Magnetic Polarity Stratigraphy and Mammalian Fauna of the Deseadan (Late Oligocene-Early Miocene) Salla Beds of Northern Bolivia

Abstract

The Salla Beds contain a rich assemblage of Deseadan mammals that traditionally has been considered of early Oligocene age. These deposits, located 90-100 km southeast of La Paz, Bolivia, consist of > 540 m of principally fluviatile clays and silts with numerous interbedded tuffs. Paleomagnetic samples were collected from 104 sites spaced at stratigraphic intervals of .apprx. 6 m. As a result of thermal demagnetization (at 400.degree. C or greater), unambiguous polarities were determined for 76 of the 104 originally sampled sites. The pattern of reversals, stage of evolution of the fossils, and preliminary isotopic dates (including a K/Ar age of 26.4 .+-. 1.0 Ma [million yr] and zircon fission track ages of cs. 22 Ma) from interbedded tuffs suggest that the Salla Beds correlate to chrons C10 through C6 of the magnetic polarity time scale, or an absolute time interval from about 28.5-24 Ma. Pyrotherium and caviomorph rodents occur throughout the sections; the primate Branisella is known to occur at .apprx. 27 Ma. The Salla faunas seem to represent an earlier part of the Deseadan than faunas of this Land Mammal Age from Argentina. These data suggest that the early Deseadan should be shifted significantly upwards (by .apprx. 10 million yr) to late Oligocene-early Miocene time.