US Geological Survey


Date of this Version



The Journal Of Geology 1985, vol. 93, p. 223-250.


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 SE of La Paz, Bolivia, consist of more than 540 m of principally fluviatile clays and silts with numerous interbedded tuffs. Paleomagnetic samples were collected from 104 sites spaced at stratigraphic intervals of about 6 m. As a result of thermal demagnetization (at 400°Co r greater), unambiguous polarities were determined for 76 of the 104 originally sampled ites. 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 and zircon fission track ages of ca. 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 to 24 Ma. Pyrotherium and caviomorph rodents occur throughout the section; the primate Branisella is known to occur at about 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 Desea-dan should be shifted significantly upwards( by ca. 10 m.y.) to late Oligocene-early Miocene time.