2.11. RUPEL GROUP
Revised: N. Vandenberghe & L. Wouters, 2011
Name
The name "Rupélien" was introduced in 1849 by Dumont. The term has been consistently used in a chronostratigraphic sense, and nowadays the Rupelian is the international stage corresponding to the Lower Oligocene (Van Simaeys & Vandenberghe, 2006). The Rupel area is the best suitable area for the lithostratigraphic description of the Rupel Group.
General characteristics
The term Rupel group is designated for the Boom Clay and the sedimentological related fine-grained and clayey deposits. The latter comprise on the one hand the super-jacent Eigenbilzen Sands, a unit much sandier than the Boom Clay, however displaying in some outcrops the typical rhythmicity of the Boom Clay. The Eigenbilzen Sands are considered as a geometrical and lateral equivalent of the Boom Clay. On the other hand in East-Limburg a sandy unit, the Kerniel Sands splits up the clay unit, the overlying clay designated as Boom Clay and the thinner underlying clay unit as the Kleine-Spouwen Clay (Van Den Broeck, 1883). The Berg Sands, occuring below the Kleine-Spouwen Clay and the Boom Clay, are littoral sands and are considered as a lateral equivalent of a part of the clay.
The age of the Rupel Group is for the larger part Rupelian. The Rupel Group was traditionally considered as Middle-Oligocene in the former threefold division of the Oligocene. In the recent twofold division of the Oligocene, the Rupel Group is equated with the Early Oligocene.
The sand layer, frequently encountered in the subsurface of the Campine area and lying upon the Eigenbilzen Sands, is named the Voort Sands. The age of this sandy unit is Chattian or Late Oligocene. The Voort Sands can be lithologically comparable to the Eigenbilzen Sands.
Type area
Dumont (1849) cited the vicinities of Rupelmonde, Boom and Hasselt as type areas for the clayey upper part of the "Rupélien", while Van Den Broeck (1883) mapped the Berg Sands, the Kleine-Spouwen Clay and the Kerniel Sands in the Bilzen area. It is reasonable to designate both areas as the type area for the Rupelian. The Boom Clay is nowadays excavated in large quarries along the rivers Rupel and Schelde, and in the vicinity of the city of Sint-Niklaas.
Occurrence
The deposits of the Rupel Group occur north of the rivers Durme, Rupel, Dijle, and of the line Leuven - Tongeren. The Rupel Group crops out along the southern rim of this area. The deposits dip to the north and are thus present in the subsurface of the Ant-werp Campine and Limburg Campine areas. The Rupel Group is deeply eroded by the overlying Diest Formation in the Hageland. As a consequence, two 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 Hageland and the Demer area in Limburg.
The Voort Sands do not crop out, and are only present in the north of Limburg. Traces or relicts of this unit have been reported in the area of Hoogstraten-Merksplas, Turnhout and north of Antwerp.
Subdivision
The Rupel Group is divided into four formations (from young to old):
The Bilzen, Boom and Voort formations are subdivided into members:
Kerniel Member, sand;
Kleine-Spauwen Member, clay;
Berg Member, sand.
Boeretang Member, siltier clay with pronounced layered structure
Putte Member, organic rich, black clays;
Terhagen Member, grey clays;
Belsele-Waas Member, silty clays.
A clayey part of the Voort Formation in the northeastern part of the country was de-scribed as the Veldhoven Clay, following a subdivision used in The Netherlands. However, the significance of the terms Voort and Veldhoven, as defined in the stratigraphic nomenclature of The Netherlands (Van Adrichem Boogaert & Kouwye, 1993), differs from this simple scheme. It is proposed to use the name Voort Formation for this unit above the Eigenbilzen Sand.as in Belgium the unit is dominated by sands; in case a thick.clay layer is observed in the unit, the clay can be designated as the Veldhoven Member. It is furthermore unclear whether two types of clay are present in this unit: marine clays deposited in the deeper parts of the basin (following the scheme by Mulder et al., 2003), and continentally influenced clays with lignites (Hager et al., 1998)
