Geochemical, palaeontological, and sedimentological approaches of a syn-orogenic clastic wedge: Implications for the provenance of the Permian (Cisuralian) Tunas Formation, Ventania System (Argentina)

BALLIVIAN, CARLOS A.; COMERIO M.; COTUREL, ELIANA P.; SATO, ANA MARIA; NAIPAUER, MAXIMILIANO

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2020

0895-9811

The Tunas Formation crops out in the eastern sector of the Ventania System (southwestern Buenos Aires Province,Argentina) and represents the last unit of the Pillahuinc´o Group (Pennsylvanian?Cisuralian). This workintegrates sedimentological, palaeontological, geochemical, and geochronological studies of the uppermostlevels of the unit with the aim to enhance its palaeoenvironmental and stratigraphic framework within theSouthwest Gondwana context. The analysed succession, of 250 m thick, is dominated by lithic arkoses andfeldspathic litharenites composed of crystalline quartz and minor feldspars and volcanic fragments, interbeddedwith muddy-siltstones and heterolithic deposits with plant fossils. The succession is organized in three thickening-and coarsening-upward packages that correspond to fluvio-deltaic deposits. The plant fossil assemblageincludes fronds of Gangamopteris obovata, Glossopteris angustifolia, Glossopteris wilsonii, and Asterotheca sp. cf.A. andersonii, in association with articulate ribbed stems of Paracalamites australis, which are here assigned to theGangamopteris Zone. The low plant-diversity of the Tunas Formation could be related to the change from postglacialto semiarid-arid conditions evidenced during the Cisuralian in the Southwest Gondwana region where thestudy area is located. The obtained geochemical results indicate low recycling and sorting of continentallyderived sediments with dominant input from the upper continental crust and acidic to intermediate magmaticprecursors related to an active continental margin. Previous studies pointed out that the North PatagonianMassif, located SW of the Ventania System, was the source of such materials. Additionally, new LA-ICP-MS U?Pbzircon ages from two tuff levels of the Tunas Formation were considered together with previous U?Pb zirconages. These suggest that the source of the ash fall deposits could be as far as the pre-Choiyoi volcanism (LateMississippian?middle Cisuralian) of the Chilean Precordillera and Frontal Cordillera and as close as the Choiyoivolcanism (middle Cisuralian?Lower Triassic) of the Las Matras and Chadileuvú blocks, all located west of theVentania System. The integrated analyses of the available U?Pb geochronological data also allowed us toconstrain the sedimentation age of the uppermost levels of the Tunas Formation to the Artinskian. However, theage interval of the stratigraphic unit should be wider.