Granulometry, carbonate and glauconite content as stratigraphic tools to distinguish the Kiel Member and lower Antwerpen Member (Berchem Formation) in the City of Antwerp area (Belgium)

Abstract

Granulometric, carbonate and glauconite analyses were performed on samples of the Kiel and Antwerpen members (Berchem Formation) in temporary exposures and boreholes in the area of the City of Antwerp. Since the upper part of the Antwerpen Member is only locally preserved from post-depositional erosion, this study focuses on the much more frequently sampled lower part of the Antwerpen Member. The granulometric results show that the Kiel Member and lower part of the Antwerpen Member are both dominated by the fine sand fraction, with an overall finer grain size for the lower Antwerpen Member compared to the Kiel Member. The Kiel Member is relatively enriched in the fine and medium sand fractions, whereas the lower Antwerpen Member is relatively enriched in the very fine sand to silt fraction. Both members show a similar, low clay content. Measurements of clay content are higher in boreholes (on average 3.9–5.2%) than in temporary exposures (less than 1%), which might be explained by the crushing of the glauconite into the clay fraction during drilling. In borehole samples, especially those analysed by sieving, the Antwerpen Member often shows a higher amount of coarse grains than the Kiel Member, which represent shell fragments present in the Antwerpen Member and nearly absent in the Kiel Member. Like the amounts of shells, also the measured carbonate content—measured outside the shell beds—is lower for the Kiel Member than for the lower Antwerpen Member (3% vs 4.2%). The average glauconite content—based on the >63 µm fraction—displays similar, strong fluctuations between 35% and 60% for both members. On top of these fluctuations, a general downward decrease in glauconite content is noticed in the Kiel Member.