The Xiuyan crater is a simple bowl-shaped crater 1800 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 Quaternary 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 (+13.76‰ versus V-PDB). The age of siderite formation is about 37000 years ago, which is later than that of lacustrine sediments (39000-50000 years ago) and that of the impact event (50000 years ago). 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.%岫岩陨石坑直径1.8 km,是一个简单碗形坑.通过在陨石坑中心实施的岩芯钻探,在被厚达107 m第四系湖相沉积物覆盖的撞击角砾岩单元顶部位置,发现少量“菱铁矿角砾岩”.这种菱铁矿角砾岩由菱铁矿微晶和矿物岩石碎屑组成.全岩碳同位素分析显示出较高的δδ13C异常,平均高达+13.76‰(V-PDB标准).菱铁矿形成时间约为37 ka,晚于撞击成坑事件(50 ka),也晚于湖泊相沉积物的沉积年龄(39～50 ka).在还原环境下,细菌分解有机质形成甲烷引起的碳同位素分馏是造成菱铁矿δ13C显著正异常的主要原因.显然,这些菱铁矿属于沉积成因.沉淀的菱铁矿胶结岩石和矿物碎屑形成菱铁矿角砾岩.
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