CALCAREOUS MICROFOSSILS ASSEMBLAGE ALLIED WITH AN ELASMOSAURID FROM LA COLONIA FORMATION, CHUBUT PROVINCE: AGE AND PALAEOENVIRONMENT

ANA PAULA CARIGNANO; LYDIA CALVO MARCILESE; JUAN PABLO PÉREZ PANERA; JOSÉ O´GORMAN

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

La Colonia Formation (Upper Cretaceous?lower Paleogene), exposed south of the Somun Curá Plateau, Chubut Province, is known for its rich fossil content, including vertebrates, plants, and microfossils. Here we present an assemblage of calcareous nannofossils, foraminifers and ostracods recovered from a level bearing the remains of a well-preserved plesiosaur at the Puesto Opaso locality (42° 49? 36.63? S, 68° 26? 43.12? W). Fragments of echinoderms, molluscs andfish teeth are part of the accompanying fauna. The plesiosaur represents an Euelasmosaurian elasmosaurid juvenile specimen with a humerus morphology that indicates Weddellonectian affinities. Despite the systematic uncertainties, it clearly differs from Kawanectes lafauqnianum and therefore it represents the second non-aristonectine elasmosaurid from Chubut. Regarding the microfossils, a moderate-low diverse and moderately abundant assemblage of nannofossils,foraminifera and ostracods with a good preservation state was recorded. The calcareous nannofossil assemblage is dominated by species of the genus Micula, with Micula staurophora being the most abundant, followed by Micula concava, Micula murus, Micula cubiformis and Micula prinsii. Other nannofossils in the assemblage are Eiffellithus gorkae, Eiffellithus turriseiffelii, Gartnerago obliquum, Kamptnerius magnificus and Cribrosphaerella daniae. This high abundance of Micula spp. is useful for regional correlation in the Southwestern Atlantic Ocean, and given the presence of Micula prinsii and Micula murus at this latitude, the assemblage is latest Maastrichtian, upper part of UC20 and younger than ~67.5 Ma. This assemblage is comparable to those recovered in the late Maastrichtian of the Salado, Colorado, and Neuquén basins, but the absenceof some typical cold-water species might indicate slightly warmer surface waters than in the surrounding areas of the late Maastrichtian Kawas interior Sea. The foraminiferal assemblage is entirely composed of benthic forms, dominated by Discorbis cf. midwayensis, Psilocitharella sp., Lagena aff. acuticosta, Favolagena ardolinoi, Guttulina communis, Lenticulina aff. gibba, Nodosaria sp., Pyrulina cylindroides, Bolivina sp., Palmula howei and Quinqueloculina sp. Most of the recorded species are typical of well-oxygenated neritic environments, fully marine in inner and mid-shelf depths. Morphotype groups of these benthic foraminifera can be correlated to microhabitat depths in the seafloor sediment; epifaunal forms such Discorbis, Lenticulina and Quinqueloculina, and shallow infaunal forms like Lagena, Guttulina, Palmula and Bolivina have been registered.The ostracod assemblage is represented by adult and juvenile valves, and a few carapaces of only two species, Cytherella cf. araucana and Anebocythereis espinosa; other ostracods, such as Actinocythereis tuberculata, Aleisocythereis? sp., Musneyella minima, Hemicytherura rionegroensis, Nigeria punctata, Hemingwayella sp. and Bertelsiana sp., are represented only by adultvalves and a few carapaces, sometimes with different colouration. This assemblage shares taxa with others previously recorded in the La Colonia Formation, at Puesto Escobar (Chubut) and in the Jagüel Formation, Neuquén Basin (Río Negro and Neuquén). These particular nannofossil and foraminiferal assemblages, with low representation of autochthonousostracods, points to a marine inner neritic paleoenvironment, with temperate surface waters, probably due to restricted connection to open ocean and intense insolation. The microfossils will be housed at the ?Dr. Eduardo A. Musacchio Repository? of the Universidad Nacional de la Patagonia San Juan Bosco.