Ocean Drilling Program, United States

Two records of the Brunhes-Matuyama (B-M) transition and one record of the upper Jaramillo transition were obtained from sediments of Ocean Drilling Program (ODP) Leg 167 along the California margin. The geomagnetic transitions were sampled with u-channels and closely spaced discrete samples. The discrete samples and u-channels from opposite halves of the core give similar virtual geomagnetic pole (VGP) paths for the upper Jaramillo reversal at Site 1020 (Gorda Ridge). For the B-M transition at Site 1020, the discrete samples and u-channels show different VGP paths. At every site, and especially for the B-M transition at Site 1014 (Tanner Basin), the VGP paths do not reach the geographic South and North Poles. The characteristic remanent magnetization is contaminated with a coring-induced overprint that has coercivities that overlap with the coercivities of the primary magnetization. The presence of a drilling remanence leads to shallow reverse and steep normal inclinations. Model calculations simulate the effects of small overprints along the axis of the sediment core or perpendicular to it. The calculations show that significant changes can occur in the transitional VGP positions because of drilling overprints. The coring-induced overprint often cannot be completely removed by alternating-field demagnetization and may give rise to false transitional directions. Other ODP reversal records should be checked for the presence of coring-induced overprints.