Palaeoenvironments and taphonomy of clypeasteroids in Miocene carbonates of the Esfahan–Sirjan Basin, central Iran

Abstract

A comprehensive study of the palaeoenvironmental and post-mortem conditions of Cenozoic clypeasteroids from the Miocene (Aquitanian) of the Qom Formation in the Esfahan–Sirjan Basin (central Iran) is based on test morphology, taphonomy, test surface preservation, echinoid abundance and echinoid sedimentary fabrics (density, orientation, and cluster), and facies analysis. The deposit-feeding clypeasteroids lived in deeper open-marine, shallow open-marine and shoal settings in a sublittoral environment. The low–moderate energy, deeper open-marine facies contain the lowest population of clypeasteroids, including rare shallow infaunal Parascutella and rare semi-infaunal Clypeaster. The moderate to high energy shallow open-marine facies contains the highest abundance and diversity of Clypeaster. The shallow open-marine and high-energy shoal facies are dominated by epibenthic Clypeaster individuals with robust, inflated and commonly dome-shaped tests. Differences in the abundance and diversity of the living clypeasteroids were related to water depth, food sources and energy levels. Different food sources, life-styles and burrowing depths are indicated by the particular morphologies of the echinoids. Dead echinoids, especially the robust and inflated Clypeaster individuals, were used as substrates and domiciles by skeletozoans and other epibiontic organisms. Disarticulation, fragmentation, bioerosion, test outline distortion and radial cracking affected the preservation of the dead clypeasteroid tests. The clypeasteroid shells in the deposits accumulated as 1—an almost autochthonous assemblage (Parascutella), 2—a moderately transported and reworked assemblage (Clypeaster and Parascutella) and 3—transported and multiple reworked assemblages (Clypeaster) affected by storms.