Paleomagnetism and correlation of some Middle Keweenawan rocks, Lake Superior

Abstract

Paleomagnetic results from the Lake Superior region appear to indicate that Middle Keweenawan formations or magnetic divisions within formations can be correlated on the basis of magnetic polarity. The Logan sills, the volcanics of the Osler Group, the lava section at Alona Bay, the lower 6000 ft (1829 m) of North Shore volcanics, the lower 1000 ft (305 m) of Cape Gargantua volcanics and the lower 3000 ft (914 m) at Mamainse Point are of reverse polarity. Normal directions of magnetization, which differ from the reverse directions of magnetization by amounts ranging from 140° to 163° occur throughout the Michipicoten Island section, and in the upper parts of the North Shore, Cape Gargantua, and Mamainse Point sections. Provided part of the section at Mamainse Point has been repeated by faulting, these new results, together with data from the literature, are compatible with a single polarity reversal in Middle Keweenawan time separating older reverse polarity rocks from stratigraphically younger normal polarity rocks. The mean direction of magnetization from a conglomerate test is defined with higher precision than would be expected from a randomly distributed population. It is believed that this mean direction reflects a secondary component of magnetization which cannot be preferentially removed by alternating field demagnetization and which has caused the non anti-parallel alignment of normal and reverse populations of magnetization. An estimate of the pole position during Middle Keweenawan time can, however, be made by calculating a pole given equal weight to the mean normal and mean reverse pole. This is calculated to be 159° W, 42° N.