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Keywords:

  • Base level;
  • deep sea;
  • depositional lobe;
  • hyperpycnal flow;
  • overbank deposits;
  • sea floor topography;
  • submarine channel

Abstract

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.