Lower middle Darriwilian (Ordovician) graptolites and index conodonts from the Central Precordillera of San Juan Province, Argentina

Abstract

This study includes 3 localities from the Jáchal area in the Central Precordillera of San Juan Province, from north to south: the Oculta creek, Las Aguaditas creek, and Cerro La Chilca sections. We deal with the graptolite faunas and conodont index species recorded from units that overlie the carbonate San Juan Formation, spanning the lower part of the Los Azules, Las Aguaditas, and Gualcamayo formations in their respective areas. The index graptolites and associated species are reported, which enable the recognition of the Levisograptus dentatus Zone in the Central Precordillera. The presence of graptolites in the limestones from the top of the San Juan Formation at the Cerro La Chilca section is documented for the first time. The record of representatives of the Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus, Histiodella, and Periodon lineages recognized in these units, linked with respective graptolite zones, provide precise information for global correlation purposes. The use of conodont and graptolite zones from different areas of the Central Precordillera enables the verification of the diachronous contact between the San Juan Formation and overlying units, which spans the lower to middle Darriwilian, in our investigated sections and classical localities previously documented.

Graptolite and conodont faunas of Upper Ordovician (Sandbian) successions of the Argentine Precordillera: Biostratigraphic implications

“…Cuerda and Furque () remarked on the occurrence of an important hiatus between the Gualcamayo ( Paraglossograptus tentaculatus Zone) and the Los Azules formations (Sandbian graptolite fauna). In a recent work, the Yangtzeplacognathus crassus conodont Zone and Levisograptus dentatus graptolite Zone were identified spanning the Gualcamayo Formation (Serra, Feltes, Ortega, & Albanesi, ), while the upper P. anserinus Zone and Nemagraptus gracilis Zone were recognized in the lower of the Los Azules Formation (Albanesi in Peralta, ; Peralta & Finney, ); thus, the gap is greater in this locality than in the previously described section, missing the Holmograptus lentus to Hustedograptus teretiusculus graptolite zones and Eoplacognathus pseudoplanus , Eoplacognathus suecicus , P. serra , and the lower subzone of the P. anserinus conodont zones (Serra et al, ). The referred regional hiatus was originally regarded as the Guandacolic orogenic event (Furque, ).…”

Conodont and graptolite biostratigraphy of the lower–middle Darriwilian (Middle Ordovician), Cerro Viejo of Huaco, Argentine Precordillera

“…The lower limit of the P. gladysae Subzone coincides with the first occurrence of the nominal species, whereas the upper limit is indicated by the lowest record of Baltoniodus medius , which is associated with P. horridus primus representing the P. horridus Subzone. Although the upper limit of this subzone could not be recognized for the Cerro Potrerillo section (Albanesi et al, ), it would be defined by the first appearance of Y. crassus (Chen & Zhang) in other Precordilleran localities (Albanesi et al, ; Serra, Albanesi, Ortega, & Bergström, ; Serra et al, ), which in turn marks the base for the homonymous zone. The upper boundary coincides with the first appearance of Eoplacognathus pseudoplanus (Viira), the eponymous species of the succeding biozone.…”

“…The lower strata of the Rinconada Formation are characterized by species that represent the upper L. variabilis Zone according to the latter author (Sarmiento, Vaccari, & Peralta, ). In the Cerro La Chilca, Central Precordillera of San Juan, the L. variabilis Zone is recognized from the upper San Juan Formation (Carrera et al, ; Heredia & Mestre, ; Serra et al, ). In turn, Feltes, Albanesi, and Bergström () analysed samples corresponding to the upper San Juan Formation and recognized the L. variabilis Zone with its subzones, at the Las Aguaditas creek section, in the Los Blanquitos range, Central Precordillera.…”

“…Considering the Las Chacritas and the Cerro La Chilca sections, the referred biozone is correlated with strata of the uppermost San Juan Formation and the lower parts of overlying units, the Las Chacritas and Las Aguaditas formations, respectively (Albanesi et al, ; Carrera et al, ; Serra et al, ; Serra et al, ). It is interesting to note that the referred strata were orginally assigned to the E. pseudoplanus Zone by Heredia () but later verified as corresponding to the Y. crassus Zone in the referred citations.…”

“…The L. dentatus Zone is recorded in the Cerro Potrerillo and the Cerro La Chilca sections, from the lower Gualcamayo Formation (Ortega et al, ; Serra et al, ). These authors estimated that this biozone is represented in correlative levels of the Oculta creek.…”