The Baltoniodus navis Zone in the Gualcamayo Formation (Middle Ordovician), Central Precordillera, Argentina

Abstract

We study the conodont fauna and its biostratigraphy recorded in the lower 3 m of the Gualcamayo Formation at the Potrerillos Creek section, northern Central Precordillera of Argentina. The analysis of two conodonts samples from levels of biostratigraphic interest, allowed us to identify 828 conodonts, assigned to 31 genera and 42 species. The recognition of Baltoniodus cf. triangularis (Lindström), Tripodus laevis s.l. Bradshaw, Baltoniodus navis (Lindström), and abundant elements of Paroistodus originalis (Sergeeva) allowed us to determine the lower part of the Baltoniodus navis Zone in the basal section of the Gualcamayo Formation. This allows us to reconsider previous biostratigraphic interpretations that assigned the lower 10 m of the Gualcamayo Formation to the Baltoniodus triangularis-Tripodus laevis Zone, to be reassigned to the Baltoniodus navis Zone. The lower part of the Baltoniodus navis Zone recognized in the present work correlates elsewhere in Argentina (Puna, Famatina, and Precordillera), as well as in China and Sweden with the lower Baltoniodus navis Zone, whereas it correlates with the upper part of the Microzarkodina flabellum-Tripodus laevis Zone in North America. Also correlates to the Isograptus maximus Zone. The occurrence of abundant conodonts representative of the Baltoniodus navis Zone in the Potrerillos Creek section, makes it suitable for further studies of a biostratigraphic interval that is underrepresented in most published sections of the Precordillera. In addition, we analyzed the preservation and CAI of the conodont elements recovered, these have a CAI of 2.5–3, indicating burial paleotemperatures of 85°−200 °C. Most gracile elements show a lower CAI (2.5) than the robust ones (CAI 3). Most elements present silica crystals attached to surfaces and the basal cavity, while few elements have recrystallized surface lamella. These fossilization features could be attributed to diagenetic effects related to intense tectonic deformation.