Harmonisation et définition des unités lithostratigraphiques briançonnaises dans les nappes penniques du Valais

Abstract

The Briançonnais lithostratigraphic units in the Valais region consist of preTriassic basements and metamorphic Mesozoic-Cenozoic covers. Structurally they constitute the «Grand St-Bernard nappe» and partly the «Combin zone» of Argand (1908,1909), now considered as middle and upper Penninic nappes. Successively the tectonic and lithostratigraphic models were refined (fig. 1; Bearth 1963, Escher 1988, Steck et al. 1999). Stratigraphic as well as structural links were established with the nappes of the Préalpes representing the nonmetamorphic sedimentary cover pushed into a more external position. All internal Penninic units underwent strong deformations and a polyphase Alpine metamorphic overprint corresponding to greenschist or even higher facies. In some pre-Permian basement units relics of a former metamorphism are preserved. In this complicated context, the main criterion permitting the distinction of lithostratigraphic entities is cartographic coherence with respect to their content and their limits. The description of the units is based on their main lithological composition, their spatial extension and the description of key lithologies and unambiguous marker levels. After the first definition of the Siviez-Mischabel and Mont Fort nappes (Escher 1988), the tectonic scheme had to be modified due to the discovery of shear zones cutting the nappe pile (fig. 2 and 3). However, the lithostratigraphic content of these two Penninic nappes constitutes a synthetic series in which the following units are defined (fig. 4): The Ergischhorn unit (fig. 5) was defined by Thélin (1983). It constitutes the most voluminous polymetamorphic gneiss mass of the Siviez-Mischabel nappe. Relics of at least two pre-Alpine metamorphic assemblages (eclogitic grade followed by amphibolitic grade) can be identified. This unit is predominantly composed of greenish quartz-micaceous paragneisses with intercalated massive or banded amphibolites coupled to thin leucocratic gneiss levels. The paragneisses probably represent Precambrian detrital series while the amphibolites are considered as tholeiitic to dacitic sills or intrusions. The association with the leucocratic gneisses probably expresses an acid-basic bimodal magmatism. Additionally metagabbro bodies (dated at 504 ± 2 Ma) are intrusive in this unit. Correlations with similar units in the upper Stalden zone, the Berisal complex, the Ruitor zone and the southern Vanoise and Ambin massifs are proposed. The Adlerflüe Formation (fig. 6) forms very characteristic ochre to brown coloured rock walls. Banded amphibolites often containing eclogite facies relics mark the base of the formation. There, amphibolites alternate with decimetric leucocratic gneiss and micaschist levels. The transition to the overlying albite-porphyroblast-bearing micaschists is progressive. Although the paragenesis of the micaschists is entirely Alpine (greenschist facies) the micaschists contain some amphibolite boudin levels, sometimes bearing retrogressed preAlpine eclogitic assemblages. In the Ruitor zone, less affected by the Alpine metamorphism, a thick series of metapelites could be correlated to the micaschists of this formation; it occupies a comparable position and displays a rich mineral paragenesis of pre-Alpine amphibolite facies. The Lirec Formation (fig. 7) shapes steep rock faces of dark green colour. The base of the formation consists of leucocratic gneiss, sometimes augengneiss, with an ochre patina. It is overlain by banded gneisses (light green and white). The main part of the formation consists of dark bottle-green gneisses and schists containing green biotite, chlorite and garnet. Various rock types are intercalated: thin white albitic levels, thicker levels of light coloured gneiss often containing small garnets, dark green amphibolite and hornblendebearing gneisses in continuous levels or boudins. An alkaline metagranite dated at 500 +3/-4 Ma forms two intrusions. One of them is accompanied by a body of metagranophyre that can be compared to the alkaline granophyre of the Mont Pourri (northern Vanoise), dated at 507 ± 9 Ma. Many small metagabbro intrusions can be observed. Correlations with similar units in the Ruitor zone and the northern Vanoise massif are proposed. The Distulberg Formation (fig. 8) was defined by Sartori (1990). The rusty brown patina of these micaschists is characteristic. In the lower part of the formation more or less graphitic and ankeritic, quartz-albite micaschists predominate. They contain levels of dark green "ovardites" (basaltic to andesitic composition) and leucocratic gneisses (rhyolitic to dacitic composition). The middle part of the formation is characterized by levels of more massive greenish ovardites, albite-porphyroblast-bearing gneisses and prasinites. The upper part of the formation is constituted of micaschists containing a graphite rich black horizon. Hololeucocratic augengneisses are of intrusive nature and derive from alkaline metagranites showing affinities to the metagranite of the Lirec Formation. Metagabbros, although often very deformed, display the same characteristics as those intruding the underlying formations. The analogy between the Distulberg Formation and the «black schists» of the Vanoise region is striking. On the right side of the Aosta Valley, the Distulberg Formation can be correlated to a part of the «zona interna» where a metagranophyre intrusion is dated at 511 ± 9 Ma. The Métailler Formation (fig. 9) is located principally in the Mont Fort nappe where it constitutes the main part of the pre-Permian basement. The contact to the underlying Distulberg Formation is sharp only when underlined by a quartzite or prasinite horizon. The transition is often diffuse due to the lack of such a separating level. The dominant rocks are greenish grey to dark green chlorite-albite gneisses and schists. Intercalated rock types are: thin continuous horizons of leucocratic albitic and quartziferous gneisses, prasinites and "ovardites", sometimes in thick masses but of limited lateral extension. Metagabbros and pillow-metabasalts can be observed here and there. The transition to the Col de Chassoure Formation is either sharp or developed as a thick transition zone. Correlations with various Briançonnais basement units can be considered, particularly the Ambin Complex, in which two metarhyolites were dated at 480-500 Ma. The Col de Chassoure Formation (fig. 10) gathers rock-types classically known as «Permo-Carboniferous» series. It usually represent a décollement level, therefore the majority of the contacts to the surrounding units display an Alpine tectonic discordance. The most common rock-type of the formation is a green quartz-sericite schist with bluish reflections, and a soapy touch of the schistosity surfaces due to sericite. Combined to occasionally predominating schistose or quartzic alternances this series may originally represent a succession of siltstones, pelites and impure sandstones. They are referred to as «background sedimentation». A laccolitic granite body accompanied by sills (Randa augengneiss, 269 ± 2 Ma) is intrusive into the succession. Well characterized sedimentary bodies of great lateral extension, intermingled with the background sedimentation, are defined as members (fig. 11). The Cleuson Member corresponds to the «quartziferous porphyries» of Schaer (1959a and b) and constitutes a marker horizon located at the base of the Col de Chassoure Formation. It contains acid volcanic levels (metarhyolites), greenish conglomerates, quartz schists and quartz-sericite schists. The top of the member corresponds to the roof of the last volcanic level. The Mondra Member generally lies above the Cleuson Member, embedded within the Col de Chassoure background sedimentation. Both the rather massive gneissic aspect and the relatively dark green colour are characteristic for the Mondra Member. It is composed of sericite-poor, chlorite-rich quartz schists, criss-crossed by quartz veins, passing sometimes into gneissic-looking chloritic quartzites. Thin intercalations of chlorite schists or "ovardites" are observed. The top of the member comprises one or several thin anthracite-rich boudin levels. The Goli d'Aget Member is defined based on the description of Derron & Jacquod (1994) and Derron et al. (2006). It consists of four sub-members (fig. 12), stratigraphically from base to top: 1) Light-coloured conglomerates and meta-arkoses, 2) alternance of albite-chlorite-bearing schists, brown dolomites and meta-arkoses, 3) polygenic conglomerates with dolomite pebbles, volcano-sedimentary metadeposits, 4) meta-ignimbrites, microcline-bearing volcanic metatuffs, green, grey or purple microquartzites, meta-arkoses with cross-bedding. A meta-ignimbrite located at the base of this last submember gave an age of 267-282 Ma. The Dent de Nendaz Member forms the principal mappable body of conglomerates and coarse sandstones (fig. 13). The lower part comprises an alternance of polygenic metaconglomerates and levels of fine-grained grey metasandstones or metapelites. Uranium-rich stratiform mineralizations intercalated within this alternance were dated at 257 ± 12 Ma. The main part of this member consists of metaconglomerates, coarse-grained meta-arkoses and light green quartzites containing dispersed quartz pebbles. Discontinuous levels of fine-grained material such as grey metasandstones, quartz-sericite schists and metapelites may be intercalated. The upper part of the member comprises even more conglomeratic and quartzic levels, organized as channelized bodies. The type of sedimentation indicates an alluvial fan.