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

Abstract

The Valanginianand Hauterivian stages were periods of transition between the relatively coldLate Jurassic and a green house world which continued in the rest of the Cretaceous, and the world seemed to have distinct climate zones, which are reflected in distinct Boreal, Tethyan and Austral marine biotas. However, the duration of these stages is presently under debate, and their numerical ages are also poorly constrained. 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 bioevents of nannofossils markers. To tackle theseand other Early Cretaceous topics we are studying the Agrio Formation of the Neuquén Basin in west-central Argentina. This basin is a retro-arc basin developed in a normal subduction segment at the foothills of the Andes. Extensive and laterally continuous outcrops and a rich fossil record, combined with ashfall tuffs interbedded in thick, expanded sedimentary successions make the basin an excellent site for stratigraphical, paleontological, and radio-isotopicstudies. The infill of the basin during the late early Valanginian to the late Hauterivian 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-marine Avilé Member in between is represented by lutites and sandstones deposited inan ephemeral lacustrine and fluvial environment and it is connected to a short episode of shallowing related to a forced regression. We performed magnetic 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 were correlated with those of classic sections of the Tethys, including the candidates 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 to independently assess the calibration of the ammonoid zones and the nannofossils bioevents in the Neuquén Basin with the -standard- chronostratigraphy in the Tethyan Mediterranean area.