DSpace at VNU: Pliocene-Quaternary evolution of the continental shelf of central Vietnam based on high resolution seismic data

Journal of Asian Earth Sciences 79 (2014) 529–539 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes Pliocene–Quaternary evolution of the continental shelf of central Vietnam based on high resolution seismic data Mai Thanh Tan a,⇑, Le Van Dung b, Le Duy Bach c, Nguyen Bieu c, Tran Nghi d, Hoang Van Long a, Phan Thien Huong a a Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Viet Nam Vietnam Petroleum Institute, 173 Trung Kinh, Yen Hoa, Cau Giay, Hanoi, Viet Nam Vietnam Union of Geological Sciences, Pham Ngu Lao, Hanoi, Viet Nam d Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam b c a r t i c l e i n f o Article history: Available online 14 August 2013 Keywords: Pliocene–Quaternary Continental shelf Seismic stratigraphy Central Vietnam a b s t r a c t The continental shelf of central Vietnam is characterized by complex geological structures in a strike-slip setting and thus by unique morphology compared to other parts of the Vietnam shelf Studying the Pliocene–Quaternary sediments in this area allows the processes that control the stratigraphic evolution to be highlighted In this paper, we present new results derived from high resolution seismic data that reveal the geological evolution of the continental shelf of central Vietnam during the Pliocene–Quaternary Our results demonstrate that the continental shelf of central Vietnam is narrow relatively to other parts of the Vietnam shelf The evolution of the shelf during the Pliocene–Quaternary was controlled by the pre-Pliocene rifting process However, fault systems below the present shelf were not strongly active during the Pliocene–Quaternary period, except for several listric faults in the slope Nonetheless, Quaternary volcanic activities occurred widely both offshore and onshore, especially near the major faults The Pliocene–Quaternary sedimentary deposits were subdivided into units based on unconformities as result of marine regression corresponding to glacial cycles Sedimentary environments during this period consist of coastal plain, fluvial, nearshore and shelf environments Most of the Holocene sediments were trapped nearshore while the outer shelf is characterized by abundant lag gravels as a result of rapid sea level rise between $20 and ka Ó 2013 Elsevier Ltd All rights reserved Introduction Pliocene–Quaternary sediments are observed extensively along the continental shelf of Vietnam These formations have been deposited under the influence of complex sea level and climate interactions during the last Ma This means that the Pliocene– Quaternary formations can be considered as a natural laboratory for understanding how paleoclimate change and coastal processes, as well as tectonic forces controlled the development of the continental margin stratigraphy High resolution of age control, coupled with better defined sea level and climate reconstructions means that we are now able to separate the competing influences better for these strata compared to older units In addition, the continental shelf and coastal area are very sensitive and vulnerable to both climate change and coastal degradation Understanding the processes that have occurred in the geological past, especially during the Pliocene–Quaternary period is important because by ⇑ Corresponding author Tel.: +84 857 2324; fax: +84 838 9633 E-mail address: mttan@fpt.vn (M.T Tan) 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.jseaes.2013.08.001 understanding how the coast and shelf have responded to earlier changes in the paleoclimate and coastal processes we may be able to predict models for natural hazard mitigation in the future The continental shelf of central Vietnam (Fig 1) is narrow and elongated, with sea floor of stair-step shape, dipping eastward to the ocean The Vietnam shelf consists of relatively thick and heterogeneous Pliocene–Quaternary successions that were deposited under different tectonic regimes (Hutchison, 2004; Tapponnier et al., 1986, 1982, 1990; Zhang and Zhong, 1996) It is unique compared to many continental margins because of the very narrow extension and the close proximity of mountains to the coast with high relief and relatively minor flood plains Some of the Pliocene–Quaternary formations may host significant petroleum potential, which makes the area attractive for petroleum exploration, as well as other economical activities such as offshore engineering and transportation A number of seismic surveys and exploration drilling campaigns have been carried out by petroleum companies during the last several decades These works allowed geoscientists to define regional geology, stratigraphy and to reconstruct evolutionary history of sedimentary basins offshore 530 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 Fig The research area in continental shelf of central Vietnam Vietnam However, major focus was on pre-Pliocene sedimentary formations, where oil and gas were expected to be discovered Therefore, geological structure and evolution of the younger Pliocene–Quaternary successions are still poorly understood Because of the limited database, stratigraphic models for the evolution of the shelf are not very accurate Because of these shortcomings striking scientific problems raised here are: (1) How have the continental shelf of central Vietnam and its sediments during the Pliocene–Quaternary? and (2) Can we use high resolution seismic data to study the Pliocene–Quaternary sediments and can the history of the continental during this period can be reconstructed thereby? Since well data and field observations are lacking, high resolution shallow seismic exploration can provide valuable geological information at higher resolution and hence it is an effective tool for studying subsurface geology In this paper, we present some new results of the evolution of the Pliocene–Quaternary continental of central Vietnam based on high-resolution 2D reflection seismic data provided by various marine research groups (Institute of Marine Geology and Geophysics, Center of Marine Geology and Mineral Resources, Institute of Marine Environments and Natural Resources, etc.) We further correlate and compare our results with the industrial multichannel 2D seismic data provided and interpreted by Petrovietnam in order to study the Pliocene–Quaternary sedimentary formations under the continental shelf of central Vietnam The primary objectives of this work were: – Defining the geological structure of the Pliocene, Pleistocene and Holocene formations off central Vietnam – Reconstructing the evolution of the continental shelf of central Vietnam during the Pliocene–Quaternary Database and methods 2.1 Seismic data The application of high resolution seismic data (HRS) with correct acquisition and processing parameters is an effective and M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 reliable tool to study the Pliocene–Quarternary sediments on the continental shelf of central Vietnam The data used in this study consist of about 5500 km of HRS profiles (Fig 2), which have been acquired by using Sparker and boomer sources The sparker used frequencies of 200–1000 Hz with penetration depth of 400– 500 m, and vertical resolution of 2–5 m while the boomer source used frequencies of 1000–10,000 Hz with penetration depth of 30–50 m, and vertical resolution of 0.5–1 m The shotpoint interval was designed as 10 and 12.5 m In this study, the industrial 2D multichannel seismic data from the previous studies (Clift et al., 2008; Hoang et al., 2010) were also referenced in order to extract the longer time-scale geological information for a better geological correlation within the regional framework The combination of different types of seismic data allows us to interpret the Pliocene–Quaternary stratigraphy more accurately 531 2.2 Seismic stratigraphy Most of sedimentary cores collected from this region by gravity corer have very limited depth and thus provide little age control for our interpretations of the sedimentary architecture Therefore interpretations of the depositional environments are primarily based on the conventional types of seismic reflection terminations and configurations (Vail et al., 1977; Veenken, 2007) Once the data had been processed our interpretation was done based on picking seismic horizons as a fundamental step in seismic stratigraphy The seismic stratigraphy together with core data is essential for the interpretation of the stratigraphic section and can be used effectively for depositional system analysis, identification of depositional cycles, as well as reconstruction of sea level change The data derived from such interpretation allows detailed depositional patterns of any identified sequence to be defined In certain Fig Basemap of the 2D seismic survey in the continental shelf of central Vietnam 532 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 Fig Seismic and interpreted geological sections of Lines SVOR93-103, SVOR-111, SVOR93-203 and VRP-93.73a showing base boundary of the Pliocene–Quaternary Seismic image acquired by Sparker is distinguished from Boomer seismic profile by higher resolution and hence clearer image (cf Fig for line locations) M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 533 Fig Structural map of the continental shelf of central Vietnam cases, sequence boundaries were dated by correlation with wellestablished regional eustatic curves (Boulay et al., 2008), which have been constrained by biostratigraphic data Based on seismic attributes and reflection configurations, we can subdivide the geological section into zones corresponding such as continental facies, near shore facies, and marine facies Each depositional environment normally corresponds to different sedimentary characteristics Neritic and shelf slope environments are generally characterized by coarse-grained sediments while the outer shelf environment demonstrates finer grained sediments Seismic reflection-free configuration together with high traveling velocity of the seismic waves can be useful parameters for identifying carbonate platforms as well as volcanic formations in the region Results 3.1 Geological setting of the Pliocene–Quaternary continental shelf of central Vietnam The development of the continental shelf of central Vietnam is considered to be closely related to the opening of the South China Sea and associated tectonism in Southeast Asia during the Cenozoic, India–Eurasia collision, sea floor spreading of the South China Sea, extension of the Red River Fault towards the south (Clift et al., 2008; Hall, 2002; Tapponnier et al., 1982) Although the mechanism of the opening of the South China Sea are still debated, these studies all suggested that the development of the Red River Fault to the South and Southeast (named as 110° Fault) played an important role in controlling evolution and extension of the continental shelf of central Vietnam In order to define larger-scale geology of the continental shelf off central Vietnam, several industrial seismic profiles were used in this study for a broader understanding of the region in the regional tectonic framework The seismic cross-sections in Fig shows that the continental shelf of Vietnam is a part of the passive margin of the South China Sea, which is characterized by a series of normal, high angle faults forming graben-like structures in the basement The Pliocene sediments were underlain by the Miocene formations, which are separated by an unconformable surface about 5.5 Ma In general, the Pliocene–Quaternary formations observed in Fig show a wedged shape, which gradually thickens towards shelf break but becomes thinner at the deeper slope Due to the fact that tectonic activities and lithology of the 534 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 Fig Depth structure map (A) and isopach map (B) of the Pliocene sediments in the continental shelf of central Vietnam Fig Depth structure map (A) and isopach map (B) of the Quaternary sediments in the continental shelf of central Vietnam M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 535 Fig The distribution of faults and basaltic formations in the continental shelf of central Vietnam basement vary from place to place the continental shelf of central Vietnam demonstrates different settings Based on structural characteristics, sedimentary architecture, the continental shelf of central Vietnam can be subdivided into three parts: Northern part: Including South Red River Basin, Da Nang Shelf and Cu Lao Re Uplift (Fig 4) The Pliocene–Quaternary structures evolved on the Neogene and Paleogene basement Depth of the base of Pliocene varies from hundreds of meters nearshore to about 4000 m in the center of the basin, while the thickness varies from tens to about 2600 m (Fig 5) The depth of the base of the Pleistocene varies from hundreds of meters nearshore to about 1600 m, while the thickness varies from tens to about 1600 m (Fig 6) It is suggested that the formation of basins has is closely linked to the regional strike-slip motion along the 110° Fault and tectonic subsidence The thin Quaternary sedimentary cover in this area suggests that the area experienced an inversion period in the Late Pliocene However, we not exclude the fact that the Red RiverYinggehai Basin was fully filled during Tertiary time and less accommodation space was available during the Quaternary Central part: Including West Phu Khanh Basin (Fig 4) Close to Quang Ngai extends a group of Tertiary basins, which contains only thin Pliocene–Quaternary successions Because sediments were mainly transported offshore to the outer shelf and continental slope thickness of the sediments in the deeper parts is much higher compared to the inner shelf In this central part, strong activities of the 110° fault created a slope, which had an important effect to the depositional environment and structure of the region As a result, observed depths of bottom of Pleistocene and Pliocene formations are $2300 m and 4200 m respectively On the Pliocene structural map (Fig 5), sedimentary thickness increases eastwards, but on the isopach map of the Quaternary sediments (Fig 6) the marine aggradational configuration along the 110° Fault widened from South to North Sediment was supplied by small scale drainage systems from the hinterland to these areas Geological cross-sections interpreted from seismic data (Fig 3a–d) and isopach maps (Figs and 6) demonstrate that the Pliocene–Quaternary sedimentary formations are very thin on the inner shelf but become much thicker on the outer shelf as 536 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 Fig High resolution seismic section and interpreted geological section of Line MDC-57-58 (see Fig for line location) a result of the bypassing process under complicated interactions between fluvial, wave, tide, and marine current forcing as well as tectonic subsidence Deceasing sediment thickness from North to South also suggests that more sediment was supplied to the shelf by major rivers e.g the Red, Ma and Lam Rivers than by local, short rivers from eastern side of the Truong Son Mountain Range Southern part: Northeast Cuu Long Basin, including Phan RangVung Tau shelf, Phan Rang uplift (Fig 4) The shelf and slope area is relatively narrow and quickly transits to the deep water of Phu Khanh Basin This area is directly controlled by the strike-slip motion on the 110° Fault, which is the southern continuity of the Red River Strike-slip fault The strike-slip motion on the 110° Fault together with the opening of the South China Sea is considered the most important factors to generate such a narrow and steep shelf As a result, sediments eroded from the Truong Son Mountain Range have been quickly bypassed to the slope 3.2 Reconstructing geological evolution of the continental shelf of central Vietnam during the Pliocene–Quaternary Although the mechanism of the opening of the South China Sea and the formation of the continental shelf of central Vietnam are still debated it has been agreed that the evolution of the continental shelf of central Vietnam was closely linked to the pre-Pliocene tectonism of western South China Sea passive margin, especially related to the Eocene–Oligocene rifting process and to the Middle Miocene deep regional unconformity in the South China Sea (Clift et al., 2008; Hall, 1996) A simplified tectonic/structural map (Fig 7) based on seismic data demonstrates that most of the major faults in the continental shelf of central Vietnam run parallel to the shore oriented in the NW–SE and/or sub-meridional direction Other minor faults developed in the NE–SW direction and are characterized by shallow depth and small offset However, the major faults occurred as M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 537 Fig High resolution seismic section and interpreted geological section of Line MDC-72-73 (see Fig for line location) normal and/or listric faults in the basement and cut through the pre-Pliocene formations under the present continental shelf while the Pliocene–Quaternary faults are not very often observed in this area (Figs and 8) These younger faults become more common in the continental slope (Fig 3) This means that tectonic activities on the continental shelf off central Vietnam have weakened dramatically since the Early Pliocene Additional normal/listric faults observed in the continental slope might have been triggered by gravity-driven force imposed on steep slopes The reactivation of the major deep faults onshore and offshore was identified by different offsets along the same fault surface Although the Pliocene–Quaternary formations have not been affected strongly by faulting, the fault reactivation has triggered volcanism with basaltic rocks, which are relatively common in the region (Fig 7) The 538 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 Table Description of seismic reflection configuration of each package Unit Seismic description S1 (N2) S4 ðQ 2b Þ Package S1 unconformably overlies on the pre-Cenozoic basement, which is characterized by onlap termination of seismic reflections across erosional surface of the basement Seismic reflection pattern of this unit demonstrates relative high-medium amplitude, good continuity, sigmoidal and oblique tangential configuration This package is separated from sequence S1 by a weakly eroded surface and it is distinguished from the lower part by weaker amplitude of seismic reflection The lower boundary of this package is marked by an erosional surface Seismic reflection configuration within this package is characterized by parallel and/or divergent reflection, high amplitude Package S4 is delineated based on the upper and lower erosional surface and its seismic configuration is similar to the unit S3 S5 ðQ 3a Þ The upper and lower boundaries of this seismic unit were picked across erosional surface and it demonstrates divergent reflections of seismic data 1À2 S6 ðQ 3b Þ À Q2 This is a transitional unit between the Pleistocene and Holocene formations The sequence shows weaker reflection amplitude, divergent configuration and thickening nearshore Although this package shows relatively similar seismic reflection configuration to the unit S6, it demonstrates thin layer, parallel reflection in the deeper part while the shallower part shows thicker formation with divergent reflection configuration S2 ðQ 11 Þ S3 ðQ 2a Þ S7 ðQ 32 Þ Fig 10 Seismic section in line BP91-118 through Well CVX-1X (see Fig for line location) latest eruption event offshore central Vietnam occurred in 1923 indicating recent volcanic activity in the region (Vo, 2001) Although the Pliocene–Quaternary tectonism is not very active in the region, tectonic subsidence together with sea level variations and sediment supply are considered as important factors controlling sedimentation processes on the continental shelf of central Vietnam By using the seismic stratigraphic scheme (Vail et al., 1977; Veenken, 2007) seismic packages corresponding to sedimentary units of the Pliocene–Quaternary were defined (Figs and 9, Table 1) The boundary between these packages are erosional surfaces resulted from regression and sea-level lowering due to glacial events (Le, 2004) Fig 10 shows that the Early Pliocene unit ðN12 Þ is characterized by sub-horizontal, parallel and continuous seismic reflection horizons These characters suggest that the paleo-continental shelf of central Vietnam during the Early Pliocene was a flat depositional environment, which resulted from a strong tectonic subsidence following an inversion by the end of the Miocene Seismic packages from the Late Pliocene to the Late Pleistocene demonstrate progradational configuration (Figs 8–10) suggesting that sediment supply was faster than the creation of accommodation space during this period The progradational pattern can be explained by the following reasons: (1) the continuous switching between the variable glacial–deglacial periods reduced the physical strength of rocks, intensified erosion of the source rocks and hence more sediments were feeding the basin (Peizhen et al., 2001; Schimanski and Stattegger, 2005; Schimanski et al., 2001); (2) tectonic subsidence and relative sea-level rise slowed down as rifting was ceased; and (3) slow continuous sea-level rise In contrast, the rapid deglacial sea level rise in Holocene (at $20–8 ka BP) has lead to a rapid transgression As a result, the whole continental shelf of central Vietnam was flooded The evidence of this event can be clearly observed by wave-cut platforms and marine terraces along the present coast Few surface sediment samples collected from the shelf ($15– 20 m deep) contain very coarse grained, iron-stained, and poorly sorted rock fragments These materials are likely reworking products, which were eroded from the pre-existing coastal/beach sedimentary rocks The presence of reworking sedimentary products in the inner shelf is attributed to the rapid deglacial sea-level rise from $20 to ka BP, most of sediments have been trapped nearshore to form a stepwise, relatively flat and horizontal sediment drapes on the present continental shelf as it is seen in Fig Conclusions The continental shelf of central Vietnam has undergone a complex geological history The sedimentary records derived from seismic and other data show that during the Pliocene–Quaternary period, the depositional pattern of sediments has been strongly variable in terms of subsidence rate, depositional environment, geological settings, sedimentary budgets, and tectonic regimes, etc Such variation and differentiation are demonstrated by the presence of a number of regional unconformities that are associated with periodic uplift, faulting, and volcanism M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539 The Pliocene and Quaternary geological structure in the continental shelf of central Vietnam was inherited from the main structural features, which were formed during the Cenozoic or earlier times The faults are mainly oriented in NW–SE and/or sub-meridional direction, and the fault motion is important in controlling sedimentation during the Pliocene–Quaternary The Pliocene–Quaternary deposits are widely distributed and tend to increase in thickness towards the deep sea Faulting activities remain active through time and play an important role in controlling the evolution of sedimentary basins and the configuration of continental Vietnam shelf Most of the major normal faults were strongly active during the earlier rifting period of the passive continental margin of western South China Sea The faulting process below the continental shelf became weaker during the Pliocene–Quaternary period, except for several listric faults observed in the continental slope Associated with the faulting activity, volcanism has been triggered and occurred widely in both onshore and offshore central Vietnam The subsidence rate of the basin during the Cenozoic era was variable in time and spaces Results from high resolution seismic data show that seismic packages could be identified The boundaries between these units are unconformities that were resulted from regression during the glacial cycles Core data derived from industrial well CVX-1X demonstrate that sedimentary formations of the continental shelf of central Vietnam during the Pliocene–Quaternary were deposited in different environments such as terrestrial coastal environment (alluvial silts, fluvial sands interbedded with thin coal), inner shelf environment (shoreface sand and conglomerates), and outer shelf environment (lag gravel, sand and fine mud) The presence of lag gravels in the inner shelf suggests a reworking process during the rapid deglacial transgression at $20–8 ka BP Apart from tectonics, sea-level variations and sediment supply are also important controls on sedimentation of the continental shelf of central Vietnam Acknowledgments This work was partly supported by the Vietnam’s National Foundation for Science and Technology Development (NAFOSTED), Project No 105.04.27.09 We would like to take this opportunity to thank NAFOSTED, geoscientists of the Hanoi University of Mining and Geology, PetroVietnam, Vietnam Academy of Sciences and Technology for giving us valuable opinions for this research We 539 are grateful to Peter Clift and Karl Stattegger for their comments that 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