Permian-Triassic lithostratigraphic study in the Northern Thrust Zone (Ora), Iraqi Kurdistan region

Abstract

The stratigraphy of Permian and Triassic successions in the Northern Thrust Zone (Ora), Iraqi Kurdistan region was studied in detail with the aid of field data and thin section analysis. For this study, two sections in the Ora region (Beduhe and Sararu) were chosen. The lithologic description and the biostratigraphic relationships were identified Late Permian (Chia Zairi Formation) to Late Triassic (Baluti Formation). Petrographic analysis based on 100 carbonate and sandy limestone samples has shown that skeletal components are dominated by relatively deep marine faunas such as thin-shelled pelagic pelecypods (Posidonia), microgastropods, ostracods, serpulid worm tubes (Spiroribs), crinoids with shallow water derivatives such as benthic foraminifera (Miliolids) and brachipods. Non-skeletal components are restricted to peloids, ooids, and intra-extra clastic particles. Petrographic facies and textural analyses have revealed that Permian and Triassic successions in the Northern Thrust Zone were deposited at a gently sloping epeiric carbonate ramp with some shoreline-detached high-energy belts especially at the upper most part of Permian Chia Zairi Formation and lower part of Early Triassic Mirga Mir Formation. The presence of collapse breccia within the upper part of Mirga Mir and Geli Khana formations indicates a late humid condition that dissolved the former evaporites. The paleoenvironmental reconstruction of Triassic successions in the Northern Thrust Zone revealed the gradual shallowing of the basin from the basinal sediments with some signs of restriction of circulations in the Lower and Middle Triassic Mirga Mir, Beduh, and Geli Khana formations to a lagoonal environment of the Baluti Formation in the Upper Triassic. On the other hand, the calcareous shale and clastic units of Late Devonian to Early Carboniferous grade up into dolomites and dolomitic limestones at the end of two sequences: the first at the end of Late Carboniferous to Late Permian represented by Chia Zairi Formation and the second at Late Triassic represented by Kurra Chine Formation.