Depositional environment of the evaporitic unit (D-member) of the Qom Formation (Central Iran)

Abstract

The Qom Formation, in the Central basin of Iran, is characterized by shallow marine limestone and lagoonal facies. The D-member, one of the six major members of the Formation, is dominated by gypsum (anhydrite). Several types of textural and structural features have been recognized in this unit. Nodular anhydrite, gypsum veins, enterolithic structure, and gypsum crust are the most common gypsum (anhydrite) facies. Petrographically, the gypsum rocks display an entire array of textures ranging from cloudy ameboid (xenotopic) to idiotopic crystals. The presence of these textures together with satin spar crystals represent the cycles of dehydration-rehydration process. Rehydration event possibly took place due to increasing heat flow related to emplacement of Tertiary volcanic bodies. Rehydration occurred when the Qom basin was uplifted and the D-member exposed to ground and/or meteoric water. The gypsum unit as well as the accompanied clay impurities are enriched in some elements (i.e., Mg, Ti, and Sr). Therefore a possible link between volcanic activity and the elements enrichment may be proposed. The unit displays some shallow water evaporite textures and it is typically cyclic, these features suggest a saltern environment type for the deposition of D-member.