Timing and stratigraphic evolution of a Miocene foreland unroofing sequence in the Austral–Magallanes Basin during Southern Patagonian Andes uplift

Abstract

Detrital zircon U–Pb geochronology is a widely used technique for interpreting sedimentary provenance in basin systems. This, combined with detailed sedimentary facies interpretation and stratigraphy evolution, has been extensively applied in foreland basin settings to understand tectonic processes by tracking the exposure and erosion of distinct sediment source areas through time. We present a case study in the Miocene foreland sedimentary record of the NW Austral–Magallanes Basin (c. 47° 30′ S), which is associated with the main phase of Andean uplift. We define three sedimentary units (SU) for palaeoenvironmental interpretation from shallow-marine deposits (SU-I) to mixed-load fluvial deposits (SU-II) to an alluvial fan (SU-III). Also, we constrain the timing of this synorogenic clastic wedge between c. 20 and c. 12 Ma applying U–Pb zircon maximum depositional ages of six sandstones and one tuff sample. From detrital zircon provenance analysis, we document a progressive upsection loss of contribution from younger units and an enrichment of older units derived from hinterland sources, suggesting the development of an unroofing erosional process owing to coeval Andean uplift of the cordilleran sources. Tectonostratigraphic stages determine the initial stages of orogenic uplift (Stage 1), the subsequent advance of the orogenic belt and progressive loss of accommodation space (Stage 2) and finally foreland clogging (Stage 3).