Extensive mass-transport deposition in the Capo d’Orlando Basin: location, 3-D geometry, facies and possible genetic processes

Downslope sedimentary processes, driven by gravitational forces, move material over the seafloor andcresediment it into deeper water. These processes are characterised by a large variety of magnitude, types of transport and deposition, and frequency. Resulting mass transport deposits (MTDs) are found on the slope, at the base of slope and on the basin floor. MTDs can represent a large portion of the stratigraphic infill of some basins around the world. This is the case of the recent sedimentary succession of the Capo d’Orlando Basin, where a remarkable variety of MTDs is present and is illustrated in this work. Our analysis focusses on: the location of the evacuation and depositional areas of the MTDs; the characterisation of the surface geomorphology of the MTDs, both in their evacuation and accumulation areas; the evaluation of the thickness and 3-D geometry of the MTDs; the determination of the MTDs’ facies. The MTDs’ evacuation areas are mainly restricted to the slope, but some of them propagate upslope to affect the shelf-edge. The largest MTD occupies the entire eastern part of the basin plain with a total volume of approximately 12 Km3. It is a frontally unconfined slump often having an erosional base and a ramp geometry in its frontal area. Intermediate-sized MTDs form an apron at the base of slope and do not reach the basin plain. Their evacuation areas are in the channel-levee wedges that consequently are experiencing a major destructional phase. Smaller scale MTDs are present in the upper slope; they are debris flow deposits generally having an axial channel and lateral and distal blocky “wings”. Larger MTDs are however also present in the upper slope of the western basin portion; they have a blocky surface and their evacuation area involves the shelf edge and is probably controlled by tectonic structures. The shelf edge is affected by sediment failure also in the eastern part of the basin. Here, landslide results in the retrogradation of canyon heads that reach the coast. They can be triggered by rapid events of high sedimentation at the shelf edge, possibly connected with hyperpycnal river discharge or oceanographic processes, and lead to flows that flush the canyons. The basin-plain MTD and the base-of-slope MTDs can result from a single failure event or from almost synchronous multiple events indicating a major pulse of widespread instability in the basin margin. Mass-transport deposits can occur at any time during a margin’s history but commonly they form thick accumulation due to accentuated seafloor instability, often associated with specific periods during a cycle of sea-level variations. Also the common occurrence of MTDs in the stratigraphy of the Capo d’Orlando Basin points to a recent intensification of sediment collapse. However, in this case, widespread mass-wasting processes have their roots in the tectonic setting of the northern Sicilian margin with active faulting, tilting, vertical movements and seismicity.