The Cerro La Chilca, Central Precordillera, San Juan Province

PERALTA, S H

In: Ordovician and Silurian of the Precordillera, San Juan, Argentina

UNT-INSUGEO

Lugar: Tucumán.; Año: 2003; p. 1 - 1

The Geological Province of Precordillera (sensu Furque & Cuerda, 1979) is a typical thin?skinned thrust?and?fold belt, about 600 km in length, extending from the south of the La Rioja Province to the south of the Mendoza River, entirely crossing the San Juan Province. It includes three morphostructural units: Eastern Precordillera (Ortiz & Zambrano, 1981), Central Precordillera (Baldis & Chebli, 1969) and Western Precordillera (see Baldis et al., 1982). From a stratigraphic point of view, the Precordillera is characterized by the broad occurrence of Paleozoic rocks, mainly from the Lower Paleozoic, which includes conspicuous Middle?Upper Ordovician graptolite?rich black shales sequences, restricted to shallow?water environments in the Eastern and Central Precordillera, whereas in the Western Precordillera they are related to deep?water environments, evidenced by hemipelagic and turbiditic deposits, intercalated with basaltic rocks related to ophiolitic complex (Ramos et al., 1984; Haller & Ramos, 1984; Ramos et al., 1986). In contrast to the well studied units of the Eastern and Central Precordillera, the Lower Paleozoic biostratigraphy of the Western Precordillera is still not well?constrained, and only sporadic fossils, mainly graptolites, are known, and a few sections were measured in detail. Several significant Middle?Upper Ordovician sections are located across the Guandacol, Jáchal and San Juan rivers, where most important exposures are represented by the Trapiche Group and the Gualcamayo, Los Azules, Las Aguaditas, La Cantera, La Invernada, Los Sombreros, and Yerba Loca formations. These units are entirely or partly, Caradoc in age, while the younger Alcaparrosa Formation is uppermost Ordovician (Brussa et al., 1999). Far away to the South, in San Isidro Creek, Mendoza province, the Empozada Formation (Middle?Upper Ordovician), includes in the lower part a Darriwilian?Caradoc graptolitic black shale sequence (Cuerda, 1979; Heredia et al., 1996), besides of Hirnantian graptolite faunas of the complanatus and ornatus zones (Mitchell et al., 1998). In some of mentioned lithostratigraphic units, graptolite faunas appear associated with trilobites, brachiopods, and other shelly fossils and microfossils, in particular conodonts. In the Los Azules Formation, the graptolite Nemagraptus gracilis was firstly recorded by Borrello & Gareca (1951), in the reference section located in the Cerro Viejo area. However, in the Ordovician of the Precordillera, the Nemagraptus gracilis Zone was formally recognized in a biostratigraphic contribution by Harrington & Leanza (1957). Later, several contributions regarding the occurrence of graptolites from the N. gracilis Zone were provided by other authors (Furque & Cuerda, 1979; Peralta, 1986, 1990; Ortega & Brussa, 1990; Brussa, 1996, 1997). On the other hand, Ordovician black shale sequences of the Precordillera and a green?house event during the N. gracilis Zone (see Berry et al., 1978; Lagget et al., 1981; Arthur, 1983; Jenkyns, 1986) have been widely discussed in previous papers (Peralta & Barrera, 1992; Heredia & Beresi, 1994).