Petrographic and geochemical evidence for the formationof primary, bacterially induced lacustrine dolomite: La Roda`white earth' (Pliocene, central Spain)

M. ANGELES GARCIÂA DEL CURA*, JOSE P. CALVO  , SALVADOR ORDO NÄ EZà ,BLAIR F. JONESx and JUAN C. CANÄ AVERASà*Instituto de GeologõÂa EconoÂmica, CSIC-UCM, Madrid, Spain Dpto PetrologõÂa y GeoquõÂmica, Fac. CC. Geolo gicas, Universidad Complutense, Madrid, Spain(E-mail: jpcalvo@eucmax.sim.ucm.es)àDpto Ciencias de la Tierra y Medio Ambiente, Universidad de Alicante, SpainxUS Geological Survey, Reston, VA 22092, USA

ABSTRACT

Upper Pliocene dolomites (`white earth') from La Roda, Spain, offer a good

opportunity to evaluate the process of dolomite formation in lakes. The

relatively young nature of the deposits could allow a link between dolomites

precipitated in modern lake systems and those present in older lacustrine

formations. The La Roda Mg-carbonates (dolomite unit) occur as a 3á5- to 4-m-

thick package of poorly indurated, white, massive dolomite beds with

interbedded thin deposits of porous carbonate displaying root and

desiccation traces as well as local lenticular gypsum moulds. The massive

dolomite beds consist mainly of loosely packed 1- to 2-lm-sized aggregates of

dolomite crystals exhibiting poorly developed faces, which usually results in a

subrounded morphology of the crystals. Minute rhombs of dolomite are sparse

within the aggregates. Both knobbly textures and clumps of spherical bodies

covering the crystal surfaces indicate that bacteria were involved in the

formation of the dolomites. In addition, aggregates of euhedral dolomite

crystals are usually present in some more clayey (sepiolite) interbeds. The thin

porous carbonate (mostly dolomite) beds exhibit both euhedral and

subrounded, bacterially induced dolomite crystals. The carbonate is mainly

Ca-dolomite (51±54 mol% CaCO3), showing a low degree of ordering (degree of

ordering ranges from 0á27 to 0á48). Calcite is present as a subordinate mineral in

some samples. Sr, Mn and Fe contents show very low correlation coef®cients

with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. d18O-

and d13C-values in dolomites range from )3á07& to 5á40& PDB (mean � 0á06,

r � 1á75) and from )6á34& to )0á39& PDB (mean � )3á55, r � 1á33)

respectively. Samples containing signi®cant amounts of both dolomite and

calcite do not in general show signi®cant enrichment or depletion in 18O

and 13C between the two minerals. The correlation coef®cient between d18O

and d13C for dolomite is extremely low and negative (r � )0á05), whereas it is

higher and positive (r � 0á47) for calcite. The lacustrine dolomite deposit from

La Roda is interpreted mainly as a result of primary precipitation of dolomite in

a shallow, hydrologically closed perennial lake. The lake was supplied by

highly saturated HCO3)/CO3

2) groundwater that leached dolomitic Mesozoic

formations. Precipitation of dolomite from alkaline lake waters took place

under a semi-arid to arid climate. However, according to our isotopic data,

strong evaporative conditions were not required for the formation of the La

Roda dolomite. A signi®cant contribution by bacteria to the formation of the

dolomites is assumed in view of both petrographic and geochemical evidence.

Sedimentology (2001) 48, 897±915

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Keywords Dolomite, lacustrine deposits, petrography, Pliocene, Spain, stableisotopes.

INTRODUCTION

Studies on the formation of sedimentary dolomitehave been in continuous progression for severaldecades. Dolomitization processes in sedimen-tary rocks are a recurrent subject for petrologistsand sedimentologists, as these processes takeplace in many distinct geological circumstances(for reviews of the `dolomite problem' as well asdolomitization models, see Fairbridge, 1957;Zenger, 1972; Lippman, 1973; Morrow, 1982a, b;Land, 1985; Machel & Mountjoy, 1986; Guna-tilaka, 1987; Hardie, 1987; Tucker & Wright, 1990;Arvidson & Mackenzie, 1999). The famous state-ment by Chilingar et al. (1979) that `there aredolomites and dolomites', also noted by Hardie(1987) in his critical review of dolomitizationmodels, clearly testi®es to the complexity of theformation patterns of dolomite in sedimentaryenvironments and explains the controversy that isfound in the literature (e.g. Zenger et al., 1980;Hardie, 1987). Occurrences of dolomite in bothmodern and ancient lakes provide opportunitiesto study the generation of dolomite. As Last(1990) points out, modern lake systems offer anexcellent chance to improve our understandingof the hydrogeochemical constraints of dolomiteformation, because of the varied chemistries andconcentrations recognized in lake waters, whichrepresent a much wider range of conditions thanthose for Mg-carbonates formed in marine envi-ronments. From nearly 50 reported occurrences ofnon-detrital lacustrine dolomite of Quaternaryage, Last (1990) concluded that most of theseoccurrences formed under saline lake conditions,which is in agreement with previous statementsby Eugster & Hardie (1978), Eugster & Kelts (1983)and Warren (1989). An additional conclusion isthat many of the recent lakes in which dolomite isformed contain NaCl-type brines, also character-ized by high carbonate alkalinities, an aspectemphasized by Hardie (1987), high Mg/Ca ratios(see also MuÈ ller et al., 1972) and commonly highsulphate concentrations. An important conclu-sion from Last's (1990) review of the recentlacustrine occurrences is that dolomite in lakesmust be mostly primary. Yet, the results obtainedfrom several studies monitored in recent south

Australian lakes (Rosen & Coshell, 1992) do notseem to support such a primary origin fordolomite, so the exact mechanism of dolomiteformation in lakes remains an open question.

Transferring these conclusions to dolomiteformation in past lake systems is speculative.Even in sedimentary formations where dolomitehas been classically studied, for instance theEocene Green River Formation of western USA(Wolfbauer & Surdam, 1974; Smoot, 1983), theorigin of dolomite remains unclear. This paperpresents a case study of lacustrine dolomitedeposits from the Pliocene of central Spain, inwhich the relatively young nature of the dolomiteallows determination of some aspects of thegenetic pattern of dolomite in lakes. Good pre-servation of the La Roda dolomite deposits, whichhave not undergone signi®cant burial diagenesisand have a powdery, non-indurated appearance,makes these materials suitable for such a goal.The study is supported by detailed petrography,mineralogy and geochemistry of the dolomitecrystal mosaics and associated clays, the latteroccurring whether disseminated within the dolo-mite or in thin beds between the carbonatesediments. Emphasis is placed on some featuresof the dolomite crystals that can be interpreted asresulting from microbial activity, this supportingthe important role of biological factors controllingthe formation of dolomite (Vasconcelos & McKen-zie, 1997; Wright, 1999).

FIELD OCCURRENCE AND SAMPLELOCATION

The Pliocene lacustrine dolomite deposits arelocated in the vicinity of the village of La Roda,Albacete province, in central-east Spain (Fig. 1).They crop out on a ¯at-lying area surrounded bysmall exposures of Mesozoic rocks that constitutethe substratum of the Neogene sediments. Thedolomite deposits are known as `white earth' inthe region where they have traditionally beenquarried to furnish local, but pro®table, paintindustries. The carbonate beds (dolomite unit),containing mainly dolomite and subordinatecalcite, reach up to 4 m in thickness and are

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observed locally to overlie well-sorted, medium-to coarse-grained sands and mudstones of ¯uvialorigin (YeÂbenes et al., 1973; Santos, 1983). Thelacustrine carbonates are in turn covered by red togreenish laminated mudstones, which are inter-bedded with carbonate and show clear palustrinefeatures (mudstone±limestone unit; Fig. 2). Themudstones, consisting mostly of illite and kao-linite with local smectite-rich horizons, showabundant pedogenic features, mainly root traces,calcite nodules with diffuse boundaries and drabhaloes, whereas the carbonates comprise indura-ted calcretes and/or root-bioturbated biomicriteand calcareous marls (Fig. 3). These units of LatePliocene age (Santos, 1983) are in turn covered

unconformably by Pleistocene red gravel andsandstone beds in which calcrete pro®les arebroadly developed (Fig. 1).

Sampling was focused on six sections croppingout in a quarried area close to La Roda. Fifty-twosamples were collected from the carbonate bedsand associated terrigenous deposits in thesesections. In outcrop, two main facies were distin-guished within the carbonate sediments of thedolomite unit: (i) massive dolomites; and (ii)porous carbonates showing varied sedimentaryfeatures (Fig. 3). The porous carbonates showslightly less induration and are characterized by agrainy and crumbly appearance resulting fromeither a rather dense network of small holes of

Fig. 1. Location of the La Rodalacustrine dolomite deposits. (A)Geological map of the study area;sampling of the dolomite sedi-ments focused on the quarries westof La Roda. (B) Cross-sectionshowing the distribution of thePliocene dolomites and associatedformations overlying Miocene andMesozoic rocks; the section cor-responds to the pro®le markedA±A¢ (which exceeds the extentof the map) in (A).

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various morphologies or lenticular gypsummoulds ranging from 0á04 mm to 2 mm in length.The porous carbonate deposits are interbedded

with more massive, white dolomite deposits ofvariable thickness, ranging from 1 dm to 1 m.Both the porous carbonates and the massive

Fig. 2. Outcrop view of thedeposits belonging to the dolomiteunit (white beds, locally showingsur®cial dark stain because oforganic patina at the lower part ofthe photograph) overlain by asequence of interbedded mudstoneand carbonates (mudstone±lime-stone unit) near La Roda. Note thatthe carbonate beds show a typicalvertical prismatic structure char-acteristic of palustrine environ-ments. The outcrop, reaching up6 m in height, corresponds tosection VI in Fig. 3.

Fig. 3. Correlation lihostratigraphic sketch of the Pliocene sediments at La Roda. The correlation level marks theseparation between lacustrine dolomite (white earth deposits) and an overlying interbedded mudstone and fresh-water limestone unit; the latter unit is especially represented in logs III, IV and VI. Locations of samples listed inTable 1 are indicated.

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dolomites exhibit local vertical disjunction in thetop beds. Calcite is also present as coarse crystal-line aggregates ®lling cracks within the dolomitegroundmass, giving way to a reticulate structure,as well as occluding voids in the porous carbon-ate. Clays, mixed with carbonate in some thininterbeds, are prominent at several levels of thesections, although they commonly occur dissem-inated within the dolomite. The clays consistmainly of sepiolite ®bres, as described below.Terrigenous grains, mainly quartz and mica, arescarce in the dolomite beds.

ANALYTICAL METHODS

All samples were systematically analysed bystandard petrography and scanning electronmicroscope (SEM) imaging (Jeol JSM-6400 andHitachi 5400 ®eld transmission apparatus). Aliz-arin red S stain of thin sections (Lindholm &Finkelman, 1972) posed severe problems forpetrographic differentiation of dolomite, as theminute crystals of this mineral also becamestained, thus preventing a reliable view of themineralogical distribution of the dolomite in thecarbonate samples. This problem was partiallysolved by obtaining polished slabs coated withcarbon, which were then observed under SEM(backscattered electron mode).

X-ray diffraction was used to determine min-eral composition in powdered carbonate samplesand insoluble residues, using quartz as an inter-nal standard. The equipment used was a PhilipsX-ray diffraction system operated at 40 kV and30 mA, and monochromated Cu±Ka radiation.Semi-quantitative estimates of relative percentag-es of minerals from whole-rock samples weremade through measurement of the intensity of thediffraction peaks by integration of the area.Values of mol% CaCO3 of the carbonate mineralswere estimated by measurement of the position ofthe d104 peak relative to a standard (Goldsmithet al., 1961). The degree of ordering of thedolomite crystals was determined by the sharp-ness and relative intensities of the ordering peaks,superstructure re¯ections corresponding to d021,d015 and d110 (Goldsmith & Graf, 1958). Thedegree of ordering is thus estimated by the ratioof the heights of the ordering peak 015 todiffraction peak 110 (Hardy & Tucker, 1988).

The clay minerals of samples containing asigni®cant percentage of clay fraction (>30% ofthe powdered whole rock) were analysed usingoriented aggregates of the <2-mm fraction. They

were treated with ethylene glycol and heated at550 °C for 2 h.

Major and trace element contents (Ca, Mg, Sr,Fe, Mn, K) from whole-rock samples collected inone of the sections (section I, Fig. 3) weredetermined by X-ray ¯uorescence using standardtechniques (Franzini et al., 1972, 1976) and byenergy-dispersive spectrometry (EDS) in samplesobserved under SEM. The silica content wasdetermined by the same method. For stableisotopic analysis, 40 samples were ground to passthrough a 100-mesh sieve; the 250-mesh fractionwas saved for analysis. Five of these samplescorrespond to limestones (exclusively formed ofcalcite) collected from beds of the mudstone±limestone unit in order to compare their isotopicsignatures with those of the dolomites. For mixedcarbonate samples, both calcite and dolomitewere analysed if the lesser mineral constitutedat least 10% of the total carbonate; otherwise,only the dominant mineral was analysed. Carbondioxide was evolved from each sample at 25 °Cusing 100% H3PO4. The gas evolved in the ®rsthour was attributed to calcite; the gas evolvedbetween 24 h and 7 days was attributed to dolo-mite. All samples were prepared and analysed atleast in duplicate. The analytical precision isgenerally � 0á10& for carbon and � 0á15& foroxygen. Oxygen values were initially reported inpermil relative to SMOW and then convertedto PDB using the equation d18 PDB � 0á97006d18 SMOW ± 29á94 (Craig, 1957).

RESULTS

Petrography

As mentioned previously, two main carbonatelithofacies were distinguished in the dolomiteunit: massive dolomite and porous carbonatedeposits, the latter showing a grainy and crumblyappearance (Fig. 3). Under the petrographicmicroscope, the massive dolomite is seen as arather homogeneous micrite with sparse quartz,feldspar and mica grains (Fig. 4A) in which smallelongate holes can be recognized very locally. Incontrast, the porous carbonate deposits show avariety of petrographic features, which allow thedistinction of three main subfacies types: (i) dolo-micrite containing moulds of lenticular gypsum, inmost cases ®lled by calcite spar; (ii) dolomicritewith abundant desiccation cracks and ®ssuresrelated to small roots, locally preserving alveolartextures; and (iii) dolomicrite with scattered plant

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remains (Fig. 4B) and occasional charophyte stemsand oogonia, the latter being poorly preserved orusually as moulds in the micrite groundmass.Calcite is locally abundant in some porous carbon-ate layers (samples 5-05 and 5-07; Table 1) where itis present as either veins formed by root bioturba-tion or rather dense aggregates of calcite-®lledmoulds after gypsum lenses.

SEM of whole-rock samples shows that themassive dolomite consists mainly of looselypacked crystalline aggregates, 1±2 lm in size,with irregular morphologies (Fig. 5A). In detail,each of these aggregates is typically composed ofinterpenetrating submicron-sized dolomiterhombs (Fig. 5B), similar to those described byTalbot & Kelts (1986) from recent sediments inLake Bosumtwi. Besides this common appear-ance of the crystals in the massive dolomitedeposit, some samples consist of crystallineaggregates, similar in size to those described

before, showing widely extended clumps ofspherical to ovate-shaped bodies, 0á1±0á3 lm insize, resembling bacteria or nannobacteria (Ro-sen & Coshell, 1992; Folk, 1993; Fig. 5C). Highlymagni®ed SEM images allow the recognition ofwell-de®ned crystalline aggregates with a knob-bly surface texture (Fig. 5D). In some clay-richlayers present within the massive dolomitedeposits, the dolomite is made of both rhombicand subrhombic, up to 1-mm-diameter crystalsmixed with clay mineral ®bres (sepiolite), re-sulting in a largely open fabric (Fig. 5E), perhapscaused by nannobacteria. In detail, some of thedolomite crystals have multiple sharply de®nededges, which suggests several growth episodes ofthe crystals (Fig. 5F).

The porous carbonate deposits show a varietyof crystalline fabrics under SEM, which is coin-cident with the various subfacies types recog-nized in these deposits (see above). SEM imagesof dolomicrite containing moulds of lenticulargypsum show that the dolomite crystals, up to1 mm in diameter, are typically rhombic anddisplay multiple growth faces. Dolomicrites con-taining plant remains or showing abundant des-iccation cracks do not have signi®cant texturaldifferences, and both subfacies types consist ofeither euhedral rhombic dolomite crystals oraggregates of subrounded crystals that arearranged according to circular (Fig. 6A) and/orrod-like (Fig. 6B) patterns.

Mineralogy

The Upper Pliocene carbonate sediments from LaRoda have varied mineralogical compositions(Table 1), which are very clearly distinguishablewhen the two stratigraphic units present in thearea are compared. Thus, carbonate from themudstone±limestone unit consists of induratedlimestones formed only of calcite (average1á3 mol% MgCO3). In contrast, the dolomite unitcomprises two carbonate lithofacies types (mas-sive dolomite and porous carbonate) in which thecarbonate mineral fraction is mostly dolomite.This mineral exceeds 90% in most of the samplescharacterized as massive dolomite. Calcite ispresent in variable proportions in some samplesfrom both the massive dolomite and the interbed-ded porous carbonate. The degree of ordering inthe richest dolomite samples, determined bymeasurement of indices 015 and 110 from X-raytraces, ranges from 0á27 to 0á48 (mean � 0á36;Table 1), indicating that the dolomites are poorlyordered (Kinsman, 1964, cited in Bathurst, 1975).

Fig. 4. Photomicrographs showing the textural char-acteristics of (A) massive dolomite deposits and (B) oneof the subfacies distinguished within porous carbonatedeposits. This subfacies corresponds to dolomicritewith scattered plant remains (arrows) encrusted bycarbonate that, where more concentrated, gives a tufa-like aspect to the deposit.

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Table 1. Mineralogical and stable isotope composition of the La Roda carbonates and associated sediments (seeFig. 3 for sample locations).

Sample FaciesDol(%) DO

Cal(%)

Q(%)

Clay(%)

Dold13CPDB

Dold18OPDB

Cald13CPDB

Cald18OPDB

1-01 M 95 0á29 5 )2á30 )0á371-02 P 95 0á32 5 )2á36 0á431-03 P 95 0á39 5 )3á63 0á561-04 M 95 0á30 5 )5á13 2á731-04b M 95 0á36 5 )3á29 1á201-05 P 95 0á35 5 )2á34 )0á021-06 P 100 0á31 tr )0á39 )1á051-07 P 100 0á30 tr )3á61 0á131-08 P 95 0á39 5 tr )4á13 )0á801-09 M 100 0á34 tr )5á01 1á111-10 P 95 0á36 5 )6á34 1á701-11 P 70 0á47 5 25 )6á00 )1á261-12 C 100 (I, K)2-01 M 80 0á39 20 tr )3á50 3á34 )3á83 2á132-02 P 50 0á48 50 tr )4á32 5á40 )4á35 0á752-03 M 100 0á27 tr tr )3á38 3á422-04 P 80 0á29 20 5 )1á49 0á35 )2á97 0á082-05 P 50 0á40 50 tr )1á38 )0á55 )4á19 )3á582-06 M 60 0á34 35 5 )1á17 )0á77 )3á37 )0á682-07 M 45 0á34 55 tr tr )3á44 )0á49 )4á53 )4á422-07b m 10 5 25 60 (Sp, P, I, K)2-08 m 35 25 40 (Sp, I, K)2-09 P 95 0á32 5 )5á29 0á702-10 IL 100 )7á68 )5á523-01 P 100 0á34 )3á75 0á743-01b M 95 0á31 5 )3á74 0á703-02 P 100 0á36 tr )3á73 0á553-03 M 100 0á36 tr )3á10 )1á803-04 P 95 0á35 )3á31 )1á703-05 M 100 0á36 )4á18 )1á734-01 il 90 104-02 m 5 50 5 40 (I, K)4-03 C 25 75 (I, K)4-03b C 20 0á39 20 60 (I, K)4-04 il 5 85 104-05 il 95 54-05b m 10 10 20 60 (I, K)5-01 IL 100 )6á49 )6á245-02 IL 5 95 )5á90 )5á895-03 IL 5 85 10 )7á03 )5á495-04 P 100 0á36 )3á05 0á865-05 P 20 75 5 )5á43 )2á11 )4á63 )3á405-06 M 100 0á35 )2á74 0á455-07 P 30 65 5 )4á68 )3á07 )4á87 )4á656-01 M 100 0á35 )3á12 )1á786-02 P 90 0á34 10 )3á27 )1á566-03 M 100 0á48 )4á26 )1á716-04 P 80 0á48 10 10 )5á00 )2á57 )5á55 )1á346-05 P 100 0á31 )2á72 )0á466-06 C 10 80 (I, K, Sm)6-07 IL 100 )7á72 )5á43

Codes for facies: M, massive dolomite; P, porous carbonate; C, clay deposits; m, marl; IL, indurated limestone. Clayminerals: I, illite; K, kaolinite; Sp, sepiolite; P, palygorskite; Sm, smectite. tr, trace (< 5%); DO, degree of ordering.

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According to the mol% CaCO3 values determinedin the dolomites (51±54 mol% CaCO3), these canbe characterized as slightly Ca-rich. Moreover, aninverse relationship between calcium content in

the dolomite and their degree of ordering(DO � )0á02 [mol% CaCO3] + 1á56) is observed,which is in agreement with that reported byHardie (1987).

Fig. 5. SEM photomicrographs of textural features shown by the massive dolomites. (A) Loosely packed 1- to 2-lm-sized dolomite crystal aggregates, which constitute the most typical fabric of the massive dolomites. (B) Enlargedview of one of the dolomite aggregates, typically composed of interpenetrating submicron-sized dolomite rhombs,resulting in a subrounded irregular morphology; note small sepiolite ®bres mixed with the dolomite. (C) 2-lm-sized,subrounded dolomite aggregates displaying abundant clumps of both spherical and oblong-shaped bodies resemblingbacteria corpuscles. (D) Magni®ed view of a dolomite aggregate showing a rather regular morphology characterizedby a knobbly surface texture in which abundant nannobacteria can be observed. (E) Rhombic to subrhombic,approximately 1-lm-sized dolomite crystals typical of some beds rich in sepiolite (observable as sparsely distributed®bres within the dolomite fabric). (F) Enlarged view of a subrhombic dolomite aggregate showing multiple sharplyde®ned edges; small spherical bodies (nannobacteria?) cover some crystals.

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The different carbonate mineralogy betweenthe mudstone±limestone unit and the dolomiteunit, together with the distinct sedimentaryfeatures observed in the two units, suggest adrastic change in the depositional conditions (seediscussion below), which is also supported by theclay mineralogy. Clay minerals are relativelyscarce within the massive dolomite and associ-ated porous carbonate deposits, except for somethin clayey interbeds. The clay minerals deter-mined in these facies consist predominantly ofsepiolite (up to 75% of the clay fraction), withminor palygorskite and usually a low content ofillite and kaolinite (Table 1). In contrast, theoverlying mudstone±limestone unit containsbeds in which clays constitute 30±80% of thebulk mineralogy. In these beds, illite is thedominant clay mineral together with kaolinite.The presence of smectite has only been recog-

nized in one layer at the lowermost part of thisunit.

Quartz is always present in signi®cant percent-ages in the clay±marly beds of the mudstone±limestone unit, where feldspars are also observedonly as traces. Quartz content in the dolomiteunit is usually very low (»5%) although quartzgrains are relatively more abundant (up to 10%)in porous carbonate beds than in the massivedolomites (Table 1).

Geochemistry

Elements

Ca, Mg, Sr, Mn, Fe, K and silica contents frommassive dolomites and associated porous carbon-ates were determined only in section I (Fig. 3,Table 2), which is representative of the variousfacies present in the La Roda carbonates. Ca, Mg,

Table 2. Chemical composition of dolomite samples from section I.

Sample SiO2 (%) Ca (%) Mg (%) Sr (p.p.m.) Mn (p.p.m.) Fe (%) K (%) Mg/Ca (Sr/Ca) ´ 104

1-01 1á69 25á46 9á52 834á00 0á01 0á23 0á09 0á37 32á761-02 1á53 25á21 9á73 832á00 0á01 0á23 0á07 0á39 33á011-03 2á06 24á69 10á11 600á00 0á01 0á25 0á10 0á41 24á301-04 1á37 25á03 10á15 521á00 0á01 0á20 0á07 0á41 20á821-04b 1á90 24á96 9á94 692á00 0á01 0á25 0á07 0á40 27á721-05 1á90 24á94 9á99 599á00 0á01 0á30 0á07 0á40 24á021-06 0á85 25á33 10á03 730á00 0á00 0á17 0á04 0á40 28á821-07 0á92 25á19 10á10 776á00 0á01 0á20 0á03 0á40 30á811-08 7á29 22á54 9á70 494á00 0á02 1á16 0á26 0á43 21á911-09 2á16 25á23 9á60 557á00 0á03 0á40 0á13 0á38 22á081-10 2á76 24á82 9á64 570á00 0á05 0á53 0á13 0á39 22á96Mean 2á22 24á85 9á86 655á00 0á02 0á36 0á10 0á40 26á29SD 1á77 0á80 0á23 123á24 0á01 0á29 0á06 0á02 4á50

Fig. 6. SEM photomicrographs of textural features observed in porous carbonates. (A) Subrounded dolomiteaggregates arranged in a circular pattern probably related to biogenic structures. (B) Rod-like arrangement of minuterhombic dolomite crystals; note that some crystals exhibit small dissolution holes (e.g. black arrow). Bacterialcorpuscles are also seen in some dolomite crystals (e.g. white arrow).

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Sr and Mn were selected for analysis because oftheir signi®cance in carbonate sedimentation anddiagenesis, whereas Fe, K and silica can help inre¯ecting the contribution of terrigenous input tothe carbonate sediments. This latter assessment isclearly corroborated by plotting silica contents vs.K and Fe in separate binary diagrams (Fig. 7A andB). In both cases, the linear correlation is high,suggesting that K and Fe are intimately associatedwith silica forming aluminosilicates. Sr exhibitslow (mean � 655 p.p.m.) but ¯uctuating values

(Table 2) that show a rather good correlation withCa contents (Fig. 7C), a behaviour that agrees withthe ®ndings of others that Sr substitutes for Carather than Mg in the dolomite lattice (Kretz,1982). The distribution coef®cients for Sr indolomite (KSr

dol) are much lower than unity (Katz& Matthews, 1977). Mn contents are generallyvery low (mean � 143 p.p.m.), except for theuppermost part of the section, where the presenceof pedogenic features is apparent (Fig. 3, Table 2).This relationship between higher Mn contentsand pedogenic features appears to be supportedby values of d13C (Fig. 8A) determined in thecarbonate samples.

Mg and Ca contents vary between 9á52% and10á15% and 22á54% and 25á46% respectively(Table 2). Mg/Ca ratios ¯uctuate in a narrowinterval (mean � 0á40 � 0á02), which con®rmsthe Ca-rich nature of the dolomites determinedby calculations from X-ray traces. Moreover,Mg/Ca ratios have been plotted vs. stable isotopesignatures (Fig. 8) in order to establish palaeoen-vironmental constraints on the formation of thelacustrine dolomite. A weak negative correlationis found between Mg/Ca ratios and d18O (Fig. 8D),which suggests that the formation of dolomitewas not constrained by highly evaporative con-ditions (see discussion below). Likewise, Mg/Caratios and d13C show a weak negative correlation(Fig. 8B).

Stable isotopes

Isotopic analysis was carried out on 40 carbonatesamples from the different sections. The whole-rock isotopic data from the lacustrine carbonateswere used to help in environmental interpreta-tions as well as in deriving valuable informationabout inorganic and/or organic constraints on theformation of dolomite. Oxygen and carbon iso-topic ratios, expressed as d18OPDB and d13CPDB arelisted in Table 1. Five samples (2-10, 5-01, 5-02,5-03 and 6-07), collected from beds of themudstone±limestone unit, were formed solely ofcalcite. A few samples (2-01, 2-02, 2-04, 2-05,2-06, 2-07, 5-05, 5-07 and 6-04) contained suf®cientcalcite and dolomite for both to be analysed; allother analyses were only for dolomite. Samplescontaining both calcite and dolomite were stud-ied under the petrographic microscope in order toidentify the primary or (early or late) diageneticcharacter of calcite and thus to understand betterthe isotopic signatures obtained from these bimi-neralic samples. Calcite appears to be mainlya late diagenetic product, occurring either as

Fig. 7. Binary plots showing geochemical af®nitiesbetween silica and K (A) and Fe (B) contents, andbetween Ca and Sr (C) contents in dolomite samplesfrom section I.

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cement partly ®lling voids (Fig. 9) or as calcitespar mosaic pseudomorphs of previous gypsumlenticles.

The isotopic compositions of the La Rodacarbonates cover a broad range of negative d13Cvalues ()7á72& to )0á39&) and of both negativeand positive d18O values ()6á24& to +5á40&;Table 1). When d18O is plotted against d13C, agroup of carbonates corresponding to induratedlimestones from the mudstone±limestone unit is

clearly differentiated (Fig. 10A). These carbon-ates are formed exclusively of calcite and showthe lightest values for both d18O and d13C. Bothmassive dolomite and porous dolomite typeshave highly variable isotope compositions (meanof d18O values � 0á06, standard deviation � 1á75;mean of d13C values � )3á55, standard devi-ation � 1á33). However, most of the d18O valuesfall into a relatively narrow domain (» )2& to» +1á5&), in which the massive dolomite andporous carbonate samples are not clearly differ-entiated. The same is seen in the distribution ofd13C values on the plot, despite the large spreadof carbon isotope compositions (Fig. 10A). Asshown in this ®gure, the isotopic compositions ofthe La Roda carbonates are clearly differentiatedfrom other reported dolomites, such as thoseoccurring in recent lacustrine environments fromAustralia (Rosen et al., 1989).

Both porous and massive dolomite samplesshow no signi®cant relation of d13C to d18O(r � )0á05), so the isotopic compositions do notfollow any simple trends. However, when exam-ined in detail (Table 1), a more de®nite trend canbe recognized, as many of the dolomite sampleswith heavy d18O values are signi®cantly depletedin heavy carbon. This is the case, for instance, insamples 1-04, 1-09, 1-10, 2-01, 2-02, 2-03 and 2-09(Fig. 10B). The negative correlation between d18Oand d13C values in the La Roda dolomites is clear

Fig. 8. Plots showing the relationships between several chemical element parameters of the dolomite samples andstable isotope compositions: (A) d13C ± Mn showing a marked negative correlation; (B) Mg/Ca vs. d13C shows nocorrelation; (C) d18O ± Sr showing some apparent negative correlation; (D) Mg/Ca vs. d18O shows no correlation.

Fig. 9. Photomicrograph of porous carbonate faciesshowing dolomicrite (darker background in the photo-graph) with voids that are partly to totally cemented bycalcite spar. The calcite spar mosaics are interpreted asa late diagenetic product after the degradation of pre-vious tufa-like remains.

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in some samples, in which the presence ofbacterial corpuscles has been recognized petro-graphically, which supports the in¯uence oforganic matter decomposition and, consequently,enrichment in 12C in the formation of the dolo-mites (see discussion below).

The bimineralic samples show various trendswith respect to enrichment or depletion in 13Cand 18O between dolomite and calcite (Fig. 10A,Table 1). Thus, dolomite in samples 2-04 (mas-sive dolomite) and 2-06 (porous carbonate) issigni®cantly enriched in 13C relative to calcite butshows similar 18O compositions. More de®nitetrends of 18O enrichment (by 1á5±3á0&) of dolo-mite relative to calcite are observed in samples2-01, 2-02, 5-05 and 5-07 (Table 1), in whichvariations in 13C are not signi®cant. Just onesample (6-04) shows an opposite pattern, i.e.calcite enriched in 18O relative to dolomite, to thecommonly observed trend.

MODEL OF DOLOMITE FORMATION

Geomorphology, hydrology and climaticsetting of La Roda during the Late Pliocene

The carbonate deposits of the Pliocene dolomiteunit of La Roda accumulated in a shallow,hydrologically closed perennial lake depression.Both the ¯uvial origin of the sandstones under-lying the carbonates (YeÂbenes et al., 1973; Santos,1983) and the evidence for shallowness shown bythe deposits of the overlying mudstone±limestoneunit support this interpretation. In addition, themassive character of the dolomite deposit and thepresence of some subaerial exposure features,especially pedogenic disjunctions developed atthe top of some sequences, suggest that thecarbonates were deposited under shallow, prob-ably oxic to suboxic bottom conditions (AnadoÂn &Utrilla, 1993). The actual extent of the depressionin which the carbonates formed cannot be de®nedprecisely, as the dolomite unit is covered byyounger sedimentary formations, and no system-atic drilling has been carried out beyond thequarries located around the village of La Roda.However, some local underground works (Santos,1983) indicate that the stratigraphic unit contain-ing the dolomites does not extend further than afew kilometres.

The area is surrounded by outcrops of Mesozoiccarbonates in which dolostones predominate.Present-day groundwater systems in the regionare mainly controlled by the distribution of the

Fig. 10. (A) d18O±d13C cross-plot for dolomite and cal-cite samples from La Roda. Isotopic values determinedin calcite from some bimineralic (dolomite±calcite)samples are also represented as pairs of samples linkedby dashed lines. A ®eld of samples showing very neg-ative d18O and d13C values (lower left) corresponds tosamples of indurated limestones from the mudstone±limestone unit. Typical isotopic compositions of Coo-rong dolomites (Rosen et al., 1989) are shown. (B)Close-up view of the lower right part of the d18O±d13Ccross-plot shown in (A); a negative correlation betweend18O and d13C values is observed in the plot (seeexplanation in the text).

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Mesozoic outcrops (DõÂaz et al., 1990). In view ofthe slow evolution of the landscape in the regionfrom the Pliocene to the present (PeÂrez GonzaÂlez,1979), it appears that the hydrologic setting hasnot changed signi®cantly, and the lake developedin La Roda during the Pliocene was fed accordingto a drainage pattern similar to that currentlyrecognized: two groundwater basins convergefrom north and south in the La Roda area(Fig. 11). Leaching of the highly permeableMesozoic carbonate rocks accounted for alkaline,highly saturated HCO3

)/CO32) groundwaters ¯ow-

ing into the lake depression, which behaved as ahydrologically closed lake basin. As indicated bydata from drillholes in the area (Santos, 1983), thegroundwaters stored in the Pliocene units of LaRoda are the calcium±magnesium±sodium bicar-bonate type. Calculations of the saturation state ofthese waters, performed with the PHRQPITZ com-puter program (Plummer et al., 1988), indicate

that present-day waters are supersaturated withrespect to dolomite and huntite, slightly saturatedwith respect to aragonite, calcite and magnesite,and undersaturated with respect to gypsum(Table 3).

Present-day regional climatic conditions of theLa Roda area are characterized as arid to semi-arid, with average annual precipitation varyingbetween 300 and 400 mm and evaporation ratesranging from 400 to 500 mm (DõÂaz et al., 1990;OrdoÂnÄez et al., 1994); average temperatures(4±6 °C in winter, > 30 °C in the summer period)are also characteristic of regions in an arid tosemi-arid belt (Grove, 1977). Palaeoclimatic datainferred from the Late Pliocene stratigraphicrecord in areas adjacent to La Roda (Mein et al.,1978; Leone, 1985; Montuire, 1999) and thoseinferred from pollen data in the southernMediterranean (Fauquette et al., 1999) indicatethat climatic conditions during this period werevariable, with several alternating warming(predominant) and cooling episodes. Variationsin the climatic conditions of the area are recordedby the lithological changes observed between thetwo stratigraphic units present in La Roda. Thus,both the predominance of magnesium carbonateand the nature of the associated clays in thedolomite unit are in contrast to the abundance ofterrigenous deposits and the freshwater natureof the limestones recognized in the mudstone±limestone unit. This suggests that warmer anddryer climatic conditions prevailed in the areaduring the deposition of the former unit, whereasmore humid conditions prevailed during theaccumulation of the latter unit.

Sedimentological model of dolomiteformation

Field sedimentological analysis of the dolomiteunit indicates that the two main facies presentshow a sequential arrangement. Massive dolo-mite, comprising most of the carbonate deposit, ispresent in the lower part of the sequences

Table 3. Saturation index values (log IAP/K) at 25 °Cfrom groundwaters sampled in recent wells drilled inthe La Roda area.

Aragonite 1á287Calcite 1á307Dolomite 3á068Gypsum )4á731Huntite 2á809Magnesite 1á160Fig. 11. Geological sketch showing the main regional

groundwater ¯ow system in the La Roda area.

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and grades transitionally upwards into porouscarbonate. The sedimentary features observed inthe latter facies (i.e. root traces, desiccation cracks,gypsum moulds and plant and/or biogenicremains) suggest that the dolomite deposit experi-enced some environmental change, perhaps as aresult of shallowing and possibly relative fresh-ening of the lake water. However, the porouscarbonate is entirely dolomite, which providesevidence that, even at these stages, the concentra-tion of water was enough to precipitate dolomite.

Primary origin of dolomite

The petrographic features shown by the dolomitedeposits argue against a detrital origin for thedolomite. This is particularly evident in themassive dolomites in which the crystallineaggregates composed of interpenetrating submi-cron-sized dolomite grains indicate multiplegrowth episodes of the crystals (Rosen & Coshell,1992; Wright, 1999). The presence of these crystalmorphologies together with euhedral dolomite inthe porous carbonate also supports the authigeniccharacter of the dolomite in this facies. However,the massive dolomites exhibit petrographic andmineralogical features that support a direct pri-mary chemical precipitate origin. These featuresare: (i) the homogeneous compositional andtextural character of the dolomite deposit; (ii)the absence of replacement textures, such assyntaxial, submicron calcite domains in thedolomite (Rosen et al., 1988); (iii) no petrographicevidence for partial dolomitization of biogeniccalcite and/or aragonite precursors (Rosen &Coshell, 1992); (iv) the small size (1±2 lm) ofthe dolomite crystal aggregates, which also showmainly subhedral morphologies (De Deckker &Last, 1989; Last & De Deckker, 1990; AnadoÂn &Utrilla, 1993); and (v) the dolomites are calciumrich and poorly ordered, which is a commonlyobserved compositional characteristic of mostprimary lacustrine dolomites (Last, 1990; Last &De Deckker, 1990).

Taking these petrographic and mineralogicalfeatures together, the massive dolomites from LaRoda appear to have formed mainly as a directprecipitate from lake waters with an elevatedcarbonate/bicarbonate concentration, the carbon-ate alkalinity being the factor that promotes theprecipitation of dolomite, even though Mg/Caratios are not high (Clayton et al., 1968; Hardie,1987). As Hardie (1987) points out, a high Mg/Caratio is not necessary to increase the dolomitesolubility product when there is plenty of solute

CO32) around, as illustrated at Deep Springs Lake

where the ratio is near 1 (Jones, 1965). In thisrespect, the catchment drainage area of the LaRoda palaeolake, underlain mainly by Mesozoicdolomite source rocks, implied a consistent ratherthan a concentrated groundwater source of Mg.

Higher carbonate/bicarbonate concentrationsand consequent increased alkalinity could havebeen derived from the relatively high tempera-tures prevailing in the arid to semi-arid region ofLa Roda for some periods throughout the Plio-cene. Evaporative concentration, leading to salinelake conditions, has been commonly invoked as adecisive mechanism in controlling the formationof dolomite, especially in cases where primarydolomite is being formed (De Deckker & Last,1989; Rosen et al., 1989; Last, 1990; Warren,1990). This is usually recorded by an enrichmentin 18O of the precipitated carbonate phases,which consequently show strong positive d18Ovalues (Rosen et al., 1988; Warren, 1990; Arenaset al., 1997). Dolomites from La Roda, however,show stable isotope compositions that are con-sistently lighter than those reported from evapor-itic dolomites elsewhere (Fig. 10A). This suggeststhat the formation of the La Roda dolomites waspromoted by moderate rather than strong evapor-itic conditions. This contention is also supportedby the scarcity or absence of preserved evaporites(i.e. Ca-sulphates) within the dolomite deposits.However, the absence of sulphate is not aconclusive criteria for discounting evaporiticconditions in the formation of dolomite, assulphate can be removed by the activity ofsulphate-reducing bacteria (Wright, 1999). Thescatter of both oxygen and carbon isotopic valuesfor dolomite, particularly for carbon, suggestssome sort of mixing between meteoric and con-centrated lake waters (Coniglio et al., 1988), aconclusion also reached by Rosen et al. (1989) fortheir type B dolomites from Coorong. Thus, theprecipitation of dolomite in La Roda mostprobably resulted from mixing of evaporativelyconcentrated surface waters developed duringshort-term periods of drier climatic conditionsand carbonate-rich groundwaters, leading tosupersaturated carbonate/bicarbonate solutionsin which dolomite precipitated directly or ininterstitial pores. Under these conditions, lack ofcovariance of d18O and d13C may be explained bythe high carbonate alkalinity of the closed-basinlake, the total CO2 concentration acting to dampthe d13C response to changes in water input andlake productivity (Li & Ku, 1999).

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Petrographic evidence from the massive dolo-mite suggests an initial ®ne, gelatinous mud(consistency reminiscent of `yoghurt') probablyin the form of protodolomite (Von der Borch &Lock, 1979), similar to that reported from recentdolomitic lake sediments (De Deckker & Last,1988). In this setting, both the submicron-sizedgrains of the dolomite aggregates and the lowordering of this mineral indicate that precipita-tion of dolomite is rapid (Wright, 1999).

Dolomite deposition took place probably underoxic to suboxic lake bottom conditions. This issupported by the absence of lamination in thedolomite deposit and can also be deduced fromthe negative correlation between Mn contents andd13C values (Lohmann, 1988; Lumsden & Lloyd,1997; Fig. 8A). However, the inhibiting effect ofSO4

� anions on dolomite precipitation (Baker &Kastner, 1981) is not considered a signi®cantfactor in the formation of dolomite in the La Rodapalaeolake for the following reasons: (i) thepresence of relics of sulphate minerals as calcitepseudomorphs after gypsum is restricted to a veryfew occurrences within the dolomite deposits; (ii)the composition of groundwater discharging inthe region today is clearly undersaturated withrespect to gypsum (see above); and (iii) observa-tions in modern lakes in which dolomite isforming indicate that even relatively high sul-phate levels of the brines do not seem to inhibitdolomite precipitation (De Deckker & Last, 1988).

Formation of the La Roda dolomites underweakly evaporitic conditions is also supported bythe mineralogy of the clays that occur associatedwith the carbonate sediments. Sepiolite is themain clay mineral within the dolomite depositsforming either thin clay interbeds or dissemin-ated as small ®bres within the dolomite aggre-gates (Fig. 5B); besides sepiolite, minor amountsof palygorskite, illite and kaolinite are alsopresent. These latter clay minerals are probablydetrital, whereas sepiolite is considered to be anauthigenic clay mineral formed by direct precipi-tation from moderately saline lake water (Jones,1986). In this respect, the absence of Mg-richsmectites (e.g. stevensite, saponite, kerolite±stevensite, etc.) in the La Roda deposit helps torule out a more saline environment (Khouryet al., 1982; Jones, 1986; Calvo et al., 1999; Chahiet al., 1999).

Bacterial contribution to dolomite formation

An alternative explanation for the scarcity orabsence of sulphates in lacustrine dolomite

deposits comes from the effect of sulphate-reducing bacteria, which, in turn, would resultin the removal of kinetic inhibitors to dolomiteformation (Hardie, 1987; Wright, 1999). More-over, bacterial activity, in either continental ormarine depositional environments, has receivedrecent attention as a mechanism involved inthe formation of this mineral (Castanier et al.,1988; Vasconcelos et al., 1995; Vasconcelos &McKenzie, 1997; Wright, 1997, 1999; Gournayet al., 1999; Burns et al., 2000).

The signi®cance of the contribution of bacteriato dolomite formation is mainly based onlaboratory experiments, recognition of microbialcommunities in recent dolomite-forming environ-ments and petrographic evidence from modernand ancient dolomite deposits. The latter aspecthas generated considerable literature about thetextural features, usually based on SEM imagery,that provide evidence for the role of bacteria inthe formation of carbonate (e.g. Buczynski &Chafetz, 1991; Folk, 1993, 1999; DeÂfarge et al.,1996). The most common petrographic evidencefor the contribution of bacteria to the formation ofdolomite, and carbonate in general, is the pres-ence of minute elliptical or subrounded corpus-cles that are interpreted to be fossilized bacterialbodies (Vasconcelos et al., 1995; Wright, 1999).Such features are clearly observed in somedolomite samples from La Roda (Fig. 5C), wherethe morphologies of grains covering dolomitecrystal surfaces are quite representative of min-eral encapsulation of bacterial cells. Besides thisevidence, detailed observation at higher magni®-cation of crystalline aggregates in the massivedolomites of La Roda (Fig. 5D) indicates thatnannobacteria (sensu Folk, 1993, 1999) appear tobe a major component involved in the formationof the dolomite aggregates. Accordingly, thecoalescing of the submicron-sized grains wouldresult in rather well-de®ned crystals whose mor-phologies clearly resemble those of the typicaldolomite rhombs. The knobbly surfaces exhibitedby the larger rhombs are probably produced bythe presence of nannobacteria, an observationalso made previously by Vasconcelos & McKenzie(1997) from modern lake environments in Brazil.However, other crystalline aggregates forming themassive dolomites of La Roda do not show suchclear `bacterial appearance', although the pres-ence of bacterial bodies is also suggested in localdomains of the subcrystals (Fig. 5F). In this case,the better de®ned crystal faces could result froman increasing diagenetic self-organization ofthe initial minute mineralized bacterial bodies

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(Wright, 1999). This neomorphic process of crys-tal growth is in agreement with that reported fromHolocene marine dolomites (Gregg et al., 1992),which suggests that recrystallization of dolomitebegins immediately after its initial precipitation(Malone et al., 1994). Observations on the Plio-cene continental dolomites of La Roda, whichhave not undergone any signi®cant burial, couldrepresent a good analogue for the recrystallizationprocesses of dolomite reported from both con-tinental and marine settings.

Further evidence for the contribution of bac-teria to the formation of the La Roda dolomitesderives from isotopic data determined for thesedeposits. Thus, the d13C values are relativelyenriched in 12C, ranging from high ()7á72&) toslightly negative ()0á39&; Table 1, Fig. 10B),which can be related to the decomposition oforganic matter. An inorganic control on carbonisotope values should result in heavier d13Cvalues correlative with stronger evaporitic condi-tions during deposition of the dolomite (Rosenet al., 1989; AnadoÂn & Utrilla, 1993). However,the d13C values characteristic of the carbonatesthat show typical pedogenic features (i.e. theindurated limestones group in Fig. 10A) areconsistently lighter and clearly plot apart fromthe dolomite facies. Accordingly, the lighter d13Cvalues of the dolomites may result from organic-ally in¯uenced processes in which anaerobicbacteria were involved.

DISCUSSION AND CONCLUDINGREMARKS

The deposits in La Roda provide an example offossil lacustrine primary dolomites that accumu-lated in offshore areas of a shallow perennial lakeundergoing episodic ¯uctuations in lake level.The deposits are relatively young (Upper Plio-cene) and were not subjected to signi®cant burial,which results in good preservation of theiroriginal depositional features. The homogeneousfabrics shown by the massive dolomites, charac-terized by small aggregates of dolomite crystalsexhibiting poorly developed faces, suggest rapidprecipitation from highly carbonate/bicarbonateconcentrated lake waters. These conditions wereapparently more stable in the offshore areas of thelake where relatively thick (up to 1 m) dolomitebeds, which show no signi®cant variations intheir structural and textural features, were depos-ited. Both the thickness and the massive characterof the dolomites are similar to those reported from

Holocene lakes in Australia and North America inwhich offshore dolomite deposition has beenrecorded (Von der Borch & Lock, 1979; DeDeckker & Last, 1988; Rosen et al., 1988; Last,1990; and references therein; Last & De Deckker,1990; Wright, 1999). However, the stable isotopecompositions of the La Roda dolomites differfrom the d13C and d18O values commonly deter-mined in sediments characterized as `evaporitic'primary dolomites (De Deckker & Last, 1989;Rosen et al., 1989; AnadoÂn & Utrilla, 1993),suggesting that precipitation of dolomite in theLa Roda palaeolake was strongly in¯uenced bymixing between somewhat concentrated lakewaters and meteoric waters supplied by ground-water ¯ow. In this regard, it should be noted thatthe La Roda deposits are unique in lackingassociation with marine sulpho-chloride or sodiccontinental sediments. Thus, these lacustrinedolomites clearly re¯ect the predominance ofthe Mesozoic dolostones as a solute source.

Owing to a lack of well-exposed outcrops anddrillhole documentation, the depositional modelof La Roda fails, however, in the de®nition of themarginal areas of the lake system. In Holocenelake dolomite-forming environments, such asthose of the south-eastern Australian region (e.g.Coorong, Pellet, Hayward, Pillie, central andwestern Victoria lakes; Last, 1990), the marginallake areas commonly show a variety of lithologiesthat include both calcium (aragonite, low- andhigh-magnesian calcite) and magnesium-richcarbonates (hydromagnesite, magnesite, huntite,nesquehonite, dolomite), besides calcium andmagnesium sulphates and chlorides (De Deckker,1988; Warren, 1999) associated with marineaerosols. Thus, these marginal areas representcomplex depositional environments in which thebiogenic contribution by cyanobacterial mats andplants is noteworthy (De Deckker, 1988; Wright,1999). In the La Roda deposits, the variedsedimentary association commonly recognizedin recent marginal lake environments is probablyrepresented by the porous carbonate facies inter-bedded with the massive dolomites, usuallycapping the carbonate sequences. Recognition ofbiogenic structures, root-related fabrics, desicca-tion cracks and occasional moulds of lenticulargypsum within the porous carbonate indicatesthat this facies formed under shallower lakelevels (even subaerial) than the massive dolomite.Accordingly, the porous carbonate facies is sim-ilar to sediments forming in lake margin areas atthe time when the offshore dolomites accumu-lated. This interpretation is compatible with

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similar mineralogical, geochemical and isotopiccompositions for the porous carbonate and themassive dolomite.

The presence and amount of organic matter inthe original dolomite deposits remains an openquestion. The bright white colour (witness thename `white earth' used for these materials) of thedolomite beds suggests net oxidizing conditionsduring or after the formation of the dolomites.Many reported occurrences of recent lacustrinesediments are associated with variable amountsof organic matter present as disseminated organicparticles or as interbedded organic-rich horizons(De Deckker, 1988; Vasconcelos & McKenzie,1997). However, organic matter is easily lost byoxidation when the deposit remains near thesurface, which was apparently the case for the LaRoda dolomite deposits. Oxidation of organicmatter leading to a drastic diagenetic change fromdark or brownish colours into a pure whitesediment has been documented in other organic-rich lacustrine deposits, such as diatomite (Calvoet al., 1998). At least some organic matter wasoriginally present within the lake sediments ofLa Roda, as indicated by abundant bacterialmorphologies in the carbonate crystals, whichprovide evidence that dolomite growth wasin¯uenced by microbial activity. In conclusion,high alkalinity and bacterial agents are thought tohave been the most important factors involved inthe formation of the La Roda dolomites.

ACKNOWLEDGEMENTS

The authors are indebted to David Benavente(University of Alicante) for helping in computertreatment of data, and to Alfonso Rodriguez(Electronic Microscopy Centre, UCM) and AndreÂsAmoroÂs (UA) for their assistance with SEMimagery. We also thank Jessica Hopple in TylerCoplen's laboratory at the USGS for aid withisotopic analyses. We are grateful to Sedimentol-ogy reviewers Dr Robert Folk and Dr William Lastfor their encouraging comments, and to Dr PeterMozley for his careful editorial work. Funding forthis work was provided by the DireccioÂn Generalde EnsenÄanza Superior of Spain through projectPB97-0244.

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