Abstract
The Ordovician System is extensively represented in the Precordillera of San Juan Province, Argentina. At the Cerro La Chilca in the Jáchal area, the limestone of the San Juan Formation is paraconformably overlain by interbedded limestone and shale of the Gualcamayo Formation. The present contribution reports new data on the conodont fauna and biostratigraphy of these darriwilian units, revising local and regional chronostratigraphic relationships. New information on the composition of conodont and graptolite associations through the stratigraphic sequence is presented. The presence of Paroistodus horridus horridus, Yangtzeplacognathus crassus, and Histiodella sinuosa constrain the uppermost strata of the San Juan Formation to the lower part of the Y. crassus Zone, according to the Baltoscandian scheme, and to the H. sinuosa Subzone of the Periodon macrodentatus Zone of the North American scheme. In the overlying Gualcamayo Formation the co-occurrence of Y. crassus with Histiodella holodentata enable the recognition of the Y. crassus Zone and the H. holodentata Subzone of the P. macrodentatus Zone. The identification of these zones allows for precise global and regional correlation. A graptolite assemblage that belongs to the epipelagic and deep-water biotopes with some components restricted to low paleolatitudes is recognized. This diverse assemblage is characteristic of the pelagic biofacies. The important diversity of graptolites in this section suggests a favorable environment for their development. Local changes in the taxonomic composition are recognized through the Gualcamayo Formation. When comparing this fauna with that of different study localities from the Central Precordillera (Cerro Potrerillo, Oculta Creek, Cerro Viejo de Huaco and Las Aguaditas Creek) slight differences in the generic composition are observed. Taxonomic differences support the preference of certain associations for particular environments; though, graptolites are more diverse in black shales facies, which represent deeper environments (the Los Azules Formation), in relation to the calcareous-shale facies of the Gualcamayo Formation from Cerro La Chilca and correlative unit at Las Aguaditas Creek.
Highlights
The Precordillera is a geological province located in the western margin of Argentina, between the Frontal Cordillera to the West and the Pampean Range to the East, covering parts of La Rioja, Mendoza and San Juan provinces (Stelzner, 1873; Furque and Cuerda, 1979; Baldis et al, 1982)
The graptolite fauna from the Cerro La Chilca section was referred in a recent study (Serra et al, 2017), where it was dealt for biostratigraphic purposes; whereas in the present paper a palaeoenvironmental appraisal is given to this fauna
A diverse conodont association was recovered from the uppermost San Juan Formation and the Gualcamayo Formation exposed at the Cerro La Chilca classical locality
Summary
The Precordillera is a geological province located in the western margin of Argentina, between the Frontal Cordillera to the West and the Pampean Range to the East, covering parts of La Rioja, Mendoza and San Juan provinces (Stelzner, 1873; Furque and Cuerda, 1979; Baldis et al, 1982). It is characterized by a ~2,000 m thick succession of Cambrian-Ordovician sedimentary rocks preserved in an extensive fold-andthrust orogenic belt (Astini et al, 1996). New information on the composition of conodont and graptolite associations through the rock succession is presented, including the description of the diverse graptolite assemblage documented by Serra et al (2017)
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