Lewis H. King

Pockmarks on the Scotian Shelf

Pockmarks are cone-shaped despressions that occur in large numbers across the LaHave clay of the Scotian Shelf. They normally range in diameter from 15 to 45 m and in depth from 5 to 10 m. They were possibly formed by ascending gas or subsurface water leakage from underlying coastal plain sediments.

Late Wisconsinan ice retreat from the Scotian Shelf

The late middle to late Wisconsinan Laurentide ice sheet terminated at the edge of the Scotian Shelf ca. 21 ka, in close temporal agreement with the H-2 Heinrich event of the North Atlantic stratigraphy. The ice began its recession along the western edge of the shelf prior to 20 ka. By 15 ka, the ice front had retreated in a northeastward direction, exposing much of the western half of the shelf, although ice appears to have remained on the inner shelf north of a morainal system for another 2 k.y. The retreat was probably controlled by an ice divide along the axis of the mainland until after 16 ka, at which time an ice-flow pattern began to emanate from local remnant ice centers on the mainland. Late Wisconsinan ice seems to have remained on much of the eastern portion of the shelf as late as 12–14.5 ka, and probably deposited several outer bank moraines. Younger Dryas ice probably was present at isolated land and coastal areas for a short period ca. 10.5 ka, but its offshore influence appears to have been restricted to floating ice and storm-related events.

Submarine End Moraines and Associated Deposits on the Scotian Shelf

A submarine end-moraine complex occurs on the Scotian Shelf south and east of Halifax at 30 to 40 km offshore. It extends as a belt of low ridges which lies parallel to the present coast. Some of the larger individual ridges extend up to 55 km in length, with a slight arcuate pattern, and are an average of 50 m in height above the underlying bedrock. The full extent of the system has not been determined; however, a cursory examination indicates its occurrence along at least 500 km of the coast, in water depths ranging from 70 to 200 m. The moraine pattern is not revealed on the published bathymetric chart, because the ridges are to a large degree sub-bottom features, masked by ponded clay and silt deposits. However, the extreme peaks of some ridges may crop out slightly above the clay and silt of the bottom, so that their pattern must be resolved by detailed geologic mapping. The materials forming the exposed portions of the ridges range from relatively unaltered to completely reworked glacial debris, depending on the depth of water in which they occur. Resolution of the moraine pattern is greatly enhanced when the ridges are mapped as sub-bottom features, using high-frequency echograms obtained at closely spaced intervals. The degree of penetration with such an echo-sounder is limited essentially to the base of the clay, but the full profile of the moraines above the underlying bedrock is revealed by low-frequency, continuous seismic reflection profiles. Sub-bottom records and textural data on bottom samples indicate the occurrence of stratified proglacial deposits associated with the moraines.

Relict iceberg furrows on the Laurentian Channel and western Grand Banks

A side-scan sonar survey along the western bank of the Laurentian Channel and on the western Grand Banks revealed the occurrence of iceberg furrows that are probably of Late Pleistocene age. The occurrence of furrows in the Gulf of St. Lawrence is significant in that it helps to date iceberg furrows along the northeast Newfoundland–Labrador margin of the northwest Atlantic, provides data on the history of deglaciation of the offshore area of the Atlantic Provinces, provides a means of evaluating sea level curves, and provides additional evidence for the broad regional extent of the Late Pleistocene shoreline at 115 to 120 m.

Relation of Plate Tectonics to the Geomorphic Evolution of the Canadian Atlantic Provinces

The plate tectonics hypothesis provides new insight into interpretation of the geomorphic history of the Atlantic Provinces. The landscape of the continental margin was probably rejuvenated in Late Triassic and Early Jurassic times as a result of regional tectonic uplift and ramping associated with continental breakup. The development of some aspects of the present geomorphic expression appears to have begun as early as Late Jurassic time. Uplift was followed by broad regional subsidence along the continental margin and adjoining ocean basin and appears to have persisted to at least late Tertiary time. Unconformities across the formations of the submerged Atlantic Coastal Plain were formed during the time of broad subsidence, and these geomorphic surfaces were probably developed by subaerial processes. The lower sea levels possibly resulted from eustatic change rather than from tectonic uplift, and were possibly controlled by tectonic events along the mid-oceanic ridges.

Continuous Seismic-Reflection Study of Orpheus Gravity Anomaly

Continuous seismic-reflection profiles across the Scotian shelf south of Cape Breton Island, Nova Scotia, reveal the presence of moderately folded Cretaceous rocks within the area represented by the Orpheus negative gravity anomaly. The folded strata are confined to the anomalous area, and contrast with the gently seaward-dipping strata typical of most other parts of the shelf. Bedrock units are recognized on the continuous seismic-reflection records by unconformable relations and differences in definition, nature, and frequency of seismic events depicting layered sequences. A preliminary geologic map was constructed from these data, the adjacent land geology, and the gravity, magnetic, and seismic-reflection data obtained by other workers. Rocks in the anomaly area are thought to range in age from pre-Pennsylvanian to probable Tertiary. The fold structures evident in the Cretaceous strata are attributed to movement along an eastward extension of the Cobequid-Chedabucto Triassic fault system that extends across Nova Scotia almost continuously from the Bay of Fundy to Chedabucto Bay. This system is postulated to extend eastward along the south side of the Orpheus anomaly, thence southeastward toward the continental slope. It may be part of a larger fracture zone continuing along the southwest margin of the Grand Banks and across the Atlantic Ocean.

A Diapiric Structure Near Sable Island - Scotian Shelf

A diapiric structure was discovered on the Scotian Shelf at a point 45 km north-northeast of Sable Island, by means of continuous seismic-reflection techniques. The structure is dome-shaped, may contain an evaporitic core, and intrudes rocks considered to be of Cretaceous age. The area of structural disturbance is at least 10 km in diameter.