San Isidro, Precordillera de Mendoza, Intra-symposium Field Trip

HEREDIA S.; BERESI M.S.

Mendoza

Simposio; 3rd. International Conodont Symposium; 2013

Pander Society

Intra-symposium Field Trip SAN ISIDRO, PRECORDILLERA OF MENDOZA Susana HEREDIA¹ and Matilde BERESI² ¹CONICET-CIGEOBIO-Instituto de Investigaciones Mineras. Universidad Nacional de San Juan. Av. Libertador y Urquiza. CP: 5400. San Juan. Argentina. sheredia@unsj.edu.ar ² CONICET-IANIGLA, CCT Mendoza, Av. Ruiz Leal s/n, CP: 5500. Mendoza. Argentina.  mberesi@mendoza-conicet.gob.ar  The classical geologic locality of San Isidro is one of the first eastern outcrops of the Mendoza Precordillera.  San Isidro is located at the foot-hill of the Precordillera overthrust front and it is 17 km away from Mendoza City.  This locality is considered 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). The Triassic and Neogene deposits are overlaying Quaternary alluvial sediments due a group of inverse faults (Divisadero Fault system).  The section starts with the Ordovician outcrops over the Triassic Cabras Formation due to an important inverse fault. 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 (macrofauna) firstly in this region. The Estancia San Isidro and Empozada formations give an excellent example for different types of allochtonous deposits: rockfalls deposits, canyon-fill deposits and diamictites. The upper overlaying succession is the Empozada Formation, of Sandbian -Hirnantian? age. Above that we will visit the Villavicencio Formation (Upper Devonian) with olistholiths of Cambrian, Ordovician and Lower Devonian ages. Estancia San Isidro Formation (Heredia and Beresi, 2004) A new stratigraphic succession is established for the Ordovician deposits of the San Isidro area. Darriwilian talus olisthostromic / olistholith deposits are cropping out in the San Isidro region. The basal siliciclastic sequence deposited in a talus with steep gradient, is now defined as the Estancia San Isidro Formation (nom. nov.). The Estancia San Isidro Formation can be correlated to other contemporaneous sequences of Cuyania that are referred to an extensive convulsive event from the upper Darriwilian. The first one, assigned to talus-basin deposits, 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 y 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. On the other hand, the allochtonous rocks have Cambrian and Ordovician fossils:  (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 (Beresi, 2013).  Dark carbonate mudstones and shales are the main lithology  (2) San Isidro Olistoliths (Bordonaro et al.,1993) trilobites (Athabasquia digesta and Glossopleura inexulcata), hyolites, inarticulate brachiopods (Obolus?) and 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 packstone and grainstone, glauconite is abundant in some levels, at the top wackestone containing well preserved trilobites are present. (3) La Cruz Olistoliths (Bordonaro, 1992) Borrello (1971) defined three trilobites? zones Cedaria, Elvinia and 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 fragments of a upper Cambrian talus. The transport for huge olistoliths is the rockfall along the slope to the basin, and maybe assigned to a major tectonic event.  Empozada Formation (Harrington and Leanza, 1957) Sandbian- Katian and possibly Hirnantian deposits of 430 m thick, characterized by two members, which are clearly distinguished in outcrops.  There is a marked change in the sedimentation between both members (major unconformity).  The Lower Member of the Empozada Formation (Middle and Upper Ordovician) is exposed discontinuously in thickness.  It composed by 305 m (on San Isidro Creek) of following lithofacies: dark conglomerate, mudstone and sandstone, paraconglomerate (?debris flow complex?) and black shale. We can recognize in the Lower Member two different sedimentary processes: The first allows us to figure out a turbidite deposits (canyon-fill deposits) belonging to Climacograptus bicornis Zone (Sandbian) (Alfaro and Fernández, 1985). Cambrian and Ordovician Blocks: a.black shale with Tetragraptus aproximatus, T. fruticosus, T. quadribrachiatus Zones (Bordonaro and Peralta, 1987); b. black mudstone with Upper Cambrian conodonts (Phakelodus tenuis Miller) and c. in the "debris flow" there is mudstone with trilobites (Trilobagnostus sp. and Pseudagnostus idalis idalis) from the Upper Cambrian (Bordonaro et al. 1993),  The second one, the black shale at the top, is interpreted as a condensed deposit (TST) from Dicellograptus complanatus and D. ornatus zones (Katian) are defined (Mitchell et al.1998).  The Upper Member is composed by three lithofacies: very fine grained sandstone and mudstone alternating carbonate paraconglomerate and carbonate sandstone and mudstone 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 to the Hirnantian 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. Three key conodonts were found out: Oepikodus evae, Amorphognathus superbus and A. ordovicicus. The last ones contains A. superbus, Aphelognathus rhodesi, Drepanoistodus suberectus, Icriodella superba, Plectodina sp.  cf.  P. tenuis, Plectodina sp., Protopanderodus liripipus, Rhodesognathus elegans, etc.