Coarse-sand beach ridges at Cowley Beach, north-eastern Australia: Their formative processes and potential as records of tropical cyclone history

Storm surges generated by tropical cyclones have been considered a primary process for building coarse-sand beach ridges along the north-eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high-resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso-tidal beach system with a >3m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700years. The ridge crests sit +35 to 51m above Australian Height Datum (ca mean sea-level). A ground-penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +25m Australian Height Datum, equivalent to the fair-weather swash limit during high tide. The lower facies comprises a sequence of seaward-dipping reflections. Beach progradation thus occurs via fair-weather-wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair-weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +25m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse-sand ridges at Cowley Beach are thus products of fair-weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.