Storm Swash Terraces: A Previously Overlooked Element of the Cliff-Shore Platform System

Abstract

Storm swash terraces are a distinctive and previously unreported component of the cliff-shore platform sedimentary system. They consist of laminated marine sands and gravels with occasional large, rounded clasts and fine muddy laminae. The planar deposits dip gently seaward, are up to 1 m thick at the seaward margin, and thin landwards, forming a wedge. They are laterally continuous over tens of meters in a coast-parallel dimension. They occur in coastal re-entrants at the rear of boulder beaches and shore platforms at elevations between 1 and 9 m above contemporary high-tide level. The terrace sediments represent deposition at the limit of wave run-up during extreme storms; during such events wave energy is largely dissipated by a shore platform and/or boulder beach but sufficient energy remains to carry water and fine sediment to higher levels, where it then accumulates. The deposition and preservation of storm swash terraces depend on a coastal topography that combines partial wave-energy dissipation on a shore platform or boulder beach with available depositional space to landwards. These deposits contain a sedimentary record of past storminess on high-energy coasts that has yet to be exploited.