The Depositional Characteristics and Generative Environment of Paleo-rock Avalanche in Wudu County,Gansu Province

Key words: 
• Middle Triassic /
• sedimentary fades and palaogengraphy /
• South China /
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Abstract: 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.