Palaeogeography and diachronous infill of an ancient deep-marine foreland basin, Upper Cretaceous Cerro Toro Formation, Magallanes Basin

Authors

  • Anne Bernhardt,

    Corresponding authorCurrent affiliation:
    1. Department of Earth and Environmental Sciences, University of Potsdam, Potsdam, Germany
    • Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA
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  • Zane R. Jobe,

    1. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA
    Current affiliation:
    1. Shell Projects and Technology, USA
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  • Marty Grove,

    1. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA
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  • Donald R. Lowe

    1. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA
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Correspondence: Anne Bernhardt, Department of Earth and Environmental Sciences, University of Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany. E-mail: anne.bernhardt@geo.uni-potsdam.de

Abstract

The details of how narrow, orogen-parallel ocean basins are filled with sediment by large axial submarine channels is important to understand because these depositional systems commonly form in through-like basins in various tectonic settings. The Magallanes foreland basin is an excellent location to study an orogen-parallel deep-marine system. Conglomerate lenses of the Upper Cretaceous Cerro Toro Formation have been previously interpreted to represent the fill of a single submarine channel (4–8 km wide, >100 km long) that funneled coarse detritus southward along the basin axis. This interpretation was based on lithologic correlations. New U/Pb dating of zircons from volcanic ashes and sandstones, coupled with strontium isotope stratigraphy, refine the controls on depositional ages and provenance. Results demonstrate that north-south oriented conglomerate lenses are contemporaneous within error limits (ca. 84–82 Ma) supporting that they represent parts of an axial channel belt. Channel deposits 20 km west of the axial location are 87–82 Ma in age. These channels are partly contemporaneous with the ones within the axial channel belt, making it likely that they represent feeders to the axial channel system. The northern Cerro Toro Formation spans a Turonian to Campanian interval (ca. 90–82 Ma) whereas the formation top, 70 km to the south, is as young as ca. 76 Ma. Kolmogorov–Smirnoff statistical analysis on detrital zircon age distributions shows that the northern uppermost Cerro Toro Formation yields a statistically different age distribution than other samples from the same formation but shows no difference relative to the overlying Tres Pasos Formation. These results suggest the partly coeval deposition of both formations. Integration of previously acquired geochronologic and stratigraphic data with new data show a pronounced southward younging pattern in all four marine formations in the Magallanes Basin. Highly diachronous infilling may be an important depositional pattern for narrow, orogen-parallel ocean basins.

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