The Applicability of Detrital Zircon in Determining Depositional Ages and Sedimentation Rates in a Retroarc Foreland Basin, Upper Cretaceous Magallanes-Austral Basin, Patagonia

Abstract:

Analysis of detrital zircon (DZ) in sedimentary rocks has become a fundamental component of many provenance studies in recent years. In addition to providing information about sediment source regions, DZ analyses help to constrain the maximum depositional age (MDA) of a sample. This is especially informative for sedimentary successions in which other age constraints are limited (e.g., few fossils or ashes). However, this method relies on hefty assumptions about sediment source areas, sediment routing systems, and the adequate representation of coeval volcanic source areas within a sample.

Upper Cretaceous strata in the Magallanes-Austral retroforeland basin of southern Patagonia record a ~34 million-year history of arc development and retroforeland subsidence. The ~6 km-thick siliciclastic package that fills the foredeep is an upward-shoaling, conformable succession of deep-marine, slope, and shelf deposits sourced primarily from the Andean arc. Deep marine shale units contain abundant ashes that corroborate MDAs established in coeval sandy facies, suggesting continued interconnectivity between the active arc and deep basin. Ashes are rarer in younger, shallow marine to terrestrial facies, but fossil data are abundant and diverse.

This study compares DZ data and rich floral/faunal fossil assemblages from fluviodeltaic strata of the Upper Cretaceous Dorotea Formation to determine the reliability of DZ MDAs. DZ data reveal an upward-younging trend within delta plain facies, as well as southward-younging of delta-front clinoforms. DZ MDAs are used to calculate first-order deltaic aggradation and progradation rates, with initial results suggesting that progradation rates were up to an order of magnitude higher than aggradation rates. These results illustrate a progressive loss of accommodation in the northern part of the basin, potentially linked to coeval thrust-front advance.