Sand beach ridges record 6000 year history of extreme tropical cyclone activity in northeastern Australia
Introduction
Multiple shore parallel ridges are common landforms along many of the world's sedimentary coasts. Their origin has been attributed to a range of processes and it is likely that different processes operate to form these landforms at different locations. A number of detailed reviews on the various origins of these landforms have been published in recent years (Tanner, 1995, Taylor and Stone, 1996, Otvos, 2000; Hesp, 2005) and there are a number of definitions as to what constitutes a beach ridge, beach ridge plain, relict foredune and plain, cheniers and other shore parallel/sub-parallel ridge features. Many have suggested for example that beach ridges consist only of wave built features (Johnson, 1919, Hesp, 2005) whereas others such as Otvos (2000) include both wave built and aeolian constructed ridges or combinations of the two depositional processes. Shore parallel ridges can also be composed of a variety of materials such as sand, shell, gravel, coral fragments and in some instances a ridge or individual ridges within a ridge plain sequence can be composed of a combination of these materials. They have been used to reconstruct various palaeoenvironmental conditions such as sea and lake levels (Stapor, 1982) and sediment delivery rates and hence climatic conditions (wetter and drier phases) within hinterland catchments (Rhodes et al., 1980, Chappell and Grindrod, 1984, Brooke et al., 2008). They have also been recognized to record episodes of higher than normal wave and or tide conditions (Reineck and Singh, 1978, Mason and Jordan, 1993) especially when the crest of the ridge sits well above normal wave and tide levels. These latter types of ridges have been used to reconstruct a record of past wave and ephemerally higher sea-level conditions but only where the ridges have been composed of coral fragments or a sand/shell mixture (Nott and Hayne, 2001, Nott, 2003) – hence materials that are unequivocally of marine origin. The possibility of developing similar records from pure sand ridges likely to have been deposited by waves and surges has not as yet been explored.
In Queensland, northeast Australia, and near shore islands along the Great Barrier Reef (GBR), many ridge plains are composed of either coral fragments (coral rubble or shingle), or shells as in the case of cheniers, or sand beach ridges containing layers of marine shells. In these instances their deposition has been regularly ascribed to tropical cyclone generated surge and wave processes (Rhodes et al., 1980, Chappell et al., 1983, Chappell and Grindrod, 1984, Chivas et al., 1986, Hayne and Chappell, 2001, Nott and Hayne, 2001, Nott, 2003) – and often to one ridge being deposited during a single storm (Rhodes et al., 1980, Hayne and Chappell, 2001, Nott and Hayne, 2001). The same is true for the shore parallel coral shingle ridges reported on coral atolls throughout the North and South Pacific (Blumenstock, 1958, Maragos et al., 1973, Scoffin, 1993). This conclusion has been commonly accepted for a number of reasons including that there have been eyewitness accounts of individual ridge formation during a single tropical cyclone (Maragos et al., 1973, Nott, 2003), the ridges contain clasts of marine origin which are generally too large to have been blown by wind and the height of these ridges is generally well above the marine inundations, including wave run up, of any meteorological conditions except for those generated by tropical cyclones.
The formation of ridge plains in northeast Australia is not just restricted to those locations where coral reefs and shell production are abundant in the near shore. In the absence of these materials the ridge plains are composed predominantly of sand. These locations experience the same process constraints as those where the coral shingle and shell bearing ridges occur i.e. a wave fetch limited environment that results in a bimodal wave regime consisting of short period (2–4 s) trade wind generated waves rarely exceeding 0.5 m height at shore and tropical cyclone generated waves associated with storm tides. Also, in many locations the sand clasts within the ridges are too coarse (1–3 mm) to have been transported by wind during non-cyclonic conditions. When gale to hurricane force winds blow onshore at any one location here the beach is invariably inundated by a storm tide and waves thereby negating the possibility of aeolian processes (as the beach would be the only source of aeolian sand) in the formation of the ridges at these times.
If waves are responsible for the formation of the sand beach ridges here then the question remains as to whether only tropical cyclone generated marine inundations are capable of reaching the crests of these ridges and are therefore responsible for their formation. We aimed to test this hypothesis by numerically modelling a range of meteorological and oceanographic conditions that could potentially result in a marine inundation capable of reaching the crests of the ridges at Cowley Beach approximately 120 km south of Cairns in northeast Queensland. Here the ‘outer barrier’ ridge plain is 2.5 km wide and contains 29 medium to coarse-grained predominantly quartz sand shore parallel ridges whose crests lie between 4 and 6 m above mean sea level and which were deposited over the past approximately 6000 years. If these ridges have been deposited by tropical cyclone induced marine inundations then they not only provide a record of such events over the latter part of the Holocene but it is also possible that other sand ridge plains in this region might have formed by like processes. If this is the case these coastal landforms could provide a much more detailed understanding of the long-term tropical cyclone climatology for this region.
Section snippets
Geologic, geomorphic and climatic setting
Cowley Beach (Fig. 1) is a 7.5 km long, arcuate shaped sand beach backed by a beach ridge plain forming an inner and outer barrier sequence (Fig. 2). The beach ridges of the inner barrier are probably Pleistocene in age. The ridges comprising the approximately 2.5 km wide outer barrier are, as shown later, Holocene in age. Salt flats and reworked beach ridge sands also occur landward of the inner barrier and these sediments merge further landward with alluvial deposits and freshwater peat swamps.
Surveying, sediment collection and sediment analysis
The beach ridge plain at Cowley Beach was surveyed using an electronic theodolite and distance measurer (total station) accurate to less than 1 mm. The site is distant from a standard topographic survey bench mark so initial levels were related to sea level at a specific time and then converted to Australian Height Datum. We expect the error using this approach to be less than ±10 cm.
Sediment samples were collected by coring the ridges using both a hand coring device and a truck mounted drill
Sediment size analysis
Sediment size data for 19 of the ridges is presented in Fig. 4. These samples were collected at 1 m depth. Median grain size for these ridges varies between 0.5 mm and 1.6 mm. Sand grains up to 5 mm diameter were common. These grains are predominantly composed of sub-angular, moderately sorted, translucent quartz with minor amounts of feldspar and mica. Graham (1993) states that the sediments in these beach ridges display characteristics in keeping with a granitic source. Samples from the aeolian
Discussion
The results suggests that the beach ridge plain at Cowley Beach appears, for the main part, to have been deposited by waves and only marine inundations generated by high intensity tropical cyclones are capable of reaching the crests of these ridges. A few of the ridges towards the rear of the Holocene sequence are capped by up to 1 m of aeolian sand. This sand is fine-grained and well sorted (Graham, 1993). The remainder of the ridges are composed of moderately sorted medium to coarse-grained
Conclusion
The textural characteristics, height above sea level and results of numerical storm surge and wave modelling suggest the elevation of the sand beach ridges at Cowley Beach is due to high magnitude/low frequency storm surges and waves generated by extreme intensity tropical cyclones. As such these beach ridges record the frequency and magnitude of high intensity cyclones over the past nearly 6000 years. This record suggests that these events have occurred considerably more frequently than that
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