Portezuelo del Tontal Formation (Lower Caradoc), Western Precordillera, San Juan Province: its biostratigraphic and paleogeographic significance
Silvio H. Peralta1, Stanley C. Finney2 and Giorgio Basilici3
1 CONICET, Universidad Nacional San Juan. E–mail: speralta@unsj–cuim.edu.ar
2 California State University, Long Beach (CSULB). E–mail: firstname.lastname@example.org
3 DGRN/IG, Unicamp, CP6152 Campinas, Brazil. E–mail: email@example.com
Key wods: Graptolites. Biostratigraphy. Upper Ordovician. Precordillera. Western Argentina.
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).
Figure 1. A. Geologic map of northen part of Sierra del Tontal. B. Antena Section, modified from Cingolani et al. (1989). *) Indicate fossiliferous levels. 1) Nemagraptus gracilis; 2) Dicellograptus alabamensis; 3) Dicellograptus gurleyi gurleyi; 4) Glossograptus sp.; 5) Pseudoclimacograptus sp.; 6) Glyptograptus sp.; 7) Cryptograptus marcidus.
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.
The La Antena Section graptolite fauna
La Antena section is named for the antenna installed for radio and TV transmissions at an elevation of 4.340 m on the western flank of the northern part of the Sierra del Tontal, ca 40 km east of Barreal village. In this section, the Portezuelo del Tontal Formation is composed of laminated and massive sandstone, interbedded with siltstone, and conspicuous levels of black shale, conglomerate, and mafic and ultramafic sill. A leuco–granitoid sill, probably Miocene in age, intrudes the Ordovician rocks, and extending continuously laterally for hundreds of meters. The succession is gently folded, with eastward vergence, and forms a series of anticlines and synclines that repeat the section. Generally, the strata strike north–south and dip 50° to 80° to the west. In spite of structural complications, stratigraphic thickness can be estimated reasonably.
Graptolite were collected extensively and systematically mainly from sandstone beds in La Antena section. Hustedograptus aff. H. teretiusculus is the most abundant species, followed by Dicellograptus gurleyi, D. cf. alabamensis, Pseudoclimacograptus sp., Cryptograptus sp., Glossograptus sp., and probable stipes of Nemagraptus sp. This assemblage, in particular the association of Dicellograptus species and Nemagraptus, suggests the Nemagraptus gracilis Zone (Finney et al., 1996). It is similar to that of the La Cantera Formation in the Eastern Precordillera of San Juan Province, except for the occurrence of Dicranograptus in the La Cantera Formation, and to assemblages from the Los Azules and Las Aguaditas formations of the Central Precordillera, and Yerba Loca Formation in Western Precordillera, except for the occurrence of Leptograptus. The differences between these localities may reflect age differences with the La Antena assemblage representing a lower level in the N. gracilis Zone than those from the La Cantera, Los Azules, Las Aguaditas and Yerba Loca formations. Or the differences may be paleoecological with the La Antena section representing a probably distal shelf setting.
Paleogeographic and geotectonic significance
Conglomerate clasts in the Portezuelo del Tontal Formation indicate several different source areas. Their lithologies are mainly limestone, together with basic volcanic and mafic rocks, sandstone, and resedimented conglomerate. Some limestone clasts contain fossils of Teiichispira argentina, orthocerids, and brachiopods suggesting the San Juan Formation as the source area. Paleocurrent pattern indicate southward transport (present coordinates). Sedimentologic and stratigraphic evidence suggests that the Portezuelo del Tontal Formation accumulated in a shelf environment (Basilici et al., a, b in review), which could mean that the continental margin was located further to the west (present coordinates). Given this depositional setting, the presence of probable Silurian sills is intriguing; they may have developed as a result of rifting along the western margin of Gondwana. The N. gracilis Zone fauna of the Portezuelo del Tontal Formation, as with those from other localities in overall Cuyania Terrane, has strong affinities with that from the Appalachian margin of Laurentia (Finney and Peralta, 2000; Peralta and Finney, 2002).
The graptolite fauna from the Portezuelo del Tontal Formation in the La Antena section correlates with the N. gracilis Biozone; it is characterized by the occurrence of D. gurleyi, D. alabamensis, H. teretiusculus, Cryptograptus sp., as well as by the absence of C. bicornis, Dicranograptus and Leptograptus. This fauna is very similar to that of the La Cantera Formation at the Sierra de Villicum, Eastern Precordillera.
Sedimentologic evidence indicate that the Portezuelo del Tontal Formation was deposited in a shelf environment. Probable Silurian mafic sills, intruding the succession, may represent rifting of the continental margin during that time. The graptolite fauna from the Portezuelo del Tontal Formation has strong affinities with that from the Appalachian margin of Laurentia.
The authors thank to San Juan University for support with the fieldwork. Technician Vicente Mulet (CONICET) helped substantially in the preparation of the figures.
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Received: February 15, 2003
Accepted: June 15, 2003