During recent years nearly 500 samples from boreholes drilled in West Flanders have been analyzed (figs 1-3). These samples appeared to contain sufficient amounts of bioclasts (1-2.4 mm) to allow a subdivision into ecozones, which formed the basis of a lithostratigraphic subdivision (fig. 4). The bioclast assemblages indicate that these boreholes are situated outside the depositional area of the Campine-Limburg basin (Maastricht-Hasselt-Antwerp). That is the reason why Arabic numerals have been used in the ecozonation, in distinction with the ecozonation of the Campine-Limburg basin (Roman numerals, fig.5). In the boreholes examined Cretaceous rests directly either on Cambro-Silurian of the Brabant Massif or on the Devono-Carboniferous of the Tournai Basin (Namur Synclinorium), north of the Bordière Fault. A limited number of samples from the Mons basin (Fig.6) appear to indicate that the West Flanders samples are more closely correspond with strata of that basin. The Nevele and Dendermonde boreholes (East Flanders) cannot directly be tied in with the ecozones. The Nevele samples may belong to ecozones 4 and 5. The Dendermonde borehole is characterized by a different bioclast assemblage and thus cannot be correlated with boreholes.Bioclast cannot be used for age determination of strata, which is why other methods are necessary. In the present case we have relied on the studies carried out by F. Robaszynski 1989 and S. Louwye 1993.