Conodonts and graptolites of the Santa Rosita Formation (Tremadocian) at the Nazareno area, Santa Victoria range, Cordillera Oriental of Salta, Argentina

GIULIANO, M.E.; ALBANESI, G.L.; ORTEGA, G.; ZEBALLO, F.J.; MONALDI, C.R.

PUBLICACION ESPECIAL - ASOCIACION PALEONTOLOGICA ARGENTINA

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

Año: 2013 vol. 13 p. 39 - 44

0328-347X

The Nazareno area is located to the southeast of the Santa Victoria range in the Cordillera Oriental of Salta, Argentina, with extensive exposures of Ordovician rocks (Ortega and Albanesi, 2002; Brussa et al., 2003; Astini, 2003; Albanesi et al., 2008, and references therein). At this area a thick siliciclastic succession of the Santa Rosita Formation from the Lower Ordovician crops out (Turner, 1960) (Fig. 1). Previous works describe a succession that consists of sandstones and shales, and subordinate calcareous rocks with abundant fossils of early Tremadocian age (e.g., trilobites, braquiopods, gastropods, conodonts, acritarchs) (Harrington and Leanza, 1957; Vilela, 1961; Manca et al., 1995). The Paltodus deltifer conodont Zone of the upper middle Tremadocian was first identified in this area by Manca et al. (1995), who reported the presence of Paltodus deltifer Lindström, Drepanodus arcuatus Pander, Acodus deltatus deltatus (Lindström), among other taxa, however, the illustrations of these specimens do not allow a definite determination. At the Nazareno area, the Santa Rosita Formation unconformably overlies the upper Cambrian marine quartzites of the Mesón Group, and is unconformably covered by Cenozoic terrigenous deposits. The upper 216 m of the Capillas river profile begins with coarse sandstone strata, ca. 67.5 m thick, at the base of a heterolytic succession, which is predominantly sandy in its lower section, and continues with a shaly succession of grey and black shales, in the upper part. Calcareous tabular and lensoid strata and concretions yielded a significant amount of microfossils, mainly conodonts (Figs. 2, 3). The composite stratigraphic column of the Figure 2 shows the lithology and biostratigraphy of the Grande Creek and Capillas River sections. In the Grande creek Minor et al. (2011) measured ca. 200 m of an heterolithic succession with the Paltodus deltifer pristinus Subzone, below the strata of the Capillas River section, which is studied in this paper. The objective of this report is the definition of the conodont and graptolite assemblages collected from the upper Santa Rosita Formation, in both sections studied at Nazareno area, which refer to a middle Tremadocian age.