The Hannut Formation is a geologic formation in the subsurface of northern Belgium. The formation consists of marine clay and silt, alternating with moer sandy layers. On top of this the lithology changes to limestone, siltstone and sandstone and the top of the formation is formed by a layer of glauconite bearing sand. The Hannut Formation was formed during the early to middle Thanetian age (Late Paleocene, about 57 million years ago). The formation is named after the town of Hannut in the province of Liège.