Mineralogical and Geochemical Evidence for Detecting K/Pg Boundary in Bade Section, Dohuk Area, Northern Iraq

다국어 초록 (Multilingual Abstract)

The Cretaceous/Paleogene boundary (K/Pg) event, marked by the mineralogical and geochemical evidence and environmental catastrophes, is a contentious topic in geology. The mineralogical and geochemical anomalies at the K/Pg boundary worldwide were caused by an extraterrestrial object impact and/or prolonged Deccan eruptions at the end of the Mesozoic. The field observations show that the proposed K/Pg layer lies within the marl layers and consists of an iron layer composed of black spherules and matrix (iron spherule-rich layer). The petrographic study suggests that the sedimentation just below and above the K/Pg occurred in low-energy water, which preserved the iron spherule-rich layer in the sediments. Moreover, the XRD results reveal that the Upper Cretaceous and Lower Paleogene layers are composed of calcite, quartz, muscovite, and anatase, while the proposed K/Pg boundary (spherules and matrix) is predominantly of goethite and trace amounts of barite, giving it the name goethite layer. Goethite dominance and a sharp drop in calcite inside the goethite layer indicate that this layer marks the K/Pg boundary. The geochemical results indicate that the platinum group elements (PGEs) may be undetectable at concentrations below 5 ppb in Iraqi K/Pg boundary sites. The REE pattern suggests that the Bade sediments share the provenance, and the Ce/Ce* ratios propose they were deposited from seawater with a slight detrital influx from neighboring continental sediments. The positive Eu anomaly (Eu/Eu*) is mostly attributed to plagioclase absence in the Bade sediments. The elevated levels of Fe2O3, Ni, Co, Cr, Sc, V, Zr, Th, U, Pb, As, S, Sb, Zn, and Cu with CaO depletion within the goethite layer suggest that this layer represents the K/Pg. Additionally, the lowest total REE (ΣREE) content and the highest levels of SO2 and SO2/MnO at the goethite layer attest to its boundary status. The sudden increase of authigenic redox-sensitive elements (element/Al) and enrichment factors of U and Mo (UEF and MoEF) indicates a change in redox conditions just within the goethite layer compared to the Upper Cretaceous and Lower Paleogene sediments, which means a rapid return to the oxygen conditions of the Upper Cretaceous after the K/Pg event. Furthermore, the sharp decline in the U/Mo ratio at the goethite layer supports the sulfidic-euxinic conditions that led to the development of pyrite. Based on this study's evidence, the goethite layer represents the K/Pg layer, and the Bade section is considered the perfect K/Pg boundary section in Iraq.