| Nanpanjiang Basin lies in the southwest margin of the Yangtze platform, which was predominated by sediments of carbonate during the Early to Late Paleozoic time, forming a alternate palaeogeographic pattern of basin and platform. It evolved platform facies during the Maokou Formation, covered by Latter Permian basic volcanic rocks with extensional tectonic framework. During the middle Triassic, another significantly extensional structure process happened, and the deep water basin facies in the Nanpanjiang areas reached maximum. Based on the study of sedimentology and field structure investigations, this article identifies a three armed rift system during the late Permian to early Triassic time in the northern Nanpanjiang Basin. Among them, the western branch of the rift is a syngenetic half graben system controlled by the Longlin fault. Its direction is from east to west in the areas to east of Longlin, gradually changes to northeast-southwest to the west of Longlin. The northern branch called the Leyun-Leyuan Rift, with two contemporaneous boundary faults, the Zhengning-Ceheng fault to the west and the Baoshu fault to the east. The eastern branch is mainly Northwest-Southeast, extending along Tianlin to Baise. The developments of the three rift branches are different. The northern branch is poorly developed, while the other two branches are well evolved, which controlled the flysch depositions during the Middle Triassic time. The space and the depth of the basin increased during the middle-Triassic and a set of deepwater flysch sediment was preserved in the basin. The environment of the basin was identified as foreland basin where there was a set of clastic and carbonate sediment in the northern part of the basin. The main study areas lie in the northern and middle part of the Nanpanjiang Basin (NPJB). Some kind of flysch sedimentary structures can be used in the field work to get paleocurrent such as small oblique bedding, small climbing ripple bedding, convolute bedding, flute cast and so on. Based on the field measurement, stratum attitude correction and the rose diagrams drawing of paleocurrent, this article discusses the paleocurrent of the sediment during the Triassic time.49 sets of paleocurrent data were exhibited here by which sedimentary provenances of NPJB can be constrained. During the middle Triassic time the sedimentary provenance of the southern part of NPJB (for example near Guangnan, Dixu, Bada, Funing) would be the Vietnam Block to the south of NPJB, the sedimentary provenance of the eastern part of NPJB (for example near Bama, Longchuan, Wangmo) would be the Jiangnan Block to the east, and the sedimentary provenance of the northern part of NPJB would be the inner Yangtze Platform to the north. The centre of subsidence is located at the areas south to Ceheng and north to Lucheng. Two palaeohigh belts were indentified between Longlin city and Xilin city, between Yangxu town and Boai town. The paleocurrent changed significantly during the late Triassic time at the northern part of the NPJB which was considered as a consequence of the tectonic fragment change.After Dongwu movement in the northern part of the Nanpanjiang Basin formed a lot of syngenetic fault system, and accompanied by the deposition of large-scale magmatic activity or pyroclastic rocks, deep and shallow strata distribution. Basin evolution is divided to build stage and, rift stage, back arc basin stage, foreland basin stage, a total of four different evolution stage for the basement of the basin, and analyzes the enrichment mechanism of manganese in the Nanpanjiang Basin. During Late devonian to early Permian in rift valley stretching deformation stage environment in the study area, The Triassic NPJB in a back-arc extensional environment, the spread of basin-magmatic hydrothermal provides the formation of sedimentary manganese content in the tectonic environment. |
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