1. Introduction

Studies on spherical calcareous microfossils present in different Ordovician and Silurian carbonate lithology around the world are scarce (Munnecke et al., 1999, 2000; Servais et al., 2009; Kröger et al., 2019). In this sense, their biological affinities remain unclear due to the limited number of diagnostic characteristics that allow accurate taxonomic assignments. Through the time this microfossil received different names, such as calcisphers (Munnecke and Servais, 2008) and calcitarchs (Versteegh et al., 2009; Servais et al., 2009; Kröger et al., 2019).

The Lower-Middle Ordovician San Juan Formation is a classic unit from the Precordillera that is composed of fossiliferous limestone, marly limestone and reef facies (Beresi, 1986; Herrera and Benedetto, 1991; Beresi and Rigby, 1993; Vaccari, 1995; Sánchez et al., 1996; Carrera, 1997; Cañas and Carrera, 2003; Benedetto et al., 2007; Mestre et al., 2020, among others). There are few descriptions and interpretations of the carbonate microfacies, as well as of the calcareous micro-organisms and problematic microfossils present in these carbonate facies (Beresi, 1986; Cañas, 1995; Soria et al., 2017; Mestre et al., 2020). Recently, Beresi and Luchinina (2018) provided new data on the calcareous microfossils of the Ordovician carbonate and mixed facies from Cuyania. These authors described calcareous microfossils from the San Juan Formation and illustrated fragments of spherical microfossil interpreted as spherical algae.

In a recent study on carbonate microfacies carried out in the lower and middle part of the San Juan Formation in the Cerro La Chilca section, several calcareous microfossils were observed which are closely similar to those spherical algae reported by Beresi and Luchinina (2018).

The aim of this contribution is to describe these spherical calcareous microfossils and provide new biostratigraphic information for the first time for the Precordillera. Also, we propose the possible paleoenvironmental implications of these calcareous microfossils in the Early Ordovician carbonate facies from the Precordillera.

2. Geological setting

The geological province of Precordillera is developed along a length of 400 km N-S with a width of 150 km E-W in western Argentina, and it extends through the provinces of La Rioja, San Juan, and Mendoza, where Cambrian-Ordovician carbonate and siliciclastic successions are developed in a shelf environment. The Lower-Middle Ordovician carbonate succession of the Precordillera is represented by the San Juan Formation (Baldis et al., 1982; Keller, 1999; Mestre and Heredia, 2013; Mestre, 2014).

The San Juan Formation crops out in several classical localities at the Central and Eastern Precordillera, and it is considered an emblematic unit from Argentina.

Keller et al. (1994) redefined the lower and middle parts of the San Juan Formation at La Silla section (Central Precordillera) and the upper part at the Cerro La Chilca section. After that, the carbonate and mixed carbonate facies in the San Juan Formation were recognized in several sections by Cañas (1999), Keller (1999), Mestre (2014) and Soria et al. (2017), among others.

The first geological contribution for the Cerro La Chilca section was carried out by Stappenbeck (1910). Then, Cuerda (1965) mentioned the presence of Caradoc deposits in this section, which were later called Cerro La Chilca “Shales” by Blasco and Ramos (1976). Finally, Furque (1979) recognized and defined the San Juan and Los Azules formations. Keller (1999) completely surveyed the Ordovician San Juan Formation from the Cerro La Chilca section. This author verified that the lowermost beds of this unit are in contact with the La Silla Formation and it is transitionally overlain by Los Azules Formation (Fig. 1).

 

Fig. 1. Location of the study area. A. Image taken from Google Earth of the study section. B. Geological map of the Cerro La Chilca section (modified from Mestre, 2012).

Lehnert (1995) retrieved the first Darriwilian conodonts from the uppermost beds of San Juan Formation in the La Chilca section, and recorded the Eoplacognathus suecicus Zone after finding the conodont Histiodella kristinae Stouge. Mestre (2012) and Mestre and Heredia (2013, 2020) recorded the Lenodus variabilis, Lenodus crassus and Lenodus pseudoplanus zones in those same levels of the San Juan Formation. On the other hand, the Oepikodus evae and Oepikodus intermedius zones were recently recognized in the San Juan Formation lower-middle beds by Moreno et al. (2020). Other paleontological studies were restricted to the last 20 or 30 m of the San Juan Formation, such as Sánchez et al. (1996), Benedetto (2015), among others.

3. Conodont biostratigraphy

The sampled stratigraphic levels in the present contribution were recently studied in relation to their conodont content by Moreno et al. (2020). These author recorded the O. evae and O. intermedius zones in the San Juan Formation lower levels, and the conodont association was composed of the following conodont taxa: Bergstroemognathus hubeiensis An, Bergstroemognathus extensus (Graves and Ellison), Juanognathus variabilis Serpagli, Microzarkodina n.sp. (sensu Pyle and Barnes 2002), “M.” buggischi Lenhert, Oepikodus intermedius Serpagli, Periodon flabellum (Lindström), Reutterodus andinus Serpagli, and Scolopodus krummi Lenhert (Moreno et al., 2020). The most relevant findings of the former contribution are related to the first records of B. hubeiensis and Microzarkodina n. sp. in the Precordillera which provides significant information related to the possible ties of the Precordillera with Gondwana, peri-Gondwana, and Northwestern of the Midcontinent regions for this time interval.

Taking into account the records of the conodont biozones registered by Moreno et al. (2020) for the same levels of the San Juan Formation studied in the present contribution, it is possible to extend the first appearance of calcitarchs into the Lower Ordovician (Fig. 2).

Fig. 2. Conodont biostratigraphic chart of the Lower-Middle Ordovician from Precordillera (Albanesi and Ortega, 2002; Soria et al., 2013; Moreno et al., 2020), correlated with those of the South China (Wang et al., 2019) and Baltoscandia (Bagnoli and Stouge, 1997).

4. Material

This study is based on laboratory analyses of thin sections. All samples are limestone collected from the San Juan Formation in the Cerro La Chilca section (Fig. 3). Seven thin sections were made in order to identify fossils and analyze the distribution of the microfacies components in the Micropaleontology Laboratory of the Instituto de Investigaciones Mineras (IIM). A petrological investigation of the thin sections was performed using Leica DM2700 microscopes and Lanset binocular microscopes.

 

Fig. 3. Stratigraphic column of the Cerro La Chilca section, Central Precordillera, showing the sampled levels and vertical distribution of the two types of calcitarch along the section.

5. Microfacies description and environment interpretation

The microfacies described below correspond to the samples taken from the San Juan Formation lower-middle beds at the Cerro La Chilca section (Fig. 3). The ca. 35 m of the stratigraphic range studied begins with 8 m of light grey massive limestone succession (Ch1 sample), followed by 6 m of grey nodular limestone with abundant gastropods in contact with a 0.20 m bentonite bed (Ch2 and Ch3 samples). These levels are followed by 3 m of grey nodular limestone with well-preserved crinoids and brachiopod shells constituting middle grey bioclastic limestone (Ch4 sample). The section continues with 8 m of limestone, interbedded with fine layers of bentonite (0.05 m thick) with large gastropods, trilobites and brachiopods. The studies levels finally with 10 m of coarse limestone interbedding with reef facies (Ch8 and Ch9 samples).

M1-Burrowed bioclastic wackestone (Ch1, Ch2, Ch3 and Ch4 samples) is characterized by poorly sorted pelmatozoans, trilobites, and gastropod fragments randomly orientated. It also contains cyanobacteria, calcareous algae, spherical calcareous microfossils and microproblematica, such as Nuia síbirica Maslov, Girvanella rafts and Girvanella intraclasts (Rong et al., 2014). Some skeletal allochems show geopetal infilled by equant, drusy and blocky spar. The matrix is micritic although it is partially recrystallized as blocky sparite. Stylolithic surface is observed (Fig. 4).

Fig. 4. Photomicrograph of the microfacies M1. A-D. Bioclastic component. E. Echinoderm ossicle, Nuia sibiria (N), trilobites, gastropod fragments and micrite matrix. The length of the scale bar is 1 mm.

Interpretation: The microfacies M1 was developed in a shallow subtidal environment. The high abundance of Nuia sibírica, calcareous algae and fine siliciclastic sediments suggest low energy conditions below wave action within the shallow euphotic zone in the inner platform. Furthermore, the presence of Girvanella intraclasts and Girvanella rafts suggest transport due to wave action and moderate energy conditions (Flügel, 2010; Rong et al., 2014; Vachard et al., 2017; Pandey and Parcha, 2018; Kröger et al., 2019).

M2-Peloidal intraclastic packstone-grainstone (Ch5 and Ch6 samples) is composed predominantly of micritic peloids of elongate and irregular shape with a size <1 mm, and subrounded oncoides with thin-cortex (Han et al., 2014). The diameter of the oncoids ranges from 0.5 to 3 mm. Their nuclei comprise pelmatozoan ossicles and Nuia sibirica coated by a thin micritic envelope. Other components include bioclast fragments, calcareous spherical microfossil and intraclast of Girvanella, with a size <1mm, moderately sorted (Fig. 5).

Fig. 5. Photomicrograph of the microfacies M2. A. bioclastic intraclast surrounded by a micrite crust, and peloids moderately-well sorted. B-D. Peloidal intraclastic packstone-grainstone with bioclastic components. The length of the scale bar is 1 mm.

M3-Intraclastic floatstone (Ch7 sample) comprises predominantly subrounded and elongated micritic intraclasts, with sizes ranging from 2mm to 1cm. Skeletal grains are micritized. The matrix is bioclastic packstone (Fig. 6).

Fig. 6. A-B. Photomicrograph of the microfacies M3. Peloidal intraclast. The length of the scale bar is 1 mm.

M4-Intraclastic wackestone-packstone (Ch8 sample) is dominated by subrounded, elongated and irregular intraclasts with a size of 0.2-1.5 mm. It also comprises skeletal components surrounded by micritic envelopes, Girvenella raft and Halysis moniliformis Høeg (Fig. 7).

Fig. 7. Photomicrograph of the microfacies M4 (intraclasitc wackestone-packstone) and M5 (bioclastic boundstone). A. Photomicrograph of the microfacies M4, subrounded, elongate intraclast and bioclastic component occurring in moderately sorted wackestone-packstone. B-D. Bioclastic component of the microfacies M4; H: Halysis moniliformis; Gr: Girvanella raft; P: micritized peloids and fragments skeletal component. E-F. Photomicrograph of the microfacies M5, bioclastic components H: Halysys moniliformis; A: calcareous algae; G: encrustating of Girvanella (horizontally growing Girvanella-like tubes). The length of the scale bar in A is 1 mm and the length of the scale bar in B-F is 200 µm .

M5-Bioclastic boundstone (Ch9 sample) represents the main microfacies of the biogenic structure. The reefs components consist of Zondarella communis (pulchrilaminids), lithistid sponges and Calathium sp. It also contains cyanobacteria, calcareous algae, Girvanella raft and Girvanella crust. Nuia sibirica, Halysis moniliformis and pelmatozoan ossicles are poorly represented (Fig. 7).

Interpretation: These microfacies represent reef and shoal facies and suggest high energy episodes produced by fair or storm weather that alternated with low energy moments in the inner open platform (Flügel, 2010; Han et al., 2014; Rong et al., 2014; Mestre et al., 2020). The relatively high abundance of Nuia sibirica and calcareous algae in the limestone is evidence of a shallow-water environment with a depositional depth of less than 25 m within the shallow euphotic zone (Granier, 2012; Kröger et al., 2019).

6. Calcareous microfossils: calcitarchs

The term “calcisphere” was used as a general term to describe the calcareous, more or less spherical microfossils, irrespective of their biological source (Flügel, 2004). Later, Versteegh et al. (2009) indicated that the term calcispheres was inconsistently used, due to the morphological and taxonomical concepts were mostly unclear, and a formal definition was needed. To solve this issue, the former authors proposed a new group called Calcitarcha (by analogy with the Acritarchs) this group is proposed as an informal category, which include all calcareous microfossils with a central cavity for which the biological affinities remain unknown. The calcitarchs should thus be considered a polyphyletic group, with most of the Mesozoic microfossils being probably calcareous cysts of dinoflagellate species. However, the biological affinities of Lower Palaeozoic calcitarchs are challenging to establish.

Munecke and Servais (2008) recorded a group of carbonate spherical microfossils from different Silurian facies from Gotland (Sweden). These microfossils were of variable sizes, between 60-100 µm, and termed them calcispheres (Young et al., 1997). These authors also illustrated littler microfossils, 10-25 µm, termed “nanospheres”. Then, Kröger et al. (2019) proposed a classification recognizing 3 types of spherical microfossils from the Late Ordovician Baltic limestone facies: type I calcitarch, type II calcitarchand type III calcitarch.

An older diagnosis carried out by Munnecke et al. (2000) included spherical to subspherical forms in the Ovommuridae family. However, those specimens illustrated by Munnecke and Servais (2008), were considered incertae sedis due to the lack of biologic affinities with some living organism groups. According to Servais et al. (2009), some calcitarchs from the Paleozoic may be interpreted as calcaerous cysts of organisms that might have been the ancestors of the dinoflagellates, whose earliest fossil record is in the Mesozoic. Nevertheless, it should be noted that many other calcitarchs should probably be attributed to other biological phyla (algal groups, foraminifers or radiolarians). The authors emphasize that it is difficult to establish biological affinities because many of the Paleozoic rocks are poorly preserved and diagenetically altered.

Kröger et al. (2019) proposed that the calcitarchs correspond to aplanosporas (green algae) and are probably related to taxa that cannot be preserved as articulate fossils, like Acetabularia which vanishes quickly. This interpretation is related to the idea that limestone matrix (mud) was originated from aragonitic algae. 

In the Precordillera, Beresi and Luchinina (2018) showed fragments of spherical microfossils from the Las Chacritas (Plate I, figs. g, f, j, l) and Talacasto (Plate II, figs. m, n, o) sections and described them as problematic spherical algae, but they were not able to identify due to the lack of morphological characteristics.

Nevertheless, the spherical microfossils here reported from the lower-middle part of the San Juan Formation are denominated calcitarch according to the classification of Kröger et al. (2019). These spherical microfossils were recognized in the Floian facies (O. evae-O. intermedius zones), and they represent the earliest worldwide record for this type of microfossil. However, the taxonomy and biological affinity of these organisms are beyond the scope of this contribution.

6.1. Classification

The Floian spherical microfossils recognized in the samples collected from the Cerro La Chilca section are described following the size criterion proposed by Kröger et al. (2019) based on the observation of thin sections only. The poor preservation of the microfossils limited the possibility to analyzed under Scanning Electron Microscope (SEM). Consequently, some features of the wall such as its width and pores are not provided.

In the studied microfossils we recognized two groups. The first one is a group that could be placed into ‘calcispheres’ with a radial shell layer (sensu Munnecke and Servais, 2008), or could be determined as calcitarch with a small size. This group of spherical microfossils (Figs. 8 and 10) exhibits smaller sizes than those described by Kröger et al. (2019) in their three categories (Type-I calcitarchs, type-II calcitarchs and type-III calcitarchs). Consequently, a fourth group in this classification is proposed termed type-0. The second one is a larger spherical microfossils group described herein (Fig. 10) which are similar in general appearance and structure to type-III calcitarchs (Kröger et al., 2019).

Fig. 8. Calcitarch from San Juan Formation (Precordillera), Floian Stage. A-D. Calcitarch type-0. The length of the scale bar in A and D is 1 mm and the length of the scale bar in B and C is 200 µm . The samples correspond to microfacies M1.

Fig. 9. Calcitarch from San Juan Formation (Precordillera), Floian Stage. A-C. Calcitarch type-0. The length of the scale bar is 200 µm . The samples correspond to microfacies M1.

Fig. 10. Calcitarch from San Juan Formation (Precordillera), Floian Stage. A-D. Calcitarch type-III. The length of the scale bar in B is 1 mm and the length of the scale bar in A, C, D is 200 µm. The samples correspond to microfacies M2.

Type-0 calcitarch: Small spheres with thin wall. Diameter ~80 μm. The specimens of figures 8A, C and 9 have a diameter of 100 μm, those of the figure 8B have a diameter of 111 μm, those of the figure 8D have a diameter of 95 μm, and those figure 8C, B have a diameter of 50 μm. They occur in burrowed bioclastic wackestone.

Type-III calcitarch: Large spheres with thin to medium wall. Diameter ~250 μm. The specimens of figure 10A, C and D have a diameter of 200 μm and those of the figure 10B have a diameter of 250 μm. They occur in peloidal intraclastic packstone-grainstone.

Based on a vertical distribution analysis of the different calcitarch types along the section, it is possible to recognize that the recovered specimens show variations in size in relation to the facies that contain them. The calcitarchs recognized in wackestone-type facies are smaller (Fig. 4) than those recovered from the packstone-grainstone facies (Fig. 5). Such a distribution pattern is recognized for the first time in the Paleozoic, showing great potential for evaluating the possible paleoenvironment preferences of these microfossils.

Taking into account their size, the San Juan Formation Floian type-0 calcitarch could be considered similar to the Silurian calcitarchs described by Munnceke and Servais (2008) and Servais et al. (2009). These authors interpreted that these specimens belong to the calcareous microplankton and consider cysts of a possible ancestor of the organic-walled dinoflagellates that probably belong to calcareous micro- and nannoplankton. However, considering their morphology, they are like the Ordovician calcitarchs described by Kröger et al. (2019), who interpreted this microfossil as possible spores of several green algae because they are associated with reef facies. Although we cannot assign the studied microspheres to any fossil group, we consider that they are related with the Kröger et al. (2019) classification as follows: we suggest that the largest microfossils correspond to type-III calcitarch and we propose a new group, type-0 calcitarch, for calcareous microfossils smaller than 100 µm.

7. Paleoenvironment interpretation of the different types of calcitarchs from the San Juan Formation

The applicability of calcareous spherical microfossils as paleoenvironmental indicators has not yet been satisfactorily investigated (Kaźmierczak and Kremer, 2005). However, according to Klovan (1964), Stanton (1963, 1967), Racki and Soboń-Podgórska (1993), and Kaźmierczak (1976), the presence of these microfossils has been interpreted as evidence of shallow-marine depositional environment within basins characterized by unstable and high eutrophication level. Flügel (2004) stated that most Palaeozoic calcispheres occurred in a shallow-marine platform and ramp carbonates, while most of the Jurassic and Cretaceous calcispheres occurred in pelagic limestones represented by calcareous dinoflagellate cysts.

The different types of calcitarchs appear associated with green calcified algae as the main skeletal component of the Baltica Limestone facies (Late Ordovician) and its associated reefs (Kröger et al., 2019). They indicate environment deposition within the shallow euphotic and euryhaline zone under low-latitude climatic conditions. The Baltic limestone facies are interpreted as representing the shallower part of an open-marine carbonate platform, which underwent high temperatures and extremely low terrigenous sediment input (Kröger et al., 2019). Moreover, the Silurian calcitarchs from Gotland (Sweden) were found in different facies associated with the ancestors of the foraminifera (Parathurammina, Caligella, etc.), Girvanella and Halysis. However, the well-preserved calcitarchs specimens were recovered only from so-called limestone-marl alternations deposit in shallow platform facies (Munnecke and Servais, 2008).

The facies and carbonate component features present in the deposits that enclose the type-0 and type-III calcitarchs in the Cerro La Chilca section indicate from shallow subtidal environment in low energy conditions to shoal deposits with an increase of energy in the carbonate platform (Fig. 11). Furthermore, the facies analyzed herein is composed of Nuia sibírica, Girvanella and other calcareous algae which allow interpreting a shallow subtidal environment within the photic zone in a tropical to subtropical climate (Flügel, 2010; Rong et al., 2014; Vachard et al., 2017; Pandey and Parcha, 2018).

Fig. 11. Carbonate platform depositional model for the San Juan Formation in the study area. The platform displays a gentle transition from shallow to deeper depositional environments. The subdivision of the carbonate platform is based on Pomar (2001). FWB: fair-weather wave base; MSL: medium sea level; M1: Burrowed bioclastic wackestone; M2: Peloidal intraclastic packstone-grainstone; M3: Intraclastic floatstone; M4: Intraclasticwackestone-packstone; M5: Bioclastic boundstone; white circle: calcitarch type-III; grey circle: calcitarch type-0.

The studied stratigraphic range begins with fine facies represented by burrowed bioclastic wackestone (M1) that has been deposited in low-energy conditions. In this facies only the type-0 calcitarch is present. The presence of carbonate mud indicates low energy conditions below fair weather waves within a shallow inner platform setting. Then, the peloidal intraclastic packstone-grainstone (M2), intraclastic floatstone (M3), intraclastic wackestone-packstone (M4) and bioclastic boundstone (M5) facies indicate high energy conditions, suggesting a shallow subtidal environment within the inner high-energy platform associated with shoal and reef facies. In these beds the type-III calcitarch is recorded. The variation in the size of the calcitarch recognized along the studied sections could be related to a gradual increase of energy within a shallow subtidal environment, from distal to proximal, including from shallow subtidal facies below wave action to shoal and reef facies. This could be interpreted as a reflect a gradual shallowing-upward evolution as it was proposed by Soria et al. (2017) and Mestre et al. (2020) for coeval levels of San Juan Formation from the Niquivil and Talacasto sections.

8. Conclusion

Based on the investigation of calcareous microfossils and their depositional facies, as well as a microfacies analysis of the lower-middle part of the San Juan Formation in the La Chilca section (Central Precordillera), conclusions are drawn as follows:

In this study we consider the classification proposed by Kröger et al. (2019) for describing our calcareous microfossils. We suggest that the largest microfossils correspond to type-III calcitarch, and we propose a new group, type-0 calcitarch, for calcareous microfossils smaller than 100 µm.

The calcitarchs recognized in wackestone facies are the Type-0 calcitarch and those recovered from the packstone-grainstone facies are the Type-III calcitarch. This distribution pattern is recognized for the first time in the Paleozoic, showing great potential for evaluating the possible paleoenvironment preferences of these microfossils.

The size variations of the calcitarch recognized along the studied section probably related to a gradual increase of energy within a shallow subtidal environment, from distal to proximal, including from shallow subtidal facies below wave action to shoal and reef facies.

The occurrence of the calcitarchs in the Floian O. evae and O. intermedius conodont zones of the San Juan Formation represents the oldest worldwide records, extending the first appearance of this type calcareous microfossil down into the Early Ordovician.

Acknowledgements
The authors wish to thank the CONICET (National Research Council of Argentina) to support this contribution through the grant PIP 2014 0058CO. We are thankful to R. Atencio for her assistance in the language improvement of the manuscript. This is a contribution to the International Geoscience Programs (IGCP) projects 653 ‘The onset of the Great Ordovician Biodiversification Event’ and 735 ‘Rocks and the rise of Ordovician life-rocks n’ROL’.

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