The first upper Silurian land-derived palynological assemblage from South America: Depositional environment and stratigraphic significance

Abstract

A thick succession of quartz-rich sandstones cropping out in the middle part of the Villacorta Creek, on the western slope of the Sierra del Famatina (Valle Hermoso, La Rioja Province, northwestern Argentina) was recently recognized as the Villacorta Formation. The sandstone package rests disconformably on greenish sandstones of the Ordovician Suri Formation and is overlain along a marked angular unconformity by rocks of the upper Carboniferous Agua Colorada Formation (Paganzo Group). The Villacorta Formation is divided into four intervals, the lower one is formed by quartz-rich sandstones showing large-scale cross-stratification; the second part is similar but includes some levels of intraformational conglomerates and shows a decrease in both grain-size and the scale of the cross-bedding sets. The third section comprises medium- and coarse-grained sandstone with intercalation of fine-grained sandstones. The upper interval corresponds to two thickening and coarsening upwards successions. This last section was deposited in a deltaic environment where the top of mouth bars and lagoon deposits preserved palynological assemblages and phytodebris. A low-diversity palynological assemblage recovered from these mudstone levels includes abundant nematophyte cuticular-like fragments, cryptospores (monads, dyads, tetrads) and trilete spores (e.g., Scylaspora scripta, Emphanisporites protophanus). The stratigraphic relationships of the succession and the palynological assemblage suggest a late Silurian age for the Villacorta Formation. The exclusive presence of land-derived components in the organic residue allows to capture a snapshot of the terrestrial vegetation and constitutes the first record of this type of palynological association and nematophyte remains in this southern region. This assemblage improves previous paleogeographic reconstruction enlarging the extent of Silurian basins in Argentina. The shallowing successions of the Famatina region pass to marine deposits in the Precordillera area, indicating a westward transition to offshore environments.