Disentangling the Middle Miocene Climatic Optimum signature from Andean Uplift control of a lacustrine system from Patagonia Argentina

SANTONJA CAMILA; SURIANO JULIETA; BENAVENTE, CECILIA; MACHADO MATIAS ; PASSALIA MAURO; BECHIS FLORENCIA

Aix les Bains

Simposio; 3rd Paleolimnology and Limnogeology International Symposium; 2025

The Ñirihuau Basin, located on the eastern side of the North Patagonian Andes, in western Argentina (41°-43°S), comprises an Oligocene-Miocene sequence constituted by the Ventana, Ñirihuau and Collón Curá formations. According to available geochronological data, the Ñirihuau Formation spans between 22 and 11 Ma (Early to Late Miocene), with a lower section deposited under an extensional tectonic regime, and an upper one recording a foreland basin stage, associated with the Andean uplift. The middle section consists of a lacustrine system that registers the transition of these two contrasting tectonic regimes and was deposited between ~15 and 13 Ma, spanning the Middle Miocene Climatic Optimum (MMCO) and the subsequent Middle Miocene Climatic Transition (MMCT). The lacustrine deposits include carbonate beds that provide insights into the climatic signature and the characteristics of the lacustrine system. This work aims to discern between the tectonic versus climatic control during basin evolution, and its relation to Andean geodynamics. Sedimentary, petrographic and geochemical analyses (C and O stable isotopes) of the carbonate beds and a complete stratigraphic analysis of the lacustrine deposits of the Ñirihuau Formation were conducted. Limestone beds occurred in a littoral lacustrine setting, with lake margin and lower delta plain facies preserved. Nine microfacies were identified: intraclastic and silty grainstone; homogeneous calcimudstone and with sparse detrital grains; bioclastic mudstone; disrupted, fenestral and birds-eye micrite; stromatolitic boundstone. The carbonate factory was dominated by bio-induced subaqueous precipitation, later affected by subaerial exposure. Carbonate stable isotope values were δ18Ocarb +1.1 to −15.6‰ and δ13Ccarb +14.2 to −4.0‰. The low correlation coefficient suggests a short residence time of paleolake waters, and the basin could be classified as either a balanced-fill or overfilled lake type. The fluctuations of values suggest possible alternating dilution and evaporation stages, proving sensibility to climate signature. Paleoflora registered in the lower and middle sections of the unit indicate that deposition would have occurred under temperate to warm and humid conditions. On the other hand, the upper section’s flora indicates a westward retreat of taxa from humid climates, suggesting and aridization due to the rain shadow effect, caused by the Miocene Andean uplift.