Integrated stratigraphy of the Agrio Formation (Neuquén Basin, Argentine Andes): towards an intercalibration with the Tethys during late Valanginian-Hauterivian times

AGUIRRE-URRETA, B.; CONCHEYRO, A.; LESCANO, M.; REBOULET, S.; MARTINEZ, M.; RAWSON, P.

Geneve

Congreso; International Meeting around the Jurassic-Cretaceous Boundary; 2018

JK2018 Organizing Committee

The Valanginianand Hauterivian stages were periods of transition between the relatively coldLate Jurassic and a greenhouse world which continued in the rest of theCretaceous, and the world seemed to have distinct climate zones, which are reflectedin distinct Boreal, Tethyan and Austral marine biotas. However, the duration ofthese stages is presently under debate, and their numerical ages are also poorlyconstrained. These uncertainties have hindered efforts to correlate andcalibrate different ammonoid zonations of the Boreal and Austral realms withthe ?standard? Tethyan Mediterranean region zonation as well as the bioeventsof nannofossils markers. To tackle theseand other Early Cretaceous topics we are studying the Agrio Formation of theNeuquén Basin in west-central Argentina. This basin is a retro-arc basindeveloped in a normal subduction segment at the foothills of the Andes. Extensiveand laterally continuous outcrops and a rich fossil record, combined with ashfall tuffs interbedded in thick, expanded sedimentary successions make thebasin an excellent site for stratigraphical, paleontological, and radio-isotopicstudies. The infill of the basin during the late early Valanginian to the lateHauterivian is represented by the Agrio Formation. We have studied this unitfor more than 20 years with bed-by-bed collection of macrofossils and samplesfor microfossils and more recently we started sampling the tuff layers. Presently,there are four high precision CA-ID TIMS U-Pb radio-isotopic ages which arewell constrained biostratigraphically by ammonoids and calcareous nannofossils.The oldest one is 130.39 ± 0.16 Ma (early Hauterivian), the second is 129.09 ±0.16 Ma (base of late Hauterivian), the third one is 127.42 ± 0.15 Ma (lateHauterivian) and the forth is 126.97 ± 0.15 Ma (late Hauterivian). We selected a stratigraphic section at the locality El Portón where thisformation is nearly 700 metres thick and represented by its three members. Thelower or Pilmatué and upper of Agua de la Mula members are both marine and composedof marl-limestone alternations, likely forced by orbital cycles. The non-marineAvilé Member in between is represented by lutites and sandstones deposited inan ephemeral lacustrine and fluvial environment and it is connected to a shortepisode of shallowing related to a forced regression. We performedmagnetic susceptibility measurements in both the Pilmatué and Agua de la MulaMembers, having obtained the first orbital time scale of the Agrio Formation. Thus, we achieved a robust combination of biostratigraphy, cyclostratigraphy andhigh-precision radio-isotopic ages for the Agrio Formation and these data werecorrelated with those of classic sections of the Tethys, including thecandidates for the base of the Hauterivian (La Charce, France) and the base ofthe Barremian (Río Argos, Spain).The astrochronological framework provided here gives an opportunity toindependently assess the calibration of the ammonoid zones and the nannofossilsbioevents in the Neuquén Basin with the ?standard? chronostratigraphy in the TethyanMediterranean area.