Early Ordovician conodonts from the Santa Rosita Formation at Pantipampa , Iruya area , northwestern Argentina *

A conodont assemblage recovered from the middle part of the Santa Rosita Formation at Pantipampa (Iruya area, northwestern Argentina) is described herein. It includes Cordylodus angulatus Pander, C. cayesi Druce and Jones, C. lindstromi Druce and Jones, C. hastatus Barnes, Furnishina furnishi Müller, Kallidontus gondwanicus Zeballo and Albanesi, Phakelodus elongatus An, Phakelodus tenuis (Zhang), Proconodontus muelleri Miller, Semiacontiodus sp. and Teridontus nakamurai Nogami, which are characteristic of the Cordylodus angulatus Zone (upper lower Tremadocian). In the Iruya area, the C. angulatus Zone correlates with the trilobite Kainella meridionalis Zone and the lower part of the Kainella teiichii Zone. Taxonomic diversity and relative frequency of conodont species enables the recognition of the Semiacontiodus-Teridontus community, which is characteristic of shallow marine environments.

(Fig. 1C) in which the trilobite Kainella meridionalis and K. teiichii zones are well exposed.Trilobites are abundant throughout this succession, whereas graptolites (Rhabdinopora Eichwald, 1855) and conodonts were reported only from the lower and middle parts of the section, respectively.The aims of this paper are to describe in detail the conodont faunas from the Santa Rosita Formation at Pantipampa, and to refine the age assignment and correlations with other Lower Ordovician localities from the Cordillera Oriental and the world.

Geological setting
The Cordillera Oriental is a high relief thrust system that is delimited to the east by the Sierras Subandinas and to the west by the Puna (Fig. 1B).Lower Paleozoic rocks are well represented in the mountain ranges of this geological province, including Cambrian quartzite of the Mesón Group and late Furongian-Early Ordovician shales and sandstones of the Santa Victoria Group (Santa Rosita and Acoite
The Pantipampa locality is about 5 km north of Iruya town (Iruya Department, Salta Province) (Fig. 1C).There, shale and sandstone of the Santa Rosita Formation constitute narrow thrust belts (in ligth grey in figure 1C) which unconformably overlie the Cambrian quartzite of the Mesón Group (Turner, 1960), and are unconformably overlain by the Upper Cretaceous-Paleogene continental sandstone of the Salta Group (Turner, 1959) and Quaternary alluvial deposits (Turner, 1964;Figueroa Caprini, 1955;FIG.2. Stratigraphic section of the middle part of the Santa Rosita Formation at Pantipampa (Iruya area, northwestern Argentina) displaying facies succession and provenance of conodont samples (6' and 6*; based on Tortello and Esteban, 2016).Vilela, 1960;Turner and Mon, 1979).As stated above, Tortello and Esteban (2016) described a stratigraphic section from Pantipampa containing high-diversity trilobite assemblages of the Kainella meridionalis and K. teiichii zones (upper lower Tremadocian) (Figs. 2 and 3).The Pantipampa section is interpreted as having formed in a wave-dominated shelf with influence of storm activity (Tortello and Esteban, 2016;Esteban et al., 2017).Three sedimentary facies are recognized throughout the succession.Facies 1 is characterized by dark and greenish gray, massive to thinly laminated shale, mudstone and silty mudstone, which are irregularly interbedded with gray, massive, very fine-grained silty sandstone in sharp-based, tabular beds 1-3 cm thick (Facies 2).Many trilobites were collected from Facies 1, in association with brachiopods, gastropods, bivalves, cephalopods and echinoderms.Dark gray to greenish gray, fine to very fine sandstone and calcareous sandstone beds containing bioclastic concentrations characterize Facies 3. Bioclastic concentrations consist mainly of fragmented brachiopods and trilobites, and generally appear at the bases of the calcareous sandstone beds, constituting lenses and layers up to 4 cm thick.Occasionally, concentrations occur at the tops of the sandstone beds.These beds were sampled for conodonts and two levels have produced positive results (Fig. 2).

Material and methods
A total of 882 conodonts were obtained from the Santa Rosita Formation in the Pantipampa locality.The samples were processed in the INSUGEO laboratory by using standard acid etching techniques (Stone, 1987).The conodonts were photographed with scanning electron microscopes of CCT-Mendoza and CIME-Tucumán (Argentina).Specimens are in the Collection Lillo-microvertebrates/Conodonts of the Instituto Superior de Correlación Geológica -INSUGEO (CONICET-Universidad Nacional de Tucumán), under the code MLC-C (2000MLC-C ( -2882)).
The assemblage studied comprises paraconodonts and euconodonts (Figs 4-5).Although the faunal diversity is low, the identified species are biostratigraphically significant.
The cordylodontid elements are well preserved, showing entire cusps and denticles with visible white matter, and a Color Alteration Index (CAI) degree of 1º-1.5º(Epstein et al., 1977).These elements are regarded as autochthonous and therefore provides essential information about the age of the assemblage.
On the other hand, simple cone elements (para and protoconodonts) show clear indications of abrasion (Fig. 5.L-O, Q) and high CAI values (4º-5º), so they may have been redeposited from older strata.Conodont-bearing sediments of the studied section were deposited in moderately high-energy conditions above storm wave base, in a favourable environment for conodont element reworking (e.g., Löfgren et al., 2005;Viira et al., 2006).Other taphonomic processes that affected the cone elements include partial dissolution of the basal margins and walls, as well as basal filling and mineral coating.The walls of some elements show small circular holes, which may have been produced by bioerosion.
The high CAI values of the simple cones could be attributed to the presence of a high carbon content which may have been transferred to the elements from the dark, organic-rich carbonate sandstone in which they were originally preserved; therefore, the CAI of these cones would not reflect the thermal history of the rocks that contain them (Epstein et al., 1977;Rejebian et al., 1987;Nowlan and Barnes, 1987).In addition, these high CAI values could be partially explained by the effect of lithostatic charge, attributed to a sedimentary cover ranging from 9,000 to 10,000 m in thickness (Epstein et al., 1977).Pressures of this magnitude are commonly associated with regional metamorphism which, in the case of the Iruya area, is of low-grade (Vilela, 1960).

Systematic paleontology
Systematic descriptions of euconodonts follow conventional notation (Sweet, 1981(Sweet, , 1988)), which defines spatial positions of M, S and P, from the front end to the rear end of the multi-elemental apparatus.Description.Simple cone element laterally compressed, with a thin, slightly reclined cusp.Element widens toward the base, which is broad and oval.The margins have well-marked ribs.Basal cavity broad and deep, occasionally filled.Outer surface with faint annulations.

Class
Remarks.Albanesi et al. (2015) recognized two types of elements for this taxon: rounded ones and bilaterally compressed ones.In our collection from Pantipampa, only the bilaterally compressed elements have been found.Discussion.Nicoll (1990) emended the diagnosis of this genus and identified six morphotypes in the structure of the apparatus; concepts that are followed herein.Description.Although only a few elements were recovered from Pantipampa, they broadly agree with the description of Cordylodus angulatus provided by Nicoll (1990): Pa element: Dolobrate ramiform element with recline cusp of oval cross section and sharp edges.The posterior process carries three long denticules with sharp edges, these denticules are located on the external margin of the process and are directed backwards, the cusp is twisted towards the inner face of the element.The base of the cuspid widens close to the union with the base forming a small platform that becomes thinner towards the end of the process.In an internal lateral view is evident one of the characteristics that defines the species, named by Lindström (1955) Phrygian cap.The basal margin is broad below the cusp and narrowed towards the process.

Cordylodus angulatus
Pb element: Ramiform element, with reclined cusp of oval cross section and sharp edges.The posterior process carries three long denticules with sharp edges; these denticules are located on the external margin of the process.The denticules and the cusp are curved towards the inner side.The anterior margin of the basal cavity is concave; the small platform is less developed than in the element Pa.The basal cavity is oval and extended along the element.The apex of the basal cavity reaches only the base of the cusp and is centrally located.
Sa element: Symmetrical elements laterally compressed with long cusp with rounded edges and sharply-edges.The outer margin of the cusp is concave and extends like a small keel beyond the basal cavity.The cusp is broad near its posterior end.The posterior process is thin and carries two denticles directed towards posterior, they are quite spaced apart.The basal cavity is wide below the cusp but it is slimming towards its anterior and posterior margins.
Sb element: Asymmetrical ramiform element.The cuspid is elongated and compressed of oval cross section, slightly curved towards interior, the anterior margin of the cusp is sharp and its basal part presents a rounded salient.The posterior process is long and carried out 4 to 6 denticles, which are located on the internal margin of the process.An inner lateral rib develops from the base of the cusp and extends laterally to form a protoprocess; this in conjunction with the posterior process and the basal margin give a triangular shape to the element.
Sc element: It is a fragment which is interpreted as a Sc element, and classified based on the anterior margin of the cusp which is keeled.This is an asymmetric element with an inwardly twisted cusp.The posterior process is fragmented but two denticles are evident.
No elements of the M and Sd morphotypes have been recovered.Description.A single element collected from Pantipampa is regarded as a Sb element of the apparatus of Cordylodus caseyi (see Nicoll, 1990).The cusp is compressed with an oval cross section and sharp anterior and posterior margins; it is twisted with respect to the posterior process, and does not have ribs or carenas.Its anterior margin is projected on a small keel.The posterior process is long and arched carrying three large denticles directed towards posterior.The cavity tip is near the anterobasal corner.

Material. One Sb element, CML-C 2017 (1).
Occurrence.Pantipampa locality (Iruya area, northwestern Argentina), sample Pan 6*.Santa Rosita Formation, upper lower Tremadocian.Barnes, 1988 Fig. 4J 1971 Description.The recovered element corresponds to the morphotype Sc described by Nicoll (1990), characterized by having a basal cavity (biapical) which extends into the first denticle of the posterior process.Element asymmetric, with inner side flattened and outer side convex.The cusp is broken and slightly twisted to the inner side; margin of cusp keeled, with its inner side extended on an inner lateral protoprocess, and the outer side with a delicate rib.Posterior process slightly arched, carrying three denticles of which the first is very close to the inner face of the cusp.Discussion.This element differs from Cordylodus prolindstromi because the latter exhibits a truncate second basal cavity.
Analysis of the taxonomic diversity and relative frequency of conodont species from Pantipampa reveals that Semiacontiodus sp.(61.5%) largely dominates the assemblages, followed far behind by Teridontus nakamurai (4.53%) and Cordylodus angulatus (1.13%), while indeterminate fragments (8.84%) and remaining species represent a very low percentage of the total fauna.This enables the recognition of the Semiacontiodus-Teridontus community, which is characteristic of shallow marine environments (Ji and Barnes, 1994;Löfgren, 1999).According to Tortello and Esteban (2016) and Esteban et al. (2017), the Pantipampa section represents a storm-and wave-dominated shelf.The conodont samples were recovered from Facies 3, which reflects sedimentation above storm wave base and moderately high-energy conditions.

Conclusions
Based on its conodont faunas, a part of the Santa Rosita Formation in Pantipampa is assigned to the upper lower Tremadocian Cordylodus angulatus Zone and correlated with other localities of the Cordillera Oriental and the world.In the studied area, the C. angulatus Zone is partially equivalent to the trilobite Kainella meridionalis Zone (uppermost part) and the Kainella teiichii Zone (lowest part).Taxonomic diversity and relative frequency of conodont species enables the recognition of the Semiacontiodus-Teridontus community, which is characteristic of shallow marine environments.
helpful comments on an early version of the manuscript.In addition, M. Ghobadi Pour and an anonymous referee are thanked for their constructive remarks on the submitted manuscript.This research was supported by the Consejo Nacional de Investigaciones Científicas y Técnicas, the Instituto Superior de Correlación Geológica (CONICET-Universidad Nacional de Tucumán), and the Universidad Nacional de La Plata, Argentina.

FIG. 3 .
FIG. 3. Correlation chart of the Lower and Middle Tremadocian trilobite and conodont zones of northwestern Argentina.The interval studied in this paper is highlighted (modified from Tortello and Esteban, 2016).
Description.Element robust, laterally compressed.The cusp is fragmented, proclined to erect with sharp flank.The anterior margin of the cusp ends in a small keel.