Stratigraphic architecture and depositional–diagenetic evolution of Oligocene–Miocene carbonate–evaporite platform in the southern margin of the Neo-Tethys Ocean, Lurestan zone of Zagros, Iran

Abstract

The Oligocene–Miocene (OM) depositional sequences (Asmari Formation) are marked by remarkable changes in thickness, lithology, depositional facies characteristics, and diagenesis history throughout the Zagros area, as the southern margin of the Neo-Tethys Ocean. An integrated sedimentological, geochemical and sequence stratigraphic analysis of these sequences is presented in two surface and subsurface sections located in the Lurestan zone. Macroscopic (field-scale) descriptions and microscopic petrographic studies represent 12 microfacies deposited on a ramp-like carbonate platform. Complex diagenesis history of the Asmari Formation encompasses hypersaline, marine, and meteoric, to shallow- and deep burial realms. Cathodoluminescence microscopy and stable isotopic (carbon and oxygen) compositions of whole-rock, micrite, and carbonate cements of different generations are used for the precise interpretation of depositional–diagenetic processes. Regional stratigraphic architecture of these strata is defined by the correlation of studied sections throughout the Zagros area, in a sequence stratigraphic framework. Development of restricted lagoons or salterns with deposition of evaporites, and hypersaline diagenetic overprints point to a completely different depositional-diagenetic evolution. The Oligocene–Miocene transition (OMT) event is correlated with global records. A 3 to 4 ‰ increase in δ18O is considered as the signature of a cooling event at the OM boundary. Variable geochemical features of the OM sequences in the Zagros area can be due to the tectonic activities and changes of paleoenvironmental conditions. Results of this study provide new insights about the Oligocene–Miocene depositional systems in the Zagros area and, in a larger scale, on the southern margin of the Neo-Tethys Ocean.