Yves Quinif

Annually Laminated Sequences in the Internal Structure of Some Belgian Stalagmites--Importance for Paleoclimatology

ABSTRACT Fifteen stalagmites from four caves and one sealed tunnel in southern Belgium are composed of alternations of annually deposited white-porous and dark-compact laminae. This is demonstrated by comparing the number of laminae with the local history of the site for modern stalagmites and with radioisotopic ages for Late Glacial and Holocene stalagmites. Annual cyclicity in the internal structure of these speleothems is explained by the highly seasonal variations of the water excess, which influences underground water flow. Comparison between climatic data and modern stalagmites of a closed tunnel shows that growth laminae can record climatic variations: (1) there is a good correlation (R = 0.84) between lamina thickness in a stalagmite and water excess; (2) during years with a high water excess, dark-compact laminae are more developed, which makes the speleothem darker. Vertical successions of several laminae represent microsequences that may have recorded climatic variations with a time resolution of 1/2 year. In a Late Glacial stalagmite, successive laminae microsequences form very regular cycles of 11 years separated by a thick darkcompact lamina. It is supposed that, as for modern stalagmites, the thick dark-compact lamina corresponds to a period of high water excess. Hence, this 11-year cycle may reflect a climatic cycle.

Geohazard map of cover-collapse sinkholes in the 'Tournaisis' area, southern Belgium

Abstract This paper reports the methodology developed to draw up a geohazard map of cover-collapse sinkhole occurrences in the ‘Tournaisis’ area. In this area, Carboniferous limestones are overlain by a Mesocenozoic cover, mainly consisting of marls, sand and clay. The thickness of this cover ranges from a few meters to more than 100 m. The surficial morphology of the area does not show any karstic evidence except for the occurrence of these collapses. From a paleogeographical point of view, a developed quaternary karst is not conceivable in the area. Recent works suggested that the collapses are set off from reactivated paleokarsts. The paleokarsts studied in the area proved to be the result of a particular weathering of the limestone. The organization of these paleokarsts seems very low and mainly guided by the limestone fracturing. As for most induced sinkholes, the reactivation of these paleokarsts is linked to the lowering of piezometric heads. In most of the area, a thick cover and intensive land use mask potential surface hints of the buried paleokarsts and of the fracturing of the bedrock. Aerial photographs and remote sensing techniques have therefore shown little results in delineating collapse hazard zones up to now. The study of the surficial morphology is also of little help. In order to draw up the geohazard map in such a difficult context, hydrogeological data and geological mapping information could only be used. These informations are based on a limited number of boreholes and piezometers and are thus, only valid on a regional scale. Records of former collapses were also available. These records were of great interest since sinkhole distribution is obviously clustered in the area. Bedrock roof and cover formation floor altitudes were digitized and adapted to produce digital thematic maps. Piezometric heads were imported from a calibrated groundwater model of the aquifer. These data and a digital elevation model of the area were integrated into a geographical information system (GIS) to produce a coherent 3-D description of the area on a regional scale. Parameters such as the dewatering of the limestone and the thickness of the cover formation where sinkholes occurred were then estimated. Density of former collapses was also computed. This showed that zones of high sinkhole occurrence coincide with zones of heavy lowering of piezometric heads. Combining the density of former collapses with the dewatering of the limestone enabled us to delineate zones of low, moderate and high collapse hazard.

Polyphased uplift and erosion of the Cévennes (southern France): An example of slow morphogenesis

The Cevennes are bordering the French Massif Central and the Gulf of Lion margin. The morphogenesis of this area results from an interaction between deep-seated and superficial processes, whose origin and timing is still discussed. We attempt a reconstruction of the surrection and erosion history of the area through a multidisciplinary approach including geology, geomorphology, thermochronology and geochronology. Thermochronology shows that the Cevennes basement underwent some 2 km denudation in mid-Cretaceous time. Analyses of the sediments preserved on uplifted surfaces and in peripheral sedimentary basins indicate a differential surface uplift of the Cevennes, of the surrounding calcareous plateaus, and of the coastal plain, that occurred in several stages during the Tertiary. Early Miocene rifting of the Gulf of Lion margin and opening of the NW Mediterranean drastically modified the drainage network. Geomorphology analyses of the incised rivers and karst network suggest that most of the incision results from uplift that occurred sometime in the Serravalian-Tortonian interval. U/Th dating of calcite concretions in karsts allows to chronologically bracket the formation of some fluvial terraces, and to find very low incision rates during the Pleistocene. Most of the morphogenesis predates the Quaternary. This ongoing study shows an example of polyphased and very slow morphogenesis, with present-day landscape including elements as old as Cretaceous.

Monitoring climatological, hydrological and geochemical parameters in the Père Noël cave (Belgium): implication for the interpretation of speleothem isotopic and geochemical time-series

Pere Noel cave climatology (air and water temperature, P CO2), hydrology (drip rate, conductivity) and geochemistry of water and calcite deposits (δ 18O, δ 13C, Mg/Ca and Sr/Ca) where studied to better interpret stable isotopic and trace element variations of speleothems. Results of an automated monitoring station and of manual sampling between 1991 and 1998 have demonstrated the highly seasonal signal of drip rate, its control by water excess and rainfall, and, at a shorter scale to air pressure changes. The modern calcite deposit study suggests a relationship between cave calcite isotopic composition (δ 18O and δ 13C) and drip rate likely due to variations in degree of isotopic equilibrium during calcite precipitation. δ 18O and δ 13C of the calcite are therefore, through drip rate, linked to water recharge. Mg/ Ca and Sr/Ca ratios of Pere Noel cave calcite, depend closely on the residence time of the water, and therefore are also linked to drip rate and therefore to water recharge. This crossed link of δ 18O and δ 13C as of Mg/Ca and Sr/Ca to water recharge may explain the very similar variations of these four parameters along the longitudinal axis of a Holocene stalagmite, but it may also be the consequence of kinetic effects during calcite precipitation as suggested by similar variations of the four parameters along a single layer of the Holocene stalagmite.

Repeated absolute gravity measurements for monitoring slow intraplate vertical deformation in western Europe

In continental plate interiors, ground surface movements are at the limit of the noise level and close to or below the accuracy of current geodetic techniques. Absolute gravity measurements are valuable to quantify slow vertical movements, as this instrument is drift free and, unlike GPS, independent of the terrestrial reference frame. Repeated absolute gravity (AG) measurements have been performed in Oostende (Belgian coastline) and at 8 stations along a southwest-northeast profile across the Belgian Ardennes and the Roer Valley Graben (Germany), in order to estimate the tectonic deformations in the area. The AG measurements, repeated once or twice a year, can resolve elusive gravity changes with a precision better than 3.7 nm/s²/yr (95% confidence interval) after 11 years, even in difficult conditions. After 8-15 years (depending on the station), we find that the gravity rates of change lie in the [-3.1, 8.1] nm/s²/yr interval and result from a combination of anthropogenic, climatic, tectonic, and Glacial Isostatic Adjustment (GIA) effects. After correcting for the GIA, the inferred gravity rates and consequently, the vertical land movements, reduce to zero within the uncertainty level at all stations except Julich (due to man-induced subsidence) and Sohier (possibly, an artifact due to the shortness of the time series at that station).

Nature and dating of karstic filling in the Hainaut Province (Belgium). Karstic, geodynamic and paleogeographic implications

The Carboniferous limestones of the Hainaut Province (Belgium) provide evidence of complex karstification. The forms and deposits recently discovered are (1) ghost-rocks with residual weathered and decayed rock, (2) fluviatile endokarsts filled with pebbles rounded by rolling, and (3) ghost-rocks taken up later by underground fluviatile erosion and containing sandy and clayey deposits. We here provide a detailed sedimentological description, a granulometric study, a palynological dating and geochronological dating (K-Ar method on grains of glauconite and ferriferous illite) of the karstic filling. This palaeokarst stratigraphy of the studied area allows us to propose the following evolution: (1) a fluviatile endokarstification in a hummocky relief in late Early Cretaceous and early Late Cretaceous, (2) the formation of ghost-rocks on a flat relief during the Cenomanian-Turonian, (3) fluviatile deposition of sands and clays filling, as a result of tectonic reactivation during the Maastrichtian-Danian. This multidisciplinary study, taking into consideration karstic, hydrodynamic, tectonic and palaeogeographic factors within a well-defined stratigraphic framework may be of use in studying of the palaeokarst successions.

Cover-collapse sinkholes in the “Tournaisis” area, southern Belgium

Abstract In the Tournaisis area (southern Belgium), cover-collapse sinkholes have been reported since the beginning of this century. In this area, the relatively flat lying carboniferous limestone is overlain by an unconformable mesocenozoic cover. Recent research has shown that these sinkholes developed from reactivated paleokarsts. The intensive pumping for domestic and industrial water supply combined with the dewatering due to deep limestone quarries operations resulted in the lowering of groundwater levels. This triggered the reactivation of paleokarstic systems and thus the occurrence of cover-collapse sinkholes. These sudden and unexpected collapses have already had major impacts in the region. In 1977, about thirty sinkholes opened up along the Escaut River downstream from the city of Tournai. As a consequence, the groundwater was polluted by an extensive loss of river water into the Carboniferous Aquifer. More recently, a house and industrial facilities were severely damaged by such cover-collapse sinkholes. In this context, a geoharzard map would be a valuable tool as far as regional development is concerned. This points at the importance to develop methods to locate and characterize hidden paleokarsts. A study site has been chosen in an area where sinkholes have been opening up for about ten years. A geophysical survey of the site which included electrical resistance mapping of the area has shown “weakness” lineaments. These lineaments are parallel to a main fault affecting the region.

Les paléokarsts du Hainaut (Belgique)

ResumeLes calcaires d’âge carbonifere de la province du Hainaut, en Belgique, presque toujours caches par une couverture meso-cenozoique et quaternaire sont intensement karstifies.Endokarsts et cryptokarsts caracterisent une morphologie karstique classique a laquelle s’ajoute une forme bien particuliere d’alteration en masse du calcaire que nous definissons sous le vocable de “fantome de roche”.Ces paleokarsts datent pour la majeure partie du Cretace. Cependant, lorsque le manteau tertiaire et quaternaire recouvre directement le Paleozoique et est plus ou moins permeable, le processus de karstification peut se poursuivre. Suite aux pompages industriels ou domestiques qui rabattent la nappe aquifere, une reactivation du processus karstique entraine la formation de fontis a jour (appeles dans la region, puits naturels).La localisation de ces formes karstiques cachees a une importance economique considerable, tant comme obstacle a l’exploitation et a la qualite de la pierre calcaire qu’a l’implantation des o...

TL stratigraphy of pre-Weichselian loess from NW Europe using feldspar coarse grains

Abstract A new feldspar relative TL dating method is proposed that enables dating of pre-Weichselian loess older than 130 ka, by applying the additive γ dose technique and the Mejdahls exponential extrapolation to alkali feldspar coarse grains. The method is applied to loess deposits from various reference loessic sections of NW Europe. Our relative TL age estimates are consistent with the information provided by the regional stratigraphy; furthermore, they lead to a clear separation of the pre-Weichselian periods of loess deposition in the time period 130 ka–300 ka.

Morphological and speleothemic development in Brujas Cave (Southern Andean Range, Argentine): palaeoenvironmental significance

Abstract Brujas Cave, in the Southern Andean Range, is a well-known endokarstic site in Argentina. However, the origin and evolution of this cave system are poorly known. Based on morphological cave features as well as characteristics of cave deposits, we propose a meteogene drawdown cave genesis, including a change from phreatic to vadose conditions related to the high rate of fluvial downcutting in the area. During the vadose period, various cave-related deposits, including authogenic calcite and gypsum speleothems, allogenic volcanic ash and external tufas were deposited. Gypsum crusts are the oldest cave deposits identified (90.2–64.3 ky BP). Their origin, deduced from isotopic characteristics ( ∂ 34 S=9.6‰), is related to the oxidation of pyrite contained in the Jurassic limestone bedrock as well as the dissolution of overlying Jurassic–Triassic evaporite formations. Gypsum crust deposition is associated with evaporation of water flowing and seeping into the cave during arid environmental conditions. Calcite deposits precipitated from flowing water under equilibrium conditions represent the main speleothem growth period (67.6–34 ky BP in age). Their stable isotope values ( ∂ 13 C=−3‰ to −5‰ and ∂ 18 O=−9‰ to −11‰) may indicate slightly humid and warm conditions related to the regional Minchin lacustrine phase and global oxygen isotope stage 3. Following this stage, a seismic event is evidenced by accumulations of broken stalactites. Seepage calcite speleothems covering cave walls were deposited under disequilibrium conditions by evaporation, probably during Holocene time. Finally, another more recent gypsum deposition period represented by gypsum balls has been differentiated. Micromorphological as well as isotopic ( ∂ 34 S=5.6‰) data indicate that these gypsum forms are related to cyclic processes (solution–deposition) from water seeping into the cave under arid conditions. In addition, intense volcanic activity in the area during Holocene time is deduced from allogenic volcanic ash and lapilli located mainly at the entrance cave. At present, limited hydrological activity is observed in the cave and small tufa deposits at karstic discharge points are evidenced. We conclude that the geomorphological evolution of Brujas Cave was controlled by climatic changes (wet and dry stages) under semiarid environmental conditions in a very active tectonic and volcanic setting during Late Quaternary time.