Eigenbilzen Formation

The information on this page is a summary description.
The full formal description is available here: Eigenbilzen Formation

Abbreviation Eg
Parent unit Rupel Group
Child units
Lithological description The Eigenbilzen Formation consists of grey to grey-green clayey fine sand and silt containing some glauconite pellets especially towards the top. It contains bioturbations but no macroscopic fossils. The Eigenbilzen Formation is well studied in the subsurface of the map sheet 25 Hasselt (Matthijs, 1999) where a consistent threefold subdivision could be demonstrated and characterized by typical resistivity and gamma-ray signatures, labeled A,B,C on Figures 19 and 20. These subdivisions A,B,C have also been identified in geophysical borehole logs outside the Hasselt map sheet and it was demonstrated that the sandy units A and B laterally grade into clay to the west (Vandenberghe et al., 2001) (Figure 4). The unit C is characterized by a coarsening upward trend with higher frequency cycles superposed very similar to the signal of the Boeretang Member of the Boom Clay Formation. Above this part C with high frequency cycles, a sandier top called unit D in Figure 4 is recognized; it was already reported by Matthijs (1999, p. 37) east of the map sheet Hasselt. Matthijs (1999) in his study on the map sheet 25 Hasselt confirmed the lateral transition of the Eigenbilzen Formation into the Boom Clay Formation also to the north (Matthijs, 1999 fig.11) (Figure 19). These units A,B,C,D in the Eigenbilzen Formation have not been ranked as official subdivisions of the formation. Geophysical logs (Figures 2, 6, 9) and the few outcrops (Vandenberghe, 1978 fig 1.1e) show the presence of more clayey and more sandy layers in the formation.
In the Mol area the Eigenbilzen Formation unit D always contains a few percent >125 µm size grains and about 20 to 75 % >62 µm grains, in contrast to the underlying Boeretang Member of the Boom Clay Formation with <1% >125 µm grains and <10% >62 µm grains (a.o. Frederickx, 2019). In the Hasselt-Diepenbeek area (Vandenberghe, 1978 fig. 1.1e) the >32 µm content is about 60% in the more clayey layers and about 80% in the sandy layers, similarly to the about 50 to 70% reported in the Mol area. In the Antwerp Campine subsurface, the Eigenbilzen Formation unit D consists of almost-10 m-sized resistivity lobes or intervals superposed on shorter alternating several-dm-sized clay and silt layers (Figure 2); the latter is confirmed by core inspection (see Weelde borehole in Figure 2) and by the geophysical pattern of the formation in the Weelde and Meer boreholes (see Vandenberghe et al., 2001 fig. 6).
Age The chronostratigraphic interpretation of the Eigenbilzen Formation is sketched in Figure 5. Based on data by Steurbaut, literature reports nannoplankton biozone NP24 in the Eigenbilzen Fm unit D (o.a. Vandenberghe et al., 2004). However the position of the NP24 range in the GTS2020 (Speijer et al. 2020 and used in Figure 5) does not fit the range usually assumed in the North Sea Basin, a discrepancy linked to the absence of the typical NP24 markers in the North Sea Basin (see discussion in De Man et al. (2010). According to Munsterman & Deckers (2020) the Eigenbilzen D Sand in the ON-Mol-1 borehole contains dinocyst zone NSO 4b-5a; these authors also invalidate the Distatodinium biffi identification by Van Simaeys et al. (2005).
Thickness On the Tongeren geological map the Eigenbilzen Formation is generally eroded and only 5 to 10 m thickness is left (Claes et al., 2001); this is the case in the eroded Albert Canal section near Eigenbilzen where thickness is about 10 m. Where the formation is complete under the Voort Sand as in the Hasselt geological map area, the Eigenbilzen Formation is about 50 m thick. In the most western occurrence area of the Eigenbilzen Formation thickness is reduced to 20 m or less.
Area of occurrence The formation crops out along the Albert Canal east of Hasselt (Figure 1a). Borehole log analysis in the Antwerp and Limburg provinces indicates the systematic presence of the Eigenbilzen Formation in the subsurface of Limburg north of its outcrop zone (Figure 21).
The most western occurrence of the Eigenbilzen Formation overlying the Boeretang Member of the Boom Clay Formation is situated in the Antwerp Campine (a.o. Weelde, Meer, Lichtaart, Geel boreholes) (Figures 1a & 1b). In this area it can be distinguished from the Boeretang Member by its higher sand content (see 11.1. ) as also expressed on geophysical logs (Figure 2). For geometric reasons this most western extension of the Eigenbilzen Formation probably corresponds to the uppermost parts of the Eigenbilzen Formation in the Limburg province (see 11.7.) (Figures 4 and 5).
Type locality No permanent outcrop of the Eigenbilzen Formation exists and the formation is seldom exposed. The sand has been studied in temporary outcrops during the works for the extension of the locks on the Albert Canal near Diepenbeek and Hasselt (Vandenberghe, 1978 fig. 1.1e), and during the enlargement of the same canal near Gellik (Steurbaut et al., 1999) (Figure 3). The latter outcrop allowed a calibration of the boundary between the Boom Clay and the Eigenbilzen Sand near the place of its original definition by Halet (1932), the village of Eigenbilzen (Vandenberghe et al., 2001).
The interpreted geophysical logs of the boreholes Genk (KB136/ 77E0300) and Houthalen (62E0269 and 62E0266) in the map sheet 25 Hasselt and figured and interpreted by Matthijs (1999, fig. 10) can be used as reference geophysical borehole logs (Figure 20). The Eigenbilzen Sand section thickens to the north and an additional unit D can be identified as shown for the Koewijde KS2 borehole in Figures 3 and 9 ; note however that to the north the Eigenbilzen Formation becomes more clay-enriched (boreholes in Vandenberghe et al., 2001 fig. 9 and Matthijs, 1999 fig. 12) (see also Figure 21). Matthijs (1999) suggests that changes in thickness and clay content can be influenced by late Oligocene differential tectonic activity in the area.
Alternative names The Eigenbilzen Formation is a sandy unit lying above the Boom Clay that was already recognized in Limburg by Van den Broeck (1884, 1893) and coded by the symbol R2d in his stratigraphic system as well as in the legends of the 1:40 000 geological maps (Anonymous 1892, 1896, 1900, 1909). On the recent 1:50 000 geological map the Eigenbilzen Formation is coded Eg.
Authors Vandenberghe, N. & Wouters, L.
Date 05/04/2024
Cite as Vandenberghe, N. & Wouters, L., 2024. The Eigenbilzen Formation, 05/04/2024. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Eigenbilzen-Formation


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