SHRIMP U?Pb ZIRCON DATING OF THE TRIASSIC PUESTO VIEJO GROUP, ARGENTINA, AND CORRELATIONS ACROSS SOUTHERN GONDWANA

OTTONE, EDUARDO GUILLERMO; MONTI MARIANA; MARSICANO, CLAUDIA; DE LA FUENTE, MARCELO; MAXIMILIANO NAIPAUER; RICHARD ARMSTRONG; MANCUSO, ADRIANA CECILIA

Mendoza

Congreso; 4° International Palaentontological Congress; 2014

IPA

The Puesto Viejo Group crops out in the San Rafael Block, southwest of San Rafael city, Mendoza, Argentina at approximately 35°S and 68°20?W. It consists of the basal mainly grayish Quebrada de los Fósiles Formation overlying by the reddish Río Seco de la Quebrada Formation. The basal unit includes both plant remains (pleuromeians and sphenopsids) and vertebrates (scattered fish scales, dicynodont synapsids and an archosaur). In contrast, the Río Seco de la Quebrada 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 assemblage zone of South Africa and thus referred to the Anisian. During recent work in the Puesto Viejo Group succession, a SHRIMP 238U/206Pb age in zircons of 235.8 ± 2.0 Ma was obtained from a rhyolitic ignimbrite interdigitated between the Quebrada de los Fósiles and the Río Seco de la Quebrada 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 Río Seco de la Quebrada beds developed during the Late Triassic (early Carnian), thus ca. 10 Ma later than the age attributed to the Cynognathus zone of South Africa. Two scenarios might explain our results. First, the Cynognathus zone of South Africa is wrongly assigned to the lower Middle Triassic (Anisan) and should be considered instead younger in age, Late Triassic (Carnian). Second, the relative age of the Cynognathus zone 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 poses serious doubts about the validity of biostratigraphic correlations based solely on tetrapod taxa, a common practice for Triassic continental successions across Gondwana.