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

Abstract

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 Pillahuincó Group (Pennsylvanian–Cisuralian). This work integrates sedimentological, palaeontological, geochemical, and geochronological studies of the uppermost levels of the unit with the aim to enhance its palaeoenvironmental and stratigraphic framework within the Southwest Gondwana context. The analysed succession, of 250 m thick, is dominated by lithic arkoses and feldspathic litharenites composed of crystalline quartz and minor feldspars and volcanic fragments, interbedded with 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 assemblage includes 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 the Gangamopteris Zone. The low plant-diversity of the Tunas Formation could be related to the change from postglacial to semiarid-arid conditions evidenced during the Cisuralian in the Southwest Gondwana region where the study area is located. The obtained geochemical results indicate low recycling and sorting of continentally derived sediments with dominant input from the upper continental crust and acidic to intermediate magmatic precursors related to an active continental margin. Previous studies pointed out that the North Patagonian Massif, located SW of the Ventania System, was the source of such materials. Additionally, new LA-ICP-MS U–Pb zircon ages from two tuff levels of the Tunas Formation were considered together with previous U–Pb zircon ages. These suggest that the source of the ash fall deposits could be as far as the pre-Choiyoi volcanism (Late Mississippian–middle Cisuralian) of the Chilean Precordillera and Frontal Cordillera and as close as the Choiyoi volcanism (middle Cisuralian–Lower Triassic) of the Las Matras and Chadileuvú blocks, all located west of the Ventania System. The integrated analyses of the available U–Pb geochronological data also allowed us to constrain the sedimentation age of the uppermost levels of the Tunas Formation to the Artinskian. However, the age interval of the stratigraphic unit should be wider.