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 (U_EF and Mo_EF) 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.

Keywords Mineralogy, geochemistry, K/Pg boundary, Cretaceous/Paleogene, Shiranish/Aaliji