birth and early evolution of a jurassic escarpment: monte kumeta western sicily

[ FACIES 46 I 273-298 : - - . _ i P I . 4 7 - 5 0 1 0 F i g s . , - - _ E r l a n g e n 2 0 0 2

Birth and Early Evolution of a Jurassic Escarpment:

Monte Kumeta, Western Sicily

Piero Di Stefano, Palermo, Andras Galacz, Budapest, Gianni Mallarino, Palermo, Andrea Mindszenty and Attila V6rEs, Budapest

KEYWORI)S: CARBONATE PLATFORM - PEI.,AGIC I>LATFORM I!SCARI>MKNTS - UNC()NFOI{N'IITtt"S - SICII,Y -JLiRASSIC

SUMMARY

The accurate reconstruction of the facies architecture in the Jurassic succession of Monte Kumeta. coupled with a detailed biostratigraphy, allow to define dynamics and genetic factors controlling the conversion o f a Bahamian- type carbonate platform to a pelagic escarpment.

A change from tidalites to oolites i.e. from the restricted, interior lagoon to a more open-marine sandy depositional environment, records the establishment of a basin south of the Monte Kumeta sector in late Hcttangian- Sinemurian times.

The oolitic limestones are overlain by earliest Carixian bioclastic grainstones and packstones with micritized grain s and by wackestones with radiolarians and sponge spicules, organized in thin sand prisms. The decrease of carbonate productivity indicated by these sediments records the dissection of the platform and the subsequent isolation of a submarine topographic high in the Monte Kumeta sector.

Though based only on indirect evidence, it is suggested that a tectonically controlled scarp must have existed between the Monte Kumeta "high" and the basin. Progressive northward retreat of this scarp resulted in the conversion of a shallow platform sector into a gradually steepening slope, along which the distribution of sediments was controlled by repeated tectonic and gravity- induced modifications of the topography of the substrate. Vertical and lateral changes and geometrical relationships of the recognized lithe facies suggest that they were deposited on a stepped surface brought about mainly by, repeatedly reactivated basin ward dipping normal faults.

This scenario is clearly reflected by the relationship of platform strata and the overlying encrinites of Carixian/ Domerian age. The encrinite bodies show again a prismatic geometry, becoming thicker towards the south and filling the first generation of neplunian (lykes.

The top of the encrinites is marked by a peculiar jaggcd dissolution surface wifh din-scale pinnacles capped by a thick ferromangancse crust. The formation of this peculiar surface could have been controlled by complex changes in water chemistry probably related to the Early Toarcian anoxic event. "Fine crust itself is dissected hy fauhs of dccimetres to metres of throw, sometimes organized into small-scale positi\, e tlowcr structures. In the hollows/depressions of this highly articu- lated substrale pelagic sediments ol'B~jocian Io ()x fordian age wcre deposited. They display a clearly onlapping relationship to the encrinites and It) the carbonate platform beds. Their thickness rarely exceeds 4 to 5 meters and they arc present also as neptunian dykes filling a dense network of lissurcs.

During l.atc Callovian and ()xfordian times synsedimentary tectonics has intensified resulting in an increase of the inclination of the slope. This led to more and more abundant, gravitationally controlled dcformations (slumping and sliding) of semi-lithified and unlilhified sediments along the Monte Kumeta escarpment.

1 INTROI)I:(;TION

The cast-west running ridge o f Monte Kumcta, rising majestically above the picturesque town of Piana degli Albancsi (= Piana dci Greci) is not only one of the spectacular highlights of the Western Sicilian scenery, but is also an cxceptional place to understand Ihe Mesozoic evolution of the wider region. Within a rather restricted area it gives the opportunity to study all-important Lower and Middle Jurassic lithofacies and associated stratal geometries and uncon fortuities offering an insight into the conversion of a carbonate plattornl into a pelagic swell-and-basin environment. The wire-cut tcchnof ogy used to extract large blocks of ornamental stone in the quarries on the mountain provides a unique, three-dimensional spcctaclc of the finest details of facies architecture and stratal geometries. These circumstances lacilita(ed the rel inc~

Addresses: Prof. P. Di Stefano, Dipartimento di Gcologia e Geodesia, tJni~ ersitgl di Palermo, Via Archirafi 22 - I - 90123 Palermo (Italy), e-mail: [email protected]; Prof. A. Galficz, Department of Paleontology, Eotvos Lorand tini\ersity, H-I117 Budapest (Hungary), Pfizm:iny PEter sEtfiny l/C, Hungary, e-mail: galaczOludens.elte,hu: Dr. G. Mallarino, l)ipartimcnto di Ge~qogia c Geodesia, L.'niversit5 di Palermo, Via Archirafi 22, 1 -90123 Palermo. Italy. c-mail: gmallarinoLa~usa.net: Prof. A. Mindszenty, [)cpartn3crU of Applied and Environmental Geology, Eotvos Lorand University H-1117 Budapest, Hungary. e-mail: an( re~ r qr s Feeble e te.hu Prof. A. VOriis, Department of Geology and Paleontology, Itungalian Natural History Museum, 1t-143l Budapest. P.O. Box 147 (Hungary), e-mail: [email protected]

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ment of previously developed reconstructions (e.g. Wendt, 1963, Catalano and D'Argenio 1982a,b, Abate et al. 1990), which have had to rely on accidental observations and analogies with studies from other localities of the same paleogeo~aphic unit.

Besides the quarrying activity mainly concentrated on the Jurassic red limestones known as "Rosso di Piana dei Greci" the site has been also an important locality for Jurassic fossils, mainly ammonites. Wendt (1963) was the first to recognize Toarcian and Middle Jurassic ammonite assemblages in loose blocks around the quarries at the southern side of the hilltop. In 1965 he listed Lower Toarciaq ammonites from a fissure-filling red limestone penetrating the Middle Liassic carbonates, and gave an extended faunal list with armnonites from the Aalenian up to the Callovian. These specimens came fl'om ferromanganese crusts attached to red nodular limestone (i.e. Rosso Ammonitico) blocks lying astray on the quarry ground.

The fundamental work of Wendt in the 1960's raised a wider interest (see Jenkyns, 1967, Jenkyns and Torrens, 1971, etc.). The perfect exposures in the quarries attracted several field excursions (e.g. Abate et al., 1982, 1990), and stimulated detailed sedimentological investigations and ammonite studies (Gal~cz, 1985, 1999; Di Stefano and Mindszenty, 2000; Di Stefano et al., 2001; Mariotti et al., 2001 ).

Our results of stratigraphical and sedimentological studies, focused on the transition between the Inici Formation and the overlying pelagic Rosso Ammonitico, have revealed that the studied formations record an example of transformation ofa Bahamian-type carbonate platform environment into a pelagic escarpment. The three-dimensional nature of the outcrop permitted the recognition of the complex lithofacies variations and the sequence of polyphase rigid/ ductile deformation structures accompanying this transformation. It became also clear that there are more than one ammonite-bearing crusts within the red limestone, and that Toarcian ammonites are associated also to the main, pinnacled hard-ground on top of the Calcari a Crinoidi. The new collections carried out parallel with sedimentological studies, helped to date the stratigraphic discontinuities within the Rosso Ammonitico inferiore, and the well-preserved, biostratigraphically diagnostic ammonites allowed dating precisely the most important episodes of the sto W .

2 G E O L O G I C A L F R A M E W O R K

Monte Kumeta is located in the central zone of an E-W trending ridge (to which we refer below as Kumcta Ridge) extending for about 20 km in western Sicily. This ridge belongs to a major structural unit of the Sicilian-Maghrebian chain in the southern sector of the Palermo Mountains (Monte Kumeta unit, Fig. l b), and it derives from the Neogene deformation of a paleogeographic unit known as the Trapanese domain (Catalano and D'Argenio, 1982a). The Late Triassic paleogeographic reconstructions account- ing for the stratigraphic and structural relationships in the Sicilian-Maghrebian belt indicate that the Trapanese do-

main formed as part of a wide Bahamian-type carbonate platlbrm along the African passive margin together with the adjacent Saccense and Hyblean domains (Catalano and D' Argenio, 1982a). In of[shore southern Sicily (Sicily Chan- nel) the continuity of this platform is well known fiom wells and seismic lines from the Malta escarpment to the Egadi Islands (Antonelli et al., 1991 ). This platform, named Siculo- Tunisian platform by Di Stefano et al. (1996), was connected northwesterly to the Panormide carbonate platform of northern Sicily, giving rise to a shallow-water embayment that was bound to the north and east by wide slopes to basinal areas, formed by the Imerese and Sicanian domains respectively (Catalano et al., 1991, 1993, 1996).

Transcurrent movements along the African margin, related to the opening of the Alpine Tethys (Stampfli and Mosar, 1999) were responsible for the dissection of the carbonate platform and the opening of intraplatform basins during latest Triassic - earliest Liassic time (In fraliassic basins, Catalano and D'Argenio, 1982b). During the Middle - Late Liassic a large part of the carbonate plafforln was drowned, and pelagic deposits blanketed a complex system of small basins, swells and tilted blocks (Jenkyns 1970a, see paleogeographic sketch in Di Stefano and Mindszenty, 2000, p.40).

The stratigraphy of Monte Kumeta has been elucidated by several studies following Caflish (1966) and Mascle (1979). Wendt (1963, 1969) has provided detailed data on the stratigraphy of the Jurassic deposits.

The basal part of the succession (Fig. 2) consists of more than 300 m of Lower Liassic peritidal to open-shelf limestones and dolomitic limestones (Inici Formation auct.) that are deeply crosscut by neptunian dykes. The Inici limestones overlie a thick, unexposed substrate of Upper Triassic platform dolostones and are unconformably overlain by the Calcari a Crinoidi, consisting of white to pink, 0 to 15-20 m thick encrinites of Middle Liassic age.

Pelagic sediments of Late Liassic - Tithonian age follow upward. The stratigraphic subdivisions and nomen- clature of these deposits in the Trapanese structural units have not been formally established yet. They show a high lateral variability in relation to their paleoenvironmental position (pelagic platforms, escarpments, basins) and have been informally named as Rosso Ammonitico (Catalano and D'Argenio, 1982a). Antonelli et al. (1991) have in- cluded all the Jurassic deposits from the Hyblean, Saccense and Trapanese domains in the Buccheri Formation (Patacca et al., 1979).

In the Monte Kumeta area the Rosso Ammonitico deposits can be easily subdivided into three units (Abate et al., 1990, Di Stefano and Mindszenty, 2000 and the present paper). a) A lower unit (RAI = Rosso Ammonitico inferiore)

formed by up to 6 m of massive, reddish, condensed limestones ranging from the Toarcian up to the Middle Oxfordian.

b) An intermediate cherty unit (MRI = membro radiolaritico) characterized by 0 to 15 m thick varicoloured bedded cherts and radiolarian marls of

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Fig. 1. a) Structural scheme of Sicily wilh location ol lhc studied area. b) Gcnerali/ed geological map of the Ktm~cta Ridge (rood. aftcr Abate el al., 1990); c) detailed geological map of the Monte Kumeta tr with localion of the sludicd outcrops.

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Middle Oxfordian - Lower Kimmeridgian age. c) An upper unit (RAS = Rosso Ammonitico superiore)

consisting of 10 to 15 m thick reddish to pink, partly nodular limestones and Saccocoma-limestone, with megabreccia beds and pebbly mudstone intercalations of Kimmcridgian - Early Tithonian age.

Upper Tithonian - Neocomian calpionellid-bearing cherty calcilutites (Lattimusa) and Lower Cretaceous to Eocene aptychus marls and calcilutites (Hybla Formation and Scaglia = Ameril]o Formation) follow upward. A deep disconformity on the Scaglia top is sealed by the Caleareniti di Corleone (Ruggieri, 1966) of Burdigalian-Langhian age, that is followed in turn by the Marne di San Cipirrello Formation (Ruggieri and Sprovieri, 1970) of Serravallian - early Tortonian age.

The Jurassic pelagic deposits are well exposed in a relatively small area on top of Monte Kumeta and on its southern slope (Fig. lc). The observation of the lateral development of these deposits is hampered by the peculiar structural setting of the area which is the result of a complex interplay between the Late Mi- ocene thrusting (Caflish, 1966, Catalano et al., 1978) related to the Maghrebian orogeny, and later (Pliocene) right-lateral transpressional deformations (Ghisetti and Vezzani, 1984). The Jurassic paleotectonic heritage, as will be discussed in the subsequent chapters, seems to have also played an important role in the tectonic evolution of this sector.

At present the Kumeta ridge appears as an E-W-trending positive flower structure. Trans- verse geological cross-sections through this structure (Abate et al.. 1990, Di Stefano and Mindszenty, 2000) show the southern flank of an anticline that is gently arched along its E-W axis and repeatedly crosscut by NW-SE and SW-NE faults. Eastward the Monte Kumeta succession is tectonically overlain by the Imerese structural units that consist of deep-water slope- to-basin Mesozoic - Tertiary carbonates and cherts and Upper Oligocene - Lower Miocene siliciclastics of the Numidian Flysch (Caflish, 1966, Abate et al., 1978). The overthrust is well exposed on the eastern sector of the ridge at Monte Leardo (Fig. l b).

North of the ridge, the Monte Kumeta succession is downfaulted and largely covered by the hnerese nappes. Also, to the south, the Mesozoic deposits are covered by Tertiary formations.

A help in the understanding the southward development of the Jurassic deposits is given by subsurface data coming from the Marineo well drilled about 5 kin ESE of Monte Kumeta. The Jurassic succession penetrated by this well has been interpreted as the filling of an infraliassic basin (Marineo basin, Catalano and D'Argenio, 1982b). It consists of about 400 m of radiolarian

and sponge spicules-bearing cherty limestones that are sand- wiched between the Inici Formation and the overlying Toarcian to Upper Jurassic cephalopod limestones. Based on its stratigraphic position, the age of this unit can be roughly attributed to the Sinemurian and Pliensbachian. Similar deposits are well known in the subsurface of eastern Sicily (Hyblean Plateau) where they are grouped in the Modica Formation and can be compared to the Corniola Formation of the Umbria-Marche Basin.

A recently published geologic section across the Kumeta Ridge and the Marineo well based on seismic profiles (Avellone et al., 1998) indicates that the severe Tertiary deformations occurring between these two sectors do not have altered substantially their mutual paleogeographic relationships. However, the amount of lateral displacement along the fault system bounding north and south the ridge, and the lateral extension of the Liassic basinal sediments in the subsurface south of the Kumeta Ridge are still poorly constrained.

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3 EARLY TO MIDDLE JURASSIC STRATIGRAPHY OF MONTE KUMETA

We will locus our attention on the stratigraphy of the Liassic-Oxfordian succession at Monte Kumeta that in- eludes the Inici platform deposits, the Calcari a Crinoidi and the lower unit of the Rosso Ammonitico.

3.1 Liassic carbonate platform deposits (lnici Formation)

The Inici Formation (Schmidt di Friedbcrg, 1964-65) is a 300-400 m thick succession oF whitish-brownish. partly dolomitic limestones that are common ly organized in shallowing upward cycles (Jcnkyns, 1970a). Locally they are transitional upward to oolitic-bioclastic limestones (Caflish, 1966, Jenkyns 1970a). The lagoonal depostis of the Inici limestones are rich in molluscs ("Megalodus" leonardii), algae (Thaumatoporella parvoresiculi[era, Cayeuxia spp., and in the upper part Paleodasycladu.s mediterraneus), and abundant benthic foraminifers as lituolids, textularids, valvulinids (Masele. 1979). The lnici Formation (equivalent to the Siracusa Formation of the Hyblean sector) generally overlies thick platform dolomites of Late Triassic age that are grouped in the Gela or Sciacca Formations (Patacca ct al., 1979, Antonelli el al. 1991). Equivalent deposits are present subsu,ilace in the foreland successions in southern Sicily and offshore (Antonelli et al., 1991),

A regional unconformity marks the top of the lnici Formation which is overlain by pelagic arnrnonitic and Bositra-limestones or crinoidal limestones. The age of the Inici Formation encompasses the Hettangian-Sincmurian. Arkell (1956, p.210) indicated a chronostratigraphic range from the Hettangian Planorbis to the Si neln urian Raricostat t,m Zones at Rocca Busambra on the basis of the rich assemblage of ammonites, gastropods, pelecypods and brachiopods described by Gemmellaro (1872-82), Fucini ( 1912) and Gugeberger (1936). Also in the Sicily Channel the top of the Inici Formation is considered as old as Late Sine- murian (Ronchi et al., 2000).

The facies characters and the depositional patterns ol'the Inici Formation show strong similarities to the Calcare Massiccio in the Apennines (Giacometti and Ronchi, 2000).

At Monte Kumeta the Inici Fornmtion crops {)tit along the northern slope where a thickness of about 300 rn is exposed. Its base is not observable here. It consists of poorly defined, metre-thick beds of whitish-grey limestones dipping 10-15 degrees southward. Three main lithofacies associations were distinguished in the lnici Formation at Monte Kumeta which are described below as Inici M I, M2 and M3 respectively.

3.1.1 Peritidal limestone (Inici M1)

In the Monte Kumeta area this unit characterizes the lower part of the Inici Formation and is exposed along the base ofthe northern slope. The t)cies recognition in this area is hampered by the cataclastic belt related to the E-W-

Fig. 3. l.ithostratigraphic relations between the lnici Formation and the overlying ('alcari a Crinoidi alld 1),./\1 at Monte Kumeta.

oriented master l+ault. Low-energy peritidal and lagoonal l itholacies such as i) stronmtolitic/lenestral/peloidal packstone-wackestonc (Pl. 47ll ), ii) oncolithic packstone/wacke- st{me and iii) algal wackestonc-packstonc with Thaumoto- porella pupwovesicul~fi, ra (Pl. 47/2), Caveuxia and benthic forams (Sipllovah',lina. P,~eudocyulammil,a. tcxtularids), can be observed, locally organized in shallowing upward cycles. The stratigraphic range of this un it at Monte Kutneta is not ,,veil constrained owing to the absence of diagnostic fossils, however a Hettangian p.t ). age is suggested by the stratigraphic position.

3.1.2 ()olitic-skelctal limestone (Mici M2)

In lnici M2 we group high-energy oolitic/skeletal cal- carenites occurring widespread on the top of Monte Kumcta. Their lower boundary is not observable because it runs along the northern subvertical slope of the mountain. The maximum thickness of lnici M2 call be roughly estimated as about 50-100 m.

The dominant lithofacies is a well-sorted grainstone consisting {71 micritizcd skeletal and non-skeletal grains in a variable ratio (Pl. 47/3-4). Non-skeletal components arc ooids (mostly micritized), along with lumps and rare peloids. The skeletal components are algae (Thaz~matOl~orella parvovesicuhJ?'ra. Paleoda.~3chzdu.s medirerraneus, Cayeu.ria). gastropods, rare bivalves, echinoderms and foraminilers (textularids. valvulinids and lituolids). In places the uppermost beds consist ol 'cm-thick alternations of medium and fine-grained, well-sorted bioclastic sands. The observation of the slratal pattern is hampered by an intense network of neptunian dykes both bed-parallel and subvcrtical, filled by pelagic calcilutitcs. 1.ocally small- scale cross-laminations can be obser',ed.

Based on the presence o F Paleodasycladus mediten~meu.v (Pia) and the absence of Agerino tnarta~za (Farinacci)

278

which appears in the overlying Inici M3, a Sinemurian age is suggested for these deposits.

The transition between Inici MI and M2 can be interpreted as the backstepping of marginal marine sand belts on the peritidal-lagoonal facies. This paleoenvironmental change in the Inici platform implies the onset of a high- energy shelf-edge in the Monte Kumeta area during Sine- murian time, reflecting the first tectonic dissection of the platform and the subsequent formation of the Marineo basin.

This event has not changed significantly the productivity of the Inici platform (probably extending here to the north) as indicated by the abundant and highly diverse skeletal fraction in Inici M2. The carbonate sands could have originated in innermost shelf areas and transported to the marginal sector where they formed a sand belt. The absence of skeletal grains derived from frame-building organisms, such as calcareous sponges or corals, suggests that patch-reefs could not develop, at least not in this area.

Similar deposits have been described by Jenkyns (1970b) at Monte Maranfusa (about 16 km south of Monte Kumeta) and interpreted as oolitic banks or sand shoals. In contrast to the Monte Kumeta ones these oolitic deposits are cycli- cally alternating with fenestral limestones. In the Monte Kumeta area we did not observe the lateral relationships of these deposits, but recent and ancient examples show that oolitic-skeletal sands are transported downslope by storm- initiated currents (Halley et al., 1983) and feed carbonate aprons in the adjacent peribasinal areas.

In Sicily, carbonate aprons fed by oolitic-skeletal sands from the Inici platform are known from Monte Genuardo (Di Stefano and Gullo, 1987) and from the Rabbito and Modica Formations of the Hyblean domain (Antonelli et al. 1991).

3.1.3 Peloidal-bioclastic limestone (Inici M3)

The lnici M2 deposits pass upward into a finer-grained peloidal/skcletal grain/wackestone in which large micritized grains (oncoids, lumps) are scattered (PI. 47/5,7). The fossil content consists of benthic forams (Lingulina, Spiril- lina, Lenticulina, Glomospira, Agerina martana (PI. 47/6), Involutina liassica), rare ostracods, bivalves and crinoids. Locally radiolarians and sponge spicules appear in a muddy matrix (Pl. 47/8). In the uppermost beds an increasing amount of crinoid ossicles is observed (P1.47/9). These deposits, here grouped in Inici M3, occur only in some seclors of Monte Kumeta, in particular along the southern slope. They were previously observed in a small abandoned quarry ("Cava A"), where they contain abundant bivalve shells ("brachiopod l imestones" in Di Stefano and Mindszenty, 2000). Their thickness reaches a few metres in this area and decreases to 0 in proximity of the Monte Kumeta top.

Following the biozonal scheme of Chiocchini et al. (1994), the presence of Agerina martana (Farinacci) suggests a Pliensbachian age for these deposits. The overlying Calcari a Crinoidi, of very probable Carixian (Early Pliens- bachian) age, confines the upper limit of the Inici M3 to the earliest Pliensbachian.

Inici M3 is a facies equivalent of the Calcare Massiccio "B" of the Umbria-Marche Basin (Centamore et al., 1971). These deposits are interpreted as markers of the incipient drowning of the highest blocks on which the carbonate platform facies persisted (Colacicchi, 1999; Galluzzo and Santantonio, 2002).

At Monte Kumeta the Inici M2/M3 transition records a drastic decrease of carbonate productivity in the Middle Liassic, possibly related to the isolation of the Kumeta marginal sector from the adjacent platform areas.

P l a t e 47

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4. Fig. 5.

Fig. 6. Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

Jurassic platform-slope transition at Monte Kumeta (western Sicily): Microfacies of the Inici Forma- tion (Lower-Middle Liassic) and the Calcari a Crinoidi (Middle Liassic)

Inici MI : Mudstone with fenestral porosity cropping out along the base of the Monte Kumeta northern slope. Scale bar 1 cm. Inici M 1 : Wackestone with Thaumatoporella parvovesiculifera (arrows), a common lithofacies in lagoonal deposits alternating with fenestral mudstone. Scale bar 1 ram. Inici M2: well-sorted grainstone with micritized ooids, coated grains, bioclasts and Paleodasycladus sp. (arrow). "Cava Palo". Scale bar 1 rmn. The same facies as above, with Thaumatoporella parvovesiculifera (arrow). Scale bar 1.5 ram. inici M3: poorly sorted grainstone with micritizcd coated grains, small-size benthic foraminifers and bioclasts. Southern slope of Monte Kumeta. Scale bar 200/.tm. Inici M3: close up of Fig. 5 showing Agerina martana (Farinacci). Scale bar 30 btm. Inici M3: poorly sorted grainstone with small-size benthic foraminifers, micritized coated grains and bioclasts. Southern slope of Monte Kumeta. Scale bar 250 btm. Inici M3: Radiolarian and sponge spicules-bearing wackestone commonly occurring in the uppermost part of Inici M3. "Cimitero dei pinnacoli". Scale bar 0.5 ram. Inici M3: wackestone with crinoid ossicles, radiolarians and sponge spicules. Southern slope ofM. Kumeta. Scale bar I ram. Calcari a Crinoidi: crinoidal grainstone with typical syntaxial overgrowth cements. "Cava Cerniglia". Scale bar 0.5 ram. Calcari a Crinoidi: crinoidal packstone with micrite intraclasts. Intergranular pore spaces are filled up by syntaxial overgrowth cements and reddish micrite. "Cava Cerniglia". Scale bar 0.5 ram.

P l a t e 47 279

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3.2 Caleari a Crinoidi

This unit consists of white or pink massive limestones formed by dominant (up to 80%) crinoid ossicles with subordinate amount of other bioclastic grains (brachiopods, benthic foraminifers, Thaumatoporella tYagments), micritized ooids and intraclasts (P1. 47/10-11). Well-rounded lithoclasts of grainstone with coated grains can be observed as well. Bedding is usually poor but locally (e.g. in "Cava A"), northward wedging stratal patterns are seen. At places. low-angle, wavy cross-stratification can be recognized suggesting the possible influence of storm events.

The Calcari a Crinoidi crop out as a wedge-shaped body along the southern slope of Monte Kumeta with a thickness of more than 20 m that drastically decreases to 0 m to the north. Sudden thickness variations can be observed along this wedge, suggesting a deposition on a stepped (faulted) substrate. Recent paleobathymetric data based on fluid inclusion analysis in the first syntaxial cement generations on the crinoid ossicles (Mallarino et al., 2001) indicate that cementation began at a water depth of about 20 to 100 m.

The relationship of this unit to the underlying rocks can be observed at "Cava A" near the top of Monte Kumeta and in a corner of the still active "Cava Cerniglia", where they unconformably overlie the Inici M3 with a sharp, (bio)erosional contact; at places they form neptunian dykes penetrating the Inici M3 (further details see below).

The upper boundary is marked by a peculiar discontinuity surface with din-sized pinnacles (see below and in Di Stefano and Mindszenty, 2000).

Jenkyns ( 1971 ) interpreted these crinoidal limestones as the result of deposition fi'om migrating sand waves or dunes on top of submarine topographic highs. However, field mapping and observations made in the last years on Monte Kumeta revealed that instead ofa subhorizontal flat substrate, we are faced with a fault-stepped paleorelief dipping gently to the south. In this model the encrinites are slope-apron type deposits related to the tectonically dissected paleo-slope of a former shalLow-water platform. The extensional tectonic process started earlier in the Jurassic, as is suggested by the appearance of the peri- platform facies of Inici M2 and M3. Due to further downfaulting and relative sea-level rise, the production of shallow water carbonates (oolitic sand) ceased and the submerged rocky surfaces became a substrate for crinoid meadows. The disarticulated crinoid ossicles (and other skeletal elelnents) were transported downslope. Part of the crinoidal sand accumulated in the depressions of the stepped escarpment, the rest arrived at the toe of the slope and deposited in the adjacent trough (Marineo basin). The erosion of the underlying rocks can also be due to the mechanical abrasion by the grain-loaded turbulent water..

Jenkyns and Torrens (1971) and Jenkyns (1971) suggested a possible Domerian age for these deposits at Monte Kumeta, but this was only indirectly based on the Toarcian age of the directly overlying formation.

We have collected brachiopods from several points and horizons of the Calcari a Crinoidi.

The identified forms are Securina cf. securi['ormis (Gemmellaro), Prionorhynchia ? cf. polypO, cha (Oppel), Cirpa cf. briseis (Gemmellaro), Cirpa sp. indet., Liospiri-

P l a t e 48

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Jurassic pelagic deposits of Monte Kumeta: Rosso Ammonitico inferiore (RAI) microfacies (Toarcian- Oxfordian)

Contact between the Inici M3 and the overlying RAIa bioclastic wackestone. The Inici M3 top is affected by intense microboring. "Cimitero dei pinnacoli". Scale bar 100 gin. Contact between the Calcari a Crinoidi (CDR) and the overlying RAIa pelagic wackestone marked by a thin Fe-Mn encrustation. Microboring is evident on top of Calcari a Crinoidi. "Cava Cemiglia". Scale bin" 100 gna. Toarcian biocastic wackestone with small fi'agments of bivalves, anamonites and Fe-Mn coated bioclasts. "Cava Cerniglia'. Scale bar I ram. RAIa: bioclastic packstone with abudant thin-shelled bivalve fragments, radiolarians and echinoderm fragments. "'Cava Cerniglia". Scale bar 1 ram. RAIb: detail of alternating laminae of peloidal/radiolarian packstone and Bositra grainstone. "Cava Cerniglia". Scale bar 250 tam. RAIc: bioclastic packstone with radiolarians, benthic foralninifers (Lenticulina), echinoderm fragments and less abundant thin bivalve shells. Fe-Mn-coated grains are also present. "'Cava Cerniglia". Scale bar 0.5 ram. Graded and laminated Bositra-rich grainstone-packstone passing upward into wackestone-mudstone. At the bottom bivalve shells are chaotically oriented and rounded, whereas upward they merge to oriented smaller fragments. This lithofacies is commonly observed at "Cava Palo'" and "Cimitero dei pinnacoli" and it is interpreted as lateral equivalent of the RAlb/c deposits. It suggests current activity along the paleoescarpment (microturbidites?). "Cava Palo". Scale bar 3 ram. RAId: Close up of an early diagenetic nodule (N) embedded in a bioclastic marly matrix (M) rich in echinoderm fragments (white spot). The boundary is overprinted by dissolution seams. "Cava Cerniglia". Scale bar 300 gin. Laminated gocthitic Fe-Mn-rich marls: laminae of yellow - brownish goetlfitic/slnectitic marly clays are separated by interlaminar spaces filled up by palisade-like calcite cements. At places the palisades are broken and slightly displaced as result of fracturing postdating the main cementation phase. Locality "Cava Cerniglia". Scale bar 2 ram.

P l a t e 48 281

282

retina angulata (Oppel) and Liospir~ferina cf. darwini (Gemmellaro).

The bulk of this fauna points to a Lower Pliensbachian (Carixian) age, though one species (Liospiriferina angulata) was known previously only from the Upper Sinemurian. A neptunian dyke of reddish, criqoidal limestones, crossing the massive Calcari a Crinoidi, provided a typical Upper Pliensbachian (Domerian) brachiopod faunula [Priono- rhynch ia c f. ha gaviensis ( B6se ). Lin guithyris a.v~asia (Zittel) and Bakonyithyris cf.pedemontana (Parona) ]. This suggests that the age of the Calcari a Crinoidi is most probably Carixian-Dome,'ian p.p.

3.3 The main hardground

In the quarries near the top of Monte Kumeta the most peculim-sedimenta W feature is the prominent ferromanganese crust between the Calcari a Crinoidi or the lnici Formation below and the different units of the Middle Jurassic Rosso Ammonitico inferiore above.

This crust covers a jagged bioeroded dissolution surface capped by a thick, bulbous, lalninated Mn-oxide crust (PI. 49/7-9). In vertical section it shows a row of elongate, rectangular pinnacles close to the base of which the outlines become rounded. Size and distribution of the pinnacles is as follows: height: 20 to 25 cm, width at the base: l0 to 20 cm, at top sometimes no more than 5 cm, spacing: 5 to 15 cm. Vertical grooves oriented parallel to the pinnacle axis can often be distinguished. (PI. 49/9). That not only bioerosion but also corrosion played an

important role in the formation of the pinnacles was confirmed by micropetrography focused on the Calcari a Crinoidi, which showed that underneath the Mn-encrusted surface intergranular porosity of the limestone is often enlarged by dissolution and filled by infiltrated red calcilutite and/or Mn-oxide.

The crust consists of very fine-grained, hollandite-type Mn-oxides and goethite (Mn/Fc = 1 ) the latter being more abundant at the bottom of the crust and being intimately intergrown with calcite. The interlaminar space is filled partly by pelagic mud, partly by calcite cement, occasionally with minor precipitations of barite. Traces of biological activity associated with the crust are shown by the abundance of worm tubes and in'egular patches of encrusting foraminifers (Tolypammina) preferentially sitting in the lobes of the wavy laminae, as docmnented by Di Stefano and Mindszenty (2000, p.53, Fig 13b).

The crust is usually thickest on the top of the pinnacles whereas in the interpinnacle space it is often split into several thinner crusts or coatings interrupted by the deposition of red carbonates in the hollows of the ,jagged surface. At places where the Calcari a Crinoidi are directly overlain by red, nodular limestone, a thin goethitic phosphate accumulation is observed.

In the interpinnacle spaces or in the fillings of slight topographic depressions of the pinuacled surfaces of the Calcari a Crinoidi, red, micritic limestone occurs (Fig. 5; Pl. 49/4). The pockets which are filled by a red, bioclastic wackestone with abundant, goethite-encrusted bio/litho-

P l a t e 49

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Figs 7, 8, 9.

Jurassic pelagic deposits of Monte Kumeta: Details of the Rosso Ammonitico inferiore deposits and of the pinnacled discontinuity surface on the Calcari a Crinoidi as observed in the "Cava Cerniglia".

Upper zone of the RAI deposits showing the contact of the nodular marly calcilutites of the RAId on the RAIc. Arrows indicate thin pinnacles of red calcilutites that characterise one of several minor discontinuity surfaces in the RAIc. Ammonites occumng below the pinnacles indicate a Middle Callovian age. See tape ruler on the right as scale. Onlap of the RAI calcilutites on a metre-scale palcorelief ofCalcari aCrinoidi. This flower-like synsedimentary structure is the result of brittle deformation partly contemporaneous with, partly post-dating, the discontinuity surface. Several Mn-Fe- encrusted surfaces whith laminated or bulbous structures follow the main hardground. Geologist as scale. Close up of the Fe-Mn-capped pinnacles, showing on top the RAta deposits (Bajocian) with resedimented ammonites having a distinct imbrication, that suggests transport from the direction of the relative paleo-high (from right to left). It is also evident that the Toarcian red micritic limestone filling up the interpinnacle spaces predate the main Fe-Mn crust development. Scale bar 20 cm. Detail of a wire-cut section of the Calcari a Crinoidi (bottom) and the overlying RAla-c succession. Two thin layers of laminated goethitic marl/clays (Igm) occur respectively on top of RAIa and within the RAIb. Scale bar 30 cm. Close up of the RAIb unit showing small-scale current cross-lamination in Bositra-rich pack/wackestone. Scale bar 15 cm. Subvertical to oblique funnel-shaped dyke about 40 cm wide, cutting across the Calcari a Crinoidi and showing polyphase fillings. The first internal sediment is a reddish mudstone that penetrates also the intergranular porosity of the Calcari a Crinoidi. Cm- to din-size subangular blocks broken off the wall suggest that the sediment was not completely lithified when affected by deformation. Scale bar 40 cm. Details of pinnacle morphologies observed along wire-cut surfaces and isolated pimmcles. Fig. 7 shows the morphology of pinnacles in transversal section (a). Scale bar 5 cm. In Fig. 8 the effect of bioerosion along the longitudinal section of pinnacles is obvious. Also fiacmres across theft" base are commonly observed. In Fig. 9 vertical grooves oriented parallel to the pinnacle axis can be distinguished. Scale bar 10 cm.

P l a t e 49 -.~83

284

clasts, yellowish when weathered, are sealed by a thick. laminated Fe-Mn oxide crust. A large part of the bioclasts consists of small fragments of thin bivalve shells (PI. 48/ 3). Occasionally, thick and heavily bioeroded, Fe oxide- coated shell fragments occur in association with other molluscs, some ostracods, small ammonite fragments (filled by internal sediments and sparry calcite), few benthic foraminifers (fragmented), and bioeroded, iron oxide- encrusted lithoclasts of mudstone or wackestone (=hardground fragments). Most bioclasts are coated by a very thin Mn-oxide fihn and Fe-Mn oxide-stain can be observed also within the red micritic matrix. This is the limestone what Jenkyns and Torrens (1971, p.94) have recorded tu Kumeta as "red fine-grained limestone". Ammonites occurring in this unit are embedded as fragments though otherwise well preserved. These tapho- nomic features indicate that in this facies not only lithoclasts and smaller bioclasts but also macrofossils were redeposited.

Locally, the same microfacies occurs as nepmnian dykes, which, in addition to the Fe-Mn oxide-coated lithoclasts, contain abundant volcanic fragments of trachytic microtexture. Occasionally hardground fl'agments sun'ouded by thick laminated goethitic crust (similm" to Fe-oolites) were also found.

Determinable ammonites are rare in the red, sometimes yellowish limestone within the thick ferromanganese crust. The red micritic limestone yielded Mn-oxide encrusted, con'oded, apparently reworked ammonites, which occur in pockets inbetween the bigger pinnacles of the hard-ground. The following ammonites have been identified: Calliphyllo- ceras sp., Holeol)hylloceras sp., Happoceras spp., Pol.yplectus pluricostatus (Haas), Polyplectus sp., Hildaites spp., Pseudomercaticeras sp. and O,v)erlioceras bicarinatum (Oppel).

This fauna clearly indicates the Lower Toarcian Serpentinum Zone. A similar assemblage indicated the

Early Toarcian age of the late generation of fissures penetrating the Calcari a Crinoidi (see Wendt 1963, p.291).

The time of the redeposition of the Lower Toarcian annnonites can be guessed from sporadic specimens ob- tained from the ferromanganese crust and the intercalated thin, yellowish calcareous layers which blanket the irregular, jagged surface of the main hardground. These limestone shreds yielded a Grammoceras and a Hammatoceras specimen, which indicate the Upper Toarcian Thouarsense Zone. It should be added that lhe main crust yielded some Bifrons and Variabilis Zone ammonites in the "Cava Palo" quarry (F. Macchioni, pers. comm. 2000).

The appearance of this prominent ferromanganese crust is not typical for Monte Kumeta only: very similar, also Toarcian hardgrounds with high, rectangular pinnacles were reported fi'om lsola di Favignana (Jenkyns, 1970), Monte Erice (Wendt, 1971), and a well-developed fen'o- manganese crust was found on Rocca Busambra (Wendt, 1963).

A model for the formation of this peculiar hard-ground was presented recently by Di Stefano and Mindszenty (2000), who proposed several explanations for the observed phenomena. However, no conclusive geochemical evidence was found to unequivocally support either of the proposed tentatives. Now, faced with precise age determi- nations for different parts of the main hardground, the previous conclusions on its origin can be completed. The age of the dissolution surface on top of the Calcari a Crinoidi is Lower Toarcian Serpentinum Zone, which is coincidentally the age of the Early Toarcian anoxic event widespread in the Western Tethyan Basins (Jenkyns and Clayton, 1986). In Early Toarcian times intermediate water masses with a general tendency to oxygen deficiency and relative CO, enrichment may have incidentally intersected the top of the submarine high of Monte Kumeta, provoking enhanced dissolution. Subsequently, a slight shift of the redox boundary (related or not to a relative sea- level change) could have permitted oxygen-dependent

P l a t e 50

Fig. 1.

Fig. 2.

Fig. 3.

Figs 4.

Figs 5, 6.

Jurassic pelagic deposits of Monte Kumeta: Discontinuities between the Rosso Ammonitico inferiore (RAI) and the underlying Inici and Calcari a Crinoidi.

"Cava Palo"- Onlap of the RAt on a stepped surface of oolitic bioclastic grainstones (Inici M2) which is cut across by an intense network of sedimentary dykes. The contact is displaced by small normal faults postdating the RAI deposits. "Cava A" - In the lower part of the wire-cut walls the sharp contact between the Inici M3 deposits and the overlying Calcari a Crinoidi is exposed. On top of the latter, a stepped uncontormity of the RAI deposits is evident. Steps are interpreted as the result ofdownslope block detachments of the topmost bed of the Calcari a Crinoidi. Scale bar I m. Slope above "Cava A": Steps on lnici M2/M3 sealed by tile RAI. Main steps (din- to m-scale) are rectilinear and roughly E-W oriented. At places broken pinnacles consisting of Mn-Fe-coated Inici limestones are embedded in the RAI. "Cava A": Close-up of the contact between the lnici M2 and the Calcari a Crinoidi. Arrows indicate bioerosional cavities on the top-Inici M3 hardground filled with crinoidal sands. Scale bar 15 cm. "Cimitero dei pinnacoli": Unconformity between the Inici M3 and the RAI deposits, locally characterized by a 40 cm thick mud-supported breccia bed. The breccia elelnents consist of din-size, Fe-Mn-coated pinnacles of Calcari a Crinoidi. Ammonites in the red calcilutite matrix indicate Upper Bajocian age (see details in Text-Fig. 6).

P l a t e 50 ~ ' "

286

f). document ing the oxygen deficient episode which pre- dated the precipitat ion of the bulk of the manganiferous crust.

Fig. 4. Stratigraphic log of a section at "'Cava Cerniglia'" (F.A. = fossil assemblage: LGM= laminated goethitic marls).

bioeroders to recolonize the already corroded surface. At the same time dissolved Mn-species, previously concentrated in the oxygen deficient water-mass, could precipitate and gradually cover the uneven surface with a thick Mn-oxide crust, which is so characterist ic of the main hardground of Monte Kumeta. Such a mechanism is cor- roborated also by the observed phosphatisat ion of the l imestone surface immediate ly adjoining the lowermost part of the crust (Di Stefano and Mindszenty, 2000, fig. 14/

3.4 Rosso Ammonitico inferiore (RAI)

The pelagic rocks above the Calcari a Crinoidi can also be label led as Buccheri Formation. This l i thostrat igraphic unit was introduced by Patacca et al. (1979) on the basis of cores dri l led in the Iblean area. It roughly corresponds to the Giardini Formation of the Pelori tani Mountains (Rigo and Barbieri ,1959). The Buccheri Formation is divided into three subunits: - lower part: reddish and greenish marls with pelagic bivalves (Toarcian-Bathonian): - middle part: radiolarian cherty and siliceous limestones (Callovian-Kimmeridgian): - tipper part: nodular marly limestones with Saccocoma, Stomiosphaera and Globochaete (uppermost Kimmerid- gian - Lower Tithonian).

Patacca et al. (1979) recognized lateral variations in thickness and facies reflecting the irregular paleotopography inherited fi-om the Early Liassic synsedimentary tectonics, and the Middle - Late Jurassic volcanic activity. They dist inguished a complete sequence of 50-100 to 650 m thickness, and a condensed, incomplete sequence which is up to 30 m thick.

As mentioned above, the "Ross() Ammonit ico" , equivalent to the Buccheri Formation deposits, can be easily differentiated into three zones also at Monte Kumeta. The red. nodular l imestone which corresponds to the lower subunit of the original subdivision of the Buccheri Forma- tion, is treated here as Rosso Ammoni t ico inferiore (RAI).

As will be described in the subsequent chapters, the vertical and lateral distribution of the RAI at Monte Kumeta reflect its deposition on a stepped paleotopography set upon the top o1' the Calcari a Crinoidi and the Inici M2 and M3. This paleosurface was repeatedly remodelled by tectonic and gravity driven deformations. Onlap geometries on this disseceted paleotopography are common, giving rise to sudden lateral decrease in thickness from 6 m down to zero (Pl. 49/2).

One of the most complete sections of RAI was observed along a wire-cut surface in the lower zone of"Cava Cemiglia" (see Chapter 4.1 and Fig. 4). Here, four main sub-units (RAla-d) can be distinguished according to their lithological/ textural characters and stratigraphic position. They are generally bounded by sharp discontinuity surfaces often marked by Fe-Mn crusts (Pl. 49/I ,4).

3.4. l Red pseudonodular cephalopod l imestone (RAIa)

This is a red calcilutite containing abundant Mn oxide- coated ammonites that overlays the main Fe-Mn crust. As the nodular structure is not evident in outcrops we adopt the term "'pseudonodular'" rather than nodnlar (sensu Martire, 1996), Nodules consist either of ammonite moulds or early diagenetic nodules.

287

f

>

E

6f

m

.= "j

t; __

L "--

E

tJ

~=~

._=

9~ 5

-- / m

0 ~ .---- .h -- ,

si~ ~ 0

288

The microfacies shows a bioclastic packstone/wackestone with abundant fiagments of thin-shelled bivalves (Bositra), radiolarians, few fiagments of thick-shelled molluscs, few benthic foraminifers and peloids (P1.48/4). At places, clue to the occurrence of microspar, the micritic matrix has a clotted appearance. Ammonites embedded in this matrix are corroded and Mn-coated indicating redeposition. Though continuous, the thickness of RAIa is apparently also controlled by the paleotopography of the underlying main discontinuity surlEce. At places, where RAIa is the thickest, it can be divided into tba'ee layers separated by Mn- coated surfaces. At least in one of these layers the imbrication shown by these redeposited ammonites strongly suggests unidirectional cmTent activity (P1.49/3).

The age of the most widespread condensation level could be dated with a rich ammonite fauna. This was one of the levels most commonly found with fossiliferous, inanga- nese-encrusted bedding planes around the quarry. The lk)l- lowing forms were identified: Phylloceras kudernatschi (Hauer), Calliphyllocems heterophylloides (Oppel/, Holco- phylloceras privasense Joly, Lvtoceras eudesianum (D' Orbigny), NamTolytocerasl~olyhelictum (B^ckh), Strigo- ceras truellei (D'Orbigny), Ox~'cerites plicatella (Gem- mellaro), Sphaeroceras tenuicostatum Sturani, Cadomites daubenyi (Gemmellaro), Ermoceras rzmcinatum Arkell, Bigotites lanquinei Nicolesco and Lepto.v~hinctes spp.

Some brachiopods were also found: Apringia cf. atla (Oppel), Apringia of. alontina (Di Stefano) and Karada- githyris gerda (Oppel). This is a fauna of Upper Bajocian Garantiana Zone.

The fauna of a few blocks consisting of numerous phylloceratids and Strigoceras truellei (D'Orbigny), Dimorphinites dimmT~hus (D 'Orb igny) and some parkinsonids, indicated, a Parkinsoni Zone assemblage.Some blocks in and around the quarry have yielded older Bajocian faunas. In a single block we found a large Skirroceras sp. suggesting a Sauzei or Humphriesianum Zone age.

A Bajocian age has been indicated from earlier stratigraphical reports on Monte Kumeta (see Wendt, 1963, 1965, 1969, Jenkyns 1970b, fig.7, Jenkyns and Torrens, 1971). However. the faunal list of Wendt (1963, p.84) contains an~nonites indicating a wider range of Bajocian levels: Sat, zei, Humphriesianum Zones and uppermost Bajocian. Wendt (op.cit) listed older Middle Jurassic ammonites as well, even fiom the Aalenian. These earlier records of faunal assemblages, which were never found again in the new excavations, suggest that along with the northward thinning of the "Rosso Ammonitico" limestone, the older t'aunal levels disappear as the condensation horizons wedge out in this direction.

In addition to the above described horizons, some records suggest more, though not so well and richly represented faunal levels. In 2001 we found a single loose specimen of O~zmiceras sp. in the quarry. This gent, s unequivocally indicates the Lower Bathonian. In the faunal list of Wendt (1965, p.290) some amlnonites (Rug~/'erites and Bulla- timorphites) suggest a Middle Bathonian age, probably the same what was evidenced by a small fauna described from an isolated locality of "Rosso Ammonitico'" limestone on

top of Monte Kumeta, about 800 m south-east fi'om the quarries described in this study (Galficz, 1985).

The evaulation of the above discussed faunal levels within RAIa suggests that the former references (Wendt, 1963, 1965, Abate et al. 1990) to a single condensation horizon with ammonites of such stratigraphic extremes as Aalenian and Middle Callovian could be corrected. Within the relatively thin red nodular limestone there are several condensation horizons, some are widespread and associated with rich ammonite faunas (e.g the Upper Bathonian one), some are scattered, appearing as smaller pockets with rare ammonites (e.g. the Lower Bathonian one).

3.4.2 Pink wavy-cross-bedded limestone (RAIb)

In the "'Cava Cerniglia" this is a lens-shaped unit that follows the Bajocian red nodular limestones. It consists of alternating laminae of coarser and finer, well-sorted, bio- elastic graiqstone/packstone/wackestone with peloids, abundant Bositra and other mollusc fragments, few benthic and planktonic foraminifers, ostracods, GIobochete. small ammonites, gastropods, echinoderm fi'agments and bioeroded lithoclasts (PI. 48/5). The coarse-grained. Bositra-rich laminae are cemented by sparry calcite, whereas the rest has a microsparitic matrix. Small-scale wavy cross-lamination interpreted as current-ripple lamination is obvious at places. Within the laminae grading is apparent. In the uppermost part we find a few cm thick layer of massive red mudstone/ wackestone locally with Protoglobigerinoids, very rare benthic foraminifers (Spirillinids), radiolaria, sponge spi- ct.les, very fine bioclastic detritus and abundant traces of bioturbation. At places this layer shows a clearly gradational contact with the wavy-cross-bedded limestone (Pl. 49/5). Where the limestone is reduced to a minimal thickness, the horizon passes into a hard~'ound with a rich ammonite fauna.

Sedimentary structures (current ripple lamination, grading), as well as the lateral and vertical facies relationships suggest that the bioclastic sand was deposited episodically from swift currents on the otherwise starving sediment surface. Similar deposits described fi'om the Jurassic of the Western Alps by Bernoulli et al. (1990, fig.2/f) were interpreted as calcareous turbidites. Sarti et al. (2000, figs. 10-11) in their paper on Cretaceous to Paleogene pelagic sediments of Taormina demonstrated that calcareous microturbidites showing similar sedimentary structures could have been repeatedly deposited also in neptunian dykes and sills at places where sedementation rates on the sea-bottom were very low. We suggest that also RAIb might have been deposited by similar microturbidity currents triggered by synsedimentary fracturing which is so characteristic throughout the Early and Middle Jurassic of the Monte Kumeta depositional environment.

Ammonites and other macrolbssils are especially common on one or two characteristic bedding planes within this unit. These bedding planes mark shorter stops in sedimentation, because their corroded, subsolved surfaces are covered in some places by calcareous worm tubes, and some hard- substrate dwelling gastropods (Neritopsis, Pyrgotmchus)

289

also occur in the fauna. The ammonites are of exceptional preservation: they are adhered to the surface of the bedding plane, with their upper surfaces covered by a thin ferromanganese film. In other places the bedding planes arc covered by Bos#ra shells.

The following ammonites were determined: Phvlloceras kudernatschi (Hauer), Calliphylloceras disputabile (Zittel), Holcophylloceras zignodianum (D'Orbigny), Ptycho- phylloceras flabellatum (Neumayr), Lytoceras sp., Strigoceras sp., Lissoceras ferro'ex (Zi tlel), Oxycerit es orbis (Giebel), Oxycerites subinflexus (De Grossouvrc). Paroecotraustes serrigerus Waagen, Prohecticoceras retrocostatum (De Grossouvre), Prohecticoce~z~s I)lana~.en.w Elmi, Epistrenoceras sp., Cadomires bremeri Tsereteli, Bullatimo rphites sp., Sphaeroptych ius sp., H omoe opl a~z u lit e s homoeomorphus Buckman and Parachq[l~atia sp.

Some additonal brachiopods are: Seprocrurella ? micula (Oppel), Striirhynchia berchta (Oppel) and Striirhym'hia ? ucinensis (Di Stefano).

This is an assemblage proving the Upper Bathonian Retrocostatum Zone, most probably its middle part.

3.4.3 Red pseudonodular marly limestone (RAIc)

This is a reddish calcilutite characterized by a pseudonodular texture due to presence of early diagcnetic nodules, ammonite moulds and bioturbation. Common microfacies is a bioclastic packstone/wackesione with radiolarians, less abundant thin-shelled bivalves, thicker, heavily bioeroded fragments of other molluscs, benthic forams, echinoderm fragments, peloids, few micritic lithoclasts, embedded in a partly microsparitic"matrix" (P1.48/6). Few microbiomolds are filled by sparry calcite and occasionally there are irregular cavities ofbiomold or bioturbation (burrow) origin, filled by internal sediments and sparry calcite. Occasionally bioeroded, rounded fragments of a red, recrystallized, iron- impregnated sediment, rich in angular chips supposedly produced by bioeroders were also observed. The upper pan of this unit is characterized by a series of Mn-coated condensation horizons. The most peculiar of such horizons, about 5 cm below the contact with the overlying red/grey nodular marls, shows widely-spaced thin (1 to 3 cm) fingcr-like pinnacles oil a clearly bioeroded surface, coated by a Ihin Mn-oxide film. The height of these pinnacles is up to 5 cm.

This horizon yielded a rich ammonite assemblage, including the following forms: Phylloceras isomorphum Gemmellaro, Phylloceras plicatum (Neumayr), Lvtoceras eudesianum (D'Orbigny), Hecticoceras (Bonarellite.v) paucifalcatum Till, Reineckeia (Tyranni[es) pictava (Bourquin), Reineckeia (Reineckeia) sp., Collotia gigm~tea (Bourquin) and Choffatia (Choffatia) spp. This is a Middlc Callovian, Anceps Zone fauna. Its level, as the uppemmst hardground within the Rosso Ammonitico. can be traced almost continuously in the quarries, but ammonites (mainly big phylloceratids and perisphinctids) appear only in some places. Nevertheless, Wendt ( 1965, p.290) also discovered this horizon, because his faunal list contains ammonites which indicate a Middle Callovian age.

In ttle lower part of the "'Cava Cerniglia" another layer of wavy cross-bedded limeslone similar to the one described under RAIb occurs as a I0 cm thick intercalation within tile higher parts o[ this unit.

3.4.4 Red/grey nodular inar[s (RAId)

In this unit nodules are very well distinguishable from the surrounding matrix (Pl. 49/l ). Nodules are due to early digcnesis randomly affecting the scdimenl. They are en- veloped by more compacted marls with iron-oxide rich clay seams (P1.48/8). The nodular slructure is generally well preserved in the lmxer zone of this unit, while in lhe middle and upper zones nodules arc inwH,~ed in synsedimentary folds (slumpings) or give rise to pebbly mudstone layers.

The microfacics consists of a wackcslone with micropeloids, shell fl'agmcnts, radiolarians, sponge spicules, bioeroded echinoderm fragments, abundant plank- Ionic and few benthic forams.

Because of the lack of math)fossils, the age of these nodular marls could not be determined yet. However, an Upper Callovian to Middle ()xlordian age is most probable.

3.4.5 Hardgrouncts wiihin RAI

Posldating the formation. Fc-Mn encrustation anti burial ofihc main hardground, wilhin RAla,b, and c. several less conspicuous discontinuity surfaces, coated with Fc-Mn- rich precipitates occur. These crusts are similar to. though generally much thinner than tile main crust covering the surface of Inici M3 or the Calcari a Crinoidi. They do not exhibit much relief, and are not necessarily laterally coil- linuous eilher. They may show a laminated or bulbous, stromatolile-like structure, often accompanied by den- dritic overgrowths. At places they are represented only by a thin layer of isolated Fe-Mn-coated inlraclasts.

As wc followed the norlhward progress orquarrying in the "Cava Cerniglia" for nearly 20 years, it became clear that the number of the condensation horizons decreases toward the top of Monte Kumeta. The faunal lists of Wendt ( 1963. pp. 84-85, 1965, p. 290), based on collections in the early 1960"s, indicate at least two Aalenian and three Baiocian horizons. When wc first visited the quarry in 1983, we identified three Bt\iocian levels of which the two Upper Bajocian were apparent ly unexposed formerly. The Bathonian and Callovian hardgrounds are probably the same which were visible for decades in and around tile quarry. These records clearly indicate the del)ositional geometry of the RAI: parallel with the northward thinning of the "'Rosso Ammonitico'" limestone, the stratigraphically older hardgrounds pinch out. and only the highest, i.e. lhe Bathonian and the Callovian ones continue.

3.4.6 Laminated goethitic Fe-Mn-rich marls

They occur as 2 to 15 cm thick intercalations within (or in between) the RAI b, c and d. They consist of thin+ about

290

1 mm, often torn-apart (sheared) laminae of yellowish brown goethitic/smectitic clay and few, likewise thin laminae of micrite, all cemented by coarse sparry calcite grown perpendicularly on the walls of the interlaminar space. Lithoclast deriving from these deposits are often embedded in neptunian dyke fillings.

In the clay Fe-Ti-rich minerals, apatite and chromite could be identified as detrital grains. A first generation of isopachous fibrous calcite could be observed around some of the torn-apart clay flakes, whereas the majority of sparry calcite cement grown in the interlaminar space is palisade- like. At places where palisades do not completely occlude the intergranular space, they are overlain by bioclastic wackestone containing thin shell fragments and few foraminifers. Occasionally, later generations of fractures, cross- cutting the laminated, calcite-cemented clay and filled either by biolastic wackestone or by mosaic-like sparry cement can also be observed.

That most of the cementation of the clay must have been early diagenetic, is shown by its tendency to form angular flakes, already with calcite veinlets, embedded, as redeposited clasts, in the overlying sediments. Postdating the major phase of cementation an event of fracturing can be observed (Pt. 48/9). The palisades are broken and slightly displaced.

Interpretation of the observed texture requires more than one episode of deformation following early compac- tion of the clay/marl deposits. The first phase of deformation may have been downslope shear, resulting in small- scale bed-parallel displacement of the ductile clay/marl laminae. At the same time fluids penetrated into the interlaminar space and from these fluids the palisade calcite could precipitate. Further episodes of shearing resulted in the break up of the cemented layer producing the re-opening of the interlaminar spaces.

4 SYNSEDIMENTARY DEFORMATIONS AND ASSOCIATED FEATURES

The active and abandoned quarries near the top of Monte Kumeta exhibit good evidence of synsedimentary deformation of all lithostratigraphic units described above. Neptunian dykes, fracturing, brecciation, slumps and debris flows, as well as the lateral and vertical facies changes clearly are features which point to the strong tectonic control of the contemporaneous sedimentation.

The abundance and combination of these features are different in each exposure, therefore they are described separately.

4.1 Neptunian dykes and other deformational structures in "Cava Cerniglia"

In this quarry the extraction of Jurassic rocks has been more or less continuous for about 20 years. The material is used as decorative stone in the construction industry. The progress of quarrying by the wire-cutting technology per-

mitted us to follow the lateral and vertical variations of RAI and its relationship to the enclosing formations since 1983. The actual quarry yard is of NS-oriented, elongate, rectangular layout with three main levels. The most interesting and longest continuous, wire-cut surface, offering the best dip- section is exposed by the long western wall of the lowermost level. Additional phenomena, including fossiliferous horizons worth observing can be found at the higher levels.

4.1.I Neptunian dykes

The first obvious event of fracturing which has affected the Calcari a Crinoidi clearly predates the formation of the main hardground. The fractures may be both vertical to subvcrtical or bedding-parallel; their width varies from cm to m scale. The contact of the fractures towards the Calcari a Crinoidi is irregular, and so is the surface of the cm- to dm-sized subangular blocks broken off the wall, suggesting that the sediment was not yet completely lithified when affected by the fracturing. Most fissures (neptunian dykes) have a polyphase filling. Individual phases may cross-cut the older ones suggesting repeated reopening of the fractures, or they may just show normal stratigraphic contacts (often) separated from each other by thin Fe-, or Mn- coated surfaces, implying gradual fill-up of the open space from above. The actual sequence of the phases recogniz- able in individual dykes exhibits considerable variation. On the highest quarry level in an about 40 cm wide subvertical, somewhat funnel-shaped dyke (P1.49/6) we have observed the following dyke-filling sucession. The first generation is a reddish mudstone. Along the walls of the fractures this reddish material penetrates the intergranular porosity of the Calcari a Crinoidi, in which, abundant, loose crinoid ossicles (derived possibly from the semi-lithified rock) are floating. The next phase is also a red sediment with occasional bioclasts (ammonite-fragments). Overlying (and cross-cutting) the bioclastic red mud, there follows a yellowish, bio- and lithoclast-rich mudstone correlated with the Toarcian micritic limestone described above. It is often separated from the older red dyke-fill by an irregular, Mn-oxide coated surface. On top of the Toarcian dyke-fill there is another mineralized (Fe- Mn-oxide rich) surface which is the equivalent of the main hardground covering the Calcari a Crinoidi. The transition from the dyke-fill to the sediment package occurring in the topographic depression right above the opening of the "funnel" is gradual, suggesting that the formation of the depression was created by the fracture underneath and that this depression has survived for some time even after the development of the main hardground.

On the quarry face the evidence of several additional generations of vertical/subvertical to bed-parallel dykes is obvious, however, often unclear cross-cutting relationships and the scarcity of identifiable, diagnostic fossils do not permit to properly establish their age. We found only a single cut block where the attached fissure-filling material contained brachiopods (i.e. Apringiacf. alontina (Di Stefano)) which confirmed a Late Bajocian age.

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4.1.2 Other deformational structures

A significant brittle deformation cvent has affected thc Calcari a Crinoidi and the main hardground. Locally it controlled also the deposition of the lower part of RAIa by creating an irregular paleotopography. One of the related features was detected and documented in lhc late 90"s (l)i Stefano and Mindszenty 1998) as a m-scale flower-like positive structure with associated clear onlap relationship of RAI lithofacies (PI. 49/2). Another spectacular feature of "Cava Cerniglia" is a later deformation event clearly postdating RAIc and resulting in ductile del'onnation of at least a part of the overlying Upper Callovian - Lower ()xfordian red/grey nodular marls (RAId) (Fig. 5). The faults are predominantly E-W to WNW-ESE striking normal faults pointing to an overall extensional regime and resulting in cm- to m-scale displacements. Minor reverse faults indicate that locally compression also occurred.

The observed ductile deformation of the nodular marl is clearly related to the above described extensional faulting event. The occurrence of slumps and associated debris flow deposits suggests that this event has rejuvenated the previously created paleotopography. Additional ly it has increased the inclination of the paleoslope and induced slumps and debris-flows accomodating on the stepped surface (Fig. 5).

4.2 Stepped unconformity and related features in "Cava A" and in "Cimitero dei pinnacoli"

"Cava A" is a small, abandoned quarry along the road about 300 m west o f "Cava Cerniglia" which is Formed by two, nearly perpendicular wire-cut surfaces facing SE and SW (Pl. 50/2).

Here the contact between Inici M3 and the overlying Calcari a Crinoidi is a sharp surface along which small-scale bioerosional cavities can be observed at places (PI. 50/4). Rare, cm-scale subvertical dykes filled with crinoidal sand- stone could be seen also on the wire-cut surface. Otherwise the contact surface is criss-crossed by several generations of cm-sized neptunian dykes filled either by a yellowish calcilutite with angular lithoclasts or by calcite cements and varicolourcd siltstone. The surface is also repeatedly cut by several subvertical faults, producing cm-scale displacements that postdate the Calcari a Crinoidi deposition. The thickness of the Calcari a Crinoidi ranges from about 1.5 to 6 m. The upper boundary of this unit is a sharp, stepped unconformity sealed by RAI deposits.

In the lower pan of the eastern side ol the quarry the RAI deposits onlap a m-scale palcorelief of the Calcari a Cri noidi. At the base of the RAI a thick Fe-Mn crust (=the main hardground) can be observed. Higher up. on the wire-cut wall a step of about 40 cm is observable. It is interpreted as the result of local downslope collapse of the topmost beds of the Calcari a Crinoidi during the deposition of tile RAI. The downslope collapse seems to have been Favoured by bed- parallel neptunian dykes in the upper part of the unit, having served as detachment surfaces (Fig. 7). The mechanical removal of joint-bounded blocks gave rise to a stepped surface sealed by the RAI limestones (stepped unconformity

Fig. 6. Stratigraphic log of a measured section at "Cimitero dci pinnacoli"

sensu, Winterer and Sarli, 1994). Thc overlying RAI contains din-scale lithoclasts either o[" Mn-Fe-coaled pinnacles or fragments of the main hardground. These lithoclasts arc obviously derived flom adiaccnl upslope areas which were exposed and eroded at those times. Sections of ammonites observed in Ihc lower RAI in this outcrop indicate a B~\focian age for the sediment filling the stepped unconformity. Higher up. along the sh)pc that overlies this quarry, well-preserved stcpped geometries of the RAI directely overlying the lnici M2 and M3 are also exposed (PI. 50/3). The din- to m-scale, south-facing steps show a roughly E-W rectilinear trend, which is the same as that of the synsedimentary fault system observed in ttle surroundings. However no exposures are visible here. and thus we could nol affirm that the observed steps are related to synsedimentary faults or Io gravity- driven collapses. Large angular fragments o[ lhc main hardground arc embedded in the RAt deposits overlying the stepped unconformily. The pinnacles consist of lnici limestone, thus suggesting thai upslope the serrate dissolution surface developed on the Inici limestones.

The gcncral relations and depositional geometries as observed in the"Cava A" and the adiacent areas clearly point to the lateral (northward) pinching out and onlap of the lnici M3 and of the Calcari a Crinoidi on a stepped surface of Inlet M2. The resulting paleotopography also in this case was continuously fejuvenatcd during the deposition of the RAI giving rise to multiple unconformities.

"Cimitero dei pinnacoli'" is a ~mall (about 6x6 m) outcrop on the genl]y sloping hill~idc, which exposes thc dircct contact between Inici M3 and the overlying RA1 with the

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Fig. 7. Sketch based on the stratigraphic record at "Cava A" and "Cimitero dei pinnacoli" outcrops, showing the incipient creation of a decimetre scale stepped unconformity (a) as result of downslope detachment of Calcari a Crinoidi blocks covered by unconsolidated RAI sediments and subsequent debris flow emplacement (b). As observed in adjacent areas (e.g. in "Cava Cerniglia"), steps were also created by repeated events of synsedimentary faulting that in some cases have triggered debris flows.

Calcari a Crinoidi missing. In this outcrop repreated episodes of submarine debris flows and slumpings in the RAI are well recorded (Fig. 6). The base of the section exposes about 1 m of the topmost zone of the Inici M3 consisting of a radiolarian and sponge spicule-beating wackestone with scattered oncoids and small benthic forams. The Inici M3 top is a sharp surface on which the RAI deposits accomodate (P1.48/1). Along the surface a step of about l dm is filled up by a reddish pelagic calcilutite with some cm-sized Fe-Mn nodules concentrated at the base. Both the Inici M3 and the step-filling calcilutite are abruptly overlain by a ca. 40 cm thick debrite (P1. 50/5) consisting of a red calcitutite matrix which supports din- sized broken pinnacles of Calcari a Crinoidi that are coated by Fe-Mn crusts (P1.50/6).

Ammonites occurring in the matrix indicate an Upper Bajocian age of this debrite bed. A few cm thick Bositra lumachella overlies this debrite bed filling also dykes in the underlying beds. Well-preserved ammonites in these fracture filling sediments indicate an Upper Bathonian (Retrocostatum Zone) age.

A 40 cm thick bed of goethitic sheared/laminated marls with abundant trace fossils follow upward. It is overlain by a centimetric alternance of reddish Bositra bioclastic grain/ packstone to thinly laminated bioclastic wacke/mudstone,

showing at places structures related to soft sediment deformations. Along this bed also a large block of Fe-Mn encrusted Calcari a Cfinoidi is present.

The last exposed unit upward is the varicoloured marly calcilutite (RAId). As in "Cava Cerniglia", this unit shows slump structures, pebbles of red calcilutites and abundant belemnites and aptychi.

The observed features indicate a long-lasting downslope mass transport of the RA1 deposits embedding also lithoclasts mainly derived from the pinnacled Calcari a Crinoidi surface.

It is suggested that the upslope morphology was a stepped surface exposing in places the Fe-Mn-encrusted pinnacled discontinuity. The pinnacled debrites point to an intense mechanical erosion of the Calcari a Crinoidi in the upslope area also after the formation of the dissolution surface (Fig 7).

4.4 Fault-controlled paleotopography in "Cava Palo"

"Cava Palo" is a small, abandoned quarry at about the middle of the slope of Monte Kumeta. Along the south- facing E-W trending quarry wall the sharp unconformity between Inici M2 and the RAI can be observed in detail (P1. 50/1 and Fig. 8). Inici M2 consists of well-sorted bioclastic/

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oolitic grainstones with Paleodasvcladus and TDaumato- porella. Reddish calcilutites from the overlying RAI pen- etrate an intense network of fractures and infiltrate in places an intergranular porosity in ttle grainstone.

Between Inici M2 and the RAI deposits neithcr lnici M3 sediments, nor Calcari a Crinoidi occur in this section. This is a common relation observed on the tipper slope of Monte Kumeta.

The Inici surface is characterized by din-scale fault steps predating the RAI deposition. Later small faults displacc thc unconformity surface and the overlying RAI. On the Inici surface a discontinuous Fe-Mn crusl occurs, from where Middle and Upper Toarcian ammonites were collected (F. Macchioni pers. comm., 2000).

Upward a bed of about 20-40 em thickness rests on this stepped morphology. It contains scattered fragments derived from the main Fe-Mn crust, and in places, angular fragments of Inici.

The articulate paleotopography in the eastern part of the quarry shows a m-scale depression filled up with alternating Bositra-rich limestone and laminated goethitic- marly/clayey horizons which laterally pinch out con> pletely. The Bositra limestone exhibits a distinct normal gradation given by concentration of randomly oriented shells at the base and passes gradually into a very fine bioclastic pack/mudstone with abundant small fragments of thin-shelled bivalves (P1.48/7). They are interpreted as channel-filling microturbidites.

On top, an about 80 cm thick bed of reddish Bos#ra l imestone follows. Higher up, along the present-day slope, thinly- laminated Bositra limestones and lhe varicoloured marly l imestone of the RAId can be also observed.

E-W trending neptunian dykes showing a polyphase filling can be observed cutting subvertically the lnici limestone both in the western and eastern termination of this quarry.

5 DISCUSSION

The Jurassic succession of Monte Kumeta reveals the way how the transformation e r a carbonate platform inlo a pelagic escarpment was controlled by tectonics and variations in carbonate productivity.

5.1 Demise of the Inici p lat form in the Monte Kumeta area

In Western Sicily, as in the Jurassic of the Western Mediterranean in general, "drowning" unconformities arc commonly manifested as sharp contacts between peritidal l imestones (Inici M 1) and the overlying pelagic deposits - as observable in the Saccense Domain (e.g. Monte Magaggiaro and Monte San Calogero) and in the Trapanese Domain (e.g. Montagna Grande). Monte Kumeta represents an exception, because transitional facies are present below the unconformity. Here, the presence of a thick unit of oolitic-skeletal l imestone (Inici M2), which overlies the peritidal deposits, indicates a high-energy margin at l~ate Hettangian - Sine- murian times. Switching from a lagoonal-tidal environment

Pig. 8. Eastern termination of the "Ca~ a Pale". showing the tmlap of the RAI on a small scale s~.cpped surlacc el (solitic hioclaslic grainsloncs (Inici M2). The contact is displaced by a small norlnal fault postdatingihe RAI. 1) Inici M2; 2) Mas,,ive reddish, bioclaslic wackestonc overlying a discontinous thick Fo-Mn crw, l wilh -Foarcian ammonites: 3) Alternating l~cJ~ilra limestones and laminated goethitic marly/clay horizons; 4) Massive tTo.~'itra I i mestones.

el a Bahamian-type carbonate platform w all oolitic margin is interpreted as a result o1" the creation of an escarpment facing a deeper inarine realm. However, t)ccuircnce of abundant and well-diversified skelctal grains suggests Ihal a ]agoonal environment persisted behind the margin, thus providing for a high rate of carbonate production.

Our data coupled with recent structural interprclations of the Monte Kulncta area (Avcllone et al., 1998) indicate thai the Monte Kumeta margin was connecicd soulhward to the Marinco basin, a deep-water Liassic basin, previously recognized by Catalano and D'Argcnio (1982b) on the base of borehole data. Thesc authors argucd lhal such a basin would be a local exprcssion of a regional strike-slip stress fictd leading Io the opening of several intraplatform pull-aparl basins along the African margin during Early IAassic times (e.g. Streppenosa Basin in Ihe Hyblean domain}. However. neither the lateral extension nor the geometry of this basin are well constrained.

On top of the oolitic unit (Inici M2) patches of fine- grained peloidal-bioclastic grainstoncs, al places merging m wackestones with radiolarians and large coaled grains (lnici M3), suggest Ihat during the earliest Carixian the productivity of the carbonate platform has decreased. The lnici M3 is very similar to the so-called Calcare Massiccio B described from the Umbria-Marchc basin by Ccntamore el al. (I 97l). As suggested by Bicc and Stewart (1990) for coeval examples in the Northern Apennines, the teclonie fragmenta- tion of the plalform, among other faclors, could have had an important role leading to perturbations in carbonate accumulation. Moreover Galluzzo and Santantonio (2002) swg- gcst that the anomalous carbonate produclion recorded by the Massiccio B could have been the consequence of lcc- Ionic isolation of seclors previously ctmnected to highly productive areas. Accordingly, 1he decreasing carbonate production recorded by the lnici M3 during earlicsl Carixian time is hcrc interpreted as aresuh era tcctonically control led isolation of Monte Kumeta flom an inlErred larger plal form sector extending to the north. Cooler water from adjacent basins could have also contributed in decreasing waler

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339; Bice and Stewart, 1990, Santantonio 1993, Zempolich, 1993, Santantonio et al., 1996; Cobianchi and Picotti, 2001 ; Mattioli et al., 2002).

Crinoidal limestones are widespread in the Lower Jurassic of the Mediterranean region. The best facies equivalent of the Calcari a Cfinoidi of Monte Kumeta is perhaps the Encrinite di Fanes of the Dolomites in North- ern Italy (Gaetani, 1975, Masetti and Bottoni, 1978) with a similar age (Pliensbachian) and analogous depositional patterns (large-scale cross-bedding) and stratigraphic setting (resting on the dissected surface of Lower Liassic Calcari Grigi platform carbonates and capped by a Toarcian hardground with ferro-manganese crust).

Erosion of part of the encrinites and the underlying Inici M3 may have been also related to the stepwise, fault- controlled subsidence possibly resulting in destabilization and removal of large masses of the not yet fully consolidated sediments.

Fig. 9. Lithostratigraphic correlation of the Jurassic succession at Monte Kumeta showing the drastic thickness increases of the l nici M3, Calcari a Crinoidi, RAI and membro radiolaritico from the present day top toward the southern slope.

temperature below the optimum for carbonate production (Centamore et al., 1971 ).

In earliest Carixian times the Monte Kumeta sector thus became a submarine structural high facing the Marineo basin to the south. Vertical and lateral changes and geometrical relationships of the overlying Calcari a Crinoidi and RAI (Fig. 9) indicate that these sediments were deposited on a southward dipping, stepped surface brought about mainly by repeatedly reactivated, basinward-dipping normal faults. This suggests that the studied area was located in the upper zone of the escarpment connecting the Monte Kumeta high and the Marineo basin during a progressive northward retreat (Fig. 10).

This scenario is clearly reflected by the relationship of Inici M3 and the overlying Carixian encrinites. The encrinite bodies show a prismatic geometry , becoming thicker towards the south and filling the first generation of neptunian dykes (Wendt, 1969). The Calcari a Crinoidi indicate that during the Carixian another carbonate source has got to be involved. Paleobathymetric data from fluid inclusions in syntaxial overgrowth cements in the Calcari a Crinoidi suggest that this facies transition took place in a depth of about 25 m, i.e. the anomalies in carbonate platform productivity occurred in shallow water (Mallarino et al., 2001). Thus some environmental perturbations seem to be responsible for the platform demise rather than extremely rapid rates of tectonic subsidence that outstripped the deposition rate of shallow water carbonate platforms (Jenkyns, 1970a p.

5.2 Imprints of the Toarcian anoxic event

The top of the Carixian/Domerian p.p. encrinites is marked by the peculiar .jagged dissolution surface with din-scale pinnacles (i.e. the main hardground). The interpinnacle space and occasional slight topographic depressions of the eroded surface of Inici M3 also are filled by pelagic sediments of Toarcian age, suggesting that the definitive switching from benthic to pelagic carbonate production could have occurred during Late Domerian times. A thick ferromanganese crust seals both the pinnacled surface and the Toarcian pockets. According to our understanding, the formation of this exceptional discontinuity surface can be explained by a complex change in water chemisty. This change caused the solution of the formerly lithified carbonate surface, followed by the precipitation of manganese, iron and phosphate minerals. This chemistry perturbations were most porbably connected to the Early Toarcian anoxic event, which only touched the slope of the Kumeta submarine high.

While true anoxic sediments are unknown from Monte Kumeta, the effects of the contemporaneus oxygen deficiency can be very well surmised. It is well known, that because of the capacity of oxygen deficient water masses to accumulate dissolved Mn -ion-species, anomalously thick Mn crusts may precipitate at redox boundaries. In fact, according to Cronan (1980) deep basins "may develop reducing conditions either within the water column or on the basin flooor" and "under these circumstances iron and manganese will not precipitate as oxides in the deeper portions of the basin". Instead they would occur in the marginal areas, where the contact with oxygenated water masses provides the necessary redox conditions for Mn- oxides formation.

In recent ocean basins, for example, maximum Mn- nodule abundance follows the known or inferred boundary of the Atlantic bottom water, and this boundary often coin- cides with the lysocline which at the same time marks a drastic decrease of the saturation level of seawater with

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Fig. 10. Cartoon dcpiclin<g the Monte K u recta escarpment du ring the Middle Jurassic. Polyphase ncptunial~ dykes, llltllipJc slopped unconformities cicalcd eitlncr by l lornla l [aults or dowr ls lo l )e cie-

tachcmenl o1 bh~cks, slumpir~gs and debris f l ows arc common featurc~- ~ihmg this shH~e. Ti~o ,-elatioi~ships of lhe escarpment v, ith the Nlar inen basin arc i h ierred

(dollied linosJ.

respect to CaCO 3 (Cronan, 1980). Anomalously thick Mn- encrustations were described from seamounts of the Atlantic and the Indian Oceans, where their slopes intersected intermediate oxygen minimum zones (Dickens and ()wen, 1994, Koschinsky et al., 1993). Oxidation and precipitation of Mn took place invariably at the redox boundary often when oxygen depleted Mn-rich waters became "mixed with cold oxygenated bottom currents" (Koschinsky et al. 1993). From the Tropic seamount offWest Africa also the association of manganiferous crust with phosphatization of the underlying hard limestone surface was described by Koschinsky et al. (1993).

In the context of the Jurassic Tethys, "'mid-waler" t,ans- port and concentration of manganese was invokcd by Jenkyns et al. (1991) to explain the formation of Mn- carbonate-rich sediments at places where ihe oxygen-minimum zone completely engulfed the depositional environ- merit of drowned platform sectors. Grusczynski (1998), discussing the validity of the stratification model ol'Hoflman et al. (1991), put forward the idea of hardground formation and C a C Q substrate corrosion as the side e[fccts of short- term mixing of stagnant bottom waters with the overlying oxic water masses.

We suggest that Monte Kumeta represents a situation analogous rather to those described from the Indian and Atlantic Oceans by Dickens and Owen (1994) and Koschinsky et al. (1993) respectively, and that the organic- rich siliceous sediments occurring in the deep water Imerese succession at Piana degli Albanesi (as described by Parisi et al., 2001) could be considered as a document of oxygen- deficient intermediate water masses coeval with the formation of the dissolution surface under discussion. These water masses, created locally or oceanwide, when incidentally

touching II~e shallowest parts olthe subsiding Monte Kumeta sector, might have restllled in the anomalous dissolution and - o n subsequent oxygenation - i n the deposilion of Ihe IhJek fcrromangancse crust. This crust marks Ihe lermination of Ihe first, shallow-waler chapter of the paleoceanographic history of Monte Kumcta. The sediincnls above the main crust record the low-rate, ephemeral sedimenlation of pelagic carbonate mud along the Kumeta slope.

5.3 Pelagic sedimentation on a stepped slope

The slope of the Kunmla high was repeatedly influenced by active Jhulting. This is best exemplified by the Toarcian main hardground which is dislocated bv abundant small faults oaf decimetrcs to metres of throw, sometimes organized into small-scale positive flower structures. In the hollows/deprcssions of this surlace pelagic sedimenis o1 Bajocian to Callovian age wcrc deposited They display a clearly onhipping relationship to the cncrinites and to lhc scdimenls of lnici M2 and M3. Their thickness rarely exceeds 4 to 5 metres, and they are prcscnl also as nepluriian dykes filling a dense network of fissures.

The distribution o1 these sediments (i.e. Rosso Ammonitico inl;eriore) on Monte Kumeta shows a general tendency of tapering northwards. This thinning is a pri mary feature of the depostional geometry, because the number (H" internal hardgrounds decreases also northward, and the stratigraphically older horizons pinch oul in northern direction, while the stratigraphically youngest ones remain as ilqost persislent, rcaclling the northernmost Iimil oldislribu- tion. This long-lasling blanketing of the dissected substrale was accompanied by repeated redistribution {H" the non-

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lithified or semiconsolidated sediments, as it is suggested by the reworked, apparently imbricated ammonites in some levels, and the appearance of cun'ent-swept grainstones in a few horizons.

6 CONCLUSIONS

Facies architecture and biostratigraphy of the Lower to Middle Jurassic succession at Monte Kumeta show that this sector of the Trapanese Domain, along the southern Tethyan margin, was initially part of a Bahamian type carbonate platform. In the Middle Jurassic, it developed into a subsiding, stepped escaq)ment of a persistent structural elevation facing the Marineo basin towards the present day south. This latter area started its career also as a part of the lowermost Jurassic Inici platform, but it became very soon a drowned then deep sunken sector, where radiolarian cherty limestones and marls deposited as early as in Sinemurian- Pliensbachian times

Major paleoceanographic changes coupled with regional extensional]transtensional tectonics and local instability appear the most important factors controlling the complex sedimentary dynamics along the escarpment. During Lale Callovian and Oxfordian times the synsedimentary tectonics intensified, and resulted in an increase of the inclination of the slope. This led to more and more abundant, gravita- t ionally-controlled deformations (slumping and sliding) of semi-lithified and un-lithified sediments along the escarpment. The subsequent change from carbonate to siliceous deposition marks the end of the second chapter of the Monte Kumeta depositional story.

As evidenced by the overlying deposits, the Liassic basin-swell topography across Monte Kumeta and Marineo sectors, lasted throughout the Jurassic into Middle Miocene times. We thus put forward that at least a part of the displacements along the fault systems bounding this ridge and actually seen, could be a paleotectonic heritage.

ACKNOWLEDGEMENTS

Discussions with N. Mariotti, U. Nicosia, G. Parisi and M. Santantonio have been very useful to develop some of the ideas of this paper. B. Gdczy kindly contributed to the determination of Toarcian ammonites. The comments of the rewievers, E. Fltigel and J. Wendt were wery helpful in improving the manuscript . We thanks the Ispet torato Ripartimentale delle Foreste (Palermo and Piana degli Albanesi) , Sig. Biscari and Sig. Cerniglia for the logistic support. Funding was from Italian MURST (ex 60% and COFIN 1997-2001, P. Di Stefano); from the Italian/Hungar- ian Scientific and Technical Cooperation project n, 1-35/98; from exchange project 'MTA/CNR' n. 5/1.

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Manuscript received November 28, 2001 Revised manuscript received February 27, 2002

birth and early evolution of a jurassic escarpment: monte kumeta western sicily

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