A study of element geochemistry of mudstones of upper Ordovician Wufeng Formation and lower Silurian Longmaxi Formation in southern Daba Mountain

Element geochemistry plays an important role in judging sedimentary environments such as redox environment and bottom water condition during deposition process of sediments. A detailed element geochemistry study of mudstones from the upper Ordovician Wufeng Formation to lower Silurian Longmaxi Formation in southern Dabashan mountain is carried out in this paper.The results show that Linxiang Formation has a higher Al₂O₃ content than Wufeng Formation,and from bottom Wufeng Formation to upper Longmaxi Formation, the contents of Al₂O₃increase gradually.Both contents of Al₂O₃ and TiO₂ have a negative correlation with TOC. Redox-sensitive elements such as U, V and Mo are enriched, indicating an anoxic environment during the deposition of the sediments.Two dramatically enriched anomalies of U, V and Mo in different strata from different profiles show a sulfidic condition. The distributions of NASC-normalized REEs display slightly right-tilted patterns, which are enriched in LREEs and depleted in HREE sand have an obvious Eu negative anomaly.The Ce negative abnormalies of Wufeng Formation and Longmaxi Formation signify an anoxic environment in bottom water and the minor Eu positive abnormalies from Liziping profile probably imply hydrothermal activities during precipitating. Covariation of ratios of redox-sensitive elements such as Mo/TOC and Mo_EF-U_EF reveal that southern Daba Mountain during Ordovician-Silurian transition is generally in an open oceanic environment with weak stagnant oxygen depletion or medium stagnant anoxia. The sedimentary environment of study area at that time is mainly controlled by fluctuations of sea level and paleo-seafloor topography,locally influenced by oceanic hydrothermal currents from Qinling Ocean in the north.