Clay that becomes more silty towards base and top (Vis et al. 2016). Rich in pyrite, poor in glauconite and calcium carbonate tends to be concentrated in the septaria layers; their presence is speculative for most of the Dutch subsurface. The silt content does not only change towards the top and the bottom of the member. Detailed studies in the Boom Clay in Belgium initiated by Vandenberghe et al. (2014) have shown that silt and clay layers alternate at a decimetre to metre scale. Moreover, the organic-matter content is highly variable and distinct bituminous layers are present. Large intervals are practically devoid of calcareous microfossils. In areas relatively close to the basin margin, the clay can be subdivided into three parts. The lower part of the clay is silty and has a blue-grey colour. Higher in the succession a great number of bituminous bands is intercalated and the colour of the clay changes to dark green-grey, dark-brown or even black. The dark clays, which stand out on gamma-ray logs, are overlain by green-grey to green clays that are more marly and slightly more silty.
Middle to outer neritic marine setting. The foraminiferal content suggests that anaerobic conditions intermittently prevailed at the sea bottom.
Conformably overlies the sandy Berg Member (previously known as Vessem Member) in most parts of the Netherlands. In places where the Berg Member is absent, the clays rest directly on the Asse or Engelsche Hoek members, which complicates definition of the boundary. However, on wire-line logs the Boom clays show somewhat higher gamma-ray response compared to the clays of the Lower North Sea Group.
Conformably overlain by the sandy deposits of the Steensel Member in the southeastern Netherlands. When the sandy member is missing, e.g. in part of the northeastern offshore, the transition with the overlying argillaceous Veldhoven Formation is unconformable. Where early Miocene erosion occurred, an unconformable contact exists with the overlying Breda Formation.