Upper Eocene-lowermost Oligocene flysch sandstones of the Dauphinois-Ultradauphinois facies and the older flysch (upper Cretaceous-middle Eocene) of the Subbrianconnais and Piedmont facies in the French Alps contain heavy mineral assemblages that are relatively stable, consisting of resistant minerals such as zircon, tourmaline, and rutile together with apatite or garnet and apatite. In contrast to the maturity of the heavy mineral suites, the light fraction is characterized by textural and compositional immaturity as evidenced by abundant large feldspar grains and nonresistant lithic fragments. The inverse maturity relationship between heavy and light constituents is explained by postulating that the sediments deposited in the flysch troughs were derived from erosion and reworking of other clastic sediments (such as the Permian- lower Triassic cover rocks) within the mobile belt rather than from the external crystalline massifs.

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