Systems tracts of the San Juan Formation (Ordovician), Argentine Precordillera: Conodont record analyses, thin sections and its intercontinental importance

Abstract

In this paper, first and last occurrences of conodont species and petrographic thin sections are used for detection of systems tracts in the San Juan Formation. After analysis of 16,432 conodont elements from Cerro La Silla and Cerro Viejo of Huaco (Los Gatos Creek), Precordillera, Argentina, two transgressive systems tracts (TST) and two highstand systems tracts (HST) are recognized in the San Juan Formation (Lower−Middle Ordovician), including the location of the maximum flooding surface corresponding to each depositional sequence. This study includes the analysis of 32 thin sections of limestones. The maximum flooding surface of the first depositional sequence is recorded in deposits of the lower Floian Prioniodus elegans Zone, whereas the second depositional sequence is located in lower Darriwilian rocks (Lenodus variabilis Zone). At Cerro La Silla and Cerro Viejo of Huaco, the global transgressions of the middle−upper Prioniodus elegans Zone (Billingen transgression) and the lower Oepikodus evae Zone (Evae transgression) are identified. In order to get contrasting results on sea-level changes from the analysis of first and last occurrences of conodont species, it is necessary to account for a high amount of conodonts from most productive samples. Conodont specimens have been recovered from average weight samples and analyses are undertaken including lithology and petrographic sections.