Abstract

Storm swash terraces mark the most distal, landward deposition of storm wave events on high-energy rock coasts. They form a potential, but as yet unexploited, sedimentary archive of past storminess. This paper describes storm swash terraces at Morgan Bay, Eastern Cape, South Africa. They form as steeply seaward dipping (30–60°), partially vegetated terraces comprising alternating beds of organic-rich sand, mud, and gravel. The terraces are up to 5 m thick, and the basal elevations range from 0.5 m amsl in the embayment, to 6.5 m amsl on the headland margins. Regardless of location, they are always fronted by a rocky shore platform on which most storm wave energy is attenuated. Six sedimentary facies are delineated on the basis of their lithological and paleontological characteristics and the nature of each bounding surface.

Deposits in the most exposed headland locations are thicker and coarser-grained than those formed in the embayment. Composition of the depositional terrace reflects the fronting shore platform, indicating a short source-to-sink transport distance. The thickest storm deposits form where the backing topography is high and accommodation is limited. Storm swash terraces have a high preservation potential with successive storms depositing material in an aggradational manner: reworked material is incorporated into subsequent depositional units. The spatial controls on facies partitioning in each terrace appear to be a function of position in the embayment, the availability of material to seaward, and the presence of accommodation space. Areas of particularly limited accommodation space produce aggradational storm sequences that form especially good archives of marine storminess.