932 Bank Edges, Saddles, and Channels
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Sites of Interest Topics & Habitats |
Geology and Seabed Morphology
The banks of the Outer Shelf are bordered by intervening deeper-water areas which include saddles and channels, submarine canyons, and the continental slope. Saddles generally have gentle relief and are shallower than about 200 m, and channels are deep, broad lowland features occurring at the depths of basins on the Middle Shelf. Saddles occur between Sable Island Bank/Western Bank, Emerald Bank, and LaHave Bank. Northeast Channel separates the Browns and Georges banks, and Laurentian Channel separates Banquereau and the eastern Scotian Shelf from banks off the coast of Newfoundland.
Submarine canyons occur along the outer edges of the Outer Shelf and extend down the continental slope. These are narrow, deep, and steep-sided features and include The Gully, and the Verrill, Dawson, Bonnecamps, Logan, Shortland, and Haldimand canyons (see T3.5). The Gully is a submarine canyon that approaches the Colorado River's Grand Canyon in depth, extending from 100 m to more than a kilometre between Sable Island Bank and Banquereau (by comparison, the Cape Breton highlands are roughly 500 m high). The Gully probably originated as a drainage channel and later developed into a canyon. The river and submarine canyon system at the mouth of the Hudson River on the east coast of the United States is an analogous feature.
The Northeast Channel joins the Outer Shelf between the Browns and Georges banks with the basins of the Gulf of Maine at depths between 200 and 300 m. Megaripples occur on the northern and eastern flanks of Northeast Channel at depths of 100-150 m, and sand waves on the bottom of Northeast Channel at depths of 230-260 m are evidently caused by tidal currents. These are some of the deepest recorded sand waves on the continental shelf, caused by strong tidal currents in the Bay of Fundy and Gulf of Maine.
The Laurentian Channel is the most impressive of these features, arising as a former river valley deepened by glacial ice, and having a sill (a shallower portion near the outer edge). This channel extends 700 km from the junction of the Saguenay and St. Lawrence rivers in Quebec to the edge of the continental shelf between Nova Scotia and Newfoundland and was cut 300 m below the rest of the shelf by the advancing ice. Down the slope from the Laurentian Channel is the Laurentian Fan, a delta-like feature containing sediments from the ancestral St. Lawrence River and from recent sediment flows.
At the edge of the Scotian Shelf, the bottom plunges downward to the continental slope. The shelf edge is marked by submarine canyons and glacial features which demonstrate the furthest extent of the ice sheets.
Sediments
Saddles between Outer Shelf Banks (Unit 931), parts of Northeast Channel, and The Gully generally have a cover of sand containing clay and silt, and frequently gravel (Sambro Sand and Gravel; see above and T3.5). The outer and inner ends of Northeast Channel also have a cover of glacial till, consisting of mixtures of significant amounts of silt and clay in addition to sand, gravel, and boulders. The glacial till is classed as Scotian Shelf Drift.
Oceanography
Saddles occur at depths of less than 200 m and form an entrance to the basins of the Middle Shelf (District 920) for subsurface water masses, typically the warmer, deeper slope water from the shelf edge. The Northeast Channel between the Browns and Georges banks, at depths of more than 200 m, is profoundly influenced by tides in the area, and significant currents occur. The deep Laurentian Channel permits incursions of deep water from the Atlantic into the Gulf of St. Lawrence.
Plants
The biomass of phytoplankton, the productivity of the waters in saddles, channels, and canyons, and seasonal patterns are similar to those of the adjacent shelves. The outer margin of the continental shelf, however, has greater plant productivity because of the interaction of shelf and slope water masses in a "frontal zone" whose position changes from year to year. The elevated productivity is used by, and is believed to enhance, populations of fish and other organisms in the area.
The edge of the Outer Shelf is exposed periodically to water masses derived from the Gulf Stream, which flows to the south. Occasionally masses of the seaweed Sargassum can be found floating in the area.
Animals
Witch Flounder is associated with deep holes and channels between the coastal banks, along the deep edges of the banks where water temperatures are suitable, and in gullies where bottom is usually clay, muddy sand, or mud. This species has localized areas of high abundance along the edge of the Laurentian Channel, between Sable Island and Banquereau, and in deep holes of Banquereau. Notable
concentrations of Atlantic Halibut occur along the edges of the Georges and Sable Island banks and Banquereau. Various flatfish species occur in areas bordering the banks. Owing to the warmer water there, the outer margin of the shelf is a principal area of concentration for Silver Hake, which move onto the Scotian Shelf as temperatures rise in summer. The main known overwintering area for Atlantic Mackerel is the continental shelf south and southwest of Georges Bank.
Short-finned Squid are usually most common along the outer edge of the Scotian Shelf in June, usually between the Emerald and LaHave banks, and in some years along the entire edge of the shelf. They spread over the shelf later in the summer and later migrate southwest down the North American east coast. The young are brought back into the area by the Gulf Stream. Juveniles live in the Gulf Stream frontal zone and slope water off the edge of the continental shelf until they reach about 10 cm in length.
Deep-sea Red Crab is abundant along the shelf edge from the Fundian Channel to Sable Island at depths of 180-550 m. Significant quantities of lobster occur at the shelf edge from Browns Bank to Sable Island Bank.
One of the two best-known areas of concentration of the Northern Bottlenose Whale is in The Gully. Sperm Whales are usually found along the edge of the continental shelf or over canyons and deep basins between banks.
Seabird concentrations are greater in the shelf edge owing to the elevated productivity there. Wintering dovekies are most common over the edges of the Scotian Shelf. On Georges Bank, Wilson's Storm-petrel is most common over the shelf break.
Cultural Environment
Like Unit 931, the major marine activities on the shelf edge are fishing and oil and gas exploration. The Northeast Channel includes the area known as the "Hell Hole," where tuna is caught in notoriously difficult sea conditions. Harvestable concentrations of deep-sea Red Crab occur along the Scotian Shelf edge from the Fundian Channel to Sable Island. In the 1960s, whalers out of Blandford caught 67 Northern Bottlenose Whales.
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