Facies variability of the Lower Miocene Houthalen Member in the southern Genk municipality (northeastern Belgium)

Abstract

The Oligocene Eigenbilzen Formation and Miocene Bolderberg Formation were studied along the Albert Canal in the southern Genk municipality, eastern Belgium. Previous studies have subdivided the Eigenbilzen Formation into geophysical subunits based on borehole logs. Correlations of borehole logs with a deep Cone Penetration Test reveal that this subdivision is also geotechnically distinct. The Eigenbilzen Formation is unconformably overlain by the Houthalen Member (lower Bolderberg Formation), which holds a late Burdigalian age according to the dinoflagellate cyst analyses on borehole samples by this study. The average glauconite, organic and carbonate content of the Houthalen Member in the study area is 8.2%, 0.5% and 3%. Within the Houthalen Member, we distinguished two different geotechnical CPT and lithological facies, herein referred to as clayey and sandy facies. Granulometric analyses indicate that both facies are dominated by fine sands, however, with the sandy facies having an average 5.1% fine fraction (<63 µm), and the clayey facies 18.8%. The clayey facies is very distinct, whereas the sandy facies is geotechnically and granulometrically similar to the superjacent Genk Member (upper Bolderberg Formation). Besides carbonate content, the main difference between the sandy Houthalen Member and the Genk Member is the larger glauconite content of the first. As the glauconite content decreases upwards in the sandy Houthalen Member, it transitions into the Genk Member. Along the 3.6 km long correlation panel, the clayey Houthalen Member thins from over six meters in the southeast towards less than one meter in the northwest, which highlights the sedimentological complexity within the unit.