SUMMARY
Fault slip induced by fluid perturbation in shale formations may only lead to a sparse seismicity. However, fault slip may strongly impact the integrity of shale formations that serve as ...caprocks for geological reservoirs holding buoyant fluids such as CO2, natural gas or hydrogen. A better understanding of the fluid reactivation processes of fault and the seismic triggering process is therefore critical for reservoir monitoring and fault stability. Here we analyse the seismic responses of a shale fault exposed to fluid pressurization during an in situ field-scale injection experiment at ∼340 m depth in the Mont Terri underground research laboratory (Switzerland). Two main types of seismic signals are observed as the fault was activated and started to slowly slip. After an aseismic phase, we observed tremor signatures and an increase in noise amplitude, which were directly associated with the slowly propagating fault slip in response to fluid injection. These signatures were later followed by micro-earthquakes that seem to occur further away from the fluid-pressurized area. We interpret these micro-earthquakes to be triggered by stress perturbations from the main slip growth. These two classes of seismic responses therefore highlight two different processes. Tremors seem to be a more direct observation for the fluid-induced slip propagation than micro-earthquakes. Even hidden in the noise, they precede earthquake failures, thus providing a useful tool for monitoring fluid leakage activated by slow deformation on low permeable shale faults, with applications for sealing integrity of caprocks.
The recent advances regarding the complex chronobiostratigraphy of Middle Miocene terrestrial deposits of southern Germany are reviewed. We propose new and revised correlations between the Swiss and ...South German faunas framework for ongoing research. We restrict our analysis to the cricetid and microtoid muroid rodents, especially the Megacricetodon and cricetodontine groups, because of their importance for this purpose. Faunal turnovers reflect global climate changes. Species level endemism is punctuated by several immigration events, and a possible westward spread of faunal associations is suggested at around 13.8 Ma and, at the end of the Middle Miocene, by introduction of Late Miocene lineages from the east.
Les récentes avancées liées à la chronobiostratigraphie complexe des dépôts terrestres du Miocène moyen du Sud de l’Allemagne sont synthétisées. Les possibles corrélations (nouvelles et modifiées) entre les faunes suisses et du Sud de l’Allemagne sont proposées, sur la base des recherches en cours. Nous limitons l’analyse aux rongeurs muridés cricétidés et microtidés, spécialement les groupes Megacricetodon et cricétodontinés, à cause de leur importance pour ce sujet. Les renouvellements fauniques reflètent les changements climatiques globaux. L’endémisme des espèces est ponctué par plusieurs évènements migratoires, et un possible déplacement vers l’ouest des associations faunistiques est suggéré à environ 13,8 Ma, ainsi qu’à la fin du Miocène moyen, par l’introduction de lignées d’origine orientale et caractéristiques du Miocène supérieur.
The geochemistry and mineralogy of organic-rich sediments of the Rietheim succession in northern Switzerland were studied to evaluate the main impacts of the early Toarcian Oceanic Anoxic Event ...(T-OAE; ~183Ma ago) on the depositional environment in the NW Tethys Ocean. The geochemical data obtained (notably, δ13Corg, δ13CCaCO3, Rock-Eval pyrolysis, TOCcff) support the hypothesis that the negative C-isotope excursion at the onset of the T-OAE mainly reflects a major perturbation in the global carbon cycle rather than local environmental changes (e.g., basinal restriction, productivity). Rock-Eval pyrolysis and δ13Corg-δ15Ntotal crossplots indicate that the organic matter was primarily of marine origin during the T-OAE, and deposited in an epicontinental sea setting. Detrital proxies notably, detritus index, siliciclastic balance and Ln(Al2O3/Na2O) suggest that the elevated pCO2 levels and greenhouse climate prevailing during the T-OAE induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates. Redox-sensitive proxies relative pyrite contents, organic and sulfide balances, V/(V+Ni) ratios, trace elements enrichment factors, trace-element-TOC co-variation patterns and TOC/Ptotal molar ratios indicate that the Rietheim Posidonia Shale sediments accumulated under oxic-to-dysoxic bottom-water conditions, with more reducing conditions during the T-OAE (anoxic and possibly euxinic). These changes in redox conditions were probably induced by substantial thermohaline stratification driven by enhanced freshwater input and the recurrent influx of brackish Arctic seawater into the NW Tethys through the Viking Corridor, in agreement with previous sedimentological and geochemical evidences from the NW Tethys Ocean. In the studied succession, such conditions were perhaps also responsible for sequestration of most of the regenerated P in the deeper water column by adsorption and/or co-precipitation into authigenic phases. Overall, the major palaeoenvironmental changes documented in this study provide a robust example of Earth’s possible responses to the rapid onset of extreme greenhouse conditions.
•The T-OAE negative CIE reflects a 13C-depleted carbon injection to ocean–atmosphere.•Detrital proxies suggest an increase in continental weathering rates during the T-OAE.•Redox proxies suggest moderate anoxic bottom-water conditions during the T-OAE.•Dysoxic-to-anoxic conditions prevailed until at least to the bifrons Zone.•P-enrichments are observed during the T-OAE.
Swelling of clay-sulfate rocks is a feared problem in tunneling in such rock, causing heave of the tunnel invert and leading to swelling pressures that seriously damage the tunnel lining. Prediction ...of the swelling potential of rocks in different tunnel sections and tools to evaluate measures that aim at reducing the swelling risk would be a major aid for designers of tunneling projects and rehabilitation works in order to respond adequately to the swelling problem. The study presents a hydrogeological approach to assess the swelling potential of clay-sulfate rocks in tunneling, using the Belchen tunnel in Switzerland as a case study. It shows that this approach can be applied to complex geological settings using structurally consistent 3D numerical models to simulate groundwater flow influenced by tunneling. The models are in particular used to evaluate hydraulic measures that aim at preventing or minimizing water inflow into clay-sulfate rocks after tunneling. Analyzed hydraulic measures include pumping shafts connected to the tunnel drainage, sealing structures applied to the rock zone around the tunnel damaged by the excavation process (excavation damaged zone), as well as groundwater level drawdown in aquifers crossing the tunnel. A sensitivity study addresses model uncertainties and evaluates the impact of model parameters on the swelling potential. Promising measures include the installation of pumping shafts in tunnel sections of clay-sulfate rocks at the border to aquifers and permeable fault zones. The study shows that monitoring of hydraulic heads as well as of groundwater volumes drained by the tunnel would increase the predictive capability of the models. Presented results are site specific. However, the approach to assess the swelling potential of clay-sulfate rocks in tunneling, and to assess hydraulic measures to respond to the swelling problem, can be transferred to other tunneling projects.
•We simulate 3D groundwater flow influenced by tunneling in complex geological setting.•We use numerical groundwater models to assess hydraulic measures that aim at counteracting rock swelling.•Water inflow into sections with swellable rock can be minimized by pumping shafts if installed at an optimal position.
The fossil record of chondrichthyans (sharks, rays and chimaeroids) principally consists of isolated teeth, spines and dermal denticles, their cartilaginous skeleton being rarely preserved. Several ...Late Jurassic chondrichthyan assemblages have been studied in Europe based on large bulk samples, mainly in England, France, Germany and Spain. The first study of this kind in Switzerland is based on controlled excavations in Kimmeridgian deposits related to the construction of the A16 motorway in the Swiss Jura (Porrentruy, NW Switzerland). This study is based on more than 2000 isolated chondrichthyan remains (teeth, dental plates, spines and dermal denticles) and adds to our knowledge of the chondrichthyan distribution at a regional scale in Europe. We describe and identify this new fauna, define a new species of hybodont with crushing‐type dentition (Asteracanthus udulfensis sp. nov.) and report for the first time the carcharhiniform Corysodon cirinensis in Switzerland. By the Late Jurassic, modern neoselachian sharks had overtaken hybodonts in European marine realms, the latter being gradually confined to brackish or freshwater environments. However, while the associated fauna of the Porrentruy platform indicates marine conditions, neoselachian sharks are surprisingly rare. The chondrichthyan assemblage is largely dominated by hybodonts, guitarfishes (rays) and chimaeroids that are all known to be euryhaline. This unexpected chondrichthyan faunal composition questions the presence of fresh to brackish water in the vicinity of the platform, and the occurrence of salinity fluctuations within a general context marine. This could explain the scarcity of neoselachian sharks and the extended success of hybodonts in the Porrentruy area as late as the Late Jurassic.
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•Enrichment in the heavy 205Tl isotope in the B soil horizons.•Redox-driven 205Tl accumulation during soil formation/rock weathering.•Thallium isotope signatures unrelated to current ...Tl speciation (soil) data.•Thallium-illite as a dominant Tl host phase.•Thallium isotopic data applicable as a proxy for redox-controlled soil processes.
Soils on the Erzmatt (Switzerland) formed on hydrothermally mineralized dolomite rock are naturally Tl-rich. In this study, we investigated if variations in the stable Tl isotope ratios in soil samples from different profiles can be linked to data on the extractability and speciation of soil Tl and whether the isotopic data allow drawing conclusions on the geochemical processes that affected Tl over the course of soil formation. In two soil profiles, we observed a marked accumulation of the heavy 205Tl isotope in the B horizons, with ε205Tl values that were up to 7 higher than in the underlying bedrock. This 205Tl enrichment, however, was neither reflected in the speciation of Tl nor its chemical fractionation. Furthermore, exchangeable soil Tl in the B horizons was found to be much isotopically lighter than the bulk soil Tl. These findings suggest that the observed isotopic shift may be linked to cyclic Tl mobilization and immobilization processes over the period of rock weathering and soil formation. Oxidative Tl uptake by Mn-oxides associated with a 205Tl enrichment, continuous weathering of the Tl(III)-containing phases, followed by a Tl(I) remobilization (leading to enrichment in 205Tl) are suggested to be responsible for the binding of the heavy Tl isotope fraction into other phases, mainly illite (a dominant Tl host), which is not normally expected. Hence, our results show that the Tl isotopic fractionation data measured in a dynamic multi-phase (soil) system can potentially serve as a proxy for tracing redox-controlled processes, but their use for phase or the sorption process identification is much more complicated.
Pre-existing faults in the mechanical basement are believed to play an important role in controlling deformation of the thin-skinned Jura Mountains fold-and-thrust belt, which constitutes the ...northernmost extension of the European Alps. We use brittle-viscous analogue models to investigate the influence of frontal and oblique basement steps on the subsequent evolution of structures during thin-skinned shortening. Vertical offset between two rigid baseplates (simulating the mechanical basement) causes the formation of reverse faults and grabens in the overlying brittle layers that are not reactivated during subsequent thin-skinned shortening. However, baseplate steps localise deformation, causing a temporary frontward propagation of deformation in an early stage and inhibiting propagation afterwards. Downward baseplate steps induce very strong deformation localisation and foster the formation of fault-bend folds. Models featuring upward steps develop step-controlled pop-up structures with imbricated fronts and viscous ramps that shorten dynamically with progressive contraction. We find that deformation localisation increases both with higher step-throws and lower obliquity (α) of the strike of the step (e.g. frontal step α = 0°). With increasing step-throws, α = 30° and α = 45° oblique upward-steps lead to a characteristic imbrication of the brittle cover with laterally confined thrust-slices and step-parallel oblique-thrusts, which rotate up to 15° about a vertical axis over time. Step-controlled backthrusts preceding the formation of thrust-slices do not show notable rotation and hence constitute excellent indicators for the orientation of oblique upward-steps. The topographic patterns of oblique-step models resemble individual thin-skinned structures of the Internal Jura (i.e. Pontarlier and Vuache fault zones, the nappe system SE of Oyonnax and the Chasseral anticline), strongly suggesting that pre-existing NNE-SSW and NW-SE striking oblique upward-steps in the basement controlled deformation in the overlying cover. Our model results may be applied to other thin-skinned fold-and-thrust belts worldwide that formed above pre-existing basement structures.
•Brittle-viscous analogue models testing frontal and oblique fault-steps.•Basement faults controlling deformation of the Jura Mountains fold-and-thrust belt.•Rigid basement steps controlling viscous oblique-ramps.•Upward and downward steps localising deformation.•Rotation of step-controlled lineaments
We describe an atypical cave network in the Jura mountains (eastern France), currently the only one known worldwide to be entirely excavated within a subglacial deposits. Extending over 600 m of ...mapped galleries, this natural cavity developed entirely within a large impermeable drumlin shaped during last glacial stage MIS 2 (Wurm). By using lidar topography, petrophysical, sedimentological and geochemical measurements, as well as U-Th dating, this cave is mainly attributed to mechanical erosion processes (piping cave). The network's morphogenesis seems to be characterized by two main phases. Firstly, thin mini-pipes were probably dug along shear planes previously formed within the drumlin. The first mini-pipes may have appeared under the ice cover during the last glaciation in response to differential hydraulic pressures on either side of the drumlin. They may also have appeared later when the drumlin was free from its glacial cover, forming a natural earth dam across a thalweg and obstructing the flow of surface water. A second phase would correspond to the development of mega-pipes, resulting from sequential capture of flows from mini-pipes. Mega-pipes appeared >2500 years ago, as indicated by U-Th dating of one speleothem. In other regions of the world, intra-till caves have been documented, but are <20 m long. This limitation is mainly attributed to instability of the materials constituting subglacial tills, disfavoring a durable existence of galleries without collapses. In the Jura calcareous mountains the subglacial till contains a high proportion of very fine calcite matrix produced by subglacial abrasion, which was indurated by compaction under several hundred meters of ice. These particular petrographic features appear to be required for the creation and the preservation of this long intra-till cave. Similar caves may likely be found in other regions of the world displaying limestone bedrock and where subglacial till with abundant carbonate matrix was formed.
This paper explores the use of adaptive support vector machines, random forests and AdaBoost for landslide susceptibility mapping in three separated regions of Canton Vaud, Switzerland, based on a ...set of geological, hydrological and morphological features. The feature selection properties of the three algorithms are studied to analyze the relevance of features in controlling the spatial distribution of landslides. The elimination of irrelevant features gives simpler, lower dimensional models while keeping the classification performance high. An object-based sampling procedure is considered to reduce the spatial autocorrelation of data and to estimate more reliably generalization skills when applying the model to predict the occurrence of new unknown landslides. The accuracy of the models, the relevance of features and the quality of landslide susceptibility maps were found to be high in the regions characterized by shallow landslides and low in the ones with deep-seated landslides. Despite providing similar skill, random forests and AdaBoost were found to be more efficient in performing feature selection than adaptive support vector machines. The results of this study reveal the strengths of the classification algorithms, but evidence: (1) the need for relying on more than one method for the identification of relevant variables; (2) the weakness of the adaptive scaling algorithm when used with landslide data; and (3) the lack of additional features which characterize the spatial distribution of deep-seated landslides.
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