Magnetism of the Keweenawan Age Chengwatana Lava Flows, northwest Wisconsin

Abstract

The Chengwatana Volcanics are subaerial plateau basalts which appear to be stratigraphically above the North Shore Volcanic Group of the Lake Superior Region. Sixty five oriented cores were collected at six sites near St. Croix Falls, Wisconsin. Magnetic cleaning of the samples was primarily step‐wise alternating field (A.F.) demagnetization to 100 mT. Thermal demagnetization and hysteresis property measurements indicate the dominant magnetic mineralogy is multidomain magnetite. However, a stable remanence is present which we attribute to grains with single domain behavior. The mean remanent direction is D=287.5°, I=45.8°, α‐95=8.0°, with a mean pole at 186.1°E., 30.9°N. This result is consistent with the 1.1 Ga. apparent pole for the North American craton. Two sites mapped as stratigraphically equivalent show distinct reversed directions. Both A.F. and thermal demagnetization isolate the same reversed direction which is interpreted as being primary. These particular flows record a short reversal in an otherwise normal sequence. This strengthens the evidence for more than one reversal in the Middle Keweenawan, and may be the same short reversal noted at Mamainse Point.