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Chinese Science Bulletin, Volume 64, Issue 15: 1620-1636(2019) https://doi.org/10.1360/N972018-01092

The disappearance of the Late Cretaceous Bangong-Nujiang residual seaway constrained by youngest marine strata in Geji area, Lhasa Terrane

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  • ReceivedNov 7, 2018
  • AcceptedFeb 27, 2019
  • PublishedApr 28, 2019

Abstract

For a long time, the key scientific issues regarding to when and how did the Bangong-Nujiang residual seaway disappear are still controversia. This paper focuses on the Late Cretaceous sedimentary strata in the Geji area by studying the stratigraphy, lithofacies and sedimentary facies. This stratigraphic unit, named here Tangza Formation was overlied unconformablely by the Jingzhushan Formation of continental conglomerates. The Tangza Formation represents the youngest marine strata both in the Lhasa terrane and Bangong-Nujiang suture zone. According to the sedimentary facies, the Tangza Formation can be divided into three members, the lower member comprises predominantly mudstone, grayish-green to fuchsia siltstone, lenticular conglomerate beds and sandstone in which parallel bedding and massive structure are commonly developed whereas cross-bedding is not well developed. The middle member mainly comprises grayish-green orbitolina limestone, marlstone, hybrid beds, grey-green shale with small laminated aggregates sandstone and siltstone. The upper member comprises purple-red pebble coarse sandstone with laminated aggregates conglomerate and sandstone in which massive structure and parallel bedding are developed. The sedimentary facies analysis of the Tangza Formation shows a transition from fan delta plain subfacies to front fan delta subfacies in the early stage, which indicates the deepening of palaeo-water depth, followed by fan-delta subfacies, which indicates shallowing of palaeo-water depth. The foraminifera fossils constrained an age of Cenomanian stage (101−94 Ma) for the depositional age of the Tangza Formation. The detrital composition shows that the lower member of the Tangza Formation is dominated by volcanic lithic fragments. The content of feldspar in the upper member decreased to 7%, and lithic fragments increased to 53%, and the proportion of sedimentary rocks increased significantly. Detrital zircons from the lower member of the Tangza Formation yield a primarily age population of 101−163 Ma (peaking at ~110 Ma), with εHf(t) values (−7.5 to +15) and additional age ranges of 424−525, 713−980, 1812−2113 and 2425−2550 Ma. The upper member of the Tangza Formation yield a primarily age population of 101–127 Ma (peaking at 110 Ma), with εHf(t) values (−6 to +10) and additional age ranges of 147−166, 211−271, 509−680, 795−1056, 1816−1879 and 2360−2513 Ma. Detrital composition, detrital zircon geochronology and Hf isotopes altogether show a significant provenance change within the Tangza Formation, from the north Lhasa Terrane and Bangong-Nujiang Suture Zone to Langshan Formation and middle Lhasa Terrane. Combined with regional paleogeography analysis, the disappearance of the Bangong-Nujiang residual seaway in the Geji area where the Tangza Formation was found occurred ca. 94 Ma. Furthmore, the residual seaway was not retreating from the east to the west, but contemporaneously retreats from the north-central Lhasa Terrane.


Funded by

国家重点研发计划(2017YFC0601405)

国家自然科学基金(91755209,41602104)


Acknowledgment

感谢南京大学赖文、许艺炜在西藏野外工作和修订过程中给予的帮助; 马安林、傅焓埔、蒋璟鑫协助本次工作的实验分析并在写作过程中进行了有益的讨论, 刘群在写作过程中给予了帮助. 感谢评审人提出的建设性的意见.


Supplement

补充材料

图S1 唐杂组中段底栖大有孔虫演化与分带特征

图S2 唐杂组有孔虫化石

图S3 唐杂组显微照片

图S4 唐杂组碎屑锆石εHf(t)图解

本文以上补充材料见网络版csb.scichina.com. 补充材料为作者提供的原始数据, 作者对其学术质量和内容负责.


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  • Figure 1

    Geological map of the Qinghai-Tibet Plateau. (a) Simplified tectonic map of the Himalaya-Tibetan Plateau after Zhu et al.[3]; (b) simplified geological map of the study area showing major terranes, suture zones and faults. JSSZ, Jinshajiang suture zone; BNSZ, Bangong-Nujiang suture zone; SNMZ, Shiquan River-Nam Co mélange zone; LMF, Luobadui-Milashan fault; IYSZ, Indus-Yarlung Zangbo suture zone

  • Figure 2

    Photographs of outcrops in Tangza Formation. (a) The contact boundary between the lower Tangza Formation and Langshan Formation is covered, which is presumed to be unconformable contact; (b) the middle Tangza Formation is marked by the appearance of limestone; (c) conformable contact between the middle section and upper section; (d) angular unconformity between Tangza Formation and Jingzhushan Formation; (e) greenish-gray mixed sedimentary and marlstone in middle section; (f) erosion surface in upper section; (g) trough cross bedding in the sandstone; (h) limestone gravels with poor sorting, Jingzhushan Formation; (i) graded bedding in upper section; (j) sand lens in upper section

  • Figure 3

    Stratigraphic logs of the three measured sections in the Tangza Formation and Jingzhushan Formation with lithofacies and sample locations. The sample number marked were used to detrital zircon dating, sandstone petrography studying and foraminifera identification

  • Figure 4

    (Color online) Ternary diagrams for sandstones of the Tangza Formation and Jingzhushan Formation. (a) Sandstone classification diagram; (b) Lv-Ls-Lm diagram[65]

  • Figure 5

    (Color online) U-Pb relative age probability density diagrams for detrital zircons from Tangza Formation. Results are compared with data from: Southern Qiangtang terrane[6670]; Bangong-Nujiang suture zone[7174]; central Lhasa terrane[41,68,69,75,76]

  • Figure 6

    (Color online) Paleogeographic cartoons of Tangza Formation in different stages. (a) The lower Tangza Formation: Marine-continental transitional stage; (b) the middle Tangza Formation: Sea level rise with limestone and mixed sedimentary; (c) the upper Tangza Formation: The seawater retreated from northern-central Lhasa

  • Table 1   Statistical table of gravel composition in the field

    点位

    灰岩

    砂岩

    火山岩

    硅质岩

    合计

    唐杂组下段

    Point1

    57

    19

    16

    3

    95

    Point2

    61

    23

    22

    6

    112

    唐杂组上段

    Point1

    65

    11

    2

    5

    83

    Point2

    80

    12

    7

    4

    103

    竟柱山组

    Point1

    205

    5

    15

    2

    227

  • Table 2   Lithofacies from Tangza Formation and their general sedimentological interpretation (according to Miall)

    岩相

    描述

    解释

    Gcm

    厚层, 颗粒支撑, 中粗粒砾岩, 分选中差, 近圆形-角砾状, 排列混杂, 层厚数十厘米~数米

    片流、富含碎屑的碎屑流、河道沉积

    Gmm

    厚层, 杂基支撑, 中粒砾岩, 分选差, 半棱角-棱角状, 排列混杂, 层厚约数十厘米~数米, 规模较Gcm小

    含泥质基质的泥石流

    Gct

    颗粒支撑, 中粒砾岩, 含砂质基质, 槽状交错层理, 分选中, 角砾-近圆状砾石, 层厚数十厘米

    辫状河道牵引流下3D沙丘迁移、较深的稳定河道

    Gch

    颗粒支撑、中细粒砾岩、分选较好、水平成层, 砾石呈叠瓦状排列

    纵向沙坝、水深较浅的牵引流

    Sm

    厚层中粗粒砂岩, 偶含砾, 层厚数十厘米, 横向延伸数十米

    富砂的泥石流

    St

    中粗粒砂岩、透镜体、槽状交错层理、层厚数十厘米

    3D波纹沙丘在单向水流中的迁移、水动力中等

    Sh

    中细粒砂岩、平行层理、10~30 cm

    强水动力条件或较浅水深、单向水流

    Sr

    细-中粒砂岩; 小型、不对称流水波痕

    较弱的水动力条件(20~40 cm/s)下砂纹迁移, 较浅水深里的单向流动

    Sp

    中-极粗粒砂岩; 板状交错层理

    中等强度水动力(40~60 cm/s)大型波纹迁移; 单向水流; 沙坝横向迁移

    Fsm

    红色厚层粉砂岩、含少量碎砾和结核、层厚约0.1~2 m、小型平行纹层

    水动力减弱流、漫滩沉积、古土壤

    Fr

    厚层-薄板状、以红色为主的杂色泥岩、生物扰动构造、含钙质结核

    水动力减弱流、漫滩沉积、古土壤

    Fm

    红色厚层泥岩、细粒粉砂岩

    水动力减弱流、漫滩沉积、洪泛平原

    M

    泥晶灰岩和黄色泥灰岩

    滨浅海灰泥

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