Coring-related deformation of Leg 208 sediments from Walvis Ridge: Implications for paleomagnetic data

Both shipboard and shore-based magnetic studies of Ocean Drilling Program (ODP) Leg 208 sediments produced poor-quality directional data. A noisy inclination signal and a pervasive declination bias demonstrate that the sediments had been remagnetized and/or deformed. In an attempt to characterize any deformation, we carried out anisotropy of magnetic susceptibility (AMS) experiments on a suite of samples from Sites 1262 and 1267. Results show that the cores have suffered at least two kinds of deformation, one producing a tilting of the foliation (by up to 40° from horizontal in some intervals), and the other producing a lineated fabric. The former is firmly linked to lithology – higher carbonate content results in a higher degree of tilting – while the latter may be more closely tied to depth, suggesting the two fabrics result from different processes. The remanence directions show no obvious correlation to either fabric, suggesting that some third mechanism may have remagnetized the sediments, possibly an isothermal remanence acquired during the drilling process. In this study, the severity of the deformation may be linked to the high carbonate content of the cores, but pinpointing precise causes for both the AMS fabric and the directional anomalies remains elusive. It is clear, however, that the deformation is complex, pervasive and has significant implications for the interpretation paleomagnetic data from Leg 208.