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Author: Jacobs (1988).

Description: marine deposit, consisting of a succession of sharp based clays, gradually coarsening upwards into sands. The formation starts with grey, glauconiferous, fine sand, growing more clayey at the top and covered by a strongly glauconiferous clay, with locally coarse glauconitic sand at the base. A homogeneous, less glauconiferous, grey blue, heavy clay is followed by a dark grey, silty fine sand, glauconiferous and micaceous. This sand is covered by a grey blue, heavy clay, which passes into a dark grey, silty fine sand. The formation ends by a grey blue, heavy clay, with perforations at the top, filled by humic, grey, fine sand and a lot of organic material, indicating a soil development or a gap in the sedimentation.

Stratotype: stratotypes have only been designated for the members.

Area: the formation outcrops in a part of the north of West- and East-Flanders and in the region between the Dender and Zenne rivers. It occurs in the subsoil in the whole northern part of West- and East-Flanders and disappears to the east in the Antwerpse Kempen; it forms outliers in the Brabant and South Flanders hillrows.

Thickness: 50 m in the northeast, but only a few meters in the southern outliers.

Members: the formation is subdivided into the Wemmel Mbr, the Asse Mbr, the Ursel Mbr, the Onderdale Mbr, the Zomergem Mbr, the Buisputten Mbr and the Onderdijke Mbr.

Age: Late Lutetian and Bartonian.

Remarks: the formation is also discussed by Jacobs (1975, 1978, 1998), Gulinck (1965b, 1969), Leriche (1921), Rutot (1882), Vincent & Lefevre (1872) and Vincent & Rutot (1878).