EDIACARAN–CAMBRIAN MICROBIAL CARBONATES OF THE AQ-KAND AREA, NW IRAN

The question of microbialite development as the Cambrian dawned is an interesting one, because, for the first time in Earth history, microbialites were accreting in the presence of macroscopic, skeleton-forming animals. The Aq-Kand section (2300 m thick) in northwestern Iran is a mixed siliciclastic-carbonate succession that includes the Kahar, Soltanieh, and Barut formations, deposited during the late Ediacaran–early Cambrian. This succession contains well-preserved microbial carbonates, including stromatolites, thrombolites, oncolites, calcified microbial microfossils, microbial mat debris, and composite microbial textures. This co-occurrence of diverse microbial forms provides an ideal opportunity for elucidating changing microbialite structures and environmental controls on their development as animals diversified. Herein we describe the distribution and abundance of these microbialites, with special attention to shifts in the geobiological conditions across the boundary. We studied growth forms and internal fabrics of the microbial carbonates at different scales, in the field, in oriented polished slabs, and in some 270 thin sections. The facies of the studied carbonates represent a shallow marine platform with deposition from the supratidal to deep subtidal zones. Stratiform stromatolites are the most common microbialites in the Ediacaran Kahar Formation, whereas, in addition to stratiform stromatolites, abundant thrombolites and calcified microbes such as Renalcis occur in the Ediacaran–Cambrian Soltanieh Formation and the overlying Barut Formation. In general, columnar and domical stromatolites are less abundant than stratiform forms in this succession. That said, the largest microbialites also occur in the Kahar Formation. Observed changes through the succession likely reflect environmental representation, metazoan evolution, and possible changes in carbonate chemistry across the Ediacaran–Cambrian boundary.