Darriwilian (Middle Ordovician) conodonts and graptolites from the Cerro La Chilca Section, Central Precordillera, Argentina

The Ordovician System is extensively represented in the Precordillera of San Juan Province, Argentina. At the Cerro La Chilca in the Jáchal area, the limestone of the San Juan Formation is paraconformably overlain by interbedded limestone and shale of the Gualcamayo Formation. The present contribution reports new data on the conodont fauna and biostratigraphy of these darriwilian units, revising local and regional chronostratigraphic relationships. New information on the composition of conodont and graptolite associations through the stratigraphic sequence is presented. The presence of Paroistodus horridus horridus, Yangtzeplacognathus crassus, and Histiodella sinuosa constrain the uppermost strata of the San Juan Formation to the lower part of the Y. crassus Zone, according to the Baltoscandian scheme, and to the H. sinuosa Subzone of the Periodon macrodentatus Zone of the North American scheme. In the overlying Gualcamayo Formation the co-occurrence of Y. crassus with Histiodella holodentata enable the recognition of the Y. crassus Zone and the H. holodentata Subzone of the P. macrodentatus Zone. The identification of these zones allows for precise global and regional correlation. A graptolite assemblage that belongs to the epipelagic and deep-water biotopes with some components restricted to low paleolatitudes is recognized. This diverse assemblage is characteristic of the pelagic biofacies. The important diversity of graptolites in this section suggests a favorable environment for their development. Local changes in the taxonomic composition are recognized through the Gualcamayo Formation. When comparing this fauna with that of different study localities from the Central Precordillera (Cerro Potrerillo, Oculta Creek, Cerro Viejo de Huaco and Las Aguaditas Creek) slight differences in the generic composition are observed. Taxonomic differences support the preference of certain associations for particular environments; though, graptolites are more diverse in black shales facies, which represent deeper environments (the Los Azules Formation), in relation to the calcareous-shale facies of the Gualcamayo Formation from Cerro La Chilca and correlative unit at Las Aguaditas Creek.


Introduction
The Precordillera is a geological province located in the western margin of Argentina, between the Frontal Cordillera to the West and the Pampean Range to the East, covering parts of La Rioja, Mendoza and San Juan provinces (Stelzner, 1873;Furque and Cuerda, 1979;Baldis et al., 1982). It is characterized by a ~2,000 m thick succession of Cambrian-Ordovician sedimentary rocks preserved in an extensive fold-andthrust orogenic belt . In the Jáchal area of San Juan Province, Lower Paleozoic strata are extensively exposed and have been the subject of many paleontological and geological investigations (e.g., Benedetto, 2003 and references therein). An important Ordovician exposure is represented in the Cerro La Chilca section, located approximately 40 km south of San José de Jáchal, in the San Juan Province, Argentina (Fig. 1).
The Ordovician rock sequence described by Cuerda (1965) at the Cerro La Chilca section was recognized as the San Juan and the Los Azules Formations (Furque, 1983;Cuerda, 1986). Later, the succession of interbedded shale and siltstone, overlying the San Juan Formation at this locality, was described by Astini and Benedetto (1992) as the Gualcamayo Formation and interpreted as transgressive deposits. These authors divided the Gualcamayo Formation into two members, equivalent respectively to the upper San Juan Formation sensu Cuerda (1986) and to the Los Azules Formation as previously identified by Furque (1983).
More recently, Peralta (1998) and Tortello and Peralta (2004) described the lower part of the Gualcamayo Formation at Cerro La Chilca as a 4.3 m thick unit with alternation of black, tabular marly limestones and dark laminated shales that overly the limestone of the San Juan Formation. The uppermost fossiliferous level of black limestone defines the upper-limit of the Gualcamayo Formation, which is unconformably overlain by the shaly Los Azules Formation (Peralta, 1998); formerly described as the upper member of the Gualcamayo Formation by Astini and Benedetto (1992). The Los Azules Formation consists of 78 m of black shales with occasional intercalations of lensoidal beds of dark limestones. A significant hiatus corresponding to the Guandacolic orogeny occurs between these units (Peralta, 2003).
In the present contribution, only the top strata of the San Juan Formation and the Gualcamayo Formation are analyzed. The former is characterized by 20 cm thick skeletal wackestones beds, which present a light gray to ochre color on weathered surfaces and dark gray in freshly broken planes. The Gualcamayo Formation, ca. 4 m in thickness, consists of 10-20 cm thick carbonate mudstones interbedded with black shales (Fig. 2). The Gualcamayo shales are light brown when weathered and black in fresh exposure surfaces, graptolite remnants are common on shale bedding planes.

Paleontological framework
Several paleontologic studies have been carried out in the Cerro La Chilca locality. For instance, conodont, sponge, bryozoan, and crinoid assemblages have been described for the uppermost part of the San Juan Formation (e.g., Lehnert, 1995;Sánchez et al., 1996;Carrera, 1997;Keller, 1999;Mestre, 2012;Carrera et al., 2013;Serra et al., 2017). In a recent contribution, graptolites were documented for the first time in the uppermost part of the formation, ca. 1 m below its contact with the Gualcamayo Formation .

Materials and methods
A total of 13 samples were collected (Fig. 3), including 5 samples from the upper part of the San Juan Formation and 8 from the Gualcamayo Formation. The samples of 2 kg each were digested in a 10% acetic acid solution, according to the method described by Stone (1987). The conodont elements picked up account for a total of 971 identifiable specimens (Table 1). These exhibit a color alteration index (CAI) of 2.5, reflecting burial paleotemperatures between 90 °C and 110 °C (Epstein et al., 1977). The graptolite fauna from the Cerro La Chilca section was referred in a recent study , where it was dealt for biostratigraphic purposes; whereas in the present paper a palaeoenvironmental appraisal is given to this fauna.
The fossil collection is housed in the Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, under the repository codes CORD-MP, for conodonts, and CORD-PZ, for graptolites.

Conodont fauna
A diverse conodont association was recovered from the study section, including the following species ( (Chen and Zhang).
This species assemblage and the presence of key index taxa such as Y. crassus, H. sinuosa and H. holodentata, constrain the contact between the San Juan and Gualcamayo formations to the Y. crassus Zone as defined for the Baltoscandian region (assessed by Löfgren and Zhang, 2003) and to the H. sinuosa and H. holodentata subzones of the P. macrodentatus Zone following the scheme of Stouge (2012) for western Newfoundland (Fig. 6). The cooccurrence of Paroistodus horridus horridus with Y. crassus identifies the base of the Y. crassus Zone at the top of the San Juan Formation. This conodont zonal determination agrees with the identification of 130 Darriwilian (Middle Ordovician) conodonts and graptolites from the Cerro La Chilca Section...   the Levisograptus dentatus Zone, confirming their mutual biostratigraphic correspondence (Albanesi and Ortega, 2016;Serra et al., 2017).

Discussion
A number of biostratigraphic studies were accomplished in the Cerro La Chilca section. Lehnert (1995) documented the E. suecicus Zone in the upper part of the San Juan Formation according to specimens that he determined as belonging to the eponymous species, from the neighboring Las Chacritas River section, and of ?Histiodella kristinae from strata of the Cerro La Chilca section (see plate 10, figs. 1, 10, 14, of the referred author). However, the specimen illustrated as ?H. kristinae is a fragmentary Pa element (lacking the base and cusp), which cannot be attributed to a particular species of the genus Histiodella. Additionally, the P elements identified as E. suecicus were reassigned to Y. crassus by Löfgren and Zhang (2003), which is consistent with our records. Mestre  5.6, Mestre, 2012) shows a cusp higher than the anterior denticles, being this relation diagnostic for the identification of H. holodentata (Stouge, 1984(Stouge, , 2012. Additionally, the stratigraphic range of H. sinuosa does not overlap the range of H. kristinae, which is a descendant of H. holodentata, a succeeding form of H. sinuosa (see Stouge, 1984). In a subsequent study, the conodont association obtained from the uppermost strata of the San Juan Formation at this locality (1 m below the contact between the San Juan and Gualcamayo formations) yielded the key species H. sinuosa, P. horridus and P. macrodentatus (Carrera et al., 2013). This association Our sample SJ-1 (ca. 1.6 m below the top of the San Juan Formation, see Fig. 3) yielded conodont elements of P. horridus secundus (Fig. 5.10), which would indicate the P. horridus Subzone of the L. variabilis Zone of the Argentine general conodont biozonation, as previously suggested by Carrera et al. (2013) and Feltes et al. (2016), and finally revised by Albanesi and Ortega (2016). Although, the co-occurrence of P. horridus horridus with Y. crassus in the same sample indicates the base of the Y. crassus Zone instead. Also, Pa elements of Y. crassus are found with H. sinuosa in this sample and in the subsequent one (1 m below the top of the San Juan Formation). The latter species is not reported, elsewhere, for the E. pseudoplanus Zone. The rich conodont association including index species such as Y. crassus, M. hagetiana, H. sinuosa, H. holodentata and P. macrodentatus, accompanied by the key graptolite species L. dentatus (Figs. 3, 7), verify the actual recognition of the Y. crassus Zone for the upper part of the San Juan Formation and the Gualcamayo Formation in the study section.

Regional correlation
The key taxa Y. crassus was first recorded in the San Juan Formation at Central Precordillera by Lehnert (1995), although this author identified it as E. suecicus Bergström, and was then documented in numerous works (e.g., Albanesi et al., 2006Albanesi et al., , 2013Heredia et al., 2005;Heredia and Mestre, 2011;Mestre and Heredia, 2013;Feltes et al., 2016;Serra et al., 2015Serra et al., , 2017. Regionally, this zone correlates with coeval strata of the Las Aguaditas Creek section (Feltes et al., 2016), Las Chacritas River section (Heredia et al., 2005;Serra et al., 2015), Cerro Potrerillo , Villicum range (Sarmiento, 1991;, Cerro Viejo de Huaco (Ottone et al., 1999;Ortega et al., 2007) and Oculta Creek , where the Y. crassus Zone is recognized in transitional facies from the San Juan Formation and overlying units. In a recent study, Mango and Albanesi (2018) recognized the L. variabilis Zone in the top strata of the San Juan Formation at the Los Gatos Creek, Cerro Viejo de Huaco area; differing with previous works (Ortega, 1987;Ottone et al., 1999;Ortega et al., 2007) that report the Y. crassus Zone in the stratigraphic sections located towards the south of Cerro Viejo de Huaco. However, the referred authors conclude that this difference is due to a diachronism occurring at the top of the San Juan Formation, where the younger strata (Y. crassus Zone) are exposed in sections located towards the south of the Cerro Viejo de Huaco (Mango and Albanesi, 2018). The conodont fauna associated with Y. crassus also allows the correlation with the Yerba Loca Formation at Ancaucha creek (Albanesi et al., 1995;Voldman et al., 2008) and with the Los Sombreros Formation at Los Túneles of Jáchal river (Voldman et al., 2009). Heredia et al. (2005) reported, with doubt, the presence of Y. crassus in the Las Chacritas River section and Albanesi et al. (2006) published the presence of this index species at Cerro Viejo de Huaco. Later, Albanesi et al. (2013) and Serra et al. (2015) verified it's the record across the contact between the San Juan Formation and the overlying Las Chacritas Formation. Mestre (2012), Mestre (2011, 2013) and Mestre and Heredia (2013) identified Y. crassus in the uppermost meters of the San Juan Formation at the El Aluvión Creek (Cerro Viejo de Huaco area), Cerro La Chilca and in the Las Chacritas River sections.  identified the Y. crassus Zone through transitional beds between the San Juan and Gualcamayo formations in the Villicum Range. On the other hand, a rich conodont fauna was documented in association with the index species Y. crassus and H. holodentata in the upper strata of the San Juan Formation at the Oculta Creek section . In a recent study, the first appearance datum of Y. crassus was recorded 15 m below the top of the San Juan Formation at the Las Aguaditas Creek section, delimiting the lower boundary of this biozone, whereas the upper limit is marked by the presence of Dzikodus tablepointensis (Stouge), 19 m above the base of the Las Aguaditas Formation (Feltes et al., 2016). These authors recognized a similar faunal relationship and recovered elements of L. variabilis, H. sinuosa, H. holodentata and P. horridus co-occurring with Y. crassus. These faunal relationships are in accordance with those obtained from the Cerro La Chilca, and reinforce the identification of the Y. crassus Zone through the uppermost strata of the San Juan Formation and the lower part of the Gualcamayo Formation.

Global correlation
The presence of Y. crassus in the Cerro La Chilca section is significant for intercontinental correlation, used as an index species in the stratigraphic schemes of Baltoscandia (Löfgren, 2004), South China (Zhang, 1998), and recently incorporated in the biozonal scheme of the Argentine Precordillera Albanesi and Ortega, 2016). The abundance of Histiodella species at the Cerro La Chilca section allow for a precise correlation with western Newfoundland (Stouge, 2012) and North China (Wang et al., 2014;Jing et al., 2016).
In China, Y. crassus appears together with P. horridus and P. macrodentatus (Chen et al., 2006). The species P. macrodentatus is recorded with H. holodentata in the Kuniutan Formation of South China (Zhang, 1998), and these species, together with H. sinuosa occur in the Dawangou Formation of the Tarim Region (Du et al., 2005). The Y. crassus Zone was also documented in the Kuniutan Formation (Dw2) from the Yichang Region of the Yangtze Platform, South China (Zhang, 1998;Wu et al., 2014).
In western Newfoundland, the Y. crassus Zone corresponds to the P. macrodentatus Zone, although restricted to the intermediate H. holodentata Subzone (Stouge, 2012). The presence of Periodon macrodentatus in the Middle Ordovician strata of the Oslobreen Group (Svalbard Archipelago) allows for correlation with the respective interval of the Laurentian margin (Lehnert et al., 2013), the authors also mention the high abundance of P. originalis. Originally, in the Argentine Precordillera, the transitional forms between Paroistodus originalis and P. horridus were recorded through the upper part of the L. variabilis Zone, in the lower member of the Gualcamayo Formation at the Cerro Potrerillo section Albanesi and Barnes, 2000), allowing for the definition of the upper interval of the L. variabilis Zone; namely, the P. horridus Subzone (Albanesi and Ortega, 2002). At the Cerro La Chilca section, the record of P. originalis, P. horridus secundus, and P. horridus horridus coexist with H. sinuosa in the upper strata of the San Juan Formation suggesting stratigraphically younger strata than observed in the Oslobreen Group.
The conodont fauna from Thompson Creek, New Zealand, is referred to the middle Darriwilian (Zhen et al., 2009), comparable with contemporaneous units of central New South Wales (Zhen and Percival, 2004). The conodont association described by the authors resembles that of the Cerro La Chilca section; namely, A. jemtlandica, C. longibasis, D. cf. reclinatus, D. tablepointensis, P. simplicissimus, V. balticus, P. macrodentatus, H. holodentata, P. originalis and P. horridus. The co-occurrence of the latter four species enables correlation of the uppermost strata of the San Juan Formation and the Gualcamayo Formation with the Thompson Creek succession in New Zealand.
As referred in Serra et al. (2017), the graptolite assemblage is dominated by the genus Pseudo-bryograptus in the lower part of the Gualcamayo Formation at the Cerro La Chilca, which decreases to the top where the genus Levisograptus becomes a common component of the fauna and some taxa, such as L. primus, P. caduceus and Pseudophyllograptus sp. appear for the first time. Although environmental conditions are interpreted to be homogeneous through the unit (Henderson et al., 2018), local changes in the taxonomic composition could be the consequence of immigration due to subtle environmental changes in the conditions of the water column, not observed in the lithofacies. Also, in this unit a constant alternation of carbonate and shale is recorded, sea level change could have affected the graptolite assemblages accounting for the local replacement of species.
The lower middle Darriwilian graptolite assemblages present in different sections of the Central Precordillera (Cerro Potrerillo, Cerro Viejo de Huaco, Quebrada Oculta, Quebrada de Las Aguaditas, and Cerro La Chilca sections) contain rich and abundant epi-and mesopelagic components that belong to the pelagic biofacies (sensu Cooper et al., 2012). However, in the different localities a slightly different association of graptolites is developed, which reflects preference to different environmental conditions for graptolite associations. Although these units record the drowning of the carbonate platform, the Los Azules Formation (Cerro Viejo section) and the Gualcamayo Formation (Cerro Potrerillo section) represent deeper depositional environment compared to that of the Las Aguaditas Formation (Las Aguaditas Creek section) and the Gualcamayo Formation (Cerro La Chilca section). It is worth mentioning the diachronous nature of the Gualcamayo Formation, where the black shales from the base of the middle member in the Cerro Potrerillo section (north of Precordillera) are equivalent in age to the alternating carbonate and black laminated shale unit in the Cerro La Chilca section (to the south in the Central Precordillera).
In the Oculta Creek, Los Cauquenes Range, the lower member of the Los Azules Formation is dominated by T. acanthonotus, and to a lesser extent by P. ensiformis and A. zhejiangensis, the first two species absent in the Las Aguaditas Formation and the Gualcamayo Formation (Cerro La Chilca section). Also, Isograptus divergens Harris and Brachiograptus etaformis Harris and Keble are only present in the Los Azules Formation, indicating deeper-water environments. On the other hand, P. parallelus is particularly abundant in the Gualcamayo Formation of the Cerro La Chilca. The species Cryptograptus antennarius (Hall) was documented in the Los Azules and Las Aguaditas formations and C. cf. antennarius in the lower member of the Gualcamayo Formation at Cerro Potrerillo section. The scheme of figure 8 represents the spatial arrangement of the main genera of lower middle Darriwilian graptolites in the in the Cerro La Chilca section and other areas of the Central Precordillera. As in the Cerro La Chilca section, the early middle Darriwilian graptolite assemblages in other sections of the Central Precordillera belong to the epipelagic and deep-water biotopes, preserved in outer-shelf facies.

Concluding Remarks
A diverse conodont association was recovered from the uppermost San Juan Formation and the Gualcamayo Formation exposed at the Cerro La Chilca classical locality. The presence of Paroistodus horridus horridus, Y. crassus and H. sinuosa allows constraining the top of the San Juan Formation to the base of the Y. crassus Zone of the Baltic scheme and to the H. sinuosa Subzone of the P. macrodentatus Zone of the North American scheme. In the overlying Gualcamayo Formation the co-existence of Y. crassus with H. holodentata enable the recognition of the Y. crassus Zone and the H. holodentata Subzone of the P. macrodentatus Zone of the relative schemes.
The identification of the Y. crassus Zone, as well as the H. sinuosa and H. holodentata subzones enable the correlation between the study interval with coeval strata at the Las Aguaditas Creek section, Las Chacritas River section, Cerro Potrerillo section, Villicum range, Cerro Viejo de Huaco, Oculta Creek, with the Yerba Loca Formation at Ancaucha creek, and with the Los Sombreros Formation at Los Túneles of Jáchal river, and globally with Baltoscandia, China, western Newfoundland, New Zealand and New South Wales.
A graptolite assemblage belonging to the epipelagic and deep-water biotopes with some components restricted to low paleolatitudes is recognized. This diverse assemblage belongs to the pelagic biofacies, in coherence with the offshore sedimentation environment suggested for the Gualcamayo Formation at the Cerro La Chilca. The local changes observed in the taxonomic composition along this unit could be the consequence of immigration due to environmental changes in the conditions of the water column, not observed in the lithofacies. On the other hand, the constant alternations of carbonate and shale of this formation driven by sea level change, could also affect the graptolite assemblages accounting for the local replacement of the graptolite species.
Slight differences in the taxonomical composition of different study localities from the Central Precordillera supports the preference of certain associations for particular environments. Graptolites are more diverse in the lower member of the Los Azules Formation, in black shales facies that represent a deeper environment, in relation to the calcareous-shale facies of the Gualcamayo and Las Aguaditas formations of the Cerro La Chilca and Las Aguaditas Creek, respectively.