The upper Olduvai geomagnetic field reversal from Death Valley, California: a fold test of transitional directions

High-resolution records of the upper Olduvai geomagnetic field reversal were obtained from two localities within steeply dipping lacustrine sediments of the Confidence Hills, southern Death Valley, California . The difference in bedding attitude between the two localities allowed us to perform a fold test of both transitional and non-transitional paleomagnetic directions. This is the first positive fold test obtained for transitional directions from any geomagnetic field reversal. In addition, variations in lithology allowed a comparison of transitional records from different sedimentary environments. Alternating-field and thermal demagnetization reveals that most samples have either single or simple two-component magnetizations. The low-coercivity, low-blocking temperature component is most likely held by large multidomain grains of magnetite, and aligns with the present-day field when uncorrected for bedding orientation. The high-coercivity component is held pr imarily by single-and pseudo-single -domain titanomagnetite, as determined from standard magnetic studies and electron microscopy. This component is interpreted as a primary magnetic signature based on the presence of stratigraphically bound reversals in the tilt-corrected data and positive reversal tests. Short-term field variations can be correlated between the two records, and differences are readily explained by the recording process inherent to the two sedimentary environments. The sampling site is located within one of the "preferred" bands of transitional virtual geomagnetic poles (VGP's) [1-3], yet the transitional VGP paths from both localities are clustered within two primary longitudinal bands which are approximately 90° away from the sampling site longitude: one lies over the Atlantic ocean (10°-50°W), and the other, antipodal to this (130°-170°E). These bands are also inconsistent with eight previous studies of the upper Olduvai transition [4-9,3,10]. When all reported studies of this reversal are examined, however, no global consistency exists. Instead, the transitional VGP's are better grouped in a sampling site reference frame.