Tetrapod tracks from the Vera Formation (Los Menucos Group, Río Negro province, Argentina) and their bearing on the chronostratigraphy of the Los Menucos Basin

PAOLO CITTON; CARLOS CÓNSOLE-GONELLA; IGNACIO DÍAZ-MARTÍNEZ; SILVINA DE VALAIS

Catania

Congreso; Congresso SGI SIMP Catania 2018; 2018

Società Geologica Italiana

The Los Menucos locality, in the north-western sector of the North Patagonian Massif (Río Negro province,Argentina), hosts a volcaniclastic succession mainly composed of dacitic to rhyolitic ignimbrites, mesosiliciclavas and subordinate sedimentary rocks made up mainly of sandstones, all pertaining to the Los MenucosGroup. Within the Los Menucos Group are included two units of lower rank, namely the Vera Formation andthe Sierra Colorada Formation. The Vera Formation is notable for having produced one of the most importantTriassic tetrapod ichnofauna of southern South America (e.g. Melchor & de Valais, 2006 and references therein).The ichnofauna, preserved both in volcaniclastic and sedimentary strata, is mostly documented both by smalland large pentadactyl tetrapod tracks. These footprints are referred to the ichnogenera Dicynodontipus andPentasauropus, respectively, and can be attributed to different therapsid trackmakers. The track-bearing levelsfrom which Dicynodontipus and Pentasauropus were reported are exposed in two distinct areas, respectively tothe west and east of the Los Menucos town. A Late Triassic age was historically proposed for the Los MenucosGroup, based on palaeofloristic and palaeoichnological data, available from the Vera Formation, and radiometricdatations available from the Sierra Colorada Formation. Recently, new geochronological evidence (Luppo etal., 2017) indicated ages ranging from the Wuchiapingian (Late Permian) to the Olenekian (Early Triassic) forthe volcanics of Los Menucos Group, strongly contrasting with the current chronostratigraphical framework.Within this context, the ichnological record turn out to be binding for assessing the age of the sedimentary andvolcaniclastic levels and estimating the starting of sedimentation in the Los Menucos Basin. The Late Triassicage for the whole Vera Formation can be questioned taking into account the global stratigraphical distributionof Dicynodontipus, spanning from the Late Permian to the early Middle Triassic (e.g. Klein & Lucas, 2010).Thus, while the Pentasauropus-bearing strata can be confidently referred to a Late Triassic age based on theglobal occurrence of the ichnogenus, sedimentary levels with Dicynodontipus, stratigraphically close to thedated volcanites, can be anticipated to Late Permian or Early Triassic times.