Abstract

Analysis of total field magnetic data, together with other geophysical evidence has led to the following conclusions concerning the structure and stratigraphy of late Precambrian Keweenawan rocks that underlie and border the northern part of Lake Superior:(1) The uppermost few hundred feet of the Osler Volcanic Series has a normal remanent magnetization. The normally magnetized mafic flows are separated from older, reversely magnetized ones which form the bulk of the Osler Series, by a thin zone of felsic igneous rocks with unknown magnetic polarity. The upper mafic unit appears from aeromagnetic data to outcrop on some of the outer islands at the mouth of Nipigon Strait, and represents the lowest part of a normally magnetized volcanic sequence that lies offshore, beneath the waters of Lake Superior.(2) The contact between the upper mafic unit and older rocks of the Osler Series is a stratigraphic rather than a faulted one. Aeromagnetic data show that mafic volcanics above and below this contact have the same strike, except toward the Thunder Bay and Schreiber regions. In these areas there is a convergence in strike between the two mafic units which may indicate an unconformity between them or a thinning of flows in the older unit. Ultimately the older, reversely magnetized unit appears to pinch out beneath the younger one to both the east and west.(3) The younger volcanics and overlying sediments occur in a large basin centered about 30 mi (50 km) WSW of the Slate Islands. Whether the older, reversely magnetized volcanics accumulated in this basin or in a smaller more northerly one is not entirely clear. The eastern margin of the main basin appears to be formed by a basement ridge extending from the Slate Islands southward to Superior Shoal, over which volcanics and younger sediments are relatively thin.

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