Holocene evolution of the paleo-Mekong incisedvalley, Mekong river delta, Vietnam

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.00085 46 HOLOCENE EVOLUTION OF THE PALEO- MEKONG INCISED- VALLEY, MEKONG RIVER DELTA, VIETNAM Ta Thi Kim Oanh 1* , Nguyen Van Lap 1 , Yoshiki Saito 2 , Marcello Gugliotta 2 , Toru Tamura 3 , Nguyen Thi Mong Lan 1 , Truong Minh Hoang 4 , Bui Thi Luan 4 1 HCMC Institute of Resources Geography 2 Estuary Research Center, Shimane University 3 Geological Survey of Japan 4 Ho Chi Minh University of Science Email: ttkoanh@hcmig.vast.vn ABSTRACT A compilation of depositional facies and 14C ages from the two new cores (DT2, and DT4 cores) considerably contribute to clarify the Holocene evolution of the paleo Mekong River (MR) incised valley, Mekong River Delta (MRD). The longitudial section documents the geometric features indicating the different gradients along the paleo-Mekong incised valley from the Cambodian lowerlands through delta plain up to the shallow offshore of the MRD. The erosional contact between pre-LGM and post-LGM deposits is found at -25 m in the upstream, and deepened over -100 m and -70 m in the medial and lower parts of the MRD. The 42.6 m thick fluvial channel facies encounterd at -97.6 m deep at DT4 core site documents the faciliated accumulation and preservation of the lowstand fluvial sediments to be the first time found in the MRD. Due to accumulation of fluvial channel facies prior ∼13.4 ka, the gradient of paleo-MR incised valley seems to be gentle ranging from -65m to -55m in the lower and medial parts, excepting an abrupt rising up to -25m in the upstream. Evolution of the paleo-MR incised valley is strongly dominated by sea-level control, sediment supply and gradient of the valley. Keywords: Incised valley, Mekong River, delta, sedimentary facies, evolution. 1. INTRODUCTION In the present-day Mekong delta area, the Holocene succession, consisting of incised- valley fill and deltaic deposits, is 70 m thick and unconformably overlays pre- LGM Pleistocene deposits. The buried incised valley was first identified in the BT2 core in the lower- delta-plain area and traced to the DT1 core (Ta et al., 2001, Nguyen et al., 2010). The valley is narrow with a width of 20-30 km and it continues upstream following the Mekong River into the Cambodian lowland (Tamura et al, 2009) and downstream crossing the present coastline and extending into the Southeast Vietnam shelf. On the shelf area, Tjallingii et al. (2010) defined at least five incised valleys that are 2- 6 km wide and up to 30 m deep. The rapid transition accurately records the postglacial transgression and moved landwards to be found at -64m in 12 ka in the incised valley. Rising sea level resulted in rapid infilling of the incised valley by aggradation of fluvially dominated sediments. Similarly to the lower delta-plain, infilling of paleo-Mekong incised valley is well documented by the transition from tidal river deposits to marsh/tidal flat and estuarine marine deposits at approximately 13.4 ka in BT2 core site (Ta et al., 2001, 2002), and marsh/ tidal flat deposits dated 11.6-10.6 ka in the DT1 core site (Nguyen et al., 2010). Moreover, mangrove marsh and tidal flat facies are widespread in the northeast part of the MRD being clear evidence of tide- dominated delta in 5.5 to 4.0 ka (Hanebuth et al., 2012). The MR channels currently have flat Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 47 topographies, till now salt- water intrusion and mangrove vegetation along the MR are occupied up to 100 km from shoreline (Gugliotta et al., 2017). This study is based on two newly collected sediment cores (DT2 and DT4) in the delta-plain area of the MRD. The core recovery was 75-80% for mud sediments and 65-70% for sand sediments. The sediment cores were split, photographed, and described sedimentologically. Slab samples (5x25x1cm) were collected from the entire Holocene succession; they were photographed, X-radiographed, and used to improve the sedimentological description. A total of 10 facies were identified, based on lithology, sedimentary structures, and fossil content. Diatom and foraminifera analyses were carried out using an optical microscope on a total of 34 and 18 samples, respectively. 2. RESULT AND DISCUSSION 2.1. Sedimentary facies of the DT2 and DT4 cores - DT2 was drilled in a natural levee, at a distance of approximately 0.3 km to the E of thepresent-day Mekong main channel, starting from an elevation of 4 m above the present meansea level. The sediment core is consists of two main unitsseparated by a marked erosional surface (incised-valley bottom) at the depth of -25.05 m below the present mean sea level. The lower unit is approximately 20 m thick andconsists of pre-LGM Pleistocene deposits. The upper unit is approximately 30 m thick,consists of post-LGM Holocene deposits, shows an overall fining-upward trend, and can besubdivided in 4 sedimentary facies, from bottom to top: fluvial channel/tidal river (DT2a),estuary/subtidal flat (DT2b), intertidal flat (DT2c), and levee/floodplain (DT2d). - DT4 was drilled in the floodplain, at a distance of approximately 1.2 km to the E of thepresent-day Mekong main channel, starting from an elevation of 3.4 m above the presentmean sea level. The sediment core consists of a singleunit, as no erosional surface was identified. The unit consist of post-LGM LatePleistocene to Holocene deposits, shows an overall fining-upward trend, and can besubdivided in 4 sedimentary facies from bottom to top: fluvial channel/tidal river (DT4a),estuary/subtidal flat (DT4b), fluvial channel/tidal river (DT4c), and floodplain (DT4d). 2.2. Stratigraphy and architecture of incised valley Section A-B was reconstructed based on two new sediment cores (DT2 and DT4) and four sediment cores from previous studies (PSG, DT1, VLM, and BT2). The section is approximately 255 km long and broadly NW-SE oriented, providing an upstream to downstream longitudinal section of the incised valley. Pre-LGM Pleistocene deposits are found in the upstream part of this section in cores PSG and DT2. The lower boundary of this unit is unknown. The maximum thickness of 20 m is observed in DT2. The erosional contact between pre-LGM and post-LGM deposits was found at depths of approximately -25 m in the upstream part of this section (i.e., PSG and DT2), whereas this surface was not encountered in the cores from the medial to downstream part (DT4, DT1, VLM, and BT2) even if the cores therein reached depths of -45 to -100 m. The post-LGM unit is approximately 30 m thick in the upstream part of the section, up to 110 m in the medial part, and up 70 m in the downstream part. In the lowest part of DT4 core, fluvial channel facies is 42 m thick and encountered from -97.6 to -64.5 m indicating paleo- MR channel deposits. In the upper part of this facies from -64.5 to -54.5 m bioturbations are common and predominance of fresh water diatoms and low frequency of marine plankton and marine brackish water diatoms that indicate the interface between fluvial and seawater. In section A–B the post-LGM unit can be subdivided in three parts: the initial fluvial valley fill, the estuarine valley fill, and the deltaic deposits. The lower part of the post-LGM unit consists of fluvial deposits locally with tidal influence, which partially filled the incised valley prior to the Holocene transgression. These deposits are seen in DT4 and BT2 only and reach a maximum thickness of 42.6 m in DT4. Ages from the overlying deposits suggest that these deposits accumulated before 13.4 ka. The medial part of the post-LGM unit consists of estuarine, marsh/tidal Hồ Chí Minh, tháng 11 năm 2019 48 flat, and shelf deposits, which filled the valley during the transgression. The isochrones show ages ranging from 11 to 9 ka. The upper part of the post-LGM unit consists of fluvio-deltaic and shelf/prodelta deposits. These deposits are seen in all cores of the A-B section and reach a maximum thickness of 36 m in the BT2 core. The isochrones show ages ranging from 8 ka to recent. 2.3. Isochrones and accumulation rates Based on the 14 C dating from two new cores and the previous cores, isochrones from two sections are identified with 1,000-year interval from 12 ka to 4 ka. It is clear that the isochrones are helpful to clarify the stacking patterns of deltaic and estuarine sediments and their accumulation rates in relation to sea-level changes. The compacted and expanded isochrones indicate low and high accumulation rates, respectively. In the period from 13.4 to 10 ka, the infilling of marsh/tidal flat deposits in BT2 is represented by confined isochrones and an accumulation rate of 1.4 mm/yr. In the subsequently, the expanded isochrones are 8.7-10.0 mm/yr from the marsh and sub-intertidal flats (DT1 and VLM cores) in the period of 11.6-10.4 ka. From 9 to 8 ka, the accumulation rate accelerated to ca. 10.6-27.2 mm/yr from PSG and DT1 cores, respectively. This accretion rates arise from differences in sedimentary dynamics andaccommodation space at the two sites, while the DT1 core accretion occurred continuously in the aggradation of delta initiation and the upper portion of the PSG core corresponded to progressively evolving into flood plain. In the subsequently, the intertidal mudflat and mangrove marsh with accumulation rate decelerated to 5.6 mm/yr from PSG from 8 to 6.3 ka that correspond to accumulation rate of 8.3 mm/yr from estuarine/ tidal flat from DT2 core. Almost incised valley completely infilled in the Cambodian lowland and upper delta plain, simultaneously, sedimentation rate increase in the lower delta plain of ca. 6.8 mm/yr from estuarine marine environment and up to 54 mm/yr from deltaic facies around 5 ka in the high standing and regression period. In the upper part of the DT4 core site, an abrupt change of isochrones occurs at between the DT2 and DT1 core sites owing to the fluvial channel/tidal river facies deposited from 4.4 to 2.2 ka. 3. CONCLUSIONS Detailed cores descriptions, depositional facies and 14 C ages from the two new cores (DT2, and DT4) considerably contribute to clarify the evolution of the paleo-MR incised valley of the MRD in Late Pleistocene to Holocene. These new core sites reach -35 to -97.6 m deep and being located in and beside the paleo-MR incised valley. The erosional contact between pre-LGM and post-LGM deposits is found at -25 m in the upstream, and deepened over -100 m and -70 m in the medial and lower parts of the MRD. It indicates that different gradients occur along the paleo-MR incised valley, and influence to infilling sediments. The fluvial channel facies composed of over 42.6 m thick from -54.5 to -97.6 m (DT4 core site), and documents the faciliated accumulation and preservation of the lowstand fluvial sediments to be the first time found in the MRD. Due to accumulation of fluvial channel facies prior ∼13.4 ka, the gradient of paleo-MR incised valley seems to be gentle ranging from -65m to -55m in the lower and medial parts, excepting an abrupt rising up to -25m in the upstream. Evolution of the paleo-MR incised valley is strongly dominated by sea-level control, sediment supply and gradient of the valley. 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