Quantitative Particle Size And Microtextural Analyses Of Glacial And Paraglacial Sediments On The Eastern Slope Of Mount Gongga

Glacial sediment in mountain areas could indicate the paleoclimate and depositional environment.In the paraglacial environment,however,discrimination of paraglacially reworked deposits from in situ glacigenic deposits has proven to be problematic.Mount Gongga is the largest area of modern glaciation and glacial and praglacial sediments are well-preserved in the valley,in the basin,and on the piedmont.Therefore,contention still exists regarding the depositional history of these paraglacial sediments(i.e.Moxi Platform).To resolve this debate,we apply particle size and quartz grain microtextures of quantitative techniques in an attempt to discriminate samples from each environment.Further assessment of particle-size and microtextural characteristics of glacial and paraglacial sediments may be most fruitful where sediments have been reworked over long distances.On the bais of our finding,the following conclusions can be drawn:(1)The glacier type and size could affect the particle size-distribution of their own till.The grain size data show that the till from temperate glacier contains more fine-fraction(medium sand,fine sand and clay)and less garvel than that of sub-continental type glacier,and the larger the size of the glacier,the higher the fine-fraction of till.Furthermore,the younger till,the higher the fine-fraction of till.We also analysis the particle size-distribution of glaciofluvial and glacial debris flow sediments,and we found that glaciofluvial sediment has higher sand fraction and lower gravel fraction than that of till and glacial debris flow,while till has higher gravel fraction and lower sand frcation than that of glacial debris flow and glaciofluvial sediments.(2)We asses the quartz grain microtextures of various glacial and paraglacial sediments.The results reveal that the quartz grains of temperate glacial till have experienced more intensive erosion processes and chemical dissolution processes,and lower chemical precipitation than that of sub-continental glacial till.In addition,the younger till has experienced more intensive erosion processes and chemical dissolution processes,and lower chemical precipitation of quart grain.Comparing with the quart gain microtextures of till,there are some different characteristic textures presenting on quartz grain of glaciofluvial and glacial debris flow sediments,e.g.no typical mechanical microtextures(stria and crushed deep hole)of glacial environments,and higher frequency of occurrence of conchoidal fracture and cleavage.(3)We compared the particle-size fraction,size parameter of different layer sedimnets of Moxi Platform with various paraglacial deposits on the eastern slolpe of Mount Gongga,we found that the basal and midle part of the Moxi Platform is close to MIS3 till,while the upper part of the Platform is similar to fluvial and glacial debris flow sediment.Furthermore,Cluster analysis was applied to sort different independent sediment into groups by quartz grain microtextures.The result indicates samples of the basal part of the Platform do cluster into MIS 3 till groups,middle part cluster into glacial debris flow groups,and the upper part into the fluvial seddiments,indicating a high degree of similarity between all samples.In combination with previous dating results,geomorphological charateristics,our further assessment of particle-size and microtextural,we consider that the the basal part of the Moxi Platform between Xinxin and the Moxi Hotel is correlative with the MIS3 till.The middle part of the Platform is glacial debris flow sediments occasionally mixed with fluvial sediments,and the reminder of the Platform is fluvial sediments.