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Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- and wave-dominated delta
Sedimentary Geology 152 (2002) 313 – 325 www.elsevier.com/locate/sedgeo
Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- and wave-dominated delta Thi Kim Oanh Ta a,*, Van Lap Nguyen a, Masaaki Tateishi b, Iwao Kobayashi b, Yoshiki Saito c, Toshio Nakamura d a
Sub-Institute of Geography, Vietnam National Center for Natural Science and Technology, 1 Mac Dinh Chi Str., 1 Dist., Ho Chi Minh City, Viet Nam b Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan c MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan d Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan Accepted 17 December 2001
Abstract The Mekong River Delta, southern Vietnam, is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world. Three cores were taken from Bentre Province in the lower delta plain, with the objective to describe the sediment facies and to clarify the changes from tide-dominated to tide-wave-dominated delta of the Mekong River Delta during the Late Holocene. Three cores (BT1, BT2, and BT3) were obtained in 1997. Holocene sediments in BT1 and BT3, with thickness ranging from 10 to 20 m, were mainly composed of deltaic sediments unconformably overlying the Late Pleistocene sediments. The BT2 core, however, consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo-Mekong River incised valley, which is more than 70 m deep and was formed during the last glacial period. The deltaic sediment facies and succession in the BT2 and BT3 cores taken from the outer delta plain, with beach ridges, were different from those of the BT1 core taken from the inland delta plain. The BT2 and BT3 cores provide a good example of tide- and wave-dominated delta sediments and were characterized by a coarsening-upward delta front facies covered by a fining-upward subtidal to intertidal flat facies, followed by a coarsening-upward foreshore/dune facies. These changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Bentre Province; Holocene; Mekong River Delta; Delta facies; Progradation rate; Delta evolution
*
Corresponding author. Tel.: +84-8-822-0829; fax: +84-8-822-
4895. E-mail addresses: [email protected] (T.K.O. Ta), [email protected] (Y. Saito).
1. Introduction The Mekong River Delta is the largest delta in Southeast Asia in terms of drainage basin size, water
0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 2 ) 0 0 0 9 8 - 2
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discharge, and sediment discharge (Milliman and Syvitski, 1992). For the last 6 ka, during the Holocene sea-level highstand, the delta has prograded more than 250 km from Cambodia to the South China Sea (Nguyen et al., 2000). Although this huge deltaic system formed during the short period of only 6 ka, its formation was not by constant progradation. The detailed topography of the delta plain indicates two zonal parts of the delta (Nguyen et al., 2000). The inner part is characterized by river-dominated features, while a well-developed beach ridge system characterizes the outer part of the delta plain along the coast. Therefore, its morphology suggests that the delta system has changed to a more wave-influenced environment during its evolution. Recent studies discussed the sediment facies and evolution of mainly tide-dominated delta systems of large rivers (e.g., Ganges – Brahmaputra, Goodbred and Kuehl, 2000; Yangtze, Hori et al., 2001, 2002). The Mekong River Delta is also a tidedominated large river delta. However, the prograding delta system has gradually changed since the last 3 ka BP and is more influenced by waves, at present, than the other large deltas such as the Ganges – Brahmaputra, Amazon, and Yangtze deltas (Galloway, 1975; Orton and Reading, 1993). Therefore, the Mekong River Delta is a good example of delta that has evolved from a tide-dominated delta to an intermediate type tide- and wave-dominated delta during the Late Holocene. Moreover, sea level is currently falling from an earlier highstand during Middle Holocene. The Mekong Delta will show a modern example of this change. This paper presents the sediment facies of the Holocene Mekong River Delta and the changes in the delta system from a tide-dominated to a tide- and wave-dominated setting based on analyses of samples from three borehole cores with high-resolution 14C chronology taken in Bentre Province, Vietnam.
2. Study area The Mekong River has its source in the mountainous area of Tibet and flows down the Indochina Peninsula into the South China Sea (Fig. 1). It is 4350 km long, with a drainage area of 810,000 km2. It is the 10th largest river in the world in terms of water discharge (470 km3 year-1) and is rated ninth in
sediment discharge (160 million tons year-1) (Milliman and Ren, 1995). The river ends in a wide delta in Vietnam, one of the largest deltas in Asia. The delta is characterized by a wide, low-lying delta plain with an area of 62,520 –93,781 m2 (Orton and Reading, 1993; Nguyen et al., 2000) that is ranked third in area in the world (Coleman and Roberts, 1989). The river has two main channels in the delta plain area, the Bassac River and the Mekong River. The Mekong River channel, furthermore, is divided into four distributaries in Bentre Province. Morphological and surficial sediment analyses of the delta have been previously published (Morgan, 1966; Gagliano and McIntire, 1968; Tran, 1973, 1986; Nguyen, 1995; Nguyen et al., 2000). Nguyen et al. (2000) and Woodroffe (2000) reviewed the characteristics of the Mekong River Delta and outlined its Holocene morphological evolution, providing a detailed geomorphological map of the Mekong. However, due to the paucity of detailed descriptions and 14 C ages from subsurface sediments, the sedimentological and stratigraphical evolution of the Mekong River Delta system has not yet been clarified. Bentre Province consists of a broad coastal plain corresponding to the lower northeastern part of the delta plain, located among the active distributaries of the Mekong River system (Fig. 1). The Bentre coastal plain contains numerous beach ridges, marking former positions of the shoreline during the last 4 ka and indicating the efficacy of wave processes at reworking the prograding delta front sediments (Woodroffe, 2000). It has been identified as an active delta lobe and a site of interaction between river and marine processes. The river’s influence extends 60 km out to sea and the marine influence reaches 80 to 100 km inland via the river channels (Gagliano and McIntire, 1968). Under the influence of the semidiurnal tide of the South China Sea, salty water intrudes into the river systems of almost the entire Bentre area. The climate of this area is subtropical and dominated by the monsoon, which is characterized by a wet season lasting from June to October and a dry season during the remaining months. Fine silt is deposited in the nearshore zone during the wet season, and some of this silt is reworked up the channels during the dry season (Wolanski et al., 1998). The mean tidal range is 2.5 F 0.1 m (Gagliano and McIntire, 1968; Wolanski et al., 1996) and the maximum tidal range is 3 –4 m
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Fig. 1. Location of the borehole sites and index map of the Mekong River Delta. The cross-section XY is shown in Fig. 2. Geographical information is simplified after Nguyen et al. (2000). Open circles indicate the borehole sites.
(Wolanski et al., 1996; Nguyen et al., 2000). Dominant southwestward longshore currents are generated by the winter monsoon (Gagliano and McIntire, 1968). The coastal area of the Mekong River Delta is classified as tide-dominated (low) to mixed energy (tide) on the mean tidal range vs. mean wave height diagram of Davis and Hayes (1984). The delta is also classified in the tide-dominated field, close to the boundary with the wave-dominated field, in Galloway’s river-tide-wave ternary diagram (Galloway, 1975).
The Holocene sea-level curve in the study area shows a highstand of sea level at 2– 4 m above the present sea level of about 6 – 7 ka, and then sea level has fallen to the present level (Nguyen et al., 2000).
3. Materials and methods In 1997, three borehole cores were taken from the lower delta plain: BT1 (latitude 10j17V1UN, longitude 106j21V34UE, altitude + 3 m above the present mean
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sea level, depth 40 m); BT2 (latitude 10j08V18UN, longitude 106j28V7UE, altitude + 2 m, depth 71 m); BT3 (latitude 10j01V5UN, longitude 106j37V44UE, altitude + 2 m, depth 29 m) (Fig. 1). The core samples were split, described, and photographed. X-radiographs of slab samples were taken throughout all split cores. Sand and mud contents were measured every 20 cm throughout the core. Sand samples were 5 cm thick and mud samples were 2 cm thick. After organic materials were removed with 10% H2O2, sands were separated on a 63-Am sieve under pored water. After measuring the dry weight of sand portion, sand contents were calculated. Fossils and microfossils such as diatoms, foraminifers, and mollusca species were identified and counted under optical microscope. Fifteen 14C ages were measured on plant fragments and molluscan shells by accelerator mass spectrometry (AMS) at the Center for Chronological Research, Nagoya University, Japan, and at Beta Analytic. Calendar ages were calculated by the INCAL98 calibration curve (Stuiver et al., 1998). A detailed description of sediment facies; diatom, mollusca, and foraminifer assemblages; and 14 C ages of the BT2 core is reported by Ta et al. (2001).
4. Results 4.1. Sedimentary facies The sediments of the BT1, BT2, and BT3 cores have been divided into 11 sedimentary facies based on the combined characteristics of grain size, color, and sedimentary structure of the sediments and the contained mollusca, foraminifers, and diatoms (Fig. 2). The identified facies are undivided Late Pleistocene sandy silt, estuarine channel/tidal river sandy silt, muddy tidal flat/salt marsh, estuarine marine sand and sandy silt, transitional sandy silt, open bay mud, sandy lag, prodelta mud, delta front sandy silt, subtidal to intertidal flat sandy silt, and subaerial delta plain facies (marsh and beach ridge). Core logs are shown in Fig. 2. A detailed description of sediment facies and foraminifer and diatom assemblages in the BT2 core is reported by Ta et al. (2001). Here, we outline the sediment facies in the three cores and the differences between the cores.
4.1.1. Undivided Late Pleistocene sandy silt facies This facies is the lowest part of the section, approximately below 10 m (below present sea level) at BT1 and approximately 19.5 m at BT3; the facies was not recovered at the BT2 site. It is mainly composed of mottled, slightly oxidized, yellowish gray stiff silt, silty clay, sandy silt with subangular quartz pebbles 0.5 –1 cm in diameter, and laterite, suggesting a period of subaerial exposure. Diatoms and foraminifers are not found in this facies. This facies is unconformably overlain by the prodelta or delta front facies, with a sharp contact in lithology and color. The overlying sediments may have been deposited in a terrestrial to coastal marine environment. Radiocarbon dates of oyster shells from similar facies in borehole samples taken between the Bassac River and the Mekong River suggest an age of 43,420 years BP (Tanabe et al., in press). 4.1.2. Estuarine channel/tidal river sandy silt facies (Latest Pleistocene transgression) This facies is only presented in the lowest part of the BT2 core from 69.0 to 62.30 m. It consists of slightly oxidized gray silty sand and fine to medium sand-bearing scattered quartz pebbles in the lower part. Sedimentary structures such as parallel lamination and lenticular bedding are found. There is an intermixture of marine plankton and marine-brackish- and freshwater diatom species. Stephanodiscus astrea, Synedra spp., and Aulacoseira granulata, indicating a freshwater habitat, are common. Foraminifer species are few, but suggest a marine, in particular a tidal, influence. This facies is interpreted as an estuary channel/ tidal river. 4.1.3. Muddy tidal flat/salt marsh facies This facies presented only at the BT2 site from 62.30 to 54.50 m. It is composed of interbedded, stiff brownish gray silty clay. Faint lamination is found in the lower portion and discontinuous parallel lamination is found in the upper part. Plant fragments and calcareous concretions are common. Several diatom species, such as Coscinodiscus spp., Thalassiosira excentrica, Nitzschia sigma, Cyclotella styrolum, and C. caspia, indicating a marine-brackish-water habitat, are found in low abundance. This facies is interpreted as a muddy tidal flat/salt marsh sediment.
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Fig. 2. Core logs and inferred depositional facies with facies distribution and isochrones in cross-section XY. Location is shown in Fig. 1.
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4.1.4. Estuarine marine sand and sandy silt facies This facies is found from 54.50 to 35.95 m only at the BT2 site. The base of this facies is characterized by intercalated yellowish gray coarse sand, silty sand, round to angular quartz pebbles, and lenticular bedding, current-ripple lamination, and smallscale cross-bedding. The lower part shows a finingupward succession consisting of brownish gray sandy silt with parallel lamination and wavy bedding. The upper part displays a coarsening-upward succession and consists of intercalated dark gray sandy silt and fine sand with parallel lamination, cross-lamination (current ripples), wavy bedding, and small-scale crossbedding. These characteristics indicate that this facies is strongly influenced by tides. There is an intermixture of marine plankton and marine-brackishand freshwater diatom species. Synedra spp., freshwater species A. granulata and S. astrea, and marine planktonic species C. radiatus, C. nodulifer, and T. excentrica are common. This facies is characterized by a high content of small-sized foraminifers. Shallowmarine genera such as Ammonia spp., Asterorotalia spp., and Quinqueloculina spp. are abundant. The presence of marine bivalves and the abundance of shallowmarine microfossils indicate that the sediments were deposited in an estuarine environment and were strongly influenced by tides. The coarsening-upward succession might indicate an estuary-mouth sandy sediment.
diatom species increase upward and reach over 70%, with high abundance of C. radiatus, C. nodulifer, T. excentrica, and Thalassionema nitzschioides, indicating an open bay or outer bay environment (Nguyen et al., 1998, 1999; Ta and Nguyen, 2000; Ta et al.,
4.1.5. Transitional sandy silt facies This facies is found from 35.95 to 33.5 m at the BT2 site and consists of intercalated dark gray clayey silt and sandy silt. Wavy bedding is common in the lower part, which changes into parallel lamination in the upper part of the interval. Shell fragments and bioturbation are common. Marine planktonic diatom and foraminifer species significantly increase. This facies is interpreted as the transition from the estuary environments of the underlying facies to the open bay environments of the overlying facies. 4.1.6. Open-bay mud facies This facies is from 35.95 to 20 m at the BT2 site and mainly consists of homogenous greenish gray mud with some faintly parallel laminae. Mud content is over 95%. Shell fragments and incipient nodules are scattered throughout the facies. Marine planktonic
Fig. 3. Downcore variation of grain-size data of BT1, BT2, and BT3 cores.
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2001). Foraminifer species, characteristic of a deepwater environment, significantly increase upward. Marine and shallow marine mollusca are also found. 4.1.7. Sandy lag facies This facies is found only in the basal part of the Holocene succession at the BT3 site from 19.9 to 18.0 m, with thickness of 1.9 m. It consists of pebbly sand and marine shell fragments overlying Pleistocene sediments with erosional contact and overlain by prodelta facies. 4.1.8. Prodelta mud facies This facies occurred at the BT2 and BT3 sites and is about 3 to 4 m thick. It consists of dark gray silty clay to very fine sand in a coarsening-upward succession. The grain size of this facies in BT3 is coarser than that in BT2. This facies characterized by interbedded sand and mud, with discontinuous and parallel lamination, suggests tidal influence. Shell fragments and incipient nodules are common. Marine plankton species of diatom are abundant but decrease in comparison to the underlying embayment mud facies, T. nitzschioides obviously decreases individually. Brackish-water species, such as Paralia sulcata and C. caspia, measurably increase upward. This indicates a shallow marine habitat that has been supplemented
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by fresh and brackish-water sources. Total number of foraminifer species markedly decreases upward. This facies is interpreted as a prodelta deposit (Coleman and Wright, 1975; Coleman, 1981). 4.1.9. Delta front sandy silt facies Prodelta mud facies is overlain by delta front sandy silt facies with the gradational contact. This facies has a thickness of 7 to 10 m at all three sites and represents a common upward succession consisting of intercalated greenish gray silt, sandy silt, and fine sand. Cross-lamination (current ripples), lenticular bedding, wavy bedding, discontinuous parallel lamination, and contorted structures are common. These structures suggest that the deposit was formed under tidal influence. The facies may reflect strong hydrodynamic conditions caused mainly by tidal currents and high depositional rates. Small shell fragments and mica flakes are scattered throughout the facies. Diatoms, foraminifer species, and mollusca reflect an increasing influence of a freshwater habitat. This facies is interpreted as a delta front facies (Coleman and Wright, 1975). 4.1.10. Subtidal to intertidal flat sandy silt facies This facies is 6 to 8 m thick and consists of laminated dark gray sandy silt and fine sand, which is
Table 1 List of 14C ages from the BT2 and BT3 cores Cores
Altitude (m)
BT2
10.2 15.4 32.54 35.1 35.15 52.38 60.87 2.94 4.97 6.52 7.52 10.09 10.46 18.0 19.45
BT3
Materials
Shell Plant material Shell Shell Shell Shell Plant material Shell Shell Shell Shell Shell Wood Shell Shell
NUTA: 14C dating in Nagoya University. Beta: 14C dating in Beta Analytic.
d13C (x)
Conventional 14 C age (BP)
Calibrated age (cal BP) Intercept
Range
1.4 27.4 0.44 0.46 1.8 2.8 26.6 4.14 1.75 1.32 2.34 1.81 28.1 0.27 0.97
3660 F 80 4590 F 90 5210 F 90 5320 F 80 4970 F 40 7590 F 60 11,340 F 115 1300 F 90 1730 F 60 2130 F 70 1990 F 80 1620 F 90 3120 F 60 4170 F 90 4030 F 70
3562 5309 5578 5658 5300 8021 13,314/13,258/13,194 868 1278 1706 1535 1177 3356 4237 4059
3660 – 3459 5451 – 5055 5646 – 5474 5747 – 5590 5321 – 5278 8110 – 7962 13,456 – 13,152 928 – 735 1319 – 1237 1806 – 1616 1629 – 1459 1266 – 1067 3385 – 3267 4383 – 4105 4141 – 3950
Sample code number
NUTA-6529 NUTA-6551 NUTA-6530 NUTA-6540 Beta-149701 Beta-149702 NUTA-6552 NUTA-6543 NUTA-6534 NUTA-6545 NUTA-6542 NUTA-6536 NUTA-6546 NUTA-6537 NUTA-6539
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characterized by wavy bedding, parallel lamination, and lenticular bedding. Shell fragments and mica flakes are common. Marine planktonic diatom species are obviously less abundant in comparison with the underlying delta front deposits, but brackish-water diatom species are very abundant. This facies is characterized by scarcely and poorly preserved benthic foraminifers. Shallow-marine foraminifer species decrease upward (Ta et al., 2001). This sedimentary facies differs somewhat between core BT1 and cores BT2 and BT3 with respect to grain size and sedimentary structures. The grain size of sediments of BT2 and BT3 is coarser than that of BT1 sediments (Fig. 3). BT1 sediments display a fining-upward succession with parallel lamination and lenticular bedding, followed by an overlying subaerial delta plain facies. BT2 and BT3 sediments also show a fining-upward succession; however, they display a wide variation in
sedimentary structures, including wavy bedding, flaser bedding, and cross-lamination (current ripples) in addition to parallel lamination and lenticular bedding, and they are covered by coarse sediments of the subaerial delta plain facies. The features of this facies are tide-influenced and it is interpreted as a subtidal to intertidal flat facies (Reineck and Singh, 1980; Li et al., 2000; Hori et al., 2001). However, the facies in BT2 and BT3 was influenced not only by tides but also, partly, by waves as evidenced by the increase in sand content and the presence of current ripples in the sedimentary structures. 4.1.11. Subaerial delta plain facies (marsh and beach ridge) The thickness of this facies is 4 to 5 m. The sediments of BT1 consist of peaty dark silty clay and sandy silt with rich organic matter and mica flakes. The facies
Fig. 4. Age-depth curves at the BT2 and BT3 sites and the related sedimentary facies. The sea-level curve for the last 20 ka is illustrated based on Hanebuth et al. (2000) and Nguyen et al. (2000).
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is characterized by discontinuous parallel lamination and lenticular bedding. Brackish- and freshwater diatom species obviously increase, while marine plankton species decrease. C. caspia, C. styrolum, S. affinis, and S. astrea appreciably increase, while C. radiatus, C. nodulifer, T. excentrica, and Actinocyclus ehrenbergii decrease. This indicates a brackish-water habitat. Foraminifers are not found in the sediments. The sediment facies in the BT2 and BT3 cores consists of well-sorted fine yellowish brown and gray sand, with some rootlets and scattered shell fragments at the BT3 site. Fresh/ brackish-water diatom species are common. This facies probably represents an intertidal to supratidal flat/salt marshy deposit at BT1. The sediment facies at the BT2 and BT3 site is interpreted as sand dune/foreshore sand-forming beach ridges.
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in this area. The deltaic sediments in the three cores formed after the MFS. The lowest part of the Holocene succession at the BT3 site consists of a sandy and shelly lag facies. Although the sediments appear to be transgressive lag sediments on the basis of facies characteristics and succession, 14C ages date the facies to 4.0– 4.2 ka BP or after the MFS. This means that these sandy lag sediments were deposited on the downlap surface after the maximum flooding, and that such sandy lags can be formed not only by transgressive erosion but also by regressive marine erosion on the shelf, or by tidal currents or by delta lobe switching when waves rework the abandoned delta lobe. This indicates that it is not easy to identify whether an erosional surface is transgressive or regressive only from the presence of a lag sediment facies without dating.
4.2. Radiocarbon dates 5.2. Accumulation curve at borehole sites The results of 14C dating are shown in Table 1 and Fig. 4. The estuarine facies of the lower half of the BT2 core shows a period of sea-level rise from 13 to about 6 ka BP, after the last glacial maximum (LGM). Deltaic and bay facies were deposited during the last approximately 5 to 5.5 ka, after the Holocene sealevel highstand (Nguyen et al., 2000).
5. Discussion 5.1. Sea-level changes and sediment facies The BT2 core shows a thick succession for the last 13 ka after the LGM, while the sediments from the other two sites are only 10 to 20 m thick and represent the Holocene succession overlying weathered Pleistocene sediments. Therefore, the incised valley formed by the paleo-Mekong River during the LGM passes under the BT2 site, and the other two sites sit above terrace-like interfluves. The incised valley was filled by estuarine sediments during the rise of sea level after the LGM. Because the onset of Holocene sea-level highstand is reported to be about 5 to 6 ka BP in southern Vietnam (Nguyen et al., 2000; Woodroffe, 2000), the boundary between the estuarine facies and the bay facies almost coincides with the highest sea level of the Holocene. The occurrence of the maximum flooding surface (MFS) also dates to around this time
The accumulation rate at the borehole sites has changed together with the changes in sediment facies and sea level (Fig. 4). In particular, the incised-valley fill consisting mainly of estuarine and bay sediments during the rise of sea level to the early sea-level highstand indicates a high accumulation rate of more than 20 m ka 1. This was the result of effective sediment trapping in the confined area of the incised valley and the upward creation of accommodation space as sea level rose and the shelf was depressed during the highstand. In the deltaic system, accumulation rates seem to increase upward. 5.3. Tide- and wave-dominated delta succession There are some differences in sediment facies and successions between core BT1 and cores BT2 and BT3. The grain size variation shows that BT1 sediment consists of finer materials as compared with BT2 and BT3 sediments (Fig. 3). Moreover, BT1 succession consists of coarsening-upward delta front facies, followed by a fining-upward subtidal –intertidal facies succession, and its facies at BT1 indicates a typical tide-dominated delta succession (Hori et al., 2001, 2002) or a fining-upward tidal flat succession (Reineck and Singh, 1980). However, the sediments at BT2, which was taken from the beach ridge plain, display different features, with a coarsening-upward
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succession that includes coarser sediment grain sizes even in its upper part and wave-generated sedimentary structures (Fig. 5). The sediment at BT3 does not show such pattern because that core was taken from inter beach ridges. The classification of the present sedimentary environment of the Mekong River Delta as a tide and wave mixed coastal environment is supported by the topography and by the sediments in the BT2 and BT3 cores. The intertidal to subtidal zone and delta front facies show tide-influenced features, while the topography of the foreshore/dune sediments, which include some wave-generated sedimentary structures in the subtidal flat facies, indicates wave influences. The wave influence does not occupy the whole intertidal zone, but the middle to upper part of the intertidal zone, foreshore (beaches). Wave influences result in the differentiation of grain size variation of subaqueous to delta front sediments because finer particles are dispersed southward by longshore currents. Typical successions of these two types of deltas, tide-dominated and wave- and tidedominated, are summarized in Fig. 5. 5.4. Delta progradation The coastal progradation rate of the Mekong River Delta has not been continuous during the sea-level highstand of the last 6 ka. Ta et al. (2001) reported a
decrease in the progradation rate, from 17 to 18 m year 1 during 5.3 to 3.5 cal ka BP to 13 –14 m year 1 during the last 3.5 cal ka, and a change in coastal plain topography from fluvial/tide-dominated features to wave-dominated (beach ridge) features. The sediment facies are consistent with these changes, as discussed above (Fig. 5). Reduced progradation rates are reflected by southward sediment dispersal by longshore currents caused by monsoon-generated waves (Ta et al., 2001). The detailed isochrones in the cross-section (Fig. 2) indicate the changes in delta progradation for the last 5 ka. By using 14C dates and the present topography of the subaqueous delta, we can calculate the progradation rates between sites BT2 and BT3, and between site BT3 and the present subaqueous topography (Fig. 6). Estimated progradation rates are 8.5 m year 1 at 20 m altitude between BT3 and the present subaqueous topography (bathymetry), 10.5 and 10.1 m year 1 at 15 m altitude between sites BT2 and BT3 and between the BT3 site and the present subaqueous topography, respectively, and 12.4 and 14.4 m year 1 at 10 m altitude between sites BT2 and BT3 and between the BT3 site and the present subaqueous topography, respectively. These data indicate that the shallower part of the delta has a high rate of migration, resulting in the change to a steeper delta front topography with delta progradation during the last 5 ka. This topographic
Fig. 5. Schematic facies-succession models for a tide-dominated and a tide- and wave-dominated delta, inferred from the Mekong River Delta succession.
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Fig. 6. Estimated progradation rate from BT2 to BT3 site and from BT3 site to present subaqueous delta. Note that shallow part indicates a higher progradation rate, resulting in steeper delta front topography with delta evolution from a tide-dominated delta to a tide- and wavedominated delta.
change can be linked to changes in sediment facies, progradation rate, and increased wave influence. The reason for the abrupt change of the delta system is estimated to be the result of the outbuilding of the delta system beyond the critical lines to be sheltered by the headland, east of Ho Chi Minh City. Climate change in the Holocene does not coincide with this delta system change in timing. After the Holocene optimum and highstand at 2– 4 m above the present sea level (Woodroffe, 2000; Nguyen et al., 2000), cooling events occurred about 4 ka BP until about 2 ka BP in the South China Sea and East China Sea (Jian et al., 2000).
6. Conclusion Detailed analyses of borehole samples taken from three sites on the coastal plain of the Bentre Provine of the Mekong River Delta show the sedimentation in the incised valley during the last transgression and the Late Holocene delta evolution.
The Pleistocene interfluvial zones are unconformably covered by Holocene regressive deltaic sediments in BT1 and BT3 sites. The incised valley that formed during the last glacial maximum was located around the BT2 site. The fining-upward succession of the transgressive incised valley is overlain by the coarsening-upward succession of regressive deltaic facies. The Late Holocene evolution of the Mekong River Delta is the change of delta system from tide-dominated to mixed tide-wave-dominated delta presented by depositional process and geomorphology. In the inland delta plain, a coarsening-upward delta front facies is overlain by a fining-upward subtidal to intertidal flat succession. In the outer delta plain with beach ridges system, a coarsening-upward delta front facies is overlain also by a fining-upward subtidal to intertidal flat succession and coarsening-upward succession, including foreshore/dune sediments in the uppermost part. These sediment facies constitute a good example of tide- and wave-dominated delta features. Wave influences are reflected in the sediment
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facies and succession, the progradation rate, and subaqueous delta topography.
Acknowledgements The authors express their gratitude to Prof. K.T. Tran and Dr. H.P. Nguyen, University of Ho Chi Minh City, for helpful advice. We thank Mrs. T.L. Bui, University of Ho Chi Minh City, for the foraminifer analysis. Critical reviews by Drs. G. Postma and S. Nichol considerably improved the manuscript. We thank these reviewers and Prof. A.D. Miall for the kind suggestions for our manuscript. This work was part of a collaborative project between the Sub-Institute of Geography, Vietnam National Center for Natural Science and Technology, the Department of Geology, Niigata University, Japan, and the Geological Survey of Japan, AIST. This research was mainly funded by the Global Environment Research Fund of the Ministry of Environment of Japan.
References Coleman, J.M., 1981. Deltas: Processes of Deposition and Models for Exploration. Burgess Publishing, Minneapolis, 124 pp. Coleman, J.M., Roberts, H.H., 1989. Deltaic coastal wetlands. Geologie en Mijnbouw 68, 1 – 24. Coleman, J.M., Wright, L.D., 1975. Modern river deltas: variability of processes and sand bodies. In: Broussard, M.L. (Ed.), Deltas, Models for Exploration. Houston Geological Society, Houston, TX, pp. 99 – 149. Davis, R.A., Hayes, M.O., 1984. What is a wave-dominated coast? Marine Geology 60, 313 – 329. Gagliano, S.M., McIntire, W.G., 1968. Reports on the Mekong Delta. Coastal Studies Inst., Louisiana State Univ. Tech. Rept., 57, 144. Galloway, W.E., 1975. Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems. In: Broussard, M.L. (Ed.), Deltas, Models for Exploration. Houston Geological Society, Houston, TX, pp. 87 – 98. Goodbred Jr., S.L., Kuehl, S.A., 2000. The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges – Brahmaputra delta. Sedimentary Geology 133, 227 – 248. Hanebuth, T., Stattegger, K., Grootes, P.M., 2000. Rapid flooding of the Sunda Shelf: a Late Glacial sea-level record. Science 288, 1033 – 1035. Hori, K., Saito, Y., Zhao, Q., Cheng, C., Wang, P., Sato, Y., Li, C., 2001. Sedimentary facies and Holocene progradation rates of
the Changjiang (Yangtze) delta, China. Geomorphology 41, 233 – 248. Hori, K., Saito, Y., Zhao, H., Wang, P., 2002. Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China. Sedimentary Geology 146, 249 – 264. Jian, Z., Wang, P., Saito, Y., Wang, J., Pflaumann, U., Oba, T., Cheng, X., 2000. Holocene variability of the Kuroshio Current in the Okinawa Trough, northwestern Pacific Ocean. Earth and Planetary Science Letters 184, 305 – 319. Li, C., Wang, P., Fan, D., Dang, B., Li, T., 2000. Open coast intertidal deposits and their preservation potential: a case study from east-central China. Sedimentology 47, 1039 – 1051. Milliman, J.D., Ren, M.-E., 1995. River flux to the sea: impact of human intervention on river systems and adjacent coastal areas. In: Eisma, D. (Ed.), Climate Change: Impact on Coastal Habitation. Lewis Publ., Boca Raton, pp. 57 – 83. Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control of sediment discharge to the oceans: the importance of small mountain rivers. Journal of Geology 100, 525 – 544. Morgan, J.P., 1966. Mekong Delta: Distribution of Physical Environments by Aerial Photographs. Coastal Studies Inst., Baton Rouge, 11 pp. Nguyen, N.H., 1995. Geological and Mineral Resources Map of Vietnam; Nambo Plain Series 1/200,000. Geological Survey of Vietnam. Hanoi. Nguyen, V.L., Tateishi, M., Kobayashi, I., 1998. Reconstruction of sedimentary environments for Late Pleistocene to Holocene coastal deposits of Lake Kamo, Sado Island, Central Japan. Quaternary Research (Japan) 37, 77 – 94. Nguyen, V.L., Kamoi, Y., Kobayashi, I., 1999. Late Pleistocene and Holocene diatom flora of the Echigo Plain and Lake Kamo, central Japan. Proc. 14th Inter. Diatom Sym. Koeltz Sci. Books, Voeingstein, pp. 551 – 563. Nguyen, V.L., Ta, T.K.O., Tateishi, M., 2000. Late Holocene depositional environments and coastal evolution of the Mekong River Delta, Southern Vietnam. Journal of Asian Earth Science 18, 427 – 439. Orton, G.J., Reading, H.G., 1993. Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size. Sedimentology 40, 475 – 512. Reineck, H.E., Singh, I.B., 1980. Depositional Sedimentary Environments. Springer, New York, 549 pp. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., van der Plicht, J., Spurk, M., 1998. INCAL98 radiocarbon age calibration, 24,000 – 0 cal BP. Radiocarbon 40, 1041 – 1083. Ta, T.K.O., Nguyen, V.L., 2000. Diatom—indicator of sedimentary environments and sea-level changes in Late Pleistocene – Holocene. Journal of Science of the Earth 22, 226 – 233 (in Vietnamese with English abstract). Ta, T.K.O., Nguyen, V.L., Tateishi, M., Kobayashi, I., Saito, Y., 2001. Sedimentary facies, diatom and foraminifer assemblages in a Late Pleistocene – Holocene incised-valley sequence from the Mekong River Delta, Bentre Province, Southern Vietnam: the BT2 core. Journal of Asian Earth Science 20, 83 – 94. Tanabe, S., Ta, T.K.O., Nguyen, V.L., Tateishi, M., Kobayashi, I., Saito, Y., 2001. Delta evolution model inferred from the Me-
T.K.O. Ta et al. / Sedimentary Geology 152 (2002) 313–325 kong Delta, southern Vietnam. In: Posamentier, H.W., et al. (Eds.), Deltas of Southeast Asia and Vicinity—Sedimentology, Stratigraphy, and Petroleum Geology. SEPM Special Publication, in press. Tran, K.T., 1973. Geology Map of South Vietnam, scale 1/500,000. Saigon Univ. (unpublished, in Vietnamese). Tran, K.T., 1986. Sedimentary map of the Mekong Plain, 1/250,000. Project of Investigating and Developing the Mekong Plain. Ho Chi Minh City Univ., 30 pp. (unpublished in Vietnamese).
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Wolanski, E., Nguyen, N.H., Le, T.D., Nguyen, H.N., Nguyen, N.T., 1996. Fine sediment dynamics in the Mekong River Estuary, Vietnam. Estuarine, Coastal and Shelf Science 43, 565 – 582. Wolanski, E., Nguyen, N.H., Spagnol, S., 1998. Sediment dynamics during low flow conditions in the Mekong River Estuary, Vietnam. Journal of Coastal Research 14, 472 – 482. Woodroffe, C.D., 2000. Deltaic and estuarine environments and their Late Quaternary dynamics on the Sunda and Sahul shelves. Journal of Asian Earth Sciences 18, 393 – 413.
Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- and wave-dominated delta Thi Kim Oanh Ta a,*, Van Lap Nguyen a, Masaaki Tateishi b, Iwao Kobayashi b, Yoshiki Saito c, Toshio Nakamura d a
Sub-Institute of Geography, Vietnam National Center for Natural Science and Technology, 1 Mac Dinh Chi Str., 1 Dist., Ho Chi Minh City, Viet Nam b Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan c MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan d Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan Accepted 17 December 2001
Abstract The Mekong River Delta, southern Vietnam, is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world. Three cores were taken from Bentre Province in the lower delta plain, with the objective to describe the sediment facies and to clarify the changes from tide-dominated to tide-wave-dominated delta of the Mekong River Delta during the Late Holocene. Three cores (BT1, BT2, and BT3) were obtained in 1997. Holocene sediments in BT1 and BT3, with thickness ranging from 10 to 20 m, were mainly composed of deltaic sediments unconformably overlying the Late Pleistocene sediments. The BT2 core, however, consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo-Mekong River incised valley, which is more than 70 m deep and was formed during the last glacial period. The deltaic sediment facies and succession in the BT2 and BT3 cores taken from the outer delta plain, with beach ridges, were different from those of the BT1 core taken from the inland delta plain. The BT2 and BT3 cores provide a good example of tide- and wave-dominated delta sediments and were characterized by a coarsening-upward delta front facies covered by a fining-upward subtidal to intertidal flat facies, followed by a coarsening-upward foreshore/dune facies. These changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Bentre Province; Holocene; Mekong River Delta; Delta facies; Progradation rate; Delta evolution
*
Corresponding author. Tel.: +84-8-822-0829; fax: +84-8-822-
4895. E-mail addresses: [email protected] (T.K.O. Ta), [email protected] (Y. Saito).
1. Introduction The Mekong River Delta is the largest delta in Southeast Asia in terms of drainage basin size, water
0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 2 ) 0 0 0 9 8 - 2
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discharge, and sediment discharge (Milliman and Syvitski, 1992). For the last 6 ka, during the Holocene sea-level highstand, the delta has prograded more than 250 km from Cambodia to the South China Sea (Nguyen et al., 2000). Although this huge deltaic system formed during the short period of only 6 ka, its formation was not by constant progradation. The detailed topography of the delta plain indicates two zonal parts of the delta (Nguyen et al., 2000). The inner part is characterized by river-dominated features, while a well-developed beach ridge system characterizes the outer part of the delta plain along the coast. Therefore, its morphology suggests that the delta system has changed to a more wave-influenced environment during its evolution. Recent studies discussed the sediment facies and evolution of mainly tide-dominated delta systems of large rivers (e.g., Ganges – Brahmaputra, Goodbred and Kuehl, 2000; Yangtze, Hori et al., 2001, 2002). The Mekong River Delta is also a tidedominated large river delta. However, the prograding delta system has gradually changed since the last 3 ka BP and is more influenced by waves, at present, than the other large deltas such as the Ganges – Brahmaputra, Amazon, and Yangtze deltas (Galloway, 1975; Orton and Reading, 1993). Therefore, the Mekong River Delta is a good example of delta that has evolved from a tide-dominated delta to an intermediate type tide- and wave-dominated delta during the Late Holocene. Moreover, sea level is currently falling from an earlier highstand during Middle Holocene. The Mekong Delta will show a modern example of this change. This paper presents the sediment facies of the Holocene Mekong River Delta and the changes in the delta system from a tide-dominated to a tide- and wave-dominated setting based on analyses of samples from three borehole cores with high-resolution 14C chronology taken in Bentre Province, Vietnam.
2. Study area The Mekong River has its source in the mountainous area of Tibet and flows down the Indochina Peninsula into the South China Sea (Fig. 1). It is 4350 km long, with a drainage area of 810,000 km2. It is the 10th largest river in the world in terms of water discharge (470 km3 year-1) and is rated ninth in
sediment discharge (160 million tons year-1) (Milliman and Ren, 1995). The river ends in a wide delta in Vietnam, one of the largest deltas in Asia. The delta is characterized by a wide, low-lying delta plain with an area of 62,520 –93,781 m2 (Orton and Reading, 1993; Nguyen et al., 2000) that is ranked third in area in the world (Coleman and Roberts, 1989). The river has two main channels in the delta plain area, the Bassac River and the Mekong River. The Mekong River channel, furthermore, is divided into four distributaries in Bentre Province. Morphological and surficial sediment analyses of the delta have been previously published (Morgan, 1966; Gagliano and McIntire, 1968; Tran, 1973, 1986; Nguyen, 1995; Nguyen et al., 2000). Nguyen et al. (2000) and Woodroffe (2000) reviewed the characteristics of the Mekong River Delta and outlined its Holocene morphological evolution, providing a detailed geomorphological map of the Mekong. However, due to the paucity of detailed descriptions and 14 C ages from subsurface sediments, the sedimentological and stratigraphical evolution of the Mekong River Delta system has not yet been clarified. Bentre Province consists of a broad coastal plain corresponding to the lower northeastern part of the delta plain, located among the active distributaries of the Mekong River system (Fig. 1). The Bentre coastal plain contains numerous beach ridges, marking former positions of the shoreline during the last 4 ka and indicating the efficacy of wave processes at reworking the prograding delta front sediments (Woodroffe, 2000). It has been identified as an active delta lobe and a site of interaction between river and marine processes. The river’s influence extends 60 km out to sea and the marine influence reaches 80 to 100 km inland via the river channels (Gagliano and McIntire, 1968). Under the influence of the semidiurnal tide of the South China Sea, salty water intrudes into the river systems of almost the entire Bentre area. The climate of this area is subtropical and dominated by the monsoon, which is characterized by a wet season lasting from June to October and a dry season during the remaining months. Fine silt is deposited in the nearshore zone during the wet season, and some of this silt is reworked up the channels during the dry season (Wolanski et al., 1998). The mean tidal range is 2.5 F 0.1 m (Gagliano and McIntire, 1968; Wolanski et al., 1996) and the maximum tidal range is 3 –4 m
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Fig. 1. Location of the borehole sites and index map of the Mekong River Delta. The cross-section XY is shown in Fig. 2. Geographical information is simplified after Nguyen et al. (2000). Open circles indicate the borehole sites.
(Wolanski et al., 1996; Nguyen et al., 2000). Dominant southwestward longshore currents are generated by the winter monsoon (Gagliano and McIntire, 1968). The coastal area of the Mekong River Delta is classified as tide-dominated (low) to mixed energy (tide) on the mean tidal range vs. mean wave height diagram of Davis and Hayes (1984). The delta is also classified in the tide-dominated field, close to the boundary with the wave-dominated field, in Galloway’s river-tide-wave ternary diagram (Galloway, 1975).
The Holocene sea-level curve in the study area shows a highstand of sea level at 2– 4 m above the present sea level of about 6 – 7 ka, and then sea level has fallen to the present level (Nguyen et al., 2000).
3. Materials and methods In 1997, three borehole cores were taken from the lower delta plain: BT1 (latitude 10j17V1UN, longitude 106j21V34UE, altitude + 3 m above the present mean
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sea level, depth 40 m); BT2 (latitude 10j08V18UN, longitude 106j28V7UE, altitude + 2 m, depth 71 m); BT3 (latitude 10j01V5UN, longitude 106j37V44UE, altitude + 2 m, depth 29 m) (Fig. 1). The core samples were split, described, and photographed. X-radiographs of slab samples were taken throughout all split cores. Sand and mud contents were measured every 20 cm throughout the core. Sand samples were 5 cm thick and mud samples were 2 cm thick. After organic materials were removed with 10% H2O2, sands were separated on a 63-Am sieve under pored water. After measuring the dry weight of sand portion, sand contents were calculated. Fossils and microfossils such as diatoms, foraminifers, and mollusca species were identified and counted under optical microscope. Fifteen 14C ages were measured on plant fragments and molluscan shells by accelerator mass spectrometry (AMS) at the Center for Chronological Research, Nagoya University, Japan, and at Beta Analytic. Calendar ages were calculated by the INCAL98 calibration curve (Stuiver et al., 1998). A detailed description of sediment facies; diatom, mollusca, and foraminifer assemblages; and 14 C ages of the BT2 core is reported by Ta et al. (2001).
4. Results 4.1. Sedimentary facies The sediments of the BT1, BT2, and BT3 cores have been divided into 11 sedimentary facies based on the combined characteristics of grain size, color, and sedimentary structure of the sediments and the contained mollusca, foraminifers, and diatoms (Fig. 2). The identified facies are undivided Late Pleistocene sandy silt, estuarine channel/tidal river sandy silt, muddy tidal flat/salt marsh, estuarine marine sand and sandy silt, transitional sandy silt, open bay mud, sandy lag, prodelta mud, delta front sandy silt, subtidal to intertidal flat sandy silt, and subaerial delta plain facies (marsh and beach ridge). Core logs are shown in Fig. 2. A detailed description of sediment facies and foraminifer and diatom assemblages in the BT2 core is reported by Ta et al. (2001). Here, we outline the sediment facies in the three cores and the differences between the cores.
4.1.1. Undivided Late Pleistocene sandy silt facies This facies is the lowest part of the section, approximately below 10 m (below present sea level) at BT1 and approximately 19.5 m at BT3; the facies was not recovered at the BT2 site. It is mainly composed of mottled, slightly oxidized, yellowish gray stiff silt, silty clay, sandy silt with subangular quartz pebbles 0.5 –1 cm in diameter, and laterite, suggesting a period of subaerial exposure. Diatoms and foraminifers are not found in this facies. This facies is unconformably overlain by the prodelta or delta front facies, with a sharp contact in lithology and color. The overlying sediments may have been deposited in a terrestrial to coastal marine environment. Radiocarbon dates of oyster shells from similar facies in borehole samples taken between the Bassac River and the Mekong River suggest an age of 43,420 years BP (Tanabe et al., in press). 4.1.2. Estuarine channel/tidal river sandy silt facies (Latest Pleistocene transgression) This facies is only presented in the lowest part of the BT2 core from 69.0 to 62.30 m. It consists of slightly oxidized gray silty sand and fine to medium sand-bearing scattered quartz pebbles in the lower part. Sedimentary structures such as parallel lamination and lenticular bedding are found. There is an intermixture of marine plankton and marine-brackish- and freshwater diatom species. Stephanodiscus astrea, Synedra spp., and Aulacoseira granulata, indicating a freshwater habitat, are common. Foraminifer species are few, but suggest a marine, in particular a tidal, influence. This facies is interpreted as an estuary channel/ tidal river. 4.1.3. Muddy tidal flat/salt marsh facies This facies presented only at the BT2 site from 62.30 to 54.50 m. It is composed of interbedded, stiff brownish gray silty clay. Faint lamination is found in the lower portion and discontinuous parallel lamination is found in the upper part. Plant fragments and calcareous concretions are common. Several diatom species, such as Coscinodiscus spp., Thalassiosira excentrica, Nitzschia sigma, Cyclotella styrolum, and C. caspia, indicating a marine-brackish-water habitat, are found in low abundance. This facies is interpreted as a muddy tidal flat/salt marsh sediment.
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Fig. 2. Core logs and inferred depositional facies with facies distribution and isochrones in cross-section XY. Location is shown in Fig. 1.
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4.1.4. Estuarine marine sand and sandy silt facies This facies is found from 54.50 to 35.95 m only at the BT2 site. The base of this facies is characterized by intercalated yellowish gray coarse sand, silty sand, round to angular quartz pebbles, and lenticular bedding, current-ripple lamination, and smallscale cross-bedding. The lower part shows a finingupward succession consisting of brownish gray sandy silt with parallel lamination and wavy bedding. The upper part displays a coarsening-upward succession and consists of intercalated dark gray sandy silt and fine sand with parallel lamination, cross-lamination (current ripples), wavy bedding, and small-scale crossbedding. These characteristics indicate that this facies is strongly influenced by tides. There is an intermixture of marine plankton and marine-brackishand freshwater diatom species. Synedra spp., freshwater species A. granulata and S. astrea, and marine planktonic species C. radiatus, C. nodulifer, and T. excentrica are common. This facies is characterized by a high content of small-sized foraminifers. Shallowmarine genera such as Ammonia spp., Asterorotalia spp., and Quinqueloculina spp. are abundant. The presence of marine bivalves and the abundance of shallowmarine microfossils indicate that the sediments were deposited in an estuarine environment and were strongly influenced by tides. The coarsening-upward succession might indicate an estuary-mouth sandy sediment.
diatom species increase upward and reach over 70%, with high abundance of C. radiatus, C. nodulifer, T. excentrica, and Thalassionema nitzschioides, indicating an open bay or outer bay environment (Nguyen et al., 1998, 1999; Ta and Nguyen, 2000; Ta et al.,
4.1.5. Transitional sandy silt facies This facies is found from 35.95 to 33.5 m at the BT2 site and consists of intercalated dark gray clayey silt and sandy silt. Wavy bedding is common in the lower part, which changes into parallel lamination in the upper part of the interval. Shell fragments and bioturbation are common. Marine planktonic diatom and foraminifer species significantly increase. This facies is interpreted as the transition from the estuary environments of the underlying facies to the open bay environments of the overlying facies. 4.1.6. Open-bay mud facies This facies is from 35.95 to 20 m at the BT2 site and mainly consists of homogenous greenish gray mud with some faintly parallel laminae. Mud content is over 95%. Shell fragments and incipient nodules are scattered throughout the facies. Marine planktonic
Fig. 3. Downcore variation of grain-size data of BT1, BT2, and BT3 cores.
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2001). Foraminifer species, characteristic of a deepwater environment, significantly increase upward. Marine and shallow marine mollusca are also found. 4.1.7. Sandy lag facies This facies is found only in the basal part of the Holocene succession at the BT3 site from 19.9 to 18.0 m, with thickness of 1.9 m. It consists of pebbly sand and marine shell fragments overlying Pleistocene sediments with erosional contact and overlain by prodelta facies. 4.1.8. Prodelta mud facies This facies occurred at the BT2 and BT3 sites and is about 3 to 4 m thick. It consists of dark gray silty clay to very fine sand in a coarsening-upward succession. The grain size of this facies in BT3 is coarser than that in BT2. This facies characterized by interbedded sand and mud, with discontinuous and parallel lamination, suggests tidal influence. Shell fragments and incipient nodules are common. Marine plankton species of diatom are abundant but decrease in comparison to the underlying embayment mud facies, T. nitzschioides obviously decreases individually. Brackish-water species, such as Paralia sulcata and C. caspia, measurably increase upward. This indicates a shallow marine habitat that has been supplemented
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by fresh and brackish-water sources. Total number of foraminifer species markedly decreases upward. This facies is interpreted as a prodelta deposit (Coleman and Wright, 1975; Coleman, 1981). 4.1.9. Delta front sandy silt facies Prodelta mud facies is overlain by delta front sandy silt facies with the gradational contact. This facies has a thickness of 7 to 10 m at all three sites and represents a common upward succession consisting of intercalated greenish gray silt, sandy silt, and fine sand. Cross-lamination (current ripples), lenticular bedding, wavy bedding, discontinuous parallel lamination, and contorted structures are common. These structures suggest that the deposit was formed under tidal influence. The facies may reflect strong hydrodynamic conditions caused mainly by tidal currents and high depositional rates. Small shell fragments and mica flakes are scattered throughout the facies. Diatoms, foraminifer species, and mollusca reflect an increasing influence of a freshwater habitat. This facies is interpreted as a delta front facies (Coleman and Wright, 1975). 4.1.10. Subtidal to intertidal flat sandy silt facies This facies is 6 to 8 m thick and consists of laminated dark gray sandy silt and fine sand, which is
Table 1 List of 14C ages from the BT2 and BT3 cores Cores
Altitude (m)
BT2
10.2 15.4 32.54 35.1 35.15 52.38 60.87 2.94 4.97 6.52 7.52 10.09 10.46 18.0 19.45
BT3
Materials
Shell Plant material Shell Shell Shell Shell Plant material Shell Shell Shell Shell Shell Wood Shell Shell
NUTA: 14C dating in Nagoya University. Beta: 14C dating in Beta Analytic.
d13C (x)
Conventional 14 C age (BP)
Calibrated age (cal BP) Intercept
Range
1.4 27.4 0.44 0.46 1.8 2.8 26.6 4.14 1.75 1.32 2.34 1.81 28.1 0.27 0.97
3660 F 80 4590 F 90 5210 F 90 5320 F 80 4970 F 40 7590 F 60 11,340 F 115 1300 F 90 1730 F 60 2130 F 70 1990 F 80 1620 F 90 3120 F 60 4170 F 90 4030 F 70
3562 5309 5578 5658 5300 8021 13,314/13,258/13,194 868 1278 1706 1535 1177 3356 4237 4059
3660 – 3459 5451 – 5055 5646 – 5474 5747 – 5590 5321 – 5278 8110 – 7962 13,456 – 13,152 928 – 735 1319 – 1237 1806 – 1616 1629 – 1459 1266 – 1067 3385 – 3267 4383 – 4105 4141 – 3950
Sample code number
NUTA-6529 NUTA-6551 NUTA-6530 NUTA-6540 Beta-149701 Beta-149702 NUTA-6552 NUTA-6543 NUTA-6534 NUTA-6545 NUTA-6542 NUTA-6536 NUTA-6546 NUTA-6537 NUTA-6539
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characterized by wavy bedding, parallel lamination, and lenticular bedding. Shell fragments and mica flakes are common. Marine planktonic diatom species are obviously less abundant in comparison with the underlying delta front deposits, but brackish-water diatom species are very abundant. This facies is characterized by scarcely and poorly preserved benthic foraminifers. Shallow-marine foraminifer species decrease upward (Ta et al., 2001). This sedimentary facies differs somewhat between core BT1 and cores BT2 and BT3 with respect to grain size and sedimentary structures. The grain size of sediments of BT2 and BT3 is coarser than that of BT1 sediments (Fig. 3). BT1 sediments display a fining-upward succession with parallel lamination and lenticular bedding, followed by an overlying subaerial delta plain facies. BT2 and BT3 sediments also show a fining-upward succession; however, they display a wide variation in
sedimentary structures, including wavy bedding, flaser bedding, and cross-lamination (current ripples) in addition to parallel lamination and lenticular bedding, and they are covered by coarse sediments of the subaerial delta plain facies. The features of this facies are tide-influenced and it is interpreted as a subtidal to intertidal flat facies (Reineck and Singh, 1980; Li et al., 2000; Hori et al., 2001). However, the facies in BT2 and BT3 was influenced not only by tides but also, partly, by waves as evidenced by the increase in sand content and the presence of current ripples in the sedimentary structures. 4.1.11. Subaerial delta plain facies (marsh and beach ridge) The thickness of this facies is 4 to 5 m. The sediments of BT1 consist of peaty dark silty clay and sandy silt with rich organic matter and mica flakes. The facies
Fig. 4. Age-depth curves at the BT2 and BT3 sites and the related sedimentary facies. The sea-level curve for the last 20 ka is illustrated based on Hanebuth et al. (2000) and Nguyen et al. (2000).
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is characterized by discontinuous parallel lamination and lenticular bedding. Brackish- and freshwater diatom species obviously increase, while marine plankton species decrease. C. caspia, C. styrolum, S. affinis, and S. astrea appreciably increase, while C. radiatus, C. nodulifer, T. excentrica, and Actinocyclus ehrenbergii decrease. This indicates a brackish-water habitat. Foraminifers are not found in the sediments. The sediment facies in the BT2 and BT3 cores consists of well-sorted fine yellowish brown and gray sand, with some rootlets and scattered shell fragments at the BT3 site. Fresh/ brackish-water diatom species are common. This facies probably represents an intertidal to supratidal flat/salt marshy deposit at BT1. The sediment facies at the BT2 and BT3 site is interpreted as sand dune/foreshore sand-forming beach ridges.
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in this area. The deltaic sediments in the three cores formed after the MFS. The lowest part of the Holocene succession at the BT3 site consists of a sandy and shelly lag facies. Although the sediments appear to be transgressive lag sediments on the basis of facies characteristics and succession, 14C ages date the facies to 4.0– 4.2 ka BP or after the MFS. This means that these sandy lag sediments were deposited on the downlap surface after the maximum flooding, and that such sandy lags can be formed not only by transgressive erosion but also by regressive marine erosion on the shelf, or by tidal currents or by delta lobe switching when waves rework the abandoned delta lobe. This indicates that it is not easy to identify whether an erosional surface is transgressive or regressive only from the presence of a lag sediment facies without dating.
4.2. Radiocarbon dates 5.2. Accumulation curve at borehole sites The results of 14C dating are shown in Table 1 and Fig. 4. The estuarine facies of the lower half of the BT2 core shows a period of sea-level rise from 13 to about 6 ka BP, after the last glacial maximum (LGM). Deltaic and bay facies were deposited during the last approximately 5 to 5.5 ka, after the Holocene sealevel highstand (Nguyen et al., 2000).
5. Discussion 5.1. Sea-level changes and sediment facies The BT2 core shows a thick succession for the last 13 ka after the LGM, while the sediments from the other two sites are only 10 to 20 m thick and represent the Holocene succession overlying weathered Pleistocene sediments. Therefore, the incised valley formed by the paleo-Mekong River during the LGM passes under the BT2 site, and the other two sites sit above terrace-like interfluves. The incised valley was filled by estuarine sediments during the rise of sea level after the LGM. Because the onset of Holocene sea-level highstand is reported to be about 5 to 6 ka BP in southern Vietnam (Nguyen et al., 2000; Woodroffe, 2000), the boundary between the estuarine facies and the bay facies almost coincides with the highest sea level of the Holocene. The occurrence of the maximum flooding surface (MFS) also dates to around this time
The accumulation rate at the borehole sites has changed together with the changes in sediment facies and sea level (Fig. 4). In particular, the incised-valley fill consisting mainly of estuarine and bay sediments during the rise of sea level to the early sea-level highstand indicates a high accumulation rate of more than 20 m ka 1. This was the result of effective sediment trapping in the confined area of the incised valley and the upward creation of accommodation space as sea level rose and the shelf was depressed during the highstand. In the deltaic system, accumulation rates seem to increase upward. 5.3. Tide- and wave-dominated delta succession There are some differences in sediment facies and successions between core BT1 and cores BT2 and BT3. The grain size variation shows that BT1 sediment consists of finer materials as compared with BT2 and BT3 sediments (Fig. 3). Moreover, BT1 succession consists of coarsening-upward delta front facies, followed by a fining-upward subtidal –intertidal facies succession, and its facies at BT1 indicates a typical tide-dominated delta succession (Hori et al., 2001, 2002) or a fining-upward tidal flat succession (Reineck and Singh, 1980). However, the sediments at BT2, which was taken from the beach ridge plain, display different features, with a coarsening-upward
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succession that includes coarser sediment grain sizes even in its upper part and wave-generated sedimentary structures (Fig. 5). The sediment at BT3 does not show such pattern because that core was taken from inter beach ridges. The classification of the present sedimentary environment of the Mekong River Delta as a tide and wave mixed coastal environment is supported by the topography and by the sediments in the BT2 and BT3 cores. The intertidal to subtidal zone and delta front facies show tide-influenced features, while the topography of the foreshore/dune sediments, which include some wave-generated sedimentary structures in the subtidal flat facies, indicates wave influences. The wave influence does not occupy the whole intertidal zone, but the middle to upper part of the intertidal zone, foreshore (beaches). Wave influences result in the differentiation of grain size variation of subaqueous to delta front sediments because finer particles are dispersed southward by longshore currents. Typical successions of these two types of deltas, tide-dominated and wave- and tidedominated, are summarized in Fig. 5. 5.4. Delta progradation The coastal progradation rate of the Mekong River Delta has not been continuous during the sea-level highstand of the last 6 ka. Ta et al. (2001) reported a
decrease in the progradation rate, from 17 to 18 m year 1 during 5.3 to 3.5 cal ka BP to 13 –14 m year 1 during the last 3.5 cal ka, and a change in coastal plain topography from fluvial/tide-dominated features to wave-dominated (beach ridge) features. The sediment facies are consistent with these changes, as discussed above (Fig. 5). Reduced progradation rates are reflected by southward sediment dispersal by longshore currents caused by monsoon-generated waves (Ta et al., 2001). The detailed isochrones in the cross-section (Fig. 2) indicate the changes in delta progradation for the last 5 ka. By using 14C dates and the present topography of the subaqueous delta, we can calculate the progradation rates between sites BT2 and BT3, and between site BT3 and the present subaqueous topography (Fig. 6). Estimated progradation rates are 8.5 m year 1 at 20 m altitude between BT3 and the present subaqueous topography (bathymetry), 10.5 and 10.1 m year 1 at 15 m altitude between sites BT2 and BT3 and between the BT3 site and the present subaqueous topography, respectively, and 12.4 and 14.4 m year 1 at 10 m altitude between sites BT2 and BT3 and between the BT3 site and the present subaqueous topography, respectively. These data indicate that the shallower part of the delta has a high rate of migration, resulting in the change to a steeper delta front topography with delta progradation during the last 5 ka. This topographic
Fig. 5. Schematic facies-succession models for a tide-dominated and a tide- and wave-dominated delta, inferred from the Mekong River Delta succession.
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Fig. 6. Estimated progradation rate from BT2 to BT3 site and from BT3 site to present subaqueous delta. Note that shallow part indicates a higher progradation rate, resulting in steeper delta front topography with delta evolution from a tide-dominated delta to a tide- and wavedominated delta.
change can be linked to changes in sediment facies, progradation rate, and increased wave influence. The reason for the abrupt change of the delta system is estimated to be the result of the outbuilding of the delta system beyond the critical lines to be sheltered by the headland, east of Ho Chi Minh City. Climate change in the Holocene does not coincide with this delta system change in timing. After the Holocene optimum and highstand at 2– 4 m above the present sea level (Woodroffe, 2000; Nguyen et al., 2000), cooling events occurred about 4 ka BP until about 2 ka BP in the South China Sea and East China Sea (Jian et al., 2000).
6. Conclusion Detailed analyses of borehole samples taken from three sites on the coastal plain of the Bentre Provine of the Mekong River Delta show the sedimentation in the incised valley during the last transgression and the Late Holocene delta evolution.
The Pleistocene interfluvial zones are unconformably covered by Holocene regressive deltaic sediments in BT1 and BT3 sites. The incised valley that formed during the last glacial maximum was located around the BT2 site. The fining-upward succession of the transgressive incised valley is overlain by the coarsening-upward succession of regressive deltaic facies. The Late Holocene evolution of the Mekong River Delta is the change of delta system from tide-dominated to mixed tide-wave-dominated delta presented by depositional process and geomorphology. In the inland delta plain, a coarsening-upward delta front facies is overlain by a fining-upward subtidal to intertidal flat succession. In the outer delta plain with beach ridges system, a coarsening-upward delta front facies is overlain also by a fining-upward subtidal to intertidal flat succession and coarsening-upward succession, including foreshore/dune sediments in the uppermost part. These sediment facies constitute a good example of tide- and wave-dominated delta features. Wave influences are reflected in the sediment
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facies and succession, the progradation rate, and subaqueous delta topography.
Acknowledgements The authors express their gratitude to Prof. K.T. Tran and Dr. H.P. Nguyen, University of Ho Chi Minh City, for helpful advice. We thank Mrs. T.L. Bui, University of Ho Chi Minh City, for the foraminifer analysis. Critical reviews by Drs. G. Postma and S. Nichol considerably improved the manuscript. We thank these reviewers and Prof. A.D. Miall for the kind suggestions for our manuscript. This work was part of a collaborative project between the Sub-Institute of Geography, Vietnam National Center for Natural Science and Technology, the Department of Geology, Niigata University, Japan, and the Geological Survey of Japan, AIST. This research was mainly funded by the Global Environment Research Fund of the Ministry of Environment of Japan.
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