Original paper

Palaeoecology of the Oligocene-Miocene Asmari Formation in the Dill Anticline (Zagros Basin, Iran)

Dill, Mohammad Allahkarampour; Seyrafian, Ali; Vaziri-Moghaddam, Hossein


Dezful Embayment of the Zagros Basin was examined. The Asmari Formation was deposited in a subtropical environment in an oligophotic to euphotic zone. The setting was a carbonate ramp with oligotrophic to mesotrophic conditions as indicated by the association of large benthic foraminifera (Heterostegina, Spiroclypeus, Amphistegina, Neorotalia, Miogypsinoides, Borelis, Archaias, Peneroplis and Dendritina) with coral debris during Chattian to Early Aquitanian and Burdigalian times. The carbonate grain associations within the Asmari Formation indicate: 1. that the lower part of the Asmari Formation (Early Chattian) was deposited in normal saline waters (middle ramp), and 2. that the middle and upper parts of the Asmari Formation (Middle Chattian-Burdigalian) experienced higher to hyper-saline waters (semi-to restricted lagoon environments of inner ramp). The diversity of large benthic foraminifera, the rarity of coral debris and the absence of green algae in the Dill Anticline area indicate a minimum surface water temperature of < 20 °C. Based on the faunal associations, the nannofor (aphotic zone-outer shelf and also foralgal (oligophotic to rarity mesophotic-middle shelf)) associations reflect a deeper water environment relative to the Asmari Formation in Khuzestan and the Coastal Fars provinces. Foramol associations (euphotic zone-inner shelf) are associated with the shallow part of the Asmari Formation and the coeval Champeh Member of the Gachsaran Formation into the Interior Fars province during the Oligocene (Rupelian-Early Chattian). Foramol associations embraced the carbonate platform from the Late Chattian until the Early Miocene.