Remagnetizaciones y migración de fluidos en la cuenca neógena de Fortuna, Cordilleras Béticas

• Autores: Miguel Garcés Crespo, W. Krijgsman
• Localización: Geotemas (Madrid), ISSN 1576-5172, Nº. 1, 1, 2000 (Ejemplar dedicado a: V Congreso Geológico de España (Alicante, 10-14 julio 2000)), págs. 105-109
• Idioma: español
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• Resumen

  • Paleomagnetic studies of the late Tortonian to Pliocene deposits of the Fortuna basin have revealed the presence of a peculiar three-component natural remanent magnetisation (NRM) in a particular stratigraphic interval of the basin infill. In the Sifón de Librilla section, a high temperature NRM component (component A) was detected in all the red and mottled silt samples. Its maximum unblocking temperature well above 600° reveals the presence o f hematite as the main remanence carrier. Component A yielded pre-folding paleomagnetic directions, recording a long sequence of up to ten polarity reversals which perfectly match with the Messinian to lower Pliocene geomagnetic polarity time scale.

    An intermediate temperature paleomagnetic magnetisation (component B) was defined in the range 250-500°C. It is dominantly present in all samples from the grey marl intervals. Component B always yields a reverse polarity and its mean direction supports a pre-tilt magnetisation. Both NRM and IRM analyses suggest that magnetite is the most likely carrier of component B.

    The fact that component B is tightly clustered to a reversed paleomagnetic direction suggests that the it must be associated to a single short-lived remagnetising event, rather than to a process operating continuously through diagenesis. The remagnetisation must have taken place in an early stage because of its pre-folding age. A likely period for this event is the reversed chron C3r, where we find a significant environmental change, as a result o f the dramatic base level drop at the end of the Messinian Salinity Crisis. A possible cause for the remagnetisation component is that this base level drop caused an increased and efficient drainage of groundwater, thus exposing a reducing environment (the grey layers) to oxidation. Oxidation of iron sulphides to magnetite would cause a newly formed chemical remanent magnetisation, acquiring the then ambient (reversed) field.