The term Rupel group is referring to the Boom Clay Formation, the main clay unit in the group, and to the other fine-grained sandy or clayey deposits which are sedimentologically, paleogeographically and geometrically related to the Boom Clay. These comprise on the one hand the superjacent much sandier Eigenbilzen Sand Formation displaying in some outcrops the typical rhythmicity of the Boom Clay and considered as a geometrical and mainly lateral equivalent of part of the Boom Clay. On the other hand in East-Limburg a sandy unit, the Kerniel Sand, splits up the main clay unit in two parts: the overlying clay designated as Boom Clay and the thinner underlying clay unit as the Kleine-Spouwen Clay (Van den Broeck, 1883a,b). Also included in the Rupel Group is the coastal Berg Sand, underlying the Kleine-Spouwen Clay; in some areas the Berg Sand also occurs below the Boom Clay where it is a paleogeographical lateral evolution of the lower part of the Boom Clay. The Kerniel Sand, Kleine-Spouwen Clay and Berg Sand are grouped as members into the Bilzen Formation.
Lithologically and from a sedimentary sequence genetic perspective the lagoonal to shallow marine Ruisbroek Sand unit underlying the Boom Clay in the northwestern part of the occurrence area of the Rupel Group could be included into the Rupel Group (Vandenberghe et al., 2002), but as it occurs in the top of an alternation of lithostratigraphic clay and sand units, originally abbreviated a (argile) and s (sable) and grouped as the Complexe argilo-sableux de Kallo by Gulinck (1969), it has been grouped as a member in the Zelzate Formation of the Tongeren Group (see NCS website). An analogous remark could be made about the Borgloon Formation of the Tongeren Group underlying the Rupel Group in its southern and eastern occurrence area as these deposits make up the initial sediments of the important transgression that will later deposit the Rupel Group sediments (Vandenberghe et al., 2003).
Age
The chronostratigraphic scheme presented in Figure 5 is based on the Geological Times Scale 2020 (Speijer et al., 2020). Note that the age for the GSSP Chattian is slightly different (27,82 Ma) in the ICS chart (2023/4). Chronostratigraphic anchoring is based on magnetostratigraphy (Lagrou et al., 2004) and on nannoplankton and dinoflagellate biostratigraphy. The identification of D. biffi in Van Simaeys et al. (2005) has not been withheld (for arguments see 5.1 in Munsterman and Deckers, 2020). The hiatus between Rupelian and Chattian is mainly situated in Rupelian time (see also Coccioni et al., 2018). In addition age intervals within the Rupel Group are based on the obliquity cycles in the ON-Mol-2D borehole (Vandenberghe et al., 2022) starting with the base of Rupel Group at 32 Ma (NP22/NP23 boundary). A silicified foraminifera event is considered the start of the tectonic tilting with uplift and erosion in the west (De Man et al., 2010). The boundary between the Rupel Group and the overlying Veldhoven Formation (line 4) is a low angle unconformity (Dusar and Vandenberghe, 2020 3.2) and in northeast Belgium at this level the RVG graben fault activity resumes depositing much thicker Chattian deposits in the graben. The hiatus between the top of the Rupel Group and the base of the Chattian Voort Sand is dated following De Man et al. (2010) and Coccioni et al. (2018). The age of the basal Voort Sand is put at the start of the Svalbardella interval including the Asterigerina horizon almost at the NP24/NP25 boundary (Coccioni et al., 2018 table 1; Speijer et al., 2020 fig.28.11). Assuming the latter boundary only slightly deviates from the North Sea NP24*/NP25* proxy (De Man et al., 2010), the Voort Sand outlier in the Antwerp area is correlated to this nannoplankton boundary (Vandenberghe et al., 2004).
Thickness
The maximal thickness in North Belgium is about 140 m (Weelde SCK 98 borehole) and about 120 m in the Mol-Dessel area (Figure 2). Compared to the North Belgium area the Rupel Group was eroded by about 80 m towards the Antwerp area (De Man et al., 2010). In the outcrop zone along the Rupel river the thickness is even further reduced to about 40 m (Reet borehole ). In the Land van Waas area only the lower part of the Rupel Group is preserved, about 12 m clay is exposed in the former clay pit at Sint-Niklaas (SVK pit). Along the Albert Canal in Southeast Limburg the Rupel Group thickness is about 45 m (Figure 3).
Area of occurrence
The deposits of the Rupel Group occur north of the rivers Durme, Rupel, Dijle, and of the line Leuven-Tongeren. Along the southern rim of this area, the Rupel Group crops out. The deposits dip to the north and are therefore present in the subsurface of the Antwerp Campine and Limburg Campine areas (Figure 1b).
Type locality
The Rupel Group is slightly dipping to the north-northeast (Figure 1b). It reaches the surface at the south-southwestern rim of its occurrence area, except in the northern Hageland, where channel erosion and channel infill has removed the Rupel Group sediments and replaced it by the Neogene Diest Formation. As a consequence, two main outcrop areas can be considered: a western area comprising the Land van Waas, the Rupel area and the Nete-Dijle interfluvium, and an eastern area comprising the southern Hageland and the Demer area in Limburg (Figure 1a).
As reference sections for the Rupel Group in the Antwerp Campine, the cored and geophysically logged boreholes at Weelde (SCK 98) and Mol (SCK-15), together with ON-Mol-1 and ON-Dessel-1, are chosen as representative for the Eigenbilzen and Boom Formations (Figure 2). In the Limburg area the geophysically logged Koewijde-KS2 borehole and the temporary outcrop along the Albert Canal near Eigenbilzen are chosen showing the Bilzen, Boom and Eigenbilzen Formations (Figure 3) (Vandenberghe et al., 2001).
Alternative names
The present LIS is a further elaboration of the previous NCS website version of the Rupel Group by the same authors. Note however that in contrast to earlier versions of the stratigraphic inventory, the Voort Sand is no longer included in the present definition of the Rupel Group. This follows from the integration of the mainly Chattian sand and clay units, such as the Voort Sand, into the Veldhoven Formation of Chattian to Aquitanian age (De Nil & Verhaegen, 2022) occurring dominantly in the RVG; hence further including these units in the Rupel Group would expand this group too far outside its core area of occurrence.
Authors
Vandenberghe, N. & Wouters, L.
Date
05/04/2024
Cite as
Vandenberghe, N. & Wouters, L., 2024. The Rupel Group, 05/04/2024. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Rupel-Group
