From the Adour valley to the Gelise valley (western part of the Gers department): an important occurrence of siliceous raw material of Senonian origin

This study refers the western part of the “Gers department” which covers the Adour valley, its tributaries (Izaute, Midour and Douze) and the Gélise valley. A geological study was carried out from 2010 to 2012. It allowed us to collect 290 alluvial siliceous pebbles from the Plio-Pleistocene sediments. At the same time, 132 flint artefacts from prehistoric sources in the Gers (Upper Palaeolithic) were collected. Petrographic, sedimentological and palaeontological studies were carried out. They allowed us to distinguish 19 lithological types:- 1 related to the Cenozoic formation;- 2 associated with Flysch deposits from the Pyrenean orogenic belt;- 10 attributed to the Senonian age (from the Coniacian to the Campanian);- 6 imputed to the Upper Maastrichtian with Lepidorbitoides.Then these types were brought together into six groups that show their stratigraphic positions and their palaeogeographic environments at different time periods.- 0.2 % of the alluvial pebbles were related to the Pyrenean Flysch;- 45 % were of Upper Maastrichtian age (Lepidorbitoides);- 54 % were of Senonian age (Coniacian to Middle Maastrichtian).The artefacts collected in the same region show a very similar proportion of siliceous pebbles (49 % of Maastrichtian with Lepidorbitoides).The result of this study is that, up to now, the western part of the Lannemezan Plateau includes alluvial areas of Plio-Pleistocene age, containing a large amount of flint pebbles of Upper Cretaceous age (Senonian and Maastrichtian). The gathering of this siliceous raw material by Palaeolithic communities is obvious. The identification of a new lithic raw material located in the southern Aquitaine Basin will have to be taken into account, while former interpretations probably have to be amended.