Lithofacies architecture within an intra-arc environment: A case study from the Permian-Triassic magmatic arc in the Chilean Frontal Cordillera (30-30.5° S)

Abstract

In the Chilean Frontal Cordillera between 28.5 and 30.5° S, two volcano-sedimentary succesions of Late Guadalupian to Middle Triassic age croup out: the El Tapado (Late Guadalupian-Lopingian) and Guanaco Sonso (Late Lopingian-Middle Triassic) formations, representing the westernmost exposures of the Choiyoi Group and post-Choiyoi magmatism. Recently, these two units have been characterized in detail, shedding light into the tectonic and climatic context of the late stages of the Choiyoi magmatism, and, more generally, the geology of intra-arc successions in the southwestern Gondwana margin. In this work, we present a stratigraphy and lithofacies analysis of three stratigraphic sections studied in the La Laguna sector (30-30.5° S), comprising rocks of the El Tapado and Guanaco Sonso formations. The former is dominated by caldera-related rhyolitic to dacitic ignimbrites accumulated in depocenters associated with the extensional activity of major structures. Lacustrine and fluvial-alluvial deposits interbedded in this rock unit were mainly controlled by volcaniclastic input and subsidence dynamics, and record the transition from relatively humid conditions to semi-arid or even arid conditions during the Late Guadalupian-Lopingian, consistent with regional paleoclimatic observations. On the other hand, the Guanaco Sonso Formation presents mainly intermediate, proximal (near-vent) volcanic products that denote a varied volcanism in terms of composition and eruptive style, likely controlled by structures and tectonics. We conclude the southwestern Gondwana margin sustained high-explosivity, silicic volcanism (El Tapado) that transitioned into a more varied, mainly intermediate volcanism (Guanaco Sonso). This activity ultimately gave way to the basaltic-andesitic to bimodal products that characterized the final part of the pre-Andean stage during the Triassic in this region.