Browsing by Author "Crook, KAW"

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    Overturned cliff-top mega-boulders at Little Beecroft Head, Jervis Bay, NSW, Australia: a mega-tsunami or aboveground bolide impact about 20ka BP?
    (Geological Society of Australia, 2010-07-05) Felton, EA; Switzer, AD; Fink, D; Crook, KAW
    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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    Overturned mega boulders on coastal cliff-tops and in bedrock river channels : can cosmogenic nuclides constrain tsunami and palaeo-flood events in Australia?
    (18th INQUA Congress, 2011-07-21) Fink, D; Fujioka, T; Mifsud, C; Nanson, GC; Felton, A; Crook, KAW; Switzer, A
    Jacks Waterhole at the Durack River in the Kimberley region of north west Australia is an exposed bedrock paleo-channel, excavated by hydraulic plucking of well-jointed bedrock. This section of the channel consist of arc-shaped disconnected stacks of imbricated meter-sized slabs dislodged from bedrock steps immediately upstream of the boulder-filled channel. The semi-arid climate is punctuated by summer tropical storms/cyclones causing occasional violent floods. High magnitude floods with high flow velocities are required to erode and transport such large rock slabs. Along the south-eastern Australian coastline, at Little Beecroft Head, large detached sandstone boulders are found unconformably on horizontal cliff top escarpments and benches of identical lithology some 20-35 meters above present day sea-level. For some boulders, local stratigraphy indicates transport from the nearby cliff face, implicating tsunami or exceptional storm events. In other cases, an interpretation of differential erodibility along bedding strata, slow emergence and preservation from the contemporary platform is applicable. For both locations, detailed geomorphic mapping, cross-bedding orientation and tracing from the identified detachment site clearly indicates that boulders have experienced at least one flipping event. Consequently, previously buried surfaces are instantaneously exposed to an enhanced production rate of cosmogenic nuclides. The possibility of dating the ‘flipping’ event depends largely on a comparison of measured cosmogenic concentrations from 4 surfaces (upper and lower boulder, shielded and exposed bedrock) to that predicted on the boulder as a function of boulder thickness. In this paper, we describe our model and its sensitivity to boulder thickness, inheritance and post-flipping time. Preliminary results of 10Be and 26Al analysis from flipped and non-flipped boulders at Jack’s Waterhole and Little Beecroft Head are given. Copyright (c) 2011 INQUA 18