Résumé
Les volumes sédimentaires accumulés dans les lacs au cours du Quaternaire ont été évalués par les méthodes électriques de la géophysique appliquée, appuyées sur l'étude géologique de forages comprenant des datations, de la palynologie et des analyses minéralogiques. Ces méthodes ne fournissant pas une solution unique à l'interprétation de leurs mesures, la meilleure succession des opérations consiste en une étude préliminaire sur le terrain suivie de quelques mesures géophysiques de reconnaissance, puis d'une campagne simultanée de prospection de surface et de forages. Une dernière phase consiste en complément de forages et/ou de géophysique. Cette procédure est indispensable à la production de résultats probants, l'obtention d'un modèle sérieux étant basé sur l'échange d'informations entre les prospections de surface et les forages. L'étude donne lieu à l'établissement de documents qualitatifs et quantitatifs qui contribuent à l'évaluation du taux de sédimentation dans le temps. Dans le bassin du lac de Chambon (Puy-de-Dôme), on a pu ainsi calculer les volumes de sédiments de quatre unités lithologiques sur quatre zones géographiques représentant une surface totale de 250 ha. Pour le lac du Tartaret, le volume total est de 11.106 m3 ce qui conduit à un taux d'érosion de 70 µm/an et de sédimentation de 1 mm/an. Le lac Chambon proprement dit est crédité du même volume de 11.106 m3 et de taux respectifs de 110 µm/an et 2 mm/an.
Abstract
Applied geophysical methods contributed to the quantitative evaluation of sedimentary rock volumes in lakes during the Pleistocene. The measurements (Fig.2) were made by electrical and electromagnetic methods in Puy-de-Dôme (France). Because of the limited resolution of geophysical methods due to their abundance in possible solutions, a multiple-method programme was carried out in well-situated boreholes, including 14C age determination, palynological and mineralogic analysis. Preliminary field and experimental study was followed by almost synchronous geophysics and drilling, succeeded by additional geophysics and drilling where necessary. This multi-method programme had the advantage of avoiding the multiple solutions produced by geophysics used alone, and resulted in a credible model based on an interchange of geologic information from drill holes and geophysical experimental results. In general electrical soundings were used with an injection dipole length of 200-300 m and consequently an investigation depth of 50 m or so was sufficient to obtain the data required. The geophysical program consisted of 52 electrical soundings, 16 Audio M-T soundings and 12 km of Radio M-T profiling. In a few localities magnetotellurics were used (Dupis et al., 1991) in the Audio M-T tests to attain greater penetration depth, Radio M-T was also used to verify-using continuous profiling - the presence of geological discontinuities or of abrupt facies variations in the sedimentary deposits. Maximum error in electrical methods was estimated at 5%, which has an insignificant effect on the quality of the model. The need for 14 C or palynologic dating methods to determine the age of the rocks was another reason for carrying out a drilling programme. Fig. 2 shows the locations of electrical soundings or Audio M-T soundings and Radio M-T profiling, and the four zones into which the basin was subdivided to take account of the sedimentary evolution in time and space. Fig. 3 shows potential dipole isoresistivity values at 2 m, 15 m and 40 m. The four zones can be distinguished by the values and by the density of the resistivity curves. Fig. 4 shows the relation-ship between electrical sounding curves and the sedimentary facies encountered in drill-holes. Geophysical interpretation was based on interpolation (or extrapolation) between drill-hole intersections. A large number of qualitative and quantitative documents were contributed as sediment budget data. Four lithologic units were distinguished in four geographic zones over a total area of 250 ha. Two different formations were identified: (1) the Tartaret formation comprising lacustrine unit (L1) overlain by a more deltaic unit (D1) dated at 12.6-8.5 Ka BP; and (2) the present Lac Chamhon formation, similary divided in L2 and D2 units, and active 2600 years ago. Figs. 5 and 6 show different sections to illustrate the geometry of the four sedimentary facies of the basin. Fig. 7 details geoelectric cross sections and situates the four phases of basin history. Fig. 8 shows the volume of the sediments of the four lithologic units in the four zones (Fig. 1). Table 1 presents an evaluation of the volumes of the four lithologic formations based on area and mean thickness. The contribution of unit L2 is very light in zones A und B but unit L1 is more equally distributed in the four zones. The rangest values are obtained, L2 in D (6,80 m thick) and for D2 and L2 (respectively 4 million and 3.5 million m3 in volume). The Tartaret formation has a total volume of 11.106 m3, giving and erosion rate of 70 µm/year and a sedimentation rate of 1 mm/year. Chambon lake itself has a volume of 11.10.6 m3 but for a period of 2 600 years only and an area of 37.7 km², leading to an erosion rate of 110 µm/year and a sedimentation rate of 2 mm/year. This second value is twice the first one: our interpretation is that clearing the land and planting both failed to protect the ground from a more intensive erosion as the forests disappeared. Applied geophysics can contribute to estimation of sediment budget if geological and geophysical data are integrated. To obtain a reliable model it is more important to understand the geological situation than to cavil dispute inversion techniques, as the total error could reach values as high as 20-30%, but without geophysics the problem has no solution at all.
Dernière mise à jour le 28.07.2015