湘西北杨家坪和鄂西峡东两条剖面的震旦寒武系界线附近均发育一层厚几厘米至几十厘米的白色粘土层，该粘土层之下普遍发育有小壳化石，在粘土层之上则未发现。地球化学分析结果显示，这一粘土层上的碳同位素及Ｉｒ，Ｏｓ，Ａｕ等稀有元素含量有明显变化。认为这是外星撞击事件引起的，它直接造成了小壳化石的绝灭。以事件地层学观点讨论了该区震旦寒武系界线的划分方案 A layer of white clay about a few centimeters to tens of centimeters thick is developed in the vicinity of the Sinian Cambrian boundary between the Yangjiaping and the western Hubei sides of the western Hunan Province. There are widespread small shell fossils beneath the clay layer, Not found above. The results of geochemical analysis show that the carbon isotopes and the contents of rare elements such as Ir, Os and Au in this clay layer have obviously changed. Think this is caused by extraterrestrial impact events, which directly caused the extinction of small shell fossils. The stratigraphic division of the Sinian Cambrian boundary in this area was discussed from the perspective of event stratigraphy