Analyse séquentielle de l'Eocène et de l'Oligocène du bassin Parisien (France)

Sequence Analysis of the Eocene-Oligocene Paris Basin, France

¹ Institut Français du Pétrole
² Université Paris VI

Résumé

Dans le Bassin parisien pendant l'Eocène et l'Oligocène, on dénombre vingt-deux discontinuités issues d'émersions correspondant aux périodes de bas-niveaux marins. La présence de fleuves est décelée dans les prismes de bordure de plate-forme et dans les prismes de haut niveau, remobilisant des stocks sableux anciens à partir des anticlinaux actifs et des bordures du bassin. Ainsi, indépendamment du volume disponible pour les sédiments, se succèdent des séquences dominées soit par des formations détritiques, soit par des formations carbonatées. La plate-forme parisienne a enregistré toutes les variations eustatiques connues, bien que les mouvements tectoniques syn-sédimentaires locaux puissent influencer fortement les conditions de sédimentation. Mieux encore, la plate-forme parisienne a subi des fluctuations marines d'ordre supérieur de très courte durée qui, si elles sont retrouvées dans d'autres domaines, pourront être attribuées à des phénomènes eustatiques.

Abstract

The Paris Basin (Map 1) is a classic example of a stable platform such as has been known throughout the World for a long time now. Aside from the Bartonian and Priabonian, all Eocene and Oligocene stages have been defined in the form of the following four international stratotypes : Sparnacian, Cuisian, Lutetian and Stampian. It should also be noted that the substages of the Bartonian (Auversian, marinesian) and the Ludian, the equivalent of the Priabonian, are also Parisian. Even though gaps have recently been redefined (C. Pomerol, 1989), though the correlations between the different formations in the basin and those of the surrounding areas (D. Curry, 1967; et al. 1969, 1978; C. Pomerol, 1977; C. Cavelier, 1979; C. Cavelier and C. Pomerol, 1986) have now been determined, and though the worldwide eustatic sea level curve was partly plotted with the help of stratotype sections (B. U. Haq et al. , 1988), the Eocene and Oligocene sequence analysis of the paris Basin has as yet only been roughly sketched out. The survey was not based on seismic profiles because the Parisian Tertiary series outcrop. Therefore, our analysis is based on field observations and the abundant bibliography dealing with the Paris Basin. Recent dating of different depositional systems for each sequence is mainly due to the determining of Wetzelliaceae (J. J. Châteauneuf and C. Gruas-Cavagnetto, 1968; J. J. Châteauneuf, 1980), calcareous nannofossils (M. P. Aubry, 1983, 1985; E. Steurbaut, 1988), Characea (J. Riveline, 1984), mollusca and mammalia (C. Cavelier, 1979, 1987). We propose a time section (Plate 1 and Figs. 1 to 21) and a paleogeographic map (Maps 2 to 18) for each depositional sequence. There are 22 unconformities corresponding to large sedimentation gaps during the Eocene and Oligocene (Plate 1). They are the results of emersions and correspond to low sea levels when the sediments were situated solely in lowstand prograduring wedges or on the Shelf margin wedge but rarely present in the Paris Basin [Sequences 1 (Fig. 1), 3 (Fig. 3), 19 (Fig. 19) and 20 (Fig. 20)]. These unconformities, bounded by depositional sequences, are often confused with transgressive surfaces on the Paris shelf and are usually revealed by erosional surfaces as well as by the presence of paleolkarsts, paleosols, eolian reworking or sandstone diagenesis. Unconformities at the Thanetian-Sparnacian, upper Cuisian-Terminal Cuisian, Cuisian-Lutetian, Mrinesian-Ludian, Ludian-Stampian and Stampian-Chattian boundaries can be recognized from the Type 1 (Plate 1). In the Paris Basin, these unconformities are major ones separating most of the stratotypic sequences. The other sequences are reparated by a Type 2 unconformity with local subaerial erosion on the edges of the Paris Gulf or on the entire area of the basin (Sequences 3, 4, 7, 8, 10, 11, 14, 17, 18, 19 and 20) depending on the eustatic fall and the subsidence. The presence of rivers, mainly issuing from the South and East of the basin, can be detected on the shelf margin wedges and in the highstand prograding wedges. However, there are no real paleovalleys underneath the unconformities, but at most an irregular surface that usually bears witness not to continental erosion but to gullying linked to the ensuing transgression. Formations subjected to erosion during lowstand periods are often quite unconsolidated. The paleoreliefs built up during emersion were easily leveled by the ensuing transgression. Likewise, outside of the tectonically active areas in the basin during the Eocene and Oligocene (Pays de Bray, Orxois dome, Rémarde anticline, Meudon anticline), which sometimes form islands (Maps 2 to 18), transgressions occurred in the rest of the basin onflat surface or one with residual reliefs of decimetric size. Like in some current drainage systems, it can be supposed that the most deeply indented paleovalleys are situated near the shelf edge during lowstand, periods. This would imply their probable existence well west of the Paris Basin near the continental slope of the werstern English Channel. An analysis of the lithology of the different depositional systems within the sequences shows an alternation or coexistance of sandy or clayey clasting sedimentation and purely carbonate sedimentation. The origin of the Parisian Tertiary sandstones has been recognized as having come successively from former Mesozoic (A. Blondeau et al. , 1964) and Cenozoic (J. Riveline-Bauer, 1970; C. Lorenz et al. , 1984) sandy sources. The remobilization of these sources was mainly linked to the episodic uplifting of anticlines (Pays de Bray and Artois regions mainly) and of basin edges, providing a source for coastal sedimentation with continental sandy or clayey influxes brought in by rivers in the South and the Champagne region (Maps 2 to 18). Thus, apart from the volume available for the sediments (accomodation), sequences succeeded one another in time while being dominated either by highly clastic formation or by essentially carbonate formations. The case of the Meretrix rustica limestones in the Vexin region, which are contemporary with the Auvers sandstones, is an exemplary one (Fig. 9, Map 10). The regions sheltered from sandy influxes (the Vexin region at the time was a marginal marine domain backed up against a chalky hinterland and isolated from the main gulf by the Pays de Bray Island and the Sables d'Auversoffshore bars) could at the same time gather in a purely carbonate sedimentation. These same limestones east of Vexin become calcareous sandstones associated with Auvers sandstones. A sequence analysis of the Eocene-Oligocene in the Paris Basin also reveals that the Soisson lignites were deposited in the midst of an upper highstand systems tract and then were marked at the level of the paleosols by the presence of a late highstand systems tract (Fig. 1, Map 2). Initially, they were in a coastal plain environment associated with prograding coastal deltaic systems and then with fluvial outwash plains. On the contrary, the Epernay lignites belong to a late lowstand wedge (Fig. 3, Map 4) and indicated the presence of palustrine environment between the emerged areas and the oyster lagoons, which themselves were sheltered by offshore bars present in the aggrading Laon sands. Elsewhere, when the gypsums in the Paris Basin are of marine origine [Lutetian gypsums (Figs. 6 and 7), fourth, third and second Gypsum Masses (Figs. 14, 15 and 16)], they belong to an early highstand systems tract. However, the first Gypsum Mass and the Marabet Bank, which are of continental origin, are part of a late highstand systems tract (Figs. 16 and 17). The great central subsidence of the basin at that time left a log of space available, but the slight subsidence of the edges, especially in the NW, and the great lowering of the sea level served to isolate a very thick continental evaporitic basin in the depocenter area. The depositional sequences thus described show, on the whole, a very close correlation with the eustatic sea level curve (B. U. Haq et al. , 1988) since this curve was plotted, among others, from stratotype sections. The Parisian platform underwent all the eustatic variations known on a worldwide scale, even though some local synsedimentary tectonic movements (A. Blondeau et al. , 1964, C. Pomerol, 1967; R. Wyns, 1978; C. Cavelier and C. Pomerol, 1979; C. Mégnien, 1980; C. Lorenz, 1984) may have strongly influence the conditions of sedimentation (Maps 2 to 18), revealing moreover the persistance during this entire period of active tectonic structures such as the Pays de Bray, and also the migration of the depocenter in the Paris Basin for each depositional sequence. Better yet, the Parisian platform underwent marine fluctuations lasting very briefly and which, if they are found in other stable domains, may be attributed to eustatic sea level phenomena.