PALEOCLIMATE FROM MIOCENE PALEOSOLS IN THE NORTHWESTERN EDGE OF THE AUSTRAL-MAGALLANES BASIN

ARAMENDÍA, I.; BOUZA, P.J.; CUITIÑO, J.; RAIGEMBORN, M.S.; GONZÁLEZ, F.; BABAY, P.; GHIGLIONE, M.

LA PLATA

Congreso; XVIII REUNIÓN ARGENTINA DE SEDIMENTOLOGÍA Y IX CONGRESO LATINOAMERICANO DE SEDIMENTOLOGÍA; 2023

ASOCIACIÓN ARGENTINA DE SEDIMENTOLOGÍA

Paleoclimate estimations (i.e. paleotemperatures and paleoprecipitations) from paleosols are important for the understanding of foreland basins evolution in compressive contexts. The study area encompasses Miocene foreland basin deposits from the northwestern edge of the Austral-Magallanes basin (AMB, c. 47° 30’ SL), which are closely related to the uplift of the Southern Patagonian Andes.Miocene beds are represented by shallow marine deposits (El Chacay Formation, 20-18 Ma) covered by continental beds. These beds named Río Zeballos Group (i.e. Río Jeinemeni, Cerro Boleadoras and Río Correntoso formations) and Santa Cruz Formation represent fluvial/alluvial deposits with paleosol development constrained to span 18–12 Ma in age. Inceptisol-like paleosol B and C horizons of the continental units were sampled for geochemical analyses in three relevant localities: i) Western Lago Buenos Aires Plateau (WLBAP); ii) Lago Posadas (LP) and iii) La Oriental lagoon (LO). Bulk geochemical data obtained using X-ray fluorescence were used to calculate paleosol weathering molecular indices on B and C horizons (CIA-K, PWI, C) and paleoclimate proxies on B horizons. Mean annual precipitation (MAP) was estimated using the CIA-K relationship of Sheldon et al. (2002), while the mean annual temperature (MAT) was estimated using the PWI relationship of Gallagher and Sheldon (2013) and C (“clayness” index) of Sheldon (2006). CIA-K and PWI values for WLBAP are 63 and 55 on average, respectively, and MAT(PWI), MAT(C) and MAP are ~11 ± 2.1 °C, 13 ± 0.6 °C and ~763 ± 182 mm/year on average, respectively. CIA-K and PWI values for LP are 62 and 50 on average, respectively, and MAT(PWI), MAT(C) and MAP are ~11 ± 2.1 °C, 13 ± 0.6 °C and ~748 ± 182 mm/year on average, respectively. CIA-K and PWI average values at LO section are 66 and 49, respectively, and MAT(PWI), MAT(C) and MAP are ~11 ± 2.1 °C, 13 ± 0.6 °C and ~815 ± 182 mm/year on average, respectively. Chemical weathering indices of the studied localities indicate a low to moderate chemical weathering and the MAT and MAP results indicate a temperate and subhumid climate. Although weathering index and paleoclimate proxies are similar throughout the analyzed localities, a slightly increase in precipitations is recorded towards the south. This is in agreement with similar rainfall and also temperatures reported by other authors in the eastern and southern part of the basin.