Geological Survey Professional Paper 754—B The Copper Harbor Conglomerate (Middle Keweenawan) on Isle Royale, Michigan, and its Regional Implications

SCOPE OF STUDY

It has been long proposed (Lane, 1898, p. 56) that part of the Keweenawan Series of the Keweenaw Peninsula (Portage Lake Volcanics² and Copper Harbor Conglomerate) extends beneath the Lake Superior syncline and is exposed again on Isle Royale (fig. 1). The basis for this interpretation is the similarity of the volcanic and sedimentary rock types and of the stratigraphic succession in both areas. The present study supports this conclusion.

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²At the request of the Geologic Names Committee of the U.S. Geological Survey and in consultation with workers in the type area (W. S. white written commun., 1972), the formation name Portage Lake Volcanics is used in this report in place of Portage Lake Lava Series in order to bring the formation name into conformity with the Code of Stratigraphic Nomenclature (Am. Comm. on Strat. Nomenclature, 1979, p. 7, 14).

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FIGURE 1.—Generalized distribution of Keweenawan rocks in the Lake Superior region. Data from Green (1971), Halls (1966), Irving (1883), White (1966), and White and Wright (1960). (click on image for a PDF version)

The Copper Harbor Conglomerate has been studied by many workers in its type area on the Keweenaw Peninsula, and much data has accumulated regarding its compositional and textural variations, direction of sediment transport, and depositional environment. The only comprehensive geologic study of Isle Royale, that of Lane (1898), places greatest emphasis on the Portage Lake Volcanics because it is the host for native copper mineralization; the Copper Harbor Conglomerate was not studied in detail. Since Lane's study, virtually no additional data regarding the Copper Harbor Conglomerate on Isle Royale has been published except for a few measurements on directions of sediment transport (White and Wright, 1960; Hamblin and Horner, 1961).

This study focuses on the nature of the sedimentary rocks above the Portage Lake Volcanics on Isle Royale, provides some basis for their comparison with the Copper Harbor Conglomerate on the south shore of Lake Superior, and contributes to the understanding of Keweenawan sedimentation in the Lake Superior region.

ACKNOWLEDGMENTS

Wolff studied the Copper Harbor Conglomerate on Isle Royale under the guidance of Professor Campbell Craddock of the University of Wisconsin; he was assisted in the field by Roger Sacia. Wolff is responsible for essentially all of the paleocurrent measurements and data on composition in this report. Financial assistance was provided by the National Science Foundation and the Wisconsin Alumni Research Foundation.

Huber, with the cooperation of the National Park Service, is mapping geologically the entire island; while engaged in studies of the Copper Harbor Conglomerate, he was assisted by David Chivington. Huber's mapping of the Portage Lake Volcanics made possible determination of the approximate base of the Copper Harbor Conglomerate between Sisikiwit Bay and Cumberland Point. Discussions and field visits with John C. Green, Harold A. Hubbard, and Walter S. White, in their respective areas of Keweenawan study, have aided considerably in understanding the regional aspects of Keweenawan geology.