1 Cricyt, Ianigla, Avda. R.Leal s/n, 5500 Mendoza, Argentina. E–mail: mberesi@lab.cricyt.edu.ar

2 Museo de Geología y Paleontología, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina. E–mail: sheredia@uncoma.edu.ar

Key words: Epiphyton. Cyanobacteria. Middle Ordovician. Mendoza. Argentina.

Outcrops of the Precordillera terrane, extend from latitude 29º S to 33º S, and correlative rocks appear near San Rafael City (35º S / 68º 20 ‘ W) in the south of Mendoza Province, Western Argentina (Figure 1a)("Cuyania" of Ramos, 1995; Keller et al., 1996). These lower Paleozoic exposures occur in the Sierra Pintada range, which represent the San Rafael Block (Criado Roqué and Ibáñez, 1979). In the San Rafael Block the lowest sedimentary unit is represented by the Ponón Trehué Formation, whose Llanvirn–Caradoc deposits suggest shallow clastic to deep carbonate facies. Astini (1999) suggests that these deposits represent an extensional episode postdating the collision of the Precordillera terrane. The Ponón Trehué Formation overlies the Precambrian basement (Cerro La Ventana Formation), which is composed of Grenvillian (Bordonaro et al., 1996) rocks: gneisses, granitoids and siliciclastic metasediments. These rocks are alternatively overlain by Ordovician (Ponón Trehué Formation) and Carboniferous strata (Pájaro Bobo Formation), which present a northwest–southeast alignment. The relationship between the continental reddish sedimentary rocks of Carboniferous age and the igneous–metamorphic Grenvillian complex is tectonic, while the contact between the latter ones and the Ordovician clastic sediments is an erosive unconformity (Criado Roqué and Ibáñez, 1979; Heredia, 1996). In a recent study of this area, Astini (2002) considered that the limestones of the Ponón Trehué Formation (sensu Bordonaro et al., 1996) were blocks and fragmentary carbonate bodies discontinuously exposed floating in arkose conglomerate, in agreement with Heredia (1998, 2001) and Beresi and Heredia (2000). We consider that the Ponón Trehué Formation represents a depositional cycle. This formation involves two different deposits: the lower one comprises coarse siliciclastic deposits (including olistoliths) and the upper one consists of fine, dark carbonate–fine clastic deposits. The biostratigraphy of these Ordovician outcrops has been based on conodont assemblages (Heredia, 1982; Bordonaro et al., 1996; Heredia, 1996; Lehnert et al., 1999; Heredia, 2001; Cingolani and Heredia, 2001). The conodont distribution in the Ponón Trehué Formation recognizes two biozones: the Pygodus serra Zone and Pygodus anserinus Zone (Figure 1b). Additional fossil materials recovered from the sampling are brachiopods, sponge spicules (Beresi and Heredia, 2000), briozoos, ostracodes, trilobites and algae fragments.

Figure 1. A. Location map of the study area in Mendoza Province, Argentina. B. Ponón Trehué Formation, La Tortuga section and sample levels.

The purpose of this study is to record for the first time the occurrence of Epiphyton in the Ordovician carbonate platform of the Cuyania terrane. This cyanobacterium was recovered from La Tortuga section (Middle Ordovician) of the Ponón Trehué region, southern Mendoza province.

This is a normally graded siliciclastic–carbonate succession, 25 m in thickness (Figure 1b). The basement of the La Tortuga section is composed of metamorphic rocks (Criado Roqué and Ibáñez, 1979), which are Grenvillian in age. An erosive surface separates it from the Ordovician sedimentary rocks. A normally graded, very thick conglomerate that contains pebbles and blocks derived from the underlying rocks occurs at the base of the unit. It passes transitionally into very thick sandstones with carbonate cement that show thick parallel laminations. They are also normally graded with thick grains of quartz chaotically interbedded among fine silts. These dark gray to black siliciclastic–carbonate beds exhibit remains of crinoids, trilobites and brachiopods on the bedding plane surface. Progressively to the top, the beds are thinner (1 to 2 cm in thickness) and laminated–normally–graded carbonate sandstones are recognized. Transitionally, thin dark limestone beds occur, and grade into slumped limestones and black shales beds, with floating isolated rounded clasts of quartz. The top of this succession is truncated by faulting.

The Ponón Trehué Formation is characterized by an increasing amount of carbonate material towards–the top. The sequence represents a transition from a predominantly coarse clastic shallow regime (Astini, 2002) to a predominantly clinoform deep–water carbonate regime (Heredia and Beresi, 2000; Beresi and Heredia, 2000; Heredia, 2001; Astini, 2002).

Sampling was restricted to carbonate layers. These limestones were subject to the standard procedure for conodonts (Stone, 1987). The insoluble residue was recovered with sieve nº100 and 200 IRAM. Only two samples (T12 and T14) yielded Epiphyton specimens which were separated using a binocular microscope. So far, 15 specimens have been collected. The preservation of the specimens is good; despite their frequently being incomplete.

Epiphyton sp.

Plate 1

1. External morphology:

a. Skeleton: calcareous dendritic, elongate, dichotomously branching

b. Stem: straight, elongate, diameter: 0.2 mm, length: 0.6 mm

c. Main Branches: diameter:0.4,0–5,0.8 mm, length .0.3–0.5 mm, angles acute: 45 º

d. Secondary branches: diameter: 0.1 mm, Length: fragmentary 0.2 mm

Occurrence: Present in samples PT 12 and PT 14 of the Ponón Trehué Formation, La Tortuga section, San Rafael Block, southern Mendoza province, Argentina.

Remarks: Specimens are well preserved, especially the one illustrated. They have a radial branching pattern with the marked distal multiplication of branches commonly observed in the genus.

Plate 1. Scanning Electron Microscope microphotograph of Epiphyton sp., from Ponón Trehué Formation, Mendoza province, Argentina. Scale bar 0.1 mm.

Accepted: June 15, 2003