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Flow processes and sedimentation in unidirectionally migrating deep-water

channels: From a three-dimensional seismic perspective

Research · September 2015

DOI: 10.13140/RG.2.1.3005.7444

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This is an Accepted Article that has been peer-reviewed and approved for publication in the Sedimentology, but has yet to undergo copy-editing and proof correction. Please cite this article as an “Accepted Article”; doi: 10.1111/sed.12233

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Received Date : 17-Dec-2014

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Article type : Original Manuscript

Flow processes and sedimentation in unidirectionally migrating

deep-water channels: From a three-dimensional seismic perspective

CHENGLIN GONG*, †, ∗, YINGMIN WANG‡, *, ∗∗, RONALD J. STEEL†, JEFF PEAKALL§, XIAOMING

ZHAO¶, QILIANG SUN††

* State Key Laboratory of Petroleum Resources and Prospecting (China University of Petroleum, Beijing), Beijing 102249,

China;

† Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA

‡ Ocean college, Zhejiang University, Hangzhou, Zhejiang, 310058, China;

§ School of Earth and Environment, University of Leeds, Leeds, West Yorkshire LS2 9JT, UK

¶ School of Geoscience and Technology, Southwest Petroleum University, Sichuan 610500, China

* Corresponding author at: Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA

E-mail address: chenglingong@hotmail.com

** Corresponding author at: Ocean college, Zhejiang University, Hangzhou, Zhejiang Province, 310058, China

E-mail address: wym3939@vip.sina.com

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†† Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao

266071, China

Associate Editor – Gary Hampson

Short Title – Unidirectionally migrating deep-water channels

ABSTRACT

Three-dimensional seismic data were used to infer how bottom currents control

unidirectional channel migration. Bottom currents flowing towards the steep bank would

deflect the upper part of sediment gravity flows at orientation of 1° to 11° to the steep

bank, yielding a helical flow circulation consisting of a faster near-surface flow towards the

steep bank and a slower basal return flow towards the gentle bank. This helical flow model

is evidenced by occurrence of bigger, muddier (suggested by low-amplitude seismic

reflections) lateral accretion deposits and gentle channel wall with downlap terminations on

the gentle bank and by smaller, sandier (indicated by high-amplitude seismic reflectors)

channel fills and steep channel walls with truncation terminations on the steep bank. This

helical flow circulation promotes asymmetrical depositional patterns with dipping accretion

sets restricted to the gentle bank, which restricts the development of sinuosity and yields

unidirectional channel migration. These results aid in obtaining a complete picture of flow

processes and sedimentation in submarine channels.

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Keywords: Unidirectionally migrating deep-water channels; helical flow circulation created

by bottom currents; sedimentation in submarine channels; flow processes in submarine

channels; West African margin

INTRODUCTION

Submarine channels have been the subject of increasingly intense study in recent

years, because they: (i) house large hydrocarbon reservoirs (e.g. Mayall & Jones, 2006; Gong

et al., 2011; Pyles et al., 2012); (ii) are a key record of climate change (e.g. Wynn et al.,

2007); and (iii) are known to be the main conduits for sediment and organic material

partitioned into the deep sea (e.g. Peakall & Sumner, 2015). Typically, sinuous submarine

channels exhibit classic ‘gull-wing’ cross-sectional geometries and laterally migrate in an

unsystematic manner (Peakall et al., 2000; Wynn et al., 2007; Janocko et al., 2013a).

However, a different type of submarine channel, characterized by unidirectional lateral

migration, a lack of associated levées and by short and relatively straight channel courses,

was recently recognized in the Pearl River Mouth slope and was termed unidirectionally

migrating deep-water channels by Gong et al. (2013) (Figs 1 and 2).

Gong et al. (2013) have suggested that unidirectionally migrating deep-water channels

in the northern South China Sea margin were created by the interaction between turbidity

flows and bottom currents resulting from the North Pacific Intermediate Water (NPIW; Figs

1 and 2). As seen in the bathymetric image presented in Fig. 1B and 1C, unidirectionally

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migrating deep-water channels do not significantly indent the shelf edge and are several

tens of kilometres long. The current study will focus on some ancient unidirectionally

migrating deep-water channels in the Lower Congo Basin along the West African margin.

Other examples of unidirectionally migrating deep-water channels have also been

recognized in the northern South China Sea margin (Zhu et al., 2010; He et al., 2013) and the

West African margin (Rasmussen 1994, Séranne & Abeigne, 1999; this study). These

previous studies documented morphological properties, architectural styles and genesis of

unidirectionally migrating deep-water channels (Rasmussen 1994, Séranne & Abeigne, 1999;

Zhu et al., 2010; Gong et al., 2013; He et al., 2013).

In recent years, great effort has been made to understand flow processes and

sedimentation in submarine channels through the use of outcrops (Pyles et al., 2012),

laboratory experiments (e.g. Kassem & Imran, 2004; Peakall et al., 2007; Straub et al., 2008;

Amos et al., 2010; Ezz et al., 2013), numerical simulations (e.g. Corney et al., 2006, 2008;

Imran et al., 2007; Abad et al., 2011; Darby & Peakall, 2012; Dorrell et al., 2013; Janocko et

al., 2013b) and direct field measurements (Parsons et al., 2010; Wei et al., 2013; Sumner et

al., 2014). To date, however, no study has documented flow processes and sedimentation

styles in unidirectionally migrating deep-water channels. The present study uses 3D seismic

data to investigate flow processes and sedimentation in this type of deep-water channel.

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GEOLOGICAL AND OCEANOGRAPHIC BACKGROUND

The study area (solid box with red outline in Fig. 3) is located offshore Angola, in the

Lower Congo Basin along the West African margin and covers an area of approximately 500

km2, with water depth ranging from 200 to 500 m. The Lower Congo Basin was created by

the opening of the South Atlantic in the Early Cretaceous (Séranne & Abeigne, 1999;

Stramma & England, 1999; Ho et al., 2012). The lower Congo Basin underwent two main

tectonic stages, namely a rifting stage from 150 Myr BP to the Early Aptian and a post-rifting

stage from the Albian to Quaternary (e.g. Valle et al., 2001; Broucke et al., 2004; Séranne &

Anka, 2005). Accordingly, the basin infill consists mainly of pre-Aptian continental synrift

and post-rift supersequences (Séranne & Abeigne, 1999; Lavier et al., 2001; Séranne & Anka,

2005). Major uplift of the hinterland occurred during the post-rifting stage exposing the

shelf, resulting in redistribution of sediment into deeper parts of the basin in the form of

turbidites and debris flow deposits (Anderson et al., 2000; Anka & Séranne, 2004; Anka et

al., 2009; Savoye et al., 2009; Ho et al., 2012). Quaternary-age deep-water channels

developed on the upper slope of the Lower Congo Basin are the focus of the present study

(Figs 3 to 5).

Three major ocean currents significantly control the present oceanographic conditions

of the study area and the West African margin (Fig. 3). Angola coastal currents with an

effective depth of <200 m are the principal processes acting on the continental shelf (Fig. 3;

Séranne & Abeigne, 1999). Eastward-flowing south equatorial counter currents also occur

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and have significant seasonal variability, reaching as deep as 250 m and northward-flowing

south equatorial currents have an effective depth of ca 350 m and a velocity of up to 10

cm/s (Fig. 3; Stramma & England, 1999; Merciera et al., 2003).

DATA SET AND METHODS

The primary source of data sets used in this work is ca 500 km2 of 3D seismic data,

acquired and provided by China Petroleum and Chemical Corporation. This study uses 3D

seismic data from well-imaged unidirectionally migrating deep-water channels from the

Lower Congo Basin (Fig. 3). Three-dimensional seismic data were migrated with a single pass

3D post-stack time migration and have a 4 ms vertical sampling rate and a bin spacing of

12.5 × 12.5 m. These data are zero-phase processed with a dominant frequency of 60 Hz,

yielding a vertical resolution of ca 15 to-20 m; they are displayed using ‘SEG reverse

polarity’, in which an increase in acoustic impedance is represented by a negative (trough)

reflection event.

This work integrates ‘classical’ 2D seismic facies analysis (Vail et al., 1997) with the 3D

seismic geomorphology approach (Posamentier et al., 2007), through which seismic

stratigraphy and sedimentology of the studied channels are quantitatively analyzed. Seismic

amplitude provides enhanced visualization of the stratigraphic architecture of small-scale

depositional elements and features, allowing accurate delineation of the external

morphology and internal architecture of deep-water channels as documented in this work.

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Two steps were used to produce flattened horizontal seismic amplitude slices. The first step

was to flatten the 3D seismic amplitude volumes using the modern seafloor (0 ms) as the

hanging horizon. The second step was to produce time slices. Flattened horizontal seismic

amplitude slices, coupled with 3D seismic transects, were used to delineate plan-view and

cross-sectional details of the studied channels. Depth measurements of the morphometric

properties of the studied channels and their associated depositional elements were

estimated, using velocities of 2000 m/s for the shallow subsurface sediment in the study

interval of interest and 1500 m/s for the sea water. Flow processes active during deposition

of the channels were interpreted on the basis of external geometries of the channels and

characteristics of reflections within them.

SEISMIC SEDIMENTOLOGY

Ancient unidirectionally migrating deep-water channels are recognized on the West

African margin (Fig. 3). Two main seismic facies (seismic facies 1 and 2) are recognized in

them, based on seismic reflection configuration (reflection continuity and amplitude),

cross-sectional geometry and stratal terminations. Please refer to Table 1 for morphological

properties of the studied channels and to Tables 2 and 3 for a complete description and

interpretation of these two seismic facies.

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Seismic stratigraphy and geomorphology

In cross-sectional view, the studied channels are not flanked by levées and are

characterized by unidirectional channel migration and an asymmetrical cross-sectional

shape (Figs 4, 5 and 6). Three unidirectionally migrating deep-water channels are recognized

in the study area of the West African margin (C1, C2 and C3) (Figs 4 and 5). ChannelC2 is

focused on here because it is the best imaged channel in our seismic dataset; C2 is

composed of three laterally stacked channel-complex sets (CCS1 to CCS3) that stack

upwardly and laterally from one channel-complex set to the next (Figs 4, 5, 6 and 8A).

Channel-complex sets recognized in the studied channels are 2.5 to 5.5 km in width (W) and

are ca 100 to 350 m in average thickness (T), giving the channels an aspect ratio (W/T) of ca

20 (Figs 4, 5, 6 and 8A; Table 1).

Each channel-complex set contains two discrete architectural elements: A lower

interval containing lenticular, subparallel to concave upward, high-amplitude, reflections

that are only located in the axis of the studied channels (seismic facies 1); and an upper

interval containing sigmoid-shaped, low amplitude reflections that dip laterally at 3º to 10º

towards the channel thalweg (seismic facies 2; Figs 6, 7 and 8A; Table 2). Seismic facies 1 has

an average width of ca 2.5 km and an average thickness of 40 to 70 m, whereas seismic

facies 2 is 3.0 to 5.5 km in width and 50 to 90 m in thickness (Figs 6, 7 and 8A; Table 3).

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Figures 9, 10 and 11A show amplitude slices through a ca 30 km long interval of C2 and

suggest that the studied channels are only slightly sinuous and that the channel-belt is

virtually straight (represented by sinuosity of about 1; the northern boundaries of the

studied channels are marked by yellow dashed lines in Figs 5A, 5B, 9, 10 and 11A; Table 1).

In plan view, seismic facies 1 is expressed as closely spaced, crescent-shaped, high

amplitude threads, whereas seismic facies 2 is seen as closely spaced, low amplitude

curvilinear threads (Figs 9, 10, and 11A; Table 2). Seismic facies 1 and 2 are preferentially

distributed toward the steep and gentle bank, respectively (Figs 9, 10 and 11A).

Sedimentological interpretation

High-amplitude reflections of seismic facies 1 display the highest amplitudes in the

studied channels, and are interpreted to result from sand in channel fills, deposited rapidly

dumping from high-density turbidity currents (Wynn et al., 2007; Gong et al., 2013; Janocko

et al., 2013a; Figs 6, 7, 8, 8A, 9A and 10A; Table 2). Seismic facies 2 exhibit low seismic

amplitudes suggestive of muddy deposits and are generally interpreted as lateral

channel-migration complexes deposited by the interaction of turbidity currents and bottom

(contour) currents (Zhu et al., 2010; Gong et al., 2013; He et al., 2013). The longer term

unidirectional migration of the studied channels and the shorter term

progradational–aggradational character of the lateral channel-migration complexes,

occurring in a consistent direction, are interpreted in terms of the persistent or sustained

action of unidirectional bottom currents (Figs 4, 5, 6 and 8A). This interpretation is based on

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the belief that unidirectional migration and aggradation of deep-water deposits is generally

considered as a ‘typical’ sedimentary response to unidirectionally flowing bottom currents

(Gong et al., 2013).

FLOW DYNAMICS AND SEDIMENTATION

Flow processes

Submarine channels are typically characterised by sediment gravity-flows (SGFs)

including turbidity currents (e.g. Peakall and Sumner, 2015), suggesting that SGFs are one of

the principal processes in the studied channels (Fx in Fig. 11B). In addition, unidirectionally

migrating deep-water channels are distributed in palaeo-water depths of 200 to 500 m (Fig.

3). The present study examines whether the unidirectional (northward) migration pattern of

the studied channels is associated with the sustained, northward directed bottom currents

of the south equatorial current that has an effective depth of ca 350 m (Figs 6 and 8A;

Stramma & England, 1999; Merciera et al., 2003). This analysis is predicated on previous

studies that have interpreted long-term unidirectional migration of deep-water deposits as

a ‘typical’ sedimentary response to bottom (contour) currents exhibiting predominantly

unidirectional flow conditions (Figs 4, 5, 6 and 8A; Fy in Fig. 11B; Gong et al., 2013).

According to conclusions reached by Merciera et al. (2003), the present authors analyse the

affect of the south equatorial current assuming maximum velocities of ca 10 cm/s flowing

across the studied channels.

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Coriolis and centrifugal forces

Coriolis forces are known to influence flow dynamics of both straight and sinuous

channels (Cossu & Wells, 2013a). Rossby number (R0) is employed to examine the effect of

Coriolis forces on secondary flow helicoid in deep-water channels and is defined as:

R0 = ∣U/Wf∣ (1) (Cossu and Wells, 2013a)

where U is the mean downstream velocity, W is the average channel width (ca 3.2 km for

the studied channels), and f is the Coriolis parameter defined as f = 2Ω sin (θ), with Ω being

the Earth’s rotation and θ being the latitude (7.16º south for the documented channels).

Therefore, when U = Fx = 0.5 m/s (as discussed later), the R0 = ∣0.5/(3200 × 2 × Ω sin

(7.16º))∣ ≈ ∣0.5/(3200 × 0.181e-4)∣ ≈ 8.62; or when U = Fx = 5 m/s, the R0 = ∣5/(3200 × 2 ×

Ω sin (7.16º))∣ ≈ ∣5/(3200 × 0.181e-4)∣≈ 86.21. In both cases, Ro is >> 1, suggesting that

Coriolis forces are negligible in determining helical flow patterns in the studied channels in

the Lower Congo Basin in low northern latitudes.

Cossu & Wells (2013b) have experimentally demonstrated that Coriolis forces in the

high northern latitudes are able to laterally deflect turbidity currents to the right-hand side

of the channel (looking downstream). This, in turn, contributes to the development of a

river-reversed helical flow circulation, which may encourage erosion on the left-hand

margins and deposition on the right-hand margins (looking downstream) (Cossu and Wells,

2013a and 2013b; Cossu et al., 2015). However, this hypothesis is challenged here by the

left-hand margin deposition versus right-hand margin erosion observed in the studied

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channels in the Northern Hemisphere (i.e. in the northern South China Sea margin), as

reflected by the fact that right-hand margins of the studied channels (looking downstream),

overall, are steeper than their left-hand counterparts (Fig. 2). Coriolis forces are therefore a

negligible term in structuring helical flow patterns in unidirectionally migrating deep-water

channels in low latitudes. In addition, the studied channels are also quite straight (sinuosity

of about1; Fig. 10A; Table 1), indicating that curvature-induced centrifugal forces are almost

absent in them.

Parameterizing bottom-current induced secondary flow

Sediment gravity flows (SGFs) in deep-water channels commonly exhibit density and

velocity stratification, typically with high velocities and high densities close to the bed

(Peakall et al., 2000), indicating that bottom currents with their relative buoyancy will have

more influence over the upper parts of the SGFs in the studied channels. The combined

effects (near-surface flow, Fi) of bottom currents (Fy) and the upper parts of SGFs (Fx) are

analyzed here in order to predict the nature of the overall helical flow. The local

palaeocurrent of Fi (ßi, in degrees) is defined as:

ß = tan-1 (Fy/Fx) (2)

where Fy ≈ 0.1 m/s, as discussed earlier, and Fx is the velocity of upper parts of SGFs.

Therefore, when Fx is 0.5 m/s, ß is equal to 11.3°; and when Fx is 5 m/s, ß is equal to 1.2°;

this suggests that, no matter how strong (up to 5 m/s) or weak (down to 0.5 m/s) SGFs may

be, bottom currents are always able to deflect the upper parts of SGFs towards the steep

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bank by ca 1° to 11° (Figs 8B and 11B). This, in turn, will lead to super elevation of the SGFs

towards the steep bank leading to a pressure-gradient force and resultant downward flow

at the steep bank, and basal-directed flow towards the gentle bank (Figs 8B and 11B). Such a

circulation pattern would lead to deposition on the gentle bank over time, and

corresponding development of a high-velocity zone on the steep bank.

The aforementioned bottom current-induced helical flow rotation within the studied

channels is supported by the following three lines of evidence: (i) lateral channel-accretion

complexes (i.e. 5.5 km in maximum width and 90 m in maximum height) in the gentle bank

are, overall, thicker and much more aerially extensive than channel fills (i.e. 4.0 km in

maximum width and 70 m in maximum height) identified in the steep bank (Figs 4, 5, 6, 8A,

9, 10 and 11A; Table 3), with lower amplitudes suggesting that these are finer grained; (ii)

sandy deposits preferentially accumulated near the steep bank, as indicated by the

high-amplitude nature of channel fills in this region (Figs 4, 5, 6, 8A, 9, 10 and 11A),

suggesting that a high-velocity core occurs along the outer steep bank; and (iii) channel

walls toward the outer steep bank are, overall, not only steeper than their gentle-bank

counterparts but are characterized by the widespread occurrence of truncation

terminations in this region (Figs 4, 5, 6, 8A, 9, 10 and 11A), also suggesting the occurrence of

a high-velocity core along the steep bank.

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Bottom current-induced sedimentation

Particles in SGFs were probably being more effectively sorted, swept and eroded by the

high-velocity core near the steep bank, resulting in intense erosion and resultant

volumetrically smaller channel fills near the steep banks (i.e. 2.5 km in average width and 50

m in average height) (Figs. 4, 5, 6, 7, 8A, 9, 10, and 11A). Finer-grained suspended particles,

in contrast, were probably deposited as a result of the low-velocity zone along the gentle

bank where there are volumetrically bigger channel-accretion complexes (i.e. 3.5 km in

average width and 60 m in average height) (Figs 4, 5, 6, 8A, 9, 10 and 11A). Bottom-current

induced helical flow circulation, as interpreted here, thus promotes steep-bank erosion

versus gentle-bank deposition. Bottom currents usually persist for very long periods of time

and show predominantly unidirectional flow conditions, constantly promoting gentle-bank

deposition and steep-bank erosion during their life spans (Figs 4, 5, 6, 8A, 9, 10 and 11A).

This, in turn, would force the studied channels to consistently migrate towards the steep

bank through time (Figs 4, 5, 6, 8A, 9, 10 and 11A).

In addition, as discussed earlier, unidirectionally migrating deep-water channels as

documented here are not flanked by coevally deposited levées (Figs 4, 5, 6, 8A, 9, 10 and

11A). This suggests that the bottom-current induced secondary flow has flow heights scaled

to the channel depth (Figs 8B and 11B). The upper part of bottom-current induced

secondary flow, therefore, would not spill out from the studied channels.

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DISCUSSION

The data and results of this study provide new ideas for sedimentological

interpretation, numerical models and physical experiments on submarine channels. Firstly,

radial pressure gradients, centrifugal forces and Coriolis forces have commonly been argued

to induce either river-like helical circulation (e.g. Kassem & Imran, 2004; Imran et al., 2007;

Abad et al., 2011) or river-reversed flow rotation (e.g. Peakall et al., 2007; Wynn et al., 2007;

Parsons et al., 2010; Pyles et al., 2012; Cossu & Wells 2013a; Sumner et al., 2014; Cossu et

al., 2015) in sinuous submarine channels, depending on the specific flow conditions (Corney

et al., 2008; Abad et al., 2011; Giorgio-Serchi et al., 2011; Dorrell et al., 2013). The current

study, from a 3D seismic perspective, demonstrates that bottom currents are able to super

elevate and downward deflect the upper part of internal SGFs at orientation of 1° to 11° to

the steep bank, resulting in a river-reversed helical flow rotation composed of faster

near-surface and slower basal return flows towards the steep and gentle bank, respectively.

Therefore, the results herein contribute to unify apparently disparate results reported from

experiments.

Secondly, modern and ancient unidirectionally migrating deep-water channels were

first recognized in the northern South China Sea margin by Gong et al. (2013), using 2D

seismic data (Figs 1 and 2). Ancient unidirectionally migrating deep-water channels are

identified in the present study, on the basis of seismic database from the Lower Congo Basin

along the West African margin (Figs 3 and 4). Unidirectionally migrating deep-water

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channels may therefore be common on continental margins worldwide, although they are

characterized by architectural styles that are dramatically different from those of

well-documented submarine canyons and/or turbidite channels. The current study

documents flow dynamics of unidirectionally migrating deep-water channels, therefore

helping to obtain a complete picture of flow processes and sedimentation in submarine

channels.

Thirdly, these results now strongly suggest that bottom currents have contributed to

the formation of a bottom current-induced helical flow circulation in deep-water channels,

where channel bends and centrifugal forces are absent (Figs 5A, 5B, 9, 10 and 11A) and

Coriolis forces are negligible. This helical flow, in turn, leads to deposition of accretionary

clinoforms continuously along one side of the channel, in marked contrast to the alternating

deposits associated with point bars in sinuous submarine channels (cf. Peakall et al., 2007;

Amos et al., 2010; Cossu & Wells, 2013a; Cossu et al., 2015). This asymmetrical depositional

pattern is analogous to that shown experimentally for high-latitude systems dominated by

Coriolis forces (Cossu & Wells, 2013a; Cossu et al., 2015), again as a result of a helical flow

that is constant in the longitudinal direction, in contrast to sinuous systems where the

orientation of the helix changes at each bend (Peakall et al., 2007; Cossu & Wells, 2013a;

Cossu et al., 2015).

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Fourthly, in contrast to settings where individual submarine channels migrated in an

unsystematic manner (Wynn et al., 2007), the present results have demonstrated that

gentle-bank deposition and steep-bank erosion induced by bottom current-induced helical

flow circulation forced deep-water channels to migrate and nest unidirectionally towards

the steep bank. This bottom current-induced helical flow sedimentation mechanism

therefore contributes to a better understanding of the mechanism for deep-water channels

growth.

Fifthly, the interplay of down-slope SGFs and along-slope bottom currents represents

one of the most controversial topics in deep-water sedimentology (Zhu et al., 2010; Gong et

al., 2013, 2015), due largely to the general energy differences between gravity flows and

bottom currents. The present results suggest that bottom currents could induce

super-elevation of the interface towards the steep bank, leading to deflection of the

downstream currents towards the steep bank by ca 1° to 11° (Figs 8B and 11B), yielding a

clear interaction of gravity flows and bottom currents. These results therefore help to better

understand the interplay of down-slope and along-slope processes.

CONCLUSIONS

This study first documents, from a three-dimensional seismic perspective, a new

mechanism of flow processes and sedimentation in deep-water channels. The results

strongly suggest that bottom currents flowing towards the steep bank can super elevate and

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downward deflect the upper part of internal sediment gravity-flows (SGFs) at an orientation

of 1° to 11° to the steep bank, yielding a helical flow circulation consisting of faster

near-surface and slower basal return flows towards the steep and gentle bank, respectively,

where curvature-induced centrifugal forces and Coriolis forces are negligible. This bottom

current-induced helical flow circulation probably promotes gentle-bank deposition and

resultant bigger and muddier (indicated by low-amplitude seismic nature) lateral

channel-accretion complexes, but favours steep-bank erosion and resultant smaller and

sandier (suggested by high-amplitude seismic properties) channel fills. This, in turn, forces

the studied channels to consistently migrate in the direction of the bottom currents. The

present results have considerable implications for modelling and interpreting flow processes

and sedimentation in deep-water channels, thus aiding a more complete paradigm of flow

processes and sedimentation in deep-water channels.

ACKNOWLEDGEMENTS

Seismic data were provided by China Petroleum & Chemical Corporation, and we thank

them for their permission to publish this paper. This research was funded by the National

Natural Science Foundation of China (No. 41372115 and No. 41402125). Weiwei Ding at the

Second Institute of Oceanography in China is thanked for providing the bathymetric image

presented in Fig. 1B. RioMAR sponsor companies are greatly acknowledged for their

discussion and generous support of C. Gong’s postdoctoral research at Jackson School of

Geosciences of University of Texas at Austin. We are indebted to journal Editor (Gary

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Hampson) and reviewer (Ven Kolla) for their critical but constructive comments that

significantly improved this paper, and to Gary Hampson and Nigel Mountney for editorial

handling. We are grateful to David R. Pyles and Mathew Wells for taking the time to plough

through an earlier version of this manuscript.

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FIGURE CAPTIONS

Fig. 1. (A) Google Earth image showing location of the bathymetric map shown in (B). (B)

Bathymetric image illustrating plan-view details of unidirectionally migrating deep-water

channels and pathways of the North Pacific Intermediate Water (NPIW) and North Pacific

Deep Water (NPDW) (modified from Gong et al., 2012, 2013, 2015). (C) Bathymetric image

illustrating the plan-view morphology of unidirectionally migrating deep-water channels

recognized in the northern South China Sea margin. The map-view location of seismic line

shown in Fig. 2 is labelled.

Fig. 2. Two-dimensional seismic transects from 3D seismic volume (line location shown in

Fig. 1) illustrating the cross-sectional seismic manifestations of seven modern and ancient

deep-water channels with unidirectional channel-growth trajectories (C1 to C7) in the

northern South China Sea margin. These unidirectionally migrating deep-water channels lack

turbidity-flow levées and consistently migrated eastward for more than 5 km. The

channel-migration directions are illustrated by arrowed dashed lines. Notice that the

left-hand (eastern) margins of these unidirectionally migrating deep-water channels (looking

downstream) are, overall, steeper than their right-hand (western) counterparts.

Fig. 3. (A): Geographical context of the study area along the West African margin. Location

of map shown in (B) is labelled. (B): Map of the West African margin showing the

oceanographic background of the study area (solid box with red outline) and the pathways

of the Angola coastal current, south equatorial currents and south equatorial counter

currents (modified from Séranne & Abeigne (1999) and Stramma & England (1999).

Approximately 300 km of the Congo Canyon off south-western Africa is visible in this view.

Regional extent of the lower Congo Fan is from Ho et al. (2012). Plan-view geomorphological

images derived from 3D amplitude volumes (Figs 5A, 5B, 9A, 10A and 11A) cover the full 3D

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seismic survey marked by the rectangle with the solid outline. Also shown are regional

plan-view locations of seismic lines presented in Fig. 4A and B.

Fig. 4. Strike-view seismic sections showing cross-sectional seismic expression of the upper

(A) and lower (B) segments of the three unidirectionally migrating deep-water channels (C1,

C2 and C3) recognized in West African margin (upper and lower panels, respectively). Note

that the studied channels seen in seismic lines presented in this figures are characterized by

unidirectional (northward) channel-growth trajectories, as indicated by dotted yellow lines.

Refer to Fig. 3 for regional plan-view location of seismic line shown in this figure.

Fig. 5. Seismic amplitude slices seen at 280 ms (A) and 300 ms (B) below the modern

seafloor (stratigraphic positions shown in Fig. 6A) illustrating plan-view seismic appearance

of unidirectionally migrating deep-water channels in the Lower Congo Basin (C2), which is

the focus of the present study. Also shown are plan-view locations of seismic profiles

presented in Figs 6A, 7A and 8A. Yellow dotted lines on (A) and (B) represent the northern

margin of the channel.

Fig. 6. Channel fills (seismic facies 1, hot colour-shaded areas) and lateral channel-accretion

complexes (seismic facies 2, cool colour-shaded areas) in uninterpreted (A) and interpreted

(B) seismic lines along depositional strike. Please refer to Fig. 5A and B for line locations and

to Figs 5A, 5B, 9, 10 and 11A for a plan view. Also shown are stratigraphic positions of time

slices presented in Figs 5A, 5B, 9A, 10A and 11A.

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Fig. 7. Channel fills (seismic facies 1, hot colour-shaded areas) and lateral channel-accretion

complexes (seismic facies 2, cool colour-shaded areas) seen in uninterpreted (A) and

interpreted (B) seismic transects along depositional dip. Plan-view location of the seismic

line presented in this figure is shown in Fig. 5A and B.

Fig. 8. (A) Strike-oriented seismic section illustrating a close-up view of the studied channels

(C2 in Fig. 5) and their associated seismic facies 1 and 2. Three types of seismic-reflection

terminations are shown. (B): Schematic diagram illustrating cross-sectional view of the

structure of bottom current-induced helical flow circulation in the studied channels.

Fig. 9. Uninterpreted (A) and interpreted (B) seismic amplitude slices taken at 270 m below

the modern seafloor (see Fig. 6A for stratigraphic position of the shown slice) showing

plan-view geomorphological expression of the studied channels and their associated

channel fills (seismic facies 1, hot colour-shaded areas) and lateral channel-accretion

complexes (seismic facies 2). Yellow dotted line represents the northern margin of the

channel.

Fig. 10. Uninterpreted (A) and interpreted (B) seismic amplitude slices seen at 250 ms below

the modern seafloor (stratigraphic position shown in Fig. 6A) showing plan-view

geomorphological manifestations of the studied channels and their associated channel fills

(seismic facies 1, hot colour-shaded areas) and lateral channel-accretion complexes (seismic

facies 2). Yellow dotted line represents the northern margin of the channel.

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Fig. 11. (A) Plan-view geomorphic image derived from 3D seismic amplitude volumes (taken

at 260 ms below the modern seafloor) illustrating plan-view expression of the studied

channels (C2, see Figs 4A, 4B, 6 and 8A for a cross-sectional view). Yellow dotted line

represents the northern margin of the channel. (B) Schematic diagrams showing plan-view

flow properties and sedimentation in the studied channels.

TABLE CAPTIONS

Table 1. Morphometric properties of channel-complex sets (CCS) developed within the

studied channels.

Table 2. Seismic facies description and interpretation.

Table 3. Morphometric properties of depositional elements within the studied channels.

Table 1 (two-column width)

CCSs Maximum

width

Average

width (W)

Maximum

thickness

Average

thickness (T)

W/T

(dimensionless)

Average

length Sinuosity

CCS1 4.49 km 2.92 km 197 m 124 m 23.55 27.43 km ca 1

CCS2 5.43 km 3.73 km 348 m 172 m 21.69 29.78 km ca 1

CCS3 4.32 km 3.32 km 165 m 112 m 29.64 28.91 km ca 1

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Table 2 (two-column width)

Seismic

facies

Cross-sectional seismic expression Plan-view

appearance Terminations Seismic examples

Amplitude Continuity Geometry

1 High Moderate Lenticular High-amplitude

threads Truncation

Figs 6, 7, 8A, 9, and

10A

2 Low High Sigmoid Low-amplitude

threads

Toplap and

downlap

Figs 6, 7, 8A, 9, and

10A

Table 3 (two-column width)

Seismic

facies Maximum width Average width Maximum height Average height

1 4.0 km 2.5 km 70 m 50 m

2 5.5 km 3.5 km 90 m 60 m

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This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

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