Recognition and Correlation of Supersequences in Lhasa Block, Southern Tibet

LI Xiang-hui WANG Cheng-shan WU Rui-zhong (Chengdu University of Technology, Chengdu 610059)  

Eleven sedimentary supersequences CQ 1～11 of marine Paleozoic and Mesozoic sediments are recognized in the Coqen basin, Lhasa Block. Detailed characteristics of sedimentation, age range, number, and T-R tecture are described in the paper, and correlations and discussions of supersequence between Tethys Himalaya and Lhasa Block are made. CQ 1 is temporally thought to be the Cambrian supersequence due to crystallinely metamorphic rocks. CQ 2 of the Ordovician supersequence is dominated by black shale and limestone, in which only regressive part is exposed and the transgressive sediment is not. CQ 3 within duration of ca. 39 Ma of the Silurian supersequence is characterized by a texture of depositional sequence sets from transgressive shalely and limy rocks to medium-thicked wackstone and packstone, within which abundant grapitutid and little thinned shelly brachiopod became shallower sea coral and conodent. The boundary between the CQ 2 and CQ 3 is placed at the Lower Ashgill of Upper Ordovician. As the Devonian supersequence in range of ca. 48 Ma, CQ4 can be broadly recognized in the Coqen basin, which is composed of transgrassive and regreassive sets. Thinned marlstone with the mixture of shale, siltstone, and sandstone and benthos associated with planktons comprises the former, and the later is of property of shallow sea dolomitized and crystalline limestone. This supersequence is separated the CQ 3 away by terrigenous sediment at the base of Devonian. CQ 5 is limited in Lower Carboniferous with an interval of ca. 33 Ma. It starts by the wormkalk at the bottom of Carboniferous and continues by bioclastic and intraclastic limestone, for which the transgrassive and regreassive depositional sets are vague to distinguish. CQ 6 ranges from upper Lower Carboniferous to lower Lower Permian with a total age of ca. 55 Ma. It is characterized by glaciomarine shale with lens of limestone and cool fauna fossils, and the boundary between CQ 6 and CQ 5 is located at the sharp contact of terrigenous rocks onto carbonate rocks. CQ 7 is the latest supersequence in Paleozoic, which has duration of age time ca. 30 Ma in most of Permian. The transgressive depositional sequence set is similar as the set in the CQ 6 and became the shallower carbonate sediments of regressive set. CQ 8 and CQ 7 are separated by disconformable and erosional surface at the base of the Triassic Quehala group. The age range of the supersequence is estimated 37 Ma. The transgressive set is not exposed, and the regression is consitituent of terrigenous turbidite of inner fan with little volcanic and carbonate rock. There is a nonconformity contact between the CQ 9 and CQ 8, above which the transgressive depositional sequence set is superposed by flysch middle and outer turbidite fan, and follows by changeable and complicated system sediment. It is ranges ca. 63 Ma and is separated over CQ 10 away by a regional unconformity. Supersequence CQ 10 is composed of kinds of limestone and clastic rocks in shallow sea environment deposited in Lower Cretaceous (50 Ma). The transgressive set is combined by hybrid sediments of shallow and volcanic rock, and the regressive by predominant carbonate rocks. The Upper Cretaceous supersequence CQ 11 is the only one superposed by much continental delta and shore facies. In general, the T-R sets are not readily differentiated either in filed or in room. Of them, three supersequences CQ 1～3 are classified in Early Paleozoic; four CQ 4～7 and four CQ 8～11 are in Late Paleozoic and Mesozoic respectively. The sedimentation, age range, number, and T-R texture of the super sequences CQ 1～3 are similar, but different are in the CQ 4～7 and CQ 8～11 of Upper Paleozoic and Mesozoic. We suppose that the turnover of supersequence from Early to Late Paleozoic could be caused by the changes of tectonic and types of sedimentary basin, but not by second-order sea-level change, because there should be a same / similar sedimentary response to eustatic change for sedimentary basins

【Fund】： 国土资源部“青藏高原重点沉积盆地油气资源潜力”项目;; 国土资源调查项目 (DKD 2 1 0 4 0 1 1 )“海相含油区对比与资源潜力分析”课题资助
【CateGory Index】： P539.2

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