2.11.2.1. Belsele-Waas Member

Name

The name refers to the S.V.K. clay pit in which the member is exposed and which is situated in Belsele -Waas, a part of the town of Sint-Niklaas (East- Flanders province).

General characteristics

The Belsele-Waas Member represents the lowermost, more silty part of the Boom Formation; it is characterised by the absence of black organic rich horizons and in particular by the occurrence of two unusual thick layers at its base basis (Vandenberghe, 1978, photo 8, p. 37). Based on the grain-size properties and evolution (see e.g. Vandenberghe et al., 2001, Fig. 2) this Member has been formally defined as the unit between the base of the Boom Clay and the base of septaria- or calcareous bed S10. However on geophysical resistivity logs, in particular the two thick silt layers at the base delineate an easily recognisable unit very usefull for corelation purposes in the subsurface. As such use of geophysical borehole logs is a common practice it is proposed to make the top of the Belsele-Waas Member coincide with the top of the thick silty layers ( many examples can be found in the figures in Vandenberghe et al.,2001).

Occurrence

The Belsele-Waas Member can be mapped in the Waasland. It is always present at the base of the Boom Clay except in the most southernly outcrop area (Leuven area) where it has been laterally replaced by a more sandy facies, the Berg Sands (see fig. 4 in Vandenberghe et al.,2001).

The Member is limited at its base by the Ruisbroek Member of the Zelzate Formation and at its top by the Terhagen Member of the Boom Clay Formation. The thickness of the Member is usually slightly less than 10 m. A detailed comparison between the Member in the boreholes Reet and Dessel-1 is given in fig.7 in Vandenberghe et al. (2001).

Stratotype

The clay exploitation pit of the company Scheerders Van Kerkhove (S.V.K.) at Sint-Niklaas, Belsele-Waas; topographic map sheet 15/5-6 (Sint-Niklaas-Temse).

Coordinates: X = 132.500; Y = 205.000; Z = + 17 m.

As the clay pit is still in exploitation the coordinates may slightly change with time.

Subdivisions

The basis is formed by a gravelly layer of mainly phosphatised internal shell moulds and worm tracks. The phosphatised sediment is the fine glauconite bearing sand of the underlying Ruisbroek Sand. In Belgium, this basal layer is only found in the Sint-Niklaas area (Vandenberghe, 1978; Janssen, 1981). This phosphorite horizon can be given a formal bed status. A detailed description of this bed and its stratigraphic significance can be found in Vandenberghe et al. (2002).

Former designations

Geological map 1/40.000: R2c p.p.

Stratigraphical Register (1929, 1932): R2b p.p.

New Geological Map 1/50.000: BmBw.

Although no formal name was given to this Member, the silty base of the Boom Clay in the type area was long recognised and indicated by the special symbol 'R2b' by Halet (1936) and Gulinck (1965) (see discussion in Vandenberghe, 1974, p. 186).

The Belsele-Waas Member was originally incorporated in the Waasland Clay (see 2.11.2.2. Terhagen Member: Former designations).