Carbon isotope chemostratigraphy of a Gondwanan Ediacaran–Cambrian transition, Soltanieh Mountains, northern Iran

Abstract

Mixed siliciclastic-carbonate rocks of the Kahar and overlying Soltanieh formations from Chopoghlu, in the Soltanieh Mountains, northern Iran, preserve an expanded stratigraphic record of Ediacaran to lower Cambrian life and environments. To date, regional placement of the Ediacaran-Cambrian boundary has been based on biostratigraphy, with contrasting views of its stratigraphic position. Here we present a high-resolution carbon isotope chemostratigraphic record to provide an independent assessment of correlation to other successions worldwide. Facies analysis indicates that Kahar siliciclastic sediments accumulated in fluvial to tidal environments, with thin microbial carbonates in tidally influenced facies. The overlying Soltanieh Formation records peritidal carbonate deposition followed by significant deepening and deposition of interlaminated mudstones and thin sandstones. δ13C values for Kahar carbonates, with a maximum age of about 550 Ma, lie within a narrow range of 0 to −2‰, save for a moderate positive excursion near the top of the formation. Above this, carbonates of the Lower Dolomite Member, Soltanieh Formation, show consistently negative δ13C values. The overlying Middle Dolomite, Middle Shale and basal part of the Upper Dolomite members feature high amplitude δ13C variations, which rise to values as high as 4‰ towards the top. Considering the integrated chemostratigraphic and paleontological records, Chopoghlu deposition appears to have begun during the late Ediacaran Period. The stratigraphically variable δ13C record of Soltanieh carbonates closely resembles lowermost Cambrian successions elsewhere, supporting placement of the E-C boundary near the top of the Lower Dolomite Member (LDM) and a Terreneuvian age for overlying Soltanieh beds. This is consistent with the presence of Protohertzina sp. and Hyolithellus sp. in or just above uppermost LDM carbonates and the lowermost occurrence of Treptichnus pedum above this. Together, bio- and chemostratigraphy help to integrate the Chopoghlu section into the global record of biospheric change as the Phanerozoic Eon began.