Error de inclinación paleomagnética en materiales aluviales del Oligoceno superior del sector Suroriental de la Cuenca del Ebro (Región Surpirenaica, NE de España)

Abstract

Alluvial red-beds of two Oligocene succesions in the Southeastem Ebro Basin (Southem Pyrenean foreland, NE Spain) show inclination shallowing, taking into account the reference Oligocene palaeolatitude expected for this region. This fact is interpreted to reflect an inclination error due to either these factors: hydrodynamic control of magnetic particles in the depositional environment, differential compaction of sediment during burial and tectonic deformation. The studied lithofacies are split into five groups: gray sandstones, red sandstones, red siltstones, red mudstones and limestones. A strong positive correlation between the relative amount of phyllosilicates and the magnitude of the inclination error has been established. Lithofacies with low percentage of phyllosilicates (limestones and gray sandstones) display a statistically not significant error of 5" with respect to the magnetic palaeofield inclination, whereas the sediments with a higher phyllosilicate content (siltstones and mudstones) show significant errors of up to 25". This observation has not a major consequence for the interpretation of the magnetic polarity but is crucial for palaeogeographic and palinspastic reconstmctions based on palaeolatitudinal data. The obtained results in this study highlight that the interpretation of paleolatitude based on paleomagnetic data is sometimes not straightfonvard and it requires a detailed evaluation of the rock magnetic parameters (AMS in particular) to reveal the presence of such inclination deflection.