Sedimentary Facies and Stable Isotope Evidence of the Middle Miocene Fatha Formation of the Zagros Foreland Basin, Kurdistan Region, NE-Iraq: Insights into the Icehouse-Warmer Transition

Integrated sedimentary facies, stable carbon, and oxygen isotope analyses were conducted on the Fatha Formation in the Kurdistan Region, Northeastern Iraq, in order to investigate the depositional environments and paleoclimate during an icehouse-to- greenhouse transition. The analysis of stable δ¹³C and δ¹⁸O isotopes was applied for the first time to reconstruct the paleoclimate. The formation mainly consists of alternations between claystone, marl and carbonate rocks with some gypsum lens intercalations. Thirteen microfacies were identified within the carbonate lithofacies, which are stacked into three facies associations: representing peritidal (FA1), characterized by supratidal sabkha to intertidal settings under saline conditions; lagoonal (FA2), representing sedimentation in restricted, low- to moderate-energy saline lagoons; and open lagoonal environments (FA3) which comprise marl lithofacies and show variations from lagoon to open marine based on fauna content. Claystone was influenced by climate-controlled oxidation and siliciclastic input, shaped by tectonics during the Zagros orogeny. Nodules and laminated gypsum within the claystone indicate a sabkha environment. Additionally, the occurrence of sandstone near the top of the formation suggests a continental, fluvial setting. The δ¹³C data are positive, and the δ¹⁸O data are negative, indicating that the paleoclimate during deposition was warm. Variability in δ¹³C values reflects fluctuating organic carbon inputs, and marine ecosystems had more nutrients available, which possibly led to the growth of marine organisms. The oxygen isotope signatures reflect episodic increases in evaporation rates, causing δ¹⁸O depletion, and are aligned with the global Middle Miocene Climatic Optimum warming trends. The deposition occurred across various environments, ranging from shallow marine to continental, and was influenced by sea-level fluctuations and a warmer climate during the Middle Miocene Climate Optimum.