1 INSUGEO – Facultad de Ciencias Naturales e I.M.L., Universidad Nacional de Tucumán, Miguel Lillo 205, 4000 Tucumán. E–mail: insugeo@unt.edu.ar

Key words: Cruziana. Skolithos. Cambrian–Ordovician. Jujuy. Argentina.

Cambro–Ordovician sequences are widely distributed in the Central Andean Basin, displaying over 8.000 m of thickness in certain areas of northern Argentina and southern Bolivia. These strata are well known by its rich paleontological material, that has helped to get a good understanding of the biostratigraphic succession. Among the most remarkable Cambro–Ordovician fauna are mentioned trilobites, brachiopods, graptolites and conodonts.

In spite of this knowledge of faunas, trace fossils have been scarcely focused when compared to the early ones. The first mention of Ordovician ichnofossils in South America was done in this Central Andean Basin, when D´Orbigny (1842) described and figured Cruziana from southern Bolivia. Later on, during the second half of the century, little attention was given to them, until systematic work start to be frequently observed in the Argentine literature during the 1970 ´s. As an exception of the early referred asseveration, there are two outstanding papers on ichnology done by Borrello (1966a, 1966b) in northern Argentina and the southern Buenos Aires ranges.

In northwestern Argentina, the Cambro–Ordovician transition is placed within the lower sector of the Santa Victoria Group (Turner, 1960), that display several formational denominations for their different lithological units. In the Quebrada de Humahuaca (Cordillera Oriental, Jujuy Province), the transitional levels between both Systems are characterized by highly fossiliferous sandstones and shales with an attractive association of relatively shallow water trace fossils. One of the most distinctive Cambro–Ordovician units in the Cordillera Oriental of Jujuy is the Casa Colorada Formation (Harrington, 1957; emmend. López and Nullo, 1969), whose type area is placed eastwards of Tilcara, in the mid sector of the Quebrada de Humahuaca.

In this opportunity, nine sections in the early referred region were studied, and their ichnofossils analyzed, from north to south: Iturbe Aguada, Pintayoc, Volcán de Yacoraite, Moya, Chucalezna, Huichaira, Casa Colorada, Rupasca and Morro Verde. Alcyonidiopsis pharmaceus, Arenicolites isp., Bergaueria isp., Bifungites isp., Cruziana furcifera, Cruziana semiplicata, Monocraterion isp., Monomorphichnus isp., Planolites montanus, Rusophycus carbonarius, Rusophycus latus, Rusophycus polonicus, Rusophycus isp., Skolithos linearis, Tomaculum problematicum and numerous undifferentiated arthropod limb marks were located in the sandstones and black and green shale levels of the unit.

The Casa Colorada Formation is the formal denomination given by López and Nullo (1969) for the unformally nominated "Casa Colorada Shales" of Harrington and Leanza (1957) cropping out in the western flank of the Sierra de Tilcara, eastwards of Tilcara (Figure 1A). Highly tectonized shales and sandstones characterize a maximum of 250 m thick sequence, situation that prevents from getting all complete sections of the formation (Figure 1B). The mean thickness in most of the analyzed localities is about 130 m.

The shales and sandstones of the Casa Colorada Formation represent the first flooding of the Cambro–Ordovician shelfs, with a general deepening of facies. Shales and rippled surface heterolithic intervals characterize the formation in the lower and upper sectors of the unit (Aceñolaza, 1996; Moya, 1998). A shoreface setting within maximum and minimum wave base level is here proposed for the formation. In a recent paper, Zeballo et al. (2002) recognize a low energy setting within a shallow platform paleoenvironment. The Casa Colorada shales and sandstones overlay the sandstones and quatrzites of the Chalhualmayoc Formation while is overlied by the sandstones of the Alfarcito Formation. Below the Casa Colorada Formation, the fluvio–estuarine Tilcara Member has been proposed and mentioned several times for the area (e.g., Moya, 1988, 1999, 2002; Buatois et al., 2000), but in this opportunity is considered equivalent to the upper part of the early referred Chalhualmayoc Formation, within the Mesón Group.

The Casa Colorada Formation is well known by its fossiliferous fauna. Trilobites, graptolites, conodonts, brachiopods and trace fossils have been mentioned in several papers (e.g., Harrington and Leanza, 1957; Moya, 1988; Aceñolaza, 1998; Tortello and Aceñolaza, 1999; Zeballo et al., 2002), recording a Cambro–Ordovician age for the same.

Figure 1. A. Locality map of the area included in this paper. 1– Iturbe Aguada, 2– Pintayoc, 3–Volcán de Yacoraite, 4– Moya, 5– Chucalezna, 6– Huichaira 7– Casa Colorada, 8– Rupasca, 9– Morro Verde. B. Integrated stratigraphic section of the Casa Colorada Formation with relative bioturbation and ichnofossil distribution.

The upper sector of the unit is characterized by the presence of the Jujuyaspis keideli subzone (Tortello et al., 2002) within the Parabolina (N.) frequens argentina trilobite biozone, while the graptolite Rhabdinopora flabelliformis and the Cordylodus conodont biozone were also found in the equivalent Lampazar Formation of the neighboring Cajas Range (Rao and Hünicken, 1995; Rao, 1999; Tortello et al., 2002).

Bioturbated levels are common all along the stratigraphy of the Casa Colorada Formation in the region (e.g., Mángano et al., 1996). The described Cruziana – Skolithos ichnoassociation is clearly developed in the heterolithic facies (Figure 1B). The presence of ichnofossils in the black starved shales of the mid–upper sector is restricted as hypichnial traces in soles of coarser tempestitic layers. Typical high energy suspensivorous trace fossils as Skolithos, Monocraterion, Arenicolites? are recorded in the thicker sediments as sandstone levels and cuartzites of the lower and upper sectors of the unit. The earlier mentioned black shale facies of the mid–upper part are the ones with a restricted record of ichnoforms, with the presence of the unfrequent Alcyonidiopsis pharmaceus and Tomaculum problematicum. Trace fossils associated to the tempestitic levels within these black shales are interpreted to be produced by opportunistic faunas. These took advantage of the momentary influx of oxygen and food supply to leave their biogenic structures in the sediments of the Casa Colorada Formation.

Cruziana–Skolithos ichnoassociations are distinguished in the black and green shales and sandstones of the Casa Colorada Formation of the Quebrada de Humahuaca. The development of both associations are tight to the environmental interpretation of the unit. Paleontological data, mainly the existence of a varied fauna dominated by olenid trilobites, supports the development of the Cruziana ichnoassociation with Cruziana furcifera, Cruziana semiplicata, Rusophycus carbonarius, Rusophycus latus, Rusophycus polonicus, Rusophycus isp., Monomorphichnus isp., Bergaueria isp., Palaeophycus isp. and numerous undifferentiated arthropod limb marks. The Skolithos ichnoassociation is better represented in the sandier levels with Arenicolites isp., Arthraria antiquata, Monocraterion isp., Planolites montanus and Skolithos linearis. The presence of the faecal related traces Alcyonidiopsis pharmaceus and Tomaculum problematicum add interesting elements to the black and green shale facies.

Both associations are mostly developed with a different timing pattern. Cruziana usally appears with an earlier setting, while Skolithos association is mostly recorded posterior to the first one.

The lack of typical deep facies horizontal trace fossils support the interpreted relatively shallow water setting of the Casa Colorada Formation. Sedimentary characters allow to recognize a normal gradational based shorface succession varying within the subtidal setting, from offshore to lower shoreface–inner shelf transition.

We greatly appreciate the reviewing of this manuscript by F. Aceñolaza and J.P. Milana. Line drawing was kindly done by E. Gómez. This contribution was economically supported by the Fundación Antorchas and INSUGEO (Conicet–Unt).

Accepted: June 15, 2003