Early to Middle Jurassic Calcareous Nannofossil Biostratigraphy: A Preliminary Scheme for South-western Gondwana

CHAUMEIL RODRÍGUEZ, MICAELA; MATTIOLI, EMAUELA; PÉREZ PANERA, JUAN PABLO

Avignon

Congreso; INA Meeting 18; 2022

Biostratigraphic studies on Early to Middle Jurassic calcareous nannofossils from the south-eastern Pacific Ocean are scarce and a local biostratigraphic scheme has not been developed yet. We present herein the calcareous nannofossil assemblages from outcrop sections named El Matuasto (I, II, and III) from Los Molles Formation, Neuquén Basin (Argentina), dated as Lower Pliensbachian (NJT4) to Early Bajocian (NJT9) according to the zonal markers identified after the Tethyan scheme by Ferreira et al. (2019). The assemblages show a high diversity index and good preservation, standing out to analyse the evolution of the group from a local to a global perspective. The Pliensbachian succession is characterized by Calyculus sp., Crepidolithus granulatus, C. crassus, Parhabdolithus liasicus, Similiscutum cruciulus group, S. finchii, S. giganteum, Tubirhabdus patulus, and First Occurrence (FO) of Crepidolithus pliensbachensis, Biscutum grande, Lotharingius barozii, Parhabdolithus robustus, and Lotharingius hauffii. The Toarcian is characterized by Axopodorhabdus atavus, Biscutum dubium, Lotharingius hauffii, Orthogonoides hamiltoniae, Similiscutum cruciulus group, Tubirhabdus patulus, and the FO of Zeugrhabdotus erectus, Carinolithus superbus, Watznaueria fossacincta, Discorhabdus striatus, D. criotus, Retecapsa incompta, and Watznaueria contracta. The Aalenian association is characterized by Biscutum intermedium, Carinolithus superbus, Calyculus sp., Discorhabdus sp., Ethmorhabdus sp., Lotharingius barozii, L. hauffii, L. sigillatus, Podorhabdus grassei, Similiscutum novum, Triscutum sullivanii, Watznaueria contracta, Zeugrhabdotus erectus, and the FO of Watznaueria britannica. Early Bajocian accounts the presence of Biscutum intermedium, Discorhabdus sp., Ethmorhabdus sp., Lotharingius hauffii, L. sigillatus, Podorhabdus grassei, Retecapsa incompta, Similiscutum novum, Watznaueria britannica, W. contracta, W. fossacincta, and Zeugrhabdotus erectus, with the Last Occurrence (LO) of Calyculus sp.The calcareous nannofossils reported in this contribution constitute the most time-expanded and continuous record of the group in the Neuquén Basin and the southwestern Gondwana. The assemblage composition presents many similarities with western Tethys assemblages, but some discrepancies also occur mainly for the FO of some species. This deserves to be further explored and a biozonation scheme for the area should be established.