Micropalentología de las sedimentitas en el contacto entre las formaciones Anacleto y Allen (área natural protegida Paso Córdoba, Río Negro): Indiferencias paleoambientales

Abstract

This study analyzes the transition between the Anacleto and Allen formations (Neuquén Basin, Late Cretaceous) in the Paso Córdoba Protected Natural Area, Río Negro Province, Argentina. The Neuquén Basin is one of the most important sedimentary basins in the country due to its complex geological history and its relevance in natural resource exploration. The Anacleto and Allen formations, belonging to the Neuquén and Malargüe Groups, respectively, represent sedimentary paleoenvironments of great paleontological and stratigraphic interest. Broadly speaking, the Anacleto Formation is characterized by predominantly fluvial deposits, while the Allen Formation reflects a transition toward coastal and eolian systems. A 9 m thick stratigraphic section including the contact between both units was measured and sampled. The base of the section consists of red, reddish-brown, and gray mudstones, with pink and whitish concretionary limestones, typically associated with the Anacleto Formation. The section continues with sandstones and mudstones of continental origin, along with marine-littoral deposits, characteristic of the Allen Formation. Five samples were collected from the first 2 m of the measured section, corresponding to fine sediments with no evident sedimentary structures, interpreted as low-energy deposits, probably from a floodplain environment. The samples were processed by wet disaggregation and sieving, removing the fraction smaller than 63 μm. Microfossils were collected using the picking technique under a binocular microscope. Charophytes, ostracods, gastropods, and bivalves were identified, analyzed, and documented through Scanning Electron Microscope (SEM) observations. Although poor preservation hindered precise species identification, the results allowed for the characterization of the main fossil groups and their relationship with the 4 paleoenvironments represented in the sequence. The presence of freshwater, brackish, and marine taxa indicates environmental fluctuations during the deposition period. Additionally, the coexistence of organisms with differing ecological requirements at the same level suggests the influence of sediment reworking phenomena and consequent time-averaging (i.e., the mixing of remains from different periods of accumulation within a single deposit). The results provide new insights into paleoenvironmental changes in the Neuquén Basin during the Late Cretaceous, reinforcing the interpretation of a sedimentary dynamic influenced by variations in salinity and depositional energy.