Sand beach ridges as longterm records

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Sand beach ridges as long-term records of
tropical cyclones

Jonathan Nott School of Earth and Environmental
Sciences James Cook University Cairns, Australia
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Key points and questions
Palaeo-records of TCs are often patchy in
geographic extent
Sand beach ridges are numerous and extensive
throughout northern Australia - they possess
characteristics suggesting they could have
developed due to TC inundations
Sand beach ridges generally thought to be
developed by wind and sometimes waves but never
seen as potentially housing long-term histories
of TCs
If they are generated by TCs then what are the
exact processes leading to their formation
If they do house records then how do these
compare with existing records
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Background information on Aust. coasts
Northern Australia dominated by sand beach ridge
plains and not so many settings conducive to back
barrier lake deposits (overwash deposits)?
Beach ridge plains develop where there is
abundant sediment supply to coast and where there
is a shallow offshore gradient
In these situations coasts prograde
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Some of the beach ridge plains are now built upon
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Sand ridge plains are numerous
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The Problem
We know that other types of ridge plains
throughout region are unequivocally deposited by
marine inundation during tropical cyclones
Such as coral shingle ridges shell ridges
sand shell ridges pumice ridges Why - because
these ridges are composed of marine derived
material
Reluctance to accept pure sand ridges as having
same origin
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Coral shingle ridges
22 coral shingle ridges
Curacoa Island
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Curacoa Island
Coral shingle ridges
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Pormpuraaw (Gulf of Carpentaria)
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Holocene ridges last 6,000 years
Pleistocene ridges 125,000 years ago
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Pumice ridge
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Cowley Beach Pure sand ridges
29 ridges 5,000 yrs
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Key question is how do the sand ridges
form? They occur world wide and have been
argued to form by Aeolian (wind) Waves
Sea level variations
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Aeolian (wind) -
They are too coarse grained
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Sea level variations Region has experienced
hydro-isostatic rebound Weight of water during
Holocene marine transgression 6,000 years ago
caused continental shelf to depress and
adjacent land to rise by 1 - 1.5 m (hence
relative sea level fell by 1 - 1.5 m since 5,500
yrs B.P.).
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So if waves appear most likely mechanism then do
they form like the other ridges?
Coral shingle ridges, shell ridges and shell /
sand ridges appear to be able to be formed
during one event
Why - because of the hydrodynamic properties of
materials i.e. they are lighter and more buoyant
than sand alone Also eye witness reports of them
being emplaced during single event
But this is not the case for pure sand ridges
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Conventional wisdom is that storms move sand
offshore
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Model of coarse-grained sand beach ridge formation
Ridges build progressively over time from a
number of inundations - initially by minor ones
then eventually by major ones
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Rm versus surge
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Forward velocity versus surge
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Track direction versus surge
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Landfall distance from site versus surge
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Wonga

Cowley Beach

Tully Heads
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Tully Heads
Dated using optically stimulated luminescence
(OSL)
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Tully Heads
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Wonga Beach (Nth)
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Wonga Beach (Sth)
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Cowley Beach
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Comparisons with other types of ridges plain
sequences
PORMPURAAW (Gulf of Carpentaria)
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Hamlin Pool, Shark Bay, W.A.
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Detrended oxygen 18 record
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Summary
Sand beaches ridges in this region (wet tropics
NE Australia) likely to be formed by tropical
cyclone induced marine inundations
As such they contain long-term records of
tropical cyclones
They hold considerable promise because they are
so extensive
They are suggesting that there has been
distinct periods of event clustering and event
absence
This is similar to other records both in
Australia and elsewhere
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Our transect (30 ridges 3,000 yrs)