Reconstructing volcanic architecture through eruptive style and spatial analysis: Cretaceous rift volcanism in the Alemanía subbasin, Salta, Argentina

Abstract

Reconstructing facies architecture in ancient volcano-sedimentary successions is often challenging due to incomplete exposure, deformation, metamorphism, and hydrothermal alteration. The Alemania Subbasin (Salta, Argentina) preserves exceptionally well both sedimentary and volcanic successions, which are exposed as subvolcanic and volcanic units due to tectonic activity. The Cretaceous northwestern basins of Argentina are interpreted as an aborted retroarc rift, comprising interconnected grabens that converge at a triple junction with a central horst (Salta–Jujuy horst). The Alemania Subbasin contains thick sedimentary-volcanic successions of the Salta Group, including the Pirgua, Balbuena, and Santa Bárbara subgroups. The Pirgua Subgroup consists of continental red beds deposited during the syn-rift stage, with volcanic activity primarily recorded by the Las Conchas Basalt (78–77 Ma) in the upper part of the succession (Las Curtiembres Formation). This study focuses on outcrops along Las Conchas Canyon, where both coherent volcanic facies (lava flows, pillow lavas, dykes) and volcaniclastic facies (diatreme breccias, pyroclastic surge deposits, peperites, sediment-infill volcanic breccias, and lahars) interbed with fluvial red beds. Variations in facies architecture reflect contrasting eruptive styles, likely controlled by changes in environmental hydrology and magma–water interaction processes. The Las Conchas Basalt records both hydromagmatic and effusive eruptions. Volcanic architecture reconstruction allowed the identification of facies corresponding to upper and lower diatreme deposits, dykes, and lava flows, associated with peperites and sediment-infill volcanic breccias. Subvolcanic units were likely emplaced in a meandering fluvial paleoenvironment, whereas lava flows advanced across sandy braided fluvial systems under semi-arid conditions. Pillow lavas within lacustrine host rocks (Morales Members) indicate local water bodies sufficient for non-explosive magma–water interactions. Changes in water availability and environmental conditions likely explain the transition from hydromagmatic to effusive eruptive styles, despite the magma remaining basanitic. This study demonstrates that paleoenvironmental and hydrological conditions strongly influenced facies development in monogenetic volcanic fields and provides insights into reconstructing ancient volcano–sedimentary systems in rift-related basins.