Sedimentary Characteristics And Tectonic Setting Of Laga Formation In Luocang Area,Tibet

This paper is based on field investigation,data sorting and analysis indoors about the Late Carboniferous-Early Permian Laga Formation in Luocang area,Tibet.Through this work,we have gained some new understanding about the stratum distribution situation,age assignment and sedimentary characteristics of the Late Carboniferous-Early Permian Laga formation in Luocang area.The Late Carboniferous-Early Permian Laga formation in study area belongs to Gangdise-Tengchong Stratigraphic Region.The Laga formation in study area is divided into 2lithologic section after a comprehensive research involving this study and previous work.The first section of Laga Formation?C₂-P₁l¹?:mainly yellow-gray medium to thick bedded gravel-bearing arenose medium-fine grained lithic quartz sandstone,glutenite,with thin intercalary strata of dark gray carbon containing silty slate,its middle and upper part is interbeds of different thickness,which is made up of gray medium or thin bedded quartz sandstone and yellow-gray thin bedded siltstone.There are a lot of coarse gravel and boulders in sandstone or slate layer,and the gravel have the characteristics of glacial boulder.The first section of Laga Formation is about1230.36m thick.The second section of Laga Formation?C₂-P₁l²?:mainly dark gray very thin carbon containing silty slate or interbedded rock association of different thickness,which is made up of gray medium or thin bedded quartz siltstone,quartz fine sandstone and dark gray very thin carbon containing silty slate.There are middle to thick bedded medium-fine grain lithic quartz sandstone,fine gravel-bearing medium-fine lithic quartz sandstone in some areas.The second section of Laga Formation is thicker than 867.79m.The age of the Laga Formation was dated to late Carboniferous epoch to Early Permian Epoch according to the fossil picked up in study area during fieldwork,Agishanoceras sp.and Early Permian Epoch Mesogondolella bisselli,and combined strip of late Carboniferous epoch to Early Permian Epoch brachiopoda Cancrinella cancriniformis-Spiriferella-Eomarginifera.The Laga Formation is marine sedimentary,it can be divided into 2 facies and 3subfacies according to the characteristics of rock associations and fossils in the study area.The 2 facies are shoreland facies and ice-shelf facies.The 3 subfacies are nearshore subfacies,interior ice-shelf subfacies and outer ice-shelf subfacies.The first section of Laga Formation?C₂-P₁l¹?is nearshore subfacies and interior ice-shelf subfacies,and the second section of Laga Formation?C₂-P₁l²?is outer ice-shelf subfacies.We analyzed the mineral composition and geochemical data of the Laga Formation sandstones collected in study area.The result shows that the tectonic setting of the Laga Formation source is recycled orogenic belt.Some of the source of the Laga Formation is provenance of blocks.The material sources of the Laga Formation is mainly recycling orogeny and blocks.Laga formation sandstone samples fall mainly in the passive continental margin area,part of the sandstone samples fall within the active continental margin area.These indicate,in the period of the Laga Formation,crustal movement had enhanced in gradually from the passive continental margin to active continental margin tectonic setting.