INTEGRATED ANALYSES OF BIOCHRONOLOGY AND RADIOISOTOPIC CHRONOLOGY IN CENOZOIC ANDEAN TERRESTRIAL SEQUENCES, AND A BRIEF UPDATE ON CALIBRATION OF SOUTH AMERICAN LAND MAMMAL "AGES" (SALMA)

More than 25 years of collaborative research in the Andes Main Range of Chile with R. Charrier, A. Wyss, D. Croft, P. Gans, geochronology graduate students J. Mosolf & T. Herriott and other scientists is creating one of the premier archives of temporally calibrated early-middle Cenozoic terrestrial mammals. Accumulation of thick volcaniclastic-rich sequences in the Andes fosters preservation of a unique record of mammalian evolution, and development of more precise and reliable terrestrial geochronologies integrating biochronology, magnetostratigraphy and high-precision radioisotopic dating (⁴⁰Ar/³⁹Ar, U-Pb). Since the last comprehensive summary of SALMA correlations in 1995, new results have refined Neogene SALMA correlations, but the greatest advances have been in beginning to calibrate previously poorly dated Paleogene intervals.

Intensive work in the Chilean Andes (particularly the Abanico Fm. and equivalents, 33-38.5ºS) has yielded >2,000 specimens from >2 dozen localities ranging in age from at least 40 to 10 Ma (late Eocene-late Miocene). Exemplar “case-studies” illustrate how these new fossils and dates provide key data for understanding mammalian evolution, documenting faunal change through time, assessing environmental transformations and biotic responses to climate change, and elucidating timing of Cenozoic Andean tectonic events. For example, the well-dated Tinguiririca Fauna (31.5-32 Ma) documented a new earliest Oligocene SALMA, and suggested faunal provinciality even by that early date. Paleoecological analyses indicate that relatively dry, open habitat, grassland/woodland environments flourished 15-20 million years earlier in South America than on other continents, probably related to climatic “deterioration” and associated paleoenvironmental events across the E/O boundary interval. More than 40 new ⁴⁰Ar/³⁹Ar and U-Pb dates in the Tinguiririca River valley further constrain fossiliferous strata ages and reveal new tectonic insights. Fossils from Laguna del Laja farther south span 5-6 SALMAs; together with ⁴⁰Ar/³⁹Ar ages, they significantly revise prior tectonic interpretations and document pronounced local faunal endemism, perhaps in response to global climate changes and regional tectonic events following the Paleogene-Neogene transition.