Stratigraphie du Lias de Champfromier (Ain)

Stratigraphy of the Champfromier Lias (Ain)
M. Meyer, C. Meister, R. Wernli
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Le pointement singulier de Trias supérieur et de Lias de Champfromier, appelé faussement diapir dans la littérature, offre un jalon des plus précieux pour la connaissance de ces roches qui affleurent mal dans le Jura méridional. La cartographie détaillée de cette structure, et l’établissement d’une série quasi complète de Lias de 170 m d’épaisseur bien datée par ammonites est présentée et un premier inventaire paléontologique les accompagne. Le Keuper apparaît tout d’abord sous la forme d’un complexe argilo-gypseux surmonté d’un intervalle marno-dolomitique pour ensuite faire place au faciès argileux à Estheridés graduellement remplacé par les grès blonds rhétiens marquant le retour à un milieu plus franchement marin. La transgression liasique très franche dépose les Calcaires gréseux à Chlamys, les Calcaires à gryphées et les Calcaires marneux à cassure conchoïdale du Lias inférieur. Les Marnes calcaires à bélemnites, les Marnes à Amalthées, les Marnes noires à nodules et la Dalle échinodermique représentent le Lias moyen. Le Lias supérieur comprend les Schistes cartons (invisibles ici mais affleurants un peu plus au nord) et les alternances micacées à bancs durs. L’Aalénien est représenté par les Calcaires gréso-micacés mais son contact avec le Toarcien est inconnu. Cette série corrélable avec les autres très rares affleurements du Jura méridional, s’intègre bien dans un schéma paléogéographique conditionné essentiellement à cette époque par une zone de faible subsidence appelée éperon lyonnais.

Mots-clés : Keuper, Rhétien, Lias, Ain, Jura chaîne.


The Triassic and Liassic outcrops of Champfromier (Prébasson) have been known for a long time; they were worked for gypsum in the past and feature in 19th Century literature (Favre, 1879; Tournier, 1887, 1897; Riche, 1894). The studied outcrops are located in the small Sandézanne valley southwest of the village of Champfromier, which is 8 km north of Bellegarde-sur-Valserine and 25 km west of Geneva (Fig. 1). The area belongs to the southern Jura mountains fold and thrust belt. The detailed stratigraphic study of the Champfromier Lias provides a precise key to understanding the history of the Jura mountains at that time. In this tectonically complex area, the description of an almost complete stratigraphic sequence, well dated by ammonites, corrects the assumptions of Bovier (1931) as well as a hypothetical log of Wernli in Charollais and Badoux (1990, Fig. 39) and for the first time offers for these units correct thickness measurements that are the sole basis for reconstructing reliable balanced profiles. The Champfromier Lias forms the core of the faulted NE-SW-trending Monnetier anticline and is truncated by the left-lateral strike-slip Sandézanne Fault (Figs. 1 and 2). This remarkable Triassic and Liassic valley is mostly covered by vegetation, but locally shows some scattered outcrops that previously gave the impression of a diapiric structure, hence the appearance of the term “Champfromier diapir” in the literature (Wernli in Charollais and Badoux 1990). Previous work by Meyer (1995) revealed that it is not a diapir, but a complicated structure that implies major shortening linked to thrusting. The stratigraphic description below relates to the Late Triassic and Liassic formations. The remaining Jurassic and Early Cretaceous units shown on the detailed geological map of Figure 2 are referred to in the explanatory notes to the geological map of Saint-Julien-en-Genevois (Donzeau et al., 1997) from which the cartographic indices are taken. Triassic formations The study of the Triassic formations is in places incomplete because of local landslides. Our observations, in conjunction with those of Tournier (1897), provide a good insight to these poorly known units. Tournier (1897) describes 6 m of metre-thick gypsum layers separated by thin clay intervals, overlain by some 28 m of coloured clays showing very thin gypsum laminations. The clays can be black, red, green or yellow, and in places seem related to cyanobacterian flats. These levels can still be partly observed in an ancient quarry. These two units may be Keuper in age on the basis of facies analogies with some wells in the external part of the Jura mountains. In the uppermost Keuper and the Rhaetian, various facies show a gradual trend from a continental environment to marine facies. We observe a 5 m-thick azoic dolomitic marl facies overlain by 3 m of thinly laminated black clays with abundant Estherids (Cyzicus [Euestheria] minuta [VON ZEITEN]); Rhaetavicula contorta (PORTLOCK) also occurs in this Keuper brackish environment. This fine-grained facies is gradually replaced by Late Rhaetian massive sandstone. Liassic formations The Hettangian is well represented by 3.5 m of sandy Chlamys limestone. The quartz content gradually decreases from 60% to 0% and is inversely proportional to the echinoderm content. The ammonites Psiloceras sp., Alsatites liasicus (d'ORBIGNY) and Waehneroceras sp. are common, as are the bivalves Pinna hartmanni ZIETEN, Plagiostoma giganteum (SOWERBY) and Chlamys valoniensis (DEFRANCE), indicating an Early to Middle Hettangian age for this unit. Farther up, we observe 20 m of the well-known Gryphea arcuata LAMARCK limestone in a succession of 5- to 60-cm-thick undulating layers; it is a bioclastic muddy facies with bivalves (Gryphea arcuata), echinoderms and ammonites (Schlotheimia sp., Pararnioceras sp. and Coroniceras sp.), indicating a Late Hettangian to Sinemurian age. The Late Sinemurian is composed of 10 m of hard, blue-grey to ochre marly limestone, commonly showing conchoidal fracture. The ammonites (Bifericeras sp., Hemimicroceras sp., Gleviceras sp., Oxynoticeras oxynotum [QUENSTEDT]) indicate that this formation extends from the Oxynotum to the Raricostatum ammonite zone. The Pliensbachian is represented by 5 m of Carixian marly facies with abundant belemnites and ammonites (Ibex to Jamesoni ammonite zones) and continues upwards with the thick Domerian units. The latter comprise 30 m of Amaltheus marl, which is dark, very homogenous and rich in ammonites (mainly Amaltheus margaritatus DE MONFORT), followed by approximately 50 m of black marl. The abundant microfauna include foraminifers (Nodosariids) and ostracods. The black marl, which is rich in ferruginous nodules that gradually increase in size from the base to the top of the formation, represents a distal marine environment. The upper levels of the Domerian show an important change in lithology with the abrupt passage into coarse bioclastic limestone showing ferruginous incrusted levels containing belemnites and ammonites (Pleuroceras spinatum [BRUGUIERE]). The Toarcian is incomplete because of the lack of outcrops of Early Toarcian “Paper shales” and seems to be mainly represented by regular alternations of black marl and mica-rich limestone in which some Middle Toarcian ammonites such as Hildoceras bifrons (BRUGUIERE) and Haugia variabilis (d'ORBIGNY) were found. The Late Aalenian facies is very similar but shows a higher quartz content and abundant sponge spicules. Discussion The described vertical succession shows a well-defined second-order sea-level trend that can easily be correlated with the charts. Nevertheless, some local particularities are superimposed onto this trend and influence the facies distribution. The sedimentation of the Liassic units was controlled by a low-subsidence zone known as the “éperon lyonnais” or “Lyonnaise spur” (Mouterde et al., 1984). Deposition of the Liassic units was therefore linked to an inherited topography with differential subsidence that controlled the sedimentation rate and the facies type (Fig. 4). The studied outcrops are paleogeographically situated in the northeastern part of this spur.

Key words: Keuper, Rhetian, Lias, Ain, Jura Mountains.

Dernière mise à jour le 02.07.2015