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Upper Cenozoic chronostratigraphy of the southwestern Amazon Basin

¹ Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA
² New Mexico Bureau of Mines and Mineral Resources, 801 Leroy Place, Socorro, New Mexico 87801, USA
³ Johnson County Community College, Overland Park, Kansas 66210, USA
⁴ Instituto Geológico, Minero y Metalúrgico (INGEMMET), San Borja, Apartado 889, Lima 41, Peru
⁵ Department of Geology, Occidental College, Los Angeles, California 90041, USA

The lack of numerical age dates for upper Cenozoic strata of the Amazon Basin has prevented resolution of its geologic history and accurate dating of important paleofaunas. Here we present results of magnetostratigraphy and ⁴⁰Ar/³⁹Ar dating of two volcanic ash deposits from the Madre de Dios Formation of eastern Peru. The two ash ages, 9.01 ± 0.28 Ma and 3.12 ± 0.02 Ma, provide the first numerical age data necessary for accurate interpretation of late Tertiary sedimentation in Amazonia and establish approximate time constraints for the last major cycle of Cenozoic deposition within the southwestern Amazon Basin. The older ash age also provides a minimum age for numerous Amazonian paleofaunas, which allows a more definitive correlation of these paleofaunas with those in other regions of South America.

Key Words: Amazon Basin • argon-argon • chronostratigraphy • magnetostratigraphy • Miocene • Peru

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