Evaluación de la señal magnética en un sondeo en sedimentos lacustres, Cuenca de Guadix-Baza (sondeo de Barranco León)

Abstract

Since N. Opdyke's pioneering study in the early 1970s, kilometers of sediment cores have been drilled and obtained, mostly in ocean basins. Much of these records have been instrumental in advancing our knowledge of the Earth's magnetic field in the past. In fact, numerous aspects of paleomagnetism such as polarity reversals, paleointensity, dipole asymmetry, etc. reside largely in the paleomagnetic signal of sediments obtained from such deep drilling, either in oceans or, to a lesser extent, in lakes. Unlike surface outcrops, where it is normally possible to recognize the lateral continuity of the strata and associated complexities, the paleomagnetic study in boreholes is limited to a few centimeters of material thickness, and determined by the diameter of the borehole. For this reason, it is essential to investigate in detail the paleomagnetic signal of these records, and take advantage of any characteristic of the sediments to ponder the robustness of the results. Our work focuses on the study of rock magnetism and paleomagnetism in a borehole of about 80 m in length, in lacustrine-alluvial sediments in the Guadix-Baza Basin (Granada), the largest paleolake in southern Europe. The sediments are of Plio-Pleistocene age, virtually horizontal, and have suffered hardly any tectonic deformation, so the expected paleomagnetic direction is well known. One of the aspects of rock magnetism that is perhaps most helpful is the anisotropy of magnetic susceptibility (AMS), which in this sedimentary context is a reflection of the depositional plane. The orientation of the magnetic ellipsoid, therefore, must include axes of minimum (sub)vertical susceptibility, as well as axes of maximum susceptibility contained predominantly in the depositional plane. Such a prediction of the spatial arrangement can be used as a basis to detect deformation induced by the mechanisms of the drilling itself, which on the other hand, very often, are not detectable with the naked eye.