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

Pliocene–Quaternary evolution of the continental shelf of centralVietnam based on high resolution seismic data Mai Thanh Tana,⇑, Le Van Dungb, Le Duy Bachc, Nguyen Bieuc, Tran Nghid, Hoa

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Pliocene–Quaternary evolution of the continental shelf of central

Vietnam based on high resolution seismic data

Mai Thanh Tana,⇑, Le Van Dungb, Le Duy Bachc, Nguyen Bieuc, Tran Nghid, Hoang Van Longa,

a

Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Viet Nam

b Vietnam Petroleum Institute, 173 Trung Kinh, Yen Hoa, Cau Giay, Hanoi, Viet Nam

c

Vietnam Union of Geological Sciences, 6 Pham Ngu Lao, Hanoi, Viet Nam

d

Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

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 Pli-ocene–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–Quater-nary 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, Quater-nary volcanic activities occurred widely both offshore and onshore, especially near the major faults The Pliocene–Quaternary sedimentary deposits were subdivided into 7 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 8 ka

1 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 5 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

conti-nental 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

continen-tal shelf and coascontinen-tal 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

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 heter-ogeneous 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 com-pared 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 explora-tion, as well as other economical activities such as offshore engi-neering 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 recon-struct evolutionary history of sedimentary basins offshore

⇑ Corresponding author Tel.: +84 4 3 857 2324; fax: +84 4 3 838 9633.

E-mail address: mttan@fpt.vn (M.T Tan).

Contents lists available atScienceDirect Journal of Asian Earth Sciences

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s e a e s

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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

Plio-cene–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

recon-structed thereby? Since well data and ﬁeld observations are

lack-ing, 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 Petro-vietnam 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

2 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

Fig 1 The research area in continental shelf of central Vietnam.

530 M.T Tan et al / Journal of Asian Earth Sciences 79 (2014) 529–539

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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

al-lows us to interpret the Pliocene–Quaternary stratigraphy more

accurately

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 effec-tively for depositional system analysis, identification of deposi-tional cycles, as well as reconstruction of sea level change The data derived from such interpretation allows detailed depo-sitional patterns of any identified sequence to be defined In certain

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Fig 3 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 2 for line locations).

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cases, sequence boundaries were dated by correlation with

well-established 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

char-acteristics Neritic and shelf slope environments are generally

characterized by coarse-grained sediments while the outer shelf

environment demonstrates ﬁner grained sediments Seismic

reﬂec-tion-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

3 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 Ceno-zoic, 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 mecha-nism 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 impor-tant 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-sec-tions inFig 3shows that the continental shelf of Vietnam is a part

of the passive margin of the South China Sea, which is character-ized 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 unconform-able surface about 5.5 Ma In general, the Pliocene–Quaternary for-mations observed inFig 3show 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

Fig 4 Structural map of the continental shelf of central Vietnam.

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Fig 5 Depth structure map (A) and isopach map (B) of the Pliocene sediments in the continental shelf of central Vietnam.

Fig 6 Depth structure map (A) and isopach map (B) of the Quaternary sediments in the continental shelf of central Vietnam.

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basement vary from place to place the continental shelf of central

Vietnam demonstrates different settings Based on structural

char-acteristics, sedimentary architecture, the continental shelf of

cen-tral 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

Pleisto-cene 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 do not exclude the fact that the Red

River-Yinggehai 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 sedi-ments were mainly transported offshore to the outer shelf and con-tinental 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 in-creases eastwards, but on the isopach map of the Quaternary sed-iments (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

(Fig 3a–d) and isopach maps (Figs 5 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

Fig 7 The distribution of faults and basaltic formations in the continental shelf of central Vietnam.

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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

Rang-Vung Tau shelf, Phan Rang uplift (Fig 4)

The shelf and slope area is relatively narrow and quickly

tran-sits 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 continen-tal 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 devel-oped in the NE–SW direction and are characterized by shallow depth and small offset However, the major faults occurred as

Fig 8 High resolution seismic section and interpreted geological section of Line MDC-57-58 (see Fig 2 for line location).

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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 3 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

dramati-cally 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 dif-ferent offsets along the same fault surface Although the Plio-cene–Quaternary formations have not been affected strongly by faulting, the fault reactivation has triggered volcanism with basal-tic rocks, which are relatively common in the region (Fig 7) The

Fig 9 High resolution seismic section and interpreted geological section of Line MDC-72-73 (see Fig 2 for line location).

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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

control-ling sedimentation processes on the continental shelf of central

Vietnam By using the seismic stratigraphic scheme (Vail et al.,

1977; Veenken, 2007) 7 seismic packages corresponding to 7

sedimentary units of the Pliocene–Quaternary were deﬁned (Figs 8

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 ðN1

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

gla-cial–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

Stat-tegger, 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 trans-gression 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 prod-ucts, which were eroded from the pre-existing coastal/beach sedi-mentary rocks The presence of reworking sedisedi-mentary products in the inner shelf is attributed to the rapid deglacial sea-level rise from 20 to 8 ka BP, most of sediments have been trapped near-shore to form a stepwise, relatively ﬂat and horizontal sediment drapes on the present continental shelf as it is seen inFig 8

4 Conclusions The continental shelf of central Vietnam has undergone a com-plex geological history The sedimentary records derived from seis-mic 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 associ-ated with periodic uplift, faulting, and volcanism

Table 1

Description of seismic reﬂection conﬁguration of each package.

Unit Seismic description

S1 (N 2 ) 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

S2 ðQ 1

Þ 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 S3 ðQ 2a

1 Þ 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 S4 ðQ 2b

1 Þ Package S4 is delineated based on the upper and lower erosional surface and its seismic conﬁguration is similar to the unit S3

S5 ðQ3a1 Þ The upper and lower boundaries of this seismic unit were picked across erosional surface and it demonstrates divergent reﬂections of seismic data S6 ðQ 3b

1 Q122 Þ This is a transitional unit between the Pleistocene and Holocene formations The sequence shows weaker reﬂection amplitude, divergent

configuration and thickening nearshore S7 ðQ3Þ 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

Fig 10 Seismic section in line BP91-118 through Well CVX-1X (see Fig 2 for line location).

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