Bilzen Formation

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

Abbreviation Bi
Parent unit Rupel Group
Child units Berg Member, Kleine-Spouwen Member, Kerniel Member
Lithological description The formation consists essentially of fine-sized and quartz-dominated sandy sediments (the Kerniel Member and the Berg Member) and a mollusc-rich (Nucula compta Goldfuss, 1837) calcareous clayey intercalation (the Kleine-Spouwen Member) between both. The formation rests on the Tongeren Group below and underlies the Boom Formation.
Age The base of the Berg Sand, the lower member of the Bilzen Formation, occurs at the start of the nannoplankton biozone NP23 (Steurbaut et al., 1999) estimated at 32 Ma. Therefore the Bilzen Formation can be situated in the lower part of this biozone as figured in Vandenberghe (2017, fig.4) (Figure 5).
Thickness Depending on the presence of all 3 composing members of the Bilzen Formation its thickness varies between 5 and 25 m.
Area of occurrence The formation crops out in the south of the Hageland and in Limburg ( Figure 1a). Further north, the formation occurs in the deeper parts of the subsurface of the eastern Campine while in the western part it is laterally relayed by the lower part of the Boom Clay Formation (Figures 8a,8b,8c,8d). Three areas can be distinguished (Figure 7): a zone southeast of the line Maaseik, Genk, Hasselt where the three members of the formation occur, a central zone around Aarschot-Leopoldsburg with only the Berg Member present and the northwest zone including Mol-Dessel, the Antwerp Campine and the Boom Formation type area, where the Bilzen Formation is laterally entirely replaced by the Belsele-Waas and Terhagen Members of the Boom Clay Formation. Note that the quality of presently available data in the boundary areas between the 3 zones makes the exact interpretation of the stratigraphic unit present remaining debatable (for example compare the Kerkhoven borehole in Figure 8a and in Vandenberghe et al. 2001 fig. 5).
Type locality A complete succession of the formation is seldom accessible in outcrop. In a detailed description of a profile between Tongeren and Waltwilder, Cadee et al. (1976) represent only the Berg Sand and the Kleine-Spouwen or Nucula Clay. Halet (1932) presents a complete profile along the Albert Canal west of Maastricht close to the profile in Vandenberghe et al. (2001, fig.10) drafted during the enlargement of the Canal section in the area of Gellik-Eigenbilzen (Steurbaut et al., 1999; Baut & Génault, 1999). Therefore the area bordering the Albert Canal northeast of the village of Bilzen is indicated as type area for the Bilzen Formation (Figure 3). A characteristic geophysical log signature of the Bilzen Formation is recorded in the borehole Koewijde-KS2 (Figures 3, 9) located north of the reference Canal area (Figure 1a).
Alternative names The formation has been coded as Bi on the 1:50 000 geological maps. Its components are coded with R (from Rupelian) on the former 1:40 000 map and in the Stratigraphic register (Anonymous, 1929 & 1932).
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Bilzen Formation, 05/04/2024. National Commission for Stratigraphy Belgium.


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