Abstract

The incision and aggradation of the Pativilca alluvial fan delta system in the western Peruvian Andes through Quaternary time can be traced in detail using well-exposed fill terraces studied by a combination of cosmogenic nuclide dating, terrace mapping and paleo-erosion rate calculations. Two alluvial terraces have been dated through depth-profile exposure dating using in-situ ¹⁰Be. The dating results return an age for the abandonment of the terrace at 200 ± 90 ka in Pativilca and 1.2 Ma ± 0.3 Ma in Barranca. These new ages complete the database of previously dated terrace fills in the valley. Together with the results of the terrace mapping and the absolute ages of the terraces, we show that the valley fills are made up of at least four terraces; two terraces near the city of Pativilca and two terraces in the city of Barranca. While previous studies have shown two periods of sediment aggradation, one period around 100 ka (Barranca) and another period around 30 ka (Pativilca), our new results show two additional periods of sediment aggradation and subsequent incision that have not been reported before. Finally, paleo-erosion rates at the time of the deposition of the terrace material were calculated and compared to the available modern estimates. The paleo-erosion rates vary from 140 ± 12 m Ma^-1 to 390 ± 40 m Ma^-1. The period of sediment accumulation prior to the abandonment of the terrace at 200 ka corresponds to a wet phase and a pulse of erosion. In contrast, the period of sediment accumulation prior to the abandonment of the terrace at 1.2 Ma does not correspond to a pulse of erosion and could rather correspond to a change of the base level possibly induced by a sea-level rise.