Magnetostratigraphy and magnetic parameters of a sedimentary sequence in Punta San Andres, Buenos Aires, Argentina

摘要

This contribution focuses on polarity changes and magnetic parameters obtained in late Cenozoic sediments exposed on the cliffs in San Andres, Chapadmalal, Buenos Aires province. The sedimentary section studied comprises the Vorohue, San Andres, Miramar and Arroyo Seco Formations. The upper layers of Vorohu6 and the base of San Andres showed records of normal polarity and were attributed to the Gauss Normal Polarity Chron (>2.6 Ma). The main portion of San Andres Formation appears to have been deposited during lower and middle Matuyama Polarity Chron, including the Normal Polarity Subchron Olduvai (1.78-2.02 Ma). Miramar Formation contains polarity changes that should be assigned to Middle Matuyama/Jaramillo (1.05 Ma) and to Jaramillo/Upper Matuyama (0.99 Ma). The upper portion of the Miramar Formation and the lower one of the Arroyo Seco Formation show reverse polarity levels and were assigned to the Upper Matuyama (>0.78 Ma). Finally, the upper part of Arroyo Seco Formation seems to have been deposited during the Brunhes polarity chron (<0.78 Ma). Noticeable differences of magnetic records are obtained when comparing loess and paleosols values with calcic horizons values. LF susceptibility values range between 380 × 10~(-8) m~3/kg and 20 × 10~(-8)m~3/kg, of which the lowest values correspond to the calcic horizons. The outcome confirms previous results in the sense that neither of the existing magnetoclimatological models (wind and pedogenetic models) adequately account for the complexities of the Pampean loess. Clearly, the entrainment of dense magnetic iron oxides minerals was more efficient during stormy, dry (glacial) periods than during humid ones. Weathering and pedogenesis produce the decrease of susceptibility values, among other epigenetic processes. Thus, it follows that the decrease of magnetic values by weathering of parent materials depends on the geographic location of loess deposits.