MAGNETOSTRATIGRAPHIC STUDY OF LAKE SEDIMENTS IN JIAOCHANG, MAOXIAN COUNTY, SICHUAN

The Jiaochang section in Maoxian County,Sichuan Province,is 101.1 m thick,in which lacustrine sediments are 98.0 m thick,consisting of light yellow-gray silt,clayey silt and silty clay.Paleomagnetic measurements indicate that all the samples show normal polarity except two samples that have negative magnetic declination.The magnetic polarity belongs to Brunhes normal polarity and the B/M boundary is absent.According to the lithology,lithofacies and landform location,the strata can be correlated with the Longjie Formation in the Jinsha River valley,Yunnan.So it is inferred that the sediments began deposition at 100-120 ka BP.In addition,the optically stimulated luminescence(OSL) age is 16.7±1.2 ka BP at 4.0 m below the surface of the sediments,so it is inferred that the deposition ended at 10 ka BP according to calculation.The deposition of the lake sediments lasted for(100,000) years.The section is one of the exposed thickest sections since the last glaciation(deglaciation) in the Minjiang River valley.