Triassic calcareous nannofossils from Arroyo Malo Formation, Neuquén Basin, Argentina. First record in South America and implications on the early evolution of the group

PÉREZ PANERA. J.P.; ANGELOZZI, G.N.; RICCARDI, A.C; DAMBORENEA, S.E.; MANCEÑIDO, M.O.

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Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

Calcareousnannofossils first appeared in the Western Tethys during the Norian. Someare considered ?calcareous cysts? related to calcareous dinoflagellates (Orthophionella, Thoracosphaera and a fewsimilar genera), but one nannolith of uncertain affinities, Prinsiosphaera triassica, and two proper coccolith genera, Crucirhabdus and Archaeozygodiscus,were found in Austria, in middle to late Norian deposits. Only P. triassica was abundant and rapidlydispersed, being the most frequent Late Triassic nannofossil. By the earlyRhaetian, another nannolith, Eoconusphaerazlambachensis, appeared and became sometimes abundant in carbonate shelves ofthe Western (Alps) and Eastern Tethys (Timor and west Australia, Exmouth and Wombatplateaus), and both above-mentioned coccolith genera became best represented inthe assemblages. However, coccoliths are very small, with low preservationpotential and always display low abundance. Up to date, outside the Tethys,calcareous nannofossils were only reported in Rhaetian beds in the NortheasternPacific Ocean (Queen Charlotte Islands, Canada). These assemblages have lowabundance, comprising Thoracosphaera sp.,Orthopithonella geometrica,Prinsiosphaera triassica, Crucirhabdusminutus and C. primulus. In lower units of the Arroyo Malo Formation, south MendozaProvince, Neuquén Basin, we found the first calcareous nannofossil record from theSoutheastern Pacific Ocean and the second outside the Tethys. The studied samplesare ?Norian to Rhaetian according to marine invertebrates (ammonites,brachiopods and bivalves). Assemblages are poorly preserved and scarce butmirror some nannofossil-events recorded in the Tethys. Lower assemblages (?Norian),yield Prinsiosphaera triassica and Prinsiosphaera sp., and the upper  assemblages (Rhaetian) yield Prinsiosphaera sp. and Eoconusphaera zlambachensis. These findingssuggest an early and continuous dispersal of calcareous nannofossils beyond theTethys. Although we did not find coccoliths yet, the presence of Eoconusphaera zlambachensis is mostencouraging for further work at this site, as it is the first mention outsidethe Tethys and a good biostratigraphic marker for the Rhaetian. Finally, asearly calcareous nannofossils (Late Triassic to Early Jurassic) are considered tohave prospered in eutrophic shelf and epeiric environments, these findings openthe question of whether there was a connection along marine shelves between theTethys and the Eastern Pacific Ocean from as early as late Norian ? Rhaetian,or if nannofossils were capable of dispersing through open ocean.