SEDIMENTARY EVOLUTION OF A DAMMED PALEOLAKE IN THE MAOXIAN BASIN ON THE UPPER REACH OF MINJIANG RIVER, SICHUAN, CHINA

Studies of lithofacies and depositional systems had been carried out for the river terraces and the dammed paleolake in Maoxian basin on the upper reach of the Minjiang River. Based on field observation, the fluvial depositional system, fan-delta depositional system, and eolian depositional system were recognized in the terrace Ⅲ of the basin. They are composed of the lithofacies of Gms、Gm、Gg、Sh、Sg、Fl、Ls and Ps. The fan-delta depositional system could be further divided into 3 subfacies assemblages, namely, the pro-fan delta assemblage, fan delta front assemblage and fan delta plain assemblage. Changes from grain flow to gravity flow and then to clastic flow were observable in the section of fan-delta depositional system. Integrated with the previous research, it was suggested that the development of fan-delta depositional system may be linked to the dammed paleolake in the Maoxian basin. The appearance of the dammed lake during this period might be related to the activity of Maoxian-Wenchuan fault. Eolian loess and paleosoil were widely distributed in the area, suggesting that the climate was dry and cold then and frequently fluctuated in the area of Maoxian basin.