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

Abstract

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 >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3Á5 to 5Á1 m 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 +2Á5 m 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 +2Á5 m 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.

Coastal ridge constructive processes at a multi‐decadal scale in Barreta Island (southern Portugal)

“…Instead, they rely on dating techniques and on well-known climatic episodes to establish a one-toone relation between climate and ridge morphology (e.g. Alternative approaches focused on the identification of erosional and depositional surfaces, left by major storms and identified in the sediment record by the presence of contrasting textures, compositions and stratigraphical characteristics (Buynevich et al, 2007;Tamura, 2012;Nott et al, 2015;Tamura et al, 2018). Lampe and Lampe (2018) found a correlation between shoreline progradation rates, ridge height and the North Atlantic Oscillation (NAO) in a German beach ridge plain.…”

Reply to the Discussion by John Nott on ‘Coarse‐sand beach ridges at Cowley Beach, north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history’ by Tamura et al. (2018), Sedimentology,65, 721–744

“…Although discussion was exchanged between Tamura (, ) and Nott () mainly regarding the presence of the aeolian sand contribution to beach‐ridge formation in this area, Tamura et al . () did not aim to criticize Nott et al . (), who considered beach‐ridge genesis at Cowley Beach with limited stratigraphic data, but to understand these unique coastal landform features within a broader framework of coastal geomorphology and sedimentology by taking advantage of valuable observations after the extraordinary tropical cyclone Yasi in 2011 (The State of Queensland, ; Nott et al ., ).…”

“…Tamura et al . () presented new stratigraphic information that refines our understanding of the processes of formation of coarse‐sand beach ridges at Cowley Beach, north‐eastern Australia. Beach ridges here have developed several metres higher than mean sea level (=0 m AHD, Australian Height Datum) and record progradation of the coast over approximately the last 7 ka.…”

“…Tamura et al . () integrated ground‐penetrating radar (GPR) surveying, trenching, auger coring, grain‐size analysis, high‐resolution optically‐stimulated luminescence dating and beach morphology characterization to clarify the compound formative processes involved in the deposition of these beach ridges, and examined their utility as records of tropical cyclone history. Although discussion was exchanged between Tamura (, ) and Nott () mainly regarding the presence of the aeolian sand contribution to beach‐ridge formation in this area, Tamura et al .…”

“…Tamura et al . () presented in unprecedented detail the complexity of ridge deposition, where each ridge is a compound feature composed of wave‐built sediments, aeolian accretion and tropical cyclone inundation deposits (Fig. ).…”