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L'influence de la geologie sur la karstification. Etude comparative entre le Massif Obarsia Closani - Piatra Mare (Roumanie) et le Massif d'Arbas (France)

Abstract : The influence of the geology over the karstification is obvious. Practically, every paper concerning a karstic zone put in evidence the role played by the geological factors for the karst initiation, evolution or the functioning of karst.
Our aim in this study is not to confirm what is already known. We intend to identify the karst peculiarities induced by the geological factors. To do that, we can recognize a peculiarity by comparing it with a reference model, which represent the standard. In the karst terrains, with their great variety, finding such real model seems to be impossible. In turn we can use a theoretical model. There is another difficulty: the choice of the model, because, practically every karstological school have his own model.
In what concerning us, we consider the karst like a geodynamic system. This concept is very closed to the functional model of Mangin (1975, 1982, 1984, 1985, 1986, 1994). In fact, the essence of the geodynamic systems is to consider the continuous relation between the morphology and the functioning in his evolution. The morphology can be considered like the spatial structure that assures the system functioning.
The next step was to choose between the karst categories defined by the model. The main criterion applied in this model is the kind of energy, which assures the karst functioning. So, we choose the gravitational karst, which means the karst where the water flowing is due to the gravitational force.
Hereafter, depending on the complexity of the criteria system we can arrive even to identify the individual karsts. Viewing the aim of this study, we preferred to make a comparison, not just referring to the model. So, we will refer to two karstic zones. The main raison to doing so is that the comparison will allow outlining the peculiarities of each zone, not only referring to the model but also referring one to the other one. In this manner we can identify the factors that are behind these peculiarities and how they acted to impose that.
To assure a meaningful sense of the comparison, we must impose some conditions to our chose. So, the two zones present similarities in what concern the geostructural environment.
Both of them are developed in folded areas with an evident well-developed tectonics. In both zones, the karstified carbonate series cover the same age interval: Jurassic . Cretaceous. Also, from a geomorphologic point of view the zones correspond to low high mountains (not exceeding 1700 m of altitude).
There is a notion that makes a sense for all .karstological schools.: the karstic massif. Why? Because all the karstologists need to place and refer their works in rapport with a spatial unit: the karstic massif. It is not obvious all the time to defining the limits of the karstic massif. They can be geomorphologic, geologic or functional limits (Goran, 2001). Important is that the karstic massif is the coherent spatial unit which show a certain homogeneity from a geomorphologic, geological or functional point of view. The two zones that we will compare represent two karstic massifs: the Arbas massif (Haute Garonne, France) and the Obarsia- Closani . Piatra Mare massif (Gorj . Méhédintzi, Romania).
The both massifs will be treated in the same manner. First, we will present the geology: carbonate series, geological structures and tectonics. This will allow us to have a first image of the potentially geologically factors.
The next step will concern the analysis of the karst features on both massifs. We will regard the morphology and the functioning too. So, we will be able to have an image over the karsts type, his functioning and development, and, finally, over the evolutionary state of the karsts. At this point, important is to identify the geological elements, which, at the massif scale, have favored the karst developing and, mainly, which imposed the karst peculiarity.
Finally, we can compare the two massifs. Starting to compare the karst features, we will be able to find the similarities and, more important, the dissimilarities from the geological point of view.
In the second chapter we will try to follow the relationship between the geology and the karst at an intermediate scale, called outcrop or intra-karstic scale. From a spatial point of view, this scale is in the range of meters.
One of the ways left to say something more objective about the karst morphology is to increase the objectivity of our analysis and interpretations. Quantifying the morphology and applying the mathematical analysis methods can make this. We tried to apply this new methods to analyze the cave walls morphology. In this category, one peculiar morphology was intensively studied: the scallops and flutes. By their properties the scallops are of great interest for our study, so, we present the basic problems and the main results regarding this morphology.
In several caves from Romania and France, from the two studied areas, developed in different geological situations, horizontal profiles of the walls were measured. In order to make a comparison, a profile in a limestone quarry was measured too. The obtained data series were analyzed with the classical method of signal treatment (spectral analysis) and other new methods (fractal and wavelets analysis).
All profiles show two different fractal dimensions that testify for relief developed at two different scales. For the small scale, the poor fractal dimension put in evidence the evolution of the wall morphology under sub-aerial conditions. The more developed morphologies (scallops) have the greatest fractal dimensions.
The spectral analysis shows the existence of some pseudo-periodicities. The log-log representations of the spectral densities make the difference between the cave profiles and the quarry measured profile. Since the quarry profile shows a pure Brownian character, the cave profiles are typical for a persistent fractionally Brownian character, which means that the existent variations are interdependent ones of others. For the scales between 3 and 7 cm, the slopes of the log-log representations allow us to make the analogy with the Kolmogorov model of the turbulence. For the small scale, the profiles are characteristic to a white noise. At this scale, even if the information exists, she cannot be interpreted because of the noise.
The wavelets analysis comes to reinforce the observations already made: the cave profiles consist in a hierarchy of morphological structures developed at different scales and interdependent ones of others.
All this observations allow us to conclude that the studied morphologies of the cave walls show the influence of a multi-scale morphogenetic process- the water flow. The original relief, depending on the geological factors, is fashioned by the flow, which arrive to integrate the initial pseudo-structures into an hierarchy of inter-dependent morphological structures. In addition, the used methods can put in evidence the influence of different morphogenetic processes.
Finally, some remarks regarding the morphogenesis of the cave walls are made.
We arrive in the third chapter at the microscopic scale - that is to consider the influence of the rock petrography on the main karstification process – the limestone dissolution.
Resuming the existing studies in this research field, we can outline the following facts: in nature, the dissolution is associated with a segregation process, the micritic fabrics are more easily attacked than the sparitic ones, an heterogeneous petrography favors the alteration process.
We can see that there is a relationship between the rock petrography and the dissolution process. In this relation we have a quantifiable variable – the dissolution, and a nominative one – the petrography. In order to make a comparison and to say something more explicit about this relationship, we will try to quantify the nominative variable.
The main idea in our thinking is to consider the limestone like a monomineral rock. Accepting that, the great varieties of the limestone derive from the micrite/sparite ratio and the spatial distribution of these elements. Their optical properties make possible the recognition of the rock petrography on images or photos realized in tons of gray.
They were used two types of images: one obtained directly on the thin sections (real surface - 1 cm2) and an other one obtained from the microscope made photos on the same thin sections (real surface – 1 mm2). These images were treated by means of some special software (image treatment, GIS system, 3d-representation) in order to obtain a series of numerical variables, which allow, as large as possible, a quantitative description of the rock petrography. They were obtained 16 variables. The multivariable analysis (analysis of principals components) allow us to see what variables are important in defining the different analyzed rocks and how to discriminate and regroup different rocks petrography. In order to have a more complete description of each rock, chemical compositions analysis were made and the content in CaO and MgO were added in the multivariable analysis.
To obtain the second member of the relation petrography – dissolution, we conceived a setup for the dissolution experience: 6 limestone tablets were exposed to the water flow during a long period time, in the same hydrodynamic conditions. For each experience we tested two different hydrodynamic conditions. The first experience was made at the Mangalia Laboratory (Romania) during 86 days, and the second one was made in the Moulis Cave (The Subterranean Laboratory, France) during 148 days. The standard tablets were different for the two experiences, and they come from the same blocks from which we made the thin sections. The samples come from the Arbas Massif (France) and Obarsia Closani – Piatra Mare Massif (Romania).
The quantitative variable – the dissolution rate – is estimated, for each rock type, as percent of the weight loss from the initial weight of the standard tablet during the dissolution experience.
The relationship between the petrography and the dissolution is analyzed by means of the already mentioned multivariable analysis, where the percent of weight loss is added to the already used petrography variables.
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Viorel Horoi. L'influence de la geologie sur la karstification. Etude comparative entre le Massif Obarsia Closani - Piatra Mare (Roumanie) et le Massif d'Arbas (France). Géologie appliquée. Université Paul Sabatier - Toulouse III, 2001. Français. ⟨tel-00166372⟩



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