PALEOMAGNETIC AGE CONSTRAINTS OF MID-CONTINENT RIFT STRATA, NORTHERN MINNESOTA

The Fond du Lac and Hinckley Formations in northern Minnesota are part of the Keweenawan Supergroup - volcanic and sedimentary units that constitute the Mid-Continent Rift system. Their age is only broadly constrained to between 1.09 Ga and 500 Ma by virtue of their stratigraphic position above the Mid-Continent Rift volcanics and below Cambrian sedimentary rock. Many workers assume they were deposited shortly after volcanism ceased, in a trough related to the failed rift. These formations are correlated to the Bayfield Group in Wisconsin and the Jacobsville Sandstone in Michigan.

To narrow this nearly 600 Ma range in potential age, we collected paleomagnetic data at seven pilot sites in the Fond du Lac Formation near Duluth, MN and the Hinckley Formation, near Sandstone, MN. We used the inclination from our results to constrain the paleolatitude at the time of deposition. By comparing those latitudes to published paleolatitudes of Laurentia from 1.09 Ga – 500 Ma, we identified potential depositional ages for the Fond du Lac and Hinckley Formations.

Initial results from the Fond du Lac Formation show shallow inclinations held by both magnetite and hematite, suggesting a ~12˚ paleolatitude for Laurentia during deposition. The preliminary results have an A₉₅ value of 26°. By contrast, the paleomagnetic results from the Hinckley Formation show steep inclinations, held by magnetite, suggesting high latitude deposition. However, the Hinckley Formation results have an A₉₅ value of 53°, rendering them problematic for interpretation. Ignoring the results from the Hinckley Formation suggests that these Precambrian units could have been deposited during two possible time periods: ~780-660 Ma and ~570-550 Ma. Both are much later than the conventional assumption imagined for Keweenawan-Supergroup strata. These preliminary results will be augmented by new data and analyses from correlative units in northern Wisconsin and Michigan in an effort to decrease A₉₅ and to better constrain the depositional age of these enigmatic rocks.