INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
A new Late Triassic age for the Puesto Viejo Group (San Rafael depocenter, Argentina): SHRIMP U?Pb zircon dating and biostratigraphic correlations across southern Gondwana.
OTTONE, E.G.; MONTI, M.; MARSICANO C.; DE LA FUENTE M.S.
JOURNAL OF SOUTH AMERICAN EARTH SCIENCES
PERGAMON-ELSEVIER SCIENCE LTD
Lugar: Amsterdam; Año: 2014 vol. 56 p. 186 - 186
The Puesto Viejo Group crops out in the San Rafael Block, southwest Mendoza, at approximately 35 S and 68200 W. It consists of the basal mainly grayish Quebrada de los Fosiles Formation (QF) overlying by the reddish Río Seco de la Quebrada Formation (RSQ). The basal unit includes both plant remains (pleuromeians and sphenopsids) and vertebrates (scattered ﬁsh scales, dicynodont synapsids and re-mains of an archosauriform). In contrast, the RSQ beds have yielded only tetrapods, although a more diverse fauna. The latter includes cynodonts as Cynognathus, Pascualognathus and Diademodon, and also dicynodonts (Vinceria and Kannemeyeria). Based on the assemblage of tetrapod taxa the bearing levels were correlated to the Cynognathus AZ of South Africa and thus referred to the Middle Triassic (Anisian). We obtained a SHRIMP 238U/206Pb age of 235.8 ± 2.0 Ma from a rhyolitic ignimbrite interdigitated be-tween the QF and RSQ formations at the Quebrada de los Fosiles section. This new radiometric date for the Puesto Viejo Group suggests that the tetrapod fauna in the RSQ beds existed, instead, during the Late Triassic (early Carnian) some 10 Ma later than the currently accepted age. Two scenarios might explain our results: ﬁrst, the Cynognathus AZ of South Africa is wrongly assigned to the lower Middle Triassic (Anisan) and should be considered younger in age, Late Triassic (Carnian); second, the relative age of the Cynognathus AZ of South Africa is correct but the inferred range of Cynognathus and Diademodon is incorrect as they were present during the Late Triassic (Carnian) at least in South America. In any case, this new date pose serious doubts about the validity of biostratigraphic correlations based solely on tetrapod taxa, a common practice for Triassic continental successions across Gondwana.