The Middle Triassic Stratigraphy and Sedimentary Paleogeography of South China

Key words: 
• Middle Triassic sedimentary /
• facies and paleogeography /
• South China

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.