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

ARAMENDIA INES; BOUZA PABLO; CUITIÑO, JOSÉ IGNACIO; M. SOL RAIGEMBORN; GONZALEZ F; VALENZUELA L; BABAY P; GHIGLIONE MATIAS

LA PLATA

Congreso; XVIII Reunión Argentina de Sedimentología : IX Congreso Latinoamericano de Sedimentología; 2023

AAS

Paleoclimate estimations (i.e. paleotemperatures and paleoprecipitations) from paleosols are importantfor the understanding of foreland basins evolution in compressive contexts. The study areaencompasses Miocene foreland basin deposits from the northwestern edge of the Austral-Magallanesbasin (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) coveredby continental beds. These beds named Río Zeballos Group (i.e. Río Jeinemeni, Cerro Boleadoras andRío Correntoso formations) and Santa Cruz Formation represent fluvial/alluvial deposits with paleosoldevelopment constrained to span 18–12 Ma in age. Inceptisol-like paleosol B and C horizons of thecontinental units were sampled for geochemical analyses in three relevant localities: i) Western LagoBuenos Aires Plateau (WLBAP); ii) Lago Posadas (LP) and iii) La Oriental lagoon (LO). Bulkgeochemical data obtained using X-ray fluorescence were used to calculate paleosol weatheringmolecular indices on B and C horizons (CIA-K, PWI, C) and paleoclimate proxies on B horizons. Meanannual precipitation (MAP) was estimated using the CIA-K relationship of Sheldon et al. (2002), whilethe mean annual temperature (MAT) was estimated using the PWI relationship of Gallagher andSheldon (2013) and C (“clayness” index) of Sheldon (2006). CIA-K and PWI values for WLBAP are 63and 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/yearon average, respectively. CIA-K and PWI average values at LO section are 66 and 49, respectively, andMAT(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 chemicalweathering and the MAT and MAP results indicate a temperate and subhumid climate. Althoughweathering index and paleoclimate proxies are similar throughout the analyzed localities, a slightlyincrease in precipitations is recorded towards the south. This is in agreement with similar rainfall andalso temperatures reported by other authors in the eastern and southern part of the basin.