Late Paleozoic deep-water stratigraphic succession in central Youjiang basin: constraints on basin evolution
Late Paleozoic deep-water stratigraphic succession in central Youjiang basin: constraints on basin evolution
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作者：
Shi Xiao-ying
右江盆地位于中国西南扬子地台西南缘，是中国南方重要的潜在油气盆地之一。晚古生代沉积环境可概括为四大类型：1）浅陆架、2）台缘斜坡、3）孤立碳酸盐岩台地、4）深水盆地。按照上述顺序，这些沉积环境具有从北到南的空间排列，从而表明边缘海到大洋盆地的框架。盆地中部上古生界以燧石、锰粘土岩和枕状玄武岩为特征，这一层序与盆地边缘有很大区别。利用层序地层学和斜坡带中识别的事件，建立了深水盆地的多重褶皱地层系统及其对比。研究表明，深水区以OIB碱性玄武岩为主，拉斑玄武岩主要集中在盆地西南部。然而，在云南、广西、越南边境地区，一些玄武岩的地球化学性质介于 OIB 和 MORB 之间。识别出以下四个岩浆期：1）早泥盆世晚期至中泥盆世早期； 2）中晚泥盆世至早石炭世； 3）中晚石炭世至早二叠世； 4）二叠纪晚期到三叠纪早期。枕状玄武岩出现在后三期，其中第三期厚度最大、分布范围最广。在我们研究的基础上，我们提出右江盆地起源于长江和印度支那地块之间的一个小型洋盆。这个小洋盆是中国西南部长宁-孟连带东部古特提斯洋的一个东支。从更广泛的角度来看，这构成了古特提斯群岛东部群岛的一部分。地层结构进一步表明右江盆地演化经历了五个阶段：1)浅表盆地(D_1晚期-D_2早期); 2）裂谷盆地（中D_2-D_3）； 3)快速扩张的洋盆(C_ 1 -P_ 2 ); 4)收缩洋盆(P_3-T_2早期); , 5) 残余盆地(T_2中-T_3早期)。我们的研究认为，从古地理上看，盆地的西南部可能属于印度支那地块的北缘，而北部可能属于扬子板块的西南边缘。
The Youjiang basin is situated on the SW margin of the Yangtze platform, southwest China, and is one of the important potential oil-gas basins in southern China. The depositional environment of Late Paleozoic sediments can be summarized as four major types: 1)shallow shelf, 2)platform margin to slope, 3)isolated carbonate platform, and 4)deep-water basin. In the above order, these depositional environments have a north to south spatial arrangement, thereby suggesting a marginal sea to oceanic basin framework. In the central parts of the basin, the Upper Paleozoic is characterized by chert, manganese claystone, and pillow basalt, a succession that is quite distinct from that at basin margins. Using the sequence stratigraphy and events recognized in the slope belt, a multi-folded stratigraphic system for the deep-water basin and its correlation has been set up. The study shows that in the deep-water areas, OIB alkaline basalt predominates, with tholeiite mainly concentrated in the SW part of the basin. Around Yunnan-Guangxi-Vietnam border areas, however, some of the basalts show a geochemistry that is transitional between OIB and MORB. Four magmatic episodes are recognized as follows:1)late Early to early Middle Devonian; 2)middle Late Devonian to Early Carboniferous; 3)middle Late Carboniferous to Early Permian; and 4)Late Permian to Early Triassic. Pillow basalt occurs in the last three episodes, of which the third one has largest thickness and most extensive occurrence. On the basis of our study we propose that the Youjiang basin originated as a small oceanic basin between the Yangtz and Indochina blocks. This small oceanic basin was an eastward branch of the eastern Paleotethys along Changning-Menglian belt, in southwest China. In a broader view, this constitutes a part of the archipelago in eastern Paleotethys. The stratigraphic architecture further suggests that the Youjiang basin evolved through five stages:1)shallow epeiric basin (late D_ 1 -early D_ 2 ); 2)rift basin (middle D_ 2 -D_ 3 ); 3)rapidly expanding oceanic basin (C_ 1 -P_ 2 ); 4)contracting oceanic basin (P_ 3 -early T_ 2 ); and , 5)remnant basin (middle T_ 2 -early T_ 3 ). Our study supports that, paleogeographically, the southwestern part of the basin may belong to the northern margin of the Indochina block, while the northern part may form southwestern margin of the Yangtz plate.