Sedimentatary Environment And Tectonic Setting Of Middle Triassic Flysch In The Nanpanjiang Basin, Southwestern China

Nanpanjiang Basin is situated in the bordering area among Guizhou Province, Yunnan Province and Guangxi Autonomous Region. Different scholars in China have different models on the tectonic settings of Nanpanjiang Basin in the Middle Triassic. It is crucial to understand the tectonic setting of Nanpanjiang Basin in the Middle Triassic because of the oil and gas exploration in the region. The author of this thesis have reviewed a lot of data collected from geological papers and reports, studied the sedimentary structures and clastic compositions of the Middle Triassic Flysch, analyzed the sedimentary environment and the tectonic setting of the Nanpanjiang Basin in the Middle Triassic. According to the newest standard, the classified the sedimentary structures in the flysch into three groups: pre-depositional structures, syn-depositional structures and post-depositional structures, and discussed the origins of all the sedimentary strtuctures. According to the sedimentary structures, the author reconstructed the framework of the paleo-currents in the Middle Triassic, demarcating the different sedimentary facies, and identifying the source area direction (SEE). After identifying the mineral composition of the flysch through the thin sections, the author quantitatively analyzed the constituents of quartz, feldspars and rock debris of the flysch sandstones. The Q-F-L composition reveals that the provenance is mainly a recycled orogenic belt. The geochemical composition and REE pattern also indicate the sources as the combinations of island arc related to a continent and an active continental margin. The author also analyzed the origin of the syn-sedimentary structures, i.e. compression structures found in the flysch, identified the compression directions acting on the basin during the Middle Triassic. It shows that there was a NE—SW compression in the Middle Triassic because the collision between Indochina and Yangtz Plate in the Middle Triassic. After analyzing the four key elements of a remnant ocean basin, the author concluded that the Nanpanjiang Basin was a typical remnant basin during the Middle Triassic. It is a key step for the basin to evolve from an open ocean basin in the Early Triassic to the foreland basin in the Later Triassic.