FIELD TRIP GUIDE ON THE SAN ISIDRO – MENDOZA

Susana HEREDIA 1

1 CONICET - Museo de Geología y Paleontología, Universidad Nacional del Comahue. Buenos Aires 1400,

(8300) Neuquén, Argentina. E-mail: sheredia@uncoma.edu.ar 

Introduction

The classical geologic locality of San Isidro is one of the first eastern outcrops of the Mendoza Precordillera. San Isidro is located in the foot-hill, on the Precordillera overthrust front and it is 17 km away from Mendoza City. This locality has been taken as one of the reference sections for the Ordovician system of Argentina.

From Mendoza City to this geological site we will cross a modern alluvial fan, the Mariño Formation (aeolian-fluvial Tertiary deposits), and Cacheuta and Cabras Formations (fluvial-lacustrine Triassic deposits).

Triassic and Tertiary deposits overlaying Quaternary alluvial sediments produce by a group of inverse faults (Divisadero Fault system). An important inverse fault associated to Divisadero S.F. allows Ordovician outcrops over Triassic deposits (Cabras Formation).

Several authors have studied this place. Among them, we must mention Borrello (1969, 1971), Harrington and Leanza (1957) and Turner (1961) who discovered and organized the Cambrian and Ordovician fossils macro fauna firstly in this region.

Empozada Formation (Harrington and Leanza, 1957)

It is a clastic unit of 430 m thick, characterized by two members, which are clearly distinguished in outcropps. There is a marked change in the sedimentation between both members (major unconformity). The basal part of Empozada Formation is under stratigraphical revision (Heredia and Beresi, in press). Empozada Formation gives an excellent example for different types of allochtonous deposits: rockfalls deposits, canyon-fill deposits and diamictites.

The Lower Member of the Empozada Formation (Middle and Upper Ordovician) is discontinuously exposed in thickness. It composed by 305 m (on San Isidro Creek) of following lithofacies: megabreccias and breccias, green shales, dark conglomerates, mudstones and sandstones, paraconglomerates ("debris flow complex") and black shales.

We can recognize in the Lower Member three different sedimentary events:

The first one is assigned to talus-basin deposits, which includes large olistoliths (see below). The autochthonous paleontological material is mostly composed of graptolites, which enable to define the Paraglossograptus tentaculatus Zone (LLanvirn): Amplexograptus aff. A. confertus Lapworth, Glossograptus hincksii hincksii (Hopkinson), Oelandograptus

austrodentatus cf. austrodentatus (Harris y Keble), fragments of cf. Glyptograptus. Recently, Brussa and Toro (2000) found out graptolites such as Isograptus caduceus caduceus (Salter), Isograptus victoriae cf. maximus Harris, Dichograptus cf. octobrachiatus (J.Hall), Pseudisograptus sp. y Xiphograptus? sp. This association indicates isograptids biofacies, which is equivalent to the Arenig.

The second allows us to figure as turbidite deposits (canyon-fill deposits) belonging to Nemagraptus gracilis/Climacograptus bicornis Zone (Caradocian) (Alfaro and Fernández, 1985).

The last one, the black shales at the top, interpreted as a condensed event (TST) from Orthograptus quadrimucronatus Zone (Upper Caradocian- Lower Ashgill) (Alfaro, 1988) and, at the top (unconformity below), Dicellograptus complanatus y D. ornatus Zones (Ashgill s.l ) are defined (Mitchell et al.1998).

On the other hand, Cambrian and Ordovician fossil material was identified on allochtonous rocks. (1) San Martin Olistoliths: trilobites (agnostiths) such as Glyptagnostus reticulatus y Aphelaspis sp. mentioned by Bordonaro et al. (1993), they defined the Upper Cambrian. There are, also, sponge spicules, sponge parts and fragment of shells. Lithology is composed mainly by dark carbonate mudstones and shales.

(2) San Isidro Olistoliths (Bordonaro et al. 1993) trilobites (Athabasquia digesta and Glossopleura inexulcata), hyolites, inarticulate brachiopods (Obolus?) y sponge spicules. San Isidro Creek: trilobites such as: Chilometopus parabolicus, Kistocare mendozanun, Alokistocare elongatum, Kootenia incerta, Zacantoides ferula, Agnostus sp., Mendogaspis sp. (Glossopleura Zone, Middle Cambrian) (Borrello, 1971). Tonkinella stephensis (Middle Cambrian) appering in

minor fragments. Lithologically is composed by packstones and grainstones, glauconite is abundant in some levels, at the top wackestones containing well preserved trilobites are present.

(3) La Cruz Olistoliths (Bordonaro, 1992) Borrello (1971) defined 3 trilobites zones Cedaria, Elvinia y Saukia from the Upper Cambrian. Heredia (1995) extended to Tremadoc the age of these olistholiths. Conodont zones: Proconodontus tenuiserratus Zone and Paltodus deltifer Zone. The lithology is the same for San Martin olistoliths. It represents a fragment of a Upper Cambrian talus.

The transport for huge olistoliths is rockfall along the slope to the basin, and maybe assigned to a major tectonic event. This is a regional event well defined along 400 km, from San Isidro to Jáchal Ordovician outcrops Cambrian and Ordovician Blocks: Black shales with Tetragraptus aproximatus, T. fruticosus, T. quadribrachiatus Zones (Bordonaro and Peralta, 1987). Black mudstones with Upper Cambrian conodonts (Phakelodus tenuis Miller) In the "debris flow" there are mudstones with trilobites (Trilobagnostus sp. and Pseudagnostus idalis idalis) from the Upper Cambrian (Bordonaro et al. 1993), The Upper Member is composed by three lithofacies: very fine grained sandstones and mudstones alternating, carbonate paraconglomerates and carbonate sandstones and mudstones alternating. The interpretation of these lithofacies is complex. They show slumping, load-casts, lenticular bedding, hummocky cross-stratification, and we can use the term "heterolitic" for describe it. They were considered as platform facies, deposited by gravity flows, originated by major storm waves (Gallardo et al., 1988). Recently they were assigned at the Ashgill marine glacial event (Keller, 1999; Beresi and Heredia, 2000).

Resedimented carbonate clasts are interpreted as old outer-middle carbonate shelf material re-transported and deposited on the platform. Two conodont zones were found out: Oepikodus evae and Amorphognathus superbus. The last one with A. superbus, Aphelognathus rhodesi, Drepanoistodus suberectus, Icriodella superba, Plectodina sp. cf. P. tenuis, Plectodina sp., Protopanderodus liripipus, Rhodesognathus elegans, etc.

At the top a hardground is developed. Devonian green shales of Canota Formation Cor Villavicencio Formation overlie it.

References

Alfaro, M., 1988. Graptolitos del Ordovícico superior (Caradociano) de la quebrada Agua de la Cruz, Precordillera de Mendoza. Ameghiniana 25(4):299-303. Buenos Aires.

Alfaro, M. and Fernández, R., 1985. Una graptofauna del Ordovícico superior (Caradociano) de Estancia Canota (provincia de Mendoza). Ameghiniana 22(1-2):63-67. Buenos Aires.

Astini, R., Ramos, V., Benedetto, J.L., Vaccari, N.E. and Cañas, F. 1986. La Precordillera: un terreno exótico a Gondwana. Actas 13º Congreso Geológico Argentino y 3º Congreso de Exploración de Hidrocarburos, 5: 293-324. Buenos Aires.

Beresi, M. and Heredia, S., 1995. Asociación de espículas de poríferos cámbricos de la Precordillera de Mendoza. Ameghiniana, 32 (4):401-405. Buenos Aires.

Bordonaro, O., 1992. El Cámbrico de Sudamérica. En: Gutiérrez Marco J.C., Saavedra y Rábano, I. (Eds.): Paleozoico Inferior de Iberoamerica. Publicación Universidad de Extremadura, 69-84.

Bordonaro, O. and Banchig, A. 1996. Estratigrafía de los olistolitos cámbricos de la Precordillera argentina. Actas 13º Congreso Geológico Argentino y 3º Congreso de Exploración de Hidrocarburos, Buenos Aires, 5: 471 – 480.

Bordonaro, O., Banchig, A y Arroqui Langer A. 1999. Síntesis estratigráfica de los depósitos del margen continental occidental paleozoico inferior de la Precordillera de Cuyo. Actas 14º Congreso Geológico Argentino, I: 393-396. Salta

Bordonaro, O., Keller, M. and Beresi, M. 1993. Reordenamiento estratigráfico del Cámbrico del área de San Isidro, Precordillera de Mendoza. Actas 12º Congreso Geológico Argentino y 2º Congreso de Exploración de Hidrocarburos, II: 12 -19. Mendoza.

Bordonaro, O. and Peralta, S., 1987. El Arenigiano Inferior de la Formación Empozada en la localidad de San Isidro, Mendoza, Argentina. Actas 10º Congreso Geológico Argentino, 3: 81-84. San Miguel de Tucumán.

Borrello, A.V., 1969. El flysh paleoídico de Mendoza. Actas 4° Jornadas Geológicas Argentinas, I: 75-89, Buenos Aires.

Borrello, A.V., 1971. The Cambrian of South America. In: Cambrian of the New World. Wiley-Interscience, Holland (Ed.), 1: 385-438. New York.

Brussa, E. D., Mitchell, C. and Astini, R. 1999. Ashgillian (Hirnantian?) graptolites from the western boundary of the Argentine Precordillera. Acta Universitatis Carolinae- Geologica 43(1/2):199-202. Prague.

Brussa E. D. and Toro B. A., 2000. Una nueva asociación de graptolitos del Arenigiano superior en el área de San Isidro, Precordillera de Mendoza. Ameghiniana 37 (4) Suplemento, 2000- Resúmenes,71 R.

Cuerda, A., 1988. Investigaciones estratigráficas en el «Grupo Villavicencio», Precordillera de Mendoza y San Juan, República Argentina. Actas V Congreso Geológico Chileno, III C: 177-187. Santiago de Chile.

Cuerda, A. and Alfaro, M.1992. Faunas Graptolíticas. Relatorio del 12º Congreso Geológico Argentino, parte II: Paleontología: 293-296. Mendoza.

Cuerda, A., Cingolani, C. and Bordonaro O., 1993. Las secuencias sedimentarias eopaleozoicas. Relatorio del 12º Congreso Geológico Argentino. Parte I: Geología: 21-30. Mendoza.

Gallardo, G. and Heredia, S., 1995. Estratigrafía y sedimentología del Miembro Inferior de la Formación Empozada (Ordovícico Medio y Superior), Precordillera Argentina. Boletín Academia Nacional de Ciencias 60 (3 - 4):449- 460. Córdoba.

Gallardo, G., Heredia, S. and Maldonado, A., 1988. Depósitos carbonáticos alóctonos, Miembro superior de la Formación Empozada, Ordovícico Superior de la Precordillera de Mendoza, Argentina. Actas 5º Congreso Geológico Chileno 1:37-53.

Harrington, H.J. and Leanza, A.F., 1957. Ordovician trilobites of Argentina. University of Kansas, Department of Geological Sciences, Special Publication 1: 1-259. Lawrence.

Heredia, S., 1993. El Llanvirniano de la Precordillera de Mendoza, Argentina. Actas 12º Congreso Geológico Argentino I:138-141. Mendoza.

Heredia, S., 1994. Conodontes y Bioestratigrafía del Cámbrico Superior de Cerro Pelado y San Isidro, Provincia de Mendoza, Argentina. Tesis Doctoral (Inédita), Universidad Nacional de Córdoba, 285 p.

Heredia, S., 1995. Conodontes cámbricos y ordovícicos en los bloques alóctonos del Conglomerado basal de la Formación Empozada, Ordovícico Medio- Superior, San Isidro, Precordillera de Mendoza, Argentina. Boletín Academia Nacional de Ciencias, 60 (3-4):235-248. Córdoba.

Heredia, S. and Beresi, M. (En prensa). La Formación Empozada y su relación con la Formación El Peñón (nom. nov.), Ordovícico de la Precordillera de Mendoza, Argentina. Revista de la Asociación Geológica Argentina.

Heredia, S. and Gallardo, G., 1996. Las Megaturbiditas de la Formación Empozada: un modelo interpretativo del Ordovícico turbidítico de la Precordillera Argentina. Revista Asociación Geológica Chilena, 23(1):17-34. Santiago de Chile.

Heredia, S., Gallardo, G. and Maldonado, A., 1990. Conodontes caradocianos en las calizas alóctonas del Miembro superior de la Formación Empozada (Ordovícico medio y superior), San Isidro (Mendoza, Argentina). Ameghiniana 27(3-4):197-206. Buenos Aires.

Keller, M. 1999. Argentine Precordillera: Sedimentary and Plate tectonic history of a Laurentian crustal fragment in South America. Geological Society of America, Special Paper: 1-118.Boulder.

Keller, M., Bordonaro O. and Beresi M., 1993. The Cambrian of San Isidro, Mendoza, Argentina: Facies and Sedimentology at the platform slope transition. Neues J. für Geologie und Paleontologie Monatshefte, 193/ 6:373-383.

Mitchell, C.E., Brussa, E.D., Toro, B., Astini, R., 1998. Late graptolites from Empozada Formation, Argentine Precordillera, an outer shelf, cool water, peri-Gondwan assemblage?. Temas Geologicos-Mineros ITGE 23:224 – 226.

Turner, J. C. M., 1959. Faunas garptolíticas de America del Sur. Rev. Asoc. Geol. Argentina 14 (1-2):5-180.