Facies sedimentology and sequence stratigraphy of the carboniferous lower section of the Ansilta Formation, Calingasta-Uspallata Basin, NW Argentina

Abstract

The Ansilta Formation is located 5 km southeast of the Astronomical Observatory El Leoncito near Barreal, San Juan Province, Argentina. It consists of glacimarine, nearshore, and fluvial systems deposited during the Late Paleozoic Ice Age along the eastern margin of the Calingasta-Uspallata Basin. Glacial environments are unique in that facies changes occur because of glacial ice margin fluctuations that may be independent from changes of eustatic sea-level. For this reason, an alternative conceptual framework outside of the traditional sequence stratigraphic approach is necessary to understand the genesis of these strata. The lower glacially influenced succession (0–427 m) records at least five glacial advances, where glacial dynamic signatures are interpreted using glacial sequence stratigraphy and glacial systems tracts (GST). The upper succession of the Ansilta Formation (427–700 m) consists of progradational shallow marine shelf, deltaic, and fluvial strata. This assemblage shows no direct evidence of ice contact deposition in the outcrop belt due to an absence of glacially shared diamictites and glacially modified clasts, representing a major deglaciation phase in western Argentina and is part of the stepped deglaciation across Gondwana that ended elsewhere during the late Permian.