Faut-il rassembler le Stéphanien supérieur et I'Autunien (Paléozoïque supérieur continental) en une seule entité lithostratugraphique ?

Do Upper Stephanian and Autunian series (continental Upper Paleozoic,) constitute a single lithostratigraphic unit?
J.-F Becq Giraudon, D. Mercier, H. Jacquemin
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Nous proposons que les formations continentales du Stéphanien supérieur et de l'Autunien ne forment qu'une seule et même entité lithostratigraphique dont le statut formel reste à définir et dont la constitution est commandée par l'évolution structurale du domaine fragile des zones internes de la Chaîne hercynienne.


Proposed in 1893 by E. Munier-Chaimas and A. De Lapparent as Upper Carboniferous and Lower Permian continental Stages, the Stephanian and the Autunian (considered nowadays as Series) have since then been considered as two distinct chronostratigraphic units well defined both palaeontologically and lithologically as well as by the presence, in some basins, of a disconformity supposed to be synchronous within the internal zones of the Variscan belt. Since thirty years, several constatations have led to a revision of this conception: - The presence of the "Autunian" fora as early as the Stephanian, as a result of different palaeoenvironments (Doubinger, Langiaux, 1982; Broutin et al., 1986; Broutin et al., 1990; Becq-Giraudon, 1993) do not allowed to fix a precise biostratigraphic limit between the two Series; - It is often very difficult to fix a lithostratigraphic limit for, in the basins where the Stephanian and the Autunian are represented, one always observes a continuous passage from one Serie to the other (Feys, 1989; Châteauneuf et Farjanel, 1989). The disconformities which should be present between the two Series are not noticed everywhere and are not synchronous when observed (Courel, 1992); They are always due to the progressive opening of a basin in a extensional tectonic regime (Becq-Giraudon et Van Den Driessche, 1993); - The Stephanian Stricto Sensu (i.e. Stephanian B to D Auct.) has a very short duration (5 Ma maximum) compared to that of the Autunian (20 Ma) (0din, 1994); - Each serie records in its lithological column the evolution of a progressively opening basin: the Stephanian represent the first step of infilling (its fluviatil nature shows that we are in presence of relics of the variscan palaeohydrographic network), the Autunianl deposits, more distals, flood over the initial valleys as a result of the widening of the basin. Thus, the continental formations of Upper Stephanian and Autunian ages are part of one continuous tectonosedimentary cycle and it is herein proposed to regroup them in a lithostratigraphic Group, which remains to be formally defined and subdivided.

Dernière mise à jour le 28.07.2015