ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

een formulated that the activity of strike-slip faults can
lead, eventually, to the rotation of preferential stream
directions.
This study has a twofold aim: to improve the knowledge of
the structural arrangement of some Italian areas affected
by tectonics active in recent or historical times, and to aid
the individuation of possible strike-slip faults of regional
importance, through the careful examination of the drainage
network patterns. To this end a methodology has been
defined to infer the existence of strike-slip faults on the basis
of the quantitative characteristics of surface drainage
and to verify whether the activity of such faults resulted in
areal variations or rotations of the preferential stream
orientations (or domains).
In the first phase of this study a sample area has been chosen;
it is located in Latium (Central Italy) and is crossed by
a left NW-SE trending strike-slip fault, known in the specific
literature as «Val Roveto Fault»,for which the results
of field survey and the related mesostructural data are avai­
1able. The quantitative characterisation of the drainage
network of this sample area has been carried out separately
for the two sectors divided by the tectonic discontinuity;
moreover within each sector both the whole drainage
network and the different stream orders have been considered
and analysed in quantitative terms. The results of
this first phase of the research have pointed out that some
significant modifications of the preferential stream orientations
occur close to the strike-slip fault.
To test the applicability of the proposed method, a second
sample area has been considered; it is located in Tuscany
(Central Italy) and is characterised by the existence of an
hypothetical strike-slip tectonic element which is more
properly considered by the Authors as a «belt of deformation
and discontinuity» rather then as a linear element. To
better define the location of this hypothetical tectonic element,
various azimuthal spectra of the channels of each
stream order have been previously elaborated and analysed;
such spectra have been automatically obtained through
suitable software and have been realised according to
azimuthal «transepts» randomly crossing the study area.
This analysis has allowed to evidence the stream channel
preferential orientations; in this way sharp deviations from
the channel main directions and domain rotations have
been singled out, which may be the surface effects of important
discontinuity lines.
The results obtained for both the considered sample
areas provide hints from the methodological point of
view. In fact it has been possible quantitatively evaluate
which of the linear and areal variations of the drainage
network main geometric characters may individuate the
possible strike-slip motion of tectonic lines of uncertain
interpretation. Moreover the proposed methodology has
turned out to be suitable to single out and locate tectonic
lines of regional importance within areas lacking of geological
and tectonic information. The developed methodology
may provide useful also for preliminary evaluations
of geological structures over wide areas by satellite
image analysis.
MOHAMED BEN BRAHIM 1
& M. THIRY 2
Un piemont detritique it silicifications differenciees:
les Hamadas tertiairesde Boudenib (Sud-Est Maroc)
1 Departement de Ceomorphologie, Universite Mohammed Premier,
b.p. 457, 60000 Oujda, Maroc
2 Centre Informatique Geologique, Ecole des Mines de Paris,
35, rue Saint-Honore, 77305 Fontainebleau Cedex, France
Le systeme de piemont atlasique du Sud-Est marocain,
d' age tertiaire, est constitue dans le secteur de Boudenib
de deux sequences distinctes. La Hamada de Boudenib
forme la sequence interieure et se caracterise par d'importants
encro tements carbonates, riches en dolomite et attapulgite,
La sequence superieure de la Hamada du Guir
ma.rque une reprise de l'erosion et le depot de materiaux
relativement peu alteres, Les deux sequences sont affectees
par deux grands types de silicifications qui nont pas la
m me signification.
Dans la Hamada de Boudenib (Eocene), les silicifications
affectent l'unite inlerieure : Boudenib I (Eocene inferieur)
ainsi que les gres cretaces sous-jacents, et jalonnent une
surface d' erosion reconnaissable dans tout le piemont. Ces
silicifications sont precoces, puisque remaniees dans les
depots ulterieurs, Les illuviations d' opale et les accumulations
granulaires intercalees entre les concretionnements
successifs de silice attestent d' environnements pedologiques.
La silicification plus forte vers la partie superieure
des coupes, ainsi que I'association frequenre des facies silicifies
avec des niveaux bioturbes et alteres sont egalement
a rapporter a des dispositions pedologiques, Ces silicifications
se developpent pendant une periode de stabilite tectonique
et s'installent dansun paysage de glacis et sous des
climats chauds a saisons alternees, Leur reconnaissance a
l'echelle regionale constitue un rep ere lithostratigraphique
precieux.
Dans la Hamadadu Guir (Neogene), les silicifications
sont presentes a tous les niveaux de la sequence: gres,
conglomerats, marnes et calcaires lacustres, mais toujours
sous forme d' «accidents siliceux» circonscrits depuis des
veines millimetriques jusqu'a des amas metriques, L'etude
micrographique de ces silicifications montre clairement
que celles-ci sont tardives, post-sedimentaires. L' essentiel
de la silice est constitue par des depots dans des vides de
dissolution et des epigenics en bordure de ces vides. Ces
depots des les vides montrent une zonation remarquable
des dilferenres phases de cristallisation de la silice, allant
de l' opale vers des quartz automorphes, et temoignent de
la circulation de solutions nourricieres, L' absence de geotropisme
dans ces depots indique un regime hydraulique
noye, d' ecoulernent de nappe. La charge en silice des eaux
est acquise par l' alteration de mineraux silicates lors de
I'ecoulcmcnt des nappes. Ces dernieres sont conditionnees
par des modeles d'entaille qui forment les exutoires
des nappes: les vallees. La superposition de plusieurs
niveaux de silicification indique des variations dans
75