Breaking up and making up – reworking of Holocene calcarenite platform into rapidly‐forming beachrock breccias on a high energy coastline (St. Lucia, South Africa)

Abstract

Beachrocks are a common characteristic of tropical and subtropical coastlines. It is known that they have a substantial influence on beach morphodynamics and they are commonly utilised as indicators of palaeo-sea levels. At the same time, facies variability in beachrocks is understudied and their effect on shoreline evolution is poorly understood. At Mission Rocks on the KwaZulu-Natal coastline of South Africa a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of disintegrating into megagravel deposits through chemical and mechanical weathering in a wave-dominated, high-energy setting. The breakdown is potentially slowed by a contemporary, fast-forming beachrock facies, that blankets the surface and fills fractures and potholes within the older platform. The accumulation and cementation of this recent beachrock is the focus of this study. The beachrock is dated by historical evidence to post-World War II. Data from field observations, petrographic and geochemical methods reveal that the cementing agents of the beachrock were precipitated from marine water in a phreatic setting despite its position above the intertidal zone. Not only does this facies have implications for the interpretation of palaeobeachrock as a sea-level indicator, it also raises further questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock.

Anomalously large marine potholes on a submerged relict shore platform: The Eastern Cape shelf of SE Africa

“…In subtropical climates, rapid cementation of the abraders at the base of the pothole may occur (Cooper et al, 2016;Falkenroth et al, 2022). This creates a hardened layer at the base of the pothole, binds the grinding agents in place and reduces grinder movement, thus reducing the efficacy of the pothole growth both laterally and vertically.…”

Coastal adaptations on the eastern seaboard of South Africa during the Pleistocene and Holocene? Current evidence and future perspectives from archaeology and marine geology

“…Beachrocks are lithified beach sediments cemented by high-Mg calcite precipitated in the mixing zone of seawater and meteoric waters (Mauz et al, 2015). They are known to form particularly rapidly along the KZN coastline, often within a span of 80 years or less (Wiles et al, 2018;Falkenroth et al, 2022). The coastal aeolianites of KZN formed through the precipitation of low-Mg calcite from CO 2 -saturated meteoric waters in coastal dunes .…”