在甘肃武都县城北第四纪地层剖面中,马兰黄土之下发育有一套以大量碎屑和部分粘土组成的混杂堆积体。其形成时的原始地貌已不复存在,成因也不明。本文在研究其沉积特征和周围地质、地貌环境的基础上,判定其属于崩滑堆积体,是基岩块体 (夹部分粘土 )高速崩塌滑动、碰撞分解堆积而成。堆积层具有上下不同的发生“层序”。作者将典型的崩滑堆积体的沉积“层序”划分为五层:A后期覆盖层或侵蚀面;B崩解—碰撞带层;C崩解—滑移带层;D滑移带层;E下伏地层。各发生层有不同的沉积特征,反映出高速运动块体经过内部碰撞分解后,上下层中存在动力过程和沉积过程的分异。作者研究了武都古崩滑体五个剖面中的粘土含量、粒径、砾石组构、沉积结构和构造的特点,并通过堆积物的沉积特征所反映的运动和动力学特性,结合周围地质、地貌环境,判定该混杂堆积体是在Q13 时期,武都城北部高耸的以变质石灰组成的基岩陡壁在暴雨的“触发”下,发生解体,形成固体碎屑流块体,由北向南高速运动堆积而成。后期由于粘土和水的加入,使崩滑体运动向泥石流体转化。
In the profile of quaternary stratum, near Northern Wudu county seat, Gansu province, there was a set of diamicton, covering by the loess, containing a lot of clastic fragment and partly clay. The landform, in which when it occurred, had been eroded and destroyed, and the cause for this diamicton is unclear. After studying on the diamicton 's characteristics and geological and geomorphologic environment, we identified this diamicton as the rock avalanche deposit. We also found that the deposit had the "sequence" in the vertical section, and divided the typical rock avalanche (containing partly soil) deposit into five "layer sequence": covers or erosive surface (A); colliding separating layer (B); separating sliding layer(C); sliding layer (D); and beneath layer (E). Each layer has special depositional characteristics. The layers indicated that the rock avalanches are interstratified when they are moving and accumulating, because of the differences in dynamical process and depositional process. After studying the soil contain, grain size, gravel fabrication, deposit structure and texture which can show the characteristics of dynamics and movement, in the five deposit section, and studying the geological and geomorphologic environment, we deduce, that, the paleo rock avalanche, in Wudu, occurred in Q31 period and caused by rainstorm. The rock blocks had been detached from the high cliff consisting of metamorphic limestone, in north of Wudou county seat, fall down, separated into piece, and moved to south, and piled up. We also found that the rock avalanche gradually changed into debris flow, because of mixing with a lot of mud and water.