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Science 25 March 1977:
Vol. 195. no. 4284, pp. 1325 - 1328
DOI: 10.1126/science.195.4284.1325
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Articles

South American Geochronology: Radiometric Time Scale for Middle to Late Tertiary Mammal-Bearing Horizons in Patagonia

LARRY G. MARSHALL 1, ROSENDO PASCUAL 2, GARNISS H. CURTIS 3, and ROBERT E. DRAKE 3

1 Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540
2 Facultad de Ciencias Naturales y Museo de La Plata, Paseo del Bosque, 1900 La Plata, Argentina
3 Department of Geology and Geophysics, University of California, Berkeley 94720

Radiometric (potassium-argon) age determinations for basalts and tuffs associated with middle to late Tertiary mammal-bearing horizons in Patagonia, southern Argentina, permit refinement of boundaries and hiatuses between beds of Deseadan (early Oligocene) through Friasian (middle to late Miocene) age. At two localities beds of Deseadan age are overlain by basalts, which gave dates of 33.6 and 35.4 million years ago; 34.0 million years ago is tentatively accepted as a terminal date for known Deseadan. At several localities beds of Colhuehuapian age are underlain by basalts, which gave dates ranging from 28.8 to 24.3 million years ago; 25.0 million years is tentatively taken as a basal age for known Colhuehuapian. The paleontological hiatus between known Deseadan and known Colhuehuapian is thus in the order of 9.0 million years. Two tuffs from the Santa Cruz Formation (Santacrucian) gave ages of 21.7 and 18.5 million years. Plagioclase and biotite concentrates of an ignimbrite from the Collón Curá Formation (Friasian) gave ages ranging from 15.4 to 14.0 million years.

Submitted on September 28, 1976
Revised on November 12, 1976


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
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