Controls on Holocene deep‐water sedimentation in the northern Gioia Basin,Tyrrhenian Sea

The northern Gioia Basin of the south‐east Tyrrhenian Sea is a slope basin, ～ 20 km wide and ～ 50 km long, with a bathymetry of ≤ 1300 m, bounded by the Calabro‐Sicilian landmass and the Aeolian Island Arc. Coarse sediment is supplied from the Calabrian margin, where the shelf is very narrow to non‐existent, whereas the wider shelf on the Sicilian margin prevents supply by storing river‐fed sediments. The basin is dominated by the Gioia–Mesima canyon/channel system paralleled by a tongue‐shaped depositional lobe. Multibeam bathymetric surveys, sea floor reflectivity data and airgun seismic profiles reveal the recent evolution of the submarine system. Slope canyons and basin‐floor levéed channels formed where major rivers built deltas at the shelfless Calabrian margin and strong hyperpycnal flows predominated. The channels are a few hundred metres wide and a few tens of metres deep, with a downslope change from a straight to meandering pattern where the slope gradient decreases from 3·2% to 1·7%. The Mesima Channel has its lower segment abandoned because of avulsion and crevasse‐splay formation at an upslope bend. The adjacent Gioia Channel has had its upper segment straightened and lower segment entrenched because of erosional deepening of the Stromboli Valley into which it debouches and which acts as the local base level. Overbank features include levées, coalescent splays and ‘yazoo’ channels; their nature and surface characteristics depend upon the magnitude and sediment grain‐size of spill‐over flows. On an adjoining narrow shelf sliver of the Calabrian margin, in contrast, the coalescing plumes of sediment suspension supplied by an array of smaller coastal streams were apparently spilling over the shelf edge, scouring a funnel‐shaped bypass depression with chutes and forming an elongate, non‐channellized depositional lobe at the slope base. The study demonstrates the impact of sediment source type, shelf width, basin‐floor gradient and base‐level change on the style of deep‐water sedimentation.     

Subaqueous density flow processes and deposits of an island volcano landslide (Stromboli Island, Italy)

Stromboli is a 3000 m high island volcano, rising to 900 m above sea-level. It is the most active volcano of the Aeolian Archipelago in the Tyrrhenian Sea (Italy). Major, large volume (1 km³) sector collapses, four occurring in the last 13 kyr, have played an important role in shaping the north-western flank (Sciara del Fuoco) of the volcano, potentially generating a high-risk tsunami hazard for the Aeolian Islands and the Italian coast. However, smaller volume, partial collapses of the Sciara del Fuoco have been shown to be more frequent tsunami-generating events. One such event occurred on 30 December 2002, when a partial collapse of the north-western flank of the island took place. The resulting landslide generated 10 m high tsunami waves that impacted the island. Multibeam bathymetry, side-scan sonar imaging and visual observations reveal that the landslide deposited 25 to 30 × 10⁶ m³ of sediment on the submerged slope offshore from the Sciara del Fuoco. Two contiguous main deposit facies are recognized: (i) a chaotic, coarse-grained (metre-sized to centimetre-sized clasts) deposit; and (ii) a sand deposit containing a lower, cross-bedded sand layer and an upper structureless pebbly sand bed capped by sea floor ripple bedforms. The sand facies develops adjacent to and partially overlying the coarse deposits. Characteristics of the deposits suggest that they were derived from cohesionless, sandy matrix density flows. Flow rheology and dynamics led to the segregation of the density flow into sand-rich and clast-rich regions. A range of density flow transitions, both in space and in time, caused principally by particle concentration and grain-size partitioning within cohesionless parent flows was identified in the deposits of this relatively small-scale submarine landslide event.