Sedimentation and Geochemistry of Upper Ordovician-Lower Silurian from Northern Chongqing, and Its Environmental Evolution: A Study of Linxiang Formation to Longmaxi Formation of Yanmai Profile from Chengkou Region
Brown-yellow marl and grey-green mudstone of Linxiang Formation above nodular limestone develops upwards successively in Chengkou, Chongqing. Grey-green mudstone changes into dark silica mudstone of Wufeng Formation-Longmaxi Formation gradually. Changes from deep to shallow, then to deep of water depth are unraveled. Much anoxic and less oxic in redox condition upwards the Yanmai profile from Linxiang Formation to Wufeng Formation-Longmaxi Formation in general is indicated with the help of element geochemical analysis. A weak enhanced oxidation exists in the middle-upper portion of Linxiang Formation, implying structural uplift of Yichang movement. Higher paleoproductivity are seen in Guanyinqiao member and Layer 5 of Linxiang Formation, which are much oxic relatively, corresponding to the ascending current and the sea-level falling at that time, which are caused by the global gradual cooling down climate. The tectonic setting of Linxiang Formation-Longmaxi Formation inclines to an active continental arc environment. Less terrigenous constituent and more volcanic clastic component upwards Yanmai profile is indicated. The silica mudstone of Wufeng Formation and Longmaxi Formation shows apparent geochemical characteristics of hot water deposition, which may be influenced by deep hydrothermal activity.