Paleomagnetic results from igneous and sedimentary rocks from the Michigan Basin borehole

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Abstract
The results of thermal and alternating field demagnetization of 13 specimens from a fully oriented mafic igneous core taken from a depth of 5.32 km near the center of the Michigan basin give a pole position at 9°S, 159°E (k = 69, α95 = 5°). This pole is close to late Hadrynian and Cambrian pole positions from the North American craton. Furthermore, one specimen was cut from each of three vertical red bed core segments, unoriented in declination, from depths of 4.59, 4.09, and 4.08 km. Thermal demagnetization revealed a large secondary magnetization in these samples, which was fully eliminated at 480°C. Further thermal demagnetization revealed closely grouped characteristic inclinations that averaged 8°. By assuming the secondary magnetizations to be parallel to the present‐day geomagnetic field direction it was possible to obtain characteristic declinations as well. The pole from the red beds is located at 7°N, 174°E and agrees well with the pole obtained from the late Keweenawan sediments (Freda sandstone and Nonesuch shale) of the Upper Peninsula in Michigan. These results suggest that the red beds acquired their characteristic magnetization before the igneous basement rock. If the igneous rock is an extrusive body of Keweenawan age, its characteristic magnetization must be entirely secondary.