Integrated biostratigraphy and geochemistry of the lower Cretaceous Radiolarian Flood Zone of the base of the Garau Formation, northwest of Zagros Mountains, Iran

The lower Garau Formation known as the Radiolaria Flood Zone (RFZ) is investigated for reconstructing palaeoceanography and controlling factors that resulted in the organic matter and radiolarian rich beds. A stratigraphic section named Tang-e-Haft located to the northwest of Zagros range with a thickness of 320 m mainly composed of argillaceous limestone, and limestone is used for this study. The RFZ is dated late Berriasian to earliest Aptian via using nannofossils, foraminifera, and palynology integrated. Semi-quantitative study of different microfossil groups and biofacies analaysis demonstrates deposition of this member in an intermediate depth under dys- to anaerobic marine condition with a shallowing upward trend. Richness of radiolarians with prevailed spumellaria assemblage and high organic matter content revealed presence of high palaeoproductivity during time of deposition. This productivity was induced by paleocurrents and upwelling systems. Values of total organic carbon (TOC), gamma ray, and amount of amorphous organic matter (AOM) in palynology slides in the lower parts of the section show a peak that indicates that palaeoproductivity was more intense in the lowermost part. Dominance of the radiolarians and consequently the RFZ unit ended up in the early Aptian. This time is coincided with Leupoldina cabri bloom and Oceanic Anoxic Event 1a (OAE1a). This event resulted in an extinction and significant reduction among radiolarians. Depopulation of radiolarians made new habitats for other organisms that planktonic foraminifera with elongated chambers were the first group to benefit the situation.