Geologica Belgica

Analysis of the groundwater resource decline in an intramountain aquifer system in Central Iran

Fri, 04 May 2012 00:00:00 +0200

The Shahrekord aquifer is located in an intramountain basin in Central Iran (90 km SW of Isfahan) and is the main resource of irrigation water for the intensively developed agriculture in the Shahrekord Plain. Early exploitation of the aquifer started back around 1950 but has intensified severely during the last decades. Irrigation water is provided by three means: spring water is tapped, water is pumped from around 650 wells and in historic times more than 100 karizes (or ghanats, deep underground channels that drain the water table and are accessed by shafts) were constructed and provide an additional source of water. However, groundwater levels have declined severely during the last decade, and although systematic piezometric monitoring already started in 1984, it stayed unclear whether the declining trend is related to increased water demand and exploitation or is due to climatic reasons, as around 2000 a severe drought lasted for three years. In this paper, exploitation and precipitation data are combined with the measured piezometric levels to analyse their relationship and help to understand the observed trend in declining groundwater storage. This aquifer is an example of a system that can easily deliver large amounts of groundwater because of a high transmissivity and considerable thickness, but has, for climatic reasons, a limited recharge. This imbalance makes the present level of exploitation unsustainable.

Redéfinition des unités structurales du front varisque utilisées dans le cadre de la nouvelle Carte géologique de Wallonie (Belgique).

Fri, 04 May 2012 00:00:00 +0200

Les nouvelles cartes géologiques de Wallonie permettent d’obtenir une vision plus globale de l’organisation des terrains paléozoïques de la zone frontale varisque dans le Sud de la Belgique. Cette bande de terrain d’orientation WSW-ENE, passant au sud d’un axe Charleroi-Namur a été reconnue à l’origine grâce aux travaux miniers, mais a été considérablement documentée par des sondages profonds et des profils sismiques. Un schéma structural cohérent du front varisque est proposé où les unités structurales suivantes sont distinguées, du nord au sud : le Massif du Brabant et la base de sa couverture non affectés par l’orogenèse varisque, le Parautochtone brabançon, les Ecailles et les massifs renversés de Haine-Sambre-Meuse (EMR-HSM) et l’Allochtone ardennais. Les terrains dévoniens non déformés durant l’épisode varisque constituent la limite nord du front varisque. Le Parautochtone brabançon comprend des terrains dévono-carbonifères plissés et/ou faillés. Les EMR-HSM sont formés par la superposition de plusieurs entités structurales (e.a. écaille de La Tombe) supportées par diverses failles de chevauchement (e.a. faille de Masse). L’Allochtone ardennais, déplacé durant l’orogenèse varisque, repose sur le Parautochtone brabançon et les EMR-HSM par l’intermédiaire d’une zone failleuse qui est définie localement sous le nom de Faille du Midi.

Redefinition of the structural units of the Variscan Front used in the context of the new geological map of Wallonia (Belgium). The new geological map of Wallonia permits to obtain a more global view of the tectonic organization of the Palaeozoic terrains around the Variscan Front in southern Belgium. This zone oriented WSW-ENE and passing south of an axis Charleroi-Namur, was firstly highlighted by the coal mining works, but has been significantly documented notably through deep boreholes and seismic profiles. A coherent structural sketch of the Variscan Front is proposed for its entire longitudinal course, where the following structural units are distinguished, from north to south: the Brabant Massif and the base of the cover unaffected by the Variscan orogeny, the Brabant Parautochthon, the Haine-Sambre-Meuse Overturned Thrust sheets (HSM-OTS) and the Ardenne Allochthon. Undeformed terrains during Variscan event constitute the northern border of the Variscan Front. The Brabant Parautochthon includes Devono-Carboniferous terrains, which are folded and faulted. The HSM-OTS are formed by the superposition of several sheets (e.g. La Tombe sheet), which are supported by various thrust faults (e.g. Masse fault). The Ardenne Allochthon, which has been displaced during the Variscan orogeny, rests on the Brabant Parautochthon and the HSM-OTS through a faulted zone, which is locally known as the Midi Fault.

U/Th dating of the Annevoie-Rouillon travertines

Tue, 13 Mar 2012 00:00:00 +0100

The carbonated travertine’s accumulations near sources or in rivers constitute very good environmental indicators (Ford & Pedley, 1996; Guendon J.-L. et al., 1997; Guendon J.-L. et al., 2003; Ollivier, 2006 ; Roiron, 1997). They can record climate variations, fluctuations of the basis level, epirogenic movement and eustatic variations. Among climate substitution variables, let’s quote the pollen collections, the plants debris and the geochemistry. Moreover, the travertines can be dated by the uranium series disequilibrium method, or radiocarbon. In Belgium, there are some sites of travertines deposits: Annevoie-Rouillon, Casteau, Chaumont-Gistoux, Hoyoux’s valley, Orval, Treignes, Vierset-Barse (Geurts, 1976). The travertines of Annevoie-Rouillon are famous because they have been the subjects of litho-stratigraphic, palynological and geochemical studies (Paepe, 1965; Bastin et al., 1968; Geurts, 1976; Janssen & Swennen, 1997). These studies have been made in the stratigraphic series of the behind-dam in the carbonated rudites and arenites carbonates and clays. A new examination of the deposit has permitted to discover the witnesses of the dam basis which present more compact facies with plant debris. Those facies are able to be dated by the uranium series disequilibrium method. The sampling has been made in two types of facies: the travertine itself and speleothems in little cavities in the travertines. The first have not permitted to have reliable ages because of impurities in the calcite. The second have been dated from the beginning of the Holocene. Those datings confirm the Holocene ages of the stratigraphic series studied by Bastin and Geurts. Moreover, those ages imply that the Meuse River occupied an altitudinal level equal or lower than its present level just after the last glaciation. Future studies would consist in drillings under the level of the river to look for the basis of the travertine edifice.

Holocene, Meuse valley, environment indicator, speleothem

Les sédiments du Dévonien de Saïda (Algérie nord occidentale): Nature et genèse

Mon, 12 Mar 2012 00:00:00 +0100

L'analyse détaillée des faciès et séquences sédimentaires de la coupe de référence de Boukourdène a permis de lier ces derniers aux conditions paléoenvironnementales régnant au sein d’un tronçon de la future chaîne hercynienne au cours du Dévonien. Les différents faciès recensés peuvent être classés en trois groupes:

- les faciès liés à des écoulements turbiditiques (chenaux et levées);

- les faciès liés à une dynamique de coulées de débris (lobes peu développés);

- les faciès de remaniement ou de courant de fond.

Les systèmes chenaux-levées forment un ensemble globalement rétrogradant. L’influence de la subsidence est étayée par de la fracturation synsédimentaire d’orientation N110°E, en régime distensif. Entre ces systèmes de chenaux-levées, se mettent en place des glissements en masse modérés initiés sur un talus continental. Les apports sont liés à une dynamique gravitaire unidirectionnelle d’orientation moyenne N130-155°E, suggérant que ces sédiments siclastiques sont issus de roches de socle. Ils proviennent de la destruction d’une chaîne de type « cordillère » faisant partie du socle panafricain de l’Atlas saharien.

A detailed analysis of sedimentary facies and sequences of the Boukourdène reference section allows linking these to the prevailing palaeoenvironmental conditions within a segment of the Hercynian belt during the Devonian. Main facies may be classified into three groups:

- turbidite facies (channels and levees);

- debris flows facies (poorly developed lobes);

- bottom current facies.

The channel-levee system forms a globally retrograding prism. The influence of subsidence is suggested by N110°E extensional synsedimentary faults. Between the channel-levee units, mass flows initiated on a continental slope are observed. Sediment accumulation resulted from a N130-155°E unidirectional supply, derived from the reworking of clastic rocks. This suggests some contemporaneous erosion of a cordillera which was part of the Panafrican basement of the Saharian Atlas.

Quantitative analysis of Elasmobranch assemblages from two successive Ypresian (early Eocene) facies at Marke, western Belgium

Mon, 12 Mar 2012 00:00:00 +0100

Temporal patterns in biodiversity are affected largely by changes in environmental conditions. Sea level fluctuations rank amongst the major factors that affect marine biodiversity or community structure on a local or regional scale, as confirmed by numerous case studies relating lithology with fossil assemblages in order to reconstruct palaeoenvironmental conditions. However up to now, few studies quantified selachian and batoid faunas (Elasmobranchii) in such a context. In the present study, we compare elasmobranch teeth from two successive facies of Ypresian (Early Eocene) age, as exposed at the Marke clay pit in the southern part of the Belgian Basin. We present a significant link between the difference in lithology of these levels and elasmobranch community structure. In general, selachians are notably more common in clayey levels, while batoids predominate in sandy levels. Following the principle of uniformitarianism, such a link indicates that the recognised patterns in elasmobranch diversity depend mainly on the preferred sea level and/or habitat requirements by a species or a species group, in analogy to what is seen in modern communities. Additional notes on the palaeoenvironment are presented, as well as a list of 36 elasmobranch taxa from Marke, including a number of new recorded taxa for the Ypresian of Belgium.

Relationship between sedimentary features and permeability at different scales in the Brussels Sands

Mon, 12 Mar 2012 00:00:00 +0100

The Brussels Sands display a complex three-dimensional subsurface architecture. This sedimentological heterogeneity induces a highly heterogeneous spatial distribution of hydrogeological parameters at different scales and may consequently influence subsurface fluid flow and solute migration. This study aims at characterizing spatial variability of permeability at different scales in the Brussels Sands. Firstly, a literature review on the permeability distribution of the Brussels Sands was performed. Secondly, a field campaign was carried out consisting of field observations of the small-scale sedimentary structures and in situ measurements of air permeability. A total of 6550 cm-scale air permeability measurements were carried out in situ in three Brussels Sands quarries in the central part of Belgium: Bierbeek, Mont‑Saint‑Guibert and Chaumont‑Gistoux. On the large basin scale, substantial differences in permeability are observed. A literature data analysis shows that there is no clear correlation between hydraulic conductivity and sedimentary facies. At the small scale, results show that permeability heterogeneity and anisotropy are strongly influenced by sedimentary heterogeneity in all three quarries. Clay-rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the cross-bedded lithofacies, where the permeability anisotropy is dominated by the foreset lamination orientation.

Geometry and kinematics of the low-grade metamorphic ‘Herbeumont shear zone’ in the High-Ardenne slate belt (Belgium)

Wed, 15 Feb 2012 00:00:00 +0100

On the geological maps of the High-Ardenne slate belt (Belgium) an EW-trending, weakly S-dipping thrust fault is classically drawn, bordering the Eifel depression to the south. The ‘type locality’ of this fault lies in a – now completely overgrown – railway section at Herbeumont. This fault is indeed known as the ‘Herbeumont fault’. A detailed structural and microstructural analysis of an outcrop area on the east bank of the Semois river, south of the railway section, allowed us to complement the geometrical and kinematic context of the classical ‘Herbeumont fault’. The rocks exposed are predominantly fine-grained siliciclastic rocks belonging to the Lower Devonian.

Besides the bedding (S₀), which is often very difficult to discern, three types of tectonic foliation can be distinguished: a primary cleavage (S₁) at small angle to the bedding, a shear band foliation (SBF = S₂) and an axial-planar, crenulation cleavage (S₃) of late, shear-related folds affecting all pre-existing structural features. The pervasiveness – or even the mere presence – of these foliations is highly dependent on lithology, layer thickness and competence contrast within the multilayer sequence, even to the degree that a strongly developed shear band foliation can easily be mistaken as bedding. Other typical structural features include folded, boudinaged layers and folded, foliation boudins, as well as chevron folds.

All these structural features can be fitted in a kinematic model of a single, polyphase progressive top-to-the-north shearing event, occurring in the late stages of the Variscan orogeny. The outcrop area studied thus offers an excellent look into a low-grade metamorphic shear zone – the ‘Herbeumont shear zone’ – that developed in mid-crustal, brittle-plastic deformation conditions in a slate belt. Evidence of purely brittle thrust faulting – the ‘Herbeumont fault’ – could, however, not be identified in the outcrop area studied.

Surface and ground water interaction processes inside the Eau Blanche river basin (Dinant synclinorium – Belgium)

Sun, 12 Feb 2012 00:00:00 +0100

Gauging river flows, measurement of in-situ parameters (such as electrical conductivity, pH and temperature) and major ions analysis were used to examine, at 8 selected sections, the groundwater inflow to the Eau Blanche River during a base flow recession period. Comparison of specific inflows between each section has permitted to identify positive and negative deviations, which were used to identify underground water inflows or outflows. Major ion analyses showed a clear evolution of river water chemical composition that could be related to the increasing amount of limestone aquifers composing the drainage area of the river. A simple two component mixing model confirmed the major influence of the limestone aquifer on the river chemistry; additional (probably anthropogenic) inputs of nitrates and chlorides were located.

A new representative of the lichid genus Ohleum (Trilobita) from the Eifelian (Middle Devonian) of southern Belgium

Sun, 12 Feb 2012 00:00:00 +0100

Un nouveau représentant du genre lichidé Ohleum (Trilobita) de l’Eifelien (Dévonien moyen) du Sud de la Belgique. Les trilobites de la famille des Lichidae sont relativement peu diversifiés au sein de la succession eifelienne du bord sud du Synclinorium de Dinant (Belgique) qui est caractérisée par une sédimentation mixte silicoclastique à carbonatée. Jusqu’à présent, ils étaient seulement représentés par des espèces appartenant aux genres Ceratarges et Eifliarges. La découverte récente d’un spécimen bien conservé au sein de la Formation de Jemelle d’âge eifelien dans la région couvinoise nous amène à proposer la première description détaillée d’un représentant ardennais du genre Ohleum (O. magreani sp. nov.).

Trilobites of the family Lichidae are relatively poorly diversified within the Eifelian mixed siliciclastic-carbonate succession of the southern margin of the Dinant Synclinorium (Belgium). Until now, they were only represented by species belonging to the genera Ceratarges and Eifliarges. The recent discovery of a well-preserved specimen within the Eifelian-aged Jemelle Formation in the Couvin area led us to propose the first detailed description of a representative of the genus Ohleum (O. magreani sp. nov.) in the Ardennes.

André Dumont medallist lecture 2011: Extension in a convergent tectonic setting: a lithospheric view on the Alboran system of SW Europe

Thu, 27 Oct 2011 00:00:00 +0200

The Betic Cordillera of southern Spain forms a clear example of a collisional orogen that has undergone large-scale late-orogenic extension while convergent motion of the bounding plates continued. The orogen provides a unique opportunity to study the tectonics of the system at different lithospheric levels. At shallow levels in the crust, fault-bounded intramontane basins, formed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal faulting, followed by a less deformed late Miocene marine succession. Extension was accommodated by coeval shortening in thin-skinned fold-and-thrust belts in the periphery of the system, and much of the region has now subsided to form a large marine basin, the Alboran basin. The thermal and deformational record of these processes is preserved in rocks from deeper crustal levels in the internal zone of the Betic Cordillera. These rocks were metamorphosed down to 50 km depth and are now exposed beneath major low-angle detachment zones that separate them from heavily faulted low-grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Peridotite massifs in the western Betics emplaced in the early Miocene provide coherent outcrops of subcontinental upper mantle that allow insight in coeval processes in the mantle lithosphere. The peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at mid-lithospheric depths possibly reflects early crustal thickening, followed by extension and subsequent heating. A sudden rise of ambient temperatures in the mantle rocks by about 400 °C suggests loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. All of these phenomena can be explained by some form of removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large-scale extension followed by thermal subsidence of the center of the system. The processes inferred here for the Alboran region are in all likelihood not unique, as many similarities can be identified with the geology of the Tibetan Plateau, but also with domains in the Variscan and the Pan-African orogenic belts where extensional processes and associated LP/HT metamorphism and magmatism can be shown to equally have occurred in a convergent tectonic setting.