Comprehensive Provenance Study In Sedimentary Basin

Because of the complexity of sedimentary rocks, provenance researches on sedimentary basin developed slowly. The traditional mothods including mineral composition analysis, heavy mineral analysis, sedimentary method, etc., were often used, while the geochemical method utilized was scarce. Previous researches on the North Jiangsu Basin were focused on paleontology, sequence stratigraphy, hydrocarbon reservoir, sedimentary facies, and so on, with low t degree of research.Constructive evolvement in the lower Yangtze region, where the North Jiangsu Basin located in, was research focus all along. However, the research of constructive evolvement since Cenozoic was scarce. At present, the Gaoyou Depression in the North Jiangsu Basin has been shown good exploration prospect, in which the Paleogene Dainan Formation (E2d) is the target of hydrocarbon exploration and research. The amount of the supplying source rocks can decide the size of sediment body. This paper will combine the traditional methods with modern means and select the E2d of the Gaoyou Depression in the north Jiangsu Basin as an example for provenance study. On one hand, making clear the provenance system of the E2d deposits would provide theoretical supports to further gas and oil exploration in this region, and reconstruct relationship between the sedimentary basin and provenance, on the orther hand, would provide new thought and act as an example for provenance research of sedimentary basin. This research would also enrich the information of source rock and offer geochemical evidence for constructive evolvement in the lower Yangtze region, which is also helpful to the oil and gas exploration in sedimentary basin or the development of provenance study.In this thesis, based on the previous fruits and sub-member is selected as basic unit, a systematic study with mineral composition, quartz observation in Cathodoluminescence (CL), isotope geochemistry and element geochemistry are carried out with abundant samples. Based on the results, we studied the provenance directions, the location of source, the type of mother rocks and the tectonic setting of the E2d Gaoyou Depression. After dealing with the data, we made a systemetic and embedded research on the source direction, location of source region, characteristics of mother rocks (type, age, and forming stage), contribution of different source region to sedimentary basin, reconstruction of the paleo-environment (weathering degree, characteristics of paleogeography and paleoclimate), and the tectonic background. Analysis on mineral composition displays the main composition of debris-feldspar rocks and feldspar-debris rocks in clastic rocks from the E2d Gaoyou Depression and the characteristics of source form recycled orogenic region. In the E2d Gaoyou Depression, there exists diverse source rocks in different area, for example, sediments in the southern and west-northern depression are from source of metamorphic rocks; deposits in the eastern and western depression are from source of igneous rocks; while the west-southern depression are effected by sources of both metamorphic rocks and igneous rocks. Source rocks in this period are mainly from four source directions:the northwestern Zheduo Salient, the eastern Wubao Salient, the southern Tongyang Uplift and the southwestern Lingtangqiao Salient. Weathering deposits in the eastern Gaoyou Depression such as Zhouzhuang area were mainly from direction of the Wubao Salient; sediments in Yong’an and the northern Fumin in the western depression were mostly from direction of the Zheduo Salient; deposits in the southern Fumin, Caozhuang, Zhenwu, Shaobo, Huangjue and the southern Majiazui came from direction of the Tongyang Uplift; And the sediments in the northern Majiazui are mainly from direction of the Lingtangqiao Salient. Not being like the stable source direction in the above areas, source directions in areas of Huazhang and Lianmengzhuang were relatively complex. In the E2d13, sediments in the eastern and the western Huazhuang area are respectively from direction of the Wubao Salient and the Zheduo Salient, while Lianmengzhuang area was influenced by provenance from direction of the Zheduo Salient. In the E2d12, the source direction changed and persisted to the E2d11, with influence of sediments from direction of the Zheduo Salient becoming greater in Huazhuang area, and the southern Lianmengzhuang area begining to receive sediments from direction of the Lingtangqiao Salient. The result of source direction in REE analysis is concord with that of mineral composition analysis. We found that the degree of fractionation between light REE and heavy REE [(La/Yb)N] can correspond well with the mineralogical maturity index (MMI). Combing with the REE gross (∑REE) and δEu values,(La/Yb)N values can well instruct the source direction. Correlative analysis show that, from delta facie to fan delta facie and to nearshore subaqueous fan facie, the average (Gd/Yb)N values present increasing trend with the decreasing average maturity of facies. This trend is also revealed in different sub-facies of the same facies.Trace and major elements data display the mature mineral composition in studied area, and show that the source rocks are mainly recycled metasediments and felsic materials, with being effected little by the basic rocks. The source area had experienced strong physical weathering and weak chemical weathering. Except for the areas of Shaobo and Huangjue, the majority of the southern step-fault zone underwent persistent weathering, because the deposits in these areas came from distant sources and underwent weak tectonic impact. On the contrast, in the northern slope zone, weathering diversified place to place and time to time, the provenance underwent strong weathering or severe tectonic movement for having caused rapid denudation and fast sedimentation in the depression. Although the E2d North Jiangsu Basin had become the inland lake, the marine characteristics in the E2d1inherit from the older E1f4, because of marine ingression, with these characteristics distributing in the paralic line (the southern Fumin-Xiaoliuzhuang-Zhenwu-Majiazui). Because of the divergence in geological history and dynamics mechamism, tectonic setting in the North Jiangsu Basin reflects the special tectonics of the eastern China, which is different from the typical Andean active continental margin of the eastern Pacific Ocean and needs deeper research. We deem that this is worth further investigating and deeper research for better understanding the tectonic evolvement of these Cenozoic rift basins in eastern China.Zircon U-Pb ages approve that there exist the Early Archean materials in Yangtze Craton and show that there are fore episodes of detrital zircons from the Late Creataceous-Paleogene studied area:1) Late Paleozoic-Mesozoic (100-300Ma):igneous rocks from the Southern Zhangbaling uplift (100-200Ma), uhp-metamorphic rocks in Dabie-Sulu orogenic belt (200-250Ma);2) Late Proterozoic (700-850Ma):the Late Proterozoic low-grade metamorphic granite widely distributed in the Dabie-Sulu orogenic belt;3) Early Proterozoic (1700-1900Ma):respond to the tectonic events in the cracking of the Yangtze platform and supercontinent;4) Late Archean-Early Proterozoic (2450-2600Ma): crystalline basement of the Yangtze platform. In addition, the zircon U-Pb ages of400-500Ma that found in E1f and K2t can respond to the tectonic events in the collision of of the Huaxia platform and Yangtze platform. Combined with the result of REE analysis, we deem that the sediments of the Late Archean-Early Proterozoic Gaoyou Depression mainly come from the internal part of studied basin and the recycled orogen around it, with the provenances mainly composed of the crystalline basement of the Yangtze platform, the shallow metamorphic basement of the Dabie-Sulu orogen, and some Mesozoic igneous rock from the southern part of Zhangbaling uplift. The source rocks had ever underwent multiple-episode global magma tectonic events. The zircon U-Pb ages in studied area reflect a well successive characteristics and recycled orogenic region. The large-scale expanding of ancient lake in E1f leads to the divergence among different areas in Gaoyou Depression.