Age constraints for the Triassic Puesto Viejo Group (San Rafael depocenter, Argentina): SHRIMP U?Pb zircon dating and correlations across southern Gondwana

EDUARDO G. OTTONE ; MARIANA MONTI ; CLAUDIA MARSICANO; MARCELO DE LA FUENTE; MAXIMILIANO NAIPAUER; RICHARD ARMSTRONG; ADRIANA MANCUSO

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 vol. 56 p. 186 - 199

0895-9811

The Puesto Viejo Group crops out in the San Rafael Block, southwest Mendoza, at approximately 35° S and 68°20? W. It consists of the basal mainly grayish Quebrada de los Fósiles 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 an archosaur). In contrast, the RSQ beds have yielded only vertebrates, although a more diverse fauna. It includes cynodonts as Cynognathus, Pascualognathus and Diademodon, and also dicynodonts (Vinceria and Kannemeyeria). Due to the tetrapod content the bearing levels were correlated to the Cynognathus AZ of South Africa and thus referred to the Anisian. A SHRIMP 238U/206Pb age of 235.8 ± 2.0 Ma was obtained from a rhyolitic ignimbrite interdigitated between the QF and RSQ formations at the Quebrada de los Fósiles section. This new radio-isotopic age for the Puesto Viejo Group suggests that the tetrapod fauna in the RSQ beds was developed, instead, during the Late Triassic (early Carnian) thus ca 10 Ma later than the age attributed based only on biostratigraphic correlations. 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.