Eustatic and oceanographic control on sedimentary evolution of middle-late Quaternary shelf margin-to-upper slope deposits on the Egadi Islands offshore (Italy)

During the past decades a number of sequence stratigraphy studies illustrated how, during Quaternary, interaction between sea level changes and sediment supply controlled the depositional evolution along continental margins, giving a cyclic signature to the sedimentary infilling. However, as both deposition and erosion are processes ultimately controlled by balance between environmental energy and sedimentary influx, also the oceanographic regime takes part in controlling the sedimentary growth along the continental margin. This is exactly what occurs on physiographic settings as islands offshore areas, where bottom currents can be very energetic. In the western Sicily offshore, southwards of Egadi Islands, the sea floor is characterized by depositional and erosional features formed under a variety of sedimentary processes and offers opportunity to investigate as sea level change and oceanographic regime combine each other to control depositional evolution. In this area, located along the clockwise flow of the Levantine Intermediate Water around the western Sicily margin, the sea floor morphology is very irregular as consequence of isolated reliefs and narrow submarine valleys, such as the Marettimo Valley which separates Favignana and Marettimo islands. We analysed and interpreted a grid of high-resolution (1 kJ Sparker) seismic reflection profiles integrated with multibeam bathy-morphologic data of selected areas; a 270 cm long gravity core has been also collected for sedimentology and biostratigraphy. The sedimentary succession accommodated along the eastern flank of the Marettimo Valley shows two different seismic units: unit A displays reflection-free seismic facies and thin, low- amplitude, inclined reflectors with downlap terminations onto the lower boundary, and erosional truncation at the upper boundary; these seismic facies are referable to oblique-tangential clinoforms and show a wedge-shaped external geometry. Unit B shows continuous, parallel, slightly concave reflectors and, towards the central sector of the Valley, continuous, sub-horizontal reflectors that form a deposit having a very broad low-mounded geometry; lateral transition in between concave and sub-horizontal reflectors is characterized by channelized erosional truncations. The two seismic units can be interpreted as the sedimentary response of different depositional processes: unit A accumulated by progradation of shallow-water deposits during eustatic forced-regression; unit B is referable to contourite drifts deposited by bottom currents through the Marettimo Valley. This unusual interbedding of very shallow water contouritic and shelf margin deposits derives from enhanced sedimentary dynamics during sea-level fall and lowstand stages when, as consequence of decreased water depth of the Marettimo Valley, bottom currents accelerated scouring channelized erosional surfaces; in the same time, deposition of forced regression across the shelf margin buried contourite drifts.