Stratigraphic architecture and evolution of the continental slope system in offshore Hainan, northern South China Sea

Stratigraphic architecture and evolution of the continental slope system in offshore Hainan, northern South China Sea

Marine Geology 247 (2008) 129 – 144 www.elsevier.com/locate/margeo Stratigraphic architecture and evolution of the continental slope system in offsho...

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Marine Geology 247 (2008) 129 – 144 www.elsevier.com/locate/margeo

Stratigraphic architecture and evolution of the continental slope system in offshore Hainan, northern South China Sea Xinong Xie a,⁎, R. Dietmar Müller b , Jianye Ren a , Tao Jiang a , Cheng Zhang a b

a Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, PR China EarthByte Group, School of Geosciences, Building H11, University of Sydney, Sydney, NSW 2006, Australia

Received 2 February 2007; received in revised form 14 August 2007; accepted 23 August 2007

Abstract We investigate the evolution of the stratigraphic architecture of two siliciclastic northern South China Sea continental slope systems with distinct structural styles, including rift-transform and rift segments. Our results reveal that the stratigraphic patterns of the slope system in the Yinggehai basin are mainly characterized by progradational slope clinoforms, which result in a rapid shift of shelf edges seaward and southeastward due to a high rate of sediment supply. In contrast, in the eastern Qiongdongnan area, the slope margins show a vertical stacking pattern or a slight shift seaward, characterized by dense gravitational faults, rollover structures, and slump deposits. The associated stratigraphic architecture in slope clinoforms is chaotic. Reactivation of pre-existing strike-slip faults in the Yinggehai area and extensional faults in the Qiongdongnan area resulted in the formation of initial slope clinoforms during the Miocene post-rifting stage. Dextral movement of the northeast marginal fault (No. 1 fault) since the Early Pliocene resulted in the formation of a deep embayment and early slope clinoforms along the No. 1 fault in the Yinggehai basin. The formation of initial slope clinoforms is closely related to movement along the underlying basement fault in the eastern Qiongdongnan area. Although the development of the slope architecture is influenced by tectonic movement and eustasy, the primary control of the geometry of slope clinoforms is through the effects of sediment supply. Plentiful sediment supply leads to rapid progradational slope clinoforms at a large distance of progradation in the Yinggehai and adjacent areas between the two basins. Sediment-starved margins occur in the eastern Qiongdongnan area because of a very wide shelf and insufficient sediment supply. © 2007 Elsevier B.V. All rights reserved. Keywords: continental slope system; slope adjustment; Pliocene; Yinggehai basin; Qiongdongnan basin; South China Sea

1. Introduction Understanding the evolution of a continental slope system can help to identify geological processes of continental margins. The area offshore Hainan Island provides a unique setting where the formation and evolution of a continental slope can be assessed through ⁎ Corresponding author. E-mail address: [email protected] (X.N. Xie). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.08.005

seismic studies of the subsurface from hydrocarbon exploration. Structural types of continental margins constrain the features of the continental shelf-slope system (Einsele, 2000). Two distinct continental margins are found in the northern South China Sea. As represented by the 200 m water depth contour at the present-day shelf edge, a wide shelf trending northeast– southwest is developed in the area offshore southeast Hainan Island; however, a narrow shelf to the south of Hainan Island widens nearly in a south–north direction

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(Schimanski and Stattegger, 2005). The distribution of the present slope appears to be dominated by different structural patterns. The former coincides with a NE-dominating structure system, the latter is in good agreement with the extension direction of the Red River Faults in the east Vietnam. However, those two systems have been separated by the Yinggehai basin which is part of a NW oriented structural system. The question is how tectonic reactivation of the Yinggehai and Qiongdongnan basins during the post-rift stage has influenced the evolution of the continental slope system. How and why did the Pliocene slope system in the Yinggehai basin migrate to its present position? Recently, hydrocarbon exploration has revealed that the slope margins began to form along the northern margins of the Baiyun sag, the Pearl River Mouth basin since 23.8 Ma (Pang et al., 2006; Peng et al., 2006). The

shelf/slope break has been stationary or has slightly moved landwards from the Early Miocene to Recent times, dominated by deep faults (Sun et al., 2005). The evolution of the shelf system, however, is not clear in the Qiongdongnan and Yinggehai basins. Many papers have documented the sedimentology, tectonics, and evolution of the northern continental marginal basins (such as, Taylor and Hayes, 1983; Pautot et al., 1986; Ru and Pigott, 1986; Erlich et al., 1990; Chen et al., 1993; Wang et al., 1995; Li et al., 1998; Lüdmann and Wong, 1999; Sarnthein and Wang, 1999; Wang et al., 1999; Zhang, 1999; Lüdmann et al., 2001), but the formation and evolution of the continental slope system in this area is not well understood. In addition, the Yinggehai and Qiongdongnan basins in the northern continental margin of the South China Sea have been explored for nearly 30 years and comprise one of the most prolific

Fig. 1. Sketch map of major tectonic features of Southeast Asia (after Leloup et al., 1995; Rangin et al., 1995; Sun et al., 2003). Tertiary sedimentary basins during the rift stage (shaded) in northern South China Sea are outlined after Li et al. (1998).

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natural gas-producing provinces in the South China Sea (Zhang et al., 1996; Hao et al., 1998; Xie et al., 2001, 2003). Exploration in the past decades has focused on Oligocene strata, where the largest gas field in the South China Sea, the Ya13–1, has been discovered (Gong and Li, 1997). Recent studies have concentrated on the Miocene and Pliocene strata, leading to the discovery of other big gas fields, such as the DF1–1, and LD8–1 in the Yinggehai basin (Dong and Huang, 2001; Huang et al., 2002). Hence, understanding the characteristics of the continental slope system is important for mapping hydrocarbon reservoirs. This paper describes the geometry and depositional characteristics of the continental slope system and discusses the slope adjustment and evolution model of slope clinoforms. The purpose of this paper is to elucidate the dominant control factors in the formation and evolution of continental slope in the area offshore Hainan Island, northern South China Sea. 2. Geological setting The study area lies in the offshore Hainan Island in the northwestern South China Sea, and covers the Yinggehai and Qiongdongnan basins (Fig. 1). These two basins are separated by a major boundary fault (the No. 1 fault), as shown in Fig. 2. Although the Yinggehai

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and Qiongdongnan basins are two Cenozoic sedimentary basins with clastic sediments of thicknesses larger than 10,000 m, they show significant differences in structural development. The northwest-oriented Yinggehai basin is characterized by a northeastern marginal fault (the No. 1 fault) that appears to connect to the Red River fault system (Fig. 1). The basin is thought to belong to a transtensional system (Chen et al., 1993), or transform-extensional system (Li et al., 1995, 1998). A sinistral oblique pull-apart model has been invoked to explain the forming mechanism of the Yinggehai basin (Guo et al., 2001). But Gong and Li (1997) and Li et al. (1998) suggested that the Yinggehai basin formed as a dextral pull-apart basin along NW–SE and N–S oriented faults. Based on research farther inland, Allen et al. (1984) and Leloup et al. (1995) suggested that the Red River fault zone experienced sinistral strike-slip from 35 to 20 Ma and shifted to dextral movement after about 5 Ma. Sun et al. (2003) used experimental evidence to indicate dextral movement of the No. 1 fault in the Yinggehai basin. In contrast, the Qiongdongnan basin extends in an east–northeast direction and shows a typical passive margin development from rifting to regional subsidence. The formation of the basin is believed to relate to the evolution of the South China Sea (Liang and Liu, 1990). Anomalous post-rift subsidence and tectonic reactivation took place

Fig. 2. Location map of the study area, showing the location of seismic sections and wells referred to in the text.

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along the northern South China Sea margin (Xie et al., 2006), which controlled the formation and evolution of continental slope system as discussed in the following section. The Yinggehai and Qiongdongnan basins underwent two tectonic evolutionary stages from Paleocene, Eocene and Early Oligocene rifting to Early Miocene– Quaternary post-rifting. The variation of sedimentary environments in the northern South China Sea is related to the tectonic evolution of those two basins. There are two abrupt environment transformations from the Paleocene to the present time, i.e. from alluvial, lacustrine to onshore, neritic to shelf-slope, abyssal facies, which are coincident with the tectonic evolutionary stages. By the Late Miocene, continental slope systems began to prevail over the northern South China Sea margin, which are coincident with the onset of tectonic

reactivation in northern continental marginal basins of the South China Sea (Xie et al., 2006). Clear anomalous post-rift subsidence resulted in the formation of the slope system in the Yinggehai and Qiongdongnan basins. Significant progradational reflection packages of slope sediments developed since the Early Pliocene in the Yinggehai and western Qiongdongnan basins. Although the Yinggehai basin is located in the present-day shelf region (Fig. 2), progradational slope clinoforms are observed along the No. 1 fault along the northeast margin of the basin during the Pliocene time. Due to the limitations of the available seismic grid in the offshore Vietnam, the detailed architecture and geometry of these intervals could not be delineated, but seismic profiles reveal a gentle slope in the southwest margin of the Yinggehai basin. It is inferred that the slope clinoforms and the relatively deep marine environment during the

Fig. 3. Sequence classification and sea-level changes of the Late Miocene Huangliu Formation and Pliocene Yinggehai Formation in the Yinggehai and Qiongdongnan basins. The ages of these sequence boundaries were adopt by Lu and Zhang (1995); Gong and Li (1997).

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Fig. 4. Regional seismic sections showing a different geometry of slope progradational clinoform. Insert shows the position of each seismic profile. (A) section in the Yinggehai area; (B) section in the adjacent area between the Yinggehai and Qiongdongnan basins; (C) section in the eastern Qiongdongnan area. The number represents sequence boundary. See Fig. 2 for the profile location.

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Pliocene are located only in the middle and southern parts of the basin adjacent to the No. 1 fault, which shows a deep embayment. The formation of the slope clinoforms must have been primarily controlled by the dextral pull-apart movements along the northeastern marginal faults (the No. 1 Fault) in the Yinggehai basin (Xie et al., 2006). 3. Methods and data sources In this study, approximately 2000 km of 2-D seismic profiles and well logs from 36 drillholes (Fig. 2) were collected in the Yinggehai and Qiongdongnan basins by the China National Offshore Oil Corporation. The basic sequence stratigraphic procedures used for interpretation of the seismic sections and the well logs are based on Exxon's sequence stratigraphy concepts (van Wagoner et al., 1990). In this area, seven sequences, corresponding to third-order sequences, can be identified in the Late Miocene and Pliocene section (Fig. 3). The Pliocene and Quaternary sequences have been further subdivided into eleven sequences (fourth-order sequences) as documented by Chen et al. (1993). Fig. 3

shows the ages and time spans of the sequences. The nomenclature of the sequences and unconformities has been adopted from Lu and Zhang (1995) and Gong and Li (1997). Sequence boundaries are defined by the occurrence of seismic features such as truncation, downcutting, onlap, a hiatus and distinct surfaces associated with facies change. These seismic sections were subsequently correlated with sonic and gamma-ray logs from the wells. The ages of these sequence boundaries during the Miocene and Pliocene were determined by planktonic foraminifera, calcareous nanoplankton, and dinoflagellates collected from more than 20 boreholes in the Yinggehai and Qiongdongnan basins (Lu and Zhang, 1995; Gong and Li, 1997). The chronostratigraphic framework for the sequence boundaries is provided by biostratigraphic analysis and isochronous correlation of seismic reflectors (unconformities). The relative sealevel curve is derived from the integrated analysis of seismic images and borehole data. The variation of third sea-level cycles obtained in this fashion is in good agreement with Haq et al's sea-level curve (Haq et al., 1987).

Fig. 5. Part of seismic profile (A) and our interpretation (B) in the Yinggehai area. The numbers represent sequence boundaries. This profile shows the bathymetric escarpments due to faulting in the Yinggehai basin. See Fig. 4 for the profile location.

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4. Characteristics and interpretation of Late Miocene–Pliocene slope system Based on the integrated analysis of seismic sections and logs in terms of the sequence geometry of the continental slope system, there are three distinct areas with different sequence geometries of slope clinoforms along the northern South China Sea margin as shown in the Fig. 4. 4.1. The Yinggehai area The progradational slope wedge began to form since the Pliocene Yinggehai Formation underlain by Late Miocene Huangliu Formation (Fig. 3). The initial formation of the continental slope clinoforms was related to the dextral movement of the No. 1 fault zone as shown in Figs. 4A and 5. Steep escarpments developed along the No. 1 fault on the northeastern margin of the

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Yinggehai basin as a result of the presence of a narrow shelf along offshore Hainan Island. Faults associated with gravitational slumping are observed in some seismic sections (Fig. 5). The trend of the early shelf break paralleled the strike of the fault. In addition, steep slopes developed adjacent to the fault due to the steepening process caused by faulting. The gradient slope decreased away from the fault due to rapid sediment accumulation. The Pliocene succession is composed of three thirdorder sequence units and six fourth-order sequence units (Fig. 3). In the upper Pliocene sequence, a wedgeshaped slope formed adjacent to faults and rapidly shifted seaward, as shown in Fig. 5. Basin floor fan and slope fan deposits formed above sequence boundaries during a lowstand systems tract (LST). Clear subaqueous erosional surfaces have been observed along the outer shelf and the shelf margin of the previous sequence (Fig. 6). Seaward dipping foresets form a

Fig. 6. Part of seismic profile (A) and our interpretation (B) in the Yinggehai area. This profile shows a rapid progradation of slope clinoform and erosional surfaces in the outer shelf and shelf break of the former sequence. See Fig. 2 for profile location.

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highstand systems tract (HST) with internal reflections of medium-amplitudes, downlapping onto the mounded basin-floor fan and slope fan deposits (Fig. 6). The slope clinoform shifted rapidly progressively southeastward with time. In the Late Pliocene sequence, slope clinoforms have shifted to the southern part of the Yinggehai basin and adjacent area between the Yinggehai and Qiongdongnan basins. The present shelf break is represented by the water depth contour of 200 m, which shifted away from the Yinggehai basin (Fig. 2); subsequently the Yinggehai area became an epicontinental shallow sea. In most of the Yinggehai basin, draped shelf strata decrease in thickness landward with sub-horizontal reflections. At the well No. LD1512 (Fig. 7) in the middle part of the Yinggehai basin, cores consist of homogeneous silty-

clay formed in inner shelf environments and well-sorted fine-grained sandstones formed in nearshore environments. Inner shelf deposits are composed of siltstones with lenticular and horizontal bedding, with burrows filled by silty sand. Nearshore deposits are composed of fine-grained sandstones interbedded with siltstones and mudstones. Ripple cross bedding and small scale cross bedding are observed in sandstones fining upward. Hummocky cross bedding occurs in the lower part of nearshore sandstones. A sand-dominated succession is bounded by a marine flooding surface with shell fragments. These features suggest that marine transgressive lag deposits formed at the bottom of sequence. Seismic profiles reveal that significant progradational slope clinoforms do not occur in the northwestern part of the Yinggehai basin, but instead we find relatively

Fig. 7. Sedimentary interpretation of inner shelf deposits from well LD1512 in the Yinggehai basin. See Fig. 2 for the location of this borehole.

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thick shelf deposits with sub-horizontal reflections and single offlaps. Based on internal downlap structures in seismic sections, multiple sediment sources have been identified. Two sets of downlap structures with opposite directions of progradation have been observed. One downlapping structure from west to east indicates a southwestern terrigeneous source from onshore Vietnam. Another one downlapping in the opposite direction indicates a northeastern source from Hainan Island. In addition, the Pliocene sediments from the axial direction of the Yinggehai basin are inferred to originate from the Red River (Gong and Li, 1997). Heavy mineral assemblages from the cores in the DF area indicate that deposits in the Late Miocene Huangliu Formation originated from both southwestern and northeastern sources (Lu, 1999). Hence, multiple sediment supplies may be the main reason for the high sedimentation rate in the Yinggehai basin. The distribution of depositional systems during the Early Pliocene has been delineated in order to provide a comprehensive understanding of the continental slope system. Abyssal environments developed in the southeastern Yinggehai basin and are located adjacent to the No. 1 fault. Seismic profiles reveal that deltaic deposits with clear foresets have only been imaged along the northeastern margin. As shown in Fig. 8, a deep embayment adjacent to the No. 1 fault opens southeastward.

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4.2. Adjacent area between the Yinggehai and Qiongdongnan basins The area between the Yinggehai and Qiongdongnan basins, separated by the No. 1 fault, shows the rapid progradation of slope clinoforms. Significant progradational laminations in slope deposits with medium to large amplitudes are observed in seismic profiles (Fig. 9). The slope gradient became less inclined with time because of increased sediment supply. In addition, the shelf breaks of Early and Late Pliocene age shift southward, where they form a wide shelf with a width of 160 km at the junction of the Yinggehai and Qiongdongnan basins (Fig. 8). The width of each slope unit is much narrower than that in the Yinggehai basin. However, the width of the shelf increases rapidly with time and is more than 200 km away from Hainan Island. 4.3. The Qiongdongnan area A relatively wide shelf formed in the Qiongdongnan basin, offshore of Hainan Island. The progradational slope clinoforms initially formed in the eastern Qiongdongnan basin during the Late Miocene (Chen et al., 1993), which is earlier than that in the Yinggehai basin. The deep fault seems to constrain the initial formation of the slope as shown in Fig. 4C, although the fault does

Fig. 8. Distribution of shelf breaks in the Pliocene and 200 m water depth contour at the present time in the Yinggehai and Qiongdongnan basins.

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Fig. 9. Seismic profile in the adjacent area between the Yinggehai and Qiongdongnan basins showing a progradation of slope clinoforms. Location of the section marks in Fig. 2.

not cross the overlapping shelf-slope deposits. The distribution of slope clinoform is related to and adjacent to the No. 2–1 fault in eastern Qiongdongnan basin. The shelf break is oriented parallel to the fault and is situated

close to the fault, as well as close to the present shelf break except in western Qiongdongnan basin (Fig. 8). The slope succession shows vertical aggradation or a slight shift seaward (Fig. 4C). The steepening of the slope

Fig. 10. Part of reflection seismic profile in the Qiongdongnan basin showing our interpretation of the stratigraphic architecture in the slope zone. Note gravity-driven faults in slope zone. Location of the section is marked in Fig. 4.

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may result from extensional fault propagation folding of the underlying faults due to flexural subsidence (Gawthorpe et al., 1997; Gawthorpe and Hardy, 2002). A number of gravitational slip faults and slump deposits formed in the slope zone during Late Pliocene and Quaternary time. These slump wedges with a chaotic seismic reflection facies have substantial relief both at their top and bottom boundaries (Fig. 10). Seismic profiles show that the shelf margin is oversteepened and sediments are bypassed into a base-ofslope position through time mainly by gravity-driven faulting and gravity flow processes. In some cases, progradational foresets in the upper slope are observed, which downlap onto the slump wedge (Fig. 10). But in most of the lower slope and base environments, slump deposits with deformation structures are dominant. This observation shows that gravity flow processes are dominant in the slope zone and sediments are bypassed onto the lower slope and basin plain. Those observations indicate that sediment supplies were insufficient, in contrast to the Yinggehai basin. 5. Discussion 5.1. Control factors on slope adjustment For a general sequence model, three control factors on the geometry and development of continental slope system (i.e. tectonic movement, eustasy, and sediment supply) have been considered (Posamentier and Vail, 1988; van Wagoner et al., 1988; Steckler et al., 1993). Tectonic movement together with eustasy determines relative sea-level change, which determines the variation of accommodation available for sediment accumulation. Both accommodation and sediment supply control the variation of systems tracts and geometry of sequence units as documented by Jervey (1988). In the slope buildup, Ross et al. (1994) emphasized the effect of equilibrium of margin physiography on gradient and geometry of the slope. Two kinds of slope evolution models have been developed: progradational and erosional margins. Progradational margins form when diffusive and sediment gravity-flow processes are in equilibrium with sediment supply, basin subsidence, and basin physiography. Erosional margins form when upper slope gradients exceed an equilibrium grade, and are characterized by erosion, slumping, and gravitational faulting, where sediments have to be bypassed to the lower slope or the base of slope via gravity-driven sediment transport processes. In this study, shelf systems offshore Hainan Island can be subdivided into two patterns. One, as represented by the Yinggehai area

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including a basin junction area, is a progradational clinoform characterized by rapid progradational foresets and rapid shift of the shelf margin during the sediment dispersion process. The eastern Qiongdongnan area can be interpreted as an erosional margin, which is characterized by gravitationally slipping faults, slump deposits and gravity flow deposits. In the buildup of the slope, gravity-driven flow processes are dominant. In the Qiongdongnan area, a wide shelf develops with a width of 200–280 km for the present-day margin. But in the Yinggehai area offshore Hainan and the Nam Con Son basin offshore Vietnam, the shelf margin is very narrow, less than 20 km wide (Schimanski and Stattegger, 2005). The No. 1 fault in the Yinggehai basin (Sun et al., 2003) and the N–S-trending strike-slip faults offshore Vietnam (Rangin et al., 1995) control the formation of the slope escarpment here, which limit the early evolution of the continental slope. Hence, narrower shelves in the study area may result from reactivation of these bounding faults near the shelf edges. On the basis of an analysis of seismic data and geophysical downhole logs, two kinds of depositional patterns differ in sediment supply and tectonic setting for the western parts of northern South China Sea ( Figs. 11, 12), resulting in the distinct patterns of evolution of slope geometry in the Yinggehai and eastern Qiongdongnan areas. As shown in Fig. 11, a typical progradational slope clinoform has been interpreted in the Yinggehai area. During a relative sea-level fall, a type I sequence boundary formed when deposition was restricted to the area seaward of the previous shelf break. A mounded basin-floor fan formed at the base of slope, and slope fan deposits onlap landward and downlap seaward on the slope of the preceding sequence, resulting in a moundlike external geometry. In this case, little deposition occurs on the shelf, and erosion of the shelf break is sometimes observed (Fig. 6). The lowstand systems tract is followed by a transgressive systems tract as relative sea-level rises and sediments reflood the entire shelf surface. Finally, progradation of the depositional shelf break resumes during the highstand systems tract (HST). In this model, the shelf system is characterized by rapid progradation of graded shelf breaks due to rapid sediment supply. In the eastern Qiongdongnan area, early sediments were deposited on the outer shelf, and were progressively aggraded onto the continental margin as shown in Fig. 12. If the rate of sea-level rise is greater than the rate of sediment supply, the basin plain in this system will be starved, and most of sediments migrate and deposit on the upper slope. When the sediment is

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Fig. 11. Schematics of evolution of the slope geometry in the Yinggehai area showing the evolution of depositional processes during a cycle of sealevel change.

transported onto the previously drowned shelf edge, the slope quickly steepens and the gradient of slope exceeds the critical angle of the equilibrium profile. At that time, the continental margin becomes erosional due to steepening of the slope gradient in response to a rapid rise in relative sea-level (Ross et al., 1994). When the upper slope gradient exceeds an equilibrium gradient, gravitational-driven faulting occurs and results in re-deposition

or slump from the upper to the lower slope. A number of such faults have been observed in the eastern Qiongdongnan area (Fig. 10). The geometry of these slopes is characterized by erosion and slumping, where upper slope sediments have been bypassed to lower slope environments via gravity-flow processes. In this case the shelf breaks show a vertical stacking pattern or a small shift seaward due to insufficient sediment supply.

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Fig. 12. Schematics of evolution of the slope geometry in the eastern Qiongdongnan area showing the evolution of depositional processes during a cycle of sea-level change.

5.2. Evolution model of continental slope system Differences in the evolution of the continental slope systems in the Yinggehai and Qiongdongnan areas are due to significant differences in sediment supply and tectonic setting. Two depositional models for the slope system evolution apply to these two areas (Fig. 13). In the Yinggehai area, Sun et al. (2003) documents that the depocenters formed during dextral-slip faulting

are located at the southeast portion of the basin and close to the eastern boundary faults based on results from experimental modeling (see her Fig. 8). This activity is driven by dextral movement of the South China block since 5 Ma (Leloup et al., 1995). Steep escarpments formed in the southeastern Yinggehai basin along the No. 1 fault due to its dextral movement as a result of the formation of a deep embayment (Fig. 13A). With rapid sediment accumulation from the northeastern,

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southwestern and axial sources, especially from both the Red River and Hainan Island, the embayment has been progressively infilled. The shelf breaks migrated rapidly southeastward in the Yinggehai basin and moved to the present-day position away from the Yinggehai basin. At same time, continental margins in a junction area between the Yinggehai and Qiongdongnan basins also shifted far away from Hainan Island due to the dextral movement of the No. 1 fault, where a maximum distance of slope progradation is up to 120 km from the Early Pliocene to the present. In the eastern Qiongdongnan area, however, the shelf edge or base of slope from Early Pliocene to present time almost keeps the same position or shifts a little seaward (Fig. 13B). It is interesting to note that an underlying fault has been observed under the slope zone (see Fig. 4C). The formation of slope may be interpreted

as a mechanism resulting from extensional fault propagation and folding of underlying faults due to flexurally-driven subsidence (Gawthorpe et al., 1997; Gawthorpe and Hardy, 2002). During anomalous postrift subsidence, the reactivation of pre-existing faults resulted in distinct differential subsidence between the hanging walls and foot walls of the faults involved, providing the framework for the formation of the initial continental slopes. In this case, the shelf margins show a vertical stacking pattern because of the very wide shelf and insufficient sediment supply as a result of the formation of an erosional margin. 6. Conclusions On the basis of an integrated analysis of seismic data, and geophysical logs from exploration wells along the

Fig. 13. Evolution model of the continental slope system in the Yinggehai and eastern Qiongdongnan basins showing the significant controls of faults on the geometry and evolution of the slope system.

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northern South China Sea margin, two distinct continental slope systems have been identified in the Yinggehai and Qiongdongnan basins with a different structural and sedimentary evolution. The Yinggehai basin belongs to a transtentional system related to the movement of the Red River fault zone. Dextral movement of the No. 1 fault connected with the Red River fault since the Early Pliocene results in the rapid subsidence of the southeastern Yinggehai basin, in which a deep embayment forms and a narrow shelf develops. The Qiongdongnan basin, however, is an extensional system in a passive margin resulting from the formation of a very wide shelf since Late Miocene. The initial slope position is clearly controlled by an underlying basement fault in this basin. The evolution of the slope in the area offshore Hainan Island can be classified into two kinds of depositional models, which have very different stratigraphic architectures and sequence geometries. One is represented by the Yinggehai area and is characterized by a series of progradational slope clinoforms with a rapid seaward shift as a result of a large sediment supply from the Red river and Hainan Island. But in the eastern Qiongdongnan slope system, slope gradients steepen with time and a large number of gravitationally-driven faults, slumps, and erosional surfaces develop within the slope zone, which indicates a deficient sediment supply and erosional slope environments. Therefore, structural reactivation and sediment supply have a significant effect on the formation and evolution of the slope system in the Yinggehai and Qiongdongnan basins. Acknowledgements The study is supported by the 973 project (grant no. 2007CB411705), the Trans-Century Training Program Foundation for Talents by the State Education Commission and the key project no: 01038 from the Ministry of Education of China. We would like to acknowledge the Institute of Petroleum Exploration and Development, Nanhai West Oil Corporation for providing geological data. The final manuscript was substantially improved following the reviews by Dr. Ho-Shing Yu and two anonymous reviewers. References Allen, C.R., Gillespie, A.R., Han, Y., Sieh, K.E., Zhang, B., Zhu, C., 1984. Red River and associated faults in Yunnan Province, China: quaternary geology, slip rates and seismic hazard. Geol. Soc. Amer. Bull. 95, 686–700.

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