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
1
Downslope-shifting pockmarks: interplay between hydrocarbon leakage, sediment
2
remobilization, slope currents and topography
3
Sutieng Ho1*, Patrice Imbert3, Martin Hovland2*, Daniel Carruthers4
4
1
5
2
6
3
7
4
8
* 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]
9 10
Abstract
11 12
Two new types of pockmarks were found in the Pliocene-Quaternary section of the
13
continental slope offshore Angola. Some features inside these pockmarks were clearly due to
14
fluid leakage, like distinct craters development that are repeated throughout their time of
15
activity; diagenetic crusts indicating that the fluids involved including hydrocarbons. Other
16
characteristics are clearly results of current activities, with two opposite effects depending on
17
topography: The first type of pockmarks, called “Advancing Pockmarks”, which
18
preferentially develop along the steepest segments of submarine valleys. They apparently
19
mimic the planar outline of buried turbidite channels. It is suspected that their infill migrates
20
downslope, mostly, as a result of vents shifting spatially from one episode to the next, with
21
only a slight participation of low-energy turbidity currents. The second type of pockmarks,
22
called “Nested Pockmarks”, which occur along the same valleys in gently sloping areas as
23
well as on the open slope. Their isolated conical infill apparently records along-slope-
24
migration in a specific depth range, likely indicating the influence of contour currents. These
25
are long-lived pockmarks and are due to the establishment of preferential fluid migration
26
along durable specific paths such as pockmark stoss sidewalls, vertically stacked erosional1
27
interface of sediment undulation, or entire pockmark bodies. The episodic developments of
28
pockmarks suggest that some external factors must trigger bursts of fluid activity, separated
29
by longer periods of passive infills.
30 31
Keywords: Pockmark infill migration, Gas leakage, Hydrocarbon, Angola, Advancing
32
Pockmark, Nested Pockmark
33 34 35
1
Introduction
36
Pockmarks are depression topographies originating from fluid expulsion at the
37
seafloor (King and MacLean, 1970; Judd and Hovland, 2007) with formation mechanisms
38
that have been widely studied. However, not many studies have addressed the sedimentary
39
processes within pockmarks. In this paper, we investigate the specific issue of pockmark
40
geometries and infills, in particular lateral infill migration and lateral shifts between
41
successive stages of pockmark activity; as well as the geological parameters that govern
42
pockmarks distribution. Classic examples of present day or buried pockmarks in literature
43
often appear in section view as v-shaped craters with a simple draping or aggrading infill (cf.
44
Hovland and Judd, 1988; Kelley et al., 1994; Judd and Hovland, 2007; Çifçi et al., 2003).
45
Slant stacking of successive pockmarks has been shown previously on seismic lines by
46
Dimitrov and Dontcheva (1994), Baraza et al. (1996), Çifçi et al. (2003), Casas et al. (2003),
47
Pilcher and Argent (2006), Dondurur and Çifçi (2008), reported by Maroga (2008), briefly
48
documented firstly by Andresen et al. (2011) and interpreted in a conceptual model by Ho et
49
al. (2012b). In contrast, most pockmark successions described so far show vertical stacking,
50
commonly with variable diameter or depth, but in pure aggradation. In that configuration, the
51
apexes of successive pockmarks are stacked vertically; they have been interpreted to record a 2
52
succession of fluid eruptions occurring at the same location over time (c.f. Hovland, 1981;
53
Çifçi et al., 2003). Pockmarks whose infill or reactivation episodes progressively migrate
54
laterally are interesting for fluid flow studies, as they may have a different significance as
55
regards fluid leakage.
56
The first strings of pockmarks (or aligned pockmarks) were discovered in the
57
Norwagien Trench by Hovland (1981). Subsequently, pockmarks aligned above buried
58
channels were reported by: Haskell et al. (1997) on the West African slope; Davies (2003),
59
who observed a series of conical fluidization structures associated with a subsurface channel
60
in the Niger Delta; Gay (2002; et al., 2003), who investigated a sinuous pockmark belt above
61
a buried turbidite channel in the Lower Congo Basin; Cauquil et al. (2003), who reported
62
pockmarks associated with a buried meandering channel in Nigeria; Coterill et al. (2005),
63
who reported pockmark trails in the Gulf of Guinea; Pilcher and Argent (2007), who studied
64
linear pockmark trains located above listric slump faults, initiated on the steepest slopes of the
65
West African continental margin; and Jobe et al. (2011), who focused on the pockmarks that
66
developed above submarine canyons in the Rio Muni Basin of West Africa. Likewise, Maroga
67
(2008) and Ho et al. (2012b) studied the formation of pockmarks above turbidite channels in
68
Offshore Angola, and Benjamin et al. (2015), who investigated pockmarks that occur along a
69
canyon of the deep Niger delta. Among these studies, pockmark arrays with slant infill
70
recording episodic fluid venting are reported in Maroga (2008) and Ho et al. (2012b).
71
However, rigorous investigation of the origin of these multi-episodic events has not been
72
carried out yet.
73
Heiniö and Davies (2009) showed channel-aligned depressions containing multiple
74
generations of laterally migrating infill sequences. The authors proposed that the migrating
75
infill sequences were induced by the activity of bottom currents (Heiniö and Davies, 2007),
76
making them a channel-confined equivalent of open slope sediment waves, and did not 3
77
recognize any pockmark or fluid escape feature in their examples. The explanation proposed
78
for the formation of pockmarks aligned above channels, is that they are caused by lateral fluid
79
drainage within the underlying turbidite channel (Gay et al., 2003; Cauquil et al., 2003), with
80
subsequent escape of overpressured pore fluids upward along the margins of the channels
81
where the overburden is least (cf. Davies, 2003; Pilcher and Argent, 2007; Jobe et al., 2011).
82
Gay (2002; et al. 2003) argued that the principal cause of the pore pressure excess within the
83
channel represents an increased fluid supply from deeper reservoirs.
84
In this paper, we report and interpret new types of migrating pockmarks lined up
85
above two channel systems. Our aim is to find the relationship between fluid leakage and
86
infill processes in relation to the depositional setting and its morphology. Conceptual models
87
for these new pockmarks, are proposed based on the geometries that show up on seismic
88
sections and two-way time (TWT) maps of selected horizons.
89 90
2
91
2.1
Seismic data and methodology Seismic data
92
The data volume used in this study consists of one main seismic survey (1310 km²)
93
that covers a higher resolution one (530 km²) in the Lower Congo basin. The water depth is
94
between ~800 and 1650 meters. The main survey has a vertical resolution of 7 ms with a bin
95
size of 6.25 x 6.25 m. Its dominant frequency is 55 – 60 Hz. The higher resolution survey has
96
the same bin size, but a vertical resolution of 5 ms. Its dominant frequency is 70 – 80 Hz. The
97
seismic data has been processed to zero phase and has been interpreted using the in-house
98
software of Total S.A., Sismage© (Guillon and Keskes, 2004). The following color code has
99
been selected for the display of our figures: seismic amplitudes are represented with a dual-
100
polarity palette with white at zero; positive amplitudes (downward increase of acoustic
101
impedance) are represented by a white to yellow to red trend, while negative amplitudes 4
102
(decrease of impedance downwards) are displayed on a grey scale, with the most negative
103
amplitude in black.
104 105
2.2
Methodology
106
The studied pockmarks are arranged in two trails, each lying above a buried turbidite
107
channel. Pockmark Trail 1 is covered by both seismic surveys; the detailed mapping of
108
individual reflections of the pockmark infill has been carried out on the higher resolution
109
survey. Pockmark Trail 2 is covered by the main survey only. Individual horizons of
110
pockmark infill in Trail 1 have been picked manually and examined under a 3D viewer. A
111
comparison of the seismic character and geometries of some pockmarks and other types of
112
venting structures has been carried out where they are covered by both surveys. The result of
113
this comparison is that, apart from vertical resolution, no geometric differences were found
114
for the same fluid venting structures covered by both surveys.
115 116
2.3
Vocabulary
117
This paper will analyse in detail local depressions on the continental slope, so that the
118
described features will have to be oriented sometimes with respect to the regional slope,
119
sometimes to the local slope of one pockmark, and sometimes to specific features inside a
120
pockmark. The following conventions will be used:
121
Upslope / downslope refer to the regional (continental) slope. Over most of the studied area,
122
downslope means SW-wards and upslope NE-wards.
123
Upcurrent (lee) / downcurrent (stoss) will be used in cases where the current direction has
124
been established only. “Progradation” will be used for downcurrent migration and
5
125
“retrogradation” for upcurrent migration where appropriate. When current direction is
126
unknown, migration of pockmark infill will be referred to as “lateral migration”.
127 128
3
129
3.1
Regional setting and oceanography Regional structure and stratigraphy
130
The study area is located offshore Angola, in the Lower Congo Basin. This continental
131
margin is the consequence of the break-up of Gondwana and opening of the South Atlantic in
132
the Early Cretaceous (Mascle and Phillips, 1972; Larson and Ladd, 1973). The pre-rift phase
133
is characterized by the development of Early Cretaceous grabens and half-grabens. The syn-
134
rift sequence of Neocomian to early Aptian age (Nombo-Makaya and Han, 2009) is overlain
135
by a succession of Aptian evaporites; above this salt comes a thick succession of Albian and
136
upper Cretaceous carbonates and Cenozoic clastic sediments (Liro and Dawson, 2000; Lavier
137
et al., 2001; Séranne and Anka, 2005). Salt tectonics has been active in the Lower Congo
138
Basin since the Late Cretaceous, which results in a broad zonation of the salt-bearing zone
139
into three belts (Broucke et al., 2004): extensional in the upslope part, with deformation
140
dominated by listric faults and rafts detaching on the salt; translational in the medial part with
141
dominant salt diapirs and normal faults; and compressional in the distal part, dominated by
142
folds and thrusts with the development of a salt canopy. The Lower Congo Basin is a prolific
143
hydrocarbon province, with oil and gas accumulated in Oligocene and Miocene turbidite
144
reservoirs in particular.
145 146
3.2
Local structure
147
The study area is located in the transition zone between the extensional and
148
translational domains (fig. 1a; Broucke et al., 2004). It is cut by a major growth fault, which 6
149
defines a footwall domain and a hanging wall domain. The fault was active during the late
150
Miocene and is sealed by Pliocene deposits (Ho, 2013). The footwall domain has been
151
extensively affected by normal faulting (fig. 2a), while the hanging wall has a rollover
152
morphology (fig. 2a). Two salt diapirs are present over the study area, identified as D1 and D2
153
on Figure 1a. Diapir (D1) has its top expressed on the present day seafloor morphology.
154 155
3.3
Local stratigraphy
156
The interval of interest where the fluid venting structures and pockmarks of interest
157
occur is comprised between the middle Miocene strata and the present day seafloor. The
158
Neogene-Quaternary sequence in this survey is dominated by well-bedded hemipelagic
159
sediments (Broucke et al., 2004; Vignau et al., 2000), commonly eroded by turbidite channel
160
complexes in the Middle to Upper Miocene interval (Ho et al., 2012a). Two turbidite channel
161
complexes cross the area of interest in the Middle-Upper Miocene; the upper one is located
162
directly below the pockmarks of interest, the other some 60 m deeper. These channels are
163
oriented NE-SW, cross the growth fault and are deviated by Diapir 1 (fig. 1). The slope
164
becomes significantly steeper ~15-20 km to the SW of the growth fault (Fig. 1b); in this
165
study, the side of channel complexes landward of this change in slope is referred to as the
166
upslope side, while the seaward part is referred to as the downslope side. Since the Upper
167
Cretaceous and Cenozoic strata are heavily deformed by salt tectonics (Liro and Dawson,
168
2000; Lavier et al., 2001), the Miocene turbidite channel reservoirs and encasing silty, clay-
169
dominated seals are also intersected by salt-related faults.
170
Polygonal, tier-bounded faults were observed in two tiers (Ho et al., 2013; 2016), one
171
within the Upper Miocene and the other covering the whole Pliocene interval (fig. 2b). The
172
studied pockmarks mainly occur within the latter.
173 7
174
3.4
Regional regime of oceanic currents
175
Oceanic currents will be proposed as a cause of pockmark migration; this section
176
therefore reviews what is known on current activity in the area. No information could be
177
found on palaeo-currents during the Miocene and Pliocene in the study area. It is known that a
178
major change in the general oceanic circulation occurred with the progressive closure of the
179
Panama Isthmus between -13 Ma and - 2.6 Ma (e.g. Haug and Tiedemann, 1998; Schneider
180
and Schmittner, 2006). However, in the absence of a better analogue, we focuses on the
181
present-day oceanic circulation and use it as a possible closest (or least remote) analogue for
182
the stratigraphic interval of interest, keeping in mind that there must have been changes, some
183
potentially significant.
184
Lots of questions still exist about the bottom currents in water depths ranging from
185
500 to 1500 m (upslope domain) along the Angola margin (Séranne and Nzé-Abeigne, 1999).
186
The deep waters in the Angola Basin are composed mainly of the dominant North Atlantic
187
Deep Water (NADW) and 20 – 30% of Antarctic Intermediate Water (AAIW) (Van
188
Bennekom and Berger, 1984). AAIW circulates northward along the Western Angola margin
189
(Boersma, 1984). It flows at a mean depth of 700 – 800 m and no deeper than 1500 m
190
(Shannon, 2009). The vertical mixing zone between AAIW and the top boundary of NADW
191
is located at 1100 – 1400 m (Berger et al., 2002); below that depth, the water mass is
192
dominated by NADW (Berger et al., 2002). The study area is therefore located in the
193
transitional zone between AAIW and NADW.
194
Coastal upwelling is estimated to have started during the latest Miocene just before
195
6 Ma (Berger et al., 2002) and presently occurs at a depth around 200 m along the coast of
196
Western Africa (c.f. Jansen et al., 1984). The upwelled water that migrates from a depth of
197
100 to 200 m up to the surface, and is suggested to be provided by the northward-flowing
198
AAIW (Séranne and Nzé Abeigne, 1999). The closest upwelling cells to the study area have 8
199
been identified to the north and south of the Congo river mouth, respectively at ~5°S and
200
~7°S (Lutjeharms and Meeuwis, 1987). Deep erosional channels with a depth of several
201
hundred meters, originating on the upper slope around -500 m and ending mid-slope around -
202
1500 m have been described offshore Congo by Séranne and Nzé-Abeigne (1999). They grew
203
along with the aggradation of the slope over the whole Miocene interval and show
204
unidirectional northward migration over that span of time. They were interpreted by Séranne
205
and Nzé-Abeigne to result from deep upwelling. This is the only published indication about
206
Miocene currents in the studied basin, but ca. 200 km to the north of our study area; no such
207
channels could be observed on the seismic dataset used for this study. In addition, high speed
208
bottom currents of 20 – 25 cm/s on the present day seafloor were measured within the studied
209
interval of 700 – 1300 m water deep, at a quasi 0° dip area adjacent to the studied channels
210
(Total internal report, 1999). To summarize, the existence of bottom currents in the Miocene
211
when the coastal morphology was similar to the present-day one is taken as a plausible
212
working hypothesis. So hereafter, we simply use the term of “bottom current” to describe
213
movements of the water mass at the bottom of the study area.
214 215
4
216
4.1
Results Pockmark morphology
217
Three main types of pockmarks have been observed in this survey. They are classified
218
according to the organisation of their infill and the evolution of their apexes within the arrays.
219
The three types of pockmarks (fig. 2b) are: 1) pockmarks with draping infill layers that stack
220
up vertically on seismic sections and are laterally continuous in the far field; they will be
221
referred to as “Vertically Stacked Pockmarks” (VSP); 2) pockmarks with infill sequences
222
showing downslope shifting over time on seismic sections; their infill is laterally continuous
223
with the far field series but is frequently truncated by younger craters on the downslope side 9
224
within the arrays; they will be referred to as “Advancing Pockmarks” (AP); 3) pockmarks
225
with infill sequences that aggrade above the basal craters without apparent lateral extension
226
into the far field and slightly migrate along the bathymetry contours; are called “Nested
227
Pockmarks” (NP). Advancing Pockmarks arrays and Vertically Stacked Pockmarks arrays
228
have been observed to develop above some bigger, earliest pockmark craters, called Basal
229
Craters (BCs) (see φ0 in fig. 3a). These craters occur at the base of the Pliocene above
230
the palaeo channel surface and are organized as a chain (fig. 3b), and will be descried in detail.
231
The first type of pockmarks (VSP) is the most common and has already been widely
232
investigated by other researchers (e.g. Hovland, 1981; Mazzotti et al., 1987; Çifçi et al., 2003;
233
Moss, 2010), so we are not going to discuss them in this study. The second type of migrating
234
pockmarks (AP) has been briefly investigated by Ho et al. (2012b), while the third type (NP)
235
has never been reported in literature, nor has any formation process been proposed for them
236
so far. In the section below, we first examine in detail the structures and morphology of the
237
two types of pockmark arrays.
238 239 240
4.2
241
4.2.1 Advancing Pockmark arrays
Pockmark types
242
Within our study area, we define an individual pockmark as a sub-conical erosion
243
surface (sub-circular in map view) that erodes underlying strata; its infill is defined as the
244
sediments that fill it up to the next episode of erosion. An “Advancing Pockmark array” is
245
defined as a stack of individual pockmarks that progressively advance downslope, with
246
younger pockmarks truncating both the stoss margin of older pockmarks and their infill
247
sequences (fig. 3a). The upslope side of individual pockmarks is draped in continuity with far
248
field reflections, while their stoss sidewall is interrupted along the axis of underlying turbidite 10
249
channels (fig. 3b-d). The horizontal shifting distance for individual pockmark arrays ranges
250
from 60 to 650 m. The diameters of individual pockmark craters generally vary from 300 to
251
600 m (fig. 4), and their depths are less than 50 ms TWT. The height of Advancing Pockmark
252
arrays is ca. 150 m on average. APs are principally found above two turbidite channels and,
253
are lined up with their axes to form pockmark trails (fig. 1). They occur in the Pliocene
254
hemipelagic interval that constitutes the upper polygonal faults tier and are commonly
255
constrained by the tier (fig. 2b; Ho et al., 2012b).
256 257 258
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
259
along the axis of a palaeo channel complex on Figure 3a illustrates the infill
260
sequences of two pockmark arrays advancing in the downslope direction. Six
261
horizons are shown, numbered h1 to h6. The five intervals they define are more or
262
less isopach all around the pockmark and inside the pockmark, with the exception of
263
the downslope side where they are thinner or absent. The isopach character and
264
continuity with the far field indicate draping and suggest that sedimentation was
265
dominated by hemipelagite deposition; however, the thinning / disappearance on the
266
downslope side indicate an additional effect.
267
The interruption of reflections on the lower downslope sidewall of individual
268
pockmarks has the following characteristics: on the upslope side of pockmark infill,
269
discontinuous reflections onlap onto the downslope sidewall (e.g. h2-5 of pockmark-
270
i in fig. 3a); on the downslope side, in contrast, discontinuous reflections appear
271
truncated (within the limits of seismic resolution) by the surface of the sidewall. The
272
maps and block-diagrams of Figure 3c show that the windows defined by reflection
273
interruption have a crescent shape in map view (fig. 3c, d). 11
274
2)
Slight thickening of the upslope packages: Figure 3a shows a slight expansion of the
275
far field succession into the pockmark, typically reaching a maximum just to the
276
upslope side of the apex. This indicates a slightly higher rate of deposition on the
277
upslope side of the pockmarks than in the background hemipelagites, therefore, there
278
is likely some influence of bottom currents. This point will be discussed in section
279
5.3 hereafter.
280
3)
Downslope migration of successive pockmark craters: pockmark development in the
281
arrays is multi-phased, i.e. each individual crater (e.g. layers 2-4 of φ in the
282
three pockmark in fig. 3a) develops tangent to, or eroding into the downslope margin
283
of the previous one, in the latter case truncating the previous infill sequence (h6 in
284
fig. 3a, d; see also arrays ii and iii in fig. 3a). Towards the top of the arrays, the
285
individual pockmarks become shallower while their apical angles increase. The
286
accommodation provided by these late craters thus progressively reduces upward,
287
until the depressions become almost filled and are sealed by Quaternary sediments.
288
The detailed architecture of an Advancing Pockmark infill is shown in Annex-1.
289 290
These younger pockmarks likely represent different episodes of fluid expulsion, and will be called “reactivation craters” or “reactivation pockmarks” hereafter.
291 292
4.2.2 Basal craters / earliest pockmarks of the arrays
293
Basal Craters are often bigger than individual pockmarks within the arrays above.
294
They have diameters ranging from 300 to more than 1000 meters with depths varying from 50
295
to 100 ms TWT. A second generation of craters are sometimes observed within the onlapped
296
infill of the BCs (e.g. 1 in fig. 3a). These craters likely provided an irregular topography for
297
initiation of the pockmarks arrays. Some of the BC are (sub) circular, while others merge into
298
large elongate features (fig. 3b). They all develop above the vertically stacked, erosional12
299
interface of sediment undulations (fig. 5d) in the upslope part of the underlying channel (fig.
300
5b-c), and truncate the top surface of undulations (fig. 5b-c; see Annex-2). Even though the
301
undulations were truncated but they can still be fairly well illustrated at certain locations.
302
Such as undulations 1, 2, 5, 6 in Figure 5b and 1-3, 5-7 in Figure 5c, showing sigmoidal
303
migration structures with generally the lee side thicker than the stoss side, the later
304
characterises the erosional-interface (fig. 5e). The high amplitude infills of these basal craters
305
are parallel and onlap against the sidewalls of the undulations (fig. 5b-d).
306 307
4.2.3 Nested Pockmarks
308
A Nested Pockmark is defined as a stack of conical infill layers that do not extend into
309
the far field (fig. 6a-b). The successive apexes of their infill migrate slightly along slope by a
310
distance of 40 – 160 m. On seismic sections, the diameter of the infill layers decreases from
311
base to top, so that the top and bottom of each unit are constrained within the limits of the
312
previous one. As a result, TWT maps at the top of the infill show a concentric pattern (fig. 6c-
313
d). The structure of this type of pockmark arrays can thus be compared to nested bowl sets. In
314
the study area, the overall diameter of this type of pockmark varies from > 300 to 600 meters,
315
with depths ranging from 90 to 150 ms TWT. Using a seismic velocity of 1700 m*s-1, which
316
is classical in the first few hundred meters below seabed, the slope angle for both sides of the
317
pockmark shown on fig. 6a is about 15 to 20°. The initial craters of this type of pockmarks are
318
rather elongate (fig. 7a) and parallel to bathymetry contours (fig. 7b). Like Advancing
319
Pockmark arrays, Nested Pockmarks are aligned along a channel complex underneath (fig. 1).
320
The main difference between the two types of pockmark arrays is that the NPs only show one
321
episode of internal truncation.
322
Infill reflections inside the initial craters of NPs show updip terminations onlapping
323
onto the margin or sidewall of the craters (fig. 6a). The uppermost part of the infill sequences 13
324
(the trough between the lee and stoss side) is filled by sub-horizontal onlapping deposits. The
325
layers inside the crater are thicker on the lee side and thinning toward the stoss side of the
326
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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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