Man. Soc. GeoL I t., (1982),7-25, 23 11 ff.
THE GEOLOGY OF THE MT. ETNA BASEMENT
by Faero LENrtNr (*)
RIASSUNTO The Iblean Foreland consists of a Dredominantlv lhick carbonalesuccession wirh repearedvolcanic iri- tercalations.This successiondatel from Nel substrato dell'Etna sono riconoscibili Lre Triassic to elementi Ouaternary. lt does not appear in rhe map of Ml. Filgipali slrutrurali : I Avampaeseibleo, I A- Erna, present (see tanfcsa buL is in lhe iubsurlace i:300,000 Gela-Cataniae la CatenaSetientrionale. scale profile). L Avampaese è costituito da una potente serie The Cela-CataniaForedeep is crtooatica estesadal Trias al Quaternirio e interes- . lhe externalpart of da rip€tute LheCalranisselra Trough and is elongaredin NE-SW -|' manifestazioni vulcaniche basiche. direcrion.Ir Esco non affìora is limited loward the SE by rhe progres- nella carta dell'Etna. ma aooare nel sive platèau a scala 1:300.000 downfaulting of the Iblean a;d-dis- Fofrlo disegnalonella caria. appears L Avanfossa toward the NW below the Sicilian fold-belt of Cela-Caraniarappresenra la pdrte the Northern pir) 6rerna della Fossadi Caltanissetta Chain. This foredeep contains the Gela ed è allungara Nappe, which reprcscnts in direzione NE-SW. Verso SE essa è delimitattdaÌ the mosi southerly extent of lhe Northern Chain rhrusts and oargine del Plateau ibleo e si immerge verso NW con is buried'under the Quaternarl deposils sisremi di faglia gradinara". sparendoal disolto "a The NorthernChain. linking èlle collri della Catena.Tl tronre piu atajjzaLo que- . LheApennines Lo lhe ol ALlasMounLains ol Norrh Africtis st€ colri è costituirodalla Falda di Cela che si inseri- namedrhe Apenni- nian-Maghrebian Chain, and consists sce fra i depositi plio-quaternari dell'avanfossa,che in of a seiies of Parte ne mascherano la presenza. k Catena Settentrionale, che rappresenta la con- tEione ha l'Appennino e le catené moniuose del Nord-Africa e che pertanto prende il nome di Catena Appenninico-Maghrebide, consisLein una serie di col- tri di ricoprimento, costiluite da unità stratigrafico- smttturali a loro volta riconducibili a ditferentiaree paleogeografiche.Le varie unità sono oggi il risultato di fasi di deformazione succeduLesidàll'Eocene al Ksy woros: Stratigraphy, tectonics, Eastem (ltratemario con un progressivo spostamenlo del fron- Sicily. L di deformazioneandando da Nord versoSud. Ven- Epoo p€rtanto brevemente descritte le varie unità ini- ziando da quelle esterne e terminando con quelle piu rDtenre- INTRODUCTORYREMARKS
ABSTRACT The volcanic complex of Mt. Etna is lo- cated between two main structural units: the In the Mt. Etna substralum three main structural Iblean Foreland and the Northern Chain. units are recognizable: the Iblean Foreland, the Gela- The Iblean Foreland, lying in the south- Catania Foredeepand the Northern Chain. easter-n.part of Sicily, does not appear in the ---Ìi) map of Mt. Etna, but lies in the subsurface, Isti,,r,o Scienze della Terra - Università di as is seen in the 1:300,000 scale profile. Catania. It repr4eíts part of the margin of the FABIO TENTINI
Ft8. I ' Schematic structural cross-sectionof Eastern Sicily, west of the Mt Etna area. A = Iblean Foreland; F = Gela-CataniaForedeep; c=celaNappe; J = Mr. Judica Unit; I= Iúerese Unit; N: Numidian p : Flvsch:-"Calabride PanormideUnit; S = Sicilide Unit; SN = Sicilide + Numidian Flysch;MS = Mt.SoroUnit; C: Units; Cl = Capo S. Andrea U.; C, = 16.t*i-luormina U.j Cr = S. Marco U.; R = Rocca Novara U.; Ca = Mandaaici U.; Cr = 45p.o*ottte Unit; RF = Reitano Flysch; O = Capo d'Orlando Flysch; VC : Variegaìed clays (Antisicilide unit); Fc = Floresta calcarenites; pr : (posrorogènous) units (afier inrrru tr vriz,uq, 1978. modified).
African Plate, which has remained relatively northeast tilting towards the southwest. Thus undisturbed save by predominantly normil towards the northeast the section becomes faulting. The progressive downfaulting of the thinner, gaps develop and locally emergence northwestern part of the Iblean Foreland by occurs, whereas to the southwest oelaeic a system of NE-SW trending faults in Cene- sedimentation is continuous. zoic times led to the formation of an asym- The pattern is little changed in the Up- metric trough: the Caltanissetta Trough, the per Miocene with volcanoclasticsof phreato- southeastern part of which is referred to as magmatic origin intercalated with coral the Gela-Catania Foredeep. bioherms. Lower Messinian limestones with The Northern Chain linking the Apen- a restricted fauna indicate a shallow-water nines to the mountains of North Africa and environment which grades up into continen- part of the Alpine orogen is commonly refer- tal conditions. red to as the Apenninian-Maghrebian Chain. From the Miocene-Pliocene boundarv the It consists of a series of thrust sheets made palaeogeographicpattem js totaìly differenr. up of sediments originally deposiîed along The present one is the result of three tectonic the old cratonic margin of Africa and prog- phases following the regional Upper Miocene ressively thrust over one another between uplift and referred to Early-Middle Pliocene, Eocene and Quaternary (fig. 1). Plio-Pleistocene and Middle Pleistocene- Recent age, which influenced the tectonic evolution of the present Ionian coast and the Syracuse-Malta Scarp (Carnoue TIIE IBLEAN FORELAND et alii,1982). l Sedimentation of the Iblean Foreland I from Triassic to Tertiary times was predomi- THE GELA-CATANIA FOREDEEP nantly one of carbonate deposition, both of limestone and dolomite. Information about all horizons below the Upper Cretaceous is only available foom boreholes. The lowest horizon penetrated by the drill is Triassic, so that nothing is known of the pre-Triassic basement. An interpretation of the subsur- face data is available îrorrl PATAc;:Aet alii (1979), who reviewed the core material. A distinction in facies has been studied in surface outcrop, in the Creaceous to near Mineo. Miocene sections, by Gnesso et atii (1979), Gnesso, Lerrrm & PEDLEY(1981), etc. In conclusion, the pattern which seems to characterize the Cenozoic of the Iblean Foreland is that of a structural hieh to the THE GEOLOGY OF TIIE MT. ETNA BASEMENT
Mt. JUDICA UNIT ( Sicanianor lmcrcrc domain) VARIEGAIEDCLAYS
Sc.gli. l.ci.. E-O CALIAVUIURQF'n. E-O
G-C
Cozzo Cucca Scction TI
Mt- Gzr'rbarrera
Succcsrion Mt. Jud.ica .s.crion (af..ì catt{taN -300fi LENTINI,tCz4) FIYSCH (or /rìUFAIA Fn.)
-o M., Scalpcl,o Sccalorr l0 FABIO LENTINI posed, for it is always buried under Quater- running approximately from South to North nary depositsand its recognition has to await orovides an account of the structural- the drill. itratigraphic units from the external to the The nappe is made up of Pliocene and internal zones. pre-Pliocenesediments and rests upon Mid- dle Pliocene layers in the northwest of the foredeep (Rabbito n. 1) to horizons as young Tse Mr. Juprcl UNrr (Leurrxr & Vezzeu, as Quaternary at its furthest advance to the 1978). (Gelan. 1). southeast The lowermost stratigraphical level of Well data indicate that the floor of the this unit consists of shales with rare interca- the nappe is made up of a car- trough under lations of sandstones and marly limestones bonate sequencetypical of the Iblean region containing faunas of the Aonoides and Sub- followed by Upper Miocene evaporites, bullatus zones (Lprrrrr, 1974). This forma- PlioceneoTrubi", and sandsand marls of the tion has been assigned to the Mufara Forma- Plio-Pleistocene.Only in the region of Cata- tion bv Scsurpt nr FnrnpeeR.c & Tnovò of the foredeep formed of nia is the base (19ó2),'but is also known in the literature as Triassic carbonatesdirectly, becausethe Cre- the uFlysch Carnicoo ("Carnian Flysch,). members are lacking and taceous-Miocene The succeeding 200-300 m thick cherty the Jurassic is very much reduced in thick- limestones witJù Halobia styriaca ar lhe base ness. This is covered by a Pliocene- and H.norica at the top are of Norian- pelitic series with intercalated Quaternary Rhetian age. They are followed by 50-100 m basic lavas (Catania No. 10 Well). of cherty shales and radiolarites with interca- lations of submarine basic volcanic rocks, Jurassic-Cretaceous in age. THE NORTIIERN CHAIN On the top of these beds rest Middle- Eocene to Lower Oligocene red marls and The Northern Chain is a fragment of the marly limestones of the Caltavuturo Forma- Apenninian-MaghrebianChain. It consistsof tion containing microfaunas with Glaborota- a pile of superposednappes of sîrucîural- lia ceftoazulen!;is at the base and with Glo- stratigraphic units, which originated from bigerina oligocenica at the top. Some rare different basins or platforms. With certain brèccias, conglomerates and slumps occur exceptions, the units which are structurally between Cretaceous radiolarites and Eocene the highest originated from the more internal marls. These breccias and conglomerates zones. The profile of the structural scheme were interpreted as a transgressive interval.
Fig. 3 - Upper Triassic cherty limestones of the Mt. Judica Unit, cropping out in the Mt. Scalpello area. THE GEOLOCY OF THE MT, ETNA BASEMENT 11
and Middle Miocene horizons and to en- croach on the external zone.
THe Iupnpse UNrr: rge Nuuroreu Fr_yscn
turo Formation is about 100 m. This unit is made up o[ a succession similar to the Mr, Judica one in Triassic- Jurassic levels, but different due to the pre_ sence of a thick flyschoid series (Numiàian Flysch), the only part out-cropping in the geological map of Mt. Etna.
Tectonically resting upon this sedimen_ tary succession is the Numidian Flysch and the Varicoloured ClayS(Variegared ót^v"t ter a tectonic phase referable to Middle_"i_
tavuturo Formation and the silty shales of the Portella Colla Formation bf Eoc.rr"_ Oligocene age (Cf Gnesso et atii, 1979).
s_ection,not more than 15 m thick, belongs to the Mt. Judica-type sequence, and repreénts the most condensedsuccession.
Fi& 4 - Jurassic-CretaceousradioÌarites with breccias and conglomerates at the top. Mt. Judica Unit. FABIO LENTfNI the Numidian Flysch, that developswidely rn the Ml" Etna map. On the basis of their structural setting three groups of outcrops, with the degree of deformation decreasing frorn South to North, can be recognizedin Eastem Sicily: 1) the area of the Madonie-NebrodiMts, where the wells Mistretta 1, Pizzo Bella Fon- tana 1, and Marangone 1 show that the ter- rigenous cover is structurally continuous with the Mesozoicsequence; 2) the area of M. Zimmara-Bronte, where although the Numidian Flysch is wide- ly exposedwith broad structures, it is pre- sent as tectonically repeated slices generally alternating with Varicoloured Clays, and in tectonic contact with Middle Miocene marls perhaps belonging to a more external unit; 3) the southern area where the Numi- dian Flysch is reduced to a number of chaotic slices mixed with Varicoloured Claysand tec- tonically superposedupon the more extemal Mt. Judica successionup to, and forming part of the Gela Nappe. In the map of Mt. Etna the Numidian Flysch outcrops only west and southwest of the volcano, even if it is probable that the Fig. 5 - Variegated (or Varicoloured) Clays of Cre- same structures continue eastwards below taceousage, belonging to the SicilideUnit. the lava flows. The outcrops North of Centuripe form rather regular monocline structures with pre- (Gratteri Formation) of Eocene-Oligocene de- valent southern vergence. In particular the posited upon the Panormide calcareous suc- structure of Grottafumata-Mt. Revisotto, cession prior to a phase of orogenic transport west of Adrano, continues westwards outside (GR sso et alii, 1.979). the Mt. Etna map and can be continuously In the upper pan of the Gratteri Forma- followed until Mt. Salici and the Gagliano tion, some quartzarenitic levels appear; that area. Such structures are ascribable to the indicates the tendency of the Numidian basin .Mt. secondgroup, the one indicated as Zim- to extend over the Panormide domain. Be- mara-Bronte area". Some regularity of the tween Upper Oligocene and Lower Miocene structu.res is found even in the Bronte areas, the Panormide Unit was erirplaced into the with large anticline and monocline bodies in Numidian Flysch horizons belonging to the which oil-fields occur. Imerese basin. Where the carbonate body South of Centuripe the Numidian Flysch closes, now it is possible to observe in the is much more chaotic and tectonically lies Numidian Flysch several sedimentary lenti- upon the coeval clays with glauconitic sand- cular intercalations of megaconglomerates stoneeof the Mt. Judica Unit. and megabreccias deriving from Triassic- Cretaceous Panormide Carbonate Platform For intercalations Tns PANoRMrpnUNrr rocks, these OcNTBEN (19ó3) used the term of Mt. San Salvatore This unit is made up of a thick sequence Wildflysch, that is built up by a combination of Triassic to Cretaceousreefoidal limestone, of sedimentation of t}te norrnal arenaceous- and of Upper Cretaceous to Eocene ( Scag- argillaceous flysch-type, and tectonic em- liaD, passing up into a pre-flysch sequence placement of large sheets and breccia beds of THÈ CEOLOGYO} THE M1, FT\A BASEME\T I3 various kinds of older rocks by orogenic can be recognized in the subsurface of the transportatron. Mt. Salici area. ABArE et alii (1981.)claim that the synse- dimenlary teclonic and eustaliclowerinq in- duced repeared discharge o[ carbonate de- bris, which accumulated over the adiacent THe SIcrr-roe Ub{rr scarps during the deposition of rhe Numidiair Flysch shales. In consequence these shales This unit crops out extensively in the with the mega-breccias represent the original Northern Chain and is well represinted in Tertiary cover of the Panormide Pìatform the map of Mt. Etna. and precede the tectonic displacement into It consists of an appreciable thickness of the Imerese basin, which occurred after Upper Cretaceous Variegated (or Varico_ Aquitanian. loured) Clays with chaotic lexture and o[ This unit does not appear in the Mt. Etna Eocene-Oligocene marly, calcarenitic and geological map: presence neverthelessits in arenaceous beds, that have been referred to the subsurlaceis only a supposirioì as can be the Polizzi Formation s.l. seen in the 1:300,000 scale geological profile. In the southern sector South of Centuripe In any case, some subsurface data suggest a in the VariegatedClays are included some prosecution of the Panormide unils or of the bìocks o[ a calcareousfacies. which is typica] megabreccias east\^ard and their presence Polizzi Formation, while northwards aiso a
- Fig. ó Eocene whitish marly limesrones of îhe pohzzi Formation (Sicilide Unit). t4 FAIIO LENTINI terrigenous facies appears, that can be refer- ence of the Variegated Clays and the Polizzi red to the Troina facies (Cf OcNreeN, 1960). Formation (s.1.)is for a large part coeval with What in the legend was named Piedi- the Mt. Soro sequence. Therefore, they be- monte Formation has, on the contrary, been long to two different basins. Moreover, the identified - during studies made after the various facies of the Polizzi Formation are - not lateral variations of a pre-flysch deposit, maD was orinted as an external Calabride'it uniì (cf Òenvrs cr?'Noet alii, 1981b); is but can often be interpreted as more or less therefore no longer to be considered a part of detrital sequences depending on the proxim- the Sicilide units. ity of the overthrust fronts. Generally Eocene The Variegated Clays sometimes contain calcarenite facies precede argillaceous- repeated large scales of Nurnidian Flysch. arenaceous facies attribuitable to Oligocene. These rest tectonically upon different levels of the same Numidian Flysch in the Northern Chain. To the South of Centuripe they rest Tne Mr. Sono UNrr directly upon the Mt. Judica Unit and com- prise part of the Gela Nappe in the Gela- The Mt. Soro Unit crops out in the north- Catania Foredeep. ern part of the peripheral belt of Mt. Etna. The Sicilide Unit is represented by ter- Resting either upon the Variegated Clays or rains deriving from more internal domains directly upon the Numidian Flysch, it is a than the Panormide areas. OclrrsBN (19ó0, sequence which passes from an alternation of 1963 and 1969) differentiated the Sicilide calcareous and argillaceous beds at the base Unit into two tectonic nappes: the lower, the into alternating argillaceous and quartzare- Troina nappe consisting of Eocene to Middle nitic beds aî the top; consequently it is possi- Miocene sequences (Polizzi Formation with ble to distinguish at least two members: othe the lateral variants); the upper, the Cesarò argillaceous-calcareous member" the lower nappe, was made up by Upper Jurassic to one, and uthe argillaceous-arelaceous mem- Cretaceous terrains (including the Mt. Soro ber" the upper one. By Vezzeur (1973) it was I lvscn' t. vertically divided into three intervals, from All these terrains were said to represent the bottom to the top: the argillaceous- an original unique sedimentary sequence dis- calcareous member, the argillaceous- membered and later overthrust. That cannot arenaceous member, and the quartzarenitic be accepted because the sedimentary sequ- member.
Fig. 7 - Stratigraphic columns of the Calabride Units. On the left there is the most external succession,made up of the Piedimonte Fm., strarigraphically overlain by the Capo d'Orlando Flysch. This terrigenous sequencehas probably never been reached by the Calabride nappes, but the sedimentological characteristics (slumps, debris flows and osedimentary klippen, belonging to the Taormina sequence) testify to the strong deformations undergone by the other units during Eocene. The Capo d'Orlando Flysch oseals" the contacts between the nappes and it is tectonically overlain by the Varicoloured Clays. The uFloresta Calcarenites, were deposited after this tectonic event. Piedimonte Fm: Ea = Eocene pelitic member; Eaa = Eocene pelitic-arenaceous member; Eac = Upper Eocene arenaceous-pelitic-conglomeraticrnember, characterized by slumps and, at the top, blocks deriving fiom the Taormina sedimentary succession;Oac = conglomeratic-arenaceousmember with large channelized bodies. At the top: the Capo d'Orlando Flysch (OM). Capo S. Andrea Uniti m1 = l't metamor- phic structural level; L: Vermcano facies (Lower Lias); Lc = massive and crinoidal limestones, and coquinas with Bositra buchi (Lias-Dogger);ra = RossoAmmonitico facies; b = nMajolica, facies; sc = Scaglia facies with (sedimentary klippen' of the Taormina sequence; m2 = 2"d metamorphic structural level. Longi-Taoftnina Unit'. b2 = 2"à rfieÌamorphic structru.al level; L = Velrucano facies (Hettangian); Li = Lower Liassic limestones and dolomites; Lm = nMedoloo facies (Middle Lias); G = Rosso Ammonitico, radiolarites (Upper Lias-Malm); Ci = Biaúcoúe facies (Malm-Lower Cretaceous); CE = Scaglia facies (Upper Cretaceous-Eocene);m3 = 3d metamorphic structural level with blocks belonging to the S. Marco sedimentary cover. The Capo d'Orlando Flysch at the top (OMc). S. Marco Uniti this sequencewas restored in the north-western part of the Pelo taíi Mts. m3 = 3'd metamorphic structural level; L = Veúucano facies; Lcd = dolomites, algal or crinoidal limes- tones with brachiopod and ammonites (Lias-Dogger); ra = Rosso Ammonilico (Upper Jurassic); sc : Scaglia facies (Cretaceous-Eocene).Rocca Novara Uniti Gcd = Upper Jurassic dolomites and algal limestones; Cm = marls and marly limestones with Aptici (Lower Cretaceous);sc = Scaglia facies; Ec = (red co[glomerate] (Èocene).Capo d'Orlando Flysch (OM): mi = 3'd meLamorphic structural level; c = conglomeratic levels; ac = arenaceous-conglomeraticÌevels; sl = slurnps; aa = argillaceous-arenaceouslevels. IHE CEOLOGY OF THE MT, ETNA BASEMENT t5
FroREsî^ F= CALCAFENTTEB tî4F-. -.-{
I
t
z conaroh.r.tic-.r.n..úr. 9 o z . É o o chalomr.tlc ò t! n.hb.r aF o o = o- Ff,liric-.8n.c.6u. n.mb.r
îz:Dl.n^r-rs,Pàot. S.cton
(.1t.. aoNARDt .t .t,,teeot
= E z 2 f = - e E o a f (.ft . ca!'raracraNo 6 2 o z
6 iJ
c,s. aadr.. ,rf't!r.i.Àocc,t.Eùt. S.o.ron
(.tr.r
Fig.7 lo FABIO LENTIM
To this sequenceof Cretaceousage, the Peturizzo Unit and the Mt. Trearie Unit can name Mt. Soro Unit has been given (LeNrrrr be different sequences of a single flysch & Vez2ntr, 1978):lt continuesupwards with basin. the Mt. Pomiere Formation made up by cal- cirudites and calcarenites probably Eocene- Oligocenein age, that is unknown in the Mt. RerrlNo FrvscH Etna map area. This does not appear in the Mt. Etna The entire sequencewas considered in map, but crops out further west near Troina. the geological literature as belonging to the It is important to note that this flysch, and (Ocumer.r, Sicilide Unit l9ó0), of which it ori Oligocene-Lower Miocene argillaceous- ginally formed the lowest stratigraphic inter- arenaceousor conglomeratic alternance, lies val (see the stratigraphic sequencesin the quite conformably upon the Polizzi Forma- Mistretta map; Vezzeur,1973); the successive tion of the Sicilide Unit in the extemal oart phasesof orogenic transport, then causedthe and unconformably both upon the (Cesarò overthrust of this formation Nappe Variegated Clays and the Mt. Soro Flysch in òr oArgille Scagliose superiori,) upon the the internal part. So that it
Fig, I - The Miocene-Pliocene succession of Centuripe. From right to left tlere crop out Tortonian clays, Messinian evaporitic levels, Lower Pliocene Trubi and ùrconformably Lower-Middle Pliocene bluish clays gmding up to the Centuripe sandstones. THE GEOTOGY OF THE MT, ETNA BASEMEN'I t7
Pescrop out. Three units are recognizable,all blocks, containing quartz pebbles, very simi- cropping out in the north-eastern sector of lar to those found in the western sectoi of the the map. They are made up by a low grade Peloritani Mts (zone of S. Agata di Militello, chrystalline basement, corresponding to S. Marco d'Alunzio, Longi, Rocche Rosse semischists and provisionally subdivided near Galati, etc.). into Metamorphites I, II and III, according to . The units are tectonigally overthrust by position. - their structural other nappes which do not appear in thL The lowest unit is called Unitò di Capo S. Andrea ar;.d is formed by the first Metamor- phic Structural Level and by a condensed and incomplete sedimentary series going from Lower Lias to Cretaceous-Eocene.Thii sequence starts from the base with continen- tal sandstones and conglomerates (uVerruca-
glomerate> probably Eocene in age, and in- terpreted as a udebris flow" and as a proxim- al deposit coeval with the early iectonic movements. co facies limestones, Majolica facies rnicritic The other nappes, the Mandanici Unit limestones and Scaglia [aciesmarls. The Taormina llnit is characferized, by a thicker and less condensed sedimentiry series going from Lower Lias to Eocene anà
The Calabride nappes were emplaced in Eocene times, and now they are covered un-
glomerates of continental environment ( age ranges fiom Upper Lias to Eocene.A thin flyschoid level occurs on the uppermost in- terual. 'îhe S. Marco tlnit is characterized by the 3rd Metamorphic Structural Level (m3), à low-grade sericite, chlorite schists inier- data and because of its characteristic tectonic- layered with quartzites and metarkoses. sedimentary evolution, this formation can be The original sedimentary cover ls repre- referred to the most external calabride sented only by occasional red calcareous sedimentation areas. l8 FABIO LENTINI THE ANTIsrcrLrDE UNrr ero FLORESTA .CAL- (Tortonian age). Brecciated Clays occur at CARENITES> the top. The formation is widespread in the SW part of the map, but is probably still pre- A small limb of this structural unit croDs sent under the lava covers. out just in lhe extreme corner of the Mt. Etna Gypsum, .basal limestone,, .Tripoli", and map. Brecciated Clats. Towards the top the forma- It is composed of varicoloured clays with tion described above passes into the f, s Ef FABIO LENTINI sandstones, quartzose sands and silty clays, to the pelagic sedimentation (the so-called which form the outcrops around the town of bv C,rrer-aNo & D'AÉceruo (1982). They ii) the passageto .flysch-type, Centuripe. Their age is most probably Upper- "àrowning,); Middle Pliocene. sedimentation (fig. 10). Bluish marly clays- In the southern sector Another fundamental event is the over- and along the Ionian coast bluish marly, thrusting of the nappes. The difficulty of re- sometimes silty, clays crop out with no clear constr-ucting the evolution is increased by the stratification and containing macrofaunas repetition of the deformative phases; some- wifh Arctica islandica and. Chlamys septemra- times one needs to separate the earlier defor- diata. MicroÎawnas with Hyalinea bahhic&, mations from the later ones. The most recent Bulimina etnea, anò Globorotalia truncqtuli- involve the previous ones and modily the noides indicaling Lower and Middle Pleis- contacts between one unit and another. At tocene age, are common.. times two overlapping and welded nappes The same pelitic sequence fills the Gela- form one single unit in subsequent thrusting. Catania Foredeep, where petroleum bore- This occurs according to a mechanism holes have gone through a thickness of from whereby the compression front is displaced ó00 to 1000 m of marly clays, sometimes both in time and space. with volcanic intercalations. South of Cata- In the same way, it seems that the fore- nia a borehole drilled about 730 m of a peli- deep migrated towards the external areas, tic sequence, only the last 120 m of which,are and was progressively created at the expense referable to Pliocene age. of the African continental margin. Values of crustal shortening have been Sands, sandstones and conglomerates. The measured; the total amount is certainly grea- most recent terms of marine Quaternary are ter than 300 or 400 km. yellow quartzose sands interbedded with Even if the movement between the diffe- well cemented sandstones and polygenetic rent structural-stratigraphic units is relative, conglomerates containing quartzarenitic it seems more convincing that the extemal clasts. Poor and stratigraphically insignifi- areas moved towards the internal ones; there cant macrofaunas with Ostrea edulis and mic- was a contemporaneous displacement of the rofaunas with Elphidium spp. and Ammonia cornpressional front whereby the external spp. occur. Their age is most probably Mid- areas were progressively deformed. That can dle-Upper Pleistocene. better explain the formation of such exten- sive nappes as the considerably plastic ones of the Variegated Clays, Mechanisms like ogravity PAIAEOTECTONTCAND PALAEOGEOGRAPHIC those used to. explain nappeso do EVOLUTION not seem convlnclng, nowever. The meaning of nallochthonouso also be- comes very elastic and difficult to define. It is The analysis of the facies and of the preferable to say that a unit is nmore or less geometrical relationships between the struc- allochthonouso or (more or less autochtho- tural stratigraphic units allows us to rewrite nous,. It can happen that some units are the palaeogeographic and palaeotectonic his- doubly allochthonous, or even more, because tory of Eastern Sicily. they have been involved in two or more tec- The whole geological feature is given by tonic deformation processes. a southverging nappe-structure (Northern For a reconstruction of the palaeogeog- Chain) piled on the foredeep (Gela-Catania raphic evolution it is possible to adopt the Foredeep) and against the Iblean Foreland. same schema as that published by Carat-aNo The Northem Chain nappes derive from the & D'ARcENro (1978 and 1982),with some dif- deformation of deposits related to different ferences, in that some structural- original palaeogeographic domains. There- stratigraphic units cropping out in North- fore, two events are of particular importance Eastern Sicily are not present further west. to define the evolution of the original basins For example the Calabride units are con- and the tectonic-sedimentary evolution: i) siderably more internal than those studied the transition from shallow water environ- underwent the early tectonic phases referred ment (i.e. the carbonate platform condition) to Eocene times. So that for that domain the THE GEOLOCY OF TIIE MT. ETNA BASEMENT 2l collisional stage began already in Early Ter- tic carbonate or non-carbonate sediments. tiary. Many conglomeratic and calcarenitic hori- zons of Early Eocene were interpreted wrongly in the previous literature as evi- 1.. Continentq.l Rlfiizg (Middle to Late dence of a general coeval transgression (Cf Triassic). for example, Ocumer, 1960). Exiept in tLe Calabride UniLs, where slrong delormalion with overthrusts give rise to a number of dif- ferent structural units, in all the other areas there is only evidence of pre-orogenetrc tecto_ nlcs. Unit). During the Carnian-Norian trmes eva- poritic to rimned carbonate platform facies 4. Continental Collision (Late Eocene prograded to fiom the basin towards the sur- Miocene). rounding emergent areas, giving rise amongst other successions to the panormide The present structural features of Sicilv platforms. In Eastern Sicily clastic deposits and in particular of North-Eastern Sicily arè in intracratonic basins sometimes grade into the result of complex and strong defoima- a carbonate platform or at other basiíal tions of the sedimentary covers deposited on cherty carbonate sequences (Mts. Iblean and different palaeogeographic dornarns, as a Mt. Judica Units). consequence of collisional orogeny. Several tectonic phases followed on each other in Eastern Sicily from Late Eocene to 2. Oceanic rifting (Jurassic to Cretaceous). Middle Miocene. It is possible to date these phases by means of the topmost levels of the The second stage distinguished by Cerl- underlying units and of the flyschoid covers raro & D'Ancpuro (1978; 1982) is that of the which rest on the nappes, (seaiing, the tecto- Tethyan Oceanic Rifting. The tensile stress nic contacts. The main phases were relatable field is expressed also by alkalibasalt volca- to the Late Eocene, to the Oligocene-Miocene nics (Mts lblean and Mt. Judica Units) and boundary, to the Lower Mioclne and to the by the normal faulting which affected the Middle-Upper Miocene. ,continental margins. A general expansion of the pelagic en- vironments is noted. Progressively the va- 5. Late Deformation (Lafe Miocene-Early rious shallow-water sequènces t.l.rd..go ,o- Pleistocene). called "drowning,. Among the more recent there are some lblean areas and the panor- To the Pliocene-Pleistocene deposits a meaning of posl-orogenic mide platform. There is much evidence to units was given by geological suggest that the Mt. Judica and Imerese the literature in the past. Now, many studies basins tended to enlarge. In the same way, lestifo during tlis time interval some Calabride units, where carbonatè- a southward progressive shortening of the sedimentary covers. The platform-type condirions prevailed, only in Caltanìssetta Basin tne Late Lreîaceous underwenl. transition [o pelagic environments (Cf R. Novara Unit in I\TZOR,Tet AIrl, t9I / ). 3. Oceanic Sealing (Late Cretaceous to Early Gela Nappe. Tertiary). It is interesting to calculate the crustal shortening occurring from Upper Miocene During this time interval important onwards. In the extemal zones of the Calta- changes occur. There is a consistent inversion nissetta Basin, as for example those of Mt. of the expansion tendency. In almost all the Crunici, shortening of one third has been me- domains evidence of active tectonics is given asuredj but the shortening calculated on the by rnegabreccias, slump structures and clas- front of the Gela nappe, in the foredeep, FABIO LENTINI (Mineo area), shows that the original exten- presence of Mt. Etna does not noticeably al- sion of 18 km has became less than l0 km, ter the pattern of the crustal structures in the only from Upper Tortonian to Present. area, as the gravimetric data also show (Cf CoLoMBre1 alii, 1979).Indeed , in a certain sense the regional geological structure be- neath Mt. Etna can be considered an east- CONCLUSION ward continuation of the Caltanissetta Trough; in fact it has a tectonic-sedimentary The vast volcanic and alluvional covers evolution and regional uplifting mechanism of the Plain of Catania do not allow any analogous to those of Central Sicily. That direct observation of the sedimentary sequ- means that a knowledge of the geological- ences forming the Etnean basement, there- structural characteristics of the Etnean sub- fore one must extrapolate, correlating with stratum is of fundamental interest for under- areas further west and colleóting all the data standing the modifications the volcano has yielded by oil boreholes and making use as unqergone. well of the general framework which is better A final consideration can be made on the known in most o[ the southern Apennines. shape of the crust-mantle surface along a N-S All these data and those collected by the direction, that is to say transversally lo the geophysical studies allowed us to produce a main structural units. The crustal thickness, bidimensional profile of the crust under the typically continental in the Iblean area (circa volcano (CRrsroFouNr et alii, 1979b). This 30 km) decreases at the Trough, and there- profile corresponds to the 1:300,000 scale fore in that case beneath Mt. Etna, and then profile printed in the geological map of Mt. abruptly increases again beneath the North- Etna, but further information has been in- ern Chain. In this latter part in particular cluded in this profile. This allows us to have low velocity layers are to be found: however, a more complete picture of the general struc- it is not safe to attribuite these to Etnean tural characteristics in which Mt, Etna is set. magma infiltration. Certain considerations can be made (see also To the thinning of the crust beneath the fig. 4 in: Lol,rnarDo & PArANÉ,this volume), Etna Mt. (where the Caltanissetta Trough There is proof of a separation surface be- continues eastwards) there is a good corres- tween the Iblean carbonate massif - and pondance with an original sedimentary cov- therefore of the African continental crust - er, deposited in wide marine basins (such as and the nappes of the Northern Chain; this the basinal sequences which occur in the Mt. separation surface dips beneath the Northern Judica Unit and Imerese Unit). That could Chain with a dip of approx.8-10'so that suggest that an oceanic, or at any rate a thin- under the Central Crater of Mt. Etna it would ned, crust was located during Mesozoic- be at a depth of about ó or 7 km below sea Cenozoic times between the Iblean continen- level (i.e. 9 or 10 km below the Central Crater tal crust (African Plate) and the units of the itself). Also the evaluation of the data col- Northern Chain; furthermore, at present, it lected by Cor-orraer,Guenna, Luonco & Scar- could have partly disappeared because of escu (1979) along the DSS profile Acireale- subduction, or have been extremely shor- Termini Imerese reveals a discontinuity at a tened and buried by the several nappes fill- depth of 8 km, interpreted by those Authors ing the trough and the foredeep. In this case, as the so-called crystalline top. The same dis- a subduction may have affected a thinned continuity is also revealed by gravimetric crust and not, as might be suggested from a analyses (Meor & Monellr 1952; Prr.rNe & first glance, a continental crust. Rapolu. 1979). A picture of apparent general compress- Another consideration to emerge ftom ion emerges in which it is difficult to set Mt. the crustal profile is that the origin and de- Etna and its eruptive events. For a better velopment of the magmatic activity must understanding of the position of the volcano necessarily be deep and therefore it is sited in the framework of the regional structure, beneath the African continental crust. one should adopt a three-dimensional profile However, the volcanic structure in the sense rather than a two-dimensional one. of several overlapping lava layers, must be The data are not sufficient to allow an considered very shallow at this scale and the easy interpretation of the regional tectonic THE CEOLOCY OF THE MT, ETNA BASEMENT pattern and of the place of Mt. Etna within borated a satisfactory model, based on a the framework of these characteristics. Some great number of data. previous Authors, among whom BenseRr e1 Certainly the presence of Mt. Etna is con- alii (1974) have made an attempt at this. nected with the considerable structural com- According to these Authors, Mt. Etna is plications met with in the Ionian border of the expression of tensile tectonics and markE Sicily as well as with the geological frame- the contact between the European and the work which appears two-dimensionally in African plates. the I :300.000scale orofile. Also Cnrsroror-rNr et atii (lg7g a) and re- The most rec"nt studies show that the cently GHrsErrr & VezzeNl (1982) tried to Strait of Messina and the Abyssal Plain of establish the relationship between tectonic Messina to the south of it were eenerated bv stmctures and the presence of the volcano. tensile fault systems connected ùith a een.i- According to them, in fact, Mt. Etna is lo- al clockwise rotation of rhe whole Calabro- cated where the Mt. Kumeta-Alcantara Fault Sicilian Arc (Grlrsarrr & Vezz,qr.rr,1979). is linked to an E-W deep-seated shear zone, The studies carried out off-shore on lhe and its major conjugate fault systems (the oMalta Escarpment> (SCANDoNEet alii, l98l) NE-SW Messina-Giardini and the NW-SE and those made on-shore on the deposits of Tindari-Letojanni faults) intersect each Miocene and Plio-Quaternary (Gcesso el alii, otner. 1979; Gusso & Lemrur, 1982) suggest that As can be seen, it is considerablv difficult the Syracuse-Malta Scarp is a NNW-SSE to ofler interpretarions withoui havine trending fault-system marking the border o[ cìeared up certain problems and having ela-- the lonian Bathyal Plain and that rhis can be tauarìoìùrí! E tAt ARTA latpes /%/ /k r-4:> _ Fig. 7i Essential schema of the relationship b€tween the most external nappes,the Gela-CataniaForedeep aDd the Iblean Foreland. The Iblean Forelanà (i.e. rhe sicilian margin of ihe africu" piat.t ì;;ff.-;t""d-;; downfaulting which has produced the Gela-catania Foredeep.The màst advanced napp.. r".r t".oni"lUv the Plio'Quaternary horizons of the Iblean sequence.Several geological profiles allovr' * t. frvp"ifr"J.'tfrài""paí sìmple flexing of the crust is sufficient Lo give rise to a superlicial-extension with norrmal faíits, ;hri; i;;; deep levels of the crust the same fìexing could be accoàpanied by compre restorations show that the Plio-Pleistocenevolcanism (v) of tire present-day ILle same area was still not a margin, but had the ròle of a Now this volcanism is no longer active, probably becau Etnean volcanism, now active, could be linked to totu trends, which cul transversally the main systems of thr FAEIO LENTINI considered a collapsed continental area. The Peloritani (Sicilia ftord-olientale).Boll. Soc. Geol. trend of the Syracuse-Malta Scarp extends as Ir.,96,331-338,4ff., 1 tav. far as the Etnean cornplex forming the oTim- BaRBERTF,, INNoCENTIF , MARINELLIG, & MAzzuolr R, (1974) - placehe: pa di Acireale, fault systems. CanaoNr,, Vulcanismo e tettonica a esempi nell'area meilitetanea. Mem. Soc. Geol. It., 13 (2), GRASSo & Lenrnrr (1982) claim that the 327-358,18 ff.,2 tab. whole trend is fairly recent in time can 4nd BoNARDTG., GruNra G., PERRoNEV , Russo M., ZuppET- still be considered active. Moreover, it seems rA A. & CrAMpoG. (1980) - Osset-vazionisull'eeolLt- to be migrating westwards and producing a defotmation in the (Southern Calabrian Arc ltaly); MEDÌ E. & MoRELLTC. (1952) - Rilieeo ífiplications a sral)iftetrico del- for seismotectotticzoníng. Tecío- la Sicilia. Ann. Ceofis., 5 (2.),209-145. nophysics, 85, 149-165. OcMerN L. (1960) - Nota tllustratita d.ello GRAssoM. & LENrrNrF.(1982) - schema geo_ Sedhtenta\t and tecto- logico della Sicilia nord-oientale. Rjv. Min. nic eyolution of piateau Sic.. úe eastenl Hjblean (5.E. 64-65, 183-222,2 í. 2 îf, Sicily) d.uing Late Cretaceous to euateftlary time. (1963) Palaeogeogr. pal^eoec' OcNrBe\_L. - Le fomtazioni lipo rgildflysch delle Palaeoclim. 39, 261-2g0,. (Sicilia Amsterdam. Madonie centro-settenrionole). illem. lst. Geol.Miner. Univ. Padova,24,58pp., 15 ff. GRAssoM., LENrrNr F. & pEDLEyH.M. (l9gD _ Late - OGNTBENL. (19ó9) - Schema introduttivo Totlon i an Lower M es s i n i an (M íocene) pala eogeo- alla geolagia del confine calabro-Iuc(1no.MeÍ\. graphy of S.E. Sicily. Infomation two íew Soc. Ceo,-I.tt.i8, -Íoftnatiofis ftoà (4),453-763,183ff., 2 of the Softino Groap. Sedimentary rav. Geoloey 32,279-3OO.6 ff. Paracca E., ScaNDoNEP,, GIUNTAG, & LrcuoRr V. (1979) - GRAssoM., LENrrNrF. & Vezzar.rrL. (197g) - Lineatnen- Mesozoic paleotectonic evoluîion of the Ra_ ti stratigtufico-strutturali delle Madonie (sicilia gusa zone lS.E. Slcily). Ceologica Romana, lt, 331-3ó9. centrc-settehtrionale).Ceologica Romana, 17, 45_ 69, I carta geologica f.t. PrNNAE. & R{poLLAA. (1929) - Struttutu cl.ostalenell'I_ Gnrsso M., LENTTNTF., LoMBARDoG. & Scmlenol G. talia fieidionale da dati gravíftetricL C.N.R. prop. - 11979) Distribuzione delle facies cretaceo- Fin. Geodinamica,pubbl. n.235, Giannini Edità- mioceniche lungo I allineamentoA ug*s!a_M - Lauro re, Napoli. (Sicilia Bolt. -Sud.-Onentale). Soc. C;ol. lt,, 9t, l25- ScorooNr P., PATAccaE., RADorcrcR., RYAN 188,9 ff., I rav. W.B.F.. CnA M.8., RawAsoNM., CHEZARH., Mrr_r_:nE.. Lr\Ì_Nf_ F. - ll974l Caruîtei stratigafici e sttutturali Mr re.rzreJ. & RossrS. ( t98l) - Mesozoícand Ce_ ctella zona di M!. Judica lsicília Orientale). Rìv. n-ozoicrocLs from Malro Escarpment lcentral Me_ Min. Sic., 145-147(25), l-20, 13 ff., 3 rav, clireftaftean).A.A.P.C. Bull., ó5, 1299_1319. LsiùrrNr F. & VEzzaNr (1975) - L. Le unità meso- ScHMrDrDr FREDBERGp. & TRovò A. (1962\ - Contribu- cenozoichedella copertura sedifientaria dcL basa- tion A l'éîude st/tlctutale du groupe du M. Judica mento-caístalli no peloi !ano ( (Sicile Sicilia nord-orientalel. orientald. Bull. Soc. Gèol. France, Ser. VlI. Boll.Soc. ceol. tt.,94,537-554,6ff. 4,754-759,| î. LENrrNrF. & lEzzliut L. (1978) - Tentat^,o di elabora- VEzzANrL. ( 1973)- Note esplicativedella Carra Galogi_ zione di uno schema strutturale della Sicilia oien_ ca d ltalia, Foglio 611 lMistretîa) scala I:50.0ó0. /ale. Mem. Soc. ceol. Ir., 19, 495-500, I f. 3t pp., 2 tf.