Pockmarks on the Scotian Shelf

King, Lewis H.; MacLean, B C

doi:10.1130/0016-7606(1970)81[3141:potss]2.0.co;2

Cited by 387 publications

(223 citation statements)

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“…In either case the upward motion of the gas or water is thought to enhance erosion around the seep and cause the formation of a conical depression. Studies of pockmarks on the continental shelf have supported the idea that they are primarily produced by the seepage of gas from the sea floor (King and MacLean, 1970); however, the role of water seepage cannot be ruled out.…”

Section: Pockmarksmentioning

confidence: 94%

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Studies of deep-sea sedimentary microtopography in the North Atlantic Ocean

Flood¹

1978

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partial fulfillment of the requirements for the degree of Doctor of Philosophy.. ABSTRACTMany of the small-scale topographic features (dimensions of centimeters to kilometers) found on the Blake-Bahama Outer Ridge (western North Atlantic, water depth greater than 4000 m) and in the Rockall Trough (northeastern North Atlantic, water depth greater than 2000 m) have been formed as bed forms of deep currents. These bed forms, all developed in cohesive sediments, include current ripples (spacings of tens of centimeters, formed transverse to the flow), longitudinal triangular ripples (spacings of meters, formed in sandy muds and parallel to the flow), furrows (spacings of tens to 100's of meters, formed parallel to the flow and presently either erosional or depositional), and regular sediment waves (spacings of a few kilometers, now found oblique to the flow and migrating either upstream or downstream).The local distribution of any given bed form is influenced by the presence of larger features. Bed forms are often found in zones which strike parallel to the regional contours.Debris flows, affecting areas of 1000's to 10,000's of square kilometers, are also present in these areas. A debris flow studied in the Rockall Trough is erosional at its shallowest depths and depositional at greater depths. Gravitational flows strike perpendicular to the contours. Pockmarks (tens of meters in diameter, marking fluid seeps) are also found on the Blake-Bahama Outer Ridge.The larger topographic features (greater than several meters) with steep slopes (greater than about 200) can be observed on surface echo-sounding profiles either as fields of regular hyperbolic echoes (e.g., echoes from regularly spaced furrows), fields of irregularly spaced, dissimilar hyperbolae (e.g., echoes from blocks, ridges, and folds in debris flows), or as regular features whose structfre is often obscured by side echoes (e.g., echoes fro .sediment waves).Although nearbottom investigations are required to describe the features, the nature of the sea floor can often be inferred from the character of the echo-sounding profile. Similar echo-sounding records in different areas of the ocean indicate the presence of similar sea-floor features.The morphology of the bed forms studied and the current and temperature structure of the overlying water column lead to conclusions about bed form origin and present-day interactions with deep currents.Furrows form as erosional bed forms during high-velocity (>20? cm/sec) current events by large, helical secondary circulations in the bottom boundary layer. Once formed, furrows may develop into depositional features, or they may continue as erosional ones, depending on the local currents and the sediment supply.Large, regular sediment waves may be formed at current speeds of 5 to 10 cm/sec by lee waves generated by topographic irregularities on the sea floor, such as submarine canyons, or by instabilities in the flow of deep, contour-following currents. Sediment waves develop where there is an abundant supply of sediment an...

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Section: Pockmarksmentioning

confidence: 94%

“…Two hypotheses have been put forward to explain the origin of these features where they are seen on the continental shelves (King and MacLean, 1970). One is that the pockmarks mark the site of gas seeps from underlying gas-rich sediments into the water column.…”

Section: Pockmarksmentioning

confidence: 99%

Studies of deep-sea sedimentary microtopography in the North Atlantic Ocean

Flood¹

1978

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“…Pockmarks no leito do mar foram primeiramente relatados por King & McLean (1970) na plataforma continental ao largo da Nova Escócia, Canadá. Relatos posteriores de outrasáreas ao redor do mundo demonstraram que pockmarks são mais que curiosidades ecológicas com apenas interesse acadêmico.…”

Section: Introductionunclassified

Estruturas rasas de gás em sedimentos no estuário Potengi (nordeste do Brasil)

Frazão¹,

Vital²

2007

Rev. Bras. Geof.

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ABSTRACT. Areas with gas accumulations and gas seeps, where gas escapes from the river-bottom to the water column, have been mapped in the fill of a fluvial valley, the Potengi estuary. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, two types of gas escapes features have been distinguished: (1) acoustic plumes and (2) pockmarks. The neotectonic activity in the semigraben of the Potengi river may be one of the responsible factors for the migration of gas coming from the seismofacies located below the present river level where it is possible to recognize the connection with the level of the gas source through the acoustic column type seismic signature.It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location.Keywords: acoustic records, shallow gas accumulations, gas seeps, Potengi estuary. RESUMO.Áreas com acumulações e escape de gás foram identificadas no estuário Potengi. As várias ocorrências de gás foram classificadas em quatro tipos, deacordo com suas assinaturas sísmicas específicas: (1) blankets ou coberturas acústicas, (2) cortinas acústicas, (3) colunas acústicas e (4) turbidez acústica. Ao mesmo tempo, dois tipos diferentes de escape de gás foram identificados: (1) plumas acústicas e (2) pockmarks . A atividade neotectônica na porção fluvial do sistema Potengi pode ser um dos responsáveis pela migração de gases provenientes das sismofácies localizadas abaixo do atual nível do rio, ondeé possível reconhecer a conexão com o nível da fonte do gás, através da assinatura sísmica do tipo coluna acústica. Conclui-se que a fácies sedimentaré provavelmente o fator determinante da acumulação e escape de gás, como também do tipo específico de acumulação ou escape em cada local.Palavras-chave: registros acústicos, acumulações de gás raso, escape de gás, estuário Potengi.Universidade Federal do Rio Grande do Norte -UFRN, Cx.

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“…Since King and MacLean (1970) first described pockmarks from the Scotian Shelf, they have been reported from many environments (Uchupi et al, 1996 and references therein). It is this diverse distribution that led Paull et al (2002) to infer that pockmarks are probably not due solely to the expulsion of gas and fluids, but are formed by a variety of mechanisms.…”

Section: Fluid Migrationmentioning

confidence: 99%