Magnetostratigraphy and environmental magnetism in a sedimentary sequence of Miramar, Buenos Aires, Argentina

Abstract

A sedimentary sequence called “Fogones de la Costa” located at 38° 17′ 35″ S, 57° 50′ 50″ W, on the coastal cliffs, south of Miramar city, was studied from geological, paleomagnetic and rock magnetic viewpoints. The stratigraphic column shows normal and reverse polarity records associated with chrons and subchrons of magnetopolarity. The younger sediments were assigned to the Brunhes Polarity Chron (<0.78 Ma) and the older sediments of the middle and lower part of the profile were assigned to Upper Matuyama (0.99–0.78 Ma), Jaramillo (1.05–0.99 Ma) and Middle Matuyama (1.78–1.05 Ma). A very short normal polarity record between cycles V and VI, at the base of the cliff, suggests the presence of a condensed record of Cobb (or Olduvai?). The oldest reverse polarity layers at the base of the profile should be related to the lower Matuyama (>1.9 Ma). On the basis of these data, the sequence should span from the Lower Pleistocene to the Holocene, and may be correlated (although partially) with the geological formations of San Andrés, Miramar, and Arroyo Seco described in coastal cliffs north of Miramar. The model of behavior of concentration parameters (κ, χ, Ms and Mr) is in agreement with that determined in other loess sections studied in northern Buenos Aires province. The values of the concentration of magnetic parameters decrease with increasing pedogenesis and weathering of the original material (loess and loess-like). This is a behavioral feature, independent of the carrier agent (wind or water). The medium silt fraction is dominant over the other grain sizes. In that fraction, the amount of magnetic minerals increases significantly. Conversely, the layers corresponding to paleochannels and paleolagoons show a decrease of magnetic minerals, the medium silt fraction also decreases, and the very fine silts and clays increase.