The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain)

Chemical Geology 200 (2003) 339 – 357 www.elsevier.com/locate/chemgeo

The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain) D.I. Cendo´n a,*, C. Ayora b,1, J.J. Pueyo c,2, C. Taberner b,1 a

School of Geosciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia b Institut de Cie`ncies de la Terra Jaume Almera, CSIC, 08028 Barcelona, Spain c Dept. de Geoquı´mica i Petrologia, Universitat de Barcelona, 08028 Barcelona, Spain Received 16 October 2002; accepted 27 May 2003

Abstract Over the last decade, our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record. The detailed findings in medium- to large-sized evaporitic basins were shadowed with a relevant question: can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin? This paper addresses that question; the results obtained are compared with results from another distant point within the basin. The general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations. The chemical evolution of an Upper Eocene evaporitic sequence from the South Pyrenean foreland basin (Spain) has been investigated along the Su´ria-19 borehole record. Detailed petrographic and mineralogical study, X-ray microanalysis of frozen primary inclusions trapped in halite (Cryo-SEM-EDS), systematic isotopic analysis (d34S and d18O in sulphates) and computerbased evaporation models have been integrated in a multi-proxy methodology. This study revealed that a variable amount of Ca excess is required throughout different parts of the marine Lower Halite Unit (LHU) for sylvite, instead of K – Mg sulphates, to form. This Ca excess is in turn different from that required for the western sector of the same evaporitic basin (Navarrese subbasin). Quick and variable changes in Ca-rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry. The general hydrological evolution of the Catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input (up to 50% of total input) had an important control on the geochemistry of the subbasin. A second stage was determined during potash precipitation, in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin. Coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources, estimated to change from a 100% (total mass of sulphate) Eocene source to 20% Eocene and 80% Triassic (Keuper) towards the latest stage of potash precipitation. The results obtained have been compared with results from the Navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole Upper Eocene South

* Corresponding author. Fax: +61-2-4221-5703. E-mail addresses: [email protected] (D.I. Cendo´n), [email protected] (C. Ayora), [email protected] (J.J. Pueyo), [email protected] (C. Taberner). 1 Fax: +34-93411-0012. 2 Fax: +34-93402-1340. 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2541(03)00195-5

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Pyrenean basin. Local geochemical variations within the Upper Eocene south Pyrenean basin are explained by the differences in paleogeographical setting of the Navarrese and Catalan subbasins. D 2003 Elsevier B.V. All rights reserved. Keywords: South Pyrenean foreland basin; Evaporites; Sylvite; Fluid inclusions; Sulphate isotopes

1. Introduction Evaporite sequences supply information on the depositional environment and hydrology of restricted basins. Studies have classically been based on mineral associations and provided general information on the type of basin, marine vs. continental, and the degree of basin restriction (Strakhov, 1970; Braitsch, 1971, among many authors). The wide use of stable-isotope geochemistry of sulphates has also helped to distinguish the origin and nature of complex processes such as recycling of older evaporites or oxidation – reduction processes (Holser and Kaplan, 1966; Nielsen, 1972; Birnbaum and Coleman, 1979; Claypool et al., 1980; Fontes et al., 1991). More recently, the direct analysis of the fluids trapped as inclusions in halite crystals has provided valuable, new first-hand information on the chemistry of brines in ancient and recent evaporite basins (Petrichenko, 1973; Holland

et al., 1986; Lazar and Holland, 1988; Das et al., 1990; Horita et al., 1991, 1996; Timofeeff et al., 2001). In recent years, the chemical and hydrological evolution of evaporite basins have been studied and used in different palaeoenvironmental reconstructions (Ayora et al., 1994a,b; Williams-Stroud, 1994; Horita et al., 1996; Fanlo and Ayora, 1998; Brennan and Lowenstein, 2002, among many authors). These studies have been based on samples collected from one to various boreholes or mine galleries, commonly in closely located areas. In general, most samples come from within the basin’s main depocenter. However, evaporite basins found in the geological record can extend from a few up to thousands of square kilometres. It is precisely in medium- to large-sized evaporitic basins that an environmental interpretation based on closely located samples could be biased. This study approaches that issue by interpreting and

Fig. 1. Geological sketch of the South Pyrenean Tertiary basin. Distribution of Upper Eocene evaporites known from outcrop, mine and borehole information. Simplified from Rosell and Anadon; in Rosell, 1990.

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comparing the evolution of the evaporite sequence in two distant points within the same basin. The Upper Eocene South Pyrenean evaporite basin is divided into two subbasins (Navarrese and Catalan) 300 km apart. Both contain a lower marine evaporite sequence and potash deposits (Fig. 1). The evolution of the Navarrese subbasin was reconstructed using the Biurrun borehole and was presented in Ayora et al. (1994b, 1995). The Catalan subbasin shows similar lithological units, although thicker than those of the Navarrese subbasin, except for the terminal evaporitic stage. The objectives of this study are threefold: (1) to reconstruct the hydrological and chemical evolution of the Upper Eocene Catalan subbasin, an active potash mining district; (2) to assess the methodology’s sensitivity to detect geochemical variations and record the effect of paleogeographic differences on a wide basin scale; and (3) to check if the evolution concluded for the Navarrese subbasin can be extended to the entire South Pyrenean basin.

2. Geological setting The South Pyrenean foreland basin formed due to the Iberian and Euroasian plate convergence from the upper Cretaceous to lower Miocene. The basin was a NW – SE-trending trough connected to the sea at its NW edge and confined between the Pyrenees to the north and the Ebro Massif to the south. The evolution of the basin is related to thrust emplacement and uplift of the Pyrenees, being filled by marine sediments with continental sediments at the top. During the Upper Eocene, a period of evaporite formation took place at the transition from marine to continental conditions (see Taberner et al., 1999; Lo´pez-Blanco et al., 2000 for a detailed chronostratigraphy). Two main evaporite depocenters developed: the Catalan subbasin to the east and the Navarrese subbasin to the west (Fig. 1). The structural setting of the whole South Pyrenean basin, and the detailed petrographical and geochemical correlation of sequences during evaporite precipitation, suggest that both depocenters belong to a unique evaporite basin (Pueyo, 1975; Ortı´ et al., 1985; Rosell and Pueyo, 1997; Cendo´n, 1999). Moreover, in both the Catalan and Navarrese sectors, the evaporite sequences occur above deltaic units of about the same age and below

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fluvio-lacustrine deposits. Thus, continuity between both depocenters and similar age for the evaporitic deposits appears conclusive. In both depocenters, the evaporite sequence shows a complete cycle of typical magnesium sulphate-free chemistry. The cycle can be divided into four units, which from base to top are as follows: (1) Basal Anhydrite Unit (BAU); (2) Lower Halite Unit (LHU); (3) Potash Unit (PU), made up of sylvite + halite at the base and carnallite + halite towards the top; and (4) Upper Halite Unit (UHU). From the margins to the centre and from early evolution to late, the basin is made up of deltaic sandstones and marls, reef platforms, stromatolitic carbonates, a belt of gypsum and a central chloride zone. By increasing the proportion of detrital interbeds in the Upper Halite Unit, the evaporite sequence gives way to clays with anhydrite nodules, which are interpreted as sabkha facies (Rosell and Ortı´, 1981). A thick and continual formation of conglomerates, sandstones and lutites of continental origin covered the evaporite sequence. The total thickness of the evaporite sequence is variable, being thicker (up to 300 m) in the Catalan subbasin. The Navarrese subbasin developed a thinner sequence (up to 150 m). Cores drilled through the Catalan depocenters indicate that the PU has a lenstype morphology located in a central position with lateral extent smaller than that of the LHU. Inside the PU, carnallite beds occupy a slightly larger area than sylvite, and the UHU covers a larger area than the PU. The borehole information available from the Navarrese subbasin confirms this described pattern.

3. Materials and methods The studied samples from the Catalan subbasin come from borehole Su´ria-19, located about 70 km northwest of Barcelona (Fig. 1). The borehole is 770 m long and intersects the whole evaporite sequence. A schematic representation of the borehole record and the location of samples collected are illustrated in Fig. 2. Forty samples from the main evaporite units were prepared for petrographic observation. Conventional optical microscopy and scanning electron microscopy with energy-dispersive microanalysis (SEM-EDS) were used to examine textures and mineral associa-

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Fig. 2. Stratigraphic section of the evaporite sequence intersected by the Su´ria-19 borehole. The location of analysed samples is detailed at the right side of the figure.

tions. Bulk sample mineralogy was determined by Xray diffraction (XRD). The bromine content in halite was determined by X-ray fluorescence (XRF). The samples were first crushed and flushed with ethanol to allow brines retained in fluid inclusions, or in the intercrystalline space, to be liberated (Moretto, 1988). This methodology ensures that the bromine analysed mainly represents that of the solid fraction. Primary fluid inclusions from growth bands in halite crystals (hoppers and plates) were selected for microanalysis. The electrolyte composition of the fluid inclusions was determined by direct X-ray microanalysis of frozen fluid inclusions (Cryo-SEMEDS), following the methodology described in Ayora et al. (1994a). This method allows the quantitative analysis of a representative group of inclusions in the

same crystal with sizes exceeding 15 Am. The Na, Mg, K, Ca, Cl and S (expressed as SO4) contents were measured in each fluid inclusion. Detection limits

Fig. 3. Conceptual model of the input and output flows in a hydrologically open basin: QE = evaporation; QSW = seawater recharge; QRW = continental surface and groundwater recharge; QHW = continental recharge recycling marginal evaporites (hal = halite, gyp = gypsum); QL = leakage by reflux or lost to aquifers; P = precipitates.

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Fig. 4. Relationship between the restriction index ( QL/QI) and the mineral paragenesis expected from evaporation of seawater with the addition of a small amount of Calcium ( QCa). Anh = anhydrite; car = carnallite; c = carbonates (calcite and dolomite); gyp = gypsum; hal = halite; syl = sylvite.

increase as the atomic number decreases. Thus, the detection limit is 0.01 mol/kg for Ca and K, 0.05 mol/ kg for SO4, and 0.3 and 0.6 mol/kg for Mg and Na, respectively. Although the analytical error varied for each element, it never exceeded 10%. Fulfilling the charge balance and the halite saturation criteria ensured the consistency of the results. The isotopic composition of sulphates—either from anhydrite layers or from sulphates dispersed in

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halite, sylvite or carnallite as accessory minerals—was determined from BaSO4 precipitated from the dissolved sulphates. The oxygen isotopic composition was determined according to the method of Longinelli and Craig (1967), and that of sulphur using the method of Filly et al. (1975). The results are reported in the classical d18Ox vs. SMOW notation for oxygen and d34Sxvs. CDT for sulphur. Analytical precision for duplicates of samples and internal standards was better than F 0.3xfor both sulphur and oxygen. Numerical calculations permit simulation of brine evolution by quantifying the amount and types of precipitates formed after the evaporation of a certain volume of brine. The calculations are based on the conceptual model of a hydrologically open basin (Sanford and Wood, 1991). An equal volume of seawater, continental water, recycled evaporite water or mixtures of the three replaces the volume of water lost through evaporation and leakage (Fig. 3). One of the key parameters affecting the mineral paragenesis precipitated is the degree of restriction. This can be easily described by the leakage ratio, which is the value of leakage outflow relative to the total inflow, QL/QI. The degree of restriction which ranges in value from 0 in a closed basin, to 1 for the open ocean, is responsible for the progressive concentration of the brine and the mineral association formed (Fig. 4). The d18O and d34S values of the sulphate in the brine were calculated knowing the composition of the recharges and the enrichment factors of the sulphates precipitated. Details of the numerical model can be found in Ayora et al. (1994b, 1995). The results of the calculations for different recharge pro-

Table 1 Solute composition and isotopic composition of sulphates dissolved in inflow waters used in the model calculations (mmol/kg H2O) QSW QRW QRW2 QHW QHW2

(SU-104)

Na

K

Mg

Ca

Cl

SO4

HCO3

d34S (x )

d18O (x )

468.0 15.8 – 5900 2290

10.2 0.5 – 0.5 730

53.2 5.7 – 5.7 2210

10.2 5.5 – 30 20

545.0 13.6 – 6000 6860

28.2 5.6 – 28 90

2.4 1.8 – 1.8 n.d.

+ 20.4 + 21.6 + 13.5 + 21.4 + 19.5

+ 9.3 + 12.2 + 11.8 + 10.6 + 7.3

(1) QSW: Average modern seawater composition at 25 jC, compositional data from (Holland et al., 1986), isotopic data corresponds to Upper Eocene seawater (Ayora et al., 1995). (2) QRW: Guadalentı´n river water (MOPU, 1990). The isotope data correspond to an average of sulphates found in margins of the Eocene basin, Torrent de Calaf section, (Ayora et al., 1995); QRW2, isotope data correspond to an average of Upper Triassic sulphates from Southern Pyrenees and Iberian Peninsula (Utrilla et al., 1992). (3) QHW: River water saturated in halite and anhydrite. The isotopic composition corresponds to the mean of analyses of sulphates intercalated in the LHU; (4) QHW2: fluid inclusion analysis in halite containing the first sylvite precipitated (sample SU-104). Isotopic composition corresponds to brine in equilibrium with the sample SU-105.

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portions and degrees of restriction can be directly compared to the observed mineral associations, the concentration of brines trapped in fluid inclusions, and to the isotopic composition of the sulphates sampled throughout the sequence. The solute concentration of QSW is that of present-day seawater (Table 1), and the d values of those estimated for the Upper Eocene from available information (Ayora et al., 1995). The solute concentration of QRW was assumed equal to that of the river Guadalentin (Table 1), as representative of rivers from arid zones and recycling sulphates. The d values have been taken from an average of sulphates outcropping at the margins of the Catalan subbasin (Table 1).

4. Results 4.1. The evaporite sequence The main characteristics of the units intersected by the Su´ria-19 borehole from base to top are described below. These units were originally described in the Catalan subbasin by Pueyo (1975) and Ortı´ et al. (1985). 4.1.1. Basal Anhydrite Unit (BAU) The Basal Anhydrite Unit contains two anhydrite layers, 2 and 9 m thick, respectively, separated by a 2m-thick marl layer, of similar appearance to the underlying marls (Fig. 2). Both anhydrite layers are made up of an alternation of anhydrite and carbonate laminae. The anhydrite laminae thicken upwards and becomes massive. Petrographic studies show that gypsum crystal outlines, now replaced by anhydrite, are common in the anhydrite unit. Some bands show flat tops suggesting dissolution processes. Framboidal pyrite aggregates, concentrated in millimetre-thick layers or dispersed, also exist within the anhydrite layers. In some samples, pyrite rims gypsum pseudomorphs with secondary anhydrite cores, suggesting sulphate reduction processes. Bottom-growth selenitic gypsum pseudomorph-crystals indicate that gypsum was the primary sulphate phase, at least in the lower part of both layers where laminae are well preserved. The BAU could be the lateral equivalent of the gypsum belt (marginal sulphates) found in higher topographic areas, mainly around the

eastern perimeter of the basin. Increasing restriction of seawater led to halite precipitation in deeper parts of the basin, leaving the marginal gypsum belt perched above the halite-forming basin. The sedimentary textures described (lamination and vertical growth of gypsum) indicate that gypsum and carbonate were the primary minerals in the BAU of the Catalan subbasin. Similar textures are also present in the samples from the Navarrese subbasin (Ayora et al., 1995; Rosell and Pueyo, 1997). 4.1.2. Lower Halite Unit (LHU) The Lower Halite Unit consists of 90 m of coarsegrained halite displaying a banded structure. Two parts can be distinguished on the basis of their macroscopic appearance. The lower part comprises up to 80 m and is formed of white-grey halite layers up to 10 cm thick, alternating with darker halite up to 3 cm thick. Microscopic observations of the lower part reveal further differences. The first few metres of the LHU show dissolution textures in the halite such as anhydrite pseudomorphs after halite with crystals growing inward to fill dissolved cavities, sulphate bands oblique to the stratification and lack of primary halite textures. All these textures are of diagenetic origin, and disappear gradually upwards. Just over 6 m above the bottom of the LHU, the presence of primary hopper textures in the crystal centres surrounded by transparent halite overgrowths is prominent. Discontinuities indicating exposure or desiccation, and textures of competitive vertical growth, such as chevron or cornet textures (Arthurton, 1973) characteristic of very shallow water, are absent. In the upper 10 m of the LHU, the halite layers are thinner and separated by continuous layers of clay and anhydrite. The halite is orange in colour due to inclusions of hematite. Under the microscope, this halite is composed of platy crystals, and hopper shapes are scarce. The change from grey –white to orange halite is completed in less than a metre with no indication of exposure or interruption in the precipitation. Anhydrite is the main sulphate mineral detected by XRD in the whole LHU. It occurs as millimetre-scale anhydrite laminae alternating with clays, prismatic crystals trapped in halite, and filling pore spaces between halite crystals. Anhydrite crystals occurring between halite are partially replaced by polyhalite in the upper LHU. Celestite appears in the lowest part of

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the LHU trapped in anhydrite, with generally euhedral – subeuhedral crystals ranging from 10 to 100 Am. Celestite and polyhalite are also found in the Biurrun borehole in the first metres of the LHU (Garcı´aVeigas, 1993), suggesting similar diagenetic processes for the LHU in both subbasins. Small quantities of sylvite are observed in the uppermost part of the LHU filling intergranular porosity between some halite crystals. 4.1.3. Potash Unit (PU) The Potash Unit consists of a lower Sylvite Member and an upper Carnallite Member. The Sylvite Member is mined for potash. In the Su´ria-19 borehole, the Sylvite Member is 5 m thick and made of centimetre-scale cycles. Each cycle is made up of a millimetre-thick clay and sulphate laminae ( + dolomite), followed by orange-red halite ( + anhydrite and/ or polyhalite + dolomite), similar to that described for the upper LHU, and finally sylvite ( + halite + anhydrite and/or polyhalite). These mineralogical sequences are of sedimentary origin and identical to those described in the Navarrese subbasin (Ayora et al., 1994b; Cendo´n et al., 1998). They are also similar to those found in the Oligocene Rhine Graben (BlancValleron and Gannat, 1985) interpreted as sedimentary in origin (Lowenstein and Spencer, 1990). The microscopic observations reveal that the sylvite beds consist of a mixture of sylvite and halite in variable proportions with sylvite enclosing halite crystals poikilitically. Sylvite has a bright red colour due to hematite inclusions. The Carnallite Member, with a thickness of 43 m, forms the major part of the Potash Unit. It is made up of carnallite beds with red halite intercalations progressively more abundant upwards. In mine galleries and other boreholes in the region, the uppermost orange-red halite in the Carnallite Member can be up to 35 m thick (Rosell and Pueyo, 1997). Carnallite beds are brecciated or massive and composed of anhedral red crystals of carnallite in a pink matrix. The matrix is a massive aggregate of carnallite with scattered clays, anhydrite and halite. Halite beds in the Carnallite Member are similar to those of the upper LHU and Sylvite Member. The increase of the halite/carnallite ratio as well as the abundance of detrital material upwards in the Potash Unit suggests an increasing continental input

345

in the evaporitic basin. In the Navarrese subbasin, the Sylvite and Carnallite Members show similar characteristics to the ones described here. 4.1.4. Upper Halite Unit (UHU) The Upper Halite Unit is poorly represented in the Catalan subbasin and consists of 3 m of banded halite and anhydrite in the Su´ria-19 borehole. In the Navarrese subbasin the UHU is up to 75 m thick. All lithofacies are similar to the upper LHU. This unit shows an alternation of well-defined centimetre-scale (2– 10 cm) halite layers with thinner (1 –3 cm) laminae of clays and anhydrite. 4.2. Bromine content in halite The bromine profile in the Su´ria-19 borehole shows three trends in the LHU (Fig. 5). The lower 12 m of the LHU has low Br contents in halite that increase steadily from 11 to 73 ppm. The middle part of the LHU displays Br values in halite ranging from 73 to 92 ppm at the top, with an average value of 80 ppm. The upper 10 m of the LHU shows a sharp increase in the Br contents from 92 to 220 ppm. The samples with high Br in halite are from the orange halite of the upper LHU. The Br evolution recorded in Su´ria-19 is similar to that of other boreholes in the subbasin (Ortı´ et al., 1985). The bromine concentrations in halite from the lower part of the LHU are below 65 ppm, a value expected for the first halite precipitated from modern seawater (McCaffrey et al., 1987). This is interpreted as the result of partial dissolution/precipitation diagenetic processes and is supported by the petrographic observations of dissolution textures at the base of the LHU. The Br content in the halite from most of the LHU, however, can be interpreted as typical of evaporation from seawater. The sharp increase in bromine in the upper LHU is associated with the increase in restriction that led to sylvite precipitation. That change is coincident with the increase in detrital components and iron oxides in the upper LHU. No samples have been analysed for Br in the halite between sylvite due to sylvite – carnallite contamination problems. A sample (SU-128) from the halite of the UHU yielded a concentration of 128 ppm. This high content can be attributed to recycling of potash

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Fig. 5. Evolution of bromine content in halite and main solutes in primary fluid inclusions in halite. Beside the trends, a simplified lithostratigraphic column indicates the main evaporitic units where samples come from.

minerals. As evaporation progresses Br is mainly accumulated in the brine, with the latest minerals to precipitate (potash-minerals) incorporating more Br than halite. The Br evolution in the Catalan subbasin is equivalent to the one registered in the Navarrese subbasin (Garcı´a-Veigas, 1993; Ayora et al., 1994b) during the LHU precipitation, except for the low values in the lower part of the Su´ria-19 LHU which are probably of diagenetic origin. Evolution of Br after potash precipitation is also equivalent to the Navarrese subbasin. 4.3. Fluid inclusions composition The major solutes in fluid inclusion brines (Na, K, Mg, Ca, Cl and SO4) were obtained from 17 halite samples. The mean of the analyses performed in each sample (between 4 and 10) and their trends are found in Table 2 and Fig. 5, respectively. Na and Mg contents show an opposite evolution from base to top of the LHU. Na is more abundant than Mg in most of the LHU with a tendency to higher Na concentrations towards the middle half of the LHU. Mg concentration increases at the top of the LHU, espe-

cially in the samples from the upper LHU, indicating the evolution to a more restricted basin. The evolution of the SO4 and Ca concentrations are opposed. In the lower half of the LHU, SO4 remained under its detection limit ( < 0.05 mol/kg H2O), whereas in the rest of the unit Ca was undetected ( < 0.01 mol/kg). Since anhydrite is present as small solid inclusions in halite throughout the sequence, the brine was assumed to be in equilibrium with anhydrite, and the Ca or SO4 concentration was calculated assuming saturation with anhydrite when it was undetected. SO4 content increases in general from SU-27 along the remaining LHU to register a sharp fall in the PU due to polyhalite precipitation. The K concentration shows a steady increase upward in the LHU with an important drop after the first precipitated sylvite. The concentration of Cl also shows an increase as evaporation progresses. The change in the lithofacies recorded in the upper LHU is also recorded by the solute evolution, which reflects significant variations from one to the other lithofacies (Fig. 5). Apart from the high Ca concentrations recorded at the base of the sequence, the solutes evolution in the Catalan subbasin present a

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347

Table 2 Average composition (mol/kg water) and standard deviation (F r) of fluid inclusions in the halite samples from the Su´ria-19 borehole SO4

Cl

K

Ca

C.B.

S.I. Syl

S.I. Car

S.I. Pol

Potash Unit (halite in Carnallite Member) SU-112 653.5 3 0.98 3.44 (0.18) (0.02)

Samples

D (m)

n

0.03 (0.01)

8.75 (0.45)

0.14 (0.001)

0.01 (0.001)


0.09 (0.08)

0.71 (0.11)

0.97 (0.11)


3.27 (0.52)

Upper part LHU, change in cyclicity SU-107 672.2 5 1.55 (0.03) SU-105 676.6 10 0.87 (0.27) SU-104 678.6 6 2.29 (0.04)

3.04 (0.04) 3.61 (0.43) 2.21 (0.04)

0.18 (0.01) 0.18 (0.08) 0.09 (0.01)

6.65 (0.03) 8.01 (0.18) 6.86 (0.06)

0.40 (0.01) 0.42 (0.17) 0.73 (0.12)

0.01 (0.001) 0.01 (0.01) 0.02 (0.001)

0.14 (0.01) 0.02 (0.06) 0.06 (0.01)


0.44 (0.02)
0.28 (0.11)
0.29 (0.07)


1.42 (0.03)
0.69 (0.18)
1.61 (0.08)


0.75 (0.06)
0.43 (0.74)
1.01 (0.16)

1.15 (0.29) 1.37 (0.05) 1.27 (0.05) 1.06 (0.02) 1.15 (0.04) 1.44 (0.13) 1.38 (0.02) 1.75 (0.01) 1.41 (0.02) 1.57 (0.23) 1.74 (0.05) 1.62 (0.18) 1.49 (0.08)

0.07 (0.02) 0.11 (0.01) 0.11 (0.01) 0.10 (0.01) 0.10 (0.01) 0.15 (0.02) 0.04 (0.00) 0.02 (0.00) 0.04 (0.00) 0.04 (0.03) 0.03 (0.02) 0.03 (0.00) 0.03 (0.04)

6.55 (0.20) 6.75 (0.48) 6.33 (0.26) 6.73 (0.28) 6.37 (0.14) 6.34 (0.48) 6.83 (0.04) 6.42 (0.21) 6.86 (0.03) 6.39 (0.39) 6.28 (0.25) 7.03 (0.25) 6.14 (0.23)

0.47 (0.14) 0.58 (0.03) 0.42 (0.04) 0.34 (0.01) 0.30 (0.01) 0.32 (0.01) 0.21 (0.01) 0.19 (0.00) 0.29 (0.02) 0.18 (0.04) 0.23 (0.03) 0.21 (0.02) 0.17 (0.02)

0.03 (0.01) 0.02 (0.00) 0.01 (0.00) 0.01 (0.00) 0.01 (0.00) 0.01 (0.00) 0.06 (0.04) 0.10 (0.01) 0.06 (0.06) 0.05 (0.01) 0.06 (0.02) 0.04 (0.01) 0.06 (0.02)


0.01 (0.06)
0.03 (0.11) 0.06 (0.04)
0.04 (0.06) 0.04 (0.02) 0.04 (0.11)
0.08 (0.01) 0.06 (0.05)
0.07 (0.02) 0.04 (0.07) 0.08 (0.02)
0.10 (0.05) 0.09 (0.05)


0.65 (0.16)
0.50 (0.03)
0.67 (0.05)
0.75 (0.03)
0.82 (0.01)
0.76 (0.03)
0.93 (0.03)
0.93 (0.01)
0.78 (0.03)
1.00 (0.13)
0.87 (0.05)
0.91 (0.03)
1.04 (0.06)


2.55 (0.06)
2.22 (0.18)
2.56 (0.13)
2.63 (0.10)
2.76 (0.02)
2.55 (0.21)
2.57 (0.05)
2.51 (0.08)
2.42 (0.03)
2.69 (0.33)
2.51 (0.04)
2.38 (0.10)
2.84 (0.15)


2.03 (0.31)
1.30 (0.15)
1.92 (0.34)
2.16 (0.38)
2.53 (0.10)
1.54 (0.20)
2.65 (0.53)
3.71 (0.05)
2.44 (0.55)
2.68 (0.31)
2.65 (0.45)
3.05 (0.13)
3.14 (0.59)

LHU SU-103

679.7

8

SU-102

682.8

9

SUA-101

684.3

8

SUA-35

690.9

6

SUC-32

697.6

3

SUA-28

710.1

7

SU-27

712.3

6

SU-26

715.7

5

SUA-26

715.7

2

SU-23

723.1

9

SU-21

730.3

4

SU-18

736.9

5

SU-17

742.9

5

Na

3.83 (0.35) 3.42 (0.31) 3.97 (0.12) 4.15 (0.34) 4.21 (0.09) 3.68 (0.33) 3.28 (0.07) 2.96 (0.12) 3.23 (0.07) 3.40 (0.43) 3.14 (0.13) 2.89 (0.27) 3.57 (0.22)

Mg

The mean of the saturation index of the analyses with respect to some evaporite minerals is also presented. D = location of the samples in the borehole represented as depth in metres from the surface. n number of fluid inclusions analysed. Syl = sylvite, Car = carnallite, Pol = polyhalite.

parallel trend to the one registered in the Navarrese subbasin. 4.4. Sulphate isotope composition The isotopic composition (d34S and d18O) of sulphates has been determined in 58 samples

obtained from the different lithological units of the Su´ria-19 borehole (Table 3). The analysed samples show a wide range of isotopic compositions. They can be grouped, however, according to the lithological unit they come from. Samples from the basal anhydrite (layer 1 and lower part of layer 2) have an isotopic composition distinctly heavier than the rest of

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348

Table 3 Isotopic composition values of sulphates from the Su´ria-19 borehole Sample

Depth (m)

Lithofacies

d34SCDT (x )

d18OSMOW (x )

Sample

Depth (m)

Lithofacies

d34SCDT (x )

d18OSMOW (x )

204 202 200 132 130 129 127 124 123 121 120 119 117 112b 109 107 105 103 101 35 30 28 24 20 19 16 13 12.14 12.13

533.60 552.70 570.30 615.40 616.30 621.30 622.50 625.70 630.00 633.90 637.30 640.80 645.90 653.50 660.50 672.20 676.60 679.70 684.30 690.90 702.20 710.10 719.90 730.50 733.50 746.90 754.20 755.30 755.31

anh noduls anh noduls anh noduls anh + marls anh + marls UHU hal + anh

+ 14.58 + 16.16 – + 15.97 + 16.43 + 18.32 + 18.90 + 20.27 + 20.47 + 21.14 + 20.36 + 20.12 + 19.96 + 20.42 + 20.24 + 21.03 + 21.11 + 21.46 + 20.86 + 21.26 + 21.26 + 21.65 + 21.40 + 21.53 + 20.98, + 22.09 + 21.71 + 21.55 + 21.86 + 22.11

+ 15.06 + 13.24 + 15.66 + 11.45 + 11.22 + 9.84 + 9.58 + 9.08 + 9.72 + 9.31 + 9.08 + 7.72 + 8.16 + 9.27 + 10.91 + 9.89 + 10.80 + 10.27 + 10.16 + 11.14 + 10.89 + 10.75 + 10.59 + 10.53 + 11.03 + 10.97 + 10.68 + 11.30 + 11.37

12.12 12.11 12.10 12.9 12.8 12.7 12.6 12.5 12.4 12.3 12.2 12.1 11 10 9 8 7 6 5 4 3 3.2 3.1 2.4 2.2 2.1 1.3 1.2 1.1

755.35 755.39 755.40 755.41 755.42 755.43 755.44 755.45 755.46 755.47 755.48 755.49 755.70 756.40 756.80 759.40 762.20 763.30 763.80 764.40 764.90 764.95 764.99 766.90 766.95 766.99 769.10 769.15 769.19

hal – anh transition

+ 21.55 + 21.63 + 21.43 + 21.86 + 22.09 + 22.38 + 22.63 + 21.60 + 21.53 + 21.71 + 21.93 + 22.21 + 22.09 + 21.28 + 21.22 – + 22.58 + 22.08 + 22.76 + 22.80 + 24.21 + 23.00 + 23.02 + 23.63 + 23.47 – – + 24.48 + 25.48

+ 11.73 + 11.31 + 11.32 + 11.67 + 11.45 + 11.85 + 11.70 – + 11.13 + 11.00 + 11.22 + 11.50 + 11.03 + 11.33 + 12.12 + 12.32 + 13.11 + 12.87 + 12.81 + 13.13 + 13.65 + 14.62 + 14.50 + 13.49 + 14.44 + 14.28 + 15.42 + 14.46 + 14.59

carnallite

UP-LHU (orange) LHU (white)

hal – anh transition

2nd anhydrite layer

1st anhydrite layer

The location of the samples in the borehole has been represented as metres below the surface. The lithofacies are also detailed.

the samples (d34S= + 25.48xto + 23.00x, and d18O=+ 15.42xto + 13.11x). The samples from the upper part of the Basal Anhydrite Unit show a general decrease in their d values with small oscillations in the anhydrite – halite transition. The samples from the LHU show generally constant isotope values (d34S=+ 21.71xto + 21.03x , and d18O= + 10.97xto + 9.89x). The lower half of the PU (Sylvite Member and lowermost Carnallite Member) shows a decreasing trend, especially in the oxygen isotope ratios (d18O= + 10.91x to + 7.72x). The decrease could have initiated in the upper LHU, although not enough samples are available. The upper half of the PU (Carnallite Member) shows a moderate increasing trend, especially evident in the oxygen isotope ratios (d18O= + 7.72x to 9.08x). After the potash formation, the samples

from the UHU and anhydrite nodules within the continental unit show an abrupt trend toward lighter sulphur and heavier oxygen isotope composition (d34S=+ 18.90xto + 14.58x, and d18O= + 9.58x to + 15.06x). The general trend of the isotopic compositions shows a decrease in the values of sulphates from the basal anhydrite up to the carnallite (Fig. 6). The decrease is not continuous, but is coincident with major changes in lithology: from basal anhydrite to halite and from halite to sylvite. The isotopic values tend to be roughly constant within each lithological unit. The sharp changes suggest two major reservoir effects associated with increases in the degree of restriction of the basin. After carnallite precipitation, the S and O isotopes show a trend to values approaching Keuper sul-

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Fig. 6. Isotopic composition of sulphates from Su´ria-19 borehole. The star represents the average values from the Torrent de Calaf section (Ayora et al., 1995). The cross and its associated error bars correspond to the isotopic composition of the first precipitated sulphate from Eocene seawater. The areas marked correspond to the range of enrichment ratios found for bacterial reduction processes in laboratory and natural environments (Mizutani and Rafter, 1973).

phates from the area, suggesting the importance of recycling.

5. Interpretation of results: brine evolution of the subbasin 5.1. Basal Anhydrite Unit (BAU) In order to simulate the BAU precipitation, an initial solution with the composition of seawater was calculated to evaporate with seawater replacing the evaporated volume ( QSW = 1). The range of possible restriction indices in which sulphate (gypsum or anhydrite) precipitates from seawater is quite broad ( QL/QI = 0.41 – 0.11, see Fig. 4). The presence of anhydrite pseudomorphs after selenitic gypsum at the bottom of both anhydrite layers suggests a tendency towards higher QL/QI values in the original brines, that is, a less restricted basin. For example, the evaporation with a QL/QI value of 0.30 reaches a steady state, under which only carbonate and gypsum precipitates. Immediately overlying the first sulphate layer is a marine marl bed which indicates a temporary loss of basin restriction. The precipitation of the

349

second anhydrite layer is simulated with the same parameters as the basal anhydrite. Upwards in the second anhydrite layer, lamination disappears together with any petrographic evidence of primary gypsum. The presence of halite laminae toward the top of the second anhydrite indicates an increase in basin restriction. These changes are simulated by gradually decreasing the QL/QI value from 0.30 to 0.06, towards a more restricted basin (see Fig. 4). The evaporation of seawater with the parameters described ( QSW = 1; QL/QI = 0.41 –0.06) and the isotope values assumed for the Upper Eocene ocean (Table 1) do not match the values analysed for first anhydrite layer (Fig. 7A). Petrographic observation of the BAU including millimetric-scale layers of pyrite as well as framboidal pyrite dispersed in between sulphates suggests that bacterial sulphate reduction may have occurred. The enrichment ratios (D34S/D18O) of those samples with respect to the average isotope composition of the BAU are between 4 and 2.5 (Fig. 6). These values are within the range of enrichment found for bacterial reduction processes in laboratory and natural environments (Mizutani and Rafter, 1973; Canfield, 2001), and may account for the heavier isotopic values found in the anhydrite layers. The upper half of the 2nd anhydrite layer has some lighter isotopic values that seem unaffected by bacterial sulphate reduction processes. The evaporation of seawater with QL/QI values decreasing gradually from 0.30 to 0.06 would lead to isotope values lighter than those observed (see dotted line in Fig. 7A). An alternative scenario is the evaporation with recharge proportions of QSW = 0.5 and QRW = 0.5. The isotope values for the recharge were those of the Upper Eocene ocean and the average of the sulphates outcropping on the margins of the subbasin, respectively (Torrent de Calaf series, Ayora et al., 1995). The result of calculations matched the isotope trend found in both the d34S and d18O values for the second part of the upper layer of the BAU (Fig. 7A). The increase in the restriction and its associated reservoir effect are responsible for the decreasing slope of the trend as evaporation progresses. 5.2. Lower Halite Unit (LHU) The progress of evaporation with a higher degree of restriction allows halite to form. Mg is used to trace

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Fig. 7. Evolution of the calculated isotopic composition of precipitated sulphate (d x ) with respect to the thickness (m) of evaporite minerals (detrital fraction excluded). Measured values for Su´ria-19 borehole are also plotted, squares correspond to d18OSMOW x , circles to d34SCDT x . (A) Hydrological parameters used in the simulations: 1st anhydrite: QSW = 1, QL/QI = 0.40; 2nd anhydrite, dotted line: QSW = 1, QL/QI = 0.30 – 0.06: 2nd anhydrite, solid line: QSW = 0.5, QRW = 0.5, QL/QI = 0.30 – 0.06. See Table 1 for isotopic composition of recharge waters. (B) Hydrological parameters used in the simulations: LHU, lower half Ca-rich QSW = 0.5, QRW = 0.50, QCa = 0.011, QL/QI = 0.04 – 0.002; dotted line, QSW = 0.8, QRW = 0.2, QCa = 0.017, QL/QI = 0.07 – 0.01. LHU, upper half SO4-rich, QSW = 0.5, QRW = 0.5, QCa = 0.008, QL/QI = 0.06 – 0.004. Upper part LHU (solid line), QSW = 0.5, QRW = 0.5, QCa = 0.007, QL/QI = 0.05 – 0.0001. See Table 1 for isotopic compositions of recharge waters. (C) Hydrological parameters used in the simulations for the Sylvite Member: ( QHW = 1, QL/QHW = 0.20 – 0.10), see text for isotopic composition of the recharge water. Carnallite Member: (1) I0 ! IF ( QHW = 1.0 – 0.3, QRW = 0.0 – 0.7, QL = 0.14 – 0.02; (2) and (3) I0 ! IF ( QH = 0.3 – 0.0, QRW = 0.7 – 0.6, QHW = 0.0 – 0.4, QL = 0.05 – 0.002. (D) General isotopic evolution with respect to thickness.

the degree of evaporation because it behaves as a conservative element until Mg sulphates or chlorides precipitate. The calculated solute evolution in a fully marine scenario does not match the analyses of fluid inclusions (Fig. 8). The predicted values for Cl and Ca are lower than observed, those of Na and SO4 are

higher, and K values are both higher and lower depending on the observed samples. Moreover, glauberite is also predicted to precipitate. From the evaporation scenario previously described, SO4 is the electrolyte with higher discrepancies. As discussed for the Navarrese subbasin

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351

Fig. 8. Calculated evolution of solutes in a completely marine basin. Analysed solutes in fluid inclusions are also plotted for comparison. Both calculated and analysed solutes are represented with regards to their Mg (mol/kg water) concentration. Hydrological parameters used in the simulation: QSW = 1, QCa = 0, QL/QI = 0.07. Mineral phases precipitated at different stages are abbreviated: anh = anhydrite; dol = dolomite; gla = glauberite; gyp = gypsum; hal = halite; hex = hexahydrite; pol = polyhalite. (1) gyp + anh + dol; (2) anh + hal + dol; (3) anh + gla + hal + dol; (4) anh + pol + hal + dol; (5) anh + hex + pol + hal + dol.

(Ayora et al., 1994b), two main hypotheses may be suggested to explain the depleted values of SO4 in the analyses: (1) bacterial sulphate reduction, and (2) the addition of Ca into the basin and removal of SO4 by precipitating Ca sulphates. The calculations including the bacterial reduction of SO4 to H2S, result in solute proportions very different from those observed in the fluid inclusions (not represented). Moreover, sulphate reduction would cause a sharp increase in the d34S of the residual brine, whereas the d34S of the sulphates interbedded in the sequences is fairly constant (Table 3). Other process such as re-oxidation of H2S, as described in the Giraud Saltworks (Pierre, 1982), would cause a sharp increase in the d18O not affecting the d34S, and this trend is not observed. Alternatively, two different processes may cause the addition of Ca and subsequent depletion in dissolved sulphate: (a) The addition of Ca from a source external to the basin, and (b) dolomitization. Diagenetic, evolved meteoric and hydrothermal solutions have been reported to have high salinity in the system Na – Ca –Cl (Holwerda and Hutchinson, 1968; Lowenstein et al., 1989; Hardie, 1990). The addition of external Ca was modelled by means of the inflow of small amounts of a hypothetical solution 1 mol/kg of CaCl2 ( QCa). Since evaporation took place under

conditions of halite saturation, the excess of NaCl in the additional solution is consumed by halite formation, and only CaCl2 deserves further consideration. Dolomitization of limestones is commonly described in restricted marine environments and evaporite basins (Hite, 1983; Williams-Stroud, 1994). This process has been modelled by means of the constant substitution of a small fraction of Mg by Ca as evaporation progresses. Besides the expected decrease in SO4, the addition of CaCl2 causes an increase of Cl in the basin. This enhances halite formation and the corresponding decrease in Na. Therefore, with the addition of a particular amount of CaCl2, the predicted values match the SO4, Cl and Na analysed in fluid inclusions. The content of SO4 and Ca defines two clear parts in the LHU solute evolution (Fig. 5). The solute proportion in the first part is better matched by the evaporation in a scenario with QSW = 0.8– 0.5, QRW = 0.2 – 0.5, QCa = 0.017 –0.011 and a restriction degree QL/ QI = 0.07– 0.01 (Fig. 9). The second part of the LHU, with higher SO4 concentrations, requires the addition of less CaCl2-rich brine, QCa = 0.013 – 0.008. The transition between the first and second stages is marked by a decrease in the degree of restriction of the brine, which did not affect the halite precipitation,

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Fig. 9. Calculated and analysed evolution of solutes along the LHU in the Su´ria-19 borehole. Hydrological parameters used in the simulations: LHU (first half Ca-rich), QSW = 0.8, QRW = 0.2, QCa = 0.017 or Ca ! Mg replacement (dolomitization) of 0.0162 mol/kg of Ca for Mg, QL/ QI = 0.07 – 0.02. Less restriction QL/QI = 0.02 – 0.05. LHU (second half SO4-rich) QSW = 0.8, QRW = 0.2, QCa = 0.013 or Ca ! Mg replacement (dolomitization) of 0.0150 mol/kg of Ca for Mg, QL/QI = 0.05 – 0.004.

but was recorded in the fluid inclusions. This was modelled allowing the QL/QI to increase slightly from 0.01 to 0.05 (see Fig. 9). In the dolomitization scenario, the addition of Ca causes anhydrite to precipitate and, consequently, SO4 in the brine to decrease. Moreover, the consumption of Mg makes the Cl and Na concentration shift to the left in the plots with respect to Mg. As a result, by selecting the appropriate amount of dolomitization, the match between the predicted and analysed SO4, Cl and Na contents is achieved. The continuous replacement of 0.0162 and 0.0150 mol/kg of Mg for Ca is required to deplete the SO4 to the values measured in the first and second stages of the LHU, respectively. The plots are identical to those obtained by means of the addition of a CaCl2-rich brine (Fig. 9). No enough evidence of dolomitization in carbonates of the Catalan subbasin is available to confirm the described scenario. The addition of Ca in general results in a precipitated mineral paragenesis that matches the observations in the Su´ria-19 borehole. Due to the SO4 depletion, glauberite fails to form, and sylvite precipitates instead of Mg sulphates. The isotopic values registered along most of the LHU (except the uppermost orange halite) are nearly constant with average values of d34S= + 21.4 F 0.2x and d18O= + 10.7 F 0.3x . These values are systematically higher (+ 1x ) than the ones registered in the

Navarrese LHU. This difference cannot be attributed to analytical problems or accuracy. Ayora et al. (1995) deduced a d value for the dissolved sulphate in seawater during the Upper Eocene of d34S= + 20.0 F 0.4xand d18O= + 8.7 F 0.6x , on the basis of measurements of the isotopic composition of sulphates from the Torrent de Calaf section, part of the marginal sulphates found in the Catalan subbasin. The higher isotopic values of the sulphates in the Su´ria-19 borehole may be explained by two options: (a) the isotopic values of Eocene seawater were slightly heavier than previously deduced, or (b) the Catalan subbasin had particularities that led to heavier isotopic values in evolved stages of precipitation. The palaeogeographical position of both subbasins, with the Navarrese closer to the ocean opening, the difficulty of explaining the lighter values obtained in the Navarrese subbasin, with isotopically heavier seawater values, and the fact that the isotopic values for the Eocene seawater are deduced from selenitic gypsum facies in the Catalan subbasin leads to favour the second hypothesis. As described above, the solute evolution was matched with evaporation calculations using a range of recharge proportions ( QSW = 0.8– 0.5 and QRW = 0.2 –0.5). The isotope ratios allow the recharge proportions to be constrained. Thus, as shown in Fig. 7B, the best fit of measured and calculated d34S and d18O values was obtained for equal proportions of continen-

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tal and seawater ( QSW = 0.5 and QRW = 0.5). Higher seawater proportion leads to lighter isotope predictions (dotted line in Fig. 7B). Higher continental water does not allow the brine to reach the fluid inclusion contents measured. These results are an approximation since they depend on the sulphate concentration and isotope ratios of the recharge (Table 1). 5.3. Potash Unit (PU) The first sylvite precipitated appears dispersed in the orange halite of the upper LHU and indicates a progressive increase in basin restriction. The lack of fluid inclusions in sylvite and carnallite, and in the associated halite precluded the solute analyses, except for one halite sample of the Carnallite Member. Nevertheless, hypothetical numerical scenarios of evolution can be established using the observed mineral paragenesis and isotope analyses as constraints. The continuation of the evaporation process forming the LHU would yield the precipitation of sylvite and then carnallite. However, the decrease of the surface area occupied by the PU compared with that of the LHU suggests a reduction of the basin volume previous to potash formation. Moreover, the higher proportion of detrital clay layers in the upper LHU suggests a higher continental influence relative to seawater. The formation of sylvite – halite interbedding in the Navarrese subbasin was attributed to periodical oscillations in the continental recharge. A similar model can be applied to form the sylvite member of the Su´ria-19 borehole (Ayora et al., 1994b).

353

Carnallite occurs above the interbedded sylvite – halite. Carnallite is more abundant in the lower 7 m with ratios of carnallite/halite of c 3.5. Higher in the section, halite becomes more abundant with carnallite/ halite ratios down to 0.24 in the upper part of the Carnallite Member. Carnallite precipitation was simulated with a scenario similar to the one precipitating sylvite but with a progressively higher continental input ( QRW) upwards in the sequence (Table 4). Some of the proposed scenarios in Table 4 reach kieserite precipitation for more restricted basins. Those scenarios are ruled out, as kieserite has not been found. Based on mineral ratios and paragenesis, variable recharge values of QHW2 = 1.0 and QRW = 0.0 at the onset of evaporation (I0) to QHW2 = 0.3 and QRW = 0.7 at the end of the evaporation (I1) explain the mineral sequence in the first carnallite precipitated. This hydrological scenario also matches the decrease of the d34S and d18O values recorded in the sulphates interbedded in the Carnallite Member (Fig. 7C). The isotope value of the recharge, QHW2, is assumed equal to the brine in equilibrium with the sulphate sample SU-105 (d34SSU-105-D= + 19.46x and d18OSU-105-D = 7.3x ), obtained from the closest sulphate analysed to sample SU-104. It is from the onset of potash precipitation that the direct influence of the seawater isotope signature disappears. The basin completely cut off from direct seawater recharge underwent a reservoir effect as shown by the decrease in d values (see 1 in Fig. 7C). The trend towards lighter values eventually stops at the upper half of the Carnallite Member, then starting a

Table 4 Potash – halite ratios and mineral paragenesis obtained for different evaporation scenarios proposed for the Carnallite Member precipitation

The composition of the initial recharge brines are that of Table 1. I0 ! I1 corresponds to the variation of the different parameters along the evaporation process. Dol = dolomite; anh = anhydrite; hal = halite; pol = polyhalite; syl = sylvite; car = carnallite; kie = kieserite; bis = bischofite.

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trend towards higher d34S and d18O values. This is modelled with the increasing influence of a recharge ( QHW) with higher isotopic compositions (Fig. 10). This recharge could have been continental water recycling previously precipitated Eocene evaporites, which were exposed as expected from the surface shrinkage of the Potash Unit. Therefore, its isotopic signature was assumed equal to the average of the LHU (d34SLHU= + 21.4x and d18OLHU = 10.6x) and its composition that of halite-saturated river water (Table 1). The hydrological scenario proposed from the mineral ratios and isotopic values of the upper part of the Carnallite Member (Table 4) was able to match the increase of the d34S and d18O values recorded in the interbedded sulphates (see 2 in Fig. 7C).

towards lighter values and the d18O increases towards heavier values. This has been interpreted as the evolution in an endorheic basin with input of continental waters recycling Keuper evaporites ( QRW2 in Table 1). This process was detected by Utrilla et al. (1992) in Miocene evaporites of the South Pyrenean foreland and has also been used to explain the isotopic evolution in the Upper Halite Unit (UHU) in the Navarrese subbasin (Ayora et al., 1995; Taberner et al., 2000). A unique Triassic signature in the recharge waters ( QRW2) would lead to a very sharp decrease in the sulphur values, thus a small proportion of recycling of Eocene marginal gypsum ( QRW1) is also assumed. Mixtures of approximately 80% of Triassic and 20% of Eocene isotopic signatures reproduce the analysed trends.

5.4. Upper Halite Unit (UHU) The Upper Halite Unit (UHU) is poorly developed in the Su´ria-19 borehole. It is formed of thin anhydrite layers or nodules, halite bands and terrigenous materials. Appropriate samples for fluid inclusion analysis were not present; therefore, the geochemical evolution of this unit is only based on the d34S and d18O values of the sulphates. A Br analysis in the UHU halite (SU-128) gave a result of 128 ppm, similar to values reported for the UHU in the Navarrese subbasin. From the uppermost sample analysed in the Carnallite Member, a sharp excursion is recorded in the d34S and d18O (see 3 in Fig. 7C). The d34S decreases

Fig. 10. Conceptual model of a closed basin. Legend: QRW = continental water inflow (rivers and groundwater); QHW = continental water inflow saturated with halite; QE = evaporation outflow; QL = outflow due to reflux and leakage to aquifers.

6. Conclusions: comparison between Navarrese and Catalan subbasins The Navarrese and Catalan subbasins experienced a parallel evolution, with three separate evaporite stages: (1) A first stage, with dominantly marine influence. (2) A second stage, with the formation of the PU, indicating extreme basin restriction and the transition from marine to continental regime. (3) A third stage, in which a continental signature is imposed on two endorheic basins which have a different sedimentary record. Despite the differences in thickness, the lithofacies in both subbasins have the same macroscopic and microscopic aspect. Thickness differences can be explained and calculated as variations in the depth of the water column and/or the time in which the evaporitic basin operated as a steady state system. A more detailed comparison can be established for each stage mentioned above. In the first stage, the SO4 concentration in the LHU is always lower in the Catalan subbasin. In the lower part of LHU, SO4 is under the detection limit ( < 0.05 m), whereas Ca is detected (up to 0.10 m). In the upper part, the SO4 is detected and rises to values of up to 0.15 m. In the Navarrese subbasin, however, the SO4 ranges from 0.14 to 0.20 m and Ca is never detected. The hydrological evolution of the LHU also differs slightly in the Catalan subbasin, where the continental recharge is more important (up to QRW = 0.50) and the addition of Ca is also higher and more variable than in the Navarrese subbasin. All these differences indicate

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less influence of marine waters in the Catalan subbasin, consistent with its paleogeographic location further away from the connection to the open ocean. Both subbasins describe a remarkable parallel evolution of the d34S and d18O values throughout the entire sequence (compare Fig. 8 in Ayora et al., 1995 and Fig. 7D). However, the d34S and d18O steady values of LHU are more than 1x higher in the Catalan than in the Navarrese subbasin. Assuming a constant isotope composition for the seawater recharge and similar restriction degree ( QL/QI) in the two subbasins, as constrained by halite precipitation and fluid inclusion composition, the higher d34S and d18O values found in the sulphates from the Catalan subbasin suggest a higher recycling of marginal sulphates by the continental recharge. In the second stage, sylvite precipitation correlates to a significant drop in the d34S and d18O values of the sulphates in both Catalan and Navarresse subbasins. This is attributed to the limited reservoir effect expected from the increase in the restriction. The drop in the d values measured is only explained if the basin is closed to a seawater supply of sulphate. During the precipitation of carnallite, a new increase in the d34S and d18O is observed in the sulphates of both basins. This is explained by the recycling of the LHU. This evolution took place in both subbasins in a remarkably similar way. In the third stage of evolution proposed, both subbasins already lost any contact, direct or indirect, with seawater, and became endorheic with only continental recharge. Then, they followed different evolution patterns. A 50– 75 m thick UHU developed in the Navarrese subbasin and only one of 2 – 3 m in the Catalan subbasin. Both subbasins show a very similar isotope evolution, despite their different evolution from the PUUHU limits. An abrupt change in the sulphate isotope values takes place at the transition from the carnallite to the UHU in both subbasins. The d34S and d18O values tend towards values typical of the Triassic evaporites found in the region. Mixtures of approximately 80% of Triassic and 20% of recycling of Eocene evaporites are required to reproduce the analysed trends in both subbasins. The results for the Catalan subbasin (Su´ria-19 borehole) show an evolution very similar to the Navarrese subbasin. This gives credit to the methodology

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of interpreting the evolution of a basin from the study of a representative unique borehole. Moreover, the methodology is sensitive enough to detect small geochemical variations between the sequences when applied to different boreholes within the same basin. This potential can be used to refine palaeoenvironmental interpretations in the studied basin. The information deduced from the Su´ria-19 borehole allowed detection of recycling of marginal sulphates prior to the LHU precipitation. In the case of the South Pyrenean foreland basin, all differences between the subbasins can be attributed to the particular paleogeography of each subbasin and not to global secular variations in seawater chemistry. Acknowledgements This research was supported by the Spanish PB930165 and PB96-0270 projects. Cryo-SEM-EDS determinations were performed with the technical advice of J. Garcı´a-Veigas and R. Fontarnau in the Serveis Cientifico-Te`cnics of the University of Barcelona. Sulphur and oxygen isotope analyses were carried out at the LODYC-Universite´ Pierre et Marie Curie in Paris. We also thank the following for discussions technical and/or logistic support during this study: C. Pierre (LODYC-Paris), L. Miralles (LIFS, Universitat de Barcelona), G. Skilbeck and E. Leitch (UTS, Sydney) and C. Jordan for her patient help. Finally, we gratefully acknowledge the review of T. Lowenstein and K.C. Benison. [LW] References Arthurton, R.S., 1973. Experimentally produced halite compared with Triassic layered halite-rock from Cheshire, England. Sedimentology 20, 145 – 160. Ayora, C., Garcı´a-Veigas, J., Pueyo, J.J., 1994a. X-ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins. Geochim. Cosmochim. Acta 58 (1), 43 – 55. Ayora, C., Garcı´a-Veigas, J., Pueyo, J.J., 1994b. The chemical and hydrological evolution of an ancient potash-forming evaporite basin as constrained by mineral sequence, fluid inclusion composition, and numerical simulation. Geochim. Cosmochim. Acta 58 (16), 3379 – 3394. Ayora, C., Taberner, C., Pierre, C., Pueyo, J.J., 1995. Modeling the sulfur and oxygen isotopic composition of sulfates through a halite – potash sequence: implications for the hydrological evo-

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lution of the Upper Eocene Southpyrenean Basin. Geochim. Cosmochim. Acta 59 (9), 1799 – 1808. Birnbaum, S.J., Coleman, M., 1979. Source of sulphur in the Ebro Basin (northern Spain) Tertiary nonmarine evaporite deposits as evidenced by sulphur isotopes. Chem. Geol. 25, 163 – 168. Blanc-Valleron, M.M., Gannat, E., 1985. Cartographie de subsurface du Salife`re supe´rieur du bassin potassique de Mulhouse (Oligoce`ne, Alsace). Bull. Soc. Geol. Fr. 8 (1), 823 – 836. Braitsch, O., 1971. Salt Deposits. Their Origin and Composition. Minerals, Rocks and Inorganic Materials. Springer-Verlag, New York. 297 pp. Brennan, S.T., Lowenstein, T.K., 2002. The major-ion composition of Silurian seawater. Geochim. Cosmochim. Acta 66 (15), 2683 – 2700. Canfield, D.E., 2001. Isotope fractionation by natural populations of sulfate-reducing bacteria. Geochim. Cosmochim. Acta 65 (7), 1117 – 1124. Cendo´n, D.I., 1999. Evolucio´n geoquı´mica de cuencas evaporı´ticas terciarias: implicaciones en la composicio´n isoto´pica del sulfato disuelto en el oce´ano durante el Terciario. PhD Thesis. Universitat de Barcelona and ICT-JA (CSIC), Barcelona. 270 pp. Cendo´n, D.I., Ayora, C., Pueyo, J.J., 1998. The origin of barren bodies in the Subiza potash deposit, Navarra, Spain: implications for sylvite formation. J. Sediment. Res. 68 (1), 43 – 52. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H., Zak, I., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol. 28, 199 – 260. Das, N., Horita, J., Holland, H.D., 1990. Chemistry of fluid inclusions in halite from the Salina Group of the Michigan Basin: implications for Late Silurian seawater and the origin of sedmentary brines. Geochim. Cosmochim. Acta 54, 319 – 327. Fanlo, I., Ayora, C., 1998. The evolution of the Lorraine evaporite basin: implications for the chemical and isotope composition of the Triassic ocean. Chem. Geol. 146, 135 – 154. Filly, A., Letolle, R., Puset, M., 1975. L’analyse isotopique du soufre. Aspects techniques. Analysis 3, 197 – 200. Fontes, J., Filly, A., Gaudant, J., Duringer, P., 1991. Origine continentale des e´vaporites pale´oge`nes de Haute Alsace: arguments pale´oe´cologiques, se´dimentologiques et isotopiques. Bull. Soc. Geol. Fr. 162 (4), 725 – 737. Garcı´a-Veigas, J., 1993. Geoquı´mica de inclusiones fluidas en formaciones salinas, Microana´lisis Cryo-SEM-EDS. PhD Thesis. Universitat de Barcelona. 260 pp. Hardie, L.A., 1990. The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis. Am. J. Sci. 290, 43 – 106. Hite, R.J., 1983. The sulfate problem in marine evaporites. In: Schreiber, B.C. (Ed.), 6th International Symposium on Salt. Salt Institute, Alexandria, VA, pp. 217 – 230. Holland, H.D., Lazar, B., McCaffrey, M., 1986. Evolution of the atmosphere and oceans. Nature 320, 27 – 33. Holser, W.T., Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulfates. Chem. Geol. 1, 93 – 135. Holwerda, J.G., Hutchinson, R.W., 1968. Potash-bearing evaporites in the Danakil area, Ethiopia. Econ. Geol. 63, 124 – 150. Horita, J., Friedman, T.J., Lazar, B., Holland, H.D., 1991. The

composition of Permian seawater. Geochim. Cosmochim. Acta 55, 417 – 432. Horita, J., Weinberg, A., Das, N., Holland, H.D., 1996. Brine inclusions in Halite and the origen of the middle Devonian Prairie Evaporites of Western Canada. J. Sediment. Res. 66 (5), 956 – 964. Lazar, B., Holland, H.D., 1988. The analysis of fluid inclusions in halite. Geochim. Cosmochim. Acta 52, 485 – 490. Longinelli, A., Craig, H., 1967. Oxygen-18 variations in sulfate ions in sea water and saline lakes. Science 156, 56 – 59. Lo´pez-Blanco, M., Marzo, M., Burbank, D.W., Verge´s, J., Roca, E., Anado´n, P., Pin˜a, J., 2000. Tectonic and climatic controls on the development of foreland fan deltas: Montserrat and San Llorencß del Munt systems (Middle Eocene, Ebro Basin, NE Spain). Sediment. Geol. 138, 17 – 39. Lowenstein, T.K., Spencer, R.J., 1990. Syndepositional origin of potash evaporites: petrographic and fluid inclusion evidence. Am. J. Sci. 290, 1 – 42. Lowenstein, T.K., Spencer, R.J., Zhang, P., 1989. Origin of ancient potash evaporites; clues from the modern nonmarine Qaidam Basin of western China. Science 245 (4922), 1090 – 1092. McCaffrey, M.A., Lazar, B., Holland, H.D., 1987. The evaporation path of seawater and the coprecipitation of Br
and K+ with halite. J. Sediment. Petrol. 57 (5), 928 – 937. Mizutani, Y., Rafter, T.A., 1973. Isotopic behavior of sulphate oxygen in the bacterial reduction of sulphate. Geochem. J. 6, 183 – 191. MOPU, 1990. Ana´lisis de calidad de aguas. MOPU (Direccio´n general de obras hidrau´licas), Madrid. Moretto, R., 1988. Observations on the incorporation of trace elements in halite of Oligocene salt beds, Bourg-en-Bresse Basin, France. Geochim. Cosmochim. Acta 52, 2809 – 2814. Nielsen, H., 1972. Sulphur isotopes and the formation of evaporite deposits. Earth Sci. 7, 91 – 102. Ortı´, F., Pueyo, J.J., Rosell, L., 1985. La halite du bassin potassique sud-pyre´ne´en (Eoce`ne supe´rieur, Espagne). Bull. Soc. Geol. Fr. 8 (6), 863 – 872. Petrichenko, O.I., 1973. Methods of study of inclusions in minerals of saline deposits. In: Roeder, E., Koslowski, A. (Eds.), Fluid Inclusion Res. Proc. of COFFI 12 (1979). Naukova Dumka, Kiev, p. 90. Pierre, C., 1982. Teneurs en isotopes stables (18O, 2H, 13C, 34S) et conditions de gene`se des e´vaporites marines: application a` quelques milieux actuels et au Messinien de la Me´diterrane´e. PhD Thesis. Universite´ de Paris-Sud Centre d’Orsay, Paris. 266 pp. Pueyo, J.J., 1975. Sedimentary structures observed in salt of the Catalonian potash basin. IX Congre`s International de Se´dimentologie. Nice (6 – 17 Juillet, 1975). Livret-guide excursion n20: ‘‘Le basin Tertiaire catalan et les gissements de potasse’’, pp. 59 – 64. Rosell, L., 1990. La Cuenca Pota´sica Surpirenaica. In: F.A.S. Ortı´, J.M. (Ed.), Formaciones evaporı´ticas de la Cuenca del Ebro y cadenas perife´ricas y de la zona de Levante. Enresa-GPPG, Barcelona, pp. 89 – 95. Rosell, L., Ortı´, F., 1981. The saline (potash) formation of the Navarra Basin (Upper Eocene, Spain). Petrology. Rev. Inst. Invest. Geol., 71 – 121.

D.I. Cendo´n et al. / Chemical Geology 200 (2003) 339–357 Rosell, L., Pueyo, J.J., 1997. Second marine evaporitic phase in the south Pyrenean Foredeep: the Priabonian potash basin (Late Eocene; autochthonous-allochthonous Zone). In: Schreiber, G.B.A.C. (Ed.), Sedimentary Deposition in Rift and Foreland Basins in France and Spain (Paleogene and Lower Neogene). Columbia Univ. Press, New York, pp. 358 – 387. Sanford, W.E., Wood, W.W., 1991. Brine evolution and mineral deposition in hydrologically open evaporate basins. Am. J. Sci. 291, 687 – 710. Strakhov, N.M., 1970. Principles of Lithogenesis, vol. 3. Oliver & Boyd. 577 pp. Taberner, C., Dinare`s-Turell, J., Gime´nez, J., Docherty, C., 1999. Basin infill architecture and evolution from magnetostratigraphic cross-basin correlations in the southeastern Pyrenean foreland basin. Geol. Soc. Amer. Bull. 111 (8), 1155 – 1174. Taberner, C., Cendo´n, D.I., Pueyo, J.J., Ayora, C., 2000. The use of

357

environmental markers to distinguish marine vs. continental deposition and to quantify the significance of recycling in evaporite basins. Sediment. Geol. 137 (3 – 4), 213 – 240. Timofeeff, M.N., Lowenstein, T.K., Brennan, S.T., Demicco, R.V., Zimmermann, H., Horita, J., von Borstel, L.E., 2001. Evaluating seawater chemistry from fluid inclusions in halite: examples from modern marine and nonmarine environments. Geochim. Cosmochim. Acta 65 (14), 2293 – 2300. Utrilla, R., Pierre, C., Orti, F., Pueyo, J.J., 1992. Oxygen and sulphur isotope compositions as indicators of the origin of Mesozoic and Cenozoic evaporites from Spain. Chem. Geol. 102, 229 – 244. Williams-Stroud, S.C., 1994. Solution to the paradox? Results of some chemical equilibrium and mass balance calculations applied to the Paradox basin evaporite deposit. Am. J. Sci. 294, 1189 – 1228.