Palaeoenvironmental and sequence-stratigraphic analysis of the Middle–Late Devonian carbonates (Bahram Formation) of Anarak, western Central Iran

Abstract

The predominantly warm greenhouse climate and high global sea level during the Middle–Late Devonian resulted in fully marine conditions along the northern Gondwana margin, including the Iranian Plate. Over three-hundred meter-thick fossiliferous Middle-to Late Devonian carbonate deposits of the Bahram Formation in the Anarak region (western Central Iran) are composed of dark grey limestone, dolostone, and interbedded shale. Seventeen microfacies have been distinguished, which correspond to four depositional sub-environments, i.e., tidal flat, lagoon, shoal, and open marine. The Middle–Late Devonian carbonate deposits were deposited on a homoclinal carbonate ramp. Seven third-order depositional sequences are identified, apparently controlled by relative sea level change. Carbon and oxygen stable isotopes, and major and trace elements of micritic carbonate samples document details about the geochemical, palaeoenvironmental, and diagenetic processes; they also indicate that the carbonates underwent meteoric diagenesis within a semi-closed diagenetic system. The low concentrations of trace elements indicative of euxinic conditions, including Mo, Cr, V, and As, suggest well-oxygenated depositional environment. Carbon isotope values show a meaningful relationship to major sequence-stratigraphic key surfaces, with more negative values occurring at the sequence boundaries and positive values corresponding to maximum flooding surfaces.