Marine Cenozoic strata crop out in the western flank of the North Patagonian Andes at Lago Ranco (40°S) and Ayacara (42°S). These deposits, known as the Estratos de Lago Ranco and Ayacara formations, consist of rhythmic successions of sandstone and siltstone. In order to unravel the sedimentary environment, age, and tectonic history of this area during the Neogene, we conducted sedimentological, ichnological, and micropalaeontological studies. In addition, we dated detrital zircons by U–Pb (LAICPMS and SHRIMP). Our results show that these strata were deposited in a deep-marine environment during the early–middle Miocene. Our findings indicate that the uplift of the western flank of the North Patagonian Andes took place after the early–middle Miocene.