Title:

Magnetostratigraphy of Paleogene Deposits in Coastal San Jorge Basin and Implications for South American Land Mammal Ages

There were many climate events during the Paleogene period (ca. 66-23 Ma) that had a major impact on biodiversity, including the aftermath of the Cretaceous-Paleogene Extinction and the Paleocene-Eocene Thermal Maximum. There are few geological and paleontological records from this time in South America, resulting in poor understanding of the effects of these climate events there. The Paleogene South American Land Mammal Ages (SALMAs) are a temporal framework that provides relative ages for South American mammal fossils; however, they are poorly correlated to the geologic timescale. The coastal San Jorge Basin in Patagonia, Argentina is one region with well-preserved fossil assemblages, but few geochronological constraints. This study applies magnetostratigraphy to better constrain the age of the fossils in the coastal San Jorge Basin and their associated SALMAs. Oriented samples were taken from the Salamanca Formation, Las Violetas Formation, Peñas Coloradas Formation, and Las Flores Formation. Samples were measured for Natural Remnant Magnetization (NRM). These data were analyzed to determine Characteristic Remnant Magnetization (ChRM). Isothermal Remnant Magnetization (IRM) analyses were also applied to determine magnetic mineralogy. IRM results indicate the prevalence of both high and low coercivity minerals. A total of six normal and eight reverse zones are preserved across four sections; however, this sequence is not continuous due to unconformities. A tentative correlation shows the preservation of the C28n/C27r reversal through the C20n/C19r reversal, representing at least 21.3 Ma. This work provides additional geochronological constraints on the fossils of the coastal San Jorge Basin and therefore the Paleogene SALMAs, allowing them to be better understood in the context of the dramatic global climate changes that occurred during the Paleogene.

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