拉萨北部旁多群坠石沉积的发现及其意义  

Discovery of dropstones in the Poindo Group, northern Lhasa, Tibet, China, and its significance.

作　　者：纪占胜[1] 姚建新[1] 武桂春[1] 蒋忠惕[2] 傅渊慧[3] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质调查局实物资料中心,河北三河燕郊065201 [3]中国地质大学地球科学与资源学院,北京100083

出　　处：《地质通报》2005年第6期542-548,共7页Geological Bulletin of China

基　　金：中国地质调查局项目(编号200313000054);国家自然科学基金项目(编号40302002)资助成果。

摘　　要：报道了新发现的旁多群中部的打鲁雄剖面及其含有典型坠石的沉积特征。该剖面旁多群中部以陆源碎屑少、悬浮泥质沉积为主的欠补偿的深水盆地相沉积为特征,同时夹有水下冰碛岩、重力流沉积等。纹层状—薄层状的粘土质岩层内分布着粒径是基质的100～1000倍的棱角状—次棱角状、长轴垂直近垂直于纹层面或层面的碎屑和砾石。砾石上下的泥质岩具有顶凸下挠的现象。碎屑和砾石及其对上下地层的影响表明其为坠石沉积。旁多群中指示冰川、冰筏沉积作用存在的坠石的发现,有力地证明了青藏高原石炭纪—二叠纪冈瓦纳相地层形成于受到冈瓦纳大陆冰川影响的海相环境。This paper mainly reports the Daluxiong section in the central part of the Poindo Group in Lhünzhub County, northern Lhasa, Tibet, and the sedimentary characteristics of the contained typical dropstones. The Daluxiong section is characterized by less compensated deep basin facies sediments dominated by suspended muddy sediments with scarce terrestrial detritus, intercalated with subaqueous till and gravity-flow deposits. Angular-subangular fragments or pebbles with the long axis perpendicular or nearly perpendicular to the laminated planes or bedding planes are distributed in laminated and thin-bedded clayey strata. Their size is 100 to 1000 or more times larger than that of the groundmass. Argillaceous rocks above and below the pebbles show the convex top and downwarped bottom. The fragments and pebbles and their influence on their overlying and underlying strata suggest that they are dropstone deposits. The discovery of dropstones in the Poindo Group that indicate the existence of glaciation and ice-rafting deposition proves that the Carboniferous-Permian strata of Gondwana affinity on the Qinghai-Tibet Plateau formed in a glaciomarine environment affected by the Gondwana glaciers.

关 键 词：拉萨北部 旁多群 冰海相沉积 杂砾岩 含砾板岩 坠石沉积 

分 类 号：P534.45[天文地球—古生物学与地层学] P534.46[天文地球—地质学]