Evaluation and models of cenozoic sedimentation in the South China Sea

ELSEVIER

Tectonophysics 235 (1994) 77-98

Evaluation and models of Cenozoic sedimentation in the South China Sea Wu Jinmin Second Marine Geological Investigation Brigade, Ministry of Geology and Mineral Resources, South China Institute of Oceanology, Chinese Academy of Sciences, 164 W Xingang Road, Guangzhou 510301, People’s Republic of China

(Accepted 13 December 1993)

Abstract The South China Sea is a marginal basin with the most complex geological structures in the west Pacific region and is characterized by the extensive distribution, large thickness, diversity and complicated evolution of the Cenozoic sediments. The author believes that studies of the sedimentary features and evolution of the Cenozoic are fundamental to understanding the Cenozoic evolution of the South China Sea, and that the definition of sedimentation models will be helpful in inferring the basin development at early stages, thus defining the basin patterns and guiding hydrocarbon exploration. This article, based on data obtained from hydrocarbon exploration in the South China Sea, together with related references, analyses the characteristics of Cenozoic sediments in the South China Sea and infers their evolution and models of formation. The Cenozoic in the South China Sea is divided into eight sedimentary provinces. A brief introduction is given to changes in sedimentary environments of the South China Sea during the Eocene, Late Oligocene-Early Miocene, Middle-Late Miocene and Pliocene-Quaternary. The line linking Taiwan and Natuna, as this article points out, was the sea-land boundary of the South China Sea in the early Cenozoic. The transgression cycle is dominant west of this line, while regression prevails to the east. During the Cenozoic, five different patterns of sedimentation existed in the South China Sea: offshore lake, shallow sea-deep sea, transgression, regression and platform. The former two belong to the lower cycle, whilst the latter three to the upper cycle. The time and spatial variations in the sedimentary sequence of these patterns is also discussed.

1. Introduction The South China Sea is a marginal basin and has the most complex tectonics found in the western Pacific Ocean. The study of its origin has recently received much attention. The study of the signature and evolution of Cenozoic sediments is fundamental to the determination of the Cenozoic evolution of the South China Sea. In the past 20 years, with the rapid development of

hydrocarbon exploration, abundant borehole and seismic data have been accumulated, which have made it possible to find solutions for the abovementioned topics. However, the lack of data published by the various countries and oil companies involved and the lack of information exchange

has formed a barrier to research. Research during the 1970s and 1980s in this region was mainly concentrated on the southern South China Sea, particularly on the stratigraphic correlation of the

0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0040-1951(93)E0277-2

U! Jinmin / Tectonophysics 235 (1994) 77-98

78

China Sea. These differences are revealed in articles of Wu (1981, 1985, 1987, 1988) and Jin (1989). This article, based on exploration results from the northern South China Sea and integrated with data already published, focuses on analyzing the sedimentary features, evolution and models of deposition of the Cenozoic South China Sea. The author believes that this study will be of

Cenozoic between basins and a study of sedimentary features in the basins (e.g., Pupilli, 1973; Woolands and Haw, 1976; Ho, 1978; Epting, 1980; ASCOPE 1981; Beddoes, 1981). Hydrocarbon exploration in the Cenozoic basin of the northern South China Sea suggests significant differences in Cenozoic sedimentary features exist between the southern and northern regions of the South

102”

i

I

in”

22” Sedimentary basin .... _.,.. m

ocean basin . ..e Oil fiild . . . . . . . . . . .x Gos fild

. . . ..*.r*..

e-t,

102”

\

I

I

KAL IMAN

1 IO”

Fig. 1. Cenozoic sedimentary basins of the South China Sea showing oif and gas fieids. After Wu (1987).

19

W Jinmin / Tectonophysics 235 (1994) 77-98

ognized (Wu, 1985, 1987, 1988) (Fig. 1). Drilling and seismic data suggest that most of these basins have well developed Cenozoic sequences (Fig. 2). In some basins the ages of the sequences are not clear because no drilling has penetrated the lower Cenozoic system. The greatest thickness of this system is over 1 X lo4 m. The unmetamorphosed upper Cenozoic sediments in the eastern basins even reach 1.5 x lo4 m. Sediments of this period in most of the sedimentary basins mainly include

use for further petroleum exploration, the study of tectonic movements and the palaeogeographic reconstruction of the South China Sea.

2. Features and distribution of the Cenozoic sediments Up to the present, about 43 Cenozoic sedimentary basins (or basin groups) have been rec102”

110”

SOUTH

118”

CHINA

TA IWAN I

22

Bank or shoal (small,lorge)

. . . . ..I.::

Reef (small,Iarge).

14

. . . . xC3

Well . . . . . . . . . . . . . . . m

u

Xisha-Nan&

A.//n

/

6

IUL

I IV

118

Fig. 2. Stratigraphic correlation of major Cenozoic Basins in the South China Sea. The locations are shown in Fig. 1.

22”

80 Y INGGEHAI

0.6 km

I +-2

Fig. 3, The distribution of Cenozoic sedimentary areas of the South China 2 = Zhu 5; 3 = Wan 2; 4 = Ying 2; 5 = Ying 9; 6 = Xiyoug 1: 7= Sampaguita;

+I

Sea. Some important wells are shown: f = CF-1; 8 = AQ-4; 9 = 16-B-1; ID = 16-C-I; If = 4-B-l.

81

W. Jinmin / Tectonophysics 235 (1994) 77-98

normal elastic deposits; volcanic detritus is present in larger amounts in the eastern basins and carbonate rocks are mainly found on the islands of Dongsha, Xisha, Zhongsha, and Nansha, and offshore of Palawan Island. Based on the distribution and features of the Cenozoic sediments, eight sedimentary provinces are defined (Fig. 3). 2.1, Beibuwan -Zhu3 province Cenozoic sediments in this province are dominated by a complete sequence of detritus, with apparent rhythms and cyclic features. The lower Tertiary mainly includes great thicknesses of continental elastics, the largest thickness on the seis-

mic profile reaches 6000 m. The upper Tertiary and Quaternary mainly comprise a relatively thin layer of neritic detritus (Fig. 4). 2.2. Northern province This province mainly includes the region north of a line strtching from Xisha Island to Bijia sediments seamount. Palaeocene(?)-Oligocene are fairly thin (about 3000 m>, with continental facies-transitional facies in the separate depressions. The early Tertiary sediments are probably partially absent. The upper Oligocene-Quaternary are represented by a transitional-broad marine facies (a thickness of 9000 m has been ob-

BRUNEI-SABAH ZENGMU WEST

MALAY

SABAH

L. 44. Miocene M. U. Miocene

L. M.

Miocm

NON-SCALE

‘..C.’

Pre - Tertiary

+>j

Fig. 4. Cenozoic stratigraphic column in northern edge of the South China Sea.

W. Jinmin / Tectonophysics 235 (1994) 77-98

82

served on the seismic profile), with the influence of marine facies increasing from west to east. Clastic rocks are dominant, with carbonate rocks locally found in the Early Miocene. The Upper Miocene is partially absent in some basins (Fig. 4).

nated by terrestrial elastics and earlier sequences have not yet been penetrated. The transgression occurred fairly late, thus the marine facies is not found until the end of the Early to the Middle Miocene. The transgression horizon ascends from east to west. The Upper Miocene is absent in the eastern region (Fig. 5).

2.3. Southwestern province 2.4. Zengmu province This province is located in the sea, west of Natuna, and has features similar to that of the Northern province. However, carbonate rocks are not well developed here; the Tertiary is domi-

This province is located in the northern Zengmu basin and Wananxi basin, no early Cenozoic stratigraphy has yet been seen. However, the

N.PALAWAN

0

0.3

t 0.6 km

LIYUE

NON-SCALE

4124m

Fig. 5. Cenozoic stratigraphic column in the southern and southwestern edge of South China Sea. Key as in Fig. 4.

83

W Jinmin / Tectonophysics 235 (1994) 77-98 CAGAYAN

0

0.6

t 1.2km

CENTRAL

VALLEY EAST

WEST

\

\

EASl

W. Jinmin / Tectonophysics 235 (1994) 77-98

84

(Palawan trough). The Palaeocene-Eocene sequence (Ranjang Group of Sarawak and Cracker Formation of Sabah) is composed of flysch and ophiolite formations, the former have undergone slight metamorphosis to phyllite and slate and all the rocks are strongly folded and tilted, forming an imbricate structure. The upper OligoceneQuaternary sequence consists of marine elastic

late Cenozoic sequence here appears rather thick and the development of Middle-Upper Miocene carbonate rocks are a clear feature (Fig. 5). 2.5. Southern province This province is located in the region south of the Zengmu submerged reef and Nansha trough

11R"

1 ln”

lrv-JO

Iooooo

Fluvial

22

. El

SOUTH

Lacustrine

El

:

.:.:::;:I:.: ....I.. .::I:: El

CHIN,

Delta Neritic

BathyaI

Emerged

landmass

400km

6

1 lo-

102” Fig. 7. Cenozoic

stratigraphic

column

on the eastern

I I8 edge of the South China Sea. Key as in Fig. 4.

85

W. Jinmin / Tectonophysics 235 (1994) 77-98

rocks, while the Middle-Upper Miocene is especially well developed and comprises regressive delta deposits (Fig. 5). 2.6. Xisha -Nansha province Island, This province includes Zhongsha northern Palawan and adjacent areas. It is char-

102”

acterized by fairly thick Cenozoic sediments and dominant terrestrial-marine elastics. The lower Cenozoic sequence appears to be absent on the platform, only occurring in depressions, while the upper Oligocene-Quaternary deposits (littoralneritic carbonate rocks) cover all or part of the platforms. In the depressions neritic-bathyal faties elastic rocks are present (Fig. 6).

118”

110”

0

ooo O. Flwial

El

Lacurtrine littoral

Ezl

Plotformal carbonate

I ::::;:‘: .*:.:._, 1.

I” -

I3 r-l

.

Neritic

Emerged landmass

?

SOUTH

CHINA

SEA

INDO -CH INA 400km

Fig. 8. Paleoenvironmental

map of the South China Sea in the Eocene.

n

= wells shown in Fig. 3.

W Jinmin / Tectonophysics 235 (1994) 77-98

86

2.7. Eastern province

observed with carbonate rocks). The maximum thickness exceeds 15,000 m (Fig. 7).

This province includes the island of Luzon and its western coastal region. Similar to the Southern province, the lower Tertiary of this province consists of folded and metamorphosed ophiolite and flysch. The upper Oligocene-Quaternary sequence is dominated by elastic rocks rich in volcanic elastics or a wide diversity of rocks (locally . n-0

22’

1:

* :

I

2.8. Deep-sea basin province This province is located in the abyssal plain and is underlain by oceanic crust. The thickness of sediments is less than 3000 m, decreasing from south and north towards the centre. The layer

. .

*no

..O

IDelta

SOUTH CHINA _-----------

400k m

INDO

_ CH

SEA

INA

6

6”

Fig. 9. Paleoenvironmental

map of the South China Sea in the late Oligocene.

W = wells shown in Fig. 3.

87

W. Jinmin / Tectonophysics 235 (1994) 77-98

soft and unconsolidated deep sea ooze and volcanic accumulations of Late Miocene-Quaternary age.

velocity and sedimentary signatures on the profile suggest that only upper Oligocene-Quaternary sequences are developed in this province. The lower part comprises a sequence of late Oligocene-Middle Miocene sediments overlying the rough oceanic basement, its thickness changes remarkably. These deposits are probably composed of terrestrial elastics, volcanic elastics and carbonate rocks. The upper part consists of mainly 102”

3. Cenozoic sedimentary evolution It is known that the sedimentary environment of the South China Sea has changed since the

110”

118”

m

CHIN,

SOUTH

22

22”

Neritic’ Plotformol corbonote

‘I * .;pShenhush.

-

-

-

--/

>--------I

ha Is.

-tr’

-

\

-

-

-

-

---\\\91

Is.-

Zhgsha

(

400km

14

INDO-CHINA

ILK\

(I!.

. ‘! -

-

6

6”

v\nfi\

I 102”

Fig. 10. Paleoenvironmental

I

qu-I

KALIMANTAN I

110”

map of the South China Sea in the Middle Miocene.

I 118”

n = wells shown

in Fig. 3.

88

K Jinmin / Tectonophysics 235 (1994) 77-98

late Oligocene, from the sedimentary environment of the Palaeocene-Eocene. The sedimentary environment in the Palaeocene-Eocene also differed between the northern and southern South China Sea. Here we divide the development of the Cenozoic sediments of the South China Sea into 4 major stages, based on time and spatial variations. 3.1. Sedimentary emironment

of the Eocene

If we draw a line between Taiwan and Natuna and place land to west and sea to the east of this !ine, we obtain a picture of the South China Sea during the Eocene (Fig. 8). On the northern Continental margin was a series of alternating rises and depressions. From west to east, the rises were: the Wanshan-Hainan rise; the ShenhuXisha rise and the Penghu-Dongsha rise. Between these rises were a series of small sags. Fluvial and lacustrine deposits are developed in these sags, which thin eastwards one by one. The present Taixinan basin may have been part of the Penghu rise at that time, only the western part of this received f’luvial and lacustrine deposits. Southwest of Hainan Island, the Yinggehai basin may have consisted of a couple of NW-SE trending lake basins. The southern margin of the South China Sea, bordering the Natuna rise, showed a remarkable difference in the sedimentary environments of its eastern and western sides. To the west, the Guff of Thailand may have been a land region, with river and lake sediments developed in the separate basins of Pattani, Malay, western Natuna and Mekong. To the east, deep-sea flyschs were developed along Sarawak, Brunei, Sabah and Southern Palawan (Haile, 1974), while neritic-bathyal elastic sediments were developed in a ring which probably covered the Liyue (Reed) Bank and at the western edge of northern Palawan. 3.2. Sedimentary environments cene-Early Miocene

of the late Oligo-

Following a structural uplift or sea-level drop (40-32 Ma), which trended S-N, the South China Sea region began to subside, with the northern

margin suffering from widespread transgression, which has persisted since the late Oligocene. The three rise zones (Wanshan-Hainan, ShenhuXisha and Penghu-Dongsha) shrunk in area and small depressions began to combine and receive littoral-neritic sand deposits (Fig. 9). The transgression generally runs from southeast to northwest, the transgressive layers young in this direction. For example, the earliest marine sediments in the Taixinan basin probably date from the NP22 Zone (Huang, 19821, the Zhujian~ou basin saw the development of sediments with an age equivalent to nannofossil Zone NP24-NN3 and the northern transgression boundary was near well Zhu-5. At that time the Zhu-1, Zhu-2 and ‘Zhu-3 depressions began to combine into a single basin (Jin, 1989). Meanwhile, seawater also entered the Qiongdongnan basin from the Xisha trough, where nannofossil Zone NP25 has been recorded. The Qiongdongnan basin was connected with the Yinggehai basin by seawater. At the northwestern brink of the Yinggehai basin, sediments of transitional facies and deltaic facies have been found in the Hanoi Depression of Vietnam (Skorduli et al., 1983). Transgression in the Beibuwan basin occurred relatively Iate because of the block formed by the Hainan Rise. A non-marine fossil assemblage, characterized by pollen spores and freshwater ostracoda are found in the upper Oligocene sequence. The marine Xiayang Formation produces NN3-NNl nannofossiIs, equivalent to upper part of the Lower Miocene (Duan and Huang, 1987). Carbonate rocks and organic reefs are widely developed on the Dongsha rise, Shenhu rise and Xisha rise and at the southern edge of the Hainan rise. Transitional sediments of fluvial-Iacustrine facies and littoral swamp facies were predominantly developed in the region west of Natuna, on the southern margin of the South China Sea (Woollands and Haw, 1976). The infiuence of marine facies increases in the upper layers. The central part of various basins are mainly developed with fine, dark mudstone, bearing coal and plant fragments. Planktonic, foraminifera-bearing marine interbeds may appear in the southeastern part of the Malay basin, 64rmitage and Viotti, 1977). Two large deltas

89

WO Jinmin / Tectonophysics235 (1994) 77-98

were developed in the Natuna basin and Mekong basin. Meanwhile, littoral carbonate rocks were dominant on rises in the Nansha region, east of Natuna Island, in the troughs and valleys between the carbonate rock platforms are neriticbathyal elastic rocks. Because of the uplift of the island of Kalimantan, the southern boundaries of the Zengmu basin and Brunei Sabah basin gradually shifted northwards. In these basins elastic sediments are found, from south to north, of coastal facies, littoral-neritic facies and bathyal facies.

AGE

1 CYCLE 1

LITHOLOGY

Pliocene

1 111

AGE

CYCLE

Early

--

II

leistocenc

I

Pliocene

Oligocene

I

p.:/

~~

Fig. 12. The upper sedimentary cycle (Cenozoic) of the northern South China Sea (Wu, 1987).

-

-

Fig. 11. Paleoenvironmental map of the South China Sea in the Pliocene. H = wells shown in Fig. 3.

3.3. Sedimentary Miocene

environment

of Middle-Late

During the Middle Miocene, the region under transgression in the northern margin of the South China Sea continued to expand whilst the area of rises shrunk (Fig. 10). The coastline stretched northward to the present 50 m bathymetric line. Most areas were dominated by elastic rocks, except for a few rises where carbonates developed. Some deltaic deposits were developed in the regressive stage. During the early stage of the Late Miocene, the northern margin of the South China Sea was uplifted extensively, most of the rises (such as Dongsha rise and Shenhu rise) again emerged and underwent subsequent erosion. A sedimentary hiatus occurred extensively along the edges

W. Jinmin / Tectonophysics 235 (19941 77-98

90

of basins, the nannofossii NN9-NNlO zone and lower NNll zone are absent (Jin, 1989). Meanwhile, on the southern margin of the South China Sea, the Nansha region (including the Nankang platform of the Zemmu basin) was still in a littoral and neritic environment. The platforms chiefly bear carbonate rocks and organic reefs encircled by elastic deposits from fairly deep water. The mouth of the Baram River in the Brunei Sabah basin developed deltaic sediments of late Middle-Late Miocene age, whilst the region off Sabah contained turbidite fan deposits (ASCOPE, 1981). In the Gulf of Thailand, the ancient coastline migrated westward to the middle part of the Malay basin. Marine sediments were developed in the eastern Mekong basin and Malay basin, the rest of the region had a transitional facies. 3.4. Pliocene-Quaternary

sedimentary environment

During the early and middle Pliocene, transgression reached a climax on the northern margin

of the South China Sea (Wu, 1985). The coastline was roughly similar to the present one. Most of the rises were submerged, except Hainan Island and Dongsha Island (Fig. 11). The PlioceneQuaternary consisted chiefly of elastic shelf deposits. Violent subsidence occurred south of Hainan Island and in southwestern Taiwan, where bathyal mudstones up to 3,000-5,000 m thick were developed (Chou, 1971, 1973). On the southern margin of the South China Sea, the ~ntinuous uplifting of Kalimantan Island caused the further northward migration of the sedimentation centres of the Zengmu and Brunei Sabah basins, where elastic sediments of littoral and neritic facies were developed. The Liyue basin and Northern Palawan basin accumulated neritic carbonate rocks. The Pattani, Malay, western Natuna and Mekong basins, west of the Natuna rise, were completely overwhelmed by shallow seas and developed elastic sediments of littoral and neritic facies. Ever since the Quaternary, the region east of the Natuna rise has been subsiding dramatically; the thickness of Quater-

Fig. 13. The upper sedimentary cycle in the Baram Delta of Kalimantan. After Ramli (1985).

W. Jinmin / Tectonophysics 235 (1994) 77-98

nary sediments can be up to 1,500-2,300 (ASCOPE, 1981).

m

3.5. Cenozoic sedimentary cycle Two sedimentary cycles - the upper cycle and lower cycle - have been determined, based on the unconformity between the lower Oligocene and upper Oligocene in the Cenozoic sediments of the South China Sea. Existing data suggest the existence of a land to sea transition in the Cenozoic sediments in the northern South China Sea basin from the lower to the upper part. The lower cycle was a entire cycle of transgression-regression whilst the upper cycle was an apparent transgressional cycle (Fig. 12). The transgression, starting in the late Oligocene, reached three climaxes: the late stage of the Early Miocene, the middle stage of the Late Miocene and the Early Pliocene. The Early Pliocene transgression was the largest and is characterized by abundant fossils and a thick mudstone layer extensively overlying the older strata. The sedimentary features of the southwestern South China Sea are similar to those of the northern South China Sea. However, the transgression here started relatively late and regression in the Middle Miocene is more probable. The formation of sedimentary basins on the southern margin of the South China Sea was later than that of the basins on the northern margin. Regional metamorphism and folding occurred in Palaeocene-Eocene sequences in northern Kalimantan and its offshore region (SEATAR, 1980). These stratigraphies formed the basement of various basins. Under the influence of continuous uplifting of the island of Kalimantan, regressive cycles were dominant in the development of sedimentary sequences in those basins. Since the Eocene, eight regressive cycles have occurred in the upper sedimentary cycle of the Baram Delta (ASCOPE, 1981). Deltaic sediments were mostly developed during the major regression in late Middle-Late Miocene (Fig. 13). 4. Cenozoic sedimentation

The diverse geological environments and complex evolution history of the South China Sea

91

have resulted in five major sedimentary patterns: offshore lake basin, neritic-deep sea; transgressive; regressive; and platform. The former two patterns are in the lower sedimentary cycle and the latter three are in the upper cycle. 4.1. Offshore lake basin This is mainly seen in some basins in the northern and southwestern South China Sea and was mainly developed during the early Cenozoic (Palaeocene-early Oligocene). The sequence changes generally from fluvial alluvial fan to lacustrine facies (including turbidite) to fluvial plain and marsh deposits, from the lower to the upper part. These basins are characterized by separate, small sags, their multiple sources, and a ringed facies distribution seen in planar view (Fig. 14). A few transgressive layers are seen in sediments of the later two stages. 4.1.1. Alluvial fan sediments This pattern of sediments are mainly seen at the steep flank of a basin, where major faults are often developed. The fault activity resulted in fairly large topographic differences. Thus alluvial fan sediments could rapidly accumulate when the river current or floods with proximal detritus entered the basin margin. The fans appear wedge-shaped on the seismic profile, with disordered reflection, variable amplitude and a divergent structure. The thickness of sand bodies changes dramatically, which reflects sediment accumulation during the initial faulting of the basin. This type of sediment is typified by the Changliu Formation of Beibuwan and the lower member of the Liushagang Formation near the Weixinan major fault. Sediments there are a suite of brownred, massive and conglomeratic, coarse elastic rocks. Besides quartz and feldspar fragments, large quantities of rock fragments are found in these elastics. They show poor sorting and spherical mud cements with iron contamination are present. Strong corrosion and growth textures appear on the surface of quartz crystals, which indicates a high-energy environment of rapid proximal accumulation. Similar sediments have

92

K Jinmin / Tect5~physics

been found in the Pearl Zhujiangkou basin (called the Shenhu Formation). 4.1.2. Lacustrine se&men ts The basin expanded and deepened as the topography changed. Small, separate sags were connected to form a large basin. Sediments in most areas of the basin usually featured fine elastic rocks of the neriti~-bathyal lacustrine facies, except for the alluvial fan sediments at the basin edge. From the basin edge to basin centre, sediments saw a decrease in sandstone and an increase in mudstone, and the thickness of the strata became stable. Reflectors with lowmedium amplitude, medium-fine continuity and subparallel-parallel signature are often seen on the seismic profile and, due to the non-reflection, a ‘blank zone’ is found when areas dominated by mudstone are encountered. For example, strong reflectors with 2-3 continuous phases have been

235 f’l9941 77-98

observed at the bottom of the Liushagang Formation in the Beibuwan basin, where sediments are interbeds of sandstone and shales. The middle part is a large suite of mudstone and shale with mainly weak reflection. Mudstone often contains index layers, such as oil-bearing shale and siderite, and a rich organic content; thus these are good hydr~arbon-bearing rocks. 41.3. Lucustrine turbidites Lacustrine turbidites have been penetrated in the lower member of the Liushagang Formation in the Beibuwan basin, among others. These rocks are generally developed in the middle part of a basin. Sandstone and sandy conglomerates show a typical 3ouma sequence and other textures. Lacustrine turbidites are also observed in the lower Tertiary in the Zhujiangkou and Qiongdongnan basins, or in upper Tertiary of the Yinggehai basin.

Fig. 14. Cenozoic sedimentary model in the offshore lake basin of the South China Sea.

93

W. Jinmin / Tectonophysics 235 (1994) 77-98

4.1.4. Fluvial deltaic plain and fluvial (lacustrinek paludal sediments

The compensatory filling of the basin and uplift of the adjacent erosive region often results in lake, swamp or fluvial plain environments, in which river (braided river and river meanders are dominant), flood plain, delta, lake and swamp subfacies are developed. An example of this is the sandstone of the upper Oligocene Gabus Formation in the western Natuna basin. Its lower part is a typical massive sandstone of a river channel, with grains changing from coarse to fine; whilst its upper part is deltaic sediments with interbedded sandstones and shales. The Weizhou Formation in the Beibuwan basin is also composed of fluvial plain sediments which were deposited under oxidizing conditions. The sedimentary environment of the Enping Formation in the Zhujiangkou basin changes upward from braided river to meandering river to fluvial sandstone.

EPOCHS

AGE

CYCLE

SE

River (lake) swamp coal layers, which are important gas-bearing layers, are fairly well developed in this environment, such as the Yacheng Formation in the Qiongdongnan basin. 4.2. Shelf neritic-deep sea sedimentation This pattern of sedimentation mainly developed during the Palaeocene-early Oligocene in the Liyue basin. Its sedimentary sequence, from the lower to the upper part, changes from littoral and neritic facies to bathyal facies, then to littoral facies, and generally appears as an entire cycle of transgression-regression. Deltaic sediments and shelf carbonate rocks may be developed in littoral and neritic facies, which are characterized by their unidirectional material source and belted distribution. It has been confirmed that the bathyal sediments of the middle part are the major gas-generating layers, while coarse elastic

NW

106YEARS

EROSION AND/ OR NON -DEPOSITION

Fig. 15. Cenozoic sedimentary model of a transgressive type basin in the South China Sea.

94

W. Jinmin / Tectonophysics 235 (1994) 77-98

sediments of the lower and upper part are the main gas-producing layers. 4.3. Transgression basin sedimentation Transgression basins are mainly seen in the northern and southwestern South China Sea. Their development occurred during the late Oligocene to present. The predecessors of those basins were mostly offshore lacustrine basins, which, after having undergone tectonic uplift, and because of large-scale subsidence, tilted seaward, causing constant transgression. The sedimentary sequence, from the bottom to the upper part, was generally: alternating marine and terrestrial faties, littoral and neritic facies, neritic facies and, finally, bathyal facies. The material provenance changed from multiple-source to single-source, dominated by terrestrial sediments. Thus the faties zones are generally parallel to the coastline (Fig. 15). 4.3.1. Marine-terrestrial alternative sediments At the beginning of the transgression, the basin appeared semi-enclosed and there were repeated transgressions and regressions of seawater. This is expressed in the stratigraphy as a few marine interbeds in terrestrial strata. Three sedimentary environments are recognized based on palaeography. The first type of sedimentary environment was located at the mouths of large rivers and is characterized by dominant deltaic and littoral sediments. The influx of massive terrestrial materials often results in a wide facies zone in planar view and prevailing elastic deposits, such as the late Oligocene Zhuhai Formation in the Zhujiangkou basin, dominated by deltaic sediments. River channel sandstone, deltaic front sandstone and sandbars, as well as mudstone and river swamp coal layers, of the bay facies and carbonate rocks are absent but dolomitic-lime sandstones, etc., developed. The rock facies show a dramatic variation both laterally and vertically. Fossils mainly include pollen spores and euryhalinous Foraminifera, nannofossils and pyrrophyta are also sparsely seen. Another type of environment occurred in the semi-enclosed bays, such as the eastern part of

the Zhujiangkou basin, which formed from the blocking of the Dongsha rise. Bay facies and littoral facies sandstones and shales are common. Meanwhile, in the Pattani basin and Malay basin in the Gulf of Thailand, due to the blocking of the Natuna rise, marine-terrestrial transitional facies were developed until the Early and Middle Miocene. Their positions are higher than that of similar sediments in various basins of the northern margin. The delta front facies of the Me Nam River during the Early Miocene might have chiefly existed in the Malay basin, while the Pattani basin during the same time might have developed delta plain facies and swamp facies. Petroleum exploration has proved that the oil and gas-producing layers in these basins are related with these facies zones. The third type of sedimentary environment is far away from larger rivers and has a fairly steep sea floor. Its sedimentary facies zone appears relatively narrow in planar view and coastal plain facies and littoral facies dominate. Both elastic and carbonate rocks are developed. An example of this is the late Oligocene Lingshui Formation, where, from north to south, sandstones and shales of coastal plain facies, organic elastic limestone of bank facies, calcareous and dolomitic sandstone and a shallow littoral facies were developed. Transgression in the Beibuwan basin started relatively late because of the barricading of the Hainan rise; alternating marine and terrestrial facies, dominated by shale and sandstone, did not develop until the beginning of the Early Miocene. Terrestrial sediments are generally characterized by a couple of features, such as the range of organisms and red or variegated mudstone layers. In addition, asphalt or coal lines are often observed in dark mudstone layers. 4.3.2. Sediments of littoral and neritic facies The littoral and neritic facies of elastic types mainly include sandstone (conglomerate) and shale. The sandstones often contain glauconite or calcareous matter. Thin coal layers in some places are interbedded in the littoral sediments and foraminifera and nannofossils are abundant. Deltaic sand bodies are often developed at large river mouths, due to local regressions, such as the

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are gradually replaced by mudstone as the water depth increases.

Hanjiang Formation in the Zhujiangkou basin. Advancing textures, such as obliquely crossingcutting layers, can be seen on the seismic profile. In planar view these layers have an arc-shaped distribution. Littoral and neritic sediments of carbonate type mainly include limestone and organic reefs. Two patterns are further divided, based on palaeogeographic environment: the first pattern of carbonate rock is developed around ancient ridges or on lower ridges (including biogenic bank facies and reef facie& This rock has better porosity and permeability and often forms a fine hydrocarbon reservoir. Another pattern is developed in the middle part of a half-enclosed basin and consists of mudstone or marl rich in organic content, which can serve as oil(gas)-bearing rock. Carbonate rocks in transgressive basins generally occur at the early stage of the transgression in places with a shortage of terrestrial elastics and

LEGEND

4.3.3. Sediments of neritic-bathyal facies Transgression in the northern South China Sea reached its climax during the early and middle Pliocene. During this period sediments dominated by neritic-bathyal mudstone may be up to thousands of metres thick. These are relatively monotonous in lithology. Abundant planktonic foraminifera and nannofossils are found in these rocks. At the base of the steep slope of the basin, turbidite sandstones are commonly seen. Neritic-bathyal sediments are mostly developed in the Yinggehai basin, Southeastern Qiongdongnan basin, Southern Zhujiangkou basin and Taixinan basin. A similar sedimentary environment may have persisted until the Quaternary. This type of extensively overlapping, thick mudstone can provide a very favourable cap rock. No bathyal

(Fig.

3-6 >

... . I

Sandstone

..-.. I - .. ... .

El

Silty mudstone/cloy

Cool /Lign i te

I

--_ _ - - -

E[

Mudstone cloy

Limestone

. . .’ .*.. . .

Shole B

El

Eizzl Oil

Shale

Coral

Marl

Limestone

Tuffoceous

Tuff

xxrdstone

Granite

I

tc

*

Volcanic

tc

tc

rock

Gron i te

gneiss

Fig. 16. Cenozoic

Siltstone

sedimentary

model of a regressive

type basin in the South China Sea.

96

OVI XONOHZ

W. Jinmin / Tectonophysics 235 (1994) 77-98

sediments were found in the southwestern part of the South China Sea, due to the Natuna rise. 4.4. Sedimentation model for a regression basin This pattern is seen in some basins in the southern and eastern South China Sea where the sedimentation centre of the basins migrates constantly in one direction. The distribution of the facies zone in planar view is roughly parallel to the coastline (Fig. 16). The sedimentary sequence, ranging from old to new, is generally: bathyal facies-neritic facies-bathyal facies-deltaic facies-littoral coastal plain facies-terrestrial facies, which is a regressive cycle. 4.4.1. Bathyal facies This facies was mainly developed during the Eocene-Oligocene and the Early and Middle Miocene. The former chiefly includes flysch sequences, such as the Ranjang Group in the Zengmu basin, the Mulu Formation and Cracker Formation in the Sabah basin and the Caraballo Group in the Cagayan basin. These rocks appear folded and metamorphosed. An example of the latter is the Setap shale in the Brunei basin. This shale consists of almost 5,000 m of an unmetamorphosed and mono-shale sequence. 4.4.2. Neritic facies This facies was mainly developed in the Lower Miocene and Pliocene and consists of elastic and carbonate rocks, such as sediments in Cycle I and Cycle II in the Balingina area of the Zengmu basin. 4.4.3. Deltaic facies The most studied Baram Delta sediments in the South China Sea were developed during the late Middle-Late Miocene regression. They consist of deltaic sandstone of the Miri Formation and Seria Formation, which is a reversed cycle composed of sandstone, siltstone and silty mudstone, coarsening upward. The sandstone contains sedimentary structural features, such as oblique bedding, cross-bedding and ripple marks. Foraminifera and plant fragments and thin lignite layers are also found.

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4.4.4. Littoral and coastal plain facies

This is mainly seen in Pliocene sequences and consists of sandstones and shales with coal interbeds. Sedimentary indicators, such as crossbedding, ripple marks, burrows and bioturbation structures, and jarosites are found. An example is the Liang Formation of the Brunei basin. 4.4.5. Volcanic rocks and volcanic detritus Volcanic rocks and volcanic detritus rocks were widely developed in the Cenozoic sequence in eastern basins of the South China Sea, which mainly include basic and intermediate lavas, volcanic agglomerate, tuffaceous sandstone and volcanic tuff. 4.5. Platform sedimentation This pattern is mainly present on high platforms caused by block uplift in the Xisha and Nansha regions. The whole process of sedimentation generally took place in a neritic-littoral environment, although there was also a certain influence from marine transgression or regression. Various sediments were formed, including platform carbonate rocks and biological reef sediments. An example of this is the Lyyue Formation in the Liyue basin (upper OligoceneQuaternary).

5. Conclusion The determination of the sedimentation pattern is helpful in inferring the early history of basin formation and can provide guidelines for hydrocarbon exploration. For example, the oldest strata drilled up to now in the Qiongdongnan basin are coal-bearing strata of fluvial and swamp facies in the Yacheng Formation. Sedimentary patterns suggest that the lacustrine facies and fluvial alluvial fan facies may have been developed before the deposition of the Yacheng Formation. Planar distribution of the facies zones and a vertical occurrence of a favourable sequence can be significant factors for choosing the region and layer of hydrocarbon exploration. Thus, constant improvement of the sedimentation

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model should be an important research.

W. Jinmin / Tectonophysics 23.5 (1994) 77-98

area for future

Acknowledgement Special thanks are owed to the Guangzhou Marine Geological Survey for some of their unpublished data used in this article; the author is also very grateful to Mr. Zhou Hua, Mr. Zheng Yi, Mr. Liu Baoming and Madame Li Guoying for their assiduous assistance.

References Armitage J.H. and Viotti C., 1977. Stratigraphic nomenclature -southern end Malay Basin. Proc. Indonesian Pet. Assoc. 6th Annu. Conv., pp. 69-94. ASCOPE, 1981. Tertiary Sedimentary Basins of the Gulf of Thailand and South China Sea: Stratigraphy, Structure and Hydrocarbon Occurrences. ASCOPE Secretariat, Jakarta, 72pp. Beddoes, L.R., Jr., 1981. Hydrocarbon plays in Tertiary basins of Southeast Asia. Energy, 6(11): 1141-1163. Chou, J.T., 1971. A preliminary study of the stratigraphy and sedimentation of the mudstone formation in the Tainan area, Southern Taiwan. Pet. Geol. Taiwan, 8: 187-219. Chou, J.T., 1973. Sedimentology and paleogeography of the Upper Cenozoic system of western Taiwan. Proc. Geol. Sot. China, 16: 111-143. Duan Weiwu and Huang Yougyang, 1987. Calcareous Nannofossil Biostratigraphy of the Tertiary in northern South China Sea. Proc. Int. Symp. on Petroleum Geology in Northern Continental Shelf area of South China Sea. Guangdong Petroleum Society and China Oil, Guangzhou, pp. 292-316. Epting, M., 1980. Sedimentology of Miocene carbonate buld, Central Luconia, offshore Sarawak. Geol. Sot. Malays. Bull., 12: 17-30. Haile, N.S., 1974. Borneo, Mesozoic-Cenozoic erogenic belts;

data for erogenic studies. In: A.M. Spencer (Editor), Geol. Sot. London Spec. Publ., 4: 333-347. Ho Kiam Fui, 1978. Stratigraphic framwork for oil exploration in Sarawak. Geol. Sot. Malays. Bull., 10: l-13. Huang Tinghang, 1982. Tertiary calcareous nonnofossil stratigrphy and sedimentation cycle in Taiwan. In: A. SalivarSali (Editor), Proc. 2nd ASCOPE Conf. (Manila, Philippines, 19811, Tech. Progr. Comm. ASCOPE ‘81, pp. 873886. Jin Qinghuan (Chief Compiler), 1989. Geology and Oil and Gas Resource of South China Sea. Geological Publishing House, 417~. (in Chinese). Pupilli, M., 1973. Geological evolution of South China Sea area; Tentative reconstruction from borderland geology and data. Proc. 2nd Annu. Convent. Indonesian Pet. Assoc. (Jakarta), pp. 223-241. Skorduli, V.D., Khudnk, M.V., Le Vai Ly, Nguen Ngok Ly, Sevostyachob, K.M., 1983. Geologicheshkoe stroenie i neftegazonosnostv khanoiskogo progiba. Geol. Nefti Gaza, 5: 55-60. SEATAR (compiler), 1980. International decade of ocean exploration-studies in East Asian tectonics and resources, a programme and review of research. 2nd SEATAR workshop (Bandung, 19781, 257~~. Woollands, N.A. and Haw, D., 1976. Tertiary stratigraphy and sedimentation in the Gulf of Thailand. SEAPEX, Offshore South East Asia Conf., pp. l-22. Wu Jinmin, 1981. Tertiary stratigraphy correlation and discussion on related questions in the Northern shelf of the South China Sea. In: Zeng Dingqian (Editor), Tertiary in the Northern Shelf of the South China Sea. Science and Technology Publishing House, pp. 179-201 (in Chinese). Wu Jinmin, 1985. The characteristics of the geological structures of the Tertiary basins on the continental margins in the Northern part of the South China Sea. In: M.J. Valencia (Editor), Geology and Hydrocarbon Potential of the South China Sea and Possibilities of Joint Development. Energy, 10(3/41: 359-372. Wu Jinmin, 1987. Types of Cenozoic sedimentary basins in South China Sea and their hydrocarbon potential. Proc. Int. Symp. on Petroleum Geology in the Northern Continental Shelf area of South China Sea. Guangdong Petroleum Society and China Oil, Guangzhou, pp. 130174. Wu Jinmin, 1988. Cenozoic basins of the South China Sea. Episodes, ll(2): 91-96.