The sedimentology of coastal boulder deposits (CBD) is little known. These deposits, and especially the largest clasts within them—which commonly weigh many 10s of tonnes—are of increasing interest because they attest to extreme energy wave events acting well above the high-tide mark (in some cases >40m above high water and >100m inland). Recent observations have made clear that storm waves (rather than tsunami) are important in both creating and activating these deposits; but beyond that we know little about their hydrodynamics, or the relative frequency of boulder movement, and how this might vary with topographic setting.Roundness is a first-order characteristic that preserves a record of deposit reworking. It can tell us whether clasts move around much after they have been created from bedrock, or whether they are emplaced into CBD with little or no subsequent transport. But because roundness has historically been difficult to measure, most roundness estimates in the literature are based on qualitative visual assessment, and are therefore of limited analytical value. Fully reproducible roundness measurements are possible, however, and in the case of boulders they can be acquired quite easily using the Kirkbride (2005) device in combination with a trigonometric formula presented here.CBD at locations in Ireland and Scotland provide a test case, demonstrating the utility of quantitative boulder roundness measurements. The 567 clasts we measured are generally angular, but a substantial proportion shows some degree of rounding. Most importantly, there are demonstrable, consistent patterns relating topography and roundness. There is a strong and unsurprising inverse correlation between clast roundness and deposit elevation. There is no first-order relationship between roundness and inland distance, but there is a power-law inverse relationship between rounding and steepness (ratio of elevation to distance inland). The greatest degree of rounding is found in CBD located at the back of gentle coastal platforms with gradual slopes. These relationships provide new insights into CBD sedimentology, which we hope will be probed and tested in future studies.
Marine Geology – Elsevier
Published: Feb 1, 2018
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