Reconstructing the depositional environment of microbial deposits of the upper Jurassic Najmah Formation in the Paveh region, Kermanshah

The Najmah Formation is composed of variable microbial build-ups disconformably resting on the dark calcareous shales and limestones of the Sargelu Formation. The transition from the middle Jurassic Sargelu to the upper Jurassic Najmah was associated with a conspicuous change in depositional environment so that the intrashelf basin of the Sargelu with the domination of black calcareous shales and dark pelagic limestones evolved to a shallow-marine ramp with carbonate deposition from mostly microbial origin. This change caused by a regression and a resultant interruption in sedimentation is evidenced by the occurrence of intraclastic breccia, fenestral fabrics, evaporite pseudomorphs as well as various microbial build-ups in the Najmah pointing to the superiority of arid climate and hypersaline conditions with paramount environmental stresses during the deposition of the formation. Facies recognized in the Najmah Formation include five microfacies (MF) and two lithofacies (LF) representing supratidal, intertidal and subtidal sub-environments. Based on microfabric viewpoint, microbialites of the formation can be divided into thrombolite and stromatolite. The formation is dominantly composed of stromatolite, which is subdivided into stratiform and domal stromatolites based on morphoscopic criteria. Moreover, these stromatolites can be microscopically subdivided into five microfabrics among which so-called “planar micrite with sparite laminae” is the most frequent microfabric. Stromatolites of the Najmah developed in a supratidal to intertidal environment. Furthermore, thrombolite with a clotted and non-laminar fabric is another build-up of the formation, deposited in a shallow subtidal zone making the basal part of the cycles of the Najmah following the bottommost red shale.