The Kortrijk Formation; abbreviation: Ko; named after the West-Flemish city of Kortrijk) is a geologic formation in the Belgian subsurface. This formation crops out in northern Hainaut, southern West- and East-Flanders and in Walloon Brabant. The formation consists of marine clay from the Ypresian age (early Eocene, about 52 million years old).
The Kortrijk Formation consists predominantly of clay, sometimes sandy or silty. The formation generally becomes sandier to the east, in Brabant and the Campine area. In some places fossils or bioturbation occur. In the westernmost part of Belgium it can be 125 meters thick, but it gradually wedges out to the east.
The Kortrijk Formation is subdivided into at least four members: the Mont Héribu Member, Orchies or Saint-Maur Member, the Moen or Roubaix Member and the Aalbeke Member. The Kortrijk Formation forms the lowest part of the Ieper Group and is stratigraphically overlain by the younger Tielt Formation (late Ypresian marine sand), part of th same group. The Kortrijk Formation lies on top of late Paleocene formations like the Tienen Formation or Hannut Formation, both part of the Landen Group.
- Laga, P.; Louwye, S. & Geets, S. (eds.); 2001: Paleogene and Neogene lithostratigraphic units (Belgium), Geologica Belgica 4(1-2), p. 135-152.