First U-Pb SHRIMP age for the Pilmatué Member (Agrio Formation) of the Neuquén Basin, Argentina: Implications for the Hauterivian lower boundary

Ammonite-based biostratigraphic schemes for the Lower Cretaceous are fairly well refined across the world, from the standard zonation in the West Mediterranean province to the Boreal and Austral provinces in the northern and southern hemispheres, respectively. However, the lack of radioisotopic ages associated to the fossil-rich, Lower Cretaceous marine successions has hindered the accurate establishment of the numerical ages for the lower boundaries of its several stages (from Berriasian to Albian). Geochronological dating by U-Pb SHRIMP of a tuff layer that occurs within beds belonging to the Holcoptychites neuquensis Zone in the Pilmatué Member of the Agrio Formation in the Austral province (Neuquén Basin, Argentina) has resulted in an absolute age of 130.0 ± 0.6 Ma (2 sigma internal errors only) or 130.0 ± 0.8 Ma (including calibration and decay constant uncertainties). This age is interpreted to represent the time of eruption and thus the timing of the pyroclastic deposit. The H. neuquensis Zone is the equivalent of the A. radiatus Zone in the West Mediterranean province. Therefore, the obtained age is the first numerical data that could help constrain the Hauterivian lower boundary. Indeed, there is reasonable agreement with the latest proposed lower boundary of the Hauterivian at ~132.9 Ma. On the other hand, the duration recently established for this stage would be hard to reconcile with the stratigraphic record of the entire Hauterivian in the study region (northeastern Neuquén Basin). Therefore, the results of this contribution could also help to assess the extent of the Hauterivian and associated stages.