Fabric Preserving and Fabric Destroying Dolomitization: A case of Seawater Dolomitization

Abstract

This study is based on field, petrographic and geochemical investigations of Hajar Supergroup autochthonous rocks: Ruus Al Jibal Group- Musandam Peninsula, and Akhdar Group- Jebel Akhdar, Oman and U.A.E., and para-autochthonous Maqam Formation-Sumeini Group-Jebel Sumeini-U.A.E. Petrographic evidence indicates that the rocks were deposited in a shallow marine shelf environment, particularly tidal flat, lagoon, reef, back-reef and shoal environments that were part of the Arabian Platform during Permian and Triassic times. However, they are almost entirely dolomitized and the rocks show different petrographic features ranging from perfect preservation of original texture by mimetic dolomitization to complete obliteration and destruction of the original limestones giving rise to inequicrystalline and equicrystalline fabrics. Dolomites analyzed by geochemical methods were categorized on the basis of textural variations; crystal size, shape and impurity or inclusion distribution within crystals, and whether these crystals are found as rock forming (replacive) or cements. The dolomites display variations in stoichiometry, ordering and trace element concentrations indicating differences in dolomitizing fluid chemistry and recrystallization stages that prevailed through time. It indicates also that although dolomitization is pervasive, dolomites are petrographically and chemically immature. All the petrographic and geochemical evidence strongly indicates seawater and/or mixing zone dolomitization which may have been initiated soon after deposition of the host sediments. Rocks showing preservation of allochems as well as the marine cements by mimetic dolomite crystals, suggest that dolomitization was early (at shallow depths) with very active marine-water circulation and occurred in a relatively short time. Evidence from crystalline dolomites indicates several crystallization events at shallow burial depths, under marine waters modified by increased temperature and mixing probably with evaporitic brines. The only fluid capable of early dolomitization in the case of the Oman Mountains dolomites was warm seawater from the Tethys Ocean which was circulating in the subsurface. KEYWORDS: