The Jurassic/Cretaceous (J/K) boundary examined: new geochronological data pose some constrains from the Southern Hemisphere.

RAMOS, V.A., M. NAIPAUER Y M.B. AGUIRRE-URRETA

Trelew

Congreso; Vº Simposio Argentino del Jurásico; 2013

Museo Paleontologico Egidio Feruglio

Systematic studies conducted in the Neuquén Basin to establish continentalization periods, either in extensional regimes during the Jurassic or at the beginning of the compressive deformation, have shown strong inconsistencies between the biostratigraphic data and the absolute values obtained when compared to the Geological Time Scale 2012 provided by the International Commission on Stratigraphy of the International Union of Geological Sciences. The results obtained by U-Pb zircon dating with Laser Ablation ICPMS, SHRIMP and CA-ID-TIMS conducted in geochronological laboratories in Porto Alegre, Canberra and Geneva respectively, show results consistent with the relative accuracy of these methods. These data indicate that the base of the Tithonian would be 145 Ma and the J/K boundary should climb up to 140 Ma. The derivation of these inconsistencies poses two alternatives. The first and less likely is that the correlations made by ammonoid zonations and calcareous nannofossil bioevents between the Neuquén Basin and the Tethys, where these limits are set, are incorrect and therefore the Tithonian would not be represented in the basin. The second, and most likely alternative, is that the Jurassic/Cretaceous boundary and the absolute ages of the Tithonian and Berriasian,present errors of about 5 Ma. When the constraints used to establish absolute geochronological control in the northern hemisphere in recent time scales are analyzed, it is evident the lack of precise dating. On this basis it is proposed that the Neuquén Basin has all the elements to determine the J/K boundary with the accuracy required by international standards.