Les altérites : l'épiderme de la Terre.

Les phénomènes d'altération supergène (météoriques) comptent parmi les processus les plus importants qui modifient la surface de notre planète. Les altérations sont le résultat des interactions entre géosphère, biosphère, hydrosphère et atmosphèr

An unexpected record of the PETM in terrestrial and organic sediments of Avesnois, between the Paris and Belgian Basins, NW Europe

International audienceThe stratigraphy of the Late Paleocene-Early Eocene has been revised in Avesnois thanks to drillings supporting a geological mapping project. Detailed sampling and analyses were performed along selected drillings, among which sedimentological, mineralogical, chemostratigraphic (δ13Corg), biostratigraphic and palynological data have been obtained. New Mbs/Fms are defined around the Paleocene-Eocene boundary (PEb) and compared to the lithostratigraphic nomenclatures of the Belgian and Paris Basins (Steurbaut, 1998; Aubry et al., 2005). Biostratigraphy based on the study of foraminifera and mammal fauna, palynological content and chemostratigraphy enable correlation to be established between adjacent basins as well as sea level and landscape evolutions to be refined during this critical interval. Upper Thanetian marine units In the AVE 007-drilling (Mormal Forest, Locquignol, between Englefontaine, Landrecies and Maroilles villages), the Coniacian chalk is overlain by a flint conglomerate, whose sandy-clayey matrix contains Thanetian pollen and dinocysts. Widespread in Avesnois and containing often reworked Upper Cretaceous foraminifera and occasionally glauconite, this unit constitutes the Vervins Mb (cf. 'Argile à silex de Vervins' interpreted as a marine conglomerate by Gosselet, 1879). It is often overlain by the Mondrepuis-Bettrechies Sand Mb (cf. 'Sable verts de l'Avesnois'), a thin sandy glauconiferous unit, more clayey and silty westward (cf. 'Tuffeau de Valenciennes' Mb). The three Mbs constitute the Hainaut-Valenciennois Fm and can be correlated to the Belgian Grandglise Mb (upper part of the Hannut Fm) and Bois-Gilles Fm, and to the Paris Basin Châlons-sur-Vesles and Bracheux Fms, all Thanetian in age. Uppermost Paleocene to lowermost Eocene terrestrial units In AVE 007 the Vervins Mb is unconformably overlain by flint gravels (Mormal Mb), then a pyritic sand (Le Quesnoy Sand Mb, cf. 'Sables et Grès du Quesnoy' of Gosselet, 1890), and a lignitic clay (Locquignol Mb), silty and ochreous at the top. All but the top ochreous paleosol contain spores and pollen grains and are devoid of dinocysts, they were thus probably deposited in terrestrial environments. The Carbon Isotope Excursion (CIE) of the PEb begins below the top of the sandy unit and continues in the lignitic clay. The palynological study confirms the Locquignol Mb's earliest Eocene age and allows correlation with the base of the Tienen Fm. To the East of AVE 007 along the Belgium-France border, fluvial fossiliferous gravels and sands have been studied in past sand quarries (Erquelinnes Sand Mb). Above a sharp unconformity, they i) overly Upper Thanetian marine sands (Bois-Gilles Fm, NP9a, then Grandglise Mb, NP8), ii) contain a mammal fauna referred as MP7 in the Mammalian biochronological scale for the European Paleogene (Missiaen et al., 2013) and iii) record the first part of the CIE onset in the basal fossiliferous gravel and marl beds. Three km west of Erquelinnes and toward AVE 007 a few drillings have recognized a sandy and pyritic unit containing resinite fragments, lignite and peat beds (Vieux Reng Mb), overlying a thin basal flint gravel bed (Mormal Mb). These five Mormal, Le Quesnoy, Locquignol, Vieux-Reng and Erquelinnes Sand Mbs form the Sambre Fm. Such fluvial gravels and sands, more or less lignitic, clayey or marly are widespread in northern France and southeastern Belgium. They belong to the so-called Upper "Landenian" and are correlated to the Tienen Fm. In AVE 007, they record the CIE onset marking the PEb and 2.5 m of Upper Paleocene fluvial units, in contrast with Belgian drillings such as Doel and Kallo where a hiatus is present at the PEb. Lower Ypresian hiatus and marine units In AVE 007, a laminated silty unit overlies the Locquignol Mb, contains dinocysts Wetzeliella sp. and agglutinated foraminifera similar to those of the base of the Orchies Clay Mb. Widespread in Avesnois-Valenciennois that marine unit is named Avesnois Mb, it is locally richer in very fine sand, and correlated to the base of the Kortrijk Clay Fm (Ieper Gp). In Avesnois, Sparnacian lagoonal units are absent upon the terrestrial ones described above, again marking a reverse trend when compared with the Belgian and adjacent Basins. This may result from restricted terrestrial environments coinciding with Variscan accidents and structural highs between the Artois anticline and Ardenne Masssif, or to their deposition followed by subsequent erosion prior to the Lower Ypresian marine transgression (cf. sea level drop 2 of Dupuis et al, 2011). Correlations and PETM impact on land All terrestrial units here described fill fluvial channels incised in marine Thanetian and older units such as the Cretaceous or the Variscan basement in Avesnois and Belgium, as already reported elsewhere in northern France (cf. sea level drop 1 of Dupuis et al., 2011). They exhibit locally cross stratification, lignite, flora or vertebrate fauna, for example at Leval, Hoegaarden and Dormaal, and/or are intensively silicified with scarce root or leaf casts. Similar fluvial sediments and silcretes are recognized in the first Sparnacian units and paleogeography of the Paris, Dieppe-Hampshire and London Basins, where the PEb is also present, the CIE being recorded on thicknesses between 3 m (as in AVE 007) to >20 m (as in Sinceny). These features may be regarded now as constant in those Sparnacian landscapes recording the PETM, with very rapid lateral and vertical facies shift, particular paleosols, a few fossiliferous fluvial to lacustrine units and variable deposition rates/sediments preservation. Terrestrial sediments, flora and fauna have been described there before the CIE onset as well, for example at Cobham, Vasterival or Rivecourt (e.g. Smith et al., 2014). Lihons, an intermediate outlier situated between AVE 007 and Sparnacian sites of those basins has further been investigated: marine Thanetian sands are overlain by thin fluvial, lacustrine then lagoonal sediments, the two latter recording the CIE of the PETM on an interval 4 m thick. Here again the basal terrestrial sediments have been deposited before the PEb. In AVE 007, depositional environments are fluvial for the Mormal and Le Quesnoy Mbs, and evolve toward flood plain then palustrine ones for the Locquignol Mb, probably here after the river bed migration, but still recording sporo-pollen fluvial inputs from the hinterland, the whole indicating a rather humid subtropical climate. Compared to the Locquignol Mb, the sporopollinic assemblage of the Mormal and Le Quesnoy Mbs (below the CIE onset) is different, particularly regarding the Normapolles distribution, the Tricolpates, Tricolporates and Monocolpates relative pollen abundances and the pollen ratio of Plicapollis pseudoexcelsus/Juglandaceae. In the three Mbs, rapid vertical variations characterize pollen and fern spores assemblages, with peaks of particular taxa reaching 25% of the total counts, such as for P. pseudoexcelsus, T. platycaryoides, C. dorogensis, P. mcgregorii, T. robustus. They suggest rapid floral changes in the catchment and/or sources changes in the surrounding landscapes compartments for the rivers supplying the fluvial sediments. Resinite particles are irregularly abundant as well as microcharcoals are occasionally abundant, notably slightly before the CIE onset and 1.5 m above it, still within the CIE. The clay mineral assemblage is homogeneous along marine Thanetian units and strongly dominated by Illite-Smectite (IS) mixed layers, whatever the facies, while it is more variable in the marine Ypresian silts (abundant IS, followed by Kaolinite, Illite, Chlorite and Vermiculite). In contrast terrestrial units are dominated by IS followed by Kaolinite, Illite and very rare Chlorite, and no variation is observed across the PEb. Those data would suggest that major environmental changes have regionally begun before the PETM, at the very end of the Paleocene, when terrestrial realm has settled, in relation with a regional uplift. Clay minerals reflect constant reworking of preexisting weathering profiles established upon Cretaceous strata and Variscan basement, while vegetation modifications are subtle and more likely attributable to changes of depositional environment

Recognizing pedogenic features in Paleogene sandstones and silcretes in Belgium: a key-feature for paleoenvironmental and sourcing material studies

International audienceA few sandstones occurring in Belgium, especially those from Paleogene strata, share many common features with pedogenic and groundwater silcretes documented in the Paris Basin in France, in South Africa and in Australia (e.g. Thiry, 1999; Summerfield, 1983; Webb & Golding, 1998,). Here we present a review of the criteria that may be used to assess the pedogenic (or "groundwater") origin for sandstones and silcretes. At field scale, pedogenic features in silcrete often consist in the development of vertical (columnar) or nodular structures and/or layers (horizons) due to vetical percolation (illuviation) and transformation of materials in the profile porosity and cracks. Groundwater silcrete often exhibit a typical mammilated surface (e.g. Nash & Ullyot, 2007). Root and rootlet casts, sometimes with silicified root or wood material, also indicate that the rock evolved at the surface or near-surface. At microscopic scale, illuviation of silt and clays in the vadose or water-table environment results in the formation of a series of finely-laminated coatings and infillings. These pedofeatures consist in "typic", "crescent", "capping", "pendent", "micropan" and "crust" coatings (Bullock et al., 1985). Irregular rounded structures or "glaebules" also develop as well as micro-columnar or prismatic textures. Many of them have been observed in pedogenic silcretes around the world (e.g. Thiry, 1999) but also in Paleogene silcrete in Belgium (e.g. Veldman et al., in review). Of particular importance is the concentration of fine-grained titanium-oxides in pedogenic silcrete. These Ti-oxides most probably originated from the weathering of clay material and were concentrated via the same pedogenic processes which concentrated silica in the silcrete profile (Thiry, 1997). In sandstones with a pure sand matrix, where no or few fine-grained material is available for illuviation, assessing the pedogenic origin for the silicification is much more difficult based solely on microtextural observations. New perspectives may arise from trace-element analysis in the quartz cement overgrowing the grains. For example, combining cathodoluminescence imaging (CL) and electron microprobe analysis (EPMA) allowed the detec on of abnormally-high concentration of Al and K in the quartz cement relative to the quartz grains in the Grandglise Thanetian sandstone in the Mons Basin (figure). This suggests that first the weathering of glauconite, which is actually observed here, is the likely source for silica and secondly, the evidence for aluminum migration would indicate strongly acidic conditions in the pore fluids

Crossing epoch and international boundaries: the earliest Eocene Erquelinnes mammal fauna from the Mons Basin and its correlation

International audienceIn 1880, the fluvial "Upper Landenian" deposits (Tienen Formation) of the Erquelinnes sand quarry in the southern part of the Mons basin near the Belgian-French border yielded their first mammal fossil, a well preserved jaw of a primitive perissodactyl. By 1927, about 40 mammal specimens had been found at Erquelinnes and had been attributed to Adapisorex, 'Protomomys', Paramys, Plesiadapis, Arctocyonides, Hyracotherium, Coryphodon and 'Oxyaena or Miacidae'. By that time however, the Erquelinnes fauna had already been eclipsed by the contemporaneous Dormaal fauna from northeastern Belgium, which yielded thousands of specimens and has since become the international MP7 reference level for the earliest Eocene of Europe. Meanwhile, attention for the Erquelinnes fauna has been limited to passing mentions of referred specimens in the formal descriptions of the new plesiadapiform Platychoerops georgei and of the miacid carnivoran Gracilocyon solei. Here we present an updated faunal list for the complete Erquelinnes mammal fauna. We show that also hyaenodontids, mesonychids, hyopsodontids, and dichobunid artiodactyls are present, and some of the earlier identifications are corrected or detailed further. This update of the Erquelinnes mammal fauna almost doubles its diversity, and strengthens the correlation with the Dormaal MP7 reference fauna. The results of the δ13Corg analysis of the strata underlying and coeval with the fluvial unit containing the Erquelinnes mammal fauna seem to independently support the faunal correlation and earliest Eocene age Sedimentological observations and δ13Corg analysis recently obtained in fluvial and lignitic sediments from the nearby Avesnois area in northern France (Quesnel et al, this meeting) moreover allow the correlation of the "Upper Landenian" fluvial deposits from the Belgian Erquelinnes locality with those of the "Sparnacian" in France. Remaining faunal differences between Erquelinnes and Dormaal are most likely the result of subtle differences in depositional environments and thus in taphonomic bias except, possibly, for the perissodactyl jaw collected in 1880. This specimen was not collected from the basal gravel bed of the Erquelinnes Sand Member like all other specimens, but from the overlying, cross-stratified sands. Its morphology is unlike that of the perissodactyl found in the basal gravel bed at Erquelinnes or of any other Dormaal aged perissodactyl. Instead, it matches Cymbalophus cuniculus from slightly younger deposits in the London Basin. The perissodactyl jaw from Erquelinnes therefore either indicates an earlier first appearance for Cymbalophus, an unexpectedly long phase of deposition for the Erquelinnes Sand Mbr, or a combination of both

Biomarker assemblages associated with the Eocene-Oligocene transition in lacustrine deposits of the Rennes Basin (France)

International audienceThe Eocene/Oligocene transition is one of the main climatic events to which more recent climate changes are often compared. Dated back to 34 Ma, it is remarkable because the Earth evolved from a greenhouse to icehouse state, permanent ice sheets appearing during the Late Eocene. In marine setting, it is marked by an increase in detrital input associated with an increase of primary productivity that resulted from well mixing of water masses (Peihzen et al., 2001). In terrestrial setting, this cooling led to major changes in animal and plant communities. For instance, palynology data show a vegetation shift from wet to dry taxa (Gregory and Chase, 1992) coincident with a large mammal turnover, the well-known "Grande coupure" (Stelhin, 1909). Although this transition is well documented in marine settings, the lack of data on lands - due to limited available outcrops - does not allow appreciating precisely its impacts on terrestrial ecosystems. Here, we have analysed the sediment infill of the Rennes Basin that consists of 300 m of organic-rich lacustrine deposits (laminated and massive) that encompass the Middle Eocene to Oligocene series, including the Eocene-Oligocene transition, the whole fully cored (CINERGY project, BRGM). About 100 samples were collected through the core enabling us to precisely decipher environmental changes that occurred during the studied interval and to focus on the Eocene/Oligocene transition. All samples were subjected to Rock-Eval and EA-irMS to measure bulk δ13C. Four over the 100 samples were analysed for their molecular content so far. For a preliminary test, we have chosen representative facies (a laminated and a massive Upper Eocene sample and a laminated and a massive Lower Oligocene sample). Lipids were extracted with an Automated Solvant Extractor (ASE200®, Dionex) using DCM:MeOH (9:1). The total extract was later separated into neutrals, acidic and polar compounds. The neutral compounds were further separated into aliphatic, aromatic, ethers and esters, ketones and alcohols. After addition of 5α-cholestane, each fraction was then analysed by Gas Chromatography-Mass spectrometry on a Trace GC Ultra gas chromatograph coupled to a TSQ Quantum XLS mass spectrometer equipped with an AS 3000 autosampler. The analysis of the different fractions of each sample revealed a high diversity of compounds that could partly be related to their source organisms or inform on the depositional environment. Hydrocarbon fraction is dominated by a series of n-alkanes that display a strong odd-over-even predominance, indicative of a good preservation. The dominance of long-chain homologues shows a strong higher plants contribution. This is confirmed by the presence of abundant and diversified higher plant pentacyclic triterpenes. These comprise angiosperms biomarkers such as oleanane, ursane and lupane, friedelin, onocerane I and diagenetic by-products (des-A-tritrepenes and aromatic derivatives). Fernene-type compounds could reveal an input from pteridophyts whereas tricyclic diterpenes indicate the presence of conifers. An algal influence is evidenced, for example, by the presence of 4-methylsteranes in the four samples. C27 to C32 hopanoids with α,β, β,β and β,α configurations show a substantial bacterial contribution. This is confirmed by high levels of hopane ketones in some beds. The comparison of angiosperm-derived biomarkers with gymnosperm-derived biomarkers indicates a stronger contribution of angiosperms in Upper Eocene samples than in Lower Oligocene samples (triterpenoids, aromatic, and rearranged-triterpenoids). This result is in agreement with palynological data (Bourdillon et al., 2012) and with the well-known cooling that is coeval with the Eocene/Oligocene transition. Reversely, the comparison between the two facies (massive and laminated) does not exhibit significant differences in the molecular assemblage. Based on this preliminary inventory of biomarkers and the richness in various biomarkers, we will proceed with the identification and quantitation on the remaining samples that cover the whole section. These preliminary results are also encouraging in the perspective of performing compound-specific isotopic analyses in order to depict, at high temporal resolution, the climatic evolution and the associated changes in plant communities during the Late Eocene and Early Oligocene and, more specifically, at the Eocene/Oligocene transition

Post-Paleozoic evolution of the northern Ardenne Massif constrained by apatite fission-track thermochronology and geological data

International audienceThe exhumation history of basement areas is poorly constrained because of large gaps in thesedimentary record. Indirect methods including low temperature thermochronology may be used to estimateexhumation but these require an inverse modeling procedure to interpret the data. Solutions from suchmodeling are not always satisfactory as they may be too broad or may conflict with independent geologicaldata. This study shows that the input of geological constraints is necessary to obtain a valuable and refinedexhumation history and to identify the presence of a former sedimentary cover presently completely eroded.Apatite fission-track (AFT) data have been acquired on the northern part of the Ardenne Massif close to theVariscan front and in the southern Brabant, in particular for the Visean ash-beds. Apatite fission-track agesfor surface samples range between 140 ± 13 and 261 ± 33Ma and confined tracks lengths are rangingbetween 12.6 ± 0.2 and 13.8 ± 0.2 mm. Thermal inversion has been realized assuming that (1) samples wereclose to the surface (20–40 °C) during Triassic times, this is supported by remnants of detrital UpperPermian–Triassic sediments preserved in the south of the Ardenne and in the east (border of the Roer Grabenand Malmédy Graben), and (2) terrestrial conditions prevailed during the Early Cretaceous for the ArdenneMassif, as indicated by radiometric ages on paleoweathering products. Inversion of the AFT datacharacterizes higher temperatures than surface temperatures during most of the Jurassic. Temperature rangeis wide but is compatible with the deposition on the northern Ardenne of a significant sedimentary cover,which has been later eroded during the Late Jurassic and/or the Early Cretaceous. Despite the presence ofsmall outliers of Late Cretaceous (Hautes Fagnes area), no evidence is recorded by the fission-track data forthe deposition of a significant chalk cover as highlighted in different parts of western Europe. These resultsquestion the existence of the London-Brabant Massif as a permanent positive structure during the Mesozoic.L’évaluation des épaisseurs érodées sur les socles n’est pas immédiate car l’absence fréquente de couverture sédimentaire rend muette leur quantification sur une grande période de temps. Des méthodes indirectes comme la thermochronologie basse température permettent d’appréhender l’érosion à condition d’inverser correctement les données par modélisation. Les résultats de l’inversion ne sont pas toujours en accord avec les données géologiques ou sont trop imprécis pour être pertinents. Cette étude montre que la prise en compte de contraintes géologiques est nécessaire pour obtenir une histoire cohérente, définir l’ampleur de l’érosion et identifier la présence d’une couverture sédimentaire aujourd’hui érodée. Des données traces de fission dans les cristaux d’apatite ont été réalisées dans le nord du massif de l’Ardenne à proximité du front varisque et au sud du Massif du Brabant, en particulier sur des échantillons de cinérites viséennes. Les âges traces de fission des échantillons prélevés à la surface varient entre 140 ± 13 et 261 ± 33 Ma et la longueur des traces confinées horizontales est comprise entre 12,6 ± 0,2 et 13,8 ± 0,2 μm. L’inversion thermique de ces données a été réalisée en prenant comme hypothèses : (1) la proximité des échantillons de la surface (20–40 °C) au cours du Trias, hypothèse étayée par des témoins de couverture du Permien supérieur et du Trias au sud et à l’est du massif de l’Ardenne (grabens de la Roer et de Malmédy) ; (2) l’existence de conditions continentales au cours du Crétacé inférieur, en accord avec des âges de cette période pour des profils d’altération datés localisés dans le massif de l’Ardenne. Le résultat de cette inversion suggère des températures assez élevées au cours du Jurassique. Ces températures sont interprétées comme le résultat du dépôt d’une couverture sédimentaire qui a ensuite été érodée au Jurassique supérieur et/ou au Crétacé inférieur. Malgré la présence de quelques témoins du Crétacé supérieur (notamment dans les Hautes Fagnes), les données ne permettent pas de détecter le dépôt d’une série épaisse datée du Crétacé supérieur comme c’est le cas dans plusieurs régions d’Europe de l’Ouest. Ces résultats remettent en question l’existence de la structure positive du massif de Londres-Brabant au cours du Jurassique en identifiant des mouvements verticaux significatifs

LIBS profiles of sedimentary sections: a new tool for paleoclimatic and paleoenvironmental reconstructions?

peer reviewedReconstructing climates and/or environments of the past requires analysis of sedimentary sections based on profiles of various geological data (geochemistry, sedimentology, mineralogy, paleontology, ...). LIBS analysis can be carried out with a fast acquisition rate on minimally-prepared samples, allowing measurement of large geological sample sets such as sedimentary sections. Here we show preliminary results obtained from a field and a core section to illustrate the potential of LIBS for paleoclimatic and paleoenvironmental studies. The studied field section consisted in 6 m thick Devonian siliciclastics with varying carbonate content used as a reference for cyclostratigraphy (New York State, USA [1]). 300 samples were analyzed manually in less than two days and the results showed very good match with the XRF measurements previously used for astrochronology, including productivity (Ca) and detrital (Ti, Al, etc.) proxies. Therefore, astronomically-forced climatic cycles could be analyzed based on LIBS data as it is reliably done with XRF. The studied core section, which consists of 150 m of Ypresian to Bartonian formations (Le Tillet borehole, France) was more challenging for LIBS measurement as it consists of both soft siliciclastic sediments and consolidated carbonate rocks [2]. Therefore, the 157 selected samples were powdered and fixed onto double-sided adhesive and automatically analyzed with the LIBS within about 1 hour. The obtained data, still under interpretation, showed that some elemental ratios such as Cs/k and Li/k exhibit interesting correlations with mineralogical and environmental data (Fig. 1)

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

Thèse publiée. Document du BRGM, 263, 268 p et 160 p d'annexes.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.La synthèse de 50 feuilles géologiques à 1/50 000, la compilation de 2500 sondages et la combinaison des cartes thématiques grâce à l'outil S.I.G. permettent d'établir la géométrie des altérites à silex (RS) à l'ouest du Bassin de Paris. Le traitement géostatistique de la variable épaisseurs de formations superficielles de plateau (RS + LP + résidus cénozoïques) et l'image de leur rugosité caractérisent l'irrégularité à échelle locale du toit de la craie, imputable aux poches karstiques. La géométrie des différents types de FS est corrélée à la morphologie des surfaces sur lesquelles elles se sont élaborées d'une part et l'épaisseur de substrat sec d'autre part. Ces diagrammes binaires révèlent deux familles de RS, associées respectivement aux paléosurfaces paléogène et plio-quaternaire. La typologie des RS, réalisée à partir des observations de terrain, des cartes de résultats analytiques de B. LAIGNEL et de l'étude des formations cénozoïques résiduelles associées, permet d'identifier plusieurs générations d'altérites à silex à l'ouest du Bassin de Paris : - Des paléo-RS remaniées sur les plates-formes d'abrasion marine du Thanétien supérieur au nord de la Seine (sauf au coeur de la Gouttière Normande) et du Rupélien supérieur en Drouais. - Des RS développées du Paléogène jusqu'à l'Actuel (faciès Thimerais et ses variantes). Elles sont les plus évoluées de toute la zone d'étude et sont localement recouvertes par des dépôts fluviatiles paléogènes et/ou silicifiées au sommet des profils à l'Eocène moyen et/ou scellées par des formations détritiques et lacustres carbonatées d'âge éocène supérieur. - Des RS développées après le dépôt et sous la couverture des Sables de Fontainebleau en Drouais, des Sables de Lozère et de St-Eustache dans le Roumois et le Pays de Caux, des alluvions pléistocènes de la Seine et l'Eure, après le décapage des argiles yprésiennes dans la Gouttière Normande et le Talou. D'âge plio-quaternaire, elles sont peu évoluées et peu épaisses, enrichies d'une part allochtone non négligeable, et le toit de la craie est très rugueux. Ces différentes altérites à silex caractérisent d'une part la surface continentale paléogène, ayant atteint un état d'équilibre, et d'autre part les surfaces pliocène et quaternaire, fortement incisées et non à l'équilibre. Les calages biostratigraphiques des silex creux des RS et paléo-RS montrent que l'altération a eu lieu in situ et que les profils de RS n'ont subi que de faibles décapages sommitaux. La géométrie actuelle des RS reflète le biseau sédimentaire des craies à silex originelles sur la bordure armoricaine du Bassin de Paris. Les microfaunes confirment l'existence du Maastrichtien dans le Bassin de Paris, jusqu'ici douteuse car occultée par les processus d'altération et d'érosion ultérieurs. Les épaisseurs de craie dissoute sont quantifiées sur une carte 3D dans les zones des différents faciès-types de RS en intégrant les calculs de bilan isosilex 1D de B. LAIGNEL. Les taux moyens de dissolution varient régionalement et au cours du temps, en fonction des contextes climatiques et de l'intensité du drainage. Ils sont plus élevés pour les RS plio-quaternaires (10 m/Ma) que pour les RS paléogènes (2 m/Ma). La Composante Verticale de la Déformation Finie de la surface continentale paléogène est calculée entre le Thanétien et l'Actuel sur une carte 3D en introduisant l'image des épaisseurs de craie dissoute. Elle varie entre - 170 m et + 170 m, pour une moyenne de - 1,5 m et une fourchette d'incertitude entre ± 20 m et ± 40 m. La confrontation de l'altitude et le faciès des sédiments ou altérites conservés avec l'altitude des paléoniveaux marins contemporains, permet ensuite de contrôler la cohérence des résultats obtenus et de valider l'image 3D de la CVDF pour toutes les zones testées. Un soulèvement général de la bordure ouest du Bassin de Paris au tout début du Tertiaire est mis en évidence, tandis que l'hypothèse d'un soulèvement récent de 100 à 150 m du Pays de Caux est invalidée. Le soulèvement s'est toutefois poursuivi au niveau de l'Eperon du Perche et plus récemment pour le Bray et le Dôme Picard. Le flambage lithosphérique déjà mis en évidence en Bretagne, Vendée et Poitou (WYNS, 1991, 1996b), et lié à la collision Afrique-Europe (compression pyrénéenne), est identifié jusqu'en Normandie. Il s'est fortement résorbé dans les grabens du Perche et le Drouais à l'Eocène supérieur et juste avant le Rupélien, en raison du passage de la compression méridienne pyrénéenne à la distension E-W oligocène. Ce facteur géodynamique interne conditionne la localisation des zones soumises à l'altération, les facteurs géodynamiques externes (climat et niveau de base) contrôlant les vitesses de l'altération. Il reste à valider et quantifier l'hypothèse de la rétroaction de la dissolution des craies sur le soulèvement, qui constituerait un exemple d'influence indirecte du climat sur la stabilité verticale de la lithosphère. Ce travail cartographique trouve enfin des applications directes dans le domaine de l'aménagement (gîtologie des réserves en silex utilisables comme granulats, prévention des effondrements), de l'environnement (gestion de la qualité de l'eau) et en géoprospective (cartes prédictives de vitesses de déformation du bâti)

Cenomanian sands and clays north of the Vesdre valley: the oldest known Cretaceous deposits in eastern Belgium

A number of motored auger holes have been drilled in 2002 and 2006 in four sand-clay deposits preserved in dissolution pockets within the Dinantian limestones of the watershed north of the Vesdre valley. These deposits of unknown age are currently classified as (Tertiary) SBL in the new geological map of Wallonia. We present detailed lithostratigraphic logs of the deposits and describe the results of sedimentological and mineralogical analyses. In particular, K-Ar dating of neoformed Mn oxides found at the base of one augerhole at Rechain yielded ages ranging from Cenomanian to Santonian, allowing us to place the Rechain and Andrimont deposits within the early Late Cretaceous. This is fully consistent with their topographic location very close beneath the trace of the pre-Cretaceous erosion surface and makes them the westernmost remains of the Hergenrath Member of the Late Cretaceous Aachen Formation. To the west, the Magnée deposit is more “typical SBL”, probably corresponding to the Late Neogene filling of a dissolution pocket by reworked weathering products of the local Cretaceous cover