The question of whether chevrons and their associated deposits are formed by tsunami is of more than just academic importance; if they are, then we may be severely underestimating the average frequency of events capable of causing destructive tsunami (be those events earthquakes, submarine landslides or, as Dallas Abbott and her colleagues propose, other bits of the solar system falling on our heads). As it turns out, however, both the dramatic origins and the proposed geologic youth of these features are still currently open to question.
Ironically enough, one big problem with associating the chevron landforms with megatsunami events is their size: the Australian ones can peak at more than 100 metres above present sea-level, which really would put the ‘mega’ in megatsunami. If you’re proposing giant waves generated by an asteroid impact, you might reasonably except it to be recorded by more than just these coastal landforms; where are the iridium spikes, or the climatic effects of debris thrown up into the upper atmosphere? Also, as Phil pointed out in the comments to the weekend’s post, there appear to be several generations of chevrons within each set, which is also somewhat hard to explain in terms of one large wave transporting all of the material inland at once. These problems make it slightly difficult to ignore the resemblence of these landforms to parabolic dunes, as is masterfully explained by Zoltan.
Then there’s last months press release from the Australian National University, which casts doubt on both the origin and timing of the West Australian ‘tsunami’ deposits:
In 2003 Australian geological researchers suggested prehistoric tsunamis over the last 10,000 years were much larger than those recorded since European settlement including findings of surges up to 20 metres in height affecting a 2500km stretch of the West Australian coastline.
But archaeologists from The Australian National University have questioned these claims, saying that some of the key evidence for ‘mega-tsunamis’ can be explained by human activity.
“Our field work would suggest that the shell and coral deposits found high on headlands in WA or further inland are evidence of Aboriginal occupation of the area, and not deposits of mega-tsunamis or other major inundations,” argues researcher Dr Tony Barham from ANU. He and his colleagues Dr Sue O’Connor and Dr Stewart Fallon also found that archaeological deposits in the area have not been disturbed by major inundation for the last 1000 years, undermining the previous theory that giant waves had flooded the area once every 400 to 500 years.
Tony Barham and his colleagues have studied the area around Cape Leveque, one of the main localities used by Nott and Bryant (2003) to argue for multiple recent tsunami events. In this area, they have found abundant – and undisturbed – signs of human habitation which can be dated as being older than the proposed inundation events, even though they would have been destroyed by waves with a run-up (height) of more than 5-10 m. Not only that, but these archaeological deposits commonly include shell middens, which raises the possibility that some of the evidence for large waves reaching far inland, and the dates for them, might in fact come from man-made debris (in fairness, Nott and Bryant do discuss this possibility, and claim that they can distinguish between anthropogenic and wave-deposited shell layers).
So what we have here are some contradictions: within the same landscape we have some researchers claiming evidence for very recent, very destructive tsunami, but other researchers finding older archaeological evidence that would not survive such an event. On the one hand, people are claiming chevrons are formed by destructive waves, whereas others point to their superficial resemblence to much more mundane landforms and ask what, exactly, makes these ones so different. In many ways, this is not a surprise: a lot of this work is very recent and has yet to make it past the ‘presented at conferences’ stage, so no-one is really in possession of all the facts and ideas. It may be possible to partly reconcile the opposing viewpoints: perhaps there is material moved by tsunami in these areas, but is it much older than has been claimed, and has been reworked into present landforms. Either way, an interesting debate is potentially brewing.