SEDIMENT PROVENANCE OF THE TRIASSIC VIEUX EMOSSON FORMATION, SOUTHWEST SWITZERLAND: INSIGHTS FROM U-PB DETRITAL ZIRCONS GEOCHRONOLOGY

The Vieux Emosson Formation lies unconformably on highly weathered crystalline basement of the Aiguilles Rouges external massif in the western Alps. It consists of up to 10 m of basal conglomerate and sandstone, overlain by up to 10 m of interbedded thin sandstone and shale with thin dolomite beds at the top. Detailed facies analysis indicates deposition in shallow braided stream system for the lower unit and in a terminal splay and playa system for the upper unit. Paleocurrent data indicate sediment transport towards the northwest. Conglomerates contain mostly angular quartz clasts, but locally there are abundant metamorphic lithic fragments. Sandstones in the basal unit are lithic arentites, but mature to sublithic arenites towards the top of the unit. Clast texture and compositions suggest derivation largely from local basement sources.

U-Pb dating (LA ICP-MS) of detrital zircons from two Triassic samples (above and below a prominent erosional surface in the lower unit) and two samples from L. Paleozoic metasedimentary basement units was utilized to further constrain source areas. All samples yielded abundant Paleozoic and Neoproterozoic, and minor amounts of earlier Precambrian ages. Neoproterozoic and E. Paleozoic ages match with the Cadomian and Pan African orogenies and reflect the northern Gondwana margin origin of the Alpine basement blocks. L. Paleozoic ages are from Variscan (Hercynian) metamorphic and igneous basement events. Similar age distributions from the L. Paleozoic and lower Triassic samples suggest similar provenance and/or significant reworking of the metasediments into the Triassic unit. In both, the presence of abundant Silurian and Devonian ages is significant; M. Paleozoic metamorphic or magmatic events are not recognized in Alpine basement. Possible interpretations are that the source rocks for these zircons are either not yet discovered, were eroded away, or were hidden by Alpine tectonism. Alternatively, a northern source area of Avalonian (Caledonian orogeny) basement may have supplied the zircons during the Variscan event and were subsequently reworked into the Triassic. The younger Triassic sample bore no M. Paleozoic or E. Neoproterozoic zircons, few Variscan zircons, but abundant Ordovician ages, indicating a significant change in provenance for the upper sandstones.