Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

During the Late Ordovician-Early Silurian period, Gondwanan glaciation has affected around the world. To infer the geologic record of the glaciation across the Ordovician-Silurian transition in South China, we measured the petrological and geochemical characteristics of 60 samples of Wufeng-Longmaxi Formations taken from four sections and Huadi no. 1 well in the Sichuan Basin. Petrologically, the Guanyinqiao Formation is composed of argillaceous sandstone, argillaceous siltstone, silty bioclastic limestone and silty limestone, combing with their sedimentary structures and fossils, which verifies a shallow water environment between the two sets of black shales in the Wufeng and Longmaxi Formations. Geochemically, the Chemical Index of Alteration (CIA) and Sr/Cu ratios indicate that palaeoclimate transformed from warm to cold and then to warm again during the Late Ordovician-Early Silurian period. The low CIA values of Guanyinqiao Formation, suggesting a short-cold climate which is consistent with Gondwana glaciation in Hirnantian period. In addition, the fluctuations of V/(V + Ni), V/Cr, V/Sc, Sr/Ba and Zr/Rb ratios from Wufeng-Longmaxi Formations implied that sea water had experienced an anoxic-oxic-anoxic process, the paleosalinity changed from sea water to brackish-marine and then to sea water, and the hydrodynamic condition had shown low-high-low characteristic. All these results indicate that because of the end Ordovician Glaciation, the area of the ice sheet increased, the climate changed and sea level fell during the Hirnantian period.