DEPOSITIONAL ARCHITECTURE OF COARSE SAND BEACH RIDGES AT COWLEY BEACH, NORTHEASTERN QUEENSLAND, AUSTRALIA

Processes associated with tropical cyclones are thought responsible for building coarse sand beach ridges along the northeastern Queensland coast, Australia. While these ridges are expected to be geological records of the past cyclone, they question the general consensus of the aeolian genesis of sandy beach ridges. We carried out the GPR survey, auger drilling, pit excavation, grain-size analysis, and OSL dating for coarse sand beach ridges at the Cowley Beach, NE Queensland to explore the ridge-forming process. The Cowley Beach is a mesotidal beach characterized by a low-tide terrace and steep beach face. Ten beach ridges are recognized along the survey transect that extends 700 m inland from the shore. 37 OSL ages are younger seawards, indicating the seaward accretion of the ridge sequence over the last 2700 years. The highest ridge is +5.1 m high above AHD (Australian Height Datum), and has OSL ages of c. 2000 years. Two GPR units are bounded by a groundwater surface at c. +1.5 m AHD. The upper unit is characterized by horizontal to sub-horizontal reflectors punctuated by seaward dipping truncation surfaces. Horizontal to hummocky reflectors in places form dome-like structure that appears to be the nucleus of a beach ridge. The shape and level (+2.5 m AHD) of the dome are similar to those of the present swash berm. The lower unit shows a sequence of reflectors that dip at an angle of present beach face. The sequence is dissected by truncation surfaces, some of which are continuous to those in the upper unit. Coarse sand mainly forms beach ridge deposits below +4.0 m AHD, while a few higher ridges have a upward fining layer composed of medium sand above +4.0 m. The Cowley Beach, like other many sandy beaches, has prograded during onshore sand accretion by fairweather waves and has been eroded by storms waves. Increased water level and high waves associated with tropical cyclones would be responsible for ridge building between +2.5 and +4.0 m AHD. Astronomical tide should be critical rather than rare, intense cyclones for frequent coastal inundations up to +4.0 m AHD, just 1.5 m higher than the high-tide swash limit. A few beach ridges higher than +4.0 m AHD suggest the possible contribution of aeolian sand to emplacing the uppermost finer layer, suggesting the ridge height is so far not a reliable direct indicator of the coastal inundation level.