AGE CONSTRAINTS ON THE PORTEZUELO DEL TONTAL FORMATION (MIDDLE-UPPER ORDOVICIAN) OF THE WESTERN ARGENTINE PRECORDILLERA: FIRST INSIGHTS FROM CONODONT BIOSTRATIGRAPHY

BANCHIG, A.L.; VOLDMAN, G.G.

AMEGHINIANA

ASOCIACION PALEONTOLOGICA ARGENTINA

Lugar: Buenos Aires; Año: 2020 vol. 57 p. 344 - 353

0002-7014

The first conodont fauna recovered from the Portezuelo del Tontal Formation(PTF) in the Sierra del Tontal, San Juan Province, is analysed, providing a precisebiostratigraphic control for the scarcely fossiliferous, thick Ordoviciansiliciclastic sequences of the western Argentine Precordillera. In the studiedCerro Cóndores stratigraphic section, the lower outcropping levels of the PTF arecharacterised by sandstone beds, with a few having minor calcareous acidic reaction,deposited under medium energy, open-marine, siliciclastic shelf conditions. Therecovered assemblage consists of 50 conodont elements, including Ansellajemtlandica, Costiconus costatus, Drepanodus arcuatus, Parapaltodus simplicissimus, Paroistodus originalis, Periodon macrodentatus, Protopanderodus graeai, and Spinodus spinatus as the mostconspicuous species. After the biostratigraphic assessment of species of the Paroistodushorridus complex, the PTF conodont asemblageis referred to the Darriwilian (Middle Ordovician) Yangtzeplacognathus crassus Zone, a critical interval in thetectonostratigraphic history of the Precordillera. The collection also containsreworked conodont specimens whose biostratigraphicranges coexist during the middle-upper Floian Stage (Lower Ordovician). Theconodont assemblage from the PTF allows for a precise correlation with theeastern and central domains of the Precordillera, as well as other regions ofthe world, improving the knowledge on the dynamics of the Ordovician Precordilleranbasin and the palaeogeographic distribution of its faunas.