088W3039.txt
ADMINISTRATIEVE & TECHNISCHE GEGEVENS
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Kaart nr.: 88W
PLAAT: Brussel
Nr.: 3039 (VI, b)
Type Boring: bouwput
Topografische kaart: 31/3
Uitgevoerd te: Brussel - Haren
Post nr.:
Adres boorplaats:
Opdrachtgever:
Boorfirma:
Boordatum: 2011
Topografie: van topokaart
Stalen door:
Boormethode:
Lengte & doormeters:
Grondwaterstanden:
1ste maal:
Bij rust:
Tijdens pompen:
Debiet:
Waterzaak nr.:
Totale diepte:
Stalen bewaard: neen
Maaiveld / ref. peil: 55 m
X: 153927
Y: 174342
NIS code:
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BOORBESCHRIJVING
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AARD DER GRONDLAGEN
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The construction pit is situated in the Eocene Brussels Sands Formation (aged 50 M years), of which
some 5-8 m are exposed below a cover of 2-3 m of Quaternary, silex-gravel based löss. The base
contact of the Formation is not exposed; it is according to data from nearby wells situated almost 30m
below the floor of the outcrop. The geological map of the area indicates the younger Lede Formation
should be present on top of the Brussels Formation, but the typical base gravel of the Lede Formation
containing rounded quartz granules and transported sandstones was not found in the outcrop, thus the
Lede Formation does not occur here. Inside the Brussels Formation, the exposure shows rapid lateral
and vertical changes of the fine-grained carbonate sand facies (containing near-horizontal layers of 10-
20 cm thick calcareous sandstones) and the fine-to-medium grained, faintly laminated to
homogeneous quartz sand facies (containing no bioturbation structures, and a few rounded siliceous
sandstone concretions). In the terms of Houthuys (2011), three sedimentary facies were found : the fine,
bioturbated, carbonate-rich Bf (decalcified in the upper part of the exposure and in the part closest to the
Woluwe valley); intercalated are lenses of the siliceous breach failure facies M. In the south-eastern part
of the outcrop, M is conformably covered by 2m of gently ESE dipping, thin cross-bedded and partly
bioturbated Xb, which is in turn covered conformably by Bf (photo 1). Elsewhere, the several metres
thick lenses of M are cut into Bf and are conformably covered by Bf. The many, mutually perpendicular
faces of the construction pit allow good observations of the geometry of facies M, which is still poorly
understood.
In the outcrops, facies M occurs both in sheet-like, laterally extensive lenses of faintly laminated to
massive quartz sand (named slope aprons in Houthuys, 2011) and in channels filled by more clearly
laminated quartz sand.
The M-sheets were found near the base of the outcrops, almost anywhere except for the southern part
closest to the Leopold III-laan. The base of M-sheets is most often not exposed, but when exposed, is
seen to be erosive in facies Bf. The M-sheets are clearly amalgamated, stacked packages (photo 2).
The top is covered conformably by Bf or Bx or Xb. The cover has ESE-dipping master bedding (photo 1),
inagreement with the "westerly lateral accretion" arrangement described in Houthuys, 2011. The
arrangement and composition of the ESE dipping, 2m thick package of Xb in the southeastern part of the
outcrop suggest it may be a local synsedimentary reworking of neighbouring M sand.
Two M-channels were found at the same vertical level as the M-sheets, in the part near the Leopold III-
laan and incising in the carbonate facies Bf. One channel was exposed by no less than 4 construction pit
faces, confirming its more or less rectilinear WNW to ESE long channel axis. The channels are 20 to 40
m wide. The walls of the incision are steep and composed of several concave facets superposed to each
other (photo 3). The incision is filled with inclined strata of clear quartz sand (photo 3), which often
contains laminae of mud flakes (photo 4), mm to cm-scale carbonate facies lumps, and broken shell
fragments. The fill strata dip often to one side of the channel and they also dip "downstream" towards
ESE (photo 5).
The M lenses are slope base deposits of collapsed shore sand, conveyed to the deeper sea floor by
breaching. Breaching supposes substantial slopes existed during the deposition. Wells in the
neighbourhood indicate the construction site is situated in a paleochannel, dubbed "Etterbeek" channel
in Houthuys (2011), fig. 2.
The M facies lenses have in the outcrop a convex-up upper surface (photo 6) which is, in a draping
manner, conformably covered by the commonly near-horizontally bedded facies Bf and/or Bx (photo 7).
This arrangement suggests post-sedimentary differential compaction. The level difference of
the originally subhorizontal top surface of M deposits exceeds 1 m in several places. One reason of the
differential compaction may be the original difference in carbonate content of facies Bf sands versus M
sands, causing different amounts of deposit volume loss due to (recent) decalcification.
The exact relationship between M-sheets and M-channels remains to be further explored. The breaching
model suggests the M-channels are feeder-canyons linking breach areas in the upper (eroded) part of
the Brussels Sands to slope-base M-aprons. It is not clear from this outcrop alone whether the M-sheets
are simply very wide channels, with dimensions exceeding the outcrops' dimensions.
Reference : Houthuys, R., 2011. A sedimentary model of the Brussels Sands, Eocene, Belgium.
Geologica Belgica, 14, 55-74
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STRATIGRAFISCHE INTERPRETATIE
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Top * Basis * STRATIGRAFISCHE EENHEDEN
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Zand Van Brussel
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AUTEUR Rik Houthuys 27.07.2011
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OPMERKINGEN
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BELGISCHE GEOLOGISCHE DIENST
88W(31/03) - 3039(VI, b) KAARTBLAD: Brussel
p. 1/1
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