Mega-pockmarks and linear pockmark trains on the west african continental margin

Abstract Seabed pockmarks, the manifestation of the natural process of fluid escape at the seabed, are a widespread feature of the equatorial West African continental margin. Pockmarks occur singly, in small groups, in large random fields and in organized arrays or ‘pockmark trains’. Pockmark trains are associated with areas of steeper seabed gradient and evolve though time to form deep gullies. Pockmark gullies may exceed 1 km in width and extend for 10–20 km down slope, and form through the interaction of slope failure and fluid escape processes. Gullies maintain a rugose internal geometry throughout their development and do not represent sediment transport pathways to the deep basin. The geological processes that form seabed pockmarks and pockmark gullies are active today and these features may represent a hazard to subsea infrastructure.

[1]  J. Charlou,et al.  Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin , 2006 .

[2]  M. Hovland,et al.  Methane-related carbonate cements in pockmarks of the North Sea , 1987 .

[3]  M. Hovland Geomorphological, geophysical, and geochemical evidence of fluid flow through the seabed , 2003 .

[4]  M. Hovland Hydrocarbon seeps in northern marine waters; their occurrence and effects , 1992 .

[5]  G. Fader Gas-related sedimentary features from the eastern Canadian continental shelf , 1991 .

[6]  A. Maestro,et al.  Geometry and structure associated to gas-charged sediments and recent growth faults in the Ebro Delta (Spain) , 2002 .

[7]  A. Elverhøi,et al.  Gas-related sea floor craters in the Barents Sea , 1993 .

[8]  P. Cochonat,et al.  Evidences of early to late fluid migration from an upper Miocene turbiditic channel revealed by 3D seismic coupled to geochemical sampling within seafloor pockmarks, Lower Congo Basin , 2006 .

[9]  T. Flodén,et al.  Gas seepages, gas eruptions and degassing structures in the seafloor along the Strömma tectonic lineament in the crystalline Stockholm Archipelago, east Sweden , 1992 .

[10]  Alan Judd,et al.  Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment , 2007 .

[11]  B. Schroot,et al.  Expressions of shallow gas in the Netherlands North Sea , 2003, Netherlands Journal of Geosciences - Geologie en Mijnbouw.

[12]  J. Cartwright,et al.  Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea , 2000 .

[13]  G. Papatheodorou,et al.  A string of large and deep gas-induced depressions (pockmarks) offshore Killini peninsula, western Greece , 2002 .

[14]  Leif Rise,et al.  Elongate depressions on the southern slope of the Norwegian Trench (Skagerrak): morphology and evolution , 1998 .

[15]  M. Hovland,et al.  Characteristic features of pockmarks on the North Sea floor and Scotian Shelf , 1984 .

[16]  G. Fader,et al.  A side-scan sonar mosaic of pockmarks on the Scotian Shelf , 1978 .

[17]  M. Hovland Large pockmarks, gas-charged sediments and possible clay diapirs in the Skagerrak , 1991 .

[18]  D. Belknap,et al.  Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine , 1994 .

[19]  N. Sultan,et al.  Analysis of submarine slumping in the Gabon continental slope , 2004 .

[20]  Susan E. Nissen,et al.  Delineation of geologic drilling hazards using 3-D seismic attributes , 1999 .

[21]  S. Harrison,et al.  AAPG Studies in Geology No. 48 / SEG Geophysical References Series No. 11, Chapter 2: Hydrocarbon Systems Analysis of the Northern Gulf of Mexico: Delineation of Hydrocarbon Migration Pathways Using Seeps and Seismic Imaging , 2002 .

[22]  M. Hovland Pockmarks and the Recent geology of the central section of the Norwegian Trench , 1982 .

[23]  K. P. Games Evidence of shallow gas above the Connemara oil accumulation, Block 26/28, Porcupine Basin , 2001, Geological Society, London, Special Publications.

[24]  Homa J. Lee,et al.  Causes of Two Slope-Failure Types in Continental-Shelf Sediment, Northeastern Gulf of Alaska , 1988 .

[25]  P. Cochonat,et al.  Polygonal faults‐furrows system related to early stages of compaction – upper Miocene to recent sediments of the Lower Congo Basin , 2004 .

[26]  T. Thorsnes,et al.  Sea-bed pockmarks related to fluid migration from Mesozoic bedrock strata in the Skagerrak offshore Norway , 1999 .

[27]  R. Lien,et al.  A Regional Shallow Stratigraphic Framework Off Mid Norway and Observations of Deep Water "Special Features" , 1998 .

[28]  M. López,et al.  Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo basin, West African margin , 2003, Geological Society, London, Special Publications.

[29]  D. Piper,et al.  Near-surface geology and sediment-failure geohazards of the central Scotian Slope , 2004 .

[30]  Steven Soter,et al.  Macroscopic seismic anomalies and submarine pockmarks in the Corinth–Patras rift, Greece , 1999 .

[31]  L. Dimitrov,et al.  Deep sea pockmark environments in the eastern Mediterranean , 2003 .

[32]  R. Pickrill Shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake, Lake Rotoiti, New Zealand , 1993 .

[33]  Michael A. Abrams,et al.  Distribution of Subsurface Hydrocarbon Seepage in Near-Surface Marine Sediments , 1993 .

[34]  B. Maclean,et al.  Pockmarks on the Scotian Shelf , 1970 .

[35]  M. Hovland,et al.  Characteristics of two natural gas seepages in the North Sea , 1985 .