Salinity changes during Late Oligocene to Early Miocene Asmari Formation deposition, Zagros Mountains, Iran

Abstract

The distributions of sediment grain types and microfacies in two outcrop sections in the central part of the Zagros Fold and Thrust Belt, Iran, are used to interpret changes in salinity during Late Oligocene (Chattian) to Early Miocene (Burdigalian) deposition of the Asmari Formation. Conditions of hypersalinity are interpreted from analogy with the modern salinity gradient in Shark Bay, Western Australia, and the time scale is derived from foraminiferal biozonation and Sr-isotope stratigraphy. The study demonstrates that frequent changes in sedimentary facies and salinity conditions took place in the central Zagros basin during the late Chattian, Aquitanian, and Burdigalian. Five main findings of the study are as follows. (1) A significant shallowing from the outer neritic zone (100 m to 200 m deep) to the innermost neritic zone (< 30 m deep) took place during the Chattian at Tang-e-Ban (imprecisely dated within the 23 Ma to 27 Ma interval) and at Tang-e-Sapou (about 24 Ma). (2) In both sections, a transition from normal marine to hypersaline conditions took place during the late Chattian following the shallowing. (3) The Aquitanian of the studied sections includes mainly hypersaline facies with a transition from normal-marine to hypersaline facies near the base and a transition from hypersaline to normal marine facies close to the top of the stage as locally recognized. (4) Episodes of severe hypersalinity (> 50 psu) occurred during Aquitanian deposition of the Asmari Formation at both localities. (5) Deposition of the upper Asmari Formation and lowest Gachsaran Formation (Burdigalian) was mainly under slightly hypersaline conditions (40 psu to 50 psu) with some normal-marine intervals at Tang-e-Sapou, and some normal-marine intervals as well as severe hypersaline episodes (> 50 psu) at Tang-e-Ban to the west. The patterns of salinity change seem to have been widespread in the central Zagros foreland basin and transitions from normal marine to hypersaline conditions may represent falls in relative sea level or be due to build up of sand banks forming sills that restricted water circulation. Changes from hypersaline to normal marine conditions may represent relative sea level rise. However, there is insufficient chronological resolution at present to allow confident correlation of the major salinity changes to sequence boundaries and surfaces previously recognized in the basin.