Multiple provenance of inter-eruptive fluvial sandstones of the Albian Cerro Castaño Member, Patagonia

Abstract

The Albian Cerro Castaño Member is a volcanogenic fluvial-alluvial succession deposited in the Somuncurá-Cañadón Asfalto Basin, extra-Andean Patagonia, Argentina. During the deposition of the unit, the basin was affected by recurrent volcaniclastic inputs from sources including either directly active volcanoes or secondary volcanogenic material from upstream positions. In this study, we analyzed the provenance of fluvial sandstones of the Albian Cerro Castaño Member, for which detailed palaeoenvironmental interpretations have been previously established suggesting accumulation during inter-eruptive phases. The methodologies included the petrographic analysis of thirty-seven thin sections with the definition of petrofacies, as well as U-Pb zircon dating of a tuff layer from the underlying stratigraphic unit, located approximately 2 m below the contact with the Cerro Castaño succession. The new U-Pb depositional-crystallization age of 114.86 ± 0.44/0.56 Ma is consistent with the previous geochronological framework and constrains the Cerro Castaño Member to the Albian. The fluvial sandstones are predominantly composed of volcanic lithic framework grains both effusive and volcaniclastic, spanning the acidic to basic geochemical spectrum, with subordinate quartz, feldspars, intraclasts, micas, olivines, pyroxenes, amphiboles, and opaques. They were classified as feldspatho-lithic to quartzo-lithic, indicating provenance from tectonic settings associated with arc (both undissected and transitional) and recycled orogen sources. Four lithic petrofacies, designated A to D, were identified to distinguish sediment sources. The data indicate that the majority of samples from the western sector of the study zone are derived from volcanic rocks with effusive and explosive origin, with a minor contribution from mafic to intermediate volcanic rocks (petrofacies A). In contrast, samples from the eastern sector indicate a source-rock lithology dominated by siliceous rocks, similarly formed by effusive and pyroclastic eruptions but with minimal input from mafic to intermediate volcanic sources (petrofacies B). Additionally, specific samples from the western basin sector display a significant contribution of volcanic lithic grains, likely indicating substantial contributions of siliceous volcaniclastic sediments (petrofacies C and D). The combination of the information obtained with the literature data set, primarily comprising paleocurrents and paleogeological reconstructions, led to the conclusion that the principal source rocks are Jurassic intraplate volcanites, which are extensively distributed in extra-Andean Patagonia. In particular, the rocks of the Lonco Trapial and Marifil Formations constitute the main sources for sandstones in the western and eastern sectors of study zone, respectively. Furthermore, in the western localities, the subordinate participation of the Paleozoic-Triassic crystalline basement as source rocks was identified, as well as the probable contribution of intrabasinal Jurassic volcanites.