Surficial geology and features of the inner shelf of eastern shore, offshore Nova Scotia

The report describes the Quaternary geology across the inner continental shelf of the Eastern Shore of Nova Scotia, stretching about 70 km from Clam Bay to Moosehead and from the coast to about 40 km offshore. A surficial geology map was constructed from a mosaic of seabed topography images from water depth soundings gridded at spatial resolutions ranging from high (5 m) to low (500 m), supplemented with a sparse network of acoustic geophysical transects and limited sample data. The coast and inner shelf morphology, to about 120 m water depth, is primarily governed by bedrock and has a far more rugged topography than the offshore. Hard meta-sedimentary rocks comprising slates and meta-sandstones of the Lower Paleozoic age Meguma Supergroup present bedding and cleavage at the seabed which impart a rough ridge and trough morphology trending parallel to the coast. The glacial depositional and erosional imprint is strong in the area; numerous broad fjord-like channels oriented normal to the coastline cut deep into bedrock and were largely mud-filled during deglaciation. Different ice flow phases are recognized from striae, drumlins and moraines. Bedrock is locally covered by ground moraine, drumlins and moraines or their erosional remnants. Outcrops with little sediment cover dominate the seabed from 70 m water depth to the headlands except where the broad channels and shallow basins developed a sand and gravel lag during the sea-level transgression. The precise low-stand of sea-level on this part of the inner shelf is not clear but has been reported as >-50 m (Forbes et al. 1991) immediately east of Halifax, and -65 to -70 m (Stea et.al. 1992b, 1994, 1995). Based on near-complete littoral washing of sediments from the bedrock surface as represented in the multibeam bathymetric images, a -70 m elevation is suggested but the shallower low-stand allows for greater sub-littoral wave-erosion action and is likely no less valid. Post-glacial marine mud, largely derived from the reworking of glacigenic sediments during transgression, dominates the offshore, below about 120 m and in harbours and near-shore basins that are relatively sheltered from storm action. Glacial reconstruction was largely established before this mapping exercise but the improved characterization of features and spatial patterns contributes further detail. Regional ice flow phases established are successfully correlated to the offshore. All but the latest (Chignecto) phase are represented by one or several glacial elements including deep mud-filled bedrock channels, drumlin, moraine and striae. There is no clear pattern as to which flow phases preferentially create or preserve glacial features. The last offshore imprint was during the Scotian Phase, nearly normal to the coastline. If a later glacial phase extended into the offshore, its margin was likely farther east.

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