Geological Survey Professional Paper 754—C The Portage Lake Volcanics (Middle Keweenawan) on Isle Royale, Michigan

HISTORICAL BACKGROUND

It has long been proposed that part of the rock sequence on the Keweenaw Peninsula extends beneath the Lake Superior basin 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 gross stratigraphic succession in both areas. As early as 1849, Charles T. Jackson (p. 474) stated that "this island has the same geological character as Keweenaw Point, and is of the same geological age." And in 1850, John W. Foster and Josiah D. Whitney (p. 81) noted that "in many respects, Isle Royale may be regarded as the counterpart of Keweenaw Point."

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

Although the reconnaissance surveys of Jackson, Foster, and Whitney led to the first suggestions of correlation between the stratigraphic sections of Isle Royale and the Keweenaw Peninsula, not until the investigations of Alfred C. Lane (1898; 1911) was a firm foundation provided for such correlations by the recognition of parallel sequences of the varied rock types. Lane (1898, p. 101) concluded that "on the whole the coincidence is fairly satisfactory, and warrants us in saying that we have represented on Isle Royale practically the whole of the Copper Range [Portage Lake Volcanics] as it exists from the Central Mine to Portage Lake," on the Keweenaw Peninsula. Since Lane's study, no additional data regarding the Portage Lake Volcanics on Isle Royale have been published.

SCOPE OF STUDY

The present report, a part of a general study of the geology of Isle Royale National Park being carried out in cooperation with the National Park Service, focuses on the stratigraphy and structure of the Portage Lake Volcanics on Isle Royale and presents additional data on correlations with the Keweenaw Peninsula and on relations to other volcanic sequences of Keweenawan age. Only compositional and petrologic data necessary to characterize the rocks are given, as the rocks are similar in all respects to rocks of the Portage Lake Volcanics on the Keweenaw Peninsula and details can be obtained from extensive studies of those rocks on the peninsula. An earlier companion report describes the overlying Copper Harbor Conglomerate on Isle Royale (Wolff and Huber, 1973). A geologic map of Isle Royale National Park at a scale of 1:62,500, published separately (Huber, 1973b), provides details of the distribution and structure of the Portage Lake Volcanics not possible to show at the scale of maps in the present report, and it is recommended that that map be used in conjunction with this report. In fact, some of the place names mentioned in this report can be found only on that geologic map or on the Topographic Map of Isle Royale National Park (U.S. Geological Survey, 1957).

ACKNOWLEDGMENTS

The generous support of the National Park Service staff made this study possible. Numerous discussions with Henry R. Cornwall and Walter S. White, drawing upon their extensive knowledge of the geology of the Keweenaw Peninsula, were very helpful. Field visits with John C. Green, Harold A. Hubbard, and Walter S. White, in their respective areas of Keweenawan study, have been of help in understanding the regional aspects of Keweenawan geology. During summers on the island, I was ably assisted, successively, by Robert J. Larson, Harrison T. Southworth, Charles E. Bartberger, and David R. Chivington.