First occurrence of Epiphyton, cyanobacteria from the middle Ordovician of the Ponón Trehue, Mendoza province, Argentina

Abstract

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.