A review of the Oligo–Miocene larger benthic foraminifera in the Zagros basin, Iran; New insights into biozonation and palaeogeographical maps

Developing a more precise time framework in any hydrocarbon-bearing basin is potentially of great value to geologists and reservoir modelers for better understanding the hydrocarbon reservoir zonation, and identifying unconformities. In the Asmari Formation, which is the most important Oligocene–Miocene carbonate reservoirs in Iran, broad time zonation is generally used based on larger benthic foraminifera (LBF). In this study, the biostratigraphic zonation of the Asmari intra-shelf basin is re- evaluated based on the spatial and temporal distribution of LBF (i.e., different species of Nummulites, Eulepidina, Archaias, Spiroclypeus, Miogypsinoides, Miogypsina, and Borelis) as well as Sr isotope datasets. This research proposes a new zonation to improve time resolution in the Oligocene and explains the effects of environmental factors on some index fossils and the limited range of some key fauna. The newly proposed zonation consists of one biofacies and nine biozones occurring throughout the Rupelian to Burdigalian allowing differentiating Oligo-Miocene stages, and tuning the chronostratigraphic framework of the Oligocene–Miocene deposits. Moreover, based on this new biozonation, regional-scale palaeogeographical maps of the Asmari intra-shelf basin during Rupelian-Burdigalian were officially established.