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Brussels Formation - replaced on 27/10/2017

REF: Rik HOUTHUYS, «A sedimentary model of the Brussels Sands, Eocene, Belgium», Geologica Belgica [En ligne], volume 14 (2011), number 1-2, URL :


Different lithological subunits are found inside the Brussel Formation, but their extent and geometric relationships remained puzzling. Houthuys (2011) described four sedimentological facies and proposed a coherent sedimentary explanation for the variation, transitions and geometry. The facies are (table 1 in Houthuys, 2011):

  • X: cross-beds, mostly between 0.5 and 1.5 m thick; medium to coarse sand; scarce bioturbation; relatively rare siliceous concretions
  • Xb: 2-3 dm thick cross-beds; beds dip 4-10% across the basin to ESE; medium sand; many burrows; many siliceous concretions
  • B: completely bioturbated; many siliceous concretions; master bedding dipping 0-4% to ESE. Subfacies are:
    • Bx: original thin cross-beds still recognizable
    • Bc: coarse sand
    • Bm: medium sand
    • Bf: fine sand, carbonate mud; contains carbonate cemented stone layers
  • M: massive structure or faint, broadly undulate lamination; medium to coarse, no fine fraction; no bioturbation other than from the top; rare siliceous concretions, often spherically shaped

Facies X, Xb, Bx, Bc, Bm and Bf are prominent members of a gradual series; gradual transitions between these exist. Only facies M is unequivocal.

To complicate matters, two lithological components occur across the sedimentary facies:

  • carbonate matter was originally present in all of the sedimentary facies, but the finest grained facies originally contained the highest percentages, sometimes over 50%. In oxidizing conditions, all facies are partially or completely decalcified. When I find a sedimentary facies in decalcified state, I add a quote (') to denote this, e.g. Bf'.
  • low contents of very fine glauconite pellets are present throughout the formation, whenever the fine sand fraction is present. All facies tend to show an increasing content in coarse, dark green, sometimes black green, glauconite pellets when going to the ESE side of the depositional basin; but the trend is not systematic and the observed variations in glauconite have not found a sedimentological explanation yet. I simply note the estimated coarse-glauconite content when making a field observation.

Some correlation between the sedimentary facies and the members proposed by Houthuys & Fobe (1988) exists, but the proposed members have never been used in practice and the gradual transitions make it difficult to use them.

From Houthuys (2011), it is also clear that the sedimentary facies cannot be mapped out: the vertical and spatial variations are too intricate and the general sedimentary model will only predict general trends but no particular occurrences.

It is thus suggested to discard a subdivision in lithostratigraphic members and to simply add observations on grain size, carbonate and glauconite content, and whenever possible on sedimentary facies such as suggested above. The latter will only be possible in good outcrops and on undisturbed cores.