Relative age determination of the Sarki Formation in the northeastern margin of the Arabian plate, revealed by foraminifera and dasycladacean algae

The Sarki Formation crops out in several cores and limbs of anticlines in the Western Zagros Fold-Thrust Belt (WZFTB) within the northeastern margin of the Arabian plate. Its precise age is controversial over the past seventy years. A biostratigraphic study of the formation was carried out in the Gulan section of the Asos anticline, within the imbricated zone of the Kurdistan Region, northeastern Iraq, to determine the relative dating of the formation. The Sarki Formation in the studied section is consists of 294 m of medium- to thick-bedded dolomitic limestone and dolomite, interbedded with thin beds of marl and shale in the lower part, and interbedded with medium-bedded limestone in the upper part. Petrographic examination of 80 thin sections from the Sarki Formation revealed several benthonic foraminifera and dasycladacean algal species from the late Triassic to the early Jurassic epochs. The Triasina hantkeni species was identified in the lower part of the Sarki Formation for the first time in the northeastern margin of the Arabian Plate, dates back to a Rhaetian age. Biostratigraphically, the formation is divided into five biozones from oldest to youngest: assemblage zone A, B and C dated to the early, middle and late Rhaetian, respectively, assemblage zone D, assigned to the Hettangian and assemblage zone E, corresponding to the lower to middle Sinemurian. The suggested new age of the Sarki Formation in the studied section is Rhaetian to early-middle Sinemurian.