西藏中部拉萨地块古生代、中生代的超层序研究

识别划分了西藏南部拉萨地块 (措勤盆地 )古生代、中生代以海相为主的沉积地层相当于二级旋回的超层序11个 (CQ _1～11),其中早古生代 3个 (CQ _1～3),晚古生代 4个 (CQ _4～7),中生代 4个 (CQCQ _8～11);描述了各个超层序的特征;不仅在地块内进行了超层序对比,而且与印度北部边缘 (特提斯喜马拉雅 )显生宙的超层序进行了比较和讨论。研究表明,除早古生代外,藏南特提斯喜马拉雅和拉萨地块 (措勤盆地 )的超层序在数量、延时、结构方面极不相同,与所处地块构造背景和沉积盆地性质发生了变化有关。提出早古生代拉萨地块与印度次大陆同属冈瓦纳相区 (克拉通上的陆表海 ),晚古生代早期以后它们已不属同一大陆,整个晚古生代拉萨地块可能为冈瓦纳与劳亚大陆之间过渡带的一部分,中生代则成为Cimmeria次大陆的南部块体,冈瓦纳大陆在西藏境内的北界应为雅鲁藏布江缝合带。

Recognition and Correlation of Supersequences in Lhasa Block, Southern Tibet

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 supersequencesCQ _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