Submerged beachrock preservation in the context of wave ravinement

• Published in: Geo-marine letters 2018-02, Vol.38 (1), p.19-32
• Main Authors: Pretorius, Lauren, Green, Andrew N., Andrew Cooper, J.
• Format: Article
• Language: English
• Subjects: Abrasion; Backscattering; Beachrock; Coastal zone management; Coasts; Comminution; Disintegration; Earth and Environmental Science; Earth Sciences; Echo sounding; Echosounding; Elongation; Geology; Gravel; Gullies; Holocene; Limestone; Offshore; Original; Outcrops; Plucking; Preservation; Reefs; Sea level; Sea level rise; Seismic surveys; Shallow water; Shorelines; Slope; Storm surges; Storms; Stratigraphy; Submerged shorelines
• ISSN: 0276-0460; 1432-1157
• DOI: 10.1007/s00367-017-0503-9

This study examines a Holocene-aged submerged shoreline, Limestone Reef, located in the shallow subtidal zone of South Africa’s east coast. It comprises an elongate, coast-oblique, slab-like outcrop of beachrock situated above the contemporary fair-weather wave base. It is currently undergoing mechanical disintegration. Its unique and rare preservation in a high-energy setting affords an opportunity to examine the mechanical processes occurring during wave ravinement associated with rising sea level. The submerged shoreline and the adjacent shoreface were examined using high-resolution seismic reflection, side-scan sonar and shallow-water multibeam echosounding techniques. Limestone Reef rests on top of unconsolidated Holocene deposits. The structure’s surface is characterised by reef-perpendicular gullies with rubble derived from the slab fringing its seaward edge. Limestone Reef slopes gently seawards and has a steep landward-facing edge where gullies are most prominently developed. Teardrop-shaped rippled scour depressions, marked by high backscatter, are located seawards of the submerged shoreline. These elongate in a seaward direction and are filled with bioclastic gravels and residual rubble from Limestone Reef. The gullies in the upstanding structure are indicative of wave plucking and abrasion of the shoreline. The material exposed by the rippled scour depressions is identical to that comprising the postglacial ravinement surface identified in the offshore stratigraphy. These deposits are considered to represent the contemporary, actively forming wave ravinement surface. The results suggest that wave ravinement of submerged shorelines is a discontinuous process dominated by the seaward entrainment of material from its landward edge controlled by high-energy drawback during storm surges. The ravinement process appears to operate at the seasonal scale and averages out over the long-term millennial scale for the continuous surface.