Submarine gullies on Italian upper slopes and their relationship with volcanic activity revisited 20 years after Bill Normark’s pioneering work

Abstract

Multibeam bathymetry and high-resolution seismic profi les depict in detail the characteristics of submarine gullies present in the upper continental slope offshore of the mouths of the Tiber and Volturno Rivers in the Tyrrhenian Sea and the Simeto River in the Ionian Sea. Upper slope gullies are interpreted as depositional features, growing because of faster aggradation on intergully areas with respect to their axes. The dispersal of river-fl ood sediment through plunging of hyperpycnal fl ows has been interpreted to be the limiting factor in sedimentation in gully axes. However, the generation of hyperpycnal fl ows requires a sediment concentration of several tens of kilograms per cubic meter in the river fl ows to overcome the higher density of seawater. This threshold is diffi cult to reach in medium-sized rivers (i.e., catchment basin of 500-5000 km 2 ), such as the Tiber and Volturno. Two alternative scenarios of enhanced sediment availability for these rivers have been proposed. The fi rst scenario is related to sea-level fall and lowstand stages, when the climate conditions may have been different and a huge amount of unconsolidated, fi ne-grained sediment deposited during the previous highstand stage may have been eroded from the river valleys, coastal plain valleys and shelf delta, as suggested in pioneering work of one of the authors (Chiocci ) and Bill Normark. This scenario might explain the formation of some of the gullies offshore of the Tiber River mouth, which developed during the last glacial maximum; however, it is unable to explain the gullies within deposits related to transgressive and highstand system tracts, such as offshore of the Simeto and Volturno River mouths. An alternative scenario is thus proposed on the basis of the relationship observed between the studied rivers and the presence of large vol canic edifi ces in their catchment basins. Explosive volcanic activity could have dras-tically modifi ed their catchment basins and caused a sudden and large supply of loose tephras, increasing the sediment load in watercourses and consequently favoring the development of gully-forming hyperpycnal fl ows. The timing of volcanic activity on the three study areas matches, in fact, the presence or absence of gullies in upper slope depositional sequences.

Characterizing the morphology of gully cross-sections based on PCA: A case of Yuanmou Dry-Hot Valley

“…Based on cross-sectional morphology, gullies have commonly been recognized as having different types, such as V-shaped, U-shaped, rectangular, triangular, trapezoidal, and wedge-shaped (Twichell and Roberts, 1982;Flood, 1983;Chiocci and Normark, 1992;Pratson and Coakley, 1996;Field et al, 1999;Spinelli and Field, 2001;Dowdeswell et al, 2008;Pratson et al, 2009;Chiocci and Casalbore, 2011;Micallef and Mountjoy, 2011;Frankl et al, 2013). Shape index (s i ) is used to distinguish between U-shaped and V-shaped cross-sections, where low values are related to V-shaped cross-sections and high values are U-shaped ones (Gabet and Bookter, 2008).…”

Coastal Evolution, Hydrothermal Migration Pathways and Soft Deformation along the Campania Continental Shelf (Southern Tyrrhenian Sea): Insights from High-Resolution Seismic Profiles

“…The transition between the shelf, gently deepening seawards, and the continental slope occurred at −150 m depth, where the shelf break was located. The slope area exhibited an uneven topography with several submarine gullies and landslide scars [56]. Most of the seabed features were located along a WNW-ESE orientation and were represented by pockmarks and elongated steps, with a direction strikingly parallel to the isobaths.…”

Multi-proxy analysis in defining sedimentary processes in very recent prodelta deposits: the Northern Phlegraean offshore example (Eastern Tyrrhenian Margin)

“…Offshore to the presentday prominent Volturno delta, which developed on the maximum flooding surface of the last glacial depositional sequence (Bellotti 2000;Amorosi et al 2012), it is possible to identify a pronounced seaward convexity of the isobaths that could be related to the ancient river mouth (Pennetta et al 1998) shifted seaward during the last glacial cycle when the sea level reached the maximum depth of 120 m (De Pippo et al 2004). The slope is characterized by a uniform profile and is incised by several submarine gullies (Chiocci and Casalbore 2011). The Volturno River is the longest river (175 km) in southern Italy and is characterized by a catchment basin of 5,634 kmq and a maximum river discharge of 102 m 3 /s (Chiocci and Casalbore 2011).…”

“…The prodelta area instead shows a slightly steep bottom as far as 0.46°with convex profile and represents an area, below the storm wave base level, of fine-grained passive deposits, delivered by fluvial water plume and reflecting episodes of minor/major river supply. In relation with this sedimentary model, past sediment deposition influx in prodelta and slope area by hyperpycnal flows were proposed by Chiocci and Casalbore (2011), linked to the sudden and large supply of loose tephras in the Volturno river catchment basin.…”

“…The slope is characterized by a uniform profile and is incised by several submarine gullies (Chiocci and Casalbore 2011). The Volturno River is the longest river (175 km) in southern Italy and is characterized by a catchment basin of 5,634 kmq and a maximum river discharge of 102 m 3 /s (Chiocci and Casalbore 2011). The Volturno River delta is characterized by a dissipative-type shoreline (Wright et al 1979;Orton and Reading 1993) with two orders of subaqueous sand bars ) and a reduced river sediment supply because of an artificial dam close to the river mouth.…”