Biostratigraphy and provincialism of conodonts from the middle-upper San Juan Formation in cerro Viejo of Huaco, Precordillera, Argentina

Abstract

The present study deals with the conodont biostratigraphy from the middle and upper parts of the San Juan Formation (Lower-Middle Ordovician) exposed at the Los Gatos creek section, west of the cerro Viejo de Huaco, Central Precordillera of San Juan Province. The numerous conodonts recovered, corresponding to 55 species, allow to recognize a series of biozones in the studied section. The biostratigraphic analysis carried out herein allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian. The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section. In general, the levels corresponding to the Baltoniodus triangularis-Tripodus laevis Zone contain more conodonts, than the records of other localities from Precordillera. These specimens are well preserved with a color alteration index (CAI) of 2-2.5, indicating overburden paleotemperatures ranging from 60° and 155° C. The large presence of fragmented conodonts, with recrystallized surfaces and crystal overgrowth, could indicate the effect of distinctive diagenetic fossil processes on the bearer limestone. The analysis of the diversity and abundance of genera and species of conodonts by chronostratigraphic intervals presents a great percentage of cosmopolitan taxa, 18% and 38% in common, at species level, only with the North-American Midcontinent Province (NAMP), and with a small percentage with the North-Atlantic Province (NAP) and the Precordillera, which allows to approximate a greater paleobiogeographic affinity of the conodonts recovered with those of the NAMP than those of the NAP for the entire interval studied. On the other hand, the Precordillera is verified as a province with its own characteristics as identified by several authors.