Southern French Alps Research Articles

Heavy Minerals and Provenance of Sands in Flysch of Central and Southern French Alps

Upper Cretaceous to lowermost Oligocene flysch formations in the southern and central French Alps contain restricted stable to ultra-stable heavy-mineral assemblages. Most common are the R-A (resistant minerals + apatite) and the G-R-A- (garnet+resistant minerals+apatite) associations that occur in the Dauphinois-Ultradauphinois, Subbrianconnais, and Piemont flysch belts. The monotony of composition and the small number of heavy minerals is in marked contrast to the diversity of light minerals in the same sandstones (immature arkoses and lithic-wackes). Thus an inverse maturity relation characterizes the heavy and light mineral fractions of flysch sandstones. Yet neither fraction appears to have been altered significantly by weathering, abrasion, or selective sorting prio to deposition, or by intrastratal solution after burial. Heavy minerals, therefore, are representative of the minerals supplied to the different flysch troughs. The eroded highlands that supplied these sands were mostly older coarse clastic formations (and not crystalline terranes) within or adjacent to the geosyncline. The most likely source during the cannibalization process was the Permian sedimentary cover of the igneous and metamorphic basement rocks. Heavy-mineral assemblages vary over short distances. This stresses the importance of local supply in the accumulation of flysch, and weighs against the hypothesis of a longitudinal transport of sand for hundreds of kilometers along this part of the Alpine arc. This observation is supported by paleocurrent directions and other field measurements. Tectonic-paleogeographic changes affecting source area are reflected without time-lag in the composition and texture of heavy minerals. The divergence between assemblages in the lowermost Oligocene flysch and the slightly younger Oligocene molasse illustrates the excellence of heavy minerals as indicators of diastrophism. The sudden mineralogic change observed between these two facies suggests that the flysch occupies a syn-orogenic position and the molasse a post-orogenic position in this part of the Alpine geosyncline.

Excursion of the Geological Society of France

A. R. LILLIE writes: The reunion extraordinaire of the Societo Gologique de France was held this year on September 10-17 in the southern French Alps. There Was an attendance of ninety-four members, who came from the following countries: France, Belgium, Britain, Czechoslovakia, India, Poland, Spain and Switzerland. The excursion, under the guidance of Profs. Gignoux, Lory, Moret, Raguin and Schneegans, was well organized and was a useful introduction to a territory of fascinating tectonic problems. The party met at Grenoble, where M. Gignoux outlined the objects of the meeting, and then went by motor-car to the Dvoluy. M. Lory there demonstrated the considerable extent of the pre-Senonian folding which he has elucidated during many years work. From Gap as centre, during the following three days, the excursion was conducted in localities situated in the ultra-Dauphinois zone and the sub-Briangonnais zone at Ancelle and in the Ubaye valley. The recent work done in this region, particularly that of Schneegans in the massif of the Grand Morgon, is very impressive in view of the complicated tectonics and the mountainous nature of the ground. Moving on to Briangon for the last three days of the excursion, the party had the opportunity of seeing something of the tectonics in the Briangonnais zone. The final day was spent at the col du Galibier, where all the tectonic elements are to be seen continued northwards into Savoie. An excellent memoire with numerous plates had been prepared by MM. Gignoux and Moret in collaboration with Lory and Schneegans, and a copy was presented to each participant. This memoire, entitled “Description Gologique du Bassin Suprieur de la Durance”, appears to include much hitherto unpublished work, and should prove a valuable summary for geologists interested in Alpine tectonics.