Erosional Records

The records of prehistoric tropical cyclones described thus far have all been depositional. At Red Cliff Point, 35 km north of Cairns, 4 terraces ranging from

Aeolian capping

-1500 yr hiatus

-1500 yr hiatus

1000

1500

2000

Fig. 11 Topography and chronology of shell ridges at Hamlin Pool

1000

1500

2000

2500

Distance from shore (m)

Fig. 11 Topography and chronology of shell ridges at Hamlin Pool

2.12 m to 6.1 m AHD are eroded into a raised lithic gravel beach deposit (Figs. 12 and 13). Nott (2003) concluded that these terraces were eroded by marine inundations most likely during tropical cyclones. These terraces are unlikely to have formed in response to falling sea-levels or recent tectonic uplift of the region. Mean sea-level has not varied by more than 1 m over the last 5,500 years along this section of coast, nor has the shoreline been uplifted by more than this amount over this time (Chappell et al., 1983). The gravel comprising the terraces was originally derived from the coarse-grained fluvial terraces and debris flows that extended farther seaward during a period of lower sea-level, most likely between 25,000-14,000 years B.P. (Thomas et al., 2001; Nott et al., 2001) and which was reworked by the Holocene marine transgression (Fig. 12). These sedimentary deposits now form sea cliffs between 5-10 m in height behind the gravel beaches. The gravel from these landforms have been re-deposited by waves to form the gravel beaches as evidenced by the presence of buried detrital corals. Sedimento-logical analyses of the gravels in the wave cut terraces show that they are generally well sorted and apart from the lower terrace show no sign of grain size variations from the toe to the crest of each terrace. This suggests that the upper three terraces have been eroded into the existing gravel deposit by waves rather than representing separate individual deposits.

As with the coral shingle ridges it is unlikely that tsunamis were responsible for erosion of the gravel terraces at Red Cliff Point. Nott (1997) proposed that tsunamis may have penetrated the Great Barrier Reef near Cairns and eroded and transported very large (>200 tonnes) lithic boulders along the coast once and possibly twice over the last millennium. Given that both tsunamis and tropical cyclone induced surges are possible in this environment, although the former are much less frequent, it is difficult to be absolutely sure that tsunamis were not responsible. However, if

tsunamis had impacted the terraces they would have likely destroyed the terrace morphology given their substantially greater velocity than surge or wind waves and ability to transport considerably larger clasts than those comprising the terraces. Hence it is suggested that tsunamis were unlikely to be responsible.

Radiocarbon ages from coral clasts buried amongst the gravels within the terraces at Red Cliff Point are presented in Table 1. They range in age from 4,200 yr B.P. to 340 yr B.P. (conventional radiocarbon ages) (Fig. 13). The older ages come from the lower terraces and the two youngest ages from the highest terrace (Terrace 4). It is suggested here that because these terraces are erosional, the lower terraces must have developed after Terrace 4 (the highest elevation terrace) and hence post-date the youngest age from this terrace; this age when calibrated at the 2a uncertainty margin occurs between AD 1815 and 1870. This then represents the time when this section of coast experienced an inundation event reaching 6.1 m AHD (Table 2). The much older radiocarbon dated coral clasts have been reworked over the millennia and reincorporated into the terraces more recently.

Fig. 13 Topography and chronology of erosional terraces at Red Cliff Point
Table 1 Radiocarbon chronology of coral clasts at Red Cliff Point

Location

Conv. Age, years B.P.

Cal. 1s, Age A.D.

Cal. 2s Age, A.D.

Terrace 1 A (2.97m)

3110 ±70

990-860 BC

1060-800 BC

Terrace 1 B (2.97m)

4070 ± 50

2220-2050 BC

2300-2000 BC

Terrace 1 C (2.97m)

4240 ± 50

2460-2320 BC

2530-2230 BC

Terrace 4 A (6.1m)

4170 ±50

2370-2200 BC

2440-2140 BC

Terrace 4 B (6.1m)

370 ± 50

1885-1950

832-1950

Terrace 4 C (6.1m)

400 ± 50

1880-1950

1815-1950

Conv. = Conventional radiocarbon age, Cal. = Calibrated radiocarbon age (at 1 sigma and 2 sigma uncertainty margins); heights in brackets are metres AHD.

Conv. = Conventional radiocarbon age, Cal. = Calibrated radiocarbon age (at 1 sigma and 2 sigma uncertainty margins); heights in brackets are metres AHD.

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