Lateral facies variations in hydromagmatic pyroclastic deposits at Linosa, Italy

The Linosa volcano, less than a million years old, located in the Sicilian Channel, is formed by a series of coalescing tuff cones, tuff rings, spatter cones, and lava flows among which the Fossa Cappellano produced a succession of surges (the "Fossa Cappellano 2" deposits). Which travelled up to 1225 m. The lapilli and coarse ashes in the deposits are generally mud-coated, accretionary lapilli are common, and the tuffs are frequently vesiculated suggesting that the surges responsible for the deposits were wet. The continuous lateral exposures, from the interior of the Fossa to the extreme edge of the island, show facies evolving in the direction of transport from a massive, in the most proximal section to one dominated by cross-laminated beds in the most distal section, passing by a parallel-laminated (plane bed) facies. In each facies, mean bed thickness, and grain size decrease regularly down current. In the cross-laminated facies, mega-ripple wavelength and amplitude decrease in the direction of transport. In the coarser sizes there is a significant increase in the direction of transport of the lower-density scoriae coupled with a decrease of the non-vesiculated lithics. In the finer sizes, there is a significant increase in relative abundance, of the lower-density feldspar crystals relative to the Fe-Mg crystals. The composition, texture, and primary structure variations of the Fossa deposits are identical to those observed in turbidite sequences, and are similar to the characteristics observed in dry surge deposits recently described from Korea. They are interpreted similarly in terms of decreasing flow power in the direction of transport. © 1992.