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TitleSurficial sediments, Grand Bank, offshore Newfoundland
AuthorSonnichsen, G V; King, E L
SourcePERD/CHC Report 31-27, 2001, 43 pages, (Open Access)
Alt SeriesGeological Survey of Canada, Contribution Series 2001060
Mediaon-line; digital
File formatpdf
ProvinceEastern offshore region; Newfoundland and Labrador
AreaGrand Banks
Lat/Long WENS -54.0000 -47.0000 49.0000 46.0000
Subjectsmarine geology; sedimentology; surficial geology/geomorphology; scouring; scour marks; glacial scours; iceberg gouging; icebergs; sediments; bottom sediments; marine sediments; tills; glacial deposits; marine sands; sea level changes; grain size analysis; silts
Illustrationstables; geological sketch maps; cross-sectionsm, stratigraphic; stratigraphic columns; seismic reflection profiles; photographs; bathymetric profiles
ProgramProgram of Energy Research and Development (PERD)
Seabed scouring by icebergs is an impeding factor to the production and transshipment of oil and gas on Grand Bank. Understanding the seabed response to iceberg impact is essential for safe and cost-effective design of subsea facilities. Until now, a regional characterization of seabed sediments and engineering properties at a practical scale has been lacking. A new surficial geology zonation is presented here together with a brief geological setting, description, and geologic unit distribution. Existing geotechnical measurements are sparse and many are extrapolated from outside the map area but grain size, density, water content and strength values are presented for most sediment units. Values vary widely, reflecting the nature of the sediment and differing depositional processes and post-depositional history. Most of the Grand Bank seabed is covered by a relatively hard till sediment (sub-glacially deposited clay to boulder range), the Grand Banks Till Formation. This originated from glacial ice cover, likely in more than one phase, the latest lasting from about 25,000 to 14,000 years before present. The Formation deposited locally in long moraine ridges and its upper surface was modified by relict icebergs and current action, creating an undulatory (1 to 3m relief) gravely or sandy gravel seabed. Reported shear strengths are as low as 20 kPa for young, unmodified deposits and as high as 515 kPa for old, glacially overridden and desiccated samples, but most seabed exposures probably range from 75 to 110 kPa. In deep-water areas (generally over 300 m) and in small, isolated basins, suspension-deposited glaciogenic silt-dominated deposits, Downing Silt Fm., overly the till: Deposits range from a few to tens of metres thick. The Downing Silt was deposited largely beyond the ice margin during retreat phases. Strength measurements from 10 to 35 kPa probably characterize the near-surface. Some have been extensively iceberg scoured but this is mainly from times of lower sea-level and abundant glacier calving. The modern iceberg regime is unlikely to impact these silts. The same holds for the overlying clay dominated muds of the Placentia Clay Fm. A sandier sub-unit of the Downing Silt occurs in water depths more likely to be scoured. Measurements are largely lacking in the Downing Silt, but relict scours dimensions are similar to those in the till at its deeper extents. The shallower extents are probably sandier and preserve fewer, shorter, and shallower scours. The Grand Banks Till and Downing Silt Formations are covered with a veneer of silty sand, Adolphus Sand, over an extensive area between about 100 to 150 m water depth, deposited as near-shore sands below the former low stand of sea-level. This unit reaches up to several metres thickness but generally is thin enough that icebergs can potentially penetrate to the underlying substrate. The Adolphus Sand has only one reported shear strength of 20 kPa. Washed sands and gravels of the Grand Banks Sand and Gravel Fm are widespread above 105 to 110 m water depths. They were subjected to coastal and shallow water processes as sea-level rose over the last 15,000 years. The rounded gravel locally overlies thin and/or patchy till deposits from which it derived through erosion and reworking. Where the till unit has been completely eroded, the gravel lies on older and partially consolidated marine sands and silts. Gravel thickness is not well documented but likely varies from several cm to 1 or 2 m. The gravel is overlain by medium and coarse sands of highly variable thickness (up to 10 m) and distribution. Cone resistance that is typically <15 Mpa characterizes these cohesionless soils. Engineering properties vary greatly, reflecting local heterogeneity. The surficial map and accompanying illustrations attempt to characterize this variability, where data control is available The new geological zonation provides a basis for future correlation of an extensive database of iceberg scour features measured on the seabed with sediment type.

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