Assessing tectonic regime variations in Miocene lacustrine deposits from the Northpatagonian Andes, Ñirihuau Basin, Argentina

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

Aix-les-Bains

Congreso; 3rd Paleolimnology and Limnogeology International Symposium; 2025

IAL-IPA Joint Meeting

The Ñirihuau Basin, located between 41° and 43° S on the eastern side of the North Patagonian Andes, Argentina, 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 (22–11 Ma). The middle section of the unit, which registers a transition from a synrift to a foreland basin tectonic stage, consists of lacustrine facies deposited between ~15 and 13 Ma, coinciding with the Middle Miocene Climatic Optimum, making it a great succession to disentangle tectonic regime effects from climate signals in its deposition. To reconstruct the paleoenvironment and asses the relative influence of tectonics and climate, a 122m thick stratigraphic section was surveyed in detail. Deposits were characterized according to lithology, color, sedimentary structures, bed geometries, and fossil content. Additionally, microfacies and geochemical analysis were conducted on carbonate beds interbedded among the fine grain deposits. A relatively low-energy lake basin was interpreted, evidenced by the great thickness of fine-grained deposits. Four facies were recognized, which were grouped into two different facies associations (FA). FA1 corresponds to a perennial lake with fluvial input, while FA2 represents the delta plain from a deltaic system. The lowermost portion of the section is constituted mainly by marginal lacustrine deposits interbedded with lower-delta-plain deposits, interpreted as laterally adjacent sub-environments. Up in the section, in the middle and upper portions, lower-delta-plain facies are recognized underlying the lake center facies. Finally, the section is capped by upper-delta-plain deposits, which contain well-preserved distributary channels and floodplain deposits. To complement this analysis, regional stratigraphic sections were surveyed and correlated along the northern sector of the basin to track spatio-temporal variations. These correlations revealed different tectonic stages of evolution, further explored through a subsidence analysis on one of the sections.