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Schistes lustrés from Corsica to Hungary : back to the original sediments and tentative dating of partly azoic metasediments

¹ Lab. de Tectonique, Université P. & M. Curie. CNRS, ESA 7072, Tour 26-0, 1^er étage, case 129, 4 place Jussieu, 75252 Paris cedex 05. Email : lemoinem{at}club-internet.fr.

Abstract

The Alpine and Corsican Schistes lustrés (SL) are nearly azoic Jurassic-Cretaceous metasediments often associated with ophiolites. They are derived from both the vanished Valais (N-Penninic) and Piemont-Ligurian (S-Penninic) oceans and from their continental margins.

Their age is generally poorly known. Because of fossils discovered by Alb. Heim and by S. Franchi at the beginning of the 20^th century, they were believed for a long time to be mostly Liassic in age. We know now that the major part of the SL is Cretaceous.

Deep-sea sediments, and particularly the SL, are made up of a hemipelagic-pelagic background (HPB) associated with detrital components of local or distant origin. The nature of the HPB, mostly conditioned by Tethyan and worldwide events, is of great help as an at least rough stratigraphic marker ; in contrast, detrital material is not a reliable marker because it may occur at different times in different places. The HPB exhibits several successive, 10 to 40 m.y. long episodes which are either predominantly argillaceous (A) or calcareous (C). During the deposition of the Juras-sic-Cretaceous SL, seven episodes can be distinguished : C1, calcareous Liassic ; A1, marly Upper Liassic ; C2, calcareous latest Liassic and early Dogger ; A2, shaly or radiolaritic late Dogger-early Malm ; C3, calcareous late Malm ; A3 shaly or marly early Cretaceous ; C4 calcareous late Cretaceous. They can be recognized, each one by its prevailing lithology, and all together by their succession in order from C1 to C4. Nearly all of these subdivisions are here and there dated by rare fossils, which allow for a rough dating of the numerous azoic SL series.

As they exhibit very different lithologies, from pelagic calcareous oozes to Black Shales and various kinds of flysch and other mass flow deposits, the SL cannot be considered as a specific, well-defined facies : they are not characteristic for a particular stage of the geodynamic evolution of the Alps.

Finally, a possible influence of worldwide events is suggested. First, the role of the depth of the CCD, governed by early late Jurassic and early late Cretaceous biotic recoveries. Secondly, the correlation with first order eustatic events : transgressions on platforms seem to be roughly coeval with A episodes in the deep sea, regressions with C episodes.

Key Words: Alps • Corsica • Schistes lustrés • Bündnerschiefer • Calcescisti • Stratigraphy • Hemipelagic-pelagic sedimentation • Tethys • Valais ocean • Piemont-Ligurian ocean • eustatism • CCD