ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

JOHN D. JANSEN & GARY J. BRIERLEY
Late Holocene superfloods and riffle-pool morphodynamics
in an arid zone gorge, Barrier Range, Australia
School of Earth Sciences, Macqarie University, 2109, Sydney, Australia
Riffle-pool landform assemblages provide useful insights
into river behaviour and history in arid zone bedrock-confined
valleys. Rather than simply focus on the magnitude
and frequency of individual 'superfloods' through time,
this study characterizes process-form relationships over the
full spectrum of flow magnitudes: from superflood boulder-trains
of imbricated clasts >2m in diameter, down to
low-energy mud facies in ephemeral waterholes. Detailed
stratigraphic and sedimentologic analyses, combined with
radiocarbon dating have revealed an 1800-year alluvial history
characterized by event-driven disequilibrious behaviour:
three major erosional flood-episodes interspersed by
long periods of relative quiescence and fine-grained sedimentation.
Sandy Creek gorge dissects a 60 km 2 upland catchment on
the north-eastern flank of the Barrier Range in arid far western
New South Wales, Australia. The well defined rifflepool
sequence strongly reflects structural control. A string
of ephemeral pools (waterholes), each occupying valley
constrictions 30-40 m wide correspond to resistant bedrock
obstructions. Pools lie within convergent flow-scoured
troughs up to 4.5 m below adjoining riffles and comprise
bed materials of coarse sands and granules. Riffles
are situated in flow-divergent valley expansions 60-80 m
wide, marked by lobate boulder bars with median clast-sizes
of 0.2-0.4 m, although some imbricated boulders exceed
2.5 m in diameter.
Geological structure, as expressed by valley width and bed
slope has produced a repeated series of landform assemblages
along the gorge - all fixed within the riffle-pool framework
(see figure below). Adjacent to each pool, finegrained
benches lie inset within the coarse-grained flood
plain and riffles. In accordance with accepted riffle-pool
hydrodynamics, these pool reaches are preferentially scoured
during large floods, but tend to aggrade during low
flows. Bench stratigraphy holds a record of cut and fill epi-
BEDROCK
sodes dating back to the last superflood clear out (>1800
a), when extreme bed shear stresses (>500 Nm- 2 ) generated
would have destroyed benches and caused scour to bedrock.
Bench excavations reveal a marked consistency of sedimentation:
the contemporary channel carries coarse sandy
gravels, and lies entrenched within a relict mud facies which
is now buried by actively aggrading (post-European)
benches narrowing the channel. The muds are no longer
accumulating in the upland valleys, but formerly accreted
on well vegetated pool floors in periods of catchment stability
between major floods. The muds are derived from
extensive late Pleistocene aeolian mantles, now largely
stripped from the steep slopes and stored in valley fills and
piedmont fans. Accelerated bench construction and the
depositional shift in pools probably reflects the increased
rates of runoff and sediment supply apparent since the invasion
by European pastoralists in the 1870's.
Landform assemblages along Sandy Creek gorge reflect an
hierarchy of stability and activation by events of widely disparate
magnitude and frequency. Active benches are juxtaposed
alongside very stable boulder riffles which lie essentially
dormant for many decades or even hundreds of
years. Morphologic change is dominated by rare, high magnitude
events that leave strong imprints on the landscape
and constrain subsequent processes.
EMMANUEL JAURAND
Comparaison de I'altitude minimale des cirques glaciaires
entre les Prealpes francaises du Sud
et I'Apennin septentrional
Laboratoire «Geomorphologie des milieux physiques mediterraneens et
semi-arides», Faculte des Lettres et Sciences Humaines,
Universite de Paris-Val-de Marne, 61 avenue du General de Gaulle,
94010 Creteil Cedex, France
Les Prealpes Francaises du Sud et l'Apennin Septentrional
(Italic) sont des moyennes montagnes mediterraneennes de
meme latitude (44°N) , situees de part et d'autre du Golfe
de Genes. Les cirques glaciaires herites se situent a des altitudes
minimales fort contrastees entre ces deux ensembles
montagneux.
Dans les Prealpes Francaises du Sud, les plus bas cirques
s'observent sur 1'ubac de lignes de cretes de 1900 m d'altitude
environ: ainsi ala Montagne de Lure, au Mont Ventoux,
ou dans les Alpes Maritimes. II faut depasser 2100 m
pour voir se des siner les premiers cirques dans les Prealpes
de Digne, plus eloignees de la mer. Au Nord de 1'Apennin,
des cirques ourlent 1'ubac des lignes de cretes a des altitudes
bien interieurcs : 1700 m dans l'Apennin de Modene,
1600 m dans 1'Apennin de Parme, 1500 m dans 1'Apennin
ligure et 1350 m dans les Alpes Apuanes. Dans ce dernier
massif, veritable barriere dressee en arriere du rivage ligu-
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