A and B: maps showing the location of the study area, between the Gris Nez Cape and the city of Wimereux. C: simplified lithostratigraphic log of the Late Jurassic formations cropping alongshore the Boulonnais (after Deconinck et al., 1996). FWB and SWB stand for fair-weather wave base and storm-weather wave base, respectively. P1 and P2 stand for the two horizons rich in phosphatized shells. D: geological setting of the Boulonnais area (after Mansy et al., 2007; Hatem et al., 2016).