an evolutionary model of the near-shore tinjar and ... · pdf filean evolutionary model of the...

International Journal of Petroleum and Geoscience Engineering (IJPGE) 2 (1): 81-91, 2014 ISSN 2289-4713 © Academic Research Online Publisher

Research paper

An Evolutionary Model of the Near-shore Tinjar and Balingian Provinces,

Sarawak, Malaysia

M. J. Mathew

a,*, N. A. Siddiqui

a,b, David Menier

a

a Department of Petroleum Geosciences, Universiti Teknologi PETRONAS, Perak, Malaysia

b NED University of Engineering and Technology, Karachi, Pakistan

* Corresponding author. Tel.: +6 013 6642998;

E-mail address: [email protected]

A b s t r a c t

Keywords:

Tectonic evolution,

Balancing and restoration,

Cross section ,

Subduction,

Wedge,

Piggyback basin.

The tectonic evolution of a basin can hold vital information regarding the

variations and changes in depositional environments. Sarawak basin has

undergone strong tectonic activity in its past. In this study, a model has been

proposed for the evolution of Tinjar and Balingian provinces in Sarawak, East

Malaysia in order to throw light on tectonic events that was responsible for

basin development. The model was developed by balancing and restoration of

cross sections obtained from published interpreted seismic profiles. The model

takes into account the subduction of proto-South China Sea oceanic crust

beneath the Borneo plate during Middle Eocene to Middle Miocene. The

resulting model depicts older sequences (T1S and T2S) were accreted on to

Borneo continental plate as a wedge and younger sequences (T3S) as

piggyback basins. Due to structural complexity and variability of the region,

each tectonostratigraphic province of Sarawak should be analysed individually

as the proposed model corresponds only to two provinces of Sarawak.

Accepted: 12March 2014 © Academic Research Online Publisher. All rights reserved.

1. Introduction

A sequence is a relatively conformable succession of genetically related strata, bounded by

unconformities or their correlative conformities [1]. The Sarawak basin consists of seven identified

sequences and eight sedimentary cycle boundaries. The cycles and their cycle boundaries in the

Sarawak basin are most likely controlled by eustatic sea level changes [2]. Approximate ages of the

top of each sequence are shown in table 1 (ages of T6S and T7S unknown). Tectonic evolution of the

basin can hold vital information regarding the variations & changes in depositional environments of

each of these sequences. Sarawak basin is believed to be one that has undergone strong tectonic

activity in its past. Various scholarly works and analysis pertaining to the evolution of the basin have

been carried out previously.

M. J. Mathew et al. / International Journal of Petroleum and Geoscience Engineering (IJPGE) 2 (1): 81-91, 2014

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Previous works had suggested the evolution of Sarawak basin commenced when the Luconia block

collided with Borneo leading to the closure of the Rajang Sea in the Late Eocene time [3]. It was also

proposed that the basin had a strike-slip origin during Oligocene time based on structural styles and

subsidence history [4]. Concluded works saw the introduction of the NW Borneo geosyncline theory

stating the origin as an elongated basin filled with intercalations of volcanic rocks and sediments. Due

to sediment loading the basin floor subsided and subsequently strong deformation of the sediments

takes places by orogenic forces. Granite emplacement at the lower sediments occurs due to

metamorphosis [5]. Tectonic evolution of NW Borneo region was illustrated previously as a result of

subduction, during Oligocene, of the South China Sea plate beneath the Borneo continental plate [6].

In the present study, a model has been proposed for the evolution of near shore regions of Tinjar and

Balingian provinces by restoring cross sections obtained from interpreted seismic profiles and data

obtained from published literature.

Table 1: Approximate ages of sequences [7]

Sequence Age in Ma

Top T5S 5.3 (Early Pliocene)

Top T4S 11 (Late Miocene)

Top T3S 18 (Early Miocene)

Top T2S 22.5 (Early Miocene)

Top T1S 37 (Late Eocene)

2. Geologic Setting of Sarawak Basin

The Sarawak basin is one of the most prolific hydrocarbon producing basins within the geologically

complex Southeast Asia. The geological complexity with regard to evolution of the basin has

prompted many previous scholarly works yielding various models. The basin forms the southern

margin of Oligocene-Recent South China Sea basin and evidences suggest it has undergone phases of

rifting and sea-floor spreading in the South China Sea marginal basin [8] and [9]. Sarawak basin is

situated onshore and near coast regions with the tectonic discontinuity, the West Baram Line

(~115°E), East of Miri, separating it from the Sabah basin, to the western extend of the Malaysian

province Sarawak (~110°E) and offshore for approximately 400km (~7°N) [10]. The basin extends

westward into Indonesia as the East Natuna basin.


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Fig. 1: Tinjar and Balingian Provinces (modified after [11]

The Sarawak basin is broadly sub divided into seven tectonostratigraphic provinces based on

structural styles and sedimentation history. They are SW Sarawak, Tatau, Balingian, Tinjar, Central

Luconia, West Luconia and North Luconia. The northern parts of the basin comprises of the Luconia

block. In the Southern parts, two wrench fault systems can be observed as a result of Luconia block

collision along with a dextral system associated with the West Balingian Line. A sinistral system

associated with strike-slip basement tectonics in East Balingian can also be observed [12].

The geology of Sarawak is recognized as belonging to two distinct provinces, corresponding to two

main geographic regions, namely West Sarawak and Central-North Sarawak. West Sarawak is the part

of the state South and West of the Lupar line. It forms part of the West Borneo Basement which

extends into Sarawak from the south. Rocks of Palaeozoic and early Mesozoic ages in Sarawak are

confined to this region. The oldest rocks in West Sarawak are considered to be pre-Upper

Carboniferous. Intrusive granitic rocks are confined mainly to this area. Faulting and folding are

common but these are mostly localized.

Central-North Sarawak is the part of the state North and East of the Lupar line. The geology of

Central-North Sarawak is younger than that of West Sarawak. The oldest rocks known in this area is

of Cretaceous age. Faulting and folding are common and appear to have affected all the rocks in the

area except the Quaternary.


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Fig. 2: a) Orientation of seismic lines used in study (modified after [4]), b) Interpreted cross section of Line 2

(L2) (modified by [4]), c) Interpreted cross section of Line 1 (L1), (modified by [4]).

b

a

c


85 | P a g e

Sarawak basin consists of seven identified sequences and eight sedimentary cycle boundaries. The

cycles and their cycle boundaries are most likely controlled by eustatic sea level changes. The age of

the basin is Late Eocene to Recent age with onshore exposures that can be followed between Sibu and

Miri and further Northeast into the Inboard Belt of the Sabah basin. The basement consists of pre-

Oligocene rocks that are exposed to the south in the Rajang Fold-Thrust Belt which extends across

Sarawak and Kalimantan, Indonesia [13].

2.1. Area of Study

Situated mainly onshore and a fragment nearshore, the Tinjar province is located between latitudes

2°25’ N and 4°15’ N and longitudes 113°20’E and 114°30’E. The province is separated from the

offshore Balingian by the NE-SW to ENE-WSW Anau Nyalau Fault. The Balingian province flanked

by the Central Luconia province to the North and Tinjar province to the South is mainly offshore. The

location of study is nearshore Balingian and Tinjar province (Figure 1) [11].

2.2. Seismic Profiles and Sequences

The near shore reprocessed lines by Sarawak Shell, obtained in between the years 1971 - 1979 and

reprocessed in 1987 by Digicon has been interpreted by Zin Ismail Che Mat where five seismic

sequences were identified. The orientation of the seismic lines and their interpreted cross section is

shown in the figure below (Figure 2, a-c).

A series of listric thrust faults due to compressional shortening and thickening of the crust along with

fault propagated folding is observed in the sequences T1S, T2S and T3S suggesting strong tectonic

activities during the Late Eocene to Middle Miocene.

3. Methodology

The evolutionary model has been constructed by cross section balancing. Cross section balancing

technique is a quantifying tool for geological structures [14]. Near shore sections have been

interpreted and the resulting cross sections are used in the study. The balanced cross section can be

tested for geological validity, structural balance and consistency of the internal geometry.

Retrodeformation of the cross section is carried out in order to refine the subsurface faulting

interpretation.

4. Results and Analysis

The balanced and restored cross section (since both seismic lines show similar structures, only Line 1

was used) (Figure 3) is shown below and is based on the structural styles, geometry and geological

validity. The proposed model encompasses the following stages.


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Fig. 3: a) T4S has been deposited during the Middle Miocene period; b) The post uplift period showing faulting

due to uplift and after erosion of T3S, T2S and T1S; c) The pre uplift period showing possibility of T3S as a

piggyback basin within an accretionary prism.

a

b

c

T1S- Middle

Eocene to

Early

Oligocene

sediments.

T2S- Early

Oligocene to

Early

Miocene

sediments.

T3S- Early

Miocene to

Middle

Miocene

sediments.

T4S- Middle

Miocene sediments.


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4.1. Middle Eocene to Early Oligocene

The Eocene period witnessed southward to South-Eastward subduction of the proto-South China Sea

oceanic crust beneath Borneo continental plate [4], [6] and [15]. During subduction, the upper crustal

sediments are scraped from the subducting plate and accreted against the overriding plate (Figure 4)

[16].

Fig. 4: Subduction of proto-South China Sea oceanic crust below Borneo.

4.2. Late Oligocene to Early Miocene

As subduction continued the sequence T2S was further combined atop on to the accreted T1S

sediments forming an accretionary wedge (Figure 5).

Fig. 5: T2S accreted along with T1S sediments as subduction continues.


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4.3. Early Miocene to Middle Miocene

This time frame was eventful in the basin formation process. Deposition of T3S sediments

commenced in the form of a piggyback basin (Figure 6). At a plate convergence point, observations in

the past from other basins, has shown the subducted lithosphere detaches from the lithosphere at the

surface often after closing of an oceanic basin. Due to decrease in slab pull forces, a period of uplift of

the surface occurs (Figure 7) [17]. Tectonic uplift of T3S sediments in the Southern and Southeastern

parts of the study location is observed from distribution pattern and topography. This affected even

the onshore area. The older sequences T2S and T1S were affected resulting in uplift and erosion of

these sediments as well [4].

Fig. 6: Piggyback basin evolution with infill of T3S sediments.

4.4. Late Miocene to Pleistocene

The younger sequences T4S and T5S were deposited atop the older three sequences and they do not

bear any structural deformation and do not hold any significant exploration prospects.

Subduction of oceanic lithosphere beneath NW Borneo, commenced between Paleocene and Middle

Eocene, extended to Early Miocene has been documented by the previous scholars [18], [19], [20],

[21], [22] and [23]. This tectonic process controlled the development of an elongated NE-SW trending

basin [24]. Deposition of the thick sedimentary sequences in the Sarawak region coincides with the

timing of subduction and basin by formation (Table 1).

By the method of cross section restoration, the proposed model has attempted to throw light on the

evolution of the near shore Tinjar and Balingian provinces as part of an accretionary wedge taking

into account the subduction process.


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Fig. 7: Uplift of surface above slab detachment (modified after Buiter, 2000).

5. Conclusions

Tectonism has played a crucial role in controlling the formation and development of the Sarawak

basin which could also be deduced from the above results. In this research, an evolutionary model for

the nearshore Tinjar and Balingian provinces has been proposed using nearshore seismic lines and

cross section balancing and restoration technique.

The resulting model depicts the subduction of proto-South China Sea oceanic crust beneath the

Borneo plate during Eocene to Middle Miocene. The older sequences (T1S and T2S) are accreted on

to the Borneo plate as a wedge and the formation of younger sequences (T3S) formed as piggyback

basins. The sequences above T3S (T4S and younger) was deposited over the piggy back basin after

the older sequences had subsided.

Through the model, an attempt has been made to illustrate the course of formation of the nearshore

areas of Tinjar and Balingian provinces. Even though, many works have been carried out in the past

on the evolution of the Sarawak basin, in light of the current analysis and the known structural

complexity & variability of the region, we recommend the evolution of each tectonic province within

Uplift

Slab detachment


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Sarawak should be discreetly and prudently studied in order to understand how tectonics has played a

major role in shaping the various vital parts of an extensive basin.

Acknowledgments

The author(s) would like to thank Prof. Dr. Manuel Pubellier, Dean of Faculty of Geosciences and

Petroleum Engineering, for his timely advice & support and Universiti Teknologi PETRONAS for

financial support.

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