Magnetic Polarity Stratigraphy and \u3csup\u3e40\u3c/sup\u3eK-\u3csup\u3e40\u3c/sup\u3eAR Dating of Late Miocene and Early Pliocene Continental Deposits, Catamarca Province, NW Argentina

Magnetostratigraphic and 40K-40Ar data on a 2300 m thick sequence of continental sediments at Puerta de Corral Quemado in Catamarca Province, NW Argentina permit calibration of land mammal faunas of late Tertiary (Huayquerian and Montehermosan) age. The sequence represents (from oldest to youngest) the Chiquimil A, Araucanense, and Corral Quemado Formations. Paleomagnetic samples were collected from 99 stratigraphic levels. Strong-field thermomagnetic and isothermal remanent magnetization experiments indicate that the dominant ferrimagnetic mineral is magnetite. Progressive alternating-field (AF) and themal demagnetization of the natural remanent magnetism (NRM) demonstrates that AF demagnetization to 20 mT peak field is sufficient to isolate the primary NRM which is of depositional origin. The resulting paleomagnetic data provide a well-defined magnetic polarity zonation, although sampling is less dense in the upper half of the section. 40K-40Ar data obtained from mineral separates of four tuffs within the section allow reliable age determinations for those levels. The combined magnetostratigraphic and 40K-40Ar data allow the magnetic polarity zonation to be correlated with the magnetic polarity time scale. This correlation indicates a nearly constant rate of sediment accumulation between ~8.0 Ma and 3.5 Ma. The boundary between the Araucanense and Corral Quemado Formations approximates the boundary between the Huayquerian and Montehermosan land mammal age faunas at this locality. The data presented here allow the boundary between the Araucanense and Corral Quemado Formations to be dated at 6.4 Ma. Combined with geochronologic data from similar age rocks and faunas from San Carlos, Mendoza Province, west-central Argentina, the geochronologic data from Puerta de Corral Quemado allow the Huayquerian-Monteher-mosan land mammal age boundary to be placed tentatively at 6.0 Ma. A specimen of the fossil land mammal Cyonasua (family Procyonidae) from unit 14 of the Araucanense Formation is dated at 7.0 to 7.5 Ma. This specimen is the earliest known representative of this North American group in South America and represents the oldest dated participant in the Great American Faunal Interchange on that continent

Geochronology of Type Santacrucian (Middle Tertiary) Land Mammal Age, Patagonia, Argentina

Mammal-bearing lacustrine and tuffaceous sediments from three localities of the Santa Cruz Formation, type fauna of the Santacrucian Land Mammal Age, in Patagonia, southern Argentina, are calibrated by radioisotope dating with the aid of magnetostratigraphy. The strata range from about 17.6 Ma to perhaps 16.0 Ma, and are thus of late-early Miocene age. The Santacrucian Land Mammal Age ranges from about 18.0 Ma to about 15.0 Ma