Portezuelo del Tontal Formation (Lower Caradoc), Western Precordillera, San Juan Province: its biostratigraphic and paleogeographic significance

PERALTA, S H; FINNEY, S. C; BASILICI, G.

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

Universidad Nacional de San Juan, Universidad Nacional de Tucumán

RESUMEN: This paper reports on an important lower Upper Ordovician graptolite fauna from the Portezuelo del Tontal Formation, in the La Antena section at Sierra del Tontal, San Juan Province, western Argentina (Figure 1). In his pioneering study, Stappenbeck (1910) reported that the Sierra del Tontal was composed of Devonian rocks, as did Amos and Rolleri (1965) and Rolleri and Baldis (1969), who also concluded that the Sierra del Tontal evolved as a structural arc, the so?called "Protoprecordillera. However, González Bonorino (1975a, b), based on sedimentologic evidence, suggested that the Sierra del Tontal is composed mainly of Ordovician rocks, a suggestion that was confirmed by Aparicio and Cuerda (1976) and Cuerda et al. (1986) who discovered that the extensive outcrops in the Sierra del Tontal provided excellent collections of Ordovician graptolites and, according to our observations, abundant trace fossils.Ordovician siliciclastic deposits exposed along the full length of the Sierra del Tontal, ca. 100 km, and average east?west extent ca. 10 km, from the Rio San Juan to the boundary between San Juan and Mendoza provinces, and far beyond into the Sierra del Tigre, to the north of the San Juan River, are a prominent and striking record of marine sedimentation. Graptolites are common to locally abundant in these Ordovician rocks, and graptolite?based correlations have been critical for resolving a broad range of tectonic, stratigraphic, and sedimentologic problems associated with the Ordovician evolution of Cuyania Terrane. Accordingly, we use graptolite biostratigraphy to address similar problems in the Sierra del Tontal.Most of these rocks belong to the Portezuelo del Tontal Formation, which is composed of five distinctive facies: sandstone, siltstone, interbedded sandstone and siltstone, black shale and conglomerate, all of which are intruded by mafic sills of probable Silurian age (Gerbi et al., 2002). Graptolites occur mainly in the interbedded sandstone and siltstone facies, occasionally in the sandstones facies, and rarely in the black shale facies. Recently, exposures of these facies in the La Antena section have been interpreted to represent deposition in a shelf environment (Basilici et al., a, b, in review), in contrast to previous interpretation of deposition in a deep?marine turbidite system, (Spalleti et al., 1989; Keller, 1999).Cuerda et al. (1986) described several graptolite assemblages from the Portezuelo del Tontal and Arroyo del Medio creek sections (Figure 1) and correlated them with the Llanvirnian to Llandelian series and possibly to the H. teretiusculus Zone. These assemblages include Reteograptus geinitzianus Hall, Cryptograptus antennarius Hall, Glossograptus cf. G. armatus Nicholson, G. hincksii (Hopkinson), Paraglossograptus aff. tentaculatus (Hall), Dicellograptus cf. D. moffatensis Carruthers, Climacograptus aff. C. parvus Hall, Climacograptus cf. C. brevis Elles andWood, Dicellograptus divaricatus var. salopiensis Elles and Wood, Amplexograptus aff. A. arctus Elles and Wood, A. minutus nov. sp., Glyptograptus cf. G. teretiusculus (Hisinger), Glyptograptus cf. G. intersitus Harris and Thomas, G. dentatus Brongniart, G. austrodentatus Harris and Keble, and Didymograptus sp. During a recent stratigraphic and sedimentologic investigation, Basilici et al. (a, b, in review) collected abundant graptolites from sandstone beds, in which graptolite are moderately well preserved, and also from black shale, in which graptolites are scarce but also well preserved. In addition, abundant trace fossils of the Cruziana Ichnofacies were found in this section.