ABSTRACT: Lift-off, de geer, ribbed, washboard, or annual moraines? Ubiquitous seabed features of geological and engineering significance on the Scotian Shelf.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Fields of linear moraines (2 - 20 m in height and 20 - 150 m in width) were first identified and found to be widespread in the basins of the Scotian Shelf, Gulf of Maine and the Grand Banks of Newfoundland during regional Quaternary mapping of the seabed off southeast Atlantic Canada in the 1970s and 80s, using high-resolution seismic reflection profilers. The ridges were mostly buried, always occurred at the surface of the uppermost till and were overlain by basal facies of the Emerald Silt glaciomarine formation. They were given the generic name "lift-off moraines" (King and Fader, 1986) as they were interpreted to form in ice fractures and crevasses as grounded glaciers first lifted-off from the seabed in response to thinning ice and sea level rise. Few were exposed at the seabed and those that were, showed a general pattern of bifurcating linear ridges often cut by iceberg furrows. New multibeam bathymetry shows that linear moraines are widespread, especially on the inner shelf where they tend to overlie bedrock. It was earlier thought that these moraines would not survive a marine transgression in a high-energy beach zone that generally covers depths above 70 m. Their occurrence in known transgressed areas has lead to a reevaluation of ideas concerning the erosive power of open ocean marine transgressions. Survival in the near shore may be related to protection in slight depressions, the presence of bedrock highs further seaward and a cover of late glacial ice. It is not completely understood how the moraines form and models range from annual frontal depositional events to formation in fields in a subglacial environment by crevasse filling through squeezing processes. In some areas there is a unique association with bedrock structure where the moraines occur only on bedrock ridges. They also overlie offshore drumlins and occur very close to the present shoreline. In some areas fields of small moraines appear to have been truncated by broader more regional larger moraines. In the Gulf of Maine they connect in deeper water to recessional ice shelf moraines on Truxton Swell. These moraines are very significant for seabed engineering projects and bottom fishing activities. Boulders are usually concentrated on moraine surfaces that make them difficult features to cross with pipelines, communication and electrical transmission cables and bottom fishing equipment. Fishing parallel to the moraine trends can minimize gear damage and increase catch.

Bibliography of Canadian Geomorphology