Early Cretaceous precise radioisotopic ages, ammonite biostratigraphy and calcareous nannofossil bioevents from the Neuquén Basin, Argentine Andes

AGUIRRE-URRETA, M.B.; NAIPAUER, M.; RAWSON, P.; LESCANO, M.; PIMENTEL, M.; RAMOS, V.; SCHMITZ, M.; CONCHEYRO, A.

Cape Town

Congreso; 35TH INTERNATIONAL GEOLOGICAL CONGRESS; 2016

Up to now, the global ?standard? Early Cretaceous stages are based on stratigraphicsections in the Mediterranean Province of the Tethys, and are mostly defined byammonite biostratigraphy and calcareous nannofossil bioevents that have beencalibrated with the M sequence of magnetic polarity chrons. The lack of preciseradiometric ages has hindered the construction of an accurate geological timescale [1], despite the efforts of the International Commission on Stratigraphy[2].The Neuquén Basin is a retro-arc basin developed in a normal subductionsegment at the foothills of the Andes. Laterally continuous outcrops and anabundant fossil record, combined with tuffaceous layers interbedded in thicksedimentary successions make the basin an excellent site for stratigraphical,paleontological, and radioisotopic studies. The infill of the basin during theLate Jurassic-Early Cretaceous is represented by both marine and continentaldeposits that are assembled in the Mendoza Group (from base to top, Tordillo,Vaca Muerta, Mulichinco and Agrio Formations).Vennari et al. [3] presented aCA-ID TIMS U-Pb age of 139.55 ± 0.18 Ma from a tuff near the base of theBerriasian. The Tithonian-Berriasian transition was recognized in the VacaMuerta Formation on the basis of ammonite zones and calcareous nannofossilbioevents which allow correlation with well-established Tethyan floras andfaunas. Although the formal definition of the base of the Berriasian is stillunder consideration, those authors proposed that the J-K boundary should beclose to 140 Ma. More recently, Aguirre-Urreta et al. [4] provided two CA-IDTIMS U-Pb ages of 129.09 ± 0.16 Ma and 127.42 ± 0.15 Ma from two distinct tuffsinterbedded within the marine sediments of the Upper Agrio Formation. Bothhorizons are well constrained biostratigraphically by ammonites and calcareousnannofossils which correlate with the ?standard? sequence of the Tethyan Realm.The lower horizon is very close to the base of the Upper Hauterivian and theupper horizon to the Hauterivian-Barremian boundary, indicating that the formerlies at c. 129.5 Ma and the latter at c. 127 Ma. Martinez et al. [5] anchoredtheir astrochronology data from two classic Mediterranean basins with our U-Pbdata base which reinforce our differences with the current IUGS geological timescale. They dated the base of the Valanginian at 137.05 ± 1.0 Ma, the base ofthe Hauterivian at 131.96 ± 1.0 Ma, and the base of the Barremian at 126.02 ±1.0 Ma.We present here a new CA-ID TIMS U-Pb age of 130.39 ± 0.16 Ma for theLower Agrio Formation of Early Hauterivian age. These new radioisotopic agesare beginning to fill a gap of over 14 million years in the numericalcalibration of the current global Early Cretaceous geological time scale[2].Research projects in progress in the Neuquén Basin on magnetostratigraphy,astrochronology and CA-ID-TIMS U?Pb zircon dating will certainly improve theEarly Cretaceous chronology presented here and will contribute to theconstruction of a more precise and stable geological time scale.