The Three Gorges valley is one of the two key capture points of the evolution of the Yangtze River, yet the formation of this valley—from the pre-Miocene to the late Pleistocene—remains uncertain. The Jianghan Basin, a late Mesozoic–Cenozoic basin located just downstream of the Three Gorges valley, is a crucial area for understanding the formation of the valley. In this study, we used heavy mineral assemblages to trace the provenance of Pliocene–Pleistocene sediments obtained from the 300-m-depth Zhoulao drillcore in the Jianghan Basin. Results show that heavy mineral concentrations, compositions, and species display a clear change at a depth of 110 m in the studied core, consistent with the change in values of magnetic indexes and trace-element geochemical indicators. The heavy mineral assemblage deposited below a depth of 110 m (lower section of the core) comprises zircon, epidote, leucoxene, rutile, anatase, pyrite, and titanite, whereas that deposited above 110 m (upper section) consists of hornblende, pyroxene, garnet, hematite-limonite, and magnetite. In addition, the heavy mineral assemblage of the upper section is similar to that of the modern surface fluvial sediments of the Yangtze, which indicates that materials of the upper core section of the Jianghan Basin were sourced from the upper Yangtze River Basin, west of the Three Gorges. After incision of the Three Gorges valley, sediments from the upper Yangtze were transported to the Jianghan Basin and deposited. Combining the results of this study with the known paleomagnetic chronology of the Jianghan Basin, we propose that the Three Gorges valley was incised at ca. 1.1 Ma.
The Indo-Asia collision significantly changed the topography and drainage network of rivers around the Tibetan Plateau. Debate continues as to when and how the current drainage system of the Yangtze River was formed. Here we use 40 Ar/ 39 Ar dating of detrital micas (muscovite and biotite) to constrain provenances of the Pliocene sediments from the Jianchuan and Yuanmou basins in SE Tibet. Muscovite and biotite data of the same Pliocene samples from the Jianchuan Basin suggest contrasting distal v. local sources, respectively. Similarly, muscovite data of the Yuanmou Basin suggest a derivation of sediments from the Yalong River, but the characteristics of the Pliocene cobbles (palaeocurrent and subrounded cobbles) suggest that these sediments are locally sourced. Sediment reworking is proposed as an explanation for the different sediment provenance signals in the Jianchuan and Yuanmou basins that have led to the controversy of an either Pleistocene or pre-Miocene age of formation of the current Yangtze. Based on sediment provenance constraints, the evolution of the Jinsha River is reconstructed. The upper Jinsha River lost its connection with the southward flowing Red River upstream from the Jianchuan basin at least before the Pliocene. At the same time a parallel site in the Yuanmou Basin shows that the Yalong River stopped flowing southward into this basin. Detrital mica from early Pleistocene sediments at the Panzhihua site between the Jianchuan and Yuanmou basins is sourced from the current Jinsha and Yalong rivers. These results would suggest that the current upper Yangtze drainage system should have been established before the Pliocene. Supplementary material: Muscovite 40 Ar/ 39 Ar data, biotite 40 Ar/ 39 Ar data and muscovite geochemistry are available at https://doi.org/10.6084/m9.figshare.c.4821573
Changes in the grain size distribution of river sediment have environmental, ecological and social implications. This study investigated the variation of the grain size of bulk samples, detrital zircons and rutiles from the mainstream and major tributaries of the Yangtze River. The mean size of bulk samples from the upper reaches is significantly higher than the mid-lower reaches. The Equivalent Spherical Diameter (ESD) of most zircons (from previous work) and rutile grains fall within the range of 32-250 μm with dominant size of 63-125 μm. Coarse-sized zircons and rutiles with ESD of 125-250 μm are found in higher proportions in the upper reaches than in the mid-lower reaches, and a significant grain size decrease is observed downstream of the Three Gorges Dam. The significantly decreasing in coarse grains downstream of the dam indicates that the massive sediment contributed by the Three Gorges Dam (TGD), especially coarse-sized sediment. Our study demonstrates that a complex sediment routing system like the Yangtze River is interrupted by the Three Gorges Dam. The problem of grain-size bias caused by human activities on age-data acquisition and interpretation of detrital minerals (rutile and zircon) from large rivers is not negligible and deserves more attention when using single grain geochronology to constrain sediment provenance and tectonic evolution.
Abstract Quartz is one of the most common rock-forming minerals and crystallizes over a wide range of temperature and pressure conditions. This diversity of quartz crystallization environments is reflected by trace-element compositional variations, which can be used to distinguish between different source sediments. Trace elements that are incorporated into the quartz lattice form corresponding paramagnetic centers (impurity centers, such as Al and Ti centers), which can be detected using the electron spin resonance (ESR) method. However, whether the quartz impurity center ESR signal intensity (quartz ESR-SI) can be used for quartz sediment provenance tracing remains uncertain. In the present study, five present-day (modern) fluvial sediments from the Songhua, Yellow, Yangtze, Huai, and Pearl rivers in China and eight ancient fluvial sand lenses from the Yichang Gravel Layer (YGL) located in the middle Yangtze River were sampled for major- and trace-element determinations by ICP-OES and ICP-MS for the purpose of provenance discrimination. A total of 1404 ESR spectra were also collected to evaluate the effect of γ-ray dose (varying from 50 to 50 000 Gy) on quartz ESR-SI to establish the relationship between quartz element contents and quartz ESR-SIs and thereby to assess the potential utility of quartz ESR-SI for sediment provenance analysis. Results indicate that: (1) quartz collected from the different studied locations can be distinguished by element contents; (2) the quartz Al center ESR-SI increases with increasing γ-ray dose from 50 to 50 000 Gy; (3) the quartz Ti center ESR-SI increases within a γ-ray dose of 10 000 Gy and decreases beyond 10 000 Gy; (4) quartz Al and Ti center ESR-SIs are closely related to the contents of Al and Ti in quartz; and (5) a plot of quartz Ti center ESR-SI vs. Al center ESR-SI using data for a γ-ray dose range of 4000–7000 Gy is the best indicator of fluvial sediment provenance using the ESR method.
长江武汉城区段呈SSW-NNE向从城市中心流过.两岸滨江平原是武汉城市的黄金地段.分布于汉阳和武昌沿岸的滨江平原，平面形状似弓形分布于晚更新世岗地与长江之间.研究发现，其形成经历了江-洲-湖-陆的转变过程，是长江中的沙洲并岸的产物.本文以武昌北滨江平原为例，对其形成过程进行了解析.长江武汉蛇山-青山段在明代江面十分开阔，左岸相对平直，因受汉江三角洲影响岸线微向江突出；右岸弯曲呈抛物线形，边界大致在现沙湖南岸-沙湖港-青山港一线.河道中沙洲发育，河型为多分汊河道.至清代中期后，沙洲先后向右岸移动，多沙洲不断聚合，河道呈东西两支分汊，西汊道为主河道.右汊道被先后靠岸的沙洲分隔为余家湖、沙湖、白杨湖等串珠状湖泊，并不断萎缩.到1899年因自然和人为作用，右汊入口被堵，沙洲与右岸陆地相联，滨江平原形成.余家湖消失，白杨湖萎缩成现在的青山港，沙湖与长江脱离成为独立湖.其河道地貌演化与滨江平原的形成过程可归结为：多沙洲分汊河道-单沙洲分汊河道与沙洲夹湖-沙洲成陆与顺直单河道.该研究对长江中下游类同地貌环境江段滨江平原形成具有启示意义.;Yangtze River runs across Wuhan city's central area along the direction of SSW-NNE. The riverside plains of Wuhan have always been considered as the city's prime locations. These plains are distributed along the riverbanks between the Pleistocene low hilly region and Yangtze River. In this study, we have discovered that the formation of riverside plains in this area are product of incorporated by sand bars into former riverbanks, experiencing four mainly multi-transportation stages:river-central bar-lake-land. We analyzed the formation factors of the northern Wuchang Plain as a classic case. By consulting the geographic maps, we found that Sheshan-Qingshan reach of Yangtze River was much wider with asymmetric banks in Ming Dynasty (1368 AD to 1644 AD), the left bank line had straighter curvature, slightly bulging into the river under affect by Hanjiang River Delta. The right bank line, however, had a parabolic shape and outline stretching along the modern southern bank of Lake Shahu, via Shahu harbor to Qingshan harbor. Multiple channel branches and sand bars developed on the Yangtze River. Until mid-Qing Dynasty, the sand bars constantly shifted towards the right bank and fused together to divide the river channel into two branches, the main drainage branch was to the west, the right branch was clogged and separated by fusing sand bars into bead-like lakes, including Lake Yujiahu, Lake Shahu and Lake Baijiahu. In the 1899 AD, causing by natural and human activities, the entrance of right branch had been blocked, the sand bars incorporated with the right bank and formed the modern riverside plain. The Lake Yujiahu disappeared, the Lake Baiyang shrunk into modern Qingshan harbor, the Lake Shahu became interior lake during the same period. Hence, the evolution of river channel and riverside plains can result into the following model:Multiple River channel branches with multiple sand bars-Dual River channel branches with single sand bar and lakes-Single straight wide river channel with sandy banks. Our study can provide rewarding insights for studying formation process of similar stretches of Yangtze River's riverside plains along the middle and lower reaches of Yangtze River.
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