Mineral separates of five tuffs interbedded in the mammal-bearing Huayquerías and Tunuyán formations in Mendoza Province, west-central Argentina, were dated by the 40K-40Ar method. At the locality of Rio Seco Ultima Aguada on the east side the Meseta Guadal three biotite concentrates of a tuff from a level that apparently corresponds to the very top of the Huayquerias Formation (or less probably to the very bottom of the Tunuyan Formation, sensu Dessanti, 1946) yielded an average age of 5.8 ± 0.1 Ma. The boundary between the Huayquerias Formation (namesake and type formation and fauna of the Huayquerian Land Mammal Age) and the Tunuyan Formation (which contains a Montehermosan age fauna at this locality) is thus about 5.8 Ma. At the locality of Rio Seco de la Salada on the west side of the Meseta Guadal, glass concentrates from two tuffs in the upper part of the Tunuyan Formation yielded ages of 2.6 ± 0.1 and 2.4 ± 0.3 Ma, respectively (the latter date is from a tuff located 90 m stratigraphically above the former). These dates indicate that the rocks and faunas from the upper part of the Tunuyan Formation are of Chapadmalalan (i.e., 2.8 to 2.5 Ma) and Uquian (i.e., 2.5 to 1.5 Ma) age. The faunas from the lower and middle parts of the Tunuyan Formation on the east side of the Meseta Guadal are apparently all of Montehermosan age (i.e., ca. 6.0 to 2.8 Ma).
Kinematics of the west-central Walker Lane: Spatially and temporally variable rotations evident in the Late Miocene Stanislaus Group