Plate tectonic history, basin development and petroleum source rock deposition onshore China

Plate tectonic history, basin development and petroleum source rock deposition onshore China M. P. Watson, A. B. Hayward, D. N. Parkinson and Zhang Zh. M. BP P e t r o l e u m D e v e l o p m e n t Ltd., P.O. Box 509, G u a n g z h o u , People's Republic of China

Received 16 March 1987; accepted 20 Apri/ 1987 China comprises a mosaic of distinct continental fragments separated by fold belts. These fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust. The major sedimentary basins onshore China can be classified into four groups. Those in western China are flexural, developing as a result of north-south compression. In contrast, those in the east are extensional and related to development of the Pacific oceanic margin. In central China, basins have a more problematic origin. Those of north central China (Ordos, Sichuan) are flexural basins controlled by eastward directed thrusting along their western margin. In contrast, basins further south (Chuxiong, Shiwandashan) are predominantly extensional and related to major strike-slip movements. By synthesizing basin stratigraphies across China in tectonostratigraphic terms (and in particular comparing the nature and timing of unconformities), it is possible to formulate a coherent model for the palaeoreconstruction of China. We identify five major tectonostratigraphic breaks which equate w i t h the collision of the following continental fragments: Tarim/North China (Carboniferous-Permian), South China Block (Permian-Triassic), Qiantang (Late Triassic-Early Jurassic), Lhasa Block (Late Jurassic-Early Cretaceous) and India (Early Tertiary). Prior to Permian times, the southern margin of Eurasia ran approximately along the northern border of modern China. The Late Carboniferous collision of Tarim/North China with Eurasia resulted in the development of a flexural basin (Junggar) and deposition of non-marine clastics. To the south of the suture, shallow marine deposition continued. In the Late Permian-Early Triassic, the progressive collision from east to west of the South China Block with the North China Block resulted in a change to fluvial/lacustrine sedimentation across the entire North China-Tarim block. Open marine carbonate deposition in the north of the South China Block passed southward into a deeper marine clastic sequence deposited in a backarc basin. Further south, a subduction zone existed along the southeastern margin of the South China Block. In western China, northward subduction throughout the Triassic resulted in the development of the Songban-Ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the Tarim basin. Oblique collision of the Qiantang Block in the Late Triassic along the east of the South China Block resulted in east-west directed thrusting which initiated the Sichuan and Ordos basins. Continued strike-slip deformation along the south western margin of the South China Block resulted in the development of basins with a significant extensional component such as Chuxiong. The collision of the Qiantang Block with the southern edge of the Tarim Basin (Early Jurassic) resulted in a renewed clastic influx in both the Tarim and Junggar basins. Along the eastern (Pacific) margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system. Subduction rollback of the extensional arc initiated rifting in the Late Jurassic in the Eren and Songliao basins. The Late Jurassic-Early Cretaceous collision of the Lhasa Block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx. In the southeast, the compressional arc phase culminated in widespread thrusting and folding of Early Cretaceous age. In the northeast, extension continued with the progressive migration of the rift system southward with time. The arrival of the Indian Block in the Early Tertiary rejuvenated the bounding thrust belts of all the western basins. In the east, the change in convergence of the Pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of Eastern China. Throughout most of China, Mesozoic and Cenozoic deposition occurred in predominantly non-marine environments. Source rocks in such settings comprise principally mudstones deposited in lakes (organic-rich mudstones). These can accumulate in both deep and shallow lakes. In order to accumulate substantial volumes, the lake must be significant in space and time. In China, lacustrine ORMs occur in both rift and flexural basins. Lacustrine ORMs deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence. In the flexural basins of western China, source rock deposition follows basin initiation by 20-30 Ma. In the extensional basins of eastern China, source rock deposition takes place 5-15 Ma after basin initiation. By contrast, semi-arid and arid climate lacustrine ORMs, whilst being best developed during the period of maximum tectonic subsidence, occur at all stages in the basin history. Keywords: Pla~ tectonics; basin development; petroleum source rock; China, onshore

0264-8172/87/030205-21 $03.00 ©1987 Butterworth & Co. (Publishers) Ltd M a r i n e and P e t r o l e u m G e o l o g y , 1987, Vol 4, A u g u s t

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Onshore China p/ate tectonic history," M. P. Watson et al. 1980; Lin et al., 1985; Zhang et al., 1984). Introduction The Phanerzoic history of China can thus be The geological evolution of China reflects the response described in terms of the successive accretion of to the prolonged and complex interaction of the continental fragments around the margins of Eurasia in Eurasian, Indian and Pacific plates along the major much the same way as is manifested by the present-day Mesozoic-Cenozoic tectonic zones of the tectonic setting (Sinha-Roy, 1982; Sengor, 1984; Circum-Pacific and Tethys-Himalaya. Indeed, the Shermer, 1984). The definition and origin of these present-day tectonic setting of China demonstrates this fragments and their history of accretion is a topic of continuing interaction (Figure 1) which can be seen in active debate in the current literature (Klimetz, 1983; neotectonic activity and associated earthquakes 1985; Zhang et al., 1984, 1985; Ji and Coney, 1985; Lin (Molnar and Tapponier, 1977; 1978; Peltzer et al., etal., 1985; Mattauer etal., 1985; Wang and Lui, 1986). 1985). The presently active convergent margins to the A better understanding of the relative positions of south and east (Figure 1) can be traced back into the different fragments prior to accretion, together with the Mesozoic-Cenozoic plate tectonic history of China. definition and timing of development of some fragment These convergent margins exhibit very different effects boundaries, awaits more detailed field mapping in at the present time, with extension in the western conjunction with more palaeomagnetic studies to Pacific (Hilde et al., 1977) and compression on the supplement the existing work (McElhinny et al., 1981; southern, Himalayan margin (Allegre et al., 1984). Lin et al., 1985). China comprises a mosaic of distinct continental Recent geological literature on China has fragments separated by major foldbelts. These concentrated on the tectonic setting of the foldbelts commonly incorporate remnants of ophiolite Mesozoic-Cenozoic basins and structural configuration sequences and high pressure-low temperature within the basins (for example, Tang, 1982; Zhai et al., metamorphic rocks suggesting the existence of sutures 1985; Chen and Dickinson, 1986; Liu, 1986) and upon resulting from the accretion of the various fragments their relationship to petroleum habitat (for example, formerly separated by intervening areas of oceanic Wang et al., 1982; Tian et al., 1983; Zha, 1984). crust (Huang, 1978; Huang et al., 1980; Xiao et al., Notable exceptions are Klimetz (1983) and Zhang et 6 0/ °

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Figure 2 M a p of China s h o w i n g m a j o r M e s o z o i c - C e n o z o i c s e d i m e n t a r y basins, and the major discrete f r a g m e n t s and separating sutures discussed in the text. Sutures are as f o l l o w s : 1 = North Junggar; 2 = Tianshan; 3 = Hegen; 4 = K u n l u n - Q i n l i n g ; 5 = Jinsha River; 6 = B a n g o n g Lake; 7 = Yarlung River; 8 = Qilianshan; 9 = Yinshan. M a j o r fold-thrust belts with no clear evidence f o r suturing are as f o l l o w s : 10 = Helanshan; 11 = L o n g m e n s h a n . M a j o r stike-slip faults are as f o l l o w s : 12 = Altyn Tagh; 13 Tanlu; 14 = Red River. Note the sense of m o v e m e n t on these faults may have varied with time al., (1984, and in press) who looked at plate tectonic development, and a recent analysis of terrane accretion by Ji and Coney (1985). These studies have concentrated on the geology of the foldbelts to elucidate plate tectonic history. By contrast, it is the aim of this paper to focus on plate tectonic history in relation to basin development from the stratigraphic as well as the tectonic viewpoint. The two are clearly intimately related and we shall attempt to address two questions. First, what the basin-fill reveals about the plate tectonic history, and secondly, how plate tectonics can be used to understand and classify the basins.

Plate tectonic history The major fragments which we recognise, together with their bounding sutures and crustal lineaments are shown in Figure 2. These are superimposed on the main Mesozoic-Cenozoic sedimentary basins which have been defined for the purposes of this study. These are discussed under basin development below, together with a more detailed discussion of the timing of the accretion events broadly outlined here. The definition of some fragment boundaries is controversial, especially where there is no clear evidence for suturing,

for example the boundaries of the North China Block and South China Block along the western margins of the Ordos and Sichuan Basins respectively. Furthermore, as will be discussed below, many of the sutures represent diachronous accretion or "scissor' closing along their length, such that designating a precise timing to a particular event necessitates a certain degree of generalization• We consider the southern margin of the Eurasian Block to be marked by the Hegen suture in the east (Zhang et al., 1984). In the west, the definition of the Eurasian margin is more controversial. The presence of a cratonic block beneath the Junggar 'Block" is conjectural (Zhang et al., 1984; Ji and Coney, 1985). Junggar is surrounded by a triangle of ophiolite belts and the collisional event associated with these is considered to be Late Carboniferous in the north and between Late Carboniferous and Permian in the south (earlier in the west than in the east: Zhang et al., 1984, Lin et al., 1985). It is possible that Junggar represents a separate microcontinent accreted during Middle-Late Carboniferous. However, these events are temporally very close and our preferred interpretation is for a marginal basin which was closed by the impact of fragments to the south of Junggar.

Marine and Petroleum Geology, 1987, Vol 4, August

207

Onshore China plate tectonic history: M. P. Watson et al. Jurassic age separating the Qiantang Block from the We interpret the Tian Shan suture (2 in Figure 2) as Songban-Ganzi terrane. To the south, the Late continuing eastward to link up with the Hegen suture to Jurassic/Early Cretaceous Bangong Lake suture form the southern margin of Eurasia in the strict sense. separates the Qiantang and Lhasa Blocks, and finally It is unclear precisely how these sutures join in north the Early Tertiary Yarlung River suture separates central China. The precise definition of the limits of the Tarim and Lhasa from the India Block. It should be noted that strike-slip tectonics have North China Blocks is problematical. We currently consider the North China Block (including the played, and continue to play, an important role in the Songliao Basin) and the Tarim Block, as a unified geological evolution of China, Many of the fragments defined above need not have accreted in their present fragment in terms of their effect on basin development. spatial positions relative to each other (Molnar and Sutures within this fragment, such as Qilianshan and Yinshan (8 and 9 respectively in Figure 2), are thought Tapponnier, 1977; Mattauer et al., 1985). to have little relevance to Mesozoic-Cenozoic basin development other than providing inherited basement inhomogeneity. Basin classification The South China Block is the name applied to the Recent advances in understanding lithospheric unified Pre-Cambrian Yangtze Craton and the South subsidence have permitted recognition of two genetic China or Huanan Foldbelt which amalgamated in the classes of basin based on subsidence mechanisms Early Paleozoic (Ji and Coney, 1985). There is much (Beaumont et al., 1982; Dewey, 1982). Rift basins debate as to the age of the suture between the North and South China Blocks (Qinling suture in Figure 2). subside due to extensional thinning of the lithosphere, which produces initial faulting and isostatic subsidence. Arguments have been advanced for a Permian age This is followed by thermal subsidence of the cooling (Zhang et al., 1984) or earlier (Mattauer et al., 1985), aesthenosphere that rose to replace the thinned whilst some authors would place it later on in the lithosphere (McKenzie, 1978). This typically results in Triassic, based largely on paleomagnetic evidence a broad concave basin overlying a narrower, (Klimetz, 1983; Lin et al., 1985; Ji and Coney, 1985). block-faulted rift. Flexural basins subside due to We believe this can be accommodated by a diachronous downwarping of the lithosphere by loading by thrust impingement of the South China Block on the North China Block, starting in the Permian in the east. This sheets in a compressional regime (Beaumont, 1981). would link up westwards with an Early Triassic age for The subsidence history of a flexural basin tends to be linear with time, directly related to episodes of the Kunlun suture (Chang Chengfa, et al., 1986). It emplacement of thrust sheets, resulting in a typically thus appears that the greater part of the Chinese wedge-shaped basin, thickening towards the thrust mainland was amalgamated by Late Permian/Early front (Dickinson, 1974). Both these basin types may Triassic times. occur in a variety of plate tectonic settings and may also The Mesozoic plate tectonic history is mainly be highly modified by strike-slip faulting. concerned with the construction of what is the There have been several recent attempts to classify present-day Tibet plateau. There is good evidence that sedimentary basins within China (Zhai et al., 1985; the accreted continental fragments of Tibet originated Chen and Dickinson, 1986; Liu, 1986; Xiang, 1986). by rifting from Gondwanaland (Xiao, et al., 1980; Jin, There is a broad agreement that basins in western 1981" Wang and Sun, 1985). They were accreted China developed by compression resulting from the against the southern margin of Eurasia as a successive collisional events along China's southwest consequence of consumption of 'palaeo-Tethys' while margin, although their development with time has not at the same time 'neo-Tethys' was being formed between the rifted blocks and the nucleus of been discussed in detail. Most authors also agree that basins in eastern China have extensional origins (for Gondwanaland (Sengor, 1984). example, Tang, 1982; Ma et al., 1983), although the The Songban-Ganzi 'terrane' is so called because driving mechanisms are a subject of debate (Molnar there is no clear evidence that it represents a discrete and Tapponier, 1977; Tapponier et al., 1982; Zhang et crustal fragment, although the presence of a small triangular block in the northeast, trapped between the al., in press). We are in general agreement with previous North and South China Blocks has been postulated classifications (Figure 3), the western basins being (Huang et al., 1980; Ma et al., 1980; Zhang et al., 1984). east-west trending flexural basins developed by This terrane, with its extremely thick Triassic turbidite north-south compression, while those in the east are deposits together with ophiolite slices may represent the deformed remnants of one or more-accretionary approximately north-south trending extensional basins. The basins in central China are more prism complexes. problematical (Ordos, Sichuan, Chuxiong etc.). To the south of the Songban-Ganzi terrane, fragSeveral authors have drawn a dividing line between ments are named after the largest continental fragment western compression and eastern extension to the west which can be recognised within them. The Qiantang of these basins (Zhang et al., 1984, and in press; Chen and Lhasa Blocks thus comprise deformed trench and and Dickinson, 1986; Lui, 1986). Indeed, Zhai et al. arc sediments in addition to relatively und~formed (1985) recognised the problematical nature of their platform deposits (Wang and Sun, 1985). The definiorigin and classed them as 'transitional'. tion of these slices which make up present-day Tibet We consider that the form of the Ordos and Sichuan is relatively well documented (Xiao et al., 1980; basins strongly suggests flexuring as a response to Andrews-Speed and Brookfield, 1982; Allegre et al., east-west compression and thrusting along their 1984; Mercier and Li, 1984; Wang and Sun, 1985; Chang western margins. Basins to the south of Sichuan (e.g. Chengfa et al., 1986). There is a southerly progression Chuxiong, Shiwandashan), appear to have a from the Jinsha River suture of Late Triassic/Early 208

Marine and Petroleum Geology, 1987, Vol 4, August

Onshore China plate tectonic history: M. P. Watson et al.

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predominantly extensional component that is interpreted to be a result of transtension associated with strike-slip movement. Each basin in the central belt of basins appears to have been initiated during approximately the same period, that is, Late Triassic. They experienced compression or extension depending upon their location along the block margin or within the block. Thus we identify two additional categories of basin lying in the central zone between eastern extensional basins and western compressional basins (Figure 3). The dividing line between these central basins and the strictly extensional basins lies to the east of Ordos and Sichuan.

breaks since Devonian times which we equate with the accretion events discussed briefly above, namely: the Tarim/North China Block collision with Eurasia, the South China Block collision, and collision of the Qiantang, Lhasa and India Blocks. A series of six palaeoreconstruction maps are presented {Figures 5-I0), which essentially bracket each of the five tectonostratigraphic events. These maps represent a 'snapshot' of a continually moving picture, and are of necessity grossly generalized. Basin initiation and classification are fitted into this regional tectonostratigraphic framework in Figure 4.

Carboniferous

Palaeoreconstruction of the Mesozoic-Cenozoic basins in China The sedimentary record preserved in the Mesozoic-Cenozoic basins of China provides the key to constructing a more refined tectonostratigraphic history. Many individual basins show features compatible with the inferred accretion history advanced here. However, by synthesizing basin stratigraphies in tectonostratigraphic terms, and in particular comparing the nature and timing of unconformities, it is possible to formulate a more coherent model for the palaeoreconstruction of China. We have identified five major tectonostratigraphic

Prior to Permian times, the southern margin of Eurasia ran approximately along the northern border of modern China (Figure 5). Volcanics, volcaniclastics and thick sequences of marine mudstones were deposited in Junggar in what we interpret to be a marginal basin. This may have opened by back-arc spreading and separated a small fragment of the Eurasian continent in the Lower Palaeozoic in response to northward subduction. In the Early Carboniferous. this marginal basin is assumed to be closing, again by northward subduction, as evidenced bv thick arc-related volcanics in the Altaishan and Karamay thrustbelt (Figure 5, section A A ' ) .

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Permian Following the Late Carboniferous collision of Tarim/North China with Junggar, and the approximately c o n t e m p o r a n e o u s closing of the Junggar marginal sea, there was initiation of the Junggar Basin, the first isolated depocentre which has retained its approximate form to the present day (Figure 6). There was now no access to open marine conditions as the thrust belt of the early Tian Shan was emplaced along the southern margin of this basin. This gave rise to the earliest recognisable flexural basin with thickening of Permian non-marine clastics into the foreland trough (Figure 6, section A A ' ) . Within the Junggar Basin, marginal coarse alluvial clastics gave way to thick lake mudstones. 212

The northern margin of the precursor of the Tarim Basin was developing as a hinterland basin on the back of the Tian Shan thrust belt. Deposition of shallow marine clastics continued in southern Tarim which is interpreted as a compressional back-arc basin behind a northward directed subduction zone in the area of the Kunlun Shan. Shallow marine sedimentation took place to the east across the Q a i d a m and Nanshan areas, and passed eastwards into non-marine fluvial sedimentation over most of the North China Block. The ocean to the north is thought to have closed progressively from west to east, with final closure in the northeast in the Late Permian (Figure 6). This correlates with an Early Permian marine sequence that is overlain by a non-marine Late Permian sequence that becomes progressively younger eastwards. Also in the Late Permian, we consider that the South China Block had begun to collide with the North China Block. Again this closure may have been diachronous, but in this case east to west, such that a westwardwidening gap remained into the Triassic. This collision

M a r i n e and P e t r o l e u m G e o l o g y , 1987, Vol 4, A u g u s t

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in the east accounts for the deposition of thick fluvial and deltaic clastics in basins developed adjacent to the collision belt in the North China Basin• Over the rest of the South China Block, shallow marine carbonates and mudstones persisted, except along the western margin where coarser clastics were deposited as well as outpourings of thick basalts (cf. the well known Omeishan basalts). These may represent attempted back-arc spreading along the margin• Reconstruction of this margin is, however, highly speculative due to later strike-slip deformation which has removed much of the evidence.

Triassic In Late Permian-Early Triassic, the South China Block became completely sutured onto the North China Block along the Qinling Suture. Sedimentation was entirely non-marine fltl~cio-lacustrine across North China/Tarim Block by this time (Figure 7). To the south, the newly accreted South China Block was a

very broad area of marine carbonate deposition. This passed southwards into very thick, deep marine clastics interpreted as the fill of back-arc basins developed behind a subduction zone around the southeast margin of the South China Block. This is the first evidence of an active margin with subduction of a precursor of the Pacific plate (Kula plate) which may have been moving northwards such that the margin became a strike-slip boundary to the north• In western China, northward subduction along the southern margin of the Tarim Block continued.- No colliding continental fragment can be identified, but huge volumes of deep marine clastics were being fed along the westward-closing ocean between the North and South China Blocks and into the triangular shaped ocean basin to the west. These clastics were derived from the Qinling collisional fold/thrust belt as well as from arc(s) along the southern margin of the Tarim Block. They were accreted into the huge accretionary prism complex of the Songban-Ganzi terrane. A modern analogy might be the Cenozoic-Recent

Marine and Petroleum Geology, 1987, Vol 4, August

213

Onshore China p/ate tectonic history." M. P. Watson

et al. f "

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accretion of the Bengal submarine fan along the A n d a m a n - N i c o b a r trench (Curray and Moore, 1974). Northward subduction, along the southern margin of the Tarim Block, gave rise to a compressional arc with northward thrusting which in turn generated a flexural basin behind the arc (retroarc basin - - Dickinson, 1974). Thus for the first time, the Tarim Basin was

confined on both sides and initiated in its present form in the Triassic. Continued northward subduction resulted in the collision of the Qiantang Block, in the Late Triassic/Early Jurassic. Throughout this time, the western margin of the South China Block was a major transcurrent or strike-slip zone. This strike-slip

214 Marine and Petroleum Geology, 1987, Vol 4, August

Onshore China plate tectonic history." M. P. Watson et al.

Jurassic

movement, combined with oblique compression caused by the collision of the Qiantang Block, produced thrusting along the western margins of Ordos and Sichuan. These two basins were thus initiated as east-west compressional basins in the Late Triassic, an event clearly seen in the Sichuan Basin with the change from marine carbonates to non-marine clastics in the Late Triassic• Strike-slip in the southwest gave rise to local extension, so that basins such as Chuxiong subsided rapidly and became filled with Late Triassic, predominantly fluvio-deltaic deposits.

By the Early Jurassic, China had taken on a form approaching that of the present day except in the southwest. Following the so-called 'Indosinian' event, most of the basins in the central and northwest areas were defined as discrete entities by this time (Figure 8). We equate this Indosinian event with collision of the Qiantang Block along the Jinsha River suture in the Late Triassic-Early Jurassic. Probably somewhat earlier, the Indo-China Blocks were accreted onto the southwest margin.

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Marine and Petroleum Geology, 1987, Vol 4, August

215

Onshore China plate tectonic history: M. P. Watson et al. China, Dongting and Nanyang (Figure 9, section BB'). The western basins already in existence, such as Junggar and Tarim, underwent rejuvenation of thrusting along their margins which in turn gave rise to Early Tertiary a further episode of flexural subsidence and a renewed pulse of clastic input. Basins such as Qaidam and The arrival of the India Block in the Early Tertiary and Nan Shan were also initiated at this time as extencollision along the Yarlung River suture (Molnar and sional intermontane basins. Probably related to Tapponier, 1975; Powell, 1979; Andrews-Speed and post orogenic extension, they comprise a series of small Brookfield, 1982; Allegre et al., 1984; Klootwijk et al., halfgrabens infilled with coarse alluvial wedges that 1985) had a profound effect. Major northward pass distally into finer fluvial and lacustrine deposits. compression rejuvenated the bounding thrust belts of The complex transcurrent margin in central China all the western basins with tremendous thicknesses of continued to evolve as first the Qiantang, and then coarse clastics pouring into the developing flexural possibly smaller fragments, moved past it in a manner basins (Figure 10, section AA'). analogous to the Mesozoic-Cenozoic terrane history of Late Cretaceous-Early Tertiary marine incursions western North America (Howell, 1985). This into southwest Tarim, which had resulted from the Late north-south trending line of compressional strike-slip Cretaceous global sea level highstand (Vail et al., may have been transmitted northwards into the 1977), were terminated as the basin was isolated from continental block along major crustal inhomogeneities the sea to the west by the advancing thrust belt. (Zhang et al., in press). As the india Block continued to move northwards, In eastern China, the initiation of an active deformation continued (and is continuing), such that subduction zone commenced in Late Triassic-Early the effects of the indentation are manifested for great Jurassic (Klimetz, 1983). An Andean-type margin distances into the China/Eurasia landmass. In developed, with widespread associated igneous activity particular, earlier lineaments and sutures have been in the Jurassic along the southeast coastal belt of China reactivated as major strike-slip faults, giving rise to the (Shi, 1979; Shi et al., in press). The Kula/Pacific plate eastward movement or 'extrusion' of crustal blocks was moving northwards such that convergence, oblique (Molnar and Tapponier, 1977: Tapponier et al., 1982). convergence and strike-slip occurred contemporanIn the central basins, there was rejuvenation and eously along the length of the margin (Figure 8). This deformation of the western margins by the led, we believe, to differences in the tectonic style compressional stress generated by the eastward along the eastern margin of China, as the movement of the blocks. Generally, however, there arc/subduction complex changed from compressional to was no major subsidence or sedimentation. extensional mode. This is a common phenomenon in Eren and Songliao Basins had by this time almost present-day convergent margins (Dewey, 1980). ceased to subside thermally. To the east, convergence In the south, the arc was compressional, giving rise to of the Pacific plate had changed to a more westerly thrusting across southern China and a broad retroarc direction. This may have caused the fundamental foreland basin (Figure 8, section BB'). In the north, change from the Andean-type convergent margin with however, we interpret initiation of rifting in the latest varying compression and extension along its length, Jurassic in the Eren and Songliao Basin to result from which had controlled the eastern margin during the an extensional arc setting. This extension would be Mesozoic. In the Early Teriary, this margin began to driven by 'subduction rollback' of the Kula/Pacific Plate appear like the present day with extension along its oceanwards towards the east (Figure 8, section CC'). entire length giving rise to widespread rifting. Retreat The early rifts of Eren and Songliao were rapidly filled of the subduction system into the oceanic plate resulted with coarse alluvial-fluvial clastics, volcaniclastics and in extensional stresses propogating into the adjacent minor lake mudstones. continental crust, such that extensional basins opened all along the eastern margin of China. Examples arc North China Basin, Subei Basin, and numerous small Cretaceous basins in the Huanan Foldbelt. These filled with coarse The arrival of the Lhasa Block in the Late continental elastics initially but exhibit a broad fining Jurassic-Early Cretaceous, and its collision along the upwards into finer grained lacustrine deposition as Bangong Lake Suture, provided the next pulse of subsidence increased and the basins became broader. renewed thrusting in the western basins. These record The extension can be followed offshore into another the cycle of flexural downwarp and coarse elastic input belt of basins on the continental shelf of eastern and from the rejuvenated mountain front (Figure 9, section southern China. Notably the Yellow Sea, and Pearl AA'), the Qaidam and Nanshan basins being initiated River Mouth Basins record stretching and rifting in the as true flexural basins for the first time. Early Tertiary followed by regional subsidence in the In the east, subduction continued along the margin, Neogene (Figure 10, section BB'). with very widespread calcalkaline volcanics and intrusives. The compressional arc phase culminated in Basin fill and source rock development widespread thrusting and folding behind the arc across southeast China at the end of Lower Cretaceous. This The preceding discussion outlining the plate tectonic event produced the major folds seen along the southern evolution of China has shown that, throughout most of margin of the Sichuan Basin. the Mesozoic and Early Cenozoic, deposition in most In the northeast, the extensional arc regime of the Chinese sedimentary basins took place in continued. Both Eren and Songliao Basins were predominantly non-marine settings. Source-rocks in undergoing active Early Cretaceous rifting, while in this setting comprise principally mudstones deposited the Late Cretaceous, active extension migrated to the in lakes (i.e. lacustrine organic rich mudstones - south with initial rifting in central basins such as North ORM). 216

Marine and Petroleum Geology, 1987, Vol 4, August

Onshore China plate tectonic history: M. P. Watson et al. q

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Potential lacustrine O R M can accumulate in both deep and shallow lakes of varying physical and chemical character, in deep lakes, oil-prone organic matter is preserved when oxygen deficiencies occur in bottom waters due to bacterial oxidation of organic matter sinking through the water column and at the sediment-water interface. This depletion is enhanced and maintained by permanent stratification of the water body. In freshwater lakes, stratification is due to temperature differences between the surface and bottom layers. In large, temperate lakes preservation of organic matter is related to the establishment and persistence of anoxic bottom waters which in turn are related to lake circulation patterns (Dean, 1981). In

deep tropical lakes, there is no regular seasonal overturn and water oxygen contents are lower because of high temperatures, bottom waters can be permanently anoxic leading to preservation of organic matter. In saline and hypersaline lakes, stratification can also be caused by differences in the chemistry of the water column. Oil-prone organic matter can also be preserved in shallow lakes, particularly when they are hypersaline. Hypersalinity inhibits the non-adaptive life forms, notably the benthic organisms and bacteria, which are the two most effective consumers of organic matter. In addition, if sulphate is absent from lake waters, degradation by sulphate-reducing bacteria will

M a r i n e and P e t r o l e u m G e o l o g y , 1987, Vol 4, A u g u s t

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220

that are generally shallow (a few metres to a few tens of metres). In such a situation, the climatic control on run-off assumes more importance than the tectonic control. In the following discussions, we use examples of both flexural and extensional basins from China to outline the interaction of tectonic and climatic control on the distribution of lacustrine source rocks and illustrate the way in which an understanding of basin evolution may be used as a tool to predict source rock distribution.

Flexural basins The formation of flexural basins controlled by thrust loading within the adjacent mountain belt (Dickinson, 1974) results in the systematic distribution of sedimentary facies that can be used as a predictive tool in source rock distribution. Initial continental collision and development of the mountain belt is marked by an influx of coarse clastics (Figure 13). This may be superceded by the development of a lacustrine filcies association deposited in a stable lake site as the mountain belt continues to grow and subsidence exceeds sediment supply (Figures 13 and 14). As continental collision finishes, flexural subsidence decreases and the basin is once again filled by a prograding fluvial system (Figure 13). The simple tectonic control described above is well documented by the early history of the Junggar basin.

Marine and Petroleum Geology, 1987, Vol 4, August

Onshore China plate tectonic history: M. P. Watson e t a

Figure 14 Schematic model for sedimentary facies distribution in a flexural basin during period of maximum subsidence

In the Junggar Basin, Lower Permian continental clastics deposited following Upper Carboniferous continental collision are overlain by lacustrine ORMs of Upper Permian age deposited in a humid lake setting. These, in turn, are overlain by a Lower Triassic fluvial clastic sequence. The climate throughout this time period is humid and the control on source rock deposition is fundamentally a tectonic one with source rock deposition taking place during the period of maximum subsidence, in this case c. 30 Ma after basin initiation. The overall geometry of a flexural basin is controlled both by the distribution of loading within the thrust belt and the viscoelastic strength of the underlying lithosphere (Beaumont, 1981). The greater the load, and the lower the viscoelastic strength of the lithosphere, the deeper will be the basin and the greater the potential for subsidence to outstrip sediment supply resulting in conditions favourable for stable lake development. In contrast, small loads and high viscoelastic strength produces shallow and broad, long wavelength basins with a much lower potential for the development of a stable lake site. The contrast between these two extremes is particularly well demonstrated in the Permian between the Junggar and Tarim basins either side of the Tian Shan thrust belt. Late Carboniferous thrusting is predominantly northward, creating a thrust load that is asymetrically distributed to the north. In addition, the lithosphere underlying the Junggar basin was thermally and mechanically weakened by pervasive igneous intrusions related to

the Tian Shan Carboniferous island arc. The distribution of thrust loading and lithospheric types results in substantially different geometries to the Permian flexural basins either side of the Tian Shah thrust belt. To the north, the Junggar Basin is a short wavelength, deep depocentre which is the site for the deposition of a significant thickness (c. 1000 m) of Late Permian ORM deposited in a humid lake setting. By contrast, the depocentre on the southern margin of the Tian Shah is a shallow, broad wavelength feature filled with (200-300 m) of alluvial fluvial clastics. Superimposed on this simple tectonic control are climatic influences that are particularly important during the later stages of basin development when tectonic subsidence has slowed. In the Tarim Basin, a Lower Jurassic alluvial-fluvial facies association, deposited in a humid climate, is overlain by Middle Jurassic organic rich lacustrine mudstone deposited in shallow hypersaline lakes. The overlying Upper Jurassic comprises a fluvial red bed sequence. Subsidence rates throughout the Jurassic are broadly similar and there is an overiding climatic control on source rock development, related to the progressive change of climatic belts and the establishment of a semi-arid subtropical climatic belt across a large area of China in the Middle to Late Jurassic (Wang, 1986). The Tertiary system in the Qaidam Basin demonstrates the way in which local climate may be affected by tectonics and the interaction of the two in controlling the position and duration of lake sites.

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Onshore China plate tectonic history: M. P. Watson et al.

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Lower Oligocene fluvial red beds that mark the onset of deformation in the Himalayas are overlain by a thick lacustrine sequence of Upper Oligocene and Lower Miocene age. The ORMs in this sequence are interpreted to have been deposited in a deep (> 50 m) freshwater to slightly saline lake(s) that covered almost the entire basin (Yang, 1984). The climate throughout this time period was humid subtropical. Continued deformation in the Himalayas resulted in an increased rate of subsidence in the Late Miocene and Pliocene. However, at the same time, the basin passed into the rain shadow of the rising Himalayas and run off into the basin was substantially reduced (Yang, 1984). The widespread lakes of the Early Miocene shrank progressively throughout the Late Miocene and Pliocene, despite the increased rates of subsidence, to leave the remnant lake sites of the present day.

Extensional basins Subsidence in extensional basins follows a pattern of rapid subsidence during rifting, followed by reduced subsidence, as rifting terminates and thermal re-equilibration of the lithosphere takes place (Mackenzie, 1978; Beaumont, 1981). During the rifting period, subsidence of the basin floor is controlled by the rate of fault movement on one or more basin bounding faults. This results in a systematic distribution

222

of sedimentary facies that can be used as a predictive tool in source rock distribution. Many extensional basins are characterized in their earliest stages by a period of widespread regional extension on a large number of relatively minor faults (e.g. North Sea, Beach 1984). In such circumstances, sedimentation keeps pace with subsidence and results in a widespread alluvial/fluvial sequence (Figure 15). As extension continues, one or several faults become the major basin bounding fault(s) with an increased rate of movement, subsidence exceeds sedimentation, allowing the development of stable lake sites and the potential for the deposition of ORM (Figures 15 and 16). The gradual decrease in extension is marked by a decrease in fault rate and subsidence. Sedimentation is once more able to keep pace with subsidence and previously stable take sites are progressively swamped by prograding fluvial systems (Figure 15). In contrast to flexural basins, the time lag between basin initiation and ORM deposition is much shorter in extensional basins, typically 5-15 Ma. In the Eren Basin, regional extension during the early Lower Cretaceous resulted in the deposition of a widespread alluvial/fluvial sequence (Wang, 1984). An increased rate of fault movement throughout the Lower Cretaceous resulted in rapid subsidence and the establishment of stable lake sites adjacent to the half-graben bounding faults. Cessation of rifting in the

Marine and Petroleum Geology, 1987, Vol 4, August

Onshore China plate tectonic history." M. P. Watson et al.

T

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Late Middle Cretaceous was accompanied by the return to deposition in a widespread fluvial system. The climate throughout this time period was humid temperate, and source rock deposition was controlled principally by the rate of subsidence. Superimposed on the simple tectonic control are important climatic controls, and the often complex interaction of the two is well demonstrated in several of the extensional basins of eastern China. In the Subei Basin, fluvial-alluvial clastics of Palaeocene age, deposited during the initial rifting, are succeeded by an Eocene humid lacustrine facies association developed during the period of maximum subsidence and half-graben development. The overlying Oligocene sequence is predominantly fluvial and marks the cessation of rifting. However, the change to a semi-arid climate in the Late Oligocene allowed the establishment of semi-arid lakes, during a period of relatively slow subsidence, and the deposition of lacustrine ORMs. A subsequent climatic change to a more humid climate, with increased runoff, in the Miocene resulted in the lakes being flooded by a widespread fluvial system. The Songliao basin of northeastern China is the only extensional basin in China that has a thick source rock sequence within the post rift section. In this case, deposition took place in an unusually large, deep, humid lake that covered almost the entire basin, as the

basin underwent thermal subsidence. Source rock deposition was terminated by the progressive filling of the basin by a number of deltaic systems as subsidence slowed (Yang et al., 1985).

Summary The primary requirement for the formation of lacustrine source rocks is that the lake environment must be of a sufficient size and duration to allow for the significant accumulation of organic matter and that the tectonic setting must allow for sufficient burial to generate hydrocarbons. We have used several examples, of both rift and flexural basins from China. to demonstrate the interaction between tectonics and climate in controlling the distribution of source rocks within the basin fill. With the exception of the Songliao Basin, lacustrine ORMs deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence during the synrift period. This overriding tectonic control provides a useful tool in source rock prediction. In many of the extensional basins of eastern China, the acme of lacustrine source rock deposition follows basin initiation by 5-15 Ma (Figure 17); in the flexural basins of western China the time scale is larger, with maximum source rock development taking place 20-30 Ma after basin initiation (Figure 17).

M a r i n e and P e t r o l e u m G e o l o g y , 1987, Vol 4, A u g u s t

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Onshore China plate tectonic history: M. P. Watson et al.

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By contrast, semi-arid and arid climate lacustrine ORMs, whilst being best developed during the period of maximum tectonic subsidence, occur at all stages in the basin's history. The primary control appears to be minor, and difficult to quantify climatic fluctuations rather than the simple tectonic control outlined above.

supply during the major period of rifting. In a semi-arid/arid climate, there is a less precise tectonic control and lacustrine ORM development is possible at all stages in the basins' history.

Acknowledgements Conclusions The evolution of the Chinese Mesozoic and Cenozoic sedimentary basins reflects the response to the prolonged and complex interaction of the Eurasian, Indian and Pacific plates. The western basins comprise a number of flexural basins initiated in the Permian to Late Triassic. Their Mesozoic and Cenozoic sedimentary fill reflects the successive accretion of continental fragments in the Tibetan-Himalayan region. Organic-rich lake mudstone deposition occurred in these basins and, under humid climatic conditions, was controlled predominantly by the rate of subsidence related to thrust loading. Stable lake sites developed 20-30 Ma after basin inception. In semi-arid and arid climate conditions, lacustrine ORM deposition is controlled more by climatic fluctuations rather than by subsidence rates. Basins in central China are also flexural basins, controlled by thrust loading along their western margins. The thrust deformation is interpreted to result from oblique collision and strike-slip deformation along the western margin of the South China Block. The basins of eastern China are extensional. The age of rift initiation youngs progressively, southward. Late Jurassic and Early Cretaceous rifting in the Eren and Songliao Basins was succeeded by Late Cretaceous and Tertiary rifting in the North China Basin and Subei/Yellow Sea Basin. This change is perhaps related to progressive subduction rollback of the Pacific oceanic plate. Within most of the extensional basins, deposition of organic-rich lake mudstones under humid climatic conditions took place, typically 5-15 Ma after basin initiation, as subsidence exceeded sediment 224

We are grateful to the British Petroleum Company p.l.c, for permission to publish this work. We thank a number of Chinese colleagues and John Dewey who took part in a recent seminar where many of the ideas in this paper were developed. We are indebted to many of our colleagues in BP, in particular C. P. Sladen and M. R. Illingworth.

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