Ordovician calcareous algae, cyanobacteria, and microproblematica from the Precordillera Argentina
Matilde S. Beresi1
1 Cricyt–Ianigla, Avda. R.Leal s/n, 5500 Mendoza, Argentina. E–mail: email@example.com
Key words: Calcareous algae. Cyanobacteria. Microproblematica. Ordovician. Precordillera.
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 grainstones, 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 Fernández, 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.
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. Nuia was found in transverse, longitudinal and tangential sections. Girvanella and Rothpletzella 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 (Asphaltina and Calciphona, Beresi, 1986), but that in this paper are mentioned only as spherical forms of probable algal origin (Plate 1, Figures h, k).
Thin sections containing individuals of the illustrated species have been deposited in the paleontological collection of the IANIGLA,CRICYT, Mendoza.
Genus Girvanella Nicholson and Etheridge, Jr. 1878
Type species: Girvanella problematica Nicholson and Etheridge, Jr. 1878 p. 23
Plate 1, figures f, g, i
Description: calcareous non–branched tubular filaments, sinuous to irregularly tangled, normally prostrate, wall thin, micritic. Girvanella forms nodules and encrusting masses consisting of individual round sinuous tube without cross partitions (Riding and Voronova, 1985).
Dimensions: external diameter of filaments: 12–37 microns
Occurrence: clasts containing tangled Girvanella tubes mainly occur in skeletal–grainstone and skeletal–peloidal packstone of the San Juan Formation in several localities of the Precordillera. It forms intraclasts (0.2–0.3 mm) on large skeletons and bioclasts of brachiopods or other skeletal fragments in grainstones or wackestone–packstones.
Discussion: the systematic position of Girvanella is somewhat problematic owing to its relative morphological simplicity and lack of internal features. It is now regarded as a cyanobacterium (Wray, 1977; Riding 1991).
Genus Rothpletzella Wood, 1948
Type species: Sphaerocodium gotlandica Rothpletz 1908, Rothpletzella gotlandica (Rothpletz) Wood, 1948, p. 19
Plate 1, figure j
Description: calcareous tubular filaments, prostrate, forming dense masses, wall thin, micritic.
Small irregularly rounded to elongate reworked algal balls with tube filaments with small diameter. In a thin section of the San Juan Formation, nodules of Rothpletzella are present with the "spaghetti"aspect of the small–intertwined calcareous tubes.
Dimension: diameter of calcareous filaments: 15–40 microns.
Occurrence: Rothpletzella occurs as clasts, nodules or algal balls of the 300–1000 microns, in skeletal–wackestones–packstone, in some samples with pelletoidal matrix, associated with Nuia.
Remarks: there has been confusion between the names Sphaerocodium Rothpletz and Rothpletzella (Wood, 1948, and Riding, 1979). The name Sphaerocodium refers to a Triassic oncoid (Riding and Fan, 2001). The fossils identified here are Rothpletzella but were previously identified as Sphaerocodium (Beresi, 1985). Rothpletzella has been regarded as a cyanobaterium or green alga, but its affinities are unclear (Riding 1991).
Genus Halysis Hoeg, 1932
Type species: Halysis moniliformis Hoeg, 1932
Plate 1, figure l
Description: calcareous rounded cells, juxtaposed in a thin undulate series; wall–structure obscure, micritic or fibrous.
Dimensions: the generally well rounded cells range in diameter from 90–175 microns, average 100 microns. Wall thickness: 21 microns.
Remarks: the specimens have cells (100 microns) similar to the type material.
Occurrence: the specimens occur in skeletal–wackestone from the San Juan Formation.
Genus Nuia Maslov, 1954
Type species: Nuia sibirica Maslov, 1954
Plate 1, figures a, b, c, d, e
Plate 1. Nuia sibirica Maslov, San Juan Formation, x 100; a, b, c, e, in transversal sections, c, d, in longitudinal sections; e, Girvanella sp. nodule and Nuia; x32. f, g, i, algal ball with Girvanella problematica, x32; j, nodule with Rothpletzella, x 50; h, k, spherical algae, x 60; l, Halysis moniliformis , x 100.
Description: calcareous radial–fibrous concentric layers, surrounding a micritic centre, rounded or elongate. In cross sections Nuia generally has a circular and ellipsoidal shape. The specimens have single layered and multilayered walls as different growth forms. Nuia has straight or curved calcareous tubes, having the distinctive dark central canal. The wall is composed of calcite prisms, radially arranged around the central canal.
Dimensions: diameters (width) of central canal: 40–90 microns, average (70); outer diameter: 100–450 microns, average (250); external length: 0,7–1,80 mm.; width single layer: 0.15–0.40 mm, average (0.25). These dimensions are based on randomly oriented thin sections. The longitudinal sections of Nuia represent fragments.
Occurrence: these specimens occur in skeletal–wackestone–packestones, from the San Juan Formation and in the Lower Member of Las Aguaditas Formation, Central Precordillera of San Juan (Cabaleri, 1986).
Remarks: Nuia is a microorganism of probable algal affinity (Toomey and Klement, 1966); it is considered problematic and has been variously assigned to green and red algae and to cyanobacteria.
The platform of the San Juan Formation, represents a relatively shallow subtidal depositional setting, with open circulation. The calcareous algae and microproblematica were commonly associated with an open marine shelly fauna of brachiopods, crinoids, sponges, trilobites, nautiloids, gastropods, bryozoans and conodonts.
Girvanella and Nuia are considered as typical inhabitans of low–latitude carbonate settings, their presence confirms the existence of warm waters during the Lower Ordovician of the Precordillera platform.
The algal microflora of the San Juan Formation is an assemblage of low specific diversity and is characteristic of a cosmopolitan flora. There are also intraclasts and spherical forms that may have been derived from calcified algae and cyanobacteria. These bioclasts and the algal microflora were important components in the construction of the carbonates of the Precordillera.
The occurrence of these cyanobacteria and microploblematica in the Precordillera extends the geographical distribution pattern of the cosmopolitan genera Nuia and Girvanella and contributes to a fuller understanding of Lower Ordovician paleobiogeography.
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Received: February 15, 2003
Accepted: June 15, 2003