The Gentbrugge Formation (Dutch: Formatie van Gentbrugge, abbreviation: Ge; named after the town of Gentbrugge in East-Flanders) is a geologic formation in the west of Belgium. The formation crops out in East- and West-Flanders and also occurs in the subsurface of the province of Antwerpen. It consists of marine clay, silt and sand, deposited in the shallow sea that covered northern Belgium during the Ypresian age (around 50 million years ago, part of the early Eocene).
The Gentbrugge Formation reaches its greatest thickness in the north of Belgium, where it can be maximally 50 meters thick. It is subdivided into three members. The base of the formation is formed by silty clay and clayey silt (Merelbeke Member). On top of this are laminae of silt (Pittem Member) and beds of very fine sand, disturbed by bioturbation (Vlierzele Member). The sands can have horizontal bedding as well as cross bedding, and are often lithified into sandstone.
The Gentbrugge Formation is part of the Ieper Group and is stratigraphically on top of the older Tielt Formation (marine silt and sand), another formation of the same group. On top of the Gentbrugge Formation are deposits of the Lutetian Zenne Group. In the north of Belgium this is often the Aalter Formation (marine clays and sands). In the south the Aalter Formation often lacks and the younger Brussel Formation (marine sand and marl) and Lede Formation (marine calcareous sand) can be found directly on top of the Gentbrugge Formation.