REPARTITION AND PALEOGEOCRAPHICAL INTERPRETATION OF VOLCANOCLASTIC AND PELAGIC SEDIMENTS OF THE LIVINALLONCO FORMATION (ITALIAN DOLOMITES)

Summary The detailed lithostratigraphy and sedimentology of the volcanoclastic and pelagic strata making up the Livinallongo Formation (Middle Triassic) in the Italian Dolomites allow us to point out two pyroclastic stages of sequential deposition ("pietra verde") and to show some relationships to extensional tectonic movements, acid volcanism and the euxinic sedimentation in narrow basins connected with mobile sills. Some comparisons with the neighbouring Carnic Alps are useful. The "pietra verde" particles are essentially of pyroclastic origin but locally also epiclastic, with a mixture of pelagic material. The magmatic characters are partially concealed by the lack of in place lava flow. But, on the basis of the existing lithic elements of the crystals with their inclusions, it is possible to say that the tuffites forming the "lower pietra verde" are coming from a water-rich magma of late differentiation and trachy-andesitic composition, and those forming the "upper pietra verde" are of rhyolitic to rhyodacitic composition. The sedimentological analysis of the pietra verde shows the various sequential modalities of their deposition, in close relationship to the seabottom morphology. The graded-bedding of particles, according to their size and density, the order of the sedimentary structures, the homogeneity and poor shaping of the particles, and the reworked intraclasts (nodule breccia) at the bottom and the occurrence of Radiolaria at the top of the sequences are characteristics of the "pyroturbidites" of the "lower pietra