Berg Member

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

Abbreviation BiBe
Parent unit Bilzen Formation
Child units
Lithological description The base of the Berg Sand Member is a regionally occurring transgressive surface characterized by black and flat flint pebbles generally about 2 to 3 cm in diameter. In the exposure area Leuven-Tienen-Tongeren, the Berg Member consists of yellowish to pale grey, slightly glauconitic, medium-grained quartz sand. It is expected that under the water table the sand may turn more greyish. In the about 6 m thick outcrop section the lower part is clearly horizontally bedded with a 150-175 μm grain-size modal value but the upper part is more homogeneous and cohesive with a modal size of 100-125 μm and clay fraction approaching 10% at the top (Gullentops, 1988 p. 242-243). Probably the twofold nature of the geophysical signal of the Berg Sand Member in some borehole logs reflects the same lithological subdivision as observed in outcrops (Figures 8c,8d,10). At particular levels (Figure 10) occur sparse molluscs, a.o. Astarte trigonella (Nyst, 1845), Glycymeris obovata (Lamarck,1819) and Arctica islandica rotundata (Agassiz, 1845) (Glibert & de Heinzelin, 1954; Glibert, 1955 & 1957 and for nomenclature see also Marquet et al., 2012).
Age The Berg Sand Member contains nannoplankton biozone NP23. The transition from the NP22 to NP23 biozone occurs precisely at the base of the member which is exactly the same biostratigraphic position of the base of the Belsele-Waas Member of the Boom Clay Formation overlying the Ruisbroek Member of the Zelzate Formatie. The age estimate for this biostratigraphic boundary is 32 Ma (Speijer et al., 2020).
Thickness The thickness of the Berg Sand Member is between about 5 and 10 m.
Area of occurrence The Berg Member occurs in outcrops and in the shallow subsurface in the Leuven-Tienen-Tongeren area, and slightly north of this area. Towards the northwest the Berg Sand Member continues in the subsurface but will be relayed by the Belsele-Waas Member of the Boom Clay Formation northwest from the area Mechelen Geel Mol onwards (Vandenberghe et al., 2002; Vandenberghe et al., 2001 figs. 4,5) (Figure 7).
Type locality Van den Broeck (1883b) refers to the “gîte classique de Berg”, an outcrop along a road on the southern slope of the small hill on which the hamlet Berg (north of Kleine Spouwen, southeast of Bilzen) is located. For the ancient references to this historical stratotype see Glibert & de Heinzelin (1954 p. 301, point 200 and map fig.4) (Coordinates: X = 233.040; Y = 171.340; Z = + 105 m ). In the archives of the Geological Survey of Belgium more recent information on investigations at this site can be found (Archives Geological Survey 93W0243).
A long standing outcrop of the Berg Sand Member is the Roelants extraction pit at Lubbeek near Leuven (Figure 10). In the subsurface, the geophysical signature of the Berg Sand Member is well expressed on resistivity and gamma-ray logs (Figures 8a, 8b, 9 and figs. 8, 9 in Vandenberghe et al., 2001).
Alternative names No other names have been used. However it should be noted that the Ruisbroek Sand unit occurring below the Belsele-Waas Member of the Boom Clay Formation has sometimes been mistaken for the Berg Sand, simply because geometrically both sand units occur under the clay.
On the geological maps 1:40 000, the Berg Sand Member has been labelled R1b and its basal gravel has been separately labelled R1a (Anonymous 1892, 1896, 1900, 1909) while in the Stratigraphic register (Anonymous 1929,1932) the Berg Sand was labelled R1. On the recent 1: 50 000 map, the Berg Sand Member is coded as BiBe.
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Berg Member, 05/04/2024. National Commission for Stratigraphy Belgium.


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