FLUVIAL ORIGIN FOR THE QUARTZOSE SANDSTONES OF THE AUTOCHTHONOUS TRIASSIC COVER OF THE AIGUILLES ROUGES MASSIF, SOUTHWESTERN SWITZERLAND

Marine or non-marine--that is the question! The depositional environment of non-fossiliferous quartzose sandstones are often interpreted as shallow marine, based largely on lithology. Other controls on composition must be considered and detailed facies analyses are necessary to distinguish depositional environments. The Triassic sequence of the Aiguilles Rouges Massif is one such controversial example. It consists of a basal unit of primarily sandstone and conglomerate and an upper fine-grained unit of shale, thin sandstones and dolomite; this paper focuses on the lower unit.

The sequence overlies highly weathered gneisses with more than 1 m of local relief, and >10 m over a few km. The lower unit, 3-10 m thick, consists of conglomerate and sandstone m-scale fining upward sequences, rarely capped by mudstone beds up to 20 cm thick. Conglomerates are sandy matrix-supported, with angular to subangular clasts up to 6 cm in size. Most clasts are quartz, but locally there are abundant metamorphic lithic fragments (MLFs) and rare mudstone rip-up clasts. Beds are 0.5 to 1 meter thick and are massive with rare cross beds and internal channel scours. Sandstones are poorly sorted, commonly pebbly, and medium to very coarse grained. Beds are typically a few to several dm thick, and fine upward from erosive bases; dm-scale (1-2 m wide) trough cross beds are common. Composition is quartzose, with some plagioclase feldspar and MLFs. Red mottled, dm-thick, sandstone beds contain cm-scale carbonate nodules and extensive desiccation cracks. Interbedded sandstone and mudstone facies transition into overlying shale. They contain abundant current and wave ripple marks, mudcracks, and mudstone rip-up clasts; rare load casts. Chirotherium reptile tracks are found on rippled beds. Paleocurrent data from troughs and ripples have unimodal pattern with northwest transport direction.

Deposition of conglomerate and sandstone facies were in shallow braided streams. Fine-grained facies are interpreted as floodplain and playa lake deposits. A fluvial interpretation is supported by: high-relief basal erosional surface, immature sediment, large angular clasts (including mudstone rip-ups), amalgamated fining-upward sequences, mottled paleosol horizons, near absence of bioturbation, and a unimodal paleocurrent pattern.