Overview of the geology of Luxembourg
The Grand-Duchy of Luxembourg shows on its small area of 2586 km2 a surprising geological diversity. It is divided into two major natural regions, which are geologically and geomorphologically different, the differences being the result of Late Tertiary differential uplift movements inducing erosional processes that uncovered the major unconformity zone between the Devonian and the Triassic rocks as seen today. The northern region, covering about a third of the surface of Luxembourg, is called ‘Eisleck' and belongs to the Ardennes - Rhenish Shield region, a part of the Hercynian mountain belt that extends through France, Belgium and Germany.
The southern region, where Mesozoic sediments overlie the Palaeozoic basement, is called ‘Gutland' and forms the northeastern tip of the Paris Basin.
Palaeozoic
The Palaeozoic rocks forming the substrate of the ‘Eisleck' originate from neritic, fine- to medium-grained, arenaceous sediments that were deposited in the Devonian epicontinental sea during Pragian and Emsian time and derive from of a northern ‘Old Red' continent. These muddy and sandy sediments, with actual thicknesses of several thousands of metres, were transformed into schists, metasandstones, slates and quartzites, bluish-grey schists being the most common rock type. During the Hercynian orogeny (Early to Mid Carboniferous time), they were affected by a generally NE-SW-trending fold system; the three main structures are, from north to south, the Bastogne anticline, the Wiltz-Eifel syncline and the Givonne anticline. Schistosity is common, associated with second- or third-order folds. Low-grade metamorphic processes are known in the northernmost part.
Mesozoic
At the end of the Palaeozoic, the Hercynides were mostly levelled to a peneplain and the Early Mesozoic depositional history of the area was strongly influenced by the ‘Eifel Depression', a N-S-striking subsidence zone separating the Ardennes from the Rhenish Shield, through which the northern German Sea gradually reached Luxembourg and the ‘Lorraine depression'. The area remained marine again until the Mid Jurassic. The deposits from these periods (Early Triassic to Middle Jurassic) reach up to 1000 metres in thickness.
Simplified geological map of Luxembourg, © SGL 2007
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The Triassic sedimentary series transgressed westwards, with N-S striking shorelines, upon the Ardennian basement and reached the Luxembourgish-Belgian border during the Late Triassic. These deposits are part of the ‘German Triassic'. The Lower Triassic succession is dominated by red sandstones, silts and conglomerates (Buntsandstein Group); this is followed mostly by dolomites and marls during the Mid Triassic (Muschelkalk Group) and by marls and dolomites during the Late Triassic (Keuper Group). In the Mid and Late Triassic, there are also gypsum deposits. In the eastern region (Moselle valley), they are developed in a fine-grained and dolomitic basin facies, while in the west, towards the Belgian border, marginal marine facies become more and more coarse-grained to conglomeratic, dolomite is uncommon and gypsum disappears.
Towards the end of the Triassic, the ‘Eifel Depression' lost its palaeogeographic influence on sedimentation, which in turn was controlled by the Paris Basin during the Jurassic, thus forming the Mesozoic ‘Luxembourg-Trier Gulf'. The shorelines along the Ardennian continent gradually turned into a NE-SW trend. The Lower and Middle Jurassic deposits consist essentially of bluish-grey shales, limestones and limestone-shale alternations (‘Blue Liassic'), but there were two major episodes where the ‘Eifel Depression' regained importance by bringing into the Paris Basin arenaceous sediments from the north, thus forming major lenticular sandstone units like the Luxembourg Sandstone (Hettangian - Sinemurian)
During Toarcian and Aalenian times, oolitic iron ore formed in a deltaic environment. These ironstones, named ‘Minette', are the northernmost part of the Lorraine iron basin. Finally, the youngest marine sediments that are found in Luxembourg are bioclastic and reefal limestones of Bajocian age.
Minette Ironstone of Middle Jurassic Age at an abandoned open-pit mine near Esch/Alzette. Photo: R.Colbach
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From the Late Jurassic onwards, the considered area has been part of a continental environment. Various surficial formations formed during the Tertiary and Quaternary including pisolithic and limonitic iron ores, pedogenetic quartzites, loams, loess, and fluvial and colluvial deposits, generally less than 10 metres thick.
The reactivation of Hercynian structures has influenced the Mesozoic deposits both during and after their sedimentation. Structurally, the region is characterized by mainly NE-SW-striking undulations with large radii of curvature, associated with a general, less than 10°, dip towards the southwest. Horsts and grabens also trending NE-SW are commonly associated with the undulations.
During the Pliocene, a general uplift of the entire Luxembourg region occurred, but this was more pronounced in the north than in the south. As a result, the Mesozoic cover was completely eroded in the north and the region was separated into the two geomorphological units by a marked flexure. The ‘Eisleck' in the north is characterized by wide, open plateaux with an average elevation of 450 metres, transected by deep, narrow, winding valleys. The ‘Gutland' shows a typical cuesta landscape due to the alternation of soft, erosion-prone units (marls) and harder, erosion-resistant units (sandstones, limestones, dolomites and iron ore horizons).
Regarding mineral resources, only a few quarries extracting different kinds of building materials are in production today, but iron ore extraction was an industry of major importance in the past. Other formerly used resources include lead, copper and antimony ores, slate in the ‘Eisleck' and gypsum in the ‘Gutland'. Bituminous Toarcian shales occur in the southwest, but their exploitation has hitherto been judged unprofitable.
Permeable sandstone, dolomite and limestone units, separated by impervious marls, form a series of stacked aquifers in the Mesozoic, by far the most important being the Luxembourg Sandstone, the source of some 60% of Luxembourg's drinking water, mostly taken from springs. On the other hand, the ‘Eisleck' region is poor in groundwater resources.
Literature for further reading:
Lucius, M. 1948: Erläuterungen zur Géologischen Karte Luxemburgs - Das Gutland. Publ.Serv.Géol.Lux. , Vol. V, 1-408 (30 fig., 10 tab.), Luxembourg.
Lucius, M. 1950: Erläuterungen zur Geologischen Karte Luxemburgs - Das Oesling. Publ.Serv.Géol.Lux. Vol. VI, 1-176 (32 fig. 2.tab.), Luxembourg.
Maquil, R. 1994: Paysages géologiques, histoire géologique de notre pays et des régions limitrophes in "Un Pays et ses Paysages". Nos Cahiers 1.94, 133-166, 6 fig. 1 tab. 1994. , Luxembourg.
Muller, A. 1980: Luxembourg in Géologie des Pays européens : France, Belgique, Luxembourg. 577-594, Paris.
Waterlot, G., Beugnies, A., Bintz, J., Hary, A. & Muller, A. 1973: Ardenne - Luxembourg. Guides géologiques régionaux, 135-205, Paris.
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© R.Maquil & R.Colbach, Service géologique, 2009