Clay-bearing Rocks Research Articles

Clay mineralogy in west Chryse Planitia, Mars: Comparison with present and future landing sites

On Mars, the well-known crustal dichotomy marks the boundary between the old southern highlands and the younger northern lowlands. Among these lowlands, Chryse Planitia resembles a quasi-circular basin surrounded by several highlands, and blends into Acidalia Planitia, another flat lowland located farther north. The transition area between these highlands and the Chryse basin is often designated as “circum-Chryse Planitia”, and is the terminus for many outflow channels. Infrared datasets display several sites therein with extensive clay-bearing outcrops, further testifying for aqueous activity on early Mars – notably around Mawrth Vallis, Oxia Planum and Xanthe Terra. In this study, we investigate clay-bearing outcrops identified along the western margins of circum-Chryse basin, often overlooked in the Martian literature. We also compare them with outcrops found in other regions along the crustal dichotomy and relevant in the Martian literature, such as Oxia Planum, Mawrth Vallis and Nili Fossae. Investigating such deposits is crucial for astrobiological perspectives, as they are appealing targets to search for organic compounds possibly stored throughout the rocks and soils. Fe,Mg-rich clays generally result from the interaction of liquid water with rocks under low temperatures, moderate pH levels and neutral to reducing conditions, factors favorable for life. Here, the clay minerals detected in west Chryse Planitia are consistent with either ferrosaponites or vermiculites associated with hydrobiotite, as recently inferred in Oxia Planum and north Xanthe Terra. Diverse alteration pathways might be involved based on either of these clay species. The clay-bearing rocks crop out in isolated hills in Lunae Planum, and along inverted channels and small craters in Tempe Terra. Further geologic investigations in circum-Chryse Planitia should certainly provide new clues on their origin and weathering conditions, while supporting the upcoming ExoMars rover mission and other future explorations.

A universal hydro-mechanical coupled behavior model for clay-bearing strata—Molecular-level simulation approach

Clay minerals in soils and rocks exhibit large volume change upon interaction with water and this behavior becomes even more complex when the strata are being stressed by the engineering and environmental loads. Therefore, a realistic prediction of the hydro-mechanical behavior of the clay-bearing strata is always a challenge due to their coupled swelling-mechanical response in the cases of geotechnical and geoenvironmental engineering problems, nuclear waste storage in clay-bearing rock repositories, shale gas extraction, and other uses of clay in the manufacturing industry. All the existing behavior models have restricted applications in the engineering and other fields of practice mainly due to the partial consideration of the structure and fabric of clay-bearing strata in the model formulation. In this study, a hydro-mechanical behavior model has been formulated using the parameters acquired from the molecular-level simulations and modeling of the volume change and stress–strain behavior of the clay-bearing structure. The Molecular Mechanics and Molecular Dynamic simulations were performed on the natural structure of the clay-bearing strata formulated using Monte Carlo technique. The mathematical model, developed from the simulation results, can predict the overall hydro-mechanical behavior of clay-bearing strata for all possible combinations of clay minerals, non-clay minerals, salts causing cementation of the soil/rock structure, confining pressures, and the induced strain levels. The developed model has successfully been validated through laboratory and field testing on the clay-bearing strata in both the elastic and elasto-plastic regions of the stress–strain behavior and also from the data of two (02) swelling clays (MX-80 and FEBEX Bentonite) from the existing literature, supporting the universal nature of the developed behavior model.

The needle penetration index for estimating the physico-mechanical properties of pyroclastic rocks

Preparation of suitable core specimens for physico-mechanical testing s not always possible, particularly for soft and clay-bearing rocks. Although several indirect test methods have been proposed to predict the properties of rocks, specimen preparation from soft rocks for some indirect tests is still difficult. For such cases, the needle penetration test has been developed. In this paper we present a study on the predictability of the physico-mechanical properties of pyroclastic rocks from the needle penetration index (NPI). The NPI, uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), density, and porosity tests were performed in the laboratory on specimens from ten different locations in Turkey. Correlations were established between the NPI values and the physico-mechanical properties. Strong correlations were observed between NPI and both UCS and BTS. General correlations were found between NPI and both density and porosity.

An Experimental Investigation on the Durability of Clay-Bearing Rocks in Different pH Conditions

An Experimental Investigation on the Durability of Clay-Bearing Rocks in Different pH Conditions

Cementation factor in clayey rock samples: investigating the role of clay content and determination using electrical rock classification

Accurate determination of cementation factor (m) has a significant role in the petrophysical characterization of underground reservoirs. For clean sandstones, the application of this parameter in Archie's Equation provides a basic framework for estimating water saturation. However, the presence of clay minerals in the rock texture could affect the evaluation process. There are different correlations from which the cementation factor can be estimated. In addition, applying rock classification techniques revealed an excellent prospect for better estimating this parameter. However, in most of the mentioned approaches, the effect of clay minerals is missed. In this study, a newly developed modified form of electrical rock typing technique (i.e. classification of rock samples according to their electrical properties) incorporating the effect of clay minerals for improving the prediction of cementation factor in clay-bearing rocks is proposed. In doing so, a new form of electrical quality index is defined based on which the modified formation resistivity factor versus porosity data for 204 clayey rock samples are classified into nine distinct rock types. Comparative studies demonstrated the priority of the suggested methodology rather than classic hydraulic flow unit-based methods such as flow zone indicator and discrete rock type. The latter showed a high degree of scattering in the classification stage. For each rock type, developed based on the proposed scheme, the equations were proposed in which the modified form of formation resistivity factor and cementation factor versus porosity were correlated. Linear relationships with reasonable values of coefficient of determination were then demonstrated for the developed equations. In addition, the effect of clay minerals on the rock's electrical quality and electrical properties (i.e., rock conductivity and cementation factor) is discussed. The results of this study offer a better framework to incorporate the effect of clay constituents in the prediction of cementation factor, as compared to classical rock typing approaches.

Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)

We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, pc, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, σ, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of pc than of σ. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures.

Field measurements, laboratory tests and empirical relations for investigating the solid-to-fluid transition of a rapid earthflow

Earthflows are widespread phenomena in the Mediterranean area. These landslides involve fine-grained soils and clay-bearing rocks, and despite their low speed are responsible for significant economic losses in vast areas. The dynamics of these landslides is still relatively obscure thus reducing our ability to forecast and mitigate their effects. In this study, we present a methodological approach for the characterization of active earthflows based on the combination of geophysical surveys, laboratory tests, and empirical formulas. Geophysical surveys consist of periodic measurements of Rayleigh wave velocities repeated over time to evaluate the change of stiffness with time of the landslide material. Laboratory tests combine Atterberg limits, fall cone and oedometric tests and allow to constrain the empirical correlations between geophysical and geotechnical properties. The method is designed to obtain relevant data when direct methods like boreholes or geotechnical soundings are not possible for safety reasons, with the aim of investigating the solid-to-fluid transition that can occur in rapid earthflows. We applied this approach to study the Montevecchio landslide (Northern Apennines of Italy), an active earthflow in young marine clays which was affected by multiple reactivations in the recent years. Results show that after a surge the earthflow material is very soft (shear wave velocity in the order of 50–60 m/s) and characterized by a high water content. However, the exact value of the Liquidity Index remains unknown due to the lack of direct measurements and to the uncertainty dictated by the empirical relationships.

Effect of Water–Rock Interaction on the Axial Capacity of Drilled Caissons Socketed in Claystone Bedrock

AbstractA large number of bridges and structures in Colorado are supported by drilled caissons and shafts socketed in weak clay-bearing rocks (e.g., Denver blue claystone and Pierre shale). During ...

Evaluation of engineering characteristics and estimation of static properties of clay-bearing rocks

Evaluation of engineering characteristics and estimation of static properties of clay-bearing rocks

Investigation of Disintegration Behaviors of Clay-Bearing Rocks by a New Methodology

Abstract Disintegration behaviors of clay-bearing rocks are special engineering problems. Samples are isolated and unconstrained in previous disintegrating-durability tests, unlike clay-bearing rocks under the natural controlled condition in the field. Then, a methodology of disintegration testing regarding natural controlled conditions was evaluated to study the disintegration behaviors of clay-bearing rocks. The study results indicated that the fissure area ratio (FAR) increases rapidly before the seventh wetting-drying cycle and then begins a stable trend. The fragment size distribution characteristics show that the proportion of small fragments gradually increases and that the proportion of large fragments gradually decreases with the increase in the number of wetting-drying cycles. Clay-bearing rock fragments larger than 2 mm disappear after seven wetting-drying cycles. The change characteristics of the disintegration resistance of the disintegrating clay-bearing rock are that, regardless of the critical fragment size diameter (Dcr) of complete disintegration, the slake durability index (I(D)) and disintegration ratio (DR) are reduced. The disintegration degree of aggregate fragments is further reflected by the DR, which corresponds with the FAR. A trend of the maximum shear stress related to the number of wetting-drying cycles has two stages: the first significant and then insignificant decrease. The critical number of wetting-drying cycles is seven, after which point the disintegrating clay-bearing rock fragments with a size coarser than 2 mm disappeared. The results indicate the highly positive correlation between strength and the DR of the disintegrating clay-bearing rock.

The effect of clay content on the relation between uniaxial compressive strength and needle penetration index for clay-bearing rocks

The needle penetration index (NPI) test is a non-destructive test that is applicable both in the field and laboratory, and does not require any special sample preparation. This test has been used for the estimation of physico-mechanical properties of soft rocks. In this study, the influence of the clay content on the relation between uniaxial compressive strength (UCS) and the NPI has been investigated for some clay-bearing rocks. The needle penetration tests were carried out at nine different gallery faces during the Cayirhan Coal Mine excavations, and the NPI values were calculated. Claystone, clayey limestone and clay blocks were collected from the locations on which the NPI tests were performed for the determination of rock strength and clay contents. The clay contents and clay fractions of the samples were determined using XRD analysis. A strong correlation has been found between the UCS and the NPI, but some of the data points were scattered. Strong correlations were also found between the NPI and both the total clay content and the smectite content. The UCS values were also strongly correlated to the total clay content and the smectite content. A multiple regression analysis was performed to determine the influence of clay content on the UCS-NPI relation and a very strong model was derived. The correlation coefficient of the multiple regression model is fairly higher than that of the UCS-NPI relation derived by using simple regression analysis. Concluding remark is that the clay content significantly affects the UCS-NPI relation in clay-bearing rocks.

Experimental Study on the Evolution of Argillization of Mudstone and Cutter Wear during the TBM Tunnelling

Argillization is a process in which clay‐bearing rocks disintegrate into the clay under the action of high temperature, pressure, and water. When tunnel boring machines (TBMs) excavate in the mudstone, argillization takes place, causing the clogging of the TBM cutterhead. As a result, the penetration rate drops gradually. Abnormal wear might occur. To investigate the evolution of argillization of mudstone and cutter wear during the TBM tunnelling, a series of rotary indentation tests were carried out on the self‐designed experimental bench for different loading times. During the test, the temperature and penetration depth of disc cutters were measured in real time. After loading, microstructures of cutting grooves, slacking mudstone, and worn cutter ring were observed by stereomicroscope. Consequently, the evolution of argillization in mudstone and cutter wear were investigated. Experimental results indicate that the argillization process of mudstone by disc cutter can be divided into three stages: mechanical cutting stage, deterioration of mudstone and the formation of slacking mudstone stage, and adherence of slacking mudstone stage. Specifically, at mechanical cutting stage, the rock was cut by cutter directly, causing high frictional heat. Then the microstructure of mudstone was deteriorated due to the water‐weakening mechanisms, temperature effect, and mechanical activation effect. Finally, the slacking mudstone was adhered to the disc cutter. Correspondingly, due to the argillization of mudstone, the disc cutter wear goes through the mechanical wear stage, argillization wear stage, and secondary wear stage in sequence. This investigation reveals the rock cutting mechanism of TBM considering the argillization of mudstone. Furthermore, it provides some references for design and operation of the TBM.

Effect of water content on the mechanical properties and deformation characteristics of the clay-bearing red sandstone

The presence of water has a great influence on the strength and deformation of clay-bearing sandstone. Most of the natural rocks suffer water softening of different saturations. In this study, due to the importance of the clay-bearing rocks in geoengineering practice, red sandstone was chosen to investigate the effect of water contents on the UCS (uniaxial compressive strength), TCS (triaxial compressive strength), BTS (Brazilian tensile strength), PLS (point load strength), and DSS (direct shear strength). The UCS and TCS are linearly decreased as the water saturation increases. When the water saturation increases from 0 to 100%, the UCS and TCS have decreased by about 52.8% and 29.3%, respectively. The BTS, PLS, and DSS are exponentially decreased with increasing water saturation, which have a reduction of about 45.5%, 66.3%, and 49%, respectively. Four typical deformation stages are observed during compression for this red sandstone samples, including an obvious compaction stage. The water saturation does not have any effect on the ratio of the strain on the transition points of these stages to the peak strain, as well as the stress. Besides, a rapid increase of the AE count occurs at the plastic yield point although the cumulative AE count has an obvious decrease trend with increasing the water saturation. Then, considering the effect of water saturation, a complete constitutive model has been proposed to well predict the stress-strain relationship of the red sandstone. The water’s weakening effect on the clay-bearing red sandstone has also been deeply discussed and developed.

The failure, slaking, and self-sealing behavior of clay-bearing rocks

Clay-bearing rocks with different thickness are commonly encountered in sedimentary rocks during many important engineering applications such as tunneling, underground and open-pit mining, and road cut or dam site constructions. Clay minerals are the main constituent of the mineralogical composition of such sedimentary rocks, and are the most important minerals that affect the strength and deformation behavior of these rocks. Due to the presence of clay minerals, the physico-mechanical features of these sensitive rocks are not constant and eventually change with variations in moisture content when subjected to natural environmental conditions. Many natural disasters and engineering projects–based problems such as landslide, slope instability, embankment failures, open-pit and underground excavation problems, and undercutting-related rockfalls generally have direct or indirect relationships with clay-bearing rocks due to their low durability and strength characteristics as well as high swelling tendency as a result of contact with water. Despite these natural disasters and engineering problems, due to their low characteristics of hydraulic conductivity and self-sealing behavior, the thermo-hydro-mechanical properties of clay-bearing rocks (e.g., Opalinus Clay and Boom Clay) have also been investigated up to now for the purpose of being used as host rocks for high-level radioactive waste. Particularly in the last two decades, the source of energy found trapped within shale formations has begun to be important in the popularity of these rocks. The reductions of conventional fossil fuels and worldwide high energy demands as well as occasional disruptions in energy supply have led scientists to do more investigation to achieve alternative energy sources from clay-bearing rocks. Considering low durability and strength dependent clay-bearing rocks–based engineering problems and the importance of these rocks for extraction shale gas and storage nuclear waste, the physico-mechanical and self-sealing characteristics of these rocks are comprehensively discussed and presented in this study.

Assessment of design parameters and the slope stability analysis of weak clay-bearing rock masses and associated spoil piles at Tunçbilek basin

Tuncbilek lignite basin situated in western part of Turkey has been increasingly excavated by open-pit and underground mining methods since 1940 to meet Turkey’s energy demand. The blasted and then the excavated stratified rock masses in this basin consist mainly of clay-bearing rocks such as claystone, mudstone, siltstone, and marl. It was found that the strength and deformation properties of these clay-bearing rocks decrease with the increase in water content. In addition, time-dependent attenuations were also recorded in the mechanical properties due to disintegration caused by physical weathering processes. Therefore, considerable time-dependent instabilities were observed in the slopes left after open-pit mining activities. In this study, it was aimed to determine the design parameters of weak rock masses and spoil piles at Tuncbilek basin and then to recommend safe slope geometry for open-pit mining activities. For this purpose, the geotechnical properties of rock masses and spoil piles were obtained based on field tests as well as characterizations and back analyses of the instabilities. This study reveals that shear strength parameters of undisturbed clay-bearing rock masses are 200 kPa and 30°. In addition, a cohesion of 150 kPa and an internal friction angle of 25° should be used for slope stability analyses of both weakly cemented conglomerate-sandstone and disturbed clay-bearing rock masses.

Effect of water saturation on uniaxial compressive strength and damage degree of clay-bearing sandstone under freeze-thaw

The previous researches mainly focused on the freeze-thaw damage of fully saturated rocks. However, most of the nature rocks experience incomplete water saturation and it may not reach the threshold of frost cracking of a void, at least 91% water saturation considering the 9% volumetric expansion of freezing water and ignoring water flow. A serious freeze-thaw deterioration of unsaturated rocks may still occur when a critical saturation is reached. This deterioration is caused by an accumulation of the long-term fatigue damage under freeze-thaw. Due to the importance of the clay-bearing rocks in geo-engineering practice, red sandstone is chosen to investigate the effect of water saturation on the physico-mechanical properties of clay-bearing rock by testing the uniaxial compressive strength (UCS) and P-wave velocity in laboratory. Besides, a fatigue damage model proposed by Liu et al. (Cold Reg Sci Technol 120:96–107, 2015) is improved and applied to evaluate the freeze-thaw damage degree and estimate the UCS after any freeze-thaw cycle. The research results suggest that freeze-thaw and water softening have an equally important influence on the clay-bearing red sandstone. In the absence of freeze-thaw, the UCS of red sandstone decreases with the increase of water saturation because of water weakening, including lubrication action, hydration of clay minerals, and dissolution of soluble minerals. However, more than 90% of UCS loss occurs at the range of the low degree of saturation below 60%. During freeze-thaw cycles, the critical saturation is about 60%, beyond which freeze-thaw damage grows quickly with increasing saturation and freeze-thaw cycles. Macro-cracks, initiating inside the clay minerals and propagating along the boundary of weather-resistant mineral grains, can be found on the surfaces of fully saturated specimens, while dried red sandstone keeps intact without obvious defects after 50 freeze-thaw cycles. Finally, the UCS of clay-bearing red sandstone with different water saturation is proved to be well predicted after any freeze-thaw cycle by the improved fatigue damage model.

Supercritical CO2 and CH4 Uptake by Illite-Smectite Clay Minerals.

Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their submicron features have direct relevance to many geoenergy applications. The quantification of gas uptake over a broad range of pressures is key toward assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals-Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-using CO2 and CH4 up to 30 MPa at 25-115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N2 (77 K) and Ar (87 K), while larger uptakes are measured with CO2 (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The lattice density functional theory model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.

Effect of Water Content on Argillization of Mudstone During the Tunnelling process

Argillization is commonly observed as excavating in mudstone stratum by tunnel boring machine. In addition to the operational and geological parameters studied by previous researchers, this phenomenon also has significant influence on the performance of tunnel boring machine, such as penetrate rate, advance rate, and utilization rate. In general, water is a key factor affecting the progress of argillization. With the aim to investigate the effect of water content on argillization of mudstone during the tunnelling, a new rolling abrasion test was conducted on rock blocks with moisture contents of 2.82%, 3.44%, 4.79%, 6.06%, and 6.75%. In the experiment, penetration depth, temperature fields of disc cutter and rock blocks, and wear loss of cutters were recorded. In addition, the microstructures of cutting groove on rock blocks and slacking mudstone were observed by OLYMPUS SZX16 stereomicroscope. According to experimental results, three stages of argillization process can be divided: (1) water evaporation of mudstone nearby the disc cutter, (2) destruction of microstructure of mudstone, and (3) formation of slaking mudstone. Uneven shrink, water-weakening effects, temperature effects, and mechanical activation are mainly contributed to the damage of microstructures of rock blocks. In addition, the variation in water content accelerates the argillization process. By comparison, it is found that wear loss of disc cutter and cutting efficiency show negative and positive correlation with the extent of argillization, respectively. However, flat wear appears due to the argillization. Therefore, in engineering practice, to obtain high work efficiency of tunnel boring machine, it is necessary to keep water content of clay-bearing rock in a reasonable range. This study reveals the argillization process and abnormal cutter wear mechanism from the microstructure’s perspective. In addition, the effects of temperature, water, and mechanical motion are simultaneously taken into consideration. The present study provides some references for reasonably improving tunnel boring machine performance in the tunnel construction.

Strength properties of fresh and weathered rocks subjected to wetting–drying cycles

Unlike previous research on soft and clay-bearing rocks, this study seeks to investigate the effect of wetting and drying (W–D) cycles on the engineering properties of two hard rocks. A series of W–D cycles were applied to fresh specimens of greywacke and basalt, and their strength, slake durability and density were progressively assessed as the number of cycles increased. It was found that unlike the basalt, the strength of greywacke rapidly decreased with increasing W–D cycles while the rock density and slake durability index remained almost the same. Analysis of the crack propagation in both rocks revealed that, compared to soft and clay-bearing material, the process of strength degradation in hard rocks was rather different, with a more pronounced effect of crack development. In addition, comparisons with the data obtained for the same type of rocks yet naturally weathered was performed to establish similarities between the properties of laboratory-deteriorated and naturally weathered rocks. The obtained results are expected to provide more practical values for such laboratory studies where the rate of rock deterioration is relatively high (several weeks) in comparison to what is commonly observed in the field (several years).

Mineralogy and geochemistry of the Tonmittelsalz (z3TM) and Tonbrockensalz (z4TS) as “zuber” equivalents in the German Zechstein (Upper Permian)

Two main lithostratigraphic units of clay-bearing rock salt, the Tonmittelsalz of the Leine formation (z3) and Tonbrockensalz of the Aller formation (z4), occur in the German Zechstein (Upper Permian) succession. These units could be equivalents of the Brown Zuber (Na3t) and Red Zuber (Na4t) in the PZ3 and PZ4 cyclothemes of the Polish Zechstein basin. Mineralogical-geochemical investigations of the Tonmittelsalz and Tonbrockensalz were carried out on samples taken from a deep borehole in the Gorleben salt dome in Northern Germany. Even though these units are characterized by a similar mineralogical composition of mainly halite with subordinate quantities of anhydrite and clay minerals, variations in mineral content and fabric were observed. The older Tonmittelsalz rocks document some primary features like chevrons in halite crystals and idiomorphic halite crystals in clay-bearing layers. A brecciated fabric and a vague layering, shown by polarizing microscopy and CT-imaging, indicate a deformation of the younger Tonbrockensalz , which is folded in the deep borehole Go1004. Nevertheless, internal fabrics of clay clasts in the z4TS show an early brecciated and folded fabric during sedimentation or diagenesis. Main component chemistry and REE are comparable in both units, but significant differences were observed for trace element and isotope data. The z3TM rocks contain higher values of trace elements like Li and higher values in REE, while the z4TS rocks are enriched in K. Isotope data of anhydrites of both units correspond to those of the Zechstein. The δ 18 O values of samples from the Tonbrockensalz display a relatively large range (8.5–11.9‰) and may indicate changing conditions during its formation. In contrast, only minor variations in the δ 34 S of samples from the Tonbrockensalz and in both isotope compositions of samples from the Tonmittelsalz have been documented.