The Tienen Formation (Dutch: Formatie van Tienen; French: Formation de Tienen; abbreviation: Ti) is a geologic formation in the subsurface of northern Belgium.
It has a late Thanetian (Paleocene) to early Ypresian (Eocene) age.
The Tienen Formation consists of clay with thin lignite layers and lithified wood, sand (sometimes with shells) and marl.
The formation is a lagerstätte for fossils of vertebrate animals.
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