--> Stratigraphic Record of Foreland Basin Dynamics, Cretaceous Magallanes-Austral Basin, Chile and Argentina

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Stratigraphic Record of Foreland Basin Dynamics, Cretaceous Magallanes-Austral Basin, Chile and Argentina

Abstract

Retroforeland basins develop from tectonic loading of continental lithosphere on the craton side of convergent margins. Cratonward propagation of the orogenic system incorporates the sedimentary fill into the fold-thrust belt, resulting in outcrop belts valuable for reconstructing tectonic evolution. However, the limited amplitude of foredeep subsidence of continental lithosphere results in basin fills dominated by facies of non-marine and shallow-marine environments related to epeiric marine incursions. As a result, investigations of high-relief retroforeland systems characterized by long-term deep-marine sedimentation are lacking. The Cretaceous Magallanes-Austral foreland basin of southern South America (49°–52°S) developed on thinned lithosphere of mixed continental/oceanic affinities and corresponding greater flexural subsidence. These geodynamic conditions led to a large-scale and relatively steep (>1° clinoforms) foredeep-parallel prograding basin-margin system. We synthesize information from a 250 km long dip-parallel transect of Magallanes-Austral stratigraphy that spans 25 Myr that includes: bed-scale sedimentology, high-resolution stratigraphic architecture, regional mapping and correlation, volcanic-ash and detrital zircon geochronology, strontium isotope stratigraphy, sandstone/mudstone composition, and flexural modeling of subsidence. At the largest scale, shallow-marine to coastal plain deposits that dominate the northern segments transition southward to intercalated shallow-marine, slope, and basin-plain deposits and, further southward, to deep-water (1000–2000 m) submarine fan, channel-levee, and slope deposits. Detrital zircon U-Pb ages indicate: (1) the presence of contemporaneous arc-derived zircons throughout the basin fill, which suggest that sediment-routing system connectivity between arc and foredeep was maintained; (2) a ∼100 Ma peak reflecting arc growth; (3) a ∼150 Ma peak appearing in younger strata, interpreted to record unroofing and exhumation of Jurassic rocks. Volcanic-ash U-Pb dates and strontium isotope stratigraphy further constrain depositional ages and detrital strontium ages provide insight into intra-basinal sediment recycling. Flexural modeling of basin deflection confirms the role of thin lithosphere in enhanced foredeep subsidence, which demonstrates that the Magallanes-Austral foreland contains a more complete record of tectonic evolution compared to unconformity-riddled shallower foreland successions.