1 Departamento Científico Paleontología Invertebrados, Museo de Ciencias Naturales, Paseo del Bosque s/nº, (1900) La Plata, Argentina. E–mail: tortello@museo.fcnym.unlp.edu.ar

2 Instituto Superior de Correlación Geológica, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, Miguel Lillo 205, (4000) Tucumán, Argentina.E–mail: insugeo@unt.edu.ar

Key words: Trilobita. Jujuyaspis. Early Ordovician. Aguilar Range. Argentina.

Cajas – El Aguilar (Jujuy Province, northwestern Argentina) is a reference area for the lower Paleozoic of South America (Aceñolaza, 1983). Its most suitable sections crop out in the northern part of the Cajas Range (e.g., Azul and Amarilla Creeks) and consist of quartzites of the Padrioc Formation (Upper Cambrian); shales with interspersed shaly sandstones of the Lampazar Fm. (Upper Cambrian); quartzitic sandstones, sandy shales and limestones of the Cardonal Fm. (uppermost Cambrian–Lower Tremadoc); and shales of the Acoite Fm. (Lower Arenig) (e.g., Aceñolaza, 1968; Ortega and Rao, 1995; Rao, 1999; Tortello et al., 1999).

In addition, intensely tectonized eopalaeozoic rocks are exposed on the eastern flank of the Aguilar Range. The best outcrops are low–grade metamorphic sediments observed at Altos del Río Despensa (Padrioc, Despensa and Acoite formations) (Aceñolaza, 1968; Alonso et al., 1982; Cortés et al., 1987). According to Cortés et al. (1987), the Despensa Formation is about 300 m thick and includes a lower unit of black shales with scarce intercalations of sandstones, and an upper unit showing a rhythmic alternation of sandstones and shales. This formation was correlated with the Lampazar and Cardonal formations (Cajas Range) (Cortés et al., 1987).

Two main facies are represented in the Río Despensa. A fine–grained facies (A) is characterized by greenish to gray laminated shales recording both low energy, suspension fall out, and episodic events of dilute turbidity. A similar facies was recorded from the Lampazar Formation in Cajas Range, but the grain of the latter is finer and it contains higher levels of organic matter.

In addition, a thick–grained facies (B) is represented by green wackes showing parallel lamination in the lower part of the beds and sinuous lamination towards the top. Beds show thin layers of pelitic material on the top, where badly preserved specimens of Rhabdinopora flabelliformis parabola (Bulman) occur. Turbidity currents are interpreted as a depositional mechanism responsible for facies B. These rocks differ from sandstones represented in the Cajas Range because the former are less mature and their matrix is more abundant.

Alonso et al. (1982) described the uppermost Cambrian trilobites Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza and Parabolina (Neoparabolina) frequens argentina (Kayser) from the lowermost levels of the Despensa Formation. The aim of this paper is to report the occurrence of Jujuyaspis keideli Kobayashi in the middle part of the unit. The material includes well–preserved 12 complete specimens assignable to different ontogenetic stages (early and late meraspids; holaspids). Material is housed in the Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, Argentina, with specimen numbers prefixed by PIL.

Figure 1. Location of the study area in the northwestern Argentina and geological map of the eastern flank of the Aguilar Range at Altos del Río Despensa.

Jujuyaspis Kobayashi is a cosmopolitan olenid that has proved to have great biostratigraphic value as an international guide for the lowermost Tremadoc in North America, China, Korea, Kazakhstan, Scandinavia, western Australia, Celtiberia and South America (e.g., see Aceñolaza and Aceñolaza, 1992). The species Jujuyaspis keideli Kobayashi has a wide geographic distribution that includes northern Norway, Colombia, Bolivia and Argentina. Since this taxon occurs with or close to Rhabdinopora flabelliformis s.l. in the upper part of the trilobite Parabolina (Neoparabolina) frequens argentina Biozone (lowest Tremadoc), it is a valuable key to define the Cambrian–Ordovician boundary in South America.

The Despensa Formation records the Cambrian–Ordovician transition and is partially correlatable with the Cardonal Formation (Amarilla Creek, Cajas Range; Rao and Ortega, 1995; Tortello et al., 1999) (cf. Cortés et al., 1987). In addition, the co–occurrence of Jujuyaspis and Rhabdinopora flabelliformis parabola provides a firm basis for correlation with other classic sections of northwestern Argentina. The studied succession is most similar to the upper part of the Lower Member of the Volcancito Formation (Volcancito River, Famatina Range, La Rioja Province; Esteban, in press; Tortello and Esteban, 1999; Albanesi et al., 1999).