A new fossil mammal assemblage from the southern Chilean Andes: implications for geology, geochronology, and tectonics

A diverse (36 taxa), new fossil terrestrial mammal assemblage has been recovered from the Santacrucian South American Land Mammal ‘Age’ (SALMA; latest Early Miocene) in the southern Andes of Chile. This is the westernmost high latitude mammal fauna known in South America and the first in a string of new mammal assemblages discovered in Chile after a lapse of nearly a century. The terrestrial mammal-bearing sequence conformably overlies a marine section of Late Oligocene to Early Miocene age. The combined marine–terrestrial sequence, as well as a locality with fossil whales and bracketing basalts, bear significantly on theories regarding the extent of the late Tertiary Patagonian epicontinental seaway and the onset of later Cenozoic phases of uplift in the southern Andes. Uplift in this region likely began by Santacrucian SALMA (∼16–17.5 Ma) time, but it remains uncertain whether this occurred in two phases (Pehuenchic and Quechuic) or one. These discoveries substantiate propositions of sharp geologic contrasts north and south of the Lago General Carrera/Lago Buenos Aires area (Magellanes basin to the south and Rı́o Mayo embayment to the north). Minimum estimates of uplift rate are approximately 0.05–0.07 mm/yr (but as high as 0.22 mm/yr), comparable to or slightly lower than those from other parts of the Andes (e.g. Bolivia). The timing and location of uplift may be correlated with major plate tectonic events associated with the Chile Margin Triple Junction.