DEDOLOMITES ASSOCIATED WITH KARSTIFICATION. AN EXAMPLE OF EARLY DEDOLOMITIZATIONIN LACUSTRINE SEQUENCES FROM THE TERTIARY MADRID BASIN, CENTRAL SPAIN

J.C. Canaveras', S. Sanchez-Moral', J.P. Calvo', M. Hoyos! and S. Ord6fiez3

IDepartamento de Geologia. Museo Nacional de Ciencias Naturales (CSIC).Jose Gutierrez Abascal, 2. 28006Madrid(Spain).

2Departamento de Petrologia y Geoquimica. Facultad de Ciencias Geologicas. Universidad Complutensede Madrid. Ciudad Universitaria sin.28040Madrid (Spain).

3Departamento de Ciencias de la Tierra y Medio Ambiente. Facultad de Ciencias. Universidad de Alicante. Campus SanVicente del Raspeig. 03690 Alicante (Spain).

ABSTRACT: Avarietyof calcitefabricsformed by dedolomitization ofmiddletoupperMiocenecontinentalsedimentsof theMadridbasin,central Spain. The dedolomites are associatedwith other carbonates that show abundant diagenetic features. The diagenetic carbonates(Diagenetic Carbonate Zone, OCZ) overlies and grades laterally into lacustrine dolomite (Lower Dolomite Unit, lDU) and evaporites(EvaporiticUnit, EU), and in turn is capped by a paleokarstsurface.

The main dedolomitefabricsincludesuturedcalcitesand radial-fibrous calcites, thelatterconsisting ofpseudospherulitemosaicsandfibrouscrusts. Other subordinatededolomitefabricsconsistof micro to mesocrystalline mosaics of rhombic,occasionallyzoned calcites as well asreworked pseudospherulite crystals. These fabrics are interpreted to have been formed from extensive calcitization of dolostones andassociatedevaporitefacies, the process being triggeredandfosteredby inputof meteoric waters throughout a karstic system.The originofthe radial- fibrous calcites is discussed more specifically in view of their similarities to other calcite fabrics (e.g. Microcodium) oftendescribed in meteoric diageneticenvironments.

Geochemical evidence indicates that dedolomitization occurred in the shallow subsurface« 40mdepth) and was achievedby oxidizingmeteoric-derived groundwaters.The diagenetic systembehavedas anopensystemfornearlyall traceelementsanalyzed;but in theshallowerzone (vadosezone) thesystemispartiallycloserespecttostrontium.Verticaltrendsof the stableisotopes alsosupporttheproposedpaleokarstmodel characterizedby an irregularshallow water table and a narrow vadose zone.

The resultsobtainedfrom the geochemicalsimulationsupportthatdedolomitization coulddevelopby twocombinedhydrographicalpatterns:authigenicrechargethroughlimestonesand allogenic recharge.The dissolution/precipitation rates calculatedfor the main minerals involvedin the process are consistentwith petrographic- and geochemicalevidence.

INTRODUCTION

The productsof dedolomitization, the concurrent dissolutionof dolomite and precipitation of calcite, has been found in awide range of diagenetic conditions. There are well­documented reports aboutproducts ofdedolomitization underburial-diagenetic conditions (Budaiet al. 1984; Sellwood etal. 1987), related to contact metamorphism (Wood &Armstrong 1975), in the transition zone between two marineenvironments (Margaritz & Kafri 1981), and related to hot,ascending calcium-rich brines(Land& Prezbindowski 1981).Nevertheless, mostofthereporteddedolomiteoccurrences areindicative of near-surface conditions: (1) associated withsubaerial-exposure surfaces in stratigraphic successions(Schmidt 1965; Braun & Friedman 1970; Swennen et al.1982; Holailetal. 1988; Kenny 1992; Friedman 1994); or (2)related to post-burial surface weathering (Mossler 1971;Chafetz 1972). Because of its palaeoenvironmentalsignificance. the identification and correct interpretation ofdedolomites are important for unraveling the depositional­and diagenetic historyof a region.

Thermodynamic-, mass-balance-, and kinetics aspects ofdedolomitization have been investigated in detailby several

Carbonates and Evaporites, v. 11,no. I, 1996,p. 85-103

procedures: (1) experimental work eitherat low temperature(De Groot 1967) or higher temperatures (Kastner 1982;Brauer & Baker 1984), and (2) geochemical studies incarbonate aquifers (e.g.,Hanshaw & Back 1979; Back et al,1983; Plummeret al.1990; Saunders & Toran 1994). In theseworks, the dedolomitization mechanism most commonlysuggested is incongruent dissolution of dolomite in presenceof calcium-sulfate-rich solutions, i.e. from dissolution ofevaporite minerals, producing calcite. According to Kastner(1982), Back et al.(1983) and Brauer & Baker (1984), thedissolution of calcium sulfateexertsa doubleinfluence on thededolomitization process: (1) to providean abundantsupplyof calcium ion in order to maintain the reactionby meansofion-common effect, (2) the sulfate ion acts as a partialinhibitorof theprocess. Therelationship betweendedolomitesand evaporites has been published in many regionalpapers(Lucia 1961; Podemski 1973; Warrack 1974; Budai et al.1984; Deike 1990; amongstothers), and even, someauthors,for instanceBack et al, (1983) state that dedolomitization inlow- temperature environments occurs only in presence ofevaporites or some other source of additional calcium ions.However, dedolomite occurrences without evidence ofevaporites havebeenalso reported(Chafetz 1972; Theriault &Hutcheon 1987).

DEOOLOMIlES ASSOCIAlED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES, SPAIN

The closerelationship betweendedolomitization and karsticprocesses has been pointed out in subaerially exposedsequences (e.g. Ng & Jones 1989; Lee & Harwood 1989;Kenny 1992; Friedman 1994) and in modern carbonateaquifers (e.g. Back et al. 1983; Saunders & Toran 1994;Bischoffet al. 1994). An example of the close relationshipbetween dedolomitization and karstic processes and productscan be clearly recognized in the Miocene sedimentary recordof the Madrid basin, central Spain. Dedolomites arewidespread atthe top of theMiocene IntermediateUnit,wherethey are associated with an intra-upper Miocene karstdeveloped on lacustrine sequences. Thepurpose of thispaperis to provide a well-eonstrained example of early, shallow­burial, evaporite-related- and karst-related dedolomitizationthat may serve as a viable analog for the interpretation ofsimilardedolomites in rock record.

GEOLOGICAL SETTING

The Tertiary Madrid basin is located in the centre of theIberianPeninsula (south-western Europe) andhas beenfilledby a succession of terrestrial deposits whosethickness locallyexceeds 3500 metres (Fig. 1). The Madrid basin is anintracratonic basin which resulted from a rather complexdeformational history due to differential- and diachronictectonic movements of theirbasin margins duringthe AlpineOrogeny (Vegas &.Banda1982). The northern basinmargin,

the Central System, is composed by Paleozoic plutonic- andmetamorphic rocks, and forms a major mountain upliftinterpreted to have been generated by basement thrusting(Warburton & Alvarez 1989). In the northeast, the IberianRange, composed of NW-SE folded Mesozoic formations,mainly carbonates, forms the basin margin. The AltomiraRange, whichlimitsthebasin to theeast, is composed mainlyof carbonate- and evaporite facies thatare considered to haveresulted from thrusting directed from E to W. The tectonicbehaviour of the Toledo Mountains, that form the southernbasinmargin, is lessknown. The ToledoMountains seemtohave been fairly active during Paleogene times and becamestableduringthe Neogene (Calvo et al. 1990).

TheNeogene sedimentary fillof thebasinisdivided intothreemainMiocene units(Lower, Intermediate andUpper) andtwoPliocene sedimentary cycles(Calvoet al. 1990). These unitsare mainly composed of lacustrine- and alluvial deposits andare separated by majorsedimentary disconcontinuities in theformof paleokarst- and/orerosional surfaces (Fig.2).

TheMiocene LowerUnit,a600to 1()()() m thicksuccession ofAgenian to Middle Aragonian (lower to middle Miocene)deposits, consists of a concentric facies arrangementcharacteristic ofanendorheic closedbasin.This unitis formedby central evaporitic saline facies, mainly anhydrite, halite,glauberite, and thenardite (Ordonez et al. 1991), which pass

CJ Quaternary deposits

~ Miocene, Upper Unit

~ Miocene, Intermediate Unit

~ Miocene, Lower Unit

E2J Lower to Upper Mioceneterrigenous sediments

I!IIIIIIlJ Paleogene (terrigenous andcarbonates)

[ill Mesozoic (mainly carbonates)

Ed Paleozoic (plutonic- and0 10 20 30 40 50 km metamorphic rocks)--

Figure 1. Geological mapof theMadrid Basin. Insetoutline mapof Spain shows regional location.

86

CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS AND OROONEZ

laterally into saline mudflat- and progressively coarseralluvialsediments (Calvoet al. 1990).

The MioceneIntermediate Unit is formed by a succession ofterrigenous and/or chemical deposits, 50-200 m thick,ranging in age from Early Aragonian to Late Vallesian(middle to upper Miocene). This unit shows a concentricsedimentary pattern with central lacustrine facies consistingexclusively of carbonate and gypsum, whichgrade laterallyinto progressively coarser alluvial facies towards the basinmargins. On the basis of sequential changes in verticalsections, marked by progradation of alluvial systems onunderlying centralchemicalfacies, twosubunits separatedbyan intra-Aragonian third-order discontinuity can berecognized (Fig. 2).The uppermost levelsof theIntermediateunit consist of lacustrine deposits that were related to anepisodeof generalised lake expansion in the region(Calvoetal. 1989). Theselacustrine beds, mainlyformed of carbonate,show a large variety of diagenetic features that resultedfromtransformation of dolostone, limestone, and gypsum. Theresulting carbonates will be referred as "diageneticcarbonates" farther along in this paper. An intra-Vallesianpaleokarstoutlinestheunconformity between theIntermediateand the Upper Miocene Units (Fig. 2). This paleokarst is ashallow, tabular-shaped karst formed early after thedeposition of the Intermediate unit and records a complex

The distribution of the diagenetic carbonates along theMiocene Intermediate Unit, and the morphologicalrelationships betweenthedifferent diagenetic carbonate typesand the paleokarstic features were first established by fieldwork. Sampling was focused on determining textural- andcompositional characteristics of the diagenetic carbonates,particulary dedolomites, their relationships with otherdiagenetic carbonate textures, and their significance in apaleokarstic context

METHODS

The Miocene Upper Unit spans Late Vallesian to LateTurolian and reaches a thickness of 50 to 60 m. The unitconsists of terrigenous fluvial deposits thatgradeupwardintoshallow fluvio-lacustrine carbonates. Thesedepositsfossilizeand/or erode the intra-Vallesian paleokarstic surface andshow an exorheic facies arrangement, which is clearlydifferent of that observedin the earlier Mioceneunits. Thissuggests thatduringtheVallesian, a majorpaleogeographicalchangein the area took place (Calvoet al. 1990;Canaveras1994).

Petrographic conclusions are based on the examination ofmore than three hundred stained- and nonstained thinsectionsbymeansofconventionalpolarized-lightmicroscopy.Alizarin red S stain was used according to the method ofLindholm & Finkelman (1972). Etched and nonetchedspecimens of rock fragments and polishedthin sections werestudiedby SEM To avoidtheformation of artifacts causedbythe polishing process (Folk 1993), the characterization ofpossiblebiogenic microstructures wasbasedonfreshlybrokensurfaces.

history of dissolution-, collapse-, and infilling processesdeveloped during a subaerial-exposure interval, which isinterpreted as a third-order regionalunconformity accordingtothehierarchy proposedbyEsteban(1991) (Canaveras 1994;Canaveras et al. 1996).

X-raydiffraction was usedto determine mineralcompositionin powdered carbonate samples and insoluble residues, usingquartz as an internal standard. Mole percent magnesiumcarbonate in calcite was estimated by measuring of the ~04

spacing (Goldsmith & Graf 1958). Major-and trace-elementcontents (Ca, Mg, Sr, Fe, Mn and Na) from whole-rocksamples were determined by X-ray fluorescence usingstandard techniques (Franzini et al. 1972; 1976) and byenergy-dispersive spectrometry (EDS) in samples observedunder SEM Oxygen- and carbon isotope values weredetermined usingstandard procedures andresultsarereportedin the delta-notation relative to the PDB standard.

50-200

600-1000

Thickness(m)

0-30

intra-Vallesianpaleokarstic 0-50

surface

Lithostratigraphic log

Pliocene

Sedimentaryunits

c.!!!"0"- Upper .. '.::sI-

'!!!c:-III11I.-

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c: 1\w IIIz '2: /\ 1\w Ql0 CI /\0 « /\(9 II::i II0

-- Paleokarst 8 Carbonates

I;,;':1 Detrital deposits 1/\ "I EvaporitesFigure 2.Stratigraphy of theNeogene sequence intheMadridBasin with indication of themain Miocene units.

Geochemical calculations to achieve a hydrogeochemicalmodeling that could explain the mechanisms ofdedolomitization has been obtained by meansofPHRQPIlZcode (version 0.2, 1990), which is a review of the original

87

DEDOLOMITES ASSOCIATED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES, SPAIN

version from Plummer et al. (1988). This code has beenadapted from the geochemical-simulation computer codePHREEQE of Parkhurstet al. (1980) in which the aquaeousmodelofPHREEQEhas beenreplaced withthePitzervirial­coefficients approach (Pitzer 1973). Both codes have beensuccesfully applied to interpret geochemical processes in avariety of well-characterized natural systems, in particular tointerpret dedolomitization processes in modem aquifers(Sanchez-Moral et al. 1993; Bischofet al. 1994; Saunders andToran 1994). PHRQPITZ possess a highreaction-modelingcapability, including calculation of aquaeous speciation andmineral-saturation index, irreversible reactions, and mineral­water mass transfer, and reaction paths. We have chosen amixed direct-inverse modeling method (plummer 1984) tocarryout thehydrogeochemical simulation: on thebasisof aninitial solution composition, the chemical evolutionary trendof thesolution isevaluated following a determined pathway orreaction path, which is conditioned by the whole set ofpetrographical-, geochemical- andgeomorphological data. Inthis way, we can predict the evolution of water compositionand establish the mineral-water mass-transfer ratios.

STRATIGRAPIllC DISTRIBUTION OFDEDOLOMITES

The dedolomites studiedin this work form part of a zoneofdiagenetic carbonates thatextends downwards to a maximum

ofabout20 to30meters below theintra-Vallesian paleokarsticsurface locatedat thetopof theMiocene Intermediate unitandextends across the east-central part of the basin. Thediagenetic carbonates embrace a greatvariety of texture- andfabric types; the dedolomites are most abundant Thethickness of thededolomite deposit rangesfrom2 to30metersthough its average thickness is about8 to 9 m.Thediageneticcarbonates occuras:(1)well-defined tabular- towavy-shapedbeds with irregular bases and -tops; their thickness average0.2 to 0.9 meters; (2) nodular bodies within marlstone- andclaystone beds; and (3) tabular- to irregular-shaped bodiesconsisting of coarsechaotic clast-supported breccias (Fig.3).The well-defined beds constituted by diagenetic carbonatesare usually located in a lowerposition respectto the brecciabodies. All types of dedolomite occurrences are associatedwith otherpaleokarstic features such as calcitic speleothems(paleoflowstones), terrigenous endokarstic infillings, andsinkholes (Cai'iaveras et al. 1996).

Theclast-supportedbreccias arequiteabundantand typical inthe studyarea. The thickness of the breccias are 5 to 10 m.Breccias bodies are laterally continuous; they can be tracedseveral kilometres laterally. The fragments within thebreccias are angular in shape, their sizes range fromcentimetres to metres; and they do not show featuresindicative of high relative inter-clast displacement Thematrix consists of a mixture ofclays,marls, andcalcitic sands

Figure 3. View ofthetopoftheMiocene Intermediate Unit atIriepal, 501an northeast of thecityofMadrid (Fig.I). Dedolomitesoccur in a diagenetic carbonate zone (DCZ) located between dolomite (Lower Dolomitic Unit, LDU) andlimestone deposits(Upper Limestone Unit, ULU). DCZ is comprised of a well-stratified lower member anda brecciated upper member.

88

CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS ANDOROONEZ

formed by dissolution- and disaggregation of diageneticcarbonates. According to Morrow (1982), thesebreccias maybe classified as mosaic packbreccia; they have beeninterpreted as evaporite dissolution-collapse brecciarelatedtoshallow karstic processes (Canaveras 1994). Although plasticdeformation of theprimarybrecciafabric canbe observed insome outcrops, the collapsebrecciaresulted mainly from apost-lithification dissolution-collapse process.

Thezoneformed of diagenetic carbonates grades downwardsinto nonaltered lacustrine carbonates (mainly dolostones) orevaporites (gypsum). In thenorthern- and eastern partsof thebasin, this zone grades upwards into bedded palustrine andshallow lacustrine limestones (Fig.4) whereas in central andsouthernparts of thebasinit isdirectly overlain byterrigenousdeposits of theMiocene UpperUnit In orderto simplify andfor a correct interpretation of further descriptions anddiscussions, wehavedefineda numberoflithological subunitsat the top of theMiocene Intermediate Unit We havetermedthemasDiagenetic Carbonate Zone (DCZ),LowerDolomiticUnit (lDU), Upper Limestone Unit (ULU) and EvaporiteUnit (EU). The spatial distribution of theselithologic unitsissketched in figure4.

In the northern- and centralareasof thebasin,the underlyingnonaltered carbonates (Lower Dolomitic Unit. LOU) consistof interbedded dolomicrites, dolomitic marlstones, micrites,claystones, and gypsum, the thickness of individual bedsreaching up 80 em, The dolomicrites present shrinkagecracks, brecciation, root casts, very rare remains ofcharophytes and ostracods, and abundant evaporites traces,

N

mainly lenticular gypsum, 0.5-10 em in length that occurseither preserved within the dolostone or as calcitepseudomorphs. The contact between the underlyingnonaltered carbonates (LOU) and the diagenetic zone (DCZ)is generally sharpandirregular, but in outcrops of thecentralpart of the basin, where the diagenetic zone is thicker, thecontact is transitional and a mineralogical- and texturaltransition between nonaltered (LOU) and diageneticcarbonates (DCZ)is observed (Fig5).

In the southern area. the underlying evaporites (EvaporiteUnit, ED) consist of decimeter-thick, sheet-like beds ofprimary gypsum with lenticular texture, bioturbationstructures (burrow structures and traces filled by anhedralgypsum cement) andscatteredchertnodules. Thecontactwiththe diagenetic carbonate zone (DCZ) is also sharp andirregular. In this area, the dedolomites are less abundant andthe breccia fragments are mainly composed of calcitizedevaporites and calcitespeleothems.

In thenorthern- and eastern partsof the basin,the limestoneslocated over the diagenetic carbonates (Upper LimestoneUnit. ULU) consist of mudstones and peloidal-intraclasticwackestones thatoccuras tabular beds,40 em to 1.2m thick,with abundant pedogenic features (root traces, bioturbation,desiccation cracks), lacustrine biota (charophytes, ostracodsand gastropods) and sparse gypsum lenticular molds (orcalcite pseudomorphs). These facies correspond to shallow­marginal lakes formed while water level fluctuated slightlyand thuscausedoccasional subaerial exposure (Alonso-Zarzaet al. 1992; Canaveras 1994). The contact between the

S

~I ;l::;:;:;!:::!;:;!;::!;:;!20 km

m Upper Limestone Unit

Ei53 Diagenetic Carbonate Zone

~ Lower Dolomitic Unit

E2S] Evaporite Unit

~ Limestones

D Clays, marls

~ Dolostones

~ Dolomitic marls

/\/\/\/\/\/\/\/\/\/\/\/\/\/\

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

/\/\/\/\/\/\/\1'1'\/\/\/\/\/\

/\ /\

Figure 4. Sketch of thespatial distribution ofthemain petrological subunits recognized at thetopof theMiocene IntermediateUnit. The logdrawn in therightside of thefigure shows thecommon stratigraphic arrangement of thesubunits in thecentralpart of theMadrid Basin.

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DEOOLOMIlES ASSOCIAlED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES,SPAIN

diagenetic carbonates (DCZ) and the overlying limestones(ULU) is usually transitional. Commonly, the lowermostlevels of the limestone facies have been affected bybrecciation; they show a gradual transition downwards intothe diagenetic carbonate breccia.

rhombic and subrhombic calcites, are muchlessabundantbuthavebeenfoundtobewidely distributed in thesequences. Allthese diagenetic fabrics show a close relationship in theirspatial distribution, so thatcommonly various textures canberecognized within the same bed.

PETROGRAPHY Sutured Calcites

Petrographically, we can differentiate two major texturalgroups of dedolomites, sutured calcites and radial-fibrouscalcites, respectively. In addition to these main dedolomitetypes, other fabrics as a result of dedolomitization, such as

Radial-fibrous Calcites

Description»- Sutured calcites are composed of xenotopicmosaics of calcite (LMC) crystals with mostly serrated- orirregular intercrystalline boundaries, givinga typicalsuturedappearance. Thecrystalsizesrangefrom 50 JJ.IIl to more than4 mm in length. In somesamples, bimodalsize distributionscan be observed (Fig.6a). The suturedcrystals are inclusionrich(mainlycomposed ofFe-oxides). Thesecrystals generallyexhibit straight extinction and show a pronouncedpseudopleocroism, which is characteristic of length-slowelongation, according tothecriteriaofFolk& Assereto (1976)and Dickson (1993). Some ghosts of millimetric- tocentimetric gypsumlenticular crystals andcalcitevoid-fillingcements are disseminated throughout the mosaics (Fig.6b).

Interpretation> Thelackof rhombic shapes,rhombiczoningoutlined by inclusions, or dolomite relicts in the dedolomitecrystals make them different from other typical fabricsresulting from dedolomitization aftera dolosparitic precursoras thosedescribed by Evamy(1967) and Budaiet al. (1984).There are several reasons to invoke a nonfabric-selectivededolomitization after a dolomicritic precursor, instead of afabric- or nonfabric-selective dedolomitization processafteradolosparitic precursor: (1) all the dolomites reported in theMadrid basin are crypto-microcrystalline in size and havebeeninterpretedas primaryor earlydiagenetic (Bellancaet al1992; Canaveras 1994), thereis noevidenceofdolosparites inthe Miocene sedimentary recordof the basin; (2) at outcropscale,thecontactbetweendedolomite (DCZ) anddolomicriteswithevaporite traces(LDU) canbefrequently observed, aswehaveindicated previously; and (3)anotherlineof argumentisthat dolosparite precursors could never be formed in thegeological contextof thebasin,wheretheMiocenesedimentsunderwent neithersignificant burialnorelevatedtemperature,both conditions having been suggested as required for thegeneration of meso-macrocrystalline xenotopic dolomites(Gregg & Sibley 1984).

Description»-Radial-fiboruscalcitesareformedofmosaicsofinterlocked low-magnesian calcite (> 3% mol MgC0Jcrystals displaying an internal fibrous microfabric ormicrostructure. According to the arrangement of the internalfibers, two end-member subgroups can be distinguished: (1)pseudospheru1itic fibrous calcites, in which the fibersradiatefrom the centres of the single subspherical crystals(pseudospherulitic fabric) (Fig. 6c); and (2) fibrous crusts,wherethe crystalsare elongated and the fibers divergefromone of thecrystaledges(Fig.6d).

-\\ \t-

A greatvariety of diagenetic fabrics is foundin theDiageneticCarbonateZone(DCZ). Mostofthemhavebeeninterpretedasbeing replacive after a dolomicritic precursor, althoughneomorphic- and replacive-after-gypsum fabrics areoccasionally present These last fabrics consist of: (1)xenotopic mesocrystalline mosaics with abundantlenticularpseudomorphsandbioturbationstructures; (2)microcrystallinemosaics with relicts of gypsum; and (3), micro­mesocrystalline mosaics with clotted or grumelose texture,which grades into lenticulargypsum or to other calcitizationfabrics at outcrop scale. They are interpreted as havingresulted from calcitization of primary lenticular gypsum atshallow burial depths associated with carbonate-richgroundwaters of meteoric origin. The formation of thesefabrics from destruction of gypsum by anaerobic bacteria(Friedman 1972; Friedmanet al. 1992) is unlikely in our casebecauseof the scarcityof organicmatterin the sediments, aswellastheabsence ofby-products, e.g.nativesulphur, thataretypically associate<! with that process.

iii I iii i i U I/----r,100 50 0 50 47 2 0

% DOLOMITE % mol MgC03

Figure 5. Vertical trends of the mineralogical compositionand % molMgCOJ ofthe carbonate phases in the transitionbetween the Lower Dolomitic Unit and the DiageneticCarbonate ZLJne.

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CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS ANDOROONEZ

The pseudospherulitic fibrous calcites consist of xenotopicmosaics of calcite crystals whose shapes are generallyspheroidal to elliptical, exhibiting irregularfaces. The sizes ofthecrystals rangein diameter from30JUIl to morethan2 mm,although this difference in size is not observed in a givensample. The mosaics containintercrystalline matrixwhichis

mainly formed of fibrous clays (sepiolite and palygorskite)with minoramountof microspar, Fe-oxides and terrigenousparticles. Sepiolite and/orpalygorskite are presentas densematsof interwoven, up to 20 um longfiberswithelliptical tocircularcrosssections, ranging indiameter from0.1 to 0.2J.111l.These fibrous minerals are also included in the internal

Figure 6. Thin-sections photomicrographs showing sutured- and radial-fibrous calcites. All photomicrographs were takenunder plane-polarized light. except (E) which was taken under crossed nicols. (A). Sutured calcite with a bimodal sizedistribution. The turbid appearance of themosaic is due to theabundance of inclusions. Scale bar is 500 um (B). Detail of aghost of void-fllling cement in theinterior of thesutured mosaic. Ghost is clearly outlined 1Yy limpid areas. Scale baris250um(C). Xenotopic mosaic compound of pseudaspherulitic crystals. Scale baris 500 J.Un (D). Detail of fibrous crust formed byinterlocking conical bundles offibrous crystals. Scale baris 250 J.Un (E). Detail of a comet-like radial fibrous crystal in thetransition zone between pseudospherulitic mosaics andfibrous crusts. Scale bar is 500 J.Un (F). General viewoffibrouscrustgrowing on radial fibrous mosaic. Scale bar is 500 J.Un.

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DEDOLOMITES ASSOCIATED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES, SPAIN

structure of thepseudospherulites andcontribute tooutlinetheradial fibrous patternof the crystals.

As mentioned previously, each pseudospherulite crystalexhibits an internal radiating pattern of fibers which is themain feature to differentiate these mosaics from the rest ofdiagenetic fabrics. The radiating pattern is displayed by thefibers extending centrifugally from the centre of the crystaltowards its borders (Fig. 6e). Inclusions formed by fibrousclays and oxides display an anastomosing- or dendriticradiating pattern which is more prominent under SEM (Fig.7a).Despiteof theapparentradialfibrous fabric, thesecrystalsarenot true spherulites. Actually theyare singlecrystalswithuniform extinction and the fibers are not crystallographicallyoriented; this is the reason we have termed this feature aspseudospherulitic fabric. SEM observations show that thepseudospherulitic crystals develop as rhomboedral calcite"first-orderfibers" (5-2Opm wide)alignedin an overlapping,radial-layered pattern with the c-axis perpendicular to thecentreof the crystaland to the growthlayering (Fig.7b). Theinternal structureof thesefirst-order fibers isformed of 0.1-0.2J.IIll in diameter "second-order fibers" with circular crosssections (Fig. 7c). These fibers are arranged in mesh-likeagglomerates (Fig. 7c) or have their long axes oriented

parallel to the longaxesof thefirst-order fibers (Fig.7d).The"second-order fibers" have been recognized in etched andnon-etched freshly broken, polished- and powdered samples,so they are not a surficial coatingor an artifactof etching orpolishing. EDS analyses avoidconfusion of thesefiberswiththe sepiolite/palygorskite crystals that form inclusionlineations and intercrystalline matrix.

The fibrous crustsconsistof interlocking conicalbundlesoffibrous calcitecrystals (Fig.6d). The bundlesare millimetric­to centimetric in thickness and showbrush extinction. Theseaggregates resemble tocone-in-cone calcitessofarreportedinthe rock record (e.g.,Woodland 1964; Tamey & Schreiber1976; Aso et al. 1992). In well-defmed massive beds, thefibrous crusts are locatedin the outer parts of the beds, veryrarely inside them; the cones are always open towards theexternal part of the beds. The fibers arearranged in feather­like (fibrous-radial) and arborescent textures, according toAso's (1991) classification (see also Aso et al. 1992, page139).

As in the sutured calcites, the radial-fibrous calcites showusually a pronounced pseudopleocroism and numerousinclusions, thatgivethema c1oudy- or turbidappearance. The

Figure 7. Scanning-electron micrographs of radial-fibrous calcites. (A). General view of a pseudospherulite. (B). Detail of aradial fibrous crystal showing its arrangement in first-order fibers. (e). Detail of second-order fibers forming meshlyagglomerates. On thebasis of the sizes- andshapes of these fibers, they are interpreted as bacterial remains or bacteria/­induced products. (D). Second-order fibers crystallographicaly oriented in theinterior offirst-orderfibers.

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CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS AND OROONEZ

pseudospherulite crystals exhibit straight-, rarely unduloseextinction. On the contrary, the fibrous crusts generallypossess undulose extinction, giving them a felty appearanceundercrossednicols.

A complete gradation both in shape and size between thepseudospherulitic fibrous calcites and the fibrous crusts isobserved (Fig.6t).Pseudospherulitic habitsgrade outwards toV-shaped (arborescent) crystals and finally into conicalbundles. In the same way, the intercrystalline matrixis moreabundanttowardsthe outer parts of the beds. The displacivecharacteristics of thecrystalsarespecially evidentin thiscase,becauseof the fibrous crustspush awayand deform the claysand marls which are interbedded between the massivededolomite beds. Palisades with beef texture, that is,composed of parallel fibrous displacive calcite, have beenlocallyrecognized withinthe pseudospherule beds.

Interpretation>- In view of the intimate geometricrelationships between the radial-fibrous and the suturedcalcites, we consider that the radial fibrous calcites alsoformedfrom dedolomitization after a dolomicritic precursor.Differencesbetweenthededolomite fabrics couldberelatedtovariation in the original composition of the bulk sediments,likelyclay content, and/or the influence of organisms in theirgrowth.

Similar pseudospherulites have been interpreted asneomorphism ofMicrocodium orMicrocodium-like bodiesina one-centimetre-thick horizon within the dolomiteCretaceous EdwardsFormation of centralTexas (Chafetz &Butler 1980). These authorsalso indicate thatbacteriawas afactor in the origin of the pseudospherulites. However,remarkable textural- and morphological differences betweenMicrocodium fabrics and pseudospherulites are found(Klappa 1978; Esteban 1982;Rossi 1993). Thus, somecrosssections of Microcodium; in particular type I of Bordegat(1974), give a false spherulitic appearance. Whichever thetype of Microcodium; fibers are considerably thicker thanthose forming the pseudospherulites. Despite differences ininterpretation, the pseudosphemlites described by Chafetz &Butler (1980) show close similarities to pseudospherulitesfromtheMadridbasin. An exampleofexceptional abundanceofpseudospherultic calciteshasbeenreportedbyRossi(1993)in peritidal sequences of the Upper Paleocene of the SouthPyrenean basin (Spain). As in our example, thepseudospherulitic calcites are related to stratiform zones ofdissolution collapse breccias and diagenetic carbonatesformedduringearlykarstification stages. Thisfabrichasbeeninterpreted to result from dedolomitization under theinfluenceof bacterial activity(Rossi& Canaveras in prep).

Both sizes- and shapes of second-order fibers in thepseudospherulites are similar to bacterial remains orbacterial-induced products described by several authors(e.g.,Folk 1993;Westall& Rince 1994). Theyarealmostidentical

93

to the M micro-rod agglomerates described by Verrechia &Verrechia (1995). Theseauthors indicatethat the originofMmicro-rods is ambiguous, they couldbe calcified bacteriaorcalcitemicronuclei in a gel.Theradial-fibrous calcitefromtheMadridbasincouldrepresent larger-andmoreevolvedshapesof M micro-rods. As in some examples described by Folk(1993), the second-order fibers couldbe addedto thegrowingmineral and often become aligned according to definedcrystallographic direction.

The patterns- and modes of occurrence of the fibrous clayminerals in both inter- and intracrystalline sites reflect adisplacive character in the radial-fibrous crystal growth.Sepioliteandpalygorskiteareinterpreted tohavebeenformedby alteration of precursor phases (trioctahedral- anddioctahedral smectites, respectively) ina Mg-richenvironmentrelatedto subaerial exposure, inflowof meteroric waters, anddedolomitization processes (Weaver& Beck 1977; Khouryetal. 1983; Calvoet al. 1995). This hypothesis is supported bythe evidence that insoluble residues of the dolomites fromLDU present higher smectite contents than dedolomites(DCZ), which are mainly composed of sepiolite andpalygorskite.

Experimental synthesis of radial-fibrous calcites has beensuccesfully reachedin siliceous gels, becauseof gels are thebest enviroment for diffusion transport that favors thedevelopment of fibrous habits (McCauley & Roy 1974;Dominguez-Bella & GarciaRuiz1987). Thepresenceofclay­rich material mixedwithradial-fibrous dedolomites supportsthis hypothesis. The radiated pattern within single calcitecrystals can be explained by the mechanism of split crystalgrowth (Kendall 1985). According to this model,fibersgrowinopticalcontinuity by precipitation at multiple separatesites(crystallites). The inclusion-defmed fibers retain near­identical lattice orientations. Thereafter, the fibers were notcrystallographically controlled. Kendall (1985) himselfpointsoutthatthespherulites ofChafetz& Butler(1980) grewaccording to this mechanism. Radial fabrics have also beeninterpreted as a product of bacterial degradation either oforganic gels(Aso etal. 1992) orcyanobacterialorganicmatterprecipitates (Friedman et al. 1985).

To conclude, we have found a close relationship betweenradial-fibrous calcites(pseudospherulites and fibrous crusts),clays, and bacterialactivity. Accordingly, we postulate thatthese fabrics formed by dedolomitization after a clay-richdolomicrite precursor, thegrowthof thefibrous fabric havingbeen fostered by bacterial activity. In this pattern, claysprovided an adequate medium, in termsof diffusion transport,for their growth. Taking the Diagenetic Carbonate Zone(DCZ) as a whole, we suggest that pseudospherulitic- andfibrous habitsrepresentanendmemberofbio-inducedcalcitefabrics whereas thesuturedcalcites, characterizedbyanhedralhabits, represent an end member of fabrics resulting frominorganic growth.

DEOOLOMITES ASSOCIATED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES. SPAIN

Figure 8. Thin-sections photomicrographs ofdedolomite fabrics. Bothphotomicrographs were taken under plane-polarizedlight. (A). Rhombic dedolomites.Idiotopic mosaics between macrolenticularpseudomorphs. Scale bar is250Jll11. (B). Rhombicand subrhombic zoned crystals forming a porphyrotopic dedolomite fabric. Some crystals have been broken and partiallyrounded. Scale bar is 500 J1m.

Other Dedolomite Fabrics

We can differentiate some other dedolomite textural typeswhich commonly are found together with those describedabove.

One of these types is observedin diagenetic carbonates withabundant calciticpseudomorphs afterlenticular gypsum (500IJ.Ill to more than 5mm in length). The matrixbetweenlensesis commonly composedof a mosaic of calcite crystals thatdisplay zonedrhombicsections andslightly curvedfaces(Fig.Sa).Thecrystalsizesaverage100to4001J.Ill. As in theradial­fibrous calcites. this fabric showsmarkedpseudopleocroismand contains a fibrous. clays-rich matrix; however. the radial­fibrous microstructure is less pronounced or absent Acomplete transition between the two fabrics is usually seen.Similarrhombiccalciteshavebeendescribed in modern- andancient calcretes (Wright & Peeters 1989) or in secondarycalcretes formed by near-surface dedolomitization ofdolocrete profiles (Khalaf & Abdal1993); but we have notfound other additional evidence to support a pedogeneticorigin for the dedolomites in the Madridbasin.

Xenotopic- andporphyrotopic mosaics composed ofrhombic­and pseudospherulitic calciteswith detrital features (broken­and roundedparticles. syntaxial cements. size sorting....) arerecognized in association with breccia and other karsticsediments such as siliciclastic infIllings. The rhombic- andsubrhombic crystals predominate. their sizes range between200 and 700 IJ.Ill (Fig. 8b). Usually these crystals showzonation as the inter-lenticular rhombs described above.Thisfabric is interpreted as having resulted from dissolution.disaggregation. and further accumulation of rhombic- andpseudospherulitic crystals in a post-lithification(dedolomitization) karstic episode.

In the southern partsof the studyarea, idiotopic-hypidiotopicmicrocrystalline mosaics composed of rhombic andsubrhombic crystals. 20 to 70 IJ.Ill thick, are relatively

94

abundant Someof thesecrystals showwell-defined rhombiccores. This fabricisusually associated withcalcitizedgypsumfabrics. In the northern partsof the area, on a regionalbasis.this fabric is practically absent

MINERALOGY AND GEOCHEMISTRY

Both the diagenetic carbonates (DCZ) and overlyinglimestones (ULU) are composed of low-Mg calcite. Asdeterminedby XRD.themole%MgC03of thecalcitesrangesfrom0 to 3.2. and averages 0.8. No significant difference inmole%MgC03 contents exitsamong the different diageneticfabrics. On thecontrary. theunderlying nonaltered carbonates(LDU) aremainlydolomitic and the mole %MgC03 contentsof the dolomites are quite variable, Where the boundarybetween DCZ and LDU is transitional. a gradient in the%MgC03 contentof thecarbonate minerals is observed (Fig.5).

Mg contents of the samples analyzedby XRF areconsistentwith percentages estimated by XRD. The Mg values rangefrom0.1 to 1.2% in dedolomites (DCZ) and micrites (ULU).and from 8 to 14 % in the underlying dolomites (LDU). Alldedolomite types show a similar range of Mg content,although Mg contents in pseudospherulites aremorevariablethanintheotherdedolomite types.Nacontentsareverylowindedolomite, dolomite as well as in the micrite fabrics, withmean values of 0.5 %, but many samples are under thedetection limit of the methodused (FRX). Sr is the elementthat presents more-variable contents with regard to the otherelements analyzed. Sr contentfromdedolomites rangesfrom25 to 420ppm.Theaverageis 178ppm. Thesevaluesare, ontheaverage, lowerthantheSr contentvaluesfromunderlyingdolomites (LDU) and overlying micrites and microsparites(ULU), which contain 430 ppm and 270 ppm, respectively(Fig. 9). The mean value of Sr in both sutured- and fibrouscalcites are aproximately 100 ppm, lower than the meanvaluesobtainedforrhombic- andmicrocrystallinededolomites,which are 200 and 259 ppm respectively. The Fe and MIl

CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS ANDORDONEZ

contents of all the dedolomite fabrics are very low (meanvalues are 0.2 % and 60 ppm, respectively), lower than thevalues from the underlying dolomites which contain, onaverage, 0.5% and 150 ppm, respectively (Fig. 9). Mncontents insutured- andfibrous dedolomites aremorevariablethan in the other dedolomite types (Fig.9).

Tbe 8180 values of dedolomites average -6.7%0 and 813C

values, -8.20/00, ranging from -6.1 to -7%0 and -7 to -9.2%0,respectively (Fig.6). 8180 and 813C values of suturedcalcitesrange from -6.1 to -70/00 and -7 to -8.50/00, respectively; 8180and 813C values of radial-fibrous calcites range from -6.3 to-6.8%0 and -7.4 to -9.2%0. respectively, although isotopevalues in fibrouscrusts are less variable than in the pseudo­spherulites. The average 8180 values of dolomites from theLowerDolomitic Unit (LDU)are -0.3%0, the average813C is-6.10/00. The isotopic values of carbonates from the UpperLimestone Unit (ULU) range from -6.4 to -6.80/00 8180 andfrom -8.2 to -8.9%0 813C (Fig. 10). Vertical trends of stableisotopes values of dedolomites are shown in figure 11; anincreaseof813C valueswithconcurrentconstant8180 values isobserved. We used theHays& Grossman (1991) equation forcontinental paleotemperatures to calculate paleotemperaturesof calcite cements and paleospeleothems from their 818Q

values. The calculated temperatures rangefrom8 to 12!lC. In

using paleotemperature equations one needs to make someassumptions: e.g., precipitation in equilibrium andpreservation of isotopic signal. We assume thatprecipitationtook place in isotopic equilibrium and take into accountpossible variations of the isotopic signal as a result ofdiagenetic processes. e.g.•recrystallization., which generallylead to a enrichment in 16() content We consider that theobtained range of temperatures represents the minimumprecipitation temperatures for calcite cements andpaleospeleothems.

Interpretation of geochemical data.-- The low incorporationof Mg in the calcite could be related to its removal fromsolution duringthe formation of authigenic magnesium clays(sepiolite or palygorskite) or to the precipitation ofmagnesium sulfate as a byproduct of the dedolomitizationprocess (Evamy 1967; Folkman 1969;Sanchez-Moral et al.1993). Low temperatures also favor minor incorporation ofMg into calcite (Mucci 1987). The Mg content of radial­fibrous calcites is generally higher. even double. that of theotherdedolomite fabrics, whichcouldbe indicative of highergrowth ratesandC0

3=/Ca'JA. ratios(Given& Wilkinson 1985).

ThelowNa contents areindicative of low-salinitiy conditions(Fritz & Katz 1972; Ishikawa & lchikumi 1985), althoughthis interpretation has beenquestioned by someauthorssuch

4000 -

-2000 -

1000 -:800 _

~x............ 600 - x

E - ~ x0- 400 -

~~ lJ.x

0- -lJ.~et'i?~. 'i? • X X

............ 0 X

L.. 200 - lJ... z::,. ~ ~ Xx(f) .lJ.

~ lJ.O x x100- • 0 'i? Upper Limestone Unit

- lJ.o. ~80 _ Diagenetic Carbo Zone60 - lJ. 0

~ 0- ~ • 0 Sutured40 - ~.~ •• • • Radial-fibrous

- ~. • 0 RhombiclJ. Microcrystalline

20 -x Lower Dolomitic Unit

10 I I I I I I I I I I I I

40 80 120 160 200 240

Mn (ppm)Figure 9. Sr-Mn diagram illustrating thegeochemical discrimination between diagenetically altered(DCZandULU) andnon­altered carbonate phases (WU) at thetopof theMiocene Intermediate Unit.

95

DISCUSSION: ORIGIN OF DEDOLOMITES

o

The several types of dedolomites previously described areassociated withotherdiagenetic carbonates that resultedfromtransformation of previous calcium-carbonate and/or sulfatefacies. Although no dolostones remainwithinthe DiageneticCarbonateZone (DCZ),priorexistenceof thislithology canbeinferred by both sttatigraphic- and petrologic criteria. In thesameway,noevaporitesaredirectly foundwithintheDCZ butthe presence of widespread laterally continuous collapsebreccias as wellas theabundance of calcitization fabrics aftergypsum,gypsumpseudomorphs, calcitebox-workandgothic­arccalcites areall features indicativeofpriorinterbedded- andassociated sulfates. As mentioned previously (see Fig. 4), thedistribution of dolostones and evaporites was far moreabundant to the souththan to the north.This different spatialdistribution of the lacustrine faciesat the top of the MioceneIntermediate Unit (Fig. 12a)obviously must have influencedthe diagenetic patterns shown by the carbonates resultingfromthe actionof meteoric surface- and near-surface waters.

paleokarstic surface

DEDOLOMI1ES ASSOCIA1ED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES, SPAIN

as Kitano et al. (1975)and Busenberg & Plummer(1985).The environmentalzonation,soindicatingmore-vadoseconditions.low Fe- and Mn contentsof the dedolomite are indicative of Stable-isotope trends are consistent with these assessmentsoxidizing conditions (Barnaby & Rimstidt 1988). Because (Fig.ll).theywererapidly precipitated outas oxides/hydroxides, FeanMn were not available to be incorporated into the calcitelattice. They occur as inclusions within the dedolomitemosaics, whetherin inter-or intracrystalline sites,and in ouropinionmustbeconsideredbyproducts ofthededolomitizationprocess.

Thestable-isotopevaluesareindicativeofmeteoric diagenesis(Hudson 1977). The verticaltrendof thestableisotopevaluesdisplaying an increaseofBI 3C values withconcurrentconstantB180 values (Fig.7) is indicative of a near-surface watertablewitha narrow vadosezone (Allan & Matthews 1982). Thisisconsistent with the development of a shallow- or low-landpaleokarstic surface(Caiiaveras 1994; Cai'laveras etal. 1996).

The low180 ,13C, Mg,andSr contents of thediagenetic fabricsindicatethat the diagenetic system was open with respect tothese elements. Under oxidizing conditions, Sr is the most­sensitive element for reflecting the degreeof closure of thediagenetic system(Banner& Hanson1990; Woo et al. 1993):the higherSr contentwouldcorrespond to a higherdegreeofclosureof the diagenetic system. The fact that the overlyingmicrites (ULU) showconsiderably higherSrcontents thanthediagenetic fabrics could be related to a meteoric-diagenetic

-8I

8018

-7 -6I I

-5 5o sutured calcites

<> radial-fibrouscalcites

0018 OC13

Figure 11. Vertical trends of SIJc and (jI80 ofsamplesfromtheDiagenetic Carbonate Zone in central part of the studyarea. The abrupt increase in (jl3C values concurrent withmore constant (jl80 values would be indicative of theminimum upper limit of thephreatic environment. The fringeof water-table oscillation has been drawn on the basis ofstratigraphic. petrological and geochemical evidence.

i-6

i-8-10-5

-1--------------water-tableOSCillation

- ---------

-7-9

25

10(/)

f--6 ~a;E

15

f--7

8C13 20

-8

--9

o

DEDOLOMITES/:::,. Sutured calciteso Pseudospheruliteso Fibrous crusts

¢ Upper Limestone Unit

'--10Figure 10. Oxygen and carbon isotopic composition of thedifferent types of dedolomites andrelated materials.

96

CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS ANDOROONEZ

Groundwater circulation

I~

r-- -~-~ I.... . ' .

li;l][ ·.7-' ·~-7'-r- A:z::--v--:

Terrigenous

Upper Limestone Unit

Lower Dolomitic Unit

Evaporite Unit

Diagenetic Carbonate Zone

Authigenic recharge ~

Allogenic recharge ~ t:tDescharge - -) Runoff

Figure 12. Idealised modelfor thedevelopment of the Diagenetic Carbonate Zone andthepaleokarst in theupperpartof theMiocene Intermediate Unit. (A). Paleogeographical sketch of theMiocene Intermediate Unit before dedolomitization andkarstification processes. (B). Hydrogeological model for dedolomitization and karstification of the top of the MioceneIntermediate Unit. (C). Final distribution of the Diagenetic Carbonate Zone.

97

An attempt to model the dedolomitization processesdeveloped in the Miocene sediments of the Madridbasin hasbeencarriedout by usinga geochemical-simulation computercode (pHRQPITZ). The main goal of this attempt is toevaluate the possible dissolution/precipitation pathways bymeteoric water ciIculating through the Miocene sedimentsand whichof themfitbetterthepetrologic features observedinthe sequences.

Thehydrochemical modelrunsonthebasisof thewholesetofstratigraphic-, petrologic- and geochemical data obtainedfromthe top ofthe Miocene IntennediateUnit Itassumesthepaleohydrological pattern obtainedabove(see Fig. 12b).Twodifferent pathways are postulated according to the basicmechanisms of recharge in the Mioceneaquifer(Fig. 13):(1)on theonehand,authigenic recharge(pathwayI inFig. 13)bydirect meteoric precipitation, ground water flow through theuppermost limestones, and furthercirculation through eitherdolostones or gypsum; (2) on the other hand, allogenicrecharge (pathwayII in Fig. 13)throughmarginal terrigenousfacies and further flow through dolostone- and gypsumsequences. The former mechanism of recharge envisagesseveralpossibilities thatdifferon the order in whichgypsumor dolostones aredissolved.

A Pco,of 10-3.5 atm is assumedfor the infilttering watersas astarting point for geochemical calculations. This valuecorresponds to the Pco, of waters that are in atmosphericequilibrium and would be consistent with the oxidizingmeteoric nature of the ground waters that causeddedolomitization in the shallowsubsurface. The other factorincluded in the simulation concerns the temperature of thecirculating waters.A rangeof paleotemperature valuesfrom8to 12DC has been determined from the isotopicdata obtainedincalcitecementsand speleothems of the MiocenepaleokarstWe chose a value of 12DC arbitrarily for the calculations;results by using a value of 8DC did not show significantvariations. BothvaluesforPco, and watertemperature remainfixed throughout the simulation. Finally, the model assumesthat meteroric waters were chemically pure before theirinfiltration.

Mass mineral transfers due to dissolution in each step of thesimulation areexpressed in cubiccentimetres of precipitated­and/or dissolved mineral per liter of infiltrated water. Thedissolution reaction of any mineral is accomplished untilequilibrium of the solution with the given mineral.

Authigenicrecharge throughlimestones(ULU).--Infiltrationalwaters will dissolve calcite thus leading to a calcium­carbonate-rich solution. Then, the solution could circulateeither through dolostone (pathway IA, Fig. 13) or gypsum

DEOOLOMITES ASSOCIATED WITH KARSTIFICATION IN LACUSTRINESEQUENCES,SPAIN

We infer that the dedolomitization process caused by the CaMg(CO~z + Cau ---> 2 Caco3 + MgUaction of meteoric waters was intimately related to thekarstification of the Miocene IntennediateUnit. so the twoprocessescould be consideredevents that were separatedintime. The spatial- and temporal relationships betweendedolomitization and karstification of gypsum and dolomiteshave been observed by several authors (Warren et al. 1990;Rossi 1993;Saunders& Toran 1994). One specificity of thecase presentedhere is that dedolomitization wouldrepresentthe initial stage of karstification, similarto that describedbyAref et al. (1986). According to this latter point (see alsosketchofFig.4), thescarcityof plastic-deformation structuresin the karstic breccia implies that brecciation was mainly apost-dedolomitization process.

The intra-Vallesian paleokarst was developed as a regionallow-landkarstprior to depositionof the Miocene UpperUnitA very detailed study of the morphologic-, petrologic- andsedimentologic features of the intra-Vallesian paleokarst(Caiiaveras 1994;Caiiaveras et al. 1996)indicates that (1)thedrainagepattern was arrangedaccording to a network: ofN-Sto NW-SE flows, with authigenic- and laterally allogenicrecharge zones in the northern parts of the basin and adischarge zone at the south (Fig. 12b);(2) the regionalwatertable was near the surface; the karstic system wascharacterized by a narrow vadose fringe (inferred fromstratigraphic and geochemical data); (3) the karstificationinprint was constrainedby the compositions of the differenthostrocks:limestone, dolostones, andgypsum. Dissolution ofgypsum is thought to be a main factor in conditioning theparticularstyleof the paleokarst

In this context. dedolomitization of carbonates took placeunder near-surface (shallow-subsurface) conditions and wasachieved by oxidizing meteroic-derived ground waters. Asindicated by thegeochemicalinprints of thededolomites, thediagenetic system behaved as an open system for all traceelements analyzed, although it was partially closed in theshallowest vadosezone as indicatedby the Sr behaviorin theprofiles.Verticaltrendsof the stableisotopes also supporttheproposed paleokarst model characterized by an irregularshallowwater table and a narrowvadosezone.

Themost-eommonmechanism suggestedfordedolomitizationis the reactionof dolomitewith Ca-enriched fluids,probablyderived from the dissolution of gypsum, producing calciumcarbonateand magnesium sulfate,which is very solubleandrarely preserved (Evamy 1967; Folkman 1969; Sanchez­Moral et al. 1993). The dissolution of interbedded- andinttasedimentary gypsum contributes to the process ofdedolomitization by providingan abundant supplyof calciumwhich maintains a low Mg/Ca ratio in the solution. Underthese conditions, dolomite is unstable and willdissolvewithconcurrent precipitation of calcitebecauseof thecommon-ioneffect.inordertomaintain equilibrium (Backet al, 1983). Thechemicalreactionof dissolution of dolomiteis as follows:

98

CANAVERAS, SANCHEZ-MORAL, CALVO, HOYOS ANDORD6NEZ

I II~IGENIC RECHA~ ~OGENIC RECH~

I METEORIC WATERS II T(OC): 12 PC0

2: 10.3

.5

1

,LIMESTONE

I I I II I I I I

I I I I

cal. dissolution 0.024 cm3

t[Ca] pH: 8.31 IBI

+GYPSUM

1\ 1\ 1\ 1\ 1\1\ 1\ 1\ 1\ 1\

1\ 1\ 1\ 1\ 1\

gyp. dissolution 1.050 cm3 ,

cal. precipitation 0.015 cm3

DOLOSTONE/ t [Cal pH: 7.82 / / /

/ / / / // / / /

IA

DOLOSTONE

dol. dissolution 0.024 ern"

cal. precipitation 0.021 ern"dol. dissolution 0.026 ern"

t [Cal pH: 8.40

DOLOSTONE

t [Cal pH: 8.40

,Ir

DOLOSTONE/GYPSUM/ / /

1\ 1\ 1\ 1\ 1\

/ / /

dedolomitization

dol. dissolution 0.406 em"cal. precipitation 0.460 ern"

.[Ca] tt[Mg] pH: 7.95

gypsum subsaturation

+DOLOSTONE/GYPSUM

/ / /1\ 1\ 1\ 1\ 1\

/ / /

,DOLOSTONE/GYPSUM

/ / /1\ 1\ 1\ 1\ 1\

/ / /

gyp. dissolution 1.629 cm3

gyp. dissolution 0.579 cm3

gyp. dissolution 1.629 cm3

dedolomitization dedolomitization dedolomitization

dol. dissolution 0.594 cm3

dol. dissolution 0.212 cm3 dol. dissolution 0.591 cm

3

cal. precipitation 0.697 cm3 cal. precipitation 0.243 cm

3cal. precipitation

30.694 cm

t[Ca] tt[Mg] pH: 7.89 t[Ca] tt[Mg] pH: 7.89 t[Ca] tt[Mg] pH: 7.89

TOTAL GYP. DISSOLVED:

TOTAL DOL. DISSOLVED:

TOTAL CAL. PRECIPITATED:

1.629 em3

0.618 em3

0.718 em3

TOTAL GYP. DISSOLVED:

TOTAL DOL. DISSOLVED:

TOTAL CAL. PRECIPITATED:

1.629 em3

0.617 ern3

0.694 ern3

Figure 13. Flow diagram usedfor the geochemical simulation of dedolomitization in the upper part of the MioceneIntermediate Unit or theMadrid basin. See text for explanation.

99

DEDOLOMIlES ASSOCIAlED WITH KARSTIFICATION IN LACUSTRINE SEQUENCES, SPAIN

of the initially dissolvedcalcite is precipitated; in this way,dedolomitization according to the general dedolomitizationreaction is not effective. Nevertheless, further dissolution ofdolostone/gypsum depositsby thecalcium-rich waterswoulddrive the reaction clearly to the right, resulting in theprecipitation of large amount of calciteafter dolomite (2.05moles of calcite per mol of dissolved dolomite).Dedolomitization ends either when gypsum stops beingdissolved or the dolomite has been completely replaced bycalcite. In this pattern, the volume of precipitated calcite is17.4% largerthanthe volumeof dissolved dolomite, which isconsistent withthe widelyobserved features of displacement,e.g. undulatory upper-and lowersurfaces of the dedolomitebeds, and deformation of the interbedded marlstones and ­claystones, in theDiagenetic Carbonate Zone.In addition, thededolomitization process leads to a largeincreaseof Mf+ inthe remaining solution whichfavors the formation of fibrousclay minerals (sepiolite, palygorskite) by authigenictransformation of smectite.

Alternatively, the calcium-carbonate-rich waters resultingfromdissolution of limestones couldcirculate directly throughgypsum (pathway m, Fig. 13) and dissolve them until thesolution reaches equilibrium. This accounts for extensiveprecipitation of calcium carbonate either as pseudomorphiccalcite after interstitial gyspum or extensive calcitization oflenticular gypsum from the Evaporite Unit (ED). Theremaining solutionis saturatedbothin gypsum andcalcite, soitsinfIltration through nearbydolostones wouldpromotewidedissolution of dolomite and concomitant precipitation ofcalcite in a stoichiometric rate of 1-1.98. In this pattern, thedissolution/precipitation rates of the minerals are lower thanthoseofpathwayIA,butan increaseofabout13.4% in volumecan be determined in the reaction dolomite-calcite. At anyrate, the remaining circulating water is undersaturated ingypsum and this could account for partial dissolution ofdolostone/gypsum deposits resulting in the formation of thecollapse breccias that commonly are recognized in theDiagenetic Carbonate Zone (Fig.4 and 12c).Brecciation wastriggered by the dissolution along bedding planes andfractures of largeinterbedded masses of sulfates. Dissolutionand collapse were gradual and more or less synchronous(Middelton 1961; Stanton 1966; Warren et al. 1990) andbrecciation was widespread, slow,and continuous.

As observed in Fig. 13, both pathways (IA and ffi) lead tosimilar volumes of the total of minerals dissolved andprecipitated under the requirements of the modeling. Thisprovides support for the hypothesis that authigenic rechargethrough the sediments of the topof the Miocene IntermediateUnit could be an effective mechanism for the formation ofdedolomites in this setting.

Allogenic recharge of the paleoaquijer,« We have alsomodeled an alternative pathway of dolostone and furtherdolostone/gypsum dissolution by considering that meteoricwaterflowed through the marginal terrigenous deposits (Fig.

12b; pathway II, Fig. 13). In this pattern, the infiltration ofmeteoric waters through limestones of the ULU is ruled out,and ground-water circulation is directly through dolostones.As observedin Fig. 13, the amountof precipitated calcite islowerbecausethewaters didnotflowdirectly intolimestones,but the obtained results for the total gypsum and dolomitedissolved are quite similarthan those in pathway I.

Theresults fromthemodeling of thededolomitization processare consistent with the stratigraphic- and petrographicevidence in the Miocene deposits. This is truedespitethefactthat PHRQPITZ is just an equilibrium model that cannotevaluate the kinetics barriers to calcite precipitation anddolomite dissolution. As pointedout by Saunders and Toran(1994), resultsareadjusted forlowcelerityof flow(slowflowconditions) thoughthiswouldconstrainonlytheearlierstagesof water circulation, because the flow rate increases as thededolomitization and karstification progress.

SUMMARY AND CONCLUSIONS

(1).-The topof theMiocene Intermediate Unitof the Madridbasin shows widespread dedolomite fabrics formed in theshallow subsurface soon after sediment deposition; thesefabrics are related to an intra-Vallesian paleokarsticunconformity.

(2).- Two major textural groups of dedolomites, suturedcalcites and radial-fibrous calcites, and other less-abundantthough widespread dedolomite fabrics havebeenrecognized.In their spatial distribution, all these diagenetic fabrics areclosely relatedship. We have interpreted them as resultingfrom dedolomitization after a dolomicrite precursor.Neomorphic- and replacive after-gypsum fabrics interpretedto result from calcitization of primary lenticular gypsum areoccasionally present

(3).-Radial-fibrous dedolomites maybe interpreted asa resultof displacement dedolomitization, with increase of volumethroughout theprocess. Therecognition of features typical ofbacterial activity led us to postulate that the radial-fibrousfabric wasbio-induced,

(4).-Geochemical evidence indicates thatbothcalcitization ofdolomite and of gypsum was achieved by the action ofoxidizing carbonate-rich ground waters of meteoric origin.Thediagenetic systembehaved asan opensystemforall traceelements analyzed; onlyin theshallowerzonewasthesystempartially closedwithrespectto Sr.

(5).- Gypsum dissolution was the driving mechanism fordedolomitization and karstification processes. Geologicalrelationships indicate that both dedolomitization andkarstification developed at shallow burialdepthsrelatedto anirregular shallow watertableanda narrow vadosezone.Thisisalsosupported by thevertical trendsof thestableisotopes inthe carbonates.

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6).- The resultsobtainedfrom the geochemical simulation ofthededolomitization processareconsistentwithstratigraphic­andpetrographic evidenceandsupportthepaleoenvironmentalconditions in whichdedolomitization took place.

ACKNOWLEDGEMENTS

Theresearch.wassupported by Spanish D.G.I.C.Y.T. projectsPB89-o032 and PB89-Q047. Mineralogical- andgeochemicalanalyseswere conductedwith the assistance of R. Gonzalez(MNCN-CSIC). J. Bedoya (MNCN-CSIC) assistedwith theSEM.This paperhas benefitted fromdiscussions withDr.C.Rossi (U.C.M). The authors are indebted to Drs. J. E.Sanders, G. M. Friedmanand an anonymous reviewerwhosesuggestions considerably improvedthe paper.

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Received: November 15, 1995Accepted: March II, 1996

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