Depositional environments of the Kalhur Member evaporites and tectonosedimentary evolution of the Zagros fold–thrust belt during Early Miocene in south westernmost of Iran

Abstract

The Oligocene–Miocene deposits of the Asmari Formation are the most important oil reservoirs in the Zagros fold–thrust belt in southwest Iran. The Aquitanian evaporites of Kalhur Member of the Asmari Formation were deposited in a depression situated nearly between the Mountain Front Fault and the Dezful Embayment Fault, which borders northern and southern margins, respectively. Detailed field, petrographic studies and wireline log analyses were used to investigate the depositional facies and sedimentary environments of the Kalhur Member. The occurrence of evaporites between pelagic facies and the presence of slump folding, considerable continuity of laminated gypsum/anhydrite along with progradation of clinoforms towards depositional side of evaporites, suggest the presence of relatively deep and isolated depressions during deposition of Kalhur Member. Different facies and thickness variation along with various geometries were formed as the result of overthrust loading event related to the Zagros deformation during Rupelian–Chattian. As the result of this tectonic event, depocentres shifted laterally and relatively narrow deep depressions were filled by evaporites under relative sea-level lowstand and arid conditions and sea-level falls during Aquitanian. Sea-level rise during Burdigalian resulted in sedimentation of the Asmari Formation carbonates throughout the Lurestan and Dezful Embayment, meanwhile with deeper facies towards north. This was along with basin reconfiguration and creation of carbonate ramp.