Paleoenvironment Implications and Genesis of Siderite in the Lower Cretaceous Nantun Formation, Hailar Basin, China

Summary In this work, the petrological characteristics, paleoenvironmental significance and genetic mechanism of siderite are studied based on core description, thin section identification, major and trace elements testing, and carbon and oxygen isotopic analysis. There are 15 siderite layers in the the Mbr 1 of Nantun black mudstone in the study area. Macroscopically, siderite beds are similar in appearance to mudstone except for the high density. Microscopically, the siderite beds mainly contain siderite, clay, quartz, calcite, and feldspar, and some contain a small amount of iron dolomite and pyrite. And they are produced in micro-powder crystals, layered form with calcite metasomatism, and associated with quartz and calcite. Siderite beds mainly contain oxides such as FeCO3, SiO2, CaCO3, Al2O3, the paleoenvironment restore results show that it was poor oxygen and reduction, fresh-brackish water during the depositional period of the Mbr 1 of Nantun Fm. Additionally, organic matter and inorganic carbonate in the sediments provide abundant CO32− for the formation of siderite. In this environment, siderite beds were formed when the reduced Fe2+ concentration reaches saturation.