A seismic study of the Mekong subaqueous delta Proximal versus … seismic... · 2019. 8. 21. · (Milliman and Syvitski, 1992) and 110 million tons in 1990s (Milliman and Farnsworth,

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Continental Shelf Research

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Research papers

A seismic study of the Mekong subaqueous delta: Proximal versus distalsediment accumulation

J. Paul Liua,⁎, David J. DeMastera, Charles A. Nittrouerb, Emily F. Eidamb, Thanh T. Nguyenc

a Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USAb School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195, USAc VAST Institute of Marine Geology and Geophysics, 18 Hoang Quoc Viet Street, Hanoi, Vietnam

A R T I C L E I N F O

Keywords:Mekong DeltaClinoformSubaqueous deltaAlong-shelf transportSediment accumulation rateChirp sonar profilesSeismic stratigraphy

A B S T R A C T

The Mekong River Delta is one of the largest in Asia. To understand its sediment distribution, thickness, massbudget, stratigraphic sequences and sediment-transport process, extensive geophysical and geochemical surveyswere conducted on the inner portions of the adjacent continental shelf. Analyses of> 80 high-resolution Chirp-sonar profiles show the Mekong River has formed a classic sigmoidal cross-shelf clinoform in the proximal areas,up to 15 m thick, with topset, foreset and bottomset facies, but constrained to water depths of< 20 m. Beyondthis depth, the East Sea/western South China Sea shelf is dominated by relict silt, sand and gravel with patches ofearly to middle Holocene mud deposits. Parallel to shore, the Mekong-derived sediment has extended> 250>300 km southwestward to the tip of the Ca Mau Peninsula, forming a distal mud depocenter up to 22 m thick,and extending into the Gulf of Thailand. A large erosional trough or channel (up to 8 m deeper than the sur-rounding seafloor and parallel to the shore) was found on the top of the clinoform, east of the Ca Mau Peninsula.

Based on the thicknesses and distribution revealed by Chirp sonar profiles, the total estimated volume of theMekong River subaqueous clinoform on the shelf is ~120 km3, which is equivalent to ~120–140 × 109 t ofsediment using an average sediment dry-bulk density of 1.0–1.2 g/cm3. Assuming the subaqueous deltaic deposithas formed within ~1000 yr, the calculated millennial-timescale average sediment discharge to the shelf couldbe 120–140 × 106 t per year. Spatially, the proximal subaqueous delta has accumulated ~45 × 109 t (~33%) ofsediment; the distal part around the Ca Mau Peninsula has received ~55 × 109 t (~42%) of sediment; and theremaining ~35 × 109 t (~25%) has accumulated within the central transition area, although the coastline andshoreface in this area are presently eroding. The spatially averaged 1000-yr-scale accumulate rate is up to 2 cm/yr.

Compared to other tide-dominated fluvial dispersal systems, the Mekong River system has a relatively young(≤1000 yr) subaqueous delta, a shallow rollover at 4–6 m water depth, gentle foreset gradients (0.03–0.57°),and a short cross-shelf dimension of 15–20 km within 20-m water depth. Like the Amazon, Po, and Yangtzerivers, the Mekong River has developed a pervasive along-shelf deposit, which in this case extends> 250 >300km to the southwest as a result of the superimposed tidal processes, wave-induced resuspension, and astrong low-flow season coastal current.

1. Introduction

Recent studies indicate that for many of the world's largest sub-aqueous deltas, the bulk of the sediment volume exists in asymmetricalprodelta lobes, and elongate or detached masses of sediment (Walshand Nittrouer, 2009; Korus and Fielding, 2015; Patruno et al., 2015).For example, the Amazon coastal mud belt extends> 1500 km north-westward to the Orinoco river mouth (Allison et al., 2000; Nittroueret al., 1986; Kuehl et al., 1986), Yangtze River sediment is transported~800 km into the Taiwan Strait (Liu et al., 2006, 2007, 2008; Xu et al.,

2012), and Yellow River sediment is deposited more than 700 km intothe south Yellow Sea (Alexander et al., 1991; Liu et al., 2004; Yang andLiu, 2007). In only a few large river systems are fluvial sediments ableto escape into deep-ocean basins via cross-shelf valleys or submarinecanyons (e.g., Ganges-Brahmaputra (G-B) (Kuehl et al., 1997; Goodbredand Kuehl, 2000), Congo (Savoye et al., 2009), Indus (Giosan et al.,2006; Clift et al., 2014), Mississippi (Ross et al., 2009), and Sepik River(Kineke et al., 2000; Walsh and Nittrouer, 2003).

The Himalayas, in Asia, are among the youngest and most activemountain ranges on the planet, with high relief, steep gradients,

http://dx.doi.org/10.1016/j.csr.2017.07.009Received 18 December 2016; Received in revised form 30 June 2017; Accepted 20 July 2017

⁎ Corresponding author.E-mail address: [email protected] (J.P. Liu).

Continental Shelf Research 147 (2017) 197–212

Available online 21 July 20170278-4343/ © 2017 Elsevier Ltd. All rights reserved.

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frequent tectonic activity, and highly erodible rocks (Clift et al., 2008).Coupled with the seasonal melting of glaciers and abundant monsoonalrainfall, the Himalayas and surrounding Tibetan Plateau contain theheadwaters to eight of the world's largest river systems (G-B, Ayeyar-wady, Thanlwin, Mekong, Red, Pearl, Yangtze and Yellow), accountingfor ~30% of the fluvial sediment flux to the global ocean (Fig. 1) (Liuet al., 2009; Milliman and Farnsworth, 2011). The Mekong River runs~4700 km from the Himalayas to the East Sea (a.k.a., South China Sea)(Fig. 1). Its annual freshwater discharge is ~470 × 109 m3, and theestimated annual sediment flux was ~130–160 million tons in 1960s(Milliman and Syvitski, 1992) and 110 million tons in 1990s (Millimanand Farnsworth, 2011). Today, with 30 large dams already constructedand ~200 new dams to be added, more significant changes are expectedin the Mekong Delta hydrological regime, coastal circulation, and se-diment dynamics (e.g., Xue et al., 2010). With the sediment flux de-creasing, the Mekong River Delta is facing severe erosion in its chan-nels, river banks and coastal zones (Noh et al., 2013; Xue et al., 2011;Shearman et al., 2013; Anthony et al., 2015; Liu et al., 2017).

Previous studies indicate that Mekong-derived sediment has a lim-ited cross-shelf distribution immediately seaward of the distributary-

channel mouths, in contrast to an extensive along-shelf, distal depositextending to the Ca Mau Peninsula (Fig. 2) (Xue et al., 2010, 2012,2014; Szczuciński et al., 2013; Unverricht et al., 2013, 2014). Strati-graphic sequence analyses along with 14C and OSL (Optically Stimu-lated Luminescence) dating indicate the current Mekong subaqueousdelta and associated nearshore area represent the most recent seawardprogradation and accumulation, which have formed only within the last~1000 yr (Fig. 3) (Nguyen et al., 2000; Ta et al., 2002, 2005; Tamuraet al., 2010, 2012a, 2012b). Most previous offshore surveys and studiesfocused on the outer shelf or distal regions near the Ca Mau Peninsula,far from the distributary mouths (see Schimanski and Stattegger, 2005;Xue et al., 2010; Unverricht et al., 2013). Only a few seismic transectsand gravity cores have been acquired adjacent to distributary mouthsbetween the My Tho and Song Hau (Bassac). For better understandingof the Mekong River Delta proximal sediment dynamics, depositionalprocesses, accumulation records, long-term sedimentary process fromthe proximal delta to the distal-along-shelf depocenter, and the overallimpacts of the decreased sediment discharge to the coastal environ-ments, we have conducted two extensive geophysical and geochemicalsurveys immediately off the Mekong River distributary channels of the

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Fig. 1. The Mekong River basin, delta and adjacentAsian large river systems. The seasonal coastal cur-rent features are based on previous studies andmodel simulations: red arrows represent the surfacecurrents under wet-season monsoon, blue arrowsindicate the currents under the prevailing dry-seasonmonsoon (Hu et al., 2000, Xue et al., 2010). (Forinterpretation of the references to color in this figurelegend, the reader is referred to the web version ofthis article.)

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Song Hau, Co Chien and My Tho (Fig. 4).

2. Background

2.1. Mekong delta plain

Located in southern Vietnam, the Mekong River Delta has an area of50,000 km2 and is ranked as the 3rd largest tide-dominated delta interms of area in the world, after the Amazon and Ganges-Brahmaputradeltas (Milliman and Syvitski, 1992) (Fig. 2). The modern Mekong Deltainitially formed during the Holocene sea-level still-stand at ~7.5 ka BP(Bird et al., 2010; Hanebuth et al., 2012; Tamura et al., 2012a, 2012b).Numerous deep boreholes have been drilled in the Mekong delta plainand extensive stratigraphic studies have been conducted (Fig. 3) (e.g.,Nguyen et al., 2000; Ta et al., 2002; Tamura et al., 2010, 2012a, 2012b;Hanebuth et al., 2012). These studies have indicated that with onset ofthe sea-level regression at ~4.8 ka BP, the delta depocenter shiftedfarther seaward and subaqueous delta progradation began (Hanebuthet al., 2012). At ~3 ka BP, the Mekong Delta experienced a phase shiftfrom “tide-dominated” to “tide-and-wave-dominated” conditions (Taet al., 2002). After 3 ka BP, the delta developed many beach ridges in itslower delta plain (Fig. 3). Since the Holocene sea-level highstand, thedelta has prograded more than 250 km from Cambodia eastward to theEast Sea at a rate of 30m/yr. The early subaqueous delta that formedduring 1.0–7.0 ka BP has been buried under the subaerial delta plain.Transects A-B and X-Y with seven boreholes across the lower delta plainhave been used to interpret depositional sequences and times lines(Fig. 3) (Ta et al., 2002, 2005). Core VC-1 (Ta et al., 2005) and anunpublished core 5-1, drilled on the coastal zone of the Ca Mau Pe-ninsula, also indicate that most subaqueous delta deposition has oc-curred within the last ~1000 yr (Fig. 2) (Nguyen and Ta, personalcommunication).

The prevailing dry-season monsoon and tides have caused the deltato develop some asymmetric bifurcations (Tamura et al., 2012a; Xueet al., 2010, 2012). From the delta apex, located just north of PhnomPenh, Cambodia, the delta grew southeastward and separated the EastSea from the Gulf of Thailand (GOT) (Tamura et al., 2010, 2012a).South of Phnom Penh, the Mekong River bifurcates into two main

channels: the Song Hau (or Bassac) in the south and the Song Tien (orMekong) in the north (Figs. 2 and 3). Overall, there are nine dis-tributary channels discharging into the sea and no major direct channelto the GOT side (see Figs. 2 and 4).

Estimations, based on hydrodynamic surveys in the lower dis-tributary channel of Song Hau, indicate that presently ~40 Mt /yr ofsediment are discharged collectively from the entire Mekong to the EastSea (Nowacki et al., 2015). This value is ~65% less than most previousestimates of Mekong sediment discharge (110–130 Mt/yr), which mightreflect the impacts of sharply increased human activities in the riverbasin, such as dam construction (Anthony et al., 2015).

2.2. Mekong subaqueous delta

Long-term shoreline analyses and coastal sedimentology studiesindicate that during the rainy season, large volumes of sediment aredischarged from the river and temporarily deposited near the rivermouth, and then during the,dry season mud and fine sand are erodedand transported southwestward (Tamura et al., 2010; Xue et al., 2012;Unverricht et al., 2014). Previous studies revealed that the Mekongsubaqueous delta reaches ~15 m in thickness and is restricted within20 m water depth. The majority of Mekong River sediment is trans-ported southwestward, and much is deposited far away from the rivermouths in the GOT shelf to water depths of< 30 m (see: Liu et al.,2009; Xue et al., 2010, 2011, 2012, 2014). Onshore drilling and stra-tigraphic studies of the lower delta plain suggest that the depocenters ofthe Mekong Delta have shifted laterally from the Vietnam-Cambodiaborder to their current position. The current subaqueous delta onlyrepresents the most-recent seaward progradation (Fig. 3) (Nguyenet al., 2000; Ta et al., 2002, 2005; Xue et al., 2010; Tamura et al.,2012a, 2012b).

Cluster analyses of surface sediments from the Mekong subaqueousdelta reveal two different sediment types within the subaqueous delta: awell sorted sandy sediment, and a poorly sorted silty sediment (Fig. 2)(Szczuciński et al., 2013; Unverricht et al., 2013, 2014). The clay, silt,and sand contents of all sediment samples averaged 26%, 62%, and12%, respectively. Spatially, subaqueous deltaic deposits along theeastern side of the delta plain were coarser than those from the western

Fig. 2. Spatial distribution of the surficial sediments around theMekong River Delta, and locations of buried paleo-river channelson the shelf (after Xue et al., 2010; Szczuciński et al., 2013,Unverricht et al., 2013; Nguyen, 2016; Bui et al., 2009). Thepositions of 3 ka and 1 ka BP paleo coastal lines are based onNguyen et al. (2000); Ta et al., (2002, 2005); Tamura et al.(2012a, 2012b)). Detailed core logs, stratigraphic facies, and de-positional time lines of transects A-B and X-Y are shown in Fig. 3.

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side in the GOT. Sediments of the subaqueous Mekong Delta consist ofwell-sorted, fine sand near the Song Hau mouth with a trend to dom-inantly fine silt toward the distal delta (Ta et al., 2005; Unverricht et al.,2014). In the GOT, well-sorted, fine silt dominates the subaqueous deltasubstrate (Xue et al., 2010, 2014; Unverricht et al., 2013). Beneath thesubaqueous clinoform, a well developed Transgressive Surface (TS) wasidentified, together with more than ten incised valleys or paleo-river-channels extending from the inner shelf to the middle and outer shelf(Fig. 2; Schimanski and Stattegger, 2005; Xue et al., 2010; Szczucińskiet al., 2013; Nguyen, 2016).

2.3. Sediment dynamics and accumulation

The Mekong River discharges ~85% of its annual water dischargebetween May and October (wet season), while only ~15% is dischargedbetween November and April in the dry season (Unverricht et al.,2013). Numerical simulations using the Delft3D model and direct ob-servations indicate sediment is transported from the lower Song Hauchannel to the sea during the high-flow season, and some sediment is

transported from the ocean back to the channel during the low-flowseason (Xing et al., 2017; Nowacki et al., 2015). Outside the dis-tributary channels, the nearshore tidal-currents and wind-driven sur-face currents play a major role in controlling suspended-particletransport throughout the topset region. For example, observed sedimentfluxes near the Song Hau distributary mouth are predominantly sea-ward during the wet season (McLachlan et al., 2017) and then farthernortheastward on the shelf (Eidam et al., 2017). Numerical simulationsusing a ROMS model also indicate that during the wet season largeamounts of fluvial sediment are delivered and deposited near the Me-kong river mouths. Then, during the dry season, a large portion ofpreviously deposited sediments is resuspended and transported awayfrom the proximal delta area (Xue et al., 2012). Near the southern endof the Ca Mau Peninsula, Unverricht et al. (2013) found that tidalprocesses in the subaqueous Mekong Delta have a significant influenceon sediment resuspension and transport direction. The prevailing re-latively high-velocity ebb-tidal current transports suspended sedimentgreater distances southwestward than slower flood-tide currents.

Radioisotope analysis of sediment cores (0.5–3 m long) collected as

Fig. 3. Sediment-core logs and depositional facies with time lines (modified after Ta et al., 2002, 2005). The red boxes show our study areas and the constrained Mekong subaqueous deltaage: ≤ 1000 yr. Core locations are shown in Fig. 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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part of this study on the Mekong subaqueous delta, show the apparentexcess 210Pb sediment accumulation rates range from>10 cm/yr nearthe Song Hau river mouth, to 1–3 cm/yr in topset and foreset bedsbetween 20 and 50 km of the river mouth, to rates as low as 0.4 cm/yrin bottomset beds (DeMaster et al., 2017; Eidam et al., 2017). The210Pb-derived sediment burial rate for the proximal deltaic depositsaccounts for 43% of the total sediment burial on the southern Vietnaminner shelf (DeMaster et al., 2017). The mechanics controlling theproximal and distal distribution, transport, accumulation, and forma-tion of the Mekong cross-shelf and along-shelf clinoform are not fullyunderstood. This paper focuses on the seismic Chirp-sonar profiles ac-quired in 2014–2015 adjacent to the Mekong distributary mouths on or

near the proximal depocenter. We also compare them with the profilesacquired in 2006–2007 from the distal depocenter, which is locatednear the Ca Mau Peninsula. We have delineated the stratigraphic se-quences of the Mekong subaqueous delta and have characterized thevarious clinoform configurations, slope gradients, total volumes andmasses, and depositional patterns between the proximal distributary-mouth region and the distal Ca Mau Peninsula region. This study willhelp us to understand better the stratigraphic features of the subaqu-eous clinoform, mechanisms of along-shelf transport, formations ofproximal and distal deltaic depocenters, and the relationship betweenshoreline retreat and offshore erosion.

Fig. 4. Locations of 2014–2015 Chirp-sonar survey lines off theMekong distributary river mouths. Selected profiles discussed inthe text are highlighted and numbered in blue. (For interpretationof the references to color in this figure legend, the reader is re-ferred to the web version of this article.)

Fig. 5. Location map of the 2006–2007 Chirp-sonar profiles in thecentral transition and distal area around the Ca Mau Peninsula.Selected profiles discussed in the text are highlighted and num-bered in blue. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of thisarticle.)

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3. Data and methods

Two research cruises were conducted in September 2014 and March2015 by a joint team of scientists from the Institute of Marine Geologyand Geophysics, Vietnam; North Carolina State University, USA; andUniversity of Washington, USA. Using an EdgeTech 0512i Chirp SonarSub-bottom Profiler (frequency range: 0.5–12 kHz), a total of 62 high-resolution seismic profiles spanning more than 1000 km on the innershelf adjacent to the modern Mekong River Delta were acquired. Mostprofiles were collected immediately seaward of the distributary chan-nels of the My Tho, Ham Luong, Co Chien and Song Hau (Fig. 4). Be-sides the Chirp sonar profiles, 32 kasten cores and 19 shipek surfacegrab samples were obtained during the two research cruises (seeDeMaster et al., 2017; Eidam et al., 2017). In addition, 17 Chirp-sonarprofiles and 20 gravity cores, acquired around the Ca Mau Peninsuladuring the 2006–2007 cruises (Liu et al., 2009; Xue et al., 2010, 2014),were included in the analyses and discussions (Fig. 5).

All seismic and navigation data were processed using EdgeTechDiscover Sub-bottom software (Version 3.6). An acoustic velocity of1500 m/s was used to calculate water depth and sediment thicknesses.All profiles have been captured and saved as image files, processedfurther under image-editing software and ArcGIS, and then ultimatelyloaded into IVS Fledermaus to construct fence diagrams. Along eachprofile, a foreset slope gradient was calculated based on the bathy-metric change for the cross-shelf horizontal distance. An overall sub-aqueous deltaic sediment distribution and isopach map was con-structed, based on the data delineated from all Chirp sonar profiles. Thesediment-thickness isopach map was imported into ArcGIS 10, and thedistribution area, distance, and volume were calculated. Analyses ofsediment cores from the top layer of the proximal clinoform yielded anaverage dry bulk density of 0.95 g/cm3, with values ranging from 0.8 to1.2 g/cm3 (DeMaster et al., 2017). Sediments around the Ca Mau Pe-ninsula showed an average dry bulk density of 1.2 g/cm3 (Szczucińskiet al., 2013). In this study, we used 1.0–1.2 g/cm3 as a dry bulk densityto calculate the sediment mass of the subaqueous clinoform.

In a fashion similar to the radio-chronological discussion (DeMasteret al., 2017), this study divided the Mekong subaqueous delta into fourareas: 1) the Northern Proximal area off the distributaries of Nha Be,My Tho Ham Luong and Co Chien; 2) the Southern Proximal area off theSong Hau distributary river mouth; 3) the Central Transition area be-tween the Soc Trang and Ca Mau; and 4) the Distal area around the CaMau Peninsula and in the eastern GOT (see Figs. 4 and 5).

At the present stage, there is no deep core over the Mekong sub-aqueous deltaic deposit available for us to decide the exact age of theclinoform. Based on onshore and nearshore drillings near the rivermouth and in the Ca Mau Peninsula and on the stratigraphic facies andcalibrated 14C ages exposed at these sites (Figs. 2 and 3), we can deducea roughly estimated age of the seaward accumulation. Overall, based onthe core logs, a small portion of the subaqueous deposits in the northerndelta might be slightly> 1000 yr, as the core BT3 reveals. But thenearshore cores located in the central (TV1) and southern parts (VC1and 5-1) reveal that all seaward accumulations were formed within~1000 yr. Therefore, the subaqueous delta on the shelf only representsrecent deltaic deposition, most likely within the last 1000 yr (Fig. 3) (Taet al., 2002, 2005).

4. Results

4.1. Area-1: the Northern Proximal subaqueous delta

High-resolution Chirp-sonar profiles acquired from the northern-most end of the study area did not show any deltaic clinoform devel-oped on the sea floor. Instead a large-scale sediment-filled valley, up to2000-m wide, was found seaward of Vung Tau (for example, in Line 46;Fig. 6). The valley incision was as much as 20 m deep, cutting into thelate-Pleistocene strata. Overlying the valley-filling fluvial sediments, a

sandy transgressive system tract (TST, up to 5 m thick) was developed.Over this transgressive deposit, some non-continuous late-Holocenepatchy marine deposits, up to 2 m thick, were observed. Previouscoring, sample analyses, x-ray, and 14C dating indicate the valley wasfilled with fluvial sediment between 13.0 ka and 9.5 ka, when sea levelrose constantly at a rate of ~10 mm/yr (Schimanski and Stattegger,2005; Tjallingii et al., 2010). The transgressive surface (TS) beneath theTST corresponded to a rapidly rising, sea-level event that occurredbetween 9.5 ka and 8.5 ka BP (Liu et al., 2004). The age of the Max-imum Flooding Surface (MFS), separating the TST deposit and late-Holocene marine deposits, is ~8.0 ka to 7.0 ka, which represents thetime when sea level reached its highest stand in the middle Holocene(Liu et al., 2004; Hanebuth et al., 2012). Sediment cores 18A and 3–73in this area indicated very slow accumulation rates of 0.39 and0.09 cm/yr (DeMaster et al., 2017).

Off the My Tho river mouth, Lines 42 and 44 (Fig. 4) showed asubaqueous delta, or clinoform, overlying a pre-Holocene relict sandydeposit (Fig. 6). The clinoform deposits were up to 15 m thick nearshore, extending 15 km seaward on the inner shelf, and reaching awater depth of 18 m. The rugged transgressive surfaces in these threeprofiles were located ~20 m below the present sea level. No apparentincised valley or TST occurred in this portion of the sea floor. In Line44, the topset extended seaward ~5.5 km, with rollover at 6-m waterdepth; the foreset facies extended 5 km farther to the bottomset at 18.5-m water depth. The clinoform foreset gradient was 0.075–0.115°. Se-diment core D45 from water depth of 14 m, over the bottomset section,has a 210Pb SAR of 2.4 cm/yr (DeMaster et al., 2017).

Chirp-sonar profiles (Lines 34 and 36) off the Ham Luong and CoChien river mouths revealed a well developed, subaqueous deltaoverlying a thin transgressive deposit. The topset ended at ~5 m waterdepth (see the rollover position at Line 36), and the bottomset ended ata water depth of 20 m (Fig. 7). In Line 36, a distinct seaward pro-gradation over the foreset portion is apparent. Some small-scale incised-valley infills occurred in Lines 2 and 32. In Line 34, a thin TST depositwas developed before MFS. The total thickness of this clinoformpackage is up to 15 m below the topset. The cross-shelf extension of thesubaqueous delta was 10 cm/yr,and on the bottomset of 3.1 cm/yr (DeMaster et al., 2017).

4.2. Area-2: Southern Proximal area showing clinoform development off theSong Hau mouth

The Song Hau is a major distributary channel of the Mekong Riversystem and carries nearly 42% of the Mekong water and sedimentdischarges (Nguyen et al., 2008). Off this river mouth, 20 chirp-sonartransects were collected between water depths of 4 m and 20 m (Fig. 4).Off the Dinh An channel in the north, Line 11 indicated there is a welldeveloped, incised paleo-channel, ~15 m deep, filled by early Holocenefluvial sediments. The channel infill was truncated by a strong trans-gressive surface and overlain by late Holocene deltaic clinoform de-posits (Fig. 8). In Lines 10 and 12 (Fig. 8), no evidence of incised paleo-channels was observed; instead, there were very strong acoustic re-flections near or under the “toe”/bottomset of the clinoform at waterdepths of 20–23 m. The reflectors could be old bank, sand-ridge dunesor a reef that developed in the early Holocene, but partially buried bythe modern Mekong deltaic deposit (see Fig. 8). Lines 10, 11 and12showed that the rollover depth in this area is ~6–7 m. On Line 10, coresKC12, KC13, and KC55 are located on the topset, foreset and bottomsetrespectively, and the associated 210Pb SARs are 2.8, 3.6 and 1.2 cm/yr.Based on the cores KC9/58, KC10/57 and KC11/56 along Line 12, the210Pb SARs over the foreset and bottomset were 3.0–3.8, 6.0–7.2, and0.44–0.76 cm/yr, respectively (DeMaster et al., 2017; Eidam et al.,2017).

Selected profile Lines 13 and 23 located immediately off the SongHau River mouth showed that the Mekong has a relatively broad topset

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~7–8 km wide from the shore, with rollover at water depth of 4 m. Theforeset extends seaward ~6 km and ends at 20-m water depth. Theforeset gradient was determined to be ~ 0.143°, based on a 15-m-thicksediment deposit and 6 km of cross-shelf distance (Fig. 9). Beneath themodern Mekong subaqueous deltaic deposit, an incised valley, filledwith fluvial sediment (Szczuciński et al., 2013), was observed in Line 13(see Fig. 9).

In Line 17, located south of the Song Hau River mouth, the Mekongsubaqueous delta continued to extend eastward and southward (Fig. 9).

The maximum thickness of the clinoform is reduced to 13–14 m,without any distinctive flat topset facies. Instead, the whole subaqueousdelta in this area showed a clinoform dominated by a pervasive foreset,extending> 10 km seaward. The angle of the foreset decreased to0.074° along this line. Beneath the clinoform, there is a TST layer up to3–5 m thick overlying incised valley infill. Farther south, Lines 21 and06–7 revealed that the clinoform extended 15 km seaward from waterdepths of 3–20 3 m to 20 m, with a very gentle slope gradient of ~0.04°(supl. Fig. 4).

Fig. 6. Chirp-sonar profile (Line 46) in the north-ernmost part of the study area, which shows a pre-dominant pre-Holocene incised valley, infilled byearly Holocene fluvial sediment, and is overlain byearly-middle Holocene transgressive deposits. Chirp-sonar profiles (Lines 42, 44) off the My Tho rivermouth in the north show a cross-shelf clinoform. Theforeset gradient was calculated based on the verticalthickness and horizontal distance. HST: HighstandSystem Tract; MFS: Maximum Flooding Surface; TS:Transgressive Surface; TST: Transgressive SystemTract.

Fig. 7. Chirp-sonar profiles off the HamLuong and Co Chien river mouths.

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4.3. Area-3: central transition on eastern side of Ca Mau Peninsula

Chirp-sonar profiles on the east side of Ca Mau Peninsula showed avery different stratigraphy than the proximal features farther north. Astrong erosional feature was found on the topset and foreset sections ofthe clinoform (see Lines 06–8 and 07–9 in Fig. 10). The 210Pb radio-chemical profiles from this area also show zero net accumulation orerosional features (Xue et al., 2010; DeMaster et al., 2017). Over theclinoform, there was a buried channel and an erosional trough (Line06–08 and 07–09). The deltaic sediment extended much farther off-shore (20–60 km), with a very gentle angle of 0.03–0.04°.

4.4. Area-4 & 5: distal accumulation around the Ca Mau Peninsula and inthe GOT

Seismic profiles (Lines 07–11, 07–13, 07–15) revealed a clinoformwith a wide topset region (extending ~15 km from shore), and rela-tively steep foreset (Fig. 11). The foreset gradient reached 0.19°, whichwas steeper than the foreset farther north (see Lines 10, 11 and 12;Fig. 8). The erosional trough observed on the shoreface off the easternside of the Ca Mau Peninsula (Fig. 10) can also be seen in Lines 07–11and 07–13. The trough extended much deeper to 8 m below the ad-jacent seabed level, or ~16 m below sea level (see Line 07–11). Farthersouth, this erosional trough became shallower and almost disappearedalong Line 07–15. Interestingly, a geomorphological change (S) wasobserved in the transect (Fig. 11). The strong erosional features on the

topset of the subaqueous delta could be related to the current severecoastal retreat east of the Ca Mau Peninsula (Anthony et al., 2015, seeDiscussion). Sediment core MKII14 from the foreset of Line 07–11showed a high 210Pb SAR of> 5.0 cm/yr (DeMaster et al., 2017). Core05 had a 210Pb SAR of 4.0 cm/yr. Farther south, over Line 07–15, theforeset had a very high accumulation rate of> 10 cm/yr. A nearby core06 in a deeper location showed a slower rate of 2.6 cm/yr (Unverrichtet al., 2013).

Selected Chirp-sonar profiles on the western side of the delta in-dicated that sediment continues to be transported past the Ca MauPeninsula and dispersed into the GOT (Fig. 12). Lines 07-6 and 07–17indicated that the subaqueous delta extends more than 30 km into 25-mwater depth at a more gentle angle of 0.115°. In this distal area, bio-genic gas charges almost the entire subaqueous delta. Northward alongthe shore, the Mekong-derived sediment gradually becomes de-pleted,< 10 m in thickness and< 10 km from shore (Line 07-2).

4.5. Subaqueous deltaic mud isopach map and sediment budget

Based on all of the Chirp-sonar profiles, an isopach map of the late-Holocene Mekong-derived sediment accumulation on the shelf wascreated (Figs. 13 and 14). Beyond the 15-m-thick proximal subaqueousdelta that has formed very close to the distributary-channel mouths, adistal depocenter (up to 22 m thick) has been growing ~200 kmsouthwestward along the shelf and surrounding the Ca Mau Peninsula.

Using ArcGIS, the area of the Mekong subaqueous delta was

Fig. 8. Chirp-sonar profiles off the Dinh AnChannel, Song Hau River, the southernproximal study area.

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determined (see Table 1). The results showed that Mekong sedimentcovers> 11,000 km2 on the inner shelf, with a calculated total volumeof ~120 km3. Based on a dry bulk density of 1.0–1.2 g/cm3 in this area(Szczuciński et al., 2013; DeMaster et al., 2017), the Mekong subaqu-eous delta has accumulated a total 120–140 × 109 t of sediment on theshelf. More specifically, the Mekong proximal subaqueous delta (areas1 & 2) accumulated ~45 × 109 t of sediment immediately off its dis-tributary channels, and ~35 × 109 t of sediment accumulated in thecentral transition area. In the distal area, ~55 × 109 t of sedimentaccumulated around the Ca Mau Peninsula and in the GOT.

5. Discussion

As one of the largest rivers in Asia, the Mekong River has beenheavily impacted by changes in weather patterns (e.g., frequency oftropical storms) and human activities (e.g., sand mining and damconstruction). As a result, sediment supply to the Mekong Delta andadjacent seas has sharply decreased by at least 30% (Darby et al.,2016), and more than half of its deltaic shorelines are eroding (Anthony

et al., 2015; Liu et al., 2017; Li et al., 2017). Investigation of linkagesbetween the diverse parts of the deltaic system, including the subaqu-eous delta, provides insight for better prediction of future coastalchanges and deltaic responses to diminished riverine sediment supply.

5.1. Distribution, stratigraphic features and sediment budget of proximaland distal deposits

The analysis of seismic profiles from the Mekong inner shelf revealsa typical clinoform structure with topset, foreset, and bottomset beds.The Holocene subaqueous delta is 15–20 m thick near shore on thetopset and extends ~8–10 km seaward, then rolls over into the foresetat a water depth of ~4–6 m, and extends another 5–6 km, graduallydiminishing into the bottomset at 20-m water depth (Figs. 13–15).

In the northern and southern proximal areas off the My Tho, HamLuong, Co Chien, and Song Hau distributary mouths, the clinoformextends< 15 km seaward between isobaths 4.5 m and 20.5 m(Figs. 6–9, 13, 14). In the central transition area, on the eastern side ofthe Ca Mau Peninsula, clinoform sediments become thinner (< 10 m)

Fig. 9. Chirp-sonar profiles off the Tran Dechannel, Song Hau River.

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but extend much farther (~20–35 km) from shore. There is no obvioustopset facies developed in this area, and erosional features are present(Fig. 10). Similar features were also found in the distal accumulation ofthe Yellow River-derived sediment off the Shandong Peninsula in theYellow Sea (Liu et al., 2004; Yang and Liu, 2007) and the accumulationof Po River sediment in the western Adriatic Sea (Cattaneo et al., 2003,2007). The area around the southern Ca Mau Peninsula has accumu-lated a very thick clinoform (up to 22 m), with steep foreset and rela-tively deep bottomset (25-m water depth). In the GOT, sediment dis-perses 20–30 km across the shelf and extends farther seaward to 26-mwater depth (Figs. 12–15). The amount of Holocene sediment near theshore gradually diminishes toward shore in the north part of the distal

delta, where there is no major distributary channel connecting theMekong River to the sea. This pattern is also similar to the distal dis-tribution of the Yellow River-derived sediment reaching the southernYellow Sea shelf (Liu et al., 2004; Yang and Liu, 2007).

Given the total sediment budget of 120–140 × 109 t and the as-sumed age (≤1000 yr) of the subaqueous delta (Ta et al., 2002; Tamuraet al., 2012a, 2012b), the averaged sediment load to the shelf over amillennial timescale is estimated to be ~120–140 × 106 t per year,which corresponds well with annual sediment discharge estimates(110–160 × 106 t per year) to the sea (Milliman and Syvitski, 1992;Milliman and Farnsworth, 2011). On a 1000-yr timescale, ~33% of theMekong-derived sediment has accumulated proximately on the

Fig. 10. Chirp-sonar profiles in the central transition area, where strong erosional reflectors occurred over topset.

Fig. 11. Chirp-sonar profiles south of the Ca Mau Peninsula, where a large topset with an erosional trough and steep foreset were found.

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adjacent inner shelf near its distributary mouths; ~42% has accumu-lated in the distal area near the Ca Mau Peninsula; and ~25% has ac-cumulated between the proximal and distal depocenters. These per-centages are comparable to the proximal/distal distributions of theYangtze River and the Yellow River Delta dispersal systems (Liu et al.,2004, 2006, 2007). Thus, a large river system can have a profoundimpact beyond its estuary and adjacent shelf. Riverine sediment canaccumulate more than a hundred to thousand kilometers away from itsriver mouth.

5.2. Clinoform topset rollover depth and foreset slope gradient

The topset in a well developed subaqueous clinoform is the zonenear shore with a gentle slope and usually a slow sediment accumula-tion rate, as it often is a region of resuspension and sediment bypassing(Nittrouer et al., 1986; Kuehl et al., 1997; Walsh et al., 2004). Theforeset is a zone farther seaward with greater accumulation rates andrelatively steep slopes, and the rollover point is the transition betweenthe topset and foreset. The position and depth of this rollover pointreflects the balances among sediment flux, shelf configuration, and

Fig. 12. Chirp-sonar profiles in the western side of the Ca Mau Peninsula, showing the distal accumulation of the Mekong-derived sediments.

Fig. 13. The overall distribution of late-Holocene clinoform de-posits around the modern Mekong River Delta, based on theseismic profiling data in Figs. 6–12 and Supplement Figs.

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offshore hydrodynamics (Driscoll and Karner, 1999; Walsh et al., 2004;Swenson et al., 2005; Patruno et al., 2015; Eidam et al., 2017). In manylarge river systems, the subaqueous clinoform has prograded 100 kmacross the shelf into water depths of 70–90 m and extends severalhundred kilometers along the shelf, e.g. Amazon (Nittrouer et al., 1986)and Yangtze (Liu et al., 2006, 2007) systems. The rollover depths fromthe above systems are generally located between the 20-m and 40-misobaths.

However, the rollover depth of the Mekong's subaqueous delta isonly 4–6 m in the proximal regions (Figs. 6–9, 13, and 15). In the distalCa Mau area, the rollover depth is ~6–8 m (Figs. 11 and 12). In thetransition areas between the Song Hau distributary and Ca Mau Pe-ninsula, the clinoform architecture differs from the sigmoidal shape inthe proximal area (Fig. 10). Instead, the stratigraphic architecture of theclinoform is more aggradational than progradational, and the topset isless developed or absent, and the rollover point is difficult to identify.This is possibly due to the along-shelf hydrodynamic processes thatdominate the sediment dispersal and accumulation. In contrast, cross-shelf advection is mainly controlled by the local oceanographic regime,including tidal currents, wind-driven currents, up- or down-wellingsystems, and the riverine input (see Amazon, Po and Yangtze studies:Nittrouer et al., 1986; Cattaneo et al., 2003, 2007; Liu et al., 2006,2007). This has been verified by numerical simulation for clinoforms in

general (see Driscoll and Karner, 1999; Harris et al., 2008; Xue et al.,2012).

The subaqueous clinoform foreset slopes near the distributarymouths range from 0.093° to 0.172°. South of the Song Hau, the foresetslope decreases to 0.03°. The steepest foreset gradient is 0.573°, whichoccurs along the eastern side of the Ca Mau Peninsula. These values fallwithin the range of 0.03–0.76° summarized by Patruno et al. (2015) fora variety of muddy deltaic environments.

5.3. Sediment accumulation rates across the subaqueous delta

Based on the maximum thickness (15–22 m) and the assumed age ofthe subaqueous deltaic deposits, the spatially averaged sediment ac-cumulation on a time scale of ~1000 yr is up to 2 cm/yr. The 210Pb-based SARs are high (> 10 cm/yr) immediately off the distributarymouths in the northeastern side of the delta (the proximal depocenter)(DeMaster et al., 2017) as well as in the southern areas around the CaMau Peninsula (Unverricht et al., 2013). Most foreset areas have 210PbSARs between 1 and 3 cm/yr, and the bottomset areas have slower ratesof ~0.5 cm/yr (DeMaster et al., 2017). Overall, the regions offshore ofthe distributary mouths and south of the Ca Mau Peninsula are the twomain depocenters on a 100-year timescale with higher accumulationrates than the central transitional area and offshore GOT area. This

Fig. 14. Isopach map of the late‐Holocene Mekong‐derived sedi-ment accumulation on the shelf of the East Sea and Gulf ofThailand. The thickness is shown in meters.

Table 1Summary of the modern Mekong subaqueous delta characteristics, based on the analysis of chirp sonar profiles.

Area Across-shelf distance from theshore (km)

Clinoform maximum waterdepth (m)

Foreset slope angle (°) Area (km2) Volume (km3) Massa (×109 ton)

Proximal (Areas 1 & 2) 1 20 −20 0.093–0.143 3090 40 40–48(33%)2 27 −22 0.143 – 0.172

Central Transition (Area3)

3 28–35 −20 0.03 – 0.143 1830 30 30–36(25%)

Distal (Areas 4 & 5) 4 10–20 −25 0.115 –0.57 6360 50 50–60(42%)5 20–30 −30 0.05–0.20

Total: ~11280 120 120–140

a Sediment mass is calculated based on the assumption of a dry-bulk sediment density of 1.0–1.2 g/cm3 (see Szczuciński et al., 2013; DeMaster et al., 2017).

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pattern matches well with the isopach map derived from Chirp sonardata (Fig. 14). The geochemical, geological, and hydrodynamic im-plications of the sediments offshore of the Mekong distributaries arediscussed in other papers in this issue (DeMaster et al., 2017; Eidamet al., 2017).

5.4. Cross-shelf versus along-shelf transport; proximal versus distal deposits

Unlike other tide-dominated, large-river deposition systems of theAmazon, Fly and Yangtze (Nittrouer et al., 1986; Walsh et al., 2004; Liuet al., 2007), the Mekong River system has accumulated a relativelysmall-scale clinoform (< 15 m thick) near its river mouths. These del-taic features extend 15 km across-isobaths, over a shallow and rela-tively flat epicontinental sea to water depths of ~20 m. Several factorshave contributed to this morphology: 1) during the Holocene, the Me-kong River subaerial delta prograded seaward very fast at a rate of~30 m/yr. The modern coastal zone represents only< 2000 yr of se-diment accumulation, and for the adjacent subaqueous delta may beonly ~1000 yr; 2) the East Sea shelf is a very shallow epicontinental sea(< 25, and the entire Holocene delta has prograded over a gentle shelf(20-m depth verse 250-km distance, see Figs. 2 and 3); and 3) strongwaves, tides and monsoon currents actively transport the proximal se-diment away from the distributary mouths (Xue et al., 2012; Eidamet al., 2017). In contrast, the along-shelf dispersal system extendssouthwestward and builds an elongated clinoform between the multipleriver mouths and the western side of the Ca Mau Peninsula. A con-ceptual model of the Mekong River subaqueous delta (Fig. 15) suggeststhat the subaqueous delta is a relatively young sediment body accu-mulating over the most-recent transgressive and flooding surfaces,

which formed during the early to middle Holocene sea-level rise.Current measurements seaward of the distributary mouths revealed

that the seasonal variations in dominant current directions may causenet sediment transport to the subaqueous delta during the high-flowwet monsoon (Eidam et al., 2017). Boundary-layer measurements fromthe foreset region near the Song Hau distributary in September 2014and March 2015 indicate that near-bed shear velocities frequently ex-ceeded 0.01–0.02 m/s, suggesting that sand and silt are often mobilizedin this river-mouth system (Eidam et al., 2017). Suspended-sedimentconcentrations across- and along-shelf indicate that most sedimenttransport is restricted to water depths< 14 m near the Song Hau dis-tributary mouth (Eidam et al., 2017). A numerical model simulationindicates that the strong dry-season monsoon and induced coastalcurrent play major roles in transporting sediment southwestward alongthe southern coast of Vietnam (Xue et al., 2012).

5.5. Comparison with other East Asian large-river delta systems

Previous studies of other large-river delta systems on East Asianmargins, such as the Yangtze (Liu et al., 2006, 2007), the Pearl (Geet al., 2014) and the Red (van Maren, 2004; Tanabe et al., 2006; Ross,2011) Rivers, indicate that, these deltas have progradaed~200–300 km laterally in the past 6 ka before reaching their presentpositions (Fig. 16). Correspondingly, their proximal depocenters alsohave shifted seaward continually during the late Holocene, and therebyformed a series of extensive subaerial delta plains. In contrast, the lo-cations of their along-shelf transport and distal depositions have beenrelatively confined or unchanged in their positions along the originalcoasts. For example, during the Holocene sea-level highstand (in the

Fig. 15. (a) Conceptual model of the Mekong River subaqueous delta distribution and formation. The red dashed line is the rollover depth and the blue line is the base of the bottomest;(b) Stratigraphic sequences and interpretation of typical proximal clinoform features based on the Chirp-sonar profiles. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

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past 6–7 ka), the Yangtze-derived sediment has been transportedsouthward>800 km along the Zhejiang-Fujian coast into the TaiwanStrait, and accumulated a>40-m distal depocenter on the inner shelfof the East China Sea (Liu et al., 2006, 2007) (Fig. 16). The Pearl Riversediment has been transported ~300 km southwestward along theGuangdong coast to the Hainan Island (Liu et al., 2009; Ge et al., 2014).In addition to a large subaerial delta plain from Hanoi to the coast, RedRiver-derived sediments have extended and accumulated southwardalong the Thanh Hoa and Nghe An coasts (Ross, 2011) (Fig. 16). In-terestingly, the shorelines adjacent to the Yangtze, Pearl and Red riveralong-shelf dispersal systems over the past 6 ka have not progradatedlaterally. Instead, most of the river-derived sediments have accumu-lated on the shelf and formed elongated clinoform deposits(300–800 km), at much deeper water depths, up to 40–90 m.

In contrast, the Mekong River has experienced a different deposi-tional process. While the Mekong River proximal deposits progradatedseaward over the past 6 ka, Mekong-derived sediment has also accu-mulated along the shelf to the southwest, forming a>200-km longmud-dominated deltaic coast (Fig. 16). With the growth and accretionof the Mekong delta plain, the along-shelf transport and distal accu-mulation also have been shifted eastward accordingly. The presentsubaqueous deltaic accumulations, both proximal and distal depo-centers, have shifted away, ~200–300 km, from their initial positions.

In contrast to the original coasts distal from the Yangtze (Zhejiang-Fujian coast), Pearl (Guangdong coast) and Red (Thanh Hoa and NgheAn coast) mouths, the Mekong distal coast around the present Ca Maupeninsula is comprised of newly deposited sediment, most likely within~ 1 ka (Ta et al., 2002, 2005). Therefore, the modern Mekong's clino-form, along-shore transport and distal accumulation are relativelyyoung and small in terms of ages and thicknesses. With limited sedi-ment, this is also the reason that the Mekong has not prograded sea-ward to greater water depths.

6. Conclusions

Our extensive Chirp-sonar surveys off the Mekong River Delta reveala subaqueous delta 10–20 m thick on the inner shelf within 20–25-mwater depth, hugging the modern shoreline and shoreface. This is arelatively rapid accumulation in terms of thickness and areal extent forthis young clinoform, which has formed in the last 1000 years over thepreviously formed middle Holocene sea-level transgressive surface andmaximum flooding surface.

Spatially averaged 1000-yr-timescale accumulation rates based onseismic sediment thicknesses and approximate ages, are up to ~2 cm/yr, which are comparable to the 210Pb-derived SAR values, i.e.1–10 cm/yr on the topset and foreset beds (DeMaster et al., 2017;

Fig. 16. Models of the Mekong River Delta showing sedimentation patterns at 6 ka BP, 3 ka BP and present in comparison with othe east Asian delta systems: Yangtze, Pearl and RedRivers (Liu et al., 2006, 2007, 2009; Ta et al., 2002, Zuo et al., 2010; Ross, 2011; Ge et al., 2014). The blue dashed lines represent the boundaries of the river-derived mud distribution onthe shelf. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Eidam et al., 2017). The total sediment volume is estimated to be~120 km3, which is equivalent to 120–140 × 109 t of sediment. Thisestimate for the past 1000 years is in agreement with the historicalannual sediment load (110–160 × 106 t; Milliman and Syvitski, 1992),considering the uncertainties of estimated age and mass volume.

The modern Mekong subaqueous clinoform extends only 15–20 kmacross the shelf; however, like other larger river systems, for examplethe Amazon, Mississippi, Nile, Po, Red, Yellow, and Yangtze, theMekong's clinoform also extends> 300km along-shelf (in this case inthe southwestward direction). The rollover point for the Mekongclinoform is relatively shallow compared to those of other large riversystems (4–6 m versus 20–40 m). The shallow rollover depth is likelycontrolled by sediment flux and nearby oceanographic conditions(waves, tides, and coastal currents; e.g., Eidam et al., 2017). Althoughthe Mekong subaqueous delta has a shallow rollover depth, the delta'sforeset gradients range from 0.03 to 0.57°, which are similar to othersubaqueous muddy deltaic clinoform systems (Patruno et al., 2015).

In summary, the Mekong River has a high sediment discharge to thesea (110–160 Mt/yr), with a fast seaward progradation rate, of up to30 m/yr for the subaerial delta plain, and produces deposits with alarge offshore sediment accumulation rate (1–10 cm/yr). The Mekongdelta has formed a classic clinoform deposit with a proximal depocenteroff its river mouth, but with limited cross-shelf distributions (~20 km).However, the strong seasonal monsoon-derived currents and tidal cur-rents have transported the Mekong sediment farther down the dispersalsystem (~250 km) from its river mouth to the Ca Mau Pensinsula andthe Gulf of Thailand, and have formed an along-shelf clinoform anddistal depocenter.

Acknowledgements

This project was funded by the US Office of Naval Research undergrants N00014-14-1–0113 to North Carolina State University (J.P. Liuand D.J. DeMaster, PIs) and grants N00014-12-1–0181, N00014-13-1–0075, N00014-13-1–0781, N00014-15-1–2014, N00014-15-1–2011,and N00014-13-1–0127 to the University of Washington (C.A. Nittrouerand A. S. Ogston, PIs). Nguyen Trung Thanh expresses thanks for thesupport from the Vietnam Academy of Science and Technology (grantVAST06.01/16–17) Vietnam. Earlier research in 2006–2007 wasfunded jointly by the Naval Oceanographic Office, (grant N62306-07-P-9S18), ONR (grant N00014-07-1-0129), and NSF (grant N00014-07-1-0129). We thank Andrea Ogston (UW), Richard Nguyen (ONR), VoLuong Hong Phuoc (VNU) and Daniel Culling (UW/Tulane University)for their assistance with many logistical issues. We also thank JenniferGlass, Deb Nittrouer and Vietnamese colleagues and students for theirfield assistance. We thank Drs. Yoshi Saito, JP Walsh and A. Ogston whoprovided constructive suggestions.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.csr.2017.07.009.

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A seismic study of the Mekong subaqueous delta: Proximal versus distal sediment accumulationIntroductionBackgroundMekong delta plainMekong subaqueous deltaSediment dynamics and accumulation

Data and methodsResultsArea-1: the Northern Proximal subaqueous deltaArea-2: Southern Proximal area showing clinoform development off the Song Hau mouthArea-3: central transition on eastern side of Ca Mau PeninsulaArea-4&5: distal accumulation around the Ca Mau Peninsula and in the GOTSubaqueous deltaic mud isopach map and sediment budget

DiscussionDistribution, stratigraphic features and sediment budget of proximal and distal depositsClinoform topset rollover depth and foreset slope gradientSediment accumulation rates across the subaqueous deltaCross-shelf versus along-shelf transport; proximal versus distal depositsComparison with other East Asian large-river delta systems

ConclusionsAcknowledgementsSupporting informationReferences