New paleomagnetic results from the Rio Dell Formation, California

A new paleomagnetic study of the Plio‐Pleistocene siltstones and mudstones of the northern California Rio Dell Formation confirms its Matuyama age but locates the Jaramillo and Olduvai normal events more accurately than does a previous paleomagnetic study. In addition, it records one of the short Réunion events not reported in the previous study. Detailed alternating field and thermal demagnetization studies, an average stratigraphic distance of 32 m between sites, two to ten samples per site, and careful consideration of the data allow the derivation of a magnetostratigraphy in a reversed polarity section plagued by normal polarity magnetic overprinting. Our magnetostratigraphy suggests that the stratigraphic location of the microfaunally determined Plio‐Pleistocene boundary is approximately 1.6 m.y.b.p. in age. The rate of sedimentation between the upper boundaries of the Jaramillo and Olduvai events is 0.7 m/1000 yrs. This agrees with the average sedimentation rates determined by microfaunal studies of the section and by the previous paleomagnetic study.