Evolutionary history and controlling factors of the shelf breaks in the Pearl River Mouth Basin, northern South China Sea

Evolutionary history and controlling factors of the shelf breaks in the Pearl River Mouth Basin, northern South China Sea

Marine and Petroleum Geology 77 (2016) 179e189 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier...

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Marine and Petroleum Geology 77 (2016) 179e189

Contents lists available at ScienceDirect

Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Research paper

Evolutionary history and controlling factors of the shelf breaks in the Pearl River Mouth Basin, northern South China Sea Jianhui Han a, b, *, 1, Guoqiang Xu a, b, **, Yangyang Li c, Haiteng Zhuo d, e a

College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, PR China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, PR China c C&C Reservoirs, 13831 Northwest Freeway, Houston, TX 77040, USA d Ocean College, Zhejiang University, Hangzhou, 310058, PR China e State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing, 102249, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 January 2016 Received in revised form 12 June 2016 Accepted 13 June 2016 Available online 15 June 2016

The migration history of shelf breaks in the stratigraphy of a basin contains significant information about sediment-budget partitioning into the deep water area, which is thus important in predicting the distribution and quality of deepwater reservoirs. Previous studies primarily focused on the sedimentarydominated shelf breaks (e.g., progradational-type), which are controlled by sediment supply and relative sea-level changes. The location and migration pattern of structure-controlled shelf breaks are comparatively less well documented. The evolutionary history of shelf breaks in the Pearl River Mouth Basin from 30 Ma to present is examined here. Shelf breaks are picked from 280 2-D depositional dip-oriented seismic profiles in the forced regressive systems tracts of 20 3rd-order sequences (SQ 1-20) in four super sequences (SSQ 1-4). Planview distribution maps indicate these shelf breaks can be divided into two groups of different character. Shelf breaks in SSQ 1 (30e23.8 Ma) have migrated progressively and considerably basinward (~50 km) along with the progradation of shelf-margin deltas, which were propelled by a very strong sediment supply even under the background of relative sea-level rise. They are consequently classified as sedimentary-dominated types. In contrast, shelf breaks in SSQ 2-4 have stayed fairly close (~20 km) to the boundary between the Panyu Low Uplift and the Baiyun Sag, which is identified as a tectonic hinge zone across which subsidence rate increased abruptly basinward. Shelf breaks were close to the hinge zone even in SQ 11 and 12, in which the depositional shoreline breaks were located ~80 km further landward. They are classified as structure-controlled types. Our study indicates that through the evolutionary history of the Pearl River Mouth Basin, the locations and migration pattern of the shelf breaks were mainly sediment-supply controlled during 30e23.8 Ma but changed to structure-controlled (i.e., tectonic hinge zone) during 23.8 Ma-Present. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Shelf break Pearl River Mouth Basin South China Sea Hinge zone

1. Introduction The shelf break, which is also commonly referred to as shelf edge or shelf margin, is defined as the point where the first major

* Corresponding author. College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, PR China. ** Corresponding author. College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, PR China. E-mail address: [email protected] (J. Han). 1 About the first author: Jianhui Han, Ph.D, male, born in 1976, works in the Chengdu University of Technology as a lecturer. He is mainly engaged in sequence stratigraphy and petroleum geoscience. http://dx.doi.org/10.1016/j.marpetgeo.2016.06.009 0264-8172/© 2016 Elsevier Ltd. All rights reserved.

change in slope gradient occurs at the outer most edge of the continental shelf (Porebski and Steel, 2003; Vanney and Stanley, 1983; Wear et al., 1974; Winker and Edwards, 1983). It separates the gently dipping continental shelf (average angle of 0.3 ) from the much steeper continental slope (average angle of 3 ) (Dietz and Menard, 1951; Pinet, 2003). On the continental shelf, depositional processes are dominated by traction flows (e.g., current, wave and tide) (Steel et al., 2008), which change to sediment gravity flows across the shelf break on the continental slope (Vanney and Stanley, 1983). Shelf-break migration through geological time and the shelfto-slope configuration are primarily controlled by the regional geological structures, by relative sea-level change and by

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sedimentary processes (Mougenot et al., 1983; Vanney and Stanley, 1983; Winker and Edwards, 1983). Mapping of ancient shelf breaks is a challenging task for several reasons: (1) complicated shelf-to-slope configuration (e.g., multiple step-like terraces); (2) structural deformation (e.g., folding, diapirism); and (3) erosion or slump (May et al., 1983; Ingersoll and Graham, 1983; Winker and Edwards, 1983). Previous studies have mostly picked shelf-break points on a dip-oriented seismic section (Gong et al., 2016), without giving much consideration to these factors, which could possibly impede the analysis results, especially in area with complex structural evolutionary histories. However, provided the structure and tilting can be restored, a quantitative method of determining the position of the shelf break was given by Olariu and Steel (2009). A review of 24 continental shelf margins worldwide by Gong et al. (2016) suggests that their migration pattern and architectural style contain significant information about sediment-budget partitioning into the deep water area and are very important in predicting the distribution and quality of deep-water reservoirs for hydrocarbon exploration. However, shelf breaks have been poorly studied given their importance (Vanney and Stanley, 1983; Wear et al., 1974), except recent research on shelf-margin clinoforms (Gong et al., 2015; Henriksen et al., 2009; Johannessen and Steel, 2005; Olariu et al., 2012). A few studies of shelf breaks have been carried out in the Pearl Mouth River Basin, northern South China Sea (Gong et al., 2015; Liu et al., 2011), which focus primarily on the sedimentary aspects of shelf-break migration and their control on reservoir characteristics in deep marine area. Different from previous studies, the current work focuses on evaluating the roles that regional structure and sedimentary processes have played during different stages of shelfbreak evolution based on the result of a new way of picking and mapping shelf breaks in the Pearl Mouth River Basin. 2. Geological background The study area is located in the southern part of the Pearl River Mouth Basin, northern South China Sea (Fig. 1). It covers four tectonic units, namely the Panyu Low Uplift, Baiyun Sag, Centre Low Uplift and Liwan Sag. The present shelf break is approximately aligned with the boundary between the Panyu Low Uplift and the Baiyun Sag (Fig. 1). The Pearl River Mouth Basin is an extensional sedimentary basin, which is characterized by half graben and horst structures. It was initiated in the Late Cretaceous as a result of continental rifting and sea floor spreading in the South China Sea area (Ding et al., 2013). The tectonic evolution of the Pearl River Mouth Basin can be divided into three stages: rifting (65e30 Ma), transition (Zhou et al., 2015) (i.e., mid-ocean ridge spreading and developing of a breakup unconformity) (30e23.8 Ma) (Ding et al., 2013) and thermal subsidence (23.8 Ma-present) (Fig. 2), the beginning of which are marked by the Shenhu event, Nanhai event and Baiyun event, respectively (Fig. 2) (Sun et al., 2008; Pang et al., 2009; Xie et al., 2014). The Pearl River Mouth Basin is primarily filled with Paleogene to Quaternary sediments, consisting of shallow lacustrine, fluvial to deltaic and deep-water mudstone and turbidite facies (Fig. 2). This study will focus particularly on the Oligocene to Quaternary sedimentary package. 3. Data Data used for this research include 280 2-D seismic profiles, which are oriented in the depositional dip direction (i.e., NW-SE) and an exploration well (well-A) (Fig. 3a and b). The 2-D seismic

data, which were acquired between 1979 and 2011 have been normalized and converted into zero phase to improve resolution. They were recorded at a 4 ms sample interval and have a primary frequency of 20e40 Hz. Given an average vertical velocity of 2000 m/s for the P wave, the 2-D seismic data is estimated to have a vertical resolution of ~20 m. The well-A is located in the northwest part of the Baiyun Sag. It has penetrated Oligocene and is drilled down to the Middle Eocene. A suite of well logs and biostratigraphy data from well-A are used for detailed lithofacies and age analysis. The well-A is tie to the seismic data through synthetic seismogram generated by acoustic and density logs. Seismic interpretation and shelf break picking are primarily done in Geoframe, with some illustrations drawn with Surfer. 4. Method and work flow 4.1. Sequence stratigraphy analysis Since several morphological shelf breaks may present in a sequence, we decided to pick the one on top of the forced regressive systems tract, mainly because it commonly represents the most seaward location in the sequence and is probably also the easiest and most obvious to be identified. This also enables meaningful comparison of shelf break locations through different sequences. To do that, firstly a high-resolution sequence stratigraphic framework is established following the four systems tract model by Hunt and Tucker (1992). Sequence boundaries are identified based on seismic reflection termination patterns (i.e., toplap and truncation below; onlap and downlap above the surface). Each sequence is divided into forced regressive, lowstand normal regressive, transgressive and highstand normal regressive systems tracts from bottom to top. 4.2. Structural deformation and erosion reconstruction Structural deformation and erosion make it difficult and in cases impossible to identify shelf breaks. In this study, we make sure shelf breaks are picked correctly by firstly reconstructing the strata to their pre-deformation and pre-erosion forms by performing seismic horizon flattening (i.e., on maximum flooding surfaces) and filling erosion by extrapolation of depositional surfaces. 4.3. Shelf break identification In this study, we define a shelf break where the dip angle of a surface increases from 0.02 ± 0.02 to >2 on a dip-oriented seismic section. Shelf breaks are mostly picked manually in previous studies, which can be time-consuming and is subject to human errors and inconsistency. One exception is by Olariu and Steel (2009), who programed to pick the shelf break at the point where the gradient ratio between that point to 30 km basinward, and between the shore line and that point is maximum. In this study, we have developed a method of calculating the dip angle of a surface with the following equation:

Ad ¼ arctanðDw =LÞ where, Ad is the dip angle, Dw is the paleobathymetry, and L is the horizontal length of the slope. Dw is calculated with the following equation:

Dw ¼ ðT  V=2Þ  C where, T is the two-way travel time of P wave through the overlying

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Fig. 1. Map showing the regional location of the study area. (a) Panyu Low Uplift; (b) Baiyun Sag; (c) Centre Low Uplift; (d) Liwan Sag. Notice that the present shelf break is approximately aligned with the boundary between the Panyu Low Uplift and the Baiyun Sag for the most part.

strata, V is the P-velocity, and C is the decompaction factor, which depends on the mud content of the overlying strata. 4.4. Sedimentary facies analysis Depositional processes change from traction current-dominated on the continental shelf to gravity flow-dominated on the continental slope. Consequently, shelf breaks normally lie seaward of coastal-shelf deltaic and shoreface deposits, but landward of gravity-flow deposits (e.g., slope fans). In this study, sedimentary facies are mapped in certain sequences by seismic facies interpretation calibrated by well data analysis.

highstand normal regressive systems tracts (Fig. 3d). 2nd-order super sequences correspond to a long-term relative sea-level change in the Pearl River Mouth Basin, with sequence boundaries coinciding with a large-scale relative sea-level fall (Fig. 2). Tectonically, SSQ 1 (30e23.8 Ma) corresponds to the transition stage (i.e., between beginning of mid-ocean ridge spreading and beginning of thermal subsidence) of the basin. SSQ 2 (23.8e13.8 Ma), SSQ 3 (13.8e8.2 Ma) and SSQ 4 (8.2 Ma-present) were deposited during the early, middle and late stages of thermal subsidence, respectively (Xie et al., 2014) (Fig. 2). The ages of sequence boundaries are determined by biostratigraphy study from well-A (Liu et al., 2011). 5.2. Shelf break locations

5. Results 5.1. Sequence stratigraphy The sedimentary interval from Early Oligocene (~30 Ma) to Present in the study area is divided into four 2nd-order super sequences (SSQ 1 to SSQ 4) and twenty 3rd-order sequences (SQ 1 to SQ 20), which are bounded by sequence boundaries (SB 30-SB 1.6) (Figs. 2 and 3c and 3d). Each sequence is subdivided into forced regressive, lowstand normal regressive, transgressive and

Shelf breaks are identified in each and all of the 20 sequences from Early Oligocene to Present by calculating the dip angles at the surface on top of the forced regressive deposits after the sequence was flattened along the maximum flooding surface on a diporiented 2-D seismic profile (Fig. 4). Same work is done on 280 dip-oriented 2-D seismic lines (Fig. 5). Shelf breaks picked on cross sections are then projected on maps and connected according to the ages of their corresponding sequences to generate shelf-break lines, giving a map view of their areal distribution (Fig. 6).

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Fig. 2. Schematic diagram showing the stratigraphy, sequence stratigraphy and tectonic evolutionary history of the Pearl River Mouth Basin. The global sea-level curve is from Haq et al. (1987). The relative sea-level change from 21 Ma to Present shown in brown solid curve is from Liu et al. (2011). The relative sea-level change from 30 Ma to 21 Ma shown in purple dashed curve is based on palaeontologic data. The sedimentary package from 30 Ma to present are divided into four super sequences, which correspond to the long term relative sea-level change cycles (i.e., the black solid curve enveloping the relative sea-level curves). (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

Shelf-break lines can be divided into two groups based on their clustering in locations including those of 30e23.8 Ma in SSQ 1 and those of 23.8 Ma-present in SSQ 2, 3 and 4 (Fig. 6), which have different characteristics. (1) SSQ 1 (30e23.8 Ma) SSQ 1 consists of five 3rd-order sequences (SQ 1e5). Five shelfbreak lines, which are named after the ages of their corresponding sequences 29 Ma, 28.4 Ma, 26.5 Ma, 25.5 Ma, and 23.8 Ma are mapped (Fig. 6a). They are collectively located in the Centre Low Uplift (Fig. 6a), having an approximate SW-NE orientation. Through geological time from 30 Ma to 23.8 Ma, there is an apparent seaward (i.e., SE) migration toward the SE. In ~6.2 million years, the shelf-break lines have migrated 40e60 km seaward (Fig. 6a), indicating a relatively rapid rate of growth of the shelf margin compared to global examples (Carvajal et al., 2009).

(2) SSQ 2e4 (23.8 Ma-Present) Four shelf-break lines from SQ 6, 7, 8 and 10 in SSQ 2, which are named after the ages of their corresponding sequences 21 Ma, 17.5 Ma, 16.5 Ma, and 13.8 Ma are mapped (Fig. 6b). Shelf-break lines in SQ 11 of SSQ 3 (12.5 Ma) and SQ 20 of SSQ 4 (Present) are also mapped (Fig. 6b). In general, they are characterized by meandering curves, having an approximate SW-NE orientation. However, the middle part of these shelf-break lines are more or less liner and coincide with the boundary between the Baiyun Sag and Panyu Low Uplift (Fig. 6b). Through geological time from 23.8 Ma to 13.8 Ma, the shelf breaks have swung back and forth around the boundary, but within a more confined range (~20 km).

5.3. Sedimentary facies Sedimentary facies are mapped based on seismic facies analysis. Deltaic and slope-fan facies are the two key facies mapped in this

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Fig. 3. Sequence stratigraphy analysis of a NW-SE seismic section across the continental shelf and slope in the study area. (a) Well-A is tied to the seismic data through synthetic seismogram (see Fig. 1 for the location of the well); (b) NW-SE seismic section across the study area (see Fig. 1 for the line of location); (c) structural and sequence stratigraphic interpretation of the seismic section in (a); (d) systems tract interpretation and seismic architecture illustration of the seismic section in (a). (FR-forced regressive; LNR-lowstand normal regressive; T-transgressive; HNR-highstand normal regressive systems tracts). b is the crustal stretching factor, with data from Zhang et al. (2014). The stretching factor increased abruptly from 1.6 to 1.8 landward of the hinge zone to 2.6e3.6 basinward of the hinge zone.

study. The deltaic facies are characterized by high amplitude continuous clinoforms prograding seaward (i.e., SE), downlapping the surface below (Fig. 7). The deepwater-fan facies are characterized by medium to high amplitude discontinuous reflections, showing landward onlapping and seaward downlapping (Fig. 7). Sedimentary facies identified on seismic sections are then projected on to maps (Fig. 6).

relationship with sedimentary facies and regional structures indicate that the characteristics and migration pattern of shelf breaks in SSQ 1 are primarily controlled by sedimentary processes, whereas they are primarily controlled by the regional structure in SSQ 2, 3, and 4.

6. Discussion

A series of shelf-margin deltas are mapped in the SSQ 1 (30e23.8 Ma) (Xu et al., 2011) (Figs. 6a and 7). They are developed in the forced regressive systems tract of its five sequences and are

The result of shelf breaks mapping and analysis of their

6.1. Sedimentary-dominated shelf breaks (SSQ 1)

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Fig. 4. Schematic diagram illustrating a typical sequence (SQ 3), its systems tracts and associated key surfaces on a dip-oriented cross section. The sedimentary package in this sequence has been flattened along a maximum flooding surface above (horizontal dashed black line) and reconstructed to its initial form before deformation. Dip angles are calculated at the surface on top of the forced regressive deposits to identify the shelf break. (CC* e correlative conformity sensu Posamentier and Allen (1999); CC** e correlative conformity sensu Hunt and Tucker (1992); MRS e maximum regressive surface; MFS e maximum flooding surface).

Fig. 5. Schematic diagram showing the locations of shelf breaks that are identified in the forced regressive systems tracts in 20 sequences on the cross section shown in Fig. 3.

characterized by seaward (SE) prograding clinoforms, which have acted as continental slopes at the time of deposition (Fig. 8). During this stage, the shelf-margin deltas in the five successive forced regressive systems tracts have prograded progressively toward the SE (Figs. 8a and 8b). The shelf breaks identified based on the topsetforeset configuration in the clinoforms coincide with depositional shoreline breaks and have moved progressively seaward along with

the deltas (Figs. 8a and 8b). Deepwater fans are mapped seaward of these shelf breaks, and these onlap the slopes formed by shelfmargin clinoforms (Figs. 6a and 7) (Liu et al., 2011; Gong et al., 2015). Consequently, the shelf breaks in SSQ 1 are classified as sedimentary-dominated and progradation type in particular (Mougenot et al., 1983). Given the fact that the relative sea level was rising during the

Fig. 6. Maps showing the areal distribution of shelf breaks. (a) Shelf-break lines of the five sequences (SQ 1e5) in SSQ 1, overlying the sedimentary facies map of SQ 2 (29e28.4 Ma); (b) Shelf-break lines of four sequences (SQ 6, 7, 8 and 10) in SSQ 2, one sequence (SQ 11) in SSQ 3 and one sequence (SQ 20) in SSQ 4, overlying the sedimentary facies map of SQ 7 (21e17.5 Ma). Notice that the shelf breaks in SSQ 1 migrated seaward overtime during 30e23.8 Ma, while they moved back and forth around the boundary of the Panyu Low Uplift and the Bayun Sag since 21 Ma. Some of the shelf break locations are modified from Liu et al. (2011).

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Fig. 7. Interpreted seismic section to show the two key seismic facies. Deltaic facies is characterized by prograding clinoforms with high-amplitude topsets passing to lowamplitude forests, which downlap the surface below. Deepwater fan facies are characterized by low to high amplitude, discontinuous seismic reflections, which show landward onlapping and seaward downlapping. See Fig. 3c for the location of the seismic section.

deposition of SSQ 1 and shelf-edge deltas were developed progressively seaward simultaneously (Fig. 2), we argue that the location and migration pattern of the shelf breaks in this stage are primarily controlled by sediment supply. Very strong sediment supply has overcome the increasing accommodation space due to relative sea-level rise and propelled the delta progradation at the shelf edges (Xu et al., 2011; Liu et al., 2011). 6.2. Structure-dominated shelf breaks (SSQ 2, 3 and 4) Since 23.8 Ma, shelf breaks have swung back and forth around the boundary between the Baiyun Sag and the Panyu Low Uplift, but in a confined range (~20 km) (Figs. 6a and 9). Shelf breaks in the SSQ 2 and 4 coincide with their corresponding depositional shoreline breaks. However, shelf breaks in SQ 11 and 12 are located ~80 km seaward of their corresponding depositional shoreline breaks (Fig. 10). Consequently, we argue that the shelf breaks in SSQ 2, 3 and 4 are not dominantly controlled by sedimentary processes. Backstripping analysis of the tectonic units in the Pearl River Mouth Basin shows that the Baiyun Sag has a much higher average subsidence rate than that of the Panyu Low Uplift since 23.8 Ma (Xie et al., 2014; Clift et al., 2015) indicating the existence of a structural hinge zone located at the boundary between the Baiyun Sag and the Panyu Low Uplift, which has probably controlled the locations and migration pattern of the shelf breaks in SSQ 2, 3, and 4. The formation of the hinge zone is closely related to the crust thickness and strength. The Moho in the Pearl River Mouth Basin is characterized by a stepped pattern as shown in the deep reflection seismic data (Huang et al., 2005). It drops in two steps from the north to the south part of the basin, resulting in an abrupt thinning of the crust at the associated two scarps, the first of which

corresponds to the boundary between the Baiyun Sag and the Panyu Low Uplift. The crustal stretching factors as defined by the ratio of pre- and post-stretching thickness are greatly influenced by the crustal strength (Clift et al., 2002). They are 1.6e1.8 to the north and 2.4e3.6 to the south of the hinge zone (Zhang et al., 2014) (Figs. 3c and 8), indicating an abrupt increase in stretching across the hinge zone toward the basin. Consequently, the shelf breaks in SSQ 2, 3 and 4 are classified as a tectonic hinge type. However, the small scale swing of the shelf breaks around the hinge zone is controlled by the sediment supply and relative sea-level change, with the latter being primarily controlled by differential subsidence across the margin (Figs. 3c and 10).

6.3. Classification of shelf breaks Genetically speaking, shelf breaks are divided into structureand sedimentary-dominated types, whereby the former are severely constrained by differential subsidence, and the latter are more free to migrate basinwards through time. Previous identified modes of migration pattern of shelf breaks include progradation, retrogradation and carbonate organic buildup (Hedberg, 1970; Mougenot et al., 1983). Previous recognized structures that can control the location and migration pattern of shelf breaks include basement ridge, fold belt, fault, diapirs (Hedberg, 1970). Based on this study in the Pearl River Mouth Basin, we propose to add a new type of structure-dominated shelf break, which is controlled by a tectonic hinge zone marking severe differential subsidence (Table 1). They are characterized by having a relatively stable location around the hinge zone, which may not coincide with the depositional shoreline break.

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Fig. 8. NW-SE seismic section (a) and interpreted results (b) across the study area in the Pearl River Mouth Basin, showing the detailed sequence stratigraphy, seismic reflection configurations and locations of shelf breaks in the five sequences of SSQ 1. The shelf breaks are identified by the topset-foreset configurations related to deltaic prograding clinoforms. They are located in the Centre Low Uplift and have migrated progressively basinward (i.e., SE).

7. Conclusions A sequence stratigraphic framework is established in the study area. The sedimentary interval between 30 Ma to Present is

divided into four super sequences (SSQ 1e4), which are subdivided into 20 3rd-order sequences (SQ 1e20). Forced regressive, lowstand normal regressive, transgressive and highstand normal regressive systems tracts are identified in each sequence.

Fig. 9. Schematic diagram showing that the shelf break is ~80 km basinward of the shoreline break in SQ 12 of SSQ 3, indicating that the location of the shelf break is not controlled by the active depositional processes (e.g., delta progradation).

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Fig. 10. Schematic diagram showing the migration pattern of shelf breaks from 30 Ma to present in the Pearl River Mouth Basin. The relative locations of shelf break are based on seismic section in Fig. 3. Two types of shelf breaks are identified: (1) progradation type (30e23.8 Ma), which is characterized by progressive seaward migration; (2) structure type (23.8ePresent), which is characterized by swing back and forth close to the hinge zone located at the boundary between the Baiyun Sag and Panyu Low Uplift.

Table 1 Summary of genetic shelf breaks types. Structure-type SB Basement ridge Folded belt Fault Diapirs Tectonic hinge

Sedimentary-type SB After Hedberg (1970)

Retrogradation (Erosion) After Mougenot et al. (1983) Progradation Carbonate organic buildup (Mougenot et al., 1983; Hedberg, 1970)

This paper

Shelf breaks are picked within the forced regressive deposits after the sequence is restored to its original geometry before deformation. Two groups or types of shelf break are identified during 30 Ma to Present in the Pearl River Mouth Basin based on their planview distributions and they are characterized by different migration patterns. Shelf breaks in the SSQ 1 (30e23.8 Ma) have migrated progressively and considerably basinward for ~50 km over ~6.2 Myrs along with the progradation of shelf-margin deltas. In comparison, shelf breaks in the SSQ 2, 3 and 4 (23.8 Ma-Present) have migrated back and forth close to the boundary between the Baiyun Sag and Panyu Low Uplift (~20 km), which is recognized as a tectonic hinge zone across which subsidence rate increased dramatically basinward. Shelf breaks are interpreted to be sedimentarycontrolled in SSQ 1 and structure controlled in SSQ 2, 3 and 4,

suggesting a change in shelf break type through the evolutionary history of the Pearl River Mouth Basin. Different from previous studies, a new type of structure named tectonic hinge zone is identified in the study in the Pearl River Mouth Basin that has controlled the location and migration pattern of the shelf breaks. Acknowledgments We thank Associate Editor Ron J Steel, and two reviewers for their critical and constructive reviews that have significantly improved the manuscript. We also thank CNOOC for providing subsurface datasets, and for the permission to publish the results of this study. This research has been jointly supported by the Doctoral Program of Higher Specialized Research Fund (20125122120022)

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