Depositional environments and Tectonic Controls on Paleocene phosphorite-rich successions in the Marbat-Traibeel Basin, Western Desert of Iraq

Abstract

This study explores the depositional environments of Paleocene phosphorite-rich carbonate and siliciclastic successions along an NNE-SSW cross-section of the Marbat-Traibeel Basin, Western Desert, Iraq. This study examines over 500 thin sections from six vertical stratigraphic sections, encompassing boreholes and outcrops along a transect from the Rutbah High to the Iraqi-Jordanian border. It investigates lithofacies distribution, depositional sequences, and basin architecture through an integrated analysis of tectonic, sedimentological, biostratigraphic, and sequence stratigraphic data, providing insights into the basin's depositional history. It aims to elucidate the geological evolution of the Paleocene Akashat Formation and the Traifawi Member/Jeed Formation within the Marbat-Traibeel Basin, located on the northern margin of the southern Tethys Ocean. This region is characterized by complex interactions between shallow marine carbonate, siliciclastic, siliceous, and phosphatic lithofacies influenced by a submerged but tectonically active structural high (Rutbah High). The study identifies 26 lithofacies classified into seven associations based on lithological and sedimentological features, representing inner, middle, and outer ramp depositional environments. The phosphorite-rich carbonate succession, particularly the Upper Danian Hirri Member, indicates deposition on an open, distally steepened ramp and hosts substantial economic phosphate reserves. Syn-depositional tectonics influenced the ramp morphology and sedimentation of the phosphorite, which resulted in lateral variations in thickness and microfacies. Tectonic instability resulted in numerous depositional hiatuses (Cretaceous/Paleogene, Lower Danian - Upper Danian; Selandian – Thanetian, Upper Paleocene – Lower Eocene boundary) and eastward thinning of depositional sequences on the Rutbah High. Seven distinct Paleocene third-order depositional sequences are identified in the western part of the study area, near the Jordan-Iraq border, passing eastward to four third-order sequences, a pattern attributed to tectonic uplift on the Rutbah High. The depositional sequences are separated by boundaries characterized by vertical facies changes, including prominent Thalassinoides burrows/firmgrounds on transgressive surfaces with significant hiatuses that correlate with known eustatic sea-level falls associated with the Arabian Platform.