Geomorphological and sedimentary records from coastal lagoon and beach-ridge systems may serve as valuable archives for the local reconstruction of key parameters influencing past shoreline configuration and trends in long-term coastal evolution. We here present observations on the sedimentology, chronology and surface properties of a modern swash-arrested macrotidal lagoon located on the northern shore of San Matías Gulf, NE Patagonia, Argentina. The study is based on remotely sensed data, GPS-RTK measurements, shallow vibracorings and a large number of samples dated using optically stimulated luminescence (OSL). Caleta de los Loros is composed of a tide-dominated, mature coastal lagoon, several sets of wave-built beach ridges, and a large field of transgressive parabolic dunes. The system is located in a morphological depression within an elongated stretch of cliffed shoreline composed of friable sand and gravel stones. Despite the prevalence of favorable conditions for the formation of prograded coastal deposits since the mid Holocene transgression, the OSL chronology established in this study yielded surprisingly young ages. Sedimentary successions in the distal part of the system formed prior to c. 2300 a BP, while the larger and more exposed western part of the system and the lagoon formed over the last c. 1000 a BP. A c. 4 km wide strandplain prograded rapidly between c. 1000 - 450 a BP, while most lagoonal deposits yielded younger, though subrecent ages between c. 600 – 70 a BP. The chronology and spatial arrangement of landforms evidence a large-scale switch-over in the directional component of sediment supply likely triggered by shifts in climate regime. The new observations fit well with recent paleoenvironmental and archaeological reconstructions from the area suggesting an association of the timing of local coastal development with global climate signals in the late Holocene. Even though we were not able to identify or benchmark the precise triggers of geomorphological change at Caleta de los Loros, our study presents a striking illustration of the potential importance of sensible environmental changes on the rapid and non-linear sedimentary development of a natural coastal system.