The European land mammal age, a continental biostratigraphic zonation for the Cenozoic, as ammonites for the Mesozoic and conodonts for the Paleozoic, classification starts after this age, the Thanetian is correlative with the MP2 to 5 of the Mammal Paleogene zone of Europe.

At this time in geologic history the climate was at an all-time high with estimated tropical temperatures of 34 °C (93 °F) and 2000 ppm atmospheric CO2.

The Tuffeau de Saint-Omer is described as a glauconitic well-sorted sandstone, with opal cement and the invertebrate fossils Pholadomya cuneata, P. konincki, Cyprina morrisi, Thracia prestwichi, Natica deshayesiana and Martesia cuneata.

The Tuffeau is exposed near Molinghem and Doulac, north of Saint-Omer, after which the formation is named.

Despite the siliciclastic lithology, the formation is locally referred to as "tuff"; volcaniclastic rock.