Biostratigraphic and paleobiogeographic significance of the Darriwilian microfossils from the top of San Juan Formation in the Los Baños de Talacasto section, Central Precordillera (Argentina)

Abstract

The microfossil hosted in the strata of the upper part of the San Juan Formation has been widely studied in several sections to the northward of the Argentinian Central Precordillera. In contrast, the coeval strata at the Los Baños de Talacasto section, in the southern part of the Central Precordillera, have scarce biostratigraphic and sedimentological data. In this work, a conodont association together with single ostracod species are documented for the first time in this section. The record of the Lenodus crassus and L. pseudoplanus zones confirms the Darriwilian age for these beds and accurately correlates them with equivalent strata of the San Juan Formation studied in several sections of the Central and Eastern Precordillera. The microfacies analysis verifies the presence of Nuia síbirica Maslov, 1954, peloids, intraclasts, cyanobacteria, calcareous algae, and a possible microbialite indicating a shallow warm-water subtidal environment, in equatorial to subequatorial climate. This suggests a low latitudes position for Precordillera during the early-middle Darriwilian. The conodont genus Aurilobodus Xiang and Zhang is recognized for the first time from the Precordillera, and the Aurilobodus leptosomatus An specimens are described and illustrated. This genus shows affinities to the warm water in shallow marine environments of North China, Central Asia, South Tibet, western Thailand, Australia, and Newfoundland, suggesting probable ties between Precordillera and these regions. The record of the ostracod Pilla nodospinosa Salas in the study section would agree with the correlation of the top of San Juan Formation with the lower levels of the Las Aguaditas Formation in the Central Precordillera, and also suggests paleobiogeographic links with Eastern Gondwana and Australia regions during the Darriwilian times.