Elsevier

Marine Geology

Volume 218, Issues 1–4, 30 June 2005, Pages 191-206
Marine Geology

Complex pockmarks with carbonate-ridges off mid-Norway: Products of sediment degassing

https://doi.org/10.1016/j.margeo.2005.04.005Get rights and content

Abstract

A new type of pockmark has been discovered off mid-Norway. They contain up to 10 m high ridges of methane-derived authigenic carbonate (MDAC) rock. The pockmarks were investigated using ROVs (remotely operated vehicles) in 2003 and 2004. The complex pockmarks are located well within the gas hydrate pressure/temperature stability field, and are associated with a regional BSR (bottom simulating reflector). Our high-resolution seismic data demonstrate that the near-surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks, and that the pipes are deeply rooted. The pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles. Other key features of the complex pockmarks include: 1) a distinct fauna with local bacterial mats, small tubeworms, stalked crinoids, and pycnogonids (sea spiders); and 2) the presence of light hydrocarbon gases (C1–C5) in clay-rich sediments. The methane δ13C values range from − 54‰ to − 69‰ PDB, suggesting the presence of both bacterial and thermogenic gases. During our two-day observation periods only micro-seepage occurred, without ebullition or other visual fluid flow. Petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane-derived aragonite within the sediments (δ13C =  52‰ to − 58‰ PDB). We suggest that the complex pockmarks formed by sudden (‘catastrophic’) fluid flow and that the large carbonate ridges represent dislodged MDACs originally formed subsurface, prior to the event. Subsequent micro-seepage is still active, and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column. Our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks. Thus our data rather suggest that the ‘catastrophic’ formation of the complex pockmarks may be a one-time-event.

Introduction

Over the last decades pockmarks have proven to be important seabed features that provide information about fluid flow on continental margins, even if their formation and dynamics are still poorly constrained. Pockmarks were first discovered off Nova Scotia, Canada, and classified as seabed gas and porewater escape features by King and MacLean (1970). Further documentation of a fluid flow origin of pockmarks came with the discovery of methane-related carbonate crusts and slabs inside North Sea pockmarks (Hovland et al., 1985). This proved that at least some of the depressions form by local mass wasting by escaping hydrocarbon gases and porewaters (Hovland et al., 1985, Hovland and Judd, 1988, Judd and Sim, 1998). The source of the gases and waters are in many cases poorly constrained, although some studies during recent years have demonstrated a genetic relationship with destabilization of gas hydrates (Vogt et al., 1999a, Vogt et al., 1999b, Paull et al., 2000, Wood et al., 2002, Bünz et al., 2003). It is suspected that pockmarks form by sudden local gas/porewater/sediment eruption, and that they periodically have short outbursts followed by long periods of quiescence or microseepage. Their detailed formation mechanism and dynamics is still largely unknown because of deficiency in long-term monitoring (c.f., Hovland and Judd, 1988).

In this paper, we present new data from some complex pockmarks, partly associated with gas hydrates in deep water off mid-Norway. Our findings may have implications for pockmark dynamics in general.

Section snippets

Methods

During spring, 2003, and summer, 2004, detailed surveys were conducted along the northeastern flank of the Storegga Slide, in the so-called Nyegga area (approx. 64° 40′ 2″ N, 05° 17′ 32″ E) (Fig. 1). They were undertaken in order to investigate suspected localized fluid flow through the seabed. Geophysical mapping, visual inspection, and seabed sampling was performed by use of the DeepOcean ROV ‘HiRov’-systems, operated from the survey/ROV-support vessels ‘Normand Tonjer’ in 2003 and ‘Edda

Geological setting

The seabed of our study area at the Nyegga region, has a general slope angle of 1° towards the west and represents the ‘shoulder’ of the continental slope leading down to abyssal depths of about 3000 m in the Norwegian Sea Basin. The region we studied (Fig. 2) lies at the border between two large sedimentary basins: the Møre Basin to the south, and the Vøring Basin to the north (Bünz et al., 2003). The basins probably developed as a result of several rifting episodes leading to Late

Oceanographical setting

The oceanographic conditions at Nyegga are rather special due to very cold Arctic bottom water in the Norwegian–Greenland deep sea basin, to the west of Nyegga. A distinct mid-water temperature gradient with a temperature decline of about 6 °C over a depth interval of only 200 m exists in the water column above the Nyegga survey area. This thermocline separates the warm, upper Atlantic water mass (7–9 °C), from the cold (− 0.5 to − 1.5 °C) Norwegian Sea Basin water mass (Fig. 4). Furthermore, the

Morphology of pockmarks

The studied pockmarks are morphologically more complex than ‘normal’ seabed pockmarks (Hovland and Judd, 1988), and occur as near-circular, up to 15 m deep and 320 m wide depressions. Their most distinctive feature is the occurrence of chaotic heaps of large carbonate rocks and slabs, which protrude from the central part of the depressions up to the mean surrounding seafloor level or, even slightly higher.

A total of four large pockmarks, named: A, C, G8, and G11 were investigated.

Seabed observations

Our survey documents the existence of an apparently pockmark-specific micro- and macrofauna (Fig. 6, Fig. 7), which includes bacterial mats (probably Beggiatoa sp), fields of small tube-worms (probably polychaetes and pogonophorans) and large (15 cm) pycnogonids (sea-spiders, suspected to be of the species Colossendeis proboscidea (T. Brattegard, personal communication, 2004). The bacterial mats were located in the deepest, soft sediment-covered portions of the G11-pockmark, and were also

Sub-bottom geology and active seeps

High-resolution SBP records were acquired across three of the pockmarks: A, C, and G8. These demonstrate that the fine acoustic upper layering gives way to a zone of unlayered (disturbed) acoustically transparent sediments inside the pockmarks. Furthermore, the finely horizontally layered sediments clearly dip down around the perimeter of the structures, indicating some sort of central subsidence or collapse. From the SBP-records it is clear that ‘younger’ material has been deposited on top of

Carbonate structure and composition

Two carbonate rocks were sampled, both of them from pockmark G11. These rocks represent two different types of carbonate lithology. However, XRD results show that both samples are dominated by aragonite. Sample A is one of at least three about 1 cm thick by 30 cm by 20 cm wide ‘lithified bedding’ wafer-shaped rocks found protruding out of the seabed inside G11 (Fig. 6d). Sample B was acquired from one of the large heaps of rough carbonate blocks and is thought to represent a typical sample from

Fauna

The relatively high density of macro-benthos and characteristic composition of the observed fauna are surprising when considering the sub-zero (− 0.7 °C) ambient seawater temperatures of the area. This fauna was only found inside the pockmarks, despite extensive visual surveys on the general seafloor of the region. However, a similar faunal composition has previously been reported from the Håkon Mosby Mud Volcano (HMMV), in the Barents Sea (Vogt et al., 1999a, Gebruk et al., 2003). We infer that

Suggested complex pockmark formation mode

Similar fluid escape features interpreted as large deep-water pockmarks associated with BSRs, have been described and discussed from offshore western Canada (Wood et al., 2002) and fossilized features have been described from the Meiklejohn Peak, Nevada (Krause, 2001). These latter ones consist of ‘piled authigenic limestones’ suspected to be of cryogenic (freeze/thaw) origin.

Those from offshore Canada occur in the Cascadia Accretionary prism. The explanation provided by Riedel et al. (2001)

Conclusions

The complex pockmarks at Nyegga contain up to 190 m long irregular ridges consisting of piled-up carbonate blocks. They are associated with micro-seepage of light hydrocarbon gases (C1–C5) and a distinct fauna, including bacterial mats, tubeworms, stalked crinoids, and pycnogonids (sea spiders). Petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane-derived aragonite within the sediments (δ13C =  52‰ to − 58‰ PDB). The pockmarks are located well within

Acknowledgements

We would like to thank Ian Grieves (DeepOcean) in help to assemble the data, and to all others onboard the ‘Normand Tonjer’ and ‘Edda Fonn’ who have helped. We acknowledge the release of seismic data from the licencees of the Ellida prospect, and also all help from our seniors in Statoil, especially Tor Inge Tjelta, who have provided the opportunity to develop this model and who have given permission to publish the results. Finally, we thank Maarten Vanneste and one anonymous reviewer for their

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