西藏西南部三叠系穷果群的地层学与岩相  

STRATIGRAPHY AND LITHOFACIES OF THE TRIASSIC QIONGGUO GROUP IN SOUTHWESTERN TIBET

作　　者：李祥辉[1] 魏玉帅[2] 陈曦[2] 王成善[2] 李亚林[2] 

机构地区：[1]南京大学地球科学与工程学院,江苏南京210046 [2]中国地质大学(北京)地球科学与资源学院,青藏高原研究中心,北京100083

出　　处：《地层学杂志》2017年第1期17-31,共15页Journal of Stratigraphy

基　　金：中国地质调查局项目"西藏1/5万仲巴县城西地区6幅区调"(编号:1212011121229);"西藏1/5万仲巴县城北地区4幅区调"项目(编号:12112011086037);国家自然科学基金项目"西藏山南地区上三叠统复理石物源区分析"(No:41072075)联合资助

摘　　要：在区域地质调查、实测地层剖面和区域对比基础上,对西藏西南部雅鲁藏布江缝合带西段的穷果群进行了研究,认为穷果群仅分布于萨嘎以西的仲巴微地体,萨嘎以东雅鲁藏布江缝合带中过去识别的穷果群实为构造混杂岩(三叠系—古近系)修康岩群的大型外来岩块;与下伏二叠系曲嘎群呈(侵蚀)假整合接触,上未见顶,与修康岩群之间可能不存在直接的整合/不整合接触关系;时代为三叠纪Induan期—Carnian期。依据岩相、层位差异,穷果群由下向上可划分为屯具场组、屯具日组和尼多曲组3个地层单元。屯具场组以石英砂岩为特征,向上夹含岩屑砂岩、粉砂岩和板岩或互层,厚30~550m,顶与屯具日组整合接触,时代暂置于早三叠世早期的Induan期,大多分布于霍尔巴—仲巴地区,其下部沉积环境为(可含障壁)滨岸,向上演变为为浅表海;屯具日组以晶粒灰岩为特征夹板岩/页岩、粉砂岩,或与之互层,产较为丰富的菊石、双壳类、牙形石等化石,厚200~800m,上未见顶或与尼多曲组整合接触,广泛分布于仲巴微地体,依据Claraia和Eumorphotis双壳动物群、Flemingites和Meekoceras菊石动物群、Neospathodus牙形石动物群,其时代可归属于早三叠世晚期的Olenekian期—中三叠世,沉积于较深陆表海环境;尼多曲组以黑色、深灰色碳质板岩为特征,厚300~500m,与下伏地层整合或假整合或断层接触,上未见顶,分布于仲巴地区,牙形石Metapolygnathus cf.polygnathiformis指示其时代为晚三叠世早期的Carnian期,沉积于深表海。On the basis of 1/50 000 regional geological mapping and stratigraphic investigation, we analyzed the stratigraphy and lithofacies of the shallow marine Triassic Qiongguo Group in southwestern Tibet, which belongs to the Zhongba microterrane, the western part of the famous Yarlung Zangbo Suture Zone. Its spatial distribution and stratigraphic relationship with other strata are revised. It is proposed that the Qiongguo Group only occurs in the Zhongba microterrane to the west of the Saga area. Strata previously described as the Qiongguo Group to the east of the Saga area represent large olistoliths of the tectonic melange Xiukang Lithic Unit （i. e. the so-called Xiukang Group）. The Qiongguo Group overlies unconformably the Permian Quga Group. Fossil ammonites, bivalves and conodonts indicate that the Qiongguo Group ranges in age from the Induan to the CarMan of the Triassic. The Qiongguo Group can be divided into three formations, including, in ascending order, Tunjuchang Formation, Tunjuri Formation, and Niduoqu Formation. The Tunjuchang Formation is 30-50 m in thickness, characterized by quartz arenite that transitions upsection into lithic sandstones, siltstones and slates deposited in shallow-water near-shore environments on an epicontinental sea. It unconformably overlies Permian strata and conformably underlies the Tunjuri Formation in the Zhongba area. It is tentatively regarded as early Triassic Induan in age. The overlying Tunjuri Formation is 200-800 m in thickness in the Zhongba microterrane and composed of crystalline limestones intercalated with slates, shales, and siltstones. It may be conformably overlain by the Niduoqu Formation. The occurrence of bivalves （e. g., Claraia and Eumorphotis）, ammonites （e. g., Flemingites and Meekoceras）, conodonts （e. g. , Neospathodus） suggests that the Tunjuri Formation is likely Olenekian-Ladinian in age and was deposited in deeper environments in an epicontinental sea. The Niduoqu Formation is 800-500 m thick in the Zhongba area, featured by dark

关 键 词：岩石地层 岩相 尼多曲组 屯具日组 屯具场组 穷果群 修康岩群 岩相 三叠系 仲巴微地体 西藏 

分 类 号：P534.51[天文地球—古生物学与地层学]