The Triassic of southwestern Switzerland – Marine or non-marine, that is the question!
Abstract It is often difficult to distinguish nearshore marine from non-marine non-fossiliferous quartz-rich sandstones, and in many cases the depositional environment is disputed. Detailed facies analysis is necessary to determine the environment and, although there are no ‘smoking gun’ features to delineate them, a group of specific attributes in combination can be used to differentiate fluvial from marine deposits. Primary characteristics of fluvial deposits are: fining-upward channel-based sequences lacking marine indicators, well-developed palaeosols, and unimodal palaeocurrent distributions. The Vieux Emosson Formation, a Triassic autochthonous unit of the western Swiss Alps, is one such controversial example. It lies unconformably above crystalline basement and consists of a basal sandstone and conglomerate-dominated unit, and an upper heterolithic sequence of thin sandstones and mudstones with thin dolostone beds near the top. The formation is overlain by allochthonous younger carbonates of the Tethys Ocean. The light color of the basal unit gives the appearance of quartz-rich sandstone. In part because of this and also the overlying marine carbonates, previous workers interpreted the Vieux Emosson Formation as having been deposited in a marginal marine environment, along the southern edge of the continent and bordering Tethys. However, petrographic analysis determined that the Vieux Emosson sandstones are poorly sorted subarkoses and arkoses, and facies analysis indicated deposition in a continental fluvial-lacustrine system with a north-trending palaeoslope off the Vindelician High, towards the Germanic Basin. Deposition of the lower unit was in braided streams, supported by: high-relief basal erosional surface, immature sediment, large angular clasts, amalgamated fining-upward sequences, mottled palaeosol horizons with carbonate nodules, absence of bioturbation, and a unimodal palaeocurrent pattern. The interbedded sandstone and mudstone (proximal) and the upper mudstone and dolostone (distal) units represent deposition in a terminal splay-playa environment. Desiccation cracks, dolostone, absence of body fossils and very rare trace fossils indicate deposition in a playa lake. Non-marine deposition in the region during the latest Early Triassic and early Middle Triassic helps constrain the location and timing of the initial marine transgression in the southern Germanic Basin.