Paléogéographie et répartition des faciès au Cénomanien dans le bassin rhodano-vocontien

During the Cenomanian times, the paleogeography of the "Rhodano-Vocontian basin" was largely controlled by great faults trending SW-NE and SE-NW, probably inherited from the late Hercynian tectonic activity. These faults played a significant role in the sedimentation. To the West and to the North, coarse detrital materials were supplied by the "Massif Central" and its sedimentary cover : they were accumulated onto a platform, parted from the Hercynian landmass by the Cévennes faults and their northern tributaries. In between the Cévennes and Nîmes faults, there was a subsiding through ("sinus rhodanien") where fine-grained terrigenous deposits were accumulated, coming from the next platform. This trough was limited to the South by the "Du-rancian high" and the depending "Ventoux-Lure ridge". In the central and eastern parts of the basin, the sediments were more definitely pelagic (shales and argillaceous limestones). Slumpings and sudden variations in thickness eventually occur in this area, reflecting the influence of the Hercynian faults. During most of the Cenomanian time span, the sedimentation was quiet, but epirogenic movements took place in the Middle Cenomanian, causing local coarse detrital influx. To the West and to the North, coarse detrital materials were supplied by the "Massif Central" and its sedimentary cover : they were accumulated onto a platform, parted from the Hercynian landmass by the Cévennes faults and their northern tributaries. In between the Cévennes and Nîmes faults, there was a subsiding through ("sinus rhodanien") where fine-grained ter¬ rigenous deposits were accumulated, coming from the next platform. This trough was limited to the South by the "Du-rancian high" and the depending "Ventoux-Lure ridge". In the central and eastern parts of the basin, the sediments were more definitely pelagic (shales and argillaceous limestones). Slumpings and sudden variations in thickness eventually occur in this area, reflecting the influence of the Hercynian faults. During most of the Cenomanian time span, the sedimen¬ tation was quiet, but epirogenic movements took place in the Middle Cenomanian, causing local coarse detrital influx.Au Cénomanien, la paléogéographie du bassin rhodano-vocontien est encore étroitement conditionnée par de grands accidents structuraux SW-NE et SE-NW, hérités du bâti hercynien. Ces accidents jouent un rôle important dans la sédimentation. En bordure du Massif Central, en grande partie émergé, s'étend une plate-forme tronquée vers l'Est par le faisceau des accidents cévenols et leurs prolongements septentrionaux. Cette plate-forme, qui se prolonge vers le Nord jusque sur l'emplacement du Vercors, est le siège d'une sédimentation essentiellement terrigène, alimentée par l'érosion du massif hercynien et de sa couverture sédimentaire. Entre les accidents cévenols et la faille de Nîmes, s'ouvre un sillon fortement subsident ("sinus rhodanien"), où s'accumulent des dépôts terrigènes fins, provenant de la bordure du Massif Central. Ce sillon se referme au Sud contre le "bombement durancien" dont l'axe Ventoux-Lure constitue l'avancée la plus septentrionale. Dans la partie centrale et orientale du bassin, les sédiments ont un caractère beaucoup plus franchement pélagique ("faciès marneux et marno-calcaire"). Dans ce secteur, de brutales variations d'épaisseur et des manifestations de glissements synsé-dimentaires traduisent un rejeu épisodique des accidents anciens. La sédimentation est assez calme durant la plus grande partie du Cénomanien, sauf dans la partie moyenne de l'étage, où des mouvements épirogéniques provoquent localement un afflux de sédiments détritiques grossiers.Porthault Bernard. Paléogéographie et répartition des faciès au Cénomanien dans le bassin rhodano-vocontien. In: Géologie Méditerranéenne. Tome 5, numéro 1, 1978. Colloque sur le Cénomanien (France – Europe Occidentale), Paris, 6-7 septembre 1976. pp. 173-182