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“Historical Impact Craters”

Guest: Dallas Abbott


Dallas Abbott and her colleagues have been exploring evidence that some historical tsunami events in the South Pacific resulted from bolide (asteroid or meteorite) impacts, rather than earthquakes as previously postulated. In her presentation, she will share some of her recent research in Australia and New Zealand to test her theories. She and her co-authors made a presentation at the Geological Society of America Annual Meeting in November 2003,  based on this Abstract:

ABBOTT, Dallas H., MATZEN, Andrew, and PEKAR, Stephen F.
Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964,
dallas@ldeo.columbia.edu and BRYANT, Edward A., Faculty of Science, University of Wollongong, Wollongong, Australia, 2522

Goff attributes coastal abandonment in New Zealand at 1500 A.D. [1]
to an earthquake-induced tsunami event.  However, the largest
historical earthquakes produced maximum tsunami runups of 40 to 60
meters.  On Stewart Island, New Zealand, beach sand is present  ~220
meters above sea level at Hellfire Hut and ~150 meters above sea
level at Mason Bay. In eastern Australia, there are mega tsunami
deposits with maximum run-ups of over 130 meters and a C-14 age of
~1500 A.D[2]. Megatsunami deposits occur on the eastern side of Lord
Howe Island in the middle of the Tasman Sea, implying a source crater
for the tsunami further east.  We named this source crater Mahuika
for the Maori god of fire.  Mahuika crater is 202 km wide and >153
meters deep.  It is on the New Zealand continental shelf at 48.3 S,
166.4E.  Several pieces of evidence point to Mahuika as the source
crater for the 1500 A.D. event. The first is that the crater lies on
a great circle path from Australia oriented at about a 45-degree
angle to the general trend of the eastern Australia coast.  Mega
tsunami deposits near Wollongong and at
Jervis Bay, Australia suggest
a tsunami wave oriented at this angle to the coast. The second is the
sub bottom depth of the impact deposits. We have found impact ejecta
in all of the dredges near the crater.  Because marine sediments are
deposited at a rate of about 1 cm per thousand years, this is
expected if the impact deposit is only 500 years old.  We are seeking
C-14 dates to confirm this.  The third is the distribution of
tektites, which are found on the opposite side of the crater from the
direction of impactor arrival[3].  Although we found impact ejecta in
many samples, only some samples contained tektites.  All
tektite-bearing samples are located SE of the crater, in the opposite
direction from SE Australia, where the impact fireball was seen by
the aborigines[2].

 References: [1] Goff, J.R. and B.G. McFadgen, Seismic driving
of nationwide changes in geomorphology and prehistoric settlement-a
15th Century New Zealand example. Quaternary Science Reviews, 2002.
21: p. 2229-2236. [2] Bryant, E., Tsunami: The Underrated Hazard.
2001, Cambridge, UK: Cambridge University Press. 320.  [3] McCall,
G.J.H., Tektites in the Geological Record. 2001, London: Geological
Society of London. 256.

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