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RIVER OF SAND – A GEOLOGICAL PERSPECTIVE ON THE EVOLUTION OF
RIVER OF SAND – A GEOLOGICAL PERSPECTIVE ON THE EVOLUTION OF
Abstract for: Fraser Island Defenders Organisation, A 2020 Vision for Fraser Island: A new focus for the Great Sandy World Heritage Area Conference, Noosa, 11-13 August 2004 with optional Field Trip on 14th RIVER OF SAND – A GEOLOGICAL PERSPECTIVE ON THE EVOLUTION OF FRASER ISLAND AND SURROUNDING SEABED Ron Boyd, Ian Goodwin, Kevin Rum ing and Shannon D avies Earth and Ocean Science Group Earth Sciences, University of Newcastle, NSW, 2308. Fraser Island consists of 113 cubic kilometers of sand above sea level. The Fraser system (Fraser Island plus B reaksea Sp it and Sh oal to a depth of 22 m below sea level) contains a total of 170 cub ic kilometers o f sand (170 ,000,000 ,000 cub ic meters). From a geological perspective it is impo rtant to id entify w here this sand came from, how the island and its surrounding seabed evolved, what the current behaviour of the sand is, and what its future development will be. From the general character of sediment transport on the eastern Australian coast, it seems reasonable to assume that virtually all the sand in the Fraser system originated from further south and was su pplied by long shore tran sport processes. This is due to the dominant south- east wave ap proach direction along a ll of SE Au stralia. Th e sand was m ainly d erived fro m sources in NSW such as the Blu e Mou ntains , and G reat Div iding R ange c atchm ents of rive rs such as the Hawkesbury, Hunter, and Clarence. Littoral drift in the lon gshore transport system is the only feasible s ource for the large coastal sand deposits of sou th-east Qu eensland . Sand is currently being transported northwa rd from NS W to the Queensland Gold Coast and beyond at rates of 500,000 cubic meters per year. Similar volumes are likely to have occurred in the past as far north as the F raser coast, giving a minimum age for the Fraser system of over 340,000 years. This transport has occurred over many sea level cycle s, and sin ce transp ort is unlikely to be as effective at low sea levels, the age of Fraser Island is probably much older than 340,000 years. This volume estimate correlates well with age estimates from uranium series dates on Fraser Island lakes of over 350,000 years, and thermoluminescence dates for the Cooloola Sand s of 730,000 years. Previous models and recent marine research indicate that Fraser Island and associated features are probably the result of sand accumulation during multiple emplacement cycles. During each sea level fall and at each sea level lowstand, sand was transported northward by wave-driven longsh ore transpo rt and sto red in be ach ridg e plain s, dune fields an d tidal d eltas. Vegetatio n helped stabilize these sand deposits o n th e exposed continental shelf. During each sea level rise, the shoreline migrated landward (transgressed), liberating large volumes of sand from the beach ridges an d dun efields. Transgression helped destabilize the san d deposits, some of wh ich were blown la ndwa rd by the w ind. The rem aining erod ed sand w as transported north along the Fraser coast in the longshore transport system. Finally, the landward advance of the shoreline ended as sea level stabilized at a highstand. By then, the dunefields landward of the shoreline on Fraser Island comprised a blanket of accumulated dunes over 50 m thick. In the last cycles, sand transported further along the shoreline accumulated in a northward m igrating spit (Breaksea Spit) at or near sea level. Each sea level cycle throughout the Pleistocene glaciations may have contributed to this cyclic history, with at least 9 major cycles adding to the final product as we see it today. Hence Fraser Island is not just one large dune island but rather the top of a large composite mass of dune sands that contains the history of numerous episodes of wind-blow n accretion linked to fluctuating sea levels. Consideration of the processes operating north of Fraser Island along Breaksea Spit and Breaksea Shoal gives an understanding of how the spit building process works. Sand supplied by the long shore tran sport syst em con tinues b eyond Sand y Cape , buildin g the sp it northw ard (see Figure 1 ). Here, it has been cut by m ultiple chan nels caused by tidal curren ts flowing in and out of Hervey Bay . Like most of the Fraser shelf, the spit b uilt originally o ver old carbonate platform s that make up the continental margin here. It has also built over the top of several live coral reefs – southw ard extensions of the present Great Barrier Reef. There appears to be at least two ph ases in the con struction of Break sea Spit, an earlie r deeper unit between the carbonate platform and 22 m water depth, and the present surface unit between 22 m depth and current sea level. An estim ate of the volum e of sand in Breaksea Spit and Shoal above 22 m wa ter depth is 8.7 5 cubic kilometers. The lower unit in the Breaksea system is at a similar level to many of the inner she lf shoreline dep osits along the south east coast of Australia that date from oxygen isotop e stage 3 (20-60,00 0 years ago). At a sand sup ply rate of 500 ,000 cub ic meters per ye ar (i.e., compara ble to the rates measured on the Gold Coast at present), it would take around 18,000 years to build the upper Breaksea s ys te m . This, together with the lack of dunes, suggests that much or all of the upper Breaksea system buil t sinc e the last l ow s ea le vel st and arou nd 1 8-2 0,00 0 yea rs ag o an d tha t toda y’s Breaksea Spit is a modern model for the process that extended Fraser Island north from Rainbow Beach. Hence, a spit sand body like the Breaksea sys tem is pred icted to underlie most of Fraser Island. After the next sea level fall and rise, the Breaksea system is also likely to be covered by a dune layer and become part of Fraser Island. Along the coast of Fraser Island today the longshore transport system is still operating effectively and transporting sediment around Double Island Point, along Fra ser Island to Breaksea Spit and beyond. However, the coastal morphology is one of cliffed dunes over much of this shoreline, with “coffee rock” dune (old B2 pod zol soil horizons) exposed on the b each face. Furtherm ore, som e dune s here are older tha n 20,0 00 yea rs and not from the last sea level cycle. Hence the eastern side of Fraser Island is experiencin g long term marine ero sion due to more sand transitin g north in th e longsho re transport system than is arriving from th e south around Double Island Point. The western side is also undergoing local erosion except where dunes have recently migrated across the island. If current cond itions of sea level h ighstand and clim atic condition s controllin g longshore transport prevail, indications are that the majority of the ocean shoreline of Fraser Island will continue to slowly erode. If global warming predictions eventuate and a rise in sea level occurs, the coastal erosion problem will accelerate.
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