ORIGINAL PAPER

Sofie Nollet Æ Christoph Hilgers Æ Janos Urai

Sealing of fluid pathways in overpressure cells: a case studyfrom the Buntsandstein in the Lower Saxony Basin (NW Germany)

Received: 20 October 2004 / Accepted: 18 March 2005 / Published online: 30 June 2005� Springer-Verlag 2005

Abstract We studied veins in the Triassic Buntsandsteinof the Lower Saxony Basin (NW Germany) with the aimof quantifying the evolution of in-situ stress, fluids andmaterial transport. Different generations of veins areobserved. The first generation formed in weakly consol-idated rocks without a significant increase in fracturepermeability and was filled syntectonically with fibrouscalcite and blocky to elongate-blocky quartz. The stableisotopic signature (d18O and d13C) indicates that thecalcite veins precipitated from connate water at temper-atures of 55–122�C. The second vein generation wassyntectonically filled with blocky anhydrite, which grewin open fractures. Fluid inclusions indicate that theanhydrite veins precipitated at a minimum temperatureof 150�C from hypersaline brines. Based on d34S mea-surements, the source of the sulphate was found in theunderlying Zechstein evaporites. The macro- and mi-crostructures indicate that all veins were formed duringsubsidence and that the anhydrite veins were formedunder conditions of overpressure, generated by inflationrather than non-equilibrium compaction. The largeamount of fluids which are formed by the dehydratinggypsum in the underlying Zechstein and are released intothe Buntsandstein during progressive burial form a likelysource of overpressures and the anhydrite forming fluids.

Keywords Veins Æ Lower Saxony Basin Æ Overpressure

Introduction

The role of fluids during the evolution of sedimentarybasins is partly recorded by cements in the rock matrixand veins. During subsidence, porosity is reduced by

compaction. Connate and diagenetic/metamorphic flu-ids are released by the dewatering of minerals—forexample the smectite–illite transition (Bruce 1984;Bjørlykke et al. 1989; Bekins et al. 1994) or the dehy-dration of gypsum (Hardie 1967; Shearman et al. 1972).Additional fluids can be released by the maturation oforganic components producing hydrocarbons (Bred-ehoeft et al. 1994; Law and Spencer 1998). Besides theseintrinsic processes, basin scale advective fluid flow ofmeteoric waters is common, usually along fractures(Gross et al. 1992, Moller et al. 1997). In a sedimentarybasin, a number of processes may produce deviationsfrom hydrostatic pore pressure and thus drive fluid flowalong a hydraulic gradient, for example topography(gravity-driven flow), compaction, thermal gradients(e.g. around salt domes due to the higher thermal con-ductivity of halite), or tectonic stresses (dilatancy andtectonic pumping).

Fluid overpressures may be generated in the presenceof a seal (such as clays and evaporites), the processesbeing either undercompaction (if the rock cannot drainfast enough during subsidence), or inflation (if fluidpressure rises after compaction by the introduction ofadditional pore fluid; Law and Spencer 1998; Townendand Zoback 2000). Extension fractures may form undermany different conditions, depending on the ratio of therock’s compressive strength and the effective stress(Ingram and Urai 1999). Extension fracturing may occurunder conditions of overpressuring when fluid pressuresare as low as 0.6 times the total vertical stress (Inge-britsen and Sanford 1998, p 104).

All of these processes could be relevant to the TriassicMain Buntsandstein in the Lower Saxony Basin, whichis characterized by Permian to Jurassic rifting andCretaceous inversion, accompanied by salt tectonics(Ziegler 1990; Brink et al. 1992; Scheck et al. 2003)(Fig. 1).

Fluid activity may be recorded in a rock whenchanges in parameters like temperature, pressure or fluidchemistry cause supersaturation and the precipitation ofsolute as cements or veins. Vein microstructures may

S. Nollet (&) Æ C. Hilgers Æ J. UraiGeologie-Endogene Dynamik, RWTH Aachen,Lochnerstr. 4-20, 52056 Aachen, GermanyE-mail: s.nollet@ged.rwth-aachen.deTel.: +49-241-8095416Fax: +49-241-8092358

Int J Earth Sci (Geol Rundsch) (2005) 94: 1039–1055DOI 10.1007/s00531-005-0492-1

vary widely with crystal habits ranging from dendritic,fibrous, elongate-blocky to blocky crystals, dependingon the boundary conditions of crystal growth (Ramsayand Huber 1983; Sunagawa 1984; Bons and Jessell 1997;Hilgers et al. 2001).

Syntectonic vein microstructures can be divided intoantitaxial, syntaxial, stretched (or ataxial crystals) andblocky veins. Antitaxial veins are often fibrous and growfrom the vein centre towards both sides of the wall.Fibres are optically continuous even when they arecurved and these fibres may sometimes be used to inferthe fracture’s opening kinematics (Ramsay and Huber1983; Urai et al. 1991; Hilgers et al. 2001). Syntaxialveins are overgrowths of wall rock material, growingfrom the walls towards the centre of the vein (Durneyand Ramsay 1973). Their microstructure is usuallyelongate-blocky rather than truly fibrous (Bons 2000).Stretched veins may contain fibrous crystals and connectfractured wall rock grains. Although they are assumedto have formed by delocalized fracturing (Passchier andTrouw 1996, p 135), at least some veins show clear evi-dence for accretion at the vein-wall interface (Hilgersand Urai 2002). From the vein microstructure, thedimensions of the fluid pathways can be estimated. Forexample, euhedral crystals grew in open fractures orvoids, while fibrous veins, even when they are centime-tre-wide, can only be formed when the incremental crackopenings are less than about 10 lm (Hilgers et al. 2001).Thus, microstructures contain significant informationon the boundary conditions during vein emplacement.

Precipitation from fluids as intergranular cements inthe Triassic sandstones in the Central European Basinwas studied by a number of researchers (Laier andNielsen 1989; Rieken and Gaupp 1991; Purvis andOkkerman 1996; Weibel 1998, Putnis and Mauthe 2001;Weibel and Friis 2004). Laier and Nielsen (1989) showedthat the Bunter Sandstone formation in Denmark,corresponding to the Buntsandstein in Germany, iscemented with halite, which is mainly present in the

sandstone layers. They suggested that the cement wasformed very late in the diagenetic history and explain thehalite cementation by downward micro-filtration ofconnate fluids. Halite cementation is located above low-permeable layers of shale, which these authors inter-preted as semi-permeable membranes.

Purvis and Okkerman (1996) observed that the res-ervoir properties in the Main Buntsandstein (offshorethe Netherlands) are reduced by cementation withdolomite, anhydrite and halite. Sulphur isotopes indicatethat the source for anhydrite and halite was the under-lying Zechstein. In North-West Germany, halitecementation is also observed in the Solling Formationwithin the Buntsandstein (Putnis and Mauthe 2001).Rieken and Gaupp (1991) showed that in the Thonsegasfield area, dolomite, anhydrite and quartz precipi-tated in veins in the Buntsandstein at temperatures of140–230�C. Based on vitrinite reflectance data, theysuggested that the veins were formed during short epi-sodes of large-scale vertical fluid flow during the UpperCretaceous (Rieken and Gaupp 1991).

In this study, we focus on sealed fractures in the MainBuntsandstein in the Lower Saxony Basin (LSB). Theaim is to unravel the complex fluid history, in particularthe fluid source, transport mechanisms and the evolutionof in-situ stress. This has important consequences for thetotal flow rate and the bulk volume of fluid flowing in asedimentary basin. We use stable isotopes and mic-rothermometry to constrain p–T conditions and timingof vein formation, and we will deduce the transportmechanism of anhydrite and the structural controlduring vein emplacement.

Geological setting

The LSB is part of the E–W trending southern Permianbasin, bordered in the north by the Ringkøbing–FynnHigh (Scheck and Bayer 1999; Kossow and Krawczyk

N

9° 10°7° 8°

Wes

erW

eser

Ems

Osnabrück

LSB

PB

Borehole 1

reverse faultnormal fault salt structure

Legend:

25 km

Leine

Borehole 3

Borehole 4

Borehole 2

LBM RM

LSB

RHCG

UK

B

NL

DLondon Amsterdam

Hamburg

Hannover

Hannover

53°

52°

Fig. 1 Overview map of thecentral and western part of thesouthern Permian Basin withmain structures (LBM LondonBrabant Massif, RM RhenishMassif, LSB Lower SaxonyBasin, CG Central Graben, PBPompeckj Block and RHRingkøbing-Fyn High). Insetshows an enlarged map of theLSB with the location of thefour sampled boreholes and themain structural elements(normal faults, reverse faultsand salt domes) (after Ziegler1990; Baldschuhn et al. 2001)

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2002) (Fig. 1). It was filled with Rotliegend clastic sed-iments and cyclic deposits of Zechstein evaporites (car-bonates, sulphates and halite). Rotliegend sandstonesand Zechstein carbonates are important reservoir rocks,with the Zechstein evaporites acting as a regional seal(Ziegler 1990). During the Triassic, NNE–SSW orientedrifting took place and continental, brackish-marine redbeds, shallow marine carbonates, sulphates and halitewere deposited in an arid to semi-arid climate(Michelsen and Clausen 2002; Szurlies et al. 2003). TheLower Triassic Buntsandstein in northern Germanyconsists of three units (Fig. 2): (1) shaly sediments in theLower Buntsandstein; (2) four depositional sub-cycles inthe Middle Buntsandstein that start with a regressivesand package formed by braided streams and sheetfloods, and close with transgressive shales, depositedunder ephemeral playa lake conditions and (3) marineshales, sulphate and halite series in the Upper Bunt-sandstein (Rot) (Herrmann et al. 1968; Ziegler 1990;Kovalevych et al. 2002; Michelsen and Clausen 2002).

During the time of rapid subsidence in the LateJurassic, WNW–ESE oriented normal faults wereformed, for example in the Gluckstadt Graben (Brinket al. 1992; Scheck et al. 2003). In the LSB, inversionstarted in the Turonian and peaked during the Santo-nian and Campanian. Total maximum inversion uplift isestimated at 2 km to locally even 8 km, causing signifi-cant erosion. The inversion created thrusts along thebasin margins and flower structures (de Jager 2003;Kockel 2003). In the southern part of the LSB, rockswere subjected to higher burial and stronger inversion,

then in the northern part (Petmecky et al. 1999).Abnormally high maturities are observed at the time ofinversion in the Bramsche area (western part of LSB),which are explained by some authors by abnormallyhigh heat flow due to magmatic intrusions, while otherssuggest that tectonic events caused deep burial and thehigh temperatures (Brink et al. 1992; Petmecky et al.1999).

Initial (passive) diapirism of Zechstein salt was initi-ated in the Late Triassic (Trusheim 1957; Brink 1984;Brink et al. 1992; Baldschuhn et al. 1998; Bayer et al.1999; Scheck et al. 2003). In the Upper Jurassic, saltmobilization continued and broke through the weakestunits of the Mesozoic cover (Jaritz 1980). From theEarly Cretaceous until Early Cenozoic, salt diapirismpersisted as indicated by salt-rim synclines (Scheck et al.2003; Mohr et al. this volume).

In this study, the Middle Buntsandstein was sampledin four boreholes in the LSB (Fig. 1), because thisformation was affected by the structural and fluid his-tory starting from early salt movement to basin inver-sion. The four boreholes are located in differenttectonic settings (Fig. 3). Borehole 1 is located in aninversion structure where faults have been reactivatedduring the inversion in the Late Cretaceous (Kockel2003). Borehole 2 is located in an anticlinal structure,related to salt doming (Rieken and Gaupp 1991). Thestructure of borehole 3 in the southern part of LSB isalso located in an inversion structure where Zechsteinsalt intruded into the overlying Triassic sedimentsalong faults (Baldschuhn et al. 2001). Borehole 4 is

Fig. 2 Stratigraphic column ofthe Middle Triassic (anhanhydrite, h halite) (afterBorchert and Muir 1964;Baldschuhn et al. 2001) andtable showing thecharacteristics of the observedveins for each formation in theMiddle Buntsandstein (cccalcite, anh anhydrite, h halite, ddolomite, qtz quartz)

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located above a salt pillow and at 4 km from a saltdome.

Methods

Standard thin sections (20–30 lm) were used formicrostructural analyses and cathodoluminescence.Cathodoluminescence was carried out on a TechnosynCold Cathodo Luminescence Model 8200 MkI under8.5–10.5 kV and 260–350 mA. Double polished, 150-lm-thick sections were prepared for fluid inclusionanalyses in anhydrite, according to the method describedin Muchez et al. (1994). Microthermometry was carriedout on a Linkham stage (K.U.Leuven, Belgium), whichwas calibrated at �56.6, �21.2, 0.0 and 374.1�C withsynthetic Syn Flinc inclusions. Homogenization tem-peratures of inclusions were measured before freezing.Freezing may cause a volume increase related to theliquid water–ice transition, which induces deformationof the crystal and a volume change in the fluid inclu-sions. This volume change results in incorrect homoge-nization temperatures (Reynolds and Goldstein 1990).For d18OV-PDB/d

13CV-PDBmeasurements, carbonatepowders reacted with 100% phosphoric acid at 75�Cusing a Kiel III online carbonate preparation line con-nected to a ThermoFinnigan 252 mass spectrometer(Institut fur Geologie und Mineralogie, Universitat Er-langen-Nurnberg, Germany). All values are reported inper million relative to V-PDB by assigning a d18O valueof �2.20& and a d13C value of +1.95& to NBS19.Reproducibility was checked by replicate analysis oflaboratory standards and is better than ± 0.03 for d13Cand ± 0.06 for d18O. Sulphur isotopes were measured inan EA-ConfloII-Finnigan Delta+ mass spectrometerafter mixing of the SO2 gas with V2O5 at 1,050�C(Department of Geology-Westfalische Wilhelms-Uni-versitat Munster, Germany). Results are reported in thestandard delta notation (d34S) as per mil difference rel-ative to Canyon Diablo Troilite (CDT). Analyticalreproducibility was generally better than ±0.4&.

Macroscopic observations

In boreholes 1, 3 and 4, the Middle Buntsandstein islocated at depths of around 2,500 m whereas in borehole2, the Middle Buntsandstein is at 3,400 m. All theboreholes are vertical and although they are located indifferent tectonic settings and up to 200 km apart, thevein types and the vein-filling minerals are very similar inthe cores studied (Fig. 2).

In all boreholes, the Solling Formation is composedof red–brown coloured, fine-grained silt- to claystone,cemented with calcite and dolomite. Green reductionspots with diameters of 1 cm are very often observed.Veins are frequent in the Solling Formation, withapproximately one vein in every 2 m core. They are filledmainly with anhydrite and calcite and rarely with onlyone of the two minerals. Occasionally, veins with calciteand halite were found in borehole 2. Veins are oriented70� to 90� to bedding. The thickness of the veins rangesfrom 0.5 cm up to 3 cm. Veins are longer than thesection exposed by the cores, and in one case where thevein is parallel to the core axis, a vein length of 3 mwas observed. The fracture morphology of the calcite-anhydrite and anhydrite veins is very regular (Fig. 4a).When the vein consists of calcite and anhydrite, thecalcite is located at the vein walls whereas the anhydriteis located in the centre of the vein. In the veins wherehalite and calcite are observed, the calcite is also locatedat the vein wall and the halite in the centre. The veinswith only calcite filling are thin (0.5 cm), have a smoothvein-wall interface in claystone and a more irregularmorphology in coarser-grained host rock. When cross-ing a layer boundary into coarser-grained host rock,they split into different mm-thin branches (Fig. 4b).

The Hardegsen Formation is only present in bore-hole 1 and consists of coarse-grained, grey-green col-oured sandstone with alternations of red–browncoloured clay-rich layers. It is often cemented withquartz and pyrite concretions are widely present in thisformation. Veins are only occasionally observed

Fig. 3 Simplified cross-sectionsof the sampled boreholes (afterBaldschuhn et al. 2001).aBorehole 1, b Borehole 2,c Borehole 3 and d Borehole 4

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(approximately 1 vein in every 10 m core), havethicknesses smaller than 1 mm and are filled withanhydrite or very rarely halite. They are oriented 85� to90� to the bedding.

The Detfurth Formation consists of red-brown,coarse-grained sandstone with clay-rich intercalations.Veins are rare, approximately one vein in a 5 m core andare filled with anhydrite and calcite. Thicknesses varybetween 0.5 cm and 1 cm and lengths are at least 30 cm(veins rarely terminate in the core). The veins are ori-ented between 65� and 90� to the bedding. In borehole 4,very thin (1 mm or smaller) halite veins are occasionallyobserved in this formation.

The Volpriehausen Formation is composed of red- togrey-coloured sandstone with clay-rich layers. Veins arerare (approximately 1 in every 30 m core) and are verythin (<1 mm). They are oriented 65� to 90� to thebedding and are mainly filled with calcite and dolomite.As in the other formation, veins are long, and rarelyterminate in the core.

Microstructural observations

Calcite veins

The vein–wall interface in calcite veins in the shales isserrated on a 10-lm scale. Fractures are intergranular,and host-rock inclusions located within the veins arecommon (Fig. 5a). When the vein crosses the interfacebetween fine- and coarse-grained host-rock material, thevein usually becomes very irregular. Quartz grains of thehost rock are incorporated into the vein, showing thatthe fracturing was delocalized (Fig. 5b). All theseaspects indicate that the host rock was only slightlyconsolidated during vein formation.

The vein–wall interface has no selvage and in somecases both fracture surfaces perfectly match. The calciteveins have a fibrous microstructure, which sometimeschanges to blocky crystals along the vein. Fibrous veinsare generally interpreted as syntectonic (Durney andRamsay 1973), which become blocky if the opening rateof the fracture becomes larger than the growth rate ofthe vein crystals (Hilgers et al. 2001). In some veins, amedian line was observed in the centre of the vein,characterized by smaller calcite crystals (Fig. 5c). Thisindicates that the crystals grew antitaxially from the veincentre towards the vein wall. In some cases, growth wasunitaxial from one fracture wall towards the other sidewithout the median line.

No zonation was observed with cathodolumines-cence, indicating that there were no significant chemicalvariations during vein growth (Fig. 5d). The calcitecements in the host rock have the same luminescencecolour as the veins.

The calcite fibres often do not connect markers suchas thin siltstone layers in the host rock and therefore donot track the opening trajectory of the fracture. This iscaused by a relatively smooth vein-wall interface andsmall opening increments (Hilgers et al. 2001; Nolletet al. 2005).

Calcite–quartz veins

Siltstone and sandstone layers contain intergranularextension veins filled with quartz and calcite, orientednormal to bedding. The microstructures show stretchedquartz crystals with wall-rock inclusions (Fig. 6a). Veinmicrostructures laterally change to elongate-blockyquartz crystals with euhedral terminations in the veincentre, which grew syntaxially on the vein–wall interface

Fig. 4 Rock samples showingthe different vein generations.a Blocky calcite-anhydrite veinwith very regular wall rockinterface. Calcite (cc) is locatedat the vein wall and anhydrite(anh) in the vein centre (sampleB3-14). b Fibrous calcite veinwith smooth wall-rock interfacein the claystone and irregularvein branches in the coarse-grained sandstone (sampleB2-08). S0=bedding plane

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towards the vein centre. Syntaxial growth resulted insignificant growth competition at the vein–wall inter-face, indicating that the vein grew syntectonically with asignificant opening (>10 lm) to the opposing wall.

Microstructures suggest that some stretched calcitecrystals grew contemporaneously with the quartz(Fig. 6b). Stretched crystals of quartz are clear evidencefor syntectonic growth. The lateral variation fromstretched crystals to elongate-blocky syntaxial growthsuggests that the open cavity growth with euhedralcrystal terminations in the veins is also syntectonic.

Locally, anhydrite fills open vugs in the syntaxial veins,indicating that quartz–calcite grew at an earlier phase.

Anhydrite veins

Anhydrite is found in extension veins oriented normal tobedding. Fracture surfaces are more regular, in contrastto the irregular morphology of the calcite veins,indicating that the rocks were more consolidated duringthis phase of vein formation.

Fig. 6 a Microstructuresshowing a stretched quartz veinwith few solid inclusionsincorporated in the quartzcrystals indicating syntectonicgrowth (thin section s61_a,normal to bedding, sampleB2-05). b Stretched quartz andcalcite crystals suggesting thatboth calcite and quartzprecipitatedcontemporaneously. Facetedquartz with euhedral crystalterminations are also observedin the same vein (thin sections61_b, normal to bedding,sample B2-05). cc calcite, qtzquartz

Fig. 5 aMicrostructure showing a calcite vein in claystone. Thevein splits in different branches in the coarse-grained siltstone (thinsection s78, normal to bedding, sample B2-08). b Calcite over-growth in sandstone showing that the vein is delocalized and doesnot cross the quartz grains of the host rock. Quartz grains arepartly dissolved by pressure solution (thin section s81, normal tobedding, sample B3-19). c Antitaxial calcite vein in claystone

showing small crystals in the centre, which grew towards both sidesof the wall. The vein grew asymmetrically (thin section s78, normalto bedding, sample B2-08). d Cathodoluminescence image of acalcite vein showing that the cements in the host rock have the sameluminescence colour as the vein. Note that fibres connect beddingon both sides of the vein in this example (thin section s78, normalto bedding, sample B2-08). cc calcite, qtz quartz

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Anhydrite veins contain aggregates of clear needle-like radiating crystals (rosettes) or single, thin needles,with well-defined cleavage (Murray 1964; Holliday 1970)(Fig. 7a). The rosettes radiate from a single blockyanhydrite crystal. Veins may have a blocky or elongate-blocky microstructure, with growth competition at thevein–wall interface (Fig. 7b).

Cross-cutting relationships of calcite veins or bran-ches and anhydrite veins indicate that calcite veinsre-opened and were filled with anhydrite (Fig.7c). Thisindicates that anhydrite veining postdates calciteveining.

Halite veins

In contrast to the other boreholes, borehole 2 containscentimetre thick halite veins in the Solling Formation.Halite is located in the centre of a calcite vein, which isoriented normal to bedding (Fig. 8a). The vein micro-structure varies laterally from fibrous to blocky within asingle vein, indicating syntectonic growth at differentopening or growth rates (Hilgers et al. 2001). Halitecontains mainly primary subgrains, recognized by theirfibrous shape. Few grains show polygonal subgrains,which are indicative of deformation due to dislocationcreep (Fig. 8b) (see also Schleder and Urai this volume).The subgrain size is then inversely proportional to thedifferential stress (Carter et al. 1993). The subgrain sizewas measured and results suggest low differential stres-ses of around 1.4 MPa.

Sulphate reduction

Green reduction spots and pyrite concretions are fre-quently observed in the shaly host rock close to anhy-drite veins. In these veins, calcite is locally present at thevein–wall interface and within the vein. The texture inthese veins points to replacement of the anhydrite crys-tals by calcite at a later stage (Fig. 7d). This may indi-cate that anhydrite was dissolved and thermochemicalsulphate reduction took place with methane to formcalcite and H2S according to the following reaction(Machel 1987, 2001;Worden et al. 1995):

CaSO4 þ CH4 ) CaCO3 þH2SþH2O:

Stable isotopes

Oxygen and carbon isotopes

The d18O and d13C isotope compositions were measuredin the calcite veins and in dolomite and calcite cementsin the host rock (Table 1, Fig. 9). Values for d18O varybetween �11.98& and �6.11&, whereas d13C valuesrange between �2.81& and 1.87& (Table 1). Signaturesfor veins and cements are very similar, indicating thatthe vein minerals have the same source as the cements inthe host rock. This is also suggested by the cathodolu-minescence colours in the calcite cements and veins.

The d18O signature in the calcite veins depends on theorigin of the precipitating fluids and on the precipitation

Fig. 7 a Microstructuresshowing anhydrite rosettecrystal indicating that thecrystals grew in an open space(thin section s67/34, normal tobedding, sample B1-03).b Growth competition betweenanhydrite crystals at the veinwall pointing to syntaxialgrowth in an open void (thinsection s67_a, normal tobedding, sample B1-03).c Cross-cutting relationshipbetween elongate-blocky calcitevein and blocky anhydriteindicating that the calcite veinwas reactivated and filled withanhydrite (thin section s66_a,normal to bedding, sample B1-02). d Anhydrite replacement bycalcite, indicated by anhydriteremnants in a blocky calcitematrix (thin section s67_1,normal to bedding, sample B1-03). cc calcite, anh anhydrite

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temperature (Faure 1998) (Fig. 10). The Buntsandsteinwas deposited in an arid climate and under these con-ditions, meteoric water has d18OSMOW values between0& and �5& (Harwood and Coleman 1983). If themeteoric water in the pores is mixed with connatewaters, resulting from mineral reactions during the earlycompaction of the sediments, the pore fluids will have amore positive d18O signature (Suchecki and Land 1983).Assuming that the pore fluids in the Buntsandstein havea d18OSMOW composition of 0& during vein growth, thecalcite veins precipitated at a temperature of 55–84�C(Fig. 10). If the pore fluids had a more connate

d18OSMOWisotopic composition of 4&, precipitationtemperatures were between 84�C and 122�C (Fig. 10).

The range of d13C around 0& in our samples can bethe result of mixing of bacterial carbon with carbonate-rich pore fluids (Irwin et al. 1977; Harwood and Cole-man 1983). If the carbon had its origin only in organicmaterial, the d13C values would be extremely negative,which is not the case in our data.

Sulphur isotopes

In borehole 1, 3 and 4 we analysed 14 samples fromanhydrite veins sampled in the Solling Formation. Valuesfor d34S range between 11.9& and 13.4&CDT (Table 2),and do not show a relation with depth. This indicates thatthe veins precipitated from a fluid with a relative uniformd34S composition. Lower Triassic seawater had a sulphurisotopic composition of 16–21.5& CDT, which is sig-nificantly higher than the values in our samples (Claypoolet al. 1980; Kramm and Wedepohl 1991; Kampschulteand Strauss 2004) (Fig.11). The Upper Permian seawaterhad a lighter sulphur isotopic composition of 12.1–14.4&CDT, which corresponds very well with our data. Thissuggests that the sulphate in the anhydrite veins was re-mobilized from the underlying Zechstein.

Fluid inclusions

Fluid inclusions with diameters between 5 lm and20 lm are common in growth zones in the large, blockyanhydrite crystals. Most inclusions contain two phases,liquid and vapour, although some inclusions with onlyone phase (liquid) were observed. These could indicate aformation below 50�C. However, it is more likely thatthe absence of a gas bubble may be due to nucleationproblems (metastable one-phase inclusions), as theseinclusions are small (<5 lm) (Roedder 1984). In bore-

Table 1 Oxygen (& V-PDB and & V-SMOW) and carbon (& V-PDB) isotopic data of carbonate minerals in veins and cements

Borehole Sample name Sample description d18O (& V-PDB) d18O (& V-SMOW) d13C (& V-PDB)

1 B1-03 Small calcite vein +calcite cement

�7.92 22.7 1.87

B1-03 Dolomite cement �6.11 24.56 1.53B1-09 Dolomite vein �9.78 20.78 1.1B1-12 Calcite vein �7.67 22.95 �0.75

2 B2-01 Calcite vein �11.06 19.45 0.49B2-02 Calcite vein �11.26 19.25 0.46B2-06 Calcite vein �11.44 19.07 0.17B2-07 Calcite vein �11.15 19.37 �0.23B2-08 Calcite vein �11.73 18.77 �0.03B2-08 Dolomite cement �11.98 18.51 0.61

3 B3-17 Dolomite cement �10.4 20.14 0.22B3-19 Small calcite vein (TSR) �10.74 19.79 �2.3B3-22 Calcite vein (TSR) �10.6 19.93 �2.81

4 B4-01 Calcite vein �9.15 21.43 1.47B4-05 Calcite vein �8.38 22.22 �0.26B4-06 Calcite vein �9.66 20.90 0.56B4-09 Calcite vein �9.97 20.58 �1.31

Fig. 8 a Halite vein in the Solling Formation with calcite (cc) onboth sides of the wall. b Halite contains primary subgrains, and isalmost free of deformation (1.4 MPa differential stress)

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hole 1, a wide range of fluid inclusion homogenizationtemperatures is observed (Fig. 12a). Such variation canbe the result of re-equilibration (stretching) of the pri-mary inclusions during further burial. In this case, thehigher temperatures do not represent the homogeniza-tion of the primary inclusions (Goldstein 1986; Burruss1987; Muchez et al. 1991). However, the lowesthomogenization temperatures (between 110�C and150�C) indicate the minimum homogenization temper-ature of the original fluid inclusions. The highest fre-quency of homogenization temperatures is observedaround 150�C (Fig. 12a). After freezing, the first melting(Tfm) is observed around �52�C, which suggests that thefluid is of the NaCl–CaCl2–H2O type (Roedder 1984;

Shepherd et al. 1985; Reynolds and Goldstein 1990).The last melting of ice is between �28.6�C and �13.9�C,pointing to salinities 2.3–3.3wt% NaCl and 14.9–21.5wt% CaCl2 (Oakes et al. 1990) (Table 3).

In borehole 3, homogenization temperatures rangebetween 100�C and 150�C, with the highest frequency at140�C (Fig. 12b). Higher homogenization temperaturesare also observed but have a smaller frequency(Fig. 12b). This suggests, as in borehole 1, stretching ofthe primary inclusions during further basin evolution.The temperature of first melting indicates fluids from aNaCl–CaCl2–H2O type, as in borehole 1. However,further heating resulted (in some cases) in final meltingof hydrohalite crystals around �24�C, after all ice wasmelted around �35�C, which points to salinities of 3.6–4.4wt% NaCl and 24.9–27.0wt% CaCl2 (Oakes et al.1990; Zwart and Touret 1994). In some inclusions, ahalite crystal was present, which dissolved between 7�Cand 24�C, pointing to salinities of 39.0–40.5wt% CaCl2(Oakes et al. 1990; Zwart and Touret 1994) (Table 3).Inclusions in anhydrite in borehole 2 and 4 could not bemeasured, because anhydrite crystals were not present ortoo small, respectively.

Discussion

Timing and paragenesis of the veins

The extensional structures in this study are characterizedby different generations of vein formation. The firstgeneration of veins consists of calcite and quartz–calciteveins. Their fibrous and stretched-crystal microstructurepoints to syntectonic vein formation. The intergranularfracture morphology, the rough shape of the vein–wallinterface, and the wall-rock inclusions of the calciteveins indicate an emplacement in a slightly consolidatedrock. Microstructural observations suggest that theanhydrite veins formed later and in a more consolidatedrock than the calcite veins. This age relationship isderived from the fracture morphology (which in turndepends on rock strength) and is also supported bycross-cutting relationships of calcite and anhydrite veins.The d18O and d13C signature in the calcite veins indicatesthat they precipitated between 55�C and 122�C frommeteoric fluids mixed with connate fluids. Homogeni-zation temperatures in fluid inclusions show that theanhydrite veins precipitated at around 150�C. Riekenand Gaupp (1991) measured homogenization tempera-tures around 140–230�C in quartz and calcite veins inthe same area, and we therefore propose that thedifferent vein generations gradually change from calcite–quartz to anhydrite veins.

The extension fractures are consistently oriented at ahigh angle to the bedding, which suggests a near verticalmaximum principal stress in the rock. This indicates thatthe veins formed during basin subsidence. Petmeckyet al. (1999) modelled subsidence curves in the LSB andshow that the basin subsided rapidly from the Late

Fig. 10 Oxygen isotopic signature against the temperature for dif-ferent isotopic compositions of the precipitating water (d18O(SMOW)

water), assuming that the fluid and the calcite veins were inequilibrium. When the precipitating fluid had a composition of0–4& SMOW, the calcite veins and cements precipitated between55�C and 122�C. Calculations based on O’Neil et al. (1969)

-12 -10 -8 -6 -4 -2

-3

-2

-1

00

1

2

cementsveinsLegend:

Fig. 9 Carbon isotopic signature (d13C) plotted against oxygenisotopic signature (d18O) for the calcite veins and cements inborehole 1–4. The data of the cements are very similar to the dataof the veins, indicating that they precipitated from the same fluids.All values are in PDB (based on d18O (SMOW)=1.03086 · d18O (PDB)

+ 30.86

1047

Jurassic to the Early Cretaceous. The Buntsandsteinreached a maximum burial of up to 7-km depth duringthe Mid Cretaceous and was uplifted to 2–3-km depthby inversion during the Late Cretaceous. Althoughmaximum burial and inversion differ in various parts ofthe LSB, similar tectonic events and fluids affected allparts of the basin. We thus take one curve, representingborehole 3, for the fluid history of the basin (Fig. 13a).The temperature data imply that the calcite veins formedat depths of 1–3 km. We did not observe compactionhalos (e.g. Price and Cosgrove 1990, p 421; Urai et al.2001) around these early veins and this implies that theporosity was already reduced significantly (see Appen-dix). The minimum depth of vein formation should

therefore correspond with a porosity of around 20%,which fits very well with a minimum depth of 1–2 km(Fig. 13b). The anhydrite veins formed at minimumdepths of 3 km during the upper Jurassic when the basinsubsided rapidly (Fig. 13a). The wide range of homog-enization temperatures in the fluid inclusions is ex-plained by stretching, which suggests that veins werebrought to higher temperatures after their formation.Thus, veins were buried to even greater depths duringfurther basin subsidence.

Table 2 Sulphur isotopic datafor all boreholes containinganhydrite veins. (a) Borehole 1,(b) borehole 3 and (c) borehole4

Borehole Sample name Sample depth (m) Stratigraphic interval d34S (& CDT)

Borehole 1 B1-01 2576.15 Solling formation 12.3B1-02 2578 Solling formation 13.4B1-03a 2580.4 Solling formation 12.7B1-03b 2580.42 Solling formation 12B1-03c 2580.44 Solling formation 12.8B1-03d 2580.46 Solling formation 11.9B1-03e 2580.48 Solling formation 13.4

Borehole 3 B3-14 2722.85 Solling Formation 11.7B3-17 2736.1 Solling Formation 12.6B3-19 2768.8 Solling Formation 11.9

Borehole 4 B4-03 2360 Solling formation 12.4B4-04 2360.5 Solling formation 12.6B4-11 2362 Solling formation 12.8B4-12 2362 Solling formation 12.1

Perm. Triassic Jurassic

Age [Ma]

0

5

10

15

20

25

30

35

260 210 160

Rot Zec Bu Mu Keuper Lias Dogger

moving average of SSS (structurally substituted sulfate)sulfate in evaporites

vein sulfate in the Buntsandstein

11.9-13.4 ‰

Legend

Fig. 11 Sulphur isotopic signature during Permian to Jurassic time(after Kampschulte and Strauss 2004) compared with results ofveins in the Buntsandstein. The data correlate with Zechsteinsulphur isotopes, which suggests that the anhydrite layers in theZechstein are the source rock for the anhydrite veins in theBuntsandstein (Rot Rotliegend, Zec Zechstein, Bu Buntsandstein,Mu Muschelkalk)

0

5

10

15

100 120 140 160 180 200 220 240

Homogenization temperature [˚C]

Freq

uenc

y

0

1

2

3

4

5

100 120 140 160 180 200 220 240

Homogenization temperature [˚C]

Freq

uenc

y

Borehole 1

Borehole 3b

a

Fig. 12 Homogenization temperatures of fluid inclusions in anhy-drite represented in a histogram for the two analysed boreholes: aBorehole 1 (n=21, three samples) and b Borehole 3 (n=57, foursamples). The wide range in homogenization temperature (in thesame sample) suggests that the primary inclusions in the veincrystals are re-equilibrated during further basin evolution. Thismeans that only the smallest values of Th represent the minimumtemperature of vein formation

1048

Mechanical aspects

All veins are extensional mode I fractures, which formwhen the differential stress r1� r3<4T (Secor 1965). Inextensional regimes, this requires r3¢=�T where r3¢ isthe minimum effective principal stress, T the tensilestrength and the maximum principal stress r1=rv , rv

the vertical stress. The vertical stress rv acting on theBuntsandstein increases during subsidence together withthe differential stress. Depending on the evolution ofrock strength and pore pressure, the rock may either notreach the failure envelope, or deform by normal faultingor by extensional fracturing (mode I). Extensionalfractures can be generated at high fluid pressure pf,

corresponding to low effective stress. The fluid pressureis described as the Hubbert–Rubey ratio (or pore fluidpressure ratio) k=pf/rv=pf/qgz, with q average rockdensity (2,200–2,500 kg/m3), g gravitational accelerationand z depth (Hubbert and Rubey 1959). The Hubbert–Rubey ratio k theoretically ranges between 0 (dry rock)and infinity. In overpressured regimes, k is higher thanhydrostatic and values may range between 0.4 and 1.0.

In brittle-failure-mode plots, the effective verticalstress rv’=rv�pf is plotted against the differential stressfor three different failure modes: (1) hydraulic extensionfractures, (2) extension shear fractures and (3) shearfractures for typical tensile strengths T of sedimentaryrocks (Sibson 2003, 2004) (Fig. 14). The graph presents

Table 3 Fluid inclusion data in anhydrite for the two boreholes

Borehole Th (�C) Tfm (�C) TmI (�C) TmHH (�C) TmH (�C) wt% NaCl wt% CaCl2

Borehole 1 110–250 � 52 Between �28.6 and �13.9 � �37 – 2.3–3.3 14.9–21.5Borehole 3 105–200 � 52 Between �25 and �22.9 � �35 – 3.6–3.78 18.7–19.6

105–200 � 52 Between �41.1 and �36.7 Between �25.7 and �22.7 – 3.6–4.4 24.9–27.0105–200 � 52 Between �41.3 and �29.0 Between �25.7 and �22.9 7–5�24.2 39.0–40.5

Th homogenization temperature, Tfm temperature of first melting, TmI temperature of ice melting, TmHH temperature of hydrohalitemelting, TmH=temperature of halite dissolution. Salinities are calculated with the program Aqso2e (Bakker 2003)

190

190

160

130

100 70

0

2

4

6

0

2

4

6

8

10

220

250

280

310

340 290 240 190 140 90 40

55°C

122°C

150°C

calcite veining anhydrite veining

Legend:

temperature of gypsum dehydration

Porosity evolution of the Buntsandstein during burial

0 50

current porosity=5%

Time [Ma]

Dep

th [k

m]

Carb. Perm. Trias. Jurassic Cretaceous Tert.a

b

Fig. 13 a Subsidence curve forthe Lower Saxony Basin (afterPetmecky et al. 1999) includingthe p–T–t conditions of veinformation. Based on the d18Odata, calcite veins formed atdepths between 1 km and2.5 km. The fluid inclusionsdata indicate that the anhydriteveins precipitated at a minimumdepth of 3 km. Veins haveformed during the basin’ssubsidence rather than duringuplift because (1) theirorientation suggests that themaximum principal stress actedvertically and (2) stretching ofthe fluid inclusions requires anincrease in p-T conditions aftervein formation. b The porosityevolution in the Buntsandsteinis plotted against the depth forcomparison of the porosityevolution with depth of veinformation. This function iscalculated from the initialporosity (typical 50% forsedimentary rocks) and thepresent porosity of theBuntsandstein (5%), reached atthe maximum burial

1049

failure stress conditions at a particular depth z andvarying fluid pressures. With this plot, we can derive theminimum fluid overpressure required to form theanhydrite veins, because (1) we only observed extensionveins and (2) we estimated the depth of vein formationfrom fluid inclusion data and a published subsidencecurve (Fig. 13). At 10 MPa tensile strength of the rock(typical for Buntsandstein sandstones) and a minimumdepth of 3.2 km for the onset of anhydrite veining,extension fractures only form at differential stressesr1�r3<4T or r1�r3<40 MPa and at k>0.63 corre-sponding to an overpressure of 18 MPa above hydro-static (Fig. 14).

Where do high fluid pressures come from?

One possible source for overpressuring of the MiddleBuntsandstein is undercompaction, which can take placeif the sediments are rapidly buried and pore fluid cannotescape fast enough (Law and Spencer 1998). Consider-ing that in most undercompacted sedimentary basinsoverpressures start at around 2 km, the presence of agood regional seal (Rot evaporites) could well have in-duced significant overpressures by undercompaction.However, this process cannot produce the fluids whichprecipitate anhydrite.

A second mechanism for the formation of overpres-sures is by dehydration of clays in the Buntsandstein(smectite-illite transformation). The release of theinterlayer water during the smectite-illite transition

starts at depths between 2,000 m and 3,000 m at tem-peratures between 60�C and 160�C (Bruce 1984; Bekinset al. 1994). However, this process is questioned in theliterature as there are also some indirect effects of illiti-zation on overpressures, such as quartz cementation,which reduce the permeability (Colten-Bradley 1987;Burrus 1998). Because the mineralogy of the Bunt-sandstein claystones at deposition is not known, it isdifficult to assess the importance of this process in pro-ducing overpressures, but again we note that this processdoes not produce fluids that precipitate anhydrite.

A third mechanism for generation of overpressures isthe thermal expansion of the fluid, which is most effec-tive in low-permeability sediments under high confiningpressures (Plumley 1980; Palciauskas and Domenico1989). We consider this mechanism rather unlikelybecause of the absence of a late thermal event.

The above described mechanisms are all possiblemechanisms for overpressuring but they do not producefluids with components that precipitate anhydrite.Therefore, although we cannot reject that the previouslydescribed mechanisms have contributed to overpres-sures, an external source of fluids is necessary to producethe anhydrite veins and this fluid may also be a potentialsource of overpressure. It is interesting to estimate thevolume of fluid required to precipitate the anhydriteveins. The fluid inclusions show that the fluid fromwhich the veins precipitated contained 0.5 M NaCl.Assuming a 1 m2 vein with 1 cm thickness, in a volumeof 1 m3, the precipitated volume corresponds with23.2 kg (density of anhydrite = 2,320 kg/m3). To pre-

Fig. 14 Brittle-failure mode plot for 3,200 m depth and averagerock density of 2,500 kg/m3. The effective vertical stress is plottedagainst the differential stress for the three different failure modes inan extensional regime (ext stress conditions where extensionalfailure occurs, ext-sh stress conditions where extensional shearfailure occurs, comp-shstress conditions where compressional shearfailure occurs). The failure of a rock depends on its tensile strengthT and therefore three different strengths are plotted (T = 5, 10 and

15 MPa). Assuming a tensile strength of 10 MPa for the Bunt-sandstein at 3,200 m depth, extensional fracturing only occurs forr1�r3< 40 MPa and therefore, a minimum overpressure of18 MPa above hydrostatic is necessary. The relation between thepore fluid pressure ratio (k) and the pore fluid pressure (pf) isplotted next to the brittle-failure mode plot and Mohr diagrams areshown to illustrate the location of the Mohr circle for the differentfailure modes

1050

cipitate a vein with this mass from a fluid which was 5%supersaturated at 150�C, 3.4·105 kg H2O is necessary,corresponding to a volume of 348 m3, assuming a den-sity of 977.7 kg/m3 (Blount and Dickson 1969; Freyerand Voigt 2004). When the supersaturation is 20%, only87 m3 of fluid is necessary. Therefore, the required fluidvolume to precipitate the observed veins is, even for 20%supersaturation, very high.

The d34S isotopes in the anhydrite veins have thesame signature as the Zechstein anhydrite. Assumingthat the vein-precipitating fluids came from the Zech-stein, we can calculate how much fluid can be pro-duced in the Zechstein. In the anhydrite of theZechstein 4 (‘‘Grenzanhydrit’’), the gypsum-anhydritetransition occurs around 70�C and releases about 60%water per unit volume gypsum (Borchert and Muir1964, p 133; Hardie 1967). These amounts of fluidscould be a source for the precipitated anhydrite veinsin the overlying sediments. However, based on thetemperature in the subsidence curve, the fluid releasefrom the ‘‘Grenzanhydrit’’ occurred before anhydritevein formation in the Buntsandstein (during Early toMid Jurassic). The fluids in gypsum layers deeper inthe Zechstein sequence were produced earlier thanthose in the uppermost Zechstein and must havedrained upwards through evaporite layers with verylow permeabilities. Here, it is interesting to speculateabout the possible retention of these fluids in theZechstein over geological time scales, starting to flowupwards at a later stage at deeper burial. This scenariowould require extremely low permeabilities of the ha-lite layers over geologically long periods. Although

halite has very low permeabilities under non-dilatantconditions (dilatancy only occurs at close to lithostaticfluid pressures; Urai et al. 1986; Fokker ; Peach andSpiers 1996; Popp et al. 2001), little is known aboutthe exact value of permeability and its anisotropy. Ofparticular importance here is the topology of the smallamount of grain boundary fluids present in the halite(Lewis and Holness 1996; Schenk and Urai 2004) andthe orientation of the principal stress under geologicalconditions, which affects the way fluids are distributedin grain boundaries during dynamic recrystallization.Stress in halite is near-isotropic (Schleder and Uraithis volume) but little is known about the orientationof the principal stress which may be an importantfactor in controlling permeability anisotropy. Withthese arguments in mind (although much more work isneeded here), it is possible that under suitable condi-tions, the brine released from gypsum can be retainedin the halite for long periods, and released to higherlevels in the basin during small changes in the state ofstress in the halite. The underlying Rotliegend andCarboniferous sediments also contain large volumes ofpore fluids (e.g. Gaupp et al. 1993). If these fluids areable to migrate through the Zechstein evaporites, theymay have transported Zechstein components into theBuntsandstein (van Bergen and de Leeuw 2001).

Irrespective of the mechanism of overpressuring, theclaystone layers of the Upper Buntsandstein and the Rotevaporites provide a seal for the overpressure cell in theBuntsandstein. The increase in fluid pressure was asso-ciated with extensional fracturing in the Middle Bunt-sandstein (Fig. 15). The solubility of anhydrite decreases

Fig. 15 Model of the pressure versus the depth during veinformation. a Pore pressure versus depth. The Middle Buntsandsteinforms a transitional zone between close to lithostatic andhydrostatic pressure. One possible scenario for the pressuredecrease is by stepwise pressure drops in the different Buntsand-stein sequences with the Rot evaporite and the top claystone layersof each sequence acting as a seal. The shaded area shows the rangeof overpressure, which is at least 18 MPa above the hydrostaticfluid pressure in the Buntsandstein. In the Rot evaporite, the fluidpressure is between hydrostatic and 18 MPa above hydrostatic. b

Horizontal pressure versus depth. The differential stress to formextension fractures can reach a maximum of 40 MPa and theshaded area displays this range. Extension fractures will form whenthe fluid pressure is 10 MPa above the horizontal stress. In theevaporite layers, the differential stress is low and therefore thehorizontal stress is close to lithostatic. (anh anhydrite layer, SSolling formation, H Hardegsen formation, D Detfurth formationand V Volpriehausen formation, rVvertical stress, rH=horizontalstress)

1051

with decreasing pressure and increases with decreasingtemperature. Therefore, we assume that the supersatu-ration of the anhydrite fluids is caused by a pressuredrop during fracturing.

The arguments above can be illustrated by a simple1D-model in which the complete Buntsandstein acted asan overpressure cell during progressive burial at 3–5-kmdepth (Fig. 15a). This model may be constrained for thepore fluid pressure by the following parameters: (1) near-lithostatic fluid pressures in the Zechstein because of thelow permeable evaporites, (2) hydrostatic fluid pressures(assumed) above Rot because of the highly permeableKeuper and Jurassic, (3) minimum fluid pressure of18 MPa above hydrostatic at a depth of 3.2 km to causeextensional fracturing, (4) pressure drops whereanhydrite precipitated (18 MPa pressure drop =20%supersaturation), (5) near-hydrostatic fluid pressuregradients along the open fractures until sealed byanhydrite.

The horizontal stress depends linearly on the effec-tive vertical stress and on the lithology. In the perme-able layers above the Buntsandstein, the horizontalstress can be calculated by rh¢=K0·rv¢ taking thecoefficient of the earth pressure at rest K0 as 0.4 (Mandl2000, p 177) (Fig. 15b). In the Buntsandstein, we can-not determine the horizontal stress exactly, becausethere are uncertainties about the fluid pressure and weonly know the fluid pressure range. However, as theminimum differential stress is 40 MPa, the range of thehorizontal stress is known. Extension fractures areexpected to form in the Buntsandstein where the fluidpressure is 10 MPa above the horizontal stress. In theRot and Zechstein evaporites, the horizontal stress isexpected to be close to the lithostatic stress, because thedifferential stress is low in these layers (Schleder andUrai, this volume).

Conclusions

In cores from the four studied boreholes, despite theirdistance of 200 km and noticeable differences in tec-tonics, similar extensional veins and vein microstruc-tures are observed. There is clear evidence for at leasttwo generations of vein formation, associated with dif-ferent parts of the burial path. The first generation ofveins formed syntectonically in subvertical fractures andis filled with fibrous calcite. Stable isotopes indicate thatthese veins precipitated from connate water (d18O = 0–4& SMOW) at temperatures between 55�C and 122�C.A generation of quartz–calcite veins is associated withthese calcite veins. No significant increase of fracturepermeability was created during the generation of theseveins.

The second generation of veins is also syntectonic,formed in steeply oriented open fractures, which areusually reactivated calcite veins. These veins are filledwith blocky anhydrite. In this second generation ofveins, fluid inclusions in anhydrite indicate precipita-

tion around 150�C, from hypersaline brines. Sulphurisotopes from anhydrites indicate that the origin of thesulphate is the underlying Zechstein. The first genera-tion calcite veins formed in a much less consolidatedrock than the second generation anhydrite veins, al-though the cohesion of the wall rock remained rela-tively low (no transgranular fractures were observedduring the whole evolution). We interpret the firstgeneration calcite veins to have formed during the earlystages of burial, between 1 km and 3 km depth, andthe second generation to have formed during progres-sive burial at a depth of minimum 3 km. However, allvein generations formed in a continuum. The secondgeneration of veins formed in a phase when the Bunt-sandstein was overpressured, the fluid pressure at least18 MPa at 3.2 km. Pore pressures in Zechstein werenear-lithostatic, dropping progressively to hydrostaticabove the Rot, providing the pressure decrease toprecipitate anhydrite veins. All mechanisms that gen-erate overpressures in sedimentary basins could havecontributed to the overpressure in the Buntsandsteinbut inflation by Zechstein fluids, released from dehy-drating gypsum (enclosed in halite), is the only requiredprocess.

Acknowledgements We thank Dr. Jentsch (EMPG) for access toBuntsandstein cores. Werner Kraus is acknowledged for thepreparation of thin sections and the fluid inclusion wafers. PhilippeMuchez (K.U.Leuven) is thanked for the use of the Linkham stageand discussions on the fluid inclusions. We are very thankful toProf. Strauss (Munster) for the analyses of the S isotopes and toDr. Joachimski (Erlangen) for the d18O and d13C isotopes. Com-ments by Anne-Marie Bouiller and an anonymous reviewer sig-nificantly improved the manuscript. This project is funded by theDFG (Hi 816/1–2) and is part of the SPP 1135 ‘‘Dynamics ofSedimentary Systems under varying Stress Conditions by Exampleof the Central European Basin System’’.

Appendix

Strain due to compaction of clays

We consider a fluid-saturated sediment, compactinguniaxially. Taking the usual notation:

Vt ¼ Vs þ Vf;

Vt ¼ total volume ½m3�; Vs ¼ solid volume ½m3�; Vf

¼ fluid volume ½m3�

Porosity / ¼ Vf

Vt

It can be shown that when the sediment changes itsporosity from /1 to /2, the volumetric stretch (sv) andthe volumetric extension (ev) are:

sv ¼Vt1

Vt2¼ Vf1 þ Vs

Vf2 þ Vs

1� /2

1� /1

and ev ¼Vt2 � Vt1

Vt1¼ sv � 1

1052

In uniaxial compaction, this leads to a finite straintensor:

D ¼1 0 00 1 00 0 1�/2

1�/1

������

������

If the veins are formed shortly after deposition (as-sume /1= 50%), and compaction continues to /2= 5%during the maximum burial at 5,000 m, Vt2 is then 52%of Vt1and this should be visible in compaction halosaround the veins (which would then act as rigid struts).As these structures are not observed, the initial porositywas smaller.

Taking /1= 20%, /2= 5%, Vt2 is reduced to 84% ofVt1. This minor volume reduction is probably not ob-servable and this fits very well with the observations ofthe veins in the Buntsandstein rocks.

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