Abstract

The seismic stratigraphy of Sable Island Bank provides a model for sand body formation in an outer continental shelf setting. The outer Scotian Shelf consists of a series of topographically isolated banks which have been influenced by Pleistocene glaciations and glacio-eustatic sea-level fluctuations. Sable Island Bank is such an example. It has the largest and thickest sand accumulation on the Scotian Shelf which emerges at 44°N 60°W as Sable Island. Seismic profiles illustrate that the Sable Island sand body covers an area of 6500 km² and is up to 51 m thick. The sand body has a complex stratigraphy because it represents a series of individual sand bodies deposited during and since the Late Wisconsinan.

The last ice advance and retreat over Sable Island Bank provided sand-sized sediments in an ice-marginal and/or proglacial environment between 27-20 ka (thousands of years ago). Sediments were transported within subglacial channels up to 100 m deep and 3 km wide. The subglacial channels were conduits for sediment-laden meltwater, depositing a thick sand body, up to 40 m thick, on the southern half of Sable Island Bank. Termination positions of the subglacial channels suggests that the last Late Wisconsinan ice extended almost to the shelf break at latitude 44°05′N.

In the Northern Spur area, a local deltaic unit up to 22 m thick lies above this glacial sequence. The stratigraphic position of this unit and the orientation of depositional foresets to the south and southwest suggest that the sequence was deposited during an ice-recession phase within a proglacial delta complex.

The glacial deposition of sand on Sable Island Bank created a sediment reservoir for the subsequent development of coastal and marine sand bodies during and after the Holocene transgression. Sea level has risen around the bank in response to ice retreat for the past 11,000 years. Transgressive reworking of the underlying Pleistocene deposits further sorted the sediment into clean coarsening-upward sequences. During the Holocene, shoreface erosion, longshore transport and storm and tide-generated currents concentrated surficial sediments to the northeast and east over Sable Island Bank. Sable Island in response, has aggraded with a sea-level rise of 20 m since 7,600 BP.

The Sable Island sand body has accumulated since the last Late Wisconsinan ice advance on the outer Scotian Shelf. The core of the major sandbody complex consists, therefore, of a Late Wisconsinan glacial sequence. It is capped by submerged transgressive sand bodies, presumably of Holocene age, and the modern eolian and beach sands on Sable Island.

It was previously thought that the Sable Island sand body was Holocene in age and therefore a post-glacial feature. The seismic stratigraphic analysis here shows that the majority of the sand body is a result of Late Wisconsinan glacial advances to the outer Scotian Shelf. It also illustrates the importance of glacial sediment supply and sea-level fluctuations to large-scale sand body formation within a continental shelf setting.