SEDIMENTOLOGIC ANALYSIS OF COASTAL BOULDER DEPOSITS IN THE NORTHEAST ATLANTIC REGION

Coastal boulder ridges are widespread in the NE Atlantic region. As we unravel the roles of storms vs. tsunami in assembly of high-energy deposits world-wide, it is helpful to have data from areas subject to big storms, but without recent tsunami. We therefore report the morphology and sedimentology of active boulder ridges from the Aran Islands and Mullet Peninsula (western Ireland) and Mainland Shetland (Scotland). Topographic analysis is based on 165 ridge transects, and grain-size data come from 54 ribbon counts at a subset of those sites.

The deposits—asymmetric coast-parallel ridges, each with a steeply dipping seaward face and a longer, gently-sloping landward side—occur up to 50 m above high water (AHW), with 13 m AHW the median elevation. They are found up to 260 m horizontal distance inland from high water, the median being 43 m inland. Typically the area seaward of the ridge is scoured clean of both clasts and vegetation. Massive clasts occur in the ridges—blocks up to 75 t were measured—but the median is in the coarse pebble to fine boulder range (4-39 cm equivalent spherical diameter). Grain-size analysis (from ribbon counts of all clasts >1 cm along 54 ridge-normal transects) reveals that the deposits are poorly to moderately-well sorted, with median standard deviation in the poorly-sorted range. Most ridges have unimodal clast-size distributions.

Ridge height, width, and largest clast size tend to decrease with elevation, but—surprisingly—those farther inland tend to be bigger, with larger clasts. Trends are noisy, but all are significant at the 0.0001 level. Slope is a strong control on ridge morphology and clast size, with the largest boulders and the biggest deposits accumulating in locations with low elevation:inland-distance ratios. When coastal slope is sufficiently low, large deposits can accumulate far inland: for example, at locations with slopes less than 0.1, ridges were found up to 260 m inland. Ridges 250-260 m inland were 3-4 m high and 30-35 m wide, with clasts up to 6 t.

This is, as far as we know, the first such dataset to have been collected. Interest in these kinds of deposit is growing, as we try to understand the ways in which high-energy waves interact with coasts and coastal materials; and this study provides baseline data with which coastal boulder deposits from other locations can be compared.