Conodont biostratigraphy from the upper San Juan Formation (Middle Ordovician) at Niquivil, Argentine Precordillera

Abstract

The present work deals with the conodont biostratigraphy from the upper San Juan Formation in the section of Niquivil, Central Precodillera of San Juan, Argentina. We study the upper 129.45 m of the San Juan Formation, starting from the upper strata of the second reef horizon up to the top of the formation. Digested limestone samples yielded 20 conodont species. The presence of Tripodus laevis Bradshaw not associated to Baltoniodus navis (Lindström) allows the recognition of the Baltoniodus triangularis-Tripodus laevis Zone, which is interpreted as correlative with the “Parapanderodus” nogamii/Parapanderodus gracilis/Ansella jemtlandica Association of Lehnert (1993, 1995; Lehnert and Keller, 1993), conversely to previous interpretations that suggested the latter as correlative to the Baltoniodus navis Zone. The zonal identification is supported by the associated conodonts Protopanderodus rectus (Lindström), Juanognathus jaanussoni Serpagli, Juanognathus n. sp., Protopanderodus gradatus (Serpagli), Rossodus barnesi Albanesi, Paltodus subaequalis Pander, Drepanodus arcuatus (Pander), Cornuodus longibasis (Lindström), Protopanderodus elongatus Serpagli, Oistodus lanceolatus Pander, Periodon flabellum (Lindström), Semiacontiodus potrerillensis Albanesi, Triangulodus brevibasis (Sergeeva), Paroistodus originalis (Sergeeva), Drepanoistodus forceps (Lindström), Oistodus multicorrugatus Harris, Parapanderodus paracornuformis (Ethington and Clark), Anodontus longus Stouge and Bagnoli, and Pteracontiodus cryptodens Mound. The petrographic microscope analysis of carbonate rocks thin sections refer to proximal middle ramp deposits.