Revised chronostratigraphic framework for the Cretaceous Magallanes-Austral Basin, Última Esperanza Province, Chile

Abstract The deposits of ancient sediment-routing systems in basins adjacent to the Andes offer key perspectives into the geologic evolution of South America, and can provide insight into basin-evolution-controlling mechanisms that operate on time scales spanning millions of years. The Andes and associated basins of southernmost South America are important archives of paleogeographic evolution in Patagonia, a region that underwent significant geologic and paleoenvironmental change during the Mesozoic and Cenozoic eras. Here, we present an updated chronostratigraphic framework for Upper Cretaceous units (Punta Barrosa, Cerro Toro, Tres Pasos, Dorotea Formations) in the Magallanes-Austral retroarc foreland basin in Ultima Esperanza Province, Chile, to assist workers with deciphering the geologic and paleoenvironmental evolution of southernmost South America. The framework combines stratigraphic information from over 60 years of research in the basin with a large suite (N = 51) of U-Pb volcanic ash and detrital zircon depositional ages, which includes new ages (N = 13) from outcrop exposures near Laguna Sofia, Chile, as well as weighted mean strontium isotope depositional ages (N = 3) from in-situ inoceramid shells. Results show that each formation contains stratigraphic architecture that is related to distinct depositional environments, all of which can be temporally constrained using the available data. The basin fill succession records deposition associated with: (1) largely unconfined submarine fan systems (Punta Barrosa Formation; 101.0 ± 2.2 Ma to 89.5 ± 1.9 Ma); (2) abyssal plain systems characterized by coarse-grained sediment starvation (lower Cerro Toro Formation; 89.5 ± 1.9 Ma to 84.7 ± 1.3 Ma); (3) conglomerate-rich deep-water channel-levee systems (upper Cerro Toro Formation; 84.7 ± 1.3 Ma to 80.5 ± 0.3 Ma); (4) a slope system characterized by widespread submarine mass-wasting (lower Tres Pasos Formation; 80.5 ± 0.3 Ma to 77.4 ± 1.1 Ma); and (5) a shelf margin system characterized by shelf-slope clinoform propagation (upper Tres Pasos and Dorotea Formations; 77.4 ± 1.1 Ma to 67.1 ± 1.8 Ma). Using the temporal constraints, we provide recommendations for use of specific ages associated with each architectural interval to facilitate future basin-analysis studies in the region.