Glacial tunnel valleys and Quaternary history of the outer Scotian shelf

Abstract

High-resolution seismic reflection data indicate the presence of huge sub-surface channel networks on the outer Nova Scotian continental shelf. Channel axes extend to over 450 m below present sea level (b.s.l.). Channel walls average 2–3 km in width. Mechanisms capable of producing such channels include fluvial, submarine canyon or glacial erosion. Considerable debate has focused on the positions of the Tertiary/Quaternary (T/Q) and Pleistocene/ Holocene (P/H) boundaries^1–4 in this region and the relationship of the T/Q boundary to the unconformity at the base of the channel networks. Uncertainty also surrounds the extent of Pleistocene ice sheets on the south-east Canadian continental margin^4–7. Here we present a new stratigraphy for the Sable Island region (Fig. 1) based on seismic profiles, with lithologic and biostratigraphic control provided by two strategically placed boreholes. This strati-graphic analysis establishes the positions of the T/Q and P/H boundaries outside the valleys at 51 m and 220 m b.s.l. respectively. Our analysis also implies that the large channel systems are tunnel valleys, cut by a sub-ice meltwater process under a pre- to early Wisconsinan ice sheet which extended close to the shelf edge.