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This is the original manuscript submitted to Marine Geology on 18th October 2016, subjected to biased review by an anonymous reviewer with delayed rejection until July 2017. Similar scientific contents and conceptual model without referencing this work should be questioned for originality. Final version published by Springer on 6th August 2018: https://link.springer.com/article/10.1007/s00531 018 1635 5 Manuscript Details Manuscript number

MARGO_2016_59

Title

Downslope-shifting pockmarks: interplay between hydrocarbon leakage, sediment remobilization, slope currents and topography

Article type

Research Paper

Abstract Two new types of pockmarks were found in the Pliocene-Quaternary section of the continental slope offshore Angola. Some features inside these pockmarks were clearly due to fluid leakage, like distinct craters development that are repeated throughout their time of activity; diagenetic crusts indicating that the fluids involved including hydrocarbons. Other characteristics are clearly results of current activities, with two opposite effects depending on topography: The first type of pockmarks, called “Advancing Pockmarks”, which preferentially develop along the steepest segments of submarine valleys. They apparently mimic the planar outline of buried turbidite channels. It is suspected that their infill migrates downslope, mostly, as a result of vents shifting spatially from one episode to the next, with only a slight participation of low-energy turbidity currents. The second type of pockmarks, called “Nested Pockmarks”, which occur along the same valleys in gently sloping areas as well as on the open slope. Their isolated conical infill apparently records along-slope-migration in a specific depth range, likely indicating the influence of contour currents. These are long-lived pockmarks and are due to the establishment of preferential fluid migration along durable specific paths such as pockmark stoss sidewalls, vertically stacked erosional-interface of sediment undulation, or entire pockmark bodies. The episodic developments of pockmarks suggest that some external factors must trigger bursts of fluid activity, separated by longer periods of passive infills. Keywords

Pockmark infill migration, Gas leakage, Angola, Advancing Pockmark, Nested Pockmark, Hydrocarbon migration

Taxonomy

Sedimentology, Earth Sciences, Seepage

Corresponding Author

Sutieng Ho

Corresponding Author's Institution

Department of Geosciences, National Taiwan University

Order of Authors

Sutieng Ho, Patrice Imbert, Martin Hovland, Daniel T. Carruthers

Suggested reviewers

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Downslope-shifting pockmarks: interplay between hydrocarbon leakage, sediment

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remobilization, slope currents and topography

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Sutieng Ho1*, Patrice Imbert3, Martin Hovland2*, Daniel Carruthers4

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* Coresponding authors: [email protected]; [email protected]

Department of Geosciences, National Taiwan University, P.O. Box 13-318, 106 Taipei, Taiwan; [email protected] Center for Geobiology, University of Bergen, Postboks 7803, N-5020 Bergen, Norway; [email protected] Total-CSTJF, Avenue Larribau, Pau 64000, France, [email protected] Previously Cardiff University, [email protected]

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Abstract

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Two new types of pockmarks were found in the Pliocene-Quaternary section of the

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continental slope offshore Angola. Some features inside these pockmarks were clearly due to

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fluid leakage, like distinct craters development that are repeated throughout their time of

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activity; diagenetic crusts indicating that the fluids involved including hydrocarbons. Other

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characteristics are clearly results of current activities, with two opposite effects depending on

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topography: The first type of pockmarks, called “Advancing Pockmarks”, which

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preferentially develop along the steepest segments of submarine valleys. They apparently

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mimic the planar outline of buried turbidite channels. It is suspected that their infill migrates

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downslope, mostly, as a result of vents shifting spatially from one episode to the next, with

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only a slight participation of low-energy turbidity currents. The second type of pockmarks,

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called “Nested Pockmarks”, which occur along the same valleys in gently sloping areas as

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well as on the open slope. Their isolated conical infill apparently records along-slope-

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migration in a specific depth range, likely indicating the influence of contour currents. These

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are long-lived pockmarks and are due to the establishment of preferential fluid migration

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along durable specific paths such as pockmark stoss sidewalls, vertically stacked erosional1

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interface of sediment undulation, or entire pockmark bodies. The episodic developments of

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pockmarks suggest that some external factors must trigger bursts of fluid activity, separated

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by longer periods of passive infills.

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Keywords: Pockmark infill migration, Gas leakage, Hydrocarbon, Angola, Advancing

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Pockmark, Nested Pockmark

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1

Introduction

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Pockmarks are depression topographies originating from fluid expulsion at the

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seafloor (King and MacLean, 1970; Judd and Hovland, 2007) with formation mechanisms

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that have been widely studied. However, not many studies have addressed the sedimentary

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processes within pockmarks. In this paper, we investigate the specific issue of pockmark

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geometries and infills, in particular lateral infill migration and lateral shifts between

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successive stages of pockmark activity; as well as the geological parameters that govern

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pockmarks distribution. Classic examples of present day or buried pockmarks in literature

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often appear in section view as v-shaped craters with a simple draping or aggrading infill (cf.

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Hovland and Judd, 1988; Kelley et al., 1994; Judd and Hovland, 2007; Çifçi et al., 2003).

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Slant stacking of successive pockmarks has been shown previously on seismic lines by

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Dimitrov and Dontcheva (1994), Baraza et al. (1996), Çifçi et al. (2003), Casas et al. (2003),

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Pilcher and Argent (2006), Dondurur and Çifçi (2008), reported by Maroga (2008), briefly

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documented firstly by Andresen et al. (2011) and interpreted in a conceptual model by Ho et

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al. (2012b). In contrast, most pockmark successions described so far show vertical stacking,

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commonly with variable diameter or depth, but in pure aggradation. In that configuration, the

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apexes of successive pockmarks are stacked vertically; they have been interpreted to record a 2

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succession of fluid eruptions occurring at the same location over time (c.f. Hovland, 1981;

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Çifçi et al., 2003). Pockmarks whose infill or reactivation episodes progressively migrate

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laterally are interesting for fluid flow studies, as they may have a different significance as

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regards fluid leakage.

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The first strings of pockmarks (or aligned pockmarks) were discovered in the

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Norwagien Trench by Hovland (1981). Subsequently, pockmarks aligned above buried

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channels were reported by: Haskell et al. (1997) on the West African slope; Davies (2003),

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who observed a series of conical fluidization structures associated with a subsurface channel

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in the Niger Delta; Gay (2002; et al., 2003), who investigated a sinuous pockmark belt above

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a buried turbidite channel in the Lower Congo Basin; Cauquil et al. (2003), who reported

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pockmarks associated with a buried meandering channel in Nigeria; Coterill et al. (2005),

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who reported pockmark trails in the Gulf of Guinea; Pilcher and Argent (2007), who studied

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linear pockmark trains located above listric slump faults, initiated on the steepest slopes of the

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West African continental margin; and Jobe et al. (2011), who focused on the pockmarks that

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developed above submarine canyons in the Rio Muni Basin of West Africa. Likewise, Maroga

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(2008) and Ho et al. (2012b) studied the formation of pockmarks above turbidite channels in

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Offshore Angola, and Benjamin et al. (2015), who investigated pockmarks that occur along a

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canyon of the deep Niger delta. Among these studies, pockmark arrays with slant infill

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recording episodic fluid venting are reported in Maroga (2008) and Ho et al. (2012b).

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However, rigorous investigation of the origin of these multi-episodic events has not been

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carried out yet.

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Heiniö and Davies (2009) showed channel-aligned depressions containing multiple

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generations of laterally migrating infill sequences. The authors proposed that the migrating

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infill sequences were induced by the activity of bottom currents (Heiniö and Davies, 2007),

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making them a channel-confined equivalent of open slope sediment waves, and did not 3

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recognize any pockmark or fluid escape feature in their examples. The explanation proposed

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for the formation of pockmarks aligned above channels, is that they are caused by lateral fluid

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drainage within the underlying turbidite channel (Gay et al., 2003; Cauquil et al., 2003), with

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subsequent escape of overpressured pore fluids upward along the margins of the channels

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where the overburden is least (cf. Davies, 2003; Pilcher and Argent, 2007; Jobe et al., 2011).

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Gay (2002; et al. 2003) argued that the principal cause of the pore pressure excess within the

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channel represents an increased fluid supply from deeper reservoirs.

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In this paper, we report and interpret new types of migrating pockmarks lined up

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above two channel systems. Our aim is to find the relationship between fluid leakage and

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infill processes in relation to the depositional setting and its morphology. Conceptual models

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for these new pockmarks, are proposed based on the geometries that show up on seismic

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sections and two-way time (TWT) maps of selected horizons.

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2.1

Seismic data and methodology Seismic data

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The data volume used in this study consists of one main seismic survey (1310 km²)

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that covers a higher resolution one (530 km²) in the Lower Congo basin. The water depth is

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between ~800 and 1650 meters. The main survey has a vertical resolution of 7 ms with a bin

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size of 6.25 x 6.25 m. Its dominant frequency is 55 – 60 Hz. The higher resolution survey has

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the same bin size, but a vertical resolution of 5 ms. Its dominant frequency is 70 – 80 Hz. The

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seismic data has been processed to zero phase and has been interpreted using the in-house

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software of Total S.A., Sismage© (Guillon and Keskes, 2004). The following color code has

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been selected for the display of our figures: seismic amplitudes are represented with a dual-

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polarity palette with white at zero; positive amplitudes (downward increase of acoustic

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impedance) are represented by a white to yellow to red trend, while negative amplitudes 4

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(decrease of impedance downwards) are displayed on a grey scale, with the most negative

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amplitude in black.

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2.2

Methodology

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The studied pockmarks are arranged in two trails, each lying above a buried turbidite

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channel. Pockmark Trail 1 is covered by both seismic surveys; the detailed mapping of

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individual reflections of the pockmark infill has been carried out on the higher resolution

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survey. Pockmark Trail 2 is covered by the main survey only. Individual horizons of

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pockmark infill in Trail 1 have been picked manually and examined under a 3D viewer. A

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comparison of the seismic character and geometries of some pockmarks and other types of

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venting structures has been carried out where they are covered by both surveys. The result of

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this comparison is that, apart from vertical resolution, no geometric differences were found

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for the same fluid venting structures covered by both surveys.

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2.3

Vocabulary

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This paper will analyse in detail local depressions on the continental slope, so that the

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described features will have to be oriented sometimes with respect to the regional slope,

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sometimes to the local slope of one pockmark, and sometimes to specific features inside a

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pockmark. The following conventions will be used:

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Upslope / downslope refer to the regional (continental) slope. Over most of the studied area,

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downslope means SW-wards and upslope NE-wards.

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Upcurrent (lee) / downcurrent (stoss) will be used in cases where the current direction has

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been established only. “Progradation” will be used for downcurrent migration and

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“retrogradation” for upcurrent migration where appropriate. When current direction is

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unknown, migration of pockmark infill will be referred to as “lateral migration”.

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3.1

Regional setting and oceanography Regional structure and stratigraphy

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The study area is located offshore Angola, in the Lower Congo Basin. This continental

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margin is the consequence of the break-up of Gondwana and opening of the South Atlantic in

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the Early Cretaceous (Mascle and Phillips, 1972; Larson and Ladd, 1973). The pre-rift phase

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is characterized by the development of Early Cretaceous grabens and half-grabens. The syn-

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rift sequence of Neocomian to early Aptian age (Nombo-Makaya and Han, 2009) is overlain

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by a succession of Aptian evaporites; above this salt comes a thick succession of Albian and

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upper Cretaceous carbonates and Cenozoic clastic sediments (Liro and Dawson, 2000; Lavier

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et al., 2001; Séranne and Anka, 2005). Salt tectonics has been active in the Lower Congo

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Basin since the Late Cretaceous, which results in a broad zonation of the salt-bearing zone

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into three belts (Broucke et al., 2004): extensional in the upslope part, with deformation

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dominated by listric faults and rafts detaching on the salt; translational in the medial part with

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dominant salt diapirs and normal faults; and compressional in the distal part, dominated by

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folds and thrusts with the development of a salt canopy. The Lower Congo Basin is a prolific

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hydrocarbon province, with oil and gas accumulated in Oligocene and Miocene turbidite

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reservoirs in particular.

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3.2

Local structure

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The study area is located in the transition zone between the extensional and

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translational domains (fig. 1a; Broucke et al., 2004). It is cut by a major growth fault, which 6

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defines a footwall domain and a hanging wall domain. The fault was active during the late

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Miocene and is sealed by Pliocene deposits (Ho, 2013). The footwall domain has been

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extensively affected by normal faulting (fig. 2a), while the hanging wall has a rollover

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morphology (fig. 2a). Two salt diapirs are present over the study area, identified as D1 and D2

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on Figure 1a. Diapir (D1) has its top expressed on the present day seafloor morphology.

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3.3

Local stratigraphy

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The interval of interest where the fluid venting structures and pockmarks of interest

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occur is comprised between the middle Miocene strata and the present day seafloor. The

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Neogene-Quaternary sequence in this survey is dominated by well-bedded hemipelagic

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sediments (Broucke et al., 2004; Vignau et al., 2000), commonly eroded by turbidite channel

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complexes in the Middle to Upper Miocene interval (Ho et al., 2012a). Two turbidite channel

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complexes cross the area of interest in the Middle-Upper Miocene; the upper one is located

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directly below the pockmarks of interest, the other some 60 m deeper. These channels are

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oriented NE-SW, cross the growth fault and are deviated by Diapir 1 (fig. 1). The slope

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becomes significantly steeper ~15-20 km to the SW of the growth fault (Fig. 1b); in this

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study, the side of channel complexes landward of this change in slope is referred to as the

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upslope side, while the seaward part is referred to as the downslope side. Since the Upper

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Cretaceous and Cenozoic strata are heavily deformed by salt tectonics (Liro and Dawson,

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2000; Lavier et al., 2001), the Miocene turbidite channel reservoirs and encasing silty, clay-

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dominated seals are also intersected by salt-related faults.

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Polygonal, tier-bounded faults were observed in two tiers (Ho et al., 2013; 2016), one

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within the Upper Miocene and the other covering the whole Pliocene interval (fig. 2b). The

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studied pockmarks mainly occur within the latter.

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3.4

Regional regime of oceanic currents

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Oceanic currents will be proposed as a cause of pockmark migration; this section

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therefore reviews what is known on current activity in the area. No information could be

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found on palaeo-currents during the Miocene and Pliocene in the study area. It is known that a

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major change in the general oceanic circulation occurred with the progressive closure of the

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Panama Isthmus between -13 Ma and - 2.6 Ma (e.g. Haug and Tiedemann, 1998; Schneider

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and Schmittner, 2006). However, in the absence of a better analogue, we focuses on the

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present-day oceanic circulation and use it as a possible closest (or least remote) analogue for

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the stratigraphic interval of interest, keeping in mind that there must have been changes, some

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potentially significant.

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Lots of questions still exist about the bottom currents in water depths ranging from

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500 to 1500 m (upslope domain) along the Angola margin (Séranne and Nzé-Abeigne, 1999).

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The deep waters in the Angola Basin are composed mainly of the dominant North Atlantic

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Deep Water (NADW) and 20 – 30% of Antarctic Intermediate Water (AAIW) (Van

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Bennekom and Berger, 1984). AAIW circulates northward along the Western Angola margin

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(Boersma, 1984). It flows at a mean depth of 700 – 800 m and no deeper than 1500 m

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(Shannon, 2009). The vertical mixing zone between AAIW and the top boundary of NADW

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is located at 1100 – 1400 m (Berger et al., 2002); below that depth, the water mass is

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dominated by NADW (Berger et al., 2002). The study area is therefore located in the

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transitional zone between AAIW and NADW.

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Coastal upwelling is estimated to have started during the latest Miocene just before

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6 Ma (Berger et al., 2002) and presently occurs at a depth around 200 m along the coast of

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Western Africa (c.f. Jansen et al., 1984). The upwelled water that migrates from a depth of

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100 to 200 m up to the surface, and is suggested to be provided by the northward-flowing

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AAIW (Séranne and Nzé Abeigne, 1999). The closest upwelling cells to the study area have 8

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been identified to the north and south of the Congo river mouth, respectively at ~5°S and

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~7°S (Lutjeharms and Meeuwis, 1987). Deep erosional channels with a depth of several

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hundred meters, originating on the upper slope around -500 m and ending mid-slope around -

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1500 m have been described offshore Congo by Séranne and Nzé-Abeigne (1999). They grew

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along with the aggradation of the slope over the whole Miocene interval and show

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unidirectional northward migration over that span of time. They were interpreted by Séranne

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and Nzé-Abeigne to result from deep upwelling. This is the only published indication about

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Miocene currents in the studied basin, but ca. 200 km to the north of our study area; no such

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channels could be observed on the seismic dataset used for this study. In addition, high speed

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bottom currents of 20 – 25 cm/s on the present day seafloor were measured within the studied

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interval of 700 – 1300 m water deep, at a quasi 0° dip area adjacent to the studied channels

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(Total internal report, 1999). To summarize, the existence of bottom currents in the Miocene

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when the coastal morphology was similar to the present-day one is taken as a plausible

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working hypothesis. So hereafter, we simply use the term of “bottom current” to describe

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movements of the water mass at the bottom of the study area.

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4.1

Results Pockmark morphology

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Three main types of pockmarks have been observed in this survey. They are classified

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according to the organisation of their infill and the evolution of their apexes within the arrays.

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The three types of pockmarks (fig. 2b) are: 1) pockmarks with draping infill layers that stack

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up vertically on seismic sections and are laterally continuous in the far field; they will be

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referred to as “Vertically Stacked Pockmarks” (VSP); 2) pockmarks with infill sequences

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showing downslope shifting over time on seismic sections; their infill is laterally continuous

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with the far field series but is frequently truncated by younger craters on the downslope side 9

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within the arrays; they will be referred to as “Advancing Pockmarks” (AP); 3) pockmarks

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with infill sequences that aggrade above the basal craters without apparent lateral extension

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into the far field and slightly migrate along the bathymetry contours; are called “Nested

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Pockmarks” (NP). Advancing Pockmarks arrays and Vertically Stacked Pockmarks arrays

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have been observed to develop above some bigger, earliest pockmark craters, called Basal

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Craters (BCs) (see   φ0 in fig. 3a). These craters occur at the base of the Pliocene above

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the palaeo channel surface and are organized as a chain (fig. 3b), and will be descried in detail.

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The first type of pockmarks (VSP) is the most common and has already been widely

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investigated by other researchers (e.g. Hovland, 1981; Mazzotti et al., 1987; Çifçi et al., 2003;

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Moss, 2010), so we are not going to discuss them in this study. The second type of migrating

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pockmarks (AP) has been briefly investigated by Ho et al. (2012b), while the third type (NP)

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has never been reported in literature, nor has any formation process been proposed for them

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so far. In the section below, we first examine in detail the structures and morphology of the

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two types of pockmark arrays.

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4.2

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4.2.1 Advancing Pockmark arrays

Pockmark types

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Within our study area, we define an individual pockmark as a sub-conical erosion

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surface (sub-circular in map view) that erodes underlying strata; its infill is defined as the

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sediments that fill it up to the next episode of erosion. An “Advancing Pockmark array” is

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defined as a stack of individual pockmarks that progressively advance downslope, with

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younger pockmarks truncating both the stoss margin of older pockmarks and their infill

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sequences (fig. 3a). The upslope side of individual pockmarks is draped in continuity with far

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field reflections, while their stoss sidewall is interrupted along the axis of underlying turbidite 10

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channels (fig. 3b-d). The horizontal shifting distance for individual pockmark arrays ranges

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from 60 to 650 m. The diameters of individual pockmark craters generally vary from 300 to

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600 m (fig. 4), and their depths are less than 50 ms TWT. The height of Advancing Pockmark

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arrays is ca. 150 m on average. APs are principally found above two turbidite channels and,

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are lined up with their axes to form pockmark trails (fig. 1). They occur in the Pliocene

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hemipelagic interval that constitutes the upper polygonal faults tier and are commonly

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constrained by the tier (fig. 2b; Ho et al., 2012b).

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As revealed by detailed horizon mapping, the cross-sectional character of the pockmark arrays is expressed by: 1)

Downslope shift of infill sequence within individual pockmarks: The seismic section

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along the axis of a palaeo channel complex on Figure 3a illustrates the infill

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sequences of two pockmark arrays advancing in the downslope direction. Six

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horizons are shown, numbered h1 to h6. The five intervals they define are more or

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less isopach all around the pockmark and inside the pockmark, with the exception of

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the downslope side where they are thinner or absent. The isopach character and

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continuity with the far field indicate draping and suggest that sedimentation was

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dominated by hemipelagite deposition; however, the thinning / disappearance on the

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downslope side indicate an additional effect.

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The interruption of reflections on the lower downslope sidewall of individual

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pockmarks has the following characteristics: on the upslope side of pockmark infill,

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discontinuous reflections onlap onto the downslope sidewall (e.g. h2-5 of pockmark-

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i in fig. 3a); on the downslope side, in contrast, discontinuous reflections appear

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truncated (within the limits of seismic resolution) by the surface of the sidewall. The

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maps and block-diagrams of Figure 3c show that the windows defined by reflection

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interruption have a crescent shape in map view (fig. 3c, d). 11

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2)

Slight thickening of the upslope packages: Figure 3a shows a slight expansion of the

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far field succession into the pockmark, typically reaching a maximum just to the

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upslope side of the apex. This indicates a slightly higher rate of deposition on the

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upslope side of the pockmarks than in the background hemipelagites, therefore, there

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is likely some influence of bottom currents. This point will be discussed in section

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5.3 hereafter.

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3)

Downslope migration of successive pockmark craters: pockmark development in the

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arrays is multi-phased, i.e. each individual crater (e.g. layers 2-4 of   φ in the

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three pockmark in fig. 3a) develops tangent to, or eroding into the downslope margin

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of the previous one, in the latter case truncating the previous infill sequence (h6 in

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fig. 3a, d; see also arrays ii and iii in fig. 3a). Towards the top of the arrays, the

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individual pockmarks become shallower while their apical angles increase. The

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accommodation provided by these late craters thus progressively reduces upward,

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until the depressions become almost filled and are sealed by Quaternary sediments.

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The detailed architecture of an Advancing Pockmark infill is shown in Annex-1.

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These younger pockmarks likely represent different episodes of fluid expulsion, and will be called “reactivation craters” or “reactivation pockmarks” hereafter.

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4.2.2 Basal craters / earliest pockmarks of the arrays

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Basal Craters are often bigger than individual pockmarks within the arrays above.

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They have diameters ranging from 300 to more than 1000 meters with depths varying from 50

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to 100 ms TWT. A second generation of craters are sometimes observed within the onlapped

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infill of the BCs (e.g. 1 in fig. 3a). These craters likely provided an irregular topography for

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initiation of the pockmarks arrays. Some of the BC are (sub) circular, while others merge into

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large elongate features (fig. 3b). They all develop above the vertically stacked, erosional12

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interface of sediment undulations (fig. 5d) in the upslope part of the underlying channel (fig.

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5b-c), and truncate the top surface of undulations (fig. 5b-c; see Annex-2). Even though the

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undulations were truncated but they can still be fairly well illustrated at certain locations.

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Such as undulations 1, 2, 5, 6 in Figure 5b and 1-3, 5-7 in Figure 5c, showing sigmoidal

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migration structures with generally the lee side thicker than the stoss side, the later

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characterises the erosional-interface (fig. 5e). The high amplitude infills of these basal craters

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are parallel and onlap against the sidewalls of the undulations (fig. 5b-d).

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4.2.3 Nested Pockmarks

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A Nested Pockmark is defined as a stack of conical infill layers that do not extend into

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the far field (fig. 6a-b). The successive apexes of their infill migrate slightly along slope by a

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distance of 40 – 160 m. On seismic sections, the diameter of the infill layers decreases from

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base to top, so that the top and bottom of each unit are constrained within the limits of the

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previous one. As a result, TWT maps at the top of the infill show a concentric pattern (fig. 6c-

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d). The structure of this type of pockmark arrays can thus be compared to nested bowl sets. In

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the study area, the overall diameter of this type of pockmark varies from > 300 to 600 meters,

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with depths ranging from 90 to 150 ms TWT. Using a seismic velocity of 1700 m*s-1, which

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is classical in the first few hundred meters below seabed, the slope angle for both sides of the

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pockmark shown on fig. 6a is about 15 to 20°. The initial craters of this type of pockmarks are

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rather elongate (fig. 7a) and parallel to bathymetry contours (fig. 7b). Like Advancing

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Pockmark arrays, Nested Pockmarks are aligned along a channel complex underneath (fig. 1).

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The main difference between the two types of pockmark arrays is that the NPs only show one

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episode of internal truncation.

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Infill reflections inside the initial craters of NPs show updip terminations onlapping

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onto the margin or sidewall of the craters (fig. 6a). The uppermost part of the infill sequences 13

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(the trough between the lee and stoss side) is filled by sub-horizontal onlapping deposits. The

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layers inside the crater are thicker on the lee side and thinning toward the stoss side of the

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slope (fig. 6a). All infill reflections are restricted inside the accommodation space created by

327

initial craters on seismic, the only exception being the first layer that drapes the whole crater

328

and extends into the far field (blue arrows; fig. 6a). The direction of infill migration is

329

systematically aligned with the long axis of the initial craters parallel to the bathymetric

330

contours (fig. 7b). Additionally, the top of each array is marked by a mounded strong positive

331

reflection (fig. 6a-b). This mounded reflection in some cases appears to crosscut background

332

horizons and to be onlapped by overlying strata (fig. 5.4; fig. 5.10a); it shows a radial pattern

333

(fig. 6e). However, this interpretation will be challenged in section 5.3.2.1.2.

334 335

4.3

Stratigraphic position of pockmark arrays

336

The first generations of pockmark craters below the Advancing Pockmark and

337

Vertically Stacked Pockmark arrays occur in the lower part of the Lower Pliocene, right

338

below the Pliocene polygonal fault tier as Nested Pockmarks do (fig. 2b). The AP arrays and

339

VSP arrays occur mainly within the Pliocene polygonal fault tier, although some AP arrays

340

can extend upward into the Quaternary or even up to the seafloor. The stratigraphic position

341

of the three types of pockmark arrays is summarised in Figure 2b.

342 343

4.4

Heights of pockmark arrays and lateral migration

344

Array heights and distances of lateral migration have been measured for 67 pockmark

345

arrays (table 1). Measurements have been taken for each array along the direction of its

346

maximum lateral migration. The height of a pockmark array is defined as the vertical distance

347

between the lowermost apex and the topmost margin of the array. The distances of horizontal

348

offset are between the topmost and lowermost apex within pockmarks arrays. 14

349 350 Vertically Stacked Advancing Pockmarks

Nested Pockmarks Pockmarks

Quantity

27

Height of arrays

Above Turbidite channel

Southern Hanging Wall

(ms TWT)

100 – 300

50 – 100

80 – 650

60 – 350

9

28

90 – 150

20 – 180

Horizontal shifting

< horizontal seismic 100 – 160

distance of infills (m)

resolution

351

Table 1. Measurements for heights of pockmarks arrays and the horizontal shifting distances

352

of their infill.

353 354

AP arrays are in general higher than the other types and their infill migrates further

355

downslope. Our observations and measurements lead us to the general conclusion that the

356

higher the arrays, the longer the lateral migration distance and the longer fluid active periods

357

(fig. 4).

358 359

4.5

Regional distribution of the pockmarks and associated features

360

Mapping the pockmarks (fig. 1a) indicates that pockmarks with significantly migrating

361

infill mainly occur on inclined topography. As shown on the TWT map of horizon 5.3 Ma (fig.

362

1a), Advancing Pockmark arrays are located: (1) above channels on the hanging wall slope

363

(dipping at c. 3°), (2) on the slope in the SE corner of the survey (dipping at c. 2.5°), (3)

364

above concentric extensional faults of the south syncline and (4) along the WSW flank of

365

Diapir 2. on inclined areas. Nested Pockmarks arrays that do not show strong infill migration

366

are located on the flanks of Syncline 1, which dip at approximately 2°. At these locations, the

15

367

migration direction of all pockmark infills is either perpendicular or parallel to the contour

368

lines of the bathymetry. Finally, Vertically Stacked Pockmark arrays occur only on low-dip

369

areas (with dip c. 1.5°) such as above rim Syncline 1 and in the least inclined regions of the

370

survey (fig. 1a). Note that the average dip of these zones was measured on seismic sections

371

(in a depth-converted volume) close to the channel axis but avoiding the pockmarks to ease

372

the evaluation.

373

The three types of pockmark arrays described above occurred above two turbidite

374

channel complexes 1 and 2 (fig. 1b). The areas associated with the channels can be subdivided

375

structurally into three zones as shown in Figure 1b. Channel Complex 1 (CC1)went around

376

the NW flank of Diapir 1 and developed throughout the Upper Miocene. Pockmark Trail 1 is

377

composed of Vertically Stacked Pockmark arrays above the upslope side of CC1, and of

378

Advancing Pockmark arrays above its downslope side (fig. 5a, 5b). Channel Complex 2

379

(CC2) is associated with the zigzag pockmark trail (fig. 1). It is composed of two sets of

380

channels, Channel North (CN) and Channel South (CS) (fig. 1b). and occurs within the Upper

381

Miocene interval on NW of syncline flank (zone 2). Pockmark Trail 2 is composed of

382

Nested Pockmarks arrays that are located only in the upslope part of Channel South (CS) in

383

the hanging wall domain (fig. 8a), and several Advancing Pockmark arrays that are located

384

above the downslope part where the two channels merge (fig. 8b).

385 386

4.6

Association with other types of gas-related seismic structures

387

Fluid-related features (gas in this study) other than pockmarks have been observed in

388

the subsurface along the turbidite channel complexes, and some of them even reach the

389

seabed. These features are: 1) negative high-amplitude anomalies (fig. 9a, c) and polarity

390

reversals (fig. 9b); 2) seismic chimneys (fig. 9c) and 3) bottom simulating reflections (BSR)

391

(fig. 9c; Maroga, 2008). 16

392

Negative high-amplitude anomalies occur especially along the crest of the levee of

393

CC2, underneath the Nested Pockmark Trail (fig. 6a) and at the base of seismic chimneys (fig.

394

9a-i). At the downslope end of Pockmarks Trail 1, local polarity inversions have been found

395

at the top of underlying CC1 (fig. 9b-i,ii) and ca. 300 ms above, at the base of polygonal fault

396

tier (fig. 9b-iii). Along the Pockmark Trail above the upslope part of CC1, chimneys emanate

397

from the underlying channel complex or from the high amplitude infill of the earliest

398

pockmarks (fig. 9c-i), then stack vertically through the whole pockmark system and finally

399

intersect a BSR. These chimneys often terminate into positive high amplitude anomalies with

400

shallower depressions (< 5ms TWT) (fig. 9a-i (section) and ii (map)).

401 402

5

403

5.1

Interpretation and discussion Role of hydrocarbons

404

Among the various fluid flow indicators described above, the presence of a BSR

405

indicates that methane gas is very likely being supplied by the hydrocarbon system, or was

406

supplied at least in a very recent past (a few hundred years, cf. Sultan et al., 2004). Evidence

407

for gas migration through the channels is supported by the negative, high-amplitude

408

reflections inside the Basal Craters beneath the BSR (fig. 9c). These amplitude anomalies are

409

interpreted as free gas accumulations (Coffeen, 1986; Sheriff, 1978). The truncation on the

410

top surface of undulations coincides with the bottom of BCs suggesting that gas eruption

411

happened within the undulation troughs. The gas-bearing BCs

412

erosional-interface of undulations in direct connection to the underlying channel. This

413

suggests that gas in the underlying channel used the erosional-interface of sediment

414

undulations as a migration path (Ho, 2013). This could result from a local increase in

415

permeability, or simply from preferential seal breaching where the overburden is thinnest; this

416

topic will be discussed in a companion paper.

occur above the tip of

17

417

The presence of shallow gas is also supported by phase reversal, which occurs in local

418

culminations of the levees of CC2 (fig. 9b). On the other hand, the negative high amplitude

419

anomalies that occur at the base of the polygonal fault tier 200 ms TWT above the reversed

420

phase patches (fig. 9b-ii) are interpreted as an indicator of a possible vertical path of gas

421

migration between the channel and pockmark. In addition, “high-amplitude bright spot”

422

chimneys such as the ones shown with blue arrows on fig. 9c-i have been interpreted as

423

methane gas conduits (Hustoft et al., 2007; Petersen et al., 2010). These gas-related features

424

provide evidence for the presence of free gas in the system where they occur. They may be

425

associated with ongoing migration or be relics of older episodes of gas migration.

426

Chimneys that terminate upward into positive high amplitude patches emanate from

427

underlying channel complexes, e.g. the levees of complex 2 (fig. 9a). These patches are

428

recognized as expression of methane-related carbonates or in association with gas hydrates

429

fed by underlying chimneys (Hustoft et al., 2007; Petersen et al., 2010; Ho et al., 2012a). This

430

vertical association is interpreted to be a result of gas escaping upward from the gas-bearing

431

levees or channel reservoirs via the vertical chimneys to the seafloor (Gay et al., 2003), where

432

they generated depressions in which methane-related carbonates precipitated.

433

In conclusion, there is converging evidence that 1) some gas is locally present in the

434

buried channels or its levees, 2) it has been actively migrating upward into the overburden

435

since shortly after burial, and that 3) methane migration is still ongoing today.

436 437

5.2

Role of marine sedimentation

438

Seismic reflection patterns reflect sedimentary processes and the energy of

439

depositional environments (Mitchum et al., 1977). In the same manner, the geometry of the

440

infill of pockmarks can provide information on the mode of sediment transport and deposition,

441

and can be used as an indicator for the direction of bottom currents at the time of their 18

442

emplacement, as shown for instance by Cattaneo et al. (2004). In order to examine models of

443

sediment deposition related to different types of currents or processes, the section below will

444

compare the infill structure of the Basal Craters, Vertically Stacked Pockmark arrays,

445

Advancing Pockmark arrays and Nested Pockmarks.

446

In the interval of interest over the study area, the dominant seismic facies is sheet-

447

drape (cf. Sangree et al., 1978; Mitchum et al., 1977; Sangree and Widmier, 1978). Individual

448

seismic reflections of Advancing Pockmark Arrays and Vertically Stack Pockmarks can be

449

followed across a wide area and the packages they define show limited thickness variations.

450

This draping character indicates that deposition depends neither on the location with respect

451

to turbidite fairways (lows) nor on the local topography of the sea bottom but settling from

452

suspension, i.e. hemipelagic deposition. Consequently, any departure from this stationary

453

character indicates influence from other sedimentary agents, for example, bottom currents in

454

the local context. The infill of Nested Pockmarks and Basal Craters in particular is marked by

455

local variations in thickness with respect to far field values and shows onlap terminations.

456

These local variations will be used to infer the role of bottom currents in the development of

457

the studied pockmark types.

458 459

5.2.1. Bottom current versus sedimentation within pockmarks

460

5.2.1.1 Draping infill of pockmarks

461

The aggrading character of Vertically Stacked Pockmark infills has been interpreted as

462

a succession of hemipelagites draping the initial crater below (Mazzotti et al., 1987). However,

463

the apex of depressional infills shifted systematically downslope within each of the

464

reactivation craters within the Advancing Pockmark arrays. This likely indicates another

465

mechanism, which involves the formation process of Advancing Pockmark arrays. On the

466

other hand, APs develop downslope of VSPs, above Channel Complex 1. The main difference 19

467

in the setting is the slope angle: the VSPs develop on a low-dip domain (ca. 1.5°), while the

468

nine APs that follow downslope develop on a significantly steeper (ca. 3°) part of the slope.

469

We therefore think that there is a genetic relationship between the down-channel migration

470

and the slope angle.

471 472

5.2.1.2 Thickness variations between pockmarks and far field

473

The infills of Basal Craterss and Nested Pockmarks are typically thicker than

474

correlative units away from the pockmarks. Some seismic units pinch out onto the crater

475

sidewalls (Fig. 6b). This thickening into the lows indicates an addition to simple settling from

476

suspension that predominates regionally, i.e. the effect of bottom currents whose sediment

477

load was partly trapped in the lows. The co-existence of turbidity and bottom currents is based

478

on the following lines of evidence:

479

- Shifting infill of Nested Pockmarks in the Lower Pliocene

480

The slight shifting infill of Nested Pockmarks (fig. 6a) is not always continuous in the

481

far field, and when continuous, is thicker than sediment drape in the far field. Shifting

482

occurred towards the SE, parallel to the contour lines of the syncline flanks (fig. 7), pointing

483

to the influence of a contour current. The direction of this current will be discussed later. In

484

addition, the sediment transport direction is likely indicated by the first infill layer of the NPs,

485

which extends preferentially on the SE side of syncline flank (fig. 7b), and deposited in the

486

same direction as the infill progradation.

487

- The elliptical bottom of Nested pockmarks

488

The existence of a contour-parallel current is also evidenced by the shape of the

489

Nested Pockmark bottom, elliptical in map view with the long axis parallel to topographic

490

contour lines (fig. 7). As demonstrated by Hovland (1984), Josenhans et al. (1978), Bøe et al.

491

(1988), the elongate plan form of pockmarks can be used to indicate the direction of bottom 20

492

currents. Thus, it is suggested that the initial morphology of NPs were modified from circular

493

to elliptical by a contour current (sensu Heezen et al., 1966) that flowed along the flank of the

494

syncline.

495

- Onlapping infill of Basal Craters

496

The Basal Craters below Advancing Pockmarks and Vertically Stacked Pockmarks

497

have a very irregular bottom, with alternating shallower and deeper depressions (fig. 5a). As a

498

result, the first layers that smooth out the intra-trail depressions still lie below the margin of

499

the pockmark set and onlap onto the upslope and downslope margins of the set. The

500

undulations of the infill that mimics the erosional base (fig. 5a) are interpreted to reflect

501

differential compaction of the infill above the pre-compacted underlying material, therefore

502

suggesting that the corresponding turbidites are dominantly muddy. Sedimentation rate in

503

each type of pockmark versus the energy of potential bottom or turbidity currents is

504

summarised in a conceptual diagram (fig. 10).

505

- Sedimentary undulations at the end of the Miocene

506

Figures 5b and 5c show sedimentary undulations below the Basal Craters, in the

507

interval between horizons 6H and 5.3 Ma, above CC1’s upslope. These undulations may look

508

at first glance like tilted blocks, but the very low angle of the surfaces (erosional-interface)

509

separating the “blocks” (6 ° to 20 °) makes it clear that they are more akin to sediment waves

510

(Migeon, 2001; Cattaneo et al., 2004) than to deformation features (Imbert and Ho, 2012). In

511

addition, their 3-D geometry (fig. 5d-e) makes it very difficult to envision that they could

512

result from deformation. The consistent erosion throughout the strata in the stoss side of

513

undulations may imply the intensified action of bottom currents toward that side. In any case,

514

the presence of the undulations is interpreted to indicate that bottom currents were actively

515

depositing and / or reworking sediments along the axis of CC1. The different sedimentary

21

516

structures below and above the BC (fig. 5b) indicate that change of current regime over time

517

(between Later Miocene and Pliocene) on the same side of channel run is possible.

518 519

5.2.1.3 Lateral migration: topography vs. bottom currents

520

The study of processes associated with the development of fine-grained sediment

521

waves initiated with the publication of the “lee wave model” by Flood (1988). This model

522

indicates that sediment deposition on fine-grained sediment waves occurs on the lee side, so

523

that sediment waves migrate upcurrent. A number of papers have been published since,

524

highlighting that such sediment waves can be produced by low-density turbidity currents on

525

the levees of submarine channels (e.g. Nakajima et al., 1988; Migeon et al., 2001) or on open

526

slopes (Wynn et al., 2000), or by contour currents on contourite drifts (Faugères et al., 1999).

527

More recently, Fildani et al. (2006) applied the concept of “cyclic step” to

528

morphologies observed in turbidite channels, initially developed by Parker (1996) to describe

529

bedforms occurring in upper flow regimes. A full review on cyclic steps vs. sediment waves

530

can be found in Cartigny et al. (2011). Heiniö and Davies (2009) proposed that undulations

531

observed along the axis of a turbidite channel offshore Brazil were upslope-migrating

532

sediment waves.

533

The infill of both Advancing Pockmarks and Nested Pockmarks shows evidence of

534

migration, downslope for the former set and alongslope for the latter. These attest to the

535

influence of bottom currents. A first point to examine is whether migration occurs

536

downcurrent (dune-type) or upcurrent (sediment wave or cyclic step-type).

537

In the case of Nested Pockmarks, the absence of reactivation craters indicates that

538

seepage is not a dominant factor for their formation. The following observations made on NPs

539

must be accounted for:

540

1) Their infill migrates slightly toward the axis of Syncline 1 (fig. 1b); 22

541

2) Their first infill layer preferentially extends towards also Syncline 1 (fig. 7b);

542

3) The long axes of their bottom are parallel to the bathymetric contours (fig. 7ab);

543

4) The coeval infills of the Basal Crateres in Zone 1 were likely deposed by

544

downslope-flowing turbidity currents (Mitchum et al., 1977).

545

The second point highlights particularly the transport direction of sediment along the

546

bathymetric contours in Zone 2. These elements suggest the probable presence at the time of

547

early infill of a slope-parallel bottom current (contour current) in Zone 2 that flowed into the

548

irregular topography of the initial craters and filled them with sediment.

549

Advancing Pockmarks develop in submarine valleys whose morphology drapes

550

underlying turbidite channels. Although the infill is dominated by draping, indicating a

551

predominance of deposition by hemipelagites, some intervals show thickening near the

552

bottom of the pockmark. The infill thickening and downslope migration within different types

553

of studied pockmarks, could thus reflect either downcurrent migration under the effect of

554

gravity flows (e.g. dilute turbidity currents), or upcurrent migration under the influence of

555

upwelling. The only evidence for upwelling to the best of our knowledge has been published

556

for specific starved channels developing in the mid-slope domain offshore Congo, described

557

by Séranne and Nzé-Abeigne (1999), while there is pervasive evidence for turbidite

558

deposition in channels all over the basin. We therefore favour the hypothesis that downslope

559

migration results from the influence of dilute downslope flowing turbidity currents.

560

As mentioned above, Nested Pockmarks migrate along slope, indicating the influence

561

of contour currents. The same issue arises, i.e. does south-eastward migration reflect

562

downcurrent migration under the influence of a south-eastward flowing contour current or

563

upcurrent migration (antidune or sediment-wave style) under the influence of a north-

564

westward flowing contour current? In the absence of direct evidence, we will simply favour

23

565

the same hypothesis as in the channels, i.e. downcurrent migration, i.e. migration would

566

represent progradation rather than backstepping.

567

To summary this section, the degree of migration shows a regional organization, with

568

upslope pockmarks being more aggrading, while those located on the flank of the syncline

569

and on the regional slope having a higher rate of migration. The difference in sediment

570

accumulation rates from different sides of pockmark sidewalls suggests that, the depositional

571

processes occurred in a regime dominated by currents impinging on the seafloor and affected

572

by the pre-existing local topographies (Brothers et al., 2011).

573 574

5.3

Mechanisms of pockmark formation

575

Nested Pockmarks and Advancing Pockmarks both occur on the most inclined parts of

576

the slope, suggesting that the inclination had an influence on the velocity of bottom currents.

577

This section proposes models for the genesis of both types of pockmarks based on their

578

geometry.

579 580

5.3.1

581

5.3.1.1 Infill of the initial pockmark

Nested Pockmarks

582

The following conceptual model is proposed: fine-grained sediments were transported

583

on seabed by different types of bottom currents (downslope-flowing turbidity currents or

584

nepheloids, or along slope-flowing contour currents). It is proposed that bottom currents

585

underwent flow separation at the rim of depressions and transformed into: a main branch of

586

current with higher velocity continued to flow towards while its secondary branch with lower

587

velocity sank into the depression (Taneda, 1979; Sinha et al., 1982; Nowell and Jumars, 1984;

588

Yao et al., 2000; Dudley and Ukeiley, 2011). Including researches in circular marine

589

depressions for benthic habitats (e.g. Yager et al., 1993; Abelson and Denny, 1997; Nowell 24

590

and Jumars, 1984), the circulations of secondary current in depressions have been well

591

studied over a half century and have been generally called ‘(lid-driven) cavity flow’ in

592

different scientific domains (e.g. industrial mechanic, civil engineering and biology; Squire,

593

1956; Greenspan, 1969; Henderson, 2001; Pey et al., 2014; Yager et al., 1993).

594

As suggested by the geometries of Nested Pockmark infills (fig. 6a), sediments

595

transported by the secondary branch were apparently deposited on the lee side of NPs, where

596

the pockmark sidewall were generally more gentle (5°-10°), while less deposition occurred on

597

the steeper stoss side (10°-15°). The curved geometry of NP infills could result from different

598

flow models of the secondary current inside pockmarks (fig. 11). Previously, Ho et al. (2012b)

599

suggested that one of the cavity flows, i.e. backward current occurs in pockmarks of this study

600

area. However, other alternative patterns exist (Manley et al., 2004). Depending on the aspect

601

ratio (height/length) of the basal pockmark craters and Reynolds number (defined by velocity,

602

density, viscosity and travel distance of fluid) (Reynolds, 1883) of the main currents (see

603

Fang et al., 1999; Zdanski et al.,2003), the secondary currents could be transformed either

604

into: 1) stream lines parallel to the pockmarks topography (Higdon, 1985; Pozrikidis, 1993;

605

Hammer et al., 2009), or into 2) vortexes of cavity flow, i.e. vertical backward circulation

606

around depressions (Taneda, 1979; Higdon, 1985; Yager et al., 1993; Pozrikidis, 1993; Kulsri

607

et al., 2007). Both possibilities have been modelled numerically by e.g. Fang et al. (1999) and

608

Zdanski et al. (2003). In the absence of palaeo oceanographic and geological data, the flow

609

model of current within our studied pockmarks cannot be verified (see Fang et al., 1999).

610

Therefore, two possible and most common flow styles (adapted from Ho, 2013) are proposed

611

as Case 1 (left-hand column of fig. 11) and Case 2 (right-hand column of fig. 11), below:

612

Case 1: when turbidity currents or nepheloids flow over the depression, the secondary

613

branch of the current with lower velocity sinks into the crater along its lee side slope and

614

flows parallel with the topography of the depression (Higdon, 1985; Pozrikidis, 1993) (fig.

25

615

11a). It suggests that the reduction in current velocity induced by separation of bottom current

616

engenders loss of particle transport capacity (Allen, 1970), and induces (the heavier) particles

617

deposition along the carried current across the streambed (Jopling, 1964; Allen, 1970; Manley

618

et al., 2004). The secondary currents thus likely left deposits on the upcurrent slope (lee side)

619

of the crater (fig. 11a). Smaller grains or hemipelagites may settle down or drape the crater

620

during low current activity periods (fig. 11b, e). Deposits are thinner on the stoss side of

621

Nested Pockmarks, and occasionally appear to be truncated (fig. 6a). This indicates that

622

erosion probably occurred after the sedimentation on the stoss side; we think such erosion was

623

caused by episodes of higher activity of the bottom currents (fig. 11d), which varied

624

throughout the pockmark filling process. Notice that, in comparison to bed-paralleled currents,

625

Manley et al. (2004) suggested that backward eddies are the likely flow patterns capable of

626

removing unconsolidated sediments from pockmarks as they are engendered by relatively

627

high velocities (fig. 11d’).

628

Case 2: Sedimentation may be related to reverse flows, i.e. vortexes in the negative

629

topography (fig. 11a’, c’). Vortexes are generated by rapid downward flowing water at the

630

stoss edge of depressions (Yager et al., 1993; Yao et al., 2000; Dudley and Ukeiley, 2011;

631

Kulsri et al., 2007). Shear stresses induced by friction between backward moving currents

632

and the side wall of the depression sidewall are higher than on the lee side where current

633

flows out (Yager et al.,1993; Sinha et al., 1982; Haigermoser et al., 2007; Pey et al., 2012;

634

Gatski and Grosh, 1983). The reduction in shear stress on the lee side of the depression

635

enhances sediment deposition in the depression (Nowell and Jumars, 1984) while erosion will

636

most likely occur on the stoss side (Jopling, 1965).

637

Finally, truncations on the stoss sidewall were likely caused either by process 1 (fig.

638

11d) or in combination with process 2 (fig. 11d’). Both cavity flows might co-exist (Manley et

639

al., 2004) during the processes of pockmark filling and alternate over time depending on 26

640

current intensity. However, the distinction between the two flow models is impossible due to

641

the absence of geological data.

642 643

5.3.1.2 Hard reflection “at the top”

644

The top of the infill of nested pockmarks is marked by high reflectivity and appears as

645

a mound onlapped by subsequent deposition (fig. 6a-b). At first glance, this would seem to

646

imply that deposition inside the pockmark “overfilled” the area of the initial low to make a

647

gentle mound, later onlapped by subsequent deposition.

648 649

Both mound growth and late onlap seem difficult to envision in our area of interest in the Pliocene for the following reasons:

650

1) Mound growth: in a siliciclastic slope setting, the dominant processes of

651

sedimentation are settling from suspension and deposition by bottom current

652

(turbidity currents and contour currents essentially). The former process results in

653

drape as dominantly observed in the interval of interest, while the latter

654

dominantly leads to deposition in local lows, hardly ever on topographic highs.

655

The mound shape observed at the top of the infill of nested pockmarks, therefore,

656

does not seem compatible with siliciclastic slope deposition

657

2) Onlap onto the “mound”: as mentioned just above, onlap is typical of gravity

658

deposits like turbidites. Figure 2b shows that the onlapping series grade laterally

659

over a few km into a polygonal fault tier, which is known to correspond regionally

660

with very fine-grained lithologies and hemipelagites. It thus seems very difficult to

661

find a mechanism that would locally make these drape-dominant series onlap just

662

over local anomalies.

663

Another possibility, which looks much more realistic to us, is the following: the high-

664

amplitude positive reflection marking the apparent top of the infill would in that hypothesis 27

665

correspond to a diagenetic horizon, methane-derived carbonates precipitated at the sulfate-

666

methane interface within the sediment (fig. 11h-i). The apparent “mound” would simply

667

reflect the upward deflection of the interface above the pockmarks, in zones of increased

668

methane flux pushing the interface towards the seabed (Paull and Ussler, 2009, their fig. 3).

669

Both the “truncation” or “differential growth” at the top of the “mound” and the “onlap” of

670

the series above would simply reflect the interference between regular sedimentation, more or

671

less draping-parallel with a slight thickness increase in the lows and a front of methane

672

derived authigenic carbonate diagenesis at the sulfate-methane interface (Hovland et al.,

673

1987). The overall higher reflectivity observed above the pockmarks than laterally to them

674

may result from scattered carbonate cementation below the final well-expressed diagenetic

675

crust. The radial pattern would in the proposed interpretation reflect fragmentation of the crust

676

by differential compaction, with more cemented and less compactable material inside than

677

outside the pockmark. Finally; the concentric pattern would simply result from successive

678

positive and negative interference of reflections from the stratigraphic succession on the one

679

hand and diagenetic crust on the other. They would therefore have no real morphological

680

significance.

681 682

5.3.2 Advancing Pockmarks

683

The formation of Advancing Pockmarks is likely more complex. Their circular

684

geometry indicates that the pockmarks themselves are not affected by bottom currents; at the

685

same time, infill reflections preferentially disappear along the downcurrent side of pockmark

686

wall. This implies that more factures might involve in AP formation.

687

28

688

5.3.3 The ruptured surfaces on stoss sidewall

689

Several observations indicate that the formation of Advancing Pockmark arrays is

690

multi-phased. The reactivation craters (e.g. P3 in fig. 3a) and the crescent-shaped disruption

691

on the stoss side (e.g. h2 - h5 in fig. 3c) are the main indications for successive episodes

692

during the pockmark development, and are the key features to understand their evolution over

693

time.

694

We acknowledge that the disruption of reflections on the downslope side could have

695

different causes, one of which being seismic resolution. The disappearance of a reflection can

696

correspond either to the real disappearance of the corresponding sedimentary layer(s), or to

697

thinning below seismic detectability. Cashew-shaped disrupted patches occur only on the

698

downslope side of pockmarks; this is compatible with erosion or starvation related to

699

downslope-flowing bottom currents. Preferential erosion could result from increased

700

turbulence of the flow inside pockmarks.

701

Erosion on the stoss sidewall of pockmarks in relation to bottom currents has been

702

proposed by Josenhans et al. (1978) and Baraza and Ercilla (1996), while in a preliminary

703

study of migrating pockmark infills Maroga (2008) mentioned the idea of downslope current

704

interfering with gas venting and disfavored sedimentation on stoss side of pockmarks.

705

Josenhans et al. (1978) hypothesized that fluid escape from the pockmark, when

706

deflected by unidirectional bottom currents, could result in erosion or non-deposition on the

707

stoss side of the pockmark. This hypothesis however would require that starvation /erosion be

708

observed from the venting point up, i.e. from the apex of the pockmark. In our case, infill

709

layers are well preserved above the apex of the pockmark (fluid vent exit), meaning that the

710

model of Josenhans et al. cannot apply here.

711

Hammer et al. (2009) proposed an alternative view, in which upwelling of bed-parallel

712

current within pockmarks could prevent settlement of fine particles but they also stated that 29

713

vortexes occurring within pockmarks remains possible in nature. Sediments being eroded and

714

inhibited by parallel streaming or vortexes within pockmarks has been on the other hand

715

suggested by Brothers et al. (2011) basing on their modelling results.

716

Downslope pockmark migrations caused by cavity flow (i.e. vortex of bottom current)

717

has been first suggested by Ho et al. (2012b). The role of cavity flow as the mechanism for

718

pockmark migration has been re-suggested briefly by Pau et al. (2014). They proposed that

719

upwelling of bed-parallel currents could be the alternative flow pattern that winnows out

720

sediment from the “downstream-pockmark-centre” which indeed corresponds to the

721

pockmark margin toward the downslope side. This conclusion is based on an analogue

722

experiment carried out on a flat-bottomed circular depression excavated on a horizontal

723

surface, downscaled from a metric-scaled pockmark; this situation is quite different from that

724

of the sub-circular Advancing Pockmarks studied here, over 100-m in diameter and located on

725

deep sea slopes. Various cavity flow studies suggest that the size and morphology of the

726

pockmark on the one hand (e.g. Yager et al., 1993), the morphology and physical properties

727

of its substratum on the other (e.g. Abelson and Denny, 1997) play an important role in

728

affecting the formation of flow pattern, as confirmed by a series of modelling experience in

729

Brothers et al. (2011).

730

Manley et al. (2004) stated that bed-paralleled water currents (not turbidity currents)

731

are not strong enough to erode sediments within pockmarks. Upwelling and the maximum

732

velocity generated during pockmark current modelling experienced so far in literature do not

733

settle/focus along the stoss sidewalls (e.g. Hammer et al., 2009; Brothers et al., 2011; Pau et

734

al., 2014) nor extend downward to the stoss side bottoms, but showing weak current speeds

735

far below the erosional domain of fine-grained sediments (Pau et al., 2014), so that it would

736

be difficult to explain the truncations along the stoss sidewall (down to the near bottom) of

737

Advancing Pockmarks with these experimental results. 30

738

In a recent study of direct pockmark observation in Burlington Bay, Manley et al.

739

(2004) concluded that erosion on pockmark sidewall by fluid seepage and vortexes are both

740

possible causes. The latter can be triggered by current speeds starting from 20 – 30 cm/s

741

(Manley et al., 2014) which reaches the erosion threshold of clay and silt in the Hjulstrom

742

diagram, and includes the range of current speeds measured in the studied area. Vortexes are

743

generated by rapid downward current on the same side where shear stress is the highest within

744

the depressional area (c.f. Sinha et al., 1982; Higdon, 1985; Haigermoser et al., 2007;

745

Brothers et al., 2011) and are identified as an important factor to resuspend fine-grained

746

sediment on depression side walls (Nowell and Jumars, 1984; Yager et al., 1993; Abelson

747

and Denny, 1997; Manley et al., 2004), so sediments on the stoss side of pockmarks opposite

748

to regional slope dips are likely being eroded or removed by vortexes. We bear in mind that a

749

more complex current pattern, a combination of several current patterns (Manley et al., 2004),

750

or other causes could induce erosion on the pockmark stoss side.

751

Erosion occurs very locally on the stoss side of Advancing Pockmarks without

752

modifying their sub-circular plan form, contrary to what was observed on Nested Pockmarks.

753

It is thus suggested that the bottom current that affected Advancing Pockmarks was not very

754

efficient, so that sidewall erosions should be caused by another factor. The analogue model by

755

De Vries et al. (2007) demonstrates that fluid seepage can reduce local sedimentation on

756

pockmark sidewalls, in accordance with the conceptual model of Manley et al. (2004) and

757

Hovland et al. (2010). We suggest that, based on the intermittent presence of crescent-shaped

758

disrupted surface and reactivation craters in the stoss side of Advancing Pockmark arrays,

759

there is an alternation between the fluid expulsive erosions

760

occurring on the stoss sidewall. We here propose a conceptual model by integrating the two

761

flow-patterns (vortex and upward-flow in pockmark) without discussing the formation of

762

cavity flow patterns.

and cavity flow erosions

31

763 764

5.3.4 Formation model of Advancing Pockmarks

765

A model for the development of Advancing Pockmark arrays is proposed, expanded

766

from Ho et al. (2012b). It is based on 1) the nucleation site and migration of pockmark craters,

767

2) the stratal geometries within the pockmark crater, and 3) their association with gas

768

accumulations in the channel complex underneath (fig. 5.18e).

769

Phase 1) Generation of initial depression: fast fluid expulsion from the underlying

770

channel creating the earliest pockmark (P0 in fig. 3a; Maroga, 2008). Later eruptions occur

771

through the seal of this pockmark, creating another depression (P1 in fig. 3a). The last

772

depression of this series provides the nucleation point for the subsequent development of

773

Advancing Pockmarks.

774

Phase 2) Cycle of sedimentation and deformation: Once a depression is created the

775

first phase of infill occurs, hemipelagic deposits drape this depression (h1 in fig. 5.3a) with

776

fine turbidite layers deposed preferential on lee side in few occasions. The reactivation of a

777

less intense fluid seepage truncate the apex of hemipelagic drape (h1 pierced by pink dot area

778

in fig. 12a) either through erosion (actual removal of sediment, real discontinuity) or reduced

779

sediment deposition (thinning of the corresponding interval below seismic resolution). The

780

second infill phase (h2) may have occurred during times of reduced bottom water current

781

activity and fluid seepage.

782

Phase 3) Cavity flows i.e. vortex or bed-parallel stream are most likely one of the

783

origins causing sediment erosions inside pockmarks (cf. Manley et al., 2004; Munro et al.,

784

2009; Ho et al., 2012b) (fig. 12b); they are generated at the pockmark rim by the separation of

785

unidirectional flow downslope (Yager et al., 1993; Shi et al., 2000; Migeon et al., 2000; Yao

786

et al., 2000; Brothers et al., 2011). Alternatively, fine-grained sediments might also be

787

reduced by resupsepension by the flows (Yao et al., 2000; Munro et al., 2009; Hammer et al., 32

788

2009). Reducing sediment infills by episodic fluid venting on pockmark sidewall (De Vries et

789

al., 2007; Manley et al., 2004; Hovland et al., 2010) is proposed as another major factor for

790

local thinning. Reductions in lithostatic pressures due to erosions caused by the activity of

791

bottom currents, could have promoted fluid escape.

792

Phase 4) Downslope migration and deposition of permeable stringers: Later phases of

793

sedimentation resulted in a draping of the initial depression (fig. 12c). Throughout the lateral

794

advancement of the pockmark, sediments filled the horseshoe-shaped area left by erosion (fig.

795

12c). During phases of seepage and/or increased bottom current activity these draped

796

sequences were periodically eroded (fig. 12d). The combination of these two processes

797

formed the infill geometries observed on Figure 3b (h2 – h3). Fluid flow pathways were

798

created along the pockmark stoss sidewall through the vertical connection of permeable, fine-

799

grained sand stringers issued from reworked sediments (fig. 12d). The top of the migration

800

path is interpreted as the contemporaneous seabed, and is located at the stoss side of the

801

uppermost depression.

802

Phase 5) Formation of reactivation crater: A new pockmark crater is formed by an

803

intense pulse of fluid expulsion (fig. 12e), possibly triggered by fluid pressure increase in the

804

underlying shallow sediments or turbidite reservoirs.

805

Reducing pressure on the seabed (i.e. lithostatic pressure), while fluid pressure

806

remained unchanged in closed reservoirs may have triggered fluid expulsion from deeper

807

sources. They overall favoured the preferential migration of gas along the vertically connected

808

permeable pathway in the stoss side of pockmark arrays. This architecture of deposits thus

809

controlled the site where a new eruption crater would be formed (fig. 12e). Repetition of

810

preferential erosion/starving, overpressure and crater formation downslope from the previous

811

one results in a lateral migration of the pockmark infill (fig. 12e).

33

812

Phase 6) Sealing: The lateral migration of the pockmark terminates when fluid

813

venting ceases. Once fluid venting stops, reactivation craters cease to form and are buried by

814

subsequent sedimentation.

815 816

6

Conclusion

817

Our results strongly suggest that the architecture of pockmarks and lateral

818

migration of infills reflect the interaction between fluid leakage, diagenesis, topography and

819

water currents (Hovland, 1984; Ho et al., 2012b; Ho, 2013). Fluid expulsion controlled the

820

creation of pre-existing pockmark topography on the seabed, which in turn perturbed the

821

bottom current and controls the location of sediment accumulation. Bottom currents

822

afterwards perturbed by pockmark topographies and transformed into different types of

823

cavity currents that affected the depositional processes of pockmark infills (Ho, 2013).

824

Pockmarks with aggrading infills occur on a relatively flat seafloor, while pockmarks with

825

migrating infills occur on an inclined seafloor, indicating that topographic dips impact

826

bottom current flow and local sediment transportation. Bottom current dynamics and seepage

827

within pockmarks modifying sediment depositions have a direct impact on the vertical seep-

828

communication of different layers in pockmark systems, as the varying degree of grain size is

829

likely to control permeability. Nucleation sites of subsequent leakage structures are thus pre-

830

configured.

831

Two unconventional gas migration pathways were formed as a result of interactions

832

between sedimentation and bottom currents. Gas migrated along: 1) the whole (Nested)

833

pockmark body, evidenced by the occurrence of carbonate crusts at the top; 2) vertically

834

stacked erosional-interface of sediment waves, evidenced by Basal Craters located at the

835

upper tips of the interfaces.

34

836

Two new types of pockmarks, "Advancing Pockmarks” and “Nested Pockmarks” have

837

been described and studied. The formation mechanism of Advancing Pockmarks is different

838

from that of Nested Pockmarks. The main differences are that no reactivation phases occur

839

for Nested Pockmarks, and their infill does not always extend into the far field. The common

840

points for both types of pockmarks are: their occurrence on topographic slopes and their

841

conical infill that dips down-current.

842

Advancing Pockmark arrays are characterised by downslope shifting resulting from

843

asymmetry of the infill on the one hand with more hemipelagic deposits on the upslope than

844

on the downslope side, and from the presence of reactivation pockmarks on the other. The

845

reactivation pockmarks truncate the stoss side margin of underlying infill sequence. Fluid

846

venting and cavity flows, are interpreted to cause discontinuities of pockmark infill on the

847

stoss side.

848

Nested Pockmarks are characterized by a series of conical infill layers which fit inside

849

the initial crater and have apexes slightly prograding along the slope, due to sediments by

850

passing the syncline flank.

851

Finally, episodic and intensity of gas venting are reflected by, successive generations

852

of reactivated Advancing Pockmark craters, dis-synchronous formations of methane-related

853

carbonates and chimneys in both Nested Pockmarks and Advancing Pockmarks locations.

854 855 856

Acknowledgment We thank Total S.A. for providing data, funding and its partner for publication

857

permission, and the Ministry Of Science and Technology of Taiwan for the grant

858

MOST1052914I002069A1. Our work is based on and extended from S.HO’s PhD. The

859

scientific work was fully carried out in Total S.A. and under its direction. S.HO thanks

860

Benoit Paternoster for his supervision on Geophysics. S.HO also thanks Cardiff University 35

861

for the partial PhD funding and JA Cartwright for supports. Thanks also for the enormous

862

support from Gordon Lawrence, Jean-Phillipe Blouet, D. Hutchings, Ludvig Löwemark, and

863

Char-Shine Liu.

864 865 866

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44

1

Figure captions

2 3

Figure 1.

4

Structural maps showing the main geological features of the study area.

5

a) Key horizon 5.3 Ma shows two pockmark trails and the distribution of different types of

6

pockmarks. Two-way time map with contour lines is overlain by semi-transparent amplitude

7

map. The direction of pockmark infill migration is indicated by symbols on this map of horizon

8

“6H”. It shows the two channel complexes underlying the two pockmark trails. A: Anticline;

9

D: Diapir; S: Syncline; C N: North Channel; C S: South Channel.

10

b) Evolution of palaeo channel complex across the different structural domains of the study

11

area. The upslope side of the channel complex (opaque green color) is defined as the segment

12

located above syncline 1 in the upper part of the hanging wall domain of the major growth fault.

13

The lower part of channel complex located above the anticline and on the slope to the SW is

14

defined as the downslope part.

15 16

Figure 2.

17

Seismic profile shows the stratigraphy and geological structures of the studied area. a) It shows

18

the palaeo Channel Complex 1 in different tectonic structure domains. The upslope side of

19

channel complex is defined in this study survey according to its location above the syncline 1

20

in the upper part of the hanging wall domain. While the part of channel complex that occur

21

above the anticline and on the hanging wall slope is defined as downslope part.

1

22

b) Stratigraphic correlations and distributions of Advancing Pockmark arrays and Vertically

23

Stacked Pockmark arrays above the Channel Complex 1, and c) Nested Pockmarks and

24

Advancing Pockmark arrays above the Channel Complex 2. See figure 1a for the line locations.

25 26

Figure 3.

27

Advancing Pockmark arrays. a) Seismic profiles show three Advancing Pockmarks i, ii and iii

28

on sections that are paralleled to the regional slope’s direction. See (b) for locations. The section

29

view of pockmark i and iii show the maximum shifting distance of infill, while the one of

30

pockmark ii shows infill layers adjacent to the maximum shifting direction. Horizons of

31

pockmark i are labelled as "h" followed by a number.

32

b) Two-way-time map superposes with dip value shows the basal craters “0” at horizon 5.3Ma

33

in profiles (a). c) 3D horizons of pockmark i in (a) are shown in two-way-time maps in

34

superposition with dip maps. The circular dot lines highline the boundary of pockmark’s

35

depressional topography. The apex of each layer is aligned with its position on the seismic

36

section at the top.

37

d) Horizon 2, 3, 4 of pockmark i in 3D views. They show the truncation area on the stoss

38

sidewalls in 3D angles.

39 40

Figure 4.

41

Statistic of Advancing Pockmark arrays heights versus the horizontal distance between the

42

topmost and lowest apexes of depressions within the arrays.

43 2

44

Figure 5.

45

Pockmark trail 1 above palaeo Channel Complex 1. This pockmark trail is composed by

46

Vertically Stacked Pockmark arrays and Advancing Pockmark arrays lined up along the axis of

47

burial channels.

48

a) Advancing Pockmark (trail) in the downslope side of this channel complex.

49

b) Vertically stacked pockmarks arrays above sedimentary undulations in the upslope side of

50

the channel complex. Notice that the major growth faults are located in the right hand side of

51

this profile. Black arrows indicate high amplitude bright spot chimneys; numbers are labels of

52

each undulations.

53

c) Seismic section that is parallel to the centre axis of the channel complex, and intersect the

54

edge of Pockmark Trail 1; It shows sedimentary wave-liked structures along the burial channels.

55

Numbers are labels of each undulations. Red arrows indicate the potential fluid migration

56

pathways. Black circles indicate the bottom horizon of the undulations.

57

d) The location of sediment undulations and basal pockmarks are shown on amplitude map of

58

5.3Ma. Yellow numbers correspond the number of sedimentary undulations shown on seismic

59

profile (c).

60

e) The internal structure of sedimentary undulations are expressed by a 3D drawing. Free gas

61

is suggested to use the vertically stacked erosional-interface between each undulations as

62

migration pathways and thus eruptional craters developed above these surfaces.

63 64

Figure 6.

3

65

Nested pockmark. a) and b) are arbitrary lines along and cross the pockmark axis. c) Two-way-

66

time map of the initial crater (basal pockmark) at 5.3Ma. d) a dip map of the top of Nested

67

Pockmark infill which shows annular structure and radial alignment. e) A dip map in

68

superposition with the time map. The morphology of the convex positive high amplitude

69

reflection at the top of the pockmark.

70 71

Figure 7.

72

Elongate morphology of the basal craters of Nested Pockmarks with longest axis parallel to

73

bathymetric contours on syncline flanks. a) Amplitude horizon of 5.5 H superpose with its

74

bathymetry contours. This horizon is located between the top surface of Channel Complex 2

75

and horizon 5.3 Ma which is truncated by Nested Pockmark. It runs across the middle level of

76

Nested Pockmarks and shows the elongate geometry of the basal craters. b) Superpositions of

77

time horizon and dip horizon of 5.3 Ma and the first infill layer of Nested Pockmarks extend on

78

the syncline flank. This infill layer shows a preferential orientation toward the syncline.

79

Stratigraphic location of this layer see fig. 6b.

80 81

Figure 8.

82

Pockmark trail 2 that occur above Channel Complex 2, is composed by Nested Pockmarks in

83

the upslope side and Advancing Pockmark arrays in the downslope side. The Channel Complex

84

2 is containing individual channel of North (CN) and South (CS). a) Nested Pockmarks locate

85

above CN above tectonic (compensation) faults at Anticline 1. b) Advancing Pockmark arrays

86

locate above CS on hanging wall slope.

87 4

88

Figure 9.

89

Other fluid venting structures in association with the pockmark trails and channel complexes.

90

a) Chimneys rooting into the levees of Channel Complex 2.

91

i) Two chimneys above a levee terminating up into seabed depressions that are illustrated on

92

the seabed map. See (ii) for the line’s location.

93

ii) Amplitude map of the present day seabed. Two chimneys terminate upward into shallow

94

depressions at the present day seabed which are associated with associate with positive high

95

amplitude anomalies and are indicated by blue arrows. These positive high amplitude anomalies

96

have been interpreted previously as methane-related carbonates or possibly in association with

97

gas hydrate.

98

b) Negative high amplitude anomalies at 5.3 Ma (base of polygonal fault tier) and stack up

99

above the polarity inversion which occurs at the top surface of Channel Complex 1.

100

i) A cross section in the downslope side of Channel Complex 1, shows negative high amplitude

101

anomalies (NHAAs) and polarity inversion above and at the channel surface (horizon 6H).

102

ii) Negative high amplitude anomalies show elongate and round shapes on amplitude horizon

103

of 5.3 Ma, and are indicated by orange colors. This amplitude map of 5.3 Ma is superposing

104

with blue contours which indicate the location of polarity inversions below the NHAAs.

105

iii) Polarity inversions show elongate and round shape on amplitude horizon of 6H, and are

106

indicated by intense black color.

107

c) Amplitude anomalies in the upslope side of Channel Complex 1. i) Seismic profile shows a

108

BSR defines the upper boundary of negative high amplitude infills within the basal pockmarks,

109

and intersects the bottom of Vertically Stacked Pockmark above. Chimneys stem from the 5

110

channel complex or negative high amplitude infills of earliest pockmarks, and go through the

111

BSR and Vertically Stacked Pockmarks above.

112

ii) Amplitude map of Lower Pliocene across the base of Vertically Stacked Pockmark (P1) and

113

the top of earliest pockmarks (P0), shows the planar geometry of negative high amplitude infill

114

inside the pockmark.

115 116

Figure 10.

117

Conceptual diagram showing the morphology of pockmark arrays as a function of the energy

118

of bottom currents and the sedimentation rate. The regional dip of the strata in areas where

119

pockmarks occur is indicated. The drawings of pockmarks are the outlines of interpreted

120

seismic reflections.

121 122

Figure 11.

123

Model for the development of Nested Pockmarks. In the first step, sediment is deposited by the

124

secondary branch of currents (cavity flows) which is separated from the main branch. Two

125

possible flow models for the secondary currents, which are suggest to depose sediment in the

126

initial depression (earliest pockmarks): the first one is depression-parallel streaming (a) and the

127

second one is vortex flow (a’). Infill sediment was either transported by (a) the laminar,

128

depression-parallel flow from the upcurrent side of the pockmark, or by (a’) vortex and

129

backward flow from the stoss margin of the pockmark; the coarser sediment grains are

130

deposited first at the bottom of the depression. b) During periods of reduced bottom current

131

activity, finer-grained sediment settles from suspension draping the previous infill deposition.

132

c - c’) Stage (a) and (a’) repeated after a new layer is deposited. During periods of increased 6

133

bottom current activity, laminar flow (d) or vortices (d’) form on the stoss slope of pockmark,

134

inducing erosion. This is evidenced by reflections truncation on the stoss sidewall of Nested

135

Pockmarks, as shown in figure 6a. e) Stage (b) repeated when the bottom current became less

136

active. f - f’) The pockmark continued to fill by the same process as in stage (c) and (c’) until

137

all accommodation space was finally be used up. g) The Nested Pockmark was thus formed. h)

138

The pockmark was sealed by hemipelagic deposits; it showed erosional characters on certain

139

infill layers in the stoss sidewall. Formation of the methane-related diagenetic carbonate patches

140

with convex structure which cross cuts the top of infill sequence and the coeval hemipelagites.

141

The convex shape of the carbonate is due to the diagenetic font got pushed up by the upward

142

migrating methane gas. i) The whole pockmark system was buried by hemipelagic sediment

143

afterward.

144 145

Figure 12.

146

Model for the development of advancing pockmarks. a) First phase of infill occur above the

147

last earliest pockmark 1 (P1). b) Fine-grained sediment on the pockmarks stoss sidewall re-

148

suspended by cavity flow (either bed-paralleled stream flow or vortex) and seepage. A

149

horseshoe-shaped truncated area was formed. c) The infill phase may occur during times of

150

reduced bottom-water current activity. Thus horseshoe-shaped void was filled by the new

151

deposits. d) Stage (a) repeated after a new layer deposited. e) Lateral migration by the creation

152

of reactivated pockmarks, which truncated the downslope flank of the preceding infill sequence.

153

Note that two cavity flows i.e. vortex and bed-paralleled stream flow are both possible patterns;

154

the vortex flow is used as demonstration in the principal drawing, while the bed-paralleled

155

stream is integrated as the alternative flow pattern.

156 7

157

Annex figure

158

Annex-1

159

The detailed seismic interpretation of an Advancing Pockmark array labelled as iii in Figure 3a.

160

Top) The interpreted seismic section of the Advancing Pockmark iii.

161

Middle) The outline for the middle portion of the pockmark. The dots line in the stoss side of

162

the pockmark infill layers represent the initial volume of infill before removed by erosion.

163

Bottom) The outline of the upper part and the middle part of the array.

164 165

Annex-2

166

Basal Craters set in the tough of sedimentary undulations located in the upslope part of Channel

167

Complex 1.

168

Basal Craters are expressed by the green color 3D horizon that intersects a seismic section

169

showing some sedimentary undulations, which are labelled by corresponding numbers in Figure

170

5c.

171

8

Highlights  Gas venting, bottom currents and topography control pockmark’s sedimentary infills  Pockmark infills shift downcurrent on incline topographies  Pockmark sidewalls eroded by current circulating inside pockmarks and sidewall seepage  Vertically stacked erosional-interface of sediment waves serve as gas migration paths  Gas used the whole pockmark as further migration paths