Shirley Anne McLaren

The seismic stratigraphy of Sable Island Bank, located 227 km east of Halifax on the Scotian Shelf, provides a model for sand body formation in a glaciated continental shelf setting. The outer Scotian Shelf consists of a series of topographically isolated banks, including Sable Island Bank, which have been influenced by Pleistocene glaciations and glacio-eustatic sea level fluctuations. Sable Island Bank has the largest known sand accumulation on the Scotian Shelf which is locally emergent as Sable Island at 44^oN 60^oW. Seismic reflection profiles illustrate that the Sable Island sand body (SISB) covers an area of 12,000 km2 and is up to 55 m thick. The composite SISB, previously interpreted as a single marine Holocene sand body, has a complex stratigraphy because it represents a series of individual sand bodies deposited during and since the Late Wisconsinan.

The SISB consists of two seismic sequences separated by transgressive unconformity, the regional RO reflector. The lower sequence 1 sand body of Late Wisconsinan age, is a poorly sorted fine to coarse-grained sand with silt, which is acoustically amorphous and associated with large-scale sub-glacial channels restricted to the N of Sable Island. The overlying sequence 2 sand body, of Holocene age, is a loose, clean, well-sorted medium to coarse-grained sand with distinct foreset reflectors.

The last ice advance and retreat over Sable Island Bank, between 30 - 11 ka, provided sand-sized sediments in an ice-marginal and proglacial subglacial channels up to 100 m deep and 3 km wide, depositing a 40 m thick sand body on the central area of Sable Island Bank. Terminal positions of the subglacial channels interpreted to represent the location of the ice margin, suggests that the last Late Wisconsinan ice extended as far as Sable Island.

At Northern Spur, a 33 m thick deltaic unit lies stratigraphically above the subglacial sequence. The location of this unit, together with eastward dipping foreset reflectors suggests that the sequence was deposited within a proglacial delta complex during a predominantly northwest to westerly retreat of ice.

The ice-marginal 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 on the bank from at least 49 m bsl during the past 11 ka in response to ice retreat. The transgressive reworking of underlying Pleistocene sand and silt by shoreface erosion, longshore transport and storm and tide-generated currents, further sorted and transported the sediment N and NE into coarsening-upward sequences located on Sable Island and East Bar.

The SISB is the sedimentary record of the Late Wisconsinan glacial event and consequence sea level rise during and after glacial retreat on the outer Scotian shelf. It also illustrates the importance of glacial sediment supply and sea level fluctuations to large-scale sand body formation of good reservoir quality within a glaciated outer continental shelf setting.

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Supervisor: Ron Boyd