The Cretaceous (66,0 – 145,0 Ma, ICS v2013/01), whose name is meaning chalk, is a geological period of generally warm climate and high sea levels. The corresponding sediments in NW Europe are indeed mostly composed of chalk. Popularly it is best known as the end of the reign of dinosaurs because the extinction event, at the termination of the Cretaceous period, has been associated with the Chicxulub meteorite impact.
In Belgium, Cretaceous deposits have the largest known areal extent of any time unit. The Cretaceous transgression has covered even the Hautes Fagnes area, as shown by residual flint deposits. Deep weathering of the peneplains, as shown by the kaolinite deposits of the High Ardennes, and continental ‘wealdian’ deposits, which have yielded the renowned Iguanodons from Bernissart, precede the marine transgressions. These have stepped over preexisting morphological barriers such as the Brabant Massif, leading to predominantly carbonate deposition over extensive areas crossing the national boundaries. The Cretaceous deposits were formed after opening of the Atlantic Ocean and form part of the undeformed sequence continuing into the Cenozoic. The base Cretaceous is everywhere unconformably overlying older strata, ranging from Jurassic to Cambrian.
Good exposures are limited to the Meuse valley between Maastricht and Visé in the east and to the upper reaches of the Haine basin in the west. Large quarries extract the chalks and calcarenites, mainly for the cement industry, in these areas. They succeed to underground quarrying for building stones and phosphates, and more locally to silicite quarrying for millstones, from the Middle Ages up to the 20th century, whereas flint mining – both open pit and underground – dates back to the Neolithicum (e.g. the Spiennes World Heritage site). Cretaceous deposits are concealed over most of the intervening land and towards the north. Also these concealed Cretaceous deposits have economic value as they are either aquifers or aquicludes. Geological hazards related to the Cretaceous deposits include collapse of underground quarries, sinkhole formation into reactivated paleokarst developed on underlying Carboniferous limestones, and rock slides on greensand turned into clay.
The lithostratigraphy of the Cretaceous mainly reflects the successive transgressions and transition from a continental regime to a fully marine, rather monotonous chalk deposit, terminated by shallowing and a rather inconspicuous transition into Paleogene deposits. As a result, the Chalk Group encompasses deposits from Cretaceous and Paleogene (Danian) times.
