Larger benthic foraminifera: a tool for biostratigraphy, facies analysis and paleoenvironmental interpretations of the Oligo-Miocene carbonates, NW Central Zagros Basin, Iran

The facies distribution and the biogenic components of the study carbonates in the Ghale Nar oil field in the central part of the Zagros basin were used to reconstruct and interpret the depositional environment of the Asmari Formation. The sediments of the formation consist mainly of limestone, dolomite, dolomitic limestone and are dominated by larger benthic foraminifers (LBF) and corallinacean assemblages. Based on the LBFs, the Asmari formation in the study section is Oligocene (Rupelian-Chattian) to Early Miocene (Burdigalian) in age. Fourteen facies representing upward gradual shallowing trend of an open marine (F1 (the Pabdeh Formation) to 3), shoal (F4-5), semi-restricted and restricted lagoon (F6-12) and near-shore lagoon (F13-14) environments were identified. The facies analysis and the paleoenvironmental interpretations revealed that the carbonate sediments of the Asmari Formation were deposited in a homoclinal ramp consisting of inner, middle and outer zones. The F1 facies (top of the Pabdeh Formation) with planktonic foraminifers represents an outer ramp. The F2-3 facies with planktonic, large and small hyaline benthic foraminifers and corallin algal indicate a distal to proximal middle ramp setting. The F 4-14 facies were characterized by the occurrence of porcelaneous benthic foraminifers representing a shallow-water setting of distal to proximal inner ramp influenced by wave and tide processes. The carbonate ramp developed in tropical waters, under oligotrophic to slightly mesotrophic conditions. Our results from the facies and paleoecological analysis revealed that the Asmari Formation was deposited on a carbonate open ramp dominated by heterozoan and subordinate photozoan skeletal assemblages.