Beached above Cape Town: well-worn cobbles talk

To understand syn-sedimentary wave-energy conditions, past coastal dynamics, and sea-level changes, this field-based study examines high-energy marine sediments of presumed Pliocene age around Cape Town, South Africa. Although these deposits were mapped over a century ago, they have not undergone modern sedimentological analysis. The deposits consist of rounded, moderately well-sorted orthoquartzite clasts ranging from cobble to boulder size (>3 m in diameter). The presence of percussion marks indicates significant sediment reworking by intense waves. Clast characteristics—size, sorting, roundness, and composition—across four locations also reflect devastatingly intense waves in powerful, multi-event storms rather than single catastrophic events like tsunamis. This interpretation aligns with evidence that southwestern Africa’s passive continental margins have been storm-dominated in post-Miocene. These insights into Cape Town's coastal dynamics, which suggest erosion of local rocky shores during "super storms," are consistent with global high-energy beach processes in the Pliocene—a period characterized by elevated global temperatures, frequent intense storms, and high sea levels. The findings refine our understanding of how past high-energy marine events shaped shorelines and provide an analogue for the impact modern sea-level rise. Accurate age dating is essential for reliably correlating these sediments with global Pliocene deposits and for reconstructing the post-Miocene shoreline history. Regardless of their exact age, the sedimentological properties and stratigraphic position of Cape Town's fossil beaches indicate that during their formation, the local sea level was up to 30 m higher than today, with the deposits having been reworked and transported in powerful marine events, which are often linked to increasing global temperatures that trigger glacio-eustatic sea level rise.