ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

were dated using lichenometry and dendrochronology.
These two techniques are particularly useful for dating surfaces
recently exposed by shrinking glaciers in regions like
Andean Patagonia where there are almost no historical and
very limited meteorological records.
However, to provide any useful results it was first necessary
to establish values for a number of variables affecting
lichen and tree dating. The variables, relating to micro environment,
included calculating a growth rate for the most
common rock-inhabiting lichen species (Placopsis perrugosa)
and obtaining reliable estimates for delays in colonization
before growth can start on freshly exposed surfaces.
A minimal dating framework of aerial photographs and a
largest lichen on a cairn erected by a previous expedition
suggested a growth rate of 4.7 mm/yr, identical to that of
its sister species P. patagonica on the western side of the
N pi (Winchester & Harrison, 1994). Colonization delays
for the lichens, varying from a few months to 18 years, were
extrapolated from the age in years provided by (a) subtracting
the date obtained from a size/growth rate correlation
of the cairn lichen from the cairn construction date
and (b) from the date provided by the largest lichens in the
youngest populations growing above the rocks exposed by
recent glacier downwasting. Other indicators (see below)
showed that, for the tree genus Nothofagus (the southern
beech), colonization here can take from 26-100 years. A
further variable required for tree dating is an estimation of
the number of years of tree growth below coring height.
This was quantified by measuring the height of small trees
and counting the number of rings at the base of their
trunks. We found that a 120 ems tall Notbofagus could
vary in age from 6-12 or more years depending on the
amount of local environmental stress.
Our initial estimate of the delay before tree colonization
was based on the discovery that there were lichens growing
on a glaciofluvial terrace beside Lago Arco which were much
older than the trees there. A 1944 aerial photograph
shows a flood covering the terrace; this was clearly a fleeting
event terminated by a glacier-dammed outburst which
killed the previouss generation of trees while the lichens
survived. The flood thus provided an event horizon and a
maximum possible delay before tree colonization here of 26
years. The 100-year delay was deduced from the difference
between lichen and tree ages in an exposed environment.
Our findings on the variability and time range of lichen
and tree colonization and tree age below coring height, illustrate
the importance of detailed field investigations aimed
at obtaining realistic estimates. Plainly, if these variables
are researched insufficiently, misleading dating may
be achieved particularly in areas of recently deglaciated
terrain where time spans are short.
As regards recent landscape change, the key finding was
the regional importance of glacier-dammed outburst
floods, with major flooding outbursts from Lago Arco in
1896, 1944 and 1958. The 1896 outburst flood was deduced
from differences in tree and lichen ages, with a largest
lichen growing on a moraine crest being some 15 years older
than the oldest trees growing near a vegetation trimline
on the valley wall 30 metres above the moraine. A rough
calculation of the difference between the volume of the
present lake contained within its valley and a lake-full situation
shown by tree trimlines on the valley walls, suggests
that the initial glacier outburst of 1985 could have released
between 300-445 million cubic metres of water, depending
on the Arco Glacier's ice front position at the time.
Since that date the tree evidence suggests that flooding
has declined, with decreasing levels of seasonal floods over
the last 40 years.
Clearly the Colonia Glacier requires watching. At present
its tongue barely closes the foot of the Arco valley, but if it
should readvance and the Arco outwash channels under
the Colonia ice become blocked then further glacier outbursts
may be expected in the future.
LISA WORRALL & TIM MUNDAY
Geomorphology applied to mineral exploration in the ancient
landscapes of Western Australia
Regolith Characterisation Program, Co-operative Research Centre
for Australian Mineral Exploration Technologies,
c/o Csiro-Em, Private Bag PO, Wembley, WA 6014 Australia
An understanding of long term landscape evolution is critical
to mineral exploration in Australia. This is because
prospective outcrop has largely been explored and companies
are now moving into regolith dominated terrains. The
term regolith is used here to describe unconsolidated rock
material at the Earth's surface whose character and/or disposition
is related to near-surface processes. Exploration
companies need to know how the regolith has evolved in
order to source pointers to primary mineralisation, and to
locate sites of secondary mineralisation.
In countries where the landscape has been shaped by glaciation
and is very young, exploration companies have
been '. able to use an understanding of glacial processes to
reconstruct dispersion trains. On Gondwanic fragments
such as Australia the landscapes are very old, and may have
been shaped by subaerial processes over billions of
years. These landscapes are polygenetic and the distribution
of regolith materials within them is a reflection of the
combined effect of numerous different processes.
In landscapes with such a long and potentially complex history
the nature of contemporary processes and the surface
distribution of regolith materials may be a poor guide to
the character of regolith materials at depth. Mapping aids
such as air-photos, TM/Spot and radiometries are useful,
but not wholly adequate, as these technologies have limited
skin depths. Airborne ElectroMagnetics (Aem) is an
important adjunct technology in this context, as it. can return
information on regolith materials and the geometry of
regolith units at regional scales and to considerable depths
(>100m).
In this paper we describe a study conducted in support of
gold exploration at Lawlers in the Yilgarn Craton, We-
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