Heterogeneous subsidence and paleogeographic elements in an extensional setting revealed through the correlation of a storm deposit unit (Aptian, E Spain)

Durante el Aptiense, en la Subcuenca de Las Parras (NW Cuenca del Maestrazgo) se depositó una unidad siliciclástica en un contexto tectónico extensional. Esta unidad se ha dividido en cuatro secuencias granocrecientes, de las cuales se analiza en detalle la tercera (S3) ya que presenta un alto potencial de correlación lateral. El análisis sedimentológico de la secuencia S3 ha permitido interpretar una evolución vertical de shoreface inferior con procesos de tormenta, a shoreface superior; también ha permitido correlacionar esta secuencia entre dos sectores de la subcuenca que presentan un desarrollo litológico considerablemente diferente. El techo de la secuencia S3 es una superfi cie erosiva menor con valor cronoestratigráfi co y se ha utilizado como datum de correlación para el análisis de la unidad siliciclástica. Las variaciones laterales de facies de la secuencia S3 permiten interpretar la proximidad de un sistema de descarga siliciclástico hacia el sureste, y se propone una zona de intersección de fallas normales, próxima al sector suroriental de la zona estudiada, como un elemento paleoestructural favorable para la entrada de un sistema de drenaje en la cuenca. En el sector suroriental, por encima del datum de correlación, la secuencia S4 presenta un desarrollo muy reducido debido a la ausencia de su parte inferior; esto ha permitido interpretar la presencia de una discontinuidad local intra-S4. Esta discontinuidad local se correlaciona con otra reconocida en el sector noroccidental. Debido a las diferencias de espesor y al grado de desarrollo de la discontinuidad intra-S4 se deduce que la historia de subsidencia de diferentes bloques de la cuenca no es exactamente la misma. Esta discontinuidad intra-S4 podría tener interés regional ya que separa sedimentos dominantemente siliciclásticos de sedimentos carbonatados y podría indicar una modifi cación importante del sistema sedimentario. Para uno de los bloques estudiados, las variaciones espaciales de espesor para cada secuencia podrían representar un desarrollo de la subsidencia alternando periodos con subsidencia diferencial atenuada y periodos con subsidencia diferencial acentuada, que pueden estar relacionados con la dinámica extensional

Profiles of chloride in matrix porewater as natural tracer for matrix diffusion in crystalline rocks

Matrix porewater from low permeable Grimsel granodiorite was successfully characterised using indirect methods applied to originally saturated core samples. Core samples were taken from a 17 m long borehole originating from a tunnel of the Grimsel Test Site into the crystalline bedrock intersecting a tectonic shear zone with a water-conducting fracture. Matrix porewater chloride profiles on the meter scale were determined on both sides of the water-conducting fracture. To evaluate transport processes within the bedrock formation, a series of diffusive model calculations were performed, which to fit the porewater data. Boundary and initial conditions were varied according to the geological conditions, whereas other required parameters such as the connected porosity and pore diffusion coefficients were determined by laboratory experiments on the cores and extrapo- lated to in situ conditions. The main conclusions can be summarized as follows: (1) Chloride porewater profiles at the meter scale can be simulated using diffusive transport models. This provides evidence that diffusive exchange with active fractures occurs over a range of a few meters in the low-permeable crystalline bedrock; (2) the best fit of the diffusion profile was achieved by a model approach, which takes asymmetric initial Cl-concentrations into account. This indicates that prior to the activation of the present water-conducting fracture, the porewater system in the bedrock was already active showing a concentration gradient in chloride; (3) the water-conducting fracture was activated at least between 850 and 1700 years before present, with a best-fit 1200 years before present; and (4) the hydraulic were affected by the construction of the rock laboratory 20 years ago, resulting in a rapid dilution of the fracture groundwater by advection

Coupled hydraulic-mechanical simulation of seasonally induced processes in the Mont Terri rock laboratory (Switzerland)

International audienceThis paper focuses on hydraulic-mechanical effects in the Mont Terri rock laboratory (Switzerland) and investigates their impact on pore pressure, the convergence of a niche and the evolution of pre-existing cracks in the wallrock of the niche. A comparison of measurements with numerical simulation results is conducted. The presented in situ measurements include long-term data on temperature, relative humidity, and niche convergence, as well as crackmeter and jointmeter measurements determining the aperture of a bedding-parallel crack. Furthermore, time-domain-reflectometry measurements were carried out in a borehole located in the niche wall close to the crack. They provide information on the water content of the claystone. The numerical simulation is carried out with OpenGeoSys. It contains a 2-dimensional coupled hydraulic-mechanical model considering orthotropy, swelling and shrinkage effects, linear elastic material behaviour and an excavation damaged zone characterized by a zone of lower rock strength. An additional focus is laid on the evolution of cracks in the wallrock of the niche. The presented model approach allows a prediction of the temporal evolution of a desaturated zone incorporating a seasonally influenced part. The comparison with measured water contents yields good agreement. Simulation results and measurements consistently predict a convergence of the niche. The evolution of the crack aperture is influenced by long-term as well as seasonallyinfluenced effects. Due to the convergence of the niche, the long-term trend is characterized by closure of the crack. Furthermore, the seasonally influenced desaturation in winter results in opening of the crack, while the resaturation in the warm and wet summer months leads to closure of the crack. Finally, the comparison of simulation results with measurements indicates qualitative agreement. As a matter of fact, the numerical model seems to represent significant effects concerning the evolution of the crack aperture of a single crack. © 2017, The Author(s)

Characterisation of pore space geometry by 14C-PMMA impregnation—development work for in situ studies

International audienceIn Finland, as in many other countries, high-level radioactive waste is planned to be disposed of in deep-lying crystalline rock. There has been a tendency to play down the potential role of the geosphere as a safety barrier in repository performance assessment. Among other reasons, current uncertainties in transport pathway definition and pore space characterisation of crystalline rock can be mentioned. Repository safety evaluation today requires going from laboratory and surface-based field work to the underground repository level. Little is known about the changes to rock transport properties during sampling and decompression. Recent investigations using resin impregnation of the rock matrix at the Grimsel Test Site imply that non-conservative errors in calculated transport properties derived from laboratory data may reach factors of 2/3. This may lead to over-conservative estimates of limited matrix diffusion depths, which may grossly underestimate geosphere radionuclide retardation.Due to the potentially great significance of pore space characterisation to safety analysis calculations, it was decided to study the rock matrix characteristics using an alternative resin technique. During last decade, the polymethylmethacrylate (PMMA) method has been developed for characterising the porosity of low-permeable granitic rocks in the laboratory. Impregnation with 14C-labelled methylmethacrylate (14C-MMA) and autoradiography allow investigation of the spatial distribution of accessible porosity at the centimetric scale. Quantitative measurement of total or mineral-specific local porosities have been also obtained using image analysis tools. Electron microscopy examinations and mercury porosimetry measurements have provided detailed information of pore and fissure apertures.The developments of the PMMA method for in situ testing at the Grimsel Test Site are reported here. Grimsel granodiorite samples were impregnated with MMA using various prehandling and impregnation conditions, resulting in variations of the measured porosity values. Vacuum drying of studied samples increased the PMMA and porosity values of water saturation gravimetry. If the rock matrix cannot be dried in situ, the intrusion behaviour of MMA in water filled pores will be a key consideration for successful in situ impregnation