Tsunami or storm? A high-level coastal boulder field on the southern tip of Eyre Peninsula, South Australia

Bourman R.P., Murray-Wallace C.V., Panda D., Buckman S., Banerjee D., Ryan D.D. & White L.T.
Australian Journal of Earth Sciences
71(1): 52–66
A high-level coastal boulder field at Whalers Way, southern Eyre Peninsula, South Australia, occurs at elevations of 20–30 m above present sea-level on a gently seaward-sloping, karst-weathered calcretecapped structural bench, formed on the Upper Pleistocene Bridgewater Formation and underlying Carnot Gneisses (Archean–Paleoproterozoic Sleaford Complex). More than 1000 ex situ boulders, cobbles and pebbles of gneiss and mafic igneous rocks, as well as fragments of calcrete, occur within the boulder field and cover an area >6000m2. All the crystalline rock clasts are derived locally from bedrock outcrops along the adjacent coastline, where metasedimentary granulite-facies of the Carnot Gneisses crop out along shore platforms and their backing cliffs, close to present sea-level. The boulder field is younger than Marine Isotope Sub-Stage 5a (MIS 5a; ca 80 ka) based on amino acid racemisation ‘whole-rock’ analyses of calcarenite of the Bridgewater Formation, which forms the calcrete bench on which the boulder field rests. The boulder field wraps around the western extremity of clifftop dunes dated at 18–17 ka by optically stimulated luminescence, implying that the boulder deposit post-dates the formation of the cliff-top dunes. Given that the clasts in the boulder field show minimal weathering rinds, sea-level during the Last Glacial Maximum was some 125 m lower than present and that between 80 and 7 ka ago, sea-level never attained present levels within the region, emplacement of the boulder field by a high-energy wave event in the Holocene highstand, following the 7000 years BP culmination of post-Glacial sea-level rise, is suggested. Storm waves, rather than a tsunami, most likely explain the emplacement of the boulder field. KEY POINTS 1. Amino acid racemisation and optically stimulated luminescence dating suggest a Holocene age younger than 7000 years for a high-level (20–30 m above present sea-level) coastal boulder field at Whalers Way, Eyre Peninsula, southern Australia. 2. Gravity-influenced sedimentation, tectonic uplift and glacial action (relict Permian or Cretaceous glacigene sediment) fail to explain the origin of the enigmatic boulder deposit. 3. The boulder deposit formed by storm waves, rather than a tsunami origin, following the 7000 year BP culmination of post-Glacial sea-level rise and onset of the Holocene sea-level highstand. Keywords: Whalers Way; Eyre Peninsula; Carnot Gneisses; coastal boulder field; storm waves; tsunami; Bridgewater Formation; Last Glacial Maximum; Holocene highstand.
Monday, 15 April 2024 17:08