Geological and tectonic investigations in the former Morsleben salt mine (Germany) as a basis for the safety assessment of a radioactive waste repository

Engineering Geology 61 (2001) 83±97

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Geological and tectonic investigations in the former Morsleben salt mine (Germany) as a basis for the safety assessment of a radioactive waste repository Joachim Behlau*, Gerhard Mingerzahn Federal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30655 Hannover, Germany

Abstract The safety assessment of a repository for radioactive waste is based on the results of model simulations and scenario analysis. To obtain a well founded geological basis for this, extensive geological and structural studies have been carried out for the Morsleben repository. The Morsleben repository for radioactive waste was constructed in Zechstein strata. The location of the Werra Formation (z1) was located in boreholes and the Stassfurt (z2), Leine (z3) and Aller (z4) Formations are exposed in the repository mine. The Zechstein Salt has a total thickness in the western part of the salt body of 580 m. In the eastern part it is only 380 m thick. The Morsleben repository is in the Allertal zone salt structure, the root zone of a deeply eroded salt body. This is evidenced by the relatively small thickness of the Zechstein Salt, as well as by the many blocks of the Hauptanhydrit (z3HA) in the salt, which are present mostly in the synclinal hinge zones, i.e. in the deeper parts of the structure, as a result of tectonic separation of ductile salt from competent anhydrite. The tectonics of the structure are documented by polyphase folds, characteristic of the deformation of ductile material. The general trend of the fold axes is horizontal, NW±SE, parallel to the trend of the Allertal zone. The western part of the salt structure is characterised by high, NE-vergent, isoclinal folds. The z2 to z4 strata involved in the folding extend from the salt table (top of the salt body) down to the lowest synclinal trough. In the eastern part the folds are open and symmetric with considerably smaller amplitude. Correspondingly, z4 strata mostly occur just below the salt table, z3 at the repository level, and z2 below that. The different styles of folding in the east and west are due to the structure of the rocks beneath the Zechstein Salt. The boundary between the two types of folds lies above a major fault in the rocks below the Zechstein Salt, separating a downthrown block in the west from an upthrown block in the east. NE-vergent, isoclinal folds occur above the downthrown block, where the major ascent of the salt occurred. In contrast, the folding in the upthrown eastern block has hardly been affected by the ascending salt; hence the wide, open, symmetrical folds. Thus, the structure of the rocks beneath the Zechstein determined the folding style in the overlying salt. The present-day fold structures chie¯y result from Late Cretaceous compression that affected the North German basin. The salt body is, therefore, primarily a tectonic structure and is not halokinetic. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Anhydritklippen; Hauptanhydrit; Geological model; Morsleben; Pre-Zechstein; Salt table

* Corresponding author. 0013-7952/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0013-795 2(01)00038-2

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1. Introduction

2. The geological/tectonic model

The Morsleben permanent repository for radioactive waste is located directly on the boundary between Saxony±Anhalt and Lower Saxony (Fig. 1). The repository is in a former potash and rock salt mine in the Allertal zone, a fault zone resulting from raft tectonics (Best, 1996), into which the Zechstein Salt migrated. The Allertal zone trends NE±SW (Fig. 1). The main migration of the salt began in the Keuper and ended in the Cretaceous (Best and Zirngast, 1999). The Morsleben repository has two shafts. The older shaft, Marie, was dug in 1897, the Bartensleben shaft in 1914. Potash and rock salt were mined until the 1960s. The Bartensleben part of the mine has been used as a repository for low and intermediate level radioactive wastes with alpha emitter activity concentrations up to 4 £ 10 8 Bp/m 3 since 1981. No wastes have been emplaced since 1998.

Commissioned by the Bundesamt fuÈr Strahlenschutz (BfS, Federal Of®ce for Radiation Protection), the Bundesanstalt fuÈr Geowissenschaften und Rohstoffe (BGR, Federal Institute for Geosciences and Natural Resources) has prepared a detailed geological/tectonic model of the salt structure as a basis for the safety assessment of the repository or for the closing of the repository. The model is also to aid planning of back®ll drifts and estimation of the necessary work. The following information was necessary for the modelling: ² the stratigraphic sequence of the salt beds; ² the tectonic structure of especially the Stassfurt potash (z2SF) and the Hauptanhydrit (z3HA), which are potential pathways and reservoirs of brine; ² the locations of the base of the Zechstein and the salt table, i.e. the thickness of the geological barrier.

Fig. 1. Location map of Morsleben site.

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2.1. The investigations In order to model the Morsleben repository, it was necessary to remap the accessible exposures in the Bartensleben and Marie parts of the mine. The data from the archives were also used, but were not suf®cient for modeling. Only petrographic and stratigraphic data for mapping the reserves of potash and rock salt were available. The geological data were limited to the mine levels and a few cross sections along the main cross cuts. This was not suf®cient to develop a comprehensive model of the complicated geological structures. The remapping was done at a scale of 1:100 along about 32 km of drifts and at a scale of 1:500 or 1:1000 along about 13 km of drifts and in chambers. Detailed mapping at a scale of 1:20 was also carried out in some parts of the mine. Cores from about 50 boreholes were reevaluated, a number of boreholes were cored for geotechnical studies, and 11 new boreholes were cored for geological studies in the eastern part of the Bartensleben part of the mine. Ground-penetrating radar (GPR) was used in the drifts and in boreholes to investigate the salt table, the base of the salt structure, and internal structure (Eisenburger and Gundelach, 2000). This part of the investigation covered 21 pro®le kilometers. The geological mapping was carried out by the Deutsche Gesellschaft zum Bau und Betrieb von Endlagern fuÈr Abfallstoffe mbH (DBE, The German Company for the Construction and Operation of Repositories for Wastes, Ltd) from November 1992 to August 1997, supervised by BGR. Most of the GPR measurements, evaluation, and geological interpretation were carried out by BGR. 2.2. The components of the geological/tectonic model Detailed geological and tectonic maps were prepared on the basis of the mapping data. A CAD program was used to process the data. The following maps were prepared at a scale of 1:2000: ² 32 geological maps of the seven main levels and the salt table, ² 21 geological pro®les perpendicular to the strike of the salt structure, ² 6 isohypse maps of the salt table,

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² as well as maps at a scale of 1:10,000, e.g. a structural map of the main fold and a map of the mine showing the locations of the map sections and pro®les.

3. Results 3.1. Stratigraphy Stassfurt to Aller beds (z2±z4) are exposed in the Morsleben mine. The Werra sequence (z1) has been studied only in borehole cores. The Aller sequence is exposed mainly in the western part of the mine. Only the basal carbonate and sulphate beds of the Werra sequence are present. The Werra rock salt bed is absent. The contact of the Werra sequence to the Zechstein is concordant, forming a prominent GPR re¯ector. The brittle deformation and faults at the base of the Zechstein are readily recognizable in the GPR data. The facies of the mapped Zechstein beds, with some exceptions, corresponds to that of the Northwest German basin (Bornemann, 1991; Jaritz, 1973; LoÈf¯er, 1962). The z2 to z4 beds are nearly complete with little deformation (Table 1). Of special interest is the Anhydritklippen of the Hauptanhydrit, which has a maximum thickness at Morsleben of about 110 m (Table 2). This bed has also been described by Schachl (1991) in the Braunschweig-LuÈneburg salt mine. The stratigraphy of Kosmahl (1969) was used for the Hauptanhydrit (z3HA). 3.1.1. Anhydritklippen of the Hauptanhydrit (z3HA) A particularly prominent feature of the Hauptanhydrit is its irregular morphology. Whereas the base of the z3HA and the underlying GebaÈnderter Deckanhydrit to the Leine Karbonat are concordant, the top of the Hauptanhydrit has a very irregular morphology. Fulda (1929) observed reef-like forms of the top of the Hauptanhydrit together with a practically undisturbed base and coined the term `Anhydritklippen of the Hauptanhydrit' for this. Hemmann (1968) studied the morphology of the top of the Hauptanhydrit in the eastern part of the Subhercynian basin. He classi®ed the thickening and thinning of the

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Table 1 Stratigra®c Stratigraphy table of at Morsleben site, after Bornemann (1991) Formation Zechstein 4 (Aller)

Zechstein 3 (Leine)

Zechstein 2 (Stassfurt)

Zechstein 1 (Werra)

Member

Symbol

Mean thickness (m)

Schnee±Rosensalz Basissalz Pegmatitanhydrit Roter Salzton

z4 z4SS/RS z4BS z4PA z4RT

.1 1±2 0.5±1 0.5±4

Tonmittelsalz Schwadensalz Anhydritmittelsalz Buntes Salz Bank-/BaÈndersalz Orangesalz Liniensalz Basissalz Hauptanhydrit Leine±Karbonat Grauer Salzton

z3 z3TM z3SS z3AM Z3BT z3BK/BD z3OS z3LS z3BS z3HA z3LK z3GT

2.7±7.4 5.2±18.7 5.2±31.2 0±2 6±15 6±12 3±22 0.2±1.5 30±50 0.3±1 2

GebaÈnderter Deckanhydrit Decksteinsalz Kali¯oÈz Stassfurt Kieseritische È bergangsschichten U Hangendsalz Hauptsalz Basissalz Basalanhydrit Stassfurt±Karbonat

z2 z2DA z2DS z2SF z2UE

1.8±2.2 2 0.1±3 (10) 0.5±3

z2HG z2HS z2BS z2BA z2SK

0.5±5 100±200 6, 8 2±3 2.5±3.5

Werra±Anhydrit Werra±Karbonat Kupferschiefer Werra±Konglomerat

z1 z1WA z1WK z1KS z1KG

45 5±10 0.05±30

Hauptanhydrit according to size, form and completeness of the Hauptanhydrit beds. The observations of Hemmann (1968) were largely con®rmed by our mapping and further details were added. 3.1.2. Characteristics of the Anhydritklippen Morphology ² The Hauptanhydrit extends nearly vertically to various distances into the Leine sequence, sometimes as far as into the Anhydritmittelsalz (Table 2).

² The outlines of the Anhydritklippen are rounded to oblong in plan. ² In cross section, rounded to mushroom-like protuberances with overturned sides that locally extend several meters laterally into the neighboring Leine beds. ² There is a lagoon-like depression at the top of the Anhydritklippe, surrounded by higher parts like an atoll. Internal structure ² The z3HA1 and z3HA7 beds have nearly their original thicknesses.

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Table 2 Stratigraphy of the Anhydritklippen, after Kosmahl (1969); Bornemann (1991) (data in bold indicate beds are absent in very large Anhydritklippen) Member Anhydritmittelsalz

Bank-/BaÈndersalz Orangesalz Liniensalz Basissalz Hauptanhydrit

Bed Anhydritmittel 6 Anhydritmittelsalz 5 Anhydritmittel 5 Anhydritmittelsalz 4 Anhydritmittel 4 Anhydritmittelsalz 3 Anhydritmittel 3 Anhydritmittelsalz 2 Anhydritmittel 2 Anhydritmittelsalz 1 Anhydritmittel 1 Buntes Salz

Anhydritschale Schwarzes TonbaÈnkchen BaÈnderanhydrit Maseranhydrit FlaserÐ, BaÈnderanhydrit BuÈndelanhydrit Lamellenanhydrit 3 Lagenanhydrit Schlierenanhydrit Flaseranhydrit Lamellenanhydrit 2 Flocken-, Flaseranhydrit Lamellenanhydrit 1

² The thickness of the z3HA8 to z3HA11 beds are much greater than the original thicknesses, which resulted in the formation of the Anhydritklippen. ² Pseudomorphs of anhydrite after gypsum up to 30 cm across occur frequently in the z3HA11 bed in the form of `crystal turf', called `selinite turf'. ² Little folding was observed in the Anhydritklippen. The z3HA12 and z3HA13 beds form the steep ¯anks of the Anhydritklippen with discordant contacts with the surrounding salt. ² In the Anhydritklippen, the z3HA12 and z3HA13 beds are present only in the lower part. ² Pseudomorphs of rock salt, kieserite, and carnallite are present in interstices in the z3HA11 bed.

Symbol

Mean thickness (m)

z3AM z3AM6/ah z3AM5/na z3AM5/ah z3AM4/na z3AM4/ah z3AM3/na z3AM3/ah z3AM2/na z3AM2/ah z3AM1/na z3AM1/ah Z3BT z3BK/BD z3OS z3LS z3BS z3HA z3HA13 z3HA12 z3HA11 z3HA10 z3HA9 z3HA8 z3HA7 z3HA6 z3HA5 z3HA4 z3HA3 z3HA2 z3HA1

0.5±2.5 1±7 0.01±0.1 0.7±5 0.3±1.6 1.5±6.2 0.4±1.2 0.4±2.8 0.01±0.14 0.4±4.6 0.01±0.3 0±2 6±15 6±12 3±22 0.2±1.5 up to 110 .1 0.01±0.05 7±50 3±4 1±12 5±13 , 0.1 4±13 3.5±8.5 3.5±4 , 0.3 2±3 0.8

3.1.3. Changes in the neighbouring and overlying Leine sequence ² The Leine rock salt beds (z3LS±z3AM2/na) have discordant contacts at the ¯anks of the Anhydritklippen and are, in part, steeply dipping. ² Within a few meters of the Anhydritklippen, the Leine rock salt contains thin layers, spots, ¯akes, and nodules of anhydrite. ² Some parts of the surrounding Liniensalz to Bank-/ BaÈndersalz beds contain blocks of anhydrite up to a cubic meter that lie completely within a single layer. ² The rock salt is, in part, colored red or grey in the vicinity of the Anhydritklippen.

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² Large Anhydritklippen that extend to the Anhydritmittelsalz (z3AM) are directly overlain by the Anhydritmittel beds 3 and 4 (z3AM3/ah and z3AM4/ah), which in such places are three to ®ve times their normal thickness. The Anhydritmittel beds (z3AM/ah) are anhydrite beds with a normal thickness of up to 2.5 m. Pseudomorphs of anhydrite after gypsum are also present in these beds. Directly above the Anhydritklippen, the pseudomorphs are up to 1 m across; in the normal facies they are only a few centimeters across. Above the Anhydritklippen, the Anhydritmittel beds 3 and 4 are mostly in direct contact with no intercalated Anhydritmittelsalz (z3AM3/na) (Table 2). 3.1.4. Genesis of the Anhydritklippen Hemmann (1968) and Schachl (1991) assumed the Anhydritklippen developed diagenetically in the form of a diapir-like ascent of the unconsolidated crystals, from which large amounts of water were released during the transformation from gypsum to anhydrite. The observations given in Section 3.1.3 lead to the conclusion that the Anhydritklippen of the Hauptanhydrit are primary sedimentary structures and that the greater thickness of the Anhydritmittel beds 3 and 4 above the Anhydritklippen is also of primary origin (Landsmann, in prep.). The most important characteristics are as follows: ² Preservation of the pseudomorphs of anhydrite after gypsum. Borchert and Baier (1953) describe the pseudomorphs of anhydrite after gypsum in the Basalanhydrit and the Hauptanhydrit and their genesis. They state that twinned swallow-tail crystals of gypsum precipitated on the paleosediment surface with later diagenesis to anhydrite with retention of the gypsum crystal structure. Because the pseudomorphs are now intact in the beds at the top of the Anhydritklippen, the Anhydritklippen cannot have formed by a diapirlike ascent. ² The individual subzones of the Hauptanhydrit in all of the exposures in the Morsleben repository can be correlated. ² Sedimentary structures, e.g. crystal turf, breccias, and turbidites, at the top of the Hauptanhydrit in the

Gorleben salt dome have been described by BaÈuerle et al. (2000). The presence of these structures is also in con¯ict with the hypothesis of a diapir-like ascent of the Hauptanhydrit.

3.2. The periphery of the salt structure 3.2.1. The ¯anks The salt structure is bounded to the SW by the Keuper (Lappwald block) and to the NE by the Bunter (Weferling Trias block) (Fig. 2). The Lappwald block separated from the Trias block during the Keuper. The Zechstein Salt began to migrate into the Allertal zone at that time, ending in the Cretaceous. Since then, the Zechstein has been subjected to uplift and subrosion (Best and Zirngast, 1999). 3.2.2. Salt table The location of the salt table was determined by numerous GPR measurements. To calibrate the GPR data, lithological logs from boreholes made to the salt table were used. The salt table is mostly horizontal to slightly wavy at an average depth of 140 m below m.s.l. A few depressions due to subrosion were observed, with a maximum depth of about 35 m below. The largest such depressions were west of the Main Syncline (Fig. 2, section B). The subrosion depressions are in the overturned Kali¯oÈz Stassfurt (z2SF, the potash bed of the z2 formation) at the salt table (Table 1). The deepest part of the depressions is always in the Kali¯oÈz Stassfurt, owing to the high solubility of the potash. The rock salt of the Hauptsalz (z2HS) caves in when the potash, which dips below it, is dissolved. The Hauptanhydrit around a subrosion depression is for this reason also below its normal level. Brine can enter the Hauptanhydrit through ®ssures in the anhydrite. Similar observations have been made in the Gorleben salt dome (Bornemann, 1991). If the beds were not overturned, or if the z2SF potash were at the salt table without the GebaÈnderter Deckanhydrit to Hauptanhydrit, no signi®cant selective subrosion would be observed. The salt table would then be at its normal level.

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Fig. 2. Geological cross sections of Morsleben site.

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3.2.3. Rotliegend The pre-Zechstein to the west of the Morsleben salt structure is at a depth of 710 m below m.s.l. and dips about 108 to the SW. To the east the base of the Zechstein is nearly horizontal at a depth of about 500 m below m.s.l. This indicates a downthrown block in the west and an upthrown block with no faults in the east. The downthrown block has a complicated `positive ¯ower structure' formed by wrench faulting during the Late Cretaceous (Best and Zirngast, 1999). The boundary between the upthrown and downthrown blocks passes between the Main Syncline and the East Anticline in the Bartensleben part of the mine and below the western limb of the East Syncline in the Marie part of the mine. 3.2.4. Thickness of the Zechstein The Zechstein has a thickness of about 580 m to the west of the Morsleben repository, decreasing in the direction of the salt structure, and a thickness of about 380 m to the east of the repository. 3.3. Tectonics Polyphase fold characteristics of ductile rock were formed during the ascent of the salt in the Allertal zone. The general strike direction of the large folds is NW±SE, in the trend direction of the Allertal zone (Fig. 2). The anticlines consist of the Stassfurt sequence (z2), the synclines consist of the Leine (z3) to Aller (z4) beds. The Hauptanhydrit of the Allertal zone is more widely distributed than in other salt structures. Its greatest distribution at the Morsleben repository is at the fourth and lower levels (Fig. 3). The reason for this is the tectonic separation of the ductile salt from the competent anhydrite, i.e. the Hauptanhydrit remains in the deeper, hinge parts of the syncline as a relatively brittle, immobile body, owing to the higher density than that of rock salt and is only sporadically dragged up by the ascending, more mobile salt. This leads to shearing of the salt at the top and bottom of the Hauptanhydrit. The main shearing horizon between the mobile Stassfurt sequence and the

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passive Leine and Aller sequences is the Kali¯oÈz Stassfurt (z2SF). It is present as kieseritic sylvinite at the lower depths of the Allertal salt structure and broken carnallite in the upper parts. In the carnallite, ¯ow folds are frequently observed. Above the Hauptanhydrit, the rock salt is sheared at the boundary to the Liniensalz. Because the rock salt beds of the Liniensalz have a higher competence than the carnallite, there is considerably less shearing in it than in the rock salt and there is only folding and thinning of the beds of the Linien Salt at the Liniensalz/Hauptanydrit boundary (Bornemann et al., 2000). It is concluded from the wide distribution of the Hauptanhydrit that the salt structure of the Allertal zone is a deeply eroded root structure (Fig. 2). This is also indicated by the large thickness of the cap rock of up to 200 m. About 73% of the salt diapir has been subroded (Best and Zirngast, 1999). 3.3.1. Fold structures in the high and subsided block The folds are isoclinal in the western part of the salt structure, becoming close in the central part and open in the eastern part (Fig. 3). The brittle Hauptanhydrit fractures during folding. The smaller the angle between the fold limbs, the smaller the blocks. Thus, in the western part of the salt structure (in the shear zone, see below), the fracturing of the Hauptanhydrit is the most extensive. 3.3.1.1. Downthrown block. The isoclinal folds in the western part of the salt structure are several hundred meters high with a width of several meters to several tens of meters. The folds have a NE vergence of about 558. The folds contain Stassfurt to Aller beds from the salt table to the deepest parts of the salt structure (Fig. 3). The z2 has broken through the z3 and z4 beds in the anticlinal folds. Especially in the Marie part of the mine the ascending z2 has dragged the z3 and z4 beds (which are exposed in the Bartensleben part of the mine), so far that z4 beds are observed only at the salt table in the GPR measurements. As a result of the ascent of the Stassfurt beds, the synclinal folds are pressed together, with considerable

Fig. 3. Geological map at the salt table.

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J. Behlau, G. Mingerzahn / Engineering Geology 61 (2001) 83±97

thinning of the strata in the folds. In the western limbs of the synclinal folds, the Leine and Aller beds are very thin. The Hauptanhydrit is still present only in the deepest parts of the synclinal folds and is often highly fragmented and distorted (e.g. in the folds to the west and in the South Syncline). Because there is hardly any difference in the competence of the strata in the western part of the salt structure, the folds are mainly shear folds. This is indicated by the foliation planes parallel to the fold axes. Moreover, there are indications of the ¯ow folds typical of a salt diapir (de Boer, 1971), in addition to the large variations in the plunge of the fold axes. The west ¯ank of the salt structure is the most strongly affected tectonically. It can be described as a shear zone on the basis of the characteristics of the folds (Fig. 4). That the west ¯ank of the salt structure is the tectonically most strongly affected is indicated by the lithology of the Kali¯oÈz Stassfurt (z2SF), which in this area consists only of kieseritic sylvinite. The west limb of the Main Syncline is a transition from the strictly isoclinal folds to the west and the open folds to the east. The entire west limb of the Main Syncline contains blocks of Hauptanhydrit. The greatest thickness of the cap rock is above this large fold. The cap rock is also rather thick above the Southern Syncline. The large cap rock thickness and the vergent, isoclinal folds demonstrate that the main migration path and ascent of the salt in the Allertal zone in the area of the Morsleben repository occurred above the downthrown block. 3.3.1.2. Upthrown block. To the east, the folds become increasingly open with no signi®cant vergence. The width of the z3±z4 synclines increases abruptly to several hundred meters. The z2 anticlines remain narrow. The outermost layers of most of the synclines consist nearly completely of Hauptanhydrit blocks. The Leine beds form several smaller folds within the large synclines. These smaller folds are also isoclinal. Near the salt table, the Aller beds are included in these folds. There are differences in competence owing to the presence of intercalated Hauptanhydrit and

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meter-thick Anhydritmittel (z3AM/ah). Flexural¯ow folding occurred at the contacts. This is indicated by the different orientations of the foliation and the slickensides at the lower contact of the Anhydritmittel. The smaller folds within the syncline are affected by shearing. The Stassfurt beds were pushed up in places through the Hauptanhydrit, forming the anticlines between the synclines. This did not occur so extensively as in the area of the downthrown block (Fig. 3a). Thus, z4 is present in large areas of the salt table in the Bartensleben part of the salt structure, z3 at the level of the mine, and z2 in the lower levels of the structure (Fig. 2). The upthrown block has been little affected tectonically, contributing little to diapir formation (Fig. 3). This is also indicated by the small thickness of the cap rock. The fold pattern indicates that the salt was subjected to only compressive stress here. 3.3.2. Folds along the strike of the salt structure If the folds are followed from the Bartensleben part of the mine in the south to the Marie area in the north, it is seen that some of the folds merge and there are fewer folds in the north. 3.3.2.1. Shear zone. The shear fold character of the folds on the west side of the salt structure extends unchanged from Bartensleben to Marie; they are somewhat wider in Bartensleben than in Marie. There are more and larger Hauptanhydrit blocks at deeper levels at Bartensleben than at Marie. From the south to the Marie shaft, the strike of the fold axes follows the general trend of the Allertal zone. The Leine and Aller beds extend up to the salt table. North of the Marie shaft, Stassfurt beds increasingly extend up to the salt table. Beginning at Marie shaft, the fold axes no longer have a NW±SE strike and instead trend WNW towards the west ¯ank of the salt structure. The fold axes plunge about 108. Further north, the folds again have general NW±SE trend. The change in the strike of the fold axes is caused by increased migration of the Zechstein in this area. At the contact of the Zechstein with the country

Fig. 4. Tectonic map at the salt table of Morsleben site.

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rock in the Marie part of the mine, RoÈt evaporite was observed, mixed with some Zechstein, showing intensive shearing. The beds near the contact dip steeply with no discordance between the Zechstein and the RoÈt. There is a continuous RoÈt anhydrite bed in the exposure. Such occurrences of blocks of salt beds of the country rock (e.g. RoÈt, Keuper, Muschelkalk) are known at the margins of other Zechstein structures. These beds are not usually very thick. 3.3.2.2. Shaft anticline. Between Marie in the NW and Bartensleben in the SE, the Shaft Anticline divides into three anticlines and two synclines. The fold axes follow the trend of the Allertal zone with little or no plunge. All the folds also have a NE vergence of about 558. The exposures show that the Shaft Anticline is internally strongly folded. The foliation indicates that these internal folds are mostly shear folds. The Shaft Anticline divides ®rst into the Shaft East Anticline II and the Shaft Anticline West, separated by the South Syncline. Further south, the South Syncline divides into the South Syncline I and the South Syncline II, separated by Shaft Anticline East I. The shear zone lies to the west of the Shaft Anticline West and the Main Syncline lies to the east of the Shaft Anticline East II (Figs. 2 and 4). The Shaft Anticline and the anticlines into which it divides consist of Stassfurt beds. The sequence from the Streifensalz, to the Kristallbrockensalz, the È bergangsschichten, Hangend Salt, the Kieseritische U to the Kali¯oÈz Stassfurt (z2SF) is exposed in the repository drifts. The Decksteinsalz and the GebaÈnderter Deckanhydrit are thinned in places. Because the Shaft Anticline extends up to the salt table and the seismic data indicates a wide base of the anticline, the KnaÈuelsalz (z2HS1) and Streifensalz (z2HS2) may be expected to be present in the deeper parts of the anticline core. The South Syncline in the mine exposures consists of Leine beds to Bank-/BaÈndersalz (z3BK/BD). Towards the salt table, Aller beds are also present. The South Syncline I is a normal syncline only at the salt table. At deeper levels it consists only of a thin kieserite bed, the remainder of the squeezed out the z2SF potash. The appearance of the South Syncline I is similar to that of the East Anticline/ Layer K system (see below). A bromine pro®le,

however, showed that there is a normal stratigraphic Stassfurt Potash sequence in both limbs of the syncline. Thus, the syncline is a fold and not a fault, as is the case with Layer K. 3.3.2.3. Main Syncline. The Main Syncline extends from the Bartensleben to the Marie part of the mine. Most of the mined area is within this syncline. All beds of the Leine sequence are exposed. On the basis of the internal isoclinal folds, Aller beds are expected at the salt table. North of the East crosscut drift in Bartensleben, the Main Syncline becomes wider, owing to the plunging of the East Anticline to the east. This provides more room for the overlying Aller beds. North of the East cross-cut in Marie, the East Syncline and Layer K fade out and the Main Syncline and the Shaft Anticline are the largest folds (Figs. 2 and 4). The Hauptanhydrit is present in the entire west limb of the Main Syncline. The subrosion depressions in the salt table mentioned above are just to the west of the west limb. There is a typical Anhydritklippe of Hauptanhydrit along the entire length of the east limb of the Main Syncline. The Hauptanhydrit, the East Anticline does not extend to the salt table because the East Anticline has not ascended as high as the salt table (Fig. 2, cross section B). The Main Syncline does not have the NW±SE strike and ca. 558 NE vergence of the other structures. The fold axis of the Main Syncline plunges to the SE at a low angle. Only in the area of the Cross Anticline in Marie is the plunge to the NW. This change is caused by the formation of the Cross Anticline (Fig. 4). The west limb of the Main Syncline is characterized by highly deformed, in places staggered blocks of Hauptanhydrit. The z2SF potash accumulated on the upstream side of the blocks. On the basis of these observations, it may be concluded that the Hauptanhydrit blocks moved horizontally towards each other. Analysis of the displacement of the Hauptanhydrit blocks indicates lateral movement with a right-lateral strike-slip fault character along the entire west limb of the syncline, from the southeastern to northern part of Bartensleben. Similar observations indicate vertical movement with a reverse fault character. Thus, there

J. Behlau, G. Mingerzahn / Engineering Geology 61 (2001) 83±97

were two movements, a horizontal and a vertical one, each affecting the other. North of Pro®le M in the Marie part of the repository (Fig. 2), the Main Syncline has been strongly tectonically compressed. The reason for this is the large width of the Rim Anticline I and the merging of the Layer K with the west limb of the Rim Anticline I. Owing to the small width of the Main Syncline, there is not enough room for the Hauptanhydrit blocks, which therefore do not extend up to the salt table. Only northwest of this area, as the Rim Anticline I becomes narrower, do the Hauptanhydrit block extend up to the salt table. The Anhydritklippen characteristic of the Main Syncline are also present in Marie. 3.3.2.4. East Anticline, Layer K. The East Anticline is present as a large fold only in the area of the East cross cuts in Bartensleben. It does not extend up to the salt table, i.e. the Hauptsalz does break through the younger, overlying z3 and z4 (Fig. 2, cross section B). The fold axis plunges relatively steeply to both the NW and SE. The East Anticline extends as a large fold about 600 m to the north and about 170 m to the south. Its core consists of Stassfurt beds. This anticline extends as a smaller fold into the southeastern part of Bartensleben. The Stassfurt beds do not break through the overlying Hauptanhydrit there. The Layer K in the Marie part of the repository is the equivalent of the East Anticline in Bartensleben (Fig. 4). The East Anticline, the Layer K and the Cross Anticline were formed by compression that took place in the North German basin during the Cretaceous (Best, 1996). The compression caused Zechstein evaporite to migrate into this area from the SW, from the area below the Lappwald block. Most of this migrated into the west and north parts of Bartensleben, causing the large folds that were already formed to be compressed, and shoved to the NW or SE. They were thus formed after the main folding. Owing to the inertia of the salt beds already present, cross folds of various sizes were formed at the margins of this accumulation area. An example is the cross folds in the southernmost and northernmost

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parts of Bartensleben. The most prominent one is the Cross Anticline in Marie (Fig. 4). Fracturing of Layer K by fault released the stress that had developed. There was about 100 m of movement along this fault. Layer K and the East Anticline are both at the boundary between the downthrown and upthrown blocks. Because the upthrown block below the Marie part of the mine is further west than that below Bartensleben (Best, 1996), the area of the downthrown block below the salt structure is smaller and there was therefore, no room for a large fold similar to the East Anticline to develop. 3.3.2.5. East Syncline. Large Anhydritklippen are present in the east and west limbs of the East Syncline in the Bartensleben part of the repository. They are major features in the East and Rim Synclines. The Hauptanhydrit has a normal thickness (about 50 m) in the 80±100 m wide core of the East Syncline. In the limbs of the syncline, the Anhydritklippen have thicknesses of up to 110 m (Fig. 2, section B). The core of the East Syncline consists of Leine beds. Owing to the stiffness of the Anhydritklippen, the Leine beds thinned considerably during the folding of the salt. This resulted in a small anticlinal fold and small synclinal folds to each side. The Anhydritmittelsalz and the Schwadensalz were included in these two small synclinal folds, which become wider above Level 2 in the mine, since the Anhydritklippen are present only in the east limb at this depth. This situation can be followed northwards as far as the northern part of Bartensleben. Aller beds are present in the East Syncline only below the salt table and can be observed only in the GPR data. The East Syncline is also exposed in the Marie part of the repository. There is an Anhydritklippe in the east limb and none in the west limb. The difference in vergence between the Main and East Synclines can also be seen in Marie, although not as large as in Bartensleben. This is because the East Syncline is narrower and the salt migrated along the Layer K fault. The carnallitite in the fault acted as lubricant (Fig. 2, cross section M). The development of the Layer K fault was due to the difference of as

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J. Behlau, G. Mingerzahn / Engineering Geology 61 (2001) 83±97

much as 160 m between the levels of the upthrown and downthrown blocks, causing the ascent of the salt to be much steeper here in Marie than in the eastern part of Bartensleben. The East Syncline merges north of the Marie shaft with the East Rim Anticline I together with Lager K (Fig. 4). 3.3.2.6. East rim folds. The East Rim Anticline I separates the East Syncline from the east rim of the salt structure, even though it is not completely developed as a large fold in the Bartensleben part of the mine. The fold axis of the East Rim Anticline I plunges generally to the SE. In the Marie part of the repository, the East Rim Anticline I has developed as a large fold, extending up to the salt table. The East Rim Syncline I and the East Rim Anticline II to the west of the East Rim Anticline I are large folds only in this area. For this reason, the Hauptanhydrit blocks extend up to the salt table only in Marie. 3.3.3. Results of the structural analysis The folds observed today in the salt are the result of the effects of wrench faulting in the North German basin during the Late Cretaceous on the folds created during the ascent of the salt (Best and Zirngast, 1999). The main folds have the same NW±SE trend as the Allertal zone. The fold system has an echelon-like character. This is particularly central and eastern parts of the salt structure. The Cretaceous compression led to the development of large folds, some of which are perpendicular to the main trend of the Allertal zone. These `cross' folds were formed after the main folding phase. At least two generations of folds were determined for the Stassfurt sequence and more than two generations for the Leine and Aller sequences. The main folds in the salt can be seen in all of the exposures. The Stassfurt sequence had a much greater mobility than the Zechstein 3 and 4, owing to its much greater thickness, and is thus responsible for the halokinetic movements leading to isoclinal folding. The z2SF potash, more mobile than the units to either side, acted as a buffer between the two, reacting with extreme thinning or by blocking up.

The plunge of the fold axes of the large folds varies only in the western part of the Morsleben repository. In the central and eastern parts, the fold axes are horizontal. Thus, the steeply plunging fold axes typical of salt diapirs (Hartwig, 1928) is absent. The small thickness of the Stassfurt sequence, the large number of Hauptanhydrit blocks, and the large thickness of the cap rock are evidence for the tectonic origin of the salt structure and that the present salt structure is a deeply eroded residual structure. The movements in the pre-Zechstein basement are re¯ected in the deformations of the Zechstein salt. This can be seen especially at the boundary between the upthrown and downthrown blocks. The movements in the pre-Zechstein basement led to differences in the folds above the upthrown and downthrown blocks and to fewer folds in the northwestern part of the salt structure. The reason for the latter is that the boundary between the upthrown and downthrown blocks approaches the west margin of the salt structure in that area, leaving little room for large folds to develop. Above the upthrown block, there was room for the east rim folds northeast of Marie to develop into large folds. A similar situation exists at the Braunschweig±LuÈneburg salt mine, where the pre-Zechstein base is also divided into an upthrown and a downthrown (Schachl, 1991). At Morsleben, however, the downthrown block is on the northeast side of the salt structure and the upthrown block on the southwest side. From Morsleben in the south to the Braunschweig±LuÈneburg in the north, the upthrown blocks form a horst between the large folds in the salt to the west and those in the east, with smaller folds above the horst. This is in agreement with the observation that the large folds in Bartensleben wedge out in the Marie part of the repository. The rim folds in the eastern part Bartensleben show little development; further north in Marie they have developed to large folds, developing further to a large fold system at the Braunschweig-LuÈneburg mine. The system of large folds in the Morsleben repository is not present at the BraunschweigLuÈneburg mine or is present only as a little developed system at the west side of the salt structure. Thus, an en echelon-like structure can be recognized over large distances in the system of large folds.

J. Behlau, G. Mingerzahn / Engineering Geology 61 (2001) 83±97

4. Summary Important questions related to the safe closing and back®lling of the repository have been answered by the geological and tectonic modeling of the Morsleben salt structure: 1. The boundaries of the salt structure were determined: the location of the base, the ¯anks, and the salt table. 2. Two styles of folding were determined: ± In the west, there is a shear zone in which the folds are isoclinal and NE vergent at about 558. The folds have a large amplitude and contain z2 to z4 beds from the level of the salt table to the deepest level of the hinge area. ± In the east, there are open, symmetric folds with nearly no vergence and smaller amplitudes. The folds contain z4 only at the level of the salt table, z3 only at the level of the repository, and z2 only below the mine. 3. The locations of beds that can potentially function as pathways for brine (the z2SF potash and the Hauptanhydrit) and the distance to the emplacement drifts were determined. 4. The genesis of the salt structure was induced tectonically. The salt migrated and ascended from the west. The eastern part of the structure was isolated from this migration and was in¯uenced only by compression.

References BaÈuerle, G., Bornemann, O., Mauthe, F., Michalzik, D., 2000. Turbidite, Breccien und Kristallrasen am Top des Hauptanhydrits (Zechstein 3) des Salzstocks Gorleben. Z. Dt. Geol. Ges. 151 (1±2), 39±125. Best, G., 1996. Floûtektonik in Norddeutschland: Erste Ergebnisse re¯exionsseismischer Untersuchungen an der Salzstruktur Oberes Allertal. Z. dt. geol. Ges. 1147 (4), 455±464.

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Best, G., Zirngast, M., 1999. Reconstruction of the structural development of the exhumed upper Allertal salt structure. J. Conf. Abs 4, 518. de Boer, H.U., 1971. GefuÈgeregelung in SalzstoÈcken und HuÈllgesteinen. Kali und Steinsalz, 5, pp. 403±425. Borchert, H., Baier, E., 1953. Zur Metamorphose ozeanischer Gipsablagerungen. N. Jb. Miner. 86, 103±154. Bornemann, O., 1991. Zur Geologie des Salzstocks Gorleben nach den Bohrergebnissen. BfS-Schriften, 4/91, 67 S., 13 Abb., 24 Anl., 5 Tab., Salzgitter. Bornemann, O., Fischbeck, R., BaÈuerle, G., 2000. Investigation of deformation textures of salt rock from various Zechstein units and their relationship to the formation of the salt diapirs in NW Germany. Proc. 8th World Salt Symposium (Salt 2000), 1 89-94, 5 Abb., Den Haag. Eisenburger, D., Gundelach, V., 2000. GPR-Measurement for Determining 3-dimensional Structures within Salt Deposits. Proc. 8th World Salt Symposium (Salt 2000), 1: 113-118, 11 Abb., Den Haag. È ber Anhydrit-Klippen. Kali und verwandte Salze Fulda, E., 1929. U 23 (9), 129±133. Hartwig, G., 1928. Schematismus der Salztektonik auf nordhannoverschen Salzaufpressungspfeilern, mechanisch-kinetisch aus dem Bilde stratigraphischer Salzkulissenfaltung abgeleite. Kali 22 (20), 310±315 see also 21: 325-329, 22: 344-347, 23: 361364, 24: 374-80. Hemmann, M., 1968. Zur Ausbildung und Genese des Leinesteinsalzes und Hauptanhydrits (Zechstein 3) im Ostteil des subherzynen Beckens. 214 S., 52 Abb., 17 Anl., 2 Tab., 21 Taf., Diss. Bergakademie Freiberg. Jaritz, W., 1973. Zur Entstehung der Salzstrukturen Nordwestdeutschlands. Geol. Jb. A10, pp. 77. Kosmahl, W., 1969. Zur Stratigraphie, Petrographie, Genese und Sedimentation des GebaÈnderten Anhydrits (Zechstein 2), Grauen Salztones und Hauptanhydrits (Zechstein 3) in Nordwestdeutschland. Beih. Geol. Jb. 71. Landsmann, O. (in prep.): Strukturell-petrographische Analyse zur Klippenbildung des Hauptanhydrits (Zechstein 3) in den Grubenfeldern der Schachtanlagen Bartensleben und Marie (Allertalzone/Subherzynes Becken). Diss., TU Braunschweig. LoÈf¯er, J. 1962. Die Kali- und SalzlagerstaÈtten des Zechsteins in der Deutschen Demokratischen Republik. -Freiberger Forschungshefte, C 97/III Geologie, 347 S., 135 Abb., 89 Tab., Berlin. Schachl, E., 1991. Das Steinsalzbergwerk Braunschweig-LuÈneburg. Schichtlagerung in der Wurzelzone des Salzstockes. Zbl. Geol. PalaÈont. 4, 1223±1245.