U.S. and Canadian Geologists Collaborate in Mapping the Georges Bank Seabed

By Page Valentine (U.S. Geological Survey) and Brian Todd (Geological Survey of Canada) Nov. / Dec. 2012

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Georges Bank is a large (42,000 square kilometers) part of the continental shelf offshore of New England that lies in both U.S. and Canadian waters. The seabed is primarily glacially derived sand and gravel deposited since the end of the last glacial maximum approximately 20,000 years ago. A series of maps showing the seabed topography of the Canadian portion of relatively shallow Georges Bank and the deeper Fundian and Northeast Channels has been compiled by geologists Brian Todd and John Shaw of the Geological Survey of Canada (GSC) and Page Valentine of the U.S. Geological Survey (USGS).

Above: Regional map showing multibeam-sonar topographic imagery of the Canadian part of Georges Bank and the Fundian and Northeast Channels. Mapped area is southeast of the Gulf of Maine and southwest of the Scotian Shelf (see inset map). Large rectangles are boundaries of the 9 sheets of the new map series. Red rectangles are locations of high-resolution images of sand waves and submarine canyons (see below). Water depths are color-coded; topographic contours are in meters. [larger version]

The area was surveyed over two field seasons using multibeam sonar technology. Todd has led the effort to map large parts of the Canadian continental margin, including the Bay of Fundy, German Bank, Browns Bank, and the Georges Bank region. Valentine has conducted research on the Georges Bank seabed in U.S. waters and has previously collaborated with Todd on geologic studies of the Canadian portion of the bank. Their mutual research interests have led Todd and Valentine to collaborate in compiling seabed maps using an approach that relies on multibeam sonar data, video and photo imagery, sediment sampling, and subbottom seismic profiling.

Above: Map and cross-section showing the morphology (shape) of mobile sand waves and megaripples on north-central Georges Bank. Inferred direction of net current and sediment transport is from northwest to southeast, as determined from the steep down-current lee faces of the sand waves. Megaripples occur on the up-current (or stoss) faces of the sand waves. Mobile sand waves in this image reach 14 meters in height (in water depths of 50–64 meters), and megaripples are less than 2 meters in height. See the regional map above for location of the image. [larger version]

The map series, to be released in January 2013, comprises 9 sheets at a scale of 1:50,000 (1 centimeter on the map represents 500 meters on the seabed). In all, 13,000 square kilometers of seabed were mapped in water depths of 42 meters to more than 1,000 meters. Multibeam bathymetric data show the topography in great detail at a horizontal resolution of 5 to 10 meters and a vertical resolution of 10 to 30 centimeters. The seabed bears the imprint of the last glaciation, when sea level was approximately 125 meters lower and the Laurentide ice sheet covered all of New England and Atlantic Canada. Glacial ice encroached on the northern and eastern margins of Georges Bank and found an outlet to the Atlantic Ocean through the Fundian and Northeast Channels, which separate Georges Bank from the Scotian Shelf to the north. Today, much of the bank is covered by sand and gravel outwash from the glaciers. These sediments have been reworked by rising sea level and modern tidal and storm currents to produce large sand waves in shallow parts of the bank. Moraines and other glacial features on the seabed show the direction of flowing ice, and numerous iceberg keel marks record the breakup (or calving) of floating ice where glaciers terminated in the ocean. Old shoreline features document rises of sea level following the melting of the regional ice sheet. On the seaward edge of Georges Bank, submarine slide features document the collapse of sediment deposits, which contributed to the formation of submarine canyons. Present-day features of deformed seabed on the bank edge identify sediments that are susceptible to future slumping and sliding events. Apart from revealing the morphology of the seabed and the recent glacial and postglacial processes that formed it, these maps provide a framework for further mapping of the geological substrates and environmental processes that characterize the region, for fishery management, and for baseline studies that would precede any future oil and gas development.

Above: Southernmost map sheet of the series shows submarine canyons incising the seaward margin of Georges Bank. Canyon walls are characterized by ridges and gullies caused by slumping of glacially derived mud. A large submarine slide is present in the head of Corsair Canyon (see inset image). Not visible at this scale are sand waves on the bank at water depths of as much as 100 meters, and iceberg keel marks and incipient slump features at the shelf break at depths of approximately 200 meters. See the regional map above for location of the image. [larger version]

The new map series will be published by the Geological Survey of Canada; the full citation is: Todd, B.J., Valentine, P.C., and Shaw, J., 2013, Shaded seafloor topography, Georges Bank, Fundian Channel, and Northeast Channel, Gulf of Maine: Geological Survey of Canada, Maps 2191A–2199A, 9 sheets, scale 1:50,000.

Multibeam bathymetric imagery of the Canadian continental margin can be viewed at http://gdr.ess.nrcan.gc.ca/multibath/e/viewer.htm. The Georges Bank imagery will be available for viewing after this map series is published in January 2013.

Geological Survey of Canada Geological Survey of Canada

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Nov. / Dec. 2012

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in this issue:

cover story:
Scientists Predict, Measure Sandy's Impacts

Post-Tropical Cyclone Sandy

Sediment Movement in the Northern Chandeleur Islands

Recovery Slows for California's Sea Otters

Mapping the Georges Bank Seabed

Native Youth in Science—Preserving Our Homelands

2011 Excellence in Partnering Award

Pacific Coastal and Marine Science Center Welcomes Andy O'Neill

Olivia Cheriton Joins Pacific Coastal and Marine Science Center

Nov. / Dec. Publications