Bulletin No. 36. The Geology of the Cuyuna District, Minnesota

Abstract

This report emphasizes initially the subdivision of the Cuyuna district into a North and a South Range, the former containing iron-bearing rocks comparable with those of the Mesabi district and its Michigan and Wisconsin equivalents and the latter being the equivalent of the younger Michigan iron-formations. The manganiferous iron ores produced on the Cuyuna Range, in east- central Minnesota, have been much desired for use in iron furnaces. The geology of the ore formations and their correlations with ore formations in other districts have been subjects of considerable disagreement, largely because the iron formations on the Cuyuna lie under 50 to 300 feet of glacial deposits. J. F. Wolff, Sr. has a lifelong familiarity with the rocks of the nearby Mesabi Range, and recognizes divisions of the iron formation into four members. He has also had years of work on the Cuyuna area and recog-nizes the same four members, with analogous subdivisions inside the members, and similar sequences of other formations above and below the iron formation. Other men have not wholly agreed on the sequence because of the scarcity of exposures, and the lack of drill cores and records over much of the area. There are also geologists who question the correlation of Mesabi and Cuyuna series, because the "South Range" iron ores on the Cuyuna lie some thousands of feet up in the slates above the main iron-bearing beds of the North Range; and no such high beds of ore have been found in the slates thousands of feet above the Mesabi ore horizons. Only a few hundred to a thousand or more feet of the thickness of the slates overlying the Mesabi iron formation have been penetrated by drills in the Mesabi district, and these were not high enough to encounter the possible South Cuyuna member. This report presents the maps and sections prepared by Wolff, and his interpretation of the sequence, and a comparison with other districts. Some drilling planned by Grout to check the underground sequence of beds was generously supported by funds allotted by the Legislature to the University for research. These two studies of detail are here reported, with scattered data from outlying areas, and suggestions of correlations with the more remote iron ore districts south of Lake Superior. The possible use of the lean manganiferous iron formation of the Cuyuna Range as an emergency resource of manganese, should foreign supplies on which we normally depend be cut off during wartime, is here recommended for further research.