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

The middle Keweenawan (Precambrian) Portage Lake Volcanics, in its type area on the Keweenaw Peninsula, is made up of a sequence of several hundred lava flows with rhyolite conglomerate separating some of the flows. Interbedded sandstone and pyroclastic rocks are less abundant. Since the first significant geologic account of the native-copper deposits in the Portage Lake Volcanics by Houghton (1841), the formation has been intensively studied by many workers. Although the latest comprehensive report is that of Butler and Burbank (1929), much of the more recent literature is covered in a review of the Keweenawan geology of the Lake Superior region by Halls (1966) and in papers on the geology of the Michigan copper district by White (1968, 1971). These reports are the source of most of the material in the following summary.

The formation name Portage Lake Lava Series was introduced (White and others, 1953) as a comprehensive name to include rocks designated the Bohemian Range, Central Mine, Ashbed, and Eagle River Groups of earlier reports. (See Lane, 1911; Butler and Burbank, 1929, p. 17-18.) At 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 (American Commission on Stratigraphic Nomenclature, 1970, p. 7, 14). The formation is locally at least 15,000 feet thick on the Keweenaw Peninsula, but the base is nowhere exposed because the lower part of the sequence is bounded by a high-angle thrust fault, the Keweenaw fault. In westernmost Michigan, structural considerations suggest that the Portage Lake Volcanics unconformably overlies older Keweenawan volcanic rocks, which form the so-called South Trap Range (Hubbard, 1968). The Portage Lake Volcanics is conformably overlain by the Copper Harbor Conglomerate, which White (1972) proposed be reassigned from the upper to the middle Keweenawan. Locally the two formations are interfingered (fig. 2). The Copper Harbor Conglomerate is overlain by the upper Keweenawan Nonesuch Shale and Freda Sandstone of the Oronto Group.

FIGURE 2.—Nomenclature for middle and upper Keweenawan rocks of northern Michigan and adjacent parts of Wisconsin (from White, 1972). The unnamed volcanic formation occurs only in the Porcupine Mountains area (fig. 1).

The lava flows are predominantly basalt or basaltic andesite, containing essential calcic plagioclase (generally labradorite), augite, and minor olivine. Compositional details of the volcanic rocks have been summarized most completely by Broderick (1935) and by Cornwall (1951a; 1951b). About 40 percent of the mafic flows have ophitic texture, coarsest in the thickest flows. The remaining flows are quite fine grained but show textural differences that depend mostly upon feldspar grain size and distribution. The uppermost 5-20 percent of most individual lava flows is conspicuously amygdaloidal and contains 5-50 per cent vesicles filled with secondary minerals. The abundance of amygdules decreases downward toward the amygdule-free massive basalt of the middle and lower part of the flow. A thin amygdaloidal zone is generally present at the base. In many of the flow tops, the vesicular crust was brecciated, probably during flow, and now consists of rubbly or fragmental material in which both amygdules within fragments and interstices between fragments are filled with secondary minerals. Many of the thicker flows have undergone magmatic differentiation, with the development of an internal zone having layers characterized by a pegmatitic texture; the Greenstone Flow is a prime example. Rhyolite flows or domes are present locally but are uncommon.

The interbedded conglomerates are thoroughly lithified and normally consist of subangular to rounded pebbles, cobbles, or boulders of rhyolite and subordinate basalt in a sand matrix. Locally, granophyre pebbles are abundant. Individual beds of conglomerate or sandstone range from less than 1 inch to more than 100 feet in thickness and tend to be lenticular. Some individual lava flows and most of the major interbedded conglomerate units can be traced for many miles along strike. They thus provide useful stratigraphic marker horizons, and many have been named (and (or) numbered) for ease of reference (fig. 3).

FIGURE 3.—Longitudinal stratigraphic section of the Portage Lake Volcanics on the Keweenaw Peninsula. Conglomerate beds are numbered according to local usage (from White, 1968). (click on image for a PDF version)

Radiometric age determinations by the uranium-lead method suggest that Keweenawan igneous activity falls within the range of 1,120-1,140 million years ago (Silver and Green, 1972). The Portage Lake Volcanics thus clearly is within a newly defined division of the Precambrian, Precambrian Y (James, 1972), which has an age range of 800-1,600 million years.