TRIASSIC SEDIMENTARY ROCKS OF SOUTHWESTERN SWITZERLAND - THE TALE OF THE SHALE

The autochthonous Triassic cover of the Aiguilles Rouges Massif overlies Paleozoic crystalline basement in a fining upward sequence of conglomerate, sandstone, interbedded sandstone and mudstone, and shale units. The sequence, up to 10 m thick, is truncated by Alpine nappes. Coarse-grained units were deposited in a shallow braided river system, however the origin of the fine-grained units is not well understood; this study focuses on the shale.

The shale unit, < 3m thick, is green in the lower part and red in the upper part, but lateral color changes occur in the same bed, indicating a diagenetic origin. The unit contains mm- to cm-scale beds of quartzose fine-medium sandstone, silt, and dolomite. Stratigraphically, the coarser beds thin and fine upward, and the unit contains similar 20-50 cm nested cycles. Abundant wave and current ripples, mud rip-up clasts, flaser bedding, parallel lamination, desiccation cracks, starved ripples and rare load casts are present. Bioturbation is rare. Thin, weakly laminated dolomite beds and cm-scale nodules are present near the top of the section.

Thin section and slab analyses of the shale revealed 2 types of graded laminae: silt -clay and sand-based. Silt-clay laminae are 0.1 to 1.5 mm thick. Sand-based laminae, 3-5 mm thick, contain fine-grained sand, and also fine up to silt- and clay-sized grains. Erosional bases, rip-up clasts, parallel laminations and ripple-cross laminations characterize the sand component. The silt-clay laminae were deposited by suspension settling in calm water, and sand -based laminae were deposited initially by traction processes and then by suspension settling. Laminae are well preserved; subvertical 0.5 mm dia. spiral burrows <2 cm long are rare.

The association of the shale with braided river deposits, lack of fossils, rare trace fossils, dolomite, desiccation cracks, suggest deposition in an ephemeral (playa) lake. Together the interbedded sandstone and mudstone (proximal) and the shale (distal) units represent deposition in a terminal splay environment. The fining and thinning upward sequence reflects an overall deepening, and the nested cycles reflect lobe switching. Results of carbon and oxygen stable isotopic analyses of the dolomite are consistent with a playa environment, but a diagenetic overprint of the dolomites can't be excluded.