Geological Environment Analysis And Evolution Characteristics Of Lijiang Lake Basin, Northwest Yunnan

Lijiang Basin is a Cenozoic fault basin located in Lijiang City,Northwest Yunnan. Lijiang Basin is adjacent to the "Jade Dragon Snow Mountain", which is famous in Western Yunnan.The Jade Dragon Snow Mountain hasthe lowest latitude mountain glaciersin the Northern Hemisphere.The Jinsha River flow through the Daju Basin to the northern of Lijiang Basin.Lijiang Basin is nearly north-southin direction, about 31 km long,the northern part is about 3-6 km in width,and 11 km in width in the south.Lijiang basin is in the central position of the Sichuan-Yunnan rhombic block structure,which develops lots of active faults.The research object of this paperis mainly about Lijiang Basin, Northwest Yunnan.Detailed field geological survey has been accomplished.The distribution range,sediments type and the mutual relationship between different sediment typesof quaternary sediments in thebasin are settled withparticular route geology survey and key point of geological investigation. The detailed information of the pivotal sediments is clear, according to several geological sections.The development of the basin evolution has been analyzed, in combination with field observation of primary faults. Achieved the following main conclusions:Sedimentary types and sediments distribution range of the Lijiang Basin are determined byanalysising the lithology, texture, structure and sedimentary environment of the sediments in the basin. The Sheshan Formationwhich is formed before the galacial epoch is mainly distributed around the Sheshan park in the southern basin.Sheshan Formationis a set of clay lacustrine deposition mixed with fine sand and gravel layer of lake and river sediments.The two phasesofmoraine of the Jade Dragon Snow Mountain distributes in the northern basin. Both phases of the moraineare all mountain glacier types,which mainly a set of melange gravelwith huge boulder.Basalt is the main ingredient of the gravel.Modern lacustrinesedimentaryis located in the southwest of the basin.The earlier distribution rangeof the lacustrinesedimentaryhas reached to the western Lijiang City and eastern piedmont region.The modernlacustrinesedimentary is mainly a set of limneticdeposits.Modern fluvial sedimentary distributes in the central basin and the north area, distribution range is constantly expanding southward. Modern fluvial sedimentaryis mainly a set offluvial gravel layer.We found thatthe Yulongshan Donglu fault is not a single normal fault nature,through field investigation on the surrounding basin fracture. Wild scratches show its coexistence with thrust and strike-slip properties.There is a new discovery in Heilongtan Park and southeast Lashi Lake.North west- south east direction of the structure has been found in those areas.That structure has clearly reflected in the Sheshan Formation.Normal faults which dip direction is northeast are developed in the Sheshan formation, southeastern Lashi Lake, which is also found in the Sheshan Formation.Synsedimentary faults are also found in Sheshan formation.Sheshan formation was affected by the NW-SE direction structure.Fold has been found in the field section.Its likely that the tectonic uplift and denudationof Sheshan formation wasbecause of the NW-SE structure.We found that Lijiang Basin is a pull-apart basin, by comprehensive survey of sediment morphology, deposition rate and the fault properties, fault compositon style of the faults surrounding Lijiang Basin.