Geochemical analysis of the Late Ordovician–early Silurian black shales from the Xindi No.2 well in the Upper Yangtze area, South China: Implications for provenance, palaeoclimate and palaeoenvironmental change

Abstract The Ordovician-Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician- early Silurian interval, we conducted a mineralogical and geochemical analysis of three formations (Wufeng, Guanyinqiao and Longmaxi formations) encountered in the Xindi No.2 well. The ternary diagram of Al2O3-(CaO*+Na2O)-K2O and the bivariate diagram of La/Th-Hf and La/Yb-∑REEs indicate that the provenance of these formations is mainly from felsic igneous rocks. The La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10 ternary diagrams and two multidimensional diagrams reveal that the samples in the study area mainly originated from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values indicate that the intensity of chemical weathering in the source area was weak to moderate. The CIA values in the lower part of the Wufeng Formation and Longmaxi Formation are relatively high (67.48-73.57), indicating that the climate was warm and humid during deposition. In contrast, the CIA value declined rapidly from the upper part of the Wufeng Formation (Metabolograptus.extraordinarius zone) to the Guanyinqiao Formation (Hirnantian stage; 58.30-64.66). During this period, the overall climate was cold and dry, but the data fluctuations show that the cold and dry climate was interrupted by a transient warm and humid climate. Palaeoredox indices (Mo concentrations and MoEF/UEF; U/Th, V/Cr, Ni/Co, and V/V+Ni values) fluctuate during the Late Ordovician- early Silurian, which were distinguished into four distinct parts, indicating two cycles of water column euxinia. The first cycle occurred during the accumulation of the Wufeng Formation, with bottom waters evolving from oxic-suboxic (part Ⅰ) to suboxic-anoxic (part Ⅱ). Regarding the period of accumulation of the Guanyinqiao Formation (part Ⅲ), most samples show low redox-sensitive trace element concentrations, suggesting oxic-suboxic conditions. The water column transitioned from oxic to euxinic in the late Hirnantian (at the base of the Longmaxi Formation). Strong euxinic bottom water persisted throughout the deposition of the Longmaxi Formation (part Ⅳ). Finally, the δ13Corg isotopic data constrain the timing of the Late Ordovician mass extinction and the evolution of temporal changes in the climatic and ocean redox conditions. Climatic fluctuations, coupled with oceanic anoxia, were likely responsible for the gradual end of the Late Ordovician biotic crisis.