The Blake Geomagnetic Field Event recorded in a sequence of marine and continental facies outcropping in the coast of Buenos Aires province, Argentina

摘要

Different authors have suggested that there is a relationship between Quaternary geomagnetic field events and climate variability. Considering that in a recent paper a detailed stratigraphic study in a section of the coast of Claromecd (Buenos Aires province) indicates Late Pleistocene paleoclimatic changes, a paleomagnetic study was performed in the same stratigraphic succession. The analyzed sedimentary section belongs to "Belgranense" unit that is composed of continental and marine deposits. According to previously published dates and stable isotope results presented in this paper, it is proposed that 6 facies (sedimentary units) of the Claromeco section developed during the interglacial Marine Isotope Stage 5 (MIS 5). These units are presumably coeval with one of the best known geomagnetic field events, the Blake Event, occurred during the MIS 5, between ca. 120 and ca. 115 ka. The studied units from the base to the top are: 1) diagenetic wackestones with clasts of sand, 2) pedogenized loessian deposits, 3) lower marine wackestones, 4) lower tidal channel deposits, 5) upper marine wackestones and 6) upper tidal channel deposits. The section is capped by a continental gravity flow much younger than MIS 5. Different facies record distinct characteristic remanent magnetizations (ChRM) suggesting that these have a not modern origin. Low-field thermomagnetic curves and hysteresis loops were performed for all the sedimentary facies that provide magnetic remanences. Special sediment magnetism experiments were made for the units with reverse and/or oblique reverse components including isothermal remanent magnetization and anhysteretic remanent magnetization to perform the Lowrie-Fuller test. In these units opaque minerals were identified using polished and thin sections together with scanning electron microscopy analysis. According to all the results it is interpreted that the Blake Geomagnetic Field Event was recorded in the pedogenized loessian deposits with magnetite crystals generated during pedogenetic processes, and in extracellular biogenic magnetite crystals in the lower marine wackestones. The reverse and/or oblique reverse ChRM components were recorded before the lower tidal channel deposits, which have normal polarity directions. The Blake Geomagnetic Field Event was recorded in Claromeco during last interglacial MIS 5, however a direct correlation cannot be made between the recorded transitional polarity and a marine transgression corresponding to an important melting of ice sheets during this marine isotope stage. The magnetic components recorded in the studied sequence could be used as a stratigraphic tool to help identify "Belgranense" unit in Argentina or correlative sedimentary facies outcropping in South America.