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 reversa l 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 pa leomagnetic directions. This is the first positive fold test obtained for transitional directions from any geomagnetic field reversal. In ad dition, variations in lithology allowed a comparison of transitional r ecords from different sedimentary environments. Alternating-field and thermal demagnetization reveals that most samples have either single o r simple two-component magnetizations. The low-coercivity, low-blockin g temperature component is most likely held by large multidomain grain s of magnetite, and aligns with the present-day field when uncorrected for bedding orientation. The high-coercivity component is held primar ily 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 po sitive reversal tests. Short-term field variations can be correlated b etween the two records, and differences are readily explained by the r ecording process inherent to the two sedimentary environments. The sam pling site is located within one of the ''preferred'' bands of transit ional virtual geomagnetic poles (VGP's) [1-3], yet the transitional VG P paths from both localities are clustered within two primary longitud inal bands which are approximately 90 degrees away from the sampling s ite longitude: one lies over the Atlantic ocean (10 degrees-50 degrees W), and the other, antipodal to this (130 degrees-170 degrees E). The se bands are also inconsistent with eight previous studies of the uppe r Olduvai transition [4-9,3,10]. When all reported studies of this rev ersal are examined, however, no global consistency exists. Instead, th e transitional VGP's are better grouped in a sampling site reference f rame.