Distinguishing the Miocene Kiel and Antwerpen Members (Berchem Formation) and their characteristic horizons using cone penetration tests in Antwerp (northern Belgium)

Abstract

The glauconitic sands in the upper part of the lower-middle Miocene Berchem Formation are subdivided into the Kiel and Antwerpen Members. Although lithological differences between both members are well known from temporary outcrops in the Antwerp city area, they are difficult to distinguish in boreholes, which hinders regional mapping of these units. In this study, we investigate whether both members can be distinguished on cone penetration tests (CPTs). For this purpose, we correlated multiple outcrops-in which the Kiel and/or Antwerpen Members have been identified-with nearby CPTs. On the CPTs, the boundary between the Kiel and Antwerpen Members is clearly identifiable as it coincides with an abrupt upwards decrease in cone resistance (q c ). The lower q c of the basal part of the Antwerpen Member is probably related to the finer grain size with more clayey admixture compared to the underlying Kiel Member. This change to a finer grain size is caused by a decrease in depositional energy and sedimentation rates as the region was transgressed during the eustatic sea-level rise at the start of the Mid-Miocene Climatic Optimum. On the CPTs, several spikes in q c values were observed within the Antwerpen and Kiel Members. These spikes could be correlated to shell beds, three horizons with sandstones and possibly a hardground. The sandstones appear to be discontinuous, whereas some of the shell beds could be traced across the entire study area. Most shell beds probably represent storm deposits within an otherwise relatively low energetic sedimentary environment. A phosphatic shell bed above the base of the Antwerpen Member is interpreted as the maximum flooding surface, lying in a zone with the lowest q c values for the Antwerpen Member, which might reflect maximum fining. The shell beds and interlayered sands of the Antwerpen Member thin in a southern direction, indicating reduced accommodation space in this direction.

Cone Penetration Test characterization of middle and upper Miocene lithostratigraphic units near Antwerp International Airport

“…On this CPT, the q c spike was not observed, which may indicate discontinuities in the sandstone horizon. Similar lateral discontinuities in the q c spikes were also observed for Kiel Member sandstone-bearing horizons in the City of Antwerp area by Deckers & Everaert (2022).…”

“…4). Based on the data presented by Deckers & Everaert (2022) for the city of Antwerp area, this interval may correlate to the Kiel Member of the Berchem Formation. Confirmation for this interpretation, at least for the upper part of the interval on correlation panel 1, is sustained by field observations from the AIA temporary outcrops.…”

“…This is then compared with field observations and laboratory analyses of these units of Goolaerts et al (2020). For the Kiel and Antwerpen members of the Berchem Formation, the CPT expression at AIA will also be compared to their CPT expression in the City of Antwerpen area as recently described by Deckers & Everaert (2022).…”

“…This allows for a detailed CPT characterization of the middle and upper Miocene sediments near AIA. This is then also compared and correlated with the results of a recently published CPT characterization of the Kiel and Antwerpen members of the lower to middle Miocene Berchem Formation in the City of Antwerp area by Deckers & Everaert (2022), further aiding to improve our understanding of all these deposits.…”

“…The third category holds the most potential as a dense network of CPTs exists, and their data can be easily harvested from the public DOV-database ('Databank Ondergrond Vlaanderen'; https:// www.dov.vlaanderen.be). In addition, the implementation of CPT data in scientific research on the Belgian Neogene reveals promising results Schiltz, 2020;Verhaegen et al, 2020;Deckers et al, 2021;Deckers & Everaert, 2022).…”