ABSTRACT: Holocene transgression and coastal retreat across a low-relief shelf, north shore of Prince Edward Island, southern Gulf of St. Lawrence, Canada.

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The North Shore of PEI consists of low sandstone headlands with a thin cover of glacial ice-contact or outwash deposits and intervening dune-capped sandy beaches and barriers protecting large estuarine embayments with tidal inlets. Multibeam bathymetry and backscatter, sidescan sonar, and seismic reflection surveys supplemented by ROV video show that the valleys occupied by present-day estuaries extend 10 km or more across the shelf to at least 35 m water depth. As-yet undated rhythmites of possible lacustrine origin occupy a closed basin at that depth, but the succession in most of the valley depressions consists of interbedded organic-rich muds and sands, interpreted as estuarine facies, directly overlying the subaerial unconformity on late-glacial diamict. These deposits are truncated by the transgressive marine unconformity, which is overlain in most cases by sand typically less than 0.5 m thick. Vibracores contain finely laminated to massive estuarine silty clays with Zostera marina L. fragments and a sparse molluscan fauna, interbedded with sands interpreted as tidal inlet and flood-delta facies. Postglacial marine regression beginning below present sea level on the central North Shore continued to approximately 8.5 ka, after which RSL rose at rates as high as 12 mm/a from a lowstand below -40 m to reach -20 m shortly before 6000 cal years BP. Pitted estuarine deposits of this age, 3 to 5 km seaward of the present coast in 20 m of water, provide localized roughness attracting concentrations of fish and attest to long-term coastal retreat averaging >0.5 m/a over 6000 years. The rate of RSL rise during this time decelerated to ~2 mm/a before rising to 3.2 mm/a (~0.3 m/century) from 1911 and 1998. Despite abundant sand at the present coast, the inner shelf is sand-starved. Most sand keeps pace with the transgression and is stored in flood-tidal deltas and large coastal dunes. Digital elevation models derived from airborne topographic LiDAR and other data provide a basis for calculation of coastal sand budgets. The multiple nearshore bar system is sand-starved with erosional lag surfaces over glacial deposits in the bar troughs. Shoreface sands are confined within subtle embayments in quasi-conical wedges thinning seaward from the outer bar to a sand veneer over channel-fill sediments or terminating against a gravel lag surface.

Bibliography of Canadian Geomorphology