The Portlandian strata of the Bas Boulonnais, France

Abstract

The Marine Portlandian strata in the Bas Boulonnais are c. 20 m thick, comprising the argillaceous glauconitic Assises de Croi overlain by the calcareous quartzose of the Grès des Oies. These are succeeded by a thin ‘Purbeckien’ algal limestone, the Calcaire des Oies (1 m), overlain by ‘Wealdien’ clays and sands (10–20 m?). The Kimmeridgian/Portlandian boundary (sensu anglico) lies within the basal few metres of the Assises de Croi and not at the Tour de Croi Nodule Bed as previously thought. The lower four Portlandian ammonite zones (Albani to Kerberus) are now proven to be present up to the Middle Grès des Oies, enabling correlation with England. The ‘Purbeckien’ limestone and ‘Wealdien’ clays are interpreted as non-marine Portlandian deposits and the ‘Wealdien’ sands as Lower Cretaceous in age. The Portlandian strata comprise an overall regressive sequence deposited in environments ranging from middle neritic to lacustrine.