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Title: Overturned mega boulders on coastal cliff-tops and in bedrock river channels : can cosmogenic nuclides constrain tsunami and palaeo-flood events in Australia?
Authors: David, F
Fujioka, T
Mifsud, C
Nanson, G
Felton, A
Crook, K
Switzer, A
Keywords: Australia
Issue Date: 21-Jul-2011
Publisher: 18th INQUA Congress
Citation: David, F., Fujioka, T., Mifsud, C., Nanson, G., Felton, A., Crook, K., Switzer, A. (2011). Overturned mega boulders on coastal cliff-tops and in bedrock river channels : can cosmogenic nuclides constrain tsunami and palaeo-flood events in Australia? 18th International Union for Quaternary Research Congress, 21th-27th July 2011, Berne Switzerland.
Abstract: 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
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