[The Journal of Geology, 2007, volume 115, p. 335–353] � 2007 by The University of Chicago. All rights reserved. 0022-1376/2007/11503-0005$15.00

335

Correlation of Diagenetic Data from Organic and Inorganic Studies inthe Apenninic-Maghrebian Fold-and-Thrust Belt:

A Case Study from Eastern Sicily

Luca Aldega, Sveva Corrado, Mario Grasso,1 and Rosanna Maniscalco1

Dipartimento di Scienze Geologiche, Universita degli Studi “Roma Tre,”Largo S. L. Murialdo 1, 00146 Roma, Italy

(e-mail: aldega@uniroma3.it)

A B S T R A C T

Temperature-dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levelsof diagenesis from the Apenninic-Maghrebian fold-and-thrust belt in eastern Sicily at the footwall of the Peloritani-Calabride Arc. Data are from units sampled along a regional transect between the Nebrodi Mountains to the northand Mount Judica to the south. These units developed in very different tectonic settings from those of oceanic topassive continental margin domains deformed during the Cenozoic mountain building and related active margindeposits. The integration of organic and inorganic thermal indicators allowed us to distinguish among different tectonicsettings, with thermal maturity generally decreasing from hinterland to foreland as a result of progressively less severethermal evolution and/or tectonic loading during the mountain building. Specifically, the highest vitrinite reflectance(VRo%) values (ca. 0.60%–0.75%) and percentages of illite layers in illite-smectite (I-S; 60%–80%) are found in trench-involved and accreted passive margin units. Lower VRo% values (0.20%–0.47%) and percentages of illite layers in I-S (30%–60%) are found in thrust-top and foredeep basin deposits and far-traveled Sicilide units that have escapedinvolvement in trench evolution. Furthermore, either sedimentary or long-lived tectonic burial (at least more than5 m.yr.) seem to have affected levels of diagenesis of the studied successions. The correlation between organic andinorganic thermal indicators is satisfactory for most of the samples derived from hemipelagic and siliciclastic deposits,whereas it is poor for some proximal siliciclastics. A tentative calculation of paleotemperatures is also proposed forthe studied tectonostratigraphic units.

Online enhancement: table 1.

Introduction

Both vitrinite reflectance (VRo%) and illite-smec-tite (I-S) are parameters that have been widely usedin petroleum geology to correlate diagenesis to hy-drocarbon generation attained during the thermal/burial evolution of sedimentary basins and oro-genic belts (Hoffman and Hower 1979; Underwoodet al. 1988; Bustin et al. 1990; Pollastro 1993; Safferet al. 2005). Nevertheless, both VRo% and I-S maybe affected by nondiagenetic factors and may showvalues that cannot be explained simply by theburial and thermal history of the hosting sediments(e.g., Uysal et al. 2000; Li et al. 2004). Furthermore,

Manuscript received February 20, 2006; accepted October 23,2006.

1 Dipartimento di Scienze Geologiche, Universita di Catania,Corso Italia 55, 95129 Catania, Italy.

one of the main problems of correlating clay min-erals and organic matter parameters in thermal re-constructions is that they may react differently tothe physical conditions of sedimentary burial. Infact, the kinetic response of clay mineral reactionsin sedimentary sequences can be significantly dif-ferent from that of vitrinite (Price and Barker 1985;Hao and Chen 1992; Hillier et al. 1995). Severalstudies in sedimentary basins with abnormallyhigh or low geothermal gradients have suggestedthat clay mineral reaction may be more sensitiveto geological heating rates than are organic mate-rials (for a review, see Frey and Robinson 1999). Incontrast, vitrinite is more reactive than clay min-erals to low temperature changes and when the du-ration of the heating event is short.

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On the one hand, VRo% has been proven to bethe most reliable technique in recording maximumpaleotemperature independent of duration of heat-ing (Barker and Goldstein 1990; Barker 1996). VRo%(Stach et al. 1982) is the most useful paleomaxi-mum geothermometer because it is highly sensi-tive to temperature and is not affected by retrogrademetamorphism (Teichmuller 1987). On the otherhand, the I-S geothermometer has been widely ap-plied for evaluating the thermal and tectonic his-tory of sedimentary basins (Nadeau and Reynolds1981; Frey and Robinson 1999) and reconstructingthe tectonic loads during the orogenic phases offold-and-thrust belts (e.g., Corrado et al. 2005). Con-sequently, if applied alone, both geothermometershave some limitations, as extensively described inthe literature (Roberts 1988; Deming et al. 1990;Allen and Allen 1993; Pollastro 1993). Further dif-ficulties are represented by the complexity of re-constructing burial and thermal histories, espe-cially in fold-and-thrust belts, where tectonicprocesses are superimposed on the depositionalevolution (Underwood et al. 1988; Saffer et al.2005). Reliable results in these cases can, however,be attained through the integration of VRo% and I-S with burial history to constrain thermal modelingof sedimentary sequences (e.g., Botti et al. 2004;Corrado et al. 2005).

In this context, several basin maturity charts cor-relating inorganic and organic thermal parameterswith stages of hydrocarbon generation have beenproposed in fold-and-thrust belt settings (e.g., Mer-riman and Kemp 1996; Frey and Robinson 1999;Kubler and Jaboyedoff 2000). Variable VRo% valuesand reaction progress of the smectite-I-S-illite-mus-covite series have been used to index diagenetic andanchizonal conditions. However, major uncertain-ties arise at low temperature ranges, particularly atthe early diagenetic zone, where VRo% and claymineral thermal parameters show lower reliability(Mukhopadhyay 1992; Merriman and Frey 1999).

In this article, we coupled VRo% and I-S in orderto investigate levels of diagenesis along a segmentof the Apenninic-Maghrebian fold-and-thrust beltcropping out in eastern Sicily (between the NebrodiMountains and Mount Judica area) that consists ofvarious tectonostratigraphic units. We also proposea conversion of data into paleotemperatures. In de-tail, the thermal maturity data derived from organicand inorganic parameters form various clusters cor-relatable with groups of tectonostratigraphic unitsdeveloped in different geodynamic settings andlater involved in the orogenic process.

Some key maturity questions, as follows, can beaddressed by these results: When were the thermal

maturity patterns acquired? Was maturity influ-enced mainly by sedimentary and/or tectonicburial? Which tectonic event during the mountainbuilding primarily influenced thermal maturity?What is the influence of change in tectonothermalregime from passive to active margin on the ther-mal evolution of sedimentary sequences? In addi-tion, the results of this study may contribute torefine existing morphotectonostratigraphic modelsof the Apenninic-Maghrebian fold-and-thrust belt(e.g., Scheck-Wenderoth et al. 2005).

Geological Setting

The Apenninic-Maghrebian orogen in the centralMediterranean region developed as a result of con-vergence between Eurasia and Africa that has beenleading since Late Cretaceous to the juxtapositionof arcuate fold-and-thrust belts and associated backarc basins (Dewey et al. 1989). Eastern Sicily is partof this collisional system, developed because of therollback of the subducting Ionian plate betweentwo segments of the continental lithosphere (fig. 1;Malinverno and Ryan 1986; Faccenna et al. 1997).In eastern Sicily, three main tectonostratigraphicdomains crop out over a short distance: the south-ern termination of the Peloritani-Calabride Arc, thesoutheastward-verging Maghrebian fold-and-thrustbelt, and the Hyblean Plateau (Lentini et al. 1996;Grasso 2001).

In detail, within the Maghrebian belt in easternSicily (fig. 1), at the footwall of the PeloritaniMountains (Lentini et al. 1995), the structurallyhighest tectonostratigraphic units derive from thedeformation of the inner preorogenic domain (Sic-ilide Complex in fig. 1; Ogniben 1960) and weregenerally involved in trench development. Unpub-lished apatite fission track analyses on one of theseunits (Mount Soro unit in fig. 1) show that it ex-perienced partial ( ) to total (60� ! T ! 120�C T 1

) annealing (M. L. Balestrieri, pers. comm.120�C2005). Units traditionally ascribed to the SicilideComplex are at present also mapped in the frontalzone of the chain (so-called far-traveled Sicilideunits in fig. 1; Carbone et al. 1990). They representthe upper Cretaceous–middle Eocene base of theTroina unit, which was completely detached fromthe basal part of the successions and transported tothe present-day front of the chain (Carbone et al.1990).

Currently, tectonically beneath the SicilideComplex lie more external tectonostratigraphicunits (e.g., Mount Judica unit in fig. 1). They formedin part at the expense of the African continentalpaleomargin and consist of rootless units, derived

Figure 1. Geological sketch map of the Apenninic-Maghrebian fold-and-thrust belt in eastern Sicily with samplingsites, redrawn and modified after Carbone et al. (1990) and Lentini et al. (2000). Regional location map compiled afterGrasso (2001).

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mainly from the deformation of pelagic Meso-Cenozoic basinal successions. The clockwise ro-tation (about 70�) that affected this area betweenLanghian and Tortonian stages indicates that strongorogenic shortening occurred in eastern Sicily as aresult of thrust emplacement (Speranza et al. 2003;Monaco et al. 2004; De Guidi and Monaco 2005).This deformation, related to plate collision, startedin the early Miocene (Catalano and D’Argenio1982; Oldow et al. 1990; Butler et al. 1992). TheNumidian flysch (cropping out in the Mount Salici,Serra del Bosco, and Nicosia units in fig. 1) repre-sents the earliest foredeep deposit at the onset ofcollision tectonics. It was affected by contractionaldeformation starting during Langhian times andwas followed by the deposition of the mid-Miocene(Gagliano marls) and the upper Tortonian (Terra-vecchia Formation) clastic deposits (Catalano et al.1996). Terrigenous sedimentation continued untilmid-Pliocene times in central Sicily and until theearly-mid-Pleistocene along the southern Sicilianmargin (Bigi et al. 1990; Butler et al. 1997). Theupper Tortonian-Pliocene succession, defined aslate thrust-top basin deposits in figure 1, underwentalmost continuous deformation in the same timespan (Butler and Lickorish 1997; Lickorish et al.1999).

In summary, two major shortening events gen-erated the present-day structural configuration ofthe orogen following continental collision. Thefirst event caused the superposition of the alloch-thonous units onto the Mesozoic-Paleogene Hyb-lean foreland carbonates through low-angle thrustsin early Miocene times (Bianchi et al. 1989; Butleret al. 1992). The second event is considered to haveoccurred in late Miocene–early Pliocene times. Itstrongly modified the geometric relationships ofthe allochthonous units, producing the internalstacking of the Mount Judica succession (Bello etal. 2000) and breakback, out of sequence, propa-gation of backthrusts in central-north Sicily (Car-bone et al. 1990; Grasso et al. 1995).

Furthermore, the Peloritani-Calabride metamor-phic massif overthrust the innermost sedimentaryunits of the analyzed transect during the early Mio-cene (Amodio-Morelli et al. 1976). In NE Sicily, thePeloritani Mountains consist of a series of south-vergent crystalline nappes that are in contact withthe underlying Apenninic-Maghrebian sedimen-tary units along the transpressive Longi-Taorminalineament (Bonardi et al. 1976). Here, the meta-morphic belt, which also shows the records of Her-cynian deformation (De Gregorio et al. 2003;Somma et al. 2005), is made up of thin continentalcrust nappes emplaced during the Upper Oligo-

cene–Aquitanian time span and postdated by Bur-digalian wedge-top basin deposits (Messina et al.2004).

Some of these crustal nappe units record Alpinemetamorphism followed by substantial exhuma-tion before their final emplacement on top of theSicilide units, as indicated by both zircon and ap-atite fission track data (Thomson 1994). This ex-humation is tectonic, and synorogenic extensioncontrols it (Cutrupia and Russo 2005). The closureof the basin where the Sicilide Complex was de-posited and the development of the foreland fold-and-thrust belt followed synorogenic extension. Inthis framework, the remains of the crystalline oro-gen, even if previously strongly thinned by syn-orogenic extensional tectonics, may have providedthe tectonic load controlling the burial and thermalevolution of the structurally underlying units.

Methods

VRo% was measured on whole-rock samples col-lected from sandstone, siltstone, and clayey litho-facies derived from most of outcropping units (table1, available in the online edition or from the Journalof Geology office). Samples were mounted in epoxyresin and polished according to standard procedu-res. VRo% analyses were then performed on ran-domly oriented grains using a Zeiss Axioplanmicroscope and conventional microphotometricmethods (e.g., under oil immersion in reflectedmonochromatic nonpolarized light). In most cases,the sample population was at least 15 readings persample on fragments never smaller than 5 nm andonly slightly fractured and/or altered. Mean reflec-tance and standard deviation values were calcu-lated for all measurements.

Qualitative identification and quantification ofI-S was performed with a Scintag X1 x-ray diffrac-togram (XRD) system (CuKa radiation, solid statedetector, spinning sample). Oriented air-dried sam-ples were scanned from 1 to 48 �2v with a step sizeof 0.05 �2v and a count time of 4 s per step at 40kV and 45 mA. The presence of expandable clayswas determined for samples treated with ethyleneglycol at 25�C for 24 h. Ethylene-glycol-solvatedsamples were scanned at the same conditions asair-dried aggregates, with a scanning interval of 1–30 �2v.

Expandability measurements were determinedaccording to Moore and Reynolds (1997) by usingthe D2v method after decomposing the compositepeaks between 9–10 and 16–17 �2v using the ScintagX1 software program with a split Pearson VIIfunction.

Figure 2. Geological section between Troina village and Mount Judica with projected data. Geological base redrawn and modified after Carbone et al. (1990).

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Figure 3. Ethylene-glycol-solvated (black line) and air-dried (gray line) diffraction patterns for the SI9 samplein the !2-mm grain-size fraction. ; ; ; ; ;I p illite K p kaolinite Ch p chlorite Qz p quartz Pl p plagioclase Lep p

. Numbers over the peaks refer to the d spacing (A) of illite (0.80)/smectite mixed layers. A, B,Lepidocrocite R p 3Composite peaks decomposed through the Scintag X1 software program with a split Pearson VII function. I 001/S002 and I 002/S 003 reflections were used for determining the illitic content in illite-smectite (I-S) mixed layersaccording to Moore and Reynolds (1997).

The I-S ordering type (Reichweite parameter, R;Jagodzinski 1949) was determined by the positionof the I 001/S 001 reflection between 5 and 8.5 �2v

(Moore and Reynolds 1997). The term R expressesthe probability, given a layer A, of finding the nextlayer to be B. The R parameter may range from 0to 3. means that there is no preferred se-R p 0quence in stacking of layers and that illite andsmectite layers are stacked randomly along the c-axis; indicates that a smectite layer is fol-R p 1lowed by an illite layer and that the order of stack-ing of layers appears in the interstratificationsequence; indicates long-range ordering andR p 3that each smectite layer is surrounded by at leastthree illite layers on each side.

Peaks in relative close position were selected forclay mineral quantitative analysis of the !2- and 2–16-mm (equivalent spherical diameter) grain-sizefractions in order to minimize the angle-dependentintensity effect. Composite peaks were decom-posed using a split Pearson VII function and theDMSNT Scintag associated program. Integratedpeak areas were transformed into mineral concen-trations by using mineral intensity factors as a cal-ibration constant (for a review, see Moore andReynolds 1997).

Nonclay minerals, such as quartz, calcite, feld-spars, and gypsum, that were recognized in the !2-and 2–16-mm grain-size fractions were not includedin the quantitative analysis of the oriented aggre-gates; thus, the given data refer to the phyllosili-cates group only. The amounts of clay minerals in

the analyzed clay-size fractions were not recalcu-lated into percentages of bulk rocks but representthe content of the specific separated phyllosili-cates-size fraction.

Sampled Units

A suite of 75 samples for XRD analysis and 24 sam-ples for organic matter optical analysis were col-lected from the various tectonostratigraphic unitsthat currently crop out in eastern Sicily along aregional N-S transect between the Nebrodi Moun-tains and Mount Judica area (fig. 1). From hinter-land to foreland, they are grouped as follows, withnames in parentheses referring to analyzed for-mations: (i) inner units and related trench deposits,comprising the Mount Soro and Troina units (Ar-gille scagliose, Mount Soro flysch, Troina-Tusaflysch, Argille varicolori); (ii) early thrust-top basindeposits (Reitano flysch); (iii) early foredeep depos-its, comprising the Nicosia, Mount Salici, and Serradel Bosco units (Numidian flysch, Gagliano marls);(iv) late thrust-top basin deposits (Pliocene claysand marls, Terravecchia Formation); (v) Africapassive-margin-derived units and synorogenic fore-land deposits comprising the Mount Judica unit(clays and glauconitic sandstones, Scaglia facieslimestones, radiolarian cherts, cherty limestones);(vi) far-traveled Sicilide units belonging to the Tro-ina unit (Argille scagliose, Polizzi Formation).

Inner Units and Related Trench Deposits. MountSoro Unit. This unit (Lentini and Vezzani 1978;

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 341

Figure 4. Ethylene-glycol-solvated (black line) and air-dried (gray line) diffraction patterns for the MS41 sample inthe !2-mm grain-size fraction. ; ; ; . Numbers over the peaks refer toI p illite K p kaolinite Ch p chlorite Qz p quartzthe d spacing (A) of illite (0.50)/smectite mixed layers. A, B, Composite peaks decomposed through the ScintagR p 0X1 software program with a split Pearson VII function. I 001/S 002 and I 002/S 003 reflections were used for determiningthe illitic content in illite-smectite (I-S) mixed layers according to Moore and Reynolds (1997).

K in fig. 1) consists of the Mount Soro flysch andthe Argille scagliose succession. The Mount Soroflysch (Hauterivian-Aptian in age) is a successionup to 1500 m thick of black and varicolored shaleswith carbonate interbeds that grade upward intoclayey- and arenaceous-rich facies. It is historicallyinterpreted as a deposit associated with early Cre-taceous orogenic movements in the western Tethys(Lentini et al. 2000). Nevertheless, recent studiessuggest that these sandstones were most likely theresult of weathering and erosion in early Creta-ceous times (Wortmann et al. 2004). The MountSoro flysch is overlain by red and green shales withsiltite and fine-grained sandstone interbeds knownas Argille scagliose.

Troina Unit. The Troina unit (according toLentini et al. [1990]; KM in fig. 1) is made up ofvaricolored clays with calcarenite and siltite inter-beds (Argille varicolori; Oligocene) that grade up-ward into a siliciclastic succession of gray marlsand shales alternating with mica-rich sandstonesand siltstones (Troina-Tusa flysch; upper Oligo-cene–lower Miocene).

Early Thrust-Top Basin Deposits. The Reitanoflysch (LMM in fig. 1) is a clayey-arenaceous-con-glomeratic succession a few hundred meters thickthat sutures tectonic contacts among differentunits of the Sicilide Complex (La Manna et al.1995). In the study area, it lies unconformably ontop of the Troina-Tusa flysch (La Manna et al. 1995;Lentini et al. 2000) and postdates the major phaseof early deformation.

Early Foredeep Deposits. The Numidian flysch

(OM in the Mount Salici and Serra del Bosco unitsand LM in the Nicosia unit in fig. 1), up to 1500 mthick, represents the Oligo-Miocene cover of dif-ferent Meso-Cenozoic sedimentary successions, as-cribed to different preorogenic domains (Giunta1985; Lentini et al. 2000). It is made up of brownishclays alternating with coarse-grained quartz-richarenites that are generally more abundant than theclayey facies. Data from numerous wells drilled ineastern Sicily and from seismic lines give a thick-ness of a few thousand meters for this formation(Bianchi et al. 1989). This thickness derives fromthe stacking of different tectonic units known asthe Nicosia, Mount Salici, and Serra del Bosco units(Giunta 1985; Carbone et al. 1990; Lentini et al.2000). The overlying deposits (Gagliano marls;MUM in fig. 1), interpreted as unconformable ontop of the Numidian flysch (Lentini et al. 2000),are Langhian-Tortonian silt-rich gray marls.

Late Thrust-Top Basin Deposits. Along the stud-ied transect, a regional E-W trending syncline isfound near the town of Centuripe. Upper Tortonianto Pliocene (UMPl and Pl in fig. 1) synorogenic de-posits fill this structure and record the complexinteraction among thrust activity, orogenic short-ening, and sea level changes (Butler and Grasso1993; Butler and Lickorish 1997).

Africa Passive-Margin-Derived Units and Synoro-genic Foreland Deposits: Mount Judica Unit. TheMount Judica unit (TkM in fig. 1) consists of Me-sozoic–lower Tertiary carbonates overlain by glau-conitic sandstones and clays of Oligocene-Serra-vallian age (Monaco et al. 2004). The Triassic and

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Figure 5. Ethylene-glycol-solvated (black line) and air-dried (gray line) diffraction patterns for the PL4 sample inthe !2-mm grain-size fraction. ; ; ; ; . Numbers over theI p illite K p kaolinite Ch p chlorite Qz p quartz Cc p calcitepeaks refer to the d spacing (A) of illite (0.3)/smectite mixed layers. A, B, Composite peaks decomposed throughR p 0the Scintag X1 software program with a split Pearson VII function. I 001/S 002 and I 002/S 003 reflections were usedfor determining the illitic content in illite-smectite (I-S) mixed layers according to Moore and Reynolds (1997).

Jurassic succession is mainly deep basinal to oce-anic facies, with cherty limestones grading up toradiolarian cherts. The Cretaceous-Eocene strataconsist of pelagic limestones and marls of the Scag-lia facies grading up into the Oligocene–middleMiocene clastic deposits (clays and glauconiticsandstones). The Mount Judica unit is deformedinto three major south-verging imbricate thrustswith a total shortening of about 20 km (fig. 2; Car-bone et al. 1990; Butler et al. 1992; Lickorish et al.1999).

Far-Traveled Sicilide Units. The far-traveled Sic-ilide units are represented by two different litho-stratigraphic units known as Argille scagliose(upper Cretaceous–lower Eocene) and Polizzi For-mation (lower-middle Eocene; Carbone et al. 1990).They are assigned to the lower portion of the Troinaand probably Nicosia units. Nevertheless, theywere detached from the original succession andtransported together with the Numidian flysch to-ward the foreland.

Results

Main results are described for the different tecto-nostratigraphic units moving from the north to-ward the south of the study area (figs. 1, 2; table1).

Inner Units and Related Trench Deposits. MountSoro Unit. Samples were collected in the neigh-borhood of San Teodoro village and include MountSoro flysch (five for XRD: SO2, SO3, SO5, SO7, SI9;five for VRo% analyses: SO1, SO2, SO3, SO6, SI10)

and Argille scagliose (one for XRD: SI7; one forVRo%: SI8).

XRD analyses reveal the presence of illite, or-dered I-S, kaolinite, and chlorite (table 1). Nonclayminerals such as quartz, plagioclase, and lepido-crocite were also detected. The I-S identified in the!2-mm grain-size fraction corresponds mainly to

structures in which the illitic layers are inR p 1the range of 70%–77%. Only sample SI9 has R p

structures and 80% of illite in I-S. Figure 3 shows3ethylene-glycol-solvated and air-dried diffractionpatterns for this sample.

Analyzed kerogen from the Mount Soro flysch isgenerally scarce and contains macerals of the hu-minite-vitrinite group, with a predominance of col-lotelinite and collinite fragments, and lesser per-cent from the inertinite group. In most samples,two separate clusters of VRo% values were recog-nized. The first one, characterized by lower VRo%values and a Gaussian distribution, is representa-tive of autochthonous wood fragments, whereas thesecond one, less regularly distributed and withhigher values, is made up of highly oxidized or re-cycled fragments. The latter group was not takeninto account because it does not provide meaning-ful information on the burial/thermal evolution ofthe analyzed stratigraphic units. Measurements onthese fragments are generally less abundant thanthose performed on autochthonous organic matter.On the other hand, kerogen in the Argille scaglioseis scarce and belongs mainly to the inertinite group;small fragments of vitrinite group macerals weredetected and measured. The VRo% values for theMount Soro unit range from 0.61% to 0.76%.

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 343

Figure 6. Ethylene-glycol-solvated (black line) and air-dried (gray line) diffraction patterns for the MS2 sample inthe !2-mm grain-size fraction. ; ; ; ; . Numbers over theI p illite K p kaolinite Ch p chlorite Qz p quartz Gy p gypsumpeaks refer to the d spacing (A) of illite (0.75)/smectite mixed layers. A, B, Composite peaks decomposedR p 1through the Scintag X1 software program with a split Pearson VII function. I 001/S 002 and I 002/S 003 reflectionswere used for determining the illitic content in illite-smectite (I-S) mixed layers according to Moore and Reynolds(1997).

Troina Unit. Thirteen samples for clay min-eralogy (TR7, TR8, TR9, SI11, MS45, MS46, SI5,SI6, TR5,TR6, SI15, SI16, SI18) and seven samplesfor VRo% (TR7, TR8, TR9, SI11, SI12, SI13, SI14)were collected in a wide area in the surroundingsof Troina village (fig. 1; table 1). Clays and shaleswere chosen mainly for expandability measure-ments, whereas sandstones, siltstones, and marlswere more suitable for VRo% analyses.

Observed I-S corresponds to structures inR p 1which the illite component is dominant. The per-centage of illite layers is in the range of 60%–77%in the !2-mm fraction. Both the Argille varicoloriand the Troina-Tusa flysch are characterized by il-lite- and I-S-rich assemblages and by the presenceof chlorite and kaolinite in both of the analyzedgrain-size fractions.

Analyzed kerogen of the Troina-Tusa flysch isgenerally abundant, homogeneous, and well pre-served. Macerals are mainly from the huminite-vitrinite group, with a predominance of collotelin-ite and telinite fragments and subordinate amountsfrom the inertinite group. Pyrite is either finely dis-persed or in small globular aggregates locally pres-ent along the rims of the vitrinite macerals. TheVRo% data are in the range of 0.61%–0.70%.

Early Thrust-Top Basin Deposits. XRD samplesare from coarse-grained and muscovite-rich sand-stone (SO10) and gray shale (SI17). Two lithology-dependent clay mineral assemblages were found.The SO10 sample is illite rich, with subordinatechlorite and I-S in both studied grain-size fractions,

whereas sample SI17 has a smectite-rich assem-blage with lesser amounts of illite, kaolinite, andchlorite (table 1). I-S, when detected, resembles

structures with an illitic content of 60%.R p 1Kerogen from four samples (SO8, SO9, SO10,

SI17) is mainly of terrestrial origin, with maceralsof the inertinite group (fusinite and semifusinite)predominant and huminite-vitrinite group macer-als (collinite and telinite) subordinate. Pyrite is fre-quently associated. The VRo% data are in the rangeof 0.35%–0.47%.

Early Foredeep Deposits. For the Numidianflysch, only clay mineralogy data (13: MS41, MS42,MS43, MS44, TR1, MS16, MS17, MS24, MS25,MS32, MS39, TR2, TR3) were determined. Clayeysamples are composed mainly of high amounts ofI-S and kaolinite and lesser amounts of illite andchlorite. The !2- and 2–16-mm grain-size fractionsdo not show any mineralogical difference but arecharacterized by slight differences in the amountsof components (table 1).

Only two samples (MS24 and TR2) are fromquartz-rich sandstones where discrete smectite andI-S mixed layers were not identified. Kaolinite, il-lite, and chlorite (present only in sample TR2) arethe recognized phyllosilicates. Nonclay mineralssuch as calcite, quartz, plagioclase, and gypsum aregenerally present in the analyzed fractions.

Observed I-S has structures with a per-R p 0centage of illite that varies from 50% to 55% (fig.4). Ordered structures were identified onlyR p 1for the MS39 sample.

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Figure 7. Ethylene-glycol-solvated (black line) and air-dried (gray line) diffraction patterns for the MS37 sample inthe !2-mm grain-size fraction. ; ; . Numbers over the peaks refer to the d spacingI p illite Qz p quartz Cc p calcite(A) of illite (0.30)/smectite mixed layers. A, B, Composite peaks decomposed through the Scintag X1 softwareR p 0program with a split Pearson VII function. I 001/S 002 and I 002/S 003 reflections were used for determining theillitic content in illite-smectite (I-S) mixed layers according to Moore and Reynolds (1997).

The Gagliano marls (SI19) have I-S similar to thatof the Numidian flysch, with an illitic content of60%. Small and fractured vitrinite fragments fromthis deposit (SI20) have a VRo% value of 0.39%.

Late Thrust-Top Basin Deposits. Late thrust-topbasin deposits have been analyzed with a suite of18 samples for VRo% (MS11, MS12, MS13, MS14,MS15) and XRD (MS14, MS15, MS40, PL1, PL2,PL3, PL4, PL5, MS12, MS13, SI21, TV1, TV2, TV4)analysis. Nine samples are from clay-rich marls andlens-shaped quartz-bearing sandstones of the Ter-ravecchia Formation, and the remaining samplescome from the overlying clay-rich deposits (Plio-cene clays and marls).

Pliocene clays and marls are characterized by aclay mineral assemblage composed mostly of I-Sand lesser amounts of illite, kaolinite, and chlorite.I-S is abundant in the finer fraction, whereas illite,kaolinite, and chlorite are more abundant in the 2–16-mm grain-size fraction (table 1). The observed I-S has stacking order in which the smectiteR p 0component is strongly dominant (fig. 5). The illitecontent is in the range of 30%–35% for both theanalyzed grain-size fractions.

The Terravecchia Formation shows an x-rayquantitative analysis similar to that of the previ-ously described deposits. I-S is always the mostabundant clay mineral; illite and chlorite haveweight percentages similar to their presence in Pli-ocene clays and marls. Based on the D2v method,the illite content in I-S is in the range of 35%–45%.

Analyzed kerogen shows VRo% values in the

range of 0.20%–0.29% (MS14, MS15) for the Plio-cene clays and marls and 0.36%–0.45% (MS12,MS13) for the Terravecchia Formation. Both unitsare generally scarce in organic matter, with a pre-dominance of macerals of the huminite-vitrinitegroup with one reworked generation and one madeup of autochthonous fragments. Pyrite may be pres-ent, while inertinite fragments are rare.

Africa Passive-Margin-Derived Units and Synoro-genic Foreland Deposits: Mount Judica Unit. A suiteof 18 samples for clay mineralogy was collected:six samples are from the innermost thrust sheet(sites 27 and 28), and the remaining pertain to sites31–35. Available data are from gray shales (MS1,MS2) interbedded in light brown cherty limestonesup to 20 cm thick, highly layered polychrome ra-diolarian cherts (SI1, MS3), finely laminated Scagliafacies red marls (SI2, SI4, MS4, MS19, MS22, MS33,MS34), and brown marls alternated with fine-grained glauconitic sandstones (MS7, MS8, MS18,MS21, MS27, MS31, MS35).

The XRD quantitative analysis of the !2- and 2–16-mm grain-size fractions displays mineralogicaldifferences between the Meso-Cenozoic pelagicsuccession and the overlying clastic deposits. Theformer has high amounts of illite and lesseramounts of I-S, kaolinite, and chlorite. The lattershows kaolinite-rich or I-S-rich assemblages andlesser amounts of illite and chlorite. A decrease inthe abundance of I-S and an increase of detrital ka-olinite and chlorite are evident in the 2–16-mmgrain-size fraction.

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 345

Figure 8. Distribution of the illite layers in illite-smectite (I-S) of the !2-mm grain-size fraction along the NebrodiMountains–Mount Judica area transect. Note how tectonostratigraphic units belonging to different geodynamic set-tings differ on the basis of the illitic content in I-S.

The percentage of illite layers in I-S ranges from55% to 75% in the inner and upper thrust sheetand from 55% to 70% in the intermediate thrustsheet and is about of 65% in the outer one. Theobserved I-S structures are and/orR p 0 R p 1structures (fig. 6). The to transitionR p 0 R p 1is found within the Oligocene-Serravallian claysand glauconitic sandstones and the Scaglia faciesmarls. The 2–16-mm grain-size fraction shows moreillitic I-S than does the finer fraction.

Far-Traveled Sicilide Units. Five samples (SI3,MS26, MS28, MS29, MS30) of the Argille scagliosewere collected in a tectonically bordered narrowbelt cropping out in sites 37 and 39. Three samples(MS36, MS37, MS38) of the Polizzi Formation arefrom the neighborhood of Castel di Judica village(site 30).

Only clay mineralogy provided results. The olderstratigraphic unit consists of highly tectonized redand green shales containing centimeter-scale lam-inae of fine-grained sandstones and white mud-

stones. Argille scagliose do not show a homogenousmineralogical assemblage, probably linked to thecolor variability and outcrop quality. In detail, redshales (MS26, MS28, SI3) are composed mainly ofdisordered I-S and subordinate illite, kaolinite, andchlorite, whereas green shales (MS29, MS30) con-tain larger amounts of kaolinite and chlorite andless I-S (table 1). The observed I-S in the !2-mmgrain-size fraction corresponds to structuresR p 0in which the smectite component is prevalent. Redshales have a range of 35%–40% illite layers in I-S, whereas green shales indicate 45% illite layers.

The Polizzi Formation is made up of marly lime-stones and marls containing chert lenses. It dis-plays I-S-rich assemblages and very small amountsof illite (table 1). Kaolinite and chlorite were notidentified. No compositional or weight percent dif-ferences were found in the analyzed grain-size frac-tions. The identified I-S structures in the !2-mmfraction are structures with an illite contentR p 0of about 30%–35% (fig. 7).

346 L . A L D E G A E T A L .

Figure 9. Correlation of organic and inorganic thermalindicators with different geodynamic settings: illite lay-ers in illite-smectite (I-S) measured in the !2-mm grain-size fraction versus vitrinite reflectance (VRo%). Note agood agreement between illite layers in I-S and VRo%values for trench involved units (high values) and latethrust-top basins (low values). Analytical data in table 2.

Discussion

Tentative Conversion into Paleotemperatures. Re-garding the use of clay-mineralogy-derived param-eters, many authors state that clay minerals aregenerally metastable and that specific and precisetemperatures or pressures cannot be ascribed tothem (Lippmann 1982; Jiang et al. 1990, 1994; Es-sene and Peacor 1995). The cause of formation ofthese metastable states below the greenschist faciesis that reactions in clay-rich sediments are sluggish.Nevertheless, the repetitive nature of these well-recognized sequences of minerals suggests thatgeothermometers should exist for rocks formingunder such low-temperature conditions and thatmineral transitions in pelites as a function of meta-morphic grade (temperature) can be referred to asa state of reaction progress. Essene and Peacor(1995) pointed out that although reactions are con-trolled by many factors (e.g., fluid composition,fluid/rock ratio, porosity, etc.), where such condi-tions are constrained to be approximately equal,there should be a correlation between reaction pro-gress and temperature. In this framework, Pollastro(1990, 1993) summarized the application of twosimple time-temperature models for I-S geother-mometry studies based primarily on the durationof heating (or residence time) at critical I-S reaction

temperatures. The first model was developed byHoffman and Hower (1979) for long-term burial dia-genetic settings. It can be applied to most sedi-mentary settings and petroleum systems of Mio-cene age or older. In this model, the major changesfrom to and from to oc-R p 0 R p 1 R p 1 R p 3cur in the temperature range of about 100�–110�Cand 170�–180�C, respectively, and a minimum heat-ing duration of 2 m.yr. is generally required (Hoff-man and Hower 1979).

The second model, which was developed forshort-lived heating events, applies to young basinsor areas characterized by relatively recent thermalactivity with high geothermal gradients or to recenthydrothermal environments. Such settings arethose where relatively young rocks were subject toburial temperatures in excess of 100�C for !2 m.yr.In this model, the conversion from toR p 0 R p

and from to occurs at about 130�–1 R p 1 R p 3140�C and 170�–180�C, respectively (Jennings andThompson 1986).

In this article, I-S data were converted into pa-leotemperatures, adopting the time-temperaturemodel of Hoffman and Hower (1979) and revisitedby Pollastro (1990, 1993) for long-term burialdiagenetic settings. Detrital illite, accumulatingmainly in the 12-mm fraction, can cause an incor-rect estimate of the illite content in I-S and there-fore of estimated paleotemperatures. As a conse-quence, only data from I-S in the !2-mm grain-sizefraction were used for paleotemperature calcula-tion. Main results are reported in table 3.

To properly convert VRo% data into paleotem-peratures, we must perform refined time-temper-ature modeling based on both geological and or-ganic maturity inputs (Tissot et al. 1987; Allen andAllen 1993). Nevertheless, sampling strategy andoutcrop conditions often did not offer the oppor-tunity to build properly constrained models. Thus,at first we used some of the equations that the lit-erature offers for paleotemperature conversion (Bar-ker and Pawlewicz 1986, 1994; Barker 1988). Theywere developed as empirical calibrations based onthe assumption that temperature has the dominantinfluence in increasing VRo% and that the effect ofheating time is usually less significant than thenoise in the experimental determination of VRo%values (Price 1983; Barker 1989, 1996).

Where both vitrinite and I-S data are available,there is a substantial agreement between calculatedpaleotemperature ranges. Further integration withgeological inputs (thickness, geothermal gradients,etc.) allowed proposing more refined paleotemper-ature ranges and, in some cases, maximum sedi-mentary and/or tectonic burial estimates.

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 347

Table 2. Organic and Inorganic Data Plotted in Figure 9

Geodynamic setting

Sample VRo% %I in I-S (!2 mm)

Vitrinite XRD Average SD No. %I R parameter

Subduction trench SI10 SI9 .76 .09 40 80 3Subduction trench SI8 SI7 .75 .21 9 77 1Subduction trench SO1 SO2 .71 .18 3 75 1Subduction trench SO2 SO2 .61 .11 6 75 1Subduction trench SO3 SO3 .69 .06 18 70 1Subduction trench SO6 SO5 .66 .12 11 75 1Subduction trench SI12 SI11 .63 .07 5 72 1Subduction trench SI13 SI11 .70 .16 26 72 1Subduction trench SI14 SI11 .65 .10 14 72 1Subduction trench TR7 TR7 .70 .11 26 70 1Subduction trench TR8 TR8 .61 .12 23 68 1Subduction trench TR9 TR9 .63 .23 23 70 1Early thrust-top basin SO8 SO10 .42 .09 7 60 1Early thrust-top basin SO10 SO10 .47 .07 11 60 1Early thrust-top basin SI17 SI17 .35 .08 52 … …Early foredeep SI20 SI19 .39 .07 41 60 0Late thrust-top basin MS14 MS14 .29 .04 15 30 0Late thrust-top basin MS15 MS15 .20 .02 9 30 0Late thrust-top basin MS12 MS12 .36 .13 12 35 0Late thrust-top basin MS13 MS13 .45 .08 3 30 0

Note. reflectance; ; diffractogram; R parameter.VR % p vitrinite I-S p illite-smectite XRD p x-ray parameter p Reichweiteo

Furthermore, I-S data (fig. 8) and their integrationwith VRo% values (fig. 9; table 2) allowed us todistinguish among different tectonic settings, withthermal maturity generally decreasing from hin-terland to foreland as a result of progressively lesssevere thermal evolution during the mountainbuilding. A positive correlation between VRo% andI-S was found (fig. 9).

The highest VRo% values (about 0.60%–0.75%)and percentages of illite layers in I-S (60%–80%)are found in trench-involved and accreted passivemargin units. Lower values (VRo%: 0.20%–0.50%;illite layers in I-S: 30%–60%) are found in thrust-top and foredeep basin deposits and far-traveled Sic-ilide units. They define different thermal maturitylevels in diagenesis corresponding to the late andearly diagenetic zone of Merriman and Frey (1999),respectively.

In detail, for the Mount Soro unit, whose maxi-mum stratigraphic thickness is about 2 km, vitrin-ite-derived temperatures are about 100�–120�C,overlapping the lower part of the clay-derived tem-perature range based on I-S ordering (100�–110�C/170�–180�C). Thus, 100�–120�C satisfies both pa-rameters.

For the Troina unit, the Troina Tusa flysch, a fewhundred meters thick, reached about 100�–110�Con the basis of both organic and inorganic thermalparameters. For the underlying Argille varicolori(maximum thickness 500 m), only clay mineralogydata are available. Even when we assume the high-est geothermal gradient for a trench environment

(about 40�C/km), the base of the Argille varicoloricannot have experienced temperatures higher than130�C.

As a whole, the Mount Soro unit and the upperportion of the Troina unit are the most thermallymature domains along the studied transect. Thus,both these units experienced a thermal evolutioncompatible with the catagenetic stage of hydrocar-bon generation (according to Scotti [2005]) and withthe first stages of the late diagenetic zone (accord-ing to Merriman and Frey [1999]).

These inner units (e.g., Mount Soro unit) repre-sent the footwall of the Peloritani-Calabride crys-talline orogen that, after thickening, experiencedsubstantial exhumation before its final emplace-ment on top of the foreland. In this framework, theremains of the crystalline orogen and their internalstacking may provide the tectonic load controllingthe burial and thermal evolution of the structurallyunderlying units (particularly the Mount Soro unit).

Nevertheless, it cannot be determined whetherthe detected maturity level for the Mount Soro andthe upper portion of the Troina unit is due only totectonic loading or to high heat flow during trenchevolution. Generally, in oceanic trenches, mea-sured geothermal gradients are highly variable(from 5� to more than 40�C/km; Allen and Allen1993; Underwood and Moore 1995) because of thegreat influence exerted by fluid circulation. Notenough data are currently available on the studiedunits to perform a reliable reconstruction of thepaleogeothermal gradient.

Table 3. Summary of Calculated/Estimated Maximum Paleotemperatures for Different Tectonostratigraphic Units

Geodynamic setting, tectonic unit,and stratigraphic unit

T from mean VRo% (�C)

T based on claymineralogy

(�C)

T integration of VRo%,I-S, and geological

inputs (�C)

Barker and Pawlewicz 1986a Barker 1988b Barker and Pawlewicz 1994c Hoffman and Hower 1979 Our proposal

Subduction trench:Mount Soro unit:

Argille scagliose 116 118 112 100–110/170–180 ∼100–120Mount Soro flysch 94–117 (109) 97–120 (112) 96–113 (106) 100–110/170–180 ∼100–120

Troina unit:Troina-Tusa 94–109 (101) 97–111 (103) 96–107 (101) 100–110/170–180 ∼100–111Argille varicolori … … … 100–110/170–180 ∼100–130

Early thrust-top basin:Reitano flysch 36–67 (53) 39–70 (55) 51–75 (64) !60 50–60

Early foredeep:Mount Salici unit:

Gagliano marls 48 50 60 60–100 60–100Numidian flysch … … … 60–100 60–100

Nicosia unit:Numidian flysch … … … 60–100 60–100

Late thrust-top basin:Clays and marls !20 !20 !40 60–100 ∼60Terravecchia Formation 39–63 (53) 42–65 (55) 53–71 (64) 60–100 60–70

Africa passive-margin-derived units andsynorogenic foreland deposits:

Mount Judica unit:Clays and glauconitic sandstones … … … 100–110 ∼100–110Scaglia facies limestones … … … 100–110 ∼100–110Radiolarian cherts … … … 100–110/170–180 ∼110–125Cherty limestones … … … 100–110/170–180 ∼110–125

External to the subduction trench:Polizzi Formation equivalent … … … 60–100 60–100Argille scagliose … … … 60–100 60–100

Note. ; reflectance; .T p temperature VR % p vitrinite I-S p illite-smectiteoa Temperatures obtained by using Barker and Pawlewicz’s (1986) equation: .T (�C) p (lnVR % � 1.40)/0.0096ob Temperatures obtained by using Barker’s (1988) equation: .T (�C) p 104(lnVR %) � 148oc Temperatures obtained by using Barker and Pawlewicz’s (1994) equation: .T (�C) p (lnVR % � 1.68)/0.0124o

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 349

This maturity level was acquired before the de-position of the unconformable early thrust-top de-posits of the Reitano flysch that show low maturitylevels ( ). This jump in thermal ma-VR % ! 0.5%o

turity suggests that Mount Soro and Troina units’thermal evolution was achieved before Burdigaliantimes.

Vitrinite-derived temperatures for the Reitanoflysch (about 50�–60�C) agree with those estimatedthrough clay mineral assemblage of shales (!60�C)but are not consistent with those of sandstones,which suggest temperatures exceeding 100�C.

Data plotted in figure 9 show two lithology-de-pendent clay mineral assemblages and a smallgroup of VRo% values (0.35%–0.47%). The chem-ical and physical conditions in shales (SI17) andsandstones (SO10) are not the same. The reactionrate of illitization can be affected by the initialsmectite or I-S composition and the relative pore-fluid ion concentration in the sedimentary rocksystem (Niu and Ishida 2000). Additionally, I-S maybe heterogeneous within any one sample due tomultiple origins of the clay (detrital vs. diageneticorigin). In our case, SO10 contains significantamounts of detrital phases (illite and chlorite). Thisdoes not allow us to exclude the presence of detritalI-S, which would record environments inheritedfrom the past, generally of higher temperature, notdirectly related to the burial history of the Reitanoflysch. K-Ar dating could help to discriminate adifferent origin of I-S (Peaver 1999; Meunier et al.2004) and to choose the authigenic population ofI-S suitable for paleotemperature calculation.Hence, only data from shale (presence of discretesmectite) have been used for paleotemperaturescalculation.

A mismatch arises from the estimated temper-atures of Gagliano marls. The I-S ratio and R p 0ordering suggest higher paleotemperatures (60�–100�C) than those calculated by Barker’s equationsfrom VRo% data (about 50�–60�C in table 3). Mea-sured vitrinite fragments are slightly fractured;therefore, calculated temperatures from organicmatter maturity of this deposit might be slightlyunderestimated. Clay mineral assemblage in theNumidian flysch suggests a temperature range sim-ilar to that of the overlying deposit. In light of thisconsideration, data from early foredeep deposits in-dicate a coherent maximum temperature between60� and 100�C. Nevertheless, the absence of suit-able organic matter for maturity analyses in theNumidian flysch prevents us from better constrain-ing this range.

A narrower temperature range can be fixed forlate thrust-top basin deposits. High expandable

I-S is associated with low VRo% values inR p 0both the analyzed formations. Barker’s equationsfail or have low reliability when applied to VRo%lower than 0.25% and around 0.30%, respectively.In the latter case, equations give temperatures!40�C for Pliocene clays and marls. These temper-atures are lower than those estimated by clay min-eral assemblage (60�–100�C) on the same unit.Nevertheless, considering that the present-daythickness of this unit is in the order of a few hun-dred meters and the onset of illitization in shalesis placed at ∼60�C (Hoffman and Hower 1979), thelower boundary of the temperature range, distinc-tive of random I-S ( ), is probably the maxi-R p 0mum temperature that Pliocene clays and marlssustained.

Furthermore, this value agrees with vitrinite-de-rived temperatures of the stratigraphically under-lying Terravecchia Formation. Collected samplesare from the upper portion of this unit. An enrich-ment of illite layers in I-S and slightly higher VRo%values were found in comparison with Plioceneclay and marls samples. Thus, a temperature of 60�–70�C seems reliable for the upper portion of theTerravecchia Formation. Sedimentary burial is con-sidered to be the main factor responsible for theacquired thermal maturity.

Data from Enna wells drilled in central Sicilyindicate a thickness of about 2000 m for the Ter-ravecchia Formation in the subsurface (Carbone etal. 1990). Therefore, the bottom of the TerravecchiaFormation should have experienced higher temper-atures.

The case of the Mount Judica area demands abrief discussion. For the Mount Judica succession,only mineralogical thermal indicators are availablebecause the slow sedimentation rate hardly favoredorganic matter preservation. The lack of discretesmectite and I-S structures indicates tem-R p 3peratures between 100�–110�C ( toR p 0 R p 1transition) and 170�–180�C ( to tran-R p 1 R p 3sition) in the late diagenetic zone. Furthermore,when we consider the thickness of the succession(!900 m; Carbone et al. 1990), the toR p 0 R p

transition within the Oligocene-Serravallian1clays and glauconitic sandstones and/or Scaglia fa-cies marls, and an assumed pre-erosional geother-mal gradient of 25�–30�C/km for these deposits (Al-len and Allen 1993; Frey and Robinson 1999), thetemperature range for the Mount Judica successioncan be constrained between 100� (top) and 125�C(bottom; table 3).

However, sedimentary burial alone cannot ex-plain these values. An additional loading of tec-tonic origin, about 3 km thick, now totally eroded,

350 L . A L D E G A E T A L .

can justify this temperature distribution. This eval-uation agrees with that of Roure et al. (1990), whostated that the Mount Judica unit was never buriedbelow 4 km, although Larroque et al. (1996) ob-served homogenization temperatures of fluid in-clusions up to 180�C, suggesting deeper burial and/or hot fluid circulation. The emplacement of innerunits in middle Miocene times (e.g., the packagemade up of far-traveled Sicilide units and Numi-dian flysch) onto the Mount Judica unit may haveprovided this tectonic loading, as already suggestedby published maps (Carbone et al. 1990).

For the far-traveled Sicilide units, a temperaturerange of 60�–100�C was estimated on the basis of

I-S that is distinctive from the rest of theR p 0Sicilide units (Hoffman and Hower 1979). This re-sult suggests that they experienced a considerablydifferent tectonic history from that of the strati-graphic units to which they were originally linked(Oligo-Miocene strata in Troina and probably Nic-osia units).

In detail, a reasonable hypothesis is that a tec-tonic decoupling occurred between the upper andthe lower portion of the Troina unit as a result ofearly orogenic deformation before Burdigaliantimes. Thus, the upper portion experienced maxi-mum paleotemperature as a result of the involve-ment into trench evolution, while the lower por-tion escaped this destiny thanks to the earlydecoupling, preserving a thermal maturity mainlyacquired in the original sedimentary basin.

Conclusions

In this study, we provided a data set of thermalmaturity indicators for the tectonostratigraphicunits that make up the backbone of the Apenninic-Maghrebian chain in eastern Sicily. The indepen-dent methodologies supplied comparable maxi-mum paleotemperatures (table 3), indicating asatisfactory match of different sets of data.

Specifically, we can conclude that along the Ne-brodi Mountains–Hyblean Plateau transect, a gen-eral trend of decreasing thermal maturity of sedi-ments is evident, moving from hinterland toforeland and from older to younger stratigraphicunits. This trend can be interpreted as being causedby differences in either tectonic loadings or pa-leothermal regimes. In this framework, (1) in theSicilide units, a difference in thermal maturity ofsediments was detected between the units thatnowadays are in the northern sector of the studyarea with maximum paleotemperatures of about100�–120�C and those cropping out to the south, at

the chain front, with maximum paleotemperaturesbetween 60� and 100�C. This data distribution sug-gests differences in the Cenozoic tectonics. Thenorthern units were involved in trench develop-ment before early Miocene times, whereas thesouthern units were tectonically decoupled fromthe original succession and then traveled far to amore external position in Langhian-Tortoniantimes. (2) In the Mount Judica succession, data in-dicate maximum paleotemperatures ranging be-tween 100� and 125�C due to tectonic burial duringLanghian-Tortonian times. (3) The siliciclastic de-posits in both the thrust-top basins and the fore-deep environment generally show lower thermalevolution, with VRo% values between 0.20 and 0.50and illite content in I-S between 30% and 60% withvariable paleotemperature ranges (see table 3).

In the end, correlation of organic and inorganicindependent thermal indicators turns out to be sat-isfactory, and a thermal zonation of the transectwas achieved. Nevertheless, some of the analyzedunits cannot be univocally constrained to narrowerpaleotemperature ranges when only one type ofthermal maturation data (either organic or inor-ganic) is available. These cases deserve a cross-check with other independent thermal indicators(e.g., fluid inclusions) to reduce the temperatureranges and better constrain the tectonosedimentaryevolution of different units. Furthermore, areas inwhich the calculated paleotemperatures are 110�Cor more need an integration with apatite fissiontrack analyses in order to check maximum paleo-temperatures and constrain the exhumation paths.

A C K N O W L E D G M E N T S

We wish to thank F. Botti for her constant supportin organic matter (OM) analysis and preparation,A. Fasto for his help in the field, and I. Federici forperforming most of the lab preparation. We aregreatly indebted to R. Tyson of the University ofNewcastle upon Tyne for providing facilities forOM maturity analyses and to C. Giampaolo, S. LoMastro, and D. Dolfi for the facilities of the XRDand preparation lab in “Roma Tre” University. F.Lentini is acknowledged for very interesting andhelpful discussions on Sicilian tectonics and stra-tigraphy. Special thanks go to R. Butler for havinginspired the use of this analytical approach to thestudy of tectonics in the Apenninic-Maghrebianfold-and-thrust belt and for supporting our workwith exciting discussions. Thanks are also due toM. L. Balestrieri, S. Mazzoli, F. Speranza, and M.Zattin for reading the original version of the man-

Journal of Geology C O R R E L A T I O N O F D I A G E N E T I C D A T A 351

uscript and providing useful suggestions. We aregrateful to A. Anderson, B. J. Sivertsen, and twoanonymous reviewers for detailed revisions andhelpful suggestions. V. Borgia is warmly acknowl-edged for the Sicilian hospitality. Research finan-

cial support came from Miur COFIN2004 (S. Cor-rado; Italian Research and University MinisteryBiannual Funding Project) and the Department ofGeological Sciences, University “Roma Tre” (S.Corrado).

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