palygorskite genesis through silicate transformation in ...hera.ugr.es/doi/15079399.pdfsouth axis,...

Palygorskite genesis through silicatetransformation in Tunisian continental

Eocene deposits

F JA M O US S I 1 A B E N A B O U D2A ND A LO P EZ - G AL I N D O2

1 Laboratoire lsquo Georessourcesrsquo INRST BP 95 2050 Hamam-Lif Tunisia and 2 Instituto Andaluz de Ciencias de laTierra CSIC-Universidad de Granada Facultad de Ciencias Fuentenueva sn 18002 Granada Spain

(Received 30 April 2002 revised 21 October 2002)

ABST RACT The mineralogical and geochemical characteristics of Eocene continental sedimentsin south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (JebelLessouda and Jebel Rheouis) which contain considerable amounts of palygorskite were studied Theclay fraction of the sediments also comprises illite kaolinite Mg smectite and Al smectite togetherwith carbonates (calcite andor dolomite) quartz gypsum and feldspars all of which are present inextremely variable proportions

The textural characteristics of the samples containing most palygorskite as well as the chemicalcomposition of the fibres and the contents of certain trace and rare earth elements suggest that thegenesis of this fibrous clay is intimately linked to the diagenetic transformation of illite mixed-layered minerals andor Al smectite as has also been observed in contemporaneous deposits inMorocco

KEYWORDS palygorskite illite Tunisia Eocene

With the aim of drawing a map of potentially usefulclays in Tunisia a systematic survey of thePalaeozoic to Quaternary sediments has beenperformed over the last few years by one of theauthors (FJ) The survey showed numerouspalygorskite-rich Eocene levels that might be ofeconomic interest

In Tunisia Eocene sediments present importantfacies variations from South to North changingfrom continental to deep marine deposits (Bishop1988 Jamoussi et al 2001a Fig 1) Thesechanges in facies and thickness are mainly due toAlpine compressive tectonic events and thehalokinetic movements of Triassic materialsaffecting the Central Atlas domain during theEocene (Bedir 1995 Boukadi amp Bedir 1996)

Continental Eocene facies studied by numerousauthors are deposited in two main areas in centralTunisia around the so-called lsquoKasserine islandrsquo(Burollet 1956) and to the south of the Gulf ofGabes and the northern part of Jeffara In centralTunisia Truc (1981) and Zouari (1984) foundEocene sediments at Jebel (J) Chaambi and Sassiet al (1984) found them on the south-eastern flankof the J Kebar In the J Lessouda and KoumineKadri et al (1986) and Kadri (1988) detected thesame facies but Jamoussi (2001) pointed out thatthey were not immediately above the Ypresiancarbonate bar In the northern area of the Chottsbelt Abdeljaoued (1983 1991 1997) defined thecontinental formation of Bouloufa dated by bulimes(continental gastropods) as Lower Eocene in JRheouis and Boudinar around the Kasserine islandcontinental deposits were also discovered and in JChamsi such sediments have been suspected(Jamoussi 2001 Jamoussi et al 2001a) Finally

E-mail albertougresDOI 1011800009855033820088

Clay Minerals (2003)38 187ndash199

2003 The Mineralogical Society

the first oil drilling carried out in the southern partof the Gulf of Gabes as well as some drilling in thenorthern part of Jeffara found continental Eocenedeposits known as the Tanit Formation (Pochitaloff1968) The mineralogy of some of these deposits hasbeen described by among others Abdeljaoued(1997) Srasra et al (1995) Ben Aboud (1998)Hachi (1998) Ben Aboud et al (1999) Fakhfakh(1999) and Jamoussi (2001)

In this paper we study the mineralogy geochem-istry and microtexture of palygorskite-rich Eocene

samples in Tunisia The better sequences arelocated in some of the above-mentioned areas(Chebket Bouloufa J Hamri J Lessouda and JRheouis) Some samples from J Boudinar were alsostudied

G E O L O G I C A L S E T T I N G

The end of the Cretaceous is marked in central andsouthern Tunisia by important palaeogeographicaland sedimentological modifications Marine sedi-

50

N

Emerged area

Phosphate deposits

Tidal zone (Sebkha)

Supratidal to tidal zone

Nummulitic limestone

Marine deposits

Sections investigated

0

Mahdia

KerkennahIsland

Sfax

Medenine

Gafsa

Gabegraves

Jerba

Tunis

ALG

ERIA

LIBYA

100 km

J Bouloufa J Hamri

J Rheacuteouis

J Lessouda

J Boudinar

FIG 1 Lithofacies map of the Lower Eocene in Tunisia (modified from Jamoussi et al 2001) and the locationsof the sections studied

188 F Jamoussi et al

mentation represented by white limestones withinoceramids and echinoderms known as the AbiodFormation (Burollet 1956) changes towards a moredetrital clayey sediment during the PalaeoceneAfter compressive tectonic movements of transversefaults during the Upper Cretaceous (Zargouni1985 Boukadi 1994 Bedir 1995) some emergedareas appeared such as the above-mentionedKasserine Island

Tertiary sequences are particularly well repre-sented in the North Chain of the Chotts and includeseveral lithostratigraphic units such as Beglia andSegui Formations consisting of sand clay andconglomerates One particular lithological memberwas defined by Abdeljaoued (1983) as the BouloufaFormation extending over a wide area on thesouthern edge of the Gafsa-Metlaoui phosphatebasin This formation appears in most cases ascalcretes and dolocretes with vacuolar appearanceAt J Chambi Sassi et al (1984) found one faunalassemblage consisting of continental gastropods(Romanella hopii Vidaliella) and helicids(Paleocyclotus) that allowed this formation to bedated as Lutecian-Bartonian

The most representative sequences of theBouloufa Formation crop out 25 km northwest ofEl Hamma town at J Bouloufa and J Hamri Theyare ~150 m thick and are made up of alternatingconglomerates marls and limestones white and redin colour with some gypsum and silex nodules

The Eocene deposit located at J Rheouis closeto a Triassic diapir (Soussi et al 1996) is situatedin central Tunisia 10 km to the south of Fa otilde d onthe boundary between the Sidi Bouzid and JGoubrar 150000 topographic maps It appears inthe intersection of NE ndashSW structures of the NorthndashSouth Axis known as J Goubrar and Boudinar andthe NW ndashSE fold of J Ksaotilde ra (Boukadi and Bedir1996) Triassic halokinetic movements alongtransverse faults led to cropping out of thesestructures particularly during the Eocene (Boukadiand Bedir 1996) The sequence is ~40 m thick andis made up of white and grey limestones and marlylimestones marls and clayey marls Some finegypsum intercalations appear occasionally

Finally J Lessouda is situated ~10 km to theNorth of Sidi Bouzid town at the western part ofthe NorthndashSouth Axis It constitutes a N40Eanticline superimposed on an Upper Cretaceousdiapiric dome (Creuzot and Ouali 1989) emergingin the centre of a vast plain It is fractured by km-long N ndashS NW ndashSE and E ndashW faults which have

synsedimentary influences on the distribution offacies and their thickness from Upper Cretaceousuntil Middle Miocene (Kadri 1988) The sequence~20 m thick is made up of alternating white redand green marls with fine intercalations of gypsum

Although the presence of bulimes in thesecontinental Eocene deposits clearly indicates aLower Eocene age recurrences of continentalfacies seem to be diachronic and closely influencedby local tectonic conditions of the geologicalsubstratum The mineralogy of the clay fractionand particularly the presence of palygorskite couldbe an element of identification and inter-relation-ship between these series which have been markedby compressive movements and halokinetics ofTriassic rocks

M E T H O D O L O G Y

The mineral phases were determined by X-raydiffraction (XRD) using Philips PW 1710 andSiemens Kristalloflex 810 diffractometers Cu-Karadiation at a scan speed of 2 to 6ordm2y min ndash1Analysis was performed on both the whole sampleand the clay fraction Oriented aggregates wereprepared for clay mineral analysis and were thentreated with ethylene glycol dimethyl-sulphoxideand heating to 550ordmC for 1 h The reflecting powersof Schultz (1964) Biscaye (1965) and Barahona(1974) were used to quantify the different mineralphases detected as well as the chemical composi-tions of samples (Lopez-Galindo et al 1996)

Major elements were analysed by atomicabsorption spectrometry using Perkin Elmer equip-ment with acetylene or protoxide acetylene flame tTrace and rare earth elements (REE) were measuredusing an ICP-MS Perkin Elmer SCIEX Elan-5000device The detection limits of the elements were10 ppb for REE and Th 100 ppb for transitionelements and Cs Rb Sr Ba and Pb and 1000 ppbfor Li and B

The scanning electron microscope (SEM) obser-vations were performed using a Zeiss DSM 950(equipped with LINK microanalysis system) Thetransmission electron microscope (TEM) observa-tions and microanalyses were carried out on theclay fraction of selected samples using a PhilipsCM 20 (equipped with an EDAX microanalysissystem)

Factor analysis (Principal Components AnalysisPCA) was used to establish the relation between thedifferent minerals and major and trace element

Palygorskite in Tunisian sediments 189

contents The factors (principal components Pcs)were selected for eigenvalues gt1 (Swan ampSandilands 1995) applying a Varimax rotation

R E S U L T S

Mineralogy

In Fig 2 the bulk and clay mineralogy of the fourmost representative sequences mentioned above aregiven Table 1 contains the average geochemicalcharacteristics of these

Two clearly distinguished groups of palygorskite-rich outcrops can be established those located insouth central Tunisia (J Bouloufa and J Hamri)which contain considerable amounts of quartzcalcite and kaolinite and those located in northcentral Tunisia (J Rheouis and J Lessouda) withdolomite as the most distinctive mineral

Thus at J Bouloufa the samples comprisephyllosilicates carbonates quartz gypsum andtraces of feldspar With the exception of thebottom of the sequence where quartz and gypsumare the main phases carbonates (calcite and

TABLE 1 Average geochemical data for the sequences studied

J Bouloufa J Rheouis J Bouloufa J RheouisJ Hamri J Lessouda J Hamri J Lessouda

Major components in the clay fraction REE contents in the clay fractionSiO2 5388 5317 5844 5672 La 2445 2964 2158 2054Al2O3 1659 2134 1068 1428 Ce 3842 5962 3231 3243Fe2O3 352 351 335 347 Pr 392 460 335 336MgO 724 521 941 777 Nd 1225 1450 1077 1080CaO 029 047 033 033 Sm 183 225 162 171Na2O 020 040 017 021 Eu 035 041 033 037K2O 225 313 141 229 Gd 126 158 116 121TiO2 018 028 014 019 Tb 019 025 018 020Mn2O3 004 004 004 004 Dy 114 145 106 123LOI 1539 1228 1558 1432 Ho 023 030 021 025

Er 065 087 058 071Trace element contents in the clay fraction Tm 011 014 010 012Ni 287 382 131 133 Yb 071 092 064 072Co 74 97 52 69 Lu 011 014 010 012Sr 98 219 50 106Ba 142 194 82 76 Structural formula of palygorskites (AEM data)V 107 119 69 95 Si 765 770 785 764Cr 101 107 80 92 Al IV 035 030 015 036Zn 118 165 78 108 Mg 191 154 176 204Cu 149 266 72 104 Fe3+ 051 053 045 058Li 40 74 78 86 Al VI 148 173 164 135Y 53 77 50 92 Ca 009 007 006 008Rb 115 124 84 93 Mn 003 003 001 002Zr 119 136 154 192 Ti 004 002 002 005Nb 219 241 294 362 K 011 017 013 011Cs 58 59 40 47Hf 31 36 37 48 Granulometry of samples ()Th 66 78 45 86 lt1 mm 7826 7614 7183 7458U 12 14 10 36 1 ndash2 mm 1584 1661 1907 1735Be 25 28 17 18 2 ndash3 mm 279 286 409 373Sc 117 138 80 105 3 ndash4 mm 159 214 212 206Ga 251 267 180 222 4 ndash5 mm 081 106 156 106Ta 16 18 20 51 5 ndash7 mm 060 087 093 092Mo 20 20 08 15 7 ndash10 mm 011 023 026 020Sn 21 23 17 18 10 ndash 20 mm 000 010 014 011Pb 93 134 03 04 gt20 mm 000 000 001 001Tl 06 09 64 94 Mean 102 107 113 109


FIG 2 Bulk and clay mineralogy of the Tunisian Eocene sediments studied


dolomite) are generally the most abundant phasessometimes reaching up to 77 and quartzrepresents ~10 (Fig 2) The clay mineralsdetected in the lt2 mm fraction are palygorskiteillite Al smectite and kaolinite Palygorskite withcontents of ~50 ndash70 is the most importantphyllosilicate illite contents range from 11 to36 smectite and kaolinite do not represent morethan 10 although at the bottom there is up to27 kaolinite

The mineral phases found at J Hamri are thesame but in different proportions Quartz (up to54) phyllosilicates and calcite are the maincomponents In the clay fraction illite is the mostabundant phase (38 ndash59) followed by palygors-kite (14 ndash40) Kaolinite normally appears in smallamounts (5 ndash10) and Al smectite is systematicallypresent reaching 28 at times

On the other hand J Rheouis and J Lessoudashow rather similar compositions which aredifferent from the southern sequences Theyconsist almost exclusively of phyllosilicates anddolomite in varying proportions with quartz andfeldspars as trace minerals In addition in JRheouis some samples contain small quantities ofclinoptilolite (lt3) The clay fraction is morevariable containing palygorskite (38 ndash96) andillite (3 ndash23) as major components Mg-smectite(not present in the southern sequences) Al smectitekaolinite (at J Lessouda) and opal are also presentbut never in significant quantities (up to 10)

Figure 3 shows the mineralogical composition ofall the samples studied Even in the same sequencethis composition is very diverse as shown by thebreadth of the fields indicating highly variableconditions of the depositional environment

regarding both the sediment supplies and theirchemical characteristics

Geochemistry

The average major component trace element andREE contents of the clay fraction of selected samplesare presented in Table 1 Southern sequences (JBoloufa and J Hamri) are richer in Al2O3 (J Hamrialso in K2O) corresponding to their higher illitecontent MgO content ranges from 521 to 941while the central sequences are richest in Mg

We obtained the correlation matrices of all themineralogical and geochemical variables (majorelements and the complete set of trace and rareearth elements) The results obtained show thatAl2O3 is closely linked to Fe2O3 (r = 097) K2O (r= 098) TiO2 (r = 098) and Mn2O3 (r = 070) aswell as to the phyllosilicates of clearly detritalorigin with r gt08 (illite interstratified illite-smectite Al smectite and kaolinite) The REEand among others transition elements (TRTE) NiCo V Cr Cu Sc behave in a similar fashionpresenting r gt08 among themselves and with theaforementioned variables

In view of the good correlations and consideringthe calculation established by Lopez Galindo et al(1996) we determined from the theoretical REEand TRTE contents that pure palygorskite would bepresent in these deposits (Fig 4) Thus the REEcontents would be ~40 ppm (Bou-Loufa and JHamri) 50 ppm (J Rheouis) and 70 ppm (JLessouda) Likewise the amount of TRTE in thispure palygorskite would vary from 125 to 175 ppm

When the REE contents of the different Tunisiandeposits are normalized to NASC we find an

Calcite + Dolomite

Phyllosilicates Quartz + Feldspar

J Bouloufa

J Hamri

J Rheouis

J Lessouda

Illite + Kaolinite

Al smectite Palygorskite + Mg smectite

J Boudinar

50

50

50

5050

FIG 3 Mineralogical composition of the samples studied


essentially horizontal pattern containing between25 and 100 of the NASC contents and that thereis a slight enrichment in light rare earth elements(LREE) (2 LaLu 45) No significant anomalieswere detected except for one case in the J Hamrideposit where the richest palygorskite sample had aclear positive anomaly in Ce

Principal component analysis was applied tomineralogical and chemical variables Threefactors explaining the 80 of the total variancewere obtained (Fig 5) Factor 1 (40) and factor 2(24) are clearly interpretable as genetic factorsand separate the clearly detrital minerals andassociated chemical elements from those compo-nents of presumably different origin (palygorskiteamorphous silica and Mg smectite) Factor 3 (16)discriminates between silicates and carbonates

Chemical composition

Table 1 summarizes the mean compositions ofthe palygorskite analysed by AEM in the differentsequences Figure 6 representing the compositionof the octahedral layer shows the wide variabilityexisting even between samples from the samedeposit Practically all the particles analysedpresent compositions intermediate between diocta-hedral and trioctahedral (this is particularly clearwhen compared with the compositional field of thedifferent types of smectites Fig 6c) We shouldalso point out that substitution of Si by Al in thetetrahedral sheet is relatively moderate (015 ndash035atoms per unit cell) there is a weak presence of Tiandor Mn in the octahedral sheet (never more than

005 atoms per structural formula) and there is asystematic presence of K and Ca in the interlayerwith predominance of the former (at times up to 04K atoms are found per unit cell)

It is interesting to note that the J Hamri depositwhich contains the most clearly detrital mineralassemblage also has the palygorskites richest in Aland with the largest amount of K in the interlayer

Micromorphology

The microtexture observed by SEM varies fromone sequence to another or even within the samesequence The most frequent manner in whichpalygorskite appears is as chaotic arrangements ofshort fibres (lt2 mm Fig 7a) or in more or less

FIG 4 REE and TRTE content of the studied samples as a function of the percentage of detrital phyllosilicates

FIG 5 R-mode factor analysis showing the contributionof statistically dominant variables (Factors 1 and 2)


planar structures with filamentous edges (Fig 7b)consisting of particles which on a small scalepresent some preferential orientation (Fig 7c)Longer fibres (up to 5 mm) are also found arrangedin bundles or sheaves and which taken as a wholehave the appearance of a mat or rug (Fig 7d)

The presence of idiomorphic dolomite crystalscovered by palygorskite fibres (Fig 7e) or growingon such fibres (Fig 7f) illustrates the differentgrowth generations of this fibrous clay dependingon the physical-chemical variations of the environ-ment

The average size-range of the clayey particlesmeasured both by laser granulometer and by imageanalysis of photomicrographs obtained by TEM issummarized in Table 1 where we can see thatgt90 of the fibres are lt2 mm

D I S C U S S I O N

There are numerous studies of the genesis ofpalygorskite in continental environments (egMillot 1964 Singer 1984 Velde 1985 Jones ampGalan 1988 among others) As is known thisfibrous clay is found especially in soils calcretesand lacustrine deposits and there is generalagreement that its formation requires alkalineconditions high Si and Mg activity and an arid orsemi-arid climate with marked dry and wetseasons

Bearing in mind the nature of the clayparagenesis (cf Chamley 1989) most of the claysfound in the Tertiary continental deposits of Tunisiaare basically detrital and their accumulation isundoubtedly linked to water andor wind supply in

FIG 6 Composition of the palygorskite octahedral sheet (a) south central sequences (b) north central sequences(c) comparison with smectite composition (d) REE distribution of a pure palygorskite normalized to NASC

FIG 7 (facing page) Microtextures of the samples studied (a) Chaotic arrangements of short fibres (J Rheouis)(b) Planar structures with filamentous edges (J Lessouda) (c) Detail from the edge of a planar aggregate(J Rheouis) (d) Imbricated fibres of palygorskite with a mat aspect (J Bouloufa) (e) Idiomorphic dolomitecrystals covered by palygorskite fibres (J Bouloufa) (f) Dolomite growing on palygorskite fibres (J Bouloufa)



closed lacustrine-type basins or in playa-lakesaccumulated during flooding episodes (Jamoussi etal 2001ab) The source area for the Tertiarydeposits must have been the surrounding formations(Jamoussi 2001) because they contain considerableamounts of clays particularly Al smectite illitekaolinite and as is the case for the Triassic diapirschlorite the alteration of which could haveprovided additional Mg (Jamoussi 2001)Moreover the genesis of both primary anddiagenetic carbonates as well as the precipitationof gypsum (present in all the sequences studied)increases the MgCa ratio of the interstitial waterguaranteeing an alkaline pH Finally in a recentstudy on climatic evolution in the Tethys during thePalaeocene-Eocene Bolle amp Adatte (2001) showedthe progressive development of arid climaticconditions in southern Tunisia during this period

The REE and trace elements data particularlythose for the first transition series (TRTE) provideinteresting clues to explain the genesis ofpalygorskite in these continental Tertiary depositsIn fact most of the natural water has extremely lowREE concentrations of ~10ndash7 ndash10ndash2 in comparisonwith the values found in the rocks (McLennan1989) This is mainly due to the fact that mobilityand fractionation of the REE are very unlikely atthe fluidrock level (Taylor amp McLennan 1985)ie the REE move from the alteration profile to thesedimentary basins without being affected by anychemical process during transport (Fleet 1984)

The behaviour of the REE during the differentalteration processes is mainly controlled byinheritance and their pattern in clayey sedimentsis clearly related to that of the mother rock (Cullerset al 1975 Bonnot-Courtois 1981 Fleet 1984)The REE are essentially adsorbed by the clays andare found in considerable quantities in clearlydetrital clays and in very small amounts in clearlyneoformed clays such as sepiolite andor stevensite(~250 ppm in the former and lt20 ppm in the latterTorres Ruiz et al 1994) In low-temperatureaqueous solutions transition elements such as NiCo V Cr Cu and Sc behave in similar fashion tothe REE and usually appear in equally lowconcentrations (McLennan 1989) In Spanishdeposits this quantity has been determined at~450 ppm in detrital minerals and lt80 ppm inneoformed phases (Torres-Ruotildeacutez et al 1994)

The REE and TRTE contents found in thepalygorskite from Tunisian deposits vary from 40to 70 ppm and from 125 to 175 ppm respectively

These values coincide on the whole with thosedetermined in other Spanish and Moroccan deposits(Ben Aboud 1998) and which are intermediatebetween those of the two mineral groups mentionedabove

The REE distribution model of a threoretical purepalygorskite (Fig 6d) shows a slight impoverish-ment in heavy REE (Gd ndashLu HREE) that can beattributed to the pH of the interstitial solutionswhich aids accumulation of light REE (La ndashEuLREE) in alkaline conditions (Nesbitt 1979) aswell as to the mobility of the HREE in naturalsolutions because of their ability to form bicarbo-nated and organic solutions more soluble than theLREE (Fleet 1984 McLennan 1989) or alsobecause of differential adsorption of lanthanidesonto hydrogenous particle surfaces due to theirdifferent polarizing power (Turner amp Whitfield1979)

As regards chemical composition palygorskite isa clay that accepts quite considerable substitutionsin the octahedral sheet its composition typicallybeing intermediate between the dioctahedral andtrioctahedral end-members (Weaver amp Pollard1973) Later studies by Paquet et al (1987)Duplay (1988) and Galan amp Carretero (1999)have shown that the variation interval in thecomposition of palygorskite is even larger thanfirst thought with many analyses clearly fallingwithin the dioctahedral domain as is frequently thecase with the Tunisian samples especially from JHamri and J Rheouis Regarding the averagecompositions summarized in Table 1 we shouldpoint out the high Fe and Al contents present in theoctahedral sheet as well as the high K and Cavalues in the interlayer the proportions of whichare similar to that detected in other aluminosili-cates such as illite andor Al smectite

As in the cases of other palygorskites from Spainand Morocco described in previous studies (BenAboud et al 1997 Ben Aboud 1998) thegeochemical and textural data suggest that thegenesis of this fibrous clay is intimately linked tothe transformation of a detrital aluminosilicateprecursor especially smectite and interstratifiedillite-smectite On this point we should mentionthat when studying the mineral assemblagesthroughout the geological record in Tunisia(Jamoussi et al 2001b) it can be seen thatduring the Eocene the proportion of smectite fallsdramatically in sequences where palygorskite ispresent This type of mechanism has often been


suggested as the most probable cause of the presenceof palygorskite eg the papers by Mart otilde n de Vidaleset al (1988) on lacustrine-palustrine Miocenesediments in the Madrid basin who suggestalteration of the structure of pre-existing mont-morillonite in a mechanism of dissolution-precipita-tion due to the presence of magnesian waters duringthe carbonatation processes involving calcreteformation by Suarez et al (1994) on theBercimuel deposit (Segovia Spain) who suggestedmechanisms of dissolution of detrital mica with Kand Al loss and Si and Mg gain giving rise tostructural formulae very similar to those describedhere or by De Pablo (1996) on Eocene palygorskite-rich mudstone in the Yucatan Peninsula wherepalygorskite formed in a tranquil marine back-reeflagoon environment of high Mg Si and Al activityby diagenesis of precursor montmorillonite fromreaction between montmorillonite dolomitic lime-stone and silicic acid On the other hand in his studyof the manner of genesis of an Al and Mgpalygorski te in the Bouloufa FormationAbdeljaoued (1997) showed that this mineral canappear both as direct precipitation from solution andas recrystallization of smectites with no particularpredominance of either of these mechanisms

The fact that idiomorphic crystals of dolomitesimilar to those described by Karakas amp Kadir(1998) were found covered with palygorskite fibresor resting on a fibre mat (their photos A and B)indicates that small chemical changes due tocl imatic fluctuations gypsum precipitation(Khademi amp Mermut 1998) or as a result ofearly diagenetic transformations in the superficiallayers of sediment could have affected the chemicalcharacteristics of the lake water thus altering theMgCa ratio and the pH of the environment andcausing different generations of carbonates and theappearance of palygorskite

C O N C L U S I O N S

The clay fraction of Tunisian Eocene continentalsediments contains considerable quantities ofpalygorskite (up to 96) Its average mineralogicalformula is intermediate between the dioctahedraland trioctahedral end-members as follows

(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O

As observed in similar Spanish and Moroccandeposits the transition trace element and REE

contents of Tunisian pure palygorskite are alsointermediate between clearly authigenic clays suchas sepiolite (lt50 ppm) and detrital clays such asillite mixed-layer I-S and Al smectite (gt400 ppm)

We suggest that this fibrous clay originatedduring the first stages of diagenesis due to thedestabilization of the existing aluminosilicates in amechanism of dissolution-precipitation that couldbe due to the presence of magnesian waters duringpost-depositional carbonation processes

ACKNOWLEDGMENTS

This research is part of the projects lsquoIdentificationcartographie et valorisation des materiaux utiles enTunisielsquo (SERST Tunisia) and CSIC 1999TN002DGI-BTE2000-0777 and research group RNM-0179 ofthe Junta de Andaluc otilde a (Spain) The authors aregrateful for the helpful comments by Prof HerveChamley (Universite Strasbourg) and Prof MD Ru otildeacutez-Cruz (Universid ad de Malaga) which helped toimprove our paper We also thank Prof IanMacCandless (Department Filolog otildeacutea InglesaUniversidad de Granada) for assisting us with theEnglish grammar

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Abdeljaoued S (1983) Etude sedimentologique etstructurale de la partie est de la Chagne Nord desChotts (Tunisie meridionale ) Special thesisUniversite Tunis Tunisia

Abdeljaoued S (1991) Les dolocretes et les calcretes duPaleocene-Eocene Tunisie meridionale PhD thesisUniversite Tunis II Tunisia

Abdeljaoued S (1997) Mode de genese des palygors-kites dans la serie continentale eocene de Tunisiemeriodionale Notes du Service Geologique deTunisie 63 15 ndash27

Barahona E (1974) Arcillas de ladriller ga de laprovincia de Granada evaluacio n de algunosensayos de mater ias pr imas PhD thesis Universidad de Granada Spain

Bedir M (1995) Mecanismes geodynamiques desbassins associes aux couloirs de coulissement de lamarge Atlasique de la Tunisie Seismo-stratigraphieseismo-tectonique et implications petrolieres PhDthesis Universite Tunis II Tunisia

Ben Aboud A (1998) Depots tertiaires de palygorskitedans des bassins circum-mediterraneens (MarocEspagne Tunisie) Mineralogie geochimie et gen-ese PhD thesis Universidad de Granada Spain

Ben Aboud A Lopez-Galindo A Fenoll Hach-Al otildeacute P ampChella otilde EH (1997) Caracteristiques mineralogiqueset geochimiques des palygorskites tertiaires drsquoIda Ou


Ga otilde lal (nord-est de Taroudannt et de Toundout(nord-est de Ouarzazate) Implications genetiquesComptes Rendu de lrsquoAcademie des Sciences Paris324 IIa 189 ndash 195

Ben Aboud A Lopez-Galindo A Fenoll Hach-Al otildeacute PJamoussi F Bedir M Abdeljaoued S amp Setti M(1999) Mineralogy and geochemistry of someTunisian palygorskitic outcrops Pp 1073 ndash1076 inMineral Deposits Processes to Processing (CJStanley et al editors) Balkema Rotterdam

Biscaye PE (1965) Mineralogy and sedimentation ofrecent deep-sea clay in the Atlantic Ocean andadjacent seas and oceans Geological Society ofAmerica Bulletin 76 803 ndash832

Bishop WP (1988) Petroleum geology of east-centralTunisia American Associat ion of PetroleumGeologists Bulletin 9 1033 ndash1058

Bolle MP amp Adatte T (2001) Palaeocene-early Eoceneclimatic evolution in the Tethyan realm clay mineralevidence Clay Minerals 36 249 ndash 261

Bonnot-Courtois C (1981) Geochimie des Terres Raresdans les principaux milieux de formation et desedimentation des argiles PhD thesis Universite deParis-Sud France

Boukadi N (1994) Structuration de lrsquoAtlas de Tunisiesignification geometrique et cinematique des nuds etdes zones drsquointerferences structurales au contact degrands couloirs tectoniques PhD thesis UniversiteTunis II Tunisia

Boukadi N amp Bedir M (1996) Lrsquohalocinese en Tunisiecontexte tectonique et chronologie des evenementsComptes Rendu de lrsquoAcademie des Sciences Paris322 IIa 587 ndash 594

Burollet PF (1956) Contribution a lrsquoetude stratigra-phique de la Tunisie Centrale Annales des Mines etGeologie 18 Tunisia

Chamley H (1989) Clay Sedimentology pp 75 ndash94Springer-Verlag Berlin

Creuzot G amp Ouali J (1989) Extension diapirisme etcompression en Tunisie centrale le Jebel Es soudaGeodynamique 4 39 ndash48

Cullers RL Chaudhuri S Arnold B Lee M amp WolfCW (1975) Rare earth distributions in clay mineralsand in the clay-sized fraction of the Lower PermianHavensville and Eskridge shales of Kansas andOklahoma Geochimica et Cosmochimica Acta 391691 ndash1703

De Pablo L (1996) Palygorskite in Eocene-Oligocenelagoonal environment Yucatan Mexico RevistaMexicana de Ciencias Geologicas 13 94 ndash103

Duplay J (1988) Geochimie des argiles et geothermo-metrie des populations monominerales de particules PhD thesis Universite Strasbourg France

Fakhfakh E (1999) Identification et caracterisation desargiles fibreuses du centre et du Sud de la TunisieDEA thesis Universite Tunis II Tunisia

Fleet AJ (1984) Aqueous and sedimentary geochem-

istry of the rare earth elements Pp 343 ndash 373 in RareEarth Element Geochemistry (P Henderson editor)Elsevier Science Pubs Amsterdam

Foster MD (1960) Interpretation of the composition oftrioctahedral micas USGS Professional Paper354-B 11 ndash49

Galan E amp Carretero I (1999) A new approach tocompositional limits for sepiolite and palygorskiteClays and Clay Minerals 47 399 ndash409

Hachi A (1998) Identification caracterisation etvalorisation des argiles Eocene-Miocene des regionsde Kasserine Feriana et Sebeitla DEA thesisUniversite Tunis II Tunisia

Jamoussi F (2001) Les argiles de Tunisie etudemineralogique geochimique geotechnique et utili-sations industrielles PhD thesis Universite Tunis ElManar Tunisia

Jamoussi F Abbes Ch Fakhfakh E Bedir MKharbachi S Soussi M Zargouni F amp Lopez-Galindo A (2001a) Decouverte de lrsquoEocene con-tinental autour de lrsquoarchipel de Kasserine aux JebelsRheouis Boudinar et Chamsi en Tunisie centro-meridionale nouvelles implications paleogeographi-ques Comptes Rendu de lrsquoAcademie des SciencesParis 333 II 329 ndash335

Jamoussi F Lopez-Galindo A Morales S andZargouni F (2001b) The chart of Tunisian claysMineralogica Polonica 32 79 ndash86

Jones BF amp Galan E (1988) Sepiolite and palygorskitePp 631 ndash674 in Hydrous Phyllosilicates (Exclusiveof Micas) (SW Bailey editor) Reviews inMineralogy 19 Mineralogical Society of AmericaWashington DC

Kadri A (1988) Evolution tectono-sedimentaire (Aptien-Quaternaire) des Dj Koumine Hamra et Lessouda(Tunisie centrale) Thesis 3rd cycle Universite ParisX Orsay France

Kadri A Matmati F Ben Ayed N amp Ben Haj Ali M(1986) Decouverte de lrsquoEocene inferieur continentalau Jebel Lessouda (Tunisie centrale) Notes duService Geologique de Tunisie 51 53 ndash 59

Karakas Z amp Kadir S (1998) Mineralogical and geneticrelationships between carbonate and sepiolite-paly-gorskite formations in the neogene lacustrine KonyaBasin Turkey Carbonates and Evaporites 13198 ndash 206

Khademi H amp Mermut AR (1998) Source ofpalygorskite in gypsiferous arid soils and associatedsediments from central Iran Clay Minerals 33561 ndash 578

Lopez-Galindo A Torres-Ruiz J and Gonzalez-LopezJM (1996) Mineral quantification in sepiolite-palygorskite deposits using X-ray diffraction andchemical data Clay Minerals 31 217 ndash224

Mart otildeacuten de Vidales JL Pozo M Medina JA amp LegueyS (1988) Formacion de sepiolita-paligorskita enlitofacies lut otilde tico-carbona ticas en el sector de Borox-


Esquivias (Cuenca de Madrid) Estudios Geologicos44 7 ndash 18

McLennan SM (1989) Rare earth elements in sedi-mentary rocks influence of provenance and sedi-mentary processes Pp 169 ndash200 in Geochemistryand Mineralogy of Rare Earth Elements (BR Lipinamp GA McKay editors) Reviews in Mineralogy 21Mineralogical Society of America Washington DC

Millot G (1964) Geologie des Argiles Masson Parispp 163 ndash209

Nesbitt HW (1979) Mobility and fractionation of rareearth elements during weathering of a granodioriteNature 279 206 ndash210

Paquet H Duplay J Valleron-Blanc MM amp MillotG (1987) Octahedral compositions of individualparticles in smectite-paly gorskite assemblagesProceedings of the International Clay Conference Denver 1985 73 ndash77

Pochitaloff A (1968) Rapport de fin du sondage Tanit 1(Rapport SEREPT unpublished) 15 pp

Sassi S Triat JM Truc G amp Millot G (1984)Decouverte de lrsquoEocene continental en Tunisiecentrale la formation du Djebel Gharbi et sesencrou tements carbonate s Comptes Rendu delrsquoAcademie des Science Paris 299 II 357 ndash 364

Schultz LG (1964) Quantitative interpretation ofmineralogical composition from X-ray and chemicaldata for the Pierre Shale United States GeologicalSurvey Professional Paper 391-C 31 pp

Singer A (1984) Pedogenic palygorskite in the aridenvironm ent Pp 169 ndash177 in Palygorskite-Sepiolite Occurrences Genesis and Uses (ASinger amp E Galan editors) Developments inSedimentology 37 Elsevier Amsterdam

Soussi M Abbes Ch Belayouni H amp Boukadi N(1996) Sedimentologie stratigraphie sequentielle etcaracteristiques geochimiques des series du Triasmoyen et superieur de lrsquoAxe nord-sud (Tunisiecentrale) Proceeding of the 5th Tunisian PetroleumExploration Conference 10 275 ndash 285

Srasra E Jamoussi F Zargouni F amp Ferid M (1995)Etude physico-chimique de la palygorskite carbona-tee de lrsquoEocene continental de Chebket BouloufaRevue du Service Geologique de Tunisie 61109 ndash119

Suarez M Robert M Elsass F amp Mart otildeacuten-Pozas JM(1994) Evidence of a precursor in the neoformationof palygorskite New data by analytical electronmicroscopy Clay Minerals 29 255 ndash264

Swan ARH amp Sandilands M (1995) Introduction toGeological Data Analysis Blackwell ScienceOxford UK pp 328 ndash397

Taylor SR and McLennan SM (1985) The ContinentalCrust its Composition and Evolution BlackwellOxford UK 312 pp

Torres-Ru otilde z J Lopez-Galindo A Gonzalez-Lopez JMamp Delgado A (1994) Geochemistry of Spanishsepiolite-palygorskite deposits genetic considera-tions based on trace elements and isotopesChemical Geology 112 221 ndash245

Truc G (1981) Encroutement calcaire (calcrete) deTunisie Premier Congres National des Sciences dela Terre Tunis pp 102 ndash103

Turner DR amp Whitfield M (1979) Control of seawatercomposition Nature 281 468 ndash469

Velde B (1985) Clay Minerals A Physico-chemicalExplanation of their Occurrences Developments inSedimentology 40 Elsevier Amsterdam

Weaver CE amp Pollard LD (1973) The Chemistry ofClay Minerals Developments in Sedimentology 15Elsevier Amsterdam

Zargouni F (1985) Tectonique de lrsquoAtlas meridional deTunisie Evolution geometrique et cinematique desstructures en zone de cisaillement PhD thesisUniversite Louis Pasteur Paris

Zouari H (1984) Etude structurale du Jebel Chaambi(Tunisie centrale) Relation entre la mineralisationet la structure Thesis 3rd cycle Universite FrancheCompte Besancon France


the first oil drilling carried out in the southern partof the Gulf of Gabes as well as some drilling in thenorthern part of Jeffara found continental Eocenedeposits known as the Tanit Formation (Pochitaloff1968) The mineralogy of some of these deposits hasbeen described by among others Abdeljaoued(1997) Srasra et al (1995) Ben Aboud (1998)Hachi (1998) Ben Aboud et al (1999) Fakhfakh(1999) and Jamoussi (2001)

In this paper we study the mineralogy geochem-istry and microtexture of palygorskite-rich Eocene

samples in Tunisia The better sequences arelocated in some of the above-mentioned areas(Chebket Bouloufa J Hamri J Lessouda and JRheouis) Some samples from J Boudinar were alsostudied

G E O L O G I C A L S E T T I N G

The end of the Cretaceous is marked in central andsouthern Tunisia by important palaeogeographicaland sedimentological modifications Marine sedi-

50

N

Emerged area

Phosphate deposits

Tidal zone (Sebkha)

Supratidal to tidal zone

Nummulitic limestone

Marine deposits

Sections investigated

0

Mahdia

KerkennahIsland

Sfax

Medenine

Gafsa

Gabegraves

Jerba

Tunis

ALG

ERIA

LIBYA

100 km

J Bouloufa J Hamri

J Rheacuteouis

J Lessouda

J Boudinar

FIG 1 Lithofacies map of the Lower Eocene in Tunisia (modified from Jamoussi et al 2001) and the locationsof the sections studied
















R E S U L T S

Mineralogy
















Geochemistry





Calcite + Dolomite


J Bouloufa

J Hamri

J Rheouis

J Lessouda

Illite + Kaolinite


J Boudinar

50

50

50

5050









Micromorphology








D I S C U S S I O N




















(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES












































































R E S U L T S

Mineralogy
















Geochemistry





Calcite + Dolomite


J Bouloufa

J Hamri

J Rheouis

J Lessouda

Illite + Kaolinite


J Boudinar

50

50

50

5050









Micromorphology








D I S C U S S I O N




















(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES





































































Geochemistry





Calcite + Dolomite


J Bouloufa

J Hamri

J Rheouis

J Lessouda

Illite + Kaolinite


J Boudinar

50

50

50

5050









Micromorphology








D I S C U S S I O N




















(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES




































































Micromorphology








D I S C U S S I O N




















(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES

































































D I S C U S S I O N




















(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES












































































(Si771Al029)O20(Mg181Al155Fe3+052

Mn002Ti003)(OH)2 (K013Ca007)(OH2)44H2O




ACKNOWLEDGMENTS


REFERENCES






























































palygorskite genesis through silicate transformation in ...hera.ugr.es/doi/15079399.pdfsouth axis,...

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