Resumen de Structural and palaeogeographical evolution of the Paris Basin during the Mid-Cimmerian events

Pierre-Charles de Graciansky, Thierry Jacquin

• English

  Presentation. The deposition of unconformable condensed layers associated with frequent sedimentary gaps record the Mid-Cimmerian events in the Paris Basin, dated as Aalenian to lowermost Bajocian.

  In order to document the Mid-Cimmerian episodes, a set of 250 commercial wells (fig. 1) together with several outcropping sections located on the southern and southeastern borders of the Paris Basin have been interpreted in terms of sequence stratigraphy. Seventeen third-order depositional sequences sensu Vail et al. [1991] have been identified and correlated. These are numbered as To1 to To7, Aa1 and Aa2, Bj1 to Bj5, Bt1 to Bt3 (fig. 2). They have been mapped out from subsurface data and from a few available outcrop data. Isopach maps corresponding to the depositional sequences illustrate both the role of synsedimentary faults and the evolution with time of depocenters (figs. 10 to 17).

  One purpose is to compare through time the depocentres location with the fault pattern (fig. 1). The well-known Jurassic Paris Basin faults make a three-branch system that converges towards the centre of the Paris Basin. These are as follows:

  NW-SE trending faults. These include the Pays de Bray, Somme River and Lower-Seine River faults. They intersect the northwestern border of the Paris Basin. They acted several times as major extensional faults from Permian through early Cretaceous times ;

  the NE-SW trending Luxembourg, Metz and other faults. They were typically extensional faults as a remote effect of the Tethyan rifting episodes. They have been active during late Toarcian through Bajocian times in Burgundy as shown from outcrop data ;

  the N-S trending group, including the Saint Martin de Bossenay, Sens, Loire and Sennely fault lines, are mainly located at the southern side of the Bray fault system. They probably correspond to the reactivation of an old Hercynian lineament such as the sillon houiller in the Massif Central. They have been documented also from outcrops in Burgundy and in the Macon area.

  Development of the main Cimmerian events in the Paris Basin During the overall regressive phase dated as Middle to Late Toarcian (fig. 3), the central Paris Basin and the northern Champagne are characterised by relatively thick deposits (fig. 11 to 14). Then, the onset of mid-Cimmerian events is marked by stratigraphic gaps and condensed layers dated as latermost Toarcian through Aalenian in the same areas (fig. 5). The correlative long-term maximum regression dated as close to the Aalenian/Bajocian boundary is expressed by maximum progradation of the carbonate platforms towards the Swabische Jura and the western Sub-Alps, i.e. the nascent Tethyan margin. At the same time the sedimentary accumulation was considerably attenuated within the whole Paris Basin during Aalenian times (fig. 9). This illustrates the structural high position of the Paris Basin during the Cimmerian events. Nevertheless, the central Paris Basin has been episodically flooded as a consequence of two, short duration and wide amplitude, transgressive events dated as middle and late Aalenian, related to Murchisonae and Concavum ammonite zones respectively (fig. 2).

  Bajocian times are marked by the onset of the Middle Jurassic transgression (fig. 7). Some faults active during Triassic and Liassic times are concealed by the Bajocian layers (fig. 8). This is accompanied by a new distribution of the depocentres location with respect to the Toarcian situation (compare figs 11--13 to fig. 7).

  The structural control of the Paris Basin development during the Mid-Cimmerian events can be illustrated on both (2D) isopach maps (figs. 5, 7 and 10 to 19) and 3D diagrams (fig. 19). They both document the evolution with time of the main groups of synsedimentary fault lines. Third-order and second-order cycles and bounding unconformities have been found to be coeval from the North Sea to the Tethyan margins including North Africa. This documents the partly eustatic control of the Mid-Cimmerian events.

• français

  Les condensations et les lacunes sédimentaires d'âge aalénien et bajocien basal caractérisent les événements mi-cimmériens dans le bassin de Paris. Afin d'analyser l'évolution de l'organisation des dépôts au travers de la discontinuité mi-cimmérienne, l'ensemble des séquences de dépôt sensu Vail et al. [1991], d'âge toarcien à bathonien moyen (soit 17 séquences notées To1 à To7, Aa1 à Aa2, Bj1 à Bj5, Bt1 à Bt3), a été cartographié à l'échelle du bassin de Paris et représenté sous la forme de cartes de courbes isopaques, ceci d'après les données des diagraphies et des affleurements.

  L'organisation des dépôt-centres pour chacune de ces séquences de dépôt fait apparaître le rôle joué par les trois directions structurales déjà bien connues : (1) NW-SE (failles du Pays de Bray et de la Somme), (2) N-S à N10 (failles de la Loire, de Sennely et de St Martin de Bossenay), (3) NE-SW (failles de Metz et du Luxembourg), héritées de la tectogenèse hercynienne La discontinuité mi-cimmérienne est caractérisée par une réorganisation des directions principales de l'allonge-ment des dépôt-centres, de NE-SW au Lias à NW-SE au Bajocien. Elle s'accompagne aussi d'une réduction considérable de l'accumulation sédimentaire à l'échelle de l'ensemble du bassin de Paris. Ceci suggère que le bassin de Paris fonctionnait globalement comme un point haut lors du passage Lias-Dogger, la sédimentation ayant été reportée à la périphérie, en direction de la marge téthysienne, du bassin germanique et de la mer du Nord.

  Le synchronisme des séquences de deuxième et de troisième ordre ainsi que des discontinuités qui les limitent, ceci d'un bout à l'autre de l'Europe et jusqu'en Afrique du Nord, traduisent l'importance du contrôle eustatique sur l'enregistrement sédimentaire au cours des événements mi-cimmériens.