2.11.4. Voort Formation
Vandenberghe, 2000.
Name
The term was introduced by Van Straelen (1923) while the describing coal wells in Limburg. Voort is a hamlet of the municipality Zolder is the Limburg province.
General characteristics
The Voort Formation consists of fine-grained and clayey, dark green glauconitic sands, rich in fossils. Gulinck (1954) remarked that the lower part of the Voort Sands passes gradually into the underlying Eigenbilzen Sands without noticeable boundary.
Occurrence
The formation is principally known from the subsurface of North and East Limburg. The upper boundary of the formation is with the Miocene Houthalen Sands, and to the west with the Miocene Berchem Formation. The lower boundary is with the Eigenbilzen Formation. The thickness of the Formation in Belgium is maximum 75 m.
During the recent geological mapping, the formation was also recorded in the north of the Antwerp province. Also along the river Scheldt - north of the city of Antwerp - relicts of the Voort Sands have been observed based on the presence of the fo-raminifer Nonion roemeri (Vandenberghe & Laga, 1986).
Stratotype
Coal mine shaft at Voort-Zolder (archives Belgian Geological Survey 62W-226); ref-erence section from -21 m to -45 m (de Heinzelin & Glibert, 1956, p. 202); geological map 25/3-4 (Beringen-Houthalen).
Co-ordinates: X = 217.330, Y = 192.725, Z = + 48,5 m.
Subdivision
The intercalation of silty clay in the Voort Sands is described as the Veldhoven Clay. This clay layer is recorded north of the Grote Brogel fault in the Roer Valley Graben.
It is common practice in The Netherlands to use the term Veldhoven Formation, sub-divided into the Voort Sands Member and the Veldhoven Clay Member (NAM and RGD Nomenclator, 1980). In the revision of the Dutch stratigraphic nomenclature by Van Adrichem Boogaert & Kouwe (1993) three depositional cycles are distinguished within the Voort Sands Member, each consisting of a clayey horizon in a dominantly sandy unit. Above the Voort Sands Member lies the Veldhoven Clay Member, essentially consisting of clay and already partly with an Aquitanian age. The Someren Sands Member is the uppermost unit.
The scheme by De Mulders et al (2003, Fig. 172) indeed shows this succession but the entire formation is called Veldhoven Formation, with the Voort and Someren units located in the upper part, and the deeper water deposits of the Veldhoven Clay in be-tween.
It is proposed to use in Belgium the denomination Voort Formation for all deposits above the Eigenbilzen Sands, since sandy sediments (i.e. the Voort Sands) dominate in the unit. When clayey intercalations are recorded, these can then be referred to as belonging to 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)
Van Simaeys (2004) divided the Chattian deposits into three sequences based on bio-stratigraphical arguments. The latter author also described two gravel layers, respec-tively at the base of the middle and upper sequence. Vandenberghe et al. (2004) corre-lated these sequences with units proposed by Hager et al. (1998).
Former designations
Geological map 1/40.000: no reference
Stratigraphical register (1929, 1932): Voort Sands: Chattian (Ch) p.p.
New Geological map 1/50.000: Vo.