Pliomeridius sulcatus Leanza and Baldis, 1975 (Trilobita–Pliomeridae) and the Sepulturas Formation in the Cordillera Oriental of Jujuy, Argentina

Florencio G. ACEÑOLAZA1

1 INSUGEO, Facultad de Ciencias Naturales e IML Universidad Nacional de Tucumán – CONICET, Miguel Lillo 205. 4000 San Miguel de Tucumán, Argentina. E–mail: facenola@satlink.com

Key words: Pliomeridius. Trilobita. Arenig. Sepulturas Formation. NW Argentina.

Recent field work carried out to the west of Purmamarca (Jujuy province) focused the type area of the "Sepulturas limestones" (Harrington, 1957). Levels bearing Pliomeridius sulcatus Leanza and Baldis associated with Hoekaspis" schlagintweiti, molluscs, brachiopods, and trace fossil are located close to La Ciénaga locality. The collected material together with some previously registered fossils, as well as other recently described specimens from Cajas region (Aceñolaza and Aceñolaza, 2002) lead us to re–state some stratigraphic and chronological concepts that nowadays are confusing due to the poor knowledge of the strata croping out in the type–area of the Sepulturas Formation, nearby La Ciénaga locality, by the international route to Chile.

Harrington (1957) describes the "Sepulturas limestones" as Light to dark limestones and marly limestone (exposed in the Upper Quebrada de Purmamarca); fossils include Hoekaspis schlagintweiti, cephalopods, pelecypods, brachiopods. Exposed thickness about 50 meters. In this description, the author does not give a precise location of the type–area that allows adjustment of the paleontologic data to the geology. According to the geographical sketch included in the book "Ordovician Trilobites of Argentina" (Harrington in Harrington and Leanza, 1957), it is possible to deduce that the exact position corresponds to the points J5, J6, J7 and J8, where J. J. Zunino and E. Maury collected Hoekaspis schlagintweiti from dark gray limestones. Likewise, according to Harrington (1957), in the area crops out yellowish–green sandy shales with intercalated layers of greenish–grey sandstones and calcareous sandstones, which he names Cieneguillas Shales, exposed a little downstream from the Sepulturas limestones.

Based on these data, and the supporting cartography, we prove that the type–area corresponds to El Patacal–La Ciénaga localities located about 200 meters south of La Ciénaga. There, a succession of calcareous sandstones and greyish limestones intercalated with green yellowish shales are exposed along the road (Figure 1). Aceñolaza (1966) made the second mention of the unit, describing the Sepulturas Formation in the western slope of the Espinazo del Diablo (Las Colas Creek, Humahuaca Departament). Greenish shaly levels with limestone intercalations bear abundant and diverse fossils, such as nautilods, rostroconchs (Gutierrez Marco and Aceñolaza, 1991) and an abundant trilobite fauna. Ampyx, Synhomalonotus kobayashi, Pliomeridius sulcatus and Hoekaspis schlagintweiti were identified in this locality, among others. Following paleontologic criteria, Aceñolaza (1966) assigns it to the Llanvirn by considering it as partially equivalent to the Santa Gertrudis Formation of the Sierras Subandinas. Subsequently, Benedetto and Malanca (1975) describe an equivalent sequence in Los Colorados (Tumbaya department), involving the Acoite and Sepulturas formations with Pliomeridius sulcatus in the middle part of the stratigraphic column, allowing a closer correlation with the Espinazo del Diablo sequence. A few years later, Martin et al., (1987) describe the northern sector of the Sierra de el Aguilar, where they analyse the stratigraphic relation between the Acoite and Sepulturas formations, and propose a basal Lumara Member for the Sepulturas Formation. Greater stratigraphic details are given by Astini and Waisfeld (1993) and Astini (1994), who postulate, following Benedetto and Malanca (1975) that the Sepulturas Formation belongs to the Upper Arenig.

Figure 1. Geological map of outcrops of Sepulturas Formation in the El Patacal–La Ciénaga area, west of Purmamarca, Jujuy (modified from Peñaloza, 2001).

The possibility that the elements of the "Hoekaspis schlaginweiti fauna" were younger than the one granted by Harrington and Leanza (1957) was stated by Waisfeld (1996, 1997). The last author correlates the Sepulturas Formation with the Santa Gertrudis Formation (Sierra de Mojotoro, Salta), suggesting a Llandeilo–Caradoc age for both units. In addition, Albanesi and Astini (2002) mentioned the presence of the teraspidomorph Sacabambaspis associated to Erismodus, Erraticodon, and Plectodina (Llandeilo–Caradoc) in the upper part of the section of the Sepulturas Formation at Quebrada El Cardonal, Los Colorados ("green member" sensu Astini, 1994). As a complement of the early referred data within the same area (Quebrada Chamarra), Albanesi and Moya (2002), records Hoekaspis n. sp. (= H. schlagintweiti, sensu Waisfeld, 1996 ) associated to conodonts similar to those of the sampled interval in the Santa Gertrudis Formation (lacking the presence of S. janvieri).

All the mentioned data is contradictory to those of Rao et al., (1994) and Ortega and Rao (1994) who in type–area of the Sepulturas Formation (La Ciénaga), assign the sequence cropping out in that position (with Thysanopyge and Zuninaspis) to the Acoite Formation (= Cieneguillas shales, sensu Harrington, 1957 ). Those limestones provided graptolites (Didymograptus (Corymbograptus) aff. D. vacillans; D. cf. simulans) and conodonts (Baltoniodus, Cornuodus and Trapezognathus) that supports a Lower Arenig age for the sequence (Late Latorpian). Adding to the latter, Rao (1999) studied the conodonts of the Sepulturas Formation in the Espinazo del Diablo section, and recognizes Trapezognathus, Baltoniodus crassulus andinus and Drepanoistodus pitjani, as those of La Ciénaga (Rao et al., 1994), dating the "Hoekaspis Fauna" as late Lower Arenig in age.

The result of our work in La Ciénaga sector enables us to recognise that this sector must be considered as type–area/locality of the "Sepulturas limestone" (Harrington, 1957). A sequence of the limestones and dark, greyish and grey whitish calcareous sandstones integrates a normal sedimentary structure of N60°W and S40°W affected by a series of fractures of equal bound. These limestones are arranged in two notable sets interbedded in the greenish and green–yellowish shales of the Cieneguillas Formation (= Acoite; = Cieneguillas Shales, sensu Harrington, 1957) that appear truncated by minor fracture in the upper sector, not affecting the overall stratigraphic interpretation.

Hoekaspis schlagintweiti and the rare Pliomeridius sulcatus were registered in the sequence, as it happens in the material described by Leanza and Baldis (1975) in the Sepulturas Formation of Cajas Range (Aceñolaza, 1966), later recognised by Benedetto and Malanca (1975) and Waisfeld (1997) in Los Colorados. Apart from the chronological value granted to the mentioned trilobites, it must be highlighted that Pliomeridius displays a short stratigraphic range, and is here proposed as a guide–fossil for the Sepulturas Formation in the Cordillera Oriental.

This Pliomeridae is characterized by subrectangular glabela, anteriorly expanded, with well stressed antero–frontal medium furrow. Three wide and deep lateral glabelar furrows. Its pigidium displays five axial rings with well–developed terminal spines of medium length. Leanza and Baldis (1975) differentiate Pliomeridius from other pliomerids by the morphological characters of the medium furrow, very different to those pliomerids species from Europe and Laurentia. Even though, both authors refer it to the Llavirn by its association with H. schlagintweiti. These strata are reassigned to the Arenig on the basis of the conodont fauna (Baltoniodus: Rao et al., 1994, Rao, 1999) and brachiopods (Desmorthis segnis assemblage: Benedetto, 2002).

Pliomeridius sulcatus is a trilobite of short temporal–stratigraphic and regional representation in northwestern Argentina, nowadays recorded in the sequences cropping out at La Ciénaga, Cajas and Los Colorados. Pliomeridius always integrates the "H. schlagintweiti fauna", whose chronological position is determined in the two first localities by the presence of Baltoniodus crassulus andinus (late Lower Arenig). These data ratify the original definition of Harrington and Leanza (1957) regarding the stratigraphic position of Hoekaspis schlagintiweiti, which differs from Hoekaspis n. sp. from the Sierras Subandinas (Waisfeld, 1996).

Chronological facts provided by mentioned fossils do not support the correlation of the Sepulturas and Santa Gertrudis formations, neither is correct to assign the name of "Sepulturas" to the "green member" (after Astini, 1994) cropping out at Los Colorados, where Erismodus, Erraticodon and Plectodina are associated with Sacabambaspis (Llandeilo–Caradoc).

The author thanks the reviewers G. Albanesi and G. Aceñolaza for constructive comments. D. Ruiz Holgado is acknowledged for the line drawings.

Accepted: June 15, 2003