Résumé
The long Grace-Autoroute section, studied in 1994 during the excavation ofthe A-16 Paris-Amiens-Boulogne motorway trench, overlies an alluvial formation representing the oldest term of the Quaternary stepped terrace system of the River Somme valley (Alluvial Formation X or Grace-Autoroute, relative altitude: + 55m). This fluvial formation, dated by ESR and ESR-U/Th from 0.9 to 1 Ma, is covered by a slope sequence exposed by road work over approximately 250 m in length and corresponding to a cumulative thickness of more than 20 m. The latter is made up of a succession of lenticular slope sub-sequences (silty sands then loess), separated by erosion discordances, and corresponding to seven successive glacial-interglacial cycles, each terminated by a brown leached palaeosol horizon (soil Grace I to VII). The pedosedimentary cyclostratigraphic approach, associated with the ESR ages obtained from the underlying alluvial sequence, makes it possible to propose a correlation with global palaeoclimatic records and the allocation of the first slope deposits to the beginning of the Cromerian Complex (Marine Isotope Stage (MIS) 18). On this site, as in the rest of the Somme valley, it is the sandy Tertiary sedimentary stock (Paleocene, Eocene) which, reworked by runoff and (or) aeolian processes, provided most of the materials forming the slope sequence; the real loess (allochthonous) only appearing during the penultimate cycle (MIS 6). The record of the Grace-Autoroute slope sequence therefore constitutes the longest sub-continuous record of the Middle Pleistocene to date both in northern France and Western Europe. It allows to propose solid correlations with the other long sequences of north-west Europe like St. Pierre-les-Elbeuf, Kesselt, Achenheim or Karlich, and to confirm the great homogeneity of the loess-paleosol sequences and their value as recorders of climatic variations in the continental domain. The Grace-Autoroute sequence is therefore proposed as a new reference record for the period between MIS 18 and 5 (> 600 ka) in Western Europe.