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Sedimentology, petrographic variability, and very-low-grade metamorphism of the Champsaur sandstone (Paleogene, Hautes-Alpes, France) : evolution of volcaniclastic foreland turbidites in the external Western Alps

Abstract : The Champsaur Sandstone, the turbidite formation under study, comprises an uppermost Eocene or Lowermost Oligocene volcanilLastic graywacke that was deposited in a migrating foreland basin at the front of an active fold- and thrust belt in the Subalpine Chains of the northern French Alps. It evolves from the slightly older and more internal Flysch des Aiguilles d'Arves into which it gradually passes towards the N . After a major erosional gap to the W, it passes into the slightly younger and more externaL St. Didier Sandstone which, in turn, eventually gives way to the Molasse Rouge. The Champsaur Sandstone is petrographically distinguished from these formations by its abundance in andesitic rock fragments which may Locally comprise the greater part of the sedimentary rock. The andesitic volcanoes that furnished the sands and gravels formed part of a much larger volcanic arc that was erected adjacent and parallel to the western edge of the Adriatic plate *in the Paleogene. Volcaniclastic intercalations of similar composition, age, and structural position are known from the Taveyanne Sandstone of the Swiss Alps, the St. Antonin Syncline of the Maritime Alps, the Petrignacola Sandstone of the northern Apennines, and the Tusa Facies of southern Italy and Sicily. In the Alps, the turbidite formations of predominantly andesitic composition progressively give way to younger and usually more external clastic formations in which the volcanic clasts are increasingly composed of ophiolitic material. These are the Val d'Illiez Sandstone of western Switzerland, the St. Didier Sandstone of the northern French Alps, and the Clumanc Sandstone of the Maritime Alps. The Champsaur Sandstone is unconformably underlain by the Priabonian Schistes à Globigérines and Calcaires à Nummulites Formations, respectively, which are strongly transgressive onto the Pelvoux Massif in the north and its deformed Mesozoic sedimentary cover in the south. On the basis of structural position and gross lithological composition, three major sandstone units have been differentiated and sequentially numbered (1 to 3) according to age, becoming progressively younger from SE to NW. Sandstone units 1 and 2, including their Mesozoic and Priabonian substrata to which they are still attached, are entirely parautochthonous. With the arrival of the deformation front shortly after their deposition, they were detached, deformed, and transported in a westerly direction ante the third sandstone unit during postnummulitic thrusting in the late Oligocene-Miocene alpine compressional regime. In the course of further underthrusting and basinward migration of the fold- and thrust belt, the area under study was successively overrun in Oligocene-Miocene times by a series of Penninic, originally more internaL nappes. This gave rise to several kilometers of overburden pressure which resulted in the development of secondary zeolite-facies assemblages in the volcaniclastic rocks.
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Alexander Waibel. Sedimentology, petrographic variability, and very-low-grade metamorphism of the Champsaur sandstone (Paleogene, Hautes-Alpes, France) : evolution of volcaniclastic foreland turbidites in the external Western Alps. Petrography. University of Geneva, 1989. English. ⟨tel-00769799⟩

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