ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

Sediment delivery from a watershed involves the processes
of erosion, transport and storage. The annual watershed
output of the inorganic material measured is considered
the sediment yield. Values of basin sediment yield
are needed for a range of management purposes including
estimates of erosion, reservoir longevity, and nutrient
and/or contaminant loadings to receiving water bodies.
For many years the movement of sediment within
watersheds has incorporated the assumption of single
grain transfers for particle sizes ranging from coarse
sands right through to silts « 63pm) and clays « Zum).
In other words, we have assumed that all inorganic, mineral
sediments are moving downstream as individual particles,
and as such these finer materials are expected to be
transported out of the river and reservoir system very
quickly as they are too fine to settle in moving waters.
Standard methods of grain size analysis (pipette, hydrometer
and sedigraph analysis) require the removal of all
organic matter and the dispersal of particles. Following
this treatment the sediments are sized. These standard
methods result in a biased representation of the size and
densities of material which are actually transported in the
stream. Using these sizes (and/or densities) for predictions
of in situ settling will result in underestimates of the
amounts stored in the channels or reservoir systems and
overestimates of the mean grain size of the stored fine
particles.
Inorganic particles in fresh water are known to flocculate,
or combine in number with other mineral or organic material
to create larger particles. This process increases the
effective size of the particle and modifies its density. Both
alter it's settling characteristics. In river systems where
flocculation occurs, mineral material in the silt and clay
size range can deposit in flow conditions which would
not allow single grain settling. While most fluvial geomorphologists
have observed the presence of fine grained
sediments stored in regions with current speeds which
should inhibit single grain settling, these observations are
often not questioned or considered in sediment transport
models. Reservoir age estimates are often wrong, one reason
being that these fine sediments which are assumed to
move through the system actually settle behind the the
dam as flocs and act to fill the reservoir more quickly.
The process of flocculation is enhancing settling within
these fluvial systems - but our field and laboratory measurements
do not consider this process.
By presenting data from the sizing of natural riverine sediments
and results from settling experiments in a variety
of Canadian rivers we would like to emphasize the
significance of the flocculation process in fluvial geomorphology.
The grain sizes of laboratory measured particles
cannot be assumed to correctly reflect in stream
processes yet these are the data that are most commonly
used to estimate transport and storage processes in river
basins.
JEAN-PIERRE PEULVAST
Late and Post-Orogenic evolution of large orogens and
the problem of planation: a review
Universite de Paris-Sorbonne, Depam, et Laboratoire de
Geodynamique interne, Cnrs, Bat. 509, 91405 Orsay Cedex, France
The ancient orogens that form the structures of large parts
of the continents are mostly bevelled by planation surfaces
more or less covered by sediments. The largest of these
'surfaces are found on Precambrian shields but well planated
surfaces are also represented in Palaeozoic to Cenozoic
orogens as well (Europe, Mediterranean Basin, Eastern
United States and Canada...). Some of them are disposed
as large simple surfaces with residual reliefs. Systems of
polygenetic or stepped surfaces are found in regions where
the basement was ultimately deformed in relation with post-orogenic
rifting or continental breaking up (rift shoulders,
passive margins). Though these surfaces have been
recognized and extensively studied for a long time, their
origin and the mechanisms by which high mountains are finally
planated are still under debate. A comparative study
of several orogens of Palaeozoic to Cenozoic ages, inactive
or still active, suggests the combinate influences of tectonic
and erosive factors.
The formation of large planation surfaces implies conditions
of equilibrium or positive balance between erosion
and vertical movements. Whatever the mechanisms involved
in this process may be, peneplanation, pediplanation,
etchplanation..., more or less complete planation can be
obtained 10 to 30 Ma after the end of orogeny, as shown
by unconformities along the Scandinavian Caledonides or
in the Variscan orogen. Mechanisms corresponding to tropical,
arid, semi-arid or marine conditions or to conditions
without present analogue, are often considered as the most
efficient, but their effects are better understood in cases
where preliminary breaking up and lowering of the relief
could occur. As shown by several examples, such a reduction
of relief is probably related to late orogenic evolution
characterized by strong faulting and high rates of erosion.
A Basin and Range type of landscape evolved in parts of
the Variscan orogen before its final planation and burying.
Elements of a somewhat similar evolution are described in
parts of the Appalachian orogen of Canada where sub-Carboniferous
landforms comprised fault scarps and residual
reliefs of reduced height finally planated or buried below
thick conglomerates brought by local and longitudinal
streams.
More recent orogens like the Mediterranean ranges in the
Aegean region and in Italy show the strong influence of extension,
doming and faulting related to gravitational spreading
and crustal thinning following major stages of crustal
shortening and thrusting. As the process of plate convergence
is still active, the development of pediments and planation
surfaces was more or less hampered by vertical mo-
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