Nannoplankton and Ammonoid evidence in the Early Aptian Oceanic Anoxic Event in the west of Kopet-Dagh Basin

Calcareous nannofossils and Ammonites have been studied in the Sarcheshmeh and Sanganeh formations at the Takal Kuh section in the western part of the Kopet-Dagh Basin. In the studied section ammonite biozones present in Sarcheshmeh Formation are Heteroceras spp. Zone, Martelite securiformis Zone, Deshayesites oglanlensis Zone, Deshayesites weissi Zone and the lower part of Deshayesite deshayesi Zone and the biozones recognized in Sanganeh Formation are the Upper part Deshayesite deshayesi Zone and Dufrenyia sp. Zone. According to the ammonite fauna present the age of the Sacheshmeh Formation is determined to be Late Barremian-Early Aptian and that of Sanganeh Formation to is Aptian. In this section the calcareous nannofossil zones Watznaueria oblonga Zone (NC5) and Chiastozygus litterarius Zone (NC6) are located within Sarcheshmeh Formation and the Upper part of Chiastozygus litterarius Zone (NC6) and the Rhagodiscus angustus Zone (NC7A) are found in the Sanganeh Formation. On the basis of calcareous nannofossil data the age of the Sarchechmeh Formation is found to be Late Barremian-Early Aptian while that of the Sanganeh Formation is estimated to be Early Aptian-early Late Aptian. Calcareous nannofossils data recorded the presence of Oceanic Anoxic Event 1a (OAE 1a) at the upper part of NC6 and the lower part of NC7A Zone (lowermost 50 meters of Sanganeh Formation meters). Ammonites are not present at the Oceanic anoxic event OAE 1a interval, but but appear at Sarcheshmeh-Sanganeh boundary, and are used for dating the event. Based on the presence of Deshayesites cf. involutus, Deshayesites cf. consobrinoides and Deshayesites deshayesi at the first part of the interval Deshayesites deshayesi ammonite biozone has been determined and accordingly the OAE 1a event is recorded within the Deshayesites deshayesi Zone of the studied section.