Submarine Depositional Terraces at Salina Island (Southern Tyrrhenian Sea)

13 Pages·2017·7.65 MB·English

geosciences
Article
Submarine Depositional Terraces at Salina Island
(Southern Tyrrhenian Sea) and Implications on the
Late-Quaternary Evolution of the Insular Shelf
DanieleCasalbore1,2,*,ClaudiaRomagnoli3,2,ChiaraAdami2,AlessandroBosman2 ID,
FrancescoFalese2,AlessandroRicchi3andFrancescoLatinoChiocci1,2
1 DipartimentoScienzedellaTerra,SapienzaUniversitàdiRoma,00185Roma,Italy;
[email protected]
2 IstitutodiGeologiaAmbientaleeGeoingegneria,ConsiglioNazionaledelleRicerche,00185Roma,Italy;
[email protected](C.R.);[email protected](C.A.);[email protected](A.B.);
[email protected](F.F.)
3 DipartimentodiScienzeBiologiche,GeologicheedAmbientali,UniversitàdiBologna,
3340126Bologna,Italy;[email protected]
* Correspondence:[email protected]
Received:16November2017;Accepted:9January2018;Published:13January2018
Abstract: The integrated analysis of high-resolution multibeam bathymetry and single-channel
seismicprofilesaroundSalinaIslandallowedustocharacterizethestratigraphicarchitectureofthe
insularshelf. Theshelfisformedbyagently-slopingerosivesurfacecarvedonthevolcanicbedrock,
mostlycoveredbysedimentsorganizedinasuiteofterracedbodies,i.e. submarinedepositional
terraces. Basedontheirpositionontheshelf,depthrangeoftheiredgeandinnergeometry,different
ordersofterracescanbedistinguished. Theshallowestterrace(near-shoreterrace)isasedimentary
progradingwedge,whoseformationcanbeassociatedtothedownwardtransportofsedimentsfrom
thesurfzoneandshorefaceduringstormyconditions. Accordingtotherangedepthoftheterrace
edge(i.e.,10–25m,compatiblewiththeestimatedpresent-day,localstorm-wavebaselevelinthe
centralandwesternMediterranean),theformationofthiswedgecanbeattributedtothepresent-day
highstand. Byassumingasimilargenesisforthedeeperterraces,mid-shelfterraceshavingtheedge
atdepthsof40–50mand70–80mcanbeattributedtothelateandearlystagesofthePost-LGM
transgression,respectively. Finally,thedeepestterrace(shelf-edgeterrace)hastheedgeatdepths
of130–160m,beingthusreferabletothelowstandoccurredatca. 20ka. Basedonthevariability
ofedgedepthinthedifferentsectors,wealsoshowhowlowstandterracescanbeusedtoprovide
insightsontherecentverticalmovementsthataffectedSalinaedificeinthelast20ka,highlighting
moregenerallytheirpossibleuseforneo-tectonicstudieselsewhere. Moreover,beingtheseterraces
associatedtodifferentpaleo-sealevels,theycanbeusedtoconstraintherelativeageofthedifferent
erosivestagesaffectingshallow-watersectors.
Keywords: prograding wedges; coastal areas; paleo sea-level; insular shelf; volcanic islands;
multibeambathymetry
1. Introduction
In the last few decades, recent advances in seafloor imagery systems enabled to extensively
map the submarine flanks of volcanic islands in different geodynamic setting, revealing a large
variabilityoflandformsduetovolcanic,tectonic,anderosive-depositionalprocesses[1–8]. Untilnow,
most of these studies focused on large-scale instability processes affecting volcanic flanks, mainly
inrelationtotheirassociatedtsunamigenicpotentialandgeohazardassessment[9–12]. Onlyinthe
Geosciences 2018,8,20;doi:10.3390/geosciences8010020 www.mdpi.com/journal/geosciences
Geosciences 2018,8,20 2of13
last years, somepapers paid attention to the shallow-water areas around volcanic islands, where
the morphological characteristics of the shelf (i.e., shelf width and depth of its edge) can be used:
(a) to reconstruct the original extension of early volcanic centres in the development of the island,
includingdeeplyerodedsubmarinecones,withrespecttosubaerialmaincentres,and(b)toassess
the vertical movements of the volcanic edifice that have occurred after the shelf formation due to
erosion[13–17]. Regardingthis,anothergeomorphological/stratigraphicmarkerforrelativepaleo-sea
levelreconstructionandassessmentofverticalmovementistheedgedepthofprogradingsedimentary
wedges,formingmorphologicalterraces(SubmarineDepositionalTerraces,SDThereafter,[18–20]).
Thesefeaturesarecommonlyfoundoninsularshelvesandtectonically-controlledmarginsandare
madebyobliqueand/orsigmoidalforesets(withslopeanglesbetween4◦ and20◦,commonlymore
than10◦),generallyslopingbasinward. Theirgenesishasbeenassociatedtothedownwardtransport
ofsedimentsfromthesurfzoneandshorefaceduringstormyconditions[21,22]. Agoodmatchhas
beenfoundbetweenthedepthoftheseterraces(commonlyintherangeof15–60m)andtheupper
10-percentileofthelocalwaveheightdistribution,evidencingthekeyroleplayedbystorm-wavebase
levelintheirformation[23].
Theaimofthispaperistocharacterize,throughtheintegrationofhigh-resolutionmultibeam
bathymetryandsingle-channelseismicprofiles,theoverallseismo-stratigraphicarchitectureofthe
insularshelfsurroundingSalinaIsland,withparticularreferencetothesuiteofsubmarinedepositional
terraces here present. Specifically, we discuss the relationship between near-shore (edge at water
depthsof10–25m)andshelf-edgeterraces(edgeatwaterdepthsgreaterthan120m)andtheiruse
aspaleosea-levelmarkers. This,inturn,canprovideinsightsontherecentverticalmovementsthat
affectedSalinaedificeinthelast20ka,aswellasonthetimingofthedifferenterosivestagesassociated
tothecanyonsthatcarvethesteepflanksofthevolcanicedifice.
2. DataandMethods
Datausedforthisresearcharehigh-resolutionmultibeambathymetryandsingle-channelseismic
profiles acquired since 1993 from the University Sapienza of Rome, University of Bologna and
CNR-IGAG.Specifically,thebathymetryistheresultofseveralsurveysmainlycarriedoutonboard
theR/VUraniaandThetis(CNR),usingmultibeamsystemsworkingatfrequencyfrom50to100kHz
forthedifferentbathymetricrange;datahasbeenrecordedusingadifferentialGPSsystem(fordetails
ondataacquisitionandprocessingsee[4]). Shallow-waterareas(approximatelyinthefirst50mwater
depths)weresurveyedaboardasmallboatusingaTeledyneReson7125(400kHz)multibeamsystem,
recordedusingaGPS-RTKpositioning(fordetailsondataacquisitionandprocessingsee[24]). Allthe
processeddataweremergedandgriddedusingthekrigingalgorithmtoproduceDigitalElevations
Models(DEMs),withcell-sizerangingfrom1mwithin100mwaterdepthto25matgreaterdepths.
Twosetsofseismicprofiles,generallyacquiredradiallytotheshelfwerecollectedin1993and
2016,respectively. ThefirstsetwasacquiredusingaSparker0.5–1kJaboardthecatamaranIncaurina
Marianna, positionedthroughGPS.BecauseofthelowaccuracyofGPSpositioning, theseprofiles
weresuccessivelyrelocatedviahomologousmatchingoffeaturesobservedonseismicprofilesand
multibeambathymetry. Thesecondsetwasacquiredin2016aboardtheR/VMinerva1usingaSparker
sourceworkingat0.5–1.5kJ;dataweregeoreferencedusingadifferentialGPS.Theverticalresolution
of Sparker profiles can be estimated in the order of some metres for both sets due to the emitted
frequencyandtotheringingeffects. Asoundvelocityof1500m/shasbeenadoptedfortime-depth
conversionintheseismicprofiles.
3. GeologicalSetting
Salina is one of the seven islands of the Aeolian Archipelago in the Southern Tyrrhenian Sea
(Figure1inset). Itis962mhighandrepresentstheuppermostpart(~16%)ofalargestratovolcano,
whosebaseliesatca. 1300mofwaterdepth(Figure1a).
Geosciences 2018,8,20 3of13
Geosciences 2018, 8, 20 3 of 14

Figure 1. (a) Shaded relief of Salina Island and surrounding shelf, showing the location of Figures 2,
Figu3r aen1d. 4(a ()bSlahcakd beodxerse)l iaenfdo tfhSea tlriancae Iosfl asneidsmainc dprsoufrilreosu (nblduien lginsehs)e lsfh,oswhonw inin Fgigtuhreesl o5c aantido n6. oSfuFbimguarriense2 –4
(blacekccbeonxtreisc )caenndtetrhs eotfr aPcizezoof sCeaipsmo iNcoprrtohfi alensd( bFlousesali ndeesll)es hFeolwcin SionuFthig aurree sal5soa nsdho6w.Snu (bfrmomar i[n1e7]e)c. c(ebn) tric
centBeartshoyfmPeitzryz o(isCoabpaothNs eoarcthh 2a0n0d mF) owsistah dtheel lmeaFpeplcinigS oouf tthhea erdegael soof tshheo iwnsnul(afrr oshmel[f1 (7IS])) .a(nbd) oBvaetrhlyyimnge try
(isobsuabthmsaerainceh d2e0p0omsit)iowniatlh tethrreacmesa (pNpSinDgT:o nfetahre-sehdogree soufbtmheariinnseu dlaerposhsietilofn(IaSl )tearnradceo;v Mer(l1y/2in)SgDsTu:b mmida-rine
depsohseitlfi osnuablmtaerrirnaec desep(oNsiStiDonTa:l nteerarra-cseh; oLrSeDsTu: bshmealfr-iendeged (elopwossittainodn)a slutbemrraarcine;e dMe(p1o/s2it)iSonDaTl :temrriadc-es)h. elf
subm(c)a rSilnoeped gerpaodsiietniot nmaalpte orrf athcee; SLaSliDnaT :esdhifeilcfe- eind gthee( lsoowutshtaernnd T)ysurrbhmenairainn eSedae (pfoors iltoicoantaiolnte srerea cthee). i(ncs)eSt)l.o pe
gradientmapoftheSalinaedificeinthesouthernTyrrhenianSea(forlocationseetheinset).
Salina together with Lipari and Vulcano is part of an elongated volcanic belt, whose formation
has been related to the occurrence of a main strike-slip NNW–SSE regional fault system, interpreted
SalinatogetherwithLipariandVulcanoispartofanelongatedvolcanicbelt,whoseformation
as the offshore prolongation of the “Tindari-Letojanni” fault, recognized onland in NE Sicily [25]. The
hasbeenrelatedtotheoccurrenceofamainstrike-slipNNW–SSEregionalfaultsystem,interpreted
volcanological evolution of Salina Island was summarized by [26], encompassing six Eruptive Epochs
as th(EeEo) fbfesthwoereenp ~r2o4l4o nangda t1i5o.n6 koaf. tThhee “pTriondduacrtsi- oLfe tthoeja onldnei”st fEaEu latr,er feocuongdn iizne tdheo NnWlan adndin NNE EsecStiocrilsy of[ 25].
TheSvaolilncaa,n wohloegrei ctahlee dveoelpultyi oenroodfedS avloinlcaanIsicla cnedntwresa sofs uPimzzmo adrii zCeodrvboy (u[2n6c]e,retaninco smtraptaigsrsainphgics ipxoEsirtuiopnt ive
Epoacnhds (rEadEi)obmeettwriec eange~)2 a4n4da nPidzz1o5 .C6akpao. (T2h4e4–p2r2o6d kuac) tasroef ftohuenodl d(FeisgtuEreE 1acr)e. Yfoouunndgeirn EtEh efoNrmWeda nthdeN E
sectMorosnotfe SFaolsinsaa ,dwelhlee Freeltchi e(1d60e–e1p2ly1 kearo) danedd Mvoolncaten diceci ePnotrrrei s(7o0f–P57iz kzao) dstirCatoorvvoolc(aunnoceesr itna itnhes tEr aatnigdr aWp hic
posiptiaornt oafn tdher aisdlaionmd, ertersipceacgtiev)ealyn d(FPigiuzrzeo 1Cc)a. pAoct(i2v4it4y– 2o2f 6thkea l)asatr eEEfo ius nlodca(tFeidg uatr eth1ec P).oYlloauran cgrearteErE inf othrme ed
theNMWon ptearFto osfs aSadlienlale, wFehlecrie( 1w6i0d–e1s2p1rekaad) rahnydolMitioc nptuemdieciePouosr rsiu(c7c0e–s5s7ioknas )wsterraet oemvopllcaacnedo ebseitnwteheen E30a nd
and 15 ka (Figure 1c). Raised marine terraces are found along the coasts of Salina, indicating an
Wpartoftheisland,respectively(Figure1c). ActivityofthelastEEislocatedatthePollaracraterin
average uplift of 0.36 m/ka for the island since the Last Interglacial [26]. Differently, the occurrence
theNWpartofSalina,wherewidespreadrhyoliticpumiceoussuccessionswereemplacedbetween
of an older MIS 7.3 terrace at lower elevations than MIS5 terraces was interpreted as the evidence of
30and15ka(Figure1c). RaisedmarineterracesarefoundalongthecoastsofSalina,indicatingan
a subsidence phase affecting the early growth stages of Salina edifice before the Last Interglacial [26].
averageupliftof0.36m/kafortheislandsincetheLastInterglacial[26]. Differently,theoccurrenceof
The submarine deeper flanks of the edifice (Figure 1a) are characterized by a complex seafloor
anolderMIS7.3terraceatlowerelevationsthanMIS5terraceswasinterpretedastheevidenceofa
morphology due to the presence of volcanic features (volcanic cones, lava flows, bedrock outcrops,
subsidencephaseaffectingtheearlygrowthstagesofSalinaedificebeforetheLastInterglacial[26].
for ~30% of the overall extension) and erosive-depositional features (landslide scars, channels, fan-shaped
Thesubmarinedeeperflanksoftheedifice(Figure1a)arecharacterizedbyacomplexseafloor
deposits and waveforms, about ~70%, [27]). The shallower part of the submarine flank is cut by an
moripnshuollaorg syhedluf e(Ftiogutrhee 1p),r ecsleeanrclye orefcvooglnciaznabiclef eoant uthree ssl(ovpoelc garnaidciecnotn emsa,pla (vFaigfluorew 1sb,)b, ebdercoacukseo ouft citrso ps,
for g~e3n0t%ly odfectlhiveitioevs e(r2a°l–l17e°x,t ebnust iotynp)icaanlldy elersoss itvhea-nd e5p°)o, siintitoernraulpftienagt utrhees m(laonrpdhsolildogeicsacla cros,ntcinhuaintyn els,
fan-bsehtawpeeedn dtehpe osstietsepaenrd (w>2a5v°)e fsourbmaesr,iaabl oauntd~ 7su0%bm,a[2ri7n]e). fTlahnekssh aolfl otwhee repdaifricteo. fItth eshsouwbsm avrairniaebfllea nk
is cumtobrpyhaonloginicsaul lcahrasrahcetlefri(sFtiicgsu; ri.ee.,1 w),icdlteha arlnyd rdeecpotghn oizf athbele shoenlf tehdeges laorpoeungdr atdheie insltamnda, psu(gFgigesutirneg1 b),
becainudsiecaotfiointss goenn thtley ddeevcelliovpitmieesnt( 2o◦f– t1h7e ◦v,oblcuatntiyc pediciafilcley [l1e7s]s. Tthhae nla5rg◦e),r ianntde rdreueppteirn sghtehlveems aorrep, ihno flaocgt,i cal
contoibnsueirtvyebde atlwoneegn thteh eWs t(ePeizpzeor C(>o2rv5o◦)), sNuEb a(Periziazloa Cnadpsou Nbomrtahr)i naendfl aSnEk (Fsoosfsath Feeelcdi iSfiocuet.hI)t ssehcotowrss ovfa trhiea ble
island, where the older volcanic activity is located (Figure 1c). Differently, narrow shelves with a
morphologicalcharacteristics;i.e.,widthanddepthoftheshelfedgearoundtheisland,suggesting
indi cationsonthedevelopmentofthevolcanicedifice[17]. Thelargeranddeepershelvesare,infact,
Geosciences 2018,8,20 4of13
observedalongtheW(PizzoCorvo),NE(PizzoCapoNorth)andSE(FossaFelciSouth)sectorsof
theisland,wheretheoldervolcanicactivityislocated(Figure1c). Differently,narrowshelveswith
a shallower edge are observed around the younger Monte dei Porri edifice (N and SW sectors of
Geosciences 2018, 8, 20 4 of 14
theisland)aswellasalongtheeasternflankoftheisland(MonteFossadelleFelci),wheretheshelf
isstsrhoanllgolwyedr iesdmgaen atrlee dobbsyerevreods iavreo-udnedp othseit iyoonuanlgperr oMceosnstees ddeui ePotorria endeiftiwceo (rNk oanfdsu SbWae sreicatlo/rssu obfm thaer ine
chainsnlaenlds)[ 2a7s] w. Tehll eaes raolsoinvge tshuer feaacseteornf tfhlaenikn osuf ltahre sihslealnfdis (Mcoovnetree dFobsysas dedelilme Feneltcsi)(,m wohsetrley tvhoel csahnelifc lias stic
strongly dismantled by erosive-depositional processes due to a network of subaerial/submarine
andbioclasticdeposits),organizedindifferentordersofsubmarinedepositionalterraces[20,22]that
channels [27]. The erosive surface of the insular shelf is covered by sediments (mostly volcaniclastic
arethefocusofthepresentpaper.
and bioclastic deposits), organized in different orders of submarine depositional terraces [20,22] that
Despitethecommonlyacceptedinfluenceofmeteo-marineconditionsintheformationofshelf
are the focus of the present paper.
adoverlyingterraces,onlyfewdataareavailablefortheAeolianIslandsinordertoquantifytheir
Despite the commonly accepted influence of meteo-marine conditions in the formation of shelf
effects. Onthewhole,themostenergeticwaveclimateinthissectorisfromwesterlydirections,since
ad overlying terraces, only few data are available for the Aeolian Islands in order to quantify their
prevailing winds are from the NW or the W [17]. The western flank of Salina is struck by storms,
effects. On the whole, the most energetic wave climate in this sector is from westerly directions, since
withpraevtyapiliincagl wfeitncdhs oafre2 f5r0o–m3 0th0en NauWti coar lthmei Wles [1a7n]d. Twhae vweessuteprnt ofla6nmk o[f1 7Sa].liWnai nisd sstrfurockm byth setoSrEmasl, swoitahff ect
theas otyupthic-aela fsettecrhn oaf n25d0e–a30st0e nrnaustiidcael omfitlhese aAndeo wliaavneIss ulapn tdos 6, ems p[1e7c]i.a Wllyinddus rfirnogmt htheea SuEt ualmson aaffnedct wthien ter,
whesroeuatsh-tehaestseorunt hanwde setaesrtnersne cstiodre iosfp tahret iaAlelyolsiahne lItselraenddbs,y etshpeecniealalryb dyuLriipngar tihIes laanudtu.mn and winter,
whereas the southwestern sector is partially sheltered by the nearby Lipari Island.
4. Results
4. Results
4.1. OverallMorphologyandSeismo-StratigraphicArchitectureoftheSalinaInsularShelf
4.1. Overall Morphology and Seismo-Stratigraphic Architecture of the Salina Insular Shelf
TheinsularshelfaroundSalinaIslandisca. 700–2000mwide,extendingfromthecoastdown
totheshTehlfe eindsguel,arw shhieclhf airsoluoncda tSeadlianta wIsalatnedr dise cpat.h 7s00ra–2n0g0i0n gmf wroimde,c eax.t1en10ditnog2 f2ro5mm th(Fe icgouarset d1oc;wsne etoa lso
Tablthee1 sohfel[f1 e7d]g).eH, wohwicehv iesr l,ovcaatleude sato wfsahteerl dfewpitdhsth raanngdindg efprotmh ocaf.t 1h1e0s tho e2l2f5e mdg (eFicgaunreb 1ec;m seaer kalesdol Tyalbolwe 1e red
inthoef [s1e7c]t)o. Hrsoawffeevceter,d vbalyuems aosfs s-hwealfs twinidgthp raoncde sdseepst,ha soff othrei nsshtealnf ecdegaelo cnagn bthee meaarskteedrnlyfl laonwkeroefdt hine tihsela nd
sectors affected by mass-wasting processes, as for instance along the eastern flank of the island
(Figure1c). Themorphologyoftheshelfisgenerallycharacterizedbyarelativelysmoothandflat
(Figure 1c). The morphology of the shelf is generally characterized by a relatively smooth and flat
surface,apartfromthecoastalareaswhereamoreruggedmorphologyispresent,aswellasbydifferent
surface, apart from the coastal areas where a more rugged morphology is present, as well as by
breaks-in-slopeathigherdepths,commonlyorientedparalleltothecontoursandcorrespondingtothe
different breaks-in-slope at higher depths, commonly oriented parallel to the contours and
edgeofsubmarinedepositionalterraces(Figures1–4).
corresponding to the edge of submarine depositional terraces (Figures 1, 2, 3 and 4).

Figure 2. (a) Shaded relief map of the area around Santa Maria di Salina village in the eastern part of
Figure2.(a)ShadedreliefmapoftheareaaroundSantaMariadiSalinavillageintheeasternpartof
Salina (location in Figure 1a), where the near-shore submarine depositional terrace (NSDT) is visible
Salina(locationinFigure1a),wherethenear-shoresubmarinedepositionalterrace(NSDT)isvisible
in the inner shelf (first 25 m of depth); (b) the bathymetric cross-section A–A1 shows the edge of the
intheinnershelf(first25mofdepth);(b)thebathymetriccross-sectionA–A1showstheedgeofthe
insular shelf and overlying NSDT.
insularshelfandoverlyingNSDT.
The overall seismo-stratigraphic architecture of the shelf, although locally variable in the
different sectors of the island, is typically characterized by an aggradational/progradational
Theoverallseismo-stratigraphicarchitectureoftheshelf,althoughlocallyvariableinthedifferent
sedimentary wedge lying upon a relatively flat erosive surface cutting the volcanic bedrock, imaged
sectors of the island, is typically characterized by an aggradational/progradational sedimentary
as having high acoustic impedance and discontinuous echo type, with small diffraction hyperbolae
wedgelyinguponarelativelyflaterosivesurfacecuttingthevolcanicbedrock,imagedashavinghigh
(Figures 5 and 6). Five seismic units were recognized within the sedimentary wedge (Figures 5 and 6):
acousticimpedanceanddiscontinuousechotype,withsmalldiffractionhyperbolae(Figures5and6).
four of these units (units 2–5) can be associated to terraced morphologies on the bathymetry (section 4.2),
Fiveseismicunitswererecognizedwithinthesedimentarywedge(Figures5and6): fouroftheseunits
whereas the lowermost unit 1 is rather discontinuous and thin (maximum thickness of 10 m). Unit 1
(units2–5)canbeassociatedtoterracedmorphologiesonthebathymetry(Section4.2),whereasthe
is commonly found directly above the volcanic bedrock, especially in correspondence of
morphological irregularities such as small depressions (SP2 in Figure 5 and SP3 in Figure 6). It is
generally characterized by continuous or semi-continuous reflectors, with variable amplitude. The
Geosciences 2018,8,20 5of13
lowermostunit1isratherdiscontinuousandthin(maximumthicknessof10m). Unit1iscommonly
founddirectlyabovethevolcanicbedrock,especiallyincorrespondenceofmorphologicalirregularities
suchGaesoscsiemncaesl 2l0d18e, p8, r2e0s sions (SP2 in Figure 5 and SP3 in Figure 6). It is generally charact5e roif z14e d by
continuousorsemi-continuousreflectors,withvariableamplitude. Theupperfourseismicunitsare
upper four seismic units are described with more detail in the next section, together with the related
describedwithmoredetailinthenextsection,togetherwiththerelatedSDTs.
SDTs.

Figure 3. (a) Shaded relief map of the SE sector of Salina (location in Figure 1a), where a large shelf
Figure3. (a)ShadedreliefmapoftheSEsectorofSalina(locationinFigure1a),wherealargeshelf
with different orders of submarine depositional terraces are present; (b) the bathymetric cross sections
withdifferentordersofsubmarinedepositionalterracesarepresent;(b)thebathymetriccrosssections
B–B1 and C–C1 evidence the edge depth of the shelf and overlying terraces. Note also the presence of
B–B1tawnod dCee–pCly1 einvdidenetnincge cthaneyeodngs ethdaet pctuht tohfet shheeslfh ienl fvearnyd shoavlelorwly-iwngatteer rarraecaess, .soNmoet eteanlss ooft hmeetperres sfeanr ceof
twodfreoempl ythien cdoeanstt.i nNgSDcaTn: ynoenars-sthhoartec suutbtmhearsinhee ldfeipnovsietiroynsahl atelrlorawce-;w Ma(t1e/r2a)SrDeaTs: ,msoidm-sehetelfn ssuobfmmareinteer sfar
fromdthepeocsoitaiosnt.aNl tSerDraTc:en; LeaSDr-Tsh: sohreelfs-uedbgme a(lroinwestdanepd)o ssuitbiomnaarilntee rdreapcoes;iMtio(n1a/l 2te)SrrDacTe:. mSPi3d -asnhde 4lf asrueb tmhea rine
depotsriaticoen oaf lthteer Srpaacrek;eLrS pDroTf:ilsehs eshlfo-ewdng ien (Floigwurset a6n. d)submarinedepositionalterrace.SP3and4arethe
traceoftheSparkerprofilesshowninFigure6.
4.2. Submarine Depositional Terraces
4.2. SubmSaruibnmeDareinpeo sditeipoonsailtiToenrarla cteersraces on the Salina shelf can be grouped in three main orders (near-
shore, mid-shelf and shelf-edge terraces) according to their position on the shelf, depth of their edge
Submarine depositional terraces on the Salina shelf can be grouped in three main orders
(rollover depth) and associated seismic unit:
(near-shore, mid-shelf and shelf-edge terraces) according to their position on the shelf, depth of
a) the shallowest terrace (near-shore submarine depositional terrace, NSDT hereafter) has the
theireeddggee a(tr wolalotevr edredpethpst hof) caan. d10a–s2s5o mci a(otend avseerisamgei c15u mni)t :and is morphologically recognizable all along
(tah)e tihsleansdh, aelvloenw ief sitt itse mrroarcee e(vnideeanrt- sinh othree esausbtemrna arninde soduetpho-esaisttieornn aslectetorrrsa (cNe,SDNTS Din TFihguerreesa f2t earn)dh 3a, sthe
edgeraetspweacttievredlye)p. Tthhsiso tefrcraa.ce1 0co–m25mmon(loyn shaovwesr aagne u1n5evmen) amnodrpihsomloogryp dhuoel otog itchael laylterrencaotgionni zoaf bslmeoaoltlha long
theisalarenads, (ecvoernreisfpiotnidsimngo rteo esvainddeyn tsienaftlhooere awsittehr nriapnpdless,o aust hd-eedasutceerdn fsreocmto rdsir(eNctS DchTecink Foirg uthrreosu2gha nd3,
backscatter data) and blocky facies (often forming small ridges parallel to the coastline) or Posidonia
respectively). Thisterracecommonlyshowsanunevenmorphologyduetothealternationofsmooth
Oceanica meadows (Figures 2, 3 and 4), widely diffused on the terrace top.
areas (corresponding to sandy seafloor with ripples, as deduced from direct check or through
Few seismic profiles cross this terrace because of its shallow depths and proximity to the
backscatterdata)andblockyfacies(oftenformingsmallridgesparalleltothecoastline)orPosidonia
coastline (tens or few hundreds of meters far from the coastline; Figures 2, 3 and 4), hindering the
Oceanicameadows(Figures2–4),widelydiffusedontheterracetop.
manoeuvrability of towed seismic devices. The available profiles for NSDT (see, for instance, SP1 in
FFeigwursee i5s manicdp SroPfi3 leins cFriogsusreth 6is) tsehrorawc eabne cuapupseermofositts ssehisamlloicw udneitp [t5h]s cahnadrapcrteorxizimedi tybyto atnh eincnoears tline
(tensporrofgerwadihnugn gdeoremdestroyf, mwiethte frosrefasertsfr doomwnthlaepc oona sat hliingeh;-aFmigpulirteusd2e –re4f)l,ehctionrd, ceorminmgothnley mreapnreoseeunvtirnagb ility
of towtheed tospe iosmf tihce duenvdiecrelsy.inTgh meiadv-sahielalfb tleerrparcoefi; luensitf o5r hNasS Da Tma(sxeime,ufmo rthinicsktnaenscse ,ofS P181 min (F1i0g mur eon5 and
SP3inavFeirgauger)e. 6 )showanuppermostseismicunit[5]characterizedbyaninnerprogradinggeometry,
withforesetsdownlaponahigh-amplitudereflector,commonlyrepresentingthetopoftheunderlying
mid-shelfterrace;unit5hasamaximumthicknessof18m(10monaverage).
Ithastobeconsideredthatthegeometryofthenear-shoreterraceisoftendifficulttodefineon
seismicprofilesbecauseitssmallthicknessispartiallymaskedbyringingeffectsgeneratedbythe
Geosciences 2018,8,20 6of13
Sparkersource,andbychaoticechotype/lowsignalpenetrationduetooccurrenceoftheblockyfacies
aGneodsciPenocseisd 2o0n1i8a, 8O, 2c0e a nicameadows(Figures2–4); 6 of 14

FFiigguurree 44.. ((aa)) SShhaaddeedd rreelileieffm maappo offth teheN NWWse scetoctroorf oSfa Slianlain(lao c(laoticoantioinnF iing uFriegu1are), 1wah),e wrehaelraer gae lasrhgeelf swhietlhf
dwiiftfher deniftfeorrednetr osrodfesrusb omf sauribnmeadreipnoe sditeiponoasilttioernraalc teesrararecepsr aersee nptr;e(sbe)ntth; e(bb)a tthhey mbaetthryicmcreotrsisc scercotsios nseDct–iDon1
sDh–oDw1s sthhoeweds gtheed eepdtgheo dfethpeths hoefl tfhaen dshoevlfe ralnyidn govteerrrlayciensg. Itnertrhaicseasr.e Ian, tthheisb aloreckay, tfhaec ibeslofcokrmy fsaacisems afollrrmidsg ae
psmaraallll erlidtogeth peacroaallsetll itnoe ,thche acroaacstetlriinzee,d cihnarcaroctsesr-sizeecdti oinn Dcr–oDss1-sbeyctaiocno nDv–eDx-1u bpyw aa crdonsvheaxp-eupsiwmailradr stohatphee
ositmheirlasru tbom tharei noethdeerp sousbitmionarailntee rdraecpeoss,ihtiaovninalg ttehreraecdegs,e haatvwinatge rthdee pedthgsea arot uwnadte1r5 dmep.NthSsD aTro:nuenadr -1sh5 omre.
sNuSbDmTa:r nineeard-sehpoorseit siounbamlaterrinraec de;epMo1sSitDioTn:aml tiedr-rsahceel;f Msu1bSmDTar: imneidd-sehpeolsfi tsiuobnmalatreirnrea cdee;pLoSsDitiTo:nsahle tlefr-eradcgee;
(LloSwDsTt:a snhde)lfs-uebdmgea r(ilnoewdsteapnodsi)t siounbamltaerrirnaec ed.eSpPo2siitsiotrnaacle toerfrtahceeS. pSPar2k iesr tprarcoefi olef sthhoe wSpnairnkFerig purroefi5l.e shown
in Figure 5.
(b)themid-shelfterraceshavetheouteredgeatdepthsaround40–50m(M1SDT)and70–80m
It has to be considered that the geometry of the near-shore terrace is often difficult to define on
(M2SDT),evenifminorandlocalslopebreaksarerecognizableonthebathymetryatdifferentdepths
seismic profiles because its small thickness is partially masked by ringing effects generated by the
(alwaysshallowerthan−120m). TheM1SDTedgeiswell-recognizableintheN,NWandSEsectors
Sparker source, and by chaotic echo type/low signal penetration due to occurrence of the blocky facies
oftheisland(Figures1c,3and4),whereastheM2SDTedgeiswell-recognizableonlyintheSandSE
and Posidonia Oceanica meadows (Figures 2, 3 and 4);
sectoroftheisland(Figure3);bothedgesarenotrecognizableintheeasternsector(Figure2).
b) the mid-shelf terraces have the outer edge at depths around 40–50 m (M1SDT) and 70–80 m
Onseismicprofiles,themid-shelfterracesM1SDTandM2SDTcanbeassociatedtotheseismic
(M2SDT), even if minor and local slope breaks are recognizable on the bathymetry at different depths
units4and3,respectively(Figures5and6).DespitebothseismicunitsoccurontheentireshelfofSalina
(always shallower than −120 m). The M1SDT edge is well-recognizable in the N, NW and SE sectors
andarecharacterizedbycontinuousandmedium-highamplitudereflectors,theyshowavariableinner
of the island (Figures 1c, 3 and 4), whereas the M2SDT edge is well-recognizable only in the S and SE
geometryinthedifferentshelfsectors(Figure5). Theirgeometrycanvaryfromprograding(SP2in
sector of the island (Figure 3); both edges are not recognizable in the eastern sector (Figure 2).
Figure5),withthetopsets/foresetsrolloverformingamarkedmorphologicaledgeoftheterraceonthe
On seismic profiles, the mid-shelf terraces M1SDT and M2SDT can be associated to the seismic
bathymetry(Wsector,Figures1cand4),tomoretabular,withtheoccasionalrecognitionofonlapping
units 4 and 3, respectively (Figures 5 and 6). Despite both seismic units occur on the entire shelf of
reflectors on the volcanic bedrock (SP1), where the terrace edge is morphologically less marked
Salina and are characterized by continuous and medium-high amplitude reflectors, they show a
(Nsector,Figure1c)ortotallyabsent,suchasintheeasternsectoroftheisland(Figures1cand2). Ina
variable inner geometry in the different shelf sectors (Figure 5). Their geometry can vary from
fewcases,themorphologicalexpressionofadeeperterracecanbemaskedbytheoverlyingterrace,
prograding (SP2 in Figure 5), with the topsets/foresets rollover forming a marked morphological edge
ifthe vertical projections of the rollover associated to the two units matches (see SP2 in Figure 5).
of the terrace on the bathymetry (W sector, Figure 1c and Figure 4), to more tabular, with the
Thethickness of these units is highly variable in the different sector of the shelf, being comprised
occasional recognition of onlapping reflectors on the volcanic bedrock (SP1), where the terrace edge
between5and20mfortheunit4andbetween10and30mfortheunit3;
is morphologically less marked (N sector, Figure 1c) or totally absent, such as in the eastern sector of
(c)theshelf-edgeterrace(LSDTinFigures3and4)isrecognizableonlyintheNWandSEsectors
the island (Figures 1c and 2). In a few cases, the morphological expression of a deeper terrace can be
of the edifice, with an edge at water depths variable from 130 (cross-section D–D1 in Figure 4) to
masked by the overlying terrace, if the vertical projections of the rollover associated to the two units
150–160m(crosssectionsB–B1andC–C1inFigure3).
matches (see SP2 in Figure 5). The thickness of these units is highly variable in the different sector of
On seismic profile, this terrace is associated to seismic unit 2, characterized by a marked
pthreo gsrhaedlfi,n bgegineogm coemtryp,rwisietdh sbteetewpefeonre 5s eatnsd(8 2◦0– 2m0◦ f)oor rthwei tuhnwite 4a kanlydt rbaentwspeaernen 1t0f aacnides 3.0W mh efroer vtherey usnteite 3p;,
theseforesetscannotbeimagedonseismicprofilessincetheyappearacousticallytransparent(seeSP4
inFigure6;seealso[28]). Thisunitmainlyliesabovethevolcanicbedrock,inthesectorswherethe
erosiveshelfedgeisdeeperthan160m(sectionB–B’inFigure3andSP2inFigure5;sectionD–D’in
Figure4andSP4inFigure6c,d). Theseismicunit2showsavariablethickness,reachingamaximum
valueof27mintheSEsector.
Geosciences 2018,8,20 7of13
Geosciences 2018, 8, 20 7 of 14

FigurFeig5u.rSep 5a. rSkpearrpkeror fiplreosfi(leas,c ()aa anndd acs)s aoncdia atessdocliinateedd rlainwe idnrgaw(bi,ndg) (sbh aonwdi ndg) sthhoewstinragt itghrea sptrhaitcigarracphhitice cture
architecture of the insular shelf on the northern (SP1) and western (SP2) part of Salina (for location
oftheinsularshelfonthenorthern(SP1)andwestern(SP2)partofSalina(forlocationseeFigure1a).
see Figure 1a). The edge of the submarine depositional terraces (i.e., the rollover depth of the
Theedgeofthesubmarinedepositionalterraces(i.e.,therolloverdepthoftheassociatedprograding
associated prograding units) are also shown. A sound velocity of 1500 m/s has been adopted for time-
units)arealsoshown. Asoundvelocityof1500m/shasbeenadoptedfortime-depthconversion.
depth conversion. Acronyms as in the previous figure, see text for details.
Acronymsasinthepreviousfigure,seetextfordetails.
c) the shelf-edge terrace (LSDT in Figures 3 and 4) is recognizable only in the NW and SE sectors
Finally,itshouldbeconsideredthateachoftheabove-mentionedterracesandassociatedseismic
of the edifice, with an edge at water depths variable from 130 (cross-section D–D1 in Figure 4) to 150–
units1c6a0n mb (ecrlooscsa lsleyctaioffnesc Bte–dB1b ayndla Cn–dCsl1i idne Fsigcaurrse (3o).f tenrepresentingtheheadofunderlyingsubmarine
canyons)Othn astecisamnicr epdroufcilee,t htheiisr twerirdacthe iasn adssmocaiakteeds stho asleloiswmeicr uthneit t2e,r rcahcaeraectdegriezewd ibthy rae smpaercktedto the
surropurnodgrinadginagre gaeso(mFiegtruyr,e wsi1tch, s2teaenpd f3o)r.esets (8°–20°) or with weakly transparent facies. Where very
Geosciences 2018, 8, 20 8 of 14
steep, these foresets cannot be imaged on seismic profiles since they appear acoustically transparent
(see SP4 in Figure 6; see also [28]). This unit mainly lies above the volcanic bedrock, in the sectors
where the erosive shelf edge is deeper than 160 m (section B–B’ in Figure 3 and SP2 in Figure 5; section
D–D’ in Figure 4 and SP4 in Figure 6c, d). The seismic unit 2 shows a variable thickness, reaching a
maximum value of 27 m in the SE sector.

Figure 6. Sparker profiles (a and c) and associated line drawing (b and d) showing the stratigraphic
Figure6.Sparkerprofiles(a,c)andassociatedlinedrawing(b,d)showingthestratigraphicarchitecture
architecture of the insular shelf on the south-eastern part of Salina (for location see Figure 1a). The
oftehdegien osfu thlaer sushbmelafroinne tdheepossoiutiothn-ael atesrtrearcnesp (ai.ret., othfeS raollilnovae(rf doerpltohc oaft ithoen assesoecFiaitgeudr pero1gar)a.dTinhge uenditgse) ofthe
s ubamrea arlisnoe shdoewpon.s Aiti soonuanldt evrerlaoccietsy (oif. e1.5,0t0h mer/so hllaosv beeredn eapdtohptoefdt fhoer taimsseo-cdieapttehd cponrovgerrsaiodnin. Agcuronnitysm)as realso
shoaws nin. Athes opurenvdiovuesl oficgiutyreo, sfe1e5 t0e0xtm fo/rs dheatasilbse. enadoptedfortime-depthconversion.Acronymsasinthe
previousfigure,seetextfordetails.
Finally, it should be considered that each of the above-mentioned terraces and associated seismic
units can be locally affected by landslide scars (often representing the head of underlying submarine
canyons) that can reduce their width and makes shallower the terrace edge with respect to the
surrounding areas (Figures 1c, 2 and 3).
5. Discussion
5.1. Late-Quaternary Evolution of Insular Shelf and Overlying Submarine Depositional Terrace
The integration of morpho-bathymetric data and single-channel seismic profiles allows us to
reconstruct the overall stratigraphic architecture of the shelf surrounding Salina Island, with
particular attention to the development of terraced prograding wedges, i.e. submarine depositional
terraces. These prograding wedges lie above an erosive surface that was carved on volcanic bedrock
(Figures 5 and 6) and interpreted as the result of cumulated marine erosion on the flanks of
subaerial/shallow-water volcanic centres during stages of reduced or inactive volcanism over
different eustatic cycles [17]. At Salina, the shelf edge is commonly at depths comparable or shallower
than the maximum depth of −127 m reached by the sea level in the last 450 ka [29], even if these values
are not corrected for glacio-hydro-isostatic effect), in agreement with the regional uplift that
characterizes the Aeolian Islands since the Last Interglacial, as witnessed by raised terraces [26].
Differently, the shelf edge is markedly deeper in the older W and SE sectors of the edifice, suggesting
that subsidence affected Salina in the early growth stages [17].
The different orders of submarine depositional terraces identified in this study have been
presumably formed during the last hemi-eustatic cycle (last 20 ka) that left its imprint over all shelf
areas. In detail, the near-shore terrace is very similar in size and depth of its edge to present-day
examples of prograding wedges observed on others insular shelves along the Mediterranean area,
such as in the other Aeolian Islands [22] and the Pontine Archipelago [18,30] as well as along
tectonically-controlled continental margins of Southern Iberia (infralittoral prograding wedge, [31])
and Southern Italy [32]. The depth range (10–25 m) of these edges is, in fact, compatible with the estimated
Geosciences 2018,8,20 8of13
5. Discussion
5.1. Late-QuaternaryEvolutionofInsularShelfandOverlyingSubmarineDepositionalTerrace
The integration of morpho-bathymetric data and single-channel seismic profiles allows us to
reconstructtheoverallstratigraphicarchitectureoftheshelfsurroundingSalinaIsland,withparticular
attentiontothedevelopmentofterracedprogradingwedges, i.e. submarinedepositionalterraces.
These prograding wedges lie above an erosive surface that was carved on volcanic bedrock
(Figures 5 and 6) and interpreted as the result of cumulated marine erosion on the flanks of
subaerial/shallow-watervolcaniccentresduringstagesofreducedorinactivevolcanismoverdifferent
eustaticcycles[17]. AtSalina,theshelfedgeiscommonlyatdepthscomparableorshallowerthanthe
maximumdepthof−127mreachedbythesealevelinthelast450ka[29],evenifthesevaluesarenot
correctedforglacio-hydro-isostaticeffect),inagreementwiththeregionalupliftthatcharacterizesthe
AeolianIslandssincetheLastInterglacial,aswitnessedbyraisedterraces[26]. Differently,theshelf
edge is markedly deeper in the older W and SE sectors of the edifice, suggesting that subsidence
affectedSalinaintheearlygrowthstages[17].
The different orders of submarine depositional terraces identified in this study have been
presumablyformedduringthelasthemi-eustaticcycle(last20ka)thatleftitsimprintoverallshelf
areas. In detail, the near-shore terrace is very similar in size and depth of its edge to present-day
examplesofprogradingwedgesobservedonothersinsularshelvesalongtheMediterraneanarea,
such as in the other Aeolian Islands [22] and the Pontine Archipelago [18,30] as well as along
tectonically-controlledcontinentalmarginsofSouthernIberia(infralittoralprogradingwedge,[31])
and Southern Italy [32]. The depth range (10–25 m) of these edges is, in fact, compatible with the
estimatedpresent-day,localstorm-wavebaselevelinthecentralandwesternMediterranean[20,21,33],
whereasitismarkedlylowerwithrespecttotheedgeofsimilarfeaturesobservedaroundoceanic
islands, wherethestorm-baselevelisdeeper[13,14,23]. Basedonsuchevidencesandconsidering
thatthepresent-daylevelhasbeenattainedsinceca. 6.5kaaccordingtothesealevelcurveof[34],
theformationofthisterracehasbeenassociatedtothepresenthighstandphaseand,moregenerally,
canbeconsideredstillunderformation(Figure7,[20]).
Byapplyingthesamegeneticmodeltothedeeperterraces,theycanbeinterpretedasprograding
wedgesformedduringstillstands,whenthesealevelwaslowerthanthepresent-day. Accordingtothe
sea-levelcurveof[34]andassumingsimilarmeteo-marineconditionsalsoforpastsea-levelconditions
(on average 15 m for the storm-wave base level), the mid-shelf terraces 1 (edge at 40–50 m water
depths)and2(edgeat70–80mwaterdepths)couldhaveformedduringthefinalandintermediate
partofthetransgressivephase(Figure7). Evenifweareawarethatmeteo-marineconditionscould
havebeendifferentduringthePleistoceneandHolocene,aswitnessedbysomestorm-wavedeposits
inthecentralandwesternMediterranean[35,36],thepossibledifferenceinthedepthrangeof±10m
isnotenoughtosignificantlychangetheirstratigraphicattribution. Finally, thedeepershelf-edge
terrace(edgeat130–160mwaterdepths)couldhaveformedduringthelastlowstandataround20ka
(Figure7),beforethesealevelrapidlyrose. However,thedepthvariabilityofitsedgeisquitelarge
andsuggeststhepossibleroleofpost-depositionalverticalmovements(discussedinthenextsection),
althoughapartofthisvariabilitycouldbealsoduetothedifferentoceanographicregimebetweenthe
twosectors.
Thisstratigraphicattributionisalsoinagreementwitha)theobservedretrogradationalpatternof
theSDTsfromtheshelfedge(LSDT)totheinnershelf(NSDT),andb)thedifferentseismicgeometry
observed for LSDT and NSDT (prograding geometry, typical of longer stillstand of the sea level,
asduringthelowstandandhighstandphases)withrespecttotheMSDTs(fromprogradingtomore
tabular,withtherecognitionofonlappingreflectorsontheravinementsurface,typicaloftransgression
phase,[22]). Differently, thelowermostunit1, showingapatchydistributionandsmallthickness,
couldbeinterpretedascomposedofregressivedeposits,formedduringthefallingstagebeforethe
Geosciences 2018, 8, 20 9 of 14
present-day, local storm-wave base level in the central and western Mediterranean [20,21,33], whereas it
is markedly lower with respect to the edge of similar features observed around oceanic islands, where
the storm-base level is deeper [13,14,23]. Based on such evidences and considering that the present-
day level has been attained since ca. 6.5 ka according to the sea level curve of [34], the formation of
this terrace has been associated to the present highstand phase and, more generally, can be
considered still under formation (Figure 7, [20]).
By applying the same genetic model to the deeper terraces, they can be interpreted as prograding
wedges formed during stillstands, when the sea level was lower than the present-day. According to
the sea-level curve of [34] and assuming similar meteo-marine conditions also for past sea-level
conditions (on average 15 m for the storm-wave base level), the mid-shelf terraces 1 (edge at 40–50 m
water depths) and 2 (edge at 70–80 m water depths) could have formed during the final and
intermediate part of the transgressive phase (Figure 7). Even if we are aware that meteo-marine
conditions could have been different during the Pleistocene and Holocene, as witnessed by some
storm-wave deposits in the central and western Mediterranean [35,36], the possible difference in the
depth range of ± 10 m is not enough to significantly change their stratigraphic attribution. Finally, the
deeper shelf-edge terrace (edge at 130–160 m water depths) could have formed during the last
Geosciences 2018,8,20 9of13
lowstand at around 20 ka (Figure 7), before the sea level rapidly rose. However, the depth variability
of its edge is quite large and suggests the possible role of post-depositional vertical movements
(LdGisMcuasnseddl oinca tlhlye pnreexset rsveecdtioonn)t, haeltshhoeulfg,hin ac oprarrets pofo nthdiesn vcaeroiafbmiloitryp hcooulolgdi cbael satlespos dourel otcoa lthdee pdrieffsesrioennts
oincetahneobgerdarpohcikc [r3e7g]i.me between the two sectors.

Figure 7. Plot of depth range of the submarine depositional terrace edges observed at Salina, with
Figure 7. Plot of depth range of the submarine depositional terrace edges observed at Salina,
respect to the sea-level curve isostatically corrected for the last 20 ka (blue line, from [34]) and that
withrespecttothesea-levelcurveisostaticallycorrectedforthelast20ka(blueline,from[34])andthat
(red line) modified for a tectonic uplift of 0.36 m/ka [26]. The inferred paleo sea-level (horizontal
(redline)modifiedforatectonicupliftof0.36m/ka[26].Theinferredpaleosea-level(horizontaldashed
dashed lines) corresponding to the different terraces is also shown; this value was obtained by
lines)correspondingtothedifferentterracesisalsoshown;thisvaluewasobtainedbysubtracting15m
subtracting 15 m (average storm-wave base level at Salina) from the average depth of the terrace’s
(averagestorm-wavebaselevelatSalina)fromtheaveragedepthoftheterrace’sedge.Notethattwo
edge. Note that two lines of inferred sea-level are drawn for LSDT in the NW and SE sectors, because
linesofinferredsea-levelaredrawnforLSDTintheNWandSEsectors,becauseofthelargedifference
of the large difference in depth range between the LSDT edge, suggesting the occurrence of
indepthrangebetweentheLSDTedge,suggestingtheoccurrenceofdifferentialverticalmovements
differential vertical movements (see text for details). Acronyms as in the previous figures; B.P. = Before
(seetextfordetails).Acronymsasinthepreviousfigures;B.P.=BeforePresent.
Present.
5.2. SubmarineDepositionalTerracesasaToolfortheAssessmentofVerticalMovementsandRelativeDatingof
ErosiveActivityofCanyons

Intheprevioussection,wehaveinferredthattheSDT’sedgecanbeusedasaproxyofsea-level
position(dashedlinesinFigure7). However,weareawarethatthisdepthisnotadirectmeasure
of the sea-level, but it is influenced by different factors, such as the storm-wave base level that
varies in response to submarine morphology, coastal physiography and exposition (fetch, winds
andwaveregime). Moreover,itsdeterminationisinfluencedbymappingprocedures(forareview
ofpossibleinaccuraciesintheestimate,see[20]). Byconsideringthevariabilityofthepresent-day
NSDT edge (10–25 m, on average 15 m), we can roughly estimate an error range in the order of
severalmeters. ThismeansthatonlylargervariationsintheedgedepthsofaSDTcouldberelated
todifferentialverticalmovementsbetweentwo(ormore)coastalsectors. Thiserrorestimationthus
suggestsexcludingtheNSDTfortheverticalmobilityassessment,becausetheamountofexpected
verticalmovementsinatimespanofabout6.5kawouldbesmall(i.e.,somemetersatmaximum)
and, consequently, within the error range. Moreover, the NSDT is considered under formation at
present,sopossiblechangesinthedepthofitsedgeduetoverticalmovementscouldbe“readjusted”
by active sedimentary dynamics [20]. Differently, the older M1SDT and M2SDT (the formation
of which is roughly corresponding to inferred sea-level around 9 and 12 ka in Figure 7) could be
Geosciences 2018,8,20 10of13
morepromisingforsuchassessment. However,mid-shelfterracesarecommonlycharacterizedby
a large variability in inner geometry (from prograding to more tabular, with onlapping reflectors)
and external shape (Section 4.1, Figures 5 and 6), posing some limits on their comparison among
different sectors. The shelf-edge (lowstand) terrace shows instead a constantly, prograding inner
geometry and a well-defined morphological edge, thus allowing a comparison at the island scale.
On the other hand, where preserved, this terrace shows a larger range of edge depth: in the NW
sectorofSalinatheedgeisaround130mwaterdepth,being20–30mshallowerthanintheSEsector
(150–160mwaterdepth), suggestingtheoccurrenceofdifferentialmovementsbetweenthesetwo
sectors. ThedepthofLSDTedgeintheNWsectoriscomparablewiththepredicteddepthofthesea
levelduringthelastlowstand(Figure7,around120mwaterdepth,withamaximumvalueof125mat
theLGM;[34]),alsoconsideringtheaverageupliftrateof0.36m/kaestimatedfortheislandsincethe
LastInterglacial[26]andaccountingforca. 7mofverticaldisplacementinthelast20ka. Differently,
the depth of LSDT edge in the SE sector is deeper than the predicted sea level during the LGM.
Inparticular,evenifwesubtractamaximumvalueof25mofstorm-wavebaselevel(derivedfromthe
rangedepthoftheNSDTedgeinordertoperformaconservativeanalysis)fromthedepthsoftheLSDT
edgeintheSEsector,weobtainaninferredsea-levelvalueof130±5m,indicatingalocalsubsidence
ofabout10moccurredinthelast20ka. ThissubsidencecouldbesupportedbythefactthattheLSDT
intheSEsectorliesaboveaninsularshelfcuttingthetopofsubmarineeccentriccenters(namedFossa
delleFelciSouthinFigure1). Thissubsidenceisalsotestifiedbytherecognitionofanearbyflat-topped
cone(Figure1c),characterizedbyarelativelyflaterosiveshelf,withedgesatdepthsaround−220m
andcoveredbyca. 25mofsedimentarycover[17], possiblyaffectedbylocalverticalmovements.
TheseeruptivecentreswereemplacedclosetotheLipari-SalinaChannel,separatingSalinafromLipari
island,wheremorethan200mofsediments(mostlywithanaggradationalpattern)wereaccumulated
inaperchedbasin(unpublisheddata),suggestingthatthissectorhasbeenalsoaffectedbyasignificant
subsidenceduringtime. Moreover,localevidenceofsubsidence,contrastingwiththegeneraluplift
trendderivedbyraisedmarineterracesforthelast127ka,havebeenalsoobservedforthelastfew
millenniaatPanarea[38]andLipari[22,39–41],insomecoastalandshallow-watersectors.
Besidesverticalmobilityassessment,submarinedepositionalterracescanbealsousedforthe
relative dating of mass-wasting events affecting the insular shelf, on the basis of the geometrical
relationshipbetweenthem. Ingeneral,submarinecanyonheadsorlandslidescarsaffectingaSDT,
and being characterized by a marked morphological freshness, are thought to represent erosive
stagesoccurringaftertheemplacementoftheterrace. Incontrast,thelowmorphologicalfreshness
of some canyon head and slide scar could be interpreted as the result of a successive draping or
burial from the SDTs, thus indicating an older erosive stage. In this regard, attention should be
paid to erosive features affecting the near-shore submarine depositional terrace (see for instance
Figures 2 and 3), since the genesis of this latter can be referred to the present high-stand, this can
representaproxyfortheoccurrenceofveryrecentmass-wastingevents. Asimilarmethodological
approachhasbeententativelyappliedtootherAeolianedifices(StromboliandLipari,[12,42]),andto
tectonically-controlledmargins[43],providingusefulinsightsforafirstgeohazardassessmentofthe
facingcoastalareas.
6. FinalRemarks
In summary, the present study allowed to characterize the complex seismo-stratigraphic
architectureoftheshelfsurroundingSalinaIsland,thatisstronglydependentontheinterplaybetween
localphysiography,verticalmovements,sedimentsupply,waveregimeandsea-levelfluctuations.
Thisarchitectureismostlymadeupbyaseriesofterracedfeatures(near-,mid-andshelf-edgeterraces)
associatedtoseismicunitswithaninnerprogradinggeometry(sometimestabular,withonlapping
reflectorsformid-shelfunits),whosegenesiscanbereferredtothedifferentsea-levelphases(highstand,
transgressiveandlowstand)developedinthelasthemi-eustaticcycle(i.e.,post20ka).