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

ARAMENDÍA, INÉS; BOUZA, P.; CUITIÑO, JOSÉ I.; RAIGEMBORN, SOL; GONZÁLEZ, FLORENCIA; VALENZUELA, LAUTARO; BABAY, PAOLA; GHIGLIONE, MATÍAS C.

La Plata

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

Paleoclimate estimations (i.e. paleotemperatures and paleoprecipitations) from paleosols areimportant for the understanding of foreland basins evolution in compressive contexts. Thestudy area encompasses Miocene foreland basin deposits from the northwestern edge of theAustral-Magallanes basin (AMB, c. 47° 30’ SL), which are closely related to the uplift of theSouthern Patagonian Andes. Miocene beds are represented by shallow marine deposits (ElChacay Formation, 20-18 Ma) covered by continental beds. These beds named Río ZeballosGroup (i.e. Río Jeinemeni, Cerro Boleadoras and Río Correntoso formations) and Santa CruzFormation represent fluvial/alluvial deposits with paleosol development constrained between18 to 12 Ma. Inceptisol-like paleosol B and C horizons of the continental units were sampledfor 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 dataobtained using X-ray fluorescence were used to calculate paleosol weathering molecularindices 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), while the mean annual temperature (MAT) was estimated using the PWI relationshipof Gallagher and Sheldon (2013) and C (“clayness” index) of Sheldon (2006). CIA-K and PWIvalues for WLBAP are 63 and 55 on average, respectively, and MAT(PWI), MAT(C) and MAPare ~11 ± 2.1 °C, 13 ± 0.6 °C and ~763 ± 182 mm/year on average, respectively. CIA-K andPWI values for LP are 62 and 50 on average, respectively, and MAT(PWI), MAT(C) and MAPare ~11 ± 2.1 °C, 13 ± 0.6 °C and ~748 ± 182 mm/year on average, respectively. CIA-K andPWI average values at LO section are 66 and 49, respectively, and MAT(PWI), MAT(C) andMAP 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.Although weathering index and paleoclimate proxies are similar throughout the analyzedlocalities, a slightly increase in precipitations is recorded towards the south. This is inagreement with similar rainfall and also temperatures reported by other authors in the easternand southern part of the basin.