Lithostratigraphy and biostratigraphy of the Asmari Formation in Jahangirabad section, North of the Zagros Basin, Iran (Kabir Kuh Anticline)

Introduction In order to study the lithostratigraphy and biostratigraphy of the Asmari Formation in Kabir Kuh Anticline in Ilam area, one outcrop section in Jahangirabad, southwest of Darreh shahr with geological coordination of  33°06'45"N , 47°21'19"E with a total thickness of 179 m were selected and sampled.Methods & MaterialsSamples of the Asmari Formation were collected systematically (~2m) and about 180 samples were prepared in the laboratory.Discussion The Asmari Foramtion in mentioned section of Kalhur Evaporite consists of 3 units, Lower Kalhur Gypsum with 9 m, Inter Kalhur with 35.5 m and Middle & Upper Asmari Limstone with 134.5 m. The Upper Kalhur Gypsum unit and Transition Beds are not present in this section. In terms of lithology, Lower Kalhur Gypsum unit mainly composed of white thick gypsum contaminated with hydrocarbon materials, Inter Kalhur Beds with limestone and gray marlstone with pelagic facies and Middle & Upper Asmari Limestone with thick, medium to thin limestones with benthic facies. The lower and upper boundaries of the Asmrai Formation in this section are conformable with Pabdeh and Gachsaran formations respectively with sharp lithological changes. Thus, the marly limestone of upper part of the Pabdeh Formation changes to gypsum of Lower Kalhur Gypsum and limestone of upper part of the Upper & Middle Asmari Limestone changes to gypsum of the Lower part of the Gachsaran Formation.In biostratigraphic study of the Asmari Formation in this section 9 species belong to 6 genera of planktonic foraminifera and 10 species belong to 21 genera of benthic foraminifera have been identified and a not-zone in Lower Kalhur Gypsum, one biozone based on planktonic foraminifera in Inter Kalhur Beds and two biozones benthic forminifera and biozonation, based on Laursen et al., 2009, in the Middle & Upper Asmari Limestone in the Asmari Formation were recognized as follows:Borelis melo curdica – Borelis melo melo Assemblage zoneMiogypsina – Elphidium sp.14 – Peneroplis farsensis Assemblage zoneGlobigerinoides subquadratus Assemblage zone Not ZonedFurthermore, one biozone based on planktonic foraminifera in the upper part of the Pabdeh Formation were identified as followsSubbotina gortanii Assemblage zoneConclusionAccording to location of Lower Kalhur Gypsum unit between upper part of the Pabdeh Formation with Late Chattian age and lower part of Inter Kalhur Beds unit with Aquitanian age,  and also by considering isotope strontium studies in Lower Kalhur Gypsum (by Van Buchem at al., 2010), the age of the Lower Kalhur Gypsum late Chattian were identified. Also, for Inter Kalhur Gypsum consisting biozone of “Globigerinoides subquadratus Assemblage zone” and 82 m of the Lower part of Middle & Upper Asmari limestone unit consisting biozone of “Miogypsina Elphidium sp.14 peneroplis farsensis Assemblage zone” with the age of Aquitanian and 50 m of upper part of Middle & Upper Asmari limestone consisting of biozone of “Borelis melo curdica Borelis mel melo Assemblage zone” with the age of Bourdigalian were determined. Totally based on biozones and planktonic and benthic foraminifera that were recognized in the Asmari Formation in jahangirabad section, Late Chattian to Bourdigalian age were suggested.