【作者】 陈文西；

【导师】 王剑； 尹福光；

【作者基本信息】 中国地质科学院 ， 古生物学与地层学， 2007， 博士

【摘要】 晚古生代末期可可西里—金沙江洋与龙木错—双湖裂谷带的闭合,北羌塘盆地形成了以前三叠系为基底的残留内陆表海演化时期,直至晚三叠世早期——肖茶卡组沉积期末残留内陆表海消亡,晚三叠世中晚期至侏罗纪进入了裂陷盆地发展阶段。论文对晚三叠世—中侏罗世早期盆地的形成与演化开展了研究,包括关键地层界面特征分析、关键地层时代归属分析、区域岩石地层、生物地层和年代地层的综合对比分析、区域沉积体系与沉积相分析、岩相古地理分析,最终建立晚三叠世—中侏罗世早期羌塘盆沉积—构造动力演化模式。1、系统的对各地层单元的界面分析发现:(1)肖茶卡组底界在盆地内部与中三叠统整合关系,在盆地北缘为沉积超覆于不同时代的前三叠系之上:(2)肖茶卡组顶界面发育古岩溶角砾岩,与上覆那底岗日组为岩溶不整合接触,说明肖茶卡组沉积时期的羌塘盆地是在早中三叠世的盆地基础发展起来的,后期出现了广泛的海退事件,使盆地结束沉积并遭受风化剥蚀作用;(3)中侏罗统雀莫错组的岩性组合特征、岩石化学特征、古生物化石混积和地层的沉积超覆现象均表现出裂陷环境下形成的伸展不整合面的特征。2、综合地层划分与对比分析认为:(1)肖茶卡组古生物化石组合反映其时代为晚三叠世卡尼期—若利期,根据其上被上三叠统那底岗日组不整合覆盖,将其时代厘定为卡尼期—若利期早期;(2)那底岗日组火山岩SHRIMP锆石U-Pb年龄在216～205 Ma之间,其时代主要为若利期,可能包含有瑞替期:(3)日干配错组底部火山岩的年龄、中上部化石组合及其被下侏罗统整合覆盖,综合反映的时代为晚三叠世若利期—瑞替期,而且那底岗日组火山岩和日干配错组火山岩的地球化学成分分析反映两者均为引张构造背景下的产物;土门地区的土门各拉组的下部岩性及界面特征、化石组合和上部可能被下侏罗统整合覆盖,综合反映其时代为若利期—若利期。因此,北羌塘的那底岗日组与南羌塘的日干配错组和土门各拉组可以对比,是“同期异相”关系。3、羌塘盆地晚三叠世—中侏罗世早期地层共计可划分出6个沉积体系14个沉积相和多个亚相。肖茶卡组主要发育扇三角洲相、三角洲相和碳酸盐岩台地相,基本继承早中三叠世古地理面貌,但早期出现广泛海侵和后期海水退出,形成早期的沉积超覆与后期遭受风化剥蚀。那底岗日组主要为火山—沉积,分为陆上喷溢、水下喷溢和火山碎屑沉积,反映裂陷盆地早期的古地理面貌。日干配错组发育有水下火山喷溢相、三角洲相、碳酸盐岩台地和陆棚相,反映盆地由早期的裂陷火山喷发—盆地基底沉降,海平面不断上升形成台地相至陆棚相的古地理面貌。下侏罗统在羌北盆地可能为近海湖泊沉积,羌南盆地为浅海陆棚沉积,主要继承早期古地理面貌。雀莫错组在羌北盆地为近海湖泊沉积,更具海相色彩:南羌塘主要为外陆棚沉积,反映羌塘盆地再次发生较强烈的裂陷作用和较大幅度的海平面上升,形成陆源剥蚀区缩小,沉积覆盖面增大的古地理面貌。4、根据地层界面特征、地层划分与对比结果和岩相古地理演化特征可将晚三叠世—中侏罗世早期羌塘盆地划分为两个不同构造性质的盆地发展时期,分别为晚三叠世卡尼期—若利早期(肖茶卡组沉积期)的残留内陆表海沉积盆地发展与消亡时期和晚三叠世若利期中晚期—中侏罗世早期的裂陷盆地的形成与发展时期。后者可进一步分为三个构造演化阶段,第一个阶段为若利期中晚期—瑞替期的强烈裂陷时期,该时期羌塘盆地形成了真正意义上的“两凹夹一隆”的构造古地理格局;第二个阶段为续第一阶段之后的早侏罗世构造相对稳定的凹陷时期;第三个阶段为中侏罗世早期续构造相对稳定的凹陷作用之后的再次较强烈的裂陷作用阶段。更多还原

【Abstract】 With the closure of KoKo Xili -Jinshajiang Ocean and Longmucuo -Shuanghu rift belt during the ending of Late Paleozoic, North Qiangtang Basin (NQB) come into the evolution phase of remnant epicontinental sea (RES), till the early stage of Late Triassic - the ending of Xiaochaka depositional stage, the RES was over. NQB was in the developing phase of rift basin from the middle stage of Late Triassic to Jurassic Period.This dissertation was focused on the research about the formation and evolution of NQB from the Late Triassic to the early middle Epoch of Jurassic Period, including integrated analyzing the characteristic of the key stratigraphic surfaces, the time of the key stratigraphy, regional rock straigraphy, biostratigraphy and chronostratigraphy, sedimentary system and depositional facies. On basis of these analysis, a tectonic evolution model have been established for NQB during from the Late Triassic to the early middle Epoch of Jurassic Period.1. After the systematic surface analysis about main stratigraphy units, we found that: (1) the bottom of Xiaochaka formation was comformity with the middle Triassic in the basin, and in the north margin it was deposited and overlapped on the different period stratigraphy before Triassic. (2) The top surface of Xiaochaka formation developed regionally karst and paleo-karst breccia, and was uncomformity with Nadigangri Formation, these things suggest that NQB in Xiaochaka depositing stage was developed from middle Triassic basin, and then an extensive regression occurred, the deposition was over and erosion come into being. (3) The lithologic assemblage, geochemistry, mixed fossils, and stratigraphy overlapping characteristic of Quemocuo Formation in early middle Jurassic indicted that the uncomformity surface was an extension-unconformity.2. Integrated analysis about stratigraphy division and correlation indicted that: (1) the paleontologic assemblage of Xiaochaka formation show its time was Carian -Norian stage of late Triassic. According to it was overlapped by late Triassic Nadigangri Formation, so Xiaochaka formation should be Carian - early Norian stage. (2) the SHRIMP Zircon U-Pb age of Nadigangri formation volcanic rocks wear vary form 216 to 205 Ma, so its time should be Norian, but maybe extended into Rhaetian Stage. (3) The age of its bottom volcanic rocks, the fossil assemblages of its middle and upper members and overlapped by lower Jurassic, indicted the age of Riganpeicuo Formation was Norian - Rhaetian stage. And the same time, the volcanic rocks geochemistry of Nadigangri and Riganpeicuo formation both reflected that they wear formed in rift tectonic settings. The lower lithologic and surface characteristic, fossil assemblages and its upper maybe was conformity with lower Jurassic, all reflected that the age of Tumengela Formation in Tumen area should be Norian - Rhaetian stage. Therefore, Nadigangri formation in NQB and Riganpeicuo formation in South Qiangtang Basin should be the relationship of "same stage and difficult facies".3. The stratigraphy of late Triassic -early middle Jurassic Peroid in Qiangtang basin could be divided 6 sedimentary systems and 14 depositional facies. Xiaochaka formation developed Fan delta facies, delta facies, and carbonate platform facies, which basically continued the paleography of early and middle Triassic, but with intensive progress in early stage and quickly regress in late stage, companied by depositional overlapping in early stage and weathering erosion in late stage respectively.Nadigangri formation mainly consisted of volcanic deposition, including continental effusive rocks, marine effusive rocks, and volcanc clastic deposition, which reflected the early paleography of rift basin. Riganpeicuo formation developed marine effusive facies, delta facies, carbonate platform facies and shelf facies, which suggest the paleography of South Qiangtang Basin (SQB) in late Triassic experienced the process of rifting, sea level rising continuously, and then developing carbonate platform and shelf facies. Lower Jurassic in NQB could be near-shore lake deposition, in SQB was shallow shelf deposition, which followed the paleography of late Triassic. During early middle Jurassic period, Quemocuo formation in NQB was offshore lake deposition with marine feature, in SQB was shelf deposition, which indicted that an intense rifting and sea level rising with more large range, and consequently, eroding area was shrunk and deposition area was expanded.4. According to the result of stratigraphic surface features, division and correlation, and the paleographic characteristic, Qiangtang Basin during late Triassic -early middle Jurassic period could be divided into two basin developing stages with difficult tectonic nature, remnent epicontinental basin stage in Carian -early Norian of late Triassic and rifting basin stage in middle Norian of late Triassic - early middle Jurassic period respectively. The latter can be divided into three tectonic evolution phases. The first is intensively extending between middle Norian to Rhaetion, and in this period the Qiangtang basin has been a tectonic paleogeography of "two depressed holder one blowups". The second that the tectonic move is gentleness and the basin floor is been depressing in the early Jurassic period. The third that the tectonic might another time become intensively extending.更多还原