Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf

Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf

Journal of Asian Earth Sciences xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.e...

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Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf Shu Gao a,⇑, Yunling Liu a, Yang Yang a, Paul J. Liu b, Yongzhan Zhang a, Ya Ping Wang a a b

Ministry of Education Key Laboratory for Coast and Island Development, Nanjing University, Nanjing 210093, China South Carolina University, USA

a r t i c l e

i n f o

Article history: Received 12 January 2015 Received in revised form 7 July 2015 Accepted 20 July 2015 Available online xxxx Keywords: Distal mud deposits Continental shelf sedimentation Evolutionary stages River delta systems Holocene China’s rivers

a b s t r a c t The sedimentary characteristics of distal mud deposits, a product of along shore transport of river-discharged suspended sediment, contain information on their different evolutionary stages. In the present study, the distal mud associated with the Pearl River, southern China, is investigated using the data sets obtained from seabed sediment sample analysis, shallow geophysical survey and 210Pb dating. The results indicate that although the mud deposit in consideration occupies an area of more than 8000 km2, the thickness is small with a young age (i.e., <102 yrs). Shallow seismic survey reveals that the Holocene strata have a thickness of around 10 m or less, with the lower layers being characterized by reworked deposits, rather than the distal mud deposits. The internal sedimentary structure shows that clinoforms are poorly developed. Compared with the distal muds of the Yangtze River (on the inner shelf of the East China Sea) and Yellow River (in the northern Yellow Sea), the distal mud here is still at its young stage. In contrast, those associated with the Yangtze and Yellow Rivers have already reached their growing and mature stages, respectively. This difference in the evolution stage is caused by the estuarine processes that control the timing and duration of the distal mud formation. Furthermore, since both river mouth deltas and distal mud deposits, at their mature stage, would be recognized as deltaic deposits in the geological record, it is necessary to establish appropriate criteria that can be used to distinguish between the two types of deposits. They contain different signals of sea level positions; hence, caution should be taken in interpreting the formation of ‘‘shelf edge deltas’’, which have been found on the outer shelf regions of the South China Sea and in many places elsewhere. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Mud deposits in coastal areas and on continental shelves result from various transport and accumulation processes (for a review, see Gao and Collins, 2014). On tidal flats, tidal currents (or combined currents and waves) cause landward transport of fine-grained sediment by a number of mechanisms, e.g., lag effects and time–velocity asymmetry (Van Straaten and Kuenen, 1958). On the continental shelf, low frequency currents driven by wind, seawater density variation and freshwater discharge lead to sediment dispersal toward deep water areas, in addition to sediment gravity flows that occur in slope areas. Because the settling velocity of fine-grained particles is low, they may reach the continental slope or beyond before reaching the bed. Thus, they may escape from the shelf and accumulate eventually onto the deep ocean bottom. Where the shelf is wide, or the seaward current is weak, ⇑ Corresponding author. E-mail address: [email protected] (S. Gao).

the fine-grained sediment may deposit on the outer shelf, forming an isolated mud patch (Gao and Jia, 2003). Some mud patches are formed temporally, subject to subsequent reworking by storms or other types of strong currents (Ogston et al., 2000). Others form permanent mud deposits, with persistent, continuous accumulation of the particles that settle from the water column. Furthermore, fine-grained materials, especially those from rivers, may be dispersed in a longshore direction, by shelf currents; before settling onto the bed the particles would have travelled a long distance (i.e., 101–102 km). The product of such a transport mechanism is the formation of distal mud deposits. A classic example is the muds associated with the Po River in the Adriatic Sea (Cattaneo et al., 2003), where the deposits are distributed on the inner shelf, extending a distance of 102 km from the river mouth. In the eastern China shelf seas, which represent a typical wide shelf with abundant sediment supply, large-scaled distal muds have been found; the Yangtze and Yellow Rivers both have a mud deposit of their own which, once again, is located 102 km away from the river mouths (Liu et al., 2004; Xu et al., 2012).

http://dx.doi.org/10.1016/j.jseaes.2015.07.024 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024


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The mud deposits represent an important sedimentary record for analyzing the past environmental changes (Lantzsch et al., 2009). For instance, they provide high resolution records (i.e. at a temporal scale of 101 yr or even within 100 yr) of the Holocene periods. Although the spatial distribution of the deposits is patchy (due to complex sea level change and transport processes), they can be connected chronologically if there is continuous terrestrial sediment supply (Gao, 2013). In the case of tidal flat deposits, their upper layers are excellent records with a high resolution (within 100 yr) and continuity (for a period of 102 yr or longer) (Gao, 2009). Similarly, the records associated with shelf mud patches and distal muds have a resolution of 101 yr, covering varied lengths of the Holocene period. Hence, in order to make full use of the sedimentary record, it is necessary to obtain an improved understanding of the spatial distribution patterns and the temporal evolution of these mud deposits. The distal muds associated with rivers have a more complicated internal structure, compared with the other mud deposits. Previous studies about the distal muds of the Yangtze and Yellow Rivers have revealed distinct clinoform patterns (Liu et al., 2004, 2006; Liu et al., 2007). In the northern South China Sea (SCS), the Pearl River, one of the largest rivers over the region, also has a distal mud area, located to the west of the river mouth (Qin, 1963; Owen, 2005). Recent investigations indicate that the major body of this mud deposit has an extensive spatial coverage, yet the late Holocene sedimentary sequence is thin, without fully-developed clinoforms; by comparing the various characteristics of this mud deposit with its counterparts of the Yangtze and Yellow Rivers, Liu et al. (2014) have proposed that the differences observed are related to the varied timing and duration of sediment infilling processes of these rivers. The present contribution is a continuous effort to address a critical issue associated with the distal mud deposit, i.e., the evolution stages of distal mud formations. Based on the data sets obtained from the northern SCS shelf, the purpose of the present study is to: reveal the sedimentological characteristics of the Pearl River distal mud; identify the present status in terms of the distal mud evolution; and discuss about the spatial–temporal distributions of the various deposits associated with a river, in response to sediment supply and sea level changes. 2. The environmental setting The northern continental shelf of the South China Sea has a width of around 200 km (Liu et al., 2002), with a bed slope of approximately 0.08° over the inner shelf waters (Feng and Zheng, 1982). The study area is located within the coastal waters to the west of the Pearl River estuary, on the northwestern inner shelf of the SCS (Fig. 1). This region is influenced by the East Asian monsoon (Liang, 1991): in winter northeasterly winds prevail over the SCS region, whilst in summer weaker southwesterly winds dominate over most parts of the SCS (Hu et al., 2000). The oceanic circulation in the SCS is characterized by seasonal variations in response to the monsoon winds, accompanied by the intrusion of the Kuroshio Current via the Luzon Strait in the north (Su, 2004; Xiu et al., 2010; Yuan et al., 2014). In the study area, the shelf circulation is dominated by currents toward the west in winter, but in summer the currents are directed toward the east; the average current speed in winter, with a magnitude of around 0.26 m s 1, is larger than that in summer (Wang, 2007). A number of rivers (i.e., the Pearl River and other small rivers) discharge into the coastal waters of the study area (Wang, 2007) (Fig. 1a). The Pearl River annually carries an amount of 54–80  106 t sediment into the SCS since the 1950s, with about 5% of the materials being transported as bed load (Zhang et al.,

2008, 2012). This quantity of sediment discharge far exceeds those of the other local rivers, e.g., the Moyang River (0.8  106 t yr 1) and the Jianjiang River (1.9  106 t yr 1), but it is smaller than the sediment discharge of the adjacent Red River in Vietnam (130  106 t yr 1) (Feng et al., 1988; Huang and Wang, 1998). The plume of the Pearl River has remarked seasonal variations (Dong et al., 2004; Liang, 1991). In the wet season (i.e., summer), the water column is stratified inside the estuary, and on the shelf the water mass is driven by the prevailing southwesterly winds toward the east; however, the suspended sediment associated with the Pearl River plume is still transported toward the west due to a coastal current (Su, 2004; Wang, 2007; Gan et al., 2009). In the dry season (i.e., winter) vertically homogeneous high salinity coastal waters occupy the eastern part of the estuary (Dong et al., 2004); the plume escapes the estuary on its western side and moves toward the west on the inner shelf (Yang et al., 2003). Thus, in both summer and winter seasons the fine-grained sediment from the Pearl River mainly travels to the southwest, with the study area being a sink of this sediment input. Since the alongshore residual current has a speed of 10 1 m s 1 in terms of order of magnitude and the suspended sediment particles have a settling velocity of 10 1–10 2 mm s 1 (assuming that the suspended particles have a median diameter of silt sizes, Xia et al., 2004), the fine-grained sediment from the Pearl river would travel for 100–102 km for 10–20 m water depths, before settling onto the bed. Further, flocculation is relatively weak outside the Pearl River estuary (Hu and Li, 2009; Hu et al., 2011). These conditions favor long distance transport of the fine-grained sediment. Northeasterly to southeasterly waves dominate over the study area (Wang, 2007). During the summer, the waves on the shelf have an average height of 0.5–2.5 m, with a maximum of 7.0– 9.5 m. The large waves are mainly associated with typhoon events. The waves are stronger in the winter season: they have an average height of 0.5–4.0 m, with a maximum of 9.0–9.5 m. There is a general trend that the wave height decreases from the Pearl River mouth toward the west. The tides of the region are irregularly semidiurnal in character (Wang, 2007). The tide range increases from the east (around 1.5 m, near the Pearl River mouth) toward the west (around 2.2 m, near the western and of the study area). On the inner shelf, the tidal current speed reaches a maximum of 0.5–1.0 mm s 1, with a time–velocity asymmetry of longer duration and weaker current during the flood phase of the tide (Feng et al., 1982, 1994).

3. Methods 3.1. Seabed sediment sampling and analysis In the summer of 2009 seabed sediment sampling was carried out using a fishing boat. Eight sediment cores with lengths varying from 0.83 to 1.25 m were collected with a gravity corer (for the sampling sites, see Fig. 1a). In laboratory, sub-samples were taken from the short cores at 1 cm intervals. Grain-size analysis for these sub-samples was carried out using a Malvern 2000 laser diffraction particle sizer; the analytical procedures are described in Liu et al. (2014). Further, bed sediment samples were also collected from the river and estuarine waters of the Pearl River and a number of local rivers (Fig. 1a). The sediment classification scheme of Folk (1954) was used here because material coarser than 2 mm, except for a few large bivalve shells, was absent in the collected sediment samples. For the analysis of 210Pb radiochronology for the short cores, the sub-samples were freeze-dried after measuring their water content and ground in an agate mortar. Sediment powder of 2–3 g was spiked with a known volume of 209Po and digested in concentrated

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024

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Fig. 1. The study area and the sites for sediment sampling and seismic survey: (a) location of gravity core samples, river bed sediment samples, and ORE shallow seismic survey lines, for the 2009 cruise; and (b) Chirp survey lines for the 2007 cruise.

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024


S. Gao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Table 1 Summary of the data sets used in the present study. Data type

Year of sampling/ measurements


Short cores ORE seismic survey Chirp seismic survey

2009 2009 2007

Seabed and river sediment sampling Seabed sediment sampling Tidal cycle measurements of current velocities, suspended sediment concentrations and water levels Short cores over the inner shelf near the Pearl River mouth

2005 2009 2009

Liu et al. (2014); this study Liu et al. (2014); this study Liu et al. (2009) and Ge et al. (2014); this study This study Liu et al. (2014) Liu et al. (2014)


Yim et al. (2006)

HF, HNO3 and HCl. Then, 209Po and 210Po were plated onto nickel planchets and counted by alpha spectroscopy to determine the total 210 Pb activity. 226Ra-supported activity was assumed to be equal to a low uniform value below the radioactive decay section of sediment cores (Palinkas and Nittrouer, 2007; Szczucin´ski et al., 2009), and the unsupported (excess) 210Pb activity was extracted from the total activities. The deposition rate (DR) was determined from the logarithmical regression profile of 210Pbex versus depth with the constant initial concentration (CIC) model (Robbins and Edgington, 1975; McKee et al., 1983; Chung et al., 2004).

types, and stratigraphic analyses using cores and sub-bottom profilers. A data set has been selected form literature and is used in the present study. These data, together with the data collected from this study, are listed in Table 1. These data sets are synthesized to establish a general picture of the geometry, internal sedimentary structures, the age of the sedimentary sequences, and the water depth of the distal mud deposit. On such a basis, the distal mud of the modern Pearl River can be compared with those of the modern Yangtze and Yellow Rivers, and with the delta systems of the paleo-Pearl River (in the forms of shelf edge deltas).

3.2. Shallow seismic survey

4. Results

Two campaigns of shallow seismic surveys were organized for the study area. The first survey was conducted in 2007, using a Chirp sonar (for the survey lines, see Fig. 1b). The data set obtained, together with a preliminary analysis, was present in Ge et al. (2014). The second survey was undertaken in the summer of 2009, along two profiles (in alongshore and across-shelf directions, respectively) (Fig. 1a). For this survey, an ORE Geoacoustic5210 sub-bottom profiler was used, with the position being fixed by a DGPS. The data have been presented by Liu et al. (2014), to reveal the thickness of the Holocene deposits of the area. In the seismic data treatment, a seismic wave velocity of 1500 m s 1 was assumed for time-to-depth conversion.

4.1. Seabed sediment characteristics

3.3. The use of available historical data sets In the past, various investigations were carried out for the study area, such as hydrodynamic and sediment dynamic observations, sediment transport and accumulation patterns, seabed sediment

The seabed sediment samples reveal a large mud patch with a mud content of P90%: it covers an area of approximately 8.87  103 km2 (Fig. 2). Grain size analysis shows that there are five major types of sediment on the northwestern inner shelf of the SCS, i.e., clayey silt, silt, sandy silt, silty sand and sand according to the Folk (1954) classification scheme. Relatively coarse sediments are found only at several discrete inshore patches, especially in the area to the south of the Jianjiang River mouth. In contrast, the fine-grained sediments are distributed more widely than the coarser fractions over the study area. This observation indicates that the northwestern inner shelf of the SCS is dominated by fine-grained sediments. The content of mud (with mean grain size 6 62.5 lm, i.e., clay and silt) for most surficial sediment samples exceeds 50%. Generally, the belt of fine-grained sediments extends roughly parallel to the shoreline, except for the shelf near Hong Kong where the muddy sediments extend across the shelf.

Fig. 2. Seabed sediment types (S = sand; TS = silty sand; ST = sandy silt; T = silt; and YT = clayey silt, on the basis of Folk (1954)).

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024


S. Gao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx Table 2 Sedimentary characteristics of the samples from local rivers and estuaries. Mean grain size (u)

Sediment type

22.19275° 22.22736° 22.57439° 22.59227° 22.74637° 22.80162° –

– 13.30 8.50 6.90 19.07 7.96 10.51

– Sandy mud Mud Mud Sandy mud mud Silt

112.00804° 112.02053° 112.00532° 111.79897°

21.82888° 21.79293° 21.78433° 21.95750°

9.19 8.04 10.94 12.52

Silt Silt Sandy mud Sandy silt

Jimei (Wuchuan city) Yanjiang road (Wuchuan city) Sanduan village Darentang village

110.73783° 110.75153° 110.62392° 110.62492°

21.47147° 21.42034° 21.34642° 21.28121°

419.47 251.87 10.37 35.24

Clayey sand Clayey sand Silt Sandy mud

Zhanjiang R.

Maxie Sanbaixi village Shimen bridge

110.42706° 110.42929° 110.38524°

21.19819° 21.27429° 21.39155°

152.66 387.53 11.16

Clayey sand Clayey sand Sandy mud

Nandujiang R.

Dongshan bridge Anding county Longtang town Nandujiang bridge

110.19549° 110.31507° 110.41974° 110.40782°

19.743510° 19.70533° 19.88632° 19.97426°

52.81 912.55 69.48 30.87

Muddy sand Sand Muddy sand Sandy mud

River system

Sampling location

Pearl R.

Hongqi farm Denglong village Zhongshan port Nansha town Hengli town Nansha port Modaomen-doumen

113.37079° 113.37783° 113.51259° 113.64861° 113.54004° 113.58312° –

Moyang R.

Nalong river Beijin village Duian-Nanbu Shuangjie town

Jianjiang R.

Although in many cases the sediment grain size is not a reliable provenance indicator, it may contain important signal of sediment sources if the source area have significant differences in grain size parameters. In the case of our study area, since the local rivers other than the Pearl River all have small sediment input and contain mainly sandy materials (Table 2), the Pearl River would provide the majority of the fine-rained materials for the distal mud. Further, the grain size information is not the only evidence: sediment dispersal in relation to shelf circulation patterns (Su, 2004; Wang, 2007; Gan et al., 2009) in the published literature provided additional evidence. Collectively, these data indicate that the Pearl River is a major source for the distal mud deposit. In the short cores, the content of mud (i.e., the mixture of clay and silt sized materials) varies in a wide range from 23.9% to 99.5%, with a mean value of 61.7% (Fig. 3). Moreover, the mud content for most sediment samples has a trend of decrease with depth, except for two stations (i.e., C01 and Z01) that are located at the edge of the mud deposit. Pure mud (with mud content > 90%) occurs mainly in the topmost thin layers. The sedimentary structure of the sediment cores is primarily characterized by inter-layered muddy and sandy sediments, with a cyclic pattern (Fig. 3). Except for C01 and Z01, the mean grain size shows a trend of fining upwards. In terms of the other grain size parameters, the most significant vertical change is associated with skewness (Liu et al., 2014): from the bottom toward the top, positively skewed patterns are replaced by negatively skewed patterns, with the boundary being at 0.5–0.6 m below the bed surface. Apparently, the course-grained materials do not result from the settling of suspended sediment within the water column, i.e., they are not directly related to the transport of the river plume from the Pearl River. The local river discharges course materials into the sea. The sediment samples from these rivers are consistent with such sediment types (Table 2). It is possible that during flooding events some sedimentary materials escape from the river mouth, being transported toward the inner shelf areas. Further, there is another possibility: some materials may have been derived from reworking of the bed. Over the inner shelf areas, the mud layer is generally thin, over lying a sandy sequence; the age of the sandy layers, on the basis of 14C dating, is often old (i.e., >6000 yr BP) (Yim et al., 2006; Ge et al., 2014). This means that the sandy material may consists of ‘‘relict sand’’ which is being modified by modern processes.

4.2. Deposition rates The 210Pb excess activities of the top layers of the cores vary from 13.2 Bq kg 1 to 86.0 Bq kg 1 (Fig. 3). The relatively high values of 54.9–86.0 Bq kg 1 occur at the stations Z03 and Z05, whereas the low values of 13.2–19.0 Bq kg 1 appear at the stations C01 and Z01. The 210Pb excess activities show a general decrease with depth. The variations in the vertical distributions of 210Pb excess activities should be related to the composition of the sediment. In general, this kind of distributions of 210Pb excess activities indicate that sedimentary environment is stable in the study area, i.e., continuous accretion has occurred in the last 102 yrs. Hence, it is appropriate to apply the CIC model to the calculation of modern deposition rates. According to the 210Pb analysis, the deposition rates vary from 1.3 mm yr 1 to 5.7 mm yr 1, with relatively high values being found at the stations Z02, Z06 and C02 (water depths 25–40 m) and relatively low values at C01 (water depth 20 m, with a low mud content), Z01 and Z05 (water depth 45–55 m). Thus, the deposition rate is relatively high at the center of the mud deposit, but decreases toward the edge of the mud belt (close to the middle shelf). Thus, because the mud layer is thin, and the deposition rate is relatively high, the period for the accumulation of the mud layer cannot be long: it has an age of 102 yrs in terms of orders of magnitude. In other words, the formation of the mud deposit only represents a recent event. Elsewhere, deposition rates of the same order of magnitude have been obtained for the region, using radiocarbon analysis. For instance, Yim (1994, 1999) and Yim et al. (2004) found that the average sedimentation rate off Hong Kong (outside the Pearl estuary) coast ranges from 0.5 to 1.4 mm yr 1. This rate fits in well with the rates of 0.8–1.0 mm yr 1 during 7.84–3.95 kyr BP (Owen et al., 1998). Similarly, Wei and Wu (2011) showed that the average accumulation rate for the Holocene was 1.3 mm yr 1 at the time of 10.0–7.5 kyr BP, 2.3 mm yr 1 for the period of 7.5–5.0 kyr BP, 1.9 mm yr 1 between 5.0 and 2.5 kyr BP, and 2.3 mm yr 1 thereafter over the entire Pearl delta. Huang et al. (1983) obtained slightly different rates, i.e., 2.7 mm yr 1 during 7.5–5.0 kyr BP, 2.2 mm yr 1 for the period of 5.0–2.5 kyr BP, and 2.2 mm yr 1 thereafter. On the open shelf, the Holocene sedimentation began from 7 to 5 kyr BP, with a deposition of around 1 mm yr 1, and the deposits were characterized by inter-layered sand and mud

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Fig. 3. The sediment composition and


Pb excess activity profiles for the gravity core samples.

containing shell debris (Wang, 2007). These studies also indicate a difference between the Pearl delta within the river mouth and the adjoining shelf regions: the former had a relatively high deposition rate and stable sediment supply, but the latter had lower deposition rates, with hiatus in sedimentation. 4.3. Seismic facies and sequence patterns The two sub-bottom seismic profiles have a length of 41 km for the N–S profile and 129 km for the E–W profile (Fig. 4), intersecting near Core C02. Three seismic facies units with concordant reflections, named U1, U2 and U3 from the top toward the base, can be recognized in the depositional sequences with a total thickness of around 50 m, based on the classic analytical procedures of seismic stratigraphy (Mitchum et al., 1977; Tang et al., 2010) and the observed reflector continuity and amplitude and their configurations and terminations. The base of each unit is shown by a heavy solid and/or dotted line. Since the unit U1 represents the Holocene post-glacial deposit, our analysis was concentrated on the characteristics of its sequences. For the N–S trending seismic profile, the

unit U1 is characterized by a high lateral continuity. Its thickness varies between 0.92 m and 6.43 m, thinning seaward (Fig. 4). Along the E–W seismic profile, the U1 unit, with a stable thickness of 6.9–10.3 m, also has high lateral continuity, except for the locations where the strata were interrupted by a paleo-valley (Fig. 4). The relatively small thickness of the Holocene deposit here is consistent with the patterns observed for ‘‘sediment starved’’ shelves (e.g., Flemming and Davis, 1994). Furthermore, the seismic record does not show well-developed clinoform structures within the thin Holocene sedimentary layers. From the Pearl River mouth toward the study area, the Chirp record also reveals the spatial distribution of the Holocene sequences. Maximum thickness occurs at the subaerial Pearl delta, with the thickness decreasing toward the study area (Fig. 1b). Along a typical transaction (highlighted in Fig. 1b), the thickness of the Holocene strata reach up to 10 m, occurring at the depo-center; in other places, the thickness is mostly smaller than 10 m. The water depth was 20 and 35 m at the northern and southern ends of the line, respectively. Ge et al. (2014) also analyzed the other Chirp profiles shown in Fig. 1b, which all show the same

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024

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Fig. 4. Seismic records of the ORE sub-bottom profiler, along the two transactions, showing thin Holocene deposits. There is a lack of well-developed clinoform structures.

characteristics. Once again, these records do not show any well-defined clinoform structures. 5. Discussions 5.1. Evolutionary stages of the distal mud clinoforms As outlined above, the distal mud in consideration has the following distinct characteristics.

First, the main body of the Holocene deposit has a small thickness, and the mud layer on the top is even thinner, i.e., of the order of 1 m. As such the original seabed morphology has not been modified yet substantially during the late Holocene period. In the direction toward the Pearl estuary, the thickness increases only slightly (Qin, 1963; Owen, 2005). Second, the 210Pb dating reveals a deposition rate of several mm yr 1. Such an order of magnitude has been confirmed also by tidal cycle measurements; the level of suspended sediment

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024


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Table 3 Comparison of the distal mud deposits associated with the Pear, Yangtze and Yellow Rivers. River

Thickness of the distal mud deposit

Age of sediment

Clinoform features

Water depth at above the mud



Generally less than 10 m

Poorly developed

20–50 m


Maximum thickness more than 40 m Maximum thickness more than 40 m

Lower part older than 6000 yrs, top mud layer 102 yrs Lower part older than 6000 yrs, top mud layer 2000 yrs Nearly continuous over the Holocene period

Mainly convergence clinoforms Convergence and parallel clinoforms

20–30 m

Liu et al. (2014) and Ge et al. (2014); this study Liu et al. (2006) and Xu et al. (2012) Liu et al. (2004)


concentration and the tidal current velocity is consistent with such an accumulation intensity (Liu et al., 2014). Hence, the temporal duration of the mud layer should be of the order of 102 yr. Third, within the thin Holocene sequences, clinoforms are poorly developed. In the best case, clinoforms representing an early stage are present (Ge et al., 2014). This situation is very different from the distal clinoforms found elsewhere, e.g., the distal mud of the Po River in the Adriatic Sea (Cattaneo et al., 2007), and that associated with the Yellow River in the Yellow Sea, northern China (Liu et al., 2004). Finally, the depo-center of the mud deposit lies over the area with a water depth of 25–40 m; outside the areas the deposition rate is reduced. Thus, there is a trend that the seabed bathymetric change would be more rapid than the surrounding areas and, over a long period of time, a sigmoid structure would become significant. To explain the young age of the mud deposit, Liu et al. (2014) have proposed that on a regional scale, the timing of the formation of distal muds associated with large rivers depends on the estuarine morphological evolution and the quantity of sediment discharge. In the case of the Pearl River, because the estuarine waters consisted of a number of large embayments during the mid-Holocene periods, the sediment from the catchment was mainly trapped in its estuarine waters (Wu and Yuan, 1995; Wu et al., 2006). The infilling of the estuarine waters is still continuing today. As a result, the quantity of fine-grained sediment that escapes from the river mouth would have been small until around 100 yrs ago. Such a mechanism can be seen more clearly when the Pearl River distal mud is compared with those of the Yangtze and Yellow Rivers. The three rivers all have large sediment discharges, with the Pearl River having the lowest discharge (i.e., up to 80  106 t yr 1). In terms of the magnitude of the estuary space, however, the Pearl estuary is the largest, with the Yellow river being the smallest. The Yellow River did not have a large embayment as its estuary. Hence, the development of the distal mud, some 500 km away from the river mouth, began almost simultaneously with the river mouth delta. Although the Yellow River often shifted its river course during the Holocene, its delta areas have continuously provided the sedimentary materials for the growth of its distal mud. Thus, thick distal sedimentary sequences were formed, with well-developed clinoform structures (Liu et al., 2004, 2007). With regard to the Yangtze River, the main body of the modern delta began to develop at around 6 kyr BP (Hori et al., 2001; Gao, 2007; Gao et al., 2011). For a period of some 4 kyr the Yangtze sediment was to a large extent trapped within its large estuary, to fill the bay-head areas and estuary itself (Wu et al., 2006). Large scale sediment escape from the estuary did not occur until 2 kyr ago, then the mud layer with high deposition rates (of the order of 101 mm yr 1) began to form (Xu et al., 2012). Before this event, there were clinoform mud deposits that resulted from the reworking of the pre-Holocene Yangtze sediment accumulated on the shelf (Gao, 2013). According to the core analyses (Xu et al.,

Around 30 m

2012), for the Yangtze distal mud, the strata have a lower part that is dominated by old sediments, probably derived from reworking of pre-Holocene deposits, and an upper part that is dominated by the modern Yangtze River, with an age of younger than 2 kyr. Thus, the ‘‘hiatus’’ is actually an indication of changes in the sediment source. With regard to the continuity of the strata, the ‘‘hiatus’’ may not be true: the old 14C age may be due to the mixing of newly input sediment with old, reworked sediments. Using appropriate dating materials (e.g., benthic foraminiferal tests), continuous sedimentary record may be established. More importantly,

Fig. 5. Schematic diagram showing the evolutionary stages of the Pearl, Yangtze and Yellow River distal muds, representing young, growing and mature stages, respectively (based on a synthesis of the results obtained from Liu et al. (2004, 2006), and this study).

Please cite this article in press as: Gao, S., et al. Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.07.024

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Fig. 6. Seismic records of the northern SCS shelf strata, with a number of clinoform systems. These ‘‘shelf edge deltas’’ have been interpreted as river deltas formed in different geological periods with different positions of sea level (from Lüdmann et al., 2001).

the strata formed in the last 2000 yrs have mainly convergence clinoforms, which is fundamentally different from the Yellow River distal mud in which both convergence and parallel clinoforms are well developed. As such, these three distal mud deposits had different temporal coverage during the Holocene, i.e., 100 yrs, 2 kyr and 6.5 kyr for the Pearl, Yangtze and Yellow Rivers, respectively. Furthermore, they have different morphological and sedimentary characteristics, in terms of thickness, the age of the mud layer, the clinoform patterns, and the water depth at the clinoform top (Table 3). In order to explain the differences in the internal structure or the clinoform pattern for the three distal muds outlined above, the factors of sediment supply, the continental shelf bathymetry and sediment dynamic processes should be considered. Since the three distal muds are all located in an environmental setting characterized by a wide shelf, active shelf currents and abundant sediment supply, it was proposed that the timing of sediment input became important, which is controlled by the estuarine infilling processes (Liu et al., 2014). This observation implies that the differences identified are actually related to the varied evolution stages. A thorough facies analysis for each of the distal mud systems will reveal the different stages of evolution for the three distal muds; here we propose that these stages can be illustrated using the clinoform morphology alone.

As shown in Fig. 5, the Pearl River distal mud represents a youth stage: the mud layer is generally thin, with the thickest part being located at the depo-center of the mud area. The position of the depo-center is a function of the speed of the shelf currents and the settling velocity of the suspended sediment. The Yangtze distal mud represents a growing stage, at which the internal structure is dominated by convergence clinoforms, which once again is related to the settling of suspended sediment. After a long period of sediment accumulation, the sedimentary layers become much thicker at the depo-center, than at the edge of the deposit; this mechanism explains the formation of the convergence clinoforms (Pirmez et al., 1998; Cattaneo et al., 2007). The Yellow River distal mud, however, has both well-developed convergence and parallel clinoforms (Liu et al., 2004), indicating that this system has reached its mature stage, i.e., the area for sediment accumulation is no longer confined within the range where settling of suspended sediment from the water column takes place. According to Gao and Collins (2014), the parallel clinoforms are formed by the movement of bottom turbid layers, which is a type of continental shelf sediment gravity flow (Sternberg et al., 1996; Wright and Friedrichs, 2006; Friedrichs and Scully, 2007). Numerical simulation and laboratory experiment both support this hypothesis (e.g., Gerber et al., 2008). According to our hypothesis, if the period of time is sufficiently long for the development of a distal mud such as the one

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associated with the Pearl River, then eventually it will evolve from the young stage toward the mature stage. 5.2. Formation of river delta systems on continental shelves The classic concept of a river delta is associated the deposits at and near the river mouth, including the subaerial deltaic plain, the delta front and the prodelta (Wright and Coleman, 1973; Coleman, 1988). A necessary condition for the formation of such a delta is that the quantity of sediment supplied by the river exceeds the amount of the materials that escape the estuarine waters to enter the open shelf waters (Gao et al., 2011). The accumulation of coarse-grained sediment occurs mostly in the mouth area, whilst fine-grained sediments are distributed in the delta front and prodelta areas; the progradation of the delta generates typical clinoform structures within the deltaic deposits (Fagherazzi and Overeem, 2007). Superimposed on this general pattern, the spatial distribution of the deltaic sediment is also influenced by the dominant hydrodynamic forcing, i.e., fluvial discharges, waves, or tidal currents (Wright, 1977, 1985). Within this framework, the Pearl River delta at its mouths represents a tidally dominated delta (Wu et al., 2006). The river delta concept outlined above does not include the distal mud on the continental shelf. However, although located in a remote area from the river mouth, the distal mud is closely related to the river, i.e., the sediment is supplied by the river and is transported and accumulated within a well-defined area. Further, at its mature stage, the distal mud would look like a subaqueous delta in the shallow seismic record, with well-developed clinoforms. For this reason, the distal mud has been viewed as a component of a river delta (Nittrouer et al., 1986; Yang and Liu, 2007). According to this modified concept, a river delta should consist of the deposits near the mouth and in the distal mud, both are associated with sediment supply by the same river. Furthermore, the river mouth may shift its position during the evolution of a river. For instance, in the Holocene the river mouth of the Yellow River changed for several times, between the Bohai Gulf and the southern Yellow Sea (Chen et al., 2012). As a result, delta systems were formed in the two different regions, both having their river mouth deposits and distal muds (Liu et al., 2007, 2010). Sea level change adds further complexity to the deltaic system. During the last glacial maximum (LGM) periods, or any historical periods when the sea level position was lower than the present-day and relatively stable, river deltas would have been developed. Indeed, in the shelf sedimentary sequences, many deltaic sequences have been identified, which are referred to as ‘‘shelf edge deltas’’ (e.g., Loboa and Ridente, 2014). In the northern SCS, shelf edge delta systems have been identified by seismic survey (Lüdmann et al., 2001). As shown in Fig. 6, these deltas are characterized by clinoforms: along a single seismic line, a number of clinoform systems are present, at different locations on the present-day shelf and different depths within the strata. Apparently, such features represent the depositional products of different sea level positions during the Quaternary periods. The question is: where was the position of sea level in relation to these deltas? The elevation of the clinoform top can be related to the sea level position, but it is necessary to distinguish between the different clinoform deposits. For a subaqueous delta close to the river mouth, the clinoform top should be close to the mean sea level at the time of the accumulation; however, for a distal mud, the position of sea level would be 20–30 m above the clinoform top, according to the modern examples of the distal muds of China’s large rivers (see above). In the case of the northern SCS, whether or not these shelf edge deltas represent river mouth deltaic deposit or distal muds may be judged by the presence or

absence of paleo-river channels on the shelf. However, in a broad sense, there is a need to establish new criteria to distinguish these structures, in order to correctly interpret the geological record of deltaic deposits. At the moment the criteria have not yet been established, but we would propose that seismic data, facies analysis and sea level information derived from other sources than the clinoforms will collectively define the different mud deposits. 6. Conclusions (1) The distal mud deposit associated with the Pearl River occupies a relatively large area (i.e., more than 8  103 km2), but only the thin mud layer at the top is associated with the direct Pearl River input, with a thickness of only 100 m. On the basis of 210Pb analysis, the age of this mud layer is younger than 102 yrs. Thus, the formation of the distal mud is a recent event. (2) The distal muds of the Pearl, Yangtze and Yellow Rivers are all formed in an environmental setting characterized by wide shelf, active shelf currents and abundant sediment supply; hence, the timing of sediment input became important, which controls the evolution stages. The distal muds of the Pearl, Yangtze and Yellow Rivers are in their young, growing and mature stages, respectively. A comparison with the Yangtze and Yellow River distal muds in terms of the mud layer thickness, internal structures associated with the clinoforms, the water depth at the clinoform top and the deposition rate/age supports this hypothesis. (3) On a wide continental shelf with abundant sediment supply, one river may have a deltaic system consisting of several components, in response to river mouth location shift and sea level changes. Furthermore, even if the river mouth and sea level are fixed, the subaqueous delta adjacent to the river mouth and the distal mud deposits may both be recognized as deltaic deposits in the geological record. Hence, appropriate criteria should be established to distinguish between these two types of delta components; this is of importance in interpreting the shelf edge deltas in the seismic record, since they represent different signals of the sea level position when the delta system was formed.

Acknowledgements The authors thank our colleagues of the MOE Key Laboratory for Coast and Island Development for their assistance in the in situ measurements, sediment sampling and laboratory analyses. This study was supported by a grant from the Natural Science Foundation of China (No. 40576023). The paper was presented during the Workshop on the Paleoenvironment, Paleoclimate and Geological Hazards over the South China Sea and the Islands (Keeleong, Taiwan, 8–9th April, 2014). SG has been supported by the South China Sea Research Centre (Nanjing University). Mr Niu Zhan-sheng helped with the preparation of some of the figures. We wish to thank the reviewers for their constructive comments on the original manuscript. References Cattaneo, A., Correggiari, A., Langone, L., Trincardi, F., 2003. The late-Holocene Gargano subaqueous delta, Adriatic shelf: sediment pathways and supply fluctuations. Mar. Geol. 193 (1–2), 61–91. Cattaneo, A., Trincardi, F., Asioli, A., Correggiari, A., 2007. The Western Adriatic shelf clinoform: energy-limited bottomset. Cont. Shelf Res. 27 (3–4), 506–525. Chen, Y.Z., Syvitski, J.P.M., Gao, S., Overeem, I., Kettner, A.J., 2012. Socio-economic impacts on flooding: a 4000 year history of the Yellow River, China. Ambio 41 (7), 682–698.

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