Early Aptian oceanic anoxic event (OAE 1a) in Northeastern Arabian Plate setting: an example from Dariyan Formation in Zagros fold–trust belt, SE Iran

Abstract

The Dariyan Formation (southwestern Iran) records the characteristic features of an oceanic anoxic event with organic- and radiolarian-rich hemipelagic intervals. A biostratigraphic study based on benthic and planktonic foraminifers provides an early Aptian age for the organic-rich succession and an Aptian age for the entire Dariyan Formation in this area. In this study, we report the first stable carbon isotope curve for the Dariyan Formation which is characterized by a pronounced negative spike (reaching −2 ‰) at the base of the section followed by a subsequent positive excursion (4 ‰) and a plateau with values fluctuating around 3 ‰. The integration of the δ13C record with the previously published litho- and biostratigraphy provides the characteristic features of the oceanic anoxic events (OAE) 1a interval. This detailed curve improved the stratigraphic resolution in this area and allowed the establishment of a temporal framework which showed good correlations with other OAE 1a sections worldwide. High production of organic matter and abundance of radiolarians and planktonic foraminifers suggest high-nutrient fluxes and meso- to eutrophic conditions at the time of deposition of the organic-rich interval of the Dariyan Formation. This is in agreement with enhanced greenhouse conditions. The facies distribution (from shallow to deep water environments) and the paleogeography of Arabian Plate during the early Aptian suggest that increasing continental runoff was a primary trigger of high trophic level conditions. Redox conditions, estimated from manganese (Mn) behavior, indicate dysoxic to anoxic conditions within the basin during OAE 1a.