930 Outer Shelf
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Topics & Habitats |
This District has been divided into two Units:
931 Outer Shelf Banks
932 Bank Edges, Saddles, and Channels
923 Valley and Plains
Geology and Seabed Morphology
The Outer Shelf is a broad zone (50-75 km wide) consisting of banks and intervening areas (saddles, channels, and one major submarine valley, The Gully) extending from Banquereau on the east to Georges Bank on the west. The main banks (Banquereau, Sable, Emerald, LaHave, Browns, and Georges in Unit 931) are relatively large, shallow (30-80 m), and more or less flat-topped. They represent features in the ancient bedrock that were overlain with glacial till and then levelled by the advancing sea following the last glaciation. Sable Island (District 890) protrudes to a height of 26 m above the surface of the Sable Island Bank and is the furthest offshore island. The relief of the banks relative to other features of the Outer Shelf is comparable to areas of the mainland today - the elevations of the outer banks are generally less than those of the Cobequid Mountains in the Wentworth area of Cumberland County and much less than the Cape Breton highlands. However, The Gully - a submarine canyon between Sable Island Bank and Banquereau - is about half as deep as the Grand Canyon in the United States.
Sediments
The bank tops contain sand and gravel deposits and, in the case of Sable Island, have been reworked and moved around to form extensive sand fields. Below a depth of about 110 m, the bottom sediment consists of sand with silt and clay mixtures. The Outer Shelf contains no basins, and the only clay deposits are found in the Laurentian Channel, which borders the eastern end of the District.
Oceanography
The shelf break front is a sharp boundary between cool, less salty coastal water and warm, more saline slope water. Shelf break fronts occur in response to tides, winds, and fluctuating offshore currents. Currents from tides can form gyres that encircle the banks and may provide a "retention area" for larval fish. Tidal action also tends to produce a mixed water column on the banks. The water column on the shallower banks may be well mixed through much of the year, while the deeper banks have a stratified water column.
Plants
The plant life is primarily phytoplanktonic, but encrusting algae may occur on suitably hard substrates in some of the bank areas. The outer edge of the continental shelf has enhanced plant productivity because of the interaction of shelf and slope waters which brings nutrients to the surface.
Animals
This District sustains a diverse fauna. The offshore banks are inhabited by many species of fish. Several species of large burrowing molluscs occur in the sandy substrate of offshore banks.
Lobster commonly move from the Inner Shelf to the Outer Shelf banks and continental slope and can occur along the Outer Shelf and upper slope from Browns Bank to southeast of Sable Island.
Cultural Environment
The Outer Shelf includes some of the most important fishing grounds for shellfish, especially scallops, and groundfish as a result of high production at the edge of the deep water. The area was heavily fished by many nations until Canada introduced the 200-mile fishing zone in 1977. Rock formations beneath the Outer Shelf and the Scotian Slope have provided most of the interest in the search for offshore hydrocarbons. Natural gas was discovered in the vicinity of Sable Island in the 1970s; the first wells to produce hydrocarbons (condensate) commercially in the early 1990s are located there. The seasonal concentration of whales, large pelagic fish, pelagic seabirds, and varied oceanic marine life offers great potential for adventure tourism. The area is traversed by several submarine cables, and two munitions dumping areas are identified on marine charts.
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