Palaeoenvironmental interpretation of the late triassic Fraele Formation : Ortles Nappe, austroalpine domain, Lombardy

The Fraele Formation crops out in the Ortles Nappe (upper Valtellina, Northern Italy), structurally pan of the Central Austroalpine Domain. It consists of fine siliciclastics alternating with carbonates, mostly limestones, rare dolostones and marls. The nature of the siliciclastics indicates a cratonic source area (Europe), where intrusive and/or orthometamorphic rocks were being eroded. The formation differs lithologically from the underlying Norian Dolomia del Cristallo, represented by early-dolomitised inner platform facies, and the overlying Early Jurassic Monte Motto Formation which consists of cherty and marly limestones deposited in a pelagic setting. The upper and lower boundaries of the Fraele Formation are sharp. Foraminifer and palynomorph assemblages from the Fraele Formation indicate a Late Norian to Rhaetian age. The sedimentary facies and faunal associations of the Fraele Formation differ from those of the underlying Dolomia del Cristallo because of different paleoenvironmental evolution. The change in environmental parameters was controlled mainly by a climatic change to more humid conditions. This favoured on one hand the mobilisation and transport by rivers of siliciclastic material from the continent to the Tethys gulf, and on the other influenced the sea-water chemistry. Freshwater influxes lowered salinity and inhibited early dolomitisation. Input of low density freshwater resulted in the establishment of a permanent water mass stratification which influenced the benthic life. This paleoenvironmental reconstruction fits with the sudden clastic input which occurred in several palaeogeographic domains of the western Tethys realm (Austroalpine, Southalpine, Apennine, Hungary, Poland, Slovakia) during the Late Norian.