931 Outer Shelf Banks
The Outer Shelf Banks include:
Geology and Seabed Morphology
The sandy components of the sand and gravels that are found on the bank tops can be shaped by wave and current activity into a variety of seabed features, including sand ridges, sand waves, ripples, and megaripples. Significant sand-wave fields are found on the western and eastern bars of Sable Island, and megaripples, sand ridges, and ribbons occur on the west Sable Island Bank (sub-Unit 931e) and Middle Bank (sub-Unit 921e). Browns Bank (sub-Unit 931b) has sand waves with megaripples on their sides. Sand waves and megaripples also occur in parts of Georges Bank (sub-Unit 931a), and large tidal ridges are found on the bank tops. Sand ridges are the largest of the features and migrate over long periods of time. Various ridges on Sable Island Bank mark the "footprint" of Sable Island moving to the east.
Patches of gravel, shell beds, and even boulders occur. Many of the surface features change with each storm or tidal event, and many of the smaller features are erased during intervening periods.
The northern edges of Sable Island Bank and Banquereau (sub-Unit 931f) have many steep-sided hanging valleys formed by glacial meltwater running over their edges. These extend onto the bank under the cover of surface sediments and are called tunnel valleys. Sediments moving off the edge of the shelf in these areas contribute material which maintains The Gully, a major submarine canyon and a probable remnant of an early drainage system. Similar movements on the outer edges of the Outer Shelf Banks, particularly during low sea level, have led to the formation of distinctive submarine canyons.
The margins of Outer Shelf Banks deeper than 110 m have sediments that are principally sand and contain small amounts of clay, silt, and frequently gravel. The surface may be flat and smooth to undulating and hummocky. Called Sambro Sand, these deposits cover the saddles adjacent to the Outer Shelf Banks in many cases.
Cod stocks from Banquereau and the Sable Island Bank migrate during the summer to the outer coast of Nova Scotia and northern Cape Breton. Some of the fish also go into the Gulf of St. Lawrence. Southern Scotian Shelf cod overwinter in deeper water around LaHave and Browns banks. Some of the cod move from deeper water to the shallower areas of the banks in summer. On Georges Bank, Atlantic Cod occur principally on the eastern portion.
Concentrations of Atlantic Halibut occur along the edges of Georges Bank, Sable Island Bank, and Banquereau, and Witch Flounder have localized areas of high abundance in the deep holes of Banquereau. Haddock aggregate around the offshore banks at the beginning of the year and move onto the banks to spawn as the water temperature rises. The sand and gravel bottom typical of the banks is suitable for haddock spawning. Pollock (Boston Bluefish) spawn on the northeastern parts of the Georges and Browns banks, at several locations on the Scotian Shelf, and on Jeffries Ledge in the Gulf of Maine and migrate as juveniles to inshore areas.
Eggs and larvae of cod, haddock, pollock, and Silver Hake are abundant on the Western and Sable Island banks; those of cod and pollock are found there during midwinter and early spring, and those of Silver Hake during midsummer.
Sea Scallop occurs on the Georges and Browns banks, particularly where the bottom consists of firm gravel, shells, and rock. Two large bivalve species - the Ocean Quahog, and Stimpson's Surf Clam - are found typically on most of the offshore banks, but they are locally abundant. The Ocean Quahog is the main species on Georges Bank and concentrations have been found on the Western and Sable Island banks. Stimpson's Surf Clam occurs on Banquereau.
Sandy areas that make up much of Sable Island Bank include organisms such as the sand dollar Echinarachnius parma and the amphipods Unciola irrorata and Leptocheirus pinguis. Sand dollars are extremely abundant in some locations. Areas of coarse substrate on the banks support the Horse Mussel, the brittlestar Ophiopholis aculeata, Sea Scallop, lobster, and Toad Crab. Whales occur in the offshore waters, and mass strandings of Atlantic Long-finned Pilot Whale have occurred on Sable Island. Humpback Whales are often associated with offshore banks, where they take advantage of spawning fish such as Capelin, herring, and Sand Lance, as well as larger zooplankton. The area of Browns Bank 60 km south of Cape Sable is visited by about 200 Northern Right Whales each year.
Common Terns, Herring Gulls, and Great Black-backed Gulls have colonies on Sable Island and use adjacent waters for feeding.
The banks are also being intensively explored for natural gas and oil with some trial production under way in the early 1990s. Plans, which include production platforms and seabed pipelines to the Nova Scotia coast, require suitable economic conditions.
There is one recorded munitions dump site (on Emerald Bank, sub-Unit 931d), and the Unit is crossed by several submarine cables.
Sites of Special Interest