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Biostratigraphy and palaeoecology of Middle–Late Ordovician conodont and graptolite faunas of the Las Chacritas River section, Precordillera of San Juan, Argentina

Published online by Cambridge University Press:  29 January 2015

FERNANDA SERRA ,
GUILLERMO L. ALBANESI ,
GLADYS ORTEGA  and
STIG M. BERGSTRÖM
FERNANDA SERRA*
Affiliation:
CICTERRA (CONICET-UNC), Av. Vélez Sarsfield 1611, X5016GCA, Argentina
GUILLERMO L. ALBANESI
Affiliation:
CICTERRA (CONICET-UNC), Av. Vélez Sarsfield 1611, X5016GCA, Argentina
GLADYS ORTEGA
Affiliation:
CONICET CIGEA, Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Córdoba CC1598, X5000JJC, Argentina
STIG M. BERGSTRÖM
Affiliation:
School of Earth Sciences, Division of Historical Geology, The Ohio State University, 125 South Oval Mall, Columbus OH 43210, USA
*
†Author for correspondence: [email protected]
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Abstract

A conodont-graptolite biostratigraphic study was carried out on the top strata of the San Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera of San Juan in western Argentina. Significant conodont records in the San Juan and Las Chacritas formations allow for the recognition of the Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (Microzarkodina hagetiana and M. ozarkodella subzones) and Eoplacognathus suecicus zones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of the Nemagraptus gracilis and the Pygodus anserinus zones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to the Pygodus serra Zone and the Pterograptus elegans and Hustedograptus teretiusculus zones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows that Periodon and Paroistodus are the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such as Histiodella, Periodon, Microzarkodina, Eoplacognathus and Baltoniodus, allow a precise global correlation with other regions such as south-central China, Baltoscandia, North America, Great Britain, Southern Australia and New Zealand. The graptolite fauna identified here are recognized worldwide in equivalent strata in the Baltic region, Great Britain, North America, China, southern Australia and New Zealand. The presence of graptolites in the ribbon limestones of the Las Chacritas Formation is documented for the first time.

Keywords

conodontsgraptolitesbiostratigraphypalaeoecologyMiddle–Upper OrdovicianCentral PrecordilleraArgentina
Type
Original Articles
Information
Geological Magazine , Volume 152 , Issue 5 , September 2015 , pp. 813 - 829
Copyright
Copyright © Cambridge University Press 2015 

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