Palæomagnetic Measurements of the Keweenawan

Abstract

RECENTLY, it has been possible to collect specimens from the Keweenawan system and measure their directions of magnetization. This system is a thick sequence of late Pre-Cambrian volcanics and sediments which outcrop along the margins of the Lake Superior syncline. It is divided into three series, Lower, Middle and Upper Keweenawan. At the type locality on the Keweenawan Peninsula, where most of the specimens were collected, only the Middle and Upper series are represented. Here the Middle Keweenawan consists of 11,000 ft. of basalt and andesite flows known as the Portage Lake Lava Series. There is a total of about three hundred successive flows with occasional interstratified layers of rhyolitic conglomerates and thin beds of sandstone. Succeeding the Portage Lake Lava Series conformably is the Upper Keweenawan, which consists of three divisions. The lowermost is the Copper Harbor Conglomerate, which consists principally of very coarse conglomerates and subordinate amounts of coarse reddish sandstone, but there are also some 500 ft. of interbedded lava flows. The total thickness of the Copper Harbor Formation is 4,500 ft. Lying conformably above the Copper Harbor is the Nonesuch Shale, which in this region consists of 700 ft. of thin bedded sandstones and siltstones. It grades upward into the third of the three divisions of the Upper Keweenawan, the Freda Sandstone. This formation consists of reddish, finegrained sandstones and siltstones. It is well bedded and consolidated, and usually only shows cross-bedding on a very small scale. One can find ripple marks, mud cracks and raindrop imprints, which clearly indicate a terrestrial deposition. On the Keweenawan Peninsula the Freda Sandstone is approximately 2,500 ft. thick, but in other localities it may reach a thickness of as much as 12,000 ft.