Erosion of rocky shore platforms by block detachment from layered stratigraphy

Danielle H Buchanan,Wayne J Stephenson,Larissa A Naylor,Martin D Hurst

doi:10.1002/esp.4797

Danielle H Buchanan, Wayne J Stephenson + Show 2 more

Open Access

https://doi.org/10.1002/esp.4797

Copy DOI

Abstract

AbstractThe majority of shore platforms form in rocks that are characterised by layered stratigraphy and pervasive jointing. Plucking of weathered, joint and bed bounded blocks is an important erosion process that existing models of platform development do not represent. Globally, measuring platform erosion rates have focused on microscale (< 1 mm) surface lowering rather than mesoscale (0.1‐1 m) block detachment, yet the latter appears to dominate the morphological development of discontinuity rich platforms. Given the sporadic nature of block detachment on platforms, observations of erosion from storm event to multi‐decadal timescales (and beyond) are required to quantify shore platform erosion rates. To this end, we collected aerial photography using an unmanned aerial vehicle to produce structure‐from‐motion‐derived digital elevation models and orthophotos. These were combined with historical aerial photographs to characterise and quantify the erosion of two actively eroding stratigraphic layers on a shore platform in Glamorgan, south Wales, UK, over 78‐years. We find that volumetric erosion rates vary over two orders of magnitude (0.1‐10 m3 yr‐1) and do not scale with the length of the record. Average rates over the full 78‐year record are 2‐5 m3 yr‐1. These rates are equivalent to 1.2‐5.3 mm yr‐1 surface lowering rates, an order of magnitude faster than previously published, both at our site and around the world in similar rock types. We show that meso‐scale platform erosion via block detachment processes is a dominant erosion process on shore platforms across seasonal to multi‐decadal timescales that have been hitherto under‐investigated. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Highlights

Rock coasts are characterised by the erosion of bedrock at the coast (Kennedy et al, 2014a)
Site A The change in extent of the stratigraphic layer at site A is shown in Figure 4, which demonstrates persistent erosion across the 78-year analysis
The results of our quantitative, volumetric meso-scale erosion rate study demonstrate that block detachment processes dominate over gradual surface lowering over a 78-year period, on the Glamorgan coast

Summary

Introduction

Rock coasts are characterised by the erosion of bedrock at the coast (Kennedy et al, 2014a). Processes of rock decay operating at the microscale (mm-cm) (e.g. salt weathering, surface swelling, and bioerosion) facilitate gradual surface lowering at the microscale (Stephenson and Kirk, 2000; Porter et al, 2010; Coombes and Naylor, 2012). These micro-scale processes are are important preparatory mechanisms enabling meso-scale (cmm) erosion to occur, when focused at sites of weakness such as joints or bedding, helping to prepare the rock for subsequent detachment (Naylor et al, 2012). Quantifying platform erosion rates by a variety of processes, requires measuring changes across a range of temporal scales, from several years to decades (Stephenson et al, 2012), or even centennial-millennial timescales inferred from cosmogenic nuclides (Choi et al, 2012; Regard et al, 2012; Hurst et al, 2016; Trenhaile, 2018)

Results

Discussion

Conclusion