The Litho/Sedimentary Facies Analysis of Lagar Formation,Xainza Area, Tibet

Key words: 
• rock assemblage /
• depositional environments /
• Lagar Formation /
• Early Permian /
• Xainza area /
• Tibet

Abstract:  Lagar Formation was developed in Lhasa Block which located between Himalaya belt south and Qiangtang Block north. It represents the glaciomarine deposits related to the Late CarboniferousEarly Permian Gondwana glaciation. However, its litho/sedimentary facies have never been clearly understood. In this paper, the litho/sedimentary facies of the Lagar Formation were analyzed based on the ZhakangMujiucuo section in Xainza County, central Tibet.
   The lithology of Lagar can be distinguished into five types of rocks assemblage. Assemblage Ⅰ could be supposed to subaqueous ice fan which compose of pebbly coarse grained sandstonesandy shoulder (diamictite), and fine pebbly sandstone gradually. Assemblage Ⅱ is composed of finegrained sandstone developed bimodal crossbedding and muds, and could be regarded as channel and interchannel. Having no bedding, extremely poor sorted, and developed complex components of grave, the rocks of Assemblage Ⅲ was supposed by "tillites". The mainly lithology of Assemblage Ⅳ is dark or grey green silts or calsiltstone, interbeded thick pebbly coarsegrained sandstone and finegrained sandstone. The remarkable character of Assemblage Ⅳ is developed soft sediment deformation in grey green finegrained sandstone, and this should to attribute to sediment gravity flows. Being developed coarse grave and pebbly sandstone in the dark siltstone, the Assemblage Ⅴ also can be attributed to channel extending so far. Assemblage Ⅵ composed of shale laminated finegrained quartz sandstone which is well roundness and sorted should be supposed by coast and shallow sea.
   According to above, depositional environment of Lagar Formation is proximal glaciomarine environment which identification factor lies in whether the depositional site is proximal or distal to a grounded ice margin (or grounding line of an ice shelf or tongue). Whether ice sheet can be form ice wall near coast tide water depend on the thermal regime of ice sheet and stability of inner ice sheet. The thermal condition at an ice base can be divided into thawed (and thawing) zones where basal ice slides over the substrate producing basal meltwaters that are integrated into a well developed drainage network, and frozen and freezing zones where no free meltwaters available. Ice shelves are also inherently unstable and during times of decreasing mass balance or rising sea level may rapidly disintegrate and revert to a grounded tidewater ice margin that terminates as a steep ice wall in shallower water along the basin margin.
   According to present literature and depositional models, Early Permian sedimentary facies belts of Indian Gondwana to Tethyan Himalaya show continental tillitesglaciofluvialproximal glaciomarinedistal icerafted debris successively from south to north. And it is said that Lhasa and South Qiangtang block also has been considered distal or superdistal glaciomarine environment.
   Sedimentation  rates in marine environments around thawed/thawing ice margins are much higher than around cold zones. The factors influenced the glacier thermal regime relate to global warming, drift to low latitude and warm currents influence. Global climate became warm with Sakmarian' deglaciation. But at the same time, Lhasa block has been in tectonic activities similar to adjacent Tengchong block and Sibumasu block where rifting from north margin of Austria.