A centimeter-scale analysis of the Hirnantian basal strata of the La Chilca Formation (Hirnantian-Early Wenlockian), Central Precordillera, San Juan Province, Argentina. Lithofacies change and paleoenvironmental significance.

GÓMEZ SÁNCHEZ, JESSICA C.; PERALTA , SILVIO H.; BERESI, M.S.

Serie Correlación Geológica

Instituto Superior de Correlación Geológica INSUGEO-Univ. Nac. Tucumán

Lugar: San Miguel de Tucumán; Año: 2021 p. 1 - 32

1514-4186

New data from a centimeter-scale analysis of the Hirnantian basal strata (M. persculptus Zone), up to 45 cm thick, of the La Chilca Formation (Hirnantian-early Wenlockian), at the Talacasto area, Central Precordillera, San Juan Province, are provided herein. A cherty pebble conglomerate and a Fe-phosphate pseudo-conglomerate, characterize the base of the La Chilca Formation, which erosively (disconformity), overlie Early Ordovician limestone of the San Juan Formation. This study has been carried out in four stratigraphic sections: Baños de Talacasto, Quebrada Ancha, Poblete Norte and Poblete Sur. Three lithofacies have been recognized: 1) Lithofacies A1- Cherty pebble conglomerate, the classical ?basal conglomerate? of the La Chilca Formation, exposed in the Los Baños, Quebrada Ancha, and Poblete Sur sections; which is overlain in sharp contact by ironstone and Fe-phosphate beds. 2) Lithofacies A2- Fe-phosphate pseudo conglomerate recognized at the base of the La Chilca Formation, in the Poblete Norte section, where the Lithofacies A1 is not observed. 3) Lithofacies B- Oolitic sandstone, recognized in the Quebrada Ancha, Poblete Norte and Poblete Sur sections, but is absent in the Los Baños de Talacasto. It is divided into two subfacies: B1, Ooid quartz sandstone; B2, Iron-rich oolites.The three studied lithofacies are thought as a result of the Hirnantian post-glacial transgression. Considering its sedimentary and stratigraphic features, the erosional surface at the base of the lithofacies A1 and A2, is thoughtful as a wave ravinement surface (wRs). In accordance with these arguments, the Lithofacies A1 and A2, are both characterized as lag deposits, but also as stratigraphic equivalents. These lithofacies are considered the result of an initial pulse of the post-glacial Hirnantian transgression, followed by a second pulse, responsible for the Fe-phosphate sandstones and oolite beds (Lithofacies B1 and B2). On the basis of the descriptive lithofacies, their stratigraphic relationship and graptolites biozones, a local stratigraphic correlation scheme between the studied sections is proposed. In accordance with previous authors, the date suggest that the uplift of the Tambolar arch controlled the tecto-sedimentary evolution of the Hirnantian-Silurian basin of Precordillera, driving lithofacies trends and morphology as result of a northwards tilting of the basin.