Study On Soft Sedimentary Deformation Structure Of Xigeda Formation,in Yongsheng County,Yunnan Province

The neotectonic movements in the middle reaches of the Jinsha River are active and the earthquakes occur frequently.Lacustrine sediments are commonly distributed on both sides of the river.According to the results of previous studies and my field observations,it is suggested that the age of the lacustrine strata may be Early-Middle Pleistocene.From the perspective of lithostratigraphic characteristics,this strata is classified into Xigeda formation.In this study,a large number of soft sedimentary deformation structures have been found in the riverside lacustrine sediments of Taoyuan Town area in the middle reaches of Jinsha River,with strong deformation and large scale.We focuses on the comprehensive analysis of six soft-sedimentary deformation profiles.And the soft sedimentary deformation structures mainly include rootless faults,liquefied clay lumps and folds.In the profiles of soft sedimentary deformation structural,there are medium and fine sand layers whose thickness is from thick to super thick.We can see the liquefaction of sand layer in the study profile,For example,there are a large number of clay debris and flow sand channels.The fault,clay block and fold structure of clay layer found in the profile are all related to the liquefied sand layer.The deformation structure among the profiles has no extension contrast,chaotic deformation characteristics and obvious stress direction,Based on the characteristics of sand liquefaction and clay deformation in the above profile,it is inferred that the deformation structure in the profile is mainly due to sand liquefaction.The liquefaction strength of sand layer determines the deformation degree of clay layer.In this paper,the possible trigger factors are analyzed based on the combination of the structural characteristics of soft sedimentary deformation,sedimentary environment and geological background of the area.According to the geomorphic location and sedimentary environment characteristics of the study area,combined with the geological background data of the study area,the possibility of triggering the deformation of the study area by collapse,lake wave,alluviation,proluvial event,volcanism,glaciation and freeze-thaw is excluded.Because of the active neotectonic movement and frequent earthquakes in the study area,and seismic are the main trigger factors for liquefaction.So it is considered that seismic action may be the main trigger factor for the strong liquefaction deformation in the study area.According to the previous studies,the relationship between the soft sedimentary deformationstructure,the liquefaction thickness and the seismic strength is discussed,the magnitude of this seismic probably reached also values of 7 or higher.