| The Baoshan Basin in Yunnan is located on the southeastern margin of the Qinghai-Tibet Plateau,between the suture zone of the Nujiang and Lancang rivers,and is a gas-bearing basin in western Yunnan.The Quaternary period in Baoshan Basin has been little studied,and there is no systematic knowledge of the Quaternary stratigraphic structure and depositional environment.Based on the analysis of the lithology,petrography and sedimentary cycles of the boreholes in Baoshan Basin,we have determined the stratigraphic basis and defined the Quaternary stratigraphic framework in Baoshan Basin;based on the indication of clay minerals and the analysis of the lithology and petrography of the boreholes in the comprehensive study area,we have obtained an understanding of the depositional environment and evolution of the Quaternary in Baoshan Basin,and have used various means to investigate and verify the direction of the Quaternary source.The following insights were obtained in this study.(1)The stratigraphic division reveals that the main genetic type of Lower Pleistocene sediments in Baoshan Basin is lacustrine;the main genetic type of Middle Pleistocene sediments is alluvial,lacustrine,alluvial and alluvial;the Upper Pleistocene is mainly lacustrine,with lacustrine accumulation and alluvial deposits exposed in the borehole near the mountain side;the main genetic type of Holocene sediments is alluvial,alluvial,lacustrine and alluvial.(2)Using sample testing techniques to obtain the clay mineral class and content of representative holes,indicating the Quaternary sedimentary environment and paleoclimatic fluctuations in Baoshan Basin.The Early Pleistocene sedimentary environment was stable,with little paleoclimatic and hydrodynamic changes and stable supply of sediment sources;the Late Pleistocene sedimentary environment was generally stable,but the sedimentary environment and source conditions changed in comparison with the previous period;the Middle Pleistocene and Holocene sedimentary environment was very turbulent,with many changes in paleoclimatic conditions and drastic changes in source supply conditions.(3)The analysis of basin type and gravelly ratio reveals that the source of the Quaternary sediment in Baoshan Basin comes from the Cambrian and Ordovician stratigraphic distribution areas on both sides of the uplift on the west side of the basin and the Puyun area on the west side adjacent to Baoshan Basin.(4)The early Pleistocene sedimentary environment of Baoshan Basin is stable and dominated by the lake phase;the middle Pleistocene period is presumed to be influenced by the Kun-Huang movement with a large area of lake receding and developing fluvial and alluvial fan phases,the late sedimentary environment is stable and the lake surface begins to expand;until the late Pleistocene period the lake surface area expands again and the sedimentary center moves north.In the Holocene period,the lake was completely withdrawn from the basin,and a set of fluvial,alluvial fan and floodplain phases were formed to cover the surface of the Pleistocene. |
