Disentangling tectonic and climatic signals from a lacustrine system during the Andean uplift and the Middle Miocene Climate Optimum (Ñirihuau Basin, Patagonia, Argentina): a multiproxy analysis

SANTONJA CAMILA; SURIANO JULIETA; BECHIS FLORENCIA; BENAVENTE, CECILIA A.

Simposio; The Virtual IAL-IPA ECR Meeting; 2025

The northern sector of the Ñirihuau Basin, located on the eastern slope of the North Patagonian Andes (41–43°S) exhibits a tectonic evolution closely linked to Andean uplift during the Neogene. This orogenic process triggered major environmental changes, including the aridification of Patagonia through the development of an orographic rain shadow. The basin records a sequence of Oligocene to Miocene volcanic rocks (Ventana Formation) overlain by Miocene volcaniclastic, clastic and carbonatic rocks (Ñirihuau and Collón Curá formations). According to available U-Pb geochronological data, the Ñirihuau Formation’s continental deposits are constrained to the Lower to Upper Miocene. Furthermore, the middle section of the unit consists of lacustrine facies deposited coeval, at least in part, with the Middle Miocene Climatic Optimum. Therefore, its filling offers a key record of tectonic and climatic shifts in the Southern Hemisphere. To unravel the tectono-sedimentary evolution and associated paleoenvironmental changes, a detailed study of the Ñirihuau Formation was conducted. Four stratigraphic sections were analyzed through a combination of sedimentological, geochronological, petrographic, and geochemical methods. Special emphasis was placed on the analysis of lacustrine sediments of the unit to reconstruct past environmental conditions and their links to tectonic processes.Ten facies associations were defined, representing alluvial fans, fluvial systems, deltas, and lacustrine environments. Notably, detailed study of lacustrine deposits and associated carbonate levels provided insights into Miocene climatic variability. U-Pb LA-ICP-IMS geochronology suggests that deposition occurred between the Early and Late Miocene (22–11 Ma). Petrographic analysis of sandstones and conglomerates, coupled with geochronological data, reveals sediment sources from the contemporaneous volcanic arc and the El Maitén Volcanic Belt. The integration of these datasets enabled the reconstruction of a three-stage tectono-sedimentary model: an extensional phase (Early Miocene), a transitional phase (Early to Middle Miocene), and a compressive synorogenic phase (Middle to Late Miocene). Paleoenvironmental reconstructions based on lacustrine multiproxy analyses indicate that temperate to humid conditions dominated during the lower and middle sections of the unit, corresponding to the Middle Miocene Climatic Optimum and the Middle Miocene Climate Transition. In contrast, the upper section reflects reduced precipitation, associated with the establishment of the Andean rain shadow.This study highlights the value of multiproxy approaches applied to lacustrine sediment records in deciphering the complex interplay between tectonics and climate in deep time.