The information on this page is a summary description.
The full formal description is available here: Bolderberg Formation
Abbreviation
Bb
Parent unit
Child units
Houthalen Member, Genk Member, Heizel Facies
Lithological description
The lithology of the Bolderberg Formation consists in the eastern part of northern Belgium at the base of dark green, often clayey, medium fine-grained sandy unit, micaceous, slightly ligniferous and glauconitic (i.e. the Houhalen Member), grading into fine-grained to fairly coarse-grained sand with lignite and gravel layers (i.e. the Genk Member) (Louwye et al. 2020). The boundary between both facies is not sharp but gradual. The basal gravel layer (the Elsloo gravel) consists of rounded pebbles and shark teeth. A third lithofacies is encountered in the very eastern part of the Limburg Province, namely a medium-grained, white sandy unit holding a lignite seam (the Kikbeek lignite) and a quartzite layer, called the Opgrimbie Facies) (Gullentops, 1963, 1972-1973; Matthijs, 1999). The isolated Heizel Facies occurs west of Brussels and consists of yellow-brown, micaceous, fine-grained sand with thin clay layers (Gulinck, 1956, 1959).
Age
The Bolderberg Formation has a confirmed late Burdigalian to early Serravallian age through dinoflagellate cyst analysis (Deckers & Louwye, 2017).
Thickness
The thickness generally increases in northeastern direction across the Campine area and reaches around 80 m just to the west or in the footwall of the border fault system of the Roer Valley Graben (Deckers et al., 2019). The thickness further increases into the Roer Valley Graben where it reaches its maximum of 155 m in the Molenbeersel borehole (DOV kb18d49w-B225; GSB 049W0225; Figure 0‑2).
Area of occurrence
The Bolderberg Formation occurs in the eastern part of northern Belgium, in exposures in the hills around and to the south of Diest (Brabant province), and in the top of the hills west of Brussels (see Figure 0‑1).
Type locality
The type locality is the village of Bolderberg, about 10 km north of Hasselt (Limburg province). The type sections of the formation are the now disappeared outcrops (road cuttings) on the Bolderberg hill.
The Gruitrode borehole (DOV kb18d48w-B186; GSB 048W0185) is a type geophysical borehole with the Bolderberg Formation between 92 m and 178 m depth on top of the Voort Formation and below the Diest Formation, according to Louwye et al. (2020).
Alternative names
Authors
Deckers, J. & Louwye, S.
Date
01/09/2023
Cite as
Deckers, J. & Louwye, S., 2023. The Bolderberg Formation, 01/09/2023. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Bolderberg-Formation
