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Cartographie numérique en géologie de surface. Application aux altérites à silex de l'Ouest du bassin de Paris

Florence Quesnel 1
1 Direction des GéoRessources, Unité Géologie de l'Aménagement des Territoires
BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
Abstract : The compilation of data from 50 geological maps (1:50,000 scale), 2,500 boreholes and their combination in the creation of thematic maps using G.I.S. software, has enabled the anatomy of the flint alterites (termed RS, residu à silex or "clay-with-flints") of the western Paris Basin to be established. Geostatistical treatment of the thickness variable of superficial plateau formations (RS + plateau loams + Cenozoic residuals) and the image of their rugosity characterise the irregularity at a local scale of the uppermost surface of the chalk, attributable to solution pipes. The anatomy of the various types of superficial formations on the one hand is correlated with the morphology of the surfaces upon which they developed, and on the other with the thickness of the dry substrate. Binary diagrams reveal two families of RS, respectively associated with Palaeogene and Plio-Quaternary palaeosurfaces. The typology of the RS formations undertaken on the basis of field observations, maps of analytical B. LAIGNEL's results, along with the study of associated residual Cenozoic formations, has enabled identification of several generations of flint alterites in the western Paris Basin : - reworked palaeo-RS occurring on the marine abrasion platforms of the Upper Thanetian to the north of the Seine (except in the Normandy trough) and the Upper Rupelian of the Drouais area. - RS developed from the Palaeogene to the present day (Thimerais facies and its variants). These are the most evolved of the whole study zone and may locally be overlain by Palaeogene fluviatile deposits, and/or be silicified at the top of the profiles during Middle Eocene, and/or be sealed by detrital or carbonate lacustrine formations of Late Eocene age. - RS which developed after the deposition and under the cover of the Fontainebleau Sands of the Drouais area, the Lozère and St-Eustache Sands of the Roumois and Pays de Caux areas, the Pleistocene alluvium of the Seine and the Eure rivers, and after the removal of the Ypresian clays of the Normandy trough and the Talou. Of Plio-Quaternary age, they are thin and poorly evolved, as well as enriched in a significant allochthonous fraction. In addition the uppermost surface of the chalk is very rugose. These various flint alterites characterise a Palaeogene continental surface having attained a state of equilibrium and strongly incised Pliocene-Quaternary surfaces in a state of non-equilibrium. Biostratigraphic dating of hollow flints of the RS and palaeo-RS shows that alteration took place in situ and that RS profiles have only been subjected to minor erosion to the upper surfaces. The current anatomy of the RS shows the sedimentary wedge of the original flint-bearing chalk lapping onto the Armorican border of the Paris Basin. Microfauna confirms the existence of the Maastrichtian in the Paris Basin, a fact hitherto doubted because of subsequent weathering, alteration and erosional processes. The thicknesses of dissolved chalk have been quantified on a 3D map in the zones of different RS facies types integrating B. Laignel's one dimensional isoflint calculations. The average rate of dissolution varies regionally and over time as a function of the climatic context and the intensity of the drainage. Dissolution was more intense for the Plio-Quaternary RS (10m/Ma) than for the Palaeogene RS (2m/Ma). The Vertical Finite Deformation Component (VFDC) of the Palaeogene continental surface has been calculated between the Thanetian and the present to produce a 3D map using an image of the thickness of dissolved chalk. The VFDC varies between + 170m and - 170m with an average of 1.5m and an incertitude spread of between ± 20m and ± 40m. Confronting altitude and facies or alterites remaining with contemporary marine palaeoelevations, enables a control of the coherence of results obtained and the validation of the VFDC 3D image for all the zones tested. A general uplift of the western Paris Basin border at the beginning of the Tertiary is demonstrated, whilst the hypothesis of a recent 100 to 150 m uplift of the Pays de Caux is shown to be unfounded. An uplift is however perceived along the "Eperon du Perche" area and more recently for the Bray and Picard Dome areas. The lithospheric doming, already demonstrated in the Brittany, Vendée and Poitou regions (Wyns 1991, 1996b) and linked to the Europe-Africa collision (Pyrenean compression), is identifiable as far north as Normandy. It is however strongly resorbed in the grabens of the Perche and Drouais areas in the Late Eocene and just prior to the Rupelian, because of the transition from southerly oriented Pyreneean compression to E-W oriented Oligocene distension. This internal geodynamic factor conditioned the location of those zones subjected to weathering, whilst the external geodynamic factors (climate and base-level) controlled the rate of weathering. There remains to validate and quantify the hypothesis of the retroaction of chalk dissolution on the uplift, which constitutes an indirect example of the influence of climate on the vertical stability of the lithosphere. Finally, the cartographic work has direct practical applications in the fields of planning such as deposit studies for flint usable as an aggregate resource and the prevention of karstic collapse, as well as in terms of environment such as water quality management and geoforcasting maps predicting the rate of basement deformation.
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Submitted on : Friday, October 5, 2012 - 5:35:16 PM
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Florence Quesnel. Cartographie numérique en géologie de surface. Application aux altérites à silex de l'Ouest du bassin de Paris. Sciences de la Terre. Université de Rouen, 1997. Français. ⟨tel-00739116⟩

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