A note of caution on the use of boulders for exposure dating of depositional surfaces.

Schmidt S., Hetzel R., Kuhlmann J., Mingorance F. & Ramos V.A.

Zusammenfassung

Exposure dating of boulders has been widely applied to determine the age of depositional surfaces under theassumption that the pre-depositional nuclide component in most boulders is negligible. Here we present acase study on fluvial terraces at the active mountain front of the eastern Andes, where this assumption isclearly invalid, because sandstone boulders (n=13) from terraces at two sites contain a highly variableinherited 10Be component and have apparent 10Be ages that exceed the age of the respective surface by up to~90 ka. Likewise, boulders from active stream channels (n=5) contain a substantial inherited 10Becomponent, equivalent to 5-48 ka of exposure. The age of the fluvial terraces is well determined by twoapproaches that allow to correct for the pre-depositional nuclide component: 10Be dating of amalgamatedpebbles and 10Be depth profiles on sand samples. At site 1, three terraces have 10Be ages of 3-5 ka (T2), 11-13 ka (T3), and 16-20 ka (T4), which are consistent with the terrace stratigraphy. The age of terrace T3 isconfirmed by a calibrated 14 C age of 12.61±0.20 ka BP obtained from a wood sample. At site 2, terrace T3 hasa 10Be age of 13-16 ka. The average inherited 10Be concentration of sand grains - determined from depthprofiles and stream sediments - is small and equivalent to 1-3 ka of exposure. In contrast, the meaninheritance of pebbles and boulders is higher and equivalent to exposure times of ~10 ka and ~30 ka,respectively. These differences in the pre-depositional nuclide component are related to the differentprovenance and transport history of sand, pebbles, and boulders. The sand is derived from rapidly erodingMiocene sediments exposed near the mountain front, whereas the pebbles and boulders originate fromTriassic sandstones in the internal part of the fold-and-thrust belt. On their way to the mountain front,boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked. Assediment deposition in intramontane basins and their subsequent excavation is common in the Andes andother fold-and-thrust belts, the presence of pre-depositional nuclide components should be evaluated whenapplying exposure dating at active mountain fronts.