CHRONOLOGY OF DEPOSITION, UNCONFORMITY DEVELOPMENT, AND PALEONTOLOGY OF A CRETACEOUS-PALEOGENE BOUNDARY SITE, MAGALLANES-AUSTRAL BASIN, PATAGONIA

The Magallanes-Austral foreland basin records orogenesis and landscape evolution in the Patagonian Andes of Chile and Argentina. Throughout the retroarc foreland basin, a regional disconformity separates Upper Cretaceous-lower Paleocene strata from overlying diachronous Eocene-Miocene deposits. We present new data from a fossiliferous mixed marine/non-marine succession in the Río de las Chinas valley of Chile (50 – 51°S) to elucidate (1) the timing and duration of unconformity development using U-Pb maximum depositional age constraints, (2) sediment provenance and dispersal patterns in the basin, and (3) the amount of sedimentary overburden removed in this region during unconformity development. We also confirm the presence of a fossiliferous southern hemisphere Cretaceous-Paleogene (K/Pg) boundary site and report vertebrate fossils recovered from the Río de las Chinas valley.

Detrital zircon U-Pb geochronology yields maximum depositional ages for the Dorotea Formation spanning the Maastrichtian to Danian (ages as young as ~65 – 63 Ma), and maximum depositional ages for Man Aike Formation samples taken directly above the unconformity yield middle Eocene ages, indicating a long-lived ~20 Myr hiatus. Vitrinite reflectance data from samples obtained from multiple coal horizons within the uppermost Dorotea Formation show consistently low vitrinite reflectance values. This suggests limited sedimentary burial, which is consistent with nondeposition or sediment bypass rather than the deposition and later erosion of a previously proposed thick package of Paleocene – middle Eocene clastic material. Fossil discoveries at the site reveal a diverse Cretaceous paleofauna, including representatives of several dinosaur clades and marine reptiles. These findings provide important comparisons with fossil assemblages from both Northern Hemisphere sites and contemporaneous deposits in Argentina, as well as new insights into the biodiversity present in South America during the latest Cretaceous.