DSpace at VNU: Thermoluminescence ages for a reworked coastal barrier, southeastern Vietnam: a preliminary report

DSpace at VNU: Thermoluminescence ages for a reworked coastal barrier, southeastern Vietnam: a preliminary report tài li...

Journal of Asian Earth Sciences 20 (2002) 535±548 www.elsevier.com/locate/jseaes Thermoluminescence ages for a reworked coastal barrier, southeastern Vietnam: a preliminary report Colin V Murray-Wallace a,*, Brian G Jones a, Tran Nghi b, David M Price a, Vu Van Vinh c, Trinh Nguyen Tinh c, Gerald C Nanson a a School of Geosciences, University of Wollongong, Wollongong, NSW, 2522, Australia Department of Lithology and Marine Geology, Vietnam National University, Hanoi, Viet Nam c Quaternary Geomorphological Center, Mineral and Geological Survey of Vietnam, South Vietnam Geological Mapping Division, 14/5 Ky Dong St., Dist 3, Ho Chi Minh City, Viet Nam b Received October 2000; revised 26 February 2001; accepted 27 March 2001 Abstract Thermoluminescence dating of quartz sand (90±125 mm) from the coastal barrier successions between Phan Thiet and Tuy Phong, southeastern Vietnam, reveals that a substantial component was deposited during the last interglacial sensu lato (Oxygen Isotope Stage 5) between 108 and 85 ka The barrier successions have subsequently, in places, experienced multiple phases of aeolian reworking during the last glacial cycle, and in particular between 62 and 19 ka, possibly related to reduced vegetation cover and landscape instability caused by climatic change The dif®culties of applying the thermoluminescence (TL) method in areas of intense tropical weathering are also examined q 2002 Elsevier Science Ltd All rights reserved Keywords: Coastal barrier deposition; Thermoluminescence dating; Quaternary; Vietnam Introduction The southeastern coastline of Vietnam between Phan Thiet and Tuy Phong is dominated by extensive sandy coastal barrier successions of presumed Late Pleistocene and Holocene age (Nghi, 1996; Fig 1) Although these successions have been examined for their economic mineral potential (Nguyen et al., 1981), little has been done to establish their age and origin The general absence of organic materials within these successions has precluded the application of radiocarbon dating or other geochronological methods for determining their age At a regional scale two prominent groups of coastal barriers can be identi®ed along parts of the central and southern coastlines of Vietnam; outer barriers of unconsolidated white sand and inner barriers of predominantly red sands with, in places, cores of older white sand The former are represented by numerous bay head barriers, spits and tombolos (Eisma, 1985) with some active dune®elds Although the older, inner barrier sands may be unconsoli* Corresponding author Tel.: 161-2-4221-4419; fax: 161-2-4221-4250 E-mail address: colin_murray-wallace@uow.edu.au (C.V MurrayWallace) dated, they generally show signs of moderate to strong induration, are characteristically red and may contain, or be capped by duricrusts, suggestive of a diagenetic origin for the red colouration of the sands Similarly, the presence of clay cutans provides further evidence for the diagenetic modi®cation of the red sand units The inner and outer barrier successions merge over extensive parts of the coastline giving rise to composite features In addition to the above, an older marginal marine sand succession onlaps the Annamite Mountains, separated from the former barriers by an extensive back-barrier mud basin (Fig 1) The dichotomy of an outer barrier of white sand of presumed Holocene age and an older, more landward Pleistocene barrier of predominantly red sand does not apply to coastal areas of Vietnam subject to active erosion At Hon Da Chau (Fig 1), for example, the succession of diagenetically strongly modi®ed Pleistocene sand crops out along the modern shoreline The working hypothesis of this pilot project is that the outer barrier white sand was deposited during the Holocene following the culmination of the post-glacial marine transgression, that in this region, is generally considered to have occurred about ka ago (Fontaine and Delibrias, 1974; Pirazzoli, 1991; Nguyen et al., 2000), and that the inner 1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd All rights reserved PII: S 1367-912 0(01)00040-2 536 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 Fig Location map of thermoluminescence dating sample sites in the Binh Thuan Province of southeastern Vietnam between Phan Thiet and Tuy Phong barrier is associated with an earlier sea level high-stand, possibly the last interglacial maximum (Oxygen Isotope Substage 5.5) following the isotopic chronology of Bassinot et al (1994) In this work, a summary is presented of our reconnaisance ®eld investigations and thermoluminescence (TL) dating of coastal barrier sands in southeastern Vietnam This report represents part of a larger project examining the Late Quaternary coastal evolution of southeastern Vietnam and further ®eldwork and geochronological studies are planned Several representative sections through the coastal barrier successions at Hon Da Chau, Suoi Tien Village, Mui Ne Beach west of Hon Rom Mountain, Phu Lac near Tuy Phong, De Them II south of the River Luy and the disused military airport at Phan Thiet were investigated in this study (Fig 1) In addition, the Holocene white sand succession at Bau Gang was also examined Geographical setting The study area is situated in Binh Thuan Province between Phan Thiet and Tuy Phong (Fig 1) Extensive accumulations of barrier sand occur along this coastline Broad alluvial coastal plains cover extensive back-barrier lagoonal mud basins, now heavily cultivated, situated landward of the barriers and extending up to the Annamite Mountains The continental shelf of the South China Sea is shallow, and up to 14 km off the south Vietnamese coastline water depth does not exceed 20 m, giving rise to a sheltered, low wave-energy coastline Apart from a narrow band of detrital sediment parallel to the modern coastline, much of the Sahul shelf directly offshore from the study area is covered with a veneer of largely relict ®ne-grained sand, minimal carbonate and some ferruginous concretions and lateritic fragments (Emery and Niino, 1963; Astakhov et al., 1990) Tides are diurnal with a strong semidiurnal component during the period of equinoxes (Eisma, 1985) Methods 3.1 Sample collection Deeply dissected stream cuttings through the coastal barrier successions of southeastern Vietnam were examined to delineate lateral facies relationships Sections were measured using a laser theodolite Samples were collected for particle-size and petrographic analysis, and for TL dating Two methods were used in the collection of TL samples from homogeneous aeolian dune sediments Those from vertical sections were obtained by cleaning the face to expose the original stratigraphy, into which a steel sampling tube was inserted, extracted with the sample and wrapped in black plastic Where vertical sections were not present, samples were obtained by hand augering followed by transferring the samples to black plastic bags by inserting the auger barrel into the bag and then releasing the sediment thus avoiding exposure to sunlight In the case of some cohesive units, a block of sediment was cut from the C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 face and wrapped in black plastic, the inner unexposed part of the block being used for dating All samples were collected from depths exceeding m Corrections for the variation of cosmic radiation with burial depth were made in accordance with Prescott and Stephan (1982) Standard petrographic analysis was carried out to determine mineral composition, shape and grain coatings The quantity of Kfeldspar is of particular importance since it has the potential to in¯uence the radiation dose and accordingly, the TL ages 3.2 Thermoluminescence dating TL samples were analysed using the combined additive and regenerative method on the 90±125 mm quartz fraction, separated from the bulk sediment sample In view of the dif®culty of ®nding an appropriate modern analogue, corrections were not made for surface residual TL The TL starting point at the time of deposition is, therefore, assumed to be that level reached following a 24 h exposure under a laboratory ultraviolet lamp (Philips MLU300W) The combination of the regenerative and additive methods has the advantage of assessing the possibility of TL sensitivity changes due to laboratory procedures The method used here ®ts the mean natural TL output to the regenerated growth curve and then plots the additive data with respect to this value (Price et al., 1999) If there is no change in TL sensitivity the two data plots coincide A minimum of 28 sample aliquots, each containing around mg of quartz of the desired grain size, were utilised in the comparison of palaeodose values All ®rst glow TL output values were normalised using a second glow procedure, thus correcting for possible variations between sample aliquots Samples were heated at a rate of 58C s 21 in a high purity nitrogen atmosphere and TL output signals were detected using an EMI 9635QB photomultiplier ®tted with a Corning 7-57 blue transmission ®lter Sample aliquots were irradiated using a Littlemore Automatic Irradiator ®tted with a 90Sr plaque source Sample annual radiation dose (mGy/a) values were determined by thick source alpha counting using a ®nely crushed portion of the received sample, previously dried to determine moisture content Potassium concentrations were measured by atomic absorption spectroscopy and corrections were made for the cosmic radiation contributions and moisture content in the ®nal radiation dose computation A full account of the laboratory procedures adopted here are given by Shepherd and Price (1990), Nanson et al (1991) and Price et al (1999) Field sites 4.1 General remarks The coastal barrier successions of southeastern Vietnam, and in particular between Phan Thiet and Tuy Phong, represent particularly large sand accumulations The average 537 height of the barrier system over much of its length between Phan Thiet and Tuy Phong is about 100±140 m above present sea level (APSL), with the highest portion reaching 160 m APSL to the south of River Luy near De Them II At its widest near Hon Moc, the sands extend some 20 km inland (Fig 1) Stratigraphically, the barrier succession closest to the modern coastline appears to be a composite feature comprising an inner core of Pleistocene white and red sand and, in more restricted pockets, an outer unconsolidated barrier of white Holocene sand The landward limit of the barrier sands appears to be constrained by the occurrence of large inselbergs of Cretaceous rhyolite and tuff (e.g Nui Ta Dom, Nui Gieng Xo, Nui Re and Hon Moc), and ¯uvial activity as de®ned by the course of the Rivers Cai, Can and Luy (Fig 1) 4.2 Phan Thiet airport The coastal red sand succession on which the disused military airport at Phan Thiet was constructed has several steeply dissected gullies on the seaward side of the airstrip Gully erosion was accentuated due to increased surface water runoff since airport construction TL sample VN#45 was collected m below the ground surface from a vertical exposure of unconsolidated, dark red (2.5YR 4/8) mediumgrained quartz sand, near the head of a major cutting adjacent to the airstrip (10854 27 00 N, 108804 21 00 E; Fig 1) 4.3 Bau Gang An extensive sheet of unconsolidated white dune sand occurs at Bau Gang near Nui Ta Dom (11804 20 00 N, 108812 06 00 E; Fig 1) The dunes are at present undergoing erosion as attested by an extensive de¯ation lag of gravel, stone and pottery artefacts, and the presence of numerous isolated vegetated knolls of dune sand, the crests of which indicate a former dune surface The dune sediment comprises clean, medium-grained, pale yellow (2.5Y 8/2) quartz sand with no visible cross-bedding The unoxidized and unconsolidated nature of these sands suggests that they were more recently deposited than the indurated Pleistocene successions Although the origin of these sands is not clear, they may represent source-bordering dunes that formed adjacent to an extensive back-barrier lagoon, an area now under extensive cultivation, or represent a product of reworking of a more extensive dune sheet seaward of this locality TL sample VN#44 was collected m below the present dune surface, from the vertical face of a small borrow pit 4.4 Suoi Tien A thick section of sand is exposed within a deeply dissected stream cutting approximately km inland from the presently eroding shoreline of Suoi Tien village (10857 16 00 N, 108815 30 00 E; Fig 1) The section along the 538 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 Fig View looking northwest at the section ca 500 m upstream from Suoi Tien Village of the Late Pleistocene red sand and underlying white sand Location of thermoluminescence samples; (a) red sand; (b) white sand The underlying grey sand was sampled from 2.5 m above the channel ¯oor modern stream channel reveals a 70 m thick accumulation of sand (Figs and 3) that at stream level is light bluish grey (8B 8/1) medium- to ®ne-grained, strongly indurated, with little discernible primary bedding This unit is overlain by a medium- to ®ne-grained free ¯owing white quartz sand The white sand has a sharp basal contact with the underlying indurated sand The top of the white sand, however, has a gradational boundary at about m above the channel ¯oor, and small mottles and rootlets extend down into this unit from the overlying red sand The red sand comprises free ¯owing, medium-grained, reddish yellow quartz (5YR 6/8) The sand particles are rounded with frosted surfaces or surfaces coated with hematite The stream is actively eroding the indurated grey sand cropping out at the base of the channel resulting in the red cover sand avalanching into the modern channel (Fig 2) Samples for TL dating were collected from each of the three sedimentary units exposed in the Suoi Tien stream cutting (Fig 3) Sample VN#14, from the basal grey sand was collected 2.5 m above the channel ¯oor From the over- Fig Schematic section of the red and white sands at Suoi Tien showing the location of thermoluminescence samples Fig Composite measured section at Mui Ne Beach west of Hon Rom Mountain lying white sand TL sample VN#37 was collected m below the gradational boundary with the overlying red sand, the latter sampled from within a deeply dissected runnel approximately 15 m above the channel ¯oor (VN#15) 4.5 4.5 Mui Ne Beach near Hon Rom Mountain A composite section was constructed based on the measurement of two sections through a predominantly red sand succession at Mui Ne Beach, west of Hon Rom Mountain A very thick succession of red sand exposed within a steeply dissected amphitheatre-like cutting km west of Hon Rom Mountain (10857 17 00 N, 108818 19 00 E; Fig 1) and a cyclic succession, in ascending order, of red, yellow, white, yellow and red sand, exposed within a small roadside runnel approximately km south of the amphitheatre at Mui Ne Beach, provide the basis for the composite section (Fig 4) The base of the exposed composite section is represented by a lower red sand which comprises a 5.5 m thick (minimum thickness as the lower bounding surface is not exposed) coarse-grained, yellowish red (5YR 5/8) quartz sand The base of the exposure is 9.5 m APSL A poorly developed ferricrete occurs in the upper portion of the unit The lower red sand is overlain by a thin unit (0.6 m) of medium- to coarse-grained, yellow (10YR 7/6) quartz C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 539 lying intermediate red sand unit Sample VN#32 was collected from near the top of the intermediate red sand and about m below the prominent truncation surface which separates these two units Over a large area of the amphitheatre exposure, the truncation surface which separates the intermediate and upper red sand units has been exhumed due to erosion of the upper red sand In places this gives rise to a well-de®ned seaward dipping bench, the presence of which appears to be related to a weakly developed ferricrete immediately beneath the truncation surface 4.6 De Them II Fig Photograph of the measured section at Hon Da Chau The section was measured from the top of the residual located slightly to the left of the centre of the photograph down to high tide level See Fig for further stratigraphical details as well as the location of the thermoluminescence samples sand The lower bounding surface has a mottled contact and the upper bounding surface is represented by a sharp contact with the overlying white sand, and a thin Fe-rich layer separates the two units The overlying white sand unit is 2.6 m thick and is in turn overlain by a 2.7 m thick unit of yellow sand An intermediate red sand unit some 26 m thick overlies the yellow sand and contains tectites in its basal portion The intermediate red sand unit comprises planar crossbedded, moderately well-indurated, medium- to ®negrained, yellowish red (5YR 5/8) quartz sand and is overlain by 41.5 m of reddish yellow (5YR 6/8) free ¯owing, medium- to coarse-grained quartz sand (upper red sand; Fig 4) Samples for TL dating were collected from the intermediate and upper red sand units exposed within the amphitheatre Sample VN#31, from the upper red sand, was collected 58.8 m APSL and 12.8 m above the contact with the under- An extensive accumulation of coastal barrier sand occurs to the south of the River Luy near De Them II (11811 30 00 N, 108828 29 00 E; Fig 1) The prominent barrier structure stands at least 140 m APSL and is likely to be younger than the extensive, indurated ¯uvial sand and gravel succession which crops out immediately adjacent to the River Luy The aeolian succession comprises yellowish red (5YR 5/8) medium- to coarse-grained quartz sand The sediment is indistinctly mottled and is free ¯owing under a case hardened surface Samples for TL dating were collected from the upper portion of the barrier structure Sample VN#29 is from a cutting within a dune and was collected below the ground surface in the lee of the barrier Sample VN#30 was collected towards the crest of the barrier and approximately 20 m above sample VN#29 4.7 Hon Da Chau A strongly dissected coastal section at Hon Da Chau (11811 00 00 N, 108837 12 00 E; Fig 1) on the eastern side of a prominent granite headland, reveals a well-developed duricrust overlying a 19 m thick succession of red and white sand (Figs and 6) The coastal cliffs are cut through Fig Shore-normal transect of the measured section through the Pleistocene sands at Hon Da Chau a Tr 2.5 0.5 0.5 1 5.5 1.5 1.5 0.5 1.5 Tr Tr Tr 72 63 59 86.5 96 73.5 81 69.5 66 86 70 100 Feldspar (%) 77 96 89.5 Quartz (%) Tr 1.5 Tr 1.5 1.5 0.5 0.5 0.5 0.5 1.5 Tr 0.5 0.5 Rock frag./chert (%) Tr ˆ Trace; A ˆ angular; SA ˆ sub-angular; SR ˆ sub-rounded; R ˆ rounded Suoi Tien VN#15 VN#37 VN#14 Hon Da Chau VN#18 VN#20 VN#22 VN#24 Mui Ne Beach west of Hom Rom Mountain VN#31 VN#32 Phu Lac VN#12 VN#12B De Them II south of River Luy VN#30 VN#29 Phan Thiet Airport VN#45 Bau Gang VN#44 Sample/location ± 28.5 34 9.5 17 28.5 24.5 27 33 38 6.5 23 9.5 FeOx/clay (%) 0.228 0.221 0.238 0.243 0.254 0.221 0.353 0.245 0.243 0.243 0.219 0.246 0.251 0.236 0.416 well mod±well mod±well mod±well well mod±well mod mod mod±well mod±well mod±well mod mod±well mod±well mod 0.221 0.179 0.225 0.227 0.248 0.185 0.342 0.205 0.193 0.147 0.121 0.156 0.229 0.227 0.393 mod±well poor mod mod mod±well poor mod poor poor poor poor poor mod mod poor Sorting Mean (mm) Mean (mm) Sorting With authigenic matrix Without authigenic matrix Table Petrological and particle size attributes of coastal barrier sediments, southeastern Vietnam between Phan Thiet and Tuy Phong a SA±SR SA±SR SA±SR SA SA±SR SA±R SA±SR SA±R SA±SR SA±R SA±SR SA±SR SR±R A±SR SA±SR Rounding 540 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 moderately to strongly indurated sand and a series of prominent pillars with rillen occur throughout a small amphitheatre, attesting to the indurated nature of these sediments The basal unit at modern beach level comprises some 6.7 m of very strongly indurated, medium- to ®ne-grained white (1 8/N) quartz sand The base is not exposed Towards the top are faint traces of bedding parallel with the overlying unit The white sand is overlain by a 2.4 m thick unit of massive, yellow sand which is, in turn, overlain by a 0.9 m unit of well-laminated yellow sand The yellow sand is overlain by a 5.7 m succession of red sand capped by a prominent duricrust The red sand is here mapped as two sub-units, de®ned as a lower and upper red sand separated by a disconformity The lower unit is 2.4 m thick and comprises very strongly indurated, yellowish red (5YR 5/6±5YR 4/6) ®ne- to medium-grained quartz sand, slightly mottled towards the top and with iron preferentially concentrated along fractures Separated by an irregular disconformity, the upper unit comprises 3.3 m of mediumto coarse-grained, homogeneous, yellowish red (5YR 5/6) quartz sand with bedding only discernible in weathered exposures The sediment is moderately indurated and jointed, but without mottles The upper red sand is overlain by a well-developed duricrust some 2.6 m thick consisting of moderate to strongly indurated pisolitic ferricrete Strong brown (7.5YR 5/6) subrounded nodules occur in a white to light grey (1 8/N± 7/N) matrix The section in places is capped by a modern cover sand of strongly bioturbated, medium- to ®ne-grained, reddish yellow (5YR 6/8) quartz sand Samples for TL analysis were collected from the upper red sand (VN#18), the top of the lower red sand (VN#20), the well-laminated yellow sand unit (VN#22) and from the top of the white sand (VN#24; Table 1; Fig 6) 4.8 Phu Lac Further samples of red sand for TL dating were obtained from a small road-side borrow pit adjacent to highway at Phu Lac village, near Tuy Phong (11813 N, 108842 E; Fig 1) The site is located within an elongate, coast-parallel sand sheet TL sample VN#12 was collected 2.5 m below the ground surface from the wall of the borrow pit, within light to yellowish red to reddish brown (5YR 5/6±5YR 4/ 4) medium-grained sand The sediment reveals faint traces of trough cross-bedding and evidence of weak bioturbation by plant roots in the upper-most part of the section The quartz grains are coated with pellicles of iron oxide and approximately 1% of the sediment comprises lithic fragments A tan coloured soil, 10±20 cm thick, has developed on the dune facies A further sample was collected from an auger hole taken within the ¯oor of the borrow pit TL sample VN#12B was collected m below the dune surface The very slightly indurated sediment comprises reddish brown (5YR 4/4) medium-grained quartz sand 541 Results 5.1 Petrology All sediment samples analysed in this study have a petrologically similar grain composition dominated by monocrystalline quartz (Table 1) Mono- and polycrystalline quartz grains are generally more common than feldspar or lithic grains and hence the majority of samples can be classi®ed as quartzarenite Feldspar (exclusively orthoclase) forms up to 5.5% of the detrital grains and is generally partly to extensively altered and replaced by secondary clay minerals Rock fragments also form a very minor component (3%) and are dominated by siltstone fragments although chert, granite, basalt, metaquartzite and other metamorphic rock fragments are also present Most nonquartzose lithic grains are partly altered and weathered In many of the measured sections the stratigraphically lower samples tend to have higher orthoclase feldspar contents than the upper samples, possibly a result of diagenetic alteration and dissolution by ¯uctuating groundwaters in the near surface environment or a function of reworking of the upper sands, although in the Hon Da Chau area it is a function of the adjacent granitoid provenance Accessory minerals are commonly present in trace amounts and include opaque grains, zircon, tourmaline, garnet, monazite and ferromagnesian minerals (epidote?) Virtually no detrital matrix was recorded in these dune samples Secondary minerals include rare micritic carbonate in two samples from the Hon Da Chau area, while clay minerals and/or iron oxides are common in most samples In all the red and yellow sands the iron oxides and clay form thin to thick grain coatings, some of which cement the smaller grains In several of the samples these secondary coatings have the appearance of soil cutans These iron oxide and clay coatings form up to 38% of the sample by volume on a porosity-free basis The clay consists predominantly of poorly crystalline kaolinite No clear systematic petrological trends are apparent from the analysed samples and the quantity of cement present probably re¯ects local and ¯uctuating groundwater table positions under the tropical weathering regime In general, the amount of iron oxides and clay determined from the petrological study exceeds the amount of ®ne material seen in the particle size analysis This indicates that not all the secondary cements were disaggregated during particle size analysis which, in turn, re¯ects slightly upon the accuracy of the particle size analyses, especially for skewness and kurtosis 5.2 Particle size analysis Particle size analysis was carried out on all the samples analysed by TL One of the major limitations of the particle size analysis is the abundance of authigenic iron oxide and clay in many samples All the yellowish and red sand b a 375 375 375 375 375 375 375 375 375 375 375 375 375 375 375 300±450 300±500 325±500 325±475 325±475 ± ± 300±500 300±500 300±500 300±500 300±500 275±500 300±500 300±500 6.3 ^ 0.7 30.5 ^ 4.2 58.2 ^ 5.4 206 24.2 ^ 2.0 187 ^ 35 90.6 ^ 15.4 144 ^ 66 91.2 ^ 11.3 196 ^ 22 124 165 79.4 ^ 7.8 36.4 ^ 4.4 254 Plateau Analysis Palaeodose region (8C) temperature (8C) (Gy) Atomic absorption spectroscopy TL ages reported with 1s uncertainty terms Suoi Tien Village VN#15 (W2212) VN#37 (W2531) VN#14 (W2211) Hon Da Chau VN#18 (W2525) VN#20 (W2526) VN#22 (W2527) VN#24 (W2528) Mui Ne Beach west of Hon Rom Mountain VN#31 (W2533) VN#32 (W2534) Phu Lac VN#12 (W2210) VN#12B (W2524) De Them II south of River Luy VN#30 (W2529) VN#29 (W2530) Phan Thiet Airport VN#45 (W2532) Bau Gang VN#44 (W2535) Sample and laboratory code 50 ^ 25 50 ^ 25 50 ^ 25 50 ^ 25 50 ^ 25 50 ^ 25 50 ^ 25 0.050 ^ 0.005 0.110 ^ 0.005 50 ^ 25 50 ^ 25 0.115 ^ 0.005 50 ^ 25 0.900 ^ 0.005 100 ^ 25 0.255 ^ 0.005 0.550 ^ 0.005 0.400 ^ 0.005 0.430 ^ 0.005 2.0 ^ 5.9 ^ 2.5 ^ 3.1 ^ 2.6 ^ 4.1 ^ 2.0 ^ 1.4 ^ 3.7 ^ 2.7 ^ 2.0 ^ 1.0 ^ 2.8 ^ 2.8 ^ 3.3 ^ Rb content Moisture content (assumed) ppm (% by mass) 0.305 ^ 0.005 50 ^ 25 0.300 ^ 0.005 50 ^ 25 0.600 ^ 0.005 50 ^ 25 1.150 ^ 0.005 100 ^ 25 0.065 ^ 0.005 0.135 ^ 0.005 0.295 ^ 0.005 K content (% by ASS) a 12.7 ^ 0.4 46.0 ^ 1.4 37.3 ^ 1.2 36.0 ^ 1.0 42.8 ^ 1.3 62.7 ^ 1.9 22.3 ^ 0.6 42.5 ^ 1.3 67.1 ^ 2.1 75.7 ^ 2.0 94.3 ^ 2.8 91.2 ^ 2.5 43.0 ^ 1.5 29.9 ^ 0.9 46.6 ^ 1.4 Speci®c activity (Bq/kg U Th) Table Results of thermoluminescence dating of quartz sand from coastal barrier sediments, southeastern Vietnam between Phan Thiet and Tuy Phong 138 ^ 25 98 ^ 25 75 ^ 25 20 ^ 10 146 ^ 25 81 ^ 25 20 ^ 10 10 ^ 105 ^ 25 87 ^ 25 56 ^ 10 42 ^ 10 10 ^ 10 ^ 10 ^ Cosmic contribution (mGy/a) 451 ^ 26 1095 ^ 31 985 ^ 30 1686 ^ 19 1269 ^ 31 1879 ^ 36 896 ^ 15 1332 ^ 21 1745 ^ 38 1912 ^ 38 2587 ^ 41 3134 ^ 38 939 ^ 22 752 ^ 16 1245 ^ 21 Annual radiation (mGy/a) 14 ^ 28 ^ 62 ^ 122 19 ^ 99 ^ 19 101 ^ 17 108 ^ 49 52 ^ 103 ^ 11 48 53 85 ^ 48 ^ 204 TL age (ka) b 542 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 543 Fig Regenerated TL growth curve at 3758C for sample VN#37 (W-2531) The natural TL (N) is minimized using a laboratory ultraviolet lamp (UV) and incrementally irradiated using a calibrated 90Sr plaque beta radiation source (b ) The laboratory-induced TL (N UV b )TL is shown plotted (A) against increasing laboratory irradiation dose The mean (n ˆ 8) natural TL value is shown plotted (1) upon the growth curve The additive TL data (N b ) is shown plotted (x) with reference to the mean N value and thus provides a check for possible TL sensitivity changes Inset shows the temperature plateau comparison derived by plotting the ratio of the natural TL (N) to that induced in the laboratory, following TL minimization, against temperature where the laboratoryinduced signal approximates the natural value The plateau region in this case extends between 300 and 5008C samples contain moderately thick iron oxide grain coatings that mask the true grain surface and cause a slight increase in the particle size of most sand grains In addition to the iron oxide, many of the oxidized samples also contain very poorly crystalline kaolinite cutans Similar clay cutans are also present in the white sands, being particularly abundant in the Mui Ne area When the samples are disaggregated using ultrasonic methods most of the clay cutans and some of the less stable altered feldspar and lithic grains break down to produce a ®ne fraction that does not appear to be indicative of depositional matrix Thus, the total particle size analysis (Table 1) probably relates at least to some extent to the severity of diagenetic alteration and soil forming processes in the tropical environment, thereby masking a realistic indication of mode of origin The reduced mean grain size, poor to moderate sorting and ®ne skewness all re¯ect the amount of diagenetic alteration If just the sand and coarser silt size particles are considered (which petrographic analysis indicates mainly consist of individual grains and iron oxide coated grains) the samples show a fairly uniform mean ®ne sand size (0.22± 0.25 mm) and are moderately to moderately-well and wellsorted (Table 1) Two samples are medium-grained and moderately sorted Although many of the grain surface textures are disguised by authigenic grain coatings, the samples from the base of the Mui Ne and Hon Da Chau sections have the greatest proportions of grains with dull surfaces These are possibly of marine origin In contrast, samples from topographically higher sections generally have more frosted and polished grains This probably indicates an aeolian environment with a greater amount of reworking of the detrital grains 5.3 Thermoluminescence dating With the exception of the TL data that re¯ect saturation, and therefore represent minimum ages, all the TL results indicate Late Pleistocene ages for the barrier sands (Table 2) The TL data indicate that saturation is attained over potentially varying intervals ranging from approximately 48±204 ka and point to the dif®culty of obtaining resolvable luminescence ages 200 ka within the study area, owing to the speci®c luminescence characteristics of the quartz (e.g apparent early TL saturation), high annual radiation doses and the intense weathering conditions experienced during diagenesis, resulting in the migration of radionuclides down pro®le by in situ leaching The data highlight, in part, the dif®culty of applying the TL method in environments where consistently high reaction temperatures and ¯uctuating groundwaters may mobilize radioisotopes and thus potentially compromise the veracity of the method (Aitken, 1985; Forman et al 2000) The results for Suoi Tien Village illustrate this point The basal grey sand yielded a result of 204 ka 544 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 Fig Regenerated TL growth curve at 3758C for sample VN#24 (W-2528) The natural TL (N) is minimized using a laboratory ultraviolet lamp (UV) and incrementally irradiated using a calibrated 90Sr plaque beta radiation source (b ) The laboratory-induced TL (N UV b )TL is shown plotted against increasing laboratory irradiation dose (A) Due to apparent TL saturation there is no discernible increase in TL beyond a palaeodose value of 165 Grays This conservative minimum palaeodose corresponds with a minimum age of 53 ka The mean (n ˆ 8) natural TL value (6.77 ^ 0.91) has not been reliably induced in this case Inset shows the temperature plateau comparison derived by plotting the ratio of the natural TL (N) to that induced in the laboratory, following TL minimization, against temperature where the laboratory-induced signal approximates the natural value The plateau region in this case appears truncated which, combined with a 13% uncertainty in the mean natural TL value, is suggestive of unreliable TL data (VN#14; W-2211) while the overlying white sand in the mottled zone beneath the red sand yielded an age of 48 ^ ka (VN#37; W-2531), and the red sand in the upper part of the succession yielded an age of 85 ^ ka (VN#15, W-2212; Fig 3) The younger age for TL sample VN#37 (W-2531) is problematic Although the post-depositional concentration of radioisotopes of the elements K, Th and U would potentially increase the environmental dose for this sediment and hence yield a younger apparent TL age, it is noted that of the three TL samples from Suoi Tien, sample VN#37 actually revealed the lowest annual radiation dose (Table 2) Although 2.5% of the sediment in sample VN#37 comprises orthoclase feldspar which may have in¯uenced the radiation dose, the measured annual dose remains the lowest of the Suoi Tien samples Sample VN#37 (W-2531) yielded a well-de®ned growth curve with an extended plateau between 300 and 5008C and does not indicate saturation (Fig 7) The basis for this age reversal is still being investigated, particularly in the context of the possibility of radioactive disequilibrium The TL results for the red sand at Hon Da Chau con®rm the Late Pleistocene age for the upper part of this succession and further illustrate the problem of obtaining TL ages on strongly diagenetically modi®ed sediments, as revealed from lower within the succession (Fig 6) Sample VN#18 from the upper red sand yielded an age of 52 ^ ka (W2525) and sample VN#20 from the top of the lower red sand yielded an age of 103 ^ 11 ka (W-2526; Fig 6) Farther down pro®le, however, the TL results show the combined effects of TL saturation and partial bleaching before deposition, the latter indicated by the truncated growth curve for sample VN#24 (Fig 8) Samples VN#22 from the welllaminated yellow sand unit and VN#24 from the top of the white sand yielded minimum ages of 48 ka and 53 ka respectively and represent conservative minimum ages (Table 2) Although sample VN#24 displays a truncated plateau indicative of partial bleaching (Fig 8), and would otherwise yield an older apparent age, the effects of TL saturation in this particular sample far exceed the former effect, thus deriving a younger minimum age Sample VN#24 comprises 5.5% orthoclase feldspar derived from the local weathering of the granite at Hon Da Chau, and would account for the particularly high annual radiation dose for this sample, resulting in the earlier attainment of TL saturation (Tables 1±2) The two TL ages for the upper and lower red sand units from the Mui Ne Beach succession near Hon Rom Mountain, yielded results consistent with respect to depth down pro®le Sample VN#31 yielded an age of 101 ^ 17 ka (W2533) and VN#32 an age of 108 ^ 49 ka (W-2534) suggestive of deposition during Stage sensu lato of the Oxygen Isotope record (Bassinot et al., 1994) The red sand from Phu Lac near Tuy Phong collected from m below the dune surface (VN#12B) yielded an C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 545 Fig Schematic shore-parallel transect between Phan Thiet and Tuy Phong showing the thickness of the Late Pleistocene coastal barrier sand deposits at individual sites, and the location of thermoluminescence samples and ages age of 99 ^ 19 ka (W-2524) whereas the overlying sample collected 2.5 m below the dune surface (VN#12) yielded a TL age of 19 ^ ka (W-2210) The similarity in sediment texture of the younger sand to the older, underlying sand points to localised dune reactivation during the Last Glacial Maximum (Oxygen Isotope Stage 2) The red sand from De Them II immediately to the south of the River Luy also yielded results suggestive of a Pleistocene age, and although the result of 122 ka (W-2530) for VN#29 points to an age older than Late Pleistocene for part of the succession, the overlying sample (VN#30) with an age of 62 ^ ka (W-2529) possibly equates with deposition during stadial Stage of the last glacial cycle Given that these two samples are only 20 m apart, vertically, suggests the presence of a diastem The stratigraphically lower sample (VN#29) contains 3% orthoclase feldspar and it is likely that this may have contributed to the higher annual radiation dose experienced by this sediment Sample VN#45 at Phan Thiet military airport yielded a TL age of 28 ^ ka (W-2532) and may also re¯ect a phase of dune reactivation during the latter part of interstadial Stage The unconsolidated white sand at Bau Gang (VN#44) yielded a TL age of 14 ^ ka (W-2535) and is attributed to reworking of the white sand during the terminal Pleistocene Discussion The coastal barrier succession of southeastern Vietnam ranges up to 20 km in width and 160 m in height Inland from the barrier system an extensive marine sand unit reaching heights of 60±80 m laps up against the older bedrock mountains but was not examined in this study (Fig 1) Late Pleistocene and Holocene erosion has occurred between the coastal barrier sands and the marine sand sheet, especially during the last low stand of sea level, creating the accommodation space for the subsequent deposition of Holocene ¯uvial valley-®ll and lagoonal sediments, which now separate the two older sand sequences (Fig 1) The sequences thin towards the margins of the coastal barrier sands, and in some areas, such as Hon Da Chau, show evidence of erosion or extensive periods of non-deposition and soil formation The sections at Phan Thiet, Suoi Tien, Mui Ne Beach near Hon Rom Mountain, De Them II and Phu Lac near Tuy Phong represent typical sequences from the main part of the barrier sand whereas the sequence at Hon Da Chau represents a thinner marginal facies (Fig 9) In places the coastal barrier sands partially cover bedrock outcrops (e.g Hon Da Chau), whereas elsewhere they cover older white sands that probably represent marine coastal sand deposits (e.g at Suoi Tien) These older sand deposits have not been investigated extensively during this study 546 C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 Fig 10 Thermoluminescence ages (ka) on quartz sand from the coastal barrier successions of southeastern Vietnam between Phan Thiet and Tuy Phong, plotted against the Oxygen Isotope chronology of Bassinot et al (1994) The plot reveals a diachronous trend in aeolian reworking throughout the Last Glacial Cycle Details of TL samples (VN numbers) are given in Table TL uncertainty terms are reported at the sigma level since two of the analysed samples (VN nos 14 and 24) did not yield de®nitive ages because of TL saturation and partial bleaching The yellow laminated sand and underlying white sand at Hon Da Chau both yielded relatively young minimum ages (ca 53±48 ka) with truncated TL plateaux suggestive of incomplete bleaching before deposition (Fig 8) In addition, down-pro®le leaching of radiogenic materials, particularly for VN#24, would account for the higher annual radiation doses evident for these samples, thus yielding even younger apparent minimum ages The TL age estimates for these samples are therefore, very conservative minimum values and re¯ect the combined effects of partial bleaching, the onset of early TL saturation and in situ leaching In contrast, the lower grey±white sand unit at the base of the Suoi Tien section yielded a TL age 204 ka which may provide a more reasonable minimum estimate for the age of this sequence (i.e possibly oxygen isotope Stage or older) The best exposures of this lower sequence were recorded just west of Hon Rom Mountain where it overlies an older red sand succession (Fig 4) The main depositional event for the red coastal barrier sands appears to be from about 108±85 ka and probably represents composite barrier formation (Fig 10) In view of the position of these deposits with respect to global estimates of Late Quaternary sea-level (Chappell et al., 1996), and the TL uncertainty terms, the successions are here correlated with the last interglacial sensu lato (Oxygen Isotope Substages 5.4±5.0) following the isotopic chronology of Bassinot et al (1994) We acknowledge, however, that as the TL uncertainty terms by convention, are reported only at the sigma level, a last interglacial age (substage 5.5) cannot be precluded for some of these deposits (e.g Mui Ne, Phu Lac and Hon Da Chau) The section just west of Hon Rom Mountain clearly shows two later periods of sand accumulation, based on the lithostratigraphical evidence, with a period of soil formation separating the two units, represented by weak ferricrete development, and the more recent formation of a prominent erosional bench In view of the large uncertainty terms, the TL ages are not signi®cantly different for these two units, and accordingly, the succession is here correlated with the last interglacial sensu lato (Fig 10; Table 2) The TL ages, therefore, suggest that ferricrete development may have occurred relatively quickly in this tropical environment (i.e potentially within a few thousand years as recorded in ¯uvial sequences of similar age from tropical northern Australia; Nanson et al., 1991) Similarly, at Hon Da Chau, the top of the lower red sand is bounded by a disconformity and minor ferricrete soil development that, based on the TL evidence, appears to have formed relatively quickly The only sample that does not ®t into the scheme of deposition during late Stage is VN#29 which yielded a minimum age of 122 ka (Stage or older) and appears to be near or at TL saturation The next main period of aeolian activity is from 62 to 28 ka spanning the terminal part of Stage and all of Stage 3, with deposits constituting an upper red sand unit at De Them II south of the River Luy and the upper part of the section at Hon Da Chau This may represent a period of extensive reworking of the upper parts of the barrier system during Stage The sands were probably already red at this stage A younger period of aeolian reworking is evident between 28 and 19 ka during Stage (Last Glacial Maximum) Such examples occur within the top m of the relevant sections at Phan Thiet and Phu Lac The red colouration of the coastal barrier sands is a diagenetic feature that probably developed soon after deposition of the barrier sands, as indicated by the presence of iron-rich C.V Murray-Wallace et al / Journal of Asian Earth Sciences 20 (2002) 535±548 weathered surfaces Iron deposition was not restricted to this period and probably continued during phases in Stages and This is clearly indicated by the well-developed iron-rich duricrust in the upper part of the red sand sequence at Hon Da Chau (i.e younger than 52 ka) It is noteworthy that, on the basis of the TL evidence, the coastal barrier successions of southeastern Vietnam appear to correlate with the later substages of Stage when sea level was up to 20 m below present sea level (Chappell et al., 1996) None of the TL age mid-points fall within the last interglacial maximum (i.e 127±122 ka) as de®ned by Bassinot et al (1994) Given that the shorelines at the time of deposition over this interval are likely to have ranged from to 20 km off the present coastline, points to a period of intense and enhanced aeolian activity in which dunes migrated considerable distances inland In a global context the coastal barrier successions of southeastern Vietnam represent thick sand accumulations whose upper surfaces extend up to 160 m APSL, and in shore-normal cross-section, between Phan Thiet and Tuy Phong attain a maximum width of about 20 km The large size of these sand deposits points to extensive deposition of sand along the Vietnamese coastal margin over a lengthy period The large barrier sand sequences in southeastern Vietnam are attributed to long-term accumulation of ¯uvially-sourced sand transported to the continental shelf by rivers during glacial lowstands, followed by wave-induced reworking and redeposition of these sediments during interstadial transgressions The extensive reworking and redeposition of aeolian sands of this marginal marine sand sheet following Stage implies intervals of reduced vegetation cover and general landscape instability to permit sand remobilization, and may relate to climate change Conclusions Extensive consolidated as well as active dune®elds, the product of coastal barrier sedimentation, occur between Phan Thiet and Tuy Phong in southeastern Vietnam The successions are divisible into an inner Pleistocene barrier of predominantly red sand with white marine sand in its basal portion, and an outer barrier of white, unconsolidated sand of presumed Holocene age which abuts, and in places onlaps, the Pleistocene successions The upper portions of the extensive accumulations of red barrier sands (dune facies) between Phan Thiet and Tuy Phong are here dated by the TL method as Stage with numeric ages ranging between 108 and 85 ka A subsequent major period of aeolian activity is represented by the reworking of the upper parts of the barrier system during Oxygen Isotope Stage with TL numeric ages spanning 62±28 ka A ®nal phase of Pleistocene aeolian reworking occurred during the Last Glacial Maximum (Oxygen Isotope Stage 2) between 28 and 19 ka 547 The pervasive reworking and redeposition of aeolian sands points to reduced vegetation cover and landscape instability which may relate to climate change Acknowledgements This research has been supported by the Australian Research Council Small Grants Scheme, the Quaternary Environments Research Centre at the University of Wollongong, the Quaternary Geomorphological Center of the South Vietnam Geological Survey and the Department of Lithology and Marine Geology, Vietnam National University, Hanoi We thank Rachel Nanson and Vanessa Squires for laboratory assistance and Richard Miller for preparing the ®gures This paper is a contribution to IGCP Project 437, `Coastal Environmental Change During Sea-Level Highstands: A Global Synthesis With Implications For Management of Future Coastal Change' References Aitken, M., 1985 Thermoluminescence Dating Academic Press, London Astakhov, A.S., Markov, Y.D., Chin, T.K., 1990 Effect of the Mekong River on Late Quaternary sedimentation in the South China Sea Lithology and Mineral Resources 24, 295±307 Bassinot, F.C., Labeyrie, L.D., Vincent, E., Quidelleur, X., Shackleton, N.J., Lancelot, Y., 1994 The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal Earth and Planetary Science Letters 126, 91±108 Chappell, J., 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Price, D.M., 1990 Thermoluminescence dating of Late Quaternary dune sand, Manawata/Horowhenua area, New Zealand: a comparison with 14C age determinations New Zealand Journal of Geology and Geophysics