摘要
The Xiuyan crater is a simple bowl-shaped crater 1 800 m in diameter. Through a core drilling at the center of the crater, a small amount of siderite breccia was found at the top of the impact breccia unit that is covered by Q uaternary lacustrine sediments of 107 m thick. The siderite breccia is composed of microcrystalline siderite and clasts of rocks and minerals. Carbon isotope analyses of the siderite exhibit extremely high δ13C values. Positive δ13C excursions in siderite are mainly explained by carbon isotopic discrimination between carbonate and organic carbon due to methanogenic bacteria decomposing organic matter in a reducing environment. The siderite should be formed by deposition. The deposited siderite cemented clasts of rocks and minerals in impact breccias, which formed the siderite breccia.