• Abstract and Figures
• Public Full-text

Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 4–12 DOI: 10.1007/s13131-013-0390-5 http://www.hyxb.org.cn E-mail: [email protected] Holocene shelf-coastal sedimentary systems associated with the Changjiang River: An overview GAO Shu1* 1 Ministry of Education Key Laboratory for Coast and Island Development, Nanjing University, Nanjing 210093, China Received 20 May 2013; accepted 18 August 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013 Abstract The fate of the terrestrial sediment supplied by rivers is a critical issue for understanding the patterns of Holocene environmental change on continental shelves. The East China Sea is a typical broad continental shelf with abundant sediment supply from large rivers. Here, a variety of sedimentary records were formed during the Holocene period. The sedimentary systems associated with these records have unique charac- teristics in terms of spatial distribution, material composition, deposition rate and the timing of deposition, which are related to active sediment transport processes induced by tides and waves, shelf circulations and sediment gravity flows. The sedimentary records thus formed are high resolution slices, i.e., each record has a temporal resolution of up to 100–10–1 a, but only covers a limited part of the Holocene time. In terms of the spatial distribution, these records are scattered over a large area on the shelf. Further studies of these systems are required to understand the underlying process-product relationships. In particular, the mid- Holocene coastal deposits on the Jiangsu coast, the early to middle Holocene sequences of the Hangzhou Bay, as well as the Holocene mud deposits off the Zhejiang-Fujian coasts, should be investigated in terms of the material supply (from both seabed reworking during the sea level rise event and river discharges), transport-accumulation processes, the sediment sequences and the future evolution of the sedimentary systems. Advanced numerical modeling techniques should be developed to meet the needs of these studies. Key words: Transport-accumulation processes, Holocene sedimentary systems, process-product relationships, sedimentary records, East China Sea Citation: Gao Shu. 2013. Holocene shelf-coastal sedimentary systems associated with the Changjiang River: An overview. Acta Oceanologica Sinica, 32(12): 4–12, doi: 10.1007/s13131-013-0390-5 1 Introduction lation and sediment gravity flow; the quantity and location of Sedimentary records are important archives for sea level, en- sediment supply affects the deposition rate and its spatial dis- vironment and ecosystem change studies. They are conceived tribution; and the transport-accumulation processes control in stratigraphic sequences, with varied spatial and temporal the seabed morphological evolution and the size and shape of distribution patterns, which are affected by sea level fluctua- the sediment body. In fact, Sloss (1963) already recognized the tions. Sloss (1963) proposed that in the geological history since importance of these factors, although detailed analysis was not the Cambrian there had been six large sea level cycles, each carried out. being manifested by a set of corresponding sequences. Efforts The East China Sea, which receives the Changjiang River dis- have been made to link the sequence distributions to the po- charge, represents a typical broad continental shelf. During the sition of sea level, e.g., the “Global eustatic chart” theory pro- Holocene, sediment supply from the catchment was abundant, posed by Vail et al. (1977). However, in this theory, other factors and intense transport and dispersion of the sedimentary mate- such as original seabed morphology on the shelf, sources of rials occurred in the shelf waters. As a result, the Holocene sedi- sediment supply and transport-accumulation processes were mentary systems associated with, or in the vicinity of, the river not sufficiently treated. As a result, any analytical results on the have a high diversity in terms of spatial distribution and tempo- basis of the Vail theory should be interpreted as maximum pos- ral evolution. They provide a typical example of the product of sible time coverage by the shelf-coast deposits formulated dur- sediment accumulation on a broad shelf with large quantities ing a sea level cycle; the real time coverage may be shorter than of sediment supply. With regard to the sedimentary record, the predicted. In other words, for any specific sedimentary system, Holocene systems of the region are of importance, but appro- which is a product of a unified geomorphological environment priate interpretation of the record depends on an improved un- with distinct hydrodynamic and sediment dynamic conditions, derstanding of the formation of the sedimentary systems. The its time coverage may not be predicted accurately by the theory. purpose of the present contribution is to provide an overview of The other factors should be taken into account: the width of the characteristics of the regional sediment transport processes the continental shelf influences the accommodation space for and the resultant Holocene deposits on the basis of a synthesis the deposit and the characteristics of tidal current, shelf circu- of the data sets obtained from the shelf and coastal environ- Foundation item: a project of the Mega-Science Program supported by the Ministry of Science and Technology of China: “Land-ocean boundary processes and their impacts on the formation of the Yangtze deposition system” under contract No. 2013CB956500. * Corresponding author, E-mail: [email protected] GAO Shu. Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 4–12 5 ments, and to identify scientific questions for future studies on developed in response to the accumulation of the materials the process-product relationships and the prediction of future derived from pre-Holocene strata reworking and modern rivers sedimentary system evolution. (mainly from the Changjiang catchment) by tidal current, shelf circulation and gravity flow. These systems include: Jiangsu 2 Regional characteristics of the Holocene sedimentary sys- coastal plain, the radial tidal ridges over the southwestern Yel- tems low Sea, Yangtze Shoal, Changjiang River delta, Hangzhou Bay With a geomorphological background of a broad shelf set- incised valley and estuarine deposits, and the mud belt on the ting, a number of Holocene sedimentary systems have been inner shelf along the Zhejiang-Fujian coastlines (Fig. 1). 120° 125° 130° E 40° N K o r e a a n ul ins P en e g P n on i and n Sh su l a 35° Profile of Fig. 2 1 2 J Jeju Island P i a r ng o 3 v i s n u c e 4 Shanghai 5 Profile of Fig. 3 30° g n e a c i n j i e v h o 6 Z r P n e ia c j in u v F o r P Taiwan Island Fig.1. Distribution of the Holocene sediment systems in the vicinity of the Changjiang River mouth (base map from Li (2009)): 1 represents Jiangsu coastal plain, 2 Radial tidal ridge system in the southwestern Yellow Sea, 3 Yangtze Shoal, 4 Changjiang River delta, 5 Hangzhou Bay incised valley and estuarine deposits, and 6 Mud deposit on the inner shelf along the Zhejiang-Fujian coastlines. 6 GAO Shu. Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 4–12 On the Jiangsu coast, the shoreline advanced towards the sea include sufficient coarse-grained sediment supply and intense slowly from the mid-Holocene (i.e., around 6 500 a B.P.) to 1127 rectilinear tidal currents (Stride, 1982). In the situation of Jiang- AD in the Song Dynasty (Gu et al., 1983). During that period of su tidal ridges, the sandy materials (mainly fine and very fine time, a coastal plain of about 10 km wide was formed. Then, the sands) were produced by the scour of the underlying strata that Huanghe River (Yellow River) discharged into the region and, consist of mud and sand materials; the tidal current off the cen- during the subsequent 700 years, the shoreline prograded for tral Jiangsu coast is strong and rectilinear, which is an ideal con- 50–60 km (Gao, 2009). Thus, the growth rates for the formation dition for accumulation of the ridges and scour of inter-ridge of the two parts of the Jiangsu coastal plain are significantly dif- channels (for a scour-accumulation pattern of the tidal ridge ferent. The tidal flat deposits formed since the 12th century are field, see Fig. 2). The total volume of sediment contained in superimposed upon the mid-Holocene deposits; the prograda- these ridges is of the order of 1011–1012 m3 (Gao, 2013a). Further, tion of the tidal flat continued, even after the Huanghe River re- because the sediment grain size here is relatively small, the ma- turned to the Bohai Sea in 1855 (Li et al., 2006; Wang et al., 2006; terial may be transported partly as suspended load, with a large Liu et al., 2011b). The accumulation rate was further enhanced transport rate. Hence, the ridges can be highly mobile, i.e., the by the colonization of the cordgrass Spartina alterniflora (Xie position of the ridge crest is instable. The wide distribution of and Gao, 2009, 2013; Li et al., 2010). At the present stage, the to- the erosion surface underneath the ridges is indicative of such tal volume of the deposit has reached an order of magnitude of migration. The fine-grained material generated by ridge growth 1011 m3. Figure 2 shows the distribution of the tidal flat deposit, is transported towards the land, eventually finds its way on the along a shore-normal transect, of the central Jiangsu coast. It adjacent tidal flat (Zhu and Xu, 1982). To the south of the tidal was proposed that the continued growth of the system depends ridge field, the tidal ellipse rate is above 0.4, which means that on the material supply from seabed erosion of the old Huanghe tidal ridges cannot form; instead, under the condition of rota- River delta and the reworking of the offshore tidal ridges (Zhu tory currents, tidal sand sheets are developed, e.g., the Yangtze and Xu, 1982).

Load more

  9

Copy Link