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

Abstract

The distribution of conodonts and graptolites from the upper 13 m of the San Juan Formation and the first metre of the lower member of the Los Azules Formation, Los Gatos and Amarilla creeks, Cerro Viejo of Huaco, Central Precordillera from San Juan, Argentina, is studied. The conodonts from the top stratum of the San Juan Formation at the Los Gatos creek section correspond to the Lenodus variabilis Zone, whereas at the Amarilla creek section, they represent the Yangtzeplacognathus crassus Zone, verifying the diachronous top surface of the San Juan Formation. In the lower member of the Los Azules Formation, 81 conodont elements are recorded from bedding plane surfaces, where the presence of Paroistodus horridus horridus (Barnes & Poplawski) allows to recognize the Y. crassus Zone. On the basis of the graptolite and conodont association, we estimate that the top of the San Juan Formation at Los Gatos creek corresponds to the Levisograptus dentatus Zone, L. dentatus Subzone, whereas at the Amarilla creek, the Arienigraptus angulatus Subzone of the same zone is represented. The presence of the graptolite A. angulatus (Mu) in the lower member of the Los Azules Formation in both sections indicates the L. dentatus Zone (A. angulatus Subzone). The records of graptolites in limestones of the San Juan Formation and of abundant conodonts on bedding plane surfaces of the lower member of the Los Azules Formation are important, taking into account the type of facies where they occur. The conodont records in the referred type of preservation are scarce in the Ordovician, which allows for a more precise biostratigraphic correlation between conodont and graptolite biostratigraphy.

New conodont records of the Los Sombreros Formation (Cambrian–Ordovician) from the Western Precordillera, Argentina: biostratigraphic and palaeoenvironmental implications

“…Thereby, the levels of the Los Sombreros Formation can be accurately correlated with dated strata from the contemporary carbonate platform (Mango et al . 2018).…”

Biostratigraphic and paleobiogeographic significance of the Darriwilian microfossils from the top of San Juan Formation in the Los Baños de Talacasto section, Central Precordillera (Argentina)

“…The Darriwilian conodont biostratigraphy from the Precordillera in western Argentina has experimented a substantial improvement in last years. Several extensive studies on conodonts recovered from different sections in Central and Eastern Precordillera (Heredia andMestre, 2011, 2013;Feltes et al, 2016;Heredia, 2013a, b, 2020a, b;Mestre, 2014;Heredia et al, 2017;Mango et al, 2019;Serra et al, 2017a) suggest affinity to Baltoscandia and South China for this time interval. Recently, Heredia et al (2017) and Mestre and Heredia (2020b) refined the Darriwilian conodont biostratigraphy for the Precordillera, recording the lower and upper boundaries of this stage and documented the following biozones (in ascending order): the Lenodus antivariabilis, Lenodus variabilis, Lenodus crassus, Lenodus pseudoplanus, Lenodus suecicus (Pygodus lunnensis and Pygodus anitae subzones), and Pygodus serra (with Eoplacognathus robustus and Eoplacognathus lindstroemi subzones).…”