1

Pockmark formation by porewater expulsion during rapid progradation in

the offshore Taranaki Basin, New Zealand

Piyaphong Chenrai1* and Mads Huuse2

1. Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330,

Thailand

2. University of Manchester, School of Earth and Environmental Sciences, Manchester, UK

* Corresponding author.

E-mail address: Piyaphong.C@chula.ac.th

Abstract

After the first discovery of seabed pockmarks in the 1960s and 1970s, many examples

of both modern and ancient pockmarks have been reported from sedimentary basins around

the world. The exact mechanisms and fluids involved in pockmark formation are still subject

to debate with many studies inferring that pockmark formation is a direct indication of

hydrocarbon expulsion. This study provides the first description of a buried Pliocene paleo-

pockmark field in the offshore Taranaki Basin, New Zealand. The paleo-pockmarks are

located approximately 1.2 - 1.3 km below the seabed and are well imaged by three-

dimensional seismic data. Their spatial density and size increases towards the distal portions

of the fan, and they vary in size between 45 - 580 m in diameter and 2 - 35 ms TWT.

Pockmark size is inversely correlated with the fan thickness in the study area. Detailed

analysis rules out any links with faulting, while the relationship with fan variations in the

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depositional thickness suggests porewater expulsion during overburden progradation as the

most likely cause of the paleo-pockmarks. A model is proposed in which rapid sediment

loading generated overpressure which was greatest on the proximal fan due to the lateral

gradient in the sediment load imposed by clinoform progradation. Fluids were consequently

forced towards the distal fan where, eventually, the pore pressure exceeded the fracture

gradient of the seal. The implications of this study are that not all pockmarks can be used as

evidence for hydrocarbon expulsion in frontier regions and care should be taken to examine

the subsurface context, plumbing systems and any associated acoustic anomalies before

concluding on the pockmark origin.

Keywords: paleo-pockmark, rapid sediment loading, seismic interpretation, Taranaki basin

1. Introduction

Pockmarks are shallow seabed depressions that have usually been interpreted as the

result of fluids escaping though the sedimentary column to the seabed (King and MacLean,

1970; Paull et al., 1995; Bøe et al., 1998; Gay et al., 2004; Schroot et al., 2005; Judd and

Hovland, 2007). The escaping fluid dislodges sediments into the water column, leaving a

crater-like depression on the seabed. The size of pockmarks may range from a few metres to

hundreds of metres wide, depending on the flux of fluid expulsion through the shallow

subsurface (e.g. Whiticar and Werner, 1981; Harrington, 1985; Hovland and Judd, 1988;

Charlou et al., 2004). Pockmarks were first reported on echograms and side-scan sonar

surveys as V-shaped notches along the sediment-water interface on the Scotian Shelf. They

were originally thought to be a result of ascending gas and/or water controlled by the

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subsurface stratigraphy and structure (King and MacLean, 1970). Since then, pockmarks have

been widely studied because of their relevance to (1) hydrocarbon exploration (Hovland and

Judd, 1988), (2) offshore geological hazards (Sill and Wheeler, 1992), (3) global warming, as

pockmarks serve as indicators of methane venting to the atmosphere (Lashof and Ahuja,

1990), and (4) local biodiversity, which is often much enhanced in and around pockmarks

(Levy and Lee, 1988).

Previous studies have suggested that pockmark formation can be controlled by several

geological and biological factors such as faults and fractures, lithology, sedimentation rate,

seabed erosion, sea-level fall and biotic activity (e.g. Eichhubl et al., 2000; Dimitrov and

Woodside, 2003; Gay et al., 2007; Andresen and Huuse, 2011; Mueller, 2015). In carbonate

environments, carbonate dissolution may create seafloor depressions similar to pockmarks

(e.g. Huuse, 1999; Bertoni and Cartwright, 2015). In addition, the evolution of sedimentary

basins may involve processes, such as overpressure development, leading to fluid expulsion

and pockmark formation (Judd and Hovland, 2007). Because various types of fluids are

expelled in different ways during basin burial and subsidence, fluid flow studies are needed

to complete our knowledge of the subsurface plumbing systems to understand basin dynamics

and to assist in resource and environmental assessments of sedimentary basins.

Paleo-pockmarks have been described as pockmarks that have been covered by

younger sediments (Long, 1992) and have been documented in several studies (e.g. Bizarro,

1998; Gemmer et al., 2002; Gay et al., 2007; Andresen et al., 2008; Andersen and Huuse,

2011). Accordingly, paleo-pockmarks have been used as evidence of past fluid flow and

escape within sedimentary basins around the world (e.g. Heggland, 1997; Andresen et al.,

2008; Andresen and Huuse, 2011; Hartwig et al., 2012; Luo et al., 2015). In the northern part

of the Parihaka three-dimensional (3D) seismic dataset, in the North Taranaki Graben,

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offshore New Zealand (Figure 1), evidence for past fluid venting at the palaeo-seabed is

manifested by the presence of numerous paleo-pockmarks.

In order to enhance our understanding of the mechanism of fluid flow and expulsion

in the study area, we integrated observations from 3D seismic data, including structure,

seismic stratigraphy and seismic geomorphology, of the paleo-pockmarks and their

surrounding stratigraphy. The results of this study provide a new model for pockmark

formation in a rapid sediment loading environment and so contribute to the ongoing

discussion on the origin of seabed pockmarks and paleo-pockmarks and their use as potential

indicators of hydrocarbon expulsion and overpressure evolution in sedimentary basins.

2. Geological Setting

The Taranaki Basin covers approximately 330,000 km2, on- and offshore the west

coast of North Island, New Zealand, and is the main petroleum producing basin in New

Zealand (Figure 1). The basin developed in the Late Cretaceous with multiple phases of

complex deformation (King and Thrasher, 1996), and consists of five main structural

provinces; the Western Platform, North Taranaki Graben, South Taranaki Graben, Tarata

thrust zone and the Southern Zone (King and Thrasher, 1996). The study area is situated in

the North Taranaki Graben province, which is bounded to the west by the Cape Egmont Fault

Zone and to the East by the Turi Fault Zone (Figure 1). The North Taranaki Graben province

developed during the late Neogene back-arc rifting, which created the ESE-dipping faults of

the Cape Egmont Fault Zone and the NW-dipping faults of the Turi Fault Zone. Faults in the

province are NE-SW trending and are associated with synthetic strike-slip faults (Pilaar and

Wakefield, 1984).

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The Taranaki Basin is dominated by terrestrial, marginal marine and shallow marine

sediments from the Cretaceous to Oligocene time (Figure 2). At the end of the Oligocene, the

convergent tectonics of the Pacific and Australian plates caused a strong differential uplift,

subsidence and deposition of deep water sediments in this region. In the Early to Late

Miocene, part of the Taranaki Basin developed a complex fold-thrust belt in a back arc basin

setting (King and Thrasher, 1992). During the Pliocene and Pleistocene, the proximal parts of

the basin were tilted, uplifted and eroded, and the Southern Alps were rapidly uplifted,

leading to rapid progradation of the Giant Foresets Formation that built up most of the

modern continental shelf and slope (Hansen and Kamp, 2006). The sedimentary succession in

the North Taranaki Graben is composed of 2 km of Cretaceous to Paleogene rocks and up to

5 km of Neogene deposits (King et al., 1996; Webster et al., 2011). The Giant Foresets

Formation, where paleo-pockmarks have been found, is comprised of a thick, shelf to slope to

basin floor succession predominantly composed of fine-grained mudstone and siltstone

derived from the South Island and transported northward by longshore drift and ocean

currents (Hansen and Kamp, 2002, 2004a, 2004b; Anell and Midtkandal, 2015; Salazar et al.,

2015). The paleo-pockmarks are located on the lower part of a clinoform set downlapping

onto a Late Miocene reflection and interpreted as a slope fan (Salazar et al., 2015).

Volcanic rocks are common in the Taranaki Basin, where the oldest volcanic activity

in the region is believed to be Early Cretaceous (Muir et al., 1995). Volcanism during the

Miocene began as a result of the subduction of the Pacific Plate beneath the Australian Plate

and led to a build-up of volcaniclastic rocks within the Taranaki Basin (Uruski and Wood,

1991). The Mohakatino volcanic centre is interpreted as a chain of submarine andesite

stratovolcanoes associated with intrusive complexes that were emplaced in the Middle to Late

Miocene (King and Thrasher, 1996). Pliocene and Recent volcanoes are part of the New

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Zealand geography and are associated with an abundant supply of iron rich sediments into the

basin during eruptions, particularly of Mount Taranaki (Campbell et al., 2012).

3. Dataset and Methods

The lithological information is based on wireline data, well reports (Arco Petroleum

NZ Inc., 1992) and previous publications (Beggs, 1990; King and Thrasher, 1996; Salazar et

al., 2015). This study utilized the Parihaka 3D seismic volume located in the offshore

Taranaki Basin (Figures 1 and 3a). The seismic volume covers a surface area of 1,735 km2

and has a total trace length of 6.144 s TWT. The bin spacing is 12.5 m in both directions with

a sample rate of 4 ms TWT. The seismic volume is a full offset, post-stack time migrated

volume. The dataset is processed as zero phase and downward acoustic impedance increases

are displayed as negative amplitude values. One exploration well, Arawa-1, is located within

the 3D seismic volume and was used for the well-to-seismic correlation (Figures 1and 3b).

The resolution of seismic data is dependent on the dominant frequency of the seismic

signal and the interval velocity of the interval of interest. The interval velocity of the

sediment package containing paleo-pockmarks is about 2.75 km/s according to velocity data

from Arawa-1 (Figure 4), and the dominant frequency of the studied interval is about 60 Hz.

Hence, the vertical resolution (a quarter of the dominant wavelength) and horizontal

resolution (half the dominant wavelength) for the interval of interest are approximately 12 m

and 23 m, respectively. In addition, the vertical detection limit is about 1.5 m (1/30

wavelength; Slatt, 2006). Most paleo-pockmarks recorded in this study are significantly

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wider than the seismic resolution, whilst the depths ranged from resolved to somewhere

between the detection and resolution limits.

Visualisation techniques, including time slices, horizon slices, seismic volume

attributes and vertical seismic displays, were chosen to visualise the changes in the seismic

geomorphology and seismic character across the study area. Seismic attribute techniques,

such as variance and dip attributes, were used for observing details of the paleo-pockmarks

and the host stratigraphy. During reconnaissance, pockmark edges were highlighted by the

variance and dip attributes in the time slices. Then, careful horizon picking was performed

across the available two-dimensional (2D) and 3D seismic data, focused on the paleo-

pockmark interval in the Early Pliocene (Figure 5).

The diameter and depth of the paleo-pockmarks on each horizon were manually

measured in order to show the buried paleo-pockmark geometry (Figure 6). Also, the fluid

and sediment losses from the paleo-pockmark craters were estimated from the pockmark

volumes. The pockmark volume was measured based on the assumption that the initial

sediment surface was undisturbed and smooth before pockmark formation. Hence, the paleo-

pockmark volumes were estimated by subtracting the high-resolution pockmark surfaces

from the smoothed surfaces that were asymptotic to the inter-pockmark areas on each surface.

The method and paleo-pockmark parameters (e.g. width and depression) used in this study

are presented in Figure 6. An area of approximately 2 km2 was selected for more detailed

analysis (Figures 3c and 6). The thickness maps between the present-day and smoothed

surfaces provide the variation in the residual topography within the examined area. Each

residual map was then converted into a sediment volume loss using the interval velocity from

the Arawa-1 well.

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4. Results

4.1 Paleo-pockmark descriptions

Seismically, the paleo-pockmark interval is characterised by an overall wedge shape

thickening southward (Figures 3c and 3d), with the paleo-pockmarks buried approximately

1.2-1.3 km below the seabed. In order to comprehensively characterise the paleo-pockmarks,

three horizons (H1, H2 and H3) were interpreted along the paleo-pockmark area within the

paleo-pockmark interval (Figures 7 and 8). The maps show that the paleo-pockmarks are

clustered near the distal pinch-out of the distal fan (Figure 8). They are generally U-shaped

(exaggeration 5-10), with round to oval shaped outlines, but most being nearly circular

(Figures 7 to 8). In general, the paleo-pockmarks have an observed depth and diameter of

approximately 2 - 35 ms TWT and 45 - 580 m, respectively. There are no acoustic

(amplitude) anomalies nor signs of vertical seismic discontinuity beneath the paleo-

pockmarks which could indicate a fluid origin from below the paleo-pockmark interval. The

paleo-pockmarks are usually associated with medium to high anomalies in both geometrical

(dip and variance) attributes. Each horizon is described in detail below.

4.1.1 H1 horizon

The H1 horizon was defined by a laterally continuous reflection and it downlaps onto

the H0 horizon. It is located at the base of the paleo-pockmark interval (Figure 7) and shows

relatively small paleo-pockmarks that can be mapped (Figures 9 and 10). The paleo-

pockmarks vary in diameter from 65-325 m with a depression range of approximately 2-20

ms TWT and slope angles of the paleo-pockmark walls between 15° and 26°. The density of

paleo-pockmarks is higher, and they are slightly larger, in the middle part of the study area

and then gradually decreases in density and size towards the distal part (Figure 8).

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4.1.2 H2 horizon

This horizon is located in the middle of the paleo-pockmark interval between the H1

and H3 horizons (Figure 7) and shows the most spectacular paleo-pockmark features

compared to the other two horizons. There is a wide range of paleo-pockmark dimensions,

ranging from approximately 45–380 m in diameter with depressions of 2–24 ms TWT

(Figures 9 and 10) and slope angles of the paleo-pockmark walls between 6°–18°. As for the

H1 horizon, the density is higher and the size of the paleo-pockmarks slightly larger in the

middle part of the study area (Figure 8). The large paleo-pockmarks are usually associated

with truncation of underlying reflections (Figure 11).

4.1.3. H3 horizon

The H3 horizon is the uppermost horizon of the paleo-pockmark interval (Figure 7).

Some of the paleo-pockmarks on H3 are co-located with those of horizon H2, and these are

usually the same paleo-pockmarks that were wider upwards (Figure 11). They are generally

deep (3–35 ms TWT) with a medium to large diameter of approximately 50–500 m (Figures

9 and 10) and slope angles of the paleo-pockmark walls between 5°–20°. A coalescence of

paleo-pockmarks can be seen in the central area. The presence of small paleo-pockmarks are

relatively rare compared to that in the H2 horizon (Figure 8). No paleo-pockmarks, nor any

other fluid flow features were observed above the H3 horizon.

4.2 Residual topography measurements

Thickness variations derived from the residual topography measurements, shown as

TWT thickness maps, clearly show a localised thickening focused over the paleo-pockmark

craters (Figure 10). H1 shows the relatively small sized paleo-pockmarks characterised by

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very shallow depressions (less than 5 ms). In contrast, H2 and H3 show the relatively large

sized paleo-pockmarks with high thickness values over the paleo-pockmark craters.

In order to evaluate the potential of pore-fluids to cause seismic-scale pockmarks, the

sediment and fluid losses during paleo-pockmark formation were estimated using some first

order volumetric calculations. Theoretically, the relationship between uncompacted and

compacted states of shale during the compaction process can be described in terms of the

porosity loss as a function of burial (Magara, 1978), as shown in Equation (1),

V(1-ɸ) = V0(1-ɸ0) (1)

Hence, V = V0(1-ɸ0)/ (1-ɸ)

Where; V = volume of shale after compaction, ɸ = porosity of shale after compaction,

V0 = volume of shale before compaction, and ɸ0 = porosity of shale before compaction.

A representative surface porosity (ɸ0) of the Giant Foresets Formation has been

reported as 0.5 (Baur, 2012) and the assumed porosity of the formation after compaction at

the paleo-pockmark depth (ɸ) is assumed to be 0.25, usually 25-35% for basin-floor fan

deposits, as based on Browne et al. (2000) and shown in Equation (2).

V = V0 x (1-0.5)/(1-0.25) , (2)

V = 0.67V0

In this case, the present-day sediment volume is reduced to 67% of the original

depositional volume, and so the fluid loss during compaction is 33% of the original volume

of the Giant Foresets Formation, which is about an order of magnitude greater than the

sediment loss estimated from the residual topography of the pockmarks (0.6-5.6%). The

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present-day sediment volume in this study is the compacted volume at the present stage of

burial. To convert this to the original-sediment volume, the sediment loss from the present-

day calculation needs to be applied to Equation (2), giving an original sediment loss of

approximately 0.4-3.7% with a fluid content of approximately 0.2-1.9%.

5. Discussions

5.1 Paleo-pockmark formation

Most of the paleo-pockmarks in the study area are located at the distal part of a

basinal wedge that resulted from an early Pliocene progradation. The mapped paleo-

pockmark distribution is restricted to a small area above a distal fan, because seismic survey

does not cover the whole part of the distal fan and the paleo-pockmark area is disturbed by

erosional truncation (Figures 3a, 3b and 8). The sedimentation rate has been estimated at 450

m/Ma during the Pliocene to Recent progradation (Arco Petroleum NZ Inc., 1992). This high

sediment rate is associated with the rapid sediment supply and progradation due to large

volumes of sediment derived from the erosion of the Southern Alps (Kamp et al., 2004;

Hansen and Kamp, 2006). The rapid sediment loading provided a high sediment thickness

and rapid increase in overburden pressure in the southwestern part, where no paleo-

pockmarks have been observed.

Rapid sediment loading is widely accepted to be a potential mechanism for

developing overpressure, pockmarks and slope failures (e.g. Rubey and Hubbert, 1959;

Smith, 1971; Fertl, 1976; Giles et al., 1998; Kjeldstad et al., 2003; Leynaud et al., 2007;

Hustoft et al., 2009). According to Dugan and Flemings (2002), overpressure is usually high

in proximal fan settings, where the sediment thickness is greater than in distal fan settings.

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Consequently, we speculate that in the study area the rapid sediment loading and

overpressure in the proximal fan caused fluids to migrate seaward within the toesets and

underlying fan (Figure 12a-c).

Sediment loading is considered the main pressure source driving fluid flow and

expulsion in this model as lateral fluid flow is an important factor in the formation and

maintenance of paleo-pockmarks in such rapid sediment loading environments (e.g.

Behrmann et al., 2006). It is generally more difficult for fluids to flow upwards, due to the

layering of aquifers and aquitards, but fluids can flow easier along the bedding as the parallel

bedding permeability can exceed the bulk vertical permeability by several orders of

magnitude (Kjeldstad et al., 2003). The lateral flow is driven by differential overpressure

within sediment layers (e.g. Darby et al., 1998), as presented in Figure 12e. In the study area,

the maximum velocities of fluid flow were controlled by a combination of driving stress and

fluid transmissivity, sedimentation rates, permeability and efficiency of fluid expulsion at the

end of the fluid pathway.

Most of the tectonic faults in the study area trend NE-SW, including the pre- and post-

Pliocene faults (Figures 3 and 5). Although there are major post-Pliocene faults within the

basin, none of the faults penetrate the paleo-pockmark area (Figures 3 and 5).

5.2 Origin of fluids

Pockmarks are often interpreted as evidence of past hydrocarbon fluid expulsion (e.g.

Gay et al., 2007; Andresen, 2012; Kluesner et al., 2013; Donda et al., 2014). Thus, pockmark

formation in petroleum fields may be biased toward evaluating hydrocarbon seep

environments. In this study, it is unclear if biogenic methane could have been generated

within the study interval. We are unable to either accept or to refute the presence of biogenic

methane, but the proposed hydraulic model demonstrates that without external fluids the

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porewater of a sedimentary formation may alone be sufficient to create pockmarks on the

seabed.

In the submarine realm, sediments are generally thought to be fully saturated. If fine

grained sediments are saturated with water at all stages of burial, any reduction in the

porosity may cause the fluid to migrate laterally and upwards according to the hydraulic

head. The loss of porewater due to compaction can be up to 70% of the initial water content

in the early stages of compaction to depths of about 1 km (Brown, 1969). Based on our

observations and data from the Arawa-1 well, paleo-pockmark occurrence is concentrated

within the distal part of the slope fan, which was dominated by claystone with thin

intercalations of coarser sandy units. This suggests that porewater within the sedimentary

formation may have been involved in paleo-pockmark formation in this area. We speculate

that the elevated pore pressure caused by rapid sedimentation in the mudstone-dominated

wedge drove the pore fluids basinwards within the coarser, more permeable slope fan

(Salazar et al., 2015), until a gradual overburden thinning eventually allowed migrating

porewaters to overcome the capillary entry pressures and/or seal strength, and so attain a

rapid porewater expulsion and pockmark formation.

The Taranaki Basin is rich in volcanic edifices, and volcanic and hydrothermal fluids

have previously been suggested as fluid sources for pockmark formation in sedimentary

basins (e.g. Pickrill, 1993). Hydrothermal fluids are not considered as a fluid source for the

paleo-pockmarks in the study area because the pockmark field is closely linked with the

occurrence of a primary slope fan rather than any igneous phenomena, and no pathways have

been resolved that link the fan to igneous features. If hydrothermal fluids had been involved

in fluid expulsion on the seabed, paleo-pockmarks should cover a more extensive area than

the observed paleo-pockmarks in this study and be more closely linked with igneous

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structures, which are clearly identified in the seismic data, and clearly distal to the paleo-

pockmark location.

Polygonal faults occur in the deeper part of the study area at approximately 2 s TWT

beneath the seabed (Figure 5). Polygonal faults have previously been suggested to transmit

fluids and form pockmarks (e.g. Gay et al., 2004), and are usually formed in the first several

hundred metres of burial (Cartwright and Dewhurst, 1998). Polygonal faults can act as a fluid

conduit during their formation, but they can be sealed later (Gay and Berndt, 2007;

Cartwright, 2011). Since the polygonal faults are buried deep under the paleo-pockmark

interval, they were most likely inactive and thus likely sealed before paleo-pockmark

formation. In the study area, the polygonal fault system extends over a much wider area than

the restricted paleo-pockmark area. The scarcity of paleo-pockmarks outside the wedge area

and the stratigraphic separation of polygonally faulted and pockmarked intervals suggests

that the fluid is more likely to have been derived from the sedimentary succession of the

wedge area.

5.3 Implications for the formation of paleo-pockmarks

Pockmarks and seeps have long been used to indicate the presence or leakage of

deeper petroleum systems (e.g. Hovland and Judd, 1988). However, since there is no

evidence of hydrocarbon migration into the overburden in this area, the paleo-pockmarks in

this study appear to be unrelated to hydrocarbon expulsion. Biogenic methane can be

generated at low temperatures from a small amount of organic matter in the shallow

subsurface. The buoyancy pressure within the sedimentary formation would be increased by

adding methane fluid in the pore space, and so could enhance the build-up of the fluid

pressure and flow within the sedimentary formation leading to pockmark formation.

However, as mentioned before, the down-dip transfer of pore fluid due to lateral gradients in

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overburden pressure would itself be sufficient to cause distal pockmark formation (Fig. 12).

The notion that paleo-pockmarks can form independently from hydrocarbon expulsion is

supported by other examples of pockmark formation in non-hydrocarbon provinces (e.g.

Pickrill, 1993; Dimitrov and Woodside, 2003; Cartwright et al., 2004; Hubscher and

Borowski, 2006).

This study provides a better understanding of the subsurface fluid plumbing systems

and likely fluid sources for pockmark formation within distal parts of the Giant Foresets

Formation. The main implication of relevance to petroleum exploration is that even clustered

pockmarks cannot be used indiscriminately as a proxy of a mature thermogenic hydrocarbon

province unless the plumbing system can be ascertained and linked with migration from

deeper levels (cf. Serié et al., 2016).

6. Conclusions

Observations based on the Parihaka 3D seismic data suggests that paleo-pockmark

formation in the Taranaki Basin (New Zealand) was related to down-dip fluid flow and

expulsion in a rapid sediment loading environment. The rapid progradation of clinoforms

above the distal fan generated a lateral gradient in pore pressure which allowed lateral

transfer in fluids basinwards due to the lateral gradient in the overburden pressure. As a result

pore fluids were driven towards the distal parts of the fan where the thinner and thus weaker

seal allowed upward expulsion and pockmark formation. Tectonic, diagenetic (polygonal)

faults and hydrothermal fluids are not considered to be important in this case because there is

no relationship between paleo-pockmark locations and underlying faults or igneous features.

The proposed model may help to explain pockmark formation in rapid sediment loading

environments and inform future studies of fluid migration and expulsion within sedimentary

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basins. As we have shown that rapid progradation can cause pockmark formation and because

porewaters are much more abundant than hydrocarbons in most basin fluid budgets, not all

pockmarks should be used as indicators of hydrocarbon seepage in frontier regions.

Acknowledgements

The authors gratefully acknowledge the financial support from Chulalongkorn University and

would like to thank the Office of Research Affairs, Chulalongkorn University for assistance

during manuscript preparation. Schlumberger and IHS generously supplied Petrel and

Kingdom licenses to the University of Manchester. The data were released by the New

Zealand government. Anonymous reviewers are thanked for their useful and constructive

comments.

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Figure 1. Location map of the Taranaki Basin and structural elements derived from King and

Thrasher (1996). Early Pliocene fans (Mangaa Formation) in the Taranaki Basin are derived

from Strogen et al. (2012). Mohakatino volcanic centre is a Middle to Late Miocene volcanic

arc aligned in a NNE–SSW trend along the axis of the Northern Graben (Hansen and Kamp,

2006). Inset map shows North Island, New Zealand. The Parihaka 3D seismic survey is

highlighted as a red polygon and 2D seismic lines are highlighted as blue lines.

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Figure 2. Synthetic stratigraphic column of the Upper Cretaceous to Quaternary fill of

Taranaki Basin, modified from King and Thrasher (1996). Study interval is shown as a red

rectangle. SR is a source rock formation of the basin.

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Figure 3. (a) Map showing the study area along the western margin of New Zealand’s North

Island. A black polygon highlights the location of the 3D seismic volume used in this study;

2D seismic lines are shown as blue lines. (b) A variance time slice at 1,460 ms through the

Parihaka 3D seismic data shows a significantly higher concentration of paleo-pockmarks

situated in the northern part of the study area (indicated by a red polygon). (c) Thickness map

of the paleo-pockmark interval based on 2D and 3D seismic interpretations. An area of

approximately 2 km2 was selected for volume loss calculations (small black rectangle). (d)

Zoom-in on the paleo-pockmark distribution area.

28

Figure 4. Well-to-seismic correlation using synthetic seismic seismogram, chronostratigraphy and some key lithostratigraphic formations are shown along with sonic, density and gamma ray logs. The paleo-pockmark interval is marked by grey area which is correlated to mudstones of the Giant Foresets Formation. Compaction-corrected sedimentation rates (metres/m.y.) for Arawa-1 well located within the Parihaka 3D seismic data derived from Arco Petroleum NZ Inc. (1992).

29

Figure 5. (a) uninterpreted and (b) interpreted vertical seismic profiles show the regional structure of the basin and the paleo-pockmark interval. The paleo-pockmark interval is highlighted as a brown color with paleo-pockmarks observed at the distal part of clinoform reflections. See seismic location in Figure 3b.

30

Figure 6. (a) TWT-structure map shows paleo-pockmarks in the examined area. (b) Smoothed

TWT-structure map or undisturbed surface in the same location. (c) Residual thickness map

after subtracting the TWT-structure map from the smoothed TWT-structure map. (d) Seismic

profile shows a paleo-pockmark. (e) Interpreted paleo-pockmark geometry from seismic

profile, where W is the width and D is the depression of the paleo-pockmark.

31Figure 7. Seismic profile shows the main features of the interpreted horizons (H0-3). The location of the seismic profile is shown in Figure 3c. The paleo-pockmark interval is characterised by wavy and truncated reflections at the paleo-pockmark walls. The red arrows represent paleo-pockmark locations.

32

Figure 8. (Top and middle) The dip and variance seismic attributes show the morphology and

size variation of the paleo-pockmarks. The paleo-pockmarks were clearly visible on these

maps as round to sub-round shapes. (Bottom) Isopach maps for the key horizons used to

study the paleo-pockmarks. The maps reveal a number of paleo-pockmarks located in the thin

distal part of the fan in the study area. The contours represent TWT depth for the horizons.

For horizon location see Figure 7.

33

Figure 9. Seismic sections across the examined area display the geometry and depth of the

paleo-pockmarks. Key horizons are highlighted in red. Seismic profiles are selected to show

the variation in the paleo-pockmarks. For area location see Figure 3c.

34

Figure 10. A series of TWT isopach maps across the paleo-pockmarks in the examined area

showing the increasing thicknesses in the vicinity of the paleo-pockmarks. H1 shows

relatively low thickness values over the paleo-pockmark craters. In contrast, H2 and H3 show

relatively high thickness values over the paleo-pockmark craters. The residual topography

patterns of H2 and H3 coincide with the paleo-pockmark geometry. For area location see

Figure 3c.

35

Figure 11. Examples of co-located paleo-pockmarks on the H2 and H3 horizons.

36

37

Figure 12. Schematic evolutionary cartoon of rapid sediment loading environment for

pockmark formation. Sediments are rapidly deposited on a saturated formation.

Sedimentation rate decreased from the proximal to the distal part, resulting in a wedge-

shaped geometry. Fluid is driven laterally (left to right) along the permeable layer and is

expelled at the toe of the slope. (a) Initial deposition of formation at a slope fan. (b) Rapid

sedimentation on top of the initial deposition. (c) Sediments continue to deposit providing a

thick seal and preventing fluid escape from the seabed. (d) Zoom-in schematic model

showing the detailed stratigraphy at the pockmark field. (e) Rapid sedimentation generates

overpressure, which is greatest on the proximal part due to the increasing overburden

pressure. The subsurface overpressure for the study area is based on Dugan and Flemings

(2002). This schematic model of rapid sediment loading environment leads to a (1)

substantial increase in overpressure and (2) culminates in the formation of pockmark craters

at the seabed.