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

Michaela Falkenroth, Andrew N. Green, Andrew Cooper, Manuel Menzel, Goesta Hoffmann

Research output: Contribution to journalArticlepeer-review

Abstract

Beachrocks are a common characteristic of tropical and subtropical coastlines. It is known that they 20 have a substantial influence on beach morphodynamics and they are often utilised as indicators of 21 palaeo sea levels. At the same time facies variability in beachrocks is understudied and their effect on 22 shoreline evolution is poorly understood. At Mission Rocks on the KwaZulu-Natal coastline of South 23 Africa a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m24 thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of 25 disintegrating into megagravel deposits through chemical and mechanical weathering in a wave26 dominated, high-energy setting. The breakdown is potentially slowed by a contemporary, fast-forming 27 beachrock facies, that blankets the surface and fills fractures and potholes within the older platform.
28 The accumulation and cementation of this recent beachrock is the focus of this study. The beachrock 29 is dated by historical evidence to post-WWII. Data from field observations, petrographic and 30 geochemical methods, reveal that the cementing agents of the beachrock were precipitated from 31 marine water in a phreatic setting despite its position above the intertidal. Not only does this facies 32 have implications for the interpretation of paleo beachrock as a sea level indicator, it also raises further 33 questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock.
Original languageEnglish
JournalSedimentology
Early online date27 Oct 2021
DOIs
Publication statusE-pub ahead of print - 27 Oct 2021

Keywords

  • Beachrock
  • South Africa
  • Coastal Geology
  • Geomorphology
  • Holocene
  • Sea-Level
  • Facies Analysis

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