Zechstein Salt Research Articles

Tomographic P wave velocity and vertical velocity gradient structure across the geothermal site Groß Schönebeck (NE German Basin): Relationship to lithology, salt tectonics, and thermal regime

Seismic wide‐angle data were collected along a 40‐km‐long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre‐Permian sediments. Wide‐angle reflections are modeled and interpreted as top Zechstein and top Pre‐Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2–3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in place at the salt lows. We interpret a narrow subvertical low‐velocity zone under the salt pillow at GrSk 3/90 as a fault in the deep Permian to Pre‐Permian. This WNW‐ESE trending fault influenced the location of the salt tectonics and led to the formation of a fault‐bounded graben in the Rotliegend sandstones with optimal mechanical conditions for geothermal production. Thermal modeling showed that salt pillows are related to chimney effects, a decrease in temperature, and increasing velocity. The assumed variations in salt lithology, density, and strain must thus be even higher to compensate for the temperature effect.

The internal geometry of salt structures – A first look using 3D seismic data from the Zechstein of the Netherlands

We present a first look at the large-scale, complexly folded and faulted internal structure of Zechstein salt bodies in NW Europe using 3D reflection seismic reflection data from two surveys on the Groningen High and the Cleaver Bank High. We focus on a relatively brittle, folded and boudinaged, claystone–carbonate–anhydrite layer (the Z3 stringer) enclosed in ductile salt. A first classification of the structures is presented and compared with observations from salt mines and analogue and numerical models. Z3 stringers not only are reservoirs for hydrocarbons but can also present a serious drilling problem in some areas. Results of this study could provide the basis for better prediction of zones of drilling problems. More generally, the techniques presented here can be used to predict the internal structure of salt bodies, to estimate the geometry of economic deposits of all kinds and locate zones suitable for storage caverns. Structures observed include an extensive network of zones with increased thickness of the stringer. These we infer to have formed by early diagenesis, karstification, gravitational sliding and associated local sedimentation. Later, this template was deformed into large-scale folds and boudins during salt tectonics. Salt flow was rarely plane strain, producing complex fold and boudin geometries. Deformation was further complicated by the stronger zones of increased thickness, which led to strongly non-cylindrical structures. We present some indications that the thicker zones also influence the locations of later suprasalt structures, suggesting a feedback between the early internal evolution of this salt giant and later salt tectonics. This study opens the possibility to study the internal structure of the Zechstein and other salt giants in 3D using this technique, exposing a previously poorly known structure which is comparable in size and complexity to the internal parts of some orogens.

Structure and evolution of the Northeastern German Basin and its transition onto the Baltic Shield

The post-Permian sequence stratigraphical and structural evolution of the Northeastern German Basin and its transition onto the Baltic Shield has been studied in the Bay of Mecklenburg (SW Baltic Sea) by means of seismic interpretation. Five major sequences have been identified: Middle Triassic, Upper Triassic, Jurassic, Cretaceous and Cenozoic. Time–isochore maps allowed the identification of several phases of salt pillow growth. The contemporaneity of active salt tectonics and the well studied tectonic evolution of the Northeastern German Basin suggest a causative correlation. The E–W directed extension during the Triassic-Early Jurassic marking the beginning break-up of Pangaea is seen as the trigger process for the first period of salt movement. A fault system outside the limit of the Zechstein evaporates is understood as the consequence of thin-skinned faulting and brittle thick-skinned deformation that accompanied this extension. The observed pronounced erosion of Upper Triassic and Lower Jurassic strata is considered to result from the uplift due to the Mid North Sea Doming event in Middle Jurassic times. The seismic data show an undisturbed Late Cretaceous succession which reflects a period of rising sea level, tectonic quiescence and no salt movement. In contrast to the salt pillows which emerged above Triassic fault systems in the westernmost Baltic and western North German Basin, the Cenozoic salt movement activity is the most pronounced. This period of reactivated salt pillow growth started coevally with the onset of the Alpine orogeny at the Cretaceous/Cenozoic transition when the Africa-Arabian plate collided with Eurasia. Generally, no significant faults were identified in the overburden of the salt floored southern Bay of Mecklenburg where ductile Zechstein salt decouples deep rooted faulting from supra-salt deformation.

Modelling groundwater contamination above the Asse 2 medium-level nuclear waste repository, Germany

The medium-level radioactive waste deposited in the Asse 2 salt mine, Germany, has a total activity of 1.2 × 1015 Bq (1 January 2002). The waste lies in a former mining chamber on the −511 m mining level close to the contact between the Mesozoic country rock and the Zechstein salt dome. The Mesozoic country rock is strongly faulted but the geometry of the fault pattern is incompletely known. Two tectonic cases are considered: a pessimistic case, which assumes that the radioactive source is linked to the surface by a single fault, and an optimistic case, which assumes that the radioactive source is not connected to any fault at all. The potential groundwater contamination by radioactive fluids expelled from the cavity is simulated with the TOUGH2 code for periods of up to 6,000 years, when the potential contamination reaches its greatest vertical extent. Two types of driving forces are considered: convergence of the rock salt surrounding the cavity and generation of H2 gas by the corrosion of iron, steel and other metallic components of the waste at low O2 fugacities. Rock-salt creep has the greatest potential impact, as it leads to a radioactive plume whose 1 Bq/L activity contour rises up to 240 m below the surface in the pessimistic tectonic case. The value of the model for the assessment of the environmental risk is limited. The knowledge of the hydrogeological situation is incomplete and the pessimistic tectonic case may not be pessimistic enough for a realistic worst-case scenario.

Structural balancing at the southern border of the North East German basin: results and challenges

Different types of salt-related structures and more or less strongly folded basement uplifts formed on the complex southern border of the North German Basin (NGB). The data resolution in this region offers excellent possibilities to study the relations of major basement thrusts with shortening in the sedimentary cover. Being still largely covered by Triassic and Jurassic sediments, the basement uplift in the south of the Lower Saxony basin (LSB) looks more subdued than its exposed eastern counterparts (Harz, Flechtingen High), but its size and vertical offset are apparently of the same order. In the Subhercynian basin and in the Northeast German Basin (NEGB) the Zechstein salt decouples the structures in the sedimentary cover from basement faults. In contrast, there is no clear evidence for regional detachments in the LSB. Cover deformation is more closely tied to underlying basement structures which are sometimes obscured by inversion (Kockel 2002). To test the hypothesis that the amount of shortening in parts of the NGB east and west of the Gifhorn fault array is roughly similar, we have investigated in detail the thin-skinned and thick-skinned deformation from the southern border of NEGB up to the complex inverted structures in the northern foreland of the Thuringian forest.

Generation of primary sylvite: the fluid inclusion data from the Upper Permian (Zechstein) evaporites, SW Poland

Abstract The chemical composition of primary inclusions in sedimentary halite from the Polish Zechstein salt deposits (Upper Permian), which represent deposition in salt lagoon to salt pan-salina conditions of two evaporitic units of the Stassfurt and Leine cyclothems, contains evidence that the included brines belong to the Na–K–Mg–Cl–SO 4 (SO 4 -rich) type. They differ from the brines typical for other Zechstein salts by having an elevated potassium content, which produced primary sylvite (daughter crystals in inclusions), caused by redeposition of locally generated potassium salts during inflow of fresh seawater into the sedimentary basin. The homogenization temperature of primary inclusions in halite with sylvite daughter crystals reflects the increased temperature (53–60 °C) of basin brines, caused by a greenhouse effect due to density stratification of the brines. Isotopic composition of the anhydrite (sulphur: 9.4–13.30‰ Canyon Diablo Troilite (CDT); oxygen: 9.44–10.4‰ Standard Middle Ocean Water (SMOW), as well as the bromine content in halite (49–77 ppm) indicated that these salts had a marine origin and were deposited at an elevated temperature, from concentrated seawater with a high potassium content, that favoured precipitation of primary sylvite.

Advance of allochthonous salt sheets in passive margins and orogens

Allochthonous salt sheets advance in four ways: (1) extrusive advance, (2) open-toed advance, (3) thrust advance, and (4) salt-wing intrusion. These mechanisms are determined primarily by the geometry and thickness of the roof that overlies the advancing sheet. An extrusive sheet spreads without a roof or with a roof of negligible mechanical strength. An open-toed sheet is partially covered by a mechanically significant roof but has an extrusive toe. An overthrusting sheet advances along a thrust fault at its leading edge, carrying its roof with it. A salt wing intrudes from the flank of a diapir into a shallower salt layer.Extrusive, open-toed, and overthrusting salt sheets are found in both passive margins and orogens. A sheet typically evolves through two or more of these mechanisms. Three lineages, or evolutionary paths, are common. Plug-fed extrusions emanate from the top of a salt dome or salt wall. Plug-fed thrusts form the base of the hanging wall of thrust faults that are rooted in the tops of salt domes or salt walls. Finally, source-fed thrusts initiate as thrust faults rooted in the autochthonous salt layer. Source-fed thrusts form the largest individual salt sheets, some covering thousands of square kilometers. Salt-wing intrusions form only under special circumstances, so they are not part of the three major lineages. These intrusions are restricted to compressionally inverted basins containing multiple salt layers and are known only in the Zechstein salt basin.

Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins

Several selected seismic lines are used to show and compare the modes of Late-Cretaceous–Early Tertiary inversion within the North German and Polish basins. These seismic data illustrate an important difference in the allocation of major zones of basement (thick-skinned) deformation and maximum uplift within both basins. The most important inversion-related uplift of the Polish Basin was localised in its axial part, the Mid-Polish Trough, whereas the basement in the axial part of the North German Basin remained virtually flat. The latter was uplifted along the SW and to a smaller degree the NE margins of the North German Basin, presently defined by the Elbe Fault System and the Grimmen High, respectively. The different location of the basement inversion and uplift within the North German and Polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the WNW-ESE trending Elbe Fault System and by the NW-SE striking Teisseyre-Tornquist Zone, the latter underlying the Mid-Polish Trough. Therefore, the inversion of the Polish and North German basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis. The inversion of the Mid-Polish Trough was connected with the reactivation of normal basement fault zones responsible for its Permo-Mesozoic subsidence. These faults zones, inverted as reverse faults, facilitated the uplift of the Mid-Polish Trough in the order of 1–3 km. In contrast, inversion of the North German Basin rarely re-used structures active during its subsidence. Basement inversion and uplift, in the range of 3–4 km, was focused at the Elbe Fault System which has remained quiescent in the Triassic and Jurassic but reproduced the direction of an earlier Variscan structural grain. In contrast, N-S oriented Mesozoic grabens and troughs in the central part of the North German Basin avoided significant inversion as they were oriented parallel to the direction of the inferred Late Cretaceous–Early Tertiary compression. The comparison of the North German and Polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field. A thick Zechstein salt layer in the central parts of the North German Basin and the Mid-Polish Trough caused mechanical decoupling between the sub-salt basement and the supra-salt sedimentary cover. Resultant thin-skinned inversion was manifested by the formation of various structures developed entirely in the supra-salt Mesozoic–Cenozoic succession. The Zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution. Only in parts of the NGB and MPT characterised by either thin or missing Zechstein evaporites, thick-skinned inversion directly controlled inversion-related deformations of the sedimentary cover. Inversion of the Permo-Mesozoic fill within the Mid-Polish Trough was achieved by a regional elevation above uplifted basement blocks. Conversely, in the North German Basin, horizontal stress must have been transferred into the salt cover across the basin from its SW margin towards the basins centre. This must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion-related basement faults are seismically detected apart from the basin margins. This strain decoupling in the interior of the North German Basin was enhanced by the presence of the Elbe Fault System which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred Late Cretaceous–Early Tertiary far-field compression.

3D structural model of the Polish Basin

A 3D structural modelling of the Permian–Mesozoic Polish Basin was performed in order to understand its structural and sedimentary evolution, which led to basin maturation (Permian–Cretaceous) and its tectonic inversion (Late Cretaceous–Paleogene). The model is built on the present-day structure of the basin and comprises 13 horizons within the Permian to Quaternary rocks. The analysis is based on 3D depth views and thickness maps. The results image the basin-scale symmetry, the perennial localization of the NW–SE-oriented basin axis, the salt movements due to tectonics and/or burial, and the transverse segmentation of the Polish Basin. From these observations, we deduce that salt structures are correlated to the main faults and tectonic events. From the model analysis, we interpret the stress conditions, the timing, and the geometry of the tectonic inversion of the Polish Basin into a NW–SE-oriented central horst (Mid-Polish Swell) bordered by two lateral troughs. Emphasis is placed on the Zechstein salt, considering its movements during the Mesozoic sedimentation and its decoupling effect during the tectonic inversion. Moreover, we point to the structural control of the Paleozoic basement and the crustal architecture (Teisseyre–Tornquist Zone) on the geometry of the Polish Basin and the Mid-Polish Swell.

Application of geochemistry in the evaluation and development of deep Rotliegend dry gas reservoirs, NW Germany

Abstract The Voelkersen field in northwestern Germany produces thermogenic dry gas from Permian Rotliegend sandstone reservoirs located at depths greater than 4700 m. In addition to this considerable depth Zechstein salt overburden plus the structural complexity and heterogeneity of the reservoirs limit the applicability of conventional methods for definition and characterization of field compartmentalization. The Rotliegend gas is sourced from Westphalian coals of semi-anthracite to anthracite rank with conventional geochemical data suggesting little variation in composition on a field-wide scale. However, detailed analysis of associated condensates and wet-gas hydrocarbons shows considerable differences between compartments and producing horizons, despite the apparent high maturity of the gas. These differences support compartment definition and identification of lateral and horizontal seals. Assuming a quasi-uniform gas composition in communicating reservoirs, variation of carbon and hydrogen isotopic data in particular confirms separation of marginal fault blocks. Additionally, these data also suggest reservoir continuity within a large field compartment and over parts of a major fault system for which the sealing properties so far were previously uncertain.

The Evolution and Growth of Central Graben Salt Structures, Salt Dome Province, Danish North Sea

Abstract The remarkable spatial resolution of 3D seismic data is particularly important in the study of salt structures, where changes in the distribution of accommodation space resulting from salt withdrawal can be mapped and related to the evolution of the individual salt structures. The Salt Dome Province of the Danish Central Graben provides an interesting example of this approach. Three adjacent salt structures (Skjold, Dan and Kraka) exhibit very different geometries, and their evolution has been the subject of some debate. The present study suggests that the Dan structure is composed almost entirely of Zechstein salt, which, facilitated by de-coupled extension during the Mid-Late Jurassic, has been intruded into and along weak planes of Triassic salt, resulting in an overall domal, circular outline. Skjold is located to the NW of Dan. It consists of Zechstein salt and forms a mature salt stock, which terminates within Upper Cretaceous strata. Skjold evolved from a linear NW-SE trending salt wall and became a point source structure during the Early Cretaceous, in common with the many other Central Graben diapers and coincident with the commencement of thermal subsidence. By contrast, Kraka, a NW-SE trending pillow structure located to the south of the Dan-Skjold alignment never became diapiric. Gravitational downbuilding of the mature Skjold diapir during rapid Cenozoic deposition was punctuated by rejuvenated (active) growth induced by regional compression, most significantly during the Mid-Miocene. This event affected also the Dan and Kraka structures, which otherwise experienced very limited growth during the Cenozoic.

Inverted basins in the Netherlands, similarities and differences

AbstractAll Dutch rift basins that formed during Jurassic and Early Cretaceous extension have been inverted during the Late Cretaceous and Early Tertiary. Several inversion pulses occurred more or less simultaneously in all basins. Analysis of vitrinite reflectance data, in combination with fission track and fluid inclusion data show that the magnitude of uplift and erosion generally did not exceed 2 km. Inversion was strongest in the Broad Fourteens, Central Netherlands and West Netherlands basins. The direction of maximum compressive stress was generally not at right angles to the pre-existing fault trends, and resulted in transpressional movements. Within the NW-SE striking basins, dextral strike-slip movements can often be interpreted, which is consistent with a general N-S to NNW-SSE direction of maximum compression related to Alpine structural events. Where no Zechstein salt is present, trends of flower structures formed through reverse reactivation of pre-existing faults. Where the Zechstein salt is thick, re-activated faults could not breach the salt, and a broad uplift of the post-salt succession resulted, while faulting below the salt caused acceleration of halokinesis. In areas where the Zechstein salt was thin, and where the offsets of reverse faults exceeded the thickness of the salt, impressive thrusts with the Zechstein salt as detachment horizon developed. The later Tertiary inversion pulses did not affect all basins, and caused broad basin uplift in the West and Central Netherlands basins while individual faults were no longer reactivated. It appears that due to crustal thickening during the first inversion pulses the crust could become stabilised such that further compression could only be accommodated by broad basin uplift.

Inversion structures in Central Europe - Expressions and reasons, an open discussion

AbstractThe diversity of morphological features of Mesozoic inversion structures in NW Germany as representatives of inversion structures in northern Europe is presented and their origin analysed and geologically dated. The particular role of salt in inverted basins and the re-shaping of pre-existing salt structures during the inversion act is demonstrated and the term ‘salt wedge’, a Zechstein salt intrusion into salt layers within the Triassic sedimentary pile, introduced. The leading theories on inversion (continent-continent collision, re-activation Variscan features) are discussed and discarded, but no new comprehensive theory was developed. The impact of inversion on HC prospectivity of sedimentary basins is debated and proposals for future interdisciplinary research are made.

Heterogeneous tectonic inversion of the Mid-Polish Trough related to crustal architecture, sedimentary patterns and structural inheritance

Using a 3-D structural model, we performed a basin-scale analysis of the tectonically inverted Mid-Polish Swell, which developed above the NW–SE-oriented Teisseyre-Tornquist Zone. The later separates the Paleozoic West European Platform from the Precambrian East European Craton. The model permits a comparison between the present depths and sedimentary thicknesses of five layers within the Permian–Mesozoic and Cenozoic successions. The inversion of the NW–SE-trending Mid-Polish Trough during the Late Cretaceous–Paleogene resulted in uplift of a central horst, the Mid-Polish Swell, bounded by two lateral troughs. These structural features are induced by squeezing of a weak crust along the Teisseyre-Tornquist Zone. The swell is characterized by an inherited segmentation which is due to NE–SW transversal faults having crustal roots. From NW to SE, we distinguish the Pomeranian, Kujavian, and Małopolska segments, that are separated by two transversal faults. During the inversion, the Zechstein salt occurring in the Pomeranian and Kujavian segments in the NW acted as decoupling level between the basement and the post-salt cover, leading to disharmonic deformation. Conversely, because no salt occurs in the SE, both basement and cover were jointly deformed. The vertical tectonic uplift at the surface is estimated to amount to 3 km in the Małopolska segment. The structural inheritance of the basement is expressed by the heterogeneous geometry of the swell and tectonic instability during Mesozoic sedimentation. The reasons for the inheritance are seen in the mosaic-type Paleozoic basement SW of the Teisseyre-Tornquist Zone, contrasting the Precambrian East European Craton which acted as a stable buttress in the NE. The horst and trough geometry of Cenozoic sediments blanketing the Mid-Polish swell reveals the ongoing intracontinental compressional stress in Poland.

Inversion-related features along the southeastern margin of the North German Basin (Elbe Fault System)

A seismostratigraphic approach constrained by well data is used for the interpretation of the deformation style along the central Elbe Fault System (EFS) within the sedimentary succession. Structural analysis allows to qualify, to quantify, and to date tectonic events. The stratigraphic interpretation is complicated by the mobilized Upper Permian Zechstein salt and by erosional events. A first-order quantification of the inversion-related uplift is estimated from vertical fault offsets that reach up to 4 km. The main uplift occurred during the Maastrichtian/Paleocene. Amounts of erosion inferred from comparing the strata thickness on top of the Flechtingen High with the surrounding basinal areas range from 3 to 4 km. The data indicate a changing deformation style: Thick-skinned deformation with southwest-dipping thrusts that vertically offset the pre-Permian basement is observed along the Flechtingen High in the central part of the EFS. Thin-skinned deformation occurs in the North German Basin where salt detaches the post-Permian cover from the barely faulted basement. It is concluded that during the Late Cretaceous/Early Tertiary inversion, the EFS responded to regional compression with uplift and formation of an internal high, the Flechtingen High. A stress-sensitive crustal weak zone beneath the EFS could be the reason for the repeated strain localization in the area.

The Andrew and Cyrus Fields, Blocks 16/27a, 16/28, UK North Sea

Abstract The Andrew and Cyrus fields lie in UK Blocks 16/27a and 16/28 at the junction of the Witch Ground Graben and the South Viking Graben. The Andrew Fields was discovered in 1974 with well 16/28-1, and Cyrus in 1979 with well 16/28-4. Both fields share a common reservoir the Paleocene Andrew Formation and both are sealed by the overlying Andrew Shale and Sele Formation. Similarly the traps for both fields formed above swells within Zechstein salt. The Andrew and Cyrus Fields differ in their petroleum content. Andrew contains both gas and oil (40API) while Cyrus contains only oil (30API). Reserves for the Andrew Field are 140 MMBBL based upon a 46% recovery factor using natural aquifer drive. The same mechanism will deliver 16.5 MMBBL from Cyrus at a 21% recovery factor. Production began from Andrew in July 1996. Cyrus production began in April 1990 using the SWOPS system until April 1992. The small field was subsequently tied back to Andrew.

Salt movements in the Northeast German Basin and its relation to major post-Permian tectonic phases—results from 3D structural modelling, backstripping and reflection seismic data

The NW–SE-striking Northeast German Basin (NEGB) forms part of the Southern Permian Basin and contains up to 8 km of Permian to Cenozoic deposits. During its polyphase evolution, mobilization of the Zechstein salt layer resulted in a complex structural configuration with thin-skinned deformation in the basin and thick-skinned deformation at the basin margins. We investigated the role of salt as a decoupling horizon between its substratum and its cover during the Mesozoic deformation by integration of 3D structural modelling, backstripping and seismic interpretation. Our results suggest that periods of Mesozoic salt movement correlate temporally with changes of the regional stress field structures. Post-depositional salt mobilisation was weakest in the area of highest initial salt thickness and thickest overburden. This also indicates that regional tectonics is responsible for the initiation of salt movements rather than stratigraphic density inversion. Salt movement mainly took place in post-Muschelkalk times. The onset of salt diapirism with the formation of N–S-oriented rim synclines in Late Triassic was synchronous with the development of the NNE–SSW-striking Rheinsberg Trough due to regional E–W extension. In the Middle and Late Jurassic, uplift affected the northern part of the basin and may have induced south-directed gravity gliding in the salt layer. In the southern part, deposition continued in the Early Cretaceous. However, rotation of salt rim synclines axes to NW–SE as well as accelerated rim syncline subsidence near the NW–SE-striking Gardelegen Fault at the southern basin margin indicates a change from E–W extension to a tectonic regime favoring the activation of NW–SE-oriented structural elements. During the Late Cretaceous–Earliest Cenozoic, diapirism was associated with regional N–S compression and progressed further north and west. The Mesozoic interval was folded with the formation of WNW-trending salt-cored anticlines parallel to inversion structures and to differentially uplifted blocks. Late Cretaceous–Early Cenozoic compression caused partial inversion of older rim synclines and reverse reactivation of some Late Triassic to Jurassic normal faults in the salt cover. Subsequent uplift and erosion affected the pre-Cenozoic layers in the entire basin. In the Cenozoic, a last phase of salt tectonic deformation was associated with regional subsidence of the basin. Diapirism of the maturest pre-Cenozoic salt structures continued with some Cenozoic rim synclines overstepping older structures. The difference between the structural wavelength of the tighter folded Mesozoic interval and the wider Cenozoic structures indicates different tectonic regimes in Late Cretaceous and Cenozoic. We suggest that horizontal strain propagation in the brittle salt cover was accommodated by viscous flow in the decoupling salt layer and thus salt motion passively balanced Late Triassic extension as well as parts of Late Cretaceous–Early Tertiary compression.

Structure and quantification of processes controlling the evolution of the inverted NE-German Basin

A NE–SW reflection seismic transect crossing the entire NE-German Basin (NEGB) was investigated in detail to study the interaction between tectonics and sedimentation during the multiphase evolution of the region. By restoring a geological section across the post-Rotliegend, the kinematic evolution was reconstructed and rates of important geological processes such as erosion or basin shortening were quantified. The present day structural style of the NEGB is the result of several major deformation events: (1) regional subsidence accompanied by local thin-skinned E–W extension during the Permo-Triassic/Jurassic was followed by basin inversions at the (2) Jurassic/Cretaceous, and at the (3) Cretaceous/Tertiary boundary. Zechstein salt played an important role since mobile Zechstein evaporites decoupled the supra-salt from the sub-salt succession and allowed the transmission of deformation over a large distance. Of major significance to the Late Cretaceous structural development was the crustal-scale upthrusting of the Calfoerde Block along the Gardelegen Fault at the southern basin margin. The accompanying horizontal shortening has a minimum value of 9 km, and led to the development of thrusts and folds within the post-Zechstein succession.

S- and O-isotopic character of dissolved sulphate in the cover rock aquifers of a Zechstein salt dome

Sulfur and O isotope analyses of dissolved SO 4 were used to constrain a hydrogeological model for the area overlying the Gorleben–Rambow Salt Structure, Northern Germany. Samples were collected from 80 wells screened at different depth-intervals. The study area consists of a set of two vertically stacked aquifer systems. Generally, the isotope data show a good spatial correlation, outlining well-defined groundwater zones containing SO 4 of characteristic isotopic composition. Highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant SO 4 isotopic compositions, which are typical of Permian Zechstein evaporites (δ 34 S=9.6–11.9‰; δ 18 O=9.5–12.1‰). Above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values (average δ 34 S=16.6‰; δ 18 O=15.3‰). The confined groundwater horizon on the top of the lower aquifer system below the low permeable Hamburg Clays is low in total dissolved solids and is characterized by an extreme 34S enrichment (average δ 34 S=39.1‰; δ 18 O=18.4‰), suggesting that bacterially mediated SO 4 reduction is a dominant geochemical process in this zone. Two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system. Sulfate in groundwaters adjacent to the river Elbe and Löcknitz has a typical meteoric isotopic signature (δ 34 S=5.2‰; δ 18 O=8.2‰), whereas the central part of the area is characterized by more elevated isotopic ratios (δ 34 S=12.7‰; δ 18 O=15.6‰). The two major SO 4 pools in the area are represented by Permian seawater SO 4 and a SO 4 of meteoric origin that has been mixed with SO 4 resulting from the oxidation of pyrite. It is suggested that the S-isotope compositions observed reflect the nature of the SO 4 source that have been modified to various extent by bacterial SO 4 reduction. Groundwaters with transitional salinity have resulted from mixing between brines and low-mineralized waters affected by bacterial SO 4 reduction.

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

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 580m. In the eastern part it is only 380m 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 chiefly result from Late Cretaceous compression that affected the North German basin. The salt body is, therefore, primarily a tectonic structure and is not halokinetic.