Magnetic mineralogy and susceptibility records in Quaternary sediments of normal (Brunhes) and reverse (Matuyama) polarity in the Juarez quarry, Buenos Aires Province

Abstract

Sediments exposed in the Juárez quarry of Buenos Aires Province (34°57'10''S and 57º53'00'' W), belong to the Pleistocene and Holocene. The sedimentary units are those of the Ensenada and Buenos Aires formations and the so-called Post-Pampean. The Ensenada Formation has reversed polarity (Matuyama) at the bottom of the exposed sequence and normal polarity (Brunhes) at the upper part, while the Buenos Aires Formation and Post-pampean sediments are of normal polarity (Brunhes). The highest susceptibility values were obtained in loess of the Ensenada Formation (189 x 10^-8 m³/kg), and the lowest in sediments belonging to the Buenos Aires Formation (11 x 10^-8 m³/kg) and in hydromorphic paleosols of the Ensenada Formation (30 to 50 x 10^-8 m³/kg), both associated with climatic conditions of higher relative humidity. An increase in magnetic values in the coarse- to medium-silt fractions reflects the dominance of vigorous winds as the main carrier of ferromagnetic minerals during arid periods (glacial periods). The frequency-dependent susceptibility (F factor) varies between 0 % and 6,45 %. The highest values, obtained in intensively weathered horizons (interglacial periods), are, however, intermediate compared with those of Siberia and China. This property allows us to suggest the existence of a third model of susceptibility behaviour related to changes of Quaternary paleoclimate: that of strong winds modified by pedological processes at middle latitudes.