LOS FORAMINIFEROS PLANCTONICOS DE LA CUENCA CRETACICOTERCIARIA EN PATAGONIA SEPTENTRIONAL (ARGENTINA), CON CONSIDERACIONES SOBRE LA ESTRATIGRAFIA DE FORTIN GENERAL ROCA (PROVINCIA DE RIO NEGRO)

Alwine Bertels

Abstract


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The present paper concerns with a study of the Upper Cretaceous (Maastrichtian) and Lower Tertiary (Lower Danian ) Planktonic Foraminifera of the Roca, Jagüel, Huantrai-co, Coli-Toro and Malargüe formations, that crop out in northern Patagonia given in detall; the section NR (northern Roca) is selected as the type section of the Roca Formation. In the studied area, pIanktonic foraminifera show an abrupt change in the Cretaceous- Tertiary boundary. The Upper Cretaceous (Maastrichtian) genera Guembelitria, Heterohelix. Arcbaeoglobigerina, Rugoglobigerina, Rugotruncana and Globotruncana are replaced in the Lower Tertiary (Lower Danian) by Globoconusa, Subbotína and Globorotalía. The JagüeI, Huantrai-co, CoH- Toro and Malargüe formations are correlative, and of Upper Cretaceous age (Lower and MiddIe Maastrichtian ) ; the Roca Formation is of Lower Tertiary (Lower Danian) age. A zonation using pIanktonic Foraminifera is presented for the Cretaceous-Tertiary boundary in northern Patagonia. The Guembelitria cretacea AssembIage Zone is correIative to the Globotruncana gansseri subzone (Pessagno, 1967) while the Globoconusa daubjergensís AssembIage Zone is corre1ative with the Globorotalia pseudobulloides and part of the Subbotina triloculinoides Zones (Berggren, 1969). Between the Upper Cretaceous and Lower Tertiary a paleontologicaI hiatus, which comprises de uppermost Maastrichtian, is registered. The paleogeographic reconstruction of the geologicaI basin in the Upper Cretaceous and Lower Tertiary is intented. In northern Patagonia the beginning of the Upper Cretaceous transgression is represented by swampy and Iacunar deposits known as Lacustrine Senonian or Loncoche. The basin of fresh and brackish waters is replaced by a neatly marine environment which reaches their greatest deepness in Middle Maastrichtian time. During the Lower Tertiary (Lower Danian) a regressive sequence is registerd. Kee1ed globotruncanids are present in the Upper Cretaceous (Middle Maastrichtian) strata; they lead to the interpretation of some paIeoclimatic factors, such as warm water conditions, as far south as to 39 at this time, and a deep no Iess than 150 m in the deepest part of the basin.

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