ELSEVIER Marine Geology 124(1995) 187-201 Differential preservation of coastal structures on paraglacial shelves: Holocene deposits of southeastern Canada D.L. Forbes, J. Shaw, R.B. Taylor zyxwvutsrqponmlkjihgfedcbaZYXWV Geological Survey of Canada, Bedford Institute of Oceanography, P. 0. Box 1006, Dartmouth, N.S. B2 Y 4A2, Canada Received 1 March 1994; revision accepted 7 July 1994 Abstract Recent work has advanced our understanding of mixed sand-gravel littoral systems in glaciated regions. New insights include the nature and causes of coastal instability and conditions for preservation of Holocene barrier deposits on paraglacial shelves. The best-preserved relict barrier structures recognized on the inner shelf of southeastern Canada are interpreted as postglacial lowstand deposits, initiated under stable to slowly rising relative sea level in areas of substantial shelf relief. A largely intact early-Holocene gravel foreland survives at 38 m present water depth, its preservation attributed in part to sediment volume and a partially protected location. The mechanism by which this structure was abandoned and overstepped is unclear, but may be related to accelerating sea-level rise. In some cases, barriers keep pace with sea-level rise for some time before being overstepped or reworked landward. The duration and extent of preservation depend in part on the deposit volume, as smaller accumulations are consumed and reworked more quickly. Deposit volume is a function of sediment supply, accommodation space, and headland spacing, among other factors. Where barriers are small and thin, beach sediments, overtaken by the transgression, may be smeared across the shoreface in a thin post-transgressive veneer. In such settings, as along the Eastern Shore of Nova Scotia, the only coastal units preserved on the shelf are back-barrier estuarine facies in flooded-valley depressions. Differential preservation is governed by shelf relief and accommodation space, coastal alignment and compartmentalisation, sediment supply and barrier volume, rate and acceleration of sea-level rise, wave energy and potential for sediment reworking on the inner shelf. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE 1. Iutroduction Southeastern Canada (Fig. 1) is a formerly glaciated region with a preponderance of mixed sand-gravel and gravel beaches largely derived from glacigenic deposits (Forbes and Taylor, 1987; Shaw et al., 1990; Forbes and Syvitski, 1994). Postglacial changes in relative sea level are known to have varied widely across the area in response to regional differences in the magnitude and timing of glacial- and hydro-isostatic loading, among other factors. While global sea level rose rapidly from about 115 m present water depth circa 16,000 yrs B.P. to < 10 m by 6000 yrs B.P. (Fairbanks, 1989), much of 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDI0025-3227(95)00040-2 southeastern Canada and adjacent regions experi- enced falling relative sea levels in early postglacial time, prior to about 9 ka (e.g. Scott and Medioli, 1980; Brookes, 1987; Belknap et al., 1987; Grant,, 1989; Shaw and Forbes, 1990a; Forbes et al., 1991a). Following an early-Holocene lowstand (e.g. Shipp et al., 1991; Forbes et al., 1993; Stea et al., 1994), relative sea levels have been rising in most parts of the region through much of the Holocene (Fig. 2), continuing to the present day (Shaw and Forbes, 1990a; Shaw et al., 1993), though some parts of eastern Newfoundland show evidence of more stable sea levels during the past 2-3 kyr B.P. (Shaw and Forbes, 1990b).