The "Tuffeau" de Saint-Omer is a geologic formation in northern France. The sandstones of the formation, named after Saint-Omer, preserve bird and primate fossils dating back to the middle Thanetian age of the Paleocene epoch of the Paleogene period, dating to about 58 Ma. The formation is locally referred to and geologically known as tuff, although the lithology of the formation is glauconitic sandstone, deposited in a shallow marine environment at the southernmost edge of the .