The Diest Formation consists of grey green to brownish, poorly sorted, fine to medium to coarse very glauconiferous sand, locally cemented by variable amounts of iron (hydr)oxide into ironstone, in particular in the outcrop areas of Hageland and Zuiderkempen. Glauconite content varies from 25% to 60%. The coarse beds often contain a subpopulation of 0.5 to 2 mm (sub)angular quartz grains. In the vertical direction, grain size is either constant or coarsening upwards. The sand shows various primary structures: large and small-scale cross-bedding, massive sand, spaced planar lamination, displaying varying intensities of bioturbation. The cross-bedded facies may also contain isolated or bundled clay laminae. In the outcrop area, macrofossils are only known from a small number of localities, and they exist solely in the form of casts. In the shallow and deeper subsurface of the Antwerp and the Antwerp Campine areas, calcareous macro- and microfossils are locally abundant.
The subdivision of the Diest Formation into members still necessitates further study (Houthuys et al. 2020; Goolaerts et al., 2020). The present views lead to maintaining the Dessel Member and Deurne Member and introducing the Hageland Diest Member, Kempen Diest Member and the Borsbeek Member.
The local facies “clayey Diest” and “Bosbeek/Opoeteren” (see location in Vandenberghe & Louwye, 2020, Fig. 3, added in annex 2) need better descriptions of extent, correlations and depositional structures. Therefore, no formal members inside the Diest Formation are proposed for these facies now.
Age
Late Miocene: Tortonian (top part of DN8 biozone) – early Messinian (at least base of DN10 biozone) (Annex 2).
Thickness
General evolution from less than 10 m in the west and southwest of its extent to almost 200 m in NE Belgium inside the Roer Valley Graben. Important thickness variations in Hageland and Zuiderkempen are related to the incised nature of the formation base. The thickness reaches more than 100 m in the centre of some of those incisions.
Area of occurrence
See Annex 1. Outcrop area from Brussels to Leuven and Hageland in Vlaams-Brabant, from Antwerpen to Zuiderkempen in the province of Antwerpen, neighbouring area in western Limburg. Subcrop area to north and northeast of the outcrop area and continuing into SE-Netherlands.
Type locality
The type locality is Diest, where exposures were available at the former town fortress. In the wide neighbourhood of Diest, exposures are temporarily offered at construction sites. The exposures represent only the Hageland Diest Member. A key permanent outcrop is present at the Kesselberg in Leuven.
No type borehole has been selected. The variation in available descriptions and geophysical log signatures reflects the geographical variation inherent of this formation.
Alternative names
disused names: Sables et grès de Diest à Terebratula perforata, Sable diestien, Diestiaanzand
Authors
Houthuys, R., Adriaens, R., Goolaerts, S., Laga, P., Louwye, S., Matthijs, J., Vandenberghe, N. & Verhaegen, J.
Date
01/09/2023
Cite as
Houthuys, R., Adriaens, R., Goolaerts, S., Laga, P., Louwye, S., Matthijs, J., Vandenberghe, N. & Verhaegen, J., 2023. The Diest Formation, 01/09/2023. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Diest-Formation
