New insights on the Silurian graptolite biostratigraphy of the La Chilca Formation, Poblete Norte section, Central Precordillera of San Juan, Argentina: faunal replacement and paleoenvironmental implications

. New data and a biostratigraphic review on Silurian graptolites of the La Chilca Formation are presented. Silurian retiolitids and monoserial graptolites are described for the first time in the lower layers of the Salto Macho Member at the Poblete Norte section, exposed in the Central Precordillera, San Juan Province, western Argentina. This member (late Hirnantian-early Wenlock) begins with a pebble paraconglomerate covered by a sedimentary succession of dark grey shales and greenish-yellowish siltstones. Two graptolite assemblages were collected therein. The first association includes Pseudoplegmatograptus reticulatus , Stimulograptus sedgwickii , Talacastograptus leanzai , Metaclimacograptus asejradi , Coronograptus sp., and biserial and monoserial indeterminate graptolites. The second contains Retiolites geinitzianus , Monograptus priodon , Stimulograptus sp., Pristiograptus sp., and unknown monoserial colonies. The late Aeronian Stimulograptus sedgwickii Zone is recorded due to the presence of the index taxon in the middle levels of the Salto Macho Member. The second graptolite fauna has been related to the early Sheinwoodian Cyrtograptus murchisoni Zone, based on the presence of palynomorph assemblages from the La Chilca Formation, previously studied in another sections. The biotic changes of the graptolite faunas, sedimentary variations, and prevalent paleoenvironmental conditions observed in the lower beds of the Salto Macho Member, could indicate the first evidence of the Sedgwickii Event in Precordillera, which is recognized globally and involves comparable features.

In the Poblete Norte section, the graptolite assemblages studied by Lopez et al. (2018Lopez et al. ( , 2020)), belong to the M. persculptus and possibly the A. atavus zones, in association with brachiopods and scolecodonts.Recently, Gómez et al. (2021) mentioned the presence of Hirnantian palynomorphs in the first levels of the Salto Macho Member at this section.

The Sedgwickii Event worldwide
During the late Aeronian sedgwickii (or Sedgwickii) Event (Štorch and Frýda, 2012), the graptolite diversity decreases markedly.Some species were extinct and other ones were gradually replaced.These faunal changes coincide with differences in lithology and sedimentation, positive excursion in the δ13C values, and fluctuations of total organic carbon (TOC).This event has been recorded in the North Hemisphere (Loydell, 2007;Melchin and Holmden, 2006;Štorch and Frýda, 2012), located temporarily in the middle part of the homonymous biozone.The graptolite extinction is coincident with a δ13C positive excursion and a lower eustatic sea level interval derived from glaciation in South America (Gondwana) (Loydell, 2007); although, its influence on the South American stratigraphy and graptolite fauna is unknown so far.

Introduction
The Precordillera Geological Province, from western Argentina, is a fold-thrust belt characterized by extensive Paleozoic carbonate and siliciclastic marine successions.It extends meridionally along the south of La Rioja, San Juan, and north of Mendoza provinces.
La Chilca Formation is an Upper Ordovician-Lower Silurian unit mainly composed of shales and sandstones.Two members can be recognized in the La Chilca Formation (Baldis et al., 1984): Salto Macho Member (late Hirnantian to early Wenlock) and Cuarcitas Azules Member (early to middle Wenlock).The basal portion of the Salto Macho Member generally consists of a pebble conglomerate with black chert clasts, covered by a dark grey shale and greenish siltstone succession.The upper part of this member grades upwards into green yellowish fine-grained sandstones of the overlying Cuarcitas Azules Member (Baldis et al., 1984).
The sedimentary record of the La Chilca Formation at the Poblete Norte section (Talacasto area) has been studied in its stratigraphic and paleoenvironmental aspects by León et al. (2016) and Asurmendi et al. (2017Asurmendi et al. ( , 2018)).These studies described reworked glacimarine deposits in high stand levels and tidal influence for the initial metres of the Salto Macho Member.
Retiolitids (Retiolitidae Lapworth, 1873) are frequent constituents of the Silurian graptolite faunas in this geological province.They are characterized by having a tubarium with poor development of fuselli and are composed mainly of cortical bandages (Maletz et al., 2017;Kozlowska et al., 2019).The colonies have lists forming a mesh-like framework, which gives it a distinctive morphology among diplograptids, known as ancora, and developed outside the thecal walls.Peralta (1985) mentioned for the first time the presence of retiolitids in the La Chilca Formation (at the Talacasto section).The author identified Retiolites sp. and assigned an early Llandoverian age due to the presence of climacograptids in the assemblage.Lenz et al. (2003a) described Pseudoplegmatograptus obesus reticulatus, later classified as P. reticulatus (Lenz et al., 2003b), together with Metaclimacograptus cf.M. asejradi.This association was collected at the Los Baños de Talacasto and Salto Macho sections, in the Talacasto area.The mentioned beds have been assigned to an early Telychian age through the presence of this graptolite association.
The present contribution deals with the graptolite biostratigraphy of the Salto Macho Member, La Chilca Formation, at Poblete Norte section.An actualized graptolite biostratigraphy scheme is proposed and specimens belonging to the Family Retiolitidae are identified and described herein.The paleontological and paleoenvironmental knowledge of the study area is enhanced, which, associated with the graptolite content, indicates possible evidence of the Sedgwickii Event in Precordillera.The new and precise data obtained by sampling the succession and the re-evaluation of data previously published by the authors, allow to develop an updated biostratigraphic scheme that enables a local, regional and global correlation with equivalent units.

Geology of the Poblete Norte section
The Poblete Norte section is located approximately 77 km north of San Juan city, at the western flank of the Talacasto Range, eastern boundary of the Central Precordillera (Fig. 1).
La Chilca Formation exposed at the Poblete Norte section, paraconformably overlies the San Juan Formation (Floian-Darriwilian) and is covered by the Los Espejos Formation (Wenlockian-Lochkovian). Graptolites analysed in the present study come from the Salto Macho Member of the La Chilca Formation.In the Poblete Norte section, this unit consists of a basal paraconglomerate covered by dark grey shales, greenish and yellowish shales, siltstones, and finegrained sandstones.At the upper part of the Salto Macho Member, yellowish siltstones with brownishgreenish concretions are found.Above these levels, overlies the Cuarcitas Azules Member.
The local stratigraphy is completed by the Talacasto Formation (Lower Devonian), the Punta Negra Formation (Lower to Upper Devonian), and Cenozoic undifferentiated deposits (Fig. 1).
In the Argentine Precordillera, the species has not been mentioned yet.Albanesi et al. (2006) described a graptolite assemblage from the La Chilca Formation at the Ancha section, Talacasto area.These authors enumerate some colony fragments of Pristiograptus aff.P. nudus, Monograptus cf.M. priodon, and Stimulograptus?sp.The latest means the only mention of the genus for Precordillera.

Local and regional graptolite correlation
Previous to the present contribution, two mentions have been made of the Stimulograptus and other graptolites related to the Stimulograptus sedgwickii Zone in the Precordillera of San Juan Province.The first one corresponds to the correlation based on palynomorphs and graptolites from the La Chilca and Los Espejos formations, in a contribution carried out by Rubinstein and Brussa (1999) In agreement with the biostratigraphic chart proposed by Loydell (2012), the presence of the S. sedgwickii Zone allows to correlate the studied strata from the La Chilca Formation, with sections from Avalonia, Baltica, Peri-Gondwana, Laurentia, and NE of Africa (Fig. 6).

Paleoenvironmental remarks
The study of Silurian graptolite faunas reveals several degrees of faunal provincialism during this period.Certain low latitude graptolite species were identified in the Llandovery succession from Cape Phillips Formation, Canada (Melchin, 1987), and strong endemism was noted at the Rhuddanian from North Africa deposits (Legrand, 2003).A pronounced provincialism is recorded at mid-Sheinwoodian (Wenlock) in response to some parameters (e.g., low and high latitude) (Lenz et al., 2012), whereas at mid-Homerian the assemblages were widely spread (eurythermal taxa).The distribution patterns of Silurian graptolite assemblages indicate faunal differentiation along paleolatitudinal belts in correspondence with paleolatitude and sea surface temperature (SST) (Goldman et al., 2013;Loydell et al., 2018).
The graptolite assemblages from La Chilca Formation at the Poblete Norte section include some taxa that are characteristic of Precordillera Geological Province, such as Talacastograptus leanzai and Lagarograptus praeacinaces (Lopez et al., 2020).T. leanzai is also recorded in Silurian beds from Bolivia (Lipeón Formation, Rickards et al., 2002).
Based on the presence of endemic graptolite taxa (T.leanzai, L. praeacinaces), pandemic taxa (S. sedgwickii, R. geinitzianus, among others), and the paleoenviromental reconstruction of the La Chilca Formation proposed by Asurmendi et al. (2018), the siliciclastic ramp facies of this unit might have been deposited in an intermediate palaeolatitudinal setting (around 40º S), according to the paleogeographic reconstruction of Cocks and Torsvik (2002).This location of Precordillera at Llandovery-Wenlock times might have enabled a moderated faunal exchange, allowing partial graptolite endemism, and the colonization of the Precordillera basin only by cosmopolitan taxa.

Sedgwickii Event: possible evidence in Precordillera
The middle part of the S. sedgwickii biozone records the Sedgwickii Event (Melchin et al., 1998) which shows the peak of the late Aeronian graptolite extinction with minimum diversity of the faunas.The stressing conditions prevailing during this event were identified worldwide, and affected the pelagic organisms, resulting in taxonomic changes in graptolite faunas, which were impoverished and showed the dominance of some species assemblages (Melchin et al., 1998;Štork and Frýda, 2012;among others).Lower sea-level conditions and positive δ13C excursion were mentioned in concordance with this event by Loydell (2007).In agreement, Štorch and Frýda (2012) identified sedimentary changes during the late Aeronian Prague Synform, evidenced by black shale deposits with high TOC (Total Organic Carbon) values, and lower graptolitic richness related to this event.
However, the sea level-Sedgwickii Event relation has no global consensus.In Baltica, Loydell (2007) describes the Sedgwickii Event as associated with a pronounced marine regression, generating erosion and a lack of deposits in some sections of the continent.On the other hand, Uriz et al. (2008) mentioned a marine transgression that covered contemporary several areas of South America (e.g., Venezuela, Perú, Bolivia, Paraguay, Argentina), coincident with a graptolite fauna from Eastern Cordillera (Northwestern Argentina), assigned by Toro (1995) to the S. sedgwickii Zone.
At the Poblete Norte section, the yellowish siltstones (level PN-011.2),which contain a graptolite fauna dominated by Normalograptus and Talacastograptus, are covered by black shales with high content of organic matter (levels PN-011.5 and PN-012), where abundant graptolites with low taxonomic diversity were identified (e.g., S. sedgwickii, T. leanzai, P. reticulatus) and indicate the S. sedgwickii Zone.These beds have been linked by Astini and Maretto (1996) with the highest relative sea level in the Silurian of the Precordillera basin.
In Poblete Norte, between the yellowish siltstones (level PN-011.2) and the black shale beds (level PN-011.5/012),lithological and taxonomic changes are recognized, which might indicate the evidence of the Sedgwickii Event in Precordillera.This would be associated with the presence of the eponymous graptolite species and also with a high sea level stage (in agreement with Uriz et al., op. cit.).

Comments on some graptolite species
A brief paleontological description of some graptolites recovered from Salto Macho Member, La Chilca Formation at the Poblete Norte section, is presented herein.These species are indicative of certain paleoambiental conditions and represent precise biostratigraphic data.(Portlock, 1843) Figure 3E-H The specimens from the La Chilca Formation are in great numbers, although complete mature specimens are absent.Generally, the colonies are in relief, filled by coloured clay or pyrite, or flat as a carbon film.The analysed material possesses a straight tubaria with slightly dorsally curved proximal end.The longest colony is 33 mm in length.The width increases from 0.49 mm at Th2 and 0.52 mm at Th3, to the maximum of 1.8 mm without spines observed in distal fragments.Thecae have an extroverted shape proximally, and a triangular shape distally, and their apertural region formed a hook.Some apertures showed spines of 0.3 mm in length, slightly proximally curved.The sicula is 1.4 mm long and its apex reaches to the top of Th1.2TRD at the proximal portion of the tubaria is 1.86 mm.Distally, 2TRD attains 2.2 mm.Thecal spacing counts 9 to 10 in 10 mm, and the thecal overlap is ½.This measures agrees with those given by Toro (1995), Chunkin and Carter (1996), Štorch and Massa (2006), and Maletz et al. (2021) for Stimulograptus sedgwickii.

Pseudoplegmatograptus reticulatus
(Boucek and Münch, 1944) Figure 3A-B The collection from the La Chilca Formation (Poblete Norte section) has fragmentary material corresponding to this species, preserved as a carbonaceous film (Fig. 3A-C).The maximum length measured is 6.42 mm.The width of the tubaria increases rapidly from 1.32 mm proximally, 2.96 mm in the middle portion, and reaches 3.67 mm distally (in specimen INGEO-PI-1913).Thecae are orthograptid in profile (see Fig. 3C).Spines are mainly straight, ventrally projected from thecal lips, and 0.72-0.85mm long.Some bifurcations appear to join with those below to form a thin lacinia.
Thecal spacing varies from 12 to 15 thecae in 10 mm.2TRD value is 1.4-1.47mm with a maximum of 1.66 mm in distal fragments.Sicula is not observed in the collected material.The material studied is assigned to P. reticulatus due to their similar dimensions, thecae morphology, presence of thecal spines, and thecal concentration (Lenz, 1982;Lenz et al., 2003a, b).

Discussion
The biostratigraphic range of two graptolite species is discussed here.While the record of Talacastograpus leanzai reaches the Atavograptus atavus biozone on Rhuddanian beds at the Los Baños de Talacasto section (as reported by Cuerda et al., 1988), in the present study its presence is verified in the younger S. sedgwickii Zone, at the Poblete Norte section.Conforming to Lenz et al. (2003a, b), Pseudoplegmatograptus reticulatus had been identified within Spirograptus turriculatus-Streptograptus crispus zones (early Telychian), whereas the interval of P. reticulatus spans to the S. sedgwickii zone (late Aeronian) at Poblete Norte, reaching older ages in the La Chilca Formation.
In a recent contribution of Lopez et al. (2020), a graptolite assemblage composed by Atavograptus atavus, Normalograptus?acceptus and Monograptus sp. were identified at the Poblete Norte section, recording the A. atavus Zone (level PN-013) and indicating a middle to late Rhuddanian age (Lower Llandovery).Additional sampling and later studies on graptolites recovered from the levels PN-011.5 and PN-012 allow to increase the knowledge and a biostratigraphic review from La Chilca Formation is made in this study.S. sedgwickii, associated with P. reticulatus, M. asejradi, and Coronograptus sp. were identified from these levels, allowing to record the S. sedgwickii Zone (late Aeronian).This assemblage, located 1.8 m below the PN-013 (A. atavus Zone of Lopez et al., 2020) indicates the latest Aeronian, and the start of the Telychian in the following levels.In this way, level PN-013 might possess an early Telychian age, the A. atavus Zone would not be located in the latest beds, and a more accurate position might be below the level PN-011 at least.Further fieldwork and a larger graptolite collection might give light on this proposal and adjustment.

Conclusions
According to new data on Silurian graptolite assemblages, a review of their biostratigraphy is proposed for the La Chilca Formation at the Poblete Norte section, Central Precordillera.
As it has been demonstrated in the present study, the biostratigraphic range of Talacastograptus leanzai spans to late Aeronian levels; in addition, the retiolitid Pseudoplegmatograptus reticulatus was also found in late Aeronian strata, given the oldest age for this species in Precordillera.
Two retiolitid species, Pseudoplegmatograptus reticulatus and Retiolites geinitzianus, are described for the first time in the same section in Precordillera.
The occurrence of Stimulograptus sedgwickii, associated with P. reticulatus, Metaclimacograptus asejradi, T. leanzai, and Coronograptus sp., allows to record the late Aeronian S. sedgwickii Zone.These levels can be correlated with equivalent beds at the Ancha and Cerro del Fuerte sections in Precordillera; with the Lipeón and Vargas Peña formations in South America; and with comparable deposits from Avalonia, Baltica, Peri-Gondwana, Laurentia, and NE of Africa sections.
FIG. 2. Stratigraphic column of the La Chilca Formation at the Poblete Norte section.The samples position and fossils are included.SJF: San Juan Formation.CAM: Cuarcitas Azules Member.