Integrated dating by high-precision geochronology and detailed biostratigraphy: a case study in the Lower Cretaceous of the Neuquén Basin, western Argentina

AGUIRRE-URRETA, B. CONCHEYRO, A. CREASERER RRAA FANNINGG CCMM LAZOO PAZOSS PP

Plymouth

Simposio; 8 th International Symposium on the Cretaceous System; 2009

8 th International Symposium on the Cretaceous System

The integration of high-resolution U-Pb geochronology and detailed biostratigraphy presents a new approach to precisely date and correlate sequences worldwide. We present a combination of ammonoid and nannoplankton biostratigraphy, an U-Pb SHRIMP zircon age and a Re-Os isochrone in a specific level ofthe Lower Cretaceous ofthe Neuquén Basin in western Argentina. The Neuquén Basin (32°-40°SL) is a retro-are basin open to the Pacific Ocean with a complex history controlled by the changing tectonic setting of the western margin of Gondwana. Its western margin was a volcanic are that had an extraordinary development from early Jurassic to Albian times. The infill of the Neuquén Basin during the Early Cretaceous is mostly marine. The Agrio Formation (Early Valanginian-Early Barremian) represents a widespread transgressive phase with the deposition ofthe shales, sandstones and limestones. It is divided in three members, Pilmatué, Avilé and Agua de la Muía. Both the lower and upper members are marine and the abundance of fossils has permitted an accurately dating by combining ammonoids and calcareous nannoplankton. The middle Avilé Member contains no marine fossils and provides an excellent marker horizon in the basin. A single tuff layer, 1.2 m thick, located 7 m above the top of the Avilé Member yielded a date of 132.5 ± 1.3 Ma (U-Pb SHRIMP zircons). This tuff is interlayered within the black shales with the ammonite Spitidiscus riccardii Leanza & Wiedmann that confirm the base of the Agua de La Muía Member in the locality Caepe Malal (37°11´S, 70°23´W). Black shales bearing S. riccardii 5.6 meters above the top of the Avilé Member in the locality Mina San Eduardo (37°31´S, 70°00´W) yielded a date of 134.4 ± 4.4 Ma (Re-Os). The Spitidiscus riccardii ammonoid zone is correlated with the Subsaynella sayni ammonoid zone of the Tethys that currently marks the base of the Late Hauterivian. in both Mina San Eduardo and Caepe Malal sections main representative nannofossils are Watznaueria bamesiae (Black) Perch-Nielsen, Watznaueria fossacincta (Black) Bown, Cyclagelosphaera margerelii Noel, Micrantholithus hoschulzi (Reinhardt) Thierstein, Micrantholithus obtusus Stradner, Zeughrabdotus embergeri (Noel) Perch-Nielsen, Glaukolithus diplogrammus Deflandre and Cruciellipsis cuvillieri (Manivit) Thierstein, with scarce nanoconids as Nannoconus steinmannii Kamptner, Nannoconus kamptneri Brónnimann, Nannoconus cornuta Deres & Achéritéguy, Nannoconus bucheri Brónnimann and Nannoconus circularis Deres & Achéritéguy. Cruciellpsis cuvillieri, a reliable nannofossil, is sparsely found in both sections in levéis with Spitidiscus. Some Re-Os data conform a 134 Ma age compatible with the U-Pb data known from the rocks, but other data scatter, possibly the result of either short-term variations in initial Os ratio in the seawater, or as a result of post-depositional alteration. Although the U-Pb age obtained of 132.5 ± 1.3 Ma has a relatively small error, the time interval bracketed by these valúes poses the base of the Late Hauterivian within the errors of the proposed valúes based on magnetostratigraphic and biostratigraphic chronology. In the corrected GTS04 the base of the Hauterivian is approximately 133.9 ± 2 Ma and the base of the Subsaynella sayni Zone is at 132 Ma.