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Large-scale seafloor waveforms on the flanks of insular volcanoes (Aeolian Archipelago, Italy), with inferences about their origin
Marine Geology 355 (2014) 318–329
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Marine Geology journal homepage: www.elsevier.com/locate/margeo
Large-scale seafloor waveforms on the flanks of insular volcanoes (Aeolian Archipelago, Italy), with inferences about their origin D. Casalbore a, C. Romagnoli. b, A. Bosman a, F.L. Chiocci c a b c
IGAG-CNR, Istituto di Geologia Ambientale e Geoingegneria, Area della Ricerca di Roma 1, P.le Aldo Moro 5, 00185 Roma, Italy Università di Bologna, Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, P.za Porta S. Donato 1, 40126 Bologna, Italy Sapienza Università di Roma, Dipartimento Scienze della Terra, P.le Aldo Moro 5, 00185 Roma, Italy
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 9 June 2014 Accepted 19 June 2014 Available online 27 June 2014 Keywords: sedimentary gravity flows cyclic steps coarse-grained sediment waves multibeam bathymetry instability processes
a b s t r a c t High-resolution multibeam data in the central and eastern part of the Aeolian volcanic arc (Southern Tyrrhenian Sea) reveal widespread large-scale waveforms on the submarine flanks of these insular volcanoes. These features show high variability in wave parameters (wavelength of 60–1600 m and wave height of 4–200 m) and morphology (from sinuous to arcuate in plan-view, and from undulating to stepped, with very steep scarps, in cross-section), indicating that different processes, at variable spatial and temporal scale, are responsible for their genesis. By relating the morphological characters of the different waveforms and the local boundary conditions (morphological setting, regional slope gradients, sediment source and dynamics) where these features form, we were able to distinguish bedforms formed by sedimentary gravity flows from bedform-like features due to gravity instability processes. In detail, bedforms have been interpreted as coarse-grained sediment waves according to their wave parameters, geometry and texture. Bedforms are always found where the slope gradients markedly decrease to values b4°–8°, promoting a hydraulic jump in the sedimentary gravity flow. Hence, we tested if the recognized coarse-grained sediment waves can be generated as cyclic steps by comparing their morphometric characteristics with the results of experimental evidence and numerical modeling proposed in literature. The results show that cyclic step is a reasonable interpretation for these features, and we discuss the effect of flow depth, flow concentration and discharge as governing parameters affecting their wavelength, wave height and shape. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since Heezen et al. (1959) first documented repetitive marine bedforms by using echo sounding profiles, these features have been described in a variety of submarine environments, from deep-sea fans to continental shelves, both on glaciated and non-glaciated margins. Over the last decade, the advances in multibeam imagery and 3-D seismic techniques, that are providing an unprecedented high-resolution imaging of seafloor and sub-seafloor features (e.g. Paull et al., 2010; Lonergan et al., 2013) have produced an exponential increase in the accurate description of submarine waveforms. Despite the high number of case-studies and the growing interest for these features, their interpretation is still difficult to be univocally defined (e.g. Lee et al., 2002; Paull et al., 2010; Cartigny et al., 2011 and reference therein). In fact they can be related to different mechanisms, such as the interaction between seafloor and dense flows (tidal, geostrophic or sediment-gravity currents, Smith et al., 2007; Wynn and Stow, 2002), the seafloor deformations due to gravity instability processes (e.g. Shillington et al., 2012)
E-mail address: [email protected] (D. Casalbore).
http://dx.doi.org/10.1016/j.margeo.2014.06.007 0025-3227/© 2014 Elsevier B.V. All rights reserved.
and\or the combination of fluidized remobilization and slumping/ breaching of canyon fills (e.g. Paull et al., 2010). High-resolution multibeam data in the central and eastern part of the Aeolian volcanic arc (Southern Tyrrhenian Sea, Fig. 1) have revealed the widespread presence of large-scale seafloor waveforms on the submarine flanks of these insular volcanoes, with wavelength ranging from hundreds of meters up to some kilometers. The aim of the paper is to describe these waveforms and their morphometric parameters, and compare them with similar features recognized in other submarine settings in order to better constrain the boundary conditions controlling their genesis and development. Based on their morphology and location, we firstly present a distinction between bedforms (repetitive seafloor waveforms formed by fluidal flow) and bedform-like morphological features due to gravity instability processes. Then, we focus our attention on the formation of bedforms and their possible interpretation as cyclic step based on the results of recent numerical modeling and experimental evidence (e.g. Postma et al., 2009; Spinewine et al., 2009; Cartigny et al., 2011). Finally, we discuss what the bedforms and changes in their morphometric parameters may tell us about the sediment dispersal and the likely characteristics (e.g. flow depth, concentration and discharge) of the flows that generated them.
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Fig. 1. Shaded relief map of the five insular volcanoes making up the central and eastern sectors of the Aeolian Archipelago, with the location of the bedform-like morphological features, bedforms, main morphological features and following figures. The inset shows the regional setting of the area, where triangles represent submarine seamounts also belonging to the Aeolian Arc.
2. Geological setting The Aeolian Arc is formed of seven insular volcanoes and several seamounts located between the Southern Tyrrhenian Sea back-arc basin and the Calabro–Peloritano Arc (Fig. 1 inset). Islands and seamounts represent the younger manifestation of volcanism related to the NW-directed subduction of the Ionian oceanic lithosphere below the Calabrian Arc (e.g. Gvirtzman and Nur, 1999). The volcanic arc can be divided into western, central and eastern sectors, characterized by different tectonic evolutions (De Astis et al., 2003). Our study area encompasses the central and eastern sectors of the archipelago (Fig. 1). The central sector includes Salina, Lipari and Vulcano islands. The volcanism started at ~ 270 and 240 ka (at Lipari and Salina, respectively, Lucchi, 2013 and references herein) and it was historically documented at Lipari (1220 AD) and Vulcano (1888–90 AD). The eastern Aeolian
sector includes Panarea and Stromboli, where subaerial volcanism started at ~ 150–200 ka (Calanchi et al., 1999) and it is still active at Stromboli, where the typical Strombolian activity persists since, at least, 2000 years (Rosi et al., 2000). The submarine setting of the Aeolian Arc has been generally illustrated in Romagnoli et al. (2013) by means of multibeam bathymetry, showing widespread mass-wasting features at different scales all around the insular volcanoes. Large-scale collapses and related debris avalanche deposits were recognized only on the eastern and northwestern flanks of Stromboli (Bosman et al., 2009; Romagnoli et al., 2009a,b; Casalbore et al., 2011), whereas smaller-scale instability is present in the submarine flanks of all the islands (Romagnoli et al., 2009b). The submarine flanks of the Aeolian volcanoes are rather steep, with slope gradients ranging from 20–30° in the upper slope to few degrees at their base (Fig. 1 ESM).
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Table 1 Synthesis of the main morphological characters of the different bedform-like features (Bl) and bedform fields (Bf) found in the central and eastern sectors of the Aeolian Archipelago. Waveform Location field
Depth range (m)
Bl1
1200–1750 8–20
Bl2 Bf1
Panarea N flank Lipari W flank Stromboli SE flank
Gradient Wavelength Height range (m) (m) (°) 1000–1600
Lateral extent (m)
Aspect ratio
Plan-view shape
50–200 1300–2000 0.07–1.25 Arcuate
Cross-section shape
Notes
Stepped (lee side of 25–30°) Undulating (b12°)
Confined within a wide scar indenting the insular shelf Lying at the base of a landslide scar complex Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; coarse-sand and gravel at surface Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; increase in wave size downslope; coarsesand and gravel at surface Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; increase in wave size downslope; coarsesand and gravel at surface Waveforms found at the base of the Panarea N flank, carved by a network of channels At surface of a semi-transparent lens on Sparker profiles; two graded levels of coarse-sands and millimetric scoriae are found in core EAC 8 Confined within a straight channelized feature
1100–1380 ≈4
400–700
8–12
1500–1700 2–5
60–400
5–20
70–800
0.02–0.08 Straight to arcuate
Undulating
800–1300 0.01–0.02 Sinuous
Bf2
Stromboli NW flank
2000–2600 3–8
70–500
5–8
100–800
0.01–0.07 Straight to arcuate
Undulating
Bf3
Stromboli N 2300–2600 2–4 flank
80–530
4–8
120–800
0.02–0.08 Straight to arcuate
Undulating
Bf4
Panarea N flank
1800–2500 3–8
300–1000
8–50
800–2000 0.02–0.07 Sinuous– arcuate
Undulating to stepped
Bf5
Vulcano SW flank
800–1200 2–6
100–300
4–20
150–700
0.02–0.07 Sinuous– arcuate
Undulating to stepped (up to 30°)
Bf6
Lipari W flank
870–1260 ≈5
160–420
14–25
300–700
0.03–0.09 Arcuate
Stepped (up to 55°)
3. Data and methods The multibeam data were collected during several oceanographic cruises carried out in the last ten years aboard small vessels in shallow-water (b100 m wd) and research vessels Thetis, Universitatis and Urania in deep water (N100 m wd). Multibeam data were acquired with systems working at different frequencies (from 50 to 455 kHz), depending on the water depth so that each bathymetric interval has been mapped at the maximum resolution. All data were RTK- or DGPSpositioned for shallow water and deep water, respectively, and processed with dedicated software, taking into account daily sound speed profiles and repeated calibration of transducers in the survey zone. Because the sounding density and the resolution decrease from the coastal sector towards the base of edifice, Digital Elevation Models (DEMs), with cell size varying from 1 m in shallow water (b 100 m wd) to 25 m (down to 2600 m wd) in deep water, were generated. Successively using the high-resolution multibeam bathymetry produced, main morphometric parameters of the waveforms were measured. Waveform parameters include: wavelength, wave height, lateral extent, length and slope gradients of stoss and lee side, aspect ratio (or wave steepness, defined as the ratio between wave height and wavelength), and symmetry index (defined as the ratio between the length of the stoss and lee side, Tanner, 1967). Moreover, the multibeam bathymetry was used as a reference base to (re)locate previously collected seismic profiles (3.5 kHz SBP and 1 kJ Sparker) and deep-towed longrange side scan sonar data (TOBI working at a frequency of 30 kHz) by matching of homologous features. In addition to ground truth the seabed morphology several seafloor dredge, core and grab samples were collected. Some of the results of this seafloor sampling have been already presented in Romagnoli et al. (2009a,b), Casalbore et al. (2010) and Romagnoli et al. (2012), whereas other ones are described for the first time in this paper.
4. Results In the Aeolian Archipelago, large bedform-like morphological features (Section 4.1) and bedforms (Section 4.2) were recognized, with
a wavelength from several tens of meters to over 1 km, wave height from some meters to tens/hundreds of meters and lateral extent from hundreds to thousands of meters (Table 1). Their crest-lines commonly display an arcuate or sinuous shape, trending roughly perpendicular to the maximum slope direction. In cross-section, their shape is variable from a stepped morphology, with steep scarps (20–55°) alternating with relatively flat surfaces, to a more undulating pattern with less steep sides (8–15°). 4.1. Bedform-like morphological features Bedform-like morphological features were identified at Panarea and Lipari (Bl, yellow areas in Fig. 1). 4.1.1. Northern flank of Panarea Here, large bedform-like morphological features (Bl1 in Figs. 1 and 2) are present in the upper part of the slope, where slope gradients range between 20° and 8°, and the flank has an overall slightly concave section. These features are found at or immediately downslope of 1.5 km-wide scars that indent the outer edge of the insular shelf. They have a wavelength of 1000–1600 m and lateral extent of 1300–2000 m (Table 1). In cross-section, they show a stepped shape, with stoss side nearly flat (up to backward sloping) and a steep lee side (25–30°), forming 50–200 m-high scarps; these steps tend to decrease in size downslope. Their aspect ratio is the highest among all the analyzed features, ranging between 0.07 and 1.25. 4.1.2. Western flank of Lipari Here, bedform-like morphological features (Bl2 in Figs. 1 and 3) have been recognized between 1100 and 1380 m wd, at the base of the landslide scars that affect the submarine flank of Banco del Bagno volcanic center. The waveforms show a wavelength of 400–700 m, wave height of 8–12 m, and lateral extent of 800–1300 m. Their aspect ratio is the lowest, ranging between 0.01 and 0.02 (Table 1). They are characterized by a prevalent undulating pattern with sinuous crestlines and lee side sloping up to 10° (5° on average), markedly
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Fig. 2. Shaded relief of the NW flank of Panarea volcano, where bedform-like morphological features (Bl) and bedform field (Bf) are recognized. Dotted lines indicate the bathymetric profiles shown in Fig. 6.
contrasting with the arcuate and stepped morphology observed for the nearby bedform field 6 (Bf6, see Section 4.2). 4.2. Bedforms Bedform fields were identified at Stromboli, Panarea, Vulcano and Lipari edifices (magenta areas in Fig. 1). 4.2.1. South-eastern, north-western and northern flanks of Stromboli Here, the bedforms commonly display a low morphological relief, but a marked backscatter variation on deep-towed side scan sonar data, with low-backscatter tones on the stoss side and highbackscatter tones on the crest-line and lee side (Fig. 4b, c, d). On the Stromboli SE flank, the bedforms are mainly confined within the thalweg of shallow erosive furrows between 1500 and 1700 m wd (Bf1 in Fig. 4b), whereas on the NW and N flanks they are found below 2000 m wd. Here, they are confined within the troughs developed on a large-scale volcaniclastic bulge created by repeated flank collapses of the NW flank (Sciara del Fuoco, SdF, fan in Fig. 4a) and in a wide valley at the foot of the saddle separating Stromboli and Strombolicchio volcanic centers (Bf2 in Fig. 4c and Bf3 in Fig. 4d, respectively). Bedforms show a wavelength of 60– 530 m, wave height of 4–10 m (apart from few cases, where they reach values up to 20 m), lateral extent of 70–800 m and aspect ratio of 0.01–0.08 (Table 1). They are straight to arcuate in planview (Fig. 4c and d), while they show a prevalent undulating pattern in cross-section, with a sub-horizontal stoss side and a lee side locally sloping to 5–25° (A-A′, B-B′, C-C′, D-D′, E-E′ in Fig. 6a). In all cases, bedforms appear below a marked decrease of slope gradients
(b 4–8°). Bedforms sometimes tend to increase their wave dimension downslope, where the gradients decrease to values of 2–3° and the seafloor morphology is more regular and flat. On deep towed sub-bottom profiles, bedforms are characterized by a very strong echo and lack of seismic penetration (Fig. 4e). Seafloor sampling performed on the bedform fields mainly recovered coarse-sand and gravel (black dots in Fig. 4a; see Romagnoli et al., 2009b; Casalbore et al., 2010 for further details). 4.2.2. Northern flank of Panarea Here, the bedforms (Bf4 in Figs. 1 and 2) develop over a wide area at the base of the volcanic flank between 1800 and 2500 m wd, where a marked decrease of slope gradients (from 8° to 3°) is present. The flank is carved by a network of channels having a width of 100– 200 m, length of 5–8 km, and depth of 20–40 m. The head of these channels indents the outer edge of a wide insular shelf developed all around Panarea island at depths of 130–150 m. Bedforms have a marked morphological relief, with a wavelength of 300–1000 m, wave height of 8–50 m, lateral extent of 800–2000 m and aspect ratio of 0.02–0.07 (Table 1). They show sinuous or arcuate shape on plan-view and have a variable cross section, from stepped to a more undulating pattern, with stoss side flat or sloping backward and lee side with gradients of 7–32° (F-F′ and G-G′ in Fig. 6). 4.2.3. South-western flank of Vulcano Here, bedforms (Bf5 in Figs. 1 and 5a) are located in the middle and lower part of the large Punta del Rosario fan, which is fed by two coastal scars that deeply indent the insular shelf edge in the southwest part of the island. The bedforms cover a total area of ~ 30 km2 between 800
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Fig. 3. Shaded relief map of the W flank of Lipari volcano, where bedform-like morphological features and bedforms are present. Dotted lines indicate the trace of bathymetric profiles shown in Fig. 6. BB: Banco del Bagno, IS: insular shelf.
Fig. 4. a) Shaded relief map of Stromboli volcano, with the location of sector collapse scar (red lines) seafloor sampling (black dots) and following figures; SdF fan: Sciara del Fuoco fan (see text for detail). In the other figures: long-range side scan sonar (TOBI) image of the south-eastern (b), north-western (c) and northern (d) flank of Stromboli, with the indication of the recognized bedform fields (Bf); light tones indicate high-backscatter values. Dotted lines indicate the bathymetric profiles shown in Fig. 6; e) deep-towed 3.5 kHz sub-bottom profile across the Bf3 (location in Fig. 4d), showing a strong seafloor echo and lack of signal penetration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and 1200 m wd, and they develop downslope of a marked decrease of slope gradients below 6°. The bedforms have a wavelength of 100– 300 m, wave height of 4–20 m, lateral extent of 150–700 m and aspect ratio of 0.02–0.07 (Table 1). In cross-section they show a variable morphology, from stepped with lee side sloping up to 32° to a more undulating pattern, with lee sides sloping to 12–20° (H-H′ and I-I′ in Fig. 6). Locally, bedforms develop as coaxial trains elongated along the maximum slope, defining a series of proto-channels on the fan surface. On Sparker profiles, the bedforms develop on the top of an acoustically semi-transparent seismic unit. A semi-continuous reflector marks up the base of the unit, several tens of milliseconds below it (Fig. 5b). EAC8 core from the deposits in the median part of the fan recovered two normal graded, 20–30 cm thick levels of coarse volcaniclastic sands and millimeter-sized scoriae in the lower part of the core; several 1–2 cm thick levels of fine sand are also present, interbedded in the plastic mud (Fig. 5c). 4.2.4. Western flank of Lipari Here, a coaxial train of bedforms is located within a straight and 500–700 m wide channelized feature between 870 and 1260 m wd (Bf6 in Figs. 1 and 3). This channel is fed by a network of narrow gullies, whose headwall indent the outer edge of the insular shelf cutting the summit of the eccentric submarine volcanic center of Banco del Bagno, located off western Lipari (BB in Fig. 3). The bedforms have a wavelength of 160–420 m, wave height of 14–25 m, lateral extent of 300–700 m, and aspect ratio of 0.03–0.09 (Table 1). These bedforms are characterized by arcuate, concave-downslope shape in plan-view and by a very steep lee side, with slope gradients up to 40° in crosssection (L-L′ in Fig. 6). 4.3. Morphometric analysis of the bedforms Of particular interest for this paper is the morphometric characterization of the bedforms, so we measured, on selected bathymetric profiles for each bedform field (Fig. 6a, excluding the bedform-like morphological features), indicative wave parameters for single bedforms. Descriptive statistics of such parameters are reported in Table 2. The comparison of median values between the different bedform fields confirms the previous general observations, with similar values of wavelength and wave height for the bedforms developed around Stromboli and Vulcano, whereas bedforms at Lipari and Panarea are characterized by three to five times higher values, respectively. Stoss side slope is quite similar for all the cases, with median values comprised between −2.5° (negative values indicate a counter-sloping surface) and 2.3°; lee side slope is comprised between 7° and 15°, except for the Lipari case, where median value is 31°. Similarly, the aspect ratio is comprised in a narrow range for the different bedform fields (0.026–0.038), except for the Lipari case where a value of 0.056 is present. Symmetry index (SI) median values are comprised between 1 and 1.45; more in detail, nearly symmetrical bedforms (SI = 0.6–1.4) are the dominant form (69%), in association with dune-like asymmetrical (SI N 1.4, 29%) and minor antidune-like asymmetrical (SI b 0.6, 2%) forms. Scatter-plots of all the measured data show, generally, a scattered distribution (Fig. 6). In most of the plots the measured parameters do not show any correlation, such as symmetry index against aspect ratio (Fig. 6c) or lee side slope against wavelength (Fig. 6f). Differently, a positive correlation can be observed between wave height and wavelength (Fig. 6b) as well as between the lee side slope against aspect ratio and wave height (Fig. 6e and g, respectively). A slightly negative correlation can be observed between the stoss side slope and the aspect ratio (Fig. 6d).
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on the submarine flanks of the insular volcanoes in the central and eastern part of the Aeolian Arc. These waveforms are not randomly distributed on the submarine flanks of the edifices, but they are mainly found within channelized features and/or on fan-shaped features, suggesting a genetic relationship with gravity-driven processes that formed them, i.e. mass-wasting processes (for bedform-like features) and sedimentgravity flows (for bedforms). Moreover, the waveform crest-lines are always aligned roughly perpendicular to the maximum slope direction. This evidence confirms the key-role played by gravity-driven processes in their formation and allows us to discard a genesis from geostrophic bottom currents, as bedforms generated by this kind of currents should have crests oblique to topographic contours and current direction (Embley and Langseth, 1977; Flood et al., 1993). Tidal currents can be discarded as well because of the microtidal regime that characterizes the study area and the bathymetric interval where the bedforms were found (N 800 m wd). The recognized waveforms show high variability (Table 1) in: a) wavelength (from tens of meters to N1 km), b) wave height (from some meters up to hundreds of meters), c) plan view geometry (from sinuous to arcuate), and d) cross-section (from undulating to stepped with very steep scarps, Fig. 6a). Therefore, it is possible to hypothesize that gravity-driven processes, occurring at variable spatial and temporal scale, are responsible for the variability of these features. Based on their morphological characters and boundary conditions, a first general distinction can be made between bedform-like morphological features (Section 5.1) and bedforms (Section 5.2). 5.1. Bedform-like morphological features Bedform-like morphological features have been recognized at Panarea and Lipari and characterized by the highest and lowest value of aspect ratio, respectively (Bl in Table 1, and Section 4.1). At Panarea the height of these features (50–200 m) is hard to attribute to sediment-gravity flows, also in consideration of the lack of relevant sediment sources able to generate them due to their proximity to the insular shelf edge (Fig. 2). Moreover, these features develop on high slope gradients (up to 20°), differently from what was observed for bedforms in other settings (see Section 5.2). Therefore, we interpret such features as the result of a slumping process, as suggested by their confinement within inward-facing lateral walls of an elongated scar (Fig. 2). Recently, a similar stepped morphology was identified in the upper part of volcanic edifices in the South Sandwich arc (Leat et al., 2010), where it has been interpreted as the result of slumping processes. At Lipari, bedform-like morphological features are found at the base of the landslide scar complex recognized on the western flank of Banco del Bagno volcanic center (Fig. 3). Seismic profiles show that the landslide scars affect the upper part of a relevant fine-grained sedimentary cover, as also supported by the recovering of hemipelagic mud from the cores collected at the scar complex (Casalbore et al., 2014). Moreover, these bedform-like morphological features are characterized by an undulating pattern and a variable plan-view morphology (sinuous) from concave down- to concave up-canyon, quite similar to the morphology created by compressive features (pressure ridges) in cohesive landslide deposits (e.g., Bøe et al., 2000; Lastras et al., 2006), and markedly contrasting with the morphology of the nearby bedform field Bf6 (Fig. 3, Sections 4.1 and 5.2). Based on the above-mentioned evidence, we interpret such waveforms as the result of compressive stress (i.e. pressure ridges) developed within the cohesive landslide deposits originating from the upslope nested scars.
5. Discussion
5.2. Bedforms
High-resolution morpho-bathymetric data show that large-scale bedforms and bedform-like morphological features are quite common
In all the other cases, we interpret the seafloor waveforms as bedforms generated by sedimentary gravity flows because:
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Fig. 5. a) Shaded relief map of the SW flank of Vulcano volcano, with the indication of the main morphological features (Bf: bedform field; IS: insular shelf; PR Punta del Rosario; LFC La Fossa Caldera; IPC Il Piano Caldera) and EAC8 sampling site; dotted lines indicate the bathymetric profiles shown in Fig. 6. b) Sparker 4.5 kJ profile (location in a) shows a seismic semitransparent unit overlaid by large diffraction hyperbolae, whose base is indicated by a black dotted line. A multibeam-derived bathymetric profile is reported above the profile (red line with an ~30 m offset) to better depict the actual morphology of the bedforms. c) Photograph (right) and stratigraphic sketch (left) of EAC8 core (location in a), where main graded coarsesandy layers are shown as dark gray, thinner fine-sandy layer as light gray and hemipelagic mud as white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a) they are found at\or close to relevant sediment sources, i.e. where a morphological continuity between the subaerial and submarine slope is present (wide depressions left by sector collapses at Stromboli in Figs. 1 and 4) or where the insular shelf is largely indented by masswasting processes or channelized features (Panarea, Vulcano and Lipari in Figs. 1, 2, 3 and 5); b) they start to develop under a marked decrease in the slope gradients (below 4–8°), similarly to what was observed for bedforms developed in other settings (e.g. Piper et al., 1985; Wynn et al., 2000; Sisavath et al., 2011). In fact, a reduction of slope gradients would force the flow
to pass the hydraulic jump at some time, during which the velocity of the flow would be reduced significantly and its thickness markedly increased, favoring the development of bedforms (e.g. Postma et al., 2009; Spinewine et al., 2009); c) in terms of wave parameters, geometry and texture, the waveforms show similarity to coarse-grained sediment waves found in proximal regions of modern and ancient turbiditic systems (e.g. Wynn et al., 2002; Hoffmann et al., 2008; Ito, 2010; Sisavath et al., 2011). As shown in the plot of Fig. 6b, the wave dimensions of the Aeolian bedforms fit well with the values measured for modern and ancient
D. Casalbore et al. / Marine Geology 355 (2014) 318–329 Fig. 6. Bathymetric profiles (a) across the different bedforms fields (vertical exaggeration 10×; location in previous figures). Scatter-plots (b–g) of the main morphometric parameters analyzed for the different bedform fields (see text for details); note that all the graphs are on log–log scale, except for graphs c and d (where negative values of stoss-side slope indicate counter-sloping surface) plotted on a linear scale. STRNW/N/S: Stromboli Nord-West/Nord/Sud; VUL: Vulcano; PAN: Panarea; LIP: Lipari; Ls/Ll: lenght of stoss and lee side. In graphs b and c, wave dimensions of modern (from Malinverno et al., 1988; Normark and Piper, 1991; Wynn et al., 2002; Smith et al., 2007) and ancient (1 from Piper and Kontopoulos, 1994; Vicente Bravo and Robles, 1995; Wynn et al., 2002, 2 from Ito, 2010) examples of coarse-grained sediment waves are also reported.
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Table 2 Descriptive statistics for the bedforms selected in the study areas (Fig. 6): Stromboli South-east (STRS), Stromboli North-west (STRNW), Stromboli North (N), Panarea (PAN), Vulcano (VUL), Lipari (LIP). Min.: minimum; Max.: maximum; St. dev.: standard deviation. L: wavelength, H wave height; Ls: length of stoss-side; SGs: slope gradients of stoss-side (negative values indicate a counter-sloping surface); Ll: length of lee-side; SGl slope gradients of lee-side; aspect ratio (H/L); S.I.: symmetry index (Ls/Ll). STRS (9 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect ratio
S.I.
PAN (10 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect ratio
S.I.
Min. Max. Mean Median St. dev.
82 211 131 131 38
1.8 5.6 3.6 3.4 1.3
40 125 74 80 26
−3 0.7 −0.17 0.4 1.4
40 86 57 53 17
4 10 7 7 2
0.019 0.034 0.027 0.026 0.005
0.78 2.25 1.35 1.45 0.46
Min. Max. Mean Median St. dev.
305 800 535 528 167
8.8 47 24 25 12
130 420 262 254 93
−7 2.6 −2.2 −2.5 3.7
172 434 273 273 98
7 17 11 10.5 3.4
0.025 0.068 0.043 0.038 0.017
0.63 1.69 1 1 0.34
STRNW (9 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
VUL (11 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
Min. Max. Mean Median St. dev.
60 166 112 101 41
2.4 9.7 4 3.5 2
32 103 59 57 24
−5 3 0.3 1.3 3
28 100 53 43 23
5.5 26 13.7 14 5.8
0.015 0.087 0.04 0.035 0.022
0.6 1.75 1.16 1.18 0.31
Min. Max. Mean Median St. dev.
122 280 175 152 51
4 11.8 7.3 7.5 2.6
68 180 100 92 33
−2 0.6 −0.29 0 0.9
49 129 75 61 27
6 32 15.7 15 6
0.021 0.069 0.042 0.038 0.012
0.54 1.96 1.39 1.44 0.36
STRN (7 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
LIP (11 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
Min. Max. Mean Median St. dev.
97 253 158 162 53
3 9.6 5 4.5 2.1
40 135 82 85 31.5
−2.3 0.4 −0.9 −1 1
49 118 76 69 23
5 18 8.6 7 4.5
0.02 0.06 0.03 0.03 0.01
0.67 1.35 1.06 1.1 0.22
Min. Max. Mean Median St. dev.
166 414 292 299 91
14 21 16.5 16 2.8
74 290 183.4 174 83
−1.8 4 1.4 2.3 2.3
64 154 109 111 33.8
12 32 24 31 10.5
0.034 0.093 0.062 0.056 0.025
0.8 3.67 1.85 1.3 1.17
coarse-grained sediment waves (e.g. Wynn et al., 2002; Ito, 2010). Their aspect ratio and symmetry index are also similar to the values reported in ancient coarse-grained sediment waves (Ito, 2010, Fig. 6c). In areas where sonar data are available the coarse-grained texture of the bedforms is evidenced by the backscatter zonation (Fig. 4). High values of reflectivity are limited on their crests and the lee sides, indicating the deposition of coarse-sediment as the result of increased flow velocities across the wave crests, similarly to what was reported for coarsegrained sediment wave along the flanks of the Canary Islands (Wynn et al., 2000). The coarse-grained texture of the bedforms is also supported by available seafloor sampling, that recovered sand and gravel down to 2600 m wd in those areas (Romagnoli et al., 2009b; Casalbore et al., 2010, Figs. 4a and 5c). Over the recent years, several examples of coarse-grained sediment waves and large erosional sediment waves have been re-interpreted as cyclic steps (e.g. Fildani et al., 2006; Lamb et al., 2008; Heino and Davies, 2009; Cartigny et al., 2011), i.e. a class of slow upslope migrating turbiditic sediment waves, where each downward step is bounded by a hydraulic jump (Kostic, 2011 and reference therein). In the next section, we test whether the cyclic step hypothesis could apply to the observed bedforms through the use of the stability diagrams proposed by Cartigny et al. (2011) and we try to discuss the variation of their wave parameters in terms of flow depth, concentration and discharge. 5.3. Inference on the origin of the bedforms as cyclic steps and reconstruction of possible paleoflow hydraulic conditions The main requirements for the generation of cyclic steps are the occurrence of a sedimentary gravity flow in supercritical condition, i.e. Froude number N 1 and possibility to form a hydraulic jump. Such conditions are fulfilled in our cases due to the steepness of the submarine volcanic flanks, with gradients N 20° in the upper slope and the presence of a marked slope break in the lower slope (below 4–8°), that should promote the development of a hydraulic jump as stated in the previous section. Available seafloor samples, acoustic facies and seismic data (Section 4) suggest that sediment-gravity flows are of high concentration, in accordance to the high sedimentation rates and slope gradients that characterize such areas. We also noted that the formation of bedforms is often associated to the concomitant development of furrows, troughs and
channel-levee morphologies, indicating a turbulent and erosive behavior of the sedimentary gravity flow, similarly to what was observed in the Laurentian Fan (Hughes Clarke et al., 1990; Piper et al., 1999) or coarsegrained fan-deltas in British Columbia (Prior and Bornhold, 1989, 1990). This erosive behavior might be due to the fact that flows with high concentration can develop, at or close to slope break, a two-phase suspension, with a lower super critical flowing traction carpet causing vigorous erosion and liquefaction of the substrate, overridden by a turbulent suspended sediment cloud (e.g. Sohn et al., 2002; Mohrig and Marr, 2003; Postma et al., 2009). This inference seems to be supported also by the recent study of the deposits related to the 2002 Stromboli submarine slide, as they derive from a sandy-matrix density flow, that segregated during the flow into a sand-rich and a clast-rich region (Marani et al., 2009). The possibility to develop two-phase suspension flows should, indeed, favor the development of a hydraulic jump and the formation of cyclic steps as shown by laboratory experiments and field evidence (e.g. Postma et al., 2009; Hughes Clarke et al., 2012, 2013. Even if no clear evidence of upstream migration of the observed bedforms is available (i.e. seismic profiles showing their internal structure or results from time-lapse bathymetric surveys, such as in the case of small-scale crescent-shaped bedforms in the Aeolian Archipelago, Casalbore et al., 2013), such migration seems to be suggested by the arcuate and downslope-concave shape of the bedform crests over the channel floor (i.e. curving away in downslope direction when approaching the canyon side). This shape might be interpreted as the result of the fastest upslope migration of the waves in the center of the channel where the flow velocity reaches the highest values (e.g. Wynn et al., 2000; Cartigny et al., 2011). This configuration is also similar to what is expected by an antidune model (e.g. Normark et al., 1980). However, we discard the interpretation of the bedforms as antidunes because these latter features typically show a more symmetrical crosssection with respect to the one's observed in the discussed cases (Section 4.3) and they are rarely preserved in nature, as they become unstable when the flow velocity decreases (i.e. during the waning phase of a turbidity current, Wynn and Stow, 2002; Cartigny et al., 2013). In contrast, different asymmetries commonly characterize cyclic steps (e.g. Cartigny et al., 2011). Supported by the above-mentioned evidence, we plotted the wave dimensions (height and length) of the Aeolian bedforms on the
D. Casalbore et al. / Marine Geology 355 (2014) 318–329
327
Fig. 7. Wave height and wavelength of the recognized bedforms plotted in the geometrical stability fields of cyclic steps realized by Cartigny et al (2011) on a log–log scale. In the graph “a” all the recognized bedforms shown in Fig. 6 are plotted, evidencing with squares the bedforms characterized by stoss and lee side slopes comparable to the values (18° and 1.7°, respectively) used in the numerical modeling of Cartigny et al. (2011). In the other graphs, only the latter bedforms are plotted, in order to infer the paleo-flow parameters that generated them. Acronyms and symbols as in Fig. 6.
geometrical stability diagrams of cyclic steps proposed by Cartigny et al. (2011)(Fig. 7). Even if some bedforms are characterized by different leeand stoss-side slope values with respect to the values used for the generation of the stability diagram (18° and 1.7°, respectively), the results clearly indicate that the observed bedforms encompass the stability field of cyclic step (Fig. 7a), strengthening the previous inferences that they can be interpreted as cyclic steps. Hence, we plotted only the bedforms characterized by similar values of stoss- and lee-side slope in the stability diagram in order to infer the related paleo-flow information (squares in Fig. 7b, c, d). The results allow us to estimate for all the cases in the diagram sedimentary gravity flows with relatively low Froude number of 1.5–2.5 (Fig. 7a), flow discharge of 30–300 m2/s
(Fig. 7b), flow velocity of 3–6 m/s (Fig. 7c) and flow depth of 5–15 m (Fig. 7d). According to the numerical modeling of Cartigny et al. (2011), the main parameters controlling the geometry (wavelength and aspect ratio) of cyclic steps are the slope gradients of stoss and lee side. Our data show that the relation between aspect ratio and lee side slope (Fig. 6e) is similar to what is expected by numerical modeling of Cartigny et al (2011). However, in the numerical modeling this change has been related to an increase of the wavelength in relation to a decrease of the lee side slope. In our graphs, we do not observe any relationship between the lee side slope and wavelength (Fig. 6f), whereas the wave height appears to be more sensitive to a change of the lee
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side slope (Fig. 6g). This evidence suggests us that a steeper lee side should increase flow acceleration, leading to a higher erosion of the depression at the base of the lee side, in turn increasing the height of the lee side and consequently decreasing the aspect ratio. Finally, we noted that wavelength and lateral extent of bedforms gradually decrease downslope as expected by several field and experimental evidence, except for some cases (Stromboli, Figs. 4c and d) where such parameters increase downslope. In such cases, the larger wavelength may be explained by an increase of sedimentary gravity flow discharge while flowing downslope, possibly in relation to seafloor erosion (Parker et al., 1986; Cartigny et al., 2011) and/or water entrainment (Komar, 1977). Alternatively, the increase of wavelength can be due to a decrease of sediment concentration in the flow at lower slope gradients, leading to the disappearance of a basal layer (e.g. Postma et al., 2009). Hence, at some point the wavelength of cyclic steps may drastically increase because of the concurrent increase of flow depth. The downslope increase in lateral extent of some bedforms may be related to the fact that the decrease of slope gradients commonly marks the passage from channelized features to a more regular and flat seafloor, where the flows are no longer constrained and can spread laterally. Similar mechanisms might be also addressed in explaining the larger wavelength and lateral extent of bedforms observed at the base of the northern flank of Panarea (Bf4, Fig. 2).
6. Conclusions High-resolution multibeam data allowed us to depict and characterize a large number of seafloor waveforms around the insular volcanoes of the central and eastern sectors of the Aeolian Archipelago. The association of waveforms with channel or fan-shaped features and their arcuate or sinuous crest-lines, which are always aligned roughly perpendicular to the maximum slope direction, allows us to correlate the genesis of waveforms with gravity-driven processes and discard the role of geostrophic or tidal currents. By relating their morphological characteristics and boundary conditions, a first distinction between bedform-like morphological features due to gravity instability processes and bedforms due to sedimentary gravity flows was proposed. Bedform-like morphological features are observed at Panarea and Lipari, having the highest and lowest aspect ratio among all the recognized waveforms, and being clearly related to upslope landslide scars and the occurrence of steep slopes (up to 20°). Differently, bedforms are found on the submarine flanks of all the volcanic edifices, at\or close to relevant sedimentary sources and always downslope of a marked decrease of slope gradients below 4–8°. According to their dimension, geometry and texture, they have been interpreted as coarse-grained sediment waves, such those found in the proximal part of modern and ancient turbiditic systems. More in detail, we propose a formation as cyclic steps based on the comparison of their geometry with numerical modeling and experimental evidence proposed in literature (e.g. Cartigny et al., 2011 and reference therein). Statistical analysis performed on the main morphometric parameters of the recognized bedforms shows a general scattered distribution of the data, even if a slightly positive relation can be observed between lee side slope against aspect ratio and wave height, whereas a slightly negative relation is present between stoss side slope and aspect ratio. In summary, this study demonstrates how large-scale bedforms are an ubiquitous feature found in modern marine volcaniclastic systems, poorly known until now due to the lack of extensive and detailed seafloor mapping. The possibility to compare a large number of bedforms present in the same volcanic area provided us useful insights on the variability in sediment dispersal and paleo-flow hydraulics by considering the effect of slope gradients, flow depth, flow concentration and discharge as governing parameters that control wavelength and height of the bedforms. The results can represent an example for comparing similar features elsewhere, both in modern marine setting than in ancient
volcaniclastic successions exposed on land, contributing to the knowledge of sedimentary processes responsible for their formation. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.margeo.2014.06.007. Acknowledgments This research was funded by the Italian MaGIC (Marine Geohazards along the Italian Coasts) Project and carried out in the framework of RITMARE (Ricerca ITaliana per il MARE) Project. Crews of R/V Urania, Universitatis and Thetis are gratefully acknowledged along with the fellow researchers and students that have took part to the many sea surveys. A special thank is due to Leonardo Macelloni for the final language check. We would also acknowledge George Postma and David J.W. Piper for their comments and suggestions that greatly improved the quality of the paper. References Bøe, R., Hovland, M., Instanes, A., Rise, L., Vasshus, S., 2000. 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Marine Geology journal homepage: www.elsevier.com/locate/margeo
Large-scale seafloor waveforms on the flanks of insular volcanoes (Aeolian Archipelago, Italy), with inferences about their origin D. Casalbore a, C. Romagnoli. b, A. Bosman a, F.L. Chiocci c a b c
IGAG-CNR, Istituto di Geologia Ambientale e Geoingegneria, Area della Ricerca di Roma 1, P.le Aldo Moro 5, 00185 Roma, Italy Università di Bologna, Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, P.za Porta S. Donato 1, 40126 Bologna, Italy Sapienza Università di Roma, Dipartimento Scienze della Terra, P.le Aldo Moro 5, 00185 Roma, Italy
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 9 June 2014 Accepted 19 June 2014 Available online 27 June 2014 Keywords: sedimentary gravity flows cyclic steps coarse-grained sediment waves multibeam bathymetry instability processes
a b s t r a c t High-resolution multibeam data in the central and eastern part of the Aeolian volcanic arc (Southern Tyrrhenian Sea) reveal widespread large-scale waveforms on the submarine flanks of these insular volcanoes. These features show high variability in wave parameters (wavelength of 60–1600 m and wave height of 4–200 m) and morphology (from sinuous to arcuate in plan-view, and from undulating to stepped, with very steep scarps, in cross-section), indicating that different processes, at variable spatial and temporal scale, are responsible for their genesis. By relating the morphological characters of the different waveforms and the local boundary conditions (morphological setting, regional slope gradients, sediment source and dynamics) where these features form, we were able to distinguish bedforms formed by sedimentary gravity flows from bedform-like features due to gravity instability processes. In detail, bedforms have been interpreted as coarse-grained sediment waves according to their wave parameters, geometry and texture. Bedforms are always found where the slope gradients markedly decrease to values b4°–8°, promoting a hydraulic jump in the sedimentary gravity flow. Hence, we tested if the recognized coarse-grained sediment waves can be generated as cyclic steps by comparing their morphometric characteristics with the results of experimental evidence and numerical modeling proposed in literature. The results show that cyclic step is a reasonable interpretation for these features, and we discuss the effect of flow depth, flow concentration and discharge as governing parameters affecting their wavelength, wave height and shape. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since Heezen et al. (1959) first documented repetitive marine bedforms by using echo sounding profiles, these features have been described in a variety of submarine environments, from deep-sea fans to continental shelves, both on glaciated and non-glaciated margins. Over the last decade, the advances in multibeam imagery and 3-D seismic techniques, that are providing an unprecedented high-resolution imaging of seafloor and sub-seafloor features (e.g. Paull et al., 2010; Lonergan et al., 2013) have produced an exponential increase in the accurate description of submarine waveforms. Despite the high number of case-studies and the growing interest for these features, their interpretation is still difficult to be univocally defined (e.g. Lee et al., 2002; Paull et al., 2010; Cartigny et al., 2011 and reference therein). In fact they can be related to different mechanisms, such as the interaction between seafloor and dense flows (tidal, geostrophic or sediment-gravity currents, Smith et al., 2007; Wynn and Stow, 2002), the seafloor deformations due to gravity instability processes (e.g. Shillington et al., 2012)
E-mail address: [email protected] (D. Casalbore).
http://dx.doi.org/10.1016/j.margeo.2014.06.007 0025-3227/© 2014 Elsevier B.V. All rights reserved.
and\or the combination of fluidized remobilization and slumping/ breaching of canyon fills (e.g. Paull et al., 2010). High-resolution multibeam data in the central and eastern part of the Aeolian volcanic arc (Southern Tyrrhenian Sea, Fig. 1) have revealed the widespread presence of large-scale seafloor waveforms on the submarine flanks of these insular volcanoes, with wavelength ranging from hundreds of meters up to some kilometers. The aim of the paper is to describe these waveforms and their morphometric parameters, and compare them with similar features recognized in other submarine settings in order to better constrain the boundary conditions controlling their genesis and development. Based on their morphology and location, we firstly present a distinction between bedforms (repetitive seafloor waveforms formed by fluidal flow) and bedform-like morphological features due to gravity instability processes. Then, we focus our attention on the formation of bedforms and their possible interpretation as cyclic step based on the results of recent numerical modeling and experimental evidence (e.g. Postma et al., 2009; Spinewine et al., 2009; Cartigny et al., 2011). Finally, we discuss what the bedforms and changes in their morphometric parameters may tell us about the sediment dispersal and the likely characteristics (e.g. flow depth, concentration and discharge) of the flows that generated them.
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Fig. 1. Shaded relief map of the five insular volcanoes making up the central and eastern sectors of the Aeolian Archipelago, with the location of the bedform-like morphological features, bedforms, main morphological features and following figures. The inset shows the regional setting of the area, where triangles represent submarine seamounts also belonging to the Aeolian Arc.
2. Geological setting The Aeolian Arc is formed of seven insular volcanoes and several seamounts located between the Southern Tyrrhenian Sea back-arc basin and the Calabro–Peloritano Arc (Fig. 1 inset). Islands and seamounts represent the younger manifestation of volcanism related to the NW-directed subduction of the Ionian oceanic lithosphere below the Calabrian Arc (e.g. Gvirtzman and Nur, 1999). The volcanic arc can be divided into western, central and eastern sectors, characterized by different tectonic evolutions (De Astis et al., 2003). Our study area encompasses the central and eastern sectors of the archipelago (Fig. 1). The central sector includes Salina, Lipari and Vulcano islands. The volcanism started at ~ 270 and 240 ka (at Lipari and Salina, respectively, Lucchi, 2013 and references herein) and it was historically documented at Lipari (1220 AD) and Vulcano (1888–90 AD). The eastern Aeolian
sector includes Panarea and Stromboli, where subaerial volcanism started at ~ 150–200 ka (Calanchi et al., 1999) and it is still active at Stromboli, where the typical Strombolian activity persists since, at least, 2000 years (Rosi et al., 2000). The submarine setting of the Aeolian Arc has been generally illustrated in Romagnoli et al. (2013) by means of multibeam bathymetry, showing widespread mass-wasting features at different scales all around the insular volcanoes. Large-scale collapses and related debris avalanche deposits were recognized only on the eastern and northwestern flanks of Stromboli (Bosman et al., 2009; Romagnoli et al., 2009a,b; Casalbore et al., 2011), whereas smaller-scale instability is present in the submarine flanks of all the islands (Romagnoli et al., 2009b). The submarine flanks of the Aeolian volcanoes are rather steep, with slope gradients ranging from 20–30° in the upper slope to few degrees at their base (Fig. 1 ESM).
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Table 1 Synthesis of the main morphological characters of the different bedform-like features (Bl) and bedform fields (Bf) found in the central and eastern sectors of the Aeolian Archipelago. Waveform Location field
Depth range (m)
Bl1
1200–1750 8–20
Bl2 Bf1
Panarea N flank Lipari W flank Stromboli SE flank
Gradient Wavelength Height range (m) (m) (°) 1000–1600
Lateral extent (m)
Aspect ratio
Plan-view shape
50–200 1300–2000 0.07–1.25 Arcuate
Cross-section shape
Notes
Stepped (lee side of 25–30°) Undulating (b12°)
Confined within a wide scar indenting the insular shelf Lying at the base of a landslide scar complex Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; coarse-sand and gravel at surface Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; increase in wave size downslope; coarsesand and gravel at surface Backscatter zonation on TOBI mosaic, strong echo on S.B.P.; increase in wave size downslope; coarsesand and gravel at surface Waveforms found at the base of the Panarea N flank, carved by a network of channels At surface of a semi-transparent lens on Sparker profiles; two graded levels of coarse-sands and millimetric scoriae are found in core EAC 8 Confined within a straight channelized feature
1100–1380 ≈4
400–700
8–12
1500–1700 2–5
60–400
5–20
70–800
0.02–0.08 Straight to arcuate
Undulating
800–1300 0.01–0.02 Sinuous
Bf2
Stromboli NW flank
2000–2600 3–8
70–500
5–8
100–800
0.01–0.07 Straight to arcuate
Undulating
Bf3
Stromboli N 2300–2600 2–4 flank
80–530
4–8
120–800
0.02–0.08 Straight to arcuate
Undulating
Bf4
Panarea N flank
1800–2500 3–8
300–1000
8–50
800–2000 0.02–0.07 Sinuous– arcuate
Undulating to stepped
Bf5
Vulcano SW flank
800–1200 2–6
100–300
4–20
150–700
0.02–0.07 Sinuous– arcuate
Undulating to stepped (up to 30°)
Bf6
Lipari W flank
870–1260 ≈5
160–420
14–25
300–700
0.03–0.09 Arcuate
Stepped (up to 55°)
3. Data and methods The multibeam data were collected during several oceanographic cruises carried out in the last ten years aboard small vessels in shallow-water (b100 m wd) and research vessels Thetis, Universitatis and Urania in deep water (N100 m wd). Multibeam data were acquired with systems working at different frequencies (from 50 to 455 kHz), depending on the water depth so that each bathymetric interval has been mapped at the maximum resolution. All data were RTK- or DGPSpositioned for shallow water and deep water, respectively, and processed with dedicated software, taking into account daily sound speed profiles and repeated calibration of transducers in the survey zone. Because the sounding density and the resolution decrease from the coastal sector towards the base of edifice, Digital Elevation Models (DEMs), with cell size varying from 1 m in shallow water (b 100 m wd) to 25 m (down to 2600 m wd) in deep water, were generated. Successively using the high-resolution multibeam bathymetry produced, main morphometric parameters of the waveforms were measured. Waveform parameters include: wavelength, wave height, lateral extent, length and slope gradients of stoss and lee side, aspect ratio (or wave steepness, defined as the ratio between wave height and wavelength), and symmetry index (defined as the ratio between the length of the stoss and lee side, Tanner, 1967). Moreover, the multibeam bathymetry was used as a reference base to (re)locate previously collected seismic profiles (3.5 kHz SBP and 1 kJ Sparker) and deep-towed longrange side scan sonar data (TOBI working at a frequency of 30 kHz) by matching of homologous features. In addition to ground truth the seabed morphology several seafloor dredge, core and grab samples were collected. Some of the results of this seafloor sampling have been already presented in Romagnoli et al. (2009a,b), Casalbore et al. (2010) and Romagnoli et al. (2012), whereas other ones are described for the first time in this paper.
4. Results In the Aeolian Archipelago, large bedform-like morphological features (Section 4.1) and bedforms (Section 4.2) were recognized, with
a wavelength from several tens of meters to over 1 km, wave height from some meters to tens/hundreds of meters and lateral extent from hundreds to thousands of meters (Table 1). Their crest-lines commonly display an arcuate or sinuous shape, trending roughly perpendicular to the maximum slope direction. In cross-section, their shape is variable from a stepped morphology, with steep scarps (20–55°) alternating with relatively flat surfaces, to a more undulating pattern with less steep sides (8–15°). 4.1. Bedform-like morphological features Bedform-like morphological features were identified at Panarea and Lipari (Bl, yellow areas in Fig. 1). 4.1.1. Northern flank of Panarea Here, large bedform-like morphological features (Bl1 in Figs. 1 and 2) are present in the upper part of the slope, where slope gradients range between 20° and 8°, and the flank has an overall slightly concave section. These features are found at or immediately downslope of 1.5 km-wide scars that indent the outer edge of the insular shelf. They have a wavelength of 1000–1600 m and lateral extent of 1300–2000 m (Table 1). In cross-section, they show a stepped shape, with stoss side nearly flat (up to backward sloping) and a steep lee side (25–30°), forming 50–200 m-high scarps; these steps tend to decrease in size downslope. Their aspect ratio is the highest among all the analyzed features, ranging between 0.07 and 1.25. 4.1.2. Western flank of Lipari Here, bedform-like morphological features (Bl2 in Figs. 1 and 3) have been recognized between 1100 and 1380 m wd, at the base of the landslide scars that affect the submarine flank of Banco del Bagno volcanic center. The waveforms show a wavelength of 400–700 m, wave height of 8–12 m, and lateral extent of 800–1300 m. Their aspect ratio is the lowest, ranging between 0.01 and 0.02 (Table 1). They are characterized by a prevalent undulating pattern with sinuous crestlines and lee side sloping up to 10° (5° on average), markedly
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Fig. 2. Shaded relief of the NW flank of Panarea volcano, where bedform-like morphological features (Bl) and bedform field (Bf) are recognized. Dotted lines indicate the bathymetric profiles shown in Fig. 6.
contrasting with the arcuate and stepped morphology observed for the nearby bedform field 6 (Bf6, see Section 4.2). 4.2. Bedforms Bedform fields were identified at Stromboli, Panarea, Vulcano and Lipari edifices (magenta areas in Fig. 1). 4.2.1. South-eastern, north-western and northern flanks of Stromboli Here, the bedforms commonly display a low morphological relief, but a marked backscatter variation on deep-towed side scan sonar data, with low-backscatter tones on the stoss side and highbackscatter tones on the crest-line and lee side (Fig. 4b, c, d). On the Stromboli SE flank, the bedforms are mainly confined within the thalweg of shallow erosive furrows between 1500 and 1700 m wd (Bf1 in Fig. 4b), whereas on the NW and N flanks they are found below 2000 m wd. Here, they are confined within the troughs developed on a large-scale volcaniclastic bulge created by repeated flank collapses of the NW flank (Sciara del Fuoco, SdF, fan in Fig. 4a) and in a wide valley at the foot of the saddle separating Stromboli and Strombolicchio volcanic centers (Bf2 in Fig. 4c and Bf3 in Fig. 4d, respectively). Bedforms show a wavelength of 60– 530 m, wave height of 4–10 m (apart from few cases, where they reach values up to 20 m), lateral extent of 70–800 m and aspect ratio of 0.01–0.08 (Table 1). They are straight to arcuate in planview (Fig. 4c and d), while they show a prevalent undulating pattern in cross-section, with a sub-horizontal stoss side and a lee side locally sloping to 5–25° (A-A′, B-B′, C-C′, D-D′, E-E′ in Fig. 6a). In all cases, bedforms appear below a marked decrease of slope gradients
(b 4–8°). Bedforms sometimes tend to increase their wave dimension downslope, where the gradients decrease to values of 2–3° and the seafloor morphology is more regular and flat. On deep towed sub-bottom profiles, bedforms are characterized by a very strong echo and lack of seismic penetration (Fig. 4e). Seafloor sampling performed on the bedform fields mainly recovered coarse-sand and gravel (black dots in Fig. 4a; see Romagnoli et al., 2009b; Casalbore et al., 2010 for further details). 4.2.2. Northern flank of Panarea Here, the bedforms (Bf4 in Figs. 1 and 2) develop over a wide area at the base of the volcanic flank between 1800 and 2500 m wd, where a marked decrease of slope gradients (from 8° to 3°) is present. The flank is carved by a network of channels having a width of 100– 200 m, length of 5–8 km, and depth of 20–40 m. The head of these channels indents the outer edge of a wide insular shelf developed all around Panarea island at depths of 130–150 m. Bedforms have a marked morphological relief, with a wavelength of 300–1000 m, wave height of 8–50 m, lateral extent of 800–2000 m and aspect ratio of 0.02–0.07 (Table 1). They show sinuous or arcuate shape on plan-view and have a variable cross section, from stepped to a more undulating pattern, with stoss side flat or sloping backward and lee side with gradients of 7–32° (F-F′ and G-G′ in Fig. 6). 4.2.3. South-western flank of Vulcano Here, bedforms (Bf5 in Figs. 1 and 5a) are located in the middle and lower part of the large Punta del Rosario fan, which is fed by two coastal scars that deeply indent the insular shelf edge in the southwest part of the island. The bedforms cover a total area of ~ 30 km2 between 800
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Fig. 3. Shaded relief map of the W flank of Lipari volcano, where bedform-like morphological features and bedforms are present. Dotted lines indicate the trace of bathymetric profiles shown in Fig. 6. BB: Banco del Bagno, IS: insular shelf.
Fig. 4. a) Shaded relief map of Stromboli volcano, with the location of sector collapse scar (red lines) seafloor sampling (black dots) and following figures; SdF fan: Sciara del Fuoco fan (see text for detail). In the other figures: long-range side scan sonar (TOBI) image of the south-eastern (b), north-western (c) and northern (d) flank of Stromboli, with the indication of the recognized bedform fields (Bf); light tones indicate high-backscatter values. Dotted lines indicate the bathymetric profiles shown in Fig. 6; e) deep-towed 3.5 kHz sub-bottom profile across the Bf3 (location in Fig. 4d), showing a strong seafloor echo and lack of signal penetration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and 1200 m wd, and they develop downslope of a marked decrease of slope gradients below 6°. The bedforms have a wavelength of 100– 300 m, wave height of 4–20 m, lateral extent of 150–700 m and aspect ratio of 0.02–0.07 (Table 1). In cross-section they show a variable morphology, from stepped with lee side sloping up to 32° to a more undulating pattern, with lee sides sloping to 12–20° (H-H′ and I-I′ in Fig. 6). Locally, bedforms develop as coaxial trains elongated along the maximum slope, defining a series of proto-channels on the fan surface. On Sparker profiles, the bedforms develop on the top of an acoustically semi-transparent seismic unit. A semi-continuous reflector marks up the base of the unit, several tens of milliseconds below it (Fig. 5b). EAC8 core from the deposits in the median part of the fan recovered two normal graded, 20–30 cm thick levels of coarse volcaniclastic sands and millimeter-sized scoriae in the lower part of the core; several 1–2 cm thick levels of fine sand are also present, interbedded in the plastic mud (Fig. 5c). 4.2.4. Western flank of Lipari Here, a coaxial train of bedforms is located within a straight and 500–700 m wide channelized feature between 870 and 1260 m wd (Bf6 in Figs. 1 and 3). This channel is fed by a network of narrow gullies, whose headwall indent the outer edge of the insular shelf cutting the summit of the eccentric submarine volcanic center of Banco del Bagno, located off western Lipari (BB in Fig. 3). The bedforms have a wavelength of 160–420 m, wave height of 14–25 m, lateral extent of 300–700 m, and aspect ratio of 0.03–0.09 (Table 1). These bedforms are characterized by arcuate, concave-downslope shape in plan-view and by a very steep lee side, with slope gradients up to 40° in crosssection (L-L′ in Fig. 6). 4.3. Morphometric analysis of the bedforms Of particular interest for this paper is the morphometric characterization of the bedforms, so we measured, on selected bathymetric profiles for each bedform field (Fig. 6a, excluding the bedform-like morphological features), indicative wave parameters for single bedforms. Descriptive statistics of such parameters are reported in Table 2. The comparison of median values between the different bedform fields confirms the previous general observations, with similar values of wavelength and wave height for the bedforms developed around Stromboli and Vulcano, whereas bedforms at Lipari and Panarea are characterized by three to five times higher values, respectively. Stoss side slope is quite similar for all the cases, with median values comprised between −2.5° (negative values indicate a counter-sloping surface) and 2.3°; lee side slope is comprised between 7° and 15°, except for the Lipari case, where median value is 31°. Similarly, the aspect ratio is comprised in a narrow range for the different bedform fields (0.026–0.038), except for the Lipari case where a value of 0.056 is present. Symmetry index (SI) median values are comprised between 1 and 1.45; more in detail, nearly symmetrical bedforms (SI = 0.6–1.4) are the dominant form (69%), in association with dune-like asymmetrical (SI N 1.4, 29%) and minor antidune-like asymmetrical (SI b 0.6, 2%) forms. Scatter-plots of all the measured data show, generally, a scattered distribution (Fig. 6). In most of the plots the measured parameters do not show any correlation, such as symmetry index against aspect ratio (Fig. 6c) or lee side slope against wavelength (Fig. 6f). Differently, a positive correlation can be observed between wave height and wavelength (Fig. 6b) as well as between the lee side slope against aspect ratio and wave height (Fig. 6e and g, respectively). A slightly negative correlation can be observed between the stoss side slope and the aspect ratio (Fig. 6d).
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on the submarine flanks of the insular volcanoes in the central and eastern part of the Aeolian Arc. These waveforms are not randomly distributed on the submarine flanks of the edifices, but they are mainly found within channelized features and/or on fan-shaped features, suggesting a genetic relationship with gravity-driven processes that formed them, i.e. mass-wasting processes (for bedform-like features) and sedimentgravity flows (for bedforms). Moreover, the waveform crest-lines are always aligned roughly perpendicular to the maximum slope direction. This evidence confirms the key-role played by gravity-driven processes in their formation and allows us to discard a genesis from geostrophic bottom currents, as bedforms generated by this kind of currents should have crests oblique to topographic contours and current direction (Embley and Langseth, 1977; Flood et al., 1993). Tidal currents can be discarded as well because of the microtidal regime that characterizes the study area and the bathymetric interval where the bedforms were found (N 800 m wd). The recognized waveforms show high variability (Table 1) in: a) wavelength (from tens of meters to N1 km), b) wave height (from some meters up to hundreds of meters), c) plan view geometry (from sinuous to arcuate), and d) cross-section (from undulating to stepped with very steep scarps, Fig. 6a). Therefore, it is possible to hypothesize that gravity-driven processes, occurring at variable spatial and temporal scale, are responsible for the variability of these features. Based on their morphological characters and boundary conditions, a first general distinction can be made between bedform-like morphological features (Section 5.1) and bedforms (Section 5.2). 5.1. Bedform-like morphological features Bedform-like morphological features have been recognized at Panarea and Lipari and characterized by the highest and lowest value of aspect ratio, respectively (Bl in Table 1, and Section 4.1). At Panarea the height of these features (50–200 m) is hard to attribute to sediment-gravity flows, also in consideration of the lack of relevant sediment sources able to generate them due to their proximity to the insular shelf edge (Fig. 2). Moreover, these features develop on high slope gradients (up to 20°), differently from what was observed for bedforms in other settings (see Section 5.2). Therefore, we interpret such features as the result of a slumping process, as suggested by their confinement within inward-facing lateral walls of an elongated scar (Fig. 2). Recently, a similar stepped morphology was identified in the upper part of volcanic edifices in the South Sandwich arc (Leat et al., 2010), where it has been interpreted as the result of slumping processes. At Lipari, bedform-like morphological features are found at the base of the landslide scar complex recognized on the western flank of Banco del Bagno volcanic center (Fig. 3). Seismic profiles show that the landslide scars affect the upper part of a relevant fine-grained sedimentary cover, as also supported by the recovering of hemipelagic mud from the cores collected at the scar complex (Casalbore et al., 2014). Moreover, these bedform-like morphological features are characterized by an undulating pattern and a variable plan-view morphology (sinuous) from concave down- to concave up-canyon, quite similar to the morphology created by compressive features (pressure ridges) in cohesive landslide deposits (e.g., Bøe et al., 2000; Lastras et al., 2006), and markedly contrasting with the morphology of the nearby bedform field Bf6 (Fig. 3, Sections 4.1 and 5.2). Based on the above-mentioned evidence, we interpret such waveforms as the result of compressive stress (i.e. pressure ridges) developed within the cohesive landslide deposits originating from the upslope nested scars.
5. Discussion
5.2. Bedforms
High-resolution morpho-bathymetric data show that large-scale bedforms and bedform-like morphological features are quite common
In all the other cases, we interpret the seafloor waveforms as bedforms generated by sedimentary gravity flows because:
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Fig. 5. a) Shaded relief map of the SW flank of Vulcano volcano, with the indication of the main morphological features (Bf: bedform field; IS: insular shelf; PR Punta del Rosario; LFC La Fossa Caldera; IPC Il Piano Caldera) and EAC8 sampling site; dotted lines indicate the bathymetric profiles shown in Fig. 6. b) Sparker 4.5 kJ profile (location in a) shows a seismic semitransparent unit overlaid by large diffraction hyperbolae, whose base is indicated by a black dotted line. A multibeam-derived bathymetric profile is reported above the profile (red line with an ~30 m offset) to better depict the actual morphology of the bedforms. c) Photograph (right) and stratigraphic sketch (left) of EAC8 core (location in a), where main graded coarsesandy layers are shown as dark gray, thinner fine-sandy layer as light gray and hemipelagic mud as white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a) they are found at\or close to relevant sediment sources, i.e. where a morphological continuity between the subaerial and submarine slope is present (wide depressions left by sector collapses at Stromboli in Figs. 1 and 4) or where the insular shelf is largely indented by masswasting processes or channelized features (Panarea, Vulcano and Lipari in Figs. 1, 2, 3 and 5); b) they start to develop under a marked decrease in the slope gradients (below 4–8°), similarly to what was observed for bedforms developed in other settings (e.g. Piper et al., 1985; Wynn et al., 2000; Sisavath et al., 2011). In fact, a reduction of slope gradients would force the flow
to pass the hydraulic jump at some time, during which the velocity of the flow would be reduced significantly and its thickness markedly increased, favoring the development of bedforms (e.g. Postma et al., 2009; Spinewine et al., 2009); c) in terms of wave parameters, geometry and texture, the waveforms show similarity to coarse-grained sediment waves found in proximal regions of modern and ancient turbiditic systems (e.g. Wynn et al., 2002; Hoffmann et al., 2008; Ito, 2010; Sisavath et al., 2011). As shown in the plot of Fig. 6b, the wave dimensions of the Aeolian bedforms fit well with the values measured for modern and ancient
D. Casalbore et al. / Marine Geology 355 (2014) 318–329 Fig. 6. Bathymetric profiles (a) across the different bedforms fields (vertical exaggeration 10×; location in previous figures). Scatter-plots (b–g) of the main morphometric parameters analyzed for the different bedform fields (see text for details); note that all the graphs are on log–log scale, except for graphs c and d (where negative values of stoss-side slope indicate counter-sloping surface) plotted on a linear scale. STRNW/N/S: Stromboli Nord-West/Nord/Sud; VUL: Vulcano; PAN: Panarea; LIP: Lipari; Ls/Ll: lenght of stoss and lee side. In graphs b and c, wave dimensions of modern (from Malinverno et al., 1988; Normark and Piper, 1991; Wynn et al., 2002; Smith et al., 2007) and ancient (1 from Piper and Kontopoulos, 1994; Vicente Bravo and Robles, 1995; Wynn et al., 2002, 2 from Ito, 2010) examples of coarse-grained sediment waves are also reported.
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Table 2 Descriptive statistics for the bedforms selected in the study areas (Fig. 6): Stromboli South-east (STRS), Stromboli North-west (STRNW), Stromboli North (N), Panarea (PAN), Vulcano (VUL), Lipari (LIP). Min.: minimum; Max.: maximum; St. dev.: standard deviation. L: wavelength, H wave height; Ls: length of stoss-side; SGs: slope gradients of stoss-side (negative values indicate a counter-sloping surface); Ll: length of lee-side; SGl slope gradients of lee-side; aspect ratio (H/L); S.I.: symmetry index (Ls/Ll). STRS (9 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect ratio
S.I.
PAN (10 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect ratio
S.I.
Min. Max. Mean Median St. dev.
82 211 131 131 38
1.8 5.6 3.6 3.4 1.3
40 125 74 80 26
−3 0.7 −0.17 0.4 1.4
40 86 57 53 17
4 10 7 7 2
0.019 0.034 0.027 0.026 0.005
0.78 2.25 1.35 1.45 0.46
Min. Max. Mean Median St. dev.
305 800 535 528 167
8.8 47 24 25 12
130 420 262 254 93
−7 2.6 −2.2 −2.5 3.7
172 434 273 273 98
7 17 11 10.5 3.4
0.025 0.068 0.043 0.038 0.017
0.63 1.69 1 1 0.34
STRNW (9 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
VUL (11 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
Min. Max. Mean Median St. dev.
60 166 112 101 41
2.4 9.7 4 3.5 2
32 103 59 57 24
−5 3 0.3 1.3 3
28 100 53 43 23
5.5 26 13.7 14 5.8
0.015 0.087 0.04 0.035 0.022
0.6 1.75 1.16 1.18 0.31
Min. Max. Mean Median St. dev.
122 280 175 152 51
4 11.8 7.3 7.5 2.6
68 180 100 92 33
−2 0.6 −0.29 0 0.9
49 129 75 61 27
6 32 15.7 15 6
0.021 0.069 0.042 0.038 0.012
0.54 1.96 1.39 1.44 0.36
STRN (7 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
LIP (11 cases)
L (m)
H (m)
Ls (m)
SGs (°)
Ll (m)
SGl (°)
Aspect Ratio
S.I.
Min. Max. Mean Median St. dev.
97 253 158 162 53
3 9.6 5 4.5 2.1
40 135 82 85 31.5
−2.3 0.4 −0.9 −1 1
49 118 76 69 23
5 18 8.6 7 4.5
0.02 0.06 0.03 0.03 0.01
0.67 1.35 1.06 1.1 0.22
Min. Max. Mean Median St. dev.
166 414 292 299 91
14 21 16.5 16 2.8
74 290 183.4 174 83
−1.8 4 1.4 2.3 2.3
64 154 109 111 33.8
12 32 24 31 10.5
0.034 0.093 0.062 0.056 0.025
0.8 3.67 1.85 1.3 1.17
coarse-grained sediment waves (e.g. Wynn et al., 2002; Ito, 2010). Their aspect ratio and symmetry index are also similar to the values reported in ancient coarse-grained sediment waves (Ito, 2010, Fig. 6c). In areas where sonar data are available the coarse-grained texture of the bedforms is evidenced by the backscatter zonation (Fig. 4). High values of reflectivity are limited on their crests and the lee sides, indicating the deposition of coarse-sediment as the result of increased flow velocities across the wave crests, similarly to what was reported for coarsegrained sediment wave along the flanks of the Canary Islands (Wynn et al., 2000). The coarse-grained texture of the bedforms is also supported by available seafloor sampling, that recovered sand and gravel down to 2600 m wd in those areas (Romagnoli et al., 2009b; Casalbore et al., 2010, Figs. 4a and 5c). Over the recent years, several examples of coarse-grained sediment waves and large erosional sediment waves have been re-interpreted as cyclic steps (e.g. Fildani et al., 2006; Lamb et al., 2008; Heino and Davies, 2009; Cartigny et al., 2011), i.e. a class of slow upslope migrating turbiditic sediment waves, where each downward step is bounded by a hydraulic jump (Kostic, 2011 and reference therein). In the next section, we test whether the cyclic step hypothesis could apply to the observed bedforms through the use of the stability diagrams proposed by Cartigny et al. (2011) and we try to discuss the variation of their wave parameters in terms of flow depth, concentration and discharge. 5.3. Inference on the origin of the bedforms as cyclic steps and reconstruction of possible paleoflow hydraulic conditions The main requirements for the generation of cyclic steps are the occurrence of a sedimentary gravity flow in supercritical condition, i.e. Froude number N 1 and possibility to form a hydraulic jump. Such conditions are fulfilled in our cases due to the steepness of the submarine volcanic flanks, with gradients N 20° in the upper slope and the presence of a marked slope break in the lower slope (below 4–8°), that should promote the development of a hydraulic jump as stated in the previous section. Available seafloor samples, acoustic facies and seismic data (Section 4) suggest that sediment-gravity flows are of high concentration, in accordance to the high sedimentation rates and slope gradients that characterize such areas. We also noted that the formation of bedforms is often associated to the concomitant development of furrows, troughs and
channel-levee morphologies, indicating a turbulent and erosive behavior of the sedimentary gravity flow, similarly to what was observed in the Laurentian Fan (Hughes Clarke et al., 1990; Piper et al., 1999) or coarsegrained fan-deltas in British Columbia (Prior and Bornhold, 1989, 1990). This erosive behavior might be due to the fact that flows with high concentration can develop, at or close to slope break, a two-phase suspension, with a lower super critical flowing traction carpet causing vigorous erosion and liquefaction of the substrate, overridden by a turbulent suspended sediment cloud (e.g. Sohn et al., 2002; Mohrig and Marr, 2003; Postma et al., 2009). This inference seems to be supported also by the recent study of the deposits related to the 2002 Stromboli submarine slide, as they derive from a sandy-matrix density flow, that segregated during the flow into a sand-rich and a clast-rich region (Marani et al., 2009). The possibility to develop two-phase suspension flows should, indeed, favor the development of a hydraulic jump and the formation of cyclic steps as shown by laboratory experiments and field evidence (e.g. Postma et al., 2009; Hughes Clarke et al., 2012, 2013. Even if no clear evidence of upstream migration of the observed bedforms is available (i.e. seismic profiles showing their internal structure or results from time-lapse bathymetric surveys, such as in the case of small-scale crescent-shaped bedforms in the Aeolian Archipelago, Casalbore et al., 2013), such migration seems to be suggested by the arcuate and downslope-concave shape of the bedform crests over the channel floor (i.e. curving away in downslope direction when approaching the canyon side). This shape might be interpreted as the result of the fastest upslope migration of the waves in the center of the channel where the flow velocity reaches the highest values (e.g. Wynn et al., 2000; Cartigny et al., 2011). This configuration is also similar to what is expected by an antidune model (e.g. Normark et al., 1980). However, we discard the interpretation of the bedforms as antidunes because these latter features typically show a more symmetrical crosssection with respect to the one's observed in the discussed cases (Section 4.3) and they are rarely preserved in nature, as they become unstable when the flow velocity decreases (i.e. during the waning phase of a turbidity current, Wynn and Stow, 2002; Cartigny et al., 2013). In contrast, different asymmetries commonly characterize cyclic steps (e.g. Cartigny et al., 2011). Supported by the above-mentioned evidence, we plotted the wave dimensions (height and length) of the Aeolian bedforms on the
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Fig. 7. Wave height and wavelength of the recognized bedforms plotted in the geometrical stability fields of cyclic steps realized by Cartigny et al (2011) on a log–log scale. In the graph “a” all the recognized bedforms shown in Fig. 6 are plotted, evidencing with squares the bedforms characterized by stoss and lee side slopes comparable to the values (18° and 1.7°, respectively) used in the numerical modeling of Cartigny et al. (2011). In the other graphs, only the latter bedforms are plotted, in order to infer the paleo-flow parameters that generated them. Acronyms and symbols as in Fig. 6.
geometrical stability diagrams of cyclic steps proposed by Cartigny et al. (2011)(Fig. 7). Even if some bedforms are characterized by different leeand stoss-side slope values with respect to the values used for the generation of the stability diagram (18° and 1.7°, respectively), the results clearly indicate that the observed bedforms encompass the stability field of cyclic step (Fig. 7a), strengthening the previous inferences that they can be interpreted as cyclic steps. Hence, we plotted only the bedforms characterized by similar values of stoss- and lee-side slope in the stability diagram in order to infer the related paleo-flow information (squares in Fig. 7b, c, d). The results allow us to estimate for all the cases in the diagram sedimentary gravity flows with relatively low Froude number of 1.5–2.5 (Fig. 7a), flow discharge of 30–300 m2/s
(Fig. 7b), flow velocity of 3–6 m/s (Fig. 7c) and flow depth of 5–15 m (Fig. 7d). According to the numerical modeling of Cartigny et al. (2011), the main parameters controlling the geometry (wavelength and aspect ratio) of cyclic steps are the slope gradients of stoss and lee side. Our data show that the relation between aspect ratio and lee side slope (Fig. 6e) is similar to what is expected by numerical modeling of Cartigny et al (2011). However, in the numerical modeling this change has been related to an increase of the wavelength in relation to a decrease of the lee side slope. In our graphs, we do not observe any relationship between the lee side slope and wavelength (Fig. 6f), whereas the wave height appears to be more sensitive to a change of the lee
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side slope (Fig. 6g). This evidence suggests us that a steeper lee side should increase flow acceleration, leading to a higher erosion of the depression at the base of the lee side, in turn increasing the height of the lee side and consequently decreasing the aspect ratio. Finally, we noted that wavelength and lateral extent of bedforms gradually decrease downslope as expected by several field and experimental evidence, except for some cases (Stromboli, Figs. 4c and d) where such parameters increase downslope. In such cases, the larger wavelength may be explained by an increase of sedimentary gravity flow discharge while flowing downslope, possibly in relation to seafloor erosion (Parker et al., 1986; Cartigny et al., 2011) and/or water entrainment (Komar, 1977). Alternatively, the increase of wavelength can be due to a decrease of sediment concentration in the flow at lower slope gradients, leading to the disappearance of a basal layer (e.g. Postma et al., 2009). Hence, at some point the wavelength of cyclic steps may drastically increase because of the concurrent increase of flow depth. The downslope increase in lateral extent of some bedforms may be related to the fact that the decrease of slope gradients commonly marks the passage from channelized features to a more regular and flat seafloor, where the flows are no longer constrained and can spread laterally. Similar mechanisms might be also addressed in explaining the larger wavelength and lateral extent of bedforms observed at the base of the northern flank of Panarea (Bf4, Fig. 2).
6. Conclusions High-resolution multibeam data allowed us to depict and characterize a large number of seafloor waveforms around the insular volcanoes of the central and eastern sectors of the Aeolian Archipelago. The association of waveforms with channel or fan-shaped features and their arcuate or sinuous crest-lines, which are always aligned roughly perpendicular to the maximum slope direction, allows us to correlate the genesis of waveforms with gravity-driven processes and discard the role of geostrophic or tidal currents. By relating their morphological characteristics and boundary conditions, a first distinction between bedform-like morphological features due to gravity instability processes and bedforms due to sedimentary gravity flows was proposed. Bedform-like morphological features are observed at Panarea and Lipari, having the highest and lowest aspect ratio among all the recognized waveforms, and being clearly related to upslope landslide scars and the occurrence of steep slopes (up to 20°). Differently, bedforms are found on the submarine flanks of all the volcanic edifices, at\or close to relevant sedimentary sources and always downslope of a marked decrease of slope gradients below 4–8°. According to their dimension, geometry and texture, they have been interpreted as coarse-grained sediment waves, such those found in the proximal part of modern and ancient turbiditic systems. More in detail, we propose a formation as cyclic steps based on the comparison of their geometry with numerical modeling and experimental evidence proposed in literature (e.g. Cartigny et al., 2011 and reference therein). Statistical analysis performed on the main morphometric parameters of the recognized bedforms shows a general scattered distribution of the data, even if a slightly positive relation can be observed between lee side slope against aspect ratio and wave height, whereas a slightly negative relation is present between stoss side slope and aspect ratio. In summary, this study demonstrates how large-scale bedforms are an ubiquitous feature found in modern marine volcaniclastic systems, poorly known until now due to the lack of extensive and detailed seafloor mapping. The possibility to compare a large number of bedforms present in the same volcanic area provided us useful insights on the variability in sediment dispersal and paleo-flow hydraulics by considering the effect of slope gradients, flow depth, flow concentration and discharge as governing parameters that control wavelength and height of the bedforms. The results can represent an example for comparing similar features elsewhere, both in modern marine setting than in ancient
volcaniclastic successions exposed on land, contributing to the knowledge of sedimentary processes responsible for their formation. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.margeo.2014.06.007. Acknowledgments This research was funded by the Italian MaGIC (Marine Geohazards along the Italian Coasts) Project and carried out in the framework of RITMARE (Ricerca ITaliana per il MARE) Project. Crews of R/V Urania, Universitatis and Thetis are gratefully acknowledged along with the fellow researchers and students that have took part to the many sea surveys. A special thank is due to Leonardo Macelloni for the final language check. We would also acknowledge George Postma and David J.W. Piper for their comments and suggestions that greatly improved the quality of the paper. References Bøe, R., Hovland, M., Instanes, A., Rise, L., Vasshus, S., 2000. 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