Overview of the geology of the Luxembourg Sandstone(s)

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)Overview of the geology of the LuxembourgSandstone(s)Robert COLBACHService géologique du Luxembourg43, bd G.-D. Charloe, Charloe, Charloe, Charloe, L-1331 Luxembourgrobert.colbach@pch.etat.luIn the northeastern tip of the Paris Basin ("Luxembourg-TrierGulf") the Lower Liassic, otherwiseformed by blueish grey limestone-shale alternations("Blue Liassic") also called "Lorrainefacies", shows lenticular insertions of arenaceoussediments referred to as the LuxembourgSandstone(s) Formation. This unit includes, fromSE to NW, the local denominations of Heange,Ernzen, Metzert, Virton, Orval or Florenvillesandstones (Fig. 1).PalaeogeographyThe lower mesozoic history of the considered areawas strongly influenced by the "Eifel Depression",a N-S striking subsidence zone separating theArdennes from the Rhenish Shield through whichthe northern german sea progressively reachedthe "Lorraine depression". On its western border,N-S oriented triassic shorelines gradually turnedto NE-SW directions as the liassic sedimentaryseries advanced upon the Ardennian continent.Sands and silts were carried southwards throughthe Eifel Depression into this flat marine area. Thismarine channel was limited to the south by theSierck Sill, the western tip of the hercynian heightsof the Hunsrück, thus limiting the depositionalarea of the sandstone to the south (Fig. 1).Sources: Berners 1983, 1985; Guérin-Franiae et al.1991.Fig. 1: Extension of the Luxembourg Sandstone(s) Formation.Ferrantia • 44 / 2005155

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)PetrographyDue to a bimodal distribution of the calciumcarbonate content, the Luxembourg Sandstoneappears in the outcrop as an alternation ofyellowish ochre, poorly cemented sandstones (10-20 % carbonate) and grey to whitish, cement-richsandy limestones (30-60 % carbonate) (Fig. 2; 3).But extreme terms with less than 1 or up to 90 % ofcarbonate also occur.Under the cover of younger sediments and inpermanently water saturated zones, where it isprotected from a deep weathering, the sandstoneappears in a blueish grey, due to the presence ofpyrite.Fig. 3: Well cemented sandstone, optical microscopeimage (natural light).Fig. 2: Outcrop showing an alternation of carbonaterich, whitish, sandy limestone and carbonate poor, yellowishsandstone, Luxembourg.The changing carbonate content has a primarysynsedimentary, bioclastic origin, but was influ-enced by secondary diagenetic redistributionprocesses. The proportion of high-carbonate layersrises from the east towards the west.Grain sizes are dominated by well-sorted, fineto medium sands; in the east, proximal faciesshow mostly fine and very fine sands, associatedto medium sands, whereas western, distal faciesshow more very fine sands and silts.Very well rounded quartzitic pebbles, intraforma-tional sandstone pebbles, fossil debris-rich sandylimestones ("lumachelle") and thin recurrences oflimestone-shale alternations also occur.Sources: Berners 1983, 1985; Guérin-Franiae et al.1991; Bock 1989.Fig. 4: Structure of the sandwaves in the Luxembourg Sandstone(s) (Bock & Muller 1989, modified). A. Lorrainefacies. B. Initial Sandwave subfacies. C. Sandbar subfacies. D. Sandbank subfacies.156 Ferrantia • 44 / 2005

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)Table 1: Sedimentological subdivisions of the Luxembourg Sandstone(s) sandwaves.Evolution stageIndividualization phaseAccumulation phaseStabilization phaseSubfacies- inter sandwave subfacies: bioturbated silts and siltysandstones, bioclastic sandy limestones ("sheet sands")- initial sandwave subfacies: fine to medium grainedsandstones with rare pebbles and low-angle cross bedding- sandbar subfacies: fine to medium grained sandstoneswith large-scale cross bedding ("sandwaves") and internalerosional surfaces- sandbank subfacies: wave and storm-influenced, bioclastrich, fine to medium sandstones with frequent erosionchannels and hardgroundsSedimentologyThe sedimentary analysis shows that the LuxembourgSandstone formed on a flat, shallow marineshelf as offshore sandwaves. These subtidal, shoreparalleldeposits are organized into 10 to 20 metersthick sequences of different subfacies, related todifferent evolution stages (Tab. 1; Fig. 4).Sources: Berners 1983, 1985; Guérin-Franiae et al.1991; Bock & Muller 1989.StratigraphyAmmonite content proved the LuxembourgSandstone formation to be diachronic: the firstsand deposits appear during the lower Heangianin central Luxembourg. The depositional areaextended towards southeastern and westernLuxembourg during the middle and upperHeangian and reached its westernmost extentonly during the Sinemurian (Fig. 5).The lower boundary of the sandstone consistsin a gradual "sanding-up" of the Lorraine facies,whereas its upper limit is oen sharp and markedby a hardground, equivalent to the Heangian-Sinemurian contact in the southeast, but geingprogressively younger towards the west. Theduration of the sand sedimentation events isevaluated to 9 Ma.The outcrops near Heange-Grande (Moselle, F)were proposed as a stratotype for the Heangianstage in 1864 by Renevier.Sources: Guérin-Franiae et al. 1991; Bock 1989.Fig. 5: Stratigraphic position of the Luxembourg Sandstone(s) Formation (Bock 1989, modified).Ferrantia • 44 / 2005157

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)Structural settingFormation thicknesses reach 100 meters in centralLuxembourg and stay more or less constant in theSW-NE direction, whereas they are subjected tochanges in the NW-SE direction, thus suggestingthat hercynian tectonic features must have beenactive already during sedimentation (Berners1985).Structurally, the Mesozoic sediments of Luxembourgare characterized by mainly SW-NE strikinghorsts and grabens, associated to large undula-tions like the Weilerbach syncline. Associated toa general SW dip towards the centre of the basin,they explain the outcropping of the Luxem-bourg Sandstone over a wide area. This seing seing seing seing isthought to be essentially due to the reactivation ofhercynian structures.Its slight dip (

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)Calcium carbonate redistribution by ground-water is also a key process in the shaping of thesandstone cliffs: holes are created by dissolution ofthe carbonate cement, which in turn can reprecip-itate on the cliff's walls, giving it a smooth aspectand creating characteristic alveoli (Fig. 7).Sources: Struffert 1994; Gronemeier 1976; vonHoyer 1971.Natural resourcesThe Luxembourg Sandstone Formation providesdifferent building materials (Wies & Wertz 2004):in Luxembourg alone, over 150 old quarries areknown and 11 extraction sites are still active today.Besides sands and different granulates, especiallythe whitish carbonate-rich sandy limestone hasbeen appreciated as building stone and freestonefor several centuries beyond the countries borders.Its main characteristics are: density 2.06-2.24, waterabsorption 3-7 mass %, compressive strength 44-57 Mpa, elastic modulus 6500 Mpa.BibliographyFig. 8: Wide open fracture with stalactites; roadcutnear Altwies.Berners H.-P. 1983. - A lower liassic offshore barenvironment. Contribution to the sedimen-tology of the Luxemburg Sandstone. Annalesde la Société Géologique de Belgique 106: 87-102.Berners H.-P. 1985. - Der Einfluss der SierckerSchwelle auf die Faziesverteilungen känozoischer Sedimente im NE des Parisermeso-Beckens: ein Sedimentationsmodell zumLuxemburger Sandstein (Lias), spezielleAspekte zur strukturellen Änderung derBeckenkonfiguration und zum mlichen Potential, Dissertation RWTH Aachen,naturräu-320 p.,100 fig., 24 tab.Bock H. 1989. - Ein Modell zur Beckenausdehnungund Fazieszonierung am Westrand der EifelerNord-Süd-Zone während der Trias und zurTransgresson des Unteren Lias am nensüdrand , Dissertation RWTH Aachen, 417Ardenp.,114 fig., 48 tab.Bock H. & Muller A. 1989. - Environnementssédimentaires et écologiques dans le Quart NEdu Bassin parisien au Trias terminal et au Liasinférieur, Ai Ai Ai Ai 3° Simposio di Ecologia e Paleon-tologia delle Communita Bentoniche: 157-178,8 fig., 2 tab.Gronemeier K.-U. 1976. - Das Grundwasser imLuxemburger Sandstein: Geologie, haushalt und Umweltbelastung am Beispiel Wasser-vonFerrantia • 44 / 2005159

R. Colbach Overview of the geology of the Luxembourg Sandstone(s)drei Grosstestflächen, Doktorat der Naturwissenscha,Johannes Gutenberg-UniversitätMainz, 195 p.Guérin-Franiae S., Armand H. & Muller A. 1991.- La formation des Grès du Luxembourg, auLias inférieur: reconstitution dynamique dupaléoenvironnement. Bulletin de la SociétéGéologique de France 162 [4]: 763-773.Owenier F. 2001. - Beiträge zur Charakterisierungdes südöstlichen Sedimentationsareals -Gandertal - der Sandsteine von Luxemburg(Unterer Lias) im Rahmen des Sedimentationsmodellsvon BERNERS (1985) unter Berücksichtigungder Erkenntnisse beim Ausbau derAutobahn Luxemburg-Saarland, DissertationRWTH Aachen, 136 p. , non publié.Struffert F.-J. 1994. - Hydrogeologische Detailuntersuchungenim Rahmen der «Solution derechange» (Ersatzlösung) in Luxemburg : eineSynthese aus Methoden der Hydraulik, Isotopenhydrologieund Hydrochemie. Publicationsdu Service Géologique du Luxembourg,Volume 28, 194 p.von Hoyer M. 1971. - Hydrogeologische undhydrochemische Untersuchungen im LuxemburgerSandstein, Publications du ServiceGéologique du Luxembourg, Volume 21, 61 p.Wies P. & Wertz P. 2004. - Les carrières de Grèsd’Ernzen (Larochee). Guide d’excursionGeologica Belgica du 30.09.2004. 13 p., nonpublié.Résumé de la présentationAperçu de la géologie des Grès de LuxembourgMots-clés: Lias; Grès de Luxembourg; Dépression Eifélienne; ridins sableuxDans le Nord-Est du Bassin de Paris, des sédimentsarénacés s'intercalent dans l'alternance de marnes etcalcaires liasiques du « faciès lorrain ». Connus, suivantles régions, sous les noms de Grès de Florenville, d'Orval,de Virton, de Metzert ou de Heange, cee formation« des Grès de Luxembourg » se répartit sur 4 pays: de larégion de Bitburg (D) dans l'Est par la Gaume (B) jusqu'àla région de Sedan-Charleville (F) à l'Ouest et jusqu'àThionville (F) au Sud.Ce grand corps sableux a été déposé durant l'Heangienet le Sinémurien sur une plate-forme marine peu profondesous forme d'un ensemble de barres et de ridins sableuxparallèles à la côte du continent hercynien (ardennais)par des courants marins en provenance du chenal de laDépression Eifélienne. En allant du SE vers le NW l'âgede cee formation varie de l'Heangien inférieur auSinémurien. Son épaisseur aeint 100 mètres au centredu Luxembourg.Partant d'une distribution bimodale du ciment carbonatéd'origine bioclastique, deux types principaux de rochespeuvent être distingués : du grès jaune ocre avec moinsde 20 % de CaCO 3, alternant avec du calcaire gréseuxblanchâtre présentant 50 à 80 % de CaCO 3. La granulométrieest dominée par des sables fins à très fins, maisdes sables moyens et des silts existent également dansles faciès proximaux, respectivement distaux. Des galetsde quartz très bien roulés, des grès lumachelliques etdes récurrences de l'alternance marno-calcaire existentégalement.Du point de vue structural, le Mésozoïque luxembourgeoisse caractérise par un système en horsts etgrabens, associés à des plis à grand rayon de courbure,d'orientations SW-NE. Ensemble avec le pendagegénéral des couches vers le centre du bassin, ces structuresexpliquent la présence de la cuesta du Grès deLuxembourg sur une grande région géographique. Deszones de fractures, principalement de direction NE etNW, affectent la formation et favorisent localement uneérosion plus importante.Grâce à sa perméabilité double de pores et de fractures,le Grès de Luxembourg est le plus important aquifère dupays, fournissant 2 /3 de l'eau potable. La dissolution et lareprécipitation du ciment calcitique par les eaux souterrainessont les processus-clé de la création d'épaissescouches de produits d'altération et du façonnage desfalaises de grès et sont, ensemble avec sa dispositionstructurale, à l'origine de paysages caractéristiques.160 Ferrantia • 44 / 2005