Download - Fe–Mn-encrusted “Kamenitza” and associated features in the Jurassic of Monte Kumeta (Sicily): subaerial and/or submarine dissolution?

Fe–Mn-encrusted “Kamenitza” and associated featuresin the Jurassic of Monte Kumeta (Sicily):subaerial and/or submarine dissolution?

P. Di Stefanoa,* , A. Mindszentyb

aDipartimento di Geologia e Geodesia, Universita` di Palermo, via Archirafi 22, 90123 Palermo, ItalybEotvos L. University, Department of Applied & Environmental Geology, Budapest, Hungary

Received 4 November 1998; accepted 4 November 1999

Abstract

An unusually jagged dissolution surface, capped by a thick Fe–Mn crust is well exposed in small quarry-cuts of the Jurassicof Monte Kumeta. It was formed on a crinoidal limestone substrate of Pliensbachian age, and is covered by Upper BajocianAmmonitico Rosso-type sediments, all cross-cut by several generations of neptunian dykes. This peculiar surface is more orless coeval with hardgrounds, Fe–Mn-capped dissolution surfaces and associated neptunian dykes described from otherlocalities of the Western Tethys and currently subject to fierce debates as to their purely submarine (or perhaps partly subaerial)origin. The major goal of this paper is to add new arguments to this debate by revealing the finest details of field relationships ata site particularly adapted to the study of this phenomenon. Field observations are supported by petrography and, to a lesserextent, by geochemistry. Results are as follows: (i) vertical dissolution grooves, pointing to dissolution by gravitationallycontrolled waters, were detected on the sides of several micro-topographic highs; (ii) extensive intergranular dissolution(predating the formation of the Fe–Mn crust) was proved in the substrate both on the micro- and meso-scale; (iii) intense(micro)bio-erosion and local phosphate enrichment were detected immediately underneath the crust; (iv) a Toarcian fauna wasidentified from the hollows of the irregular surface; (v) synsedimentary faults and fractures clearly predating the major Fe–Mn-encrusted surface were observed, and (vi) a meso-scale synsedimentary growth structure, post dating the Fe–Mn crust, whichcontrolled the Liassic depositional environment of Monte Kumeta is documented. Our conclusion is that the studied surfacerecords at least three separate events of dissolution and precipitation/sedimentation each having either erased or overprinted theeffects of the previous one and therefore not permitting the exact reconstruction of all the details of the complex story. To formthe irregular surface, in addition to a transient phase of subaerial exposure, a complex history of bio-erosion and submarinedissolution by fluids of widely different chemical composition is proposed. To permit the mixing of sea-water with fault-controlled waters of higher temperature and with groundwaters introduced by deep circulation, a scenario of down-faultedblocks and an adjoining, distant subaerially exposed region is invoked. Such a region provided the hydraulic drive for thepostulated circulation. The ultimate cause for the unusual phenomena under scrutiny was the combined effect of tectonics (thelocal manifestation of Early Liassic rifting in the Western Tethys) and the well-known Pliensbachian–Toarcian sea level-rise.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Jurassic; Sicily; pelagic sedimentation; hardground; discontinuities

Sedimentary Geology 132 (2000) 37–68

0037-0738/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0037-0738(99)00128-1

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* Corresponding author.E-mail addresses:[email protected] (P. Di Stefano), [email protected] (A. Mindszenty).

1. Introduction

Ferro-manganese crusts and nodules, commonlyassociated with prominent events of neptunian dykeand sill formation, have been described from pelagicenvironments of the Jurassic of the Western Mediter-ranean, reviewed in Jenkyns (1986), and additionallycited by, Mindszenty et al. (1986), Vera et al. (1987,1988), Garcia-Hernandez et al. (1988) and Prescott(1988). They preferentially occur at the base of, orwithin, Early to Middle Jurassic condensed Ammoni-tico Rosso-type successions at a time whensedimentation in the western sector of Tethys wasstrongly influenced by rifting, rift-related volcanism,subordinated carbonate deposition and consequentdissolution and hardground formation. Up to veryrecently, there was a general agreement regardingtheir submarine origin.

Vera et al. (1988), Jimenez de Cisneros et al. (1991)and Molina et al. (1991) described a case from theJurassic of the Subbetic Zone of the Iberian peninsulawhere, laterally equivalent to a ferro-manganeseencrusted surface, they identified supposedly con-temporaneous subaerial exposure features. Theseincluded karstification and local accumulation ofbauxites. Temporary exposure of parts of a shallowcarbonate shelf at times of overall extension andaccelerated subsidence were explained by block-rotation along listric faults cross cutting the shelfdomain (Vera et al., 1988, Fig. 17.12).

According to these authors, bauxites formed in thehighest elevated parts of the exposed shelf, whereaslow-lying coastal domains were characterized by akamenitza-like micro-relief, and the adjoiningsubmarine environment was subject to erosion,submarine hardground formation and Fe–Mn encrus-tation. When, on subsequent subsidence, the platformbecame completely submerged and drowned pelagicsediments covered the previously formed micro-relief.

Winterer and Sarti (1994) have strongly criticizedthe interpretation of Vera et al. (1988) and concludedthat the overwhelming majority of features could beexplained simply by extensional tectonics and con-current submarine processes.

A situation, similar in certain respects to thatdescribed by Vera et al. (1988) can be observed alsoin the Jurassic of the Trapanese domain of WesternSicily, at Monte Kumeta. However relationships to a

nearby contemporaneous subaerial episode areequivocal. There is compelling evidence for: (1)anomalously intense dissolution preceding the majorphase of Fe–Mn-encrustation and the opening ofneptunian dykes; and (2) the tectonically controlledgrowth of a minor submarine topographic high, accom-panied by fracturing, re-sedimentation, stratal onlapand concurrent hardground/rockground1 formation.

The phenomenon was first observed by Wendt(1965) and reiterated by Jenkyns (1970, Fig. 7, p.253) who noted an irregular Fe–Mn-coated surfacebetween the Liassic and the Dogger of MonteKumeta, but did not provide any detail.

Kindle (1990) provided some analytical data oncalcite cements collected from neptunian dykes anddyke-filling pelagic sediments of Monte Kumeta.

Di Stefano (1990a) agreed with Wendt (1965) andJenkyns (1967, 1970) that there is little doubt aboutthe Fe–Mn-crusts were of submarine origin.However, he pointed out that the morphology of theunderlying substrate showed a striking similarity tokarstic “kamenitza” surfaces (e.g. Esteban andKlappa, 1983; Choquette and James, 1988; Ford andWilliams, 1989).

By combining new field observations with theresults of laboratory studies we aim, in this paper:(a) to contribute to the understanding of the sequenceof events which resulted in the formation of what wecall an Fe–Mn capped (submarine) kamenitzasurface; and (b) re-evaluate the issue of “submarinevs. subaerial”, in the context of the well-establishedTethys-wide drowning of shallow carbonate platformsin Early to Middle Jurassic times.

2. Methods

Field observations were made in a series of smallquarries. Wire-cut faces were photographed and eithercomputer-processed or manually traced to give linedrawings. Samples were mainly taken from cut-slabs,restoring the original stratigraphic position of eachspecimen by comparing slabs and cut face on the spot.

Petrographic description and geochemical charac-terization of samples were carried out by opticalmicroscopy. Petrographic observations were aided

P. Di Stefano, A. Mindszenty / Sedimentary Geology 132 (2000) 37–6838

1 Terminology according to Clari et al. (1995).

by SEM-EDAX and microprobe in the case of thinsections, whereas bulk samples of the crust wereanalysed by AAS and ICP for major and traceelements, and by XRD for the major minerals. Alimited number of carbonate sub-samples wereanalysed for stable O and C isotopes. Stratigraphicposition was established and confirmed by biostrati-graphic means (mainly ammonites).

Instrumental analyses (optical microscopy, SEMand microprobe) were undertaken in the laboratoriesof the Institute of Geology of Eo¨tvos L. University,Budapest, the University of Gartenbau, Budapest(ICP) and at the Nuclear Research Institute of theHungarian Academy of Sciences, Debrecen (stableisotopes).

3. Geological framework

3.1. Tectonostratigraphic setting

The studied succession is located near the top ofMonte Kumeta, an E–W trending ridge, about 20 kmlong, occurring in the southern part of the Palermo

Mountains, in western Sicily (Fig. 1). Monte Kumetabelongs to a major structural unit of the intermediatezone of the Sicilian–Maghrebian chain. This unit isderived from the Neogene deformation of a widepalaeogeographic domain pertaining to the Africanpassive margin, known as the Trapanese Domain.(Catalano and D’Argenio, 1982a). The stratigraphicand structural relationships in the Maghrebian belt,indicate that during the Late Triassic the Trapanesedomain formed part of a wide carbonate platform,extending to the southeast in the Sicily channel andthe Hyblean plateau, and northwesterly to thePanormide domain of northern Sicily. To the eastthis platform was bound by a wide slope to a basinalarea, formed by the Imerese and Sicanian domains(Catalano et al., 1991, 1993, 1996).

Transcurrent motions along the African margin,related to the opening of the Neotethys, were respon-sible from the Late Norian for the dissection of thecarbonate platform and the opening of intraplatformbasins during latest Triassic-earliest Liassic time(Catalano and D’Argenio, 1982b). During the MiddleLiassic a large part of the carbonate platform drownedand Ammonitico Rosso deposits blanketed a complex

P. Di Stefano, A. Mindszenty / Sedimentary Geology 132 (2000) 37–68 39

Fig. 1. Major structural domains of the Central Mediterranean and structural map of Sicily (based on AA.VV. Structural Model of Italy, sheetno. 6) with location of the studied area and the localities quoted in the text.

system of small basins, swells and tilted block slopes(Fig. 2).

A different evolution is recorded by the sedimen-tary successions pertaining to the Panormide domainin the north (Fig. 3). The Upper Triassic margin of thisplatform was tilted, subaerially exposed and subse-quently drowned during the earliest Liassic, and wasconverted to a slope environment. A tectonic controlin the evolution of this slope is suggested by severalneptunian dykes of Jurassic age (Vo¨ros et al., 1986).The presence of a master fault along the platform

margin is documented by a hugein situ breccia withAmmonitico Rosso matrix (Billiemi limestone) (DiStefano, 1990b).

Tectonic activity coupled with sea-level changes inthe inner sectors of the Panormide platform, resultedin tilted blocks with prolonged subaerial exposure oftheir highest elevated sectors during the Jurassic. Thisis well documented by an angular unconformitybetween peritidal cycles of Late Triassic age and theonlapping Upper Jurassic sediments. The unconformityis marked by bauxites (Ferla and Bommarito, 1988;


Fig. 2. Tentative restoration of Middle Jurassic palaeogeography of the Sicilian–Pelagian area: Similar to other Tethyan platforms the extensiveLate Triassic Siculo-Tunisian Platform (sensuDi Stefano et al., 1996), comprising the Panormide, Trapanese, Saccense and Hyblean domainsand bordering the African margin of Tethys, was also converted into a complex mosaic of small basins and swells during the Middle Liassictimes. This basin and swell topography and the associated slopes were mostly covered by Ammonitico Rosso-type sediments. It was only insome sectors of the Panormide platform that the top of large tilted blocks became subaerially exposed and had been the site of bauxite formationfrom? Early to Middle Jurassic times. Simultaneously in the adjacent non-exposed sectors sedimentation of Ammonitico Rosso was essentiallycontinuous.

Simone et al., 1992) which are exposed at MonteGallo, in the northernmost sector of the PalermoMountains (Fig. 4).

The stratigraphy of the Trapanese domain, partly

corresponding either to the “Vicari zone” of Broquet(1968) and Mascle (1979), or to the “Trapanese basin”of Giunta and Liguori (1973), has been outlined byseveral authors (Wendt, 1965, 1969; Jenkyns, 1967,


Fig. 3. Chronostratigraphy of the Panormide, Imerese and Trapanese successions of western Sicily. (modif. from Catalano et al., 1996; timescale according to Harland et al., 1990).

1970; Giunta and Liguori, 1972, 1975; Mascle, 1979;Catalano and D’Argenio, 1982a,b).

The “Trapanese type” succession is characterizedby several thousand metres of Upper Triassic peritidaldolostones, followed by 200–300 m of Lower Liassicperitidal and open-shelf limestones (Inici Formation).These deposits are unconformably followed in turn bythe Ammonitico Rosso of Late Liassic–Tithonianage, at places, with interposition of discontinuouslenses of brachiopod/crinoidal limestones and

volcanics. Calpionellid cherty calcilutites (Chiara-monte Formation, also known asLattimusa) andCretaceous to Eocene pelagic cherty calcilutites andmarls (Hybla and Amerillo (� Scaglia) FormationsAuct.) follow upward (Figs. 3–5).

3.2. The Jurassic succession of Monte Kumeta

The Jurassic lithostratigraphy of Monte Kumeta isshown by Fig. 6. The oldest outcropping deposits are


Fig. 4. Monte Gallo area, near Palermo. Unconformity marked by bauxites between Upper Triassic and Upper Jurassic peritidal cycles, along asuccession pertaining to inner zones of the Panormide Platform.

Lower Liassic peritidal limestones of the IniciFormation. These deposits are cross cut by largeneptunian dykes with polyphase fillings. In the upper-most zone these peritidal limestones gradually giveway to grainstones with coated grains. Skeletalgrains consisting of benthic foraminifera (mostlyValvulinids), gastropods, echinoderms, rare crinoids,calcareous algae such as (Cayeuxiasp.,Thaumatopor-ella parvovesiculifera(Raineri) and fragments ofPalaeodasycladus mediterraneus(Pia), are the mainconstituents, associated with micritized ooids andbotryoidal lumps. The age of these beds is not wellconstrained by biomarkers; however, in the adjacentstructure of Rocca Busambra, located about 20 km


Fig. 5. Lithostratigraphy of the Monte Kumeta unit.

Fig. 6. Jurassic lithostratigraphy of Monte Kumeta.


Fig. 7. (a) Geological map and (b) cross section of Monte Kumeta and location of the studied outcrops.

south of Monte Kumeta and pertaining also to theTrapanese domain, a rich ammonite fauna gives aSinemurian age (Gugemberger, 1936; Warman andArkell, 1954). The gradual transition of peritidaldeposits to open-shelf grainstones could have beenrelated to changes of the physiography of thecarbonate platform in a ramp setting, as a probableconsequence of tectonic tilting.

Lenticular bodies of brachiopod-bearing skeletalgrainstones up to a few metres thick, here informally

named Brachiopod limestone, overlie these deposits.They show a composition similar to the open-shelfgrainstone and may be the lateral facies equivalentsof the latter in the deeper ramp sectors. However, therelationship of these brachiopod-bearing sedimentswith the underlying formations is not directly obser-vable at Monte Kumeta.

White to pink encrinites with a massive structurefollow upward. These deposits, informally named“Crinoidal limestone” by several authors, also show


Fig. 8. Quarry A 1: (a) contact between the Brachiopod limestone and the Crinoidal limestone; (b) detail of the contact showing several phasesof fracturing and filling: (1) Brachiopod limestone; (2) Fracture filling calcilutite ff-I; (3) Fracture-filling crinoidal sand ff-II; (4) Fracture fillingfibrous calcite ff-III and internal sediment ff-IIIa; (5) Angular fragments of Brachiopod limestone and crinoidal sand (fracture fill ff-IV); (6)Eroded surface of Brachiopod limestone; and (7) Crinoidal limestone.

lenticular geometries with a maximum thickness ofabout 20 m. Their relationship with the underlyingrocks could be observed only in a small quarry nearthe top of Monte Kumeta (Fig. 8), where they uncon-formably overlie the Brachiopod limestone with asharp, erosional contact (see detailed description inSection 4.1.2). These deposits consist of up to 80%crinoid ossicles with subordinate other bioclasticgrains (benthic foraminifers, Thaumatoporellafragments), micritized ooids, and intraclasts. Also,well-rounded lithoclasts of grainstone with coatedgrains, derived from the underlying formation, canbe observed. Jenkyns and Torrens (1969) and Jenkyns(1970) indicated a probable Domerian age for thesedeposits in the Monte Kumeta area. Jenkyns (1970)interpreted these deposits as the result of depositionfrom migrating dunes or sand waves on top ofseamounts. Another possibility could be that the encri-nites are slope-apron type deposits related to tectoni-cally controlled subsidence of the formerly shallow-water platform. Whichever depositional mechanismwe accept, the erosion of the underlying rock, as weobserved in the small quarry, may have been related tothe mechanical abrasion by the moving crinoidal sands.

The Ammonitico Rosso deposits follow uncon-formably upward. Their base is marked by a thickferromanganese crust (see Section 4.1.3.1) that has aregional extent. The Ammonitico Rosso of MonteKumeta can easily be subdivided into a lower andupper member separated by an intermediate chertymember (Fig. 6).

The lower member, here informally named LowerAmmonitico Rosso (LAR) consists of 3–4 m ofmassive reddish condensed limestone spanning fromthe Toarcian up to the Oxfordian p.p.

The intermediate cherty member (ICM) consists of a10–15 m thick complex of varicoloured bedded chertsand radiolarian marls. No biostratigraphic data areavailable for this unit although a Late Oxfordian–Kimmeridgian age was indicated by Wendt (1969).The contact between the ICM and the LAR is poorlyexposed. Along the wall of Quarry “A”, the uppermostzone of the LAR, immediately below the ICM, showsslumpings and pebbly mudstone levels. Moreover thecontact surface is repeatedly downfaulted, pointing toan infra ICM tectonic event.

The Upper Ammonitico Rosso (UAR) is wellexposed on top of Monte Kumeta. Here 10–15 m of

reddish to pink, partly nodular limestone of Kimmer-idgian to Early Tithonian age are intercalated betweenthe ICM and the overlying calpionellid limestones(Lattimusa). Several facies types alternate along theUAR succession.

A well-exposed section along the wall of anabandoned quarry on top of Monte Kumeta showsfrom the bottom: a coarse megabreccia up to 1 mthick, resting on the top zone of the ICM and com-prising large angular fragments of platform lime-stones (from the Inici Formation) and minorlithoclasts of LAR. A 3 m thick and massive bed ofreddish mudstone with rareSaccocoma followsupward. This bed is pervaded by a network of sheetcracks filled by sparry calcite. Large angularfragments of Inici limestone up to 1 m in diameterare also embedded in these limestones. Above thisbed about 3 m of pebbly mudstone, consisting ofslumped nodular wackestones, occur. The upper partof the UAR consists of about 10 m of crinoidal lime-stones whithGlobochaete, Saccocoma, radiolariansand, in the topmost beds,Pygope. The presence ofmegabreccia and pebbly mudstones, resting on aslightly folded substrate and sheet cracks, interpretedas due to an incipient downslope creep, suggest aslope environment for the Monte Kumeta zone duringthe deposition of UAR.

4. Monte Kumeta

4.1. Field observations

4.1.1. General remarksThe Fe–Mn-encrusted surface associated with the

basal Ammonitico Rosso is well exposed in two smallquarries at about 200 m below the summit of MonteKumeta (see Fig. 7).

Quarry “A1”: in the lower part of this small quarrya fine-grained shallow-water bioclastic packstone/grainstone has lenticular intercalations of brachiopodand bivalve shells. It is overlain by coarse Domeriancalcarenites (� encrinites). The boundary betweenthe brachiopod-bearing limestone and the encrinitesis a sharp, uneven bioeroded and/or mechanicallyeroded discontinuity surface (� a “rockground”sensuClari et al., 1995). In certain places, the limestoneexhibits a well-developed brecciation limited to a


zone of about 25–30 cm immediately below thecontact with the overlying encrinites. The matrix ofthe breccia consists of infiltrated material of theoverlying encrinites (Fig. 8). The reason for thisbrecciation (erosional?) is not clear. On top of theencrinites there follows the serrate, Fe–Mn-encrusted rockground (details below) overlain byabout 3.5 m of Ammonitico Rosso (“LAR”, Toarcianto Oxfordian).

Quarry “A2”: the brachiopod-bearing limestone isnot exposed here. The succession begins withencrinites, followed by the rockground and then bythe Ammonitico Rosso. The exposed successionends with Oxfordian–Lower Kimmeridgian radio-larian marls and bedded cherts (Fig. 9).

4.1.2. Fractures, dykes and cavitiesSeveral sets of fractures (both bed-parallel and

vertical to subvertical) penetrate the encrinites andalso the Ammonitico Rosso. Some of these fractureshave strikingly sharp contacts with the host. They arefilled by a wide variety of pelagic material and byangular fragments broken off from their walls togive the impression of a “fitted breccia”. Othersdisplay effects of repeated dissolution, minor collapse,cementation and internal sedimentation as well (Figs.8(a), (b) and 10(a), (c)). The majority of features asso-ciated with these fractures (open spaces, normaldisplacements, collapse phenomena, etc.) suggestsfracturing in an overall tensional regime, although insome cases compressional relationships were alsoobserved.

One of the earliest phases of fracturing cross-cutssubhorizontally the brachiopod-bearing limestone andis filled by fitted, angular fragments of the host rockembedded in a fine, pink calcilutite (ff-I in Fig. 8). Thesedimentary dykes thus formed have sharp contactswith the host rock and show no cement growth preced-ing or interrupting the deposition of the dyke-fill.They are clearly cross-cut by a subsequent set of like-wise subhorizontal fractures filled by the overlyingencrinites and again by angular fragments of theenclosing limestone (ff-II). In both cases filling musthave taken place immediately after the opening of thefractures by the classic mechanism of neptunian dykeformation (suction of the overlying unconsolidatedsediment), because fracture-walls do not showdissolution or cementation and the fracture-fill is notstratified.

The encrinite-filled dykes are crosscut by asubsequent generation of fractures, which are,however, clearly enlarged by dissolution. Theirwalls are coated first by a thin (1–2 mm) isopachlayer of black calcite, followed by a white radial-fibrous mass either filling all the remaining space, orinterrupted by an event of internal sedimentationresulting in a greyish-green, or red laminatedmechanical infill followed by a last generation ofwhite fibrous calcite (ff-III, -IIIa).

Crosscutting ff-III, there is another set of fractures,filled again by encrinites (ff-IV) and distinguish-able from ff-II only by its crosscutting relationshipto ff-III.

The last major fracturing event resulted in little


Fig. 9. Panoramic view of Quarry A 2 as in 1991 showing the contact between Crinoidal limestone and L.A.R deposits (a) Crinoidal limestone(CL) occurs as discontinuous lenses, 5–20 m thick, on top of Lower Liassic platform limestones (Inici Formation) not exposed here. The LowerAmmonitico Rosso (LAR) consists of 3–4 m of massive limestone covering a time span from Toarcian up to Oxfordian. Upward a 10–15 mthick succession of radiolarites and radiolarian marls (ICM) of Oxfordian to Lower Kimmeridgian age follow. (b) Outcrop view of thehardground capped “kamenitza” on top of the Crinoidal limestone.


Fig. 10. Quarry A2: The unconformity between the Crinoidal limestone and LAR as exposed by a quarry-face in 1997: (a) Vertical section of thehardground-capped “kamenitza” on top of the Crinoidal limestone of Pliensbachian age, which forms a couple of metres high palaeorelief here.The small-scale palaeotopoghraphic high (a flower-like synsedimentary growth structure) is the result of brittle deformation partly contem-poraneous with and partly post-dating the dissolution surface. The palaeorelief is onlapped by the lower Ammonitico Rosso (L.A.R.). The age ofthe sediment covering the principal hardground is Late Bajocian (Humpresianum zone) as shown by Wendt (1965); Jenkyns (1970) and Galacz(personal communication). Several Mn–Fe encrusted surfaces with laminated or bulbous structures follow the principal hardground along theAmmonitico Rosso section (SC-1 to SC-3). (b) Close-up of the above hardground-capped “kamenitza”. Immediately on top of this serratesurface there is a 20 cm thick layer of re-sedimented ammonites, which show a distinct imbrication suggesting transport from the direction of therelative palaeo-high. Incipient Fe–Mn crusts (satellite crust “SC-1”) intercalated in, and covering the ammonite-bed indicate that the process ofre-sedimentation was interrupted by episodes of non deposition and Fe–Mn precipitation. (c) Coastal meso-karst relief developed on exposedbeach-rocks on the island of San Salvador, Bahamas. (d) Lateral (Southward) continuation of the above section showing a network ofdissolutionally enlarged fractures crosscutting the underlying Crinoidal limestone and filled by pelagic mud.

subvertical normal faults crosscutting all previouslyformed fracture-fills and bringing about a series ofminor displacements along the contact betweenencrinites and the overlying Ammonitico Rosso.Some of these little displacements were later clearlyrenewed because they can easily be traced upwardswithin the Ammonitico Rosso; others fade quickly,suggesting, that this phase of minor normal faulting

was partly antecedent to and partly contemporaneouswith the deposition of the basal layers of theAmmonitico Rosso. The scale of the displacementsis strikingly similar to the scale of the “kamenitza”topography observed in association with the Fe–Mnencrusted rockground (see below).

Higher up, within the Ammonitico Rosso, there isample evidence for additional recurrent episodes


Fig. 11. Details of the serrate Mn-encrusted interface between the encrinites and the overlying Ammonitico Rosso sediments (partly observed onthe smooth surfae of large wire-cut-blocks astray in the quarry) (a) Eroded surface showing several pinnacles developed on Crinoidal limestones(1), capped by the Fe–Mn crust, followed in turn by the Ammonitico Rosso deposits (2). In the interpinnacle spaces a pinkish–yellowish pelagicwackestone can be observed (3), sealed upward by the principal hardground. Hildoceratids recovered from this interpinnacle sediment gave aToarcian age (Falciferum/Bifronszones). In the lower part a neptunian dyke crosscutting the Crinoidal limestone can be observed (4). It is filled-up by Ammonitico Rosso deposits with Late Bajocian brachiopods (Apringia cf. alontina, det. A. Voros). (b) Vertical section of thin andelongate pinnacles showing irregular corrosion phenomena along their walls and evidence of bio-erosion. (c) Vertical section showing corrosionphenomena particularly concentrated in the basal zone of the pinnacles, resulting in the weakening of the structure and in some cases, ultimatelyin rupture and toppling of the originally vertical elements (arrow). (d) Transversal section of pinnacles (a) surrounded by the Fe–Mn crust (b)and by the Ammonitico Rosso deposits (c). In transversal sections the pinnacles show a highly irregular, corrosion-controlled morphology. Thepinnacle surface is often sculptured by longitudinal grooves oriented parallel to their axis, giving the impression of dissolution by gravity-controlled water flow. (e) Hard-ground-capped pinnacles at Rocca Busambra (Piano Pilato) near Corleone. In this locality the peculiardiscontinuity surface is developed on platform limestones of the Inici formation.

of small-scale brittle deformation post-dating theformation of the principal mineralized rockgroundand essentially contemporaneous with the depositionof the Ammonitico Rosso itself (for details seeSection 4.1.4).

4.1.3. Discontinuity surfaces

4.1.3.1. The “principal” Fe–Mn encrusted rock-ground. In Quarry A2, the serrate Mn-encrusted

interface between the encrinites and the overlyingpelagic sediments can be studied in detail (Figs. 10and 11). In vertical section (Fig. 11(a)), this surfacepresents itself as a row of sharp, elongate, rectangular/rectilinear pinnacles (height: 20–25 cm, width at thebase: 10–20 cm, at top sometimes not more than5 cm, spacing: 5–15 cm). As a rule, close to thebase of the pinnacles, the rectangular outlines becomesmoothed and curvilinear.

Lateral extension of the phenomenon is difficult to


Fig. 12. (a) Grain-corrosion in the Crinoidal limestone underneath the crust. The intergranular spaces are filled by Mn-oxide and calcisiltite.Thin section KP2/A. (b) Mesoscale irregularity of the “Kamenitza”-surface (may be the result of sponge (?) bio-erosion. Bioerosion on themicro-scale (fungal/algal borings) is also obvious. (c) Grain corrosion in a red calcisiltite-filled cavity (in Crinoidal limestone). Thin sectionKP2/C. (d) Close-up of vertical wall of pinnacle. To the left the Crinoidal limestone, corroded surface, with microborings. To the right redpelagic biomicrite with internal hardgrounds.Thin section 90-06-28/2/B. (e) Dissolution cavities in encrinite filled by red calcisiltite. Notedogtooth-calcite rimming the micritic peloid(s) and syntaxial calcite grown on crinoid ossicles. Corrosion of early cement is obvious along thecavity walls. Thin section 90-06-28/3/A.

judge, because of the two-dimensional nature of thequarried wall and the limited area exposed by themining operations (details of the Fe–Mn encrustedsurface are almost impossible to discern on theweathered discontinuous outcrop of the grassyhillside). Similar features, though not as spectacu-lar as here, are long since known from other Trapa-nese-type successions (quarries of Montagna Grandeand Rocca Busambra, mentioned by Wendt, 1969;Jenkyns, 1970; Martire et al., 1998) indicating thatthe peculiar surface may have been more widespreadthan suggested by the relatively isolated ridge ofMonte Kumeta.

The pinnacles, though generally characterized bysurprisingly smooth sides, may show smaller or largerirregular corrosion phenomena along their verticalwalls and locally on their top. This corrosion mayresult in “overhangs” or sharp pin-pointed peaks inthe vertical section (Fig. 11(a)). Where affecting thebase of the pinnacles, corrosion has resulted inmerging overhangs, weakening of the structure andin some cases even in consequent rupture and topplingof the originally vertical elements (Fig. 11(c)). Whenlooked at from above, the morphology of the pinnaclesis even more irregular and clearly corrosion-controlled(Fig. 11(d)). Their surface is often sculptured by long-itudinal grooves oriented parallel to the axis of thepinnacles giving the impression of dissolution bygravity-controlled water-flow—a phenomenon well-known and extensively discussed by karst morpholo-gists in relation to various small-scale karst features,like kamenitza, rillen-karren, etc. (Sweeting, 1973;Schneider, 1976; Bo¨gli, 1978; Desrocher and James,1988; Ford and Williams, 1989).

It is interesting to note that, despite the clear-cutrectilinear network they form in the vertical section,only few pinnacles show a direct continuation towardsthe fractures criss-crossing the substrate; thus the ideaof a direct tectonic control of this strange micro-topo-graphy, even if tempting because of the regularity ofthe phenomenon, is only partly sustainable.

In contact with the overlying Fe–Mn crust, atplaces the encrinite is Fe–Mn-stained. Staining occursalways in the intergranular space, apparently substi-tuting the original matrix (or cement), and rapidlyfades with distance from the crust (Fig. 12(a)). Atother places, the contact between encrinite and crustis sharp; the encrinite shows no perceptible alteration,

at least not at the hand-specimen scale (for moredetails see Section 4.2 below).

The interpinnacle space is filled by pinkish–yellowish pelagic wackestone rich in iron-coatedbioclasts and lithoclasts (hardground fragments) upto 1 cm in diameter. Occasionally broken pieces ofthe pinnacles (recognizable by their grooved,Mn-coated surface) could be identified among theclasts. The distribution of the clasts shows a vaguestratification suggesting preferential deposition inthe interpinnacle lows. Hildoceratids recovered fromamong the “rubble” and identified by A. Galacz(personal communication) gave a Toarcian age(Falciferum/Bifronszones) in accordance with theobservations of Wendt (1965), who assigned thesame age to the earliest fracture-fills in the Pliens-bachian substrate of the Monte Kumeta unit. Thisshows that the formation of the serrate dissolutionsurface, which controlled the deposition of the inter-pinnacle rubble probably, did not take much longerthan a few Ammonite zones.

The deposition of the interpinnacle sediment wasapparently a very slow process interrupted by periodsof non-deposition when the slightly concave sedimentsurface became coated by a thin (,1 mm) film of Fe-or Fe–Mn-oxide (Fig. 12(b) and (d)). The frequencyand the thickness of the thin coatings (incipient hard-grounds) increase upward and terminate in the maincrust, covering and “smoothing out” the still irregularsurface of the “pinnacle field”. The average thicknessof this terminal crust is about 20 cm. It is generallyblack and exhibits a conspicuous laminated structureending up in a bulbous upper surface in the lobes ofwhich every now and then again preferential accumu-lation of Fe–Mn coated intraclasts and bioclastsembedded in yellowish–pinkish pelagic mud can beobserved. At places, e.g. in quarry A1, encrustations,belonging to the principal hard-ground but beingobviously richer in goethite than in Mn-oxides werealso observed. Goethite generally predates theprecipitation of the major Mn-oxide phase. The ageof the sediment covering the Mn-encrusted surface isLate Bajocian (Humphresianum zone) as shown byWendt (1965), Jenkyns (1970) and Galacz (personalcommunication).

4.1.3.2. Hardgrounds within the Ammonitico Rosso.Following the formation, Fe–Mn encrustation and


burial of the principal rockground, during thedeposition of the overlying Ammonitico Rosso anumber of additional “satellite” discontinuitysurfaces were formed and encrusted by thin Fe–Mn-rich precipitates. The relief coated by these thin crustsis, however, never as spectacular as in the case of theprincipal hardground and also their thickness isinvariably minor. (Fig. 10(a)). They may show alaminated or bulbous, stromatolite-like structure.Locally, along strike, cm-size “bulbs” are separatedby uncoated or very thinly coated sections pointing toa discontinuous Fe–Mn oxide film covering theuneven, bioturbated sediment surface. In placesthese “satellite” crusts are represented simply by athin layer of unconnected, Mn-coated intraclasts,suggesting that crust formation may have begunwith encrustation and subsequent gradualcoalescence of intraclasts “adrift” on the surface ofthe strongly bioturbated, nodular AR.

4.1.4. Tectonically controlled-small-scale lithofaciesvariations observed within the basal tract of LAR

4.1.4.1. Vertical variations. In quarry A2 wherestratal geometries suggest 3–5 m of palaeorelief, theAR displays conspicuous facies and thicknessvariations (Fig. 10(a)). Immediately overlying theprincipal crust, in front of a prominent relativepalaeo-high there is a 20 cm thick layer of well-sorted,2–3 cm size, worn, encrusted, shingled ammonites,which show a distinct imbrication suggesting trans-port from the direction of the relative palaeo-high(Fig. 10(b)). Incipient Fe–Mn crusts (satellite crust“SC-1”) intercalated in and covering the ammonite-bed indicate that the process of re-sedimentation wasinterrupted by episodes of non-deposition and Fe–Mnprecipitation. This basal bed is overlain by about2.5 m of intensely bioturbated, nodular AR on thetop of which there is an irregular 5–10 cm thicklaminated marly layer stained by goethite and towardsthe top showing stromatolitic structures and repeatedthin bulbous Mn-oxide encrustations (“SC-2”). Theinterlaminar space is filled by sparry calcite andlocally by pink pelagic mud. Overlying the goethiticmarl horizon about 10 cm of pink pelagic mudfollows, with laminae rich in small,1 cm-sizedMn-coated intraclasts and bioclasts. The uppersurface of this muddy layer exhibits a peculiar

scalloped micro-relief of 1–3 cm amplitude. Topo-graphic highs of this micro-relief are covered by athin, often bulbous Mn-oxide crust (“SC-3”). Higherup in the succession, the last exposed Mn-coated hard-ground (“SC-4”) occurs in quarry A2, capping anabout 60 cm thick AR bed above “SC-3”.

4.1.4.2. Lateral variations.Lateral variations of theabove described facies succession are obviouslyvery closely related to those fractures, faults anddykes which cross-cut the studied sequence and arethe result of repeated episodes of brittle deformationbefore, during and after the deposition of the LAR.The marginal fault separating the above-mentionedprominent topographic high from the relative lowand resulting in a displacement of about 1.2 m ofthe principal crust is of particular importance in thisrespect.

The geometry of this topographic high suggests aminor synsedimentary flower structure repeatedlyactive during the deposition of the Liassic sedimen-tary succession exposed on Monte Kumeta. Gradualfading of the movement is shown by the upwardsdecreasing displacement (in the uppermost “satellitecrust” it is already less than 10 cm).

In quarry “A2” thickness variations of the basalammonite bed, the nodular AR, the goethitic-marlylayer and SC-1 are clearly related to the abovementioned structure (Fig.10(a)). The basal bed, withthe shingled ammonites, described from thetopographic low, is absent on the topographic highand also at the uppermost level of the quarry. SC-1is observable on the flank of the topographic high butwithin a distance of about 1.5 m it merges with theprincipal crust. The nodular bed, though observableon one side of the topographic high, is remarkablythinner there and its thickness abruptly diminishesfrom about 50 cm to zero towards the supposedcrest of the high. At the uppermost level of the quarrythe basal bed and the nodular bed are represented byone single, Ammonite-rich layer about 5–10 cm thickwith bioclasts lying flat on the bedding surface. Thegoethitic-marly horizon displays a peculiar flexuralbend above the marginal fault and merges with theprincipal crust in the crestal zone of the high. TheMn-coated intraclast-rich horizon and SC-2 areclearly displaced by the same marginal fault, but thedisplacement is only about 60 cm here, and


diminishes towards the crestal zone, where they, too,are close and to merge with the principal crust. At theuppermost level of the quarry they reappear togetherwith SC-4 and the underlying AR, but merge again inthe corner of the working face.

4.2. Laboratory investigations

4.2.1. Mineralogy, petrography and geochemistry ofthe “principal” rockground

4.2.1.1. The substrate (� underlying rock suite,“UR”). It is a well-sorted, well-cemented, compact,bioclastic packstone/grainstone (biosparite) con-sisting mainly of crinoid ossicles accompanied byabundant micritic intraclasts, micrite-envelopedbenthic foraminifera, calcareous algae such asT.parvovesiculifera(Raineri) and some lithic fragments,possibly originating, at least partly, from the under-lying brachiopod-bearing calcarenites. The crinoidossicles and other echinoderm debris are surroundedby clear syntaxial calcite cement, whereas the micriticlithoclasts and also some of the foraminifera andcalcareous algae show clear signs of transport andrelated erosion/corrosion on their surface. Some ofthe rare micritic peloids are coated by a discontinuous

layer of acicular cement. (Recrystallized remnants ofearly submarine aragonite cement?) Intergranularpore space is sealed by a later generation of coarsepoikilotopic and/or microsparry calcite. Though bothtexture and cementation point to a relatively highenergy open-shelf depositional environment, thepresence of micritic grains, and local re-sedimentedbenthic foraminifera suggests that some lower-energylagoon must have existed contemporaneously nearby(or higher up on the ramp) and thus in the MonteKumeta unit the deposition of the encrinites mayhave pre-dated final isolation and acceleratedsubsidence of the platform (cf. with Jenkyns, 1970).

The encrinites are altered within a few tens ofcentimetres to the Fe–Mn-encrusted surface. Theyshow clear signs of dissolution of both the last pore-filling cement and part of the syntaxial rim-cement ofthe crinoid ossicles (Fig. 12(c) and (e)). Micriticpeloids are also commonly corroded. Part of the inter-granular porosity thus recreated was subsequentlyfilled by reddish calcilutite, devoid of any kind ofmicrofossils and tentatively interpreted as “internalsediment” (� dissolution residue?). Locally a singlelayer of tiny dog-tooth cement (transparent or iron-stained crystals), apparently predating the depositionof the calcilutite, was also observed on the cavity


Fig. 13. (a) Worm tubes in intergranular cavity-fill from Crinoidal limestones, immediately underlying the crust. Thin section 90-06-28/3. (b)Worm tubes and abundant encrusting foraminifera within the Fe–Mn crust. Thin section 90-06-28/2/A. (c) Dyke-fill consisting of pelagicmuddy matrix supporting Fe-oxide coated volcanic exstraclast of trachytic texture, altered pyroxene grains (in the middle) and K-feldspar (topright).Thin section 628/2. (d) Volcanic extraclast of trachytic texture from dyke-filling pelagic mud. Thin section 628/10. (e) Fe-coated ironoolite and volcanic extraclasts from pelagic-dyke-fill. Thin section 90-06-28/1A.


Fig. 14. (a) Fe–Mn crust overlying the Crinoidal limestone. Within the crust the white patches are co-precipitated barite aggregates. Back-Scattered Electron Image. Sample 628/5. (b) Detail of the photo to the left, showing the barite aggregates (white areas). Back-Scattered ElectronImage. Sample 628/5. (c) BaKa showing the distribution of Ba within the crust. Back-Scattered Electron Image. Sample 628/5. (d) Barite fromthe intergranular space of the encrinite underlying the Fe–Mn-encrusted surface. Back-Scattered Electron Image. Sample 90-06-28/5 (e) Ironstain (incipient hardground) along the boundary between Crinoidal limestone (top) and Ammonitico Rosso (bottom). Fe-enrichment is bound tomicroborings and other bio-erosional features. Back-Scattered Electron Image. Sample KP-1. (f) PKa showing the enrichment of phosphorousalong the incipient hardground surface. Back-Scattered Electron Image. Sample KP-1.

walls (Fig. 12(e)). EDAX-coupled SEM analysis hasshown that the red calcilutite is relatively rich in clay(mostly of illitic composition) and contains alsodetectable amounts of Ti-rich grains. Tiny fragmentsof K-felspar, biotite and quartz could also beidentified in it.

Similar micro-dissolution features and associatedcementation were described from echinoderm/bryozoan packstones and grainstones of Mississippianage of New Mexico and confidently interpreted aspalaeokarst-related phenomena by Meyers (1988).On the contrary, Melim et al. (1995), having studiedmarine, burial diagenetic assemblages of the shallowsubsurface of the Great Bahama Bank, suggested thatdog-tooth spar may grow on aragonitic substratesunder purely submarine conditions as well.

The rest of the dissolution porosity is partly orcompletely filled by a likewise fine-grained reddish–pinkish mud, the pelagic origin of which is in somecases clearly shown by fossils (foraminifera, worm-tubes), and locally also by bio-erosion of the pore-walls (Figs. 12(b), (d) and 13(a)). In places wherethe two kinds of pore-fill are in contact the interfaceis invariably enriched in iron oxide. Pore-fillings notcontaining fossils but similar to those of obviouslypelagic origin were also considered pelagic when

showing a thin (2–4mm) orange-coloured Ca-phosphate coating on the pore-walls. 100–200mm-sized skeletal crystals of barite (Fig. 14(d)) werealso occasionally found (and confirmed by themicroprobe) within these equivocal fillings). It hasto be noted however, that where neither fossils norphosphate coatings or microborings could beobserved distinction between pelagic mud-fill andinternal sediment was essentially impossible.

4.2.1.2. The Fe–Mn crust.The crust displays adistinctly laminated bulbous structure consisting oftenths of mm size concentric brownish-colouredlaminae richer in iron-oxide (goethite) andintimately intergrown with calcite at the bottom ofthe crust. Upwards the laminae become darker-coloured and apparently more and more manganese-rich. The interlaminar space is filled partly by a finemicrosparry calcite cement, partly by pelagic mud.Biological activity associated with the precipitationof the crust is clearly shown by the abundance ofworm tubes and irregular patches of encrustingforaminifers (Tolypammina) preferentially sitting inthe lobes of the wavy iron rich laminae (much likethose described by Wendt (1965, 1969), Drittenbass(1979) and Mindszenty et al. (1986), from theNorthern Calcareous Alps and the TransdanubianCentral Range, respectively) (Fig. 13(b)). SEM-analysis of the walls of these foraminifers shows anenrichment of clay on their surface. The concentricstructure of the opaque Mn-rich parts of the crust isless obvious in the petrographic microscope; however,it is clearly visible when studied under the SEM.Because of its ultrafine grain size, preciseidentification of the Mn-oxide phase byconventional powder diffractometry was impossible.Diffractograms suggest that most of it is probablybound to a hollandite-type structure. Microprobeanalyses proved the presence of collomorphousbarite filling the interlaminar space of theuppermost, Mn-rich parts of the crust (Fig. 14(a)–(c)). Clay minerals recovered from the crust andkindly analysed by I. Dodony (TEM Laboratory ofthe Department of Mineralogy of the Eo¨tvos L.University, Budapest) proved to be illiteaccompanied by subordinate amounts of halloysite,and are considered to be of detrital origin.

Chemical composition of the crust (summarized in


Table 1Chemical composition of the Fe–Mn-oxide crust of the principalhardground (major elements by AAS, trace elements by ICP)

Sample no. 628/4A (Kumeta) 628/4 (Kumeta)

SiO2% 2.94 3.39TiO2% 0.62 0.78Al2O3% 0.49 0.65Fe2O3% 33.55 33.24MnO% 28.52 22.16Mn/Fe , 1 , 1MgO% 0.66 0.96CaO% 12.58 18.5Na2O% 0.11 0.14K2O% 0.08 0.04P2O5% 0.15 0.5L.O.I. 16.33 20.0Co ppm 2192 2318Ni ppm 1604 1658Cu ppm 907 594Zn ppm 535 550Ba ppm 5454 5026Cr ppm , 10 , 10

Table 1) shows that it compares very well with otherfossil Fe–Mn deposits reported from similar Jurassicpelagic environments of the Tethyan region and alsowith the average of “hydrogenous” crusts recoveredfrom “continental borderland areas/marginalseamounts and banks” of recent oceanic realms(Cronan, 1980, see Table 2). Their Mn/Fe ratio (,1)and trace element contents (high Co, Ni) show noidentifiable hydrothermal contribution. The con-spicuous enrichment of barite is poorly understood,as is its occurrence in some other non-hydrothermalFe–Mn crusts reported in the literature (Cronan, 1980;Paull et al., 1984). One possibility is that it is relatedto the abundant biological activity. The decay of theproduced biomass may have led to anomalousconcentrations of Ba in the pore-water and thus toearly diagenetic precipitation of barite.

4.2.2. Crust/substrate interface (�the discontinuitysurface, “DS”)

The contact between crust and the underlyingsubstrate is abrupt on the micro-scale. It may beplain or irregular, showing corrosion/bio-erosionrelated pitting of the encrinite (possibly resulted bybrowsing/rasping) (Fig. 10(b)). Traces of iron-stainedmicroborings (the majority delicate enough to beassigned to fungal activity) are particularly abundant,but always restricted to a 10–20 mm thick zoneimmediately below the truncated, apparently hardsurface (Fig. 12(b)). Likewise restricted in thickness,

discontinuous iron staining and a diffuse impregnationby Ca(PO4) is also frequently observed in the topmostparts of the substrate (Figs. 12(a), (b) and 14(e), (f)).Predating the deposition of the Fe–Mn crust proper, acoating, rich in clay, iron oxide and Ti-oxide, a fewmicrometres thick, can be observed, uniformlycovering the corroded truncated surface of crinoidossicles, micritic lithoclasts and also the red micriticpore-fill of the encrinite. It suggests that even thismicritic pore-fill was formed and hardened beforethe deposition of the bulk of the laminated Fe–Mncrust above. The concentration of clay and Ti-oxideat the base of the crust should be the result ofcarbonate dissolution accompanied by residualaccumulation of the detrital fraction of the carbonate.A micro-fracture cross-cutting both crinoid fragmentsand the red pore-filling confirms that the crust wasdeposited on an already lithified surface whichpreviously was subject to rigid deformation. Thisfracture was filled by grey internal sediment, then itbecame truncated and microbored, to the same extentas the truncated crinoid fragments and pore-fillsaround, and is now covered by the Fe–Mn crustabove.

The bottom of the crust is invariably rich in ironand is characterized by the abundance of biologicalactivity. The “catalyzing” effect of iron-oxides ininducing subsequent precipitation of Mn-oxideswas suggested by Wendt (1969), Burns and Burns(1977), Cronan (1980) and others and widely


Table 2A series of chemical analyses (by AAS), of crusts and nodules collected from recent oceanic environments, from the Galpagos hydrotermal field(DSDP 424 and 424A according to Cronan, 1980, p. 204)

Fe–Mn concretions fromGalpagos hydrotermal field

Fe–Mn concretions fromtransform fault “A” FamousAccording to Hoffert et al.(1978) in Cronan, 1980, p. 200

Pacific Mn-nodules(oceanicseamounts, basins)

Marginal seamounts,banks (according toCronan, 1977 inCronan, 1980, p. 141)

Ni 81 570 6340 2960Co 16 91 3350 2560Zn 126 126 680Cu 39 275 3920 780Mn 33.0 wt% 19.78 wt% 15.65 wt%Fe 7.25 11.96 wt% 19.32 wt%Mn/Fe q 1 . 1 , 1Ba 589Mn3O4 37.2 wt%Fe2O3 16.3 wt%

confirmed by observations on both fossil and recentFe–Mn nodules and crusts (e.g. Mindszenty et al.,1986).

In certain places this biological activity apparentlyfostered the penetration of iron-oxide into thesubstrate: preferentially along grain boundaries orrandomly. The “intrusion” of iron-stained colonies ofmicroborers and other unidentified endolithic organ-isms in the substrate can be observed within a distanceof about 10–300mm from the interface with thecrust (Fig. 12(b)). Microborings are common notonly at locations where the hard surface is coveredimmediately by the crust but also at places where theencrinite is in direct contact with the pelagic mudabove.

4.2.2.1. Stable isotopes from the associatedcarbonates.To complete the data set of Kindle(1990) on micrites and calcite cements of Liassicneptunian dyke-fills from Monte Kumeta, a limitednumber of stable isotope analyses were performedalso in the frames of the present study.

From the encrinite, 10 micro-samples (10–15 mgeach) were taken, the pore-filling red calcilutite, theinfiltrated pelagic mud, the calcite cement of theprincipal crust and from an obviously late vein-fillingcalcite. The micro-samples were drilled with a slow-revolution dental drill from two polished slabs repre-senting two juxtaposed Mn-coated pinnacles showingstraight vertical and extremely crenulated horizontaloutlines. They were analysed by mass spectrometry atATOMKI the Nuclear Laboratories of the HungarianAcademy of Sciences in Debrecen, Hungary.

Analytical conditions were as follows. Samplepreparation: 24 h of 100% phosphoric acid treatmentunder temperature-stabilized conditions at 258C;measurements undertaken on the mass spectro-graph of ATOMKI specially developed for theanalysis of light isotopes; standard: international“KH2” (PDB); analytical precision: 0.2‰; analyst:Dr E. Hertelendi).

The results are summarized in Table 3 and Fig. 15.It can be seen at the first glance that all measured

points safely plot within the general field of marinecalcites as fixed by Jenkyns and Clayton (1986) for theEarly Jurassic, and they also fit very well into thecorrelation diagram of Kindle (1990). There areonly two points slightly astray, one of them represent-ing the late calcite vein, the other the calcite cement ofthe crust. This latter is much less enriched in the heavyisotope than the others, but even like this, it does notleave any doubt about the marine origin of thatcement. This is in accordance with the observationsof Jenkyns (1978), Cronan (1980), Cronan et al.(1991), and others, according to which early cementa-tion of hardground-related Fe–Mn-crusts and nodules,as a norm, takes place under submarine phreaticconditions essentially at the sediment surface.

The acquired data do not support the idea of anylong-lasting subaerial exposure preceding the forma-tion of the principal crust. On the contrary, they caneasily be interpreted as the results of early cementa-tion on the sea-floor. It should be noted, however, thatsystematic isotope geochemical studies of stacked


Table 3(a) Stable isotope analyses of micro-samples from the encriniteunderlying the kamenitza surface and from the red to orange micri-tic material filling the intergranular space. For comparison analyti-cal results published by Kindle (1990) from fibrous calcites and redpelagic micrites of Monte Kumeta are also shown (b and c)

Sample no. d 13C d 18O Remarks

(a) Stable isotope analyses28/3/1 2.86 21 Bulk Crinoidal lst.K-3 2.72 20.68 Bulk Crinoidal lst.28/3/p 2.43 20.59 Bulk Crinoidal lst.28/3/2 2.88 20.99 Red matrix in Crinoidal lst.K-2 2.86 20.7 Red matrix in Crinoidal lst.K-1 2.86 20.71 Orange matrix in Crinoidal lst.28/3/3 2.8 20.13 Bulk Liassic micrite (above

crust)28/3/3A 2.54 Bulk Liassic micrite (above

Crinoidal lst.)28/3/4 3.22 20.06 Coarse sparry calcite28/3/5 1.35 20.6 Sparry calcite in Mn-crust(b) Fibrous calcite (Kindle, 1990)330-Kc 1.97 21.75331-Kc 2.26 21.54332-Kc 2.54 21.11(c) Red micrite (Kindle, 1990)336-K 0.82 24.66338-K 2.76 20.11340-K 2.34 0.04341-K 2.29 0.12342-K 1.63 24.74348-K 1.72 23.27(d) Early Jurassic calcite in equilibrium with sea-water (Jenkynsand Clayton, 1986)

1.5–4.0 21.0

palaeosol sequences occurring in shallow-watercarbonate successions, have shown that in order toattain appreciable stable isotope anomalies in thefossil record, a longer-than-ephemeral episode ofsubaeral exposure is necessary. Without the latter noproper re-equilibration with meteoric waters ispossible, and the eventual isotope-signature may belost during subsequent stages of burial diagenesis(e.g. Joachimski, 1994). In other words, although theanalyses summarized in Table 3 and Fig. 15 cannotprove that a subaerial episode was responsible for theformation of the serrate topography, they cannotexclude it either.

Another point to be taken into considerationhere is that when tracing the composition of fluidsresponsible for any kind of dissolution on thebasis of the analysis of what was left over onthe dissolved surface, geochemistry may be incon-clusive. Cavities formed during the early stages ofdissolution may become filled by precipitates ofsediments later, when the chemistry of solutionsis already very different from that of the primaryfluid the nature of which would therefore remainobscure.

4.2.3. Micropetrography of selected horizons of theoverlying rock suite (“OR”)

4.2.3.1. Lithoclast-rich wackestone from the inter-pinnacle space, the basal ammonite bed, and bed-parallel neptunian dykes. The matrix of theserocks is all macroscopically similar. It is a red micriticpelagic wackestone, particularly rich in planktonicmicro-fossils, juvenile Ammonites and thin-shelledBositra-type pelagic bivalves. Thin sections revealed,that enclosed in this matrix there are irregular clastsup to 5 mm or even centimetre size, mostly of hard-ground or rockground origin. Some of them are coatedby concentrically laminated iron-oxide (mainlygoethitic) crusts, several millimetres thick, to suchan extent that they may qualify as iron-oolites ornodules (Fig. 13(e)). Their often broken surface-layers and their sharp contact with the surroundingpelagic mud clearly shows that they were depositedat their present place after some transport. Fe–Mn-coated volcanic fragments of trachytic micro-textureand felspar macro-crystals criss-crossed by calciteveins were also encountered in higher sections ofthe basal Ammonite bed and in the bed-parallel


Fig. 15. Scatter diagram ofd 18O andd 13C of micro-samples from the encrinite underlying the kamenitza surface and from the red to orangemicritic material filling the intergranular space in it. For comparison also analysis results published by Kindle (1990) from pelagic micrites andfissure filling fibrous calcite cements from the Early Jurassic of Monte Kumeta are displayed.

dyke-fills. This confirms the observations of Jenkynsand Torrens (1969) and Jenkyns (1970) regarding thesedimentary record of Early Jurassic volcanic activityin the Trapanese domain in general, and at MonteKumeta in particular (Fig. 13(c)–(e)). No volcanicfragments were found, however, in the interpinnaclerubble, indicating that the beginnings of extrusivevolcanic activity must have post-dated the formationof the principal hardground.

4.2.3.2. The marly horizon below “SC-2”.This marlyhorizon is essentially a strongly compacted bivalvelumachella the original matrix of which must havebeen rich in clay, to permit greater compactionwithin this level, than in the overlying andunderlying more calcareous lithologies. Shells arerecrystallized, broken, and overgrown, generally onone side only, by palisade cement-calcite. The clayis smectitic, very fine-grained, goethite-stained andlaminated. The calcareous laminae are stronglyrecrystallized, they consist of a mosaic of slightlyelongate, almost fiber-like calcite crystals. Theargillaceous laminae consist of tiny parallel orientedclay flakes and iron oxide micro-clasts rich in Ti.Lamination is distorted by cement-growth. Post-dating the major phase of cementation an event offracturing can be observed. The palisades are brokenand slightly displaced. In the cross-cutting micro-fractures infiltration of reddish or greyishfossiliferous pelagic mud could be detected, in somecases spreading also laterally along the interlaminarspace.

Interpretation of the observed textures requiresmore than one episode of deformation followingearly compaction of the clay. The first phase ofdeformation may have been shearing, resulting insmall-scale bed-parallel displacement of the ductileclay laminae and at the same time the introductionof fluids into the interlaminar space from which thepalisade calcite could precipitate. The next phasecould have been a seismic shock, resulting in thebreak-up of the whole cemented layer and also theMn-oxide crust (SC-2) on its top. In this way a bed-parallel neptunian dyke was formed. Palisades andformer fracture-fills of the marly horizon becamecrushed, the interlaminar space partly reopened andthen all was infilled by soft sediment forced in fromsomewhere above.

4.3. Interpretation of the observed features

Field observations and laboratory studies describedabove facilitate the following tentative reconstructionof the sequence of events which led to the formationof the peculiar Fe–Mn-encrusted surface andassociated phenomena in the Lower Jurassic succes-sion of Monte Kumeta.

1st event:Repeated brittle deformation resulting inthe formation of tensional (open) fractures in theLower Liassic Brachiopod limestone. Polyphasefilling of the fractures by pink crinoidal sand,grey–green internal sediment and white RFC.

2nd event:Early submarine (or partly meteoric)phreatic lithification of crinoidal sands overlying theBrachiopod limestone (local early fibrous rim-cementdeveloped on micritic peloids, abundant clear, at placesasymmetric, syntaxial overgrowths around the crinoidossicles; precipitation of coarse mosaic-like sparrycalcite fill sealing part of the intergranular space).

3rd event: Beginning of the formation of theserrate, dissolution-controlled micro-topography (asa result of a possible brief, subaerial episode).Repeated dissolution on the micro- and meso-scalewithin the encrinite: partial dissolution of the inter-granular cement, corrosion of the syntaxial rims andgrains, formation of dm-scale cavities partly filled bycollapse-breccia and internal(?) sediment (facilitatedeither by an ephemeral fresh-water lens, or bysubmarine(?) mixing of waters of different chemicalcomposition). Some fine dog tooth-like cement-growth on cavity-walls and grain surfaces; depositionof reddish calcisiltitic cavity-fills. Further down,below the serrate surface, instead of cavity filling, athin reddish coating forms and covers the first-genera-tion syntaxial cement and itself becomes sealed by asubsequent sparry mosaic.

4th event:Modification of the hard, cemented surfaceby additional dissolution and/or bio-erosion(?) on themeso-scale (endolithic bioeroders and grazers) followedby bio-erosion on the micro-scale (boring by cyano-bacteria(?) and fungi). The latter was accompanied byphosphate-impregnation and subsequent Fe–Mn encrus-tation, apparently due to a lack of carbonate sedimentsupply, contemporary (relative?) increase of availableparticulate Fe and Mn oxides and the colonization ofthe serrate surface by abundant epifauna and the asso-ciated microbial communities. Sand grains and red


cavity-fills are equally affected by Fe-oxide filledmicrobial/fungal borings. Worm-tubes found inmicro-scale intergranular cavities and pore-spacesseveral mm below the actual hard surface showthat biological activity penetrated deeply into thesubstrate wherever partially or completely unsealedintergranular pores offered free space.

5th event:beginning of Ammonitico Rosso-typesedimentation. Preferential accumulation of Fe–Mn-encrusted hardground fragments within the inter-pinnacle space. Deposition of fine pelagic mud fromtime to time interrupted by the formation of minor“internal” hardgrounds. Intense early submarinelithification (micritic cementation).

6th event:Repeated episodes of brittle deformation(mainly tension) both on the micro- and meso-scale.Formation of neptunian dykes, partial re-sedimenta-tion of the Ammonitico Rosso-type sediment accom-panied by the accumulation of loose Ammonite debrisin topographic lows. Accumulation of volcanic micro-extraclasts of trachytic composition.

7th event:Continued episodic brittle deformation(tension and shear), neptunian dyke formation.Repeated, less intense episodes of formation of disso-lutional micro-relief and Fe–Mn-encrustation.

5. Discussion

5.1. Origin of the serrate surface on top of theencrinite: alternatives

By morphological analogy, the meso- and micro-relief, and the associated collapse-breccia-filled disso-lution cavities underneath would suggest an episodeof subaerial exposure and karstic dissolution. This issupported also by micro-corrosion features, internalsediments and void-rimming dog-tooth cementassigned to event 2 above.

Kamenitza2-fields, modified by subsequent bio-erosion, strikingly similar to the one described fromthe Lower Jurassic of Monte Kumeta, are oftenencountered in low-level coastal settings, e.g. onmany of the carbonate beaches of the Bahamas as

shown by Fig. 10(c) and described by Ford andWilliams (1989), Rasmussen and Neumann (1988)and others. However, the data provided by stableisotope analysis and classical petrography are eitherequivocal or negative in this respect: No shallow-water sediments entrapped in the topographic lowsof the micro-relief, or remains of coastal bioeroderscould be detected and the isotope record does notsupport interaction with meteoric waters. No unequi-vocal signs of beach-rock type cementation on themicro-scale could be observed either.

It should be pointed out, however, that, accordingto Rasmussen and Neumann (1988), the completepreservation of a subaerial overprint in coastalsettings is greatly biased by the intensity of bio-erosion and the resulting substrate removal by anerosive transgression. Having studied the effects ofantecedent banktop geometry on Holocene floodingand substrate modification on the Northern GreatBahama Bank they observed substantial bioerosivetruncation of the previously subaerially exposedsediment surface. Clari et al. (1995) arrived at thesame conclusion when studying polygenetic disconti-nuities in Mesozoic carbonates of the Apennines. Thismeans that if the shallow crinoidal sand bank of theLower Jurassic of Monte Kumeta was briefly exposedby a relative sea-level fall, the subaerial meso-reliefthus formed, may not have been preserved comple-tely, but rather modified by bio-erosion when sea levelbegan to rise again. If that (presumably eustatic) riseof sea level was combined with a sudden accelerationof tectonic subsidence of the shallow-water shelf, thenwe may expect that water depths around the lowerlimits of the photic zone could have been reachedbefore the re-establishment of the carbonate factory(� lag-effect). Such a scenario may justify theabsence of shallow-water sediments in the cover ofthe hard karstified, bioeroded surface.

The lack of stable isotope evidence supportinginteraction with fresh or at least mixing zone waters,however, remains to be understood. For the time beingthis is the only argument against the subaerialexposure hypothesis, which is difficult to defend.

When rejecting the hypothesis of a brief subaerialexposure, the only logical alternative to explain theformation of the peculiar serrate relief isdissolutionon the sea-floor, a hypothesis equally difficult to proveor disprove.


2 Kamenitza (Slowenian “kamenica“): dish-shaped solution panformed on limestone substrate, ideally with slightly overhangingsides and flat bottom. Diameter between 0.1 and 1.0 m; depth rarelyexceeding 15 cm (Bo¨gli, 1978).

Theoretically there are three possibilities to bringabout undersaturation, the prerequisite of dissolution,on the sea-floor: (i) contact with undersaturatedcurrents; (ii) contact (mixing) with submarinedischarge of waters of chemical composition differentfrom that of the surrounding sea-water (so-calledsubmarine cold seeps); or (iii) contact with ascendinghydrothermal solutions, which on cooling maybecome undersaturated. The difficulty lies in the factthat while circumstantial geological evidence does notallow the exclusion of any of the above possibilities,there is no conclusive evidence to show that one or theother was of prime importance when sculpturing thepeculiar “pseudo” kamenitza.

The arguments and counter-arguments are asfollows.

5.1.1. Undersaturated currentsThe earliest generation of neptunian dykes,

observed in the Brachiopod limestone, suggests thatalready the deposition of the encrinites very probablyhad a tectonic control. Likewise the depositon ofAmmonitico Rosso-type sediments on top of theFe–Mn-encrusted hardground surface is repeatedlyinterrupted by synsedimentary fracturing, a clearsign that the platform drowning resulted fromtectonically controlled subsidence. Current sweepingof the surface of drowning blocks is a common sideeffect of differentially subsiding former carbonateplatforms (Jenkyns 1986; Clari et al., 1995), so thisis a possibility for Monte Kumeta, too. The lack of finemud-size particles at the contact between bedrock andFe–Mn crust may also indicate some current effect.Water undersaturation may have been the result ofrapid fault-controlled subsidence of the Kumeta areato depths where water temperatures were sufficientlylow to bring about dissolution.

A fact that deserves mention here is that the disso-lution surface, though extremely serrate and irregular,shows apparently no textural control. The dissolu-tional (or rather bioerosive?) meso- and micro-topo-graphy equally crosscuts echinoid debris, sparrycalcite cement and the red calcisilitite-fill of the inter-granular space. Likewise, micro-borers do not showany obvious preference for any particular bioclasts orcements of the hard substrate. In other words dissolu-tion seems to have been water-controlled rather thanmineral or grain size controlled (sensuChoquette and

James, 1988) supporting the idea, that by the time thefinal dissolutional and bioerosive meso- and micro-relief was formed the substrate has already beenmineralogically stabilized. What remains to beunderstood therefore is, why a mineralogicallyhomogeneous substrate should have resulted in sucha jagged irregular surface if dissolution was broughtabout by undersaturated currents uniformly sweepingacross the sediment-free rocky sea-bottom.

5.1.2. Submarine discharge of non-marine watersSubmarine discharge of waters of salinities widely

different from that of normal sea-water have beendescribed from the southeastern Atlantic coast of theUS (Florida, Hatteras shelf, Blake Plateau) (Matsonand Sanford, in Manheim, 1967; Paull et al., 1984;Paull and Neumann 1987; Commeau et al., 1987).

5.1.2.1. Fresh water.Manheim (1967) describedfresh-water discharge and associated large-scaledissolution on the sea-floor observed at 510 m depthand over 200 km from shore during the dive of thesubmarineAluminaut in 1967 (Markel in Manheim,1967). At several other places along the southeasternAtlantic coast of Florida discharge from fresh andbrackish aquifers well below sea level was observed.Based on information provided mainly by the JOIDESprogram Manheim predicted that“in addition to well-defined outflow…there may be seepage and minorflow which…may give rise to “halos” of unusualsediment properties.”Enos (1988) proposed that asimilar mechanism (“subsurface flow of meteoricwater that emerged as a submarine spring”) mighthave been the cause of early diagenetic dissolution/cementation in Cretaceous sediments of the Poza Ricatrend, Mexico. In that case the hypotheticgroundwater system responsible for the observedearly diagenetic features was supposed to have beenof the diffuse flow type in a porous aquifer. To ourknowledge, however, the effects of fresh- or brackish-water discharged from fractured palaeo-aquifers onthe sea-floor have not yet been documented indetails from the stratigraphic record.

We think, that dissolution by mixing of sea-waterwith fresh or brackish waters debouching on the sea-floor via tensional fractures and coming into contactwith the hard substrate may easily have been one ofthe factors to contribute to the formation of the


micro-topography of the bioeroded/dissolved surfaceon Monte Kumeta. The association of the dissolutionwith a submarine seep may explain why the highlydeveloped serrate topography is somewhat limited inits areal extent, even though Fe–Mn coatedhardgrounds of more-or less the same age arewidespread all over the Jurassic of the Tethyanrealm but mainly with a much more subdued asso-ciated micro-relief.

As a possible fresh-water source a scenario similarto the one suggested by Vera et al. (1988) in theSubbetic area may be hypothesized. As wasmentioned in Section 3 and shown in Fig. 4, bauxiteslocally developed within the Jurassic succession ofthe Panormide platform to the north clearly showthat in the Sicilian sector of the African shelf, longersubaerial exposure was a possibility in Early Jurassictimes, even though the general tectonic regime wasthat of extension and subsidence. The time and thehumid climate needed to form bauxites (see Mind-szenty et al., 1996) would have been sufficient tocreate a regional freshwater lens sizeable enough toresult in groundwater discharge below sea level.

The lack of evidence of bauxites in the Trapanesedomain is certainly a serious counter-argument here,but we should not forget, that we can observe onlysmall areas of this wide Mesozoic palaeogeographicdomain because of the Neogene thrust tectonicsrelated to the formation of the Sicilian–Maghrebianchain. Traces of a local, Jurassic bauxite event couldeasily have been destroyed by subsequent phases ofintense orogenic deformation.

5.1.2.2. Brines.Paull et al. (1984) have observedcontinental margin cold brine seeps at the base ofthe Florida Escarpment during one of the dives ofAlvin. The micro-topography of the seep sites (Paullet al., 1984, Fig. 3, p. 966) was strikingly similar to theone observed at Monte Kumeta. The chemistry of theassociated sediments, however, clearly shows thatthe analogy is far from perfect. In the case of theFlorida scarp seeps elevated salinities and highsulphur content of the brines resulted in extremecorrosion at places where H2S dissolved in theseep-water came in to contact with sea-water andbegan to oxidize. The corrosive, acidic environmentwas assumed by Paull et al., to have resultedalso in mobilization of available Fe-sources and

direct formation of pyrite around the seep-sites. 38%of the sediment thus formed, was pyrite occurringas authigenic cement. Associated with the cold seepsan unusual abundance of mussels, gastropods, tubeworms, serpulid polychetes, clams, etc. was observed.The observed taxa showed a close resemblance to thosedescribed as chemosynthesizing communities fromalong the East Pacific Rise (Hessler and Smithey,1984, in Paull et al., 1984). An unusual mineralaccompanying the observed seep-community washigh-Sr barite found as a coating on the periostracumof mussel shells.

Despite the obviously high organic productivityrelated to the serrate micro-and meso-relief of theJurassic of Monte Kumeta and the presence of low-Sr barite in the Fe–Mn crust overlying it, we think thatthe total absence of pyrite and the concomitantpresence of Fe and Mn-oxides suggest that conditionsmust have been oxidizing on the sea-floor here.Though transient episodes of mild reduction may beinferred from the local phosphate impregnation, formost of the time seep chemistry was probably notreducing enough to substantially change the regime.

Tempting though the morphological resemblanceof the corroded rock surface may be to the oneobserved at Monte Kumeta, we think that the brineseep analogy does not properly work in our case.Major arguments are the lack of pyrite and the lackof the ultra-light carbon-isotope anomaly recentlydescribed in association with actual and possiblefossil cold seep sites both from the Pacific (recent)and the Mediterranean (fossil) (e.g. Cavagna et al.,1998; Greinert and Bohrmann, 1998).

5.1.3. Ascending “hydrothermal” solutionsThis is a hypothesis often reiterated when dissolu-

tion phenomena and carbonate cementation arecoupled with anomalous stable isotope compositionsand with Fe–Mn-mineralization in association withneptunian dykes (Kindle, 1990; Winterer et al.,1991; Winterer and Sarti, 1994 and many others).The isotope signal (light oxygen and relativelyheavy (marine) carbon) of dyke-filling calcite cementsis generally interpreted by supposing precipitationfrom moderately evolved pore waters at (slightly)elevated temperatures. Tensional fractures related tostrike-slip activity and/or listric faults are generallysupposed to serve as conducting channel-ways for


the fluid flow (Sibson, 1987; Ramboz, 1989 in Kindle,1990). As to fluid source and migration mechanism,Suchecki and Hubert (1984) proposed that formationwaters of compacting basinal sediments may befocussed to fracture-zones of basin margins, whereasKindle (1990), referring to the model of Hsu¨ (1983)put forward the idea of sea-water being warmed upwhile percolating downward along open fracture-zones, ultimately resulting in convective fluid flowand the associated anomalous dissolution/precipita-tion phenomena. Sibson (1987) claimed that seismicshocks may create considerable underpressure in openpore spaces and therefore resulting in the injection ofmineralizing fluids into higher levels of thestratigraphic column. Kindle (1990) discussing thepossible causes of mineralization along neptuniandykes of the Jurassic of the Northern CalcareousAlps, proposed that pore-waters may have beenheated up also conductively, because of the heatingeffect of nearby igneous activity.

In the case of Monte Kumeta we have goodevidence of recurrent synsedimentary tectonic activityboth in the wider surroundings and in the immediatevicinity of the studied locality. As it was pointed outin the introduction in Lower Liassic times the area ofthe formerly coherent Triassic carbonate platform wasaffected by a large scale transtensional regime, whichresulted in the opening up of major pull-apart basins(Catalano and D’Argenio, 1982b; Di Stefano andGullo, 1986). Disintegration of the platform wasaccompanied in the more advanced stages of riftingfirst by trachytic, then by basaltic volcanism in thebasins (Wendt, 1965; Jenkyns and Torrens, 1969and others).

The evidence of this volcanic activity is representedby abundant trachytic extraclasts detected on MonteKumeta in Ammonitico Rosso-type sediments fillingneptunian dykes post-dating the “principal” rock-ground but definitely not younger than Bajocian.Though we have no direct evidence of volcano-sediments strictly contemporaneous with the forma-tion of the serrate surface and the principalhardground, we may speculate that, as a result ofextension-related attenuation of the crust, a markedpositive heatflow anomaly could have affected thearea already well before the beginning of extrusivevolcanism. This way anomalously warm pore-waterscould have been produced and conducted along

fracture zones far into the interior of the disintegratingplatform as well.

We do not think that the fracture-system and thesynsedimentary flower-structure cross-cutting theTriassic/Liassic succession of Monte Kumeta shouldbe one of the master strike-slips postulated byCatalano and D’Argenio (1982b), but we do suggestthat the observed features could have been hydro-logically linked to one of those distant major faultsformed during the early stages of disintegration of theonce coherent carbonate platform. Prolonged episodicdischarge and spreading on the sediment surface offluids chemically and/or thermally slightly differentfrom the composition of normal sea-water may havecontributed, to the final development of the anoma-lous dissolution phenomena observed in the Liassic ofMonte Kumeta.

The almost normal marine isotope signal observedin carbonate phases sampled in the vicinity of theserrate surface and the overlying Fe–Mn crustsuggests that the source was distant and thus isotopeexchange between the fluids and the enclosing hostrocks efficient.

5.2. Significance of the “principal” Fe–Mn-encrustedrockground

Mineralogy geochemistry and micro-texturessuggest that the closest analogues of the main Fe–Mn crust of Monte Kumeta are those formed in con-tinental borderland environments of recent oceanicrealms, where carbonate sediment supply is reducedand metals are supplied as fine particulate Fe–Mnoxides mainly from terrigeneous sources washed outinto the pelagic realm (cf. with Cronan and Tooms,1967, 1969; Cronan, 1980; Bonatti et al., 1972). ThisFe–Mn content, diluted when the rate of carbonatesedimentation is high, may become concentratedand form hydrogenous precipitates on the sedimentsurface at times of non deposition or eventual dissolu-tion of carbonates. Increased fluxes of Fe and Mneither directly from submarine volcanic sources orindirectly by interaction of sea-water with newlyformed ocean crustal material is a possibility,particularly in oceanic realms with active rifting.However, the separation of endogenic from exogenicMn in the eventual precipitate is mostly difficult if notimpossible. The ratio of Mn to Fe is often used as a


source indicator�Mn=Fe, 1 indicating a terrigeneoussource while Mn=Fe. 1 is thought to suggest ahydrothermal origin. (Cronan, 1980).

By their Mn/Fe ratio the Kumeta crusts wouldqualify as hydrogenous precipitates from apredominantly terrigeneous source. With the excep-tion of Ba, trace elements detected in the crust wouldalso comply with a hydrogenous origin. The co-entration of Ba, an order of magnitude greater thanin most continental borderland settings, is probablythe result of the unusually intense biological activityassociated with the crusts.

The event-scale abundance of similar, though notstrictly contemporaneous, Fe–Mn coated hard-grounds/rockgrounds all over the drowning carbonateshelves of the Alpine–Mediterranean area in EarlyJurassic times has been related to the opening ofNeotethys by Bernoulli and Jenkyns (1974),Laubscher and Bernoulli (1977), Jenkyns (1978,1986) and several others. Not withstanding theobvious synchroneity of Fe–Mn encrustation with thebeginnings of rifting, a direct relationshipbetween ferromanganese encrustations and submarinevolcanism in the Alpine–Mediterranean realm wassuggested by Germann (1971) and Prescott (1988)only for the northern Calcareous Alps and Mallorca,respectively, in all the other cases an hydrogenousorigin of the crusts was postulated.

It is interesting to note that despite the obvioussynsedimentary tectonic activity and the related fluidcirculation, no geochemical signal indicating directhydrothermal contribution could be detected in thethick Fe–Mn crust covering the serrate hardgroundof Monte Kumeta either. The reason for the apparentcontradiction may be that in the Sicilian–Maghrebianrealm, the first rift-related extrusive volcanic eventspost-dated the formation of the principal hard-ground. This is supported also by the fact thatthe first trachytic volcanoclasts occur in thoseneptunian dyke fills, which cross-cut the Mn-encrusted surface.

We suggest that the formation of the Fe–Mn crustcovering the serrate rockground of Monte Kumeta isthe manifestation of the Tethys-wide diachronousoceanographic event which resulted in the formationof Fe–Mn coated hardgrounds in many other drown-ing platform domains in Early Jurassic times. Therepeated occurrence of the satellite hardgrounds and

crusts higher up in the stratigraphic column of MonteKumeta shows that the conditions of crust formationpersisted from Early Toarcian up to Late Bathoniantimes, but actual encrustation was from time to timeinterrupted by episodic increase of the rate ofcarbonate (re)sedimentation (possibly related to thelocal revival of tectonic activity).

The peculiarity of the Kumeta crust is due to thefact that the underlying serrate surface is a poly-genetic disontinuity surface, the formation of whichis directly related to the Early Jurassic palaeotectonichistory of the Monte Kumeta unit. We think thatincipient faulting either coincided with some minoreustatic sea-level fall, or temporarily outran theeffects of the beginnings of the eustatic sea-levelrise recorded world-wide at around the Pliens-bachian/Toarcian boundary (cf. Hallam, 1988). As aresult, the former crinoidal sand shoal of the shallow,now tilted surface of the Kumeta unit became brieflyexposed and subjected to ephemeral karstic dissolu-tion. As tectonic subsidence continued and relativesea- level rose the lithified sediment could havebeen attacked by coastal bioeroders and the imprintsof the brief exposure phase partially or, in places,completely erased. Accelerated subsidence mayhave prevented the shallow water carbonate factoryfrom restarting carbonate production again and thedrowned rock surface was left exposed on the seabottom. Tensional forces active in the wider surround-ings resulted in faulting and fracturing in the Kumetaunit, too. Fluid circulation along the newly formedfractures may have led to mixing of fluids of differentcomposition and corrosive waters spreading all overthe already irregular rock surface contributed to thefinal development of the peculiar serrate topography.As to the source of fluids, we may hypothesize that theadjoining, supposedly still exposed sectors of thedisintegrating platform may have created a greatenough hydraulic head to feed a deep hydrologicalcirculation system and this way freshwater couldhave percolated downwards along faults and fractures,eventually mixing with sea-water penetrating thesame fractures from the basin-side. On furthersea-level rise the platform submerged completelyand deep groundwater circulation faded. The dissolu-tional surface became subsequently covered by Fe Mnoxides. After this probably relatively long quietperiod, characterized by the formation of the thick


principal crust and the beginnings of AmmoniticoRosso sedimentation, deformation along previouslyformed faults and fractures continued. Minordisplacements locally resulted in partial inversionand the formation of the small-scale growth structureobserved in quarry A2. This involved repeatedre-sedimentation of pelagic mud and fossils, ahigher rate of sediment accumulation in the areaof the relative topographic low and continuing

hardground formation and Fe–Mn-encrustation onthe topographic high, bringing about the verticaland lateral variations observed in the studiedquarries.

6. Conclusion

In conclusion, the peculiar hardground-capped


Fig. 16. Cartoon showing the possible early Jurassic evolution of Monte Kumeta.

dissolution surface of Monte Kumeta is principally anattribute of platform drowning induced by thecombined effects of local tectonics and the globalPliensbachian–Toarcian sea-level rise. While thehardground is clearly of submarine origin no unequi-vocal geochemical, mineralogical or biologicalevidence could be presented either for or against thesubmarine/subaerial origin of the dissolution surfaceunderlying the crust. We suggest that the equivocalityof the data is the direct reflection of the super-imposition of a series of erosional and corrosionalevents (from submarine to subaerial and then tosubmarine again, like that described in Sections5.1.2.1 and 5.1.3, each efficiently erasing or modi-fying the effects of the previous one and thusrendering it difficult to unravel all the details ofthe complex story.

Though fully aware of the above equivocality ofour data, we still think that the observed unusuallyintense multiphase dissolution and the obvioussynsedimentary tectonic activity together with allthe detailed information collected in the presentstudy permit the following tentative reconstructionof the sequence of events (see also Fig. 16):

Towards the end of the Pliensbachian, when localtectonics (block rotation) outran the effects of thebeginning eustatic sea-level rise, a brief phase ofsubaerial exposure may have resulted in ephemeraldissolution. Subsidence and drowning commencedwhen the rate of global sea-level rise acceleratedand, in phase with this, downfaulting continued.Downfaulting must have created a network of deep-reaching open fractures through which an effectivehydrological circulation may have been established.We propose that for some time this circulation playedan important role in enhancing the dissolutionprocesses, which affected the crinoiodal limestone.On further subsidence the formation of the hard-ground and then the deposition of the LAR,controlled by the synsedimentary growing meso-scale structure, took place in a conventionalsubmarine environment.

Acknowledgements

The authors wish to express their thanks to Dr E.Hertelendi (Debrecen) for the stable isotope analyses,

Dr K. Gal-Solymos (Budapest) for her constructivecollaboration in the electron microscope laboratory,to L. Hoffman (Budapest) for the chemical analyses,and to Prof A. Gala´cz and Dr A. Voros (Budapest),and Prof. G. Carannante (Napoli) for useful discus-sions. The authors are particularly indebted to M.Tucker (Durham) and P. Clari (Torino) for the carefulreview of the manuscript and to B. Sellwood(Reading) for having substantially improved theEnglish. Thanks are due to Prof. R. Catalano, Prof.G. Lo Cicero and Dr C. Pecoraro for their logisticalsupport. Thin sections were made by S. Kraus.

Financial support was provided by Project 1-35/98of the Italian–Hungarian Technical and ScientificCooperation Program (20%); Project 05/1 of theMTA-CNR Exchange Program (20%), by OTKAT-019309 (Hungary) to A.M. (5%) and by MURST(cofin. 1997, P.D.) project “Jurassic stratigraphicevolution of the platform-basin systems from Sicily”.

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