931 Outer Shelf Banks
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Sites of Interest Topics & Habitats |
The Outer Shelf Banks include:
931a East Georges Bank
931b Browns/Baccaro Banks
931c LaHave Bank
931d Emerald Bank
931e Sable Island Bank
931f Banquereau Bank
Geology and Seabed Morphology
The Outer Shelf Banks were initially bedrock features known as cuestas, typically formed in coastal-plain environments by erosion during early geological periods when they were not submerged. A modern example of a cuesta is Prince Edward Island. Their appearance has been transformed by deposition of glacial till, which has been reworked by the sea to form the present-day surfaces. The banks have moderate relief, generally between 100 and 150 m, and are thus comparable in range to the elevations found on the mainland today. For comparison, the elevations of the Cobequid Mountains in the Wentworth area of Cumberland County are between 200 and 300 m above the surrounding coastal lowland; and North Mountain, between the Annapolis Valley and Bay of Fundy, reaches elevations of about 200 m.
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.
Sediments
The surfaces of the Outer Shelf Banks shallower than about 110 m consist chiefly of sands and gravels in various combinations in a layer generally less than 15 m deep. In some areas (such as the top of Emerald Bank, sub-Unit 931d), gravel predominates, but Sable Island Bank is mostly covered in sand. Where gravel is found, it can form a protective pavement of rounded stones embedded in the bottom. The sand tends to be smooth, hard, and flat with a variety of surface bedforms. Both types of bottom are classified as Sable Island Sand and Gravel.
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.
Oceanography
Currents derived from tides can form gyres that encircle the banks and may provide a "retention area" for larvae. Tidal action tends to produce a mixed water column on 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. It is in these areas that shelf-break mixing processes occur.
Plants
The biomass of phytoplankton, the productivity, and the seasonal pattern of the waters over the shelves are similar to those found between the banks. Thus there is no greater food supply for other organisms here than over adjacent banks and saddles. The outer edges of the banks are an exception, as phytoplankton productivity is greater in the zone of interaction of shelf and slope waters.
Animals
More plant material reaches the seabed on the banks than in the adjoining areas and, consequently, vertebrate and invertebrate animal populations, including groundfish which feed near the bottom, are more significant on the offshore banks than in adjoining areas.
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.
Cultural Environment
The fishing banks have been heavily used for ground (bottom) fishing with both trawl and line since the seventeenth century. In the period following the Second World War, there was a deadly increase in fishing by many nations under the international regulations of the ICNAF (International Commission on Northern Atlantic Fisheries). However, declining stocks resulted in the establishment of a 200-mile fishing zone in 1977. Cod stocks on Banquereau (sub-Unit 931f) have declined severely and cod fishing is now closed in most areas. Main products include scallops, offshore lobsters, and ocean clams. The Georges Bank scallop stock is the largest scallop resource in the world. Concentrations of Stimpson's Surf Clam on Banquereau formed the basis for a fishery in the late 1980s.
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
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