Geochronology, Stratigraphy, and Provenance of the Early Fill of the Magallanes–Austral Basin, Southern Patagonia: Diachronous Initiation of a Retroarc Foreland Basin

Abstract:

The Magallanes–Austral Basin (MAB) is preserved along a >1000 kilometer north–south trending outcrop belt in the southern Patagonia region of Argentina and Chile. The stratigraphic evolution of the MAB has been well documented in the Chilean sector of the basin, however its along-strike counterpart in Argentina is poorly constrained. We present new stratigraphic and geochronologic data from the early basin fill (Aptian–Turonian) from the Argentine sector (49–51°S) of the MAB to document spatial variability in stratigraphic facies and timing of deposition. The initiation of the MAB is marked by the transition from mudstone to coarse-clastic deposition which is characterized by the consistent presence of thick sandstone beds. These sandy facies are interpreted to represent turbidity current deposits in a submarine fan system. This study documents that such facies are present as far north as El Chalten, Argentina (49°S), indicating that facies-equivalent rocks can be traced along-strike for at least 5 degrees of latitude, based on correlation with strata as far south as the Cordillera Darwin (54°S). Detrital zircon U-Pb ages (477 grains from 6 samples) from sandstone within the Argentine sector reveal similar trends to those documented in the Chilean sector. Age populations primarily consist of Aptian–Campanian (126–75 Ma) arc-related grains as well as Cambrian–Triassic ages (600–200 Ma) derived from the East Andean Metamorphic Complex. Jurassic to earliest Cretaceous ages (200–126) representing rift-phase volcanism and early arc magmatism, typically account for less than 5% of the ages. Finally, 6 new U-Pb ages from ash beds, coupled with maximum depositional ages from detrital zircon populations, reveal a southward younging trend in depositional ages. Interpreted ages range from 115 Ma to 95 Ma in the northern sector, but based on previous studies, are not older than 92 Ma and 89 Ma in the central and southern sectors, respectively. The diachronous delivery of coarse detritus into the basin may reflect orogen-parallel variations in pre-existing structures and crustal composition from the earlier tectonic history of this region. These results highlight the influence of tectonic inheritance on patterns of foreland basin initiation in Cordilleran-style margins.