Reconstruction of the holocene paleo-geography of the the Mekong river delta, Vietnam

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.00086 50 RECONSTRUCTION OF THE HOLOCENE PALEO-GEOGRAPHY OF THE THE MEKONG RIVER DELTA, VIETNAM Nguyen Van Lap 1* , Ta Thi Kim Oanh 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:nvlap@hcmig.vast.vn ABSTRACT The paleo-geographical reconstructions are obtained from the interpolated MRD sea-level curve, depositional facies and Pleistocene surface topography of the former and new cores in the MRD and surrouding shallow offshore. The paleo-geography of the MRD is distinguished by four stages corresponding to sea-levels of -32m/10 ka, -4m/8.0ka, +1.6m/6.0ka and +1m/4.0 ka. Keywords: Holocene, paleo-geography, sedimentary facies, delta. 1. INTRODUCTION Sedimentary facies responsed to Holocene sea-level change have been focussed not only in the subaerian delta plain but also in the subaqueous delta plain. Paleo- MR valley is defined by the stratigraphic units, geomorphologic features, depositional characteristics and ages of cores from mainland and offshore. Relationships among stratigraphic records in the mainland and offshore provided the relative timing of events and the geomorphological features revealed processes by that incised valley formed. The basal boundary of the incised valley is around -25 m inCambodian lowland (Tamura et al., 2009), lowering to over -97 m (BT4 core) in the upper delta plain, reaching- 70 m (BT2 core) (Ta et al., 2001) nearby recent shoreline, and finally -67 m in the offshore (68-3, 69-2 cores; Tjallinggii, 2010). The depositional facies and their ages from the cores are well correlated throughout the sections from Cambodian lowland to the recent coastline of MRD, and also coincided with cores from offshore, indicating that the incised valley were formed before 13.4 ka. The morphology of incised valley is resolved by the data of cores in the mainland, and high- resolution seismic profiles and cores in the offshore (Tjallinggii, 2010) confirm a system of Mekong river paleovalley. Valley incision depths and widths vary, with the deepest incision penetrating more -97 m and widths typically ranging from 4-15 km, reaching a maximum of 20 km. The incised valley that extends from Cambodian lowland initially flows along a large valley towards the SE, then turns to the E trend towards the offshore. The principal axes of the main incised valley (Mekong and Bassac rivers) are oriented NW-SE, roughly perpendicular to regional bathymetric contours. The main incised valley occurs at the mouth of MR and merges into an incision depth of 30-40 m (69-2, 68-3 cores) (Tjallingii et al., 2010) to extend further offshore in W-E trend. This result is consistent with previous studies that reported the formation of incised valley of the MR connecting the mainland and offshore at around 13.5 ka in the eastern shelf of the East Sea. 2. RESULTS AND DISCUSSION The stratigraphic record provides a remarkably well developed succession and can be divided into 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 Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 51 partially filled the incisedvalley prior∼13.4 ka to Holocene transgression (Ta et al., 2001, 2002; Nguyen et al., 2010). These deposits are seen in DT4 and BT2 and reach a maximum thickness of 42.6 m and -97.6 m deep in DT4. The medial part of the post-LGM unit consists of estuarine, marsh/tidal flat, and shelf deposits, which filled the valley during the transgression. These deposits are maximum thickness of 27 m in the DT4 core, and also seen in all cores, and 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 maximum thickness of 36 m in the BT2 core, and ages ranging from 8 ka to recent. When sea-level rise had ended around 6.0 - 5.5 ka and continued fluvial sediment input, resulted in the change from a transgressive to a regressive system with downstream migration of the pro-delta and delta front deposits. The prograding subaqueous delta deposits migrated southward into the shallowing embayment/ shelf where subaquous delta occupy -18 to -22 m from DT1 and VLM cores, meanwhile shelf deposits are encountered in -30 to -42 m from VL1 and BT2 cores. The regression resulted in considerable silting-up of the pro- delta and delta front deposits and the overlying sub-to intertidal flats. In the upper part of the DT4 core, the fluvial channel/ tidal river facies is approximately 27.5 m thick from -27 m to 0 m, and dated 4.4 to 2.2 ka. The paleo-geographical reconstructions are obtained from the interpolated MRD sea-level curve, depositional facies and Pleistocene surface topography of the former and new cores in the MRD and surrouding shallow offshore. The paleo-geography of the MRD is distinguished by four stages corresponding to sea-levels of -32 m/10 ka, -4m/8.0ka, +1.6m/6.0ka and +1m/4.0 ka. - Around 10 ka sea-level was -32 m bpsl, paleoshoreline is approximately 45- 50 km long eastwards from the present shoreline. Paleo MR valley is about 25- 30 km wide running nortwest – southeast direction from Campodian lowlands (PSG core site) to the medial part of the channel (VLM core site) and then changed to west- east direction entering into the sea near Vung Tau city. The paleo-valley could be divided into three parts: fluvial channel took place in the upstream part, then river channel occupied the medial part (DT1 and VLM core sites) and embayment occupied the lower part of valley (BT2, VL1 core sites). The BT2 core site is located in the center of embayment showing the deepest part because of 23m paleowater depth, meanwhile the VL1 core site is at 5m paleowater depth of this embayment. This paleo MR valley accords with the paleo valley in the shallow offshore detected in 10, 20, 30 paleowater depth from 69-2, 70-2, 68-3 core sites and further side of deep water of 85 m from 91-2 and 93-3 core sites. - Around 8 ka, sea-level is -4 m bpsl, due to upstream incursion of seawater almost the medial and lower parts of paleo-MR valley and the present lower delta-plain are submerged under seawater and paleoshoreline is located in the present upper delta plain where the Late Pleistocene bsaement is shallow- Plain of Red and Long Xuyen Quarangle. In the mainland, tidal river and mangrove/ tidal flat took places in the upstream part of the paleo-MR valley (DT2, PSG core sites), conversely, the medial part of the is submerged under seawater. Based on the paleo-bathymetry, seafloor is a very gentle slope of 17 cm/ km excepting the elongate depression of the paleo-MR valley concentrated by 20 to 40 m paleo-bathymetries. The paleo-bathymetry of 30 m also suggests the west-east trend of paleo-MR valley entering into the shallow offshore (73-2 and 72-2 cores). Deposition is almost concentrated in and arround the paleo-MR valley. Subaqueous delta is formed in -17 m (DT1 core site), and shelf occupied at -22 and -49 m deep (VLM and BT2 cores). The remainder of seafloor has not deposition. The paleo-bathymetry of 10 m is nearly paralell line to the paleo-shoreline and indicates a large shallow seafloor on the south and southwest parts excepting the deep and elongate area of the paleo-MR valley. - Around 6 ka, sea-level is maximum of +1.6 m apsl., paleoshoreline moved a little northwards and located also in the present upper delta plain. In the mainland, the paleo- MR valley is divided into three parts: the flood plain and fluvial channel are in the upstream part (PSG core site), and river channel in the medial part. Mangrove/tidal flat occupied almost the present upper deltaplain where the basement is shallow and paleo-waterdepth to be -3 to -3.5 m deep. Based on Hồ Chí Minh, tháng 11 năm 2019 52 the paleo-bathymetry, seafloor is very gentle slope around 23 cm/km to the east and southeast trends excepting the elongate depression of the paleo-MR valley bordered by the 30 m paleo- bathymetry. The paleo-bathymetries of 10 and 20 m are nearly paralell lines to the paleo-shoreline and indicates a large shallow seafloor on the south and southwest parts. It is silimar to those of the 8 ka, deposition is almost concentrated in and along the paleo-MR valley. The subaqueous delta is expanded from DT1 to VLM core sites (18-22 m water depth) according with the embayment/shelf the BT2 core site (42 m water depth). - Around 4 ka, sea-level is +1 m apsl., due to prograding delta pushed seaward the paleo shoreline moved southwards and get its location beside the Cai Lay beachrigde. In the mainland, the paleo-MR valley is identified by flood plain (PSG core site), fluvial channel in the upstream part and river channel (DT1 core site) near river mouth in the medial part. Floodplain is mainly occupied the large area of recent upper delta plain. Mangrove/tidal flats occupied a lowland behind the paleo shoreline. Based on the paleo-bathymetry, the slope of seafloor increases to 23 cm/km between the paleo shoreline and paleo-bathymetry of 20 m. The subaqueous delta is widespread over 90 km long seawards and occupies almost area within paleo-bathymetry of 20 m, and it includes the paleo-MR valley. It indicated the paleo-MR valley is already fulfilled by subaqueous delta reaching the paleo- bathymetry of 20 m, and father the paleo-MR valley is identified and connected through a paleo- valey entering the shallow offshore. 3. CONCLUSIONS The palaeogeography reconstruction of the MR incised valley and MRD is detected infour stages corresponding to sea-levels of -32m/10ka, -4m/8.0ka, +1.6m/6.0ka and +1m/4.0ka. In each stage, positions of shorelines, paleo-bathymetries within 10 m interval in the shallow shelf. It indicates a fulfilment of the MR incised valley contains a valley-fill succession with an incised lowstand fluvial system at its base, filled with a transgressive- regressive sediment wedge. The middle to late Holocene highstand and following rapid progradation of the delta occurred and the muddy shelf to pro-delta were filled since 5.5 ka. In the upper and lower delta plain, around 5.5 to 3 ka, continuous fluvial sediment supply caused formation of delta progradation. The position of coastlines of the four stages and evolution of the subearial and subaquous delta are pronouncedly documented and according to the paleo-bathymetries in 10 m interval. 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