Conodont biostratigraphy and global correlation of the middle Darriwilian-lower Sandbian (Ordovician) Las Aguaditas Formation, Precordillera of San Juan, Argentina

Abstract

ABSTRACT. Middle Darriwilian to lower Sandbian conodonts were recorded from the Las Aguaditas Formation at its type section in the Argentine Precordillera. A total of 9,974 conodont specimens were recovered from 46 carbonate samples, which represent 68 species of 38 genera. A biostratigraphic study verified a middle Darriwilian age for the interval spanning the contact between the San Juan and the Las Aguaditas formations. The following zones are determined in the study section: the Lenodus variabilis Zone, with the Periodon gladysae and Paroistodus horridus subzones following the Precordilleran scheme; the L. variabilis, Yangtzeplacognathus crassus, and the Eoplacognathus pseudoplanus zones with the Microzarkodina hagetiana and M. ozarkodella subzones, and the Pygodus anserinus Zone, according to the Scandinavian scheme; the Periodon macrodentatus Zone, with the Histiodella sinuosa, H. holodentata and H. cf. holodentata subzones, and the P. zgierzensis Zone with the H. kristinae Subzone that correlates the North American scheme. A stratigraphic gap was recognized between the lower and middle members of the Las Aguaditas Formation. It comprises the Eoplacognathus suecicus and Pygodus serra zones, and the lower subzone of the Pygodus anserinus Zone. The variation of conodont diversity through the study section conforms to shallowing and deepening patterns, which accompanies the changes of the provenance lithology. Three conodont assemblages were quantitatively recognized: a) Diverse conodont association, b) Low diversity conodont association and c) Recovery phase association. We propose to use the North American biozonal scheme of conodonts for the Central Precordillera because of the affinity of documented index taxa, which provides a more accurate intercontinental correlation for the global Middle Ordovician Series. Keywords: Conodont, Darriwilian-Sandbian, Biostratigraphy, Paleoecology, Precordillera, Argentina. RESUMEN. Bioestratigrafía de conodontes y correlación global del Darriwiliano medio-Sandbiano inferior (Ordovícico) Formación Las Aguaditas, precordillera de San Juan, Argentina. En la Formación Las Aguaditas, en su sección tipo en la precordillera Argentina, se registraron conodontes del Darriwiliano medio al Sandbiano bajo. Se recuperaron un total de 9.974 especímenes de 46 muestras carbonáticas; estos representan 68 especies pertenecientes a 38 géneros. Se llevó a cabo un estudio bioestratigráfico para verificar la edad del intervalo que incluye el contacto entra las formaciones San Juan y Las Aguaditas. La presencia de taxones guía indican una edad darriwiliana media para este intervalo. Las siguientes biozonas fueron determinadas en el área de estudio: Zona de Lenodus variabilis, con las subzonas de Periodon gladysae y Paroistodus horridus, correspondientes al esquema establecido para la precordillera; las zonas de L. variabilis, Yangtzeplacognathus crassus y la de Eoplacognathus pseudoplanus con la subzonas de Microzarkodina hagetiana y M. ozarkodella, y la Zona de Pygodus anserinus, según el ...

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

“…Albanesi and Ortega () proposed a lower middle Darriwilian age for the top of the San Juan Formation at the Las Aguaditas Creek section based on the identification of the Lenodus variabilis Zone. Recently, this record was updated to the Yangtzeplacognathus crassus Zone in the contact interval between the San Juan and the Las Aguaditas formations (Albanesi & Ortega, ; Feltes, Albanesi, & Bergström, ; Feltes et al, ).…”

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

“…Lehnert, Bergström, Keller, & Bordonaro, ) were identified by Albanesi and Ortega () in the upper part of the unit. More recently, Feltes et al () documented P . anserinus Lamont & Lindström, , throughout the entire middle and upper members, and Heredia, Mestre, Soria, and Kaufmann () recovered P. anserinus in the upper 50 m of the Las Aguaditas Formation.…”

“…Recently, Feltes, Albanesi, and Bergström () described a lower member (43‐m thick), of lower to middle Darriwilian age. In the present study, we recognize a middle member (138‐m thick) that comprises a lower middle member (62‐m thick) and an upper middle member (76‐m thick); and an upper member (120‐m thick).…”

“…In the Argentine Precordillera, the N. gracilis Zone is documented from several localities of the San Juan Province; particularly, in the Central Precordillera from the Los Azules Formation at the Cerro La Chilca area (Blasco & Ramos, ; Peralta, ), the Las Aguaditas Formation at the Los Blanquitos Range (Brussa, , ; Feltes et al, ; Ortega & Albanesi, ) and the Las Chacritas River section (Serra et al, ) and in the Sierra de La Invernada Formation at the La Invernada Range (Ortega, Albanesi, Banchig, & Peralta, ; Peralta & Finney, ). In the Eastern Precordillera, graptolites of the N. gracilis Zone occur in the La Cantera Formation, at the Don Braulio creek, on the eastern slope of the Villicum Range (Baldis et al, ; Peralta, , ).…”

“…Several conodont studies were accomplished from the Las Aguaditas Creek section (e.g., Eberlein, ; Keller et al, ; Lehnert, ; Sarmiento, Rao, & Herrera, ), the latest works from the Las Aguaditas Formation identified several conodont zones of the Baltic and North American schemes (e.g., Albanesi et al, ; Feltes et al, ).…”