Distribution, geomorphology and lithofacies of cliff-top storm deposits: Examples from the high-energy coasts of Scotland and Ireland

摘要

Cliff-top storm deposits (CTSDs) occur on cliffs at elevations of up to 50 m above sea level at exposed sites on the deep-water coasts of the British Isles. This study examines the distribution, geomorphology and lithofacies of CTSDs at sites from Shetland, Orkney, Caithness and the Outer Hebrides in Scotland and from the Aran Islands in Galway Bay in Ireland. CTSDs are generated largely by the quarrying of blocks from the cliff top, and transported by green water bores across cliff-top platforms and ramps to be deposited in backing ridges or as debris spreads. Maximum boulder sizes reach 48 m~3 but it is likely that much larger blocks can be quarried, prior to disintegration during transport. Eye-witness accounts and field mapping demonstrate that formation and modification of CTSDs has continued during major storms over recent decades. Recent CTSDs bury a range of man-made debris but older deposits lack this and instead show weathering effects that indicate a longer residence time. In Shetland OSL dates on intercalated sands suggest that the oldest CTSDs date from approx 800 AD. Radiocarbon dates on shell buried inside CTSD ridges relate to major storms between 1700 and 1900. CTSDs represent an overlooked archive of storm sedimentation that has great potential for the elucidation of local storm chronologies. CTSDs also provide evidence of wave impacts on any part of the cliff face or top, in fundamental contrast to the concentration of wave action at the cliff foot implicit in traditional models of the erosion of rock coasts.