Till palemagnetism in northeastern Kansas.

Abstract

Recent chronometric studies of Early Pleistocene deposits in the plains of central North America have resulted in considerable revision of the presently used terms for the North American Pleistocene stages. Modern investigations have suggested these terms need to be either redefined and given suitable stratotypes or abandoned. Paleomagnetic studies have not previously been conducted on tills in northeastern Kansas to verify their age. During the fall of 1987, 51 samples of Lower Kansas Till were used for this study. Thirty-five samples were surface samples from the Kansas Drift stratotype in Atchison Couty; another 16 were subsurface samples from test-drilling sites in Nemaha county, Kansas. The natural remnant magnetism for all samples was measured on a "Molspin" spinner magnetometer. Samples were subjected to alternating field (a.f.) demagnetization at 15, 50, and 100 Oersteds. The data were analyzed using the microcomputer program PALMAG. The output included the declination and inclination before and after each a.f. demagnetization step, plus the intensity for each sample. Paleomagnetic data show that the Lower Kansas Till has stable primary remnant magnetism with normal polarity, except where obvious structural deformation is present. The normal polarity of the Lower Kansas Till restricts its age to the Brunhes Epoch, which began about 700,00 years ago. Consequently, age of the Kansas Drift is between 600,000 and 700,000 years old. In addition, as the Lower Kansas Till shows a positive polarity, the Fremont Till at Cedar Bluffs, Nebraska, which also has a positive polarity, should be of Kansan age. The Nebraskan Till which has reversed polarity, is not present in northeastern Kansas.