The Formation Age And Environmental Indications Of Longjie Silt Stratum In Yuanmou County Of Yunnan Province

The Longjie silt stratum is a series of silt, silty clay and clay horizontal layers, whose color contains grayblack, luidity and gray. It distributed along the valley of Jinshajiang River from Sanduizi of Panzhihua City to Baimakou of Wuding County, and well-developed in the Longjie basin in Yunnan Province. The formation age and process of the Longjie silt stratum will be importance in the research of the evolutionary process of Jingshajiang River and the regional tectonic activities.Based on the systemic field survey, the Longjie, Bainiwan, Jiangtou and Baimakou natural stratigraphic profiles have been collected, and these four profiles is about116m thick altogether. The Longjie profile is40.6m thick, it is showed the parallel unconformable contact relationship between the top layer and the overlying fluvial sand gravel. The Bainiwan profile is40m thick and capped by the red regolith, there exited an angular unconformity between the Longjie silt stratum and its underlying Yuanmou Formation. The Jiangtou profile is29.4m thick and capped by the red regolith, Baimakou profile is6m thich and covered by the fluvial sand gravel.The relatively intact stratigraphic sequence has been acquired by the Longjie drill. Including the overlying fluvial sand gravel and the underlying coarse sand, the total thickness is101.18m, and the Longjie silt stratum is in unconformable contact with the overlying and underlying layers.Through the study of the magnetostratigraphy, OSL and14C dating work, it is showed that the formation age of Longjie silt stratum is about151-30ka BP.The analysis of grain size indicated that the Longjie silt stratum was a set of lacustrine sediment, and its sedimentary environment could be mainly divided into6stages, which include the forming stage (151■148.5ka BP), lake expansion stage (148.5～132ka BP), deep lake stage (132±91.5ka BP), shrinking stage (91.5-67.2ka BP), shallow lake stage (67.2±39.4ka) and transformation stage (39.4±30ka BP).The regional climate and environment character form the151ka BP could be roughly divided into5periods, based on the study of climatic proxies which include chromaticity and carbon content. These5periods include the warm wet period (151±132ka BP), warmer wetter period (132～91.5ka BP), cold wet period (91.5～67.2ka BP), cold dry period (67.2±39.4ka BP) and warm dry period (39.4±30ka BP). At the same time, through the analysis of regional climatic and environmental change process, the climatic proxies also indicated some climatic events, such as the warm dry event which occurred at about141.2ka BP and the relatively warm wet events that occurred at60±63,50and44ka BP. The climatic change of research area shows good synchronism with the global change in generally, but it also showed some regional differences:the rapid cold at116.5ka BP, the warmer at100±95ka BP and70±59ka BP.