FISSION-TRACK DATING OF THE PUNTA DE VACAS GLACIATION IN THE RIO-MENDOZA VALLEY, ARGENTINA

Fission-track dating of three tephra samples related to mappable drift s and nonglacial sediments, including alluvium, in the Rio Mendoza val ley, Argentina, places limiting ages on the oldest two glacial events. A tephra-layer interstratified with fan alluvium that surrounds and c uts the outermost Uspallata moraine has an age of 170,000 +/- 50,000 y r, implying that the drift predates marine oxygen isotope stage (OIS) 6. A tephra dated at 134,000 +/- 32,000 yr, that was deposited on allu vial fan sediments, underlies Punta de Vacas drift, which is inferred to equate with Isotope Stage 6. The Punta de Vacas outwash, which like ly correlates with the Rio Colorado drift, overlies another tephra uni t that dates to 260,000 +/- 150,000 yr. Although the error limits of t he dates preclude definitive correlations, all three tephra units may have been deposited during an interval prior to the maximum advance of the Punta de Vacas glaciation during OIS 6 or maybe in the Stage 7 in terglacial period. (C) 1998 Elsevier Science Ltd. All rigts reserved.