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The Tongeren Formation (Dutch: Formatie van Tongeren; abbreviation: TO) is a geologic formation in the subsurface of the Netherlands. The formation consists of shallow marine, epicontinental and continental sediments, predominantly clay and sand from the late Eocene and early Oligocene epochs (between 37 and 30 million years old). The Tongeren Formation is part of the Middle North Sea Group and correlates with the Tongeren Group from Belgian stratigraphy.
The name was introduced by Belgian geologist André Hubert Dumont in 1849.