Predominantly carbonate rocks. The bulk comprises white, buff, cream and light-grey, hard, fine-grained, bioclastic limestones and marly limestones. Originally, these limestones had a more chalky nature, but as a result of deep burial they were compacted strongly and became denser. Locally, marls, calcareous claystones and glauconitic sands occur. Chert concretions, isolated or in layers parallel to the bedding, are common. Along the basin margin in the SE Netherlands coarser bioclastic limestones and sand tongues are present. Sand is also present near inversion structures (e.g. Central Graben, Peel Block).
Except in the SE Netherlands, entirely marine. Upper Cretaceous chalks are found over large areas of the earth's surface and reflect relatively stable and uniform conditions during a long period of time. Sedimentological features associated with downslope transport and abundant planktonic fossils suggest a certain minimum water depth, whereas the presence of light-dependent benthic organisms, such as Bryozoa, is indicative of a shallower environment. Ostracods with eyes, found throughout the sequence, indicate deposition within the photic zone. Most authors agree that, generally, the depositional depth of the Chalk sediments must have been between 50 and 300 m. The lenses of coarse, bioclastic limestones and tongues of glauconitic sandstone found along the basin edge (e.g. South Limburg) reflect deposition in a shallower marine environment. Hardgrounds, present throughout the sequence, are related to subrosion, non-deposition or abrasion caused by either sea-level fluctuations or small-scale tectonic uplift.
Base of the limestone succession on top of the (mainly red brown) marls of the Holland Formation. On wire-line logs this boundary is usually marked by a pronounced decrease in gamma-ray radiation and an increase in resistivity and sonic velocity.
Base of the sands, marls and clays of the Lower North Sea Group (Upper Paleocene) or, where this group is absent, of the Middle North Sea Group.
