Abstract

Provenance studies are widely used in foreland basin settings to evaluate temporal changes in sediment source areas, pinpoint the composition of outcropping units during deposition, and shed light on the interactions between source-area composition, depositional environments, tectonics, and paleoclimate. We present a case example from the Miocene foreland basin of the Austral–Magallanes in southern Patagonia, Argentina (46.5–47.9° south latitude), providing provenance studies of conglomerates, sandstones, and mudstones, combined with previously published detrital-zircon U–Pb geochronology and sedimentological data. Based on this information, we document a change in volcaniclastic source areas consistent with an unroofing sequence during the main phase of Andean tectonic uplift. The studied Miocene beds are up to 1000 m thick and represent a synorogenic environmental transition from shallow marine (El Chacay Formation, ∼ 20–18 Ma) to fluvial–alluvial (Río Zeballos Group, ∼ 18–12 Ma), and ultimately to fluvial (Santa Cruz Formation, ∼ 18–14 Ma) paleoenvironments. Litharenite and feldspathic litharenite thin-section samples (n = 51) plot within the magmatic-arc and recycled-orogen tectonic fields. Six conglomerate samples indicate a similar setting. Furthermore, X-ray diffraction analysis of 36 mudstone samples identifies a well-crystallized smectite clay mineral, commonly associated with illite, mixed-layer illite–smectite, chlorite, minor calcite, and zeolites, suggesting the alteration of labile volcanic material from orogenic sources. Our compositional evidence, together with previous detrital U–Pb zircon data, characterizes the studied unroofing sequence exposing Mesozoic to Paleozoic rocks, and serves as a case study for other orogenic basins, where the progressive tectonic exhumation of source areas plays a dominant role.