Biostratigraphy, paleoecology, and paleoenvironmental reconstruction of the Asmari formation in Zagros basin, southwest Iran

Abstract

The present work is focused on the biostratigraphic analysis, paleoecological interpretations, and paleoenvironmental reconstruction of sediments of the Asmari Formation. The Asmari Formation, which is mainly composed of carbonates, has been studied in the Rig anticline in the Izeh zone, Zagros Basin, Iran. The Asmari Formation at the study area is Rupelian to Burdigalian in age, based on the occurrence of 21 foraminiferal species belonging to 21 genera which led to identification of five assemblage zones including (1) Nummulites vascus-Nummulites fichteli, (2) Lepidocyclina-Operculina-Ditrupa, (3) Archaias asmaricus-Archaias hensoni-Miogypsinoides complanatus, (4) Miogypsina sp.-Elphidium sp. 14-Peneroplis farsenensis, and (5) Borelis melo group-Meandropsina iranica. Detailed petrographic analysis led to the recognition of 13 facies that are belonging to four depositional environments including tidal flat, lagoon, shoal, and open marine environments. These carbonate facies were deposited in shallow marine carbonate platform as a homoclinal ramp type. The lower part of the Asmari Formation deposited in an open marine environment with normal water salinity, while the middle and upper parts of the Asmari Formation were deposited in a shallower water with alternating intermittent normal and higher water salinity. Occurrence of larger benthic foraminifera such as Nummulites, Operculina, Heterostegina, Amphistegina, Archaias, Peneroplis and Borelis, coral and red algae reflect mesophotic to oligophotic conditions in subtropical to tropical environments with a high nutrient stability in a mesotrophic to oligotrophic conditions existed during deposition of the Asmari Formation.