Boulder emplacement on a tectonically stable, wave-dominated coastline, Mission Rocks, northern KwaZulu-Natal, South Africa

The presence of large boulders along rocky coastlines is a useful indicator as to the occurrence and nature of past storms and/or tsunami. Field observations from Mission Rocks, northern KwaZulu-Natal, South Africa allowed for the assessment of historical high-magnitude wave events in the area. A total of 143 anomalously large boulders were assessed in terms of their dimensions, morphologies and distribution along a regionally developed + 3 m raised shore platform. Based on long–intermediate- and short-axes relationships, we calculate boulder masses of up to 9.03 t that were found to have been transported horizontally shoreward for up to 65 m. Based on geomorphological evidence, we infer that the majority of boulders encountered were from a joint-bound pre-transport setting. Additionally, overturning of the boulders was apparent. We assess the boulder field numerically via the numerical model of Pignatelli et al. (2009) which we deemed most applicable for slab-like boulders such as those encountered here. Results yielded a maximum breaking storm wave height of 11.36 m or a maximum tsunami height of 2.84 m. An analysis of swell data revealed that the modelled storm wave heights fell within the range defined by the storm wave records for the area within the past 32 years. Boulder alignments were likewise compatible with the wave regime of the northern KwaZulu-Natal coastline. Additional review of offshore seismic records and landslide hazards on the tectonically stable passive margin show that the tsunami hazard for landslides within the area is negligible. The findings suggest a storm provenance as the causal mechanism for boulder emplacement at Mission Rocks.

► We analyse slab-like boulders from the tectonically stable KwaZulu-Natal margin.
► Equations that weight the c-axis best fit observed storm wave data.
► We show that such deposits are a likely product of storm waves.
► Based on shape, such boulder deposits can easily accrue on stable margins.