Sedimentology of Avanah Formation (Middle-Upper Eocene) from Selected Sections within Iraqi Kurdistan Region

Sedimentary features, petrography, and microfacies analysis of the Avanah Formation are investigated in the current study. Two exposed outcrops (Sartak-Bamo and Belula) for the Avanah Formation are studied in detail, located in the Zagros High Folded Zone (ZHFZ), and two subsurface sections (K90 and K306) within the Zagros Low Folded Zone (ZLFZ). The Avanah Formation is composed of multi-lithological sequences (limestone, dolomitic limestone, dolostone, and marly limestone). It has been subdivided into five units based on its composition. The Khurmala Formation underlies the Avanah Formation at all the studied sections unconformably. The Pila Spi Formation at the Sartak-Bamo section conformably overlies the Avanah Formation, but the Jaddala Formation conformably overlies the Avanah Formation at the Belula section. The Fatha Formation unconformably overlies the Avanah Formation at both subsurface sections. A variety of large benthic foraminifera species have been identified, such as Nummulite spp.; Orbitolite complanatus (Lamarck); Alveolina spp.; Discocyclina spp. and Assilina spp. In addition, some other small benthic foraminifera, such as Miliolids, Rotalids, a few Mollusca, and Algae with their bioclasts, are identified in the studied sections. Based on detailed microfacies analysis, four main microfacies (Mudstone, Wackestone, Packstone, and Packstone-Grainstone) and eleven submicrofacies are determined. Three facies’ associations are identified, which lead to interpreting that the Avanah Formation was deposited in a semi-lagoon, shoal (bank) to shallow open marine environment of the inner ramp and extending to the middle ramp setting.