Magnetostratigraphy of Cenozoic sediments in western Buenos Aires

Abstract

A palaeomagnetic study of Quaternary sediments in western Buenos Aires city, was carried out in order to know their age and the time span during which the different processes of deposition and evolution took place. This study carried out on two profiles called CAS and GAO, recognized the upper 8m of the loessic plain sediments as belonging to the Brunhes Magnetic Polarity Zone (younger than 0.78 Ma), and the underlying sediments to the Matuyama Magnetic Polarity Zone. This significant extension in time supports geochemical and mineralogical studies that suggest the existence of different areas of origin of the sediments in the Pampa Ondulada, and modifies the traditional concept of their homogeneity. The change of magnetic polarity was found above a buried soil, at 13,30 m a.s.l., probably equivalent to the Hisisa Geosol, identified in other areas of the province as a regional palaeosurface and proposed as the upper boundary of the Matuyama Magnetic Polarity Zone. Taking into account the morphological features of the studied materials and the fact that this magnetic polarity boundary has been repeatedly found in the upper section of the Ensenada Formation, it is possible to correlate the sediments from the lower section of the CAS profile with this formation, whereas sediments of the upper level, as well as the ones recognized in the GAO profile, correlate with the Buenos Aires Formation.