Geochemistry of carbonate rocks and ferruginous horizons in the verrucano in southern Tuscany - paleoenvironmental and paleogeographic implications

Mineralogy and chemistry of carbonate rocks and of ferruginous shales and ironstones contained in the Permo(?)-Triassic continental sedimentary sequence from southern Tuscany (Italy) support detailed paleoenvironmental and paleoclimatic reconstructions. The composition of carbonate rocks rich in Mn and Ba and low in Sr is consistent with a marine tidal depositional environment of lagoonal type where high-Mg calcite deposition predominates and mildly evaporitic conditions favoured the formation of anorganic micritic limestones, sometimes containing minor amounts of evaporitic elements (Ba). The low Sr levels suggest post-depositional diagenetic processes in which fresh water acts as diagenizing solution. The ferruginous shales and ironstones seem to be related to the development of paleosoils under tropical weathering regimes favouring the selective mobilization of more soluble elements (Na, Ca) and of iron which reprecipitates near the soil surfaces according to the formation mechanism of the ironstone caps ("cuirasses ferrugineuses") in present tropical areas. A paleogeographic reconstruction of the Early- Middle Triassic environment of southern Tuscany is presented. The Verrucano sedimentary sequence was deposited in continental basins (graben) developed during a distensive phase after the Hercynian orogeny not far from the coast, under semi-arid conditions. The continental sedimentation was interrupted by limited marine ingressions during which coastal lagoons were established that were suitable for limited carbonate sedimentation.