Textural pattern of secondary gypsum in the Basal Anhydrite of the Asmari Formation, SW Iran

Abstract

The Oligocene–Miocene deposits of the Asmari Formation are located in SW of the Zagros Mountains of Iran. The lower part of the Asmari Formation consists of an evaporite bed that is termed Basal Anhydrite. Seven sections from four different anticlines were selected and sampled for petrographic and mineralogical analyses .The petrographical examinations were carried out using an optical polarized light microscope and SEM analyses. To determine the mineralogical composition, evaporite samples were analyzed by X-Ray Diffraction. Particular petrographic characteristics, at the macro- and microscales, especially common pseudomorphic features and mineralogical analysis of the Basal Anhydrite showed that anhydrite was transformed to gypsum during exhumation. Diagenetic processes caused an increase in the pure gypsum and a simultaneous decrease in the residual anhydrite. Two main textures, fine- crystalline (alabastrine) and coarse crystalline (porphyroblast), were distinguished in samples indicating replacement of anhydrite by gypsum. The textures related to the tectonic movements in evaporite rocks were observed in the thin sections. The effect of tectonic varies due to the type of gypsum texture. Microcrystalline gypsum is mostly recrystallized to form coarse-crystalline and aligned-flowing texture. Coarse crystalline gypsum which contains anhydrite is more resistant against the tectonic effects because it shows only a little rotation.