ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

lOAN MAC & lOAN IRIMUS
The evolution of the riverbeds in conditions of
tectonic basin situated on a platform morphostructure:
the Transylvanian Depression
«Babes-Bolyai» University, Faculty of Geography,
Clinicilor 5-7 Street, Cluj-Napoca, 3400, Romania
The fluvial riverbeds are subjected to a complex controll,
with both variables dependent on the fluvial system and independent
of this one.
The Transylvanian Depression, situated in the middle of
the Carpathian orogene, but on a structure of platform, covered
by a thick sedimentary layer, presents a totally specific
situation, of morphohidrodynamic controll. First of all
the tectono-structural variable is expressed by two tendencies
which affect both the phragmented basement and the
layer, which is distarted by epirogenetic movements a.nd
diapir folds: the raising of territorial compartments which
affect segments of valleys and subsident movements, which
impose other relations between the erosional processes
and the accumulation ones. In such circumstances the riverbeds
present contrastant behaviours on neighbouring
spaces: strongly alluviated segments in alternation with
segments in which the main rock comes directly at the surface;
portions in which appear floodplains, but also portions
where these ones are lacking; portions with highly
developed meanders, in which the corridors of meandration
are well distinguished and portions in which the riverbeds
follow an evolution with a relative uniform profile.
Consequently, the tectonic reflex of both the basement
and the layer, is expressed in the present physiognomy,
structure and dynamic of the fluvial riverbeds, in differentiated
shapes, from place to place.
TOMMASO MACALUSO 1 & DGO SAURO 2
Aspects of weathering and landforms evolution
on gypsum slopes and ridges of Sicily
1Dipartimento di Geologia, Universita di Palermo,
corso Tukory 131,90134 Palermo, Italy
2 Dipartimento di Geografia, Universita di Padova,
via del Santo 26, 35123 Padova, Italy
Most of the papers dealing with the geomorphological evolution
of surfaces in gypsum rocks develop the analysis of
the erosional forms originated by mass wasting, by fluvial
and by karstic processes. In particular it is possible to find
information about landslides, fluvial valleys, blind valleys
and dolines.
Aim of this paper is to discuss relatively neglected aspects
related with the weathering latu sensu of the outer layer of
the gypsum rock, especially in correspondence of the de-
nuded slopes and ridges.
In western Sicily we have found ideal conditions for the
study of these aspects. In fact there are large rocky surfaces
without a soil cover as a consequence of the soil erosion induced
by forest clearing, fires, sheep and goat grazing. Macro-crystalline,
alabstrine gypsum, laminated balatine and
arenitic gypsum outcrops widely. Between the most interesting
areas we remember the large slope of Serra Balate,
south of Palma di Montechiaro and some reliefs in the neighbouring
of Sant'Angelo Muxaro (f.i, Monte Mpiso).
On most of the surfaces it is possible to recognise a
«weathering crust» characterised by a polygonal fissuring
and other small forms indicating phenomena of increase of
volume of the outer rock mass for a thickness between some
decimetres and some metres. The development of this
crust is not controlled by the bedding or other structural
features, even if sometimes one or more sides of certain
polygons correspond to previous fracture lines.
Inside the crust there is a clear evidence of the tendency to
the sealing both of the previous and of the new formed fissures.
The morphological types recognised on the rocky surfaces
are: a) the rocky polygons, often with bended fringes, b)
the gypsum bubbles, c) the pressure ridges, d) the pressure
humps, e) the pancakes, f) the pressure pans and the pressure
half pans, g) the steps.
The size of these forms is between some decimetres and
some tens of meters. The largest bubbles are more than 10
meters in diameters and 50 ern in high.
In some areas it is possible to recognise «mega - bubbles»
in form of small domelike hills with diameters of some tens
of meters and highs of some meters. On some of these
forms a well developed polygonal crust is also recognisable.
A spectacular group of mega-bubbles constitutes a
small ridge near to the houses «Salamona» not far from
Muxariello on the road to Sant'Angelo Muxaro.
The summits of many hills in gypsum may appear similar
to the mega-bubbles. These domelike forms show different
sizes, from some tens to some hundreds of meters in diameters.
The domelike forms are not controlled by the dip
of the gypsum beds; in fact those have been found also on
blocks with steeply deeping or vertical beds. On most of
the dome summits a polygonal fissuring with pressure
structures underline that the outer layer is evolving as a
«weathering crust» characterised by processes of increase
of volume.
However, it is not possible to explain the evolution of these
domelike summits only with the bulging phenomena of
the outer layer. The genesis of this type of form is surely
the expression of a homogeneous behaviour of the rock in
comparison with the erosional processes. These forms may
remember some domelike summits in granite rocks. Probably,
it is just the formation of the «weathering gypsum crust»
which favours the development of the domelike forms
trough the creation of isotropic field of stresses, with reference
to the central part of the relief. The weathering crust
minimise, in this way, the influence of the pre-existing
structural elements, like the bedding planes and the
fractures.
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