Evaluating Quaternary activity versus inactivity on faults and folds using geomorphological mapping and trenching: Seismic hazard implications

The incorporation of active faults in seismic hazard analyses may have a significant impact on the feasibility, design and cost of major engineering projects (e.g., nuclear facilities, dams), especially when located in the site vicinity. The regulatory definition of active versus inactive fault is generally based on whether the fault has ruptured or not after a specific chronological bound (i.e. fault recency). This work presents a methodology, mainly based on geomorphological mapping and trenching, for determining whether specific faults can be considered as active or inactive. The approach is illustrated through the analysis of several faults located in the Spanish Pyrenees (Loiti, Leyre, La Trinidad, Ruesta faults). The 29 km long Loiti Thrust was included in the Neotectonic Map of Spain as a probable neotectonic structure. Previous works, based on geomorphological investigations, incorporated the 28 km long Leyre Thrust as a significant seismic source in a probabilistic seismic hazard analysis, which challenged the seismic design of nearby large dams. The production of detailed geomorphological strip maps along the faults allowed the recognition of specific sites where the faults are covered by Quaternary deposits. The establishment of chronosequences (pediments-terrace sequences)and the available geochronological data helped identifying the most adequate morpho-stratigraphic units for satisfactorily evaluating fault activity vs. inactivity. The excavation of trenches at the selected sites provided unambiguous information on the presence or lack of deformation in the Quaternary cover overlying the fault, and the origin of scarps (tectonic versus erosional). Trenches were also useful for collecting samples and reliably measuring the relative height of terraces overlain by thick colluvium. The evidence gathered by these methods were complemented with the numerical dating of non-deformed slope deposits covering a fault, the analysis of the longitudinal profiles of old pediment surfaces located in the proximity of a fault, the examination of a cave situated next to a fault searching for speleoseismological evidence, and regional geodetic and seismotectonic data (GPS measurements, earthquake focal mechanisms). The integration of all the data, and especially the trenches dug in non-deformed old terrace deposits (>100 ka)truncating the faults, indicates that the analysed faults can be considered inactive and that previous neotectonic postulations were based on non-valid geomorphological interpretations

Hurricanes and climate in the Caribbean during the past 3700 years BP

International audienceA multiproxy analysis of lacustrine sediments cored in Grand-Case Pond at Saint-Martin, north of the Lesser Antilles archipelago, reveals three distinct climatic periods for the last 3700 years. From 3700 to ~2500 yr cal. BP and from 1150 yr cal. BP to the present, carbonate mud deposition occurred in connection with pond lowstands. These periods were also punctuated by severe drought events, marked by gypsum laminae, and hurricane landfalls, leading to marine sand inputs into the pond. The intermediate time interval, from 2500 to 1150 yr cal. BP, is typified by black organic mud deposition, suggesting that hypoxic to anoxic conditions prevailed at the pond bottom. These were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today. The carbon isotopic composition of the ostracod Perissocytheridea bisulcata shows that the lowest δ13C values are recorded during the hypoxic periods, as a consequence of bacterial recycling of isotopically depleted organic matter. Such a climatic history agrees closely with that documented from other records in the Caribbean area, such as the Cariaco Basin, central coast of Belize or Barbados. By constrast, discrepancies seem to emerge from the comparison between hurricane activity recorded at Saint-Martin on the one hand and Vieques (Puerto Rico) on the other hand. We explain this apparent contradiction by a balance between two distinct storm paths in response to latitudinal shifts of the Intertropical Convergence Zone (ITCZ). Stronger storm activity over the Gulf coast and the inner Caribbean Sea is favoured by a southern position of the ITCZ in connection with dry climatic conditions. Plausible links with the North Atlantic Oscillation (NAO) are also suggested

Sedimentological and palaeohydrological characterization of Late Pleistocene and Holocene tufa mound palaeolakes using trenching methods in the Spanish Pyrenees

Lakes developed in the inner depressions of tufa mounds are rare geomorphic features and still poorly understood. Sedimentation in this unusual type of endorheic lake with a very restricted catchment area is highly sensitive to environmental and hydrological changes. The Isona tufa mound complex, north-eastern Iberian Peninsula, is associated with the discharge zone of a confined artesian aquifer and comprises 11 tufa mounds consisting of an annular rimstone enclosing a central depression filled with lake deposits. Data gathered from trenches excavated in four palaeolakes located within three different morphostratigraphic units permitted a precise analysis of the geometrical characteristics and stratigraphic relationships of the deposits and provided a sedimentation model for the Late Quaternary infilling of the spring-fed lakes. The work illustrates that trenches allow a precise characterization of the stratigraphic arrangements, lateral facies changes and deformation structures, which are not apparent in studies relying solely on borehole records, and facilitate sampling for dating and geochemical analyses. The five sedimentary facies described represent different evolutionary stages of the lakes, including: (i) carbonate-rich palustrine deposits probably related to periods with strong hydrological seasonality; (ii) massive highly bioturbated organic ooze; (iii) banded organic carbonate-rich facies associated with an increase in the regional effective moisture; (iv) finegrained quartz-rich aeolian/slope-wash sediments; and (v) colluvial facies deposited following the desiccation of the lakes located at higher altitudes. Geochemical and sedimentological analyses of the lacustrine sequences provided information on the palaeohydrological evolution of the Isona tufa mound complex and the palaeoenvironmental conditions of the area over the last 28 ka. Radiometric dating suggests that deposition occurred simultaneously at ca 22 ka in palaeolakes situated at different elevations. A drop in the piezometric level prompted by the opening of springs at lower altitudes probably caused the deactivation of the upper springs and the desiccation of the lakes. Arid conditions prevailed in the area during the Late Glacial and the early Holocene (28·0 to 8·5 ka BP). More humid conditions recorded from 8·5 to 4·2 ka and again since 1·7 ka are in accordance with palaeoenvironmental reconstructions available in the Western Mediterranean since the Last Glacial Maximum

Identifying the boundaries of sinkholes and subsidence areas via trenching and establishing setback distances

One of the most effective mitigation strategies in sinkhole areas is the exclusion of sinkholes and their vicinity to construction. The application of this preventive measure requires precise mapping of the boundaries of the areas affected by subsidence and the establishment of adequate setback distances, which is an important policy issue with significant economic implications. Through the investigation of several buried sinkholes in the mantled evaporite karst of the Ebro Valley by trenching, this work illustrates that the actual extent of the subsidence areas may be much larger than that inferred from surface mapping and geophysical surveys. The objective and accurate subsurface information acquired from trenches on the outer edge of the deformed ground revealed sinkhole radii 2–3 times larger than initially estimated, increasing one order of magnitude the sinkhole area. Trenches can therefore help to reduce mapping uncertainties and the size of setbacks. Moreover, the trenching technique, in combination with geochronological data and retrodeformation analyses, provides critical information on the subsidence phenomena and the characteristics of the sinkholes relevant to hazard assessment. Since recommended setback distances found in the existing literature are highly variable and rather arbitrary, we include a discussion here on the main factors that should be considered when defining setback zones for sinkholes

Sinkholes in hypogene versus epigene karst systems, illustrated with the hypogene gypsum karst of the Sant Miquel de Campmajor Valley, NE Spain

The Garrotxa-Banyoles artesian aquifer system in NE Spain includes zones with epigene and hypogene evaporite karst and sinkhole development. The sinkhole fields related to hypogene evaporite dissolution are associated with the discharge zones of the aquifer, where groundwater from a confined limestone rises traversing an overlying gypsum formation capped by thick marls. This work analyses the sinkhole field developed in the Sant Miquel de Campmajor Valley (SMCV), a discharge area of the aquifer, where a cartographic inventory including 94 sinkholes has been produced. Sinkholes in this hypogene gypsum karst are mainly large caprock-collapse sinkholes rooted in deep-seated cavities generated by interstratal hypogene karstification, as corroborated by an electrical resistivity imaging (ERI) survey. They show a non-clustered distribution within the sinkhole field and the variations in size and density across the area are mainly controlled by the thickness of the marl caprock. The magnitude and frequency relationships of the sinkholes can be modelled with a high goodness of fit with a log-normal function, like in other regions of the world. However, in this deep-seated hypogene system the depressions show much larger dimensions. Potential differences between the sinkholes developed in hypogene and epigene karst environments are discussed, including their distribution with respect to the groundwater flow system, their spatial patterns and dimensions

Reemergence of dengue in Cuba: a 1997 epidemic in Santiago de Cuba.

After 15 years of absence, dengue reemerged in the municipality of Santiago de Cuba because of increasing migration to the area by people from disease-endemic regions, a high level of vector infestation, and the breakdown of eradication measures. The 1997 epidemic was detected early through an active surveillance system. Of 2,946 laboratory-confirmed cases, 205 were dengue hemorrhagic fever, and 12 were fatal. No deaths were reported in persons under 16 years of age. Now the epidemic is fully controlled

4D monitoring of active sinkholes with a Terrestrial Laser Scanner (TLS): A Case study in the evaporite karst of the Ebro Valley, NE Spain

This work explores, for the first time, the application of a Terrestrial Laser Scanner (TLS) and a comparison of point clouds in the 4D monitoring of active sinkholes. The approach is tested in three highly-active sinkholes related to the dissolution of salt-bearing evaporites overlain by unconsolidated alluvium. The sinkholes are located in urbanized areas and have caused severe damage to critical infrastructure (flood-control dike, a major highway). The 3D displacement models derived from the comparison of point clouds with exceptionally high spatial resolution allow complex spatial and temporal subsidence patterns within one of the sinkholes to be resolved. Detected changes in the subsidence activity (e.g., sinkhole expansion, translation of the maximum subsidence zone, development of incipient secondary collapses) are related to potential controlling factors such as floods, water table changes or remedial measures. In contrast, with detailed mapping and high-precision leveling, the displacement models, covering a relatively short time span of around 6 months, do not capture the subtle subsidence (< 0.6-1 cm) that affects the marginal zones of the sinkholes, precluding precise mapping of the edges of the subsidence areas. However, the performance of TLS can be adversely affected by some methodological limitations and local conditions: (1) limited accuracy in large investigation areas that require the acquisition of a high number of scans, increasing the registration error; (2) surface changes unrelated to sinkhole activity (e.g., vegetation, loose material); (3) traffic-related vibrations and wind blast that affect the stability of the scanner

La longue séquence de Marchésieux: reconstitution de paléoenvironnements marins durant le premier cycle glaciaire de l'hémisphère nord

International audienceA l'échelle des cinq derniers millions d'années, les enregistrements paléoclimatiques restitués par les sédiments océaniques illustrent le contrôle des paramètres orbitaux (précession, obliquité et excentricité) sur le climat global. La tendance au refroidissement qui accompagne la fin du Néogène est ainsi marquée, vers -2,4 Ma, par le développement rapide des calottes de glace de l'hémisphère nord (Shackleton et al., 1984). Alors que l'histoire du climat global restituée par les enregistrements océaniques est sans cesse précisée, les données concernant l'évolution des environnements continentaux et côtiers au cours de ces changements restent essentiellement fragmentaires. Le forage effectué à Marchésieux (Manche; Normandie) a permis de réaliser une étude pluridisciplinaire de la signature de ce premier cycle glaciaire de l'hémisphère nord (Prétiglien). Les premiers résultats stratigraphiques, paléoenvironnementaux (eustatisme, température, paléobathymétrie) sont présentés

Hydrogeochemical characterization of an evaporite karst area affected by sinkholes (Ebro Valley, NE Spain)

The main processes controlling the hydrochemistry of an alluvium-covered evaporite karst area with high sinkhole risk (Ebro Valley, NE Spain) are examined by means of multivariate analyses (Principal Component Analysis and Hierarchical Cluster Analysis), ion correlations and geochemical speciation-solubility calculations. The hydrogeochemistry of the studied system seems to be governed by the interaction between the groundwater from the salt-bearing evaporitic karst aquifer and from the overlying Ebro River alluvial aquifer. The observed hydrochemical features in the alluvial-karst aquifer system are mainly determined by the relative contribution of gypsum/anhydrite and halite dissolution, showing a wide spectrum from relatively fresh recharge waters (mainly irrigation waters) to highly evolved groundwater from the evaporitic aquifer. The variability of these contributions is especially evident at sinkhole ponds which, in some cases, seem to be associated with discharge areas of the karst aquifer in the valley bottom alluvium. Calculated saturation indexes suggest that, in contrast to gypsum, the amounts of halite in the sampled portions of evaporitic aquifer are not large enough to attain equilibrium, which is consistent with the predominance of gypsum/anhydrite reported for these materials. Furthermore, the observed Na:Cl and Ca:SO4 correlations and stoichiometries suggest that other possible processes, such as glauberite dissolution or Na/Ca-exchange, generally play a minor role (compared to halite and gypsum dissolution) in this system. Another important process in the system is the dissolution of carbonate minerals (dolomite and, possibly, calcite) fostered by the input of CO2(g), which is probably produced by pedogenic processes. Dolomite dissolution seems to be particularly relevant in the evaporitic materials probably due to dedolomitisation triggered by gypsum/anhydrite dissolution

La longue séquence pliocène de Marchésieux - Résultats analytiques et premiers résultats

National audienceLe sondage intégralement carotté de Marchésieux a permis d'atteindre l'objectif fixé dans le projet AR42 du BRGM : l'analyse paléoclimatique du premier épisode glaciaire de l'hémisphère nord (Prétiglien, -2,4 Ma) et probablement le premier rafraîchissement de -3,1 Ma. Les 159,3 m de sédiments datés du Reuvérien et du Prétiglien ont déjà livré une quantité importante d'informations aussi bien climatiques que paléoenvironnementales. De par sa position géographique privilégiée et de par son environnement côtier, la "Longue sequence" de Marchésieux a précisé les modalités de la séquence de dégradation climatique en milieu continental et marin à nos latitudes moyennes. La méthodologie fortement pluridisciplinaire, le plus souvent il très haute résolution, permet d'affiner notablement les modalités de la réponse de chaque environnement au changement climatique, et apparaît, en ce sens, beaucoup plus riche que les études monodisciplinaires classiques. Ainsi, les réponses zoologiques, phytosociologiques, sedimentologiques et géochimiques au stimulus climatique externe peuvent être comparées et calibrées