Biostratigraphy of the Asmari Formation in the Dehdasht region (Zagros Basin, Southwest of Iran)

The Oligo/Miocene carbonate succession of the Asmari Formation from Zagros foreland Basin is cropped out in Dehdasht area (southeast of the Izeh zone). In compare to depositional area of the Asmari intra-shelf basin (more than 1500 kilometre2) and available data, the study area is nearly untouched and need more investigation. This study mostly focused on biostratigraphy of the Asmari Formation though four naturally well-exposed outcrops (Ganaveh, Siang, Arand and Dehdasht). A precise biostratigraphic interpretation of the Asmari Formation was compiled on the basis of the larger benthic foraminifera biozones allowing subdivision of the successions into four well-dated assemblages. Assemblage 1 only is present in the Ganaveh and Siang sections, indicating the Rupelian-Chattian? age. This assemblage toward the NW of the study area (Arand and Dehdasht) replaced by pelagic marl of the Pabdeh Formation. Assemblage 2 is restricted to the Chattian due to the first occurrence of Spiroclypeus blanckenhorni and also flourishing of the Archaias species. The low diversity of biotic contents and existence of Miogypsina and Favreina asmarica mark Assemblage 3 referring to the Aquitanian. Appearance of the Borelis melo group as a main characteristic of Assemblage 4 indicates Burdigalian. This investigation proves that the carbonates of the Asmari Formation in the studied area started to deposit in a proximal domain of a carbonate platform from Rupelian, whereas it was started in distal position from Chattian.