Petrography and geochemical composition of the middle Eocene, the Shahabazan Formation at Kialu Section, Zagros Basin, Southwestern Iran

Abstract

The Shahbazan Formation at Kialu section in the Zagros Basin, south-west of Iran, is composed mainly of carbonates with the age of middle Eocene. This formation, with a thickness of 240 m, consists of thick carbonates which pervasively dolomitized. On the basis of petrographic and geochemical features, four different replacive and cement dolomite phases have been recognized. Cathodoluminescence and scanning electron microscope were used to study time of cementation and diagenetic feature as well as to illustrate fine dolomite crystals which are not visible under common microscope. Cathodoluminescence figures show growth zonation and a rimmed bright green luminescence of syntaxial overgrown dolomite as a diagenetic cement as well as some kinds of lattice defects in some cement crystals. Inside-out dolomites seen by applying scanning electron microscope have been interpreted as a result of meteoric diagenesis, caused dissolution of dolomites during exposure. Geochemical analysis of major elements including Mg2+ and Ca2+ and trace elements such as Sr, Na, Mn2+ and Fe2+ with a trend of decreasing in Sr and increasing in Na, Mn2+ and Fe2+ during burial diagenesis, along with the cement zonation, visible under cathodoluminescence microscope, indicate dolomite purification and recrystallization under reducing condition.