Fluid flow structures and processes ; indications from the North Norwegian continental margin

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1 NORWEGIAN JOURNAL OF GEOLOGY Fluid flow structures and processes ; indications from the North Norwegian continental margin 57 Fluid flow structures and processes; indications from the North Norwegian continental margin Liv Plassen & Jochen Knies Plassen, L. & Knies, J. 2009: Fluid flow structures and processes; indications from the North Norwegian continental margin. Norsk Geologisk Tidsskrift, Vol. 89, pp ISSN X. On the basis of multibeam echo-sounder bathymetric data and high-resolution seismic records about 285 pockmarks and 25 dome structures are mapped in the outer parts of Andfjorden, northern Norway. These features indicate that near-vertical fluid migration takes place (or has taken place) in the shallow subsurface. Enhanced density of pockmarks occurs along the margins of the trough, where glacial diamicton (till) generally lacking organic material - constitutes the seafloor. Many of the pockmarks are related to zones of normal faults/structural lineaments in the sedimentary bedrock, suggesting that liquids and/or gases migrate from these to higher levels. A petrogenic origin of the fluids is therefore considered to be more than likely. Liv Plassen, Jochen Knies, Geological Survey of Norway, N-7491 Trondheim, Norway (Liv.Plassen@NGU.NO). Introduction Gas seepages and fluid escape structures have been reported from continental shelf areas worldwide (e.g. King & MacLean 1970; Solheim & Elverhøi 1985; Field & Jennings 1987; Hovland & Judd 1988; Fader 1991; Kelley et al. 1994; Rise et al. 1999; Judd & Hovland 2007; Chand et al. 2008). This type of fluid flow may be characterised as cold seeps, which are slow emissions of a mixture of fluids often containing hydrocarbons through the seafloor, and commonly observed on continental margins (e.g. Judd 2003). Fluid escape structures can occur on land as well, often as mud volcanoes (e.g. Kopf 2003). Focused fluid flow expresses itself through a wide range of geological phenomena, such as mud volcanoes, pockmarks, pipe structures, diapirs and gas hydrates (e.g. Henriet & Mienert 1998; Judd & Hovland 2007). However, descriptions of such fluid flow structures in former glaciated margins and shelves characterized by glacigenic sediments are scarce. Fig. 1: Location maps. Box in B refers to the study area in Fig. 2.

2 58 L. Plassen & J. Knies NORWEGIAN JOURNAL OF GEOLOGY Fig. 2: Shaded relief map of the bathymetry showing distribution of ca. 285 pockmarks (red circles) and 25 dome structures (yellow circles) on the seafloor of the outer parts of Andfjorden. Note highest occurrence along the margins of the trough. Contour interval is 100 m. Boxes refer to the following figures. Indications of shallow gas, gas hydrates and related fluid flow features are reported from the Barents Sea (Solheim & Elverhøi 1985; Solheim & Elverhøi 1993; Lammers et al. 1995; Laberg & Andreassen 1996; Laberg et al. 1998; Long et al. 1998; Vogt et al. 1999; Chand et al. 2008). Fluid flow features had not yet been described from Andfjorden, northern Norway (Fig. 1), despite an extensive marine geological program performed by the University of Tromsø (e.g. Vorren et al. 1983; Hald & Vorren 1984; Vorren et al. 1988; Plassen & Vorren 2002; Vorren & Plassen 2002; Ebbesen & Hald 2004). The advent, however, of swath bathymetry imaging combined with high-resolution seismic data is as shown in this study, the key to identifying and discussing fluid flow structures in the Andfjorden area (Fig. 1). The study was mainly based on multibeam echo-sounder bathymetric data provided by the Norwegian Hydrographic Service (SKSK) (Fig. 2), and high-resolution seismic data (Topas) provided by the Norwegian Defence Research Establishment (FFI). The bathymetric data were gridded with a cell size of 50 m and visualised using the software program ER Mapper 7.0. In this paper we present ca km2 bathymetric data and high-resolution seismic records that show numerous fluid flow features in the outer parts of Andfjorden, indicating the presence of natural gas seepages (Fig. 2). Our aim is to provide information about the morphology, distribution and origin of these fluid flow related features.

3 NORWEGIAN JOURNAL OF GEOLOGY Fluid flow structures and processes ; indications from the North Norwegian continental margin 59 Fig. 3A. Map showing distribution of pockmarks (red dots) and dome structures (blue dots) related to bedrock type and structural elements in the Andfjorden area. The pockmarks and dome structures occur in areas above subcropping Cretaceous and Tertiary sedimentary rocks, and many occur close to, or along normal faults/structural lineaments. Bedrock map from Henningsen & Tveten (1998) and Zwaan et al. (1998). Normal faults/structural lineaments (grey bold lines) after Bergh et al. (2007). Fig. 3B. Cross-section showing the lithostratigraphic succession in the northeastern part of the study area (from Zwaan et al. 1998).

4 60 L. Plassen & J. Knies NORWEGIAN JOURNAL OF GEOLOGY Fig. 4A. Map showing occurrence of pockmarks (red dots) and dome structures (yellow dots) related to sediment distribution and morphological elements of the outer parts of Andfjorden. Most of the pockmarks and dome structures are found in areas where glacial diamicton (till) constitute the seafloor. Seafloor map modified from Plassen & Vorren (2002) and Vorren & Plassen (2002). Fig. 4B. Seismic (Sparker) profile across the shelf edge showing Quaternary glacigenic sediments that superimpose the sedimentary bedrock (from Vorren & Plassen 2002).

5 NORWEGIAN JOURNAL OF GEOLOGY Fluid flow structures and processes ; indications from the North Norwegian continental margin 61 Fig. 5A-D. A) Shaded relief image and bathymetric cross-section of a distinct V-shaped pockmark at the shelf edge. B) Shaded relief image and seismic cross-profile (Topas) of a distinct V-shaped pockmark at the northern margin of the trough. The pockmark cuts through the youngest unit of glacial diamicton that superimposes on an older unit of glacial diamicton (described by Plassen & Vorren 2002). C) Shaded relief image and bathymetric cross-section showing a high-density area of pockmarks at the eastern margin of the trough. D) Shaded relief image of dome structures and pockmarks at the southwestern part of the outer shelf. The bathymetric cross-section shows a distinct dome structure.

6 62 L. Plassen & J. Knies NORWEGIAN JOURNAL OF GEOLOGY Physiographic setting Andfjorden is a north-south trending cross-shelf trough located on the North Norwegian continental shelf (Fig. 1). The length is about 80 km and the maximum water depth exceeds 500 m in the central and inner parts, whilst the shelf edge has a depth of ca. 240 m. The bedrock of the shelf comprises Jurassic, Cretaceous and Tertiary sedimentary rocks, whereas older crystalline rocks occur inshore (Henningsen & Tveten 1998; Zwaan et al. 1998) (Fig. 3 A & B). The main part of Andfjorden is a sedimentary basin of Mesozoic and Cenozoic rocks downfaulted within crystalline rocks along normal faults/structural lineaments characterised by SW-NE and N-S trends (Bergh et al. 2007). In the outer parts of Andfjorden, Quaternary sediments drape the bedrock (Fig. 4 A & B). A ca. 200 m thick depocentre of glacigenic sediments occurs near the shelf edge (Rokoengen et al. 1979; Vorren et al. 1984; Vorren & Plassen 2002). In the inner, deeper parts of the Andfjorden trough, up to ca. 50 m of Late Weichselian glacimarine and Holocene marine sediments exist (Plassen & Vorren 2002). During the Last Glacial Maximum, the Fennoscandian ice sheet reached the shelf edge of Andfjorden. Deglaciation took place between ca and C kyr BP (Vorren & Plassen 2002). Andfjorden is presently influenced by temperate, saline Atlantic Water transported by the Norwegian Current and by less saline coastal water in the Norwegian Coastal Current (Sundby 1984; Hopkins 1991). A strong bottom-current system, established at the Late Weichselian-Holocene transition, partly eroded Late Weichselian sediments and resulted in erosion/nondeposition of Holocene sediments in the outer parts of Andfjorden (Plassen & Vorren 2002). Results Shaded relief images of the bathymetric data reveal numerous circular features on the seafloor of the outer parts of Andfjorden (Figs. 2 & 5). Bathymetric profiles across the features show that the majority represent depressions on the seafloor, and these structures are interpreted as pockmarks. A few of these features have a positive, convex shape, and are termed dome structures. Pockmarks About 285 pockmarks are identified in the study area (Fig. 2). The size of these vary from depths of ca. 0.5 m and ca. 50 m across, to depths of >4 m and ca. 150 m across (Figs. 5A, 5B & 5C). Bathymetric sections and transverse seismic lines across these pockmarks show them to be v-shaped with almost symmetrical geometry. Based on our data, there is no indication of any infilling in the pockmarks. Most pockmarks are found along the lateral trough margins. The bathymetric ridge at the eastern margin can be characterized as a high-density area (Fig. 2). There, about 70 pockmarks are counted within 25 km 2 (Fig. 5C). Most of the pockmarks occur at water depths of m. All pockmarks are located in areas above hidden outcrops of Cretaceous and Tertiary sedimentary rocks, and many occur close to, or along normal faults/structural lineaments (Fig. 3). Glacial diamicton (till) typically constitutes the seafloor in areas where pockmarks are observed (Fig. 4). Dome structures Dome structures on the seafloor occur only at the southwestern part of the outer shelf (Fig. 2). Here, approximately 25 dome structures were mapped in an area of ca. 60 km 2. The domes are ca. 1 m high and 100 m across (Fig. 5D). Nearly all dome structures lie in water depths of m, and in an area above hidden outcrops of Cretaceous sedimentary rocks (Fig. 3). Many of them are related to a zone of normal faulting. Glacial diamicton (till) constitutes the seafloor where the dome structures are located (Fig. 4). Discussion The presence of numerous pockmarks and some dome structures on the seafloor of the outer Andfjorden area indicates near-vertical sub-surface fluid migration and escape. Pockmarks and dome structures are mainly found along the margins of the Andfjorden trough, in water depths between m. Lack of any infilling of the pockmarks indicates that the fluid escape features are active or have been active until recently. Most of the pockmarks are found on the outer shelf where strong bottom-current systems established at the Late Weichselian-Holocene transition caused sediment erosion and non-deposition of Holocene sediments (Plassen & Vorren 2002). Hence, the pockmarks may have formed, and been active, during the Late Weichselian, after the ice had left the outer parts of Andfjorden. During the Holocene, they may have been starved for sedimentation due to bottom current induced erosional processes. Most of the pockmarks in Andfjorden are found in areas where glacial diamicton constitutes the seafloor. General lack of organic matter in glacigenic sediments suggests that gas does not originate in situ (e.g. Plassen & Vorren 2003). Instead, many of the pockmarks appear related to fault zones/structural lineaments in the sedimen-

7 NORWEGIAN JOURNAL OF GEOLOGY Fluid flow structures and processes ; indications from the North Norwegian continental margin 63 tary bedrock, indicating that fluids may have migrated upwards from deeper sources. A petrogenic origin of the fluids is therefore more than likely; however, groundtruthing by geochemical sampling is necessary to confirm this assumption. Pockmarks are commonly associated with gas venting but few attempts have been undertaken to verify this assumption. Nevertheless, acoustic signatures attributed to gas in the subsurface are well described (e.g. Hovland & Judd 1988; Plassen & Vorren 2003). A comprehensive mapping program including systematic geochemical investigations conducted in the Big Sur pockmark field, off California, gave no evidence for active gas or other fluid venting (in Quaternary sediments) (Paull et al. 2002). These authors then raise the question as to how valid the assumption is that the existence of well-defined fields of pockmarks on the seafloor implies active fluid or gas venting, and if perhaps other and perhaps purely sedimentological mechanisms should be considered. In Andfjorden, the presence of fluid flow structures on the seafloor indicates that fluids originate from deeper sources. Hence, it seems reasonable that the distribution of pockmarks and dome structures may reflect gas migrating to the surface along normal faults/structural lineaments in the sedimentary bedrock. This seems to be the most likely mode of formation of the observed seafloor features. Conclusions Numerous pockmarks and some dome structures in the outer parts of Andfjorden indicate that near-vertical fluid migration took place (or has taken place) in the shallow sub-surface. Enhanced density of pockmarks occurs along the margins of the trough, where glacial diamicton (till) generally lacking organic material constitutes the seafloor. Many of the pockmarks are related to zones of normal faults/structural lineaments in the sedimentary bedrock, indicating that liquids and/or gases migrate from deeper sources. A petrogenic origin of the fluids is therefore considered to be more than likely. Acknowledgements We are grateful to the Norwegian Hydrographic Service for access to their multibeam echo-sounder data, and to the Norwegian Defence Research Establishment for the right to use their high-resolution seismic data. The paper benefited from reviews by Martin Hovland and Maarten Vanneste. We extend our most sincere thanks to these persons. 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