Conodonts from siliciclastic rocks: a case study from the Portezuelo del Tontal Formation, Ordovician of the Western Argentine Precordillera

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

Simposio; 2020 Virtual Annual Meeting of IGCP 653; 2020

Conodontinvestigations in the Ordovician of the Argentine Precordillera experienced duringthe past three decades, major developments in taxonomy, biostratigraphy, palaeoecology,palaeogeography and paleothermometry. Conodont studies flourished granted by thefossiliferous character of the limestones of the San Juan Formation and theoverlying calcareous units of the central and eastern Precordilleran domains. Incontrast, conodont reports from the deeper siliciclastic sedimentarysuccessions of the Western Precordillera, which involve slope to ocean floorsedimentary rocks including pillow lavas and mafic-ultramafic bodies in itswesternmost sections, are noticeably scarce, essentially restricted to the mixedclastic-calcareous turbidites of the Yerba Loca Formation. Thus, the general absenceof appropriate facies for sampling conodonts, along with the laboratorydifficulties in recovering conodonts from siliciclastic rocks, difficults the assessmentof the conodont biostratigraphic framework of the Precordilleran basin. In anattempt to precise the age of deposition of the ca. 2 km-thick siliciclastic Portezuelodel Tontal Formation, we thoroughly searched for rocks samples with calcareousreaction in the Telégrafo Creek and the Cerro Cóndores stratigraphic sections,on the western and eastern slopes of the Sierra del Tontal, respectively. Afterprocessing 10 rock samples in total (2-3 kg each) following standard laboratoryprocedures, only 2 samples from Cerro Cóndores were productive, yielding merely50 conodont elements. The recovered assemblage includes Ansellajemtlandica(Löfgren), Costiconus costatus(Dzik), Drepanodus arcuatus Pander, Parapaltodus simplicissimus Stouge, Paroistodus originalis (Sergeeva), P. h. horridus (Barnes & Poplawski), P. h. secundus Albanesi, Periodon macrodentatus (Graves yEllison), Protopanderodus graeai(Hamar), and Spinodus spinatus(Hadding), which typically coexists in the Darriwilian (Middle Ordovician) Yangtzeplacognathuscrassus Zone(Dw1?Dw2), a critical interval in the tectonostratigraphic history of thePrecordillera. This zone has wide geographical distribution, involving most ofthe transitional interval from the San Juan Formation to the overlying LasAguaditas, Las Chacritas, Los Azules, and Gualcamayo formations at the easternand central domains of the Precordillera. Recognition of the Y. crassus Zone in the Portezuelo delTontal Formation also allows for a tight intercontinental correlation withSouth China and Baltoscandia. In view of our results, the great importance ofconducting conodont investigations in siliciclastic sedimentary deposits isreaffirmed.