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

BERESI M.S. AND HEREDIA S.E

Serie Correlación Geológica

INSUGEO, Serie Correlación Geológica

Lugar: Tucumán; Año: 2003 vol. 17 p. 257 - 262

1514-4186

In: Albanesi G. L., Beresi M. S. and Peralta S. H. (Eds.). INSUGEO, Serie Correlacion Geologica, 17: 257 - 261 Ordovician from the Andes                  Tucuman, 2003 - ISSN 1514-4186 - ISSN on-line 1666-9479 First occurrence of Epiphyton, cyanobacteria from the Middle Ordovician of the Ponon Trehue, Mendoza Province, Argentina Matilde S. BERESI 1 and Susana HEREDIA 2 Key words: Epiphyton. Cyanobacteria. Middle Ordovician. Mendoza. Argentina. Introduction Outcrops of the Precordillera terrane, extend from latitude 290 S to 330 S, and correlative rocks appear near San Rafael City (350 S / 680 20 ´ W) in the south of Mendoza Province, Western Argentina (Figure 1a)("Cuyania" of Ramos, 1995; Keller et a1., 1996). These lower Paleozoic exposures occur in the Sierra Pintada range, which represent the San Rafael Block (Criado Roque and Ibanez, 1979). In the San Rafael Block the lowest sedimentary unit is represented by the Ponón Trehue 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 PonDn Trehue Formation overlies the Precambrian basement (Cerro La Ven­tana Formation), which is composed of Grenvillian (Bordonaro et a1., 1996) rocks: gneisses, granitoids and siliciclastic metasediments. These rocks are alternatively overlain by Ordovician (Ponón Trehue 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 Roque and Ibanez, 1979; Heredia, 1996). In a recent study of this area, Astini (2002) considered that the limestones of the Ponón Trehue Formation (sensu Bordonaro et a1., 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 PonDn Trehue 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 PonDn Trehue Formation recognizes two biozones: the Pygodus serra Zone and Pigodus australodentatus Zone (Figure 1 b). Additional fossil materials recovered 1 CRICYT,IANIGLA. Avda. R.Leal sin, 5500 Mendoza, Argentina. E-mail: mberesi@lab.cricyr.edu.ar 2 Museo de Geologia y Paleontologia, Universidad Nacional del Comahue, Buenos Aires 1400,8300 Neuquen, Argentina. E-mail: sheredia@uncoma.edu.ar