The Evolution Of The Late Cenozoic Sediment Provenances Of The Yangtze Estuary

As one of the important parts of global plate tectonic movements, the intensive Himalayan orogeny is often considered as being a great impact on the uplift of the Tibetan Plateau and the origin of the Asian monsoon. With the intensive uplift of the Tibetan Plateau, there occurred great landform change in the Asian area. The great elevation difference makes the Tibetan Plateau as the main sediment source area in Asia. This high altitude area is also the source of the major Asian river systems.The uplift of the Tibetan Plateau not only changed the route of hydro-thermal circulation but also the atmospheric circulation. The Neotectonics plays a decisive role on the origin of the Asian monsoon. Meanwhile, tectonic movement can cause a series of chain reaction including morphological changes, climate change, river evolution and so on. The information of these geographical phenomena has been recorded in sediments which were transported to the estuarine depocenter by rivers. The research on depocenter can reveal tectonic activity, climate change and landform evolution, and also reflect the effect of human activity.As the depocenter of the late Cenozoic, the Yangtze delta and its adajecent area have been fed by the sediments sourced from its river-basin. This serves as a unique channel for the present study. There have been hundreds of meters of thick terrigenous sediment desposited in the Yangtze delta since the Late Pliocene. Serving as an ideal recorder, this depocenter kept a large amount of information on paleoclimate and the nature of sediment sources in relation to landform evolution. In order to investigate sediment provenance and omorphological evolution of the study area, it is most important to take integrated diagnostic indexes as for geo-environmental change study. In the upper Yangtze, the E'meishan basalt is the unique exposure rock. This type of rock can yield a plenty of pyroxene and Ti-Fe oxides. In the middle-lower reaches, however, rock types are completely different. These areas are dominated by sedimentary, metamorphic and intermediate-acid igneous rocks, in which heavy minerals and geochemical elements are so different from the upper reach. Different source rocks have unique mineralogical and geochemical characteristics, and can produce different minerals and geo-elements through the weathering processes. Therefore, with the development of tectonic movement, delta area is constantly changing; the sediment nature generated by different source rocks was eventually deposited in the depocenter via sediment transport. The analysis of detrital minerals of the present study will contribute to the understanding of sediment provenance in the deltaic strata.In this study, the surface sediment samples of the Yangtze River basin and sediment core samples of the Yangtze Delta were collected. Two late Cenozoic sediment cores were acquired in the river mouth area (Chongming Island) and in the Northern Jiansu plain. In our study,55 surface samples of the tributaries and mainstream of the Yangtze were selected for heavy minerals analysis; 9 for zircon morphology analysis and 12 for fine grain magnetite geochemical test. Grain size analysis, foraminifera identification and superconducting paleomagnetic dating were made for the two sediment cores, respectively. In this research, mineralogical characteristics, zircon shape features and geochemical properties of detrital magnetite in the main tributaries of the Yangtze River covering the entire river basin are investigated to identify the diagnostic indexes of different reaches. And then, these indexes were applied to the Plio-Quaternary core sediments to interpret the change in the sediment provenance in relation to geomorphological evolution of the study area.The result illustrates that the characteristics of the heavy mineral, zircon morphology and magnetite geochemistry vary significantly in different reaches of the Yangtze basin:In the upper Yangtze tributaries, heavy-mineral assemblages generally include amphibol-epidote-magnetite, and pyroxene is the highest among all samples and thus is regarded as a diagnostic mineral. The proportion of long elongation (>3) accounts for only 3.7%. Roundness of the zircon grains sums up to 37.8%. Short-elongated and round zircon grains significantly increased reflect the long sediment transport. Magnetite in the upper Yangtze contains high Ti, Mg and V. Besides Cr of the magnetite is also higher in the upper Yangtze tributaries than the middle-lower tributaries, but the discrepancy is much lower than that of Ti, Mg and V. High content of pyroxene and trace elements (Ti, Mg, V and Cr) in magnetite is the important signal of the upper Yangtze.Assemblages of the middle Yangtze tributaries (the Hanjiang River, the Dongting and Poyang Lake) are characterized by ilmenite, epidote and amphibole. Zircon, tourmaline and metamorphic minerals are the diagnostic heavy minerals in the middle basin. High content of altered minerals (limonite and leucoxene) in the middle-lower tributaries would partly relate to the intense chemical weathering in the middle-lower Yangtze. Short-elongated zircon grains increase significantly in the middle tributaries. There is a small percentage of long-elongated zircon. The proportion of long elongation (>3) is more than 8%. Subangular and rounded zircon grains significantly increase up to one third. The roundness of the zircon grains is overwhelming indicating that a long sediment transport.The trace elements (such as Ti, Mg, Cr and V) in the magnetite are low in concentration. Large distributions of metamorphic, intermediate-acid igneous and sedimentary rocks are the reason for why almost all trace elements in magnetite grains are minor in the middle Yangtze tributaries.In the rivers on the delta plain, heavy minerals assemblages of the Tiaoxi River are of similar assemblage of epidote-amphibole-ilmenite. Fluorite content is the highest among all samples of the Yangtze basin which can reach to 8.2%. Features of the zircon roundness in the local rivers are also strikingly different from others. The proportion of long elongation (>3) sums up to 17.3%. The proportion of long elongation (>3) is less than 10%. The proportion of rounded zircon grains in the local rivers takes only 4.3%. Local zircon with slender poorly rounded crystal morphology can be used as a unique index for the local river provenance. In these, trace elements (Ti, Mg, Cr and V) in the magnetite are also low, indicating large distributions of metamorphic, intermediate-acid igneous and sedimentary rocks are in the local area.Based on the vertical distribution of heavy mineral and elemental variations of magnetite in Core LQ24, three distinct zones can be divided as follow:1) The Pliocene (>2.5 Ma), heavy-mineral assemblages and diagnostic heavy mineral are totally distinctive from the underlying Quaternary layer. This stratum includes high content of stable minerals (ilmenite, zircon and tourmaline) and altered minerals (limonite and leucoxene). The diagnostic mineral in this layer is fluorite with relatively high concentration that reaches 3.2%. Heavy mineral characteristic was closely matched to the local river (the Tiaoxi River). Statistical analysis showed the Pliocene samples featured by poorly rounded zircon grains with angular-subangular euhedral crystal. In these samples, zircon with angular and subangular accounts for more than 95%. Rarely rounded zircon grains are observed in this zone. The content of long elongation (>3) is high up to 15%. The feature of zircon shape also matches to the local river samples.2) The Early and middle stage of the Early Pleistocene (2.5-1.2 Ma). A sudden change in provenance occurred at depth of 336.1 m, where the boundary of the Pliocene and the Pleistocene is located. Heavy mineral assemblages change to amphibole-epidote-ilmenite. Heavy minerals of this layer are diagnosed by amphibole and garnet. Limonite; leucoxene, tourmaline and zircon content reduce dramatically. The feature of heavy mineral indicates that the provenance of this stratum is originated from the area where large distributions of metamorphic, intermediate-acid igneous and sedimentary rocks are located. Completely different from the samples in the Pliocene, the elongations of zircon grains become small. In these samples long elongation (>3.0) consists of ca.8%. Meanwhile, rounded zircon grains increase significantly to ca.20%. Similar with the characteristics of felsic rocks, the trace elements content of magnetite is generally lower. All of these characteristics of heavy minerals match the intermediate acid magmatic rocks and metamorphic rocks, indicating in the early Quaternary an extended sediment provenance derived from the Zhe-Min (Zhejiang-Fujian) uplift in the east Yangtze estuary.3) The late stage of Early Pleistocene to the Holocene (1.2 Ma-the present). In this section, the heavy-mineral assemblages include amphibole, epidote and ilmenite. However the diagnostic heavy minerals are changed to pyroxene and magnetite. The evident increases of pyroxene illustrate the felsic igneous source to be imput. In this zone, zircon grains with short elongation continue to increase. The proportion of this group (>3.0) decreases significantly. Besides, the roundness in this zone was the best among these strata. The ratios between the rounded and angular were higher than that of the underlying strata. In addition, magnetite grains with high content of Ti, Mg and V start to occur and remain steadily. The distinct characteristics of heavy minerals, zircon morphology and magnetite geochemistry in the end stage of the early Pleistocene are similar to those of the upper Yangtze. These indexes all implied that new sediment provenance from the upper Yangtze tributaries had probably joined the delta area since the 1.2 Ma ago.There are different vertical variations of heavy minerals indexes between the North Jiangsu Basin and the Yangtze River mouth area. Three distinct zones can also be divided in the Core ZKJ39 as follow according to the vertical distribution of heavy mineral characteristics in Core ZKJ39:1) In the Mid-Pliocene (>3.3 Ma), heavy-mineral assemblages were similar to Core LQ24, but the diagnostic heavy mineral changed to garnet, indicating different provenance from Core LQ24. Zircon shapes in the bottom samples were characterized by poorly long elongation, similar to the local river samples. The proportion of rounded zircon grains in the local rivers accounts for 16.7%, reflecting increases distance of sediment transport. Moreover, there is a little magnate in the strata and the magnetic susceptibility is also very low. Based on these evidences we thought that in the late stage of the Pliocene sediment source of the North Jiangsu Basin was extended to the Zhe-Min Uplift.2) In the late Pliocene (3.3-2.3 Ma), heavy mineral assemblages changed to epidote-limonite-ilmenite. The characteristic of the heavy mineral indicated that the sediment provenance was from metamorphic, intermediate-acid igneous and sedimentary rocks. Zircon morphology in the upper layer changed gradually, and the proportion of short-elongated and rounded increased. These features of zircon grains imply that the transport distance of the detrital sediment became longer. So we thought that in the late stage of the Pliocene sediment source of the North Jiangsu Basin was the mixture from the local, Zhe-Min Uplift and middle Yangtze drainage.3) The Late Pliocene to Early Pleistocene (2.3 Ma-the present). Different from the samples of the underlying strata, the heavy-mineral assemblages changed to amphibole and epidote; diagnostic heavy minerals are pyroxene and magnetite. Short-elongated and rounded zircon grains increase gradually, which was in accordance to the upper Yangtze and the estuary. Features of trace elements of magnetite are also similar to the surface samples of the upper Yangtze River. The sediment signals of the upper Yangtze imply that the catchment of the North Jiangsu Basin was expanded to the upper Yangtze basin and the Yangtze River was connected to the sea via the Jiangsu Basin since then.The provenance analysis demonstrates that the sediment provenance of the Yangtze delta shifted from local sources to the Zhe-Min Uplift and the middle-upper Yangtze basin. However, the provenance evolution in the present and the past depocenter were obviously distinct. The differences of sedimentary evolution attributed to the Neotectonic movement characterized by the uplift of the Tibetan plateau and the subsidence of eastern China coast during the Plio-Quaternary time period. From the late Pliocene, depocenter of the Yangtze delta gradually moved southward. The Yangtze paleo-channel also moved accordingly southward from the North Jiangsu Basin to the estuarine area. Hence, the Yangtze River connection to the sea through the Jiangsu Basin occurred in the early phase of the Quaternary (ca.2.3 Ma). From then on, the sediment source of upper Yangtze River was transported to the estuary area and formed a broad estuary-delta-shelf system.