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

VIZÁN, HAROLDO; AZCUY, CARLOS L.; TÓFALO, OFELIA RITA; GEUNA, SILVANA E.; AMENABAR, CECILIA R.; ORGEIRA, MARÍA JULIA; RENDA, EMILIANO M.; CARREÑO, CLARA BRAÑA; PANARELLO, HÉCTOR; CASTRO, LILIANA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 92 p. 506 - 522

0895-9811

Different authors have suggested that there is a relationship between Quaternary geomagnetic field events and climate variability. Considering that in a recent paper a detailedstratigraphic study in a section of the coast of Claromeco (Buenos Airesprovince) indicates Late Pleistocene paleoclimatic changes, a paleomagneticstudy was performed in the same stratigraphic succession. The analyzedsedimentary section belongs to ?Belgranense? unit that is composed of continental and marine deposits. Accordingto previously published dates and stable isotope results presented in thispaper, it is proposed that 6 facies (sedimentary units) of the Claromecosection developed during the interglacial Marine Isotope Stage 5 (MIS 5). Theseunits are presumably coeval with one of the best known geomagnetic field events, the Blake Event, occurred duringthe MIS 5, between ca. 120 and ca. 115 ka. The studied units from the base to the top are: 1) diageneticwackestones with clasts of sand, 2) pedogenized loessian deposits, 3) lowermarine wackestones, 4) lower tidal channel deposits, 5) upper marine wackestonesand 6) upper tidal channel deposits. The section is capped by a continentalgravity flow much younger than MIS 5. Different facies record distinctcharacteristic remanent magnetizations (ChRM) suggesting that these have a notmodern origin. Low-field thermomagnetic curves and hysteresis loops wereperformed for all the sedimentary facies that provide magnetic remanences.Special sediment magnetism experiments were made for the units with reverseand/or oblique reverse components including isothermal remanent  magnetization and anhysteretic remanentmagnetization to perform the Lowrie-Fuller test. In these units opaque mineralswere identified using polished and thin sections together with scanningelectron microscopy analysis. According to all the results it is interpretedthat the Blake Geomagnetic Field Event was recorded in the pedogenized loessian deposits with magnetite crystals generated during pedogeneticprocesses, and in extracellular biogenic magnetite crystals in the lower marinewackestones. The reverse and/or oblique reverse ChRM components were recordedbefore the lower tidal channel deposits, which have normal polarity directions.The Blake Geomagnetic Field Event was recorded in Claromeco during lastinterglacial MIS 5, however a direct correlation cannot be made between therecorded transitional polarity and a marine transgression corresponding to an important melting of ice sheets during this marine isotope stage. Themagnetic components recorded in the studied sequence could be used as astratigraphic tool to help identify ?Belgranense? unit in Argentina or correlative sedimentary facies outcropping inSouth America.