Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine

Kelley, Joseph T.; Dickson, Stephen M; Belknap, Daniel F.; Barnhardt, Walter A.; Henderson, M.

doi:10.1130/0091-7613(1994)022<0059:gsbpef>2.3.co;2

Cited by 119 publications

(73 citation statements)

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“…Inconsistent evidence for venting (Kelley et al, 1994;Ussler et al, 2003), a dearth of methanotropes (Wildish et al, 2008), and nominal seafloor change (Brothers et al, 2011a) suggests these pockmark fields currently experience minimal fluid-escape activity. Geochemical sampling (Ussler et al, 2003) suggest that the region's methane is microbial in origin, although the stratigraphic source of the gas remains undetermined.…”

Section: Regional Settingmentioning

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%

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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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Section: Regional Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

et al. 2012

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“…In addition, pockmarks identified in the concurrent bathymetric trace had slopes of 30°. Such steep slopes in a muddy estuarine seafloor would presumably slump without reinforcement by venting fluids, and Kelley et al (1994) interpreted the morphology as an indication of relatively frequent venting activity. Indeed, Andrews et al (2010) reported pockmark walls commonly exceeding even 40°in the bay.…”

Section: Physical Settingmentioning

confidence: 99%

“…Because of the ubiquity of shallow natural gas, methane escape has often been hypothesized as a key mechanism for pockmark formation in the area (e.g., Scanlon and Knebel 1989;Kelley et al 1994). Rogers et al (2006) proposed that lakes, marshes, and bogs that existed offshore when sea level was lower than at present provided the organic Fig.…”

Section: Physical Settingmentioning

confidence: 99%

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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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Paleodeltas and Preservation Potential on a Paraglacial Coast — Evolution of Eastern Penobscot Bay, Maine

Belknap

Gontz

Kelley

Coastal Systems and Continental Margins

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Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine

Cited by 119 publications

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Shallow stratigraphic control on pockmark distribution in north temperate estuaries

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

Paleodeltas and Preservation Potential on a Paraglacial Coast — Evolution of Eastern Penobscot Bay, Maine

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