The Khami group within the petroliferous Zagros Basin of Iran includes the Surmeh, Hith, Fahliyan, Gadvan and Dariyan formations. The Dariyan Formation, previously known as Orbitolina limestone is very significant as a reservoir rock in the Zagros Basin. In this study, the age of the Dariyan Formation is examined based on biostratigraphic and paleontological investigations. 236 m of Dariyan Formation sediments in the sections at Kuh-e Rahmat(north) and Nour-Abad(west) of Iran were selected to study its biostratigraphy.According to the lithostratigraphic investigations, the Dariyan Formation in these sections is subdivided into three: lower, middle and upper, which include thin-bedded to medium-bedded and massive limestone. Micropaleontologic study of the Dariyan Formation has led to the recognition of two biozones in the Nour-Abad area and one biozone in the Kuh-e Rahmat stratigraphic column. Diagnostic foraminifera in these sections are as follows: Globigerinelloides algerianus Cushman and Ten Dam, Globigerinelloides ferreolensis Moullade, Globigerinelloides maridalensis(Bolli), Globigerinelloides paragottisi Verga and Permoli Silva, Hedbergella aptiana Bartenstein, Hedbergella excelsa Longoria, Hedbergella luterbacheri Longoria,Hedbergella occulta Longoria, Hedbergella ruka(Banner et al.), Hedbergella similis Longoria, Hedbergella trocoidea(Gandolfi), Muricohedbergella delrioensis(Carsey), Pseudoplanomalina cheniourensis Sigal, Conicorbitolina conica d'Archiac, Iraqia sp., Mesorbitolina parva Douglass, Mesorbitolina pervia Douglass, Mesorbitolina ovalis Gorog and Arnaud-Vanneau, Mesorbitolina subconcava Leymerie, Mesorbitolina texana(Roemer), Mesorbitolina ovalis-pervia Cherchi and Schroeder, Debarina hahounerensis Fourcade et al.,Marssonella trochus(d'Orbigny), Nezzazata sp., Orbitolina sp., Textularia sp.Based on the identified foraminifera, the age of the Dariyan Formation in the Kuh-e Rahmat area is assigned to Aptian, and in the Nour-Abad area is determined as upper Aptian.
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