Li Congxian

The fills and stratigraphic sequences in the Qiantangjiang incised paleovalley, China

ABSTRACT The tide-dominated Qiantangjiang Estuary can be zoned into three sections: upper normal river section, middle estuary funnel, and lower estuary mouth. Strong rejuvenation and incision of the Qiantangjiang River formed a type I sequence boundary and a paleovalley in the last glaciation. During the subsequent transgression and highstand, the incised paleovalley was filled and flooded, forming a complete lowstand-transgression-highstand sedimentary sequence including sequence boundary, lowstand systems tract, transgressive systems tract, maximum flooding surface, and highstand systems tract. The fluvial-channel sandy gravels and gravelly sands at the bottom of the Qiantangjiang incised paleovalley are composed of early lag sediments deposited during river incision, and later aggradational sediments resulted from baselevel rise whose demarcation is the transgressive surface. The aggradational sediments are distinct from the lag sediments in their aggradational parasequence assemblage, fining.up. ward sequence, and relatively later formation. The formation and evolution of the Qiantangjiang Estuary can be divided into four stages: Last Glaciation (20,000-15,000 yr B.P.): formation of incised paleovalley. (2) Early postglacial transgression (15,000-7500 yr B.P.): filling of the paleovalley. (3) Maximum transgression (7500-6000 yr B.P.): formation of the bay. (4) Highstand period (6000 yr B.P. to present): evolution of the estuary. Qiantangjiang River sediments contribute only a small part of the enormous volume of deposits trapped in the Estuary, whereas the adjacent Yangtze River is an indirect supplier of major sediments. The specific characteristics of the sedimentary sources in the estuary result in its unique sediment pattern, i.e., sandy gravels, gravelly sands, fine sands, and muddy sands from upstream to downstream, which is obviously different from the normal coarse-fine-coarse sediment distribution pattern in estuarine areas.

The influences of suspended load on the sedimentation in the coastal zones and continental shelves of China

Abstract The coastline of China intersects belts of subsidence and uplift. River sediments mainly discharge to the coastal zones of subsidence belts, where they contribute more than 90% of the total suspended load of river sediment transported into the coastal zones. Rivers discharging into the coastal zones of the uplift belts contribute no more than 10% of the total. In coastal zone subsidence belts, tidal-flat deposits develop, and Quaternary strata are quite thick. In coastal zone uplift belts, in contrast, beach sand and gravel and aeolian sand develop. Only where great amounts of fine sediments are provided to coastal uplift zones by the adjacent major rivers can tidal-flat deposits develop. Quaternary strata are thin in uplift belts, and patchy relict sediments are present on the continental shelves. On the continental shelves of the southern part of the East China Sea and the South China Sea, where discharge of river sediments is fairly small, relict sediments form a broad continuous deposit. Marine processes can transport the sediments of large rivers into the mouths of small adjacent rivers where various sand and mud bodies form. So far in the coastal zones of China no coastal depositional body is known to have formed from sediments derived directly from the continental shelves.

Monazite age spectra in the Late Cenozoic strata of the Changjiang delta and its implication on the Changjiang run-through time

The Late Cenozoic strata are 313 m thick, revealed by the drilling core PD-99 in the south Changjiang delta. Monazite chemical dating shows that 350–500-Ma monazites predominate in the Pliocene and 100–275-Ma monazites in the Quaternary, indicating a great change of their provenance. The first presence horizon of monazites younger than 25 Ma is just above the Matruyama/Gauss boundary (~2.58 Ma), whch is exactly when uplift of the Tibetan Plateau began to influence deposition in the East China Sea. Variations in contents of monazites younger than 25 Ma can be divided into two sections. The Early-Middle Pleistocene with less <25 Ma monazites corresponds with rapid uplift of the Tibetan Plateau, and the Late Pleistocene with more <25 Ma monazites parallels the peak uplift of the Tibetan Plateau. This study demonstrates that chemical dating of monazites in the river-mouth strata is a useful method to explore changes of river drainage basins, and deconvolute multistage tectonic and magmatic activity histories in the provenance areas.

Hydrological Regime and Morphodynamics of the Yangtze River Mouth Area

The features of the hydrological regime, modern morphological structure, fluviomorphological processes, and the history of formation of the Yangtze River mouth area are discussed. The influence of the huge river water and sediment runoff and tides on the hydrological and morphological processes in the Yangtze River mouth is shown. The possible impact of construction of the large waterworks “Three Gorges” on the hydrological and morphological processes in the Yangtze River mouth is roughly evaluated.

Stratigraphy of the late quaternary barrier–lagoon depositional systems along the coast of China

Abstract Four sedimentary facies assemblages of barrier-lagoon systems along the coast of China can be distinguished. Facies assemblage 1 is underlain by terrestrial layers and overlain by marine layers, with marine influences increasing from the bottom of the sequence to the top. This facies assemblage is formed during transgression and is called the transgressive facies assemblage. Facies assemblage 2 is underlain by marine layers and overlain by terrestrial layers, with a decreasing tendency of marine influences from the bottom of the sequence to the top. This facies assemblage is formed during regression and is called regressive facies assemblage. Facies assemblage 3 is both underlain and overlain by terrestrial layers. This assemblage is formed when the coastline remains stationary, and is referred to as stationary facies assemblage. Facies assemblage 4 is modified from the above assemblage, and involves the overlying of barrier islands upon lagoonal sediments. This type of facies assemblage is formed during local transgression and is caused by changing sediment supplies; it is classified as locally transgressive facies assemblage. Transgressive facies assemblage occurs in the lower part of the postglacial strata in coastal and deltaic plains up to the boundary of maximum postglacial transgression. Regressive facies assemblage occurs in the upper part of Holocene strata between the boundary of maximum postglacial transgression and the present coastal line. Stationary facies assemblage occurs in the narrow zone near the maximum transgressive limit. The coast of China cuts across several tectonic subsidence and uplift belts. More than 90% of the total riverine sediments discharge into the subsidence belts, forming broad coastal plains and thick depositional strata. In such areas, favorable conditions exist for the development and preservation of various kinds of barrier-lagoon systems. In the uplift belts, due to the poor sediment supplies, stationary and locally transgressive facies assemblages tend to be developed. The best sites for the development of barrier-lagoon systems are in the transitional areas between uplift and subsidence belts.

Paleocurrent of the tidal sand body in Subei coastal plain

Based on analysis of magnetic febric and magnetic relict, the paleocurrents racorded in drilling cores in Subei coastal plain are bidirectional. NE-SW in the northern core and NW-SE in the southern core. The paleacurrent pattern in the area is similar to that of the tidal current system in the South Yellow Sea. So it is inferred that the convergent-divergent tidal current system existed during stage of sand body formation, and they should be radial in plane.

Characteristic elemental compositions of the Yangtze River and Yellow River surface sediments and their geological background

Significant differences are noticed in major and trace element compositions between the Yangtze River and the Yellow River surface sediments. The former sediments are rich in some major elements such as K, Fe, Mg, Al, and most of the trace elements which show wide variations in element concentrations, whereas the Yellow River sediments only have higher Ca, Na, Sr, Ba, Th, Ga, Zr, Hf contents and show slight variations in element contents. In the Yangtze River Basin are widely distributed intermediate-acid igneous rocks and complicated source rocks together with strong chemical weathering which determine the elemental compositions of the Yangtze River sediments, while the elemental compositions of the Yellow River sediments are decided by the chemical composition of loess from the Loess Plateau and intense physical weathering. Cu, Zn, Sc, Ti, Fe, V, Ni, Cr, Co, Li and Be can be used to distinguish the Yangtze River sediments from the Yellow River sediments and be treated as tracers for both the sediments to study the processes of their mixing and diffusion in the coastal zones of China.

The characteristics and significance of magnetic fabrics of muddy tidal flat

ConclusionsThe magnetic fabric parameterK of the tidal flat with the same material sources in the wide coastal zone is a high-resolution indicator for the recognization of sedimentary dynamically active zones of tidal flat, which indicates that the magnetic fabric method could be widely applied to the study of sedimentology, comparative sedimentology and sedimentary dynamics.The high and low values of magnetic fabric parametersq, F andL occur alternately, which indicates the presence of microbedding structures, such as small-scale sequences consisting of a single muddy layer and a sandy layer and small-scale sequence series. This indicates that the magnetic fabric analysis could be a new effective method for the study of sedimentary microstructure sequence series of tidal flats.The statistical results of parametersK, L, F andq have revealed that correlations between the variations of magnetic fabric parameters of sediments and the sedimentary dynamic environments are a new way for the studies of tidal flat types and sedimentation.

Riverine sediments and chinese coastline changes

The tectonic uplifting and depression in the coastal zones results in the anormal distribution of riverine sediments. The coastal zones of tectonic depression areas received about 95% of the riveine sediments, whereas the tectonic uplift belts received only 5% of that, which is the main reason for the variety of the coastline types and the changes in the coastline. On the basis of this anormal distribution of riverine sediments in Chinese coastline, this paper discusses the scope and time of the maximum transgression, the trend, rate and period of the coastline changes, and the impact of riverine sediments on the future changes in coastline.

Roundness Parameters in Barrier-Lagoon Systems in Northeastern Shandong Peninsula, China: Limitations to Environmental Discrimination

Twenty-five samples from six subenvironments in the barrier-lagoon systems in northeastern Shandong province, China, are examined. A statistical method is used to study the roundness variation of grains of different sizes. Roundness of very fine pebble and very coarse sand varies significantly in different subenvironments. It is possible to discriminate among aqueous depositional environments using the roundness of grains of these sizes. Roundness of grains finer than 0.84 φ is not distinguishable in different subenvironments.