Integrated AMS and textural analyses as objective tools to define paleocurrent directions in turbidite beds

Turbidite systems, volumetrically the most important accumulations of sand in deep sea, are characterized by a great variability in size, geometry, facies, and stacking patterns. The development of depositional models at the basin scale is essential to understand this variability. Models require an accurate knowledge of the palaeocurrent directions within the turbidite systems. Traditionally, sedimentological current indicators (flute marks, ripple marks, etc.) are used to obtain paleocurrent directions, but these are not always present in outcrop sections and are virtually absent from drill cores. This limitation raises the need to identify an alternative, objective method to define paleocurrent directions in turbidite successions. We have applied the anisotropy of magnetic susceptibility (AMS) to nearly 550 samples collected in the Marnoso Arenacea Formation (northern Apennine, Italy; Ricci Lucchi and Valmori, 1980), a well-known foredeep basin of Miocene age that comprises well-exposed stratigraphic sections. The method is based on the fact that a current is able to orient para- and ferromagnetic grains and minerals. The AMS ellipsoid often reflects the orientation imparted by the current to such grains. In order to calibrate the method, we have compared the AMS data to paleocurrent directions measured from flute casts at the base of each sampled bed, finding that the AMS ellipsoids are consistently oriented sub-parallel to flow direction estimated by such sedimentological indicators. We have also carried out textural analysis (image analysis on thin sections, neutron diffraction, and tomography) to relate the magnetic signal to mineralogic and shape fabrics. Thin sections have been cut along defined directions: parallel and perpendicular to the flow direction. Image analysis has been carried on in order to obtain a bi-dimensional representation of grains’ orientation. Neutron diffraction is an experimental technique for quantitative texture analysis. With this analysis, three-dimensional crystallographic information has been collected, and the presence of crystal preferential orientations have been visualized through polar figures. Tomography allows to assemble a series of closely spaced X-ray sections into a three-dimensional image where different mineralogical phases can be visualized on a grey scale. Bi-dimensional image analysis on thin section, and three-dimensional textural analyses (neutron diffraction and tomography) indicate that paramagnetic muscovite controls the observed current-induced AMS fabric. Our integrated AMS and textural analyses provide an objective, quantitative way to define paleocurrent direction in absence of sedimentological indicators. References Ricci Lucchi F., and Valmori E. (1980). Basin-wide turbidites in a Miocene, over-supplied deep-sea plain: a geometrical analysis. Sedimentology, 27, pp. 241-270.