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Title: Overturned cliff-top mega-boulders at Little Beecroft Head, Jervis Bay, NSW, Australia: a mega-tsunami or aboveground bolide impact about 20ka BP?
Authors: Felton, EA
Switzer, AD
Fink, D
Crook, KAW
Keywords: Australia
Sedimentary Basins
Beryllium 10
Aluminium 26
Issue Date: 5-Jul-2010
Citation: Felton, E. A., Switzer, A. D., Fink, D., & Crook, K. A. W. (2010). Overturned cliff-top mega-boulders at Little Beecroft Head, Jervis Bay, NSW, Australia: a mega-tsunami or aboveground bolide impact about 20ka BP? Australian Earth Sciences Convention 2010 (AESC 2010), 4th – 8th July 2010. Canberra, Australia: National Convention Centre.
Abstract: The largest of several detached boulders on top of the 33 m high cliffs at Little Beecroft Head measures 3.5 x 2.1 x 1.6 m and weighs ~31 tonnes. Sedimentary structures (cross‐bedding) show that this boulder has overturned. The boulder and the nearby cliff are pebbly quartz sandstone of the Permian Snapper Point Formation in the southern Sydney Basin. A comparison with the local stratigraphy indicates that the boulder was detached from the nearby north‐east facing cliff face, in which a bed of identical lithology crops are at a lower elevation. A Holocene tsunami has been invoked by other authors for emplacement of the boulder onto the cliff top. Using the isotopes 10Be and 26Al, we determined the cosmogenic exposure ages of (a) the currently exposed upper surface of the mega‐boulder; (b) the lower surface which is in partial contact with the cliff‐top platform upon which the mega‐boulder rests; and (c) the cliff‐top platform surface itself, ~3 meters landward of the boulder. 10Be and 26Al exposure ages for all three samples are consistent. Simple age modelling (zero‐erosion case) shows that today’s lower surface of the boulder was exposed for ~63 ka prior to the overturning event and that today’s upper surface has been exposed for ~19 ka, which represents the detachment age or the elapsed time since the overturning event exposed the top surface of the boulder. The adjacent platform surface has a zero‐erosion exposure age of 84 ka. At the time of global Last Glacial Maximum, 20–22 ka, sea level was 120–130 m lower than at present and the land-ocean boundary lay some 20 km east of the site. This precludes boulder emplacement and overturning by a storm wave or even by a ‘normal’ tsunami wave. We suggest that the event which dislodged the boulder from the vertical cliff face, lifted and overturned it and emplaced it ~2 meters further inland could be (1) a mega‐tsunami resulting from a large bolide impact in the distant ocean; or (2) a mid‐air cometary explosion similar to that which is thought to have occurred at Tunguska, Russia, in 1908.
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