RAMOS, V.A.; AGUIRRE-URRETA, B.; NAIPAUER, M.
Recent biostratigraphic studies based on precise calcareous nannofossil determinations were able to recognize several bioevents that are used in the Tethys realm of the northern hemisphere to identify the J-K boundary (Scasso y Concheyro, 1999; Kietzmann et al., 2011, Vennari et al., 2013). The precise zonation produced by the calcareous nannofossils correlates with the present biozonation to set the proposed J-K boundary in the Tethys (Wimbledon et al., 2011, and citations therein). This boundary is confirmed by the classic ammonite zonation developed through several generations of paleontologists in the Neuquén Basin that were able to correlate the ammonite biozones of Neuquén with the standard biozones of the Tethys (Riccardi, 2008; Aguirre-Urreta et al., 2011, and citations therein). However, in the Tethys the absolute age was based only on the identification of the magnetic polarity chrones, which in turn were dated assuming constant spreading rates of magnetic anomalies in the Pacific Ocean floor (Oggs and Hinnov, 2012). The present J-K boundary is based on the Ar-Ar dating of some altered oceanic basalts from an ODP well drilled in the Shatsky Rise in the middle of the Pacific Ocean (Mahoney et al., 2005). Nevertheless, the biostratigraphic control of this well is poor and does not accomplish the present criteria to define the boundary. Most of the recent studies recognize that the Jurassic-Cretaceous (J-K) boundary is poorly constrained, and is the only major Phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section. Our present study obtained the first precise U-Pb chemical-abrasion isotope-dilution thermal ionization mass spectrometry (CA-ID-TIMS) ages in zircons from an ash-fall tuff layer of the Vaca Muerta Formation, near the base of the Berriasian. Based on the sedimentation rates obtained by cyclostratigraphy in neighboring areas we established a 140 Ma age for the J-K boundary. This new age confirms previous data obtained by U-Pb Laser Ablation and SHRIMP data in zircons, which also show a five million years mismatch with the standards presented by the Geological Time Scale (2013), where this boundary is 145 Ma old. The thick successions of fossiliferous marine beds of Tithonian-Berriasian age in the Andes, together with frequent interbedded tuff layers offer a great opportunity to establish and define the Global Boundary Stratotype Section and Points (GSSP) for the Jurassic-Cretaceous boundary. The precise age of that boundary will allow us to correlate for the first time the volcanic and plutonic activity based on geochronological data with the different processes of that age identified in the retroarc sedimentary basins.