2.8.2. Brussel Formation - Br - replaced on 27/10/2017
2.8.2. Brussel Formation - Br
I suggest to exceptionally allow the equivalent names Brussel – Bruxelles – Brussels Formation. If a choice is needed, I suggest to use the English name.
Authors: Dumont (1839), Gulinck & Haquaert (1954), Houthuys (2011). Note: more references are needed, especially on biostratigraphy
Description: fine to medium, locally coarse sands characterized by the presence of silicified, often capriciously shaped, very hard sandstones. The formation is calcareous; the carbonate content is strongly correlated with grain size, with the fine sands containing the highest percentages, sometimes over 50%. In many outcrop locations, the formation is partially or completely decalcified. In these locations, the siliceous sandstones are often fragile. In the strongly calcareous, fine to very fine grained facies, often horizontal layers of calcareous sandstone occur; they are locally known as the Diegem and Gobertange stone. The finer grained facies are stronger bioturbated. Some medium-to-coarse grained facies lack any trace fossils and are proof of fast sedimentation stages. The formation contains reworked glauconite. The percentage of coarse, dark green reworked glauconite pellets increases from west to east.
The formation shows an internal architecture of low-angle master beds that dip to ESE. These bedding surfaces define clinoforms, that have in general finer-grained toes and coarsening-upwards slope deposits, that are often bioturbated, thin cross-beds. Thick, up to nearly 3m high cross-beds are found in some elongate-depression fills. A process named “breaching” caused thick, massive canyon and clinoform base deposits of medium-grained sand that represents the product of a transfer of shallow shore sand to deeper environments. The formation as a whole is interpreted as a high-stand west-to-east fill of a 40km wide, 120km long marine embayment, tributary to the Southern North Sea, that represents the final fill of the large-scale Ypresian depositional cycle (see discussion in Houthuys, 2011).
I can provide a grain-size map, glauconite content map, occurrence of calcareous sandstone map, occurrence of massive facies, and occurrence of subpopulation of coarse, angular grains.
Stratotype: a lectostratotype is defined in Jodoigne (Zétrud - Lumay area). Sheet 32/7-8 (Meldert - Tienen). Co-ordinates: x = 185.375, y = 161.2, z = +90 m. The stratotype in Zétrud-Lumay is typical for fine-grained calcareaous sands superposed on coarse grained, glauconitic channel fill sands. I suggest to add the Brussels area, where construction sites regularly expose Brussels Sands, as the stratotype area for the normal succession of facies. The Al’Brûl sandpit in Chaumont-Gistoux can be added as a relatively permanent outcrop of this fill style. The Bouillon sandpit in Bierbeek is the type for the highly glauconiferous, thick-cross bedded final fill stages of the formation.
Area: this formation has a characteristic, restricted outcrop area in Brabant and the north of Hainaut and Namur. In the province of Antwerp, the formation’s subcrop is the northnortheastward prolongation of the outcrop area.
Thickness: 20 to 40 m, locally over 80 m. The formation is the fill of a complexely incised marine embayment; the thickest deposits are depression fills.
Members: no members are defined though sedimentary facies can often be recognized (X, Xb, Bx, Bc, Bm, Bf and M). See text on members.
Age: Transition Ypresian to Early Lutetian.
Remarks: see the exact lateral relationship to (nearly) contemporaneous formations remains to be settled.