Belsele-Waas Member

The information on this page is a summary description.
The full formal description is available here: Belsele-Waas Member

Abbreviation BmBw
Parent unit Boom Formation
Child units
Lithological description The Belsele-Waas Member consists of very silty grey clay, very low in carbonate and in organic matter and forms the base of the Boom Clay Formation. In the Waasland area (Figure 1a) the silt-enriched clay is divided in two parts by a thinner 50-60 cm clay horizon (bed 3 heading IV in Figure 12) (Vandenberghe, 1978 photo 9; Vandenberghe, 1980 p 162;Vandenberghe 2017 fig. 8). In the top of the lower thick silt layer occasionally small and poorly consolidated septaria-like carbonate nodules have been observed (labelled S05 in Figure 12). Compared to the Waasland area, in the subsurface of the Campine area an additional thick silty layer overlain by another thinner clay bed occurs at the base of the geophysically interpreted Belsele-Waas Member (labelled 0 in Figure 11).
Grain-size analyses of the member in the Waas area in Vandenberghe and Van Echelpoel (1987, fig. 12) show the sand fraction to be limited to 20-30% except for the very basal part. Analyses in the On-Mol-1 borehole show an upward decreasing sand content, from about 70% at the very base to about 5% in the top (Frederickx,2019 fig.35); the coarsest samples occur in the level 0 at the base of the Belsele-Waas Member in the Mol-Dessel area (Figure 11).
Compared to the overlying part of the Boom Clay, the alternation of clay and silty beds is missing in the Belsele-Waas Member. Still banding as stacked silt layers is present (Vandenberghe 1978; Vandenberghe, 2017 fig. 8) and its origin is explained in Vandenberghe et al. (2014 §4.2.1.). Characteristic regionally-consistent flat gully levels indicating wave erosion at the sea bottom are present in the silty clay (Vandenberghe, 1978 photo 8 p 37; Vandenberghe , 2017 fig. 10) and occasionally even a gutter fill was observed ( Vandenberghe et al., 2002 fig. 3). Beds of Pycnodonta callifera (Lamarck, 1810) have been reported although these oysters were also found in the Ruisbroek Sand (Vandenberghe and Van Echelpoel, 1987). According to Marquet & Herman (2012) Arctica islandica rotundata (Agassiz, 1845) is characteristic for the lowermost part of the Belsele-Waas Member.
Age The transition between the underlying Ruisbroek Sand and the Belsele-Waas Member coincides with the transition of the NP 22 to NP23 nannoplankton biozones dated at 32 Ma (Figure 5).
Thickness The thickness of the Belsele-Waas Member is usually slightly less than 10 m.
Area of occurrence The Belsele-Waas Member is accessible in the shallow subsurface of the Waasland and Rupel area and it can be systematically mapped on the Mechelen and Aarschot map sheets 1:50 000 (Buffel et al., 2009; Schiltz et al., 1993). To the north, in the deeper subsurface, the Belsele-Waas Member overlying Ruisbroek Sand has been observed in the Herentals and the Geel-Mol-Dessel-Balen area (Figures 7,8).
Type locality In the past several decades, the now abandoned clay exploitation pits of the company Scheerders-Van Kerkhove (S.V.K.) at Sint-Niklaas, Belsele-Waas, have been the reference for studies of the Belsele-Waas Member (topographic map sheet 15/5-6 Sint-Niklaas-Temse, last open pit located at X = 132.500 , Y = 205.000, Z = + 17 m). Sections, analyses and photographs can be found in Vandenberghe (1978; 2017) and Vandenberghe & Van Echelpoel (1987).
The cored and geophysically logged Reet borehole is the type section of the Belsele-Waas Member in the Boom Clay Formation reference area (Figure 11).The geophysical silt and clay intervals labelled 1, 2, 3 and 4 in Figure 11 can be well traced to the actual grain-size analysis of the member in Vandenberghe & Van Echelpoel (1987 fig. 12); note that the labelling of the layers in Figure 11 is not related to the labelling systems in reference Figure 12 and only serves the purpose of comparing the Reet and ON-Dessel-1 boreholes in this particular Figure 11. The reference section of the member in the Campine subsurface is based on the ON-Mol-1 and ON-Dessel-1 boreholes presented in Figure 12. The relationship between the Rupel area and the Mol-Dessel area is illustrated in Figure 11 displaying the thickening of the Belsele-Waas Member towards the northeast as discussed in 1. (from Vandenberghe et al., 2001, fig.7).
Alternative names In a now obsolete two-fold subdivision of the Boom Clay, Vandenberghe (1974, fig. 8.1. and p. 186; Vandenberghe, 1978) distinguished a lower grey clay and an upper black clay, respectively named Waasland clay and Putte clay. The Waasland clay included the present Belsele-Waas and Terhagen Members. The silty base of the Boom Clay Formation to which the Belsele-Waas Member refers has been long recognized and although no formal name was given to this silty base in the type area of the Boom Clay Formation it has been indicated by the special symbol ‘R2b’ by Halet (1936) and Gulinck (1965) (discussion in Vandenberghe, 1974, p. 186). The silty base of the Boom Clay Formation is included pro parte in the unit labelled R2c on the geological map 1:40 000 (Anonymous, 1892,1896,1900,1909) and pro parte in the unit labelled R2b in the Stratigraphic Register (Anonymous, 1929, 1932). On the recent 1: 50 000 geological maps, the Belsele-Waas Member is coded BmBw.
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Belsele-Waas Member, 05/04/2024. National Commission for Stratigraphy Belgium.


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