Stratigraphy, conodont biostratigraphy and correlation of the San Juan Formation (Lower-Middle Ordovician) in the Villicum Range, Eastern Precordillera, Argentina

MESTRE, ANA; HEREDIA, SUSANA; MORENO, FLORENCIA; GÓMEZ, MARÍA JOSÉ

NEWSLETTERS ON STRATIGRAPHY

GEBRUDER BORNTRAEGER

Año: 2024

0078-0421

New stratigraphic and conodont data of the San Juan Formation carbonate rocks in the Don Braulio section (Villicum Range, Eastern Precordillera, Argentina) are presented. These new data allow proposing the conodont biostratigraphy of this unit in the Eastern Precordillera, as well as its correlation with coeval strata from several sections of the Central Precordillera. The San Juan Formation is 105 m of thick in the study section. A hardground marks its lower boundary with the La Silla Formation and its upper strata are transitional with the lower beds of the Los Azules Formation. The recovered conodonts allow the recognition of five Lower and Middle Ordovician conodont biozones, the Oepikodus evae and O. intermedius from the Floian stage and the Lenodus antivariabilis, L. variabilis, and L. crassus from the Darriwilian stage. Unfortunately, conodont data from the latest Floian to earliest Darriwilian interval are scarce or null due to the unfavorable lithofacies for conodont recovery in this interval. The Floian conodont species Nasusgnathus dolonus (An), Cooperignathus nyinti (Cooper), and Gothodus costulatus (Lindström) were recovered for the first time in the Precordillera. The two first species provided a link with the Australasian and Midcontinent shallow warm-water domain and the last species evidences, an early connection with the Andean basin and the Baltoscadian shallow cool-water domain. Two reef horizons were recognized: a middle reef horizon, latest Floian in age, composed of laminar pulchrilaminids and scarce sponges, and an upper reef horizon, which has columnar pulchrilaminids as the main reef-builder and developed during the latest Dapingian-lowest Darriwilian. The stratigraphic and biostratigraphic correlation of the San Juan Formation between the Don Braulio section and several sections from the Central Precordillera provided important novel data on the relation and temporal distribution of the carbonate facies on the Ordovician platform in the Precordillera. The data also showed that, toward the east, there is a significant truncation of the early Floian conodont zones, as well as a progressive decrease in thicknesses in the late Floian and Darriwilian conodont biozones. In addition, this new stratigraphic information displays the regional and vertical distribution of successive reef horizons registered in the San Juan Formation which are controlled by a eustatic sea-level change that consisted of two cycles, transgressive-regressive and transgressive.