Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea

Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea

Accepted Manuscript Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern...

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Accepted Manuscript Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea Wei Zhou, Xianzhi Gao, Yingmin Wang, Haiteng Zhuo, Weilin Zhu, Qiang Xu, Yongfeng Wang PII:

S0264-8172(15)30094-5

DOI:

10.1016/j.marpetgeo.2015.09.006

Reference:

JMPG 2352

To appear in:

Marine and Petroleum Geology

Received Date: 3 December 2014 Revised Date:

3 September 2015

Accepted Date: 15 September 2015

Please cite this article as: Zhou, W., Gao, X., Wang, Y., Zhuo, H., Zhu, W., Xu, Q., Wang, Y., Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea, Marine and Petroleum Geology (2015), doi: 10.1016/ j.marpetgeo.2015.09.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea

Yongfeng Wangf

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Wei Zhoua,b,*, Xianzhi Gaoa,b, Yingmin Wangb, c, **, Haiteng Zhuoc, Weilin Zhud, Qiang Xue,

a College of Geosciences, China University of Petroleum, Beijing 102249, China

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102249, China

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b State Key Laboratory of Petroleum Resources and Prospecting (China University of Petroleum, Beijing), Beijing

c Ocean College, Zhejiang University, Hangzhou 310058, China

d China National Offshore Oil Corporation, Beijing 100010, China

e China National Offshore Oil Corporation Research Institute, Beijing 100027, China

Abstract

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f Sinopec Management Institute, Beijing 100012, China

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Multi-phase deepwater fan systems have formed in the Baiyun Sag of the Pearl River Mouth Basin on the northern margin of the South China Sea since the beginning of the Miocene.

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Integrated analysis of 2D/3D seismic, well log, core, and biostratigraphic data shows that the early Miocene Pearl River deepwater fan deposits consist of four 3rd-order sequences, SQ23.8, SQ21, SQ17.5 and SQ16.5. Each of them is bounded by regional discontinuity surfaces (3rd-order sequence boundaries). Of particular interest are the sandy deepwater fan deposits in the HST of SQ23.8 and LST of SQ21, which are absent in SQ17.5 and SQ16.5. The SQ23.8 deepwater *

Corresponding author. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), 18 Fuxue Road, Changping District, Beijing 102249, China. Tel: +86 15210871535. E-mail address: [email protected] (W. Zhou) ** Correponding author. Ocean College, Zhejiang University, 886 Yuhangtang Road, Hangzhou, Zhejiang Province, 310058, China. Tel: +86 13501103904. E-mail address: [email protected] (Y.Wang)

ACCEPTED MANUSCRIPT deposits are relatively thin sheets, densely spaced sandy debris flow channels and MTDs, whereas in SQ21 they manifest as thicker sheets and larger channels. In terms of lithofacies, SQ23.8 deepwater deposits are massive sandstones, in contrast to SQ21 deposits of massive sandstones,

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normally graded sandstones, inversely graded sandstones, and locally interbedded ripple laminated siltstones. It is interpreted that SQ23.8 deepwater sands result from homochronous slumping of

of the ancient SQ23.8 highstand shelf margin delta.

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the shelf margin delta front, while SQ21 deepwater sands result from the erosion and re-deposition

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The Paleocene-Early Oligocene rift stage (57.5 Ma ~ 32 Ma) was characterized by extensive development of half-grabens and normal faults. Since 32 Ma, the drift stage has been characterized by thermal subsidence and weak tectonic activity. The distributions of deepwater sands in SQ23.8 and SQ21 are interpreted to have been controlled by seabed topography that was

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inherited from buried rift-related tectonic features. We propose that the primary controlling factor for the development of the early Miocene Pearl River Deepwater Fan System was likely sediment supply of the Pearl River influenced by the Baiyun Event and the uplift of Qinghai-Tibet Plateau.

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The specific location of the sandy deepwater fan was due to tectonic influence on paleotopography

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of the shelf. The interpreted two depositional models can help predict the types of deepwater depositional elements are in the Baiyun Sag of the Pearl River Mouth Basin and where deepwater sands are likely to occur.

Keywords: Pearl River Deepwater Fan System; Baiyun Event; shelf margin delta; inherited slope topography; early Miocene; Pearl River Mouth Basin

1. Introduction The significant oil/gas exploration potential in deepwater reservoir elements such as

ACCEPTED MANUSCRIPT channel-fills, levees, sheets and mass-transport deposits has attracted extensive attention from both academia and industry (e.g. Posamentier and Kolla, 2003; Posamentier, 2003; Prather, 2003; Prather et al., 1998; Mayall et al., 2006; Weimer and Slatt, 2004, 2007). Regional basin tectonics,

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sediment supply and sea level fluctuations are regarded as the three main controlling factors of the deepwater clastic system, and their controls are commonly interdependent (Stow et al., 1984; Mutti and Normark, 1991; Reading and Richards, 1994; Richards et al., 1998). Over the last three

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decades, there has been persistent focus on relative sea level in order to predict the delivery and

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sedimentary formation of sandy deepwater deposits (Posamentier et al., 1988; Posamentier and Allen, 1999; Catuneanu et al., 2009; Helland-Hansen, 2009), despite the possibility that sediment supply might be the key driver for shelf margin progradation and delivery of deepwater sand even during periods of rising sea level (Covault et al., 2007; Carvajal et al., 2009). High cross-shelf

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sediment flux is very important for the occurrence of deepwater sands during rising sea level, especially in the region where a shelf margin delta develops (Deibert et al., 2003; Krassay and Totterdell, 2003; Carvajal and Steel, 2006, 2009; Comblells-Bigott and Galloway, 2006; Porebski

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and Steel, 2003, 2006; Carvajal et al., 2009).

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In tectonically complex regions, the sea floor tectonic activity (e.g. salt diapirism, normal faulting, thrust faulting, etc.) and slope topography have significant effects on the routing (Kneller and McCaffrey, 1999; Huyghe et al., 2004; Callec et al., 2010; Kane et al., 2010), storage of sediment (Pickering and Corregiror, 2005; Shultz and Hubbard, 2005; Gervais et al., 2006; Jackson et al., 2008; Alves et al., 2009; Hubbard et al., 2009), and deepwater sedimentary processes (Prather et al., 1998; Posamentier and Kolla, 2003; Prather, 2003; Adeogba, et al., 2005; Heini and Davies, 2007). These effects can be summarized as topographic variations that can

ACCEPTED MANUSCRIPT cause complete ponding or deflection of sediment gravity flows and form the spatial confinement of the associated depositional units (Jackson et al., 2008). In addition, the old basin configuration (deep buried structure) may have some effects on the younger turbidite complex (Fugelli and

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Olsen, 2007). Previous studies have recognised that the inherited submarine inflexions controlled by buried faults influence deepwater turbidite systems (Higgs, 1988; Loncke et al., 2006; Fugelli and Olsen, 2007; Mayall et al., 2010). Thus, the spatial-temporal evolution of the slope system

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directly affects the scale, geometry and distribution of deepwater hydrocarbon reservoirs

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(especially high-quality reservoirs).

Located at the continental margin of the northern South China Sea (SCS), the Pearl River Mouth Basin (PRMB) is a tectonically complex region which has some differences from the Atlantic-type and the GOM (Gulf of Mexico)-type continental margin basins and characterized by:

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(1) a multitude of tectonic units and faulted blocks; (2) non-mobile substrates; and (3) basin development is influenced by the evolution of the marginal sea (South China Sea) (Figs. 1 and 2). The Baiyun Sag of the PRMB (Fig. 1) has been a deepwater continental slope environment and

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within it the Rearl River Deepwater Fan System has developed since the early Miocene (Peng et

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al., 2004, 2005; Pang et al., 2007a). Previous studies concluded that there were six phases of sandy deepwater fan system occurred in the Baiyun Sag of the PRMB corresponding to six instances of third-order sea-level falling below the shelf break between 21 Ma and 10.5 Ma (Peng et al., 2004, 2005; Pang et al., 2007a). However, oil and gas exploration activities in the Baiyun Sag have revealed that only the drops of sea-level at 21 Ma and 13.8 Ma resulted in deepwater turbidite sands, and the deepwater turbidite sands at 21 Ma have been only discovered in the east Baiyun Sag (location is shown in Fig. 1A) (Wang et al., 2012). Accordingly, using integrated high-quality

ACCEPTED MANUSCRIPT 2D/3D seismic, well log, core and biostratigraphy data, the current study tries to: (1) discuss possible controlling factors on the development of the early Miocene Pearl River Deepwater Fan System using a Source-Sink System framework; (2) describe the characteristics of the early

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Miocene Pearl River Deepwater Fan System in a sequence stratigraphic framework; (3) investigate the submarine slope topography as a control on the deepwater sedimentation; and (4) propose depositional models for the early Miocene Pearl River Deepwater Fan System.

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2. Geological setting

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2.1 Structural Framework 2.1.1 Basin evolution

Similar to many other passive continental margin basins, the evolution of the PRMB has experienced rift (57.5 Ma ~ 32 Ma), transition (32 Ma ~ 23.8 Ma) and drift (23.8 Ma ~ present)

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stages (Fig. 2). The Paleocene-early Oligocene rift stage was characterized by the extensive development of half-graben structures (Huang et al., 2005; Sun et al., 2005, 2009). The transition stage that occurred during the late Oligocene was defined by a significant decrease in the intensity

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of active normal faults (Pang et al., 2008). The southward transition of the mid-oceanic ridge of

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the SCS at 23.8 Ma, named the Baiyun Event, led the shelf break to skip to the northern margin of the Baiyun Sag from the southern margin of the Southern Uplift Belt, and as a result the depositional environment of the Baiyun Sag changed from shallow water to upper continental slope (Pang et al., 2007b). This tectonic event marked the beginning of the drift stage of the PRMB (Pang et al., 2008; Zhang, 2010). 2.1.2 Baiyun Event and Uplift of Qinghai-Tibet Plateau The Baiyun Event occurred at the end of the Oligocene (ca. 23.8 Ma), had a region of

ACCEPTED MANUSCRIPT influence covering the South China Sea and East Asia (Pang et al., 2007b). The Oligocene-Miocene transition was a period of great changes in the geological environment, including tectonics, climate and sea level. On the aspect of tectonics: (1) there was a readjustment

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of the boundary between the Southeast Asia plate and Southwest Pacific plate, collision between Australia and the Philippine Sea Arc and collision between Ontong Java Plateau and Melanesian Arc (Hall, 2002); (2) rapid uplift of the northern part of the Qinghai-Tibet Plateau (Edward and

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Trevor, 1997); and (3) transformation of the Red River Fault and a shift in the strike of the

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mid-ocean ridge of the South China Sea from east-west direction to south-west direction (Taylor and Hayes, 1983; Briais et al., 1993). On the aspects of climate and sea level, they were mainly reflected in the cooling events during the Miocene, the dryness and desertification of Asia, and the dramatically declining global sea level (Guo et al., 2002; Cai et al., 2008; Zhong et al., 1998).

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2.1.3 Magmatic activity

Magmatic activity in the PRMB can be divided into three stages, Paleocene-Eocene, Oligocene-middle Miocene and late Miocene-Quaternary (Fig. 2), according to boreholes and

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seismic data (Li and Rao, 1994; Li and Liang, 1994; Zou et al., 1995; Yan et al., 2006).

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Intermediate-felsic volcanic rocks of the first stage constructed domes in the basin with ages of 57 ~ 49 Ma (Li and Rao, 1994; Li and Liang, 1994). The second stage of magmatism, including basalt and intermediate eruptive rocks, was accommodated through fissures or fault intersections within extensionally faulted depressions, generally occurred around the Shenhu Uplift area (Yan et al., 2006). In general, both the first and the second stages of magmatism were of limited duration and extent (Yan et al., 2006). The final stage of magmatism is mainly identified on seismic data as a suite of intrusive bodies. Seismic evidence of intrusive and extrusive rocks is widespread along

ACCEPTED MANUSCRIPT the lower slope of the northern margin of South China Sea, especially around the continent-ocean boundary (Lüdmann and Wong, 1999; Yan et al., 2001). The Dongsha Event is regarded as an important trigger factor for the magmatic activity of the final stage (Lüdmann and Wong, 1999;

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Lüdmann et al., 2001). For the current study interval, only the second magmatic event might have influenced early Miocene sedimentation, but the influence would have been very limited (Yan et al., 2006).

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2.2 Stratigraphic Framework

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Figure 2 shows the sequence-stratigraphic scheme of the PRMB. During the rift stage, depositions of fluvial-lacustrine sandstones, mudstones and coal occurred within the study area (Shenhu Formation, Wenchang Formation and Enping Formation). The mudstones and coal layers in Wenchang Formation and Enping Formation are the main hydrocarbon source rocks of the

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PRMB (Zhu et al., 2009). After the rift stage, a giant neritic shelf delta system deposited within the study area (Zhuhai Formation). And the transitional sandstones make up approximately 50 ~ 60% of the rock volume, cover a large area and are regarded as the most important reservoir rocks

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within this basin (Zhu et al., 2009). In the drift stage, deepwater depositional systems formed

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within the Baiyun Sag of the PRMB (Zhujiang Formation, Hanjiang Formation, Yuehai Formation, Wanshan Formation and Quaternary Formation). The current study interval, the Zhujiang Formation, which is the oldest segment of the Miocene sequence, is divided into four three-order sedimentary sequences (Posamentier et al., 1988), SQ23.8, SQ21, SQ17.5 and SQ16.5, based on the three-order relative sea-level fluctuation curve of the PRMB (Fig. 2; Pang et al. 2008).

3. Data and Methods Both multichannel 2D and high-quality 3D seismic data are used in this study (Fig. 1B and

ACCEPTED MANUSCRIPT C). The 3-D seismic volume has a dominant frequency of 30 Hz in the interval of interest (the Early Miocene layer) and covers an area of 4000 km2. The line spacing is 25 m, the trace spacing is 12.5 m and the sample rate is 4 ms. Well data used for this study include nineteen shallow water

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and nine deepwater wells, which contain a whole suite of well logs and biostratigraphy data (Fig. 1B and C). Meanwhile, the cores collected from two deepwater wells (L311 and L212) are available.

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Firstly, regional sequence stratigraphy-sedimentary analysis were used to reveal the

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development of Source-Sink System (S2S System), such as the shallow water Pearl River delta on the shelf (Source) and the sandy deepwater fan on the slope (Sink). The wireline and core data after seismic-well tie analysis were used to identify the lithology, predict the possible sedimentary facies and analyse the sequence stratigraphy of the early Miocene Pearl River Deepwater Fan

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System. After seismic-well tie, seismic geomorphological analysis were used to identify the deepwater depositional elements in the deepwater settings (Posamentier and Kolla, 2003; Weimer and Slatt, 2007). Channels can be identified as V- or U-shaped downcutting seismic facies from

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the cross section view and they can have linear to sinuous geometries from plan-view. When filled

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with sandy sediments, the channel-fill deposits will have a high-amplitude characteristic. The sheet sands have high-amplitude, high-continuity reflectors from the section view and lobate patterns at the terminus of channels from the plan-view. In terms of well-log motifs, the box-like well-log motif indicates sheet sands or amalgamated channel-fills, the dactylic well-log motif indicates levee deposits, the upward-fining well-log motif indicates inner to middle fan channel-fills and the upward-coarsening well-log motif indicates outer fan lobes (Weimer and Slatt, 2007). The depositional facies were identified in the attribute extractions, such as root mean

ACCEPTED MANUSCRIPT square (RMS) amplitude and coherence attribute extractions, and were verified through an examination of seismic profiles and well control where available.

4. Results

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4.1 General description of the early Miocene Pearl River Source-Sink System Figure 3A is a 2D seismic profile superimposed with wells across the shallow water ZHU I Depression, Panyu Low-Uplift and the deepwater Baiyun Sag along the Pearl River source

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direction. There are many NW-trending normal faults on the Panyu Low-Uplift. Figure 3B and 3C

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show early Miocene depositional sequence correlations and well-ties along the sediment transport direction and along the shelf margin, respectively. These two well-ties show that the early Miocene sands are abundant on the shallow water shelf but rare on the deep water slope (Fig. 3B). In the HST of SQ23.8, the Pearl River Delta prograded to the shelf margin to form a shelf-edge

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delta (Fig. 3B and C). But, in the upper three third-order sequences (SQ21, SQ17.5 and SQ16.5), the Pearl River deltas were hardly able to prograde to the shelf margin to form shelf-edge deltas; thus, the sands are rare on the shelf margin and the deltas could not supply enough materials to the

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adjacent continental slope to form deepwater fans (Fig. 3B and C). There are probably some

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incised valleys preserved within SB21 along the shelf margin (Fig. 3C). The vigorous activity of the boundary fault between the ZHU I Depression and the Dongsha Uplift (Fig. 4) proves the continuous uplift of the Dongsha Massif during the early Miocene (23.8 Ma ~ 15.5 Ma) and prompted the Dongsha Uplift to become a carbonate platform characterized by development of reefs after 18.5 Ma (Figs. 3C and 4). The sequence division is described in chapter 4.2. 4.2 Deepwater sequence stratigraphic framework and seismic-well tie analysis

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major discontinuity surfaces, T6 through T4, are also recognized based on detailed analysis of the seismic data and each of these five discontinuity surfaces shows clear truncation below and onlap (or conformity) above. The deepwater successions (or major depositional cycles) are commonly

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characterized by gravity flow deposits at the bottom overlain by hemipelagic drape deposits

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(Posamentier and Kolla, 2003; Adeogba et al., 2005), so these five discontinuity surfaces are sequence boundaries (Fig. 5). And these five discontinuity surfaces divide the Early Miocene sequence into four 3rd-order sequences, named SQ23.8, SQ21, SQ17.5 and SQ16.5. 4.2.2 Seismic-well tie analysis

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Well L311 provides core and biostratigraphy information for detailed analysis. High-quality digitized sonic and density logs from this well allow seismic-well tie analysis in order to check seismic interpretations (Fig. 6). The integrated seismic-well tie analysis suggests that sequence

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boundaries T6, SB21, SB17.5, SB16.5 and T4 appropriately correspond to 23.8 Ma, 21 Ma, 17.5

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Ma, 16.5 Ma and 15.5 Ma, respectively (Fig. 6). Meanwhile, the maximum flooding surfaces MFS22, MFS18.5, MFS17 and MFS16 appropriately correspond to 22 Ma, 18.5 Ma, 17 Ma and 16 Ma, respectively (Fig. 6). Seismic-well tie also indicates that the high-amplitude reflection overlying SB21 is the seismic response of the sandy deposits in SQ21, suggesting that high-amplitude events in the deepwater setting most probably represent sandy facies (Fig. 6). 4.3 Deepwater fan within SQ23.8

ACCEPTED MANUSCRIPT 4.3.1 Morphology and architecture The time thickness of SQ23.8 is approximately 50 ~ 450 msec (TWT), which generally decreases southward (Figs. 5B and 7D). Within the sequence stratigraphic framework, the

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deepwater sandy deposits primarily occur in the HST of SQ23.8 and manifest as thin sheets and densely spaced channels, which are identified from the seismic section views and coherent layer attribute extractions (Fig. 5; Fig. 8B-D). The densely spaced channels are nearly N-S oriented and

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have plan-shapes from straight to low-sinuosity (Fig. 8B and C). Among them, the straight and

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approximately straight channels developed upon the basement high in the centre of the 3D survey; whereas the low-sinuosity channels are distributed upon the trough in the east (Fig. 8B and C). Spacing intervals of the straight and approximately straight channels are approximately 100 ~ 2000 m, and the width and length of a single channel are approximately 100 ~ 200 m and 3 ~ 30

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km, respectively (Fig. 8B and C). From the plan-view, heads of densely spaced channels reach the northern boundary faults of the basement high, but there are some exceptions which cross these

4.3.2 Lithofacies

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faults (Fig. 8B, C and D).

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In well-logs the SQ23.8 sand bodies have box-like (or amalgamated) well-log motifs, dactylic motifs, upward-fining motifs and upward-coarsening motifs, such as the sand bodies in wells L211, L212, L321 and L911 (Fig. 9). The thicknesses of the sand bodies range from 0 to 15 m (Fig. 9).

The cores in well L212 consist of deepwater massive sandstones in the HST of SQ23.8 (Fig. 10; Table 1). The massive sandstones are locally rich in mud clasts some of which are interpreted as ripped-up mud clasts, and they are considered to be deepwater sandy debris flow deposits

ACCEPTED MANUSCRIPT (Shanmugam, 1996; Shanmugam et al., 2009). 4.4 Deepwater fan within SQ21 4.4.1 Morphology and architecture

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Within the sequence stratigraphic framework, the SQ21 deepwater sands primarily developed in the LST (Figs. 5 and 6). Approximately eight sheets (S1-S8) and eight channel belts (C1-C8) are identified in this current 3D survey (Figs. 8A and 11). This study divides the sandy deposits in

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(Turbidite Complex B), TCC (Turbidite Complex C).

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the LST of SQ21 into three turbidite complexes, named TCA (Turbidite Complex A), TCB

TCA is located in the east of the 3D survey, which composes S1 upstream and C1 downstream (Figs. 11 and 12).

TCB is located in the centre of the 3D survey, which composes S2 in the upper-segment,

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S3-S4 and C2-C5 in the middle segment, S5, S7, C3, C7, C8 in the lower segment (Figs. 11 and 13). And it mainly appears as sheet sands and channel-fills ponded in the localised low-relief zones (Figs. 7D, 7F, 11 and 13). In the downflow area, a high sinuosity leveed channel (C8), a

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migratory aggradational channel (C3) and a terminal fan (S7) are present (Figs. 11 and 14).

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TCC developed in the west part of the 3D survey (Figs. 11 and 14). In the upstream section, there are many straight or low-sinuosity erosional channels (C6) (Figs. 5C and 11). And in the downstream area, the deepwater depositional elements are two lobate sheets (S6 and S8) manifested as a high-amplitude, high-continuity bright spot feature in the seismic section and plan views (Figs. 11 and 14). 4.4.2 Lithofacies SQ21 sand bodies have box-like (or amalgamated) well-log motifs (Fig. 9A) indicating sheet

ACCEPTED MANUSCRIPT sands or amalgamated channel fills. The thicknesses of the sand bodies range from 0 to 35 m (Fig. 9A; Table 2). The cores in the well L311 are comprised of massive sandstones, normally graded sandstones,

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inversely graded sandstones, mudstones and locally interbedded ripple laminated siltstones in the LST of SQ21 (Fig. 10; Table 1). The discriminated sediment gravity flow deposits including sandy debris flow deposits, high-density turbidity deposits and low-density turbidity deposits (Fig. 10;

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Table 1). The isolated sigmoidal cross-bedding and ripple-laminated siltstones that are identified

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in L311 likely originated from waning low-density turbidity currents because low-density turbidity currents could result in tractive current deposits (Lowe, 1982). 4.5 Deepwater deposits within SQ17.5 and SQ16.5

Within the upper two third-order sequences SQ17.5 and SQ16.5 are only very small, sporadic

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channels and sheets. These channels and sheets can be identified by local high-amplitude reflections in the seismic profiles (Figs. 5, 6, 12 and 13). So in this current study, the deposits in SQ17.5 and SQ16.5 are mainly regarded as deepwater fine-grained hemipelagic drape deposits.

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4.6 Fault structures and slope topography

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4.6.1 Fault structures

The Meso-Cenozoic tectonic map of the PRMB shows that there were six major fault belts

(Fig. 15A). Figure 15B shows the fault systems of the rift stage of the current study area which are interpreted from the 3D seismic volume. The marked tectonic feature of the rift stage in the 3D survey is the basement normal faults, which form graben and half-graben structural system (Figs. 5, 7A, 12 and 13). There are four dominant fault strikes, NEE-SWW direction, NNW-SSE direction, NWW-SEE direction and NNE-SSW direction (Fig. 15B). The common tectonic feature

ACCEPTED MANUSCRIPT of the late Oligocene and early Miocene is minor fault re-activation of the early NEE-SWW and NNE-SSW trending main faults (Figs. 5, 8, 12 and 13). 4.6.2 Slope topography

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Since the sandy deepwater fan system mainly occurs in the lower two third-order sequences (SQ23.8 and SQ21), we only examine the morphology of the slopes during SQ23.8 and SQ21 in this current study.

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The slope topography during SQ23.8 is best described by an isochron of the SQ23.8 interval

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in combination with a time-structure map of seismic horizon SB23.8 (Fig. 7B and D). Seismic horizon SB23.8 indicates that the overall slope dip is towards S or SE, although there are a number of approximately NEE-SWW-trending and N-S-trending anticlines (bypass area) and synclines (minibasins 1 and 2) (Figs. 5A and D; Figs. 7D and 13).

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Similarly, the slope topography during SQ21 is best described using an isochron of the SB21-MFS18.5 interval in combination with a time-structure map of seismic horizon SB21 (Fig. 7C and F). Seismic horizon MFS18.5 roughly merges with the underlying SB21 in the north of the

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study area (Figs. 5A and B, 12A and 13A). Thus, the syn-depositional topography of the SQ21

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sequence mainly appears as the local development of topographic relief (minibasins 1-5 and bypass area) within a gentle slope background (Figs. 5B, 7, 12A and 13).

5. Discussion

5.1 Controlling factors on the development of the early Miocene Pearl River Deepwater Fan System Previous studies concluded that there were four phases of the sandy Pearl River Deepwater Fan System in the Baiyun Sag of the PRMB which were correlated with four drops of relative sea

ACCEPTED MANUSCRIPT level within the early Miocene sequence (Peng et al., 2004, 2005; Pang et al., 2007a). Our results indicate that the sandy deepwater fan system in the early Miocene mainly occurs in the lower two third-order sequences (SQ23.8 and SQ21). So, it is necessary and important to deduce the

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controlling factors on the development of the early Miocene Pearl River Deepwater Fan System in the PRMB.

5.1.1 Sediment supply of the Pearl River influenced by the Baiyun Event and the uplift of

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Qinghai-Tibet Plateau

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The uplift and erosion of the eastern Qinghai-Tibet Plateau determined formation of large-scale drainage systems (e.g., Red River and Pearl River) which drained into the South China Sea (Clark et al., 2004). The Pearl River originated from the Yunnan Plateau and has played a role as one of the major sediment input points of the northern continental margin of the South China

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Sea since the beginning of continental rifting (Fig. 1). There is a consensus that the main source of clastic sediment of the northern margin of the South China Sea is from the Pearl River and neighbouring drainages throughout most of the Neogene (Clift et al., 2002; Li et al., 2003). In the

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geological history, the primary source of sediments within the northern continental margin of the

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South China Sea did not stay the same. Wan et al. (2010) has revealed that the primary source for sediments at ODP Sites 1146 and 1148 (Fig. 1A) before 3 Ma was mainly from the Pearl River, however, sediment source to ODP Sites 1144-1148 (Fig. 1A) since 3 Ma is mainly from Taiwan. The sediment supply of the Pearl River has been controlled by regional tectonic events. The provenance analysis of the sands from ODP Site 1148 and the wells PY7, L311, L312 and L313 (Fig. 1) of the PRMB have revealed that the main sediment source area of the Pearl River expanded westward during the Baiyun Event (at ~ 23.8 Ma), which changed from the Oligocene’s

ACCEPTED MANUSCRIPT South China coastal Yanshanian granite to the Miocene’s eastern edge of the Qinghai-Tibet Plateau (Clift et al., 2002; Pang et al., 2007b; Sao et al., 2008; Li et al., 2013). In addition, both the grain size of the samples and the terrigenous sedimentation rate reflected in ODP Site 1148

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increased after ~ 23.8 Ma, and these increases approximately end at ~ 21 Ma (Sao et al., 2004). Under the background of transgression during the early Miocene, the ancient Pearl River delta fed sediment to the Baiyun Sag to form a submarine fan. However, only during the HST of

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SQ23.8 could the Pearl River delta (the highstand delta is also termed the ‘supply-driven delta’

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(Porebski and Steel, 2006)) prograde to the shelf margin to deliver sediment to the sandy deepwater fan system in the adjacent deepwater area (Fig. 3B and C), and so sandy deepwater fans within SQ23.8 and SQ21 are a response to the Baiyun Event. In addition, the deepwater sands within SQ23.8 and SQ21 accord with the rapid uplift period of Qinghai-Tibet Plateau (20 ~ 25 Ma)

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(Edward and Trevor, 1997). Thus, from the point of “source-sink system”, sediment supply of the Pearl River influenced by the Baiyun Event and uplift of Qinghai-Tibet Plateau is the first-order controlling factor on the development of the early Miocene deepwater fan system in this area.

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5.1.2 Shelf paleotopography

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In order to feed sediment to a deepwater fan system in the Baiyun Sag, the Pearl River delta must have prograded across the wide ZHU I Depression and the Panyu Low-Uplift/Dongsha Uplift to the shelf margin to form a shelf margin delta (Figs. 1 and 3). The ZHU I Depression is the first discharge area of clastic sediments of the Pearl River delta and the negative relief of the ZHU I Depression would have been filled before the prograding delta could reach the Panyu Low-Uplift and the Dongsha Uplift (Figs. 1, 3 and 4). Secondly, there are many large-scale, NW-trending, long-lived normal faults within the Panyu Low-Uplift (Figs. 1B, 3 and 15A) that

ACCEPTED MANUSCRIPT generated paleo-topography that could impede progradation of the ancient Pearl River delta, especially in SQ21 (Fig. 3B). In addition, on the relatively higher topography of the Dongsha Uplift has been a carbonate platform characterized by development of reefs since 18.5 Ma,

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indicating that siliciclastic sediment was directed elsewhere (Figs. 1, 3C and 4). The higher relief of the shelf margin is more favourable for the subaerial erosion during the period of sea level low, especially the shelf margin area which is located in the north of the 3D study area (Figs 1, 3 and 4).

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Nevertheless, the terrain of the shelf margin west of the Panyun Low-Uplift was lower than that of

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the east during the early Miocene Sequence, as shown in Fig. 3C and no deepwater turbidite sands at 21 Ma has been discovered within the west Baiyun Sag. Therefore, the shelf paleotopography might be a key controlling factor for impeding the development of the early Miocene submarine fan system in this area.

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5.1.3 Sea-level fluctuations

Migration of shorelines from the innermost shelf out to the shelf margin produces inner-shelf, mid-shelf and shelf-margin deltas (Porebski and Steel, 2006). Among these delta types, the

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shelf-margin delta has the most important relationship with the deepwater sands (Porebski and

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Steel, 2003, 2006). The sea level curve of the PRMB has some differences from the global eustatic curve (Haq et al., 1987), which is mostly characterized by a transgression after the Oligocene (Fig. 2). In SQ17.5 and SQ16.5, the maximum transgression reached the ZHU I Depression, but in SQ23.8 and SQ21, it could only reach to the Panyu Low-Uplift (Fig. 2). Therefore, when entering regressive periods, the real widths of the paleo shelf (or the distance between shoreline and shelf break) in SQ23.8 and SQ21 were much shorter than those of SQ17.5 and SQ16.5. The strong transgressions in SQ17.5 and SQ16.5 led to an excess amount of detrital materials transported by

ACCEPTED MANUSCRIPT the delta deposited on the inner and middle shelf (Fig. 3B), leading to a mud-rich, aggradation-dominated shelf margin (Fig. 3). An aggradation-dominated shelf is not conducive to the formation of a deepwater fan (Carvajal et al., 2009) and so sea level fluctuations may have

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been a key factor on the development of deepwater fan systems in this area. However, Sylvester et al. (2012) has shown some important implications for autocyclic lobe switching and factors controlling stacking patterns that are not driven by sea-level change. Considering that deepwater

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et al., 2007), we must also explore other factors.

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turbidite sands can be formed even during sea-level highstand (Carvajal and Steel, 2006; Covault

5.2 Inherited inflexed slope topography as a control on the distribution of deepwater sands Slope topography has been proven to play an important role in sediment routing and deposition process in multiple types of sedimentary basins, such as passive margin basins (e.g.,

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Prather et al., 1998; Anderson et al., 2000; Winker and Booth, 2000; Heiniö and Davies, 2007; Jackson et al., 2008), extensional basins (e.g., Nelson et al., 1999; Kneller and McCaffrey, 1999; Khalil and McClay, 2009; Kane et al., 2010), compressional basins (e.g., Grecula et al., 2003;

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Mayall et al., 2010) and pull-apart basins (e.g., Haughton, 2000; Hodgson and Haughton, 2004). A

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significant consideration in all of these basins is the rate of structural growth vs. the character and rate of sediment supply and sedimentation. In this study, there is a matching relationship among the slope topography, sediment routing and sediment dispersal system in the Baiyun Sag of the PRMB (Figs. 8, 11 and 15B). The distributions of deepwater sands within SQ23.8 and SQ21 are clearly influenced by the inherited inflexed seabed topography that is essentially controlled by buried tectonic features that formed during the rift stage. The idea that inherited topography influences sedimentation has been mentioned in other research results as well (Davies et al., 2004;

ACCEPTED MANUSCRIPT Fugelli and Olsen, 2007; Vinnels et al., 2010). Jackson et al. (2008) has considered that the depositional topography on the slope environment is formed through the differential compaction of mud-rich strata across the underlying fault blocks. In our study, characters of slope topography

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at 23.8 Ma and 21 Ma are mainly inherited from structures formed due to thermal subsidence and minor fault activities of the Baiyun Sag during the late Oligocene and early Miocene (Figs. 5, 7, 12 and 13). Like the slope topography described in Jackson et al. (2008), the inherited

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syn-depositional topography in this current study area might also have originated from the

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differential compaction of the mud-rich strata across the underlying fault blocks. Since the sandy debris flow channels in the HST of SQ23.8 are mainly developed upon the basement high inherited from rift stage tectonics, we conclude that this zone is a focused energy release area of the gravity flows. Accumulative flows might form with an increase in slope

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gradient or a convergence of flow through a seafloor confinement in the downstream direction; depletive flows might form with a decrease in slope gradient or a divergence of flow over an unconfined seafloor in the downstream direction (Kneller, 1995; Weimer and Slatt, 2007). In the

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current study, the topography of the zone of the densely spaced channels had weakly positive

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paleotopography, and the slope gradient increases along the down-current direction (Figs. 5B, 13; Fig. 7B and D). These conditions of slope topography are beneficial for the formation of accumulative flow to form densely spaced, high-energy sandy debris flow channels and gullies. The inherited slope topography controlling the distribution of deepwater sands is expressed

more obviously in the LST of SQ21. For TCA, the orientation of the axial channel is consistent with buried boundary faults, the deepest erosional point is developed upon the inflexed topographic high and the thick channel-fill deposits along the channel profile are deposited in the

ACCEPTED MANUSCRIPT local low-lying regions, all of them are considered to be high influenced by the underlying fault blocks (Figs. 11, 12A and 15B). Alterations of channel shapes are also found to share a connection with the changes in confinement conditions, which are influenced by the buried tectonic features

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(Fig. 12C and D). For TCB, channels and sheet sands are deposited in the thin slope accommodation created by the wide basement high, these channels and sheets are similar to the transient fan termed by Adeogba et al. (2005) and change to a leveed channel, a migratory

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aggradational channel and a terminal fan in the downstream region as a response to lower

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confinement conditions (Figs. 5C, 8A, 11 and 15B). C2 developed in the southern region of a structural transfer zone (Figs. 11 and 15B). By affecting the local topographic slope and accommodation, the structural transfer zone captured sediment and determined the sediment input-points and delivery conduits (Fugelli and Olsen, 2007; Khalil and McClay, 2009). Fault

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blocks that were buried near the seabed are found to affect the overlying younger seabed inflexed topography to form local narrow belts of relatively high accommodation, which were consequently favourable pathways for the transportation of sediment (Figs. 5D, 11 and 15B). TCC

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is a simple conduit-sink system controlled by the gentle slope topography, due to the

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westward-directed fading out of the amplitude of the basement high (Figs. 5B, 7A, 8A, 11 and 15B). As a whole, depositional processes of these three turbidite complexes in the LST of SQ21 all follow the “fill-spill model” (Prather et al., 1998) where the turbidite flow interacts with the slope topography, while differences among them are determined by the diverse syn-depositional slope topography inherited from buried tectonic features. 5.3 Genesis of the deepwater densely spaced channels in SQ23.8 The densely spaced channels within SQ23.8 have not been recognized in previous studies.

ACCEPTED MANUSCRIPT Firstly, the densely spaced channels are aligned parallel to the slope dip direction, so we can conclude that these channels have originated from the effect of gravity flow. In addition, the seismic reflection configuration of densely spaced channels in the cross section view is

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represented by a discontinuous-chaotic seismic reflection (Fig. 5A, C and D), while chaotic seismic facies in the deepwater environment are often interpreted to be the slump and debris flow deposits (MTDs) (Posamentier and Kolla, 2003; Mayall et al., 2006). Furthermore, numerous

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incised phenomenon could be identified within SQ23.8, indicating simultaneous formation of the

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channels and MTDs (Fig. 5A, C and E). The lithofacies of the core samples in well L212 is primarily massive sandstones that are rich in mud clasts, ripped-up mud clasts and scour surfaces, which are interpreted to be sandy debris flow deposits (Fig. 10 and Table 1). The L212 well intersects the channel within SQ23.8 (Fig. 8B and C) and so we consider that the channel-fill is

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deposited from sandy debris flow (cf. Shanmugam, 1996; Shanmugam et al., 2009), rather than fine-grained sheet-like turbidity currents (Lonergan et al., 2013). Secondly, the Baiyun Event at 23.8 Ma resulted in transgression and landward retreat of the

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shoreline (Fig. 2). After that event, shelf breaks remained stable near the northern margin of the

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Baiyun Sag (Fig. 2). At the regional scale, the ancient Pearl River Delta prograded to the shelf margin to form a shelf-edge delta in the HST of SQ23.8 (Fig. 3), hence we can identify a significant progradational reflection configuration within the HST of SQ23.8 in the seismic profiles along the Pearl River source direction (Figs. 5E, 12A and 13). The time thickness of the HST of SQ23.8 is wedge-shaped decreasing southward, rather than lenticular, and this suggests that the main body of the HST of SQ23.8 in the Baiyun Sag is a shelf-edge delta (Fig.3 and 5B). Porebski and Steel (2006) consider that the shelf margin delta can have a thick succession of

ACCEPTED MANUSCRIPT sandy turbidites on its delta front because of the rapid accumulation causing instability of the steep clinoforms, and that the sandy turbidites have a characteristic of upward-thickening lobes. The distribution of the densely spaced channels is so close to the shelf edge and the southward-directed

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thickness reduction of the HST of SQ23.8, so we propose that the densely spaced channels originated from the slumping of the mouth bar of the SQ23.8 highstand shelf margin delta. We speculate that the deepwater fan at 23.8 Ma probably expanded to the west of the Baiyun Sag

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edge of the west Panyun Low-Uplift (Fig. 3C).

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because the shelf margin delta at the highstand of SQ23.8 was well developed along the south

5.4 Depositional models of sandy deepwater fans in SQ23.8 and SQ21

Based on the discussions about the controlling factors for the distribution of deepwater sands in SQ23.8 and SQ21, we establish two depositional models of the early Miocene Pearl River

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Deepwater fan System in the Pearl River Mouth Basin (Fig. 16). These two models are based on a comprehensive consideration of the type, source of sediment supply and the character of slope topography.

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The ancient Pearl River formed a large scale delta on the shelf in the HST of SQ23.8 and this

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delta prograded across the shelf to form a shelf-margin delta. The SQ23.8 deepwater fan was a highstand deepwater fan, which was sourced from slumping of the sand-rich shelf-margin delta front, and was dominated by sandy debris flow channels (Fig. 16A). When the sea level fell below the shelf break at 21 Ma, the shelf margin suffered subaerial

erosion (Fig. 16B). The highly mature sands that had been deposited in the HST of SQ23.8 on the shelf margin were eroded and transported to the Baiyun Sag to form the SQ21 lowstand deepwater fan (Fig. 16B).

ACCEPTED MANUSCRIPT In addition, the inflexed seabed topography, which was developed due to the effects of buried faults and paleohighs, influenced the spatial distribution of the intra-slope accommodation. The gravity flow process followed the fill-spill model, and resulted in the particular distribution of

to either the presence or absence of deepwater sands.

6. Conclusions

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deepwater sands in this area (Fig. 16A and B). Overall, the shelf margin delta was most important

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Using high-quality 2D/3D seismic, well log, core and biostratigraphic data, this study

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documents the seismic geomorphology, lithology and controlling factors of the early Miocene Pearl River Deepwater fan System in the Pearl River Mouth Basin. The integrated data show the following:

(1) At the regional scale, the two primary controlling factors on the development of the early

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Miocene Deepwater Fan system in this area are 1) the sediment supply of the ancient Pearl River influenced by Baiyun Event and uplift of Qinghai-Tibet Plateau, and 2) tectonic influence on paleotopography of the shelf. Sea level fluctuations seem to have had a

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secondary influence on deepwater sedimentation in the study area.

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(2) The early Miocene Pearl River Deepwater Fan System consists of four 3rd-order sequences, including SQ23.8, SQ21, SQ17.5 and SQ16.5. Each of them is bounded by regionally discontinuity surfaces (3rd-order sequence boundaries).

(3) The sandy deepwater fan system is primarily developed in the HST of SQ23.8 and LST of SQ21, but absent in SQ17.5 and SQ16.5. The SQ23.8 deepwater deposits are thin sheets, densely spaced sandy debris flow channels and MTDs, whereas the SQ21 deposits contain relatively larger sheets and channels. In terms of lithofacies, SQ23.8 deepwater deposits are

ACCEPTED MANUSCRIPT massive sandstones. SQ21 deposits are massive sandstones, normally graded sandstones, inversely graded sandstones and locally interbedded ripple laminated siltstones. (4) The SQ23.8 deepwater sands resulted from the homochromous slumping of the shelf margin

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delta front, whereas SQ21 deepwater sands resulted from the erosion and redeposition of the ancient SQ23.8 highstand delta.

(5) After the area entered the drift stage, weaker tectonic activity and thermal subsidence resulted

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in gentle continental slope topography. The slope topography was controlled by the buried

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structures from the rift stage. The distributions of sands in the deepwater environment of SQ23.8 and SQ21 are likely controlled by the inherited seabed inflexed topography. The inherited topography could affect the dominant flow pathway, sediment dispersal, and the sediment-input points of the gravity flow.

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(6) This study reconstructs depositional models for the SQ23.8 and SQ21 Pearl River Deepwater Fans in the Pearl River Mouth Basin. The shelf margin delta was the most important factor to either the presence or absence of deepwater sands. The seafloor inflexions were controlled by

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buried tectonic features and controlled the local distribution of deepwater sands. These

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depositional models should help to define where deepwater sands are likely to occur, and what type of deepwater depositional elements are in Baiyun Sag of the Pearl River Mouth Basin.

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Acknowledgements: This study is sponsored by the National Natural Science Foundation of China (No. 41372115 and No. 41302147). We gratefully thank CNOOC Research Institute for providing the subsurface data and permitting publishing of the results of this study. The insightful reviews and constructive suggestions by Editor Dr. Thomas Hadlari and anonymous reviewers significantly improved the final manuscript, for which we are grateful.

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Lithofacies

Description

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Table 1 Typical lithofacies description and interpretation based upon the cored intervals in the wells described in the text Interpretation

The sandstone is fine- to coarse-grained. No obviously graded bedding, and the internal material distribution is very homogeneous, locally enriched in mud clasts, ripped-up mud clasts and micro-depositional fault in the local area.

L2 normally graded sandstones

The sandstone is fine- to coarse-grained. Scoured surface on the bottom, normal grading and rich in bioclastics. Coarse-grained deposits are followed by fine-grained deposits.

Turbidite deposits (Kneller, 1995). Normal grading is interpreted as the product of a single depositional event (Shanmugam, 2002).

L3 inversely graded sandstones

The sandstone is mainly medium- to coarse-grained. Lithologic surface of discontinuity is on the top with diapire and scouring on the bottom surface and is rich in bioclastics.

High-density turbidity deposits (Lowe, 1982); Sandy debris flow deposits (Shanmugam, 1996)

L4 ripple laminated siltstones

The rippled laminated siltstones which are about 3.5-cm thick. Obvious sigmoidal cross bedding, and the dip of the foreset bed is approximately 30 degrees.

L4 likely originated from waning low-density turbidity currents (Lowe, 1982)

L5 mudstones

Flat bedding and interbedded deformed bedding ribbon siltstone in the local area.

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L1 massive sandstones

Sandy debris flow deposits (Shanmugam, 1996; Shanmugam et al., 2009)

Deep-water draping deposits interbedded low-density turbidity deposits.

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lithology

possible depositional facies

L211 L212* L321 L322 L311* L312 L313 L911

23 0 31 16 17 35 2 10

Medium sandstone Mudstone/Silty mudstone Medium sandstone Medium sandstone Medium sandstone Medium sandstone Medium sandstone Medium sandstone

channel infill levee/overbank sheet sands sheet sands sheet sands sheet sands sheet sands sheet sands

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* marks out the well with available core data.

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well name

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the early Miocene Pearl River Deepwater Fan System mainly developed in the east area of the Baiyun Sag. Shelf breaks at 23.8 Ma, 21 Ma and 15.5 Ma are modified from Liu et al. (2011). Abbreviations: PRMB=Pearl River Mouth Basin; NUB=Northern Uplift Belt; SUB=Southern

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Uplift Belt; ZHU I=Zhu I Depression; ZHU II=Zhu II Depression; ZHU III=Zhu III Depression;

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BS=Baiyun Sag; CS=Chaoshan Depression; LS=Liwan Sag. (B) and (C) Study area and locations of figures used below.

Fig. 2 Sequence-stratigraphic framework of the PRMB (modified form Pang et al., 2008). The red

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line shows approximated locations of shelf breaks. Colours of the lithology column: brown=basement rock; violet=erosion; yellow=sandstone; grey=mudstone; light grey=carbonate. The relative sea level curve of the PRMB is from Qing (2002) and the global eustatic curve is

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from Haq et al. (1987).

Fig. 3 (A) Through-well seismic profile across the shallow water ZHU I depression, Panyu Low-Uplift and deepwater Baiyun Sag along the Pearl River source direction. Note there are many NW-trending normal faults existing on the Panyu Low-Uplift. (B) The well-tie sequence stratigraphy and sedimentary system analysis through the early Miocene Pearl River depositional system along the direction of Pearl River material transportation. The sands on the shallow water shelf are abundant but on the deepwater slope are rare. The Pearl River delta could not be able to

ACCEPTED MANUSCRIPT prograde to the shelf margin in the upper three third-order sequences. (C) The well-tie sequence stratigraphy and sedimentary system analysis along the shelf margin of the PRMB showing that the sands on the shelf margin are very rare in the upper three third-order sequences, particularly

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the shelf margin northern of the 3D study area. Locations are shown in Fig. 1B.

Fig. 4 Seismic profile showing that since 18.5 Ma, the Dongsha Uplift has been a carbonate

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platform upon which the Pearl River delta could not develop. Location of the profile is shown in

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Fig. 1B.

Fig. 5 Seismic profiles along depositional strike (A) and dip (B) showing typical stratal termination relationships and stacking patterns in the deepwater area of the Baiyun Sag. See Fig.

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1C for locations. (C) The incised and chaotic seismic reflection configurations within SQ23.8. (D) The inherited development of the slope topography under the rift stage’s tectonic geomorphology. Note the coupling between channels upon SB21 and underlying rift stage’s synclines which are

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controlled by the buried faults. (E) The progradation seismic reflection within the HST of SQ23.8

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in the seismic profile along the depositional dip. Abbreviations: O=onlap; T=truncation; TL=toplap; DL=downlap.

Fig. 6 Seismic-well tie, facies and stratigraphic architecture based on integrated seismic, well logs, core, and biostratigraphy data. See Fig. 1C for location of the well L311. Each of the four 3rd-order sequences is comprised of a fining-upward succession overlying a coarsening-upward succession, the vertical stacking of which turns out to be a 2nd-order sequence. Note the high-amplitude

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systems tract.

Fig. 7 (A) Time-structure map of seismic horizon T7 (breakup unconformity). Note the

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distribution of the basement high. (B) Time-structure map of seismic horizon SB23.8. The

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morphological feature of SB23.8 is much similar as that of T7. (C) Time-structure map of seismic horizon SB21. (D) Time-isochore map of SQ23.8. Note the thinnest area of the formation is in the southwest of the well L212. (E) Local time-isochore map of SQ23.8, location is shown in Fig. 7A. (F) Local time-isochore map of horizon layer limited by SB21 (bottom) and MFS18.5 (top)

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showing that there are two depocenters on the local slope. Location is shown in Fig. 7A. Also note the minibasins (1-5) and bypass area labelled in these maps.

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Fig. 8 Coherence attribution extractions along seismic horizons: SB21 (A), SB21+40 ms (B),

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SB21+80 ms (C) and SB21+120 ms (D), as well as their correspondent geologic sketch maps. These maps show the distributions of the faults, channels and possible sheets in SQ23.8 and lowstand of SQ21. (E) Seismic profile showing positions of these horizon slices, location is shown in Fig. 1C.

Fig. 9 (A) Correlation profile through key wells of lower interval of Zhujiang Formation flattened on top MFS18.5. (B) Structural cross section parallel to the correlation profile showing that the

ACCEPTED MANUSCRIPT inherited development of the slope topography under the rift stage’s anticline and syncline topography. See Fig 1C for location of the cross section.

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Fig. 10 Core photographs of wells L311 and L212, and their facies codes. See Table 1 for details in lithofacies.

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Fig. 11 (A) Root mean square (RMS) amplitude attribute extraction along the seismic horizon

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SB21 showing the distribution of deepwater sands (mainly sandy channel-fills and sheet sands) at 21 Ma. In this map, eight sheets (S1-S8) and eight channel belts (C1-C8) are identified by relative high RMS amplitude attribute.

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Fig. 12 (A) Axial seismic profile along Turibidite Complex A, location is shown in Fig. 1C. Note the buried composite half-graben structure of rift stage, the basement high, and the shear tectonic system of the later period. Also note changes in the channel fill thickness and the location of the

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deepest gravity flow erosional point along the channel profile. The fault activity is very weak

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within the layer limited by T7 (bottom) and T4 (top). (B) ~ (D) Transverse seismic profiles across Turbidite Complex A showing configurations of sheets and channels.

Fig. 13 (A) N-S oriented seismic profile showing the rift stage’s composite half-graben structure and the later period’s shear tectonic system, location is in Fig. 1C. Rectangle outlined in black shows detailed seismic line in (B). Under the well L212

there is a basement high. Also note the

localised low-relief zone near the well L211 and the seismic event pinch-out.

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Fig. 14 Attribute maps (A) and (B) and representative seismic sections (C). The coherence attribute (A) and RMS amplitude attribute (B) are extracted along seismic horizon SB21. Location

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is shown in Fig. 1C. These attribute maps and seismic sections illustrate that in the south of the study area, the main turbidite facies in the LST of SQ21 are sheets, a leveed channel and a lateral

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migratory aggradational channel.

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Fig. 15 (A) Main tectonic map of the Meso-Cenozoic in the PRMB (modified from Chen et. al., 2005). Note there are many NW-trending normal faults existing upon the Panyu Low-Uplift and the Dongsha Uplift. Abbreviations: NUB=Northern Uplift Belt; SUB=Southern Uplift Belt; ZHU I=Zhu I Depression; ZHU II=Zhu II Depression; ZHU III=Zhu III Depression; CS=Chaoshan

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Depression; BS=Baiyun Sag; LS=Liwan Sag. (B) The fault system of the rift stage of the current study area within the Baiyun Sag overlapped on the RMS amplitude map which is extracted along

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SB21.

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Fig. 16 (A) Depositional model of the Pearl River deepwater fan in SQ23.8. The SQ23.8 deepwater fan is a highstand fan which is sourced from the slumping of the sand-rich highstand shelf margin delta front. (B) Depositional model of the Pearl River deepwater fan in SQ21. The ancient Pearl River formed a huge scale delta on the shelf in HST of SQ23.8. When the sea level fell below the shelf break at 21 Ma, the sands deposited in the HST of SQ23.8 on the shelf margin suffered erosion and were transported to the Baiyun deepwater area to form the lowstand deepwater fan. The inflexed seabed topography controlled the spatial distribution of the intra-slope

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The early Miocene Deepwater Fan System consists of four 3rd-order sequences. Features of the early Miocene Deepwater Fan System are descripted in detail. Controlling factors on the development of deepwater fan system are discussed. Slope topography as a control on deepwater sedimentation is investigated. Depositional models of SQ23.8 and SQ21 sandy deepwater fan are proposed.

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1. 2. 3. 4. 5.