Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China

Muritlr urxi Prtrol~rn

PII: SO264-8172(97100027-5

Geoloy?~, Vol. 14. No. 7,‘8. pp. 95 l-972. 1997 I(’ 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0264-X I72197 $17.00 + 0.00

ELSEVIER

Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China M. B. Allen* Cambridge University,

and D. I. M. Macdonald

Arctic Shelf Programme, Department of Earth Sciences, Cambridge Downing Street, Cambridge, CB2 3EQ, UK

Zhao Xun Chinese Academy

S. J. Vincent Cambridge University,

of Geological

Sciences, Beijing, China

and C. Brouet-Menzies

Arctic Shelf Programme, Department of Earth Sciences, Cambridge Downing Street, Cambridge, CB2 3EQ, UK

Received 26 August

1996; revised 19 March 1997; accepted

19 April 1997

The Bohai Basin is one of a family of early Cenozoic extensional basins that lie along the eastern margin of Asia from Russia to Vietnam. initial extension was probably triggered by subduction roll-back of the oceanic Pacific Plate from the Asian continent. There were two phases in the Bohai Basin’s rift history. The earlier, Paleocene-early Eocene phase resulted in the deposition of the Kongdian Formation and the fourth (lowest) member of the Shahejie Formation in a series of elongate half grabens. These half grabens have master faults with a NNE-SSW orientation. Secondary normal faults are typically clockwise oblique to the master faults, indicating a component of dextral transtension. Deposition was focused in the west and south of the present basin. These rocks are mainly alluvial fan and fluvial red beds. The architecture of the basin underwent an important change at ca. 43-45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half grabens as the Kongdian Formation, but the Bozhong Depression in the central part of the basin originated at this time, and became the major depocentre. The Bozhong Depression superficially resembles a pull-apart basin. It formed when continued transtension of the earlier Tertiary fault systems to the east and west created an extensional overlap between them. During this phase, the basin as a whole had a geometry with elements typical of both pull-apart and transtensional basins. Regional extension in many east Asian basins ended at the end of the Oligocene, probably because of the onset of transpression within eastern Asia, caused by the collision of Australia with the Philippine Sea Plate. Dextral transpression caused minor inversion of some of the earlier normal faults in Bohai, but as a whole the basin began to subside in a postrift phase of thermal subsidence that has lasted until the present day. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: transtension; fault; basin; Bohai; China

The Bohai Basin,? northeast China, belongs to a family of extensional basins of similar age and structural style that exist along the eastern side of the Asian continent from Russia to Vietnam (Fiyzrrc> I). Rifting began across a vast area of eastern China and adjacent areas between the latest Cretaceous and the Oligocene. This created an array of basins both onshore (Bohai, Jianghan, Nanyang.

Zhoukou, Subei/North Jiangsu) and otrshore (North Yellow Sea, South Yellow Sea. East China Sea, Pearl River Mouth, Beibu Gulf, Yinggehai, Qiongdongnan, South China Sea). In the northern part of this extensional province, the age of the initial rifting becomes progressively younger from east to west: Late Cretaceous in the East China Sea, Paleocene in the Subei-South Yellow Sea Basin and PaleoceneeEocene in the Bohai Basin (Hu Jianyi rt d., 1989; Zhang Yuchang et al., 1989; Zhou Zhiwu et ~1.. 1989). The most important tectonic control on extension was probably subduction roll-back of the Pacific plate relative to the eastern margin of Asia (Watson rt al., 1987). although this is hard to quantify. Northrup et cd. (199$ related the style of late Mesozoic-Cenozoic tectonics

*Author to whom all correspondence should be addressed. tAlternative names exist. Bdui Buj, Bmin IS sometimes used to cover the entire basin. but other authors specifically use it for the offshore portion. Bol~ri SCN also refers strictly to the region of the basin that is marine at present. The Bohai Basin is often known collectively with the Zhoukou and Nanyang basms as the Huabei (North China) Basin (Dai Jinxing and Xia Yinghe. 1990: Chang Chengyong, 1991: !?qov~ I )

951

952

Cenozoic extension,

and inversion: M. B. Allen et al.

thermal subsidence

120°

125’

rly Tertiary extens ional basins

0

km

500

‘r’:-..‘rJ

/ Figure

1 Location

of Tertiary

rift basins

in eastern

and northeastern

along the eastern Asian margin to the velocity of the Pacific Plate relative to the Asian Plate. They suggested that regional extension occurred during relatively slow convergence in the early Tertiary (approximately 30 40 mm yr-’ in the Eocene). that caused a decrease in the horizontal compressive stress transmitted between the plates. The Bohai Basin has a north-south length of c. 1,000 km. and a total area of c. 100,000 km’. To the northeast and east are the mountains of the Liaodong and Shandong peninsulas (Figztw 2). To the west. there are a series of ranges that separate the coastal plain from Nei Menggu (Inner Mongolia). In the central area of the

-I

/

I

China

basin the Bohai Sea is shallow and almost completely land-locked. consisting of a series of bays which radiate from a central depression. The onshore part of the basin is tlat and low-lying and includes the flood plain of the Huang He (Yellow River) near its southern margin. The modern delta of the Huang He builds out into the Bohai Sea. The outline of the basin is a large ‘dog-leg’, with a rhomboidal central area and narrow extensions 10 both the northeast and southwest. Four major internal uplifts (Chengning. Cangxian. Neihuang and Xingheng) define six major sub-basins: the Liaohe Depression in the north, the Jizhong, Huanghua, Bozhong and Jiyang depressions

Cenozoic extension, 114OE

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and inversion: A4 B. Allen et al.

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Provincial borde

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setting of the Bohai Basin

in the central area, and the Linqing Depression to the south (Fiyure 3). Over fifty smaller sub-basins (‘sags’) are recognised (Figure 3). In brief, the basin consists of Paleogene rifts, which were filled by a thick non-marine elastic succession. An unconformity at the top of the syn-rift sediments separates them from Miocene-Recent strata, which were deposited during post-rift thermal subsidence. Several of the eastern Asian rift basins contain major oil and gas fields, and Bohai is second in oil production

only to Songliao amongst China’s basins. Tertiary synrift strata form the major hydrocarbon source and reservoir rocks of the basin. Additional reservoir rocks occur in the footwall crests of Tertiary fault blocks; these structures are known by Chinese geologists as ‘buried hill’ reservoirs. Reservoir lithologies include Lower Palaeozoic and Upper Proterozoic carbonates, and Archaean to Lower Proterozoic crystalline basement (e.g. Liu Xingli, 1989). Existing descriptions of the Bohai Basin tend to

954

Cenozoic

extension,

thermal

subsidence

and inversion:

M. 9. Allen et al.

m

Basement high

-

Normal Fault

-

Thrust

,, /

Marginof basement high

Figure 3 Tertiary structures of the Bohai Basin. Uplifts are in italics, depressions are in largest type, sags and rises are in small type. Names of sags follow Chang Chengyong (1991). names of rises are taken from many sources. Structure compiled from the following sources: Li Desheng (1981); Wang Shangwen (1983); Qian Kai and Chen Yunlin (1987); Zheng Qing and Xin Quanlin (1987); Tang Zhi et a/. (1988); Liu Zerong et a/. (1989); Chang Chengyong (1991); Chen Fajing and Zhang Shulin (1991); Li Shaoguang and Wu Tao (1991); Lu Banggan (1991); Qi Jiafu and Chen Fajing (1992); Yan Junjun and Ma Qiangui (1992); Feng Zhenfu (1993); Li Yancheng (1993); Wang Xinyun and Zhang Yamin (1993)

account for its structure in one of two ways: (1) orthogonal extension, creating NNE trending normal faults as the result of ESE-WSW extension (e.g. Hu Jianyi et al., 1989); (2) as a dextral pull-apart structure created at the fault-overstep of the Taihang Shan and Tan-Lu (Tancheng-Lujiang) Fault systems at the western and eastern margins of the basin (e.g. Klimetz, 1983). This paper accounts for the structure and stratigraphy of the basin as the result of combination of transtension and strikeslip faulting, and suggests that the geometry of the deformation zone changed at ca. 4345 Ma (middle Eocene).

Our account is based on a review of the structure and stratigraphy of the basin, utilising the considerable volume of subsurface data that have been released in recent years. Figure 3 is a new summary structural map of the basin, compiled from these data; this map served as a framework for the interpretation of individual areas. Hydrocarbon exploration continues in the basin, increasingly with the involvement of international companies. A key aim of this paper is to provide a model for the overall architecture of the basin which will help predictions of the structure. stratigraphy and hydro-

Cenozoic carbon potential tigations.

Pre-Cenozoic

of local areas

extension,

in more

focused

thermal inves-

regional tectonics

The Bohai Basin lies entirely within the North China Block. This cratonic nucleus consists of Archaean and Proterozoic metamorphic basement, which crops out to the west and north of the basin, in Hebei Province, and to the east and south in Shandong Province. The North China Block formed part of Gondwanaland during the late Proterozoic and early Palaeozoic (Lin Jinlu et ~1.. 1985). Pre-Mesozoic structural trends in the basement of the Bohai Basin are dominantly east-west (Li Yancheng, 1993). At the northern margin of the North China Block the YinshanTumen fault system marks the collision zone between the North China Block and a region of Palaeozoic accretionary complexes known as the North China Foldbelt (Klimetz, 1983; Figure 4). This event is variously dated as mid-Carboniferous (Yang Zunyi et al., 1986) or Permian (Sengor et al., 1993). Southward-directed thrusting generated by this collision may have created a midLate Carboniferous foreland basin within the North non-marine sediments were China Block. Clastic. deposited in this region, and represent the first significant deposition since the Middle Ordovician. The depocentre migrated southwards with time (Hu Jianyi et ul., 1989). Approximately 200 km south of the Bohai Basin, the Qinling-Dabie Shan range forms the collision zone between the North China Block and the South China Block to the south and southeast (Figure 4). Sengor et cd. (1988) summarised the tectonics of the range and favoured a Triassic age for the collision, which is supported by more recent work on the geochronology of the orogen (Ames et al., 1996). Middle Triassic thrusting, folding and granitic plutonism, have been recognised in the Yanshan foldbelt north of the Bohai Basin (Hu Jianyi et al., 1989). Triassic sedimentation within the Bohai Basin area was limited to the deposition of 1000 m of continental elastics in its southwestern part. There is evidence for further convergence between the North China Block and the South China Block, possibly Late Jurassic-Early Cretaceous in age, that was accommodated by thrusting in the Dabie Shan and in the Shandong Peninsula (Okay and Sengor, 1992). The Tan-Lu Fault (Xu Jiawei, 1993) acted as a transfer zone between these thrust zones. Although most of the Mesozoic displacement on the Tan-Lu Fault took place along the segment between the Dabie Shan and Shandong, the fault is commonly depicted as a linear structure further into northeastern China, and as forming the eastern border to the Cenozoic Bohai Basin (Xu Jiawei, 1993). More detailed maps of the sub-surface structure of eastern Bohai indicate that this over-simplifies a complex array of early Tertiary normal and strike-slip faults (Figure 3). Late Jurassic-Early Cretaceous non-marine elastic sedimentation occurred across a vast area of central and eastern China, and adjacent areas of Mongolia and northeastern Russia. The Songliao, Zeya-Bureya, Erlian, Hailar and Govi basins all originated at this time (Figure 4). Sixty minor basins in this region are localised pullaparts (Li Sitian et al., 1984). Jurassic and Cretaceous strata have been recorded beneath Tertiary syn-rift sediments in the Bohai Basin (Liu Hefu. 1986; Lu Banggan,

subsidence

and inversion:

M. B. Allen et al.

955

1991). Their original distribution and structural controls are not usually clear. In the northwest of the basin and adjacent exposed areas of the Taihang Shan these rocks are folded in en echelon arrays that indicate roughly northhsouth sinistral transpression (Liu Hefu, 1986). Middle and Upper Jurassic strata are preserved in the folds. There are Late JurassiccEarly Cretaceous metamorphic core complexes in the Xishan. about 60 km northwest of Beijing. and in western Nei Menggu, close to the Mongolian border (e.g. Zheng Yadong and Zhang, 1993). Beneath the Tertiary Jiyang Depression, Upper Jurassic and Lower Cretaceous strata appear to have been deposited in half grabens which were reactivated in the early Tertiary (Li Guoyu and Lu Minggang, 1988). Mesozoic extensional basins across much of eastern China have been attributed to the effects of subduction roll-back at the Asian-Pacific convergent plate boundary (Watson et al., 1987) with suggested complications from putative ridge subduction or mantle plume impact (e.g. Xie Mingqian, 1987). Extensive evidence for regional strike-slip faulting, strike-slip basin development, as well as thrusting in the Dabie Shan and Shandong, suggest a more complicated tectonic regime, with both extensional and compressional events occurring along a strike-slip dominated active continental margin.

Tertiary stratigraphy Overview Tertiary strata rest unconformably on a variety of older pre-rift strata and are covered conformably or disconformably by Quaternary sediments. The succession is typically 4000&7000 m thick. Lithologies are dominated by terrestrial elastic rocks in two main associations: (1) sandstones and organic-rich mudstones formed in lacustrine and transitional lacustrine/fluviatile or deltaic environments; (2) purple-red and mottled sandstones and mudstones that formed in alluvial environments. Basaltic lavas were extruded locally throughout the Tertiary. The Cenozoic succession has been divided into six ‘formations’; this standard stratigraphic scheme is applied throughout the basin (Figure 5). However, it should be noted that formations and members as defined in the Bohai Basin do not correspond to standard international definitions of lithostratigraphic units, but may include a wide variety of lithologies in different parts of the same sag. For example. the Sha-3 Member contains rocks as different as lacustrine turbidites and sub-aerial alluvial fan conglomerates within a single sag. They are closer in spirit to chronostratigraphic sequences than to lithostratigraphic formations. Members are named by an abbreviation of the formation name plus a designating number. Thus Kong-3 represents the lowest member in the Kongdian Formation and Kong-l the highest. Tertiary lithofacies display a strong lateral variability, this being especially true in the early stages of Cenozoic rifting. Areas close to the centres of individual depressions tend to contain a higher proportion of dark mudstone and sandstone, formed in deep lacustrine or transitional environments, whilst in more peripheral areas, sandstones that were deposited in alluvial or shallow lake environments are more important. In addition to these lateral facies transitions, the division of the Bohai Basin into a series of separate rifted sub-basins was responsible for the development of large-scale variations

956

Cenozoic

extension,

thermal

subsidence

115DE

IlODE

and inversion: 1209

M. f3. Allen et al. 125%

\

N

i

N

Mesozoic foreland basin

El

Late Mesozoic rift basin

\

‘1, National border

\

Modern coastline

Figure 4 Major pre-Cenozoic Cretaceous extension

tault

systems

wlthln

and

aalacent

in subsidence and sedimentation patterns between each of these regions. The Liaohe Depression is a particular case in point. with its facies associations commonly being different from those of other areas for much of the Eocene-Oligocene syn-rift succession.

i

\

I/

I to the

North

China

Block,

and areas

affected

by Late Jurassic-Early

Despite the high lateral variability of depositional environments within the Bohai Basin. large-scale trends within the sedimentary record are apparent through time (Fiyu~ 5). There were two major phases of extension and subsidence, marked in the stratigraphy by the bases of

Cenozoic

extension,

thermal

subsidence

-

x -

Period/ Epoch >uaternal

:otmatio

tiembc

Typical sickness mge (m

Ainghua zhen

bliocene

suantao

M. B. Allen et al.

957

-r Schematic stratigraphy

Depositional

Basin evolution

environment

summary

1 ‘_.

Jingyuai

Wocene

and inversion:

‘.‘.;; . ..)

:..i: ‘,’ > ,:._

.,..

‘.

T.

“‘Y

.:

Varied alluvial and deli& Resumption of IimiIed dearal shike-slip faulting; focalised increase in sedimentation rate

:y.‘::.:‘,

Meandering rfver. fluvial overbank and hcustrine

-10

Braided river Minor, intermittent normal faulting

20 ,.

; :..,., .:_::

(‘.

..,

Onset of thermal subsidence Regional inversion

Fluvlal, delIaic and shallow lacustrine, becoming increasingly fluvial dominated

)ongyin( 30

Ngocens

Reduction in rifting, and final infilling of separate Paleogene syn-rift subbasins

long-: long-:

Sha-I

Shallow - deep lacustrlne. with evaporiIes and reef carbonates. Minor fan deltas

iO-600

Sha-2

$0 -

Qvial

jhahejie

Decrease in elastic input due to reduced topography. Expansion of lacustrine conditions

- deltaic and shallow lacustrine

3eep

Sha-3

water lacustrine with turbllites. :xtensive anoxia. Fan delIas

Sha-4

4lluviaJ fan - saline lake environments NiIh reef and mfcritic limestone and ?vaporiIes. Localised turbidites

Regression - because of decrease in tectonism? Major increase in transtension; Bozhong forms as central pull-apart. First sedimentation in many areas, but ollap at fault blodcs active since early Eocene

3cene

:ong-1

deposition

50 -

Increasing aridity promotes

Fluvial, alluvial fan and satkha

saline

lake development

across :ongdiar

much o Long-2

the Bat lai Basil

Fluvial, delIa plain and II moderately deep lake

Initial dextral

Nluvial fan, ephemeral stream and shallow lacustrine

:ong-:!

transtension

$0 aleocens

.ate Mesozoic ranspresslonal deformation

)

Organic-rich mudrocks

i

Finegrained

ETJTJJ

k G%ntermediate lcanics

elastics

int?“‘*fate’

Sandstones Figure

5 Tectonic-stratigraphic

-~nw-c

chart for the Bohai

Basin.

Compiled

from

data sources

A a VW

cited

in

Ir Isolated hall arabens. mainlv in Jizhong. Hua-nghua r&d Lin$ng depressions. Associated aasic-intermediate volcanism

Crystalline basement Unconformity Major

hydro&+on

source/reservoir in the text

,_

unlr

958

Cenozoic

Table 1 Age schemes

extension, for Cenozoic

thermal formations

subsidence

and inversion:

of the Bohai Basin; table entries

M. 9. Allen et al.

are ages in millions

of years

of the base of each formation

or member Li Desheng al. (1988) (LangfangGu’an) Strat. unit Pingyuan Fm. Minhuazhen Fm. Guantao Fm. Dong-1 Member Dong-2 Member Dong-3 Member Sha-1 Member Sha-2 Member Sha-3 Member Sha-4 Member Kong-l Member Kong-2 Member Kong-3 Member ‘Yao Yimin

et a/. (1995)

2 6 14

30 32.5 35 38 42

et Xu Huaimin al. (1993) (Bozhong)

et

25

27.6 33.5 34.7 40

(Jiyang)

Zhang Qingchun ef al. (1994) (Damintun)

24.6

24.7

24.6 30.8

36.9 38.4 40 43 45.9

32.6 38

Dai Xianzhong eta/. (1993)

38 42

Yao Yimin et al. (1994) (whole basin)’

Wang Tonghe (1995) (whole basin)

Guo Suiping et al. (1996) (Dongying)

23.3 30.3

24.6

43 50.5

35.4 42.5 45.5 50

65

56.5

50.5 50 gave

65 a K-Ar age of volcanics

65 near the base of the Kong-3

the PaleoceneeEocene Kong-3 Member and the middle Eocene Sha-3 Member. Other. lesser, trends (including possible temporary marine connectivity) are also documented before more uniform conditions developed during Neogenc post-rift times. with the establishment of widespread Auvial environments. Localised unconformities and disconformities occur within the Paleogene succession between different formations and between members of the same formation. These appear to be related to hiatuses or rotations on individual faults rather than regional events. Marine incursions were short-lived; eustatic changes were not an important control on basin-wide stratigraphy.

Tertiary sedimentation in the Bohai Basin began with the deposition of the Kong-3 Member of the Kongdian Formation (F@w 5). Although some authors (e.g. Xu Huaimin c’t cd., 1993) place the onset of deposition at 65 Ma, i.e. right at the base of the Paleocene. it is more likely to be later in the Paleocene or in the early Eocene. Yao Yimin et cd. (1995) quoted a K-Ar age for basic volcanics in the Kong-3 Member of 55.7Ma (no errors given). This age puts the Kong-3 member close to the Paleocene Eocene boundary (Harland ct rrl., 1990; Tuhlc

1). There was deposition of alluvial red-bed successions. which pass laterally into shallow lacustrine facies. This took place in ;I number of separate, sub-basinal depoccntres (F&~cs 6 and ELI), principally in the west of the basin, in the Jizhong. Huanghua and Linqing depressions, with little deposition in the Bozhong, Liaohe and northern Jiyang depressions. As rifting waned. a series of regressional facies developed. typified by alluvial and sabkha associations (f+q~c, -5). Thicknesses of over 4000m are recorded from these dcpocentres. but this progressively decreases towards the middle of the basin. with only 194m in the Bonan Rise and 50 m in parts of the Bozhong Sag. The formation is absent from many wells in the Bohai Sea area. Strata1 thicknesses increase towards the hangingwall of each half graben that W;IS active during Kongdian deposition. In detail, the Kongdian Formation consists of three members. The Kong-3 Member was deposited mainly in

Member

32.8 38 42

as 55.7 Ma.

alluvial fan, ephemeral stream and shallow lake environments (Chen Changming et cd., 1984). Fans were deposited at half graben margins. while ephemeral streams were sited in the basin axes. sometimes feeding minor lakes. The climate was arid and oxidising conditions predominated. The Kong-3 Member is typically 245 500m thick, although it is completely missing from some basin uplifts. The Kong-2 Member consists of dark grey mudstones. interbedded with thin coal seams, carbonaceous shale, oil-bearing shale and minor basalts, whilst purple+brown mudstones are common towards its top. In the Liaohc Depression, dolomitic limestone and calcareous shale were deposited in addition to mudstone. The Kong-? Member is conformable with the underlying Kong-3 Member. Thicknesses typically vary between 400-700 m. although deposition was apparently lacking or erosion occured on many of the uplifted blocks. Dark mudstones and oil shales contained in this member represent an increase in the importance of anoxic lacustrine conditions, with the oil shales forming a recogniscd hydrocarbon source rock (Chang Chengyong. 199 I ). The upper member of the formation (Kong-l Member) is composed of intercalations of mudstones, lithic arkoses and sandy conglomerates. At the top of the member, lithologies become dominated by gypsum-bearing mudstones in the middle of the depocentres, with these passing laterally into red mudstone and sandy mudstone towards the basin margins. The member is up to 15OOm thick. and has its depocentre in the Huanghua Depression. The Kong-l deposits represent an alluvial fan-evaporitic playa lake facies association.

The age and division of this formation are debated. Some authors (e.g. Chang Chengyong, 1991) describe four members. with the oldest (Sha-4) being ascribed an Eocene age, whilst the rest is placed in the Oligocene. Other papers (e.g. Xu Huaida c’t c/l., 1986; Hu Jianyi et (I/.. 1989) have four members, but place them entirely in the Oligocene. In the Langfang-Gu’an Sag. Li Desheng (I/ (I/. (198X) record the EoceneeOligocene boundary within the Sha-4 Member, so dividing the member into a lower and upper part. Yet another scheme (e.g. Ye Hong

Cenozoic

extension,

thermal

subsidence

and inversion:

M. 5. Allen et al.

959

r-

-

-.s-

lsopachs(km)

-

Normal fault

-

Modemcoastline

‘, Shenyanj

100 km

0 Beijing

1

120

E

Figure 6 lsopach map for the Eocene Kongdian Formation. Note the lack of significant deposltlon In tne Boznong Depression area. After Li Desheng (1981), with modifications from Xu Jie eta/. (1985). No value was given for several contours in the original diagram. Faults are shown schematically

et al., 1985) has only three members, with the Sha4 Member of other classifications grouped with the Kongdian Formation or included in an enlarged Sha-3 Member. Where absolute ages are given, most or all of the Shahejie Formation is Eocene, using the timescale of Harland et al. (1990). This is illustrated by the range of values in Table I; note that few Chinese papers quote the origin for the particular scheme used, and that Chinese geologists do not always use internationally recognised values for the ages of standard stages or periods. For example, Li Desheng ef al. (1988) quoted the base of the Oligocene at 40 Ma, almost 5 million years earlier than international usage. This paper adopts a scheme that defines four members, and places the formation in the Eocene with the exception of the upper part of the Sha-I Member (Figure 5). In seismic interpretations (Lu Banggan, 1991) the Sha-4 Member is commonly grouped with the underlying Kongdian Formation, with no distinct boundary between the two units, whereas there is a strong reflector at the base of the Sha-3 Member. The Shahejie Formation was deposited across a wide area of the Bohai Basin, and for the first time in the Cenozoic, significant thicknesses of sediment accumulated in the Bozhong Depression and the northern part

of the Huanghua (Figures 7 and 8). The formation thicknesses of over 3000 m (Figure 8).

reaches

Sha-4 Member In general, the lower part of this member consists of red mudstones interbedded with thin sandstones. There is a localised disconformity at the top of the lower part of the member, above which there are bluish-grey mudstones, siltstones and sandstones with gypsum and halite horizons. Near the top of the member there are also carbonates and oil shales. Over the entire basin this member is typically 350-950 m thick, with the thickest deposits in the northern part of the Dongying Sag. There was an increase in the area that underwent extension and subsidence, with littoral and shallow lacustrine deposition in significant parts of the Bozhong Depression for the first time (Yao Yimin et al., 1994; Figure 7b). Alluvial and relatively shallow lacustrine conditions developed with the deposition of saline lake limestoneevaporate facies associations. There were transitionalmarine conditions in the Jiyang and Liaohe depressions, with well developed, low diversity foraminifera assemblages. Many of the facies in this member are similar to the underlying Kongdian Formation, but it was deposited

960

Cenozoic

extension,

thermal

subsidence

and inversion:

b

e

Sha-1 Member

Figure 7 Depositional environments for the Kongdian and Shahejie mentation for the Sha-3 Member. From Yao Yimin et al. (1994)

M. B. Allen et al.

Sha-4 Member

0L-E-Y

formations.

/&

Note

the

increase

in water

depth

and area

of sedl

Cenozoic

extension,

thermal

subsidence

and inversion:

M. B. Allen et al.

961

Area of deposition > 2 km strata

Figure 8 lsopach structure

adapted

map for the Shahejie from Figure 3

Formation,

for central

and northern

across wider areas of the basin (Fiyurr 7). For this reason, we regard the Sha-4 Member as transitional between the Kongdian Formation and the overlying Sha-3 Member (see later discussion). ShLl-3 Merllher The Sha-3 Member consists predominantly of dark sandstones and mudstones, interbedded with sandy shales. fine-grained sandstones and a few layers of limestone at its base. This member contains the most important hydrocarbon source rocks in the basin. The Sha-3 Member occurs in sags across the entire Bohai Basin (Figure 7~). Thicknesses are typically 40&1500m. The highest sedimentation (and, by implication, subsidence) rates within the Tertiary Bohai Basin were during the deposition of the Sha-3 Member (Table 2; Li Desheng et al.,

parts of the

Bohai Basin. After Xu Jie et al. (1985), with the

1988). The base of the Sha-3 Member is commonly a prominent seismic reflector, more distinctive than the boundary between the Sha-4 Member and the Kongdian Formation (Figure 9). Several facies associations have been identified, reflecting various lacustrine and nearshore environments (Figure 7~). Intense rifting and a change to wetter, more humid conditions resulted in a dramatic expansion of lacustrine conditions. which covered about 60&80% of sag areas at this time (Chang Chengyong, 1991). An initial deep water lacustrine turbidite facies association passed up into a shallower water fluvialPdeltaic succession as subsidence decreased. The deep lacustrine facies association is extensively developed (e.g. in the Dongying, Liaoxi, Dongpu and Qikou sags). This association consists of typical turbiditic

Table 2 Stratigraphy and sedimentation rates of the Langfang-Gu’an Sag, Jizhong Depression. From Li Desheng et a/. (1988). Note that these authors place the base of the Oligocene at 40 Ma. This is c. 5Ma earlier than most sources place this boundary. They also place the Shahejie Formation predominantly in the Oligocene (compare with Table 1)

Series

Stratigraphic

Quaternary Pliocene Miocene

Pingyuan Fm. Minhuazhen Fm. Guantao Fm.

Oliocene

Dongying Fm. Sha-1 M. Sha-2 M. Upper Sha-3 M. Middle Sha-3 M. Lower Sha-3 M. Upper Sha-4 M. Lower Sha-4 M. Kongdian Fm.

Eocene

unit

Time interval (millions of years B.P.)

Maximum thickness (m)

o-2 2-6 6-14 14-24 24-30 30-32.5 32.5-35 35-36 36-37 37-38 38-40 40-42 42-50

1340 469 0 660 645 550 782 894 2200 1300 1200 444

449

Sedimentation rate (mm yr 0.22 0.34 0.05 no deposition 0.11 0.26 0.22 0.78 0.89 2.20 0.65 0.60 0.06

‘)

962

Cenozoic extension, NW

=z -

-

z-

1

and inversion: M. 6. Allen et al.

SE -

-__IP-p-

0

thermal subsidence

---

lkm

I

Figure 9 Interpreted seismic section for the Bieguzhuang trap in the Hexiwu buried hill belt, Jizhong Depression. Stippled area indicates Cambro-Ordovician carbonates in footwall crests. T,=basal Tertiary reflector; T,=base of Sha-3 Member; T,=base of Neogene. Strong reflectors below T, are igneous rocks within the Sha-4 Member. The Sha-1 Member and the Dongying Formation are absent in this area, presumably as a result of late Oligocene to early Miocene exhumation and erosion. From Lu Banggan (1991). Location shown in Figure3

sequences within deep lacustrine, organic rich mudstone background sediments. Internally, the deposits display well-developed rhythmic bedding, analogous to Bouma sequences. Over large parts of many sags, e.g. Dongying and Dongpu (Wang Xinyun and Zhang Yamin, 1993), deep water lacustrine mudrocks were deposited away from the influence of the basin-margin sourced turbidite fans (Figure 10). Figure 7c shows that during the deposition of this member there was not only an increase in water depth in the basin, but a major increase in the area undergoing with large areas of the Bozhong sedimentation, Depression receiving sediment for the first time in the Tertiary. The significance of this expansion is discussed below. Sha-2 Menlbet The Sha-2 Member typically consists of grey to dark grey mudstones interbedded with bioclastic limestones and organic-rich shales in the sags, which pass into arkoses and conglomerates in marginal areas. The Sha-2 Member overlies the Sha-3 Member conformably in some places, but there is a prominent disconformity or unconformity across large parts of the basin, including much of the Jiyang Depression (Yao Yimin et al., 1995) and the Liaohe Depression (Nummedal et al., 1995), whilst the Sha-2 Member is absent altogether in the Damintun Sag (Tong Xiaoguang and Huang Zuan, 1991). Across the basin as a whole, it forms one of the most important oil reservoirs. This member varies markedly in thickness; in the Bohai Sea region it is 600&800m thick, but in the southern parts of the basin only 30-80 m thick. Within the Sha-2 Member there is a much higher proportion of coarse sandstones. with intercalated red mudrocks, when compared to the underlying Sha-3 Member. This represents a regressive event, with alluvial, deltaic

--L

Normal fault

#

Sediment flow

m

Sub-aqueous fan

0

Semi-deep lacustrine

Figure 10 Sedimentary environments of the Sha-3 Member in the eastern part of the Dongpu Sag and their relationships to contemporary faults. Note the dextral sense of motion implied by the orientation of secondary normal faults in the southern part of the map. Location shown in Figure 3. After Wang Xinyun and Zhang Yamin (1993)

and shallow lacustrine environments all represented (Figure 74. Lacustrine environments only occupied about 10P20% of the total depositional area (Chang Chengyong, 1991). Deltaic facies are particularly important, with delta-plain, delta-front and pro-delta facies all recognised. Subsidence rates are much reduced compared

Cenozoic

extension,

thermal

to the Sha-3 Member (T&de 2; Li Desheng et al., 1988). In sequence stratigraphic terminology, the unconformity at the base of the Sha-2 Member has been identified as a sequence boundary (Nummedal et ul., 1995), separating a transgressive and possibly a highstand systems tract within the Sha-3 Member from a lowstand systems tract (the Sha-2 Member). %a-I Meniher The upper part of Shahejie Formation (Sha-1 Member) is typically composed of dark mudstones, with some carbonates at its base. More terrigenous elastic rocks occur in marginal areas of the depressions. In the sags in or near the Bohai Sea, deeper water interbeds of turbiditic dark sandstones and mudstones are developed. Here, sandstones account for 30% of the total stratigraphic thickness, mainly composed of lithic gravel-bearing arkoses and fine- to medium-grained sandstones. The member is widely distributed across the basin, and overlies older strata mainly conformably. In the sags in or near the Bohai Sea, it is a minimum of 200 m thick; in more peripheral sags it is generally 50-l 50 m thick. The thickness of the Sha- I Member in the Liaohe Depression is particularly variable, from (r973 m thick. The Sha-1 Member forms a transgressive unit, and has been identified as a transgressive systems tract by Nummedal rt al. (1995). It commonly onlaps previous members of the Shahejie Formation onto the crests of footwall blocks (Zha Quanheng, 1984; see also Figure 7~). However, because subsidence was relatively uniform (particularly away from the Bohai Sea region), shallow lacustrine facies associations dominate the Sha-1 Member, such that although oil shales are extensive they are normally of poorer quality and thinner than those deposited during Sha-3 times. There were marine incursions at this time in the Huanghua, Liaohe and Jiyang depressions. Oligocene:

Dongying

Fornmtion

The Dongying Formation lies conformably on the Shahejie Formation. It ranges in thickness from 2001640 m, with both lithofacies and thicknesses changing markedly both laterally and vertically. Deltaic facies associations are important in all three members of the Dongying Formation, in all the major depressions. These rocks represent a general regression from the lacustrinedominated sedimentation of the Shahejie Formation due to the final infilling of the syn-rift topography (Figure 5). The elastic rocks typically form coarsening-upward packages. Each package, or cyclotherm, may reach 200 m in thickness. Associated lithofacies are deep-lacustrine basinwards and fluvial towards the basin margin hinterland. Restricted marine conditions locally developed during the deposition of the Dong-3 Member. This member represents a transition from the lacustrine dominated conditions of the top of the Shahejie Formation to the deltaic environments higher in the Dongying Formation. At the end of the Oligocene Period, a phase of regional exhumation and erosion affected the Dongying Formation to various degrees, and is discussed in more detail below. Miocene-Quaternary Forrmtions Post-Oligocene formations are more uniform in thickness and facies across the basin than older Tertiary units, this

subsidence

and inversion:

M. B. Allen et al.

963

being a consequence of their deposition during thermal subsidence, with only minor faulting compared to earlier Tertiary rifting. During deposition of the Miocene Guantao Formation, most of the basin was occupied by an extensive braided fluvial system, whilst fringing alluvial fans were deposited in front of the Yanshan Mountains in the region of the Beitang Sag. These rocks lie unconformably over older strata. The proportion of coarse elastics decreases towards the south and west. The Pliocene Minghuazhen Formation lies conformably on the Guantao Formation. It consists of silty mudstones. with intercalations of sandstone, and several basalt layers. The formation coarsens upwards, and is between 600 and 3,OOOm thick. The succession thickens towards the present-day Bohai Sea. Deposition occurred within meandering river systems and on overbank floodplains. Since the Pleistocene, the Bohai Basin has continued to subside slowly, with an accumulation of 100-200 m of loess and alluvium covering most of the basin: these rocks are grouped as the Pingyuan Formation. There are discrete depocentres of Quaternary fluvial sedimentation in the northwest of the basin, however, that contain up to 1,OOOm of sediment (Chen Wangping and Nabelek, 1988).

Structure Uplifted Precambrian basement blocks surround the Bohai Basin: the Taihang Shan to its west, the Yanshan to its north, Luxi to its south, and Jiaoliao to its east (Figure 3). The boundaries between the Bohai Basin and these basement blocks are a series of normal faults that have had a polycyclic motion history. Both the Taihang Shan and the Tan-Lu fault systems continue beyond the boundaries of the basin itself. Taken together, these marginal faults give the Bohai Basin its distinctive ‘dog-leg’ outline (Fi~gztres3 and 4). The six depressions and four uplifts of the basin are fault-bounded. These depressions and uplifts are subdivided into numerous sags and ‘rises’ (Figure 3) that correspond in general to grabens/half grabens and horsts respectively. The nomenclature adopted for the sags in this paper comes from Chang Chengyong (1991). but names for the rises are taken from many sources. Fuult distribution The Tan-Lu fault system has been traced for more than 2000 km through eastern Asia (Xu Jiawei, 1993). and is depicted as forming the eastern boundary to the Bohai Basin for c. 500 km, although the detailed Tertiary structure in this region is of en tchelon normal faults and localised strike-slip segments (Figure 3). Movement on the Tan-Lu fault system since the early Tertiary has been dextral (Yan Junjun and Ma Qiangui, 1992). On 4th February 1975, there was a magnitude 7.3 earthquake east of the Liaohe Depression, with an epicentre at 40 35’ N 122’ 45’ E. This had a dextral fault plane solution on a NNE-striking plane (Ma Xingyuan, 1986). Other earthquakes, for which there are no fault plane solutions, have been recorded in historic times along the fault system at the eastern margin of the basin and beyond. At the western margin of the basin, the Taihang Shan fault system has a southern segment that trends northsouth and a northern segment that trends northeast. Dex-

964

Cenozoic

extension,

thermal

subsidence

tral motion has been recorded from an earthquake in 1966 on a parallel fault on the northern side of the Xingheng Uplift, but not directly for the Taihang Shan fault system itself (e.g. Ma Xingyuan, 1986). The maximum Cenozoic throw on both Tan-Lu and Taihang Shan fault systems is approximately 6 km, down to the basin (Li Desheng, 1981). Zhao Zhongyan and Windley (1990) demonstrated that the Taihang Shan fault system at the margin of the the Xushui Sag is a segmented planar structure. It dips ESE at about 40-45 until an abrupt change at a depth equivalent to 2.3 seconds two-way travel time, where it shallows to about 25’ Normal fault systems at the margins of the Luxi and Yanshan uplifts dip towards the interior of the basin. The throw on the marginal faults north and south of the basin is less than for the Tan-Lu and Taihang Shan fault systems. The Lanliao Fault at the western side of the Luxi Uplift has a component of dextral transcurrent motion in its southern part at least: minor faults between strands of the main fault system appear to form a dextral strikeslip duplex (Fipw IO; Wang Xinyun and Zhang Yamin, 1993). The Zhangjiakou-Bohai seismic zone (Fiyurr 3) is a region of historical seismicity that was noted as a sinistral fault system by Liu Guodong (I 987). but without further detail. In the eastern part of this zone, in the Tangshan region (Fipw 3), the 1976 earthquake sequence was of dextral slip on NNE trending faults. with minor components of normal faulting and thrusting at extensional and compressional fault oversteps respectively (Nabelek et c/l., 1987). The details of the structure in this area are not well-constrained, and Fiyuw 3 no doubt greatly simplifies this area. On some sketch maps of the basin (e.g. Yao Yimin CI L/I., 1994; see Figure 7), this region is shown as containing a number of stepped fault segments, that mimic the offset of the Liaohe and Jizhong depressions. Faults that define the boundaries of the sags and rises within the basin have a variety of orientations. In the sags close to the Tan-Lu and Taihang Shan fault systems they commonly trend northeast or NNE, sub-parallel to the major basin-bounding faults. Sags in the Jiyang and Bozhong depressions are commonly orientated eastwest, or, more rarely, WNW ~-ESE (Figwc 3). Many published sections are orientated east-west, and so emphasise a dominant role for normal faults with a northeast southwest strike, while neglecting the east -west structures (e.g. Ye Hong LJIr/l., 1985).

The oldest syn-rift deposits preserved at the bottom ot Cenozoic grabens and half grabens belong to the Paleocene --Eocene Kongdian Formation. These rocks were deposited in all of the main rift zones of the Bohai Basin with the exception of the larger part of the BoLhong Depression and the northern part of the Jiyang Depression. Subsidence was greatest in the Huanghua and Linqing depressions and the western part of the Jizhong Depression, where c. 4 km of Kongdian Formation is preserved (FiywcJ 6). Most of the fault systems active during this interval are oriented northeast-southwest or NNE-SSW. An exception is the Dongying Sag in the Jiyang Depression, which underwent extension on east-west trending 1:dults. bounded to the west by the Chengning Uplift and to the east by the Tan-Lu Fault (Fi,ywr.r 3 and h).

and inversion:

M. B. Allen et al.

During the middle Eocene there was no major propagation of faults northwards from the Paleocene and early Eocene rifts in the Jizhong Depression (compare Figures 6 and 8). In contrast, at the eastern side of the basin the faults at the eastern margin of the Liaohe Depression propagated southwards, to define the eastern margin of the Bozhong Depression. Significant extension and subsidence within the Bozhong Depression began at this time, with the Shahejie Formation, and in particular the Sha-3 Member (Fi~gurrs 3, 7c and 8; Yan Junjun and Ma Qiangui, 1992). Published seismic sections for the Liaohe Depression commonly show the Shahejie Formation as the first synrift formation (see Lu Banggan (1991) for good quality data), giving the impression that the Kongdian Formation was not deposited in this area. However, there arc several accounts of typical Kongdian Formation strata in the Liaohe Depression (e.g. Yao Yimin tit (I/., 1994). albeit in depocentres far smaller than those developed later (Ficpw 7l4). Li Desheng it a/. ( 1988) calculated subsidence rates for the Cenozoic strata of the Langfang-Gu’an Sag in the Jizhong Depression (Tuhlr _?). The greatest subsidence and fastest subsidence rates in this sag occurred during deposition of the Sha-3 Member; nearly 4000 m ofsedimentary rock (present thickness) accumulated in what these authors estimated to be three million years, between 35 and 38 Ma. Most other estimates for the duration of deposition of the Sha-3 Member put it between 3 and 5 million years (Tcrhke I). Major extension in most areas ended at the end of the Oligocene (top of the Dongying Formation), making the syn-rift phase about 30 million years in duration. Normal faults locally cut Neogene strata. but more typically the Neogene succession blankets faults which were active in the Paleogene. Thus the basin’s continued subsidence during the Neogene and Quaternary is best explained as post-rift, thermal subsidence, similar to the late stages of many large extensional basins. This post-rift phase has lasted about 25 million years to date.

Individual sags within the Bohai Basin are typically formed from a number of adjacent half grabens, with symmetrical grabens. such as the Beijing Sag, being less common. The structural highs (rises) are not completely undeformed in their interiors, but the densities and throws ofthe faults are far smaller than in the sags. Where a progression in the faulting is discernible, younger faults have propagated in the footwalls of older half grabens. i.e. away from the master fault that separates the sag from the adjacent rise (Zhao Zhongyan and Windlcy. 1990). A variety of fault geometries have been described. Arrays of half grabens are typically developed on planal faults with the longest and deepest fault at the margin of the sag (Figlr~cj 1 I). These master faults sometimes appear to have listric profiles on sections based on seismic lines. However. it is not clear how well constrained the reconstructions are at these depths (often more than 7 km): wells do not penetrate to such depths and seismic migration techniques are not normally published. The listric profiles may therefore be artefacts of the data processing. Where t‘aults flatten out within the Tertiary sedimentary succession there can be more confidence about the listric geometry (Fkywc l-7).

Cenozoic extension,

thermal subsidence

and inversion: M. 9. Allen et al.

965 s

N

Okm

0

Neogene and Ouatemafy Dongying Formadon Shahejie Formation Kwgdian Formation

1-_3

Pm.ktialy

Ei

Precambrian basement

strata

Figure 11 Cross-section

through the Jiyang Depression.

Location shown on Figure 3. From Zhai Guangming

eta/. (1988)

Figure 12 Cross-section

through the Liaohe Depression. Location shown on Figure 3. Note the ‘swallow-tail’ structures in the upper parts of the half grabens. After Qi Jiafu and Chen Faqing (1992) and Yang Kesheng and Yuan Bingheng (1993)

The spacing between the major faults within a depression is typically 15-25 km, but smaller faults, often antithetic to the larger structures, are commonly only I2 km apart. These are the smallest structures depicted on published seismic sections and interpretations; still smaller faults almost certainly deform the half grabens defined by these faults. Little work has been done on possible transfer zones between opposing sets of half grabens. There are northwest or WNW trending faults in the Jizhong Depression that lie between or at the terminations of northeast or NNE trending normal faults (Figure 3). A similar feature is present to the southwest of the Bodong Sag. These faults are depicted as sinistral strike-slip structures by Wang Tonghe (1995). but it is not clear what data were used to determine the sense of fault motion. En echelon arrays of normal faults occur within many sags, including Liaoxi, Dongying, Dongpu and Baxian (Figures 3. 10 and 13; e.g. Lu Banggan, 1991; Wang Xinyun and Zhang Yamin, 1993). These arrays occur both as splays off larger faults (e.g. Liaoxi) or without any direct connection with larger faults (e.g. Dongying). The secondary structures are consistently oriented clockwise from major faults at the sag margins, indicating that extension was accompanied by a component of dextral transtension. This transtension operated throughout the rifting history of the basin: all three syn-rift formations are present in half grabens defined by these en echelon arrays in the Jizhong, Huanghua and Linqing depressions. An unusual structural style is developed in the western part of the Liaohe Depression (Figure 12). Early. westdipping planar faults that deform the Sha-3 and Sha-4 members of the Shahejie Formation die out up-section

while a new set of east-dipping faults, sometimes with listric cross sections, control the deposition of younger sediments. This style of deformation has been termed ‘swallow tail faulting’ by Chinese geologists. A similar style of deformation was noted by Zha Quanheng (I 984) in the Jizhong Depression.

Estimates

of crustal

extemion

Attempts to calculate the total amount of Cenozoic extension for the Bohai Basin have used a variety of methods: balanced section restorations, comparisons of present crustal thickness in the basin and surrounding areas, and estimates based on calculations of the tectonic subsidence after backstripping analysis (Li Desheng, 1982; Hellinger et a/., 1985; Shedlock et al.. 1985). All of these analyses fall into the range of lo-30% extension, but the estimates based on crustal restorations fall below those made by other methods (Hellinger et cd., 1985). This is probably due to the representative section chosen. which was east-west across the northern part of the Huanghua Depression and the Bozhong Depression. Figure 3 shows that this is parallel to the strike of the major structures of this region. and so results in an underestimate of the extension. Published seismic sections that have been converted into true-depth sections allow calculations of the /3 factor to be made using the method of Jackson and White (1989) where b = [sin &/sin 0,] (Figure 14). Tuhle 3 presents a selection of estimates made using this method. The estimates in Tuhle 3 are broadly in line with previous calculations, but they emphasise that the greatest extension has occurred in the Bozhong Depression; this is consistent with the presence of the thinnest crust in the

966

Cenozoic extension,

thermal subsidence

Table 3 Extension estimates for half grabens in the Bohai Based on the method of Jackson and White (1989) Location

/{factor

Liaoxi Sag Liaoxi Sag Liaozhong (Central) Sag Jizhong Jizhong Jizhong Jizhong ZhanhuaSag Bozhong

Source

of data

Basin.

used for calculation

1.3 1.4 1.2

Qi Jiafu Qi Jiafu Qi Jiafu

and Chen Fajing and Chen Fajing and Chen Fajing

(1992) (1992) (1992)

1.3 1.1 1.4 1.3 1.3 1.8

Tang Zhi eta/. (1988) Tang Zhi et al. (1988) Tang Zhi et al. (1988) Lu Banggan (1991) Lu Banggan (1991) Wang Shanshu et al. (1992)

basin in this area: 28 km compared with 36-42 km at the basin margins (Liu Guodong, 1987).

There is an important regional unconformity above the Dongying Formation (i.e. above the syn-rift succession). This unconformity records a more complicated event than a straightforward transition from syn-rift to post-

N

z

-

I

5km

Early Tertiary normal faults ??

Well

Figure 13 Examples of dextral transtensional Baxian Sag, Jizhong Depression. Location After Lu Banggan (1991)

fault array from the shown on figure 3.

and inversion: M. B. Allen et al. rift thermal subsidence: there was regional uplift and erosion of the Dongying Formation prior to Guantao deposition. Estimates of exhumation vary, with contributions from recent fission track studies suggesting that in the early Miocene there was about 1300 1450m of exhumation and erosion in the Dongpu Sag in the south of the basin (Wang Lizhi ct ill., 1994), ‘more than 200 m’ in the Dongying Sag (Guo Suiping et a/., 1996), c. 1,000 m in the Huangkou Sag (Liang Zhigang, 1996). In all cases the basal Neogene stratigraphy is undisturbed. but upper portions of the syn-rift sedimentary succession are exhumed (F&I.P 9). Few authors provide a more detailed stratigraphy than placing the youngest syn-rift rocks of the Dongying Formation in the Oligocene. and the post-unconformity strata of the Guantao Formation in the Miocene (F&u~e 5). For the Langfang-Gu’an Sag, Li Desheng et d. ( 1988) placed the Dongying Formation in the upper part of the Oligocene (30.-34 Ma), but recorded sedimentation restarting with the deposition of the Guantao Formation at I4 Ma. If a similar chronostratigraphy is true for other regions of the basin, it implies a IO million year gap between syn-rift and post-rift sedimentation. The Neogene and Quaternary have not been free from faulting. but deformation has been drastically reduced. with most faults either entirely inactive since the Oligocene or showing only minor amounts of Neogene extension in comparison with their early Tertiary histories (Figwt~ I I). There has also been localised, but significant compressional or lranspressional deformation. For example, there is evidence for thrusting prior to the deposition of the Miocene Guantao Formation in the Liaohe Depression (Lu Banggan, 1991; Fi,yure 15). This particular thrust developed at a compressional overlap between a left-stepping pair of dextral faults (Wang Tonghe, 1988). Also in the Liaohe Depression, the western margin of the Damintun Sag is a thrust that strikes northeast and overthrusts to the southeast (Ge Taisheng et Ill., 1992). Within and around the Langfang-Gu’an Sag in the northern Jizhong Depression. the Shahejie and Dongying strata are commonly truncated by the basal Neogene (Fi,pre 16). This is a distinctive feature of unconformity the northwestern part of the Jizhong Depression, and seems to be a result of late Oligocene to early Miocene inversion of early Tertiary normal faults, such as the Daxing Fault. At the western margin of the Jizhong Depression, the east-dipping normal fault at the western margin of the Xushui Sag became a thrust (Zhao Zhongyan and Windley. 1990). and partially exhumed the synrift succession in its hangingwall prior to the onset of Neogene sedimentation. Zhao Zhongyan and Windley (1990) described Quaternary thrusting, including some inversion of previous normal faults, from the southern end of the LangfangGu’an and Baxian sags in the Jizhong Depression. Because the seismic sections presented do not subdivide the NeogeneeQuaternary succession, and because the faults die out more than 500m below the surface. the deformation may also be partly Neogene. or entirely Neogene. Zhao Zhongyan and Windley (1990) attributed this deformation to east~ west compression across the basin. caused by a long-distance effect of the India-Asia collision. However. this analysis relied on the asymmetric moment tensor calculated for the Bohai region by Molnar and Deng Qidong (1984). who obtained a pattern of

Cenozoic

extension,

thermal

subsidence

and inversion:

967

M. B. Allen et al.

~~~~

aseismic

Moho

-

pa

-

.,..

aseismic Moho Figure 14 Illustration of the method of Jackson and White (1989) for estimating the extension in a half graben system. In (a), OOis the angle between the surface and the incipient fault plane. In (b), H, is the angle between the surface and the fault plane at the end of rifting

NW 0 6)

l-

SE

8

a

e

ci

fl

M

0 km

Neogene + Quaternary

1

2

: 3

t

a

0

4

Figure 15 Mid-Tertiary thrusting (1991) and Wang Tonghe (1988)

within the western

+++++++*+++++++++++t

A

0

+++++

Neogene and Quaternary Sha-2 Member 10 Dongying Formation

part of the Liaohe Depression.

+ + $iium&hen’ Ris6 ++*+**+++ +++*+++++++*+*++

Location shown on Figure 3. From Lu Banggan

+

I

10 km

I

Sha-3 Member Kcngdian Fomwdton and Sha-4 Member D

Pm-Tertiary strata

m

Precambrian igneous and metamorphic basement

Figure 16 Cross-section though the northern part of the Jizhong Depression. Note the truncation of the Shahejie formations beneath the regional Neogene unconformity. Location shown on Figure 3. From Tang Zhi et a/. (1988)

and Dongying

968

Cenozoic

extension,

thermal

subsidence

approximately equal north south extension and east west compression. A similar exercise was performed by Chen Wangping and Nabelek (1988), who used an improved analysis of the Bohai Sea shock of 1969, and showed that the regional strain due to seismic slip is principally dextral simple shear on NNE trending faults. Thus the Quaternary deformation may be caused by transpressional structures developed locally on strike-slip faults rather than a regional, east-west, compressional event. A rejuvenation of sedimentation took place in the Quaternary. Isolated Quaternary depocentres in the northwest of the basin have received as much as IO00 m of sediment (Ye Hong et d., 19X5). Chen Wangping and Nabelek (I 988) proposed that this Quaternary subsidence has taken place in a series of pull-apart basins. developed between right-lateral right-stepping faults in this region, that trend northeast or NNE. The disastrous Tangshan earthquakes of 1976 represented activity on one of these structures. One of these shocks was a thrust. interpreted by Nabelek et cd. (1987) as the result of a rare left-stepping. compressional overstep between two dextral fault segments. There is other evidence for localised Quaternary compressional deformation in and around the Bohai Basin. An earthquake in November 1983 near the Lanliao Fault was caused by a thrust (Dziewonski et a/.. 1984). We suggest that the basin was thrown into mild compression or transpression at the end of the Oligocene, and has undergone punctuated compressional events between the end of the Oligocene and the present. This compression is important for several reasons. First, it creates the possibility of hydrocarbon traps typical in inverted fault systems. Second, the resultant uplift must be incorporated into subsidence and maturation models as the thermal subsidence history of the basin may deviate from theoretical models that assume a neutral state of stress in the post-rift phase.

Discussion There are a number of structural arguments that suggest a dextral strike-slip regime for the tectonic setting of the Bohai Basin: the overall shape of the basin, fault plane solutions for major earthquakes in the region and fault arrays that indicate a component of dextral shear. Whereas the overall shape of the basin is similar to a very large dextral pull-apart. there are some features that do not fit a simple pull-apart model, in particular the significant WNE-ESE extension across large regions of the basin especially the Linqing. Jizhong. Huanghua and Liaohe depressions. Although these regions are sometimes simplified as single dextral strike-slip faults on small-scale maps, their widths are about 100 km not much less than the Central Graben of the North Sea. We account for the distribution of structures and sediments within the Bohai Basin in a two-phase deformation model. illustrated in Figwr 17. In the late Paleocene and early Eocene, extension was concentrated in the western part of the basin. principally in the Jizhong and Huanghua depressions and the western part of the Jiyang (Fi,qwc 17~0. Many of these rifts are orienDepression tated approximately NNE SSW. There was a component of dextral transtension to the rifting, expressed by en Cchelon secondary faults within each major sub-basin which are individually clockwise oblique to the normal

and inversion:

M. B. Allen

et al.

faults at the sub-basin margins. Minor rifting occured in the Liaohe Depression. and almost none in the Bozhong Depression during this time. The Kongdian Formation was deposited during this phase. In middle Eocene times (ca. 43-45 Ma), faults in the Liaohe Depression propagated southwards, possibly reactivating late Mesozoic structures in this area: Lower Cretaceous sediments in eastern Liaoning Province are exposed in small, fault-bounded outliers that cut across the basement structural grain (Bureau of Geology and Mineral Resources of Liaoning Province, 19X9). Splays of secondary faults that branch otT the main structures suggest that this middle Eocene deformation was also dextrally transtensional. Fault propagation linked the easternmost faults of the Liaohe and Jiyang depressions (Fi+~c 3). and created an overlap zone between the TanLu and Taihang Shan fault systems. This overlap Lone was analogous to a classic pull-apart basin developed in an overlap between simple strike-slip faults: it underwent major north south extension to create the Bozhong Depression (Ficpm 17h). This model accounts for the initiation of sedimentation in the Bozhong Depression at this time. for the prominent east--west orientation of faults in the depression which is anomalous to the peripheral rifts, and for the high amount of extension in this area compared to other rifts. Sedimentation continued in rifts outside the Bozhong Depression in much the same manner as during the early Eocene, although the increase in subsidence rates during the deposition of the Sha-3 Member and the basin-wide switch to anoxic. deep water sedimentation suggests that there was a general increase in the extensional strain rate at this time. We suggest two possible causes for the termination of rifting at the end of the Oligocene!‘early Miocene. First, the early Miocene was an important time for changes in rt (I/., 1986; tectonic style elsewhere in Asia (Tapponnier Avouac c’t (I/.. 1993), with shifts in the location and style of compressional deformation related to the continuing. post-collisional India/Asia convergence. Deformation began, or greatly intensified in regions of Central Asia far removed from the Himalayas (Hendrix (21(I/., 1994). The end of extension in Bohai and other eastern Chinese basins may be a long-distance effect of this tectonic realignment, with continued motion on the Tan-Lu and other major faults becoming transprcssional rather than transtensional. Second, the OligoceneeMiocene boundary was the time at which the northern Australian continental margin collided with the Philippine Sea Plate (Fi,quw lX). which began to rotate clockwise (Hall, 1996). This rotation. and subsequent collisions of arc fragments on the Philippine Sea Plate with the Asian margin, could have thrown the earlier Tertiary extensional basins of China into mild compression. witnessed by Neogene thrusts in several of the basins besides Bohai, such as the South Yellow Sea (Lu Banggan, I99 I ) and East China Sea (Wang Guangming et (I/.. 1995). and the end or great diminuation of rifting in other basins where thrusting has not been unambiguously identified (Jianghan, Zhoukou, Beibu Gulf. Qiongdongnan). Note that where early Tertiary rifting was intense enough to product an ocean ridge system. in the South China Sea, Sea of Japan and Kurile Basin, seafloor spreading continued in each case well into the Miocene. Whatever caused the end of rifting in the continental basins was not strong enough to terminate sea-tloor spreading.

Cenozoic a Paleocene-early

Eocene:

extension,

Kongdian

thermal

Formation

.

subsidence

M. B. Alien et al.

and inversion:

b Middle Eocene: 3rd Member

Shahejie

969

Formation,

Jiyang depressions begins at this time

Figure 17 Schematic diagram for the Tertiary evolution of the Bohai Basin. (a) (Paleocene-Eocene) sedimentation occurs as result of dextral transtension in narrow, isolated rifts in the Jizhong, Liaohe, Linqing and southern Huanghua depressions. (b) (middle Eocene) rifting has propagated southwards from the Liaohe Depression, and possibly northwards from the eastern Jiyang Depression, to create an extensional overlap that is the Bozhong Depression. Extension on east-west normal faults in this region led to rapid subsidence and deposition of the thickest successions of the Shahejie Formation known from the Bohai Basin

We prefer the Australian collision hypothesis, because (1) the eastern Chinese basins are closer to the Philippine Sea Plate than compressional ranges in Central Asia active in the early Miocene, (2) the exact timing and nature of the mid-Tertiary reorganisation of the IndiaAsia collision zone are not yet precisely understood, (3) the early Miocene halt in rifting was abrupt, given present stratigraphic resolution, in separate basins for over 2500 km along the eastern Asian margin. This is better explained by a readjustment of the Asia-Philippine Sea Plate boundary than an effect propagating from the India-Asia collision zone.

Conclusions A wealth of published data on the Tertiary rift basins of eastern China has become available in the last ten years, as a consequence of their importance in regional geology and hydrocarbon exploration. Clearly, much work remains to be done, both in understanding the geology of individual basins and the regional controls on basin development. From a synthesis of data on the Bohai Basin, we conclude that transtension played an important part in its evolution, both in an early phase in isolated half grabens, and in a second phase when the area undergoing deformation increased greatly to include the Bozhong Depression in the middle of the present basin. This region resembles a large, complex pull-apart structure between narrower transtensional zones to the east and west. Its development coincides with the propagation of faults southwards from the Liaohe Depression; we suggest that the two events are linked, with the initiation of the Bozhong Depression occurring once an extensional overlap was created between the Liaohe and Huanghua depressions. Significant extension began in the Bozhong Depression with the deposition of the Sha-3 Member.

as deduced from isopach and facies maps. Deep-water lacustrine rocks of this member are distinct from alluvial facies in the underlying Sha-4 Member and Kongdian Formation elsewhere in the basin, and so probably there was a basin-wide increase in extensional strain rates at this time. Extension took place over c. 30 million years, roughly from the late Paleocene-early Eocene to the end of the Oligocene, with the boundary between the two phases of extension described above occuring near the beginning of the middle Eocene (ca. 4345 Ma). The end-Oligocene termination of significant rifting was part of a regional event, that affected other basins in eastern China. Two broad possible causes can be identified: a far-field effect of the India-Asia collision, or a reorganisation of plate motions between Asia and the oceanic plates to the east. We favour the second cause, and single out the collision of Australia with the Philippine Sea Plate as the likely specific event that induced mild compression along much of the east Asian margin in the Neogene (Fiyure 18). This deformation is superimposed on. and commonly masked by, thermal subsidence that followed the end of rifting. This post-rift phase has lasted for ca. 25 million years, and continues at present.

Acknowledgements This work originated as part of the CASP China Basins Project. which has been supported by the following companies: AGIP, Amoco, Anderman-Smith, Apache. Arco, Chevron. Conoco, Deminex, Exxon, JNOC, Louisiana Land and Exploration. Mobil, Phillips, Texaco and Unocal. We thank David Roberts and Steve Bergman for constructive reviews, and Kong Fanchen for useful discussions. This is Cambridge University Department of Earth Sciences Contribution No. 4918.

970

Cenozoic extension,

thermal subsidence

and inversion: M. 6. Allen et al. SO-E /

loJo

120-

/

/

WY

160'

\

\

Earl Tertiary rift basin, last extension c.25Lla Early Tertiary basin, last extension other than 25Ma Oceanic crust in minor basin Continental crust Arc magmatism Oceanic spreading centre Plate motion sense

Pacific Plate

r-y \ \

. ,....ppine

?

Caroline ate

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AUSTRALI’A’

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Figure 18 Tectonic reconstruction of the east Asian margin at 25 Ma. Collision of Australia and the Philippine Sea Plate causes clockwise rotation and northward motion of the Philippine Sea Plate. This causes mild compression in east Asian early Tertiary rift basins, but fails to end sea-floor spreading in the South China Sea, Sea of Japan and Kurile Basin. Plate boundaries adapted from Hall (1996) and Jolivet et a/. (1989)

Cenozoic extension,

thermal subsidence

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