Guillermo F. ACEÑOLAZA1, Lucía ARÁOZ1, María del Milagro VERGEL2, M. Franco TORTELLO3 and Sergio M. NIEVA1

1 INSUGEO, CONICET/UNT, Facultad de Ciencias Naturales e IML Miguel Lillo 205, 4000 Tucumán, Argentina. E–mail: insugeo@unt.edu.ar; acecha@unt.edu.ar

2 CONICET, Facultad de Ciencias Naturales e IML, Miguel Lillo 205, 4000 Tucumán, Argentina. E–mail: insugeo@unt.edu.ar

3 CONICET. Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Plata. Paseo del Bosque s/n, 1900 La Plata, Argentina. E–mail: tortello@infovia.com.ar

Key words: Paleontology. Biostratigraphy. Ordovician. Cordillera Oriental. Argentina.

The Sierra de Zenta is one of the most remarkable physiografic units of NW Argentina. Covering a surface of over 15.000 km2 in the northern provinces of Salta and Jujuy, reaches in certain areas up to 5.000 meters above sea level. Surprisingly, there is a restricted knowledge on many geological aspects of the range, being most of the available data related to mining projects. This virtual lack of stratigraphic and biostratigraphic information gave as a result several mapping problems, where inaccurate dated sequences are common elements within the range.

Well developed Ordovician strata display over 1000 meters of thickness of sandstones and shales, cropping out extensively all along the central sector of the range. The ages varies from the Cambro/Ordovician strata of the Abra de Zenta up to the Upper Tremadocian – Mid Arenig (?) in the neighbouring Santa Ana and Caspalá localities.

Within the framework of an integrated project that will help on a better understanding of the high mountain chain of Zenta, different sections of Ordovician strata have been measured, and their paleontological data analysed. In this opportunity, the highly fossiliferous Abra de Zenta is reviewed by means of the fossil material located in the strata.

Located 59 km eastwards to Humahuaca, by the road between Puerta de Zenta and Santa Ana (23º 10´ 20´ and 65º 02´96´´), displays a 149 m thick succession assigned to the lower part of Santa Rosita Formation, represented by greyish and greenish sandstones and shales. The fossil fauna is composed by trilobites, molluscs, palynomorphs and trace fossils dating the unit as Tremadocian (Figure 1). The sequence displays a general shallowing upwards arrangement, starting with laminated black and grey shales with few intercalation of fine sandstone layers getting coarser to the mid sector of the sequence. The upper part of the sequence displays an heterolitic succession of greenish shales and fine sandstones. Shoreface setting, close to the maximum wave base is here interpreted for the lower sector of the log (lower offshore/upper offshore), changing upwards to an eustatic related shallower setting (upper offshore, transitionally to lower shoreface). Tempestitic coquinoid pulses appear above the lower shally interval and within the lower sector of the shallower rippled sandstone levels. The lower sector of the sequence is disrupted by a fault zone that repeats strata, while the upper part of the succession, is tectonically in contact with Silurian (?) sandstones.

A paleontological detailed sampling of the strata allowed to date the sequence as early to Middle Tremadocian in age. Trilobites, palynomorphs, molluscs, gastropods and trace fossils were identified, presenting the first description of Lower Ordovician fossils in the range.

Figure 1. A. Location map of the Abra de Zenta, in the Cordillera Oriental of Jujuy and Salta Provinces. B. Schematic section of Tremadocian strata cropping out in the locality, with distribution of fossils and location of fossiliferous levels.

Among the trilobite fauna, the association of Leptoplastides marianus (Hoek), Asaphellus catamarcensis Kobayashi and Kainella characterizes the Lower Ordovician in the Central Andean Basin of South America. Harrington and Leanza (1957) documented these taxa from several localities in Salta and Jujuy provinces, characterizing the Kainella meridionalis biozone. Classical localities representative of this biozone in northern Argentina are Santa Victoria, Colorada Creek, Nazareno River, La Caldera, Angosto de Pascha and Lampazar in Salta; as well as Rupasca, Casa Colorada, San Gregorio (Alfarcito) and Pocoy creeks (Huichaira) in Jujuy (Harrington and Leanza, 1957). This trilobite biozone can be partially correlated with some conodont and graptolite biozones in the Cordillera Oriental, highlighting the Cordylodus angulatus biozone, the "Acanthodus lineatus Association" (Ortega and Rao, 1995; Rao and Tortello, 1998; Rao, 1999; Tortello and Rao, 2000), the Rhabdinopora flabelliformis flabelliformis zone and the "Association 3" of Erdtmann (Moya et al., 1994; Ortega and Rao, 1995).

Palynomorph data are rare in the lowermost Ordovician beds of the Central Andean Basin. The lower sector of the sequence yielded an abundant although low to moderate diversified acritarch assemblage characterized by the dominant presence of Saharidia spp. (76,2%), and several taxa belonging to diacromorphitae and herkomorphitae forms. The following taxa were identified: Acanthodiacrodium achrassi, A. crassus, A. formosum, A. hirsutum, A. ignoratum, A. tuberatum, A. cf. angustum, Cristallinium cambriense, Cymatiogalea velifera, Leiosphaeridia spp., Lophosphaeridium spp., Polygonium dentatum, P. gracile, P. symbolum, cf. P. pungens, Saharidia downie, S. fragilis, Stelliferidium sp., Vulcanisphaera africana, V. britannica, V. nebulosa, V. tuberata, and V. turbata (Aráoz and Vergel, 2001; Aráoz, 2002).

The above mentioned assemblage was compared with previously established palynological zonations of Upper Cambrian and Ordovician sequences from different paleogeographic areas (e.g., from Algerian Sahara: Jardine et al., 1974; Vecoli, 1999; eastern Newfoundland: Martin in: Martin and Dean, 1981; 1988).

The Sierra de Zenta assemblage shows some similarities with the early Tremadocian palynological zones from Algeria. The presence of S. fragilis and S. downie allows a preliminary correlation with the palynological B0 zone. On the other hand, the presence of V. africana, V. nebulosa, P. dentatum, C. velifera and the existence of Herkomorphitae forms show affinities with the B1 and B2 Tremadoc palynological zones of Jardiné et al. (1974).

Recently, Vecoli (1999) informally introduced a new stratigraphical zonation summarizing and comparing other previously defined Cambrian–Ordovician palynozones. Within this framework, the Sierra de Zenta assemblage shows some similarities with the Acanthodiacrodium angustum–Vulcanisphaera britannica assemblage, attributed to the Lower Tremadocian, above the earliest Tremadoc as determined by correlation with the Moroccan zones 2 and 3 of Elaouad–Debbaj (1988). The presence of A. angustum, P. gracile, V. britannica, the dominance of diacrodians and some herkomorphitids forms support this correlation, allowing to place it within the Lower Tremadocian.

Some taxa are common to those of eastern Newfoundland, with part of the A5 and A6 microfloras (uppermost Cambrian – early Tremadocian: A. achrassi, C. cambriense, S. fragilis, V. africana) (Martin and Dean, 1988).

The dominance of simple forms as Saharidia and leiosphaerids associated to low diversity microfloras, supports a shallow environmental setting as in Tongiorgi et al. (1998), Martin (1993), Molyneux et al. (1996) and Vecoli (2000).

Moreover, according to Vavrdová (1997) provincialism of the organic–walled microfossils of earliest Ordovician age (Tremadocian) is less evident, and not acritarch provinces have been proposed for the Late Cambrian–Tremadocian interval.

Among additional fossils the perigondwanan rostroconch mollusc Ribeiria sp. and the gastropod Peelerophon sp. were also recorded in a remarkable low stratigraphic position, sharing the early mentioned Lower Tremadocian trilobite association.

Plate 1. A. Asaphellus catamarcensis (x 3.5; PIL 15.127). B. Asaphellus catamarcensis (x 5; PIL 15.133). C. Kainella sp. (x 6.5). E. Leptoplastides marianus (x 4.5; PIL 15.130). F. Kainella sp. (x 5.5; PIL 15.113). G. Acanthodiacrodium achrassi (x 375). H. Acanthodiacrodium crassus (x 375). I. Saharidia fragilis (x 200). J. Vulcanisphera britanica (x 200). K. Polygonium gracile (x 250). L. Cymatiogalea velifera (x 375). M. Cristallinium cambriense (x 275). N. Vulcanisphaera africana (x 200). O. Vulcanisphaera turbata (x 250). P. Acanthodiacrodium cf. angustum (x 500). Q. Bolonia isp. (x 0.8). R. Monomorphichnus isp. (x 0.5). S. Palaeophycus tubularis (x 0,3).

Trace fossils are common, and in a general sense they are assignable to the "Cruziana ichnofacies", being related to a low to mid energy paleoenvironmental setting. Asaphoidichnus trifidum, Bolonia isp., Diplichnites isp., Bergaueria isp., Isopodichnus isp., Monomorphichnus isp., Petalichnus isp., Palaeophycus tubularis, Rusophycus cf. latus, Rusophycus didymus and Skolithos linearis were identified as the most common ichnotaxa (Aceñolaza et al., 2001).

Trilobites of the Abra de Zenta are assignable to the Kainella meridionalis Biozone, which is correlated with a late Lower to Middle Tremadocian. Palynomorphs also indicate a lowermost Ordovician age. The absence of typical Cambrian taxa, unknown from post–Late Cambrian rocks supports an Ordovician correspondence for this palynological association from the Abra de Zenta. These assemblages provide relevant biostratigraphical data to the lower levels of Santa Rosita Formation in the Cordillera Oriental of northern Argentina.

The trace fossils of the Abra de Zenta display the typical ichnoassociation representative of the Cambro–Ordovician transition in NW Argentina, with the addition of some new traces for the Cambro–Ordovician ichnology in the region as Asaphoidichnus trifidum, Bolonia isp. and Petalichnus isp. Traces belong to the "Cruziana ichnofacies" within a relatively shallow, on shore environmental setting.

The additional Tremadocian record of Ribeiria sp. and Peelerophon sp. adds a new locality for these remarkable perigondwanan forms of rostroconch and Gastropod.

A shore–face setting, from lower offshore transitionnally changing to a lower shoreface paleoenvironment is here proposed for the unit cropping out at the Abra de Zenta locality.

Authors thank F. Aceñolaza, J. C. Gutiérrez–Marco, A. Uchman and M. Schlirf for critically reading this contribution and the ichnological advise. Financial support was given by the Fundación Antorchas and the Instituto Superior de Correlación Geológica (CONICET–UNT), Argentina.

Accepted: June 15, 2003