Pockmarks in the inner Oslofjord, Norway

Webb, Karen E.; Hammer, Øyvind; Lepland, Aivo; Gray, John S.

doi:10.1007/s00367-008-0127-1

Cited by 40 publications

(38 citation statements)

References 71 publications

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“…Pockmark forms observed in the Belfast Bay pockmark field and spatial clustering are common to other fields (Hovland and Judd, 1988;Pilcher and Argent, 2007;Webb et al, 2009). Extracted pockmarks in Belfast Bay have a consistent depth/diameter ratio across the field (Fig.…”

Section: Pockmark Morphologymentioning

confidence: 95%

“…Despite several studies of the relation between pockmarks and host sediment thickness (Josenhans et al, 1978;Fader, 1991;Rogers et al, 2006;Webb et al, 2009), pockmark morphology does not necessarily constrain a correlation between pockmark size and host sediment thickness. Pockmarks in any one field may span a range of morphologies (Judd and Hovland, 2007).…”

Section: Pockmark Morphologymentioning

confidence: 99%

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Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

2010

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Seafloor pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion, but few numerical analyses of their morphology and spatial distribution of pockmarks exist. We introduce a quantitative definition of pockmark morphology and, based on this definition, propose a three-step geomorphometric method to identify and extract pockmarks from high-resolution swath bathymetry. We apply this GIS-implemented approach to 25 km 2 of bathymetry collected in the Belfast Bay, Maine USA pockmark field. Our model extracted 1767 pockmarks and found a linear pockmark depth-to-diameter ratio for pockmarks field-wide. Mean pockmark depth is 7.6 m and mean diameter is 84.8 m. Pockmark distribution is non-random, and nearly half of the field's pockmarks occur in chains. The most prominent chains are oriented semi-normal to the steepest gradient in Holocene sediment thickness. A descriptive model yields field-wide spatial statistics indicating that pockmarks are distributed in non-random clusters.Results enable quantitative comparison of pockmarks in fields worldwide as well as similar concave features, such as impact craters, dolines, or salt pools. Published by Elsevier B.V. IntroductionFirst identified in muddy sediments of the Scotian Shelf (King and MacLean, 1970), pockmarks are seafloor depressions that are found worldwide in a variety of geologic settings (Hovland and Judd, 1988;Judd and Hovland, 2007). These craters can measure hundreds of meters in diameter, may occur in chains kilometers long and, where present in extensive fields, may dominate the seafloor surface (Fader, 1991;Rogers et al., 2006;Pilcher and Argent, 2007). Despite global distribution and general association with seafloor fluid escape, the mechanisms for pockmark formation and evolution remain uncertain (Ussler et al., 2003).Analysis of pockmark morphology and spatial distribution relative to antecedent geology and subsurface fluids (e.g., methane) can provide insight into fluid-migration pathways, pockmark field evolution, and possible mechanisms for pockmark generation and maintenance. In the absence of high-resolution seafloor bathymetry data, previous characterizations of entire pockmark fields relied upon visual interpretation of acoustic backscatter data for pockmark delineation, size statistics and spatial distribution (Fader, 1991;Kelley et al., 1994;Gontz et al., 2002;Rogers et al., 2006). Although acoustic backscatter data were the best available in the cited studies, interpreting size dimensions of concave features, such as pockmarks from these data is often ambiguous (Song, 2007). High-resolution bathymetry data collected by multibeam echosounder and swath sonar technologies enable the study of seafloor morphology to reach scales and resolutions similar to studies in subaerial geomorphology based on digital elevation models (DEMs) (Hughes Clarke et al., 1996). With one exception ) these new technologies have not been applied to an entire pockmark field. Instead, whole-field spatial and morphologic analysis has ebbed, r...

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Section: Pockmark Morphologymentioning

confidence: 95%

Section: Pockmark Morphologymentioning

confidence: 99%

Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

2010

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“…In addition, the pockmarks may not continue offshore due to the diminution in Holocene sediment thickness, but more extensive seismic coverage is necessary to confirm this. The factors that appear to control pockmark distribution in the examined temperate estuaries may also explain pockmark distribution in other structurally-controlled coastal settings such as fjords, or Rias (Iglesias and Garcia-Gil, 2007;Webb et al, 2009). …”

Section: Pockmarks' Relation To Stratigraphymentioning

confidence: 99%

“…However, large pockmarks also persist in environments lacking those characteristics (e.g., Kelley et al, 1994;Webb et al, 2009), and that presently exhibit minimal fluid venting (Ussler et al, 2003). Constrained to fine-grained substrates and typically associated with subsurface gas (Judd and Hovland, 2007), the factors that determine why pockmarks scar some gassy seafloors while other muddy embayments remain relatively flat are unknown.…”

Section: Introductionmentioning

confidence: 99%

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

et al. 2012

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Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for pockmark formation are not obvious. Nor is it known why pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for pockmark formation; and 2) the region's physiography controls pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5-10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r 2 =0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for pockmark formation in nearshore coastal environments.Published by Elsevier B.V.

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“…Today, pockmark fields are known to occur in a variety of settings worldwide (e.g., Hovland and Judd 1988;Fleischer et al 2001;Mazurenko and Soloviev 2003), including deltas (Nelson et al 1979), fjords (e.g., Webb et al 2009), rias (e.g., Iglesias and García-Gil 2007), and previously glaciated estuaries (e.g., Fader 1991), as well as in areas of petroleum production (e.g., Tjelta et al 2007) and gas hydrate occurrence (e.g., Vaular et al 2010). Fluid escape is commonly invoked for pockmark formation and maintenance (cf.…”

Section: Introductionmentioning

confidence: 99%

More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity

et al. 2011

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Mechanisms and timescales responsible for pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in pockmark frequency or distribution over a 9-year period (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). In that time pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore 137 Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to pockmark formation and failure do not occur frequently within this field, and (2) pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within pockmarks induced by near-bed currents.

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Pockmarks in the inner Oslofjord, Norway

Cited by 40 publications

References 71 publications

Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity

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