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
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摘要: 通过沉积及元素地球化学分析，重庆城口地区燕麦剖面临湘组自底部小瘤状灰岩向上沉积岩性依次为褐黄色泥灰岩、暗灰绿色泥岩，逐渐过渡为五峰组—龙马溪组深灰色硅质泥岩。水体深度呈现出由深变浅再变深的特征，向上总体水体还原性增强、氧化性减弱，在临湘组中上部存在微弱的氧化性增强的特征，指示区域构造短暂抬升，水体变浅，为宜昌上升运动的早期响应。古生产力指标显示氧化条件较好的观音桥段以及临湘组下部（层5）具有更高的生物生产率，与冰期气候变冷海平面下降导致上升洋流盛行以及水体氧化还原条件改善有关。燕麦剖面临湘组—五峰组—龙马溪组倾向于活动型大陆岛弧环境沉积，自下向上陆源组分含量逐渐降低，火山碎屑组分含量逐渐升高。剖面上部五峰组和龙马溪组底部硅质泥岩热水沉积地球化学特征较为明显，可能受到深部热液活动的影响。Abstract: 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.
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