Australian Mega Tsunami field evidence
The challenge was to pursue the sources of this evidence to the ocean and detect the signatures of catastrophic tsunami in the coastal landscape. This landscape is also one subject to some of the most intense tropical storms in the world (Nott 2004) associated with winds in excess of 300 km hr21 and storm surges of 3.6 m (Bureau of Meteorology 2000). Two sites stand out as showing evidence of tsunami. The first is located at Cape Voltaire directly west of Kalumburu. Here, waves beyond the capacity of cyclones have truncated the ends of headlands. This erosion was not controlled by bedrock lithology or structure as exemplified by the erosion into columnar basalt on the headland.
Tsunami erosion on the exposed side of the headland created a ramp that cuts across the dominant structural control that normally would have influenced coastal landforms. This ramp terminates about 20 m above sea level. It would be tempting to attribute the excavation of the ramp to storm waves but for one additional factor. Little debris evacuated from the ramp is present either on the ramp surface or offshore. Instead, the columnar basalt has been broken into 5 m lengths, tossed over the 40 m high headland and deposited on the sheltered lee slope above the influence of storm waves such that individual blocks reflect the direction of flow, 3508 to the NW (Fig. 7b). The second site is further south at Walcott Inlet where the best Aboriginal legend for a tsunami exists. Here, at Collier Bay, a tsunami has infilled an embayment with a mixture of sand, gravel and shell that rises at least 6 m above the swash or storm surge limit of tropical cyclones. Everywhere in the Kimberley, the coastline evinces either the erosional effect of a catastrophic wave or its depositional residue in sheltered embayments.
Australian Mega Tsunami chronology
It is possible to date the timing of this mega-tsunami in the Kimberley using radiocarbon dating of shell. Again, the same methodology as was used in the Tasman Sea region was used to calibrate the ages. Thirteen dates have already been reported for the comet-induced mega-tsunami detected south of the Kimberley (Bryant & Nott 2001; Nott & Bryant 2003). These are presented in the top panel of Figure 8. The most recurrent age centres between AD 1620 and 1730 with a defined peak at AD 1690. The date from Cape Voltaire (bottom of Fig. 8) peaks earlier at 800 AD and may either be a separate tsunami event or the result of dating old shell. The date from Collier Bay corresponds with the prominent peak in NW Australia indicating that the effects of a mega-tsunami that occurred around the seventeenth century can now be traced along 1500 km of coastline. This age agrees with the age of the Wandjina paintings. Attempts are being made to retrieve more datable material from the Kimberley coastline to refine the chronology.
Based on the evidence presented here, and because Aboriginal legends concentrate on the three main elements of a comet impact in the ocean: the comet itself, tsunami and flooding rains, this seventeenth century tsunami has been labelled the Wandjina event. No impact crater has yet been found, although attempts are being made to find it, if it exists. However, this is not a limitation to our research because both Aboriginal and Maori legends favour the explosion of meteoritic debris in the atmosphere, rather than an actual impact with the earth’s surface. Bolides can also generate significant tsunami (Chyba et al. 1993; Verschuur 1996). The Wandjina event generated the biggest and most widespread mega-tsunami yet found in the Australian region. The wave reached a maximum of 35 km inland in the Great Sandy Desert, deposited sands up to 40 m deep on the lee side of headlands and laid down bedded gravels on the landward side of 40 m hills situated over 5 km inland (Bryant & Nott 2001; Nott & Bryant 2003). These aspects are an order of magnitude greater than that produced by any historic volcanic or earthquake generated tsunami originating from Indonesia (Nott & Bryant 2003). The spectacular nature of this cosmic phenomenon has dominated Aboriginal mythology for the past four centuries. Only by interpreting the proper significance of this rich oral history and artwork, especially in the Kimberley, can the true origin of this tsunami be recognized.