Queesnland Coral Rubble Ridges

Coral rubble/shingle ridges occur in locations where coral reefs occur close to shore. Most of the paleorecords derived from these ridges comes from sites along the Great Barrier Reef region offshore from Queensland's east coast. Coral rubble ridges also occur on the Abrolhos Islands off the southwest Western Australian coast but to date no detailed work has been published from this site. The ridges are formed when coral fragments are eroded from near-shore reefs by wave action during a tempest and transported either onshore or offshore (Hughes,1999; Davies, 1983; Baines et al., 1974; Rasser and Riegl, 2002). Fragments can also be transported from existing accumulations of coral rubble in the offshore zone. These offshore accumulations result from erosional processes such as biodegradation and wave action during both storms and fair weather conditions. (Rasser and Riegl, 2002; Hughes, 1999). It is thought that the angle of the offshore reef slope plays a role in whether the eroded fragments are transported predominantly offshore or onshore. Steep reef fore-slopes favour offshore transport of fragments, often to depths of greater than 50 m which is too deep to be reworked and transported by storm waves. Shallow sloping, and particularly wide, reef fronts favour transport onshore and the formation of coral rubble ridges. However, some sites, such as Curacoa Island in the central Great Barrier Reef, Australia (Figs. 2 and 3) that are fronted by narrow, steep reef slopes have extensive coral rubble ridge development on land (Hayne and Chappell, 2001; Nott and Hayne, 2001). These sites with presumably minimal accumulation of coral rubble in the shallow waters of the reef and maximum accumulation of rubble in the deeper offshore waters below wave base suggest that the onshore ridges could have formed from predominantly live coral fragments broken off during the storm. At other sites, however, there can be little doubt that onshore ridges were formed from the reworking of existing accumulations of rubble in the shallow waters offshore.

Along the east coast of Queensland coral rubble ridges often accumulate on the sheltered side of islands (Fig. 3), presumably because on the exposed sides they are constantly removed by the largest or most intense tropical cyclones affecting a

Fig. 2 Location of sites in northeast Queensland

region. The sheltered sides will experience diminished wave energy, but at the same time a substantial surge. Because the wave energy is reduced, the likelihood of the ridge being removed during subsequent cyclones is lessened. Where the preservation potential for ridges is high, such as on the lee side of islands, a number of ridges are sometimes able to accumulate over time. Curacoa Island on the Great Barrier Reef (GBR) has 22 consecutive coral rubble ridges paralleling the shore on its northwestern or sheltered side (Fig. 4). Individual ridges extend for over 100 m along shore and rise to over 5 m above the mid-tide level (the tidal range here is approximately 3 m) or Australian Height Datum (AHD). The ridges were deposited by successive cyclones so that new ridges are deposited seaward of the previously emplaced ridge. The radiocarbon age of the ridges increases progressively with distance inland. The average interval between ridge emplacement here is 280 years over the past 5,000 years.

Lady Elliot Island at the southern end of the GBR also has an extensive sequence of coral shingle ridges. About fifteen successive ridges are preserved here with an average interval between ridge emplacement of 253 years (dated using radiocarbon) over the past 3,200 years (Chivas et al., 1986; Nott and Hayne, 2001). Other ridge sequences have been radiocarbon dated at Normandy Island, Fitzroy Island and Double Island in the central north GBR region (Nott, 2003). These latter sites have

Fig. 3 Oblique aerial vie of Curacoa Island Central Great barrier Reef. Open ocean is toward background. Location of cross-section shown in Fig. 4 shown by white line (Photo D. Hopley)

0 Distance I m) from shore 200 400

Fig. 4 Stratigraphy and chronology of shingle ridges at Curacoa Island

0 Distance I m) from shore 200 400

Fig. 4 Stratigraphy and chronology of shingle ridges at Curacoa Island considerably fewer ridges in their sequences and do not provide the length of record found at Curacoa and Lady Elliot Islands. They do show though that the average interval between ridge emplacement is, like the other sites, between 200-300 years (Nott and Hayne, 2001; Nott, 2003). Wallaby Island in the southern Gulf of Carpentaria has a sequence of 14 coral shingle ridges spanning the past 4,100 years with an average interval between ridge emplacement of 180 years.

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