Abstract

In deep-sea fans, fan lobes form from the stacking of the deposits of turbidity currents and other sediment gravity flows beyond the channel mouth. Slopes lateral or oblique to the main direction of turbidity currents exiting channel confinement can profoundly affect flow behavior and the distribution of facies and lithology in fan lobes. In this paper, we report on the analysis of multibeam and CHIRP subbottom data aimed at the study of the effects of a lateral slope on turbidity-current behavior and resultant geomorphology and stratigraphy of a modern fan lobe. The Villafranca fan lobe, the subject of this study, develops on the 0.5° dipping seafloor of the Gioia intraslope basin in the southeastern Tyrrhenian Sea. The Villafranca is a transient fan lobe that has its downslope limit coinciding with a 200-m-high bathymetric step formed by the erosional flank of the Stromboli slope valley, perpendicular to the fan-lobe trend. A “lobe complex” hierarchical level is assigned to the Villafranca fan lobe. The lobe complex develops beyond the mouth of the Villafranca leveed channel, which trends N and is oblique to the regional NW-dipping slope. The depositional topography created by an adjacent channel levee wedge to the west is the cause of the observed obliquity. The western and the eastern “lobes” constitute the Villafranca lobe complex. Both lobes have mainly NW-trending channels, and therefore they do not conform to the classical divergent pattern of channels observed in deep-sea fan lobes. The channel trend is a result of the inability of flows to freely spread sideways, due to the presence of the lateral slope and the dip of the seafloor at the channel mouth. The western lobe has an up-dip apex with respect to the eastern one and is characterized by channels that nucleate on the downslope side of a channel-mouth bar and that die out distally before reaching the flank of the Stromboli slope valley. The eastern lobe, in contrast, has channels that are depositional upslope but on approaching the flank of Stromboli slope valley become mainly erosional or bypass features. The two lobes develop concomitantly, therefore an evolution due to compensational stacking with the two lobes being in different stages of development is here disregarded. It is therefore feasible that turbidity currents, which due to different initiating mechanisms have distinct magnitude and efficiency, deposit their load either in the eastern or in the western lobe. Another possibility is that single turbidity currents exiting the Villafranca channel mouth form two separate flow portions with different properties, which independently but simultaneously feed the eastern and the western lobe. In both cases, the effect of the lateral slope and the obliquity between the dip of the slope and the trend of the channel mouth are here considered as the primary factors controlling the differences in depositional style of the two lobes.