CAMBRIAN AND ORDOVICIAN CALCAREOUS ALGAE, CYANOBACTERIA, AND ALGAE OF UNCERTAIN POSITION FROM THE ARGENTINEAN PRECORDILLERA: A REVIEW

BERESI M.S.; LUCHININA V.

MENDOZA

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

The Palaeontological Society

CAMBRIAN AND ORDOVICIAN CALCAREOUS ALGAE, CYANOBACTERIA, AND ALGAE OF UNCERTAIN POSITION FROM THE ARGENTINEAN PRECORDILLERA: A REVIEW   Matilde S. Beresi¹ and Veronica A. Luchinina²   ¹IANIGLA-CONICET/ CCT Mendoza, Av. Ruiz Leal s/n, C.P.5500. Mendoza. Argentina. mberesi@mendoza-conicet.gob.ar   ²A .A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian, Academy of Sciences, pr. Akademika Koptyuga 3. Novosibirisk, 630090,  Russia.   Calcareous algae, cyanobacteria and algae of uncertain position are present in the Middle Cambrian and can be locally abundant in Lower-Middle Ordovician and Lower-Upper Ordovician carbonate platform sequences in the Argentine Precordillera terrane, western Argentina. Outcrops of this terrane extend from 29º S to 33º S latitude, and correlative rocks appear near San Rafael City (35º S /68º 20 ' W) in the south of Mendoza Province. Renalcis group, Girvanella (Cyanobacteria) and a few specimens of the genus Solenopora (Rhodophyta,) were described from the Middle Cambrian shallow carbonates of the La Laja Formation (Bolaspidella Zone), and Girvanella mentioned from the Los Sombreros Formation, Precordillera of the San Juan Province. Also, Girvanella is common in the Olistholiths of the San Isidro area, Mendoza, Precordillera. Several Ordovician (Dapingian-Darriwilian) calcified cyanobacteria (Girvanella problematica); algae of uncertain position (Rothpletzella sp.; Halysis monoliformis and Nuia sibirica) were described from the shallow-subtidal carbonate platform of the San Juan Formation in the central and eastern ranges of the Precordillera of San Juan Province. Also, individual segments and fragments assigned to Chlorophyta, Family Dasycladaceae (Vermiporella? or Mastopora?) and globular solid forms with an exterior dark film referred with doubts to algae, were mentioned from this formation. These morphological algal types of different sizes may account for 35 % of the particles in many thin sections from this formation, and are associated with distinct limestone textures. Girvanella and Rothpletzella are present as oncoids or intraclasts in algal oncoid grainstones and Nuia and Girvanella mainly in bioclastic-peloidal wackestones. In a deeper-water slope sequence of the Las Aguaditas Formation (Darriwilian-Sandbian), Central Precordillera of San Juan, Nuia and Girvanella are the most common. In the San Rafael Block, Mendoza Province, a fragment of a dentritic thallus with lateral branches was recovered from the shallow clastic-carbonate facies of the Ponón Trehué Formation (Darriwilian-Sandbian). Its determination is doubtful. A new discovery, of an elongate algal septate thallus with branches, was recovered from clastic sediments of the La Cantera Formation (Sandbian) at the Villicúm Range, Eastern Precordillera of San Juan. It could be assigned to the Rhodophyta or to Chabakovia Vologdin. According to new interpretations, Chabakovia in fact, characterizes different stages of the life cycle of algae of the genus Epiphyton. The near-equatorial position of the Precordillera during the Middle Cambrian and Lower-Middle Ordovician times, favored the flourish of these ?flora? assemblages in the euphotic zone of tropical to warm-water carbonate platforms.