Essai sur les mécanismes de fragmentation des marnes des Baronnies (France)/Trials carried out on the disintegration mechanisms of marls in the Baronnies Mountains (Southern Alps, France) - article ; n°1 ; vol.2, pg 23-49

Patrice Garnier, Michel Lecompte

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Géomorphologie : relief, processus, environnement - Année 1996 - Volume 2 - Numéro 1 - Pages 23-49
Résumé Des recherches ont été entreprises sur la fragmentation des marnes dans les ravines des Baronnies. La décompression superficielle pourrait expliquer la fragmentation initiale de la roche compacte telle qu'on l'observe, après denudation de celle-ci, dans des fosses abritées de l'action directe des agents atmosphériques. L'eau joue un rôle essentiel dans la comminution des débris engendrés par cette fragmentation primaire et, par conséquent, sur l'ablation et l'allégement de la couverture détritique. Comme celui-ci retentit mécaniquement sur la facilité d'ouverture des fissures de la roche sous-jacente, l'action superficielle de l'eau contrôle ainsi de manière rétroactive le processus de désagrégation en profondeur. La sensibilité à l'hydroclastie des débris des marnes oxfordiennes et gargasiennes, les secondes pourtant nettement plus calcaires, expliquerait la rapidité de leur érosion, par rapport à celle des marnes cénomaniennes. Ces différences de sensibilité sont à rechercher dans l'histoire pétrographique des marnes, y compris certaines microstructures à découvrir.
Abstract Research has been undertaken on the disintegration of marls in the gullies of the Baronnies Mountains. Release of surface pressure might explain the initial disintegration of cohesive rock, as observed after surface erosion of the latter, in trenches protected from the direct action of weathering agents. Water plays an essential role in the comminution of debris resulting from this incipient disintegration, and thus in the denudation of the rock-mantle and the relief of the load on it. Since this has a mechanical effect on the extent of joint widening in the underlying rock, the action of water on the surface makes it possible to retrospectively monitor the process of disintegration within. The rapid erosion of both oxfordian and gargasian marls, even though the latter have a much higher calcium carbonate content than the former, is much more rapid than that of cenomanian marls, and can probably be explained by their propensity to disintegrate through hydration. Differences in the levels of sensitivity to such disintegration no doubt lie in the petrological history of the marls, possibly in certain microstructures which have yet to be discovered.
27 pages
Source : Persée ; Ministère de la jeunesse, de l’éducation nationale et de la recherche, Direction de l’enseignement supérieur, Sous-direction des bibliothèques et de la documentation.

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• Publié par: GEOMORPHOLOGIE_RELIEF_PROCESSUS_ENVIRONNEMENT
• Publié le: 01 janvier 1996
• Nombre de lectures: 32
• Langue: Français
• Poids de l'ouvrage: 3 Mo

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Patrice Garnier
Michel Lecompte
Essai sur les mécanismes de fragmentation des marnes des
Baronnies (France)/Trials carried out on the disintegration
mechanisms of marls in the Baronnies Mountains (Southern
Alps, France)
In: Géomorphologie : relief, processus, environnement. Juillet 1996, vol. 2, n°1. pp. 23-49.
Citer ce document / Cite this document :
Garnier Patrice, Lecompte Michel. Essai sur les mécanismes de fragmentation des marnes des Baronnies (France)/Trials
carried out on the disintegration mechanisms of marls in the Baronnies Mountains (Southern Alps, France). In: Géomorphologie
: relief, processus, environnement. Juillet 1996, vol. 2, n°1. pp. 23-49.
doi : 10.3406/morfo.1996.868
http://www.persee.fr/web/revues/home/prescript/article/morfo_1266-5304_1996_num_2_1_868Résumé
Résumé Des recherches ont été entreprises sur la fragmentation des marnes dans les ravines des
Baronnies. La décompression superficielle pourrait expliquer la fragmentation initiale de la roche
compacte telle qu'on l'observe, après denudation de celle-ci, dans des fosses abritées de l'action
directe des agents atmosphériques. L'eau joue un rôle essentiel dans la comminution des débris
engendrés par cette fragmentation primaire et, par conséquent, sur l'ablation et l'allégement de la
couverture détritique. Comme celui-ci retentit mécaniquement sur la facilité d'ouverture des fissures de
la roche sous-jacente, l'action superficielle de l'eau contrôle ainsi de manière rétroactive le processus
de désagrégation en profondeur. La sensibilité à l'hydroclastie des débris des marnes oxfordiennes et
gargasiennes, les secondes pourtant nettement plus calcaires, expliquerait la rapidité de leur érosion,
par rapport à celle des marnes cénomaniennes. Ces différences de sensibilité sont à rechercher dans
l'histoire pétrographique des marnes, y compris certaines microstructures à découvrir.
Abstract Research has been undertaken on the disintegration of marls in the gullies of the Baronnies
Mountains. Release of surface pressure might explain the initial disintegration of cohesive rock, as
observed after surface erosion of the latter, in trenches protected from the direct action of weathering
agents. Water plays an essential role in the comminution of debris resulting from this incipient
disintegration, and thus in the denudation of the rock-mantle and the relief of the load on it. Since this
has a mechanical effect on the extent of joint widening in the underlying rock, the action of water on the
surface makes it possible to retrospectively monitor the process of disintegration within. The rapid
erosion of both oxfordian and gargasian marls, even though the latter have a much higher calcium
carbonate content than the former, is much more rapid than that of cenomanian marls, and can
probably be explained by their propensity to disintegrate through hydration. Differences in the levels of
sensitivity to such disintegration no doubt lie in the petrological history of the marls, possibly in certain
microstructures which have yet to be discovered.Essai sur les mécanismes de fragmentation
des marnes des Baronnies (France)
Trials carried out on the disintegration mechanisms
of marls in the Baronnies Mountains (Southern Alps, France).
Patrice Garnier et Michel Lecompte
Résumé
Des recherches ont été entreprises sur la fragmentation des marnes dans les ravines des Baronnies. La
décompression superficielle pourrait expliquer la initiale de la roche compacte telle qu'on
l'observe, après denudation de celle-ci, dans des fosses abritées de l'action directe des agents atmosphér
iques. L'eau joue un rôle essentiel dans la comminution des débris engendrés par cette fragmentation
primaire et, par conséquent, sur l'ablation et l'allégement de la couverture détritique. Comme celui-ci
retentit mécaniquement sur la facilité d'ouverture des fissures de la roche sous-jacente, l'action superfi
cielle de l'eau contrôle ainsi de manière rétroactive le processus de désagrégation en profondeur. La
sensibilité à l'hydroclastie des débris des marnes oxfordiennes et gargasiennes, les secondes pourtant net
tement plus calcaires, expliquerait la rapidité de leur érosion, par rapport à celle des marnes
cénomaniennes. Ces différences de sensibilité sont à rechercher dans l'histoire pétrographique des marnes,
y compris certaines microstructures à découvrir.
Mots clés : Marnes, érosion, décompression, hydroclastie, météorisation, méditerranéen, montagne
Abstract
Research has been undertaken on the disintegration of marls in the gullies of the Baronnies
Mountains. Release of surface pressure might explain the initial disintegration of cohesive rock, as obser
ved after surface erosion of the latter, in trenches protected from the direct action of weathering agents.
Water plays an essential role in the comminution of debris resulting from this incipient disintegration, and
thus in the denudation of the rock-mantle and the relief of the load on it. Since this has a mechanical effect
on the extent of joint widening in the underlying rock, the action of water on the surface makes it possible
to retrospectively monitor the process of disintegration within. The rapid erosion of both oxfordian and
gargasian marls, even though the latter have a much higher calcium carbonate content than the former,
is much more rapid than that of cenomanian marls, and can probably be explained by their propensity to
disintegrate through hydration. Differences in the levels of sensitivity to such disintegration no doubt lie
in the petrological history of the marls, possibly in certain microstructures which have yet to be discover
ed.
Key words : Marls, erosion, release of pressure, disintegration by hydration, weathering, Mediterranean,
mountain.
Abridged English version
This article completes a series of publications concerning the gullying of marls in the
Baronnies Mountains produced under the auspices of the Centre National de la Recherche
Scientifique (CNRS) (cf. Dumas et al., 1987 ; Chodzko et al, 1991 ; Chodzko et Lecompte, 1992 ;
Lhénaffet al, 1993 ; Alexandre, 1995). It attempts to explain the phenomenon of marl disinte
gration at depths of a few dozen decimetres below the surface.
' Laboratoire de Géographie Physique, Université de Paris VII, 2 Place Jussieu 75005 Paris. CNRS (G.O. 849),
Université de Paris- Val de Marne, Créteil.
Géomorphologie : Relief, processus, environnement, 1996, n°l pp. 23-50 24 Patrice Garnier et Michel Lecompte
Current state of research and experiments
The study presented here includes petrological data concerning oxfordian, gargasian and
cenomanian marls in this particular region of France, their sedimentology and fabric, and the
dynamics of their discompacting (fig. 1, tab. 1, 2, 3). The sources used are the data bases of
Georef(USA), Geoarchives (UK), Geobase (UK), Pascal (France), and Horizon (ORSTOM,
France). Experiments are being conducted both in the field and in the laboratory : the first
concern the final, or "current" release of pressure in compact rock in situ; the second the action
of water on fragments released. Such action is likely to interreact with physical and transpor
tation processes and determine the extent of subsequent reduction of debris cover.
Fundamental research in the field has consisted in creating covered decompression trenches
in order to observe the effects of relieving the load on rock protected from weathering and infi
ltration. Areas of compact rock of approximately one metre square were exposed by the removal
of mantle and regolith, and then sheltered by sheets of corrugated iron anchored down at the
top and at the sides. Outcrops of both cenomanian ("Chassenaye" site) and oxfordian ("Séderon" site) marls have been studied.
In the laboratory, thin plates were produced in order to observe the mineralogical charact
eristics, grain-size distribution and weathering of the three types of marl surfaces. Various
hydric measurements made it possible to determine spatio-temporal variations in the volume of
water in the rock, regolith and mantle. The degree of fragmentation by hydration was calcula
ted using the measuring protocol of the Paris Laboratoire des Ponts et Chaussées (LCPC).
Supplementary analysis of the geometry, porosity (measurement ofmicroporosity using mer
cury) and structure of the clay minerals (using electronic scan microscopy) is now necessary.
Principal findings
With regard to the release of pressure, the very removal of mantle and regolith leads to the
rapid crumbling of the exposed rock. In natural conditions, this decompression is regulated by
the constant erosion of the debris mantle. The disintegration of the marls is facilitated by rock-
jointing which results from the release of internal pressures. The forms of the tectonic figures
thus created will depend on the petrological history o