Brussels Formation

The information on this page is a summary description.
The full formal description is available here: Brussels Formation

Abbreviation Br
Parent unit Zenne Group
Child units Machelen Member, Neerijse Member, Bierbeek Member
Lithological description The Brussels Formation consists of white to pale yellowish, greenish yellow or greyish green sand. In beds that were originally rich in calcium carbonate and that have decalcified, the colour is now
brownish yellow or greenish brown. The grain size varies from fine- to coarse-sized sand. Often, a small fraction of clay and silt is present. When not decalcified, the carbonate is often of clay and silt size. The deposit is often homogenized by bioturbation; in most outcrops, a large-scale internal master bedding slightly dipping to ESE can be recognized. Many outcrops show dm- to several metres thick cross beds. Some beds are faintly wavy laminated to completely structureless. Apart from the latter beds, burrows are common. A base gravel is only found in some parts of the main outcrop and subcrop area, especially in the southeast sectors. It is probably not continuous and may be thicker in erosional base incisions (see below). In the western half and most of the subcropping northern part of the basin, there is generally no base gravel. Quartz grains dominate the deposit. They are mostly well rounded. Locally admixtures of angular grains are present of which it is assumed they are derived from quartzitic Paleozoic subcrops in the eastern part of the Brabant Massif, eroded at the time of the Brussels Sand deposition. The heavy minerals are dominated by ubiquists, mostly tourmaline, then zircon; the parametamorphic group contains typically large pleochroic andalousite grains (Geets et al., 1985). The characteristic composition is largely grain-size determined. The original deposit is calciferous; the carbonate content is strongly reverse-correlated with grain size, i.e. the finer sized sand beds have higher calcium carbonate contents. The carbonate is mostly very fine grained but also mollusc shells or shell fragments can be preserved. In the outcrop area the sand can be locally or completely decalcified. The colour has then turned to brownish yellow or greenish brown. Fine-sized pale green glauconite pellets occur in very small quantities throughout the Formation, whenever the fine sand fraction is present; darker and larger glauconite pellets can occur in the coarser-sized sand beds. Such coarse dark glauconite is lacking in the west and south of the outcrop area, even when coarse sand is found. The presence of coarse glauconite is limited to the east and northeast of the occurrence area and there its content tends to increase towards the east (ann. 1). At the base, coarse dark green glauconite is found over a wider area, but also restricted to the eastern part of the basin. Occurrence of individual slab- or capriciously shaped cemented zones is a very common aspect of this Formation. Silicified concretions are so frequent that their presence is almost diagnostic of the Brussels Formation. They are not conclusively diagnostic knowing that, at rare locations inside the Brussels Formation, no concretions are found at all, while on the other hand, concretions are not unique to the Formation (e.g. silicified concretions occur also in the Mont Panisel Member of the Lower Eocene Hyon Formation, but this Member occurs almost exclusively at locations where the Brussels Formation is lacking). In the Brussels Formation, the siliceous concretions are often very irregularly shaped, like hard cores with short stubs, sometimes with separately cemented burrow cores (“grès fistuleux”); but also many decimetres wide and about one decimetre thick, very hard, slab-shaped concretions are common (“grès lustrés”). Sometimes, the concretions are remarkably spherical (this characterizes facies M, see below). When not decalcified, the finer grained facies of the Brussels Formation often contain carbonate sandstones. They have often been used as construction stone and sometimes have been given specific names (see “Use”). A summary of the Brussels Formation sandstones with a relevant bibliography is given in Dusar et al. (2009). In decalcified areas, the carbonate sandstones may have completely disintegrated while siliceous sandstones may have turned friable or crumbly or have even decayed into white ghost spots in the matrix of loose, yellowish-brown sand. In outcrop areas, secondary limonite cementation occurs locally in subhorizontal beds of a few dm thickness, up to several metres thick massive iron-cemented sandstone, such as at Ottenburg, Chaumont-Gistoux, the upper Woluwe Valley and the Sonian Forest (Stainier, 1924) southeast of Brussels and Braine-l’Alleud.
Age Chronostratigraphy, based in particular on nannoplankton analysis, situates the Brussels Formation at the latest Ypresian and the early Lutetian (NP13, mainly NP14a, NP14b) (Herman et al., 2000); the Brussels sand Formation is thus part of biozones that also contain (part of) the Vlierzele Sand Member of the Gentbrugge Formation and the Aalter Formation. A sequence stratigraphic model relating these lithostratigraphic units is given in Vandenberghe et al. (2004). The exact lateral relationship to (nearly) contemporaneous formations remains to be settled.
Area of occurrence The Brussels Sand Formation crops out in a strip in Central Belgium, between the Zenne and Gete valleys, in Brabant, NE Hainaut and N Namur, where it fills a 40 km wide complex erosional incision with its long axis oriented SSW-NNE (ann. 1). This erosional incised complex is embedded in a package of conformable Eocene strata that slightly dips to N – NNE where it is buried under younger strata; the erosional incision of the Brussels Formation extends over at least 120 km long. It is found in subcrop in the north of Vlaams-Brabant, the province of Antwerpen and the west of Limburg. In the south, the continuous occurrence of the Formation is replaced by some isolated patches, which extend the outcrop area unto Nalinnes south of the Sambre river. Apart from the central, contiguous area, two remarkable outliers are present inside the Cassel hill and the Mont des Récollets in northern France.
Type locality
Alternative names Brussels Sand, Formatie van Brussel, Zand van Brussel, Formation de Bruxelles, Sable de Bruxelles
Authors Houthuys, R., Vandenberghe, N. & Matthijs, J.
Date 12/01/2023
Cite as Houthuys, R., Vandenberghe, N. & Matthijs, J., 2023. The Brussels Formation, 12/01/2023. National Commission for Stratigraphy Belgium.


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