921 Middle Shelf Banks
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Topics & Habitats |
Banks on the Middle Shelf include:
921a Grand Manan Bank
921b Roseway Bank
921c Sambro Bank
921d MacKenzie Spot
921e Middle Bank
921f Canso Bank
921g Misaine Bank
921h Artimon Bank
Geology and Seabed Morphology
The Middle Shelf Banks are generally underlain by bedrock features in the Meguma formation and overlain by Quaternary and Cretaceous material. Middle Shelf banks vary in depth, surface sediments, and morphology. The Roseway and Sambro banks, which separate the Roseway, LaHave, and Emerald basins in Unit 922, are steep-sided, flat-topped mesas at 80-100 m depths. Other banks (Middle, Canso, Misaine, and the western part of Banquereau) appear to be cuestas with more gradually sloping margins. The western part of the Middle Bank is part of the Country Harbour moraine, one of a series of submarine glacial features found on the inner edge of the Middle Shelf (see T3.3 and Unit 922). The surface of Misaine Bank on the northeast end of the Scotian Shelf is extensively incised by channels believed to have resulted from the melting of the ice sheet. Other bank tops were levelled during the post-glacial sea-level advance as they became successive beach zones.
Sediments
Bottom sediments on the tops of the banks are a coarse deposit known as Sable Island Sand and Gravel, which contains sand and rounded gravel in various mixtures at the surface and has glacial till (Scotian Shelf Drift) beneath. Most of the smaller banks (Sambro, Roseway, The Patch) have a cover of predominantly gravel containing various proportions of sand at depths shallower than 110 m. In contrast, the larger Middle and Misaine banks have, in addition to areas of gravel bottom, a surface cover of chiefly sands with gravel mixed in. Both types of bottom are part of the Sable Island Sand and Gravel formation. The gravel can form a protective pavement of rounded stones embedded in the bottom. The sand tends to be smooth, hard, and flat and to have a variety of surfaces.
The margins of the banks at depths below 110 m have principally sandy sediments that contain small amounts of clay and silt, and frequently gravel. The surface may be flat and smooth to undulating and hummocky. These deposits are called Sambro Sand.
Oceanography
Currents derived from tides form a gyre around the banks and provide a potential though yet unproven means of keeping the larvae of fish species in the vicinity as they mature. Tidal action also 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.
Plants
Plant life follows that of the District description. There does not appear to be a difference in plant production on banks and adjacent basins and channels.
Animals
The coarse sand and gravels of bank tops favour large bivalve species such as the Ocean Quahog and Stimpson's Surf Clam which are less abundant on the Roseway and Middle banks. Concentrations of Sea Scallop also occur on some of the banks of the eastern Scotian Shelf. Bottom invertebrate communities characterized by the Horse Mussel, the Brittle Star Ophiopholis aculeata, Sea Scallop, lobster, and the Toad Crab are expected to occur on the coarse substrate in these areas. Sandier areas such as Canso Bank include organisms such as the sand dollar Echinarachnius parma and the amphipods Unciola irrorata and Leptocheirus pinguis. Sand Lance also favour this type of substrate.
Storm-petrels, shearwaters, jaegers, and phalaropes are found in the summer, and murres and kittiwakes are present in the winter.
Cultural Environment
Small banks on the Middle Shelf are important for trawling and line fishing from vessels from many small ports. The various local names (e.g., Mackenzie Spot, The Patch, The Bull Pen) come from long use. Some of the earliest records of molluscs reported by J.R. Willis in the 1860s were brought to him from Sambro Bank by local Halifax fishermen.
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