1 Departamento Paleontología Invertebrados, Museo de La Plata, Paseo del Bosque s/n°, 1900 La Plata, Argentina. E–mail: tortello@museo.fcnym.unlp.edu.ar

2 INGEO – Instituto de Geología, Facultad de Ciencias Exactas, Físicas y Naturales, UNSJ, Av. Ignacio de La Rosa y Meglioli, 5400 San Juan, Argentina.

Key words: Trilobites. Early Llanvirn. Gualcamayo Formation. La Chilca. San Juan.

The Gualcamayo Formation is widely represented in the Central Precordillera, Western Argentina, and only in Sierra de Villicum in the Eastern Precordillera. Although this unit is mainly composed of graptolite–rich black shales, its lower part (Lower Member, Peralta, 1993) is characterized by a rhythmic alternation of mudstones and black shales ("transfacies calcáreo–pelítica" or "transition zone", Baldis and Beresi, 1981), which overlies the open sea limestones of the San Juan Formation. This calcareous–pelitic sequence is interpreted as a shelf transgressive succession assignable to the Nileid Biofacies (Fortey, 1975a; Baldis and Beresi, 1981; Baldis, 1995).

According to Baldis (1995), the trilobites of the lower member of the Gualcamayo Formation belong to the Kainisiniella cuyana Zone (Late Arenig–Early Llanvirn), which is characterized by the occurrence of Kainisiniella cuyana Baldis and González, associated with Australoharpes cordilleranus Baldis and González, Bienvillia turneri Baldis and Pöthe, Neptunagnostella superba Shergold, and Mendolaspis sanjuaninus Baldis. The trilobites described in most detail come from the Sierra de Villicum (Baldis and González, 1986, 1987; Waisfeld et al., 2001), Las Aguaditas Formation in the Cordón de Los Blanquitos (Baldis and Pöthe, 1995; Baldis et al., 1995; Edgecombe et al., 1998, 1999a, 1999b; Chatterton et al., 1999; Waisfeld et al., 2001), and SW Guandacol (Benedetto et al., 1986; Waisfeld et al., 2001).

Although the Cerro La Chilca is considered as one of the type localities of the Kainisiniella cuyana Zone (Baldis, 1995), its trilobite faunas, with the exception of the cheirurid Macrogrammus pengi Edgecombe et al. (1999b), have not been studied in detail. The aim of this paper is to report on the trilobites of the Lower Member of the Gualcamayo Formation in the Cerro La Chilca section and to discuss their biostratigraphic and paleoecologic implications. Since this section represents a continuous, well exposed Ordovician succession containing varied, very well–preserved graptolites and shelly fossils, it serves as a regional reference for the Lower Paleozoic of South America.

The section studied is located on the western flank of the Cerro La Chilca, 28 km to the West of Tucunuco locality, San Juan Province, in the Central Precordillera setting. Cuerda (1986), Astini and Benedetto (1992) and Peralta (1998) provided a complete stratigraphic description of the Lower Paleozoic of the area, which includes the San Juan (Arenig), Gualcamayo (Llanvirn), Los Azules (Lower Caradoc), Don Braulio (Ashgill), La Chilca (Ashgill? to Early Wenlock), Los Espejos (Wenlock? to Ludlow), Talacasto (Lower Devonian) and Punta Negra (Lower to Middle? Devonian) Formations. Towards the top, this sequence is erosively truncated by diamictites of the Guandacol Formation (Upper Carboniferous).

The Lower Member of the Gualcamayo Formation is 4.31 m thick and composed of an alternation of thin to medium layers of black, tabular marly limestones and dark–coloured, laminated shales bearing abundant shelly faunas and graptolites. The thickness of the beds are variable, ranging from 5 to 28 cm. The whole member shows a limestone/shale ratio of 62% – 38% on average. Its lower limit is separated by a paraconcordance from the San Juan Fm., whereas the top is marked by the uppermost fossiliferous black limestone, which underlies the graptolitic black shales of the Los Azules Fm. (Peralta, 1998) of Lower Caradoc age.

The limestone beds contain conodont faunas and abundant shelly–fossils dominated by trilobites and brachiopods, whereas the shales mostly bear graptolites. Cuerda (1986) recognized Paraglossograptus tentaculatus (Hall) associated with Tetragraptus bigsbyi (Hall), Glyptograptus austrodentatus austrodentatus Harris and Keble, and Glossograptus hincksii (Hopkinson) in La Chilca section, and assigned the transfacies to the eponymous biozone (Lower Llanvirn). In addition, conodonts were tentatively assigned to the Eoplacognathus suecicus Zone (Keller, 1999).

Trilobites from the succession include Geragnostus sp., Neptunagnostella superba Shergold, Kweichowilla sp. nov. A, Porterfieldia sp., Nileus depressus n. subsp. A, Mendolaspis salagastensis Rusconi, Carolinites n. sp. A, Carolinites aff. pardensis Legg, Macrogrammus pengi Edgecombe et al., and Illaenidae gen. et sp. indet. Material is housed in the Instituto de Geología (INGEO), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Argentina, with specimen numbers prefixed by UNSJ–PI. The nileid community, originally described from the Arenig–Llanvirn of Spitsbergen (Fortey, 1975a), is mainly characterized by the occurrence of raphiophorids, nileids, shumardids and metagnostids. The genus Mendolaspis largely dominates the assemblages of La Chilca, representing more than 75% of the total fauna. Benthic nileids, shumardids and olenids were especially adapted to outer shelf, low–oxygen environments. Similarly, the long genal spines of the blind raphiophorids must have favoured the permanence of these animal on the soft floor, preventing a sinking in the mud.

Correlations based on species which show wide geographical distributions and independence of benthic communities are more reliable than those based on comparisons of benthic assemblages occurring in similar biofacies (Fortey, 1975b). The occurrence of the pelagic genus Carolinites from Cerro La Chilca section provides valuable biostratigraphic information. Carolinites n. sp. A has been previously recognized from the Lower Llanvirn of the Keele Range (northwestern Yukon Territory, Canada), in association with conodonts tentatively assigned to the Pygodus serratus Zone (Eoplacognathus suecicus Subzone) (Dean, 1973), and from the lower Middle Ordovician of Western Newfoundland (=Carolinites sp. 1 of Whittington, 1965). Besides, Carolinites aff. pardensis compares more closely with material from the Goldwyer Formation (Canning Basin, western Australia), from levels assigned to the Lower Llanvirn (Legg, 1976), whereas the metagnostid Geragnostus sp. is most similar to material from the upper Arenig of the Guandacol area (Argentine Precordillera) and the lower Llanvirn of Canada (Geragnostus cf. longicolis, E. suecicus Subzone).