Dark greenish-grey and blue-grey, plastic clay. Locally indications of bioturbation, and glauconitic (in particular at the base) and some micaceous. Generally, it is slightly to non calcareous. Notably the upper part of the member is sandy and free of calcium carbonate in a proximal position. In the eastern Netherlands, close to the palaeo-coastline, this part is very sandy, has a brownish colour and contains lignite fragments. The border zone of the basin comprises shells and Nummulites at the base.
Open-marine unit, possibly deposited in rather shallow water (Water depth of Oosteind, Ieper and Asse: up to 200 m).
The lower boundary is located at the transition to the Brussels Sandstone Member or Brussels Marl Member. This is generally a clear break, but it may be somewhat gradual in places.
Where the succession is not truncated by later erosion, the Asse Member is in most of the Netherlands on- and offshore area, unconformably overlain by the Rupel Formation. Where the latter starts with the sandy Berg Member, as is the case in the central Netherlands, the upper boundary is well defined by the rather sharp transition to this overlying sandy unit. Where the Berg Member is absent, the Boom Member rests directly on the Asse or Engelschhoek members, complicating the definition of the upper boundary on lithological grounds. However, on wire-line logs, the Boom Member shows a somewhat higher gamma-ray response compared to the clays of the Lower North Sea Group. Biostratigraphic analysis is commonly applied to verify this transition.
