Résumé
Une paléoaltérite, caractérisée par une accumulation de gravillons ferrugineux formant une carapace rouge sang de plusieurs mètres d’épaisseur, existe en plusieurs secteurs du Périgord Noir et du Quercy. Elle est décrite ici sous le nom de Formation de Rouffignac. Sa teneur moyenne en oxy-hydroxydes de fer est de 33 % et celle en alumine de 11 %. Elle évolue localement en cuirasse enrichie en hématite. La reconstitution de la paléosurface de ce manteau d’une ancienne latérite datant de la fin Bartonien / début Priabonien, qui recoupe en discordance toutes les formations antérieures, tant éocènes que crétacées ou jurassiques, est affectée de dépressions festonnant une pente générale vers l’ouest en bordure d’un plateau ondulé, héritage probable des effets de l’orogenèse pyrénéenne.
Mots-clés : Altérite, Ferrallitisation, Cuirasse ferrugineuse, Eocène, Périgord, Quercy, Bassin d’Aquitaine
Abstract
The Eocene ferralitic duricrust, here described as the Rouffignac Formation, is widespread throughout several areas of the Périgord and Quercy regions and is particularly developed around Rouffignac in the Dordogne department. It belongs to the so-called “Siderolithic” facies of former authors. Formation thickness, which varies according to bedrock lithology, ranges from 5 m at Rouffignac to 10 m at Gavaudun. The ferruginous Rouffignac Formation unconformably overlies a variety of older formations including Middle Eocene fluviatile sands, Early Eocene kaolinitic clays, Palaeocene alterites, Santonian or Campanian chalky limestones, Cenomanian sandstones and Late Jurassic marly limestones. Reference sections have enabled the ferruginous process to be dated as between Late Bartonian and earliest Priabonian; it ended with sedimentation of the oldest molassic deposits (Palaeotherium-bearing clays). The Rouffignac Formation is a weathered profile particularly characterized by the blood-red colour of its clayey-silty matrix and the presence of a ferralitic gravelly duricrust. The gravelly nodules display iron hydroxide contents ranging from 28 to 38% and a rather constant aluminium oxide content of about 11%. The nodules are locally bound by a ferruginous duricrust of absolute accumulation, in which the Fe2O3 content can reach 45% and the Al2O3 content nowhere exceeds 16%. The ferruginous cement is composed particularly of hematite, which gives the duricrust its bright red colour. The evolution of this paleoweathering process is consistent with the evolution of the Eocene climate from contrasted tropical warm and humid to more arid conditions. Several type sections measured and described between Rouffignac and Floressas, to the south of Fumel, display the relationships between the ferralitic weathered profiles, iron gravels and gravelly duricrusts. Only some truncated in situ profiles remain of the intense lateritic pedogenesis at the end Bartonian – earliest Priabonian; these contain gravelly accumulations and ferralitic duricrusts that could serve as marker beds. The topography of this weathered paleosurface has been reconstructed by mapping the altitudes of the truncated profiles, and those of the base of the unconformably overlying Oligocene fluviatile deposits. The paleosurface, reaching +300 m NGF at Rouffignac, corresponds to a westward-dipping undulating plateau of Campanian to Eocene bedrock. Along 90 km to the southwest of Rouffignac, the surface forms a steep slope that drops sharply down to +130 m NGF and on which the molassic deposits pinch out. Some troughs forming a regular festoon pattern at the top of this slope could be interpreted as tectonic downthrown areas related to the Pyrenean orogenic phase.
Key words: Alterite, Ferralitization, Ferruginous duricrust, Eocene, Périgord, Quercy, Aquitaine basin.
Dernière mise à jour le 02.07.2015