922 Middle Shelf Basins
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Topics & Habitats |
Basins on the Middle Shelf include:
922a East Jordan Basin
922b East Georges Basin
922c LaHave Basin
922d Emerald Basin
922e St. Anns Basin
Geology and Seabed Morphology
These basins are extensive depressions in the Middle Shelf and are similar to lowlands. They have been extensively filled and smoothed by sedimentation and the action of the ocean. Collectively, their landward edges form a trough that runs parallel to the Atlantic coast of Nova Scotia and is analogous to lowland areas of the emerged coastal plain south of New Jersey.
Basins in the Gulf of Maine tend to be larger and deeper, and are frequently separated by sill-like features typified by the Truxton Swell between Jordan and Crowell basins. Other prominent basins in the Gulf of Maine are Grand Manan Basin (at the mouth of the Bay of Fundy) and Georges Basin (off the northern edge of Georges Bank).
Submarine glacial end moraines occur on the landward flanks of the Middle Shelf basins on the Scotian Shelf, and in the form of the Fundian Moraine that reaches between Crowell and Georges basins in the Gulf of Maine. This complex of moraines extends more than 800 km, from the Gulf of Maine to the Laurentian Channel. These moraines differ from those found on land because they were formed while an ice sheet floated over seawater, and the morainic material was deposited into marine conditions. Some of these moraines have furrows from ancient icebergs that grounded when the water was shallower.
Sediments
The main sediment of the basins is a grey clay known on the Scotian Shelf as LaHave Clay, which formed during glacial retreat and settled to the seabed in the reduced currents and waves of the basins. Occasionally boulders pierce the cover where they were dropped from the bottoms of melting glaciers, and crater-shaped depressions known as pockmarks are found where natural gas from subsurface rock formations bubbles to the surface. Beneath the clay are layers of silt (Emerald Silt) carried by meltwater from the advancing glaciers and spread throughout the basins. In some places, Emerald Silt has been exposed at the surface. The upper levels of the basins frequently have deposits of glacial till, which also occurs in the moraines in the inner flanks of the basins. The glacial till is classed as Scotian Shelf Drift and was not modified by the last advance of the sea across the continental shelf.
Oceanography
Middle Shelf basins are connected with the Outer Shelf (District 930) and the edge of the continental shelf through "saddles," areas of intermediate depth which separate the banks of the Outer Shelf. The saddles are at depths of less than 200 m but generally more than 100 m and form an entrance to the basins for subsurface water masses. Frequently, storms will force warmer, deeper slope water from the shelf edge into the basins. Middle Shelf basins in the Gulf of Maine have a deeper connection with the Outer Shelf through the Northeast Channel between the Browns and Georges banks, at depths over 200 m. Owing to the greater influence of the tides in the area, significant currents and a larger transport of water occurs through the Northeast Channel into the basins.
Plants
Movements of water through the Outer Shelf saddles into the basins of the Middle Shelf bring periodic influxes of nutrients which help to sustain phytoplankton populations. Basin bottoms are too deep to sustain plant growth. Plant productivity in the water over the basins is similar to that over the banks.
Animals
The deep basins on the Scotian Shelf contain high concentrations of the copepods Calanus glacialis and C. hyperboreus at depths below 200 m, and the populations are greater than on the adjacent shelves. These basins make it possible for
C. finmarchicus to dominate the shelf zooplankton for most of the year. Two of the largest basins, Emerald and LaHave, contain large populations of Silver Hake, and a large euphausiid (krill) population occurs in Emerald Basin. The juvenile Silver Hake feed principally on young euphausiids (Meganyctiphanes norvegica). Red Hake are common in deeper portions of the southwestern Scotian Shelf and the Gulf of Maine, and Witch Flounder occur in deep holes and channels between the coastal banks and along the deep edges of the banks where water temperatures are suitable.
Invertebrates on the soft bottom of the basins include the Brittle Star (Ophiura sarsi), the Heart Urchin, the Mud Star (Ctenodiscus crispatus), Northern Shrimp, the anthozoan Pennatula aculeata, Snow Crab, Jonah Crab, and Tusk Shell (Dentalium spp.). Polychaete worms which live in bottom sediments of the basins are part of the Labrador faunal group, while those in shallower water are Acadian, or warmer water forms.
Snow Crab occur on muddy or sandy mud bottoms at depths of 45-245 m around Cape Breton Island. Significant populations of shrimp (Pandalus borealis and P. montagui) occur over deep muddy bottoms in the basins between Middle, Canso, and Misaine banks, and in basins north of Misaine Bank).
The endangered Northern Right Whale is often seen on the Scotian Shelf, especially in or near Roseway Basin (Unit 923) between Browns and Baccaro banks.
Cultural Environment
The deeper basins are used to some extent for fishing, including for Northern (Pink) Shrimp. The most important fishery is that for Snow Crab around Cape Breton Island. There are munitions dumping areas (e.g., sub-Unit 922d, Emerald Basin).
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