Oligocene biostratigraphy, depositional patterns, and stable isotope values in the eastern Zagros basin: Novel insight into the signature of regional and global events

Abstract

Biostratigraphy, depositional environment, sequence stratigraphy, and paleoclimate iterpretation of the Asmari carbonate reservoir are the subject of the current study. Two, earlier uninvestigated stratigraphic sections are studied and correlated over the eastern Zagros basin. The vertical distribution of foraminifera (benthic and planktic) reveals that the Asmari Formation deposited during the Rupelian-Chattian. Three shallow benthic foraminifera biozones are established, namely the Globigerina-Turborotalia cerroazulensis-Hantkenina Assemblage Zone, the Nummulites vascus-Nummulites fichteli Assemblage Zone, and the Archaias asmaricus- Archaias hensoni-Miogypsinoides complanatus Assemblage Zone. Ten microfacies types associated with the inner, middle and outer ramp of a carbonate platform are identified. Three depositional sequences are recognized in the eastern Zagros basin. The oldest sequence is Rupelian in age, and two others are of Rupelian-Chattian and Chattian in age. Two maximum flooding surfaces occur in the eastern Zagros basin. The isotope startigraphical data show that the more positive values of the δ18O and δ13C coincide with the maximum flooding surfaces, while these values show depletion over the sequence boundaries. The exception is observed at the upper boundary of the succession where the hypersalinity conditions affected the trend. The stable isotope values obtained from bulk samples enable to identify the Oligocene glacial events such as Oi2 and, Oi2a (Rupelian) and Oi2b (Chattian). The examination of the grain associations of the MF3 and MF5 prove these cooling events. We suggest the middle ramp microfacies and MFS2 of Sq.2 as signatures for tracing these cooling events throughout the eastern Zagros basin.