Optically Stimulated Luminescence and in situ 10Be / 26Al cosmogenic dating of the Upper and Lower Units from ‘La Falaise de la Mine d'Or’ at Pénestin (SW Brittany, France) within the cron-BRET Project.

The sedimentary units overlying the so-called ‘Falaise de la Mine d'Or’ on the South-East coast of Brittany (France) have been studied for decades to reconstruct the evolution of fossil fluvial valleys in Brittany during the Pliocene and Quaternary (Guillocheau et al., 1998; Menier et al., 2006). However, published numerical ages are insufficient to provide a precise age of each of the units described, whose chronology relies on correlations with ESR dating of fluvial sediments from the interior of Central Brittany (Laurent et al., 1996). Thanks to the cron-BRET Project of the MSCA-Bienvenüe Bretagne Programme carried out by the Geo-Ocean Laboratory of the Université de Bretagne Sud in collaboration with the Cosmogenic Nuclide Laboratory of the University of Cologne (Germany), it has been possible to date the lower unit (U1), mainly composed of quartzite gravels and pebbles. In situ ¹⁰Be and ²⁶Al concentrations produced within the quartz of these clasts become controlled by differential rates of decay when shielded from production at the surface (Dunai, 2010). The fact that the sediments are buried under a sedimentary shield of more than three metres, allows for the calculation of a burial age from the concentration of ¹⁰Be and ²⁶Al by using the isochron method (Balco and Rovey, 2008; Granger et al., 2022). Preliminary results provide numerical data that place the formation of this unit 2.72 ± 0.19 million years ago, at the Plio-Quaternary boundary. Our study also includes the dating of the upper unit (U3) mainly composed of sand-sized materials (90-2000 µm), by analysis of the optically stimulated luminescence (OSL) signal of quartz (Murray et al., 2021) at the RenDaL Luminescence Laboratory (Géosciences- Univ. Rennes). The calculation of the palaeodose using Bayesian procedures (BayLum; Philippe et al., 2019) and of the natural dose rate from high-resolution gamma spectrometry (HRGs) measurements provides a burial age range between 263 and 408 ky. These data will be complemented by the dating of the materials composing unit U2 by analysing the infrared stimulated luminescence signal (IRSL) of potassium feldspar to extend the available dates and the knowledge of the landscape evolution of this coastal area linked to glacioeustatic oscillations and neotectonics during the Pleistocene.