dolomites. The low strontium content in the Khuff dolomite could have been due to depletion during diagenetic modification of the initially precipitated non-ideal Khuff dolomite. Jubaila carbonates were formed in a normal marine environment, where the source of Mg+2 responsible for the dolomitization of its upper part is related to the prograding of the supratidal sabkha front formed at the terminal Jubaila stages. The Jubaila dolomites are relatively poorly ordered with a CaCO3 content of 7 mole % and an average Sr-content of 241 ppm.