Synoptic interpretation of seismic reflection and refraction data

Summary. The structure of the upper lithosphere beneath southern Germany, northern Switzerland and west-central Utah (U.S.A.) has been investigated in detail by various geophysical methods. A synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide-angle reflection surveys, combined with near-normal incidence reflection observations, now permits the elucidation of the fine structure in a more quantitative and unified manner. With this scheme it is possible to unambiguously identify low-velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process.     

Mesozoic (Lower Jurassic) red stromatactis limestones from the Southern Alps (Arzo, Switzerland): calcite mineral authigenesis and syneresis-type deformation

The Broccatello lithological unit (Lower Jurassic, Hettangian to lower parts of Upper Sinemurian) near the village of Arzo (southern Alps, southern Switzerland) is a mound-shaped carbonate deposit that contains patches of red stromatactis limestone. Within the largely bioclastic Broccatello unit, the stromatactis limestone is distinguished by its early-diagenetic cavity system, a relatively fine-grained texture, and an in-situ assemblage of calcified siliceous sponges (various demosponges and hexactinellids). A complex shallow subsurface diagenetic pathway can be reconstructed from sediment petrography in combination with comparative geochemical analysis (carbon and oxygen isotopes; trace and rare earth elements, REE + Y). This pathway includes organic matter transformation, aragonite and skeletal opal dissolution, patchy calcification and lithification, sediment shrinkage, sagging and collapse, partial REE remobilization, and multiple sediment infiltration. These processes occurred under normal-marine, essentially oxic conditions and were independent from local, recurring syn-sedimentary faulting. It is concluded that the stromatactis results from a combination of calcite mineral authigenesis and syneresis-type deformation. The natural stromatactis phenomenon may thus be best explained by maturation processes of particulate polymer gels expected to form in fine-grained carbonate sediments in the shallow subsurface. Conditions favorable for the evolution of stromatactis appear to be particularly frequent during drowning of tropical or subtropical carbonate platforms.

Dual-porosity modeling of groundwater recharge: testing a quick calibration using in situ moisture measurements,Areuse River Delta,Switzerland

A simple method for calibrating the dual-porosity MACRO model via in situ TDR measurements during a brief infiltration run (2.8 h) is proposed with the aim of estimating local groundwater recharge (GR). The recharge was modeled firstly by considering the entire 3 m of unsaturated soil, and secondly by considering only the topsoil to the zero-flux plane (0–0.70 m). The modeled recharge was compared against the GR obtained from field measurements. Measured GR was 313 mm during a 1-year period (15 October 1990–15 October 1991). The best simulation results were obtained when considering the entire unsaturated soil under equilibrium conditions excluding the macropore flow effect (330 mm), whereas under non-equilibrium conditions GR was overestimated (378 mm).Sensitivity analyses showed that the investigation of the topsoil is sufficient in estimating local GR in this case, since the water stored below this depth appears to be below the typical rooting depth of the vegetation and is not available for evapotranspiration. The modeled recharge under equilibrium conditions for the 0.7-m-topsoil layer was found to be 364 mm, which is in acceptable agreement with measurements.

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Resumen Se propone un método sencillo para calibrar el modelo de doble porosidad "MACRO" mediante medidas in-situ obtenidas por TDR durante un breve ensayo de infiltración (2,8 horas), con el objetivo de estimar la recarga local al acuífero. Ésta ha sido modelada de dos formas: considerando los 3 m de suelo no saturado y empleando sólo desde la capa superior hasta el plano de flujo nulo (de 0 a 0,70 m). Se compara la recarga modelada con la recarga local medida en campo, la cual fue de 313 mm durante un ciclo anual (del 15 de octubre de 1990 al 15 de octubre de 1991). Las mejores simulaciones corresponden a la hipótesis de columna entera no saturada en condiciones de equilibrio, excluyendo el efecto de macroporos (valor de 330 mm), mientras que el resultado obtenido para condiciones de no equilibrio en la recarga local está sobreestimado (378 mm).Los análisis de sensibilidad muestran que la investigación del horizonte superior del suelo es suficiente para estimar la recarga local en este caso, ya que el agua almacenada por debajo de esta profundidad parece estar fuera del alcance típico de las raíces de la vegetación y no puede ser evapotranspirada. La recarga modelada en condiciones de equilibrio para la capa superior de 0,70 m de espesor es de 364 mm, valor aceptable respecto a las medidas.

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Résumé Une méthode simple pour la calibration du modèle à double porosité MACRO par des mesures TDR in situ durant un bref essai d'infiltration (2.8 h) a été proposée pour l'estimation locale de la recharge de la nappe (RN). La RN a été d'abord simulée en tenant compte de toute la zone non saturée (3 m) et ensuite, en considérant uniquement la couverture du sol entre zéro et le plan du flux nul (0.70 m). La RN simulée a été comparée à la RN observée. La RN mesurée durant une année (15 octobre 1990–15 octobre 1991) était de 313 mm. Les meilleures simulations ont été obtenues en tenant compte de toute la zone non saturée sous les conditions d'équilibre excluant le flux préférentiel (330 mm). Sous les conditions de non équilibre, la RN a été surestimée (378 mm).Les analyses de sensitivité ont montré que l'investigation de la couverture du sol est suffisante pour l'estimation locale de la RN du fait que l'eau traversant le plan du flux nul se trouverait sous la zone des racines et échapperait à l'évapotranspiration. La RN simulée sur les 0.70 m du sol sous les conditions d'équilibre était de 364 mm, ce qui est comparable aux mesures.

     

Relationship between precipitation phase and air temperature: comparison between the Bolivian Andes and the Swiss Alps / Relation entre phase de précipitation et température de l'air: comparaison entre les Andes Boliviennes et les Alpes Suisses

Abstract

Determining the precipitation phase—rain or snow—is an important factor in modelling discharge in mountainous basins. In a study carried out in the outer tropical Andes Cordillera of Bolivia, half-hourly determination of precipitation phase was obtained by applying a suitable expert system, taking 11 meteorological parameters into consideration that are measured over 21 months at an altitude close to 4800 m. Straightforward relationships between the determined precipitation phase and observed air temperature were analysed in histograms that contain percentage occurrences of snowfall, rainfall and mixed precipitation events for 0.5°C air temperature increments. The graph shows a nearly identical distribution of percentage occurrences of snowfall in the Andes to that on a 1600-m high site in the Swiss Alps. This result suggests that, for hydrological modelling purposes in the outer tropical Andes, the same rain/snow threshold temperature as in the compared Swiss site can be applied.     

New data on the age of the installation of Urgonian-type carbonates along the northern Tethyan margin: biostratigraphy of the Chopf Member (Helvetic Alps,eastern Switzerland)

The Chopf Member is a glauconitic, phosphate-bearing succession that occurs in the distal part of the Helvetic Alps (eastern Switzerland). The recent discovery of age-diagnostic ammonites within this horizon allows for its attribution to the lower part of the Gerhardtia sartousiana zone (middle Late Barremian). This new age corresponds to a maximal age for the onset of the Schrattenkalk Fm. in this area, and is coeval with the onset of the Urgonian facies in other parts of the western Tethyan realm. This new age allows also for a more precise dating of Late Barremian ${\delta }^{13}\text{C}$ curves. To cite this article: S. Bodin et al., C. R. Geoscience 338 (2006).     

Cosmogenic nuclide-based investigation of spatial erosion and hillslope channel coupling in the transient foreland of the Swiss Alps

Transient landscape disequilibrium is a common response to climatic fluctuations between glacial and interglacial conditions. Such landscapes are best suited to the investigation of catchment-wide response to changes in incision. The geomorphology of the Trub and Grosse Fontanne, adjacent stream systems in the Napf region of the Swiss Molasse, was analyzed using a 2-m LIDAR DEM. The two catchments were impacted by the Last Glacial Maximum, LGM, even though the glaciers never overrode this region. They did, however, cause base levels to drop by as much as 80 m. Despite their similar tectonic, lithologic and climatic settings, these two basins show very different responses to the changing boundary conditions. Stream profiles in the Trub tend to be smooth, while in the Fontanne, numerous knickzones are visible. Similarly, cut-and-fill terraces are abundant in the Trub watershed, but absent in the Fontanne, where deep valleys have been incised. The Trub appears to be a coupled hillslope–channel system because the morphometrics throughout the basin are uniform. The morphology of hillslopes upstream of the knickzones in the Fontanne is identical to that of the Trub basin, but different downstream of the knickzones, suggesting that the lower reaches of the Fontanne have been decoupled from the hillslopes. However, the rapid incision of the Fontanne is having little effect on the adjacent upper hillslopes.We tested this interpretation using cosmogenic ¹⁰Be-derived basin-averaged denudation rates and terrace dating. The coupled nature of the Trub basin is supported by the similarity of denudation rates, 350 ± 50 mm ky^− 1, at a variety of spatial scales. Upstream of the knickzones, rates in the Fontanne, 380 ± 50 mm ky^− 1, match those of the Trub. Downstream of the knickzones, denudation rates increase to 540 ± 100 mm ky^− 1. The elevated rates in the downstream areas of the Fontanne are due to rapid incision causing a decoupling of the hillslope from the channel. Basin response time and the magnitude of base level drop exert the principal control over the difference in geomorphic response between the two basins. The timing of the filling of the Trub valley, 17 ± 2 ka, and the initial incision of the Fontanne, 16 ± 3 ka, were calculated, verifying that these are responses to late glacial perturbations. Unique lithologic controls allow for one of the fastest regolith production rates yet to be reported,  380 mm ky^− 1.     

Damage field and site effects: multidisciplinary studies of the 1964 earthquake series in Central Switzerland

Central Switzerland shows comparatively high seismic activity by Swiss standards. Many historical earthquakes are known and several of them caused damage. The last major event dates back to 1964 and has the characteristics of an earthquake swarm. Among dozens of felt shocks were two main shocks (Mw = 5 and 5.7) that moderately damaged a limited area with hundreds of buildings suffering loss. Our aim here was to reconstruct the damage field and to analyze whether it was influenced by site effects. Given the existence of a contemporary damage assessment and other historical sources, we could describe the damage field in detail. For about 95% of the affected buildings, we could reconstruct the location and extent of loss, using assessments from the European Macroseismic Scale (EMS 98). Spatial analysis of the resulting data showed that most losses were concentrated in the villages of Sarnen and Kerns. Damage to residential houses and barns was by far most frequent (90%), but expensive losses to the relatively few sacral buildings were responsible for almost 50% of the repair costs. We compared the damage data with deposit thickness and soil composition and carried out field experiments using H/V spectral ratios to measure the fundamental frequency of ground resonance at 75 sites to estimate the frequency band in which amplification occurs. Our results show that locations on both thick fluviatile sediments and large alluvial cones showed higher intensities than did other ground types. Moreover, at some sites, intensity was probably increased by a layer of weathered rock below thin deposits.     

Expression of the Toarcian Oceanic Anoxic Event: New insights from a Swiss transect

A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock‐Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north–south transect, from the Jura through the Alpine Tethys (Sub‐Briançonnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon‐isotope excursion in carbonate (ca 2 to 4‰) and organic matter (ca 4 to 5‰) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (≤11 wt.%) in the Jura, to thin‐bedded marl (≤5 wt.% total organic carbon) in the Sub‐Briançonnais Basin and to hemipelagic reddish marly limestone (total organic carbon <0·05 wt.%) in equivalent levels from the Lombardian Basin. The carbon‐isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub‐Briançonnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely‐bedded laminae and homogeneous mud layers containing rip‐up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian–Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea‐level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings.