Downstream evolution of the Stromboli slope valley (southeastern Tyrrhenian Sea)

Marine Geology 243 (2007) 180 – 199 www.elsevier.com/locate/margeo

Downstream evolution of the Stromboli slope valley (southeastern Tyrrhenian Sea) Fabiano Gamberi ⁎, Michael Marani Istituto di Scienze Marine, Sezione Geologia Marina Bologna, Via Gobetti 101, 40129, Bologna, Italy Received 11 November 2005; received in revised form 26 April 2007; accepted 2 May 2007

Abstract The Stromboli Slope Valley is located in the southeastern Tyrrhenian Sea and has a length in the order of 120 km. It crosses different physiographic domains characterized by highly varied gradients and by a different character of past and recent tectonic activity in the Sicilian and Calabrian margins. Furthermore, the course of the Stromboli Valley is strongly controlled by the distribution of the Aeolian Island arc volcanoes located offshore Sicily and Calabria. Numerous tributaries join the Stromboli Valley from the Aeolian arc slopes and from the Sicilian and Calabrian margins. Six reaches, showing unique morphology and stratigraphic architecture have been identified along the course of the Stromboli Valley, through the analysis of multibeam bathymetric data and seismic lines. The first reach has a canyon morphology with frequent landslides and chaotic deposits infilling the valley floor which is also characterized by three plunge pools. The second reach is characterized by an aggradational infill consisting of high amplitude reflections and by the development of a levee. The third reach presents an inner meandering thalweg associated with discontinuous and thin lateral accreting packages within an entrenched valley. The fourth reach is characterized by a high degree of incision, an inner entrenched thalweg and by the lack of any infill. The fifth reach is an erosional feature that however is at present experiencing deposition within the valley floor. The sixth reach has a depositional multithalweg valley floor often with a braiding pattern. The results of the interpretation have been used to infer the regime of sediment gravity flows in the different reaches of the Stromboli valley. Tectonic and volcanic features are the main elements that control the location of the Stromboli slope valley. In addition, tectonic structures have a large impact on the pattern of erosional and depositional processes along the Stromboli Valley and in the consequent distribution of geomorphic elements and sedimentary architecture. © 2007 Elsevier B.V. All rights reserved. Keywords: submarine slope valley; sediment gravity flow; longitudinal depth profile; base level; structural topography and sedimentary processes; extensional faults; Tyrrhenian Sea

1. Introduction Submarine slope valleys are the component of deepsea depositional systems that guide the transfer of sediments from the shelf to the deep basin plain. Over ⁎ Corresponding author. E-mail address: [email protected] (F. Gamberi). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.05.006

thousands of years, through the depositional and erosional action of turbidity currents they tend towards an equilibrium profile, characterized by a concave-up depth profile (Pirmez et al., 2000). Slope valleys that have reached an equilibrium profile are at grade and carry sediments with minimum aggradation and degradation (Kneller, 2003; Pirmez et al., 2000). Erosional processes occur on slope valleys that are above the graded profile

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whereas depositional processes take place in those that are below the equilibrium profile (Pirmez et al., 2000; Kneller, 2003). The gradient of the slope, the local or ultimate base level of the system and the character of the prevailing flows that are channelled within the valley, control the equilibrium profile of a slope valley and thus dictate whether it is the site of erosional, depositional, or bypass processes (Pirmez et al., 2000; Kneller, 2003; Smith, 2004). Thus, variations of sediment flux and flow character and behaviour, in turn mainly dependent upon sea level changes, are a prime control on the temporal evolution of slope valleys (Babonneau et al., 2002; Kneller, 2003; Samuel et al., 2003; Smith, 2004). They combine with the effects of tectonic activity and with the degree of sub-basin confinement, reflecting the interplay between tectonic deformation and sedimentation rates, in moving a slope valley from a depositional to an erosional regime and vice versa (Pirmez et al., 2000). In addition, since the timing, character and rate of tectonic deformation are often unevenly distributed along the

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path of a slope valley, a downstream pattern of contemporaneous aggradation and degradation can ensue. The analysis of the variations of the grade conditions along slope valleys is best carried out with the interpretation of multibeam bathymetric and seismic data on present-day examples. This type of study is herewith presented with the aim of explaining the controls on the pattern of erosion and deposition along the course of the modern Stromboli Slope valley. 2. Area description The Stromboli Slope Valley (SV) runs in the southeastern Tyrrhenian Sea and has a length of around 120 km (Gamberi and Marani, 2004a) (Figs. 1 and 2). Along its course it crosses different physiographic regions characterized by highly varied gradients and evidence at the seafloor of tectonic structures (Figs. 1 and 2). The SV starts on the slope of the Sicilian margin, at the junction of two distinct canyons, and runs west of the Capo di Milazzo promontory with a NNW direction

Fig. 1. Bathymetric map (contour interval 100 m; bold contours every 500 m) of the southeastern Tyrrhenian Sea and of the SV (bold, dotted line). The SV originates in the Sicilian margin and crosses the Gioia Basin and the distal part of the Calabrian margin before reaching the around 3000 m deep Marsili Basin. The Aeolian Island arc is present to the west of the SV. The box corresponds to the area of Fig. 2.

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Fig. 2. Shaded relief map from multibeam bathymetric data of the SV (see location in Fig. 1). The first reach of the SV runs west of the Capo di Milazzo Promontory within a graben originated by NNW trending faults (bold black lines). Two extensional faults (dashed bold lines) marks the boundary between the Gioia Basin and the Calabrian margin. North of Stromboli Island, the SV turns toward the west and runs within a depressed area bounded to the north by faults (dashed bold lines). Finally, the SV enters the Marsili Basin where a deep-sea fan is developed at a depth of about 3000 m. Numerous tributaries enter the SV from the Aeolian Arc and from the Sicilian and Calabrian margins. The boxes correspond with the areas of the figures showing the detailed bathymetric maps of the six reaches of the SV.

(Fig. 2). It then runs with a NE-trend along the axis of the Gioia Basin, a gently dipping intraslope basin bounded westward by the slope of the Aeolian island

arc. Further downslope, after crossing the extensional faults that limit the Gioia Basin, the SV enters the Calabrian margin where it maintains a NE- trending

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Fig. 3. Longitudinal depth profile of the SV from depth measurements every 10 m contour. The vertical dashed lines are the limits between the different reaches of the SV indicated by the corresponding Roman numbers. The crosses are the depth of the right valley margin (top of the levee in the second reach). The position of the main extensional faults (F) that crosscut the SV is indicated by the arrows in the top portion of the graph. The major knickpoints discussed in the text are also shown. The dark grey field on the left corresponds with the reaches of the Gioia Basin, whereas the light grey field to the right corresponds with the reaches of the Calabrian margin.

course, still confined to the west by the slope of the Aeolian Island arc (Fig. 2). Finally, in the distal part of the Calabrian margin, at a depth of around 2300 m, in coincidence with a breach in the volcanic Aeolian arc north of Stromboli Island, the SV takes a westward

course and debauches in the plain of the Marsili Basin, at a depth of around 3000 m where it feeds the Marsili deep-sea fan (Gamberi et al., 2006) (Figs. 1 and 2). Further details of the tectonic and physiographic setting of the regions crossed by the SV will be reported

Table 1 Main morphologic data and seismic facies of the valley floor infill for the six reaches of the SV (the two values refer to the proximal and distal part of the same reach respectively) Reach

Valley floor depth (m)

Valley relief (m)

Width of valley floor (m)

Width of valley (m)

Average gradient (deg)

Maximum gradient (deg)

Main geomorphic elements

Seismic facies of valley infill

Plunge pools, landslide scars on flanks Levee, thalwegs and bars, abandoned portion Terraces, meandering thalweg, point bars Entrenched thalweg, terraces, landslide scars on flanks Meandering thalweg, depositional bodies from lateral input Braiding thalwegs and longitudinal bars

Chaotic

I

700–950

500–150

700

5000

1.53

1.91

II

950–1300

100

4700–1800

5400–3500

1.1

2.08

III

1300–1490

150–200

1000

2700

0.7

0.98

IV

1490–1900

450

540–1000

2800–1600

1.75

3.58

V

1900–2310

400–150

1600–3800

4000–2300

0.8

0.93

VI

2310–3000

120–250

6000–600

7000–1300

0.88

1.1

Discontinuous, high amplitude Lateral accretion packages None Discontinuous, high amplitude Discontinuous, high amplitude

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in the sections dealing with the description of the single reaches of the slope valley. 3. Data set and methods Multibeam (Simrad EM12) bathymetric data, acquired in 1996 and 1999 by the Institute for Marine Geology of Bologna cover the course of the SV downslope from a water depth of around 700 m (Fig. 2). Down to a depth of around 2000 m, bathymetric maps were obtained by gridding the multibeam data with an interval of 25 m; below 2000 m water depth, bathymetric charts were produced with a gridding interval of 50 m. Grids were obtained with an in-house software (Ligi and Bortoluzzi, 1989). The bathymetric maps have been used to construct the longitudinal depth profile of

the SV with measurements at every 10 m contour (Fig. 3). Concomitantly, the valley relief, on the right side, was measured at contour increments of 50 m along the valley axis and then plotted at the corresponding points along the longitudinal depth profile (Fig. 3). Considering that the SV follows an almost linear path over much of its course, the valley relief graph also gives a good approximation of the gradient of the slope surrounding the SV (Fig. 3). The bathymetric data has led to the identification, along the course of the SV, of different reaches, each characterized by specific geomorphic elements (Table 1). The single reaches also stand apart in having distinct valley incision and width and for differences in gradient (Table 1). Finally, a low density grid of 30 kJ Sparker and single channel airgun lines, both parallel

Fig. 4. Bathymetric map of the first reach of the SV (10 m contour interval; bold contours every 100 m; see location in Fig. 2). The SV is confined within a graben between the Capo di Milazzo and the Tindari ridges. The 200 m high, NW-trending escarpment, west of the SV, corresponds to the fault (bold, dashed line) that bounds the graben to the west. The eastern fault of the graben coincides with the eastern flank of the SV (see Fig. 5). Sediment failures, shown by landslide scars, affect discrete seafloor portions, with surface areas up to 5 km2, of the western flank of the SV. Masswasting also occurs as a more ubiquitous process on the steep eastern flank. Three plunge pools are present in the valley floor. The bold line corresponds with the seismic profile of Fig. 5.

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and transverse to the SV, shows that the single reaches are also characterized by varying subseafloor stratigraphic architectures (Table 1). Fusion of the bathymetric and seismic data has led to the determination of the downslope pattern of erosion and deposition of the flows that run within the SV and to infer the cause of the changing character of the flows. 4. Data interpretation

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In the flanks of the valley, mass wasting is a frequent process with downslope movements that affect sediment packages as thick as 300 ms. The failed material is found both as disrupted bodies along the flanks of the valley and as a transparent or chaotic valley floor infill of around 200 ms in thickness (Fig. 5). Gradient variations of the first reach of the SV (Fig. 3) are mainly connected to three asymmetric plunge pools, with steeper downslope sides, 300–1600 m wide, and up to 50 m deep (Fig. 4).

4.1. The first reach: the canyon 4.2. The second reach: the leveed valley The first reach of the SV, from the upslope limit of our bathymetric data, has the morphology of a submarine canyon (Table 1). It is carved into the surrounding slopes with a depth that gradually diminishes downslope (Table 1, Figs. 3 and 4). Not all of the relief and width of the valley are due to erosional processes. Both the eastern and the western flanks of the valley correspond in fact to extensional faults that confine the valley within a NNW trending graben and contribute to much of its relief (Figs. 4 and 5).

The reach begins with a marked and sudden reduction of the valley gradient downslope from the deepest plunge pool (Fig. 3), and with the enlargement of the valley floor to 2 km occurring at the base of a major E–W extensional fault (Table 1, Fig. 6). The Baia di Levante Valley joins the SV in the distal part of the reach (Figs. 2 and 6). Beside the Baia di Levante Valley, numerous canyons from the slope of Vulcano enter the left side of the SV (Fig. 6). As shown by the bathymetric

Fig. 5. Seismic line BG8, crossing the first reach of the SV (see location in Fig. 4). The extensional fault (bold line) that bounds to the east the NNWtrending graben, where the upper reach of the SV is confined, is responsible for the observed step in the basement (dashed, bold line). The fault is sealed by the recentmost sediments. The sedimentary package of the eastern flank of the valley has a transparent seismic facies resulting from downslope movements and sediment disruption. An evacuation surface is evident in the upper part of the valley flank. Sediment failures from the valley flanks contribute to the valley floor infill consisting of a 200 ms thick package with a transparent seismic facies. A landslide deposit, flanked by erosional scars, is present in the top portion of the Capo di Milazzo Ridge.

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Fig. 6. Bathymetric map of the second reach of the SV (10 m contour interval; bold contours every 100 m; see location in Fig. 2). The reach starts at the base of a major E-W trending extensional fault (bold, dashed line) that corresponds with the slope of the Capo di Milazzo Ridge (see also Fig. 1). Downslope from the fault, a depositional levee flanks the SV to the east. The Baia di Levante Valley is the largest tributary to the SV from the Vulcano Island slope (see also Fig. 2). Two inner thalwegs (bold dotted lines) are present in the proximal valley floor separated by a longitudinal bar. Upslope from the Niceto channel junction, slump scars cause the removal of sediment packages as wide as 1 km from the inner side of the levee. The bold line is the trace of the seismic line shown in Fig. 7.

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map (Fig. 6) and by the seismic line of Fig. 7, a depositional levee is present in the right side of the valley breached only by the entry point of the Niceto channel. The valley gradient is constant until the distal part of the sector, comprised between the Niceto channel and the Baia di Levante Valley entry points, where a knickpoint (Knickpoint 1) coincides with a gradient increase (Fig. 3). Upslope from knickpoint 1, the floor of the valley is occupied by two distinct thalwegs separated by a longitudinal bar (Fig. 6). In the knickpoint 1 area, the two thalwegs connect into a single, very subdued one that runs along the axis of the valley floor; further downslope, close to the junction with the Baia di Levante Valley a prominent ridge isolates an abandoned valley portion to the east of the presently active section (Fig. 6). Besides the construction of the right levee through overbanking flows, deposition also occurs within the valley floor that presents an aggradational infill (Fig. 7). 4.3. The third reach: the meandering thalweg within an entrenched valley In the first part of the third reach, the SV has an ENE direction, then, after a turn, it takes up a northeastward trend, along the axis of the Gioia basin (Figs. 2 and 8). The gradient is relatively homogeneous (Fig. 3) and lower than the second reach (Table 1). Two terraces on the left side and one on the right side, downslope from

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the northeastward turn, are evident (Fig. 8). We interpret the terraces as representing paleo valley floors, thus recording a complex history of valley incision, entrenchment and narrowing. An inner thalweg meanders within the valley floor with an average sinuosity index of 1.26, but with values as high as 1.36 in the upper part of the reach. Point bars flank the meander bends (Fig. 8). The seismic line of Fig. 9, crossing the uppermost meander, shows that the bar is the result of lateral accretion on the inner side of the thalweg bend. 4.4. The fourth reach: the incised valley A high gradient (Fig. 3), a high relief of the valley walls (Table 1, Fig. 10) and the absence of any valley floor infill, as shown by the seismic line of Fig. 11, are the main features of the reach. A prominent knickpoint (knickpoint 2), represents the upslope limit of the reach (Figs. 3 and 10) and is followed downslope by a steep portion with an inner thalweg that is entrenched up to 100 m (Fig. 11). Upslope from the entry point of the Gioia/Mesima channel/canyon system, a less steep area is present (Fig. 10). Further downslope, the SV is again characterized by a very high gradient that is maintained down to the end of the reach. This high gradient area starts in coincidence with a knickpoint (Knickpoint 3) located upslope from two faults that crosscut the valley and which are visible on the bathymetric map (Fig. 10) and in the seismic lines of Figs. 12 and 13. Thalweg

Fig. 7. Seismic line BC7 crossing the proximal part of the second reach of the SV characterized by a depositional right levee (see location in Fig. 6). The valley lies to the east of the western extensional fault (bold line) of the NNW trending graben controlling the first reach of the SV. The pattern of the high amplitude reflections of the channel floor fill indicates that channel aggradation has taken place concomitantly with a westward migration. Levee instability is shown by a chaotic body on the inner levee side. Within the valley floor a terrace is present to the east of the inner thalweg.

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Fig. 8. Bathymetric map (10 m contour interval; bold contours every 100 m; see location in Fig. 2) of the third reach of the SV, running at the base of the Panarea volcanic slope and characterized by terraces (dashed, bold lines) and an inner meandering thalweg (dotted bold line). The Baia di Levante Valley joins the SV at the beginning of the reach. Point bars are as high as 50 m above the surrounding thalweg floor and span the valley for around 800 m confining the thalweg floor to 200 m. Gullies are developed on the downslope continuation of a channel that runs on the Sicilian slope. The bold line is the trace of the seismic line of Fig. 9.

entrenchment results in two terraces cut on both sides of the valley. 4.5. The fifth reach: lateral input and aggrading valley floor The gradient of the reach is in general low but short, steeper segments are also present (Fig. 3). The height of the confining walls (Table 1) diminishes gradually downslope till the end of the reach that coincides with the westward turn of the SV (Fig. 14). In parallel, a widening of the SV is apparent (Fig. 14). A flat-bottomed featureless valley characterizes the first part of the reach (Fig. 14). Further downslope, the Basiluzzo canyon, the Angitola slope valley and the Strombolic-

chio canyon deposit sediment within the SV floor causing the thalweg to develop a sinuous pattern. Both in the upper and lower parts of the reach the valley floor presents an infill mainly consisting of high amplitude reflections above an erosional surface (Figs. 15 and 16). 4.6. The sixth reach: the multithalweg aggrading valley floor The major feature of the reach is the recti-angular course of the right, northern side of the valley (Figs. 17 and 18) and the large width of the valley floor (Table 1). It likely corresponds to faults separating the SV from the Lametini–Alcione flat located at an upper structural level (Mussoni, 2005). The left margin of the valley

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Fig. 9. Seismic line BG2 crossing the third reach of the SV characterized by a meandering inner thalweg (see location in Fig. 8). The right, unterracced flank of the SV corresponds to the first outer bend of the inner thalweg. The valley fill consists of reflections dipping toward the outer side of the bend and is likely the result of lateral accreting package deposition. The step-like morphology of the basal erosional surface of the SV shows that terraces were present also at lower levels now being covered by the valley infill.

corresponds, on the contrary to the volcaniclastic apron of the Stromboli edifice that is prograding over the valley floor (Gamberi et al., 2006). The reach is characterized by aggradation within the valley floor that is often the site of a multithalweg pattern. In particular, in the proximal part of the reach, corresponding to the area of the westward turn, Gamberi and Marani (2004b), through the interpretation of deep tow sidescan sonar data, have recognized a braiding pattern of thalwegs developed between longitudinal bars with a seafloor sculpted by sediment waves and giant scours. Downslope from the bend, due to the great waterdepth, the multibeam data show the morphologic details of the valley floor less clearly. However, the SV has a multithalweg floor (Fig. 18) and a seismic line shows that it is floored by a 200 ms thick package of highly discontinuous, high amplitude reflections (Mussoni, 2005). Finally, downslope from a last knickpoint (Knickpoint 4), the SV becomes again very narrow and incised (Fig. 3). The incision and narrowing is likely the response of the valley to the damming effect of a structural high that from the south side of the valley connects with the Alcione-Lametini flat (Fig. 18). 5. Discussion

The structural grain of the Sicilian margin controls the location of the first, NNW-trending reach of the SV, that runs within a graben with the same orientation (Figs. 2, 4 and 5). The graben is transverse to the margin and continues into the Castroreale graben on land (Lentini, 2000) where the Termini River is the largest sediment feeder to the northeastern Sicilian margin. The sediment input connected with the Termini river can thus have contributed, particularly during the periods of low sea level, to the initiation of the valley cutting processes and to its subsequent development. The direction of the subsequent, NE-trending reaches of the valley, down to the westward turn north of Stromboli Island is controlled by the alignment of the edifices of the Aeolian island arc (Fig. 2). The breaching of the Aeolian volcanic arc between Stromboli Island and the Lametini Seamount allow, on the contrary, the valley to take a slope transverse path down to the Marsili Basin (Figs. 2 and 17). In addition, the, E-W trending sixth reach of the SV is also controlled by the pattern of tectonic and volcanic features. The E-W slope portion, where the SV runs, is in fact a structural depression bounded to the north by extensional faults (Mussoni, 2005), evident also in the right-angular pattern of the northern flank of the valley, and to the south by the distal slopes of the Stromboli slope (Figs. 1, 2 and 18).

5.1. Controls on the course of the Stromboli valley The course of submarine slope valleys is mainly controlled by slope topography, reflecting active or past tectonics, sediment supply and sea level variations (Laursen and Normark, 2002). Besides these factors, in the study area, the distribution of the volcanic edifices of the Aeolian arc has a major influence in determining the path of the SV (Figs. 1 and 2).

5.2. Stromboli Valley longitudinal depth profile development Graded turbidity current channels are characterized by relatively smooth, concave-up longitudinal profiles; complex longitudinal depth profiles similar to that of the SV (Fig. 3), are indicative of channels that are out of grade owing to gradient changes imposed by external

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Fig. 10. Bathymetric map of the fourth reach of the SV characterized by the highest degree of incision (10 m contour interval; bold contours every 100 m; see location in Fig. 2). Two areas of very high gradient in the upper and the distal parts are separated by a less steep area located upslope from the Gioia/ Mesima channel/canyon system entry point. As a cosequence a knickpoint (Knickpoint 2) is located at the beginning of the reach and one further downslope (Knickpoint 3) (see also Fig. 3). An inner thalweg (dotted line) is entrenched up to 100 m in much of the valley floor. Terraces are the result of the entrenchment of the valley floor. Indentation of the valley flanks, with a lateral width of around 400 m, are slump headwalls. Two morphologic steps in the slopes surrounding the SV, correspond, to the Fault 1 and Fault 2 (bold dashed lines). The bold lines are the traces of the seismic profiles of Figs. 11–13.

Fig. 11. Seismic line Bg3 (see location in Fig. 10) crossing the SV close to the Knickpoint 2. Note the complete absence of any infill within the deeply incised SV. The line crosscuts the SV 10 km upslope from the F1 and F2 faults, thus showing that the erosion and entrenchment connected to the fault activity has propagated upslope for a comparable distance.

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Fig. 12. Seismic line Tir 66 (see location in Fig. 10) showing the fault F1 offsetting the basement in coincidence with the SV and separating the Gioia Basin from the Calabrian margin. The recentmost infill of the Gioia basin is not confined by the fault. The line also shows the terrace above the valley floor.

factors (Pirmez et al., 2000). In this case, the systems react by smoothing the topography through a differential distribution of erosion and deposition along its course, in order to reach an equilibrium profile. Although the longitudinal depth profile of the SV is very complex, it can be divided into two, roughly concave-up profiles (Fig. 3). The proximal profile comprises the reaches I to III while the distal one comprises the reaches IV to VI (Fig. 3). Both profiles are characterized by erosional processes in the upper parts (reaches I and I V) and by deposition in the lower parts (reaches II–III and V–VI). In both profiles, the erosional and depositional reaches are separated by extensional faults that crosscut the SV (Fig. 3). We therefore infer that the location of

crosscutting tectonic structures is the major controlling factor defining the longitudinal depth profile of the SV. In particular, the longitudinal depth profile of the SV is similar to that of systems crossing topographically complex slopes that, although characterized by complex longitudinal profiles, tend to locally develop gradually varying concave-up profiles within the single sub-basins (Pirmez et al., 2000). 5.3. Controls on the processes within the Stromboli Valley and related morphology Along the flanks of the first reach of the SV, frequent failures affect sedimentary packages as thick as 200 ms

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Fig. 13. Seismic line Tir 67 showing the F2 fault on the left side of the fourth reach of the SV (see location in Fig. 10). The fault causes a prominent step at the seafloor pointing to a recent or still on- going activity. To the southwest of the F2 the basement lies at around 2 s, whereas to the northeast it is deeper than 3 s and is not imaged in the seismic line below the SV infill.

and seafloor areas as large as 5 km2 (Figs. 4 and 5). Some of the failed sediments accumulate in the flanks; others however, reach the floor of the valley (Fig. 5). Thus, besides being a conduit for sediments fed by the coastal areas, the first reach of the SV is also a source of landslide related surge-like flows that are depositing on the valley floor but that can also likely evolve into turbidity currents and move down the SV to feed the deeper parts of the system. In fact, the sediment waves shown by Gamberi and Marani (2004b) are the evidence

that turbidity currents are actively shaping the levee of the second reach immediately downslope from the first reach. Three plunge pools with a maximum negative relief of 30 m and as wide as 1600 m are a characteristic tract of the first reach of the SV (Fig. 4). Similar plunge pools have been documented in areas of sudden gradient decrease at canyon mouths at the base of the New Jersey and California continental slope (Lee et al., 2002). Lee et al. (2002) suggested that both flow impact

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Fig. 14. Bathymetric map of the fifth reach of the SV running in the distal part of the Calabrian margin (10 m contour interval; bold contours every 100 m; see location in Fig. 2). The Basiluzzo canyon, smaller gullies and the Strombolicchio canyon enter the SV from the Aeolian volcanic slope. The Angitola slope valley joins the Stromboli valley from the Calabrian margin. All the lateral sediment fairways originate depositional mounds within the valley floor. In correspondence to the depositional mounds, the inner thalweg (dotted line) is forced to run in the opposite side of the valley floor. Bold lines are the seismic line of Figs. 15 and 16.

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Fig. 15. Seismic line BG5 crossing the fifth reach of the Stromboli Valley (see location in Fig. 14). One of the gullies of the Aeolian volcanic slope is imaged to the left of the valley. The SV infill, consisting of an at least 100 ms thick package of high amplitude seismic reflections, lies above an erosional surface that connects with the present-day valley flank. The slightly elevated area adjacent to the left valley margin corresponds to the depositional mound fed to the valley floor from the Aeolian volcanic slope.

mechanisms and hydraulic jumps, due to the abrupt gradient decrease, can result in plunge pool formation. In order to explain the row of three plunge pools in the SV a repetition of these processes must occur along the course of the valley. We envisage that an uneven distribution of the frequent slump or debris flow deposits can create defects and gradient variations along the SV floor that are then progressively accentuated by successive flows, leading to plunge pool formation. Modelling results by Pratson et al. (2000) show that a similar accentuation of topographic depressions leading ultimately to plunge pool formation can be achieved

through successive turbidity current flows. Thus, due to already mentioned evidence for turbidity currents immediately downslope from the plunge pools, this mechanism is most likely at work in the SV. The onset of a depositional character of the SV, at the beginning of the second reach, occurs at the foot of the major E–W trending fault that marks the base of slope and interrupts the NNW-trending graben that confines the first reach (Fig. 6). The sudden gradient decrease and the dying away of the confinement effect of the eastern graben margin are both likely to be responsible for the slowing and spreading of the turbidity current flows thus

Fig. 16. Final part of the seismic line BG 6 crossing the left margin of the SV and running longitudinally within the valley floor (see location in Fig. 14). The valley floor infill lies above an erosional surface that connects with the left flank of the valley. The fill is characterized by discontinuous high amplitude reflections and has a thickness of about 200 ms.

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Fig. 17. Bathymetric map of the sixth reach of the SV (10 m contour interval; bold contours every 100 m; see location in Fig. 2) running toward the west between the slope of Stromboli Island and the Lametini Seamount. Longitudinal bars (dashed lines along the axis) and thalwegs (dotted, bold lines) with depth of around 20 m characterize the floor of the SV. They are confined between erosional walls as high as 50 m.

allowing the onset of a depositional regime. Here, the SV becomes aggradational and develops a depositional levee on its right side (Figs. 6 and 7). Aggradation has occurred concomitantly with a westward migration of the leveed channel (Fig. 7). The third reach of the SV is characterized by a single meandering thalweg confined within walls as high as 200 m (Table 1, Fig. 8). It is developed in the axial part of the Gioia Basin that was previously the site of lobe deposition fed by the SV and by the other channels of the Sicilian margin (Gamberi

and Marani, 2006). An erosional phase and the incision of the third reach of the SV through progressive entrenchment and narrowing, witnessed by the mostly terraced valley flanks (Figs. 8 and 9), must therefore have followed a depositional period in this sector of the basin. To create erosion on a previously depositional area a lowering of the base level or a change in flow parameters is needed (Pirmez et al., 2000; Babonneau et al., 2002; Kneller, 2003; Samuel et al., 2003; Smith, 2004; Adeogba et al., 2005). Lowering of the base level

196 F. Gamberi, M. Marani / Marine Geology 243 (2007) 180–199 Fig. 18. Bathymetric map of the distal part of the sixth reach of the SV (10 m contour interval; bold contours every 100 m; see location in Fig. 2). The northern margin of the SV is characterized by straight segments with both NW–SE and NE–SW direction that likely correspond to extensional faults. In the proximal part, the valley floor is up to 10 km wide and is the site of small thalwegs (dotted bold lines). In the distal part, on the contrary, the valley narrows and becomes incised between the Alcione Lametini Flat to the north and a ridge of volcanic origin to the south. The proximal part of the Marsili fan (see Fig. 1), is shown on the left side of the figure.

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occurs in confined intraslope basins, when the upper basin is completely filled and undergoes erosion due to the establishment of a new base level coinciding with the deeper basin. Although bounded downslope by extensional faults, the flat portion of the Gioia basin, where the third reach of the SV is developed, is not completely isolated from the deeper basin portion downslope; at least in the recent history of the basin infill, sediments do not show evidence of confinement (Fig. 12), ruling out base level lowering as the cause of the entrenchment of the third reach of the SV. The alteration of the equilibrium profile, connected with changes in flow parameters is the other process that can switch a system from a depositional to an erosional setting. In general, changes in flow parameters responsible for variations in the sedimentary regime of slope valleys and channels are thought to be related to different stages of sea level stand (Babonneau et al., 2002; Kneller, 2003; Samuel et al., 2003; Smith, 2004). In the study area, in addition to sea level variations, the effects of the sedimentary dynamics of the Baia di Levante Valley represent a viable alternative for modifying the equilibrium profile of the SV. The Baia di Levante Valley is a major erosional feature that runs in the Vulcano volcaniclastic apron and that has its apex in the area of the Fossa volcano caldera (Fig. 2) (Gamberi, 2001). It joins the SV at the beginning of the third reach (Fig. 8). At about 15 ka the Fossa caldera was the site of voluminous submarine hydromagmatic eruptions with the production of large volumes of pyroclastic deposits (De Astis et al., 1997a, b). Since out-sized flows can create erosion surfaces in systems that in the longer term are graded, we suggest that submarine pyroclastic gravity flows originating from the Fossa cone generated the incision of the third reach of the SV. A similar erosional behaviour of submarine pyroclastic gravity density flows led to the formation of channelling in the distal part of the Myojinsho cone, at water depths of between 900–1500 m, 3–7 km away from the eruption area (Fiske et al., 1998). The new equilibrium profile controlled by the character of the flows from the Baia di Levante can also explain the knickpoint 1, the valley incision and the abandonment of the right side of the valley that occurs in the distal part of the second reach (Figs. 3 and 6). All these processes in fact can be the reaction of the distal part of the leveed valley (second reach) that evolves in response to the establishment of a local, deeper base level, due to the incision of the third reach of the SV. On the contrary, the development of the meandering thalweg and the deposition of lateral accretion packages that at present characterize the third reach of the SV

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(Figs. 8 and 9) can indicate that after the erosional phase, a new graded profile was reached, as also displayed by the convex-up profile of this tract of the valley (Fig. 3). In the third reach, therefore, flows are presently in equilibrium and they mostly bypass the valley forming only thin lateral accretion packages (Fig. 9) at the inner side of the bends of the meandering thalweg. A large degree of incision and the lack of any infill form the evidence that erosional processes are prevailing in the fourth reach of the SV (Table 1, Figs. 10 and 11). The reach has the highest gradients of the SV in coincidence with two distinct knickpoints (knickpoints 2 and 3) upslope from a relay ramp between two orthogonal extensional faults (Figs. 2, 3 and 10). The degree of incision and the abrupt knickpoint 2 at the onset of the tract with the development of a narrow entrenched thalweg that has no continuation upslope, point to an evolution of the fourth tract independent from that of the upslope reach. A stratigraphic architecture, with deposition in low gradient areas and contemporaneous erosion and incision of canyon-like features in steeper areas is a typical tract of depositional systems in topographically complex slopes (Huyghe et al., 2004; Smith, 2004; Adeogba et al., 2005). Similarly, the fault controlled cutting of the fourth reach of the SV, corresponding with the steepest gradient of the adjacent slope (Fig. 3), could therefore be coeval with the deposition of lobes in the upslope flat portion of the basin. We suggest that the flows that were depositing lobes in the flat area of the Gioia basin underwent a rapid acceleration and became erosional in the steep slope portion created by the faults. In more detail, a process of headward erosion induced by downslope-eroding sediment flows, similar to that proposed by Pratson et al. (1994) and by Pratson and Coakley (1996), is envisaged here for the establishment and successive evolution of the erosional canyon-like fourth reach of the SV. The presence of the knickpoint 3 downslope from the knickpoint 2 (Figs. 3 and 10), is thus likely the indication that the incision of the valley took place during two distinct episodes of local base level lowering that in turn could be related to discrete phases of fault activity. The seismic data do not allow to deduce a clear inference about this point. The fifth reach of the SV is still characterized by high relief erosional flanks, but it is experiencing aggradation through the deposition of as much as 200 ms of sediments within the valley floor (Table 1, Figs. 14–16). Depositional processes occurring in the hanging wall of extensional faults and coeval with erosion on the footwall block have been described from the Niger delta continental slope and explained as resulting from the

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creation of accommodation space at the base of the faults (Adeogba et al., 2005). Thus, we infer that the lowering of the hangingwall block creates accommodation space and promotes the depositional attitude of the fifth reach of the SV. Like the fifth reach, the sixth reach is characterized by an aggradational valley floor (Gamberi and Marani, 2004b). In the area of the bend, Gamberi and Marani (2004b) described a depositional style resulting from a braiding pattern of thalwegs confined within erosional flanks. Further downslope, the seismic data are very sparse and do not allow to penetrate below the fill of the valley that is very coarse–grained due to a large contribution of volcaniclastics from the Stromboli volcano. However, a seismic line presented by Mussoni (2005), fails in evidencing any extensive erosional surface below the valley fill in the distal part of the sixth reach, and points to a tectonic origin for the northern margin of the valley as also evidenced by its right-angular pattern (Fig. 18). We can therefore deduce that this area has always been the site of depositional or bypass flow evidencing that the sixth reach is at grade or slightly below grade. 6. Conclusions 1) The proximal part of the Stromboli valley is confined within a graben that continues onshore into an area where the largest river of the Sicilian margin is developed. Thus, the on-land geology, controlling the route of the more frequent and voluminous sediment gravity flows, in particular during low sea level stands, can have contributed to the initial slope valley cutting processes. 2) Besides being a conduit for sediment gravity flows delivered from the coastal areas, the proximal part of the Stromboli Valley is a source of surge-like flows, originating through widespread landslides that occur along its flanks, that evolve into turbidity currents downslope. 3) Contemporaneous depositional and erosional reaches are found along the 120 km long Stromboli slope valley. The main depositional reaches are downslope from the E–W trending fault that marks the transition between the slope and the base-of-slope in the Gioia Basin and in the hanging wall of the two faults located at the boundary between the Gioia Basin and the Calabrian Margin. Thus, the creation of accomodation space in the hanging wall of extensional tectonic structures is the main control on the location of the depositional areas. Concomitantly, erosion with a high degree of slope valley incision is occurring in the footwall of the same structures. The results thus

confirm that predicting the pattern of deposition and erosion along the course of submarine valleys in topographically complex slope areas requires a good knowledge of the tectonic setting of the region. 4) Depositional processes within the different reaches of the Stromboli valley result in: a) a chaotic valley floor infill, consisting mainly of landslide deposits (first reach); b) an aggradational leveed valley (second reach); c) lateral accretion packages within an entrenched valley (third reach); d) an aggradational valley fill confined within erosional walls (fifth reach); e) an aggradational area confined within a structural depression (sixth reach). Erosion, on the contrary, dominates the fourth reach. As a consequence, a highly varied grain size, geometry, stacking pattern and continuity of the resulting sediment gravity flow deposits must be expected along the course of the Stromboli Valley. 5) Modification of flow characteristics due to the lateral input of out-sized sediment gravity flows from the Baia di Levante tributary have altered the equilibrium of the third reach of the Stromboli Valley leading to erosion in a previously depositional area. Thus, the resultant dynamics of sediment gravity flows that derive from the junction of different drainage trunks in slope valleys is another factor that has a large impact on the location and timing of depositional and erosional processes. Acknowledgments We thank Marco Ligi for his help in the construction of the bathymetric maps. The manuscript greatly benefited from the suggestions of two anonymous reviewers and of Tjeerd van Weering the Editor of this volume. This is contribution 1567 of the Sezione Geologia Marina of the Istituto di Scienze Marine. References Adeogba, A.A., McHargue, T.R., Graham, S.A., 2005. Transient fan architecture and depositional controls from near-surface 3-D seismic data, Niger delta continental slope. Am. Assoc. Pet. Geol. Bull. 89, 627–643. Babonneau, N., Savoye, B., Cremer, M., Klein, B., 2002. Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan. Mar. Pet. Geol. 19, 445–467. De Astis, G., Dellino, P., De Rosa, R., La Volpe, L., 1997a. Eruptive and emplacement mechanism of widespread fine-grained pyroclastic deposits on Vulcano Island (Italy). Bull. Volcan. 59, 87–102. De Astis, G., La Volpe, L., Peccerillo, A., Civetta, L., 1997b. Volcanologic and petrologic evolution of Vulcano island (Aeolian arc, southern Tyrrhenian Sea). J. Geophys. Res. 102, 8021–8050.

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