金沙江寨子村滑坡坝堰塞湖沉积及其对昔格达组地层成因的启示

Deposits of Zhaizicun Landslide-dammed Lake along Jinsha River and Its Implication for the Genesis of Xigeda Formation

金沙江干流右岸寨子村滑坡后缘与前缘高差638m,滑坡体积约2.5×108m3,曾诱发严重的滑坡堵江事件,左岸坡残留滑坡坝顶面高出江面118m,方量约180×104m3。寨子村滑坡坝堰塞湖沉积沿江连续分布长度46km,平均宽度2.6km,对金沙江干流及其支流的追踪使得堰塞湖沉积平面上呈现树枝状结构。堰塞湖沉积分布于1180～1500m标高之间,主要由纹层状粉土、粉质粘土、粘土及粉细砂构成,偶夹砂卵石层,以水平层理为主,粉细砂层中见有小型交错层理,湖相沉积特征显著,而树枝状平面分布格局进一步证实其形成于堰塞湖环境,属于典型的昔格达组地层。大部分昔格达组地层应形成于滑坡坝堰塞湖,堰塞湖成因模式可以很好地解释昔格达组地层的露头区平面形态、埋藏特征、与冲积砂卵石层之间的成生关系、对河流的高度依赖性、不同地区昔格达组地层之间的沉积规模差异及空间离散性。

Height difference between the crown and the toe of rupture surface of Zhaizicun landslide along the right bank of Jinsha River is 638 m,with about 2.5×108 m3 total rock displaced.The giant rock landslide complex caused serious landslide-damming and the crest of the residual landslide dam with a volume of 180×104 m3 on the left bank is 118 m above the river.Deposits of the landslide-dammed lake with a average width of 2.6 km stretch 46 km along Jinsha Rive and the outcrop boundary takes on a branchy structure.The lacustrine deposits between 1180~1500 m above sea level consist of horizontally laminated clay and silt and sandwich fluvial sand—gravel layers.Fine-sandy,ripple cross-laminated and cross-stratified beds show the existence of bottom currents in the landslide-dammed lake.Deposits of Zhaizicun landslide-dammed lake is a representative case of the Xigeda Formation.Deposits of landslide-dammed lakes compose 80% of the Xigeda Formation and landslide dammed lake genesis model for the Xigeda Formation may reasonably explain various its plane outcrop shapes,profile features,the genesis relationship between the Xigeda Formation and fluvial gravel beds,highly dependence on rivers,dimension difference and space discontinuity among different regions.