Wisconsinan Glaciation On the Continental Shelf - Southeast Atlantic Canada

Abstract

Through the interpretation of high resolution seismic reflection profiles of the Scotian Shelf, eastern Gulf of Maine, and the Grand Banks of Newfoundland, and the employment of a model for a marine ice shelf (Carey and Ahmad, 1961), we have developed a conceptual model for the deposition of glacial and glaciomarine deposits from these areas. The Scotian Shelf Drift was derived from subglacial melt-out debris from a neutral to negatively buoyant active ice shelf in direct contact with the seabed. Emerald Silt formed from subglacial melt-out 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 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. Type areas representative of the offshore Wisconsinan section were chosen on the basis of (1) what appeared to be the most complete seismostratigraphic sections representing both the Emerald Silt (glaciomarine) and Scotian Shelf Drift (glacial till) Formations, (2) their accessibility to sampling by piston corer, (3) the occurrence of unique structural and stratigraphic characteristics which were useful in the development of our conceptual models concerning glacial deposition in marine areas, and (4) their correlative relationships with the glacial geology of the entire shelf so that groundtruth information, for example radiocarbon dates, could be extrapolated within and beyond the type areas using continuous seismic reflection control. We have used the Carey and Ahmad 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. 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 46000 yBP. At this time a till blanket with lift-off moraines on its surface and associated glaciomarine sediment was deposited. During the period 46000 to 32000 yBP 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 have receded along the Bay of Fundy reentrant to deposit the Salmon River beds (38000 yBP) north of Yarmouth. During the latter part of the Middle Wisconsinan and Late Wisconsinan (32000 to 16000 yBP) 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 ( 17000 yBP) occurs on Truxton 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 15100 to 14465 yBP, and for the Late Wisconsinan-Holocene transgression is well expressed along the entire shelf.