Biostratigraphy, paleoenvironment and foraminiferal associations of the Rupelian-Chattian sediments in Zagros Basin, SW Iran

• Oligocene sediments of the Asmari Formation sediments are important for petroleum exploration in Zagros Basin.
• Paleoecology and paleoenvironment reconstructions according to the presence of larger benthic foraminifera.
• Schematic diagram for interpreting foraminiferal associations in the Interior Fars sub-Basin.

In this research larger benthic foraminiferal distribution and their paleoenvironmental characteristics are used to introduce biostratigraphic zonation, paleoenvironmental reconstruction and paleoecological interpretation of the Oligocene Asmari Formation in Fars Province. Two stratigraphic successions were examined for these purposes. The first (Khollar Section) is Rupelian in age and the second (Siakh Section) is of Chattian age. Recognized assemblage zones are: 1-Nummulites vascus-Nummulites fichteli and 2- Archaias asmaricus/hensoni-Miogypsinoides complanatus. Four microfacies types are identified according to the occurrence of the main biogenic components. They were arranged along the inner part of a carbonate platform. A shallowing upward trend in microfacies arrangement from Rupelian to Chattian times is considered according to the occurrence of larger benthic foraminifera. Two foraminiferal associations are recognized in the investigated sections. The identified foraminiferal associations represent a salinity value of 40–50 psu and a depth range of lower than 40 m, warm tropical and subtropical waters with temperature of 18–25 °C at Rupelian time. More restricted conditions through Chattian Stage has resulted in a shallower depth and higher salinity of more than 50 psu, with water temperature being higher than 20 °C in the oligotrophic to mesotrophic conditions. Restricted conditions in marine circulation is suggested to have controlled these associations.

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