Ordovician calcareous algae, cyanobacteria, and microproblematica from the Precordillera Argentina

BERESI, M. S.

Serie de Correlación Geológica

INSUGEO

Lugar: Tucumán; Año: 2003 vol. 17 p. 251 - 256

1514-4186

In: Albanesi G. L, Beresi M. S. and Peralta S. H. (Eds.). INSUGEO, Serie Correlacion Geologica, 17: 251 - 255 Ordovician from the Andes                 Tucuman, 2003 - ISSN 1514-4186 - ISSN on-line 1666-9479 Ordovician calcareous algae, cyanobacteria, and microproblematica from the Precordillera Argentina Matilde S. BERES!´ Key words: Calcareous algae. Cyanobacteria. Microproblematica. Ordovician. Precordillera. Introduction Calcareous algae, cyanobacteria and microproblematica are present and are locally abundant in the Lower-Middle Ordovician carbonate platform sequences in the Argentine Precordillera terrane, western Argentina. Calcified cyanobacteria (Girvanella problematica) and Microproblematica (Rothpletzella sp.; Halysis monoliformis and Nuia sibirica) are descri­be from the San Juan Formation (Arenig-Lower Llanvirn). Algal grains may account for 35 % of the particles in many thin sections from this formation. These particles of algae are present in intraclastic grains tones, and bioclastic, peloidal wackestones. Nuia and intraclasts with Girvanella are the most common throughout the limestone successions. Gnoli and Serpagli (1979) mentioned Nuia for the first time at the locality of Pachaco in the San Juan Fm. Nuia, Girvanella and other associated forms, were described in petrographic thin sections of diverse sections from the San Juan Formation (Beresi, 1981, 1985, 1986, 1991). Nuia was mentioned from the Las Aguaditas Fm. (Middle Ordovician) (Cabaleri, 1985) and Girvanella was mentioned coming from the Los Sombreros Formation, western Precordillera of San Juan, as reworked carbonates (Bercowski and Fernandez, 1988). Nuia and Girvanella have recently been determined from the Cambrian-Ordovician transition of the Volcancito Formation, Famatina System (Astini, 2002). This paper reports and describes problematic microorganisms and cyanobacteria from the Lower-Middle Ordovician carbonate sediments of the Precordillera, western Argentina. Material Petrographic analysis of the thin sections from several measured sections of the San Juan Formation, revealed the presence of four genera, based on the examination of specimens in longitudinal and transverse sections that in general show good preservation. These calcareous algae constitute a wide range of morphological types with different sizes. Nllia was found in transverse, longitudinal and tangential sections. Gintlnella and Ro/hple~lIa are present as masses and oncoids or intraclasts. Also, there are globular solid forms with an exterior dark fIlm that were assigned with doubt to algae (Asphallina and Cakipholla, Beresi, 1986), but that in this paper are mentioned only as spherical forms of probable algal origin (plate I, Figures h, k). I CRICYT-IANIGLA, Avda. R.Leal sin, 5500 Mendoza, Argentina. E-mail: mberesi@lab.cricyt.edu.ar