Trilobites from the Lower Member of the Gualcamayo Formation (Lower Llanvirn) in Cerro La Chilca, Argentine Precordillera: biostratigraphic and paleoecologic implications

TORTELLO, M.F.; PERALTA, S H

San Juan

Congreso; 9th International Symposium on the Ordovician System, 6th International Graptolite Conference and 2003 Field Meeting Subcommission on Silurian Stratigraphy, San Juan; 2003

Univ. Nac. San Juan-Univ. Nac. Tucumán

ABSTRACT: 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.