Depositional and erosional bedforms in Late Pleistocene-Holocene pro-delta deposits of the Gulf of Patti (southern Tyrrhenian margin, Italy)

Marine Geology 385 (2017) 216–227

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Depositional and erosional bedforms in Late Pleistocene-Holocene pro-delta deposits of the Gulf of Patti (southern Tyrrhenian margin, Italy) Daniele Casalbore a,b,⁎, Domenico Ridente b, Alessandro Bosman b, Francesco L. Chiocci a,b a b

Sapienza Università di Roma, Dipartimento Scienze della Terra, Piazzale Aldo Moro 5, Roma, Italy Istituto di Geologia Ambientale e Geoingegneria (Consiglio Nazionale delle Ricerche), Area della Ricerca di Roma 1, Montelibretti, Via Salaria Km 29,300, Monterotondo, Roma, Italy

a r t i c l e

i n f o

Article history: Received 1 August 2016 Received in revised form 13 January 2017 Accepted 22 January 2017 Available online 24 January 2017 Keywords: Gullies Waveforms Flash-floods Multibeam bathymetry Seismic-stratigraphy

a b s t r a c t Multibeam bathymetry, high-resolution seismic profiles and seafloor samples have been analyzed to characterize depositional and erosional dynamics recorded pro-deltaic deposits and outer shelf sediments along of a sector of the NE Sicilian margin (southern Tyrrhenian Sea). The deltaic deposits cover an area of ca. 15 km2 in front of the Mazzarrà River, and are morphologically characterized by waveforms trending overall along strike and incised cross-strike gullies of variable length. The gullies are shallow and characterized by small, coaxial erosive scours in the inner-middle shelf, whereas they become larger and deeper in the outer shelf-upper slope, in relation to the marked increase of slope gradients at the shelf break. Here, the wider gullies are characterized by a frame of crescent-shaped bedforms interpreted as cyclic steps, indicating the occurrence of sedimentary gravity flows in supercritical regime. Prodelta waveforms are widespread between −50 and −120 m, with wave lengths of 34–110 m and wave heights of 0.5–3 m. Morphometric characterization and spatial distribution of the waveforms suggests a main role in their genesis played by hyperpycnal flows, although we cannot discard the possible effect of internal waves or slow deformation processes (i.e., creep). Depositional and erosional features similar to those observed on the seafloor are evident in the subsurface stratigraphy, revealing the onset and growth of the Mazzarrà Delta since the Last Glacial Maximum. The post-glacial sea level rise caused lateral shifts of the Mazzarrà River mouth controlling migration of depositional lobes and intensity of seafloor incision and sediment reworking, ultimately resulting in the observed wavy bedforms extending from the inner shelf to the upper slope. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Geomorphic shaping of the seafloor at different spatial and temporal scales and in different marine environments generally reflects a strong interaction between oceanographic and sedimentary processes under long-term tectonic deformation. As a result, erosional features scaling from gullies to deeply incised channels (e.g., Burger et al., 2001; Ridente et al., 2007; Chiocci and Casalbore, 2011; Harris and Whiteway, 2011) and bedform fields including megaripples to large sediment waves (e.g. Lo Iacono et al., 2014; Lobo et al., 2015; Symons et al., 2016) have been frequently observed on modern continental margins. The continental shelf and upper slope of the NE Sicilian margin, in the southern Tyrrhenian Sea (Fig. 1), provide an insightful example of multi-scale geomorphic features reflecting the interaction between recent and long-term sedimentary processes (Gamberi et al., 2014,

⁎ Corresponding author at: Sapienza Università di Roma, Dipartimento Scienze della Terra, Piazzale Aldo Moro 5, Roma, Italy. E-mail address: [email protected] (D. Casalbore).

http://dx.doi.org/10.1016/j.margeo.2017.01.007 0025-3227/© 2017 Elsevier B.V. All rights reserved.

2015). This part of the Tyrrhenian margin is affected by differential regional uplift rates and deformation along regional tectonic lineaments (Ferranti et al., 2006; Sulli et al., 2013) causing massive input of sediment and frequent mass-wasting on the continental margin. Largely due to this active tectonic setting, the NE Sicilian continental shelf is narrow or absent and its edge typically cut back by canyon heads (Gamberi et al., 2015). Within this margin, we analyzed an area in the Gulf of Patti (Figs. 1 and 2) where the shelf is relatively wider and less carved by canyons compared to the nearby sectors. Here, the large input of terrestrial sediment funneled to a short and steep river stream led to the growth of a submarine delta characterized by different bedforms indicative of high-energy erosional and depositional events. Cross-strike shallow and sub-parallel incisions are superimposed on gentle depositional waveforms present in front of a mountainous river locally known as “Fiumara” (Sabato and Tropeano, 2004). Fiumara is a kind of gravelbed rivers, very steep and short, having a great transport and erosion capacity because of their flow regime, dominated by long periods of inactivity alternated with short intervals of intense water supply and flash floods. During these events, large amount of sediment and debris are released at the river mouth and possibly evolves into hyperpycnal flows (e.g., Casalbore et al., 2011).

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Fig. 1. a) Shaded relief map of the onland sector (azimuthal enlightening, data downloaded from Aster GDEM, https://lpdaac.usgs.gov/) and isobaths of the NE Sicily offshore (location in the Fig. 1b), where the fluvial drainage is indicated by the blue lines; the bold magenta line indicates the Mazzarrà Fiumara. The light-blue line across the continental shelf indicates the trace of the longitudinal profile (A–B) of Mazzarrà Fiumara and associated submarine delta shown on the lower right; b) Simplified structural map of Southern Italy; KCC: Kabilo-Calabride Chain Units; AMC: Appennine-Maghrebian Chain Units; ETF: External Thrust System Units; PBF: Pelagian Block Foreland Units; QV: Quaternary Volcanoes. (Source: modified from Lentini et al., 2006).

Fig. 2. Shaded relief map (a) and backscatter mosaic (b, high backscatter values are in lighter tones) of the submarine deltaic system facing the Mazzarrà Fiumara mouth. Note the difference of the Fiumara mouth geometry between the 1884 (a) and 2009 (b) geological map (data from http://193.206.192.231/carta_geologica_italia/tavoletta.php?foglio=253; http://www.isprambiente.gov.it/Media/carg/587_600_MILAZZO_BARCELLONA/Foglio.html, respectively). W, C, and E indicate the western, central and eastern part, in which the Mazzarrà submarine delta can be recognized based on the different geomorphology; yellow lines indicate the location of the bathymetric profiles shown in Figs. 2 (P0–P4, vertical exaggeration 30×) and 5 (P5–P10); red dots in Fig. 2b indicate the location of seafloor samples.

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Shelf stratigraphic units also reveal an internal alternation of regular and chaotic or wavy reflectors, faintly displaced by high-angle crestparallel planes, corresponding to the depositional waveforms. Similar morpho-stratigraphic features have been extensively described from Holocene pro-deltaic deposits on many continental margins (e.g. Urgeles et al., 2011 and reference therein). Their origin has been variably interpreted, and attributed to: a) sediment deformation related to creep and/or early incipient instability (e.g. Díaz and Ercilla, 1993; Chiocci et al., 1996; Sacchi et al., 2005; Correggiari et al., 2001; Lee et al., 2002; Lykousis et al., 2003); b) sediment waves induced by bottom currents and/or hyperpycnal flows (e.g., Prior and Bornhold 1990; Trincardi and Normark, 1988; Lee et al., 2002; Urgeles et al., 2007; Lobo et al., 2015); c) a combination of both deformation and sedimentary bedforms, the former creating seafloor irregularities that enhance the latter (e.g. Cattaneo et al., 2004; Marsset et al., 2004; Berndt et al., 2006; Sultan et al., 2008), and more recently to d) internal waves (e.g. Droghei et al., 2016; Ribó et al., 2016). In this paper, we investigate on the origin and mutual relationship between the observed erosional (gullies) and depositional (waveforms) features, on the background of the Late Pleistocene-Holocene sea level rise and highstand that caused widening of the shelf, river mouth shifts and consequent readjustment of sediment sourcing to the continental shelf and upper slope. By this, we aim at improving the understanding of active processes affecting sediment pathways and source-to-sink transport under a dynamic environment impacted by short and steep river courses. The results can have also important implications for geohazard management of coastal areas, including the preservation of infrastructures there present (e.g. harbors, platforms, cables, pipelines). Recent studies have, in fact, demonstrated the power of submarine flows in breaking submarine cables and removing sediments around seafloor infrastructures from coastal environment to deep water (e.g., Carter et al., 2012; Pope et al., 2016). 2. Geological setting and study area The study area is located in the Gulf of Patti, extending between Cape Milazzo to the E, and Cape Calavà to the W, along the NE Sicilian continental margin (Fig. 1). This part of the Tyrrhenian Sea lays on a complex geodynamic boundary (e.g. Cuffaro et al., 2011), between the rifted Sicilian continental margin (to the west) and the “Calabria-Peloritani Arc”, formed by the Calabria Peninsula and the Peloritani Mountains (NE tip of Sicily; Fig. 1b). It faces the Aeolian Volcanic Arc, which includes several seamounts and insular volcanoes (Romagnoli et al., 2013). The Calabria-Peloritani Arc is an allochthonous terrain of the Alpine domain (Kabilo-Calabride nappes), overriding the Apennine units to the SE, along the accretionary prism of the narrow, NW-subducting Ionian plate (e.g. Polonia et al., 2011; Catalano et al., 2003). The coastal mountains are rather close to the Tyrrhenian margin and are made up of deformed metamorphic-crystalline basement rocks, discontinuously covered by Mesozoic limestones and Cenozoic flysch deposits (Lentini et al., 2006). The Calabria-Peloritani block is affected by regional uplift at rates in the order of ca. 1 mm/yr since the Pleistocene (e.g. Antonioli et al., 2006), with higher local uplift associated with co-seismic vertical movements (Catalano et al., 2003; Scicchitano et al., 2011); seismicity in this area is documented both by instrumental and historical records (CPTI Working Group, 2004; Chiarabba et al., 2005). Because of the rapid uplift, several “Fiumara” streams with short and steep course deeply incise the coastal highlands (blue lines in Fig. 1a). One of the main water course in the Gulf of Patti is the Mazzarrà Fiumara (bold magenta line in Fig. 1a), having a length of N 25 km, a drainage area of ca. 120 km2 (maximum elevation around 1200 m) and average slope gradients of ca. 5°. The drainage basin is mainly characterized by strongly fractured basement rocks and flyschoid deposits favoring widespread shallow instability processes. Maximum river discharge for the Mazzarrà Fiumara has been estimated between 500 and

777 m3/s considering a recurrence time for intense flash-flood of 50 and 300 years, respectively (http://www.sitr.regione.sicilia.it/pai/ bacini.htm). The longitudinal profile of the Mazzarrà Fiumara shows a concave-upward shape (A–B in Fig. 1), with slope gradients decreasing from 25° in the upper reach to 0.3° in the ca. 2 km wide coastal plain. Beyond the coastline, the cross-section of the submarine delta linked to the Mazzarrà Fiumara shows a steeper (ca. 2°) and almost linear shape (A–B in Fig. 1) down to the shelf edge located at ca. −150 m. The facing offshore is a relatively low-energy shelf area, dominated by storm waves with a maximum significant height of 3–4 m (Istituto Idrografico della Marina, 1982) and a maximum tide of 0.6 m. 3. Data and methods Geophysical and sedimentological data have been collected during four oceanographic cruises aboard R/V Urania and Minerva1 (CNR), between 2010 and 2015. Morpho-bathymetric data were acquired at depths between − 20 and − 1000 m using a Kongsberg EM710 multibeam system, operating at a frequency of 70/100 kHz. Data were DGPS-positioned and processed with dedicated software, taking into account daily sound speed profiles and patch test of transducers in the survey zone. As sounding density and resolution decrease with depth, Digital Elevation Models (DEMs) were generated with cell-size varying from 1 m in shallow water (≤−100 m) to 20 m in deeper water (down to −1000 m). Multibeam backscatter data were also processed using Caris Hips and Sips Geocoder (Time series), applying corrections for a) beam pattern to remove acoustic artifacts from the imagery caused by imperfections in the sonar, b) angle varying gain to remove the angular response of sediment from the imagery, normalizing the mean angular intensities, ping-by-ping, with a moving average filter, and c) despeckle to homogenize isolated light and/or dark pixels. A multibeam backscatter mosaic was realized with a pixel sizes from 0.2 to 1 m, allowing qualitative inference of sediment type distribution relative to mesoscale erosive-depositional features. Subsurface stratigraphy was inspected with a ca. 100 km grid of single-channel, high resolution seismic reflection profiles, with average spacing of ca. 300 m. Seismic profiles were acquired with a hullmounted Teledyne BENTHOS III CHIRP system, working with a frequency modulation of 2–20 kHz and yielding a 0.5-m-vertical resolution. Constant sound velocities in water and sediment (1500 m/s) were used for time-depth conversions. Finally, 6 Van Veen grab samples (red point in Fig. 2b) provided mini-cores (ca. 30 cm) of seafloor sediments that were in turn subsampled for sedimentological analysis. Grain-size analyses were performed through sieving and laser particle sizer. 4. Results 4.1. Morpho-bathymetry of the Gulf of Patti The study area is characterized by a narrow continental shelf (maximum width of 6 km), having dip gradients of 1°–2° and outer edge at depths of −130 to −150 m (Figs. 1 and 2). The shelf further narrows towards Capo Milazzo and Capo Calavà promontories (Fig. 1a), E and W of the study area, respectively. Here, several canyon heads carve the shelf forcing the shelf edge to retreat in very shallow-water (b−30 m), only few hundreds of meters from the coast (Fig. 1). On the relatively wider shelf in the Gulf of Patti, a 5 km wide deltaic system develops at the mouth of Mazzarrà Fiumara (Fig. 2). In cross-section, the submarine deltaic system has an overall planar to slightly concave-up shape (profile A-B in Fig. 1), whereas in plan-view, this system has an overall seaward-convex shape and extends over an area of ca. 15 km2. The submarine delta can be morphologically divided in three overlapping areas defined as Western, Central and Eastern (W, C and E in Fig. 2a); these show convexity that gradually decreases both seaward (P0–P4 in

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Fig. 2) and westward, where the seafloor becomes smoother and flat (P2–P3 in Fig. 2). The submarine delta as a whole is characterized by a rugged surface as the result of wavy bedforms cut by narrow (b70–80 m wide) and shallow (few meters deep) gullies (Figs. 2 and 3). Landward, the prodelta attains an overall regular surface, except for the presence of a 3 m deep, 1 km long and 100 m wide, U-shaped channel, termed Channel A (Figs. 2, 3 and 4). This channel is close to, and aligned with the present-day mouth of the Fiumara Mazzarrà (Fig. 2). Multibeam backscatter data show that the submarine delta is characterized by a general low/medium backscatter. A series of very lowbackscatter stripes (darker tones in Fig. 2b) are associated to downslope-elongated morphological lows and gullies, including Channel A. Only two high backscatter areas, ca. 300 m long and 100 m wide, are present in the shallow-water regions on the western sector of the submarine delta (Fig. 2b). 4.2. Seafloor waveforms and gullies Seafloor waveforms, even if mainly concentrated in the submarine delta, are recognizable at depths between − 50 and − 260 m (limit of the study area) and appear characterized by crest-lines roughly oriented perpendicular to the maximum dip gradients (Figs. 2, 3 and 4). Based on their size, plan-view shape and cross geometry, two main types of seafloor waveforms can be distinguished: unconfined prodelta waveforms (Type A) and channel confined waveforms (Type B) (Fig. 3 and Table 1). Type A waveforms occur at depths between − 50 and − 120 m (Figs. 2a and 3) and have wavelength (L) between ca. 34 and 110 m, height (H) ranging between ca. 0.5 to 3 m, and lateral extent of few hundreds of meters; their L/H ratio yields values between 20 and 190 (frequently b 100). Their wave crests are sinuous or straight in planview (Figs. 2 and 3), and they frequently display a landward asymmetry in cross-section, with a seaward side sloping of 3–6° (P5 and P6 in

Fig. 4. zoom (location in Fig. 2) of the type B waveforms recognized within the thalweg of the gullies (for B1 and B2 see text for details); W, C and E indicate the western, central and eastern part of the submarine delta, respectively.

Fig. 5). In general, wave dimension gradually increases down to ca. − 90 m (compare P5, P6 and P7 in Fig. 5), then decreasing at greater depths (compare P7 and P8 in Fig. 5). An increasing trend of wave dimensions is also observed from W to E (Table 1), with the maximum values recorded at the termination of Channel A (symmetric waveforms in Fig. 4). Here, waveforms reach wavelength of 100 m and wave height

Fig. 3. 3D view of the Mazzarrà prodelta (shelf margin in foreground), evidencing the distribution of type A and type B waveforms and the network of gullies (vertical exaggeration 7×).

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Table 1 Dimensional and statistical parameters and values used to characterize seafloor waveforms as Type A (unconfined) and Type B (confined) in the study area (see text for details). Min.:minimum; Max.:maximum; St. dev.: standard deviation; L:wavelength; H: wave height; SGl: slope gradients of lee-side. A-Unconfined waveforms (L1, 24 cases)

L (m)

H (m)

SGl (°)

L/H

A-Unconfined waveforms (L2, 25 cases)

L (m)

H (m)

SGl (°)

L/H

Min. Max. Mean Median St. dev.

38 103 69 71 17

0.5 2.8 1.6 1.5 0.6

3 18 7 6 4

19 85 50 47 18

Min. Max. Mean Median St. dev.

34 110 64 65 18

0.4 2.7 1.1 1.0 0.5

1 11 5 4 4

35 133 65 61 22

A-Unconfined waveforms (L3, 22 cases) Min. Max. Mean Median St. dev.

L (m) 38 106 70 67 18

H (m) 0.3 1.2 0.7 0.7 0.2

SGl (°) 1.4 4.8 2.7 2.5 1.1

L/H 63 190 110 106 32

B-Confined bedforms (27 cases) Min. Max. Mean Median St. dev.

L (m) 22 91 54 52 22

H (m) 0.5 4.5 2.3 2.3 1.2

SGl (°) 7 24 12 11 4

L/H 14 48 26 25 9

of ca. 3 m, resulting mostly symmetric on cross-section and having seaward side sloping up to 18° (P7 in Fig. 5). In plan-view, we observe an overall progressive northwards deflection of the waveforms at depths N ca. 80 m. Type B waveforms are mainly confined within the thalweg of gullies on the outer shelf and upper slope, between −100 and −250 m (Fig. 3 and waveforms B1 in Fig. 4). Compared to Type A waveforms, they show overall larger variability in dimensions. Their lateral extent often matches the width of the hosting gully, spanning from ca. 80 m, in those located on the outer shelf, to more than 300 m in those more deeply cut on the upper slope (Fig. 2); accordingly, a parallel increase

of wavelength is observed on the upper slope (Fig. 2a and P10 in Fig. 5; Table 1). In plan-view, these waveforms show crescentic shape, whereas, on cross-section, they frequently show a downslope asymmetry, e.g. a longer downslope flank compared to the upslope flank (P9 in Fig. 5). Similar, but smaller crescent-shaped waveforms are also recognizable within the thalweg of the gullies cutting the previous unconfined waveforms between −60 and −100 m (waveforms B2 in Fig. 4). Scatter-plots data of all measured parameters show an overall interspersed distribution (Fig. 5), although a correlation can be observed between wave height and wavelength, as well as between wave height

Fig. 5. Bathymetric cross-sections (above) of the waveforms (vertical exaggeration 10×; location in Fig. 2a). Below, scatter-plots of morphometric parameters used to describe the unconfined (type A) and confined (type B) waveforms. The R-squared coefficients of determination have been calculated for all the bedforms shown in the three graphs: waveforms A in the eastern (E) part of the prodelta have values of 0.28, 0.37, 0.001, respectively; waveforms B in the central (C) part of the prodelta have values of 0.6, 0.6, 0.3, respectively; waveforms A in the western (W) part of the prodelta have values of 0.25, 0.6, 0.02, respectively; waveforms B at the shelf edge have values of 0.6, 0.2, 0.003, respectively; waveforms B in the upper slope have values of 0.7, 0.46, 0.23, respectively.

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and the lee side slope (except for Type B waveforms on the upper slope); in contrast, no correlation is observed between wavelength and the lee side slope. The prodeltaic deposits are cut by arrays of gullies that increase in depth when reaching the shelf break and upper slope (Figs. 2 and 3). Gullies increase in number on the outer shelf, where they have straight-parallel courses lengthening up to 1300 m and with an average spacing of ca. 120 m. Incision depth ranges between 2 and 20 m (P2, P3 and P4 in Fig. 2c). The wider gullies may host a frame of Type B waveforms, while the narrower ones show a smoother thalweg. Most of the shelf-edge and slope gullies are in morphological continuity with the gullies observed on the inner/middle shelf (Figs. 3 and 4); these gullies are 800 to1700 m long, some tens of meters wide, and a few meters deep (P2 in Fig. 2). Multibeam backscatter mosaic show high backscatter stripes (lighter tones) along with gullies on the outer shelf, whereas the gullies in the inner/middle shelf do not display any significant backscatter variability and are generally characterized by medium-low backscatter values (Fig. 2b). 4.3. Late Pleistocene-Holocene shelf stratigraphy Seismic-stratigraphic analysis allowed to identify the post-LGM (Last Glacial Maximum) stratigraphic succession lying above an unconformity (red reflector in Figs. 6, 7, 8 and 9) that is thought to be formed during sub-aerial erosion following the last glacial sea level fall and subsequent transgressive reworking. This deposit forms a sedimentary wedge up to 50 m thick, locally composed of three to five seismic subunits, merging into two larger-scale seismic units characterized by different seismic reflector patterns (Fig. 6). Overall, transparent seismic facies and/or sub-parallel reflectors prevail in the lower part of this stratigraphic interval, which is made of 3–4 sub-units with onlapping geometries in cross-strike sections (blue and light-blue sub-units in Figs. 6, 7, 8 and 9). The upper unit consists of 1–2 sub-units forming a progradational clinoform (green tone sub-units in Figs. 6,7, 8, and 9) with high-amplitude and high-continuity reflectors. These seismic units are very similar to those documented on other deltas developed on Tyrrhenian continental shelves, where comparable lower and upper units have been interpreted as Late-Pleistocene-Holocene transgressive

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(TST) and highstand (HST) deposits, respectively (e.g. Chiocci, 2000; Ridente et al., 2012; Lobo and Ridente, 2013 and reference therein). The progradational clinoform developed off the Mazzarrà Fiumara, during this same Late Pleistocene-Holocene interval, shows wavy reflectors and irregular seismic facies; at places a wavy pattern is observed within stratigraphic intervals overlying thin, seismically transparent intervals (Fig. 6). The wavy pattern affects entirely the uppermost subunit of the HST progradational clinoform, but sometimes it is also present in the lower sub-unit (Figs. 6, 7, 8 and 9). On the shelf, sediments are frequently gas-charged landward, up to the base part of the uppermost sub-unit (Fig. 6). Evidence of buried channels and gullies are locally preserved along the basal unconformity and at different levels within the intervals of both the TST and HST deposits (orange polygons in Figs. 7, 8 and 9). Incisions on the basal unconformity are generally deeper and broader, while they almost entirely disappear in the TST units, except for their basal part in the outer shelf (Figs. 7 and 9). Incisions occur again upsection in the HST units both on the inner and outer shelf (Figs. 8 and 9). These shelf incisions display two different patterns: one in the form of shallow gullies with erosional flanks (prevalently along the unconformity and lower TST units, and at the seafloor on top of HST deposits on the outer shelf; e.g. Fig. 9); or as channels flanked by levees (mainly in the HST interval on the inner shelf; e.g. Fig. 8). Channel incision and levee growth is particularly evident in the case of Channel A, developing in front of the present-day mouth of Mazzarrà Fiumara (Fig. 8). Overall, the deposits affected by incisions show rapid lateral pinchout and overlapping (Figs. 7–8), indicative of lateral migration of channels and lobes during the post-glacial sea level rise. 4.4. Seafloor sediment composition Seafloor sampling was targeted on the different morphological features observed in the study area (Fig. 2b). Two grab samples from the eastern part of the prodelta (B5 and B6 at −67 m and −82 m, respectively) recovered sediments with a sandy silt composition (Table 2). Sample B4 at − 62 m, outside the prodelta deposits, also recovered sandy silt sediments but with higher mud and silt content (Table 2). Sample B7 from the wider gully in the outer shelf contained a lower

Fig. 6. Along-slope seismic profile across the shelf and sequence stratigraphic interpretation of shelf deposits formed after the Last Glacial Maximum (location in Fig. 1a). Erosion during glacial sea level fall and lowstand is recorded by a shelf-wide unconformity that was also reworked during follow sea-level rise (red line). The transgressive units recording the sea level rise (TST, blue tones) have a transparent seismic facies discontinuously filling the irregular topography along the basal unconformity and passing vertically into onlapping units with subparallel reflectors. At the end of the sea level rise and during the highstand (HST), low-angle downlapping geometries develop (green tones), indicating renewed progradation of shelf deposits; a wavy pattern characterizes the seismic horizons composing the uppermost HST deposits.

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Fig. 7. Oblique seismic profile from the middle-outer shelf and sequence-stratigraphic interpretation (location in Fig. 1a). Major incisions are evidenced in orange, and show an overall upward narrowing trend. Note the thickening of late TST and HST units (green tones) in the central sector of the profile, corresponding to the area of mayor pro-delta growth. Discontinuous, high-amplitude reflectors and lateral onlapping geometry within the uppermost units characterizes the prodeltaic deposition.

sandy interval (sand percentage up to 72%, mean of 2.88) and an upper layer of sandy silt (Table 2). Downslope of site B7, sample B8 at −170 m recovered sandy silt with grain-size composition similar to that found in the upper part of B7 (Table 2). Finally, sample B9 at −224 m from inside a gully in the upper slope (−224 m) recovered muddy silt (Table 2). 5. Discussions 5.1. The Mazzarrà prodelta system The submarine deltaic system formed off the Mazzarrà Fiumara covers a surface of ca 15 km2, thus comparable to other Mediterranean submarine deltaic systems that are fed by steep, mountainous streams (e.g. Bárcenas et al., 2015 and reference therein, Fig. 10). However, in

contrast to the lobate and linguoid shape of deltaic deposits commonly associated to short and steep rivers, those in the Gulf of Patti show an overall elongated plan-view shape, recalling that of deltaic systems associated to larger rivers (e.g. the Tiber, Ebro and Po rivers; Bellotti et al., 1994; Díaz et al., 1996; Cattaneo et al., 2003, Bosman et al., 2014). If we refer to the 1884 geological map of the area (Fig. 2a), it is evident how the Mazzarrà Fiumara was previously characterized by two main branches that roughly fit the total width of the submarine delta, so forming a compound submarine delta. Since the 1960s, the Mazzarrà Fiumara has been confined within the main eastern channel, as shown by the 2009 geological map (Fig. 2b) and available aerial photos (Fig. 11). This modern confinement within the eastern side and the avulsion of the western (old) branch could explain the marked decrease in delta convexity (compare P2 and P3 in Fig. 2c) and size of the

Fig. 8. Along-strike seismic profile from the inner shelf and sequence-stratigraphic interpretation (location in Figs. 1a, 3 and 4). Incisions (orange) within lower TST units are almost absent and concentrate in the upper late TST-HST units, where they also attain channel-levee geometry (i.e. Channel A in Figs. 2, 3 and 4). Also note the frequent lateral onlapping of reflectors and overlapping of internal units within HST deposits.

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Fig. 9. Along-strike seismic profile from the outer shelf (location in Fig. 1a). Note that seafloor incisions on the shelf edge (to the left) deeply cut into late Holocene HST deposits, indicating that erosional processes are there active in recent times.

waveforms A (compare profile P5, P6 and P7 in Fig. 2c; see also Table 1) moving westward, similarly to what was recognized for the Guadalfeo prodelta along the Iberian Margin (Lobo et al., 2015). The large and elongated plan-view shape of Mazzarrà prodelta can be also related to the interplay between shore-normal riverine dynamics and lateral oceanographic currents. The former process is essentially associated to flood events and river mouth shifting, promoting deposition of fluvial sediments through a compensation mechanism, by which successive filling unit are preferentially accommodated in the space left by the previous one. The oceanographic process promotes sediment advection and is likely reflected by a progressive eastward deflection of the prodeltaic deposits and overlying waveforms from the inner-middle to the outer shelf (Fig. 2c). Indeed, the general eastward path of locally predominant shelf and long-shore currents in the area (e.g. Istituto Idrografico della Marina, 1982) supports the effect of a lateral component in deflecting geomorphic features that otherwise would be oriented perpendicular to the riverine fluxes and regional dip gradients. The prodelta is dominated by fine-grained sediments, inferred on the basis of the medium-low backscatter data (Fig. 2b), and seismic facies (Figs. 6, 7, 8, 9); this inference is supported by the prevalently silty sediments recovered from the sparse seafloor samples (Table 2). This fact would indicate a main influence of river-derived sediment plumes in shelf sedimentation, similarly to what was documented on other shelves off steep and mountainous rivers, as for instance on the southern Iberian shelf (Bárcenas et al., 2011), the Eel Shelf in California (Goff et al., 1999), and the Waiaipu shelf in New Zealand (Wadman and McNinch, 2008). In the study area, the abundant fine-grained component transported by the Mazzarrà Fiumara (large part of the drainage basin being covered by shale-like deposits) can be funneled at greater

depths both as hyperpycnal and hypopycnal flows, making up the prodelta foresets. The dip of these foresets (1°–2°) is, however, steeper than foresets observed in muddy prodeltas offshore large rivers (usually below 0.2°, e.g. Liu et al., 2006), probably reflecting a greater role of hyperpycnal flows during flash-flood events with respect to larger rivers, where the generation of these flows is more difficult (Mulder and Syvitski, 1995).

5.2. Genesis of seafloor waveforms and gullies In the study area, the gullies can be interpreted as the result of erosive activity exerted on the seafloor by the hyperpycnal flows generated during flash-floods at the Mazzarrà Fiumara, as supported by their radial dispersion with respect to the two river mouths (Fig. 2a). Hyperpycnal flows are, in fact, likely to occur in the area because of the hydrological (dominated by a torrential regime) and lithological nature of the drainage basin of the Mazzarrà Fiumara (Section 2). In this regard, the submarine Channel A (Fig. 2), aligned with the presentday Mazzarrà Fiumara mouth and equally wide, constitutes a pattern of fluvial submarine erosion frequently observed in modern deltas dominated by hyperpycnal flows (i.e., Mitchell, 2005; Piper and Normark, 2009; Hughes Clarke et al., 2014). The formation of this wider channel in comparison to the other gullies could have been also favored by the anthropogenic confinement of the Fiumara stream in the main eastern branch since the 1960s (Fig. 2b), by this leading to the recent focusing of the hyperpycnal flows along this direction. This could also explain the development of a depositional lobe at the end of this channel, the only one clearly recognizable from seafloor morphology (Figs. 3 and 4; profiles P2 and P3 in Fig. 2).

Table 2 Location (UTM 33 WGS 84 metric coordinates), geomorphic environment, depth and grain size composition of seafloor sediment samples (see also Figs. 1 and 2). “Base” and “top” refers to double sampling of mini-cores extracted from each grab sample. Grab/mini-core samples

Easting

Northing

Provenance

Depth

%mud

%silt

%sand

Mean size (Φ)

B4 B5 (base) B5 (top) B6 B7 (base) B7 (top) B8 (base) B8 (top) B9

508354 511368 511368 510973 509888 509888 509878 509878 509724

4221608 4223147 4223147 4223455 4224790 4224790 4225310 4225310 4226729

Smooth seafloor Prodelta lobe 3 Prodelta lobe 3 Prodelta lobe 3 Outer shelf gully Outer shelf gully Outer shelf gully Outer shelf gully Upper slope gully

−62 m −67 m −67 m −82 m −137 m −137 m −170 m −170 m −224 m

16,96 11,48 13,61 16,79 5,95 18,46 17,26 24,91 26,78

70,68 54,72 56,64 66,46 22,01 57,96 57,36 67,29 69,79

12,36 33,8 29,75 16,75 72,04 23,58 25,38 7,8 ,43

6.16 5,37 5,63 6,02 2,88 5,39 5,19 6,72 6,89

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Fig. 10. Scatter-plot of wave height/wavelength from Mazzarrà prodelta waveforms (black, red and blue circles) compared with values from similar wavy bedforms observed in Southern Iberia prodeltaic systems (left); W, C and E are referred to the western, central and eastern part of the Mazzarrà submarine delta, respectively. The graph in the left is a zoom of the red rectangle shown in the graph on the right, where the comparison of wave height/wavelength ratio reported for waveforms on different Mediterranean prodeltas is also shown. (Source: modified from Urgeles et al., 2011).

The recognition of the crescent-shaped waveforms B within the thalweg of the gullies in the outer shelf and upper slope (Figs. 2, 3 and 4) is another characteristic commonly associated to the interaction between sedimentary gravity flows and seafloor. These features are very similar in shape and size to crescent-shaped waveforms observed within active canyon/channel heads, where they have been frequently interpreted as cyclic steps (Paull et al., 2010; Romagnoli et al., 2012;

Chiocci et al., 2013, Babonneau et al., 2013; Casalbore et al., 2014; Hughes Clarke et al., 2014). These are a class of upslope-migrating sediment waves generated by turbidity currents, in which each downward step is bounded by a hydraulic jump (Kostic, 2011 and references therein). Generating cyclic steps requires the occurrence of a gravity flows in alternating subcritical and supercritical condition and the occurrence of a hydraulic jump. Such conditions are fulfilled in our

Fig. 11. Time lapse (2002–2014) of the present-day Mazzarrà fiumara mouth (aerial photo downloaded from Google Earth). In stages a and b, the fiumara mouth is rather stable, with slightly swift of the active water course; in stage c (after a high-energy flash-flood event occurred in November 2011) the fiumara mouth was able to generate a marked progradation of the coastline, but also to create a new westward, ephemeral fiumara branch. In stage d, part of the fiumara mouth built during the November 2011 flash-flood was largely eroded by waves.

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case, where seafloor waveforms occur beyond a marked shelf break, along slope gradients higher than 4°. The interpretation of the unconfined prodelta waveforms (A) is more controversial due to the multiple causes by which sediment layers may attain their wavy geometry, as stated in the introduction. The most likely hypothesis is that also in this case the waveforms are generated by the activity of the hyperpycnal flows, in agreement with the interpretation of analogous features identified on the Guadalfeo prodelta (Lobo et al., 2015), where a comparable geologic and oceanographic setting is present. This inference is also supported by the low L/H ratio of the waveforms (generally b100, Table 1), similar to the values observed for sediment waves generated by hyperpycnal flows on other Mediterranean prodelta (Urgeles et al., 2011). Moreover, these waveforms attain the higher wave heights and are more symmetric in cross-section just seaward of the Channel A and consequently of the present-day Mazzarrà river mouth (Figs. 3 and 4, P7 in Fig. 5). Despite the above-mentioned evidence, however, it is still unclear how these sedimentary gravity flows are able both to carve the gullies and generate the unconfined waveforms. Alternative explanation for the genesis of these waveforms could be related to creep-like processes or the breaking of internal waves. In the former case, the high sedimentation rate (considering that ca. 25 m of prodeltaic deposits were emplaced in the last 6 ka, Fig. 7), the presence of gas within prodelta sediments and the stratigraphic partitioning of TST and HST units (Figs. 5–8), may all act as predisposing factors for slow deformation processes in an area where also seismicity may induce overpressure within sediments. Nevertheless, we also observed that the waveforms often show well-defined crest, with a landward asymmetry (P5 and P6 in Fig. 5), which could be indicative of upcurrent migration during growth of sediment waves (Lee et al., 2002; Cattaneo et al., 2004). In contrast, creep folds more commonly show broader, poorly defined crests separated by narrow troughs (e.g., Lee and Chough 2001). Moreover, the highest H values of the waveforms tend to occur in middle-outer shelf and then decrease down-dip (compare P6, P7 and P8 in Fig. 5), in contrast with submarine creep deposits that commonly show the highest H values in the distal bottomsets of the wavy deposits. In the second case, the breaking of internal waves has been recently proposed as a plausible genetic mechanism for the development of sediment waves with crests parallel to the isobaths in micro-tidal environments (i.e., Ribó et al., 2016 and reference therein; Urgeles et al., 2011). This applies in particular to cases where water stratification occurs and variations in the slope gradient favors breaking of internal waves. However, the lack of sediment waves away from the delta-influenced area along with the reduced lateral continuity of their crest-lines (few hundreds of meters) is apparently in contrast with what was commonly observed in the case of sediment waves generated by internal waves, typically displaying longer (i.e., kilometric scale) linear crests (Puig et al., 2007). In order to reconcile the pattern of sediment waves observed in the Gulf of Patti with an internal wave forcing mechanism, an extreme condition is required, which is that, even on such short distances as those investigated, the lateral extent of the forming sediment waves was limited by the availability of mud, sufficient to the process only in the proximity of the river mouth. 5.3. Spatial distribution of bedforms The distribution of gullies and waveforms in the study area is most likely controlled by dip gradients and overall energy and frequency of sediment fluxes from Mazzarrà Fiumara, which determine the overall partitioning of erosional and depositional processes across the shelf and slope. From a morphological and seismo-stratigraphic point of view, gullies and waveforms A seem to be not morphologically correlated each other. Gullies are more deeply incised at the shelf edge, suggesting acceleration of hyperpycnal flows as increasing dip gradients enhance flux velocity and seafloor erosion. This increase in velocity of the gravity

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flows should promote both the development of cyclic steps at the thalweg of gullies (see Section 5.1) and the progressive re-establishment of an equilibrium profile along their axis. The feathering of these incisions shortly landward of the shelf edge, virtually lacking any connection with the shallower gullies, suggests that the middle shelf is essentially an area of overall fine-grained sediment deposition and reworking. This mainly results in the growth of unconfined prodelta waveforms, in relation either with hyperpycnal flows, creep or internal waves, as discussed in the previous section. Only after high-energy flash-flood events could sedimentary gravity flows by-pass the shelf and transport coarser material to the upper slope. This inference is supported by the difference between low/medium values of backscatter on the prodelta and high values observed only within gullies at the shelf-edge; accordingly, sparse data on sediment distribution indicate overall silty sediments on the prodelta and sandy sediments in the shelf-edge gullies. The possible trace of these high-energy sediment flows across the inner-middle shelf is provided by the network of shallow and ephemeral gullies observed here. These features also display a coaxial train of small, erosive scours (waveforms B2) that can be interpreted as the result of seafloor “delamination”, i.e., removal of sheet portions of a layered seafloor due to the passage of an erosive sedimentary flows, similarly to what was depicted off the Giampilieri Fiumara in the Western Messina Strait during the 2009 flash-flood (e.g. Casalbore et al., 2011). A comparison of aerial photos, available from Google Earth shows that severe flash-flood events have recurrently affected the Mazzarrà Fiumara, as for instance on November 2011 (Fig. 11b and c), when the coastal area prograded for ca. 100 m and a new, though ephemeral, branch of the Mazzarrà Fiumara formed to the west of the present one. 5.4. Glacial to post-glacial evolution of gullies and wavy bedforms More in general, the fluvial influence on seafloor erosion and remolding by superposition of wavy bedforms and shallow linear incisions may have affected shelf sedimentation on longer-time intervals compared to what can be observed in relation to the modern seafloor and location of Mazzarrà Fiumara mouth. Seismic profiles indicate that the post-glacial (ca. the past 20 ka) shelf stratigraphy records the vertical and lateral evolution of incisions and wavy bedforms in the study area. The deeper and broader shape of incisions on the LGM unconformity can be probably interpreted as remnants of larger channels that have been partially removed by transgressive erosional processes. Transgressive deposits are instead poorly affected by incision, indicating an interval of subdued impact of fluvial erosional processes on the shelf (Fig. 8), except for some incisions found in deposits of the lower unit laying immediately above the LGM unconformity on the outer shelf, thus indicating that incision of TST units occurred largely during the early sea level rise (Fig. 7). Incisions increase in number during the Holocene stratigraphic interval, both on the inner (Fig. 8) and outer shelf (Fig. 9), indicating renewed fluvial activity during late sea level rise and highstand. During the highstand renewed progradation of deltaic system occurs, accompanied by along-shelf partitioning into laterally overlapping units, interpreted as migrating prodelta lobes also affected by shelf-wide development of unconfined waveforms (Figs. 7–9). This lateral partitioning indicates that the growth of the Mazzarrà prodelta was mainly controlled by climatic-eustatic sea level changes resulting in the: a) lateral shift of the Mazzarrà Fiumara and exploitation of the accommodation space made available adjacent to older depositional lobes; b) changes in fluvial dynamics and supply regimes that resulted in periods of enhanced seafloor incision and sediment reworking into wavy bedforms from inner shelf to slope settings. 6. Conclusions Our analysis provides a detailed morpho-stratigraphic characterization of a submarine prodelta formed off the mouth of a steep and mountainous river course in NE Sicily. Similar rivers are frequent in southern

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Italy and are known as typically prone to intense and recurrent flashflood events during which a large amount of sediment is rapidly transported into the sea. Flash-floods evolving in hyperpycnal flows are thought to have played a major role in the growth of the Mazzarrà Fiumara submarine delta, and their recent to present-day activity is recorded by a network of gullies and unconfined prodelta waveforms shaping the seafloor in the Gulf of Patti. The gullies are likely related to the interaction between hyperpycnal flows and seafloor, becoming more erosive at the shelf break in response to an increase of slope gradients, where they are also able to develops small cyclic steps indicative of flows occurring in supercritical regime. The undulated waveforms can be interpreted as sediment waves also forming during hyperpycnal flows, notwithstanding a possible effect from other processes that are less likely and/or more difficult to constrain on the basis of our data, as for instance creep-like deformation or the breaking of internal waves. The Mazzarrà submarine prodelta is similar in scale to other prodeltaic deposits related to short and steep rivers, except for a more elongated plan-view shape. This peculiarity can be interpreted as the result of multiple river outlets active in the past, both at the scale of the post-LGM evolution and of the last century human impact, overall contributing to the build up of a compound delta, under the control of shore-normal riverine dynamics and sediment advection by oceanographic currents. The longer term evolution of the Mazzarrà prodelta is evidenced by seismic-stratigraphic data, showing a lateral migration and overlapping of multiple, small channels and depositional lobes, as fluvial activity progressively and variably responded to climatic-eustatic sea level changes. In recent times, anthropogenic impact also played a role in shaping the delta, determining a preferential development of the eastern part after a forced confinement of the river course since the 1960s. This artificial change in river course is in agreement with the presence of a main channel and depositional lobe only off the present-day river outlet, along with the lower wave heights of the unconfined waveforms facing the historical river outlet. In summary, the present study can provide insights for better understanding sediment erosion and dispersal by hyperpycnal flows on the shelf, representing a case-study for the comparison with similar setting (narrow shelves rimming tectonically-controlled coasts) as well as well as for a possible interpretation of ancient prodeltas deposits incorporated in foredeep-foreland successions during low-subsidence and uplifting phases. Moreover, it is also important to consider that similar coastal areas are often strongly exploited by human settlements, making the understanding of riverine-flows dynamics and the evaluation of their impact on the marine environment fundamental for assessing geo-hazard and addressing cost-risk analysis for infrastructures here present. Acknowledgments This research was funded by the Italian Project MaGIC (Marine Geohazards along the Italian Coasts) and carried out in the framework of the flagship project RITMARE (Ricerca ITaliana per il MARE) Project. Crews of R/V Urania and Minervauno (CNR) are gratefully acknowledged, together with the participants to the many surveys. Finally, we thank the Editor Michele Rebesco and the reviewers Claudio Lo Iacono and Serge Bernè for their insightful comments and suggestions. References Antonioli, F., Ferranti, L., Lambeck, K., Kershaw, S., Verrubbi, V., DaiPra, G., 2006. Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea). Tectonophysics 422, 23–40. Babonneau, N., Delacourt, C., Cancouët, R., Sisavath, E., Bachèlery, P., Mazuel, A., Jorry, S.J., Deschamps, A., Ammann, J., Villeneuve, N., 2013. Direct sediment transfer from land to deep-sea: insights into shallow multibeam bathymetry at La Réunion Island. Mar. Geol. 346, 47–57. Bárcenas, P., Lobo, F.J., Macías, J., Fernández-Salas, L.M., del Río, V.D., 2011. Spatial variability of surficial sediments on the northern shelf of the Alboran Sea: the effects of hydrodynamic forcing and supply of sediment by rivers/Variabilidad espacial de los

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