Late Quaternary Glaciation Successions and Their Dating on the Eastern Piedmont of Kongbugang Peak in the Eastern Himalayas
WU Zhong-hai1,ZHANG Yong-shuang1,ZHAO Xi-tao2,MA Zhi-bang2,YE Pei-sheng(1.Institute of Geomechanics,CAGS,Beijing 100081,China;2.Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029)
Glacial successions on the eastern piedmont of Kongbugang Peak in the eastern Himalayas,located in the northwestern Cona county of Tibet and adjacent to north-trending Cona-Nariyong Co graben to east,are reassessed and preliminary glacial chronologies are presented for the region.Based on field investigation to the associated glacial landforms and detailed analysis on remote images,the glacial landforms are divided into four sets of tillscorresponding with so-called Nyanyxungla Ice Age,Jilongshi Ice Age,Ronbushi Ice Age and Holocene glaciations,respectively,from oldest to youngest in the area in the light of morphostratigraphy and relative weathering.The oldestGlacial Stage(orthe first tills)is represented by subdued and rounded moraine ridgesor platforms that contain boulders commonly exceeding several meters in diameter.These usually occur on high surfaces(about 4 900~5 000 m a.s.l.) above the main valleys and often are best seen on the interfluves between the lateral moraines of the second tills along the eastern piedmont of Kongbugang Peak,and deeply weathered on thetop of moraines platform.Becausethe tills have been dissected and partly overlaid by younger moraines,the reserved parts of the oldest tills are very limited in region,but it is believedthat the Glaciation maybe the largest in extentin light of evidence for piedmont glaciers,whichfill the whole Cona-Nariyong Co basin and joint with the glaciersfrom eastern mountains of the Cona-Nariyong Co basin.The second tills arerepresented by a few end moraines extendinginto Cona-Nariyong Co basin at an altitude about 4 800 m a.s.lin north of the basin,and some hummock moraines of piedmont glaciers at an altitude about 4 700~4 600 m a.s.l in the central basin.They extend to 10~11 km far from the present ridge of Kongbugang Peak and represent the last most prominent glaciation in the region.The boulders on these moraines are generally 1~3 m in diameter;they are limitedly weathered on surface.The tills may be divided into two sets of moraines on basis of the weathering and end moraines landform around Yaba Co in southern basin.The older moraines,extendingabout 4 600 m a.s.l.,have deeper weathering layer and are more subdued on the top of tills;the younger end moraines about 4 700 m a.s.l.have slighter weathering and few sharp glacial landform.The third tills are represented by aseries of more sharp,crested moraines,overlying on the second tills,within 4~6 kilometers of the presentridge of Kongbugang Peakand only haveslight weathering and relatively fresh meter-size boulders on their surfaces.The youngest tills consist of few lateral-frontal moraines within 1~2 km of present ice margin,and the ice-dammed lakes often occur behind the end moraines.The till identification is easieron remote image,because of the increasing albedo resulted from scarcely weathering.To constrainthe dating of glaciations,samples for U-series and thermo-luminescence(TL)dating were taken in the Cona-Nariyong Co basin.Eight samplesof calcareous sands and coats for U-series and coarse-fine grained sands for OSL dating wererespectively collected from natural and artificial exposures either within or associated with the moraines(see Fig.1 and Table 1).The four dating results from the second tills show that the last most remarkable glacial advance occurred in marine isotope stage(MIS) 6.The older and younger moraines of the tills maybe relate with MIS 6.4 and MIS 6.2,respectively.The results indicate that the first moraines are older than MIS 6,but no exactly datingfor the tills.Two TL datingfrom youngest fluvioglacial terrace related with the third tills indicate the glaciation occurred in MIS 2,maybe involvingMIS 4.One U-series age result from lacustrine of Nariyong Co showsthat the highest terrace of the lake was at least formed in MIS 4 and the result is coincide with the evidence for the highstand paleo-shorelines dissecting the second tills and indicatesthe lake was resultedfrom the glaciers dammed outlets of basin during penultimate glaciations.The fresh and youngest tills were resulted from Holocene glaciations.The researchresults suggest that glacial advances are related with cold stage of marine isotope stage,but the tendency of extent change of glaciers was gradually decreased since MIS 6 and the most prominent change of glacier extent occurred in MIS 5 in the region.
【Fund】： 国家自然科学基金项目(40501006);; 中国地质科学院地质力学研究所基本科研业务费项目(DZLXJK200706);; 科技部国际合作项目(2006DFA21320);; 中国地质调查局“滇藏铁路沿线地壳稳定性调查评价”项目(1212010541404)资助
【CateGory Index】： G634.55
【CateGory Index】： G634.55