FORMATION AND GEOLOGICAL SETTING OF THE ACCUMULATIONS IN JINPINGZI REACH OF THE JINSHAJIANG RIVER
金沙江河谷金坪子堆积体成因及其地质意义*

The complex deposit in the valley of Jinshajiang River at Jinpingzi(Yunnan Province, Southwest China) is located at the southeastern margin of the Tibetan Plateau as well as the northern margin of the Yunnan Plateau. The total volume of the complex deposit is estimated to be 7×108m3 and several geomorphic units can be outlined for the deposit body and its vicinity. Based on the field investigation and available data from several drills, silos and stulms implemented on different parts of the deposit body, we carry out a comprehensive studies, including geomorphology, sedimentology and chronology, on the main features and the genesis of the complex deposit. Engineering works expose alternating layers of colluvia and alluvia within the complex deposit. Thermoluminescence(TL) dating provides a temporal frame for the process of the river incision and fluvial sedimentation in the past. The samples in the bottom and in the top of the deposit give TL ages of ca.197kaB.P. and 137kaB.P. respectively, implying that the forming of this deposit took about 60ka.Drills reveal the basal morphology of the complex deposit. The profile shows that the floor altitude varies from 810m in the east, 710m in the middle to 1010m in the west. The complex deposit infilled an ancient deep trough at Jinpingzi. There are many nickpoints along the Jinshajiang River, and Jinpingzi is situated downstream of a nickpoint with an intensive downcutting resulting in the formation of its deep trough. When the steep slopes of the deep trough were destabilized, collapse would have happened and detritus subsequently filled the deep trough. When the collapse was of large scale, the Jinshajiang River might have been dammed. A new river channel would have developed above the colluvia and incised the east bank, with new fluvial sediments formed, which resulted in the alternating layers of colluvia and alluvia of the deep trough.