The Jurassic/Cretaceous boundary in the Neuquén Basin, southern Andes: new geochronological and palaeontological data

AGUIRRE-URRETA, B.; VENNARI, V.V.; LESCANO, M.; NAIPAUER, M.; CONCHEYRO, A.; RAMOS, V.

Ankara

Simposio; 9th International Symposium on the Cretaceous System; 2013

The Jurassic-Cretaceous (J/K) boundary is the only major Phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section and its present established age is based on a number of assumptions, including the relative duration of ammonite zones, the first appearance datum (FADs) of calcareous nannofossil markers, the constant spreading rates of magnetic anomalies, and the extrapolation of Ar-Ar isotopic data. Precise radio-isotopic U-Pb data are unavailable for the Berriasian, and drawing the boundary between the Jurassic and Cretaceous systems is now a days a matter of global scale discussions. We report here the recent finding of a hundredmetre- thick sequence of marine black shales in the Andean region of the Neuquén Basin of western Argentina, interbedded with ash-fall tuffs that provides important data on this boundary. This succession, part of the Vaca Muerta Formation, bears a characteristic ammonite fauna together with calcareous nannofossils, which have a wellestablished correlation with classic Tethyan faunas and nannofloras. This transition in the Andes, as recognized on the basis of ammonite zones and nannofossil bioevents, is, for the first time, constrained by precise U-Pb zircon dating. The new U-Pb zircon age of 139.55 ± 0.18 Ma obtained by chemical-abrasion isotope-dilution thermal ionization mass spectrometry (CA-ID-TIMS) is near the base of the Berriasian. Several calcareous nannofossil markers previously identified in the Tethys are recognized in the studied section. The first occurrence (FO) of Umbria granulosa is considered the principal event of subzone NJK-B and correlated with the uppermost Tithonian. The appearanceof Rhagodiscus asper is a further marker for this subzone as it is considered a secondary bioevent for its upper part. Another Late Tithonian marker is the FO of Nannoconus wintereri, a bioevent of subzone NJK-C while the FO of Nannoconus kamptneri minor defines the base of subzone NJK-D, assigned to the Berriasian. Currently, this event is considered a reliable marker of the Tithonian-Berriasian boundary. In the study section, the last occurrence (LO) of Polycostella senaria and Nannoconus wintereri and the FO of Nannoconus steinmannii minor confirm the Berriasian age for the studied interval. The preserved ammonites are grouped in the Substeueroceras koeneni biozone traditionally ascribed to the Late Tithonian, although it may reach the earliest Berriasian, and in the Argentiniceras noduliferum biozone of the Early Berriasian. Asthe obtained age is more than 5 Ma younger than the age of the Jurassic-Cretaceous boundary in the currently accepted geological time scale of the International Commission on Stratigraphy, this contribution shows that there are still existing problems and discrepancies in its determination and emphasizes the need of more accurate absolute data coupled with precise biostratigraphy.