Dark-grey, green and brown, slightly calcareous clays, with an intercalated, glauconitic sand to sandstone body, which grades distally (north and northwest) into a marly unit. The lowermost part of the formation is characterised by tuffaceous clays and silts and is fine-grained (63-210 µm) sandy in a proximal position (mid and southern part NL).
Marine deposit, with a transgressive basal part (reaching south of Paris). Mainly inner to outer neritic conditions. Max. waterdepth 200 m. Estuarine conditions are interpreted for the Brussels Sand Member when the North Sea basin was still in connection with the Paris Basin (Houthuys, 1990). Possible continental setting in the southwest of the Netherlands onshore.
Generally, the lowermost boundary is characterised by a sharp transition from the clay of the Liessel Member to the sandy or tuffaceous base of the Dongen Formation. The contact is conformable to mildly unconformable. In the southern Netherlands, where the underlying Landen Formation has a regressive sandy top, the boundary is less clear. In the easternmost parts of the provinces of Gelderland and Overijssel, the formation unconformably overlies Mesozoic deposits.
In areas where the succession is virtually complete, the upper boundary of the formation is marked by the usually clear transition from its upper argillaceous part to the unconformably overlying sands of the Berg Member of the Rupel Formation. Around the eroded ‘Southern Early Tertiary High’ and its northwestern extension into the North Sea, an unconformable contact with the overlying formations exists. Locally clay may be absent at the top of the unit and calcareous sand is covered by slightly or non-calcareous sand of the Rupel Formation. In those areas where the Berg Member is absent, the Boom Member rests directly on the clays of the Dongen Formation. In that case, determination of the boundary on lithological grounds may be difficult. However, on wire-line logs the Boom Member shows a somewhat higher gamma-ray response compared to clay of the Lower North Sea group. Biostratigraphical analysis is commonly applied to verify the transition.