Astrochronology of the Agrio Formation (Neuquén Basin, Argentina): toward an intercalibration of the ammonite zones in the Early Cretaceous

Abstract

Interbasinal correlations at high resolution requirethe precise definition of the chronological relationships between thebiostratigraphic markers, from a faunal province to another. In the Early Cretaceous,the Kilian Group tends to establish correlation charts between the Andean andthe Tethyan ammonite zones (Reboulet et al., 2014). Attempts to providedurations and numerical ages of the ammonite zones in the Valanginian and theHauterivian stages have been made, based on cyclostratigraphy in the Tethyanarea (Martinez et al., 2015), and on the U-Pb method on volcanic ashes in theNeuquén Basin (Aguirre-Urreta et al., 2015). Because of the scarcity of ashlayers in the Tethyan area, the durations calculated for the ammonite zonescannot be anchored on numerical ages without correlations with the Andeanseries. Conversely, the Andean series have not been dated by cyclostratigraphy.The hemipelagic sediments of the Neuquén Basin in the Upper Valanginian-Hauterivianform the Agrio Formation. This formation is composed of marl-limestonealternations, likely forced by the orbital cycles (Sagasti, 2005).We propose here the first orbital time scale ofthe Agrio Formation and its subdivisions (biozones) based on magneticsusceptibility measurements (MS). The MS measurements reflect themarl-limestone alternations, with higher values for the marls, rich iniron-bearing clay minerals than in the limestone beds. The first results ofspectral analyses tend to show a pervasive expression of the eccentricitycycles. Notably the time of deposit of the lower part of the Agrio Formation isestimated as 5.4 myr. Such a long duration is in disagreement with the currentgeological time scale (Gradstein et al., 2012), but agrees with the calibrationof the Valanginian-Hauterivian times performed in the Tethyan basins (Martinezet al., 2015).