Lithostratigraphy and Biostratigraphy of the Aghchagil and Apsheron Formations based on Calcareous Nannoplankton in Gorgan Plain: Application for paleogeographical reconstruction of South Caspian Bain

Carbonate and siliciclastic Aghchagil and Apsheron Formations has been studied in a subsurface section in the Gorgan Plain. The Aghchagil Fm. consists of lower sandstone and upper carbonate units in the studied area. Sandstones petrofacies include litharenite, sublitharenite and feldspatic litharenite. The grain-supported facies is dominated in this Fm. and displays abundant porosity as moldic, vuggy and fenestral forms specially in the carbonate sediments, which indicates considerable lithological differences in Iran and the neighboring countries like Azerbaijan and Turkmenistan. These features candidate the Aghchagil sediments as prone reservoir succession, hence emphasizes a comprehensive geological and exploration study in the studied area. The Apsheron sediments can be sub-divided into lower sandstone and upper mudstone units and consist of sandstone with litharenite and sublitharenite petrofacies and ooidal/bioclastic limestone and gray/brown mudstone/claystone. Based on the youngest nannofossil species the late Pliocene-lower Pleistocene and middle Pleistocene ages are defined for the in the Aghchagil and Apsheron Fm., respectively. Nannofossils distribution of the studied succession displays the SCB was connected to the Black Sea and Mediterranean Basin in the lower Pleistocene and middle Pleistocene and was isolated in the late Pliocene-earliest Pleistocene.