摘要:
中三叠世是中国南方沉积盆地从海相沉积向陆相沉积转变的重要时期, 因此阐明该期的地层结构和沉积古地理分异对于揭示该区域构造和地质发展历史是相当重要的。本文将中国南方划分为四个沉积区 :东南沉积区、下扬子沉积区、上扬子沉积区及右江沉积区。同时对各沉积区序列、岩相生物群的分布和分异进行了总结。从而认为当时东南沉积区与下扬子沉积区是一个直接关联的沉积古地理整体, 而上扬子沉积区与右江沉积区为另个沉积古地理整体。前者在中三叠世时是一个南东高, 北西低的向北西方向倾斜的陆缘海沉积盆地, 从安尼期至拉丁期, 沉积相带不断向北西方向迁移, 并在安尼期与拉丁期之交完成从海向陆的转变。
Abstract:
The Middle Triassic was the changing turning period of South China from marine sedimentary basin to continental deposits or even erosion. In this paper, South China is subdivided into four sedimentary provinces: Cathaysia sedimentary province, Low Yangtze sedimentary province, Upper Yangtze sedimentary province and Youjiang sedimentary province, among which the distribution and variation of the lithostratigraphic sequences, lithofacies and biotas are summarized. Each sedimentary province has similar stratigraphic sequence and unique lithofacies though the difference between the Anisian and the Ladinian is apparent. In the Anisian, carbonate rocks are mainly on Yangtze block, while clastic sediments are mostly distributed in Cathaysia and Youjiang sedimentary provinces. The mixed sequences of carbonate and clastic rocks mainly occur at the transition areas between the sedimentary provinces. In the Ladinian, carbonate sediments were limited, mainly in Upper Yangtze sedimentary province. Cathaysia, Lower Yangtze and Youjiang provinces were occupied by clastic sediments. In addition, biotas were of distinctive assemblages and associated biotic groups in various lithofacies. In Youjiang province, Daonella group and ammonoids are the most distinctive biota, while Upper Yangtze province is characteristic of widespread Costatoria subgroup, Cathaysia and Lower Yangtze provinces are predominated by Asoella subgroup. So there is a close relationship between the biotic paleoecology and the lithofacies as well as the sedimentary facies in South China during the Middle Triassic. The Cathaysia block and the Lower Yangtze sedimentary province belonged to a single sedimentary paleogeographic unit while the Upper Yangtze and Youjiang sedimentary provinces were another affiliated unit in the Middle Triassic. The paleogeographic pattern of the former was a continent marginal sedimentary basin declining northwestward. From the Anisian to the Ladinian, the sedimentary facies zones migrated northwestward rapidly. The whole region finished the transformation from sea to land in the latest Anisian. In the latter, the Middle Triassic paleogeography was a typical pattern of continental margin deepening from north to south, accompanied by an accordant differentiation of sedimentary facies. From the Anisian to the Ladinian, the enormous shallow carbonate platform, the main body of Upper Yangtze province, became shallower and saltier, and the clastic turbidites, the main body of Youjiang province, gradually expanded over the carbonate platform. Consequently, the Middle Triassic completely records the course of each sedimentary province from sea to land and their relationship, which is important to study the regional tectonics and geological history of South China. There exists a great interrelationship between the biotic paleoecology and the lithofacies as well as the sedimentary facies in South China during the Middle Triassic. The variation of ecological types and the change of biotic composition well show the history of the regional paleogeographic change and the development of sedimentary basin in this region. Cathaysia Lower Yangtze and Upper Yangtze-Youjiang were two separate sedimentary basin and have quite different evolutionary histories, which indicates that they had different dynamics in the transition from sea to land.
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