Depositional history and diagenetic evolution of evaporite deposits of the Sachun Formation in Siyah anticline section–south east of Sarvestan

  The Paleocene- Early Eocene Sachun Formation in southeast Zagros basin of Iran (Fars sub-basin) is composed of marl, carbonates and evaporates facies (Amiri Bakhtiyar, 2007). The first Cenozoic evaporate succession of Zagros basin in Iran is present in the Sachun Formation. This formation is at type section is composed of marl, carbonate and evaporite (Motiie, 2003; Amiri Bakhtiar, 2007). According to Heydari (2008), during this period Arabian plate and Zagros basin were at the 300 N latitude, so temperature condition was suitable for the formation of evaporate and these deposits have formed along with shallow marine carbonates of Umm er Radhuma (Paleocene), Rus (Early Eocene) and Dihban (Paleocene-Eocene) Formations (Zigler, 2001; Alavi, 2004; Ghazban, 2007). The Sachun Formation has not been studied in detail and can only been refer to works of Arzaghi et al. (2012) and Shabafrooz et al (2013). The aim of this study is to investigate depositional history and diagenetic evolution of evaporite deposits of the Sachun Formation in Siyah anticline section located about 12 kilometers south east of Sarvestan city. After identification of the lower and upper boundaries of this formation, 300 samples were colocted for petrographic studies with sampling interval of 1-2 meters. In order to identify evaporate minerals and intensity of evaporation, ten samples were selected to analyze by XRD (Sw 1800). Additional studies of diagenetic processes heve been performed by Su 3500 scanning electron microscope (SEM) on ten samples at central laboratory of Shahid Beheshti University. Ten Polished section were prepared in central laboratory of National Iranin South Oil Company (NISOC) to study carbonate-evaporite alternation. In this study, structure and structural classification of evaporate have been performed according to Maiklam et al. (1969) and Warren (2006) works. Discussion and Evaporates deposits of the Sachun Formation in Siyah anticline have formed as three types: primary, secondary and tertiary. Primary evaporates are beds and laminates that have formed in sabaqoues environment (salina) due to evaporation. Along with primary evaporates, carbonate also deposited in lagoonal and sabkha environments. Secondary evaporates formed during eogenetic and mesogenetic stages. Nodules sulphates (anhydrite & gypsum) have displaced in marl-carbonate matrix as enterolithic and chicken wire structures. Transformation of gypsum to anhydrite have taken placed in mesogenetic stage due to incrassation of pressure and temperature. Tertiary evaporates or telogenetic evaporites formed in meteoric environment. The most important diagenetic processes in this stage include hydration of anhydrite and formation of bassanite along with secondary gypsum with granoblastic, porphyroblastic and alabastrian textures. Gypsum viens in evaporate and carbonate sediments have been filled by satinspare and sigmoidal gypsum cements. Besides, gypsum cements have formed in carbonate facies. Another diagenetic processes are calcification, dissolution and dissolution breccia. Keywords: Siyah Anticline; Sachun Formation; Gypsum; Anhydrite; Bssanite.
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