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

Abstract

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.