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Published July 1995 | public
Journal Article

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 primarily 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.

Additional Information

© 1995 Elsevier Science B.V. Received 8 August 1994; accepted 22 May 1995. We first thank Bob Adams for leading us to the Confidence Hills and pointing out their potential. We are grateful to Kathi Beratan, Jean Hsieh and Bruce Murray for all of their important work on the sedimentary record, and to Chris Pluhar for the initial paleomagnetic survey. Special thanks are due to Liz Warner Holt for field assistance and proofreading, and to Toshitsugu Yamazaki for assisting in magnetic separations and for performing TEM work. Helpful comments on the manuscript were received from two anonymous reviewers. We appreciate the National Park Service allowing us to take paleomagnetic samples within the former Death Valley National Monument. We made extensive use of software provided by Craig Jones (cjones@mantle.colorado. edu) for the analysis and presentation of paleomagnetic data, and the GMT package [39] for global projections of VGP's. We appreciate obtaining the reversal atlas [37] from Joanna Athanassopoulos. Work on this project was funded by NSF grant No. EAR-9019289. This is California Institute of Technology Contribution No. 5436.[MK]

Additional details

August 20, 2023
October 20, 2023