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Western Mediterranean |
1 Departament de Geologia Dinàmica, Geofísica i Paleontologia, Universitat de Barcelona, Zona Universitària de Pedralbes, 08071-Barcelona, Spain
2 Institute of Earth Sciences, CSIC, Martí i Franqués s/n, 08028-Barcelona, Spain
Palaeogeographic reconstructions based on kinematic and other integrated models are rarely in perfect agreement with palaeomagnetic data. Furthermore, the reliability of rocks that record the geomagnetic field is often questioned, particularly for detrital remanent magnetization (DRM). Among the diverse types of magnetization bias, inclination shallowing caused by several mechanisms such as gravitational torques upon deposition and post-depositional compaction can be a major source of error in palaeolatitudinal estimations. Inclination shallowing for a variety of fine-grained alluvial sediments from the Neogene Vallès-Penedès Basin (NE Spain) has been observed to be dependent on sedimentary facies. The correlation of inclination shallowing to sediment fabric is investigated in samples from 11 stratigraphic profiles. Each profile has a dominant lithology, from thinly laminated to edafized silts and massive muds and breccias. A measure of particle alignment in sediment can be determined by means of the degree of anisotropy (P) and shape symmetry (T) AMS parameters. Since each sedimentary facies shows a particular range of P and T, a representative mean AMS ellipsoid can be estimated from each lithologically homogeneous profile. Estimated mean remanence inclination (INRM) has a strongly consistent relationship to the mean AMS ellipsoid and a positive linear correlation is inferred between ln (P) and ln tan (INRM). Given the extreme case of no sediment anisotropy, i.e. P = 1, inclination error would be cancelled and INRM would record the true magnetic field inclination of the sampling site. The regression function predicts a corrected magnetic inclination of 60°, consistent with that of the expected geomagnetic dipole for the studied site and age.
