Past relative sea-level indicators in coarse-grained paraglacial coastal systems from Eastern Canada

Coastlines in eastern Canada are dominantly paraglacial, meaning that the geomorphology and morphodynamics of the coasts are largely governed by the presence of glacigenic deposits related to the Laurentide Ice Sheet. These deposits act as antecedent topography and sediment sources for these coastal systems. Chedabucto Bay (Nova Scotia, Canada) is site to uniquely preserved drowned barrier beach features, which are hypothesised to form through shoreline retreat and barrier overstepping. In this process, relative sea-level (RSL) rise forces the barrier to migrate landward, then, when the conditions allow, to be preserved in place. Due to the nature of these paraglacial beaches to organize into coarse clastic barriers, they are quite resistant to shoreline migration through wave action thus have a bias towards overstepping when compared with sandier systems. This high degree of preservation is useful for reconstructing the post-glacial sea-level history as these features are good indicators of past RSL. Here we investigate the external morphology and internal architecture of modern and drowned barrier-beach systems by using ground-penetrating radar and LiDAR, for the former, and multibeam bathymetry and seismic reflection data, for the latter, to study their differences. Offshore seismic mapping has revealed buried barrier-beach systems at ~46 metres below present-day sea level and former paleo-estuaries dated to have formed by 10.51 ka cal BP. Preliminary morphometric analysis of drowned barrier systems indicate maximum berm heights of ~5 m from toe of slope, which is comparable to those observed in the modern system. The height of the modern barrier systems varies alongshore depending on beach aspect and the dominant direction of currents and waves (drift-aligned versus swash aligned systems). Recently surveyed paraglacial barrier elevation data along northern Chedabucto Bay show a ~3 metre difference in maximum berm crest elevation between swash and drift aligned systems. With this we emphasize the need to differentiate the expected indicative range for these relict paraglacial RSL indicators based on surficial morphology and internal geometries known from modern systems. The results of this work will help inform and guide science and policy on managing shoreline retreat through overstepping and help in characterizing sediment type distribution in coastal-shallow marine paraglacial environments.