Using detrital zircon U-Pb ages to calculate Late Cretaceous sedimentation rates in the Magallanes-Austral basin, Patagonia

Abstract

Determining both short- and long-term sedimentation rates is becoming increasingly important in geomorphic (exhumation and sediment flux), structural (subsidence/flexure) and natural resource (predictive modelling) studies. Determining sedimentation rates for ancient sedimentary sequences is often hampered by poor understanding of stratigraphic architecture, long-term variability in large-scale sediment dispersal patterns and inconsistent availability of absolute age data. Uranium-Lead (U-Pb) detrital zircon (DZ) geochronology is not only a popular method to determine the provenance of siliciclastic sedimentary rocks but also helps delimit the age of sedimentary sequences, especially in basins associated with protracted volcanism. This study assesses the reliability of U-Pb DZ ages as proxies for depositional ages of Upper Cretaceous strata in the Magallanes-Austral retroarc foreland basin of Patagonia. Progressive younging of maximum depositional ages (MDAs) calculated from young zircon populations in the Upper Cretaceous Dorotea Formation suggests that the MDAs are potential proxies for absolute age, and constrain the age of the Dorotea Formation to be ca. 82-69 Ma. Even if the MDAs do not truly represent ages of contemporaneous volcanic eruptions in the arc, they may still indicate progressive-but-lagged delivery of increasingly younger volcanogenic zircon to the basin. In this case, MDAs may still be a means to determine long-term (1-2Myr) average sedimentation rates. Burial history models built using the MDAs reveal high aggradation rates during an initial, deep-marine phase of the basin. As the basin shoaled to shelfal depths, aggradation rates decreased significantly and were outpaced by progradation of the deposystem. This transition is likely linked to eastward propagation of the Magallanes fold-thrust belt during Campanian-Maastrichtian time, and demonstrates the influence of predecessor basin history on foreland basin dynamics.