Late Holocene Evolution of the Mekong Subaqueous Delta, Southern Vietnam Zuo Xue a, ⁎, J. Paul Liu a, ⁎, Dave DeMaster a , Lap Van Nguyen b , Thi Kim Oanh Ta b a Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA b HoChiMinh City Institute of Resources Geography, Vietnam Academy of Science and Technology, Ho Chi Minh City, Vietnam abstract article info Article history: Received 22 January 2009 Received in revised form 9 November 2009 Accepted 13 December 2009 Available online 23 December 2009 Communicated by J.T. Wells Keywords: subaqueous delta mud wedge delta evolution sediment budget South China Sea As Asia's third largest river, with regard to sediment load, the Mekong River delivers approximately 160 million tons of sediment per year to the South China Sea. High-resolution seismic profiling and coring during 2006 and 2007 cruises revealed a low gradient, subaqueous delta system, up to 20 m thick, surrounding the modern Mekong River Delta (MRD) in the west of the South China Sea. Based on clinoform structure, grain size, 210 Pb, AMS 14 C, and δ 13 C results, the subaqueous delta is divided into four zones defined by different sedimentary processes and depositional features. Over the past 3000 yr, the evolution of the MRD has shown a morphological asymmetry indicated by a large down-drift area and a rapid progradation around Cape Camau, ∼200 km downstream from the river mouth. This asymmetric feature is consistent with increased wave influence. The strong southwestward coastal current, strengthened by the strong NE monsoon, plays an important role locally in longshore transport of resuspended sediments into the Gulf of Thailand. A late Holocene sediment budget for the MRD has been determined, based on the area and thickness of deltaic sediment. Approximately 80% of Mekong delivered sediment has been trapped within the delta area, which, together with a falling sea-level, resulted in a rapidly prograding MRD over the past 3000 yr. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Most of the world's deltaic systems began their formation between 7400 and 9500 cal yr BP as a result of decelerating sea-level rise (Stanley and Warne, 1994). These deltaic systems are characterized by different stratigraphy controlled by variations in relative sea level, fluvial inputs, marine dynamics, morphology, and tectonics. Conceptual processes- based models for deltaic deposition include: 1) river-dominated/ influenced, such as the Mississippi, Yellow, and Po deltas, 2) wave- dominated/influenced, such as the Nile and Danube deltas, 3) tide- dominated/influenced, such as the Amazon, Yangtze, and Fly deltas (Galloway, 1975), and 4) deltas dominated by the combination of the former three processes, such as the Mekong Delta (Ta et al., 2002a,b). The evolution of a deltaic system is a non-steady process and is usually characterized by lobe switching, such as in the Mississippi (Roberts, 1997, 1998) and Po deltas (Correggiari et al., 2005), and even changes of dominant process, such as in the Mekong Delta (Ta et al., 2002a). As part of the prograding depositional units of the deltaic systems, subaqueous deltas and clinoform structures have been documented in numerous deltaic systems including the Amazon (Nittrouer et al., 1986, 1996), Yellow (Liu et al., 2004, 2007a), Yangtze (Chen et al., 2000; Liu et al., 2007b), Po/Adriatic Sea (Cattaneo et al., 2003), Ganges-Brahmaputra (Kuehl et al., 1997; Goodbred and Kuehl, 1999, 2000), and Fly River/Gulf of Papua (Walsh et al., 2004; Slingerland et al., 2008). Late Quaternary sediment budgets have been established, based on the volume estimation of these subaqueous deltas and clinoform structures. Although historically the term “clinoform” has referred to the foreset part of a deltaic system, recent usage of the term refers to the topset– foreset–bottomset morphology of deltaic systems. The term “com- pound-clinoform” has been proposed to describe a subaerial/subaque- ous delta couplet (Nittrouer et al., 1996; Swenson, 2005). Determined by multiple factors such as marine hydrodynamics, fluvial sediment inputs, eustatic sea level, and subsidence, the development and character of subaqueous deltas vary among different locations. In general, energetic marine environments, such as the Amazon Shelf, Gulf of Bengal, or Gulf of Papua are ideal for subaqueous delta development, whereas low energy environments, such as the Gulf of Mexico are less suited for development of such a feature (Swenson, 2005). Studies of the sedimentation processes on the continental shelf off the Mekong River Delta (MRD) are limited. Seismic and sediment core studies only have been conducted either along the continental shelf edge (Schimanski and Stattegger, 2005) or to the south around the Sunda Shelf, where the paleo-shoreline was located during the Last Glacial Maximum (LGM) (Hanebuth et al., 2000, 2002, 2003, 2009; Hanebuth and Stattegger, 2004). This paper will present the results of a seismic and sediment core field study of the MRD's coastal area between 2006 and 2007, with specific interests focused on the morphology and sedimentatary processes of the subaqueous delta. Marine Geology 269 (2010) 46–60 ⁎ Corresponding authors. Tel.: +1 919 515 7767; fax: +1 919 515 7802. E-mail addresses: zxue@ncsu.edu (Z. Xue), jpliu@ncsu.edu (J.P. Liu). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.12.005 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo