Architecture of a modern transient slope fan (Villafranca fan, Gioia basin–Southeastern Tyrrhenian Sea)

Abstract

The high resolution of geophysical data applied to the study of the modern seafloor provides excellent details on the architecture of deep-water fans. Numerous researches have addressed the study of modern ponded and terminal deep-water fans. In contrast, thorough studies of present-day examples of transient slope fans, i.e. those fans that are connected to a deeper depositional area, are not available. Multibeam bathymetry and CHIRP profiles have been used in the study of the Villafranca transient slope fan located in the northeastern Sicilian margin. Hanging above the 300-m high erosional flank of the Stromboli axial valley, perpendicular to the fan elongation, it represents a particular kind of transient fan. The fan is composed of a leveed channel followed downslope by a depositional lobe. The relief of the eastern levee above the channel axis changes due to longitudinal variations in the rate of sediment overbanking; despite the low sinuosity of the channel, an increased rate of overbank flow is observed in the outside of the channel bends. Downslope from a knickpoint, an increase in levee relief is the result of the entrenchment of the central tract of the channel. The erosional deepening of the channel followed the failure of a mass-transport deposit (MTD) that lowered the distal slope portion, also causing a step along the channel course. The MTD is characterized by an extensional headwall domain and by a slightly deformed main body that has suffered very restricted downslope movement. It was caused by the gliding of the levee wedge as a consequence of its undermining, due to the excavation of the Stromboli valley in the distal slope. Retrogressive sediment failures are widespread in the eastern levee; they form high mobility MTDs such as a debris flow that spread in the eastern portion of the depositional lobe. The Villafranca depositional lobe has an internal architecture that largely differs from that of classic fan models. Straight, approximately 500-m-wide and 10 m deep channels and longitudinal bars are in fact the main elements that compose the Villafranca channel-mouth depositional lobe. The channels narrow and deepen downslope, pointing to a progressive establishment of lateral flow non-uniformity in approaching the step at the edge of the depositional lobe. Thick transparent layers or thin well layered deposits, sometimes arranged in smaller scale bedforms, accumulate on the bars. Conversely, prevailing erosional or bypass processes are responsible for reduced sedimentation in the channels. The spatial arrangement of channels and bars within the depositional lobe results in a downslope increase in stratigraphic complexity. The architecture of the Villafranca fan lobe has the potential to be applied as analog for seismic or outcrop-based studies of fans developed in topographically complex slopes and connected to a deeper base level.