Machelen Member

The information on this page is a summary description.
The full formal description is available here: Machelen Member

Parent unit Brussels Formation
Child units
Lithological description Very fine to fine sand. The central grain size value (mode or median) is finer than 150-175 μm. The Member contains a subdominant admixture (much less than 2%) of very fine, light green glauconite pellets. In its original appearance (i.e., not decalcified), the sand is white to pale yellow, the carbonate content is higher than 15% and consists of fine mud. Carbonate content may locally reach high values, even exceeding 50%. The sand is fully or partially bioturbated and may contain very fine to cm-scale marly mud laminae. Macrofossils may be present. In flush wells, this facies is easily recognized by the white colour of the flushing fluid. In outcrop, it is easily recognized by the fact that on touching the fingers are stained white.In this Member, subhorizontal, ca. 1 to 2 dm thick stone beds, cemented by carbonate and by some opal (Fobe, 1986), are common. The stone beds have a high total carbonate content, even up to 80%, that occurs as cement, as marly laminations, as fossils and as rounded bioclast grains that are mixed with the quartz grains (Dusar et al., 2009, p. 242 & 330). When they occur, often several such subhorizontal sandstone beds are found over the vertical profile, separated by 0.5 to 1 m of uncemented sand. These carbonate sandstones have in the past been used as building stones (ann. 2). At the historic type locality, Brussels and its immediate surroundings, this unit is predominantly found near the top of the Formation (Dumont, 1839; Rutot & Van den Broeck, 1883). In the area NE of Brussels, the sandstone is called “Diegem sandstone”, in the area between Tienen and Jodoigne, the “Gobertange sandstone” is found, while in the wide area around Braine-l’Alleud and Nivelles, a similar stone called “grès calcareux du Bruxellien” (the name of “central Brabant sandstone” is proposed here) occurs. The Diegem sandstone mostly represents stone from the Brussels Formation but also partially from the Lede Formation sandstone, where it is found on top of the Brussels Formation. The stone layers in both formations have a similar aspect but Lede stones have less well sorted quartz grains, may contain 1-2 mm quartz grains and have more varied fossil remains. The Gobertange sandstone has in the 19th and first half of the 20th century been quarried and exported as building stone on an almost industrial scale (Tordoir, 2000). This stone is characterized by very thin white marl laminae perforated by burrows, filled with glauconite-rich medium to coarse sand. South and southeast of Brussels, in the wide area around Braine-l’Alleud and Nivelles, and locally around Grez-Doiceau (ann. 2), carbonate sandstones are found, which have an aspect very similar to the Diegem sandstone though in this area some layers attain a thickness of over 3 dm and the proportion of medium sand may be higher. Local variants may be found, e.g. finely laminated calcareous sandstone at Rood Klooster/Rouge Cloître (Oudergem/Auderghem). Capriciously shaped, hard, often small (at most 1 dm) siliceous concretions (“grès fistuleux” and “pierres de grottes”) are frequent in this facies; in fact, their presence is a diagnostic for separating it from surrounding, similar strata such as the Lede Formation. In calcareous facies, the siliceous concretions may contain carbonate elements. The Member is often decalcified. Decalcification caused a change in colour: it is yellowish or greenish brown due to the fact that non soluble grains such as fine-grained glauconite are concentrated and grains are limonite coated. Decalcification is found only in the outcrop area and there tends to occur primarily near the flanks of the valleys (geomorphology) and near the surface (vadose zone). Decalcification fronts are generally very sharp and may form capricious surfaces. Decalcification is caused by dissolution of carbonates in permeating water. The process is recent (related to the present landscape evolution) and is still ongoing.As it is decalcified, the facies no longer contains carbonate cemented sandstones and the chalcedony cemented concretions have turned brittle. In outcrop, the occurrence of capricious brittle siliceous sandstones or ghosts provides a good diagnostic. An important consequence of this superficial weathering is that the sand becomes undercompacted and loses nearly all compressional strength. The alteration depth is in the metric scale but may exceed 10 m and its distribution is erratic. Recording the alteration is of great importance for geotechnical applications.
Area of occurrence
Type locality Problematic, not permanently exposed. Building sites near Diegem and in Braine-l’Alleud often show the calcareous facies of this Member. The Waversesteenweg entrance to the RBINS museum at Brussels (Lambert 72 X 150546, Y 169493, outcrop from about 61 to 70 m TAW) also showed this facies, but the exposure is now walled up. It would make a meaningful stratotype. An alternative could be the Hussompont sandpit west of Jodoigne (X 183440, Y 158190, outcrop from about 87.5 to 104 m TAW), but at present, access to the site is difficult to obtain. The abandoned sand extraction pit at Waterloo in the Soignes Forest, situated between the R0 and the N5b (Chaussée de Tervuren), X 152955, Y 155810, shows the decalcified facies of this Member but the outcrop would need to be refreshed.
Alternative names
Authors Houthuys, R., Vandenberghe, N. & Matthijs, J.
Date 12/01/2023
Cite as Houthuys, R., Vandenberghe, N. & Matthijs, J., 2023. The Machelen Member, 12/01/2023. National Commission for Stratigraphy Belgium.


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