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Tanabe,S.,etal.HoloceneevolutionoftheSongHong(RedRiver)deltasystem,northernVietnam.SedimentaryGeology

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Sedimentary Geology 1

Holocene evolution of the Song Hong (Red River) delta system,

northern Vietnam

Susumu Tanabe a,*, Yoshiki Saito a, Quang Lan Vu b, Till J.J. Hanebuth c,

Quang Lan Ngo b, Akihisa Kitamura d

a AIST, Geological Survey of Japan, Institute of Geology and Geoinformation, Central 7, Higashi 1-1-1, Tsukuba 305-8567, Japanb Northern Mapping Division, Department of Geology and Minerals of Vietnam, Ai Mo 1, Gia Lam, Hanoi, Vietnam

c Department of Geosciences, University of Bremen, PO Box 330440, 28334 Bremen, Germanyd Institute of Geosciences, Shizuoka University, Ohya 836, Shizuoka 422-8529, Japan

Received 30 June 2005; received in revised form 28 November 2005; accepted 7 December 2005

Abstract

The Song Hong (Red River) delta, located on the western coast of the Gulf of Bac Bo (Tonkin) in the South China Sea, formed as a

result of the Song Hong sediment discharge throughout the Holocene. The river’s sedimentary basin upstream from the delta plain is

not large. The delta plain comprises emerged tidal/mangrove flats formed during the sea-level highstand at +2–3 m (6–4 cal. kyr BP)

and a beach-ridge strandplain, with straight-to-lobate beach ridges, on the landward and seaward sides of the delta plain, respectively.

The delta affords us the opportunity to examine river-mouth morphodynamics comprehensively in relation to sediment discharge and

sea-level changes. In this paper, we describe the Holocene evolution of the Song Hong delta system and the river-mouth

morphodynamics on the basis of seven sediment cores, each 30–70 m long, taken from the delta plain during 1999–2001 and 101

radiocarbon dates obtained from the core sediments.

Sediments from the seven cores, consisting of incised-valley fills since the last glacial maximum, can be divided, in ascending

order, into fluvial sediments composed of gravelly sand and mottled clay, tide-influenced estuarine sediments containing shell and

wood fragments, and deltaic sediments composed of tide-influenced sand and mud deposits, in which the contents of sand and

wood fragments increase upward. By applying sequence stratigraphic concepts, a transgressive surface can be identified at the

gravelly sand-mottled clay boundary, and the maximum flooding surface can be identified at the estuarine–deltaic sediments

boundary. The lowstand systems tract, transgressive systems tract, and highstand systems tract record ~15, 15–9, and 9–0 cal. kyr

BP, respectively.

During the past 9 kyr, the majority of the sediment discharged by the Song Hong accumulated in the incised valley and

enhanced progradation at the river mouth. The river mouth prograded from the bay head toward the gulf, and its morphology

changed from funnel-shaped (9–6 cal. kyr BP) to straight (6–2 cal. kyr BP) and finally to lobate (2–0 cal. kyr BP). The

morphological change from a funnel-shaped to straight coast is interpreted to have been closely related to a hydrologic regime

shift from a tide-dominated bay-head setting to a wave-influenced open-coast setting. The cessation of the sea-level rise at 6 cal.

kyr BP may have played only a small role in the change of river-mouth morphology because the progradation rate decelerated from

22 to 4 m/yr, instead of accelerating at that time. The river-mouth morphodynamic change from straight to lobate was largely

0037-0738/$ - s

doi:10.1016/j.se

* Correspondi

E-mail addr

87 (2006) 29–61

ee front matter D 2005 Elsevier B.V. All rights reserved.

dgeo.2005.12.004

ng author. Tel.: +81 29 861 3515; fax: +81 29 861 3579.

ess: s.tanabe@aist.go.jp (S. Tanabe).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6130

caused by an increase in the Song Hong sediment discharge from 17–27 (9–2 cal. kyr BP) to 49 million t/yr (2–0 cal. kyr BP) as a

result of anthropogenic deforestation along the upper reaches of the Song Hong.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Holocene; Delta; River-mouth morphology; Sea-level change; Sediment discharge; Regime shift

1. Introduction

Deltas are defined as discrete shoreline protuber-

ances formed where rivers enter oceans, semi-enclosed

seas, lakes, or lagoons, and supply sediment more

rapidly than can be redistributed by basinal processes

(Elliott, 1986). Based on present-day coastal hydrody-

Fig. 1. Quaternary geology and topography of the Song Hong delta and adja

the delta plain into fluvial-, tide-, and wave-dominated systems (Mathers et

corrected to depth below mean sea level (MSL) by adopting the tidal datu

Marine Environments and Resources, Vietnam Academy of Science and Tec

the study area. Elevation and bathymetric data and the distribution of tidal

Department of Geography of Vietnam (1993, 1998a,b,c).

namics, the morphology and sedimentary facies of

marine deltas can be classified as fluvial-, wave-, or

tide-dominated (Galloway, 1975). However, recent

studies on Holocene deltas have shown that their mor-

phology and sedimentary facies in relation to delta

progradation have not been consistent through the Ho-

locene because of changes in coastal settings and sed-

cent areas. Thin dotted lines indicate the geomorphological division of

al., 1996: Mathers and Zalasiewicz, 1999). Bathymetric contours were

m at Hai Phong (1.86 m below MSL) (Tran Duc Thanh, Institute of

hnology, personal communication, 2002). Inset shows the location of

flats and marsh are based on 1 /250000 map sheets published by the

Fig. 2. Location map showing the approximate area of the Song Hong

drainage area (modified after Tanabe et al., 2003b). The extent of the

Red River fault system is based on Rangin et al. (1995) and Harrison

et al. (1996). The Song Hong Basin is after Nielsen et al. (1999). The

dotted oval indicates the area where the annual precipitation exceeds

1600 mm (Vietnam National Committee for International Hydrolog-

ical Programme, 1994).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 31

iment supply (e.g., Saito et al., 2001; Tanabe et al.,

2003a).

Modern marine deltas initiated at the same time as the

deceleration of the postglacial sea-level rise during the

early Holocene (Stanley and Warne, 1994). During the

middle Holocene sea-level highstand, the morphology,

sedimentary facies, and progradation rates of some deltas

changed as a result of changes in the coastal hydrody-

namics caused by delta progradation and changes in the

river sediment supply (Rao et al., 1990; Bellotti et al.,

1994; Cencini, 1998; Amorosi and Milli, 2001; Saito et

al., 2001; Ta et al., 2002a; Tanabe et al., 2003a). For

example, the Holocene delta plain topography of the

Mekong delta changed from tidal and mangrove flats

to beach-ridge strandplain (Ta et al., 2002a; Tanabe et al.,

2003a). The sedimentary facies of the delta front became

coarser-grained, and the progradation rate of theMekong

delta decreased at 3.0–2.5 cal. kyr BP, in large part owing

to changes in the coastal oceanographic setting from a

tide-dominated bay head to a wave-influenced open

coast. The morphology of the Huanghe (Yellow River)

delta also changed from straight to lobate as a result of an

increase of river sediment discharge caused by human

activities in its drainage basin during the last thousand

years (Saito et al., 2001). An increase of sediment dis-

charge during the late Holocene also created deltaic

protrusions of the shorelines at the mouths of the Po,

Tevere, and Krishna rivers (Rao et al., 1990; Bellotti et

al., 1994; Cencini, 1998; Amorosi and Milli, 2001).

Many factors control the morphodynamics and sedimen-

tary facies of deltas. Therefore, comprehensive, quanti-

tative analyses of variable effects such as coastal

hydrodynamics, sediment discharge, and sea-level fluc-

tuation are required to better understand delta evolution.

The Song Hong (Red River) delta, located in the

western coast of the Gulf of Bac Bo (Tonkin) of the

South China Sea, is the fourth-largest delta in Southeast

Asia, after the Mekong, Irrawaddy, and Chao Phraya

deltas, in terms of delta plain area. The delta initiated in

the vicinity of Hanoi about 9 cal. kyr BP, and it subse-

quently prograded and expanded to reach its present

area of 10300 km2 (Tanabe et al., 2003b; Hori et al.,

2004) (Fig. 1). Because a beach-ridge strandplain is

found only in the outer part of the delta plain along

the present coastline, the coastal setting of the delta

must have changed from tide-dominated to wave-influ-

enced during its evolution (Tanabe et al., 2003b). The

Song Hong, which originates in the mountains of Yun-

nan Province in China, flows in a straight line toward

the gulf, regulated by the NW–SE alignment of the Red

River fault system (Fig. 2). Because its catchment does

not include any secondary sedimentary basin and be-

cause no offshore sediment dispersal to the deep ocean

has occurred, by quantifying the deltaic sediments of

the Song Hong, we can calculate its Holocene sediment

discharge.

In this paper, we present new data from four newly

collected drilling cores taken from the Song Hong delta.

We then compile data on sedimentary facies and radio-

carbon dates from three previously described sediment

cores taken from the delta previously (Fig. 1). Based on

the sedimentary facies data and radiocarbon dates from

all the sediment cores and on bifurcation patterns of the

beach ridges, the paleogeography and evolution of the

Song Hong delta and sediment discharge during the last

9 kyr are discussed. Finally, we examine the river-

mouth morphodynamics in relation to changes in the

coastal hydrodynamics, sediment discharge, and sea

level during the Holocene.

2. Regional setting

2.1. Geological setting

The Song Hong delta is surrounded by mountains

composed of Precambrian crystalline rocks and Paleo-

zoic to Mesozoic sedimentary rocks, and is situated in a

Neogene NW–SE-trending sedimentary basin approxi-

mately 500 km long and 50–60 km wide (Fig. 2)

(Mathers et al., 1996; Mathers and Zalasiewicz, 1999;

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6132

Nielsen et al., 1999). The NW–SE alignment of the Red

River fault system (Rangin et al., 1995) regulates the

distribution of mountainous areas, the sedimentary

basin, the drainage area, and the straight course of the

Song Hong. The basin is filled with Neogene and Qua-

ternary sediments to a thickness of more than 3 km, and

the subsidence rate of the basin is 0.04–0.12 mm/yr

(Mathers et al., 1996; Mathers and Zalasiewicz, 1999;

Tran and Dinh, 2000).

2.2. Quaternary stratigraphy

The Quaternary sediments, which unconformably

overlie Neogene deposits, are composed mainly of

sands and gravels with subordinate lenses of silt and

clay, and the sediments thicken seaward to a maximum

thickness of 200 m beneath the coastal area of the

delta (Mathers et al., 1996; Mathers and Zalasiewicz,

1999).

The Song Hong incised valley, formed during the last

glacial maximum (LGM), is located southwest of the

present Song Hong channel (Fig. 3) (Tanabe et al.,

2003b). The narrow, elongate valley is oriented NW–

SE and is approximately 20 kmwide and more than 80m

deep. It is filled with lowstand, transgressive, and high-

stand deposits consisting of gravel, sand, and clay, re-

spectively (Mathers et al., 1996; Mathers and

Zalasiewicz, 1999), whereas the surrounding sediments

are composed mainly of highstand deposits of massive

clay intercalated with peaty organic layers (Lam and

Boyd, 2000, 2003).

Fig. 3. Basal topography of the Song Hong incised valley, formed

during the LGM, predicted from the groundwater table in the Qua-

ternary sediments (modified after Tanabe et al., 2003b).

2.3. Geographical setting

The landward limit of the Song Hong delta is defined

by those of the Holocene mangrove clay and mid-Holo-

cene marine terraces (Fig. 1) (Tanabe et al., 2003b).

The Song Hong delta plain has been divided into

wave-, tide-, and fluvial-dominated systems on the

basis of surface topography and hydraulic processes

(Fig. 1) (Mathers et al., 1996; Mathers and Zalasiewicz,

1999). The wave-dominated system is located in the

southeastern part of the delta, where wave energy gen-

erated by summer monsoon winds is relatively strong.

The system is characterized by alternating beach ridges

and inter-ridge marshes. The tide-dominated system has

developed in the northeastern part of the delta, where

Hainan Island shelters the coast from strong waves

(Fig. 2). The system comprises abandoned and active

tidal flats, marshes, and tidal creeks/channels. The flu-

vial-dominated system is composed of meandering riv-

ers, meandering levee belts, floodplain, and fluvial

terraces. It is located in the western portion of the

delta, where the fluvial flux is relatively stronger than

in the tide or wave systems.

The subaqueous part of the delta is divided into delta

front and prodelta on the basis of the subaqueous

topography (Fig. 1) (Tanabe et al., 2003c). The delta

front lies from 0 to 22 m below mean sea level, and the

prodelta extends offshore from the delta front. The delta

front is subdivided into the delta front platform and

delta front slope. The delta front platform is the area

above the slope break, and it has a water depth of 8 m

and a gradient of b0.9 /1000. The delta front slope has

a relatively steep face with a gradient of N3.0 /1000.

2.4. Hydrology

The Song Hong drains an area of 160�103 km2

(Milliman et al., 1995). It originates in the mountains

of Yunnan Province, China, at an elevation above

2000 m (Fig. 2). The Song Hong flows 1200 km

before it empties into the Gulf of Bac Bo (Gulf of

Tonkin) in the South China Sea. At present, the total

sediment and water discharge of the Song Hong is

100–130 million t/yr and 120 km3/yr, respectively

(Milliman et al., 1995; Pruszak et al., 2002), and

the average sediment concentration in the river is

1.08 kg/m3. The water discharge varies seasonally

because most of the drainage area is subject to a

subtropical monsoon climate regime. The discharge at

Hanoi station reaches a maximum in July–August

(about 23000 m3/s) and a minimum during the dry

season (January–May) (typically 700 m3/s). Approx-

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 33

imately 90% of the annual sediment discharge occurs

during the summer monsoon season, at which time

the sediment concentration may reach 12 kg/m3

(Mathers et al., 1996; Mathers and Zalasiewicz,

1999).

In the delta plain, the river diverges into two major

distributaries in the vicinity of Hanoi: the Song Hong to

the southwest and the Thai Binh River to the northeast

(Fig. 1). The Thai Binh River carries 20% of the total

water discharge (General Department of Land Admin-

istration, 1996).

2.5. Oceanography

The mean tidal range is 2.0–2.6 m (Coleman and

Wright, 1975; Tran Duc Thanh, Institute of Marine

Environment and Resources, Vietnam Academy of Sci-

ence and Technology, personal communication, 2000),

and the maximum tidal range is 3.2–4.0 m along the

Song Hong delta coast (Mathers et al., 1996; Mathers

and Zalasiewicz, 1999; Tran and Dinh, 2000). In the

summer monsoon season, tidal influences within the

delta are restricted because of the overwhelming effect

of the high freshwater discharge, but in the dry season,

tidal effects are evident in both major distributaries

almost as far inland as Hanoi (Mathers et al., 1996;

Mathers and Zalasiewicz, 1999).

Along the delta coast, the mean and maximum wave

heights are 0.88 and 5.0 m, respectively (Tran and Dinh,

Fig. 4. Sea-level curve for the western margin of the South China Sea during

this curve are a combination of those for the Sunda Shelf between 20 and 11

and 6 cal. kyr BP (Geyh et al., 1979), and the Song Hong delta region betwe

sea-level jump between 15 and 14 cal. kyr BP corresponds to MWP1A.

2000). Strong southwest winds during the summer mon-

soon tend to produce NNW-directed wave action in the

Gulf of Bac Bo. Throughout most of the rest of the year,

winds are from the ENE, and then the delta coastline is

well protected by the Chinese mainland and Hainan

Island (Mathers et al., 1996; Mathers and Zalasiewicz,

1999). NNW-directed waves are responsible for the

dominant bifurcation patterns of longshore currents

and beach ridges along the coast in the wave-dominated

system (Fig. 1) (Mathers et al., 1996).

In accordance with the classification scheme of

Davis and Hayes (1984), the Song Hong deltaic coast

is considered a mixed energy (tide-dominated) coast.

2.6. Sea-level curves for the Song Hong delta region

Figs. 4 and 5 show the sea-level curve for the Song

Hong delta region over the past 20 kyr. The sea-level

curves were constructed from a combination of those

for the Sunda Shelf between 20 and 11 cal. kyr BP

(Hanebuth et al., 2000), the Strait of Malacca between

11 and 6 cal. kyr BP (Geyh et al., 1979), and in and

around the Song Hong delta between 6 and 0 cal. kyr

BP (Tran and Ngo, 2000; Lam and Boyd, 2001). The

paleobathymetries (or mean tidal ranges) are regarded

as bF2.0 m in the Sunda Shelf (Hanebuth et al., 2000),

bF1.3 m along the Strait of Malacca (Geyh et al.,

1979), and F2.0–2.6 m along the Song Hong delta

coast (Coleman and Wright, 1975; Tran Duc Thanh,

the past 20 kyr (modified after Tanabe et al., 2003b). The data used for

cal. kyr BP (Hanebuth et al., 2000), the Strait of Malacca between 11

en 6 and 0 cal. kyr BP (Tran and Ngo, 2000; Lam and Boyd, 2001). A

Fig. 5. Sea-level curve in the Song Hong delta region during the past

8 kyr (modified after Tanabe et al., 2003b). Heights of marine notches

described by Lam and Boyd (2001) were corrected to heights above

mean sea level by adopting the height between the tidal datum and the

mean sea level in Ha Long Bay (1.86 m) (Tran Duc Thanh, Institute of

Marine Environments and Resources, Vietnam Academy of Science

and Technology, personal communication, 2002). The sea-level curve

is shown as a thick shaded zone, taking into account the accretion

zone of oyster shells in modern notches in Ha Long Bay. Modern

oyster shells accrete in a zone between 0.5 m above and below the

present mean sea level.

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6134

Institute of Marine Environment and Resources, Viet-

nam Academy of Science and Technology, personal

communication, 2000), respectively.

During the LGM, the sea level was about 120 m

below the present level. It reached approximately 50,

30, 15, and 5 m below the present level at about 11, 10,

9, and 8 cal. kyr BP, respectively. The sea reached its

present level at ca. 7 cal. kyr BP. After attaining a high

of 2–3 m above the present level at 6–4 cal. kyr BP, sea

level fell, at first rapidly and later gradually, reaching

the present level from 4 to 0 cal. kyr BP.

3. Materials and methods

Seven rotary-drilled sediment cores (ND-1, DT, CC,

VN,HV,NB, andGA)were obtained from the SongHong

delta plain in 1999–2001 (Fig. 1). The sedimentary units

and facies and radiocarbon dates from the ND-1, DT, and

CC cores obtained in 1999–2000 have been reported by

Tanabe et al. (2003c), Tanabe et al. (2003b), and Hori et

al. (2004), respectively. In this paper, we summarize the

Table 1

List of cores taken from the Song Hong delta, Vietnam, and their locations

Core Latitude (N) Longitude (E) Altitude (m) Penetration depth

ND-1 20822V22W 106808V48W ca. 1–2 70.0

DT 20837V59W 105859V20W ca. 3–4 41.3

CC 20841V05W 105854V13W ca. 3 29.4

VN 20824V37W 106822V39W 0.6F0.1 35.0

HV 20813V27W 106819V49W 0.6F0.1 60.0

NB 20820V05W 106827V09W 1.2F0.3 45.0

GA 20815V26W 106830V57W 1.1F0.2 40.2

sedimentary facies and units and radiocarbon dates from

both the three previously reported cores and the VN, HV,

NB, and GA cores, which were obtained in 2001. The

sediments from below 32 m in the HV core are not dealt

with here, but are described by Hanebuth et al. (2006).

The ND-1, DT, and CC cores were obtained from the

floodplain in the fluvial-dominated system, and the VN,

HV, NB, and GA cores were obtained from the inter-

ridge marshes alternating with beach ridges in the wave-

dominated system (Fig. 1). The location, altitude, pene-

tration depth, and percentage recovery of each core are

summarized in Table 1.

In the laboratory, the sediment cores were split, de-

scribed, and photographed. X-radiographs were taken of

slab samples (6 cm wide�20 or 25 cm long�1 cm

thick) from the split core. Sand and mud contents were

measured by using a 63-Am sieve for 5-cm-thick samples

collected every 20 cm. In all, 101 accelerator mass

spectrometry (AMS) radiocarbon dates were obtained

on molluscan shells, an echinoderm, crab shell, peaty

organic matter, and wood fragments by Beta Analytic

Inc., including 56 new radiocarbon dates from the four

new cores. Calibrated 14C ages were calculated accord-

ing to CALIB v.4.4 (Stuiver and Reimer, 1993) using the

data set of Stuiver et al. (1998). For the calculation of

ages from molluscan shells, the echinoderm, and crab

shell,DR (the difference between the regional and global

marine 14C age) (Stuiver and Braziunas, 1993) was

regarded as �25F20 yr (Southon et al., 2002), and

the marine carbon component as 100%. All ages in this

manuscript are reported as calibrated 14C ages (calendar

years before present) unless otherwise noted as years

before present (conventional 14C ages).

4. Sedimentary units and facies

The ND-1, CC, DT, VN, HV, NB, and GA core

sediments can be divided into four sedimentary units,

in ascending order: 0 (late Pleistocene shallow-marine

sediments), 1 (latest Pleistocene fluvial sediments), 2

(Holocene estuarine sediments), and 3 (Holocene del-

(m) Core recovery (%) Drilling period Reference

65.5 Dec–99 Tanabe et al. (2003c)

65.0 Dec–00 Tanabe et al. (2003b)

60.0 Dec–01 Hori et al. (2004)

94.3 Nov–01 This study

76.2 Nov–01 This study

88.0 Nov–01 This study

75.7 Nov–01 This study

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 35

taic sediments). Units 1, 2, and 3, dated to b15 cal. kyr

BP, consist of two (channel-fill and floodplain sedi-

ments), six (tide-influenced channel-fill to coastal

marsh, lagoon, flood tidal delta, tidal flat and salt

marsh, sub- to intertidal flat, and estuary front sedi-

ments) and seven (tide-influenced channel-fill, shelf to

prodelta, delta front slope, delta front platform, tidal

flat, abandoned channel-fill sediments, and floodplain

sediments and surface soil) sedimentary facies, respec-

tively (Table 2 and Fig. 6). Each sedimentary unit is

characterized by a combination of its lithology, colour,

sedimentary structures, textures, contact character, lith-

ological succession, fossil components, grain size

(shown as sand and mud content), and radiocarbon

dates. Unit 0 is found only in the HV core, where it

is overlain unconformably by Unit 3. At the other sites,

none of the cores penetrated to Unit 0. Details of the

sedimentary units are described below.

4.1. Unit 0 (late Pleistocene shallow-marine sediments)

Depth in core: HV core, 32.0–27.9 m.

This unit consists of tan-colored, laminated sand

(depth in core: 32.0–28.9 m) and bioturbated clay

(depth in core: 28.9–27.9 m) (Fig. 7A). The lithology

is stiff compared with that of the overlying units. The

laminated sand is composed of alternating very fine sand

and clay laminae 3–10 and 3–5 mm thick, respectively.

Vertical burrows, 3–30 cm long and 1–5 cm in diameter,

occur at the top of this unit, and are filled with black clay

fromUnit 3. Plant fragments rarely occur in this unit. It is

dated to 45210F700 yr BP (Hanebuth et al., 2006).

Interpretation: The burrows seldom occur in non-

marine environments (Pemberton et al., 1992). There-

Table 2

Sedimentary units and facies identified in the CC, DT, ND-1, VN, HV

Sedimentary units Sedimentary facies

0 Late Pleistocene shallow-marine sediments

1 Fluvial sediments 1.1 Channel-fill sediments

1.2 Floodplain sediments

2 Estuarine sediments 2.1 Tide-influenced chann

2.2 Lagoon sediments

2.3 Flood tidal delta sedim

2.4 Tidal flat and salt mar

2.5 Sub- to intertidal flat s

2.6 Estuary front sediment

3 Deltaic sediments 3.1 Tide-influenced chann

3.2 Shelf to prodelta sedim

3.3 Delta front slope sedim

3.4 Delta front platform se

3.5 Tidal flat sediments

3.6 Abandoned channel-fi

3.7 Floodplain sediments

fore, this unit is interpreted as late Pleistocene shallow-

marine sediments. The tan color and the stiff lithology

of the sediments might indicate that they were oxidized

during lateritic weathering after their emergence (Parton

et al., 1995). Hanebuth et al. (2006) provide a more

detailed description and discussion of the age of this

unit.

4.2. Unit 1 (fluvial sediments)

Depth in core: ND-1 core, 70.0–45.0 m.

This unit consists of pebbly sand (Facies 1.1) and

mottled clay (Facies 1.2) in ascending order. The sedi-

ments display an overall fining-upward succession (Fig.

6A). The unit contains abundant organic materials com-

posed of plant fragments but no shell fragments. Mud

clasts are common. This unit yielded two 14C dates of

14950F580 and 14840F580 cal. yr BP.

Interpretation: This unit is interpreted as fluvial sedi-

ments, because the sediments are not bioturbated and the

abundant plant fragments indicate that the sediment was

deposited under predominantly terrestrial conditions. Fur-

thermore, a fining-upward succession is characteristic of

lateral accretion in a meandering river system (Bernard et

al., 1962; Allen, 1963; Visher, 1965; Miall, 1992).

Details of facies 1.1 and 1.2 composing this unit are

described below.

4.2.1. Facies 1.1 (channel-fill sediments)

Depth in core: ND-1 core, 70.0–54.0 m.

This facies displays a fining-upward succession con-

sisting of pebbly sand (Fig. 7B) and poorly sorted

medium to very fine sand. Sedimentary structures iden-

tified in this facies are planar cross-bedding, dipping at

, NB, and GA cores, taken from the Song Hong delta, Vietnam

Cores

HV

ND-1

ND-1

el-fill to coastal marsh sediments CC, DT

DT, ND-1

ents ND-1

sh sediments DT, ND-1

ediments VN, NB, GA

s NB, GA

el-fill sediments CC, DT

ents ND-1, VN, NB, GA

ents ND-1, VN, HV, NB, GA

diments ND-1, VN, HV, NB, GA

CC, VN, HV, NB, GA

ll sediments DT, ND-1

and surface soil CC, DT, ND-1, VN, HV, NB, GA

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6136

approximately 208, and current ripple cross-lamination.

Interlaminated sand and mud, and plant fragments

occur within the interval from 56.6 to 56.5 m.

Fig. 6. Sedimentary columns of the ND-1, CC, DT, VN, HV, NB, and GA core

al. (2003c). (B) CC andDTcores, respectively, modified after Hori et al. (2004)

Interpretation: The lithology and sedimentary struc-

tures of Facies 1.1 can be classified according to the

facies codes Sp (medium to very coarse sand, includ-

s. (A) Sedimentary column of the ND-1 core is modified after Tanabe et

and Tanabe et al. (2003b). (C) VN andHV cores. (D) NB andGA cores.

Fig. 6 (continued).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 37

ing pebbles), Sr (very fine to coarse sand), and Fl

(sand, silt, and mud) of Miall (1992), in ascending

order. This combination of Sp, Sr, and Fl is character-

istic of channel (CH) (Allen, 1983; Miall, 1992) and of

waning-flood deposits (Miall, 1992). Thus, this facies

is interpreted as channel-fill sediments. CH is defined

for small channels within a channel complex or for

component elements of main-channel-fill deposits

(Miall, 1992).

4.2.2. Facies 1.2 (floodplain sediments)

Depth in core: ND-1 core, 54.0–45.0 m.

Facies 1.2 consists mainly of reddish mottled silty

clay (Fig. 7C). Abundant rootlets have iron encrusta-

tion. The silty clay is intercalated with well-sorted

very fine to fine sand beds in the interval from 48.1

to 46.8 m.

Interpretation: Facies 1.2 is interpreted as floodplain

sediments. The colour and the texture of the mottled clay

Fig. 6 (continued).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6138

can be interpreted as backswamp deposits (Fsc) (Miall,

1992) or floodplain deposits with lateritic soil develop-

ment (Collinson, 1996). The sand bed is interpreted as

crevasse splay sediments (Miall, 1992; Collinson, 1996).

The term bflood plainQ as used by Collinson (1996) can

include various subenvironments such as backswamp

and crevasse splay. Therefore, this facies is interpreted

as floodplain sediments.

4.3. Unit 2 (estuarine sediments)

Depth in core: ND-1 core, 45.0–20.5 m; DT core,

41.3–22.6 m; CC core, 29.4–23.8 m; VN core, 35.0–

29.8 m; NB core, 45.0–34.5 m; GA core, 40.2–36.6 m.

This unit is characterized by thinly and rhythmically

interlaminated or bedded sand and mud, bidirectional

ripple cross-lamination, and paired mud drapes. Abun-

dant wood and plant fragments and molluscan shells of

Potamocorbula sp. and Corbicula sp. occur in the

sediments. This unit consists of, in ascending order in

each core, clay with abundant plant fragments, well-

sorted sand, and laminated shelly sand in the ND-1 core

(Fig. 6A), interbedded sand and mud and clay with

abundant plant fragments in the DT core (Fig. 6B),

interbedded sand and mud in the CC core (Fig. 6B),

bioturbated silt in the VN core (Fig. 6C), bioturbated

silt and laminated clay in the NB core (Fig. 6D), and

laminated clay in the GA core (Fig. 6D). The unit

Fig. 6 (continued).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 39

yielded 14C dates ranging from 11430F200 to

9210F180 cal. yr BP.

Interpretation: The rhythmically interlaminated/bed-

ded sand and mud, bidirectional ripple cross-lamina-

tion, and paired mud drapes of this unit indicate that the

sediments were deposited in environments strongly

influenced by tides (Reineck and Singh, 1980; Nio

and Yang, 1991; de Boer, 1998). The wood/plant frag-

ments and the molluscan shells, respectively, indicate

terrestrial input and a brackish-water environment. The

mixture of terrestrial input with brackish water and tidal

influences suggests that this sediment was deposited in

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6140

a mixed zone with marine and fluvial influences such as

in an estuarine environment.

Details of facies 2.1–2.6 composing this unit are

described below.

4.3.1. Facies 2.1 (tide-influenced channel-fill to coastal

marsh sediments)

Depth in core: CC core, 29.4–23.8 m; DT core,

41.3–30.0 m.

Facies 2.1 shows an overall fining-upward succes-

sion from medium sand to laminated clay (Fig. 7D).

The sand contains mud drapes (b5 mm in thickness)

and mud clasts (b25 mm in diameter). Ripple cross-

laminations identified in the radiographs of the sand

contain bidirectional or multidirectional foresets. Peaty

organic layers and very fine sand layers less than 10 mm

thick are interlaminated with the clay. Rootlets occur at

the top of this facies.

Interpretation: Facies 2.1 is interpreted as tide-influ-

enced channel-fill to coastal marsh sediments. An over-

all fining-upward lithological succession and the

occurrence of mud drapes and bidirectional ripple

cross-lamination are common in tidal creek and tidal

flat sediments (Reineck and Singh, 1980; Dalrymple,

1992). Peaty laminated clay or clay with roots, which

occurs at the top of this facies, is a common feature of

floodplain and coastal marsh environments (Frey and

Basan, 1985; Miall, 1992; Collinson, 1996). It is most

appropriate to interpret this facies as coastal marsh

sediments because it is overlain by the lagoon sedi-

ments of Facies 2.2.

4.3.2. Facies 2.2 (lagoon sediments)

Depth in core: DT core, 30.0–22.6 m; ND-1 core,

45.0–36.0 m.

This facies is characterized by dark reddish silty

clay and bluish gray massive clay (Fig. 7E) with

abundant plant fragments and minor burrows. Thin

Fig. 7. Photographs of typical sedimentary facies of the ND-1, DT, CC, VN,

bar is 10 cm. (A) Unit 0, HV core, 27.65–28.15 m depth. Tan-colored silty s

62.50 m depth. Gravelly sand. Parallel laminations are partly contorted owing

depth. Red to black organic clay. (D) Facies 2.1, DT core, 34.80–35.30 m d

thick. (E) Facies 2.2, DT core, 25.85–26.25 m depth. Gray massive clay. (F

with peaty layers containing wood and plant fragments. Burrows and shells

Rhythmically interlaminated sand and mud. The sand and mud laminations

2.5, VN core, 30.00–30.50 m depth. Bioturbated sandy clay. (I) Facies 2.6,

Facies 3.1, DT core, 6.45–7.20 m depth. Sand–mud alternation. The sand

rhythmically laminated with thin silt and peaty layers. (K) Facies 3.2, ND-1

3.3, ND-1 core, 11.80–12.40 m depth. Sand–mud alternation with calcareou

core, 5.40–6.15 m depth. Medium sand interbedded with black organic clay.

rhythmically laminated clay. Shelly layer identified at the bottom. (O) Facies

organic layer at the top. (P) Facies 3.5, CC core, 3.05–3.65 m depth. Interla

1.55 m depth. Gray mottled clay. (r) Facies 3.7, CC core, 0.30–1.00 m dep

peaty organic layers, 5 mm thick, and calcareous

concretions (35–55 mm in diameter) are common in

the dark reddish silty clay.

Interpretation: The characteristics of this facies re-

semble those of lagoonal deposits reported by Reinson

(1992). The peaty layer may have formed in a salt

marsh at the margin of a lagoon (Reinson, 1992).

4.3.3. Facies 2.3 (flood-tidal delta sediments)

Depth in core: ND-1 core, 36.0–27.0 m.

Facies 2.3 displays a coarsening-upward succession

from laminated fine-medium sand (Fig. 7F) to well-

sorted medium sand. The laminated sand consists of

very fine to medium sand with clay laminae and peaty

organic layers. The clay laminations, 1–5 mm thick,

occasionally display paired mud drapes. Trough cross-

bedding that dips at 208 to 408 occurs in the well-sorted

medium sand. Shell fragments of Potamocorbula amur-

ensis (Shrenck) and Corbicula sp. are scattered

throughout this facies.

Interpretation: The upward-coarsening succession

of this facies resembles that of the flood-tidal delta

sediments described from the late Pleistocene paleo-

Tokyo Bay in Japan (Murakoshi and Masuda, 1991).

4.3.4. Facies 2.4 (tidal flat and salt marsh sediments)

Depth in core: ND-1 core, 27.0–20.5 m.

This facies is characterized by gray, laminated silty

clay, occasionally rhythmically interlaminated with

very fine sand laminae 1–3 mm thick (Fig. 7G).

Bivalves of P. amurensis (Shrenck) and Corbicula sp.

are present in 3- to 5-cm-thick shelly layers (shell

concentrations) in the silty clay. Some of the bivalves

are articulated. Occasional small burrows (b5 mm in

diameter), plant fragments, and rootlets are found in

this facies.

Interpretation: This facies can be interpreted as

tidal flat sediments. Rhythmically laminated clay is

HV, NB, and GA cores. The top is at the upper left in all figures. Scale

and overlain by black prodelta clay. (B) Facies 1.1, ND-1 core, 61.70–

to deformation during coring. (C) Facies 1.2, ND-1 core, 48.8–49.2 m

epth. Sand–mud alternation. Sand and mud layers are each 1–10 cm

) Facies 2.3, ND-1 core, 33.40–34.20 m depth. Sand–mud alternation

are identified at the top. (G) Facies 2.4, ND-1, 25.40–25.80 m depth.

are 1–5 mm thick. Shell fragments are identified at the top. (H) Facies

NB core, 40.25–41.00 m depth. Shelly silt with faint sand lamina. (J)

and mud are 1–8 and 1–20 cm thick, respectively. Mud beds are

core, 16.20–16.8 m depth. Gray clay with shell fragments. (L) Facies

s concretions (colored black in the photograph). (M) Facies 3.4, VN

(N) Facies 3.4, HV core, 4.40–5.10 m depth. Medium sand partly with

3.4, GA core, 8.40–9.00 m depth. Sandy clay–clay alternation. Peaty

minated sand and mud with rootlets. (Q) Facies 3.6, ND-1 core, 1.15–

th. Red mottled clay.

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 41

a common feature of tidal deposits or tidal rhythmites

(Reineck and Singh, 1980; Nio and Yang, 1991). The

concentration of in situ P. amurensis (Shrenck) and

Corbicula sp. shells may suggest a primary biogenetic

concentration (Kidwell, 1991) in a brackish-water

environment. Rootlets are commonly found in salt

Fig. 7 (continued).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6142

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 43

marsh and upper tidal flat sediments (Frey and Basan,

1985).

4.3.5. Facies 2.5 (sub- to intertidal flat sediments)

Depth in core: VN core, 35.0–29.8 m; NB core,

45.0–41.6 m.

This facies is characterized by reddish gray, inten-

sively bioturbated silt (Fig. 7H). A large portion of this

facies is bioturbated, but very fine sand and clay lami-

nations remain as faint lenses 3–8 mm thick. In the

lower part of the VN core, from 32.5 to 33.0 m, clay

laminations 3–5 mm thick are preserved. They alternate

with very fine sand layers 3–10 mm thick, forming

lenticular bedding. The contact at the base of the sand

and silt layers is sharp, whereas at the top, the transition

to clay is gradual. Shelly layers 2–3 cm thick consisting

of jointed bivalves of Potamocorbula laevis and Cor-

bicula fluminea are also found. Wood/plant fragments

are scattered throughout this facies.

Interpretation: This facies is interpreted as subtidal

to intertidal flat sediments because of the lenticular

bedding, which is characteristic of those environments

(Reineck and Singh, 1980). Sand is transported and

deposited by currents whereas mud in suspension set-

tles onto the sand during slack-water periods (Reineck

and Wunderlich, 1968). Shelly layers of P. laevis and C.

fluminea indicate a brackish-water environment. A sim-

ilar lithology is reported from modern tidal flat sedi-

ments in Gomso Bay, Korea (Kim et al., 1999).

4.3.6. Facies 2.6 (estuary front sediments)

Depth in core: NB core, 41.6–34.5 m; GA core,

40.2–36.6 m.

This facies consists of reddish brown clay (Fig. 7I).

Parallel and lenticular laminations composed of silt, 3–

8 mm thick, rarely intercalate the clay. Articulated

shells of the bivalve P. laevis are scattered within the

clay, probably in life position. The mud content of this

facies is almost 100%. In the NB core, the contact with

the underlying Facies 2.5 is gradational, and wood/

plant fragments are decreased relative to Facies 2.5

(Fig. 6D).

Interpretation: The high mud content indicates that

most sediments of this facies were deposited from

suspension. Suspended clay in the present Song Hong

is red (Mathers and Zalasiewicz, 1999) because it is

derived from the lateritic soils along the river’s upper

reaches. Therefore, the red clay in this facies might

indicate a strong fluvial influence. Silt layers overlying

the clay with gradual boundaries might have been

deposited from hypopycnal flow during floods (Read-

ing and Collinson, 1996). In situ P. laevis indicates that

the water depth did not exceed 20 to 30 m. On the basis

of the facies succession, this facies is considered to

have been deposited seaward of Facies 2.5 (sub- to

intertidal flat sediments) and landward of Facies 3.2

(sea-floor and prodelta sediments). A similar lithology

has been reported from latest Pleistocene incised-valley

fill sediments in the Changjiang (Yangtze River) delta

in China, which have been interpreted as estuary front

sediments (Hori et al., 2001a).

4.4. Unit 3 (deltaic sediments)

Depth in core: ND-1 core, 20.5–0 m; DT core,

22.6–0.0 m; CC core, 23.8–0 m; VN core, 29.8–0.0 m;

NB core, 34.5–0.0 m; HV core, 27.9–0.0 m; GA core,

36.6–0.0 m.

This unit in the CC and DT cores is characterized by

laminated sand containing bidirectional ripple cross-

lamination and abundant shell fragments (Fig. 6B).

On the other hand, in the ND-1, VN, HV, NB, and

GA cores, this unit consists of an overall coarsening-

upward lithological succession from massive clay to

interlaminated sand and mud or massive sand (Fig.

6A, 6C, and 6D). Molluscan shells vary upward from

those with a shallow-marine habitat, including Sca-

pharca sp., to those with an intertidal habitat, including

Potamocorbula sp. The number of burrows and inten-

sity of bioturbation decrease upward, whereas the num-

ber of wood and plant fragments increases upward. The

mud content decreases from 100% to 0–40% upward.

In ascending order in each core, this unit consists of

massive clay, laminated clay, interbedded or laminated

sand and mud, interbedded sand and mud, and red

mottled clay in the ND-1 core; laminated sand, inter-

bedded sand and mud, and red mottled clay in the DT

core; laminated sand, grayish-brown silty clay, and red

mottled clay in the CC core; and massive clay, lami-

nated clay, interbedded or laminated sand and mud,

brown mottled silty clay, and red mottled clay in the

VN, HV, NB, and GA cores. This unit yielded 14C dates

ranging from 10090F110 cal. yr BP to modern.

Interpretation: This unit is interpreted as deltaic sedi-

ments because an upward-coarsening and shallowing

lithological succession is typical of prograding deltaic

deposits (Scruton, 1960; Visher, 1965; Coleman and

Wright, 1975). Vertical variations of bioturbation and

contents of wood and plant fragments are similar to those

of modern deltaic deposits reported from the Mekong

delta in Vietnam (Tanabe et al., 2003a) and the Chao

Phraya delta in Thailand (Tanabe et al., 2003d).

Details of facies 3.1–3.8 composing this unit are

described below.

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6144

4.4.1. Facies 3.1 (tide-influenced channel-fill

sediments)

Depth in core: CC core, 23.8–3.3 m, DT core, 22.6–

6.4 m.

This facies consists of well-sorted fine to medium

sand partly interlaminated/bedded with clay and silt

(Fig. 7J). The medium sand contains abundant shell

fragments of Potamocorbula sp., Corbicula sp., and

Mactridae sp., which are mostly broken into thin

pieces less than 5 mm in diameter. Ripple cross-

lamination with bidirectional foresets and cross-lami-

nations dipping approximately 108 occur in the sands.

Clay and silt laminations/beds range in thickness

from b1 mm to 12 cm. They occasionally create

bbundle sequencesQ 3–30 cm thick in the medium

sand. The basal portion of the DT core (depth in

core: 22.6–22.4 m) contains oyster shell fragments,

quartz and feldspar grains, and calcareous concretions

of various sizes, ranging from very coarse sand to

pebbles. Calcareous concretions are well rounded

compared with those obtained from the underlying

Facies 2.2. The top portion of the DT core (depth

in core: 7.2–6.4 m) consists of rhythmically interla-

minated sand, mud, and peaty layers, between 1 and

5 mm thick (Fig. 7J).

Interpretation: The lithologies of this facies resem-

ble those reported from modern tide-influenced chan-

nel-fill deposits of the Fly River delta in the Gulf of

Papua (Dalrymple et al., 2003) and the Colorado River

delta in the Gulf of California (Meckel, 1975; Galloway

and Hobday, 1996). Well-rounded calcareous concre-

tions and oyster shell fragments in the DT core indicate

that the sediments were deposited in a tide-influenced

channel cutting into the underlying Facies 2.2. The

lithological succession of Facies 3.1 is relatively

thick compared with the succession of the channel-

fill deposits of the Fly and Colorado rivers, but the

heterolithic nature of the interbedded/laminated sand

and mud well resembles the sedimentary deposits of

those rivers. The rhythmically interlaminated sand,

mud, and peaty layers are regarded as tidal bar or

tidal flat sediments, which cap the channel-fill se-

quence (Dalrymple et al., 2003). The occurrence of

Potamocorbula sp. and Corbicula sp. also indicates a

brackish-water environment, which supports the inter-

pretation that the unit was deposited in a tide-influ-

enced environment.

4.4.2. Facies 3.2 (shelf to prodelta sediments)

Depth in core: ND-1 core, 20.5–16.2 m, VN core,

29.8–18.7 m; NB core, 34.5–26.3 m; HV core, 27.9–

22.5 m; GA core, 36.6–30.9 m.

This unit is characterized by blue to gray bioturbated

clay (Fig. 7K). Parallel lamination/bedding, 1–40 mm

thick and consisting of very fine sand and silt, is

occasionally preserved in the clay. Tests of the benthic

foraminifer Ammonia beccarii and shell fragments are

scattered in the clay. Mud content increases upward

from 80% to 100%. In the VN, NB, and GA cores,

the number of foraminifera and shell fragments

decreases upward and the frequency of sand and silt

layers increases upward.

Interpretation: This facies is interpreted as shelf to

prodelta sediments. The fining-upward lithological suc-

cession with the decrease in shell fragments and the

increase in sand/silt layers may reflect an increase in

fluvial influence. The increased mud content upward

also indicates an increase in riverine mud. A similar

lithology has been reported from the Holocene shelf to

prodelta sediments of the Mekong and Chao Phraya

deltas (Tanabe et al., 2003a,d). The lowest part of these

sediments is interpreted as condensed, having been

deposited in a sediment-starved offshore environment

(Tanabe et al., 2003a,d).

4.4.3. Facies 3.3 (delta front slope sediments)

Depth in core: ND-1 core, 16.2–6.8 m; VN core,

18.7–11.5 m; NB core, 26.3–10.0 m; HV core, 22.5–

9.5 m; GA core, 30.9–9.0 m.

This facies is characterized by reddish gray or black,

laminated silty clay (Fig. 7L). The boundary with un-

derlying Facies 3.2 is gradual, and the lithology grad-

ually coarsens upward. Well-sorted very fine sand and

silt layers, 0.1–10 cm thick, are commonly intercalated

with the silty clay. Burrows, molluscan shells, crab

shells, wood and plant fragments, and calcareous con-

cretions are common throughout this facies (Fig. 7L).

Paired mud drapes and rhythmically laminated silt and

clay, each layer 1–3 mm thick, are visible in the upper

portions of the ND-1, VN, NB, and GA cores.

Interpretation: The upward-coarsening succession

with abundant molluscan shells, crab shells, and

wood and plant fragments is typical of a delta front

facies (Scruton, 1960; Elliott, 1986; Coleman, 1981;

Reading and Collinson, 1996). The rhythmically lam-

inated silt and clay and the paired mud drapes show

that tidal currents influenced the upper portions of the

ND-1, VN, NB, and GA cores (Reineck and Singh,

1980; de Boer, 1998). The gradual contact with the

underlying Facies 3.2 suggests that these sediments

were deposited in the landward or shallower region

of the prodelta. The basal depth of this facies in the

cores is approximately 20 m below the present sea

level, which is comparable to the lower limit of the

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 45

present delta front slope (Fig. 1). Therefore, this facies

is interpreted as delta front slope sediments. Massive

sandy silt in the GA core (depth in core: 28.8–19.3m)

(Fig. 6D) is interpreted as hyperpycnal-flow deposits

preserved in a delta front.

4.4.4. Facies 3.4 (delta front platform sediments)

Depth in core: ND-1 core, 6.8–3.3 m; VN core, 11.5–

3.0 m; NB core, 10.0–2.3 m; HV core, 9.5–2.3 m; GA

core, 9.0–2.8 m.

This facies consists of interbedded/laminated sand

and mud. The contact with the underlying Facies 3.3 is

gradational except in the VN core, where a sharp

boundary with well-sorted very fine sand bearing abun-

dant shell fragments is observed. Ripple cross-lamina-

tion and bivalve shells of Potamocorbula sp. from an

intertidal to shallow-marine habitat are common

throughout this facies. Paired mud drapes are recog-

nized in the ND-1 core, and bidirectional cross-lamina-

tion is found in the HV core. Wave ripple cross-

lamination is identified in the GA core. Bioturbation

is rare compared with Facies 3.3.

Interpretation: Sedimentary structures indicate that

this facies was deposited in a tide- and wave-influ-

enced environment. From the lithological succession

from Facies 3.3 and the present-day offshore profile,

this facies corresponds to a delta front platform

environment.

This facies can be divided into three types based on

the overall lithological succession: a coarsening-upward

type (Type A), a fining-upward type (Type B), and an

intermediate type (Type C).

Type A is characteristic in the VN core (Fig. 6C).

The lithology consists of alternating bed of well-sorted

sand and sandy clay, 5–65 and 30–90 cm thick, respec-

tively (Fig. 7M). The sand bed thickens and coarsens

upward, from very fine to medium sand. Parallel and

low-angle (b58) cross-lamination is observed in the

sand. The sandy clay contains abundant plant frag-

ments, causing it to have a dark color (Fig. 7M).

Interpretation: Well-sorted sand indicates removal

of fine-grained materials by waves under relatively

high-energy conditions. The upward-coarsening succes-

sion shows that the energy of the environment increased

upward. A similar lithology has been reported from the

delta front platform sediments of the wave- and tide-

influenced deltaic deposits in the Mekong delta (Ta et

al., 2002a,b; Tanabe et al., 2003a). Therefore, Type A is

interpreted as wave-influenced delta front platform

sediments.

Type B is characteristic in the ND-1, NB, and HV

cores (Fig. 6A, D, C). This lithology consists of alter-

nating beds of laminated clay and well-sorted sand, 1–

30 and 1–40 cm thick, respectively (Fig. 7N). The

contacts at the base of the sand layers are sharp. The

sand beds show a thinning- and fining-upward succes-

sion. Bidirectional ripple cross-lamination, current rip-

ple cross-laminations, and paired mud drapes are

common in the laminated clay.

Interpretation: The bidirectional ripple cross-lami-

nation, paired mud drapes, and lenticular bedding

indicate that this lithology was strongly affected by

tides. Furthermore, an overall fining-upward litholog-

ical succession is typical in regressive subtidal to

intertidal flat deposits of a tide-influenced delta

(Hori et al., 2001b, 2002a,b; Ta et al., 2002a,b;

Tanabe et al., 2003a; Dalrymple et al., 2003). The

coarsest portion of the deltaic succession corresponds

to the boundary zone between delta front and subtidal

flat sediments, which is correlative to the boundary

between the delta front platform and the delta front

slope of the Song Hong delta. Therefore, this lithology

is interpreted as tide-influenced delta front platform

sediments.

Type C is characteristic in the GA core (Fig. 6D).

This lithology consists of silty sand (Fig. 7O) sharply

overlain by red laminated mud. The silty sand is a

mixture of clay and very fine sand intercalated with

thin clay layers. The fine sand at the top is relatively

well-sorted, and wave-ripple lamination is visible in the

X-radiograph. The laminated mud consists of alternat-

ing layers of lenticular sand and silt, 5–20 mm thick,

and massive clay, 0.5–10 cm thick.

Interpretation: The poorly sorted sand might be

derived from dense hyperpycnal flow during flooding.

Based on the depth distribution, this lithology is com-

parable to that of a subaqueous bar lying about 10 km

seaward from the Song Hong river mouth (Tran, 1993).

The laminated mud was deposited under tidal influence

(Reineck and Singh, 1980). Therefore, it can be as-

sumed that the poorly sorted sand was overlain by

prograded mud-dominated sub-tidal flat sediments.

Compared with types A and B, Type C is more strongly

affected by fluvial influence. Therefore, this lithology

can be interpreted as fluvial-influenced delta front plat-

form sediments.

4.4.5. Facies 3.5 (tidal flat sediments)

Depth in core: CC core, 3.3–1.8 m; VN core, 3.0–

0.5 m; NB core, 2.3–1.5 m; HV core, 2.3–0.9 m; GA

core, 2.8–1.0 m.

This facies consists of thinly laminated dark red and

black silty clay with abundant plant fragments and

rootlets in the CC, NB, HV, and GA cores (Fig. 7P).

Table 3

Summary of AMS 14C dates from the sediment cores taken from the Song Hong delta

Depth in

core (m)

Altitude

(m)

Material Species d13C

(x)

Conventional 14C

age (yr BP)

Calibrated 14C age Sample code

(BETA-)1r range (cal. yr BP)

ND-1

3.22 �1.72 Wood �30.2 440F50 521–472 142396

4.82 �3.32 Molluscan shell Potamocorbula amurensis

(Schrenck)

�5.1 6040F30 6471–6395 142397

5.05 �3.55 Molluscan shell Potamocorbula amurensis

(Schrenck)

�8.4 4930F40 5295–5224 142398

6.73 �5.23 Molluscan shell Potamocorbula amurensis

(Schrenck)

�5.4 4970F40 5315–5266 142399

10.88 �9.38 Molluscan shell Solidicorbula erythrodon

(Lamarck)

�2.7 5780F40 6244–6159 142400

13.95 �12.45 Molluscan shell Mactra (Mactra)

veneriformis Reeve

�2.9 6060F40 6496–6401 142401

15.29 �13.79 Molluscan shell Varicorbula sp. cf.

V. rotalis (Hinds)

�1.3 6170F40 6639–6527 142402

16.66 �15.16 Shell fragments – �2.4 6420F40 6925–6797 142403

19.34 �17.84 Shell fragments – �1.0 6860F80 7424–7276 142404

20.45 �18.95 Molluscan shell Placamen tiara (Dillwyn) �0.9 7230F80 7742–7593 142405

22.4 �20.9 Molluscan shell Estellarca olivacea

(Reeve)

�5.8 8340F120 8945–8685 142406

23.35 �21.85 Molluscan shell Potamocorbula amurensis

(Schrenck)

�4.8 8530F80 9263–8909 142407

25.6 �24.1 Molluscan shell Corbicula sp. cf.

C. fluminea (Muller)

�8.6 9020F100 9824–9445 142408

27.75 �26.25 Molluscan shell Potamocorbula amurensis

(Schrenck)

�6.1 8560F70 9267–8929 142409

30.55 �29.05 Wood – �28.8 8360F70 9473–9280 142410

33.8 �32.3 Molluscan shell Corbicula sp. cf.

C. fluminea (Muller)

�10.9 9330F100 10275–9814 142411

37.15 �35.65 Molluscan shell Anisocorbula

venusta(Gould)

�11.8 9040F80 9827–9464 142412

40.2 �38.7 Wood – �26.7 9330F80 10670–10242 142413

43.4 �41.9 Wood – �30.2 9970F90 11627–11231 142414

56.35 �54.85 Wood – �27.0 12470F110 15413–14263 142417

67.05 �65.55 Wood – �28.3 12650F110 15526–14372 142418

DT

2.68 0.82 Molluscan shell Corbicula sp. �7.9 490F40 234–0 157762

3.25 0.25 Molluscan shell Corbicula sp. �6.6 540F40 263–132 157763

5.1 �1.6 Wood – �28.7 1710F40 1692–1549 159435

6.37 �2.87 Peaty organic – �28.7 8250F50 9398–9093 157764

11.43 �7.93 Shell fragments – �9.8 7260F60 7786–7664 157765

13.13 �9.63 Shell fragments – �9.6 7010F50 7562–7460 157766

14.52 �11.02 Molluscan shell Mactridae sp. �7.5 7300F50 7822–7700 157767

18.9 �15.4 Molluscan shell Mactridae sp. �10.4 6940F50 7498–7414 157768

20.37 �16.87 Molluscan shell Mactridae sp. �10.4 7450F50 7725–7634 157769

22.6 �19.1 Molluscan shell Oyster �3.9 7020F50 7566–7469 159436

30.7 �27.2 Wood – �28.0 8840F50 10147–9779 159438

36.61 �33.11 Wood – �29.7 8940F60 10191–9919 159439

39.16 �35.66 Wood – �29.4 9040F50 10234–10186 157773

41.17 �37.67 Wood – �29.3 9210F50 10479–10242 159440

CC

2.25 0.75 Wood – �28.8 5200F40 5988–5915 164861

5.13 �2.13 Wood – �28.3 6320F40 7269–7212 164862

11.63 �8.63 Molluscan shell Potamocorbula laevis �9.8 7660F40 8169–8056 164863

12.7 �9.7 Molluscan shell Corbicula fluminea �10.8 8170F40 8816–8588 164864

14.33 �11.33 Wood – �28.0 7240F40 8146–7976 164865

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6146

Table 3 (continued)

Depth in

core (m)

Altitude

(m)

Material Species d13C

(x)

Conventional 14C

age (yr BP)

Calibrated 14C age Sample code

(BETA-)1r range (cal. yr BP)

CC

17.6 �14.6 Wood – �28.9 7280F40 8163–8014 164866

23.9 �20.9 Wood – �27.4 8220F40 9394–9034 168813

25.73 �22.73 Wood – �27.8 8220F50 9395–9033 164867

27.05 �24.05 Wood – �30.2 8390F40 9473–9328 168814

27.96 �24.96 Wood – �30.7 8490F40 9530–9473 168815

VN

2.5 �1.9 Peaty organic – �29.3 1800F40 1816–1632 164811

4.3 �3.7 Shell fragments Potamocorbula laevis NA 2310F40 1984–1881 164812

7.3 �6.7 Bivalve Potamocorbula laevis �3.5 3690F40 3674–3563 164813

11.1 �10.5 Jointed bivalve Cycladicama cumingii �0.9 2590F40 2327–2273 164814

13.8 �13.2 Wood – �29.9 3440F40 3810–3637 164815

13.8 �13.2 Bivalve Potamocorbula laevis �1.6 3440F40 3372–3299 164816

16.0 �15.4 Wood – �30.3 3630F40 4055–3872 164817

17.7 �17.1 Jointed bivalve – �6.6 3990F40 4077–3951 164818

21.9 �21.3 Wood – �28.0 6790F40 7673–7588 164819

24.4 �23.8 Jointed bivalve Arcopsis interplicata +0.8 4420F40 4648–4521 164820

27.8 �27.2 Bivalve Mabellarca consociata +0.3 4960F40 5327–5280 164821

29.8 �29.2 Bivalve Scapharca kagoshimensis �2.1 8070F40 8592–8471 164822

30.1 �29.5 Wood – �29.0 8970F40 10207–9981 164823

30.1 �29.5 Gastropod – �1.5 8320F50 8924–8803 164824

34.7 �34.1 Bivalve Potamocorbula laevis �10.6 9210F40 10239–9720 164825

NB

3.5 �2.3 Wood – �29.6 970F40 931–795 164794

5.4 �4.2 Wood – �27.8 5750F40 6635–6488 164795

6.5 �5.3 Bivalve – NA 1690F40 1289–1231 164796

11.0 �9.8 Wood – �31.9 1270F40 1264–1172 164797

13.8 �12.6 Wood – �31.2 4990F50 5294–4991 164798

16.7 �15.5 Jointed bivalve Dosiniella sp. �0.4 1730F40 1319–1260 164799

22.4 �21.2 Bivalve – NA 2150F40 1813–1700 164800

24.8 �23.6 Bivalve – �7.4 2210F40 1872–1780 164801

27.6 �26.4 Jointed bivalve Arcopsis interplicata NA 2640F40 2349–2304 164802

30.4 �29.2 Jointed bivalve Scapharca troscheli �0.5 3320F40 3250–3136 164803

31.8 �30.6 Bivalve – �2.0 3860F40 3887–3779 164804

34.1 �32.9 Bivalve Arcopsis interplicata �0.5 5650F40 6155–5985 164805

34.7 �33.5 Bivalve – NA 5740F40 6214–6152 164806

38.6 �37.4 Jointed bivalve Potamocorbula laevis �5.2 9010F40 9822–9479 164807

41.7 �40.5 Jointed bivalve Corbicula fluminea NA 9910F40 11118–10385 164809

44.5 �43.3 Wood – �30.2 8980F40 10209–10153 164810

HV

4.4 �3.8 Bivalve – NA 1470F40 1068–967 164826

6.7 �6.1 Bivalve Potamocorbula laevis �5.6 1420F40 1031–929 164827

8.6 �8.0 Bivalve Potamocorbula laevis �4.6 1270F40 895–771 164828

13.0 �12.4 Bivalve – �4.0 1420F40 1031–929 164829

16.2 �15.6 Crab shell – NA 1680F40 1284–1223 164830

19.3 �18.7 Jointed bivalve Arcopsis interplicata �2.0 1800F40 1395–1301 164831

22.5 �21.9 Bivalve Yoldia sp. �2.6 2870F40 2720–2660 164832

23.5 �22.9 Wood – �28.9 5660F40 6487–6405 164833

25.4 �24.8 Wood – �28.7 3510F40 3833–3698 164834

27.8 �27.2 Bivalve Scapharca

kagoshimensis

�2.5 4410F40 4635–4514 164835

31.6 �31.0 Wood – �29.7 45210F700 – 164836

(continued on next page)

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 47

Table 3 (continued)

Depth in

core (m)

Altitude

(m)

Material Species d13C

(x)

Conventional 14C

age (yr BP)

Calibrated 14C age Sample code

(BETA-)1r range (cal. yr BP)

GA

2.4 �1.3 Gastropod Radix auricularia

swinhoei

�14.9 130F40 modern 164844

4.0 �2.9 Wood – �27.8 470F40 530–505 164845

8.5 �7.4 Peaty organic – �26.8 1480F40 1409–1311 164846

11.4 �10.3 Wood – �29.1 290F40 428–298 164847

18.6 �17.5 Shell fragments – �2.0 740F40 451–328 164848

19.8 �18.7 Bivalve Yoldia sp. �2.3 1030F40 649–567 164849

23.1 �22.0 Wood – �13.3 1250F40 1260–1146 164850

26.1 �25.0 Wood – �28.3 1800F40 1816–1632 164851

29.3 �28.2 Bivalve Yoldia sp. �0.2 1330F40 928–871 164852

31.0 �29.9 Wood – �28.3 1650F40 1591–1521 164853

33.5 �32.4 Echinoderm – �6.3 1960F50 1586–1479 164854

34.4 �33.3 Jointed bivalve Gafrarium dispar �0.6 2240F40 1903–1813 164855

36.2 �35.1 Bivalve Scapharca kagoshimensis �1.4 3420F40 3355–3259 164856

38.0 �36.9 Jointed bivalve Corbicula fluminea �13.7 9740F50 10617–10309 164857

NA, A d13 C is not measured because of the very small size of the sample. However, the conventional 14 C age has been corrected for all isotopic

fractionation effects. 14 C dates from the ND-1, DT, and CC cores are from Tanabe et al. (2003c), Tanabe et al. (2003b), and Hori et al. (2004),

respectively. A Pleistocene date at the bottom of the HV core is after Hanebuth et al. (2006).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6148

In the VN core, it consists of fine sand intercalated with

peaty layers 1–5 cm thick. Tubular burrows, 2 mm in

diameter, and faint silt lenses, 3–10 mm thick, occur in

the silty clay.

Interpretation: This facies is considered to be com-

posed of middle to upper intertidal flat sediments.

Abundant plant fragments and rootlets and thinly lam-

inated silty clay indicate vegetation in mangrove and

Fig. 8. Accumulation curves (age–depth plots) of the CC, DT, ND-1, VN, H

The sea-level curve is based on Fig. 4. W, radiocarbon dates obtained from

tide-influenced environments. The laminated sand in

the VN core correlates with sediments along the land-

ward margin of beach ridges, based on the present

geography.

4.4.6. Facies 3.6 (abandoned channel-fill sediments)

Depth in core: DT core, 6.4–2.3 m; ND-1 core,

3.3–0.9 m.

V, NB, and GA cores. Age uncertainties correspond to 1r estimates.

plant/wood fragments.

Fig. 9. Sequence stratigraphic interpretation of the post-LGM sediments of the longitudinal sections A–B and C–D. 14C dates and time lines are calibrated 14C ages (cal. kyr BP). SB, sequence

boundary; TS, transgressive surface, IFS, initial flooding surface; MxFS, maximum flooding surface; LST, lowstand systems tract; TST, transgressive systems tract; HST, highstand systems tract.

S.Tanabeet

al./Sedimentary

Geology187(2006)29–61

49

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6150

This facies shows an overall fining-upward succession

from well-sorted medium sand to reddish gray, mottled

silt (Fig. 7Q). Mud clasts (b15 mm in diameter) and

parallel lamination occur in the sand. Burrows and in

situ jointed Corbicula sp. are common in the grayish

red clay at the top of this facies.

Interpretation: A series of channel levees beside the

DT site and along the modern distributary indicate that

the DT core site is located on a filled, cut-off mean-

dering channel of the Song Hong distributary. Based on

an aerial photograph of the ND-1 site and its vicinity,

the ND-1 site is also located on an abandoned and

filled cut-off meandering channel of the Song Hong

distributary (Haruyama et al., 2000). The presence of

burrows and Corbicula sp. in life positions suggests

that the sediments were influenced by brackish water.

As tidal effects penetrate all of the major distributaries

almost as far inland as Hanoi during the dry season

(Mathers et al., 1996; Mathers and Zalasiewicz, 1999),

brackish water prevails in the modern distributary

channel near the core sites. Therefore, this facies is

interpreted as channel-fill sediments of a modern Song

Hong distributary.

4.4.7. Facies 3.7 (floodplain sediments and surface

soil)

Depth in core: CC core, 1.8–0 m; DT core, 2.3–0.0

m; ND-1 core, 0.9–0 m; VN core, 0.5–0.0 m; NB core,

1.5–0.0 m; HV core, 0.9–0.0 m; GA core, 1.0–0.0 m.

This facies consists of reddish brown mottled clay,

which corresponds to a lateritic weathering profile de-

veloped in floodplain and channel-levee sediments at

the land surface of the core sites (Fig. 7R). Abundant

rootlets with iron encrustation and gypsum crystalliza-

tion are common in this facies.

5. Radiocarbon dates

In all, 101 14C dates were obtained and calibrated.

All of the data are shown in Table 3. 14C dates obtained

from units 1, 2, and 3 fell within the latest Pleistocene

or Holocene. Individually, units 1, 2, and 3 date roughly

to 15–11, 11–9, and 9–0 cal. kyr BP, respectively.

Fig. 8 shows age–depth plots with accumulation

curves for the ND-1, DT, CC, VN, HV, NB, and GA

cores. The accumulation curves do not take into ac-

count sediment compaction effects. The accumulation

curves indicate that the calibrated results of most 14C

dates obtained from plant and wood fragments (organic

carbon) tend to be older than those obtained from

molluscan shells, the echinoderm, or the crab shell

(carbonate). Dates on molluscan shells and wood frag-

ments obtained from the same horizons in the VN core

indicate that wood fragments show ages about 400–

1300 years older than those obtained from shell frag-

ments. As the sediment cores contain no slump or slide

deposits, it is difficult to postulate a stratigraphic inver-

sion. Most organic carbon dates might be older because

the materials were reworked from older fluvial deposits

upstream from the core sites that included old detrital

carbon.

6. Sequence stratigraphy

The sequence stratigraphic interpretation is based on

the core lithology, facies interpretation, isochrons, and

stacking patterns of the deltaic and estuarine sediments

along with the sea-level curve for the western coast of

the South China Sea since the LGM (Figs. 4 and 5).

The facies distribution and sequence stratigraphic inter-

pretation along longitudinal sections AB and CD are

shown in Fig. 9.

6.1. Sequence boundary

The Sequence Boundary (SB) is observed as an

unconformity between latest Pleistocene–Holocene flu-

vial–estuarine–deltaic sediments (units 1, 2, and 3) and

the underlying late Pleistocene sediments (Unit 0).

Lateritic weathering of interfluve sediments at the top

of Unit 0 in the HV core indicates subaerial exposure

during the sea-level lowstand of the LGM. The thalweg

of the fluvial incision formed during the LGM is locat-

ed beneath the southern part of the present delta plain

(Fig. 3). Though the SB cannot be identified in the

cores located in this incised valley, its depth was more

than 70 m below the present sea level, based on the

ND-1 core data. As underlying marine successions

below the incised valley fills and Holocene deltaic

sediments are correlated with Marine Isotope Stage

(MIS) 3 during the last glacial (Hanebuth et al.,

2006), the SB must be formed during the falling sea

level into the LGM. Overlying sediments consisting of

fluvial, estuarine, and deltaic sediments compose a

single depositional sequence reflecting one regres-

sion–transgression–regression cycle since the LGM.

6.2. Transgressive surface

A transgressive surface (TS) is defined as a flooding

surface separating a progradational or aggradational

lowstand systems tract (LST) from a retrogradational

transgressive systems tract (TST) (Van Wagoner et al.,

1988). Allen and Posamentier (1993) placed a TS at the

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 51

boundary between the tidal–estuarine and fluvial sedi-

ments in the Gironde estuary, France, which they

regarded as an initial marine flooding surface. In con-

trast, Hori et al. (2002b) placed a TS around the bound-

ary between sandy fluvial sediments and basal fluvial

gravels in the paleo-Changjiang incised-valley fill, be-

cause the thick aggradational fluvial sediments of the

upper part of the sequence were dated at around 12 cal.

kyr BP, which corresponds to the middle part of the last

sea-level rise after the LGM. They showed that a thick

fluvial sequence was overlain by an estuarine sequence

later during the period of sea-level rise.

In the Song Hong delta area, only the ND-1 core,

which is in the main incised valley, records information

about the transition from the LST to the TST. As in the

case of the paleo-Changjiang, a rather thick (20 m)

fluvial sequence (Unit 1) in the ND-1 core consists of

two parts, a lower coarse-grained channel-fill and an

upper fine-grained floodplain deposit. The age of the

lower part is ~14.8 or 14.9 cal. kyr BP; these dates fall

within the sea-level lowstand of the LGM (Fig. 4). On

the other hand, the age of the upper part must be before

11–12 cal. kyr BP, because the overlying estuarine

sediments were dated to 11.4 cal. kyr BP. As there is

no erosional surface between these intervals, the fine-

grained floodplain sediments must have been deposited

between 14.8 and 11–12 cal. kyr BP. During this period,

a rapid rise in sea level of more than 30 m, known as

Melt Water Pulse (MWP) 1A (Fairbanks, 1989), oc-

curred (Fig. 4) and the paleoshoreline moved from the

outer shelf area of the Gulf of Bac Bo to the vicinity of

the ND-1 site. Therefore, the landward shoreline migra-

tion (transgression) and the deposition of fine-grained

floodplain sediments were concurrent. Thus, the TS

should be located near the facies boundary between

fluvial channel-fill and floodplain sediments, which

might reflect a change from an amalgamated braided

river system to an aggradational meandering river sys-

tem. The braided river system may contain sediments

deposited during the sea-level fall into the LGM (~21

cal. kyr BP) (Hanebuth et al., 2000) and those deposited

during the sea-level lowstand of the LGM (~21–15 cal.

kyr BP). However, as there is no age data from the lower

parts of this system, further dating will be necessary. A

rapid rise in sea level affects not only the coastal area but

also the fluvial plain with respect to sediment stacking

and abrupt facies changes. Accommodation space on a

fluvial plain created by a rise in sea level is effectively

filled by fluvial sediments just as the accommodation

space on a coastal plain is filled by fluvial-coastal sedi-

ments. On the other hand, a fluvial–estuarine contact at

about 40 m depth in the ND-1 core has been identified as

the initial flooding surface (Zaitlin et al., 1994; Hori et

al., 2002b).

6.3. Maximum flooding surface

The Maximum Flooding Surface (MxFS), which is

defined as the flooding surface that separates the retro-

gradational transgressive systems tract (TST) from the

progradational highstand systems tract (HST) (Van

Wagoner et al., 1988), was identified between estuarine

sediments and downlapping deltaic sediments in the

cores. Its age is 8.8–8.5 cal. kyr BP (ca. 9 cal. kyr

BP). Below the MxFS, sedimentary environments indi-

cate a deepening-upward succession. The environment

changed from a floodplain to a flood-tidal delta in the

ND-1 core, from a tide-influenced channel to a lagoon

(central basin) in the DT core, and from a sub- to

intertidal flat to an estuary front in the NB core. All

these environments changed suddenly to a shelf envi-

ronment above the MxFS. On the other hand, sedimen-

tary environments indicate a shallowing-upward

succession above the MxFS. Shelf to prodelta environ-

ments changed to delta plain in the ND-1, VN, HV, NB,

and GA cores. In the VN, HV, NB, and GA cores, the

MxFS was identified at the base of the shelly layers in

the shelf and prodelta deposits (Fig. 6C and D). The

shelly layers overlie estuarine sediments with sharp

erosional surfaces and gradually shallow-upward to

the delta front sediments. These erosional surfaces are

interpreted as ravinement surfaces formed during a

transgression. The ages of the shelly layers just above

the erosional surface show 8.5, 6.2, 4.6, and 3.3 cal. kyr

BP at each site, respectively, and a hiatus with a dura-

tion of 0.4 to 5.6 kyr. Its duration increases basinward,

where the period of sediment starvation was relatively

longer (Jarvey, 1988; Loutit et al., 1988; Cattaneo and

Steel, 2003). These data indicate that transgressive sand

sheets overlying a ravinement surface (e.g., Saito,

1994) are not obvious in this region and the observed

patterns might be rather interpreted as regressive sand

sheets overlying a marine erosion surface (Tanabe et al.,

2003a; Ta et al., 2005). The MxFS is situated within the

hiatus between the transgressive estuarine system and

regressive delta system at VN, HV, NB, and GA core

sites.

The MxFS and the maximum paleo-water depth in a

sequence should occur at the same time (Posamentier et

al., 1988). However, a time gap of about 1–2 kyr

between the MxFS and the maximum paleo-water

depth has been preliminarily reported from the Song

Hong delta based on data from three cores (Hori et al.,

2004). The seven cores discussed here clearly show this

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6152

gap between proximal and distal offshore sites. The

differences between the Holocene sea-level curve and

the accumulation curves reflect the changes in paleo-

Fig. 10. Paleogeographic map illustrating the evolution of the Song Hong d

newly collected data.

water depth at each core site, if we neglect sediment

compaction effects (Fig. 8). The maximum paleo-water

depth and its age at each site were ca. 10 m at 8.1 cal.

elta during the past 9 kyr. Modified after Tanabe et al. (2003b) using

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 53

kyr BP at the CC site, ca. 18 m at 7 cal. kyr BP at the

ND-1 site, and ca. 30 m at ca. 6–7 cal. kyr BP at the VN

site. The maximum paleo-water depth was recorded

approximately 2–3 kyr later than the time of the

MxFS at the VN site; the highest sea level occurred

in the middle Holocene, because accumulation rates

were lower than the rate of sea-level rise. However,

because the ND-1 site is close to the location of the

paleo-river mouth, where the accumulation rate was

relatively high, the gap is shorter than that recorded at

the VN site. Moreover, it is less than 1 kyr at the CC

site. Thus, the time gap between the MxFS and the

maximum paleo-water depth depends on both the rate

of sediment accumulation and that of sea-level rise. The

maximum paleo-water depth was reached after the

formation of the MxFS and before the time of the

highest sea level, and the timing of its occurrence at a

site depends on the accumulation rate.

The same phenomenon, a time gap ranging from 1.2

to 2.1 kyr, has been reported in the paleo-Changjiang

incised-valley fill (Hori et al., 2002b). Such a time gap

means that delta progradation began during the last

Fig. 11. Paleoshorelines at the Song Hong distributary mouths during

the last 5 kyr.

phase of sea-level rise (e.g., during 6–7 cal. kyr BP in

the Song Hong delta: Figs. 4 and 5), and that the ages of

the maximum paleo-water depth in different succes-

sions are not synchronous but depend on the location

and the relationship between rates of sediment accumu-

lation and sea-level rise. This differential accumulation

from the proximal to the distal part of a delta system

during the last phase of sea-level rise is responsible for

the delta front slope topography.

As the delta prograded during the rise of sea level

during 9–6 cal. kyr BP, its shoreline migrated seaward.

On the other hand, mangroves developed in marginal

areas surrounding the river mouth; the associated man-

grove clay is widespread and displays an onlapping-

stacking pattern. Marine inundation, as indicated by

mangrove vegetation, of the marginal parts of a delta

can also occur above the MxFS.

6.4. Lowstand, transgressive, and highstand systems

tracts

A lowstand systems tract (LST) usually consists of

the basal portion of fluvial sediments overlying the SB.

The coarse-grained fluvial sediments in the Song Hong

example are also regarded as the LST. The minimum

sea level was reached during the LGM in the South

China Sea at about 21 cal. kyr BP, and it remained

relatively stable in that low position until ca. 15 cal. kyr

BP (Hanebuth et al., 2000). MWP1A occurred at 14.7–

14.1 cal kyr BP (Fairbanks, 1989), coinciding with the

facies change from coarse-grained channel-fill (Facies

1.1) to floodplain sediments (Facies 1.2).

The transgressive systems tract (TST) consists

mainly of aggradational and retrogradational fluvial

and estuarine sediments in the Song Hong delta

area. Thick floodplain sediments in the incised valley

constitute the lower portion. Transgressive estuarine

sediments are widely distributed and were recognized

at six sites, excepting only the HV site. Lagoonal

facies are found at inner, landward parts. Because

the transgressive succession at ND-1 site is an up-

ward-coarsening succession from lagoon to flood tidal

delta sediments, the transgressive system is interpreted

as an estuarine system with an estuary-mouth sand

body. The age of the TST is approximately 9–15

cal. kyr BP (more precisely, between 8.5–8.8 and ca.

14.7–14.1 cal kyr BP).

The highstand systems tract (HST) consists of pro-

gradational deltaic sediments dated to 9–0 cal. kyr BP.

The HST corresponds to the last phase of sea-level rise

after the LGM (9–6 cal. kyr BP) and the subsequent

period characterized by a stable to falling sea level (6–0

Table 4

Sediment volumes and sediment discharges of the Song Hong during the past 9 kyr

Age (cal. kyr BP) Land area (km2) Depth (m) Volume (km3) Sediment discharge (million t/yr)

9–4 6400 10–15 64–96 17–25

4–2 1400 20–30 28–42 18–27

2–0 2500 30 75 49

Total 10300 – 167–213 24–31

Values were determined using a dry bulk density of 1.3 g/cm3 .

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6154

cal. kyr BP). The deltaic sediments prograded seaward

during the latter period.

7. Paleogeography

The paleogeography of the Song Hong delta in

relation to Holocene sea-level changes can be divided

into three stages on the basis of Tanabe et al.

(2003b), Hori et al. (2004), newly collected data

from the VN, HV, NB, and GA cores, and bifurcation

patterns of the beach ridges on the delta plain. They

are Stage I (9–6 cal. kyr BP), Stage II (6–4 cal. kyr

BP), and Stage III (4–0 cal. kyr BP). Fig. 10 illus-

trates the paleogeography and sedimentary environ-

ments characterizing Stages I, II, and III. This figure

is based on Tanabe et al. (2003b), data from the CC

core (Hori et al., 2004) and from the newly collected

cores.

Fig. 12. Comparison between the paleoclimatic proxies (A, B, C, D) for Sou

Song Hong during the past 15 kyr. (A) Ice-core record from Guliya, Tibet (Th

(Jian et al., 1996); (C and D) pollen records from lake sediments in south

respectively, (E) Song Hong sediment discharge during the past 9 kyr.

foraminifera. YD, Younger Dryas event; NG, Neoglaciation; PM, Pulleniat

As a result of the deceleration of sea-level rise at ca.

9 cal. kyr BP, a sand body such as a river-mouth bar

was present in the bay-head portion of the Song Hong

drowned valley (SHDV) during Stage I. It prograded

toward the Gulf of Bac Bo and filled the valley (Fig.

10A, B, C and D). The riverine sand started to accu-

mulate at 9 cal. kyr BP in the CC site (Hori et al.,

2004), and it prograded toward the Gulf and filled the

SHDV during 9–6 cal. kyr BP. On the other hand, VN,

HV, NB, and GA sites were still in shelf to prodelta

environments during this period. During Stage II, sea

level was stable at a height of +2–3 m above the present

sea level, and mangrove flats widely occupied the

present delta plain (Fig. 10D and E). At 4.9 cal. kyr

BP, a series of beach ridges, extending from Go Trung

to the vicinity of the ND-1 site developed. At 4.0 cal.

kyr BP, delta front clinoforms approached the VN site.

During Stage III, in response to rapid sea level lowering

theast Asia to South China and the paleo-sediment discharge (E) of the

ompson et al., 1998); (B) foraminifer record from the South China Sea

east China (Jarvis, 1993) and southeast Cambodia (Maxwell, 2001),

Pulleniatina obliquiloculata percentage is relative to all planktonic

ina minimum event (Jian et al., 1996; Wang, 1999).

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 55

to the present level, the mangrove flats emerged to form

the mid-Holocene marine terraces. During this period,

the VN, HV, NB, and GA sites became a beach-ridge

strandplain (Fig. 10E and F).

Fig. 11 illustrates the shoreline migration patterns of

the Song Hong distributary mouths during Stage III.

This figure is based on the depositional ages at �2 and

�15 m below the present sea level in the ND-1, VN,

HV, NB, and GA cores and the bifurcation patterns of

the beach ridges on the delta plain. The depositional

ages were calculated from the accumulation curves

(Fig. 8), and they are regarded as the depositional

ages of the lower limits of the tidal flat and the

delta front platform sediments, respectively. The bifur-

cation patterns of the beach ridges were identified

from SPOT satellite images. The pattern of change

of the paleoshoreline suggests that a relatively straight

shoreline became lobate at about 2 cal. kyr BP, and at

least three deltaic lobes (lobes A, B, and C) formed in

Fig. 13. Evolution of the Song Hong river mouth along the Song Hong distri

evolution of the river mouth. Delta front progradation rates during 9–6 and 6

below the present sea level in the CC and ND-1 cores and those in the ND-1

BP is an average rate of those from the HVand NB sites to the modern slope

mouth changed its morphology from (1) a tide-dominated funnel shape to (

interactive lobate shape as a result of its progradation basinward and incre

depositional systems is modified after Galloway (1975) and Wright (1985).

the mouths of the Song Hong distributaries during the

last 1 kyr. The bifurcation patterns of the beach ridges

suggest that lobes A and B are older than lobe C.

Shoreline progradation rates forming lobe A, B, and C

during the past 1 kyr are calculated to be N3.0, N0.75,

and N1.25 m/yr, respectively. The high progradation

rate of lobe A may be due to land reclamation and the

construction of sea dykes along the coast by the Day

River mouth (Haruyama and Vu, 2002; Pruszak et al.,

2002).

8. Variation of fluvial sediment discharge

The variation in Song Hong sediment discharge

during the last 9 kyr can be calculated based on the

shoreline positions on the paleogeographic maps (Figs.

10 and 11) and the thicknesses of sediments constitut-

ing the Song Hong delta plain and the subaqueous delta

in the Gulf of Bac Bo. Table 4 summarizes the annual

butaries during the past 9 kyr. (A) A schematic diagram illustrating the

–2 cal. kyr BP are calculated based on the depositional ages at �10 m

and HV cores, respectively. The progradation rate during 2–0 cal. kyr

break of the delta front (AB and CD transects in Fig. 9). (B) The river

2) a wave-influenced straight shape, to (3) a fluvial-, wave-, and tide-

ased sediment discharge. Triangular classification diagram of deltaic

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6156

sediment discharge of the Song Hong during 9–4, 4–2,

and 2–0 cal. kyr BP. It is assumed that the Song Hong

delta started to prograde at 9 cal. kyr BP, and the

seaward limit of the mid-Holocene marine terraces is

regarded as the paleoshoreline at 4 cal. kyr BP. The

thickness of the deltaic sediments trapped during 9–4

cal. kyr BP is regarded as 10 to 15 m on the basis of the

shape of the Song Hong incised valley and the thickness

of the deltaic sediments along the Song Hong (20 m) and

Thai Binh (5–30 m) rivers (Lam and Boyd, 2000, 2003).

The thickness of the deltaic sediments trapped between

the paleoshorelines from 4 to 2 cal. kyr BP is 20 to 30 m,

and that of the sediments between the paleoshorelines

from 2 to 0 cal. kyr BP is regarded as 30 m on the basis of

the thickness of the deltaic sediments and the present

offshore limit of the prodelta. The bulk density of the

deltaic sediments is estimated as 1.3 g/cm3.

The resulting calculated paleo-sediment discharges

show variations during the past 9 kyr. The sediment

discharge was 17–25 million t/yr during 9–4 cal. kyr

BP, 18–27 million t/yr during 4–2 cal. kyr BP, and 49

million t/yr during 2–0 cal. kyr BP. The sediment

discharge during 9–2 cal. kyr BP did not vary greatly,

but it increased drastically during the last 2 kyr. It was

still less than half of the sediment discharge measured

during the 1960s–1990s of 100–130 million t/yr, how-

ever (Milliman et al., 1995; Pruszak et al., 2002). This

means that sediment discharge of the Song Hong has

accelerated to the present level during the last 2 kyr.

The drastic increase in sediment discharge during the

last 2 kyr might be related to human activities such as

cultivation in the upper Song Hong catchment because

the sediment discharge increase is independent of cli-

matic records (Jarvis, 1993; Jian et al., 1996; Thompson

et al., 1998; Maxwell, 2001) collected from Southeast

and East Asia (Fig. 12). Neotectonics along the Red

River Fault System (RRFS) (Fig. 2) might have also

played a role in increasing the Song Hong sediment

discharge. However, it is difficult to consider that the

mass movements in the upper reaches of the Song

Hong, which were triggered by fault movements/earth-

quake, occurred frequently only in the last 2 kyr. Rath-

er, it is likely that the recurrence intervals of

earthquakes along the RRFS during the last millennia

(General Department of Land Administration, 1996)

were consistent with those throughout the Holocene.

On the other hand, people started to farm the moun-

tainous area where the annual precipitation rate is high-

est, up to 1600 mm, near the northwest border between

Vietnam and China in the Song Hong drainage at about

2 cal. kyr BP (Fig. 2) (Vietnam National Committee for

International Hydrological Program, 1994). At nearly

the same time, sites of occupation of the Bronze Age

Dong Son culture (3–2 cal. kyr BP) became widespread

on the present delta plain (Ogura, 1997), and it can be

inferred that people deforested the upper to middle parts

of the Song Hong catchment. Pollen data recorded in

the ND-1 core also show vegetation changes reflecting

deforestation by human activities during the last 3 kyr

(Li et al., 2006). The present extremely high sediment

discharge is thought to be the result of these human

activities, along with others, for example, dyke con-

struction, which began along the Song Hong in the

eleventh century (Ogura, 1997).

9. Morphodynamics of the Song Hong river mouth

Paleogeographic maps (Figs. 10 and 11) show that

most of the sediments discharged by the Song Hong

have accumulated along the Song Hong distributaries

within the SHDV during the past 9 kyr. The shoreline

prograded more than 100 km along the Song Hong

distributaries, but less than 50 km along the Thai Binh

distributaries. This means that the area along the Song

Hong distributaries has been the actively prograding

part and the area along the Thai Binh distributaries

was the marginal part of the delta during the past

9 kyr (Tanabe et al., 2003b). Fig. 13 summarizes the

morphodynamics and the acceleration/deceleration of

progradation rate at the distributary mouths of the

Song Hong. During the past 9 kyr, the Song Hong

river-mouth morphology changed from funnel-shaped

(9–6 cal. kyr BP), to straight (6–2 cal. kyr BP), to

lobate (2–0 cal. kyr BP), and the delta front prograda-

tion rate at 10 m below present sea level decelerated

from 22 to 4 m/yr at around 6 cal. kyr BP and then

accelerated from 4 to 24 m/yr at around 2 cal. kyr BP

(Fig. 13A).

The river-mouth morphological change, from fun-

nel-shaped to straight, and the deceleration of the pro-

gradation rate, which occurred at around 6 cal. kyr BP,

were mostly caused by a change in coastal oceano-

graphic conditions resulting from the river-mouth pro-

gradation. As the river mouth prograded basinward,

filling the SHDV, the tidal amplitude and the velocity

of tidal currents confined by the valley topography

became relatively weak, and the wave height and ve-

locity of longshore currents confined by the shoreline

topography became relatively strong along the coast.

The cessation of the sea-level rise at 6 cal. kyr BP may

have enhanced the progradation of the delta system

(Hori et al., 2004). However, the deceleration of delta

progradation rate is inconsistent with the deceleration

of sea-level rise. Furthermore, no major change oc-

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–61 57

curred in the Song Hong sediment discharge at around

6 cal. kyr BP (Fig. 12). Therefore, the morphological

change of the river mouth might have resulted from the

change of coastal topography and oceanographic con-

ditions from a narrow confined bay to a wider open bay.

A similar phenomenon affected the Mekong delta in

southern Vietnam at 3.0–2.5 cal. kyr BP (Ta et al.,

2002a; Tanabe et al., 2003a). Because of the delta

progradation toward an open coast from a sheltered

inner bay, the delta system was more open to wave

attack, and sediments were dispersed by wave-induced

longshore currents, resulting in the decrease in the

progradation rate from 30–40 to 11–20 m/yr (Tanabe

et al., 2003a). Relatively strong waves along the open

coast created beach ridges on the delta plain and well-

sorted, coarse sandy facies in the delta front sediments

(Tanabe et al., 2003a). The examples from the Song

Hong and the Mekong deltas indicate that a delta

system can evolve as a result of changes to the coastal

setting in relation to delta progradation into the open

ocean, resulting in its evolving into a more wave-dom-

inated delta from a tide-dominated delta (Fig. 13B). A

similar delta evolution from tide-dominated to wave-

influenced may have happened in other deltas that have

prograded from a narrow bay to an open, tide-influ-

enced coast, such as the Niger, Irrawaddy, and Orinoco

deltas, which are composed of series of beach ridges

and cheniers only along the present coastline (Allen,

1965; Rodolfo, 1975; Warne et al., 2002).

The morphological change from straight to lobate,

and the acceleration of the progradation rate that oc-

curred at around 2 cal. kyr BP are in large part due to an

increase in the Song Hong sediment discharge. Based

on the river-mouth morphology, the Song Hong distrib-

utary mouths became more protuberant and river-influ-

enced than they were before 2 cal. kyr BP (Fig. 13B).

As mentioned above, the Song Hong sediment dis-

charge during the last 2 kyr increased to nearly double

that during 9–2 cal. kyr BP as a result of anthropogenic

influences. The change of delta shape implies that for

the last 2 kyr the sediment volume supplied to the river

mouth through the river was much higher than the

amount of sediment reworked or redistributed by

waves and longshore currents. Several deltas changed

their morphology and progradation rates during the late

Holocene as a result of an increase in sediment dis-

charge and anthropogenic influences. In the case of the

Huanghe delta, the straight shoreline became lobate and

the rates of shoreline progradation became rapid at

about 1 cal. kyr BP because of a rapid increase in

sediment discharge induced by human activities (culti-

vation and vegetation change) on the Loess Plateau,

along the middle reaches of the Huanghe (Milliman et

al., 1987; Saito et al., 2001; Yi et al., 2003). In the Po

and Tevere catchments, human activities caused sub-

stantial changes in river dynamics, leading to the mor-

phological change of the deltas from arcuate to cuspate

(Bellotti et al., 1994; Cencini, 1998; Amorosi and Milli,

2001). In the case of the Krishna delta in India, the

shoreline progradation rate became rapid and the delta

morphology changed from straight to lobate as a result

of increased sediment discharge at 2 kyr BP (Rao et al.,

1990). Thus, changes in sediment discharge as well as

changes in coastal oceanographic settings have affected

delta morphology and progradation rates during the

Holocene.

10. Conclusions

(1) A detailed facies analysis and high-resolution

AMS 14C dating of seven sediment cores

obtained form the Song Hong delta plain revealed

that sediments at these sites can be classified into

four units: Unit 0 (late Pleistocene shallow-ma-

rine sediments), Unit 1 (fluvial sediments), Unit 2

(estuarine sediments), and Unit 3 (deltaic sedi-

ments), in ascending order. Units 1, 2, and 3,

dated to b15 cal. kyr BP and consist of two

(channel-fill and floodplain sediments), six

(tide-influenced channel-fill to coastal marsh, la-

goon, flood tidal delta, tidal flat and salt marsh,

sub- to intertidal flat, and estuary front sediments)

and seven (tide-influenced channel-fill, shelf to

prodelta, delta front slope, delta front platform,

tidal flat, abandoned channel-fill sediments, and

floodplain sediments and surface soil) sedimen-

tary facies, respectively.

(2) A sequence stratigraphic analysis was performed

on units 1, 2, and 3, which filled the Song Hong

incised valley since the last glacial maximum

(LGM) based on the stacking patterns of the

sediment facies and the lithology, isochrons, and

sea-level changes along the western coast of the

South China Sea. Units 1–3 can be divided into

lowstand (LST), transgressive (TST), and high-

stand (HST) systems tracts. The transgressive

surface (TS) between the LST and the TST lies

in the middle part of the fluvial sediments and

dates to ca. 15 cal. kyr BP. The maximum flood-

ing surface (MxFS) between the TST and HST is

at the top of the estuarine sediments and dates to

ca. 9 cal. kyr BP. The TST consists of aggrada-

tional and retrogradational estuarine sediments,

whereas the HST consists of progradational del-

S. Tanabe et al. / Sedimentary Geology 187 (2006) 29–6158

taic sediments. The LST, TST, and HST can be

dated to ca. ~15, 15–9, and 9–0 cal. kyr BP,

respectively.

(3) During the past 9 kyr, the majority of the sedi-

ment discharged by Song Hong accumulated and

enhanced river-mouth progradation along the

Song Hong distributaries within the Song Hong

drowned valley (SHDV). The river mouth pro-

graded from the bay head of the SHDV toward

the Gulf of Bac Bo, changing its morphology

from funnel-shaped (9–6 cal. kyr BP) to straight

(6–2 cal. kyr BP) to lobate (2–0 cal. kyr BP). The

morphological change from funnel-shaped to

straight was largely the result of filling of the

SHDV and a coastal hydrologic change from a

tide-dominated setting to a wave-influenced set-

ting. The cessation of sea-level rise at 6 cal. kyr

BP may have played only a small role in the

change of river-mouth morphology because the

river-mouth progradation rate decelerated from

22 to 4 m/yr instead of accelerating. The river-

mouth morphodynamic change from straight to

lobate was largely due to the increase of the Song

Hong sediment discharge from 17–27 (9–2 cal.

kyr BP) to 49 million t/yr (2–0 cal. kyr BP)

caused by anthropogenic deforestation along the

upper reaches of the Song Hong.

Acknowledgments

We thank members of the staff of the International

Cooperation Division of the Department of Geology and

Minerals of Vietnam (DGMV) for their kind support in

arranging all of the cooperative activities. Prof. C. Field-

ing and two anonymous reviewers are thanked for con-

structive reviews of the submitted manuscripts. This

research was conducted as part of a Geological Survey

of Japan (GSJ)-DGMV joint research program on Deltas

in Vietnam from 2001 to 2003, financially supported by

the Asian Delta Project of AIST and the Global Envi-

ronmental Research Fund of the Ministry of Environ-

ment of Japan. This paper is presented under the

UNESCO/International Union of Geological Sciences

(IUGS)-supported International Geoscience Programme

(IGCP) Project No. 475 bDeltas in the Monsoon Asia-

Pacific Region: DeltaMAPQ.

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