Western Sichuan Litang Basin Pleistocene Stratum And Sedimentary Facies Analysis

Litang is located on the southeast margin of Qinhai-Tibet plateau, belonging to the west region of Sichuan province. It is a basin in the north of Sichuan-Yunnan rhombus-shape block, and also is the latest tectonic change unit surrounded by various NW series active faults and ruptures in nearly SN direction, such as Garze-Litang fault zone, Xianshuihe fault zone, Anninghe fault zone, Zemuhe fault zone, Xiaojiang fault zone, Red river fault zone and Jinsha river fault zone. Tectonic activity in Litang fault zone is intense and strong since quaternary period.Based on a study project “Comprehensive study on the determination of major active faults and active structural system in the southeast margin of Qinhai-Tibet plateau”, this thesis does research into Litang region, especially on a basin within Sichuan-Yunnan block(sub-block in Western Sichuan Province), and mainly focuses on the study of Quaternary lacustrine sediments of Litang area. The paper conducts a research on lacustrine deposits of Litang basin according to the materials of field investigation and survey. Obviously, it’s also based on previous studies of it, as well as taking the analysis of stratums and sedimentary facies as its main contents, combining multiple profile measurements to analysis, adopting ESR technique to sample till dating and so on. Finally, the research methods and theories contain sedimentology, geomorphologyand stratigraphy. Cunge village and Benge village are the two places discussed in the paper, which respectively located in the west by north and the south by east of Litang town. By comparing the two stratigraphic divisions, the characteristics of sedimentary facies are analyzed in the paper. Followings are the results and progress we made through the survey:(1)Integrating previous research results, dating data and formation conditions of different profiles discussed in the paper, the outcome of the research can be concluded that stratums of Cunge Village and Benge Village are mainly middle pleistocene in age, and lake sediments of them are mainly composed of clay, sand, sand and gravel. According to their lithologic characters, sedimentary facies of the discussed stratums can be classified into shore lake facies, shallow lake facies and deep lacustrine facies with alternative evolvement in them over the time.(2)Sedimentary sequences in Benge village are usually shore lake facies, especially those lie in the CG3 profile where have more gravel layers with the thickness of 6.5m and thin clay layers. All these show that Cunge village is more close to piedmont and at the edge of paleo-lake sedimentary in late middle Pleistocene. By comparing the stratums, the result we conclude is more and more lacustrine deposits are found from west to east. While sedimentary sequences in Benge village are usually deep lake facies and shallow lake facies, and they have fairly thick clay pan, which resulting in more close to the center of the basin.(3) BG3 profile can be divided into five sedimentary cycles. From the early Pleistocene to early Late Pleistocene, ancient lake has experienced the demise or decline during the five tests.