On the Scotian Shelf, eastern Gulf of Maine, and the Grand Banks of Newfoundland, the surficial succession occurs as a thin blanket (approximately 50m thick) across the submerged coastal plain surface, and is composed of five formations: Scotian Shelf Drift (glacial till), Emerald Silt ( glaciomarine), Sam bro Sand, LaHave Clay, and Sable Island Sand and Gravel. The Scotian Shelf Drift was derived from sub glacial meltoul debris from a neutral to negatively buoyant active ice shelf in direct contact with the seabed. Emerald Silt formed from subglacial meltout debris from a pinned but floating ice shelf. The debris is thought to have settled through a water column of variable thickness to form the conformable, rhythmically banded deposits which mimic a highly irregular substrate which is recognized over very broad areas. Horizontal migration of the ice-seabed contact (buoyancy line), induced by changes in ice thickness and changes in relative sea level, leads to the development of thick regional moraines (regional subglacial ice shelf moraines) interbedded with the glaciomarine deposits. Wedge-shaped till deposits (till tongues) are often formed at the distal side of the moraines through advance and subsequent retreat of the buoyancy line. The configuration of the seabed beneath an ice shelf is another important factor in the stratigraphic development of these marine deposits. We have used the Carey and Ahmad ice model as an aid in interpreting the seismostratigraphy of the surficial sediments on the continental shelf and integrated these studies with sample data to postulate the glacial history. This stratigraphy is correlated with the adjacent land area. In late Early Wisconsinan time the entire shelf was occupied by an ice sheet (the Scotian Shelf-Grand Banks advance). This wasted to an ice shelf which became buoyant in the deeper basins at about 46 OOO BP. At this time a till blanket with numerous discrete ridges (lift-off moraines) on its surface and associated glaciomarine sediment was deposited. During the period 46 OOO to 32 OOO BP the buoyancy line oscillated intermittently, resulting in the development of till tongues at the periphery of the banks and outer edge of the inner shelf which were intercalated with glaciomarine sediments in the basins. At the western end of the Scotian Shelf the ice may ha1·e receded along the Bay of Fundy re-entrant to deposit the Salmon River beds (38 OOO BP) north of Yarmouth. During the latter part of the Middle Wisconsinan and Late Wisconsinan (32 OOO to 16 OOO BP) the buoyancy line receded to the coastal areas except in the eastern Gulf of Maine where the ice shelf again grounded to deposit till in the areas peripheral to Georges Basin, the southern flank of Browns Bank, and the Fundian Moraine of Sewell Ridge. The youngest moraine (= 17 OOO BP) occurs on Trnxton Swell. Subsequently the buoyancy line receded to the coastal areas of Maine and New Brunswick, and the resulting glaciomarine deposits emerged through glacioisostatic rebound. Evidence for the Late Wisconsinan sea level, which we date within the range of 15 OOO to 14 500 BP, and for 1he Late Wisconsinan-Holocene transgression is well expressed along the entire shelf.
Pockmarks, cone-shaped depressions that occur in unconsolidated fine sediments at the seabed, are generally thought to be formed by gas ascending from underlying sediments. A mosaic of pockmarks in Emerald Basin on the Scotian Shelf was constructed and the problems of mosaic construction are discussed. Within the study area pockmarks have an average depth of 6 m, an average diameter of 85 m and cover up to 18% of the seabed. Pockmarks that occur in thin clay (LaHave clay) are generally < 5 m deep and those in thick clay are up to 15 m deep. Many pockmarks are elongate in shape and show a preferred orientation, both of which may be due to the influence of bottom currents. Within the study area, the process of pockmark formation has displaced 2.5 × 105 m3 km−2 of sediment. Pockmarks on the Scotian Shelf appear to be inactive (relict) and in cases where they are buried they are referred to as ancient.
This paper presents the results of a comparative study of pockmarks and associated features appearing on both sides of the North Atlantic: on the Scotian Shelf off Nova Scotia and in the northern North Sea. Pockmarks are formed in seabed material consisting of soft silty clay. The seismic, sonar and lithologic characteristics of the sediments on the Scotian Shelf are remarkably similar to those found in the northern North Sea. Sediment clouds suspended in the water column immediately over the seabed have previously been observed on side‐scan records associated with gas‐charged sediments on corresponding shallow‐seismic records. These and similar observations strongly suggest that most pockmarks are caused by gas efflux through the seafloor. However, the detailed mechanism of formation and the origin of the gas in the sediments is still unknown.
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