Submarine canyons and channels of the Tyrrhenian Sea: from geological observations to oceanographic, biological and hazards studies

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1 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 Submarine canyons and channels of the Tyrrhenian Sea: from geological observations to oceanographic, biological and hazards studies Fabiano Gamberi stituto di Scienze Marine, SMAR, Consiglio Nazionale delle Ricerche, CNR, Bologna, taly NTRODUCTON The Tyrrhenian Sea sits in a complex geodynamic region dominated by the convergence between the African and Eurasian plates that results in the building of the Apenninic-Maghrebian chain and in back-arc basin opening. The two young basin plains of the Vavilov and Marsili back-arc regions, located at depths of more than 3000 m, are surrounded by intraslope basins developed on the eastern Sardinian, the northern Sicilian and the eastern talian mainland margins (Fig. 1). 42 Corsica 40 fyjtjthr.:n&a.n $LA SWATH aal HYM'TtY loum.t.d'ufh.,.' J" Figure 1. Bathymetric map of the Tyrrhenian Sea. 165 CESM Workshop Monographs n"47

2 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 The Sardinian margin represents the passive margin of the Tyrrhenian Sea back-arc system and is characterized by negligible overall vertical movements, low seismicity, a relatively large shelf and a low-relief hinterland. The northern Sicilian and the western Calabrian margins are tectonically active, are characterized by high rates of vertical movements, have narrow continental shelves and an high-relief hinterland. As a consequence, the Tyrrhenian Sea represents a natural laboratory to test how different geodynamic settings impact the development and the architecture of submarine drainage networks from source to sink. The Tyrrhenian Sea canyons show different characteristics of their head regions, some of them located very close to the present-day coastline and thus representing an element of hazards to infrastructures. The different positions of the canyons heads also imply that feeding systems to the deep sea are varied, being represented by direct hyperpycnal flow of riverborne sediment, by coastal sediment reworking due to oceanographic processes and by landslide often triggered by earthquakes. n addition, the different water depth of the heads of the canyons and their location with respect to shoreline physiography must impact on coastal oceanographic processes, creating local perturbations that can modify their regional characters. The canyon and channel systems of the Tyrrhenian Sea are both inactive and active- in the latter case showing a variety of erosional and depositional behaviours. n addition, they show the development of highly varied internal geomorphic elements that reveal a large environmental diversity that must be taken into account when studying biological responses to sedimentary and oceanographic processes and when working on deep-sea habitat mapping. CANYON HEADS: '1E MPAcr OF OCEANOGRAPHY, ONSHORE GEOLOGY, CLMATE AND SEA LEVEL VARATONS ON SEDMENT ROUfJNG n the northern part of the Sardinia margin, the continental shelf is up to 20 km wide. Canyons have heads that incise the shelf for a length of about 7 km. The shelf break is located at a depth of 150 m, whereas the canyon heads reach depth of only 100m (Fig. 2) Figure 2. Shaded relief image of the northern portion of the eastern Sardinian margin. The coastal systems developed during the last low stand of sea level are located at a depth of 120 m, thus during the last low stand of sea level rivers directly fed sediment to the heads of the canyons. Elongated depositional bodies, resulting from the reworking of coastal sediment due to long-shore currents are also present close to the canyon heads, showing that oceanographic processes contributed as well to the sediment budget delivered to the canyon heads during the last sea level low stand. The heads of the canyons are flanked landward by a continuous littoral barrier formed when the sea level was about 90 to 80 metres below present day. They bounded a lagoon CESM Workshop Monographs n Multiprint

3 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 seaward and therefore isolated the canyon heads from any input of riverbome sediment, thus dictating the onset of canyon deactivation during the flrst phase of the late sea level rise. n the central sector of the Sardinian margin, the Orosei-Gonone canyon system represents an exception having its head very close to the shoreline (Fig. 3). Figure 3. The Gonone canyon head located very close to the coast and to harbour infrastructures. The Orosei-Gonone canyon system shows that the processes of canyon head retrogradation can be important and can be a source of hazards to coastal infrastructures even in passive margins. t also shows that the extent of canyon head retrogradation is not always directly related to its connection to large drainage area on land, since the Orosei-Gonone canyon head faces a very small river. n the Sicilian margin many of the canyons have their heads very close to the coastline (Fig. 4) Figure 4. Shaded relief image of the northeastern Sicilian margin. n this case it appears that at the present time the canyon heads are often directly fed by riverbome sediments particularly during the flash flood events that in recent years have been a typical catastrophic process in the area. 167 CESM Workshop Monographs n 47 Multiprint

4 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 Figure 5. Shaded relief image of the head of the Milazzo canyon. n many canyons of the Sicilian margin the canyon heads occupy a wide seafloor portion and can be an hazard to coastal infrastructure as shown in the image of Fig. 5 where channels that are located at the heads of the Milazzo canyons are adjacent to the piers of the Milazzo industrial area. n addition it appears that in some cases the canyon heads act as sink of sediment, causing sediment starvation along the shallow water areas and favouring coastal erosion and retreat. CANYON COURSE: DFFERENT GEOMORPillC ELEMENTS REPRESENT A VARETY OF ENVRONMENTAL SETNGS The submarine channels and canyons of the Tyrrhenian Sea are characterized by both erosional and depositional attitude and features, sometimes varying along very short distance within the same system. Local closed depressions correspond to areas of focussing flow erosional behaviour in plunge pools, and form circular or elongated localized features. They can be up to 3 km long and up to 50 m deep and thus give rise to longitudinally isolated environments along a channel or canyon course. Figure 6. Shaded relief image of one sector of the Caprera channel in the Sardinian margin. CESM Workshop Monographs n Multiprint

5 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 A further common erosional element that causes longitudinal environmental variations along channels and canyons consists of cross channel escarpments that coincide with gradient increase and are often associated with smaller scale erosional crescentic scours (Fig. 6). These erosional escarpments can have variable dimensions, going from small, 10m high escarpments, to large, up to 100m high features. n some cases they separate two sectors of a channel or a canyon with depositional attitude, thus representing areas of longitudinal environmental discontinuity. Erosional channels show in general steep flanks and a V -shaped axial incision; however, deposition can occur locally also in this type of conduits. This is for example the case of channels in which incision is accompanied by a progressive narrowing that leaves terraces hanging at various heights above the entrenching channel floor (Fig. 6). Deposition of mainly fine grained and thin bedded turbidites occur in the terraces from the upper, low-energy part of flows funnelled within the channel; coarser and thicker bedded facies deposited by the higher energy lower part of flows are however found when the terraces are at a low height above the channel axis. Depositional channels are also characterized by a variety of internal elements. Meandering channels show depositional elements that are developed mainly on the inside of bends that are often comparable to river point bars (Fig. 7). Figure 7. Bathymetric map of a sector of the Stromboli slope valley. Meandering depositional channels are often characterized by a single channel element, but they also display sometimes the development of multi-channel tracts, likely as a result of flow pathway divergence connected with the dynamics of sinuosity development. Single thread mendering channels also show longitudinal variations with formations of knickpoints and depressions of variable sizes. This pattern appears to be dictated by the varying character of flows along a single meandering pathway creating variable environmental settings. Multiple-channel tracts develop also within straight channels or canyons. They are often associated with mid-channel and side-attached bars. n some of which, seismic lines appear to substantiate that lateral accretion processes are the principal growth motif (Fig. 8). 169 CESM Workshop Monographs n 47

6 SUBMARNE CANYON DYNAMCS - Sorrento, taly, April 2015 ~ -~~ =~ = ~~ - ;... ~--- ;~- i~--~~ um p ~ Figure 8. Bathymetric map of one distal sector of the Stromboli slope valley This setting appears to be best developed in relatively large and low gradient channels, often in coincidence with conduit enlargement. Flat, relatively featureless canyon and channel floor are also characteristic of some systems. Also in this case a gradual transition between a channel-axis and a channel-margin depositional setting is sometimes evident. Channel margin environments have a seismic facies that is indicative of thinner and finer grained deposits deposited by lower energy flow portions than their correlative coarser-grained facies within the axial part of the conduit. Smaller scale geomorphic elements, such as sediment waves and scours are developed in many tracts of canyons and channels. Sediment waves appear ubiquitous, developed on the heads of canyons and channels. Their wavelength can give indication of the grain-size of the corresponding deposits and of the variations of energy level at the seafloor. Scours have a variety of forms and dimensions and represent localized high-energy erosional environments within otherwise lower energy depositional settings. *this chapter is to be cited as: Gamberi F Submarine canyons and channels of the Tyrrhenian Sea: from geological observations to oceanographic, biological and hazards studies. pp n CESM Monograph 47 [F. Briand ed.] Submarine canyon dynamics in the Mediterranean and tributary seas -An integrated geological, oceanographic and biological perspective, 232 p. CESM Publisher, Monaco. CESM Workshop Monographs n