Andrea Concheyro

Advances in the biostratigraphy of the Agrio Formation (Lower Cretaceous) of the Neuquén Basin, Argentina: ammonites, palynomorphs, and calcareous nannofossils

Abstract Detailed stratigraphic logging and systematic studies on late Valanginian–early Barremian ammonites from the Agrio Formation of the Neuquen Basin of west-central Argentina, have provided a detailed local ammonite zonation. Here we combine the ammonite data with an initial study of calcareous nannofossils and palynomorphs from two sections to produce a provisional integrated biostratigraphy for the Agrio Formation. While the palynomorphs are essentially of local value only, especially the continental forms, the nannofossils and the ammonites allow some correlations to be made with the European Mediterranean `standard sequences.

Filling the Gap: New Precise Early Cretaceous Radioisotopic Ages from the Andes

Two tuffs in the Lower Cretaceous Agrio Formation, Neuquen Basin, provided U–Pb zircon radioisotopic ages of 129.09±0.16 Ma and 127.42±0.15 Ma. Both horizons are well constrained biostratigraphically by ammonites and nannofossils and can be correlated with the ‘standard’ sequence of the Mediterranean Province. The lower horizon is very close to the base of the Upper Hauterivian and the upper horizon to the Hauterivian/Barremian boundary, indicating that the former lies at c. 129.5 Ma and the latter at c. 127 Ma. These new radioisotopic ages fill a gap of over 8 million years in the numerical calibration of the current global Early Cretaceous geological time scale.

Bioestratigrafía y paleoambiente de la Formación Agrio (Cretácico Inferior), en la Provincia de Mendoza, Cuenca Neuquina, Argentina

Biostratigraphy and palaeoenvironment of the Agrio Formation (Lower Cretaceous), Mendoza Province, Neuquen Basin, Argentina. A bioestratigraphical and palaeoenvironmental study of the Agrio Formation, at the Cuesta del Chihuido section, southern Mendoza Province (35o45S/69o34W), Argentina, is presented. Thirty seven species of nannofossils, thirty four of foraminifers and six of ostracods have been recognized. Nannofossils association corresponding to Zones CC4A y CC4B indicates an Upper Valanginian-Upper Hauterivian age. At the base of the section, the presence of the benthonic foraminifer Epistomina loncochensis Ballent and radiolaria suggests eutrophic conditions of surface waters and oxygen depressed sea-bottom conditions. In the middle part of the section, diversity of lagenid foraminifers and nannofossils indicates a shelf and nearshore environment with clear and well-oxygenated waters.

Nautilids from the Upper Cretaceous of the James Ross Basin, Antarctic Peninsula

To date, Cretaceous nautilids from the Antarctic Peninsula have received little attention and only a single species had been reported, Eutrephoceras simile Spath, from Seymour, Snow Hill, and James Ross islands. Currently, it is considered a synonym of Eutrephoceras subplicatum (Steinmann), which has also been described from the Upper Cretaceous of central Chile, southern Argentina and Angola. Here, we report and describe E. subplicatum in detail, based on specimens from the Lower Campanian–Maastrichtian of Vega, Seymour and James Ross islands, presenting, for the first time, embryonic conch features related to the palaeoecology of these organisms. The nauta of this species had a diameter of approximately 30 mm with 5–6 septa. In addition, we describe a new species, Eutrephoceras antarcticum, and one specimen assigned to the same genus in open nomenclature, both recovered from the Lower Campanian beds of James Ross Island.

NANOCÓNIDOS DEL GRUPO MENDOZA (CRETÁCICO INFERIOR) EN LA PROVINCIA DEL NEUQUÉN, REPÚBLICA ARGENTINA: TAXONOMÍA, CRONOESTRATIGRAFÍA E IMPLICANCIAS PALEOGEOGRÁFICAS

Abstract. n NANNOCONIDS OF THE MENDOZA GROUP (LOWER CRETACEOUS) NEUQUEN PROVINCE, REPUBLICA ARGENTINA: TAXONOMY, CRONOSTRATIGRAPHY AND PALEOGEOGRAPHIC SIGNIFICANCE. Late Jurassic-Early Cretaceous nannoconid assemblages are reported from 11 sections in the Neuquén Basin. Six species and 11 subspecies of Nannoconus Kamptner are studied systematically. Among them, Nannoconus kamptneri minor Bralower, Nannoconus kamptneri kamptneri Brönnimann, Nannoconus bucheri Brönnimann, Nannoconus circularis Deres and Achéritéguy and Nannoconus ligius Applegate and Bergen are highlighted as markers for the studied interval. Nannoconus elongatus minutus Van Niel, Nannoconus quadriangulus apertus Deflandre y Deflandre-Rigaud, Nannoconus truitti frequens Deres and Achéritéguy, Nannoconus truitti rectangularis Deres and Achéritéguy, Nannoconus truitti truitti Brönnimann are described for the first time in the Neuquén Basin. Nannoconus steinmannii minor Deres and Achéritéguy is reported for the first time in the Southern Hemisphere. The biostratigraphic analysis suggests an ampliation of the stratigraphic ranges of Nannoconus elongatus minutus, Nannoconus quadriangulus quadriangulus Deflandre and Deflandre-Rigaud, Nannoconus quadricanalis Bown and Concheyro, Nannoconus truitti frequens and Nannoconus truitti rectangularis. Changes in the composition and diversity of the recorded nannoconid assemblages suggest variations in the paleobiogeographic and paleoecological conditions that may be helpful in the characterization of the sedimentary environments represented in the Vaca Muerta and Agrio Formations.

Calcareous Nannofossils from the La Barca Formation (Paleocene/Eocene Boundary), Tierra Del Fuego, Argentina

The La Barca Formation, an organic-carbon rich sedimentary succession exposed along the Atlantic coast of Peninsula Mitre (Tierra del fuego, Argentina), was initially assigned to the late Paleocene, based on foraminifera and dinocysts. Planktonic foraminifera or calcareous nannofossils were not previously recorded in the two members of this formation. New samples of the upper member provided calcareous nannofossils that correlate with the late Paleocene-early Eocene assemblages in the Southern Hemisphere, spanning nannofossil zones NP9a/NP9b. The Paleocene/Eocene boundary in the La Barca Formation could be placed between the last occurrences of Fasciculithus richardii group and F. alanii, and the first occurrence of Romboaster cf. R. cuspis.

Biostratigraphy and palaeoenvironment of the Agrio Formation (Lower Cretaceous), Mendoza Province, Neuquén Basin, Argentina.

Se caracteriza bioestratigrafica y paleoambientalmente a la Formacion Agrio en la seccion Cuesta del Chihuido, en el sur de la Provincia de Mendoza (35o45S/69o34W), Argentina. Se han reconocido treinta y siete especies de nanofosiles, treinta y cuatro de foraminiferos y seis de ostracodos. Los nanofosiles permiten asignar la edad al Valanginiano superior-Hauteriviano superior e incluir la asociacion en las Zonas CC4A y CC4B. En la base de la seccion, la asociacion del foraminifero bentonico Epistomina loncochensis Ballent y radiolarios sugieren condiciones altamente eutroficas en la columna de agua con abundante desarrollo de microbiota y fondos deficitarios en oxigeno. Hacia la parte media, la diversidad de foraminiferos lagenidos y de nanofosiles senala condiciones marinas normales de plataforma, con aguas claras y bien oxigenadas. Abundantes polimorfinidos, spirillinidos e involutinidos hacia el techo de la seccion indican una marcada tendencia a la somerizacion. Las intercalaciones calcareas-pelitico arenosas que representan depositos tempestiticos distales contienen una microfauna aloctona de foraminiferos adheridos y caparazones adultos de ostracodos cytheroideos.